Degradation behavior of crushed-compacted sludge solidified with industrial by-products and reinforced with waste pulp fibers under wet-dry cycling

Crushed-compacted solidified sludge (CCSS) is a novel fill material produced by first chemically solidifying dredged sludge to a target strength, followed by crushing and compaction. A systematic investigation into the wet-dry durability of CCSS and the corresponding mitigation strategies is of great significance for its engineering application. In this study, CCSS specimens solidified with an industrial by-product-based curing agent (GCP, composing ground-granulated blast-furnace slag, calcium carbide slag, and phosphogypsum), as well as GCP combined with waste pulp fibers (WPF), were subjected to cyclic wetting and drying tests, with ordinary Portland cement (OPC)-solidified CCSS serving as a comparison. Variations in macroscopic appearance, mass and volume, unconfined compressive strength (UCS), splitting tensile strength (STS), deformation modulus (E 50 ), and fracture energy (W), along with microstructural evolution characterized by X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), and low-field nuclear magnetic resonance (LF-NMR), were analyzed to elucidate the degradation behavior of GCP-CCSS and the reinforcing mechanisms of WPF under wet-dry cycling. Results showed that GCP-CCSS exhibited lower mass and volume losses than OPC-CCSS under wet-dry cycling, and these losses were further mitigated by the incorporation of WPF. Throughout the cycling process, the UCS, E 50 , STS, and W of GCP-CCSS remained obviously higher than those of OPC-CCSS, while both mechanical stability and fracture energy were effectively enhanced by fiber reinforcement. Microstructural analyses revealed that GCP-CCSS initially contained abundant C-(A)-S-H gel and ettringite (AFt), which partially dissolved and disintegrated after 10 cycles, resulting in pore structure coarsening. In contrast, the incorporation of WPF effectively delayed microcrack initiation and propagation by enveloping soil aggregates and redistributing stress through interfacial friction among fibers, soil particles, and hydration products, thereby mitigating the detrimental effects of wet-dry deterioration.