Polycondensation reaction effect on the thermal behavior of metakaolin-based potassium geopolymers

Abstract: This study aims to demonstrate the differences between two series of metakaolin-based geopolymers obtained from silicate solutions with different potassium concentrations (5 and 6 mol/L). First, the differences between the two solutions were identified by performing 29Si MAS-NMR, Raman and infrared spectroscopies. The reactivity of the solution was found to depend on its degree of condensation; the less condensed Si species there are, the higher the reactivity. Then, the fresh, solution-based geopolymer pastes, a commercial metakaolin (Si/Al = 1.44), sands and carbonate fillers, were subjected to in situ FT–IR spectroscopy and a DTA–TGA analysis to evaluate their reaction kinetics, rate and energy. Finally, consolidated samples of the same geopolymers were investigated with mercury intrusion porosity and endogenous shrinkage tests to evaluate the evolution of the structure over time. The results showed that the composition of the geopolymer influences its characteristics, as geopolymers based on silicate solutions with low concentrations of potassium have slow reaction rates and high reaction initiation energy requirements. Moreover, the structure of the analyzed geopolymer is denser with less porosity than the structures of geopolymers made with a siliceous solution that have a higher potassium content; its pore sizes are also approximately 40% smaller than those of the other geopolymer. The same behavior is noted for shrinkage, as it is predominant in geopolymers with lower Si/K molar ratios. In conclusion, it is possible to predict the characteristics of a geopolymer by analyzing its raw materials. Graphical abstract: [Figure not available: see fulltext.] © 2023, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.