Application of calcium nitrate as calcium source on self-healing concrete with non-ureolytic bacteria

Self-healing concrete has been investigated over recent years, with microbially induced calcite precipitation (MICP) is adopted as one of the potential pathways to heal cracks in concrete structure. However, several factors may affect the performance of bacteria based self-healing, e.g. temperature, pH value and moisture content etc. Furthermore, as calcium source may affect bacterial activities, calcium source type is another potential factor. The presented study investigated the possibility of using calcium nitrate as the calcium source for the non-ureolytic bacterium, for application in self-healing concrete. This combination was confirmed to be feasible through microbiology test and it was the first to combine calcium nitrate with non-ureolytic bacteria in the self-healing concrete. Bacterial spores were encapsulated in aerated concrete
granules coated with PVA. Calcium nitrate (5% by mass of cement) and yeast extract (1% by mass of cement) were directly added into cement matrix instead of encapsulating in any carrier which simplified the manufacturing process and had less negative effect on mechanical performance of concrete. Samples were subjected to a wet-dry cycle healing regime after cracking. Precipitated calcite crystals on crack surface were visible under an optical microscope after 1-week of healing. After 28-days of healing, crack size was reduced from approximately 0.4 mm to about 10 µm, whereas reference samples (standard cement mortar) showed no obvious healing and only slight healing was apparent in control samples without bacteria. Scanning Electron Micrography (SEM) and Energy dispersive spectra (EDS) evidenced the presence of calcite precipitated within the crack. Water permeability tests showed an improvement in permeability relative to cracked samples of close to 90% for bacterial samples after 28-days healing, whereas reference samples and control
samples only showed improvements of 23% and 34% respectively.