Chemical and Structural Microanalysis of Aluminosilicate Geopolymers Synthesized by Sodium Silicate Activation of Metakaolinite

An earlier study by the authors on the chemical optimization of compressive strength in aluminosilicate inorganic polymers (geopolymers), produced by sodium silicate activation of metakaolinite (MK), has been extended to provide chemical analysis of the principal microstructural features using scanning electron microscopy/energy dispersive spectrometer (SEM/EDS) measurements. SEM imaging reveals the presence of a two-phase microstructure; the matrix phase being the fully formed inorganic polymer, while the grain phase is reminiscent of, but chemically dissimilar to, the MK precursor. The current study has provided the following insights into microchemical optimization of compressive strength: (1) The overall chemical composition of the starting materials does not accurately predict the chemical composition of the geopolymer matrix due to incomplete dissolution of the starting material. (2) The ideal Na:Al molar ratio of 1.0 required for charge balance in the bonding network has been observed for the matrix. (3) As the nominal Si:Al molar ratio for the final material increases, the grain Si:Al ratio increases steadily from the MK value, without resulting in the full dissolution of the grain. These results underline the importance of using SEM/EDS microchemical analysis for designing chemical processing conditions for strength-optimum geopolymers, rather than relying on imaging or bulk chemistry in isolation, which may provide misleading results.