Study on the synergistic effects of red mud and recycled coarse aggregate to improve the mechanical properties of ultra-high performance concrete

To promote ultra-high performance concrete (UHPC) as an advanced material with greater economic and environmental benefits, this study combined recycled coarse aggregates (RCA) and red mud (RM) to further reduce the proportions of mineral material. Compared to natural coarse aggregates (NCA), the introduction of RCA can lead to the formation of numerous weaker interfacial transition zones (ITZs) between RCA and the new cement matrix due to the presence of old adhered mortar, resulting in insufficient mechanical properties of RCA-UHPC. Herein, the addition of 10 wt% RM did not improve the mechanical properties of NCA-UHPC, but significantly narrowed the strength gap between RCA-UHPC and NCA-UHPC, reducing the difference in compressive and splitting tensile strength at 28 d from 32.6 MPa and 4.7 MPa to 2.9 MPa and 1.1 MPa, respectively. Microscopic results indicate that ultrafine RM particles can fill the ITZs between RCA and new cement mortar, reducing their width to an almost invisible level. In detail, RM particles can serve as fillers and nucleation sites to promote the formation of high-crystallinity CaCO 3 crystals that can fill micropores and improve the microstructure between the old adhered mortar and the RM layer. Additionally, the bonds between the RM layer and the new cement mortar can be slightly enhanced by the hydration reactions induced by RM’s pozzolanic activity. The increase in hydration products from the continuous pozzolanic reaction of RM contributes to improved bonding strength, as reflected in the higher mechanical properties observed at 56 days compared to the reference group without RM. In this study, the combination of 20 wt% RCA and 10 wt% RM in the UHPC matrix under standard curing achieves a trade-off, offering a promising strategy for the combined reuse of construction and solid waste in the construction industry.