Effect of carbonation on bacteria-based self-healing of cementitious composites

Self-healing cementitious composites are being developed to respond to the high cost of repair and maintenance of infrastructure. A promising solution is the use of bacteria to induce calcium carbonate precipitation within cracks when they occur and prevent further deterioration. Previous work has shown successful bacteria-mediated self-healing of cementitious composites at early-ages, in conditions where the material was uncarbonated prior to cracking. However, as cementitious composites often crack when they have reached a more aged state and are likely carbonated at the time of crack formation, these previous experiments did not fully reflect the real-world situation. In the present study, we show that for cementitious composites that do not carbonate prior to cracking the calcium hydroxide created as a hydration product is a sufficient source of Ca2+ ions to provide effective bacteria-induced healing. We note that supplying an extra source of Ca2+ ions at the moment of cracking, delivered via encapsulation, further enhances the degree of healing. Importantly however, in carbonated mortars calcium hydroxide is not available as a source of Ca2+ ions. Consequently, we show for the first time that bacteria-based self-healing in mortars that have carbonated prior to cracking is almost totally dependent on the availability of Ca2+ ions released from an encapsulated source. Our study therefore provides important insights for the rational design of self-healing concrete, where the conditions of the concrete during service life need to be taken into consideration when choosing between direct addition or encapsulation of calcium sources to ensure optimal performance.