Gypsum plaster speaks to types of gypsum plaster, what is gypsum plaster and why gypsum plaster.
This explains that gypsum is an abundant mineral in nature and is a crystalline hydrous calcium sulfate. It is quarried, crushed, dried, ground to a fine powder and heated to 350 F (175 C) in a process known as calcining to drive off about three quarters of its water of hydration.
The calcined gypsum, ground to a fine white powder, is known as plaster of Paris. When plaster of Paris is mixed with water, it rehydrates and recrystallizes rapidly to return to its original solid state. As it hardens, it gives off heat and expands slightly.
Gypsum is a major component of interior finish materials in most buildings. It has but one major disadvantage which is its solubility in water.
Among its advantages are that it is durable and light in weight compared to many other materials. It resists the passage of sound better than most materials. It has a very fine grain, is easily worked in either its wet or dry state and can be fashioned into surfaces that range from smooth to heavily textured. But above all it is inexpensive and it is highly resistant to fire.
When a gypsum building component is subjected to the intense heat of a fire, a thin surface layer is calcined and gradually disintegrates. In the process, it absorbs considerable heat and gives off steam, both of which cool the fire. Layer by layer, the fire works its way through the gypsum, but the process is slow.
The uncalcined gypsum never reaches a temperature more than a few degrees above the boiling point of water, so areas behind the gypsum component are well protected from the fire’s heat. Any required degree of fire resistance can be created by increasing the thickness of the gypsum as necessary.
The fire resistance of gypsum can also be increased by adding lightweight aggregates to reduce its thermal conductivity and by adding reinforcing fibers to retain the calcined gypsum in place as a fire barrier.
For use in construction, calcined gypsum is carefully formulated with various admixtures to control its setting time and other properties. Gypsum is made by mixing the appropriate dry plaster formulation with water and an aggregate, either fine sand or a lightweight aggregate such as perlite or vermiculite. Because of its expansion during setting, gypsum is very resistant to cracking.