Influence of Bacillus species on mechanical and microstructural properties of concrete

This study investigates the self-healing efficacy and mechanical performance of concrete incorporating four Bacillus species [B. subtilis (BS), B. cereus (BC), B. halodurans (BH), and B. licheniformis (BL)] at cell concentrations of 10<^>8 and 10<^>9 cells/mL. The influence of different curing regimes (calcium lactate, water, and ambient conditions) on the crack healing efficiency and strength recovery is evaluated. Compressive, tensile, and flexural strengths are assessed before and after healing, and microstructural characterization is performed using XRD and SEM-EDX analyses. The results demonstrate that the incorporation of Bacillus species significantly enhances the self-healing capabilities of concrete, with B. halodurans exhibiting the highest crack healing efficiency of 90% and compressive strength recovery of over 97% under calcium lactate curing at 10<^>9 cells/mL. Tensile and flexural strength recoveries also reach 97% and 99%, respectively, for B. halodurans. XRD and SEM-EDX confirm the pore-filling and crack-healing ability of bacterially precipitated calcite, with B. halodurans showing the densest calcite crystallization and the highest calcite content of 6.2%. EDS data reveals C-S-H gel formation, with B. halodurans exhibiting the maximum gel deposition and the lowest Ca/Si ratio of 1.54. The synergistic effects of calcite precipitation, C-S-H gel formation, and biofilm integration lead to significant improvements in mechanical properties and microstructure. These findings highlight the potential of Bacillus-based self-healing concrete as a sustainable and resilient construction material, capable of autonomously repairing cracks and enhancing the durability of structures. The multifaceted strength recovery mechanisms induced by Bacillus bacteria, particularly B. halodurans, demonstrate the promising prospects of microbial self-healing in concrete technology.