This study aimed to assess the viability of utilizing lime–fly ash (LF) and red mud (RM) in the modification of silty soil (LF-RMS) for subgrade filling. The primary objective of this research was to analyze the mechanical characteristics and examine the curing mechanisms associated with said modified materials. Different curing times were utilized in the analysis of mechanical properties (e.g., via unconfined compression testing), microstructure (via scanning electron microscopy, X-ray diffraction, and thermogravimetric-differential thermal analysis), and environmental indices (via assessment of corrosivity, heavy metal concentration, and radioactivity) with various dosages of red mud (D_RM) and Lime–fly ash (D_LF). Analyses of the curing mechanisms, failure modes, microstructures, and degrees of environmental impact associated with LF-RMS were also undertaken. The tests indicated that the unconfined compressive strength (UCS) exhibited an initial increase followed by a decrease as the D_RM and D_LF levels increased. Additionally, the strength of LF-RMS increased with an increase in curing time. It is worth noting that the specimen composed of 20% LF and 23% RM (D_20%LF+23%RM) demonstrated a maximum UCS value of 4.72 MPa after 90 days of curing, which indicates that it has the strongest ability to resist deformation. The strength of the specimen cured for 90 days was 1.4 times higher than that of the specimen cured for 7 days (1.97 MPa). Furthermore, the toxic concentration and radionuclide index of LF-RMS were significantly reduced compared to those of pure RM. The overall concentration of heavy metals in the D_20%LF+23%RM specimen decreased by more than 60% after curing for 28 days. The internal irradiation index and the external irradiation index decreased by 1.63 and 1.69, respectively. The hydration products in LF-RMS play a key role in the solidification of heavy metals, and the alkaline environment provided by RM also contributes to the precipitation and replacement of heavy metals. In this study, red mud, fly ash and lime were used to modify silty soil. The central tenets of sustainable development may be achieved through the reuse of RM as a road filler.

Red mud (RM) is a kind of solid waste produced during the bauxite refining process, which can cause significant environmental pollution when stored in large quantities. To address this issue and to improve the reuse rate of RM, this study investigates the feasibility of using RM as a roadbed filling material, specifically in combination with cement, using modified silty sand (P.O-RMS). Therefore, mechanical and microscopic tests are conducted with different RM contents (W_RM), cement contents (W_P.O), and curing ages to analyze the P.O-RMS′ strength, deformation characteristics, and microstructure formation mechanism. Additionally, the radioactivity and heavy metal concentrations of P.O-RMS are also detected. Mechanical test results showed that the unconfined compressive strength (UCS) of P.O-RMS does not increase monotonically with the increase of W_RM, and the peak strength appears at W_RM = 20%. Although adding RM has a negative impact on the toughness of P.O-RMS at the initial curing ages of 7 days and 28 days, they still meet the requirements of the Chinese standard for high-grade highway base strength. Scanning electron microscope testing shows that the gel products produced by cement hydration increase with the amount of RM, and reach their maximum at W_RM = 20%. Therefore, W_RM = 20% is recommended as the optimum admixture of P.O-RMS. The usage of RM as a filling material for roadbed construction in this study not only aligns with the principles of sustainable development, but also provides a crucial theoretical foundation for the effective utilization of RM resources.