Influence of water content and dry density on pore size distribution and swelling pressure of two Indian bentonites

The swelling pressure of compacted bentonites plays an important role in the design of deep geological repositories. The current study highlighted the influence of compacted specimen dry density and water content on pore size distribution (PSD) and swelling pressure of two Indian bentonites. Constant volume swelling pressure tests were carried out on one divalent bentonite and one monovalent bentonite at three different dry densities such as 1.4, 1.6, and 1.8 Mg/m³ by varying three initial water contents of 8%, 12%, and 16%, respectively. The change in pore size distribution and morphology of the bentonites were investigated using mercury intrusion porosimetry (MIP) and field emission scanning electron microscopy (FESEM). The results indicate that the compacted dry density has significant role on macropore alteration of both bentonites; however, its effect is marginal on micropores. The macropore volume was noted to significantly decrease in monovalent bentonite due to increase in water content from 8 to 16%. The hydration phenomenon in compacted bentonites was found to be strongly dependent on both the clay fabric and the pore diameter. The high macropore volume of monovalent bentonite exhibited collapse behaviour during swelling process and developed a new family of big pores. However, collapse behaviour was not observed in divalent bentonite due to its small pore diameter. At a given dry density, the compacted divalent bentonite predominantly having Ca-Mg montmorillonite and palygorskite possesses a higher swelling pressure than monovalent bentonite, which has Na-montmorillonite as a predominant component. The effect of initial water content was noted to be negligible on swelling pressure of the monovalent bentonite at a lower dry density of 1.4 Mg/m³. On other hand, irrespective of dry density, the swelling pressure of divalent bentonite decreased with an increase in initial water content; however greater percentage reduction was noted at higher dry densities for both bentonites.