Pick up the correct statement from the following:

Gypsum (calcium sulphate) acts as a retarder

Calcium chloride acts as a retarder

Gypsum (calcium sulphate) acts as an accelerator

Pick up the correct statement: A) Calcium chloride acts as retarder, B) Gypsum acts as accelerator, C) Gypsum acts as 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.