A Binary Microorganism Self-Healing Agent for Concrete Cracks Comprising Bacillus pasteurii and Saccharomyces cerevisiae

The use of microbially induced calcium carbonate precipitation (MICP) to self-repair concrete cracks has received extensive attention. The production of mineralization precipitates reflects the repair capability of the self-healing agents. However, using a single type of microorganism in the self-healing agent, such as Bacillus pasteurii, usually produces a low amount of mineralization products in the direction of crack depth; therefore, the cracks cannot be well repaired. In this work, a binary microorganism self-healing agent was developed, and its crack repair capability was investigated. In the binary microorganism system, Bacillus pasteurii and Saccharomyces cerevisiae were mixed at six mixing ratios, 10:0, 8:2, 6:4, 4:6, 2:8, and 0:10. The results show that the highest concentration of the microorganism cells, the highest weight of the mineralization precipitates, and the purest calcite crystals were produced when the mixing ratio was 6:4. Besides, after 28 days of repair, cracks in mortar specimens were repaired with the binary microorganism self-healing agent, with the mixing ratio of 6:4 showing the highest area percentage of repair (97.1%) and the strongest capability to repair deep cracks (9–12 mm from the specimen surface). The synergic mineralization mechanism is that Bacillus pasteurii plays a major role in the closure of the fractured surface, while Saccharomyces cerevisiae promotes the production of carbonate ions by decomposing glucose under oxygen-poor conditions, resulting in the formation of calcium carbonate precipitates and facilitating the repair of deep cracks where there is a lack of oxygen. This work provides a promising binary microorganism self-healing agent and an understanding of the mechanism involved in the mineralization process, and experimentally confirms its superior self-healing capability over the single-type microorganism system considering the repair depth.