FORMATION OF HYDROTALCITE-LIKE COMPOUNDS DURING R7T7 NUCLEAR WASTE GLASS AND BASALTIC GLASS ALTERATION

Alteration experiments have been performed using R7T7 and synthetic ba saltic glasses in MgCl2-CaCl2 salt solution at 190-degrees-C. The dura tion of experiments ranged from 0.25 to 463 days. The alteration produ cts were studied by Scanning Electron Microscope (SEM), Scanning Trans mission Electron Microscope (STEM), X-ray diffraction (XRD) and Electr on Spectrometry for Chemical Analysis (ESCA). For both glasses, the ea rly alteration product is a hydrotalcite-like compound [Mg6Al2CO3(OH)1 6.4H2O] in which HPO4(2-), SO4(2-) and Cl- substitute for CO3(2-). The measured basal spacing is 7.68 angstrom for the hydrotalcite formed f rom R7T7 glass and 7.62 angstrom for the hydrotalcite formed from basa ltic glass which reflect the high Al/Al + Mg ratios x (0.34 less-than- or-equal-to x less-than-or-equal-to 0.46). The chemical microanalyses show that the hydrotalcite is subsequently covered by a silica-rich ge l which evolves into saponite after a few months. These results suppor t the use of basaltic glasses alteration patterns in Mg-rich solution, to understand the long-term behavior of R7T7 nuclear waste glass.