Page 34 - Curriculum Visions Dynamic Book

Plastering. Plaster uses water to help it set. If it
is not made wet enough
or the wall not prewetted sufficiently, the plaster will not set properly. (Below) Adding a base coat of plaster to insulating bricks. (Left) Finishing off the wall with a thin surface coating.
Plaster
Plaster is the material most widely used to provide a durable skin to the inside of buildings. Plaster is gypsum (calcium sulphate CaSO4).
Gypsum is a naturally occurring white mineral. It is found in thick beds where it was once deposited as ancient lake beds evaporated. Gypsum is plentiful and easy and cheap to quarry.
Its main use is as a wall liner. But it is also made into wallboards and used to fill cracks. When it is poured into moulds, it is known as plaster of Paris.
Plaster is made by heating gypsum to remove some water. It is then crushed into a fine powder. When water is added, the plaster takes up the water and then sets hard, usually within a couple of hours.
Many grades of plaster are produced by mixing gypsum with a range of synthetic additives. For example, plasters are made that can be used on very porous surfaces, on surfaces that are not very porous,
as a base coat (which contains a lot of crushed quartz), and as a much finer finishing plaster (which contains less quartz that has also been crushed to a finer size).
When surfacing a wall, a plasterer will commonly put on a surface of the coarser plaster about 11 mm thick and then, when it is dry, go over it with a 3-mm coat of finishing plaster.
(Below) Gypsum crystal.
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