In situ observations of the swelling of bentonite aggregates in NaCl solution

The microstructural evolution and hydraulic conductivity of bentonite in NaCl solution is investigated by in situ laser scanning microscopy (LSM). Upon exposure to saline solution, the bentonite aggregate swells rapidly, followed by a longer period of more gradual swelling. The expansion ratio of an aggregate reached a steady state 15 min after initial exposure to the solution. The steady-state expansion ratio decreased sharply with increasing NaCl concentration up to 0.1 mol/dm3 NaCl and then decreased gradually above 0.3 mol/dm3 NaCl. The results indicate that the fraction of macropores among bentonite aggregates increases with NaCl concentration, which is likely to account for increase in hydraulic conductivity under highly saline conditions.