Effect of Freeze–Thaw and Wetting–Drying Cycles on the Hydraulic Conductivity of Modified Tailings

Mine tailings have shown viability as the fine–grained layer in a capillary barrier structure for controlling acid mine drainage in a circular economy. Their saturated hydraulic conductivities (k_sat) under wetting–drying cycles and freeze–thaw cycles remain unexplored. In this study, modified tailings with a weight ratio of 95:5 (tailings/hydrodesulfurization (HDS) clay from waste–water treatment) and an initial water content of 12% were used. The k_sat of specimens was measured after up to 15 wetting–drying cycles, each lasting 24 h, with a drying temperature of 105 °C. The k_sat for wetting–drying cycles decreased from 3.9 × 10⁻⁶ m/s to 9.5 × 10⁻⁷ m/s in the first three cycles and then stabilized in the subsequent wetting–drying cycles (i.e., 5.7 × 10⁻⁷ m/s–6.3 × 10⁻⁷ m/s). Increased fine particles due to particle breakage are the primary mechanism for the k_sat trend. In addition, the migration of fines and their preferential deposition near the pore throat area may also promote this decreasing trend through the shrinking and potentially clogging–up of pore throats. This could be explained by the movement of the meniscus, increased salinity, and, subsequently, the shrinkage of the electrical diffuse layer during the drying cycle. Similar specimens were tested to measure k_sat under up to 15 freeze–thaw cycles with temperatures circling between −20 °C and 20 °C at 12 h intervals. Compared to the untreated specimen (i.e., 3.8 × 10⁻⁶ m/s), the k_sat after three freeze–thaw cycles decreased by 77.6% (i.e., 8.5 × 10⁻⁷ m/s) and then remained almost unchanged (i.e., 5.6 × 10⁻⁷ m/s–8.9 × 10⁻⁷ m/s) in subsequent freeze–thaw cycles. The increased fine grain content (i.e., 3.1%) can be used to explain the decreased k_sat trend. Moreover, the migration of fines toward the pore throat area, driven by the advancing and receding of ice lens fronts and subsequent deposition at the pore throat, may also contribute to this trend.