Influence of Dosage of Iron Ore Tailings–Based Nonsintered Artificial Aggregates on the Strength and Microstructure of Ultrahigh-Performance Concrete

By integrating experiments with microanalysis, this study investigates the influence of replacing coarse quartz sand with iron ore tailings (IOT)–based nonsintered artificial aggregates on the strength, relative humidity (RH), and microstructure of ultrahigh-performance concrete (UHPC). Furthermore, it elucidates the mechanism by which the replacement ratio of artificial aggregates affects the strength of UHPC. The results indicate that when artificial aggregates replace coarse quartz sand at ratios ranging from 0% to 60%, both compressive and flexural strengths of UHPC initially increase and subsequently decrease as the replacement ratio increases, the maximum strength is achieved at a replacement ratio of 15%, and 28-day compressive and flexural strengths are 131.6 and 18.7 MPa, respectively. The incorporation of IOT artificial aggregates can significantly retard the reduction in RH within UHPC; for UHPC with 15% and 30% artificial aggregate substitution, the 28-day RH had increases of 3.58% and 8.55% compared with the sample without IOT artificial aggregates. Microstructural analysis reveals no significant differences in the UHPC matrix across varying replacement ratios. However, artificial aggregates significantly enhance the interfacial transition zone (ITZ) between the UHPC matrix and aggregates. Notably, the ITZ width between the UHPC matrix and quartz sand is minimized when the replacement ratio of artificial aggregates is 15%. Additionally, the replacement ratio alters the pore size distribution of the UHPC matrix. As the replacement ratio increases, the pore size distribution initially shifts toward smaller pores and then toward larger pores. Correspondingly, the fractal dimension of the pore structure first increases and then decreases with the rise in the replacement ratio. This study has economic feasibility such as wide sources of raw materials, strong technical adaptability, extensive application fields, and great potential for industrial synergy, which offers a practical solution for waste management while simultaneously reducing the demand for natural sand resources. Additionally, the findings regarding the optimal replacement ratio and its influence on UHPC strength and microstructure provide valuable guidance for the design and production of high-performance concretes, thereby contributing to a reduced environmental footprint. Finally, the insights into the mechanisms by which artificial aggregates affect the interfacial transition zone and pore structure enhance understanding of the material science of UHPC.