Residual Bringelly Shale (RBS), common across Western Sydney, is a highly expansive soil that poses significant challenges due to moisture-induced volume changes. This study explores a sustainable stabilisation method combining alkaline activation with a reduced lime dosage to enhance soil performance while minimising lime usage. Sodium hydroxide and lime were applied simultaneously, and X-ray diffraction, X-ray mapping (XRM), swelling tests, and unconfined compressive strength (UCS) measurements were used to evaluate mineralogical changes, gel formation, and mechanical improvements. XRM provided detailed insights into the location, distribution, and quantification of calcium–aluminosilicate and sodium–aluminosilicate binding geopolymers (binding gels) within the soil matrix. The presence of these gels correlated strongly with reductions in swelling and improvements in strength. The combined alkaline–lime treatment promoted a denser, more stable soil fabric, demonstrating that lime quantities can be reduced without compromising stabilisation effectiveness. These findings highlight the potential for cost-effective and environmentally sustainable stabilisation of expansive soils, such as RBS, in Western Sydney and similar regions.