Cement is made by crushing and heating limestone with small amounts of other natural materials, such as clay or shale, in a rotating kiln to a temperature of 1450°c. This changes the stone chemically into a hard substance called clinker, essentially changing calcium carbonate (CaCO3) to calcium oxide (Ca0). 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.
The description that follows is of the limestone-based process.