Infrastructure and Construction Materials Guide — Gypsum

Commodity Description

Gypsum is a versatile mineral with hundreds of uses for thousands of years. The earliest use of gypsum plaster dates to about 8000 BCE with the discovery of its use in Anatolia (Turkey). Gypsum plaster was used as early as 7000 BCE as a construction material in Egypt. Gypsum was first discovered and utilized in North America in Nova Scotia, Canada in 1770. Surface exposures of gypsum were dug and used on agricultural fields. Early exports of raw gypsum were eventually used in the US for growing legumes, such as clover, soybeans, and especially peanuts in the far south. Benjamin Franklin is reported to have spread ground gypsum on clover fields and showed potential customers the much healthier clover plants by spelling out “Land Plaster Used Here.” Over time, many agricultural uses of raw, ground gypsum were developed, and it is a significant soil amendment and water clarifier. Gypsum helps to break up clayey soils, and through an ion exchange process removes sodium and substitutes calcium in alkaline soils.

Gypsum was mined in southwestern Virginia from about 1830 until 2000—a period of about 170 years. Several long-term, underground mining operations in New York, Kansas, and Ontario ceased operations in the early part of the 21st century when reserves were exhausted.

Geology and Mineralogy

Gypsum (CaSO4•2H2O), calcium sulfate dihydrate, has the unique property of releasing 1.5 molecules of water when heated to about 320-350oF. The resulting chemical compound, calcium sulfate hemihydrate (CaSO4•½H2O) will, in turn, react with water to reform gypsum. This property is the basis for gypsum’s use in making wallboard panels, molds for ceramics, such as dishes, sinks, and toilets, dental and orthopedic plasters, and for many other uses.

Commercial deposits of gypsum may be almost pure, or contain variable amounts of impurities such as limestone, dolomite, clay, anhydrite, and soluble salts of potassium, sodium, and magnesium. Primary gypsum deposits consist of rock gypsum (alabaster). Selenite, satin spar, and gypsite are secondary varieties of gypsum. Anhydrite may occur as either primary or secondary minerals in a deposit, depending on its geological history.

Petrographically, most rock gypsum has a medium to coarse crystalline texture. Some deposits contain large gypsum crystals, known as poikiloblasts because they contain crystals that have not been metamorphosed within a metamorphic rock. Gypsum in the Upper Miocene Boleo Formation in Baja California del Sur, Mexico contains selenite crystals up to about 10-inches in width. The Mississippian Windsor Group in Nova Scotia, Canada, contains abundant scattered poikiloblasts.

Production and Pricing

For United States production statistics and sale price, “Gypsum,” go to p. 86 of the USGS Commodity Summaries 2024.

For World Mine Production and Reserves, “Gypsum,” go to p. 87 of the USGS Commodity Summaries 2024.

Uses

There are over 400 different uses and products made from gypsum. The largest uses are:
• Wallboard – There are many varieties of gypsum wallboard for both interior and exterior uses.

Typical gypsum wallboard insulation

 

• Portland Cement – Every ton of Portland cement requires 3-5% by weight of gypsum. Gypsum is ground with clinker to form Portland cement. The gypsum controls the setting time of the cement.
• Agricultural – Soil amendments, settling pond clarifiers
• Molding Plasters – Ceramic plumbing fixtures, plates and dinnerware, statuary, construction plasters, etc.
• Dental and Orthopedic Plasters

Substitutes

Cement and lime

Major Producers

U.S.: United States Gypsum Company, Georgia-Pacific, National Gypsum, CertainTeed
International: Knauf, Saint Gobain

Mining Methods

Most of the world’s gypsum is produced by surface-mining operations. In 2022, only three underground gypsum mines were in operation in North America, one each in Indiana and Iowa in the United States and one in Ontario, Canada. Gypsum is quarried from near-surface deposits. Overburden consisting of glacial materials (till, sand, clay), shale, mudstone, siltstone, sandstone, sand and gravel, or limestone, is removed. Final cleanup of the exposed surface of the gypsum is important depending on the final manufactured products, such as food and pharmaceutical grade gypsum, orthopedic plasters, and specialized gypsum-based cements used in many industries. Underground mining of gypsum is conducted by the room-and-pillar method using equipment and techniques commonly used in mining rock salt, potash, trona, and limestone.

Gypsum mine during product storage

 

If the quarried rock is to be used in agricultural products, Portland cement rock, or wallboard, then much of the impurities can be removed in the finer fractions during crushing and screening. Conversely, gypsum used for high-quality, high-value-added products requires more stringent cleaning. For example, articulated hydraulic excavators with multiple, interchangeable bucket widths will scrape clay from the gypsum surface and fractures that extend deep into the gypsum.

Drilling and blasting is the primary method of quarrying gypsum. Quarry benches are generally about 25 feet in height. Hydraulic rotary drilling and auger drilling are commonly used. Gypsum is soft but relatively difficult to blast due to its high elasticity which is similar to a block of hard rubber. Blasting energy is absorbed by the gypsum and holes are spaced relatively close together to distribute the explosive forces throughout the rock mass. Quarry haulage trucks or over-the-road dump trucks transport the mined gypsum from the quarry site to the primary crusher.

Another method of extracting gypsum from quarries is the use of a surface miner. This is an adaptation of highway-resurfacing technology in which a horizontally rotating mandrel with cutting teeth chips away at the asphalt and either discharges the broken material in long rows or directly into a haulage truck. Specialized machinery using this technology has been developed to quarry coal, gypsum, and limestone. The size, spacing, and arrangement of the cutting teeth on the mandrel are important

Mineral Processing

Gypsum processing can be divided into three basic steps: (1) rock preparation, (2) calcining, and (3) formulating and manufacturing. This is illustrated graphically with the flow chart in Figure 1. The specifics of each step will vary with the quality of the gypsum and with the type of final product.

Figure 1. Gypsum process flow diagram (Henkels, 2006)

 

1) Rock Preparation

Primary crushing is the first step in processing mined gypsum. Little to no processing may be required for agricultural and cement products, other than size reduction and classification. For cement products, particle size requirements vary with the individual cement plants, but commonly are in the range of 1.5 to 2.0-inches top size to 0.25 to 0.5-inches bottom size. Gypsum or a blend of gypsum and anhydrite is mixed with clinker to form cement. Agricultural gypsum products may be pulverized, granular, or pelletized depending on the application. Finely pulverized gypsum may be applied by the addition to irrigation water.

Gypsum is further pulverized for feed into a calcining kettle or flash calciner. For most of the last 100 years, the most commonly used mill for reducing gypsum to a fine powder has been the Raymond mill. Raymond mills consist of a vertically rotating shaft with articulated arms ending with steel rollers. The body of the mill consists of a steel shell with a thick steel ring inserted around the perimeter near the bottom. As the vertical shaft rotates, the articulated arms with circular rollers swing outward to make contact with the thick steel outer ring. Gypsum is introduced above the ring and is pulverized between the ring and rotating circular rollers. Air is introduced at the bottom of the mill and the pulverized gypsum is blown upward and air-classified. Oversize material is recirculated through the mill for further size reduction.

2) Calcining

Gypsum is heated to thermally breakdown its crystal structure and chemically release the combined molecules of water (water of crystallization). Traditionally, calcining has been performed in vertical kettles, fired by wood, coal, oil, or gas. Calcining kettles originated about 1870 and technological innovations have been continuous. Gypsum pulverized to a particle size of 90% -100 mesh (~0.006-inch) is introduced into the top of the kettle. This material is called “land plaster” in the gypsum industry. Calcining by vertical kettles may be designed for continuous operation or in batches dependent upon the final use of the product.

Figure 2. General cross-section of a calcining kiln (Henkels, 2006)

 

Calcium sulfate hemihydrate is commonly called “stucco” or “Plaster of Paris.” Although the term “stucco” is used throughout the gypsum industry, this term should not be confused with the Portland cement and sand mixture also known as stucco that is used for exterior application on buildings.

Batch calcining is used to produce stucco of high plasticity, high strength, and high density—characteristics that are desirable for construction and industrial plaster.

Most modern gypsum processing is performed in machinery that combines the pulverization and calcining steps in the transformation of gypsum to hemihydrate. Equipment manufacturers such as Pfeiffer and Claudius-Peters specialize in this technology, known as “flash calcining.” This technology is performed by calcining the pulverized gypsum in a stream of hot air and capturing it in a cyclone or baghouse dust collector either during or after final grinding. High-temperature hammer mills or roller mills have seen increased use as hot air-swept grinder-calciners for efficient stucco production. These mills are fed at a size of less than two inches gypsum rock.

Pressure calcining in steam autoclaves, with or without the addition of a crystal modifier, is used to make high-strength plasters and cements for certain industrial uses. The gypsum is introduced into the vessel either as a crushed, sized rock or as a slurry; the stucco is dried as soon as calcination is completed. This method produces all alpha hemihydrate and is relatively expensive. Its use is limited to those products requiring high-strength and/or fast-setting specialized plaster or cement.

3) Formulation and Manufacturing

Two forms of calcium sulfate hemihydrate are known as “alpha” and “beta.” Alpha hemihydrate is characterized by dense, well-formed crystals. Beta hemihydrate particles are less well formed and are often splintered.

The largest use of beta hemihydrate is in the manufacture of wallboard and building markets. Beta hemihydrate gypsum is suitable for construction applications where early high-strength development is necessary. Because alpha hemihydrate makes a denser, higher-strength cast, it is preferred for industrial uses where these characteristics are important. Alpha hemihydrate may be blended with Portland cement to make quick-setting, high-strength products for repairing roads, bridges, runways, and floors within buildings such as warehouses. These products minimize the disruption of traffic and industrial operations.

Another significant use is in the manufacture of specialized cements for oil and gas wells, geothermal wells and injection wells.

The number of known uses of gypsum exceeds 400, varying at its simplest use as ground gypsum used in agriculture to specialized plasters used in the medical and aerospace industries. However, its use in construction products is widespread. The development and continual improvement of gypsum wallboard in the early 1900s allowed the rapid construction of homes, schools, and other structures at the end of World War II. The need of gypsum to make Portland cement likewise aided in the construction of infrastructure, such as roads, bridges, etc.

Flue Gas Desulfurization (FGD) gypsum
To limit the amount of SO2 and fly ash produced in coal-fired power plants, Flue Gas Desulfurization (FGD) units are used. These units are for air pollution control for coal-fired power plants. The chemical composition of the gypsum sludge is dependent on the amount of oxygen available during the reaction. In the absence of oxygen, the reaction produces calcium sulfite hemihydrate [CaSO3•½H2O]. In the presence of oxygen, the reaction produces calcium sulfate dihydrate [CaSO4•2H2O] as a usable gypsum. FGD gypsum contains impurities that are not found in mined gypsum. Fly ash is one impurity, and can result in accelerated wear to the production machinery and physical defects in the final products. Common market specifications established by North American wallboard manufacturers limit the amount of fly ash allowed in the FGD gypsum used in wallboard to 1%. Approximately 50% of all gypsum used in the manufacture of gypsum board in the United States is FGD gypsum (epa.gov).

References

Henkels, Paul J., Jr.,2006, Gypsum plasters and wallboards, in Industrial Minerals and Rocks, 7th edition, Society for Mining, Metallurgy, and Exploration, Inc., pp. 1143-1152.

Sharpe, Roger D. and Greg G. Cork, 2006, Gypsum and anhydrite, in Industrial Minerals and Rocks, 7th edition, Society for Mining, Metallurgy, and Exploration, Inc., pp. 519-540.

United States Geological Survey (USGS) Commodity Summaries 2023.  https://pubs.usgs.gov/publication/mcs2023. NOTE:  2023 version reports 2022 statistics. 

United States Geological Survey (USGS) Commodity Summaries 2024.  https://pubs.usgs.gov/publication/mcs2024. 2024 version reports 2023 statistics. 

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