Admixtures MCQ with answer and solution

Concept

Calcium chloride is an accelerator. It speeds up setting and hardening, so it is not a retarder.

Step-by-step solution

1. Retarders slow down the setting of cement. 2. Common retarders are sugar, starch, cellulose products, and calcium sulphate (gypsum). 3. Calcium chloride (CaCl₂) speeds up hydration, so it is an accelerator. 4. So the option that is NOT a retarder is calcium chloride.

Explanation

Retarding admixtures delay the setting time of concrete. They are useful in hot weather or for long transport. Calcium chloride does the opposite: it makes concrete set and gain strength faster.

Why the other options are wrong

A: Sugar is a strong retarder. B: Calcium sulphate (gypsum) is added to cement to retard setting. D: Starch (a carbohydrate) acts as a retarder.

Shortcut

CaCl₂ = most common accelerator. Sugar, starch, gypsum = retarders.

Common mistake

Thinking all calcium salts behave the same. Gypsum retards setting, but calcium chloride accelerates it.