PURPOSE: A slurry that is composed of a raw material of silicic acid, that of magnesia and water is subjected to hydrothermal synthesis to produce magnesium silicate where the raw material of silicic acid and the reaction conditions are specified to give a formed product of high strength and low dry shrinkage without reduction in water repellency.

CONSTITUTION: An amorphous raw material for silicic acid, preferably ferrosilicon dust or silicon dust as a by-product in the production of silicon-iron or metallic silicon, a raw material for magnesia and water are mixed to prepare a slurry. The slurry is pressurized and heated under stirring or molded or casted, then pressurized and heated to effect hydrothermal synthesis to produce magnesium silicate. At this time, the saturated vapor pressure, i.e., the reaction pressure, is set to lower than 9kg/cm2 at 179°C. The use of amorphous silica such as white carbon or kieseguhr as the raw material for silicic acid is undesirable, because the resultant magnesium silicate tends to lower its water repellency.