Synergistic enhancement of sulfate resistance in recycled aggregate concrete through multi-pathway blocking via carbonation treatment and fly ash incorporation

Due to the large amount of construction waste generated, the use of recycled aggregates from waste concrete as a substitute for natural aggregates in concrete production has become a hot research topic. However, the poor durability, particularly the inadequate sulfate resistance, limits the widespread use of recycled aggregate concrete. This study aims to investigate the synergistic effect of recycled aggregate carbonation and fly ash incorporation on the sulfate resistance of recycled aggregate concrete. In this work, a sulfate wet-dry cycle test was employed to assess the sulfate resistance of recycled aggregate concrete with varying carbonation pressures of the aggregate (0.1, 0.3, and 0.5 MPa) and fly ash contents (10 %, 20 %, and 30 %). Additionally, a novel experimental approach was applied to analyse the sulfate ion transport pathways in recycled aggregate concrete. The results indicate that both recycled aggregate carbonation and fly ash incorporation effectively enhance the sulfate resistance of recycled aggregate concrete. Carbonation treatment improves sulfate resistance in pathways involving old mortar, with an optimal pressure of 0.5 MPa. Fly ash enhances resistance in pathways involving new mortar, with an optimal content of 20 %. These methods show good compatibility. In sulfate wet-dry cycle test, the treated group (0.5 MPa carbonation and 20 % fly ash) exhibited a 146.9 % lower mass loss rate, a 2487 % higher relative compressive strength, and fewer harmful pores, similar to natural aggregate concrete. These findings confirm the effectiveness of multi-path synergistic blocking and provide important theoretical support for the engineering application of recycled aggregate concrete. • Carbonation enhances sulfate resistance by sealing old mortar cracks in recycled aggregates. • Fly ash reduces sulfate ingress via pore refinement and Ca(OH) 2 consumption in new mortar. • Combined carbonation and fly ash synergistically block all sulfate transport pathways. • Treated recycled concrete matches natural aggregate concrete's durability under sulfate attack.