Mitigation of Expansive Soil Through Controlled Thermal Treatment: Geotechnical and Microstructural Assessment

Expansive soils present a significant geotechnical challenge due to their pronounced volume changes with moisture variations, leading to substantial infrastructure damage. This study investigates the efficacy of thermal stabilization in mitigating the swell potential and compressibility of a high-plasticity, kaolinite-rich clay from Al Ghat, Saudi Arabia. As well, the changes in basic properties including consistency limits, specific gravity, and compaction characteristics were studied and highlighted. Microstructural studies using X-ray diffraction (XRD), Scanning electron microscopy (SEM), and Energy-dispersive X-ray spectroscopic (EDX) were performed to trace the structural changes and interpret the achieved improvement. Soil specimens were subjected to heat treatment at levels of 200 °C, 400 °C, and 600 °C for two hours, after which their geotechnical and microstructural properties were comprehensively evaluated. The results demonstrate a direct correlation between increasing temperature and the reduction in expansive behavior. Treatment at 600 °C caused a substantial decrease in the plasticity index from 27.00 to 2.94. Correspondingly, oedometer tests showed that the free swell was reduced from 6% to nearly zero, and the swelling pressure was eliminated, dropping from 250 kPa to 0 kPa. XRD analysis confirmed kaolinite decomposition through dehydroxylation, producing metakaolin with diminished water absorption capacity. SEM further revealed significant particle aggregation and the formation of a coarser soil fabric. The findings confirm that heat treatment at temperatures of 400 °C and above is a highly effective method for permanently stabilizing kaolinitic expansive soils, rendering them suitable for construction applications.