Initial and long-term dissolution rates of aluminosilicate glasses enriched with Ti, Zr and Nd

The alteration mechanism and rate of three aluminosilicate glasses were inv estigated experimentally in aqueous media between 90 and 200 degrees C. In order to assess their containment properties with regard to minor actinides for the purpose of developing new radionuclide containment matrices, the t hree glasses were enriched with neodymium to simulate the trivalent actinid es. The proportions of the major glass network formers, silicon and aluminu m, were comparable to those found in tholeitic basalt glasses. The composit ion differences for the other elements (Ca, Zr, Ti, Nd) revealed the role o f glass network modifiers in aqueous corrosion resistance of silicate glass es. Two types of experiments were performed: open-system leaching to determ ine the dissolution rate constants at 90, 150 and 200 degrees C; and closed -system tests at high (200 cm(-1)) SA/V ratios (glass-surface-area-to-leach ing-solution-volume) for 3 months at 90C to simulate long-term behavior. Th e open-system test results showed that, regardless of the aluminosilicate g lass composition (not only the test glasses, but also tholeitic basalt glas ses and nuclear aluminoborosilicate glasses), the initial dissolution rates are on the same order of magnitude in neutral or slightly basic media, wit h a common activation energy of 60 +/- 5 kJ mol(-1). This step appears to b e controlled exclusively by hydrolysis of the Si-O and/or Al-O bonds, irres pective of the nature of the network modifying or intermediate components. Otherwise, with renewal of the leaching solution, the formation of alterati on films at the 'glass/solution' interface appears to limit glass alteratio n; however, the protective effect of these layers depends on the experiment al conditions and on the glass composition. Finally, in closed-system condi tions studied at 90 degrees C, the three test glasses exhibit very low alte ration rates (three to four orders of magnitude lower than the initial rate s) without the development of a thick and abundant altered layer. Two hypot heses are discussed to account for these low rates: a protective aluminosil icate surface gel less than 50 nm thick and thus with a low silicon diffusi on coefficient (similar to 10(-13) cm(2) s(-1)); and a chemical affinity ef fect with respect to the glass itself, as the solubility products attribute d to glass are determined from the thermodynamic model proposed by Paul (19 77) [Paul, A., 1977. Chemical durability of glasses: a thermodynamic approa ch. J. Mater. Sci. 12, 2246-2268.]. (C) 1999 Elsevier Science B.V. All righ ts reserved.