Abstract <p>Large quantities of waste rocks and tailings generated from iron ore mining present serious environmental challenges. Recycling these materials in green concrete provides an effective approach to reducing solid waste generation and lowering carbon emissions. However, the application of iron ore waste rock powder (IWRP) as a supplementary cementitious material (SCM) remains limited, and its influence on early-age strength, particularly within the interfacial transition zone (ITZ), has not yet been fully clarified. This study investigates the effects of IWRP and partial replacement of IWRP by lithium slag (LS) on early-age concrete performance, focusing on ITZ evolution. Backscattered electron (BSE) imaging, phase quantification, energy-dispersive X-ray spectroscopy (EDS), thermogravimetric analysis (TG), and X-ray diffraction (XRD) were employed to evaluate ITZ porosity, phase distribution, elemental ratios, and hydration products. The single IWRP system increased ITZ porosity and heterogeneity, weakening interfacial bonding and reducing early strength by about 17%. At the same total replacement level of 20%, partial replacement of IWRP by LS decreased ITZ porosity, improved unhydrated particle distribution, lowered Ca/Si, increased Al/Si, and promoted ettringite formation and chemically bound water, leading to partial strength recovery by approximately 8%. This work elucidates the ITZ-controlled mechanism governing early-age strength and supports sustainable utilization of all-sized iron ore tailings in low-carbon green concrete, providing a microstructural basis for practical industrial applications.</p> Graphical Abstract <p></p> Highlights <p><UnorderedList Mark="Bullet"> <ItemContent> <p>Sustainable utilization of iron ore tailings as a resource for green concrete, addressing mining waste disposal challenges.</p> </ItemContent> <ItemContent> <p>Innovative binary SCM system: Combining iron ore tailings with lithium slag to enhance early-age strength and improve ITZ microstructure.</p> </ItemContent> <ItemContent> <p>Advanced BSE–EDS imaging to quantify the impact of tailings on ITZ porosity, phase composition, and elemental ratios.</p> </ItemContent> <ItemContent> <p>Enhanced early-age strength through improved interfacial transition zone (ITZ) properties, offering a solution for mining waste valorization.</p> </ItemContent> <ItemContent> <p>New insights into the ITZ role in tailings-based concrete, promoting sustainable and low-carbon building material design.</p> </ItemContent> </UnorderedList></p>