PURPOSE: To reduce weight and to enhance strength by kneading a compsn. based on a hydraulic inorg. material with spherical particles of a specified org. compd. and carrying out molding and curing with steam.

CONSTITUTION: A compsn. based on a hydraulic inorg. material and contg. aggregate, reinforcing fibers, cellulose, etc., is kneaded with spherical particles of an org. compd. having 0.1-15μm particle size added by 1-20 pts.wt. per 100 pts.wt. of the hydraulic inorg. material. The org. compd. is selected from among cetyl alcohol, phenol, thymol, lauric acid and palmitic acid. The resulting kneaded material is molded at ordinary temp., cured in the air at ordinary temp. and caked by drying. This caked body is cured with steam at a temp. between the m.p. of the spherical particles and 80°C for 4-24hr.