JEE Main 2022 · 29 July · evening · Single correct · +4 / −1
Portland cement contains 'X' to enhance the setting time. What is 'X'?
- (a) CaSO₄ · 1/2 H₂ O
- (b) CaSO₄ · 2 H₂ O
- (c) CaSO₄
- (d) CaCO₃
Answer and solution
Answer (b) CaSO₄ · 2 H₂ O
Solution
Chemistry ma’am · 4 steps
Enhancing the setting time means slowing the set down, and the slowing agent is plain gypsum.
- Read the phrase. To enhance the setting time is to make the cement take longer to set. Without help, cement sets too fast to be mixed, poured and shaped.
- Why it sets fast. Clinker contains tricalcium aluminate, 3CaO.Al₂O₃, which reacts with water almost at once. That quick reaction can make the paste stiffen within minutes. This is called a flash set.
- What gypsum does. About 2 to 3% of gypsum, CaSO₄.2H₂O, is ground into the clinker. It reacts with the aluminate and forms a coating that slows its hydration. The cement then sets slowly and hardens properly.
Substance Common name Role CaSO₄.1/2H₂O plaster of Paris sets very fast on its own CaSO₄.2H₂O gypsum added to clinker to slow setting CaSO₄ dead burnt plaster does not set with water CaCO₃ limestone raw material burnt in the kiln - So. X is gypsum, CaSO₄.2H₂O, which is option (b).
The idea to keep. Gypsum is the retarder in cement. Keep the three calcium sulphates apart: dihydrate is gypsum, hemihydrate is plaster of Paris, anhydrous is dead burnt plaster.
Answer from the NTA final answer key. Solution by PrepWiser.