Synergistic reuse of red mud and circulating fluidised bed fly ash in self-compacting recycled concrete

The utilisation of waste materials in construction reduces natural resource depletion and mitigates waste accumulation. This study investigates self-compacting recycled concrete (SCRC) prepared using circulating fluidised bed (CFB) fly ash as a supplementary cementitious material (SCM) and red mud (RM) as a viscositymodifying agent (VMA). The fresh and hardened properties of SCRC, including compressive strength, carbonation resistance, drying shrinkage, surface resistivity, and leaching behaviour, were evaluated. Economic and environmental benefits were also assessed in terms of raw material cost and carbon emissions. Results show that RM effectively reduces segregation and mitigates the negative effects of CFB fly ash on carbonation resistance. Incorporating 15 wt% CFB fly ash increased the segregation rate by 6.1 % and the 28-day carbonation depth by 3.57 mm, while 15 wt% RM reduced these values by 7.5 % and 0.81 mm, respectively. Additionally, the combined use of CFB fly ash and RM significantly enhanced surface resistivity, with SCRC containing 15 wt% CFB fly ash and 20 wt% RM achieving an increase from 13.6 kS2 center dot cm to 49.4 kS2 center dot cm. However, this resistivity was lower than that of SCRC with 20 wt% CFB fly ash (62.3 kS2 center dot cm). The prepared SCRC also demonstrated excellent solidification of Na and heavy metals, with leaching concentrations meeting China's groundwater safety standards. SCRC with 20 wt% RM exhibited the highest leaching levels after 28 days, with 153.8 mg/L for Na and 0.00953 mg/L, 0.00374 mg/L, and 0.00428 mg/L for Cr, Pb, and As, respectively. While SCRC prepared with CFB fly ash and RM showed significant carbon emission reductions compared to the control, cost savings were limited due to increased use of polycarboxylate superplasticisers at high dosages. This study provides comprehensive data to support the engineering application of CFB fly ash and RM in SCRC.