Influences of different calcium sources on the early age cracks of self-healing cementitious mortar

Abstract

Self-healing schematic diagram of mortar cracks. • Studied the changes of Ca 2+ concentration and calcification rate in alkali solution. • Early age cracks of mortar could be well repaired by microorganisms. • The maximum average healing depth of early age cracks could reach 4.0 mm. • The self-healing mechanism of mortar cracks was discussed. • Calcium lactate was more suitable as an external calcium source than calcium nitrate. Microbial induced calcium carbonate precipitation (MICP) has become an efficient and environmentally friendly technology for repair of cracks in cement-based materials. In this research, the alkali-tolerant microbial spores was prepared by spray drying, and two kinds of microbial self-healing agents were used to study the mineralization reaction in alkaline environment, then they were used in the self-healing of the early age cracks with a width of 0.2−0.3 mm. The self-healing efficiency of cracks was characterized by the area repair ratio, permeability coefficient and healing depth. Meanwhile, the morphologies and polymorphs of precipitates were analyzed by SEM equipped with an EDS and XRD. The results indicated that the calcification rate using calcium lactate as calcium source was higher than that of calcium nitrate, and had good self-healing efficiency on mortar cracks. However, compared with the specimens adding N-Ca and spores, the specimens adding L-Ca and spores had lower permeability coefficient and deeper healing depth, their average healing depth was 1.9 mm and 4.0 mm, respectively. In addition, the precipitates generated were most at the crack surface, with the increase of the crack depth, the precipitates gradually decreased. XRD analysis showed that the precipitates at the crack mouth were calcite. Finally, the reasons for the different healing depth of the microbial self-healing agents were analyzed and the self-healing mechanism was discussed.