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

Intelligent technologies powering clean incineration of municipal solid waste: A system review

Cleanliness has been paramount for municipal solid waste incineration (MSWI) systems. In recent years, the rapid advancement of intelligent technologies has fostered unprecedented opportunities for enhancing the cleanliness of MSWI systems. This paper offers a review and analysis of cutting-edge intelligent technologies in MSWI, which include process monitoring, intelligent algorithms, combustion control, flue gas treatment, and particulate control. The objective is to summarize current applications of these techniques and to forecast future directions. Regarding process monitoring, intelligent image analysis has facilitated real-time tracking of combustion conditions. For intelligent algorithms, machine learning models have shown advantages in accurately forecasting key process parameters and pollutant concentrations. In terms of combustion control, intelligent systems have achieved consistent prediction and regulation of temperature, oxygen content, and other parameters. Intelligent monitoring and forecasting of carbon monoxide and dioxins for flue gas treatment have exhibited satisfactory performance. Concerning particulate control, multi-objective optimization facilitates the sustainable utilization of fly ash. Despite remarkable progress, challenges remain in improving process stability and monitoring instrumentation of intelligent MSWI technologies. By systematically summarizing current applications, this timely review offers valuable insights into the future upgrade of intelligent MSWI systems.

Characterization of novel polysulfide polymer coated fly ash and its application in mitigating diffusion of contaminants

Fly ash consists of a considerable amount of hazardous elements with high mobility, posing substantial environmental risks during storage in surface impoundments and landfills. This hinders its efficient reuse in construction or material industries. To enhance the versatility of fly ash applications, a novel surface modification technique, termed SuMo, has been developed to create a hydrophobic polysulfide polymer coating on the surface of fly ash particles. The physicochemical properties of SuMo fly ash samples were examined using atomic force microscopy (AFM), environmental scanning electron microscopy (ESEM), thermal gravimetric analysis (TGA), Fourier Transform Infrared spectroscopy (FTIR), and leaching of hazardous elements was tested under practical environmental conditions (pH 4-12) based on the EPA's leaching environmental assessment framework (LEAF). The successful coating of polysulfide polymer on fly ash surface was verified through an increased percentage of C, S, and O in elemental mapping, coupled with the identification of S-O, CO, and C-H functional groups consistent with the chemical structure of polysulfide polymer. While the SuMo fly ash particles maintained their spherical shape, they exhibited increased surface roughness, robust hydrophobicity, and thermal stability up to 250 °C. Notably, owing to the coating's resilience against water leaching, the SuMo fly ash demonstrated a substantial reduction (up to 60-fold) in leachate concentrations of multiple concerning elements, including B, Be, Ba, Mn, Zn, As, Cr, Hg, etc., under various pH conditions compared to the uncoated fly ash. Furthermore, the polysulphide polymer coating effectively prevented Hg volatilization from fly ash below 163 °C. This study highlights the efficacy of the developed polysulfide polymer coating in mitigating the diffusion of hazardous elements from fly ash, thereby enhancing its potential reutilization in material, construction, and agriculture industries.

The leaching behavior of hazardous element under different leaching procedure utilizing slag-fly ash-based agent: Chromium, antimony, and lead

Low-carbon cementitious materials based on blast furnace slag (BFS) and municipal solid waste incineration (MSWI) fly ash play a pivotal role in the construction industry by substituting cement clinker. This innovation significantly reduces CO2 emissions and enables the extensive utilization of both industrial solid waste and hazardous urban waste on a large scale. However, the application of MSWI fly ash as a precursor for alkali-activated cementitious materials presents a significant leaching risk of heavy metal during the extended reaction process, posing a critical barrier to the efficient and widespread utilization of these solid waste. Three static leaching methods [horizontal vibration (HV), sulphuric acid & nitric acid (SN), and acetic acid buffer solution (AAB)], along with acid neutralization capacity (ANC) leaching tests, were applied in BFS-fly ash-based cementitious materials (BFCM) to assess the leaching behavior of high-risk elements-Cr, Sb, and Pb-within MSWI fly ash. The A4 matrix (BFS: MSWI fly ash:FGDG = 70:20:10) exhibits a compressive strength of 72.51 MPa at 180 day, with the leaching concentrations of target elements remaining below the standard limit under chemical attack (H+ and OH-). The critical pH determined is 9.2 from the ANC leaching test results. Visual MINTEQ simulation illustrates the occurrence states of Cr, Sb, and Pb as (CrO4)2-, [Sb(OH)6]-, and Pb(OH)3- within the BFCM system, respectively. The "double salt effect", intended to enhance the dissociation degree of BFS, acts as the driving force behind the long-term hydration reaction. It also serves as an assurance in controlling the long-term leaching risk of object elements. The dissociation degree of BFS within A4 matrix increased by 38.71 %, with the relative content of the typical low-solubility double salt "Ettringite" reaching 29 % at 180 d. This study provides novel theoretical and data-driven evidence to investigate the leaching behavior associated with MSWI fly ash and the accomplishment of replacing cement clinker with low-carbon BFCM.

Medical waste incineration fly ash-based magnesium potassium phosphate cement: Calcium-reinforced chlorine solidification/stabilization mechanism and optimized carbon reduction process strategy

The traditional solidification/stabilization (S/S) technology, Ordinary Portland Cement (OPC), has been widely criticized due to its poor resistance to chloride and significant carbon emissions. Herein, a S/S strategy based on magnesium potassium phosphate cement (MKPC) was developed for the medical waste incineration fly ash (MFA) disposal, which harmonized the chlorine stabilization rate and potential carbon emissions. The in-situ XRD results indicated that the Cl- was efficiently immobilized in the MKPC system with coexisting Ca2+ by the formation of stable Ca5(PO4)3Cl through direct precipitation or intermediate transformation (the Cl- immobilization rate was up to 77.29%). Additionally, the MFA-based MKPC also demonstrated a compressive strength of up to 39.6 MPa, along with an immobilization rate exceeding 90% for heavy metals. Notably, despite the deterioration of the aforementioned S/S performances with increasing MFA incorporation, the potential carbon emissions associated with the entire S/S process were significantly reduced. According to the Life Cycle Assessment, the potential carbon emissions decreased to 8.35 × 102 kg CO2-eq when the MFA reached the blending equilibrium point (17.68 wt.%), while the Cl- immobilization rate still remained above 65%, achieving an acceptable equilibrium. This work proposes a low-carbon preparation strategy for MKPC that realizes chlorine stabilization, which is instructive for the design of S/S materials.

Synergistic mechanism of iron manganese supported biochar for arsenic remediation and enzyme activity in contaminated soil

This study prepared and characterized bamboo-derived biochar loaded with different ratios of iron and manganese; evaluated its remediation performance in arsenic-contaminated soil by studying the changes in various environmental factors, arsenic speciation, and arsenic leaching amount in the soil after adding different materials; proposed the optimal ratio and mechanism of iron-manganese removal of arsenic; and explained the multivariate relationship between enzyme activity and soil environmental factors based on biological information. Treatment with Fe-Mn-modified biochar increased the organic matter, cation exchange capacity, and N, P, K, and other nutrient contents. During the remediation process, O-containing functional groups such as Mn-O/As and Fe-O/As were formed on the surface of the biochar, promoting the transformation of As from the mobile fraction to the residual fraction and reducing the phytotoxicity of As, and the remediation ability for As was superior to that of Fe-modified biochar. Mn is indispensable in the FeMn-BC synergistic remediation of As, as it can increase the adsorption sites and the number of functional groups for trace metals on the surface of biochar. In addition to electrostatic attraction, the synergistic mechanism of ferromanganese-modified biochar for arsenic mainly involves redox and complexation. Mn oxidizes As(Ⅲ) to more inert As(V). In this reaction process, Mn(Ⅳ) is reduced to Mn(Ⅲ) and Mn(II), promoting the formation of Fe(Ⅲ) and the conversion of As into Fe-As complexes, while As is fixed due to the formation of ternary surface complexes. Moreover, the effect of adding Fe-Mn-modified biochar on soil enzyme activity was correlated with changes in soil environmental factors; catalase was correlated with soil pH; neutral phosphatase was correlated with soil organic matter; urease was correlated with ammonia nitrogen, and sucrase activity was not significant. This study highlights the potential value of FM1:3-BC as a remediation agent in arsenic-contaminated neutral soils.

Solidification of heavy metals in municipal solid waste incineration washed fly ash by asphalt mixture

Using asphalt mixture to solidify heavy metals in municipal solid waste incineration fly ash can reduce pollution and realize resource utilization. In this study, the physical and chemical properties of washed fly ash were analyzed, and washed fly ash was added to asphalt mixture as filler instead of mineral powder. The study involved analyzing the mechanical attributes of asphalt mixtures containing washed fly ash, along with examining the characteristics of asphalt binder that incorporates the washed fly ash. Subsequently, assess the potential leaching hazards associated with asphalt mixture incorporating washed fly ash. The test results showed that washed fly ash was a Si-Al-Ca system material, which had small particle size, large specific surface area and many pores. It increased the contact area with asphalt, which improved encapsulation of asphalt and aggregates. The optimal dosage of washed fly ash is 2.5%. At this dosage, the mixture attains optimal high-temperature performance, while both low-temperature performance and the characteristics of washed fly ash asphalt binder align with requirements. Asphalt mixture has solidification on heavy metals, with strongest solidification for Zn, followed by Cu, Cr. A prediction model of leaching amount versus time was constructed for Pb, Ba and Ni, which have weak solidified ability. The cumulative leaching amount of the road within 15 years of service life was calculated through the model, and it was obtained that the addition of washed fly ash will not cause pollution to environment. Overall, this study showed that asphalt mixtures can be used for stabilization/solidification of washed fly ash while saving natural mineral, providing a theoretical basis for the resource application of washed fly ash in asphalt road construction.

Leaching toxicity and deformation failure characteristics of phosphogypsum-based cemented paste backfill under chemical solution erosion

In order to explore the potential environmental and safety risks of phosphogypsum-based cemented paste backfill (PCPB) in mines, aiming at the actual problems of different acidity and alkalinity of the groundwater environment where PCPB is located, the chemical solution erosion test, element concentration determination test, uniaxial compressive strength (UCS) test, and microscopic analysis test of PCPB were carried out. The effects of three different chemical solutions, HCl solution, NaOH solution, and pure water on the leaching toxicity and deformation failure characteristics of PCPB were analyzed. The kinetic equations of pH value of PCPB in the HCl and NaOH solutions, the leaching models of total P and fluoride, and the UCS erosion model of PCPB were established. The research shows that the pH value of PCPB is weak alkaline or alkalinity, when it reaches dynamic equilibrium in different chemical solutions. The leaching concentration of total P is higher than the Class III standard of surface water; the leaching concentration of fluoride is higher than the Class III standard of surface water, the Class III standard of groundwater, and the Class I standard of sewage. In the early stage of chemical solution erosion, scanning electron microscope (SEM) images show that the hydration product C-S-H gel and Aft are intertwined and firmly combined. The research results have important engineering practice and application value in mine environmental governance and safety management.

Mineral phase transition characteristics and its effects on the stabilization of heavy metals in industrial hazardous wastes incineration (IHWI) fly ash via microwave-assisted hydrothermal treatment

Toxic heavy metals in industrial hazardous waste incineration (IHWI) fly ash can be effectively stabilized by using microwave-assisted hydrothermal technology. However, few works have focused on the relationship between mineralogical conversion and stability of heavy metals of fly ash during hydrothermal process. This study investigated the effect of mineral phase transition process on the stabilization and migration behavior of heavy metals in IHWI fly ash using coal fly ash as silicon‑aluminum additive. Mineral composition analysis reveals that after microwave-assisted hydrothermal treatment (MAHT) of IHWI fly ash, zeolite-like minerals (e.g., tobermorite, katoite and sodalite), secondary aluminosilicate minerals (e.g., prehnite and anorthite) and other newly-formed minerals (e.g., wollastonite, pectolite and larnite) were found. The leaching concentrations of heavy metals (Cr, Ni, Cu, Zn, Cd and Pb) in IHWI fly ash decrease sharply after MAHT with the most obvious decreases in Cu, Pb and Zn. Spearman correlation analysis show significantly negative correlation between the content of zeolite-like minerals and the leaching concentrations of most heavy metals (e.g., Ni, Cu, Zn, Cd and Pb). These results suggest that the immobilization effects of heavy metals in IHWI fly ash can be effectively enhanced by promoting the formation of zeolite-like minerals during the MAHT. This study is expected to further promote the development of IHWI fly ash harmless treatment technology.

Effect and mechanism of nano-alumina on early hydration properties and heavy metals solidification/stabilization of alkali-activated MSWI fly ash solidified body

Municipal solid waste incineration (MSWI) fly ash has serious pollution. It needs to be solidification/stabilization (S/S) to sanitary landfill as quickly as possible. In order to achieve the objective, the early hydration properties of alkali-activated MSWI fly ash solidified body were investigated in this paper. Meanwhile, nano-alumina was utilized as an agent to optimize the early performance. Therefore, the mechanical properties, environmental safety, hydration process and mechanisms of heavy metals S/S were explored. The results showed that after adding nano-alumina, the leaching concentration of Pb and Zn in solidified bodies after 3 d curing was significantly reduced by 49.7-63% and 65.8-76.1%, respectively, and the compressive strength was enhanced by 10.2-55.9%. Nano-alumina improved the hydration process, and the predominant hydration products in solidified bodies were C-S-H gels and C-A-S-H gels. Meanwhile, nano-alumina could obviously increase the most stable chemical speciation (residual state) ratio of heavy metals in solidified bodies. Pore structure data showed that, due to the filling effect and pozzolanic effect of nano-alumina, the porosity has been reduced and the ratio of harmless pore structure has been increased. Therefore, it can be concluded that solidified bodies mainly solidify MSWI fly ash by physical adsorption, physical encapsulation and chemical bonding.

Strength Investigation and Prediction of Superfine Tailings Cemented Paste Backfill Based on Experiments and Intelligent Methods

The utilization of solid waste for filling mining presents substantial economic and environmental advantages, making it the primary focus of current filling mining technology development. To enhance the mechanical properties of superfine tailings cemented paste backfill (SCPB), this study conducted response surface methodology experiments to investigate the impact of various factors on the strength of SCPB, including the composite cementitious material, consisting of cement and slag powder, and the tailings' grain size. Additionally, various microanalysis techniques were used to investigate the microstructure of SCPB and the development mechanisms of its hydration products. Furthermore, machine learning was utilized to predict the strength of SCPB under multi-factor effects. The findings reveal that the combined effect of slag powder dosage and slurry mass fraction has the most significant influence on strength, while the coupling effect of slurry mass fraction and underflow productivity has the lowest impact on strength. Moreover, SCPB with 20% slag powder has the highest amount of hydration products and the most complete structure. When compared to other commonly used prediction models, the long-short term memory neural network (LSTM) constructed in this study had the highest prediction accuracy for SCPB strength under multi-factor conditions, with root mean square error (RMSE), correlation coefficient (R), and variance account for (VAF) reaching 0.1396, 0.9131, and 81.8747, respectively. By optimizing the LSTM using the sparrow search algorithm (SSA), the RMSE, R, and VAF improved by 88.6%, 9.4%, and 21.9%, respectively. The research results can provide guidance for the efficient filling of superfine tailings.

Synthesis of geopolymer using municipal solid waste incineration fly ash and steel slag: Hydration properties and immobilization of heavy metals

In this study, a novel method for the disposal of municipal solid waste incineration fly ash (MSWIFA) was proposed. By applying geopolymer technology, steel slag (SS) and MSWIFA were used together as precursors to synthesize a cementitious material with sufficient strength that is useable in construction. The effects of the dosages of SS and alkaline activator on the properties of the geopolymer were investigated. Compressive testing was used to characterize the mechanical properties of the geopolymer. X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used for microscopic analysis. Leaching tests were performed to assess the immobilization effect of the geopolymer on heavy metals. The results showed that the compressive strength of the geopolymer reached 23.03 MPa at 56 d with 20% SS and 11% Na₂O admixture. Highly polymerized hydration products, such as C-(A)-S-H gels and N-A-S-H gels, contributed to the compact microstructure, which provided mechanical strength and limited the migration and leaching of heavy metals in the geopolymer matrix. In terms of the results, this work is significant for the development of MSWIFA management.

Application of desulfurization gypsum as activator for modified magnesium slag-fly ash cemented paste backfill material

Recycling industrial solid waste for mine backfill is one of the best ways to achieve green production in multiple industries. In this paper, the desulfurization gypsum (DG) as an activator is combined with the modified magnesium slag-fly ash cementitious paste backfill (MFPB) technology for the co-disposal of solid waste and goaf treatment, and the influence of DG on the performance of MFPB was comprehensively analyzed through rheological properties, mechanical properties, durability, microscopic analysis and environmental characteristics experiments. The results show that the fresh MFPB mortar conforms to the Herschel-Bulkley model at different maximum shear rate (γ̇_max) conditions. When the γ̇_max is 100 s^-1, the mortar exhibits shear-thickening properties. The apparent viscosity, yield stress and static yield stress of mortar decreased first and then increased with the increase of DG content, and all had the minimum value when DG was 2.5 %. The thixotropy of the mortar was significantly increased with the addition of DG, and the change in thixotropy was significantly correlated with the difference between the two yield stresses. Both the rheological and mini-slump results demonstrate that DG can improve the flowability of MFPB mortars. In addition, the UCS of D0 under steam curing and standard water curing conditions for 28 d were 4.342 MPa and 2.827 MPa, and the sample containing DG were 6.109-8.241 MPa and 6.669-9.492 MPa, respectively. The addition of DG not only improves the strength of MFPB, but also improves the durability of MFPB. Microscopic analysis (XRD, SEM, and TG-DTG) indicated that this was mainly because DG promoted the hydration reaction of the MMS-FA system and accelerated the generation of C-S(A)-H and AFt. Finally, in the results of in situ leaching based on durability and leaching based on standard HJ 557, all the indexes of MFPB meet the standard of class III groundwater in GB/T 14848-2017, and it has an effective stabilization/solidification effect on heavy metals (As, Cu, Ni, Ba, Zn and Mo, etc.). To sum up, the collaboration of DG and MFPB technology can not only efficiently clean and utilize a variety of solid wastes (MMS, FA and DG), but also greatly improve the performance of MFPB to promote its application.

Study on mechanical properties and damage characteristics of rice straw fiber-reinforced cemented tailings backfill based on energy evolution

Low-cost and underutilized plant fibers can affect the mechanical behavior of cementitious materials such as cemented tailings backfill (CTB). This paper attempts to explore the mechanical properties and damage evolution characteristics of rice straw fiber (RFS)-reinforced CTB (RSFCTB) from the perspective of energy. A series of mechanical and microscopic tests were carried out on CTB and RSFCTB samples. On this basis, the energy evolution law and of the filling body under different stress paths were analyzed. Meanwhile, a damage variable based on dissipation energy was established, and the damage evolution process of the filling body was discussed. The results show that uniaxial compressive strength (UCS) of filling body first grew and then dropped with the enhancement of RSF content, and indirect tensile strength (ITS) was positively correlated with RSF content. Scanning electron microscope showed that RSF was encapsulated by hydration products, which promoted the bridging effect of RSF. The bridging effect of RSF improved the integrity of RSFCTB after compression failure and resulted in bending and asymmetric tensile cracks after tensile failure. The energy storage limit and dissipation energy of the filling body under different stress paths were enhanced due to the incorporation of RSF. The damage curve based on dissipation energy showed three stages of slow, steady, and fast damage under compressive loading. The damage curve of RSFCTB was located below CTB depending on the crack arresting effect of RSF. Moreover, the damage curve under tensile load shows three stages: slow, stable damage, and sudden increase in damage. The damage value of RSFCTB at the mutation point was increased, and the ability of RSFCTB to resist tensile damage was enhanced. The energy evolution and acoustic emission parameters were combined, and their development trends were similar, which proved that it was reasonable to characterize the damage of filling body based on the dissipated energy.

Geopolymer composite spheres derived from graphene-modified fly ash/slag: Facile synthesis and removal of lead ions in wastewater

Geopolymer composite spheres derived from potassium-activated graphene-modified slag/fly ash powder were produced in a polyethylene glycol (PEG 400) solvent. The effect of graphene type (graphene oxide (GO) and few-layered graphene (GNP)) on the pore structure and lead ions (Pb²⁺) removal performance of the spheres were evaluated. The results showed that the composite spheres modified with GOs (0.1-0.4 wt%) and GNPs (1-4 wt%) could be spheroidized with an improved performance to adsorb Pb²⁺ in solution. The graphene-containing spheres reached a maximum BET surface area of 68.85 m²/g. Pseudo-second-order and Langmuir isotherm models could express the adsorption process, which was controlled by both monolayer adsorption and chemisorption. The obtained spheres also showed high adsorption capacities for Ni²⁺, Cu²⁺, Zn²⁺ and Cd²⁺ ions. Chemical, physical, electrostatic, ion exchange and cation-π interaction were attributed to the adsorption mechanism of the spheres. The spheres showed good cycling ability compared to those without graphene, which had potential application in heavy metal wastewater treatment.

The Preparation Process and Hydration Mechanism of Steel Slag-Based Ultra-Fine Tailing Cementitious Filler

Steel slag, desulphurised ash, desulphurised gypsum and ultra-fine iron tailing sand are common industrial solid wastes with low utilisation rates. Herein, industrial solid wastes (steel slag, desulphurised gypsum and desulphurised ash) were used as the main raw materials to prepare a gelling material and ultra-fine tailing was used as an aggregate to prepare a new type of cementing filler for mine filling. The optimal composition of the cementing filler was 75% steel slag, 16.5% desulphurised gypsum, 8.75% desulphurised ash, 1:4 binders and tailing mass ration and 70% concentration. The compressive strength of the 28-day sample reached 1.24 MPa, meeting the mine-filling requirements, while that of the 90-day sample was 3.16 MPa. The microscopic analysis results showed that a small amount of C₃A reacted with the sulphate in the desulphurised gypsum to form ettringite at the early stage of hydration after the steel slag was activated by the desulphurisation by-products. In addition, C₂S produced hydrated calcium silicate gel in an alkaline environment. As hydration proceeded, the sulphite in the desulphurised ash was converted to provide sulphate for the later sustained reaction. Under the long-term joint action of alkali and sulphate, the reactive silica-oxygen tetrahedra and alumina-oxygen tetrahedra depolymerised and then polymerised, further promoting the hydration reaction to generate hydrated calcium silicate gel and ettringite. The low-carbon and low-cost filler studied in this paper represents a new methodology for the synergistic utilisation of multiple forms of solid waste.

Improving stabilization/solidification of MSWI fly ash with coal gangue based geopolymer via increasing active calcium content

Municipal solid waste incineration fly ash (MSWI FA) is categorized as a hazardous waste, which demands environmentally acceptable treatment due to its easy leachability toxic of heavy metals. This study investigated an innovative and improved method for stabilization/solidification (S/S) of MSWI FA with coal gangue based geopolymer by the addition of active calcium content. The specimen with addition of calcium oxide up to 10 % reached the compressive strength of 2.14 MPa at 28 d. The addition of 30 % calcium oxide resulted in the highest immobilization efficiencies of Cd (98.96 %) and Pb (99.19 %). X-ray Diffraction (XRD), Fourier Transform Infrared Spectrometry (FTIR), Scanning Electron Microscope (SEM), and thermogravimetric (TG) analysis indicated the generation of calcium-containing hydration products was promoted after the improvement of calcium content in binder. Heavy metals were stabilized through the chemical adsorption and ions exchange of amorphous hydration products. On the whole, this study illustrated that the incorporation of active calcium content can improve efficiently S/S of hazardous ash waste such as MSWI FA.

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.

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.

Stabilization of Waste-to-Energy (WTE) fly ash for disposal in landfills or use as cement substitute

This study investigated two approaches for managing Waste-to-Energy (WTE) fly ash (FA): (i) phosphoric acid stabilization of FA and disposal in non-hazardous landfills, so that it can pass the U.S. TCLP procedure and meet the U.S. Resource Conservation and Recovery Act (RCRA) standards; (ii) use of FA or phosphoric acid stabilized fly ash (PFA) as cement substitute in construction for avoiding disposal in landfills and reducing the consumption of Portland cement. The effect of stabilization was identified by TCLP tests and XRD quantification (QXRD), which showed that the economically optimal concentration for PFA to pass the RCRA was 1 mol/L H₃PO₄ (equivalent to 0.4 mol of H₃PO₄/kg of FA). Zn/Pb-phosphates were formed in treated ash by using high concentration H₃PO₄ (e.g., 3 mol/L). Thus, the hazardous FA was chemically stabilized to PFA, that were both discussed as cement substitute. QXRD and SEM results showed that both FA and PFA (1 mol/L H₃PO₄) chemically reacted with cement and water. Up to 25 vol% of the cement can be replaced by FA or PFA, with similar mechanical performance of cement mortars than that of reference. Testing by LEAF Method 1313-pH dependence showed that the FA and PFA cement mortars exhibited the same leachability of heavy metals; therefore, this study demonstrated the technical feasibility of utilizing either raw FA or stabilized PFA as supplementary cementitious material. The leachability of heavy metals in optimal FA or PFA 25 vol% cement mortar was under the U.K. WAC non-hazardous limits.

A preliminary study of aeolian sand-cement-modified gasification slag-paste backfill: Fluidity, microstructure, and leaching risks

To realize low-cost green backfill mining, this paper proposes a novel model of aeolian sand-cement-modified gasification slag-paste backfill (ACGPB). This model realizes the safe disposal and resource utilization of hazardous solid wastes. A comprehensive experiment (including slump test, uniaxial compressive strength tests, microscopic test, and leaching toxicity tests) was conducted to explore how the mechanism of ACGPB depends on activator type and dosage. The results showed that fresh ACGPB slurry can be expressed by the Herschel-Bulkley model (R² ≥ 0.965 in all recipes). With Na₂SO₄ as activator type, the yield stress, apparent viscosity, thixotropy, and slump of ACGPB slurry increased with increasing activator dosage. With CaO as activator type, the yield stress, apparent viscosity, thixotropy, and slump of ACGPB slurry fluctuated with increasing activator dosage. The mechanical properties of all recipes (not including Control group and C-C1) met the mechanical requirement (3 d ≥ 0.5 MPa and 28 d ≥ 1.0 MPa). In addition, the concentrations of all heavy metals remained within the range specified by the national standard. Specifically, the activator exerted a positive effect on the stabilization/solidification of heavy metal ions (Cu, Cd, Ba, Ni, Cr, Se, and As). Finally, FTIR, TG-DTG, SEM, and hydration heat were used to analyze the microstructure of ACGPB. The research results provide a creative way for the resource utilization of solid waste.

Microstructure Analysis and Effects of Single and Mixed Activators on Setting Time and Strength of Coal Gangue-Based Geopolymers

Geopolymer is a green non-metallic material with high strength and favorable properties in resistance to corrosion, fire, and high temperature, which makes it a potential substitute for Portland cement. The existing studies have primarily focused on the preparation of geopolymers using silico-alumina materials such as fly ash, red mud, metakaolin, volcanic ash, and blast furnace slag to develop geopolymers. This study explores the potential of using ultrafine calcined coal gangue and ground granulated blast furnace slag to develop a new geopolymer with the activation of a single activator (sodium hydroxide) or mixed activator (sodium hydroxide, liquid sodium silicate, and desulfurization gypsum). The setting time and strength of the geopolymers were investigated, followed by the mineral, functional groups, microstructure, and elements analyses using X-ray diffraction, Fourier transform infrared diffraction, scanning electron microscope, and energy dispersive spectrometer to elucidate the effect of different activators on geopolymers. The results showed that the optimum molarity of NaOH single activator was 2 mol/L, the initial setting time and final setting time were 37 min and 47 min, respectively, and the compressive and flexural strengths at 28 days were 23.2 MPa and 7.5 MPa. The optimal mixing ratio of the mixed activator was 6% desulfurization gypsum, 0.6 Na₂SiO₃ modulus, and 16% SS activator; the initial setting time and final setting time were 100 min and 325 min, respectively, and the compressive and flexural strengths at 28 days were 40.1 MPa and 7.8 MPa. The coal gangue geopolymers were mainly C–A–S–H, N–A-S-H, and C–N–A–S–H gels. The mixed activator tended to yield higher strengths than the single activator, the reason is that the hydration reaction was violent and produced more gels. Meanwhile, the relation between setting time and activator and the relation between strength and activator were also obtained, which provide theoretical support for predicting the setting time of coal gangue base polymer and the ratio of alkali activator for geopolymers with a certain strength.

A comparative study on characteristics and leaching toxicity of fluidized bed and grate furnace MSWI fly ash

Grate furnace and fluidized bed are the most widely used technologies for municipal solid waste incineration (MSWI), which play significant roles in characteristics of MSWI fly ash. A comparative study of the physicochemical characteristics, microstructure morphology and leaching toxicity of fluidized bed and grate furnace MSWI fly ash was conducted in this work, and some resource utilization and disposal treatments were proposed. Results showed that calcium salt and chlorine salt formed the dominant components of MSWI fly ash. CaO-SiO₂-Al₂O₃ ternary system indicated that MSWI fly ash had potential pozzolanic activity, similar to coal fly ash and blast furnace slag. The total Pb, Cd and Zn contents in fluidized bed MSWI fly ash was only 1/2, 1/3 and 2/3 of grate furnace MSWI fly ash, respectively. Leachability of Pb in MSWI fly ash collected from Dalian, Shanghai, Zhuji and Hangzhou was 4.25 mg/L, 3.83 mg/L, 3.84 mg/L and 3.68 mg/L, 28.3, 25.5, 25.6 and 24.5 times as much as the national standard limitation (GB16889-2008), respectively. However, grate furnace MSWI fly ash with high chloride and unstable chemical speciation distribution of heavy metals would pose more environmental risk toits immobilization and disposal. Fluidized bed fly ash is a promising candidate for preparing the Portland cement clinker, microcrystalline glass and ceramics due to its high Si and Al content. Grate furnace MSWI fly ash is more appropriate for Alinite cement preparation because of high chloride content.

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.

Designing novel magnesium oxysulfate cement for stabilization/solidification of municipal solid waste incineration fly ash

Municipal solid waste incineration (MSWI) fly ash is a typical hazardous waste worldwide. In this study, an innovative magnesium oxysulfate cement (MOSC) binder was designed for stabilization/solidification (S/S) of MSWI fly ash, focusing on the interactions between MOSC binder and typical metallic cations (Pb²⁺)/oxyanions (AsO₃^3-). Experimental results showed that Pb and As slightly inhibited the reaction of high-sulfate 5MS system but significantly suppressed the reaction process of low-sulfate 10MS system. The 5MS binder system exhibited excellent immobilization efficiencies (99.8%) for both Pb and As. The extended X-ray absorption fine structure spectra suggested that Pb²⁺ coordinated with SO₄^2-/OH^- in the MOSC system and substituted Mg²⁺ ion sites in the internal structure of 5Mg(OH)₂·MgSO₄.7H₂O (5-1-7) phase. In contrast, the AsO₃^3- substituted SO₄^2- sites with the formation of inner-sphere complexes with Mg²⁺ in the large interlayer space of the 5-1-7 structure. Subsequent MSWI fly ash S/S experiments showed that a small amount of reactive Si in MSWI fly ash interfered with the MOSC reaction and adversely influenced the immobilization efficiencies of Pb, As, and other elements. Through the use of 33 wt% tailored MOSC binder for MSWI fly ash treatment, a satisfying S/S performance could be achieved.

Stability evaluation of potentially toxic elements in MSWI fly ash during carbonation in view of two leaching scenarios

Carbonation treatment (CT) by alkaline fly ash (FA) affects the stability of potentially toxic elements (PTEs). This study investigated the leachability and environmental risk of six PTEs contained in FA during natural and accelerated carbonation (NC, AC) using two typical leaching scenarios with distilled water (DW) and acetic acid (AA). The leaching of Pb/Cu/Cr/Ni in solidified/stabilized FA decreased due to CT in DW leaching, but the leaching of Pb/Zn/Cu/Cd increased due to CT in AA leaching. The leaching of the six PTEs (especially Pb/Cd) in AA leaching was significantly higher than that in DW leaching. CT was a promoting factor to increase the environmental risk level of PTEs in FA leachate, especially in AA leaching with H⁺ input. In the early stage of NC, under DW leaching tests, the environmental risk level of PTEs in FA leachate can be weakened due to the formation of carbonate minerals in the FA matrix. However, excessive NC increases the environmental risk of leached PTEs due to the decalcification of carbonate minerals. Both NC and AC increased the potential environmental risk of PTEs contained in the carbonated FA matrix. The nucleation and dissolution of carbonate minerals were interdependent with the immobilization and leaching of PTEs, which played a dominant role in the CT and leaching tests respectively. They jointly affected the occurrence behavior of PTEs in the FA matrix in CT tests and the leachability of PTEs in leaching tests. This study demonstrates that it is more scientific to evaluate the leachability of PTEs in carbonated FA according to the actual disposal scenarios.

Characterization of Macro Mechanical Properties and Microstructures of Cement-Based Composites Prepared from Fly Ash, Gypsum and Steel Slag

Using solid wastes (SWs) as backfilling material to fill underground mined-out areas (UMOAs) solved the environmental problems caused by SWs and reduced the backfilling cost. In this study, fly ash (FA), gypsum and steel slag (SS) were used to prepare cement-based composites (CBC). The uniaxial compression, computed tomography (CT) and scanning electron microscope (SEM) laboratory experiments were conducted to explore the macro and micromechanical properties of CBC. The findings showed that the uniaxial compressive strength (UCS) of CBC with a curing time of 7 d could reach 6.54 MPa. The increase of SS content reduced the UCS of CBC, while the gypsum and FA content could increase the UCS of CBC. Microscopic studies have shown that the SS particles in CBC have noticeable sedimentation, and the increase of SS content causes the failure mode of CBC from tensile to tensile-shear. These research results can provide a scientific reference for the preparation of backfilling materials.

Stabilization and passivation of multiple heavy metals in soil facilitating by pinecone-based biochar: Mechanisms and microbial community evolution

Soil contamination by multiple heavy metals and As is one of the major environmental hazards recognized worldwide. In this study, pinecone-biochar was used for stabilization and passivation of Pb, Cu, Zn, Cr, and As in contaminated soil around a smelter in Hubei province, China. The stabilization rate of heavy metals in soil can exceed 99%, and the leaching amount can meet the national standard of China (GB/T 5085.3-2007, less than 5, 100, 100, 15, and 5 mg/L, respectively.) within 90 days. The study confirmed that the addition of pinecone-biochar and the coexistence of indigenous microorganisms can effectively reduce the bioavailability of heavy metals. Among the heavy metals, As(III) can be oxidized to As(V) and then stabilized, and other heavy metals can be stabilized in a complex and chelated state characterized by X-ray photoelectron spectroscopy. After pinecone-biochar was added, the abundance of microbial community and intensity of metabolic activities became vigorous, the types and contents of dissolved organic matter increased significantly. A novel innovation is that the addition of pinecone-biochar increased the Bacillus and Acinetobacter in soil, which enhanced the function of inorganic ion transport and metabolism to promote the passivation and stabilization of heavy metals throughout the remediation process.

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.

The mechanism of hydrating and solidifying green mine fill materials using circulating fluidized bed fly ash-slag-based agent

This study focused on classifying and disposing Circulating fluidized bed (CFB) fly ashes from the level of its origin, and proposed an optimal formulation system for clinker-free cemented backfill materials. CFB fly ash-blast furnace slag (BFS)-based cemented backfill materials with unequal strength grades are used in different locations of the goaf that require more than 1 Mpa and 4 Mpa, respectively, and the leaching levels of all toxic components are lower than the underground III water quality standard limit when the additional amount of CFB fly ash does not exceed 60 wt.%. The stable S/S of Cl^- is due to the combined effect of chemical fixation of HCC and physical adsorption of the C-S-H/C-A-S-H phase. B2(20 wt.% CFB fly ash) exhibits more functional hydration products and higher degree of polymerization with the hydration age extension. Ettringite is the major effective product of CFB fly ash-BFS-based cemented system due to low level of chlorine environment and HCC transformation. CFB fly ash with appropriate active Al₂O₃ can dissolve and promote [AlO₄]^5- to substitute [SiO₄]^4- to form the C-A-S-H phase with longer chains and higher degree of polymerization with increase in Al/Si ratio of C-A-S-H/C-S-H phase.

Treatment of municipal solid waste incineration fly ash: State-of-the-art technologies and future perspectives

Municipal solid waste incineration (MSWI) fly ash is considered as a hazardous waste that requires specific treatment before disposal. The principal treatments encompass thermal treatment, stabilization/solidification, and resource recovery. To maximize environmental, social, and economic benefits, the development of low-carbon and sustainable treatment technologies for MSWI fly ash has attracted extensive interests in recent years. This paper critically reviewed the state-of-the-art treatment technologies and novel resource utilization approaches for the MSWI fly ash. Innovative technologies and future perspectives of MSWI fly ash management were highlighted. Moreover, the latest understanding of immobilization mechanisms and the use of advanced characterization technologies were elaborated to foster future design of treatment technologies and the actualization of sustainable management for MSWI fly ash.

Preparation of Cemented Oil Shale Residue-Steel Slag-Ground Granulated Blast Furnace Slag Backfill and Its Environmental Impact

A new environmentally friendly cemented oil shale residue-steel slag-ground granulated blast furnace slag backfill (COSGB) was prepared using oil shale residue (OSR), steel slag (SS) and ground granulated blast furnace slag (GGBS) as constituent materials. Based on univariate analysis and the Box-Behnken design (BBD) response surface method, the three responses of the 28 days unconfined compressive strength (UCS), slump and cost were used to optimize the mix ratio. Using a combination of scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and mercury intrusion porosimetry (MIP), the reaction products, microscopic morphology and pore structure of the specimens with the optimal mix ratio at different curing ages were analyzed. The influence of heavy metal ions from the raw materials and the COSGB mixtures on the groundwater environment was studied by leaching tests. The research demonstrates that the optimal mix ratio is GGBS mixing amount 4.85%, mass ratio of SS to OSR 0.82, and solid mass concentration 67.69%. At shorter curing age, the hydration products are mainly calcium alumino silicate hydrate (C-A-S-H) and calcium silicate hydrate (C-S-H) gels. With the increase of curing age, ettringite (AFt) and C-S-H gels become the main source of the UCS. Meanwhile, the porosity of the filler decreases continuously. The leaching concentration of heavy metal ions from the COSGB mixtures is all lower than the leaching concentration of raw materials and meet the requirements of the Chinese groundwater quality standard (GB/T 14848-2017). Therefore, this new COSGB cannot pollute the groundwater environment and meets backfill requirements. The proposed technology is a reliable and environmentally friendly alternative for recycling OSR and SS while simultaneously supporting cemented paste backfill (CPB).

Analysis of Strength and Microstructural Characteristics of Mine Backfills Containing Fly Ash and Desulfurized Gypsum

The utilization of solid wastes (SWs) as a potential resource for backfilling is not only conducive to environmental protection but also reduces the surface storage of waste. Two types of SWs, including fly ash (FA) and desulfurized gypsum (DG), were used to prepare cementitious backfilling materials for underground mined-out areas. Ordinary Portland cement (OPC) was used as cement in mine backfill. To better investigate the feasibility of preparing backfill materials, some laboratory tests, such as uniaxial compressive strength (UCS), scanning electron microscopy (SEM), and energy dissipation theory, were conducted to explore both strength and microstructural properties of backfilling. Results have demonstrated that the main components of FA and DG in this study are oxides, with few toxic and heavy metal components. The ideal ratio of OPC:FA:DG is 1:6:2 and the corresponding UCS values are 2.5 and 4.2 MPa when the curing time are 7 days and 14 days, respectively. Moreover, the average UCS value of backfilling samples gradually decreased when the proportion of DG in the mixture increased. The main failure modes of various backfilling materials are tensile and shearing cracks. In addition, the corresponding relations among total input energy, dissipated energy and strain energy, and stress–strain curve were investigated. The spatial distribution of oxygen, aluminum, silicon, calcium, iron and magnesium elements, and hydration product are explored from the microstructure’s perspective. The findings of this study provide both invaluable information and industrial applications for the efficient management of solid waste, based on sustainable development and circular economy.