This study investigates the potential application of a blend, termed GGRM, consisting of red mud (RM) and ground-granulated blast furnace slag (GGBS), for stabilizing subgrade expansive soil. RM, an industrial waste from aluminium refineries, poses environmental concerns due to its high alkalinity and presence of heavy metals. Despite its increased utilization in construction sector, research on its role in soil stabilization is limited. With this in mind, RM has been used as an activator for GGBS, to create synergy between these industrial wastes with an objective to utilize this blend for stabilization of black cotton soil (BCS). Therefore, laboratory investigations were conducted to assess the strength of BCS stabilized with GGRM comprising varying proportions of GGBS and RM (0:100, 70:30, 50:50, 30:70, and 100:0 by weight). Further, the optimal GGRM quantities were evaluated by mixing it in different proportions (5-30% by weight). This study also examined the effect of curing on strength properties and leaching behaviour and investigated the associated mechanisms through microstructural studies (XRD, XRF, SEM, and FTIR analysis). The leachate potential was assessed using ICP-OES analysis. Results indicated a maximum sevenfold improvement in unconfined compressive strength of BCS, from 131 to 920 kPa, after 28 days of curing in 70:30 combinations with 25% GGRM content. Furthermore, leaching of heavy metals from stabilized soils are within the permissible limits of hazardous waste management regulations. In conclusion, RM-activated GGBS blends emerged as a potentially sustainable binder, enhancing the strength of expansive soil for subgrade applications.

Red mud, a by-product generated during the extraction of aluminium from bauxite ore, poses challenges to the alumina industry due to its inherent sodicity, alkalinity and heavy metal content. Consequently, studies related to its bulk utilization and valorization have gained attention in the construction sector to promote sustainability. However, utilization of red mud in stabilization of expansive soils using alkali activation is seldom explored. Therefore, this study focuses on improving the geotechnical properties of an expansive soil (black cotton soil, BCS) through chemical stabilization by using blends of two distinct industrial wastes, viz. red mud and GGBS, termed as GGRM, activated with sodium hydroxide (NaOH) solution. The strength, stiffness and durability characteristics of these compacted blends were assessed based on a series of laboratory investigations like unconfined compressive strength, ultrasonic pulse velocity and wet-dry cycles tests. Leachate analysis was also performed to assess the geo-environmental issues of soil ameliorated with blends of alkali-activated GGRM blends. These blended specimens were moulded with different molar concentrations of NaOH solutions (i.e. 2, 5 and 10 M). Further, microstructural studies were carried out through XRD, SEM-EDS and FTIR analysis. The results show that heavy metal contents in alkali-activated specimens are within the permissible limits of USEPA guidelines. Based upon the assessments of strength, durability and P-wave velocity after 28 days of curing period, 25 and 30% binder contents of GGRM100:00, GGRM70:30 and GGRM50:50 corresponding to 5 M and 10 NaOH, were found suitable for subgrade applications in accordance with IRC 37 guidelines.