Self-Healing of Later-Age Cracks in Cement-Based Materials by Encapsulation-Based Bacteria

Bacteria-induced calcium carbonate precipitation has been considered as an intelligent and environment friendly technology to repair cracks in cement-based materials. However, because of the high alkalinity and harsh physical and chemical environment inside the cement-based materials, even alkali-tolerant microbial spores can hardly survive for a long time in this environment, which affects the self-healing effect of later age cracks of cement-based materials. Two low alkaline materials, potassium magnesium phosphate (MKPC) and sulphoaluminate cement (SC) were screened to protect microbial spores in this paper, and the protective effects of the two materials on spores were compared within 180 days. The results showed that SC had a better protection effect than MKPC in simulated porous solution of cement-based materials because of its compact porosity. The area repair rate and resistance to water permeability repair rate were used to characterize the self-healing effects of cracks. After 180 days, the area repair rate of the specimens with unencapsulated spores, MKPC encapsulated spores and SC encapsulated spores were 18%, 92%, and 96%, respectively, and the corresponding resistance to water permeability repair rate were 14%, 78%, and 85%, respectively. The mineral precipitations at the crack area were analyzed by Scanning Electron Microscope (SEM) equipped with an Energy Dispersive X-ray Spectrometry (EDS) and X-ray diffraction (XRD). These results indicated that the filling materials at the crack area were ${\mathrm{CaCO}}_3$ with compound polymorph of calcite and vaterite. The research suggests the carrier technology can enhance the self-healing effects of later age cracks of cement-based materials, which provides a new idea and method for the long-term self-healing ability of concrete.