A water-based concrete or masonry sealant includes an acrylic- or silicone-modified reaction product of an unsaturated drying oil and an alkyd, the reaction product being dispersed in aqueous solution; and a dispersion of metal salt catalyst in aqueous solution. A method of treating a previously sealed concrete or masonry substrate, includes the steps of: applying to the previously sealed concrete or masonry substrate a sealant comprising an acrylic- or silicone-modified reaction product of an unsaturated drying oil and an alkyd, the reaction product being dispersed in aqueous solution; an aqueous dispersion of metal salt catalyst, and water; and allowing the sealant to cure.