Microbially Induced Calcite Precipitation for Autonomous Crack Healing in Cementitious Composites: Strain Screening, Transport Recovery, and Microstructural Confirmation

Concrete is vulnerable to cracking due to mechanical, thermal, and environmental loading, which, as exacerbated by time, leads to unavoidable deterioration facilitated by water penetration and aggressive ions. Current repairs are mostly temporary fixes, which fail to restore microstructural integrity, highlighting the necessity for self-healing solutions with autonomous action. Microbially Induced Calcite Precipitation (MICP) is a biomineralization-type self-healing solution; however, much of the literature surrounding MICP utilizes commercial ureolytic strains and fails to perform extensive assessments of strains-to-strains or correlations of microbial characteristics and engineering performance. This study aims to bridge those gaps by isolating multiple indigenous ureolytic Bacillus spp. Strains from a natural construction site and subjecting them to a comprehensive, multi-parameter evaluation that is characterized by pH tolerance, growth kinetics, urease activity, and CaCO₃ precipitation potential. The most promising strain (Isolate-1) was used as a treatment in mortar and concrete samples to assess ultimate compressive strength recovery and self-healing. Statistical analysis, confidence intervals, and significance tests revealed significant differences, which determined that Isolate-1 performed better than Bacillus pasteurii and Bacillus sphaericus in all cases of mineral precipitation, enhanced rehabilitation of cracks, and strength restoration. Structural integrity was validated by SEM and XRD through dense calcification treatment on all surfaces exposed. These findings not only serve as a comparative performance baseline for in-situ strains compared against well-documented commercial strains but also provide an effective, multi-faceted assessment protocol for practical, field-scale applicability in determining operable strains for self-healing concrete.