Chemical Regulation of Multiscale Crack Healing in Cementitious Materials by Active Components of Crystalline Admixtures

Crystalline admixtures facilitate crack self-healing in cementitious materials via precipitation and ion migration, yet the specific chemical roles of individual active components remain poorly understood, limiting insight into the underlying healing mechanisms. To address this issue, sodium carbonate, sodium silicate, and calcium formate were selected as representative active components of crystalline admixtures and comparatively investigated through single-component analysis. The self-healing behavior of mortar was evaluated based on crack width evolution, relative water flow ratio, compressive strength retention, chloride migration coefficient, pore structure characteristics, and the structure and composition of healing products. The results indicated that all three functional admixtures promoted crack healing by inducing precipitation reactions, while exhibiting distinct roles at different scales. Sodium silicate showed the most pronounced effect on internal microcrack healing and pore structure refinement. After preloading and curing, the compressive strength retention of mortar containing sodium silicate increased to 90.4%, and the chloride migration coefficient decreased to 5.43 × 10–12 m2/s, accompanied by a noticeable reduction in large pores. In contrast, calcium formate performed better in macroscopic crack closure. Mortar incorporating calcium formate achieved complete healing of cracks with dimensions between 0.10 and 0.36 mm, while the relative water flow ratio decreased to 4.8% and the cumulative water absorption was decreased by 35.4% compared with the ordinary mortar. XRD analysis identified CaCO3 as a major crystalline phase in the crack regions. The incorporation of functional admixtures led to a more continuous and denser deposition layer, among which calcium formate formed a more intact healing layer at the crack surface, contributing to more stable crack closure.