Abstract: A process for producing in situ foams based on alkali metal silicates characterized in that an aqueous alkali metal silicate solution on the one hand and a hardener solution on the other hand are accomodated in separate pressurized containers A and B, prior to use the hardener solution in container B is added to and mixed with the silicate solution in container A in the presence of a suitable emulsifier for foam formation and a propellent gas liquid under high pressure between 0.degree. and 50.degree. C., and, on actuation of the discharge valve of container A, an alkali metal silicate foam is discharged. The alkali metal silicate foams formed are distinguished by non-inflammability, high temperature stability and good heat-insulating properties and are used as a filling, construction and insulating material or the like in the building industry.