$word_{count}$: 220

Plaster of Paris $\frac{CaSO4.1}{2H2O}$: Made by heating gypsum (CaSO4.2H2O) at 120 C. Sets by rehydration: CaSO4.1/2H2O + 3/2H2O → CaSO4.2H2O (slight expansion — ideal for moulds). Above 200 C → dead burnt plaster (anhydrous CaSO4, cannot set).

Portland cement: Limestone + clay heated at ~1500 C in rotary kiln → clinker → ground with 2-3% gypsum. Composition: ~60% CaO, ~20% SiO2, ~5-10% Al2O3. Gypsum retards setting (prevents flash set from C3A). Setting involves hydration of calcium silicates — exothermic, takes 7-28 days for full strength.

CaO (quicklime): CaCO3 → CaO + CO2 (lime kiln). Slaking: CaO + H2O → Ca(OH)2 (exothermic). Used in steel, cement, water treatment.

Lime water test: Ca(OH)2(aq) + CO2 → CaCO3 (milky) + H2O. Excess CO2: CaCO3 + CO2 + H2O → Ca(HCO3)2 (clear). This reversible solubility is key.

BaSO4: Extremely insoluble (Ksp ~10^-10). Safe for X-ray barium meal despite $Ba^{2+}$ toxicity. Never releases $Ba^{2+}$ even in stomach acid.

MgSO4.7H2O (Epsom salt): Saline purgative — osmotic action draws water into intestine.