Optimization of deposition process for a productive and cohesive bio-CaCO₃ to repair concrete existing cracks

Microbial-induced carbonate precipitation (MICP) is being investigated to repair concrete cracks because of its good durability and compatibility with cementitious matrix. However, during the in-situ application, the repairing often lasts weeks, even months. And the strength regain is quite low. The repairing time is largely determined by the CaCO₃ yield, and the strength regain after the repair is closely related to the cohesion and bonding strength of CaCO₃ itself. Thus, the purpose of this paper is to obtain an efficient precipitation of bio-CaCO₃ with both high yield and good cohesion to improve the in-situ repairing efficiency. Firstly, the most influential factors on urease activity were screened and the precipitation kinetics were detailly investigated. The results show that the CaCO₃ with the largest yield and cohesion was obtained when the bacterial concentration was 10⁷ cells/mL and the concentration of urea and calcium was both 0.5 M at 20 °C. This weight loss of bio-CaCO₃ was 9.24% under ultrasonic attack. Secondly, two models were established to quantify or semi-quantify the relationship between the most influential factors and the yield and cohesion of precipitates, respectively. The results showed the order of contribution for bio-CaCO₃ precipitation was calcium ions concentration > bacterial concentration > urea concentration > temperature > initial pH. According to these models, the required yield and cohesion of CaCO₃ by engineering could be obtained by adjusting affecting factors. Models were proposed for guiding the application of MICP in practical engineering. KEY POINTS: • Screened the most affecting factors on urease activity and investigated the precipitation kinetics. • Obtained optimal conditions of bio-CaCO. • Established two models in order to give some guidance for practical civil engineering.