Hardenable foam compositions comprising oxygen-releasing foaming agents, a hardener consisting of an alkali silicate solution, a fine particle solid material which reacts with said hardener, namely an oxide mixture consisting of SiO.sub.2 and Al.sub.2 O.sub.3 extracted from a filter dust, an electrofilter ash from high-temperature power plants, and calcinated bauxite or amorphous silica, to which reaction accelerators are added, are improved, firmer, faster hardening foams. Reducing agents, in particular sulphur compounds with a reducing action are preferred. Oxidizing agents or fine particle substances such as soot or color pigments are effective.