Effect of sodium occupancy and solute concentration on the swelling pressure and hydraulic conductivity of Boom Clay

The poorly indurated Boom Clay (BC) has been studied as a potential host formation for the geological disposal of radioactive waste including the intermediate-level long-lived bituminized radioactive waste called Eurobitum. In case of leaching of Eurobitum, fluids with altered chemical compositions would be expected to infiltrate locally into the BC. To evaluate the effect of salinity on the BC hydromechanical behaviour, a series of one-dimensional constant-volume swelling tests were conducted on intact BC samples that were first percolated with mixed sodium-calcium-nitrate solutions ((Na, Ca)NO ₃) with varying sodium concentrations, resulting in varying Na ⁺ occupancies in the clay. Results showed that the swelling pressure of BC decreased with an increase in the solute concentration and sodium occupancy. Additionally, higher Na ⁺ concentrations led to a faster stabilization of the swelling pressure. The hydraulic conductivity of BC increased with increasing solute concentration and sodium occupancy. Moreover, the sodium occupancy and concentration affected the swelling pressure and hydraulic conductivity in a similar way. This phenomenon could be elucidated by two primary mechanisms: (i) the influence of solute concentration on the diffuse double layer, and (ii) the effect of cation type and hydrated radius within the interbasal space of montmorillonite sheets, especially concerning sodium occupancy.