Study on semi-dynamic leaching and microstructure characteristics of MSWI fly ash solidified sediment

municipal solid waste incineration (MSWI) fly ash partially replaces cement to solidify sediment, and then can be used as intermediate cover materials in landfill as one of the resources utilization ways of MSWI fly ash and sediment. The strength and the semi-dynamic leaching characteristics of MSWI fly ash solidified sediment under hydrochloric acid attack at different pH were studied by means of unconfined compressive strength (UCS), semi-dynamic leaching, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Thermogravimetric analysis (TGA). Results revealed that the UCS strength increased as the curing age and cement content increased. When the curing content is 50% and the replacement ratio of MSWI fly ash is 75% and 80%, the UCS of 7 d can be greater than 50 kPa. The primary contribution to the strength development was from silicic acid gels such as calcium silicate hydrate (C-S-H) and carbonates. Notably, the leaching behavior of Zn and Cu within the solidified sediment underwent substantial alterations. The leaching amount of heavy metals in a strong acidic environment (pH = 2) is significantly greater than that in a weak acidic (pH = 4) and neutral (pH = 7) environment. Conversely, minimal disparities were observed in the leaching characteristics of Zn and Cu between the weakly acidic and neutral environments. Ca(OH)₂, C-S-H and carbonate exhibits a remarkable acid-resistant buffering capacity in the solidified sediment. The obvious diffusion coefficient (D^obs) was less than 10^-9 m²/s in semi-dynamic leaching tests. Moreover, the mobility of Zn and Cu surpassing 12.5, coupled with a leaching index exceeding 8, further attests to the favorable S/S outcome achieved. Based on these findings, the solidified material is confidently recommended to be used as suitable landfill middle soil cover material.