The gypsum used to make drywall comes either from natural gypsum (quarried rock) or from a by-product of coal-burning power plants. Guess which gypsum source is considered more environmentally friendly? If you guessed natural gypsum, guess again. Most green building enthusiasts prefer drywall made from flue-gas desulfurization (FGD) sludge. Because of Clean Air Act requirements, power plants burning high-sulfur coal use scrubbers to remove much of the sulfur dioxide, a component of acid rain, from their flue gas. The scrubbing process results in the production of 22 million tons of FGD sludge a year. Because landfill costs are rising, coal-burning plants are under pressure to find a market for their FGD sludge, which can