Gypsum

Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O.[3] It is widely mined and is used as a fertilizer, and as the main constituent in many forms of plaster, blackboard chalk and wallboard. 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 Mohs scale of mineral hardness. It forms as an evaporite mineral and as a hydration product of anhydrite.

 

Etymology and history

The word gypsum is derived from the Greek word γύψος (gypsos), “chalk” or “plaster”.[4] Because the quarries of the Montmartre district of Paris have long furnished burnt gypsum (calcined gypsum) used for various purposes, this dehydrated gypsum became known as plaster of Paris. Upon addition of water, after a few tens of minutes plaster of Paris becomes regular gypsum (dihydrate) again, causing the material to harden or “set” in ways that are useful for casting and construction.

Gypsum was known in Old English as spærstān, “spear stone”, referring to its crystalline projections. (Thus, the word spar in mineralogy is by way of comparison to gypsum, referring to any non-ore mineral or crystal that forms in spearlike projections). Gypsum may act as a source of sulfur for plant growth, which was discovered by J. M. Mayer, and in the early 19th century, it was regarded as an almost miraculous fertilizer. American farmers were so anxious to acquire it that a lively smuggling trade with Nova Scotia evolved, resulting in the so-called “Plaster War” of 1820.[5] In the 19th century, it was also known as lime sulphate or sulphate of lime.

 

Physical properties

Gypsum is moderately water-soluble (~2.0–2.5 g/l at 25 °C)[6] and, in contrast to most other salts, it exhibits retrograde solubility, becoming less soluble at higher temperatures. When gypsum is heated in air it loses water and converts first to calcium sulfate hemihydrate, (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.[6]

Gypsum crystals are found to contain anion water and hydrogen bonding.[7]

Crystal varieties

Main article: Selenite (mineral)

Gypsum occurs in nature as flattened and often twinned crystals, and transparent, cleavable masses called selenite. Selenite contains no significant selenium; rather, both substances were named for the ancient Greek word for the Moon.

Selenite may also occur in a silky, fibrous form, in which case it is commonly called “satin spar”. Finally, it may also be granular or quite compact. In hand-sized samples, it can be anywhere from transparent to opaque. A very fine-grained white or lightly tinted variety of gypsum, called alabaster, is prized for ornamental work of various sorts. In arid areas, gypsum can occur in a flower-like form, typically opaque, with embedded sand grains called desert rose. It also forms some of the largest crystals found in nature, up to 12 m (39 ft) long, in the form of selenite.[8]

Occurrence

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.[9] 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.

Because gypsum dissolves over time in water, gypsum is rarely found in the form of sand. However, the unique conditions of the White Sands National Monument in the US state of New Mexico have created a 710 km2 (270 sq mi) expanse of white gypsum sand, enough to supply the construction industry with drywall for 1,000 years.[10] Commercial exploitation of the area, strongly opposed by area residents, was permanently prevented in 1933 when president Herbert Hoover declared the gypsum dunes a protected national monument.

Gypsum is also formed as a by-product of sulfide oxidation, amongst others by pyrite oxidation, when the sulfuric acid generated reacts with calcium carbonate. Its presence indicates oxidizing conditions. Under reducing conditions, the sulfates it contains can be reduced back to sulfide by sulfate reducing bacteria. Electric power stations burning coal with flue gas desulfurization produce large quantities of gypsum as a byproduct from the scrubbers.

Orbital pictures from the Mars Reconnaissance Orbiter (MRO) have indicated the existence of gypsum dunes in the northern polar region of Mars,[11] which were later confirmed at ground level by the Mars Exploration Rover (MER) Opportunity.[12]

 

 

Mining

Commercial quantities of gypsum are found in the cities of Araripina and Grajaú in Brazil; in Pakistan, Jamaica, Iran (world’s third largest producer), Thailand, Spain (the main producer in Europe), Germany, Italy, England, Ireland, Canada[14] and the United States. Large open pit quarries are located in many places including Plaster City, California, United States, and East Kutai, Kalimantan, Indonesia. Several small mines also exist in places such as Kalannie in Western Australia, where gypsum is sold to private buyers for changing the pH levels of soil for agricultural purposes.

Crystals of gypsum up to 11 m (36 ft) long have been found in the caves of the Naica Mine of Chihuahua, Mexico. The crystals thrived in the cave’s extremely rare and stable natural environment. Temperatures stayed at 58 °C (136 °F), and the cave was filled with mineral-rich water that drove the crystals’ growth. The largest of those crystals weighs 55 tons and is around 500,000 years old.[15]

Synthesis

Synthetic gypsum is recovered via flue-gas desulfurization at some coal-fired power plants. It can be used interchangeably with natural gypsum in some applications.

Gypsum also precipitates onto brackish water membranes, a phenomenon known as mineral salt scaling, such as during brackish water desalination of water with high concentrations of calcium and sulfate. Scaling decreases membrane life and productivity. This is one of the main obstacles in brackish water membrane desalination processes, such as reverse osmosis or nanofiltration. Other forms of scaling, such as calcite scaling, depending on the water source, can also be important considerations in distillation, as well as in heat exchangers, where either the salt solubility or concentration can change rapidly.

A new study has suggested that the formation of gypsum starts as tiny crystals of a mineral called bassanite (CaSO4·0.5H2O).[16] This process occurs via a three-stage pathway: (1) homogeneous nucleation of nanocrystalline bassanite; (2) self-assembly of bassanite into aggregates, and (3) transformation of bassanite into gypsum.

Occupational safety

People can be exposed to gypsum in the workplace by breathing it in, skin contact, and eye contact.

United States

The Occupational Safety and Health Administration (OSHA) has set the legal limit (permissible exposure limit) for gypsum exposure in the workplace as TWA 15 mg/m3 for total exposure and TWA 5 mg/m3 for respiratory exposure over an 8-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of TWA 10 mg/m3 for total exposure and TWA 5 mg/m3 for respiratory exposure over an 8-hour workday.[17]

Uses

Gypsum is used in a wide variety of applications:

  • Gypsum board[18] is primarily used as a finish for walls and ceilings, and is known in construction as drywall, wallboard, sheetrock or plasterboard.
  • 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.[19]
  • 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.[20]
  • 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[21]
  • Used in baking as a dough conditioner, reducing stickiness, and as a baked-goods source of dietary calcium.[22] The primary component of mineral yeast food.[23]
  • 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
  • Test have shown that gypsum can be used to remove pollutants such as lead[24] or arsenic[25][26] from contaminated waters.

 

 

What is Gypsum?

Gypsum is an evaporite mineral most commonly found in layered sedimentary deposits in association with halite, anhydrite, sulfur, calcite and dolomite. Gypsum (CaSO4.2H2O) is very similar to Anhydrite (CaSO4). The chemical difference is that gypsum contains two waters and anhydrite is without water. Gypsum is the most common sulfate mineral.

Physical Properties of Gypsum

Chemical Classification sulfate
Color clear, colorless, white, gray, yellow, red, brown
Streak white
Luster vitreous, silky, sugary
Diaphaneity transparent to translucent
Cleavage perfect
Mohs Hardness 2
Specific Gravity 2.3
Diagnostic Properties cleavage, specific gravity, low hardness
Chemical Composition hydrous calcium sulfate, CaSO4.2H2O
Crystal System monoclinic
Uses Use to manufacture dry wall, plaster, joint compound. An agricultural soil treatment.

 

Uses of Gypsum?

Gypsum uses include: manufacture of wallboard, cement, plaster of Paris, soil conditioning, a hardening retarder in Portland cement. Varieties of gypsum known as “satin spar” and “alabaster” are used for a variety of ornamental purposes, however their low hardness limits their durability.

 

 

 

What is Gypsum?

Chemically known as “calcium sulfate dihydrate,” gypsum contains calcium, sulfur bound to oxygen, and water. Gypsum is an abundant mineral and takes forms including alabaster—a material used in decoration and construction as far back as ancient Egypt. The White Sands National Monument in New Mexico is the world’s largest gypsum dunefield.

This non-toxic mineral can be helpful to humans, animals, plant life, and the environment. While the majority of gypsum produced in North America is used to manufacture gypsum board or building plasters, gypsum is used in many other ways.

Two Types of Gypsum: Natural and FGD

Natural gypsum, occurs in sedimentary rock formations, and is found in over 85 countries. The United States, Canada and Mexico have some of the largest reserves of high-quality gypsum. Gypsum is mined in 17 states. Iowa, Texas, Utah, and New Mexico are particularly important producers.

One hundred pounds of gypsum rock contains approximately 21 pounds (or 10 quarts) of chemically combined water. Gypsum rock is mined or quarried, crushed and ground into a fine powder.  In a process called calcining, the powder is heating to approximately 350 degrees F, driving off three fourths of the chemically combined water. The calcined gypsum, or hemihydrate, becomes the base for gypsum plaster, gypsum board and other gypsum products.

Flue-Gas Desulfurization (FGD) gypsum has been used to manufacture gypsum board for more than 30 years.  A by-product of desulfurization of flue gas from the stacks of fossil-fueled power plants, emissions captured from smoke stacks can be purified into a hard substance and manufactured into gypsum. Today, almost half of all gypsum used in the United States is FGD gypsum. Natural gypsum and FGD gypsum have the same chemical composition, they are calcium sulfate dihydrate (CaSO4·2H2O). FGD gypsum production and sales encourages power producers to capture “waste” for reuse, rather than merely storing it.

The production of this extremely pure type of gypsum reduces environmental pollution. In 2010, the U.S. gypsum industry diverted nearly 8 million short tons of FGD gypsum to board manufacturing that otherwise would have been sent to local landfills.

The increased use of FGD gypsum has also encouraged new plants to be sited much closer to major population centers. This close proximity between manufacturing facilities and distribution centers saves energy and decreases pollution. Today, a significant percentage of gypsum products distributed in North America are manufactured near installation sites.

Because of its environmental benefits, both the Environmental Protection Agency and the U.S. Green Building Council use FGD gypsum board in their office buildings.

Synthetic gypsum that is suitable for use in wallboard includes flue-gas desulfurization (FGD) gypsum, fluorogypsum, citrogypsum, and titanogypsum. Titanogypsum is a by-product from manufacturing titanium dioxide.

Some types of FGD gypsum are generally considered unsuitable for use in gypsum board due to potential environmental hazards; for example, phosphogypsum may contain radon and radio nuclides. Synthetic gypsum with potentially harmful materials is not used to manufacture gypsum board. Members of the Gypsum Association do not use phosphogypsum to manufacture any gypsum-based product.

By using synthetic gypsum in its manufacturing process, the gypsum industry contributes to a cleaner environment in at least two ways. The majority of synthetic gypsum used by the industry is generated to keep the air clean; it is also an otherwise useless material that would take up valuable space in landfills if not used in the manufacture of wallboard. Both the natural and the synthetic gypsum used in gypsum board are considered to be non-toxic and safe.

Certain impurities occasionally occur with natural as well as synthetic gypsum. The impurities are generally inert and harmless and typically consist of clay, anhydrite, or limestone in natural gypsum and fly ash in synthetic gypsum. Each individual source must be analyzed separately to assess its particular suitability which may vary depending on purity levels of the specific materials that have mixed with the gypsum at that source. Traditionally, most plants that incorporated synthetic gypsum into their board products relied on a mixture of synthetic and natural ore; however, modern plants can manufacture wallboard without using any natural gypsum.

 

 

 

Formula:

CaSO4 · 2H2O

System:

Monoclinic

Colour:

Colourless to white, …

Hardness:

2