Which of the following oxide is in the highest percentage in ordinary portland cement?
🏗️ The Building Blocks of Cement: Chemical Composition
The properties of cement are determined by its chemical makeup. It is manufactured from raw materials rich in lime, silica, alumina, and iron oxide. The proportions of these oxides are carefully controlled to produce a cement with desired characteristics like strength, setting time, and durability.
The most important constituent, by far, is Lime (Calcium Oxide), which forms the basis for the primary strength-giving compounds in cement.
📝 Detailed Analysis of the Options
(a) Iron oxide (Fe₂O₃)
Iron oxide is a minor component, typically making up only 0.1% to 4% of the cement. It primarily acts as a flux in the kiln, helping to lower the clinkering temperature. It also imparts the characteristic grey color to cement.
(b) Magnesium oxide (MgO)
Magnesia is another minor constituent, present in quantities of about 1% to 3%. While a small amount is acceptable, an excess of MgO can cause unsoundness, leading to delayed expansion and cracking of the hardened concrete.
(d) Aluminium oxide (Al₂O₃)
Alumina constitutes about 3% to 8% of cement. It plays a key role in the setting process. It lowers the clinkering temperature and is responsible for the quick setting of cement by forming Tricalcium Aluminate (C₃A).
(c) Calcium oxide (CaO)
This is the correct answer. Calcium oxide, commonly known as lime, is the principal component of Portland cement, making up 60% to 65% of its total weight. It is the primary ingredient that combines with silica to form the main strength-providing compounds: Tricalcium Silicate (C₃S) and Dicalcium Silicate (C₂S). An adequate amount of lime is essential for producing high-quality, strong cement.
📊 Chemical Composition of Ordinary Portland Cement
| Oxide | Chemical Formula | Percentage Range | Average % |
|---|---|---|---|
| Calcium Oxide (Lime) | CaO | 60 - 65% | 63% |
| Silica | SiO₂ | 17 - 25% | 20% |
| Alumina | Al₂O₃ | 3 - 8% | 6% |
| Iron Oxide | Fe₂O₃ | 0.1 - 4% | 2% |
| Magnesium Oxide | MgO | 1 - 3% | 1.5% |
💡 Study Tips
- Lime is Prime: Remember that Lime (Calcium Oxide) is the number one ingredient in cement by a very large margin. It's the foundation of cement's strength.
- The "Big Four": After Lime, the order of abundance is generally Silica > Alumina > Iron Oxide. Knowing this sequence can help you answer many related questions.
- Balance is Key: While lime is crucial, too much can cause the cement to become unsound (expand and disintegrate). The proportions of all oxides are carefully balanced.