Self-Healing Mechanisms in Geopolymer Composites: Microstructural Characterization and Performance Metrics

Self-healing geopolymer composites represent a frontier in sustainable construction materials, capable of autonomous crack repair through intrinsic chemical reactions or extrinsic agent delivery, restoring structural integrity without external intervention. The main aim of this review is to comprehensively analyze self-healing mechanisms, performance characteristics, and further research directions in this area. The article draws from the state-of-the-art literature review with critical evaluation, enriched by representative case studies. The following issues are discussed: (1) Microstructural mechanisms of self-healing, including intrinsic (continued geopolymerization of unreacted precursors, Ca-rich phases forming CSH-like products, carbonation-induced precipitation) and extrinsic (microcapsules, vascular networks, mineral admixtures like Na2SiO3 or MgO), and their microstructural evolution during healing. (2) The currently used methods of investigating self-healing and their limitations. (3) The challenges, such as deep crack healing (>500 μm), multiple healing cycles, and environmental robustness. (4) Applications, including durable pavements and 3D-printed elements prone to shrinkage cracking. (5) Future directions emphasizing hybrid mechanisms, in situ monitoring (acoustic emission, ultrasonic testing), and life-cycle assessment. The main findings show that self-healing geopolymers advance circular economy principles by extending service life and minimizing repair costs in low-carbon infrastructure.