Supercritical hydrothermal flow synthesis of xonotlite nanofibers

This article reports a satisfactory and innovative method for the synthesis of xonotlite using a flow reactor and supercritical water. This study widens the variety of inorganic nanofibers produced in record breaking times by means of continuous reactors working under supercritical water conditions. In particular, the synthesis time of xonotlite, which takes normally more than 5 h, was reduced to only 20s by carrying out the reaction at 400 °C and 23.5 MPa. Resulting product was studied by several characterization techniques: x-ray diffraction, transmission electron microscopy, 29Si and 1H nuclear magnetic resonance and infrared spectroscopy. Furthermore, obtained product consisted of highly pure and crystalline flat nanofibers of 1–10 μm long with a length to diameter ratio of the order of 100. Also, the typical deviation from the ideal structure observed by nuclear magnetic resonance and the presence of Si-OH were explained in terms of surface defects. This work reinforces the interests of using supercritical conditions for the fast synthesis of crystalline nano-calcium silicates which, due to the number of potential industrial applications and the scalability of the technology, might represent technological breakthrough.
This study was carried out under the umbrella of the BASKRETE initiative and supported by the Basque Government under the ELKARTEK Program (project SUPER). In addition, Marta Diez is grateful to the University of the Basque Country (UPV/EHU) and the University of Bordeaux for her pre-doctoral fellowship, within the framework of the Cross-Border Euroregional Campus of International Excellence IDEX Bordeaux–Euskampus.