CaSO₄ — Calcium Sulfate

Calcium sulfate (CaSO₄) is an inorganic compound that naturally occurs as the minerals gypsum (CaSO₄·2H₂O), bassanite (CaSO₄·0.5H₂O), and anhydrite (CaSO₄). It is a white crystalline substance widely used in construction, agriculture, and medicine. In its various hydration forms, calcium sulfate plays key roles in products such as plaster of Paris, drywall, and surgical casts.

This compound is formed by the reaction between calcium and sulfuric acid or sulfate salts. It is known for its low solubility in water and strong ionic bonding. Because of its versatility, calcium sulfate is used in making cement, as a coagulant in tofu preparation, and as a drying agent in laboratories. Its unique property of reversible hydration makes it extremely useful in construction and molding applications.

Calcium sulfate consists of one calcium ion (Ca²⁺) and one sulfate ion (SO₄²⁻). The sulfate ion has a tetrahedral geometry, with the sulfur atom bonded to four oxygen atoms through covalent bonds. These ions are held together by strong electrostatic forces, giving the compound its ionic character.

In its hydrated forms, water molecules are incorporated into the crystal lattice. For example, in gypsum (CaSO₄·2H₂O), two water molecules are integrated per formula unit, while in plaster of Paris (CaSO₄·0.5H₂O), only half a molecule of water is present per calcium sulfate unit. The reversible interconversion between these hydration states is the basis for many industrial applications.

Calcium sulfate can be prepared both naturally and synthetically through several chemical reactions:

• 1. Reaction Between Calcium Carbonate and Sulfuric Acid: This is the most common laboratory method to produce calcium sulfate.

• 2. Reaction Between Calcium Hydroxide and Sulfuric Acid:

• 3. Industrial Production: In industry, calcium sulfate is produced as a by-product in the manufacture of phosphoric acid (from phosphate rock and sulfuric acid) and in the desulfurization of flue gases.

Natural gypsum deposits are mined and then heated to form plaster of Paris. The process of controlled heating removes part of the water of crystallization, converting the dihydrate into the hemihydrate form.

Physical Properties:

• White crystalline solid that is odorless and tasteless.
• Exhibits moderate hardness and low solubility in water.
• The dihydrate form (gypsum) contains about 20% water by mass.
• Converts to plaster of Paris when heated to 373 K (100°C).

Chemical Properties:

• 1. Reaction with Water: The hemihydrate form (plaster of Paris) reacts with water to form gypsum again.

\( 2(CaSO_4·0.5H_2O) + 3H_2O \rightarrow 2(CaSO_4·2H_2O) \)

• 2. Reaction with Acids: Calcium sulfate is insoluble in most acids except concentrated sulfuric acid.
• 3. Reaction with Carbonates: It reacts with sodium carbonate to produce calcium carbonate and sodium sulfate.

\( CaSO_4 + Na_2CO_3 \rightarrow CaCO_3 + Na_2SO_4 \)

• 4. Thermal Decomposition: Upon heating above 1000°C, calcium sulfate decomposes into calcium oxide and sulfur trioxide.

\( CaSO_4 \xrightarrow{heat} CaO + SO_3 \uparrow \)

Calcium sulfate is a versatile compound with widespread applications across various industries:

• 1. Construction Industry: Used as a key ingredient in plaster of Paris, wallboard (drywall), and cement. Gypsum-based plasters are known for their quick-setting properties and smooth finish.
• 2. Medical Applications: Used in orthopedic casts, dental molds, and surgical implants as a biocompatible and biodegradable material.
• 3. Agriculture: Improves soil structure and provides calcium and sulfur nutrients. It helps in reducing soil salinity and enhancing crop yield.
• 4. Food Industry: Serves as a coagulant in tofu production and as a firming agent in canned vegetables (approved as food additive E516).
• 5. Drying Agent: The anhydrous form acts as a desiccant to remove moisture from organic liquids and gases.
• 6. Paper and Paint Industry: Used as a filler material to improve whiteness, smoothness, and opacity.
• 7. Sculpting and Art: Used in mold making, statues, and decorative elements because of its easy shaping and quick setting characteristics.

Calcium sulfate is generally safe and non-toxic but should be handled with care in industrial or laboratory environments.

Health Hazards:

• Inhalation of dust may cause mild respiratory irritation.
• Prolonged contact can cause skin dryness or minor irritation.
• Ingestion of large amounts may lead to gastrointestinal discomfort.
• Exposure to decomposed fumes (SO₃) can be harmful.

Safety Precautions:

• Wear protective gloves, goggles, and masks when handling powdered calcium sulfate.
• Ensure adequate ventilation during processing or mixing.
• Store in a cool, dry environment away from acids.
• Clean spills promptly to avoid dust inhalation or slipping hazards.

Despite minor irritant properties, calcium sulfate is considered safe for environmental use and biodegradable, making it an eco-friendly compound.