Hydrodynamic hysteresis exists widely in agglomerated heaps with well-developed intra-pores, and it directly affects solute transports and bioleaching reaction. In this paper, the dynamic liquid retention behavior under different heap porosity and irrigation condition is quantified via a novel real-time, in-situ liquid retention characterizing system (RILRCS), and the potential effects of initial liquid retention on solute transport and leaching reaction are carefully discussed. The results show that the immobile liquid is dominant in agglomerated heaps. The ratio of immobile and mobile liquid (η) dynamically changes due to mineral dissolution and new flow path appearances. The η normally increases and mobile liquid occupies a higher proportion due to acidic leaching reactions, especially at a smaller R_g (10.32 mm) and a larger u (0.10 mm/s). The dynamic liquid retention is more sensitive to the diameter of packed feeds (R_g) and superficial flow rate (u) instead of leaching reactions. This might be because the damage of leaching reaction on minerals pores/voids is limited and cannot extensively change the potential pore channels or fluid flow paths. Based on pulse tracing and conductivity tests, we reveal that the solute resides longer under a slower u and smaller packed R_g condition, which corresponds well with desirable copper leaching efficiency. Specifically, the liquid hysteresis behavior is more obvious at a lower u (0.01 mm/s) and smaller R_g (10.32 mm). This paper gives a good reference to ascertain the liquid retention and hydrodynamic hysteresis and promote mineral leaching performance.

Backfilling mining methods control the surrounding pressure and ground subsidence by backfilling goaf and managing the ground pressure, providing a safety guarantee for mining in complex environments and serving as a key means of achieving the deep mining of metal minerals. However, in the design of backfill strength, material mix ratios are determined under indoor standard constant temperature and humidity conditions, which differ significantly from the in situ curing environment. Strength measurements obtained from field samples are notably higher than those from indoor test specimens. To address this issue, this study designed a curing device simulating the in situ thermal-hydraulic multi-field environment of the mining site and tested the strength and porosity of the backfill under different curing temperatures, curing pressures, and pore water pressures. The results indicate that curing pressure and pore water pressure significantly altered the pore structure of the specimens. Specifically, when the curing pressure increased to 750 kPa, the maximum pore diameter decreased from 3110.52 nm to approximately 2055 nm, accompanied by a continuous reduction in porosity. Pore water pressure exhibited a positive linear correlation with specimen porosity, which increased continuously as the pore water pressure rose. With increasing curing temperature, the strength of the backfilled specimens first increased and then decreased, reaching a maximum at 45 °C. As the curing pressure increased, the strength of the backfilled specimens rose, but the rate of increase gradually slowed. With increasing pore water pressure, the strength of the backfilled specimens showed a gradual decreasing trend.

To effectively apply slag-gypsum binder (SGB) to most modern mines. An experimental study is conducted to investigate the feasibility and reliability of slag-gypsum binder (SGB) to replace cement in the mine backfill to effectively apply SGB in modern mines. The optimum parameters of aggregate and SGB fineness were investigated on a laboratory basis and the effect of concentration on the workability was analyzed. Also, industrial tests were carried out to study the cementitious properties of SGB and compared to cement. Finally, the hydration properties of SGB were analyzed using XRD and SEM. The results indicated that the main hydration products of SGB are C-S-H gel and ettringite, while Ca(OH)₂ is consumed during the hydration process. In addition, the structure formed in the later stage of SGB is more compact. SGB is beneficial to the strength of the waste rock mixture. The sample prepared with the waste rock-rod milling sand-river sand mixture has a maximum strength of 7.54 MPa at 28 d. Reducing fineness is beneficial to form early strength. However, a specific surface area exceeding 510 m²/kg will harm the later strength. Surface area is less (400 m²/kg), the sample strength decreases when the dosage increases. Comparing the workability of SGB slurry and cement slurry, it is found that there is no essential difference between them under different mass concentration conditions. SGB slurry shows better fluidity and stability at a mass concentration of 80–82%. Industrial backfill experiments show the 3-, 7-, and 28-d strengths of SGB samples under 80% mass concentration conditions are 0.91 times, 1.43 times, and 1.2 times the cement samples. Therefore, using SGB instead of cement to backfill is a good application prospect.

Failure to fully backfill the goaf may result in increased exposure of roof strata, significantly raising the risk of roof collapses in mining zones and potentially causing surface subsidence, thereby endangering the safety of mining personnel. To address this issue, expansive agents are utilized to produce active roof-contact backfill (ARCB) materials, which promote localized self-compaction of backfill materials in unroof-contact areas through hydration reactions. In this study, an isothermal calorimeter was employed to measure the ARCB hydration heat release rate curves of three types of expansive agents, CaO-based, MgO-based, and ettringite-based, at dosages ranging from 6% to 12%. Hydration kinetic parameters were calculated based on the Krstulovic–Dabic model. The influence of expansive agent type and dosage on these parameters was analyzed, and the hydration mechanism of ARCB materials was investigated. The results indicate that the hydration process of grouting materials using all three expansive agents follows five distinct stages: rapid reaction, induction, acceleration, deceleration, and decay. However, increasing the dosage of the CaO-based expansive agent will enhance heat release and prolong the duration of the acceleration stage. When the dosage is 12%, the total heat release reaches 327.4 J·g⁻¹. At the same dose, the sample doped with MgO-based expansive agent was only 254.3 J·g⁻¹, which was 22.3% lower than that of CaO-based, and the occurrence time of the second heat release peak was earlier. In contrast, the ettringite-based expansive agent shows a decreasing trend in heat release with increasing dosage. Furthermore, the use of CaO-based and MgO-based expansive agents allows the hydration process to bypass the phase boundary reaction (I) stage and directly enter the diffusion (D) stage. Ettringite-based expansive agents still undergo three stages, but exhibit a shortened nucleation and growth (NG) stage and an extended induction stage. Additionally, different expansive agents have varying effects on the crystal growth index (n), reaction rate constant, and degree of hydration.