Waste-to-resource geopolymer technology: mechanical, durability, and microstructural performance of red Mud–fly ash-stabilized black cotton soil

Black cotton soil is highly expansive and moisture-sensitive, resulting in low strength, poor bearing capacity, and volume instability in transportation infrastructure. This study adopted a two-stage approach to optimize a red mud–fly ash geopolymer binder and subsequently evaluate its effectiveness for black cotton soil stabilization. Geopolymer binders containing red mud-to-fly ash ratios of 90:10, 80:20, and 70:30 were activated using 10 M NaOH and Na₂SiO₃/NaOH mass ratios of 0.5, 1.0, and 2.0. The optimum binder, 80RM + 20FA at a Na₂SiO₃/NaOH ratio of 2.0, achieved 28-day compressive strengths of 5.81 MPa under ambient curing and 6.59 MPa at 60 °C. When incorporated into black cotton soil, an optimum dosage of 30% geopolymer produced the best overall engineering response. The 70BCS + 30GP mix achieved 28-day UCS values of 315 and 365 kPa and STS values of 118 and 132 kPa under ambient and 60 °C curing, respectively, representing increases of 80.0% in UCS and 126.9% in STS relative to untreated soil under ambient curing. The optimum mix also achieved unsoaked and soaked CBR values of 5.90% and 3.71%, respectively, while reducing the free swell index from 52.17% to 21.74%. After twelve wet–dry cycles, it exhibited only 1.82% mass loss and retained approximately 87% of its original UCS. XRD, SEM, and EDX observations confirmed amorphous aluminosilicate reaction-product development, matrix densification, pore filling, and improved particle bonding. Overall, the 70BCS + 30GP mix demonstrated promising laboratory-scale potential for selected low-volume road, shoulder, embankment, and subgrade-improvement applications, subject to further long-term and field-scale validation.