The expansive nature of soils containing high amounts of montmorillonite mineral can be altered through chemical stabilization, resulting in a material suitable for construction purposes. In this paper, the physicochemical behaviour of phosphoric acid treated samples was studied using X-ray Diffractometry (XRD), Energy Dispersive X-ray Spectrometry (EDAX), Field Emission Scanning Electron Microscopy (FESEM), Fourier Transform Infrared Spectroscopy (FTIR), and Solid-State Nuclear Magnetic Resonance spectroscopy (SS-NMR). It was found that in acid stabilized montmorillonitic soil, clay alumina was more susceptible to dissolution and mostly responsible for the formation of aluminate phosphate hydrate compounds that bonded the soil particles together. Furthermore, since the action of stabilizer was mainly “surface-associated”, the efficiency of phosphoric acid treatment in terms of strength improvement was rather limited.