Building Construction Handbook, Eighth Edition

Plaster ~ this is a wet mixed material applied to internal walls as a

finish to fill in any irregularities in the wall surface and to provide

a smooth continuous surface suitable for direct decoration. The

plaster finish also needs to have a good resistance to impact

damage. The material used to fulfil these requirements is gypsum

plaster. Gypsum is a crystalline combination of calcium sulphate

and water. The raw material is crushed, screened and heated to

dehydrate the gypsum and this process together with various

additives defines its type as set out in BS EN 13279-1: Gypsum

binders and gypsum plasters. Definitions and requirement.

Raw material (gypsum) is

heated to 150 to 170ƒC to drive

off 75% of the combined water

Expanded perlite and other

additives added giving ...

One coat plaster;

Renovating grade plaster;

Spray plaster.

Usual lightweight additives:-

Expanded perlite giving ...

Browning plaster type C3/20.

Exfoliated vermiculite +

perlite + rust inhibitor giving ...

Metal lath plaster and

Exfoliated vermiculite giving ...

Bonding plaster types B4/20/2

and C3/20.

Retarder added giving ...

Retarded hemi hydrate finish

plaster type B1/20/2

Lightweight aggregates added

giving ... Premixed lightweight

plaster

Hemi-hydrate Plaster of Paris

Plaster of Paris is quick setting

plaster (5 to 10 minutes) and is

therefore not suitable for walls

but can be used for filling cracks

and cast or run mouldings.

Browning and Bonding plasters are

used as undercoats to Premixed

lightweight plasters.

All plaster should be stored in dry conditions since any absorption

of moisture before mixing may shorten the normal setting time of

about one and a half hours which can reduce the strength of the

set plaster. Gypsum plasters are not suitable for use in

temperatures exceeding 43°C and should not be applied to frozen

backgrounds.

A good key to the background and between successive coats is

essential for successful plastering. Generally brick and block walls

provide the key whereas concrete unless cast against rough

formwork will need to be treated to provide the key.

Plasters