Effect of CaO Content on Structure of Zirconolite-barium Borosilicate Glass-ceramics

Glass-ceramics belonging to SiO₂-Na₂O-B₂O₃-BaO-CaO-ZrO₂-TiO₂ system were prepared by a melting-heat treatment method. The differential thermal analysis (DTA), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to research the effects of the CaO content (CaO, TiO₂ and ZrO₂ mole ratio is x∶2∶1, x=0.5-6) on crystalline phases and microstructure of samples when the concentration of nucleating agents (CaO, TiO₂ and ZrO₂) is 45%. The leaching resistance of zirconolite-barium borosilicate glass-ceramics of C₂ (x=2) was characterized by product consistency test (PCT). The results show that the glass network structure is mainly formed of SiO₄, BO₃ and BO₄, the increase of calcium concentration leads to more B ions exist in the form of BO₄ tetrahedral units in the glass network. The glass transition temperature T_g gradually increases and the exothermic peak temperature gradually decreases while the strength of exothermic peak increases with the CaO content. The other crystalline phase (ie. TiO₂ and ZrO₂) forms in addition to CaZrTi₂O₇ phase with x＜2 while only CaZrTi₂O₇ phase forms with x=2 and 4. The stellate CaZrTi₂O₇ and columnar CaTiO₃ phases are observed for the sample with x=6. The PCT experiment of glass-ceramics shows that the normalized leaching rates of B, Na and Ca gradually decrease with increasing leaching time and remain almost unchanged after 28 d, about 8.4, 7.8, 2.2 mg/(m²•d), respectively, which are the same order of magnitude of borosilicate glass waste.