## 3 Results and Discussions  
### Influence of FA Content on Mechanical Properties of Concrete  
After the standard curing for 3, 7, and 28 days, the concrete specimens were tested for flexural strength and compressive strength based on different FA content (0-50%). The results are shown in Figure 4. In addition, the standard error lines were added in Figure 4, to describe the differences in the results of the three parallel tests.  
Results show that under the condition of keeping the concrete mix unchanged, and the standard curing for 3 days flexural strength of concrete slowly decreased with the increase of FA content. In particular, the flexural strength of the curve decreases rapidly after the inflection point of 20% content. When the FA content is 20%, the flexural strength is 3.09 MPa. When the FA content is 50%, the flexural strength is 1.76 MPa, which is also the minimum flexural strength of concrete after 3 days of standard curing. The flexural strength of concrete after 7 days of standard curing increases slowly with the increase  
Figure 4: Effect curve of FA content on mechanical properties of concrete: (**a**) flexural strength; (**b**) compressive strength.  
Figure 3: FRINGE CLASS diffraction instrument.  
of the FA content and then decreases rapidly. When the FA content is 20%, the flexural strength is 4.68 MPa, which is also the maximum flexural strength of concrete after 7 days of standard curing. As can be seen from Figure 4b, after the incorporation of FA, the variation law of concrete's compressive strength is consistent with that of its SF on the whole. Basically, it showed a trend of a slow increase at first and then a gradual decrease. When the FA content is 20%, compressive strength at 3 days of standard curing age is 11.48 MPa, at 7 days of standard curing age is 19.46 Mpa, and at 28 days of standard curing age is 27.81 MPa. When the FA content is 50%, compressive strength at 3 days of standard curing age is 7.08 MPa, compressive strength at 7 days of standard curing age is 8.31 Mpa, and compressive strength at 28 days of standard curing age is 13.87 MPa.  
Compared with the concrete specimens without FA, the flexural strength of concrete after 3 days of standard curing and the compressive strength of concrete after FA incorporation show little change. Compared with the untreated concrete, after 28 days of standard curing, the flexural strength and the compressive strength of concrete with 20% FA increased by 0.82 MPa and 4.32 MPa, respectively. With the increase of the standard curing age, the volcanic ash effect of FA is further released, which can not only keep the flexural strength of concrete but also improve the overall mechanical properties of concrete due to the active characteristics of FA. Therefore, from the perspective of industrial solid waste utilization and reducing carbon emissions in cement production, the experimental results show that the optimal content of FA is 20%.  
FA as an admixture of concrete has been extensively studied, and most of the studies are consistent with the law of change in the compressive and flexural strength of concrete after the addition of FA. As the age increases, it basically shows a trend of fast growth at the beginning and slow growth at the end. At the same time, the experimental results show that the relative percentage values of the compressive strength of cement concrete, Portland concrete, and high-performance concrete with FA content of 15% and 20% are significantly larger than those of the other mixtures. The optimal FA content in this study is 20%, which is in line with most experimental results on the mechanical properties of concrete materials with FA content [39; 40].