Cement is made by crushing and heating limestone or chalk with small amounts of other natural materials, such as clay or shale, in a rotating kiln to a temperature of 1450o Celsius.
This chemically combines the stones into a hard substance called clinker, essentially changing calcium carbonate (CaCO3) to calcium oxide (CaO) which then reacts with silica (SiO2) to form calcium silicates. This is ground to a powder with about five per cent gypsum, added to control the setting time of the end-product.
The manufacturing process depends on the moisture content of the raw materials. Chalk has a high moisture content and chalk-based processes tend to be less energy efficient than those based on hard limestone.
MPA Cement is part of the Mineral Products Association, the trade association for the aggregates, asphalt, cement, concrete, dimension stone, lime, mortar and silica sand industries.
MPA Cement, Gillingham House, 38-44 Gillingham Street, London SW1V 1HU
T: 0207 963 8000