Products of Hydration of Cement
During Hydration process, several hydrated compounds are formed most important of which are, Calcium silicate hydrate, calcium hydroxide and calcium aluminium hydrates which is important for strength gain.
Calcium silicate hydrate:
This is not only the most abundant reaction product, occupying about 50% of the paste volume, but it is also responsible for most of the engineering properties of cement paste. It is often abbreviated, using cement chemists’ notation, to “C-S-H,” the dashes indicating that no strict ratio of SiO2 to CaO is inferred. C-S-H forms a continuous layer that binds together the original cement particles into a cohesive whole which results in its strong bonding capacity. The Si/Ca ratio is somewhat variable but typically approximately 0.45-0.50 in hydrated Portland cement but up to perhaps about 0.6 if slag or fly ash or micro silica is present, depending on the proportions.
The other products of hydration of C3S and C2S are calcium hydroxide. In contrast to the C-S-H, the calcium hydroxide is a compound with distinctive hexagonal prism morphology. It constitutes 20 to 25 per cent of the volume of solids in the hydrated paste. The lack of durability of concrete is on account of the presence of calcium hydroxide. The calcium hydroxide also reacts with sulphates present in soils or water to form calcium sulphate which further reacts with C3A and cause deterioration of concrete. This is known as sulphate attack. To reduce the quantity of Ca (OH)2 in concrete and to overcome its bad effects by converting it into a cementitious product is an advancement in concrete technology. The use of blending materials such as fly ash, silica fume, and such other pozzolanic materials are the steps to overcome the bad effect of Ca(OH)2 in concrete. However, Ca(OH)2 is alkaline in nature due to which it resists corrosion in steel.
Calcium aluminum hydrates:
These are formed due to hydration of C3A compounds. The hydrated aluminates do not contribute anything to the strength of concrete. On the other hand, their presence is harmful to the durability of concrete, particularly where the concrete is likely to be attacked by sulphates. As it hydrates very fast it may contribute a little to the early strength.