why is gypsum always interground with cement clinker during the final stages of portland cement manufacture?
Gypsum is added to cement to control the setting. Cement contains C3A and C3S as the major constituents. These compound react very rapidly under water. This is not desirable as it reduces the workability of the cement past. In order to control this early setting, gypsum is added to form a secondary compound called Ettringite which kind of shields the C3A and C3S from reacting rapidly with the water.
Because they are trying to dispose of Gypsum and this is one of the few ways that it can be blended with something else in large quantities. Gypsum is produced as a by-product form the de-sulphurisation of flue gasses from power plants that still burn dirty fuels like coal or oil residues. There is often thousands of tons of this stuff produced every day and it has to be dumped somewhere. Land fill sites are filling up rapidly and charge big fees for disposal so it is very attractivce to belnd it into cement products. Gypsum is also used as the main part of plaster board which is another place to get rid of the unwanted material.
