What is the approximate composition of sulphur trioxide (SO₃) present in ordinary Portland cement?
Try it first: pick an option, then read the explanation.
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.