Pick up the correct statement from the following:
Correct Answer: C. Gypsum (calcium sulphate) acts as a retarder
📚 Detailed Explanation: Gypsum = Retarder; CaCl2 = Accelerator
Why C is correct: Gypsum (calcium sulphate, CaSO4·2H2O) acts as a retarder in cement. It reacts with C3A to form ettringite, which coats the C3A particles and slows their hydration, delaying the initial set. Calcium chloride (option A) is an accelerator — it speeds up hydration. Both A and B are wrong; only C is correct.
Gypsum as a Retarder
| Aspect | Detail |
|---|---|
| Chemical name | Calcium sulphate dihydrate: CaSO4·2H2O |
| Common name | Gypsum |
| Function | Retarder — slows initial hydration of C3A; prevents flash set |
| Added at | 2–3% by mass during grinding of OPC clinker |
| Without gypsum | C3A hydrates explosively → flash set in 2–5 minutes; cement unusable |
| With gypsum | Ettringite coats C3A; initial set delayed to minimum 30 minutes (IS 269) |
Calcium Chloride as an Accelerator
| Aspect | Detail |
|---|---|
| Chemical formula | CaCl2 |
| Function | Accelerator — speeds up hydration; reduces setting time; promotes early strength |
| Safe dosage | 1–2% by wt. of cement (IS 456 limits Cl¹¯ to 0.3% of cement mass in RC) |
| Harmful if excessive | >2% detrimental; >4% severely harmful; banned in pre-stressed concrete |
Summary
| Statement | Correct? |
|---|---|
| A. CaCl2 acts as a retarder | ✗ Wrong — CaCl2 is an ACCELERATOR |
| B. Gypsum acts as an accelerator | ✗ Wrong — gypsum is a RETARDER |
| C. Gypsum acts as a retarder | ✓ Correct |
| D. None of these | ✗ Wrong — C is correct |
- Gypsum = retarder: prevents flash set; added 2–3% during cement grinding.
- CaCl2 = accelerator: reduces setting time; used in cold weather; limited to 2% by wt. cement in RC.