An innovative sustainable solution: Recycling shield-discharge waste soil as fine aggregate to produce eco-friendly geopolymer-based flowable backfill materials

The disposal of Shield-Discharge Waste Soil (SDWS) is substantial, yet their recycling rate remains low, necessitating the exploration of new recycling methods for effective waste management. This study examines the potential use of SDWS as a fine aggregate in the production of Controlled Low-Strength Material (CLSM), incorporating slag and fly ash as precursors. Key properties such as strength, flowability, setting time, microstructure, chemical composition, CO₂ emissions, energy consumption, and cost were assessed and analyzed. Additionally, the environmental impact and material-related strategies were discussed. The results reveal that the developed CLSM exhibits competitive performance, with compressive strengths ranging from 2.830 MPa to 4.121 MPa, achieving over 1.0 MPa strength within 24 hours. The material demonstrates high flowability, exceeding 200 mm within 30 minutes, and has a setting time between 2.10 and 4.23 hours, offering advantages in both setting time and early strength. SDWS contributes to extending the coagulation process and enhancing the flowability. Optimal strength is observed when SDWS constitutes approximately 30 % of the binders or when the alkali equivalent is around 7 %. Compared to traditional cement-based CLSMs, incorporating SDWS results in reduced CO₂ emissions and lower energy consumption. When considering savings from reduced waste disposal costs, the overall material cost remains competitive. Furthermore, higher SDWS content leads to enhanced environmental benefits, and it is recommended to keep the alkali equivalent below 7 % for optimal performance. In conclusion, the developed CLSM presents significant potential for wide-scale applications and offers a sustainable solution for recycling SDWS.