Cement when mixed with water forms a plastic mass called cement paste. During hydration reaction, gel and crystalline products are formed. The inter-locking of the crystals binds the inert particles of the aggregates into a compact rock like material.

This process of solidification comprises of

(i) setting and then

(ii) hardening

Setting is defined as stiffening of the original plastic mass due to initial gel formation. Hardening is development of strength, due to crystallization.

Due to the gradual progress of crystallization in the interior mass of cement, hardening starts after setting. The strength developed by cement paste at any time depends upon the amount of gel formed and the extent of crystallization. The setting and hardening of cement is due to the formation of inter locking crystals reinforced by rigid gels formed by the hydration and hydrolysis of the constitutional compounds.

Reactions involved in setting and hardening of cement:- When cement is mixed with water, the paste becomes rigid within a short time which is known as initial setting. This is due to the hydration of tricalcium aluminates and gel formation of tetra calcium alumina ferrite.

Dicalcium silicate also hydrolyses to tobermonite gel which contributes to initial setting.

Final setting and hardening of cement paste is due to the formation of tobermonite gel and crystallization of calcium hydroxide and hydrated tricalcium aluminate.

During setting and hardening of cement, some amount of heat is liberated due to hydration and hydrolysis reactions. The quantity of heat evolved during Complete hydration of cement is 500 KJ/Kg.

Function of gypsum in cement :-

Tri calcium aluminate (C3A) combines with water very rapidly.

After the initial setting, the paste becomes soft and the added gypsum retards the dissolution of C3A by forming insoluble calcium sulpho aluminate.