Effect of Na＋ on xonotlite crystals in hydrothermal synthesis

The effect of Na＋ ion concentration on the crystalline phase composition and morphology of xonotlite crystals pre- pared in a CaO-SiO2-H20 system via hydrothermal synthesis was analyzed. X-ray diffraction （XRD） and scanning electron mi- croscopy （SEM） results indicate that Na＋ ion concentration has a significant impact on the composition and morphology of crystalline phases in the products under the initial conditions of a molar ratio of CaO/SiO2 of 1.0 and a reactant concentration of 0.05 mol.L-1 at 225℃ for 15 h. The main crystalline phase in the products has a phase transition from xonotlite to pectolite, and the morphology changes from fibrous to broomlike shape with the Na＋ ion concentration increasing. Therefore, the content of Na~O in the raw material should be less than 5wt% for preparing pure xonotlite crystals via hydrothermal synthesis in a CaO-SiO2-H20 system.