Gypsum Production Process
The mineral gypsum is calcium sulphate dihydrate (CaSO4.2H2O) from
a chemical point of view. The production process starts with the
mining of the mineral, which is then ground to a powder. This
powder can be sold as a component to cement.
Inside the gypsum production process, the powder is dried until a
water content of 0.5% is reached. The resulting raw can be sold as
a soil conditioner, then called land plaster. If the raw gypsum is
heated (“calcined”) at 150°C to 165°C, three-quarters of its
combined water is removed to produce hemi-hydrate plaster
(CaSO4,1/2H2O), commonly known as stucco or ‘Plaster of Paris’.
When this powder is mixed with water the resulting paste sets hard
as the water recombines to produce gypsum again. Higher calcination
temperatures produce socalled anhydrite, which has a lower reaction
with water. In a last production step, the plaster can be mixed
with water and other components (additives, accelerators, etc.) to
produce gypsum blocks, or, if pressed between two sheets of paper,
plasterboard. gypsum reinforced with glass-fibre (Glass reinforced
gypsum – GRG) is used to increase its fire-resistance.
An important alternative to the use of natural gypsum is gypsum
that comes from a flue gas desulphurization plant (FGD) of the
power station industry. This FGD gypsum is the end product of a wet
purification procedure with natural lime, formed in the same but
speeded-up process as natural gypsum. This synthetic gypsum has a
higher purity (gypsum content of 96%) than most natural gypsum
(80%). On the other hand, it has also a higher wet content (8- 10%)
than natural gypsum (1-3%), leading to a higher energy input for
drying.
