Admixtures MCQ with answer and solution

Concept

Chemical admixture classification of plasticizers

Formula

Water Reduction % = ((W_control - W_admixed) / W_control) × 100

Step-by-step solution

1. Concrete admixtures are categorized as either chemical or mineral admixtures. 2. Water-reducing admixtures (plasticizers and superplasticizers) are organic chemical compounds added in minor liquid or powder dosages. 3. They function by dispersing cement particles through electro-repulsive or steric hindrance mechanisms. 4. Because they do not constitute bulk pozzolanic cementitious replacements, they are classified as chemical admixtures. Answer: B

Explanation

Water-reducing plasticizers are synthetic or modified organic chemical formulations added to concrete in precise dosages to disperse flocculated cement grains. Because they function chemically at the molecular interface without acting as supplementary cementitious bulk materials, they are categorized as chemical admixtures. They allow significant water reduction while maintaining workability.

Why the other options are wrong

A) Mineral admixtures are finely divided pozzolanic materials added in large percentages by weight of cement. C) Air-entraining agents are a separate class of chemical admixtures designed to create stable micro-bubbles. D) Fibre reinforcements are discrete fibers added to provide post-cracking tensile toughness.

Shortcut

Water reducers, retarders, accelerators = Chemical; Pozzolans, fly ash, slag = Mineral.

Common mistake

Confusing chemical admixtures with mineral admixtures like fly ash, silica fume, and GGBS.