Calcium Sulfate
Properties
| State | Solid (white powder or crystalline) |
| Color | White |
| Solubility | Slightly soluble in water (2.4 g/L at 20°C) |
| Melting Point | 1460°C (anhydrite) |
| Boiling Point | Decomposes before boiling |
About Calcium Sulfate
Calcium sulfate is interesting chemically because it is the same compound in three practically different forms: the dihydrate (gypsum, CaSO4·2H2O), the hemihydrate (plaster of Paris, CaSO4·0.5H2O), and the anhydrous mineral anhydrite. The phase boundary that matters is around 42 °C in aqueous suspension — below that, the dihydrate is the stable phase, above it anhydrite begins to take over. The hemihydrate sits in between as a metastable intermediate, and the entire plasterboard industry runs on driving water out at about 150 °C to make hemihydrate, pressing it between paper sheets, and letting it rehydrate to the dihydrate during cure. Anhydrous CaSO4 also has a thermodynamic quirk that makes it useful: it is a strong desiccant — sold under the trade name Drierite, often dyed blue with cobalt chloride as a moisture indicator that turns pink when spent — yet it does not deliquesce and is essentially neutral, which makes it suitable for drying gases and solvents that more aggressive desiccants like P2O5 or KOH would attack. The compound is also the dominant cause of permanent water hardness because, unlike Ca(HCO3)2, it does not decompose on boiling. In Portland cement, 3 to 5 percent gypsum is added to clinker as a set retarder — without it, fresh cement would flash-set within minutes of mixing.
Where you'll encounter it
If you have ever changed the indicating Drierite in a desiccator and watched the granules turn from blue to pink, that is anhydrous CaSO4 picking up water. Walk into a modern building and you are surrounded by the dihydrate as drywall. Run a tofu kitchen and you weigh out food-grade calcium sulfate (E516) by the gram per kilogram of soy milk to coagulate the proteins. In a soft-water lab, the limit of CaSO4 solubility — about 2.4 g/L at 20 °C — is what sets the upper bound on Ca²⁺ that scrubbing alone can remove from feed water before you reach for ion exchange or RO.
Common Uses
- Core of drywall and plasterboard, with the dihydrate's bound water giving fire-retardant performance
- Plaster of Paris for orthopedic casts, dental stone, sculpture molds, and theatrical scenery
- Indicating desiccant (Drierite) for drying gases and solvents in laboratory practice
- Tofu coagulant E516 in soy-protein food manufacturing at gram-per-kilogram dosing
- Set retarder at 3 to 5 percent in Portland cement clinker to control flash setting
Safety Information
Bulk material is essentially inert — non-toxic by ingestion at food-additive levels, GRAS as E516, no GHS hazard classification under CLP. The two practical hazards are dust exposure during drywall sanding (regulated as nuisance dust, OSHA PEL 15 mg/m³ total / 5 mg/m³ respirable), and thermal burns from the exotherm during plaster setting in thick orthopedic casts, where temperatures can reach 60 to 70 °C if applied without padding. Anhydrous Drierite spent of moisture sometimes contains a few percent CoCl2 as the indicator dye, which is itself a Cat. 1B carcinogen — segregate exhausted Drierite from regular gypsum waste streams.
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.