## 4 Conclusions  
In this paper, bonding property evaluation indexes were used to determine how the bonding properties of NSPE\(-\)ECC and DSPE\(-\)ECC changed over time by the uniaxial tensile test. The results of the calculation of the bond performance evaluation index and the results of the uniaxial tensile test were examined using microscopic testing. The main conclusions are as follows:  
* The addition of desert sand enhanced the fiber/matrix's bonding properties. The cracking loads of the DSPE\(-\)ECC at 3, 7, 14, and 28 days increased by 16.72%, 28%, 23.23%, and 10.05%, respectively, compared with those of the NSPE\(-\)ECC of the same age, indicating that the addition of 30% desert sand can increase the cracking load of PE\(-\)ECC.
* It can be seen from the bond properties evaluation index that the bond strength of NSPE\(-\)ECC is better than that of DSPE\(-\)ECC at 3 and 28 days. Although the bond  
Figure 12: C\(-\)S\(-\)H gel generation process.  
strength of DSPE\(-\)ECC is 36.11% lower than that of NSPE\(-\)ECC at 3 days, at 28 days, the bond strength of DSPE\(-\)ECC increased by 56.51% compared with that at 3 days, which is only slightly less than that of NSPE\(-\)ECC. The growth rate of bond strength started to slow from 7 days. The number of cracks in DSPE\(-\)ECC at 28 days was much higher than that at 3 days, which showed that the development of multiple cracks in DSPE\(-\)ECC became better as the curing ages increased. Moreover, the bonding properties of DSPE\(-\)ECC on day 28 were slightly lower than those of NSPE\(-\)ECC in terms of bond strength, and the bonding properties became steadier over time.
* The micro\(-\)test results are consistent with the calculation results of the bonding property evaluation indexes and tensile test results at 3 and 28 days in terms of fiber roughness, matrix compactness, and hydration degree. This result shows that the calculation results of bond performance evaluation indexes can reflect the changes in the bonding properties of NSPE\(-\)ECC and DSPE\(-\)ECC.
* Desert sand slowed the rate at which the combination of C\({}_{2}\)S, C\({}_{3}\)S, and water molecules in the matrix combined to form C\(-\)S\(-\)H owing to its low water content and high water absorption. Therefore, the formation of C\(-\)S\(-\)H gel in the NSPE\(-\)ECC matrix was faster than that in the DSPE\(-\)ECC matrix.
* The initial tensile load of DSPE\(-\)ECC was low; however, the ultimate tensile load of DSPE\(-\)ECC increased substantially as curing ages increased, demonstrating obvious multi\(-\)crack steady\(-\)state cracking, and the bonding properties tended to stabilize, providing theoretical support for the practical engineering application of this material.  
Conceptualization, Y.N. and F.H.; methodology, Y.N.; software, Y.N. and X.Y.; validation, Y.N. and Q.L.; formal analysis, Y.N.; investigation, Y.N. and Q.L.; resources, F.H.; data curation, Y.N. and Q.L.; writing\(-\)original draft preparation, Y.N.; writing\(-\)review and editing, Y.N. and F.H.; visualization, F.H. and X.Y.; supervision, F.H.; project administration, F.H.; funding acquisition, F.H. All authors have read and agreed to the published version of the manuscript.  
This study was financially supported by the National Natural Science Foundation, Youth Science Foundation of China (No. 51708479).  
Not applicable.  
All data generated and analyzed during this study are included in this article.  
The authors declare no conflict of interest.