Laser scanning confocal microscopic analysis of metakaolin-based geopolymers

Laser scanning confocal microscopy (LSCM) is a microscopy technique that has the capability of transmitting light through thin sections of material, to visualize a 2-dimensional plane of the sample interior. Computer software is then used to form 3-dimensional representations of the sample from multiple 2-dimensional scans. In this study, LSCM was employed to resolve the distribution of the formed geopolymer phase and metakaolin precursors in geopolymers with varying molar ratios (M₂O·Al ₂O₃ ·xSiO₂, where x = 0, 1, 2 and M = Na⁺, K⁺, or Na/K = 1) and make correlations between the observed distributions and the degree of reaction between the two precursors. LSCM analysis agreed with results in NMR and microstructural studies presented in literature- increasing concentrations of silica and higher sodium to potassium ratios resulted in less reaction between metakaolin particles and alkalisilicate solutions. When potassium is used, the resultant microstructure consisted of finer precipitate sizes and porosity regardless of the amount of silica added. The use of LSCM allowed for a 3-D microstructural perspective of pores, unreacted metakaolin, and formed geopolymer precipitates.