Sustainable construction from industrial waste: synergistic stabilization of granite residual soil using red mud byproduct and nano-silica

• First study on red mud/nano-silica for granite soil stabilization. • Red mud as curing agent enables solid waste reuse. • 27% nano-SiO₂ + 1% red mud boosts UCS 38.7%, UTS 54.8%, cohesion 33.7%, CBR 240%. • 27% nano-SiO₂ + 5% red mud increases friction angle 43.9%. • FTIR confirms denser structure, stronger cementation in nano-modified soil. To develop sustainable construction materials from granite residual soil, this study investigates the synergistic stabilization effects of red mud and nano-silica for improving its mechanical characteristics for geotechnical applications like artificial slopes, foundations, and roadbeds. The influence of these two materials on the mechanical characteristics of the granite residual soil was investigated by unconfined compressive strength test, Brazilian splitting test, direct shear test, dynamic cone penetration tests. Besides, the enhancement mechanism of the above two materials was studied by Fourier transform infrared spectroscopy experiments. Research results indicate that: (1) When the nano-silica concentration was 27 % and the red mud content was 1 %, the optimum improvement was achieved. Compared with the unimproved soil (1179 kPa, 74.0 kPa, 243.36 kPa, and 22 %), the compressive strength, tensile strength, cohesion, and California bearing ratio of the improved soil increased by 38.7 %, 54.8 %, 33.7 %, and 75 %, respectively. These values were notably higher than those obtained with 27 % nano-silica alone (37.4 %, 49.5 %, 30.7 %, and 66 %). (2) When the nano-silica concentration was 27 % and the red mud content was 5%, the internal friction angle of the soil increased by 43.9 % compared with the unimproved soil (37.4°) and by 25.8 % compared with the addition of nano-silica alone. Research results indicate that a mixed improver of 1 % red mud and 27 % nano-silica can not only improve the mechanical characteristics of granite residual soil, but also open up a new avenue for the application of industrial waste.