FIELD: manufacture of building materials. SUBSTANCE: invention relates to improved composite material especially suited to manufacture building products. Method comprises adding aqueous siloxane emulsion to aqueous suspension of calcium sulfate material and host particles wherein indicated suspension is found at temperature allowing existence of calcium sulfate semihydrate crystals, said suspension containing at least one hydrogen-modified siloxane and being stable under conditions allowing existence of calcium sulfate semihydrate crystals+ADs- placing siloxane emulsion onto even porous molding surface to form cake before temperature of the latter falls below temperature, at which calcium sulfate semihydrate is rehydrated into calcium sulfate dihydrate+ADs- removing major part of water from the cake through above- mentioned porous surface and cooling the cake to rehydration temperature+ADs- pressing the cake into sheet and removing excess water, during which operation calcium sulfate semihydrate crystals adjacent to host particles are rehydrated in situ into calcium sulfate dihydrate crystals+ADs- and drying thus obtained sheet to remove remaining free water and to bring temperature of internal part of sheet to a value high enough to harden siloxane. EFFECT: increased waterproofness of material. 40 cl, 1 tbl, 3 ex
