Patent ID: 7892472

Claim:
A method for making a water-resistant gypsum-based board comprising: (A) mixing a siloxane emulsion with the gauging water used to prepare said gypsum-based board; (B) mixing a small amount of dead-burned magnesium oxide with calcined gypsum, wherein the amount of magnesium oxide is from 0.1 to about 0.5 wt % of the gypsum; (C) mixing said siloxane emulsion/gauging water mixture with said calcined gypsum/magnesium oxide mixture to form an aqueous slurry; (D) allowing said dead-burned magnesium oxide to catalyze cross-linking of said siloxane; and (E) shaping said slurry and allowing said shaped slurry to set to form a set gypsum-based, water-resistant board.