## 3 Result Analysis
### Scanning Electron Microscope  
In order to understand the mechanism of geopolymer strength formation, the geopolymer portion of the maintenance-completed mix was selected for elemental composition analysis and elemental distribution analysis; the composition information is first detected, as shown in Figure 7. In addition, considering that the distribution uniformity of the cementitious material has an impact on the strength that cannot be ignored, the cementitious material powder is magnified by 1000-times for single element surface scanning, as shown in Figure 8.  
Figure 7 shows the test results of the elemental composition of the geopolymer. From the diagram, the following can be seen: the geopolymer composition consists mainly of O, C, Al, Si, Mg, Ca, K, Na and Fe, which is basically the same as the composition of cement [31]. Therefore, the geopolymer has an acceptable composition basis for being used as cementitious material. From Figure 8, it can be seen that the distribution of each element component of the geopolymer cementitious material is relatively uniform, which further indicates that the geopolymer cementitious material has great dispersibility after the hydration reaction and can be fully cemented with the aggregate as a cementitious material, thereby affecting the strength of the mixture. Figure 9 shows an image of the mixture samples without geopolymer, 20% and 40% geopolymer content after 3000-times magnification by SEM. It can be seen that in the samples without a geopolymer, the products generated by cement hydration are connected with each other and cover the surface of the aggregate, making the surface relatively complete. However, with an increase in the geopolymer dosage, the hydration product and calcium aluminate hydrate began to appear on the surface of the sample. When the dosage  
Figure 8: Element surface distribution diagram of geopolymer. Figure 7: Composition of geopolymer. of the geopolymer was raised from 20% to 40%, the surface hydration products changed from flake accumulation to granular accumulation. This is mainly because the reaction mechanism of the geopolymer is divided into three steps [32]. First, under alkaline conditions, Al-O-Si and Si-O-Si in the raw materials are dissociated to form silicate and aluminate ions. Then, these ions are gelled, Al(OH)\({}_{4}^{-}\) Al(OH)\({}_{5}^{2-}\), Al\({}^{3+}\), Ca\({}^{2+}\) and so on. Finally, silicate ions and aluminate ions recrystallize to form zeolite-like substances. Under the influence of alkaline excitation materials, 20% of the monomers in the geopolymer, such as Si, Al, etc., are separated from the material and become root ions. These monomers then react with dehydrovalidation and interact with alkali metal ions to recombine to form silicates and aluminosilicates. These chemical changes make the geopolymer hydration reaction products connect and harden to form a plate-like structure, resulting in solidification and hardening effects. Therefore, the mechanical properties of the mixture with 20% geopolymer content are better than those of the mixture with no geopolymer dosage. When 40% geopolymer is added, many massive and granular substances accumulate on the specimen surface. This is due to the high content of the geopolymer. On the one hand, the incomplete hydration reaction of some geopolymers makes the hydration products in the mixture accumulate in the form of particles or agglomerates and does not form a plate structure that can increase the strength. On the other hand, the incorporation of too much geopolymer increases the heat released by the hydration reaction, and the cement and geopolymer in the mixture shrink via heat release, resulting in shrinkage cracks [33], which, in turn, causes more pores to form inside the mixture, affecting the strength, and ultimately leading to a loss of strength in the mixture with 40% geopolymer content. Figure 10 shows an image of the mixture samples with 20% geopolymer content after 3000 freezing and thawing cycles using SEM.