Building Materials, Third Edition

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X Proper attention to the rheological properties of a mixture can

effectively reduce construction and material costs. The properties of the materials used

and field conditions have a great influence.

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The sample of concrete used for analysis should be true representative of the material under
consideration. Several portions weighing at least 5 kg each are taken, broken up, crushed to
about 10 mm size and reduced to a fineness of approximately IS Sieve 8 in a suitable device.
100 g of this is dried at 105°C for 2 hours.
2g of each prepared sample is weighted in three 250 ml beakers. Care is taken to see that the
sample does not adheres to the beakers or lumps formed. 100 ml of 3.3N Hydrochloric acid is
added slowly and stirred. After the evolution of carbon dioxide has ceased and the reaction is
apparently completed, it is heated gently for few minutes and contents of beaker are allowed
to settle down. It is then decanted through an ignited and weighted gooch crucible which
contains a mat of short asbestos shreds, practically insoluble in hydrochloric acid. It is then
filtered through Whatman filter paper No. 42. Once the filtration has begun, care is taken so
that the mat and accumulated residue do not dry out completely untill the filtration process is
completed. The session is regulated so as to maintain rapid rate of dropping during filtration.
The residue retained in the beaker is washed by decantation twice with hot water. 75 ml of 1N
Sodium hydroxide is added to the residue while string and heating to about 75°C. The residue
is then transferred to the crucible and washed with at least 60 ml of hot water.
Now the filtration contains silica in the form of silicic acid in the true solution or in suspension
in the hydrochloric acid medium. If the aggregate of the original sample are largely calcareous
or dolomatic, 10 ml of hydrochloric acid having specific gravity 1.19 is added. It is then
transferred to the suitable beaker with several rinsing of the suitable flask. This is evaporated
to dryness with great care, heating it below 120°C for one hour, and moistening with 10 ml
concentrated hydrochloric acid. It is evaporated and heated again and is taken of filtration in
75 ml of 2 N Hydrochloric acid heated to boiling. It is filtered through an ash less filter paper
No. 42 and the residue is washed with 50 ml of hot 1N hydrochloric acid and then with hot
water untill the washings are free from chlorides. The evaporation and filtering process is
repeated to recover the small amounts of silica dissolved and these are added to the first residue.
When the aggregates used in the concrete being analyzed are available a blank test is run on
these aggregates to determine their content of silica. This content of the soluble silica is then
used as a correction factor and subtracted from the total soluble silica found in the concrete, the
difference being due to the cement contained in the specimen.
The percentage of cement in the sample is calculated by dividing the percentage of silica
found by the factor 0.214, provided the silica content of the cement is not known to be different
from this value.

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Admixtures are the materials other than the three basic ingredients of cement concrete—
cement, aggregate and water—added to the concrete mix before or during mixing to improve
certain of its properties in fresh or hardened state. The properties commonly modified are rate
of hydration or setting time, workability, dispersion and air entrainment.