Thermal behaviour of inorganic geopolymers and composites derived from sodium polysialate

Abstract Inorganic polymers based on alumina and silica polysialate units were synthesised at room temperature from metakaolinite and sodium silicate in a highly alkaline medium, followed by curing and drying at 65 °C. When properly cured, these polymers exhibit remarkable thermal stability; after losing their hydration water at about 200 °C, they retain their X-ray-amorphous tetrahedral Al and Si network up to the onset of melting at 1300 °C. A small amount of mullite and corundum formed at 1200–1300 °C may result from the presence of a trace of unreacted metakaolinite. Similar experiments with poorly-curing formulations containing higher Na and Si contents show that their unpolymerised components form crystalline nepheline (NaAlSiO 4 ) at 800 °C, prior to melting at about 1100 °C. A series of geopolymer composites were prepared containing 10–20 vol.% of various granular inorganic fillers ranging from waste demolition materials through mineral tailings to engineering ceramics. The physical and thermal properties (bulk density, compressive strength and thermal expansion) of these composites were measured. The thermal expansion is influenced by the properties of the filler, but all the samples showed only slight expansion up to ∼800 °C on the first heating cycle. Microcracking of the composite bodies during drying can be minimised by the addition of a small amount of glycerol.