Gypsum is a particularly useful processed material. Its main application is as a building
material, mostly produced as so-called plaster of Paris for plastering walls and making
decorative features in buildings. However gypsum also has a diversity of other uses including
making writing chalk, soil conditioning for agriculture, making moulds for pottery, as an
additive in the manufacture of Ordinary Portland Cement (OPC), in surgical splints, in
increasing the hardness of water, as an additive in certain foods such as tofu and in some
types of medicines and pharmaceuticals. Its construction uses can also include external
applications, such as in making walling blocks, where the climate is generally dry and not
prone to the dampness that damages common types of gypsum.
Gypsum can be processed relatively simply with basic equipment and techniques as well as
on a large and industrialized scale as is done in some countries such as the United Kingdom.
It is made from deposits of gypsum rock or sand. Raw gypsum is heated to drive off a portion
of the chemically combined water. The resulting gypsum plaster, when re-combined with
water, sets like cement, only much faster.
The main advantage of gypsum plaster
over some other binders is that
temperatures of only 120 to 160°C are
required during production, so high-
temperature kilns are not needed.
Gypsum rock (calcium sulphate
dihydrate – CaSO4.2H2O) was formed in
geological time through the evaporation
of seawater. It is often laid down in
beds, ranging in thickness from a few
centimetres to several tens of metres.
Because gypsum rock is slightly soluble
in water it is not usually found above
ground in wet or damp areas, but may
Figure 1: Mould for making gypsum plaster wall
be found underground, where it is not
affected by the water table. In dry regions it may also be found on the surface, sometimes in
the form of gypsum sand. The presence of gypsum on or just below the surface of the ground
is often indicated by changes in vegetation; some plants thrive on gypsum-rich (alkaline) soil
whereas others are not at all tolerant of those conditions.
Gypsum rock is usually white or colourless, although it may sometimes have grey, yellow,
pink, or brown hues. Gypsum is much softer than minerals of similar colour, such as calcite
or quartz, and is the only one that can be scratched with a fingernail. If a piece of gypsum
rock is held over a flame it will turn cloudy and opaque and give off water.
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