Which of the following oxides is present in highest proportion in Ordinary Portland cement?
🔬 Understanding Cement Composition
Ordinary Portland Cement (OPC) is not a single chemical compound but a mixture of several oxides. The properties of cement, such as strength, setting time, and durability, are determined by the proportions of these constituent oxides. The four most important oxides are Lime (CaO), Silica (SiO₂), Alumina (Al₂O₃), and Iron Oxide (Fe₂O₃). These four make up over 90% of the cement's weight.
📝 Detailed Analysis of the Oxides
(d) CaO (Lime)
This is the correct answer. Lime (Calcium Oxide, CaO) is by far the most abundant component in Ordinary Portland Cement, typically making up 60% to 65% of its total weight. Lime is the primary ingredient responsible for the strength of the cement. An excess of lime can make the cement unsound (prone to expansion), while a deficiency reduces strength.
(c) SiO₂ (Silica)
This is incorrect. Silica (Silicon Dioxide, SiO₂) is the second most abundant component, typically present in the range of 17% to 25%. Silica also contributes to the strength of the cement. While it is a major component, its proportion is significantly less than that of lime.
(b) MgO (Magnesia)
This is incorrect. Magnesia (Magnesium Oxide, MgO) is a minor component, usually present in amounts of 0.1% to 4%. While small amounts can contribute to strength, an excess of MgO can cause the cement to become unsound, similar to excess lime.
(a) Na₂O (Sodium Oxide)
This is incorrect. Sodium Oxide (Na₂O), along with Potassium Oxide (K₂O), are known as alkalis. They are present in very small quantities, typically less than 1% combined. They do not contribute to strength and can cause undesirable reactions with certain types of aggregates (alkali-aggregate reaction).
📊 Summary: Typical Composition of Ordinary Portland Cement
| Component | Chemical Formula | Percentage by Weight |
|---|---|---|
| Lime | CaO | 60 – 65% |
| Silica | SiO₂ | 17 – 25% |
| Alumina | Al₂O₃ | 3 – 8% |
| Iron Oxide | Fe₂O₃ | 0.5 – 6% |
| Magnesia | MgO | 0.1 – 4% |
| Sulphur Trioxide | SO₃ | 1 – 3% |
| Alkalis (Soda & Potash) | Na₂O, K₂O | < 1.3% |
💡 Study Tips
- Remember the "Big Two": Lime (CaO) is #1 (around 62%) and Silica (SiO₂) is #2 (around 22%). Together, they make up about 85% of cement. If you remember these two, you can answer most composition questions.
- Use an Acronym: A simple way to remember the order of abundance for the top four is L-S-A-I (Lime, Silica, Alumina, Iron).
- Minor Components are "Unsound": Remember that the minor components like Magnesia (MgO) and excess Lime (CaO) can cause the cement to be "unsound" if their proportions are too high.