What is the approximate composition of sulphur trioxide (SO₃) present in ordinary Portland cement?
🔬 Understanding Minor Constituents in Cement
While the bulk of cement is made up of Lime and Silica, several minor constituents play crucial roles. Sulphur Trioxide (SO₃) is one such component. It is not a raw material but is introduced into the cement primarily through the addition of gypsum (Calcium Sulphate) during the final grinding stage. A small amount of SO₃ is essential to control the setting time, but an excess amount can be harmful, causing unsoundness (destructive expansion) in the hardened concrete.
📝 Detailed Analysis of the Composition Ranges
(a) 1% to 2%
This is the correct answer. The standard specifications for Ordinary Portland Cement limit the total Sulphur Trioxide (SO₃) content to a very small range, typically 1% to 3%. The option "1% to 2%" falls squarely within this acceptable range and is the best choice among the given options. This small amount is sufficient to regulate the setting of C₃A without causing long-term durability issues.
(b) 60% to 65%
This is incorrect. This range represents the percentage of Lime (CaO), the most abundant component in cement.
(c) 17% to 25%
This is incorrect. This range represents the percentage of Silica (SiO₂), the second most abundant component.
(d) 3.5% to 9%
This is incorrect. This range is too high. A value above 3.5% would likely cause the cement to fail the soundness test, making it unsuitable for use.
📊 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
- Focus on the Minor Players: While Lime and Silica are the main components, it's important to know the approximate percentages of the minor ones like SO₃, MgO, and Alkalis, as they are often the subject of questions.
- SO₃ comes from Gypsum: Remember that the source of Sulphur Trioxide in cement is the gypsum added to control the setting time.
- Too Much is Unsound: Associate excess SO₃ (along with excess Lime and Magnesia) with the problem of unsoundness.