A solid, cellular, heat- and sound-insulating alkali silicate material which may be sawn into slabs or blocks is made from an aqueous solution of alkali silicate in which the molecular ratio of silica: alkali oxide is about 3. This silicate solution is coagulated either by the addition of a coagulating agent, namely an alcohol, a ketone or an alkali chloride, or by neutralization with acid (e.g. hydrochloric acid) of not more than one third of the initial alkali oxide content, and the coagulated mixture is separated, e.g. by pressure, from any uncombined water, coagulating agent, eliminated alkali oxide or salt and then heated for about 24 hours at a temperature between 200 DEG and 450 DEG C. until it has a bulk density of 0.3 to 0.4 gms./cm3 and a solid fine macrocellular structure. If sodium silicate solution is subjected to the process, the resulting material has a melting point of about 900 DEG C.