Calcium Sulfate

OVERVIEW

Calcium sulfate (KAL-see-um SUL-fate) occurs in three forms:

• anhydrous calcium sulfate (CaSO₄); the anhydrous form of calcium sulfate is available in two forms, known as insoluble anhydrite and soluble anhydrite;
• calcium sulfate dihydrate (CaSO₄·2H₂O), also known as mineral white, terra alba, light spar, precipitated calcium sulfate, native calcium sulfate, and by other names;
• calcium sulfate hemihydrate (CaSO₄·1/2H₂O), also known as plaster of Paris and dried gypsum;

KEY FACTS

OTHER NAMES:

Anhydrous gypsum; also see Overview for synonyms of hydrates

FORMULA:

CaSO₄

ELEMENTS:

Calcium, sulfur, oxygen

COMPOUND TYPE:

Salt (inorganic)

STATE:

Solid

MOLECULAR WEIGHT:

136.14 g/mol

MELTING POINT:

1460°C (2660°F)

BOILING POINT:

Not applicable

SOLUBILITY:

Insoluble in water and most organic solvents

The physical properties of the three forms of calcium sulfate differ somewhat from each other, but their chemical properties are essentially the same. Anhydrous calcium sulfate and calcium hemihydrate are fine white odorless powders or crystalline solids, while the dihydrate may occur either as a powder or as white lumps. Both hydrates are converted to the anhydrous form upon heating as, for example: CaSO₄·2H₂O → CaSO₄ + 2H₂O.

Anhydrous calcium sulfate is essentially insoluble in water. As their names suggest, the soluble form of the compound (soluble anhydrite) is somewhat more soluble in water than is the insoluble form (insoluble anhydrite). The dihydrate and hemihydrate are only slightly soluble in water. When water is added to the hemihydrate, a reaction occurs that results in the formation of a hard, solid mass (plaster of Paris) used in making casts, such as those used to hold broken bones in place. Neither the anhydrous form of calcium sulfate or the dihydrate reacts with water in this way.

Both the anhydrous and dihydrate forms of calcium sulfate occur naturally in the form of the minerals anhydrite, angelite, muriacite, and karstenite (CaSO₄); and gypsum (CaSO₄·2H₂O). These minerals have been known to humans and used by them for thousands of years. The method for converting natural gypsum to the hemihydrate (plaster of Paris) has also been known and used for a very long period of time. Archaeologists have learned that the Egyptians developed a method for converting gypsum to plaster of Paris, which was then used as mortars to join blocks in buildings, more than 5,000 years ago.

HOW IT IS MADE

Gypsum is an abundant mineral providing a ready natural supply of calcium sulfate. The mineral usually consists of a mixture of the anhydrous and dihydrate forms of calcium sulfate, which can be separated into its component parts. Clay, sand, limestone, and other impurities are also present in most gypsum deposits. The process of separation and purification begins by crushing natural gypsum and heating it in an open kettle to temperatures of 100°C to 125°C (212°F to 257°F) for a few hours. Heating converts the gypsum to the hemihydrate or anhydrous calcium sulfate depending on the temperature of the reaction. The solubility of the final product can also be controlled by the temperature used in the heating container and the time during which the material is heated.

Large amounts of calcium sulfate are also produced as the by-product of other reactions or by synthetic processes. The production of phosphoric acid (H₃PO₄) from sulfuric acid (H₂SO₄) and phosphate rock, for example, results in the formation of calcium sulfate, which can be recovered and purified. Calcium sulfate can also be produced in the reaction between other calcium compounds, such as calcium hydroxide (Ca(OH)₂) and sulfuric acid.

COMMON USES AND POTENTIAL HAZARDS

The most widely used form of calcium sulfate is the dihydrate, gypsum, which is an important raw material in the construction industry. It is used in the manufacture of Portland cement, in specialized plasters (known as gypsum plasters) for walls, in the production of wallboard, and in cement blocks and mortars. Gypsum is also used extensively in agriculture as a conditioning agent that adds both calcium ions (Ca2+) and sulfate ions (SO₄2-) to the soil. The compound is also used as a raw material in the synthesis of other calcium compounds and in the production of plaster of Paris.

The anhydrous form of calcium sulfate also has a number of practical applications, the most important of which are in the manufacture of cement and as a filler in the production of paper. A filler adds body to paper, making it firmer, brighter, and easier to write, draw, and print on. Soluble anhydrite is used as a desiccant, which is a material that removes water from other substances. It is usually sold under the trade name of Drierite®.

The primary use of calcium hemihydrate is, of course, in the production of plaster of Paris. Plaster of Paris has a number of important applications, such as:

• In the construction of arts and crafts projects, such as masks, ceramics, and pottery;
• For the production of plaster casts used to immobilize broken bones;
• As a building material in the manufacture of stucco, drywall, division panels, ceiling tablets, plaster board, and wall plaster.

Interesting Facts

• Calcium sulfate has been used in China for more than 2,000 years to thicken soy milk in the production of tofu.
• One reason that calcium sulfate hemihydrate is called plaster of Paris is that large deposits of gypsum (from which plaster of Paris is made) exist near the city of Paris, France. These deposits were long mined to obtain the hemihydrate form of calcium sulfate.
• Ancient Romans used plaster of Paris to cast copies of Greek statues.

Some other important applications of calcium sulfate include:

• As a firming agent in foods such as canned vegetables, soft-serve ice cream, regular ice cream, frostings, and gelatins;
• As an additive to animal feeds to provide the calcium and sulfate that animals need in their diets;

• In the production of polishing powders;
• As a paint pigment (white); and
• In a variety of metallurgical processes, such as the conversion of zinc minerals to zinc metal.

Although nontoxic, calcium sulfate can irritate the respiratory tract if inhaled. It may cause symptoms such as coughing, shortness of breath, and nosebleeds. The compound can also irritate the skin and eyes, causing redness and pain. Ingestion of the compound can cause stomach pains, nausea, and vomiting.

Words to Know

DESICCANT

A material that removes water from other substances.

SYNTHESIS

A chemical reaction in which some desired chemical product is made from simple beginning chemicals, or reactants.

FOR FURTHER INFORMATION

"A Brief History of Plaster and Gypsum." Association of Lifecasters International. http://www.artmolds.com/ali/history_plaster.html (accessed on December 10, 2005).

"Calcium Sulfate." National Organic Program. Agricultural Marketing Service. http://www.ams.usda.gov/nop/NationalList/TAPReviews/CaSO4.pdf (accessed on December 10, 2005).

"Calcium Sulfate, Anhydrous, Powder." J. T. Baker. http://www.jtbaker.com/msds/englishhtml/c0497.htm (accessed on December 10, 2005).

Dontsova, Katerina, et al. "Gypsum for Agricultural Use in Ohio-Sources and Quality of Available Products." Ohio State University Extension Fact Sheet. http://ohioline.osu.edu/anr-fact/0020.html (accessed on December 10, 2005).

"Gypsum Statistics and Information." U.S. Geological Survey. http://minerals.usgs.gov/minerals/pubs/commodity/gypsum/ (accessed on December 10, 2005).