Gypsum Processing and Use
Practical Action
is to mix the gypsum stone and fuel in a mound or in a shallow pit in the ground and burn it.
Medium-scale batch production might be carried out in an excavated hillside kiln, a shaft kiln
with alternate fuel and stone layers, or a permanent walled kiln. An alternative method is to
heat the gypsum in large pans or on a flat metal plate positioned above a kiln. Industrial
production may be carried out in a purpose-built enclosed batch kiln, a continuously fed
vertical shaft kiln, a specially designed large kettle, or in rotary kilns. Kiln-based systems are
more efficient than burning the gypsum in mounds or pits, and even a small kiln may use less
than half of the fuel of a pit or mound.
It is easiest to judge when enough heat has been applied if an indirectly heated pan or metal
plate is used. When the temperature of the surface is increased steam will be produced and
the material seems to boil. The temperature is maintained until this ‘first boil’ is completed,
which removes all but a quarter of the water, and leaves hemi-hydrate plaster. If the
temperature is allowed to rise further this will start to convert to anhydrous plaster. Allowing
the material to cool naturally after
firing will help to remove some
Hopper
residual water left in the mass. It
is also important to stir the material
continuously to help the steam
Steam
vents
escape. If the material is heated in
a kiln then it is more difficult to
know when the removal of water has
taken place, and fuel usage and
loading would need to be judged
from experience.
As a rough guide, a field kiln
Thermometers
might require 0.2 tonnes of wood
for every tonne of raw gypsum
burned, or 70 litres of fuel oil,
Unloading
eye
although kiln efficiencies will vary
Stir holes
Crushed raw
gypsum
Furnace
widely and so would the optimum
amount of fuel to use.
Fuel
In some small batch kilns gypsum
Figure 5: Even a small kiln, such as this hopper
rock is burned in the form of lumps, fed furnace-heated batch kiln, may use less than
with the larger lumps nearest the
half of the fuel of a pit kiln
fire supporting the smaller stones
further away. After burning, the
gypsum would still need to be
crushed, screened and, possibly, ground down further in a mill.
Using gypsum plaster
Plaster of Paris is mixed with sand and water to
produce a mix suitable for plastering walls and
ceilings, for external rendering in dry climates, for
mortar, and for making building blocks. It can also be
used as a soil stabilizer in stabilized soil blocks. If
mixed into lime-based plasters and mortars, it will give
a fast initial set compared with the much slower-
setting lime.
Gypsum-based mortars and plasters are typically made
up of one volume of gypsum to two or three of sand,
with enough water added to make the mix sufficiently
workable. As the mix may start to stiffen only five
minutes after adding the water, it is important to make
4
Masonry cylinder 1 – 1.5m diameter
Raw gypsum
Fuel
Door for
removing fired
gypsum
Figure 6: A Continuously fed
vertical shaft kiln is an efficient
way of processing large amounts
of gypsum.