Admixtures MCQ with answer and solution

Concept

Spherical fly ash ball-bearing effect

Formula

NONE

Step-by-step solution

1. Fly ash particles collected from electrostatic precipitators are predominantly microscopic glassy spheres (cenospheres). 2. When incorporated into concrete mixes, these rounded smooth spheres lubricate aggregate particles and cement clusters. 3. This action is known as the ball-bearing effect, which reduces inter-particle friction during mixing and placement. 4. Thus, the spherical particle shape improves the workability of fresh concrete. Answer: B

Explanation

Pulverized fuel ash consists primarily of microscopic solid and hollow spherical glassy particles. These smooth spherical particles produce a ball-bearing lubricating effect among cement grains and aggregate particles in fresh concrete. This reduces internal friction and significantly enhances workability without increasing water demand.

Why the other options are wrong

A) High water demand reduces slump and workability rather than improving it. C) Fly ash particles are spherical and rounded, whereas angular shapes increase frictional resistance. D) High unburnt carbon content absorbs water and chemical admixtures, degrading workability.

Shortcut

Fly ash = Smooth spheres -> Ball-bearing effect -> Superior workability.

Common mistake

Assuming fly ash has angular particles or high water demand similar to crushed rock dust.