Microstructure and mechanical properties of controlled low-strength materials with recycled coarse aggregate and metro shield spoil for backfill applications

In this study, a sustainable approach is proposed for reusing metro shield spoil (MSS) by the incorporation of with demolition and renovation waste (DRW) to produce controlled low-strength material (CLSM) that complies with established engineering standards. With an orthogonal experimental design, the effects of various DRW content on the workability and mechanical properties of CLSM are systematically investigated. The findings demonstrate that the inclusion of DRW can remarkably enhance the particle distribution, leading to improvements in flowability and compressive strength. Particularly, the flowability of mixture can be increased from 155 mm to 230 mm, and the 28-day compressive strength reaches 1.81 MPa. Microstructure observation reveals that the introduction of DRW can bring about the change of pore structure, resulting in a more refined and optimized matrix. Additionally, a higher presence of calcium-silicate-hydrate (C–S–H) gel and ettringite can be detected, which is attributed to the sulfate content in DRW. This sulfate-induced formation leads to an increase in strength, further validating the suitability of DRW-modified MSS as a promising, eco-friendly solution to produce CLSM. This work provides the potential of this innovative material as a viable, sustainable construction solution to address both waste recycling and performance optimization in civil engineering applications.