Due to the presence of _______, the cement derives the characteristic grey colour.
🎨 Understanding Cement Composition and Color
The final properties of Portland cement, including its color, are determined by the chemical composition of the raw materials used in its manufacture. While the main components like lime and silica are responsible for strength, minor components play crucial roles in the manufacturing process and in determining characteristics like color.
🔬 Detailed Analysis of the Options
D. iron oxide
This is the correct answer. Iron Oxide (Fe₂O₃) is the primary ingredient responsible for imparting the characteristic greyish-green color to Ordinary Portland Cement. It is typically present in amounts ranging from 0.5% to 6%. Besides providing color, iron oxide also acts as a flux during the clinkering process, helping to lower the fusion temperature of the raw materials. It also contributes to the hardness and strength of the cement.
A. gypsum
This is incorrect. Gypsum (CaSO₄·2H₂O) is a white mineral that is added to the cement clinker *after* it has been burned and cooled. Its sole purpose is to regulate the setting time by controlling the rapid hydration of Tricalcium Aluminate (C₃A). It does not contribute to the grey color.
B. magnesia
This is incorrect. Magnesia (MgO) is present in small quantities in cement. While it can act as a flux at high temperatures, its primary role is not to provide color. In fact, an excess of magnesia is undesirable as it can lead to unsoundness (volume expansion) in the hardened concrete.
C. silica
This is incorrect. Silica (SiO₂) is a major component of cement (as C₃S and C₂S) and is fundamental to its strength-giving properties. However, in its pure form, silica is colorless or white and does not contribute to the grey color of the cement.
📊 Summary: Function of Cement Components
| Component | Typical % | Primary Function |
|---|---|---|
| Lime (CaO) | 60-67% | Controls strength and soundness. |
| Silica (SiOâ‚‚) | 17-25% | Provides strength. |
| Alumina (Al₂O₃) | 3-8% | Responsible for quick setting. |
| Iron Oxide (Fe₂O₃) | 0.5-6% | Provides color, hardness, and strength. |
| Gypsum (CaSOâ‚„) | 3-5% | Controls initial setting time. |
💡 Study Tips
- Iron = Color: The easiest way to remember this is to associate iron with color. Just as rust (an iron oxide) gives a reddish-brown color, the iron oxide in cement gives it its characteristic grey color.
- White Cement: To produce white cement, the amount of iron oxide and manganese oxide are kept extremely low (less than 0.1%). This highlights the role of iron oxide as the primary coloring agent in regular grey cement.
- Gypsum is an Add-on: Remember that gypsum is added at the end of the process. The clinker is already grey before gypsum is introduced.