## 5 Conclusions  
A multi-scale moisture diffusion coefficient model of concrete is established for the non-uniformity of moisture migration in concrete in this paper. The influence of microstructure on the moisture migration of concrete is discussed. The main conclusions are as follows:  
1. The moisture diffusion coefficient model of concrete considering microstructure quantifies the role of Knudsen diffusion in moisture migration, which can effectively con  
Figure 14: The effect of aggregate volume fraction on the concrete moisture diffusion coefficient. Figure 13: The effect of ITZ thickness on the concrete moisture diffusion coefficient. sider the effects of pore size distribution, aggregate content, and excessive interface on moisture migration;
2. The pore size distribution has a great influence on the humidity diffusion coefficient, and the humidity is more likely to migrate through the pores of 10-100 nm. Knudsen diffusion reduces the moisture diffusion coefficient of concrete to a certain extent, and its influence increases with the increase in the water-cement ratio and decreases with the increase in relative humidity, which also leads to the increase in the moisture diffusion coefficient with the increase in relative humidity to a certain extent;
3. The transient moisture migration of concrete has obvious non-uniformity. There is a \"flow around\" phenomenon at the edge of aggregate, and water is more likely to be transmitted through the excessive interface. With the increase in the aggregate volume fraction, the excessive interface with a higher moisture diffusion coefficient also increases, but the influence of the additional excessive interface on the overall moisture diffusion coefficient is less than that of the aggregate \"dilution\" effect;
4. The \"S\"-type growth relationship between the moisture diffusion coefficient and relative humidity of concrete includes the difference in the moisture diffusion coefficient between the wetting and drying process, which makes wetting faster than drying. At the same time, the jump growth of the moisture diffusion coefficient is also the main reason for the \"inflection point\" of the humidity distribution inside the concrete during the wetting and drying process. In this study, the effect of microstructure on moisture migration in concrete is investigated by a multi-scale moisture diffusion model based on Knudsen diffusion, which can consider the cracking damage of pores induced by dry shrinkage from non-uniform moisture transfer and provide a theoretical basis for improving pore structure and moisture migration to reduce dry shrinkage and enhance the durability of concrete. In addition, for concrete subjected to gas or ion corrosion, the non-uniform transport and diffusion of ions and gas in the concrete can be studied according to the multi-scale diffusion model proposed in this study and the effect of concrete microstructure on the diffusion of ions and gas can be obtained. In view of possible future developments, the variation in environmental temperature and the changes in concrete components and microstructures during the service have a significant influence on moisture transfer in concrete. Therefore, in future research, changes in temperature and the effect of load should be considered to further develop the multi-scale diffusion model. Additionally, experiments on moisture transfer should be conducted by considering the changes in concrete components and microstructures. The porosity, pore structure, and mineral composition can be experimentally measured. The experimental results can be used to validate and improve the multi-scale moisture diffusion model proposed in this study. Furthermore, the effect of temperature variation, load action, component, and microstructure changes on the moisture transfer in concrete can be investigated, which can provide a theoretical basis for reducing the dry shrinkage cracking and improving the durability of concrete in the service period. Conceptualization, B.T. and B.C. ; methodology, B.C. ; validation, X.Z. and Z.P.