Thermal behavior of waterglass: foaming and xerogel-to-glass evolution

Structural properties of aqueous Na-silicates at both room and high temperature (below 500°C) have been investigated to better understand the microscopic evolution of structure and the macroscopic phenomenon of foaming. NMR spectroscopy has provided a direct quantification of structural units defined as Qⁿ. Solid-state NMR along with Raman spectroscopy has indicated the extent of local structural reorganization on heating the silicates. Heating an aqueous Na-silicate leads to structural changes due to the removal of water, where the quantity of leftover water in the system obeys an Arrhenian evolution with an activation energy of 30 kJ.mol⁻¹. TGA, NMR and Raman measurements suggest that the structure becomes similar to the one of hydrated silicate glasses studied in geochemistry. Increasing the Na concentration results in a larger quantity of water retained after a pre-drying treatment, which correlates with a lower softening temperature of the material and is, macroscopically, related to foaming of the silicate.