Influence of Glass Composition on Microstructure and Leaching Behavior of Gd2Ti2O7 Pyrochlore Glass-Ceramics

Internal crystallization is a significant process in the preparation of glass-ceramics (GCs). The glass phase is one of the important factors affecting the internal crystallization process within GCs. In this study, Gd 2 Ti 2 O 7 pyrochlore GCs with different borosilicate glass systems were prepared via an oxide route and consolidated via a cold-pressing and sintering process to be used as waste forms for actinide immobilization. After sintering at 1250°C, it was confirmed that the pyrochlore phase (Gd 2 Ti 2 O 7 ) was the major immobilization phase, while the apatite phase (Gd 9.33 (SiO 4 ) 6 O 2 ) was a minor phase. The influence of glass composition on the minor phase formation and pyrochlore grain growth was also investigated. The content of the minor phase (Gd 9.33 (SiO 4 ) 6 O 2 ) in GCs showed a positive correlation with the content of non-bridging oxygen (NBO) in the glass systems. Meanwhile, the average grain size of the pyrochlore phase also exhibited a positive correlation with the NBO content, highlighting the influence of glass composition on the grain growth process. In addition, static leaching tests confirmed that all GCs exhibited high chemical durability with normalized Gd and Ti leach rates ranging from ∼10 -4 -10 -5 g m -2 d -1 after 28 days. However, preferential leaching of the glass phase was observed in GCs during the leaching tests. Glasses with higher NBO content exhibited more severe alterations from leaching, evidenced by significantly higher leach rates of glass-forming elements such as Si and B. Surface analysis revealed that the GCs surface experienced breakage and peeling, with damage on the glass phase ranging from ∼200 nm to 700 nm.