The setting time of Cement is increased by adding :

Solution

Chemistry ma’am · 4 steps

Gypsum is the retarder: about 2 to 3% is ground into the clinker so cement does not set too fast.

1. What sets cement. Cement clinker contains tricalcium aluminate, 3CaO.Al₂O₃. On its own it reacts with water almost at once, so the paste would harden before it could be mixed and laid.
2. Role of gypsum. Gypsum, CaSO₄.2H₂O, is added to the clinker during final grinding. The sulphate reacts with the aluminate and coats it, which slows down its hydration. This increases the setting time.
3. Other options. Limestone (CaCO₃), clay and silica are raw materials. They are heated together in the kiln to make the clinker itself. They are not additives that control setting.
4. So. The setting time of cement is increased by adding gypsum, option (a).

The idea to keep. Remember the split: limestone, clay and silica go into the kiln to make clinker. Gypsum is added after, during grinding, to slow down the setting.

Answer from the NTA final answer key. Solution by PrepWiser.