What is calcium sulfate?

calcium sulfate is a ternary calcium salt, alkaline earth metal (Mr Becambara), sulfur and oxygen. Its chemical formula is CaSO 4 , which means that for each Ca 2+ cation there is an SO 2- anion interacting with it. It is a compound with a wide distribution in nature.

Its most abundant forms are CaSO 4 · 2H 2 O (gypsum) and the anhydrous form CaSO 4 (anhydrite). There is also a third form: plaster of Paris, produced by heating the plaster (hemidrate, CaSO 4 · 1 / 2H 2 O).

Calcium sulfate is a very useful compound in the human being, since it is used in numerous functions that include road construction, home beautification and even bone repair.

Chemical structure of calcium sulfate

Calcium sulfate

The upper image shows the orthorhombic unit cell for CaSO 4 . Here it is assumed that the interactions are purely electrostatic; that is, the Ca 2+ cations attract the tetrahedral anions SO 2– .

Considering the previous point, now the Ca 2+ ions accept dative bonds (contributed by the O) and the unit cell is transformed, as indicated in the image below:

Calcium sulfate

Consequently, the CaO 8 polyhedron is formed (the green Ca 2+ sphere surrounded by eight red O spheres from nearby SO 4 tetrahedra ). Calcium polyhedra and sulfate tetrahedra; this is the crystal structure of anhydrous CaSO 4 .

Additionally, when the crystals are hydrated – forming the di-hydrated salt or the hemidrate (CaSO 4 · 1/2 H 2 O) – the structure expands to incorporate the water molecules.

These molecules can intercalate and coordinate with calcium; that is, they replace one or two sulfate groups.

On the other hand, not all the water becomes part of the CaO 8 polyhedron . Some, on the other hand, form hydrogen bonds with sulfates. These serve as a junction for two zigzag sections, products of the arrangement of the ions in the crystal.

Appearance of calcium sulfate

Molecular formula

CaSO4 · nH2O.

Anhydrous molecular weight

136.134 g / mol.


It is odorless.


In the case of anhydrite, it looks like a white powder or solid orthorhombic or monoclinic crystals. The crystals are of a variable color: they can be white or with a bluish, greyish, or reddish tint; it can also be brick red.


2.96 g / cm3 (anhydrous form). 2.32 g / cm3 (dihydrate form).

Melting point

1450 ° C (2840 ° F). Characteristic of the strong electrostatic interactions between the divalent ions Ca 2+ and SO 2- .


0.2-0.3% in water at 25 ° C. It is poorly soluble in water and insoluble in ethanol.


Stable at room temperature.

Uses of calcium sulfate

In construction and in art

It is used in the elaboration of stucco to frize the walls of homes and other constructions that contribute to its beautification. In addition, reliefs are made by means of molds in the ceilings and window frames. The plaster is also in the ceilings.

Calcium sulfate is used to help solve the problem that occurs in the hydration of concrete, thus collaborating in the construction of roads, avenues, etc.

With the plaster sculptures are made, especially religious figures, and in cemeteries they are used on tombstones.



Experimentally, sterile calcium sulfate pieces have been used in veterinary medicine to repair bone defects or cavities, such as those left by wounds or tumors.

Plaster of Paris can be used to repair bone defects because of its unique ability to stimulate osteogenesis. X-ray studies and technetium (Tc99m) medronate support the use of plaster of Paris as an alloplastic and its osteogenic capacity when implanted in the frontal sinus.

Bone regeneration has been demonstrated in six dogs in a period of 4 to 6 months. Calcium sulfate began to be used in this field in 1957, in the form of plaster of paris tablets, being able to fill defects in the bones of dogs.

The bone substitution of calcium sulfate is comparable to that observed in autogenic bone.

Ruhaimi (2001) applied calcium sulfate to a recently destroyed rabbit jaw bone, observing an increase in osteogenesis and bone calcification.


Calcium sulfate is used in medicine to immobilize dislocated joints and fractured bones, as well as being used as an excipient in the manufacture of tablets.


In dentistry it is used as a basis for making dental prostheses, in restorations and impressions of teeth.

In food processing

It is used as a coagulant in the production of tofu, a food made with soy and widely consumed in eastern countries as a substitute for meat. In addition, it has been used as a firming of food and in the treatment of flours.

As a fertilizer and conditioner for crop soils

Gypsum (CaSO 4 · 2H 2 O) was used as a fertilizer in Europe since the 18th century, having the advantage over the use of lime as a source of calcium with greater mobility.

Calcium must be available to the roots of the plants for its adequate supply. So the addition of calcium improves horticultural and groundnut (groundnut) crops.

Peanut root rot caused by biological pathogens, as well as watermelon and tomato blossom end rot, are partially controlled with applications of agricultural gypsum.

Gypsum helps to reduce the dispersion of clay, which causes the formation of crusts on the soil. By reducing the crusts formed on the ground, the plaster facilitates the exit of the seedlings. Likewise, it increases the entry of air and water to the ground.

Gypsum helps to improve the soil by mitigating the acidity and toxicity of aluminum, thus making the crop suitable for sodium soils.

In the production of other compounds

Calcium sulfate reacts with ammonium bicarbonate to form ammonium sulfate. It has also been used in the sulfuric acid production process.

Anhydrous calcium sulfate is mixed with shale or lean and, as the mixture is heated, sulfur trioxide is released in gaseous form. Sulfur oxide is a precursor to sulfuric acid.

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