Admixtures MCQ with answer and solution

Concept

Calcium chloride is a common accelerator, but its chloride ions cause corrosion of steel in concrete.

Step-by-step solution

1. Accelerators speed up setting and early strength gain of concrete. 2. Common accelerators are calcium chloride, silicates, fluorosilicates, carbonates and triethanolamine. 3. Calcium chloride adds free chloride ions to concrete. 4. Chloride ions break the protective film on steel and cause corrosion. 5. So calcium chloride is harmful for RCC and is not allowed in prestressed concrete.

Explanation

Calcium chloride is a cheap and strong accelerator. But it brings chloride ions into the concrete, and these cause rusting of reinforcement and prestressing steel. So it is not used in RCC or prestressed concrete.

Why the other options are wrong

A: Silicates do not add chlorides, so they do not cause steel corrosion. B: Fluorosilicates are chloride-free and are not harmful to steel. D: Carbonates (sodium/potassium carbonate) are chloride-free accelerators and do not attack steel.

Shortcut

Chloride = corrosion. Any chloride admixture is a danger for steel.

Common mistake

Thinking all accelerators are equally safe. Only chloride-based ones cause steel corrosion.