Page 179 - Handbook of Thermal Analysis of Construction Materials

162 Chapter 4 - Introduction to Concrete Admixtures

Volatilized silica, obtained as a by-product in the metallic silicon
or ferrosilicon industry, is known as silica fume. It has been advocated for
incorporation into cement to obtain high strength. It acts as a pozzolan and
is also known to accelerate the hydration of tricalcium silicate. Therefore,
it can be treated as an accelerator.
There has been continued interest in developing an organic-based
accelerator. Ramachandran and coworkers studied the effect of o, m, and p-
nitrobenzoic acids on the hydration of tricalcium silicate. [69] The m and p
nitrobenzoic acids acted as accelerators. The acceleratory effect was
attributed to the complex formation between the organic compound and the
C-S-H phase on the surface of the tricalcium silicate phase.
Some work has been carried out using a finely ground portland
cement hydrate at a dosage of 2% as a “seeding agent.” [70] It is claimed that
2% of seeding is equivalent to 2% CaCl in its accelerating effect.
2
It appears from the published reports that in spite of intensive
efforts, few chemicals perform as well as calcium chloride in terms of
accelerating the hydration of cement, and developing early strengths when
used at equal dosages.

3.0 WATER REDUCERS AND RETARDERS

3.1 Introduction

Most water reducers and retarders are based on similar formu-
lations, and they are treated together in this chapter. These admixtures can
be divided into four categories, viz., normal water reducers, water-reduc-
ing–retarding admixtures, water-reducing–accelerating admixtures, and
retarders.
A normal water reducer lowers the water requirement to attain a
given slump. Thus, for the same slump and a constant cement content use
of lower w/c ratios, results in general improvement in strength, perme-
ability, and durability. Alternatively, the desired slump is achieved with-
out a change in w/c ratio by lowering the cement content. A water-reducing
admixture may also be used to increase the slump to facilitate placements.
According to ASTM, water reducers should be able to lower the water
requirements by 5% of the control. The water reduction depends on the