## 3 Test Result Analysis
### Unconfined Compression Strength (UCS)
#### 3.1.2 Physical Explanation of Sand on Strengthening Mechanism of Cemented Soil  
The strengthening mechanism of sand particles on cemented soil is mainly manifested in three aspects: (1) the modulus replacement effect (sand particles with large stiffness replaces soft soil with small stiffness), (2) the sand particle-cemented soil interface cementation effect, and (3) the microcrack expansion obstructing effect.  
In order to analyze the microscopic control mechanism of sand particles on the solidification effect of the cemented soil, scanning electron microscope tests were performed on the CS-SS samples (medium sand, 15% sand particle content, and curing age 28 d) after conducting the UCS test, and the test results are shown in Figure 3. At lower magnification, it can be seen that the sand particles are embedded in the cemented soil, forming a tighter bond with the surrounding soil. The skeleton system originally constructed by soil particles is transformed into a support system with a sand particle-centered aggregate structure as the main skeleton. The sand particles were observed on a large scale, and different interfaces appeared around the sand particles. The surface of the sand particles was uniformly covered with C-S-H gel. The cemented soil and the sand particles form a stable structure through the hydration product C-S-H gel, which is also the reason why the sand particles promote the strength of the cemented soil to be greatly improved. Due to the large density and strength of sand particles, it plays the role of coarse aggregate in cemented soil. For  
Figure 2: Relationship between the UCS of the CS-SS samples and the sand particle content; (**a**) curing age 7 d; (**b**) curing age 14 d; (**c**) curing age 28 d.  
the early-age (7 d) samples, the degree of hydration of the cement was low, the interface cementation strength of the interfaces between the sand particles and cemented soil was low, and the strengthening effect of sand particles on the cemented soil was mainly induced by the modulus replacement effect. With the increase in sand particle content, the degree of the modulus replacement effect increased, and the compaction effect of sand particles on the cemented soil was intensified. The UCS of the CS-SS samples increased with the increase of sand particle content, but the increased amplitude was limited. Meanwhile, it was slightly affected by the sand particle size. For the long-age (28 d) samples, the hydration of cement was more adequate, and a stronger interface cementation was achieved between the sand particles and the cemented soil through the cement hydration products, as shown in Figure 3c. The deformation coordination between the sand and cement soil was strengthened; that is, the high strength of sand can be transferred through interface cementation, and the modulus substitution effect of the sand is enhanced [14]. The UCS of the CS-SS samples increased with the increase of sand particle content, and the increase in amplitude was promising. The smaller the sand particles, the greater the specific surface area, and the greater the interface area, the more pronounced was the reinforcing effect of modulus replacement. The later UCS of the sample increased with the decrease in sand diameter.