<p>This study investigates the stabilization of Sludge using a hybrid biogelation method with sodium alginate (SA) and cement. The optimal SA dosage was determined through unconfined compressive strength (UCS) and consolidated undrained triaxial (CU) tests. Subsequently, CU tests under varying numbers of wetting-drying cycles and confining pressures were conducted to analyze the stress-strain behavior of the stabilized soil. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses were employed to examine the changes in pore structure, mineral composition, and microstructure, elucidating the micro-mechanisms of the SA-cement stabilization. Results indicate that the compressive strength of the stabilized soil initially increases and then decreases with increasing SA dosage, while continuously rising with curing age. The combination of 10% SA and 8% cement yielded the best stabilization performance. The stress-strain curves shifted from strain-softening to strain-hardening types with increasing SA content, and the peak shear strength increased progressively. After six wetting-drying cycles, the strength reduction of the stabilized soil was lower than that of cement-stabilized soil, with significantly reduced water absorption and mass loss rates, demonstrating superior resistance to wetting-drying cycles. SEM observations revealed that the pores and cracks in the stabilized soil were filled with gel when SA was added, resulting in a more compact structure. XRD analysis showed no new mineral phases, but the diffraction peaks became flatter, indicating reduced crystallinity. These findings provide insights into the mechanical properties of SA-cement soil.</p>