Effect of Alumina Release Rate on the Mechanism of Geopolymer Gel Formation

The effect of the rate of alumina release during the reaction of a one-part (just-add-water) geopolymer mix on growing geopolymer gels is investigated by coupling time-resolved and spatially resolved infrared spectroscopic analysis. The rate of alumina release from different precursors has previously been identified as a critical controlling factor in the formation of mechanically strong and durable geopolymers; however, its influence on the nanostructure of the geopolymer gel has never before been directly analyzed. Gel microstructure and nanostructure are able to be observed by synchrotron radiation-based infrared microscopy (SR-FTIR) with hierarchical clustering analysis, conducted in conjunction with the in situ attenuated total reflectance (ATR) FTIR technique to provide temporal resolution. The SR-FTIR technique provides the opportunity to analyze the chemistry of the heterogeneous geopolymer binder at a level of detail that previously has not been available. Although spatially averaged (ATR-FTIR) infrared results show similar spectra for well-cured samples with different alumina release rates, SR-FTIR shows that the gels arc markedly different, with less unreacted silica particles remaining in geopolymer gels with slow alumina release. but greater homogeneity when alumina is released more rapidly. Rapid release of alumina is shown to impede the dissolution of silica particles in the early stages of the reaction; thus, the participation of alumina in forming geopolymer gels appears to be more beneficial to geopolymer nanostructural development when it becomes available in the later stages of the reaction process.