- Lime (CaO): The chief constituent of cement is lime. Its proportion varies from 60 to 67 per cent. The lime in excess makes the cement unsound and causes the cement to expand and disintegrate and also retards the setting property. On the other hand, if lime is in deficiency, it reduces the strength of cement.
- Silica (SiO2): It forms 17 to 25 per cent of Ordinary Portland Cement. It imparts strength to the cement due to the formation of dicalcium and tricalcium silicates. Excess of silica increases the strength of cement, but at the same time the setting time is prolonged.
- Alumina (Al2O3): It acts as a flux and lowers the clinkering temperature. It imparts quick setting property to cement. If in excess, it weakens the strength of cement.
- Calcium sulphate (CaSO4): This ingredient is in the form of gypsum. It is generally added in very small amounts (2 per cent of wt.) to cement towards the last stage of manufacture with a view of retarding the setting time of cement.
- Iron oxide (Fe2O3): This is responsible for imparting the characteristic grey colour to cement. Its percentage varies from 0.5 to 6 per cent.
- Magnesia (MgO): Magnesia varies from 0.1 to 45 per cent. Excess of magnesia reduces the soundness of cement. It imparts hardness and colour to the cement.
- Sulphur: It varies from 1 to 2.5 per cent. If it is in excess, it makes the cement unsound.
- Alkalies: Most of the alkalies present in raw materials are carried away by the flue gases during heating. If they are in excess in cement, they result in alkali-aggregate reaction, efflorescence and staining when used in masonry work.
4.2.1 Harmful constituents of cement
The presence of alkali oxides like K2O and Na2O and magnesium oxides like MgO adversely affects the quality of cement. If the amount of alkali oxides exceeds 1 per cent, it leads to the failure of concrete. If the content of magnesium oxide exceeds 5 per cent, it causes cracks after mortar and the concrete hardens. Table 4.1 shows the admissible average (in %) and limits (in %) of ingredients in ordinary cement.
Table 4.1 Admissible Average (in %) and Limits (in %) of Ingredients in Ordinary Cement
| Ingredient | Limits (%) | Average (%) |
|---|---|---|
| 1. Lime | 60–66 | 63.5 |
| 2. Silica | 18–25 | 22.5 |
| 3. Alumina | 3–8 | 6 |
| 4. Iron oxide | 0.5–5 | 2.5 |
| 5. Magnesia | 1–5 | 1 |
| 6. Sodium and potassium oxides | 0.5–5 | 1 |
| 7. Sulphuric anhydride | 0.5–5 | 1 |
INGREDIENTS OF CEMENT