Development of sustainable ultra-high-performance concrete (UHPC) by synergistic utilization of red mud and limestone powder

Ultra -high-performance concrete (UHPC) has been extensively studied and applied worldwide due to its outstanding mechanical and durability performance over ordinary concrete. However, UHPC is a cementitious material with high cement content, which can lead to costly production and an excessive environmental footprint. The utilization of supplementary cementitious materials (SCMs) in UHPC can effectively reduce the consumption of cement and improve its sustainability. Red mud (RM) is a solid residue discharged from the alumina production process. It has been frequently added into normal concrete to partially replace cement. However, recent studies showed that incorporating RM into UHPC could undermine its performance due to the low reactivity of RM. It is generally recognized that the alumina in cementitious material can synergistically interact with calcium carbonate to promote hydration products and enhance the performance of the matrix. Since RM is abundant in aluminum phases, to achieve more effective utilization of RM in UHPC, a sustainable UHPC is developed in this study by the synergistic use of RM and limestone powder (LP) to partially substitute cement. It not only significantly reduces the cement consumption in UHPC but also provides a novel avenue for the reuse of solid waste materials. In this study, the effects of RM fractions on the mechanical, hydration, and microstructural properties of UHPC were investigated through mechanical tests, calorimetry analysis, thermogravimetric analysis, XRD, SEM, and water -permeable porosity test. The results showed that the early -age strength of UHPC incorporated with RM and LP was lower than the reference mix, but its strength developed more rapidly in the later stage due to the synergistic reaction among RM, LP, and cement. The strength of UHPC with 10 % RM and 10 % LP substitution even surpassed the strength of the reference mix after 28 days. As the RM fraction increased to 20 %, the UHPC achieved a comparable 28d strength to the reference mix, whereas other studies reported that the UHPC with the equivalent proportion of RM but lacking LP showed a 15 % strength reduction as opposed to the control mix. This further demonstrates the synergistic effects of LP and RM as a composite substitute for cement. The study also found with the increase of RM dosage, the main exothermic peak occurred earlier and the hydration reaction was accelerated owing to the nucleation effect and high alkalinity of RM. Moreover, the coupled addition of RM and LP could lower the porosity of the UHPC matrix due to RM's filling effect and the synergistic effect among RM, LP, and cement. A prediction model was established at the end of this study to correlate the porosity and strength of UHPC containing RM and LP.