Raw materials of cement
Cement needs lime (calcium), silica, alumina and a little iron oxide.
- Limestone (calcium carbonate, CaCO3) gives the lime. About 3 parts are used.
- Clay gives silica, alumina and iron. About 1 part is used.
- Gypsum (calcium sulfate) is added at the very end, in small amount (about 3 to 5 percent).
Some factories also use shale, sand or industrial by-products such as slag instead of part of the clay.
Manufacturing process, step by step
- Crush and grind. Limestone and clay are crushed, then ground into fine raw meal and mixed in the right ratio.
- Burn in the kiln. The meal slides down a long, slightly tilted, slowly turning steel tube. Flame at the low end heats it to about 1450 °C. First, limestone breaks down: CaCO3 gives CaO + CO2. Then lime joins silica, alumina and iron to form hard grey lumps called clinker.
- Cool. Clinker is cooled quickly with air.
- Grind with gypsum. Clinker is ground to a very fine powder. Gypsum is added so that cement does not set too fast.
- Pack. The powder is stored in silos and filled into bags.
Two ways exist: the dry process (dry meal, saves fuel) and the wet process (meal mixed with water into slurry, older, needs more fuel).
Setting and hardening
When water is added, cement does not just dry. It reacts with water. This is hydration. The new compounds grow as tiny crystals that lock sand and stones together.
- Setting: the paste stops being soft. It takes about 30 minutes to some hours.
- Hardening: strength keeps growing. Most of it comes in 7 days, and the standard test is at 28 days.
- Without gypsum, cement would stiffen almost at once (a flash set) and could not be worked.
- The reaction gives out heat, so big pours get warm.
Properties and uses of cement
Properties: fine grey powder; sets and hardens with water, even under water; very strong when squeezed (compression), weak when pulled (tension); does not burn; lasts for decades.
Uses:
- Mortar (cement + sand + water): joins bricks, plaster on walls.
- Concrete (cement + sand + stones + water): floors, roads, bridges, dams.
- Reinforced concrete: concrete with steel bars, so it also resists pulling.
- Pipes, blocks, tiles and poles.
Care: keep bags dry, wear gloves and a mask (cement dust and wet mix irritate skin and lungs). Making cement gives off CO2 from the limestone and the fuel, so engineers try to use less clinker.
Try it
In the 3D, move the days slider from 0 to 28 and read the strength. Then predict: at 7 days, is it more or less than half? Check. At home, mix a spoon of plaster of Paris or clay with water and watch how it stiffens; ask an adult to show a cured wall with a wet cloth.
Key formulas and definitions
- Limestone + heat: CaCO3 → CaO + CO2
- Cement = ground clinker + about 5% gypsum
- Concrete = cement + sand + stones + water
- Raw meal burns at about 1450 °C
- Standard strength test: 28 days
Worked examples
1. Name the two main raw materials of cement and what each gives.
Limestone gives lime (calcium). Clay gives silica, alumina and iron oxide.
2. A factory makes 100 kg of cement powder with 5 percent gypsum. How much clinker is used?
Gypsum = 5 kg. Clinker = 100 − 5 = 95 kg.
3. Why is a freshly laid floor kept wet for several days?
Hardening is a reaction with water. If the floor dries out, the reaction stops early and the floor stays weak and cracks. Wet curing keeps the reaction going.
4. A mix uses 1 part cement, 2 parts sand and 4 parts stones by volume. How many parts of cement are in 14 parts of dry mix?
Total parts = 1 + 2 + 4 = 7. So 14 parts is two batches, and cement = 2 × 1 = 2 parts.
Common mistakes
- Saying concrete and cement are the same. Cement is the binder powder; concrete is cement plus sand, stones and water.
- Thinking cement hardens by drying. It hardens by reacting with water, even under water.
- Forgetting gypsum. It is added to slow the setting, not to make cement stronger.
- Saying clay is burned alone. The limestone and clay meal is burned together to make clinker.