*3.3. Gas Permeability*

Figure 6 shows the relationship between the gas permeability and moisture content. It is noted that the gas permeability of the soils was measured immediately after drying for various times. It is observed the gas permeability of compacted clay increased with the rise in moisture. The relationship of GCL Kp and moisture content can be reflected in three stages in Figure 6. The first trend in stage I represents Kp decreasing sharply with increasing moisture content ranging from 4% to 30%. The second trend in stage II shows Kp very slightly decreasing with the increasing moisture content of 30~50%. The third trend in stage III represents Kp decreasing sharply with the increasing moisture content of 50~100%. According to the comparison between this study and geosynthetic clay liners in previous studies, as shown in Figure 6, the gas permeability of both CCC and GCL decreases with the rise in moisture content. In addition, the test results of Rouf et al. [11] showed that when the moisture content of the GCL was 5~100%, the gas permeability at 20 kPa overburden pressure varied from 10−<sup>13</sup> m<sup>2</sup> to 10−<sup>16</sup> m2, much lower than that of compacted clay. Only when the ZP content reached 1.0% was the gas permeability of ZP-modified compacted clay lower than that of the GCL, with moisture content in the range of 20% to 28%.

**Figure 6.** Relationship between the gas permeability and moisture content of compacted clay after ZP modification [11,13,60].

The CCC sample total drying time is 17.5 h. The CCC's final state after drying can be compared to unveil the barrier performance under different ZP contents. Figure 7 shows the influence of ZP content on the gas permeability of the compacted clay after 17.5 h of drying. It can be observed that the moisture content of the clay enhanced as the content of ZP increased, while the gas permeability Kp gradually decreased. The moisture content and gas permeability showed an apparent negative correlation, meaning that gas permeability decreased with the rise in moisture content [11–13]. When the ZP content was 1.0%, the gas permeability of the clay after drying was 1.12 × <sup>10</sup>−<sup>11</sup> m2, which was 97% lower than that of the ZP admixture content of 0.2%. As proved, ZP can increase the moisture content of clay after drying, thereby reducing its gas permeability. When the PAM content is 1%, the gas barrier performance of clay can be effectively improved by about one order of magnitude.

**Figure 7.** Influence of ZP content on gas permeability of the compacted clay.

#### *3.4. Gas Diffusion Coefficient*

Figure 8 displays the relationship between the gas diffusion coefficient and moisture content (after different times of drying with the initial moisture content of 25.6%) of the compacted clay after ZP modification. The relationship of GCL Dp and moisture content can be divided into two stages in Figure 8. The first trend in stage I represents Dp, which has no changes with the increasing moisture content, ranging from 8% to 42%. The second trend in stage II shows Dp, which decreases sharply with the increasing moisture content of 58~100%. The relationship between CCC Dp and moisture content follows the linear decreasing law; Dp decreases sharply with the increasing moisture content of 0~25.6%.

**Figure 8.** Relationship between the gas diffusion coefficient and moisture content of the compacted clay after PAM modification [11,57].

As the content of ZP ranged from 0.2% to 1.0%, the gas diffusion coefficient decreased with the rise in moisture. According to the analyses of the gas diffusion test results of Rouf and Bouazza [19,56] on the GCL, when the moisture content of the GCL was 8~100%, the gas diffusion coefficient was between 10−<sup>6</sup> and 10−<sup>9</sup> m2/s, much smaller than that of compacted clay with the same moisture content.

As shown in Figure 8, when the moisture content was 25.6%, the gas diffusion coefficients of clay mixed with ZP at the contents of 0.2%, 0.4%, 0.6%, 0.8%, and 1.0% remained in the same order of magnitude, which was 4.56 × <sup>10</sup>−<sup>6</sup> m2/s, 5.05 × <sup>10</sup>−<sup>6</sup> m2/s, 4.72 × <sup>10</sup>−<sup>6</sup> <sup>m</sup>2/s, 4.56 × <sup>10</sup>−<sup>6</sup> <sup>m</sup>2/s, and 4.51 × <sup>10</sup>−<sup>6</sup> <sup>m</sup>2/s, respectively. When the drying starts, the CCC samples present different gas barrier properties. The Dp of the CCC sample with 0.2% ZP content after 17.5 h drying is 1.37 × <sup>10</sup>−<sup>4</sup> m2/s, while the Dp of the 1.0% ZP content sample is 3.06 × <sup>10</sup>−<sup>5</sup> m2/s. This is because the CCC samples mixed with high content ZP have high WRC, which can reduce the microstructure of clay particles and enhance the barrier property of CCC.

The CCC sample's total drying time is 17.5 h. After drying, the CCC's final state can be compared to unveil the barrier performance under different ZP contents. Figure 9 presents the relationship between the gas diffusion coefficient and moisture content of the compacted clay after drying for 17.5 h with the change in ZP content. It can be observed that the Dp decreased with increasing moisture content. When the ZP content was 1%, the gas diffusion coefficient of the clay was 3.06 × <sup>10</sup>−<sup>5</sup> m2/s, which was only 35% of that at the ZP content of 0.2%. Hence, ZP can effectively improve compacted clay's gas-barrier and anti-diffusion performance, and its modification effect increases with the rise in ZP admixture content.

**Figure 9.** Influence of ZP content on the gas diffusion coefficient of the compacted clay.

#### **4. Conclusions**

This study investigated the gas barrier performance of CCC of an industrial contaminated site that was modified by zwitterion polyacrylamide (ZP). The water retention capacity (WRC) test, liquid limit (LL) test, gas permeability test, and gas diffusion test were conducted to unveil the barrier mechanism. Based on the results, the following conclusions can be drawn:


Further studies are warranted to explore the economic efficiency and long-term stability of ZP-modified CCCs. The data in this study offer modified materials as gas barriers in applying geotechnical engineering.

**Supplementary Materials:** The supporting information can be downloaded at: https://www.mdpi. com/article/10.3390/app12168379/s1. Figure S1: Clay cracking with different PAMs. Figure S2: Gas permeability and gas diffusion coefficient of clay under different PAMs. Table S1: The results of each sample.

**Author Contributions:** Data curation, Y.-Z.B., J.-M.W. and H.-L.W.; Funding acquisition, Y.-J.D. and Y.-Z.B.; Investigation, Y.-Z.B., J.-M.W. and H.-L.W.; Supervision, Y.-J.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was financially supported by the National Key Research and Development Program (Grant Nos. 2018YFC1803100), Jiangsu Province Key Research and Development Program of China (SBE2022740941), National Natural Science Foundation of China (Grant Nos. 41877248 and 42177133), Scientific Research Foundation of Graduate School of Southeast University (Grant No. YBJJ 1844), and Postgraduate Research and Practice Innovation Program of Jiangsu Province (Grant No. KYCX17\_0130).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data that support the findings of this study are available from the first author, Yuzhang Bi, upon reasonable request.

**Acknowledgments:** We thank the anonymous referee for their careful reading and for providing insightful comments to improve the initial version of this paper. The corresponding author (Yan-Jun Du) would like to acknowledge the Jiangsu Province Key Research and Development Program of China (project name: Key Technology Research on Multi-Coordinated Low Carbon Cover Barrier for VOC Sites in Yangtze River Delta).

**Conflicts of Interest:** The authors declare no competing interests.
