Post-fire curing and autogenous self-healing in alkali-activated slag: Microstructures and healing mechanisms

Among various ways to counter the effects of cracking and damage induced by exposure to elevated temperatures, self-healing appears to be a promising technique for maintaining construction materials' structural integrity and longevity. This research investigates the effectiveness and potential of autogenous self-healing for alkali-activated concrete performance recovery after exposure to elevated temperatures up to 800 °C. Two post-fire curing methods were examined: exposure to ambient air and immersion in sodium hydroxide (NaOH), to assess their effectiveness in promoting self-healing in AAS. To achieve better understanding of the mechanisms behind post-fire AAS's self-healing, a comprehensive evaluation of crack healing, microstructural changes, and the durability of the treated materials was conducted through a combination of visual inspection, ultrasonic pulse velocity, water sorptivity, rapid chloride permeability tests, scanning electron microscopy-energy dispersive X-ray, and X-ray diffraction. Findings reveal that NaOH immersion significantly enhances self-healing, evidenced by reduced crack widths and improved material properties. Changes in the amount of hydration products and various shapes of self-healing product crystals, such as trigonal, cubical, and rectangular, were detected. However, the dominant healing product was calcium carbonate (CaCO 3 ), which precipitates through a combination of hydration reactions and carbonation processes, crucially contributing to the closure of microcracks and the restoration of the material's integrity. This process underscores the critical role of chemical reactions in AAS's self-healing mechanism, offering insights into its potential for enhancing the durability and resilience of construction materials. This emphasized the potential for leveraging AAS properties by self-healing and mitigating fire damage, thus extending the lifespan of structures and reducing maintenance costs.