Dolomite′s macro⁃microscopic mechanism of mechanical property deterioration under high⁃humidity conditions

To explore the effects of high⁃humidity on the mechanical properties of rock pillars in deep mining，some standard specimens were produced with cores collected from dolomite pillars below - 850 m depth at the Chashanling coal mine. To further study the dynamic response characteristics of water absorption rate and pore struc⁃ ture，the pore size，microstructure and mineral components of dolomite with 60 d limited axial displacement under four high⁃humidity conditions (70%，80%，90%，100% RH) were tracked and measured by nuclear magnetic reso⁃ nance ( NMR ) ， scanning electron microscopy ( SEM ) ， energy dispersive spectroscopy ( EDS ) and X⁃ray diffraction (XRD). Combined with laboratory results of uniaxial and triaxial compression tests，the strength soften⁃ ing mechanism of dolomite after water swelling was summarized. The results show that 1 higher environmental humid⁃ ity causes faster circumferential strain at the early stage，and it needs more time to be stabilized. A significant stress re⁃ laxation occurs after the expansion force achieves its maximum value. 2 Higher humidity and longer action time weak⁃ en the strength and accelerate the deformation of dolomite. Under a near⁃site humidity condition (90% RH)，the inter⁃ nal friction angle declines more evenly over 60 d. In contrast，the cohesion changes less at the first and last 20 d and drops rapidly between 20~40 d.3 Double peaks appear in NMR T2 spectrum curve at the beginning of water ab⁃ sorption. The relaxation time of peak value continuously moves to the right. The originally compact layered struc⁃ ture develops cracks and small mesopores expand and connect to form large pores. According to the characteristics of water absorption curve and T2 spectrum area curve，a high⁃humidity condition exerts some influences in three stages: rapid water absorption， slow water absorption and stable hydration. 4 After dolomite absorbs water under a high⁃humidity condition，the mechanical parameters are weakened by the lubricating effect of bound water film and the swelling effect of minerals，which finally changes the limited failure mode of the dolomite.