Cement milling is usually carried out using ball mills with two or more separate chambers containing different sizes of grinding media (steel balls).
Grinding clinker requires a lot of energy. How easy a particular clinker is to grind (“grindability”) is difficult to predict, but rapid cooling of the clinker is thought to improve grindability due to the presence of microcracks in alite and to the finer crystal size of the flux phases. It is frequently observed that belite crystals, which have a characteristic round shape, tend to separate and form single crystal grains during grinding.
As part of the grinding process, calcium sulfate is added as a set regulator, usually in the form of gypsum (CaSO4.2H2O). Natural anhydrite may also be added to discourage lumpiness of the gypsum due to its water content.
Since the clinker gets hot in the mill due to the heat generated by grinding, gypsum can be partly dehydrated. It then forms hemihydrate, or plaster of Paris – 2CaSO4.H2O. On further heating, hemihydrate dehydrates further to a form of calcium sulfate known as soluble anhydrite (~CaSO4). This has a similar solubility in water to hemihydrate, which in turn has a higher solubility than either gypsum or natural anhydrite.
Cement mills need to be cooled to limit the temperature rise of the cement. This is done by a mixture of both air-cooling and water-cooling, including spraying water inside the mill.
The relative proportions and different solubilities of these various types of calcium sulfate are of importance in controlling the rate the rate of C3A hydration and consequently of cement set retardation. Problems associated with setting and strength characteristics of concrete can often be traced to changes in the quantity of gypsum and hemihydrate, or with variations in cooling rate of the clinker in the kiln and subsequent changes in the proportions or size of the C3A crystals.
For set regulation, the most important feature of aluminate is not necessarily the absolute amount present, but the amount of surface which is available to water for reaction. This will be governed by many factors, such as the surface area of the cement, the grinding characteristics of the different phases and also the size of the aluminate crystals. Over-large crystals can lead to erratic setting characteristics.