## 3 Results and Discussion
### Mechanical Properties of Concrete
#### 3.1.3 The Fracture Performance  
Figure 9 plots the load-crack opening displacement (P-CMOD) curves for concrete with different mineral admixture content after 3, 7, 14, and 28 days, respectively. It can be seen that the peak loads of concrete with different mineral admixture content increased with age, indicating that the concrete continued to hydrate within 28 days, generating more hydration products and increasing bond strength, which was manifested as an increase in concrete strength. The peak loads of the mineral admixture-modified concrete were all smaller than those of the OC group, except for the F15S20 group, where the 28-day peak loads were greater than those of the OC group, which was due to the low initial activity of fly ash and slag, which contributed to the limited strength of the concrete. As can be seen  
Figure 8: Flexural strength for the four groups of concrete under erosion of SO4\({}^{2-}\) and Cl\({}^{-}\) and a large temperature difference. from Figure 9b, the peak load values for the four groups of concrete on day 7 were 3.52 MPa, 2.85 MPa, 2.73 MPa, and 3.03 MPa for OC, F10S20, F15S15, and F15S20, respectively, and the gap in peak loads continued to decrease as the age increased, with the peak loads on day 28 being 4.20 MPa, 4.01 MPa, 3.91 MPa, and 4.24 MPa for F10S20, F15S15, and F15S20, respectively, which was due to the secondary hydration of active silica and alumina in the fly ash and slag in the concrete as the hydration reaction proceeded, consuming calcium hydroxide, improving the spatial composition of the interfacial transition zone, reducing the porosity of the concrete, and improving the performance of the concrete. It could also be found that the slope of the rising section of the concrete load-CMOD (P-CMOD) curve increased with age, indicating that the concrete had an increasing modulus of elasticity and improved resistance to deformation. Comparing concrete with different mineral admixture content, it was found that the slope of the rising section of the P-CMOD curve for the OC group was higher than that for mineral admixture concrete. The reason for this was that SO4^2- from the salt solution simulating saline soil erosion entered the concrete and reacted with the cement hydration products to form swelling products such as ettringite and gypsum, which filled the voids and made the concrete denser. Figure 10a,b plot the variation curve of fracture toughness with age for concrete mixed with different mineral admixture content. It can be seen that the fracture toughness of concrete increased with age, the change trends of instable fracture toughness (\(K^{\mathrm{S}}_{\mathrm{IC}}\)) and initial fracture toughness (\(K^{\mathrm{Q}}_{\mathrm{IC}}\)) were the same, and the initial fracture toughness of concrete in each group on day 3 was between 0.6 and 0.7 MPa.m\({}^{0.5}\). The differences between  
Figure 9: P-CMOD curves of concrete at different ages: (**a**) day 3; (**b**) day 7; (**c**) day 14; (**d**) day 28.  
instable fracture toughness and initial fracture toughness were OC: 0.378, F10S20: 0.203, F15S15: 0.266, and F15S20: 0.266. In practical engineering, when cracks are created in concrete, the larger this difference is, the greater the load required to carry the concrete structure from cracking to instability and the greater the ability of the concrete to resist crack expansion. On day 14, the OC group had the greatest initial fracture toughness and the F15S20 group had the greatest instable fracture toughness. Combining the analysis of the concrete P-CMOD curves and the fracture toughness formulae, it is clear that the main reason is that these two groups developed their own strength rapidly and were subjected to higher loads at fracture.