Preliminary investigation of high-water content dredged sediment treated with chemical-physical combined method at low cement content.

Land reclamation projects are increasingly incorporating dredged sediment from waterways. The high water content of dredged sediment is a major issue, making the dewatering process difficult and time-consuming. The chemical-physical combined method (CP) is therefore used in this study, which simultaneously uses vacuum dewatering by utilising vacuum pressure (VP) in conjunction with prefabricated horizontal drain (PHD) and Portland cement (PC)-based solidification/stabilisation (SS), thereby significantly reducing the duration of treatment of DS with high water content. The effectiveness and feasibility of the chemical-physical combined method with Portland cement (PC) as a binder are evaluated and compared with the traditional PC-based solidification/stabilisation (SS) method. A number of experimental tests were performed to accomplish the objectives of the study, such as unconfined compressive strength (USS), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The experimental results indicated that the CP method showed better performance compared to the traditional SS method in treating high water content DS at low cement content. The water content of DS treated with the chemical-physical combined method was reduced by half in just about 3 days, and the final rate of settlement was 2.9 times higher than with SS-treated DS. The USC results showed that the strength of CP cases was 4.8 times higher than SS-treated DS after 56 days of curing age. The microstructural tests revealed the development of CSH and CASH as major hydration products of both CP and SS cases. Moreover, CP cases exhibited a densely stabilised matrix compared to SS cases.

Assessing long-term performance of stabilized Zn-contaminated dredged sediment slurry treated with the PHDVPSS method.

Solidification/stabilization (S/S) has been widely used for effective treatment of dredged sediment (DS) for many years, with the objective of improving the mechanical properties of DS through the addition of inorganic cementitious materials. Most previous studies have reported the short-term performance of S/S. However, long-term effectiveness is critical, as contaminants remain underground and are subjected to a variety of environmental stresses that can degrade S/S materials. In this regard, this experimental work investigated the long-term efficacy of solidification/stabilization of dredged contaminated sediments (DCS) treated with a new integrated method, namely PHDVPSS, which uses a prefabricated horizontal drain (PHD) assisted by vacuum pressure (VP) as well as solidification/stabilization. The DCS were treated with Portland cement (PC) as binder in the PHDVPSS method (abbreviated as VP-PC) at different zinc (Zn) concentration levels and compared with the traditional cement-based solidification/stabilization method (abbreviated as SS-PC). A series of experimental tests such as unconfined compressive strength (UCS), toxicity characteristics leaching procedure (TCLP), X-ray diffraction (XRD) and scanning electron microscopy in conjunction with energy-dispersive spectroscopy (SEM-EDS) were performed to assess the long-term strength, leaching and microstructural characteristics of high-water-content DCS, respectively. The UCS test results indicated that the strength of VP-PC mixes increased significantly with curing time compared to the limited strength development of SS-PC mixes. After 180-day curing, VP-PC mixes exhibited 3.5-5.5 times higher UCS values than the SS-PC mixes. Furthermore, when compared to the SS-PC mixes, the VP-PC mixes had 14.7-36.4% lower leached Zn concentrations at different Zn levels. This is attributed to an increase in the least reactive F4 (residual) fraction and a decrease in the most mobile F1 (acid-soluble) fraction as confirmed by the BCR method. Microstructural tests including XRD and SEM-EDS revealed that calcium silicate hydrate-like compounds were identified as the main hydration products of both the VP-PC and SS-PC mixes. However, portlandite, a major hydration product of PC, was not detected in either case, which is attributed to the retardant effect of Zn on cement hydration. Overall, the experimental results showed that the PHDVPSS method, when compared to the conventional solidification/stabilization method, is a viable choice for treating high-water-content DCS at different Zn concentration levels with low cement content.