Swelling pressures of compacted Ca-bentonite

One of the major functions of a bentonite buffer for a HLW repository is to provide the buffer with a self-sealing capacity through its ability to swell on contact with free water. Swelling pressure measurements were carried out to investigate the swelling behavior of Ca-bentonite. Experimental results showed that the swelling pressure increased with an increase in dry density, and its dependence on dry density increased at higher dry densities beyond 1.6 Mg/m3. The effect of salinity on the swelling pressure for the Ca-bentonite was different from that for Na-bentonite. The swelling pressure of the Ca-bentonite was higher in a diluted concentration of NaCl solution (0.04 M) compared with de-mineralized water, and it then decreased with an increase in the concentration of the NaCl solution beyond 0.04 M. Such a swelling behavior of the Ca-bentonite was explained by the ion-exchange of Ca2 + for Na+ from the solution followed by hydration with water molecules and osmosis due to the differences in concentrations of the NaCl solutions.