An Experimental Investigation of the Effects of Dry–Wet Cycles and pH Values on Gangue Waste-Based Roadway Shotcrete: Mechanical Performance and Microstructural Analysis

The mechanical durability of gangue-based roadway shotcrete material (GRSM) in aqueous environments was systematically investigated by evaluating the effects of immersion duration, dry–wet cycles, and pH variations on its uniaxial compressive strength (UCS). The results indicate that prolonged immersion significantly degrades the mechanical performance of GRSM. After 28 days of immersion, the UCS decreased by 8.68 MPa (22%) compared with specimens under standard curing conditions. In contrast, limited dry–wet cycling (up to two cycles) enhanced the UCS to 36.05 MPa by promoting continued hydration and pore refinement, whereas additional cycling led to progressive deterioration. GRSM exhibited pronounced pH sensitivity: acidic environments induced the most severe strength loss, followed by alkaline conditions, whereas neutral to weakly alkaline environments (pH 8–12) resulted in relatively stable mechanical performance. Mercury intrusion porosimetry (MIP) confirmed that pore structure evolution governed strength variation, with acidic exposure and dry–wet cycles producing the greatest increases in porosity. Mechanically activated gangue (MA-gangue) was prepared by ball milling and partially substituted for cement. Although MA-GRSM exhibited lower UCS than conventional GRSM under all conditions, both materials demonstrated similar environmental response patterns. These findings elucidate the coupled physicochemical mechanisms governing the durability of gangue-based shotcrete materials in underground water-bearing environments.