Properties of Ultra-High Performance Concrete incorporating iron tailings powder and iron tailings sand

Iron tailings (IT) is a kind of solid waste, which is usually divided into iron tailings powder (ITP) and iron tailings sand (ITS). It has been reported that IT has potentials to be used in ultra high performance concrete (UHPC). However, the recovery rate of IT is relatively low, and rare research mentioned the co-utilization of ITP and ITS to get full-scale use of IT. Therefore, in this study, ITP and ITS with different replacement ratio were used to replace cement and quartz sand, respectively, in UHPC. The effects of ITP and ITS substitution on the mechanical properties, microstructure, hydration products, durability and environmental evaluation of UHPC mixtures were studied. The optimal ITP replacement ratio is 20% that 20% ITP-contained UHPC mixtures had better mechanical properties. Further increasing the ITS replacement ratio, the mechanical properties initially increased and then suffered a reduction. The UHPC with 20% ITP and 50% ITS had the maximum compressive strength (145.6 MPa) and flexural toughness (110.99 N·m). Appropriate addition of IT could improve the microstructures of UHPC mixtures by its filler effect and pozzolanic potential. The average pore diameter was reduced from 97.00 nm to 27.04 nm, and the ratio of harmful pores (> 100 nm) was reduced from 85.7% to 60.9% when ITP and ITS were added. In the durability tests, the addition of IT has less effect on the electric flux passed. Owing to the pores refining effect of IT, the drying shrinkage of UHPC mixtures was reduced. According to the environmental evaluation, the amount of leached heavy metal ions can be neglected. The IT contained UHPC had relatively low green degree and ITS/ITP surplus in 1 m3 UHPC production which implies its good environmental performance. Overall, ITP and ITS can be co-used in developing environmentally friendly UHPC to minimize the environmental burdens.