Micro-macro experimental investigation into the thermomechanical volumetric response of non-active clay

The thermomechanical behaviour of clayey soils is critical for various geotechnical and geoenvironmental applications, yet the underlying particle-scale mechanisms remain debated. This study presents a multi-scale investigation into the microstructural origins of the distinct volumetric responses observed in normally consolidated (NC) and overconsolidated (OC) clays subjected to thermal loading. Temperature-controlled isotropic compression tests were coupled with Mercury Intrusion Porosimetry (MIP) on reconstituted samples of Speswhite Kaolin clay. The macroscopic results confirm that NC non-active clay exhibits irreversible contraction upon heating, while highly OC non-active clay undergoes reversible expansion. MIP analyses reveal that thermal contraction in NC kaolin is linked to a reduction in macropore volume, driven by irreversible slippage at edge-to-face particle contacts due to a temperature-induced weakening of electrochemical attractions. In contrast, the thermal expansion of OC clay is associated with an elastic increase in the separation distance between face-to-face particle configurations. These findings provide direct experimental support for recent particle-scale modelling based on 3D DLVO interactions and demonstrate that thermal loading under constant stress activates the same microscale processes that drive mechanically induced volumetric deformation. The high level of experimental control makes the resulting dataset particularly suited for validation of particle-scale numerical models and for informing constitutive formulations that explicitly link thermomechanical behaviour to interparticle forces and contact mechanics.