Development of excavatable and eco-friendly controlled low-strength material fully utilizing recycled fresh concrete waste and by-products

The recycling of fresh returned concrete (RECON) waste from ready-mixed concrete production represents a key challenge in advancing sustainable construction practices. This study presents a novel, eco-friendly controlled low-strength material (CLSM) produced entirely from fresh RECON waste materials and an industrial by-product binder, contributing to sustainable construction by diverting waste from landfills, eliminating the use of virgin materials, and supporting circular-economy practices. A four-stage experimental program was conducted to (i) optimize recycled aggregate content, (ii) assess partial replacement using concrete sludge powder (CSP), (iii) determine the optimal binder dosage, and (iv) evaluate the effects of a super-retardant admixture. The results demonstrate that replacing up to 20% of recycled fine aggregate with CSP enhances mixture stability by reducing bleeding and improving strength. A comprehensive re-excavatability assessment, based on 28-day compressive strength, removability modulus, and long-term strength development at 56 and 91 days, identified 40 kg/m³ as the optimal binder content. Increasing the super-retardant admixture dosage effectively delayed hardening and improved workability without increasing bleeding. Durability evaluation under twelve wet–dry cycles showed adequate resistance to degradation, with a mass loss of 11.77% and a residual compressive strength of 206.49 kN/m², exceeding the minimum requirement of 200 kN/m² for buried pipe backfilling. Environmental safety was confirmed by a hexavalent chromium leaching concentration of 0.007 mg/L, well below the regulatory limit of 0.05 mg/L. • Effective utilization of returned fresh concrete promotes waste minimization. • The optimal binder content was 40 kg/m 3 , according to re-excavability criteria. • Super-retardant admixture delayed the hardening process. • Minimal leaching of hexavalent chromium is necessary for practical application.