GYPSUM PLASTER — a non-hydraulic binder created through the grinding and calcination of gypsum rock.

Generally, two types of gypsum plaster should be distinguished: hemihydrate plaster, commonly known as plaster of Paris, which rapidly solidifies within minutes when mixed with water, and anhydrous calcium sulphate, which sets more slowly and is used to make casts and stucco decorations. While the physical properties of these two forms of calcium sulphate may display slight variations, their chemical properties remain fundamentally the same.

Unlike cement plaster, gypsum plaster does not need special processes to attain a smooth surface. It is lighter and sets quicker.

However, tilers see gypsum plaster as a weak material: when in direct contact with cement based adhesive, it forms a layer of ettringite that can prevent the adhesive from properly adhering to the plaster. From a tiler’s perspective, therefore, gypsum plaster is not the best substrate due to its limited load-bearing capacity of 20 kg/m² making it unsuitable for many tile installations.

It is important to keep in mind, also, that gypsum plasters generally prove unsuitable for re-plastering old buildings or undertaking conservation work. They possess excessive hardness and brittleness, hindering their ability to flex and adapt to building movements. Moreover, most of them will break down in the presence of moisture. Those plasters that resist dampness incorporate water repellents, effectively sealing the wall's surface and impeding its ‘breathability’.