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.
Chemically, the synthetic gypsum produced from power plant scrubbers is nearly identical to virgin gypsum that is extracted from gypsum mines. Drywall manufacturers recognized in the 1990s that they could reduce costs by switching to synthetic gypsum, and they’ve also been able to tout the environmental benefits of keeping this waste product out of landfills. Of the 21 drywall plants operated by USG (North America’s largest drywall manufacturer), nine use exclusively synthetic gypsum, and six use a blend of virgin and synthetic gypsum. Most of the drywall factories that have been built in the past ten years in North America are using synthetic gypsum.
Heavy metal contamination has become a concern with fly ash from coal power plants—should we also be concerned about synthetic gypsum, another coal combustion byproduct? While fly ash is directly produced from coal combustion and physically removed from stacks, synthetic gypsum is produced using just one constituent of the flue gases—sulfur dioxide—so it has much less potential for heavy metal contamination.
Nonetheless, the Healthy Building Network recently cited drywall plant emissions data suggesting that high mercury levels might occur in synthetic gypsum that comes from power plants burning high-mercury coal. More detailed studies and transparency are needed to address these concerns. UL Environment is developing a standard for drywall, ULE 100, that addresses—among other factors—mercury emissions. Low mercury emissions will earn a point toward different levels of green certification for drywall. While mercury emissions won’t be a “knock-out” criterion for drywall in ULE 100, the standard will put this issue on the table and force drywall manufacturers to address it.
While we look forward to ULE 100, EBN still considers drywall made from synthetic gypsum to be safe, and we still consider this drywall to be a greener option than drywall made from virgin gypsum.