Abstract: The interfacial transition zone (ITZ) between aggregates and cement paste in concrete constitutes a weak region, weakening both strength and durability.To address the limited research on the relationship between Ca(OH)₂ crystals (hereafter referred to as CH crystals) microcosmic characterization in the ITZ and material mechanical properties, this study employed FTIR, XRD, and TG techniques to quantitatively characterize the orientation index, average crystal size, and content of interfacial CH crystals.The evolution of CH crystal structure was elucidated, thereby establishing a correlation between the CH crystal orientation index and mechanical performance.Key findings reveal that the ITZ exhibits CH crystal enrichment, oriented growth, and large crystal dimensions.With increasing curing age, CH crystal content rises while the orientation index gradually declines.The incorporation of fly ash and silica fume reduces the orientation index, size, and content of CH crystals.Furthermore, empirical relationships between the CH crystal orientation index and flexural/compressive strength were fitted for mixtures with different supplementary cementitious materials.