Effect of C–A–S–H formed by iron ore tailings ceramsite on the micromechanical properties of the interfacial transition zone in concrete

To clarify the effect of iron ore tailings (IOTs) ceramsite on the interfacial transition zone (ITZ) in concrete, this study systematically investigates the microstructural evolution and micromechanical properties of the ITZ in IOTs ceramsite concrete (LC). Results show that a continuous Ca–Si–Al-rich interfacial region associated with C–A–S–H-rich/Al-bearing C–S–H products develops near the IOTs ceramsite surface, corresponding to a denser and more uniform ITZ than that in normal concrete (NC). Nanoindentation results show that the HD C–S–H fraction in LC is higher than that in NC at 28 days, while the low-modulus interfacial region observed in NC is substantially reduced in LC. With curing up to 90 days, the ITZ in LC becomes progressively more compact and exhibits a more uniform local stiffness distribution. The effective elastic modulus of the C–S–H/C–(A)–S–H assemblage, calculated using both self-consistent and Mori–Tanaka multiscale homogenization models, is approximately 18% higher in LC than in NC after 90 days of curing. The observed interfacial modification is associated with the combined effects of moisture redistribution by the porous ceramsite and interfacial reactions involving Si- and Al-bearing species. These findings provide a quantitative micromechanical basis for understanding the interfacial behavior of IOTs ceramsite concrete and for further evaluating its application in cementitious composites.