Microstructure of sodium polysialate siloxo geopolymer

Scanning electron micrographs of a well-cured polysialate siloxo geopolymer (silica:alumina ratio 2:1) prepared from metakaolinite reveal a microstructure consisting of a glass-like matrix containing metakaolinite relicts and impurity quartz grains. This inhomogeneity probably results from viscosity increases during preparation militating against efficient mixing of the components, but does not appear to seriously degrade the physical properties of the product. The matrix composition, determined by EDAX, conforms closely to the expected molar ratio and is unchanged by heating at 1200 °C, which however, brings about the crystallization of mullite needles in areas of the geopolymer matrix, the dissolution of the quartz impurities and the depletion of silica from the metakaolinite relicts, leaving behind fine alumina grains. These previously unsuspected thermal reactions help to explain the exceptional stability of the geopolymer matrix. Transmission electron micrographs confirm the essentially amorphous nature of both the as-prepared and heated geopolymer samples, but unexpectedly reveal in the latter nanometre-sized features resembling spinodal decomposition structures in glass.