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Last edited 29 Dec 2023

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Gypsum plaster

1 What is gypsum plaster?
2 Types of gypsum plaster
3 Plaster applications
4 Related articles on Designing Buildings

[edit] What is gypsum plaster?

Gypsum plaster is a building product which has gypsum as its base, often mixed with other elements, and is one of the most common finishes for interior walls. It may be based on naturally occurring gypsum or synthetic gypsum, or a combination of both.

[edit] Types of gypsum plaster

[edit] Standard gypsum plaster

Standard gypsum plaster is produced by heating gypsum to around 150°C. When mixed with water, the dry plaster powder re-forms into gypsum. Unmodified plaster starts to set about 10 minutes after mixing, but it will not be fully set until 72 hours have elapsed. Gypsum is a hydrous mineral which has two water molecules attached (CaSO4·2H2O) and as plaster has good fire-resistant qualities.

[edit] Anhydrite gypsum plaster

Anhydrite gypsum plaster is manufactured by heating gypsum to around 1700°C, it is a harder and denser type of plaster than gypsum and is normally found in powder form, often with a bluish tint. Both gypsum and anhydrite are used as soil additives as well as in plasters and additives to clinker which is used in the manufacture of Portland cement. Anhydrite is an anhydrous mineral and therefore has no water molecules attached (CaSO4) and is used as a a drying agent in plasters and cements.

[edit] Hemihydrate gypsum plaster

Hemihydrate gypsum plaster is produced when gypsum is heated to more than 1700°C. It is often referred to as Plaster of Paris or calcium sulphate hemihydrate because the two formula units of CaSO4 share one molecule of H2O. It is commonly used as a binder for gypsum plaster.

[edit] Hydroxypropyl methyl cellulose (HPMC / MHPC) and methyl cellulose (MC)

Hydroxypropyl methyl cellulose and methyl cellulose might also be referred to as cellulose ethers, and are additives that are used in different types of plasters. They add characteristics to the plaster such as:

Water retention; with a high water-holding capacity cellulose can help improve resistance, to prevent cracking, maintaining surface uniformity, lubrication, and adhesion.
Strength; it increases thickness, tensile strength, flexibility as well as adhesion qualities and the weathering strength of gypsum based plasters.
Stabilisation; as it forms a film over the surface it improve the stability of plaster in the longer term.

[edit] Plaster applications

There are many different mixes, grades and type of gypsum plaster (as well as other types of plaster such as lime, earth or clay plasters). The different mixes and types have different characteristics and qualities and therefore different applications, some of which are listed below:

Casting gypsum plasters, sometimes referred to as stone plasters are used for fibrous plaster work, glass reinforced structures, and to cast decorative plasterwork pieces such as coves and ceiling roses or sculptures.
Undercoat gypsum plasters, sometimes referred to a brownings due to their colour act as a bonding coat because they are normally the first coat of plaster that bonds to surfaces being finished, such as brick or blockwork. They are also referred to as thistle or hardwall.
Finish gypsum plasters, also referred to as a finish coat are fine plasters that give a hard smooth finish, similar to casting plasters. They are often a light pink in colour due to aggregates and processing, which may include clay, and lime. They dry to a brownish pink colour. They are sometimes referred to as carlite.
One coat gypsum plasters are specific mixes that might be more costly but have both the ability to bond well and maintain a smooth surface. They are normally white in colour and dry to a hard finish.

Other types of plaster finish include machine applied gypsum plasters, which are applied with a specifically-designed plaster tool. The mixes and additives are usually designed around the needs of the machine to function efficiently.

Other terms might be used to describe the finished application of plaster which may include lime, clay, earth, different additives and mixtures, but also different tools and methods of finishing the plaster.

Venetian plaster is fired limestone and water, with no aggregates in the mix to give a smooth dark pink finish.
Tadelakt, sometimes called Moroccan plaster is a form of lime plaster, which gives a rustic light coloured finish.
Marmorino plaster is made from lime putty, or slaked lime, with the addition of ground marble to create a stone-like finish.
Stucco plaster, might be used to describe external plaster with a rougher texture but also decorative interior plaster.

Other terms that might be found relating to both internal and external plaster finishes are pebble dash, depeter, exposed, textured, scrapped, rough cast and sand faced.