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What is Plaster?
Basically, plaster is made up of three main ingredients: an aggregate, a binding agent and water. Aggregate is small particles of a hard material. Sand is an example of an aggregate and makes up the bulk of plaster mortar. Some of the Italian plasters use marble dust as their aggregate. Binding agents, or cementitious materials, such as cement, lime or gypsum, when mixed with water form the glue that makes the aggregate stick together and become one solid unit. Portland cement is a binding agent used primarily on mixtures for exterior wall surfaces and gypsum primarily used as a binder for interior surface mixtures.
To the basic plaster recipe, other ingredients can be added which will give the plaster different qualities. For example, retarders or accelerators can be added to slow down or speed up the rate of set. Sometimes, plaster of paris is added to a mortar to accelerate how quickly it sets up and color is also often added so that there is no further need to paint the surface.
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