## 3 Test Result Analysis
### Dry-Wet Cycle Test
#### 3.2.1 Influence of Sand on The Dry-Wet Cycle Characteristics of Cemented Soil
The XRD test results in Figure 10 show that the hydration product was C-S-H gel, and its diffraction peak decreased considerably with the increase of the number of dry-wet cycles, indicating that the C-S-H gel decreased with the increase of the number of dry-wet cycles. After one dry-wet cycle, with the decrease of hydration products, large microcracks appeared, as shown in Figure 7b, resulting in microstructure damage and strength degradation. After two dry-wet cycles, the microcracks in the samples continued to develop, the cemented soil aggregates cemented on the surface of the sand particles gradually peeled off (as shown in Figure 7c), the overall cementitious structure of the samples was destroyed, and the strength greatly decreased, which was consistent with the macroscopic test results given in Figure 5c. Figure 8 shows the SEM images of the CS-SS samples with different sand particle sizes after two dry-wet cycles under freshwater conditions. After two dry-wet cycles, small and dense microcracks appeared in the cemented soil near the sand particles in the CS-SS sample with fine sand (as shown in Figure 8a), and the cementation of the sample was good, and the strength was high. In the medium sand samples, continuous large and long microcracks appeared in the cemented soil around the sand particles (as shown in Figure 8b), the cementation effect between sand particles and the cemented soil aggregates decreased, and the strength of the sample decreased. In the coarse sand samples, the cemented soil aggregates fell down from the sand particle surface (as shown in Figure 8c), the samples had large fractures, the microstructure was destroyed, and the strength was greatly reduced. Figure 9 shows the SEM images of the CS-SS samples with fine sand after two dry-wet cycles under seawater and freshwater conditions. Table 3 shows the high concentration of corrosive ions (CI\({}^{-}\)) in seawater, and the chloride ions (CI\({}^{-}\)) reacted with tricalcium silicate (3 CaO-SiO\({}_{2}\)) in the cement in the hydration reaction to generate [PERSON]'s salts (3 CaO-Al\({}_{2}\)O\({}_{3}\)-CaCl\({}_{2}\)-10H\({}_{2}\)O) without cementation [28; 29]. On the one hand, this reaction consumed tricalcium silicate in the cement and reduced the production of cement hydration products (C-S-H gel). On the other hand, the [PERSON]'s salt covered the surface of cement particles and soil particles, which inhibited the continuation of the cement hydration reaction and reduced the cementation effect and strength of the CS-SS samples. Figure 10a,b show the mineral composition of the control group of the samples and the first and second rounds of freshwater/seawater dry-wet cycle tests, respectively. Figure 10c shows the mineral composition of samples from the control group and the two-cycle freshwater/seawater dry-wet cycle test. The main differences are the No.5 diffraction peak and the No.6 diffraction peak; that is, the composition of C-S-H gel and Friedel's salt changed greatly. Among them, the C-S-H gel content of the control group was the highest, followed by the freshwater circulation group, and the seawater circulation group was the lowest. Figure 10a showed that the Friedel's salt diffraction peaks of the freshwater dry-wet cycle samples were not obvious and remained unchanged with the number of dry-wet cycles. It can be seen from Figure 10b that the Friedel's salt increases as the number of dry-wet cycles of the sample increases, indicating that the Friedel's salt continued to be generated as the number of dry-wet cycles increases. The hydration product C-S-H gel is a crystalline phase that can be found in the XRD pattern. It is generally present in the diffraction peak of 20\({}^{\circ}\)-40\({}^{\circ}\), which is consistent with the results of [PERSON] et al. and [PERSON] et al. [30; 31]. It can be seen from Figure 10c that [PERSON]'s salt occurs only in a large number of samples in the seawater circulation group.