Mudflat sediments can pose dual challenges of engineering diseases and pollution risks due to unfavourable mechanical performances and potential heavy metal enrichment, impacting coastal engineering construction, ecological environments, and human health. Commonly used Portland cement has significant restrictions in ensuring mechanical stability and environmental sustainability during the remediation of heavy metalcontaminated mudflats. This study investigates a novel approach using chitosan-enhance alkali-activated geopolymer (CS-AGP), composed of slag, fly ash, and desulphurised gypsum, for solidifying/stabilizing highly toxic and concentrated Cu-/Cr(VI)-polluted sediments. The unconfined compressive strength, durability, and leaching toxicity of these sediments are assessed across varying binder incorporations, contamination concentrations, curing periods, and dry-wet cycles. The results demonstrate that the CS-AGP remarkedly increases both early and long-term strength as well as environmental stability of Cu-/Cr(VI)-polluted sediments, even after suffering serious dry-wet alternations and pollutant accumulation, far surpassing the USEPA strength criterion (0.35 MPa) for safe landfill and suiting for in-situ engineering applications. Moreover, the CS-AGP solidified/stabilized contaminated sediments exhibit excellent acid resistance and minimal environmental risk and leaching concentrations meet Cu <= 1.5 mg/L and Cr(VI) <= 0.1 mg/L, as these metals primarily redistribute to the residual fraction. Microstructure evolution reveals CS-AGP generates significant amounts of calcium silicate hydrate, calcium aluminium silicate hydrate, and ettringite to compact sediment skeleton structures, which is the improvement source of mechanical performance. Simultaneously, the comprehensive physical encapsulation, chemical bonding, and coordination effects promote the transformation of Cu/Cr(VI) into a low availability state. The study offers new insights for efficient remediation and safe development of coastal mudflats.

To address the challenges posed by the significant quantity of ammonia-alkali white mud, this study explores the preparation of fluid solidified soil using ammonia-alkali white mud, mineral powder, and fly ash. The findings reveal that ammonia-alkali white mud primarily comprises sulfate, carbonate, and soluble chloride salt, with an alkaline solution and a well-developed pore structure. Optimal fluid solidified soil formulation, comprising 30% white mud, 30% salt mud, 25% mineral powder, 10% fly ash, and 5% calcium oxide, yields a slurry fluidity of 176 mm and a compressive strength of 3.98 MPa at 28 days. Microscopic analysis highlights AFt and C-S-H gel as the principal hydration products of fluid solidified soil. The fine particles of calcium carbonate in ammonia-alkali white mud fill the structural pores and intertwine with the hydration products, facilitating the formation of a dense structure, which constitutes the primary source of strength in fluid solidified soil. Furthermore, the heavy metal content of the solidified soil aligns with the first type of land use requirements outlined in the GB 36600-2018 standard, and the toxicity of the leaching solution adheres to the emission concentration limit stipulated by GB 8978-1996.

Municipal solid waste incineration fly ash (MSWIFA) can be reused as a positive additive to strengthen soft soil. In this study, MSWIFA was initially used as a supplementary solidification material in combination with ordinary Portland cement to prepare fly ash cement-stabilized soil (FACS) with silty sand and silty clay, respectively. The ratio of MWSIFA to total mass was 5%, 10%, and 15%, and the cement content was set as 10% and 15%. The mechanical properties of FACS were evaluated by unconfined compressive strength test. The heavy metal-leaching test was conducted to estimate the environmental risk of FACS. The scanning electron microscope was used to test the micro-structure of FACS. The X-ray diffraction was performed to analyze material composition of FACS. The result indicates that the collaborative solidification of soft soil with MSWIFA and cement is feasible. Regarding the silty clay, the FA had positive effects on the silty clay in the service age (between 50 and 100% with 15% MSWIFA), as the MSWIFA reformulated the initial silty clay structure, resulting in interconnection and pore fill between particles. It can be founded that C-S-H and ettringite are the main products of MSWIFA and cement hydration, which are formed by the hydration of C3S and C2S. Regarding the silty sand, the MSWIFA decreased the peak strength (between 35 and 48% with 15% MSWIFA) but increased the ductility of the stabilized cement. Under the same mix proportions, the leaching toxicities of Zn and Pb in FACS of silty clay were obviously lower than were those of silty sand. Generally, the leaching concentrations of tested metals under all the mix proportions were well below the limit value set by GB 18598-2019 for hazardous waste landfill. Thus, the reuse of MSWIFA in cement-stabilized soil would be one of the effective methods in soft soil treatment and solid waste reduction.