Formation and Progression of Sheet-Like Spalling in Earthen Heritage Under Freeze–Thaw Cycles

Numerous earthen heritage sites with high historical, scientific, and cultural value are widely distributed across northwestern China. Under repeated freeze–thaw cycles, the earthen heritage sites undergo severe deterioration, including sheet-like spalling, crack propagation, basal undercutting, and partial collapse, among which sheet-like spalling is the most typical and destructive failure mode. In this study, laboratory tests on earthen specimens were conducted under controlled freeze–thaw cycles. By combining three-dimensional laser scanning with depth-of-field digital microscopy, the formation and evolution of sheet-like spalling were continuously tracked and documented. Concurrently, multiple indicators were monitored, including the spatiotemporal migration of moisture, the redistribution of soluble salts, the evolution of specific surface area and mean pore size, and the attenuation of elastic wave velocity. The results indicate that sheet-like spalling can be generalized into a four-stage progressive sequence of “powdering – agglomeration – crusting – spalling.” During this process, the coupled water–salt system persistently migrates from the interior towards the surface, leading to continuous enrichment of salts and the development of a high-salinity surface hard crust. This is accompanied by a microstructural transition from particle disintegration and refinement to salt-cemented aggregation and interfacial deterioration at the crust – substrate boundary.