JEE Main 2023 · 8 April · morning · Single correct · +4 / −1
What is the purpose of adding gypsum to cement?
- (a) To facilitate the hydration of cement
- (b) To speed up the process of setting
- (c) To give a hard mass
- (d) To slow down the process of setting
Answer and solution
Answer (d) To slow down the process of setting
Solution
Chemistry ma’am · 4 steps
Gypsum is the brake in cement: it stops the aluminate from setting the paste before you can use it.
- The fast culprit. Cement clinker contains tricalcium aluminate, 3CaO.Al₂O₃. It reacts with water very quickly, so without help the paste would harden within minutes. This is called flash setting.
- What gypsum does. About 2 to 3% gypsum, CaSO₄.2H₂O, is ground in with the clinker. It reacts with the aluminate to form calcium sulphoaluminate, which coats the aluminate grains and slows down their reaction with water.
- Why that helps. With setting delayed, there is enough time to mix, carry and lay the concrete. The slower silicate hydration then gives the final strength.
- So. Gypsum is added to slow down the process of setting, which is option (d).
The idea to keep. In cement, gypsum is a retarder. Remember the pair: the aluminate sets too fast, and gypsum calms it down. The strength comes later from the silicates.
Answer from the NTA final answer key. Solution by PrepWiser.