Synthesis and characterisation of wollastonite with aluminium and fluoride ions

Abstract

This work aims to investigate the influence of aluminium and fluoride ions on the mechanism of wollastonite formation during a two-step synthesis. For the synthesis, three mixtures with different chemical compositions were prepared. The first step of the synthesis was carried out under hydrothermal conditions (stirring – 50 rpm; 200 °C, 4–48 h); the second step was carried out in a furnace with a temperature range of 900–1100 °C for 1 h. The chemical composition of the primary mixture has been determined to exert a significant influence on the mineralogical composition of the hydrothermal synthesis products, since tobermorite and xonotlite dominated in the pure system, while tobermorite and cuspidine dominated in the mixture with aluminium and fluoride ions. After calcination of the precursors, pure wollastonite was obtained using a mixture of amorphous silica and calcium oxide, while in the other two mixtures, it was identified together with wollastonite cuspidine and fluormayenite or gehlenite. Dilatometric and BET analyses of the products showed that Al3+ and F− ions caused densification and loss of porosity during the formation of wollastonite. The synthesised products were characterised by STA, XRD, DIL and BET analyses.