A Study on the Pore and Strength Characteristics of an Eco-Friendly Sprayed Ultra High Performance Concrete with Manufactured Sand

Conventional shotcrete systems face critical limitations in adverse geological environments, including a delayed strength development (<5 MPa at 3 h), excessive rebounds (15–25%), and permeable macropore networks (>50 μm), often resulting in support failure for deeply buried tunnels. To address these challenges, this study systematically investigates the mechanical properties and pore characteristics of a manufactured sand-based sprayed UHPC at different spraying positions under simulated tunnel conditions. Our results demonstrate that the high-pressure air (0.8 MPa) driven spraying process optimizes its pore distribution, reducing large pores (>10 μm) and increasing harmless pores (<100 nm). Furthermore, the sprayed UHPC incorporating manufactured sand derived from tunnel slag not only maintains a 28-day compressive strength of 110.9 MPa but also reduces material costs and enhances sustainability. Field tests validate its low rebound rate (<5%) and rapid strength development (achieving a compressive strength of 30 MPa within 1 day), confirming its adaptability to complex geological conditions such as high-stress zones, thereby providing a novel method for support in complex geological conditions.