The synthesis of an in situ inorganic foam based on alkaline polysialate was achieved at a slightly elevated temperature by alkaline activation of raw minerals and industrial waste based on a geopolymer synthesis process. To understand the formation of the foam, the differences induced by the variations in the alkaline element (sodium or potassium) and water content were investigated throughout XRD, in situ FTIR-ATR, volume expansion and thermal analysis coupled with mass spectrometry. The amount of water is the determining parameter for volume expansion but does not interfere with the structure formation of foams. The comparison between geopolymers and foams has elucidated the formation of at least two amorphous networks in the case of sodium foam and three in the particular case of potassium as the alkaline element.