*3.4. SEM, BET, MIP, and TGA Tests Results*

SEM images for the compacted clay specimens (a) Unamended; (b) Attapulgiteamended; (c) Diatomite-amended; and (d) Dual-additives-amended are shown in Figure 10. It is seen from Figure 10a 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 10.** SEM images of compacted clay specimens.

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 μm and a pressure range of 0.1 pasi to 61,000 pasi. The average PSD of specimens amended by dual-additives, attapulgite, diatomite, 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 11a 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 11b 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 11.** MIP results of amended compacted clay specimens: (**a**) cumulative intruded pore volume vs. pore size diameter; and (**b**) incremental intruded pore volume.

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, diatomite-amended, and dual-additives-amended clay were 0.00721 cm3/g·nm, 0.00707 cm3/g·nm, and 0.00724 cm3/g·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. The volume of microporous and mesoporous attapulgite and dual-additives-amended clay increased significantly. Therefore, the dual-additives decreased the microporous size diameter of clay powder and increased its specific surface area.

**Table 4.** The BET test results.


The TGA test results of clay amended by different additives are shown in Figure 13. The water in the clay was classified into tightly bound water (TBW) connected with clay minerals by hydrogen bond, loosely bound water (LBW) connected with clay minerals by molecular force [52,53], and free water; the limit of water decomposition temperature was 120~230 ◦C, 75~120 ◦C, and 25~75 ◦C respectively [54]. The DTG curve was obtained from the differentiation of the TG curve. Each minimum point at the valley on the DTG curve represents the water decomposition point. Figure 13a shows that the temperature limits of TBW, LBW, and free water of unamended clay were 29.4 ◦C, 101.55 ◦C, and 184.41 ◦C respectively. The LBW temperature limit of attapulgite-amended clay was 106.68 ◦C, an increased of 5.05% compared with unamended clay, and its TBW temperature limit was 210.46 ◦C, an increase of 14%. The LBW temperature limit of diatomite-amended clay was

105.10 ◦C, an increase of 3.5%, and that of dual-additives-amended clay increased by 8.9%. The TBW and free water have no relationship with the moisture retention performance of clay. The LBW is double-diffuse layers (DDLs) of clay minerals [55,56], which is the critical factor in the moisture retention performance of clay. Figures 4,5 and 13, present that the trend of the temperature limit of clay amended by different additives is consistent with the results of moisture retention percent and liquid limit. It illustrates that the moisture retention percent of amended clay increases with the increasing the temperature limit of LBW. That is attributed to the fact that the thermal energy required to lose the same amount of LBW is greater with the increasing temperature limit; therefore, its moisture retention performance is enhanced at the same temperature (60 ◦C).

**Figure 12.** BET tests results of clay specimens amended by attapulgite, diatomite, dual-additives, and unamended.

**Figure 13.** TG tests results of specimens: (**a**) Unamended; (**b**) Attapulgite-amended; (**c**) Diatomiteamended; and (**d**) Dual-additives-amended.
