Gypsum – CaSO4 . 2H2O
Added to cement to increase the setting time of cement so that it can be easily worked with, transported & placed.
If we don’t add gypsum to cement, cement dries or hardens very quickly as it is mixed with water.
By adding gypsum, it gives high strength & hardness to the cement.
Cement + water is a exothermic process, by adding gypsum, it acts as a coolant.
C3A is the compound responsible for initial setting of cement & is the first compound to react with water.
Gypsum reacts with C3A & turns it into Calcium sulphoaluminate
Since Gypsum reacts with C3A & C3A reduces.
Thus C3A is not available for reaction with water & hence setting action reduces.
Reduction in setting action means elongation of setting of cement i.e., setting time increases.