Calcium Sulfate Dihydrate (CaSO₄·2H₂O), commonly known as gypsum, is a widely used mineral compound. It is a white or colorless crystalline solid found extensively across the globe. The material’s utility stems from its unique composition and reaction to heat, making it a foundation for many products in construction and other industries.
Chemical Structure and Geological Source
Calcium sulfate dihydrate’s structure is defined by the presence of two water molecules bound within its crystalline lattice. The “dihydrate” designation indicates that each calcium sulfate molecule is associated with two molecules of water. This specific hydration state grants the mineral its distinctive properties.
Gypsum is primarily sourced from geological deposits where it forms as an evaporite mineral. These sedimentary rock formations were created millions of years ago as ancient seas and saline lakes slowly evaporated. The crystallization and concentration of dissolved salts resulted in the massive beds of gypsum found worldwide.
The mineral exhibits a monoclinic crystal system, influencing its physical appearance. Gypsum deposits are extracted using large-scale mining operations to meet high industrial demand. Calcium Sulfate Dihydrate is also produced as a by-product in certain industrial processes, such as flue-gas desulfurization.
Understanding Key Physical Characteristics
One of the defining characteristics of Calcium Sulfate Dihydrate is its relative softness, registering a Mohs hardness value of only 2. This low hardness means the mineral is easily scratched, contributing to its workability in industrial applications. At ambient temperatures, the mineral is chemically stable and exhibits low solubility in water.
The most valuable property is its reaction to heat in a process called calcination, which transforms its hydration state. When heated between 100°C and 150°C, Calcium Sulfate Dihydrate undergoes partial dehydration. This process causes gypsum to lose three-quarters of its bound water, resulting in calcium sulfate hemihydrate (CaSO₄·½H₂O), commonly known as Plaster of Paris.
This hemihydrate is a fine, white powder that retains the capacity to absorb water. When mixed with water, Plaster of Paris undergoes a reverse reaction called rehydration. The water molecules re-incorporate into the crystalline structure, causing the material to rapidly revert back to the rigid, set form of Calcium Sulfate Dihydrate. This exothermic reaction forms a strong crystal lattice, which is the core scientific principle behind its use in casting and construction.
Primary Commercial Uses
The unique setting property of calcined gypsum makes it indispensable for construction materials globally. Its most prominent application is in the manufacturing of drywall, where the rehydration of the hemihydrate provides a fire-resistant core for wallboard products. The process is also the basis for stucco and various plaster products used for interior finishing and molding.
In agriculture, Calcium Sulfate Dihydrate is widely used as a soil amendment to improve soil structure and composition. Known as agricultural gypsum, it provides both calcium and sulfur, which are nutrients essential for plant growth. It helps loosen compact soils and can reduce aluminum toxicity in acidic soils.
The material also has a range of specialized uses across several industries. In the food industry, high-purity calcium sulfate dihydrate acts as a firming agent and coagulant, notably used in tofu production. Furthermore, the ability of Plaster of Paris to set quickly is utilized in medical and dental fields to create molds, casts for immobilizing broken bones, and dental impressions.