Gypsum plaster

Gypsum plaster is produced by heating natural gypsum (calcium sulfate dihydrate, CaSO₄·2H₂O) to drive off part of the water of crystallization, yielding calcium sulfate hemihydrate (CaSO₄·½H₂O), commonly known as plaster of Paris. When mixed with water, the hemihydrate rehydrates to form interlocking crystals of gypsum, which give the material its strength and hardness.¹ The setting time can be controlled by additives: accelerators like potassium sulfate speed up the reaction, while retarders such as glue or citric acid slow it down. The setting reaction is exothermic and slightly expands (about 0.5–1%), which helps fill molds and create sharp details in ornamental work.²

Gypsum plaster is primarily used for interior wall and ceiling finishes, offering a smooth, paintable surface that is resistant to cracking and shrinkage. It is also the basis for plasterboard (drywall), where a gypsum core is sandwiched between paper liners. The material's fire resistance is notable: the chemically bound water in gypsum acts as a heat sink, delaying temperature rise and providing valuable time in a fire.³ Gypsum plaster also regulates indoor humidity by absorbing and releasing moisture, contributing to a comfortable environment. In restoration work, it is favored for its compatibility with historic substrates, though it is not suitable for exterior use or persistently damp areas because it is slightly water-soluble.

Gypsum plaster has been used for millennia; ancient Egyptians employed it in pyramid construction and for wall finishes, and the name "plaster of Paris" derives from large gypsum deposits in the Montmartre district of Paris, which supplied the material for building and art in the 18th century.⁴ Modern production involves mining gypsum, crushing, and calcining in rotary kilns or flash calciners. Synthetic gypsum, a byproduct of flue-gas desulfurization at coal-fired power plants, now accounts for a significant share of raw material, reducing environmental impact. Advances in additives have led to lightweight, high-strength plasters and improved workability, while recycling of gypsum waste from construction sites is increasingly practiced.

Beyond conventional use, gypsum plaster has niche applications: it is used in orthopedic casts (as plaster of Paris bandages), in dental molds, and in art for sculptures and decorative elements. In agriculture, gypsum (not plaster) is used as a soil conditioner, but the same mineral is also used in tofu production as a coagulant. A lesser-known fact is that gypsum plaster can be used to create "scagliola," an imitation marble technique that dates to the 17th century, where colored gypsum plaster is applied and polished to mimic stone. Additionally, gypsum plaster has been used in the conservation of historic buildings because it is reversible and does not introduce salts that could damage masonry. The material's low embodied energy compared to cement plaster makes it a sustainable choice in green building certifications.

Calcination

Heating a mineral to drive off volatile components, such as water, to change its chemical composition.

Hemihydrate

The partially dehydrated form of gypsum, CaSO4·½H2O, which is the active component of plaster of Paris.

Plasterboard

A panel made of a gypsum core sandwiched between paper liners, used for walls and ceilings.

Flue-gas desulfurization

A process that removes sulfur dioxide from exhaust gases of coal-fired power plants, producing synthetic gypsum as a byproduct.