Abstract: This study addresses the challenge of disposing of mud waste generated by underground engineering projects in soft-soil regions. A solid alkali-activated geopolymer based on slag and fly ash was used to stabilize mud with an initial moisture content of 80%, thereby converting it into recycled fine aggregates. First, at the macroscopic scale, this study evaluated the effects of key geopolymer mixture parameters—including precursor proportion, precursor content, the molar ratio of solid sodium silicate, alkali-activator content, and the water-to-precursor ratio—on the pore distribution, connectivity, and particle-packing behavior of the recycled fine aggregates. These findings provide a theoretical basis for their lightweight engineering application of these aggregates. Subsequently, drop-hammer impact tests were conducted to investigate how these parameters affect the impact-breakage behavior of the aggregate fills. The analysis focused on post-impact changes in particle-size distribution, fractal dimension, and relative breakage index. At the microscopic level, scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to characterize the micromorphology and chemical composition of individual recycled fine-aggregate particles. These observations clarified the mechanisms governing the impact-breakage behavior of recycled fine aggregates with different mixture designs and established a cross-scale correlation between macroscopic performance and microstructure. The results show that although all trial fills satisfy the lightweight requirements for protective cushion layers (bulk density <1 200 kg/m3), their basic physical properties and impact-breakage behavior differ significantly. These differences are governed primarily by the type of geopolymer gel formed and by the morphology and spatial distribution of the pores. Notably, when the precursor contains 90% slag and 10% fly ash at a dosage of 30%, together with solid sodium silicate at a molar ratio of 0.8 and an alkali-activator content of 15.85%, direct dry mixing with mud at an initial moisture content of 80% yields recycled fine aggregates with excellent physical properties and outstanding resistance to impact breakage. The findings of this study identify a promising granular material for the protective cushion layers of underground structures. They also promote the high-value utilization of mud resources and demonstrate significant engineering and environmental benefits.

Key words: geopolymer, mud, recycled fine aggregate, basic physical properties, impact-breakage characteristics, macro–micro correlations