What is plaster of paris made of

What is plaster made of?

Gypsum: a common mineral with a long history

Gypsum was used as a building material as early as the Neolithic Age. In ancient Greece, too, people were already well acquainted with the methods of manufacturing and processing plaster as a raw material for splendid ornaments on buildings. Although gypsum is not weatherproof even when it has set, gypsum with its specific material properties was able to convince, not least because of its favorable availability in large quantities.

While gypsum in today's parlance can mean both the gypsum powder and the hardened material, the term “gypsum spar” should actually be used purely geologically for the naturally degraded material. From a chemical point of view, the starting product for gypsum production can also be referred to as water-containing calcium sulfate.

The chemical processes involved in processing gypsum

The naturally occurring gypsum spar is calcium sulfate dihydrate of different purity or mixed with other materials. This consists of the combination of calcium sulfate with 2 water molecules and is described with the chemical formula Ca [SO4] · 2H2O. In principle, various sources can be considered for the material:

  • Open pit
  • Underground mining
  • By-products from chemical processes and the desulphurisation of coal exhaust gases

Plaster of paris, which is white in its pure form, consists of monoclinic crystal systems and is characterized by the fact that so-called crystal water is stored through the crystallization and the plaster of paris only becomes really hard as a result. When burning plaster of paris, the calcium sulfate dihydrate is heated to temperatures between 120 and 130 degrees. This removes water of crystallization from the substance and reduces it to hemihydrate. This creates the potential to turn gypsum into a fast-hardening casting and leveling compound by mixing it with water.

Plaster of paris in different variations

Nowadays, in addition to different types of plaster for different areas of application, there is also gypsum plaster. Certain proportions of other ingredients can be added to these product variants in order to achieve specially desired properties such as faster or delayed setting.

Author: Sara Müller - Note: All information is provided without guarantee.
Article image: PetchyPetchyFOTO / Shutterstock
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