Question

What is the approximate composition of silicon dioxide (SiO₂) present in ordinary Portland cement?
1. 40 - 50%
2. 17 - 25%
3. 3 - 8%
4. 60 - 67%

Answer 1

Correct Answer - Option 2 : 17 - 25%

Concept:

The following table shows the composition of cement and its properties.

Ingredients	Within limit	If in excess
Lime (62 – 65%)	Controls strength and soundness	Makes the cement unsound and it expands and disintegrates
Silica (17 - 25 %)	Adds strength to cement, as it forms C2S and C3S	Causes slow setting of cement
Alumina (3 - 8 %)	Provides quick setting of cement and lowers the clinkering temp	Lowers the strength
Iron oxide (3 - 4 %)	Provides color, hardness, strength and helps infusion of raw material
Magnesia (1 - 3 %)	Provides hardness and color Decreases shrinkage	Makes the cement unsound
Alkalies (0.2 - 1 %)	Should be in limit	Alkali aggregate reaction Efflorescence and staining