Fiberboard with improved water resistance

A method of making a water resistant fiber board comprising:

• combining calcium sulfate dihydrate and host particles with water to form a slurry;
• heating the slurry under pressure to calcine the calcium sulfate dihydrate to form alpha-calcined calcium sulfate hemihydrate;
• relieving the pressure;
• adding a silicone compound and magnesium oxide to the slurry;
• dewatering the slurry to form a filter cake;
• forming the cake into a desired shape;
• and
• allowing the cake to set.

A fiberboard article comprising:

• host particles distributed throughout said article, having a plurality of calcium sulfate dihydrate crystals formed in at least one of the group consisting of voids, crevices, pits, cracks, fissures, hollow cores and other surface imperfections in said host particles;
• an interlocking matrix of calcium sulfate dihydrate crystals distributed throughout said article, including one or more crystals from said plurality of calcium sulfate dihydrate crystals formed in said host particles;
• a silicone polymer matrix distributed throughout said article around said calcium sulfate matrix;
• and
• magnesium oxide distributed throughout said article surrounded by said silicone polymer matrix.