Effects of NaOH amount on condensation mechanism to form aluminosilicate, case study of geopolymer gel synthesized via sol–gel method

Abstract

To balance cost and performance of geopolymers, alkalinity of activating solution is critical. Alkalinity affects condensation that determines the final gel structures, but this effect is confounded by dissolution and is not understood from direct experimental evidence. In this study, we investigated effects of alkalinity on condensation for gels synthesized via a sol–gel method that eliminates dissolution process. As alkalinity increased, particle sizes of the gels increased as indicated by SEM, Si/Al ratios of the gels decreased but polymerization extent increased as supported by FTIR, 27Al and 23Na NMR, and composition analysis. The mechanism for the effects of alkalinity was proposed accordingly: (1) increasing alkalinity lowers the Si/Al ratio (i.e., more incorporation of Al) of the resulting products probably by affecting charging conditions of the Si and Al units; (2) the presence of Al(OH)4− units promotes their condensation with nearby species to increase the extent of polymerization; (3) enhanced condensation increases particle sizes of the gels even at microstructural level. This understanding on condensation independent of dissolution provides ways to control gel structures and Si/Al ratios and thus tailor properties accordingly, as well as to suggest a strategy (by altering Si/Al ratios during condensation) to develop kinetics-controlling admixtures.