Synergistic Mechanism of Precipitation-Type and Chelation-Type Crystalline Admixtures for Enhanced Self-Healing of Concrete

Crystalline admixtures promote self-healing in concrete by activating chemical reactions that seal cracks and restore the integrity of the matrix, but the distinct mechanisms of different types make it difficult to achieve both rapid sealing and long-term healing within a single system. To address this challenge, precipitation-type crystalline admixtures (PCAs: sodium bicarbonate, sodium fluorosilicate, and sodium silicate) and chelation-type crystalline admixtures (CCAs: sodium gluconate, sodium maleate, and sodium citrate) were combined to develop a PCA–CCA synergistic healing system for concrete. The healing performance of concrete containing this synergistic system was systematically investigated based on crack morphology, closure ratio, water permeability, water absorption, compressive strength retention, chloride diffusion coefficient, and the microstructural characteristics of repair products. Results demonstrated that the PCA–CCA system substantially improved the self-healing ability of the concrete. After 28 days, ordinary concrete exhibited incomplete healing for cracks narrower than 0.20 mm, whereas the sodium maleate–sodium silicate combination achieved full recovery of cracks up to 0.52 mm. In concrete containing this combination, the water permeability decreased to 0, and water absorption was 37% lower than that of ordinary concrete. For predamaged concrete, the compressive strength retention reached 96.7%, while the chloride diffusion coefficient was only 5.2% higher than that of undamaged concrete. Microscopic analyses revealed that the C–S–H gels generated by sodium silicate and the CaCO 3 deposits induced by sodium maleate were evenly distributed and tightly compacted, which enhanced the overall self-healing performance of the concrete.