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Full text of "Cement, concrete and bricks"

CEMENT, CONCRETE AND BRICKS 



OUTLINES OF INDUSTRIAL 
CHEMISTRY. 

A SERIES OF TEXT-BOOKS INTRO- 
DUCTORY TO THE CHEMISTRY 
OF THE NATIONAL 
INDUSTRIES. 

EDITED BY 
GUY D. BENGOUGH, M.A., D.Sc. 




Disintegrator for Coarse Grinding. 




Mill for Fine Grinding. (Gebr. Pfeiffer. 



Frontispiece.] 



OUTLINES OF INDUSTRIAL CHEMISTRY. 

CEMENT, CONCRETE 
AND BRICKS 



ALFRED B. SEARLE 

LECTURER ON BRICKMAKING UNDER THE CANTOR BEQUEST ; CONSULTING EXPERT 
IN THE CEMENT AND CLAY PRODUCTS INDUSTRIES 

AUTHOR OF "BRITISH CLAYS, SHALES AND SANDS," "THE NATURAL HISTORY OF CLAY, 

"MODERN BRICKMAKING," "THE CLAYWORKER'S HANDBOOK"; 

JOINT AUTHOR OF " REINFORCED CONCRETE," ETC. 




NEW YORK 

D. VAN NOSTRAND CO. 

TWENTY-FIVE PARK PLACE 
1914 



\ 



PREFACE 

OF all building materials which are not found in a state of 
nature, the most important are, unquestionably, cement, 
concrete and bricks. The first of these includes a large variety 
of materials used to bind together particles of stone, sand, and 
other naturally occurring materials ; the second used in a 
broad sense includes all kinds of artificial stones made by 
cementing various materials together without the aid of heat, 
and the last bricks includes an even larger number of 
different articles, distinguished by their general form and by 
the fact that heat has been used to render their shape per- 
manent. It is a mistake to suppose that all bricks are made 
of clay ; at the present time they are made from a number of 
other materials, such as destructor refuse, sand, slag, etc. 
Indeed, the composition of some bricks so closely resembles 
that of concrete as to render it necessary to include them as 
one of the forms taken by this material. 

It is important, in considering the chemical and other 
properties of these three typical building materials, to observe 
the genetic relationships between them. To neglect this is to 
enter upon a course of study which is exceptionally difficult, 
and to follow a pathway of thought along which many men 
have lost their way. So long as cement and concrete are 
considered as having nothing in common with bricks, and vice 
versa, it is almost impossible to understand the constitution 
of any of these materials. Separately, they lead to no 
important conclusions, but considered together they throw a 
light on each other's characteristics which is as important as 
it is unexpected. 

Until a few years ago the brick industry of this country had 
no men of sufficient scientific training to study adequately the 
constitution of the materials used or the chemical and physical 
changes which occur during manufacture. Consequently, the 



293076 



viii PREFACE 

industry was largely worked by " rule of thumb," and men had 
often to pay very dearly for their experience, simply because 
there was no source from which to obtain guidance on the 
complex technical problems associated with their work. 

The manufacturers of cement have been more fortunate, for 
they realised at an early stage that success or failure depended 
largely on maintaining a mixture of constant chemical com- 
position ; they found that tests were necessary at so many 
stages in the manufacture that the employment of several 
chemists became essential. With this scientific assistance the 
chemical and physical laws affecting the production of cement 
were studied with very gratifying results, and though much 
remains to be done, the importance of a knowledge of chemistry 
in the manufacture of cement has been fully established. 

Concrete is in an intermediate stage so far as the application 
of science to its production is concerned. It does not appear 
to lend itself to such definite study as cement, on the one hand, 
or the changes produced by heat in brick materials on the 
other, yet the physical properties of concrete, and the light it 
throws on many of the problems met with in the manufacture 
of bricks, are sufficiently important to render its study 
invaluable, whilst the increasing use of concrete by engineers 
and builders renders a thorough knowledge of its charac- 
teristics, under various circumstances, quite essential. 

With these facts in mind the purpose of the present volume 
is to show the intimate relationships which exist between 
cement and bricks (with concrete as a relative of both), and at 
the same time to indicate some of the directions in which a 
further study of any of these materials will be likely to prove 
of value. In carrying out this work it may be necessary, at 
times, to question statements generally regarded as facts, and 
to show that some of these are erroneous ; at the same time, 
the position taken by the author is one of progressive con- 
servatism, in which the old ideas are retained as long as is 
reasonably possible, and are only abandoned where the evidence 
against them is conclusive. 

For the use of illustrations the author is indebted to the 
various firms whose names are mentioned in connection with 
the systems of reinforcement described, to Messrs. J. Whitehead 



PREFACE ix 

& Co., Ltd., of Preston ; Whittaker & Co., Ltd., of Accrington ; 
Sutcliffe, Speakman & Co., Ltd., of Leigh; Geb. Pfeiffer, of 
Kaiserslautern ; the Associated Portland Cement Manufac- 
turers, Ltd., the British Engineering Standards Committee, and 
to numerous other writers and firms duly acknowledged in the 
text. For much assistance in writing, and also in revising the 
proofs, the author is also indebted to Mr. J. W. Merchant and 
other members of his staff. 

ALFRED B. SEARLE. 

THE WHITE BUILDING, 
- SHEFFIELD. 

March, 1913. 



XI 



CONTENTS 



CHAP. PAGE 

PREFACE vii 

I. THE RAW MATERIALS FOR CEMENTS 1 

II. METHODS OF CEMENT MANUFACTURE .... 20 

III. THE CHEMICAL AND PHYSICAL CHANGES IN CEMENTS . 39 

IV. THE CHANGES WHICH OCCUR IN SETTING AND 

HARDENING 81 

V. TESTING THE PROPERTIES OF CEMENTS .... 96 

VI. THE COMPONENTS OF CONCRETE AND THEIR PROPERTIES 146 
VII. THE PREPARATION OF CONCRETE . . . . .162 

VIII. REINFORCED CONCRETE 206 

IX. SPECIAL PROPERTIES OF CONCRETE . . . . 254 

X. TESTING CONCRETE . .277 

XI. THE RAW MATERIALS FOR BRICKS 236 

XII. METHODS OF BRICKMAKING . . . . . . 319 

XIII. THE CHEMICAL AND OTHER CHANGES IN DRYING AND 

BURNING BRICKS ... , 363 

XIV. THE PROPERTIES OF BRICKS 376 

XV. SILICEOUS BRICKS 395 

XVI. BASIC AND NEUTRAL BRICKS 401 

INDEX . 403 



CEMENT, CONCRETE AND 

BRICKS 



CHAPTER I 

THE RAW MATERIALS FOR CEMENTS 

IN the present volume, the term cement is used with reference 
to those materials which are employed to effect adhesion 
between stones, sand and other aggregates used in the con- 
struction of buildings, reservoirs, etc. Cements of organic 
origin, such as fish glue, seccotine, etc., are purposely excluded, 
together with those cements (Keene's cement, Parian cement, 
etc.) which have gypsum as their chief constituent, and the 
cements composed of oxy-chlorides and oxy-phosphates 
(Porcelain cements). 

In the limited sense in which the term cement is used 
in the following pages, it refers to a variety of substances 
composed essentially of clay and lime, and includes Portland 
cement, Roman cement, trass, hydraulic lime, pozzolana, etc. 
With the exception of the first-named, all these cements are 
made from naturally occurring materials without any ad- 
mixture, and no serious attempt is made to ensure their having 
any fixed composition or more than roughly uniform properties. 
Portland cement, on the contrary, is made by carefully mixing 
clay with limestone or chalk, or materials corresponding to 
these substances, such as marls and loams, in definite propor- 
tions so as to secure a product of perfectly uniform composition 
and properties, and special care is taken by analysis and other 
methods of testing to ensure this uniformity in the product. 
This is one reason why the manufacture of Portland cement 
now exceeds in importance that of all other hydraulic cements : 
the product is uniform instead of variable in its properties, 
it is greatly superior because the defects due to unsuitable 

c. B 



2 THE BAW MATERIALS FOR CEMENTS 

composition are removed, and, being made from materials 
which occur in extremely large quantities in several important 
industrial areas, its use enables an artificial product of known 
composition and properties to be used in place of an inferior 
cement made from natural materials, which can never be so 
reliable. 

Cements of the kinds herein considered are conveniently 
termed hydraulic cements, as their hardness, when set, is not 
appreciably affected by immersion in water. Zulkowsky has 
suggested the term hydraulite (with the corresponding adjective, 
hydraulic) as convenient for all cements of this class. 

There are no naturally occurring cements, though some 
metamorphosed volcanic lavas may be regarded as feeble 
cements. Consequently, cements are all prepared by various 
methods of treatment. 

The raw materials used in the manufacture of cements are 
clays, chalk, limestone, volcanic tuifs and lavas, various muds 
more or less related to these substances and sundry by-products 
of other industries, as slags. These raw materials may be 
classified as follows : 

(a) Those which require to be mixed with one or more 
substances and then heated and ground before a cement is 
obtained (clays and limestones). 

(b) Those which merely require to be heated and ground 
in order to form a cement (natural cements and hydraulic limes). 

(c) Those which require no heating, but must be mixed with 
a complementary substance, such as lime, before a cement is 
produced (pozzolanas) . 

(d) Those which merely require to be ground (slags). 

(a) The Raw Materials o/ Portland Cements. 
The raw materials used in the manufacture of Portland 
cement are essentially lime and clay a compound of these 
two substances, in which the lime plays the part of a base 
and the clay that of an acid, being formed when they are 
heated together to a sufficiently high temperature. As 
combination does not take place until a temperature is reached 
which is higher than that at which calcium carbonate is 
converted into quicklime, it is customary to use some naturally 



CHALK 



occurring form of calcium carbonate, such as limestone or 
chalk, in place of lime itself. 

The lime compounds used in the manufacture of hydraulic 
cements furnish the basic portion of the product. It is essential 
that they shall produce quicklime at a temperature lower than 
that at which the cement-clinker is produced, and for this 
reason only the naturally occurring calcium carbonates are 
employed. 

LIME COMPOUNDS. 

The chief lime compounds used in the manufacture of 
Portland cement are as follows : 

Chalk is a soft limestone, and is composed mainly of minute 
sea-shells, but these have been so damaged by earth-move- 
ments, etc., that it is only occasionally that their definite 
structure can be recognised. As found in the south-eastern 
counties, chalk is a fairly pure form of calcium carbonate, 
which occurs as white, compact masses which are readily 
crushed to powder. Flints and other stones and impurities 
occur in the chalk, and these should be removed by stirring 
the chalk with water in a wash-mill, the creamy white fluid 
so obtained being run off and the impurities remaining behind 
in the mill. 

Chalk is largely used in Kent and Sussex, where it occurs, 
and considerable quantities of it are sent to Northumberland 
and Yorkshire, where it is used in connection with a local 
clay. 

ANALYSES OF CHALK (after drying at 110 C.). 





Medway 


Gravesend 


Purfleet 


Ilarefleld 


Kitchen 


Cambridge 




Upper 


Upper 


Upper 


Upper 


Middle 


Lower 




Chalk. 


Chalk. 


Chalk. 


Chalk. 


Chalk. 


Chalk. 


Silica .... 


2-34 


1-12 


1-22 


1-86 


0-69 


3-57 


Alumina 


1-84 


0-21 


0-58 


0-63 


0-57 


0-56 


Iron oxide 


1-49 


0-13 


0-11 


0-30 


0-37 


0-14 


Calcium carbonate . 


92-18 


97-88 


97-82 


97-01 


96-81 


91-94 


Magnesium carbonate 


1-16 


0-41 


0-25 


0-15 


1-42 


1-37 


Other substances 


0-99 


0-25 


0-02 


0-05 


0-14 


2-42 



Wherever chalk and a suitable clay are found in sufficiently 
close association, Portland cement may be manufactured. 

B2 



4 THE RAW MATERIALS FOR CEMENTS 

Where the two materials do not occur together, the nature of 
the demand for the cement will largely determine the most 
suitable site for the works. In the south-eastern counties the 
works are close to the chalk, and the clay is brought to them 
from some distance ; at Newcastle and Hull, on the contrary, 
the works are situated on the clay and the chalk is brought 
from the south-eastern counties. 

Limestone, like chalk, consists chiefly of calcium carbonate ; 
it has been coloured by trifling proportions of organic matter. 
It is much harder than chalk, and must be crushed before use. 
This makes its use more costly than that of chalk. 

The limestone chiefly used for cement manufacture is of 
Liassic origin, and occurs in the Midlands and South Wales 
in association with shale. Limestones which are free from 
shale or clay are inconvenient because of the cost of bringing 
the two essential materials together ; hence, argillaceous 
limestones are almost invariably selected. 

Some oolitic limestones are used, notably in Northamp- 
tonshire. 

Liassic limestones are characterised by great regularity in 
composition when sufficiently large quantities are examined ; 
smaller quantities show considerable variations in the different 
strata, numerous bands of clay or shale alternating with those 
of limestone. In some quarries thirty or more different beds^ 
may clearly be recognised. 

The following analyses are typical of the Lower Lias lime- 
stones used for cement making : 



ANALYSES OF LIAS LIMESTONES (after drying at 110 C.). 





Warwick- 
shire. 


Shropshire. 


South 
Wales. 


Northamp- 
tonshire. 


Silica .... 


13-25 


8-43 


1-28 


6-85 


Alumina 


5-72 


5-27 


0-51 


2-74 


Iron oxide 


1'97 


1-76 


1-42 


2-85 


Calcium carbonate . 


77-46 


81-82 


94-76 


86-12 


Magnesium carbonate 


1-35 


2-47 


1-62 


0-67 


Other substances 


0-25 


0*25 


0-41 


0-77 



LIME COMPOUNDS AND CLAYS 5 

Great care must be taken not to use dolomitic limestone 
which contains much magnesia, as this substance is deleterious 
to the resultant cement. 

Argillaceous limestones are the raw material from which 
hydraulic limes are made (p. 13). 

If the limestone is sufficiently free from clay or shale it 
will, when burned, produce quicklime. If the latter is slaked 
and then ground with sand and water it forms ordinary 
builders' mortar. The quality of such mortar depends on the 
purity of the limestone, satisfactory burning, and thorough 
mixing or tempering with sand. Many builders at the present 
time do not pay sufficient attention to these details and so 
obtain a mortar of inferior quality. 



CLAYS, SHALES, MARLS AND MUDS. 

The clays and similar materials used in the manufacture of 
Portland cement furnish the acid constituent which combines 
with the lime to form the final product. The essential con- 
stituent of all clays is aluminosilicic acid, but whether there 
is one or more varieties of this substance is, at present, 
uncertain. In the author's opinion there are many alumino- 
silicic acids which have so similar a chemical composition that 
some of them cannot readily be distinguished by analysis. 
Such compounds are known as isomers ; they differ greatly in 
many ways from each other, especially in physical properties. 
Many clays used in cement manufacture are probably isomeric, 
their apparent similarity in analysis being merely a coincidence. 
The study of isomers forms an important branch of organic 
chemistry ; but little attention has, as yet, been paid to complex 
inorganic isomeric substances. (See Chapter III.). 

Even if several distinct aluminosilicic acids should be 
found to be the essential constituents of various clays, their 
composition and properties must be very similar, as the 
product obtained by refining most, if not all clays has a com- 
position corresponding to the formula xH 2 . yAl^O^ . zSiO 2 . 
Thus, carefully washed china clay, which is the purest clay 
known, is composed of ; 



THE RAW MATERIALS FOR CEMENTS 

Alumina . . 39 '45 

Silica .... 46-64 

.Water 13' 91 



or HiSi 2 Al 2 O 9 . lOO'OO 

Together with the aluminosilicic acid, clays contain very 
variable proportions of impurities, the chief of which are free 
silica, iron oxide and sulphide, lime and magnesia compounds, 
together with various minerals such as mica, felspar and other 
siliceous rocks. 

The clays chiefly used for the manufacture of Portland 
cement are those commonly known as surface clays. Their 
composition is unimportant providing that they do not contain 
an excessive amount of coarse sand. Lean or slightly plastic 
clays are better than those of a more sticky character, as they 
are more easily treated ; the highly plastic clays are usually 
somewhat richer in aluminosilicic acid. 

It is only natural that those clays should chiefly be used 
which are situated, in convenient localities, and at the present 
time the gault clay of Sussex, a surface clay in Northumberland 
of apparently glacial origin, and an alluvial clay at Hull are 
the ones most extensively employed. In other localities more 
conveniently situated, clays are employed on a small scale. 
Thus, there is no reason, except lack of demand for cement in the 
localities where they occur, why the extensive deposits of Triassic 
clays should not be more largely used than is at present the case. 

The Gault clay occurs in the lower portion of the chalk 
formation, immediately above the Greensand or between the 
upper and lower Greensand. It is a very stiff, dark blue or 
black clay, which often becomes brown on weathering. It 
extends from Folkestone on the east to Eastbourne on the 
west, and also occurs in Norfolk, Cambridgeshire, Hertford- 
shire, Bedfordshire, Buckinghamshire, and to a small extent 
on the east coast of Yorkshire, near Filey. For cement 
making it is chiefly used in Sussex and Kent. It contains a 
considerable amount of calcium carbonate (chalk) occasionally 
as much as 33 per cent, and seldom less than 13 per cent. 
together with nodules of pyrites and calcium phosphate. 
These nodules should be removed by washing the clay. 



CLAYS, SHALES, MARLS AND MUDS 



The composition of gault clay varies considerably ; the 
following shows the composition of samples taken from the 
upper reaches of the Medway : 

Silica 44-23 

Alumina ..... 14*85 

Iron oxide . . . . . 5 '93 

Calcium carbonate . . . 26 '54 

Magnesium carbonate . . . 6 '06 

Alkalies 0'51 

Water, etc 1-88 

Alluvial clays are found on the sites of former lakes or river 
beds and near to the banks of existing rivers. They occur 
in various parts of the country, particularly in East Yorkshire, 
Lincolnshire, Cambridgeshire, Norfolk, Suffolk, Essex, Kent, 
Sussex, Somerset and Lancashire, and to an even larger extent 
in central Ireland. Small areas occur in other low-lying 
districts. These clays have usually been deposited under 
such conditions that they are relatively light and somewhat 
open in texture, though some of them are highly plastic. 
They are seldom free from appreciable proportions of very 
finely divided calcium carbonate, which is an advantage to 
the cement manufacturer, but is objectionable to some brick- 
makers. As the term " alluvial " implies, these clays have 
been collected from a variety of sources and their composition 
varies in each locality in which they occur. The chief alluvial 
clays used for cement making show the following composition 
on analysis : 

ALLUVIAL CLAYS AND MUDS (after drying at 110 C.). 





Hull. 


Thames Estuary Mud . 
a. b. 


Medway Mud. 


Silica .... 


51-32 


60-12 


73-68 


51-98 


Alumina 


20-76 


10-81 


7-46 


15-63 


Iron oxide 


6-89 


7-46 


11-13 


9-12 


Calcium carbonate . 


10-44 


8-31 


2-36 


4'08 


Magnesium carbonate 


7-81 


1-86 


1-61 


2-76 


Alkalies .... 


1-86 


0-62 


1-84 


0-92 


Water, etc. 


0-92 


10-82 


1-92 


15-51 



THE RAW MATERIALS FOR CEMENTS 



Alluvial clays are essentially superficial in character and 
may be distinguished from the other great class of surface clays 
(the boulder clays) by the absence of large stones in which 
the boulder clays abound. 

Loams are naturally occurring mixtures of sand and clay. 
They are of little use for cement manufacture on account of the 
large proportion of sand they contain, but clayey loams are 
occasionally used with advantage. Strictly speaking, any clay 
which contains free silica (sand) is a loam, but the term is 
only applied to those with a low plasticity and open texture. 

Shales are naturally indurated clays which have been com- 
pressed to almost stony hardness by the forces of nature, but 
the term is also applied to any rock which can be split into 
thin layers, so that some shales contain very little clay. 

Slate waste a variety of shale has been used in America 
for the manufacture of cement. 

' ANALYSES OF SHALES (after drying at 110 C.). 





Warwick- 
shire. 


Shropshire. 


South 
Wales. 


Northamp- 
tonshire. 


SilW . 




31-74 


8-07 


58-17 


8-41 


Alumina 


. ' . . ...) 9*23 


5-23 


14-34 


5-39 


Iron oxide 


3-14 


1-84 


5-11 


1-72 


Calcium carbonate . 


47-41 


80-17 


12-61 


77-29 


Magnesium 
Alkalies . 


carbonate 


7'35 
0-37 


3-73 


5-98 
2-47 


5'86 
0-97 


Water, etc. 


0'76 


0-96 


1-32 


0-36 



Clay shales occur in enormous quantities in Great Britain, 
particularly in Coal Measure, Permian, Jurassic and Wealden 
regions, and are of very diverse character and composition. 
Some shales are so closely interstratified with limestone that 
it is difficult to separate these substances, but this is not 
important where the material is to be used for cement manu- 
facture, providing that sufficient limestone is available. For 
cements, the sandy and bituminous shales are of little value. 
Shales are costly to grind, and so are only used for cement 
when found in close association with limestone, the Liassic beds 



CLAYS, SHALES, MARLS AND MUDS 9 

of the Midlands and especially of Warwickshire and the shales 
of South Wales being chiefly employed for this purpose. 

The chemical composition of shales varies at different 
depths in the deposit as well as in different areas, but if 
sufficiently large quantities are obtained at a time and a little 
care is taken to mix the strata in approximately constant 
proportions, a sufficiently uniform product may be obtained 
without difficulty. 

For brickmaking, the shales must be free from large percen- 
tages of lime and pyrites, but for cement making the lime is an 
advantage. The pyrites is objectionable in both cases on 
account of its discolouring power. The organic matter present 
in shales sometimes exceeding 5 per cent. is an advantage 
in so far as it saves fuel, but it tends to cause an overheating 
of the material in the kilns. 

Muds are essentially impure clays which have not been 
compacted by pressure. Those found in the estuaries of the 
Thames and Medway are largely used for cement manufacture 
on account of the readiness with which they can be mixed 
with washed chalk. Some muds consist almost exclusively of 
lime dust, and if these are to be used for cement, clay must 
be added to them (see Alluvial Clays, p. 7). 

The composition of the muds used varies greatly, and, like 
that of the clays previously mentioned, is of little importance 
providing that the muds contain sufficient real clay. On 
account of the ease with which they can be converted into a 
slurry, muds are generally mixed with chalk slurry in the 
manufacture of cement. 

Marls are naturally occurring mixtures of clay and calcium 
carbonate (chalk), but the term is also used in a misleading 
manner for friable rocks which are free from lime compounds. 
The chief marls used in the manufacture of Portland cement 
are friable limestones of fine texture and lacking the compact- 
ness and coherence of ordinary limestones. Cement makers 
specially favour those found in Cambridgeshire between the 
Chalk and the Greensand formations, but marls from other 
districts such as those in the centre of Ireland may be used 
with equal advantage when the demand for cement in their 
locality is sufficiently great, 



10 THE RAW MATERIALS FOR CEMENTS 

Marls have, apparently, been formed by the natural 
admixture of clayey and chalkey slimes ; streams carrying the 
finely divided clay or chalk in suspension meeting at some 
locality and depositing their burdens in the form of a mixture. 
There is, however, some considerable divergence of opinion as 
to their true origin, more especially as many marls contain 
more silica than corresponds to a mixture of clay and chalk. 
This additional silica may be in the free state (as appears most 
likely), but Le Chatelier's experiments lend support to the 
view that marls contain an aluminosilicic acid which is richer 
in silica than is that usually found in " clays." If the mode of 
formation indicated above is correct it is clear that marls in 
different localities must vary greatly in composition ; some of 
them being rich in clay and poor in lime, whilst others are so 
poor in clay as to be regarded as impure lime compounds. 
For cement manufacture, the most suitable marls are those 
which have a composition resembling that of raw Portland 
cement slurry. 

For Portland cement manufacture, the composition of the 
marl must be ascertained frequently, so that the proportions 
of the other ingredients of the " mix " may be accurately 
adjusted. For this reason, many cement manufacturers prefer 
to use a relatively pure chalk on which they can rely, rather 
than a marl with an irregular composition. 

It is very important that the marl should be fine in texture 
and free from hard lumps, for pieces the size of a pea will 
seriously increase the cost of manufacture, and if left in the 
material they are very liable to produce an irregular cement. 
Marls differ greatly in this respect and great care is, therefore, 
needed in their selection. 

Marls for cement manufacture differ from those used for 
brickmaking on account of the much larger percentage of 
lime permissible in the former. For brickmaking, marls with 
60 per cent, of calcium carbonate are almost useless, but for 
cement they are more suitable than those containing at most 
12 per cent, of chalk, which is the largest proportion acceptable 
to a brickmaker. In this way a material useless to one branch 
of clay- working is of great advantage to another branch. 

Pure marls are white in colour, but most marls are grey on 



CLAYS, SHALES, MARLS AND MUDS 



11 



account of the organic matter they contain. This burns away 
on heating, and is of no importance to manufacturers. 

Marls which burn red are seldom useful for cement manu- 
facture unless they are rich in clay and sufficient limestone or 
chalk is available in the same locality. 

Marls from the Permian formation are usually rich in 
magnesia, and should be avoided by cement manufacturers. 
Liassic marls, on the contrary, are valuable. 

The " Midland Marls" are not true marls (but Triassic 
clays), and do not contain enough lime to be so named. They 
are greatly prized for the manufacture of red bricks and terra 
cotta, but before they can be used for cement manufacture 
they must have nearly three times their weight of limestone 
or chalk added to them. The Cambridge marls, on the 
contrary, are argillaceous limestones with a composition so 
similar to that of unburned cement slurry that they are used 
for natural cements. 

ANALYSES OF MARLS USED FOR CEMENT 
(after drying at 110 C.). 





Cambridge. 


Petersfield. 


Silica 




18-16 


26-82 


Alumina 


. 


7'82 


2-14 


Iron oxide 


. 


0-94 


3-47 


Calcium carbonate 


68'16 


52-86 


Magnesium 


carbonate . 


2-77 


1-49 


Water, etc. 


. 


2-15 


13-22 



Broadly speaking, the clays used in cement manufacture 
should be highly siliceous, and should contain 60 to 70 per cent, 
of silica and 6 to 20 per cent, of alumina. Magnesia should 
not exceed 3 per cent., and the alumina and iron oxide together 
should not exceed half the silica. Clays very rich in iron 
should be avoided as they produce a dark coloured cement ; 
those with less than 10 per cent, of iron oxide are not objec- 
tionable in this respect. Also, clays containing stones should 
be avoided as they unduly increase the cost of grinding. 



12 THE RAW MATERIALS FOR CEMENTS 

All clays on heating to a dark red heat are decomposed, the 
aluminosilicic acid splitting up into free silica, alumina and 
water, or else forming a compound in which the silica and 
alumina are exceptionally able to combine with lime ; if the 
temperature and proportions of each of these substances are 
suitable a clinker is formed which, on grinding, produces 
Portland cement. The reactions which occur during the 
formation of this clinker are described in Chapter III. 

Further particulars of the occurrence of clays, shales and 
muds suitable for the manufacture of Portland cement* will 
be found in the author's " British Clays," published by Messrs. 
C. Griffin & Co., London, W.C. 

The proportions in which the clay or its equivalent and the 
lime compound are mixed to produce Portland cement depends 
on the precise composition of the actual materials selected. 
The mixture (before burning) should contain 75 to 77 per cent, 
of calcium carbonate and 23 to 25 per cent, of real clay, the 
best proportions being found by trial and adhered to as closely 
as possible in the course of manufacture. It is clear that if 
the " clay " contains lime compounds (like the marls), less 
limestone or chalk will be needed than when a lime-free clay 
or shale is used. The great value of Portland cement lies in 
the accuracy with which the materials are mixed so as to obtain 
a product the composition of which is exceedingly uniform. 
This uniformity can only be secured by paying the closest 
attention to the composition of the mixture before it is burned, 
and, in all cement works of importance, analyses of the mixture 
are made daily, or even more frequently. 

(b) The Raw Materials used in the Manufacture of 
Natural Cements and Hydraulic Ijimes. 

The materials used for the production of natural cements, 
chiefly clayey limestones or highly calcareous marls, are 
characterised by containing calcium carbonate (chalk) and 
aluminosilicic acid (clay) in proportions similar to those in 
Portland cement. Those usually employed in this country 
are in the form of calcareous nodules or septaria 1 which are 

1 Septaria are nodules of impure limestone which occur in some clays and derive 
their name from characteristic divisional lines (septae). These lines appear to be 



NATURAL CEMENTS 



13 



dredged from the sea-bottom near Harwich, Sheppey, etc., or 
are obtained from dry land near the coast, as was formerly 
the case at Speeton in Yorkshire. Calcareous nodules from 
beds of Kimeridge, Greensand and Liassic formations, and 
some of the Cambridgeshire marls (p. 9) are also used for 
this purpose. 

ANALYSES OF NATURAL CEMENT NODULES. 





Sheppey 
Septaria. 


Harwich 
Septaria. 


Speeton 
Nodules. 


Tournai 
Earth, i 


Silica .... 


17'84 


20-74 


20-43 


2128 


Alumina 


6'42 


4-21 


8-81 


35 


Iron oxide 


4-13 


7'85 


6-87 


1-21 


Calcium carbonate . 


63-76 


58-79 


56-36 


6063 


Magnesium carbonate 


4-37 


5-46 


3-42 


i-i i 


Water, etc. 


3*48 


2-95 


4'11 





, Where a bed of rocky material is of suitable composition 
(i.e., with a composition similar to that of the mixture of clay 
and chalk used for Portland cement) to be calcined without 
any admixture, the product is termed rock cement. Large 
quantities of this kind of cement are produced annually in 
America, but the troubles caused by irregularities in com- 
position have made many manufacturers use this material 
as the basis for Portland cement, the composition being 
adjusted from time to time by the addition of clay or limestone 
as may be required. 

As the materials are used without any purification, rock 
and natural cements usually suffer from the presence of an 
excess of iron oxide, which colours them dark brown, and of 
inconvenient proportions of magnesia, etc. 

HYDRAULIC LIMES. 

The properties of hydraulic lime were first recognised by 
John Smeaton in 1760, who, when investigating the properties 

shrinkage cracks, which have become filled with crystallised calcium carbonate. 
The use of septaria for cement manufacture was first patented by James Parker 
in 1796 ; it has since been known as Roman cement. 

1 The material of which Belgian cement (p. 31) is made. 



14 THE RAW MATERIALS FOR CEMENTS 

of different limestones for use in ths Eddystone lighthouse, 
found that the Aberthaw limestone and others rich in clay were 
more resistant to immersion in water than purer limestones. 
A hydraulic lime is one prepared by calcining an argillaceous 
limestone, the clay present entering into combination with a 
portion of the lime and forming what may be regarded as a 
mixture of Portland cement and quicklime. Its value as a 
hydraulite must, therefore, depend on the extent to which 
the lime and clay have combined. 

Hydraulic limes differ from ordinary lime in slaking Jess 
readily and in setting to a hard stony mass when immersed 
in water. 

The chief raw material in this country for hydraulic limes is 
the blue Liassic limestone of Warwickshire, South Wales, etc. 
(p. 4), but other argillaceous limestones may be used. As 
will be understood from the previous section on the raw 
materials used for Portland cement, a much superior cement 
is obtained when the composition of the material is adjusted 
so as to give a product of approximately the same composition 
as Portland cement. 

The composition of the limestones used for making hydraulic 
limes must lie between (1) pure limes which are free from clay, 
and (2) marls or mixtures of clay and chalk which contain no 
excess of lime. It has been found that argillaceous limestones 
with 70 to 80 per cent, of calcium carbonate, 10 to 17 per cent, 
of silica and not more than 3 per cent, of iron and alumina are 
best, as, in the hydraulic limes made from these, most of the clay 
is combined with lime, yet there is sufficient free lime present 
to cause the material to slake satisfactorily. Hydraulic limes 
may also be produced by under-burning a rock which would, 
at a higher temperature, produce an excellent natural cement, 
but these are very inferior and unsatisfactory. 

(c) Volcanic Lavas, Tuffs and Trass. 

Both the Greeks and Romans were aware that the addition 
of certain materials of volcanic origin, in a finely ground 
condition, to mortar had the effect of making it hydraulic. 
Such lavas and tuffs have already been heated previous to 



LAVAS, TUFFS AND TRASS 15 

their discharge from the volcano, and when mixed with lime 
they form pozzolanic cement or pozzolana, from the occurrence 
near Pozzoli, in the Bay of Naples, of the most typical material 
of this kind. 

Pozzolana is chiefly obtained in Italy, in south-eastern 
France, and in the Azores. It is usually found near the 
siirface, but some of the Italian workings are several hundred 
feet in depth. 

All the natural pozzolanas are volcanic lavas which have 
undergone subsequent changes (now attributed to the action 
of superheated steam and carbon dioxide), whereby they have 
been reduced to a fine sand and have gained hydraulic proper- 
ties. Their composition is very variable, but their essential 
constituents are the same as those of clays which have been 
heated to dull redness. Their value depends on the amount 
of silica and alumina (possibly also ferric oxide) present in a 
form in which it can produce hydraulitic compounds when 
the pozzolana is mixed with lime and water. Though generally 
amorphous, pozzolanas not infrequently contain crystals of 
various igneous rocks. 

Artificial pozzolanas have been made since Roman times by 
heating clays to redness or, more frequently, by crushing 
burnt clay ballast, broken bricks, or tiles to a fine powder. 
Broken pottery (potsherds) is occasionally used, but these 
should be of porous material, not of vitrified stoneware or 
porcelain . 

The completeness with which brick or tile dust can be used 
to replace natural pozzolana is strongly confirmative of the 
view, already expressed, that natural pozzolanas and trass 
have the same composition and general properties as calcined 
clay. 

Tosca is a pozzolana or volcanic ash obtained from Teneriffe, 
in the Canary Islands, and chiefly used in Spain. 

Trass is also a metamorphosed volcanic lava, the most 
important deposits being found near the Rhine. It resembles 
pozzolana in many ways, and has a similar composition, but is 
quite distinct from it. 

Santorin earth is another similar material obtained from the 
Greek island of that name. It is usually slightly more siliceous 



16 THE RAW MATERIALS FOR CEMENTS 

and contains rather less alumina than Rhenish trass or true 
pozzolana earth. 

(d) Slags 

are really glasses and are in the state of an extremely viscous 
fluid, the rigidity of which is apparently equal to that of a 
solid, though it is devoid of any crystalline structure. All 
glasses are in a state of instability and tend to crystallise just- 
as a slowly cooling liquid does, only far more slowly. The 
conversion into the more stable crystalline form is hindered 
by the enormous viscosity of the material ; if this is reduced 
(as by heating the substance to below its melting point), 
crystallisation proceeds more rapidly. 

It is extremely difficult to ascertain what compounds actually 
exist in granulated slags, as prolonged heating may cause a 
molecular learrangement of their constituents, whereby the 
crystals so formed would not represent the composition of 
the slag out of which they grew. In any case, the crystals 
found in granulated slags lack the characteristic forms of the 
calcium orthosilicate they most closely resemble (gehlenite 
and melilite), and their identity has not been satisfactorily 
established. The commonly accepted view is that granulated 
basic slags contain a-dicalcium orthosilicate, but this is based 
on evidence which is by no means conclusive. 

W. and D. Asch have assigned to such slags formulae bearing 
a resemblance to those used by them for clays and cements. 

Some slags obtained from blast furnaces as a by-product in 
the manufacture of pig iron, contain the same constituent oxides 
as Portland cement, but not necessarily in the same propor- 
tions. In a well-managed ironworks the composition of such 
slags is very constant, and, as they are of small value and 
require a large amount of storage space, many attempts have 
been made to convert them into Portland cement or similar 
hydraulites. As blast-furnace slags are very hard and difficult 
to grind they are run direct from the furnace into water and 
so become reduced to a coarse powder or granulated. 

Blast furnace slags may be divided into three classes : 

(a) Slags of such a composition that when granulated and 



SLAGS 17 

ground they form a kind of Portland cement, best termed 
slag cement, as it contains less lime and more alumina than 
true Portland cement. 

(b) Slags which, when granulated and ground, form a 
pozzolanic material, i.e., a material which must be mixed with 
lime before it becomes hydraulic. 

(c) Slags of an acid character which must be mixed with 
limestone and calcined. Such slags simply replace the clay 
ordinarily used in the manufacture of Portland cement. 

Although it is conceivable that cements may be produced 
from basic blast-furnace slags with a composition and properties 
closely resembling those of true Portland cement, it is difficult 
to do this at a sufficiently low cost to make the manufacture 
commercially profitable in countries where suitable clays are 
available. In the United States this method is largely used 
as a means of getting rid of the enormous quantities of slag 
produced in the large ironworks, the charges for tipping and 
storing on land being thereby avoided. The slag is very 
cheap though the grinding is costly, and the fact that it is 
a by-product prevents its composition being materially altered 
in the furnace. The addition of more limestone to the contents 
of the furnace will usually result in the production of a mass 
which is too viscous to be run off, it being seldom possible to 
produce a fluid slag containing more than 50 per cent, of lime, 
whilst a good Portland cement contains more than 60 per cent, 
of this base. 

The production of a pozzolanic slag is much easier than that 
of a Portland cement from the same slag, and large quantities 
of such are now made. To obtain satisfactory results, the 
slags must have a composition within somewhat narrow limits, 
such as the following : silica 30 to 36 per cent., alumina and 
iron oxide 12 to 17 per cent., lime 48 to 50 per cent., and 
magnesia under 3 per cent. Like all pozzolanas, such slags 
only form cements when ground with lime and water. 

The utilisation of blast-furnace slag in cement manufacture 
has attracted much attention for a long time, but the difficulties 
in handling the material are so great that much remains to be 
done. At present, only basic slags can be used for the purpose, 
and the methods of granulation and of reduction to powder 

c. c 



18 THE RAW MATERIALS FOR CEMENTS 

present difficulties almost as great as do the variations in the 
composition of the slags. 

(e) Sundry Raw Materials used in the Manufacture 
of Cements. 

IRON ORE. 

Iron ore has been largely used in the manufacture of a kind 
of Portland cement used in the Panama Canal and for various 
marine works. The iron ore is used in place of clay, and the 
process of manufacture is precisely the same as that used for 
producing Portland cement from shale and limestone. The 
cement produced has a composition closely resembling that of 
Portland cement in which the alumina has been replaced by 
iron oxide. W. Michaelis has published the following analysis of 

IRON-ORE CEMENT : 

Silica 23-26 

Alumina ..... 1-67 

Iron oxide . . . . . 8-20 

Lime 64-84 

Magnesia . . . . . 0-66 

Sulphur trioxide (S0 3 ) . . . 1-08 



99-71 
ALKALI-WASTE. 

For many years the waste produced by alkali works was 
unusable ; it consists chiefly of calcium carbonate with a 
considerable proportion of sulphides. Now that the sulphur 
can be removed, the purified " waste " in the form of a fine 
slurry is admirably adapted for the manufacture of Portland 
cement. For this purpose it is " blunged " in a wash-mill 
with a suitable quantity of clay and water, and the mixed 
slurry is run off to a dryer. Alkali-waste containing a large 
percentage of calcium sulphate is unsuitable for cement 
manufacture. 

The Le Blanc process waste is inferior to ammonia process 
waste for the manufacture of Portland cement. 



ALKALI-WASTE 19 

The following is an analysis of alkali-waste : 

Silica 1-98 

Alumina . . . . . 1-41 

Iron oxide ..... 1-38 

Calcium carbonate . . . 86-27 

Magnesia . . . . 2-84 

Sulphuric acid . . . 1-26 

Potash and soda . . . . 0-65 

Water, etc. .... 4-21 

If sufficient care is taken in freeing the waste from sulphur 
compounds, in adding the correct proportion of clay, and in 
securing a finely-ground and well-burned clinker, the .use of 
alkali-waste is highly satisfactory. Unfortunately, the waste, 
after purification, is often more costly than if limestone is 
used. 



c 2 



CHAPTER II 

METHODS OF CEMENT MANUFACTURE 

HYDRAULIC cements are manufactured by a variety of 
processes in which the underlying principles are practically 
identical ; they consist of (a) the preparation of a suitable 
and uniform material, (b) heating it to a suitable temperature, 
and (c) reducing the calcined product (clinker) to the state 
of a very fine powder. In some cases (as in natural and rock 
cements) only one material is used, and a large part of the 
first stage of manufacture may be omitted ; in others (as the 
pozzolanas) the material is obtained after natural calcination 
by volcanic action and the second stage may be omitted : but 
in spite of these modifications the general principles apply in 
all cases. 

PORTLAND CEMENT. 

Portland cements are chiefly made by mixing some form of 
limestone, chalk or other lime-compound with some form of 
aluminosilicic acid (clay), the materials used being chosen for 
their cheapness, accessibility and general convenience. The 
most important of these materials have already been described 
(pp. 3, 5, et seq.). 

The development of machinery used in the cement industry 
has brought about so great an improvement in the preparations 
of the raw materials that it is no longer difficult to produce a 
good Portland cement from any suitable materials. The 
selection of the best method of working is, consequently, much 
easier now than formerly, though the difficulties experienced 
in some cases are made far more serious in consequence of the 
stress of competition. A scheme of general applicability to 
be employed by all firms can never be given, because in each 
case local conditions must receive proper consideration. 

According as the raw materials are of a soft and open, or 



MANUFACTURE OF PORTLAND CEMENT 21 

hard and compact nature, two entirely different methods of 
manufacture are employed, and are respectively known as the 
" wet " process and the " dry " process. 

In the " wet " process, which is the older of the two, the 
materials must be sufficiently soft to form a cream or slurry 
when vigorously stirred with water. Hard, rocky materials 
cannot, therefore, be treated in this manner, and the wet 
process is largely confined to the use of chalk with clays, marls or 
muds. These are thrown, in suitable proportions, into a wash- 
mill a circular brickwork tank about fourteen feet diameter, 
built below the surface of the ground, which is fitted with 
rotating arms or harrows so arranged that they completely 
sweep every part of the mill and thoroughly stir and mix the 
contents. Sufficient water is added and the harrows rotated 
by steam or electrical power, whereby the chalk and clay are 
rapidly churned into a slurry. Some wash-mills are arranged 
to work continuously, the solid materials and water being 
added at regular intervals and the slurry flowing away in a 
continuous stream. Other mills are charged, set in motion 
so as to convert the whole material into a slurry, and then the 
harrows are stopped and the slurry is run off through a grating. 
The intermittent system of working is slower, but wastes less 
material and is considered to produce a more uniform mixture. 

In either case the flints, stones and other coarse impurities 
in the raw materials sink to the bottom of the mill, and are 
either removed continuously by means of a bucket elevator, 
or they are removed from the mill after the slurry has been 
run off. 

In order that the output of the wash-mill may be as large 
as possible it is customary to break up lumps of chalk more 
than three inches diameter with small picks or hammers or 
to pass the chalk over a very coarse screen, the rejected portion 
being crushed by rollers and again passed on to the screen. 
To keep the slurry passing out of the mill free from coarse 
material a fine grid or gauze is placed in the outlet pipe. In 
some works catch pits for any coarse sand in the slurry are also 
used, the slurry flowing through these as soon as it has left 
the mill. Some firms also grind their slurry in a tube mill in 
order to ensure the complete absence of coarse particles. The 



22 METHODS OF CEMENT MANUFACTURE 

slurry is next run into settling tanks or " backs " where the 
solid portion settles and the clear watei is run off. As soon 
as the material in the " backs " is fairly stiff and has attained 
the consistency of butter, it is taken to the mixer and then 
to the drying floors. 

It is of the greatest importance that the mixture should be 
quite uniform in composition, as small variations will, in some 
cases, create serious differences in the product. Its fineness 
and its chalk-content must therefore be tested several times 
daily. 

As the settling of the material in the " backs " is a slow 





FIG. 1. Johnson's Kiln. 

process and tends to cause a separation of the various ingre- 
dients, whilst most of the water remaining in the paste must 
be all dried out before the material can be sent to the kilns, 
many attempts have been made to reduce the amount of water 
used whilst still obtaining a material so fine that only 4 per 
cent, will remain on a sieve with 180 holes per linear inch. 
The process which has proved most successful is based on a 
suggestion of Goreham, who recommended the addition of 
only about 15 per cent, of water to the materials, which are 
therefore converted into a much thicker slurry. The wash- 
mill is the same as that previously described, but a grating 
with | -inch openings is used instead of the fine sieve, and the 
slurry is ground between mill stones or in a tube-mill consisting 



MANUFACTURE OF PORTLAND CEMENT 23 



of a cylinder rotating about its longitudinal axis and containing 
heavy steel balls. To ensure the absence of coarse particles 
of flint, etc., the slurry is then sieved before being passed to 
the mixing tanks and thence to the rotary kilns. 

Where one material is much harder than the other it is 
treated separ- 
ately, and the 
slurries from each 
mill are mixed 
in suitable pro- 
portions. This 
arrangement se- 
cures a larger 
output of a more 
uniform character, 
as if the clay is 
washed separately 
from the chalk 
the proportions 
of each may be 
more accurately 
gauged. The 
grinding of the 
mixed slurry in 
a special mill has 
only been prac- 
tised in recent 
years. It in- 
creases the cost 
of manufacture, 
but gives so 
superior a product 
that it should seldom, if ever, be omitted. 

The paste produced from the slurry must be dried to a 
solid mass if stationary kilns are used, as the material must be 
sufficiently firm to resist the crushing action of the material 
above it. Some firms press the paste into bricks and dry 
these, but the usual practice has been to spread the paste on 
a level floor heated by waste gases from the kilns (Fig. 1). 




FIG. 2. Modern Shaft Kiln. 



24 METHODS OF CEMENT MANUFACTURE 



Owing to the shrinkage it undergoes on drying, the material 
cracks into pieces of a convenient size and these are placed in 
* the kiln. Two forms 

of stationary kiln are 
used the shaft kiln 
(Fig. 2), of which there 
are numerous patterns, 
differing from each 
other chiefly in the 
arrangements provided 
for preventing the waste 
of heat. A typical 
example is Johnson's 
kiln (Fig. 1), in which 
the slurry is dried in 
the part A by waste 
heat from the kiln B. 
Another is the Hoff- 
mann kiln (Fig. 3), in 
which the material is 
slaked in a series of 
chambers, the heat 
from one being used 
to warm up the others 
so that it is used to 
the best advantage. 
Both these classes of 
kiln are now being re- 
placed by rotary kilns 
(p. 26), so that no de- 
tailed description of 
them is needed. 

The dry process is 
suitable for almost 
every kind of material, 
though sticky, highly plastic clays are troublesome unless 
previously heated to destroy their plasticity. This heating 
is usually termed " drying," though it does more than merely 
drive off the moisture in the clays. The material is usually 




FIG. 3. Hoffmann Kiln. 



MANUFACTURE OF PORTLAND CEMENT 25 

passed through a preliminary crusher which reduces it to 
pieces of a convenient size for " drying." The " dried " 
materials are then mixed in suitable proportions in a mixing 
drum and passed into storage bins. From these bins the 
material passes to the grinding mills, where it is ground so fine 
that not more than 4 per cent, will remain on a No. 180 sieve, 
though the actual fineness is a matter for experiment, some 
comparatively coarse materials making an excellent cement. 
The mills used for this purpose are edge runners, ball mills, 
mill stones, centrifugal mills, or disintegrators, the last named 
being only suitable for coarse grinding. Various screening or 
sifting devices are employed to separate the fine material and 
to return the coarser product to the mill to be still further 
reduced. 

The ground material, termed raw meal, is stored in silos or 
bins, each sufficiently large to hold about six hours' output. 
To secure a uniform product some means of mixing the material 
is employed in these silos, a species of bucket elevator, which 
removes the material from the bottom and returns it to the 
top of the silo, being generally used. The material in the silos 
must be tested for fineness, and its composition must be adjusted 
by the addition of clay or limestone powder if it does riot 
correspond exactly to that required to make good cement. 

From the mixer the raw material is passed to a rotary kiln 
(Fig. 4), which consists of an inclined steel tube lined with 
firebricks and cement clinker, 100 ft. or more in length and 
6 ft. or more in diameter. The raw meal is fed in at the top 
whilst the fuel, in the form of dust, is blown in at the other 
end by a blast of air more than sufficient for its combustion. 
As the kiln revolves the material travels slowly down the 
tube, becoming hotter and hotter until it reaches a state of 
partial fusion or sintering, and is eventually discharged from 
the lower end of the kiln in a white hot condition. Variations 
in the shape of the kiln have been made from time to time, the 
modern tendency being to use kilns which are of larger diameter 
in the zone of greatest heat than they are at either end. The 
speed of rotation is quite slow about thirty revolutions per 
hour and is regulated to suit the material being heated. 
The final temperature reached in the kiln is about 1410 C. 



26 METHODS OF CEMENT MANUFACTURE 

The material discharged from the lower end of the kiln is 
termed clinker, and is received into a cooling device which 
usually consists of a rotating, inclined steel tube 30 to 50 ft. 
long and 5 ft. wide, which is partially lined with firebrick and 
fitted with baffles. As the cooler rotates, these baffles lift 
the clinker and allow it to fall in the form of a cascade, so that 
it is brought into close contact with a current of air which is 
sent through the cooler by means of a fan. The design ami 




i 



FIG. 4. Pfeiffer's Rotary Kiln. 



construction of a rotary kiln and its accessory coal-grinding 
plant require a large amount of skill. A number of firms have 
specialised in their manufacture, have brought them to a high 
degree of perfection, and have adopted many devices of 
considerable importance in obtaining a first-class product in 
the most economical manner. These details are beyond the 
scope of the present volume. 

Cement clinker from stationary kilns is in the form of irregular 
lumps ; that from rotary kilns is in grains rather larger than 
peas. If correctly burned it is a dark grey or blue grey sub- 



MANUFACTURE OF PORTLAND CEMENT 27 

stance, extremely hard and full of minute pores. Insufficiently 
burned clinker is buff coloured ; it must be separated and re- 
burned. Over-heated clinker can only be produced when the 
material has been in contact with a siliceous kiln lining, as 
clinker alone is not affected by the greatest attainable heat 
in a cement kiln. In the presence of silica, on the contrary, 
a more siliceous silicate is formed which is not hydraulic, and 
is therefore useless for cement. 

The cooled clinker is usually passed into a storage bin, from 
which it is drawn as required by the men in charge of the 
grinding plant. It is a curious fact, and one which in the 
early days of rotary kilns caused great inconvenience, that 
merely to reduce rotary kiln clinker to a fine powder will not 
produce a satisfactory cement ; such a powder sets immedi- 
ately, and some means must be used, therefore, to delay its 
setting for a convenient time. This is usually accomplished 
by the addition of about 2 per cent, of water, preferably in the 
form of steam, and 1 per cent, of gypsum. The grinding 
machines are chiefly tube or ball mills consisting of rotary 
cylinders containing steel balls which fall like a cascade when 
the mill is in action, and, in falling, crush the material to 
powder. This powder is then passed into a separator in which 
it falls on a rapidly rotating plate, the coarse powder being 
returned to the mill whilst the fine powder is carried along by 
a current of air and is eventually deposited in sacks or casks 
or in a silo. Sieves may be used in place of separators, but 
are understood to give less fine flour in the cement. 

Many arguments have been brought forward as to the 
relative advantages of the dry and wet processes, but the 
opinion is still held by many people that the wet process is 
preferable wherever it can be applied. A careful investigation 
of both processes will fail to show any material differences in 
the final product, and it may be taken for granted that with 
equal care and skill either method of working will produce 
good results. The choice of one method should, therefore, 
depend on the costs of manufacture and, as these will differ 
in different localities, it is usually necessary to employ an 
impartial and independent expert to go fully into the question. 
Broadly speaking, the wet process is the cheaper for moist, 



28 METHODS OF CEMENT MANUFACTURE 

soft, raw materials such as chalk and marly clays ; its chief 
drawback being the high cost of burning due to the evaporation 
of the added water. If the dry process is used, the raw 
materials contain only 10 to 15 per cent, of moisture, and this 
is removed by the waste gases of the kiln without any cost. 
With the wet process, on the contrary, the grinding is much 
less expensive ; but this small saving cannot counterbalance 
the high cost of fuel for drying out the added water. Moreover, 
the saving in power will be nullified if the wet mills require 
more repairs than the dry ones. A well-managed plant working 
in the wet way averages 5 Ibs. of medium quality coal to each 
100 Ibs. of cement more than when the dry process is used, as 
the evaporation of the water in the wet process requires as 
much heat as will clinker the dry raw material. This is 
equivalent to almost 20 per cent, of the coal used. 

In the earlier days of rotary kilns, when they were built 
too short for materials treated by the dry process, the coal 
burned was greater in the dry than in the wet process, and the 
influence of the fuel ashes was so great that many firms were 
driven to use the wet process. Recently, the dimensions of 
rotary kilns have increased enormously, and the modern kilns 
can deal with dry-process material in a perfectly satisfactory- 
manner. Other important reasons for using the wet process 
are the absence of drying plant for the raw material and absence 
of dust, but neither of these are of primary importance. 

The idea that the wet process effects a better mixing of hard 
materials and improves the quality of the cement is no longer 
tenable, and the belief that less power is required is equally 
inaccurate. The power required is only less when the output 
of a wet mill is compared with that of a ball or tube mill of 
an old pattern ; the modern dry-process machines operate 
very advantageously and satisfy all requirements with regard 
to uniformity and fineness of product and power consumption, 
providing the materials are suitable ; wet materials being 
best treated by the wet process. For hard materials, such as 
limestone and shale, the dry process is preferable. In this 
country, the softer materials are available in such large 
quantities that the wet process is more generally used ; in 
America, on the contrary, the harder materials are the 



SAND CEMENT 29 

more frequently employed, and the dry process is preferred 
there. 

SAND CEMENT. 

Sand Cement is a mixture of equal weights of sand and 
Portland cement, the two materials having been ground 
together. Sand cement has a tensile strength almost as great 
as that of neat Portland cement, but its value is greatly 
exaggerated by tensile and other tests, as will be seen by using 
sand cement mixed with three times its weight of sand. It 
will then be found that the strength of the cement-sand 
mixture is much lower than that of mixtures of Portland 
cement and sand in the same proportion. The fact is that 
tests of the tensile strength of neat cements are almost meaning- 
less, and only mixtures of cement and sand in the proportion 
of 1 : 3 give really satisfactory results. Tests of neat cement 
are no longer made on the Continent (see p. 139). 

NATURAL CEMENTS. 

Long before Portland cement had been invented, many 
so-called " natural cements " were in use. These were made 
by heating certain naturally occurring materials (p. 13) and 
grinding the calcined product. These " natural " or " rock " 
cements are far less uniform in composition than are Portland 
cements, because no care is taken to adjust the composition 
of the raw materials used. They are also inferior because 
they are generally under-burned the temperature in the 
furnace being insufficient to cause complete combination of 
the clay and lime or their equivalents as it is under 1200 C. 
instead of 1400 C. or above. 

It is essential that the materials from which they are made 
should contain clay and limestone (or the equivalent alumino- 
silicic acid and calcium carbonate) in suitable proportions, 
and the value of the cements produced depends largely on 
the composition of the raw rock. 

The methods employed in the manufacture of natural and 
rock cements resemble those used for Portland cement, 
except that there is only one raw material instead of two, and 



30 METHODS OF CEMENT MANUFACTURE 

that no efforts are made to test and adjust the composition 
of the material during manufacture. The raw rock or septaria 
(p. 13) are placed in the kiln without being crushed, stationary 
kilns (Figs. 2 and 3) being employed. The calcined material 
is then ground to powder in a similar manner to cement, 
though the grinding is seldom so complete. 

The clinker drawn from the kiln must be sorted, the light- 
coloured under-burned pieces being separated from the darker 
clinker and used only for inferior work or returned to the kiln 
to be re-heated. Some manufacturers claim that the clinkered 
material is less satisfactory than a rather more lightly burned 
product. These differences in behaviour are probably due 
to differences in the composition of the clinker : the greater 
the proportion of lime the higher must be the temperature 
inside the kiln to secure an adequate combination. If, on 
the contrary, the burning has been properly executed, a cement 
with a large proportion of lime will be stronger than one 
containing less lime. If the proportion of lime (CaO) in the 
raw rock or marl is between 1-8 and 2-4 times that of the 
silica and alumina, a good natural cement will be obtained at 
a temperature of about 1150 C., but if the lime exceeds four 
times the " silica + alumina," the temperature needed will be 
as high as that required for Portland cement. The absence 
of accurate knowledge of the composition of the materials 
and variations in the temperature of the kilns used, usually 
results in a considerable proportion of over- and under-burned 
material being produced, and it is not unusual for one quarter 
of the contents of a kiln to be rejected. 

Natural cements are usually much coarser than Portland 
cements, but during the last few years finer grinding has been 
customary so as to be better able to compete with Portland 
cement. 

The great drawback to natural and rock cements is their 
unreliability. At the present time, their chief purpose appears 
to be to form a cheap rival to Portland cement. The superiority 
of the latter is so great, however, that manufacturers are 
finding it will pay them better to take more pains to secure a 
uniform product of a composition and properties practically 
identical with those of Portland cement. Some of them have, 



MANUFACTURE OF NATURAL CEMENT 31 

therefore, installed arrangements for testing and adjusting the 
composition of the raw material and of treating it in the same 
manner as in making Portland cement. The process is certainly 
more costly, but the better prices obtained fully warrant the 
additional expense. With natural materials so nearly correct 
in composition it seems unfortunate that firms should continue 
to produce so inferior a product as natural cement when they 
might so advantageously manufacture Portland cement. To 
do this it is essential that the material should be ground to 
powder and thoroughly mixed before it enters the kiln, as the 
direct calcination of relatively large lumps is one of the chief 
causes of irregular composition. The two chief constituents 
of the material are not in sufficiently intimate contact to 
produce a uniform product unless the raw material has first 
been reduced to powder. 

Roman cement (p. 13) is one of the oldest of the natural 
cements, but its name is quite misleading, as the material 
bears no resemblance to the mortar used by the -ancient 
Romans. It was first made in England in 1796, the raw 
material being the sept aria (p. 13) or clayey nodules dredged 
from the sea near Harwich and Sheppey. These nodules are 
calcined lightly in a shaft kiln (Fig. 2) and are then reduced 
to a rather coarse powder. 

Whilst useful where Portland cement is not available, Roman 
cements can only be regarded as crude inferior products of a 
similar type, their disadvantages being due to their irregularity 
in composition and the coarseness of the product, but their 
low cost about half that of Portland cement causes them 
to be largely used in some localities. 

Belgian cement sometimes, but erroneously, sold as Belgian 
Portland cement is a natural cement manufactured in the 
district of Tournai, where apparently inexhaustible quarries 
of clayey limestone occur. This material (see analysis on 
p. 13) when calcined, very closely resembles Portland cement 
clinker, and is only distinguished from it with difficulty. 

Belgian cement is made with greater care than most natural 
cements, but it is, nevertheless, very inferior to Portland 
cement on account of the lower temperature at which it is 
burned and the coarseness of the final product. Good 



32 METHODS OF CEMENT MANUFACTURE 

genuine Portland cement is undoubtedly produced in Belgium, 
but what is known as " Belgian cement " is quite different, 
and is notoriously inferior, very unreliable, and often even 
dangerous. The prudent professional man should never allow 
its use on works under his control. This " Belgian cement " 
is no better than a hydraulic lime, and is made in the same 
manner. There is no careful mixing of two separate raw 
materials, with all the refinements of scientific control at every 
stage of the process and at every hour of the day in order to 
remedy the ever-present variations in the chemical composition 
of the materials employed, and to secure a uniform chemical 
composition of the resulting product, by which alone a uniform 
quality can be obtained. The Belgian rock, which varies 
greatly in its chemical composition and is usually deficient in 
lime, is taken just as it comes from the quarry, burned and 
ground exactly as if it were to be sold as " hydraulic lime " ; 
indeed, much of it is sold under that name in its own country. 
But some wily vendors, keenly alive to the value of a name, 
know by experience that many persons who would not touch 
it under a true description will buy readily enough if it be sold 
as " Portland cement," or " best Portland cement," and they 
therefore pack it in casks exactly like that used for genuine 
Portland cement, are ever ready to attach any label preferred 
by the purchaser, and more often than not print the label 
in the English language, call their firm by an English name, and 
do everything they can to make the guileless consumer imagine 
he is buying genuine Portland cement produced by English 
makers, whose reputation throughout the world still stands 
as a guarantee of high and reliable quality. In short, the 
sooner the use of the word " Portland " in connection with such 
Belgian cement is stopped, the better. 

QUICK AND HYDRAULIC LIMES. 

Hydraulic limes are prepared by burning limestones con- 
taining clay (p. 4) in a manner precisely similar to that used 
in the manufacture of quick-lime. The material is placed 
in a shaft kiln with alternate layers of coal, and the burned 
lime is drawn out at the bottom of the kiln. In order to avoid 



MANUFACTURE OF HYDRAULIC LIME 33 

the admixture of ashes from the fuel with the burned lime, 
gas may be used instead of coal, and where the output is large 
various methods are used for keeping the kilns continuously 
at work. With smaller kilns, the usual custom is to fill, burn 
and empty them, treating each kiln separately from the rest. 
This arrangement has the advantage of keeping the lime in 
larger lumps than when it has to travel down a tall shaft as 
is the case when the kilns are worked continuously. 
Horizontal draught kilns of the Hoffmann type (Fig. 3) are 
also used for this purpose. It is important to avoid over- 
heating, whereby the lime becomes partially vitrified or 
sintered. In burning pure or fat lime this over-heating does 
not readily occur, but the proportion of clay and free silica 
in hydraulic limes renders special precautions necessary or 
the lime will be dead-burnt. 

The difficulty in burning hydraulic limes is increased by the 
fact that they require a higher temperature of calcination than 
does common (fat) lime. 

After burning, the material is carefully slaked by the addition 
of a suitable quantity of water, and the fine powder thereby 
produced is then ready for sale. Properly-made hydraulic 
lime, therefore, needs no grinding and can thus be produced 
more cheaply than Portland cement. Ordinarily, however, 
lumps of material remain after slaking and must be separated 
by screening (see Grappier cement, p. 34). 

The chief difference between hydraulic lime and quick-lime 
is the clay in the former which prevents the hydraulic lime 
from slaking readily, but enables it to set when immersed 
in water. 

The raw materials used for hydraulic limes have been 
described on pp. 4, et seq. 

Hydraulic limes consist essentially of mixtures of " Portland 
cement " with considerable quantities of quick-lime, the 
proportions of each being dependent on the amount of clay 
in the original limestone and on the temperature attained 
during the burning. Some hydraulic limes made from blue 
Lias limestone have a composition corresponding almost 
exactly with that of Portland cement, but others correspond 
more nearly with a mixture of 70 per cent, of Portland cement 

c. D 



34 METHODS OF CEMENT MANUFACTURE 

and 30 per cent, of free lime. There is, at present, no limit 
of composition whereby hydraulic limes can be distinguished 
from other cements, and they are best defined as made from 
argillaceous limestones and as containing sufficient cementitious 
material to give hydraulic properties to the product and 
sufficient free lime to enable the material to slake on the 
addition of water. The advantage of the free lime present is 
that the material can be reduced to powder simply by the 
addition of water (slaking), and so does not need to be ground 
as does Portland cement clinker. It is, however, important 
that no more free lime should be present than is essential for 
this slaking, as an excess of lime merely weakens the cementing 
value of the material. Finely-ground grappiers (below) are 
usually added to increase the hydraulic properties of the 
lime. 

One of the most famous hydraulic limes is exported from 
Teil, in the south of France, and is considered to be specially 
suitable for sea walls and marine work. The following is an 
analysis of Teil hydraulic lime before slaking : 

Silica . . . . . 26-69 

Alumina . . . T . 4-24 

Iron oxide . . . . . trace 

Lime . . . / . 68-55 

Magnesia . .. . . . 0-52 

Most English hydraulic limes are only moderately hydraulitic 
and are much feebler than some of the French and German 
products, owing to their much larger proportion of free lime. 
They are improved by the addition of 5 per cent, of finely- 
ground plaster of Paris, and the product is then known as 
Scott's cement or selenitic cement. 

Grappier cement consists of the ground lumps or nodules 
which remain when hydraulic limes are screened. It is a true 
cement, though of a composition somewhat different from that 
of Portland cement, being usually rather low in alumina and 
lime. Grappier lumps consist chiefly of the hydraulite and of 
unburned limestone ; if the latter is present in a large propor- 
tion the cement will be useless. The following is an analysis 
of a, typical grappier cement : 



POZZOLANAS 35 

Silica ..... 26-5 

Alumina ..... 2-5 

Iron oxide . %,, . . . 1*5 

Lime . ... . . 63-0 

Magnesia ..... 1-0 

Sulphur trioxide (S0 3 ) , 0-5 
Carbon dioxide (CO.,) . 
Water 
(See also p. 52). 

POZZOLANAS. 

Pozzolanas are not true cements, but only become so when 
mixed with lime and water. The raw materials composing 
pozzolanas are, essentially, clays which have been heated to 
redness either by natural forces, such as volcanic action, or 
artificially in kilns. Pozzolanas are, as regards their origin, 
of three classes : 

(a) The direct products of volcanic action, usually found on 
the slopes of volcanoes, such as pozzolana proper, santorin, 
tosca, tetin and trass (p. 15). These pozzolanas bear a close 
resemblance to ashes and slags. 

(b) Products of the decomposition of certain igneous rocks. 
These are but feeble hydraulites. 

(c) Artificial pozzolanas obtained by crushing lightly-burned 
clay, ballast, tiles, bricks, etc. Care should be taken to avoid 
clays which have been heated to partial vitrification. Some 
blast-furnace slags, when ground, are also pozzolanic. 

SLAG CEMENTS. 

Certain basic blast-furnace slags, when granulated by sudden 
cooling with water and then ground to a fine powder, form 
valuable cements. The sulphur present in the slags is largely 
removed in the form of sulphuretted hydrogen gas during the 
granulation. Small percentages of various other elements 
are also present, but do not appreciably affect the hydraulicity 
of the cement. 

The use of slag as a raw material for cements which, in many 
ways, resemble Portland cements, has already been discussed 

D2 



36 METHODS OF CEMENT MANUFACTURE 

(p. 16), and some of the chief differences have been pointed 
out. All slags are not suitable for cement manufacture, and 
it is convenient to divide those made from blast-furnace slag 
into three classes : 

(1) Pozzolanic slag cements which consist of ground slag of 
a pozzolanic nature, to which sufficient slaked lime is added 
during the grinding of the material to bring the total content 
of calcium oxide up to 63 to 66 per cent. Such a cement is 
but little better than a hydraulic lime. 

(2) Plaster-slag cements made by mixing rapidly cooled and 
granulated slags of innate hydraulicity with a considerable 
proportion of plaster of Paris. Such cements contain little 
free lime, and are peculiarly resistant to acids and magnesium 
salts. There appears to be a future for slags of this neutral 
type preferably with a neutral, sulphur-free substitute for 
the plaster for maritime work. (See Asch's suggestion in 
a later section dealing with the effect of sea water on cement.) 

(3) Iron Portland cement , which is used in large quantities 
in Germany and in the Far East. The title is far from being 
a satisfactory one, as the material is not Portland cement at 
all, but is produced from basic blast-furnace slag. Basic slag 
is mixed with a suitable proportion of limestone, the material 
being then ground and burned in the usual manner. The 
clinker is mixed with nearly half its weight of granulated slag 
and then ground to a fine powder. 

Iron Portland cement differs from true Portland cement in 
the materials from which it is made and in some of the properties 
it possesses, though in many respects the two materials closely 
resemble each other. 

The composition of Iron Portland cement varies considerably 
in different localities ; a fair average is : 

per cent. 

Silica 20 to 25 

Alumina and iron oxide . . 9 ,, 15 

Lime . . . . 54 ,, 60 

Magnesia .... 0-6 ,, 5-0 

Sulphur tri-oxide (SO S ) . . 0-8 2-7 

Alkalies . 2 



MANUFACTURE OF SLAG CEMENT 37 

The term " iron-Portland cement " is also applied to blast- 
furnace slag which has been mixed with twice its weight, or 
more, of true Portland cement. It then resembles " sand 
cement " (p. 29), but is rather stronger. It is, of course, 
weaker than pure cement. 

The Iron Portland cement above mentioned must not be 
confused with the Iron Ore cement mentioned on p. 18. 

Slag cements are essentially mixtures of slaked lime and 
slag, and are therefore of the nature of pozzolanic cements, 
the slags being regarded as a kind of artificial pozzolana. 
Some firms produce a kind of Portland cement in which slag 
takes the place of the clay and part of the limestone or chalk, 
and in this way less fuel is used than in the manufacture of 
ordinary Portland cement, the temperature at which the 
slag-lime mixture clinkers being lower than that needed in a 
Portland cement kiln. The disadvantage of slag used in this 
manner is that the granulation of the slag introduces a large 
amount (20 to 40 per cent.) of water which must be driven off 
by heat before the mixture can be calcined. 

The slag is run from the furnace in the form of a white hot 
molten stream, which is reduced to porous grains of slag-sand 
by contact with water. Various methods of effecting this 
granulation are in use, one of the most satisfactory consisting 
in allowing the stream of slag to flow into a trough containing 
a rapid stream of water. Sometimes a jet of steam or water 
is allowed to play on the slag before it reaches the trough. 

Granulation plays an important part in the manufacture of 
slag cement, for it not only reduces the material to a coarse 
powder in a simple and cheap manner, but the water appears 
to have a chemical action, as the granulated slag has much 
stronger hydraulic properties than slag which has been more 
slowly cooled. In fact, the hydraulic properties of slowly- 
cooled slag are almost negligible. 

The porous granulated slag cannot be ground direct on 
account of the water it contains. It must, therefore, be dried 
at a temperature below a red heat in an automatic dryer. 
Such a dryer consists preferably of two concentric pipes slightly 
inclined from the horizontal. The material is passed through 
the annular space between the pipes and hot air is passed over 



38 METHODS OF CEMENT MANUFACTURE 

it, so that the wet material enters the machine at one end and 
emerges sufficiently dry at the other. 

The dried slag is next mixed with a suitable proportion of 
slaked lime, usually about 35 parts of lime to 100 of slag 
being used. The mixture is then reduced to a fine powder in 
a tube-mill. The mixing is usually effected automatically, 
and without requiring any special attention, by charging the 
mills used for fine grinding with the various raw materials in 
the desired proportions. The grinding machinery is the same 
as is used for Portland cement (p. 25). 

Slag-cements set so slowly that they must usually be 
accelerated by the addition of calcined silica, highly aluminous 
slag, or caustic alkali. 

Slag-cements are much used as adulterants of, or substitutes 
for, Portland cement, but differ from the latter in the lower 
proportion of lime and alumina they contain and in the pro- 
portion of calcium sulphide present. The chief distinction to 
be found is in the 4 to 8 per cent, loss which occurs on the 
ignition of slag cements, due to the water they contain. 




A ROTARY KILN B 



OAL HOPPCH 



^H Rlf-LED COOLINO CYLIN^R? 




Diagram of Modern Rotary Kiln and Cooling Cylinders. 



CHAPTER III 

THE CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 

THE chemical and physical changes which occur in the 
manufacture and use of cements are both complex and difficult 
to investigate. They may be more easily studied by separating 
them into groups : (a) the changes which occur in the manu- 
facture of cement from the raw materials, and (6) the changes 
occurring when the cement is used. 

Changes in Manufacture. From what has been stated in 
previous chapters, it will be understood that the various 
cements described are formed essentially from an acid substance 
corresponding to aluminosilicic acid in combination with a 
basic substance. The acid and base occur quite separately 
in the form of clay, pozzolanas, etc., and chalk or limestone 
respectively, or as a mixture (marl, argillaceous limestone, and 
the raw materials used for natural cements). Combination 
only occurs when the mixture is heated to a suitable tempera- 
ture, and it is during this heating that the most important 
" changes during manufacture " occur. 

No perfectly pure clay occurs in nature, and this still further 
complicates the problem. Most of the clays used in the 
manufacture of cement appear to consist of a mixture of what 
may be termed clay substance (aluminosilicic acid) together 
with free silica and other (non-plastic) minerals. As naturally- 
produced mixtures must vary greatly in composition, no single 
chemical formula can be assigned to natural clays. The best 
way is to consider the essential constituent of each of such 
clays as the chief factor in the acid portion of the cement, the 
free silica and other minerals being considered S3parately. In 
other words it is necessary first to consider the action of the 
heat on each of the constituents of the raw materials apart 
from each other, and then to study what reactions occur when 
the substances produced by the action of heat are brought into 



40 CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 

intimate contact with each other. In the case of cement- 
burning, both these sets of changes occur at the same time in 
different portions of the material, and their separate considera- 
tion is only a convenient means of explaining what actually 
occurs. It is also convenient to distinguish as far as possible 
between the chemical changes which take place on heating the 
raw materials and the purely physical changes which occur 
simultaneously. 

The Chemical Action of Heat on Clay Substance. 

The first effect of heat on clay is to drive out any moisture 
it may contain, either in the free state or in the form of water 
absorbed by any colloidal matter present. Some clays contain 
as much as 30 per cent, of such water without appearing to be 
really wet. The changes which occur during this drying are 
purely physical so long as the temperature is not appreciably 
above 100 C., and will be described later. 

If the temperature is raised, chemical decomposition occurs 
with appreciable rapidity, and at 500 to 800 C. a quick 
evolution of water occurs. The precise nature of the decom- 
position, of which this is a sign, is not clearly understood. 
Rebuffat, Mellor and Holdcroft, and others maintain that the 
clay molecule is completely decomposed into a mixture of free 
oxides silica, alumina and water ; but other chemists maintain 
that an anhydride or similar compound of alumina and silica 
is formed. Thus, W. and D. Asch believe that clays are 
aluminosilicic acids, or the corresponding hydrates, in which 
the various atoms are arranged in a series of hexagons and 
pentagons resembling the well-known " benzene ring " of 
organic chemistry and they represent a molecule of the purest 
clay obtainable (refined china clay) as 
(OH) 2 OH OH 




CHEMICAL CONSTITUTION OF CEMENT 4i 



Other and less pure clays have corresponding formulae, 
depending on their composition (see p. 5). 

A complete statement of Asch's theory is too lengthy and 
complex to be included in the present volume, 1 but its general 
correctness has been confirmed in various ways. It is of great 
value in explaining the changes which occur whilst the materials 
are in the kiln. As, according to Asch's theory, clays are 
merely crude aluminosilicic acids, the reactions which occur 
on heating a clay with a suitable proportion of lime consists 
chiefly in displacing some of the hydrogen atoms in the clay 
by lime or magnesia, just as the corresponding hydrogen in a 
complex organic acid is replaced by a base when the acid is 
neutralised. Any study of the reactions is complicated by the 
fact that the clays and bases (lime or chalk) are far from pure. 
The precise nature of the compound formed must depend 
largely on the extent to which the reaction is allowed to occur 
as well as on the particular aluminosilicic acid originally 
present. Fortunately for the manufacturer all aluminosilicic 
acids (clays) act as though they are partially decomposed at 
high temperatures, and form one of three types of stable 
compounds, viz. : 



1. 



12Si0 =8i All All Si 




Si Al Al 
~~\/\/ 



8i 



3. 



l2Si0 2 = | Si 
\/ 



Al Al 



Thus, a clay of the type 6Al 2 B l2Si0 2 l2H 2 will, on heating, 
lose water and silica, or water, silica and alumina, according 
to the temperature and duration of heating, and will form one 
of the types above mentioned. 

1 Readers with. a sufficient knowledge of chemistry should consult "The Sili- 
cates," by W. & D. Asch, published by Constable & Co., Ltd., London, 



42 CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 

The effect of this particular decomposition is to convert all 
clays into one or more of the above types of compounds, and, 
consequently, enables commercially useful Portland cements 
to be made from almost any material containing a sufficient 
proportion of clay. 

Whatever substances are produced by the direct action of 
heat, it is generally recognised that their nearest natural 
equivalents are the trasses and pozzolanas already described 
(p. 14) and corresponding (e.g.) to 3#,0 . RO . 3AW 3 . ISSiO.,. 

That a decomposition of the clay molecule, and not a mere 
evolution of water of crystallisation occurs when clay is heated 
cannot be doubted, and Sokoloff has shown that the production 
of a pozzolanic material of maximum hydraulicity occurs 
precisely at the point where a clay loses the whole of its con- 
stitutional water. This production of pozzolanas by heating 
clays to a temperature of about 500 C. appears to be charac- 
teristic of most, if not all, clays, and is strong evidence of the 
existence of a definite type of essential constituent or " clay 
substance " in all clays, even though its composition and 
the arrangement of the atoms within the molecule may vary 
considerably. 

The action of more intense heat on clays is extremely difficult 
to study. The only definitely crystalline substance which is 
produced by heating pure clay to the highest available tem- 
perature and allowing it to cool is sillimanite (SiAl 2 5 ). This 
substance appears to' be formed at 1200 C., but whether it is 
a decomposition product of a more complex alumino-silica 
compound or whether it is produced by the recombination of 
silica and alumina set free at a temperature of 800 C. has not 
yet been determined, though the latter appears to be the more 
probable explanation. Indeed, Mellor and Holdcroft regard 
the formation of sillimanite at 1200 C. as strong evidence of 
the complete dissociation of clay into free silica and alumina 
at a lower temperature. There is, however, remarkably little 
evidence as to the true composition of this crystalline substance. 
It resembles sillimanite in several respects, but is not im- 
probably one of the three compounds whose structural formulae 
are given on p. 41. The quantities of crystals available for 
analysis are far too small for a clear distinction to be made 



ACTION OF HEAT ON SILICA 43 

between SiAl. 2 5 and the three types of compounds mentioned, 
though the formation of the latter is, theoretically, far more 
probable than the complete dissociation of the clay into free 
silica and alumina, and its recombination into sillimanite. 

In the present volume, the product of the direct action of 
heat on clay alone is termed calcined clay. 

The Chemical Action of Heat on Free Silica. 

The .chemical action of heat on free silica is inappreciable at 
temperatures below 800 C., but above this temperature a 
considerable increase in the volume of the silica occurs and 
tridymite, which is apparently a polymerised form of silica, 
i.e., xSi0. 2 , is produced. (For the action of heat on a mixture 
of free silica and lime, see p. 66). 

The proportion of free silica in a well-made Portland cement 
is extremely small ; together with all the other insoluble 
matter it should not exceed 3 per cent. As it is quite inert 
when the cement is in use the presence of so small a proportion 
is quite devoid of importance. 

In the raw materials used for the manufacture of cements a 
notable proportion of colloidal silica (Si0 2 xH 2 0) may be present, 
but is converted into ordinary, amorphous silica on heating. 
At one time much stress was laid on the distinction between 
colloidal (soluble) and amorphous (insoluble) silica owing to 
the greater reactivity of the former, but this distinction is 
valueless in the case of all cements in the manufacture of which 
a high temperature has been employed, providing that the 
material is ground sufficiently fine. 

The effect of Heat on Free Alumina. 

The effect of heat on free alumina is to make it insoluble in 
acids and very resistant to the action of alkalies. It therefore 
becomes inert. It is doubtful whether free alumina ever 
exists really in cements to any appreciable extent. When 
free alumina is added in small quantities (4 per cent.) to some 
samples of commercial Portland cement it causes expansion 
on gauging ; on other samples it appears to have no action of 



44 CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 

this kind even when briquettes made of the mixture are kept 
in boiling water for several hours. 

Other minerals in clay will undergo various physical and 
chemical changes when exposed to the action of heat at 
temperatures reached in the manufacture of cement. Some 
of the complex aluminosilicates appear to decompose with the 
formation of simpler silicates ; others lose water, and must, 
therefore, be regarded as undergoing some decomposition, 
though the nature of this is far from being well understood. 
As the temperatures reached particularly in rotary kilns 
are very high (about 1400 C.), it is not unreasonable to suppose 
that any calcium aluminosilicates present in a clay produce 
substances which bear a close resemblance to the essential 
constituent of cements, and that the sodium, potassium and 
magnesium aluminosilicates produce analogous substances 
which may, however, be devoid of hydraulic properties. 

As the proportion of " other minerals " in clays and similar 
substances used for cement making is seldom large, the direct 
chemical action of heat upon them may be neglected ; the 
total effects of their presence is noted on a later page. The 
chemical changes induced by the reaction of these substances 
on one another are described on p. 72. 

The Chemical Action of Heat on Limestone. 

The chemical action of heat on limestone is comparatively 
easy to understand. Chalk and limestone are essentially 
composed of calcium carbonate which, on heating to 700 C. 
or above, dissociates into free lime (CaO) and carbon dioxide 
(C0 2 ) ; the latter, being a gas, escapes and leaves the free lime 
behind. The extent to which this decomposition occurs 
depends on the pressure of the carbon dioxide produced ; if 
this gas is allowed to escape the whole of the carbonate will 
be converted into oxide, but if some of the carbon dioxide 
remains in the kiln or other appliance in which the heating 
occurs, it will, in time, produce such a pressure that no further 
decomposition of the carbonate will take place. This is 
commonly expressed by the following equation : 

CaC0 3 I > CaO + CO 2 , 

which indicates the reversibility of the reaction. 






ACTION OF HEAT ON LIMESTONE 45 

Lime which has been heated to above 1000 C. loses part of 
its power to slake when water is added to it, but this is believed 
to be due to a reduction in the surface area of the material 
rather than to any molecular rearrangement. 

Free lime does not occur in properly made Portland cement ; 
many statements to the contrary are based upon erroneous 
conclusions drawn from experimental observations. This is 
due to the ease with which Portland cement is hydrolysed when 
treated with water, whereby lime, originally in combination, 
is set free. The presence of free lime in cement would be very 
disadvantageous, as when the cement is in use the lime hydrates, 
expands, and may easily cause dangerous cracks in the struc- 
ture. Comparative tests on good Portland cements, with and 
without the addition of 6 per cent, of lime, showed that the 
whole of the expansion of the lime occurred when the briquettes 
were exposed for twenty-four hours to a moist atmosphere. 
The same results are produced in an insufficiently-burned 
cement, i.e., one in which all the lime and clay have not entered 
into combination, so that free lime and free " calcined clay " 
are present. 

It is because of the danger caused by the presence of free 
lime that a limit to the proportion of lime is usually imposed. 
The best means of controlling its presence is the " expansion 
test " described in a later chapter. If a cement can pass this 
test the proportion of free lime in it will be insignificant. 

If a cement mixture containing an excess of chalk or lime- 
stone is heated so strongly that the excess of lime produced is 
converted into the slow-slaking, dense modification, or into 
the cubic crystalline form produced by prolonged exposure at 
a temperature exceeding 1400 C., the cement produced will 
crack when it is gauged and placed in water. The lime then 
slakes so slowly that the cement sets before the slaking is 
complete, and the increased volume of the slaked lime brings 
about the cracking, or even complete disintegration, of the 
mass. Very small percentages of this crystalline lime will 
render a cement unsound, but as its presence is exceedingly 
difficult to detect, it is customary to omit all search for it and 
to test the cement directly as to its soundness and expansibility 
in water. (See p. 124). 



46 CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 

In the rotary and Hoffmann kilns used for cement manu- 
facture there is generally sufficient draught for the complete 
removal of all the carbon dioxide liberated, but in some of the 
shaft kilns a small but recognisable proportion of undecomposed 
(or re-formed) calcium carbonate is usually present. 

The impurities present in the chalk or limestone will behave 
so far as any chemical changes are concerned like the 
similar constituents of the clays or like calcium carbonate. 
Thus, magnesium carbonate is decomposed, forming free 
magnesia and carbon dioxide. 

Chemical Reactions between Clay and Lime. 

The most important chemical changes which occur in the 
manufacture of cements are not the direct chemical action of 
heat on the various substances used, each being considered 
separately, but are due to the various reactions of the various 
substances upon each other. Thus, either a pure clay or lime, 
when heated separately, is quite infusible at all industrial 
temperatures, but when a mixture of clay and lime in suitable 
proportions is heated, it melts at a temperature of 1400 C. or 
lower, and the product is entirely different in chemical and 
physical properties from the original mixture. Owing to the 
variety of substances present in natural clays and chalk or 
limestone, the various reactions of these upon each other are 
extremely difficult to study. Some of the reactions at present 
regarded as of minor importance may be proved, later, to be of 
greater significance ; at the moment of writing, however, the 
following are considered to be the chief reactions which occur : 

Those portions of the contents of a kiln which have attained 
a temperature of 1000 C. or above will not consist of the 
original clay and chalk or limestone fed into the kiln, but of a 
mixture of " calcined clay " 1 and free lime, together with 
such free silica (including tridymite) and other " calcined 
minerals " as may be present adventitiously. At the tempera- 
ture mentioned some amount of fusion will have taken place, 
particularly among the " calcined minerals," and the glassy 

1 It is clear that with the present divergence of opinion as to the nature of this 
" calcined clay" (p. 43) it is inadvisable to regard it definitely as either a mixture 
of free silica and alumina or as an aluminosilicic complex. 



REACTIONS BETWEEN CLAY AND LIME 47 

substances so produced will bring the particles of lime and 
" calcined clay " into such intimate contact with each other 
that various reactions will commence. Indeed, one object of 
the fine grinding and thorough mixing of the raw materials is 
to secure the most intimate contact possible between the various 
materials. 

That no fusion is necessary is clearly observable if a mixture 
of pure china clay and pure lime is heated in a Doelter's 
microscope, when it will be found that no appreciable fusion 
occurs until a temperature of 1300 C. is reached, though the 
complete solubility of the product heated to a lower tempera- 
ture will show that combination of the two substances has 
occurred. Moreover, as shown later, J. W. Cobb has con- 
clusively proved that reactions between lime, silica and 
alumina can occur with the production of a mass completely 
soluble in hydrochloric acid at temperatures far below that 
at which even partial fusion takes place. The partial fusion 
which occurs, with some clays, at a temperature of 1000 C. is 
due to the impurities present, and has no essential connection 
with the progress of the main reaction whereby cement clinker 
is formed, though it may have a physical effect in increasing 
the rapidity of the reaction by bringing the particles into 
more intimate contact with each other. 

If the " calcined clay " is regarded as a simple mixture of 
silica and alumina, both in the free state, the chief action of 
the lime will be to form : 

(a) calcium silicates (xCaO,ySi0 2 ) 

(b) calcium aluminates (xCa 

(c) calcium aluminosilicates 

These substances will be mixed together in proportions 
depending on the relative amounts of lime and " clay," on 
the duration of the heating, and on other conditions required 
for the formation of each of these classes of substances. 

From a simple mixture of lime, silica and alumina, it is most 
natural to suppose that the product would consist largely of 
a mixture of one or more calcium silicates with one or more 
calcium aluminates, and that the proportion of calcium 
aluminosilicates would be very small. The apparent simplicity 



48 CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 

of this arrangement has made many investigators content to 
regard cements as mixtures of simple binary compounds, and 
especially as solid solutions of a calcium aluminate in a calcium 
silicate. This theory of solution is, indeed, so popular that 
very few writers realise the peculiarly slender foundations on 
which it is based and the enormous lacunce between the assump- 
tions made as to the theoretically possible existence of certain 
substances and the complete failure to produce these substances 
under the conditions existing in the manufacture of cements. 

If, on the contrary, the " calcined clay " is considered to be 
an aluminosilicic complex or anhydride, it is more probable 
that the greater part of the product would consist of ternary 
compounds, i.e., calcium aluminosilicates, and that the pro- 
portion of binary calcium silicates and calcium aluminates 
would be small. The presence of all three classes of substance x 
is, of course, quite probable, whichever theory as to the chemical 
constitution of " calcined clay " is adopted ; it is only the 
relative proportions of each which is important. Again, it is 
quite possible that the action of the lime on any aluminosilicic 
anhydrides present may result in a decomposition of the 
complex and the consequent formation of simple binary 
silicates and aluminates, though there are serious objections 
to the view that this decomposition occurs to any great extent. 

Three entirely different series of methods of investigation 
have been adopted in studying the constitution of cements. 
The first of these comprises analytical methods applied to the 
cements themselves and including the study of the physical 
properties as well as their behaviour towards chemical reagents. 
The second method of investigation comprises synthetic 
methods of research which consist essentially in endeavour- 
ing to prepare the apparent or presupposed constituents of 
cements, and in comparing the properties of these synthetic 
products with cements prepared for purposes of commerce. 
The third method consists in observing the reactions which 
occur when the raw materials are heated or when the clinker 
is cooled. In this connexion it is important to observe that 
great care must be taken to avoid overheating, with the 

1 This is due to the use of impure materials. 



METHODS OF INVESTIGATION 49 

resultant fusion of the clinker, for this produces substances 
which do not necessarily exist in ordinary cement clinker. 
Dittler and Herold go so far as to state that observations of 
what occurs when well-made and carefully-burned cement 
clinkers are allowed to cool, can never show the constitution 
of the clinker satisfactorily as the formation of crystals does 
not take place like that in most other systems of two or more 
components, for instead of the substances in excess separating 
first, it is the substance which has the greatest rate of crystal- 
lisation which first becomes crystalline. The formation of the 
various compounds does not occur during the cooling, but 
during the heating, the fusion and crystallisation occurring 
simultaneously. For these reasons the results of observations 
made on cooling clinker should be accepted with great reserve. 

In studying what occurs when the raw materials are heated 
under conditions where the reactions can be observed, the 
changes in the electrical conductivity of the mixture are of 
great value. It is well known that the extent to which a 
substance will conduct electricity is a measure of its dissocia- 
tion into ions, and it is an important fact that when clays and 
cements are heated above 700 C. their electrical conductivity 
increases rapidly with increasing temperature to 900 C., after 
which it is rather slower ; at temperatures of 1400 C. to 
1600 C. it is as high as that of aqueous solutions of corre- 
sponding salts. This behaviour implies that at the highest 
temperature reached in rotary kilns, cement clinker is com- 
pletely dissociated into its separate ions, though the constitu- 
tion of these ions has not yet been ascertained. 

If Asch's theory (p. 55) is correct, these ions would be 
Ca and 6Al 2 3 l2Si0 2 . No other published theory explains in 
a simple manner what ions are produced by this dissociation. 

Each of the lines of investigation mentioned above is 
important and neither is reliable without the others, yet so 
difficult is the study of cements and so great is the influence 
of the theories of the earlier investigators that, in spite of the 
enormous amount of evidence available, it is difficult to reach 
conclusions which will be accepted by all chemists interested 
in cements. 

Thus, a microscopic examination of a thin section of clinker, 

c, E 



50 CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 



or preferably a similar study of a piece of first-class clinker 
which has been polished and then etched with very dilute acid 
or water, shows that such clinker is chiefly composed of a 
single crystalline constituent, the particles of which are 
separated by a much smaller quantity of other material. 
Clinker from a rotary kiln usually contains less intercrystalline 
matter than that obtained from stationary kilns, as the burning 
is more efficient in the former. As the crystals of the principal 
constituent form so large a proportion of the whole material 

in really well-made 
clinker, it is not un- 
reasonable to sup- 
pose that these 
crystals represent the 
really essential con- 
stituent, and that the 
remaining materials 
are of an entirely 
adventitious charac- 
ter and are due to 
impurities in the raw 
materials or to im- 
perfections in the 
processes of manu- 
facture. 

Le Chatelier and 
Tornebohm were 
among the first in- 
vestigators to use 
the microscope in investigating the nature of cement clinker 
in the manner described above. They observed four different 
kinds of crystals, for which Tornebohm proposed the names 
alite, belite, celite and /elite, respectively. By far the most 
important of these are alite and celite, particularly the 
former, which constitutes the principal portion of the 
material, the celite (with occasionally a little belite and felite) 
forming a filling material or matrix between the grains of 
alite. 

1 Courtesy of C. H. Desch, Esq. 




FIG. 5. Cement Clinker x 180 diams. 1 

(Lightly etched with very dilute hydrochloric 
acid.) 



ALITE, BELITE, CELITE AND FELITE 51 

Belite resembles alite in some respects, but is a dirty green 
colour, is characteristically striated, and gives brilliant inter- 
ference colours. It has never been isolated in a crystalline 
form nor in a state sufficiently pure for analysis, but is generally 
understood to be calcium ortho-silicate, 2CaOSi0 2 . 

Calcium ortho-silicate, prepared synthetically by heating a 
mixture of lime and silica in equivalent proportions to 1150 C., 
melts at 2074 C. It is almost devoid of hydraulicity and 
rapidly falls to powder on exposure to air. According to Day 
and Shepherd, there are three calcium ortho-silicates, depending 
on the temperature of the material, the a-form, which crystal- 
lises in monoclinic prisms with a hardness of 5 to 6 on Moh's 
scale and is only stable above 1410 C. ; the /3-form, which is 
produced when the a-form is cooled from 1410 C. to about 
675 C., and is ortho-rhombic with a specific gravity of 3-27 ; 
and the y-form, which is produced by cooling to temperatures 
below 675 C. The y-form has a specific gravity of 2-97, so 
that its formation is accompanied by an increase in volume 
which accounts for the material disintegrating as the rapidly- 
cooled a-form is slowly converted into the /3-ortho-silicate. 

Various attempts have been made to explain the difference 
between the slowly cooled, or y-ortho-silicate, and the hy- 
draulic, or a-ortho-silicate, obtained by suddenly quenching 
the molten mass, by representing them as of different molecular 
arrangements. Such attempts are, however, founded on data 
which are far too slight to justify the use of different structural 
formulae. 

It should be observed that the calcium ortho-silicates obtained 
by Day and Shepherd were produced by heating mixtures of 
lime and silica to complete fusion and then allowing the molten 
mass to cool to various temperatures. Slags are subjected to 
this treatment, so that the work of these investigators is 
valuable when applied to slags ; but in the manufacture of 
other cements complete fusion is never reached, and the 
conditions are so entirely different as not to warrant the 
application of these investigations to Portland cement (p. 49). 

Celite is recognised by its deep brownish-orange colour. It 
is of lower fusing point than alite and gives brilliant colours 
when examined between crossed nicol prisms, Richardson 

E 2 



52 CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 

claims to have identified celite with dicalcium aluminate 
(2CaOAl 2 3 ) in solution in dicalcium silicate (2CaOSi0 2 ). 

Celite has never been isolated and examined apart from the 
other constituents of cement. Its composition is, therefore, 
entirely problematical, and definite statements with regard to 
it should be accepted with reserve. 

Felite forms colourless rhombic crystals in some partially 
decomposed blast-furnace slags, but is by no means common. 
Its composition is not accurately known, though Kappen and 
others believe it to be a non-hydraulic form of calcium ortho- 
silicate. Some chemists maintain that it is a magnesium ortho- 
silicate, or a double silicate of magnesium and calcium. 

Alite crystals belong to the rhombic system, and tend to 
assume hexagonal forms, but their properties are by no means 
clearly established. C. Richardson claims to have identified 
alite as a solid solution of tricalcium aluminate (3CaOAl 2 0^) in 
tricalcium silicate (3CaOSi0 2 ). Such a solid solution cannot 
exist in the form of such definite crystals as those generally 
recognised as alite. Le Chatelier has found that certain 
grappiers (p. 34) consist almost entirely of alite, and contain : 

Per cent. 

Lime . , . . . . 66-0 

Silica . .'. ? . . . 26-0 

Alumina . . . . V .~ 3-5 

Ferrous oxide (FeO) . . . . " 1-0 

Water, etc. . . "" . " . . 3-5 
CaO : Si0 2 ratio = 2-75 : 1 mols. 

This is, however, far too low in alumina to be comparable 
to the best Portland cement. 

Le Chatelier regarded the alumina and water in cement as 
impurities, and concentrated his attention exclusively on the 
lime and silica present. He therefore endeavoured to prepare 
a synthetic cement in which the ratio of CaO : 8i0 2 =3:1, 
on the assumption that the lower ratio, 2-75 : 1, in grappiers 
and in some of the best Portland cement is due to impurities. 
He found that a calcined mixture containing lime and silica 
in the proportion of 3 molecules to 1 (3Ca08i0 2 ) remained 



TRICALCIUM SILICATE 53 

constant in volume on setting, but hardened remarkably 
slowly. The brothers Newberry confirmed this, and showed 
that with even a slight excess of lime (SJCaO . Si0 2 ) the product 
did not form a sound cement, but cracked when kept under 
water. Le Chatelier also found that commercial cements with 
only sufficient lime to correspond to a ratio of 2-5CaOSi0. 2 
were inferior in strength, liable to disintegrate and deficient 
in alite. 

Nevertheless, the evidence is almost conclusive against the 
existence of 3CaOSi0. 2 in notable quantity in cements, or even 
of the possibility of its formation by heating pure lime and 
silica in suitable proportions. Day, Shepherd and Wright. have 
found that specimens which were thought to be the synthetic 
trisilicate were really composed of an intimate mixture of 
lime and calcium ortho-silicate crystals, which may easily be 
mistaken for a homogeneous substance. 

Some chemists have suggested that the microscopical 
appearance of alite crystals implies that profound modifications 
have been induced in consequence of the presence of adven- 
titious substances in the cement, as a result of which the 
3CaOSi0. 2 has been rendered stable. Such a supposition is 
purely speculative and unnecessary. The repeated unsuccess- 
ful attempts by such competent authorities as Le Chatelier 
and others to produce a really satisfactory cement from pure 
lime and silica indicate that some constituent other than lime 
and silica is essential. The clearly recognisable differences 
between cements made from basic slag and limestone and those 
made from clay and limestone also indicate that the particular 
state of combination, or the existence in the free state, of the 
alumina and silica present is of great importance as regards 
the structure of the resultant cement. The constancy of the 
ratio CaO : 8i0 2 =3:1 appears to be merely a coincidence 
and not to indicate the whole of the facts, for this ratio would 
be the same in a compound with the formula 3xCaO . yAl. 2 0. 3 . 
xSi0 2 , but the properties of such a compound would be entirely 
different from those of SxCaO . xSi0 2 on account of the alumina 
present. It is precisely because investigators have been 
obsessed with the idea of the existence of 3CaO . Si0. 2 in cements 
that most of them have overlooked the possible existence of 



54 CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 

calcium alumino-silicates, whilst most of those who have 
recognised the possibility of such triple compounds have not 
pursued their researches far enough to prove their case. It is 
one of the curiosities of modern chemistry that so many 
investigators have so willingly accepted statements as to the 
existence of certain silicates and aluminates in spite of the 
fact that these substances cannot be prepared under conditions 
met with in the manufacture of cements. Yet these considera- 
tions have been almost entirely overlooked by those engaged 
in investigations of the chemical constituents of cements. 

Thus, to account for the presence of alumina in all satis- 
factory cements, various investigators have prepared calcium 
aluminates corresponding to the ratio 2CaO . Al. 2 O.^ and 
finding that these set very rapidly, producing cements of 
constant volume when set and of good hardening properties, 
they at once assumed that such binary aluminates are present 
in Portland cement in simple admixture with the alite already 
mentioned. Unfortunately, no one has yet been able to isolate 
such calcium aluminates in appreciable quantities from 
commercially valuable cements, and as calcium aluminates, 
when pure, turn an alcoholic solution of phenolphthalein red, 
their presence in Portland cement would be readily detected 
by this indicator. Well made and freshly burned Portland 
cements do not, however, produce any colour with this solution, 
so that it is improbable that free calcium aluminates occur in 
these cements. Thus, although the theory that cements are 
composed of simple mixtures of substances, such as 3CaO . Si0 2 
and 2CaO . ALO^, is in some respects simple and convenient, 
its disadvantages are very great. 

Some of the largest alite crystals yet obtained were produced 
by Schmidt and Unger by heating Portland cement to complete 
fusion in an electric arc. The whole mass became crystalline 
on cooling, and consisted of a largely preponderating proportion 
of crystals all of one kind, together with a microcrystalline 
aggregate of a different substance, possibly celite, but more 
probably a heterogeneous mixture of crystals containing all 
the " impurities " in the cement. The largest and most 
perfect crystals proved to be identical in physical characters 
with alite. On analysis they were found to contain : 



TRICALCIUM SILICATE 



55 



(a) 



Lime ...... 67-43 per cent. 

Silica . . . . . 23-43 

Alumina . . . . . 3-78 

Ferrous oxide . . . . 2-32 ,, 

Magnesia . . . . . 2-44 ,, 

Water, etc. . . . . . 0-60 

These results agree sufficiently closely with those found by 
Le Chatelier (p. 52), but are lower in alumina than are the 
best Portland cements. 

W. and D. Asch have shown that, in all probability, there 
is no single substance alite which is the essential constituent 
of all Portland cements, but that these materials contain one 
or more calcium alumino-silicates of a highly complex struc- 
tural composition. They consider that Portland cements are 
basic salts of these aluminosilicic acids, and that they 
consist of a series of hexagonally or pentagonally arranged 
groups of silicon and aluminium atoms to which are attached 
a number of side chains, the latter containing the greater part 
of the base. 

In Great Britain, the clays and shales generally used for 
cement-making are highly siliceous, and probably have a con- 
stitution corresponding to (a) H ls AlQSi l2 42 , or (b) H 1& Al^8i u 0^ f 
though other types are possible. In accordance with Asch's 
theory these typical clays have the following constitutional 
formulae l : 

(OH)z OH (OH) 2 







o 



(OH),= 






C 


) 














SiOAl 


AlOSi 


\ 


(OH) 


\\ 
( 


, 


( 


1 


06 


> 


!' = 




1 / 








\l 




i 




SiOAl 


AlOSi Si = 


(OH). 


/ 


\ 


/ 


/ 


\ 




/ 




o 


( 


9 
















\Al/ 

1 








\Si/ 


/ 






OH 








(OR), 







(OH) 

1 The lime, magnesia and alkalies present in the clays replace equivalent 
hydroxyl (OH] groups and the iron compounds as described on p. 74, but for 
clearness the formulae shown are those for pure clays of the same type. 



56 CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 



O 



O 



o, 
o- 



o 
^ 






o 

1 



I 



\ 



o o 



^ 



O 



$' 



These may be represented more simply, as 



I 

/\ 

8i\Al 

\/ 
I 




Si 



=( Si 



Al 



(a) 



(6) 



THE ALUMINO-SILICATE THEORY 



57 



On heating with lime, combination occurs which, in the case 
of formula (a), may be represented by the following equation : 

HisAlvSiwOu + 38<7aO = Ha&Al$iuf)n + 8# 2 

water being liberated owing to the hydroxyl (OH) groups 
which give the clay its acid character being replaced by basic 
groups and forming a cement with the structural formula on 
p. 58, which may be represented in an abbreviated form by : 



5CaO.CaO.5CaO. 
1 



4CaO 
4CaO = 




= 4CaO 
= 4CaO 



5CaOCa05CaO 

Clays of type (b) form cements with formulae similar to the 
second one on p. 59. 

According to this theory, there is an extremely large number 
of calcium alumino-silicates of very similar percentage com- 
position, yet having noticeably different chemical structures. 
This number is indefinitely increased by the presence of 
magnesia, potash, soda and iron oxide in the raw materials 
from which cements are made, these oxides forming still more 
complex groupings around the central hexagon or pentagon 
rings. Any potash or soda present may replace part or all of 
the CaO attached to the ^4Z-ring ; any magnesia will usually 
replace one or more of the CaO molecules at the sides of the 
formula. These complications have, however, been omitted 
for the sake of clearness. 

The following are typical examples of Portland cements : 

(2) (2) OK (2) (2) 



4= 




+ 0'5Na(K)CO< 



ONa 4 

. 3lCaO . MgO . 



c 

o 
o 
e 

6 



58 CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 

/'0\ 



O 

o 

js 



o o 

0_ 



^ O CQ 

1^^4 



<S-_0--^ 



/*\ 






-o ' 



o 



5 



o 
6 



o 
6 



THE ALUMINO-SILICATE THEORY 

5 OK 5 



59 



4 o_ 
4 o_ 




,=4 
Si + 0-SNaCl 

_ 4 o 



5 OK 5 
= 3oCaO . MgO . K^O . 

where each hexagonal ring represents 6Si0. 2 or 3A1 2 O 3 , and the 
numbers indicate the corresponding number of molecules of 
CaO or MgO which have replaced OH groups in the clay. A 
somewhat different type of cement is 




39<7aO . 

in which the hexagonal ring represents 3A1 2 3 and each 
pentagon represents 5Si0. 2 , the numbers having the same 
meaning as before. 

If this theory is correct, what occurs when a mixture 
of clay and calcium carbonate is heated under the conditions 
usual in the manufacture of Portland cement is that the 
hydroxyl groups attached to the silicon atoms in the clay are 
replaced by anhydrobasic groups derived from the lime. This 
implies that on heating clays the molecule is not broken up 
into its constituent oxides, but that a single compound is 
formed (p. 41), or the material is dissociated into its respective 



60 CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 



ions. These ions may, at a later stage in the use of the cement, 
re-act as though a single compound was present. The loss 
of resistance to, or conversely the increase of conductivity of, 
electricity suffered by the raw materials in the manufacture of 
Portland cements is shown in Fig. 6, which represents the 
results obtained by E. Dittler and K. Herold on (1) a dry marl, 
(2) another dry marl, (3) a cement mix of chalk and marl made 
by the dry method, (4) a cement mix of limestone and clay 

made by the wet 
method, and (5) a 
mixture of pure cal- 
cium carbonate and 
kaolin (china clay). 
The general simi- 
larity of the curves 
is very striking, and 
it appears highly 
probable that the 
differences observ- 
able which are 
greatest at the lower 
temperatures - - are 
chiefly due to the 
variations in the 
composition of the 
materials rather than 
to any differences in 
the reactions which 
occur. 

(Zentr. Chem. Anal. Hydraul. 
have found that the electrical 




800 850 900 050 1000 1050 1100 1150 1200 12501300 1350 WHf 

FIG. 6. Electrical Conductivity of Kaw 
Cement Mixes. 



E. Dittler and L. Jesser 
Zemente, 1910, pp. 7178) 
conductivity increases gradually until a temperature of about 
1375 C. is reached, when there is a break in the graph repre- 
senting it accompanied by an endothermic reaction and slight 
fusion, and when the temperature reaches 1425 to 1450 C. an 
exothermic reaction occurs suddenly with the immediate 
formation of crystals of alite (?) with a little celite. The 
change in conductivity and formation of the alite (?) is shown 
in Fig. 7, which is the resistance graph of a clay-limestone 



ELECTRICAL CONDUCTIVITY OF CEMENTS 61 



cement maintained for three hours at a temperature of 
1430 C. 

Such a break in the conductivity curve of many minerals is 
well known to coincide with their melting points, and clearly 
points to the formation of a crystalline compound of definite 
composition and with a sharply marked melting point of 
1425 C. This result is in complete opposition to the view 
that cements are composed of a mixture of tricalcium silicate 
and dicalcium aluminate, and shows that a cement clinker 

is a definite 

chemical indi- 
vidual and not Ohms 
a solid solution 
of two or more 
substances. In 
this respect it 
confirms Asch's 
theory that the 
chief constitu- 
ent of cements 
is a definite 
calcium alu- 
mino- silicate. 

If alite has 
the definite 
c o mposition 
such as those 
assigned to the 
chief con- 



700 



600 



500 



400 



300 



200 



100 




190 210 230 250 270 290 310 330 mm-. 
FIG. 7. Electrical Conductivity of Clay -lime 
Mixture. 



stituent in cements by W. and D. Asch and there seems 
little reason to doubt that its composition is quite definite 
for any given cement, whatever may be the arrangement 
of the atoms within the molecule it is clear that the 
best qualities of cement will be those in which the composition 
most closely resembles that of the particular alumino-silicate 
(alite ?) present in the largest proportion. It has long been 
recognised that very small variations in the percentage of lime 
cause serious differences in the strength and properties of the 
cement, and that the limits of composition of the mixture of 



62 CHEMIAL AND PHYSICAL CHANGES IN CEMENTS 

raw materials are very narrow. In endeavouring to obtain 
some simple guide of the limits of composition of cements, it 
has been customary to follow Le Chatelier's assumption that 
Portland cements are composed of 3CaOSi0. 2 and 3CaOAl. 2 0.j 
and Newberry 's assumption that they are composed of 
3CaOSi0. 2 and 2CaOAL 2 0$. On these assumptions a cement 
will consist of a molecules of 3CaOSi0. 2 , and b molecules of 
either 3CaOAW. 3 or 2CaOAl. 2 0^. From this it follows that 
the ratio between the lime and (silica + alumina) in cements 
should remain constant, and the ideal proportion of these oxides 
would then be 
(1) according to Le Chatelier : 

molecules CaO 



molecules Si0 2 + molecules 
(2) according to Newberry : 

3a -j- 26 molecules CaO 



a molecules SiO^ + b molecules 



= 3 



In spite of the fact that ^CaOAl 2 3 has not been proved to 
exist in cements and that the constant 3 is too high, the equation 
suggested by Newberry has not been adopted, and Le Chatelier's 
is still regarded in many quarters as indicating the maximum 
proportion of lime which can exist in a sound cement. In the 
British standard specification for Portland cement a somewhat 
lower constant is given, viz. : 

molecules CaO 2- 85 



molecules $*O a + molecules Al 2 0t 

This demands a somewhat lower lime content than that 
suggested by Le Chatelier. 

Where the use of molecular equivalents is inconvenient, 
the limit is expressed as 

% lime = 2-8 X % silica + 1-1 X % alumina 

The exact proportion of lime which can be used will depend on 
the care and skill used in the manufacture of the cement. If 
the raw materials are extremely finely ground, thoroughly 
mixed and properly burned, a higher proportion of lime may be 



FORMULA FOR COMPOSITION 



63 



present than in a less skilfully prepared cement. The larger 
the proportion of lime present, combined as alumino-silicate, 



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I 2345 6 7 8 9 10 II 12 13 14 IS 16 17 18 13 20 

mols. Si 2 

FIG. 8. Empirical Limits of Composition of Portland Cements. 

the stronger will be the cement. Lime in other forms is less 
valuable, and uncombined or free lime is dangerous. 

As the proportions of lime, silica and alumina all vary 
in different samples of commercial cement, it is convenient, 



64 CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 

in classifying samples, to express the proportions in molecular 
equivalents, the number of molecules of alumina being taken 
as unity, thus 

UCaO AW-. 



In this way one of the three variables is removed and the 
remaining two may be plotted on a chart as ordinates and 
abscissae, respectively. Thus, accepting the limits in the 
British standard specification given above, the molecular 
ratio of each of the oxides is 

2-85 (a + b) molecules CaO 
a molecules Si0 2 
b molecules Al<) 3 

and by taking 6=1, the maximum permissible limit of lime = 
2-85 (a -{- 1) molecules to each a molecules of silica and 1 
molecule of alumina. The disadvantages of assuming the 
existence of 3CaOSi0 2 and 2CaOAl 2 O s in cements are, in this 
way, avoided, and the influence of each of the constituent 
oxides is most easily perceived. Fig. 8 is a chart of this 
kind, based on the ratios 



xCaO ySi0 2 1-00 

in which the various values of x are plotted on the ordinate and 
those of y on the abscissae. The value for alumina being made 
constant it does not require to be plotted. In order to show 
the limits within which commercially useful cements lie, the 
curve giving the maximum limits for lime recognised by the 
British standard specification based on the ratios 

2-85 (a + 1) CaO, aSi0 2 , 1-00 A1 2 3 

is shown. The lowest proportions of lime generally recognised 
as permissible is that given in the hydraulic modulus, a term 
introduced by W. Michaelis, and adopted in the German 
standard specification. 



Hydraulic Modulus = % sUica + % ^^ + % ferric 



German experience has shown that the hydraulic modulus 
should never be less than 1-7. In any case, the hydraulic 



HYDRAULIC MODULUS 65 

modulus is not alone sufficient for determining the proportions 
of each of the ingredients used in making Portland cement. 

The foregoing limits are too wide in some respects, for it is 
well known that a variation of 2 per cent, in the proportion 
of calcium carbonate in the slurry or mixture before it enters 
the kiln will make all the difference between a sound and an 
unsound cement. The limits mentioned, and shown in 'Fig. 8, 
must, therefore, be regarded as purely approximate, and as 
of academic rather than empiric value. 

In .connection with this diagram it is interesting to note that 
according to Asch's theory all cements should lie on one of the 
four vertical .lines marked A. The height above the base line 
at which they should be placed depends on the lime-content, 
which according to the same theory may vary within limits 
similar to those indicated by the dotted lines on the chart, 
i.e., between three and eighteen molecules of lime. 

The commercial limits of composition usually recognised are 
somewhat larger than those corresponding to Asch's theory. 
This is due to the impurities in the materials used commercially. 

In the limiting formulae mentioned on pp. 62 64, no definite 
ratio between the silica and alumina is stated, yet this is of 
great importance, as cement rich in silica is deemed superior to 
that deficient in it. Moreover, cement mixtures low in silica 
when burned in rotary kilns cause the formation of adherent 
rings of clinker which choke the kiln. 0. Dormann has found 
that the best cements are obtained commercially when the 
limits are R^O^ : Si0. 2 == 1 : 2-5 to 3-0 expressed in absolute 
weight, or 1 molecule A1 2 0.^ to .each 4-25 to 5-1 molecules 
Si0. 2 , and a study of numerous published analyses of German, 
American and British cements confirms this ratio, very few 
high-class cements with an A1 2 3 : Si0 2 ratio lower than 3-66 
(molecules) having been found. According to the theory of 
the constitution of cements given on pp. 55 59 the Al. 2 3 : Si0 2 
ratio should be definitely 4 or 5 (molecules). 

In the chart shown on p. 63, the minimum values of lime are 
made to include an allowance for the iron oxide by assuming 
that it is equivalent to that of the alumina present. 

The lines on the chart show the relatively narrow range of 
composition permissible in cements, but it must not be for- 

c. F 



66 CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 

gotten that they are not defined with absolute accuracy, and 
future investigations into the constitution of cements may 
result in an even narrower limit of composition being imposed. 
The assumptions on which they are based are by no means 
free from objection ; thus, it is not by any means correct to 
add together the percentages of alumina, ferric oxide and silica 
as is done in the formula for the hydraulic modulus, though, 
in the ordinary manufacture of cement, variations in the 
proportion of alumina and iron are sufficiently small to permit 
such formulae to be conveniently used in spite of their theoretical 
inaccuracies. For the same reasons the lines on the chart 
cannot rightly be extended very far beyond the positions 
shown, as it has been found that cements containing a ratio 
of only 2-5 molecules of lime to 1 molecule of silica are unsound. 

Chemical Reactions between Silica and Lime. 

Owing to the proportion of free silica in the raw materials 
and the possible decomposition of the clay into a mixture of 
free silica and alumina, there is always some free silica present 
in the contents of the cement kiln during an early stage of the 
heating. 

As well-made Portland cements leave only an insignificant 
proportion of insoluble matter when treated with hydrochloric 
acid, the study of the silicates present is simplified by the 
exclusion of all possible silicates which are insoluble in this 
acid. For this reason, synthetic experiments in which the 
substances actually present in the raw materials used (or 
supposed to be produced by the direct action of heat on these 
materials) are heated in groups of twos and threes, are of 
the utmost importance. The products obtained from pure 
materials can be examined with accuracy and without the 
disturbing factors present when less pure materials are used. 
Experimenting in this manner, J. W. Cobb has found that on 
heating a mixture of limestone or chalk and finely powdered 
quartz, a reaction occurs at 800 C. (i.e., at or below the tem- 
perature at which free lime is formed), and the combination of 
the lime and silica takes place with increasing rapidity as the 
temperature is raised, a soluble silicate being formed. At 



REACTIONS BETWEEN SILICA AND LIME 67 

temperatures below 1250 C. no fusion is observable, though 
the formation of the soluble silicate CaOSi0 2 is quite definite. 
At 1400 C. (or just below the highest temperature in a rotary 
kiln) the formation of calcium mono-silicate is practically 
complete. Any calcium sulphate present is also decomposed 
by the silica, calcium mono-silicate being formed at tempera- 
tures above 1005 C. 

Cobb also found that if sufficient silica is present, the pro- 
portion of silica and lime has no influence on the result, and 
that the compound CaOSi0 2 is invariably formed. When the 
original mixture contained the materials in the ratio SCaO -j- 
8i0 2 he observed the formation of a more basic silicate 
(2CaO . Si0 2 ) at first, and that this persists in the presence of 
sufficient lime ; otherwise CaO . Si0 2 is formed. Under no 
circumstances could Cobb produce a calcium silicate containing 
more lime than 2CaOSi0. 2 . 

The apparent impossibility of producing a compound 
corresponding to 3CaOSi0. 2 makes it very difficult to accept 
the largely-held view that Portland cements contain a large 
proportion of this substance. As already indicated, in such a 
view the alumina present in cements is overlooked, or is, at 
least, regarded as forming entirely different compounds. 

So far as cements are concerned, it appears improbable that 
more than an insignificant proportion of any calcium silicate 
as basic as, or more basic than, 2CaOSi0 2 can be present, and 
that such reactions as may occur between free lime and free 
silica in the cement must finally result in the formation of 
calcium mono-silicate (CaOSi0 2 ): The fact discovered by 
Boudouard, that CaOSi0 2 corresponds to the most fusible 
mixture producible from lime and silica alone, implies that 
this substance, if present, forms part of the inter-crystalline 
material observable in all Portland cements, and as it is devoid 
of hydraulic properties it is of no value in cements. 

Calcium ortho-silicate 2CaOSi0 2 is present in cements and 
slags not containing sufficient silica, or which have not been 
heated sufficiently long to form the meta-silicate CaOSi0 2 . 
The ortho-silicate is described under the term belite (p. 51). 

Many chemists interested in cements have attached great 
importance to the investigations of Shepherd, Day, R-ankine. 



68 CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 

Wright and others on the products formed by fusing binary 
mixtures of lime and silica, or lime and alumina, and ternary 
mixtures of lime, alumina and silica, and allowing the fused 
mass to cool slowly. Under such conditions various crystalline 
substances are found in the mass produced, the most important 
being in the case of lime-silica mixtures a, ft and y 
Ca 2 SiO i , a and ft CaSi0 3 , tridymite, quartz and free lime. 
In burning cements, however, complete fusion of the mass is 
never reached, and the products formed in a mixture which 
has only been partially fused are not the same as those produced 
in a completely fused mass of the same original composition. 
For this reason the application of the work of the above- 
mentioned investigators to the constitution of cement clinker 
is somewhat misplaced. Indeed, no less an authority than 
C. Doelter (Zeitsch. /. Elektro Chem., Bd. 17 (1911), p. 795) has 
reached the conclusion that " the application of the phase rule 
to fused mixtures of silicate components is erroneous," and 
that such substances " must not be treated as metallic alloys, 
as this leads to erroneous conclusions which only increase the 
difficulties experienced in the application of physical chemistry 
to mineralogical problems." Doelter has also shown, experi- 
mentally, that silicate mixtures when heated above their 
melting point produce quite different substances from those 
which are formed when no over-heating has occurred. The 
study of such completely fused mixtures is, therefore, of little 
value in studying the constitution of cements, and is most 
likely, as Doelter suggests, to lead to false conclusions. 

The size of the grains of free silica has an important influence 
on the reaction. 0. Dormann has found that unless the free 
silica is fine enough not to leave a residue of more than 20 per 
cent, on a No. 180 sieve, a considerable part of it will remain 
uncombined. 

Experience shows that, within certain limits, cements with 
a high proportion of silica are stronger than others, the ultimate 
strength increasing approximately with the percentage of 
silica. This is usually attributed to the larger proportion of 
tri-calcium silicate in such cements, but, in view of the doubts 
as to the existence of tri-calcium silicate in cements, a more 
probable explanation is that, within the limits indicated, the 



REACTIONS BETWEEN LIME AND ALUMINA 69 

ultimate strength really depends on the nature as well as on 
the amount of alumino-silicate formed. 

An increase in the percentage of silica in a cement will 
usually be accompanied by a decrease in the alumina and a 
reduction in the speed of setting of the cement. This is 
apparently due to the smaller proportion of the alumino- 
silicate, which is the essential constituent of the cement, and 
to the correspondingly larger proportion of inert matter 
present. 

Chemical Eeactions between Lime and Alumina. 

It is by no means certain that the contents of a cement kiln 
ever contain an appreciable amount of free alumina, though 
it may possibly be produced by the direct action of heat on 
clay. Such free alumina as does occur may enter into com- 
bination with both lime and silica, forming an alumino-silicate, 
or it may combine solely with lime to form one of two calcium 
aluminates, CaOAl 2 3 or CaO 2A1 2 S , both of which are soluble 
(with decomposition) in hydrochloric acid. 

The reaction between lime and alumina commences (according 
to J. W. Cobb) at 850 C., occurs rapidly at 1100 C., and is 
practically complete at 1300 C. According to the proportion 
of lime and alumina either CaOAl 2 O 3 (which is soluble in acid), 
2CaOAl. 2 O s (which is very slowly soluble) or an insoluble 
calcium aluminate of unknown composition is produced. 
Cobb has been unable to produce a calcium aluminate corre- 
sponding to 3 CaOAl. 2 O s and soluble in hydrochloric acid. The 
^CaOAl. 2 3 obtained from molten mixtures at 1531 C. by 
Shepherd and others is not produced under conditions existing 
in cement kilns, and, as already explained (p. 68), it is erroneous 
to suppose that a substance produced when a completely fused 
and usually overheated mixture is allowed to cool is neces- 
sarily formed when the same raw materials are heated to only 
partial fusion. Many binary silicates and aluminates possessing 
hydraulic properties appear to exist, but in all probability a 
large number of these are incapable of production by the 
methods at present employed in the manufacture of cements. 
The tri-calcium aluminate and tri-calcium silicate, so generally 



70 CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 

considered to be essential constituents of cements, both appear 
to be of this nature. The compound 5CaO3Al. 2 3 , which rnelts 
at 1386 C., may be present in cements, but has not yet been 
identified therein. 

The importance of such calcium aluminates as may be present 
depends upon the proportions in which they occur in cements. 
Those who maintain that cements consist chiefly of a mixture 
of tri-calcium silicate and tri-calcium aluminate, naturally 
consider that a large proportion of the latter aluminate must 
be present, notwithstanding the fact that it has not yet been 
definitely isolated from commercial cements. 

The di-calcium compound 2CaOAl.,O s prepared by Newberry 
is a quick-setting hydraulite with constant volume and good 
hardening properties. If present in a commercial cement it 
would not affect the quality of the material, though it might 
increase the rate of setting. Hence, it is generally considered 
desirable to have the proportion of alumina as low as possible, 
so as to secure greater ultimate strength and slowness of 
setting, but it must not be in too small a proportion, or the 
material will not be properly burned at the temperature 
ordinarily used. 

In other words, if too little alumina is present the amount 
of alumino-silicate produced will be insufficient to produce a 
valuable cement, and a mixture of various feeble hydraulites 
with only a little true cement will be formed. If the com- 
position of cement is represented as x molecules CaO, Si0. 2 + y 
molecules 2(7aO, ALO B it is difficult to conceive how the presence 
of alumina can reduce the temperature at which the cement 
clinker is formed, because a mixture of di-calcium aluminate 
and tri-calcium silicate, when heated, does not form a clinker at 
the temperature reached in cement kilns. The behaviour of 
a completely fused mixture which has been heated to a much 
higher temperature and then allowed to cool does not have 
any bearing on this question, and the almost inevitable con- 
clusion is that the alumina enters into combination with both 
the lime and silica and produces a definite alumino-silicate, and 
not a solid solution. It is precisely because this alumino- 
silicate is an essential constituent of the cement that the 
percentage of alumina in the raw mixture must not fall below 



REACTIONS BETWEEN LIME, ALUMINA & SILICA 71 

a certain limit, the exact value of which has not been definitely 
ascertained, though it appears to be approximately one- 
twentieth of the weight of the lime in the cement. If, on the 
contrary, too large a proportion of alumina is present, the 
excess above that needed for the alumino-silicate will induce 
the formation of calcium aluminates, which produce a sticky 
and adherent clinker of less strength than that formed from 
alumino-silicate alone. If the 2CaOAl. 2 0. 3 theory is held, it is 
difficult to understand why an increase in the proportion of 
alumina above one-third of the weight of the silica present 
should produce a weaker cement than is formed when the 
normal proportion of alumina is present, as pure di-calcium 
aluminate forms a stronger cement than pure tri-calcium 
silicate. The chief evidence against the existence of calcium 
aluminates in appreciable quantities in commercial cements 
is (a) the appearance of the latter under the microscope, 
(b) the fact that the " alite " crystals contain practically all 
the alumina in the cement, apparently in the form of a com- 
pound of lime alumina and silica, and (c) the absence of any 
colouration when Portland cements, are heated with an alcoholic 
solution of phenolphthalein, whereas both the synthetical 
calcium aluminates produce a strong colouration with this 
indicator. 

Chemical Reactions between Lime, Alumina and Silica. 

J. W. Cobb has found that when a ternary mixture of lime, 
alumina and silica, corresponding to CaO -f- Al. 2 0. 3 -j- lQSiO. 2 , 
is heated to temperatures ordinarily reached in cement manu- 
facture, the reaction begins at 800 C. and proceeds slowly up 
to 1300 C., at which temperature a siliceous mass is pro- 
duced which is unaffected by hydrochloric acid. A similar 
mixture, but containing a much larger proportion of lime, 
porresponding to SCaO -\- Al. 2 0. 3 + 3Si0. 2 , and intended to 
represent Portland cement (though it has at least 3 per cent, 
more alumina and 3 per cent, less silica than the average 
cement), forms, at 1250 C., a product which is soluble in 
hydrochloric acid, but is not a useful cement and contains 
much free lime. If it is really a compound it has a 



v 72 CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 

composition corresponding to 2-1 CaO, 0-4 ^4/.,0 3 , SiO.,. J. W. 
Cobb inclines to the view that the insoluble products are true 
ternary compounds, whilst the soluble ones are mixtures of 
binary silicates and aluminates, though the evidence afforded 
by his experiments is by no means conclusive on this 
point. 

A further idea of the great complexity of a study of these 
compounds may be gained from the fact that in the product 
formed by heating CaO -f- Al.fi z + 10 Si0. 2 the alumina acts 
as a base and is replaced by Na.fi, but in the analogous 
product from Na.fi -\- CaO -f- IQA1 2 3 , the alumina acts 
precisely like silica, i.e., as an acid, the last named compound 
bearing the most striking resemblance in many of its properties 
to that from CaO -f Alfi.^ WSiO.,. 

The inter-reactions of other substances in cements are extremely 
difficult to ascertain. To a large extent the felspathic, 
micaceous and other complex minerals which may be present, 
form, either alone or in combination with the lime present, a 
considerable portion of the amorphous inter-crystalline matter 
always observable when a polished piece of cement clinker is 
etched with water and examined under the microscope. 

The influence of the small proportions of these various 
silicates cannot be ascertained with accuracy, as the apparently 
complex products partake of the nature of viscous glass or 
slags, and do not readily crystallise in the form of definite 
compounds. Moreover, attempts to cause the amorphous, 
glassy matter to crystallise invariably effect changes in its 
composition, so that an examination of the crystals so formed 
is of little real value. Cements made some years ago, for which 
stationary kilns were used, show a considerably higher pro- 
portion of insoluble matter and of imperfectly burned material, 
most of which is inert and useless in the cement. The use of 
rotary kilns, working at higher temperatures, enables a superior 
cement with a much smaller proportion of undesirable and 
inert matter to be produced, facilitates the formation of those 
compounds which have a hydraulic value, correspondingly 
reduces the proportion of uncombined oxides and useless 
products, and increases the rate at which the cement sets. 
Some of the oxides stated in analyses as being present in 



EFFECTS OF MAGNESIA 73 

cement are of sufficient interest for their reactions to be 
described briefly. 

Magnesia is only present in small quantities in the better 
class commercial cements, as the use of magnesic materials 
is prohibited in all official specifications for Portland cement, 
but larger proportions are present in many natural cements. 
It combines with alumina and silica in a manner analogous to 
lime, but there is a great difference of opinion as to the value 
of magnesic cements. Magnesia needs a much higher kiln 
temperature than lime in order that it may combine with 
silica and alumina ; hence an excess of it in a cement mixture 
is undesirable, as it will largely exist in an uncombined state 
unless an unusually high kiln temperature is employed. 

Newberry has found that magnesia up to 20 per cent, 
produces a satisfactory cement if due care be taken in mixing 
and burning, and the relatively high proportion of magnesia 
in many satisfactory natural cements shows that magnesia 
is not in itself a disadvantage, providing that the cement is 
properly burned. Therein lies the difficulty. 

Tests on cement to which magnesia has been added just 
previous to gauging, show that magnesia causes expansion, 
but much less rapidly than in the case of free lime, and the 
damage effected is correspondingly* less. 

Since the failure of a French bridge and a portion of the 
Aberdeen harbour works, it has been customary to limit the 
percentage of magnesia compounds to a maximum of 3 per 
cent. MgO. 

The lime-magnesia cements differ in several important 
respects from Portland cements, but are not, at present, of 
sufficient commercial importance to warrant further description 
here. 

" Alkalies " chiefly potash and soda silicates and alumino- 
silicates are only present to a small extent, as a large pro- 
portion of the alkaline oxides are volatilised during the burning 
when rotary kilns are used, so that when present only in small 
quantity in the raw mixture they are unimportant. According 
to Hillebrand the alkalies be'gin to volatilise before all the 
sulphur tri-oxide has been expelled, i.e., at a temperature of 
about 1150 C. Being very fusible, particularly when mixed 



74 CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 

with lime, alumina and silica, they form a part of the amorphous 
glassy material of an inert nature observable in all cements. 

Iron compounds, analogous to those of alumina, appear to 
be formed in the manufacture of cements, but their nature is 
by no means well known. The reducing atmosphere inside 
most cement kilns causes the formation of ferrous compounds, in 
which the iron oxide behaves precisely like a base, though 
calcium ferrites (xCaO,yFe. 2 O s ) appear to exist in small quantities 
in some cements, these proportions being much smaller than 
is generally supposed. Under ordinary conditions, the ferrites 
are devoid of hydraulic properties, but a mixture corresponding 
to 2CaO,Fe 2 3 was prepared by Newberry and produced a 
black slag on heating ; the ground slag did not set when mixed 
with water in the cold, but on exposure to a temperature of 
100 C. it rapidly formed a very hard mass of constant volume. 

Newberry also found that a cement in which iron oxide 
entirely replaced the alumina usually present sets slowly to 
a sound mass. This points to the existence of ferro-silicates, 
rather than ferrites in cements. These calcium ferro-silicates 
are apparently of the type xCaO,yFe.>0.^z8iO.>, but very little 
investigation has been made of their properties. They appear 
to have a close resemblance to the corresponding alumino- 
silicates and to be formed in a similar manner, but are much 
less hydraulitic. " The fusibility of ferrous alumino-silicates, 
their strong tendency to form glasses or slags, and the ease with 
which they are formed at the temperatures employed in cement 
burning, make it highly probable that these compounds 
contain the greater part of the iron present in the cement. 
Their nature is such, however, that it is almost impossible to 
isolate them. Ferrous alumino-silicates prepared synthetically 
have a close resemblance to cements, but are, on account of 
their dark colour and the rapidity with which they set, almost 
useless for commercial cements. 

A variety of Portland cement, in which iron ore replaces the 
clay ordinarily used, is employed in considerable quantities 
on the Continent and in the Panama Canal on account of its 
resistance to sea water. In this case the iron appears to 
replace the alumina in ordinary Portland cement and to form 
a completely analogous compound. It has not yet been 



EFFECTS OF ALKALIES AND IRON 75 

conclusively shown whether such iron ore cement contains a 
calcium ferro-silicate of the hexite-pentite type or whether it 
is a mixture of calcium silicates and ferrites, but the existence 
of a ferro-silicate is the more probable. 

There is a custom among writers on cement to associate 
together the alumina and iron expressed in the form of ferric 
oxide (Fe. 2 0. 3 ) as though they were mutually replaceable. 
Although ferric oxide is, to some extent, capable of replacing 
alumina in a hydraulite, it very seldom does so in commercial 
cements, because the reducing action of the kiln gases cause 
the formation of ferrous oxide, which is instantaneously 
converted into ferrous silicate or ferrous alumino-silicate, and 
then takes little or no part in the chemical changes which 
occur in the kiln. Very few Portland cements contain an 
appreciable proportion of ferric oxide (Fe. 2 s ) (notwithstanding 
the fact that this substance is reported in most analyses) ; 
the greater part of the iron is in the ferrous state, and exists 
in combination with silica and alumina as a glassy mass or 
slag. It would be more correct, in most cements, to regard 
the iron as allied to the lime and magnesia as a fairly powerful 
base and not, as is so often the custom, to associate it with 
the alumina and silica as though it were an acid. That a 
small proportion of the iron in a cement is present in the form 
of an acid radical is not improbable, but the greater part of it 
is certainly not in this form. Hence, in empirical formulae 
designed to express the limits of composition of a cement 
mixture, the iron oxide (unless more than 10 per cent, is 
present) is preferably omitted, the " constant " in the formula 
being altered accordingly, if necessary. 

The chief objections to a large proportion of iron compounds 
in a cement are the very dark colour of the cement, the reduced 
hydraulicity, and the lower fusing point of the materials as a 
whole, with a consequent change in the conditions under which 
the cement is burned, and an increased tendency to the decom- 
position of the cementitious compounds desired and the 
formation of solid solutions of simpler silicates. 

Sulphates and sulphides chiefly calcium sulphide CaS, 
ferrous sulphide FeS, and calcium sulphate CaS0 4 are often 
present in small proportions in cements and in larger proper- 



76 CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 

tions in slags. They are removed from the latter by quenching 
the molten slag in water ; the sulphur is then evolved as 
sulphuretted hydrogen, H.>S. Sulphides occur in the raw 
materials used for cement-making, but the greater part found 
in cement is due to the action of the sulphur in the fuel used 
for heating the kiln. J. W. Cobb has found that a mixture of 
pure chalk, alumina and quartz, in proportions corresponding 
to Portland cement, absorbed 17 per cent, of sulphur tri-oxide 
during a twenty-four hours' heating in a works furnace, 
the greater part of this being absorbed below a temperature 
of 800 C. The reducing atmosphere present in most cement 
kilns prevents the absorption of so high a proportion of sulphur 
tri-oxide by the cement, and any calcium sulphate produced 
during the earlier stages of the burning would be again dis- 
sociated at a temperature above 1100C. Nevertheless, it is 
important to avoid the use of materials and fuel containing 
sulphur as far as it is possible to do so. The chief danger 
arising from the presence of sulphides is their liability to 
become oxidised, forming sulphates and, at the same time, 
expanding and tending to disintegrate the cement. For this 
reason, a higher proportion than 2f per cent, of sulphur as 
tri-oxide (SO%) is usually prohibited in specifications. The 
addition of slag to Portland cement usually raises the pro- 
portion of sulphur trioxide beyond the limit just mentioned 
as permitted by the British Standard Specification. 

In order to regulate the setting of cement, the addition of 
2 per cent, of water and 2 per cent, of anhydrous gypsum 
(CaSO^) is permitted in the British Standard Specification. The 
water is applied in the form of steam and the gypsum in the 
form of plaster of Paris. Hence, an examination of most 
Portland cements will show a somewhat higher proportion of 
calcium sulphate than is derivable from the raw materials. 
This additional calcium sulphate is not included in the 
limit of 2|" per cent, previously mentioned. The addition 
of anhydrous gypsum has a greater retarding effect on the 
setting than the equivalent amount of plaster of Paris. If, 
however, the cement is aerated, the gypsum becomes hydrated, 
and its retarding effect is correspondingly diminished. 

The addition of gypsum or plaster of Paris, or the presence 



EFFECTS OF SULPHUR COMPOUNDS 77 

of free calcium sulphate in a cement, may be shown by adding 
5 per cent, of barium carbonate to the cement, gauging the 
mixture and testing its expansion after it has been boiled in 
water (Le Chatelier's test). The barium carbonate decomposes 
the calcium sulphate and a notable increase in the expansion 
of the cement is observable. 

Chemical Changes in Manufacturing Natural Cements 
and Hydraulic Lime. 

In the so-called Roman and natural cements made from 
naturally occurring substances (p. 13), in which the proportions 
of lime and aluminosilicic acid are such as enable useful 
cements to be made, the chemical changes which occur are 
precisely the same as those described in the foregoing pages. 
The final product is very inferior on account of (a) the lower 
temperature in the kilns whereby the reactions between the 
various substances do not proceed so completely and (b) the 
absence of any adjustment of the composition of the mixture, 
which is seldom exactly correct. 

Although the sintering point is seldom reached in burning 
natural cements, this does not prove that certain silicates and 
aluminates have not been formed, for, as already noted, 
J. W. Cobb has ascertained that calcium silicates and 
aluminates (and also alumino-silicates) are formed at tempera- 
tures far below their melting points. 

Unfortunately, natural cements do not usually form a 
clinker of sufficient rigidity to enable them to be polished, 
etched and examined microscopically, so that their true 
composition must largely be argued from analogy. The 
better qualities, which yield a stronger clinker, bear a close 
resemblance to Portland cement, and the same changes may, 
therefore, be assumed to occur as take place in the manufacture 
of the latter cement. 

For the same reason, the nature of the changes which occur 
in the burning of hydraulic limes cannot be stated with 
accuracy. These limes are clearly mixtures of free lime with 
some form of pozzolana, or of free lime and a kind of Portland 
cement. 



78 CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 

Such natural cements,, therefore, contain free chalk as well 
as free pozzolanic material, and are equivalent to a mixture 
of Portland cement containing an indefinite quantity of raw 
materials, and of the products, such as lime, obtainable from 
these by heating at a relatively low temperature. 

Chemical Changes in Manufacturing Pozzolanas and 
Slag Cements. 

As already explained (p. 16), granulated slags differ from 
other raw materials used in the manufacture of cement in 
that they have been heated to fusion and then cooled rapidly 
by quenching with water. 

Like the natural pozzolanas, the granulated slags are usually 
mixed with lime, and the changes which occur have not been 
studied to the extent as those occurring in the burning of 
Portland cements. That any form of combination occurs 
between the lime and the pozzolana or slag when the materials 
are in the dry state is scarcely to be expected. In all proba- 
bility the reaction which occurs takes place as soon as water 
is added to the cement. The nature of this reaction is described 
more fully later under the heading " Setting and Hardening." 

Some basic slags, when quenched and granulated, contain 
sufficient lime to produce useful cements, though much less 
than is found in Portland cement. The fact that the produc- 
tion of a slag involves the complete fusion of the material, 
accounts for the reaction being more complete, so that a 
smaller proportion of lime is sufficient. Such cements are, 
however, quite different in constitution from Portland cements, 
and partake largely of the nature of unstable glasses, which 
are readily decomposed by water. The absence of all crystal- 
lisation in such cements makes it impossible to state what 
compounds are contained in them ; they may be solid solutions 
of various calcium silicates and aluminates or may consist 
chiefly of a calcium alumino- silicate, which is so viscous that 
it retains small quantities of other substances in solution. 
The partial crystallisation which occurs when such slag cements 
are maintained for a long time at a temperature just below the 
melting point is generally considered to imply the existence 



PHYSICAL CHANGES DURING MANUFACTURE 79 

of several binary compounds rather than one impure ternary 
one, but Doelter's and Dittler's researches point to the opposite 
conclusion. 

The Physical Changes during Manufacture. 

The physical changes which occur during the conversion of 
the raw materials into cement are less interesting than the 
chemical changes with which they are so closely associated. 
The most obvious is the gradual conversion of irregular lumps 
of clay or shale and chalk or limestone into a slurry in the wash 
mills, or a powder in the grinding mills. The slurry is then 
dried and passed to the kilns, whilst the powdered mixture is 
either made into briquettes and passed into a stationary kiln, 
or it is taken direct to a rotary kiln as described in Chapter II. 

It is in the kiln that the chief physical as well as the chief 
chemical changes occurring during manufacture take place, the 
material being converted from a friable material, or loosely 
adherent powder, into a hard, dark grey clinker with lighter, 
softer portions when the burning has been less complete. A 
microscopical investigation of this clinker will show, as already 
explained (p. 50), that it consists largely of a preponderating 
crystalline constituent (alite ?) and of a much smaller propor- 
tion of a glassy or slag-like material, the nature of which is 
not easily ascertained. The composition and texture of this 
glassy material vary in different cements, and it is probably 
not unreasonable to suppose that it constitutes the residium 
of all the ingredients left in the raw materials after the forma- 
tion of the alite or cement proper. 

On observing a sample of raw mix heated gradually to 
1450 C. by means of a Doelter's microscope, no visible change 
occurs until a temperature of about 1375 C. is reached, slight 
fusion then occurs, but is barely appreciable until the tempera- 
ture rises to 1410 C. or thereabouts, when a sudden formation 
of (alite ?) crystals takes place simultaneously with the fusion 
of the remaining constituents of the material. The purer the 
raw materials, i.e., the nearer they approach to aluminosilicic 
acid and calcium carbonate, the smaller will be the amount of 
fusion observable, and when the purest possible materials are 



80 CHEMICAL AND PHYSICAL CHANGES IN CEMENTS 

used it is difficult to see any fusion at all until just at the 
moment when the crystals are formed. With very impure 
materials, on the contrary, or where the proportion of lime is 
too large and the clay is highly siliceous, the production of a 
fused material in relatively large quantities may be observed 
at a temperature of about 1300 C. 

Contrary to the statements of some writers who have argued 
from analogy rather than from direct observation, the cement 
materials do not fuse gradually, the fused portion attacking the 
remainder and bringing more and more of the material into 
the molten state. On the contrary, the combination between 
the lime and the aluminosilicic acid occurs without fusion, and 
as soon as the temperature is reached at which fusion does 
occur the compound formed (alite ?) begins to crystallise with 
great rapidity. It is only with very impure materials or in 
very abnormal mixtures that a large amount of glassy material 
is produced. 

The clinker itself, in spite of its hard and vitrified appearance, 
is an extremely porous material with a true specific gravity of 
3 to 3*4 as compared with 2-5 in the case of the raw mix 
before burning. 

The slag cements are usually harder and less porous, whilst 
the natural cements, pozzolanic cements and hydraulic limes 
are quite soft and more free from noticeable pores. 

Whilst the raw materials are not chemically changed in con- 
tact with water, the clinker is decomposed in a manner presently 
to be described, and whilst dilute acids only dissolve the cal- 
careous portion of the raw mix and leave the clayey portion as an 
insoluble residue, the same acids will dissolve clinker completely. 

The changes in the optical character of the materials are 
those which naturally result from the conversion of a mixture 
of amorphous materials into a vitrified mass composed of 
crystals with glassy inter-crystalline matter. 

The physical properties of clinker are of small importance 
so long as the clinker is maintained in the form of irregular 
lumps, or is ground to the fine powder in which commercial 
cements are sold ready for use. When mixed with water, 
however, the changes which occur are so striking and so 
important that they must be described in a separate chapter. 



CHAPTER IV 

THE CHANGES WHICH OCCUR IN SETTING AND HARDENING 

THE chemical and physical changes which occur when a 
cement is mixed with water are both numerous and important, 
but are so complex that the physics and chemistry of some of 
them cannot readily be considered separately. They are 
comprised in the various phenomena which are known collec- 
tively as setting and hardening. 

When a cement is mixed with a suitable proportion of water, 1 
it is converted into a plastic paste which may readily be 
moulded into any simple shape. After a time, the mass loses 
its plasticity and becomes more rigid and the water disappears 
from its surface ; it is then said to have " set." After a much 
longer period it becomes intensely hard and mechanically 
resistant. The period between the first addition of water and 
this loss of mobility is termed the initial set. With a quick- 
setting cement the change is very noticeable, but with others 
it is so indistinct as to be almost unrecognisable. In some 
stages this division into two separate stages is strongly marked, 
but in the plasters only one stage is observable. It is important 
not to continue the gauging after the initial set has commenced 
or the interlocking network of the particles will be destroyed, 
and the final strength of the cement correspondingly reduced. 
For this reason, cements should be gauged in small quantities 
at a time and placed in position, or moulded, before an 
appreciable amount of setting occurs. 

The proportion of water affects the time of setting, an excess 
of water retarding and a very dry gauging accelerating the 
setting. An increase of temperature also increases the speed 
of setting and hardening ; hence, cements set more rapidly 
in hot climates than in cooler ones, and some difficulty is 

1 The process of mixing water and cement to form a paste is known technically 
as gauging. 

C. G 



82 CHANGES IN SETTING AND HARDENING 

experienced in hot weather on account of the cement setting 
much more quickly than usual. 

The conversion of the mixture of cement and water into a 
mass of a hard, stony nature may be due to one or more of the 
following changes, which may proceed simultaneously : 

(a) The formation of a crystalline magma from a super- 
saturated solution. 

(b) The desiccation of a colloidal substance or gel. 

(c) The reaction of various substances upon each other, or 
with water, giving rise to a product which is either crystalline, as 
in (a), or colloidal, and is later desiccated as in (b). 

Crystallisation from a super-saturated solution is charac- 
teristic of the setting of plaster of Paris and allied substances, 
including some of the simpler meta-silicates, such as BaOSiO.>, 
CaOSi0 2 , etc. The first stage, or " setting," of a Portland 
cement appears to be of this nature, crystallised calcium 
hydrate being formed. In such cases the substance dissolves 
in the water present, more material entering into solution than 
can long remain in that state. A hydrate is then formed by 
combination of the basic portion of the substance with the 
water, and as this hydrate crystallises out, more of the 
anhydrous material is dissolved, the process being continued, 
though at a rapidly diminishing rate, until all the water is 
combined or all the material is hydrated. If sufficient water 
is present to separate the crystals from each other, they will 
not form a cement, though the chemical changes which take 
place are identical ; the setting of the mass under the conditions 
described being merely a question of the spacial relationship 
of the crystals. 

The rate at which such setting occurs depends on the purity 
of the solution ; the presence of comparatively small quantities 
of lime and other colloidal substances in solution will retard 
the setting, or may even prevent its taking place. 

Colloidal substances form hard, stone-like products in an 
entirely different manner, no chemical reaction taking place. 
When a substance is composed of extremely minute particles 
of an amorphous character it possesses a number of peculiar 
properties which distinguish it from the same substances when 
in a crystalline state. Gelatin or glue is a typical colloid, and 



CHANGES IN SETTING AND HARDENING 83 

its colloidal properties are easily recognised, but calcium 
hydrate (Ca0. 2 H. 2 ), clay, silica, alumina, and many other 
substances are, under certain conditions, of the same nature. 

All colloids may exist in three forms, (a) one in which 
extremely fine particles are suspended in a liquid ; this mixture 
bears a close resemblance to a true solution, as it can be 
filtered without removing the colloidal substances from it (such 
a pseudo-solution being termed a sol) ; and (b) a coagulated 
form obtained by heating the sol or by adding acids and certain 
salts whereby the substance becomes thick and slimy, and a 
plastic, gelatinous solid or gel is then removable from it by 
filtration or deposition. Some of these gels and sols are 
convertible into each other an indefinite number of times, but 
other gels (or coagulated form) cannot be converted into the 
sol form, and are then said to be " fixed " or " irreversible." 
(c) A third form consists of an amorphous one corresponding 
to the dried gel ; it may be of a horny or a granular nature 
or in the form of a fine powder. Unless "fixed" it may be 
converted into a sol on treatment with water, though some 
colloids in this form are readily hydrolysed by such treatment. 

The behaviour of a colloid is most readily understood by 
observing that of gelatin or glue. If this substance is placed 
in water it gradually swells and softens, the increase in volume 
being very large. If sufficient water is present the gelatin will 
appear to pass into solution (sol form), otherwise it will form 
an exceedingly soft jelly (gel form). If this jelly is removed 
from the water and allowed to dry, it will gradually shrink 
and harden until a strong, horn-like mass is produced. If the 
drying is carried still further by the aid of heat, the dried 
material may be rendered irreversible, and will not soften again 
when placed in water. Instead of drying the sol, the colloid 
may be coagulated by the addition of various chemicals, 
chiefly of an acid nature. 

Lime, silica, and possibly some alumino- or other silicates 
are the chief colloids found in cements and mortars previous 
to the addition of water. The existence of free lime is 
natural to hydraulic lime and mortar, but it should not occur 
in cement which has been carefully made and well burned. 
Free lime retards the setting of a cement or mortar containing 

G 2 



84 CHANGES IN SETTING AND HARDENING 

it, as on treating the material with water some of this lime 
enters into solution and thereby furnishes one of the products 
of the hydrolysis. The presence of such a reaction-product 
always reduces the rate of a chemical reaction, as it tends to 
establish an equilibrium earlier than would occur if such a 
product were entirely produced by the reaction itself. The 
free lime (if any) in cement and mortar must not be confused 
with that produced by the action of water on cement. 

The colloidal silica present in some cements and mortars 
before they are mixed with water is never large in amount, 
and is due to the decomposition of adventitious silicates or 
aluminosilicic acids in the kiln. The behaviour of colloidal 
silica obtained in this manner may be observed under the 
microscope, as described later, though it cannot, with this 
instrument, be distinguished from any colloidal alumino- 
silicic acid which may be produced by the hydrolysis of an 
alumino-silicate by the water added to the material. 

The chief changes in the setting and hardening of cements, 
mortar, or other hydraulites, may be most conveniently 
observed in connection with Portland cement, those in other 
cements and hydraulites being analogous, but less easily studied 
on account of the greater irregularities in the composition of 
the materials. 

As already explanied, Portland cement consists of a crystal- 
line constituent, which is a calcium alumino-silicate, together 
with a much smaller proportion of a slag-like or glassy mass, 
which is an aggregate of the various " impurities " contained 
in the raw materials used. 

When the cement is mixed with water, allowed to harden, 
and then polished and examined under the microscope, it will 
be found that about half of it consists oi the unaltered grains 
of cement and the remainder of colloidal or gelatinous material. 
The latter may be readily distinguished by soaking the specimen 
in an aniline dye, such as methyline blue or eosin (red ink), 
and washing out the unabsorbed dye. The colloidal matter 
retains the dye so persistently that it cannot be removed by 
simple washing, and in this way the particles of colloid may be 
readily distinguished under the microscope by their colour. 
The proportion of unaltered cement depends on the fineness 



CHANGES IN SETTING AND HARDENING 85 

to which the cement has been ground, the finest particles being 
the most readily affected by the water, but in commercial 
cements it is found that there is always 30 to 50 per cent, of 
unaltered cement in the fully hardened neat cement. By the 
addition of sand or other suitable inert material the particles 
of cement are distributed more widely, so that in mixtures of 
this kind a more complete use of the cement is made than 
when it is the sole constituent of the material. The finer the 
grains of cement, the greater the proportion of inert material 
which may be used, and the more complete the hydrolysis. 
It is important to observe that this unaltered cement takes 
no part in the hardening (unless it forms a nucleus to the 
colloid, for which sand is equally efficient). Failure to 
remember this has led to several fanciful attempts to explain 
what occurs during the hardening of cements, and has especially 
led to unnecessary complex theories as to the reason that a 
hardened cement will set a second time if re-ground and then 
again gauged with water. Careful experiments with the aid 
of the microscope have shown that so long as unaltered cement 
is present the hardened mass may be re-ground and re-gauged 
an indefinite number of times, the limit being fixed by the 
amount of cement which is hydrolysed each time the mass is 
mixed with water. Even with the most finely ground cements 
and the most carefully made mixtures of these with sand, the 
whole of the cement is never hydrolysed the first time the 
mixture is gauged with water. 

Those portions of the cement which have been altered by 
contact with water are found,, on examination, to have under- 
gone a chemical decomposition, free calcium hydrate (Ca0. 2 H. 2 ) 
and a new series of compounds (hydro-silicates and hydro- 
alumino-silicates) being formed. Water acts both as a base and 
an acid and decomposes the cement, liberating lime and colloidal 
silica, colloidal aluminosilicic acid and, possibly, colloidal 
calcium silicates. The lime crystallises as hydrate from the 
solution, and the remaining substances rapidly assume a 
gelatinous form. Careful examination under a microscope will 
show that the crystallisation of the lime occurs somewhat 
rapidly, especially with quick-setting cements. At the same 
time, though much more slowly, the other minute particles 



86 CHANGES IN SETTING AND HARDENING 

of decomposed cement material show their characteristically 
colloidal nature inasmuch as they gradually swell and lose 
their well-defined shape (just as gelatin does in water) and 
produce a gelatinous transparent substance. This colloidal 
material gradually increases in density and hardness as the 
lime set free by the hydrolysis is absorbed by the gel, and this 
desiccation of the colloid continues even though the cement is 
immersed in water. That the effect of water on cement is 
chemical and not entirely physical is shown by the rise in 
temperature being roughly inversely proportional to the time 
of setting of the cement. Slow-setting cements have only a 
trifling rise in temperature. For a long time no satisfactory 
explanation could be given for this rise in temperature, but 
it is now recognised that it occurs at the moment of hydration 
of the lime liberated from the silicate molecule in cements which 
contain no free lime. In hydraulic limes (in which free lime 
as well as " cement " is present) a development of heat occurs 
at first ; this is due to the hydration of the lime. Then, after 
an interval, a second development of heat is observable 
corresponding to the chief one occurring in Portland and other 
lime-free cements, and produced at the moment when the 
lime is first separated from the alumino-silicate (alite ?) 
molecule, and forms free calcium hydrate (Ca0. 2 H.>). It is not 
improbable that the water first attacks the glassy constituents 
of the cement and that the rise in temperature associated with 
the initial set is due to this chemical reaction on the amorphous 
material. The action of water on the crystalline alite is 
probably slower, and this may account for the second rise in 
temperature during setting. It should be observed, however, 
that the hardening of cements, as distinct from the setting, 
does not admit of very accurate and precise study, especially 
as both it and the setting occur, to some extent, 
simultaneously. 

The conflicting views as to the constitution of Portland 
cements have their counterpart in the variety of substances 
which are supposed to occur in a cement which has been mixed 
with water and then allowed to harden completely. Those 
chemists who maintain that cements are solid solutions of 
calcium aluminate and silicate aver that the action of water 



CHANGES IN SETTING AND HARDENING 87 

on cement is to produce a mixture of colloidal and crystalline 
tri-calcium aluminate (3CaOAl.,0.^) 9 colloidal and crystalline 
calcium hydroxide (CaO 2 H. 2 ), and colloidal calcium silicate. 
The existence in hardened cements of free calcium hydroxide 
(Ca0. 2 H.,) in crystalline form was discovered by Le Chatelier, 
but his explanation of the reaction of cement with water cannot 
be accepted, as it refers exclusively to the behaviour of tri- 
calcium silicate (which has never been definitely isolated from 
cement), and omits all reference to the alumina present. 
Moreover, the analogous barium silicates are hydrolysed and 
form a hard mass which differs from Portland cement in 
being crystalline and not colloidal, and in being useless 
as a hydraulite on account of the product being soluble 
in water. In view of the fact that the chief constituent 
of cements is a calcium alumino-silicate, it appears to be 
far more probable that the result of the action of water 
will be to form such compounds as are shown on 
p. 88, in which a portion of the lime has become 
hydrolysed and has been liberated as Ca(OH) 2 . The total 
proportion of lirne thus set free need not be large, and there 
are reasons for supposing that only a proportion of the lime 
attached to the silicon rings, together with the whole of. that 
attached to the alumina ring (p. 55) is removed from the 
molecule. 

The simplest and most obvious theory explaining the setting 
and hardening of cements is that which regards these pheno- 
mena as hydration processes of the cement molecule. In 
fact, it follows directly from Asch's theory of the chemical 
constitution of cements (p. 55) that the addition or substitu- 
tion of a large number of hydroxyl groups is. possible, and W. 
and D. Asch, Feichtinger, and others have also found experi- 
mentally that substances which can add hydroxyl (OH) groups 
at a definite rate to their molecule are hydraulites. The 
addition of water must be progressive and must not occur too 
rapidly. The experimental evidence suggests that any sub- 
stance in which the addition of the OH -groups occurs slowly 
at first and progresses at a suitable rate until, finally, a very 
large number of OH-groups have been added, will form a 
harder and denser cement than if a smaller quantity of water 



88 CHANGES IN SETTING AND HARDENING 



had entered into the reaction or if the reaction had occurred 
more rapidly. 1 

If the constitutional formula of a typical cement is repre- 
sented in accordance with Asch's theory as 

5CaO. KO. 5CaO. 

4CaO. 




5CaO. KO. 5CaO. 

FORMULA A. 

it can, on treatment with water, form numerous hydrated 
compounds, 2 such as 



HO . Ca . O 
HO . Ca . O 



HO .Ca . O\ 
HO .Ca . O/ 

HO .Ca . O\ 
HO .Ca . O/ 



Si 



HO .Ca .0 
HO .Ca .0 



OK 



Al 



OK 



. Ca . OH. 
. Ca . OH. 



Si 



O . Ca . OH. 

\O . Ca . OH. 

/O . Ca . OH. 
. Ca . OH. 



O . Ca . OH. 
O . Ca . OH. 



FORMULA B. 



and will require twenty-eight molecules, or 13-8 per cent, of 
water a figure which agrees remarkably closely with that found 
experimentally by Zulkowski in a " perfect " cement when set. 
The dissociation of the alumino- silicate molecule marks the 
commencement of the conversion of the mixture of cement 
and water from a plastic paste to a solid mass. This is known 
as the " initial set," the time required for it to occur being 
termed the " time of setting." Cements differ greatly in this 

1 The hardening of trasses and pozzolanas with lime and water may be 
explained in an analogous manner. These "calcined clays " (p. 43) first react 
with lime and then add hydroxyl groups to the calcium salt first formed. 

2 For other typical hydrated* compounds, see Asch's Silicates (Constable & Co., 
Ltd., London 1913), 



CHANGES IN SETTING AND HARDENING 89 

respect, some being very much quicker than others. The 
earlier cements (which were under-burned in parts and so 
contained free lime) and the hydraulic limes set slowly, as the 
free lime in them dissolves in the water and hinders the 
hydrolysis of the crystals, because lime is one of the products of 
this hydrolysis. In more modern Portland cements the burning 
is more efficiently carried out, no free lime occurs in them, 
and the clinker is much more finely ground, with the result 
that they set much more rapidly, and a small percentage of 
plaster or gypsum must usually bemadded to retard the setting 
or it would take place before the mixing of the cement and water 
is complete. 

In a carefully made cement, which has been well-burned in 
a rotary kiln, the effect of water is tri-fold : 

(a) The alumino-silicate molecule is hydrolysed and is 
decomposed, free calcium hydroxide (Ca0. 2 H. 2 ), and such 
substances as those described on a previous page being 
produced. The products are in each case colloidal, but may, 
later, become crystalline. The colloidal matter gradually 
absorbs the free calcium hydroxide and hardens, and the 
network of crystalline matter formed as either a primary or 
secondary product of the hydrolysis adds to the strength of 
the material. 

(b) Any calcium ortho-silicate present in the cement is 
hydrolysed, calcium hydroxide and a hydrated silicate being 
produced 

2CaO . Si0 2 + (x + 1) H^O = CaO . Si0 2 . xH^O + Ca (OH) 2 

(c) With alumino-silicates the corresponding equation is 
expressed by one of the following types : 

(1) C 



(2) Ca, 5 AkSi lb O M + 36# 2 = Ca l9 

It will, however, differ slightly according to the particular 

calcium alumino-silicate present and to the extent of the' 

hydration. 1 

1 That the proportion of lime may vary within wider limits than is generally 
recognised is implied by some experiments of Fremy (Compt. rend : 67, 1205), and 
Zulkowsky (Sonderabd. 1908), in which as much as 14 per cent, of lime was 
removed from some Portland cements without destroying their power to harden 
when mixed with water, 



90 CHANGES IN SETTING AND HARDENING 

Any ferrates, aluminates and ferro-silicates will be 
hydrolysed in a similar manner, and will form either colloidal 
products or will add to the complexity of the interlocking 
silicate crystals. It was at one time thought that quick- 
setting cements owed their property to the aluminates they 
contained. This is only indirectly the case. Cements which 
are rich in alumina are those which contain most alumino- 
silicate, and they set more rapidly because of this and of the 
relative absence of retarding substances. Some of the Roman 
cements set rapidly because of the large proportion of alumino- 
silicate present. Hence, it is not unreasonable to assume that 
the greater part of the strength of hardened cements is due to 
the colloidal matter produced by the action of the water 
(including the absorbed calcium hydrate) rather than to the 
formation of any network of crystals which may occur. A 
microscopical examination of hardened cements which have 
been polished confirms this view, the total proportion of 
crystalline matter being small compared with that of the 
colloid or .amorphous matter present. 

The opinion held by some chemists that the constituents of 
hardened cements are free lime, free silica and free alumina, 
and that the " initial set " marks the hydration of the 
aluminates, and the " final set " that of the silicates, does not 
coincide with many of the facts. It is quite true that these 
substances all occur in hardened cements and mortars, but 
they are in proportions which are far too small to correspond 
to the strength of the material as a whole. To a very large 
extent the existence of free alumina and free silica in large 
proportions is merely academic deduction from the supposed 
existence of correspondingly large quantities of tri-calcium 
silicate and di- or tri-calcium aluminate in cement clinker, 
and the general supposition that only binary compounds of 
lime, silica and alumina exist in cements. -The proved existence 
of large quantities of alumino-silicate, and especially the 
recognition of the chief constituent of cement as a definite 
alumino-silicate, deprives the explanation of the process of 
setting and hardening usually found in text-books of much of 
its assumed value. The products of the action of water 
on cements are not free silica and alumina, but complex 



CHANGES IN SETTING AND HARDENING 91 

aluminosilicic acids, some of which contain much combined 
lime. 

It was, at one time, thought that the hardening of cements 
and mortars was due to the direct combination of free lime with 
silica and the formation of calcium silicate. No combination 
can occur between sand and lime at ordinary temperatures, 
but active aluminosilicic acid (such as occurs in pozzolanic 
materials) will absorb free lime and form a hard colloidal 
mass which increases in hardness as the water in it is removed. 
The great hardness of many ancient mortars is due to the use 
of ground tiles and other pozzolanic material (capable of 
providing active colloidal silica, alumina and aluminosilicic 
complexes) in addition to the sand ordinarily used. 

The action of carbon dioxide (in the atmosphere) on cements 
and mortars is always of a secondary character so far as 
hardening is concerned. The primary action of carbon dioxide 
is on the free calcium hydroxide present 

Ca(OH}* + CO* = CaC0 3 + H 2 0. 

The lime thus carbonated is rendered insoluble and useless 
as far as any further reaction is concerned, but the calcium 
carbonate rapidly assumes a crystalline form, and the inter- 
laced network of crystals slightly increases the strength of the 
whole mass. The greater part of the hardness and strength is, 
however, due to the colloidal matter present, as already 
explained, the action of carbon dioxide being almost entirely 
confined to the surface of the material and rarely penetrating 
into the interior even after 2,000 years, as may be seen by 
examining the walls of ancient buildings. 

Some carbon dioxide (about 5 per cent, under favourable 
conditions) is absorbed by most cements when they are stored 
or " matured." The amount absorbed was, at one time, 
thought to correspond to the free lime present in the cement, 
but more recent investigations have shown that well-made 
Portland cements contain no free lime, and indicate that the 
action of carbon dioxide which requires the simultaneous 
presence of water is due to a slight decomposition of the 
cement. 

As the finest particles will react the most rapidly, a definite 



92 CHANGES IN SETTING AND HARDENING 

degree of fineness is necessary to the production of a satisfac- 
tory hydrated product, but too fine a material will react so 
rapidly as to form a feeble cement, unless its action is 
retarded by the addition of a suitable agent. 

RETARDATION OF SETTING. 

The retardation of setting becomes increasingly important 
as the manufacture of cements is improved. Portland cements 
burned in stationary kilns are comparatively slow-setting, but 
those burned in rotary kilns set too rapidly for convenient 
and satisfactory working. The rate of setting of a cement 
may be decreased in two ways : (a) the cement may be exposed 
to the air (i.e., aerated), or (b) a small proportion of a retarding 
agent may be added, some form of calcium sulphate being 
usually employed. Other salts may be used instead ; thus, 
P. Rohland found the time of setting of cement was shortened 
by adding calcium chloride, aluminium chloride, potassium 
sulphide, sodium carbonate, potassium carbonate, aluminium 
sulphate and alum. The time is lengthened by potassium 
dichromate, boric acid, borax, sodium sulphate, potassium 
sulphate and calcium sulphate. Exposure to air is really 
equivalent to exposure to moisture, and steam is, therefore, 
used as a retarding agent with good effect. This is due to the 
hydration of a small portion of the cement and the consequent 
liberation of a little free calcium hydrate. When the clinker 
is treated with steam so that about 1 per cent, water is absorbed, 
the proportion of calcium sulphate required is reduced to 
about one-half that otherwise needed. 

The precise nature of the action of calcium sulphate on 
cement is not fully understood. As only 2 to 3 per cent, is 
required, its action may be largely catalytic. Various other 
salts produce a retardation when used in small quantities and 
an acceleration when larger proportions are used, though the 
results of investigations on this subject are far from conclusive 
and are, in some instances, mutually incompatible. 

E. Candlot has also stated that a double salt corresponding 
to the formula 3CaOAl. 2 O^CaSO^H,0 is formed by the inter- 
action of calcium sulphate and cement, and that this compound 
is insoluble in water, and so converts any aluminate into a form 



RETARDATION OF SETTING 93 

in which it takes no part in the setting. This explanation is 
widely accepted, but can hardly be said to meet the facts. 
Candlot has endeavoured to remove some of the objections to 
this theory by postulating that a certain quantity of free lime 
is necessary in order that the gypsum may have its effect. 
It is true that a partially hydrated cement is retarded by a 
much smaller quantity of calcium sulphate than a cement 
which has not been treated with steam or moisture, but this 
does not necessarily prove Candlot 's theory. A more probable 
explanation is that the calcium sulphate dissolves at such a 
rate that its saturated solution prevents the hydrolysis of 
the cement by enabling only a very small proportion of lime 
to be liberated and dissolved at a time. Whichever theory 
be adopted, it is a curious fact that the highly soluble calcium 
chloride has an even stronger retarding action than calcium 
sulphate, but magnesium chloride accelerates the setting. 
This implies that it is the acid portion of the material ($0 4 , 
Cl, etc.) which is the active agent, and that some combination 
of these ions and the cement molecule occurs. The destructive 
action of sulphate solutions, and particularly of sea-water on 
cement immersed in them, confirms the opinion that some 
reaction occurs between the $0 4 -ion and the cement. 

In a hydrated or hardened Portland cement such as is 
represented by Formula B (p. 88), the two OK groups are 
readily replaceable by monovalent acid radicals such as 
S0 2 .OH, so that when such a cement comes in contact with a 
solution of calcium sulphate there is a great probability that 
an alumino-silicate will be formed in which this replacement 
has occurred. Such alumino silicates are well known, and 
include many sodalites and ultramarines. As the replace- 
ment is accompanied by a change of volume, it will, if at all 
extensive, tend to cause the cracking and ultimate disintegra- 
tion of the cement. Arguing in this manner, W. & D. Asch 
have suggested that the only way to prevent adventitious 
sulphates (including those in sea water) from affecting con- 
crete structures is to avoid all cements of the type a, p. 56, in 
which these readily replaceable groups attached to the 
aluminium hexite are present, and to use exclusively cements 
in which they do not exist, e.g., cements of the type 6, p. 56.. 



94 CHANGES IN SETTING AND HARDENING 

The retarding action of calcium sulphate is diminished by 
storing the cement. This has been explained as being due to 
the gradual carbonation of the lime set free by the sulphate 
and moisture, but it is equally probable that a combination 
between the sulphate and the cement occurs on long storage 
or on exposure to the atmosphere. 

Calcium sulphate is usually added to cement clinker in the 
form of gypsum, the materials being ground together. Finely 
powdered gypsum may also be added to the ground cement, 
but it is preferable to use plaster of Paris, which is obtainable 
in a much finer state of powder than is gypsum. The amount 
of either gypsum or plaster needed depends on the proportion 
of CaSO^ present and on the fineness of its particles ; not on 
any imaginary difference in the chemical activity of these two 
substances. 

Experiments as to the effect of the addition of 2 to 3 per 
cent, of calcium sulphate to various quick-setting cements 
show curiously irregular results, and indicate that the action 
of this substance is by no means so simple as is sometimes 
supposed. Some of the discrepancies may be due to lack of 
uniformity in the cement itself or to irregular admixture of 
the retarding agent. Whatever the cause, the disadvantages 
of adding such retarders should not be overlooked, and the 
amount used should be kept as small as possible. 

The whole subject of the action of retarding agents and 
the changes which take place when the cements are stored is 
worthy of further investigation. The inherent difficulties of 
the subject are, however, very serious. 

Aeration, or exposure of cement to the atmosphere, reduces 
the rate of setting and so acts as a retarder. It also hydrates 
any finely divided particles of free lime which may be present, 
and so reduces their tendency to expand or " blow " at a 
later stage in the use of the cement. The changes which occur 
in aeration are similar to those effected by water, but the 
small proportion of water present in the atmosphere makes 
the changes much slower. The carbon dioxide in the air 
converts any hydrated lime into microscopic crystals of 
calcium carbonate and so renders them inert. Excessive 
exposure to air reduces the value of the cement by effecting 



AERATION OF CEMENT 95 

the hydrolysis and carbonation of the finest and therefore 
most valuable particles. 

The earlier cements were improved by aeration, which 
hydrated any quicklime present and so reduced it to a fine 
powder, but well-made modern cements are not improved by 
this treatment. Nevertheless, it is a wise precaution, when 
testing cements which yield unsatisfactory results, to expose 
them to the air for three or four days and then test again ; 
they will then, in many cases, yield satisfactory results. 

NORMAL RATES OF SETTING. 

For information on the rates at which setting and hardening 
occur, the reader should refer to p. 109, et seq. 






CHAPTER V. 

TESTING THE PROPERTIES OF CEMENTS. 

THE primary object of all cement-testing is to determine 
whether the material is satisfactory in two important particu- 
lars strength and soundness. Other' tests for fineness, 
specific gravity, the time of setting and chemical analysis 
are only of value as additional information on the general 
suitability of the material. 

Of all materials which are regularly tested in chemical and 
physical laboratories there are none in which the tests are more 
dependent on the judgment and skill of the operator than 
cements, and even in the best equipped testing stations it is 
impossible to avoid a large personal equation. For this 
reason tests carried out by amateurs are usually of little value 
until the necessary manipulative skill has been attained. 

It is of the greatest importance in studying results obtained 
in testing cements that the precise manner in which the tests 
have been performed should be known. Thus, differences in 
the proportion of water used in the gauging will cause great 
discrepancies, and may lead to erroneous conclusions. Where 
the tests are supposed to have been made in accordance with 
a standard specification, care should be taken to see that the 
instructions have been fully carried out. Even then, there is 
still room for widely differing results, because two testers, 
working quite independently on the same cement, may have 
different ideas as to the precise meaning of " normal con- 
sistency " as applied to gauged cements. 

Users of cement are frequently, though quite unconsciously, 
very unfair in their manner of judging the value of different 
cements. Some of them prefer to make what they call a 
" practical test," that is to say, they observe the behaviour of 
cements from different sources when in use for actual construc- 
tional work. This, at first sight, appears to be the best of all 



TESTING CEMENTS 97 

tests, but it overlooks the enormous influence of the " personal 
equation." A little carelessness or lack of skill on the part of 
the workmen employed will result in the production of inferior 
work, and may lead to the condemnation of a cement of 
exceptionally good quality. There are many ways in which 
good cements may be spoiled by faulty manipulation and by 
the use of unsuitable aggregates, and to obtain uniformly 
satisfactory results needs the unceasing exercise of vigilance, 
care and skill on the part of everyone concerned. 

Other users base their judgment on unimportant matters 
and conclude that a cement which hardens more slowly than 
another must be inferior, or they condemn a cement which, 
after twenty-eight days, does not show a large increase in 
strength when its original strength is greatly above the normal. 
Only as users are prepared to study cements scientifically 
will the true value be appreciated and the best results 
obtained. 

The improvements effected in the manufacture of cements, 
and particularly of Portland cement, are due, in large measure, 
to the imposition of standard specifications for cements in all 
the more important countries. These standards differ some- 
what from each other, partly on account of the differences in 
the climatic conditions, and partly because of minor variations 
in the manner in which some of the tests are carried out. 
These variations are rapidly disappearing in consequence of 
the work of the International Association for Testing Materials. 
In the following pages only brief notes are given as to the 
various properties with the limits set in the specifications, and 
as these limits are varied from time to time and the details of 
the official tests are occasionally altered, it is desirable that 
the specification in force at any particular time should be 
consulted. The British Standard Specification may be obtained 
from the Engineering Standards Committee, 28, Victoria 
Street, Westminster, London, S.W., price 5s. 3d., post free. 
This relates exclusively to Portland cement, no official standard 
for other cements having yet been prepared. 

In drawing up this specification for use by engineers, the chief 
aim has been to select properties which will indicate a lack 
of soundness in the cement when in use, together with such 

0. H 



98 TESTING THE PROPERTIES OF CEMENTS 

other properties as will ensure the closest similarity between 
various batches of cement or cements made by different firms. 
It is recognised that great variations exist in the composition 
of the raw materials used, and the specification is therefore 
arranged so as to secure a maximum of uniformity with a 
minimum of disturbance to existing manufacturers. 

Not only is care and skill needed in carrying out the tests 
themselves, but the manner in which the samples are 
taken from the bulk must be such as will yield " fair " 
samples. 

The procedure recommended in the British Standard Specifi- 
cation is generally adopted in Great Britain, viz. : 

" Each sample for testing shall consist of approximately equal 
proportions selected from twelve different positions in the 
heap or heaps when the cement is loose, or from twelve different 
bags, barrels, or other packages, when the cement is not loose, 
or where there is a less number than twelve different bags, 
barrels, or other packages, then from each bag, barrel, or other 
package. Every care shall be taken in the selection, so that 
a fair average sample may be taken. 

" When more than 250 tons of cement is to be sampled at 
one time separate samples shall be taken from each 250 tons or 
part thereof." 

CHEMICAL COMPOSITION. 

The chemical composition of Portland cements has been 
limited by the definition of Portland cement as " the product 
resulting from the burning of an intimate admixture of 
calcareous and argillaceous materials as principal ingredients, 
which burning is carried to the point of incipient fusion, the 
clinker produced being ground to a fine powder." In the most 
recent British Standard Specification the composition is still 

CaO 
further regulated by the ratio r^ , TTTT (expressed in 

OtC/g -\- A-lvU^ 

molecular equivalents), being limited between 2-85 as a maxi- 
mum and 2-0 as a minimum. This limited range of com- 
position is intended to exclude cements containing granulated 
blast-furnace slag. (See p. 62.) 



COMPOSITION OF PORTLAND CEMENT 99 

The maximum amount of the following constituents is also 
fixed at the figures stated below : 

Per cent. 

Insoluble residue . . . . 1-5 
Magnesia ..... 3-0 
Loss on ignition l . . . .2-0 
Sulphur trioxide (S0. 3 ) . . . 2-75 

APPARENT DENSITY. 

The apparent density of a cement is the ratio of the weight 
of a given volume of cement to its volume, but the expression 
is of small value unless the method of filling the measuring 
vessel with cement is known. If, for example, the powder is 
introduced into a glass flask, the latter being tapped gently from 
time to time, a very different figure will be obtained for the 
apparent density than if no tapping or shaking is permitted. 
The usual practice is to employ a funnel provided with a 
stopper at its lower end. A sufficient quantity of cement is 
placed in this funnel and is allowed to run into a cylindrical 
measure placed beneath until the latter is filled to overflowing. 
The excess of cement is removed from the receiver by drawing 
a straight-edge across the top of the latter so as to leave it 
exactly full. The receiver with its contents is then weighed. 

The receiver may be of any convenient capacity. In former 
days it held exactly one bushel (= 8 gallons), but at the 
present time it is more frequently 1 litre. From the larger 
measure the " weight per bushel " is easily ascertained ; from 
the latter the " litre-weight " is equally easy. It is desirable 
to check the capacity of the receiver by weighing or measuring 
the water it holds when filled exactly to the brim. 

The object of ascertaining the apparent density is to gain 
some idea as to whether the cement has been under-burned, 
as the more complete the burning the greater will be the weight 
per bushel or litre-weight. This test is complicated by the 
fact that the fineness and age of the cement both reduce the 
apparent density, so that it is of little use in comparing cements 

1 Unless it can be shown that the cement has been ground for more than four 
weeks. 

H2 



100 TESTING THE PROPERTIES OF CEMENTS 

from different sources, but is of value to the manufacturer 
who, by its means, is able to check the correctness or other- 
wise of the calcination of the cement. With the increasing 
use of rotary kilns and the consequent reduction in the 
proportion of under-fired clinker, the necessity for determining 
the apparent density is gradually disappearing. 

The usefulness of this ratio is also limited by the fact that 
the weight of ten bushels, or litres, or any other volume, does 
not correspond to that of a smaller number. The size and shape 
of the measuring vessel have a great influence on the relative 
positions of the particles. For the same reason cement cannot 
be measured instead of weighed by dividing the weight required 
by the apparent density. The volume thus calculated differs 
sufficiently from the true volume to cause an appreciable 
difference in the strength of the mortar for which the cement 
is used. Where the cement all comes from the same works, 
however, and the true relation between weights and measures 
on a large scale has been found, the apparent density may be 
used with more accuracy. 

The weight per bushel of a good Portland cement will vary 
between 95 and 115 Ibs., and the litre- weight from 1,010 to 
1,400 grammes, but, as already mentioned, these figures are 
reduced by grinding the cement more finely, and by storing 
it for some time, when the particles become partially hydrated 
and carbonated. 

The specific gravity test has for several years replaced the 
weight-per-bushel and litre-weight tests in this country, as the 
latter are unreliable with very finely ground cements. 

SPECIFIC GRAVITY. 

The specific gravity of any substance is the ratio of the 
weight of that substance to the weight of an equal volume of 
water. In the case of fine powders, such as cements, the 
specific gravity differs from the apparent density because the 
former relates to the volume- weight of the individual particles, 
whilst the latter relates to the volume-weight of the mass, and 
includes the space between the particles which is occupied by 
air. Hence, to determine the specific gravity it is necessary 



ASCERTAINING SPECIFIC GRAVITY 101 

to measure the total volume of all the particles, each one being 
considered separately. To do this, there is introduced into 
a flask or bottle with a very narrow neck 1 sufficient paraffin, 
turpentine, or other convenient fluid, which is without action 
on the cement, until it reaches the prearranged mark on the 
neck of the vessel. The vessel with its contents is then weighed 
accurately, and the weight noted. The vessel is then partly 
emptied and a definite weight of cement introduced through 
a funnel. The vessel is then refilled to the mark with the 
fluid, tapped gently to loosen air-bubbles, and its weight 
again noted. If 

F = the weight (in grammes) of the empty vessel, 
T = the weight of the vessel when filled with fluid, 
W the weight of the vessel when containing cement and 

also fluid, 

C = the weight of cement introduced into the vessel, 
S = the specific gravity of the fluid used, 

OS 

then the specific gravity of cement = ^ ^ ^ 

It is by no means easy to determine the specific gravity of 
cement, as the air adheres very closely to the particles, and 
some of the latter are liable to be carried to the top of the fluid, 
and even to rise above it. Only by very careful tapping in 
a horizontal direction can the cement be kept in its place below 
the surface of the liquid. On no account must the vessel be 
shaken vertically, of an accurate determination will be rendered 
impossible on account of the cement which will lodge on the 
upper part of the neck of the vessel. Several patterns are in 
use, but one of the most convenient is that devised by W. H. 
Stanger and B. Blount, which is a modification of one used by 
Le Ch atelier. It consists of a flattened flask with a narrow 
neck graduated in one-tenths of a c.c. The capacity of the 
flask to the lowest graduation is 64 c.c., and this is marked 14, 
the remainder of the larger graduations being marked succes- 
sively 15, 16, 17 and 18, so that the capacity of the flask to 
the top graduation is exactly 68 c.c. This flask is carefully 

1 The use of an ordinary stoppered specific gravity bottle is inadvisable on 
account of the floating of the finest cement particles. 



102 TESTING THE PROPERTIES OF CEMENTS 

dried and exactly 50 c.c. of paraffin is introduced into it by 
means of a pipette, great care being taken not to wet the 
graduated portion of the neck of the flask. Then exactly 
50 grammes of cement is added through a funnel, and the 
flask is gently tapped to remove air-bubbles. The level of 
the liquid is then read on the graduations. This number 
divided into 50 will give the specific gravity of the cement. 
Thus if the liquid reached to 15-8 the specific gravity of the 
cement is 50 -r- 15-8 = 3-16. The cumbersome calculation 
which is necessary when a specially designed vessel is not used 
may, by this means, be avoided. The cement should be 
introduced after the paraffin, as otherwise it is difficult to get 
a sharp reading. 

Unlike the apparent density (p. 99), the specific gravity is 
not affected by the fineness of the cement, but the specific 
gravity diminishes as the age of the cement increases, in 
consequence of the absorption and chemical combination of 
moisture and carbon dioxide from the atmosphere, whereby 
partial hydration and carbonation of the cement are effected. 

The chief uses of the specific gravity are : (a) To distinguish 
Portland cement from natural cement and particularly from 
that form of the latter known as Belgian cement (p. 31). 
Genuine Portland cement has a specific gravity between 3'0 
and 3'4, whilst natural cement has a specific gravity below 
3-0. 

(b) The specific gravity is also used as a test of the value of 
a cement in relation to the extent to which the clinker 
has been burned, but the difference between the specific gravity 
of under-burned and normal clinker is too slight to be relied 
upon. It not infrequently happens that a cement of low 
specific gravity is of greater strength than one of high specific 
gravity, so that no important conclusions should be based on 
this test. 

(c) In testing for adulterants in cement the specific gravity 
is of little value unless the added material is present in a 
very large proportion. The specific gravity of Kentish rag 
a sandy limestone at one time much used as an adulterant 
is 2-9 ; that of basic slag is still closer to that of cement, 
so that the differences are, for most purposes, insignificant. 



ASCERTAINING SPECIFIC GRAVITY 103 

In short, although Portland cement has a specific gravity of 
3-00 to 3-40, which is higher than that of other cements, and this 
enables a somewhat denser mortar to be produced, a low 
specific gravity does not necessarily indicate an inferior cement, 
as the absorption of water and carbonic acid from the 
atmosphere will cause a considerable reduction of the specific 
gravity and yet may not lower the value of the cement. 

The British Standard Specification imposes a minimum 
specific gravity of 3*15 for fresh Portland cement and 3-10 
for cement which has been ground more than four weeks 
previous to testing. The lower limit for older cement is to 
allow for the change in specific gravity which occurs when 
cement is hydrated and part of the lime present is converted 
into calcium carbonate on exposure or storage. The effect of 
the age of the cement on the specific gravity may largely be 
eliminated by heating the cement to a temperature of 1,000 C. 
(bright red heat) for a short time. This treatment drives off 
the water and carbonic acid which have been absorbed, but 
the cement is not really re-converted into a properly burned 
cement, as the hydration effected by the moisture in the 
atmosphere to which the cement was originally exposed 
causes a decomposition of the cement which reheating at the 
temperature mentioned does not restore. For most purposes, 
however, the error introduced into the specific gravity figure 
is so small that it may be neglected. 

FINENESS. 

The size of the particles of cement is a matter of the greatest 
importance, as the reactions between the cement and water 
which give the material its chief value depend upon it. 
The test for fineness is also highly important, because fine 
cement has a much greater binding power, and much larger 
proportions of aggregate may therefore be used than with a 
coarser cement, or, conversely, the strength of the material 
will be much greater for the usual proportions of aggregate 
if a finely ground cement is used. 

It is sometimes stated that the " flour " or finest particles 
contain the whole of the cementitious material. This is not 
strictly correct, though sufficiently so for many purposes. 



104 TESTING THE PROPERTIES OF CEMENTS 

Careful tests of the coarser particles will show that they are 
cementitious, but that they are less rapidly attacked by the 
water used in gauging. On grinding the coarser particles to 
flour they have the same cementitious value as the fine particles 
from which they have been separated. The difference in their 
behaviour is entirely due to the relative amount of surface 
exposed and not to any other chemical or physical difference. 
With coarse particles the relative surface area is much less than 
with finer ones, so that the water can only react to a much 
smaller extent, and the final product is much weaker than 
would be the case if finer cement were used. In addition to 
this, the finer particles can be distributed over a much larger 
quantity of aggregate when the cement is made into mortar 
or concrete, so that the finer a cement is ground the less will be 
the proportion of cement needed to produce a mortar of given 
strength. Commercially this is very important, as the cement 
is by far the most costly ingredient. 

There is, however, another reason why cement should be 
finely ground, namely, its much greater freedom from 
" blowing " and cracking, particularly if it be underburned or 
overlimed. This has clearly been shown by D. B. Butler, who 
found that a number of cements, in the state in which they 
were received from the manufacturer, formed pats which were 
badly blown under trying conditions, yet the same cements 
when re-ground, so as to pass completely through a No. 180 
sieve, gave perfectly sound pats. 

According to W. Michaelis, only those particles are of 
value which pass through a 305 X 305 sieve. Hence the old 
methods of grinding gave only 50 per cent., but the best 
modern ones yield not less than 70 to 75 per cent, of the only 
valuable constituent of cement. 

In the face of these results, it is not surprising that the 
compressive strength of mixtures of fine cement with three 
parts of normal sand exceeds that of normal cement and 
normal sand by over 1,400 Ibs. per square inch. A still finer 
raw material will increase the strength still more. 

Better quality, higher commercial value, with moderate 
increase in the cost of production, are the chief advantages 
resulting from fine grinding. 



THE FINENESS OF CEMENT 



105 



The only drawback to fine grinding is the increased rate at 
which the cement sets ; this is usually overcome by treating 
the hot clinker with steam and adding about 2 per cent, of 
calcium sulphate or other retarder (p. 92). 

To produce a cement of very great fineness is necessarily 
costly, and there is, therefore, a tendency not to grind more 
finely than the user considers necessary. Some years ago a 
residue of 10 per cent, on a No. 50 sieve and 20 per cent, on a 
No. 76 sieve was con- 
sidered to be good 
grinding and is now 
customary for some 
of the cheaper 
cements. For Port- 
land cement of good 
quality, however, the 
leading makers now 
grind so that there is 
less than 3 per cent, 
on a No. 76 sieve, 
and the tendency is 
to demand increas- 
ingly fine grinding. 

In order to obtain 
so fine a product 
r a pi dly - driven 
grinding machines 
must be excluded, as 
they are not suitable 
for very fine grinding. 

Tube-mills and millstones can grind very fine, but for a 
residue of, say, 10 per cent, on a 175 X 175 sieve their output 
is so small that they cannot be used commercially ; if a still 
finer product is required, e.g., 2 per cent, on a 175 X 175 sieve, 
the output is insignificant. There can, on the contrary, be 
no question as to the ability of ball-mills to grind to any 
desired degree of fineness, whenever a suitable separating or 
sifting device is available. Such an arrangement must not 
operate in the rough and ready manner of a sieve, but must 




FIG. 9. Air Separator (Gebr. Pfeiffer). 



106 TESTING THE PROPERTIES OF CEMENTS 

only remove the very finest particles and must return the 
remainder to the mill so that it may be crushed still finer. 
Such an apparatus appears to exist in the " Selector " a 
form of air-separator (Fig. 9) which can easily produce a 
cement with as little as 2 per cent, residue on a 175 X 175 sieve 
with an output of 80 per cent, of that obtained when cement of 
normal fineness is ground. The product of such a device, 
when tested, gave Michaelis the following results : 





Test 1. 


Test 2. 






Per cent. 


Per cent. 


Residue on 


a 75 X 75 sieve 





Between a 


75 X 75 and 167 X 167 sieve . 


2 0-5 


;> 


167 X 167 and 305 x 305 sieve . 


28 


24-5 


?> 


305 X 305 and 610 X 610 sieve . 


28 


25 


Through a 


610 X 610 


42 


50 



Modern Portland cements contain about 55 per cent, of 
flour separable by a current of air, the remainder being in the 
form of a very fine grit. In cement plants using air-separators 
there is a somewhat larger proportion of flour than when 
screens are used. 

The fineness of a cement is ascertained by sifting the material 
through carefully standardised sieves. In all the chief coun- 
tries of the world two kinds of standard sieves are used ; the 
coarser has seventy-six holes per linear inch or 900 per sq. cm., 
and the finer has 180 holes per linear inch or 4,900 per sq. cm. 
Finer sieves are also used in testing laboratories for special 
investigations, though it is almost impossible to use sieves with 
more than 250 holes per linear inch on account of the clogging 
which ensues. 

In selecting a sieve it is of the greatest importance that all 
the holes should be exactly the same size, as otherwise the 
particles which pass through the sieve will be so irregular as 
to make the results useless. The recognised standard is to 
make the holes twice as wide as the diameter of the wire. 
For the No. 76 sieve the British standard size of wire is 0-0044 
inch diameter, and for the No. 180 sieve it is 0-0022 inch. 



THE FINENESS OF CEMENT 107 

The sieves and gauze ordinarily sold by wire merchants are 
quite useless, the wires being too irregularly spaced, and some 
of the gauze is twilled instead of being evenly woven. Messrs. 
Greening & Sons, Limited, of Warrington, are regarded as the 
semi-official makers of suitable gauze. The manner in which 
the gauze is attached to the frame is important. It must not 
be stretched or strained over a circular frame, as this alters the 
shape of the holes and destroys the value of the gauze. The 
proper method is to use a square frame of wood or metal about 
three inches deep, and to attach the gauze to this by means of 
a thin supplementary frame or slips of wood screwed to the 
former one. It is very convenient to make the sieves fit into 
each other, the upper, coarser one being provided with a close- 
fitting lid, and the lowest with a box to receive the finest 
material. This arrangement enables the sifting to be carried 
out without creating any dust, and is more rapid than when 
each sieve is used separately. 

To drive the fine particles through the sieve it is necessary 
to shake it continuously, an operation which requires a certain 
amount of skill. In the British Standard Specification it is 
directed that 100 grammes or 4 ounces of cement is to be 
continuously sifted for a period of fifteen minutes on each sieve. 
The area of the sieves is not stated, but the mesh is Nos. 76 
and 180, respectively. The residue on the coarser sieve must 
not exceed 3 per cent., and that on the finer sieve 1-8 per cent. 
Mechanical contrivances for shaking the sieves are frequently 
used, but do not produce such satisfactory results as hand 
shaking, the vibration being much sharper when the sieve is 
mechanically shaken. The use of a fine sieve is so tedious that 
several attempts have been made to separate the finest particles 
by other methods. The use of a washing or elutriating appa- 
ratus with paraffin as the levigating fluid is seldom practical, 
as it involves the use of enormous volumes of paraffin which 
cannot be readily purified for repeated use. Attempts to 
separate the finer particles by a process of sedimentation in 
paraffin have also proved unsatisfactory. Results of reasonable 
reliability have been obtained by Gary and Lindner, and 
independently by Cushmann and Hubbard, who passed a 
current of air through a series of three vessels of different sizes. 



108 TESTING THE PEOPERTIES OF CEMENTS 

The diameter of each vessel is arranged to correspond to a 
convenient speed of air and to give a product of which the 
particles are within very narrow limits of size. The air is 
admitted at a pressure of 100 mm. water column to the bottom 
of the first vessel, and passing through it is then carried to the 
bottom of the second vessel, and so on throughout the whole 
apparatus. A convenient quantity of cement (usually 20 
grammes) is introduced into the first vessel and is separated 
by the air-current, the particles being carried along in propor- 
tion to their fineness. The largest particles remain in the 
first vessel, the smallest pass through the apparatus into a 
collecting vessel, and particles of intermediate fineness are 
left in the second and third vessels respectively. When 
carefully used, this apparatus, which is known as a flouro- 
meter," gives fairly concordant results, though these are always 
subject to a loss of about 5 per cent, of the original material. 
The flourometer is of insignificant value in distinguishing 
cements and adulterants ; its chief value lies in showing the 
thoroughness or otherwise of the grinding. 

The author has obtained highly satisfactory results with a 
modification of a form of centrifugal apparatus patented by 
W. J. Gee. In this appliance paraffin of a definite density is 
mixed with the cement to form a thin slip or cream which is 
then run into the top of a rapidly rotating cylinder. Clear 
paraffin passes out at the bottom of the apparatus and on 
opening the latter the cement is found to be graded accurately, 
the finest particles being at the lower end. The separation is 
remarkably sharp and repeated tests have confirmed its relia- 
bility and superiority to the " flourometer " described above. 

No complete standard of fineness has yet been formulated, 
those in use merely limiting the proportion of useless, coarser 
particles and paying no attention to the size of particle which 
is actually the most efficient. D. B. Butler has made experi- 
ments which appear to indicate that cement particles which 
pass a No. 120 sieve, but are retained on a No. 180 sieve, are 
sufficiently small, but the demands of users since those experi- 
ments were made have resulted in the best commercial brands 
of cements being so fine that only about 5 per cent, is left on 
a No. 180 sieve, 



THE RATE OF SETTING 109 

For the British Standard Specification, Portland cement 
shall be ground so fine that the residue left on a No. 76 sieve 
must not exceed 3 per cent., and that on a No. 180 sieve shall 
not exceed 18 per cent. All the better brands of Portland 
cement conform to these limits, and a number of them leave 
only a trace on the No. 76 sieve and 2 per cent, or less on the 
No. 180 sieve. 

RATE OF SETTING. 

When Portland cement is mixed with water l a plastic paste 
is formed, which soon loses its plasticity, stiffens and " sets," 
and, later, hardens to a stonelike mass, which was supposed 
by the inventor to resemble Portland stone. (See Chapter IV.) 

Setting and hardening are two entirely different properties, 
and seem to have little, if any, connection with each other. 
It is, however, generally true that quick-setting cements 
harden more slowly than slow-setting ones. 

The speed at which a cement sets when gauged with water 
is no criterion of its ultimate strength, except in so far as 
quick-setting cements are very difficult to work and so may 
produce a weak material. If the workman should continue 
the mixing of the cement and water after the initial set has 
commenced, the ultimate strength of the material will be 
seriously reduced on account of the destruction of the crystalline 
network formed in the first stage of the setting. With modern 
quick-setting cements, excessive gauging resulting in working 
through the initial set is responsible for much faulty work in 
concrete construction. This is a defect which is extremely 
difficult to avoid, and is one of the soundest reasons for using 
a slow-setting cement whenever possible. 

As previously mentioned (p. 81), two distinct stages are 
recognised in the setting of cements : the first, or initial set, is 
when the pasty mass becomes just " solid," and the second, or 
final set, when the cement mass is sufficiently hardened to 
resist scratching by the thumb-nail or by some more accurate 
method of applying a light but definite pressure, such as a 
Vicat's needle. 

This is terxned gaugin g. 



110 TESTING THE PROPERTIES OF CEMENTS 

The point at which the initial set occurs is often difficult to 
recognise with quick-setting cements unless some definite 
method is adopted for ascertaining it. The one in general use 
consists in placing a pat of the cement paste on a glass plate. 
The Vicat needle is then applied ; if it penetrates the pat 
completely no setting has occurred, but if the needle sinks into 
the pat, but fails to penetrate it, the initial set has begun. 

In stationary kilns the proportion of fuel ash which becomes 
mixed with the cement is sufficient to make the latter slow- 
setting. Cement which has been burned in rotary kilns 
contains much less fuel ash and sets almost instantaneously, 
unless a suitable amount of a retarding agent is present. 

Cement manufacturers supply quick, medium and slow- 
setting cements, and occasionally a lot may be delivered of 
a different rate of setting to that to which the user is 
accustomed. 

Owing to the serious consequences which may follow the 
use of a quick-setting cement without its rate of setting being 
observed, it is important that each bag of cement should be 
tested before use. 

The three rates of setting are defined in the British Standard 
Specification as follows : 

Quick. Initial setting time not less than two minutes. 
Final setting time not less than ten minutes, nor more 
than thirty minutes. 

Medium. Initial setting time not less than ten minutes. 
Final setting time not less than half an hour, nor more 
than two hours. 

Slow. Initial setting time not less than twenty minutes. 
Final setting time not less than two hours, nor more than 
seven hours. 

It should be observed that quick-setting cements have a 
lower tensile strength and a lower compressive strength than 
those which set more slowly, but this is not invariably the 
case. 

The final set is said to occur when the Vicat needle, having 
been gently lowered on to the pat, fails to make an impression 
on it. The needle should be applied at sufficiently frequent 
intervals usually every ten minutes to a different part of 



THE RATE OF SETTING 



111 



Weight 
300 grms. 



the pat. Skilled workers usually invert the pat before testing 
for the final set, as the side uppermost when moulded is usually 
covered with a misleading scum which is much softer than the 
cement. 

The Vicat needle (Figs. 10 and II) 1 ordinarily used consists of 
a round steel bar which, with its flat head, weighs exactly 
300 grammes. At the lower end of this rod a needle or wire 
exactly 1 sq. mm. in cross section is clamped. The rod carries 
an indicator which moves over a graduated 
scale attached to the frame. The cement 
is held by a split ring 8 cm. in diameter, 
4 cm. high (E), resting on a glass plate. 

The cement confined in the ring resting 
on the plate is placed under the rod 
bearing the needle, which is then gently 
brought into contact with the surface of 
the cement and quickly released and 
allowed to sink into the cement. 2 This 
process is repeated until the needle, when 
brought into contact with the cement, 
does not pierce it completely, and the 
period between the time when the cement 
is filled into the mould and the time at 
which the needle ceases to pierce the 
cement completely is the initial setting 
time above referred to. 

Various auxiliary devices, the object 
of which is to make the Vicat needle 
automatic and self -registering, have been 
devised, but none of them are so satis- 
factory as the simpler form described. 3 

A test which would be better than the use of a Vicat needle 
would consist in measuring the pressure needed to drill a 
standard distance into the block of hardened cement. Such 




FIG. 10. Vicat's 
Needle (front view). 



1 The illustrations are of the British Standard Specification pattern. 

2 Care must be taken that the needle C rests with its full weight on the pat. 

8 The Vicat needle may, if desired, be fitted with a mechanical attachment, such 
as a " dash-pot." so as to ensure the steady and gentle application of the point of 
the needle to the surface of the pat and thereby render the test independent of the 
hand of the operator. 



TESTING THE PROPERTIES OF CEMENTS 



Weight 
^300 grammes 



a test would be particularly useful in distinguishing defective 
cements which harden only on the surface and leave a soft 
interior. 

A disadvantage applying to all mechanical methods of 
ascertaining the time of setting and hardening is the fact that 
the processes which occur in the cement are chemical, and it 
might be supposed that they would be more accurately 
measured by thermal than by mechanical methods. For this 

reason it is interesting to note 
that a method originally used 
by Faija, but abandoned by 
him in 1884, was revived in 
a modified form by Gary in 
1906. H. Faija observed that 
when the greased bulb of a 
thermometer was placed in a 
pat of cement immediately 
after gauging, the tempera- 
ture rose until a maximum 
was reached, after which it 
slowly sank to the original 
temperature. Gary, however, 
observed that at the moment 
corresponding to the final set 
the temperature again rises 
appreciably. 1 This method, 
whilst apparently of great 
promise, is affected by so 
minor considerations, 
the quantity of the 




many 
Fio.ll.-VIcaf. Needle (side view). guch 



material used and the rate of setting, that much further 
investigation is necessary before it can be brought into 
general use. Moreover, its indications do not always agree 
with the generally accepted Vicat needle test, the second rise in 
temperature in some cements occurring over an hour after the 
" final set " shown by the needle. H. Faija, who investigated 

1 W. Ostwald, in 1883, drew attention to the rise in temperature 5 7 days 
after the first set. W. and D. Asch consider that it is due to the separation and 
hydration of calcium oxide from the alumino-siiicate molecule. 



THE RATE OF SETTING 



113 



the thermal method very thoroughly, found its indications 
were so irregular that he abandoned it in favour of the needle, 
as constant results can be obtained with the latter. The 
following table will show the discrepancies between the two 
methods : 





NEEDLE METHOD. 


THERMAL METHOD. 


- Cement. 


Initial 
Set in 
minutes. 


Final 
Set in 
minutes. 


Increase 
in tem- 
perature 
during 

setting. 


Time to 
reach first 
maximum 
tempera- 
ture. 


Time to 
reach 
second 
maximum 
tempera- 
ture. 


A 


1 


5 


39 


5 


110 


B . ..'. . 


8 


17 


33 


12 


224 


C . 


6 


15 


30 


14 


176 


D . 


2 


8 


21 


7 


132 


E . 


8 


23 


25 


11 


114 


F (overlimed 












and under- 












burned) 


15 


250 


14 


25 


147 



It is very important, in gauging cements which are 
afterwards to be tested, that the water used should 
be free from salts, as these would alter the rate of 
setting. 

The use of well water of exceptionally low temperature 
will cause test pieces to show a low tensile and compressive 
strength, a difference of only 3 or 4 degrees below the 
normal being quite sufficient for this purpose (p. 131) ; hence, 
the necessity for testing the temperature of all water used for 
gauging. The mixture of cement and water, or of cement, sand 
and water, should be of the proper consistency. Unfortunately, 
it is very difficult to define the limits of consistency, and the 
amateur should, therefore, make a number of tests on well- 
known brands of cement and compare his results with those 
published by the manufacturer. In this way he will soon 
learn to judge what is the correct consistency far better than 
by attempting to use prescribed limits. For the same reason, 

c. i 



114 TESTING THE PROPERTIES OF CEMENTS 

the committee responsible for the British Standard Specification 
express themselves in exceedingly cautious terms, merely 
providing that " the cement shall be mixed with such a 
proportion of water that the mixture shall be plastic 




FIG. 12. Nicol's Spissograph. 

when filled into the mould," and adding the proviso that 
" the gauging shall be completed before signs of setting 



occur. 



In Germany an excess of water is used, a syrup being first 



THE SPISSOGRAPH 115 

formed which runs off the trowel in long threads. To this 
more cement is added in small quantities with vigorous 
trowelling until the mixture slimes and ceases to adhere to the 
mixing board. 

Where a more accurate guide to the consistency of the paste 
is required than that offered by noticing its behaviour during 
the gauging, the makers of one form of Vicat needle (Messrs. 
Adie, of London) provide a short cylinder 1 cm. in diameter 
which replaces the ordinary needle. The cement paste is 
gauged until it has reached what is considered to be the 
desired consistency, the time taken being carefully observed. 
It is then placed in the mould supplied with the instrument 
and tested. The cylinder should sink to a depth of 6 mm. 
above the level of the glass plate a scale being provided on 
the instrument to show the depth it has sunk. If the cylinder 
sinks further in the paste the latter is too thin ; if it does not 
sink so far the paste is too thick, and fresh proportions of water 
and cement, or cement and sand, must be tried until the 
correct consistency is obtained. 

An ingenious device for automatically recording the initial 
and final setting points is Nicol's Spissograph obtainable from 
A. & J. Smith, Maxwell House, Aberdeen (Fig. 12). This 
consists of a Vicat needle suspended from a cord and lowered 
on to the cement at regular intervals by clockwork. A 
supplementary mechanism ensures that the needle is applied 
to a different part of the cement each time it is lowered. 
The depth to which the needle sinks and the time of each test 
are marked on a revolving chart. 

The gauging ought not to occupy more than three minutes 
with slow-setting cements, or more than one minute with 
quick-setting ones. It is essential that the temperature of the 
cement, water and room in which the gauging is performed 
should be between 15 and 16 C., as variations in the 
temperature have a marked effect on the rate of setting. 
Yet comparatively little attention is paid to this matter. 
If the room is too warm the cements will be found to 
be quicker setting than they should be, whilst in a cold 
testing room a quick-setting cement which might cause 
serious trouble in use may be overlooked. The following 

I 2 



116 TESTING THE PROPERTIES OF CEMENTS 



table by D. B. Butler shows the variations in five typical 
cements : 





Temperature Centigrade. 


Sample 
No. 


38 


27 


16 5 


38 


27 


16 


5 




Initial set in minutes. 


Set hard in hours. 


1 


H 


4 


6 


13 


U 


H 


2 


2J 


2 


3 


5 


6 


8 


i 


ii 


If 


2 i 


3 


4 


10 


15 


20 


| 


} 


11 


61 


8 


10 


15 


35 


40 


I 


i 


!J 


If 


12 


15 


35 


70 


360 


8f 


6 


7 


22 



The increased rate at which cements set at slightly higher 
temperatures than the normal makes it necessary in the 
tropics to employ cements which would here be very slow 
in setting. 

SOUNDNESS. 

In order that a cement may be useful and satisfactory, it 
must not undergo any changes in volume when in use under 
any probable conditions of exposure. If the cement shrinks 
unduly during setting it will produce cracks, whilst if it expands 
after setting a different kind of cracking is produced, which 
is known technically as " blowing." Excessive contraction is 
seldom observed in Portland cements, the minute cracks 
produced by the small contraction which occurs in most cements 
being almost entirely superficial, and have no appreciable effect 
on the strength of the cement. Expansion after setting 
(" blowing ") is, on the contrary, one of the commonest defects 
of badly made or inferior cements, and as the results of such 
expansion are serious, and may even result in the destruction 
of a building with loss of life, it is of the greatest importance 
to ascertain by appropriate tests whether such expansion is 
likely to occur. A cement which cracks or twists after setting 
is said to be unsound. 

Unsoundness is usually attributed to the too tardy hydration 



THE SOUNDNESS OF CEMENTS 117 

of some of the constituents of a cement. It may, to some 
extent, be a result of imperfect mixing or gauging of the 
cement with water, or to the occurrence in the cement of 
undesirable substances. Thus, it is well known that quick- 
lime has a powerfully expansive force when moistened, and 
some other calcareous compounds possess the same property. 
Quite recently, however, Hans Kuhl has found, experimentally, 
that normal Portland cements (with and without gypsum), 
dead burned lime and quick-lime have a smaller volume when 
mixed with water than the sum of the volumes of the solid 
substance and the water, the contraction being greater with 
cements than with quick-lime. Cements which " blow " 
badly after setting were found by Kuhl to have a much smaller 
contraction (in some cases they expanded), from which he 
concludes that the true cause of " blowing " and expansion 
in cement is to be found in the formation of crystals from a 
supersaturated solution and in the pressure due to this 
crystallisation. 

In former years much of the unsoundness of cements was 
due to the use of too much lime in the raw materials ; this is 
now a far less frequent cause. 

A cement in which there is an appreciable proportion of free 
lime or an excessive proportion of magnesia or of sulphates is 
usually unsound, and though each of these substances under- 
goes a different chemical reaction with water, the final physical 
effect expansion and possible destruction of the material 
is the same. 

The action of magnesia in unsound cement is not clearly 
understood. The strict limitation as to the proportion of 
magnesia permissible is due to the collapse of certain bridges and 
other structures, including the Cassel Town Hall, in which 
magnesian limestone was used in the manufacture of the 
cement. In these cases the defects were attributed to the 
magnesia present, though it is by no means improbable that 
this is erroneous, as excellent samples of cement have been 
prepared from magnesian limestone. It is, however, necessary 
to burn cements containing magnesia at a higher temperature 
than when no magnesia is present (see p. 73). 

Magnesia requires a much higher temperature before it 



118 TESTING THE PROPERTIES OF CEMENTS 

combines with clay to form a cement ; if strongly heated, yet 
insufficiently so to effect combination, it will prove dangerous 
on account of the great expansion of highly calcined magnesia 
in the presence of water. Lightly calcined magnesia has no 
influence on cement. The increasing use of cements containing 
a large proportion of magnesia indicates that the danger of 
this oxide is far less than is commonly supposed, if only the 
conditions of manufacture, and particularly of burning, are 
correct. Improperly burned cements whether made of purely 
calcareous or magnesian limestones will be defective, the 
latter being particularly so. Hence the limit of 3 per cent, 
imposed in the British specification is a wise one. 

The action of sulphates on cement is discussed later, in the 
section on concrete, as it is of great importance in connection 
with maritime work. 

The presence of free lime or of lime in an unsuitable state of 
combination is usually regarded as the chief and commonest 
cause of unsoundness in cements. Properly prepared Portland 
cement contains no free lime, but although the original propor- 
tion of lime in the cement-mix may have been correct, it is 
not unusual, particularly when some of the earlier methods of 
settling, drying and burning are used, for the final clinker to 
be far from uniform in composition. With rotary kilns fed 
with slurry there is less likelihood of the various materials 
becoming unmixed, but in the intermittent, stationary kilns 
fed with broken lumps of deposited material, the extent of the 
irregularity is considerable, and, in some cases, is serious. 
This irregularity in composition is partly due to defective 
mixing appliances and partly to the natural tendency of 
materials mixed in the " wet process " to " settle out " in the 
wash-backs or settling tanks. No amount of grinding and 
mixing of the clinker will entirely remove the lack of homo- 
geneity, for the well-mixed clinker will be composed partly 
of true cement and partly of calcined but uncombined materials, 
and can thus destroy the value of the whole material. Under- 
burning may also account for the presence of free lime in the 
cement, and almost always occurs when stationary kilns are 
used, owing to irregularities in the draught ; the injurious 
action of this is avoided by carefully sorting out the clinker 



TESTS FOR SOUNDNESS 119 

before sending it to the mills. Under-burning occurs to only 
an insignificant extent in well -managed rotary kilns. Free 
lime in Portland cement is seldom due to wrong proportions 
of the raw ingredients, special care being taken to avoid this. 
It may be avoided by carefully checking the proportions of 
the raw mix and by securing as uniform a mixing and burning 
as possible, but as there is always a chance of free lime being 
present it is advisable to test all batches of cement as to their 
soundness. 

The general opinion that free lime is the cause of unsoundness 
in Portland cements is disputed by H. E. Kiefer, who found 
that, with sufficiently fine grinding, a mixture of cement and 
quick-lime containing 25 per cent, of lime will pass the ordinary 
tests for soundness. To avoid blowing, all that is necessary 
is that the lime shall be so finely ground that it becomes 
hydrated immediately. Kiefer's investigations seem to show 
that the phenomenon of " seasoning " is not so much one of 
hydrating the free lime as a decrepitation process in which 
the glassy particles of the cement are broken up. This is 
shown by the increased percentage of fine particles in a cement 
which has been stored in vacuo for some weeks. If Kiefer's 
views are correct the widely-held theory that unsoundness is 
due to free lime is not well founded. 

The soundness of a cement is difficult to ascertain with 
great accuracy, as the majority of the Portland cements now 
on the market are of such a character that they all pass any 
ordinary test for unsoundness. A very large number of 
different tests have been proposed as indicating the soundness 
or otherwise of a cement, and some of these are so severe as 
to make it questionable whether they really measure the 
soundness at all. In nearly all soundness tests the cement is 
subjected to very trying conditions, and any changes in it 
(such as an increase in volume, cracking, etc.) are noted. 
A cement in which no change can be found is regarded as sound 
under the conditions of the test. 

A test which is remarkably accurate, considering its sim- 
plicity, consists in gauging some of the cement to be tested 
with sufficient water to form a slurry or cream. This is then 
poured into a test-tube until the latter is full. The test-tube 



120 TESTING THE PROPERTIES OF CEMENTS 

with its contents is hung in a tank of cold water for one or 
more days. If the cement expands, due to imperfect burning 
or over-liming, it will crack the test-tube ; if it contracts, 
because the raw materials do not contain sufficient lime, the 
cement will shrink and become loose in the tube. Unfortun- 
ately the test-tube with its contents cannot be immersed in 
hot water, as the glass and cement expand unequally, and the 
former is cracked even with sound cements. 

The contraction of cements after setting is tested by means 
of a tapered metal mould which is filled with cement paste. 
If the cement shrinks or contracts it will gradually become 
loose, whilst a cement of constant volume will remain so tight 
in the mould as to be difficult to remove. There is no official 
test for contraction, but the mould used for determining the 
rate of hardening is generally made tapered and is commonly 
used for ascertaining whether a cement shrinks. 

Owing to the naturally slow hydration and other changes 
which occur during the hardening of cement it is almost 
hopeless to expect that any tests for soundness can be made 
with much rapidity. Moreover, it does not necessarily follow 
that because a cement can withstand the action of boiling 
water for several hours it will therefore resist exposure for 
several years ; in other words, an accelerated test does not 
necessarily give the same results as actual use, during which 
the cement hardens slowly in air. The present tests must, 
therefore, be regarded as tentative in character. The value of 
such accelerated tests is disputed by many cement manufac- 
turers and chemists, and to such a height did controversy rise 
at one time with regard to Le Chatelier's test (p. 125) that an 
International Congress Committee was appointed to investigate 
it thoroughly, and found it quite trustworthy and capable of 
detecting unsound cements which were passed as good by a 
number of other tests. Even at the present time the German 
manufacturers object to this test, on the ground that it gives 
different results with the same cement when tested in different 
places, and that it has failed to condemn some cements which 
swell when tested by the usual German method, viz., keeping 
pats in cold water for a month. 

Although all hot- water tests do undoubtedly reject some 



HOT WATER TESTS 121 

sound cements, from the user's point of view there are so 
many firms manufacturing cement which will stand the action 
of boiling water that these naturally have the preference. 
Some injustice is done to those manufacturers of sound cements 
which will not stand these tests, but under present com- 
mercial conditions this appears to be unavoidable. The 
student should, however, always bear in mind that a cement 
is not necessarily unsound because it cannot stand the Le 
Chatelier test ; at the same time, all cements which do pass 
this test can be relied on as being sound in use so far as the 
existence in them of expansive ingredients is concerned. 

The whole attitude of those responsible for the officially 
recognised tests is somewhat inconsistent so far as accelerated 
tests are concerned, and in different countries widely differing 
opinions are held. Thus, in the German Standard Rules 
recently issued, accelerated tests are entirely ignored, and in 
the American Standard Specification the following significant 
paragraph is included : "In the present state of our knowledge 
it cannot be said that cement should necessarily be condemned 
simply for failure to pass the accelerated tests ; nor can a 
cement be considered entirely satisfactory simply because it 
has passed these tests." 

Whatever excellent reasons there may be for or against 
accelerated tests, it is clearly unwise for engineers and others 
engaged in the use of cement to be too dogmatic on the subject. 
It is a well-known fact that fifteen years ago not 10 per cent, of 
Portland cement manufactured in England or elsewhere would 
withstand boiling- water tests, and the obvious inference is 
that if these are a true test for soundness, 90 per cent, of the 
cement used fifteen years ago was unsound. This, in view of 
the hundreds of thousands of tons then used for important 
engineering work throughout the world, is rather startling and, 
having regard to the excellent condition of such work at the 
present time, is a view that cannot be seriously maintained. 
The risk of using unsound cement is so great, however, that 
it is questionable whether any test can be really too severe, 
providing that it is a true test for soundness and not for some 
unimportant property. 

The earliest method of testing the soundness of a cement 



122 TESTING THE PROPERTIES OF CEMENTS 

consisted in making the cement paste into a thin pat with 
tapering edges and in placing it in water as soon as it is set. 
If at the end of a week it developed no cracks or twists it was 
considered to be sound. This is known as the " plunge test," 
but is now seldom used ; it has been found in practice to be 
too lenient, as exposure for a longer period frequently developed 
cracks, and it is unfair to cements which set very slowly. It 
has, therefore, been modified to avoid this objection. Thus, 
in what is known as the " cold water test," the cement pat 
is kept for twenty-four hours in moist air and is then placed 
in water for twenty-eight days. This modification requires 
an inconveniently long time, and is considered to be too lenient ; 
its severity has therefore been increased in various ways. At 
the other extreme is Erdmenger's test, in which the test 
pieces are heated in an autoclave under a pressure of 560 Ibs. 
per square inch ; this test has been regarded as unnecessarily 
severe. 

Of the accelerated tests, that devised by H. Faija was 
exceedingly popular for some years. It consists in main- 
taining a freshly gauged pat in water vapour at a temperature 
of 38 to 40 C. for about seven hours, or until thoroughly 
set, and then immersing it in water of a temperature of 46 to 
49 C. for the remainder of the twenty-four hours. A sketch 
of the apparatus used is shown in Fig. 13. It consists of a 
double-walled vessel in which the space between the walls is 
filled with water to act as a temperature equaliser. The 
inner vessel is only partially filled with water so that pats of 
cement placed on the shelf shown are immersed in vapour. 
According to D. B. Butler, who has an exceptionally thorough 
acquaintance with this test, a cement can be relied upon with 
perfect confidence if, after being treated in the manner 
described, it shows no signs of cracking or blowing at the end 
of twenty-four hours and adheres firmly to the glass plate on 
which it was made. Butler insists that the narrow limits of 
temperature prescribed are essential ; if a lower temperature 
than 46 C. is permitted a faulty cement will go undetected, 
whilst if exposed to too high a temperature some cements will 
be condemned which would prove satisfactory in use. The 
great advantage of the Faija test is that it requires only 



HOT WATER TESTS 



123 



twenty-four hours to sort out almost all the defective cements 
submitted to it, and where testing machines and plant are not 
available it is generally quite satisfactory. During the past 
few years, however, the demand for more severe tests, in which 
the cement is exposed to the action of boiling instead of merely 
warm water, has resulted in the Faija test falling into disuse, 
the Le Chatelier test being included in the British Standard 
Specification. 

The most characteristic feature of Faija's test, viz., exposure 
of the freshly-made pat to warm moist air in order to accelerate 
the setting and hardening, has 
been abandoned in more recent 
tests in favour of a longer 
exposure to cold moist air. 
This is regarded by many of 
those interested in cement as 
unfortunate, for extensive ex- 
periments have shown that 
the higher temperature has a 
very important effect on the 
cement. 

DevaVs Hot Water Test has 
also been the subject of 
much controversy. It con- 
sisted originally in allowing 
a pat of cement to remain 
in moist air for twenty-four 
hours and then immersing it 
in hot water at a temperature of 80 C. for six days 
or more, after which treatment sound cements are to 
show no signs of twisting, cracking or blowing. The tensile 
strength of test pieces subject to Deval's test is equal to that 
gained after twenty-seven days' immersion in cold water. 
More recently, the temperature of the water used in the test 
has been raised to boiling point, and the time has been shortened 
to three hours, both of which modifications will greatly simplify 
the test. In this modified form the test is much used in 
Germany under the somewhat inappropriate term, " Darr- 
probe," which really signifies a drying test. Twelve or fourteen 



jfe 



FIG. 13. H. Faija's Test for 
Soundness. 



124 TESTING THE PROPERTIES OF CEMENTS 

years ago Deval's test was considered to be unduly severe, but 
so greatly has the manufacture of cement improved within the 
last decade that in 1909 the International Association for 
Testing Materials found that all good cements stood it easily, 
but that it was uncertain in indicating some doubtful ones. 
On the recommendation of this association it was therefore 
abandoned in Great Britain in favour of the Le Chatelier test. 

Bauschinger's Method. One of the most accurate methods 
of measuring the expansion or contraction of cement is that 
devised by Bauschinger, who uses a special micrometer calliper, 
in which the test pieces consist of prisms or square bars 100 mm. 
long and 22 mm. by 22 mm. cross section ; the delicacy of the 
instrument is such that variations in the length of the bar to 
within 2Uo mm., or 0-0005 per cent., can be determined with 
certainty. It requires, however, such very careful expert 
handling, and is somewhat expensive (the equipment, including 
moulds, costing about 12), that it is only used for research pur- 
poses when an unusually high degree of exactitude is required. 

Le Chatelier' s Test is based on a somewhat different principle 
to those previously mentioned. Instead of the treated samples 
being examined for cracks they are measured before and after 
heating in water and the amount of expansion is noted. Le 
Chatelier's test is therefore much more sensitive than the 
earlier ones, and is correspondingly more severe. The total 
increase in volume is exceedingly small, and is almost impossible 
of direct measurement except with the use of exceedingly 
delicate appliances, which are more suited to the purposes of 
scientific research than to the needs of the cement manufacturer 
and user. This difficulty has, however, been overcome in a 
very ingenious manner by the invention by Le Chatelier of a 
special calliper (Fig. 14) which greatly magnifies the expan- 
sion. Le Chatelier's calliper x consists of a brass cylinder 0-5 
mm. (-02 inch) in thickness forming a mould 30 mm. (1 T 3 ^ inch) 
internal diameter, and 30 mm. (1 T 3 6 inch) high. This ring is 
split, and on each side of the split an indicator 165 mm. 
(6J inches) long is attached. The free ends of these indicators 
are pointed so as to facilitate accurate and rapid reading. 

1 This description of the instrument and test is taken from the British 
Standard Specification. 



LE CHATELIER'S EXPANSION TEST 



125 



In carrying out the test the calliper is placed on a small 
sheet of glass and is filled with freshly-gauged cement, the 
edges of the cylinder being kept together during this operation 
by means of a clip or piece of fine string. The cylinder is then 
covered with another piece of glass on which a small weight is 
placed, and the whole appliance is then placed in cold water 
(15 C.)for twenty-four hours. The calliper is then taken out 
of the water, the clip or string fastening removed, and the 
distance apart of the points of the indicators is accurately 
measured, and the calliper with its contents is then placed in 
cold water which is heated at such a rate that it boils in about 
half an hour and is kept boiling for six hours. The calliper is 







^A 



Split cylinder of spring brass or other 
suitable metal about Vz m / m in thickness 



Glass 



i Glass 



. FIG. 14. Le Chatelier's Test. 

again removed from the water, allowed to cool, and the distance 
apart of the pointed ends of the indicators is again measured. 
The increase in their distance is proportionate to the expansion 
of the cement. The British Standard Specification imposes 
the following limits of expansion under the foregoing 
conditions : The difference between the two measurements 
must not exceed the following limits, namely, 10 mm. when 
the cement has been spread in a layer three inches thick and 
exposed to the air for twenty-four hours, or, if this fails, 
5 mm. after the cement has been exposed to the air for seven 
days in the same manner. 

In cold water, iron cements expand rather more than Portland 



126 TESTING THE PROPERTIES OF CEMENTS 

cements, a mixture of hydraulic lime and sand (1:3) shows the 
same expansion as Portland cement, whilst a mixture composed 
of four parts of trass, three of lime and two of sand only expands 
half as much as Portland cement. Roman cements usually 
show an expansion of 20 mm., and hydraulic limes an expansion 
of 4 mm. in the Le Chatelier test. None of these substitutes 
for Portland cement can withstand long exposure to boiling 
water ; they crack and disintegrate. 

As previously mentioned (p. 120), Le Chatelier's test is by 
no means generally accepted, notwithstanding its inclusion in 
several standard specifications. It yields some curiously 
anomalous results at times, such as greater expansion in cold 
water than after boiling for six hours. Some of the difficulties 
experienced in its use are undoubtedly due to the delicacy of 
the measurements to be made ; others are due to an insufficient 
allowance for certain characteristics in the cements themselves. 
Thus, cements which set very slowly will yield bad results if 
tested in the ordinary manner by Le Chatelier's method, but 
if the same cements are allowed to harden properly before 
being tested the results will be normal, i.e., there will be only 
a trifling increase in volume. This test is, therefore, unsuitable 
for cements which do not set hard in twenty-four hours. The 
results of the Le Chatelier test should not, for these reasons, 
be interpreted too rigidly. 

The soundness of cement is increased by fine grinding, but 
in many cases users do not avail themselves of the extra 
fineness (which should enable them to use less cement), but 
continue to use the same proportion as in years gone by when 
only coarse cements were available. 

COMPRESSIVE STRENGTH. 

The value of a cement depends chiefly on its power to bind 
particles of inert material together so as to form a compact 
mass of great strength. The chief mechanical resistance 
required by structural materials in which cement is used is 
that to compression, and, accordingly, the compressive strength 
of cement, and more particularly of concrete or cement-sand 
mortar, is of the greatest importance. 



COMPRESSIVE STRENGTH OF CEMENTS 127 

The compressive strength of a cement, concrete or mortar is 
ascertained by crushing cubes of the material in a hydraulic 
press (Fig. 93) , the pressure applied being measured by means 
of a sensitive gauge. The cubes usually measure three inches, or 
70-7 mm., each side for cement, and 6-inch cubes for concrete. 

The Tests Committee of the Concrete Institute specify a 
crushing test in addition to the tests in the British Standard 
Specification. This test is carried out as follows : 3-inch 
cubes consisting of three parts by weight of standard sand 
to one part of cement shall be made in the manner described 
for cement-sand mixtures used in the tensile test. The crushing 
strength shall not be less than ten times the tensile strength 
after twenty-eight days required by the British Standard 
Specification. 

It is difficult to obtain uniform results in tests of the com- 
pressive strength of cement, as apparently trifling differences 
in the arrangement of the cubes or in the distribution of the 
pressure produce marked variations in the result. To equalise 
the pressure as far as possible, thin boards of soft wood are 
placed above and below the cube to be tested. If the crushing 
strain has been properly distributed the cube will, when crushed, 
leave two fairly perfect pyramids. 

Tests of compressive strength form part of the official tests 
on the Continent, but in Great Britain they are seldom made, 
the difficulties in the way of obtaining concordant results being 
considered to be too great for the test to be brought into 
general use. The unsatisfactoriness of the results of tensile 
tests and the obvious advantages of compressive tests are, 
however, becoming more generally recognised, and it is probable 
that before many years a minimum compressive strength 
(probably 250 Ibs. per square inch) will be included in the 
British Standard Specification. However, it has been found 
that the tensile strength is so closely proportionate to the com- 
pressive strength in well-made cements, being usually one- 
tenth 1 of the latter, and tests of tensile strength are so much 
easier to make, that they are generally substituted for com- 
pressive tests. 

1 The ratio of tensile to compressive strength increases with time, so that in very 
old mortars it may be as high as 1-18. 



128 TESTING THE PROPERTIES OF CEMENTS 



TENSILE STRENGTH. 

At the present time, the test which is regarded as of the 
chief importance is that of tensile strength. Although it is 
very convenient, the test itself is so illogical (cement structures 
seldom, if ever, being subjected to tensile stresses) and the 
results are so erratic that it cannot be regarded as really 
satisfactory. It is commonly supposed that the tensile 
strength is directly proportionate to the compressive strength 
of the material, but this ratio is only approximate and varies 
considerably with different cements. This has been fully 
established by results obtained by Tetmajor. Hence, the test 

of tensile strength is 
only used because of 
its general conveni- 
ence and should be 
replaced by com- 
pressive tests when- 
ever possible. 

As will be under- 
s tood from the 
statements made in 
respect of setting 
and hardening in 
140 weeks, a previous chapter, 
cements gradually 



Ibs/sq.fn 
600 



4-00 



200 



20 4.0 60 80 100 120 

FIG. 15. Increase of Tensile Strength with 
Time (Unwin). 



increase in strength 
as they increase in hardness. The increase is very rapid at first, 
but becomes increasingly slow with the age of the gauged 
material, and though it does not reach a complete maximum, 
even after several years, the strength reached after two years 
is so near to the final maximum that it may be regarded as 
identical for most purposes. The chemical and physical changes 
which occur during hardening are extremely complex, and it is 
therefore necessary to determine the strength of a cement on at 
least two different dates, which should be as widely separated 
as possible. From a number of such tests a graph may then 
be drawn and the probable maximum strength of the cement 
ascertained by extrapolation. Two such graphs obtained by 



TESTING TENSILE STRENGTH 129 

W. C. Unwin, in 1886, are shown in Fig. 15. The increase in 
strength is so rapid during the first week that it cannot be used 
for estimating the maximum strength likely to be developed ; 
after the expiration of a year the rate of increase is very slow, 
and the total strength then approaches the maximum. 

It will be thus understood that the total strength is likely 
to be greatest when the rate of increase does not diminish 
very rapidly during the weeks following the first. Hence, the 
custom has arisen of ascertaining the tensile strength at the end 
of three, seven and twenty-eight days, respectively, from the 
time of gauging. Much longer intervals are desirable, but are 
impracticable under most circumstances, and, so far, the three 
periods just mentioned appear to give a sufficient clue as to 
the probable maximum strength. It is not desirable to 
calculate the maximum strength likely to be developed, but 
to see that the strength at certain periods after gauging is 
above certain minimum limits. The most suitable limits 
recognised at the present time are prescribed in the standard 
specifications of the chief civilised countries where Portland 
cement is used. Thus, the British Standard Specification 
demands that test pieces (erroneously termed " briquettes "), 
made as directed (see p. 132) and tested in a suitable machine, 
in which the load is applied steadily at the rate of 100 Ibs. in 
twelve seconds, shall give the following results : 

FOR NEAT l CEMENT : 

25 per cent, when the seven-day test is above 400 Ibs. and not above 450 Ibs. 
20 450 500 

15 500 550 

10 550 600 

5 600 

FOR 1 : 3 CEMENT-SAND MORTAR : 

25 per cent, when the seven-day test is above 200 Ibs. and not above 250 Ibs. 
15 250 300 

10 300 350 

5 350 

Measurements of the tensile strength of test pieces which 
have been " aged " artificially by immersion in hot water 1^-ve 

1 By neat cement is meant a mixture of cement and water alone, no sand or other 
aggregate being added. The objections to tests of the tensile strength of neat 
cements are stated later under sand-cement tests. 

C. K 



130 TESTING THE PROPERTIES OF CEMENTS 

not given satisfactory results, the strength of the " accelerated " 
test pieces never showing any definite relation to those of the 
same cements kept under normal conditions for more lengthy 
periods. Attempts to measure the strength of " accelerated " 
test pieces have now been abandoned. Accelerated tests, 
omitting the determination of the tensile strength, are, however, 
valuable for indicating the presence of expanding constituents 
(p. 120). 

The complexity of the reaction which occurs during the 
hardening of cement is so great that the utmost care must be 
taken in gauging the cement (or cement-sand mixture) with 
water if uniform results are to be obtained. For the same 
reason at least six test pieces must be used for each test, 
as single tests are often far from correct. The following 
precautions are essential : 

(a) The proportion of water used must be correct within somewhat 
narrow limits. This has already been mentioned with regard 
to the setting and hardening (soundness) of cements (p. 113), 
but it is particularly important in connection with the tensile 
tests. No definite proportions of water can be specified, as 
cements differ greatly in the amounts they require. It may, 
however, be taken that the best-known brands of Portland 
cement require about 18 to 25 per cent, of water for neat 
cement, and about 12 per cent, for the usual 3 : 1 mixture of 
sand and cement. The smaller the proportion of water the 
better will be the results. Some amount of experience and 
skill is needed to know whether sufficient or too much water 
has been added. . If the former, the cement will crumble 
under the trowel used for mixing it and cannot be made to 
take a smooth surface ; with too much water, on the contrary, 
the paste will be so fluid that it can be poured from one vessel 
to another. It will usually be found that if the water just 
rises to the surface of the cement when the mixture has been 
well smoothed with a trowel, the mixture has the right con- 
sistencv 'It will then be plastic without being unduly dry 
^2 ifuid. This plastic mass can be mixed and moulded to the 
greatest advantage and is therefore preferred to a drier mixture 
possibly containing only 10 per cent, of water, though the 
latter may give a stronger product. Such dry mixtures give 



TESTING TENSILE STRENGTH 131 

such irregular results, however, that they cannot be relied 
upon. 

The British Standard Specification gives no precise directions 
as to the proportion of water, but merely states that it must 
be such that " after filling into the mould the mixture shall be 
plastic." 

(b) The temperature of both water and cement must be normal, 
i.e., between 14J and 18 C. (58 to 64 F.). If the tem- 
perature of the water is only 10 C. above normal the strength 
of the cement may be reduced 20 per cent. 

Lack of increase in strength on storage is also frequently 
traceable to the cement being tested in too hot a room, wherein 
the test pieces harden so rapidly that they really form 
accelerated tests. This trouble 
is particularly noticeable in 
summer in buildings with iron 
or glass roofs. 

(c) The duration of the gauging 
must be controlled. Steinbriick 
has found that the strength of 
cement increases with the 
thoroughness and duration of 
the gauging so long as the 

commencement of the initial set FlG " 16Fai^Mechanical 
is not reached. This merely 

means that the better the mixing the stronger will be the 
cement, and for this reason, and because the gauging of many 
cements becomes extremely fatiguing, the author has made 
extensive use of a simple mechanical gauger devised by 
H. Faija (Fig. 16). This consists of a circular pan about one 
foot in diameter, within which revolve the arms of a stirrer. 
These arms revolve round their own axis in one direction and 
round the pan in the reverse, this motion being given them by 
an internally toothed wheel, which actuates the pinion of the 
stirring spindle. The modus operandi is as follows : After 
having ascertained, by means of a preliminary hand-gauged 
pat, how much water the cement under treatment requires, 
sufficient cement to fill a nest of moulds is put into the gauger, 
and the correct amount of water added all at once. The 

K 2 




132 TESTING THE PROPERTIES OF CEMENTS 



handle of the machine is then turned fairly quickly for a half 
or three-quarters of a minute, by which time it will be found 
that the cement is thoroughly incorporated with the water, 
and the mass is in a proper condition to be turned out on the 
gauging plate or bench and filled into the moulds. In gauging 
cement and sand in this machine, for making sand briquettes, it 
is necessary, of course, first thoroughly to mix the sand and 



approximately 
0-10 approximately 




FIG. 17. Dimensions of British Standard Test Piece. 

cement in the dry state, after which the same routine may be 
followed. By the use of this machine two or three pounds of 
cement at a time can be efficiently gauged in a few minutes, 
and personal experience has proved it to be of the greatest 
value in avoiding the labour and wrist work necessary to bring 
the cement to a proper consistency with a trowel. 

The whole operation of filling a nest of moulds, from the 
time of adding the water, should not exceed five or six minutes, 
and after being smoothed off with a trowel, the moulds should 
be placed on one side until the briquettes are sufficiently set 
to be removed. 



TESTING TENSILE STRENGTH 133 

During recent years the Steinbriick-Schmelzer machine has 
been brought into extensive use for gauging cement for testing 
purposes. This machine consists chiefly of a single edge- 
runner moving in a rotating annular trough. Both roller and 
trough move in the same direction but at different speeds, the 
paste being turned over by two curved scrapers. The action 
of this machine is very satisfactory, about twenty rotations of 
the pan being ample to mix the cement. The roller and 
scrapers are then moved out of the way, giving ready access 
to the paste, and permitting of a rapid cleaning of the 
machine. 

If a cement is very quick setting it may be necessary to 
gauge it by hand in very small quantities at a time, or it may 
be spread out in a thin layer and exposure to the air to aerate 
for two or three days ; this 
exposure will make it slower 
setting. The addition of 
any retarding agent to a 
cement for the purpose of 
facilitating the testing is 
expressly forbidden in the 
British Standard Specifica- 
tion. On no account must FIG. 18. Mould for Testing Cement, 
the gauging be continued 

after the initial set has commenced, or the strength of the 
material will be irretrievably reduced. In gauging cements 
of which the rate of setting is unknown it is, therefore, desirable 
to test the rate of setting. as described on p. 109. 

(d) The mould must be of standard size and shape. That now 
universally adopted was devised by Grant, but the shape of the 
ends has been modified. Fig. 17 shows the standard dimensions, 
and Fig. 18 the mould. The mould and plate on which it 
stands should be slightly oiled before use. 

(e) The moulds must all be filled in a uniform manner. The 
usual method consists in placing the mould on a glass or smooth 
metal plate, taking up on a trowel more paste than will fill 
the mould, throwing the paste into the latter, and then tamping 
it with the trowel until the mould is filled evenly, and the 
water rises to the surface, giving it a shiny appearance. The 




134 TESTING THE PROPERTIES OF CEMENTS 

superfluous paste is then cut off by drawing the edge of the 
trowel across the top of the mould. 

(/) The test piece must be stored under standard conditions. 
It is important that the moulds and their contents should be 

PLAN 



i-oo 







ELEVATION 



L_.0-7f..JL i 20 -*.- 075 




FIG. 19. Jaws of Tensile Machine. 

placed on a bench which is free from vibration, or the strength 
of the test pieces will be impaired. If kept on the same bench 
or table as that used for filling other moulds, low results will 
be obtained, and too much care cannot be taken in this respect, 
particularly when tests are made of mixtures of cement and 



TESTING TENSILE STRENGTH 



135 



sand. It should also be noted that all test pieces used for 
strength tests should be kept immersed in water until ready for 
testing. This keeps them at a uniform temperature and also 
makes the test rather more severe than when the test pieces 
are kept in air. 




Fi<?. 20. Tensile Test Machine. (Adite.) 

The British Standard Specification demands that when the 
cement has set sufficiently to enable the test piece to be 
removed from the mould without injury, it is so removed and 
kept in a damp atmosphere for the remainder of the twenty- 
four hours after gauging. It is then to be placed in clean fresh 
water and allowed to remain there until required for breaking. 



136 TESTING THE PROPERTIES OF CEMENTS 

The water used for this purpose is to be changed every seven 
days, and is to be maintained throughout at a temperature of 
14J to 18 C. (58 to 64 F.). 

The test pieces are to be tested for tensile strength at seven 
and twenty-eight days after gauging, six pieces being used 
for each period. 

(g) The jaws of the testing machine must be of standard size 
and shape. Those most generally used are the ones recognised 
by the British Standard Specification, and shown in Fig. 19. 
The test pieces are greased slightly where they come in contact 
with the jaws. 




I 

FIG. 21. Tensile Test Machine. (8 alter <fr Co.) 

The testing machine may be of any convenient pattern ; 
one of the most compact is the compound lever originally 
devised by Adie (Fig. 20), but several other patterns are in 
use. So long as the load is applied " steadily and uniformly, 
starting from zero and increasing at the rate of 100 Ibs. in 
twelve seconds " the particular type of machine is unim- 
portant. 

Some means of arresting the indicator or of enabling the 
machine to work automatically is desirable. In the machine 
shown in Fig. 21, the weight corresponding to the breaking 
strain is applied in the form of shot, which runs from a reservoir 



TESTING TENSILE STRENGTH 137 

into a vessel attached to the beam of the machine. When 
the test piece breaks, this vessel falls, strikes a lever below it 
and instantly cuts off the supply of shot. The shot in the 
receiving vessel is then weighed and multiplied by the necessary 
factor to express the breaking strain in pounds per square inch. 
In some testing machines, water is used instead of shot, and 
in others a weight (Fig. 20) slides along the beam of the 
machine, its position at the moment of breaking indicating 
the tensile strength of the test piece. 

Great care should be taken to place the test piece properly 
in the jaws of the machine, with the contact between the two 
evenly distributed. After each piece has been broken it 
should be examined to ensure that no side strain or irregular 
pressure has been applied. Unless very great care is taken 
when placing the test piece in the machine one or more results 
will be much below the average ; this is frequently due to a 
defectively shaped test piece, or to its not having been placed 
truly in the machine. 

(h) The rate at which the load is applied must be the same in 
all tests. D. B. Butler tested over 800 samples at different 
rates of applying the load, and found that a rapid application 
greatly increased the apparent strength. He and Faija 
therefore adopted as a standard a uniform increase in load of 
100 Ibs. in fifteen seconds, which was largely adopted, but was 
afterwards altered to 100 Ibs. in twelve seconds when the 
British Standard Specification was drawn up. 

The minimum permissible tensile strength of test pieces 
made of neat cement is defined by the British Standard 
Specification as follows : 

" The average breaking strength of the briquettes seven 
days after gauging must not be less than 400 Ibs. per square 
inch of section. 

" The average breaking stress of the briquettes twenty-eight 
days after gauging must show an increase on the breaking stress 
at seven days after gauging of not less than 

25 per cent, when the seven-day test is above 400 Ibs. and not above 450 Ibs, 
20 ., 450 ,. 500 
15 500 550 
10 550 ., 600 
5 600 , 



138 TESTING THE PROPERTIES OF CEMENTS 

(i) The age of the cement and the amount of aeration it has 
undergone will affect the tensile strength. Modern cements are 
not, as a rule, improved by aerating, but rather the contrary. 
No really satisfactory explanation for this has been offered. 

(j) The conclusions drawn from different tests may be inaccurate. 
Thus, it is customary to average the results of a number of 
tests of the strength of cements and mortars, but the accuracy 
of such an average is seldom great. Cement manufacturers 
consider that the highest result of a series represents the value 
of a cement most accurately, and attribute the lower results 
to discrepancies in the gauging, moulding or fitting into the 
testing machine. 

CEMENT-SAND TESTS. 

The tensile strength of neat cement is a purely arbitrary 
factor, as cement alone is never used to withstand heavy 
structural strains, but for this purpose is always mixed with 
several times its weight of inert materials. Indeed, tests of 
the tensile strength of neat cement will frequently lead to 
erroneous conclusions in the absence of corresponding tests 
on mixtures of cement and sand. Thus, a very finely-ground 
cement if tested neat will appear to be inferior to a coarser 
sample, because, in the latter, the coarser particles will behave 
more like an inert material. Mixtures of cement and sand, 
on the contrary, show the great advantage to be derived from 
fine grinding. 

The futility of tensile tests on neat cement is shown by the 
fact that, if a mixture of cement with an equal weight of 
sand is finely ground, the product, when tested neat, has the 
same tensile strength as pure Portland cement. With the 
modern demand for a very finely-ground cement, therefore, in 
place of neat tests an admixture of sand should be employed 
in estimating the value of a cement. 

In the " neat" tensile test the full value of the cement as a 
concreting material, i.e. its cementing power, never comes into 
play ; and though it is generally recognised that a finer cement 
is a more valuable product, yet a coarse sample in a neat test 
will give results as to tensile strength equal to those obtained 
by a fine cement. On the other hand, the difference between 



TESTING TENSILE STRENGTH 139 

the constructive values of a coarse and of a fine cement will be 
most noticeable in a test for tensile strength if carried out with 
a mixture of sand and cement in the proportion of 3 : 1. 

The value of sand tests in the place of neat tests is becoming 
more appreciated day by day, and tensile tests on neat cement 
are not recognised in the German, Austrian and Swiss Standard 
Specifications. 

In order to obtain concordant and comparable results it is 
necessary to employ a " standard sand "; that recognised in the 
British Standard Specification being obtained from Leighton 
Buzzard. This sand is " thoroughly washed, dried and passed 
through a sieve of 20 X 20 meshes per square inch, and must 
be retained on a sieve of 30 X 30 meshes per square inch, the 
wires of the sieves being -0164 inch and -0108 inch in diameter, 
respectively." The German standard sand is rather coarser, as 
it passes through a plate perforated with circular holes -054 inch 
diameter, but not through holes -031 inch diameter. The 
French, Austrian, Swiss and American standard sands are of 
the same fineness as the British. The Russian sand is rather 
finer. There is an objection to the use of a different material 
in the tests to that which will be employed in the actual 
structure, namely, the differences between the two materials 
will prevent a true comparison of the results. It is, indeed, 
possible that a cement will be condemned when tested in 
admixture with standard sand, but will prove highly satis- 
factory .when tested as a portion of an actual structure. This 
discrepancy cannot, at present, be avoided without great 
difficulty ; substituting the sand and aggregate to be used in 
the actual structure does not solve the problem, as low tests 
on a given cement might then be attributed to unsuitable 
aggregates rather than to defective cement. At the same time 
the discrepancy between cement-sand mixtures and concrete 
made from the same cement is, in some cases, so serious as to 
make tests on the actual concrete imperative in all important 
structures where failure of the material might involve loss of 
life. 

The tensile strength of mixtures of cement and sand is very 
valuable in detecting adulteration in the form of inert material, 
such as Kentish rag (limestone) added to cement to cheapen 



140 TESTING THE PROPERTIES OF CEMENTS 

it. So far as the test on neat cement is concerned, the addition 
of an inert material will frequently increase the tensile strength, 
but the admixture will readily be detected by the low results 
obtained when the sand-cement mixture is tested. 

The great drawbacks to testing mixtures of sand and cement 
are (a) the irregularities in the results unless skilled men are 
employed, and (b) the length of time required for the test. 
The latter difficulty is not appreciable when the tests described 
in the British Standard Specification are used, as the time 
required for the tensile test of the neat cement is made the 
same as that for the cement-sand mixture, viz., twenty-eight 
days. 

The variations in the results obtained by different testers, 
or even by the same man on different occasions, are much 
greater than in the case of neat cement unless great manipulative 
skill is used in the gauging and filling of the moulds. With 
skilled operators the differences are not important. They are 
due almost entirely to the greater difficulty in gauging a 
cement-sand mixture and in handling it afterwards. 

The following extract from the British Standard Specifica- 
tion represents the best modern practice : 

' The cement shall be tested by submitting to a tensile 
stress briquettes prepared from one part by weight of cement 
to three parts by weight of dry standard sand, the said 
briquettes being of the shape described for the neat cement 
tests. 

" The mixture of cement and sand shall be gauged with so 
much water as to be moist throughout, but no surplus of water 
shall appear when the mixture is gently beaten with a trowel 
into the mould. Clean appliances shall be used for gauging, 
and the temperature of the water and that of the test room at 
the time the said operations are performed shall be from 
58 to 64 F., and no ingredient other than cement, sand, and 
clean, fresh water shall be introduced in making the test. The 
mixture gauged as above, shall be filled, without mechanical 
ramming, into moulds of the form shown in Fig. 18 (p. 133), 
each mould resting upon a non-porous plate until the mixture 
has set. When the mixture has set sufficiently to enable the 
mould to be removed without injury to the briquettes, such 



TESTING TENSILE STRENGTH 141 

removal is to be effected. Each said briquette shall be kept in 
a damp atmosphere for twenty-four hours after gauging, when 
it shall be placed in clean, fresh water and allowed to remain 
there until required for breaking, the water in which the test 
briquettes are submerged being renewed every seven days, 
and the temperature thereof maintained between 58 and 64 F. 

" The briquettes shall be tested for breaking at seven and 
twenty-eight days after gauging, six briquettes for each period. 
The average tensile stress of the six briquettes shall be taken 
as the tensile stress for each period. For breaking, the 
briquette shall be held in strong metal jaws, of the shape 
shown in Fig. 19 (p. 134), the briquettes being slightly greased 
where gripped by the jaws. The load must be steadily and 
uniformly applied, starting from zero, increasing at the rate 
of 100 Ibs. in twelve seconds. 

" The average breaking stress of the cement and sand 
briquettes seven days after gauging must be not less than 
150 Ibs. per square inch of section. 

" The average breaking stress of the briquettes twenty- 
eight days after gauging must not be less than 250 Ibs. per 
square inch of section, and the increase in breaking stress from 
seven to twenty-eight days must not be less than : 

25 per cent, when the seven-dav test is above 200 Ibs. and not above 250 Ibs. 
15 250 300 

10 300 350 

55 55 55 55 350 ,, 

All the best British Portland cements give much higher 
results than those just mentioned, and a large number tested 
under the author's supervision yielded results with an average 
of:- 

After seven days. After twenty-eight days. 

Neat cement, 660 Ibs. per square inch. 800 Ibs. per square inch 

Cement-Sand { 2 - ft - 

1-3 I " " " " 

The German standard is confined to 1:3 cement-sand 
mixtures, which must have a tensile strength exceeding 
228 Ibs. per square inch after twenty-eight days, and a com- 
pressive strength of 2,280 Ibs. per square inch. 

The French specification requires the gauging to be done 



142 TESTING THE PROPERTIES OF CEMENTS 

with sea-water, and gives the minimum tensile strength as 
114 Ibs. per square inch after seven days, and 214 Ibs. per 
square inch after twenty-eight days, with a further proviso 
that the increase in strength between seven and twenty-eight 
days must be at least 24 J Ibs. per square inch. A further 
clause in the French specification, which is not always insisted 
upon, requires the strength of the cement-sand test pieces to 
be at least 256 Ibs. per square inch at the end of eighty-four 
days, and in any case greater than at the end of twenty-eight 
days. 

Although mechanical ramming is not allowed in the above 
specification, it is employed on the Continent and (for their 
own satisfaction) by a few firms in Great Britain. Massive 
iron moulds must then be used in which a piston is fitted, and 
a considerable number of blows usually 150 are then 
delivered to the top of this piston at the rate of one blow per 
second, either by means of a small tilt-hammer devised by 
Boehme, or by a miniature pile-driving rammer designed by 
Klebe. The latter is prescribed in the Austrian and Swiss 
specifications. Presses of either the " screw " or " hydraulic " 
type have been used to consolidate the cement-sand mixture 
in the moulds, but they give lower results than do mechanical 
hammers, and have not been adopted in any official specifica- 
tions. 

The tensile strength of slag cements, mortars, hydraulic 
lime, pozzolanas, etc., is tested in a manner similar to that 
just described, though slight modifications are necessary on 
account of the different nature of the materials. Slag cement 
should give tensile strength results almost identical with those 
of Portland cement. Feeble hydraulic limes have a tensile 
strength of 70 to 100 Ibs. per square inch, but the stronger 
limes show results up to 140 Ibs. per square inch. Roman 
cements, rock cements and natural cements vary greatly, but 
should not have a tensile strength below 130 Ibs. per square 
inch ; very few of them reach as high as 200 Ibs. per square 
inch, which may be regarded as the minimum for a 1 : 3 
Portland cement-sand mixture. Pozzolanas (including trass) 
must be mixed with lime as well as sand before being tested. 
According to M. Gary, a mixture of equal volumes of trass, 



TESTING RESISTANCE TO BENDING 



143 



standard lime paste and standard sand should have a tensile 
strength of at least 200 Ibs. per square inch. The standard 
lime paste used is made by mixing slaked or hydrated lime 
with an equal weight of water. 



TRANSVERSE BENDING STRENGTH. 

In most cements the bending strength is fairly proportionate 
to the compressive and tensile strengths, and, as it is not difficult 
to measure, its determination is becoming increasingly popular, 
especially as the variations 
between different tests are less 
than in those of tensile and 
compressive strengths. 

Though not yet recognised 
in any official specifications, 
the resistance of cements and 
cement - sand mixtures to 
bending stresses have been 
extensively studied. Instead 
of the test pieces previously 
described, prisms (usually 
16 x 4 x 4 cm.) are employed, 
the cement and sand mixture 
being gauged with one-tenth 
of its weight of water. The 
prisms are tested by three 
knife edges two below and 
one above the prism which 




FIG. 22. Schiile's Machine for 
Transverse Bending Tests. 



are fitted to the jaws in a tensile strength testing machine 
(Fig. 22), the load being applied in the same manner as when 
testing tensile strength. 

0. Frey has found that the bending strength of a number of 
German standard cement-sand mixtures tested by him is 
about one-fifth of the compressive strength or double the 
tensile strength, but the British cements examined by the 
author show lower figures. The conclusions which may be 
drawn as to the value of a bending test may be summarised 
in the following manner : 



144 TESTING THE PROPERTIES OF CEMENTS 

1. The testing of cements by means of prisms made from 
plastic mortar is distinguished by its extreme simplicit}^ and 
by the great advantage that both bending and compression 
tests can be obtained from the same block-samples. This 
testing process is the solution of the question as to equal 
capacity of the tensile and compression test samples. 

2. The individual differences compared with the mean are 
smaller by this method than by the use of tensile test samples 
of 8 shape. For compression tests there is no considerable 
difference between the individual deviations and the mean 
values given by the results with cubes or with prisms. 

3. The lower figures given by the prism method correspond 
better with the strengths obtained in practice than do the 
high figures obtained with material of earth-moist consistency 
subjected to hard ramming. 

4. The difficulty of exactly determining the mixing- water and 
the work of ramming is solved in a satisfactory manner by 
using the same weights of the cement, sand and water in every 
test. 

5. The use of prisms gives the compression tests the import- 
ance which they deserve, which far exceeds that of the tensile 
and bending strengths, both in regularity and practical 
importance. 

SHEARING STRENGTH. 

The resistance of blocks made of cement and sand to a 
shearing stress would furnish valuable information if only it 
could be measured easily and accurately. At present, however, 
this is not the case, but as this subject is receiving a considerable 
amount of attention, a satisfactory method may shortly be 
discovered. Meanwhile, bending tests (p. 143) are the nearest 
approximation. 

OTHER TESTS. 

A number of other properties of cements and cement-sand 
mixtures have been proposed as the basis of valuations, but 
have never been extensively used. 

The adhesion of cement to metal is an important property as, 
in reinforced concrete, it tends to counteract the sliding of the 



VARIOUS TESTS 145 

metallic bars embedded in the material. This adhesion or 
resistance to gliding is a resultant of several forces, including 
the resistance of the cement to shearing, the pressure exercised 
between the metal and the cement, the alterations in volume 
accompanying the hardening of the cement and the action of 
external loads. Unfortunately, it is, at present, impossible to 
measure this adhesion accurately. 

Adhesiveness is tested by placing a mould on the surface 
to which it is desired the cement shall adhere (stone, brick, etc.), 
and then filling the cement, cement-sand mixture or mortar as 
usual. When the material is sufficiently hard the mould is 
removed and the test piece is subjected to tensile strain in the 
ordinary testing machine. The most satisfactory results are 
obtained when the test piece has the shape and dimensions 
shown in Fig. 17. Tests of adhesion are, usually, tests of the 
" other " material rather than of the cement, and are not 
required except in very unusual circumstances. 

The compactness or apparent density of a test piece is deter- 
mined by means of a volumenometer, 1 in which the volume 
of the test piece is measured. The volume in c.c., divided by 
the weight of the test piece in grammes, is the apparent density, 
and may be regarded as a measure of the compactness. 

The porosity of a test piece is determined by weighing it, 
then immersing it in a vessel from which all the air can be 
exhausted. Enough water must then be admitted to 
completely cover the test piece. The air is exhausted so as to 
cause the water to enter all the open pores in the test piece ; 
the latter is then wiped dry and then re- weighed. The 
increase in weight will be the water absorbed by the pores in 
the test piece. 

The porosity of test pieces made of neat cement is extremely 
low ; that of concrete aggregates is much higher and is 
preferably determined, as described in the section on concrete. 

1 For a description of this apparatus, see the author's " British Clays, Shales and 
Sands," C. Griffin & Co., London, 1911. 



CHAPTER VI 

THE COMPONENTS OF CONCRETE AND THEIR PROPERTIES 

CONCRETE may be defined as an inert material, the particles 
of which are united by cement to form a hard, stony mass 
useful for all the ordinary purposes of natural building stones. 

The use of concrete has been extended enormously during 
the past few years, and now covers a wide sphere of usefulness 
in the construction of bridges, breakwaters, docks, canals, 
dams, reservoirs, paving stones, partitions, roofing tiles, 
building blocks, boats, rafts, conduits, water mains, sewers, 
telegraph poles, fences, caissons, pipes, etc., etc. Each year 
new uses are made of this remarkable material, and our 
knowledge of its properties is increased. 

The inert material is usually composed of sand of different 
degrees of fineness and a coarser material termed an aggregate. 
Other materials are, however, introduced in special cases to 
confer additional properties on the concrete. Thus, pozzolanas 
(p. 161) are sometimes added to increase the hardness of the 
mass, whilst very finely ground rock, dust or clay is sometimes 
used to secure a denser concrete. Clay is inadvisable for this 
purpose on account of its plasticity and water-repelling power. 

The components of concrete are usually four in number : 
(a) cement, (b) sand, (c) aggregate, and (d) water. To these 
must be added metal (usually steel) in the case of reinforced 
concrete. 

The apparent simplicity of concrete has led to its abuse in 
many ways, and there is widespread belief that any inert 
material and almost any kind of lime or cement may be used 
with satisfactory results. This is altogether wrong, for in 
reality the production of a satisfactory concrete is by no means 
simple, and involves considerable knowledge, continual super- 
vision and conscientious work. 



COMPONENTS OF CONCRETE 147 

CEMENT. 

Many kinds of cementitious material may be employed for 
concrete, the strength of the structure depending on that of 
the cement and of the aggregate. Lime and hydraulic lime 
concretes are largely used for foundation work on account of 
the low cost ; natural cements (p. 29) are used for somewhat 
stronger work, and Portland cement (p. 20) where the best and 
strongest concrete is required, and for all reinforced concrete. 

The strength and reliability of Portland cement are such 
that it is rapidly replacing other kinds in all the most important 
work. Care should, however, be taken that only Portland 
cement of good quality, and preferably that complying with 
the requirements of the British Standard Specification (p. 97) 
should be used. For greater accuracy of working as well as 
convenience, the cement should be delivered in bags or barrels 
containing a definite weight of cement. This weight and the 
maker's name should be marked legibly on the bags or barrels. 

WATER, 

The water ordinarily available is suitable, unless it contains 
a notable quantity of humic acid or other organic matter which 
retards the setting of the cement. The water used should 
always be clean ; that from ponds and streams is liable to 
contain clay and other detrimental matter in suspension. 
In case of doubt it should be run into a settling tank or filtered 
through sand. The water should therefore be tested to ascer- 
tain the effect (if any) of its constituents on the rate at which 
the cement sets. Hard water should be avoided wherever 
possible as the action of the salts dissolved in it is always 
uncertain and often detrimental. Carefully collected rain 
water is the most generally suitable kind of water ; that from 
wells sunk in the chalk is the most unsatisfactory of all kinds 
of fresh water. Sewage and other effluents should never 
be used. 

AGGREGATES. 

Many varieties of stone, broken bricks, coke, clinker, ashes 
and other substances of a stony character may be used as 

L 2 



148 COMPONENTS OF CONCRETE AND PROPERTIES 

aggregates in concrete, the selection of any particular piece 
of work depending on the purpose for which the concrete is 
to be used, the strength it is desired the concrete should possess, 
the accessibility and cost of each aggregate-material and 
in many instances the whim of the architect or engineer in 
charge of the work. Strictly speaking, the whole of the solid 
non-cementitious material used in concrete constitutes the 
aggregate, but this term is usually applied solely to the coarser 
portion, the finer being termed " sand." 

Aggregates used in concrete may be divided into four 
groups : (a) natural stones, such as granite, limestone, etc. ; 

(b) artificial stones, such as burned clay, broken bricks, etc. ; 

(c) by-products of various kinds, such as coke, blast-furnace 
slag, clinker or ashes ; and (d) pozzolanas or trass (p. 159). 

Natural stones must usually be broken into pieces of con- 
venient size, but those which occur in the form of gravel have 
the advantage that, if free from sand and clay, they are ready 
for use without any further preparation. The most suitable 
are angular fragments of moderate density ; rounded pebbles 
from the sea-shore or river beds (ballast) are less satisfactory, 
but are used in large quantities on account of their convenience. 
Fortunately, the difference between the useful strength of 
concrete made with pebbles and that made with angular 
pieces is not sufficient to be of importance in most instances. 

What are known to geologists as igneous rocks furnish 
many excellent aggregates, though the basalts, traps, felsites 
and denser lavas have too high a specific gravity (2-9 to 3-2) 
to be desirable. The lighter lavas, such as pumice stone, on 
the contrary, have so low a crushing strength that they can 
only be used for partitions and other work where lightness 
rather than strength is required. Where pumice stone is not 
available, a good, light concrete of a similar nature may be 
made from hard coke. 1 Some of the basalts have the further 
disadvantage of a glossy surface, to which the cement does not 
adhere satisfactorily. 

Granite, if washed free from adventitious substances and 
powder, is a very suitable aggregate for almost any class of 

1 The risk of fire must not be overlooked. This danger is often exaggerated 
(see p. 150). 



NATURE OF AGGREGATES 149 

structure. Granite-concrete is dense and rather heavy (i.e., of 
rather high specific gravity), but is particularly useful for large 
structures and for maritime work. It is costly, except in those 
localities where granite occurs, but is universally recognised as 
the best aggregate. 

As the presence of fine adherent granite dust is detrimental, 
the granite chippings should be thoroughly washed before use. 
In quarries when fans or dust extractors are used the granite 
is more free from dust, and is to that extent superior in the 
absence of facilities for washing the material. 

Limestones are amongst the most valuable sedimentary 
rocks which may be used as aggregates, and are so widely 
distributed as to be generally available. The harder varieties 
with a specific gravity of 2-7 are the most suitable for the 
purpose. The most artistic results are obtained with the 
particular variety of limestone known as Portland stone, on 
account of the close resemblance it bears to Portland cement, 
thereby enabling the concrete to be carved and dressed in a 
manner impossible when an aggregate of a different colour is 
used. 

The chief disadvantage of limestone as an aggregate is the 
decomposition it undergoes when subjected to the action of 
fire, resulting in the shrinkage and rapid collapse of the 
structure. 

Sandstones, quartzites and other siliceous stones are generally 
very suitable as aggregates, but shales, slates and micaceous 
sandstones should be avoided, as the flat fragments they 
produce do not form a strong concrete. 

In some districts particularly in the eastern counties 
flint pebbles are the only stones available. They make an 
excellent concrete for use at ordinary temperatures, but one 
which is apt to " fly " and split when heated. This tendency 
is reduced by crushing the flints in a stonebreaker, but their 
great hardness makes this costly. 

The artificial stones used as aggregates consist chiefly of 
broken bricks, terra-cotta or burnt clay ballast. Inferior 
material should be avoided, as it has a low crushing strength 
and is very liable to crumble. The materials selected should 
be hard and preferably should show signs of vitrification. 



150 COMPONENTS OF CONCRETE AND PROPERTIES 

Soft and crumbly material should be avoided. In addition 
to their primary nature as hard and inert materials, all products 
composed of burnt clay have cementitious properties when 
mixed with lime (see pozzolanas, p. 35), and so tend to increase 
the strength of the concrete. This is particularly the case 
where the material is ground to the state of sand (p. 159). 

If broken bricks are used they should be carefully cleaned 
from any adherent mortar and dust. Broken pottery, tiles 
and pipes are unsuitable, as the pieces " bridge over " each 
other and form excessively large voids. 

Bricks from some localities are occasionally found to cause 
" blowing " or cracking in the concrete, but the reason for this 
has never been satisfactorily explained. The statement some- 
times made that Fletton bricks contain an excessive amount 
of sulphur does not apply to most of the bricks from the 
Fletton district, though it is an unquestionable fact that, in 
several instances, concrete made of bricks alleged to have 
been made near Fletton has proved unstable. Whether its 
failure be due to the method employed in making the concrete 
or to the broken bricks used is, at present, impossible to say. 

Coke-breeze, when sufficiently resistant to crushing, forms 
a good aggregate, but the softer coke which is sometimes 
substituted is dangerous. Coke has been chiefly used as an 
aggregate in concrete floors and walls, as it makes a light 
construction into which nails may be easily and securely 
driven. It has somewhat undeservedly fallen into disrepute 
because of its prohibition in Germany where its quality is 
distinctly inferior to that of good English coke. 

It has also been suggested that coke forms an inflammable 
aggregate, but the tests made by the Fire Prevention Committee 
with various kinds of floors exposed for three hours to a rapidly 
increasing and ultimate temperature of 1,900 F. followed by 
a spray of cold water, placed coke breeze and burnt clay ballast 
as first, and showed them to be (after the tests) quite free 
from cracks and deflection, and much superior to granite so 
far as these particular tests were concerned. 

The fear of expansion and disintegration of coke-concrete 
in ordinary use also appears to be largely imaginary. The 
chief disadvantage of coke is the presence of oxidisable sulphur 



NATURE OF AGGREGATES 151 

compounds in it ; these expand and rupture the concrete, 
sometimes to an alarming extent. What is known as " steel 
coke " is usually very free from sulphur compounds, and may 
be highly recommended where a light aggregate is desired, but 
coke rich in sulphur is undesirable, and ashes are a particularly 
unsuitable material to mix with the coke. Coke breeze should 
not be used for reinforced concrete. 

Ashes are similar to coke, but usually contain a large pro- 
portion of sulphur and so are undesirable, though much used 
on account of the low cost. Ashes from locomotive boilers 
are considered to be superior to domestic ashes, the steam in 
the forced draught being supposed to effect a volatilisation of 
the sulphur present in the coal. Care should be taken that 
the ashes are free from admixture with hydraulic lime, as lias 
limestone is mixed with some of the coal used by engine 
drivers. 

Clinker and furnace slag are stronger aggregates than coke or 
ashes, but suffer from the presence of sulphur compounds which, 
when oxidised, expand and may cause the destruction of the con- 
crete. Provided that the proportion of sulphur is sufficiently 
small to be practically harmless, and that the clinker is hard and 
free from dust, shale, lime, ash, metal scrap or scale and basic 
slag, slags form valuable aggregate material, their general 
availability and low cost rendering them very attractive to 
builders and contractors. The fact should not be overlooked, 
however, that many slag heaps vary very greatly in com- 
position, and that tests do not necessarily represent the percen- 
tage of sulphur in any and every part of the heap. Hence, 
there is always some risk in using slag unless its composition 
is exceptionally uniform. The commercial attractiveness of 
slags as aggregates must not be allowed to obscure the danger 
of collapse which accompanies their use, except when the 
construction of a building has been under the most strict 
supervision. 

Clinker and slags should not be used for reinforced concrete, 
except under protest. 

If the particles composing the aggregate are of various sizes 
the mass will be more compact than if all the particles are 
large and uniform in size. Thus a mixture of broken stone. 



152 COMPONENTS OF CONCRETE AND PROPERTIES 



cement and sand will occupy a smaller volume than when each 
of the materials are kept separately. This is clearly shown in 
Figs. 23 26. In Fig. 24 is shown a plan of a cubical box 
containing twenty-seven spheres of equal size ; there is a 
considerable amount of space between each of these spheres, 
even though the box is apparently " full." In Fig. 25 the 
same box is shown with a number of smaller spheres packed 





J L 




Gemsnt Sa.nd Stone Concrete 

FIG. 23. Space occupied by Concrete and its components. 

in between the larger ones, so that the box now contains 
considerably more material than it did before. At the same 
time it is clear that there is still space enough to contain a 
comparatively large quantity of sand and fine powder before 
all the spaces or voids between the particles would be com- 
pletely filled. If instead of truly spherical particles, irregularly 
shaped ones are used, the same facts will be observed, and this 






FIG. 24. 



FIG. 25. 



FIG. 26. 



is what actually occurs in the -preparation of concrete (Fig. 26). 
From this it is clear that most aggregates must be graded or 
separated into fragments of suitable sizes, as if this is not done 
either a weak concrete will be obtained or a very large amount 
of cement will be wasted. As the cost of the additional cement 
is far greater than the expense of grading the aggregate, a 
careful grading of the material effects a considerable saving 



NATURE OF AGGREGATES 153 

in cost. It is possible to make a difference of nearly 100 per 
cent, in the strength of a concrete by careful attention to the 
grading. If the aggregate is composed of particles all of one 
size, the amount of voids will be about 50 per cent. ; if it is 
composed of two sizes of particles, the proportion of voids 
will shrink to between 30 and 40 per cent. ; by still more 
variation in the size of the particles, it is possible to get the 
amount of voids down to 20 to 25 per cent., and in a very 
weak proportion, such as 1 : 15, there is no reason why one 
should not include an amount of fine sand so as to reduce the 
percentage of voids still lower. To use a lot of sand will not 
always so reduce the percentage of voids, because sand, which is 
composed of a fairly regular form of grain, will likewise contain 
50 per cent, of voids. A gradation of size is necessary in either 
large or small material in order to reduce the amount of voids, 
it being the gradation in size which enables the particles to 
arrange themselves more closely together and leave a smaller 
proportion of interstices. It is easy to see that in a properly 
graded material wherein the voids are a minimum, the cement 
will go much further and the concrete will be denser and 
stronger. The principle applies equally to a poor concrete, 
such as 1:5:1, or a rich mixture, such as 1:1:2 (see 
p. 167). 

Many gravels require no grading, as the particles they 
contain are all of sizes and in proportion suitable for use as 
aggregates ; this is one of the advantages of such gravels. 
Other gravels and most of the larger materials must first be 
crushed to fragments of a suitable size, the crushed product 
being then divided by sieves into particles of certain pre- 
arranged sizes. 

The size of the largest particles permissible in an aggregate 
depends to some extent on the size of the mass of concrete to 
be produced. For most purposes the aggregate should pass 
completely through a hole one inch diameter, though for 
very large blocks some of it may be large enough to just pass 
through a 3-inch hole. The Second Report of the Com- 
mittee of the Royal Institute of British Architects states that 
I inch is the usual maximum size allowable. In very large 
masses of concrete, such as are used in maritime work, large 



154 COMPONENTS OF CONCRETE AND PROPERTIES 

blocks of stone are embedded at intervals. The usual practice 
is to screen through sieves of the meshes as under : 

For Artificial -paving blocks . J inch to f inch. 
Floors . , . 1 | 

Walls . . ; ' .1 2 inches. 
Foundations . . 2| inches. 

" Plums " or large stones used in ordinary foundation work 
should be carefully deposited well away from one another, and 
from the sides and angles of the mass ; they are never allowable 
in reinforced work. 

All material which will pass through a hole J-inch diameter 
should be removed by sifting or washing or a combination of 
these processes, as the presence of loam and clay is highly 
detrimental, and sand of unusual fineness has been known to 
cause a failure in the concrete. An aggregate which contains 
more than 3 per cent, of clay should, generally speaking, be 
rejected. 

It is sometimes urged that the removal of sand from 
aggregates is undesirable, especially as some sand must be 
added to the concrete mixture in order to fill the voids in the 
coarser aggregate. For this reason, some engineers wash the 
aggregate so as to remove earth, dust, clay and other very 
fine particles, but endeavour not to remove the coarser sand. 
If the aggregate is of a very uniform character this method may 
be adopted with success, but in most instances it will be found 
that the distribution of the sand in the aggregate is so irregular 
that it is almost impossible to ascertain accurately the propor- 
tion present. The removal of all particles which will pass 
through a hole J-inch diameter secures an aggregate of much 
greater uniformity as regards the voids and greatly assists 
in the production of a concrete of maximum strength. 

Whether a particular aggregate requires washing may be 
ascertained in a rough, but usually sufficient manner by a 
method recommended by the Associated Portland Cement 
Manufacturers, Limited. A tall glass cylinder, graduated in 
c.c. and of at least 50 c.c. capacity, is half filled with aggregate 
and water is then added to fill the cylinder about three-quarters 
full. The cylinder is then closed with a cork or rubber stopper. 



NATURE OF AGGREGATES 



155 



and it is then shaken violently so as to wash out all clay and 
sand. The cylinder is then placed on a bench and its contents 
are allowed to settle. After a few minutes the sand and most 
of the clay will have settled on top of the aggregate, and its 
proportion to the latter can be roughly gauged by comparing 
the respective volumes of each. The apparatus required is 
shown in Fig. 27. 

It should be observed that the foregoing method does not 
show the presence of sulphurous slag, coke breeze and certain 
other deleterious ingredients of a coarse nature. If these are 
suspected, the 
aggregate must be 
subjected to a more 
searching investiga- 
tion. 

The removal of 
the fine material is 
best effected by 
washing in a shallow 
trough, the bottom 
of which is per- 
forated. The output 
is increased if the 

trough is vibrated PlG . 27, Apparatus for determining whether 
in a manner similar aggregates require washing before use. 

to the jiggers used (By courtesy of the Associated Portland Cement 

, . , , Manufacturers, Ltd. ) 

tor washing coal and 

ores. The method frequently adopted of throwing the 
material against a steeply inclined riddle is not satisfactory, and 
fails to remove a large proportion of the finer material. Where 
no jigger is available, metal wheelbarrows, the sides and bottom 
of which are perforated with J-inch holes, may be substituted. 
The concrete aggregate is delivered in piles near the work, and 
carried from them to the mixer in wheelbarrows. Between 
the mixer and the storage piles a water-pipe is connected up, 
with a large perforated nozzle at its discharge end. Each 
wheelbarrow load of gravel on its way to the mixer is rolled 
under the nozzle and streams of water discharged upon it, the 
material being churned about with a spade to expose the 




156 COMPONENTS OF CONCRETE AND PROPERTIES 

lower part of the load to the cleansing action of the water. The 
water and the loam pass out through the perforations in the 
wheelbarrow, and the clean gravel is then carried to the mixer 
and used. 

Aggregates should always be wetted thoroughly before being 
used, and the additional labour of screening is so insignificant 
in relation to the advantages gained that it should never be 
omitted. This is particularly the case where the aggregate is 
composed of crushed limestone, the fine dust of which is 
peculiarly detrimental and may reduce the ultimate strength 
of the concrete by as much as 40 per cent. 

It is a curious fact that the larger the proportion of fine 
aggregate the weaker will be the concrete. Thus, a concrete 
made of small pieces of sandstone and cement will be much 
stronger than one made of the same materials, but with the 
stone reduced to powder. Consequently, it is desirable to use 
an aggregate of a coarse nature and also with as few voids 
as possible. In order to ensure this, the aggregate should 
all be passed through a screen with holes of a prearranged 
size (say 1J inch diameter), then washed and passed over 
another screen with holes J inch diameter. 

As suitably graded aggregates yield stronger concretes than 
those in which the fragments of aggregate are all the same 
size, the Testing Committee of the Concrete Institute have 
suggested that the aggregate should " be sifted to the following 
degrees and the percentage of voids ascertained of (1) the 
whole, and (2) of each separate grading : 

To pass an aperture of To be retained on an aperture of 

inch by f inch f inch by f inch 

~z h i* " 's " 



No definite proportions of either size of particle or of voids have 
yet been specified. It is, however, recognised that the more 
varied the sizes the denser and stronger will be the concrete, 
and the smaller the proportions of cement and sand needed. 

The proportion of each grading to the whole and the specific 
gravity of the aggregate should also be ascertained. The 
aggregate thus obtained should then be tested to ascertain the 



MEASURING THE PROPORTION OF VOIDS 157 

proportion of voids present. This should not exceed 45 per 
cent, and will seldom be less than 25 per cent. 

The percentage of voids in an aggregate or sand is deter- 
mined in a manner .similar to the following, various small 
refinements being possible where a more accurate determina- 
tion is required: The aggregate to be tested is well wetted 
and allowed to drain on a sloping sheet of glass so as to 
remove all surplus water from the surface without withdrawing 
any from the pores. Two precisely similar graduated glass 
cylinders, each of about 1,000 c.c. capacity, are then placed 
ready for use. In the first cylinder is placed a convenient 
quantity of the aggregate. This is shaken and tamped down 
so as to make it as compact as possible without breaking the 
fragments. The volume of the aggregate in the cylinder is 
then carefully noted; it should be about 600 c.c., but must 
be measured as exactly as possible. In the second cylinder 
is placed 400 c.c. of water. The measured quantity of 
aggregate is then completely transferred to the second cylinder, 
and any air bubbles in the latter are removed by probing 
carefully with a wire. The contents of the second cylinder 
are then shaken so that no air spaces exist in the aggregate, 
and after a few moments settling the height of the water level 
in this cylinder is noted by means of the graduations on the 
glass. The difference between the volume of aggregate and 
water before mixing and after mixing will be the volume of 
the voids in the quantity of aggregate tested, and from this 
the percentage of voids may be easily calculated. Thus, 
supposing that the volume of aggregate used was 640 c.c., the 
volume of water 400 c.c. (making a total volume before mixing 
of 1,040 c.c.) y and that the volume in the second cylinder 
(after mixing) was 787 c.c., then the difference in volume on 
mixing (due to the voids) would be 253 c.c., and the percentage 
of voids will be 



253 X 100 

= 39*5 per cent. 

640 



The apparatus used is shown in Fig. 28 by courtesy of the 
Associated Portland Cement Manufacturers, Ltd. 

If the aggregate is porous (like coke) it must be well-soaked 



158 COMPONENTS OF CONCRETE AND PROPERTIES 



in water before being put into the measuring vessel, as it is 
the space between the particles and not the porosity of the 
particles themselves which it is desired to measure. Materials 
of a highly porous nature, yet with somewhat large pores, are 
troublesome, as each piece must be immersed in paraffin wax 
before being tested, for unless the pores are sealed in this way 
the water will run out of them in transferring the aggregate 
to the measuring vessel and an erroneous result will be obtained. 
Fortunately, in the majority of materials used as aggregates, 
the porosity is so small that its influence may be neglected in 
making the test, provided the aggregate is well wetted. 

The proportion of voids is important in two ways : first, it 

serves as an index 
of the suitability 
of the aggregate as 
regards the size of 
the particles, and 
secondly, it serves 
as a measure of the 
amount of sand 
which must be 
added to the con- 
crete, as will be 
described later. 
The sand used in 

concrete consists 
FIG. 28. Apparatus for determining proportion essent j a n v o f an 
of voids in aggregates. J 

inert material, the 

particles of which are sufficiently small to occupy the 
spaces or voids between the larger fragments of aggregate. 
The composition of the sand is relatively unimportant 
providing that it is free from clay. The latter is usually 
removed by washing the sand in a stream of water either in 
large tanks or, preferably, in a rotating drum ; the clay, 
roots, grass, seeds, etc., are carried away by the water and 
the clean sand remains behind. Washing sand does not 
always improve it, however, as in some cases the finest 
particles thus removed are of value. 

Most contractors are willing to pay a good price for cement, 




MEASURING THE PROPORTION OF VOIDS 159 

but are anxious to use the cheapest sand available. This is 
a mistake, as the final strength of the concrete will be seriously 
reduced if an unsuitable sand is employed. 

Sands may be divided into three groups : (a) siliceous, 
(b) calcareous, and (c) pozzolanic. 

The siliceous sands are by far the most widely used, and 
consist of the material abraded by the action of wind, weather 
and water from the siliceous rocks. The purest siliceous sands 
consist almost exclusively of fragments of quartz and, under 
the microscope, may be seen to be composed of small, clear 
glassy particles with either sharp or rounded edges according to 
the conditions to which they have been subjected. River sand, 
the particles of which are sharper and more angular than 
those of sea sand, is preferable for use in concrete. 

The general properties of siliceous sands are well known. 
Those chiefly used in concrete are : 

(a) Pit sand (other than that of glacial origin). 

(6) River sand. 

(c) Sea sand. 

(d) Grit or sand from crushed coarse material. This may be 
of a mixed nature and contain calcareous matter. 

Calcareous sands do not occur naturally in a sufficiently 
pure state to be used in concrete. They are prepared by 
crushing limestone rocks to powder and screening out the 
coarser particles. It has been found that calcareous sands 
yield stronger yet lighter concretes than do purely siliceous 
ones, but the much lower cost of the latter makes it only 
natural that siliceous sands should chiefly be employed. 

Pozzolanic sands are made by crushing burnt clay products, 
such as broken bricks, terra-cotta, or clay which has been 
calcined specially or by grinding natural pozzolanas or trass 
(p. 35) to a fine powder. These sands have the advantage of 
forming cements in the presence of the free lime produced by 
the hydrolysis of the cement, and thereby forming a stronger 
concrete than when entirely inert sand is used. This fact was 
well known to the ancient Romans, who mixed ground pot- 
sherds with their mortar to increase its strength. 

The great disadvantage of Portland cement is the formation 
of free lime when the cement sets. This free lime is gradually 



160 COMPONENTS OF CONCRETE AND PROPERTIES 

washed out of the structure especially in maritime works 
and leaves a porous mass which is liable to corrosion and 
decay in proportion to its porosity. Indeed, there are few 
works of importance, in which Portland cement has been 
used, which do not show the serious results of this loss of lime. 

In order to prevent these defects, some material must be 
added which will combine with the lime set free by the 
hydration of the cement. The ideal substance for this pur- 
pose is a trass or pozzolana, as these unite with lime to form 
a new cement. The proportion of trass to be added should 
be slightly in excess of that required to neutralise the lime set 
free in the cement. This is, theoretically, rather more than 
one-quarter of the Portland cement. Hence, the best propor- 
tion of trass is half that of the Portland cement in the 
concrete mixture. This trass may legitimately take the place 
of part of the sand used in making the concrete. 

Concrete composed of 1 measure of cement, J measure of 
trass, and 5 measures of (sand -f aggregate) has proved 
particularly durable and resistant to sea water. It is not 
improbable, in fact, that the simultaneous use of both 
Portland cement and trass may go far towards solving the 
problems raised by the action of sea water on concrete. 

The use of pozzolanic sands in concrete has not been extensive 
in this country, notwithstanding their obvious advantages. 1 

The shape and hardness of the sand grains is often more 
important than their composition. Flat grains, derived from 
micaceous stones or shales, are liable to form a weak concrete 
on account of their shape. More spherical grains with sharp 
angular projections are the most suitable. 

The size of the sand particles should not be too small. The 
British specification for Standard Sand limits the size of the 
grains to those which will pass through a No. 20 sieve, but will 
be retained on a No. 30 sieve (p. 139). The Standing Committee 
of the Concrete Institute fix the upper limit of sand as that 
which passes through a J-inch by J-inch aperture, and the 
lower limit as that which is retained on a ^-inch by ^-inch 
aperture. The Second Report of the Committee of the Royal 

1 Further information on this important subject will be found in " A Manual for 
Masons," by J. A. van der Kloes and A. B. Searle. (London: J. & A. Churchill.) 



SIZE OF SAND GRAINS 161 

Institute of British Architects specify a " sand composed of 
hard grains of various sizes up to particles which will pass a 
|-inch square mesh, but of which at least 75 per cent, should 
pass a J-inch square mesh. Fine sand alone is not suitable." 
The Report also states that " the value of a sand cannot always 
be judged from its fineness, and tests of the mortar prepared 
with the cement and the proposed sand should always be 
made." 

As coarse sand has fewer voids than fine sand, it is always 
preferable, and, in addition to yielding a stronger material, 
it requires less cement. 

Graded sands give better results than those in which all the 
grains are of approximately the same size, as in well-graded 
sands there are fewer voids. For this reason the Tests Com- 
mittee of the Concrete Institute have suggested that the 
" sand " shall be sifted to the following degrees, and the 
percentage of voids ascertained of (1) the whole, and (2) of 
each separate grading : 

To pass an aperture of To be retained on an aperture of 
J inch by | inch inch by | inch 

8" > TS 16 

Tff Tff T5 1 -! aV 

sV 3*5 sV > ihf > 

The proportion of each grading to the whole and the specific 
gravity of the sand should also be ascertained. No definite 
proportions have been fixed. 

The proportion of voids in sand varies with the size of the 
grains. In sand suitable for concrete it varies from 23 to 40 
per cent. The voids are determined in precisely the same 
manner as those in the aggregate (p. 157). 

The tensile strength of test pieces made of standard cement 
and the proposed sand should be ascertained, so as to ensure 
that the concrete is not weakened by the use of unsuitable 
sand. The best method of determining the tensile strength 
is that described on p. 138, but substituting the sand to be 
tested for the Leighton Buzzard sand there mentioned. 



c. M 



CHAPTER VII 

THE PREPARATION OF CONCRETE 

THE manufacture of a concrete article and the erection of 
a structure of concrete is similar to the making of a casting 
in a foundry. Forms or patterns are built to correspond 
exactly with the shape of the finished work, the reinforcing 
steel (if any) is set in place, and the concrete is poured into the 
forms. The whole structure is thus built or moulded into the 
finished form as a single piece or monolith. 1 The concrete is 
allowed to set a requisite length of time, the forms are removed, 
and the building or article stands complete a structure 
carved, as it were, out of solid rock. 

Hence, the work of construction or erection consists primarily 
of four distinct operations : (1) the erection of the forms, 
centering or false work ; (2) the placing of the reinforcing 
steel (in reinforced concrete) ; (3) the mixing and pouring or 
placing of the concrete within the forms ; and (4) the removal 
of the forms or centering. 

As the forms represent the mould from which the finished 
structure is made, great care is used to make these exact and 
true to line. They must be built rigid and thoroughly braced 
so as to bear the weight of the plastic concrete without deflec- 
tion. In order to give a smooth finish, surface-planed boards 
are used, and the corners of all columns and beam boxes must 
be chamfered. All joints should be set closely together so as 
to make the forms fairly water-tight. 

The steel is set accurately in place in accordance with 
detailed drawings prepared for the purpose by the architect, 
engineer or firm of concrete specialists, and these drawings 
should be followed explicitly. 

In the production of concrete from its various components 
aggregates, sand, cement and water it is of the greatest 

1 From monos = one, or a single, lithos a stone or rock. 



PROPORTIONS OF INGREDIENTS 163 

importance that every available means should be adopted to 
secure a product of the highest quality. To this end the 
greatest care and skill should be used in the selection of the 
materials, in mixing them thoroughly in the most suitable 
proportions and in applying the concrete thus produced with 
all necessary speed. It is equally important that the climatic 
and other conditions should be suitable, for the production of 
concrete in times of frost is always detrimental to its quality 
and strength, though if certain precautions, such as those 
described on pp. 187, et seq., are taken, the loss of strength may 
be unimportant. 

The results of carelessness, ignorance or lack of skill in the 
production and shaping of concrete are so serious and are 
occasionally attended with so great a loss of life that no con- 
demnation of them can be too strong, and no precautions to 
prevent their occurrence can be too severe. 

PROPORTIONS. 

The proportions in which the various components of concrete 
should be mixed together must be ascertained separately for 
the aggregate used in each case. The strength and cost of 
the concrete will depend largely on the correctness or otherwise 
of these proportions, yet it is surprising how little attention is 
paid to this matter by many users. Just because some 
builders have found that for certain aggregates used by them 
a given proportion of sand and cement gave the best results, 
numerous others have concluded that for other aggregates the 
same proportions are the most suitable. 

It is generally agreed that the strongest concrete is that 
which contains the smallest proportion of voids, and the 
primary object of grading the sand and aggregate is to secure 
this. At the same time, the proportion of cement present 
must be sufficient to coat every particle of material in the 
concrete, and in this way to secure the firm adhesion of all 
the particles to each other. The builder or engineer who uses 
concrete has to avoid, on the one hand, the production of too 
weak a material by the use of too little cement and, on the 
other, the useless employment of cement (which is expensive) 

M 2 



164 THE PREPARATION OF CONCRETE 

merely to fill the voids or spaces between the particles. It is 
sometimes stated that only those particles of the cement which 
are left over when the voids have all been filled can be used as 
cementitious material, but this is not necessarily the case, as 
a strong concrete may sometimes be obtained notwithstanding 
the presence in it of a large percentage of voids. 

That the percentage of voids cannot in any way be judged 
by inspection is clearly shown in Figs. 29 and 30. The 
limestone shown in Fig. 29 had 37-5 per cent, of voids, and 
consisted of grains of the following sizes : 

Per cent. 
Eetained on No. 10 sieve 44 



20 ,, 

30 

40 

50 

Passed thrqugh a No. 50 sieve 



21-4 
8-5 
4-0 
3-4 

18-7 



whilst 8 per cent, of that shown in Fig. 30 is passed through 
a No. 50 sieve. Yet the fine stone contained 40-5 per cent, 
of voids, required 9 per cent, more cement than the coarser 
stone, and the resultant concrete had only one-third the 
tensile strength of that made from the coarser, but better 
graded stone. 

The minimum proportion of cement is that which will 
cover the whole surface of each particle of aggregate and of 
each grain of sand with a coating of sufficient thickness to 
cause the particles of aggregate and sand to adhere so firmly 
to each other as to form a hard, stone-like mass. This mini- 
mum is unattainable in practice, as it would involve coating 
each grain and particle separately with cement, and making 
no allowance for cement filling up any voids in the material. 
In practice these voids are seldom, if ever, filled completely 
with cement, and it has been found, as the result of 
innumerable tests, that the strongest concrete is prepared as 
follows : 

The aggregate carefully selected, and with particles within 
the limits of size mentioned on pp. 154, 156 is tested to 
ascertain the percentage of voids in it (p. 157). The proportion 
of voids in the sand must also be ascertained. The quantity 
of sand (consisting of suitably sized grains, as described on 



PROPORTIONS OF INGREDIENTS 



165 



-- 



p. 160) to be added to this aggregate is equal in volume to 
the total volume of voids in the aggregate used. 1 

The quantity of the cement to be added is then equal 
to the volume of the voids in the sand used plus an addi- 
tional quantity to effect the cementation of the various par- 
ticles. The " allowance " is 
usually 10 per cent, of the 
cement required to fill the 
voids. (Some engineers 
prefer 15 per cent.) In no 
case should the amount 
of cement used be less 
than will fill the voids in 
the volume of sand used 
in the concrete. It is 
generally advisable to use 
a larger proportion, as the 
strength of the concrete is 
thereby greatly increased. 

Thus, if a given aggre- 
gate has 40 per cent, of 
voids and the sand 
be used with it has 



per cent, of voids, 
each 100 measures 
aggregate there will 
required .40 measures 



to 
33 
to 
of 
be 
of 




i A 33 
sand and - 

100 



X 40 = 13-2 f 



13-2 
~LO~ 



FIG. 29. Limsstoiie. 



= 14-5 measures of cement. 



In other words, the proportions of aggregate, sand and cement 
will be 100 : 40 : 14-5, or 7 : 2f : 1. 

It must be observed that the volume of cement required is 
not that of the dry material as received from the manufacturers, 
but that of the cement after it has set and hardened. This 
must be determined by making a test piece of neat cement and 
water, using an accurately weighed quantity of cement, and 
measuring the volume of the piece after a sufficient length of 



1 It is sometimes preferable to replace part of the sand by finely ground trass 
or pozsolana, as described on p. 159. 



166 



THE PREPARATION OF CONCRETE 



time (say after twenty-eight days 1 ). It will then be possible 
to calculate the volume which will be occupied by any given 
weight of cement under the conditions in which it exists in 
concrete. It is usual to reckon 1 cubic foot of dry cement 
powder as equivalent to 0'85 cubic foot of cement in actual 
use. 

Where large quantities of concrete are used it is better to 
reverse the calculations and to start with a bag of cement as 
the unit. The weight of Portland cement is usually marked 

clearly on the bags in 
which it is sold, so that 
no weighing is necessary. 
For the purposes of pro- 
portioning the amount of 
cement to be added, one 
cubic foot of Portland 
cement may be said to 
weigh 90 Ibs. Thus, if 
an aggregate has 50 per 
cent, of voids and a sand 
has 45 per cent, of voids, 
the proportions of aggre- 
gate and sand and cement 
will be 100 : 50 : 24-75, 
or for a bag containing 
100 Ibs. of cement (= 1-111 
cubic feet) there will be 
required 4-5 cubic feet of 
aggregate and 2*25 cubic 
feet of sand. 

The advantage of speci- 
fying the cement by weight and not by measurement is that 
the makers are prepared to guarantee the weight of material 
contained in each bag, whereas a contractor can easily make 
a difference of 5 to 10 per cent, in the weight of cement 
contained in a given volume. 

1 In neat cement some of the cement acts as an inert material, so that the volume 
found in this manner is less than that which would be occupied by cement in 
concrete. This error may be neglected, as it is on the safe side. 




me 

FIG. 30. Fine Limestone. 



PROPORTIONS OF INGREDIENTS 



167 



Parts 


Parts Parts 


Parts. 


Mortar. 


Cement. Sand. 


Mortar 


1-20 


1 + 2 = 


2'35 


1'50 


1 + 2| = 


2'70 


1*90 


14-3 = 


3'00 



The joint committee formed under the auspices of the Royal 
Institute of British Architects in 1907 recommended that 
the proportions of cement and sand should be settled with 
reference to the strength of concrete required, and the volume 
of mortar produced by the admixture of the sand and cement 
proposed to be used should be determined in each case. On 
small works, it suggests, the following figures may be taken 
as a guide, and are probably approximately correct for medium 
siliceous sand : 

Parts Parts 

Cement. Sand. 

1 + \ = 
1 + 1 = 

i+i!== 

These proportions are only correct for Portland cement ; 
when Zirae-concretes are required the volume of sand plus 
aggregate must never be greater than six times that of the 
cement. The much greater cementing power of Portland 
cement enables strong concretes to be made which contain 
only 6| per cent, of cement. 

The proportions of the components of concrete are usually 
expressed in the form of a double ratio without any words. 
Thus, a 1:2:4 mixture is understood to be composed of one 
measure of cement, two measures of sand, and four measures 
of aggregate. The following mixtures are in common use : 

(a) In a rich mixture for columns and other structural parts 
subjected to high stresses or requiring exceptional water- 
tightness, 1 : 1J : 3. 

(b) In a standard mixture for reinforced floors, beams, and 
columns, for arches, for reinforced engine or machine founda- 
tions subject to vibrations, for tanks, sewers, conduits, and 
other water-tight work, the proportions should be 1 : 2 : 4. 

(c) A medium mixture for ordinary machine foundations, 
retaining walls, abutments, piers, thin foundation walls, 
building walls, ordinary floors, side walks, and sewers with 
heavy walls : proportions, 1 : 2J : 5. 

(d) A lean mixture for unimportant work in masses, for 
heavy walls, for large foundations supporting a stationary load, 
and for backing for stone masonry : proportions, 1:3:6, 



168 



THE PREPARATION OF CONCRETE 



In order to obtain the best results with the least wastage of 
cement, however, the proportions of aggregate, sand and cement 
should be calculated in the manner described on p. 164. 

To measure the aggregate and sand, boxes of suitable sizes 
are used. Sometimes a barrel with both ends removed is used 
as a measure. It is placed on the platform or mixing board 
and is filled with the material. The barrel is then lifted off 
and the correct measure of material remains on the board. 
It is incorrect to place a second, smaller barrel, on top of the 
first one and to fill the smaller barrel with sand, as some of the 
sand trickles down into the voids of the coarser aggregate and 
wrong proportions are obtained. 

It is convenient to make the measuring boxes of such a 
capacity that they correspond exactly to the sand and aggregate 
needed for two bags of cement, or, if much larger quantities 
are needed, to four bags of cement. Larger quantities than 
this are inadvisable except under special circumstances. The 
following table shows proportions of materials and the sizes 
of the measuring boxes : 

TABLE SHOWING THE QUANTITIES OF MATERIALS AND THE 
RESULTING AMOUNT OF CONCRETE FOR TWO-BAG BATCH 
(EQUAL TO 168} LBS. CEMENT) (WILSON AND GAYLORD). 



Proportions 
by parts. 


Two-bag Batch. 






IE 

> 


Materials. 




Size of Measuring Boxes. 
Inside Measurements. 


III 

Big 






& 

C5 








* 8b 


Cement. 


TS 
1 


Stone or 


Cement. 


a 

1 


o_- 

II 

o 


Concrete 


Sand. 


Stone or Gravel. 


x'iig 

ii : 

5"o = 








Bags. 


Cu. ft. 


Cu. ft. 


Cu. It. 






Gals. 


1 


2 


4 


2 


31 


H 


8A 


2' x 2' x Hi" 


2' X 4' X 11|" 


10 


1 


3 


6 


2 


53 


ll\ 


12 


2' x 3' x 1H" 


3' x 4' x ll|" 


is| 



For measuring water a tall, narrow bucket or cylinder is 
the most suitable. This bucket should be checked as to 
capacity and should be marked to distinguish it from all 
others. No other bucket should be used for measuring. 



MEASURING OUT THE INGREDIENTS 



169 



It is easy to make a mistake in proportioning concrete 
mixtures and it is, therefore, necessary to use some experi- 
mental method in order to ascertain if the proportions of each 
ingredient in the concrete is correct. To do this a rough, but 
usually sufficiently accurate method recommended by the 
Associated Portland Cement Manufacturers, Ltd., may be 
used. The apparatus required is shown in Fig. 31 to consist 
of a large and small measure of metal, a combined funnel and 
strainer fitted with gauze of J-inch mesh, two 500 c.c. graduated 
glasses and a vessel capable of delivering a fine stream of 
water. This last is used for washing the material. 

To check a mixing of concrete 
a sample is drawn from the heap 
in one of the measures (the large 
one being used if concrete is 
poor in cement, or contains large 
aggregate). The strainer is held 
over one of the graduated glasses 
and the sample placed therein 
(the measure being washed out 
to ensure inclusion of all cement 
and sand). The large stones are 
caught on the mesh of the 
strainer, and the cement and 
sand pass through into the glass, 
this separation being assisted by 
stirring and washing down with 
water from the aspirator. The large aggregate left in the 
strainer is measured in the second glass to determine whether 
it is approximately correct. The agitation of the cement and 
sand by the water in the first graduated glass will cause them 
to separate ; the sand settling more rapidly than the cement. 
After standing for about 15 minutes, the approximate pro- 
portions can be readily seen by means of the graduations. 
Slight allowance must be made for the fact that the cement 
will not settle quite so compactly as the sand. 

The testing of two samples of equal quantity from different 
parts of the heap will show whether the mixing is uniform. 

This test will not show minor and unimportant variations 




FIG. 31. Apparatus for 
checking the composition 
of Concrete Mixtures. 



170 THE PREPARATION OF CONCRETE 

from the specified proportions, but its use will prevent such 
serious trouble as occurred recently, when after a new sea-wall 
had been demolished by a storm, it was found by analysis that 
the concrete, instead of being 4 : 1 as specified, was approxi- 
mately 9:1. 

This test cannot be applied to concrete which has set, but 
only to freshly made mixtures. 

CONSISTENCY. 

The amount of water required depends on the consistency 
desired, on the temperature at the time of mixing and also 
on the materials composing the aggregate. Four different 
consistencies are recognised by users of concrete : 

(1) Grout consisting of cement, sand and water in the form of 
a thick cream or slurry. Very little aggregate can be present 
unless its particles are small. 

(2) Very wet mixture, consisting of concrete wet enough to 
flow off a shovel yet not so fluid as grout. This mixture is 
largely used for those portions of reinforced concrete where 
the metal work is close together. 

(3) Dry mixture having a consistency resembling damp 
earth. It is used for foundations and wherever the concrete is 
required to set rapidly. This mixture must be thoroughly 
rammed or tamped so as to secure a uniform distribution of 
the water in it. It should be observed that the dry mixture 
is not really dry, but may have had 6 to 12 per cent, of water 
added to it. It sets with inconvenient rapidity, and owing to 
the tamping needed it is liable to contain an excessive pro- 
portion of voids unless worked by skilled men. 

(4) Medium, or ordinary mixture, in which the material is 
plastic or jelly-like. To remove air bubbles and fill the voids 
it is necessary to ram or tamp this mixture lightly. It is 
used for all the ordinary purposes of concrete. It has the 
consistency which is the safest for men of average skill. 25 to 
30 per cent, of water is usually necessary, but the proportions 
vary greatly. One large firm of concrete users employ one 
gallon of water to each cubic foot of dry material (=16 per 
cent.), but in warm weather this is increased to 1J gallons 



CONSISTENCY OF CONCRETE 171 

(= 25 per cent.). Another equally important firm generally 
uses 20 per cent, of the volume of cement plus sand, or about 
10 per cent, of the whole mixture. 

Notwithstanding these variations in the consistency of the 
mixture, it is generally agreed that no more water should 
be added than is necessary to effect the desired chemical 
changes in the cement, plus an added amount just sufficient 
to enable the particles to slide over each other and to form a 
plastic mass. As some aggregates are very porous they should 
be thoroughly soaked, and the surplus water drained off before 
adding the water necessary to act on the cement. 

The quantity of water to be used must be found by tests, 1 
but it is generally agreed that the mass should be sufficiently 
plastic to be packed easily into the required position, and yet 
should not be so wet as to allow any dripping of the cement, 
warter or sand. This point is shown when, after ramming the 
mass, the water just shows on the surface. 

A standing committee of the Concrete Institute has suggested 
the following specification for the consistency of concrete : 

For mass concrete the quantity of water added to the other con- 
stituents shall be sufficient to make a plastic mixture which, after 
thorough ramming, will quiver like a jelly. 

For reinforced concrete the quantity of water added to the other 
constituents shall be such that the plastic mixture is capable of being 
rammed into all parts of the moulds and between the bars of the 
reinforcement. In dry weather the quantity of water shall be increased 
in order to allow for evaporation. 

Some architects and engineers, on the contrary, stipulate 
that the mixture shall not " quake " or quiver like a jelly when 
rammed. This is to avoid the production of a " mushy " mass, 
.such as is generally employed in the United States. In Great 
Britain a mass of so sloppy a consistency that it is difficult to 
keep on a shovel is considered unsuitable and liable to cause 
an undue segregation of the cement near the bottom of the 
material. A slight excess of water is better than too small a 
quantity, though under normal conditions the best results are 
obtained when there is neither an excess or a deficiency of 
water in the mixture. A concrete which has been worked in 

1 See "A Manual for Masons," by J. A. van der Kloes and A. B Searle 
(J. & A. Churchill, London). 



172 THE PREPARATION OF CONCRETE 

too dry a condition does not pack properly, and so is weaker 
than one in which more water has been used. 

The difference between the strength of dry and wet mixtures 
in the early stages of hardening is strongly in favour of the 
former, but after several months the strength of both mixtures 
is approximately equal. 

There are a few rough-and-ready methods of telling whether 
the consistency of concrete is right. If the concrete is placed 
in a barrow, by the time it is wheeled into its place it should 
not have taken a horizontal surface. A shovelful of concrete 
from the bank held at a slight angle, should show no signs of 
the cement dropping away. Also a hole made in the concrete 
in the barrow should never be filled in by any ordinary amount 
of vibration. 

The proportion of water which is used in laboratory tests is 
usually too low for mixing on a larger scale. In the tests, the 
concrete is mixed on impervious glass plates and is moulded 
in non-porous metal moulds, whilst in the works the mixing 
board and the moulds or centering both absorb a large amount 
of water, the exact amount of which it is almost impossible 
to ascertain, as it varies with the dryness of the timber. 

The nature of the structure has some importance on the 
proportion of water which is most suitable. Thus, in beams 
particularly near the points of support the concrete needs to 
be very fluid to pass between the closely-set reinforcing bars. 
In plain masses of concrete, on the contrary, a much drier 
mixture is preferable, and where the thickness of the concrete 
is very great (as in foundations) the last portions must be 
almost dry to counteract the effect of excess of water in the 
other portion which is brought to the surface in ramming or 
tamping. 

When used for submarine work, as in docks, harbours, etc., 
the mixture must appear to be dry, though in reality it contains 
a considerable proportion (612%) of water. Perfectly dry 
mixtures of aggregate, sand and cement should never be 
employed. 

In moulded concrete, such as is used for building blocks, the 
mixture is kept as dry as possible in order that it may be rapidly 
turned out of the moulds. Even in such work, however, it is 



CONSISTENCY OF CONCRETE 173 

undesirable to have too little water, as a somewhat larger 
proportion produces a stronger article with a better surface. 

The best consistency of a concrete mixture, like that of a 
clay used by the potter, is easily recognised by men accustomed 
to working the material, but it is almost impossible to describe 
it with accuracy or to express the proportions of solid material 
and water in terms of definite figures (see p. 171). 

Most of the water added to a concrete mixture is purely 
mechanical in its action ; the proportion required to effect 
the hydrolysis of the cement being small. 1 Nevertheless, 
this additional water performs several very important duties, 
and to limit its amount unduly is to reduce the strength 
of the concrete. Thus, whilst it is usually necessary to add 
about 10 per cent, of water to the concrete in order to obtain 
a workable mass, a considerable proportion of this water is 
removed on tamping. Some experiments made to ascertain 
this proportion showed that it is usually about one-quarter of 
the water originally added, but with very wet mixtures it may 
amount to half. Most of the water remaining immediately 
after tamping is removed by evaporation as the structure dries, 
and only about 1 per cent, is left in combination in the dry 
concrete. 

MIXING. 

The various components of concrete must be thoroughly and 
rapidly mixed in order to form a uniform mass before the cement 
has begun to set. To prolong the mixing is seriously detri- 
mental to the concrete, just as cement which has been 
" worked " too much in making tests (p. 133) is greatly 
reduced in strength. Two main methods of mixing are 
employed, viz., by hand and by means of mechanical mixers. 
The latter have the advantage of ensuring a fairly thorough 
mixture and avoid any possibility of " scamping " the labour, 
such as not infrequently occurs in hand mixing. When skilled 
and conscientious men are employed, however, the concrete 
they produce is better than that produced by any machine, 
but the labour of mixing large quantities by hand is so great 

1 It has been shown experimentally that the water required for com 
hydration is about 14 per cent, of the weight of the cement used (see p. 89). 



174 THE PREPARATION OF CONCRETE 

and it is so difficult to rely on the men usually employed for 
this purpose, that machine-mixing is now employed on all 
large work. Whichever method is used, it is of the greatest 
importance that the proportions of the various ingredients are 
as accurate as possible, and that the measures used are correct 
in size and are properly employed. The mixing must be so 
thorough that the product is uniform in colour and in texture. 

Hand mixing is the most effective if properly carried out, 
but this can only be done when relatively small quantities of 
concrete are required. The materials should be turned over 
three or four times and well mixed in the dry state, these 
operations being repeated to an equal extent after the addition 
of the water. The mixing should be made on a water-tight and 
non-porous platform about nine feet by ten feet, constructed 
of one-inch boards cleated together and provided with a frame 
to keep any surplus water from running off the platform. The 
surface of this platform should be planed smooth. This 
mixing board should be placed as near as possible to the spot 
where the cement is to be used, and in such a position that the 
men, in mixing, move their shovels along, and not across, the 
joints between the boards. The board must be packed so 
that it is solid and level before use. Any tendency to sag in 
the centre must be overcome by packing with sand or ballast. 

A suitable quantity of sand should be measured by means 
of the measuring box (p. 168). The sand is shovelled in without 
any beating or packing until the box is more than full. The 
excess of sand is then removed by laying a stout lath or iron 
bar on top of the box and drawing it along. By repeating this 
action two or three times, the box will be filled just level with 
its upper edges. The measured sand is then turned out on to 
the measuring board and spread into a 3-inch or 4-inch 
layer. On top of the sand the requisite number of cement 
bags is emptied as evenly as possible. Two men, standing at 
opposite sides of the board, then mix the sand and cement 
together, turning the material over and over so as to mix it 
thoroughly. The sand and cement should be turned over 
thrice, a shovelful at a time, after which it should be sufficiently 
mixed for the aggregate and water to be added. It is next 
spread as evenly as possible on the board, and the larger 



MIXING CONCRETE 175 

measuring box (p. 168) is then filled with aggregate in the 
same manner as the sand, care being taken that the box is 
filled " just level " and not " heaped up." The measured 
quantity of aggregate is then turned onto the sand and cement 
mixture as evenly as possible. About three-quarters of the 
water likely to be needed is next thrown on to the aggregate, 
and the whole mass is then turned over and over, one shovelful 
at a time, water being added to the drier portions until the 
whole of the water has been added. With skilled men the 
mixing will be complete after three turnings, but if it shows 
streaks or dry portions it must be turned again. It is then 
shovelled into a compact pile and is ready for use. It must 
not be kept long, nor must much time have been occupied in 
the mixing, or the cement will have commenced to set and the 
concrete will be spoiled. 

Re-mixed concrete must never be used. 

Where the aggregate consists of a naturally occurring mixture 
of sandy gravel, this should be measured out and spread on 
the board, wetted thoroughly, and then covered with the 
proper quantity of cement. The mixture is then turned over 
three separate times, as before, the additional water not used 
for the sand being added during the turning. It requires a 
considerable amount of skill to mix cement with sandy gravel, 
and it is usually better to screen the materials and mix the 
sand and cement and add the gravel and water later. 

It is never satisfactory to mix the sand and aggregate and 
to add the cement later ; the sand and cement should first 
be mixed, as this secures a more homogeneous product and 
yields a stronger concrete. 

Too much care cannot be taken in securing the adequate 
supervision of the men employed in mixing concrete, and in 
all cases the men themselves should be exceptionally reliable 
and trustworthy. The mixing of concrete is hard and laborious 
work, and is severely straining to the wrists. Moreover, the 
greater part of the labour can be avoided by moving the 
material without turning it over. This is a dodge to which 
careless and unscrupulous men resort, yet it is fatal to the 
successful mixing of the materials, and is one of the strongest 
arguments in favour of the use of mechanical mixers. 



176 THE PREPARATION OF CONCRETE 

Each day, at the conclusion of the mixing, the board should 
be carefully cleaned and freed from all cement, aggregate and 
sand. This is best accomplished by first sweeping and then 
scrubbing it. If the board is not properly cleaned the 
shovelling will be made much harder on later days. 

Mechanical mixers may be divided roughly into two main 
divisions : (1) continuous mixers, in which the material is 
continuously fed and issued from the machine ; and (2) batch 
mixers, in which a certain quantity of concrete is fed and mixed 
and then discharged as a batch. There are advocates for both 
types of machine, and certainly, if operated correctly, both 
can do excellent work. 

Batch mixers are filled with measured quantities of the 
materials, are then set in operation for a given time and finally 
discharge a batch of concrete ready for use. 

The designing of a concrete mixer which will give the 
requisite number and kind of movements required for the 
making of good concrete, is by no means a simple matter, as 
the long trail of failures has taught the manufacturer and 
contractor to their cost. A concrete mixer must be designed 
to operate under the severest conditions, and the mechanical 
construction should therefore be of the best, while the great 
rapidity of output required for modern building operations 
calls for a machine which will mix and discharge a batch in 
the shortest possible time. 

In the Smith mixer (the T. L. Smith Co., Ltd.) and the 
roll mixer (Builders and Contractors' Plant, Ltd.) the box 
consists of a double cone which can be revolved on its own 
axis to discharge the material at one side ; in the Ransome 
mixer the rotating box is a short cylinder with blades to mix 
the materials. The Express mixer (U. K. Winget Concrete 
Machine Co.) consists of the open circular pan with blades to 
mix the materials, the discharge being effected by opening 
doors in the bottom. A small mixer consisting of an open box 
or trough with paddles is also made by the Ransome Van Mehr 
Machinery Co. The objection to paddles is the liability of 
stones to jam them, but this may be avoided by the provision 
of safety springs. 

It is important that the discharge from machines should be 



MECHANICAL MIXERS 



177 



rapid or the tendency of the materials to become un-mixed will 
reduce the strength of the cement. 

In the construction of the Panama Canal a machine known 
as the " Chicago improved cube mixer " is used. This is a 
simple adaptation of the first form of machine used extensively 
in the concrete industry namely, the cubical box, journalled 
at diagonally opposite corners, and having a door on one 
side through which the charge of cement, sand, stone, and 
water were filled ; the machine was rotated for several 




FIG. 32. Chicago Cube Mixer, used in Panama Canal. 

minutes, and the batch of mixed concrete then emptied 
out. Though mechanically crude, these machines produced an 
excellent concrete, the main objections to them being that, 
to discharge, the cube had to be stopped with the door at the 
bottom and, to be recharged, had to be turned until the side 
containing the door came to the top further time, of course, 
being lost in unclamping and reclamping the door. 

In the improved forms of this machine (Figs. 32 37), 
which preserves the principle of treating the batch as a unit, 

C. N 



178 THE PREPARATION OF CONCRETE 

and include both longitudinal and rotary movements, and mix 
by knead'ng and not by stirring, many interesting modifica- 
tions have been introduced. For example, the shaft of the 
older machines is replaced by hollow trunnions riding on 
rollers, and made sufficiently large to serve as openings for 
charging and discharging. To rotate the cube, a strong 
circumferential rack is fastened around it at right angles to 
and midway between the trunnions ; and this rack, geared with 
a pinion shaft, is so operated by the engine shaft that all 
gearing is removed as far as possible from the material which 




FJG. 33. T. L. Smith Co., Ltd., Power Mixer. 

flies about during the charging and discharging operations. 
An automatic dumping device has been provided, and, to 
ensure the construction of a larger cube in the same space, and 
eliminate any opportunity for pocketing the fine mortar, the 
former sharp corners and edges are rounded. 

The cube in Fig. 32 is provided with breaker rods, from 
| inch to 1 inch in diameter, so placed across each of the six 
central corners of the cube that as the machine revolves they 
slice through the mass of concrete, each following a different 



MECHANICAL MIXERS 



179 



line, and effectually breaking any hard lumps or cakes found- 
in the materials. Otherwise, the interior of the cube is free 
from paddles, deflectors, or other obstructions. 

In the mixer made by the T. L. Smith Co., Ltd. (Figs. 33 
and 34), the drum consists of two cones and contains blades 
arranged spirally. In the Victoria mixer supplied by the same 
firm (Figs. 35 and 36) the drum is cylindrical, with four 
deflecting blades which subject the concrete to twelve distinct 
mixing actions for each revolution of the machine. A dis- 
charging spout (Fig. 36) can be swung partly into the cylinder 
when required. The use of the skip (Fig. 35) enables the 
mixer to be loaded 
from the ground level. 
Above the machine is 
a tank (Fig. 35) for 
the automatic supply 
of water to the 
concrete. 

The Ransome-ver 
Mehr Machinery Co., 
Ltd., make a large 
number of different 
types of concrete 
mixers, and their 
machines are exten- 
sively used in all 
parts of the United 
Kingdom. Fig. 37 
shows one of them 
with an engine and boiler directly coupled to the mixer. 

Where batches of only two cubic feet each are required, or 
where a power-driven mixer is not available, a hand mixer 
may be used. 

In the type of hand mixer (Fig. 38) supplied by the Ransome- 
ver Mehr Machinery Co., Ltd., the aggregate is fed into an 
elevating skip by the operator, the skip itself having a capacity of 
two cubic feet of material. The mouth of the skip is extended 
in the shape of a chute, in order to ensure no spilling of material 
when the skip discharges into the mixing drum, the necessary 




FIG. 34. Cut-away View showing interior 
of Fig. 33. 



180 



THE PREPARATION OF CONCRETE 




FIG. 35. Victoria Mixer, with Skip 
for filling. 



water being added simultaneously. Materials entering the 

drum immediately get into contact with the mixing blades, 

which are of a special 
form, designed to 
ensure the whole of the 
batch being thoroughly 
mixed. 

The mixing opera- 
tion being complete 
(this usually occupies 
about 30 seconds), the 
drum itself is inverted 
by means of an aux- 
iliary handle, pinion 
wheel, and rack. As 
the drum is inverted it 
discharges its contents, 
being assisted during 
the whole period by 
the action of the 

paddles themselves. The paddles, in addition to facilitating 

discharge, at the same time automatically clean the drum. 

The mixer is of such 

dimensions that a ":. "', '-"" : - ~~ 

standard navvy 

barrow can be readily W^ 

placed beneath the : 

drum, in order to 

receive the batch 

when discharged. 

This eliminates all 

necessity for lifting 

material into the 

barrows after it is 

mixed, as is the case 

with ordinary hand 

mixing. 

A batch mixer of a different type is known as the " Express " 

mixer, and is sold by the U. K. Winget Concrete Machine Co., 




FIG. 36. Victoria Mixer (in use). 



MECHANICAL MIXERS 181 

Ltd., of Newcastle-on-Tyne. In designing this machine, the 
idea kept in view was that of mixing the material by mechanical 
means in a manner precisely similar to hand-mixing properly 
conducted. The machine (Figs. 39, 40) will mix concrete in which 
the aggregate is not larger than one and a half inch gauge ; it 
is therefore particularly suitable for concrete block and rein- 
forced work. The maximum charge is five cubic feet ; t his 
quantity is mixed dry, then water is added and mixed with 




KKJ. 37. Banabme-ver Mdir Power Mixer. 

the concrete, the finished product being delivered from the 
machine on to the ground in sixty seconds. The " Kx press " 
mixer will mix either dry, semi -dry, or wet, and as it is a batch 
mixer the proportions of the different materials may be altered 
at will to suit requirements. The power required to drive is 
about 6 b.h.p., so that the " Express " mixer is eheap to drive ; 
it has nothing to get out of order, and if the ploughs get worn 
out they can be replaced at a small expense. Each part of 
the machine is accessible for cleaning, and it does not clog up. 



182 



THE PREPARATION OF CONCRETE 



Fig. 39 gives a general view of the machine, which consists 
of a stationary pan 6 feet 6 inches diameter and 1 foot 9 inches 
deep. The capstan is keyed on to a vertical shaft, driven from 
below by mitre gearing, and carries revolving arms, to which 
are attached adjustable plough-shaped beaters ; each plough 
carries in its rear an iron rake. The action of the machine is 
to imitate hand-mixing, but the number of times the materials 




FIG. 38. Hand Mixer for Concrete. 

are turned over in the pan is enormously greater than in the 
most patient hand- mixing. This is caused by the combined 
action of the ploughs turning over the material, and the rake 
immediately following raking it out ready for the next succeed- 
ing plough to turn over. In some mixers of this type the 
materials are piled up in great furrows where the large material 
falls from the top of the furrow to the bottom, exactly as 
happens in badly conducted hand mixing, but in the " Express" 



MECHANICAL MIXERS 



183 




FIG. 39. Top view ol 
Express Mixer. 



mixer the material is kept level in the pan. This machine 

mixes without balling the material a most objectionable 

thing in concrete block work. 

If a continuous mixer is used it must be so constructed that 

it cannot be tampered with. Some patented mixers of the 

continuous type do not mix the 

material sufficiently, and so 

produce a poor and irregular 

concrete. For this reason batch 

mixers are often preferred. With 

the latter, the same precaution is 

necessary, as some of them permit 

the material to be fed in at the 

same time as other material is 

being discharged, thereby intro- 
ducing some imperfectly mixed 

material into the waggon or 

wheelbarrow intended to receive the mixed concrete. 

In Barker and Hunter's mixer, the simultaneous filling and 

discharging of the machine is made impossible by the necessity 

of rotating the machine in one direction when filling and 

mixing, and in the opposite one when discharging. 

A discussion of the 
relative merits of the 
various types of mixers 
is beyond the scope of 
the present volume, but 
care should be taken in 
selecting a machine to 
choose one in which there 
is no likelihood of the 
particles being separated 

FIG. 40. Side view of Express Mixer, instead of being mixed 

more completely. There 

is some danger of this in machines fitted with internal blades. 

It is also necessary to keep the machine in good working order. 
As it is essential that the output of the mixer should approxi- 
mate that at which it is rated by the makers, these machines 
should always be purchased under guarantee from a reliable 




184 THE PREPARATION OF CONCRETE 

firm. The effect of the work of mixing is very severe on the 
machines, and it is often aggravated by neglect. The best 
way is to put a competent man in charge of the mixer and 
to make him responsible for it. He should be allowed at 
least half an hour a day for cleaning the machines, tightening 
nuts and keys, and attending to the bearings, etc. 

Whatever method or machine is used it is wise always to 
mix the cement and aggregate well before adding the water, 
then to mix thoroughly after the water has been added, great 
care being taken not to work through the initial set. 

It is desirable not to mix concrete during frost, but if this 
is unavoidable the use of warm water for gauging and of warm 
cement and aggregate will reduce the risks of working at 
undesirably low temperatures. Some suggestions with regard 
to this will be found later under the caption, " Placing the 
Concrete." 

MOULDS AND CENTERING. 

Freshly mixed concrete is a plastic material which can be 
moulded into any shapes consistent with the size of its particles. 
To obtain any given shape, however, some kind of mould or 
form l is necessary, and the concrete paste is poured into this 
and allowed to set. The hard mass is then removed from the 
mould, or, more commonly, the mould is taken away piece- 
meal, leaving the moulded mass standing and increasing in 
strength and hardness with the lapse of time. 

For most structural purposes, the forms used in concrete 
construction are built of timber, the boards being used 
separately or in groups (termed shutters) where a number of 
forms of the same size are required for consecutive use. Great 
ingenuity is sometimes exercised in providing forms for 
structures of special shapes, but for most purposes temporary 
walls of wood are built on the spot where the concrete wall or 
other structure is desired. These wooden " walls " or shutters 

1 It is interesting to note that the word form was introduced into this country 
from the United States. It owes its origin to the German emigrants, the word 
Form being the German equivalent of the English word mould. In modern concrete 
construction the use of the word mould is limited to the moulds used for com- 
paratively small articles, and indicates a permanent piece of apparatus. Where a 
" mould " is built up and taken down after use it is termed a, form. Sometimes the 
terms shuttering or centering are used instead of mould and form. 



FORMS 



185 



are placed the same distance apart as the desired thickness of 
the concrete, and they are supported by props and angle pieces 
so as to remain secure against the thrust of the concrete. 

The construction of the form for a square column and floor 
is shown in Fig. 41. 

Three important precautions must be taken : the forms must 
be removable without jarring the concrete, there must be no 
leakage between the boards constituting the form, and the 
whole structure must be stiff enough not to bulge or alter in 
shape during use. To aid this, perforated wooden plugs or 
separators, with bolts through them, may be used, the bolts 
and wood being removable, and the holes in the concrete filled 
up. Much concrete work which would otherwise be excellent 
is spoiled by insecure and insufficiently rigid forms and 




FIG. 41. Arrangement of Forms. 

centering, and it is therefore of great importance that special 
attention should be paid to this part of the work. 

Boards one inch in thickness are usually strong enough, but 
they must be supported by a sufficient number of shores or 
buttresses, the latter being also of wood and placed about two 
feet apart. Thick boards last longer and are cheaper in the end. 

The cost of forms is very great, and unless care is taken in 
constructing them much waste may result. By careful 
planning beforehand, most of the boards may be used 
repeatedly, and for small work they need cost practically 
nothing, being made of odds and ends of timber, which is as 
valuable afterwards as before it was used. 

The construction of the form requires great care, as mistakes 
cannot be remedied. The space inside should also be kept 



186 THE PREPARATION OF CONCRETE 

clean, any chips, etc. being removed at once. Where possible 
the shape should be such that the angles of the concrete are 
rounded. 

The essential characteristics of good forms are rapidity and 
ease of construction, great stability in use and rapidity and 
ease of removal without tearing or jarring the concrete. 

The wood should be wetted before pouring in the concrete, 
so as to prevent the adhesion of the latter. Unless this is 
done, some of the concrete will be torn away when the forms 
are taken down. For special work the forms may be lime- 
washed, oiled or coated with soft soap. Creosote and kerosene 
oil are useless, but linseed and black or cylinder oils are 
excellent. Even when oil is used, it is desirable, just before 
pouring in the concrete to flush the form with water. 

The designing and construction of forms is in itself a large 
subject, and to enter upon it in further detail is beyond the 
scope of the present volume. It should be observed, however, 
that the use of forms would be greatly facilitated and the cost 
greatly reduced if greater standardisation were possible, though 
the awkwardly-shaped and restricted sites upon which it is 
so often necessary to work appear to prohibit this in the 
majority of cases. Even under good conditions the centering 
costs one-third of the total price for the concrete, and may 
easily be more than this, so that any saving which may be 
effected is of considerable influence. At present the chief 
economy is realised by avoiding all unnecessary cutting of 
the boards and by planning out the work in such a manner 
that the long lengths of timber can be used repeatedly. By 
making beams, floor panels and columns of certain standard 
sizes, and adhering to these as far as ever possible, the cost of 
forms may be reduced to 5 per cent, for firms in regular work. 
The timber is not damaged by the concrete, but in most 
instances it is cut up into short, useless pieces which cannot 
be used again, and have to be replaced by new ones. Architects 
and engineers may often effect a great saving by using pillars 
of standard size and placing them standard distances apart. 
The question of the use of standard forms is one to which the 
student of concrete construction may well devote a large 
amount of thought and care. 



PLACING CONCRETE 187 

The operation which, in the metal industries, is known as 
casting, and in the plastic industries, as moulding, is, in concrete 
construction, termed placing. In each case it is the operation 
which gives the material the shape which it is desired it shall 
possess. 

PLACING CONCRETE. 

Immediately after the concrete has been properly mixed it 
should be placed in the forms or moulds or on the spot in which 
it is to be used. There should be no delay in placing the 
concrete or the consequences may be serious. It is, therefore, 
wise to mix the material in only small amounts, and on no 
account to use concrete which has begun to set. A slow-setting 
cement should be used when the concrete has to be taken a 
considerable distance from the place of mixing to where it 
can be poured into the forms. The concrete paste may be 
handled and " placed " in any convenient manner, providing 
that it does not begin to set before it is in position and that 
no un-mixing of the material occurs. Wherever possible, the 
concrete should be shovelled direct from the mixing board or 
machine into the forms, but where large quantities are required 
it is usually necessary to transport it in barrows to that part 
of the structure where it is required. 

A wheelbarrow holding two cubic feet of concrete is an 
exceedingly heavy load, and where the concrete is very wet, 
a load of one cubic foot is not uncommon, since the ordinary 
steel or wooden barrow has a body or bowl too shallow to 
prevent the wet concrete from overflowing. To reduce the cost 
of transporting concrete, the Ransome-ver Mehr Machinery Co., 
Ltd., have designed an all-steel cart (Fig. 42) that holds six cubic 
feet (water measure). One man can push or pull this cart 
over a plank runway, even when the cart is level full of concrete. 
In other words, one man transports from three to six times as 
much concrete as he could transport in a wheelbarrow. This 
remarkable result is due to the wheels of the cart being much 
larger than those of a wheelbarrow, and, therefore, more easy 
running is secured ; no weight is thrown on to the man's 
hands as in the case of a wheelbarrow, but he is free to use all 
his strength in pushing or pulling the cart, and as no concrete 



188 



THE PREPARATION OF CONCRETE 



is slopped on to the run-planks where these carts are used, 
only half the effort is needed to push a cart over clean planks 
that is necessary when going over dirty ones. In addition to 
the larger loads moved per man, there is an important economic 
advantage in being able to discharge the batch from a concrete 
mixer in much less time where these carts are used instead of 
wheelbarrows, as a mixer can be discharged into these carts 
in one-third the time required with wheelbarrows. 




FIG. 42. Cart for Concrete. 

The amount of concrete added at a time should not be more 
than will produce a layer about six inches in thickness, or 
three inches in the neighbourhood of reinforcement. Other 
things being equal, the strength of concrete depends on its 
compactness or density. Hence it is desirable to use some 
means of increasing this. A vertical spade is then inserted into 
this layer close to the inner face of the form, and is worked 
up and down so as to push the aggregate away from the form, 
release any air bubbles and produce an even face. The size 



PLACING CONCRETE 



189 



of the spade will depend on the space into which it is introduced ; 
a long board four inches by one inch, sharpened to a chisel 
edge, is exceedingly useful for this purpose, though for broader 
work an ordinary spade may be used. Spading in this manner 
needs considerable skill if a dry concrete mixture is used ; with 
a very sloppy mixture, on the contrary, no such treatment is 
needed, though it is, in all cases, a wise precaution. A ram 
or tamping tool (Fig. 43) is then held just above the top of 
the concrete layer and is brought 
down into it in a succession of 
blows until all the material is com- 
pact and the surplus water has 
risen to the surface. Hard ramming 
is seldom necessary with a well- 
made concrete, though the drier 
the mixture the harder must be 
the blows. Indeed, very heavy 
blows tend to do more harm than 
good, what is required being a 
series of tamps or taps of just 
sufficient force to secure the various 
particles all fitting into their places 
without leaving any voids. Many 
light taps are far more useful than 
a few heavy ones in consolidating 
the concrete. Some firms prefer 
to use a wet mixture and not to 
tamp at all, but to rely on the 
natural fluidity of the concrete 
aided by the use of a special 
spade (Fig. 44). Vibrators - 

operated in a similar manner to pneumatic hammers are 
applied to each side of the form or shuttering by the 
Vibrocel Co., Ltd. This special mode of tamping is claimed 
by the patentees to give a more impervious concrete than is 
obtainable by any other method. The resumption of placing 
(after an interval) and the attachment of new concrete to old 
is also greatly facilitated by vibrating instead of the more 
usual tamping. 




FIG. 43. Tamping Tool. 



190 THE PREPARATION OF CONCRETE 

If the work of placing the concrete is suspended, all necessary 
grooves for joining future work must be made before the 
concrete is set. When the placing is resumed, the previous 
concrete must be wetted, roughened, cleaned of all foreign 
material and covered with mortar, consisting of 
one part of Portland cement with not more 
than two parts of sand so as to form a layer of 
mortar half an inch thick. Care should be 
taken that joints of this kind are made in 
positions where they will have the least 
possible effect on the strength of the structure. 
Thus, footings and floors should be placed the 
full thickness at one operation ; columns 
should only be stopped (if at all) at . the 
underside of the lowest projection of the 
capital ; constructional joints in beams and 
girders should be vertical and within the middle 
third of the span, and similar joints in slabs 
should be near the centre of the span. If 
" plums " or large masses of stone, or packing 
is used, the pieces should be placed in position 
before the concrete is added. These large 
masses should be at least two inches apart, 
and should not be within two inches of the 
face or back of the form. The concrete should 
be rubbed around them with a spade, and 
that immediately above and around them 
should be well tamped. 

As the strength of the structure depends 
largely on the care and skill exercised in the 
placing of the concrete, this should have all 
FIG. 44. Spade the attention it needs. Frost has a very 
(Ross ) >nC1 serious effect on plastic concrete, and no placing 
should be done in frosty weather. Under some 
circumstances, however, working at a temperature below 32 F. 
is unavoidable, and the concrete must therefore be protected 
against the action of frost until it has set and hardened. 
Where the concrete will not be seen the addition of common 
salt or calcium chloride to the water used for mixing will 



PLACING CONCRETE 191 

reduce the freezing point of the water, and so permit the 
placing to go on as usual. Where a scummed surface 
on the concrete must be avoided, it is necessary to heat the 
materials of which the concrete is made and to shelter the 
concrete until it has set and become fairly hard. The tempera- 
ture of the materials should not be higher than that of boiling 
water, and the water itself should generally be only lukewarm, 
and certainly not above 100 F. Immediately the warm 
materials have been mixed and the concrete placed in position, 
it must be protected by canvas tents and fires or hot pipes. 

In Canada, good results are obtained by spreading the whole 
of the materials on a steam-heated floor and by using hot 
water for mixing. The mixing drum is also heated by a 
steam jacket. 

The forms are made of sheet iron and are double, steam being 
led between the walls so as to heat the core or space to be 
occupied later by the concrete. During this preliminary 
warming the forms are covered with tarpaulin, which is 
removed a little at a time during the placing of the concrete. 

Concrete made of warm materials during frost is never 
considered to be quite as satisfactory as that made at the 
normal temperature. Hence, the wisest course is to follow 
the recommendations of the joint committee under the auspices 
of the Royal Institute of British Architects, viz., to suspend 
all work during frosty weather, to protect new work at night 
when frost is expected, to leave the centering in position for 
a fortnight longer than ordinary, and not to remove it until 
all signs of frost have departed. 

Placing in water. Concrete should not, as a rule, be allowed 
to set under water. Where this is unavoidable, special pre- 
cautions must be adopted, one of the most important being 
to prevent the cement from floating away. The use of a 
drop-bottom bucket facilitates rather than prevents this loss 
of concrete. In no case should soft concrete be allowed to 
drop through water. 

One of the most usual methods is to fill the concrete into 
bags, sewing up the mouths of these and depositing the whole 
under water. This method involves the services of divers and 
is cumbersome, slow and expensive ; it is rapidly becoming 



192 THE PREPARATION OF CONCRETE 

obsolete except for repair work and for work under moving 
water. 

The use of a tremie or pipe with its upper end projecting out 
of the water appears to have many advantages. The concrete 
is poured in at the top of the tube as fast as it escapes from the 
lower end. In practice, this method presents several serious 
difficulties, as it is by no means easy to prevent the water from 
rising inside the tube and floating the cement away from the 
concrete. The motion of the water near the bottom of the 
tube has a similar action. What is required is to have the 
lower end of the tube buried in concrete for two to five feet, 
so as to form an effective seal against the outside water ; when 
this can be secured the use of tremies is highly advantageous. 
The tremie is raised a few inches at a time as the work 
progresses. 

An ingenious device, used by the Vibrocel Co., Ltd., consists 
of polygonal cells of concrete which are made on land and then 
floated to the place where they are to be used. These floating 
cells are then kept vertical in the water and the bottom is 
blown out by means of a gelignite charge previously placed 
therein. The cells then sink to the bottom of the water and 
form a permanent tremie which is later filled with concrete. 

The concrete should be wetter than that employed on dry 
land, in order that it may flow properly. Care should be taken 
not to disturb the freshly-set concrete, and the deposition of 
the concrete should be continuous so as to avoid the necessity 
for cleaning the surface of the different lots. 

Where coffer-dams are used they should be sufficiently 
water-tight to prevent a current of water through the pit, and 
to keep any water in the pit quite still. 

SETTING AND HARDENING. 

The chemical and physical changes which take place during 
the setting and hardening of concrete are practically the same 
as those occurring in cement. If the concrete has been 
correctly proportioned and prepared, however, there will be 
far fewer unhydrolysed particles of cement in the concrete 
than when neat cement is used. In other words the cemen- 



SETTING AND HARDENING OF CONCRETE 193 

titious power of the cement is more fully utilised than when 
no sand or aggregate is present. Apart from this, as the 
reactions which occur take place exclusively between the 
cement and water, there is no need to describe them further ; 
the reader who desires to refresh his memory concerning them 
should see pp. 81 et seq. 

If the " sand " used in the concrete is made by crushing 
bricks and burnt clay ballast to powder, it will be, of itself, 
cementitious, and will unite with the free lime formed by 
hydrolysis of the Portland cement, and will form a pozzolanic 
cement whieh will, in its turn, be hydrolysed in a similar 
manner, and will set and finally harden into a stony mass. 
Properly burned clay when reduced to powder, therefore, 
increases the strength of all concrete in which it takes the 
place of silicious sand (see p. 159). 

Concrete which is placed under water sometimes takes 
much longer to set. This is due to the wetness of the mixture 
rather than to the influence of the water in which it is immersed. 

STRIKING CENTERING. 

The removal of the forms, leaving the concrete mass in situ, 
is known technically as " striking the centering." This 
operation usually takes place as soon as the concrete has 
hardened sufficiently for the support of the forms to be no 
longer necessary, that is about eight to ten' days after 
" placing." The length of time which elapses must, however, 
be left to the discretion of the man in charge of the work, and 
in frosty weather the forms may remain for three weeks or 
more. The longer the forms remain in place the safer will 
be the concrete. Some of the simpler forms may, in summer, 
be removed as early as three days after placing, but this is 
somewhat risky. 

The side casing of beams, the casing of columns, and for 
the soffits of floor-slabs of less than five feet span, may usually 
be removed after eight days ; the casing of soffits of beams 
and floors of greater span should not be removed for at least 
fourteen days, whilst for arches of large span the forms should 
not be touched for at least a month. 

c. o 



194 THE PREPARATION OF CONCRETE 

The striking of centering is a most important and responsible 
duty, as if done too soon or unskilfully it may result in the 
collapse of the structure and in serious loss of life. Many of 
the fatal accidents in connection with concrete particularly 
in America have been caused by too early removal of the 
centering and by loading the structure before the concrete 
was properly hardened. Concrete is often used for bridges, 
floors, and in other positions where the weight of material 
with no support immediately beneath it is very great, so that 
the dead weight of the structure alone may be more than 
half that of the live load it is designed to carry. Consequently, 
the premature removal of the forms is always risky, and too 
much care and skill cannot be exercised in preventing accidents 
due to this cause. 

The greatest care should also be taken in removing the forms 
not to jar, shake or tear the concrete, as it will not be fully 
hardened and may easily be damaged. After the removal of 
the forms, the concrete should be protected from the sun's 
rays, rain, dust and wind by canvas, burlap or sheeting, and 
its surface should be kept wet by sprinkling water on it twice 
daily for five or six days. This treatment is necessary to 
prevent the outside of the mass drying more rapidly than the 
inside, and so causing strains. In summer weather and tropical 
climates this watering of the surfaces should be done with 
care and intelligence. If sheeting or burlap is used, this 
should be wetted as well as the surface of the concrete. 

The forms, after removal from the structure, should be at 
once cleaned by means of a short-handled hoe, care being taken 
not to gouge the wood. 

SURFACE TREATMENT. 

Concrete is subject to many conditions which make it 
difficult to obtain a satisfactory finish. Every irregularity and 
almost every joint in the forms leaves an imprint. Patches 
of exposed aggregate show here and there, and variations of 
colour occur in streaks and layers. Thus, a discoloration of 
the material may be due to (a) partial bleaching of the lime 
compounds, (b) the formation of efflorescence or " scum," 



SURFACE TREATMENT OF CONCRETE 195 

(c) the inclusion of organic matter in the water used, and (d) a 
" laitence " face, caused by an excess of water in some portion 
of the concrete mixture which has floated some of the lighter 
cement particles to the surface and produced a thin im- 
permeable coating. Roughness or irregularity of the surface 
may be due to careless work, too long a time between watering 
the form and placing the concrete, insufficient " spading " or 
rough removal of the forms and shuttering, the accidental 
inclusion of sawdust, chippings, etc. 

Untouched concrete work may normally have one of three 
distinctive surfaces : 

(1) Dense surfaces produced by careful proportioning of the 
materials. These are the best of all surfaces on concrete, and 
are obtained by using a medium wet mixture and carefully 
spading (p. 189) so as to get a rich mixture in contact with the 
form. Then, if the forms are removed with sufficient care, the 
surface of the material will be such that it cannot be much 
improved by later treatment, particularly if the concrete has 
been properly proportioned in the first place. 

(2) Porous surfaces caused by the leanness or dryness of the 
mixture, by bad mixing or insufficient tamping, by adultera- 
tion or careless working. The chief cause is attempting to 
reduce the cost of working. 

(3) Crocodile surfaces due to the cement " floating " to the 
surface, then shrinking and cracking, or " alligating " on 
account of the difference in the contraction of the cement and 
concrete. In bad cases the surface spalls or peels away, but 
usually it is covered with minute hair-lines, which bear a 
fancied resemblance to a crocodile skin. This surface is usually 
due to working with too wet a mixture. Surfaces coated with 
neat cement often produce these hair-lines. Such surfaces 
and lines are always due to excessive shrinkage. 

The treatment of the surface of concrete is conveniently 
divided into four main groups : 

(1) Cleaning the surface so as to remove dirt, irregularities, 
etc. This may be effected by brushing, chipping, rubbing 
(with carborundum bricks), sand-blasting, etc., or by washing 
with soap and water, acetic or hydrochloric acid and water, or 
plain water. Efflorescence or scum is usually removed by 

02 



196 THE PREPARATION OF CONCRETE 

washing with weak hydrochloric acid, followed by plenty of 
water applied by means of a hose. If the surface is to be 
cleaned by brushing it is usually necessary to do so whilst 
the concrete is still green 1 , as otherwise the process would be 
too laborious. Hence, the forms must be removed within 
twenty-four hours of placing the concrete. This .limits the 
applicability of simple brushing. 

Hard concrete may be cleaned with acid or with a bush- 
hammer or a pneumatic hammer, but the last-named removes 
the cement surface and so reduces the water-proofness of the 
concrete an objection which applies to all methods of cleaning 
concrete surfaces. 

(2) Filling in the surface voids, without discoloration, in 
order to produce a more pleasing surface. Plaster is unsuitable 
for this purpose as it falls away when the concrete becomes 
damp, and grout is unstable though largely used. A 1:3:5 
mixture with fine aggregate is generally considered to be one 
of the most suitable ; it is made very thin and is applied with 
a whitewash brush. The chief disadvantage of most fillings 
applied to concrete lies in their great liability to scale and 
peel off. This does not always commence at once, but may 
begin after two or three years. 

The only surface fillings which are likely to be permanent 
must contain a pigment which is resistant to the action of 
the sun as well as to damp, which is sufficiently heavy to 
fill the voids and prevent absorption, and of such a nature that 
the texture of the finished surface bears a sufficiently close 
resemblance to that of the original concrete. 

Ordinary concrete cannot be given a polished surface 
because the particles of cement crystals are too soft in propor- 
tion to the aggregate, and are easily reduced to powder without 
securing a smooth, hard surface capable of reflecting light. 
Rich concrete, in which the aggregate is in the state of a very 
fine powder, placed in a glass-lined mould gives the nearest 
approach to a polish at present attainable. 

(3) Treatment of the surface with a coloured material. 
Specially prepared paints are sold for this purpose, but black 
or white marble and other substances may be introduced in 

1 Concrete which has not reached its greatest hardness is said to be green. 



SURFACE TREATMENT OF CONCRETE 197 

the material nearest to the forms. The face is " cleaned up " 
as soon as possible after the centering has been removed. If 
necessary, distance pieces may be used in the forms and the 
surface concrete may in this way be cast quite independently 
of the backing. 

Specially made cement mixtures sometimes known as 
granolithic facings are sometimes poured in a narrow space 
arranged between the concrete and the form. There is, 
however, considerable difficulty in getting the face to adhere 
to the concrete without alligating or spalling, owing to the 
difference in contraction between the two mixtures. The 
facing mixtures may contain red granite or marble chippings 
in order to give a surface of a colour different from the natural 
grey of concrete. Other colouring materials are used for the 
same purpose ; they are added in suitable proportions to the 
cement-sand mixture used for facing. Those most usually 
employed are : raw iron oxide for bright red, roasted iron 
oxide for brown, ultramarine for bright blue, yellow ochre for 
buff to yellow, carbon black or lampblack for grey to dark 
slate, manganese dioxide for black (11 Ibs. per bag of cement). 
A mixture of equal parts of carbon black and red iron ore for 
dull reds. 

Facings of terra cotta either plain or glazed are also 
extensively employed, as they give a particularly pleasing 
appearance and add a warmth of tone not otherwise obtainable. 

Most facings only make concrete damp-proof, and not 
always that, as dampness may rise through the foundations 
by capillary attraction in the case of very porous concrete. 
For this reason it is generally better to adopt some means of 
rendering the whole mass waterproof, and not to rely too much 
upon a surface-finish. (See footnote on p. 171.) 

(4) Treatment with a view to preserving the concrete from 
the action of the weather, viz. : 

(a) Damp-proofing with only partial obliteration of the 

surface and preservation of decorative feature. 

(b) Waterproofing with complete obliteration of the surface. 
The term damp-proofing should be confined to methods and 

appliances used for keeping water and dampness out of the 
superstructures of buildings, the term " waterproofing " being 



198 THE PREPARATION OF CONCRETE 

used for treating work subject to hydrostatic pressure and for 
vessels intended to contain or retain water. Three distinct 
classes of damp-proofing materials are used : 

(1) Transparent coatings. 

(2) Opaque cement coatings. 

(3) Special bituminous coatings. 

Transparent coatings include those which do not change the 
appearance of the surface treated. 

The most frequently used materials are : soaps, oils and 
various waxes together with fluates, water-glass, casein paints 
and bitumens. 

The old Sylvester process, although now practically obsolete, 
was one of the earliest efforts in this kind of damp-proofing. 
This process involves alternate treatments of the surface with 
solutions of soap and alum, and depends for its efficiency upon 
the formation of aluminium salts of the fatty acid contained 
in the soap, which are insoluble and possess a very distinct 
but temporary water-repellent action. This process is not 
economical, as it is necessary to repeat the operations a number 
of times to produce sufficient insoluble soap. 

In the treatment of the porous surface with hot paraffin, 
the exposed surfaces are carefully heated and coated with 
melted paraffin wax applied with a brush. By this means the 
paraffin penetrates to a considerable depth before it chills and 
is thereby deposited in the pores. Fairly successful results 
can be obtained by this method, but the expense makes its use 
quite prohibitive in most cases. 

Water-glass has also been applied with some success, but it 
is difficult to get it into the pores of the concrete. Oxalate of 
soda and various zinc compounds are also used. 

Most of the transparent liquid coatings which are applied 
to the surface with a brush, like paint, and offered as infallible 
remedies for dampness and porosity, consist of a paraffin or wax 
of low melting point, dissolved in a light volatile oil. They 
depend for their efficiency upon the deposition of the wax or 
paraffin in the pores of the concrete. Some of these water- 
proofings contain over 95 per cent, of volatile constituents and 
a very small amount of solid base, only the latter forming the 
waterproofing agent. 



DAMP -PROOFING CONCRETE 199 

Quite recently, some progressive manufacturers have been 
able to produce synthetic water-repellent bases which form far 
stronger solutions in volatile vehicles, and these are more 
satisfactory. 

Under the term fluates, various soluble silico-fluorides are 
largely in use for rendering concrete water-proof and for 
increasing the hardness of its surface. Aluminium, magnesium 
and zinc silico-fluorides are the ones chiefly used for this pur- 
pose, the concrete being either soaked in a solution of fluate 
or painted with the latter. These fluates act in a manner 
similar to water-glass, but form a harder product. 

There is a very distinct field for all such coatings, as they 
are the only means available for treating the exterior surfaces 
of porous stone and concrete in existing structures without 
altering their appearance. 

Opaque Coatings for Cement include paints and coatings of 
plain cement grout, with the attendant difficulty of obtaining 
a perfect bond and the tendency of the coating to absorb 
water. Several manufacturers have produced coatings made 
with a Portland cement base, which show a perfect bond on 
the surface coated and are perfectly repellent and damp- 
proof. These products contain no oil and possess none of the 
characteristic qualities of oil paints. It is well known that 
an oil paint must not be applied directly to a concrete surface, 
as the vegetable oils used react with the alkali in the cement, 
forming a soap, and cause the disintegration of the coating. 
This may be avoided by the employment of a fluate or of a 
coating of casein paint previous to the use of the oil paint. 
Oil paints also dry with a distinct gloss, which is very ob- 
jectionable on a concrete surface, where the coating should 
retain the characteristic texture of the surface coated. Casein 
paints followed by treatment with formaldehyde are becoming 
increasingly popular. 

Where colour is of no importance, as in some underground 
work, there is no better paint than ordinary tar, applied 
hot. 

Cement coatings have a very general application for making 
cement surfaces, such as stucco, cement blocks, etc., uniform 
in colour and are also used to replace the somewhat cheerless 



200 THE PREPARATION OF CONCRETE 

and unattractive surfaces of untreated concrete with a soft- 
toned surface which is thoroughly damp-proof. 

Special Bitumens are not applied to the exposed surface, 
but to the interior of concrete walls. They are black in 
appearance and are made of various waterproof gums. They 
are applied with a brush, and, besides forming a damp-proof 
surface, they provide a good bond for a coat of plaster applied 
directly to them. 

These products eliminate the necessity of furring and lathing, 
and so increase the available space and remove all the disagree- 
able features of the air space. Although the film is a fairly 
good non-conductor, it has not the same insulating efficiency 
as an air space, and is not recommended where there is serious 
condensation on the inner surface. 

The term waterproofing should be confined to treating 
structures subject to water pressure and those designed to 
retain water, but not to prevent mere dampness. One of the 
two following methods is generally employed : 

(1) " Integral," or rigid method in which a waterproofing 
compound is incorporated in the concrete mass, rendering the 
same waterproof within itself. 

(2) " Membrane," or bituminous shield method, in which 
the concrete work is insulated from contact with the water by 
interposing a continuous, waterproof, bituminous shield. 

The " Integral " method involves the addition of a compound 
to the composition of the concrete during the mixing or 
placing, and this compound thus becomes an integral part of 
the mass of substances. Two classes, characterised by the 
physical condition in which they are added to the concrete, 
are used : 

(1) Finely powdered dry compounds added to the dry 
cement in the proportion of about 2 per cent. 

(2) Liquids or pastes added to the water used to temper the 
dry mixture of cement and aggregate. 

The compounds in the first class are usually hydrated lime 
with a greater or less amount of the lime salts of fatty acids. 

The first essential for success lies in obtaining an even, 
homogeneous distribution of the waterproofing compound. 
This is very difficult on account of these dry compounds. 



WATERPROOFING CONCRETE 201 

No matter how great an effort is made to mix a repellent 
compound with dry cement and then with dampened sand, as 
soon as the water is added the repellent property of the com- 
pound will manifest itself, and as the fluidity of the concrete 
increases, and there will be a tendency for the repellent com- 
pound to be concentrated in other sections, thereby making 
an even, homogeneous distribution impossible. Although the 
repellent feature is an excellent property for a compound to 
possess when in place in the mass of concrete, its very nature 
makes even distribution difficult, and thereby defeats its 
intended purpose (see p. 205). 

In the case of compounds that are added directly to the water, 
on the contrary, there is no difficulty in obtaining an even 
distribution, as the water acts as a vehicle or carrier and 
evenly distributes them. Hence, as far as the homogeneous 
disposition of the waterproofing agent is concerned, compounds 
that are originally miscible with water have a decided advan- 
tage over dry compounds of a repellent nature. A suitable 
compound for this class of work should not contain any organic 
constituents or other materials capable of interfering with the 
strength of the concrete, as it should not be necessary to 
sacrifice strength to obtain waterproofing efficiency. Yet 
results have been obtained which indicate a loss of over 50 per 
cent, in the strength of concrete when emulsified oils were 
contained in the compound used for waterproofing. 

The " Integral " method of waterproofing has a very 
general application to waterproofing conditions, though it 
cannot be used in cases when there is a liability for the con- 
tinual development of cracks in the work, as these would, of 
course, destroy the waterproofing efficiency. The "Integral" 
method is very largely employed for substructural work, 
cisterns, reservoirs, etc., which are designed for containing 
water. The waterproofing compounds can be used throughout 
the mass of concrete, or in cases where this precedure would 
be impracticable, on account of the cost, it can be concentrated 
in a plaster coat on the surface of the structure (see p. 205). 

The Coating of waterproofed cement mortar should be 
prepared by thoroughly tempering to required consistency a 
dry mixture of one part of cement to two parts of sand 



202 THE PREPARATION OF CONCRETE 

with water, to which the waterproofing compound has been 
added in the proportion directed by the manufacturer. The 
sand must be clean and spherical and well graded. Before 
plastering such cement mortar on to old concrete, the surface 
of the latter should be treated as follows : 

(a) The old surface must be cleaned very thoroughly with 
a heavy wire broom so as to remove all dust and dirt. A jet 
of steam should, if available, be employed to clean the 
wall. 

(b) To the mechanically-cleaned surface a liberal coat of 
1 : 10 solution of hydrochloric acid is applied with a large 
brush. The acid remains until it has exhausted itself, which 
will require at least ten minutes. A second liberal coating 
of acid solution should then be applied before removing the 
first, and a third coat if the two applications have not satis- 
factorily exposed the aggregate and entirely removed the 
skin of hardened cement. 

(c) With a hose, under good pressure, the surface should 
be washed in one direction so as to remove the salts resulting 
from the action of the acid. This washing is continued until 
the salts and all loose particles are removed and the old concrete 
is thoroughly soaked to its full hydrometric capacity. 

(d) To the cleaned and saturated surface a coating of pure 
cement is applied, mixed to the constituency of thick cream 
with water, and to which the waterproofing agent has been 
added in the proportion directed by the manufacturer, the 
coating being rubbed in vigorously with a strong fibre brush 
so as to fill all the crevices and cavities produced by the 
action of the acid. 

Immediately after applying the above slush coat, the first 
coating of waterproof cement mortar should be applied 
(thickness, three-eighths of an inch) directly upon the slush 
coating and well trowelled into every void or crevice of the 
surface. Before this first coat has reached its final set, a 
second and final coat should be applied to an equal thickness, 
so as to make the full average thickness three-quarters of an 
inch. The finishing coat should be floated to an even surface, 
and subsequently trowelled free from any porous imperfections. 
If the conditions of the work make it impracticable to apply 



WATERPROOFING CONCRETE 203 

a finishing coat before the scratch coat has set, the latter 
must be dampened and slush coated before the finishing 
coat is applied. 

Floors should be treated and prepared exactly as indicated 
above for walls, and finished with the waterproofed mortar 
to a thickness of two inches. Special care should be exercised 
to bond the wall coating to the floor coating, so as to make 
the waterproof coat absolutely continuous. 

The " Membrane " method of waterproofing differs distinctly 
from the "Integral," in that it does not attempt to treat the 
concrete, but rather to insulate it from contact with water 
by enveloping the structure in a continuous bituminous shield. 
The fact that the " Membrane " is not a unit or rigid part of 
the structure permits a certain freedom of movement and 
action in the structure, without impairing the efficiency of the 
waterproofing. This feature of the " Membrane " system 
makes it specially suitable for waterproofing work not fully 
reinforced and liable to settlement or subject to vibration or 
shock, such as railway bridges, culverts, etc. 

Coatings of burlap and coal-tar felts have been extensively 
used for this purpose, and there are now on the market specially 
manufactured felts which are both saturated and coated with 
bitumen, and possess great pliability and strength. 

The bitumens most generally used for cementing the felt 
together in constructing the membranes are coal-tar pitch, 
commercial asphalts and special asphalt compositions. 
Although coal-tar pitch, on account of its cheapness, has been, 
and still is, being very extensively employed for waterproofing, 
many engineers regard the coal-tar pitch produced to-day by 
modern methods of gas production as inferior to that produced 
by the older process in vogue when pitch was first used - for 
waterproofing. 

The asphalts are more suitable for waterproofing as they 
possess greater elasticity and permanence, but care should 
be taken to obtain a material of as low a melting point as the 
nature of the work will permit. 

This not only ensures greater elasticity when subjected to 
cold temperatures, but it is much more freely and easily 
applied. Special asphalts are manufactured from a hard 



204 THE PREPARATION OF CONCRETE 

hydrocarbon, such as gilsonite, tempered with petroleum 
residuums to impart the necessary elasticity. 

Oils, Animal and vegetable oils which are liable to turn 
rancid have a corrosive action on cement and should not be 
kept in concrete tanks. Care should also be taken not to 
spill such oils on foundations or floors made of concrete, as 
the acid produced when the oil turns rancid will decompose 
the cement. Mineral oils are free from this objection. 

The waterproofing materials used for concrete are of two 
kinds : (a) those which are merely pore-fillers, and (b) those 
which fill the pores and also have a repellent action towards 
water. Amongst the best pore-fillers are rock dust, slaked 
lime, and china clay ; their particles are so fine that they 
penetrate almost all the pores, and as they are not greasy they 
become uniformly distributed throughout the material. 

Oils and soaps, on the contrary, tend to accumulate in small 
pasty mas^s which do not readily break up, and render it 
almost impossible to obtain homogeneous concrete. To this 
extent they are a distinct disadvantage, and many of the 
claims made for them are more in the nature of " selling 
arguments " than facts of technical importance. Moreover, 
the addition of oil or soap to a concrete mixture greatly delays 
its setting, and does not confer any advantage adequate to 
the risk thereby involved. Soluble soaps, and particularly 
soft soap (potash soap), are better waterproofing agents than 
oils, because the soap may be dissolved in the water used and 
so becomes more evenly distributed throughout the concrete. 
Useful proportions are 6 to 9 Ibs. of soap per cubic yard of 
concrete. The lime set free by the hydrolysis of the cement 
forms the insoluble lime soap, and this is the compound which 
fills the pores and repels water. 

Waterproofing concrete is best effected by rendering the 
whole mass as impermeable as possible, and not merely con- 
fining the treatment to the surface. The power of water to 
penetrate concrete depends on the number and size of the 
pores in the latter ; hence a concrete in which all these pores 
or voids are completely filled with inert material or are partially 
filled with a water-resisting substance will be waterproof. 

If the necessary care and skill have been taken in preparing 



WATERPROOFING CONCRETE 205 

it from properly graded materials, the ordinary concrete will 
be sufficiently waterproof for most purposes. The object of 
water-repellents is simply to occupy voids in the concrete which 
are produced by imperfect grading or mixing of the concrete. 

Most of the substances sold for waterproofing concrete are 
of a soapy or oleaginous nature, and water-glass (a soluble 
sodium silicate) is also used, though it is costly. 

It should never be forgotten that the addition of oil or soap 
or any non-cementitious material to concrete reduces the 
strength of the structure, and is to this extent disadvantageous. 
Some oils reduce the strength much more than is commonly 
supposed. 

One of the most effective methods of preparing a waterproof 
concrete consists in carefully grading and proportioning the 
materials so as to have as few voids as possible, and to mix 
them together in a dry state. Then, instead of some sand 
there is used a mixture of trass or burned clay in such 
proportions that the amount added will be approximately 
half that of the cement used. The trass or burned clay powder 
acts as a pozzolana, fixes the lime set free by the cement and, 
if the grading and proportioning have been properly effected, 
renders the concrete quite waterproof. The substitution of a 
mixture of equal parts of china clay and lime, though some- 
times recommended, is far less effective. 

Cloyd M. Chapman has found that in many concretes the 
permeability to water is due to the use of a mixture of aggregate, 
sand, cement and water, which is either too dry or too wet 
one extreme being as bad as the other so far as water-proofness 
is concerned. His investigations showed that the most 
waterproof concretes he was able to produce were those in 
which the water represents 13 to 17 per cent, of the weight of 
the mixture, but these figures may differ with different cements 
and aggregates. 



CHAPTER VIII 

REINFORCED CONCRETE 

THE term " Reinforced Concrete " or " Ferro-Concrete " l is 
applied to structures consisting of a combination of concrete 
and metal (usually steel, and termed the " reinforcement ") of 
such a nature that the two materials act as one, which is 
stronger and more durable than either alone. Concrete is not 
very suitable for withstanding tensional stresses ; steel, on 
the contrary, is not sufficiently cheap to be used alone. Steel 
alone is not very resistant to weather, and its surface must 
be protected. Concrete preserves the steel from deterioration 
and, in case of fire, from expansion. A combination of steel 
and concrete is therefore capable of meeting the demands for 
structures which will resist both these stresses. In short, 
reinforced concrete combines the structural qualities of steel 
and timber with the durability of good masonry. It is subject 
to no form of deterioration which cannot be avoided by 
reasonable precautions, and is free from many of the limitations 
of masonry in mass. Because of the greater latitude it affords 
in the design and execution of structures, it often yields the 
best and most economical solution, and in some cases the only 
practicable solution, of the most difficult problems of building 
construction. 

The great advantage of reinforced concrete lies in the fact 
that it is capable of withstanding stresses due to transverse 
strains, tension, and shearing. All the forms that could be 
executed in steel or timber can be closely imitated in reinforced 
concrete, which is immune from corrosion and decay. This 

1 The student should remember that the terms " Ferrocrete " and "Steelcrete" 
have been registered for a Portland cement and do not refer to any form of 
reinforced concrete. 



STEEL IN REINFORCED CONCRETE 207 

makes it possible to adopt designs wherein the structure acts 
by its structural resistance and not by dead weight, and even 
the material to be retained and held back may be made by 
this means to add to the stability of the work as a whole. 
Dead weights on foundations are diminished, difficult excava- 
tion is often avoided or lessened, and total costs often 
greatly decreased, as compared with structures formed of 
masonry in mass ; in many cases reinforced concrete affords 
a variety of desirable solutions not practicable in any other 
material. 

The saving in the thickness of inverts of locks and dams, or 
in retaining walls of all kinds, the use of caissons filled with 
dead materials in lieu of solid masonry walls, the use of rein- 
forced concrete piles to anchor a light structure to the dead 
mass below, and the many other useful devices and applica- 
tions, all open up the possibility of practically limitless 
applications of reinforced concrete to hydraulic structures 
so as to attain both greater efficiency and a diminished 
cost. 

Unlike timber, iron, and steel, which rapidly perish by 
natural decay or corrosion, and unlike many stones, which, 
more slowly, but none the less surely, disintegrate and crumble 
away as the result of atmospheric influences, good concrete 
increases in durability and strength by continued exposure. 
The same property is still more highly developed in reinforced 
concrete, owing to the superior quality and the scientific 
proportions of the constituent materials, and in consequence 
of the close attention devoted to the thorough mixing of the 
ingredients, and the methods of depositing and tamping the 
resulting concrete, which are ensured when the execution 
of works is confined to recognised contractors whose 
experience and standing give assurance of their perfect 
reliability. 

When properly designed and executed it is, therefore, among 
the most valuable materials available for use in structural 
and hydraulic works. 

The chief objection to reinforced concrete is an aesthetic 
one ; the appearance of the finished work is dull and 
colourless, without any of the distinctive tones and colour 



208 REINFORCED CONCRETE 

associated with brick and stone ; yet, when colour is of 
minor importance, as in piers, jetties, foundations, etc., 
reinforced concrete has claims far in advance of any other 
material. 

Steel, when under compression, is about thirty times as 
strong as concrete, and when under tension is about 
300 times as strong as concrete. As steel costs about 
fifty times as much as an equal volume of concrete, it 
is possible to use a combination of the two which is 
cheaper (considering the stresses to be resisted) than either 
material alone. 

Hence, if the members of the structures are arranged in 
such a way that the compressive stresses are all borne by the 

concrete and the 

>^F 1 7T?^*&tftW'^ tensile stresses by the 

' steel, each material 

b will be used for the 

''^#W%&ty:>^ - fvij t'**$$p purpose for which it 

yi/iS^^i'^ is the cheapest and 

^ 1L''. .''.'*.. ''. * ' ''.- -"*.. ''.*;'-.' -I.:* '' <-' $'/. '-VI;-.V A *' '*.'>': '* -...- IrX? ii T j .Cj.j.^J] rpv 



>^r -ty the best fitted. The 

. shearing resistance of 

l| | | | |0 |l \3 \* y 

SCAU-FECT concrete is also very 

FIG. 45. Beams, of equal strength, of i nar |p nil ptp o nr l 
(a) Keinforcedand (b) Ordinary Concrete, inadequate, ana 

hence, in the most 

approved designs, the shearing stresses are borne by small 
steel rods and stirrups embedded in the concrete. 

The remarkable effect of steel in increasing the strength of 
concrete is strikingly illustrated in Fig. 45, which shows two 
beams designed to carry ordinary floor loads, the one made 
entirely of concrete and the other of concrete with a sheet of 
expanded metal embedded in the tensile portion of the beam. 
The saving in mere weight of concrete alone is obvious ; and 
when it is remembered that the adoption of floor beams entirely 
of concrete means an increase in thickness of nine inches, or 
supposing five to eight floors, an increase in the total height 
of the building (with extra cost of higher and heavier walls, 
together with heavier foundations to carry them) of from 
four to six feet, it is clear that even as regards initial outlay 
for materials, the introduction of steel reinforcement into 



AGGREGATES FOR REINFORCED CONCRETE 209 

concrete construction is of very great importance. Another 
most remarkable fact is that the weight of steel, if properly 
disposed, is so small as almost to be insignificant. Comparing 
areas of steel and concrete exposed in cross-section, the steel 
is sometimes only J per cent., and rarely rises above 1 or 1 J per 
cent, of the area of the concrete. 

The concrete must be of first-class quality, and the aggregate 
must be smaller than that used for mass concrete. No pieces 
of .aggregate which will not pass through a hole IJ inches 
diameter should be used. 

Both the aggregate and sand must be carefully selected, 
especially when a combination of strength and fire resistance 
is desired. A hard aggregate, such as river ballast, pit gravel, 
blue bricks, granite, etc., is preferable, as a soft one, such as 
clinker, red brick, sandstone, etc., means a weaker concrete. 
The stones should be angular and of an irregular nature, both 
in shape and size. Flaky aggregates should be avoided as 
they lie too close together. 

For encasing steelwork, flooring and similar work, brick or 
well-burned furnace clinker, or a mixture of the two, affords 
an excellent fire-resisting aggregate, but must be hard and free 
from combustible materials, old mortar, ashes and dust. 

The natural aggregates, while they have greater compres- 
sional strength, are liable to splinter and " fly " under intense 
heat ; limestone will shrink when exposed to fierce fire, and 
sandstone is so variable when heated that it should be avoided 
where fire resistance is required. 

Cinders and coke breeze are not recommended for reinforced 
concrete work, and their use should be restricted to filling 
purposes, such as bedding to which to nail floor boards. 

The sand used should be hard and gritty, with grains of 
various shapes and sizes. Sharp, angular sand is preferable 
for reinforced concrete work. Crushed limestone should be 
avoided, and clayey sand should be entirely prohibited unless 
the clay is first removed by very thorough washing. Sea 
sand sometimes causes an efflorescence or scum to appear on 
the finished work. Very fine or " blown " sand should not 
be used. 

Both the aggregate and the sand should be clean, sharp and 

c, p 



210 REINFORCED CONCRETE 

free from all foreign substances, and, if necessary, should be 
washed before use. 

Materials containing more than, say, 1 per cent, of sulphates 
or other corrosive substances, should be strictly avoided, and 
when any doubt exists on this point it is wise to have a chemical 
analysis of them made to ensure the absence of such ingredients. 
Some bricks notably some made at Fletton, near Peter- 
borough have proved unsatisfactory as aggregates on account 
of the sulphur compounds they contain. 

The cement employed must be Portland cement which 
conforms to the Standard Specification (p. 97), as no other is 
of sufficiently good quality. The risks in defective reinforced 
concrete are so enormous that no pains should be spared to 
prevent them. 

The following mixtures, all parts by measure, are typical of 
those used by the most reliable firms : 

For floors, walls, etc. : 

( | to 3 1 inches thick) (thicker than 3 J inches) 

3 parts of aggregate. 4 parts of aggregate. 

If parts of sand. 2 parts of sand. 

1 part of Portland cement. 1 part of Portland cement. 

For general and heavy concrete work : 

3 parts of aggregate. \ f4 parts of aggregate. 

2J parts of sand. [or| 2 parts of sand. 

1 part of Portland cement.) (l part of Portland cement. 

For tanks, etc., where the concrete is required to resist liquid 
pressures : 

3 parts of aggregate. 

2 parts of sand. 

1 part of Portland cement. 

The practice of using fixed proportions of aggregates, sand 
and cement is particularly unsatisfactory and dangerous ; in 
every case the proportions should be selected after ascertaining 
the percentage of voids in the materials as described on p. 157. 

The mild steel used does not vary in quality so much as the 
concrete. In tension the strains are proportional to the stress 
below the elastic limit, beyond which it is unsafe to stretch it. 



STEEL FOR REINFORCEMENT 211 

The mild steel used for reinforcement usually has an elastic 
limit corresponding to a stress of 32,000 to 50,000 Ibs. per 
square inch. The working stress specified in reinforced concrete 
is usually about half this, namely, 16,000 to 25,000 Ibs. per 
square inch. The working stress in steel beams under com- 
pression may be taken at 16,000 Ibs. per square inch, and for 
columns at 12,000 Ibs. per square inch, or still lower if there is 
any probability of buckling. 

The coefficient of expansion of steel and concrete are almost 
identical at ordinary temperatures, otherwise serious internal 
strains would be produced. The actual coefficients of expan- 
sion vary with different specimens, but on an average steel 
expands -00065 per cent., and the concrete -00060 per cent, 
for a rise in temperature of each degree Fahrenheit. 

Steel with a high percentage of carbon is unsuitable for use 
in reinforced concrete, as it is brittle (particularly after ham- 
mering), is more costly and more difficult to work than mild 
steel. Steel containing less than 0-3 per cent, of carbon is 
not open to this objection, and consequently mild steel is 
almost invariably employed. 

The Committee of the Concrete Institute recommended 
that the steel used shall have the following properties : 

(a) It shall attain an ultimate tensile strength of not less than 
60,000 Ibs. per square inch. 

(b) It shall withstand a stress of at least 34, 000 Ibs. per square inch 
before showing any appreciable permanent set. 

(c) The contraction of area at fracture shall be not less than 45 per 
cent., or the elongation in the case of bars of one inch diameter and 
under shall be not less than 25 per cent., measured on a length equal to 
eight times the diameter of the bar tested. 

The elongation shall be measured in the case of bars over one inch 
diameter on a length equal to four diameters of the bar, and shall be 
not less than 30 per cent. 

(d) All steel shall stand bending cold to an angle of 180 degrees 
around a diameter equal to that of the piece tested, without fracturing 
the skin of the bent portion. 

(e) The steel shall be free from scabs and flaws, and must be clean 
and free from rust. It must not be painted or oiled, but a wash of 
Portland cement grout is desirable. 

The Joint Committee under the auspices of the Royal 
Institute of British Architects has made very similar recom- 
mendations, but in (b) this committee places the elastic limit 
at not less than 10 per cent, nor more thun 60 per cent, of the 

P 2 



212 REINFORCED CONCRETE 

ultimate strength, and the minimum elongation (c) at 22 per 
cent., and requires the steel to stand the other tests specified 
in the British Standard Specification for Structural Steel. 

Welding is to be avoided wherever possible ; if necessary, it 
should be at the points of minimum stress. 

Some of the firms specialising in concrete construction have 
even stricter conditions in their specifications. The following 
extracts from specifications issued by several leading firms 
give some idea of these additional requirements : 

British Reinforced Concrete Engineering Co., Ltd. 

All rods, plates, bars or braces to be of mild steel, manufactured on 
open-hearth basic or acid Siemen's process, uniform in quality, and 
entirely free from defects. 

Ultimate tensile strength not less than 28 nor more than 32 tons per 
square inch. 

All steel on delivery to be cleaned and stored in a dry place. 

All stirrups and hoops to accurately fit rods and bars to be bent to 
proper shape. 

Trussed Concrete Steel Co., Ltd. (Kahn System). 

No reinforcing steel shall be considered that does not provide for 
shearing stresses as well as direct tension. 

These shear -resisting members must be inclined at an angle of 
45 degrees, pointing up and towards the supports of the structure. 

Shear members shall be rigidly attached to main tension members. 

Sufficient steel to be placed that concrete shall be obliged to resist 
only direct compression and shearing stresses up to 50 Ibs. per square 
inch. 

No steel shall have at any point less than one inch concrete covering. 

In no case will steel of a higher elastic limit than 45,000 Ibs. be 
considered. Same shall have a tensile strength of from 60,000 to 
70,000 Ibs. per square inch, with elongation not less than 20 per cent, 
in eight inches. 

British Concrete- Steel Co. 

The indented steel bars to be of best quality : tensile strength 
38 to 42 tons per square inch. Elastic limit not less than 50,000 Ibs. 
(22 tons) per square inch. 

Ample supply of soft iron wire is to be provided for lapping steel 
bars at joints and at points where they cross each other. 

L. 0. Mouchel and Partners (Hennebique System). 

Steel to be in the form of round bars and strip, obtained from makers 
of good repute, and to be mild steel produced by the open hearth, basic 
or acid process. Neither Bessemer steel nor high carbon-steel to be 
employed. 

Consider e Construction Co., Ltd. 

Steel must be of British manufacture and have tensile strength not 
less than 28 tons or more than 32 tons per square inch, and show 



COMMERCIAL SPECIFICATIONS 213 

contraction of area at fracture of 50 per cent, and 40 per cent, respec- 
tively, and appearance of fracture not to show more than 5 per cent, 
and 10 per cent, granular surface respectively. 

Expanded Metal Co., Ltd. 

Mixture of concrete not less than 1:2:4 by volume. Working 
stress not exceeding 16,000 Ibs. per square inch in tension in the 
expanded steel, and 500 Ibs. per square inch extreme surface com- 
pression in the concrete. Ratio of the moduli of elasticity of steel and 
concrete taken as 15. 

Edmond Coignet, Ltd. 

Annealed wire used for binding together various bars of framework 
at intersection should be about ^-inch diameter. 

Binding to be done as tightly as possible and cutting pliers used. 

It is of the greatest importance that concrete structures 
should not be overloaded, especially when their design is such 
that a rigid economy in material has been anticipated. 

Loads in buildings and other structures are of two kinds : 
dead loads which are constant, and live loads which are moved 
from time to time, or may even be of a purely momentary 
character, such as a train passing at a high speed over a bridge. 
Live loads usually tend to cause vibrations in the structure, 
and consequently their effect is greater than their actual weight. 
This effect is most conveniently expressed in the form of a 
ratio of which the equivalent dead load forms one term. 
Thus a floor intended to carry a crowd of people would have 
an equivalent dead weight of 120 Ibs. per square foot, whilst 
that of a train travelling over a bridge would correspond to 
a dead load of 500 Ibs. per square foot. 

When calculating loads, the weight of the structure and all 
fixed loads and the equivalent of any thrusts and other 
forces must be included in the dead load, the weight of rein- 
forced concrete being usually taken at 150 Ibs. per cubic foot. 

The following working stresses represent those commonly 
employed : 

Ibs. per square inch 
Steel in tension . . 16,000 



Steel in compression 

Steel in shear .... 

Concrete in compression (bending) 

Concrete in compression (columns, etc.) 

Concrete in shear 

Adhesion of concrete to steel 



12,000 

8,000 

600 

500 

60 

100 



214 REINFORCED CONCRETE 

If the concrete has a crushing strength above 2,400 Ibs. per 
square inch after twenty-eight days, the working stress in 
compression for beams may be taken as one-fourth, and for 
columns, etc. as one-fifth, of its crushing strength. It is only 
fair to point out, however, that it is the elastic limit of the 
steel and not its ultimate strength which forms the critical 
factor, as at the yielding point of the steel the whole member 
will fail on account of the concrete being unable to stretch as 
much as the steel has done. 

The following figures represent the value ordinarily assumed 
for equivalent dead loads on floors : 

Ibs. per square foot. 

Crowd of people ..... 120 

Dwelling-houses, hotels, etc. . . . 80 to 120 
Theatres, churches, etc. .... 100 , 150 



Drill halls and ballrooms . . . .140 

Stores, warehouses, and light factories . 100 

Heavy factories and workshops . . 200 

Roofs . 30 



160 

200 

400 

50 



Factors of Safety. Before the design of a concrete structure 
is prepared, the following particulars should be definitely 
settled : (a) the live or maximum load per unit, which should 
not exceed one-half the elastic limit of the steel used for 
reinforcement ; (b) the factor of safety for the live load, which 
is usually taken as 4, but sometimes as 5 or 6 ; (c) the ratio of 
live to dead load, usually taken as 2 : 1 ; (d) the factor of safety 
for the dead load, usually assumed as three-quarters of that 
of the live load ; (e) the test load, which is usually one and a 
half times the live load. 

Where the factor of safety is stated in relation to the tensile 
strength of the material the elastic limit of the steel should 
be doubled. For example, with a steel whose elastic limit is 
one-half its tensile strength, the maximum live load would 
be one- quarter of the load representing its tensile strength, 
and if 4 is taken as the factor of safety, the greatest live 
load permissible would be one-quarter of the factor of safety 
multiplied by the figure obtained for the live load. 

The safe load for Portland cement concrete made in the 
usual proportions is from 6 to 8 tons per foot super. 

For grey stone lime concrete, it is 1 to 2 tons. 

For blue lias lime concrete, it is 2 to 3 tons. 



FACTOR OF SAFETY 



215 



To obtain satisfactory results without using an unnecessary 
quantity of material, the bending moments, shearing forces, 
and other stresses to which the structure will be subjected 
must be calculated. These calculations must be studied from 
a text-book on " The Theory of Structures," or " Strength of 
Materials," as they are entirely a matter for structural engineers 
and are beyond the scope of the present work. For the con- 
venience of the reader the following rules and recommendations 
of the Joint Committee formed under the auspices of the 
Royal Institute of British Architects are printed here ; the 
student should study them carefully. 



METHODS OF CALCULATION. 

1. Loads. In designing any structure there must be taken into 
account : 

(a) The weight of the structure. 

(6) Any other permanent load, such as flooring, plaster, etc. 

(c) The accidental load. 

(d) In some cases also an allowance for vibration and shock. 

Of all probable distributions of the load, that is to be assumed in 
calculation, which will cause the greatest straining action. 

(i.) The weight of the concrete and steel structure may be taken at 
150 Ibs. per cubic foot. 



Neutral - - -X- 




FIG. 46. Diagram showing Principal Lines of Stress in Loaded Beam. 




FIG. 47. Shear Diagram for Beam under gradually increasing Load. 

(ii.) In structures subjected to very varying loads and more or less 
vibration and shock, as, for instance, the floors of public halls, factories, 
or workshops, the allowance for shock may be taken equal to half the 
accidental load. In structures subjected to considerable vibration and 
shock, such as floors carrying machinery, the roofs of vaults under 
passageways and courtyards, the allowance for shock may be taken 
equal to the accidental load. 

(iii.) In the case of columns or piers in buildings, which support 



216 REINFORCED CONCRETE 

three or more floors, the load at different levels may be estimated in 
this way : 

For the part of the roof or top floor supported, the full accidental 
load assumed for the floor and roof is to be taken. 

For the next floor below the top floor 10 per cent, less than the 
accidental load assumed for that floor. 

For the next floor 20 per cent, less, and so on to the floor at which 
the reduction amounts to 50 per cent, of the assumed load on the floor. 

For all lower floors the accidental load on the columns may be taken 
at 50 per cent, of the loads assumed in calculating those floors. 

2. Spans. These may be taken as follows : For beams the distance 
from centre to centre of bearings. For slabs supported at the ends, 
the clear span + the thickness of slab. For slabs continuous over 
more than one span, the distance from centre to centre of beams. 

3. Bending Moments. In the most ordinary case of a uniformly 
distributed load of w Ibs. per inch run of span, the bending moments 
will be as follows : 

(a) Beam or Slab simply supported at the ends. Greatest bending 
moment at centre of span of I inches is equal to wl* -=- 8 inch Ibs. 

(b) Beam continuous over several Spans, or Encastre or fixed in direction 
at each end. The greatest bending moments are at the end of the span, 
and the beam should be reinforced at its upper side near the ends. 
If continuity can be perfectly relied on, the bending moment at the 
centre of the span is wl 2 -*- 24, and that over the supports wl 2 -=- 12. If 
the continuity is in any way imperfect, the bending moment at the 
centre will, in general, be greater, and that at the supports less, but the 
case is a very indefinite one. It appears desirable that in building 
construction generally the centre bending moment should not be taken 
less than wl 2 -=-12. The bending moment at the ends depends greatly 
on the fixedness of the ends in level and direction. When continuity 
and fixing of the ends, whether perfect or imperfect, is allowed for in 
determining the bending moment near the middle of the span, the 
beam or slab must be designed and reinforced to resist the corresponding 
bending moments at the ends. When the load is not uniformly 
distributed, the bending moments must be calculated on the ordinary 
statical principles. 

4. Stresses. The internal stresses are determined, as in the case of 
a homogeneous beam, on these approximate assumptions : 

(a) The coefficient of elasticity in compression of stone or gravel 
concrete, not weaker than 1 : 2 : 4, is treated as constant, and taken 
at one-fifteenth of the coefficient of elasticity of steel. 

Ibs. per sq. in. 

Coefficient for concrete = E c = 2,000,000 
steel = E 8 = 30,000,000 



It follows that at any given distance from the neutral axis the stress 
per square inch on steel will be fifteen times as great as on concrete. 

(b) The resistance of concrete to tension is neglected, and the steel 
reinforcement is assumed to carry all the tension. 

(c) The stress on the steel reinforcement is taken as uniform on a 
cross-section, and that on the concrete as uniformly varying. 

5. Working Stresses. If the concrete is of such a quality that its 
crushing strength is 2,400 to 3,000 Ibs. per square inch alter twenty- 



METHODS OF CALCULATION 217 

eight days, and the steel has a tenacity of not less than 60,000 Ibs. per 
square inch, the following stresses may be allowed : 

Ibs. per sq. in. 

Concrete in compression in beams subjected to bending . 600 
Concrete in columns under simple compression . . 500 
Concrete in shear in beams ..... 60 
Adhesion 1 of concrete to metal . . . . .100 
Steel in tension 15,000 to 17,000 

When the proportions of the concrete differ from those stated above, 
the stress in compression allowed in beams may be taken at one-fourth, 
and that in columns at one-fifth of the crushing stress of cubes of the 
concrete of sufficient size at twenty-eight days after gauging. If 
stronger steel is used than that stated above, the allowable tensile 
stress may be taken at half the stress at the yield point of the steel. 

When the foregoing rules are familiar, the formulae used by 
specialists in concrete construction 2 may then be studied with 
advantage, though many of these simplified formulae contain 
constants and other somewhat empirical matter which, in the 
hands of a man of experience, are useful, but, if used by a 
beginner, may lead to a considerable risk of error. 

Specialists in concrete construction have, by the aid of 
such calculations, devised various ingenious arrangements of 
the steel within the concrete so as to produce structures of 
enormous strength at a relatively low cost. These " systems " 
of reinforcement usually bear the name of their inventor, or give 
some indication of the shape of the reinforcement in their 
titles. Whilst each system usually has some advantage over 
others, no definite means has yet been formed whereby all 
points, including economy, may be considered and balanced 
against each other. Consequently, it is very difficult for an 
engineer or architect who has not specialised in reinforced 
concrete to determine the relative merits of various systems. 
To attempt to decide on a question of cost leaving the respon- 
sibility of selecting the proper factors of safety to the firms 

1 It is desirable that the reinforcing rods should be so designed that the adhesion 
is sufficient to resist the shear between the metal and concrete. Precautions should 
in every case be taken by splitting or bending the rod ends, or otherwise, to provide 
additional security against the sliding of the rods in the concrete. [It should, 
however, be noted that this treatment has no effect on the greater part of the 
lenath of smooth bars, but only near the ends where the adhesion is least liable to 
be destroyed. A. B. S.] 

2 The excellent theoretical analysis in the handbook supplied by the manu- 
facturers of the Kahn trussed bar is well worth special study, as it is a particularly 
clear enposition of the " Straight Line Formula," which is rapidly becoming of 
general acceptance. 



218 REINFORCED CONCRETE 

making the tenders is, of all ways, the most unsatisfactory, 
and is sure, at some time, to lead to trouble. 

The generally accepted method of reinforcement consists in 
the insertion of thin horizontal bars or rods and strips of steel 
in just those places, and in those places alone, where the 
resistance of the concrete requires to be supplemented in order 
that it may withstand tensile stresses. These bars may be 
bent at each end so as to provide an increased resistance to 
the shearing force supplied by the load, or they may be fitted 
with " shearing members " in the shape of attached bars, loops, 
stirrups, etc. The shape of these attachments and of the bars 
themselves is the subject of various proprietory rights, and 
forms the chief distinction between the different " systems " 
of reinforced concrete. 



FIG. 48. Bending Moment of Beam under gradually increasing Load. 

These shear members are necessarily of short length, since 
they are limited by the depth of the beam itself, which, as a 
rule, does not exceed one or two feet. 

It is, therefore, essential that these ".shear members" 
should be rigidly attached to the main tensional members, 
and it is also of great importance that a good mechanical 
bond should exist between the steel and the concrete along 
the shear bars. For this reason the R.I.B.A. Committee 
Rules (p. 215) stipulate that : "As the resistance of the shear 
members to the pull depends on the adhesion and the anchorage 
at the ends, it is desirable to use bars of a small diameter, and 
to anchor the stirrups at both their ends." If a smooth bar 
is bent up as a shear bar the adhesion of the concrete to the 
short length of steel available is insufficient to develop the 
full strength of the metal before the latter will pull out of the 
concrete. A large proportion of the metal in the shear bars 
is thus wasted, and to obtain the requisite strength far more 
steel has to be provided than is actually necessary. If, how- 



SHEAR MEMBERS 219 

ever, indented bars (p. 235) are used, the full strength of the 
steel is brought into action. Some of the main tensional 
members can themselves be bent up towards the ends of the 
beam into the correct position for taking the shearing stresses. 
These " shear members " are not merely connected rigidly to 
the main bars, but are actual portions of the same bars, the 
anchorage extending throughout their length (Fig. 49). 

The L.C.C. rules refer to this property of indented bars 
when they demand that all shear bars shall have a " mechanical 
anchorage at both ends, or they shall have a mechanical bond 
with the concrete throughout their length." 

In ordinary commercial round or square bars, small sections 
are generally favoured, as they give a larger proportionate 




FIG. 49. Indented Bais bent to form bliear Members. 

surface for adhesion, and are more easily manipulated. Hoops, 
bands, and flats of small section are used, but rounds and 
squares are the sections generally employed in reinforced 
concrete work. Other sections, more particularly T-bars, are 
specially fitted for reinforcing arches and tall chimneys, while 
the old type of steel and concrete floor, of which there is a 
considerable amount still constructed, has small I joists. 

Some patent bars for reinforcement are moulded in various 
shapes and sizes with the idea of providing the mechanical 
bond, though numerous authoritative investigations have 
shown these to be unnecessary in many cases, owing to the 
natural adhesion of the concrete to the steel. 

The arrangement of these bars as well as their shapes, 



220 REINFORCED CONCRETE 

constitute the foundations of the various " systems " now in 
use. Each fundamentally different structure requires a 
different arrangement ; thus, independent spread foundations 
are now reinforced with bars, although the older form of steel 
joists, crossing each other, is still much used. Where raft 
foundations are used, a reinforced concrete raft, thoroughly 
framed together with -beams forming ribs, can be constructed 
very much thinner than one which is not reinforced, and gives 
security against unequal loading and unequal support from 
the subsoil, and against shocks and vibrations of great magni- 
tude, so that such rafts are particularly applicable in districts 
subject to earthquakes. 

In reinforced concrete work, most buildings are carried out 
on the frame principle, the loads being carried from the beams 
on to columns, and the walls are mere partitions between them. 

In ordinary cases, there is very little lateral pressure on 
walls, and an ordinary square mesh- work embedded is sufficient. 

Many retaining walls have been built in reinforced concrete. 
Steel reduces the thickness and the amount of material to be 
excavated, because buttresses, with thin intermediate slabs 
and a projecting foot, can be designed to be thoroughly stable 
against overturning and shear. There are no special systems 
with regard to reinforcing retaining walls that call for notice. 

A cantilever retaining wall is a striking example of the 
enormous economy of reinforced concrete as compared with 
mass concrete, brickwork or masonry, the precise type of wall 
being selected according to the requirements of the case. For 
instance, in the retaining walls of the Royal Insurance Offices, 
in Piccadilly, the thrust of the earth is taken by a vertical 
wall, and a horizontal slab occurs on the opposite face to the 
earth pressure. The horizontal beams at the top do not 
afford any horizontal support to the wall. This wall, 
constructed in 1908 of concrete reinforced with indented bars, 
was the first of its type to be built, but several have since 
been constructed. Counterforts could, of course, be used, but 
the cantilever type adopted in this instance affords a clear 
run immediately behind the wall, and affects an enormous 
saving in space as compared with a counterfort wall, or still 
more in comparison with a mass concrete or masonry wall, 



RETAINING WALLS 



221 



which would have been approximately ten feet wide at the 
base instead of two feet six inches as built. Another excellent 
'example of the use of reinforced concrete (with indented bars) 
in place of mass concrete is the retaining wall for Self ridges, 
which is probably the deepest in London, being sixty feet deep 
and twelve feet thick at the base. The load on the piers 
which support the building is carried by a heavy reinforced 
concrete slab. This wall provides three basement floors below 
street level ; it is of cantilever type, similar to that in the 
Royal Insurance Offices just described. 

Columns, piers, posts, or stanchions are constructed of 
reinforced concrete, the reinforcement being used to reduce 
the section of the concrete, to bind it together, and to prevent 



1 i I I i 



f ' 


/ 




\ 


1 f 


! 




\ 


1 I 








\ \ 








\ ' 








\ 


\ 




. 


\ 

\ 


\ 




/ 



-7 
/ 
1 
I 
\ 




\ 

i 
/ 


1 

1 
\ 
\ 
\ 




\ 
i 
i 

/ 



FIG. 50. No Lateral 
Ties. 



FIG. 51. One 
Lateral Tie. 







> 
i 

i 
i 

> 

/ 














i 




IG. 52. Sever* 
Lateral Ties. 



bulging. For these articles the concrete usually consists of 
a 1:2:4 mixture. A column provided with longitudinal 
reinforcement only would bulge or swell when overloaded 
(Fig. 50). Hence, these longitudinal bars must be tied 
together by a series of transverse bonds. Four types of these 
are used : (a) circular hoops or rings placed at a convenient 
distance apart, (6) rectilinear ties made of wire, similarly 
placed (Fig. 51), (c) a spiral wire (Fig. 52), and (d) a network 
surrounding the vertical bars. 

The use of these ties was originated by F. Hennebique, and 
has been applied extensively with satisfactory results. For 
difficult cases it is well to bear in mind that there is evidence 
to show that the greatest strength is obtained with longitudinal 



222 



REINFORCED CONCRETE 




FIG. 54. 



bars surrounded by spiral reinforcement, the distance between 
the coils being small enough to resist the lateral or radial 
expansion of the concrete (this is the basis of the Considere 
System). Jointed circular hoops and horizontal wire ties are 
slightly inferior, though for ordinary cases they are of more 
than ample strength. 

Tests of concrete columns with short lengths of longitudinal 
reinforcement in combination with continuous transverse 
reinforcement show that this is the weakest form. The 
reinforcements consist of longitudinal bars and horizontal 
ligatures, the latter being formed 
either of links or spirally-wound 
rods of smaller section than the 
longitudinal bars. 

The use of hoops, network, or 
of a spiral wire without longitudinal 
bars has proved unsatisfactory 
when tested, the metal in this form 
of reinforcement being able to 
develop only a small fraction (about 2 per 
cent.) of its full strength, and is therefore 
wastefully applied. The use of longitudinal 
bars of ample size running the whole length 
of the column and surrounded by suitable ties 
affords an ample reserve of strength. 

The use of longitudinal bars fitted with wings or inclined 
members, but without ties, is not satisfactory for column 
construction. If ties are used the inclined members are 
unnecessary, and without the ties the column is weak. 

The objection to a mesh or network for reinforcing columns 
is the difficulty of making a secure joint along the edges of the 
material and of tamping the concrete into the meshes of the 
network. This difficulty is often exaggerated by the represen- 
tatives of other systems of reinforcement, though it does 
undoubtedly exist. Attempts to overcome it by using a more 
fluid concrete should be resisted strenuously (see p. 234). 

To overcome the inherent weaknesses of the foregoing 
systems especially the strains created in the metal by longi- 
tudinal compression on account of the setting of the concrete, 



FIG. 53. 

Hennebique 

Column. 



COLUMNS, PIERS AND STANCHIONS 223 

and the consequent tensional stress in the concrete the 
reinforced metal type of column (Fig. 55) has been devised 
(British Patent 27529, 1910). 

It consists of a single axially-disposed steel cylinder, either 
hollow or solid, or a star-shaped rolled steel section, which 
furnishes the longitudinal reinforcement, and serves as anchor- 
age for four steel spirals which are disposed eccentrically 
around it. These spirals embrace the core and on their inner 
side bear hard against it ; they intermesh with one another, 




FIG. 55.- Core with Anchored Spirals. End View. 
(Courtesy of Reinforced Metal, Ltd.) 

giving lateral reinforcement at every point throughout the 
column and constitute shear-resisting members of a most 
effective character. The area of steel when a solid core is used 
is 12-56 per cent., that when a hollow core is used 2'4 per 
cent, of the area of the column within the reinforcement, but 
the percentage of steel reinforcing each compartment relatively 
to the area of its contents is about five times as great as would 
be the case with the ordinary spirally-hooped column were the 



224 



REINFORCED CONCRETE 




same total weight of spiralling employed in both. The peculiar 
characteristic of this arrangement is that, as shown by 
Professor A. Gray's test, its modulus of elasticity under 

progressive loading increases with 
increase of compressive stress a 
property possessed by no other 
combination of materials yet in- 
vestigated. 

The compressive strength of a re- 
inforced metal column is stated by 
the owners of the patents to be 
twice that of spirally-hooped con- 
crete column and three times that 
of a steel stanchion encased in 
concrete in which the same weight 
of longitudinal steel and concrete is 
used. 

It is claimed that, as the materials 
do not cost more to buy nor the 
column more to make than others, 
the cost of the Reinforced Metal 
Column per ton of load capacity per 
about one-half that of reinforced 




FIG. 56. Column Base 
(Hennebi^ue). 



foot of length is only 
concrete columns. 

Column bases may advantageously be reinforced in two 
horizontal directions, 
each at right angles to 
the other (Figs. 56 
and 57) so as to take 
up all tensional strain 
which would other- 
wise result in a 
spreading of the base. 





/O" 














w\ 


\ 


8 






: 


Stirrups 






:: H* : * 


': 






]^-^,-^!ii m 



Beams, girders and 
stanchions are usually 



FIG. 57. Column Base ((Joignet). 



made of a 1:2:4 mixture reinforced with one-inch rods of 
mild steel, with a network reinforcement or with patent bars. 
The reinforcement of beams one of the most important 
branches of concrete structural work is usually placed near 



BEAMS 225 

to the lower side of the beam, except above the supports 
where it is bent upwards so as to lie near the top of the 
beam. 

The reason for this is that the parts of a beam which lie in 
the centre of the span between the columns have the lower 
part in tension and the upper part in compression. For a 
short distance over each column, however, the nature of the 
stresses is reversed, and the upper part of the beam is in tension 
and the lower part in compression. 

A further provision of steel is necessary by way of web 
reinforcement to resist shear. The function of web reinforce- 
ment is to connect together the compression and tension areas 
in a rigid manner in precisely the same way as^the bars of a 
lattice girder or the members of a truss girder. 




FIG. 58. 

The shear which these members resist is the greatest at the 
ends of a uniformly loaded beam and diminishes towards the 
centre. It is therefore necessary that shear reinforcement 
should be heavier at the ends than at the centre, or if the shear 
bars are of the same section throughout, they should be 
placed at gradually decreasing distances from the centre to the 
ends of the beam. The three principal points of reinforcement 
in beams are, therefore : 

(a) The main tension bars at the bottom of the beam. 

(b) The tension bars at the top of the beam at points of 
contraflexure over columns. 

(c) The web or shear reinforcement connecting the tension 
and compression areas together. 

Where the beam is large and T-shaped, supplementary 
reinforcing bars are also placed in the head of the T and at 
right angles to the others. The same arrangement is used in 
reinforced floors carried on beams (Fig. 58), 

c. Q 



226 



REINFORCED CONCRETE 



As the actual amount of diagonal tension at any point in 
a beam cannot be determined with accuracy, it is customary 
to calculate the vertical shearing stress, and to use that as a 
convenient measure of the diagonal tensile stress. Hence the 
survival of the terms " shearing failure " and " shear reinforce- 
ment," though in reality the form of failure in beam tests, 
commonly described as a " shearing failure," is almost 
invariably a " diagonal tension failure." 

The investigations of M. Feret, Professor Spofforth, Professor 
Talbot and others, have shown that the shearing strength of 
concrete may be taken as two-thirds of its compressive strength, 
so that there is little risk of failure by vertical shear, even in 
a plain concrete beam, but failure will occur by diagonal 




Section on A-b. Section on C'D Section en E-F. 

FIG. 59. Reinforced Beam (Hennebique System). 

tension when the vertical shearing stress reaches a very 
moderate intensity, which Professor Talbot has found to vary 
from 70 Ibs. to 140 Ibs. per square inch, according to the quality 
of the concrete used. Thus, although the shearing stress may 
not in itself be sufficient to affect the concrete, a comparatively 
small stress of this kind generally indicates the presence of a 
dangerous amount of diagonal tension. Hence, it is evident 
that the shearing stress of concrete regarded as the measure 
of diagonal tension must be kept within limits which are 
so small as to be practically negligible. Thus, the practice of 
making no allowance for the shearing resistance of concrete is 
fully justified, and the recommendation, made by some other 
engineers, that concrete in shear may be subject to a stress of 
50 Ibs. or 60 Ibs. per square inch, would leave too small a 
margin of safety against failure by diagonal tension. 



BEAMS 



227 



Whilst failure by diagonal tension may be expected to occur 
in a beam having no web reinforcement if the shearing stress 
intensity were to attain about 100 Ibs. per square inch, recent 
well-authenticated tests have shown that beams when properly 
reinforced are capable of withstanding shearing stresses of 
more than 600 Ibs. per square inch 
without giving the slightest indication 
of failure by diagonal tension. 

One of the earliest, and still one 
of the most prominent, systems of 
reinforcing beams is that invented in 
1892 by F. Hennebique (Fig. 59), and 
used in the General Post Office, London, 
and numerous other large buildings. 
Round bars are used, arranged as 
shown. The bars are hung from 
strips bent in the form of stirrups, the latter taking the 
shear, and the inclined bars the diagonal tension. These 
stirrups are placed uniformly, except at the ends of the span 
where the shear stress is greater ; there they are placed closer 
together (Figs. 60 and 61). 

The bent up ends of the bars lie fairly across the lines of 
rupture near the supports of the beam, and afford in them- 
selves very secure anchorage. The vertical stirrups are formed 




FIG. 60. Hennebique 
Stirrup and Bar. 




FIG. 61. Bars and Stirrups (Hennebique System). 

so that they can be spaced at proper intervals along the beam 
to provide for variations of stress from point to point. They 
cross numerous lines of maximum tension (Fig. 68), are, there- 
fore, of great efficiency, and being vertical they facilitate the 
operation of ramming the concrete without causing the risk of 
displacement. Being made with a simple spring clip at the 
lower end, the stirrups are automatically held in position on 
the main bars, thereby obviating the necessity for temporary 
wedges (Fig. 63) or ties. Finally, the upper ends of each 

Q2 



228 



REINFORCED CONCRETE 



stirrup are bent over at right angles so as to ensure perfect 
anchorage in the concrete, and to make each stirrup an efficient 
connection between the tension and compression areas of the 
beam in which they are embedded. 

In discussing the relative efficiency of inclined and vertical 
stirrups, Professor Turneaure has pointed out that while an 
inclined stirrup, or the bent end of a horizontal bar, is in a 
position to take stress immediately, a vertical stirrup is more 
effective in resisting vertical distortion of the concrete. In 
his opinion, the stirrups should be looped around the horizontal 
bars so as to be firmly anchored at their lower end where the 
stress is a maximum, but that attachment to the bars is not 
necessary, as the object of the stirrup is to prevent vertical 
or nearly vertical distortion. 

The Hennebique vertical stirrups also form an effective 
web-connection between the tension and compression portions 




FIG. 62. Reinforcement for Beam with Compression Bar and 
Double Stirrups (Hennebique). 

in reinforced beams. It has been shown (Fig. 68) that the lines 
of maximum tension assume diagonal directions towards the 
ends of a beam, and these can be resolved into two components, 
one vertical and the other horizontal, the former being taken 
entirely by the stirrups and the latter by the horizontal bars. 
This fact is made use of in advocating the adoption of vertical 
stirrups, as in the Hennebique system, in preference to inclined 
ones. 

The mechanical bond in the Hennebique system is secured 
by flattening and opening out the ends of all bars so as to form 
a secure anchorage, and even in the most simple beams at 
least half the bars are bent up towards the supports, thus 
giving further security. 

Where unusually heavy loads have to be carried by beams, 
whose dimensions must be kept within comparatively small 
limits in order to comply with structural or architectural 



HENNEBIQUE SYSTEM 229 

requirements, and in some structures where the spans of 
continuous beams are liable to variable and unequal loadings, 
such as the main beams in bridges, viaducts, wharves, piers and 
jetties subject to heavy rolling loads, as of railway rolling- 
stock or other vehicles carrying considerable weights, it is 
desirable to use compressional reinforcement. In the Henne- 
bique system the beams are provided with stirrups, as before 
described, for withstanding tension on diagonal planes, and 
in addition with a series of inverted stirrups passing over the 
upper bars and anchored in the lower part of the beam. Both 
the compression and tension bars are carried across the supports 
or through columns, and so perfect connection is provided 
between adjoining spans. 

The complete arrangement, illustrated in Fig. 62, shows that 
the systenTof main bars and double stirrups ensures ample 




FIG. 63. Keedon Bar for Beams. 

resistance to horizontal tension and compression, and to tension 
and compression in diagonal directions, while at the same time 
it constitutes, after incorporation in the surrounding concrete, 
a truss of great rigidity and extraordinary capacity for with- 
standing deflection. 

Although one of the earliest forms of reinforcement, the 
general arrangement of the bars, stirrups and ties originated 
by F. Hennebique still form the basis of the most important 
" systems " now in use. The variations made by other 
engineers and patentees occur chiefly in the shape and mode of 
attachment of the shear members, in the use of bars^ of special 
surface intended to secure greater adhesion between the 
concrete and steel, and in the elimination of certain members 
or the reduction in size of others, with a view to reducing the 



230 REINFORCED CONCRETE 

amount of steel used. Some of these " improvements " are 
of great value, e.g., the rigid attachment of the shear members, 
and the indentation of the surface of the main bars but they 
are usually accompanied by other disadvantages, so that the 
engineer or architect must study the whole matter with full 
regard to local requirements before selecting any " system " 
or arrangement to suit a particular case. 

If workmen could be relied upon to follow instructions 
implicitly, and if the time occupied were of secondary impor- 
tance, there can be no question that the arrangement of the 
shear members to suit each case as it may arise would be 
best, and the Hennebique system would then be the best 
basis on which to work. As the risk of inserting the shear 
members in the wrong places, and of displacing them during 
tamping, to say nothing of other carelessness, must be reckoned 
with in actual practice, the use of one of the following systems 




FIG. 64. Coignet Reinforcement for Beams (latest type). 

is often convenient and may, on occasion, prove even more 
satisfactory. This is not so much due to any defect in the 
Hennebique system, but to the workmen using it, and the 
aim of the engineer and architect using concrete should always 
be to secure the best results whilst leaving as little as possible 
to the intelligence or integrity of the workmen employed. 

For all ordinary structures, the designs of reinforcement 
suggested by different firms of " concrete specialists " approach 
so nearly to a common standard that there is little to choose 
between them. Each of these firms can offer an abundance 
of evidence of such good work that on this alone no decisive 
choice can be made. 

For special purposes, on the contrary, each of the leading 
designs has some characteristics of value not possessed by the 
others, and the architect or engineer must, therefore, study 
these variations in detail, and choose the system which is 
best adapted to the special needs of the case. 



COIGNET SYSTEM 



231 



In the system invented by E. Coignet (Figs. 64 and 65), round 
bars are also used together with transverse rods of smaller 
diameter on the upper or compression side. This enables the 
main beams to be made first, then hoisted into position and the 
floor slabs fitted afterwards. The stirrups in this system consist 
of round bars three-sixteenths to one- quarter inch in diameter, 
the ends of which are twisted to form a loop. These loops are 
placed over both top and bottom bars, and are tied in place 
with wire. 

The Kahn Trussed Bar (Fig. 66) is of special shape, the section 
of the main portion being that of a diamond. This bar is 
provided at frequent intervals with supplementary bars bent 




FIG. 65. Section of Coignet Floor (old type). 



at an angle of 45 degrees so as to lie along the principal lines 
of stress (Fig. 67), and at right angles to the main lines of 
rupture. 

The objection to this arrangement is that the lines of maxi- 
mum tension only assume the angle of 45 degrees at the neutral 
axis and thereby reduce the value of the bent bars, and in 
practice they are almost certain to be bent during the ramming 
or tamping of the concrete unless an undesirably fluid mass 
is used. 

When the supplementary bars or stirrups are inclined and 
rigidly connected to the bar, thus delivering their strain into 
it, the tensile stress then existing in the horizontal reinforce- 
ment is not only that caused by the adhesion of the concrete 



232 REINFORCED CONCRETE 

to it, but also the summation of the horizontal components 
of the strain in each of the diagonals. We then notice that 
the principles of truss action begin to appear (see Fig. 68). 
By embedding the bars above described in concrete, a com- 
posite truss is formed in which the tension members are steel, 
and the missing compression members are furnished by the 
concrete. 



Note 

rigid 
connection 



FIG. 66. Kahn Trussed Bar. 

The rigid connection of the bent bars with the large hori- 
zontal bar is claimed as a special advantage by the makers, 
who rightly state that the transfer of the stress from the shear 
members to the main member can only be accomplished by 
some definite connection between them which can only be 
obtained by a rigid attachment. 

It is claimed by some engineers that the concrete surrounding 



KAHN BARS 



233 



Gcction of Bor 



the bars will prevent the slipping of the loose stirrups or tied- 
on bars, but actual tests have proved that slight slipping does 
sometimes occur. The objection to all bars in which the 
auxiliary reinforcement is placed at fixed points is that the 
position of every bar, rod, or strip of steel, if of uniform 
section, ought to be settled 
by the designers of such 
bars conformably with the 
stress diagram for each e ^FIG 67^ 

structural member, or, if 

the auxiliary or web reinforcement is in the form of bars 
placed at equal distances apart, then the cross-sectional 
area of the bars should be progressively varied from point to 
point. Neither of these methods of variation is, however, 
possible in the case of patent bars where the web reinforcement 
is formed by shearing and bending up part of a projecting 
flange rolled with the main bar. 

A practical disadvantage of bars with web members attached 
at the sides is the excessive width of the bars in proportion 



'.>r>ci of ^rinci.oa/ compresses Sfrt 33 




i 1 

Lrrics of prinr/uol 




. 68. Showing position of Shear Members in Kahn Bar 
and Lines of Stress in Beam. 



to their tensile resistance. In consequence, bars of this type 
cannot be used with economy in beams having to carry heavy 
loads, as the width of the beams must be very great to enable 
the requisite number of bars to be applied, the excessive width 
naturally representing waste of material and labour. More- 
over, the width of the bars is apt to constitute an obstacle 
to the flow of concrete, making it difficult, if not impossible, 
to fill every part of the moulds, and to obviate the presence of 



234 REINFORCED CONCRETE 

voids between and under the reinforcement. The use of a 
very fluid concrete only introduces more serious difficulties, 
as it fills the moulds with concrete of irregular composition, 
because the aggregate and heavier particles of sand must 
necessarily settle to the bottom. 

The most rational method in the design of reinforcement, 
whether in beams, columns, piles, or other structural details, 
is to employ forms of steel which permit the engineer to vary 
the spacing of the main and auxiliary bars at pleasure, and 
to determine by calculation the number and sectional area of 
the stirrups as demanded by the intensity and distribution of 
the stresses in every part of the construction. 

That is the method adopted in the Hennebique System ; it 




PIG. 69. Moss Bar. 

is generally approved by some of the most eminent authorities 
and confirmed by the many perfectly satisfactory structures 
in which it has been used, which occur in every part of the world. 
Yet there are many engineers and architects who feel that they 
cannot trust the workmen engaged in placing the concrete to 
take sufficient care in putting the stirrups in the corrrct 
positions and in ensuring their non-disturbance during tamping. 
Such engineers and they are very numerous are naturally 
willing to forego some of the minor advantages of the loose 
stirrups in order to secure the shear members from slipping 
out of place and so rendering possible a serious collapse of the 
whole structure. Consequently, they prefer fixed bars of the 
Kahn, Moss or similar type. 

When used for columns, Kahn bars are placed vertically, 
one near each corner of a square a little smaller than the 



MOSS SYSTEM 



235 



average area of the column, with the trusses or stirrups pro- 
jecting towards the interior of the column (p. 222). 

In the Moss system (Figs. 69 and 70) the bar is of I section 
with a large bottom flange ; inverted stirrups are fastened along 
the bar, and at an angle to it so as to form trusses with some 
resemblance to the Kahn bars. The arrangement of the 
stirrups depends upon the loading of the bar. 

The Indented Bar system uses bars of square or round section 
with projections or indents on each side (Fig. 49 and 71), these 
indents being sufficiently deep to prevent the concrete slipping 
along the bar. These bars are made of steel with a somewhat 
higher proportion of carbon than is usual in reinforcement, as 
such steel can be stressed more severely. The ends of .the bars 
are bent where required to take additional shear stresses. 




None THE POSITIVE BOND 
WITH THE RIGID CONNECTION 



FIG. 70. Isometric View of Moss's Patent Girder Reinforcement. 

In addition to the advantages mentioned on p. 219, the 
makers of the indented bars claim that with their bars " there 
is so close a bond between the concrete and the steel that it 
prevents undue extension of the concrete towards the beam 
ends, and the concrete is thus bound together where it is 
exposed to the greatest shearing stress and diagonal tension, 
so that it is able to resist shear of itself to a far greater 
extent than is the case when diagonal tension cracks can 
take place in the concrete and thus reduce its resistance. 
There is no doubt that the concrete itself plays a very 
important part in resisting shear when it is absolutely prevented 
from cracking by the use of a mechanical bond bar." 

A further advantage claimed in favour of the use of indented 
bars is that, with them, " tests and actual practice conclusively 



236 



REINFORCED CONCRETE 



prove that no cracks of sufficient size to admit moisture to the 
steel can occur in the concrete until the yield point of the steel 
has been reached. At this point, however, the whole member 
will be practically disintegrated owing to the complete inability 
of the concrete to stretch to the same extent as the steel. 
In other words, the yield point of steel reinforcement is the 
critical point at which the structure will fail, and it is utterly 
fallacious to consider the ultimate or breaking strength of the 
steel as the critical point." 

The objection is sometimes raised that the ridges and sharp 
angles on indented bars injure the concrete when the structural 

members are in a 
state of strain. The 
effect is, however, too 
small to be appre- 
ciated in practical 
construction, and no 
failures have yet 
occurred through it. 

Spiral Bars have 
spiral grooves run- 
ning along their outer 
surface and have the 
advantage over 

smooth, round bars of rendering crooking or fishtailing the 
ends of tension bars unnecessary. The spiral grooves secure 
a strong mechanical bond and enable the bars to be used for 
working stresses up to 30,000 Ibs. per square inch or 50 per 
cent, more than is the case with smooth bars ; consequently, 
a large reduction may be made in the quantity of metal 
used. The lateral bending resistance is also much greater 
than that of smooth bars. 

In the Expanded Metal system the reinforcement consists of 
mesh work (Fig. 72) made by cutting slots in a sheet of metal 
and then pulling it transversely until a network is formed. 

The usual types of expanded steel used in reinforced concrete 
construction are the 3-inch diamond mesh, and the rib mesh, 
a few 1 J-inch and 6-inch diamond meshes being also occasionally 
used for such work. The lighter weights of the |-inch and 




FIG. 71. Indented Bars. 



EXPANDED METAL 237 

1 |-inch diamond meshes are frequently used in concrete for 
encasing steelwork. 

The " 3-inch diamond mesh " measures three inches by 
eight inches from centre to centre of its intersections or junc- 
tions. The size of the mesh is constant, but according to the 
thickness of the sheet from which it is made the strands vary 
in sectional area, and thus several weights are produced. 
Each intersection is twice the sectional area of its strand, and 
there are eight strands per foot run short way of mesh. 

It is a curious fact that tests prove that the process of 
expanding raises the elastic limit and increases the ultimate 
strength, thereby greatly improving it. 

In the rib mesh the ribs are 
constant in cross-section, but 
are spaced at varying centres. 
The total area is available for 
reinforcement in both the 
" diamond mesh " and " rib 
mesh." 

The rib mesh (Fig. 73) 
expanded steel consists of a FIG. 72 -Expanded Metal. 

Diamond Mesh, 
series of straight ribs, or mam 

tension members, which in the process of expansion are 
left rigidly connected by light cross ties which act as 
spacers. It is essentially a bar reinforcement without some 
of the disadvantages of the latter, for it is obviously an improve- 
ment on ordinary bars for slab reinforcement to have them 
attached together as in a sheet of rib mesh expanded steel. 
It is cut in various meshes, in sheets up to twenty-four feet 
six inches long, which are made from one original section of 
steel by cutting the light cross ties shorter or longer, so as to 
allow of the ribs being opened out less or more widely. While 
rib mesh expanded steel differs from diamond mesh expanded 
steel, in that the meshes are square instead of diamond shape, 
calculations of safe working loads for the two materials are 
based on the same tensile strength, so that the one mesh may 
be substituted for the other so long as the same sectional area 
is used. 

The advantages of expanded steel as reinforcement for 




238 REINFORCED CONCRETE 

concrete make it specially useful for plain and curved areas. 
It is supplied in flat sheets ready for use ; it packs closely, and 
is easily transported, and quickly handled ; it is simple, 
economical and effective. The expanded sheets are machine- 




FIG. 73. Expanded Metal, Rib Mesh. 

made, and although of network formation there are no loose 
strands, as the junctions between the meshes remain uncut 
during the process of manufacture, and thus the strands or 
members are all rigidly connected, and have continuous fibres. 



NETTING REINFORCEMENT 239 

The meshes key into each other and consequently interlock 
where the sheets overlap at joints, thus the reinforcement may 
be made continuous no matter how large the area be treated. 
Excellent mechanical, as well as cross bond, and anchorage 
are obtained with expanded steel, and owing to its peculiarity 
it cannot slip within the concrete, for this is most efficiently 
locked within the meshes. 

The special feature of expanded steel is that it is a solid 
sheet of network formation wherein all the strands or members 
are all rigidly connected, and when in position cannot be 
displaced by the laying and tamping of concrete. It is there- 
fore quite reliable as a reinforcement as the steel goes where 
it is planned to go without requiring skilful setting out on the 
part of the workmen. 

In this respect it is superior to loose bars placed by measure- 
ment and tied at intersections with wire, for if one bar is lower 
than another it is evident that the former will be more highly 
stressed than the latter, and the full value of the total reinforce- 
ment will not be obtained. 

The distribution of stress by expanded steel is such that 
wherever a load may come there is steel to transmit it in all 
directions, so that a load does not affect merely the portion of 
slab directly under it. When a concentrated load comes on 
a slab reinforced with separate bars, only the bar under the 
load is affected ; with the expanded steel the mesh distributes 
the stress in all directions. Expanded metal sheets are manu- 
factured in various sizes up to sixteen feet the long way of 
the meshes, and there are some seventy-six varieties with 
meshes one and three sixths inches to six inches wide. The 
manufacturers claim that expanded metal saves about 75 per 
cent, of the bulk of the concrete when used as a tension bond. 
It is specially used as lathing for partitions and in floors, but 
is equally available for beams, stanchions and columns, bridge 
work, reservoirs, conduits, sewer pipes and retaining walls. 
For beams, a square bar is rolled with a lateral rib ; the latter 
expands upwards so as to form a mesh which takes the place of 
the stirrups in other systems. 

Richard Johnson, Clapham and Morris, Ltd., use a form of 
wire netting (Fig. 74) as reinforcement. For many purposes 



240 



REINFORCED CONCRETE 



this is quite satisfactory, but for heavy work drawn wire 
network is best avoided. 

Another form of reinforcement recently introduced into 
this country is the " triangle mesh " supplied by the United 
States Products Co. It is made of hard drawn cold steel wire 
with an elastic limit of 22 tons per square inch. There are 
no welds in this reinforcement, which, being of the hinged 
joint type, is flexible without producing initial stresses when 
bent (Fig. 75). 

A large number of other shapes of reinforcing bars and 
meshes have been devised, but the foregoing are sufficient for 
the student to gain some idea of the arrangements most 
commonly used. 

The reinforcement in arches and arched bridges constructed 
of reinforced concrete serves three purposes, namely, to take 




FIG. 74. Johnson's Netting Keinforcement. 



compression, to resist tension, and prevent shearing and 
temperature cracks. The reinforcement is designed in con- 
formity with the modern theory of the elastic arch, and hinges 
are often introduced so as to ensure the line of stress passing 
through given points. The design should prevent tension in 
the arch ring, and the reinforcement chiefly serve to assist in 
the resistance of the compressive stresses ; consequently, large 
T-shaped steel members are frequently adopted. 

Trussed bridges of various forms are constructed in rein- 
forced concrete, but no special principles are introduced in 
their design, the members consisting of posts, perpendicular 
or inclined, and beams of cantilevers. 

For bridges, etc., reinforced concrete has one marked 
advantage over most other materials, in that it may indicate 
conditions of maximum allowable tension in its embedded 



ARCHES AND BRIDGES 



241 



steel before actual danger exists. This advantage rests in the 
fact that the coefficient of elasticity of the concrete and of 
the embedded steel do not bear the same ratio as their allowable 
stresses. When the embedded steel is stressed to 5,000 Ibs. 
per square inch, invisible cracks occur in the surrounding 
concrete. At 10,000 to 15,000 Ibs. per square inch these 
cracks become visible. At 20,000 Ibs. per square inch they 
frequently become objectionable. 

In arch design, where the stresses in the arch are magnified 
by the behaviour of the spandrel walls, cracking of the concrete 




FIG. 75. Triangle Mesh Eeinforcement. 

serves the purpose of an extensometer to detect excessive 
stresses. If an arch is designed too flat at the crown, cracks 
will appear in the spandrel near the ends of the span. If too 
flat at the haunches, cracks will appear in the coping over the 
crown or through the arch ring at the haunches directly under 
the spandrel only, and not extending far into the soffit of the 
arch. The tensile stress in the arch itself is rarely sufficient 
to show cracks actually penetrating the arch ring. Small 
cracks, particularly in the spandrels, are no indications of 
failure, being merely the magnified effects of movements in 
the arch ring, but a properly designed and erected arch will 

C. B 



242 



REINFORCED CONCRETE 



be free from such cracks, if provided with expansion joints 
in the spandrels above each springing. 

Reinforced concrete is now extensively used in the construc- 
tion of conduits, water mains, and sewers. The latter are usually 
constructed in situ that is, the concrete is mixed and placed 
to set in the position it will finally occupy, although a great 
number of concrete sewer pipes of large diameter are manu- 
factured and sold as an ordinary market commodity. Water 
and drain pipes are occasionally constructed in situ, but are 
usually made in moulds. The reinforcement is generally of 
meshwork, preferably with the warp of spiral form, the strands 
perfectly crossing the pipe at an angle (Fig. 76), and not running 
longitudinally and at right angles to the length. The spiral 
reinforcement serves to resist the bursting pressure. Expanded 

metal and wire 
meshwork have 
been largely used 
for this purpose. 
In the Bonna 
system, special 
cruciform bars are 
wound spirally 
inside and outside 

a thin sheet of 
FIG. 76. Coignet Pipe. ^^ the gteel 

serving to prevent contamination by penetration of external 
moisture in the soil or percolation under great pressure. 

After being made, concrete pipes must usually be kept at 
a temperature of 50 F. or above, protected from direct sun- 
light and air currents, for at least seven days, during which 
period they must be kept moist by sprinkling with water. 
They should not be removed to the open air until they are at 
least a month old. If the pipes are to be " steam cured " 
they are placed in an autoclave and subjected to the action 
of moist steam for about forty-eight hours, if the steam is at 
or below 212 F., or for a much shorter period if the steam is 
under considerable pressure, as described later under " Lime 
Sand Bricks." 

For docks, reservoirs, aqueducts and water tanks of all sizes, 




PILES 



243 



reinforced concrete has proved about 15 per cent, cheaper 

than mass concrete or bricks, and about 20 per cent, cheaper 

than stones, and in addition there are practically no charges 

for maintenance or repair. 

Silos, magazines for explosives, 

coal bunkers, as well as tanks 

for oil, brine, and many other 

fluids, are proving perfectly 

satisfactory providing that 

they are well designed and 

the concrete is properly made 

and placed. 

Piles are constructed of 

concrete in a manner similar 

to columns when they are cast 

in place, but piles cast in a 

mould are not usually rein- 
forced so strongly. Concrete 

piles are superior to wood 

both as regards strength and 

durability, and are much 

cheaper than piles constructed 

exclusively of steel. 

The essentials of a good pile are the following : 
(a) That it shall be capable of being driven into the ground 

(either soft or hard, wet or 
dry) to such a depth as will 
enable the buried portion to 
support the weight of some 
superstructure, or to withstand 
I force applied against it in a 
I horizontal direction, or to resist 
forces applied in any other 
desired direction. 

(6) That the projecting 
portion of the pile shall be 

capable of supporting axial vertical loads with perfect safety, 

and of supporting eccentric vertical loads without appreciable 

flexure. 

R2 




FIG. 77. Overloaded Column, 
with Insufficient Longitudinal 
Reinforcement. 




FIG. 78. 



244 



REINFORCED CONCRETE 



(c) That, if the pile is to be moulded before use and not 
" cast in place," it shall possess rigidity and elastic strength 
sufficient to permit it to be transported, slung, and driven 
without injury. 

Piles may be circular, square, or any other cross-section, 
the usual shape for foundation being that of a square with 
chamfered edges, whilst rectangular piles are used for sheeting. 

The reinforcement usually consists of stout longitudinal bars 
with suitable wire ties, the construction being very similar to 
that of columns, and similar considerations apply to them 
(see p. 221). The metal exposed in cross -section varies from 
2 to 5 per cent., and is almost invariably of round rods, clamped 
and tied as in the columns. 

The following table gives particulars of Hennebique piles 
made and used at Southampton : 



Section of Pile 
in inches. 


Reinforcement. 


Percentage of 
Steel in 
Cross-section. 


12 X 12 


4 rods 1| inches diameter. 


5 


14 X 14 
15 X 15 
16 X 12 


-"-4 " 

> -*- U !> ?> 

1 5 

55 X 8 5> > 


2 i 
2 1 
4J 



It is important that the longitudinal rods should be of ample 
diameter, as thin rods may prove disastrous (Fig. 77). 

As the power required to drive a pile is largely, if not entirely, 
due to friction between its surface and that of the surrounding 
earth, a given volume of concrete in the form of a hollow 
cylinder should be more effective than if applied as a solid 
cylinder, or conversely, concrete may be saved by using a 
hollow pile of the same external dimensions as a solid one. 

This point is illustrated in Fig. 78, where the left-hand 
diagram represents the cross-section of a 13-4 inch diameter 
solid cylinder with the area of 141 square inches and the 
circumference of 42 inches; and the right-hand diagram 
represents the cross-section of an 18-inch diameter hollow 
cylinder with the net area of 141 square inches and the circum- 



PILES 



245 




^- ,-. JCi. 



ference of 56-5 inches. Thus, for the same area of material, 
the circumference of the hollow cylinder is over 33J per cent, 
more than the circumference of the solid cylinder, and the 
bearing power of a hollow pile can readily be 
made one-third greater than that of a solid pile 
without increasing the cost in the slightest degree. 
L. J. Mouchel has therefore effected a notable 
improvement by the construction of hollow piles 
with a reinforcement based on the Hennebique 
system. 

For practical reasons it is generally desirable 
to employ piles of rectangular form, and for this 
reason the Mouchel patent hollow piles are 
usually made with the cross-section shown in 
Fig. 79, thereby sacrificing a small proportion of 
the saving that could be effected by 
the adoption of the circular form in 
order to gain advantages which 
appeal to the engineer using piles as 
structural members. 

The head and point of the pile are 
usually protected by steel caps. 
When casting piles in place the re- 
inforcement is first erected and is 
surrounded by the necessary centering 
or forms. The concrete is put in in 
small quantities and carefully tamped 
around the reinforcement. It is most 
convenient to erect three sides of 
the form and to build up the fourth as 
the addition of concrete proceeds. 
After three days the forms are 
removed and the pile is left for 
several weeks in order that it may harden properly. 
Coignet Pile. After ^his ^ is ready for driving. The piles must be 
made of exactly the required length as it is costly 
to cut them. Piles which have been treated in steam for three 
days are ready for driving at the end of this period ; the steam 
or high temperature greatly increase the rate of hardening. 






FIG. 79. 

Mouchel 

Hollow Pile. 



246 



REINFORCED CONCRETE 



Chimneys constructed of reinforced concrete can be erected 
by well-organised firms in about half the time and for about 
half the cost of brick chimneys of the same size. The larger 
the chimney the greater is the saving, as a large brick shaft 




FIG. 81. 
Hennebique 
Pile, 



FIG. 82. Hennebique 
Sheet Pile. 



FIG. 83. Pile with Solid 
Core. 



CHIMNEYS 



247 




must have a very thick base in order to 
provide the necessary support. The weight 
and space occupied by a concrete chimney 
are also less than a brick shaft, this being 
important where the foundation is treacherous, 
and it is also claimed that a reinforced con- 
crete chimney, if properly designed, has a 
greater stability than one of brick or stone 
on account of its monolithic character. The 
resistance of concrete to the heat and abrasive 
action of flue gases appears to meet all 
requirements, and experimental blocks of 
concrete placed in brick chimneys were found 
to have a greater strength than similar blocks 
stored for an equal time under water. It is, 
however, necessary that the concrete should 
be well set before being heated ; two months 
or more should be allowed to lapse before 
the chimney is used. 

A temperature of 520 C. is considered to be 
the highest safe temperature for large concrete 
chimneys, though most concrete can be heated 
on one side only to 900 C. for several hours 
without ill effects. It is, however, essential 
that only sand be used as aggregate, as stone 
" flies " under the action of heat. 

There have been several serious failures of 
concrete chimneys, especially in the United 
States. In almost every case these have 
been traced to faulty workmanship in con- 
struction and not to errors in design. This 
is due to the use of a safety factor of five 
for transverse resistance and to allowing for 
stresses set up by a wind travelling at the 
rate of 100 miles per hour a velocity greater 
than that of a cyclone. A maximum wind 
pressure of 50 Ibs. per square foot on a square FIG. 84. Con- 
shaft, and two-thirds of this on a round one is jidere Pile with 
i I-, bpiral Rein- 
ample allowance. forcement. 



248 



REINFORCED CONCRETE 



Engine and dynamo beds and foundations for machinery are 
more free from vibration when made of concrete than of brick- 
work or stone. The bed can also be constructed to any shape 
at a cheaper rate than when other materials are used. 

Foundation rafts in boggy or " quick " ground are made of 
reinforced concrete, and thus enable large buildings to be 
erected on what would otherwise be too treacherous a 
foundation. 

Floors made of concrete are usually constructed of iron girders 
or concrete beams (Fig. 85) placed fairly close together, the 
intervening space being occupied by slabs 3| inches to 6 inches 
thick, formed of a 1:2:5 or other suitable mixture, and 
reinforced with T Vinch rods placed 4 inches apart, or with 




-Expanded Metal Lathing 
to be fixed when Plastering is required 



//$ mesh Expanded Steel - 
for Concrete Encasing 




-Reinforced Concrete Beam 
Expanded Sted Bars 



Rein forced Concrete Beam 
Reinforcing Rods 



FIG. 85. Use of Expanded Metal for Beams and Floors. 

network reinforcement so that the whole structure can carry 
a load of 3 cwt. per square foot in addition to the weight of 
the floor itself for factories, or 1| cwt. per square foot for 
houses and public buildings. Many designs of floors are in 
use, some of them for very large spans. 

Paving blocks and floor slabs made of concrete are used 
increasingly in districts where natural stone is costly. Curbs 
for pavements are cheaper when made of concrete than of 
sandstone, and are equally durable. 

Building blocks of concrete are also made in large quantities, 
the claim being made that as a builder can lay fifty blocks 
2 feet X 1 foot X 9 inches per day, the use of such blocks is 
cheaper than ordinary mass concrete. The plastic concrete is 



FLOORS AND BLOCKS 



249 




placed in moulds (Figs. 87, 88) of cast iron or mild steel, 
and is gently tamped until the surplus water rises to the 
surface. After being left for a short time until the cement 
has set, the sides of the mould are allowed to fall and the 
block or slab is removed and stored in a cool shed until it is 
hard enough for 
use. The square 
slabs used for 
paving are fre- 
quently made in 
a strong wooden 1 
or metal frame, 
which is laid on 
a piece of 
matting or 
canvas on a 
smooth level 
floor. The con- 
crete is placed 
in this frame, 
tamped care- 
fully, and the 
surplus material 
removed with a 
long - bladed 
knife. After 
the cement has 
set, the frame 

is removed leaving the paving block on the canvas. If a 
sufficiently fine aggregate and a fairly rich mixture is used 
the frame may usually be lifted off as soon as the tamping 
is finished. The canvas gives a pleasing texture to the surface 
of the slab and renders it non-slipping. Such slabs are not 
reinforced in the ordinary sense of the term, the metal (if any) 
embedded in them being intended to prevent them spalling, 
and not to increase their tensile strength. 

When the number of blocks or slabs is sufficiently large a 
machine is desirable. Care should be taken in selecting one 
for this purpose, as some of those on the market are far from 




FIG. 86. Concrete Building Blocks. 



250 



REINFORCED CONCRETE 



satisfactory. Among the best is the " Winget " concrete block 
machine, which consists of a mould box with hinged sides and 
ends carried in the frame hung in trunnions, as shown, with core 
blocks set inside. When the lever at the right-hand side of the 
illustration is pulled down the bottom of the mould box (which 
is formed with a loose pallet inserted for each block) is lifted up, 
and at the same time the sides of the box fall outward, leaving 
the finished block on the pallet ready to be carried off. 
When it is required to use the machine in the face-down 

position the frame 
carrying the mould box 
is swung over in the 
trunnions into a hori- 
zontal position with the 
same lever on releasing 
the stop controlled by 
the small horizontal 
lever at the right-hand 
end of the machine. 
The cores are carried on 
a fixed bed plate under 
the pallet forming the 
bottom of the mould 
box. 

The movements neces- 
sary to make a block 
are the fewest and 
simplest possible, so that the maximum output is secured 
with a minimum cost for labour. If the instructions given 
with the machine are followed, two men can make about 400 
blocks or 800 slabs per day. 

Concrete roads are usually made of mass concrete without 
reinforcement, except for the paving curbs, but there is a 
movement in favour of making the whole road of reinforced 
concrete, and using the hollow space beneath it for sewers, 
gas pipes, etc. 

Lintels have steel joist bars or rods embedded in them, but 
the distribution of these varies greatly in different cases. 
A 1:2:4 mixture is generally employed. 




FIG. 87. Mould for Slabs. 



STAIRS AND STEPS 



Tiles made of cement are used for both roofing and flooring. 
They are made in metal moulds consisting of a lower box or 
container into which an upper plate or plunger fits closely. 
The plunger is lifted, its lower surface and the inside of the box 
are oiled or wetted, the box is filled with concrete paste, and 




FIG. 88. " Winget " Block-making Machine. 

the plunger is brought down so as to compress the mixture, 
the tile is turned out on to a bench, and in a few hours is ready 
for use. About 500 tiles can be made in a day by one man. 

Stairways and steps made on the site are reinforced longi- 
tudinally in a manner similar to beams and transversely near 
the front of the tread, with vertically placed studs in order to 



252 



REINFORCED CONCRETE 



reduce the tendency of this portion of the steps to spall or 
break away under traffic. The treads and risers are also cast 
solid, and sold ready to be fixed as stone stairs. 

Railway sleepers are used in large numbers in the United 
States. Though slightly more costly than wood in the first 
instance, they are reported to be more durable, and so become 
more economical. 

Telegraph poles and tramway standards are cheaper in concrete 
than in steel. They are of tubular form, and must be carefully 
made in such a manner that the reinforcement does not slip 
during the tamping. 



Nl EM Lathing 



f\l 92 Expanded Metal Lathing 




FIG. 89. Use of Expanded Metal in Stairs. 



Pit props, gate posts, boats, pontoons, garden ornaments, and 
many other useful and ornamental articles are also made of 
concrete. In fact, new uses of this remarkable substance are 
constantly being found. Some of these uses are fantastic, in 
others the naturally suitable materials are discarded in favour 
of concrete, merely because of a desire to use the latter material, 
but in the majority of cases concrete may rightly be used in 
situations where other materials are less readily available, and 
in circumstances where concrete is particularly suitable. 

In all concrete work it is essential that ample precautions 
shall be taken to secure the concrete being properly made and 
placed, to prevent the forms being taken away too hurriedly. 



CAUSES OF FAILURE 253 

and to ensure that no pains are spared to keep the surface of 
the material properly wetted when such treatment is necessary. 
The repeated failures of concrete construction have, in almost 
every case, been due to the use of concrete under unsuitable 
conditions, or to the improper manipulation of the materials 
and forms rather than to inherent disability in the design of 
the structures. Fortunately, concrete structural work tends 
increasingly to become a separate occupation, and this will 
reduce the risks of its use, and men will work more and more 
skilfully as they become accustomed to the material. The 
simplicity of many forms of concrete construction makes it 
specially attractive to farmers and others in isolated situations, 
but the risks of imperfect mixing, etc., then become more 
serious. Concrete is an invaluable material, yet those who see 
its advantages, but not its drawbacks, may easily find them- 
selves and others seriously inconvenienced, especially in the 
case of buildings erected by men unskilled in this branch 
of work. 

At the same time, even among the most skilled workers there 
is still room for much improvement in the preparation of 
concrete, particularly with regard to its strength and permea- 
bility. Grading the aggregate more carefully and into a 
larger number of portions than is at present customary, will 
probably prove to be the most efficacious, but the cost of this 
increased grading is so frequently found to be greater than that 
of an additional quantity of cement that it is difficult to increase 
the number of grades above that now in use. The study of 
the proportion of voids in the concrete and of the materials 
composing it will naturally give the clue to obtaining stronger 
and more impermeable concretes, and will not improbably 
result in a decrease in the use of waterproofing materials. 
That the concrete of the future will be superior to that of the 
past cannot be doubted by those who are closely in touch with 
the many attempts now being made to increase the stringency 
of the present standard specifications. 



CHAPTER IX 

SPECIAL PROPERTIES OF CONCRETE 

THE properties of concrete vary with its age and composition 
as well as with the quality of the cement used. 

The crushing strength of concrete is roughly proportional to 
the amount of cement in the material, the maximum strength 
being reached when all the voids are filled and each particle 
of material is coated with cement. As explained in a previous 
chapter, this usually corresponds to a weight of cement equal 
to ii times the amount of voids in the quantity of sand 
used in a batch of concrete. It is not usually wise to employ 
a concrete with a compressive strength after twenty-eight 
days which is below 2,400lbs. per square inch. (For further 
particulars see p. 126.) 

The tensile strength of concrete may be tested in the same way 
as that of cement (p. 128), but is usually assumed to be one- 
tenth that of the crushing strength. 

The shear strength of concrete is not known with accuracy, 
and appears to depend largely on the nature of the aggregate. 
Professor A. W. Talbot finds it to be about half the compressive 
strength, but one-tenth of the latter is the figure usually 
assumed. 

The effect of testing concrete in shear and also in lines of 
fracture, are shown in Fig. 90. The blocks tested were 
9 inches X 12 inches X 12 inches. The result obtained in the first 
case is very low and possibly due to some latent imperfection. 

The transverse strength of concrete has also been tested 
in certain instances. The results obtained, after reduction to 
their equivalent values, for a beam twelve inches square in 
cross-section, and twelve inches long between supports, are 
exhibited in Fig. 91. The weight given is the breaking load 
applied centrally. 

Shear tests are peculiarly difficult on account of the material 
bending instead of simply shearing, and it is not impossible 
that future experiments will give even higher results. 



ADHESION 



255 



The adhesion between concrete and steel is remarkably great, 
so that there need be no fear of any slipping occurring so long 
as the reinforcement is properly designed. It is upon this 
adhesion that much of the value of reinforcement depends. 



.- 9" 




10 Tons 



12s Tons 



2 to 1 



2 to ) 



FIG. 99. Shearing Strength of Concrete and^Nature of Fracture 
under Shearing Stress. (H. E. 'Jones.) 

The adhesion of concrete to steel cannot alone be depended 
upon to transmit the stresses from the reinforcement to the 
surrounding concrete. 

When the surface of the steel is smooth and free from obvious 
indentations or projections, all adhesion must be due to the 
cement particles enter- 
ing the microscopical 
irregularities on the 
surface of the metal, 
so that whilst the 
adhesion, under such 
circumstances, is often 
remarkable it may be 
greatly increased by 



20 



i 10 



Proportion of Aggregate to Matrix 

FIG. 91. Transverse Strength of 
Concrete. (H. E. Jones.) 



the use of a more 

irregular surface. 

Moreover, Professor Popplewell has shown that when steel 

bars are under stress the reduction in area which they 



256 SPECIAL PROPERTIES OF CONCRETE 

suffer destroys the adhesion if the bars of the steel are 
smooth, whilst the bar with ridges, indentations or other 
irregularities of surface retains its grip on the concrete 
until the latter is actually broken. Vibration of the concrete 
mass, as a whole, also reduces adhesion, and may be serious 
if completely smooth bars with no shear members are 
used. It is now well known that the shocks and vibration 
inseparable from the average building are sufficient to reduce 
appreciably and sometimes to destroy the adhesion when 
plain rods or bars are used. To guard against this risk 
many devices are employed by engineers who use plain bars, 
such as splitting and bending the bars at their ends to obtain 
anchorage. Such methods are not wholly satisfactory, and 
are often inadequate, and many types of deformed bars have 
been devised to overcome these objections. For some purposes 
there can be no doubt that the best form of reinforcement is 
a rigid network giving a dependable mechanical and cross 
bond ; for others, indented or ridged bars are preferable. 

Adhesion is measured by embedding a steel rod in a rectan- 
gular block of concrete and, after a suitable time, measuring 
the force required to pull it out. The area of the surface of 
the embedded bar must also be ascertained. The Joint 
Committee under the auspices of the Royal Institute of British 
Architects recommends a working adhesive stress of 100 Ibs. 
per square inch, though the actual force required to separate 
a rod from the concrete in which it is embedded varies from 
550 to 650 Ibs. per square inch. 

The adhesion is increased by coating the steel reinforcement 
before use with a thin slurry made of cement and water, and 
by employing ribbed or corrugated reinforcement bars instead 
of smooth ones. 

For information concerning the loads which can be carried 
by reinforced concrete see p. 213. 

In addition to its strength, many of the advantages of 
concrete as a structural material depend on certain of its 
properties, such as resistance to fire, sea water, shocks, etc., 
and it is therefore desirable to mention these special properties 
in somewhat greater detail. 

Fire Resistance. The behaviour of concrete buildings when 



FIRE RESISTANCE 257 

subject to fire has been the subject of numerous investigations 
and tests. Small changes in temperature, such as occur daily 
in tropical climates, cause a superficial cracking of the concrete, 
which is of no structural importance except as regards the 
appearance of the building. Much greater heat, as when a 
structure is "on fire," effects several changes, the chief of 
which are (1) the expansion of the material, which may 
endanger the stability of the structure ; (2) chemical dehydra- 
tion ; and (3) destruction of the cement and aggregates. All 
steel structures tend to expand when heated, and this has, in 
the past, resulted in much loss of property as the girders have 
pushed out walls in consequence of the expansion of the metal. 
In reinforced concrete the steel is so covered with concrete 
which has a low heat conductivity that its expansion is 
reduced to a minimum, but the durability of the material 
during a conflagration is entirely dependent on there being a 
sufficient thickness of concrete around the metallic reinforce- 
ment. The opinion expressed by the R.I.B.A. Committee 
that " Usually a cover of \ inch on slabs, or 1 inch on beams, 
is sufficient," is much too low where the fire is likely to be of 
long duration. The tests of the British Fire Prevention 
Committee have shown that 2 inches of concrete is much more 
likely to prove a safe minimum, with 2 \ inches for columns, and 
3 inches for beams. If the steel is thinly covered or is exposed 
at any part of the heated structure, its temperature will rise 
so rapidly that the concrete itself may be strained sufficiently 
to cause it to rupture. This caused the destruction of several 
concrete buildings in the San Francisco and Messina disasters. 

The action of heat on neat Portland cement is such that at 
temperatures of 300 or above, the cement is dehydrated and 
rapidly crumbles to powder. In well-made concrete structures, 
however, the heat-conductivity is so low that this integration 
is confined to the surface unless an unsuitable aggregate has 
been used. Hence the disintegration of concrete by fire is of 
academic rather than practical importance, few conflagrations 
lasting sufficiently long for serious destruction of the concrete 
to occur by the direct action of the heat. 

Unsuitable aggregates greatly reduce the resistance of 
concrete to fire. The igneous rocks form the most resistant 

c. s 



258 SPECIAL PROPERTIES OF CONCRETE 

aggregates, then broken bricks, sandstones, gravel, limestone, 
and furnace slag. Ballast especially if containing flint 
pebbles cracks badly on being heated. Coke and cinders are 
the least satisfactory so far as resistance to fire is concerned. 

Measurements of the heat conductivities of various aggregates 
made by Professor Woolson have shown that gravel has a 
high conductivity, limestone comes next, then igneous rocks 
and coke breeze come last with the lowest power of heat 
transmission. Care must, however, be taken that the breeze is 
almost free from sulphur. If pan breeze or cinders be 
substituted, destruction of the concrete through oxidation of 
the sulphur compounds is likely to occur (p. 151). The coke 
must not contain more than 5 per cent, of bituminous matter 
or it may, when thoroughly heated, continue to burn of its 
own accord. 

The heat conductivity of concrete made with sandstone as 
aggregate has been found by Professor C. L. Norton to be 
0021 to -0029 calories per square centimetre per centimetre 
per second per degree C.. or 150 to 200 B.T.U. per degree F. 
per square foot per inch thick per twenty-four hours. 
Untamped and coke breeze concrete give about half the above 
results on account of their lower density. 

Investigations of concrete structures after severe conflagra- 
tions show that whilst limestone, sandstone and gravel aggre- 
gates suffer under the action of severe fires and must be renewed, 
yet coke breeze is frequently only injured superficially and 
does not lose strength. Unexpected results of this kind make 
it difficult to decide which is the most suitable aggregate to 
resist fire. 

The stability of the structure during conflagration is also 
increased by connecting the reinforcement on the lower side 
of beams or floor slabs to that on the upper sides, as the latter 
are seldom injured by fire. 

In buildings more than usually liable to conflagration, the 
walls, ceilings and floors should be covered with a fire-resisting 
plaster, as this, if injured, can be cheaply renewed ; terra-cotta 
facings may be used with equal success though they are more 
costly. The terra-cotta must be suitable for the purpose, as 
if it is dense in texture it will fly to pieces when heated and will 



FIRE RESISTANCE 



259 



prove quite unreliable. Only porous terra-cotta should be 
employed, and the pieces should be of moderate thickness. 
Much of the " hollow tile " used in the United States has failed 
under fire because of its excessively thin web. The most 
serious disadvantage of terra-cotta for protecting steel work is 
the difference in expansibility of the two materials, whereby 
the steel not infrequently becomes exposed, particularly if, 
under the combined action of fire and a high wind, the terra- 
cotta cracks or falls away. 

In considering the action of fire on concrete, that of the water 
applied to quench the fire must not be overlooked. This is 
frequently more severe than that of the fire itself, as the hot 
concrete, when suddenly quenched, is extremely liable to 
crack and spall. 

The British Fire Prevention Committee has proposed the 
following standard requirement for floors intended to resist fire, 
and grants three groups of certificate according as the protection 
afforded by the material is " temporary," " partial," or " full." 


STANDARD CLASSIFICATION FOR FLOORS. 



Protection 


Least Dura- 
tion of Test. 


Minimum 

Temperature 
of Fire to be 
reached 
during Test. 


Minimum 
Load per 
Superficial 
Foot Dis- 
tributed. 


Minimum 
Superficial 
Area under 
Test. 


Minimum 
Time for 
Application 
of Water 
under 
Pressure. 


Temporary^; 


45 min. 
60 , 


1500 F. 
1500 F. 


Optional. 


100 sq. ft. 
200 


2 min. 


t Class A . 


90 , 


1800 F. 


112 Ibs. 


100 




al * ( Class B. 


120 , 


1800 F. 


168 


200 




n (Class A. 


150 . 


1800 F. 


224 


100 




\ClassB. 


240 , 


1800 F. 


280 


200 





Closeness of surface texture is often an important factor in 
determining the heat-resistance of a concrete structure. 
Hence, concretes containing a larger proportion of cement will 
usually prove better than leaner ones, a concrete made from a 
wet mixture will prove more resistant than one made with less 
water, and a fine aggregate will prove better than a coarser 
one. In this connection it is important to note that concrete 
surfaces which have been treated with fluid water-glass mixed 

S 2 



260 SPECIAL PROPERTIES OF CONCRETE 

with three times its weight of water, the treatment being 
repeated after twenty-four hours, appear to have a much greater 
resistance to fire than the same concrete which has not been 
so treated, as the siliceous coating greatly reduces the rate of 
dehydration. 

Sharp corners in concrete tend to be more seriously affected 
by fire than those which are rounded, as the sharp angles 
break or spall off more readily. 

Resistance to fire is of minor importance in some buildings, 
whilst it is highly essential in others, so that different forms of 
concrete must be used to meet different requirements. This 
is the more necessary as a floor which affords full protection 
against fire may not be strong enough to meet the requirements 
of a warehouse in which the risk of damage by fire is trifling. 

Professor C. L. Norton has found that a reinforced concrete 
beam 6 x 6 x 48 inches, when heated for an hour in a fire 
which was hot enough to fuse the surface, broke under a 
compression load of 2,750 Ibs. A similar beam, similarly 
heated for two hours, broke under a load of 1,950 Ibs., and a 
third beam which was not heated at all broke under a load of 
5,700 Ibs. He also found that larger beams are weakened in 
a lesser proportion. Other experiments confirm the above, 
and show that concrete still possesses a notable degree of 
strength even after four hours continuous exposure to a 
temperature of about 1,600 C. 

The fact should never be overlooked that no building is 
absolutely fire-proof, and the use of such a term is liable to 
lead to unnecessary carelessness. The best that can be done 
is to make a structure as fire-resisting as possible, and to arrange 
a system of alarms so that the fire brigade can be on the spot 
before a serious conflagration is produced. 

Resistance to Shocks. Next in importance to resistance to 
fire comes the ability of a concrete structure to withstand 
repeated shocks. Such shocks need not necessarily be severe, 
for comparatively small ones of great frequency will destroy 
some structures. Earthquake tremors of moderate dimensions 
are usually resisted by large monolithic buildings, and in 
countries liable to this form of disturbance reifnorced concrete 
is particularly valuable. Most of the shock-resisting structures 



RESISTANCE TO SHOCKS 261 

are, however, erected to withstand the vibration of traffic over 
a bridge or of machinery in a factory. 

The ability of reinforced concrete to resist repeated and 
powerful shocks is shown in the thousands of concrete piles 
which have been successfully driven into moderately hard 
strata. In such cases the weight of the pile driven is usually 
between two and three tons, and the blows are repeated so 
frequently as to keep the pile always moving. 

The ability of concrete structures to resist such shocks 
depends primarily on the adhesion between the concrete and 
the steel reinforcement, and on the suitability of the design 
of the reinforcing members. Apart from these no special 
precautions are necessary. The extensive use of piles made of 
reinforced concrete is conclusive evidence of the ability of 
this material to resist shocks. 

Permeability frequently, but quite erroneously spoken of as 
" porosity " is the power possessed by a substance to allow 
water or other fluid to penetrate through it. Porosity consists 
in the possession of pores or voids, and on immersing a porous 
substance in water the pores will become more or less filled 
and a corresponding quantity of water absorbed. This is, 
however, an entirely different property from the permeability 
of a material, such as a roofing tile or slab of concrete in which 
water dropped on one side will gradually percolate through the 
material and appear in the form of drops on the other side. 
Permeability appears to have no definite relationship to 
porosity, and some concretes of low porosity are far more 
permeable than others. The amount of permeability depends, 
in fact, on the number and area of the passages through the 
material, and not on the total volume of pores. 

Most concrete is slightly porous, but if its constituents have 
been well graded and proportioned, concrete should be prac- 
tically impermeable. 

Permeability is a serious defect in concrete intended for water 
tanks, reservoir and marine embankments, etc. Hence, for 
such purposes it is necessary to take special care in grading and 
proportioning the aggregate, sand, cement and water, 
and for some purposes other materials are added to make 
the material waterproof. The methods for doing this 



262 SPECIAL PROPERTIES OF CONCRETE 

and the principles which underlie them are described on 
pp. 197205. 

Resistance to Corrosion. It is one of the curious properties 
of steel that when embedded in concrete it does not rust. 
Indeed, a thin coating of rust which may exist when the steel 
is embedded will be found, after a time, to have disappeared 
completely. This is generally explained as being due to the 
iron oxide comprising the rust combining with the lime set 
free during the setting of the cement and forming a hydrated 
calcium ferrite which acts as a protecting agent. 

The following instances investigated by R. G. Clark 
show in a striking manner the highly protective action of 
concrete. 

On the river Thames a reinforced concrete pile had to be 
withdrawn as the result of a very severe collision. The pile 
had been driven about three years to a very hard set, as it 
carried a heavy load. After being pulled up, the pile was laid 
on the bank for inspection. At various places along the length 
of the pile the concrete was cut away and the steel exposed. 
In each case the steel had not the slightest signs of rust. On 
a pier further down the same river a reinforced concrete tie- 
beam, that had been in position about eighteen months, had 
to be cut away to make provision for a diagonal brace. The 
tie-beam was midway between high and low water mark, so 
that it had a severe test, being alternately wet and dry. On 
examination the steelwork was found as good as when put in, 
and the original rust had disappeared. It might easily be 
imagined in what condition the steel would have been if it 
had not been protected by the concrete. Again, at Southamp- 
ton in 1898, several pile heads were cut off and thrown on the 
foreshore, so that they were alternately exposed to the air and 
covered by the tide. They were examined some seven years 
after by several well-known engineers, who found that the 
exposed steelwork had greatly rusted and deteriorated, whereas 
by chipping away the concrete the bars which were embedded 
in the concrete were found to be as free from rust as on the 
day when they were first used. 

The resistance of concrete itself to the corrosive action of 
acids is quite a different matter. Most acids both mineral 



RESISTANCE TO CORROSION 263 

and organic decompose Portland cement and therefore bring 
about the destruction of any concrete to which they may be 
exposed. Such corrosion is particularly noticeable in drain 
and sewerage pipes, especially where the sewage is obtained 
from chemical and other factories. Sometimes the acid does 
not occur in the original fluid, but is produced by the oxidation 
of sulphur compounds, etc. 

Coating the concrete with tar or other acid-proof material 
is the only method of preventing corrosion by acid, though the 
impregnation of the concrete with some fatty or oleaginous 
substance is sometimes of assistance. The only really satis- 
factory " remedy " consists in facing the concrete with an 
entirely acid-proof material, or in replacing it by acid-proof ware 
as salt-glazed pipes. 

The occasional absorption of trifling quantities of very 
dilute acids is less serious, and is frequently neglected. 

Fresh sewage is, ordinarily, alkaline and without action on 
concrete ; it is only when the waste acid liquors from factories 
and trade effluents are admitted that sewage becomes corrosive. 
Only two courses are then open : (a) to neutralise the sewage 
before admitting it to the sewers, or (b) to construct the sewers 
of a different material. Acid-proof paints, such as coal tar, 
are only of slight value, as they are soon worn away by the 
abrasive action of the flowing fluids. 

Sewage which has been treated in bacteria beds, on the 
contrary, is very liable to develop acid sulphur compounds, 
which corrode the concrete, particularly at the mean water- 
level. This corrosion may usually be prevented by coating 
the pipes with a coal-tar preparation, but this must be 
frequently inspected and renewed as occasion requires. 

The admission of hot water to sewerage appears to increase 
the production of sulphuretted hydrogen and sulphur dioxide, 
both of which have a marked deleterious effect on concrete. 
Wine musts and some other liquids of an acid character 
appear to be without action on concrete, but Rohland has 
shown that beer attacks concrete to a serious extent. 

Even fresh water containing an appreciable amount of carbon 
dioxide in solution will, in time, bring about the destruction 
of concrete, The carbon dioxide removes the lime from the 



264 SPECIAL PROPERTIES OF CONCRETE 

concrete and so reduces its strength and increases its porosity 
at the same time. 

Quite pure water will also, in time, remove an appreciable 
quantity of lime from concrete, but if the concrete is made 
fairly compact, and especially if its surface is water-proofed, the 
amount of lime removed may usually be neglected. In many 
cases the removal of the lime may be prevented by adding 
trass in place of part of the sand (pp. 159, 205). 

Sea water appears to have a peculiarly destructive action 
on some kinds of concrete, and its behaviour is, therefore, of 
great importance to those engaged in the construction of 
harbours, docks, breakwaters and other maritime work. 

The action of sea water is partly physical, and the impact 
force of the waves and the grinding action of sand, shingle and 
pebbles, are not infrequently of greater importance than the 
purely chemical corrosion. Against much of this purely 
physical action man is powerless ; he can only reduce it by 
the use of hard aggregates and of dense, well-compacted 
concrete sufficiently rich in cement to resist for a reasonable 
number of years. That such remarkably resistant marine 
works can be constructed is, in fact, one of the wonders of 
modern engineering. Strength and density are the two chief 
properties on which modern engineers rely in constructing 
concrete which will withstand the physical action of the waves. 
Experiments on a large scale in Germany and Copenhagen 
have shown that the succession of saturation, drying, freezing, 
etc., to which marine work is subjected by the changes in the 
water level due to the tides, also has a most important effect 
in causing the destruction of the masonry. The action of 
frost is particularly powerful, and the utter impossibility of 
protecting embankments and other marine works from its 
action makes this all the more serious. The general action of 
frost on concrete structures is described on p. 271. 

A further factor which has not received the attention it 
deserves is the percolation of salt water into the interior of 
the concrete, and the crystallisation of the salt as the water 
evaporates. The effect of this crystallising in cement blocks, 
which are only immersed at long intervals, is similar to that 
of frost. 



EFFECT OF SEA WATER 265 

The chemical action of sea water is due to that of a variety 
of substances, of which carbon, sulphur and magnesia 
compounds are usually considered to be the most important. 

Sea water in the neighbourhood of land usually contains a 
notable amount of carbon dioxide, and this dissolves out a 
portion of the lime in the concrete, leaving the mass more open 
and accessible. Generally speaking, however, the corrosive 
action of sea water is confined to the material quite close to 
the surface of the concrete, as a re-deposition of some of the 
dissolved matter not infrequently occurs in the pores of the 
concrete. Excessive action in a compact concrete is also 
prevented by the slowness with which the water can enter 
the material. 

The constituent of sea water, which is generally regarded as 
being the most detrimental to concrete, is magnesia in the form 
of magnesium sulphate, or chloride, as these salts decompose 
cement, forming calcium sulphate and chloride. The calcium 
sulphate may then form crystalline calcium aluminium sulphate 
with considerable increase in volume, and thereby tend to 
destroy the concrete. Hence, the effect of sea water is greatest 
when the concrete is porous, and is at a minimum when the 
concrete is impervious. 

C. von Blaese has found that half the lime in a cement is 
removed as calcium chloride when the cement is treated with 
an excess of a 6 per cent, solution of magnesium chloride. 
So intense a reaction is disputed by Kallauner, though the 
latter agrees that all magnesium salts decompose cement, the 
chief reaction occurring between them and the calcium hydrate 
formed during the setting, colloidal magnesium hydroxide 
and a calcium salt being produced. In contradiction to von 
Blaese, Kallauner also observed that where the calcium salt 
is almost insoluble in water the volume of the cement was 
altered and cracks formed ; when soluble calcium salts are 
produced the volume of the cement is not affected, but its 
strength is greatly reduced. 

In other words, it is not the small proportion of sulphate in 
the original cement which is important in submarine work, 
but the permeability of the concrete which permits the 
magnesium sulphate in the sea water to react with the cement, 



266 SPECIAL PROPERTIES OF CONCRETE 

If this is the true explanation of the destructive action 
of sea water, the latter may be overcome by the use of 
an impervious concrete, prepared from a more finely-ground 
cement and a more carefully graded and proportioned 
concrete. 

The precise reactions produced by magnesia compounds have 
not, however, been studied with great accuracy, and there is 
ample room for a further investigation of the subject. More- 
over, the calcium sulphate in sea water appears to have a 
direct chemical action on cement, according to E. Candlot a 
calcium sulpho-aluminate being formed. Cements which 
contain little or no alumina (such as Teil hydraulic lime or 
grappier cement) appear to be less affected by calcium and 
magnesium sulphates, and the conclusion has been drawn that 
the action of these sulphates must be largely confined to the 
alumina in the cement. For this reason iron cements, which 
are free from alumina, are sometimes used for maritime 
work. W. and D. Asch, on the contrary, maintain that the 
action of sea water depends, to a larger extent than is 
generally supposed, on the constitution of the cement, 
and that highly aluminous cements are quite resistant to 
magnesium and calcium sulphates, providing that such 
cements possess the correct constitution. In accordance 
with the theory of these investigators (p. 55) most 
Portland cements contain hydroxyl groups attached to the 
aluminum rings, these being combined with lime or other 
base, thus : 

5CaO KG 5CaO 

-ICaO 



Si 



Al 



Si 



/ 
5CaO KG 5CaO 



In such a compound the lime attached to the aluminium 
ring behaves differently from that attached to the silicon 
rings. Thus, on treating such a cement with water, part of 
the lime is hydrolysed and separated as calcium hydrate, thus 



EFFECT OF SEA WATER 



267 



The compound formed is, according to Asch's theory, of the 
type 

HO.Ca.O O.Ca.OH 



HO' 


Ca'.O 




O.Ca 


'.OH 






OK 


' 




HO.Ca 


.o\ 


x\/\/\/0. 


Ca.OH 


HO.Ca 


.o/ 


\0. 


Ca.OH 


HO.Ca 


0\ 


/o. 


Ca.OH 


HO.Ca 


.o/ 


\/\/\/\0. 


Ca.OH 


HO. 


Ca . 01 


OK 


^0 .Ca. OH 


HO 


.Ca. 





O .Ca 


.OH 



The two OK-groups attached to the aluminium ring have a 
strong tendency to react with groups of acid radicals, such as 
- S0 2 OH, SO'OH, etc., so that the destructive action of the 
sulphates in sea water is a necessary consequence of Asch's 
theory. From this it follows that if hydraulic cements can 
be prepared which cannot contain these two hydroxyl groups, 
they would be resistant to sulphates. Such cements would 
probably be of the following types : 





Si Al 



TYPE A. 



TYPE B. 



TYPE C. 



(See p. 56) W. and D. Asch's conclusions are too new, as yet 
to be generally accepted. In this connection it is interesting 
to note that Schuljatschenko, J. A. van der Kloes and the 
author have each independently found that the replacement 
of some of the sand in the cement by trass improves concrete 
for maritime work. 

The difficulty of ascertaining accurately the action of acid 
radicals (including sulphates, etc., in sea water) on cement is 
greatly increased by the relative impermeability of the concrete, 



268 SPECIAL PROPERTIES OF CONCRETE 

This has the effect of reducing the apparent action of the 
solution, and has led to the conclusion that sea water per se 
has no action on concrete. Thus, the results of the large scale 
tests carried out by the Scandinavian Portland cement manu- 
facturers are regarded as showing that, provided a good quality 
of Portland cement is used and that the mortar is rich (1 of 
cement to 2 of sand) and compact, the chemical action of 
sea water will be inappreciable in ten years. A loose mortar 
or one made with hydraulic lime, on the contrary, is soon 
disintegrated, except in unusually mild climates, such as the 
Mediterranean. 

In order to withstand the severe physical conditions, concrete 
blocks for marine work should be allowed to harden in air for 
as long as possible before being immersed in the sea. The 
results of some important German tests show that a hardening 
period of a year is desirable. A still more recent report of 
experiments by R. L. Humphrey states that " while opinions 
differ as to its durability in fresh and sea water, concrete 
mixed and placed so that the resulting mass is of maximum 
density affords ample resistance to the action of both fresh 
and sea water, especially if allowed to harden before exposure." 

The consensus of opinion on this point seems to be as follows : 

(a) " Little, if any, damage is done to even the poorest 
concrete structure below water. 

(b) " The principal destructive actions occur between tides, 
generally at the mean-tide line. 

(c) " Where the concrete has disintegrated in sea water this 
is undoubtedly caused by freezing, for the reason that in 
tropical waters, where there is no freezing, the structures do 
not show this deterioration, excepting in cases where sea water 
has been used in the mixing and the concrete has been placed 
through sea water, thereby producing an inferior structure 
unsuited to resist sea water action. 

(d) " While there may be some chemical action after the 
concrete is weakened through freezing, it is a fact that there 
is little or no chemical action on a properly proportioned, 
mixed, and placed concrete. 

(e) " It seems to be a fact that it is desirable that the cement 
used in making concrete to be exposed to sea water shall 



EFFECT OF SEA WATER 269 

contain sufficient hydraulic materials as will satisfy whatever 
excess lime there may be in the hardened cement. This is 
accomplished in many parts of Europe through the addition of 
trass or pozzolana to hydraulic cement. The trass combines 
with the lime set free from the cement, and so increases its 
strength and renders it stable in sea water. 

" It is generally understood that the concrete must be rich in 
slow-setting cement and must not contain less cement than the 
proportions : 

(a) 2 cement, 1 trass, 6 (sand and aggregate) ; 
(/>) 2 cement, 3 (sand and aggregate). 

(/) "It is quite apparent that one of the prime essentials 
for a concrete structure that will be immune against sea water 
action is that the surface shall be dense and impervious. An 
attempt has been made to secure this condition by applying 
mortar under an air pressure of upwards of SOlbs., and while 
this undoubtedly increases the density as compared to hand 
methods, nevertheless it remains to be seen whether it achieves 
the object desired. 

(g) " Good practice demands that the concrete shall be mixed 
a sufficient length of time, without too much water, so that 
there results a mass of viscous consistency which will flow 
readily and yet the ingredients not separate. If such a 
concrete is deposited under conditions which will prevent the 
sea water permeating it before it has set, such concrete affords 
excellent resistance to sea water. Another method proposed 
has been to deposit the concrete in tremie, as with this form 
of construction only the upper surface comes in contact with 
the water, and there results a concrete which is not affected 
by sea water action. (See p. 192.) 

(h) " When reinforced concrete is used in sea water it is 
essential that the aggregate shall be a hard, dense material 
of low absorption, and that the reinforcement be protected 
by a coating of at least one inch of trass-like mortar." 

The corrosion of reinforced concrete is much more serious 
than that of plain mass concrete, as once the corrosive agent 
attacks the steel reinforcement the process of rusting will 
continue at an increased rate. As previously explained, 



270 SPECIAL PROPERTIES OF CONCRETE 

steel and iron are ordinarily protected from rusting by a coating 
of concrete, but this does not apply to an exceptionally open 
and porous material such as is produced by corrosion on the 
concrete itself. It is also a fact that if concrete is mixed 
with sea water, or with sea sand, or if it has salt mixed with it, 
and it is subsequently exposed to dampness, the reinforcement 
will corrode. This was shown by examples on a rather extended 
scale some years ago when J. S. Sewell had two experimental 
slabs of reinforced concrete made of identical composition, 
except that one was mixed with sea water and one with fresh 
water. They were exposed to the weather on a roof in 
Washington, D.C., for some months. At the end of that time 
the reinforcement in the sea water slab was badly corroded, 
while that in the other was entirely untouched. It is of 
importance, therefore, that the ingredients used in mixing 
concrete for hydraulic works should contain no corrosive 
material in themselves if the concrete is to be reinforced. 

The electrolytic corrosion of the steel reinforcement in concrete 
is a matter requiring serious attention. The external symptoms 
usually take the form of deep cracks in the concrete, usually 
on the under side of beams or vertically in columns and 
extending to the reinforcement. Wherever the steel is visible 
it will be found to be rusty. Sometimes the damage is so 
great that larger flakes of concrete fall away, exposing the 
reinforcement. Leakage from electric lighting or tramway 
circuits is the usual cause. H. P. Brown has found that 
rubber covered wires in japanned steel conduits are not 
satisfactory in preventing leakage. He also recommends 
that the intermediate wire in three-wire systems and the 
secondary circuit of a transformer should not be earthed within 
a concrete building. All the reinforcing steel in a structure 
should be in electrical connection by means of binding wire. 
Gas, water and steam pipes should be electrically insulated 
where they pass through concrete walls, etc. In new buildings 
of steel or reinforced concrete the foundation steel should rest 
on good concrete. 

Providing, however, that the concrete is sufficiently compact 
and that acids and other corrosive agents are kept away 
from it, there is no likelihood of ordinary, well-made concrete 



EFFECT OF FROST 271 

being sufficiently porous to permit the steel reinforcement 
to become rusty. 

Frost. The resistance of hardened concrete to frost depends 
largely on the density or compactness of the concrete. If the 
material is so impervious that no appreciable amount of water 
can penetrate it, the action of frost will be infinitesimal. If, 
on the contrary, the permeability of the concrete is great, 
water will penetrate easily and, on freezing, will expand and 
so break up the bond between the particles forming the concrete. 
Each repetition of the frost increases the effect until finally, 
if the frosts are sufficiently numerous and severe, the concrete 
will be completely disintegrated. 

As well-made concrete is not particularly permeable, the 
action of frost is unimportant except where the concrete is 
used in harbours, piers, etc., and in other exceptionally exposed 
positions. What is required is a well-proportioned mixture 
to which a liberal, though not excessive, amount of water has 
been added, the concrete being carefully tamped and protected 
from anything which will wash out any ingredients before the 
cement is properly hardened. Such a mixture, especially if it 
contains trass, has been found in innumerable instances to 
fulfil all requirements respecting resistance to frost. 

The durability of concrete has long been established beyond 
all doubt, for in spite of the fact that early Roman and other 
buildings were made from materials obviously inferior to modern 
Portland cement, they still exist after 2,000 or more years of 
exposure. With the elaborate precautions now taken and 
the superior cement now available, it should not be difficult 
to ensure modern and equally durable structures being 
erected whenever they are considered necessary. 

The fact that the strength of concrete is increased by age 
is a further proof of its durability. The rapid increase in 
strength is indicated by the steepness of the curve in Fig. 92. 
In this diagram the area enclosed by the dotted lines includes 
the results of a large number of tests by Johnson, whilst the 
continuous curve indicates the average of these results. It 
will be observed that the strength is expressed as a ratio of 
tensile strength : compressive strength. 

In the case of reinforced concrete, the durability is conditioned 



272 



SPECIAL PROPERTIES OF CONCRETE 



by the completeness with which the reinforcements is covered. 
If it is exposed it will soon effect the destruction of the whole 
edifice, but if properly embedded in sound concrete it will last 
indefinitely. The only danger that threatens it thereafter is 
the danger of cracks, which will destroy the integrity of the 
concrete and open up a way for atmospheric moisture or 
water to gain direct access to the reinforcement. Such cracks 
might be due to shrinkage in setting, to expansion and con- 
traction under changes of temperature, or to deformation 
under stress. If the components of the concrete are well 



fi 9 



f- 



24- 6 8 10 

FIG. 92. Eatio of Strength to Age. (Johnson. 



12 

months. 



mixed in the right proportions and kept wet while setting 
there is small danger of shrinkage cracks, and they are rarely 
seen in practice. Light reinforcement is sometimes used with 
a view to preventing them, but it is not easy to see how it- 
would be effective, for the shrinkage of the concrete would set 
up compressive stresses in the reinforcement, and it is not 
usually heavy enough to resist them effectively. The danger 
of such cracks is more imaginary than real, and an abundant 
supply of water during the mixing, and whilst the concrete 



CRACKS IN CONCRETE 273 

is hardening together, with protection from evaporation during 
setting, will ensure a successful result. The reinforcement will 
generally be heavy enough to prevent shrinkage cracks, even 
in exceptional cases. 

Cracks due to expansion and contraction after setting are 
brought about probably by a slight slipping of the mass on 
its bed during expansion, and by the excess of frictional 
resistance over the tensile strength of the concrete during the 
subsequent contraction. This trouble can be overcome by 
proper reinforcement, but it is better to divide a long wall or 
other structure into sections, so that each can act as a unit. 

Cracks due to deformation under stress only occur when the 
reinforcement is stressed so that the strain exceeds the limit 
of extensibility of the concrete. This can be avoided by 
proper design and workmanship. This danger makes it inad- 
visable to utilise the high working stresses, otherwise permissible 
in high carbon steel, since the modulus of elasticity is no 
greater than with low carbon or medium steel. If working 
stresses are kept well within the limits allowable for mild 
steel there. is no danger of cracks in the concrete. 

With regard to the tendency to crack, on account of irregular 
settlement in the foundations, concrete has a great advantage 
over other materials, as the whole structure may be made 
monolithic, and so could take tension as well as compression. 
Any tendency to cant sets up tension in some member, which 
in ordinary construction makes itself evident by cracks and 
fissures. 

Efflorescence or " scum " on concrete is due to the presence 
of soluble salts in the aggregate, sand or water, and, much less 
frequently, in the cement. These soluble salts are carried to 
the surface of the concrete during the drying and are deposited 
there as the water evaporates. Occasionally, efflorescence in 
concrete is caused by water containing soluble salts, being 
drawn up from the foundations by capillary attraction. 

The only means of preventing efflorescence consists in using 
materials as free as possible from soluble salts. 

The discoloration of concrete is usually due to oil or to dirt 
on the forms, but in some cases it is caused by impurities in 
the aggregate, which rise to the surface and form stains. 

c. T 



274- SPECIAL PROPERTIES OF CONCRETE 

Failures in concrete structures have been both numerous 
and serious, so that a few words with reference to them may 
be included here. The causes of failures may be divided into 
two groups unpreventable and preventable. 

Failures from unavoidable causes include earthquakes, 
inundation, lightning, tempest, fire, explosions and exceptional 
shocks. As they are, by their nature, unavoidable nothing 
further need be said about them beyond the remark that 
concrete structures, if designed for the purpose, will withstand 
most of these forces as well as or better than any other building 
material. 

Failures from preventable causes are almost invariably due 
to mistakes or carelessness in construction. If, for instance, 
the shuttering or forms are too weak, they may collapse and 
bring about the partial destruction of the building. Badly 
arranged pillars have been a frequent cause of collapse, and 
mistakes in the erection of the building have been even more 
serious. What, for instance, can be thought of the contractor 
who built a bridge of reinforced concrete and omitted to put 
the stirrups on the reinforcement ? Other mistakes not 
infrequently observed particularly when the work is under 
the charge of men who are not skilled in concrete construction 
or the use of dirty water, unsuitable aggregate, unwashed 
or badly washed sand or low grade cement. If the concrete 
is placed improperly, and particularly when the adhesion 
between two successive layers is imperfect (p. 190), failures 
are almost sure to result. 

Some of the common causes of failure in the making of 
concrete are as follows : 

(a) Too much water during mixing, or water carelessly 
applied, or an insufficient quantity of water. 

(6) An insufficiently graded aggregate, particularly one 
containing only very coarse material, or one with too much 
sand or loam. 

(c) Incomplete incorporation of the aggregate with the 
cement. 

(d) Allowing the concrete to stand until the setting action 
has commenced and then regauging before use, or using up 
old concrete. 



CAUSES OF FAILURE 275 

(e) Bad cement. A branded British cement is usually 
reliable, but it must not be too quick setting or lumpy or caked. 

(/) Unsuitable aggregates. It is particularly necessary to 
avoid using any aggregate that may be handy. The best for 
the purpose should be chosen. It is also unwise to accept 
aggregates on the basis of small samples. Natural aggregates 
are risky on account of the variations in the proportion of sand 
they contain, and should be screened before use (p. 156). 

(g) Rendering cement work on dry foundations and without 
thoroughly saturating the surface with water. 

(h) Dirty aggregate or water containing earthy matter, clay, 
loam, or strongly coloured water. 

() Carelessness in proportioning mixtures. 

(j) Excessive ramming or tamping. 

(k) Weak centering, sparse timbering and badly arranged 
forms. 

(I) Premature removal of forms. 

(m) Excessive trowelling or floating of cement surfaces. 

(n) Erroneous design or arrangement of the reinforcement. 

Failures due to overlooking springs or subsoil water, to errors 
in calculating the strength and arrangement of the various 
portions of the structure, or to making insufficient allowance 
for the face of the sea or wind, are by no means unknown, 
and the danger of failure by electric currents has already 
been mentioned (p. 270). 

Ignorance and hurry are, in fact, the two great causes of 
concrete failures. There is frequently an undue stress put 
upon the contractors or on the workman to use as little shutter- 
ing timber as possible, and to remove the forms too soon. 
It must be remembered that concrete does not gain its total 
strength immediately, but requires several days before it is 
strong enough to enable the supports to be removed with 
safety. Shortage of timber and undue haste in construction 
are therefore extremely serious, especially if the responsible 
persons are not too well informed as to the properties of 
concrete. 

It is a noteworthy fact that practically no " mysterious " 
failures have ever occurred in concrete structures long after 
they have been completed, so that it may be assumed that in 

T 2 



276 SPECIAL PKOPERTIES OF CONCRETE 

almost every case the failure has been due to some error in 
construction. The carelessness and ignorance of some con- 
tractors who undertake concrete work, and the indifference 
they show with regard to inspection during construction, make 
it essential that all concrete work should be placed in the hands 
of skilled and trustworthy men. This is confirmed by the 
fact that all localities in which a good code of regulations is 
enforced have proved free from great disasters of a preventable 
nature. 

The conclusion seems justified that the objections which 
have been urged against the use of concrete as a structural 
material are either imaginary or can be overcome by practicable 
methods, especially as many of them arose when the subject 
was not so well understood as at present. That this conclusion 
is justified is abundantly proved by the increasing and successful 
use of the material in permanent structures of the most varied 
kinds in all parts of the world. That concrete is not suitable 
for every kind of imaginable purpose is not surprising, but its 
uses are so multifarious that there is no need to attempt to 
employ it for purposes or in situations for which it is not 
eminently suitable. 



CHAPTER X 

TESTING CONCRETE 

THE greater part of the testing applied to concrete is in 
relation to the raw materials of which it is made. 

Cement. The tests generally used for Portland cement 
have been described in Chapter V. 

Aggregates. These are seldom subjected to special tests, 
though it is very desirable that they should be tested in a 
manner similar to cement. The proportion of aggregate of 
various grades or sizes is usually specified by the engineer or 
architect in charge of the work, and precautions should be taken 
to ensure these proportions being maintained. 

The limits of size of particles in the various grades of 
aggregate should also be checked, as otherwise very inferior 
concrete may be produced. It is particularly desirable that 
all sand should be removed from the coarser grades of 
aggregate. 

The specification of a minimum crushing strength for the 
aggregate is seldom made, it being generally assumed that 
if the finished concrete stands the necessary tests, the materials 
of which it is composed must be ipse facto satisfactory. The 
usual limitations are stated on p. 154, but far too little atten- 
tion is paid to the correct grading of the aggregates. 

Sand. The sand must be tested to ensure its freedom 
from clay and dust, it being now recognised that all material 
which will pass through an aperture ^ inch by ^\y inch should 
be rejected as harmful. Dust and extremely fine sand greatly 
reduce the strength of any concrete in which they may be 
present. 

Steel. The tests imposed on the steel used for reinforced 
concrete have been mentioned on p. 211. It is necessary 
to check the sizes carefully, as any reduction in the cross-section 



278 



TESTING CONCRETE 



may be very serious. Every piece of steel used should also 
be carefully examined for flaws and irregularities. Steel 
which shows a granular fracture should be avoided. 

The crushing strength of concrete is the chief test imposed. 
The test pieces are cubes with either 3-inch, 4-inch or 6-inch 
sides, smaller ones being unreliable. The test pieces are made 
on the works at the same time as the various batches of concrete 
are made. The test-cubes should be kept slightly damped 




FIG. 93. Machine for Crushing Tests. 

for seven days. For tests to be made after longer periods, 
the cubes should be kept under cover so as to protect them 
from dust, rain and direct sunlight. 

Care is needed to keep the amount of tamping as similar 
as possible to that employed on the larger masses of concrete. 

Sometimes blocks of concrete are cut from work actually 



CRUSHING STRENGTH 279 

under construction, but this very greatly increases the cost 
of the test on account of the difficulty of cutting the sample. 

The Joint Committee under the auspices of the Royal 
Institute of British Architects has recommended that the 
cubes should be made " before the detailed designs for an 
important piece of work are prepared," and that the tests 
should be made twenty-eight days after moulding. At least 
six cubes should be used in each test. " In the case of concrete 
made in proportions of 1 cement, 2 sand, 4 hard stone, the 
strength should not be less than 1,800 Ibs. per square inch." 
Such a concrete should develop a strength of at least 2,400 Ibs. 
per square inch after ninety days. 

Other authorities recommend tests to be made at the end of 
seven, twenty-eight, fifty-six, ninety and 365 days, and some 
at the end of two, three, four, or five years. Owing to the 
speed with which large concrete buildings are erected a seven- 
days' test is desirable, and though it is claimed that its results 
are unreliable, and that at least twenty-eight days should 
elapse before testing, there seems much probability of a 
short-time test being brought into regular use. 

Tests on cubes or other small samples are seldom very 
reliable ; tests on full-sized columns, beams or blocks are much 
more accurate and preferable, but they require such special 
and powerful machinery, as to necessitate the samples being 
sent to a testing station fitted for the purpose. 

In testing columns of reinforced concrete for crushing strength 
it will usually be found that they break at the end which was 
uppermost during manufacture, thus showing that the solidity 
of a concrete column diminishes towards the upper end. This 
is confirmed by tests of the specific gravity of the various parts 
of the columns, the upper end having a much lower density 
than the lower one. 

In testing floors and slabs care must be taken to distribute 
the load uniformly. Bars of pig iron and building bricks both 
tend to form arches and cause an irregular distribution of 
the load. 

Loading Tests. In view of the general unsatisfactoriness of 
testing cubes of concrete, F. von Emperger proposed, in 1903, 
that they should be replaced by loading tests on specially 



280 



TESTING CONCRETE 



prepared reinforced concrete beams. The Emperger test is 
intended to be carried out on the spot, the test beams being 
prepared from the materials there in use, and the testing 
apparatus being fitted up close at hand from parts which are 
packed in portable cases, which also contain bent steel rods 
as models to be copied by the workmen preparing the test 
beams. It is best to prepare four such beams for each test, 
two of which are reinforced with a single longitudinal rod 
(Type I.), and two with two parallel rods (Type II.). The 




FIG. 94. Chart for use with Emperger's Test. 

beams are 2-30 metres (6 feet 8 inches) long, 7 cm. (2f inches) 
broad, and 10 cm. (4 inches) deep. The reinforcing rods are 
12 mm. (| inch) in diameter, and are bent up and turned over 
at the ends, as shown at the top of Fig. 94. The maximum 
stress obta nable in concrete by the use of this type of beam is 
considerably in excess of that ever attained in actual practice. 
The error due to accidental shifting of the reinforcing rods 
during the preparation of the beams does not exceed 2 per 
cent, of the stress. The beams are so chosen that the breaking 

p 
moment (M ) is equal to half the breaking load, i.e.. M = . 



EMPERGERVS LOADING TEST 



281 



The moulds are double, made of wood, lined with sheet iron. 
They are oiled and fitted together by means of distance pieces ; 
two loops to serve as handles are placed in position, and the 
sides are accurately adjusted by means of a gauge until the 
middle third has exactly the correct width. The reinforcing 
rods are then laid in position on the distance irons and handles. 
The concrete is introduced, and tamped in the same way as 
in the construction of a floor. The moulds may be removed 
in two or three days, after which care is taken that the beams 
are exposed to the same conditions of temperature and weather 




136 



FIG. 95. Emperger's Loading Test. 



as the work to be controlled. If four beams are used, two are 
tested after three weeks, and two after six weeks, but for rapid 
tests intended to determine whether centering may be removed, 
a single beam is sufficient. 

The framework used in making the loading test is shown in 
Fig. 95. The two hardwood knife edges, with iron edges, are 
placed exactly 2 metres apart, and the load is then hung from 
two wooden riders, placed as shown at a distance apart of 
50cm. (1 foot 8 inches). The chains must allow the loading 



282 TESTING CONCRETE 

platform to swing freely, without allowing too great a fall 
when destruction occurs. It is advisable to place a board 
(not a thick plank) immediately under the beam to receive 
the broken halves and prevent complete collapse. A vertical 
scale may be attached to this board at the middle point to 
measure the deflection before fracture. The load is applied 
by means of bricks, which are added symmetrically, according 
to a definite scheme, by two workmen, and are counted until 
fracture occurs. The breaking load is then made up of the 
weights of the bricks and the loading apparatus, and two- 
thirds of the weight of the beam. (When loading is 
applied at four points, corresponding with a test under 
distributed load, the whole weight of the beam must be 
included.) 

Calling the breaking load P, the compressive stress (<r 7 >) 
reached in the concrete is, for Type I. of reinforcement, cr y > = 
0-384 P, and for Type II., cr B = 0-3285 P. It remains to be 
determined, however, whether this stress was the cause of 
fracture that is, whether the maximum compressive strength 
was utilised. This is most readily determined by inserting 
p 

M = -JT in the graphical diagram here shown. It will be 

Zi 

seen from this that for the best qualities of concrete it is 
advisable to use Type II., with 4 per cent, of reinforcement. 

In the diagram (Fig. 94), the percentage of reinforcement is 
plotted as abscissae, and the values of the breaking moment, 
M , as ordinates. The strength of the steel is taken as its 
elastic limit (which is generally 3,500 kgs. per square centi- 
metre or 49,700 Ibs. per square inch) ; it is shown as a thick 

P 
line on the diagram. If the value of M = falls below the 

steel curve, fracture is due to failure of the concrete, and the 
strength of this may be determined by the aid of the second 
group of curves ; if above, fracture is due to the steel, and it 
cannot be known whether the maximum strength of the 
concrete has been reached or not. Actually, failure of the 
steel is very rarely observed. In the neighbourhood of its 
elastic limit, the steel unloads itself at the expense of the 
concrete, and the resulting shifting of the neutral axis upwards, 



EMPERGER'S LOADING TEST 283 

and increase of the compressive stress in the concrete, causes 
fracture of the concrete, really from a secondary cause. 

A comparison of Dr. von Emperger's results with those 
obtained by Professor Morsch and by Professor von Bach, 
shows that greater uniformity is found in the bending tests 
than in the compression tests with cubes. 

The average variation in the results of the bending tests is 
about 7 per cent, to 8 per cent, of the mean value, these varia- 
tions being attributable to differences of temperature, moistness 
of air, etc., and in some cases also to the fracture of the beam 
taking place at different parts. The corresponding compression 
tests with cubes exhibit far greater irregularities and may 
easily reach 25 per cent. It is important that the reinforce- 
ment should be accurately placed, as quite small errors in this 
respect cause a serious loss of strength. Excluding the most 
irregular values, the ratio of the breaking stress in bending 
tests to that in compression tests is from 1-3 to 1-6 : 1. 

The cheapness and simplicity of the test, as well as its 
trustworthiness, are sufficient reasons for its introduction as 
a means of systematic control of building operations and for the 
regular courses of instruction in several important colleges. 
It has been appreciated wherever used, by engineers, con- 
tractors and workmen alike. 

Concrete structures, particularly floors, are frequently tested 
by the direct application of a load one-and-a-half times that 
which they have been designed to carry, the deflection being 
noted. The great disadvantage of this method of testing 
is that the structure may be strained and permanently injured. 
Hence, it is generally desirable only to apply the designed 
load. If no undue deflection then occurs, further loading 
can serve no useful purpose, and may prove a serious dis- 
advantage. 

The Joint Committee under the auspices of the Royal 
Institute of British Architects emphatically declare that no 
load tests on the structure itself should be made until at least 
two months after the laying of the concrete, and that even then 
the test load must not exceed one-and-a-half times the acci- 
dental load, nor two-thirds of the elastic limit of the steel 
reinforcement . 



284 TESTING CONCRETE 

Deflection. The maximum amount of deflection permissible 
can be calculated from the bending moment, the elastic 
modulus, and the equivalent moment of inertia, but this 
calculation is beyond the scope of the present work. 

Permanent deflection which continues after the load has 
been removed is usually a sign that the material has been 
overloaded, though in some instances it is caused by a 
" settling " of the material on the bedding or supports, and is 
then of minor significance. An extremely slight permanent 
deflection (not exceeding J inch) is in most cases unavoidable. 

Experiments made by Van Ornum in 1907 show that 
repeated applications of a load increase the permanent deflec- 
tion, but do not alter the elastic deflection. The greater part 
of the permanent set occurs on the first application of the load, 
but each subsequent application increases it slightly. H. C. 
Berry and A. T. Goldbeck have confirmed this, and find that 
the ultimate strength, the maximum deflection, and the 
adhesion of the steel and concrete are not materially affected 
by one-and-three-quarter million repetitions of high, but safe 
loads. 

Arguments and recommendations based on deflections are 
often erroneous unless made by thoroughly competent engineers, 
as the deflection varies according to the distribution of the load, 
the manner of supporting the test piece, and the relation of 
the size of the load to that of the test piece. 

Tests of the permeability of concrete are made by measuring 
the amount of water which passes through a slab or disc of 
the concrete in a given time, under a predetermined pressure. 
The slab or disc is clamped on to the end of a pipe into which 
water is pumped under pressure. The water passing through 
the concrete is collected in a measuring glass. Unfortunately, 
such a method of testing is far from satisfactory, as it necessi- 
tates the use of thin slabs and cannot be applied to blocks of 
the same thickness as the concrete structure. It is also difficult 
to use aggregate of the same coarseness as that used on the 
larger scale. 

More reliable results are obtained by mixing the concrete 
and placing it in a large iron tank made strong enough to resist 
the testing pressure, and tamping and finishing the surface 



LOADING TESTS 285 

exactly as though the iron tank were a " form." Water is 
supplied under pressure, and the water which passes through 
the concrete is collected. Testing with an excessive pressure 
is useless, as, if the pressure is sufficiently great, solution of 
the hydrated cement will occur, and all concretes will be found 
to be permeable. A pressure of 20 Ibs. per square inch is the 
maximum ordinarily used. 

The permeability of new concrete is not necessarily an indica- 
tion of bad workmanship. Many tanks will exude a small 
quantity of water when first filled, but this " weeping " ceases 
after two or three months, on account of the re-deposition of 
some of the constituents previously dissolved by the water. 

Where permeability is to be tested, apart from pressure, a 
method suggested by Le Chatelier may be used. The test 
piece is immersed in a solution of calcium sulphide, and after 
a suitable length of time it is taken out, wiped dry and broken. 
The iron compounds in the concrete will be converted into iron 
sulphide, recognisable as a black or dark green stain, wherever 
the solution has penetrated. Well-made concrete is quite 
impermeable when tested in this manner. 

Tests for concrete to be used under sea water are peculiarly 
difficult and costly. It has been shown repeatedly that test 
pieces immersed in still sea water are useless. Moreover, 
the test pieces must be immersed for several years, as the 
changes occurring during the first twelve months are irregular, 
and conclusions drawn from them are illusory. 

An exceedingly valuable record of numerous tests will be 
found in " Concrete and Constructional Engineering," Vols. IV., 
V., VI., VII. and VIII. (1909 to 1913). 



BRICKS 
CHAPTER XI 

THE RAW MATERIALS FOR BRICKS 

BRICKS are ordinarily made of clay or shale, or of a mixture 
of these with sand. Hence, the raw materials used in brick- 
making include all the clays possessing such a degree of plas- 
ticity as will enable them to be formed into bricks, which are, 
at the same time, sufficiently cheap to make the manufacture 
of bricks reasonably profitable. Bricks are also made of sand, 
as described in a later chapter, and these are known as " sand 
lime bricks." 

Bride Clays. No rigid scientific definition of brick clays 
can be formulated, as almost all clays can (technically) be 
made into bricks. Whether a particular clay can be com- 
mercially used for this purpose depends on a variety of other 
considerations, the chief of which is the saleable value of the 
clay for more remunerative purposes. Thus, a fireclay 
valuable for its ability to resist high temperatures may be 
used for the manufacture of building bricks in a district where 
the demand for the latter is proportionately greater than that 
for furnace bricks. Again, some clays as London clay- 
make excellent bricks when mixed with a suitable proportion 
of sand, but large quantities are rendered quite useless for this 
purpose by the absence of sand from the localities in which 
they occur, and by the selling price of bricks being too low to 
admit of the transportation of sand from other districts. 

For the manufacture of bricks it is essential that the clay 
shall not contract unduly during drying, so that clays with a 
high degree of shrinkage are excluded from use as brickmaking 
materials unless a sufficient quantity of sand is available at 
a cost not exceeding that of the clay, so that from the two 
materials a mixture with the desired degree of shrinkage may 
be obtained. 



THE RAW MATERIALS FOR BRICKS 287 

For brickmaking purposes, clays are classified according 
to certain of their physical characteristics, and not according 
to their chemical composition. Indeed, the latter is of very 
small significance, except in so far as it enables the physical 
properties of the clays to be predicted. The difficulties 
attending the interpretation of a chemical analysis of clay 
are, moreover, so extraordinarily great that it is often best for 
the brickmaker to discard it altogether and to rely upon 
physical tests. 

The chemical composition of the clays used for brickmaking 
is, in fact, so complex that it is doubtful whether it is even 
approximately understood by some of the most eminent 
mineralogists and chemists at the present time. Some indica- 
tion of the present state of knowledge on this subject is given 
on pp. 5, 40, et seq, but the large proportions of sand and other 
substances found in most brickmaking earths make it almost 
impossible to investigate, with accuracy, the real nature of the 
true clays they contain. Fortunately, for the building and 
allied trades, this lack of knowledge of the constitution of the 
clay molecule does not seriously affect the brickmaker, as he 
deals with the physical rather than the chemical properties 
of clays. 

For the production of good bricks, a clay must, when burned, 
possess a suitable colour, hardness and porosity ; it must be 
accurate in shape, free from warping, and of sufficient strength 
to carry any loads likely to be placed upon it. In addition 
to this it must be sufficiently refractory to withstand the 
action of any heat to which it may be subjected. In order 
to produce good bricks, clays must, therefore, be of such a 
nature when raw that they can possess these properties when 
burned. 

The colour of a raw clay is of little or no importance to the 
brickmaker and need not be described further. The colour of 
the burned clay, on the contrary, is often of the greatest 
importance, especially where re*d bricks are required for use 
on residential property or blue bricks are needed for engineering 
works. Not a few clays are excellent so far as the technical 
manufacture of bricks is concerned, but the bricks produced 
from them have so unpleasant a colour that the clays themselves 



288 THE RAW MATERIALS FOR BRICKS 

are practically valueless. In some instances such clays may 
be purified or otherwise treated so as to remove the discolouring 
materials, but in other cases this treatment may prove so 
expensive as to be impractical. 

So important is the colour of the burned clays, i.e., of the 
bricks and other goods produced from them, that it is con- 
venient to classify clays according to the colour of the finished 
goods. 

Red-burning clays are those from which most of the building 
bricks used in the Midlands and the North of England and the 
facing bricks of the South are produced. When free from 
discolouring substances, the bricks made from these clays are 
of a uniform " terra-cotta " colour, but unless made from 
carefully selected materials and prepared and burned with the 
greatest care, some irregularity in colour is sure to occur. 

The red colour is usually attributed to ferric oxide (red 
oxide of iron) distributed throughout the material in the form 
of so fine a powder that no artificially prepared ferric oxide 
can be used to replace it or to convert another class of clay 
into a really satisfactory red-burning one. The amount of 
ferric oxide which must be present in a clay in order to produce 
a good, red brick depends more on the fineness of the oxide 
than on the actual proportion present a very small quantity 
of an extremely fine powder exercising a far greater colouring 
effect than a much larger proportion of a coarser powder. 
Clays which show less than 5 to 6 per cent, of ferric oxide on 
analysis seldom produce bricks of a pleasing red colour, 
though as little as 3 per cent, will sometimes produce a good 
red brick. 

There is good reason to suppose that no ferric oxide occurs 
as such in the raw clay, but that the iron is present in the form 
of an almost colourless hydro-ferrosilicate or ferrosilicic acid, 
such as nontronite (Fe.f) ^2Si0. 2 2H. 2 0) which, on heating to dull 
redness, decomposes into free ferric oxide, silica and water. 
Limonite a yellow hydrated oxide of iron also occurs in 
many clays, and on heating to redness it also forms red ferric 
oxide. 

C. F. Binns and others consider that the colour of red bricks 
and terra-cotta is due to colloidal iron oxide having been 



RED-BURNING CLAYS 289 

precipitated in the raw clay. They attribute the formation 
of this colloidal matter to the oxidation of pyrites with ferrous 
sulphate, the latter, being soluble, permeating the clay and, 
on further oxidation, precipitating oxide of iron. Alterna- 
tively, any ferrous carbonate present in the clay may be 
dissolved by water containing free carbonic acid, and after- 
wards oxidised to ferric hydrate or oxide. Ferrous carbonate 
(siderite) when mixed with clay does not produce a good red 
brick. 

It must be clearly understood that the red colour is due to 
the action of heat on the clays, as some clays which are red, 
when first dug, do not make good red bricks, but usually 
produce unpleasantly discoloured ones. 

Purple bricks are produced by the reducing action of some 
substance on the iron compound, whereby a bluish ferrous 
silicate is produced. The most usual reagent for this purpose 
is carbon, which may occur naturally in the clay or may be 
introduced in the form of coal dust, cinder dust (" soil "), 
or sawdust. The addition to the clay of one-fiftieth of its 
weight of manganese dioxide, or a rather larger proportion of 
;; Weldon mud," will also produce a purple colour in bricks 
made of red-burning clay. 

Blue bricks are more grey than blue in colour. Like the 
purple bricks just mentioned, the colour is produced by the 
reduction of the iron in a red-burning clay to ferrous silicate, 
on account of the presence of some form of carbon in the clay 
or of a strongly reducing atmosphere in the kiln. The clays used 
for blue bricks must be specially suitable for the purpose, and 
must contain a sufficiently large proportion of iron (corre- 
sponding to at least 5 per cent, of ferric oxide) together with 
sufficient lime and alkalies to form a brick which vitrifies 
easily without losing its shape in the kiln. The so-called marls 
of Staffordshire are specially useful for the production of blue, 
bricks, and with them the blue colour is produced wholly in 
the firing of the kiln. Some other clays of an entirely different 
composition and origin may be converted into blue bricks by 
special methods, such as injecting heavy oil into the kiln just 
before finishing the firing, or by adopting other means of 
producing a strongly reducing atmosphere. 

c. U 



290 THE HAW MATERIALS FOR BRICKS 

Yellow bricks are produced by the action of heat on clays 
containing only a small proportion of iron. The intensity of 
the colour is sometimes increased by mixing the clay with 
cinder dust or other combustible material containing sulphur. 
Some yellow bricks are made from clays containing a sufficient 
proportion of iron to burn red were it not for the simultaneous 
presence of chalk in the clay (see " White Bricks "). The 
malm bricks, which are considered in the London district to 
be the best building bricks, owe this yellow colour to their 
production from a mixture of red-burning clay, chalk and 
cinder dust (" soil "), the bricks being burned in contact with 
the fuel in a clamp. " London stocks " are prepared in the 
same manner, but are regarded as slightly inferior in quality 
to malm bricks ; it is, however, not unusual to include both 
malms and stocks under the term " stocks." 

Buff bricks are made by heating clays, containing only a 
small proportion of iron, in kilns and out of direct contact with 
fuel. The proportion of iron (expressed as ferric oxide) must 
usually be less than 3 per cent., though a somewhat larger 
quantity may be present if it is in the form of iron pyrites 
(FeS 2 ) which forms black spots and not a red colouration. The 
lower grade fire-clays and certain vitrifiable clays found in 
the neighbourhood of Little Bytham are the best known 
buff -burning clays. 

The cause of the buff colour has not been ascertained. That 
it is not due merely to the presence of a small quantity of iron 
compounds may be proved by mixing a red-burning and a 
white-burning clay together. The bricks made of such a 
mixture are not buff-coloured, but pale red. There is reason 
to suppose that alumina plays an important part in the forma- 
tion of the buff colour, but the nature of its action is still 
unknown. The observation of Seger, that all buff clays contain 
a large proportion of alumina and a small proportion of iron, 
has been confirmed repeatedly, and quite recently Binns and 
Makeley have produced good buff-burning test pieces by the 
addition of alumina to red-burning clays. 

White-burning clays are of two classes : (a) those which are 
so free from iron compounds that they are naturally white 
when drawn from the kilns, and (b) clays containing iron 



WHITE-BURNING CLAYS 291 

compounds together with a sufficient proportion of chalk 
to prevent the development of the red colour. The first 
class are typified by the clays used for domestic pottery and 
china ware, which are too valuable to be used for brickmaking, 
and the second class by the white Suffolk bricks and the 
bricks made from the alluvial clays in the centre of Ireland 
and elsewhere. 

Clays in which the correct proportions of chalk and iron 
occur naturally are found in various parts of the country, 
but many white bricks are made by adding a sufficient quantity 
of chalk to a red-burning clay. If such an artificial mixture 
is examined during the burning it will be found to develop 
a pale red colour at a very low temperature, this colour dis- 
appearing at about 850 C. heat on account of the interaction 
between the iron compound and the chalk. 

The amount of chalk required depends upon the colouring 
power of the iron compounds in the clay. Usually 10 per cent, 
.of the weight of the clay is ample, and for bricks intended 
to withstand the action of the weather more than 12 per cent, 
of chalk can seldom be used : for inside work, on the contrary, 
as much as 25 per cent, of chalk may be present in a clay. 

White bricks are usually made in localities where the clays 
naturally contain chalk, so that the proportion of added chalk 
is usually small. The proportion required must usually be 
ascertained by making bricks with different proportions of 
clay and chalk, and observing which are the best when drawn 
from the kiln. An excess of chalk will weaken the brick and 
will prevent its being durable, whilst if too little chalk is added 
the clay will burn to a pale red or " salmon " colour, according 
to the proportion of free iron oxide present. 

Grey bricks are popular in some parts of Lancashire, where 
they are considered superior to red bricks. As a matter of 
fact, these grey bricks are merely red bricks which have been 
badly scummed by improper treatment during the earlier stages 
of heating in the kiln. This scum is purely superficial and 
has nothing whatever to do with the strength or other engineer- 
ing qualities of the bricks, nor does it indicate that the bricks 
have been well burned. Indeed, the preference of some 
architects and engineers for these scummed bricks is all the 

U 2 



292 THE RAW MATERIALS FOR BRICKS 

more curious in that it is based on an entirely false conception 
of the cause of the scum. 

In other parts of the country grey bricks are produced by 
firing red-burning clays under reducing conditions (see " Blue 
Bricks "). 

The texture of a clay is frequently an important indication 
of its nature, though this cannot be relied upon entirely. 

In respect of texture, clays are classified as (a) shales, 
(b) marls, (c) loams, (d) boulder clays, and (e) plastic clays, 
whilst a number of clays which are not comprised in this 
classification, but are not used in brickmaking, bear special 
names. 

Clay shales are clays which have been subjected to enormous 
pressure since their deposition, with the result that they have 
become harcj or indurated until they resemble a soft stone. 
Some shales owe their hardness to the interpenetration of a 
cementitious solution of a siliceous nature. 

It is important to remember that- the term " shale " relates 
solely to the texture and not to the composition of the material. 
There are, in fact, many shales which are almost devoid of 
clay, and consist entirely of siliceous matter. 

Shales are characterised by the manner in which they are 
split up into layers when tapped at right angles to the line of 
their deposition. 

The clay shales are extensively used for the manufacture 
of bricks by mechanical methods, particularly in the North, 
the Midlands and in South Wales. When observed in situ, 
or when first dug, these shales do not appear to possess the 
characteristic properties of clay, but when ground and mixed 
with water their indurated texture is destroyed and their 
original argillaceous characters are restored. When properly 
selected and suitably treated, clay shales make first-cla^s 
bricks and terra-cotta, and, like most marine-deposited clays, 
their colour when burned is particularly rich and uniform. 

Marls or malms are natural mixtures of clay and chalk, 
and may usually be recognised by their friable nature, their 
texture being quite different from that of other clays. 

The term " marl " is sometimes used for any material of a 
friable nature, with a texture resembling the true marls, 



CLAY SHALES AND MARLS 293 

but such an extension of the term is very confusing and should 
be avoided. All the true marls contain a considerable propor- 
tion of chalk, and therefore effervesce violently when a few 
drops of hydrochloric acid are allowed to fall on them. The 
so-called marls of the Midlands and some parts of Wales, 
on the contrary, are almost free from chalk and are harder 
than the true marls. They are used for blue bricks. 

True marls are largely used for white and yellow (stock) 
bricks, their composition and colour when burned being 
modified, where necessary, by the addition of a further pro- 
portion of chalk. Clays to which an addition of chalk has 
been made are frequently termed artificial malms. 

As it is necessary that no coarse grains of chalk should be 
mixed with the clay, the material must be treated in a wash- 
mill in order to remove all the larger particles. Unless this is 
done, the larger particles of chalk are liable to split the bricks 
(see " Boulder Clays "). 

The true marls chiefly occur in the neighbourhood of the 
chalk deposits in the south eastern counties, but the red marls 
which, as already explained, do not contain chalk occur 
chiefly in the Midlands, in North-east Yorkshire and in Wales. 
The Midland marls belong to the Triassic clays, and are famous 
for the excellent terra-cotta colour of the wares produced from 
them. Ruabon terra-cotta and Leicester red bricks and tiles 
are typical products of the Midland red marls. 

Further particulars respecting marls will be found in the 
author's " British Clays, Shales and Sands " (Charles Griffin 
and Co., Ltd., London). 

Loams are essentially mixtures of sand and clay, and if 
stirred up with water and allowed to settle for a few minutes 
leave a residue readily recognised as sand. The term loam 
is, however, applied in a larger sense to any earth which is 
not distinctly a sand or clay, but appears to partake of the 
character of both. Thus, the red marls of the Midlands (supra) 
are loamy in character, whilst not possessing the peculiar 
texture of a true loam, and consequently loams and marls 
are frequently confused. The distinction is clear when the 
composition of each is considered, and confusion is only the 
result of regarding the terms marl and loam as having reference 



294 THE RAW MATERIALS FOR BRICKS 

exclusively to texture. Even then, there is a distinct difference 
in texture between a loam and a marl, though this is often 
overlooked. 

Loams or sandy clays are of great value for brickmaking, 
as they do not shrink unduly and yet have sufficient plasticity 
to be made into bricks and other simple shapes. Where the 
proportion of clay in a loam is less than one-quarter of the 
total material the loam is seldom suitable for brickmaking, 
unless it is mixed with a plastic clay. Consequently, sandy 
loams are of minor value to brickmakers except as a diluent, 
though the addition of washed chalk will sometimes enable 
excellent bricks to be produced, the chalk acting as a binding 
agent. 

Loams are frequently found in association with gravel ; 
the latter must be removed before the loams can be satisfactorily 
used for brickmaking. Merely to crush the gravel to powder is 
not sufficient, unless the loam is rich in clay. 

Boulder clays form a considerable part of the glacial drift 
deposited over the whole of the North and Midlands, and are 
a product of the Great Ice Age. The term " boulder clays " 
is sometimes erroneously used for the whole deposit ; it ought 
really to be confined to the argillaceous portions, the term 
applied to the whole being drift, or more correctly, glacial drift. 
Boulder clays are characterised (as their name implies) by 
their association with boulders and smaller stones, many of 
these consisting of limestone brought by the ice over very 
great distances, and the occurrence of characteristic stones 
in or close to the clay affords on 3 of the readiest means of 
identifying it. It would be too sweeping an assertion to state 
that all stony day is of glacial origin, but the statement is 
sufficiently truthful to cover the majority of cases where 
a plastic clay in Great Britain contains stones. 

Boulder clays are usually lightly plastic and tough, if care 
be taken to select portions free from stone. Some of them 
shrink greatly in drying, and require to be mixed with sand in 
order to produce sound bricks. The presence in them of stones 
too small to be removed readily by hand picking is a serious 
disadvantage to the brickmaker desirous of using this material. 
He must either remove the stones and gravel completely, by 



BOULDER CLAYS 295 

washing the clay, or he must have the larger stones removed 
by hand or some simple mechanical means, and must then crush 
the gravel and smaller stones to so fine a powder that they 
can do no further harm. Unfortunately, both these methods 
add largely to the cost of brickmaking, and many firms using 
boulder clay and other stony clays endeavour to economise 
by crushing and mixing the whole of the material without 
subjecting it to any preliminary cleaning or freeing from 
stones. Where the proportion of stones is small and they are 
of a siliceous character, the number of bricks spoiled by a simple 
crushing of the raw material may be insignificant, but if 
limestone is present, so large a percentage of the bricks produced 
will be cracked, " blown " and spoiled, that the matter becomes 
exceedingly serious. 

When a particle of limestone is subjected to the ordinary 
heat of a brick kiln it loses carbon dioxide and is converted 
into quicklime. If this particle of quicklime is situated at 
or near the surface of a brick it gradually absorbs water from 
the atmosphere, becomes hydrated and swells, thereby exerting 
so great a pressure that the surface of the brick may be cracked. 
If the lime is on the surface of the brick the hydrated product 
forms a loose white powder, and the brick is then said to be 
" blown." If the particles of lime are so far from the surface that 
they cannot crack or weaken the brick, little or no harm is done 
and, generally speaking, the action of all particles of lime at a 
greater depth than one inch from the surface may be ignored. 

The only true remedy for these defects is the prevention of 
their occurrence by the removal of the limestone previous to 
the clay being made into bricks. Unfortunately, this is, in 
most cases, impossible, though much may be done by passing 
the material through a clay cleaner, which consists of a 
perforated drum through which the soft clay is forced, whilst 
the stones and gravel larger than the perforations remain 
behind. As the perforations cannot be made less than 2*5 inch 
in diameter if a reasonable output is to be obtained, such an 
appliance does not remove the smallest particles of limestone ; 
but the proportion of those left in the clay which are able to 
destroy the bricks is so small as to make the use of such a 
clay-cleaner invaluable, 



296 THE RAW MATERIALS FOR BRICKS 

An alternative means for preventing the effect of limestone 
on clay consists in grinding the material so fine that the 
particles will be distributed and so cannot cause the disintegra- 
tion of the bricks. Such bricks are quite sound, but have an 
unpleasantly spotted appearance, due to the white particles 
of lime. If it is possible to burn the bricks at a temperature 
at which the lime will combine with the clay and form a slag, 
the spottiness will be removed and an even stronger brick 
will be produced. Unfortunately, most of the British boulder 
clays cannot be heated to a sufficiently high temperature 
without losing their shape. 

Notwithstanding the defects caused by the stones and gravel 
associated with them, the boulder clays are very largely used 
for the manufacture of common bricks, whilst facing bricks 
are made in considerable quantities from selected portions 
of these clays. If care is taken in excavating a boulder clay, 
it will usually be found that the stones and gravel occur in 
seams and pockets, and that by careful oversight a clay may 
be obtained which is sufficiently free from stones and gravel 
to make excellent bricks. 

Plastic clays are the most readily identified on account of 
their peculiar texture. They are usually composed of much 
finer particles than are marls and loams, and most of them leave 
an insignificant residue when mixed with water and rubbed 
on a No. 200 sieve. This is not due to any difference in the 
clay itself, but rather to the coarser nature of the chalk, sand 
and other ingredients of less plastic clays. 

Plastic clays occur in almost every part of the United 
Kingdom, and most of them may be used for brickmaking, 
providing that the shrinkage is not too great. In some localities 
this may be reduced by the addition of crushed rock or sand or 
even of clay burned specially for the purpose, but where none 
of these materials can be used, highly plastic clays are of small 
value for brick and pottery manufacture. 

The term " plastic " is somewhat indefinite, and no means 
has yet been found whereby plasticity can be adequately 
and satisfactorily represented numerically. Indeed, the term 
plasticity appears to be almost incapable of exact definition, 
its use by potters and other clay workers being different from 



PLASTIC CLAYS 297 

the meaning attached to it by artists, engineers, and others 
working with different materials. The study of plasticity is 
highly complex and technical, and as it has been dealt with 
exhaustively in the author's " British Clays, Shales and Sands," 
its discussion need not be detailed here. 

Suffice it to say that a plastic clay is one which can be 
formed into any desired shape by means of the fingers or light 
tools, this shape being retained indefinitely and rendered 
permanent by heating the clay to a suitable temperature in 
a kiln. The plasticity is thus seen to be closely related to the 
complexity of shape of articles which can be made from a 
given clay. A highly plastic clay can be formed into the most 
complex curves and patterns, whilst a slightly plastic clay can 
only be made into simple forms, such as bricks and bars. 
Hence, clays are grouped according to their plasticity, the 
highly plastic clays being termed tough, strong or foul, and less 
plastic clays as lean, mild or weak. These terms are by no 
means exactly definable, but they serve to distinguish between 
many different clays, and to indicate their apparent nature. 

The apparent plasticity of a clay depends largely on the 
condition in which the clay is found, and may vary from day 
to day, or even from hour to hour. Thus, a clay when found 
may appear to be lean and almost devoid of plasticity, yet if 
suitably moistened or mixed with a little water it will become 
as plastic as a highly plastic clay. It is therefore necessary, in 
describing the plasticity of a clay, to state the conditions under 
which the description is applicable, as otherwise the same clay, 
on one occasion, may be described as highly plastic and on 
another as of a mild nature the state of the atmosphere 
making a great difference in the appearance of the material. 

Many methods have been proposed for comparing the 
plasticity of clays, but none of them are entirely satisfactory, 
as this property is not of an elementary nature, but is closely 
associated with the binding power, viscosity, adhesion, cohesion, 
absorption, impermeability, and other properties of the 
particles. Attempts to measure plasticity will usually be 
found, on investigation, to consist, in reality, in the measure- 
ment of one of these other properties, and not in that of the 
plasticity itself. The result is that numbers supposed to 



298 THE RAW MATERIALS FOR BRICKS 

represent plasticity obtained by different investigators differ 
widely from each other and, which is more important, lead to 
conclusions inconsistent with what is ordinarily understood as 
plasticity by practical clay workers. Thus, the plasticity 
numbers of a certain chemist in London represent china clay 
as less plastic than ball clay, whereas all potters hold precisely 
the reverse opinion. 

If, as appears probable, the plasticity of clay is due to a 
variety of minor properties, it can only be adequately measured 
by some means in which all these minor properties are included . 
Such a method would appear to be very complex, and almost 
impossible in its entirety. The plasticity of a clay may, how- 
ever, be measured with an accuracy sufficient for most practical 
purposes, as follows : 

(a) A sample of clay is mixed with sufficient water to form a 
paste of good modelling consistency ; that is to say, a paste 
which is sufficiently moist to be formed into any desired shape 
and yet not so moist as to adhere to the fingers. A weighed 
portion of this paste is then dried at 105 C. and the proportion 
of water to dry clay is ascertained. A less accurate method 
consists in weighing the clay and measuring the water required 
to mike a paste of the desired consistency, but allowance 
must then be made for the proportion of moisture in the 
original clay. Clay which has been dried artificially must not 
be used for this test, unless the plasticity of such dried clay is 
to be determined instead of that of the raw clay. 

(6) Some of the clay paste produced as just described, is 
placed in a small expression machine (a sausage mincer of the 
old-fashioned type being suitable), and in this way a cylinder 
of clay paste about three inches long and one inch diameter 
is formed. This cylinder is fitted with clamps at each end, 
and two marks exactly two inches apart are then made on it. 
Its tensile strength is determined by attaching one clamp to 
a support and applying a suitable weight to the other. The 
weight, which is exactly sufficient to rupture the cylinder, is 
noted, the two broken pieces of clay are then fitted together, 
and the percentage of extension in length is then measured. 

The product of the percentage of water required in (a), the 
percentage extension and the tensile strength expressed in 



PLASTICITY 299 

kilogrammes per square centimetre is a figure which has been 
shown by Zschokke, Rasenow and others to agree with the 
relative plasticity of clays as far as this can be judged by 
practical potters. 

This method is imperfect, inasmuch as it does not give 
sufficient prominence to the power of certain clays to retain 
their plasticity when mixed with sand or rock-dust, but it 
has proved in the author's experience to be the least objec- 
tionable of any method yet published, and its value may 
readily be increased by applying it to mixtures of any clay to 
be tested with sand or other non-plastic material. 

In the production of bricks and tiles the plasticity of a clay 
is seldom developed to its fullest extent. Indeed, to do this 
would usually render the clay useless for these particular 
purposes, as the paste would shrink so much in drying and 
burning as to warp or crack. Where pottery or terra-cotta is 
being manufactured a higher degree of plasticity is usually 
necessary, and is obtained by a more thorough mixing with 
water and by storing the clay paste under conditions likely to 
increase its plasticity. This " souring " or " maturing " is a 
slow process, and some porcelain clay-mixtures require to be 
stored for several years before they are fit for use. 

The causes of plasticity being imperfectly understood, it is 
clear that the best method of increasing the plasticity of lean 
clays is still not known with certainty. If, as appears probable, 
the plasticity of clays is due to the hydrolytic action of water 
on the clays themselves, it is probable that any increase in 
plasticity must be effected by increasing this action of water. 
If, as suggested by Rohland and others, plasticity is a charac- 
teristic of colloidal substances mixed with inert particles, it 
will be increased by any treatment which will increase the 
proportion of colfoids, e.g., by storing in a moist condition in 
a cool place. An artificial or pseudo-plasticity may be con- 
ferred on clay by the addition of various vegetable colloids, 
such as starch paste, or of other colloids, such as 
glue, but the use of such materials is seldom practicable in 
brickmaking, as it is too costly. 

The reduction of the plasticity of clays may be effected in 
several ways, of which the most important are : (a) drying, 



300 THE RAW MATERIALS FOR BRICKS 

(6) heating to redness, (c) mixing with sand or other inert 
material, and (d) by the addition of a minute quantity of a 
suitable chemical. Of these methods, drying is only temporary, 
the clay again becoming plastic on further treatment with 
water. Heating to a temperature short of redness will reduce 
the plasticity of clay in proportion to the amount of water 
removed. If only the free water is driven off, the clay will again 
become plastic when moistened, but not if the clay molecule 
has been partially destroyed as only the complete molecules can 
become plastic. When a clay has been heated to redness it 
cannot again become plastic ; its chemical constitution has 
been destroyed and it is no longer a " clay " (p. 40). 

Clays which have had their plasticity reduced by mixing 
with sand may have it restored by any simple process of 
elutriation or washing which will remove the added material, 
but those which have been treated with chemicals behave 
differently. Thus, the addition of a few drops of baryta 
solution, or of a solution of caustic soda or potash, will convert 
a stiff plastic paste into a fluid slip or cream, but the addition 
of a quantity of acid just sufficient to neutralise the alkali 
previously added will re-convert the fluid slip into a stiff 
paste. 

The effect of merely a few drops of acid and alkali may be 
shown in a striking manner in the following experiment, which 
is well worth making by everyone interested in clays : 

Solutions of hydrochloric acid and of caustic soda are prepared 
of such a strength that a given volume of one of them exactly 
neutralises an equal volume of the other. (The " normal " 
solutions sold by manufacturing chemists are suitable.) A 
stiff clay paste is made by mixing -a little dry clay with water 
in a shallow cup or basin, and its stiffness noted. A few drops 
(accurately measured) of the alkaline solution are then added, 
and the mixing is continued until the mass becomes sufficiently 
fluid to pour into another vessel. When its fluidity has been 
clearly demonstrated, the same volume of the acid solution 
(a few drops) is added and the mixing still further continued, 
when it will be found that the clay again becomes stiff and 
pasty and cannot be poured from one vessel to another. 
Strictly speaking, the addition of acid or alkaline solutions only 



PLASTICITY 301 

alters the viscosity of the clay, but this property is so closely 
allied to plasticity that anything which affects the one must 
have some influence though not necessarily a proportionate 
one on the other. 

It has also been suggested that the power possessed by clays 
of adsorbing dyes from solution might be made a measure of 
plasticity, but experience has shown that adsorption and 
plasticity are not related closely enough for this purpose, 
though in many clays the relationship is remarkable. 

At the present time there is no entirely satisfactory method 
of determining the plasticity of clays, the methods now in 
use being merely approximations, some of which are far from 
accurate, except when used to compare several clays of the 
same type. 

The shrinkage of clay pastes is an important factor when 
classifying clays with regard to their suitability for brick- 
making. If the volume of a piece of clay paste is accurately 
measured before and after drying, it will be found to have 
diminished roughly in proportion to the plasticity of the clay 
and to the amount of water present in the paste. If the test 
piece is then burned in a kiln and is re-measured when cold, a 
further reduction in volume will have occurred ; this is known 
as kiln shrinkage. 

The shrinkage on drying which clay pastes undergo is due 
to the removal of the water surrounding (and possibly pene- 
trating) each particle, with the result that, as this water 
evaporates, the particles are brought nearer together, until 
finally they are as close as their shapes permit. As the particles 
are irregular in shape, complete contact over the whole of 
their surfaces is impossible,- and some pore spaces or voids are 
bound to occur. 

Any colloidal material present in the clay will absorb water 
with which it comes into contact -just as dry glue swells when 
immersed in water and forms a bulky gelatinous mass. Hence, 
the volume of the individual particles of colloidal matter in 
the clay is increased when it is mixed with water and made 
into a paste. When the clay paste is dried, this water is 
removed and the colloidal particles shrink to their original 
(dry) volume. If it be true that the plasticity of clays is due 



302 THE RAW MATERIALS FOR BRICKS 

to the proportion of active colloidal matter present, then the 
shrinkage they undergo on drying must bear some relation 
both to the colloidal matter and to their plasticity. Other 
causes of shrinkage such as the surrounding of clay particles 
into a film or covering of water are also present, and render 
it impossible to measure the influence of the colloidal matter, 
so that the plasticity and shrinkage are not directly propor- 
tional, yet, broadly, the more plastic the clay the greater will 
be the shrinkage of the clay paste and vice versa. 

Excessive shrinkage of a clay paste involves so great a 
movement of the particles that it is almost impossible to dry 
some clays without cracking them. Even when no heat is 
used, the irregular rate at which various parts of the material 
dry is a prolific cause of cracks, and one of the most difficult 
and worrying problems of the clayworker consists in drying 
bricks and other articles at a reasonable speed without intro- 
ducing internal strains and stresses which result in the fracture 
of the articles. The problem is still further complicated by 
the fact that it is often impossible to distinguish the cracked 
articles before they are fixed, so that it is only after the com- 
pletion of the manufacture when they are drawn out of the 
kilns that the defective articles can be separated from the 
rest. 

The shrinkage which would occur if the clay were taken 
direct from the pit and dried, is of small importance to the 
brick manufacturer, and though it is frequently reported in 
tests of clays it does not give the information required, except 
in those cases where the material is worked without the addition 
of any water. The clay or other material must first be made 
into a paste of the consistency it will have when moulded or 
otherwise shaped (see p. 332), and it should then be moulded 
into the form of a small brick or bar. The internal measure- 
ments of the mould will then give the size of the test-piece. 
After drying, until the whole of the free water is removed, 
the test-piece is again measured and the shrinkage calculated 
as (a) percentage of the original volume, (6) percentage of 
the original length, or (c) in inches per linear foot. All three 
units of shrinkage are in use, but the one most commonly 
employed in practice is the last, viz., the reduction in length 



THE SHRINKAGE OF CLAY 303 

which would be observed if a bar or block exactly one foot 
in length were made of the clay paste and then dried. 

The volume shrinkage (a) may be roughly calculated by 
multiplying the lineal shrinkage (b or c) by three, but this is 
not exact enough for accurate work. 

It is generally found that clays with a shrinkage exceeding 
one-and-a-half inches per linear foot are unsuitable for brick- 
making, unless their shrinkage can be reduced by the addition 
of a non-plastic material. The majority of brick manufacturers 
prefer a clay with a shrinkage of one inch per linear foot. 
Smaller shrinkage usually indicates that the plasticity of the 
clay is insufficiently developed or that too much sand or other 
inert material is present, with the result that the bricks will 
probably be soft and too easily crushed to be used to more than 
a very limited extent. At the same time, the fact must not 
be overlooked that some clays with an exceedingly low shrink- 
age are made into exceptionally strong bricks, some special 
reactions occurring in the kiln which overcome the weakness 
due to lack of plasticity in the clay. 

In the manufacture of architectural terra-cotta, in which 
a large number of pieces of different shapes must fit accurately 
together, it is highly important that accurate allowances 
shall be made for shrinkage. Where this is not the case the 
pieces fit so badly that the strength of the structure is seriously 
reduced, and its aesthetic value is largely destroyed. As the 
calculations for shrinkage are tedious when large quantities 
of work are to be executed, manufacturers of architectural 
terra-cotta usually provide their modellers with specially 
constructed measuring scales by means of which all calculations 
for shrinkage are avoided. 

The shrinkage of clays is an interesting study, inasmuch as 
it occurs at some stages with great regularity, and at others 
in a curiously irregular manner. Carefully recorded measure- 
ments of clays at various stages during drying and burning seem 
to show that the shrinkage proceeds proportionately to the 
loss of water during the early stages of drying, but as soon as 
the particles have come as closely into contact with each other 
as their shape permits, shrinkage ceases, though the loss of 
water continues. This indicates the close of the first and most 



304 THE RAW MATERIALS FOR BRICKS 

difficult stage of drying, for it is in the removal of the greater 
part of the absorbed or colloidal water, and the accompanying 
shrinkage, that the greater part of the cracking and warping of 
the goods occurs. Some clays are so delicate at this stage that 
the only way to dry them is to cover them closely with tarpaulin 
and to heat them with wet steam. The moisture in the steam 
prevents them drying, whilst being heated, and when they 
are at a sufficiently high temperature for the water in them 
to be converted into vapour, the steam supply is cut off and 
the bricks are uncovered and heated indirectly. This 
" sweating " is tedious and costly, but it is the only means 
available with certain clays. ' Most clays are less troublesome 
and provided they are protected from draughts or irregular 
heating, they may be dried in sheds of which the floors are 
heated with steam or fuel, and in summer large numbers of 
bricks are dried in the open air a slow process requiring 
several weeks. 

In the second stage of the drying, when no shrinkage 
accompanies the removal of the water, the goods may be heated 
more strongly so as to dry them with fair rapidity without 
incurring any serious risk of warping or cracking. At the end 
of this second stage the bricks or other articles are placed in 
the kiln, and there undergo a further contraction (" kiln 
shrinkage ") due to the decomposition of the clay molecule 
and the evolution of water. If the goods are damp when 
placed in the kiln, a further drying first occurs, and this 
necessitates a very gentle warming of the kiln during the first 
two or more days ; otherwise the evolution of steam will be 
so rapid that the goods will be cracked and broken by the 
expansive force of the imprisoned steam. When the heating 
is sufficiently slow, the steam escapes steadily, and without 
doing any damage, leaving a porous material devoid of all 
plasticity and resembling an extremely soft stone. 

The action of heat on clays to be used for bricks is also 
important. Some clays contain so large a proportion of metallic 
salts and oxides other than alumina and silica, that they cannot 
be burned on a large scale without serious loss of shape. The 
precise changes which occur when clay is heated must be left 
to a later chapter; it is sufficient for the moment to state 



ACTION OF HEAT ON CLAYS 305 

that a satisfactory clay for brickmaking, etc., must be able 
to withstand the heat of a kiln sufficiently long for a hard, 
compact material of the desired characteristics to be produced. 
To some extent the temperatures reached in the kilns may 
be adjusted to suit the clays being heated, but this can only 
be done to a limited extent, and where the composition of the 
material made into bricks is very irregular, it will be difficult, 
or, in some cases, impossible, to secure a large proportion 
of saleable goods. 

When overheated, bricks become vitrified, slag-like and very 
irregular in shape. They are deficient in porosity and, conse- 
quently, are very difficult for the bricklayer to use. Such 
bricks are known by a variety of names many of them of 
purely local use such as crozzles, burrs and clinkers. 

Firebricks are distinguished from others by their exceptional 
resistance to high temperatures. Most red-burning bricks 
will run to a shapeless mass if maintained for several hours 
at a temperature of 1,200 C., but firebrick will not lose 
its shape at 1,600 C., and the better qualities are in regular 
use at much higher temperatures. It is almost impossible 
to overheat firebricks during manufacture, but the clays from 
which they are made are too valuable to be used for ordinary 
building bricks. 

Impurities in Clays. Having realised that there are a number 
of ways of classifying clays according to one or more of their 
important properties, the reader will readily understand that 
an exact and complete classification is at present unattainable. 
All that can be stated is that most clays appear to consist of 
mixtures of stones, gravel, sand, silt, rock-flour and other 
inert materials all of which are obviously not of the nature 
of clay with a substance of a more or less plastic nature, 
the composition of which resembles that of an aluminosilicic 
acid (p. 40). 

According to the impurities present, a clay will be useful 
or worthless for certain purposes, and no single clay can be 
equally valuable for ail the purposes for which clays are used. 
Thus, an almost pure clay would produce white bricks, the 
production of red bricks and terra-cotta necessitating the 
presence of certain impurities, notably iron compounds. Again, 

c< x 



306 THE RAW MATERIALS FOR BRICKS 

a pure clay, if plastic, would shrink too much for ordinary use 
and would have to be diluted with sand or other non-plastic 
material. Hence, pure clay which does not occur in Nature 
is of less value to the brickmaker than the impure clays which 
serve his purposes so admirably. "Pure clay" is, in fact, 
little more than an abstract idea, for the most carefully refined 
plastic clays are invariably so different in properties from the 
refined china clays of apparently the same composition as to 
leave it a matter of conjecture whether they are two forms of 
identically the same substance or not. What appears to be 
most probable is that they are different aluminosilicic acids of 
highly complex structure. 

Whatever may be the real nature of the essential substance 
of all " clays " and brick earths, it is important, in some cases, 
to ascertain, at least roughly, the amount of clay substance 
present. This is a matter for the expert in clays and cannot 
be ascertained by the ordinary clayworker, nor is it shown 
by the analysis made by public analysts and other chemists 
with no special knowledge of clays. A method of so-called 
" rational analysis " sometimes used for this purpose is particu- 
larly misleading and erroneous when applied to brickmaking 
materials. 

It is convenient, and for most purposes sufficiently accurate, 
to consider clays and brick earths as being composed of a certain 
amount of real clay a together with silt, sand, chalk, gravel and 
stones. According to the nature and proportions of these 
adventitious ingredients the clays will prove satisfactory or 
otherwise for the manufacture of any given articles. 

In addition to the occurrence of impurities in these recog- 
nised forms, however, clays also contain impurities which are 
more conveniently considered under their separate names 
rather than as constituents of sand, etc. The most important 
of these impurities are : 

Free silica present as sand, or in a colloidal form readily 

1 The author has suggested the word pelinite derived from a Greek word mean- 
ing " of the nature of clay " for the clayey substance found in all plastic clays. 
The identity or otherwise of pelinite and kaolinite the latter being the essential 
constituent of kaolins and china clays remains to be proved, though the relation- 
ship between the two appears to be very close. Kaolinite is, however, almost 
devoid of plasticity. (See the author's " Natural History of Clay," Cambridge 
University Press.) 



IMPURITIES IN CLAYS 307 

soluble in a solution of caustic soda. This colloidal silica 
absorbs water readily and shrinks greatly on drying, and so 
is liable to cause trouble in manufacture. The free silica 
present as grains of quartz or sand, merely serves as a diluent 
of the clay, reducing its plasticity and shrinkage. If these 
silica grains are impure they will affect the clay according to 
the impurities they contain (see later). 

If a clay is very impure and readily fusible, the addition of 
sand may increase its power of heat-resistance as free silica is, 
in itself, highly refractory. 

Free alumina is, in many respects, like free silica so far 
as its behaviour in brick clays is concerned. It increases the 
heat-resistance of clays containing it, and appears to aid in 
the formation of buff -coloured bricks. Free alumina occurs 
in British clays to so small an extent that it is of little import- 
ance, but in some tropical countries its presence is highly 
significant. 

Lime compounds occur chiefly in the form of calcium car- 
bonate (limestone and chalk) and calcium sulphate (gypsum). 
Both these substances are converted into quicklime on pro- 
longed heating in a brick kiln, but the conversion is not always 
complete. Lime may also occur in the form of a calcium 
alumino-silicate as a species of felspar. It then acts as a flux, 
binding the surrounding particles together as described 
under (a) below. 

Lime compounds have three important characteristics 
when they occur in clays : 

(a) On heating the clay, the lime unites with the clay 
substance and with any free silica or free alumina present, 
and forms a viscous glassy mass, which cements the less fusible 
particles together into a hard vitrified and very strong mass. 
Where sufficient lime is present the mass may become so 
viscous as to lose its shape and form burrs, or clinkers (p. 377) 
instead of well shaped and sound bricks. 

(b) Soluble lime compounds (chiefly the sulphate) dissolve 
in the water used in making the clay paste, and are brought 
to the surface of the goods during the drying. As the water 
evaporates it leaves the salts on the surface in the form of a 
thin white deposit or scum, which is much more noticeable 

X 2 



308 THE RAW MATERIALS FOR BRICKS 

on the burned than on the green bricks. The only means of 
preventing this scum consists in converting the soluble salts 
into insoluble ones by the addition of barium carbonate or some 
other suitable chemical to the clay previous to mixing with 
water. The scum does no harm to the bricks except that it 
is unsightly and detracts from their selling value. 

(c) Small nodules of limestone, situated near the surface of 
a brick which is not heated sufficiently in the kiln to cause a 
combination of the lime with the other constituents, will become 
hydrated on exposure to moist air, and will then swell and crack 
the bricks, or they will " blow " and form ugly hollow places. 
This defect is peculiarly characteristic of some boulder clays 
(p. 294) and many thousands of bricks are damaged annually 
in this manner. Two remedies are employed : ( 1 ) grinding the 
material so fine that the particles of lime will be too small 
to harm the bricks, together with an increased finishing 
temperature in the kiln which will cause them to combine with 
the silica, etc., in the bricks and to become harmless ; (2) the 
bricks may be immersed in water as soon as they are drawn 
from the kiln. This sudden slaking of the lime prevents the 
bricks from cracking, but has the disadvantage that they 
are made heavy with the water they contain, and are not so 
readily purchased by builders. 

Magnesium compounds so closely resemble those of lime 
that there is no need, in brickmaking, to distinguish them. 
In the form of mica (a magnesium silicate) magnesium com- 
pounds occur in a large variety of clays and may usually be 
recognised by a silvery sheen, which is characteristic of mica 
particles. There are several kinds of mica, all of which are 
in the form of very thin plates, which do little or no damage 
when present in small quantities, but in larger ones they tend 
to weaken a structure of articles made from the clay in which 
they occur. 

The influence of mica in the kiln is of small importance in 
ordinary brickmaking ; it tends to make the clay-mass fusible, 
but at the ordinary temperatures at which bricks are burned 
its action is scarcely noticeable. 

Sodium and potassium compounds are generally regarded 
together under the term alkalies, though, in reality, the forms 



IMPURITIES IN CLAYS 309 

in which they occur are salts, such as felspar, muscovite, mica, 
and other minerals. Some clays also contain sea salt (sodium 
chloride), sodium sulphate, potassium chloride and sulphate, 
and other soluble salts. These form a scum, as described in 
(b), p. 307. 

Usually, sodium and potassium compounds are among the 
most fusible constituents of a brickmaking material, and they 
therefore bind the more refractory particles together. When 
present in very small proportions, they increase the strength 
of the bricks, but in large proportions they bring about so 
rapid a vitrification and fluidity of the clay that an excessive 
number of shapeless goods is produced. 

Iron compounds in brick clays appear to have been derived 
from a number of minerals, of which the most important are : 
(a) certain ill-defined ferrosilicates which form free ferric oxide 
when the clay is heated, (b) pyrites and other forms of ferric 
sulphide, (c) limonite and other forms of ferric hydrate, and 
(d) other iron compounds occurring in very small quantities, 
and of such a nature that they may be regarded by the brick 
manufacturer as though replaced by compounds in one of the 
three previous groups. 

Silicates containing iron are not easily identified in clays, 
but are of two classes those in which the iron remains in the 
form of a silicate after being heated to redness, and those 
which are decomposed on heating, red ferric oxide being one 
of the products. In the first class of compounds, the iron 
is in the form of a base and may be regarded as having partially 
displaced one or more of the other metallic bases as in the micas. 
In the second group the iron forms part of a complex acid 
group, as in nontronite and other ferrosilicates which are 
analogous to clays, and on heating form free silica and iron 
oxide. 

Sulphide of iron (FeS 2 ) occurs in four well-known forms in 
clays : 

Pyrite as nodular or kidney-shaped masses, which, on frac- 
ture, are seen to consist of minute brassy cubes, or as the 
minute cubes separately. The brass-like lustre is very charac- 
teristic, and pyrite is occasionally mistaken for metallic 
gold. 



310 THE HAW MATERIALS FOR BRICKS 

Marcasite, as fibrous masses and twin-rhombic crystals, and 
occasionally as nodules. 

Mundic, as root or twig-like masses of great relative weight, 
which, on fracture, are found to consist of either pyrite or 
marcasite. 

Chalcopyrite, as nodules resembling pyrite, but differing in 
containing copper sulphide as well as iron sulphide. 

All forms of iron sulphide are objectionable in brick clays, as 
they form black spots or " splashes " of black slag on the 
surface of the fired goods. They occur less frequently in the 
red-burning clays than in the fire clay and buff-burning ones, 
the pyrites originally present in the first-named having, it is 
believed, been oxidised previous to, or shortly after, its intro- 
duction into the clay. 

Hydrates of iron of which limonite is the best example 
yield red-ferric oxide on heating, and evolve water. The 
proportion of water in chemical combination with the iron 
appears to vary greatly, and the formula 2Fe. 2 O^H. 2 0, usually 
attributed to limonite, cannot be relied upon. These hydrates 
usually occur in nodules which may be picked out of the clay- 
especially when it has been exposed to the weather but they 
also occur distributed throughout the clay mass in so fine a 
state of division as to give the raw clay a yellow, brown, or 
even red colour. The proportion of combined water in 
limonite and allied compounds appears to influence the colour ; 
this water may not be in strict chemical combination, however, 
but rather as a colloidal material. 

Ferrous compounds are not readily noticeable in raw clays, 
but in those which have been heated they form bluish-grey or 
black slags, to which the well-known " blue Staffordshire 
bricks " owe their characteristic colour, the ferrous compounds 
being formed in the kiln by the action of the reducing gases 
from the fuel. 

Carbonaceous matter, derived from plants and animal remains, 
occurs in many clays, and usually gives them a greyish or brown 
colour. If this impurity has become carbonised the clay may 
be quite black, though the total proportion of carbon may not 
exceed 5 or 6 per cent. 

All carbonaceous matter burns away when clays are slowly 



IMPURITIES IN CLAYS 311 

heated in contact with air, so that it has no direct influence on 
the colour of the finished goods. Indirectly, it tends to produce 
a reducing atmosphere in the kiln, and so may effect the 
reduction of some ferric oxide and the formation of the bluish 
ferrous silicate. 

When carbonaceous matter is burned out of a clay, the latter 
becomes porous ; hence, sawdust, etc. is sometimes added to 
a clay when specially porous bricks are desired. 

Other impurities are occasionally important in clays to be 
used for special purposes. They usually reduce the heat 
resistance of a clay (as titanium oxide), or affect its colour 
when burned (as tourmaline), but are not ordinarily of import- 
ance in brickmaking, and, therefore, need no further mention 
in the present volume. Further information concerning them 
will be found in the author's " British Clays, Shales and 
Sands." 

The following clays are of sufficient importance in brick- 
making to be described here in addition to those previously 
mentioned : 

Agglomerate clay is a mixture of angular stones and fragments 
of rock cemented together with a plastic clay. It is of little 
or no value. 

Alluvial clay collects in valleys in various parts of the 
country, and is usually of very fine texture. In composition 
and general character it is liable to vary very greatly within 
small areas, and is, therefore, less trustworthy than marine 
deposited clays. Alluvial clay is often rich in chalk and lime- 
stone dust, and is then of insignificant value. 

The term " alluvial clay " is often used to distinguish light 
fine clays of low plasticity from the compact, strongly plastic 
clays, and from the loams and shales. 

Ball clays are not intentionally made into bricks unless the 
clays are of exceptionally low quality. The better qualities 
are too valuable, and are used for pottery manufacture. 

Brick earth is a term used to denote any material containing 
clay which can be made into good bricks. Some natural 
products contain so little clay that they cannot suitably be. 
termed " clays," and yet they can be made into good bricks. 
Many loams and true marls are of this class, but the term may 



312 THE RAW MATERIALS FOR BRICKS 

be applied with accuracy to any clay or clay mixture which 
can be made into good, merchantable bricks. 

Clunches are clays which are mined and not quarried, and 
may be used for a common brick shale or clay or for a high- 
class firebrick. The term has nothing to do with their 
composition. 

Conglomerate days are similar to agglomerate clays (q.v.), 
but the rock particles are rounded instead of angular. They 
are of little or no value for brickmaking. 

Drift days are commonly termed " Boulder clays," and form 
the chief brickmaking material in the north of England, where 
they occur in enormous quantities as the residue. As explained 
on p. 296, these clays make good building bricks when suffi- 
ciently free from stones, but they are liable to be spoiled by 
fragments of limestone in the gravels with which they are 
associated. With care this difficulty may be overcome, and 
good, hard bricks produced. 

Fat days are those which are oleaginous in consistency and 
usually possess a high degree of plasticity. When used alone 
they are seldom satisfactory for brickmaking, but on the 
addition of grog, sand, or very lean clay, they form excellent 
bricks. 

Firedays are those which can withstand exposure to a 
temperature rather higher than that of molten steel without 
showing signs of fusion, and are, consequently, valuable for 
furnaces, kilns, and other structures where a refractory material 
is required. 

Fireclays are mined from pits sunk in the Coal Measures, and 
are closely associated with all the best known seams of coal. 
The manufacture of firebricks, retorts, etc., is, therefore, largely 
undertaken by colliery proprietors, or by firms working in 
connection with them. The best fireclays occur in the Stour- 
bridge, Yorkshire, Northumberland and Scotch coalfields, but 
others of almost equal quality are found in other districts, 
particularly in Wales. 

The chief characteristics required in a fireclay are resistance 
to heat and to sudden changes of temperature, but other 
requisite properties are resistance to the corrosive action of 
slags, dross and metallic compounds, and to the abrasive 



FIRECLAYS 313 

action of lumps of lime, stone or ore and of flue-gases. As it 
is impossible to obtain a fireclay which can be used for all the 
purposes for which firebricks, etc. are used, it is necessary to 
select certain clays for certain purposes. For this reason the 
fireclays from different localities are specially adapted for 
different purposes. The colour of raw fireclays is a dirty grey, 
which becomes buff or stone colour on heating, the fired product 
being usually spotted with grains of ferrous silicate due to the 
pyrites in the clay (p. 309). 

Fireclays are largely used in the manufacture of crucibles, 
for which purpose they are frequently mixed with plumbago, 
which renders them less sensitive to sudden changes in tempera- 
ture and tends to maintain a reducing atmosphere in contact 
with the contents of the crucibles. This is important in some 
metallurgical operations. 

Allied to the fireclays, though not really a clay, but a 
siliceous rock with a low clay content, is the material known as 
ganister, which usually occurs in the coalfields below the coal 
seams : it is occasionally found nearer the surface, and in 
such cases the previously superincumbent coal has probably 
been removed by denudation. 

The subject of fireclays is a very large one, and the reader 
desiring more information should therefore consult the author's 
" British Clays, Shales and Sands." 

Fusible clays may be regarded as the opposite of fireclays, 
as they undergo partial fusion, and form a hard, vitrified mass 
when heated. If such clays retain their shape at a temperature 
approaching the melting point of steel, they are exceedingly 
valuable, and under the term " stoneware " are employed on 
a very extensive scale (see " Verifiable Clays "). 

Engineering, clinker and paving bricks are made of some- 
what fusible clays which are able to retain their shape at a 
high temperature, notwithstanding the larger amount of 
vitrification which takes place. They owe their strength to 
the tenacity with which the infusible particles in the clay are 
bound together by the more fusible constituents. 

Gault is a stiff, dark-coloured clay which occurs in the 
Greensand formation in the south-eastern counties, and extends 
westward towards Midhurst and southward to Eastbourne. 



314 THE RAW MATERIALS FOR BRICKS 

Owing to the calcium carbonate it contains, bricks made from 
it are white. Some gault bricks are reddish in colour, but white 
ones may be made by the addition of chalk. Wherever it 
crops out near the surface of the ground, gault clay is found 
suitable for brickmaking, the exceptions to this statement being 
few, and chiefly in some parts of Cambridgeshire where the 
clay is more unctuous (see p. 6). 

Though largely used for brickmaking especially after the 
addition of chalk gault clay cannot be regarded as a high-class 
material, though for common bricks it is excellent. 

Grog is simply clay which has been burned in a kiln and then 
reduced to powder. The clay may be calcined specially, or 
the grog may be made by crushing broken bricks. According 
to the nature of the original clay, so will the grog be refractory 
or fusible, soft or hard, buff-coloured or red. The chief uses 
of grog are to increase the heat -resistance of a fusible clay 
and to reduce the plasticity and shrinkage of a clay which 
would otherwise be difficult to work. 

Kaolin or china clay (the latter being one kind of kaolin) is 
not used for brickmaking, though the sandy residues obtained 
by washing are made into bricks in some localities. The value 
of good kaolin to paper makers and others is so great and 
its plasticity is so low that it is not likely to meet with 
extended use in the manufacture of building bricks in this 
country. 

Laminated clays are those composed of layers or flakes and 
are troublesome to use, as they tend to split along the lines of 
deposition. When hard, they form shales (p. 316) to which 
this objection is less applicable, as the harder material can be 
reduced to particles so small that their lamination becomes 
imperceptible. 

Lean clays may be regarded as the opposite of " fat " ones 
(p. 312) and have only slight plasticity. Clays of moderate 
leanness are the most useful for brickmaking, as they do not 
shrink unduly in drying. Indeed, many plastic clays which, 
by themselves, would be useless, may be made valuable by 
the addition of sufficient sand or grog to convert them into 
moderately lean clays. Clays which are so lean that they do 
not readily retain the shape into which they have been formed 



VARIOUS CLAYS 315 

are of little value unless some of the sand or other non-plastic 
material they contain can be removed cheaply. 

London clay is one of the best known clays, and is at the same 
time one of the most risky for the manufacture of bricks. 
It occurs chiefly around London and occupies a very extensive 
area. Unfortunately, it is so sticky and contractile that 
alone it is practically useless for brickmaking, but in localities 
where it occurs in close association with sand, it is a valuable 
material for this purpose. It is to be regretted that in many 
localities where it would be most useful the absence of sand 
renders it of no value. 

The portions of London clay which are worked are mixed 
with sifted cinder dust (technically termed soil) and chalk, 
whereby the contraction is reduced and bricks obtained which 
have been long famous for their ability to resist as no other 
building material appears to do the corrosive action of the 
atmosphere and climate of the Metropolis. 

Marine deposited clays are those which were originally 
deposited on an ocean bed, but now form dry land. They 
occur in many parts of Great Britain, that known geologically 
as the Oxford clay being one of the most important, particularly 
near Peterborough and Fletton, where some of the largest 
brickworks in the country are situated. 

The Midland marls are Triassic clays of a friable texture 
and moderate plasticity, and are not true marls (p. 9). 
They are well known as the raw material from which the 
excellent terra-cotta and red facing bricks in the Midlands are 
made. These clays cover a very extensive area, and are 
largely used in places so far apart as Nottingham, Leicester, 
Shropshire and Wrexham. 

Reading clays form the western end of the London basin 
and are much esteemed for the manufacture of red tiles and 
bricks. They are of moderate plasticity and not particularly 
difficult to work, providing that they are carefully selected. 

Refractory clays have been sufficiently described under 
" Fire clays " (p. 312). 

Rock clays are indurated clay deposits which have attained 
a hardness and form corresponding to that of rocks, owing 
to the pressure of neighbouring strata and the interpenetration 



316 THE RAW MATERIALS FOR BRICKS 

of cementitious solutions. Shales, slates and fireclays are 
typical rock clays. 

Sandy clays are, .as their name implies, mixtures of clay and 
sand which contain so large a proportion of the latter substance 
as to have a sandy nature. As " loams " they are valuable, 
but if too rich in sand (" clayey sands ") they are useless for 
brickmaking, as they do not possess sufficient cohesion. 

Shales are laminated, indurated clays which must be crushed 
to powder and kneaded with water before they become plastic. 
They occur in many localities usually at some depth below 
the surface of the ground and their composition varies greatly. 
Some shales form buff, and others red bricks, and a few shales 
contain so much fusible matter that blue engineering bricks 
can be made from them. 

Most shales contain only a small percentage of carbonaceous 
matter, but others contain so much shale oil that they are very 
difficult to use for brickmaking, and a specially designed kiln 
is usually necessary for them. 

The shales chiefly used for brickmaking occur in the Oxford 
clay, Lias, Wealden and Coal Measure formations. The loca- 
tion and extent of these can be seen in any good geological 
atlas (see also p. 8). 

Silt is an extremely fine sand which usually contains sufficient 
clay to form a plastic mass. It is chiefly found on the sites of 
ancient rivers and in low-lying districts, and is found in large 
quantities in the eastern counties. The silt found near Hull 
(and termed warp) has been largely used for brickmaking with 
considerable success, but it needs special knowledge and skill 
to obtain good results, and would probably prove disastrous 
to anyone not acquainted with its peculiar characteristics. 

Slates are very hard clays, which have undergone a partial 
recrystallisation. They are not extensively used for brick- 
making, though the accumulation of rubbish in slate quarries 
has caused many attempts to be made to convert this material 
into bricks. The material is not well adapted to this purpose 
and could only be used commercially in localities where ordinary 
bricks were unobtainable. 

Soil is the uppermost layer of earthly material, and is the 
"home" of plants. It does not consist entirely of mineral 



VARIOUS CLAYS 317 

matter, but usually contains a considerable proportion of 
materials derived from the decay of plants as well as of animal 
excreta and artificial fertilisers. On this account it is not 
usually suitable for inclusion in the material employed for 
brickmaking, and should, in most cases, be kept separate. 
The soil overlying a useful deposit of clay is commonly termed 
the overburden or callow, and is generally removed before the 
underlying clay is dug. 

Strong days are highly plastic and shrink, crack and warp 
extensively when made into bricks and dried. They are 
objectionable to most brickmakers, who commonly term 
them foul days. If mixed with sand or grog, they usually 
make good brick earths, but the difficulty and cost of obtaining 
a satisfactory mixture often prohibits their use. 

Surface days are, strictly, any clays which occur near the 
surface of the ground and immediately below the soil. The 
term is, however, used colloquially by many brickmakers to 
indicate a red-burning clay as distinct from a buff-burning 
material obtained from a greater depth. This use of the 
term has led to much confusion in the past and should be 
avoided. 

Tender days are those which crack or warp when made into 
bricks. They are usually characterised by a high shrinkage 
and great plasticity. The tenderness may be reduced by the 
addition of sand or chalk, but some tender clays are so peculiarly 
constituted that the addition of non-plastic materials makes 
them so weak and friable as to be useless. Tender clays are 
the ruin of brickmakers who do not know how to deal with 
them, as they require expert knowledge before their use can 
be satisfactory. 

Till day is the plastic clay forming the lower portion of the 
boulder clay (p. 294), and is valued for brickmaking on account 
of its freedom from stones and gravel. The term is, however, 
applied in a loose manner to boulder clay generally, in which 
case its special significance is lost. 

Verifiable days are used in brickmaking for the production 
of bricks of great strength and imperviousness to water and 
other fluids. Vitrification is the state in which a portion of 
the clay fuses and binds the remaining particles firmly together, 



318 THE RAW MATERIALS FOR BRICKS 

the fused portion filling some or all of the pores previously 
existing in the material. The extent to which vitrification 
occurs depends on the nature of the fusible material present 
and on the temperature and duration of the heating ; it is 
said to be complete when the whole of the pores have been 
completely filled with fused material. Nearly all clays can 
be vitrified if a sufficiently high temperature is reached, but- 
many of them lose their shape before vitrification is complete. 
The commercial value of vitrifiable clays, therefore, depends 
on a considerable time elapsing between the production of a 
sufficient amount of fused material to close a sufficient number 
of pores and the commencing of a noticeable loss of shape. 
Clays which begin to vitrify and then to lose shape almost 
instantaneously or without any appreciable rise in temperature 
are useless commercially as vitrifiable clays. The time of 
heating or the rising temperature required between these two 
changes is termed the " range of vitrification," and it is an 
important factor in deciding the value of a clay. Clays in 
which the more readily fusible portion is rich in potash, soda or 
lime, usually have a short range of vitrification, and are 
therefore less valuable than clays in which the chief flux is 
magnesia, and therefore possess a longer range of vitrification. 

The vitrifiable clays chiefly used in brickmaking are found 
in the Midlands, where they form the buff paving bricks of 
the Little Bytham district and the famous blue bricks of 
Staffordshire, but equally good bricks can be obtained from 
selected seams in most of the coalfields. The purer stoneware 
clays of Dorset, Devon and Cheshire are too valuable for this 
purpose, except where bricks impervious to strong acids are 
required for use in chemical works and for other special 
purposes. 

Yellow clays, i.e., those which are yellow when freshly dug, 
are usually strong clays (p. 317) and difficult to work without 
the addition of sand, but the yellow colour gives so little 
indication of their nature that further tests are necessary 
before their value can be ascertained. 



CHAPTER XII 

METHODS OF BRICKMAKING 

FROM what has been stated on the foregoing pages, the 
reader will easily perceive that the variety of clays available 
and their complex nature make the manufacture of bricks a 
work requiring more skill than is generally supposed. 

Capital required. Before commencing the manufacture of 
bricks, and allied articles, there are two essentials which must 
be considered, viz., capital and knowledge. Without sufficient 
capital the risks of failure are very great, because the manu- 
facture of bricks is subject to many vicissitudes which cannot 
be overcome without ample financial backing. 

The amount of capital needed depends greatly on the 
locality of the works ; in an undeveloped area, where common 
bricks will be all that is desired, the money needed will be 
small a few hundred pounds but for a plant with an annual 
output of a million or more bricks, the capital required will be 
much greater. In the case of an " average " works producing 
20,000 bricks per day, it may easily reach to 10,000, and many 
works with this output have a capital more than four times 
this amount. It is only fair to add, however, that the capital 
invested in a large number of brickworks is to .a considerable 
extent " lost," it having been spent in continuous attempts 
to improve the product by methods which a man with sufficient 
technical and scientific knowledge could have predicted would 
be futile. 

The cases where men have made fortunes out of brickmaking 
become fewer each year, and an investigation of most of them 
will show that whilst these brick manufacturers may not have 
had much cash of their own, yet the credit they have been 
able to obtain and other facilities they have possessed have 
had the same effect as the possession of considerable capital. 

A man who would start a brickworks in the United Kingdom 



320 METHODS OF BRICKMAKING 

at the present time without ample capital in some form or 
other would be almost certain to fail. With sufficient capital, 
however, there, are excellent prospects in a number of 
localities, particularly in the Midlands. 

Technical Knowledge Needed. Most brickmakers who started 
in business forty years or more ago did so under conditions 
very different from those at the present time. The use of 
machinery for this purpose was practically unknown, and the 
profits obtained were much higher than at the present time. 
With little or no literature on the subject the brickmakers of 
half a century ago were only able to use a very limited number 
of clay deposits, and those of a peculiarly favourable character. 

The introduction of machinery effected a complete revolution 
in the technical equipment required, it increased enormously 
the number of clays and earths which could be made into 
bricks, and created an impression as false as it is widespread 
that, so long as an earthy material feels plastic when mixed 
with a little water, good bricks can be made from it by any 
man possessing the necessary plant. 

This belief has caused the loss of hundreds of thousands of 
pounds, for the manufacture of bricks is an industry requiring 
much technical knowledge, and it is quite a mistake to imagine 
as many engineers do that all that is requisite are a few 
moulds or a machine, a kiln, and a few labourers. With good 
fortune the clay available may happen to be one which is 
easily worked in the manner which appeals most to the pros- 
pective brickmaker, but the enormous number of derelict 
works and of machines which fail to find purchasers prove 
that those who regard brickmaking merely as an elementary 
branch of engineering run risks of the most serious financial 
character. 

In the author's experience as an expert adviser for many 
years past, he has found numberless cases of firms who were 
entirely mistaken as to the correct methods of working the 
particular clay available to them, and who, in consequence, 
have lost enormous sums of money which might have been 
saved had they obtained reliable and independent expert 
advice before purchasing plant or ordering the erection of 
kilns. 



TECHNICAL KNOWLEDGE NEEDED 321 

In order that a brickworks may be a commercial success, it 
is necessary that only a reasonable amount of skill shall be 
needed in the production of bricks, and it is precisely because 
of failure to realise this fact that so many brickworks prove to 
be failures. Given ample time and labour, a few good 
merchantable bricks can be made with almost any machinery 
on the market, but this is far different from the conditions 
which must prevail in a brickworks working commercially. 
The result is that many prospective brick manufacturers are 
entirely misled by the sample bricks made from their clay 
by enterprising firms of machinery makers and kiln builders, 
and find, when too late, that they have prosecuted their 
enquiries in the wrong direction. 

Almost equally serious and erroneous are so-called " tests " 
made by " practical men " on small quantities of clay. These 
" match box tests " (so called because a match box is frequently 
used in place of a proper mould) may or may not yield results 
worth the labour expended on them, but in any case it is exceed- 
ingly foolish to erect a works on so slight a result. The pro- 
duction of even 1,000 bricks by a friendly brickmaker is by no 
means conclusive evidence of the nature of the clay, as there 
are many other matters to be considered in manufacturing, 
of which these tests give no indication. 

Finally, it is almost useless to have an analysis made of the 
clay. Certain chemical and physical tests are essential, but 
a chemical analysis as conducted by a works chemist or a public 
analyst is of scarcely any value, as it does not give the informa- 
tion required. 

The only satisfactory method of ascertaining whether a 
given clay is likely to be suitable for the manufacture of bricks 
is to have it examined by an expert in clay testing, who is 
financially independent of all machinery and kiln constructing 
firms, and who is known not to accept secret commissions, 
rebates or other inducements which will bias his recommenda- 
tions. The fees charged by such a man will be saved in the 
avoidance of unnecessary machinery and in the absence of 
those annoying " extras " by which the original estimate 
of costs is usually increased so largely, but which are almost 
unavoidable in the absence of such skilled technical supervision. 

C. Y 



322 METHODS OF BRICKMAKING 

The man who wishes to build a house employs, if he is wise, 
an architect to advise and assist him in its design and erection, 
and to check the tendency to extravagance on the part of 
both owner and builder. The man who is ill is most likely 
to recover if he seeks the aid of a medical man, for an able 
physician, who has made a special study of the subject, wastes 
no time in effecting a cure. " A man who is his own lawyer 
has a fool for a client " is a proverb famous alike for its general 
applicability and its truthfulness. Yet an error in the selection 
of brick machinery or kilns, or in the valuation of a clay pro- 
perty, may easily involve the loss of several thousands of 
pounds, which the employment of a reliable expert in clay 
working would have saved. 

Many instances might be quoted to illustrate this, but the 
following recent one, from the author's personal experience, 
must suffice. A certain landowner found a shale on his land, 
which he believed to be valuable, especially as the neighbour- 
hood was one in which there was an increasing demand for 
bricks. A company was formed, machinery purchased, kilns 
erected and work begun. But, alas ! the output was less than 
half that anticipated, and the costs were slightly higher than 
the selling price of bricks in the neighbourhood ! Many weary 
months of working failed to improve the conditions and eventu- 
ally an expert was called in to report upon the whole works. 
He found, as was only to be expected, that the machinery 
installed had been seriously overrated for the particular 
material to be treated, though the output promised could 
readily have been obtained with a different clay. He learned 
that various firms had tendered for the supply of machinery, 
and that the accepted tender was for the machinery not 
best fitted for the work. The kilns had been purchased in an 
indirect manner, the ordinary price for eight kilns had been paid 
for the five kilns erected (the balance representing a commis- 
sion to the firm introducing the kiln builders, and a "little 
extra " for the builders because there was no competition). 
Reconstruction of the works was imperative, but was rendered 
difficult because the shareholders, having been " once bitten " 
were " twice shy." Eventually, however, the rearrangement 
of the plant was completed, and it has been working satisfac- 



TECHNICAL KNOWLEDGE NEEDED 323 

torily ever since. Had the present method of working been 
installed in the first instance as would certainly have been 
the case had the expert been consulted at a sufficiently early 
stage the total amount saved would have been 9,000 plus 
the losses due to working with unsuitable machinery. 

This is not an isolated example, but is typical of many in 
various parts of the United Kingdom. It is not fair to blame 
the firm who supplied the machinery and kilns ; their first 
business was to sell their own goods and to make out the best 
case for these. To have recommended the inclusion of plant 
they did not make was no part of their work, hence their advice 
was necessarily and unavoidably biassed, whereas an expert 
expressly chosen because of his independence and freedom 
from bias of this kind would have specified the plant and kilns 
most suitable for that particular material. 

The following description of methods of manufacturing 
bricks is only intended to outline the most important processes, 
and the reader who wishes for further information and for 
more illustrations of the machinery, etc. employed, should 
consult the author's " Modern Brickmaking " (Scott, 
Greenwood & Son), the author's " Clay workers' Handbook " 
(Griffin & Co.) or " Bricks and Tiles," by Dobson and Searle 
(Crosby, Lockwood & Co.). 

Mining and Quarrying. The first operation in the manufac- 
ture of bricks is the mining or quarrying of the clay or brick 
earth and its delivery to the machinery which prepares it 
for use. A description of the various methods of working in 
clay pits and mines would require a volume to itself, and it 
must here suffice to state that in mining the ordinary colliery 
methods are employed, and that in quarrying the clay is usually 
obtained by means of picks and shovels, the excavation being 
carried out in a series of shelves or ledges. Very hard materials 
are loosened by blasting with gelignite or other " safety " 
explosives . 

Steam navvies and ditch cutters are used where the material 
is sufficiently soft and uniform, but their use is impracticable 
in many brickyards on account of the need for selecting certain 
portions and discarding others from the quarry face. 

Selection of suitable materials and rejection of unsuitable 

Y 2 



324 METHODS OF BRICKMAKING 

ones will make all the difference between good and bad bricks, 
and in most quarries too much care cannot be taken to keep 
detrimental material away from the useful clay. It is, gener- 
ally speaking, foolish to mix the materials indiscriminately 
together and expect good bricks to be made, though this is 
done successfully in some yards where the conditions of 
deposition of the various materials has been peculiar. The 
wise plan is to keep various materials separate, loading them 
into separate waggons and then mixing them in the desired 
proportions in the machines. Such treatment secures a more 
uniform product than is possible when they are mixed in the pit. 

At one time, wheelbarrows were used to convey the clay 
to the machines ; these are now used in some small works, 
but in the larger ones waggons are preferred. These waggons 
have a capacity of 5 to 15 cwts. of clay, the smaller ones being 
used on steep inclines and the larger ones for general work. 
The waggons travel on rails, turntables or " points " being used 
at junctions, so as to provide a good surface on which the wheels 
may rotate. These rails are of narrow gauge and are usually 
of a semi-portable character, so that they may be extended 
rapidly to convenient parts of the clay pit. 

The loaded waggons are pushed by one or more men from 
the working face in the pit until they reach the hauling 
mechanism, or ponies may be employed to do the whole of the 
haulage if the clay hole is not very deep. Usually, the best 
method of haulage consists in the use of an endless rope or 
chain to which the waggons are attached by some simple 
form of clip. In some works the use of a simple haulage rope 
or a main and tail system, such as is used in collieries, is 
preferred, the arrangement most suitable in any particular case 
depending on the length of haulage, the inclines to be nego- 
tiated, the changes in direction of travel, and other purely 
local conditions. 

Overhead ropeways are by far the most economical method 
of conveying clay to the works where the distance and 
quantity are large, but most brickworks are too small for an 
overhead ropeway to be used. 

Many brick manufacturers spend far more in haulage costs 
than is really necessary, because of the directions in which 



MINING AND QUARRYING 325 

the rails have been laid, but the reduction of these costs is 
too complex a problem for a solution to be attempted here. 
Suffice it to say that the shorter and more direct the course along 
which the waggons travel providing the inclination is not too 
steep and the nearer the further end of the hauling plant is to 
the working face, the cheaper will be the cost of transportation. 

As soon as the waggons with their load of clay, sand or other 
material reach the near or machine end of their journey, the 
second stage of the manufacture is reached. 

Clay Preparation. The conversion of the freshly-mined or 
quarried material into one which is suitable for the direct 
production of bricks is by no means a simple task. Clays and 
brick-earths vary so much in composition and physical nature 
that a treatment which is ample for one may be quite insuffi- 
cient or even unsuitable in character for another, and any 
short description of the processes of preparation must neces- 
sarily be merely indicative and incomplete. 

The object of all preparation processes is the production of 
a plastic paste of such a consistency that it may readily be 
formed into bricks or other articles of any desired size and 
shape. 1 In order that this object may be attained, the material 
may require to be crushed to powder and then kneaded or 
mixed with a suitable quantity of water, the latter treatment 
being carried out in such a manner as will secure a paste of 
as uniform a composition and texture as possible. Insufficient 
or careless treatment in the preparation of the paste is one of 
the most frequent causes of defective goods. 

Some clays occur naturally as a paste, which only requires 
to be kneaded to make it uniform and of the desired consis- 
tency ; these are the easiest clays to manipulate, but the 
quantity available is limited. Other clays require the addition 
of water arid possibly of sand, chalk, cinder dust or other 
non-plastic materials, and some of them require to be crushed 
or pressed into thin sheets before being kneaded ; unless this 
is done the water added will not mix properly, and they cannot 
be made into a suitable paste. 

1 The only exception to this is in what is known as the " dry dust," or " semi- 
dry " process, in which the finely powdered material is compressed in powerful 
presses to the desired shape. This method of manufacture is described later. 



326 METHODS OF BRICKMAKING 

Marls, loams and friable brick-earths require a much larger 
proportion of water and a more thorough kneading before a 
uniform paste is produced. Some of these materials also 
require to be crushed or ground to powder and sifted before 
being mixed with water. If this grinding is omitted the 
" paste " will be so irregular in texture as to be quite useless 
for brickmaking. 

Shales, also clays containing stones or pieces of hard material, 
must always be ground to powder and sifted the coarse 
particles being returned to the mill and reground before 
they can be kneaded with water to form a suitable paste. If 
the clay is naturally of a pasty nature, with small stones or 
gravel embedded in it, the preliminary crushing is sometimes 
extremely difficult, the unctuous nature of the clay making it 
almost impossible to crush the other ingredients sufficiently 
small. 

During the past few years many clays of this character have 
been made available by means of a day cleaner, which consists 
of a cylinder with a series of small apertures through which 
the soft, plastic material and the finer grains of gravel, sand, 
etc., are forced, whilst the coarser particles are retained because 
they are too large to pass through the apertures. Clay-cleaning 
machines are of several types, but the separating principle just 
mentioned is their chief feature. 

Clays which contain sand as well as stones or gravel cannot 
be sufficiently purified in this manner, but must be washed by 
stirring the material with water in a large trough or shallow 
well, the solid portion being broken up by means of rotary 
beaters or " hurdles " until a cream or slurry is formed. This 
slurry is allowed to remain stationary for a few moments, 
during which much of the sand settles out, and the fluid is 
then run off to settling pits, where, after some time, the clear 
supernatant water is cautiously run off, and the clay paste 
remaining in the pits is allowed to stiffen. When sufficiently 
dry it is then taken to the mixing machines in order to render 
its texture as uniform as possible. No washing, however 
perfect, can completely remove the impurities from clay, but 
the treatment just described is extensively used by bookmakers 
in the southern counties, particularly in those districts where 



PREPARATION OF THE CLAY 327 

chalk is mixed with the clay. In such a case the chalk is 
made into a slurry in a separate wash-mill, and this slurry is 
then run into the clay-mill and afterwards to the settling pits. 
Before any clay or brick-earth is crushed, however, it should 
be examined in order to ascertain whether the same effect and 
other improvements in its nature cannot be obtained by 
exposure to the weather. Many hard clays and earths are 
reduced to a comparatively soft material when exposed to 
frost, and not a few of them fall almost to powder if merely 
exposed for a few days to the action of the atmosphere. This 
exposure (or weathering) not only facilitates the crushing and 
mixing of the clay, but it frequently brings about chemical 
and physical changes of the greatest importance to the brick 
manufacturer. The precise nature of some of these changes 
is obscure, but it appears to be a kind of oxidation combined 
with the production of internal stresses and strains which 
cause the particles of clay to separate from each other and to 
form a loosely coherent mass. Weathering also effects the 
purification of some clays by causing the solution of some of 
the impurities, and, as the water evaporates, some of these 
are carried to the surface and form concretions or a scum which 
can be picked or scraped from the surface. 

The crushing machinery used in brickmaking is of three chief 
types, consisting of : 

(a) One or more pairs of horizontal rolls, each about 18 inches 
diameter and 24 inches wide, placed side by side in such a 
manner that the raw material falls on them and is crushed 
to the desired fineness by passing between them. Ordinarily 
these rolls should not be further apart than the thickness 
of a penny, but some clays require them to be set almost in 
contact. The rolls tend to spring apart slightly in use, and 
as they become worn they permit larger pieces to pass between 
them, so that they cannot be relied upon to reduce the material 
to particles much less than J inch in thickness, though the 
greater part will be much smaller. Where the clay contains 
stones or hard lumps the output of the machine can be increased 
by the use of two, three or even four pairs of rolls, placed 
one above the other, the rolls in each pair being set rather 
further apart than in the pair below. Thus, the lowest rolls 



328 



METHODS OF BRICKMAKING. 



may be -fy inch apart, the middle pair J inch apart, and the 
ones in the upper pair may be an inch or more from each other. 
If the clay is exceptionally tenacious an additional pair of 
spiked, grooved or studded rolls may be necessary, as lumps 
of such material would slip on smooth rolls and would not be 
crushed. Such rolls are termed kibblers or wolves. 

(b) The second type of crushing machine consists of a pair 
of rollers each about 5 feet in diameter and 9 inches wide, 




FIG. 96. Crushing Rolls. 

fixed at opposite ends of a single horizontal shaft and rotating 
on a pan or bed in such a manner that any material on the 
latter is crushed as the rollers pass over it. Such an arrange- 
ment is termed an edge-runner grinding mill, and it is specially 
adapted for crushing dry or slightly moist rocks, hard clays 
and shales. Two patterns of edge-runner mills are in use ; 
in one, the bed is fixed and the rollers run over it, dragging 
behind them scrapers which remove the crushed material 
and conduct it to an elevator, whilst in the second form of 



PREPARATION OF THE CLAY 329 

mill the rollers are carried loosely in a framework and merely 
rotate by friction, whilst the pan or bed is rotated rapidly 
by mechanical power. This pan is perforated or provided 
with slots over a considerable part of its base, and as the 
material is crushed between the rolls and the pan, it passes 
through these perforations and falls into a well, fitted with an 
elevator. 

The size and weight of the rollers, the speed at which the 
rollers or pan is driven, together with the size of the apertures 
in the base, determine the output of such mills, the nature 
of the material to be crushed being also an important factor. 

Edge-runner mills of these patterns are not suitable for very 
pasty materials, though if the slots are sufficiently large 
(each about 3 inches by J inch) they form admirable preliminary 
crushers where very adhesive clays 
are worked. The most suitable 
clay for crushing in an edge-runner 
mill is a friable marl, loam or soft 
shale which is not moist enough 
to adhere too closely to the pan 
or rolls. Some shales and fireclays 
are so dry that a considerable 
amount of water may be added 

to them during grinding with- FIG 97. Edge-runner 

,,. ' . & ,., , Grinding Mill, 

out their becoming perceptibly 

plastic or adhesive. The use of this water prevents the loss 
of the finest particles of clay which would otherwise escape 
in the form of fine dust. 

It is seldom possible to have the perforations in the pan of 
an edge-runner mill less than J inch diameter, and smaller 
ones so rapidly become larger with wear and tear that |-inch 
holes are rightly regarded as the least size practicable. Much 
of the crushed material is far finer than this, and some users 
of bricks demand such a texture that the whole of the material 
must be passed through a sieve with ten, twelve, fourteen or 
even eighteen holes per linear inch. It is therefore customary 
to lift the powder which issues from the edge-runner mill to 
a screen or riddle or suitable mesh, which is fixed on an upper 
floor well above the grinding plant. The fine material which 




330 METHODS OF BRICKMAKING 

has passed through the screen falls on to a floor or into a receiv- 
ing hopper and the coarse material or tailings is returned to 
the mill for further treatment. 

In order to obtain a uniform paste it is necessary to mix the 
clay or brickmaking material with water in a very thorough 
manner. Hence, the general necessity for first crushing the 
clay into thin sheets or of grinding it to powder as described. 
The water and material so prepared (if necessary) may then 
be kneaded together with a spade, but the process is slow 




FIG. 98. Vertical Pug-mill. 

and imperfect. A better result is obtained by treading it 
with bare feet or by turning a number of horses on to it and 
keeping them moving about until the mass is sufficiently 
uniform ; this method is also too imperfect in highly civilised 
lands. 

It is usual, in this country, to employ a pug-mill or mixing 
machine driven by horse-power or mechanically according 
to the output required. Such a pug-mill consists of a closed 
cylinder with an inlet at one end and an outlet at the other, 



MIXING THE CLAY 



331 



and provided with a shaft which runs right through the centre 
and is fitted with blades or mixing knives. These knives are 
specially designed to cut the clay contained in the cylinder and 
to mix it as it travels 
from one end of the 
machine to the other. 
Pug-mills were for- 
merly of the vertical 
type, the clay, etc., 
being fed in at the top 
of the cylinder and 
passing out near the 
bottom, but at the 
present time a very 
large number of hori- 
zontal pug-mills are 
in use and have several 
advantages when em- 
ployed in connection 
with other machinery. 
Moreover, by making 
the pug-mill horizon- 
tal a portion of the 
cylindrical casing may 
be omitted and an 
open or trough-mixer 
produced, with the 
advantage that the 
whole of the mixing 
operations may be 
observed and any 
defective blades re- 
placed. It is also 
much easier to regu- 
late the addition of Fl<3 ' 99.-Open or Trough-mixer. 

water and, therefore, the consistency of the paste, when an 
open mixer is employed. 

The speed of rotation of the shaft, the shape and number 
of the blades, and the length of the pug-mill will determine 




332 METHODS OF BRICKMAKING 

the extent of the kneading and the resultant texture of the 
pasty mass. Clays and mixtures which are difficult to work 
may require to be passed several times through the pug-mill, 
or through two or even three pug-mills in succession. With 
most brickmaking clays which have been adequately crushed 
or ground, a well designed pug-mill six feet in length will be 
found ample, but the variation in character of clays in 
different localities is so great that no definite limit of length 
can be laid down. 

If an exceptionally thorough kneading of a somewhat lean 
clay is necessary in order to produce a clay of the desired 
plasticity as is the case with some mixtures of grog and 
fireclay an .ordinary pug-mill is not always efficient, and it 
is then better to employ a pan mill or tempering mill. This 
machine consists of an edge-runner mill with a revolving pan 
similar to that used for grinding, but of lighter construction 
and with no perforations in the pan. Machines of this type 
are well known under the term mortar mills, as they are largely 
used for mixing mortar. The clay together with such other 
materials as are to be mixed with it, and the necessary quantity 
of water, are placed in the pan and the mill is set in action for 
twenty minutes or more, according to the amount of tempering 
required. At the end of a suitable time a peculiarly shaped 
shovel mounted on a swivel is employed to empty the mill, 
after which a fresh charge is added. The machine works 
intermittently, and the quality of the product will depend 
chiefly on the manner in which the materials constituting the 
charge are added. With a little care the paste obtained is 
remarkably uniform, and is somewhat more plastic than when 
the same materials have been treated in a pug-mill. The use 
of a tempering mill is, however, considerably more expensive, 
so that pug-mills are preferred wherever practicable. 

Consistency. The production of a paste of the required 
consistency completes the second stage of brickmaking. The 
consistency of this paste varies greatly with different clays 
some needing to be made into a sloppy material scarcely stiffer 
than freshly-made mortar, whilst others are so stiff that 
considerable pressure has to be exercised in order to make any 
impression upon them, 



MOULDING BY HAND 333 

The softer the paste the easier it is to produce, so that in 
districts where little or no machinery is available, and the 
bricks are simply moulded by hand, the paste will be extremely 
soft. Where powerful mechanical mixers are available, 
however, it is more economical to employ a stiff er paste. 

Methods of Shaping the Clay. As already stated, there are 
a number of different methods in use for converting the 
prepared clay or mixed material into bricks, and these may 
now be considered in order. 

In consequence of these differences in consistency, various 
methods of converting the paste into bricks and other articles 
are frequently distinguished by the kind of clay paste used. 
Thus 

(a) The plastic methods of brickmaking employ a plastic 
paste, as in the manufacture of hand-made and wire-cut bricks. 

(b) The semi-plastic methods of brickmaking employ a stiff 
paste, and are sometimes termed " stiff plastic " processes. 

(c) The semi-dry process of brickmaking consists in the use 
of a moistened powder which is almost devoid of plasticity. 

(d) In the dry or dust process an almost dry powder is used. 
For hand-made bricks the paste is made into the shape of 

bricks by means of wooden or metal moulds, consisting of a 
stout box, without lid or bottom, which rests on a piece of 
hardwood fastened to a rough table. The base piece is some- 
times covered with a special kind of cloth, to which the paste 
adheres so slightly that the mould with its contents can be 
readily lifted off. Without the use of such a cloth or of sand 
sprinkled on the table the clay paste would adhere tenaciously 
to the table. If the bricks are to have a frog or cavity on one 
side, a piece of wood or brass is fixed to the table or base piece, 
and guides or pegs must then be used to secure the mould 
always being correctly placed on the bench. It is less easy 
to produce a frog on a slop-moulded than on a sand-moulded 
brick. 

The moulder prepares the mould, places it in a convenient 
position on the table, and then takes up a convenient quantity 
of the soft paste in both hands, raises it above his head and 
throws it down with great force into an empty mould placed 
on a bench or table in front of him. He then presses the paste 



334 METHODS OF BRICKMAKING 

well into the corners of the mould, scrapes off any superfluous 
paste with a wooden blade or " strike," and with a dextrous 
turn of the wrist he empties the contents of the mould on to a 
small board or pallet placed convenient for its reception by 
the moulder's assistant. The brick thus produced is carried 
away to be dried, either by hand or on a barrow of special 
construction, so as to avoid undue vibration of the bricks. 
Meanwhile, the moulder dips his mould in water so as to wet 
it thoroughly (slop moulding), or first in water and then in 
sand (sand moulding). If sand is used it will cling to the 
surfaces of the clay in contact with the mould and will produce 
a rough-faced brick, the colour of which will depend upon that 
of the sand when burned. As red-burning sand is generally 
employed for this purpose, sand-faced or sand-moulded bricks 
are usually of a good red colour when sold, whilst slop-moulded 
bricks are the same colour as the burned clay of which they 
are made. 

Although the difference between sand- and slop-moulding 
appears to be slight, in practice they necessitate an entirely 
different arrangement of the works. Slop-moulded bricks are 
so soft that they must usually be carried one at a time and 
placed about an inch apart on a level floor until they have 
hardened slightly and can then be taken to the hacks. Sand- 
moulded bricks, on the contrary, are stiffer and stronger, and 
can be taken in quantities on barrows and stacked directly 
on the hacks one sand-moulder keeping two men and three 
barrows constantly employed in the transport of the bricks. 

When slop-moulded bricks are made, the drying floor must 
be close to the moulding bench it is, indeed, customary for 
the bench to be moved to different parts of the floor two or 
three times each day but sand-moulded bricks can be taken 
on a barrow for any reasonable distance. In works provided 
with a steam-heated drying floor, slop -moulding is generally 
used, particularly if, as in the case of bricks for inside work, 
the colour of the finished bricks is of little importance. 

The rate at which hand-made bricks can be moulded is very 
great, a fair average being 36,000 or more sand-moulded, or 
9,000 to 10,000 slop-moulded, bricks per week. 

Each brick when freshly moulded contains about 1 Ib. of 



MOULDING BY HAND 335 

water, and this must be removed by drying in such a manner 
that the brick is not damaged. Slop-moulded bricks are 
usually allowed to remain on the flat or floor for about six 
days, after which they may be stacked in an open fashion 
in long rows about six bricks high, so as to make more room 
for fresh bricks. They remain stacked for several weeks 
until dry enough to be burned. Sand-moulded bricks are 
stiffer and are arranged in hacks (Fig. 110) immediately after 
they have left the mould, and take much longer to dry. These 
hacks consist of long rows of bricks set openly one above the 
other to a height of about two feet, and are covered with gable- 
shaped boards to keep off the rain. The sides of the hacks are 
also protected, when necessary, with loo-boards, matting or 
straw in order that the bricks may not be damaged by frost, 
draughts or water. The bricks must usually be taken down 
and rearranged at least once during the drying, arid if pressed 
bricks are desired a portable press is taken to the hacks and the 
bricks pressed and replaced. 

The drying of bricks always requires care and attention ; 
apparently insignificant draughts will crack many bricks, 
and even if the sun shines on some bricks during drying 
the damage will be serious. Hence, a considerable proportion 
of the anxiety experienced by the owner of a yard where 
hand-made bricks are produced is due to the difficulty of 
avoiding loss during the drying, especially if the clay is a tender 
one. 

The thoroughly dry bricks are next taken to a clamp or kiln 
to be burned as described later. 

The difficulties experienced in obtaining skilled brick-moulders 
has led to the introduction of machines in which the hand- 
moulding process is closely imitated. These machines are 
described in the author's " Modern Brickmaking " (Scott 
Greenwood & Son), but they have not been used extensively, 
as others working on entirely different principles have a much 
larger output. 

Wire-cut bricks are produced by machinery ; they are not 
moulded, but are shaped by expression through a suitable die 
in a manner greatly resembling the production of sausages. 
The plastic paste (prepared in one of the ways previously 



336 METHODS OF BRICKMAKING 

described) is passed from the mixing machine into a pug-mill . 
the die being attached to the exit end of the latter, and thus 
forming a mouthpiece, through which the clay exudes in the 
shape of a band or column 9| inches by 4| inches, i.e., 
whose width is the length of a brick, and whose thickness is 
the width of a brick. This band is cut into convenient 
lengths by means of a wire stretched tightly in a frame, and 
each section is again cut into pieces about 2-J inches or 
3 inches wide by a series of other stretched wires. In this 
manner the bricks are produced six or more at a time, and 
are taken away on long boards or pallets to be dried. 

This process of brickmaking is exceedingly simple in theory, 
but there are numerous matters in connection with it which 
require skill and care if good bricks are to be produced. Thus, 
the construction and maintenance of the mouthpiece need 
constant attention, or the clay band will be irregular in shape 
and having serrated edges. Some clays are extremely trouble- 
some in this respect and have to be passed between a pair of 
expression rolls, placed between the pug-mill and the die, 
before good bricks can be obtained. 

The arrangement of the wires on the cutting table also 
admits of numerous modifications, some of which are far better 
than others. The usual plan in this country is to keep the wires 
fixed and to push the clay column sideways through them by 
means of a push plate, but much neater bricks can be obtained 
lay moving the frame carrying the wires in a diagonal direction 
towards the table, as in most of the Continental machines. 

Any stones present in the clay band may catch the wires 
and tear the bricks, so that the wire-cut process is not well 
adapted for very rough clays, though excellent for most others. 

Owing to the manner of their production, wire-cut bricks 
cannot be provided with frogs or depressions unless the bricks 
are passed through a re -press. 

In order to reduce the space occupied by the plant, some 
of the makers of brick machinery combine the crushing rolls, 
mixer, pug-mill, mouthpiece, and expression rolls on a single 
framework so that the whole plant has the appearance of a 
single machine. 

Bricks made by the wire-cut process are very soft, and must 



THE WIRE-CUT PROCESS 



337 



usually be laid out on a drying floor or kept separate on the 
racks of a drying tunnel or " stove " in order that they may 




become dry and hard. The most generally employed arrange- 
ment for drying consists of a large concreted floor in a corre- 
ct z 






338 METHODS OF BRICKMAKING 

spondingly large and well-ventilated shed. Beneath the floor 
is a series of flues heated by. steam or fires in such a manner 
that the temperature of the floor is as uniform as possible. 
Steam has several advantages over fires, particularly in the 
regulation of the temperature of various parts of the floor. 
Whichever source of heat is employed, the bricks are placed 
singly on the floor about f inch apart and care is taken to avoid 
draughts and to raise the temperature very steadily. In three 
to five days the bricks are usually sufficiently dry to be taken 
to the kiln. 

Where the output of bricks is sufficiently large a tunnel 
dryer may be employed and, if rightly constructed, will be 
more efficient than a drying floor. Unfortunately, however, 
many of the tunnel dryers now in use are far from satisfactory 
owing to the lack of knowledge, on the part of both designers 
and users, of the principles underlying the construction. In a 
tunnel dryer the bricks are placed on cars and enter one end 
of a tunnel, travel along it to the other end and finally emerge, 
after twenty-four to seventy hours, in a dry state. The sim- 
plicity of the operation, the reduction in the amount of handling, 
and the lower cost of heating are all in favour of the use of 
tunnel dryers, but at this and in all stages of brickmaking, 
the manufacture is not as simple as it appears to be, and both 
care and skill are needed in the management of the temperature 
and ventilation of the tunnels. 

The use of some means of drying plastic clays before they 
enter the kiln is imperative, as otherwise the bricks would 
crack and fall to pieces in the kiln. The details of design in 
a drying plant suited to a particular clay must be adapted to 
the special needs of that clay ; it is no more reasonable to 
expect to dry a clay efficiently in a dryer which has not been 
made to suit it than it is for a man to expect to be well dressed 
in a ready-made suit of clothes, or for him to be suited with the 
first hat he tries on. 

If, in spite of all care in drying and in the use of a dryer 
of suitable design, the proportion of bricks which crack con- 
tinues to be large, there is a probability that the clay is too 
plastic and that it requires to be diluted with sand or some 
other non-plastic material. The impossibility of obtaining 



THE WIRE-CUT PROCESS 339 

sand at a sufficiently cheap rate is one of the chief reasons 
why numerous clays otherwise suitable cannot be used 
for brickmaking. 

If the cracks appear to emanate from the edges of any brand 
or other distinguishing mark stamped on the bricks or formed 
on them during the moulding, the texture of the material 
requires adjustment. If a coarse material is used, cracks are 
almost certain to be formed wherever there is an indentation 
in the bricks. The coarser particles act as centres of radiation 
for the cracks. 

After being dried, the bricks are taken to a kiln and burned 
in a manner to be described later. There is often much 
unrecognised carelessness in drying which results in the pro- 
duction of numerous hair-like cracks in the bricks. These 
cracks are almost invisible in the unfired bricks, and their 
occurrence in the finished bricks is, for this reason, often 
wrongly attributed to the action of the kiln. Further details 
of the manufacture of bricks from a plastic paste will be found 
in the author's " Modern Brickmaking " (Scott, Greenwood & 
Son). 

In the semi-plastic or stiff-plastic methods of brickmaking a 
paste of such stiffness is employed that very considerable 
pressure has to be used in order to obtain the imprint of a 
thumb or finger. This very stiff paste is usually prepared 
from a powdered shale or other indurated clay, as the variations 
in stiffness of clays quarried in a stiff plastic condition make 
their use inconvenient. 

The crushed, screened and powdered clay (p. 329) is received 
in an open mixer, and is there kneaded with the requisite 
quantity of water and passed into a small but powerful pug- 
mill which compresses it into metal moulds. These moulds 
may be arranged on the top of a rotating table or on the 
circumference of a drum, both these constructions having 
proved satisfactory. The rough-shaped bricks or clots are 
then removed from the moulds, one at a time, and are pressed 
accurately to shape in a plunger-press, which is attached to 
the same framework. The shape of the moulds and the 
arrangements provided for removing the clots from them has 
a great influence on the power required by the machine, it 

z 2 



340 



METHODS OF BRICKMAKING 



being generally found that the simpler the clot mould the 
better. Thus, in the Bradley and Craven stiff -plastic brick 
machine the moulds are rectangular depressions in a horizontal 
steel disc ; in the Scholefield machine they form similar 
depressions in the circumference of a drum, and in the Fawcett 
machine they form the " spaces " between the " cogs " in 
a peculiarly shaped " cogged wheel." In the two first- 




FIG. 101. Bradley^and Craven Stiff-plastic Brick-machine. 

mentioned machines the clay is pressed upwards or outwards, 
but in the last-named one it is pressed longitudinally. 

Each machine is rated at about 12,000 bricks per day, but 
the actual output depends on the nature of the clay. 

The advantages of using so stiff a paste are twofold ; the 
clay is obtained in a condition suitable for immediate pressing 
into shape, and the bricks may usually be sent direct to the 



THE WIRE-CUT PROCESS 341 

kiln without the need of drying. A considerable saving is 
thereby effected, as it is difficult to watch the drying of plastic 
bricks so closely as to be able to re-press them when all are in 
the best condition for this operation, and, further, the cost of 
a dryer is completely avoided, though against this there is the 
cost of additional fuel required in the kiln. Where a continuous 
kiln is not available these bricks must, in some instances, be 
dried before being placed in intermittent kilns. The burning 
is carried out as described later. 

Further details of the machines used for making stiff-plastic 




FIG. 102. Fawcett Stiff -plastic Brick Machine. 

bricks will be found in the author's " Modern Brickmaking " 
(Scott, Greenwood & Son). 

In the semi-dry process the material is used in the form of 
a powder which contains just sufficient moisture to make it 
" cake." It can only be used with shales and dry clay which 
are almost devoid of plasticity. The powder is obtained by 
crushing the material with edge runners, as already described 
(p. 329), and is fed into the boxes of plunger presses of a 
particularly powerful type, in which it is compressed into 
bricks. Several types of press are in use ; in each case they 
must be capable of exerting an enormous pressure and of 
giving several compressions in succession, as a single pressure, 



342 



METHODS OF BRICKMAKING 



however great, will not produce a sound brick. It appears 
to be necessary to press once, release the pressure and allow 
air to escape, re-press, remove from the mould, and again 
re-press either once or twice before a reliable brick can be made 
from some materials. 

The semi-dry process has gained its chief reputation in the 
neighbourhood of Accrington (where it is now being replaced 




FIG. 103. Whittaker Plunger Press. 

by stiff-plastic machines) and Peterborough, where enormous 
quantities are made annually. It is claimed that, where it is 
applicable, this is the cheapest of all brickmaking processes 
when large quantities are required, though the bricks produced 
are less readily purchased by builders on account of their low 
porosity and their consequent tendency to ' : float " on mortar. 
This process ought never to be installed except under expert 
advice which is quite independent of that of the various 



RE-PRESSING BRICKS. 343 

makers of machinery, or serious disappointment may result. 
It is therefore unnecessary to describe it in greater detail here, 
but a fuller description of it will be found in the author's 
" Modern Brickmaking " (Scott, Greenwood & Son). As 
suggested, bricks made by the semi-dry process contain so 
little moisture that they are sent direct to the kiln and do not 
need to be dried. 

The dry dust process is seldom used for bricks, as the diffi- 
culties experienced in obtaining sound bricks are very great. 
For tiles and other thin articles it is largely used. As the 
name implies, the clay or shale is ground to the form of a dust 
and this is placed in the box of the press and is duly compressed 
into the desired shape in a manner similar to bricks made by 
the semi-dry process. The difficulty of removing all air from 
between the particles and of exercising a perfectly uniform 
pressure over every part of the brick is so great as to make the 
use of a semi-dry material necessary for brickmaking. Indeed, 
the machines which are supposed to be making bricks from dry 
dust are, in almost every case, working with a slightly moistened 
material by means of the semi-dry process. 

Re-pressing Bricks. Facing bricks, or those in which great 
exactitude of shape and size is essential, are not infrequently 
placed in a re-press, and any irregularities in form are thereby 
corrected. Re-pressing is costly, as even with a power-driven 
press two strong youths cannot deal with more than 2,000 
bricks a day, and even at this rate a considerable number of 
bricks will be defective. Unless the clay is in exactly the right 
condition the number of bricks which can be satisfactorily 
re-pressed will be very small. Too moist a paste will adhere 
too closely to the press box and plunger, no matter how well 
they are oiled, and too dry a clay will break and crack, thereby 
producing bricks of an unsightly appearance when burned. 
For this reason, where re-pressing is practised, both youths and 
foremen must be very alert as to the condition of the clay, and 
must be regardless of overtime if a large number of bricks is in 
the desired condition. 

Much difference of opinion exists as to the real value of 
re-pressing. If the alteration in the shape of the brick or clot 
is appreciable, the structure of the brick will be damaged 



344 METHODS OF BRICKMAKING 

and the crushing strength seriously reduced. Where appear- 
ance and accuracy of shape are of such importance that a 
slight loss of strength need not be considered, re-pressing 
may be desirable. It is, however, quite a mistake to suppose 
that re -pressing really improves bricks ; it may give them a 
better appearance, but it can only reduce their strength. 
Hence, the able brick manufacturer endeavours to make his 
bricks right at first and so to avoid the necessity of re-pressing. 

Burning Bricks. In some countries, where the climatic 
conditions are favourable, bricks are simply placed in the sun, 
the rays from which are sufficiently intense to effect a sufficient 
hardening of the mass. Sun-baked bricks have so low a resist- 
ance that they are far from durable, so that one of the early 
results of civilisation is the substitution of bricks baked by 
means of fuel. In cooler climates the use of fuel is a necessity 
in the production of sound bricks. 

Pieces of clay in the shape of bricks, but which have not 
been heated are termed green bricks, this term being analogous 
to that applied to freshly cut wood. They cannot be used 
for building in moist climates, as they would be washed to pieces 
by a heavy shower of rain beating upon them. To render 
them permanent they must be heated to a temperature 
sufficiently high to make them durable. 

When a piece of plastic clay is first allowed to dry thoroughly 
and is then heated slowly and steadily to a bright red heat, 
a number of remarkably interesting changes take place in 
both its chemical and physical properties. These are described 
more fully in a separate chapter, but the appliances used for 
effecting these changes may be briefly described here. 

Two chief groups of appliances are used for heating bricks : 

(a) Clamps, which consist of a peculiar stacking of the bricks 
and fuel, covering the outside of the stack, or clamp, with clay 
paste and then lighting the fuel. Under favourable conditions 
the heat produced will burn the raw clay into good and service- 
able bricks. 

Clamps are particularly suitable for small outputs, as there 
is no outlay for the construction of a kiln, but the bricks 
produced whilst strong and durable have not the pleasant 
appearance of kiln-burned bricks. 



BURNING BRICKS 345 

The construction of a clamp requires too much skill for a 
detailed description to be given here. It consists essentially 
in building a series of flues, and above these the bricks are 
stacked close together, the direction of the courses changing 
frequently. Coke breeze is laid in the flues and fine coke 
or cinders is sprinkled over each course of bricks ; moreover, 
the bricks to be burned in clamps usually have a considerable 
percentage of coke or cinder dust mixed with the clay before 
it is used ; this forms a further supply of fuel, with the result 
that when a clamp is burning briskly each brick becomes a 
sort of fireball, and the heating is so effective that only a very 
small proportion of fuel is needed. " London stocks " are 
typical clamp-burned bricks. 

(b) Kilns or ovens, which consist of brickwork chambers 
into which the bricks are placed, the fuel being usually kept 
separate and burned in specially designed fireplaces or furnaces. 
Bricks burned in kilns have a more uniform and pleasing 
colour than those heated in clamps, and are of more uniform 
strength, but the cost of the kiln itself and of the additional 
fuel required enables clamp-burned bricks still to hold their 
own in districts where the clay or brick-earth is favourable. 
The number of different patterns of kilns is incalculable, 
but they can all be grouped under four heads, the kilns in each 
group differing from each other in details which, however, 
important in practice, cannot be adequately described here. 

(1) Single up-draught kilns of rectangular shape, commonly 
termed Scotch kilns, are formed by building four walls around 
a suitably sized space, and providing a narrow doorway at 
each end and fireplaces along each of the longer sides. The 
top of the kiln is quite open and usually there is no chimney ; 
though a series of small stacks one to each fire may be 
constructed if necessary. These kilns are filled with bricks 
carried through the doors or (in America) by means of a crane 
through the open top of the kiln. The bricks are stacked in 
a special manner about one finger's width apart, the courses 
being crossed so as to obtain the most uniform distribution 
of the heat. The top of the kiln is covered with old bricks 
and earth or ashes. Kilns of this type have long been in regular 
use in almost every county north of the Trent, but they are 



346 



METHODS OF BRICKMAKTNG 



fuel wasters and are being replaced by continuous kilns wherever 
the output justifies the erection of the latter. 

(2) Single down-draught kilns are either circular or rectangular 
in plan, and are covered with an arched roof or dome. The 




FIG. 104. Single Up -draught Kiln. 

fires are placed at regular intervals in the walls and the fire 
gases first rise up to the roof of the kiln and are then deflected 
downwards and distributed among the goods until they eventu- 
ally pass out through flues in the floor of the kiln to the chimney 




FIG. 105. Single Down-draught Kiln. 

stack. The great advantage of a well-built down-draught 
kiln is that facing bricks of best quality can be burned in it, 
the colour of the bricks not being damaged by the flame as 
in kilns with up-draught or horizontal draught. Consequently, 



BURNING BRICKS 



347 



for small yards where facing bricks, tiles and terra-cotta are 
made, single down-draught kilns are essential for these goods. 
In larger works, continuous kilns of a corresponding type are 
preferable. 

(3) Single horizontal draught kilns are usually known as 
" Newcastle kilns " and are rectangular in plan. In outward 
appearance they resemble a rectangular down-draught kiln 
or a Scotch kiln, to which a roof has been attached, but the 
fireplaces are arranged differently. In a Newcastle kiln the 
fires are all at one end, and the flames and hot gases travel 
horizontally through the goods to the opposite end, where they 
pass into a flue leading to the chimney. It is customary to 




' Fire-places at end ' 

FIG. 106. Newcastle Kiln. 

arrange Newcastle kilns in series back to back, and when very 
large ones are used fires are built at both ends whilst the waste 
gases pass away at the centre. A separate chimney may be 
built for each kiln, but where several kilns are in use it is better 
to employ a single chimney stack for all the kilns. Newcastle 
kilns are largely used for burning firebricks, though circular 
down-draught kilns are sometimes preferred for this purpose. 
(4) Continuous kilns are really down-draught and horizontal- 
draught kilns, in which the waste gases from one kiln are not 
sent direct to the chimney, but are passed through a number of 
other kilns or " chambers " until the temperature of the gases 
is too low to be of further use. In these kilns the gases passing 



348 METHODS OF BRICKMAKING 

up the chimney should never have a temperature exceeding 
130 C., whereas those from single kilns frequently have a 
temperature of 800 C. or even higher, thus wasting about 
half the fuel used. In a continuous kiln, on the contrary, this 
loss of fuel is prevented, as the heat which is not required to 
burn the goods to which it is first applied is utilised to warm 
up goods in other parts of the structure. 

This method of heating is primarily due to Siemens, who 
adopted it in connection with the steel-melting furnace which 
bears his name. He observed that a large amount of heat was 
passing away from the furnace unused, and sought to utilise 
this by passing the waste gases through masses of checkered 
brickwork. When one such mass of brickwork was sufficiently 
heated the waste gases were diverted to another " regenera- 
tor " and air was drawn in the opposite direction through the 
hot brickwork. In this way hot air was supplied for the 
combustion of the fuel, and a great saving in heat was effected. 

Hoffmann, who modified Siemens' furnace and applied it to 
brick-burning, soon saw that, instead of heating up permanent 
structures with the waste gases, the best result could be 
secured by using the bricks to be burned as regenerators, and 
accordingly devised the continuous kiln which bears his name. 
Many modifications of Hoffmann's original kiln have been 
made, and some of them are great improvements on it where 
the colour of the finished bricks is of importance, but in none 
of these improvements is the departure from the Siemens- 
Hoffmann regenerative principle very great, and most of the 
newer kilns may justly be regarded as adaptations of this 
principle to suit the special circumstances either as to the 
product or the fuel. 

Wherever the output of a brickyard exceeds 15,000 bricks 
per day or 450,000 per year and in some cases for even smaller 
outputs a properly designed continuous kiln will be found to 
require only one-third to one-half the fuel needed by the 
corresponding number of single kilns, whilst the product is of 
equal value in every respect. There is a general impression 
that continuous kilns can only be used for common bricks ; 
this is quite erroneous, as the best terra-cotta and facing bricks 
may be advantageously burned in continuous kilns. The 



BURNING BRICKS 349 

nature of the product depends entirely on the suitability of 
the kiln for the purpose, and not on the " continuous " as 
distinct from " single " kilns. The selection of a kiln is, of 
course, a matter requiring expert knowledge, and even kiln 
builders themselves cannot be relied upon too implicitly in 
this direction, as they are naturally biassed in favour of the 
particular designs used by their firms and cannot be expected 
to admit that the kilns built by any other firm are more suitable 
for a particular case. 

The following pages do not aim at presenting more than a 
mere outline of typical continuous kilns for various classes of 
bricks. They merely show the general principles of construc- 
tion, and will require modification to local circumstances and 
requirements. 

The original Hoffmann kilns were circular in plan, but it is 
found more convenient to adopt the shape shown in Fig. 3. 
The Hoffmann kiln, 1 as now used, consists of a long brickwork 
structure containing a kind of endless tunnel from which a 
number of flues lead to a long central " main flue," the latter 
being connected directly to the chimney. Each of these flues 
is controlled by a separate damper. 

In the " roof " of the '"' tunnel " is a series of rows of 5-inch 
openings, each row being about three feet apart and consisting 
of three, four or five holes. These openings are covered with 
air-tight metal caps, and are known as feed holes ; through 
them the fuel is introduced into those portions of the kiln, 
whilst cold air is admitted through those parts which require 
cooling. Larger openings termed " wickets " or " door 
gaps " are made in the outer walls of the kiln and serve for 
the admission and removal of the bricks to be burned. The 
number of these wickets depends on the size of the kiln : 
usually there is one to each fourteen feet of linear kiln wall. 
A Hoffmann kiln of convenient size will have sixteen wickets 
and an equal number of dampers leading to the main flue. 

As a matter of convenience, it is desirable to regard such a 
kiln as composed of a certain number of units or chambers, 
each of which corresponds to one wicket and one flue damper. 

1 Hoffmann's patents have long since expired, and most firms of kiln builders are 
prepared to build " Hoffmann kilns."' 



350 METHODS OF BRICKMAKING 

Thus, a kiln with sixteen wickets is termed a sixteen-chamber 
kiln, and is treated as though it were actually partitioned off 
into sixteen separate compartments, though, in reality, no 
such partitions exist in a true Hoffmann kiln. 1 A smaller 
number than sixteen wickets is seldom desirable, though many 
kilns have only twelve or fourteen. They are, however, less 
economical in fuel than sixteen-chamber kilns, as the larger 
number of chambers permits a greater utilisation of the heat. 
It is, indeed, a very false idea of economy to erect a continuous 
kiln with a small number of chambers. If the output is to 
be small, the kiln should be made narrower than otherwise, 
i.e., the distance from the wickets to the centre of the kiln 
should be made less, but the effective perimeter of the kiln 
should never be less than 224 feet. In other words, the output 
of a continuous kiln should not be made to depend on the 
number of the chambers, but on their width. Failure to 
recognise this simple fact is the cause of much of the waste of 
fuel, and most of the disappointment which has attended the 
erection of continuous kilns in some localities. 

The bricks are placed in the " tunnel " of a kiln, such as the 
one described, with their longer sides parallel to the length of 
the kiln, but a few courses of bricks at right angles to the 
others are valuable as " ties." The bricks are placed about 
J inch or | inch apart, so that the gases may travel between 
them and heat them uniformly. For the same reason a space 
of one or more inches is left between each blade or row of bricks. 
Immediately under each feed hole the bricks are arranged to 
form " fireplaces " in which the fuel can burn ; these fire- 
places may take the form of hollow shafts built of bricks, with 
occasional projecting bricks to prevent all the fuel from falling 
directly to the bottom, or a trench or space the full width of 
the chamber may be left for the same purpose. The former 
method is usually regarded as being the more economical, as 
it wastes less space, but as one properly constructed trench to 
each fourteen feet of tunnel length is usually sufficient, there 
is very little difference in fuel consumption between the two 
constructions. By thus keeping the fuel entirely out of 

1 For a description of continuous kilns in which permanent partitions are used, 
see later under the caption Chamber Kilns. 



BURNING BRICKS 



351 



contact with the bricks to be burned, the colour of the latter 
is greatly improved, and whereas only common bricks can be 
burned in the original Hoffmann kilns, facing bricks can be 
burned in those provided with troughs and grates. The use 
of grates also enable the kilns to be fired from the front, 
i.e., from the ground level if this is preferred. As already 
remarked, there are many modifications of the Hoffmann 
kiln, in all the more important of which trenches, troughs 
or grates for the fuel are employed as in the Guthrie and 
Belgian kilns. 

Assuming that the kiln is in full work, what takes place is, 
approximately, as follows : the fuel is fed into the feed holes 
covering three chambers (Nos. 1, 2 and 3) or about forty feet 

4 Air travels 

towards 




**WJ Hot Gases tra vet > *-Air enters 

here and is warmed. 

FIG. 107. The Round of the Kiln. 

of tunnel length, a light charge of fuel being placed in each hole 
every quarter of an hour. It is essential that the amount of 
fuel used should not be too large ; sufficient to fill an ordinary 
quart jug is ample, though in practice a very small shovel is 
the most convenient instrument for introducing the fuel. 
In a properly managed kiln the three chambers to which fuel 
is added will all be at a red-heat, and No. 1 will be nearly 
finished. The hot gases from the burning fuel will be carried 
by the draught through the five succeeding chambers (Nos. 4, 
5, 6, 7 and 8) and will gradually heat (i.e., pre-heat) them 
without their requiring any attention. After this, the gases 
will be of so low a temperature that they are no longer useful 
and are taken through the flue in chamber No. 8 into the main 
flue and so to the chimney. All the dampers in chambers 



352 METHODS OF BRICKMAKING 

1 to 7 are meanwhile kept closed, so that all the available heat 
is used in warming the bricks to be burned. 

Chambers Nos. 9, 10 and 11 contain freshly-set bricks and 
these must be separated from the remainder of the kiln by 
partitions of paper or metal running across the whole of each 
side of the chamber, and their temperature must usually be 
raised to at least 120 C. by a separate supply of heat ; to 
heat them by waste gases would usually cause them to be badly 
scummed and so spoiled, though for some purposes this would 
not matter, and they may then be taken at once into what 
is termed the " round of the kiln " without any preliminary 
heating. Ordinarily, however, the bricks must be heated 
by as pure air as possible, until their temperature is such that 
no condensation products can form upon them ; 120 C. being 
generally a suitable temperature for this purpose. The purest 
warm air obtainable is that which is drawn through the cham- 
bers containing cooling bricks, and many kilns have specially 
arranged flues for the supply of warm air for this purpose. 
Another, but less satisfactory method of warming the bricks 
to 120 C. consists in lighting a small fire in the wicket and 
allowing air to pass over this into the chamber to be warmed. 
During this warming of the bricks the moisture present in 
them is driven off, and on cool days it forms a white smoke, 
whence this first stage of the burning is frequently termed 
water-smoking or shortly, smoking. 

As soon as the bricks have reached a temperature of about 
120 C. the partition between No. 8 and 9 is removed (or, 
if of paper, is torn) so as to admit the hot gases. The damper 
in No. 8 is closed, the supply of warm air to No. 9 is shut off 
and any opening made in connection with the wicket fire is 
closed. The hot gases from the fuel then pass into No. 9 
chamber and the latter is then said to be " taken into the round 
of the kiln." Meanwhile, chamber No. 12 has been filled, 
and the " smoking " of this chamber is, therefore, commenced 
at once. Chamber No. 13 is, meanwhile, empty or being 
emptied, Chambers 14, 15 and 16, contain finished bricks 
which are cooling, this being accomplished automatically by 
the draught of the kiln which draws air through the open 
doorway of No. 13 through the bricks. The air thus admitted 



BURNING BRICKS 353 

first comes into contact with almost cool bricks, and becomes 
gradually hotter in its journey until, when it reaches the burning 
fuel, it is of the same temperature as the hottest bricks in the 
kiln and ensures, with careful management, a very complete 
combustion of the fuel with scarcely any avoidable waste of 
heat. 

Any description of the working of a continuous kiln must, 
necessarily, appear complicated, in reality these kilns are quite 
simple. As soon as a chamber is filled, its contents are first 
warmed by hot air or a wicket fire, and then it is taken into the 
round of the kiln as described. It then needs no further atten- 
tion until it has become so hot that a little fuel must be fed 
into it in order to complete the burning. As soon as the 
contents of this chamber have been heated sufficiently, the 
addition of coal to it is stopped, another chamber is taken 
into the round of the kiln, and so on ; one chamber being 
emptied and another being filled continuously, and the fire 
travelling round and round the kiln in a perfectly regular and 
continuous manner. The work of the firemen is much lighter 
than for single kilns of equal output, and so long as the draught 
created by the chimney remains steady and the chambers 
are filled, emptied and fired regularly, there is little or no 
trouble. 

Unfortunately, climatic changes greatly affect the draught 
and render constant watchfulness on the burner essential, 
and even with all the care possible, irregular heating will occur 
in stormy weather so long as a chimney is used to create 
the draught. For this reason, a number of firms have 
installed large fans usually over six feet diameter and in 
this way obtain a more powerful and perfectly steady draught. 
When carefully managed, their use for this purpose is highly 
advantageous, but like all other machinery, a fan requires 
to be understood, and well cared for, or it may cause trouble. 
The few failures which have arisen from the use of fans 
have, so far as the author has been able to investigate them, 
been due to three causes, none of which are the fault of the fan 
itself : (1) the use of too small a fan, (2) careless or improper 
management, and (3) failure to use and follow the indications 
of a recording draught gauge. A fan, being a far more powerful 

c. A A 



354 METHODS OF BRICKMAKING 

draught producer than a chimney, requires to be kept under 
proper control ; it then works in the most satisfactory manner 
possible. 

When it is desired to make bricks in large numbers over a 
period not exceeding five or six years the colour of the bricks 
being unimportant so long as they are strong and well shaped 
a much cheaper kiln may be constructed by omitting the 
arched roof and replacing it by a platting or cover of bricks 
laid flat and close together, and covering them with a 3-inch 
or 4-inch layer of cinder dust, sand or ballast. Holes are, 
of course, left in the platting for the insertion of feed-caps, 
it being usually convenient to employ square caps in this case. 
In a temporary kiln of this character the simplest possible form 
of construction should be used, the most convenient plan being 
a large rectangle 112 feet X 18 feet internally, with stout walls 
well buttressed outside. Eight openings, each wide enough 
to admit a cart, are left in each side in order that the kiln may 
be filled and emptied expeditiously ; these openings are closed 
by temporary brickwork, well coated with clay paste when the 
" chamber " to which they correspond is filled with fresh 
bricks. 

Instead of the massive centre usual to permanent kilns, a 
single brickwork partition may be built longitudinally down 
the centre of the kiln, and a flue carried down each side of the 
kiln about one foot below the ground level may be connected 
to suitable openings (controlled by dampers) in the outer 
walls of the kiln and to the fan used to create the draught. 

Kilns of this archless type have been used (with greater or 
less modification) with great success in the colonies and in 
the tropics, the saving in fuel as compared with ordinary kilns 
being fully 50 per cent, and sometimes 75 per cent. Two such 
kilns are in use in Great Britain at the present time under 
licence from the patentee, H. Harrison. 

Well-built Hoffmann kilns will usually require 2J to 3 cwt. 
of coal per thousand bricks burned, but much depends on the 
quality of the coal used, on the nature of the clay and on the 
amount of vitrification needed in the goods. If grates or 
troughs and hot-air flues are used, the quantity of fuel will 
rise to 3 to 4 cwt. per thousand bricks, but should seldom 



BURNING BRICKS 355 

exceed the higher figure. The fuel consumption of most 
Hoffmann kilns is, therefore, about half that of single kilns 
when a normal brick clay is being burned. 

It may here be noted that the number of patents which have 
been taken out for some small modification of the Hoffmann 
kiln is very large. Consequently, there are many kilns 
advertised under various distinctive names which are. in 
reality, nothing but Hoffmann kilns with a flue for supplying 
hot air to the chambers to be " smoked," and grates or troughs 
for the fuel instead of the original shafts. The distinctive 
features of all these separate modifications of Hoffmann's 
original kiln would occupy more space than can be devoted 
to them in the present volume. Readers who wish to study 
the details more freely will find them set out very fully in the 
author's " Kilns and Kiln Burning." Two modifications may, 
however, be mentioned here on account of their different 
construction in several important particulars, and particularly 
because they are really composed of a series of chambers 
connected in such a manner as to work continuously. For 
this reason they are conveniently considered as a different 
type of kiln, and are suitably termed chamber kilns. 

The " Staffordshire " kiln, in general shape, resembles the 
modern Hoffmann kiln, but internally it is divided into a 
number of chambers by permanent partitions. These 
partitions have vertical slits in them which can be closed by 
means of vertical dampers when it is required to shut off a 
chamber for water-smoking, annealing or other purposes. A 
number of special flues are also constructed in the roof and 
floor of the kiln so as to provide an ample supply of hot air 
for smoking, aiding combustion or for prolonged heating in 
a current of hot air, such as is needed with certain brick clays 
which, otherwise, form black cores or " hearts." As the 
amount of air which can be heated by drawing it through the 
chambers containing cooling bricks is not sufficient for all 
these purposes, the Staffordshire kiln is provided with additional 
flues in the arched roof of the kiln through which an additional 
supply of air can be drawn and heated. It should, however, be 
pointed out that the use of these flues involves the consumption 
of an additional amount of fuel in order to replace the heat 

AA2 



356 METHODS OF BRICKMAKING 

supplied to the air by them. The convenience of this method 
of heating the air is, however, so great that it is, in the end, one 
of the most satisfactory ways of obtaining a sufficiently large 
supply of hot air. A number of other kilns have accessory 
flues of this character, thus, in Brown's kiln the flues are 
placed beneath the floor instead of in the roof. 

With some clays it is necessary to provide for the rapid 
removal of large volumes of steam produced during the 
" smoking," and here again the " Staffordshire " kiln is well 
prepared. As in some cases the steam is best removed from 
the upper part of the kiln, and in others from near the floor 
level, arrangements for both are provided, and the steam 
evolved by the goods is therefore removed in any direction 
desired and at any speed which may be considered suitable. 
This kiln has, in fact, long been regarded as the most completely 
fitted continuous kiln at present in use, and in it large numbers 
of best facing bricks, terra-cotta and other valuable clay 
products are burned with complete success, the results being 
in every way equal to those obtained by the best single down- 
draught kilns and this, notwithstanding the fact that the 
draught in the Staffordshire kiln is horizontal rather than 
" down-draught " in direction, though it can be made com- 
pletely down-draught if required. The fuel consumption of a 
Staffordshire kiln is about J to 1 cwt. more than that of a 
plain Hoffmann kiln, but as the latter cannot be used for 
facing bricks and terra-cotta, the additional fuel consumption 
may be regarded as necessary. 

Where a completely down-draught kiln is required without 
the additional flues provided in the kiln just mentioned and 
yet with the low fuel consumption of a continuous kiln, the 
" Ruabon " kiln will prove very suitable. This consists of a 
number of down-draught kilns arranged consecutively, the 
waste gases from one kiln passing directly into the next and 
through as many subsequent ones as may be considered 
desirable. By arranging the kilns in this manner most of the 
advantages of a continuous kiln are obtained in combination 
with the excellent colour and strength of goods burned in 
single down-draught kilns. The Ruabon kiln requires about 
1 J to 2 cwt. per thousand bricks more than the plain Hoffmann 



BURNING BRICKS 357 

kiln, or about fths of that of separate down-draught kilns 
of equal capacity. 

From what has been stated about continuous and chamber 
kilns, the reader will understand that the greatest advantages 
are derived from those kilns in which the gases from the kiln 
travel the longest distance over and among bricks before they 
enter the exit flue, and those in which the distance the incoming 
air has to travel among cooling bricks before it reaches the 
burning fuel. There is a great tendency in most British works 
to use kilns in which these distances are far too short, and 
consequently the amount of fuel burned is greater than would 
be the case if the kilns had been more advantageously designed , 
For instance, in twelve-chamber and fourteen-chamber kilns 
a wastage of J to 1 cwt. of coal per thousand bricks is common, 
and would have been avoided had the kiln been built with 
sixteen or eighteen chambers. 

Most continuous kilns in the United Kingdom are too wide, 
and therefore too short, this defect having originated from the 
fact that it is easier to get a good draught with a short, wide 
kiln than with a long, narrow one. Since the substitution of 
fans for chimneys in creating the draught, the use of such 
wide kilns is no longer necessary for bricks, especially as 
narrower kilns have several advantages. Hence, about 
thirty years ago, Jacob Biihrer effected a great saving in fuel 
and a large increase in output by the use of a continuous kiln 
with a tunnel about twice as long and half as wide as those 
commonly in use. In order to overcome the difficulties of 
construction and loss of heat incidental to an extremely long 
and narrow kiln, Biihrer arranged his tunnel in a zigzag 
manner (Fig. 108) so that whilst externally his kiln is square 
in plan, its effective tunnel length is almost double that of a 
Hoffmann kiln covering the same area. In this manner a 
continuous kiln can be built for common bricks with a fuel 
consumption of about 2 to 2J cwt. per thousand, and as the 
fire travels very rapidly forward in so narrow a kiln, the 
conveniences and advantages which accrue from this are 
readily obtained. For various reasons Biihrer's kilns have 
made no headway in the United Kingdom, there being only 
one and that a new one in use at the present time, whereas 



358 



METHODS OF BRICKMAKING 



in Europe and abroad generally, they have been built in large 
numbers. 

The use of a tunnel kiln in which the bricks to be burned are 

placed on small waggons and 
run slowly through a tunnel, 
the centre of which is heated 
to the finishing temperature 
of the bricks, has met with 
considerable success abroad, 
but has not been used in the 
United Kingdom except for 
pottery and other wares 
which are more expensive 
than bricks. The cost of 
repairs of the central portion 
of the tunnel when such solid 
materials as bricks are being 
burned, is greater than the 
cost of repairs of other types 
of continuous kilns. 

Kilns in which gas is used 
instead of solid fuel are 
slowly increasing in popu- 
larity. They are no more 
economical than continuous 
kilns in which coal is used, 
so far as fuel consumption is 
concerned, but they are easier 
to regulate and to maintain 
at a constant temperature, 
and with some clays this is 
very important. 

The changes which occur 
to bricks and other articles 
whilst they are in the kilns, 
form the subject of the following chapter. It is here suffi- 
cient to remark that the heating must be slow and steady 
from start to finish, as too rapid a rise in temperature or 
sudden changes in the temperature will result in cracked, 




BURNING BRICKS 359 

warped or twisted goods. Similarly, the cooling must 
also be slow and uniform, great care being taken to avoid 
cold air impinging directly on to very hot goods. It is also 
obvious that the effect of the heat must be watched very care- 
fully ; if insufficiently heated, the goods will be weak and 
porous, whilst if over burned they will be misshapen. The 
man in charge of the kiln ascertains the completion of the burn- 
ing- by means of trials, shrinkage measurements or Seger cones. 
Pyrometers are sometimes used to determine the temperature 
actually reached, but what is required is the effect of the heat 
which is a function of the time of heating as well as of the 
temperature and this cannot be ascertained by a pyrometer 
such as is used in other industries. 

Trials consist of bricks or other pieces of clay which are 
placed in such parts of the kiln as to be easily removable 
whilst the kiln is being fired. One or more of these trials is 
withdrawn at intervals and is carefully examined. If it 
possesses all the desired characteristics of the finished brick 
the burning is considered to be finished ; otherwise, it is 
continued until a satisfactorily fired trial is obtained. In 
most cases, the properties of trial bricks are not sufficiently 
distinctive to be clearly discernible, but with some clays, 
trials are invaluable. For instance, some bricks, when broken, 
show a black core or " heart," due to too rapid or incomplete 
burning. Such cores are objectionable in several ways, and 
they can be removed or prevented by careful heating with an 
ample supply of air at a temperature corresponding to a dull- 
red heat. When a clay has a tendency to produce bricks with 
this defect, the withdrawal of trials at intervals is one of the 
simplest and most efficacious methods of ascertaining whether 
the formation of dark cores is being prevented or whether the 
cores are being properly burned out. 

Trials are also useful, though to a much smaller extent, 
when bricks of a certain colour are required. Unfortunately, 
the rapid cooling of the trials which is unavoidable has a 
marked influence on the colour and renders accurate comparison 
impossible with some clays. 

Glazed bricks are almost invariably burned with the aid of 
trials, as the appearance of the glaze can be readily judged in 



360 METHODS OF BRICKMAKING 

this manner and the firing regulated accordingly. Pieces of 
clay of convenient shape are covered with glaze in the same 
manner as the bricks usually by dipping them into a cream 
or slurry made by mixing the glaze materials with water. 
Several of these trial pieces are placed in various parts of the 
kiln and are withdrawn and examined at intervals. A trial 
which has once been withdrawn should never be replaced ; 
hence the need of a number of trials in each kiln or chamber. 

Shrinkage measurements form an admirable method of 
determining the effect of the firing. All clays shrink when 
heated, and if the conditions of manufacture are kept reason- 
ably constant, the amount of shrinkage will be uniform. The 
total shrinkage which occurs in the kiln is not great, averaging 
about | inch per linear foot of material, so that a large amount 
of material is needed for an accurate measurement. The 
method usually adopted is very ingenious and consists in 
inserting an iron through the roof of the kiln until it touches 
the top of the bricks being burned. As the height to which the 
bricks are stacked in the kiln is known, their height during and 
after burning can readily be ascertained by noting how far 
the iron rod must be inserted. The pressure of the bricks 
in the upper part of the kiln affects the contraction of the lower 
ones to a slight extent, so that the shrinkage measured in this 
manner is somewhat greater than the average shrinkage of 
each brick. It will usually be found that the iron " shrinkage 
rod " can be inserted about three to five inches further into the 
kiln after the burning is complete, than it can at the start ; 
the " settlement " of bricks in different districts varies, however, 
and in some localities a shrinkage of nine or even ten inches 
is not unusual. Bricks fired in clamps with a layer of fuel 
between the courses appear to shrink much more than those 
burned in kilns, because, as the fuel burns away, the bricks sink 
and take its place. 

Measurements of the shrinkage of trial pieces are less satis- 
factory, as the roughness of the surface makes the errors of 
measurement on the smaller pieces very great. 

Seger cones are small pyramids made of special materials. 
When placed in a kiln these pyramids bend over and form an 
arch, the heat-effect corresponding to a particular cone being 



BURNING BRICKS 361 

quite constant. The cones are numbered and form a very 
extensive series, those most used for bricks being : 

No. 015a to No. 2a for building bricks. 

No. la to 14 for sintered bricks, paviours and clinkers. 

No. 6a to 9 for glazed bricks. 

No. 6 to 20 for fire-bricks. 

Strictly speaking, the Seger cones do not correspond to any 
definite temperatures, but to heat effects on the materials 
of which they are made. For this reason they are more valu- 
able than simple temperature indicators, such as pyrometers. 
If, however, the heating is steady and the rise in temperature 
corresponds to that at which the cones have been tested by the 
manufacturers about | C. per minute the cones correspond 
with sufficient accuracy to the tables of temperature supplied 
with them. For slower or more rapid heating, an error, 
depending on the time of heating, is introduced ; this does not 
interfere with the use of cones in kilns, as in the latter the rate 
of heating should be as constant as possible each time the kiln 
is burned, and cones can, therefore, be relied upon to indicate 
whether the bricks and other goods have been sufficiently 
heated. 

The use of these cones is quite simple : a trial is first made to 
ascertain which cone corresponds to the correct finishing point 
of the kiln, and afterwards the cones are arranged in groups of 
three in various parts of the kiln. In each group one cone 
bends at about 20 C. below the correct finishing point of the 
kiln, the second cone indicates this finishing point, and the 
third, which should bend about 20 C. above the second, is 
used as a precaution to show that the kiln has not been over- 
heated. The cones are placed so that they can be viewed 
through spy-holes in the walls or top of the kiln, and by their 
use the progress of the burning can be watched with the greatest 
ease. If a clay is difficult to manage at a temperature much 
below the finishing point, the use of additional cones corre- 
sponding to the lower temperature enables the burner to know 
when any desired stage of the burning above the darkest 
visible red heat has been reached. 

A wise burner will not rely solely on trials or shrinkage or 
cones, but will employ all three methods of controlling the 



362 METHODS OF BRICKMAKING 

kiln, and will, in addition, be able to tell by the colour of the 
inside of the kiln and by its general appearance whether the 
burning is or is not proceeding satisfactorily. 

The manufacture of bricks by machinery is not difficult so 
far as the production of the undried and unburned bricks is 
concerned though even here more technical skill and experience 
are necessary than is generally imagined. The chief difficulties 
of manufacture occur in the drying and in the burning, and 
so complex are the changes which occur in these two stages 
of manufacture that even when the greatest care is taken by 
the most skilful expert an occasional batch of defective bricks 
is produced. 

If a firm manufacturing facing bricks can continue week in 
week out for several years without producing more than 
4 per cent, of their output which is unsaleable as facing bricks, 
such a firm may be considered very fortunate. It is difficult 
to state the average ratio of facing bricks actually obtained 
to those made, but it will seldom exceed 90 per cent., most of 
the remainder being sold as common bricks, and a few being 
quite unsaleable except as brickbats. In the manufacture of 
common building bricks the ratio of saleable bricks to bricks 
made is about 97 per cent, in well-managed works, but with 
inferior management or unskilful burners it may drop, at 
times, to as low as 50 per cent. 

In the manufacture of firebricks, which are usually moulded 
by hand, dried on a steam-heated floor and burned at cones 
6 to 20, according to the quality of the material used, the 
proportion of first-class bricks to the total output is about 
the same as that for facing bricks. Some firms are, however, 
more careful in sorting, and in this way they are able to supply 
bricks for various purposes at prices which represent a loss 
of not more than Ij to 2 per cent, of the total number of 
bricks made. 



CHAPTER XIII 

THE CHEMICAL AND OTHER CHANGES IN DRYING AND BURNING 

BRICKS 

IT has been explained in a previous chapter that the " clays " 
used for the manufacture of building bricks are far from pure. 
They may, indeed, be regarded as mixtures of (a) true clay, 
(b) sand, and (c) fusible minerals. 

In the green or unburned bricks these materials are in the 
form of a compressed powder containing grains of widely 
different sizes, from the coarse pieces of stone or gravel of 
J inch diameter to the particles of clay which are so fine that 
their shape cannot be clearly distinguished under the most 
powerful microscope. In addition to these materials, most 
raw bricks contain a certain amount of water which has been 
added to secure the proper cohesion of the solid particles. 
This water, which in plastic bricks may easily amount to 
1 Ib. in each brick made, must be completely removed during 
the drying. 

In the freshly-made brick, each solid particle may be regarded 
as surrounded more or less perfectly by a film of water which 
keeps each of the solid particles slightly separated from each 
other. When a brick is dried, however, this water is removed, 
and consequently the solid particles move nearer together. 
This is shown diagrammatically in Fig. 109, in which the solid 
particles are represented by black discs, the space between 
them in the left side (A) of the illustration indicating the 
water which separates them. When this water has been 
removed by drying, the particles take up the position shown 
in the right side (B) of the diagram, and it will at once be 
observed that the total volume of the material has been reduced 
by the removal of the water. In other words the brick has 
shrunk on drying. 



364 CHANGES IN DRYING AND BURNING BRICKS 



Whether the water in the freshly-made bricks is really in 
the form of a film, as suggested above, or whether it is in a 
state similar to that contained in glue which has been soaked 
for several hours, is a matter concerning which there is a 
divergence of opinion. Those who regard clays as colloidal 
substances naturally maintain that the bulk of the water is 
retained within the " meshes " of the colloidal aggregate, 
whilst others hold that the water which causes the shrinkage 
is almost, if not entirely, superficial, and acts as a film surround- 
ing each particle. 

In all probability the water occurs both as a film surrounding 
some of the particles and also in an adsorbed or enmeshed 
condition. Such water is conveniently termed shrinkage water. 





FIG. 109. 



B 



There is, however, a small proportion of water which occupies 
the pores or spaces between the particles, even when the latter 
are packed as closely as possible ; this is conveniently termed 
the pore water. 

If the diagram B is observed closely it will be noticed that 
there are a number of white spaces still left between the black 
discs. These contain the " pore water " just mentioned, and 
correspond to a stage in the drying at which the particles 
cannot move closer together, and yet some water is still 
present in the brick ; that is to say, the shrinkage ceases 
before all the water has been evaporated. This stage is very 
important to brick and terra-cotta manufacturers, though less 
attention is paid to it than its importance deserves. During 
the first stage of drying, when the particles are in motion, on 



CHANGES IN DRYING BRICKS 365 

account of the evaporation of the water which separates them 
from each other, there is a great danger of cracking and 
twisting. Some bricks are, indeed, so sensitive that they 
present great difficulties, and the clays from which they are 
made are practically worthless, because of the large proportion 
of cracked and warped bricks produced. As soon as the 
shrinkage ceases, however, the remainder of the water present 
may be readily evaporated, the sensitiveness of the material 
having almost entirely disappeared. Thus, bricks which have 
to be kept covered and fully protected from heat and draughts 
during the first stage of drying may be placed on a hot floor 
or even on an iron plate heated by a gas burner without damage 
during the removal of the remainder of the water. The wise 
manufacturer, therefore, ascertains as accurately as possible 
when the shrinkage during drying has ceased, and he then 
completes the removal of the water at a more rapid rate, 
usually by sending the bricks to. the kiln. 

To remove the whole of the water from the bricks in a dryer 
is to work more slowly and less economically than when all 
the shrinkage water is taken out in a dryer and the remainder 
of the water is removed in the kiln. 

Previous to the bricks entering the kiln, the only changes 
which occur in them are the removal of the water used to 
facilitate the shaping of the clay and the reduction in the size 
of the bricks due to the particles drawing closer to each other. 
Bricks made by the semi-dry process contain so little water 
that they are sent direct to the kilns, as the drying shrinkage 
they undergo is practically negligible. Bricks made of softer 
and more plastic clay require to be dried in hacks (p. 335), on 
a steam-heated floor or in a special dryer before they can be 
sent to the kilns. In short, the changes in drying are 
purely of a physical character, unless it may be assumed that 
the clay is a swollen colloid, the water enmeshed in it 
being in the form of a loosely combined compound and not 
simply in an adsorbed form. This loose combination is quite 
possible and has some evidence in its favour, but the view 
that the clay simply changes physically during drying is more 
readily understood, and as it explains most, if not all, 
the phenomena observed during drying, it is generally 



366 CHANGES IN DRYING AND BURNING BRICKS 

accepted, notwithstanding the evidence in favour of the other 
theory. 

It is after the bricks have entered the kilns that the greatest 
and most important changes occur. Before doing so, the bricks 
are soft, friable, grey masses, which must be handled with 
great care or corners and arrises will be broken off. If placed 
in a bowl of water such bricks would soften and, in time, would 
fall to powder or to a sticky shapeless mass of loosely coherent 
mud. When removed from the kiln, the bricks have increased 
enormously in hardness ; instead of being weak and friable 
their resistance to crushing is exceptionally great, and is equal 




FIG. 110. Bricks arranged in Hack. 

or even superior to that of stone ; they are unaffected by water 
and atmospheric conditions, and form one of the most durable 
materials known. What, then, are the causes which have 
brought about so remarkable a change in the nature of the 
materials ? How is it that such changes can occur as the 
result of heating the bricks ? 

In the first place, these great changes in the physical 
properties of the bricks are the result of a series of chemical 
changes of considerable complexity. As soon as the bricks 
are warmed to a slightly higher temperature to that to which 
they have previously been exposed, the remainder of the 



CHANGES IN BURNING BRICKS 367 

moisture or pore water in them is evaporated. This may be 
completely removed at or below a temperature of 120 C., 
i.e., during the " smoking " period. 

As the temperature rises above 120 C. the clay and some of 
the fusible minerals begin to decompose, and at 500 C. the 
decomposition is sufficiently rapid for the steam produced to 
become noticeable. No matter how carefully a clay is dried, 
it will be found on heating it to a temperature of 500 to 600 C. 
to evolve a considerable amount of water. This water is not 
added to the clay, for it is an essential part of its constitution ; 
indeed, it does not exist as water in the clay, but the elements 
of which the clay molecule are composed are so arranged that 
on reaching the temperature mentioned the clay is decomposed, 
and one of the products of its decomposition is water. The 
amount of this combined water in a brick depends on the amount 
of true clay present ; ordinarily it is not more than 5 or 6 per 
cent, of the dried brick, but in a pure clay, such as the finest 
grades of china clay, it reaches 13 per cent. 

With a steadily increasing temperature the decomposition 
of the clay and other hydrous minerals continues, water being 
evolved in the form of steam, rapidly at 500 to 600 C., and more 
slowly at higher temperatures as the amount of undecomposed 
clay diminishes. 

As soon as the whole of the clay has been decomposed, the 
brick consists of a light porous mixture of aluminosilicic 
anhydride (i.e., the calcined or dehydrated clay), 1 sand and 
dehydrated fusible minerals, with negligible proportions of 
hydrous minerals which have not yet been affected by the 
heat. If removed from the kiln at this stage the bricks would 
be weaker and more friable than at first, but a little harder 
so far as the average particles are concerned. The colour would 
be a dirty, grey, with irregular patches of black sooty matter 
derived from the vegetable matter contained in all clays. 

As the temperature increases still more and reaches a red 
heat, the vegetable and other organic matter present begins 
to burn, and in some cases its combustion may require special 

1 The precise constitution of this material is so complex that it cannot be 
described fully here. Some indications of its nature have been given on 
pp. 40-43. 



368 CHANGES IN DRYING AND BURNING BRICKS 

attention. For instance, some of the shales used for brick- 
making contain considerable quantities of paraffin and allied 
substances which, on heating, form readily combustible shale 
oil, and if carelessly managed this may burn too rapidly and 
result in the bricks being seriously overheated and so spoiled. 
In most brick clays, however, the proportion of combustible 
matter is too small to be serious in its consequences, though 
it is a prolific source of blue-black cores and hearts. In 
any case the kiln must be kept at a dull red heat until the 
whole of this combustible matter has been burned away, the 
supply of fuel being restricted and that of the air being 
controlled so as to secure complete combustion without an 
undue rise in temperature. If at this stage of the burning 
the temperature rises too high, vitrification will set in and it 
will be impossible to burn out the combustible matter. 

Between the end of the removal of the shrinkage water in 
drying and the complete decomposition of the clay and the 
combustion of the organic matter present, the bricks undergo 
a second contraction or kiln shrinkage which is usually about 
equal in amount to (though frequently less than) that of the 
shrinkage in drying. This kiln shrinkage is partly due to the 
decomposition of the clay and other hydrous minerals present, 
whereby the particles become still smaller, and therefore occupy 
still less space than before. The kiln shrinkage is also due, in 
part, to fusion of some of the particles, as described later. 
Both these forms of kiln shrinkage are shown diagrammatically 
in Fig. 1 11, in which B is reproduced from Fig. 109 and indicates 
the particles which have shrunk in drying, whilst C shows 
further contraction which occurs in the kiln, due to decomposi- 
tion of the clay, and D the material after partial fusion, the 
fused portion having filled some of the pore spaces, as shown. 

The kiln shrinkage, which commences in the manner 
described, continues until the heating is finished, because as the 
temperature increases still further the more fusible materials 
begin to melt and then flow into the pores or interstices between 
the particles. This fusion increases as the temperature rises 
or the heating is prolonged. 

Grains of sand, and particles of stone, gravel and various 
minerals other than clay, do not show the same kind of shrink- 



CHANGES IN BURNING BRICKS 



369 







age as the clay grains. Many minerals only shrink when they 

reach a temperature near to the finishing point of the firing, and 

a few retain their volume throughout. The chief shrinkage 

occurs in the case of limestone 

which, on heating, is decomposed 

into carbonic acid gas, which 

escapes, and quicklime which 

remains in the bricks. All forms 

of limestone are objectionable as a 

constituent of bricks, but their 

presence is particularly detrimental 

when they occur in the form of 

race or septaria nodules. The 

quicklime formed in the kiln slakes 

slowly when the bricks are exposed 

to the air, and swells as it absorbs 

atmospheric moisture and acids. 

The white patches of lime near the 

surface of the bricks are soon 

washed out and leave unpleasant 

cavities ; those somewhat below 

the surface will create strains in 

the bricks, which may cause parts 

of the latter to crack or spall off, 

and in very bad cases the whole of 

the bricks may be reduced to a 

mass of useless material. These 

phenomena are known technically 

as blowing, and are, as already 

stated, due to the lime present in 

the clay. If the lime compound 

is in a very finely divided state, 

as in washed chalk, the bricks 

will not blow ; indeed, this defect may sometimes be prevented 

by grinding the clay (and consequently the lime compound) to 

an exceedingly fine powder. 

Very small grains of lime, soda or potash combine with the 
clay at high temperatures and form a fused, glassy material. 
If the proportion of this is sufficiently large the bricks 

c. B B 





FIG. 111. 



370 CHANGES IN DRYING AND BURNING BRICKS 

will lose their shape and may even fuse to a shapeless 
slag. 

Iron compounds, when heated to a temperature at which all 
shrinkage in the bricks ceases, will produce the red compound 
which is the characteristic colouring material of red bricks, 
unless the presence of Lime or reducing gases prevents the 
formation of this compound. 

Pyrites, or iron disulphide, on the contrary, produces black 
slag-like spots of a ferrous silicate or ferrous alumino-silicate, 
and only under abnormal conditions does it form a red 
compound. 

Soluble salts in the clay will be brought to the surface by 
the moisture, as it evaporates during the drying and early 
stage of burning and will form an efflorescence or scum on 
the bricks. If the temperature is sufficiently high these salts 
will combine with the clay and will then form a slight coating 
of glaze, the scum simultaneously disappearing. The produc- 
tion of this scum may be prevented, in many instances, by 
rendering the salts insoluble, as by the addition of barium 
carbonate to the raw clay. 

Minerals of the felspar type fuse independently of the clay if 
the temperature reached in the kiln is sufficiently high, but 
in the manufacture of most building bricks it is seldom more 
than a very slight fusion of these minerals occurs except in the 
presence of some other more fusible minerals which, by their 
chemical action, lower the temperature required for the fusion 
of the felspar, etc. 

If the bricks were to be removed from the kiln very soon 
after the first signs of fusion of some of the particles had 
commenced, they would be of moderate strength and only 
useful for inside work ; their colour would be pale or irregular, 
and they would be typical baked bricks. 

Such bricks are used in large quantities for the interiors of 
buildings or where they can be covered with stucco or other 
waterproof material. The white bricks of Suffolk, the primrose- 
coloured bricks of other localities, " rubbers," " cutters," 
bath-bricks and all firebricks are of this type, as are many 
red building bricks. They are readily distinguished from more 
fully-burned bricks by the dull sound produced when two baked 



COLOUR OF BRICKS 371 

bricks are struck together a sound very different from the 
clear, metallic note of a fully vitrified brick. 

Another characteristic of baked bricks is their high porosity, 
whereby they can absorb upwards of 15 per cent, of water and 
yet appear to be perfectly dry. Bricklayers prefer such bricks, 
as the mortar adheres to them more rapidly, with little or no 
liability to " float." 

In the manufacture of red facing bricks, for building 
purposes, and terra-cotta, the colour of the burned material 
is of great importance, and as soon as the articles have attained 
sufficient strength, the manufacturer concentrates all his 
attention in obtaining a pleasant colour. For this purpose 
he may prolong the heating indefinitely after vitrification or 
the melting of the more readily fusible particles has commenced, 
and will stop the heating as soon as a satisfactory colour has 
been obtained. 

The red colour of bricks and terra-cotta is primarily due to 
iron compounds in the clay, these forming iron oxide or other 
polymerised compounds of a red colour when the clay is 
heated. As the temperature in the kiln rises, or the heating 
is prolonged, the colour increases in intensity until a maximum 
is reached, after which the fused material present affects the 
colour, converting it into a dark and unpleasant brown shade 
if the atmosphere inside the kiln is oxidising, or into a bluish 
grey or black if reducing gases are present in the kiln. Even- 
tually, if the temperature rises still further, or the heating is 
excessively prolonged, the amount of fused matter produced 
destroys the strength of the remainder and the brick becomes 
a shapeless mass of slag. The burner must, therefore, stop 
the heating of the kiln as soon as the contents have reached 
the desired colour, or even slightly before this, because of the 
effect of the heat in the kiln whilst the fires are dying down. 

When dark-coloured bricks are required, the addition of 
2 to 4 per cent, of manganese dioxide will often be found to 
produce the desired tint. When purple and irregular tones 
are desired the clay must be specially selected and must be 
fired with too little and too much air alternately, i.e., the 
atmosphere inside the kilns must be alternately oxidising and 
reducing. The tints produced on burning some clays are a 

B B 2 



372 CHANGES IN DRYING AND BURNING BRICKS 

special characteristic of the materials from certain loaclities 
and cannot be obtained artificially with other clays. 

Insufficient heating will produce bricks of too pale a colour ; 
excessive heating, on the contrary, will result in bricks of an 
unpleasant tint. Where a clay can be found which can be 
made into bricks possessing all the desired mechanical 
properties, such as strength, lightness, porosity, in addition 
to an excellent colour, it may be said to form the ideal brick 
earth. Usually, however, some property must, in part, be 
sacrificed in favour of the others. Thus, many bricks in the 
Midlands and North of England are heated to a temperature 
which is rather too high for the production of the most pleasing 
colour, but the added strength gained by such treatment is 
considered to more than compensate this. It is always a 
temptation to the manufacturer of red bricks to pay the first 
attention to colour, as it is this property which attracts the 
attention of architects, though the strength of the bricks is, 
in most cases, of much greater importance. 

Some very pretty colour effects are obtained by the use of 
reducing gases ; many of them being largely of the nature of 
" accidents." Thus, the admixture of a little coal or coke dust 
with the clay before it is made into bricks will often produce 
pleasing purple tones in the bricks. Sometimes organic matter 
and pyrites are present in sufficient quantities to produce 
these plays of colour without any effort on the part of the 
burner, but more frequently they are produced, as suggested, 
by the deliberate addition of organic matter or by purposely 
working with a very smoky fire towards the end of the 
" baking " stage. 

In the manufacture of " blue bricks " in Staffordshire and 
elsewhere, the material used is so rich in iron oxide and in 
fusible matter that when smoky or reducing fires are used the 
iron becomes reduced and forms a ferrous silicate or ferrous 
alumino- silicate of the desired " blue " colour. The nature of 
the material is such that when this colouring substance has 
once been formed it is " fixed " by the fusible matter present, 
and so is not re-oxidised during the remainder of the heating. 
In clays which are not suitable for the manufacture of blue 
bricks, on the contrary, there is a great tendency for the 






FINAL STAGE IN BURNING BRICKS 373 

bluish silicate to be re-oxidised into the red compound to 
which red bricks owe their colour. Indeed, if a clay which is 
ordinarily used for blue bricks is fired under purely oxidising 
conditions, excellent red bricks may be produced. 

Another curious fact about the colour of bricks is that if 
15 per cent, of chalk is mixed with a red-burning clay, the 
bricks withdrawn from the kiln at a temperature below about 
750 C. will be red, but those heated to a higher temperature 
will be white. A similar result will be obtained if a clay 
naturally containing both chalk and iron oxide is used, the 
chalk, iron and clay uniting to form a white alumino- silicate. 

Engineering bricks and' others, in which the greatest possible 
strength is required, must be heated to such an extent that the 
fused material in them fills all the pores and unites the 
remaining particles into a very strong whole. Such bricks 
can only be produced by prolonged heating at sufficiently 
high temperatures, but care is required not to let the partial 
fusion, or vitrification, proceed too rapidly lest the bricks 
should lose their shape. The metallic oxides present in the 
clay are of great importance in this respect, as a clay containing 
magnesia will lose its shape far less readily with an equal 
amount of vitrification than will one containing an equivalent 
proportion of lime or alkalies. 

The final stage of burning consists, therefore, in raising the 
temperature slowly and steadily until a sufficient amount of 
fused matter has been produced to give the bricks the necessary 
strength when cold. The amount of this fusion or vitrification 
is usually ascertained by drawing trials, breaking them and 
examining the texture of the fractured surface. Two pieces 
of clay which have been fired until vitrified will also ring with 
a metallic note when struck together, the dullness of the tone 
being some guide to the insufficiency of the heating. 

Where the heating is pushed to the fullest extent possible 
without loss of shape, the material will be found to resemble an 
opaque glass or slag in character. It will be excessively hard, 
entirely impervious to water and highly resistant to corrosive 
acids. Its colour will be dark, approaching a brownish black, 
a slag grey, or what is known technically as a clinker or 
Staffordshire blue, and its density will be appreciably increased. 



374 CHANGES IN DRYING AND BURNING BRICKS 

In size, articles which have been burned to complete vitrifica- 
tion will be somewhat less than those which have been merely 
baked, but the. difference is not very great. The formation of 
gases in the interior of the material tends to cause swelling ; 
this, which may prove a serious defect, may be recognised 
by the shape of the pores observable on the fractured surface 
of the article when broken. 

The most perfect vitrified texture is that obtained in hard- 
fired porcelains, the raw materials for which are more carefully 
prepared than would be remunerative for bricks. 

It is frequently stated in books and lectures on building 
materials (obviously by those who have little or no real know- 
ledge of brickburning) that heating to one temperature will 
produce red bricks and to another temperature will produce 
blue ones. This is quite erroneous, as the difference in colour 
does not depend on temperature, but on the nature of the 
atmosphere inside the kiln. Moreover, clays which contain a 
considerable proportion of fluxing materials, such as chalk, 
limestone, magnesia, soda and potash compounds, will fuse 
and form a shapeless mass at temperatures far below those 
used for burning other brick clays. 

The temperature at which a sufficiently intense colour is 
developed varies greatly with different materials ; in some 
bricks it is reached at a temperature of about 800 C. whilst 
for other bricks a temperature of over 1,000 C. is needed, and a 
certain amount of fusion occurs before the full red colour is 
produced. As the colour of burned bricks is due to the 
chemical decomposition of the clay, it cannot be predicted 
from the appearance of the raw clay. Thus, a yellow clay 
will be red when burned, a grey clay may burn to a yellow, 
red or blue colour, and so on, and white bricks may be made 
from clays of almost any colour except a bright red one. The 
spotty appearance of some firebricks is due to the grains of 
pyrites in the clay, and instead of being regarded as a sign of 
inferiority it is rather an indication that the bricks have 
proved their refractoriness by the temperature to which they 
have been heated during the course of their manufacture. 

The best qualities of firebricks are heated to temperatures 
higher than those used in burning any other kind of clay 



FINAL STAGE IN BURNING BRICKS 375 

goods, but they are so resistant to heat that complete vitrifica- 
tion is never reached. It is essential that firebricks should 
not shrink appreciably when in use, particularly when they 
are employed in furnace linings ; hence, they should be heated 
in the kiln to a higher temperature than they are ever likely 
to reach afterwards. Many manufacturers do not heat their 
firebricks to temperatures above 1310 C., and many users 
consider that the spaces left by the shrinkage of the bricks 
in use are unavoidable. This is not the case, as if the bricks 
are burned at a sufficiently high temperature which obviously 
must depend on that of the furnace in which they will be used 
the shrinkage is so small as to be immeasurable. For annealing, 
brass melting and much other work, the temperature reached 
in the kilns will be sufficient to prevent the bricks shrinking 
still further in use, but for steel manufacture, regenerators, 
coke ovens, etc., harder-fired bricks must be employed. In 
no case, however, can firebricks be said to be " fully burned " 
in the same sense in which this phrase is applied to engineering 
and other well- vitrified bricks. The resistance to heat of most 
firebricks is so great that the amount of vitrification produced 
is, necessarily, very small. Where bricks of only a moderate 
refractoriness are required in combination with a high degree 
of resistance to abrasion or to crushing, it is often possible to 
use vitrified bricks made of the lower-grade fireclays or of 
clays which vitrify readily, but still retain their shape at high 
temperatures. 

Firebricks made of silica instead of clay are described in 
a later chapter. They are characterised by their expanding 
when heated instead of contracting like bricks made of clay. 



CHAPTER XIV. 

THE PROPERTIES OF BRICKS 

THE properties of bricks are chiefly of a physical character, 
their chemical characteristics being confined to their resistance 
to acids and to the action of water and other atmospheric 
conditions. 

The chemical composition of bricks is seldom of any 
significance, though some architects and engineers attach to 
it a wholly unwarranted importance. As previously stated, 
the analysis of a clay is only of value for investigational 
purposes, and even then it is of much less importance than other 
tests. The same remark is equally applicable to bricks. 

The one possible exception where an analysis may prove 
useful is in the case of firebricks, as a clay with more than 
2 per cent, of lime, magnesia and alkalies can scarcely be 
expected to be highly refractory. Even in this case, however, 
a direct determination of the refractoriness is of greater value 
than an analysis. 

Bricks are commonly classified according to their texture 
and colour, though the mode of manufacture, hardness and 
some other properties must also be taken into consideration, 
as will be observed in the following definitions. 

Cutters and rubbers are the softest bricks used for building 
purposes. They have a uniform, sandy texture, and are easily 
cut or rubbed to any desired shape. Their chief use is for 
arches and other gauged work. The best bricks of this class 
are made of sandy clay, lightly burned in clamps. 

Malms are yellow bricks, and are regarded in the neighbour- 
hood of London as one of the best kinds of building bricks. 
They are composed of clay and chalk to which cinder dust 
has been added and are burned in clamps, with the result that 
they have an irregular colour and texture, with a considerable 
amount of vitrified matter. 



VARIOUS BRICKS 377 

Stocks are common building bricks, and vary greatly in 
texture and other properties, according to the materials used 
and the method of manufacture. They are bricks left when 
the most regularly coloured ones have been sorted out, and 
are used for inside work, cellars, etc., but not for facing work. 
In some localities, however, the whole of the contents of a 
kiln are sold as " stocks," the builders selecting such facing 
bricks as they require after the bricks have been delivered to 
the site. Rough stocks, as their name implies, are somewhat 
irregular in colour, surface and ' shape, and are an inferior 
kind of stock brick used (on account of cheapness) for founda- 
tions and other work where appearance is of small importance. 
Grey stocks are so termed because of their colour ; apart from 
this, they are good bricks. Grizzles are underburned bricks ; 
they have a loose, porous texture, and are soft and not very 
durable when exposed to the weather. Crozzles, burrs and 
clinkers are bricks which have partly lost their shape through 
overheating. They have a vitrified texture and are heavy, 
dense, and " ring " when struck. The term clinker is also 
used for well- vitrified paving bricks of a good quality. This 
double use of the term is liable to prove confusing. Bats or 
brickbats are broken bricks or those which are so misshapen 
as to be quite useless. They are the " unsaleable residue " of 
the brickyard. Shuffs, shivers and shakes are bricks which, 
in course of manufacture, have become cracked either internally 
or externally, and are consequently unsound. No matter how 
uniform their texture apart from the cracks, or how good their 
colour, they are practically useless except for low walls and 
other work where little or no strength is needed. 

Engineering bricks and paving bricks have a close, well- 
vitrified texture, and are noted for their durability, impervious- 
ness and strength. On account of their clinking or ringing 
note when struck, these bricks are sometimes termed clinkers. 

In some localities the bricks which have been laid flat in 
a clamp forming the "paving" are termed paviours. 
They are not appreciably different from the stocks with which 
they are associated, and must not be confused with pavers, 
which are bricks used in the construction of floors anc\ 
pavements. 



378 THE PROPERTIES OF BRICKS 

The shapes and sizes of bricks are exceedingly varied, 
according to the custom of the locality and the purposes for 
which they are. used. Thus, the standard brick, as denned 
by the Royal Institute of British Architects in 1904, measures 
length between 8| and 9 inches ; breadth, between 4 ^ 
and 4 inches ; thickness, between 2| and 2j J inches, but 
bricks of larger and smaller sizes are made in many works. 
Thinner bricks (some of them only one inch in thickness) are 
also used in large quantities, as are bricks with one corner cut 
off or ornamented (squints), bricks with one rounded corner 
(bullnoses), bricks with an ornamental end (jambs), or with an 
ornamental face (diamond stretcher, dog-tooth stretcher, half- 
moon stretcher, string course bricks, etc.). Some bricks are also 
made of special shapes for arches, channels, copings, etc. ; 
others are perforated to make them lighter or in order that 
they may serve as ventilators, etc. The surface is sometimes 
scored to prevent slipping, as in paving and stable bricks, or 
a depression, or frog, is made in one or both sides of the brick, 
partly in order that the mortar may grip more firmly and partly 
to reduce the weight of the brick without affecting its size. 

Ornamental bricks are made in an endless variety of shapes 
and sizes, the modelled work on some being exceedingly 
elaborate. It is usually better, however, to have the more 
highly decorative work on larger pieces (terra-cotta), as the 
joints of ordinary brickwork are a disfigurement to modelled 
or carved work. The modelling should be completed before 
the material is burned, as if sculptured afterwards the outer 
" skin " which forms the most durable surface of the material is 
destroyed by the carving. 

Irregularity in the size of bricks is a source of great annoyance 
to bricklayers, and causes unsightly work. It is sometimes 
due to the use of inaccurate moulds or to allowing one or more 
moulds or presses to become unduly worn, whilst others in 
use at the same time are of the correct size. It is occasionally 
due to the different temperatures reached in different parts of 
a kiln or to variations in the temperature of the same kiln on 
different occasions. Some irregular bricks are produced in 
almost every kiln, but if the proportion is large it indicates 
carelessness or ignorance in manufacture ; they are removed 



THE COLOUR OF BRICKS 379 

from the better qualities by sorting the bricks as they come 
from the kiln. 

The surface of bricks is of moderate smoothness ; that of 
terra-cotta is smoother, and glazed bricks have a surface 
resembling that of glass. Too smooth a surface is considered 
undesirable for building bricks, as a moderate degree of rough- 
ness increases the adhesion of the mortar and enables the bricks 
to present a more pleasing appearance than would otherwise 
be the case. For the last-named reason, some brickmakers 
have for many years past covered their bricks with red- 
burning sand (sand-faced bricks, p. 334), whilst others use some 
form of vibrating wire to produce a special form of roughness 
of face. 

In glazed bricks, the surface should be as glossy as possible, 
free from pinholes and cracks, and, if the bricks are to be 
exposed to trying conditions, the glaze should be impermeable 
to red ink. 

The colour of bricks varies with the purposes for which they 
are required. As stated on a previous page, the majority of 
brick manufacturers endeavour to obtain bricks of as uniform 
a colour as possible, though some architects prefer a play of 
colours produced by irregularities in the composition of the 
material and in the stoking of the fires. A perfectly uniform 
colour can only be obtained with materials of great fineness 
which are extremely well mixed. Coarse particles, imperfect 
mixing and materials of a different composition to the main 
mass tend to produce irregular colouring. 

A whitish scum, or efflorescence, on the surface of bricks may 
be produced by soluble salts in the clay or by condensation 
products which are deposited whilst the bricks are in the kiln. 
These salts dissolve in the water used to make the clay plastic, 
the solution is gradually drawn to the surface as the bricks 
dry, and as the water evaporates the salts are deposited on 
the surface of the bricks. 

Some bricks have a white, or grey face or exterior, but are 
red internally. The white, or grey, coating is due to the 
kiln gases condensing on the bricks. The moisture in these 
gases is condensed into minute drops of water, and any sulphuric 
or other acids present are dissolved by these drops. The 



380 THE PROPERTIES OF BRICKS 

dilute acid solution, so formed, dissolves some of the bases in 
the clay and forms a scum similar to that described in the 
foregoing paragraph, but usually distributed more uniformly 
over the surface of the bricks. Bricks of this character do 
not receive any more heating than red bricks, as the grey or 
white coating is produced at temperatures far below red heat. 
Notwithstanding this fact, many architects consider these 
bricks to be better and stronger than if the same material 
was burned so as to produce bricks of a uniform red colour. 
The origin of this erroneous opinion is obscure, but is apparently 
due to confusion of scummed red bricks with grey engineering 
bricks of an entirely different character. 

When the natural colour of bricks is too irregular and dis- 
pleasing for facing purposes, the bricks are sometimes coated 
with a special engobe made of a red-burning clay of superior 
colour. These coated bricks are naturally regarded as inferior, 
as the coating tends to wear and chip off leaving the discoloured 
interior visible. In a few cases, bricks are dipped in a special 
slurry after they have been withdrawn from the kiln, in order 
to give them a red surface. Such bricks may be compared 
to bricks which are merely painted to hide their defects. 

The hardness of bricks varies greatly. Most building bricks 
are rather harder than sandstone, and the best vitrified paving 
and engineering bricks are too hard to be scratched by an 
ordinary steel knife. 

Soft bricks are regarded as having been insufficiently heated, 
and are termed " underburned." They occur in those parts 
of the kiln where the average temperature is not reached, and 
are particularly numerous in kilns which are badly designed 
or which have been improperly fired. Sometimes bricks which 
are seriously underfired may be returned to the kiln and burned 
a second time, but this obviously causes a serious increase in 
the cost of manufacture. 

Soft, friable bricks are weak and therefore of more limited 
use than stronger bricks in which the particles are more firmly 
cemented together by the fused material present. 

The resistance to abrasion shown by the more vitrified bricks 
is very considerable, and is an important property where 
bricks are used for pavements, roadways, etc. In the United 



THE DENSITY OF BRICKS 381 

States, paving bricks are tested by placing them in a barrel 
or tumbler with heavy pieces of iron and rotating for a definite 
time, usually 1,000 revolutions at the rate of thirty per minute. 
The loss in weight undergone by the bricks is regarded as 
indicating their lack of durability. In Germany the brick is 
held by a weight on to the surface of a grinding table and the 
amount worn away is ascertained. Neither of these tests is 
really accurate, though useful as a rough method of sorting 
out the worthless bricks. Resistance to wear and tear is, 
largely, dependent on the toughness of the bricks, and no 
means have yet been devised for accurately measuring this 
property. It appears, however, that the more fused material 
a brick contains, i.e., the more completely it is vitrified the 
greater will be its toughness, though some very completely 
vitrified bricks are too brittle to be of value. Bricks made of 
glass are quite useless for paving roads because of their 
brittleness. 

Brittleness is an undesirable property in bricks, and is most 
noticeable in those which have been heated excessively, though 
in some of the softest and underburned bricks the particles have 
so little power of cohesion that the bricks appear to be brittle. 
Bricks which have been properly burned, but cooled too 
rapidly, are usually brittle because of the strains set up by 
the rapid cooling. 

The density of bricks is the ratio of their weight to that of an 
equal volume of water, but the term is sometimes used in a 
different sense with reference to their texture. The porous 
nature of most bricks makes it difficult to ascertain their 
density with accuracy, and it is better to use the term specific 
gravity in reference to the true density of the material itself, 
apart from the pores it may contain. The apparent 
density may then be understood to relate to the weight of a 
brick including its pores, relative to that of an equal volume 
of water. The practical value of a study of changes in the 
specific gravity and apparent density of bricks during various 
stages of burning is important, as it enables the degree of 
vitrification at different temperatures to be compared, and, 
apart from any consideration of the colour of the finished 
brick, shows the temperature at which the heating of the kiln 



382 THE PROPERTIES OF BRICKS 

should stop and the nature of the heating which is most 
suitable for any particular material. These tests are, however, 
best relegated to an expert in clay testing, or erroneous con- 
clusions may be reached. Apart from this and from specula- 
tions on the precise chemical constitution of the substances 
which exist in burned clays, the determination of specific 
gravity and apparent density are of minor importance. 

Heavy bricks are usually, but not necessarily, strong ones, 
and are of a more vitrified nature than the lighter porous 
bricks. 

Light bricks are valuable in some localities because of the 
lower cost of carriage per thousand bricks. The cause of their 
low apparent density is their great porosity ; this may be a 
natural property of the clay, but it is usually due to the addition 
of sawdust or some other combustible material of the clay. 
This combustible matter burns away in the kiln and leaves a 
brick of normal volume, but highly porous, and so of very small 
weight. The properties of these bricks are such that they 
can only be used for inside work where great strength is not 
required. 

The texture of bricks is best observed by breaking them and 
studying the fractured surface. It will then be seen that the 
texture is that of an amorphous material of a somewhat 
spongy or porous character, the particles being held firmly 
together by a glassy substance which is formed by the action 
of heat on the more fusible portion of the material. Hence, 
the texture of bricks in some ways resembles that of building 
stones, but is more homogeneous and the particles of material 
are usually smaller. 

Bricks of perfectly uniform texture are costly to produce 
from some materials, and when shales and other indurated 
clays or boulder clays are used, the bricks frequently contain 
pieces as large as peas, which produce an irregular texture ; 
this is often an indication of weakness. 

A well-made brick should present as uniform an appearance 
as possible when it is broken, and the fractured faces are 
examined with the unaided eye. If a good magnifying lens of 
the Coddington type, or a microscope, is used, the irregularities 
in the material will be made more apparent, localised patches 



THE POROSITY OF BRICKS 383 

of fused material being found irregularly distributed throughout 
the brick. In a low grade of brick the presence of various 
materials other than clay may usually be recognised without 
much difficulty, though the definite identification of them 
with certain minerals is not always possible. 

Though not often found, the best texture of a brick is that 
which consists of a complex, felted mass of minute crystals, 
due to maintaining the bricks for a considerable time at a 
temperature somewhat above that at which vitrification is 
first noticeable. The risk of spoiling the bricks completely 
makes the majority of manufacturers unwilling to prolong the 
heating, and consequently, in most bricks, this crystalline, 
felted texture is replaced by a main mass of amorphous 
particles cemented together by a kind of glass. 

Apart from showing any great irregularities in the texture, a 
microscopic examination is of little value to the engineer and 
builder, as the porosity and crushing strength yield similar 
information in a simple form. 

The porosity of bricks is an important characteristic in two 
respects : the more porous a brick the less will be the cost of 
transport, and the better will be its ventilating power ; on 
the other hand, the more vitrified and dense a brick the better 
will it be able to resist the action of frost and rain. It is 
therefore necessary for the manufacturer to produce bricks 
which shall not be so porous that walls made of them remain 
damp, or bricks which are so impervious that no air can pass 
through them. For ordinary building bricks a porosity 
corresponding to the absorption of water equal to 6 to 15 per 
cent, of the weight of the bricks is found to be generally 
satisfactory, but for bridges, sewers and other engineering 
work bricks which are completely impervious are preferable. 

The porosity is usually determined by weighing three bricks, 
separately immersing each in water in such a manner that a 
piece of the brick, about 9 x 4J x J inch, is above the water 
level. After half an hour the brick is immersed completely 
and is left for twenty-four hours. At the end of this time each 
brick is taken out, rapidly wiped dry with a smooth cloth, so 
as to remove the water adherent to the surface, and is re- 
weighed. The increase in weight is due to the water absorbed 



384 THE PROPERTIES OF BRICKS 

by the pores in the brick, and is calculated to a percentage on 
the weight of the original brick. Thus, if a brick weighs 
9J Ibs. == 148 ounces before immersion, and 163| ounces 
afterwards, its porosity will be 163J -- 148 = 15 J ounces in 
148 ounces, or 10 J per cent. The object of a partial immersion 
at first is to permit the air to be displaced from the pores by 
the water. A more accurate method consists in breaking the 
brick, immersing it in water and extracting the air with a 
vacuum pump, but this does not so nearly represent the 
porosity under the normal conditions of use as the form of 
test first described, as the protective action of the dense 
" skin " on the surface of the brick is nullified by breaking 
the brick. 

The porosity of samples cut from the interior of bricks is 
often useful for studying the extent to which the burning has 
been carried. If sufficient care be taken in the determina- 
tion, the amount of porosity will indicate the amount of 
fused matter which has been formed in the brick, and will, 
therefore, show the conditions under which bricks of the 
greatest strength can be made from the material under 
examination. So many considerations enter into this question, 
however, that it is best left in the hands of a competent expert, 
as, otherwise, very erroneous conclusions may be drawn. 

The permeability to water shown by some bricks is important 
on some occasions. For ordinary buildings, bricks should be 
permeable to air, but less so to water, in order that the buildings 
may " breathe " through the brickwork and yet not have 
damp walls. The best bricks for this purpose are those which 
have a sufficient porosity to retain more water than will 
impinge on them by the heaviest rainfall to which they are 
subject, without the pores being so large that the inside of the 
wall becomes damp. Walls built of such bricks will dry 
between the showers and will preserve a dry and warm interior 
to the building of which they form a part. 

The strength of bricks is invariably expressed in terms of 
the weight required to crush them. Bricks vary greatly in 
this respect, those which are the most vitrified (i.e., the ones 
which have been the most intensely heated) being the strongest. 
In this connection it is important to remember that the strength 



THE STRENGTH OF BRICKS 385 

of even a poor brick is much greater than that of a well-built 
wall made of good bricks, as the mortar used in jointing is 
not as strong as the bricks. The demands of architects that 
bricks shall show a minimum crushing strength are, therefore, 
somewhat beside the mark, and are chiefly of value in checking 
the general nature of the bricks specified rather than in showing 
their actual strength when in use. 

The strength of bricks is affected by the precise chemical 
and physical nature of the raw materials, the amount of water 
added, the general treatment in mixing, shaping, and burning, 
as well as by the manner of cooling. Hence, no two bricks 
have precisely the same crushing strength. Figures which state 
that " Staffordshire blue bricks " have a crushing strength of 
800 tons per square foot are, therefore, misleading to the 
extent that some blue bricks made in Staffordshire are much 
weaker (300 tons per square foot) and others are much stronger 
than the figure mentioned. Even in the same works the 
strength of the bricks varies greatly at different times, and 
to assume that because certain bricks made at one time have 
a satisfactory crushing strength, therefore all the bricks made 
by the firm will be equally strong, is entirely to misunderstand 
the facts. 

Comparison of the crushing strengths of bricks of different 
characters and from different parts of the country is, therefore, 
of little value. The following figures, obtained from a very 
large number of samples by the author, in the course of his 
practice as a consulting expert in brick manufacture, etc., 
represent about as fair an average as the widely divergent results 
obtained from different bricks of the same class permits : 

Tons per 
Crushing strength. square foot. 

London grey stock bricks . , 92 

Suffolk white bricks (Gault) , . 139 

Essex red sand stocks .... 94 

Leicester red bricks (wire-cut) . . 273 

South Yorkshire bricks (stiff-plastic) . 543 
Fletton bricks (semi-dry process) . 253 

Staffordshire blue bricks . . . 790 

Rubbers and cutters (very variable) . 70 
C. C C 



386 THE PROPERTIES OF BRICKS 

In Germany no building bricks may be used which have a 
crushing strength below 136 tons per square foot. 

The principal assurance required by the architect and 
engineer is not the total strength of the bricks when new, but 
their strength after they have been in use for some time. 

The durability of bricks is of greater importance than their 
strength, for if a brick decays rapidly, its primary strength is 
of no importance. Broadly speaking, the durability of a 
brick will be greatest when the porosity is least, but as com- 
pletely impervious bricks are undesirable for many structures, 
too much stress must not be laid on a low percentage of porosity. 
Again, some bricks, notwithstanding their impermeability, are 
so weak as to be far from durable under a heavy load, and 
others are so affected by vibrations as to be useless where 
heavy machinery is employed. The durability of bricks 
depends primarily on the closeness of their texture and on 
their resistance to the action of very dilute acids. 

Air itself has no power to destroy bricks, but the gases, water 
and salts it contains can do so. Thus, the small proportion of 
carbonic acid present in the air is sufficient to attack any free 
lime in the bricks and, eventually, to cause it to " blow," 
forming unsightly white patches and cavities on the surface, 
or actually cracking the bricks. No matter how fine may be 
the particles of lime in the brick, if the latter is porous they 
will become carbonated in the course of time. 

The action of rain is also much greater on a porous brick 
than on an impervious one. The water has a solvent action 
on some of the materials, including the lime, and so washes 
them away ; at the same time it has a softening action on 
imperfectly burned clay, rendering it easier for subsequent 
rains to remove such material. If a brick absorbs a certain 
amount of rain and is then exposed to frost, the water will 
expand and so set up strains which, in time, will cause the 
brick to crumble away. Vegetable matter, such as lichens, 
mosses and algse, also effect the destruction of bricks on to 
which they settle from the air, the rain keeping them moist 
and permitting them to thrive if the bricks are sufficiently 
porous and rough of surface. Some bricks have such small 
pores and so smooth a surface that the lichens, etc., cannot 



SCUM IN BRICKS 387 

adhere to them ; such bricks do not vegetate. The " scum " 
formed by vegetation can readily be recognised under the 
microscope. 

Bricks behave very erratically so far as durability is con- 
cerned, and cases are known where the same manufacturer has 
supplied bricks for two houses in the same town and one 
house has suffered far more than the other from the weather. 

The sheltered or exposed nature of the site and the direction 
and intensity of the prevailing winds are often important 
factors in such a case. Slight, but oft-repeated changes in 
temperature are another cause of the destruction of some 
bricks, the more compact and vitrified kinds suffering the most 
damage. In very porous bricks there is sufficient space 
between the particles to absorb the expansion due to heat or to 
allow for the contraction on cooling, but impervious bricks 
cannot do this. The damage caused by changes in temperature 
is most serious in the case of bricks and terra-cotta which have 
a thin dense " skin " on the surface ; this " skin " does not 
expand and contract at the same rate as the body behind it and 
so tends to peel or " shell " off, leaving a rough under-surface 
which rapidly becomes dirty and unsightly. This expansion, 
which is not infrequently permanent, is most noticeable in 
copings. 

A cause of decay which is not usually important as regards 
the durability of the bricks, but is very detrimental to their 
appearance, is the formation of scum or efflorescence. If this 
does not occur on the bricks when first laid, it will usually be 
derived from the mortar or from the foundation on which the 
bricks are laid. In any case this scum or efflorescence (pro- 
viding that it is not of a vegetable nature) is due to soluble 
salts which have gained access to the bricks. Usually they 
have been dissolved in the water mixed with the mortar and 
have dried out on the surface of the bricks as the mortar dried, 
or the rain falling on the wall has dissolved the salts out of the 
water and has brought them to the surface of the bricks by 
capillary attraction, as the rain-water evaporated. An alter- 
native and by no means infrequent cause of scum is the 
presence, in the foundations, of soluble salts. If the brickwork 
is not provided with an adequate damp-proof course, the 

c c 2 



388 THE PROPERTIES OF BRICKS 

ground water will dissolve these salts and will rise up in the 
brickwork, and later, as it dries away, it will leave them as a 
scum on the surface. The irregularity of the patches of scum 
on bricks is often a clear sign of variations in their porosity. 

Notwithstanding all that has been stated above with regard 
to causes which lessen the durability of bricks, the fact still 
remains that well-made bricks are the most durable building 
material known. Granite and all other stones are by their 
very nature subject to decay by the agency of the weather, 
but bricks which have been properly made and burned have 
been found to outlast the hardest stones. The remarkable 
state of preservation of many ancient Chaldean, Assyrian, 
Egyptian, Indian and other buildings is due, in large measure, 
to the fact that they were built of burned clay. The climatic 
conditions of modern industrial towns are much more severe 
than those of ancient days, owing to the peculiarly corrosive 
action of the acids contained in smoke and in furnace gases, 
yet even at the present day the evidence is all in favour of 
good bricks as by far the most durable building material, and, 
in many localities, one of the cheapest. 

Resistance to strong acids is a property possessed in a marked 
degree by many vitreous bricks and by all those which have 
been glazed by the aid of salt. Porous bricks, if free from 
lime, will resist the action of strong acids for a time, but not 
for long. The more free the clay is from iron, lime and 
magnesia compounds and alkalies, and the more completely 
it is vitrified, the greater will be its resistance to acids. 

Bricks which are rendered impervious by a coating of glaze 
will only resist the action of acids so long as the glaze is 
undamaged. If it cracks, spalls or is broken, the resistance of 
the brick will be very small. Hence, the use of glazed bricks 
is limited to those cases where there is little likelihood of the 
surface being damaged. 

The refractoriness or resistance to very high temperatures 
possessed by firebricks deserves brief mention here. Strictly 
speaking, the refractoriness of a clay is its power of resisting 
the action of heat alone, quite apart from any accessory 
conditions, but the term is generally used to imply resistance 
to heat when used in furnaces, boilers, fireplaces, etc., in which 






THE REFRACTORINESS OF BRICKS 389 

the bricks come in contact with coal and coke ashes, dust of 
various kinds, slags or metal drosses, and with other sub- 
stances, all of which tend to reduce their resistance to heat. 
The coal in contact with the bricks brings about reducing 
conditions, and effects an undue slagging of the iron com- 
pounds ; the ashes, slags, dross, dust, etc., combine with the 
clay and form a more fusible material, thereby spoiling the 
bricks and rendering them useless. 

Some firebricks are very sensitive to sudden changes in 
temperature, and crack and spall (i.e., throw off portions of 
material from the surface) if cooled suddenly, as when a furnace 
is emptied, or the fires in the boiler are withdrawn. This 
sensitiveness is not due to lack of true refractoriness, but to 
the texture of the material. It is generally found that highly 
porous firebricks are capable of withstanding the most rapid 
changes in temperature, whilst the same bricks, when heated 
more strongly, and thereby rendered more dense, are found to 
spall when cooled suddenly. 

For lining furnaces used in metallurgical operations, fire- 
bricks must be highly resistant to the metal and the corrosive 
slags and dross, and at the same time the bricks must not spall 
or crack with the sudden drop in temperature when the furnace 
is emptied. Such firebricks are, therefore, required to possess 
two diametrically opposite characteristics for resistance to 
corrosion they must be dense, and for resistance to sudden 
changes in temperature they must be as porous as possible. 
A compromise is usually effected by making the bricks porous 
with a dense face or surface. 

As shrinkage during use would be serious, many furnace 
builders use firebricks made of silica instead of clay, as silica 
bricks expand on heating instead of shrinking. They are, 
however, less refractory than the best fireclay bricks. Their 
manufacture is described on p. 399. 

The selection of firebricks for various purposes would be 
greatly facilitated if their properties were better known to the 
users. At present, the only basis of specification, and that 
of only a semi-official character and by no means widely 
adopted, is the one published by the Institution of Gas 
Engineers. 



390 THE PROPERTIES OF BRICKS 

This provides that a material made from fireclay shall contain 
approximately not more than 75 per cent, of silica. It is known, 
however, that there are, in fe&rtain areas, fireclays containing as much 
as 80 per cent, silica, and material made from such clays shall be 
considered to conform to this specification if it passes the tests herein 
specified. 

Clause 1. Refractoriness. Two grades of material are covered by the 
specification : 

(1) Material which shows no sign of fusion when heated to a 
temperature of not less than Seger cone 30 (about 1,670 C.). 

(2) Material which shows no sign of fusion when heated to a tem- 
perature of not less than Seger cone 26 (about 1,580 C.). 

The test shall be carried out in an oxidising atmosphere, the 
temperature of the furnace being increased at the rate of about 50 C. 
per five minutes. 

The "new scale" of Seger cones is to be used. 

The material is to be chipped to the form and size of a Seger cone 
and tested against standard Seger cones (small size). 

A preliminary trial is first made with a piece of the material chipped 
into the approximate form of a cone. This should be cemented on to 
a refractory disc or slab with a mixture of alumina and best china clay, 
together with Seger xxmes 28, 30 and 32 (small size). It is essential, 
however, that the temperature should be maintained constant ; and 
if it is necessary to remove plugs, etc., for the purpose of obtaining 
the temperature, great care should be taken to avoid cooling the furnace 
by such means. The cones should, in all cases, be placed relative to 
the sample so that both are subjected to the same temperature. These 
cones are selected because they cover the range of first-class clays. 
Best china clay fuses between cones 35 and 36 ; and all British fireclays 
fall below this point. If cones 28 and 30 fall, the furnace should be 
cooled, and the material under investigation examined. If it exhibits 
no sign of fusion, the trial should be repeated with cones 31, 32 and 33. 
When cone 32 squats, the piece should be again examined, and if it 
shows signs of fusion, the trial should be repeated with cones 30, 31 
and 32. By this method of approximation, it is possible to decide 
whether the piece vitrified between cones 30 and 31 or between[cones 
31 and 32. A similar method should be adopted when testing second- 
grade material. 

It may be noted that clays and related materials have no sharply 
defined melting point, and the definition of refractoriness here adopted 
refers to the temperature at which the angular edges of the material 
under investigation begin to lose their angularity when heated. 

Clause 2. Chemical Analysis. A complete chemical analysis of the 
materials is to be provided when required by the engineer (or purchaser), 
for his personal information only. 

The silica should be determined by two evaporations with an inter- 
vening filtration ; and the alumina, lime and magnesia by two 
precipitations. The amount of titanic oxide should be indicated, and 
not included with alumina and iron. The potash and soda should be 
separately determined. 

Clause 3. Surfaces and Texture. The material shall be evenly 
burnt throughout, and the texture regular, containing no holes or 
flaws. All surfaces shall be reasonably true arid free from flaws or 
winding. 

Clause 4. Contraction or Expansion. A test piece, when heated to 



SPECIFICATION FOR FIREBRICKS 391 

a temperature of Seger cone 14 for two hours, shall not show more 
than the following linear contraction or expansion : 

Per cent. 

No. 1 grade .... 1 
No. 2 . . . 1-25 

After the test temperature has been reached, the furnace shall be 
maintained constant at that temperature throughout the testing period. 

The test piece shall be 4| inches long and 4^ inches wide, the ends 
being ground flat, and the contraction measured by means of Vernier 
calipers reading to O'l mm. a suitable mark being made on the test 
piece, so that the calipers may be placed in the same position before 
and after firing. 

The contraction referred to is linear, and is equal to 

change in length X 100 
original length of test piece* 

A pyrometer is required to ensure the temperature being maintained 
within the proper limits. 

Clause 5. Variations from Specified Measurements. In the case of 
ordinary bricks, 9 inches by 4| inches, by 3 inches or 2-| inches thick, 
there shall not be more than + li per cent, variation in length, nor 
more than + per cent, variation in width or thickness, and in all cases 
the bricks shall work out their own bond, with not more than |-inch 
allowance for joint. In the case of special bricks, blocks, or tiles, 
there shall not be more than + 2 per cent, variation from any of the 
specified dimensions. 

Clause 6. Crushing Strength. The material shall be capable of 
withstanding a crushing strain of not less than l,8001bs. per square 
inch, 1 when applied to whole bricks placed with their long side vertical 
between the jaws of the machine, giving a vertical thrust. The most 
important factors requiring attention are : 

(1) The two ends of the brick which come in contact with the jaws 
of the machine must be either ground or sawn flat and parallel, so as 
to receive a vertical thrust. 

(2) An average of not less than three bricks must be used, because 
flaws, etc., may give an abnormal result, which might not be detected 
if only one brick be used. 

Clause 7. Cementing Clay or " Fireclay Mortar." This shall be 
machine ground, and, at the discretion of the manufacturer, may 
contain a suitable percentage of fine grog ; but in all cases the cement 
clay shall be quite suitable for the purpose of binding together the 
bricks, blocks, or tiles for which it is supplied, and shall be capable 
of withstanding the same test for refractoriness. 

Clause 8. Marking of Material. All bricks, blocks, or tiles shall be 
distinctly marked by means of a figure 1 or 2 (not less than one inch 
long) stamped on them to indicate the grade to which they belong, and 
it shall be understood that any material not so marked shall be ungraded, 
and is not purchased in accordance with the terms of this specification. 

Clause 9. Inspection and Testing. The engineer (or purchaser), or 
his agreed representative, shall have access to the works of the maker 
at any reasonable time, and shall be at liberty to inspect the manu- 
facture at any stage, and to reject any material which does not conform 

1 This figure is exceedingly low, and is only 85 per cent, of the minimum 
crushing strength permitted for common building bricks in Germany. A. B. S. 



392 THE PROPERTIES OF BRICKS 

to the terms of this specification. Pieces may be selected for the 
purposes of testing, either before or after delivery, but, in either case, 
a representative of the maker shall, if he choose, be present when such 
selection is made, and shall be supplied with a similar piece of the 
retort material to. that taken for the purpose of testing. 

If the engineer (or purchaser) and the maker are not prepared to 
accept each other's tests, they shall agree to submit the samples for 
testing to an independent authority to be mutually agreed upon, and 
the engineer (or purchaser) reserves to himself the right, if the material 
does not conform to the tests laid down in the specification, to reject 
any or all the material in the consignment from which the test pieces 
were taken. 

The cost of these independent tests and of any retort lengtLs or 
tiles damaged before delivery for obtaining test pieces, shall be equally 
divided between the purchaser and the maker if the test proves satis- 
factory, and if unsatisfactory such cost, and that for all other subsequent 
tests required on this account from the same consignment, shall be 
borne by the makers. 

The cost of any tests or of any material damaged for the purpose of 
obtaining test pieces after delivery shall be borne by the purchaser 
in the event of the test being satisfactory, and, if unsatisfactory, by 
the manufacturers, in a similar manner to that specified for the tests 
prior to delivery. 

It is suggested that, until all the manufacturers have suitable arrange- 
ments and appliances for constantly testing their goods, it may be 
possible to render them some assistance by allowing a fairly large 
sample of their material to be sent in for testing and general approval 
before extensive deliveries are made. This is in no way, however, to 
be construed as removing the right of the purchaser to test material 
in any subsequent consignment. 

Such a specification cannot, by its very nature, meet the 
needs of more than a few users of firebricks, for the following 
reasons : 

(a) The specification is so lenient that many unsatisfactory 
firebricks conform to it. 

(6) It does not recognise the relative importance of various 
properties under given conditions of use. 

(c) It has been based upon a too limited knowledge and 
experience of the purposes for which firebricks are used and 
the conditions under which they are employed. 

In selecting firebricks for a particular purpose it is important 
to remember that the actual resistance to heat alone may not 
be the most important factor. It has already been stated 
that the strength of a building brick depends very largely on 
the amount of vitrification which has taken place in it i.e., on 
the extent to which the fused matter has bound the less fusible 
particles together. The same is equally true of firebricks so 



SPECIFICATION FOR FIREBRICKS 393 

far as the load-carrying power of the cold bricks is concerned. 
When they are heated, however, the glassy material in them 
softens and is a source of weakness. If, therefore, the tempera- 
ture to which the firebricks will be exposed in use is not very 
high say not exceeding 1,100 C. it is preferable to use a 
firebrick which shows a low refractoriness, corresponding to 
cone 26 to 29, in preference to a highly refractory one (cone 34 
or above). Such a second-grade or third-grade brick will, if 
it has been properly burned, contain more fused matter than 
a better grade of brick, and will be better able to resist the 
action of various abrasives and reducers with which it may come 
into contact, and will be better able to withstand rough usage. 
If, on the contrary, the temperatures in the furnace reach 
1,400 C. or above, the choice of a firebrick is much more 
difficult. It is then necessary to ascertain by experiment 
what importance should be attached to strength and heat 
resistance respectively, it being always borne in mind that as 
the resistance to heat is increased the strength is diminished, 
and vice versa. 

For this reason, it is usually wise to build different portions 
of the furnace with different kinds of firebricks. Thus, the 
dome or arch will require to be built of bricks whose chief 
characteristic is strength to resist the crushing tendency due 
to expansion and contraction, combined with ability to with- 
stand sudden changes of temperature ; in these bricks resistance 
to heat per se is of secondary importance. The walls of the 
furnace will require to be more heat-resisting, whilst still 
retaining as much mechanical strength as possible. In the 
hearth or hottest part of the furnace, where the greatest possible 
resistance to heat is required, some strength must be sacrificed 
to secure refractoriness. 

It is futile, if the best results are desired at very high tempera- 
tures, to expect to have the furnace built throughout of the 
same materials, and for the same reason it is equally futile 
to expect any official or semi-official specification to be of value 
except in 'excluding materials which have so low a resistance 
to heat as to be unworthy of the adjective " refractory." To 
secure the maximum of durability each portion of a furnace 
requires a different specification, and even then, unless the 



394 THE PROPERTIES OF BRICKS 

type of furnace and its uses are fully recognised, the specification 
will be of small value. 

It is the absence of any proper basis of specification which 
is the cause of so much contention and dissatisfaction between 
the manufacturers and certain users of firebricks. In many 
instances neither the user nor the manufacturer knows what 
properties the bricks are required to possess, or, if a selection 
of properties must be made, which are the most important 
ones. In other cases, the manufacturer could be of great 
assistance if only the user would describe more fully the precise 
purpose for which the bricks are required and for which part 
of the furnace. In the most difficult cases, the best results 
are obtained by consulting an expert who is independent alike 
of the manufacturer and the user, and whose training and 
experience are such that he knows what neither of the other 
parties concerned can ascertain. The rapidly increasing 
demand for furnaces to work commercially at temperatures 
unattainable ten years ago has made the consultation of such 
independent experts an absolute necessity in many cases and 
desirable in many more. 

Other properties of bricks and clays which have been burned 
in kilns will be found described in the author's " Modern 
Brickmaking," in his " British Clays, Shales and Sands," and 
in " Bricks and Tiles," by Dobson and Searle. 



CHAPTER XV 

SILICEOUS BRICKS 

BRICKS made of clay, or of earths containing a large propor- 
tion of clay, are by far the strongest and most durable as a 
building material, but in localities where clay is scarce and 
sand is plentiful, the latter may be used. Sand possesses so 
little cohesion that the grains must be cemented together by 
the aid of some added material. The nature of this added 
material determines the properties of the bricks made. 

Siliceous bricks are of three main kinds, termed respectively 
(a) lime-sand bricks, (6) cement-sand bricks, and (c) silica fire- 
bricks. 

Lime-sand bricks are made by mixing sand or crushed 
sandstone with a " milk of lime." This material, which 
consists of quicklime suspended in water, all lumps being 
removed by passing it through a fine sieve (No. 50), is mixed 
directly with the sand so as to form a very stiff coherent mass. 
Much better results may be obtained by grinding the quicklime 
with rather more than an equal weight of sand in a ball mill 
until the mixture is fine enough to pass completely through a 
No. 50 sieve. It is then mixed with the remainder of the 
(unground) sand, water is added and the mixture heated so 
that it "boils" whilst passing through a further mixing 
machine. The proportion of lime required in either case is 
between 6 and 10 per cent, of the weight of the sand, but the 
proportion which gives the best result must be determined by 
experiment and rigidly maintained. It is essential that no 
unslaked lime shall be present in the mixture, as this would 
make the bricks unsound and weak. Hence, it is a wise 
precaution to store the mixture of lime, sand and water in 
large bins or silos for twenty -four to seventy -two hours, 
in order that the water may be uniformly distributed and the 



396 



SILICEOUS BRICKS 







LIME-SAND BRICKS 397 

lime completely slaked. The stored mixture is formed into 
bricks in machines capable of exerting a pressure of 100 to 
150 tons, and similar to those used for making " dry " or 
" dust " bricks (p. 343). The bricks so produced are then 
placed on waggons and heated with steam at about 125 Ibs. 
per square inch pressure for eight to ten hours in an autoclave 
or hardening chamber. On removal from this chamber they 
are ready for use, but their strength is increased and their 
quality is improved by storing them in open sheds for several 
weeks, or even months, before they are used. 

There are many details which require skilled attention in the 
manufacture of these bricks, and it is by no means so easy to 
make them remuneratively as may appear to be the case from 
this description. With suitable sand and strict and skilful 
management, however, the production of lime-sand bricks is 
by no means so difficult as that of bricks made of clay, as the 
very difficult process of burning is entirely avoided. 

Lime-sand bricks are usually white, or as nearly so as the 
natural colour of the sand used in their manufacture permits 
them to be. They should have a crushing strength of at 
least 128 tons per square foot, and are, therefore, quite as 
strong as the average building bricks used in the south of 
England. They are seldom so strong as the best bricks made 
by the stiff -plastic process in the Northern Midlands. 

As " sand " is a term used to indicate the physical nature and 
not the composition of materials to which it is applied, the 
term " lime-sand bricks " is frequently used for bricks made 
from ground slag, boiler and destructor refuse or similar 
siliceous materials. These clinker bricks are made in the 
same manner as bricks made from sand and lime, but the 
chemical reaction which occurs when the crushed slag or ashes 
are mixed with water renders a thorough heating, mixing and 
slaking even more essential than when a purely siliceous sand 
is used. Clinker bricks and slabs form a useful means of 
converting an inconvenient waste product into a commercially 
valuable one, and they are, therefore, being made in increas- 
ingly large quantities by corporations and firms with sufficiently 
large supplies of clinker to make their utilisation possible as 
a commercially profitable process. 



398 



SILICEOUS BRICKS 



Cement-sand bricks are those in which the particles of sand 
or other crushed siliceous material are bound together with 




a, 
65 



a 



Portland and other suitable cement. The sand is screened or 
washed so as to remove gravel and other coarse particles, and 
is then mixed with about one-third of its weight of Portland 



SILICA BRICKS 399 

cement and with sufficient water to produce a suitable paste. 
The mixture which is really a concrete (see p. 250) is then 
mixed by hand or in a mixing machine, and is filled into moulds 
and tamped until a film of water rises to and covers the surface. 
The mould is then removed and the brick allowed to stand for 
a few weeks until it is fully matured. Cement-sand bricks 
are sometimes made in power -presses (Fig. 112), but they are 
seldom so satisfactory as those made in hand presses (Fig. 88) ; 
excessive pressure appears to disturb the arrangement of the 
particles during hardening. As the process of moulding is 
somewhat slow, it is customary either to mould six or more 
bricks at a time or to form larger blocks, about 1J cubic feet 
each (Fig. 86). These may be made with a face cut to imitate 
worked stone, by the insertion of a suitable die in the mould. 

The " sand " used in cement-sand bricks need not consist 
of a siliceous sand ; boiler and destructor refuse, ground 
furnace slag and various other waste products may be used, 
providing that they do not contain raw clay. If more than 
about 3 per cent, of raw clay is present the bricks or blocks will 
be too weak. 

For a full description of the processes and reactions which 
occur in the manufacture of cement-sand bricks, the reader 
should consult the section on concrete, pp. 248, et seq. The 
use of these bricks is distinctly limited in scope, as the erection 
of a monolithic concrete structure is cheaper except in the 
case of small repairs because- in addition to the cost of making 
the bricks or blocks there is the expense of laying them. The 
chief use of such bricks or blocks is thus confined to localities 
or circumstances where concrete monolithic work is undesirable 
or impracticable. 

Silica firebricks sometimes termed " Silica bricks," Dinas 
bricks or Ganister bricks are chiefly used in metallurgical 
furnaces, for which purpose some firms prefer them to bricks 
made of fireclay. In general composition they resemble 
lime-sand bricks (p. 395), but contain less lime, and, in order 
to prevent them from shrinking when in use in the hottest 
parts of the furnace, they are fired in kilns previous to being 
sent out by the manufacturer. 

They are chiefly made from a pure siliceous rock or from a 



400 SILICA BRICKS 

similar rock, known as ganister, which contains about 10 per 
cent, of clay, and occurs in the Coal Measures. The clay in 
the ganister bricks is often sufficient to provide all the binding 
material necessary to cement the particles together, but the 
purer siliceous rocks require the addition of about 2 per cent, 
of their weight of quicklime. The proportion of lime should 
be kept as low as is possibly consistent with the strength of 
the bricks, as it reduces their refractoriness (pp. 307, 388). 

The crushed rock is mixed with milk of lime (p. 395) and 
water so as to form a stiff coherent mass similar to that used 
in the manufacture of lime-sand bricks. The paste is then 
taken direct from the mixer to hand moulds, where it is formed 
into bricks under a slight pressure. The bricks are then dried 
in a warm room and are afterwards taken to the kiln, where 
they are burned under such conditions that cone 18 is bent 
after about three days. Some firms are content to bend 
cone 14, but the additional heating secures several advantages 
and greatly increases the durability of the bricks. 

Such bricks, when broken, have a texture resembling sand- 
stone, the lime having combined with some of the silica to 
form a fusible compound, which, on cooling, cements the grains 
of silica together. Though not so refractory as the best 
fireclay bricks, silica bricks are superior to low-grade firebricks 
so far as mere resistance to heat is concerned. They are, 
however, extremely sensitive to sudden changes in temperature, 
and crack, spall and peel badly when cooled suddenly. Unlike 
fireclay bricks, they expand instead of shrinking when heated. 



CHAPTER XVI 

BASIC AND NEUTRAL BRICKS 

FOR some purposes it is important that the bricks used 
should have a definitely basic or neutral character. This is 
particularly the case in the manufacture of certain chemicals 
and of certain metals and alloys. 

Basic bricks are specially useful for furnaces in which slags 
rich in lime are produced, as they are unattacked by such 
slags, whereas acid bricks (made of fireclay or silica) would 
rapidly be corroded, owing to the combination of the acid and 
the base. Basic bricks are made of magnesia or lime, but the 
latter are so weak as to be seldom employed. 

Magnesite bricks, which are the most extensively used basic 
bricks, are made by cautiously burning magnesite in a shaft 
kiln similar to those used for burning lime. The magnesite 
is thus decomposed, evolving carbonic acid gas, and forming 
magnesia. Some of the material is drawn from the kiln after 
a short exposure at a red heat, but for the remainder the 
heating is continued until the magnesia sinters and forms a 
slaggy mass. The caustic magnesia and the sintered magnesia 
are then mixed in suitable proportions and ground to a fine 
powder ; a little water is added and mixed thoroughly with 
the materials so as to form a stiff paste. The paste is then 
compressed in powerful hydraulic presses and the bricks so 
produced are dried and then burned in suitable kilns. The 
temperature required for burning magnesia bricks is so high 
(cone 18 to 23) that gas-fired kilns fitted with regenerators are 
the most economical, though round down-draught kilns are 
also extensively used for this purpose. 

The manufacture of magnesite bricks is one of peculiar 
difficulty, as the sintered magnesia is difficult to grind, the 
bricks as they come from the drying chamber are exceedingly 
tender and must be handled with great care, and the high 
temperatures in the kilns are by no means easy to maintain. 

c. D D 



402 BASIC AND NEUTRAL BRICKS 

The production of these bricks is, therefore, confined to a 
limited number of firms. 

Magnesite bricks are particularly sensitive to silica and 
clay, and with these materials form a glassy slag at high 
temperatures. It is, for this reason, very difficult to use both 
magnesite and clay- or silica-bricks in the same structure. 

Bauxite bricks are commonly regarded as basic, though in 
many ways they partake of a neutral character. Bauxite is 
the mineral name for a variety of impure alumina, which 
usually contains considerable proportions of iron oxide and 
combined water. The bauxite is ground to powder, mixed 
with a little clay and water to form a stiff paste, and s then 
moulded in a manner similar to hand-made bricks (p. 333). 
The burning may be effected in any kiln suitable for firebricks, 
but the greatest possible care must be taken to avoid a shortage 
of air. If there is a reducing atmosphere in the kiln the iron 
compounds in the bauxite will be reduced, and bricks of very 
low heat resistance will be produced. Bauxite shrinks greatly 
during the burning, and if this is carried out at too low a 
temperature the bricks may shrink badly when in use. The 
great shrinkage makes it very difficult to produce bauxite bricks 
of a uniform size, and the men engaged in emptying the kilns 
must be instructed to sort them carefully with the aid of 
gauges. Unless this is done it will be impossible to keep the 
joints in the brickwork uniform. 

Neutral bricks are those which are not affected by either basic 
substances, such as lime or magnesia, or by acid substances, such 
as clay or silica. They are more costly than other bricks, and 
are only used for purposes for which the others are unsuitable. 

The majority of neutral bricks are made of chromite, and 
are composed of chromium oxide with some iron oxide. As 
this material has no inherent binding power, it is usually 
mixed with twice its weight of fireclay. Chromic bricks are 
made in the same manner as ordinary firebricks, though some 
makers prefer to compress them in powerful presses instead 
of moulding them by hand. 

Several other materials are made into bricks for special 
purposes, but their use is too limited to warrant their description 
in the present volume. 



INDEX 



O-ORTHO- SILICATE, 51 

Abrasion, resistance to (bricks), 380 
Accelerated tests, 121 
Accrington, 342 
Adhesion, 1, 229 

between concrete and steel, 255 

of cement to metal, 144 

of concrete to metal, 217, 218 
Adie's machine, 135 
Adulterants in cement, 38, 102 
Aeration, 74 
Agglomerate clay, 311 
Aggregate, 146 

grading, 156 

measuring, 168 

washing, 154 
Aggregates for concrete, 147 

for reinforced concrete, 209 
testing, 277 

Air, exposure of cement to, 92 
Air separator, 105 
Alite, 50, 52 
Alkali-waste, 18 
Alkalies, 73 

in clays, 308 
Alligating, 195 
Alluvial clay, 6, 7, 311 
Alum, 92 
Alumina, 75 

and lime, reactions between, 69 

effect of heat on, 43 

free, 307 

free, in cement, 43, 90 

in cements, 69, 70 

lime and silica, reactions between, 
71 

^silica ratio, 65 
Aluminate theory, 71 
Aluminates, 54, 69, 90 
Aluminium chloride, 92 
sulphate, 92 
Aluminosilicates, 47, 48, 55, 74, 78, 83, 

85, 87, 89, 90, 93 
Aluminosilicic acid, 5, 39, 55 
American Standard Specification, 121 
Analysis of clay, 321 
Anchored spirals, 223 
Apparent density, 99, 381 
Aqueducts, 242 



Arches, 241 

Architects' Institute recommendations, 
211 

Archless kiln, 354 

Argillaceous limestone, 5 

Artificial aggregates, 149 

Asch, W. & D., 87, 93, 112, 266 

Asch's theory, 40, 55, 61, 65, 267 

Ash, proportion of, 110 

Ashes as aggregate, 151 

Associated Portland Cement Manufac- 
turers, Limited, 154, 155, 157, 169 



/3-ORTHO-SILJCATE, 51 

Bach, von, 283 
Baked bricks, 370 
Ball clays, 311 
mills, 105 

Ballast, 148, 149, 193, 258 
Barker and Hunter, 183 
Bars for reinforcement, 219 
smooth, 217 
with wings, 222 
Basic bricks, 401 
Batch mixers, 176 
Bauschinger's method, 124 
Bauxite bricks, 402 
Beam, continuous, 216 

loaded, 215 

Beams, 216, 218, 224, 225, 233, 248 
Belgian cement, 13, 31 

kilns, 351 
Belite, 50, 51 
Bending moments, 216, 218 

strength, 142 
Berry, H. C., 284 
Binary silicates, 69 
Binne, C. F., 288 
Binns and Makeley, 290 
Bins, 25 
Bitumens, 200 
Blaese, C. von, 265 
Blast furnace slag, 17 
Block-making machine, 251 
Blocks, 248 
Blount, B., 101 
Blowing in bricks, 295, 369 

of cement, 94, 104, 116, 119 

D D 2 



404 



INDEX 



Blowing of concrete, 150 

Blue bricks, 289, 372 

Boats, 252 

Bonna system, 242 

Boracic acid, 92 

Borax, 92 

Boudouard, 67 

Boulder clays, 294 

Bradley and Craven stiff-plastic brick 

machine, 340 

Breaking strength of briquettes, 137 
Brickbats, 377 
Brick clays, 286 

earth, 311, 326 

Brickmaking, methods of, 319 
Bricks, 286 
basic, 401 
bauxite, 402 
chromic, 402 
Fletton, 150, 210 
for aggregate, 149 
glazed, 359 
magnesite, 401 
neutral, 401, 402 
properties of, 376 
siliceous, 395 
Bridges, 240 

British Concrete-Steel Co., 212 
British Fire Prevention Committee. 259 
British Reinforced Concrete Engineer- 
ing Co., Ltd., 212 

British Standard Specification, 97, 98, 
103, 107, 109, 111, 118, 125, 127, 129, 
131, 132, 133, 135, 137, 139, 140, 147, 
212 

Brittleness of bricks, 381 
Brown, H. P., 270 
Buff bricks, 290 
Buhrer's kiln, 357 
Builders' and Contractors' Plant, Ltd., 

176 

Building blocks, 248 
Bulging, 221 
Bullnoses, 378 
Burning, 33 

bricks, 344 
changes in, 363 
cement, 23, 68 
Burrs, 305, 377 
Butler, D. B., 104, 108, 116, 122, 137 



CALCAREOUS SANDS, 159 

Calcined clay, 43, 46, 47, 48, 88, 367 

Calcium aluminates, 47, 52, 54, 69, 70, 

71 

aluminosilicates, 47, 55 
carbonate. See Chalk, Lime- 
stone and Lime, 
chloride, 92, 93 



Calcium ferrite, 262 

meta-silicate, 67 
mono-silicate, 67 
ortho-silicate, 51, 89 
phosphate, 6 
silicates, 47, 52, 91 
sulphate, 67, 75, 76, 92, 93, 94 
sulphate, action of, on cement, 

92 

sulphide, lo 
Callow, 317 
CaO : SiO ratio, 53 
Candlot, E., 92, 266 
Cantilever wall, 220 
Capital required for bricks, 319 
Carbon dioxide, action of, 91, 94 
Carbonaceous matter in clays, 310 
Cart for concrete, 188 
Celite, 50, 51 
Cement, 1 

clinker. See Clinker. 

coatings, 199 

for concrete, 147 

for reinforced concrete, 210 

hardened, constituents of, 90 

in concrete, 165 

manufacture, 20 

mixture, effect of heat on, 45 

sand bricks, 395, 398 

sand tests, 138 

testing, 96 
Cements, organic, 1 
Centering, 184 

striking, 193 
Chalcopyrite, 310 
Chalk, 13 

and clay, reactions between, 46 

effect of heat on, 44 

in bricks, 291, 373 
Chamber kilns, 355 
Changes to bricks in burning, 358 
Chapman, C. M., 205 
Checking mixture of concrete, 169 
Chemical changes in cements, 39 
Chemical composition, 98 
Chicago cube mixer, 177 
Chimneys of concrete, 246 
China clay, 5, 314 
Chromic bricks, 402 
Cinders for reinforced concrete, 209 
Clamps, 344 
Clark, R. G., 262 
Clay and lime, reactions between, 46, 57 

effect of heat on, 40, 366 

preparation of, 325 
Clays, 1, 2, 5, 39, 55, 286 

action of heat on, 304 

occurrence of, 12 

plastic, 296 

various, 315 



INDEX 



405 



Cleaning the surface of concrete, 195 
Climates, effect of, on setting, 81 
Clinker, 26, 30, 50, 80, 118 
as aggregate, 151 
bricks, 305, 377, 397 
physical properties of, 80 
Clots, 339 
Chinches, 312 
Coal bunkers, 243 
Cobb, J. W., 66, 67, 69, 72, 76, 77 
Coefficient of elasticity, 216 
Coffer-dams, 192 

Coignet, Edmond, Limited, 213, 231 
pile, 245 
pipe, 242 

reinforcement, 230 
Coke-breeze aggregate, 150, 258 

for reinforced concrete, 209 
in clays, 299 

Colloidal material in clay, 301 
silica, 84 
substance, 82 
Colloids, 83 

Colour of brick clays, 287 
of bricks, 371, 379 
of terra- cotta, 371 
Colouring concrete, 196 
Column base, 224 
Columns, 215, 221 

testing, 279 

Combined water in clay, 367 
Commercial specifications, 213 
Compactness, 145 
Components of concrete, 146 
Composition of bricks, 376 

of cements, limits of, 61, 62 
of concrete mixtures, 

checking, 169 
of Portland cement, 98 
Compression bar, 228 
Compressive strength, 126, 224 
Concrete, 146 
blocks, 249 

Institute, recommendations, 211 
placing, 187 
preparation of, 162 
reinforced, 206 
spading, 189 
testing, 277 
Conduits, 242 
Cones, Seger, 360 
Conglomerate clays, 312 
Considere Construction Co., Ltd., 212 
pile, 247 
system, 222 
Consistency of concrete, 170 

of paste for bricks, 332 
Continuous kilns, 347 
Contraction of cement, 120 
Cooling clinker, 26 



Core, metallic, 223 

Cores, 368 

Corrosion, electrolytic, 270 

of reinforced concrete, 269 
resistance to, 262 
Costs, 217, 324 
Cracking (bricks), 369 
of cement, 116 
of concrete, 150, 195, 273 
Cracks, 236, 272 

Crocodile surfaces of concrete, 195 
Crozzles, 305, 377 
Crushing, 25 

machinery, 327 

strength, 214, 278 

strength of bricks, 385 

strength of concrete, 254 

tests, 278 
Crystallisation, 82 
Crystals, 54 

formation of, 49 

in cement, 50 
Cushmann, 107 
Cutters, 370, 376 



DAMP-PROOFING CONCRETE, 197 
" Darrprobe," 123 
Day, 67 

Day and Shepherd, 51 
Day, Shepherd and Wright, 53 
Dead- burnt lime, 33 
loads, 213 

loads, equivalent, 214 
Deflection under load, 284 
Dense surfaces of concrete, 195 
Density, apparent, 99, 145 

of bricks, 381 
Desch, C. H., 50 
Deval's test, 123 
" Diamond " stretcher, 378 
Di-calcium aluminate, 52, 70, 90 

silicate, 52 
Dinas bricks, 399 
Discoloration of concrete, 194, 273 
Ditch cutters, 323 
Dittler, E., 60, 79 
Docks, 242 
Doetler, C., 68, 79 
" Dog-tooth " stretcher, 378 
Dolomitic limestone, 5 
Dormann, 0., 65, 68 
Down-draught kilns, 346 
Drift clays, 312 
Dry concrete mixture, 170 

dust process, 343 

process, 24, 27 
Drying, 24 

bricks, 304, 335, 362 

changes in, 363 



406 



INDEX 



Drying clays, 338 
Durability of bricks, 386 

of concrete, 271 
Dye-test, 84 

Dyes absorbed by clays, 301 
Dynamo beds, 248 



EARTHQUAKE tremors, resistance to, 200 
Edge-runner grinding mill, 328 
Efflorescence, 195, 273, 379, 387 
Elasticity coefficient. 216 
Electrical conductivity of cement mixes, 

60 

Electrolytic corrosion, 270 
Emperger, F. von, 279 
Encastre, 216 
Engine beds, 248 
Engineering bricks, 373, 377 
Expanded metal, 248 

Metal Company, Limited, 

213 

metal system, 236 
Expansion of cement, 94, 116, 125 
of coke-concrete, 150 



FACING BKICKS, 343, 362 
concrete, 197 
with terra-cotta, 258 
Factors of safety, 214 
Faija, H., 112, 122, 131 
Faija's mechanical gauger, 131 
Failure, causes of, 253, 274, 275 
Fat clays, 312 

.lime, 33 

Fawcett stiff-plastic brick machine, 341 
Feichtinger, 87 
Felite, 50, 52 
Felspar, 370 

Felspathic matter, effect of, 72 
Feret, M., 226 
Ferrates, 90 
Ferric oxide, 75 
Ferrocrete, 206 
Ferro-silicates, 74, 90 
Ferrous carbonate, 289 
compounds, 74 
sulphide, 75 

Filling in the surface of concrete, 196 
Final set, 90, 110 
Fineness, 68, 103 
Fire Prevention Committee tests, 150 

resistance of concrete, 256 

resistance ; standard classification 

for floors, 259 
Firebricks, 305, 362, 375, 395 

durability of, 393 

selection of, 389 

silica, 395, 399 



Fireclays, 312, 313 
Fish glue, 1 

Fletton bricks, 150, 210 
Flints, 3 

for aggregate, 149 
Floor slabs, 248 
Floors, 214, 215, 216, 225, 231, 248 

concrete, 210 

testing, 279 
" Flour," 103, 106 
Flourometer, 108 
Fluates, 199 

" Flying" of aggregates, 209 
Forms, 184, 185, 186, 191 
Formulae of cement, 88 

of Portland cements, 57 
Foul clays, 297, 317 
Foundations for machinery, 248 
Free lime in Portland cement, 45 
Fremy, 89 
French standard for tensile strength, 

141 

Frey, O., 143 

Frog in bricks, 333, 336, 378 
Frost, effect of, 190, 271 
Fuel consumption in brick kilns, 354 
Furnace slag as aggregate, 151 
Furnaces, lining, 389 
Fusible clays, 313 
Fusion, 46, 47, 54, 68, 69, 70, 80 



7-CmTHO-SILICATE, 51 

Canister, 313 

bricks, 399 

Garden ornaments, 252 
Gary, 107, 112 

Gas Engineers, Institution of, 389 
Gas-fired kilns, 358 
Gate posts, 252 
Gauging, 81, 113, 115, 131 
Gault clay, 6, 313 
Gee, W. J., 108 
Gel, 82, 83 
German standard for tensile strength, 

141 

Standard Rules, 121 
Girders, 224 

Glassy matter, 72. 74, 80 
Glazed bricks, 359, 379 
Gliding, resistance to, 145 
Goldbeck, A. T., 284 
Goreham, process of, 22 
Graded sands, 160 
Grading, 261 

aggregates, 153, 253 
Grains, size of, 68 
Granite, 148 
Granolithic facings. 197 
Granulation of slag. 37 



INDEX 



407 



Grappier cement, 34, 266 
Grappiers, 34, 52 
Green bricks, 344 

concrete, 196 
Grey bricks, 291 
stocks, 377 
Grinding, 23, 118 
object of, 47 
Grizzles, 377 
Grog, 314 
Grout, 170 
Guthrie's kiln, 351 
Gypsum, 76, 93, 94 



HACKS, 335 

Hair-lines in concrete, 195 
" Half-moon " stretcher, 378 
Hand-made bricks, 333 
Hard materials, treating, 23 
Hardening, 109 

chambers for bricks, 398 
changes in, 81 
of concrete, 192 
Hardness of bricks, 380 
Haulage, 324 
Hearts in bricks, 368 
Heat, action of, on clays, 40, 304 
on limestone, 44 
on silica, 43 
conductivity, 258 
development of, in setting, 86 
effect of, on alumina, 43 
on bricks, 366 
on cement mix, 69, 

79 

on lime and silica, 66 
resistance to, by bricks, 388 
Heating, insufficient, 372 
Heavy bricks, 382 

concrete work. 210 
Hennebique column, 222 

base, 224 
F., 221, 227 
piles, 246 
stirrups, 227, 228 
system, 212, 226, 229, 230, 

234, 245 
Herold, K., 60 
Hillebrand, 73 
Hoffmann kiln, 24, 46, 348 
Horizontal draught kilns, 347 
Hot water tests, 120 
Hubbard, 107 
Humphrey, R. L., 268 
Hydraulic cements, 1, 2 

lime, 1, 2, 12, 13, 77, 86, 260 
burning, 33 
concrete, 147 
manufacture of, 32 



Hydraulic lime, tensile strength of, 142 

modulus, 64 
Hydraulite, 2 
Hydro-silicates, 85 



IMPURITIES, 3, 84 

effect of, 72 

in cement, 52 

in chalk, 21, 46 

in clays, 305, 307 
Inclined members, 222 
Indented bars, 219, 235 
Inert material in concrete, 146 
Initial set, 81, 88, 90 
Insoluble residue, 99 
Integral waterproofing, 200 
Inter-reactions in cements, 72 
Iron compounds, 74 
in clays, 309 

in cement, 74 

ore, 18 

ore waste, 18 

Portland cement, 36 
Irregularity in bricks, 378 



JESSER, L., 60 

Johnson, R., Clapham and Morris, Ltd., 

239, 240 

Johnson's kiln, 22 
Jones, H. R., 255 



KAHN SYSTEM, 212 

trussed bar, 217, 231, 232 
Kallauner, 265 
Kaolin, 314 
Kaolinite, 306 
Keedon bar, 229, 234 
Keene's cement, 1 
Kentish rag, 139 

specific gravity of, 102 
Kibblers, 328 
Kiefer, H. E., 119 
Kiln, Hoffmann, 24 

Johnson's, 22 

rotary, 25 

shaft, 23 
Kilns, 23, 33, 46, 72, 76, 79, 110, 345 

changes occurring in, 366 
Kloes, J. A. van der, 267 
Kiihl, H., 117 



LAMINATED clays, 314 

Lateral ties, 221 

Lavas, 2, 14 

Lean clays, 297, 314 

Lean mixture concrete, 167 



408 



INDEX 



Le Chatelier, 50, 52, 55, 101, 120, 121, 

285 

Le Chatelier' s test, 124 
Leicester red bricks, 293 
Lias limestone, 4, 14 
Liassic marls, 11 
Light bricks, 382 
Lime, 1, 2 

action of heat on, 45 

alumina and silica, reactions be- 
tween, 71 

-alumina ratio, 53 

and clay, reactions between, 46, 57 

and silica, effect of heat on, 66 
reactions between, 66 

carbonated, 91 

carbonation of, 94 

compounds, 3 
in clay, 307 

concrete, 147, 167, 214 

effect of, on setting, 83 

(free) in cement, 45, 117, 118, 119 

hydraulic. See Hydraulic lime. 

minimum proportion of, 65 

sand bricks, 395 
Limestone, 1, 2, 4, 165 

action of heat on, 44 

in bricks clays, 295, 369 
Limestones for aggregate, 149 
Lindner, 107 

Lines of stress in beam, 233 
Lintels, 250 
Litre weight, 99 
Live loads, 213 
Loads, calculating, 213, 215 
heavy, 228 
in buildings, 213 
varying, 215 
Loading tests, 279 
Loams, 8, 293, 326 
London clay, 315 
L.C.C. rules, 219 
London stocks, 345 
Loss in manufacture of bricks, 362 

of shape (bricks), 370 

on ignition, 99 



MAGAZINES, 243 

Magnesia, 5, 46, 57, 73, 99, 117, 265 

compounds, reactions of, 266 

effects of, 73 
Magnesite bricks, 401 
Magnesium chloride, 93 

compounds in clays, 308 
Malm bricks, 290 
Malms, 292, 376 
Manufacture, changes in, 39 
Marcasite, 310 
Marine deposited clays, 315 



Maritime work, 267 
Marls, 5, 9, 292, 326 

for brickmaking, 10 

for cement manufacture, 10 

of Staffordshire, 289 

red, 293 
Materials for cements, 1 

selection of, 323 
Meal, 25 
Mechanical bond, 228 

mixers, 176 
Medium mixture concrete, 169 

setting, 110 

Melting point and electrical con- 
ductivity, 61 

Membrane waterproofing, 200, 203 
Meta-silicates, 67, 82 
Methods of cement manufacture, 20 
Michaelis, W., 104, 106 
Microscopical examination, 49 

study of cement, 84 
Midland marls, 315 
Mild clays, 297 
Mills, 25 
Millstones, 105 
Mineral matter, effect of, 72J 
Minerals in clay, 44 
Mining, 323 
Mixer for clay, 331 
Mixers, 25, 176183 
Mixing clay for bricks, 331 

concrete, 173 
Mixture theory, 48 
Moisture, exposure to, 92 
Mono-silicate, 67 
Morsch, 283 
Mortar, 5 

mill, 332 

tensile strength of, 142 
Moss bar, 234, 235 
Mouchel, L. G., and Partners, 212 

hollow pile, 245 
Moulding bricks, 333 
Moulds, 184 
Muds, 2, 7, 9 
Mundic, 310 



NATURAL cement, 2, 12, 77 

manufacture of, 29 
Neat cement, 129 
Neat tensile test, 138 
Netting, 239 

Network for reinforcing, 22 
Neutral bricks, 401, 402 
Newberry Bros., 53, 73, 74 
Newcastle kilns, 347 
Nodules, 6, 13 
Nontronite, 288 
Norton, C. L., 258, 260 



INDEX 



409 



OILS in concrete, 204 
Ordinary concrete mixture, 170 
Ore, 18 

Ornamental bricks, 378 
Ormim, Van, 284 
Ortho-silicates, 51, 89 
Ostwald, W., 112 
Overburden, 317 
Over-heating, 33 
Overlimed cement, 104 
Overloaded columns, 221 
Oxy-chloride cements, 1 
Oxy-phosphate cements, 1 



PAINT applied to concrete, 199 
Pan mill, 332 
Paving blocks, 248 
bricks, 377 
Paviours, 377 
Parian cement, 1 
Pebbles, 148 
Peeling, 195, 196 

of bricks, 387 
Pelinite, 306 
Permeability of bricks, 384 

of concrete, 261, 265 
tests of, 284 
Permean marls, 11 
Peterborough, 342 
Physical changes in cements, 39 
Piers, 215, 221 
Piles, concrete,' 243 
Pipes, concrete, 242 
Pit props, 252 
Placing concrete, 187 

in water, 191 
Plaster of Paris, 76, 94 

slag cements, 36 
Plastic clays, 296 

methods of brickmaking, 333 
" Plums," 154 
Plunge test, 122 
Polished concrete, 196 
Pontoons, 252 
Popplewell, 255 
Pore water, 364 
Porosity, 145 

of aggregates, 158 

of bricks, 383 

Porous surfaces of concrete, 195 
Portland cement, 1 

action of sea water on, 266 

adulterants of, 38 

manufacture of, 20 
Potash, 73 
Potassium carbonate, 92 

compounds in clay, 308 
di-chromate, 92 
sulphate, 92 



Potassium sulphide, 92 
Pozzolana, 1, 2, 15, 35, 42, 77, 78, 88, 
146, 205 

tensile strength of, 142 
Pozzolanic sands, 159 
slag, 17 

cements, 36 
Practical test, 96 
Preheating bricks, 351 
Preparation of concrete, 162 
Properties of concrete, 254 
Proportions, 30, 62, 68, 71 

for beams, etc., 224 

for reinforced concrete, 210 

in concrete work, 252 

of clay and lime, 12 

of components of concrete, 163 
Pugmill, 330 
" Pure clay," 306 
Purple bricks, 289, 371 
Pyrites, 6, 290, 309, 370 



QUARRYING, 323 
Quartz, 66, 68 
Quicklime, 5, 33 

manufacture of, 32 
Quick setting, 110 

cement, 81, 90, 94, 109 



RACE in bricks, 369 
Rafts, 248 

Railway sleepers, 252 
Ramming, 189 

mechanical, 142 
Rankine, 67 
Ransome ver Mehr Machinery Company, 

176, 179, 181, 182, 187 
Rasenow, 299 

Rate of setting, 82, 92, 95, 109 
Rational analysis, 306 
Raw materials for bricks, 286 
Raw meal, 25 
Reading clays, 315 
Red-burning clays, 288 
Reducing atmosphere, 76, 372 
Refractoriness of bricks, 388 
Refractory clays, 315 
Re-heating, 103 
Reinforced concrete, 206 

Metal, Limited, 223 
Re-pressing bricks, 343 
Reservoirs, 242 
Resistance to abrasion of bricks, 380 

to corrosion, 262 

to shocks, 260 

to strong acids of bricks, 388 
Retaining walls, 220 
Retardation of setting, 92 



410 



INDEX 



Rib mesh reinforcement, 237 
Rich mixture concrete, 167 
Richardson, 51 
Roads, concrete, 250 
Rock cement, 13 

manufacture of, 29 

tensile strength of, 142 
Rock clays, 315 
Rohland, P., 92 
Roman cement, 1, 13, 31, 77, 90 

tensile strength of, 142 
Roof, 216 
Ropeways, 324 
Ross, 190 

Rotary kiln, 25, 46, 72 
Rough stocks, 377 
Roughness, 195 
R.I.B.A. Committee, 211, 218, 257, 279, 

283 

standard for bricks, 378 
" Ruabon " kiln, 356 
Ruabon terra-cotta clay, 293 
" Rubbers," 370, 376 



SAFE load for concrete, 214 

for lime-concrete, 214 
Safety factors, 214 
Salter tensile test machine, 136 
Salts, effect of, 113 

soluble, 370 
Sampling, 98 
Sand, 146, 150, 159 

cement, 29 

cementitious, 193 

for reinforced concrete, 209 

measuring, 168 

moulding, 334 

standard, 139 

testing, 277 

Sandstones for aggregate, 149 
Sandy clays, 294, 316, 326 
Santorin earth, 15 
Schmidt and Unger, 54 
Schule's machine, 143 
Schuljatschenko, 267 
Scotch kilns, 345 
Scott's cement, 34 
Scum, 195, 293, 309, 379, 387 
Scummed bricks, 291 
Sea sand, 209 

water, 93 

action of, 264 
Seasoning, 119 
Seccotine, 1 
Seger cones, 360 
Selenitic cement, 34 
Semi-dry process of brickinaking, 333, 
341 



Semi-plastic methods of brickmaking. 

333, 339 
Septaria, 12 

in bricks, 369 
Setting of cement, 76 

of concrete, 192 
rate of, 81, 82, 109 
retardation of, 92 

Sewage, action of, on concrete, 263 
Sewell, J. S., 270 
Sewers, 242 
Shaft kiln, 23 
Shakes, 377 
Shales, 5, 8, 55, 286, 292, 314, 316, 

326 

for aggregate, 149 
occurrence of, 12 
Shapes of bricks, 378 
Shaping the clay, 333 
Shear bar, 218 

diagram, 215 
members, 219, 233 
reinforcement, 225 
resistance to, 145, 235 
tests, 254 

Shearing strength, 144, 254, 255 
stress of concrete, 226 
Shelling of bricks, 387 
Shepherd, 67, 69 
Shivers, 377 

Shocks, resistance to, 260 
Shrinkage measurements, 360 

of clay, 301, 302, 363, 389 
of concrete, 195 
water, 364 
ShufiEs, 377 
Shuttering, 184 
Shutters, 184 
Siderite, 289 
Sifting, 105 
Silica, action of heat on, 43 

alumina and lime, reactions 

between, 71 

and lime, reactions between, 66 
bricks, 399 
colloidal, 84 
firebricks, 395, 399 
free, 90, 306 
in Portland cement, 67 
Silicates, 51, 52, 67, 69, 72 

containing iron, 309 
Siliceous bricks, 395 
Sillimanite, 42 
Silos, 25, 243 
Silt, 316 

Size of sand particles, 160 
Sizes of bricks, 378 

of particles in aggregate, 151, 153 
Slabs, 216, 248 
testing, 279 



INDEX 



411 



Slag cements, 78 

manufacture of, 35 
tensile strength of, 142 
Slag, specific gravity of, 102 
Slags, 2, 16, 51, 67, 76, 78, 258 
Slate waste, 8 
Slates, 316 
Sleepers, 252 

Sliding of reinforcement, 144 
Slipping of stirrups or bars, 233 
Slop moulding, 334 
Slow setting, 110 

cements, 86, 92, 109, 269 
Slurry, 21 

Smith, T. L., Co., Ltd., 176, 178, 179 
"Smoking " bricks, 352 
Soaps in concrete, 204 
Soda, 73 
Sodium carbonate, 92 

compounds in clay, 308 
sulphate, 92 
Soil, 316 
Sol, 83 
Solid solution, 70, 86 

theory, 48 
Soluble salts, 370 
silica, 43 
Soundness of cements, 116, 117 

tests, 119 

Spade for concrete, 190 
Spading, 195 
Spalling (concrete), 195 

(bricks), 369 
Spans, 216 
Specific gravity, 100 

of bricks, 381 

Specification for firebricks, 391 
Specifications, commercial, 213 
Spiral bars, 236 

reinforcement, 242 
Spissograph, 114 
Splintering, 209 
Spofforth, 226 
Spots on bricks, 370 
Squints, 378 
Staffordshire bricks, 372 

kiln, 355 
Stairways, 251 
Stanchions, 221, 224 
Standard mixture concrete, 167 
sand, 139 

specifications for cements, 97 

Stanger, W. H., 101 

Stationary kilns, 72 

Steam, effect of, 92 

navvies, 323 

Steel for reinforcement, 211 
in concrete, 206 
testing, 277 
Steelcrete, 206 



Steinbriick, 131 

-Schmelzer machine, 133 
Steps, 251 

Stiff-plastic process, 333, 339 
Stirrups, 218 

Hennebique, 227, 228 
Stocks, 377 
Stones, 148 
Stony clay, 294 
Storage, 25 

Straight line formula, 217 
Strength of bricks, 384 
of cement, 90 
of concrete, 254, 271 
of mixtures, 104 
to age, ratio of, 227 
Stress, distribution of, 239 
lines of, 215 
shearing, 255 
Stresses, internal, 216 

working, 213, 216 
Striking centering, 193 
String course bricks, 378 
Strong clays, 297, 317 
Submarine work, 265 
Suffolk bricks, 370 
Sulphates, 67, 75, 117, 211, 265 
effect of, 93 
in clay, 307 
Sulphide of iron, 309 
Sulphides, 75 
Sulphur, 258 

compounds in concrete, 151 
in concrete, 150 
tri-oxide, 76, 99 
Surface clays, 6, 317 
fillings, 196 

treatment of concrete, 194 
Swelling, 221 

Sylvester process of damp-proofing, 198 
Systems of reinforcement, 217 



TALBOT, 226 

Tamping, 189 
tool, 189 

Tanks, concrete, 210, 242 

Tar, 199 

Technical knowledge needed by brick- 
makers, 320 

Teil lime, 34 

Telegraph poles, 252 

Temperature, effect of, 115 

required in burning 
bricks, 374 

Tempering mill, 332 

Tender clays, 317 

Tensile strength, 128, 161 

of concrete, 254 
of mixtures, 139 



412 



INDEX 



Tensile strength of slag cements, 142 
Tension bars, 225 

diagonal, 226 

Terra-cotta, architectural, 303 
facing with, 258 
for aggregate, 149 
Testing aggregates, 154, 277 
cements, 96 
clay for bricks, 321 
concrete, 277 
Tests, loading, 279 

of concrete columns, 222 
Tetmajor, 128 
Texture of bricks, 382 

of clay, 292 
Thermal method, 113 
Tiles, cement, 251 
Till clay, 317 

Time of setting, 81, 88, 112 
Tornebohm, 50 
Tosca, 15 
Tough clays, 297 
Toughness of bricks, 381 
Tournai cement, 31 
Tramway standards, 252 
Transverse bending strength, 143 
bonds, 221 
strength of concrete, 254, 

Trass, 1, 14, 15, 42, 88, 205, 269, 271 

tensile strength of, 142 
Tremie, use of, 192 
Trials, 359 

Triangle mesh reinforcement, 240 
Triassic clays, 315 
marls, 11 
Tri-calcium aluminate, 69, 90 

silicate, 52, 67, 69, 90 
Tridymite, 68 
Truss girder, 225 
Trussed bar, 231 

Trussed Concrete Steel Co., Ltd., 212 
Tube-mills, 105 
Tuffs, 2, 14 
Tunnel dryer, 338 
Turneaure, 228 



UNDER-BURNING, 119 
Unsoundness, 116 
Unwin, 128 
Updraught kilns, 345 



VIBRATION of concrete, 256 



Vibrators, 189 
Vibrocel Co., Ltd., 189, 192 
Vicat needle, 110, 111 
Vitrifiable clays, 317 
Voids, 152, 157, 163 

in sand, 161 
Volcanic lavas, 2, 14 
tuffs, 2 



WALLS, concrete, 210 
Water absorption, 145 

action of, on cement, 85 

combined, in brick, 367 

for concrete, 147 

fresh, action of, on concrete, 263 

in bricks, 363 

-mains, 242 

-proofing concrete, 197 

proportion of, in concrete, 172 

repellents, 204 

-smoking bricks, 352 

tanks, 242 
Weak clays, 297 
Weathering, 327 
Web-connection, 228 
Web members, 233 

reinforcement, 225 
Weight per bushel, 99 

per cubic foot, 215 
Wet concrete mixture, 170 

process, 21, 27, 118 
White bricks, 290 

burning clays, 290 
Whittaker plunger press, 342 
Wilson and Gay lord, 168 
Wine musts, action of, on concrete, 263 
" Winget " concrete blocks, 250 
Winget Concrete Machine Company, 

176, 180, 251 
Wire-cut bricks, 335 

process, 337 
Wire netting, 239 
" Wolves," 328 
Woolson, 258 
Wright, 68 



YELLOW bricks, 290 
clays, 318 



ZSCHOKKE, 299 
Zulkowski, 88, 89 



BRADBURY, AGNKW, & CO. LI>., PRINTERS, LONDON AND TONBR1DGE. 



VAN NOSTRAND'S 
"WESTMINSTER" SERIES 

Bound in Uniform Style. 
Fully Illustrated. Price S2.OO net each. 

Gas Engines* By W. J. MARSHALL, Assoc. M.I.Mech.E., 
and CAPT. H. RIALL SANKEY, R.E. (Ret.). M.Inst.C.E., 
M.I.Mech.E. 300 Pages, 127 Illustrations. 
LIST OF CONTENTS : Theory of the Gas Engine. The Otto Cycle. The 
Two Stroke Cycle. Water Cooling of Gas Engine Parts. Ignition. 
Operating Gas Engines. The Arrangement of a Gas Engine Instal- 
lation. The Testing of Gas Engines. Governing. Gas and Gas 
Producers. Index. 

Textiles* By A. F. BARKER, M.Sc., with Chapters on the 
Mercerized and Artificial Fibres, and the Dyeing of 
Textile Materials by W. M. GARDNER, M.Sc., F.C.S. ; 
Silk Throwing and Spinning, by R. SNOW ; the Cotton 
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BRADBURY. 370 Pages. 86 Illustrations. 

CONTENTS : The History of the Textile Industries ; also of Textile 
Inventions and Inventors. The Wool, Silk, Cotton, Flax, etc., 
Growing Industries. The Mercerized and Artificial Fibres em- 
ployed in the Textile Industries. The Dyeing of Textile Materials. 
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Soils and Manures* By J. ALAN MURRAY, B.Sc. 367 

Pages. 33 Illustrations. 

CONTENTS : Introductory. The Origin of Soils. Physical Proper- 
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Coal. By JAMES TONGE, M.I.M.E., F.G.S., etc. (Lecturer 
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LIST OF CONTENTS : History. Occurrence. Mode of Formation 
of Coal Seams. Fossils of the Coal Measures. Botany of the 
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Foreign Coals and their Values. Uses of Coal. The Production 
of Heat from Coal. Waste of Coal. The Preparation of Coal 
for the Market. Coaling Stations of the World. Index. 

Iron and Steel. By J. H. STANSBIE, B.Sc. (Lond.), F.I.C. 

385 Pages. With 86 Illustrations. 

LIST OF CONTENTS : Introductory. Iron Ores. Combustible and 
other materials used in Iron and Steel Manufacture. Primitive 
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facture. The Refining of Pig Iron in Small Charges. Crucible 
and Weld Steel. The Bessemer Process. The Open Hearth 
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Timber* By J. R. BATERDEN, Assoc.M.Inst.C.E. 334 

Pages. 54 Illustrations. 

CONTENTS : Timber. The World's Forest Supply. Quantities of 
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bers of India, Burma, and Andaman Islands. Timber of the 
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Natural Sources of Power. By ROBERT S. BALL, B.Sc., 
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CONTENTS : Preface. Units with Metric Equivalents and Abbre- 
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Various Types of Turbine. Construction of Water Power Plants. 
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of Wind Power to Industry. The Modern Windmill. Con- 
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A, B,C Index. 

Electric Lamps, By MAURICE SOLOMON, A.C.G.I., 

A.M.I.E.E. 339 Pages. 112 Illustrations. 
CONTENTS : The Principles of Artificial Illumination. The Produc- 
tion of Artificial Illumination. Photometry. Methods of Testing. 
Carbon Filament Lamps. The Nernst Lamp. Metallic Filament 
Lamps. The Electric Arc. The Manufacture and Testing of Arc 
Lamp Carbons. Arc Lamps. Miscellaneous Lamps. Compari- 
son of Lamps of Different Types. 

Liquid and Gaseous Fuels, and the Part they play 
in Modern Power Production. By Professor 
VIVIAN B. LEWES, F.I.C., F.C.S., Prof, of Chemistry, 
Royal Naval College, Greenwich. 350 Pages. With 54 
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LIST OF CONTENTS : Lavoisier's Discovery of the Nature of Com- 
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Electric Power and Traction. By F. H. DAVIES, 

A.M.T.E.E. 299 Pages. With 66 Illustrations. 
LIST OF CONTENTS : Introduction. The Generation and Distri- 
bution of Power. The Electric Motor. The Application of 
Electric Power. Electric Power in Collieries. Electric Power 
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Electric Power in the Printing Trade. Electric Power at Sea. 
Electric Power on Canals. Electric Traction. The Overhead 
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ways. Glossary. Index. 

Decorative Glass Processes. By ARTHUR Louis 

DUTHIE. 279 Pages. 38 Illustrations. 

CONTENTS : Introduction. Various Kinds of Glass in Use : Their 
Characteristics, Comparative Price, etc. Leaded Lights. Stained 
Glass. Embossed Glass. Brilliant Cutting and Bevelling. Sand- 
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Town Gas and its Uses for the Production of 
Light, Heat, and Motive Power. By W. H. Y. 
WEBBER, C.E. 282 Pages. With 71 Illustrations. 
LIST OF CONTENTS : The Nature and Properties of Town Gas. The 
History and Manufacture of Town Gas. The Bye-Products of 
Coal Gas Manufacture. Gas Lights and Lighting. Practical 
Gas Lighting. The Cost of Gas Lighting. Heating and Warm- 
ing by Gas. Cooking by Gas. The Healthfulness and Safety 
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Electro-Metallurgy. By J. B. C. KERSHAW, F.I.C. 

318 Pages. With 61 Illustrations. 

CONTENTS : Introduction and Historical Survey. Aluminium. 
Production. Details of Processes and Works. Costs. Utiliza- 
tion. Future of the Metal. Bullion and Gold. Silver Refining 
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cesses. Dry Processes. Electro-Thermal Processes. Electro 
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Radio-Telegraphy. By C. C. F. MONCKTON, M.I.E.E. 

389 Pages. With 173 Diagrams and Illustrations. 
CONTENTS : Preface. Electric Phenomena. Electric Vibrations. 
Electro-Magnetic Waves. Modified Hertz Waves used in Radio- 
Telegraphy. Apparatus used for Charging the Oscillator. The 
Electric Oscillator : Methods of Arrangement, Practical Details. 
The Receiver : Methods of Arrangement, The Detecting Ap- 
paratus, and other details. Measurements in Radio-Telegraphy. 
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System. Radio - Telegraph Station at Nauen : Telefunken 
System. Station at Lyngby : Poulsen System. The Lodge- 

( 4 ) 



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Muirhead System, the Marconi System, Telefunken System, and 
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India-Rubber and its Manufacture, with Chapters 
on Gutta-Percha and Balata. By H. L. TERRY, 
F.I.C., Assoc.Inst.M.M. 303 Pages. With Illustrations. 

LIST OF CONTENTS : Preface. Introduction : Historical and 
General. Raw Rubber. Botanical Origin. Tapping the Trees. 
Coagulation. Principal Raw Rubbers of Commerce. Pseudo- 
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and Physical Properties. Vulcanization. India-rubber Planta- 
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Solvents and their Recovery. Rubber Solution. Fine Cut Sheet 
and Articles made therefrom. Elastic Thread. Mechanical 
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The Testing of Rubber Goods. Gutta-Percha. Balata. Biblio- 
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The Railway Locomotive, What It Is, and Why It is 
What It Is. By VAUGHAN PENDRED, M.Inst.M.E., 
Mem.Inst.M.I. 321 Pages. 94 Illustrations. 

CONTENTS : The Locomotive Engine as a Vehicle Frames. Bogies. 
The Action of the Bogie. Centre of Gravity. Wheels. Wheel 
and Rail. Adhesion. Propulsion. Counter-Balancing. The Loco- 
motive as a Steam Generator The Boiler. The Construction of the 
Boiler. Stay Bolts. The Fire-Box. The Design of Boilers. 
Combustion. Fuel. The Front End. The Blast Pipe. Steam 
Water. Priming. The Quality of Steam. Superheating. Boiler 
Fittings. The Injector. The Locomotive as a Steam Engine 
Cylinders and Valves. Friction. Valve Gear. Expansion. The 
Stephenson Link Motion. Walschaert's and Joy's Gears. Slide 
Valves. Compounding. Piston Valves. The Indicator. Ten- 
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The Work of the Locomotive. 

Glass Manufacture. By WALTER ROSENHAIN, Superin- 
tendent of the Department of Metallurgy in the National 
Physical Laboratory, late Scientific Adviser in the Glass 
Works of Messrs. Chance Bros. & Co. 280 Pages. With 
Illustrations. 

CONTENTS: Preface. Definitions. Physical and Chemical Qualities, 
Mechanical, Thermal, and Electrical Properties. Transparency 

( 5 ) 



THE "WESTMINSTER" SERIES 

and Colour. Raw materials of manufacture. Crucibles and 
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Precious Stones. By W. GOODCHILD, M.B., B.Ch. 319 
Pages. With 42 Illustrations. With a Chapter on 
Artificial Stones. By ROBERT DYKES. 

LIST OF CONTENTS : Introductory and Historical. Genesis rf 
Precious Stones. Physical Properties. The Cutting and Polish- 
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Precious Stones. The Diamond. Fluor Spar and the Forms of 
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INTRODUCTION TO THE 

Chemistry and Physics of Building Materials. 
By ALAN E. MUNBY, M.A. 365 Pages. Illustrated. 

CONTENTS : Elementary Science : Natural Laws and Scientific In- 
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and Combustion. Nature and Measurement of Heat and Its 
Effects on Materials. Chemical Signs and Calculations. Water 
and Its Impurities. Sulphur and the Nature of Acids and Bases. 
Coal and Its Products. Outlines of Geology. Building Materials : 
The Constituents of Stones, Clays and Cementing Materials. Clas- 
sification, Examination and Testing of Stones, Brick and Other 
Clays. Kiln Reactions and the Properties of Burnt Clays. Plasters 
and Limes. Cements. Theories upon the Setting of Plasters and 
Hydraulic Materials. Artificial Stone. Oxychloride Cement. 
Asphaite. General Properties of Metals. Iron and Steel. Other 
Metals and Alloys. Timber. Paints : Oils, Thinners and Varnishes ; 
Bases, Pigments and Driers. 

Patents, Designs and Trade Marks : The Law 
and Commercial Usage. By KENNETH R. SWAN, 
B.A. (Oxon.), of the Inner Temple, Barrister-at-Law. 
402 Pages. 

CONTENTS : Table of Cases Cited Part I. Letters Patent. Intro- 
duction. General. Historical. I., II., III. Invention, Novelty, 



THE ' WESTMINSTER " SERIES 

Subject Matter, and Utility the Essentials of Patentable Invention. 
IV. Specification. V. Construction of Specification. VI. Who 
May Apply for a Patent. VII. Application and Grant. VIII. 
Opposition. IX. Patent Rights. Legal Value. Commercial 
Value. X. Amendment. XI. Infringement of Patent. XII. 
Action for Infringement. XIII. Action to Restrain Threats. 
XIV. Negotiation of Patents by Sale and Licence. XV. Limita- 
tions on Patent Right. XVI. Revocation. XVII. Prolonga- 
tion. XVIII. Miscellaneous. XIX. Foreign Patents. XX. 
Foreign Patent Laws : United States of America. Germany. 
France. Table of Cost, etc., of Foreign Patents. APPENDIX A. 
i. Table of Forms and Fees. 2. Cost of Obtaining a British 
Patent. 3. Convention Countries. Part II. Copyright in 
Design. Introduction. I. Registrable Designs. II. Registra- 
tion. III. Marking. IV. Infringement. APPENDIX B. i. 
Table of Forms and Fees. 2. Classification of Goods. Part 
III. Trade Marks. Introduction. I. Meaning of Trade Mark. 
II. Qualification for Registration. III. Restrictions on Regis- 
tration. IV. Registration. V. Effect of Registration. VI. 
Miscellaneous. APPENDIX C. Table of Forms and Fees. INDICES. 
i. Patents. 2. Designs. 3. Trade Marks. 

The Book: Its History and Development. By 
CYRIL DAVENPORT, V.D., F.S.A. 266 Pages. With 
7 Plates and 126 Figures in the text. 

LIST OF CONTENTS : Early Records. Rolls, Books and Book 
bindings. Paper. Printing. Illustrations. Miscellanea. 
Leathers. The Ornamentation of Leather Bookbindings without 
Gold. The Ornamentation of Leather Bookbindings with Gold. 
Bibliography. Index. 



The Manufacture of Paper. By R. W. SINDALL, F.C.S., 
Consulting Chemist to the Wood Pulp and Paper Trades ; 
Lecturer on Paper-making for the Hertfordshire County 
Council, the Bucks County Council, the Printing and 
Stationery Trades at Exeter Hall (1903-4), the Institute 
of Printers ; Technical Adviser to the Government of 
India, 1905. 275 Pages. 58 Illustrations. 

CONTENTS : Preface. List of Illustrations. Historical Notice. Cel- 
lulose and Paper-making Fibres. The Manufacture of Paper from 
Rags, Esparto and Straw. Wood Pulp and Wood Pulp Papers. 
Brown Papers and Boards. Special kinds of Paper. Chemicals 
used in Paper-making. The Process of " Beating." The Dye- 
ing and Colouring of Paper Pulp. Paper Mill Machinery. The 
Deterioration of Paper. Bibliography. Index. 

( 7 ) 



THE "WESTMINSTER" SERIES 



Wood Pulp and its Applications* By C. F. CROSS, 
B.Sc., F.I.C., E. J. BEVAN, F.I.C., and R. W. SINDALL, 
F.C.S. 266 pages. 36 Illustrations. 

CONTENTS: The Structural Elements of Wood. Cellulose as a 
Chemical. Sources of Supply. Mechanical Wood Pulp. Chemical 
Wood Pulp. The Bleaching of Wood Pulp. News and Printings. 
Wood Pulp Boards. Utilisation of Wood Waste. Testing of 
Wood Pulp for Moisture. Wood Pulp and the Textile Industries. 
Bibliography. Index. 



Photography: its Principles and Applications. 
By ALFRED WATKINS, F.R.P.S. 342 pages. 98 Illus- 
trations. 

CONTENTS : First Principles. Lenses. Exposure Influences. Prac- 
tical Exposure. Development Influences. Practical Develop- 
ment. Cameras and Dark Room. Orthochromatic Photography. 
Printing Processes. Hand Camera Work. Enlarging and Slide 
Making. Colour Photography. General Applications. Record 
Applications, Science Applications. Plate Speed Testing. Pro- 
cess Work. Addenda. Index. 



IN PREPARATION. 

Commercial Paints and Painting. By A. S. JENN- 
INGS, Hon. Consulting Examiner, City and Guilds of 
London Institute. 

Brewing and Distilling* By JAMES GRANT, F.S.C 



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