Journal of Materials Science, Vol.45, No.14, 3707-3713, 2010
Synthesis, characterisation and thermal behaviour of lithium aluminosilicate inorganic polymers
Lithium aluminosilicate inorganic polymers were synthesised from dehydroxylated kaolin-type clay (halloysite) by the conventional method under highly alkaline conditions with lithium hydroxide or lithium silicate solutions of two different Li2O/SiO2 molar ratios. Variants were also developed of a solid-state synthesis method involving the thermal reaction of dehydroxylated halloysite with LiOH followed by hydration of the product. The molar compositions of the materials prepared by all three methods (SiO2/Al2O3 = 2.41-3.27, Li2O/SiO2 = 0.30-0.61, and H2O/Li2O = 9.33-10.40) fall within the range of compositions previously reported to produce viable geo-polymers. Curing at 40 degrees C produces solid samples of varying viability depending on the amount of synthesis water. The cured materials are not characteristically X-ray amorphous, but contain the lithium zeolites Li-ABW and fibrous Li-EDI, the latter in the materials synthesised by solid-state reaction. The Al-27 and Si-29 MAS NMR spectra of the cured materials contain narrow resonances more characteristic of zeolites than of inorganic polymers. Heating the synthesised products at <800 degrees C produces beta-eucryptite, LiAlSiO4. In a further series of thermal reactions, beta-spodumene, LiAlSi2O6 is formed at 900 degrees C, decomposing at 1100 degrees C to form additional beta-eucryptite. At 1275 degrees C, beta-spodumene reappears in the samples of higher silica content. Judicious manipulation of the composition and thermal treatment of the Li-zeolites formed in these lithium aluminosilicate syntheses could make them useful precursors to beta-eucryptite and beta-spodumene ceramics.