Ground granulated blast furnace slag (GGBS), calcium carbide slag (CS), and phosphogypsum (PG) were combined in a mass ratio of 60:30:10 (abbreviated as GCP) to solidify dredged sludge (DS) with high water content. The long-term strength characteristics of solidified DS under varying curing agent dosage and initial water contents, as well as its durability under complex environmental conditions, were investigated via a series of mechanical and microstructural tests. The superior performance of GCP-solidified DS (SDS-G) in terms of strength and durability was demonstrated in comparison to solidified DS using ordinary Portland cement (SDS-O). The results indicated that the unconfined compressive strength (UCS) of SDS-G was approximately 3.0-4.5 times greater than that of SDS-O at the same dosage and curing ages, exhibiting a consistent increase in strength even beyond 28 days of curing. Additionally, the strength and deformation modulus (E50) of SDS-G increased initially and then decreased during wet-dry cycles, with reductions in mass, volume, and strength significantly were smaller than those observed in SDS-O. Furthermore, the reductions in UCS and E50 induced by freeze-thaw cycles were considerably smaller for SDS-G than for SDS-O, with strength losses of 50.7 % and 88.3 %, respectively, after 13 freeze-thaw cycles. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses revealed that the enhancements observed in SDS-G were attributed to the formation of ettringite (AFt), which effectively fills larger pores between agglomerated soil particles, thereby creating a denser and more stable microstructure in conjunction with hydrated calcium aluminosilicate (C- (A)-S-H) gels.

Recently, the disposal of high-water-content dredged sludge through geotextile tubes with vacuum-assisted prefabricated horizontal drains (PHDs) has gained growing popularity for its convenience and efficiency. However, existing simulations for this typical coupled filtration-consolidation process using conventional consolidation models exhibit deficiencies due to the absence of effective stress and invalidity of Darcy's law in the particle-wandering filtration phase. In this study, based on the compressional rheology theory, a two-dimensional coupled filtration-consolidation model constituted by the compressive yield stress Py(phi) and hindered setting factor r(phi) is developed to elucidate the solid-solid and solid-fluid interactions during slurry dewatering. A novel approach for measuring consistent Py(phi) and r(phi) relationships is proposed. The numerical solution is derived utilizing the alternating direction implicit difference method and verified against a classical one-dimensional compressional rheology model and a field trial. Further analysis suggests that constitutive parameters, e.g., gel point, affect the dewatering efficiency by determining the relative degree of soil disorder, obstruction to particle movement, and vacuum transmission effect, while design parameters, e.g., PHD spacing and tube height, impact the magnitude and radiation range of vacuum pressure to influence the overall efficiency.

Hundreds of millions of tons of dredged sludge are generated by waterway dredging worldwide every year. Traditional disposal of dredged sludge, such as in-situ stockpiling and offshore dumping, cannot avoid the waste of land resource and the pollution to marine environment. Sludge stabilization/solidification treatment currently used can achieve the reuse of drudged sludge but requires large investment and time. Therefore, how to turn waste into treasure in an effective, environmentally friendly and cheap way is a notable problem. In this study, the variation of strength of solidified sludge cured in air with water-cement ratio, water content and curing time by unconfined compression test was investigated, and the inner mechanism of strength influenced by watercement ratio and water content was revealed by XRD test, which offered an optimal working condition. Also, solidified sludge with the maximum strength in the optimal working condition was immersed into seawater at different times, which showed the 7d strength after mixing completion for 8 h immersed into seawater could reach 20.60 MPa (1.37 times of the strength in air), and the prediction formulas considering all the parameters mentioned above were established. At last, a field test of solidified dredged sludge for protection of submarine pipelines was carried out in Bohai Bay, China, which demonstrated the feasibility of mixing dredged sludge with cement on board and solidifying in seawater environment. Compared to the traditional subsea pipeline protection solutions, the cost of using solidified sludge to protect subsea pipelines is 25 % and 39 % less than the cost of using sandbags and concrete mats, respectively. This study provides a more economic and environmentally friendly idea for dredged sludge treatment and subsea pipeline protection than the conventional methods, which provides a new source of green ocean building materials, reduces the pollution of the marine environment by the discharge of dredged sludge, turns waste into treasure and has wide applications in ocean engineering.

This study proposed an improved bio-carbonation of reactive magnesia cement (RMC) method for dredged sludge stabilization using the urea pre-hydrolysis strategy. Based on unconfined compression strength (UCS), pickling-drainage, and scanning electron microscopy (SEM) tests, the effects of prehydrolysis duration (T), urease activity (UA) and curing age (CA) on the mechanical properties and microstructural characteristics of bio-carbonized samples were systematically investigated and analyzed. The results demonstrated that the proposed method could significantly enhance urea hydrolysis and RMC bio-carbonation to achieve efficient stabilization of dredged sludge with 80% high water content. A significant strength increment of up to about 1063.36 kPa was obtained for the bio-carbonized samples after just 7 d of curing, which was 2.64 times higher than that of the 28-day cured ordinary Portland cement-reinforced samples. Both elevated T and UA could notably increase urea utilization ratio and carbonate ion yield, but the resulting surge in supersaturation also affected the precipitation patterns of hydrated magnesia carbonates (HMCs), which weakened the cementation effect of HMCs on soil particles and further inhibited strength enhancement of bio-carbonized samples. The optimum formula was determined to be the case of T = 24 h and UA = 10 U/mL for dredged sludge stabilization. A 7-day CA was enough for bio-carbonized samples to obtain stable strength, albeit slightly affected by UA. The benefits of high efficiency and water stability presented the potential of this method in achieving dredged sludge stabilization and resource utilization. This investigation provides informative ideas and valuable insights on implementing advanced bio-geotechnical techniques to achieve efficient stabilization of soft soil, such as dredged sludge. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

In recent years, dredging projects in rivers and lakes have generated large volumes of sludge that exhibit high water content and low permeability. This dredged sludge needs to be treated quickly to reduce its volume, thus reducing transportation costs and environmental impacts. Flocculation combined with electroosmotic vacuum preloading is a new technology for dewatering sludge. At present, the influence of flocculants on the electrokinetic properties of sludge has not been thoroughly studied, and composite forms of these have not been applied in electroosmotic vacuum precompression. Therefore, inorganic flocculant and organic flocculant were combined to form a composite flocculant, which was used in electroosmosis vacuum preloading to increase the water discharge effect of dredged sludge. Based on analyzing the mechanism of flocculants, two kinds of composite flocculants, PAC-APAM and FeCl3-APAM, were configured, and the optimal ratio of inorganic flocculant and organic flocculant in the composite flocculant was determined with a settling column test. The properties, including pore size distribution, electric conductivity, and electroosmotic permeability coefficient, of the sludge mixed with flocculants PAC, FeCl3, APAM, PAC-APAM, and FeCl3-APAM were analyzed by NMR tests and Miler Soil Box tests. In addition, a model test of electroosmotic vacuum preloading treatment of dredged sludge was conducted, and the influences of different types of flocculants on the surface settlement, water discharge, current, pore water pressure, and shear strength of the sludge were compared and analyzed. The results showed that the composite flocculants suggested in this study were able to increase the electroosmotic permeability coefficient of sludge more significantly than single-type flocculants. In situations where electroosmotic vacuum preloading combined with composite flocculants was used to treat sludge, water discharge, and consolidation settlement were larger, and the shear strength of the treated sludge was higher. Compared with FeCl3-APAM, PAC-APAM showed better performance in electroosmotic vacuum preloading.

Carbonation stabilization presents an efficacious and eco-friendly soil stabilization method with extensive applicability. Because market costs play a crucial role in determining the recycling of dredged sludge, this research investigates the potential use of solid waste steel slag as a cost-effective alternative stabilizing agent. The objective is to promote the wider adoption of carbonation stabilization technology for the reuse of dredged sludge. This investigation examined the alteration patterns of unconfined compressive strength and CO2 uptake in steel slag carbonation stabilized soils (CS) under varying steel slag contents, stabilization moisture contents, and carbonation durations. Furthermore, a detailed analysis of micro-mineral composition and micro-pore structure evolution during the carbonation process was conducted employing X-ray diffraction (XRD), thermogravimetric and derivative thermogravimetric analysis (TG-DTG), and mercury intrusion porosimetry (MIP). The outcomes revealed a positive linear association between the unconfined compressive strength of CS specimens and their CO2 uptake across different steel slag contents and carbonation durations. Through XRD and TGDTG assessments, the carbonation byproducts in CS specimens were identified as calcite-type CaCO3, with an increasing concentration of CaCO3 correlating with the increase of steel slag content and carbonation duration. MIP testing revealed that a 72-hour carbonation period could diminish the total porosity of steel slag dredged soils from 40.96% to 34.53%. The strength of CS specimens predominantly derives from the carbonation reaction of steel slag, which enhances the soil's mechanical attributes through the cementing and pore-filling impacts of CaCO3 crystal growth on the soil matrix.