Non-monotonic evolution and spatial reorganization mechanism of thermally induced micro-damage in sandstone

This study employs transformative nanoscale-spatial methodology, which reveals the complete thermal damage evolution mechanism of sandstone from 25 to 700 ℃, fundamentally challenging the conventional linear degradation paradigm. Spatial autocorrelation analysis is innovatively applied to atomic force microscopy data, establishing that thermal damage follows a non-monotonic pathway governed by spatial heterogeneity evolution rather than simple progressive weakening. Four critical temperature thresholds are identified: 200 ℃ marks the initiation of localized damage through expansion of weak zones with 61.4% stiffness reduction; 400 ℃ features unexpected transient hardening due to clay mineral dehydroxylation and pore restructuring, reaching the peak adhesion force; 500 ℃ represents not only increased damage but also the thermal decomposition of the 400 ℃ hardened structure, evidenced by maximum spatial heterogeneity and negative adhesion; and the 700 ℃ following quartz phase transition establishes a completely reconfigured structure with a novel "harder-in-lower-areas" pattern. Crucially, this research demonstrates that the spatial clustering of mechanical properties evolves from balanced high-low zones at 200 ℃ to a percolating low-stiffness network at 500 ℃, establishing spatial reorganization as the governing principle of thermal damage. The findings provide a quantitative framework that can accurately predict rock behavior in geothermal systems and underground energy storage applications.