The geochemical interactions of simulated borosilicate waste glass, granite and water at 1000 - 3500C and 50 MPa

Four experiments involving a simulated borosilicate glass, granite and deionised water have been carried out at 1000, 1500, 2000, and 3500C at a total pressure of 50 MPa to simulate the near-field geochemistry of a high level waste repository in granite. Experiments were conducted for run durations of 200 days (1000, 1500, and 2000C) and 30 days (3500C). The high temperature speciation characteristics and degrees of mineral saturation of the fluids were investigated using the geochemical software packages, EQ3/6 and SOLMNQ. Although generated fluids were all alkaline, hydrogen ion activity continued to increase with time suggesting that pH was limited by the rate of precipitation of hydroxyl-consuming mineral phases, principally smectite. The oxygen activity of experimental fluid phases were broadly 'reducing' at 2000 and 3500C and oxidising' at 1000 and 1500C. Consideration of XRD, SEM and modelling data suggest the importance of secondary solids in controlling the aqueous phase concentrations of various chemical components. There is a general coherence of geochemical processes operating across the temperature range investigated, suggesting that the observed differences between results at different temperatures are attributable to differences in reaction kinetics rather than fundamental differences in reaction mechanisms