PROBLEM TO BE SOLVED: To provide a water-hardenable hardened body which has sufficient strength, is usable as reinforced concrete and can reduce discharge of CO.SOLUTION: A water-hardenable hardened body is obtained by kneading industrial by-products, e.g. fly ash and blast furnace slag, fine powder of concrete sludge, aggregate, and, if necessary, additives. The fine powder of concrete sludge is obtained from residual concrete or return concrete. Sludge water is obtained by adding water to residual concrete or return concrete to form a slurry and removing ballast, sand and fine sand from the slurry; the sludge water is dehydrated to obtain a dehydrated cake; and the dehydrated cake is charged into a specified rotary drum to be blown with hot air and crushed and dried to obtain fine powder of concrete sludge. The fine powder of concrete sludge preferably has a specific surface area of 8,000 cm/g or smaller.
