Study on freeze-thaw damage of limestone cultural relics under gradient sodium sulfate concentrations: Multiscale characterization via nanoindentation

Limestone-based cultural relics long-term exposed to natural environments suffer significant synergistic damage from sodium sulfate crystallization-induced expansion and freeze-thaw cycling. To simulate the coupled erosion effects of sulfate attack and freeze-thaw cycles, this study replicates the environmental conditions typical of Grotto Temple cultural relics. Nanoindentation testing and the Mori-Tanaka homogenization model were employed to analyze the microscopic and macroscopic mechanical properties of limestone specimens. By examining microstructural pore evolution, the study provides a cross-scale analysis of the mechanical degradation mechanisms in limestone relics. The results indicate that under coupled sodium sulfate gradient and freeze-thaw action, the macroscopic mechanical parameters of limestone specimens exhibit rapid deterioration during initial cycles, followed by a reduced decline rate and stabilization after 50–60 cycles. Both microscopic and macroscopic mechanical properties degradation positively correlate with sodium sulfate concentration. Higher solution concentrations exacerbate freeze-thaw damage. Compared to elastic modulus, hardness and fracture toughness demonstrate more pronounced deterioration under freeze-thaw cycling. Microstructural analysis reveals that freeze-thaw cycling promotes the enlargement of small pores into larger pores and the development of cracks. Sulfate solution immersion accelerates pore and crack propagation, with this effect intensifying at higher concentrations. The microstructural evolution aligns well with the observed degradation in mechanical properties, providing mutual validation of the damage mechanisms.