Dual Effects of Delayed Activation and Ca²⁺ Adsorption: Investigation of the Self-Healing Function of Expansive Soil in Solid Waste-Based Cementitious Materials

The cost and environmental friendliness of materials are key factors limiting the widespread application of self-healing cementitious materials. This study investigated the influence of low-cost, pollution-free expansive soil (ES) on the crack self-healing properties of cementitious materials prepared from industrial solid wastes: circulating fluidized bed fly ash (CFBFA) and calcium carbide slag (CS). Results indicate that the system incorporating 10% ES achieved a strength recovery rate of 92.7% relative to its original strength after 28 days of self-healing. It could close cracks up to 75.23 μm wide, with ultrasonic pulse velocity (UPV) increasing over time as cracks progressively narrowed. SEM revealed the crack surface covered by hydration products. XRD and TG analysis identified repair products including AFt, C-S-H, and CaCO₃. By comparing ES's delayed activation and Ca²⁺ adsorption experiments, it was verified that adding ES provides more unhydrated particles during cracking. Simultaneously, montmorillonite adsorbs Ca²⁺ through electrostatic interactions, causing it to accumulate around the ES particles, while the hydration products deposit on the surface of the cracks. The synergistic action of these two processes enhances the self-healing capacity.