A moisture resistant gypsum-based product, e.g., a gypsum board, is made by adding a small amount of a siloxane to the aqueous slurry used to make the gypsum-based product along with a small amount of a dead burned magnesium oxide catalyst to enhance the curing of the siloxane. In the preferred embodiment, the siloxane is formed into an aqueous emulsion in situ with no chemical emulsifier.