Reversibility of micro and macrostructure in compacted bentonite under chemo-mechanical couplings

Compacted bentonite was selected as buffer material in deep geological repository to deal with high level nuclear waste (HLW). During the long-term operations of repositories, volume change characteristics of compacted bentonite are affected by several factors, for example: (1) Change of the concentration in infiltration liquid caused by the variation of groundwater level; (2) Vary of additional stress owing to the unpredictable geological tectonic activity and the stress generated by the swelling of buffer material in confined condition. Hence, it is very crucial to study the effects of chemo-mechanical couplings on volume change characteristics of compacted bentonite. To simulate different chemo-mechanical coupled conditions, serial swell-compression-rebound tests were performed infiltrated by distilled water (DIW) and NaCl solutions with different concentrations. Multi-step salinization-desalination tests were carried out as well. Volume change behaviors of compacted bentonite were studied from microstructure to macrostructure. Experimental results showed that: (1) Final swelling strain and compression index of compacted bentonite decreased with increasing NaCl solution concentration; (2) Comparing the volume change behaviors of compacted bentonite underwent desalination-salinization cycles to those did not undergo these cycles, it was found that the plastic property at the macro level exhibits irreversible property to a certain extent. While, microstructural characteristics are somewhat reversible and elastic.