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Coal fly ash (CFA) is a type of solid waste produced in the process of coal combustion, which is rich in silicon oxide, aluminum oxide and a small number of heavy metals and radioactive elements. Therefore, CFA is considered a secondary resource with high recovery value. Currently, CFA is mainly reused in the fields of building materials, mine backfilling, soil conditioners and fertilizers, among which the production of building materials is one of the most important ways to realize large-scale utilization of CFA. This paper introduces the physical and chemical properties, classification and environmental impact of CFA and summarizes the utilization status of CFA in building materials such as cement, concrete, ceramics and geopolymers, as well as the management policy of CFA. In addition, the existing problems in the utilization of CFA as a building material are analyzed, and their development prospects are discussed. [ABSTRACT FROM AUTHOR]

Safety and efficient dispose of municipal solid waste incineration (MSWI) fly ash with high toxicity has emerged as a worldwide challenge. Vitrification provides the advantages of capacity reduction, detoxification, and solidification of heavy metals, which has the potential to dispose of hazardous waste on a large scale. Herein, co–vitrification of MSWI fly ash and bottom slag has been accomplished based on the characteristics of calcium and silicon composition. A novel approach for producing glass ceramic foams by alkaline activation–crystallization was developed to realize the disposal of the obtained glass. The effect of MSWI fly ash/bottom slag ratios on the glass network, crystallization ability of the basic glass, pore structure, and physical properties of the porous green body was investigated. The results revealed that with increasing MSWI fly ash proportion, the Si–O of [SiO 4 ] in the basic glass changed significantly and the crystallization ability steadily reduced. Si–O and Al–O in basic glass are easy to corrode under alkaline conditions, releasing Ca2+ and forming a low solubility product, calcium silicate hydrate. When the crystallization temperature increases from 950 ℃ to 1150 ℃, it is more conducive to the precipitation of the gehlenite phase. Extending the crystallization time promotes three–dimensional growth of crystals that are coupled with each other to form a network structure and a multi–stage pore structure. The pore structure was developed with the help of NH 3 and H 2 generated by the secondary aluminum ash (SAA). Through the preparation of glass ceramic foams, the raw materials were detoxified. The toxic heavy metals showed extremely low leaching concentrations, which were smaller than the limit of TCLP. The prepared samples had 70.22–80.61% of porosity, 0.78–1.19 g/cm3 of low bulk density, and 0.54–7.86 MPa of compressive strength. [Display omitted] • Co-vitrification of high proportions of MSWI fly ash and bottom slag was studied. • The change of glass characteristics by different MSWI fly ash content was determined. • Secondary aluminum ash (SAA) was employed as a foaming agent at room temperature. • Co–detoxifying hazardous solid wastes was successfully realized. [ABSTRACT FROM AUTHOR]

Municipal solid waste incineration (MSWI) has already been extensively applied to facilitate volume reduction and energy recovery. Known as toxic solid waste for high content dioxins, MSWI fly ash is accumulated accordingly. Dioxins in MSWI fly ash are primarily derived from homogeneous reaction and heterogeneous reaction in the range of 200–800 °C, including cyclization, chlorination, catalytic and de novo synthesis of precursors. Degradation of dioxins is considered necessary for the disposal of MSWI fly ash harmless. Among the alternative degradation technologies, plasma, mechanochemistry, hydrothermal, photocatalytic and biodegradation have demonstrated an excellent detoxification effect and are conducive to the application of MSWI fly ash as a sort of resource. In this paper, a systematic review is conducted of the mechanism and process parameters of these five methods, with the emphasis placed on their potential for the large-scale treatment of MSWI fly ash. The study aims at indicating the current research status for scholars, and at identifying a treatment for MSWI fly ash that can be performed in a safer and more efficient way. Finally, their respective characteristics, challenges and future improvements are discussed as well. Image 1 • Influence of factors on formation of dioxins in MSWI fly ash are commented. • Five degradation technologies and its mechanisms for dioxins are reviewed. • Plasma and hydrothermal technology have potential for treating MSWI fly ash. • Dechlorination is the most common degradation mechanism of dioxins. [ABSTRACT FROM AUTHOR]

[Display omitted] • Heavy metals in MSWI fly ash were reduced into alloy by SAD. • Reduction rates of iron and zinc were 67 and 100% • Transparent glass for building was made from MSWI fly ash. • 115 ∼ 213 dollars were earned after disposing of 500 kg of MSWI fly ash. Municipal solid waste incineration (MSWI) fly ash is a hazardous waste containing heavy metals. Secondary aluminum dross (SAD) is a hazardous waste discharged from aluminum smelting, containing active aluminum nitride (AlN). In this work, heavy metals from MSWI fly ash were reduced into alloy by AlN from SAD, and the slag was manufactured into transparent glass for building. Reduction of iron and zinc was 67 and 100 %, respectively. Reduction mechanism was explored after applying XRD, XRF and thermodynamics analysis. It was found that the reduction reaction was an ion reaction. The AlN and heavy metal oxide transformed into anionic group containing nitrogen and heavy metal cation, after entering slag. The heavy metals were reduced into alloy after electron was transferred from anionic group to cation. In addition, the reduced iron and zinc could merge into alloy, which inhibited evaporation of zinc. Yellow transparent glass was obtained after the reduction process. Yellow was come from titanium oxide, which could not be reduced by AlN. Microhardness, density and water absorption of the transparent glass were 741 HV, 2.86 g·cm−3 and 0.04 %, respectively. Leaching content of Ni, Cu, Zn and Pb of the glass were 0.1, <0.1, 0.6 and < 0.1 mg/L, respectively, all below the TCLP limit. About 115 ∼ 213 dollars were earned after manufacturing 500 kg of MSWI fly ash into transparent glass. This work provided a novel idea of recycling solid waste into alloy and transparent glass for building. [ABSTRACT FROM AUTHOR]

In consideration of recycling solid waste to achieve high value-added products, glass-ceramics have been fabricated from municipal solid waste incineration (MSWI) fly ash, pickling sludge (PS), and waste glass (WG) by melting at 1450 °C firstly to achieve parent glass and then crystallizing at 850 °C. Results demonstrated that heavy metals have been well solidified in the prepared glass-ceramics, and relatively/extremely low leaching concentrations of heavy metals have been detected. The synthetic toxicity index of heavy metals has been greatly reduced from 7-18 to <3.2 after crystallization treatment, and the leaching concentrations of Cr, Ni, Zn, Cu, and Pb are 0.15, 0.05, 0.26, 0.12, 0.19 mg L-1 respectively. Chemical morphology analysis, principal component analysis, TEM and EPMA were utilized to clarify the migration, transformation, and solidification mechanism of heavy metals from the as-received solid wastes. The major heavy metals, Cr and Ni which is responsible for the most toxicity, mainly exist in form of the oxidation state and residual state in parent glass, while the residual state in the glass-ceramics. The solidification performance was mostly positively correlated with the form of residue state, which the stability of heavy metals in glass-ceramics is improved. The solidification mechanism of heavy metals in glass-ceramics could be explained by the combination of chemical solidification/stabilization and physical coating. The TEM and EPMA confirmed that Cr and Ni mainly exist in the spinel crystalline (NiCr 2 O 4 , Fe 0.99 Ni 0.01 Fe 1.97 Cr 0.03 O 4) by solid solution or chemical substitution, and a small amount of Cr in the diopside phase. Pb, Cu, and Zn are homogenously dispersed in the glass-ceramics, which is considered as physical coating solidification. [Display omitted] [ABSTRACT FROM AUTHOR]

Geopolymer possesses good immobilization capacity for Pb. There are two rival interpretations regarding the immobilization mechanism of Pb. This research investigates the behavior of 3 Pb compounds in geopolymer and clarifies the immobilization mechanism. When Pb contamination is added in the form, soluble in sodium hydroxide solution, the Pb is converted to an amorphous form and participates in the formation of geopolymer network. Successful immobilization of these species relies on chemical bonding and physical encapsulation. On the contrary, the Pb compound inert to sodium hydroxide solution is segregated from the binder and trapped by physical encapsulation. [ABSTRACT FROM AUTHOR]

As a hazardous waste, municipal solid waste incineration (MSWI) fly ash poses a serious threat to the environment, and considerable efforts have been made to achieve its harmless treatment. Herein, a novel approach for handling MSWI fly ash has been proposed to prepare porous glass–ceramics by alkali activation–crystallization, which overcomes the difficulty of controlling air bubbles under high temperatures. MSWI bottom ash rich in Al 2 O 3 and SiO 2 are co-treating as raw material to develop glass framework structure. The alkali activation mechanism shows that the development of MSWI ashes based basic glass from monomer to silica–aluminate gel under alkaline condition. The influence of H 2 O 2 addition on the morphology and physical properties of porous glass–ceramics were evaluated. Porous glass–ceramics with uniform hierarchical pore structure have been fabricated at 3 mol/L NaOH, a solid loading of 55 wt%, and 1–4 wt.% H 2 O 2 , then crystallization treatment at 1150 °C for 2 h, the porosity and the bulk density of which varies from 77.22 to 82.94% and 0.68–0.91 g/cm3 respectively, while their relatively high compressive strength ranges from 1.62 MPa to 3.85 MPa. Attributed to the hierarchical porous structure, uniform distribution of pores and excellent physical properties, porous glass–ceramics has potential applications in construction, filtration, and adsorption materials, providing a convenient and feasible method for the efficient and comprehensive utilization of MSWI ashes. [Display omitted] • A novel method for developing hierarchically porous glass ceramics is proposed. • The alkali-activation mechanism of basic glass prepared by MSWI ashes has been studied. • Hydration products can promote the formation of sub-micron pore. • The porous glass-ceramics has excellent properties and environmentally acceptable. [ABSTRACT FROM AUTHOR]

Municipal solid waste incinerated (MSWI) fly ash and pickling sludge contain heavy metals such as Cr, Pb, Ni, etc., which are harmful to the environment but also suitable to be the nucleating agents in glass-ceramics preparation. In this paper, MSWI fly ash, pickling sludge, and waste glass were melted at 1400 °C for 3 h, and the obtained parent glass was nucleated and crystallized at 800 °C for 0.5 h. Glass-ceramics was successfully prepared by the one-step process. The effects of the content of pickling sludge on crystalline properties of parent glass and the physicochemical properties of glass-ceramics have been systematically discussed. The gap between nucleation temperature (T n) and crystallization temperature (T c) can be narrowed with higher content of pickling sludge, which caused fracture of the glass network (Si-O) and generation of the nonbridging oxygen (NBO), promoting the formation of diopside phase. As a result, nucleation and crystallization can be finished at the same temperature, called one-step process. The sample PS22 with 22 wt% pickling sludge added performed the best crystallization ability for having the narrowest gap (ΔT = 177 °C) between T c and glass transition temperature (T g) and the highest ratio of NBO to total oxygen (T) (NBO/T = 1.72). The Vickers' hardness and bending resistance of the products are 13.11 GPa and 135.84 MPa, respectively. The glass-ceramics are composed of homogeneously distributed diopside, donathite-spinel and glass phase. The toxic heavy metals are effectively stabilized by glass-ceramics. This study provides insights into more efficient and cleaner utilization of MSWI fly ash, pickling sludge and waste glass as glass-ceramic materials. Image 1 • Glass-ceramics were made from MSWI fly ash and pickling sludge and waste glass. • Properties of glass-ceramics by one-step were satisfied with the industrial standard. • Pickling sludge is an efficient nucleating agent in terms of glass crystallization. • Pickling sludge depolymerizes the glass network and decreases the thermal stability. [ABSTRACT FROM AUTHOR]