Cement mill temperatures normally lie in the range 100–130°C.

Under these conditions the calcium sulfate dihydrate (gypsum) added to the mill undergoes dehydration first to hemihydrate (CaSO4·0.5H2O) and then to soluble anhydrite (CaSO4).

These dehydrated forms of gypsum are present in commercial plasters and it is the formation of an interlocking mass of gypsum crystals which is responsible for the hardening of plaster once mixed with water.

The dehydrated forms of gypsum dissolve more rapidly than gypsum and this is beneficial in ensuring that sufficient Ca2+ and SO42– ions are available in solution to control the initial reactivity of C3A by forming a protective layer of ettringite.

An inadequate supply of soluble calcium sulfate can result in a rapid loss of workability known as flash

set.

This is accompanied by the release of heat and is irreversible.

However, if too high a level of dehydrated gypsum is present, then crystals of gypsum crystallize from solution and cause a plaster or false set. This is known as false set because if mixing continues, or is resumed, the initial level of workability is restored.