Expansive soils present significant challenges to ground stability worldwide, particularly in arid and semi-arid regions. Residual Bringelly Shale (RBS), an expansive soil prevalent in Australia, exemplifies these geotechnical problems. While traditional calcium-based stabilisers are commonly utilised for soil stabilisation, they often raise environmental concerns and may induce secondary swelling. This study explores an environmentally sustainable approach for RBS stabilisation through direct alkaline activation using sodium hydroxide (NaOH) without external precursors. Experimental investigations were conducted to evaluate the effects of varying NaOH molarities (8 M, 10 M, and 12 M) and dosages (2-10% by dry mass) on soil performance. The treatment effectiveness was assessed through comprehensive characterisation including swelling behaviour analysis, compressive strength testing, and microstructural examination using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray mapping (XRM), Fourier-transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). The results demonstrated that direct alkali-activated RBS exhibited measurable improvements in engineering properties, specifically reducing the swelling potential from 10.3% to 3.0% and increasing the unconfined compressive strength (UCS) from 454.4 kPa to 593.1 kPa. Microstructural analysis revealed the development of a denser matrix with reduced reactive mineral content through the formation of sodium aluminosilicate hydrate (N-A-S-H) gels. These findings demonstrated that alkaline activation is a promising and sustainable alternative to conventional stabilisers for expansive soil treatment.