## 3 Results and Discussion
### Microstructure of Concrete
#### 3.2.3 Pore Structure Analysis  
The macroscopic properties and microscopic test results of the different groups of concrete show that the F15S20 group had better performance in the 28-day range, so the pore structure parameters of the OC group and the F15S20 group were analyzed here, mainly including porosity, average air-void size, and pore spacing coefficient. Figure 13a plots the porosity of the OC group and the F15S20 group at different ages. It can be seen that the porosity of concrete with different mineral admixture content showed a decreasing trend with increasing age. As the cement hydrated, the internal pores were continuously filled with hydration products and the porosity decreased. The porosity of the OC group was significantly higher than that of the F15S20 group at day 3 of age because the fly ash and slag added to the F15S20 group filled the concrete pores. Moreover, 14 days later, the porosity of the F15S20 group was less than that of the OC group because the fly ash and slag added to the F15S20 group caused the internal structure of the concrete to become dense by day 14, making it more difficult for the salt solution to invade the concrete, whereas in the OC group, due to the early porosity, more sulfate ions were invaded, and the cement  
Figure 12: XRD pattern of concrete on the 28 th day. hydration products produced swelling erosion products such as ettringite and gypsum, which reduced the porosity of the concrete. Figure 13b shows the average air-void size of the concrete in the OC and F15S20 groups at different ages. The average air-void size in the OC group and the F15S20 group decreased and then increased as the age increased. The average air-void size of the concrete in both groups was the smallest on day 14 because the concrete was no longer cured by removing the geotextiles after day 14. The moisture inside the concrete evaporated faster under external conditions and the concrete underwent drying shrinkage. Micro-voids and micro-cracks were created, resulting in an increase in the average air-void size of the concrete. Figure 13c plots the pore spacing coefficients for the OC group and the F15S20 group at different ages, which did not vary equally with age. The larger the pore spacing coefficient, the smaller the number of pores within a certain range. The increase in the pore spacing coefficient of the F15S20 group with age indicated that as the cement continued to hydrate, the internal pores of the F15S20 group continued to be filled with hydration products and became less porous. The pore spacing coefficient of the OC group decreased on day 7 and day 28; presumably, the rapid hydration reaction of the concrete from 3 to 7 days resulted in a large number of calcium hydroxide crystals generated in the interface transition zone, which were significantly oriented, resulting in an increase in concrete pore volume and an increase in the number of pores. Figure 14 plots the percentage distribution of pore size for the OC group and the F15S20 group. From Figure 14a, it can be seen that the number of 1-30 \(\upmu\)m pores in the OC group increased and then decreased as the age increased, the percentage of 1-30 \(\upmu\)m pores increased before the 14 th day due to the hydration of cement and slag, and the products filled the pores, showing the transformation of pores larger than 30 \(\upmu\)m to 1-30 \(\upmu\)m pores. Later, as the environment changed and the water in the concrete evaporated, the concrete developed temperature shrinkage and drying shrinkage, and the shrinkage stress in the weak areas was greater than the tensile strength, producing micro-cracks. The number of 1-30 \(\upmu\)m pores increased with age in the F15S20 group. Compared to the OC group, the percentage of 1-30 \(\upmu\)m pores was larger in the F15S20 group at day 3, mainly due to the filling effect of the mineral admixture, which reduced the percentage of large pores.