PURPOSE: To provide the titled rust preventive compsn. excellent in rust proofing and adhesion to reinforcing bars, by incorporating silica rock powder, slaked lime, asphalt emulsions, styrene copolymer latices, water, and surfactants.

CONSTITUTION: A compsn. which comprises 15W25pts.wt. silica rock powder, 25W35pts.wt. slaked lime, 8W14pts.wt. asphalt emulsion (org. solids 40W45wt%), and 10W25pts.wt. org. solids in styrene copolymer latex (styrene content 65% or higher, org. solids having reactive groups 40W60wt%), the balance being water, surfactants, and inorg. fillers to be added if necessary. A styrene/butadiene copolymer with styrene content of 65% or above, org. solids of 40W60%, and modified substrates of 2W7% is pref. as styrene copolymer latices to be used, and the amt. of surfactants added to said compsn. is pref. 1% or less.