Which of the following is commonly used as a retarder in cement?

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.