Experimental investigation on thermal strengthening behaviour of marble in the mild temperature range under cyclic loading

High-temperature conditions in current deep-earth engineering predominantly hover within the 300 °C mark. While several studies have shown granite and marble to display thermally enhanced properties within the mild temperature range, the thermo-mechanical behavior of marble under cyclic loading and unloading in this temperature range remains largely unexplored. This study addressed the issue by applying six heat treatments to marble, spanning from the control group (room temperature at 25 °C) through increments of 50 °C, 100 °C, 150 °C, 200 °C, to 250 °C. The outcomes of the triaxial cyclic loading and unloading tests following these treatments were detailed herein. Electron microscopy scanning (SEM) was employed to examine the fracture mechanisms of marble post-failure. The findings revealed that temperatures within the mild range indeed exert a strengthening effect on marble under cyclic loading, evidenced by an increase in peak strength from 25 °C to 200 °C. Beyond this, from 200 °C to 250 °C, a weakening trend emerged. Additionally, both the maximum and minimum horizontal strains in the marble post-heat treatment across varied mild temperatures demonstrated a linear increase, while the dissipation energy and damping ratio initially rises and subsequently fell. Analysis using Spearman's correlation coefficient indicated that the average loading modulus served as the optimal equivalent Young's modulus for characterizing the damage evolution in marble following heat treatment within the mild temperature range. Observations from SEM of the microstructure showed a significant reduction in transgranular cracks, intragranular cracks, and micropores as the temperature increased from 25 °C to 200 °C. However, at a temperature of 250 °C, the beneficial effects of thermal expansion shifted towards to damaging effects.