Enhancing microbial induced calcium carbonate precipitation (MICCP) in self-healing concrete by addressing cell encapsulation: Investigating the impact of exogenous carbonate ions

This study investigates the impact of calcium ion-rich media on biomineralization yield during microbially-induced calcium carbonate precipitation (MICCP) reactions, presenting a novel approach to enhance crack-filling kinetics crucial for concrete self-healing. Our findings reveal that bacterial cells, nucleation sites for calcium carbonate precipitation dominating their surfaces, become encapsulated by calcium carbonate crystals, particularly in calcium-rich environments. This encapsulation was hypothesized to impede cell access to nutrients and urea, resulting in significant reductions in cell growth (98.4%), urea hydrolysis (84.6%), and calcium carbonate production (81.2%), delaying MICCP initiation by up to 10 days. Moreover, the consumption of dissolved carbonate ions due to calcium carbonate precipitation diminishes media buffering capacity, further retarding bacterial growth. Conversely, adding exogenous sodium carbonate accelerated bacterial growth (up to 122-fold), urea hydrolysis, and biomineralization (up to 10-fold), expediting crack-filling in cement mortar specimens. We also found that higher initial cell concentrations (OD600 of 1) provided more nucleation sites, reducing cell encapsulation and synergistically enhancing MICCP kinetics while maintaining media buffering capacity. These findings offer mechanistic insight into the importance of mitigating cell encapsulation during MICCP to enhance concrete crack-filling. • Cell encapsulation by calcium carbonate crystals restricts nutrient uptake and reduces bacterial growth rates, urea hydrolysis, and MICCP performance. • The lack of soluble carbonate ions during cell encapsulation, by reducing the buffering capacity of the culture media, negatively impacts the ability of cells to regulate the internal pH. • Exogenous carbonate ions maintain buffering capacity, promote higher growth rates, and enhance biomineralization by increasing the number of active cells not fully encapsulated by calcium carbonate.