Mechanical behavior of crushed and compacted solidified dredged sludge solidified by industrial by-products

The disposal of dredged sludge (DS) has become a global issue, while the demand for embankment construction fill materials increases. To address the challenges associated with solidified DS (SDS) in embankment construction, the utilization of crushed and compacted solidified DS (CCSDS) was proposed. In this study, a novel curing agent (GCP), composed of industrial by-products including ground granulated blast furnace slag (GGBS), calcium carbide slag (CS), and phosphogypsum (PG), was employed to solidify high water-content DS. A series of unconfined compressive strength (UCS) and direct shear tests were performed to investigate the impacts of curing agent amount, leaving time, initial water content of DS, and subsequent curing time on the mechanical behaviors of CCSDS. The strength of CCSDS was compared to that of SDS. Additionally, scanning electron microscopy, X-ray diffraction and low field nuclear magnetic resonance were used to analyze the microstructures of representative samples. Results indicated that the solidification effect of GCP significantly surpassed that of slag Portland cement (SPC) in DS with high water content. After 28 days, the UCS of GCP-solidified DS was approximately 4.0–7.5 times that of SPC-solidified DS. When the UCS of SDS was below 600 kPa, the UCS of CCSDS and the strength reduction factor increased as SDS strength increased. Conversely, the UCS of CCSDS remained around 200 kPa when the UCS of SDS exceeded 600 kPa, with a notable decline in the strength reduction factor as SDS strength increased. Moreover, CCSDS with shorter leaving times and higher amounts of curing agent demonstrated more pronounced strength increases over the subsequent curing periods, with the final UCS reaching 40–70 % of that of SDS. For the shear strength parameters, cohesion increased linearly with higher amounts of curing agent, longer leaving times, and extended subsequent curing periods. Nevertheless, the internal friction angle initially increased and then stabilized as leaving and curing times were prolonged with a fixed amount of curing agent. Furthermore, a logarithmic growth relationship was observed between the cohesion and UCS, whereas a bilinear relationship was noted between the internal friction angle and UCS. Microstructural analysis revealed that CS and PG effectively activated the GGBS, thus enhancing the hydration reaction and promoting the formation of AFt and C-(A)-S-H gels, which significantly improved the strength of high water-content SDS. Additionally, the strength reduction in CCSDS is primarily due to the loss of structural integrity in SDS after crushing and compaction, while the strength development in CCSDS during the subsequent curing periods is attributed to ongoing hydration and pozzolanic reactions.