Admixtures MCQ with answer and solution

Concept

Silica fume acts as a highly reactive pozzolana that enhances concrete strength and durability through micro-filling and pozzolanic reactions, but decreases workability due to high specific surface area.

Step-by-step solution

1. Understand the physical and chemical nature of silica fume: extremely fine spherical particles (high surface area). 2. Analyze its effect on strength: reacts with free lime (calcium hydroxide) to produce dense C-S-H gel, leading to improved strength. 3. Analyze its effect on workability: high surface area absorbs and demands more mixing water, leading to reduced workability if water-cement ratio is kept constant.

Explanation

Silica fume is a very fine pozzolanic material with a high surface area. When used to replace a small percentage of cement (such as 5%), it reacts with calcium hydroxide to form additional calcium-silicate-hydrate (C-S-H) gel, significantly improving both early and ultimate compressive strength. However, due to its extremely high fineness and surface area, it drastically increases the water demand, thereby reducing the workability of the concrete unless superplasticizers are used.

Why the other options are wrong

A: Strength is improved, not reduced. B: Strength is improved and workability is reduced. D: Workability is reduced due to high fineness.

Shortcut

Silica fume = High strength, Low workability (needs superplasticizer).

Common mistake

Assuming all mineral admixtures or supplementary cementitious materials improve or maintain workability similar to fly ash.