The mechanism action of crystalline admixtures on hydration, microstructure, and self-healing of cementitious materials

This study investigates the self-healing potential of crystalline admixtures (CAs) in cement pastes at dosages of 0.25%, 0.5%, 1%, 3%, and 5% by weight of CEM I 42.5. The effects of CAs on hydration kinetics, physico-mechanical properties, and self-healing efficiency were analyzed using TAM calorimetry for heat evolution analysis, X-ray diffraction (XRD) for phase identification, scanning electron microscopy (SEM) for microstructural observation, thermogravimetric (TG) analysis for quantifying calcium formations, and physico-mechanical testing, open porosity, density, and capillary absorption. TAM calorimetry revealed decreased total heat after 50 hours when CA content increases, indicating modifications in the hydration process. XRD identified early age formed sodium-aluminum-silicate hydrate phases as CAs interaction products. Increased amorphous content and reduced calcite crystallinity in CA-modified samples highlighted the interactions of CAs with the cement matrix, enhancing the self-healing process. Pre-damaged specimens (up to 90% of 28-day compressive strength) demonstrated crack closure and improved microstructure at 0.5% and 1% CA dosages, while higher dosages (3% and 5%) negatively affected mechanical properties. Thermogravimetric analysis supported these findings, showing that increased CA concentrations reduced portlandite content and promoted calcite formation, especially during early hydration. The study focuses on the mechanism of CAs when added to the cement matrix and the optimum dosage. To enhance cement durability and self-healing efficiency, 0.5% and 1% CAs dosages were the most effective, as they maintained strength and enhanced the crack closure through calcite, aragonite and C-S-H formation. At the same time, higher concentrations disrupted hydration and compromised the integrity of the cement matrix. • Crystalline admixtures (CAs) enhance self-healing, achieving up to 91% crack closure for 350 μm cracks at 1% dosage. • CA dosages of 1% wt. promote CaCO 3 and C-S-H gel formation, densifying the matrix and reducing porosity. • XRD analysis showed amorphous phases in CAs-modified healing agents, enhancing hydration and self-healing efficiency. • Sodium-aluminum-silicate hydrate crystalline phase identified at 24 hours was unstable and decomposed at 3 days recordings. • CA dosages of 3% wt. and higher disrupt hydration, forming sodium-calcium-aluminum silicate phases that reduce calcite crystallinity, increase amorphous content and induce heterogeneity.