The present invention is directed to a hardening composition characterized by comprising (A) a slag composition containing 0 to 37 wt. % of CaO, 23 to 90 wt. % of SiO.sub.2, and 0 to 40 wt. % of Al.sub.2 O.sub.3 and 0.1 to 50 wt. % of one or more compounds selected from the group consisting of iron oxide, alkali components, phosphorous components, titanium dioxide, manganese oxide, and magnesium oxide, and/or 0.1 to 10 wt. % of a halogen; and (B) an alkaline activator. The invention is also directed to a hardened product obtained by mixing the hardening composition with water. The properties of the hardened product are suitable for use as various concrete materials, and therefore, refuse-incinerated ashes sewage sludge, etc. are effectively utilized and recycled as a resource.