Which of the following is commonly used as a retarder in cement?
Correct Answer: B. Gypsum
📚 Detailed Explanation: Gypsum — The Standard Retarder in Cement
Why B (Gypsum) is correct: Gypsum (calcium sulphate dihydrate, CaSO4·2H2O) is added to all Portland cement during clinker grinding at 2–3% by mass. Its purpose is to regulate the setting time by retarding the rapid hydration of tricalcium aluminate (C3A) and preventing flash set. Calcium sulphate (option A) is the same compound as gypsum, but gypsum is the specific mineral name for CaSO4·2H2O and the correct terminology here.
How Gypsum Retards Cement Setting
| Step | Chemistry |
|---|---|
| 1. Without gypsum: flash set | C3A + H2O → rapid hydration → stiff within 2–5 minutes; unusable |
| 2. Gypsum added at 2–3% | CaSO4·2H2O dissolves in water → provides SO42− ions |
| 3. Ettringite forms (initial retardation) | C3A + 3CaSO4·2H2O + 26H2O → ettringite (AFt); coats C3A particles |
| 4. Coating protects C3A | Ettringite shell slows water access to C3A; hydration rate controlled; initial set delayed to 30+ min |
| 5. Gypsum depletes; final set | When all SO42− consumed, ettringite converts to monosulphate (AFm); C3A hydration resumes; final set occurs |
Comparison of Options
| Option | What It Is | Use in Concrete |
|---|---|---|
| A. Calcium sulphate | CaSO4 (anhydrous) or CaSO4·2H2O (gypsum); technically correct but less specific | Also a retarder; but gypsum (B) is the more specific and common answer |
| B. Gypsum | CaSO4·2H2O — standard retarder; added 2–3% during clinker grinding | Correct; the universal retarder in all Portland cements |
| C. Potassium carbide | K2C2; a carbide compound; not used in cement | Not a concrete admixture |
| D. Sodium chloride | NaCl; common salt; can be a mild accelerator at low doses; harmful chloride source | Not a retarder; actually mildly accelerating and corrosive to steel |
- Gypsum (CaSO4·2H2O) is the standard retarder added to all Portland cement at 2–3% during grinding.
- Without gypsum, cement would flash-set within minutes of mixing — completely unusable.
- CaCl2 is the opposite: an accelerator that reduces setting time.