A very small amount of ______ is useful in making sound cement. If it is in excess, it causes the cement to become unsound.
🔬 Understanding Soundness of Cement
Soundness is a crucial property of cement that refers to its ability to maintain a constant volume after setting. A cement that expands significantly after hardening is called "unsound." This expansion can cause cracks and lead to the failure of the structure. The primary causes of unsoundness are excessive amounts of three components: Free Lime (CaO), Magnesia (MgO), and Sulphates (SO₃).
📝 Detailed Analysis of the Constituents
(b) sulphur
This is the correct answer. Sulphur is added to cement in the form of gypsum (Calcium Sulphate). A small, controlled amount of gypsum (which contains sulphur) is essential and useful because it acts as a retarder, preventing the cement from setting instantly (flash set). However, if the total sulphur content (as SO₃) exceeds the permissible limit (usually around 3-3.5%), it can lead to a delayed formation of expansive compounds like ettringite, causing the cement to become unsound. This perfectly fits the description of being useful in small amounts but harmful in excess.
(a) iron oxide
This is incorrect. Iron oxide acts as a flux in the kiln and gives cement its color. It does not have a significant role in soundness.
(c) silica
This is incorrect. Silica is a primary strength-giving component and does not cause unsoundness.
(d) alkali
This is incorrect. While excess alkalis can cause a different durability problem called Alkali-Aggregate Reaction, they are not the primary cause of unsoundness related to volume expansion of the cement paste itself.
📊 Summary: Causes of Unsoundness
| Constituent | Role / Effect | Test Method |
|---|---|---|
| Excess Sulphur (as SO₃) | Useful in small amounts (as gypsum) to control set, but causes expansion in excess. | Chemical analysis (no specific physical test) |
| Free Lime (CaO) | Always harmful in excess, causes expansion. | Le-Chatelier Apparatus |
| Magnesia (MgO) | Always harmful in excess, causes very slow expansion. | Autoclave Test |
💡 Study Tips
- The "Too Much of a Good Thing" Rule: Remember that Gypsum (Sulphur) is the only substance intentionally added that can cause unsoundness if you add too much. It's helpful but must be controlled.
- The Unsound Trio: The three main culprits for unsoundness are Lime, Magnesia, and Sulphur.
- Link the Tests: Associate the tests with the cause: Le-Chatelier for Lime, Autoclave for the combined effect of Lime and Magnesia.