Light weight metal fire door core

A building material composition prepared by dewatering, drying and curing a fluid aqueous slurry of calcined gypsum, paper fibers, a water insoluble organic binder, fiber reinforcement and a set retarder wherein said aqueous mixture contains on a dry weight basis, about 50 to 72 percent of the calcined gypsum, about 25 to 45 percent of the paper fiber, about 0.5 to 5 percent of the water insoluble organic binder, about 0.5 to 2 percent of the fiber reinforcement and up to about 0.3 percent of the set retarder.

A method for making a fire door core from a fluid aqueous mixture of materials, which method comprises mixing water, calcined gypsum, paper fibers, a water insoluble organic binder, fiber reinforcement and a set retarder, wherein, on a water-free, dry weight basis, the aqueous mixture contains about 50 to 72 percent of the calcined gypsum, about 25 to 45 percent of the paper fiber, about 0.5 to 5 percent of the water insoluble organic binder, about 0.2 to 2 percent of the fiber reinforcement and up to about 0.3 percent of the set retarder, the water being present in an amount in excess of the stoichiometric amount needed to hydrate the calcined gypsum and sufficient to provide a fluid slurry, placing a charge of the fluid slurry in a pressure mold, applying sufficient pressure to the fluid slurry in the mold to dewater the fluid slurry and maintaining the pressure until a self-supporting compressed charge comprising a press-molded slab is produced, removing the press-molded slab from the mold and thereafter drying the slab by heating it sufficiently to remove excess water.