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 prepared in a CaO-SiO₂-H₂O system via hydrothermal synthesis was analyzed. X-ray diffraction (XRD) and scanning electron microscopy (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 Na₂O in the raw material should be less than 5wt% for preparing pure xonotlite crystals via hydrothermal synthesis in a CaO-SiO₂-H₂O system.