## 3 Results and Discussion
### SEM, BET, MIP, and TGA Tests Results  
SEM images for the compacted clay specimens (a) Unamended; (b) Attapulgite-amended; (c) Diatomite-amended; and (d) Dual-additives-amended are shown in Figure 10. It is seen from Figure 10 that the flaky, unamended compacted clay particles polymerize to form agglomerates. There are gaps between adjacent agglomerates for gas and water molecules to pass through. Figure 10b shows that the acicular attapulgite particles attached to the surface of the clay and filled the gaps, which reduced the gaps between clay particles and increased the water retention percent. Figure 10c presents that the size of diatomite particles with micropores was larger than clay particles. The pores between the clay particles decreased, and the micropores on the surface of the diatomite made the gas migration path more complex. However, the microporous structure was larger than those of \"zeolite molecular sieve\", which had a limited effect on moisture molecules adsorption. Therefore, it had a limited effect on the enhancement of moisture retention performance. Figure 10d shows that the dual-additives filled and decreased the pores between clay particles and the micropores structure made the gas migration more complex and difficult. Figure 9: The flexible wall penetration test results of amended compacted clay. Figure 11 shows the cumulative intruded pore volume and incremental intruded pore volume vs. pore size diameter (PSD) of amended compacted clay specimens with different additives (dosage 5%). The distribution is shown for a diameter range of 3 nm to 425 \(\upmu\)m and a pressure range of 0.1 psi to 61,000 psi. The average PSD of specimens amended by dual-additives, attapulgite, diatomic, and unamended were 30.6, 31.19, 35.02, and 47.28 nm respectively. Porosities were 19.4%, 21.5%, 27.2%, and 47.3% respectively. The average PSD and porosity of dual-additives-amended compacted clay decreased by 55% and 144% respectively compared with unamended ones. Figure 10(a) presents that the cumulative intruded pore volume of all amended specimens was lower than unamended ones, regardless of PSD. The cumulative intruded pore volume of dual-additives-amended was the lowest, which decreased by 60% at PSD of 5 nm compared with unamended. It illustrates that the dual-additives filled the gaps and pores of compacted clay significantly. Figure 10(b) shows that the incremental intruded pore volume of unamended compacted clay increased significantly at a PSD of 100 nm. The peak of curves of amended compacted clay were in the range of 5-100 nm, especially at 15 and 40 nm. The incremental intruded pore volume of all specimens was almost zero in the range of 500-100,000 nm. It indicates that almost all of the pores in amended compacted clay had a diameter range of 5-500 nm. Therefore, the dual-additives could enhance moisture retention and gas barrier performance by decreasing the gaps and pores of compacted clay. Figure 10: SEM images of compacted clay specimens. The BET test results of clay powder amended by different additives are shown in Figure 12. The PSDs of less than 2 nm, 2-50 nm, and larger than 50 nm were defined as microporous, mesoporous, and macroporous respectively. It is seen that the microporous amount of amended clay increased significantly compared with unamended clay, and the mesoporous and microporous volume increases were not significant. The microporous volume of attapulgite-amended, diatomic-amended, and dual-additives-amended clay were 0.00721 cm\({}^{3}\)/g\(\cdot\)nm, 0.00707 cm\({}^{3}\)/g\(\cdot\)nm, and 0.00724 cm\({}^{3}\)/g\(\cdot\)nm respectively when the pore size diameter was 2 nm, increases of 9%, 7%, and 9% respectively compared with unamended clay. So, the dual-additives could enhance the moisture retention performance of compacted clay. The specific surface area and Density Functional Theory (DFT) pore distribution results are presented in Table 4. It was seen that the specific surface area of clay amended by additives increased 13-15% compared with unamended clay.