Sequential microbial modification of high-sulfur coal gangue mortar for enhanced durability: Sulfur regulation and calcium carbonate mineralization

High-sulfur coal gangue utilization in cementitious materials is constrained by mobile sulfate release, volume instability, and durability loss. A function-ordered strategy that regulates mobile sulfur before carbonate mineralization remains insufficiently evaluated. This study developed a sequential microbial pretreatment in which high-sulfur coal gangue was treated with Citrobacter freundii (CF) for 7 d and then Bacillus pseudofirmus (BP) for 7 d, dried at 60 °C for 24 h, and used to replace 10 vol% of standard sand in mortar. Sulfate release and the mechanical, transport, freeze–thaw, pore, and phase responses of the mortar were evaluated. During CF pretreatment, the leachate SO₄²⁻ concentration decreased from 134.41 mg·L⁻¹ at 1 d to 73.66 mg·L⁻¹ at 7 d. At 28 d, S-CF + BP achieved a compressive strength of 44.1 MPa, 3.76% higher than that of untreated high-sulfur coal gangue mortar (MHSCG), while remaining below the single-strain groups. Compared with MHSCG, S-CF + BP reduced drying shrinkage and the chloride migration coefficient by 28.43% and 60.26%, respectively, and increased ultrasonic pulse velocity by 10.54% to 4.195 km·s⁻¹ . After 125 freeze–thaw cycles, its mass-change ratio was 6.93%, and its relative dynamic elastic modulus remained at 89.5%. SEM–EDS and XRD observations showed less pronounced AFt-associated features, while the apparent TG-derived CaCO₃-equivalent content of S-CF + BP was 8.69%, 11.70% higher than that of MHSCG. These findings establish a function-ordered microbial framework connecting aggregate-scale sulfur regulation and carbonate mineralization with mortar-scale transport and freeze–thaw performance, providing a durability-oriented basis for utilizing high-sulfur coal gangue in cementitious materials.