Immobilisation of high-arsenic-containing tailings by using metallurgical slag-cementing materials

Effect of long-term dry-wet circulations on the Solidification/stabilization of Municipal solid waste incineration fly ash using a novel cementitious material

Solidification/stabilization (S/S) is a typical technique to immobilize toxic heavy metals in Municipal solid waste incineration fly ash (MSWI FA). This study utilized blast furnace slag, steel slag, desulfurization gypsum, and phosphoric acid sludge to develop a novel metallurgical slag based cementing material (MSCM). Its S/S effects of MSWI FA and long-term S/S effectiveness under dry-wet circulations (DWC) were evaluated and compared with ordinary Portland cement (OPC). The MSCM-FA block with 25 wt.% MSCM content achieved 28-day compressive strength of 9.38 MPa, indicating its high hydration reactivity. The leaching concentrations of Pb, Zn and Cd were just 51.4, 1895.8 and 36.1 μg/L, respectively, well below the limit standard of Municipal solid wastes in China (GB 16889-2008). After 30 times' DWC, leaching concentrations of Pb, Zn and Cd for MSCM-FA blocks increased up to 130.7, 9107.4 and 156.8 μg/L, respectively, but considerably lower than those for OPC-FA blocks (689, 11,870.6 and 185.2 μg/L, respectively). The XRD and chemical speciation analysis revealed the desorption of Pb, Zn and Cd attached to surface of C-S-H crystalline structure during the DWC. The XPS and SEM-EDS analysis confirmed the formation of Pb-O-Si and Zn-O-Si bonds via isomorphous replacement of C-A-S-H in binder-FA blocks. Ettringite crystalline structure in OPC-FA block was severely destructed during the DWC, resulting in the reduced contents of PbSO4 and CaZn2Si2O7·H2O and the higher leachability of Pb2+ and Zn2+.

Investigation of Preparation and Shrinkage Characteristics of Multi-Source Solid Waste-Based Cementitious Materials

Cement-stabilized macadam (CEM-SM) base layers on highways are prone to early shrinkage cracking in extremely cold and arid regions, mainly caused by the large drying shrinkage of traditional cement-stabilized base materials. A multi-component solid waste cementitious material (SWCM) was designed based on the response surface method. The synergistic reaction mechanism of SWCM was analyzed using X-ray diffractometer (XRD), Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TG). A shrinkage testing system was developed to evaluate the anti-cracking characteristics of stable macadam using multiple solid waste cementitious materials (SWCM-SM), and the strength growth law and frost resistance were analyzed. The results show that the Box-Behnken response surface model was used to obtain the optimal parameter combination for SWCM, including 60% slag, 30% steel slag, and 10% desulfurization gypsum. The compressive strength and flexural strength of SWCM-SM were 24.1% and 26.7% higher than those of CEM-SM after curing 180 days. The frost resistance of SWCM-SM was basically equivalent to that of CEM-SM, and the dry shrinkage strain of SWCM-SM was reduced by 30.7% compared to CEM-SM. It can be concluded that steel slag and desulfurization gypsum stimulate the hydration reaction of slag, thereby improving the bonding strength. Compared to CEM-SM, SWCM-SM exhibits slower hydration reaction and longer hydration duration, exhibiting characteristics of low early strength and high later strength. The early microstrain of the semi-rigid base layer is mainly caused by the occurrence of early water loss shrinkage, and the water loss rate of SWCM-SM is lower than that of CEM-SM. This study concludes that SWCM has good early crack resistance performance for stabilized crushed stones.

Influence of pH on the leaching behavior of a solidified arsenic contaminated soil

Stabilization/solidification is widely used for treatment of arsenic (As)-contaminated soils. The stability of the soil may deteriorate significantly when exposed to acid or alkaline leachate. In this study, semi-dynamic leaching tests under different pH were carried out to investigate the leaching behavior of As from the solidified soils. Spectroscopic and microscopic analyses were performed to reveal the related mechanisms. The results showed that the leaching of As was closely correlated with the pH of the leachate, because the encapsulation effect of the cementitious matrix and the chemical speciation and valence of As were all significantly influenced by pH. In the strongly acidic leachant (pH 3.0), the leached As concentration increased by an order of magnitude, and the effective diffusion coefficient of As reached 3.71 × 10-13 m2/s. This is because that pores and cracks increased owing to the acidic corrosion of CSH, such that the physical encapsulation effect was reduced and the mobility of As increased. The leachability index showed that the solidified soil was unsuitable for 'controlled utilization' under strongly acidic conditions. The leached As concentration was the lowest in the weakly alkaline leachant (pH 9.0) because under weakly alkaline conditions the hydration process of the cement was facilitated, and more CSH gels were attached to the surface of the soil particles, forming a tighter structure for As encapsulation. However, as pH increased from 9.0-11.0 the leached As concentration increased due to an increased content of As(III)-O in the solidified soil.

Immobilization of Cr(VI)-containing tailings by using slag-cementing materials for cemented paste backfill: influence of sulfate and limestone addition

Heavy metals in mine tailings lead to serious environmental problems. Cemented paste backfill (CPB) is widely used for treating the mine tailing. The high cost of ordinary Portland cement (OPC) reduces the profit of mine production. The work investigates the treatment of Cr(VI)-containing tailings by using slag-based cementitious materials for CPB. Flue gas desulfurization gypsum (FGDG) and limestone were used to modify the properties of samples. Results showed that the coupling addition of 6 wt% FGDG and 3 wt% limestone (A6L3) led to the highest compressive strength of CPB samples, which also presented satisfactory immobilization effects for Cr(VI). The compressive strength of CPB samples using A6L3 as a binder was comparable to the OPC-based sample, reaching about 5.53 MPa; the immobilization efficiency for Cr(VI) was about 99.5%. The effects of FGDG and limestone were twofold: the addition of FGDG favored the formation of ettringite and then contributed to a more compact structure; besides, incorporating limestone increased the packing density of the CPB system by decreasing the loosening and wedge effect.

Iron-calcium reinforced solidification of arsenic alkali residue in geopolymer composite: Wide pH stabilization and its mechanism

Arsenic-alkali residue (AAR) from antimony production can pose significant health and environmental hazards due to the risk of arsenic (As) leaching. In this study, geopolymer composite synthesized from fly ash (FA) was investigated for efficient stabilization of high-arsenic-containing AAR (As₂O₃ of 22.74 wt%). Two industrial wastes, e.g., granulated blast furnace slag (GBFS) with active calcium composition and water-quenched slag (WQS) from lead-zinc smelting with active iron composition, were investigated for the reinforcement of AAR geopolymer solidification. A wide pH stabilization (from pH = 3-pH = 12) of AAR with the geopolymer composite was successfully achieved, and As leaching concentration of geopolymer with the addition of 5 wt% AAR was significantly reduced from 2343.73 mg/L (AAR) to that below 0.18 mg/L, which successfully meet the regulatory limit of Chinese domestic waste landfill (GB, 18598-2019, 1.2 mg/L) and hazardous waste landfill (GB16889-2008, 0.3 mg/L). Johnbaumite (Ca₅(AsO₄)₃(OH)) was formed in geopolymer composite and leached samples with initial pH from 2.6 to 6 (final pH from 5.54 to 13.15). Magnetite and iron hydroxide phases with strong adsorption and/or As co-precipitation capability were also observed. As stabilization was also achieved with iron oxidation from As(III) to As(V). This study solves the problem of unstable As leaching at different pH for the solidification of arsenic-bearing solid waste, and provides a promising and practical strategy for efficient solidification/stabilization of AAR as well as other similar arsenic-bearing solid wastes with geopolymer composite.

Preparation and performance of composite activated slag-based binder for cemented paste backfill

The utilization of blast furnace slag (BFS) and fly ash (FA) to replace ordinary portland cement (OPC) has become a hot topic in the preparation of low-cost cemented paste backfill (CPB). This study has prepared a composite activated slag-based binder (CASB) using BFS and FA as the basic raw materials and desulfurization gypsum (DG) and cement clinker (CC) as the activator. The optimum ratio of CASB was determined based on the orthogonal test and the efficacy coefficient method. The hydration products and hydration mechanism of CASB materials were further investigated using XRD, TG, and SEM tests; on this basis, the compressive strength of hardened CASB-CPB under different working conditions and the rheological properties of fresh slurry were investigated, and the cost analysis and environmental effects of CASB were carried out. The results show that the optimum ratio of CASB was 15:12:13:60 for FA: CC: DG: BFS; the hydration mechanism of CASB was the coupled alkali-sulfate activation of CC and DG, and the main hydration products were hydrated calcium silicate gels (C-S-H gels) and ettringite (AFt); increasing the mass concentration (Cw) at a constant cement-aggregate ratio (C/A), which caused a significant improvement in the compressive strength at 7 and 28 d while reduced the flowability of the slurry; CASB considerably reduced the filling cost compared to OPC, and effectively immobilization the heavy metals in the tailings. This paper has developed a cement alternative binder of CASB, which has considerable significance for the comprehensive utilization of solid waste, reduction of filling costs, and improvement of economic and ecological benefits of the mine.

Arsenic(V) immobilization in fly ash and mine tailing-based geopolymers: Performance and mechanism insight

Global mining activities produce thousands of millions of toxic-bearing mine tailing (MT) wastes each year. Storage of the mine tailings not only encroaches upon large areas of cropland but also arouses additional ecological and environmental risks. Herein we demonstrate that geopolymerization of a mixture of the toxic-bearing mine tailings and the coal fly ash (FA) can effectively immobilize exogenous arsenic (As) species in addition to inherent As from the raw materials. The geopolymers also possess high compressive strengths (e.g., >25 MPa for specimens with 54 wt% FA and activated with 10 M sodium hydroxide (NaOH)), allowing them to be further used as low-carbon, cement-free building materials. The geopolymer strength was found to depend clearly upon the NaOH concentration, the FA content, and the curing time, with the maximum being 37.07 MPa for a specimen with 54 wt% FA, 0.03 wt% As, activated with 10 M NaOH and cured for 28 days. Leaching tests showed that all specimens achieved an immobilization efficiency as high as 95.4% toward As, and that both the short-term and long-term leachabilities of all toxic elements are far below the standard maximum contaminant levels. Microstructural analyses indicate that calcite, calcium silicate, and calcium silicate hydroxide are likely to play a crucial role in immobilizing As species and heavy metals of concern in the geopolymer matrixes. Given the superior mechanical strengths and long-term stabilities, the FA/MT-based geopolymers demonstrate a promising low-carbon material for both the remediation of As-bearing lands and the construction industry.

Efficient remediation of heavily As(III)-contaminated soil using a pre-oxidation and stabilization/solidification technique

The high mobility of As(III) makes it difficult to remediate heavily As(III)-contaminated soil. A novel remediation technique that combines pre-oxidation and stabilization/solidification (PO + S/S) is proposed in this study to remediate heavily As(III)-contaminated soil. After oxidizing As(III) in the contaminated soil using Fenton's reagent, FeCl₃·6H₂O was used as a chemical stabilizing agent to reduce the toxicity and mobility of As. Finally, Portland cement (PC) was used for solidification. The effects and mechanisms of the proposed technique were studied using unconfined compressive strength tests, leaching tests, sequential extraction procedure (SEP), and a series of spectroscopic/microscopic investigations. The experimental results showed that the addition of FeCl₃·6H₂O increased the strength of the curing body because the hydration degree of PC and pore structure were improved. Portland cement can increase the pH of the curing body. At a 1:1 Fe to As molar ratio and a 15 wt% PC dosage, the leached As concentration decreased to 3.25 mg L^-1, and the remediation efficiency reached 99.54%. The SEP results showed that the PO + S/S treatment converted As into more stable phases and effectively reduced the potential mobile phase risk. The majority of As was bound to hydrated iron oxides; however, the increased pH affected the Fe-As interactions and prompted the release of As from the surface of the hydrated iron oxides. Spectroscopic/microscopic investigations indicated that the PO + S/S treatment converted As(III) to less toxic and less mobile As(V) and then immobilized by the encapsulation of calcium silicate hydrate and ion exchange of ettringite. This study provides a scientific basis and theoretical support for the effective remediation of heavily As(III)-contaminated soil.

Simultaneous stabilization/solidification of arsenic in acidic wastewater and tin mine tailings with synthetic multiple solid waste base geopolymer

Stabilization/Solidification (S/S) is considered as a feasible technology for the treatment of arsenic (As) in acidic wastewater (AW) and tin mine tailings (TMTs); however, high cost, high carbon footprint, and strict reaction conditions are the main limitations. Herein, a novel alkali-activated geopolymer material (AAGM) for S/S As was synthesized by combining AW, TMT, gypsum (GP), and metakaolin (MK). At room temperature, an initial As concentration of 3914 mg/L, a NaOH content of 4.98%, and an MK content of 20% decreased the As leaching concentration to 1.55 mg/L (<5 mg/L). The main S/S mechanisms of As included physical encapsulation of C-(A)-S-H and geopolymer structures, ion exchange of ettringite, and formation of Fe-As and Ca-As precipitates. Further studies showed that increasing initial As concentration and MK content facilitated the formation of Ca-As precipitates and C-(A)-S-H gels. The semi-dynamic leaching tests revealed that the leaching mechanism of As was surface wash-off. The effective diffusion coefficients of the samples were less than 10^-13 cm²/s, and the respective leachability indexes were greater than 9, indicating that AAGM was effective in preventing the leaching of As. Therefore, this study provides a green and low cost solution for the synergistic utilization of AW, TMT, GP, and MK.

Recycling of arsenic-containing biohydrometallurgy waste to produce a binder for cemented paste backfill: Co-treatment with oil shale residue

The high cost of ordinary Portland cement (OPC) limits the broad usage of cemented paste backfill (CPB). Additionally, improper disposal of arsenic-containing biohydrometallurgy waste (BW) can cause tremendous pollution to the environment. Consequently, BW is used to prepare an alternative cementitious material for CPB in this study. The effect of calcined oil shale residue (COSR) on the binder's characteristics was studied. The reaction kinetics of the binder in the presence of COSR were studied via the isothermal calorimeter test and the Krstulovic-Dabic model; mechanical strength and hydration product modifications due to the addition of COSR were also investigated. The leaching of hazardous elements from the binder was also investigated. The results showed that adding COSR reduced the flowability of fresh slurry and early-age compressive strength; however, adding 20 wt% COSR resulted in the highest later age compressive strength, thereby reaching ∼43.65 MPa after 60 days. The compressive strength of the CPB sample using the COSR20 as a binder may reach ∼87% of the OPC-based CPB sample. Furthermore, the presence of COSR had no significant effect on the phase assembles but changed the amount of ettringite (AFt) and calcium silicate aluminate hydrate (C-A-S-H). The results of this study show that the prepared binder could be used as an alternative to OPC in CPB.

Valorizing (cleaned) sulfidic mine waste as a resource for construction materials

Proper management and storage of mine waste, e.g., tailings and waste rock, is one of the main issues that mining industries face. Additionally, there is already an uncountable amount of existent historical mine waste, which may, even centuries after the closure of the mine, still be leaching contaminants into the environment. One solution to minimize the risks associated with the mine waste, with also potential economic benefits, is through the valorization of the waste. This can be done by first recovering valuable metals and removing hazardous contaminants. Then, the remaining residue can be valorized into green construction materials, such as geopolymers, ceramics or cement. For some mine waste materials, such as those with only trace levels of metals that are not economically viable to extract, the "waste" can be reused directly without this additional cleaning step. In the present study, mine waste originating from three different sites was characterized and compared with the cleaned mine waste (i.e., cleaned by bioleaching or flotation methods) and with different types of green construction materials containing 13-80 wt% (cleaned and uncleaned) mine waste. Particular emphasis was given to the mobilization of metal(loid)s from the mine waste and construction materials (i.e., ceramics, alkali-activated materials and cement) under different conditions, through a series of leaching tests (i.e., EN 12457-2, US EPA's Toxicity Characteristic Leaching Procedure, and a pH-dependent leaching test). The leaching tests were applied to either mimic current 'natural' conditions at the mining site, conditions in a landfill (end of life) or extreme conditions (i.e., extremely acidic or alkaline pH). Most of the original mine waste samples contain high levels of Pb (18-3160 mg/kg), Zn (66-10500 mg/kg), and As (10-4620 mg/kg). . The cleaning methods were not always efficient in removing the metal(loid)s and sulfur. In some cases, the cleaned mine waste samples even contained higher total metal(loid) and sulfur concentrations than the original mine waste samples. Based on the leaching studies, some alkali-activated materials, ceramics, and cement effectively immobilized certain metals (e.g., <0.5 mg/kg of Pb and <4 mg/kg of Zn). Also, longer curing times of the alkali-activated materials, in most cases, improved the immobilization of metal(loid)s. Additionally, for ceramics, the temperature at which the test pieces were fired (up to 1060 °C), also played a major role in decreasing the mobility of some metal(loid)s, while increasing others (e.g., As, potentially via the structural rearrangement of As and Fe). Overall, through this detailed characterization, the environmental impact from the mine waste to the downstream products was evaluated, determining which valorization methods are the most viable to close the circular economy loop.

Advances in the use of recycled non-ferrous slag as a resource for non-ferrous metal mine site remediation

The growing global demand for non-ferrous metals has led to serious environmental issues involving uncovered mine site slag dumps that threaten the surrounding soils, surface waters, groundwater, and the atmosphere. Remediation of these slags using substitute cement materials for ordinary Portland cement (OPC) and precursors for alkali-activated materials (AAMs) can convert hazardous solid wastes into valuable construction materials, as well as to attain the desired solidification and stabilization (S/S) of heavy metal(loid)s (HM). This review discusses the current research on the effect of non-ferrous slags on the reaction mechanisms of the OPC and AAM. The S/S of HM from the non-ferrous slags in AAM and OPC is also reviewed. HM can be stabilized in these materials based on the complex salt effect and isomorphic effects. The major challenges faced in AAMs and OPC for HM stabilization include the long-term durability of the matrix (e.g., sulfate attack, stability of volume). The existing knowledge gaps and future trends for the sustainable application of non-ferrous slags are also discussed.

Harmless Treatment of High Arsenic Tin Tailings and Environmental Durability Assessment

The treatment of arsenic (As) in tin tailings (TT) has been an urgent environmental problem, and stabilization/solidification (S/S) treatment is considered an effective technology to eliminate contamination of As. In this study, we developed a low-carbon and low-alkalinity material to S/S of As, and the results showed that the leaching concentration of As after treatment was lower than the Chinese soil environmental quality standard (0.1 mg/L). Based on a series of characterization tests, we found that OH- promoted the dissolution of As(III)-S, Fe-As(V), and amorphous As(III)-O species and formed Ca-As(III) and Ca-(V) species with Ca2+. Simultaneously, hydration produces calcium silicate hydrate (C-S-H) gel and ettringite by the form of adsorption and ion exchange to achieve S/S of As. We also assessed the durability of this material to acidity and temperature, and showed that the leaching concentration of As was below 0.1 mg/L at pH = 1-5 and temperature 20-60 °C. The method proposed in this study, S/S of As, has excellent effect and environmental durability, providing a new solution for harmless treatment of TT and its practical application.

Study on the Hydration Reaction of Typical Clay Minerals under Alkali and Sulfate Compound Activation

Sand, stone, tailings and other aggregates often contain a small amount of clay mineral and their hydration activity is low, thereby lowering concrete performance indexes while negatively affecting their resource utilisation. In this study, clay minerals, calcium hydroxide and desulfurised gypsum were used to prepare cementitious materials to examine kaolinite, montmorillonite, illite and chlorite clay mineral contents under compound activation. The effects of curing temperature and water reducer on clay samples were analysed. The results showed that the compressive strength of kaolinite samples cured at 25 °C and 55 °C reached 1.09 and 4.93 MPa in 28 days and increased by 43% and 12%, respectively, after adding a 0.3% water reducer. Montmorillonite was activated and its compressive strength reached 5.33 MPa after curing at 55 °C in 28 days. Illite exhibited some activity and its compressive strength reached 1.43 MPa after curing at 55 °C in 28 days and the strength increased slightly after adding a water reducer. The chlorite sample had no strength after activation under the same conditions. Furthermore, X-ray diffraction and scanning electron microscope and energy-dispersive spectroscopy microstructure analyses showed that after alkali and sulfate activation, the hydration products of activated clay minerals mainly included ettringite, hydrated calcium aluminate and hydrated calcium silicate. The increase in curing temperature accelerated the reaction speed and improved the early strength. However, the effect on chlorite minerals was not obvious.

The mechanistic insights into the leaching behaviors of potentially toxic elements from the indigenous zinc smelting slags under the slag dumping site scenario

Since lager quantities of the zinc (Zn) smelting slags were traditionally dumped at the indigenous Zn smelting sites, the release characterization of potentially toxic elements (PTEs) from the Zn smelting slags under various environmental conditions were of great significance for an environmental risk analysis. The acidification of the Zn smelting slags to pH= 4 and 6 would result in the leaching concentrations of Cd and Mn exceeding the fourth-class standard of surface water quality standard in China (GB3838-2002). Notably, most metals exhibited an amphoteric leaching pattern, where the highest leached concentrations of As, Cd, Cu, Mn, Pb, and Zn were 4.15, 4.21, 140.0, 78.1, 156.9 and 477.0 mg/L, respectively. In addition, the highest release of toxic metals within 96 h reached 0.17 % of As, 3.50 % of Cd, 2.77 % of Cu, 6.92 % of Mn, 0.13 % of Pb, and 2.57 % of Zn, respectively. The combined results of various characterization techniques suggested that the PTEs remobilization effected by rhizosphere-like organic acids were mainly controlled by the precipitation of newly formed Fe, Mn and Al (hydr) oxides and the complexation of organic ligands. The present study results could provide valuable insights into the long-term leaching behaviors of PTEs from the Zn smelting slags to reduce ecological hazard.

Effects of slag-based cementitious material on the mechanical behavior and heavy metal immobilization of mine tailings based cemented paste backfill

Slag-based cementitious material was synthesized from blast furnace slag, clinker, gypsum, and activator to replace cement in cemented paste backfill (CPB). We researched the influence of slag-based cementitious material dosages and curing times on the properties of CPB, including unconfined compressive strength tests, leachate toxicity and chemical speciation of heavy metal as well as microstructural tests and analyses. The results indicated that the addition of slag-based cementitious material improved the compressive strength of the CPB, which attained the compressive strength requirements (≥1.0 MPa) at 28 days. The leachate concentrations of Pb, Cr, Cu, and Cd in CPB decreased as the slag-based cementitious material dosage and curing period increased, which met the standard (GB 5085.3-2007). The dosage of 10% slag-based cementitious material could effectively immobilize the heavy metals in the tailings, and the immobilization performance was similar to that of 20% cement, which indicated the amount of slag-based cementitious material was only half the quantity of cement in CPB. Microstructural analysis showed the hydration products included calcium silicate hydrate, ettringite, and portlandite, which could enhance the bonding force between the tailing grains.

Solidification/Stabilization of MSWI Fly Ash Using a Novel Metallurgical Slag-Based Cementitious Material

Four industrial wastes, i.e., blast furnace slag, steel slag, desulfurization ash, and phosphoric acid sludge, were used to prepare a low-carbon binder, metallurgical slag-based cementitious material (MSCM). The feasibility of solidification/stabilization of municipal solid waste incineration (MSWI) fly ashes by MSCM were evaluated, and the immobilization mechanisms of heavy metals were proposed. The MSCM paste achieved 28-day strength of 35.2 MPa, showing its high-hydration reactivity. While the fly ash content was as high as 80 wt.%, the 28-day strength of MSCM-fly ash blocks reached 2.2 MPa, and the leaching concentrations of Pb, Zn, Cr, and Hg were much lower than the limit values of the Chinese landfill standard (GB 16889-2008). The immobilization rates of each heavy metal reached 98.75–99.99%, while four kinds of MSWI fly ashes were solidified by MSWI at fly ash content of 60 wt.%. The 28-day strength of binder-fly ash blocks had an increase of 104.92–127.96% by using MSCM to replace ordinary Portland cement (OPC). Correspondingly, the lower leachability of heavy metals was achieved by using MSCM compared to OPC. The mechanisms of solidification/stabilization treatment of MSWI fly ash by MSCM were investigated by XRD, SEM, and TG-DSC. Numerous hydrates, such as calcium silicate hydrate (C-S-H), ettringite (AFt), and Friedel’s salt, were observed in hardened MSCM-fly ash pastes. Heavy metals from both MSWI fly ash and MSCM could be effectively immobilized via adsorption, cation exchange, precipitation, and physical encapsulation.

Effect of Freeze–Thaw Cycles on Mechanical and Microstructural Properties of Tailings Reinforced with Cement-Based Material

In China, more than 10,000 Tailings storage facilities (TSF) have been created on the ground surface through mineral mining processes, these TSF occupy a large amount of land. The strength of the tailings is too low to be able to stand on its own without strengthening. In order to save land resources and alleviate the damage to the environment caused by mineral mining, it is necessary to reinforce the TSF so that they can store more tailings. China is one of the countries with the largest area of permafrost and seasonal frozen regions, accounting for about 75% of the country’s total land area. The problem can be exacerbated in these regions where the freeze–thaw effect can further degrade the strength of tailings. A review of the literature suggests that there is little research on the mechanical and microstructural properties of tailings reinforced with cement-based materials under freeze–thaw conditions, especially when the tailings are to be discharged to land for sustainable development. This study investigates the effect of freeze–thaw cycles on the mechanical properties and microstructural changes of tailings reinforced with cement-based materials to mitigate environmental hazards. Unconfined compressive strength (UCS) tests, scanning electron microscopic images, X-Ray Diffraction tests, thermogravimetry tests and mercury intrusion porosimetry tests were conducted on samples of tailings. The results from this study show that freeze–thaw cycles reduce the UCS of all the tested samples eventually, but the frozen temperature does not significantly affect the UCS. The larger number of freeze–thaw cycles, the more damage is to the surface morphology and the matrix of the tailings. The results presented in the paper can help engineers and managers to effectively transport the TSF to other locations to minimize environmental hazards to achieve sustainable production of mineral mining processes.

The mechanistic understanding of potential bioaccessibility of toxic heavy metals in the indigenous zinc smelting slags with multidisciplinary characterization

Smelting slags is a well-known industrial solid waste, while there were limited studies on the key factors controlling the potential health risks caused by these smelting slags. In this work, the metal bioaccessibility in the size fractionated-zinc smelting slags was examined using various In vitro assays, in combination with multidisciplinary methods. The results indicated that the bioaccessible fractions of heavy metals showed a significant difference, but no statistical difference among different particle sizes of the zinc smelting slags. The bioaccessible metal fractions in the gastric (GP) and gastrointestinal (GIP) phases were 0 (Cr) - 91.39% (Cd)) and 0 (Cr) - 47.80% (Ni). Among the studied metals, Cd, Cu, Mn, Pb and Zn were the most bioaccessible to human. The Pearson correlation analysis showed that the carbonate bound phases of heavy metals were responsible for their bioaccessibility in GP and GIP. Moreover, the combined results of multidisciplinary characterization also further implied that the solubility behaviors of toxic elements in the smelting slags were dominated by soluble metal bearing- mineral phases and absorbable Fe, Mn and Al-rich minerals and metal bearing-precipitates during SBRC extractions. Therefore, these study results provide a insight into the potential controls of metal bioaccessibility in the zinc smelting slags, which was of great significance from the aspects of their resource recycling and risk management.

Effect of calcium formate as an accelerator on dilatancy deformation, strength and microstructure of cemented tailings backfill

Recycling mining wastes to produce cemented tailings backfill (CTB) is the optimal approach to eliminate the environmental pollution caused by their accumulation. However, its low strength limits its application. Using calcium formate (CF) as an accelerator for improving its mechanical properties is of great significance to promote sustainable development. The effects of CF dosage and curing time on dilatancy deformation, compressive strength and microstructure of CTB were investigated through mechanical compression, scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) tests. The strengthening and deterioration mechanisms of CF dosage on CTB were revealed, and its engineering practicability was systematically evaluated. The results show that the variation of volumetric strain in the dilatancy deformation stage firstly increase and then decrease with the increases of CF dosage and curing time. The relationship between CF dosage and compressive strength can be characterized by quadratic polynomial, and the optimal CF dosage characterizing the superior mechanical property of CTB is between 1.60 and 1.84. The supplement of CF reduces the size and distribution of microcracks and micropores, thereby optimizing the microstructure of CTB. Nevertheless, the excessive dosages of CF deteriorate the microstructure of CTB and produce serious defects, which cannot be effectively filled by hydration products, thus weakening the strength property of CTB. This study provides an effective accelerator for improving the mechanical properties of CTB, which is of great significance to promote the recycling of tailings.

Study on Solidification and Stabilization of Antimony-Containing Tailings with Metallurgical Slag-Based Binders

Blast furnace slag (BFS), steel slag (SS), and flue gas desulfurized gypsum (FGDG) were used to prepare metallurgical slag-based binder (MSB), which was afterwards mixed with high-antimony-containing mine tailings to form green mining fill samples (MBTs) for Sb solidification/stabilization (S/S). Results showed that all MBT samples met the requirement for mining backfills. In particular, the unconfined compressive strength of MBTs increased with the curing time, exceeding that of ordinary Portland cement (OPC). Moreover, MBTs exhibited the better antimony solidifying properties, and their immobilization efficiency could reach 99%, as compared to that of OPC. KSb(OH)₆ was used to prepare pure MSB paste for solidifying mechanism analysis. Characteristics of metallurgical slag-based binder (MSB) solidified/stabilized antimony (Sb) were investigated via X-ray diffraction (XRD), field emission scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). According to the results, the main hydration products of MSB were C-S-H gel and ettringite. Among them, C-S-H gel had an obvious adsorption and physical sealing effect on Sb, and the incorporation of Sb would reduce the degree of C-S-H gel polymerization. Besides, ettringite was found to exert little impact on the solidification and stabilization of Sb. However, due to the complex composition of MSB, it was hard to conclude whether Sb entered the ettringite lattice.

Comparison of limestone calcined clay cement and ordinary Portland cement for stabilization/solidification of Pb-Zn smelter residue

Decreasing carbon emissions by replacing Portland cement (PC) with supplementary cementitious materials (SCMs), such as low-grade limestone (LS) and calcined clays (CC), has tremendous potential for stabilization/solidification (S/S) of industrial hazardous waste primarily with heavy metals. Recently, a low-carbon-based cementitious binder, namely, limestone calcined clay cement (LC), has emerged as an alternative for S/S treatment of wastes. However, comprehensive comparison between LC and PC application in solidifying/stabilizing wastes has not been conducted. This study aims to investigate the S/S efficiency of Pb-Zn smelter residue (LZSR) comprising heavy metals lead (Pb), zinc (Zn), and cadmium (Cd) at higher concentrations. LZSR is treated with LC and PC for capturing strength and leaching toxicity. The test results indicate that low-grade CC and LS in the LC binder can promote the alkaline environment, and act as fillers in solidifying heavy metals. The toxicity characteristic leaching procedure leaching concentrations of untreated (UT) LZSR were 503 mg/kg, 1266 mg/kg, and 251 mg/kg for Pb, Zn, and Cd, respectively. After a 28-day curing, the leaching concentrations in LC-treated LZSR reduced to 4.33 mg/kg, 189.68 mg/kg, and 0.46 mg/kg, while the leaching concentrations of PC-treated LZSR reduced to 29 mg/kg, 338 mg/kg, and 6 mg/kg for Pb, Zn, and Cd, respectively. The maximum immobilization efficiencies for Pb, Zn, and Cd reached 85%, 99%, and 99%, respectively. Moreover, the insoluble phases for Pb, Zn, and Cd obtained from the sequential extraction test results were 63.5%, 72.1%, and 42.4% for LC-treated LZSR and 35.7%, 38%, and 43% for PC-treated LZSR with binder content of 8% binder and curing time of 28 days. Increasing curing time and binder content reduced leaching concentrations, and the underneath mechanisms were interpreted by XRD, SEM-EDS, and FTIR analyses. Overall, the results indicate that Pb, Zn, and Cd can be successfully immobilized using 8% LC binder by transforming soluble heavy metals to insoluble hydroxides and their complexes.

Solidification performances of contaminants by red mud-based cementitious paste filling material and leaching behavior of contaminants in different pH and redox potential environments

Considering the urgent need for disposal of red mud and the comprehensive treatment of coal mined-out areas, this paper presented red mud-based cementitious paste filling material (RMFM) to achieve the purpose of green filling treatment. However, the solidification performance of alkaline RMFM for contaminants can be affected when in contact with acid goaf water in practice, which may in turn cause secondary pollution to the surroundings. The leaching tests of RMFM under different pH and redox potential (E_h) conditions were designed to investigate the effects of environmental elements on the solidification performance of RMFM, and primarily investigated the treatment effectiveness of RMFM on goaf water. The test results manifest that the acidic and oxidizing environments could damage the hydration products generated by alkali and sulfate activation, thus affecting the solidification performance, while the alkaline and reducing environments could effectively prevent the release of the contaminants by enhancing the degree of alkali activation and inhibiting oxidation acid forming process. In the possible exposure environment, RMFM could effectively stabilize its own pollutants without secondary pollution. In addition, the powder RMFM samples had significant removal effects on heavy metals, the values of Cu, Pb, and As removal efficiency all reached more than 96.15%.

Research Progress on Controlled Low-Strength Materials: Metallurgical Waste Slag as Cementitious Materials

Increasing global cement and steel consumption means that a significant amount of greenhouse gases and metallurgical wastes are discharged every year. Using metallurgical waste as supplementary cementitious materials (SCMs) shows promise as a strategy for reducing greenhouse gas emissions by reducing cement production. This strategy also contributes to the utilization and management of waste resources. Controlled low-strength materials (CLSMs) are a type of backfill material consisting of industrial by-products that do not meet specification requirements. The preparation of CLSMs using metallurgical waste slag as the auxiliary cementing material instead of cement itself is a key feature of the sustainable development of the construction industry. Therefore, this paper reviews the recent research progress on the use of metallurgical waste residues (including blast furnace slag, steel slag, red mud, and copper slag) as SCMs to partially replace cement, as well as the use of alkali-activated metallurgical waste residues as cementitious materials to completely replace cement for the production of CLSMs. The general background information, mechanical features, and properties of pozzolanic metallurgical slag are introduced, and the relationship and mechanism of metallurgical slag on the performance and mechanical properties of CLSMs are analyzed. The analysis and observations in this article offer a new resource for SCM development, describe a basis for using metallurgical waste slag as a cementitious material for CLSM preparation, and offer a strategy for reducing the environmental problems associated with the treatment of metallurgical waste.

Long-term leachability of Sb in smelting residue stabilized by reactive magnesia under accelerated exposure to strong acid rain

This study investigated the long-term leachability of antimony (Sb) in a smelting residue (39519 mg/kg) solidified/stabilized by reactive magnesia (MgO). Different dosages of MgO (0% as control, 2%, 5%, and 10% on a dry basis) were compared, and the long-term performance was evaluated by an accelerated exposure test consist of 20 consecutive leaching steps with simulated strong acid rain (SAR, HNO₃: H₂SO₄ = 1:2, pH = 3.20) as the extractant. Notably, the MgO treatments efficiently reduced the Sb leachability. Compared to the original slag (8.3 mg/L), the leaching concentrations based on a Chinese standard HJ/T299-2007 were reduced by 58%, 79%, 85%, and 86% at MgO dosages of 0%, 2%, 5%, and 10%, respectively. Because the studied slag was rich in oxides like SiO₂, CaO, and MgO, the hydration reactions probably happened during the aging processes with oxic water. It was inferred that the formed hydration products have a self-solidification/stabilization function to suppress the Sb leaching from the solid phase. The mineralogical characterization results proved that the hydrated Mg(OH)₂ played an essential role in the decrease of Sb leachability. Besides, the MgO addition promoted the hydration of this smelting slag and formed new hydrate gels that immobilize Sb in this slag. Our results confirmed that MgO-amended slags were resistant to continuous SAR corrosion. Compared to the control, the dosage of 5% MgO could effectively reduce the cumulatively released Sb by 57%, with only 0.46% of total Sb could be leached. The decomposition of Mg(OH)₂ and hydrate gels determined the re-release of Sb in a long term. Our work has demonstrated that reactive MgO amendment could be potentially selected as an effective strategy for the treatment of Sb-containing smelting residues in field conditions.

The rheological, mechanical and heavy metal leaching properties of cemented paste backfill under the influence of anionic polyacrylamide

Anionic polyacrylamide (APAM) has widely been employed in backfill mining to accelerate the sedimentation of fine tailings particles and increase the concentration of tailings slurry. However, APAM inevitably remains in thickened tailings, leading to a nonnegligible influence on the rheological, mechanical, and heavy metal leaching properties of tailings-based cemented paste backfill (CPB). In an effort to solve these issues, the influences of APAM on CPB properties were examined in the present study. Experimental tests such as rheology, uniaxial compressive strength (UCS), toxicity leaching, and microscopy were conducted. The results showed that the presence of APAM first significantly increased the yield stress and viscosity of CPB slurry. APAM slightly improved the early UCS of CPB curing for 7 days but hindered the UCS development of samples cured for 28 days. Moreover, the presence of APAM restrained the hydration reaction, reduced the amounts of hydrated products, increased pore size, and loosed the microstructure of the test samples. Finally, the addition of APAM effectively reduced the leaching of Ag and As, while incremented that of Cu and slightly affected the leaching of Ba. In sum, these findings look promising for the safe production and environmental protection of the mining industry.

Red mud-metakaolin based cementitious material for remediation of arsenic pollution: Stabilization mechanism and leaching behavior of arsenic in lollingite

The proper treatment of lollingite is of great significance due to its rapid oxidation leading to release of arsenic into the environment. Herein, a green multi-solid waste geopolymer, consisting of red mud, metakaolin, blast furnace slag, and flue gas desulfurization gypsum, was developed. The obtained red mud-metakaolin-based (RMM) geopolymer demonstrated good arsenic retention capability. The results showed that the replacement of SO₄^2- in ettringite with AsO₄^2- via ion exchange, formation of Ca-As and Fe-As precipitates, and physical encapsulation with aluminosilicate gel were the main mechanisms that prevented the release of arsenic. Further dissolution of ettringite in RMM was alleviated by adding a suitable amount of Ca(OH)₂ and controlling the pH of the leachate. TCLP results verified that RMM materials possessed an outstanding ability to stabilize arsenic, with a leaching rate below the permitted value of 5 mg/L for safe disposal. The low leachability of the RMM geopolymers (<0.50 mg/L) is potentially related to the pH buffering capacity of the hydration products at a pH range of 2-5. RMM geopolymers showed a high compressive strength (>15 MPa) and low arsenic leaching concentration (<2.66 mg/L) after 28 days of curing. These results demonstrate the potential of RMM geopolymers to be utilized as an environmentally friendly backfilling cementitious material for sustainable remediation of arsenic pollution.

Long-term stability of arsenic calcium residue (ACR) treated with FeSO4 and H2SO4: Function of H+ and Fe(Ⅱ)

Arsenic calcium residue (ACR) generated from the As-bearing wastewater treatment is highly hazardous due to high content of available As, which was seeking a suitable method for safe disposal such as stabilization treatment. In this study, the stabilization of available As in ACR was performed by combined treatment with FeSO₄ and H₂SO₄. After stabilization treatment, the As leaching concentrations extracted by China Standard Leaching Test (CSLT, HJ/T299-2007) decreased significantly from 162 mg/L to less than the Chinese regulation limit of 1.2 mg/L. And FeSO₄-H₂SO₄ treated ACR could maintain good long-term stability even after cured for 365 days. The stabilization mechanism for available As in ACR using leaching tests, sequential extraction analysis, XPS, XRD, and SEM-EDS was investigated. H⁺ from H₂SO₄ and Fe(Ⅱ) hydrolysis was committed to the full release of available As. Reactive oxygen species (ROSs) produced from Fe(Ⅱ) oxygenation drove the oxidation of As(Ⅲ) to As(Ⅴ). The release As was stabilized by forming stable Fe-O-As complexes (FeAsO₄·xFe(OH)₃). Moreover, this study also presented an effective and feasible method for ACR disposal.

Solidification/Stabilization of Arsenic-Containing Tailings by Steel Slag-Based Binders with High Efficiency and Low Carbon Footprint

The disposal of nonferrous metal tailings poses a global economic and environmental problem. After employing a clinker-free steel slag-based binder (SSB) for the solidification/stabilization (S/S) of arsenic-containing tailings (AT), the effectiveness, leaching risk, and leaching mechanism of the SSB S/S treated AT (SST) were investigated via the Chinese leaching tests HJ/T299-2007 and HJ557-2010 and the leaching tests series of the multi-process Leaching Environmental Assessment Framework (LEAF). The test results were compared with those of ordinary Portland cement S/S treated AT (PST) and showed that the arsenic (As) curing rates for SST and PST samples were in the range of 96.80–98.89% and 99.52–99.2%, respectively, whereby the leached-As concentration was strongly dependent on the pH of the leachate. The LEAF test results showed that the liquid–solid partitioning limit of As leaching from AT, SST, and PST was controlled by solubility, and the highest concentrations of leached As were 7.56, 0.34, and 0.33 mg/L, respectively. The As leaching mechanism of monolithic SST was controlled by diffusion, and the mean observed diffusion coefficient of 9.35 × 10⁻¹⁵ cm²/s was higher than that of PST (1.55 × 10⁻¹⁶ cm²/s). The findings of this study could facilitate the utilization of SSB in S/S processes, replacing cement to reduce CO₂ emissions.

Use of CO2 to Cure Steel Slag and Gypsum-Based Material

To improve the utilization of steel slag (SS) in CO2 capture and making building materials, the paper mainly discussed the effects of desulphurization gypsum (DG) and w/s ratio on strength development and CO2 capture capability of high Al content SS. It showed that 10 wt% DG enhanced the strength of hydration-curing SS by 262% at 28 days. Similarly, adding 6 wt% DG in carbonation-curing SS contributed to increases in strength and CO2 uptake by 283% and 33.54%, reaching 42.68 MPa and 19.12%, respectively. Strength decreases and CO2 uptake increases with w/s. Microanalysis (QXRD, SEM-EDS, TG-DTG, FTIR, XPS, and MIP) revealed that the main hydration products of SS were C-S-H gel and C4AH13, which transformed to ettringite with DG addition. The carbonation products were mainly calcite and aragonite. Additionally, the amount of aragonite, mechanically weaker than calcite, decreased and calcite increased significantly when DG was added in carbonation-curing samples, providing a denser structure and higher strength than those without DG. Furthermore, high Al 2p binding energies revealed the formation of monocarboaluminate in the DG-added carbonation samples, corresponding to higher CO2 uptake. This study provides guidance for the preparation of SS-DG carbide building materials.

Stabilization mechanism of arsenic-sulfide slag by density functional theory calculation of arsenic-sulfide clusters

Stabilization of arsenic sulfur slag (As‒S slag) is of high importance to prevent the release of deadly As pollutants into environment. However, the molecular understanding on the stability of As‒S slag is missing, which in turn restricts the development of robust approach to solve the challenge. In this work, we investigated the structure-stability relationship of As‒S slag with adopting various As‒S clusters as prototypes by density functional theory (DFT). Results showed that the configuration of S multimers-covering-(As₂S₃)_n is the most stable structure amongst the candidates by the analysis of energies and bonding characteristics. The high stability is explained by orbital composition that the 4p-orbital (As) binding with 3p-orbital (S) decreases energy level of highest occupied molecular orbital (HOMO). Inspired from the calculations, an excess-S-based hydrothermal method was successfully proposed and achieved to promote the stabilization of As‒S slag. Typically, the As concentration from the leaching test of stabilized As‒S slag is only 0.8 mg/L, which is much lower than the value from other stabilized slag.

Sustainable stabilization/solidification of arsenic-containing soil by blast slag and cement blends

Arsenic (As) is a naturally occurring trace element that may pose a threat to human health and the ecosystem, while effective remediation and sustainable reuse of As-containing soil is a challenge. This study investigated the geoenvironmental characteristics of a geogenic As-rich soil, and green binders (ground granulated blast slag (GGBS) and cement blends) were employed for the stabilization/solidification (S/S) of the soil under field-relevant conditions. Results indicate that the use of 10% binder could effectively immobilize As and chemical stabilization/physical encapsulation jointly determined the leaching characteristics of the S/S soils. The geogenic As could be effectively immobilized at the pH range of 5.5-6.5. The increasing use of GGBS enhanced the strength of the 28-d cured S/S soils because of long-term pozzolanic reaction, but also slightly improved the As leachability. Besides, the moisture content of the contaminated soils should be suitably adjusted to allow for desirable compaction of S/S soils, which resulted in high compressive strength and low of As leachability. Results show that soil moisture content of 20% was the most appropriate, which resulted in the highest strength and relatively lower As leaching. In summary, this study presents a sustainable S/S binder for recycling As-contaminated soil by using a combination of cement and GGBS.

Feasibility of using fly ash-slag-based binder for mine backfilling and its associated leaching risks

As a binder to completely replace Portland cement for mine backfilling, the use of clinker-free cementitious materials combined with municipal solid waste incineration (MSWI) fly ash is proposed to achieve the targets of low-cost green backfilling, safe disposal and resource utilisation of bulk urban hazardous waste and metallurgical solid waste. This study balances the positive and negative effects of adding MSWI fly ash to the backfill by controlling its quantity in the binders, thus establishing an optimal concentration of 49 wt.% steel slag (SS), 21 wt.% blast furnace slag (BFS), 10 wt.% MSWI fly ash and 20 wt.% flue gas desulfurisation (FGD) gypsum. It is also reported that the filling performance of slurry (A2) satisfied strength requirements and is very suitable for long-distance transportation according to filling parameters. The leaching levels of the target elements (Cr, Ni, Zn, As, Cd, Sb, Pb, Hg and dioxins) for A2 matrix are lower than the required maximum concentration limits for the underground class Ⅲ water standard. Furthermore, the risk of leaching harmful constituents is mainly controlled by the pH of the environmental and the excellent buffering capacity of the matrix can reduce the potential leaching risk. The encapsulation, precipitation and adsorption of low-solubility double salts, such as hydrate calcium chloroaluminate (HCC) and ettringite, are the solidification/stabilisation (S/S) mechanism of series A on harmful substances. In addition, the high degree of polymerization(Ca/Si = 1.18 < 1.2, at 90d), the formation of long-chain C-S-H gels in binder A2-2, the dense pore structure lead to very stable growth in strength and control of leaching risks in subsequent periods.

Optimal Mixture Designs for Heavy Metal Encapsulation in Municipal Solid Waste Incineration Fly Ash

Mixing municipal solid waste incineration fly ash (MSWIFA) with industrial by-products such as ground granulated blast furnace slag (GGBFS) and ladle furnace slag (LFS) can lead to a hardened system which can encapsulate the heavy metals present in the MSWIFA. The objective of this study is to find optimal mixture designs to effectively encapsulate these heavy metals. The nature of the hydrates and the strength of the mixtures are studied to develop a sustainable and practical construction material incorporating MSWIFA. Heavy metals including Cr, Cu, Zn and Cd are safely encapsulated in several developed mixtures with leachate concentration below EPA drinking water limit. The encapsulation behavior is complex and depends on metal type, age of testing, and hydration products. In general, mixtures containing LFS have more aluminate hydrates, and show greater encapsulation capacity for most heavy metals. However, they also generally show significant Sb leaching. Mixtures which show satisfactory encapsulation for all ions and adequate strength development are identified. Three ideal mixtures, including one containing zero cement, are identified which satisfy both leaching and strength requirements.

Influence of calcium hydroxide addition on arsenic leaching and solidification/stabilisation behaviour of metallurgical-slag-based green mining fill

In this study, metallurgical-slag-based binder (MSB) with different dosages of calcium hydroxide (CH) was mixed with high-arsenic-containing mine tailings (HAMT) to form green mining fill samples (GMFs) for As solidification/stabilisation (S/S). The As leaching characteristics of the GMFs were evaluated using pH-dependent leaching tests, semi-dynamic leaching tests and toxicity leaching tests. The effective diffusion coefficient (D_e) decreased from 6.98 × 10^-14 to 5.90 × 10^-15 cm²/s and the leachability index (LI) increased from 13.53 to 14.73 after 3 wt.% CH was added to the GMFs. The GMFs containing 0 wt.% CH (GMF-0C) and those containing 3 wt.% CH (GMF-3C) reached pH = 2 with acid addition amounts of 9.0 meq/g-dry and 9.3 meq/g-dry at 90 d curing time, and the maximum As leaching concentrations of GMF-0C and GMF-3C reached 10.47 mg/L and 7.47 mg/L, respectively, indicating that GMF-3C exhibited better acid neutralisation and As retention capacities than GMF-0C. Further, a Tescan Integrated Mineral Analyser (TIMA) was used to analyse the dominant hydration products of GMF-3C, which revealed that calcium silicate hydrate, CASH, ettringite and zeolite phases represented approximately 22.5 wt.% of the products. These results provide an understanding regarding the safe large-scale utilisation of GMFs.

Stabilization of lead contaminated soil with traditional and alternative binders

The application of an innovative solidification/stabilization (S/S) process was investigated for the remediation of Pb contaminated soil. The performance of Pb stabilization was evaluated by comparing the use of calcium aluminate cement (CAC) and an alkali activated metakaolin binder vs the Ordinary Portland Cement (OPC). The phase composition of the stabilized products was investigated by XRD and correlated to the internal microstructure obtained by SEM-EDX imaging. Leaching tests were performed to ascertain the effectiveness of the proposed binders in the S/S of the contaminated soil, and Pb release was evaluated for each binding system. The overall results proved that multiple mechanisms are involved in Pb retention and that key parameters regulating the stabilization performance are strongly dependent on the type of applied binder system. Pb was found to be associated to C-S-H in the case of OPC, whereas ettringite played a key role in the retention of this contaminant using the CAC binder. The use of a NaOH activated metakaolin resulted in almost total retention of Pb, despite a lack of solidification, highlighting the importance of pH in the regulation of the leaching behavior.

Investigation into the semi-dynamic leaching characteristics of arsenic and antimony from solidified/stabilized tailings using metallurgical slag-based binders

The leaching characteristics of metallurgical slag-based binders (MSB) solidified/stabilized tailings containing arsenic (As) and antimony (Sb), were investigated via a series of semi-dynamic leaching tests using three kinds of leachant, for the simulation of actual leaching conditions. The effectiveness of solidification/stabilization (S/S) treatment was evaluated by measuring the observed diffusion coefficients (D_obs). It was found that MSB efficiently prevented As and Sb leaching, providing D_obs values in the range of 10^-15 to 10^-13 cm²/s and 10^-11 to 10^-9 cm²/s, respectively, with the exception that the leaching mechanism of As was dissolution rather than diffusion under acetic acid leaching conditions. Physical encapsulation was found to be the dominant mechanism for Sb immobilization, while the dominant mechanism of As immobilization was precipitation in the monolithic MSB S/S treated tailings (MST). Results showed that the concentrations of leached As, Sb, Ca and Si, were affected by leachant pH and total acidity as well as the MSB constituent ratio. The effect of these parameters may be attributed to the stability of hydration products and their influence on the buffering capacity and structure of matrices, and the leachant pH and total acidity having the greatest influence on leaching characteristics.

Mechanistic insights into red mud, blast furnace slag, or metakaolin-assisted stabilization/solidification of arsenic-contaminated sediment

Elevated level of arsenic (As) in marine sediment via deposition and accumulation presents long-term ecological risks. This study proposed a sustainable stabilization/solidification (S/S) of As-contaminated sediment via novel valorization of red mud waste, blast furnace slag and calcined clay mineral, which were selected to mitigate the increased leaching of As under alkaline environment of S/S treatment. Quantitative X-ray diffraction and thermogravimetric analyses illustrated that stable Ca-As complexes (e.g., Ca₅(AsO₄)₃OH) could be formed at the expense of Ca(OH)₂ consumption, which inevitably hindered the hydration process and S/S efficiency. The ²⁹Si nuclear magnetic resonance analysis revealed that incorporation of metakaolin for As immobilization resulted in a low degree of hydration and polymerization, whereas addition of red mud promoted Fe-As complexation and demonstrated excellent compatibility with As. Transmission electron microscopy and elemental mapping further confirmed the precipitation of crystalline Ca-As and amorphous Fe-As compounds. Therefore, red mud-incorporated S/S binder achieved the highest efficiency of As immobilization (99.9%), which proved to be applicable for both in-situ and ex-situ S/S of As-contaminated sediment. These results advance our mechanistic understanding for the design of green and sustainable remediation approach for effective As immobilization.

Recent advances in flue gas desulfurization gypsum processes and applications - A review

Flue gas desulfurization gypsum (FGDG) is an industrial by-product generated during the flue gas desulfurization process in coal-fired power plants. Due to its abundance, chemical and physical properties, FGDG has been used in several beneficial applications. However, during the past decade, the rate of beneficially used FGDG has gradually decreased, while its production has drastically increased. The presence of hazardous elements such as arsenic, mercury, cadmium, lead, and selenium in FGDG has reduced its beneficial value. Nevertheless, due to the recent developments in flue gas desulfurization processes, the "modern" FGDG contains lesser amounts of these elements, thus increasing its beneficial value and appeal to be included in other products. Hence, there are novel and traditional FGDG applications in different reuse scenarios investigated recently that have been deemed to pose minimal environmental concern - these need to be better understood. This review summarizes beneficial FGDG applications that have been deemed to pose minimal environmental concern, emphasizing their principles, research gaps, and potential developments, with the aim of increasing the reuse rate of FGDG.

Effects of acid and phosphate on arsenic solidification in a phosphogypsum-based cement backfill process

Phosphogypsum (PG) produced during phosphoric acid production contains significant amounts of arsenic and can potentially cause adverse environmental and health effects. Cement backfill technology is an effective management technique that is used to store PG to prevent such problems. The goal of this paper is to study the influencing factors and mechanism of arsenic stabilization in a PG-based cement backfill process. First, a leaching toxicity test was conducted, which showed that the arsenic concentration in PG batches ranged from 129.1 μg L⁻¹ to 407.1 μg L⁻¹, which were all far above the standard limit (10 μg L⁻¹) set by GB/T 14848-93. In addition, the arsenic content was higher in samples with larger PG particles. Secondly, hydrogen and phosphate ions were added to the backfill to investigate how they influenced arsenic solidification, and the results indicated that phosphate ions, rather than hydrogen ions, delayed the arsenic solidification process. This suggests that controlling the soluble phosphate in PG will help reduce arsenic pollution during backfilling. A toxicity leaching test was carried out after backfill samples were cured for 28 d. All arsenic concentrations were below the standard limit, indicating that the cement backfill technology ensured the long-term solidification and stabilization of arsenic.

Phosphogypsum (PG) produced during phosphoric acid production contains significant amounts of arsenic and can potentially cause adverse environmental and health effects.