Experimental investigations and microscopic analyses of chemical effects and dry density on the swelling behavior of compacted bentonite

Compacted bentonite is used as a buffer or backfill material in deep geological repositories. However, the seepage of pore water containing various salt and minerals can affect the hydro-mechanical performance of the bentonite. To investigate these effects, samples of compacted bentonite with initial dry densities of 1.50 Mg·m⁻³ and 1.70 Mg·m⁻³ were subject to saturation by distilled water, and solutions of NaCl and NaOH (0.1 M and 1.0 M), and the changes in swelling pressure and microstructure were monitored. The swelling pressure was seen to reduce with greater concentrations of the salt solution or when the initial dry density was decreased. At lower salt concentrations, the variation of the swelling pressure exhibited a double-peak, while further increases in the salt concentration caused the swelling pressure curve to exhibit only a single-peak. Compared to samples with a 1.7 Mg·m⁻³ dry density, the large swelling pressure decreased following the initial peak, and a smaller second peak was observed for samples with a 1.5 Mg·m⁻³dry density. The swelling pressure of samples saturated with NaOH solutions was notably lower, particularly after the second peak in the double-peaked curves. Applicable empirical kinetic equations were proposed to fit the relationship between the swelling pressure (p, MPa) and time (t, h) of the compacted sample saturated with DW and aqueous NaCl and NaOH. Microscopic analyses provide evidence that this reduction of swelling pressure could be attributed to the osmotic consolidation, ionic exchange, and mineral dissolution of bentonite.