## 4 Conclusions  
In this study, the compressive strength, flexural strength, and fracture performance of concrete modified by a mineral admixture in an environment with a large temperature difference and saline soil erosion were tested, and the microstructure, phase composition, and pore structure of the concrete were observed, the chloride ion content at different depths of concrete was measured, and the fracture resistance mechanism of fly ash and slag was also analyzed, which provides a reference for engineering applications. The main conclusions are as follows:  
1. Both the compressive strength and the flexural strength of concrete increase with age. The results showed that the addition of the mineral admixture reduced the compressive strength of the concrete; compared with the OC group, the compressive strength of the F10S20, F15S15, and F15S20 groups decreased by 12.8%, 8.4%, and 9.4%, respectively, and the flexural strength of mineral admixture concrete was lower than that of ordinary concrete before 14 days, but its strength growth rate was greater than that of ordinary concrete, where the average growth rates in compressive strength from day 3 to day 14 for the four groups of concrete were: OC: 0.0406 MPa/d, F10S20: 0.0679 MPa/d, F15S15: 0.0744 MPa/d, F15S20: 0.0580 MPa/d. The average growth rates in compressive strength from day 3 to day 14 for the F10S20,F15S15, and F15S20 groups were 67.24%, 83.25%, and 42.86% higher than those of the OC group, respectively. 2. Through the P-CMOD curve of concrete, it was found that the addition of a mineral admixture reduces the early fracture performance of concrete but has a more obvious effect on the later fracture performance of concrete. Moreover, the fracture toughness and fracture energy of concrete increase with age. Adding a mineral admixture can effectively improve the fracture energy of concrete after 28 days, and the fracture toughness of concrete mixed with 15% fly ash and 20% slag is the highest. 3. The microstructure of the concrete was observed using scanning electron microscopy, and it was found that as the age increased, the hydration products of the concrete changed from individually forming clusters to gradually bonding to form a laminar structure, and the fly ash was in a good state of bonding with the cement paste. The width of cracks in the transition zone at the interface of concrete at the age of day 28 decreased significantly. From the XRD diffraction patterns of the different groups of concrete at day 28, it could be seen that F15S20 concrete had the lowest C2S/C3S diffraction peaks and the concrete was more hydrated and had the best performance. 4. By analyzing the pore structure parameters of the OC group and the F15S20 group, it was found that with the increase in age, the porosity of the F15S20 group was slightly higher than that of the OC group, but the average air-void size was lower and the pore spacing coefficient was larger. The percentage values of the OC group and the F15S20 group from 1 \(\upmu\)m to 30 \(\upmu\)m on day 28 were 13% and 48%, respectively. The pore structure of the F15S20 group was better. 5. The addition of a mineral admixture can effectively reduce the Cl\({}^{-}\) content in the concrete surface layer. The inhibition effect of fly ash on Cl\({}^{-}\) infiltration is better than that of slag. By observing the XRD diffraction patterns at the 28 th day in the different groups of concrete, it could be seen that the ettrinite (AFt) content of concrete mixed with 15% fly ash and 20% slag was the lowest, indicating that its resistance to sulfate was stronger than ordinary concrete. 6. Under the conditions of large temperature differences, the macro-mechanical properties and microstructure of the concrete are combined, indicating that the concrete modified by mineral admixtures has higher resistance to this harsh environment than ordinary concrete, and the optimum admixture of mineral admixtures is 15% fly ash and 20% slag.