Crystallization‐induced suppression of intumescence in aqueous alkali silicates

Abstract

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.