Primer

Synthetic gypsum, which is now used in about 30% of drywall, is a byproduct of coal-fired power plants. It is sometimes confused with fly ash—another coal combustion product that’s been in the news recently—but the two have very little in common.

Synthetic gypsum is also called flue-gas-desulfurization (FGD) gypsum. It is produced through a chemical reaction in the chemical scrubbers that remove sulfur from the flue gases of coal-fired power plants. Sulfur dioxide from power plants is the leading cause of acid rain, and the Clean Air Act of 1970 required power plants to reduce their sulfur dioxide emissions.

Chemical scrubbers remove sulfur dioxide (SO2) from power plant emissions by passing the flue gases through a slurry of limestone or calcium carbonate (CaCO3). The chemistry of this process is really elegant: sulfur dioxide in the flue gases reacts with the calcium carbonate to produce calcium sulfite (CaSO3), and this calcium sulfite is converted into gypsum (CaSO4 · 2H2O) by oxidizing it with water.