ESEM observations of compacted bentonite submitted to hydration/dehydration conditions

The MX80 bentonite (bentonite of Wyoming) contains about 85% of montmorillonite and 15% of accessory minerals. The dominant presence of montmorillonite in this clay mineral could cause it to perform exceptionally well as an engineered barrier for a radioactive waste repository because of its swelling properties. In the current study the MX80 bentonite was mechanically compacted by a uniaxial system at four different pressures (21, 35, 49, and 63 MPa) in order to obtain four different physical densities. Then each sample was submitted to a hydration/dehydration cycle into an environmental scanning electron microscopy (ESEM) chamber in order to observe in situ the structural modifications after one hydration/dehydration cycle. This instrument allows the possibility to observe geological samples in their natural state without preliminary preparation or modification. In addition, the classical methods (BET measurements, SEM, Hg-porosimetry and isothermal adsorption of water vapour) were carried out to characterize the texture of compacted samples before hydration/dehydration in the ESEM. ESEM observations clearly show that the mechanical compaction influences the textural behavior when the bentonite MX80 is submitted to a hydration/dehydration cycle. In general, the inter-aggregate pores are open during hydration stage and sometimes there is an aggregate cracking. This phenomenon is more significant when mechanical compaction increases since the cohesion force of inter-aggregates at high mechanical compaction “49 and 63 MPa” limits the free swelling of MX80 bentonite. In addition, Hg-porosimetry shows that only the macro-porosity (porous size >50 nm) and eventually the meso-porosity (porous size between 2 and 50 nm) are affected by the mechanical compaction. In fact, the total porosity decreases when the mechanical compaction increases. On other hand, BET measurements and adsorption isotherms of water vapour show a non-significant impact on the microporosity.