USG Closes Southard Gypsum Plant Raising Costs for Specialty Plasters
USG will idle its Southard plant, tightening the supply of high-purity specialty gypsum and raising plaster sourcing costs.
Alpha-Hemihydrate
Chemical Structure
High-strength industrial plaster relies on a specific crystalline form of calcium sulfate that offers superior density and mechanical properties. This calcium sulfate derivative, known as alpha-hemihydrate, is produced through the hydrothermal treatment of gypsum under pressure. The process yields regularly shaped, non-porous needle-like crystals that require minimal water to form a workable slurry.
Industrial Processing
Autoclave processing regulates the thermal dehydration of gypsum to achieve the desired crystalline structure. To generate alpha-hemihydrate, raw calcium sulfate dihydrate undergoes heating in saturated steam or in aqueous salt solutions at temperatures ranging from one hundred and ten to one hundred and forty degrees Celsius. Controlled crystallization produces compact, well-defined prisms that exhibit low water absorption during subsequent mixing.
This morphology ensures that the resulting cured cast exhibits high compressive strength, making it suitable for precision metal casting molds and dental models.
Performance Limit
Thermal stability determines the boundary conditions under which the cured material maintains its structural integrity. Dehydration occurs if the set alpha-hemihydrate encounters temperatures exceeding sixty degrees Celsius for extended periods. This degradation causes the dihydrate crystals to revert to an anhydrite phase, reducing the mechanical strength of the product and limiting its use in high-temperature environments.