Admixtures MCQ with answer and solution

Concept

Rapid early strength acceleration in precast concrete

Formula

NONE

Step-by-step solution

1. Rapid formwork turnaround in precast production requires achieving target demolding compressive strength within 12 to 24 hours. 2. High early-strength cement (Rapid Hardening Portland Cement) is ground to high fineness and has high C3S content. 3. Accelerating admixtures (such as calcium chloride or calcium formate) accelerate the early rate of hydration. 4. Hence, high early-strength cement combined with an accelerator is the most suitable combination. Answer: B

Explanation

Precast concrete manufacturing requires rapid early strength development to enable early stripping and rapid cycle reuse of formwork. Combining high early-strength cement (such as Rapid Hardening Portland Cement with high C3S and fine grinding) with an accelerating admixture (such as calcium chloride or calcium formate) maximizes 24-hour compressive strength.

Why the other options are wrong

A) Fly ash retards initial hydration and slows early-age strength gain. C) Retarders delay setting and reduce early strength. D) Normal OPC with air-entraining agents does not provide accelerated early strength.

Shortcut

Rapid early strength = High early-strength cement + Accelerating admixture.

Common mistake

Selecting fly ash mixtures, which actually retard early strength gain due to slow pozzolanic activity.