19 Cement
Cement is one of the most important construction materials used in modern engineering and infrastructure development. It is regarded as the backbone of the construction industry.
It is the second most consumed material in the world after petroleum, which highlights its global importance.
Cement is defined as a binder, a substance capable of holding different materials together by setting and hardening. It is widely used in building construction, road construction, bridges, dams, and other civil engineering works.
Cement is never used alone. It is used in combination with other materials:
- With sand and gravel → Concrete
- With sand, gravel, and steel rods → Reinforced Cement Concrete (RCC)
Cement is a fine, soft, grey powder. When mixed with water, it forms a paste that undergoes a chemical reaction known as hydration. During hydration, cement reacts with water to form microscopic crystals and gel-like substances that interlock, causing the mixture to set and harden into a solid mass.
This property of setting even under water makes cement a hydraulic cement.
The most commonly used cement is Portland cement, named after the resemblance of its hardened form to Portland stone found in England.
Important Characteristics
- Does not have a fixed chemical formula
- It is a mixture of several compounds
- Hardens due to chemical reaction, not drying
Composition of Cement
| Component | Function | Percentage |
|---|---|---|
| CaO (Lime) | Provides strength and durability | 50–60% |
| SiO₂ (Silica) | Provides hardness | 20–25% |
| Al₂O₃ (Alumina) | Controls setting time | 5–10% |
| Fe₂O₃ | Provides colour and strength | 1–2% |
| MgO | Provides hardness | 1–2% |
| SO₃ | Controls setting | 1–2% |
The manufacture of cement requires raw materials that provide essential oxides like calcium, silica, alumina, and iron.
Main Raw Materials
1. Calcareous Materials
- Provide calcium oxide (CaO)
- Main source: Limestone (CaCO₃)
- Other sources: Chalk, marble
2. Argillaceous Materials
- Provide silica (SiO₂), alumina (Al₂O₃), and iron oxide (Fe₂O₃)
- Examples: Clay, shale
Other Important Materials
- Gypsum: Added later to control setting time
- Coal: Used as fuel in kiln
- Fly ash: Improves quality and reduces cost
Raw materials are extracted by blasting, drilling, and excavation.
Although these materials are natural, cement itself is an artificially manufactured product.
Flow Sheet Diagram
Step-by-Step Explanation
1. Crushing and Grinding
Raw materials are crushed into small pieces and then ground into fine powder.
Wet Process: Materials mixed with water → slurry (~40% water)
Dry Process: Materials dried → powdered raw meal
2. Blending
Materials are mixed in correct proportions to ensure uniform composition.
3. Heating (Calcination)
- Occurs in rotary kiln
- Temperature: 1400–1600°C
- CO₂ and water removed
- Chemical reactions occur
4. Clinker Formation
Dark grey nodules called clinker are formed.
5. Final Grinding
Clinker is ground with gypsum to form cement.
| Property | OPC | PPC |
|---|---|---|
| Composition | No additives | Contains fly ash |
| Setting time | Fast | Slow |
| Strength | High early strength | High long-term strength |
| Cost | High | Low |
| Environmental impact | Higher CO₂ emission | Eco-friendly |
During heating in the kiln, several chemical reactions occur:
- 2CaO + SiO₂ → 2CaO·SiO₂ (Dicalcium silicate)
- 3CaO + SiO₂ → 3CaO·SiO₂ (Tricalcium silicate)
- 3CaO + Al₂O₃ → 3CaO·Al₂O₃ (Tricalcium aluminate)
These compounds are responsible for the strength and setting properties of cement.
The reactions are exothermic, so temperature must be controlled carefully.
Gypsum is added to regulate setting time and prevent rapid hardening.
The cement industry in Nepal began in the 1950s.
- First factory: Himal Cement Company Ltd. (1967)
- Closed in 2002 due to pollution issues
- Hetauda Cement Industry (1976)
- Udayapur Cement Factory (1987)
After economic liberalization in the 1990s, many private industries were established.
Current Status:
- ~55 cement industries
- ~160 million sacks annually
- ~22 million tons production capacity
Raw materials like limestone are locally available, while gypsum and coal are imported.
The cement industry plays a vital role in infrastructure development such as roads, buildings, and bridges.