Hydro-mechanical evolution during saturation of a two-component bentonite barrier for radioactive waste disposal

Bentonite columns consisting of two parts (a compacted block and a mixture of pellets and powder, with similar dry density and water content) were placed in an instrumented cell and saturated under constant volume conditions. MX-80 bentonite and deionised water were used. The water intake, development of radial and pore pressure and relative humidity evolution were followed. When the tests were dismantled, the columns were subsampled to determine the final physical state and microstructural characteristics of the bentonite (pore size distribution, specific surface area, basal spacing of smectite). The results of two tests differing only in the disposition of block and mixture inside the cell are compared. The analysis shows that the microstructure of the two materials tends to homogenise, and that the performance of two-component buffers during hydration is optimal and essentially similar to that of a one-component buffer. The radial pressure development was not steady (including transient periods of stabilisation or decrease and an initial collapse of the mixture), and was linked to the increase in relative humidity at every location along the bentonite column. Inhomogeneities due to hydration persisted after quasi-full saturation and pressure stabilisation, with higher water content and lower dry density in the proximity to the hydration surface. The interface between the two parts did not affect the physical evolution of the barrier and was healed after saturation. • Hydro-mechanical and fabric evolution of binary bentonite barriers during saturation • Pellets/powder mixtures have initially large macroporosity and preferential pathways • Water redistribution and pressure build-up continues after bentonite full saturation • Equilibrium radial pressures are exponentially related to dry density • Water content and dry density gradients persisted after quasi-full saturation • The interface between the two parts was healed after saturation