Byproduct gypsum is the ultimate recyclable. It comes from a process used to remove sulfur dioxide from emissions at coal-fired power plants. After the power plant removes fly ash and other impurities from the coal combustion process, the remaining stack emissions are fed through a limestone slurry which removes sulfur dioxide. The byproduct of this process is calcium sulfate – the chemical name for gypsum rock.

Four National Gypsum plants produce wallboard exclusively with byproduct gypsum: Shippingport, PA, near Pittsburgh; Apollo Beach, FL, near Tampa; Mt. Holly, NC, near Charlotte and the company's Westwego Plant, near New Orleans.