Synthesis and characterisation of materials based on inorganic polymers of alumina and silica: sodium polysialate polymers

Abstract

Inorganic polymers based on alumina and silica polysialate units were synthesised from dehydroxylated aluminosilicate clay (metakaolinite) condensed with sodium silicate in a highly alkaline environment. Reaction of the aluminosilicate with alkali polysilicates yields polymeric Si–O–Al three-dimensional structures with charge-balancing positive ions such as hydrated Na+ in the framework cavities. A statistical study of the effect on the polymerisation process of the molar ratio of the component oxides and the water content of the mixture showed the latter to be a critical parameter. The polymerisation mechanism and structures of the products were investigated using NMR, XRD and FTIR spectroscopy. 29Si liquid-state NMR shows that some compositions do not cure properly because of incomplete reaction of the sodium silicate with the metakaolinite. FTIR confirms that during drying of the incompletely cured samples, Na migrates to the surface where it undergoes atmospheric carbonation. The cured polymers were found to be essentially X-ray amorphous, with bulk densities of 1.3–1.9. During polymerisation the coordination of Al in the metakaolinite reactant (IV, V and VI) changes almost completely to IV in all the polymer compositions. The environment of the Na is unchanged irrespective of the polymer composition. The solid-state 29Si NMR spectra indicate a range of Si–O–Al environments. Typical mechanical properties of the best polymers were: Mohs hardness >7, Vickers hardness about 54, and compressive strength (after drying for 1 h at 65°C) 48.1 MPa.