Concept:Gypsum is added to cement as a retarder to control the setting time.
Explanation:When cement is mixed with water, hydration reactions start and the cement hardens.
Without gypsum, the setting occurs too quickly, a phenomenon called “flash set”.
Gypsum (calcium sulfate dihydrate,
CaSO4⋅2H2O) reacts with tricalcium aluminate in cement to form ettringite.
This reaction slows down the overall hydration process.
As a result, the initial setting time is delayed, giving more working time for construction.
Thus, the addition of gypsum increases the setting time of cement.
Gypsum does not affect the colour or surface shine of the final product.
Answer:The correct option is C: increases setting time of cement.