Paradoxical effects of superabsorbent polymers on coalfield fire area slopes: water retention limitations versus mechanical reinforcement

Revegetation in the arid coalfield fire area is challenging due to water scarcity, high salinity, and soil erosion. While superabsorbent polymers (SAPs) are generally effective for soil improvement, their impact in this specific environment is under-researched. This study tested three SAPs (A, B, and C) at various concentrations (0%, 0.1%, 0.3%, 0.5%, and 1%) to assess their effects on soil hydraulic properties, Soil–Water Characteristic Curve (SWCC), soil aggregate microstructure, and soil skidding resistance. Results indicated that (a) SAPs significantly increased the soil’s largest water-holding ratio (by 4.41%-68.82%, p < 0.05) but only marginally extended water retention time (by 1–2 d). (b) The cycle of water absorption and loss led to a permanent reduction in the SAPs’ water absorption capacity (5.61%-30.60%), worsened by salinity. (c) When the matric suction was below the permanent wilting point (1500 kPa), SAPs significantly enhanced the soil water retention capacity (19.99%-248.09%, p < 0.05). (d) SAPs also improved soil aggregate formation and anti-sliding performance, increasing the critical sliding angle from 45° to 60°, with the Type B treatment group showing significantly less sliding than the control. From a “water-structure-mechanics” multi-dimensional perspective, this study reveals the dilemmas of SAPs in extreme environments—namely, short water retention time and structural deterioration—while also identifying their strengthening effect on slope stabilization. Furthermore, it proposes the design concept of a “salt-resistant and structurally stable” SAP, providing a theoretical basis and technical reference for the development and engineering application of ecological restoration materials in coalfield fire areas.