PURPOSE: To produce a gypsum board of pulp sludge for high-strength building material as a means for reusing papermaking pulp sludge.

CONSTITUTION: In producing building gypsum board of pulp sludge by blending papermaking pulp sludge (aggregate having 0.5-10μm used in papermaking from 25-150μm fibrous material, such as clay, containing 40-60% free water) with gypsum, pressurizing and molding, blending of hydrous fine fibers of the papermaking pulp sludge and gypsum is carried out by applying shear force from a funicular-II region to a capillary region. Free water in the hydrous fine fibers of the pulp sludge is replaced with saturated water of gypsum by generated instant shear force, crystallization of gypsum dihydrate is completed in the fine fibers in the papermaking pulp sludge by mechanochemical reaction between the fine fibers and hemihydrate gypsum particles to efficiently react free water in the hydrous fine fibers with gypsum and to bring about complete adhesion of the fine fibers to the gypsum.