Electrochemical Impedance Spectroscopy Investigation and Results Interpretation of Rammed Earth from Pingyao Ancient City Walls (UNESCO World Heritage Site) Under freeze-Thaw Cycles

To investigate the deterioration mechanisms of rammed earth materials from the UNESCO World Heritage Pingyao Ancient City walls under freeze-thaw cycles, this study examines raw soil and 3:7 lime soil specimens through unconfined compressive strength test, electrochemical impedance spectroscopy (EIS) test, and scanning electron microscopy (SEM). Raw soil and 3:7 lime soil specimens were subjected to 15 freeze-thaw cycles. Raw soil exhibited 77.9% strength loss, with Nyquist plot showing reduced bulk resistance (20.1% decrease) and Bode plot indicating dielectric degradation. SEM revealed porous structure and broken interparticle bonds. In contrast, lime soil retained structural stability, with only 32.8% strength loss and minimal electrochemical variations due to CaCO₃ and C-S-H gels formed via lime carbonation and pozzolanic reactions. EIS-derived equivalent circuit model (R_s (CR_ct)) demonstrated strong correlation (R² > 0.97) between unconfined compressive strength and R_s, validating EIS as a non-destructive tool for assessing freeze-thaw damage. SEM confirmed lime soil’s dense microstructure with localized cracks suppressed by cementation products. This study pioneers EIS application in earthen heritage conservation, highlighting lime soil’s superior durability, self-healing capacity, and compatibility with heritage preservation principles. Results provide theoretical and technical foundations for preventive conservation of rammed earth structures. The study pioneers the application of EIS technology for non-destructive evaluation of freeze-thaw damage in earthen heritage materials, providing theoretical and technical foundations for preventive conservation of rammed earth sites.Lime soil specimen retains 67.2% strength after 15 freeze-thaw cycles (vs. 22.1% for raw soil).The established equivalent circuit model (R_s(CR_ct)) demonstrates strong correlation between unconfined compressive strength (q_u) and R_s parameters.SEM observations further demonstrated the effect of freeze-thaw cycles on the microstructure of both soil types. The study pioneers the application of EIS technology for non-destructive evaluation of freeze-thaw damage in earthen heritage materials, providing theoretical and technical foundations for preventive conservation of rammed earth sites. Lime soil specimen retains 67.2% strength after 15 freeze-thaw cycles (vs. 22.1% for raw soil). The established equivalent circuit model (R_s(CR_ct)) demonstrates strong correlation between unconfined compressive strength (q_u) and R_s parameters. SEM observations further demonstrated the effect of freeze-thaw cycles on the microstructure of both soil types.