Short and long-term behavior of compacted bentonites in contact with solutions of different salinities: a safety issue fort he long-term closure of repositories for radioactive waste?

Results of a 3 years study regarding the changes of MX-80 bentonite after reaction with solutions of different ionic strength and pH, occurring in repositories in granite, in clay and in salt formations are presented. Each solution reacted differently with the bentonite and affected differently the resulting mineralogy and swelling pressure (SWP). SWP were highest in contact with water (over 4 MPa), significantly lower in contact with low ionic strength solutions (around 2 MPa) and lowest in contact with high saline brines (mostly under 1 MPa). Alteration and partial dissolution of montmorillonite was observed. The Mg, Al and Si contents in the solutions increased with reaction time. Concomitantly in the octahedral layers of montmorillonite Mg was substituted by Al and the interlayer charge decreased. The correlation between the alteration process, the charge reduction and SWP is presented. The key for the observed mineralogical alteration may be the acidity of water in the interlayer space. This may explain why the alteration in compacted bentonite is faster than in the non compacted. All results can be explained by processes which lead to a pyrophyllitization/kaolinitization and Si-excess of the montmorillonite particles. These processes may lead in the long run to a significant or even total loss of swelling capacity of compacted bentonites, if high saline solutions and cement are involved.