Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard/sidewalk chalk, and drywall. A massive fine-grained white or lightly tinted variety of gypsum, called alabaster, has been used for sculpture by many cultures including Ancient Egypt, Mesopotamia, Ancient Rome, the Byzantine Empire, and the Nottingham alabasters of Medieval England. Gypsum also crystallizes as translucent crystals of selenite. It forms as an evaporite mineral and as a hydration product of anhydrite.
The Mohs scale of mineral hardness defines gypsum as hardness value 2 based on scratch hardness comparison.
Gypsum is used in a wide variety of applications:
- Gypsum board is primarily used as a finish for walls and ceilings, and is known in construction as plasterboard, sheet rock, or drywall.
- Gypsum blocks are used like concrete blocks in building construction.
- Gypsum mortar is an ancient mortar used in building construction.
- Plaster ingredients are used in surgical splints, casting moulds and modeling.
- Fertilizer and soil conditioner: In the late 18th and early 19th centuries, Nova Scotia gypsum, often referred to as plaster, was a highly sought fertilizer for wheat fields in the United States. It is also used in ameliorating high-sodium soils, such as in the Zuiderzee Works.
- Used for reclamation of saline soils: when gypsum is added to sodic and acidic soil the highly soluble form of boron (sodium metaborate) is converted to the less soluble calcium metaborate and exchangeable sodium percentage is also reduced by gypsum application.
- A binder in fast-dry tennis court clay.
- As alabaster, a material for sculpture, it was used especially in the ancient world before steel was developed, when its relative softness made it much easier to carve.
- A wood substitute in the ancient world: For example, when wood became scarce due to deforestation on Bronze Age Crete, gypsum was employed in building construction at locations where wood was previously used.
- A tofu (soy bean curd) coagulant, making it ultimately a major source of dietary calcium, especially in Asian cultures, which traditionally use few dairy products.
- Adding hardness to water used for brewing.
- Used in baking as a dough conditioner, reducing stickiness, and as a baked-goods source of dietary calcium. The primary component of mineral yeast food.
- A component of Portland cement used to prevent flash setting of concrete.
- Soil/water potential monitoring (soil moisture).
- A common ingredient in making mead.
- In the medieval period, scribes and illuminators mixed it with lead carbonate (powdered white lead) to make gesso, which was applied to illuminated letters and gilded with gold in illuminated manuscripts.
- In foot creams, shampoos and many other hair products.
- A medicinal agent in traditional Chinese medicine called shi gao.
- Impression plasters in dentistry.
- Used in mushroom cultivation to stop grains from clumping together.
- Tests have shown that gypsum can be used to remove pollutants such as lead or arsenic from contaminated waters.