PROBLEM TO BE SOLVED: To prevent a concrete surface modifier even in a large lump from being left behind undissolved and the surface of a modified concrete structure from being made brittle by a method wherein the concrete surface modifier is made into the lump containing at least iron sulfate and magnesium sulfate. SOLUTION: A concrete surface modifier is made into a lump containing at least iron sulfate and magnesium sulfate functioning as a heat source through the generation of heat caused by its contact with the water content and alkali component of the concrete and as a filler suppressing the content of the iron sulfate in the large lump. The powder of the iron sulfate and the magnesium sulfate is granulated into lumps, each of which weighs about 0.5-5 g, since too small or too large recessed parts formed by too small or too large lumps are not effective for the improvement of the adhering and proliferating conditions of algae and microbes. Since the change in the content of the iron sulfate beings a brittle surface modification and an insufficient modification, the iron sulfate and the magnesium sulfate are respectively set within the range of about 20-60 wt.%. Thus, the poor dissolution due to insufficient amount of heat and the embrittlement of the modified surface due to excess iron content can be avoided.
