Thermo-mechanical behavior of clays: Divergent responses of kaolinite and montmorillonite in temperature-controlled shear tests

This study examines the thermo-mechanical behavior of cohesive soils, focusing on the influence of temperature and clay mineralogy on shear strength and stiffness. Temperature-controlled direct shear tests were conducted on montmorillonite-rich bentonite and kaolinite under normal stresses of 100–300 kPa and temperatures ranging from 5 °C to 80 °C. Specimens were compacted at optimum moisture content to maximum dry density, then saturated and consolidated prior to testing. Kaolinite exhibited thermal stability, with negligible changes in strength and stiffness across the temperature range. This behavior is attributed to its low surface activity, limited interlayer water, and stable 1:1 lattice structure, which minimize thermo-hydro-mechanical interactions. Montmorillonite, in contrast, displayed a biphasic thermal response: strength and stiffness increased with temperature up to ∼60 °C, due to reduced pore water viscosity and partial dehydration enhancing interparticle bonding. Beyond this threshold, a decline in mechanical properties was observed, likely caused by the disruption of diffuse double layers and microstructural weakening. These contrasting responses underscore the critical role of mineralogy in governing soil behavior under thermal loads. The findings have direct implications for the design and safety of energy geostructures, nuclear waste containment systems, and other geotechnical applications exposed to elevated temperatures.