What is the approximate composition of silicon dioxide (SiO₂) present in ordinary Portland cement?
🔬 Understanding Cement Composition
Ordinary Portland Cement (OPC) is a carefully controlled chemical blend of several oxides, each playing a vital role. The main ingredients are derived from calcareous materials (like limestone, rich in calcium carbonate) and argillaceous materials (like clay or shale, rich in silica and alumina). The proportions of these oxides determine the final properties of the cement, including its strength, setting time, and durability.
📝 Detailed Analysis of the Composition
(b) 17-25%
This is the correct answer. Silicon Dioxide (SiO₂), commonly known as Silica, is the second most abundant constituent in Ordinary Portland Cement. It typically makes up about 17% to 25% of the cement by weight. Silica is crucial as it combines with lime (CaO) at high temperatures in the kiln to form Dicalcium Silicate (C₂S) and Tricalcium Silicate (C₃S), the two compounds primarily responsible for the strength of the cement.
(a) 60-67%
This is incorrect. This range represents the percentage of Lime (Calcium Oxide, CaO), which is the most abundant component in cement.
(c) 40-50%
This is incorrect. This range does not correspond to any of the major constituents of OPC. It is too low for lime and too high for silica.
(d) 3-8%
This is incorrect. This range represents the percentage of Alumina (Aluminum Oxide, Al₂O₃), the third most abundant component in cement.
📊 Summary: Composition of Ordinary Portland Cement
| Constituent | Chemical Formula | Percentage by Weight |
|---|---|---|
| Lime | CaO | 60 – 65% |
| Silica | SiO₂ | 17 – 25% |
| Alumina | Al₂O₃ | 3 – 8% |
| Iron Oxide | Fe₂O₃ | 0.5 – 6% |
| Magnesium Oxide | MgO | 0.1 – 4% |
| Sulphur Trioxide | SO₃ | 1 – 3% |
| Alkalis (Soda & Potash) | Na₂O + K₂O | 0.5 – 1.3% |
💡 Study Tips
- Remember the Top Four: The easiest way to remember the composition is to know the order of the four most abundant ingredients: Lime > Silica > Alumina > Iron Oxide.
- Use an Acronym: A popular mnemonic is L-S-A-I (Lime, Silica, Alumina, Iron). This helps recall the order of decreasing percentage.
- Approximate Percentages: For quick recall, remember approximate values: Lime (~62%), Silica (~22%), Alumina (~5%), Iron (~3%). This will help you quickly identify the correct range in multiple-choice questions.