Bio-protection of cementitious materials below ground: The significance of natural soil environments

This study explores the potential impact of natural soil on concrete crack self-healing in sub-surface structures. Three types of pre-cracked cement mortar samples were prepared for laboratory experiments, with some samples inoculated with bacterial healing agents, others supplemented with nutrients to attract indigenous soil bacteria, and plain mortar served as controls. The samples were placed within saturated soil under two conditions: slightly organic natural soil and sterilised soil. After 100 days, crack closure was evaluated through microscopic inspection, water absorption tests, and SEM-EDX scanning. The results indicated calcite precipitation on crack surfaces across all samples, but with varying ratios of crack closure (16%–81%). Notably, most samples incubated in natural soil exhibited an overall increase (up to 59.4%) in healing ratio compared to those in sterilised soil, highlighting the potential of indigenous soil microorganisms and their microbial activity in enhancing the biogenic mineral precipitation and thus protecting sub-surface concrete structures.