Meso-characterization of the effective thermal conductivity of selected typical geomaterials in an underground coal mine

To comprehensively understand the bulk macro conduction characteristics of typical geomaterials existing in deep underground coal mines, i.e. coal, rock, and concrete (shotcrete), meso-scale composition (solid mineral and fluids such as water and air) and structure (particle and pore size, shape, and contact form) analyses were conducted via water absorbability and thermal conductivity measurements, scanning electron microscope, and X-ray diffraction technologies. The results indicate that coal, rock, and concrete's respective meso-scale composition and structures have different degrees of influence on their macro bulk effective thermal conductivity at various conditions. First, the fluid (water and air) composition significantly impacts thermal conductivity at a certain bulk effective porosity: coal < rock < concrete. The ranking of thermal conductivity shows two different forms: concrete < coal < rock in a dry state and coal < concrete < rock in a water-saturated state. Second, the surface morphology and face porosity obtained by scanning electron microscope imaging can explain thermal conductivity fluctuations at various moisture contents and magnification times. The face porosity as well as its quadrant anisotropy degree can also be achieved via scanning electron microscope digital image processing technology, and both of them have a certain relationship with magnification times (scale effect) and effective volumetric porosity, as obtained by water absorbability experiments. In addition, apart from the fluid composition, the mineral composition (species and content of mineral) of the solid skeleton or matrix also significantly influences the macro-scale thermal conductivity of geomaterials. Furthermore, a quantitative comparison analysis can be performed by X-ray diffraction, but further research is needed to improve its accuracy. This study's detailed meso-characterization of the macro effective thermal conductivity for typical geomaterials in an underground coal mine may be of reference value to establish the relationship between different properties of differing scales.