An experimental study on the effects of drying-wetting cycles to the mechanical properties of red mud

Red mud, a hazardous industrial waste generated during alumina production, faces great challenges in safe stacking and utilization due to its complex chemical composition and poor engineering properties. Long-term stacking of red mud is accompanied by repeated natural drying-wetting cycles, which significantly alter its strength characteristics and further threaten the stability of red mud stockpiles. In this study, Bayer red mud (BRM) and mixed red mud (MRM) from the No.5 stockpile of Guizhou Aluminum Plant were tested under 0, 1, 3, 7, 12 drying-wetting cycles. Their physical, microscopic, and mechanical properties were investigated. Stockpile stability under extreme rainfall was evaluated using Geo-Studio software (SEEP/W and SLOPE/W modules). The results showed that unconfined compressive strength (q_u) and cohesion (c) of BRM decrease with cycles, while those of MRM increased significantly. Specially, the (q_u) of MRM rise from 115.78 kPa to 150.38 kPa and the cohesion (c) doubles from 61.67 kPa to 121.91 kPa. The internal friction angle (φ) of BRM increases slightly, while that of MRM decreases slightly. Under extreme rainfall conditions, safety factors of red mud are 1.205, 1.219, 1.240, 1.263 and 1.288, respectively, all greater than the critical value of 1.0 and increasing with cycles. This study demonstrates that stacking MRM on BRM is feasible and effective for improving the long term stockpile stability, which provide theoretical support and engineering guidance for the safe stacking and resource utilization of red mud.