Gypsum Processing and Use
Practical Action
Some gypsum sand deposits contain only about 60 per cent gypsum, and these are not very
suitable for producing a plaster; those containing more than 80 per cent would be most
suitable.
Gypsum is also produced as a by-product of the phosphate fertilizer industry, but in some
cases this is slightly radioactive and little use is made of it at present, except as an additive
in the manufacture of Portland cement. More recently by-product gypsum is also available
from flue gas desulphurisation; a process for cleaning up emissions from chimneys to reduce
the quantity of sulphurous gases flowing into the atmosphere.
Dehydration: Rock into plaster
Gypsum rock is converted into gypsum plaster by driving off some of the chemically combined
water. Heating gypsum at 120°C for one hour results in a hemi-hydrate (CaSO4.1⁄2H2O) –
with three quarters of the water removed. Gypsum hemi-hydrate is also known as Plaster of
Paris. Prolonged heating over several hours results in the formation of anhydrite with
practically none of the chemically combined water left. Anhydrite sets more slowly and is a
slightly stronger plaster than hemi-hydrate, but with the drawback of added production cost.
In practice, a simple production system would most likely give a mixture of the hemi-hydrate
and anhydrite phases. Much of the commercial plaster produced industrially today is Plaster
of Paris. Gypsum plaster sets by chemically combining with water to form solid calcium
sulphate dihydrate.
A special type of plaster, called alpha hemi-hydrate, can be produced by reacting the raw
material under high pressure steam in strictly controlled conditions. This plaster sets harder
and is used in dental work and as a modelling plaster, but is impossible to produce without
expensive production equipment.
Steel plate
Cooling area
Fuel
Figure 2: One simple production method is to heat the gypsum on a flat metal plate above a
kiln
Properties of gypsum plaster
The properties of gypsum plaster will vary depending on the method of production. With
relatively simple burning and processing techniques the plaster that will result will have
varying amounts of hemi-hydrate, anhydrite, ash, and possibly some sand, silt, and clay.
In general, gypsum plaster sets very rapidly when mixed with water, so only small quantities
should be mixed at a time and used almost immediately. Gypsum plaster will begin to stiffen
in about five minutes and become completely rigid in under twenty. The addition of retarders
can delay this process by as much as half an hour. Lemon juice, borax, sugar, molasses, fish
liquor, and keratin (made from animal hoofs and horns) can all be used, and the actions of
some of these are enhanced by a small quantity of building lime.
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