betahemihydrate

Beta-hemihydrate, also written as beta-hemihydrate, refers to the beta crystalline form of calcium sulfate hemihydrate (CaSO4·0.5H2O). It is produced by calcination of gypsum (CaSO4·2H2O), a process that removes part of the water of crystallization.

Formation and properties

Beta-hemihydrate is typically obtained by heating gypsum in air at moderate temperatures, roughly in the range of 120 to 180°C. The resulting powder is porous and consists of fine crystals, often described as needle-like or tabular in shape. Compared with the alpha-hemihydrate form, beta-hemihydrate is generally more porous, less dense, and dissolves more readily in water. When mixed with water, it hydrates back to calcium sulfate dihydrate (CaSO4·2H2O) and hardens through an exothermic setting reaction.

Uses and applications

Beta-hemihydrate is most commonly associated with plaster of Paris, a widely used material in construction, sculpture, and medical or dental applications. It is employed for casts, molds, and models, as well as for making architectural plaster coatings and decorative elements. In dental and industrial contexts, beta-hemihydrate provides convenient workability and rapid setting, though the resulting material is typically less strong and more porous than formulations based on the alpha form.

Relation to other forms

The calcium sulfate hemihydrate system includes at least two polymorphs: beta-hemihydrate and alpha-hemihydrate. Alpha-hemihydrate is produced under higher temperature and pressure (often in autoclaves) and generally yields a denser, stronger material with different setting characteristics. Beta-hemihydrate remains the conventional, widely used form for general plastering and casting needs.

Storage and handling

Beta-hemihydrate should be kept dry and in sealed containers to prevent hydration or clumping from atmospheric moisture.