Field validation and mechanism of volcanic ash-lime-MICP composite binder for concrete crack repair in seawater environments

To address the crack repair requirements of concrete in marine revetment structures, this study proposes a composite cementitious system by integrating volcanic ash, hydrated lime, and microbially induced carbonate precipitation (MICP). The mix proportion was optimized using a Box-Behnken response surface methodology, while the cementation mechanisms were investigated via microstructural characterization. Marine revetment concrete cracks were used as the repair target, and the repair effectiveness was evaluated through macroscopic physical and mechanical tests. The results indicate that: (1) the optimal mixture contains 25.8% hydrated lime, a liquid-to-solid ratio of 1.28, and a bacterial concentration of OD 600 =1.95; under this mixture, the fluidity is 241.3 mm, and the 7 d and 28 d compressive strengths reach 3.08 MPa and 5.53 MPa, respectively. (2) Engineering performance verification shows that, after repair, the ultrasonic transit time decreases by 64.04% - 68.08%, the compressive strength recovery increases by 90.27%, the interfacial shear strength becomes 2.0 - 2.5 times that of the unrepaired state, and the permeability coefficient is reduced from 10⁻⁴ m/s to 10⁻¹¹ m/s. (3) The crack-repair mechanism of the composite system can be attributed to three aspects: (i) hydroxyl sites on volcanic ash and C-(A)-S-H gel surfaces promote bacterial enrichment and stable attachment, thereby enhancing the efficiency of MICP precipitation; (ii) C-(A)-S-H gel and products such as layered double hydroxides (LDHs) improve the microenvironment for bio-reactions and reduce the risk of secondary deterioration; (iii) gel-carbonate hybrid products synergistically fill pores and construct a dense composite cementation network, strengthening interfacial interlocking and load transfer. These findings can provide a reference for material design and engineering applications in the repair of cracks in marine revetment concrete.