Can X-ray computed tomography (CT) be used to determine the pore-size distribution of intact loess?

Given that micro-CT scanners have started being used for loess investigation, this study examines the reliability of the micro-CT technique for characterizing intact loess, aiming at the pore-size distribution (PSD) that is the most basic quantitative parameter of the soil structure. Intact loess from Xi'an, China was scanned with a micro-CT scanner at two resolutions, 40 and 10 mu m. ImageJ software was used to process the image stacks to construct the three-dimensional (3D) pore networks and compute the sizes of 3D pores. The PSD determined by micro-CT was compared with that determined using mercury intrusion porosimetry (MIP) for the same loess. In addition, the microstructure of the same intact loess was observed using scanning electron microscopy to facilitate the discussion. The results show that the structure of intact loess is macroporous and macropores with the dimensions up to a few hundred microns are not rare and unevenly distributed. The micro-CT technique underestimates the loess porosity, because the pores with the dimensions smaller than the resolution cannot be identified. Intra-aggregate pores cannot be detected by this technique at present. The estimation accuracy may be improved a little by increasing the scanning resolution, while lack of representativeness of the aggregate-scale specimen and the resolution limit can still result in great underestimation of porosity and significant misrepresentation of the intact loess PSD. In summary, the PSD that covers a wide range and reveals the essentials of the pore structure of intact loess could not be accurately obtained by micro-CT. A combination of MIP and micro-CT is expected for a thorough evaluation of the pore structure of intact loess. In addition, a combination of CT systems with different resolution capabilities can serve for a multi-scale investigation of the pore network of intact loess.