Crystallization-induced suppression of intumescence in aqueous alkali silicates

Thermal properties of aqueous alkali silicates have been investigated to establish a fundamental understanding of the structural state of silicate network and its impact on intumescence, up to 450°C. Foaming behavior is dependent upon the type of alkali ion (Na, K, Li) as well as composition as evidenced from a combination of tools involving thermogravimetric analysis, nuclear magnetic resonance, and X-ray diffraction. Na silicates show extensive foaming, Li-silicates do not foam, whereas K-silicates exhibit an intermediate behavior depending upon the starting state and heating kinetics. Crystallization is observed for K- and Li-silicates but not in the case of Na-silicates and appears to be a limiting factor for macroscopic structural expansion. Quantitative analysis of different species reveals the network to be relatively mobile, in terms of its ability to reorganize, in pre-dried (at 150°C) Na-silicates. A reduction in this mobility is seen for K-silicates as crystallization reduces K ions and silanols in the amorphous phase at and above 150°C, thus reducing the extent of foaming. In contrast, phase separation coupled with crystallization resulting in a complete exit of Li ions from the amorphous phase tends to suppress completely the intumescent phenomenon.