Superheated steam-induced phase transformation of layered aluminosilicates

Abstract

This study demonstrated that phase transformations of solid-state materials can be accelerated by processing under superheated steam―a dry vapor relevant to both Earth's crust and industrial environments. Here, layered aluminosilicates were used as a model system to investigate the thermal behavior of inorganic materials under superheated steam. Specifically, kaolinite―a two-dimensional materials with Al and Si homogeneously within its stacked layers―was calcined under superheated steam and compared to conventional calcination. In the 400–600 °C range, dehydroxylation of kaolinite was shifted to higher temperature under superheated steam. This result indicated the hydroxyl-bearing inorganic structures can be preserved at relatively higher temperatures in superheated steam environments. Above 1000 °C, the transformation of metakaolinite―an amorphous layered aluminosilicate formed via dehydroxylation of kaolinite―into mullite and cristobalite was accelerated, with both phase appearing at temperatures ∼100 °C lower than in air. Solid-state ²⁹Si nuclear magnetic resonance spectra revealed increased Si content in mullite formed under superheated steam, confirming enhanced mullitization. However, complementary analyses using X-ray diffraction and scanning electron microscopy showed that cristobalite particles grew more rapidly than mullite, suppressing its crystallinity under superheated steam conditions. These results reveal that superheated steam not only facilitates phase transformation in layered aluminosilicates but also modulates the competitive crystallization dynamics between mullite and cristobalite offering new opportunities for advanced inorganic processing.