Experimental study on the deterioration mechanisms of physical and mechanical properties of red sandstone after thermal-acid coupling treatment

Sandstone, a primary construction material for many renowned historical structures, may experience long-term stability degradation due to combined effects of fire and acidic hydrochemical environments. To effectively protect and restore these historical buildings, it is crucial to investigate the deterioration patterns and mechanisms of mechanical properties of sandstone under thermo-acid (t-a) coupling conditions. In this study, red sandstone specimens were subjected to coupled treatment of high temperatures (300°C and 600°C) and sulfuric acid (H₂SO₄) solutions at pH=2. Subsequently, the physical properties (including appearance, mass, P-wave velocity, and porosity) and mechanical properties (including static and dynamic compressive and tensile strengths) of the treated specimens were evaluated. Techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray computed tomography (CT) were utilized to investigate the mineralogy, microstructure, and crack propagation of specimens. The results revealed that the t-a coupling treatment led to a significant increase in mass loss and porosity, along with a decrease in P-wave velocity. Above the threshold temperature, high temperature became the dominant damage factor, and damage exhibited a distinct size effect. The static and dynamic mechanical properties of red sandstone decreased significantly after t-a coupling treatment, with the failure mode being more complex. After immersion in coupling with H₂SO₄ solution at 600°C, the amount of gypsum formed on the specimen surface decreased, and the static peak stress (σ_t) and dynamic peak stress (σ_d) decreased by 41.18 % and 26.21 %, respectively. High temperatures caused the connections between mineral particles to weaken, primarily affecting minerals other than feldspars, while immersion in H₂SO₄ solution initiated intense reactions between calcite, goethite, hematite, and chlorite with H⁺ ions. After t-a coupling treatment, the crack morphology became more intricate, with an increase in the three-dimensional (3D) crack volume.