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 selected as research objects. Laboratory experiments were conducted to systematically investigate the variations in physical, microscopic and mechanical properties of BRM and MRM subjected to different numbers of drying-wetting cycles (0, 1, 3, 7, 12). Furthermore, the stability of red mud stockpiles under extreme rainfall conditions after different drying-wetting circles was quantitatively evaluated using Geo-Studio software (SEEP/W and SLOPE/W modules). The results showed that with the increase in drying-wetting cycles, the unconfined compressive strength (q _u ) and cohesion (c) of BRM decreased gradually, while those of MRM increased significantly: the (q _u ) of MRM increased from 115.78 kPa to 150.38 kPa and the cohesion increased from 61.67 kPa to 121.91 kPa (nearly doubled). The internal friction angle (φ) of BRM increased gradually with the number of drying-wetting circles, whereas that of MRM showed a decreasing trend. Under extreme rainfall conditions, the safety factors of red mud stockpiles after 0, 1, 3, 7 and 12 drying-wetting circles were 1.205, 1.219, 1.240, 1.263 and 1.288, respectively, all greater than the critical value of 1.0 and increasing with the number of cycles. This study demonstrates that stacking well-behaved MRM on the original BRM stockpile is a feasible and effective engineering strategy, which can improve the stability of red mud stockpiles under the action of drying-wetting cycles and provide theoretical support and engineering guidance for the safe stacking and resource utilization of red mud.