Influence of multiple porous structure evolution on physical–mechanical properties and damage mechanism of sandstone heated continuously/periodically with the identical heating time

This paper studied the physical–mechanical properties and damage mechanism based on porous structure evolution after continuous/periodic heating with identical 10 h of heating. Nuclear magnetic resonance was used to investigate the pore characteristics and microscopic damage of thermally treated sandstone. This research performed Brazilian tensile strength (BTS) and unconfined compressive strength (UCS) tests. The digital image correlation and acoustic emission (AE) techniques were applied to track the strain evolution and monitor the acoustic signal feature during the UCS loading process. The results suggested that, at 200 °C, the periodical heating–cooling specimens show thermal strengthening after multiple thermal cycles, resulting in higher UCS than the continuously heated specimens. However, the UCS of periodical heating specimens is lower than that of the continuously heating ones at 400 ~ 800 °C. In addition, the BTS and UCS initially increased at 200 ~ 400 °C due to thermal-induced enhancement and then decreased at higher temperatures for the continuously heating–cooling specimens. The evolution of mechanical damage based on AE confirmed that periodic heating promotes cracks more in the same heating time. The periodic treatment stimulates the initiation, propagation, and coalescence of nanopores, but mesopores and macropores play a leading role in the mechanical properties. The porous structure evolution is a dynamic process, and the multiple porosities show different characteristics under different temperatures and thermal conditions. Thermal fatigue increases the uniformity coefficient of pores. Repeated treatment at 200 °C enhances the brittleness of sandstone, while periodic heating at high temperatures results in brittle-ductility and failure mode transition.