Abstract The advancement of self-healing concrete represents a transformative step toward sustainable infrastructure, yet the integration of microbial agents with engineered admixtures under marine conditions remains underexplored. This study investigated the synergistic effects of Bacillus sphaericus and polymeric air-entraining agents (AEAs) on the self-healing performance and durability of high-strength concrete (HSC) exposed to saline environments. Two strains of B. sphaericus (UPMB-10 and ATCC 14577) were evaluated for viability, sporulation, and mineralization potential under varying salinity levels, with ATCC 14577 selected for its superior resistance and CaCO 3 yield. Freeze-dried spores enriched with calcium lactate and urea were incorporated into HSC mixes containing different AEA dosages (2–6%), enabling microbial survival within air-void niches. Concrete specimens were subjected to cyclic curing in both tap water and artificial seawater to simulate fluctuating marine exposure. Healing efficiencies reached up to 71 % in tap water, while seawater immersion produced complete crack closure by 28 days and up to 96 % recovery in water tightness by 56 days. X-ray diffraction revealed mineralogical adaptation in seawater, with polymorphic phases such as aragonite and diopside forming alongside calcite, driven by the presence of Ca 2+ and Mg 2+ ions. These diverse precipitates contributed to enhanced crack sealing compared to the predominantly calcitic deposits in tap water. The findings demonstrate that moderate polymeric AEA dosages preserve structural-grade strength (>100 MPa) while supporting microbial viability and self-healing activity. By linking bacterial metabolism with saline-induced mineral diversity, this study introduces an integrative microbial-admixture framework for designing next-generation self-healing HSC tailored for marine infrastructures. The synergy between optimized bacterial strains and controlled air-void incorporation (by suitable polymeric AEAs) offers a durable, autonomous repair mechanism capable of withstanding aggressive coastal environments.
Al-Yaari et al. (Thu,) studied this question.