Previous study has revealed the potential of using un-calcinated kaolinite as a precursor for alkali-activated binders (U-KAB) to stabilise expansive soils. However, challenges remain due to slow gel development and excessive residual alkali. In this context, the present study introduces the incorporation of ground granulated blast-furnace slag (GGBS) as a supplementary precursor to enhance the alkali activation process, particularly by promoting the consumption of excess alkali and facilitating the polycondensation of dissolved aluminosilicate species derived from un-calcinated kaolinite. A series of binder slurries were prepared at a fixed water-to-precursor (w/p) mass ratio of 0.4 using 16 M NaOH solution, and the kaolinite-to-GGBS ratios were varied systematically (20:80, 40:60, and 60:40 by mass) to investigate the synergistic effects on reactivity and binder formation. Such slurries were applied at a dosage of 6% to treat locally sourced expansive soil, residual Bringelly Shale (RBS), in Western Sydney. Prior to treatment, the soil was prepared under wet and saturated conditions with a moisture content of 60%. Then, two experimental programs were conducted on the treated soils, involving swelling potential assessment and microstructural characterisation. The findings provide valuable insights into optimising un-calcinated clay-based alkali-activation systems through the incorporation of other reactive industrial by-product wastes, thereby contributing to the development of sustainable, low-carbon binders for expansive soil stabilisation.