Hydration of Basic Compounds of Cement:
The basic compounds of cement namely, tri-calcium silicate (C3S), di-calcium silicate (C2S), tri-calcium aluminate (C3A) and tetra-alumino-ferrite (C4AF) are also called as Bouges' Compounds. Those compounds hydrate within different ways, which has been discussed in the subsequent sections.
Among the four basic compounds of cement, tri-calcium aluminate (C3A) hydrates very fast therefore, produce maximum amount of heat which is harmful for the durability of concrete. A product of hydration of C3A contributes to the early strength of concrete. Hydration of C3A being extremely rapid might cause flesh set. To prevent flesh set of cement, gypsum is added within the cement at the time of grinding. While water is added in the cement, the gypsum, due to its chemical composition, reacts with C3A, with the result less C3A is available to react with water, and hence setting process gets retarded.
Hydration of tri-calcium silicate (C3S) and di-calcium silicate (C2S) are mainly responsible for governing the early and later strength of concrete respectively, because rate of hydration of C3S is faster than that of C2S. But due to the fast rate of hydration, C3S produced more amount of heat of hydration that is harmful for the concrete. Hydration of that compound also generates huge amount of calcium-hydroxide, i.e. Ca (OH)2, which is also harmful for the concrete because this product is soluble in water which afterwards gets leached out on the surface of concrete leaving pores inside the concrete. In general, one can say that hydration products of C2S are more effective in improving the strength of concrete as compared to those of C3S.
In view of the above discussions it is obvious that cement with high contents of C3S is preferred for cold weather concreting. Reducing the percentage contents of C3S and C3A can reduce the total heat of hydration and the rate of heat generation.