Study of the solubility of xonotlite

Xonotlite (Ca6Si6O17(OH)2) is a hydrated crystalline calcium silicate with CaO/SiO₂ = 1, which forms under hy‐drothermal conditions at temperature above ∼100°C. It has good thermal stability, it dehydrates to wollastonite only at 770 to 800°C and shows a good resistance to high pressures. Deposits of xonotlite are rare in nature, but it can easily obtained by hydrothermal synthesis in the temperature range of 180 to 350°C. Several studies have been conducted to better understand the formation conditions of xonotlite in terms of optimum temperature, duration and starting materials, but very little is known about its solubility under different conditions. The two solubility data sets available in literature differ by as much as 10 log units with no clear reason for this difference. The aim of the present study is to determine the solubility of xonotlite at 20°C and to compare this data with the available data in literature. To achieve this goal, solubility experiments from undersaturation were carried out at ambient temperature (20°C). Thermodynamic modelling was used to calculate the solubility product of xonotlite based on the measured total aqueous concentrations. The derived solubility of xonotlite is comparable to the solubility previous reported for synthetic xonotlite at 25°C. The synthesized xonotlite is several log units more stable than the solubility reported in literature for natural xonotlite, which seems to describe rather the solubility of amorphous C‐S‐H.