Control of the polymorphism of calcium carbonate produced by self-healing in the cracked part of cementitious materials

Cracking is an inherent development in reinforced concrete structures and can lead to serious damages during their service period. The repeated occurrence of such damages can enlarge the cracks, thereby allowing other deteriorating elements such as CO_² and Cl⁻ to further infiltrate the concrete, which can seriously compromise the concrete structure. This study focuses on the type of calcium carbonate (CaCO₃) crystals generated by the self-healing phenomenon. Owing to polymorphism, CaCO₃ has three types of crystal forms-calcite, vaterite, and aragonite-whose formation can be controlled by the temperature and pH. Vaterite has the highest density among these crystals, and it is expected to be capable of self-healing. Therefore, experiments were conducted to establish the conditions required to promote the generation of vaterite. A saturated Ca(OH)₂ solution with CO₂ nanobubbles (CN) was employed for effective self-healing. The temperature was controlled at 20, 40, and 60 ^◦C, and the pH was controlled at 9.0, 10.5, and 12.0. The results showed that the self-healing of cracks occurred both on the surface and internally, and the main product of the self-healing phenomenon was vaterite in CaCO₃ crystals at a pH of 9.0 and a temperature of 40 ^◦C. Furthermore, the addition of a saturated Ca(OH)₂ solution with CO₂ nanobubbles (CN) resulted in the most effective self-healing of the surface and internal cracks.