Characterization of microstructural features of Tamusu mudstone

Tamusu mudstone formation, located in the Alxa area in western Inner Mongolia, is considered a potential host formation for high-level radioactive waste (HLW) underground disposal in China. In this study, complementary analyses with X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), mercury intrusion porosimetry (MIP), and N2 physisorption isotherm were conducted on the Tamusu mudstone to characterize its physical characteristics and microstructural features, such as mineral compositions and pore structure. Several minerals, including carbonates, feldspar, clays and analcime, were identified in Tamusu mudstone by XRD. Images from FE-SEM show that pores in the Tamusu mudstone were dominantly on nanometer scale and generally located within their mineral matrix or at the interface with non-porous minerals. The combination of the MIP and N2 physisorption curves indicated that the Tamusu mudstone has diverse pore sizes, a porosity varying from 2.34% to 2.84%, and a total pore volume in the range of 0.0065–0.0222 cm3/g with the average pore diameter ranging from 9.6 nm to 19.23 nm. The specific surface area measured by MIP (2.572–5.861 m2/g) was generally higher than that by N2 physisorption (1.29–3.04 m2/g), due to the pore network effect, pore shape (e.g. ink-bottle shape), or technique limits. The results related to pore information can be applied as an input in the future to model single- or multi-phase fluid flow and the transport of radionuclides in porous geomedium by migration and diffusion.