The raw materials needed to produce cement (calcium carbonate, silica, alumina, and iron ore) are generally extracted from limestone rock, chalk, shale, or clay. These raw materials are won from the quarry by either extraction or blasting.
These naturally occurring minerals are then crushed through a milling process. At this stage, additional minerals are added to ensure the correct chemical composition for making cement. These minerals can be obtained from the waste or by-products of other industries, such as paper ash. Milling produces a fine powder, known as raw meal, which is preheated and then sent to the kiln for further processing.
The kiln is at the heart of the manufacturing process. Once inside the kiln, the raw meal is heated to around 1,500 degrees C – a similar temperature to that of molten lava. At this temperature, chemical reactions take place to form cement clinker, which contains hydraulic calcium silicates.
In order to heat the materials to this very high temperature, a 2,000-degree C flame is required, which can be produced through the use of fossil and waste-derived fuels. The kiln itself is angled by 3 degrees to the horizontal to allow the material to pass through it, over a period of 20 to 30 minutes.
Upon exiting the kiln, the clinker is cooled and stored, ready for grinding, to produce cement.
A small amount of gypsum (3 percent to 5 percent) is added to the clinker to regulate how the cement will set. The mixture is then very finely ground to obtain “pure cement.” During this phase, different mineral materials, called “additions,” may be added alongside the gypsum. Used in varying proportions, these additions, which are of natural or industrial origin, give the cement specific properties, such as reduced permeability, greater resistance to sulfates and aggressive environments, improved workability, or higher-quality finishes.
Finally, the cement is stored in silos before being shipped in bulk or in bags to the sites where it will be used.