Magnesium analogues of aluminosilicate inorganic polymers (geopolymers) from magnesium minerals

Abstract

Attempts to synthesise magnesium-containing analogues of aluminosilicate geopolymers from the 1:1 and 2:1 layer magnesiosilicate minerals chrysotile and talc, as well as the magnesium mineral sepiolite are reported. The effect of pre-treating these starting minerals by grinding and/or dehydroxylation was also investigated by XRD, ²⁹Si and natural-abundance ²⁵Mg solid-state magic angle spinning (MAS) NMR spectroscopy. The products from sepiolite most closely resembled an aluminosilicate geopolymer, setting at 40 °C to an X-ray amorphous product containing a broad characteristic ²⁹Si MAS NMR resonance at −90 ppm. The ²⁵Mg MAS NMR spectrum of this product also showed evidence that some of the Mg was located in tetrahedral sites, as expected for a conventional geopolymer. A similar ²⁵Mg MAS NMR result was obtained for chrysotile, but talc proved to be extremely resistant to geopolymer synthesis, requiring treatment at 120 °C for 3 days to set to a friable material retaining the XRD and NMR characteristics of the original talc or its crystalline dehydroxylation products. This lack of reactivity may be related to the 2:1 layer-lattice talc structure, or to the fact that a suitably reactive amorphous product is not formed upon dehydroxylation.