Calcium Sulfate Dihydrate
Properties
| State | Solid at room temperature |
| Color | White to colorless |
| Solubility | Slightly soluble in water (0.21 g/100 mL at 20 °C) |
| Melting Point | Loses water at 150 °C (forms plaster of Paris) |
About Calcium Sulfate Dihydrate
Gypsum is the chemistry of a reversible dehydration done at modest temperatures, and that single trick — heating CaSO4·2H2O to about 150 °C to drive off three quarters of the bound water and yield the hemihydrate CaSO4·0.5H2O — is what underwrites most of the global plasterboard industry. Add water back to the hemihydrate and gypsum re-crystallizes as a mat of interlocking acicular needles that lock the slurry into a rigid solid in roughly 20 to 30 minutes, releasing about 17 kJ per mole as it sets. The reaction is mildly exothermic, which is why a thick orthopedic cast warms noticeably against the skin while curing. Mineralogically, the dihydrate is the stable form below 42 °C in contact with water; selenite, satin spar, and alabaster are all the same compound in different crystal habits. The Egyptians used burned gypsum plaster on the interior walls of pyramids, and the Bayer-process aluminum industry produces it as a desulfurization byproduct from flue-gas scrubbers — synthetic gypsum from coal-fired power plants now feeds a large fraction of drywall manufacturing in North America. Food-grade material (E516) is the standard tofu coagulant in firm-pressed tofu, where Ca²⁺ cross-links soy protein into a curd.
Where you'll encounter it
Walk into any modern building and you are surrounded by this compound — drywall is roughly 70 to 90 percent gypsum sandwiched between paper, with the bound water acting as a passive fire retardant because driving it off absorbs heat before the wall structure behind it begins to fail. In a teaching lab the setting reaction is the classic demo for an exothermic crystallization: weigh out 50 g of plaster of Paris, add 30 mL of water, and the beaker is warm to the touch within minutes. Orthopedic casts, dental stone for tooth impressions, and the white scenery flats used in theater are all the same chemistry. Agricultural-grade gypsum is sold by the tonne to flocculate sodic clay soils and supply S without raising pH.
Common Uses
- Core material for drywall and plasterboard, with bound water acting as a passive fire retardant
- Plaster of Paris for orthopedic casts, dental stone, and decorative moldings
- Soil amendment to flocculate sodic clay soils and supply Ca and S without changing pH
- Tofu coagulant (E516) where Ca²⁺ cross-links soy protein into a firm curd
- Set retarder added at 3 to 5 percent in Portland cement to control flash setting
Safety Information
Bulk gypsum is essentially benign — non-toxic, non-flammable, GRAS for food use as E516. The relevant occupational hazard is inhalation of fine dust during cutting or sanding drywall, which is regulated as a particulate not otherwise classified (OSHA PEL 15 mg/m³ total dust, 5 mg/m³ respirable). The exothermic setting reaction can reach 60 to 70 °C in thick orthopedic casts, enough to cause burns if applied directly to skin without padding — a documented injury pattern in pediatric ER literature. No GHS hazard classification under the standard CLP criteria.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.