## 3 Results and Discussion
### Microscopic of Sands and Bonding Sections
#### 3.5.2 Matrix Transition Zone and Hydration Product Analysis  
Samples were collected from the failure surface of the tensile specimen (with many fibers attached to the surface) for SEM and XRD analyses to analyze and verify the results of the uniaxial tensile test and adhesive force analysis. The results are depicted in Figures 9 and 10. Figure 9: _Cont._  
A comparison of Figure 9a,e reveals that the connection between the matrix and fiber was tighter in Figure 9e, indicating that the addition of desert sand strengthens the bond between the fiber and matrix. The DSPE\(-\)ECC matrix was dense, and the matrix strength was more remarkable, which made the DSPE\(-\)ECC crack load stronger than the NSPE\(-\)ECC crack load at the same age. However, the DSPE\(-\)ECC specimen was more susceptible to damage after the matrix was fractured owing to the smaller particle size and smooth surface of desert sand. Consequently, the ultimate load and strain of DSPE\(-\)ECC at 3 days were less than those of NSPE\(-\)ECC, as were the bonding properties. The hydration degree of NSPE\(-\)ECC and DSPE\(-\)ECC was almost the same, and a large amount of C\(-\)S\(-\)H gel was formed on the surface of the matrix and attached to the fibers, as shown in Figure 9c,g. The 28\(-\)day\(-\)old DSPE\(-\)ECC and NSPE\(-\)ECC fibers formed an excellent bridge with the matrix; therefore, the ultimate loads of DSPE\(-\)ECC and NSPE\(-\)ECC were the same. Figure 10: Mineral composition at different ages: (**a**) 3 d, (**b**) 28 d.  
Figure 9: Microstructures of the specimens: (**a**) 3 d transition zone of NSPE\(-\)ECC, (**b**) 3 d matrix of NSPE\(-\)ECC, (**c**) 28 d transition zone of NSPE\(-\)ECC, (**d**) 28 d matrix of NSPE\(-\)ECC, (**e**) 3 d transition zone of DSPE\(-\)ECC, (**f**) 3 d matrix of DSPE\(-\)ECC, (**g**) 28 d transition zone of DSPE\(-\)ECC, (**h**) 28 d matrix of DSPE\(-\)ECC. Figure 10a showed that the proportions of C\({}_{3}\)S and C\({}_{2}\)S in the hydration process of the PE\(-\)ECC matrix after 3 days were greater than those of other substances. The matrix contains the C\(-\)A\(-\)H gel produced by the initial hydration of tricalcium aluminate and a small amount of CH. Figure 9b,f shows that a remarkable amount of cement raw materials need to be fully hydrated in the formed matrix, confirming that the matrix at 3 days was in the initial stage of hydration, during which NSPE\(-\)ECC was in the fiber roughness stage. The characterization effects of cement hydration, particle size, and compactness were more substantial for NSPE\(-\)ECC than for DSPE\(-\)ECC, and the bonding strength of NSPE\(-\)ECC was greater than that of DSPE\(-\)ECC. It can be seen from Figure 9d,h that the 28\(-\)day\(-\)old NSPE\(-\)ECC hydration particles form a complete cement matrix surface, whereas the DSPE\(-\)ECC hydration particles form fibrous networks with numerous micropores. At the same time, Figure 10b shows that the diffraction results of the two PE\(-\)ECCs at 28 days were identical, indicating that their hydration and bonding properties were comparable. This suggests that the NSPE\(-\)ECC fibers are more firmly bonded to the matrix, making the fibers more challenging to extract. Consequently, DSPE\(-\)ECC's limit strain at the same age would be less than that of the NSPE\(-\)ECC. The C\(-\)S\(-\)H gel was viscous in the solid and liquid states and could form strong bonds with fine aggregate; this property was the primary factor influencing the bonding properties between the PE\(-\)ECC fiber and matrix. The SEM test times were extended, as depicted in Figure 11, to determine the formation process of C\(-\)S\(-\)H gel in NSPE\(-\)ECC and DSPE\(-\)ECC at varying ages. The C\(-\)S\(-\)H gel of NSPE\(-\)ECC at 3 and 28 days corresponded to two forms: flocculating and block\(-\)polymerizing. The C\(-\)S\(-\)H gel of DSPE\(-\)ECC also corresponded to two conditions: flocculent and a fibrous and blocky combination.