Converting Iron Mine Tailings into Composite Aggregates for Ultrahigh-Performance Mortar

Iron tailings (IT) are by-products of iron ore mining, which have the characteristics of high stockpile and low cost. Recycling IT as alternatives for high-quality quartz sand (QS) in ultrahigh-performance mortar (UHPM) is an environmentally friendly and sustainable approach. However, the addition of IT changes the continuity of the material gradation, which in turn affects the dense structure of the UHPM. Therefore, in this paper, the effects of composite aggregates composed of different blends (0%–100%) of IT and QS on the working properties, compressive strength, chloride ion penetration, toxic leaching, and microstructure of UHPM were investigated. The chloride ion permeability and toxicity leaching of UHPM were tested using the electric flux method and the inductively coupled plasma method. The microstructure of UHPM was characterized using the mercury intrusion method and electron microscope. The results showed that a composite aggregate consisting of 60% IT and 40% QS could achieve the best strength of UHPM. When QS was completely replaced by IT, similar strength levels to the control group (100% QS) could still be achieved, while the electric flux was reduced by 24.8% and the leaching of harmful ions was negligible. What counts is that the IT replacement rate should be controlled within 60% to maintain the necessary working performance. The results of the close packing model indicate that the UHPM material is most closely packed when the IT content is about 60%. The random aggregate distribution model also indicates that the range of 60%–80% IT admixture is the optimal gradation interval for UHPM, and the results of the two models are consistent with the basic performance law.