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

Crystallization‐induced suppression of intumescence in aqueous alkali silicates

“…Li‐silicates also show a similar behavior in terms of the onset in temperature for the start of dehydration with a higher value, that is, 200°C seen for Li 2 O/SiO 2 = 0.29 (the mass loss behavior seems to be slightly different when compared to Na‐ or K‐silicates). This difference in onset of dehydration temperature is linked probably to the network being more mobile on increasing the alkali concentration as shown in our previous study on Na‐silicates 11 . Furthermore, a higher water content is retained in the system on increasing alkali content 11,37 resulting in a reduced onset temperature of dehydration as observed especially in the case of Na‐ and K‐silicates (see Figure S2).…”

“…Crystallization of the system increases as a function of temperature for both the molar ratios suggesting the existence of the disproportionation reaction: 2Q 3 (glassy) ↔ Q 2 (crystalline) + Q 4 (glassy). Globally, as observed in case of Na‐ 11 and K‐silicates, the system tends to evolve toward a more condensed state on increasing temperature. Dissolution of the crystalline phase is observed for Li 2 O/SiO 2 = 0.4 at 450°C similar to what has been reported for glasses at very high temperatures (800°C) 50 suggesting a behavior relatively similar to binary glasses, though not a lot of information is available on such glass systems with added water content.…”

“…Aqueous alkali silicates contain a silicate network dissolved in water as represented schematically in Figure 1A. These solutions are composed of several proton‐related species, namely, free water, solvating water (H‐bonded water molecules), and silanols 10,11 . Thermally treating these silicate solutions results in the removal of water directly influencing the macro and microscopic structural properties (Figure 1B) shows a schematic illustration of the expected state of the network on dehydration.…”

“…Na‐silicates have shown an Arrhenius evolution (below 500°C) with $${E}_a$$ values approaching 30 kJ/mol for both the molar ratios 11 (as shown in Figure S3), which has been previously observed for glass melts 39,40 . The existence of a single $${E}_a$$ is a consequence of an equilibrium where silanols are released first followed by their conversion into solvating water molecules which then leave the system due to evaporation 11 . K‐ and Li‐silicates, on the other hand, tend to show multiple activation energies.…”

“…Thermal behavior of Na‐based silicates has been quite extensively studied, focusing mainly on the macroscopic thermal evolution and microscopic properties at relatively lower temperatures. These systems have been found to exhibit considerable intumescence, that is, foaming upon thermal treatment, associated to the removal of water 9–13 . However, only recently have we reported an in‐depth understanding of the thermal evolution of Na‐silicates by linking the macroscopic structural expansion to the changes induced at the microscopic scale 11 .…”