## 5 Conclusions  
Cement mortars are porous bodies whose pore voids are regarded as interconnected, which can affect the rate of diffusion. Additionally, there are regions in the dry mortar body where fissures exist, such as the interfaces between the sand or aggregate and the mixed binder (cement) when it dries, and these fissures could be connected. All factors have a bearing on the diffusion of gases in the mortar; however, there could be a resistance to diffusion by the solids in the mortar, and the fissures and interconnected pores could allow the flow of gaseous (as well as liquid) substances at the depth of these mortars, resulting in deeper solutes in the gas in the mortar, which increases the penetration depth of the gas and reduces the directional distance for solutes to diffuse from the gas to the mortar solid body. In contrast, OPC mortars with replacement ratios of \(\gamma\)-C\({}_{2}\)S fall under this category of porous mortars, and the fact that the binder mixture contains OPC, which is a hydraulic substance, and \(\gamma\)-C\({}_{2}\)S, which is not hydraulic, can complicate diffusion. This questions the behavior of such mortars to diffusion, especially when the gas diffusing is CO\({}_{2}\), which reacts with the phases in the OPC/\(\gamma\)-C\({}_{2}\)S mortars, causing densification. This study is the first to test the conventional laws of diffusion (Fick's laws) for OPC/\(\gamma\)-C\({}_{2}\)S paste specimens. It was confirmed that the cement concrete with \(\gamma\)-C\({}_{2}\)S can absorb a certain amount of CO\({}_{2}\) for a long time. The following conclusions were drawn from the phenomenon of diffusion of CO\({}_{2}\) into the pastes and the carbonation process:  
1. The diffusion of CO\({}_{2}\) in mortars and pastes whose binders are made from OPC and \(\gamma\)-C\({}_{2}\)S generates more calcite (CaCO\({}_{3}\)) than that generated by the carbonation of mortars and pastes, whose binder is only OPC. 2. The source of Ca in the above carbonation is CH, C\({}_{3}\)S, C\({}_{2}\)S, C-S-H, and \(\gamma\)-C\({}_{2}\)S, as a higher intensity of calcite was observed when the ratios of \(\gamma\)-C\({}_{2}\)S were varied in the experiments. 3. The generated CaCO\({}_{3}\) deposits in the pore structures of the mortars and pastes result in densification. 4. The deposited CaCO\({}_{3}\) in the pores affects the rates of diffusion of CO\({}_{2}\) in the mortars and pastes, resulting in the densification of such bodies and a decrease in the rate of diffusion, leading to the shutdown of the diffusion process. The diffusion of CO\({}_{2}\) in hardened cement pastes made from OPC and \(\gamma\)-C\({}_{2}\)S follows [PERSON]'s second law, wherein there is a change in the concentration of CO\({}_{2}\) diffusing at a particular distance with time. OPC with \(\gamma\)-C\({}_{2}\)S has the capacity to capture CO\({}_{2}\). In future work, we will study the CO\({}_{2}\) absorption in a hardened cement body with \(\gamma\)-C\({}_{2}\)S according to the change in time and CO\({}_{2}\) concentration. We suggest a mechanism for CO\({}_{2}\) absorption and CO\({}_{2}\) storage in hardened cement bodies according to the long-term hydration progress of the cement body. Investigation, visualization S.-M.K. and W.-G.L. ; writing--review and editing, supervision M.-S.S.; validation, K.-K.M. All authors have read and agreed to the published version of the manuscript. This work was supported by a Korea Agency for Infrastructure Technology Advancement (KAIA) grant funded by the Ministry of Land, Infrastructure, and Transport (Grant 21 CTAP-C163942-01) and by the Technology development Program (S3174481) funded by the Ministry of SMEs and Startups (MSS, Republic of Korea). Not applicable. All data on which the results described in the manuscript are based can be used without restriction. The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of the data; in the writing of the manuscript; or in the decision to publish the results.