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In this instalment of Concrete for Beginners, we learn about the substance that binds all the ingredients together – cement.


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Cement Compounds

What is Cement?

A lot of people are unclear about the difference between cement and concrete and use the words interchangeably. Cement is the binder used in the creation of concrete. When cement is combined with sand, aggregate particles and water, the result is concrete.

How is it Made?

Cement is a manufactured substance. It is made by combining limestone (CaCO3) with silicates (shale and sand),aluminates (such as bauxite) and iron oxide at approximately 1450 °C to form clinker. The clinker is then interground with gypsum to control the setting of cement. If cement is manufactured with low levels of iron and magnesium, the result is white cement.

Two main reactions occur during cement manufacturing. The first is calcination of limestone during which CO2 is driven off from the limestone to produce CaO (lime). The second is that the lime reacts with the silicates and aluminates to form 4 main compounds of cement (C3S, C2S, C3A and C4AF).

Cement Compounds

• C3S (Tricalcium Silicate) is the compound responsible for early strength and durability.

• C2S (Diacalcium Silicate) is the compound responsible for late strength and durability.

• C3A (Tricalcium Aluminate) is a compound that, when used in large quantities, can reduce workability and affect setting time.

• C4AF (Tetraclacium Aluminoferrite) is the compound that gives cement its grey colour.

The amount of free lime and sulphates affects the quality of cement. To produce a consistently good quality product, the quantity of both substances is carefully controlled within specific limits.


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