Colorimetric test for organic impurities in sands

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B 32

:

COLORIMETRIC TEST

FOR

ORGANIC IMPURITIES IN SANDS

BY
Duff A. Abrams

Professor in Charge of Laboratory;^- ^,
AND

Oscar E. Harder

Chemist

Circular No. 1
Structural Materials Research Laboratory

Cooperation of
Portland Cement Association

and

Lewis Institute

Experimental work carried out in cooperation with
Committee C-9 on Concrete and Concrete Aggre-
gates, of the American Society for Testing Materials.
Published by permission.

LEWIS INSTITUTE

Chicago
February, 1917

43

Colorimetric Test for Organic Impurities
in Sands

INTRODUCTION..

During the past few years the subject of impurities in sands
for mortar and concrete has been receiving rapidly increasing
attention from testing engineers and inspectors. Experience
has shown that on account of the presence of impurities, certain
natural sands do not give satisfactory results when used in con-
crete, and many failures and much inferior work have been
attributed to impurities in the fine aggregate. However, when
it comes to determining before the sand is used in construction
or subjected to tests in mortar, whether or not the sand is suit-
able for fine aggregate, there has been no well-established cri-
terion regarding the impurities. Practically all specifications
have required that the sand be clean; the so-called dirty sands
have generally been in bad repute. Organic matter has been the
principal source of trouble, yet many publications stating that
as much as 10 per cent of loan in sand was not harmful and
might even increase the strength of the mortar, are to be found
in the engineering journals.

An investigation initiated by Committee C-9 on Concrete and
Concrete Aggregates, of the American Society for Testing Ma-
terials, showed the desirability of a simple and reliable test which
would make it possible to detect sands which contained harmful
impurities before they were used in concrete. Commercially, it
was also desirable to develop some remedial measure which would
make it possible to use sands which in their present condition,
on account of impurities, are not satisfactory for concrete.

Engineers and chemists have recognized the danger of using
sands which contained appreciable amounts of organic matter
which may come from surface loan or other sources of contami-
nation, and have used various methods for detecting such impur-
ities. A method which has probably been most used depends
upon the loss on ignition. Obviously the loss-on-ignition test
drives off, besides the organic matter, the water of crystalliza-
tion, hydration, etc., and the carbon dioxide (C02). In certain
"limestone sands" the loss on ignition due to carbon dioxide
alone may amount to more than 10 per cent, and yet the sand may
be entirely free from organic matter. The determination of the
organic matter by combustion has little advantage over the loss-
on-ignition method, unless a correction be made for the car-
bonates present. Methods which depend upon the moist oxida-
tion of the organic matter have some advantage, but are not
entirely satisfactory. None of these methods determines directly
the amount of organic matter in the sands. Most of them deter-
mine the amount of carbon by first converting it into carbon
dioxide. The organic matter is then calculated by multiplying

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the per cent of carbon by some factor, the accuracy of which is
often questionable.

The colorimetric test for organic impurities in sands represents
one phase of the work which has been done at the Structural
Materials Research Laboratory, Lewis Institute, Chicago, under
the direction of the Committee mentioned above. The fact that
the sand has passed the colorimetric test does not by itself prove
that the sand is suitable for mortar or concrete, since its fineness
and other characteristics must also be considered as they affect
the strength. The result of the researches, however, indicate that
it is a reliable test for organic impurities, which are the most
common source of danger in a sand of satisfactory granulome-
tric composition.

It is intended at this time to give only a description of the
methods of making the test so that it may be tried out further in
commercial and private laboratories. The experimental data
obtained in the course of these studies will furnish the subject-
matter of a subsequent report.

It will be of assistance to the Committee having this investi-
gation in charge, if other experimenters will report the details
of their tests and observations to the Chairman, Mr. Sanford
E. Thompson, 136 Federal Street, Boston, Mass., or to the
authors.

The colorimetric test may be described briefly as follows:

A sample of sand is digested at ordinary temperature in a
solution of sodium hydroxide (NaOH). If the sand contains
certain organic materials, thought to be largely of a humus
nature, the filtered solution resulting from this treatment will
found to be of a color ranging from light yellow up through
the reds to that which appears almost black. The depth of
color has been found to furnish a measure of the effect of the
impurities on the strength of mortars made from such sands.
The depth of color may be measured by comparison with proper
color standards.

Two methods of procedure have been developed: (1) For
Field Tests; (2) For Laboratory Tests.

METHOD FOR FIELD TESTS.

Fill a 12-oz. graduated prescription bottle to the 4J-oz. mark
with the sand to be tested. Add a 3% solution of sodium hydrox-
ide until the volume of the sand and solution, after shaking,
amounts to 7 oz. Shake thoroughly and let stand over night.
Observe the color of the clear supernatant liquid.

In approximate field tests it is not necessary to make com-
parison with color standards. If the clear supernatant liquid is
colorless, or has a light yellow color, the sand may be considered
satisfactory in so far as organic impurities are concerned. On
the other hand, if a dark-colored solution, ranging from dark

2

reds to black is obtained the sand should be rejected or used
only after it has been subjected to the usual mortar strength
tests.

Field tests made in this way are not expected to give quanti-
tative results, but will be found useful in:

(1) Prospecting for sand supplies;

(2) Checking the quality of sand received on the job;

(3) Preliminary examination of sands in the laboratory.

An approximate volumetric determination of the silt in sand
can be made by measuring or estimating the thickness of the
layer of fine material which settles on top of the sand. The
per cent of silt by volume has been found to vary from 1 to 2
times the per cent by weight.

METHOD FOR LABORATORY TESTS.

To a 200-gram sample of dry sand add 100 cc. of a 3% solution
of sodium hydroxide (NaOH) and digest at ordinary tempera-
ture, with occasional stirring, for 24 hours. Filter this solution
through a good grade of filter paper ; refilter if necessary. The
filtrate must be clear. Place 10 cc. of the clear filtrate in a 50 cc.
Nessler cylinder and dilute to 50 cc. with distilled water. Shake
thoroughly and let stand until all foam and bubbles disappear.
Determine the color value of this cylinder by comparing it with
cylinders containing standard solutions of alkaline sodium tan-
nate. Compare the colors by looking through the full depth of
the solution with the cylinders held toward a good natural light.

Standard Tannic Acid Solution for Color Comparison. The
preparation of the standard tannic acid solution for comparing
the color of the filtrate should be begun at the same time as
the treatment of 'the sand. Add 10 cc. of a 2% solution of tannic
acid in 10% alcohol to 90 cc. of a 3% solution of sodium hydrox-
ide. The sodium hydroxide combines with the tannic acid to form
sodium tannate. Let the solution stand 24 hours at room tem-
perature. Place 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 cc., respectively,
of this solution in 50 cc. Nessler cylinders and dilute to the mark
with distilled water. The amounts of tannic acid in the different
cylinders will then be as shown in the following table :

Alkaline Sodium
Tannate in each
cylinder— cc. ... 123456789 10

Tannic acid in
each cylinder —
milligrams 2 4 6 8 10 12 14 16 18 20

Color value in
parts of tannic
acid per million
of sand by weight 100 200 300 400 500 600 700 800 900 1000

It is desirable to have good sunlight for comparing the colors ;
if sunlight is not available, the amount of tannic acid in each
of the cylinders containing the standard solutions may be de-
creased by one-half and the other values in the table modified
accordingly.

In case the solution obtained by digesting the sand with the
sodium hydroxide is very dark, use less than 10 cc. for the com-
parison and make the necessary modifications in the calculation
of the color values. With very light-colored solutions use more
than 10 cc. of the filtrate for the comparisons. The depth of
color of the solution decreases on standing, and for that reason
the solution should be made up fresh for each day's work.

Method of Calculation. An example will make clear the meth-
od of calculating the color value of a sand. Suppose that 10 cc.
of clear filtrate obtained by digesting the sand with 100 cc. of
a 3% solution of sodium hydroxide when diluted to 50 cc. cor-
responds in color to the Nessler cylinder containing 12 milligrams
of tannic acid, or 6 cc. of the alkaline tannate solution. The
sand will then have a color value of 600. The 10 cc. of the fil-
trate placed in the Nessler cylinder is 1/10 of the 100 cc. of 3%
sodium hydroxide solution which was added to the sand, and the
sample of sand (200 grams) is 1/5 of a kilogram; therefore, the
milligrams of tame acid per kilogram of the sand, by weight,
are 12 x 10 x 5 = 600 ; or the tannic acid equivalent when ex-
pressed in parts per million of the sand, by weight, is 600.

BASIS OF COLORIMETRIC TEST.

The colorimetric test has been applied to sands from about
40 widely distributed deposits in 20 different states. The research
to date has brought out the following :

1. Examinations of deposits of defective sands show that
surface loan is the principal sxmrce of contamination.

2. All natural sands which have been found to be defective
on account of the presence of organic impurities have responded
to the colorimetric test with sodium hydroxide, and all sands
which have given high color values have shown low values in
mortar tests.

3. Sands which were similarly graded by screening out and
recombining the different sizes to a definite sieve analysis, showed
a fairly definite relation between the compressive strengths of
1-3 mortars at 7 and 28 days and the color values of the sands.

4. The motar-making quality of sands known to contain
organic impurities has been much improved by removing the
organic matter, either by repeatedly digesting them with sodium
hydroxide and then washing free from alkali, or by driving off
the organic impurities by ignition.

5. When the sodium hydroxide extracts from sands which
mortar tests had shown to be defective were purified and applied
as coatings on high-grade sand, that sand was made "defective"
or gave much reduced mortar strength.

6. It is impracticable to give exact values for the relation
between the color value of a sand and the strength of mortars
made from the same sand. However, the tests made thus far
show this relation to be about as follows :

Reduction in

Color Values Compressive Strength

of sand of 1-3 Mortar

Per cent

250 10-20

500 15-30

1000 20-40

2000 25-50

3000 30-60

The reduction in strength is based on compression tests at ages
of 7 days, 28 days, and 3 months, of 1-3 mortars made from
the same sand, before and after coating with different per cents
of organic impurities which had been extracted from defective
sands and purified. Tests on mortars made from defective
sands as received and after removal of the organic impurities,
either by repeated extractions with sodium hydroxide or by
ignition, showed that in some cases the reduction in strength for
a given color value was even greater than the higher values
given above. Frequently the test pieces completely disinte-
grated in the storage water. The higher reductions in strength
for a given color value for a natural sand as compared with the
same color value for an artifically coated sand is probably due
to the artificial coating being more easily removed by the sodium
hydroxide.

Sufficient data are not available to indicate whether or not the
effect of a given quantity of organic impurities varies with the
grading of the sand. The reduction in strength seems to de-
crease slightly with the age of the test pieces.

APPARATUS.

The apparatus required for making the colorimetric test of
sands will vary according to the nature of the investigation.
Two lists are given below: (1) Apparatus for Field Tests;
(2) Apparatus for Laboratory Tests. Sufficient apparatus
has been included in each list for tests on 5 samples at a time.

Approximate
For Field Tests. Cost

5 12-oz. graduated prescription bottles $ .25

Stock of 3% solution of sodium hydroxide (dis-
solve 1 oz. of sodium hydroxide in enough
water to make 32 oz.) 50

$ .75

For Laboratory Tests.

15 Nessler cylinders $ 7.50

5 3-in. glass funnels 1.00

10 250-cc. beakers 2.00

1 test tube support 50

100 qualitative filter papers, 11 cm. diam. (good
grade) 50

1 100-cc. graduated cylinder 75

2 oz. 2% solution of tannic acid in 10% alcohol. .50
1 Ib. sodium hydroxide (NaOH) (sufficient for

150 tests) 50

1 32-oz. bottle containing 3% solution sodium

hydroxide 25

1 10-cc. Mohr 's pipette, graduated in 1/10 cc. . . .50

$14.00

CONCLUDING REMARKS.

Impurities in sands other than organic, such as clay or similar
admixtures, if present in considerable quantities, may be ex-
pected to affect the strength of the concrete. However, the
examination of a large number of defective sands has shown
that it is the organic impurities of a humus nature which are
responsible for the abnormally low strengths of such sands. The
presence of these impurities can be detected by the methods
described above.

It has been found that the colorimetric test can be made with
a greater degree of uniformity by different operators than can
the strength tests of mortars from the same sands.

The fact that the colors produced by digesting certain natural
sands with sodium hydroxide are similar to those produced by
treating tannic acid in the same manner must not be intepreted
to mean that the organic matter in sands is necessarily tannic
acid, or even a tannate. The chemical compounds which make
up the organic material are probably numerous and complicated.

The time actually required for a single determination by the
laboratory method is about -J hour. If tests on as many as 5
samples can be made at once, the total time consumed need not
exceed 1 hour. The time required from the beginning to the
end of the test is about 24 hours.

Inexperienced operators without technical training or previous
experience in work of this character have ' secured satisfactory
results at their first trials.

The strength test of mortar is used as the criterion of the
effect of impurities in sand. In making strength tests of sand

mortars and interpreting the results of such tests, the following
points require particular attention:

(a) Sampling of sand,

(b) Grading of sand,

(c) Quantity of water used in mixing,

(d) Methods of mixing, molding and testing,

(e) Form and size of test piece.

(a) Great care is necessary to insure that the sample tested
is representative of the larger lot of sand. Good judgment
and considerable experience are necessary to secure representa-
tive samples from an undeveloped deposit. Some form of
sampler should be used in the laboratory in selecting a test sample
from a larger lot.

(b) The size and grading has an important influence on
the concrete and mortar-making qualities of sands. A sand
graded from coarse to fine, with the coarse particles up to about
j in. in diameter, is desirable for use in concrete. Fine sands
when mixed to the same plasticity, give mortar strengths much
below those of coarse, well-graded sands. The influence of the
grading of sand on the strength of the mortar must not be con-
fused with the effect of impurities.

(c) The quantity of water used in mortar tests has just as
important an influence on the strength as the amount of ce-
ment. The quantity of water required for normal consistency
of mortar made from a given cement is a function of the grading
of the sand.

(d) The methods used in mixing, molding the test pieces,
storing and testing the specimens, etc., used in standard tests
of cement should be followed in making strength tests of sand
mortars.

(e) Compression tests of mortar are believed to be more
significant than tension tests. A 2 by 4-in. cylinder has been
found to give satisfactory results as a compression test piece.
This is the form of test piece recommended by the American
Society for Testing Materials in their proposed tentative speci-
fication for compression tests of cement.

The funds for this investigation were furnished by Committee
C-9 on Concrete and Concrete Aggregates, of the American So-
ciety for Testing Materials, Sanford E. Thompson, Boston, Mass.,
Chairman, F. W. Kelley, Albany, N. Y., Chairman of Sub- Com-
mittee V, on Impurities Affecting Fine Aggregates for Concrete.

The study of impurities in natural sands is being continued
as a part of the work of the Structural Materials Research
Laboratory, Lewis Institute, Chicago. Special attention will
be given to further improvements in the colorimetric test, and
to discovering remedial measures which can be used in a com-
merical way for counteracting the effects of such impurities.

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2 1933

MAR 3 19

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