## 3 Results and Discussion
### Mip
This is because in the early stage of erosion, sulfate ions enter the interior of cement-based materials and undergo chemical reactions with calcium silicate hydrate and calcium hydroxide, etc., generating gypsum, ettrinigite, and other substances. These products occupy the specimen's internal pores of the specimen, thereby reducing its porosity. In the intermediate and later stages of erosion, while these internal pores remain filled with reaction products, additional erosion products continue to be generated. When the expansive force exerted by these products exceeds the tensile strength of the cement-based material, cracks may develop, along with spalling at edges and corners, ultimately leading to an increased in porosity. Figure 10 illustrates the variation in pore size distribution for M3 compared to C3 without erosion. Figure 11 indicates significant differences in pore characteristics between mortar and concrete with identical water-cement ratios. Regarding pores exceeding 1000 nm, C3 exhibits larger pores than M3; however, for pores ranging from 100 nm to 1000 nm, the sizes of the pores in M3 and C3 pores are comparable. Notably, when the pore diameter is less than 100 nm, the pore volume of M3 surpasses that of C3 due to the incorporation of aggregates in the concrete, which leads to coarsening within its voids. Figure 8: Cumulative feed curves for M3 with different erosion durations. Figure 9: Pore size distribution curves for different erosion durations of M3. Furthermore, Figure 11 presents changes in porosity for mortar specimens in group M3 after exposure to sulfate erosion at intervals of 0 days, 60 days, and 120 days. As depicted in Figure 11, the porosity initially decreases before subsequently increasing as erosion progresses. After a period of 60 days under erosive conditions, there is a recorded decrease in porosity of approximately 1.01%. Additionally, it is observed that concrete with an equivalent water-cement ratio possesses a porosity that is approximately 6.48% greater than that of cement mortar. When considering pore diameters greater than 10,000 nm, the porosity of concrete can be estimated to be about four to six times higher than that of mortar. The volume of pores with diameters ranging from 100 to 1000 nm increased by 84% after 60 days of erosion, followed by an additional increase of 29% after 120 days compared to the initial volume. This trend suggests an initial rise in pore volume, which is subsequently followed by a decline as erosive aging progresses. In contrast, the pore volume within the diameter range of 1000 to 10,000 nm exhibits a consistent upward trend with extended erosion duration, while pores exceeding 10,000 nm demonstrate an initial decrease before experiencing an increase. The change in pore volume for sizes between 10 and 100 nm without erosion corresponds to an initial increase followed by a reduction in capillary pressure during water transfer, thereby directly influencing capillary water absorption characteristics. Additionally, the unit pore volume for sizes less than 10 nm continues to rise with increasing erosive age due to the dissolution of CSH gel. Table 3 shows the pore size characterization parameters.It can be seen that the mortar specimens, with the increase in erosion porosity, show a trend of decreasing first and then  
Figure 11: Pore change stacking diagram of M3 after sulfate erosion degradation. Figure 10: Pore change stacking diagram of M3 and C3. increasing, and the flexural and compressive strength corresponds to that described in Section 3.1, showing a trend of increasing first and then decreasing. After 60d of erosion, the porosity decreases, the flexural and compressive strength increases, the percentage of pore sizes less than 10 nm and 100-10,000 nm increases, and the percentage of pore sizes of 10-100 nm and greater than 10,000 nm decreases.