Influence of calcium concentration and pH value on the efficacy of MICP in self-healing cementitious materials

Elucidating the influence of calcium concentration and pH value on the efficacy of MICP is important for tailoring the pore/crack solution to achieve improved self-healing performance. In this study, the urease activity and MICP of three microbial strains Sporosarcina pasteurii ( Sp ), Bacillus cereus ( Bc ), and Bacillus subtili ( Bs ) in environments with various calcium concentration and pH value were compared. Moreover, the effect of calcium acetate addition and bacteria encapsulation with CSA cement pastes on the self-healing performance was studied. The results revealed that the activity of urease from Sp was highest despite the negative effect of increasing pH from 9.5 to 13.0 on urease activity. The efficacy of MICP was governed by calcium concentration rather than pH value in the pore/crack solution and thus a high alkali environment within cementitious material is acceptable for urease-producing bacteria. The quick crack closure within 48 hours achieved in cement pastes with calcium acetate verified the vital role of soluble calcium in self-healing by MICP. Encapsulation with CSA cement paste was found able to confine MICP within crack, which is probably helpful for healing wide cracks. The microstructure analysis showed that calcite was the dominant phase in products from MICP, which could lack bonding with the crack face.