SWELLING AND PERMEABILITY OF COMPACTED BENTONITE WHEN HYDRATED WITH A MULTICOMPONENT HYPERSALINE PORE FLUID

Abstract

The swelling properties and permeability of compacted bentonite when hydrated to a multicomponent, hypersaline fluid (332 g/L total dissolved solids; 6.6 mol/L ionic strength) were quantified with laboratory experiments. The model water was designed to mimic the constituents and concentrations of Cobourg limestone of the Michigan Basin to investigate its effects on the engineering properties of compacted bentonite that are relevant for the buffer and seal components in nuclear waste disposal facility. Swell pressure decreased while the permeability increased when hydrated with the model water. Swell pressure under conditions of zero volume increase showed a rapid increase to a peak swell pressure within 10 hours from the onset of hydration with the model water, followed by a continual decrease over time from further chemical interactions. Peak swell pressure when hydrated with the model water was 45% of that with deionized water, decreasing to only 10% after 1 year at a dry density of 1.6 Mg/m3. Swell pressure increased with increasing dry density of the bentonite and reducing ionic strength of the model water. The hydraulic conductivity with respect to model water was at least 110 times greater than that for deionized water (at 1.6 Mg/m3), but low hydraulic conductivity was still attained with an average of 3x10-11 m/s after 1.7 years. There was no evidence of mineralogical change of the bentonite, but cation exchange of potassium and magnesium from the model water with the natural sodium from bentonite was the dominant reason for change in properties. Flow values, local density and macropore scale imaging showed that the bentonite was able to swell to eliminate an initial radial gap corresponding to 10% of total volume, despite being hydrated and exposed to the model water for more than a year (at 1.65 and 1.8 Mg/m3). While the permeability increased relative to samples with the same initial dry density having no initial gap, good hydraulic performance was still attained for the highest initial dry density tested as its permeability increased by only 13% of that for the samples with the same final dry density but without an initial gap.