## 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:  
1. ZP could increase the moisture content of the compacted clay. The WRC is related to the linear expansion of polymer molecules after water adsorption. The moisture content of the clay after drying increases with the rise in the ZP admixture content. The modification had a prominent effect at a ZP content of 1%, and the moisture content after drying increased by 193% more than at the ZP content of 0.2%. 2. The WRC of compacted clay significantly improves after adding ZP. When the admixture content is 0.8% and 1.0%, the water loss rate after the clay is dried for 17.5 h at 100 \({}^{\circ}\)C is 64% and 54%. Its WRC is better than ordinary water-retention agents for agriculture and forestry. 3. The effects of ZP, i.e., reducing the clay gas permeability \(\rm K_{p}\) and gas diffusion coefficient \(\rm D_{p}\), are not obvious. When the content is 1.0%, the \(\rm K_{p}\) of CCC is about one order of magnitude lower than that at the ZP content of 0.2%. Its \(\rm D_{p}\) is only 35% at the admixture content of 0.2%. For CCC with the same moisture content, the \(\rm K_{p}\) and \(\rm D_{p}\) decrease with the rise in ZP content. 4. The reasons for the slight improvement in the gas barrier performance of modified clay are as follows. ZP modification enhances the WRC of CCC after drying, thereby reducing the \(\rm K_{p}\) and \(\rm D_{p}\). 5. ZP can increase the LL of compacted clay. When the admixture content varies from 0.2 to 1.0%, LL increases by 45-55% more than unmodified clay. 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. The supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app12168379/s1](https://www.mdpi.com/article/10.3390/app12168379/s1)]([https://www.mdpi.com/article/10.3390/app12168379/s1](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. 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. Figure 9: Influence of ZP content on the gas diffusion coefficient of the compacted clay. **Funding:** This study was financially supported by the National Key Research and Development Program (Grant Nos. 2018 YFC1803100), 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). **Instructional 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, [PERSON], 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 ([PERSON]) 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).