*2.5. Scanning Electron Microscopy (SEM) and Mercury Intrusion Porosimetry (MIP) Tests*

Image analyses using SEM (SIGMA 500, ZEISS-Tech, Jena, Germany) and pore size distribution using MIP (AutoPore V 9620, Micromeritics Instrument Corporation, Atlanta, America) were performed on the amended compacted clay specimens to evaluate the microstructural-amended mechanism. First, the attapulgite-amended, diatomite-amended, dual-additives-amended, and unamended compacted clay specimens were cut into 1 cm<sup>3</sup> pieces, the dosages of additives were all 5% (dry weight basis), and the ratio of dualadditives was 4 (dry weight of attapulgite 4% to dry weight of diatomite 1%). Then, the SEM and MIP specimens were frozen using liquid nitrogen for 5 min at a temperature of −120 ◦C. Sublimation of water was conducted at a temperature of −80 ◦C for 24 h in a vacuum freeze-drying unit at −18 N (Nanjing Xianou Instruments Manufacture Co., Ltd., Nanjing, China). After freeze-drying, the frozen SEM specimens were cut into small blocks with a natural fracture surface area of approximately 0.25 cm2, and these blocks were coated with a thin gold layer and then subjected to SEM analyses. The freeze-dried MIP specimens were subjected to MIP analyses in compliance with ASTM D 4404-18.
