Polymerization of siloxane is improved using a gypsum-based slurry that
includes stucco, Class C fly ash, magnesium oxide and an emulsion of
siloxane and water. This slurry is used in a method of making
water-resistant gypsum articles that includes making an emulsion of
siloxane and water, then combining the slurry with a dry mixture of
stucco, magnesium oxide and Class C fly ash. The slurry is then shaped as
desired and the stucco is allowed to set and the siloxane polymerizes.
The resulting product is useful for making a water-resistant gypsum panel
having a core that includes interwoven matrices of calcium sulfate
dihydrate crystals and a silicone resin, where the interwoven matrices
have dispersed throughout them a catalyst comprising magnesium oxide and
components from a Class C fly ash.