Eggshell powder modified sustainable ultra-high performance fibre-reinforced concrete for transportation preservation and rehabilitation

Ultra-high-performance fibre-reinforced concrete (UHPFRC) exhibits exceptional strength and durability but utilises high cement volume, resulting in substantial CO 2 emission and high production cost. Eggshell powder (ESP), an abundant agro-industrial waste rich in CaCO 3 , offers a low-cost and low-carbon cement replacement that can promote hydration and microstructure development. However, the performance of ESP in UHPFRC, particularly with bulk replacement volume in structural repair application, remain unclear. This study investigates the macroscopic engineering performance and the microstructural mechanisms of UHPFRC, with different ESP replacement volumes. Mechanical tests (i.e., compressive strength, pull-off, slant shear) and microstructural characterisation (i.e., XRD, TGA, MIP, X-CT, SEM) were therefore conducted to elucidate the interfacial bonding behaviour and microstructure evolution, respectively. Results show that moderate ESP replacement levels (≤35%) can maintain compressive and bond strength while refining the pore structure; whereas any excessive replacement level will increase the porosity in UHPFRC matrix and reduce the interfacial integrity between the repair overlay and the old concrete substrate in bridges. This work provides foundational insight into use of high-volume ESP in high-performance cementitious systems, enabling the design of eco-efficient, durable repair materials for demanding structural applications.