## 4 Conclusions  
In this paper, a novel formulation of sprayed UHPC suitable for large-scale applications has been successfully developed. The mechanical properties and pore characteristics of sprayed UHPC were systematically investigated through controlled laboratory spray tests in a simulated tunnel environment and field applications. The main theoretical and practical contributions are summarized as follows:  
1. The mechanical properties of the sprayed UHPC material were examined through a series of strength tests. Experimental results demonstrated that by incorporating manufactured sand into the UHPC matrix, the mechanical properties of sprayed UHPC are comparable to those of cast UHPC across various spraying orientations, confirming its structural reliability for practical applications. This study offers robust theoretical underpinnings and practical guidance for the development of sustainable green sprayed UHPC, effectively driving down UHPC costs and advancing its progress in environmental protection and resource recycling.
2. The pore characteristics of the sprayed UHPC material were revealed through advanced microstructural analysis using SEM. The pore structure was primarily characterized by isolated and non-interconnected micropores with diameters smaller than 100 nm. The high pressure air-driven spraying technique effectively prevented the formation of harmful macropores larger than 10 um, thereby enhancing the mechanical properties of the material. This process simultaneously reduced early age porosity and improved pore connectivity in the UHPC, providing a critical reference for advancing research on enhancing UHPC durability through process optimization.
3. The early age strength development and low rebound rate of sprayed UHPC were validated through field tests. A compressive strength of 30 MPa was achieved within 24 h, reaching 60 MPa within 48 h, while the rebound rate remained consistently below 5%. The systematic refinement of indoor spraying process parameters, which integrates material fundamentals with engineering considerations, establishes a robust theoreti  
Figure 7: Mechanical property development curve of on-site sprayed UHPC.  
cal framework and actionable technical guidelines for implementing sprayed UHPC technologies. This study offers pivotal insights into the technological evolution of sprayed UHPC systems, thereby expanding their applicability in specialized domains such as tunneling and underground construction.  
Z.W.: Writing--original draft, Formal analysis, Data curation, Conceptualization. X.T.: Writing--review & editing, Methodology, Data curation, Conceptualization. J.Y.: Writing--review & editing. C.W.: Writing--review & editing, Conceptualization. Y.L.: Project administration. All authors have read and agreed to the published version of the manuscript.  
The authors gratefully acknowledge support provided by the National Natural Science Foundation of China (Grant No. 52279119, 42293355, 51991392); the Science and Technology Planning Project of Xizang Autonomous Region, China (Grant No. XZ202201 ZY0021G, No. XZ202401 ZY0085); and the Youth Innovation Promotion Association CAS.  
Not applicable.  
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.  
The authors declare no conflict of interest.