Effect of clays on the formation kinetics of xonotlite

Xonotlite (Ca 6 Si 6 O 17 (OH) 2 ) formation is highly dependent on the reactivity of the raw materials used during hydrothermal synthesis. The raw materials, typically silica (SiO 2 ) and lime (CaO), often contain secondary phases such as clay and Mg-bearing compounds which can further influence the synthesis of xonotlite. While small amounts of Mg stabilize xonotlite, the role of clay which may introduce Al, Fe and Si, remains poorly understood. This study investigates the impact of different quantities of clay during xonotlite synthesis. Xonotlite was obtained through hydrothermal synthesis carried out in an autoclave with stirring at 180 °C. The reaction kinetics was studied by stopping the synthesis at different time intervals and analyzing the obtained solids by XRD, TGA and FT-IR. Results indicate that the presence of aluminum from clay delays xonotlite formation and promotes instead the crystallization of (Al-)tobermorite, as confirmed by thermodynamic modelling. The effect increases with increasing quantities of clay or aluminum.