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Gypsum is a soft sulfate mineral composed of calcium sulfate di-hydrate, with the chemical formula CaSO4·2H2O. It can be used as a fertilizer, is the main constituent in many forms of plaster and is widely mined. 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, Byzantine empire and the Nottingham alabasters of medieval England. It is the definition of a hardness of 2 on the Mohr’s scale of mineral hardness. It forms as an evaporite mineral and as a hydration product of anhydrite.
Gypsum is a common mineral, with thick and extensive evaporite beds in association with sedimentary rocks. Deposits are known to occur in strata from as far back as the Archaean eon. Gypsum is deposited from lake and sea water, as well as in hot springs, from volcanic vapors, and sulfate solutions in veins. Hydrothermal anhydrite in veins is commonly hydrated to gypsum by groundwater in near-surface exposures. It is often associated with the minerals halite and sulfur. Pure gypsum is white, but other substances found as impurities may give a wide range of colors to local deposits. Gypsum dissolves over time in water, gypsum is rarely found in the form of sand.
Physical Properties
Gypsum is moderately water-soluble (~2.0–2.5 g/l at 25°C) and, in contrast to most other salts, it exhibits a retrograde solubility, becoming less soluble at higher temperatures. When gypsum is heated in air it loses water and converts first to calcium sulfate hemi hydrate, (bassanite, often simply called “plaster”) and, if heated further, to anhydrous calcium sulfate (anhydrite). As for anhydrite, its solubility in saline solutions and in brines is also strongly dependent on NaCl concentration. Gypsum crystals are found to contain anion water and hydrogen bonding.
Advantages of gypsum
- Economical
- Easy workability
- Fire resistant
- Light weight
- Sound insulation
- Strength and durability
- Provides smooth surface
- Sharp finishing
Gypsum plaster / Plaster of Paris
Gypsum plaster, or the plaster of Paris, is produced by heating gypsum to about 300 °F (150 °C)
CaSO4·2H2O + heat → CaSO4·0.5H2O + 1.5H2O (released as steam).
When the dry plaster powder is mixed with water, it re-forms into gypsum. The setting of unmodified plaster starts about 10 minutes after mixing and is complete in about 45 minutes; but not fully set for 72 hours. If plaster or gypsum is heated above 392°F (200°C), anhydrite is formed, which will also re-form as gypsum if mixed with water.
A large gypsum deposit at Montmartre in Paris led “calcined gypsum” (roasted gypsum or gypsum plaster) to be commonly known as “plaster of Paris”. Plasterers often use gypsum to simulate the appearance of surfaces of wood, stone, or metal, on movie and theatrical sets for example. Nowadays, theatrical plasterers often use expanded polystyrene, although the job title remains unchanged.
Plaster of Paris can be used to impregnate gauze bandages to make a sculpting material called modroc. It is used similarly to clay, as it is easily shaped when wet, yet sets into a resilient and lightweight structure. This is the material that was (and sometimes still is) used to make classic plaster orthopedic casts to protect limbs with broken bones, the medical use having been partly inspired by the artistic use (see orthopedic cast). Set modroc is an early example of a composite material.
Plaster boards
Plaster boards are made of large sheets of gypsum plaster faced on both sides with stout paper as reinforcement. These boards are used for partitions and the lining of walls, ceilings, roofs and roofs. The properties of plasterboard can be modified to meet the specific demands and requirements such as fire resistance, humidity resistance, shock resistance, etc.
Pyrocell
When water is added to finely ground gypsum powder, gas is liberated and the volume expands by almost 3 to 4 times and hardens after some time. This mixture is known as pyrocell and is light, cellular and fire resistant. It is commonly used in acoustical and insulating purposes in building construction
