The setting time of Cement is increased by adding

The setting of cement involves the hydration of calcium aluminates and silicates, which is an exothermic process that occurs rapidly.

Gypsum (CaSO₄·2H₂O) is added to cement to increase the setting time (slow down the setting process). It reacts with tricalcium aluminate (C₃A) to form calcium sulfoaluminate (ettringite), which coats the C₃A particles and prevents their rapid hydration, thereby increasing the setting time.

Without gypsum, cement would set too quickly (flash setting), making it difficult to work with.

The correct answer is Gypsum.