It is basically formed by a mixture of clay and limestone (calcium carbonate rock), which manufacturers call “flour”. “This mixture is taken to the oven at a temperature of 1450ºC, where it remains until the two elements merge”, says chemist Carlos Eduardo Tango, from the Technological Research Institute (IPT), in São Paulo. The result is small acorns that are called clinker. They are added with gypsum (the raw material for gypsum) and finally ground to a powder. The basic cement formula is ready. To harden and be used in construction, this powder needs to receive water, forming a paste. The chemical reactions that cause hardening reduce the water inside the paste, and a good part of this liquid also evaporates with the heat generated by the reaction – the loss is so great that the cement can crack. Sand goes into the mix to prevent this from happening.
As it is an inert material, it does not chemically react with the other elements of the formula – on the contrary, it thickens the mixture, preventing the cement from cracking. This combination of cement, sand and water is called mortar. But when the volume to be filled is too large, even the mortar can crack easily – in this case, we add gravel (ground stone) and we have concrete.
Cement powder is the result of a long and complex industrial process
1 – Trucks take the two cement raw materials to the factory (limestone and clay), initially stored in a shed
2 – The two materials are mixed and ground to a fine powder
3 – The resulting powder is stored in silos so that it is mixed until it is very homogeneous
4 – The mixture is taken to the so-called preheating tower, where it is subjected to temperatures of up to 900ºC
5 – In this huge cylindrical rotary kiln, at a temperature of 1450ºC, the mixture of limestone and clay rotates until it melts. Result: it condenses into acorns, called clinker
6 – Upon exiting the kiln, the clinker is stored in this circular deposit and left to cool down
7 – Now, the cement is almost ready. In this step, just add gypsum, the same material used to make plaster.
8 – The mixture is ground again until it becomes powder
9 – The final product is stored in silos and then packaged in bags for distribution
Cement is made from three ingredients
Clay + limestone + clinker + gypsum = cement