The effects of biomineralization on the localised phase and microstructure evolutions of bacteria-based self-healing cementitious composites

Microbially induced calcite precipitation (MICP) is one of the most effective mechanisms to achieving self-healing abilities in cementitious composites. However, there has only been limited understanding of the effect of the MICP process on the mineralogy and microstructure of the cementitious matrix closely mixed with the healing products. This study systematically assessed the effect of biomineralization on the localised cementitious binders at micro and atomic level combining different characterisation techniques (i.e. XRD, FTIR and μCT). The results show that, in addition to the formation of CaCO3 polymorphs that close the crack space, the MICP process will also modify the phase assemblages near the healed cracks. For the first time we observed that when the most common source of calcium for the MICP process (calcium hydroxide) is limited, ettringite and C–S–H can also act as the providers of the calcium for the biomineralization process to take place. The detailed microstructure characterisations support that, apart from the dense thin layer (around 0.5 mm) of healing products formed on the surface of the cracks, loose particle-like calcium carbonate crystals can also form in pores and voids, suggesting that healing can also be generated in deeper sections of the crack. The outcomes of this study advance the fundamental understanding of the MICP process in Portland cement binders, and will also assist the further evaluation of the durability performances of these self-healed cementitious composites.