Thermodynamics of the hydrothermal synthesis of xonotlite

Xonotlite, as a multifunctional inorganic material, has been widely used in the fields of building insulation, friction braking, and bionic composite materials. However, the main method of producing xonotlite, the dynamic hydrothermal method, is regarded as a black box process. Optimization of synthesis conditions can only be achieved through trial-and-error experimentations, and there are inconsistencies in the experimental results reported in the literature. In this work, we established a thermodynamic model of the Ca(OH)₂-SiO₂-H₂O system under hydrothermal conditions, and investigated influencing factors of the xonotlite synthesis. The results show that, the predicted lowest temperature of xonotlite synthesis is approximately 170 °C. Furthermore, an optimum condition to synthesis xonotlite with a purity of 99% is proposed as follows: reaction temperature, 200 °C; Ca/Si ratio, 0.9-1.0; water-solid ratio, no more than 20; pH, 7-8. This new synthesis process has been confirmed by experiments.