Investigation into the mercury intrusion porosimetry (MIP) and micro-computed tomography (CT) methods for determining the pore size distribution of MX80 bentonite pellet

The pore size distribution (PSD) plays an important role in the hydro-mechanical behaviour of porous materials. In this study, the PSD of MX80 bentonite pellets under various imposed suctions from 262 to 1.5 MPa was determined by both mercury intrusion porosimetry (MIP) and micro-computed tomography (mu CT). The void ratios e(18-344) in the overlapping pore range (18-344 mu m) identified by MIP and mu CT were compared to clarify the difference between the two techniques. Results showed that the MIP-detection e(18-344) nearly overlapped with the mu CT-detection one at suction s >= 57 MPa, indicating the reliability of MIP and mu CT results for pellets at s >= 57 MPa. However, the mu CT-detection e(18-344) became larger than the MIP-detection one at 9 MPa <= s <= 38 MPa, while the changes of MIP-detection and mu CT-detection e(18-344) with suctions were opposite at s <= 4.2 MPa. As the difficulty of interpreting the variation in mu CT attenuation coefficient at low suctions undermined the reliability of mu CT thresholding, the segmentation of mu CT images was suggested to be determined by making the mu CT-detection e(18-344) equal to the MIP-detection one. Because the reliability of MIP results at s <= 4.2 MPa was reduced by the difficulty of keeping the integrity of freeze-dried samples, the MIP-detection e(18-344) determined in a previous study without the sample integrity problem was chosen as reference for the mu CT thresholding at 1.5 MPa <= s <= 4.2 MPa. The corrected mu CT-detection e(18-344) calculated by the new thresholds matched well with the MIP-detection e(18-344), completing the mu CT-detection at suctions smaller than 57 MPa. Since the MIP-detection PSD only covers the pores smaller than 344 mu m, the mu CT-detection PSD with pores larger than 344 mu m was then integrated into the MIP-detection PSD, enlarging the MIP-detection to millimetric pores.