Microstructure and micromechanical properties of the interfacial transition zone in concrete incorporating lime mud-based artificial aggregates

Lime mud-based artificial aggregate (LAA) is a promising substitute for mitigating the shortage of natural aggregate; however, the lack of comprehensive study on its interface interaction with cement matrix hinders its application in concrete. This study aims to investigate the microstructural and micromechanical properties of the interfacial transition zone (ITZ) in lime mud-based artificial aggregate concrete (LAAC) and explore the relationship between two of them. The microstructure and mineralogical phase characteristics are qualitatively and quantitatively characterized using backscattered electron microscopy (BSEM), energy-dispersive X-ray spectrometer (EDS), and X-ray diffraction (XRD), while the micromechanical properties of the ITZ are thoroughly investigated via nanoindentation. Key results reveal that LAAC's ITZ exhibits a 34.4 % lower porosity, 35.6 % higher elastic modulus, and 74.7 % greater hardness compared to natural aggregate concrete (NAC). The Ca/Si molar ratio at the LAAC interface increases from 1.0 to 1.3 as curing time extended from 3 to 28 days, accompanied by a narrower ITZ width (50 μm at 28 days) than that in both NAC and early-age LAAC. These improvements are attributed to the internal curing effect of LAAC through a dual-phase mechanism. Although interlocking sites and C-S-H gels enhance the interfacial bonding, LAA-induced Ca²⁺ supersaturation forming excessive CH crystals remains problematic. This study advances the understanding of the ITZ in calcium-rich artificial aggregate concrete, and establishes a foundation for the application of LAA in concrete.