Abstract: The invention relates to a furnace having an inner wall, an outer wall and a heat insulation arranged in between. The heat insulation is foamed up in situ, and hardened, from a mass which contains,per 100 parts by weight of a reactive solid (stone-forming component),40-250 parts by weight of a water-containing hardener which effects the hardening reaction of the reactive solid (stone-forming component) in the alkaline region,40-250 parts by weight of fillers and a blowing agent.The reactive solid used is preferably a reactive solid from the group comprisingI finely dispersed oxide mixture containing amorphous silica and alumina,II a glassy, amorphous electrostatic precipitator ash,III ground calcined bauxite,IV electrostatic precipitator ash from lignite-fired power stations,V undissolved, amorphous SiO.sub.2, especially from an amorphous, disperse-pulverulent, dehydrated or water-containing silica or from high-temperature processes (silica fume), andVI metakaolin.