Solid Waste Incineration Fly Ash Research Articles

Mitigating heavy metal volatilization during thermal treatment of MSWI fly ash by using iron(III) sulfate as a chlorine depleting agent

In the thermal treatment of municipal solid waste incineration fly ash (FA), the presence of chlorides leads to the pronounced volatilization of heavy metals at high temperature, making heavy metals stabilization challenging. Conventional washing processes struggle to remove chlorides completely, and even minor residual chlorides can lead to significant heavy metal volatilization. This study innovatively applied iron(III) sulfate as a chlorine depleting agent, which can form FeCl3 (boiling point 316 °C) and volatilize to remove the residual chlorides at below 500 °C, thus preventing the chlorination and volatilization of heavy metals at 600–1000 °C. Using water-washed FA to produce lightweight aggregate (LWA) preparation, after adding iron(III) sulfate, the volatilization rates of Pb and Cd at 1140 °C decreased to 5.4% and 9.3%, respectively, a reduction of 82.8% and 84.1% compared to before its addition. The LWA met standard requirements in both performance and heavy metal leaching toxicity. The mechanism was further studied through thermodynamic equilibrium calculations and heating experiments of pure chemicals. This study presents novel approaches and insights for suppressing the volatilization of heavy metals in FA at high temperature, thereby promoting the advancement of thermal treatment techniques and the safe, resourceful disposal of FA.

Use of municipal solid waste incineration fly ash as a supplementary cementitious material: CO2 mineralization coupled with mechanochemical pretreatment

The use of municipal solid waste incineration fly ash, commonly referred to as “fly ash”, as a supplementary cementitious material (SCM), has been explored to mitigate the CO2 emissions resulting from cement production. Nevertheless, the incorporation of fly ash as an SCM in mortar has been shown to weaken its compressive strength and increase the risk of heavy metal leaching. In light of these challenges, this study aims to comprehensively evaluate the influence of CO2 pressure, temperature, and residual water/binder ratio on the CO2 uptake and compressive strength of mortar when combined with fly ash. Additionally, this study systematically examines the feasibility of mechanochemical pretreatment, which enhances the homogenization of fly ash and augments the density of the mortar's microstructure. The results indicate that the use of mechanochemical pretreatment leads to a notable 43.6% increase in 28-day compressive strength and diminishes the leaching of As, Ba, Ni, Pb, Se, and Zn by 17.9-77.8%. Finally, a reaction kinetics model is proposed to elucidate the CO2 sequestration process under varying conditions. These findings offer valuable guidance for incorporating fly ash as an SCM and CO2 sequestrator in mortar.

Effect of particle size of NanoSiO2 on cement-based stabilization/solidification of municipal solid waste incineration fly ash

As a part of by-product generated during the incineration, the municipal solid waste incineration fly ash (MFA) is a secondary pollutant. The ordinary Portland cement (OPC)-based stabilization/solidification (S/S) is a prevalent and robust technique for the treatment of MFA, which can not only effectively dispose the MFA, but also decrease the demand of OPC and reduce the greenhouse gas emissions. However, when MFA was directly utilized to partly replace the cement with a replacement ratio of 15%, the MFA-OPC based composites exhibited inferior mechanical properties. The nano silica (NS) was considered to be introduced to compensate for the strength loss caused by the MFA. In this research, the NS with varying dosages and particle sizes were integrated into the MFA-OPC composites and in total seven groups of the composites were prepared. The mechanical properties of the composites were examined and the results indicated that the composites containing 2 wt% 100 nm NS presented relatively superior mechanical properties. The incorporation of NS could increase the compressive strength of MFA-OPC composites by 37.5% at later age. Then, thermodynamic modelling and the micro-structural analysis including hydration heat, XRD, TG, MIP and SEM were coupling applied to probe into the NS modification mechanism. Experimental results indicated that the addition of NS enhanced the strength of OPC containing MFA at both early and later ages via pozzolanic reaction, hydration acceleration and micro filling effects. Overall considering the macro performance improvement and microstructure modification effect, the 2 wt% NS with particle size of 100 nm was recommended to be utilized. Therefore, this study expands the emerging application of NS and demonstrates that NS-augmented binders can ensure low-carbon and high-performance S/S of hazardous materials.

Immobilization mechanisms of heavy metals by utilizing natural cow bone waste for municipal solid waste incineration fly ash treatment.

Municipal solid waste incinerator (MSWI) fly ash poses intricate compositional challenges and potential environmental hazards. Effective management of such hazardous waste is imperative to mitigate the release of toxic compounds into the environment. Solidification/stabilization (S/S) processes have emerged as a viable strategy to transform MSWI fly ash from incineration waste into a safer and more environmentally benign material. This study aims to comprehensively investigate the potential of utilizing cow bone waste to stabilize heavy metals, focusing on Pb, within municipal solid waste incineration fly ash. Experimental investigations encompassed cow bone-to-fly ash weight ratios ranging from 0.0 (control group) to 7:3, a settling time of 2h, and a liquid-to-solid (L/S) ratio of 1.0mL/g. Cow bone waste exhibited pronounced efficacy, particularly within the short settling time, yielding a remarkable Pb removal efficiency of up to 99% at a cow bone waste dose of merely 2% and an L/S ratio of 1.0mL/g. Concurrently, other heavy metals such as Cd, Cu, and Zn were effectively stabilized with a cow bone waste dose of 1.5% during the same 2-h settling period. The results underscore the pivotal roles of ash/bone ratio and settling time in augmenting Pb stabilization in MSWI fly ash. The application of cow bone waste is anticipated to offer a cost-effective and environmentally sound approach, aligning with sustainable waste management principles.

Bio-treatment of municipal solid waste incineration fly ash: A sustainable path for recyclability

This study investigates the impact of bio-treatment on Municipal Solid Waste Incineration Fly Ash (MSWIFA), comparing bio-treated MSWIFA (BFA) to raw MSWIFA (RFA). BFA exhibited a larger particle size and the presence of microbial-induced calcium carbonate precipitation (MICP) gels. Both RFA and BFA maintained highly alkaline pH levels, with mineral analysis showing reduced chlorine content in BFA and the presence of Monocarboaluminate (Mc), suggesting potential concrete strength improvements. Bio-treatment effectively reduced polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) levels, meeting standards. Biomineralization stabilized heavy metals in MSWIFA, reducing leaching concentrations and meeting national standards. Blending BFA with magnesium phosphate cement (MPC) improved compressive strength, especially in early stages, further enhanced by calcium acetate. SEM images confirmed MICP gels on paste surfaces, contributing to increased strength. Analytical techniques (X-ray diffraction, Fourier transform infrared, and thermal gravimetric analysis) supported superior performance in hydration and strength. SEM-EDS analysis showed effective solidification/stabilization of heavy metals, reduced chlorine content, and enhanced biomineralization. Leaching tests confirmed reduced heavy metal concentrations in leachate. In summary, bio-treatment enhances the properties and environmental impact of MSWIFA, offering sustainable waste management opportunities.

Research on oxygen enrichment for municipal solid waste fly ash melting: A pilot-scale study on natural gas and coal as the melting fuel

High energy consumption is the main obstacle of melting/vitrification technology for the disposal of municipal solid waste incineration fly ash (MSWIFA) for industrial applications. To reduce energy consumption and lower operating costs, oxygen enrichment melting was proposed and studied in this work. This research was conducted in a pilot-scale melting furnace, and three melting conditions were compared and discussed. The results showed that 66% of natural gas was saved and the operating cost was reduced by 55% when oxygen enrichment technology was applied to MSWIFA melting. When coal was used as the fuel with the oxygen enrichment melting technology, the operating cost was even lower at 66.39 dollar/ton of fly ash. Because MSWIFA was a Ca-rich material, the relatively high content of Si and Al in the coal fly ash promoted the formation of vitrificated slag, leading to a reduction in the overall pollution toxicity index (OPTI) of MSWIFA by 99.98%. Meanwhile, SO2, HCl, and secondary fly ash were the main pollutants during MSWIFA melting, and when coal was used as the fuel, the emissions of SO2 and HCl could be reduced and the OPTI of secondary fly ash was suppressed. These results suggested that to obtain the lowest operating cost and reduce secondary pollution during MSWIFA melting, the best option consisted of oxygen enrichment technology with coal as the fuel.

Acid controlled washing of municipal solid waste incineration fly ash: Extraction of calcium inhibiting heavy metals and reaction kinetics

Washing method has attracted much attention in the research of municipal solid waste incineration (MSWI) fly ash treatment and resource utilization. However, the controlled leaching of heavy metals and the extraction of recyclable calcium in the washing process are still blank. Acid controlled washing was conducted with different acids, concentrations, times and temperatures to extract calcium while inhibiting heavy metals. The mechanism was investigated by reaction kinetics calculation and washed fly ash characterization. The high Ca concentration of 37,420 mg/L while the low heavy metal concentrations of around or <1 mg/L were achieved at 25 °C for 60 min under a liquid-solid ratio (L/S) of 3/1 in 1.5 M HCl. The reaction kinetics of acid controlled washing conformed the layer diffusion control. The results of X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectrum (EDS) analysis indicated that the rate-limiting step was the diffusion of ions through the product layer. Simultaneously, the washing solution enriched in Ca, Na and K and the washed fly ash, which met the standard requirements (HJ 1134–2020) for leach toxicity, both had the potential for further resource utilization.

PH-controlled sulfuric acid washing enhancing heavy metal stabilization in lightweight aggregate production from municipal solid waste incineration fly ash

Municipal solid waste incineration fly ash (FA) is a pressing issue in urban development. In FA treatment, washing is a common dechlorination pre-treatment. During washing, considerable heavy metal would leach due to the abundant alkaline substances, posing challenges to the heavy metal stabilization. Also, with high alkalinity, FA has potential to co-dispose of waste sulfuric acid (hazardous waste). Addressing these, our study presented a novel pH-controlled sulfuric acid washing for FA dechlorination during the sintering preparation of lightweight aggregate (LWA). It was aimed at enhancing heavy metals stabilization and enabling the co-recycling of FA and waste sulfuric acid. By controlling a pH of 10.50 during sulfuric acid washing, the leaching rate of Pb, Cd, Zn, Cu, Ba, and Cr was below 2%, which was a marked improvement over conventional water washing. Sulfuric acid washed FA (SFA) can be used to produce standard-compliant LWA by sintering with bentonite and SiC. The SFA-based LWA stabilized heavy metals more effectively than LWA using FA and water-washed FA. At a 30 wt% addition rate, the stabilized rate of Pb and Cd increased from 11.7% (using FA) to 72.4% (using SFA), and from 6.3% (using FA) to 49.0% (using SFA), respectively. During washing and sintering, XPS analysis further investigated the transformation of Pb, Cu, and Zn into stable states (Pb-glass, Cu2O/CuO, and ZnAl2O4), enhancing their stabilization. Our study introduces a novel method for co-recycling FA and waste sulfuric acid, offering enhanced environmental sustainability and deeper insights into the mechanism of heavy metal stabilization.

Novel cost-effective oxygen-enriched melting method for MSWI fly ash

ABSTRACT Herein, a novel oxygen-enriched melting process for fly ash, which uses the biogas produced from the leachate of municipal solid waste incineration (MSWI) plants, is proposed to reduce the high cost of conventional fly ash – melting technology. The fly ash composition was estimated via X-ray fluorescence analysis; the six constituent elements detected in fly ash in the decreasing order of their content were calcium, chlorine, silicon, sulfur, sodium, and potassium. Based on literature and actual production data, the average yield of the leachate was 15% of the total waste entering the MSWI plants and the COD of leachate was 30,000–75,000 mg/L. The amount of biogas that can be used per ton of fly ash was calculated to be 62.0–157.0 m3. The analysis of melting thermal equilibrium revealed the amount of biogas required per ton of fly ash as 57.8 m3. The aforementioned research findings indicate that the biogas produced by MSWI plants can successfully meet the demands of the oxygen-enriched melting of fly ash produced in these plants. By establishing an oxygen-enriched-melting pilot platform, the pilot tests of melting were conducted on fly ash; the results revealed the good melting effects of fly ash. The X-ray diffraction analysis of the slag demonstrated that the content of the vitreous body met the technical requirements for glassy substances. Furthermore, the leaching toxicity test results revealed that heavy metals were well solidified in the slag. This study presents a novel fly ash – melting scheme for MSWI fly ash, namely, biogas oxygen-enriched melting strategy, which has the advantages of technical feasibility and cost-effectiveness. The proposed technique exhibits considerable prospects for widespread application in MSWI plants in China and can play an important role in the safe disposal of fly ash. Implications: In this paper, a low-cost melting method of municipal solid waste incineration(MSWI) fly ash is proposed. This method uses the biogas generated by MSWI plant itself as fuel for melting. Through research, it has been found that the production of biogas can meet the demand for fly ash melting. Adopting biogas as a molten fuel can significantly reduce the cost of melting, thereby significantly reducing the cost of fly ash melting. This study established a pilot scale platform for the melting of biogas and conducted pilot scale experiments on fly ash and additives. The experimental results showed that the melting system operated well and achieved the vitrification of fly ash. The leaching test results of the molten slag showed that heavy metals were well solidified in the slag. The research results can be extended to the MSWI plant for application, which can significantly reduce the cost of fly ash melting disposal, and has broad application prospects.

Simultaneous realization of heavy metal Cd solidification and chloride separation in municipal solid waste incineration fly ash: Mechanism and DFT analysis

The solidification of heavy metals and the separation-recycling of chloride salts are effective approaches for the safe disposal of municipal solid waste incineration (MSWI) fly ash. This study achieved the solidification of the excessive heavy metal Cd by transforming MSWI fly ash into a solidified body mainly composed of ettringite, while also purifying the calcium and sulfate ions in the chloride solution. The research results demonstrate that the introduction of aluminum ions rapidly decreases the pH value of the MSWI fly ash reaction system. By adjusting the reaction system to a pH value of 11.50, the conversion rate of calcium ions reaches 99.68%, the separation rate of chloride reaches 95.99%, and the solidification rate of heavy metal Cd reaches 98.92%. Density functional theory (DFT) calculations indicate that the vacancy formation at the Ca-2 position of ettringite and Cd entering the vacancy has a higher probability. The combination of DFT calculations and experiments validates that heavy metal Cd leads to charge redistribution, increased interplanar spacing, and decreased thermal stability of ettringite. The concentration of calcium ions in the solution decreases to 22.64 mg/L, achieving efficient recovery of sodium chloride and potassium chloride in a shorter process. The ettringite-based solidified body contains 0.87% chloride ions, showing potential for resource utilization in cement-based materials.

A novel amine functionalized porous geopolymer spheres from municipal solid waste incineration fly ash for CO2 capture

Municipal solid waste (MSW) incineration fly ash (FA) is classified as hazardous waste, and strategies for recycling FA have attracted attention. In this study, the porous geopolymer spheres (PGS) were prepared from FA by the foaming-suspension-solidification method, and then the PGS were functionalized with tetraethylenepentamine (TEPA) to capture CO2. The results showed that washing pretreatment and the addition of H2O2 foaming agent enhanced the pore volume and specific surface area of PGS. The CO2 adsorption capacity of amine-functionalized PGS exhibited a trend of increasing and then decreasing in the range of 35–80 °C. The maximum adsorption capacity of TEPA-WPGS3 was 2.55 mmol/g at 65 °C higher than expected for the average of TEPA and PGS. This was because PGS improved the dispersion of TEPA, thus exposing more active sites of TEPA and making it more likely to interact with CO2. The adsorption efficiency of amine-functionalized PGS decreased by only 2.4% after 10 cycles, indicating that it has excellent regeneration performance. In addition, amine-functionalized PGS, which showed excellent CO2 adsorption capacity, had a significant ability to selectively adsorb CO2 and the adsorption capacity of the rapid stage accounted for approximately 80% of the saturated adsorption capacity. This study shows that FA-derived geopolymers have excellent CO2 adsorption properties and provides a new method for the resource utilization of FA.

Reactivity of MSWI-fly ash in Mg-K-phosphate cement

In this study, the behaviour and reactivity of Municipal Solid Waste Incineration Fly Ash (MSWI-FA), introduced in the formulation of magnesium potassium phosphate cement (MKPC), was investigated by considering the waste either as fully inert or reactive. MSWI-FA induced structural and compositional modifications in MKPC as a consequence of dissolution/precipitation processes which involved many MSWI-FA elements (e.g., Ca, Mg, Al, Si, Zn) and led to the formation of mostly amorphous phosphate secondary products. The reaction path has been described in terms of the early fast dissolution of MSWI-FA, with precipitation of very low solubility phases, and subsequent late precipitation due to pH changes and water subtraction during MKPC gelification. The increase in amorphous content peaked to 50 wt.% and it has been related to the improved behaviour with respect to the leaching of heavy metals (reduced by 70-99%), pointing to this cement as an excellent matrix for their chemical stabilization. The obtained MKPC microstructure exhibited better mechanical performance, with an improvement of up to 60% in compressive strength. All in all, the results indicated that the incorporation in MKPC is a viable recycling opportunity for MSWI-FA, although, for an effective cement formulation, its reactivity must be taken into account.

A novel process for the desalination of MSWI fly ash via in-situ leachate concentrate washing and residue solution electrolysis

Desalination of municipal solid waste incineration (MSWI) fly ash is necessary prior to its disposal. Water washing followed by evaporation crystallization for salt extraction is a popular desalination process for MSWI fly ash, while facing large water consumption, high evaporation energy consumption, and difficulties in crystalline NaCl selling. Leachate concentrate is another hazardous waste produced in MSWI plant which is hard to be degraded. In this work, a novel process for the desalination of MSWI fly ash via leachate concentrate washing and residue solution electrolysis was proposed. The effects of liquid-to-solid (L/S) ratio and washing step were discussed. Low-carbon electrolysis strategy for the high-salt washed leachate concentrate was explored. Results showed that leachate concentrate washing displayed the similar desalination performance to water washing and the preferred L/S ratio was 3. The dissolution of Cl in fly ash exceeded 92 % after its twice washing. The desalination capacity of leachate concentrate decreased sharply with increasing using times while its salinity increased steadily. With CO2 injection in a regulating chamber, stable electrolysis of the washed leachate concentrate was achieved. The findings are expected to provide guidance on the treatment of MSWI fly ash, leachate concentrate as well as high-salt wastewater in other fields.

Process optimization and mechanism for removal of high-concentration chlorine from municipal solid waste incineration fly ash washing wastewater by Friedel's salt

Waterwashing is an important pretreatment method for the reuse of municipal solid waste incineration (MSWI) fly ash. However, the presence of high levels of chlorides and small amounts of sulfides in the waterwashing solution makes it difficult to treat and reuse. Therefore, in this study, a calcium sulfate- Friedel's salt precipitation method was used to dechlorinate and desulfurize in the MSWI fly ash washing solution. This paper mainly focused on the chloride removal, and the effects of factors such as reagent ratios, temperature, and reaction time on chloride removal rate and the two-stage dechlorination and desulfurization process were optimized. The experimental results indicated that when the ratio in the first stage was n(Ca):n(Al):n(Cl) = 3:1.5:1, and in the second stage, n(Ca):n(Al):n(Cl) = 8:4:1, a chloride removal rate of up to 90.5% and a sulfide removal rate of over 99.88% could be obtained. The deposited particles were analyzed by using FE-SEM, XRD, and FTIR to investigate their size, morphology, phase composition, and functional groups. The study revealed that excessively high or low reagent ratios could reduce the interlayer spacing of Friedel's salt. Additionally, high temperatures led to the decomposition of Friedel's salt, and the dechlorination efficiency was affected.

Carbothermal treatment of municipal solid waste incineration fly ash: Purification and valuable elements extraction

Thermal separation of heavy metals from municipal solid waste incineration fly ash (MSWI FA) is a promising approach for both fly ash purification and heavy metal recovery. However, its commercial viability is hindered by excessive energy consumption. In this study, we introduce carbothermal reduction for MSWI FA treatment as an innovative strategy to enhance heavy metal volatilization and reduce the required heating temperature. By incorporating carbon (FA/Carbon = 1:0.2), the volatilization fractions of Pb, Cd and Zn reached 96.5 %, 100 %, and 63.9 %, respectively, when subjected to 900 °C for 2 h. Remarkably, these volatilization amount surpassed the volatilization fractions attained when raw fly ash (RFA) was independently calcined at 1000 °C for 2 h: Pb (90.2 %), Cd (92.9 %) and Zn (30.2 %). Conversely, Cu volatilization was impeded by carbon inclusion. Comparable trends were shown in washing fly ash (WFA). Notably, concentrated carbothermal reduction led to substantial concentrations of Pb (2.5–7.1 %) and Zn (4.9–20.7 %), rendering them amenable for recycling through metallurgical routes. Intriguingly, higher carbon content (FA/Carbon = 1:0.3) hindered rather than enhanced heavy metal volatilization from RFA. We found that condensation of NaCl and KCl occurred in carbon pores, causing the formation of molten eutectic which trapped the heavy metals at high temperatures. Therefore, more carbon showed more enhancement of heavy metal volatilization from WFA which was free of NaCl and KCl. Ultimately, two comprehensive pathways for the resourceful utilization of fly ash based on carbon thermal reduction were proposed and compared. This study demonstrates carbothermal reduction as an effective approach for simultaneous purification and metal recovery of MSWI FA, exhibiting promising potential for industrial application.

Preparation of municipal solid waste incineration fly ash/ granite sawing mud ceramsite and the morphological transformation and migration properties of chlorine

Municipal solid waste incineration (MSWI) fly ash is a hazardous waste containing high chlorine and harmful substances generated during the waste incineration disposal, and its resource utilization has a positive effect on reducing environmental pollution. In this study, the feasibility of preparing lightweight MSWI fly ash/granite sawing mud ceramsite (MG ceramsite) was investigated by evaluating the influence of Al2O3 addition, MSWI fly ash content and sintering temperature on the ceramsite properties. The microstructure of MG ceramsite was investigated by using SEM, the chlorine morphological transformation and migration behaviors were simultaneously explored by using the tube furnace experiment, XRD and XRF analyses. The experimental results show that the maximum MSWI fly ash content is about 30 wt%∼35 wt%, with the Al2O3 addition of at least 10 %. By controlling the MSWI fly ash content of 30 wt%, MG ceramsite can be obtained with bulk density of 986 kg/m3, cylindrical compressive strength of 19.67 MPa, 1 h water absorption of 0.31 %, and chlorine content of 0 after sintering at 1150 °C for 20 min. Chlorine in MG ceramsite enters into the tail gas or secondary fly ash in the form of chlorine salts and chlorine-containing gas when the sintering temperature is above 800 °C. The MG ceramsite prepared from MSWI fly ash meets the lightweight aggregate standard and are environmentally friendly. However, the disposal of tail gas and secondary fly ash needs attention when the MSWI fly ash is used as one of the main raw materials to prepare ceramsite.

Manufacture of alkali-activated cementitious materials using municipal solid waste incineration fly ash (MSWIFA): The effect of the Si/Al molar ratio on fresh and hardened properties

Due to the escalating production of municipal solid waste and the widespread adoption of incineration technology, there is a substantial demand for secure disposal of municipal solid waste incineration fly ash (MSWIFA). In this research, alkali-activated MSWIFA via combination with metakaolin (MK) and cement was used to produce an eco-friendly composite. The influence of Na2O content and modulus (Ms) of alkali activator on the early compressive strength and setting time of the composites was investigated, and 4 wt% Na2O content was found to be the most appropriate alkali content. Then, the influence of the Si/Al ratio (2.35–3.18) on the fresh properties (initial setting time, rheology properties, and hydration heat) and hardened properties (compressive strength and shrinkage performance) of composite materials was studied. The microscopic mechanism of the effect of the Si/Al ratio on material properties was explained by XRD, TG, and SEM-EDX. The results showed that the compressive strength of the composite can reach 16.4 MPa at 28 days, which met the quality standard of MU15 (GB/T 25779–2010) of load-bearing concrete perforated brick. With the increase of the Si/Al ratio, the setting time of the slurry is prolonged and the rheological property has deteriorated. The composites with a lower Si/Al ratio (2.35) have the densest structure and the lowest degree of drying shrinkage, which leads to a higher strength. This study provides a potential solution for MSWIFA solidification in low-carbon construction applications.

Killing two birds with one stone: Preparation of ceramsite high-strength lightweight aggregate via co-sintering of dredged sediment and municipal solid waste incinerated fly ash

The disposal of dredged sediment (DS) and municipal solid waste incineration fly ash (MSWIFA) has become a notable concern. In this study, a novel ceramsite high-strength lightweight aggregate (HLWA) was developed by the co-sintering of DS and MSWIFA. The optimal HLWA (DS-10FA-1160) was synthesized using a mixture of 90 % DS and 10 % MSWIFA at 1160 °C. DS-10FA-1160 exhibited a high cylinder compressive strength (42.82 MPa) and negligible leaching concentrations of heavy metals (HMs). The “killing two birds with one stone” mechanism was adopted as follows: MSWIFA promoted the generation of strength enhancing phases (e.g., rankinite and amorphous phases) while phillipsite-K and the disconnected pore structure of DS-10FA-1160 steadily encapsulated HMs.

Hydration characteristic and strengthening mechanism of municipal solid waste incineration fly ash cementitious composites modified by nano SiO2

As a by-product from the incineration of municipal solid waste (MSW), the incineration fly ash (MSWIFA) possesses potential hydration reaction activity and could be as an alternative supplementary cementitious material (SCM). In this study, 85 wt% cement and 15 wt% MSWIFA were blended to prepare the MSWIFA cement-based composites. Nano SiO2 (NS) was added at varying dosages to strengthen the performance of the composite materials. The fresh and mechanical properties of the nano SiO2-modified MSWIFA cement-based composite (NMCC) were examined to determine the optimal NS dosage. Moreover, the hydration behavior, phase composition, pore structure, and microstructure morphology of the NMCC were examined to dissect the hydration characteristic of NMCC and the strengthening mechanism of NS. The results showed that NS significantly accelerated the hydration process of the NMCC, reflected by a higher hydration degree, the formation of more C-(A)-S-H gels formed, and superior mechanical properties. MSWIFA bestowed the NMCC with a high alkali content, which was beneficial to accelerating the hydration process. However, a low silicon content in MSWIFA failed to provide sufficient [SiO4]4- to react with Ca(OH)2 to produce C-(A)-S-H gels. This problem was effectively addressed by the addition of an optimal dosage of NS, which was rich in silicon element, so as to compensate for the decline in MSWIFA performance caused by insufficient Si elements in the composite. The NMCC sample, which consisted of 15 wt% MSWIFA and 1.0 wt% NS, exhibited the highest percentage (53 %) of harmless pores after 28 days. Additionally, its compressive strength reached 96.78 % (42.9 MPa) of the pure cement control indicating superior mechanical properties a compact microstructure. The utilization of NS as a viable material holds great potential in augmenting the application of MSWIFA as a SCM within the construction and building industry.

Solidification of municipal solid waste incineration fly ash with alkali-activated technology

Alkali-activation is effective municipal solid waste incineration fly ash (MSWIFA) solidification/stabilization (S/S) technology. Percolation and migration of heavy metals in MSWIFA S/S matrix is a complicated and slow process. Here, several alkali-activated MSWIFA samples are selected to comparatively investigate the long-term leaching behavior and environmental availability of Pb, Zn and Cd when exposed in different erosion environment. Acid environment posed the more serious destroy to MSWIFA S/S matrices. RAC demonstrated that potential risk level of heavy metals is higher in acid rain environment, and Cd, Zn showed the prominent risk. When soaked in acid rain solution, the surface of alkali-activated MSWIFA S/S matrices was cracked seriously and a large number of hardened slurry peeled off. However, more stable structural properties and lower heavy metal leachability can be found in alkali-activated MSWIFA/aluminosilicate. The immobilization efficiency of Pb, Zn and Cd were all above 99.0%. Microstructure and morphology results indicated that there is new phase Friedel's salts generated and much more amorphous substance such as C-(A)-S-H gel with incorporation of aluminosilicate, which all contributed much to the formation of compact and stable microstructure, then significantly facilitated the encapsulation of heavy metal. These findings will provide theoretical basis and new insight for resource utilization and security landfill of MSWIFA.