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 xH20 . yAl^O^ . zSiO2.
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 HiSi2Al2O9. 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
21—28
Alumina
6'42
4-21
8-81
3—5
Iron oxide
4-13
7'85
6-87
1-21
Calcium carbonate .
63-76
58-79
56-36
60—63
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 (S03) . . . 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 (S03) , 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 (SOS) . . 0-8 „ 2-7
Alkalies . 0 „ 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)2OH 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.
l2Si02 = | Si
\/
Al Al
Thus, a clay of the type 6Al20Bl2Si02l2H20 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 . 3AW3 . 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 (SiAl205). 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.205 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 (Si02xH20) 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
(C02) ; 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 :—
CaC03 I > CaO + CO2,
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,ySi02)
(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 6Al203l2Si02. 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, 2CaOSi02.
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
(2CaOAl203) in solution in dicalcium silicate (2CaOSi02).
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 (3CaOAl20^) in
tricalcium silicate (3CaOSi02). 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 : Si02 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 : 8i02 =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 (3Ca08i02) 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 . Si02) 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 : 8i02 =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.20.3 .
xSi02, but the properties of such a compound would be entirely
different from those of SxCaO . xSi02 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.2O.^ 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 . Si02
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) HlsAlQSil2042, or (b) H1&Al^8iu0^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
0
o
(OH),=
0
0 C
)
0
0
Si—O—Al
Al—O—Si
\
(OH)
\\
0 0 (
,
(
1
06
>
!'=
1 /
\l
i
Si—O—Al
Al—O—Si Si =
(OH).
/
\
/
/
\
/
o
0 (
9
0
0
\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 0
O
o,
o-
o
^
o
£1
I
\
o o
^
O
$'
These may be represented more simply, as
I
/\
8i\Al
\/
I
Si
=( Si
Al
(a)
(6)
THE ALUMINO-SILICATE THEORY
57
On heating with lime, combination occurs which, in the case
of formula (a), may be represented by the following equation :—
HisAlvSiwOu + 38<7aO = H£a&Al$iuf)n + 8#20
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
4o_
4o_
,=4°
Si + 0-SNaCl
_4o
5° OK 5°
= 3oCaO . MgO . K^O .
where each hexagonal ring represents 6Si0.2 or 3A12O3, 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 3A1203 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. 71—78)
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 CHEMI€AL 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.20.j
and Newberry 's assumption that they are composed of
3CaOSi0.2 and 2CaOAL20$. On these assumptions a cement
will consist of a molecules of 3CaOSi0.2, and b molecules of
either 3CaOAW.3 or 2CaOAl.20^. 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 Si02 + molecules
(2) according to Newberry :
3a -j- 26 molecules CaO
a molecules SiO^ + b molecules
= 3
In spite of the fact that ^CaOAl203 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 $*Oa + molecules Al20t
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 02
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 Si02
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 3CaOSi02 and 2CaOAl2Os 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 ySi02 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, aSi02, 1-00 A1203
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 A120.^ 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 A1203 : Si02 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.203 : Si02
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 CaOSi02 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 CaOSi02 is invariably formed. When the
original mixture contained the materials in the ratio SCaO -j-
8i02 he observed the formation of a more basic silicate
(2CaO . Si02) at first, and that this persists in the presence of
sufficient lime ; otherwise CaO . Si02 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, 2CaOSi02 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 (CaOSi02): The fact discovered by
Boudouard, that CaOSi02 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 2CaOSi02 is present in cements and
slags not containing sufficient silica, or which have not been
heated sufficiently long to form the meta-silicate CaOSi02.
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
Ca2SiOi, a and ft CaSi03, 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, CaOAl203 or CaO 2A120S, 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 CaOAl2O3 (which is soluble in acid),
2CaOAl.2Os (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.2Os and soluble in hydrochloric acid. The
^CaOAl.203 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.203, 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.,Os 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, ALOB 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.20.3 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.20.3 -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.20.3 + 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/.,03, 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- IQA1203, 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.2Os) 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,Fe203 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.20.3) 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.20s) (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 CaS04 — 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 1100°C. 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.>,
CaOSi02, 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 (Ca0.2H.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 (Ca0.2H.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 (Ca0.2H.>). 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 (CaO2H.2), and colloidal calcium silicate.
The existence in hardened cements of free calcium hydroxide
(Ca0.2H.,) 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
0 . Ca . OH.
0 . 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 (Ca0.2H.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 . Si02 + (x + 1) H^O = CaO . Si02 . xH^O + Ca (OH)2
(c) With alumino-silicates the corresponding equation is
expressed by one of the following types :—
(1) C
(2) Ca,5AkSilbOM + 36#20 = Cal9
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* = CaC03 + H20.
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.2O^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 ($04,
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 $04-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
S02.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 neck1 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
0 0
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
2i
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. (1T3^ inch)
internal diameter, and 30 mm. (1T36 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 Vzm/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-
tenth1 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 14J0 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" 16—Fai^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
14J0 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 «-0
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 essentjanv of 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 » T51-! » 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 days1). 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 (6—12%) 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 green1, 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
>^F17T?^*&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;-.VA *••'••• '*•.'•>': '*• »-..•.- 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 inar|pnilptp onrl
(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 wl2 -*- 24, and that over the supports wl2 -=- 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 wl2 -=-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 = Ec = 2,000,000
„ steel = E8 = 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
Adhesion1 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 construction2 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
\
\
\
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i
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/
FIG. 50.— No Lateral
Ties.
FIG. 51.— One
Lateral Tie.
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i
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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"
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:
Stirrups
::H*: ••••*
':
]^-»^,-^!iim
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> »>
2i
21
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 TVinch 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.
N°l 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. 197—205.
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.
0
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
- S02OH, —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-
0 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 (M0) is equal to half the breaking load, i.e.. M0 = — .
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 (<r7>)
reached in the concrete is, for Type I. of reinforcement, cry> =
0-384 P, and for Type II., crB= 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
M0 = -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,
M0, 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 M0 = — 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) ^2Si0.22H.20) 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
(FeS2) 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 (FeS2) 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.2O^H.20, 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, 1—3
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, 176—183
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
Industry, by W. H. COOK ; the Linen Industry, by F.
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.
The Principles of Spinning. Processes preparatory to Spinning.
The Principles of Weaving. The Principles of Designing and
Colouring. The Principles of Finishing. Textile Calculations.
The Woollen Industry. The Worsted Industry. The Dress
Goods, Stuff, and Linings Industry. The Tapestry and Carpet
Industry. Silk Throwing and Spinning. The Cotton Industry.
The Linen Industry historically and commercially considered.
Recent Developments and the Future of the Textile Industries.
Index.
Soils and Manures* By J. ALAN MURRAY, B.Sc. 367
Pages. 33 Illustrations.
CONTENTS : Introductory. The Origin of Soils. Physical Proper-
ties of Soils. Chemistry of Soils. Biology of Soils. Fertility.
Principles of Manuring. Phosphatic Manures. Phosphonitro-
genous Manures. Nitrogenous Manures. Potash Manures.
Compound and Miscellaneous Manures. General Manures. Farm-
yard Manure. Valuation of Manures. Composition and Manural
Value of Various Farm Foods.
THE " WESTMINSTER " SERIES
Coal. By JAMES TONGE, M.I.M.E., F.G.S., etc. (Lecturer
on Mining at Victoria University, Manchester). 283
Pages. With 46 Illustrations, many of them showing the
Fossils found in the Coal Measures.
LIST OF CONTENTS : History. Occurrence. Mode of Formation
of Coal Seams. Fossils of the Coal Measures. Botany of the
Coal-Measure Plants. Coalfields' of the British Isles. Foreign
Coalfields. The Classification of Coals. The Valuation of Coal.
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
Methods of Iron and Steel Production. Pig Iron and its Manu-
facture. The Refining of Pig Iron in Small Charges. Crucible
and Weld Steel. The Bessemer Process. The Open Hearth
Process. Mechanical Treatment of Iron and Steel. Physical
and Mechanical Properties of Iron and Steel. Iron and Steel
under the Microscope. Heat Treatment of Iron and Steel. Elec-
tric Smelting. Special Steels. Index.
Timber* By J. R. BATERDEN, Assoc.M.Inst.C.E. 334
Pages. 54 Illustrations.
CONTENTS : Timber. The World's Forest Supply. Quantities of
Timber used. Timber imports into Great Britain. European
Timber. Timber of the United States and Canada. Timbers
of South America, Central America, and West India Islands. Tim-
bers of India, Burma, and Andaman Islands. Timber of the
Straits Settlements, Malay Peninsula, Japan and South and
West Africa. Australian Timbers. Timbers of New Zealand
and Tasmania. Causes of Decay and Destruction of Timber.
Seasoning and Impregnation of Timber. Defects in Timber and
General Notes. Strength and Testing of Timber. " Figure " in
Timber. Appendix. Bibliography.
Natural Sources of Power. By ROBERT S. BALL, B.Sc.,
A.M.Inst.C.E. 362 Pages. With 104 Diagrams and
Illustrations.
CONTENTS : Preface. Units with Metric Equivalents and Abbre-
viations. Length and Distance. Surface and Area. Volumes.
Weights or Measures. Pressures. Linear Velocities, Angular
Velocities. Acceleration. Energy. Power. Introductory
Water Power and Methods of Measuring. Application of Water
Power to the Propulsion of Machinery. The Hydraulic Turbine.
( 2 )
THE "WESTMINSTER" SERIES
Various Types of Turbine. Construction of Water Power Plants.
Water Power Installations. The Regulation of Turbines. Wind
Pressure, Velocity, and Methods of Measuring. The Application
of Wind Power to Industry. The Modern Windmill. Con-
structional Details. Power of Modern Windmills. Appendices.
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
Illustrations.
LIST OF CONTENTS : Lavoisier's Discovery of the Nature of Com-
bustion, etc. The Cycle of Animal and Vegetable Life. Method
of determining Calorific Value. The Discovery of Petroleum
in America. Oil Lamps, etc. The History of Coal Gas. Calorific
Value of Coal Gas and its Constituents. The History of Water
Gas. Incomplete Combustion. Comparison of the Thermal
Values of our Fuels, etc. Appendix. Bibliography. Index.
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
in Engineering Workshops. Electric Power in Textile Factories.
Electric Power in the Printing Trade. Electric Power at Sea.
Electric Power on Canals. Electric Traction. The Overhead
System and Track Work. The Conduit System. The Surface
Contact System. Car Building and Equipment. Electric Rail-
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-
Blast and Crystalline Glass. Gilding. Silvering and Mosaic.
Proprietary Processes. Patents. Glossary.
( 3 )
THE "WESTMINSTER" SERIES
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
of Gas in all its uses. Town Gas for Power Generation, including
Private Electricity Supply. The Legal Relations of Gas Sup-
pliers, Consumers, and the Public. Index.
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
Process. Gold Refining Processes. Gold Extraction Processes.
Calcium Carbide and Acetylene Gas. The Carbide Furnace and
Process. Production. Utilization. Carborundum. Details of
Manufacture. Properties and Uses. Copper. Copper Refin-
ing. Descriptions of Refineries. Costs. Properties and Utiliza-
tion. The Elmore and similar Processes. Electrolytic Extrac-
tion Processes. Electro-Metallurgical Concentration Processes.
Ferro-alloys. Descriptions of Works. Utilization. Glass and
Quartz Glass. Graphite. Details of Process. Utilization. Iron
and Steel. Descriptions of Furnaces and Processes. Yields and
Costs. Comparative Costs. Lead. The Salom Process. The Betts
Refining Process. The Betts Reduction Process. White Lead Pro-
cesses. Miscellaneous Products. Calcium. Carbon Rhulphide.
Carbon Tetra-Chloride. Diamantine. Magnesium. Phosphorus.
Silicon and its Compounds. Nickel. Wet Processes. Dry
Processes. Sodium. Descriptions of Cells and Procco.,e;. Tin'.
Alkaline Processes for Tin Stripping. Acid Processes for Tin
Stripping. Salt Processes for Tin Stripping. Zinc. Wet Pro
cesses. Dry Processes. Electro-Thermal Processes. Electro
Galvanizing. Glossary. Name Index.
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.
The Experimental Station at Elmers End : Lodge-Muirhead
System. Radio - Telegraph Station at Nauen : Telefunken
System. Station at Lyngby : Poulsen System. The Lodge-
( 4 )
THE "WESTMINSTER" SERIES
Muirhead System, the Marconi System, Telefunken System, and
Poulsen System. Portable Stations. Radio-Telephony. Ap-
pendices : The Morse Alphabet. Electrical Units used in this
Book. International Control of Radio-Telegraphy. Index.
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-
Rubbers. Congo Rubber. General Considerations. Chemical
and Physical Properties. Vulcanization. India-rubber Planta-
tions, india-rubber Substitutes. Reclaimed Rubber. Washing
and Drying of Raw Rubber. Compounding of Rubber. Rubber
Solvents and their Recovery. Rubber Solution. Fine Cut Sheet
and Articles made therefrom. Elastic Thread. Mechanical
Rubber Goods. Sundry Rubber Articles. India-rubber Proofed
Textures. Tyres. India-rubber Boots and Shoes. Rubber for
Insulated Wires. Vulcanite Contracts for India-rubber Goods.
The Testing of Rubber Goods. Gutta-Percha. Balata. Biblio-
graphy. Index.
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-
ders. Tank Engines. Lubrication. Brakes. The Running Shed.
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
Furnaces for Fusion. Process of Fusion. Processes used in
Working of Glass. Bottle. Blown and Pressed. Rolled or
Plate. Sheet and Crown. Coloured. Optical Glass : Nature
and Properties, Manufacture. Miscellaneous Products. Ap-
pendix. Bibliography of Glass Manufacture. Index
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-
ing of Gems. Imitation Gems and the Artificial Production of
Precious Stones. The Diamond. Fluor Spar and the Forms of
Silica. Corundum, including Ruby and Sapphire. Spinel and
Chrysoberyl. The Carbonates and the Felspars. The Pyroxene
and Amphibole Groups. Beryl, Cordierite, Lapis Lazuli and the
Garnets. Olivine, Topaz, Tourmaline and other Silicates. Phos-
phates, Sulphates, and Carbon Compounds.
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-
vestigations. Measurement and the Properties of Matter. Air
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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