Green innovation: Clean conversion of shield-discharged waste soil and industrial solid waste into environmentally friendly high-performance flowable fill

To address the high resource consumption and environmental burden of traditional fill materials, this study presents an environmentally friendly flowable fill using shield-discharged waste soil (SDWS), ground granulated blast-furnace slag (GGBFS), and fly ash as the main raw materials activated by a NaOH–Na₂SiO₃ composite. Microstructural analysis, performance testing, and life cycle assessment (LCA) were conducted to systematically evaluate the reaction mechanisms, engineering properties, environmental impacts, and economic viability of the developed material. The results show that the prepared flowable fill has excellent workability and mechanical properties, with an initial flowability of 602–755 mm and a 28-day compressive strength of 6.43–8.98 MPa. The aggregate-to-binder ratio and alkali equivalent exert a more significant effect on the compressive strength than the liquid-to-solid ratio and activator modulus do. Microstructural analysis indicates that GGBFS and fly ash, under alkali activation, synergistically form dense C–S–H and C–A–S–H gels, improving the properties of the material. The LCA results reveal that the flowable fill achieves carbon dioxide equivalent (CO₂-eq) emissions of 136–214 kg CO₂-eq/m³, an eco-strength efficiency (ESE) of 0.038–0.053 MPa/(kg CO₂-eq·m⁻³), and a cumulative energy demand (CED) of 1095–1537 MJ/m³. Both the ESE and CED metrics outperform those of conventional high-strength ordinary Portland cement-based controlled low-strength materials (CLSMs). The raw material cost is 224–307 CNY/m³, and the use of SDWS further reduces costs. This study provides an innovative and sustainable solution for flowable fill, supporting the green transformation and sustainable development of the construction industry.