A novel iron-based phosphate cement (IPC), derived from iron-rich smelting slag (ISS), was developed as a sustainable and efficient binder for the stabilization/solidification of trivalent chromium (Cr3+). The mechanical properties, hydration behavior, microstructure, leaching toxicity, chromium chemical forms, and environmental safety of chromium-stabilized iron phosphate cement (CIPC) were thoroughly evaluated. The results showed that, with a mass ratio of ISS to ammonium dihydrogen phosphate (ADP) of 2.0, and even with the addition of 20 % chromium nitrate nonahydrate (CN), the compressive strength of CIPC reached 4.2 MPa after curing for 28 d. Furthermore, chromium leaching was well below 1 mg/L, significantly lower than the GB 5085.3-2007 standard limit of 15 mg/L, demonstrating the effective encapsulation of Cr3+ due to IPC's high early strength. In the IPC system, Cr3+ was primarily stabilized by forming CrPO4 and CrxFe1-x(OH)3 co-precipitates, which were further solidified through the physical encapsulation of IPC hydration products, such as (NH4)2Fe(PO3OH)2 center dot 4H2O, (NH4) (Mg,Ca)PO4 center dot H2O, and FePO4. This process resulted in a solidification efficiency of up to 99 %. BCR analysis confirmed that more than 98 % of the chromium in the CIPC remained in a stable residual form. Finally, the ecological risk index (PERT) was found to be 23.52, far below the safety threshold of 150, indicating the solidified material's long-term environmental safety. This study provides an innovative approach for the reutilization of ISS while effectively stabilizing/solidifying chromium.

Deep cement mixing (DCM) is a popular in situ soil stabilization method, while the investigation on long-term coupled consolidation and contaminant leaching behavior of cement-stabilized contaminated soil is limited. In this study, axisymmetric physical model tests were conducted to investigate the coupled behaviors of a composite ground, which consisted of a central column made of cement-stabilized arsenic-contaminated marine deposits and surrounding untreated marine deposits. The test results revealed the settlement development of composite ground and the mechanism of load transfer between the DCM column and surrounding soils with increasing loading. The presence of arsenic decreased the strength and stiffness of the DCM column through the reaction between arsenic and hydration and pozzolanic reaction products. With the increase of the water/cement ratio in the DCM column, the concentration level of arsenic in the draining-out water of the composite ground increased significantly, while that in the surrounding soil showed no obvious change, indicating that arsenic mainly migrated directly through the DCM column. A theoretical axisymmetric consolidation model coupling solute transport for composite ground was established and subsequently applied to analyze the test data. The numerical model accurately depicted the pore water pressure, settlement, and spatiotemporal distribution of arsenic concentration in the physical model.

This study aims to evaluate the possibility of reusing treated marine clayey soils by stabilization/solidification (S/S) technology as geomaterial in reclamation projects from the aspects of engineering strength, chemical modification and environmental risk assessment. The lime-activated incinerated sewage sludge ash (ISSA) together with ground granulated blast furnace slag (GGBS) was employed as the binder. The multi-controlling factors including water content, curing time, salinity, and chemical compositions of mixing solution were taken into account to identify the S/S treated Hong Kong marine deposit (HKMD) slurry based on the strength tests, pH measurement, thermo-gravimetric (TG) analysis, X-ray diffractometer (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy coupled with energy-dispersive spectrometry (SEM-EDS) and toxicity characteristic leaching procedure (TCLP) tests, etc. The results show that the S/S treatment using lime-activated ISSA-GGBS can effectively enhance the strength of marine soil at the initial water content of 110% and 200%. The water content and curing time have a significant impact on the S/S treated HKMD. The pH of treated soils is higher than 11.1, which proves an alkaline environment for the reactions in the treated soil. A special case is the treated HKMD at 200% water content hydrated by MgCl2 solution, which has a low pH of 10.23 and maintains a slurry state. Based on the TCLP results, the leaching concentration of heavy metals from S/S treated HKMD is environmentally safe and meets Hong Kong standard for reusing treated soil with a low level of <0.2 mg/L. The content of main products such as calcium/magnesium silicate hydrate, ettringite or Friedel's salt depends on the chemical additions (e.g. distilled water, seawater, NaCl and Na2SO4). The products in the specimens mixed with MgCl2 solutions are mainly composed of Mg(OH)(2), M-S-H and MgCO3, which is distinct with the neoformations in the other cases. Therefore, this study proves that the S/S treated soil slurry could be reused as geomaterials in reclamation projects, and the S/S process is greatly affected by water content, curing time and solution compositions, etc. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published 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/).

To optimize the use of chlorine saline soils commonly found in many coastal areas, ground granulated blast furnace slag (GGBS) and calcium carbide residue (CCR) were used in this study to stabilize/solidify these soils. This study aims at evaluating the suitability of GGBS-CCR as industrial by-products in improving the mechanical behaviors of chlorine saline soil in comparison with the use of Portland cement (PC) as a traditional binder. The optimal proportion of the binder was determined by the unconfined compressive strength, conductivity, and leaching characteristics. Moreover, the water stability coefficients, collapse coefficients and microscopic characteristics of the solidified soil were evaluated. The results reveal that when the ratio of GGBS to CCR in the binder is 4:1, the 28-day unconfined compressive strength reaches 4.53 MPa, and the leaching of chloride ions is reduced by 94.1 %. The excellent water stability and reduced collapsibility further indicate that GGBS-CCR is a preferable binder for solidifying saline soil compared to PC. Furthermore, microscopic analysis revealed that chloride ions in the saline soil were involved in the hydration reaction to form Friedel's salt.

This paper presents a study on the stabilization of hazardous tin mine tailings (TMT) using a metakaolin-based geopolymer binder for their potential reuse as geomaterials in geotechnical works. The extensive laboratory testing evaluated the mechanical properties, such as unconfined compressive strength, and the durability properties, including mass loss during freezing-thawing and wetting-drying cycles. Environmental assessment included the analysis of leached heavy metal concentration using Toxicity Characteristic Leaching Procedure (TCLP). Additionally, Scanning Electron Microscopy (SEM) was conducted to investigate the microstructure of the stabilized TMT. Satisfactory results show that improvements in mechanical and durability properties depend on variations in metakaolin content, NaOH molarity, and compaction density. The novel porosity/binder index (eta/Biv) has proven to be effective in predicting the behavior of mixtures. Additionally, it demonstrated that freezing-thawing cycles have a more adverse impact on the durability of the examined mixtures. Laboratory results for mechanical strength, durability, and immobilization of hazardous heavy metals demonstrate the potential performance of TMTs for safe reuse in geotechnical works, specifically as a geomaterial for subbase and base layers of pavement exposure to severe environment and climate of the Andean highlands.

An imbalance between available resource reserves generated by human activities and disposal measures causes a series of harmful problems related to water, soil and human health, such as those from hazardous metal ions (HMs) and their carrier materials (LMs). As excellent intermediates with adjustable spatial structures and many active sites, geopolymers can be directly used as adsorbents to remove HMs from wastewater or produced by LMs via in situ stabilization/solidification methods. This paper first reviews the common methods used to optimize the pore structures of geopolymers and then reports the common methods for optimizing LM-based geopolymers. For geopolymers-based adsorbents, the pore structure is vital in the adsorption of the target ions, and the application of functional auxiliary materials among adsorbents has been summarized. The feasibility of using geopolymers for in situ stabilization/solidification of HMs is highlighted, but instability and low mechanical strength remain significant factors hindering their development. Finally, the mechanism for bonding between HMs and geopolymers is summarized, and future developments, challenges, and possible solutions are briefly described.

Cement-based solidification/stabilization (S/S) techniques have been widely used to produce stable forms of contaminated soils and reduce the mobility of contaminants into the environment. However, information on the long-term performances of S/S under environmental conditions (i.e., variable loading and atmospheric carbon dioxide) remains sparse. In this study, a triaxial test setup was modified to simulate environmental conditions. The permeability and compressive strength of silica sand solidified with portland cement were measured at different stages of four scenarios involving carbonation only, axial strain only, carbonation followed by axial strain, and axial strain followed by carbonation. X-ray computed tomography (CT) was used to characterize the internal structure of the samples. Permeability and compressive strength results indicate that the axial strain accelerated the damage to the S/S specimens and increased their permeability. The deterioration due to the mechanical strain decreased in the presence of carbon dioxide. Consistent changes in microstructure were observed with the CT scan. The results indicate that the influence of stressors on the void size distribution, compressive strength, and permeability is complex and characterized by interactions between the stressors.