## 4 Discussion  
### The Effect of Pore Size and Water-to-Cement Ratio on Moisture Transfer  
Considering that the moisture diffusion coefficient of concrete is the accumulation of moisture migration in a single pore, the relationship between the moisture diffusion coefficient of a single pore and the pore size is shown in Figure 9, when the RH is 1 according to the moisture diffusion coefficient model in this paper. The results show that the moisture diffusion coefficient of a single pore increases with the increase in pore size, and finally tends to be stable. The growth rate of the diffusion coefficient increases first and then decreases. This is mainly due to the fact that when the pore size is small (about <7 nm), the moisture migration in the pores is mainly Knudsen diffusion, and the diffusion coefficient is mainly determined by the pore size. Between the pore size of about 7-700 nm, it is a mixed diffusion. The size of the diffusion coefficient is determined by the pore size and the free path. With the increase in the pore size, the proportion of Knudsen diffusion gradually decreases, and the proportion of free diffusion gradually increases. When the pore size is large (about >700 nm), the moisture migration in the pores is mainly free diffusion, and the diffusion coefficient is only determined by the free path. At the same time, according to the pore size distribution (PSD) curve of 28 days in reference [39] (Figure 10), the equivalent moisture diffusion coefficient of cement paste under different water-cement ratios was calculated, as shown in Figure 11. The results show that the moisture diffusion coefficient of cement paste increases with the increase in the water-cement ratio at the micro scale. Additionally, with the increase in RH, the \"S\"-type growth law is presented, which is consistent with the macroscopic law of concrete. It is noteworthy that the free path of water molecules increases with the decrease in RH, so the proportion of Knudsen diffusion in the pores of cement paste increases when the RH is low. Because the moisture diffusion coefficient of Knudsen diffusion is much smaller than that of free diffusion, Knudsen diffusion is also one of the reasons why the moisture diffusion coefficient increases with the increase in RH. In addition, combined with the effect of pore size on the moisture diffusion coefficient in Figure 9, the results in Figure 11 also show that the micro-scale moisture diffusion coefficient is greatly affected by the pore size distribution of cement paste. Figure 8: Average RH evolution of 0.30 aggregate volume fraction concrete at different times: (**a**) average RH of wetting process; (**b**) average RH of drying process. Figure 11: The effect of water-to-cement ratios on cement paste moisture diffusion coefficient. Figure 10: PSD of different water-to-cement ratios [35]. Figure 9: Moisture diffusion coefficient of a single pore (RH = 1). In order to further illustrate the influence of pore size distribution on moisture migration, the contribution of pores with different pore size ranges to the overall moisture diffusion coefficient is calculated according to the moisture diffusion coefficient model (i.e., \([D_{ce\text{-}dp1}-D_{ce\text{-}dp2}]/D_{ce}\), considering that when RH is about greater than 0.6, \(D_{ce}\) begins to increase, so the effective moisture diffusion coefficients when RH is 0.6, 0.8, and 1 are selected as the benchmarks, respectively), as shown in Figure 12. The results show that under the influence of the free path difference in water molecules at different RHs, with the increase in RH, the migration proportion of water molecules in macropores increases, that is, the influence of Knudsen diffusion decreases gradually, and the proportion of free diffusion increases gradually.