Time- and concentration-dependent shear strength evolvement of compacted GMZ bentonite in alkaline conditions

Understanding the shear behaviors of compacted bentonite has been imperative to evaluate the mechanical stability of the buffer/backfill materials, especially under the influence of the alkaline solutions generated by the degradation of cementitious materials. By soaking the compacted bentonite in the alkaline solutions for different periods (e.g., 60, 90, and 120 days) at constant volume condition, this study investigates the impact of different concentrations of NaOH solutions (e.g., 0 M, 0.1 M, 0.5 M, and 1.0 M) on the shear strength of compacted Gaomiaozi (GMZ) bentonite via direct shear tests. The microstructure and the mineralogical evolvement of the compacted bentonite were investigated through scanning electron microscope (SEM) and X-ray diffraction (XRD) tests, respectively. Results demonstrated that the peak shear strength of compacted bentonite generally increased with the concentrations of NaOH for a short soaking period (e.g., 60 days) with the friction angle increased from 3.66 degrees to 12.24 degrees. However, this trend became less pronounced over longer periods (e.g., 90 and 120 days), with the peak shear strength initially increasing and then decreasing after reaching a critical NaOH concentration (i.e., 0.5 M), which suggests the impact of alkaline solution on the shear strength of compacted bentonite is both time- and concentration-dependent. Analyses from SEM and XRD tests indicated that the microstructure degradation and mineral dissolution were induced by the alkaline solutions.The mechanical behaviors of compacted bentonite suggest the complex interplay between osmotic suction and mineral dissolution influencing the shear strength of compacted bentonite. Based on the fractal dimensions of the compacted bentonite, a modified model is proposed to account for both the osmotic and dissolution effects of the alkaline solutions on the compacted bentonite.