The structural evolution of in-situ inorganic foam based on sodium or/and potassium is investigated. The synthesis of foam based on an alkaline polysialate, is achieved at slightly elevated temperature by the alkaline activation of raw minerals and industrial waste. The structural evolution is studied through differential thermal analysis/thermogravimetric analysis (DTA-TGA) coupled with mass spectrometry, in-situ X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and 29Si, 27Al magic-angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR). The role of alkali cations is observed, and they are shown to have a significant effect on structure changes with temperature, leading to the onset of a crystalline phase in potassium-based foam. Shifts of MDI reflect the competition that takes place during heating between the evolution of the zeolite phase and the amorphization of the material.