Mechanical Properties and Microstructure of Bentonite-Cement Solidified Sludge

Introduction: River dredging generates large volumes of high-water-content sludge, and reuse-oriented treatment can reduce disposal pressure and environmental risk. This study quantifies the influence of bentonite dosage on the strength, deformability, and pore structure of cementstabilized dredged silty sludge, and examines its field applicability at a groundwater-sensitive metro portal. Methods: Specimens were prepared with 6–25% cement and 0–12% calcium bentonite. Unconfined compressive strength (UCS), failure strain, and secant modulus E50 were measured at 7, 28, and 90 days. The 28-day microstructural interpretation was based on SEM, XRD phase-related features, and mercury intrusion porosimetry (MIP) pore metrics. Engineering verification was conducted using 28-day cores from a triple-axis mixed portal zone, and a coupled seepage-deformation model was calibrated using the measured UCS and permeability. Results: UCS increased with bentonite content to an optimum at 9% and declined at higher contents; the 7-day UCS reached approximately 45% of the 90-day value, and the 28-day UCS exceeded approximately 75%. MIP showed reduced accessible pore volume at moderate bentonite contents, consistent with gains in UCS and E50, while SEM indicated a denser and more continuously cemented fabric. In the field verification, the 28-day core UCS was 0.6–1.0 MPa in weak reinforcement zones and >1.0 MPa in strong reinforcement zones, with a maximum value of 1.48 MPa. Permeability was on the order of 10−6 cm/s, with a minimum value of 3.27 × 10−8 cm/s. The coupled seepage-deformation model reproduced the monitored settlement trend, with discrepancies typically within 30%. Discussion: The results indicate that moderate bentonite addition can refine the pore structure and improve the mechanical performance of cement-stabilized dredged silty sludge. The consistency between the laboratory observations and field verification suggests that bentonite contributes positively to both strength development and seepage control in groundwater-sensitive reinforcement applications. Conclusion: Overall, 9% bentonite provides a practical balance between pore refinement and mechanical performance, and the combined laboratory and field evidence supports its use for portal reinforcement where seepage control is critical.