Combined impact of aging and saline solution on the swelling characteristics of compacted GMZ bentonite

Abstract

The swelling characteristics of compacted bentonite preferred as an ideal buffer material are of pivotal importance to the sealing functionality of the multi-barrier system in the context of nuclear waste disposal. The aging process resulted from the intermediate storage period in real case is typically ignored, while the impact of saline solution on bentonite should be generally accounted. To get a more comprehensive understanding on the swelling, this study presents an experimental investigation on the impacts of potential aging time (0–90 days) and saline solution (salinity: 0–12.3 g/L) on the swelling pressure and deformation of compacted GMZ bentonite at an initial dry density of 1.70 Mg/m3. Combined uses of Mercury Intrusion Porosimetry (MIP) and Scanning Electron Microscopy (SEM) were also adopted to capture the microstructure characteristics, aiming to make a reasonable interpretation from a microscopic view. Test results show that the swelling potential decreases with increasing aging time and/or solution salinity, while the aging impact appears to be ignorable over 30 days. The exfoliation of clay particles/aggregates is expected to happen before saturation due to aging, and this feature was also found to be more obvious in aged specimens, with smaller inter-aggregates and more uniform pore spaces. Clear contraction of clay aggregates is observed after immersion in saline solution, which can be attributed to the decrease in the repulsion between particles and limited inhibition of expansion in interlayer spaces, leading to a decrease in swelling potential. Specially, the aging effect is believed to be related with the phenomenon of cation demixing, during which the monovalence and bivalence cations tend to be separated from each other, resulting in a structure modification and hence a weaker swelling potential. This study highlights the importance to carefully evaluate the engineering properties of compacted bentonite considering the whole process after its manufacture.