Hydration of Cement & Evolution of Heat
When water is added to cement, the cement hydrates and during the chemical reactions which take place while the cement is setting an increase in temperature occurs and a considerable quantity of heat is generated. Hydration of cement is incomplete without an adequate quantity of water. Heat and humidity accelerate hydration of cement. The amount of heat and the rate at which it is generated depends mainly on the type (chemical composition) of the cement and affects the rate of hardening. The greater the heat generated the more rapid the rate of hardening. Shrinkage occurs on subsequent cooling of the cement mortar or concrete resulting in cracks. The more rapid the rate of hardening the more susceptible is a concrete to shrinkage cracks.
The amount of water required to hydrate cement is about 25 per cent of the weight of the cement. The amount of mixing water is rarely less than twice this quantity.
If water/cement ratio is less than 0.4 to 0.5 çomplete hydration of cement will not occur. Roughly water/cement ratio is 0.60 for a 1:2:4 mix, 0.5 for 1:1.5:3 mix, and 0.45 for 1:1:2 mix.