Calcium sulphate (\(\text{CaSO}_4\)) is a naturally occurring inorganic salt found extensively across the globe. This white, odorless compound has been utilized by human civilization for millennia, often in its natural hydrated form. As a substance that is sparingly soluble in water, it contributes to the “permanent hardness” of water and serves as a source of calcium and sulfur in various industrial applications. Its ubiquity and chemical properties make it a foundational material in industries ranging from construction to food production.
Defining Calcium Sulphate Hydration States
The versatility of calcium sulphate relates to its ability to incorporate water molecules into its crystal structure, a characteristic known as hydration. The amount of chemically bound water determines the specific form of the compound and its resulting properties. The most common natural state is gypsum, or calcium sulphate dihydrate (\(\text{CaSO}_4 \cdot 2\text{H}_2\text{O}\)). This soft mineral form, where two water molecules are associated with each \(\text{CaSO}_4\) unit, is typically mined from evaporite deposits.
When gypsum is subjected to heat, it undergoes controlled dehydration to form intermediate states. Heating gypsum between 120 and 180 degrees Celsius drives off three-quarters of the water content, resulting in calcium sulphate hemihydrate (\(\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}\)). This semi-hydrated form is widely known as Plaster of Paris. The hemihydrate’s high reactivity is the basis for many of its applications.
If the heating process is carried out above 200 degrees Celsius, all the water molecules are removed. The resulting compound is anhydrous calcium sulphate (\(\text{CaSO}_4\)), commonly called anhydrite. Anhydrite is the least reactive of the three forms and is sometimes used as a drying agent, though it slowly rehydrates back into gypsum when exposed to moisture.
Primary Role in Building and Construction Materials
The primary use of calcium sulphate stems from the reversible reaction between gypsum and its hemihydrate form. Naturally mined gypsum dihydrate is first crushed and then heated in a precise process called calcination, which converts it into Plaster of Paris. This process must be controlled, as excessive heat would produce the unreactive anhydrite form.
When Plaster of Paris is mixed with water, it rapidly reverts to the solid gypsum dihydrate state. This exothermic reaction releases heat and causes the mixture to set quickly into a hard mass. Solidification occurs because the hemihydrate crystals dissolve and then rapidly precipitate as interlocking, needle-like gypsum crystals, which gives the material its strength.
This quick-setting property and the slight expansion during hydration make it ideal for casting and molding applications. In modern construction, the largest use is in the production of drywall, often called gypsum board or plasterboard. Drywall consists of a core of set gypsum sandwiched between thick sheets of paper, providing a fire-resistant and structurally sound material for walls and ceilings. The same reaction is the foundation for stucco and various building plasters used to create smooth interior surfaces.
Applications in Food, Medicine, and Agriculture
Beyond construction, calcium sulphate demonstrates versatility in the food, medical, and agricultural sectors. In the food industry, it is recognized as the additive \(\text{E}516\) and serves multiple functions. It is widely used as a coagulant in tofu production, where calcium ions cause soy proteins to gel and solidify into firm curds.
The compound also acts as a dough conditioner in baking, strengthening the gluten structure and providing a source of calcium for yeast. It is also employed as a firming agent to help canned vegetables maintain their texture and crispness during processing. Its neutral flavor and high calcium content make it a common fortifier in flour and cereals to boost nutritional value.
In medicine and dentistry, the quick-setting nature of Plaster of Paris has been historically utilized for forming casts to immobilize broken bones. More recently, medical-grade calcium sulphate is used as a synthetic bone graft substitute in orthopedic and periodontal surgery. It is highly biocompatible and osteoconductive, providing a scaffold for new bone growth, and is designed to resorb completely into the body over several weeks.
In agriculture, calcium sulphate is applied as a soil amendment rather than a fertilizer. Its primary benefit is improving the physical structure of the soil, especially in clay-heavy or sodic soils, by promoting aggregation and increasing water infiltration. Gypsum addition is also effective for mitigating aluminum toxicity in acidic subsoils, as the sulphate ions bind with toxic aluminum ions, enhancing deep root growth without altering the soil’s overall pH.