Cement MCQ with answer and solution

Concept

Iron oxide (Fe₂O₃) in cement gives the grey colour, adds strength and hardness, and acts as a flux during burning.

Step-by-step solution

1. Recall the main ingredients of cement: lime, silica, alumina, iron oxide, magnesia, gypsum, alkalis. 2. Iron oxide gives colour, hardness and strength, and helps fusion during burning. 3. Iron oxide forms the ferrite phase C₄AF, which gives the grey colour. 4. So the grey colour comes from iron oxide. 5. Answer is (C).

Explanation

Iron oxide present in cement gives it the typical grey colour. It also gives strength and hardness and acts as a flux during burning. White cement is made with very low iron oxide, so it is white.

Why the other options are wrong

(A) Gypsum controls the initial setting time. (B) Silica gives strength through C₂S and C₃S. (D) Magnesia gives some colour and hardness in small amounts, but excess causes unsoundness. It is not the source of the grey colour.

Shortcut

White cement has very little iron oxide, so iron oxide = grey colour.

Common mistake

Choosing magnesia or gypsum by thinking of colour or setting functions.