Last reviewed 16 Sept 2026 · 11 min read
Portland cement
Cement is a finely ground hydraulic binder that sets and hardens by chemical reaction with water and remains stable under water. Joseph Aspdin patented Portland cement in 1824, naming it after its resemblance to Portland stone.
Raw materials
| Material | Examples | Supplies |
|---|---|---|
| Calcareous | Limestone, chalk, marl, shells | Lime (CaO) |
| Argillaceous | Clay, shale, slate | Silica (SiO₂), alumina (Al₂O₃), iron oxide (Fe₂O₃) |
| Corrective materials | Bauxite, iron ore, sand | Adjust alumina, iron or silica |
| Gypsum (added to clinker during grinding) | Calcium sulphate | Retards setting — prevents flash set due to C₃A |
| Fuel | Coal, petcoke, alternative fuels | Heat |
Manufacture
Processes
| Wet process | Dry process |
|---|---|
| Raw materials ground with water to form a slurry (about 35–40% water) | Raw materials dried and ground to fine raw meal |
| High fuel consumption (evaporation of water) | Lower fuel consumption — used by modern plants with preheaters and precalciners |
| Better homogenisation (older technology) | Needs efficient blending; faster |
| Longer kilns | Shorter kilns with suspension preheaters |
Burning in the rotary kiln
The kiln is a long inclined steel cylinder lined with refractory bricks, rotating slowly; material moves down towards the hot end.
| Zone | Temperature (approx.) | Reactions |
|---|---|---|
| Drying zone | Up to about 100–200 °C | Evaporation of free water |
| Preheating / calcination zone | About 600–900 °C | Dehydration of clay; calcination of limestone (CaCO₃ → CaO + CO₂) |
| Burning (clinkering) zone | About 1400–1500 °C | Lime combines with silica, alumina and iron oxide to form clinker compounds; partial fusion |
| Cooling zone | Clinker cooled rapidly | Rapid cooling improves quality (glassy phases, less free lime) |
The clinker (dark nodules, a few mm to 25 mm) is cooled, stored and ground with about 3–5% gypsum (and other additions for blended cements) to a fine powder, then stored in silos and packed (commonly 50 kg bags) or dispatched in bulk.
Composition of cement
Oxide composition (typical ranges for OPC)
| Oxide | Approx. % | Remarks |
|---|---|---|
| Lime (CaO) | 60–67 | Excess causes unsoundness (free lime); deficiency reduces strength |
| Silica (SiO₂) | 17–25 | Strength (forms silicates) |
| Alumina (Al₂O₃) | 3–8 | Quick setting; lowers clinkering temperature |
| Iron oxide (Fe₂O₃) | 0.5–6 | Colour, fluxing, contributes to C₄AF |
| Magnesia (MgO) | 0.1–4 | Excess causes unsoundness (limited by standards) |
| Sulphur trioxide (SO₃) | 1–3 | From gypsum — controls setting; excess causes unsoundness |
| Alkalis (Na₂O, K₂O) | 0.2–1.3 | Efflorescence, alkali–aggregate reaction |
Bogue compounds
| Compound | Formula (cement notation) | Approx. % in OPC | Properties |
|---|---|---|---|
| Tricalcium silicate (alite) | C₃S — 3CaO·SiO₂ | 40–60 | Hydrates rapidly — early strength (first 7–14 days); high heat of hydration (about 500 J/g) |
| Dicalcium silicate (belite) | C₂S — 2CaO·SiO₂ | 15–30 | Hydrates slowly — later strength; low heat (about 260 J/g); better resistance to chemical attack |
| Tricalcium aluminate | C₃A — 3CaO·Al₂O₃ | 5–10 | Reacts very fast — flash set (controlled by gypsum); highest heat (about 870 J/g); vulnerable to sulphate attack |
| Tetracalcium aluminoferrite (celite) | C₄AF — 4CaO·Al₂O₃·Fe₂O₃ | 8–12 | Low strength contribution; gives grey colour; moderate heat |
FormulaBogue's equations (percentages by mass; no free lime)