Swelling pressures of MX-80 bentonite in solutions of different ionic strength

Abstract

Bentonites are part of the engineered barrier system (EBS) of repositories for radioactive wastes. High salinity and high pH of the pore space solutions may cause changes in the mineralogical composition and of the swelling capacity of the bentonites, possibly affecting the long term performance of the EBS. Results of a 3 years study on the changes of MX-80 bentonite properties after reaction with solutions of different ionic strength and pH are presented. The effects of seven characteristic saline solutions potentially occurring in repositories in granite, clay and salt formations were investigated, with comparison with one “pure water” experiment. Each solution reacted differently with the bentonite, resulting in different mineralogy and swelling pressure (SWP). SWP were highest in contact with water (>4 MPa), significantly lower in contact with low ionic strength solutions (∼2 MPa) and lowest in contact with high saline brines (mostly under 1 MPa). An alteration and partial dissolution of montmorillonite was observed. 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. Later, a loss of some octahedral layers led to an excess of Si in the montmorillonite particles. The correlation between the alteration process, the charge reduction and SWP is presented. Tentative explanations are given for the activation of bentonites with acids. Key factor for the observed mineralogical alteration may be the acidity of water in the interlayer space. This also may explain that the alteration in compacted experiments with a high solid–liquid ratio (less water, higher acidity) is faster than in batch experiments with a lower solid–liquid ratio. All results can be explained by processes which lead to a pyrophyllitization/kaolinitization and Si-excess of the montmorillonite particles. An illitization seems less likely. The expected kaolinitization/pyrophyllitization and Si-excess 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. Therefore, the question to be answered is not, whether or not bentonites are stable under the conditions of a repository in salt, but rather, what degree of swelling capacity is needed for what length of time.