Plasters that are used in the construction industry are made of gypsum and anhydrite. Anhydrite is not commonly used anymore.
Gypsum has calcium sulphate and crystallising waters in it and it is a soft rock. The gypsum is crushed and then heated until most of the waters of crystallisation have gone. The material that is left is then grinded. The finished material is known as plaster of paris (hemi-hydrate plaster).
The plaster can be made back into gypsum again just by adding water because of the waters of crystallisation. The plaster will only take in as much as water as was in it when it was first found as gypsum.
Plaster of paris is a Class A plaster, but class A plasters dry out too quickly so a retarded gets added during the manufacturing process. This slows down the setting time and it then becomes a Class B plaster.
Class C plaster are called anhydrous gypsum plaster and all of the waters of crystallization were removed when being manufactured. They set slowly and have a harder finish than hemi-hydrate plasters.
Class D plasters are heated at a very high temperature and have certain materials added to make it set faster. These plasters are used in places such as external angles and reveals where it is more likely to get damaged because class D plasters are harder than the others. These plasters are also called gypsum cement and they can not be used on plasterboard.
The plaster must not be around any moisture before being used. If a small amount gets damp then the whole batch will be affected when it comes to the setting time which is why it must be stored away from the ground and under a cover.
Certain plasters should never be mixed together, for example class a plasters should not be mixed with class b plasters.
All tools and mixing equipment should be kept clean and the water used when mixing should be clean because any impurity will affect the setting times and strength of the plaster.