Unit 19: Cement
Introduction
Cement is a binding material that hardens when mixed with water, forming a solid mass through hydration reactions. The most common type is Portland cement, a hydraulic cement that sets and gains strength even under water.
Key Points
- Binding action results from calcium silicate hydrates (C‑S‑H) and calcium hydroxide.
- Portland cement derives its name from its resemblance to Portland stone.
Raw Materials for Cement Production
The composition of cement clinker is controlled by blending specific raw materials that supply the four major oxides: lime (CaO), silica (SiO₂), alumina (Al₂O₃), and iron oxide (Fe₂O₃). Gypsum is added later to regulate setting time.
Typical Oxide Ranges (by mass)
| Oxide | Source Material | Typical Percentage |
|---|---|---|
| Lime (CaO) | Limestone (CaCO₃) | 60‑67 % |
| Silica (SiO₂) | Clay, shale | 17‑25 % |
| Alumina (Al₂O₃) | Clay, bauxite | 3‑8 % |
| Iron oxide (Fe₂O₃) | Iron ore, red earth | 0.5‑6 % |
| Gypsum (CaSO₄·2H₂O) | Added during final grinding | 3‑5 % |
Chemical Reactions in the Kiln
- Calcination (decomposition of limestone):
CaCO₃ (s) → CaO (s) + CO₂ (g) - Clinker formation** (solid‑state reactions at ~1450 °C):
2 CaO + SiO₂ → Ca₂SiO₄ (belite) CaO + SiO₂ → CaSiO₃ (wollastonite) 3 CaO + SiO₂ → Ca₃SiO₅ (alite, C₃S) 2 CaO + Al₂O₃ + Fe₂O₃ → 2 CaO·Al₂O₃·Fe₂O₃ (tetracalcium aluminoferrite, C₄AF)
These reactions produce the four main clinker minerals: alite (C₃S), belite (C₂S), aluminate (C₃A), and ferrite (C₄AF).
Main Steps in Cement Production
- Crushing and Grinding – Limestone and clay are crushed to ≈10 mm size, then ground in ball mills to a fine powder known as raw meal.
- Blending and Proportioning – The raw meal is homogenized to achieve the target oxide composition (see table above).
- Pre‑heating and Calcination – The meal passes through a series of cyclones and a pre‑calciner where CaCO₃ decomposes (≈800‑900 °C), releasing CO₂.
- Clinker Burning (Rotary Kiln) – The pre‑heated meal enters a long, inclined rotary kiln rotating at 1‑2 rpm. Temperature peaks at 1400‑1500 °C, where solid‑state reactions form clinker nodules.
- Cooling – Hot clinker falls into a grate cooler where ambient air reduces its temperature to ≈100 °C, recovering heat for the pre‑heater.
- Final Grinding – Cooled clinker is ground with 3‑5 % gypsum in a cement mill to produce the final powder. Gypsum retards the flash set caused by C₃A.
- Storage and Packaging – The cement is stored in silos and packed in bags or dispatched in bulk.
Types of Cement
Ordinary Portland Cement (OPC)
OPC is the standard Portland cement containing ≈95 % clinker and 5 % gypsum. It develops compressive strength of about 20 MPa after 7 days and 30‑40 MPa after 28 days.
Portland Pozzolana Cement (PPC)
PPC blends OPC with a pozzolanic material such as fly ash (15‑35 % by weight) or volcanic ash. The pozzolan reacts with calcium hydroxide released during hydration to form additional C‑S‑H, improving durability and reducing permeability.
- Advantages: lower heat of hydration, better resistance to sulfates, reduced CO₂ footprint.
- Typical strength: comparable to OPC at later ages (≥28 days) with improved long‑term performance.
Other Variants (brief)
- Portland Slag Cement (PSC): OPC + granulated blast‑furnace slag.
- White Cement: Low iron oxide raw materials, used for architectural finishes.
- Rapid Hardening Cement: Higher C₃S content, finer grinding.