Synthesis, characterisation and thermal behaviour of lithium aluminosilicate inorganic polymers

Abstract

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 geopolymers. Curing at 40 °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 ²⁷Al and ²⁹Si MAS NMR spectra of the cured materials contain narrow resonances more characteristic of zeolites than of inorganic polymers. Heating the synthesised products at <800 °C produces β-eucryptite, LiAlSiO₄. In a further series of thermal reactions, β-spodumene, LiAlSi₂O₆ is formed at 900 °C, decomposing at 1100 °C to form additional β-eucryptite. At 1275 °C, β-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 β-eucryptite and β-spodumene ceramics.