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.
The preparation of geopolymers from tailings rich in quartz typically requires activation to enhance final strength, though the underlying mechanism remains unclear. Quartz, as a common inert gangue mineral in tailings, was selected as the research subject. Quartz with varying reactivity was prepared through mechanical activation of different particle sizes and used for geopolymer synthesis, aiming to elucidate the strengthening mechanism of activated quartz on geopolymers. When the activation time was identical, quartz particles of different sizes exhibited the same soluble silicon content. However, due to the increased surface-active groups after activation, geopolymers derived from larger quartz particles (5–8 mm) achieved compressive strengths of 57.24–64.73 MPa, while those from smaller particles (0.08–0.12 mm) only reached 43.83–50.42 MPa. After activation, the main active groups on the quartz surface were the Q n (n = 1, 2, 3) structures containing non-bridging oxygens and silicon radicals. The Q 1 and Q 2 structures mainly provide soluble silicon in geopolymerization and have little effect on the strength of the geopolymer. While the Q 3 structure can combine with the geopolymer gel through Si-O-Si bonds, reducing the porosity at the interface between quartz and geopolymer, significantly enhancing the strength of geopolymer. The results indicate that, compared to soluble silicon content of quartz, the content of active groups on its surface is the main factor influencing the performance of geopolymer. For the activation of tailings, increasing the reactivity of their surface is more effective in improving geopolymer strength than enhancing the leaching of soluble silicon and aluminum through overall activation.
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