Microstructural evolution and strength development of circulating fluidized bed ash activated by Bayer red mud and semi-dry desulfurization ash

In this study, red mud (RM) and semi-dry desulfurization ash (SDA) were selected as representative alkaline and sulfate solid wastes, respectively, to investigate their synergistic effects on the reactivity of silica-alumina-rich circulating fluidized bed fly ash (CFB-FA). The influence of different solid waste ratios on the setting time and compressive strength of cementitious materials was evaluated. Furthermore, microscopic characterization was conducted using X-ray diffraction, fourier transform infrared, thermogravimetric analysis, and scanning electron microscopy. The experimental results demonstrated that the control group (without RM or SDA) exhibited a 28-day compressive strength of only 23.3 MPa, accompanied by prolonged setting times. While single doping SDA accelerated setting time, and single doping RM improved the 7-day compressive strength by 29.17 %, both approaches showed limitations. Specifically, the 28-day compressive strength of SDA-doped samples decreased by 51.50 %. Meanwhile, single doping RM extended the initial setting time. Notably, RM significantly enhanced early strength development. Compound doping of RM and SDA emerged as the optimal strategy. This approach substantially shortened setting times and improved 28-day strength. The optimal mass ratio (CFB-FA:SDA: RM = 1:3:2) achieved a 30 % enhancement in 28-day compressive strength compared to the control group. Mechanistically, the synergistic effects of RM and SDA included: activation of quartz in CFB-FA, promoting the formation of C/N-(A)-S-H gels; acceleration of Ca(OH)₂ participation in hydration reactions; facilitation of calcium sulfate in SDA to form hydration products; generation new hydration products such as calcite (CaCO₃), which further enhanced mechanical performance through pore-filling and microstructure densification.