Recycling of coal gangue in alpine mining areas: Mechanism of SAPs regulating the pore structure to improve water retention capacity and their optimizing effect

The Muli mining area suffers severe ecological damage, primarily due to unregulated mining in the early stage. Coal gangue, as the main waste in the mining area, is characterized by poor water retention capacity and rapid water evaporation, significantly limiting ecological restoration. To improve the water retention capacity of the coal gangue matrix, coal gangue collected from the Juhugeng mining area of Muli Coalfield in Qinghai Province is considered the research object in this study. The influence of superabsorbent polymers (SAPs) on the water retention capacity of coal gangue is systematically explored by constructing different particle size gradations and adding SAPs with a gradient mass fraction of 0–0.4%. With the aid of saturated water content (SWC), capillary water content (CWC), field capacity (FC), infiltration-evaporation dynamics, and freeze-thaw cycle experiments, the regulatory mechanism of water characteristics is analyzed using variance and multiple regression analysis. The results of this study show that: (1) Water retention capacity is synergistically regulated by multiple factors. It is positively correlated with SAP mass fraction and particle size gradation (where SWC increases by 12.10%), but negatively correlates with bulk density and the number of freeze-thaw cycles (where SWC decreases by 5.03%). SAPs improve water retention capacity by increasing capillary porosity, but at the cost of aeration. (2) SAPs play a vital role in inhibiting water loss: It extends the time of wetting front migration to a maximum of 2,220 min in the case of 0.4% SAP (194% higher than that of the control group). It also increases the cumulative infiltration volume, creating a dynamic balance of slow infiltration and high water retention capacity. (3) 0.3% SAP is the critical threshold: The three water characteristic values all reach a comparatively good level, and the water retention ratio increases significantly to 0.67. The combination of “the gradation of the medium-content balanced group + 0.3% SAP + a bulk density of 1.4 g/cm³” achieves the optimal water retention + aeration + freeze-thaw resistance performance. This study proposes an engineered formulation suitable for the ecological restoration of coal gangue piles in alpine mining areas by revealing the synergistic mechanism of SAP-gradation-pore reconstruction. It provides a scalable solution for vegetation reconstruction and solid waste recycling in mining areas. • SAPs enhance coal gangue’s water retention by 12.10% via capillary porosity increase, but at a controlled cost to aeration. • 0.3% SAP is the critical dosage for optimal water retention (0.67 ratio), infiltration-evaporation balance, and freeze-thaw resistance. • Medium-content balanced gradation (25% fine + 30% medium particles) achieves bimodal pore structure, maximizing water-gas synergy. • SAPs inhibit water loss by extending wetting front migration time to 2,220 min (+194% vs. control) under alpine conditions. • The engineered coal gangue-SAP matrix enables cost-effective ecological restoration by reducing the need for topsoiling.