Long-term behaviour of the Wyoming bentonite MX-80 in high saline solutions

Abstract

The mineralogical and chemical changes of bentonites were studied under the boundary conditions of a repository in salt formations by means of a literature review, a natural analogue study, and laboratory experiments. In the natural analogue study the expandability of clays from deep boreholes in the East Slovakian Basin was found to be dependent not only on depth and temperature but also on the salinity of the pore waters. In laboratory experiments the fraction <2 μm of the MX-80 bentonite was reacted with two high saline solutions, a NaCl- and a MgCl 2-rich brine at 25, 90 and 150 °C, at three different pH levels, 1, 6.5, and 13. In all experiments, montmorillonite remained at all temperatures and all pH the predominant mineral phase over 580 days, with full ethylene–glycol expandability to 17 Å. However, significant changes could be detected by looking at parameters like morphology, crystallinity, particle height, particle surface, interlayer charge, and chemistry of octahedral layers. The experimental results of this study suggest that under repository conditions a certain percentage the MX-80 montmorillonites will be transformed in kaolinites and pyrophyllites rather than in illites. The investigated reactions not only caused mineralogical changes but also considerable changes of the water uptake capacity. The MX-80 montmorillonites in salt solutions lost about half of their water uptake capacity compared with pure water. The reduction was found to be dependent on the reaction time and temperature. Smectites in salt solutions with cement lost their water uptake capacity almost completely.