Pore characterization of mine shales by low pressure nitrogen adsorption and mercury intrusion porosimetry: Implication on water retention behavior of shales

The moisture-induced shale deterioration is considered one of the primary roof stability issues in underground coal mines. Understanding of the pore morphology of shale will provide clues to its water retention behavior and strength degradation due to seasonal changes in humidity in coal mines. In this study, low pressure nitrogen adsorption and high pressure mercury intrusion porosimetry were employed to characterize pore structure of one fireclay and four coal mine shales. The results suggest that the porosity and specific surface area (SSA) for all five samples are dominated by mesopores (2-50 nm). Based on the obtained pore size distributions, water retention curves are built and are closely associated to water adsorption behavior. Due to the dominant mesopores, the studied fireclay and shales behave a strong water retention capacity with matric suction reaching ~ 100-150 MPa at degree of saturation equals ~3%. Larger porosity and/or SSA tend to have higher water adsorption. Despite a relatively low porosity of the fireclay, the large matric suction over the whole range of degree of saturation corresponding to the high proportion of micro/mesopores lead to a strong water retention capacity, which in turn contributes to water adsorption.