Low-Carbon Valorization of Construction Spoil into High-Value Recycled Aggregates for Geopolymer Concrete

This study investigates the production of recycled aggregates (RAs) derived from construction spoil (CS) and their influence on the mechanical properties of geopolymer concrete. Two manufacturing routes, disc pelletization and crushing granulation, were employed to produce CS-based RAs. The resulting RAs were characterized in terms of particle size distribution and geopolymer compressive strength development. Geopolymer concretes incorporating disc-pelletized and crushed aggregates achieved 7-day compressive strengths of 31.0–32.5 MPa and 37.9–38.4 MPa, 21-day compressive strengths of 31.6–36.5 MPa and 40.8–41.5 MPa, 28-day compressive strengths of 36.9–37.1 MPa and 42.3–43.5 MPa, respectively. These results confirm the technical feasibility of using CS as a high-value RA resource in structural geopolymer concrete. At the same time, the approach offers environmental and economic benefits by reducing the reliance on conventional natural aggregates and lowering the associated carbon footprint. Compared with disc-pelletized RAs, crushed RAs exhibit superior performance in improving concrete compressive strength, which is attributed to their angular morphology and higher apparent density that enhance the overall structural integrity of the concrete matrix. In contrast, disc-pelletized RAs display higher porosity and smoother surfaces, which tend to induce stress concentration and thus reduce the mechanical performance of geopolymer concrete. Overall, the findings provide practical guidance for the valorization of construction spoil through RAs production. They demonstrate that crushed CS-derived RAs can effectively replace natural aggregates in structural concrete, thereby mitigating the impacts of aggregate mining and contributing to circular economy and low-carbon construction objectives.