The thermo-mechanical and microstructural character of sodium-poly(sialate-siloxo) (Na-PSS) geopolymers synthesised from dehydroxylated kaolinite (metakaolinite) have been investigated. Thermal analysis by means of TG-DTA showed a single endothermic peak at 135-140 degrees C due to dehydration (water evolution) from the geopolymer framework. Thermal expansion measurements show that geopolymer suffers 2% shrinkage below 250 degrees C and is then dimensionally stable up to 800 degrees C. The inclusion of aggregate (alpha-quartz or granite) was found to reduce the shrinkage by 1% although the presence of the quartz limits the working temperature range of the composite due a to disruptive phase change. Thermal conductivity and compressive strength of Na-PSS geopolymers varied with change in chemical composition of the geopolymer as well as the amount and type of aggregate. Investigation of the microstructure by electron microscopy showed that the ratio of the starting materials influences the homogeneity of the geopolymer microstructure, which in turn leads to differences in thermal conductivity and compressive strength.