## 5 Microscopic Mechanism of Composite-Cemented Soil  
### SEM Tests  
The samples of ordinary and composite-cemented soil (cured for 7, 28, and 60 d) that were crushed in the UCS tests were reserved and then observed through the SEM to compare their microstructures at 3 kx and 10 kx magnifications. The specific microstructures are illustrated in Figures 8-13. Figure 11: Composite-cemented soil cured for 28 d. (**a**) (3.00 kx magnification) and (**b**) (10.00 kx magnification). Figure 12: Ordinary-cemented soil cured for 60 d. (**a**) (3.00 kx magnification) and (**b**) (10.00 kx magnification). Figure 10: Ordinary-cemented soil cured for 28 d. (**a**) (3.00 kx magnification) and (**b**) (10.00 kx magnification). As shown in Figures 8a and 9a, the ordinary- and composite-cemented soil samples both produce a certain amount of hydration products after being cured for 7 d, while they both have rough surfaces and large amounts of pores between soil particles. Combined with Figures 8b and 9b, the two types of cemented soils both contain numerous pores in their microstructures, and certain amounts of bonded materials are found on the surface of soil particles, including slender acicular crystals, lamellar crystals, and reticular colloids. Therefore, it is inferred that the acicular and rodlike structures are Aft crystals, lamellar ones are Ca(OH)\({}_{2}\) crystals, and reticular amorphous colloidal structures are C-S-H gel. It can be seen from Figures 10a and 11a that after being cured for 28 d, the ordinary- and composite-cemented soil samples both produce lots of hydration products, and their soil particles are further cemented. The surface roughness decreases, and the pores reduce, accompanied by the production of plenty of reticular structures in contact with each other. By combining with Figures 10b and 11b, it is evident that the two types of cemented soil have reduced pores in microstructures and hydration products are bonded and wrap the surface of soil particles. More rodlike Aft crystals are generated between soil particles, and a lot of irregular colloidal C-S-H gel is distributed around soil particles to render soil particles to be bonded with each other. As a result, the agglomerated structure is formed initially. It can be seen from Figures 12a and 13a that the cementation between soil particles and hydration products is further enhanced in both the ordinary- and composite-cemented soil cured for 60 d. It is difficult to distinguish soil particles, the surface roughness further decreases, and a compact structure is formed initially. Combining Figures 12b and 13b indicates that the C-S-H gel formed a reticular structure that covers a large area, which contains cross-distributed short, coarse columnar ettrinigite crystals. A large quantity of soil particles has been cemented as a whole so that the compactness and integrity of the structure are improved significantly compared with those after a curing period of 7 d. Therefore, the mechanical properties and resistance to wetting-drying and freeze-thaw cycles of the two groups of cemented soils are both strengthened significantly with the increase in the curing period. A comparison of Figures 8 and 9 shows that the two types of cemented soil after being cured for 7 d both produce a certain amount of hydrates. While compared with the ordinary-cemented soil, more hydrates are generated in the composite-cemented soil, in which a lot of amorphous colloidal C-S-H gel also appears. Correspondingly, the ordinary-cemented soil produces a certain amount of tabular Ca(OH)\({}_{2}\) crystals inserted in the gel. By comparing Figure 10 with Figure 11, the hydration products of the two types of cemented soil cured for 28 d are found to both increase significantly compared with those after a curing period of 7 d. More C-S-H gel is produced in the composite-cemented soil than in the ordinary one.