Effect of microbial mineralization characteristics on crack repair and structural recovery of coal-based solid waste concrete

Coal-based solid waste concrete (CSWC) has attracted increasing attention for solid waste utilization; however, its porous structure and weak crack resistance limit long-term durability. Microbial mineralization provides an environmentally friendly approach for crack repair, while the relationship between microbial mineralization characteristics and structural recovery of CSWC remains unclear. This study investigated the crack repair behavior and mechanical performance recovery of CSWC treated with two microbial mineralization systems, Bacillus megaterium ( B.M. ) and Sporosarcina pasteurii ( S.P. ). Prefabricated cracked specimens were treated with cementation solutions at concentrations of 0.5, 1.0, and 1.5 mol·L −1 . These concentrations were selected to represent different substrate availability conditions and to evaluate whether increasing cementation-solution concentration resulted in a corresponding improvement in microbial mineralization and repair performance. The crack healing ratio (CHR) was calculated from surface crack-width measurements and therefore represents the apparent closure degree of exposed crack openings rather than direct evidence of complete internal crack filling. Ultrasonic pulse velocity (UPV) and compressive strength recovery were used as complementary indicators to evaluate internal structural continuity and mechanical restoration. The phase composition, thermal decomposition behavior, micromorphology, and elemental composition of the mineralized deposits were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM–EDS). The results showed that both microbial systems promoted crack repair and structural recovery, but their mineralization characteristics resulted in distinct repair behaviors. The B.M.-based system exhibited gradual mineral accumulation and sustained crack filling, whereas the S.P.-based system produced more rapid crack closure and earlier mineral accumulation. After 28 d of treatment, the crack healing ratio of the B.M. and S.P. systems reached approximately 40% and 55.9–62.1%, respectively, while the compressive strength recovery ratios were 19–21% and 26–28%. Microstructural analyses associated the higher repair performance of the S.P.-based system with denser and more continuous carbonate-rich deposits. Because different calcium sources were used in the two systems, the observed differences represent their combined system-level responses. This study establishes a relationship between microbial mineralization characteristics, CaCO 3 deposition behavior, and structural recovery of CSWC, providing new insights into the mechanism of microbial-assisted crack repair for coal-based solid waste concrete.