In simple, you can say that heat of hydration is the liberation of heat during its curing period. You know that the reaction between cement and water is an exothermic reaction.
The exothermic reaction liberates a considerable amount of heat during initial stages. The first stage is very important during concreting because the rate of development of concrete strength is widely depending upon the first few days of curing.
When mixing cement with water rapid heat of evolution take place and this rapid heat last for few minutes. This liberation of heat takes place due to the solution reaction between aluminates and sulfates.
This initial heat ceases when the solubility of aluminates decreases. The second stage of formation of heat is occurred due to the formation of ettringite and reaction of C3S. Tricalcium silica is one of the bogues compounds present in concrete.
Why do you need to take care of heat of hydration?
When constructing large structures such as bridge piers, dams, mass concrete etc..There will be a large variation of temperature in the inside and outer surface of these structures during the hydration process
These temperature variations can cause cracking of structures which leads to a reduction of strength and durability of the building.
These variations can be reduced by using low heat cement, using ice instead of water and using fly and slag. Low hat cement contains comparatively smaller amount of C3S. Hence heat of hydration is very less in low heat cement. Fly ash and slag has the capability to reduce the hydration heat. Also, you can cover the concrete surface with clothes which can absorb water by preventing evaporation loss.