On the use of recycled UHPC to reduce cement demand in UHPC mixes: mechanical and durability validation

Ultra-High Performance Concrete (UHPC) offers superior durability and strength, as compared to ordinary concrete solutions, but its inborn environmental footprint is dictated by high cement content and the environmental impact of raw material extraction, which would require a heavily optimized structural and process design to be levelled off. With the aim of assessing effectiveness of strategies aimed at reducing the embodied carbon footprint of UHPC mixes, this study investigates two recycled UHPCs (R-UHPC) designed by replacing all natural aggregates with crushed UHPC and partially substituting Portland cement (30 %) with recycled material. One mix used ungraded crushed UHPC; the other included additional fine fractions (≤75 μm) obtained through further processing. The partial replacement of cement constitutes a novelty alongside the widely established aggregate replacement in high performance cementitious materials. Both mixes achieved superior compressive strength and comparable flexural strength to the reference UHPC, while demonstrating effective autogenous self-healing under repeated NaCl exposure, with full recovery of crack sealing, sorptivity, strength, and stiffness over six months. However, the mix containing recycled fines showed reduced performance under repeated damage-healing cycles, mainly attributed to its higher water absorption. A cradle-to-gate life cycle assessment using the CML-IA method and a functional unit of 1 m3 – subsequently normalized over long-term compressive and flexural strength – revealed that mechanical performance strongly influences environmental impacts. Notably, the additional processing and increased input volumes required for fine fractions led to higher impacts across all categories. This work lays the foundation for a rational and engineering-wise effective promotion of the circular economy concept in the design and production of highly durable cement-based materials and structures by demonstrating unexplored and effective recycling strategies for UHPC elements at the end of their service life, facilitated by their unaltered condition even after prolonged use.