Mechanical damage evolution and mechanism of sandstone with prefabricated parallel double fissures under high-humidity condition

To explore deeply high-humidity effects on mechanical properties of fractured pillars, sandstone pillar in the -750 m room of the Wengfu phosphate mine was cored and modeled into standard specimens with prefabricated parallel double fissures. Then, the specimens were placed in a self-designed and fabricated humidity control device, which simulated the relative humidity condition on-site (90% RH). The strength deformation, crack evolution, failure mode and microscopic damage mechanism of the samples were analyzed by uniaxial compression test, nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The results show that: (1) The sandstone structural looseness is positively correlated to high-humidity action. Upon loading into the humidity device, the well-arranged lamellar and flake-shaped mineral crystals develop into spongy or flocculent ones with secondary pores due to the loss of cement wrappings. The longer immersion time causes further loosening of the structure, which creates fuzzy interfaces between layers and increases the number of micro-cracks and micro-pores. (2) The failure mode and crack propagation of sandstones under compression are determined by the high-humidity condition; with the longer exposure time of high-humidity, the crack penetration and crack initiation stresses decrease, the newly macroscopic crack growth drops and the failure mode moves from shear to tensile damage. (3) Water-rock chemical reactions occur within the fractured sandstone under the high-humidity condition, which weakens the friction between grains and fracture surfaces, accelerating the damage and failure of the fractured sandstone pillar.