Effect of CaO Content on Crystalline Phases and Chemical Stability of Lanthanum Borosilicate Glass-Ceramics

Abstract: The Na2O-CaO-La2O3-B2O3-SiO2 glass were prepared by melt-quenching method. Then, the silicate oxyapatite borosilicate glass-ceramics were obtained by heat treatment of the prepared glass. After that, the X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and the product consistency test (PCT) were used to explore the effect of CaO substitution for SiO2 on phases, microstructure and chemical stability of borosilicate glass-ceramics. The results show that with the increase of CaO content, the diffraction peaks of silicate oxyapatite crystalline phase are enhanced, and the others are weakened until disappearing. When the mole fraction of CaO is 15%, only the CaLa4(SiO4)3O crystalline phase is detected in the glass-ceramics samples. The crystalline phases types, shapes, sizes and distributions of glass-ceramics are affected by CaO content, and the cluster and growth of ceramics crystalline phases are caused by the change of CaO content. After immersing by the method of PCT for 28 d, the normalized leaching rates (g·m-2·d-1) of Si, Ca and La of all samples remain below the order of 10-3, indicating that the glass-ceramics samples have excellent chemical stability. In addition, the glass-ceramics samples containing the CaO mole fraction of 15% exhibit the best chemical stability. The results show that the borosilicate glass-ceramics containing silicate oxyapatite are potential substrates for immobilization high-level radioactive wastes containing La-rich and some actinide rich.

Key words: CaO, borosilicate glass-ceramics, silicate oxyapatite, crystalline phase, leaching rate, chemistry stability

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