Fly Ash Particle Size Research Articles

Effects of Fly Ash Particle Size and Chemical Activators on the Hydration of High-Volume Fly Ash Mortars

This study investigated the effects of the fly ash (FA) particle size and chemical activators on the hydration reactions of high-volume fly ash (HVFA) cement pastes and the mechanical properties of HVFA mortars, with five types of FA with varying proportions of particles smaller than 10 μm prepared to assess the influence of particle size. In addition, the effect of chemical admixtures on the early-age hydration of the HVFA cement paste was evaluated. Quartz powder with the same fineness as that of normal FA was prepared to specifically examine the pozzolanic reaction of FA. The hydration reactions of the HVFA cement pastes were analyzed using isothermal calorimetry to measure the heat of hydration for 72 h. The results showed that an increase in the contents of FA particles with a diameter lower than 10 μm increases specific heat flow. Chemical activators, including Na2SO4, triethanolamine (TEA), and triisopropanolamine (TIPA), promote early-age hydration of HVFA cement pastes. The mechanical properties of the HVFA mortar were evaluated by measuring its compressive strength at 3 d, 7 d, and 28 d, with the findings revealing that the compressive strength of HVFA mortar improved with an increased proportion of FA particles smaller than 10 μm. Na2SO4, TEA, and TIPA consistently increased the compressive strength of the HVFA mortars.

Honeycombed pomegranate-like sludge ceramsite particles: Preparation with fly ash floating beads as the pore-forming template and performance optimization

Ceramsite used in construction and building are typically limited by high water absorption rates, the contradiction between lightweight and high strength, and severe performance fluctuations. This can be attributed to the fact that ceramsites are currently prepared by high-temperature sintering expansion, wherein viscosity and surface tension of the liquid phases shall perfectly match with the gas generation rate. On the other hand, sand washing sludge and fly ash are common solid wastes in construction and building and their environmentally friendly disposal are of great significance. In this study, honeycombed pomegranate-like sludge ceramsites with low density (packing density = 733.2–968.3 kg/m3), high strength (cylinder compressive strength = 11.2–18.8 MPa), and low water absorption rate (one-hour water absorption rate = 1.2–2.8 %) were prepared with sand washing sludge and fly ash floating beads as the raw material and the pore-forming template, respectively. Also, the effects of particle size and content of fly ash floating beads on the structure and performance of the as-prepared ceramsite were investigated. The results indicated that the pore structure of the as-prepared ceramsite was directly dependent on size and content of the fly ash floating beads, suggesting that the performance of the as-prepared ceramsite can be customized by tuning size and content of the beads. Additionally, lightweight aggregate concrete prepared by using the as-prepared ceramsites exhibited improved 7-day and 28-day compressive strengths and reduced thermal conductivity compared with concrete prepared by using commercially available ceramsites. Overall, this study presents the preparation of ceramsite particles with both high strength and low density by using fly ash and sand washing sludge, both of which are abundant solid wastes in constructions, thus contributing to solid waste recycling in construction.

The Evaluation and Detection of the Chemical Bond Between Silane Coupling Agent and Silver Layer on Alkali Activated Fly Ash.

In this study, wettability was employed to evaluate the effect of alkali activation by NaOH on different fly ash (FA) particle sizes. The results indicated that the surface wettability of FA particles with 13.8 μm increased from 0.025 g2/s to 0.034 g2/s after activation by the NaOH solution, which is suitable for silane modification and electroless plating. X-ray photoelectron spectroscopy (XPS) was used to analyze whether three kinds of silane coupling agents coated on FA surfaces could detect the chemical bonds between silane coupling agents coated on the FA surface and silver layers by shortening the plating time. The XPS results demonstrated that N-Ag coordination bonds can be detected by reducing silver plating time to 2 min for Ag-plated FA modified by N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (KH792). However, there were no chemical bonds detected for Ag-plated FA modified by γ-(2,3-epoxypropoxy)propytrimethoxysilane (KH560) and methyltrimethoxysilane (MTMS), even when the satellite peak of Ag disappeared after plating for 80 s. The SEM showed that Ag particles agglomerated on FA surfaces, and even a bare surface was found after modification by KH560 and MTMS, which further proved no chemical bonds between silver layers and the silane coupling agents.

The Influence of Particle Size and Calcium Content on Performance Characteristics of Metakaolin- and Fly-Ash-Based Geopolymer Gels.

This research systematically investigates the influence of raw material particle size and calcium content on the geopolymerization process to gain insight into the physical and mechanical properties of geopolymer gels, including setting time, fluidity, pore structure, compressive strength, and leaching characteristics of encapsulated Cr3+ heavy metal ions. Utilizing a diverse range of particle sizes of metakaolin (MK; 3.75, 7.5, and 12 µm) and fly ash (FA; 18, 45, and 75 µm), along with varied calcium levels, this study assesses the dual impact of these factors on the final properties of both metakaolin- and fly-ash-based geopolymers. Employing sophisticated analytical techniques such as Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), and Nuclear Magnetic Resonance (NMR), the research meticulously documents alterations in chemical bonding, micro-morphology, and pore structures. Key findings reveal that reducing the size of MK and FA particles to 3.75 and 18 µm, respectively, enhances the compressive strength of their matrices by 128.37 and 297.58%, respectively, compared to their original values (63.59 and 33.87 MPa, respectively) at larger particle sizes. While smaller particle sizes significantly bolster compressive strength, they adversely affect slurry flow and reduce the leaching rates of Cr3+ from MK- and FA-based matrices, reaching 0.42 and 0.75 mg/L at 3.75 and 18 µm, respectively. Conversely, increased calcium content markedly enhances setting times and contributes to the formation of dense microstructures through the production of calcium aluminate silicate hydrate (C-A-S-H) gels, thus improving the overall curing performance and durability of the materials. These insights underline the importance of fine-tuning particle size and calcium content to optimize geopolymer formulations, offering substantial benefits for varied engineering applications and promoting more sustainable construction practices.

Comparative experimental study on the co-firing characteristics of water electrolysis gas (HHO) and lean coal/lignite with different injection modes in a one-dimensional furnace

Zero-carbon fuels (H2/NH3) replacing coal co-firing is a promising way to address carbon emissions. Hydrogen co-firing helps improve combustion stability at low loads and control pollutant emissions. This study used a safer water electrolysis gas (HHO) for the multi-coal co-firing adaptability test. The effects of HHO injection mode and flow rate on combustion intensity, flue gas emissions, and fly ash characteristics were investigated. The results showed that HHO and lean coal/lignite co-firing could significantly increase the combustion temperature. The maximum temperature increases after co-firing were 108 °C and 95 °C. As the HHO increased, the CO2 in lignite co-firing increased, and CO and unburnt carbon decreased, while the opposite results were observed for lean coal (LC). At the 2200L/h HHO co-firing, CO2 decreases by 14.46 % and 27.22 % for LC premixed co-firing (PC) and staged co-firing (SC). Compared to SC, PC is more conducive to promoting the complete combustion of coke. The NOX emissions of HHO/LC co-firing are lower than those of pure coal, reaching the lowest value of ∼ 9 ppm under SC. As the HHO increases, the NOX of Lignite-PC first increases and then slowly decreases, while Lignite-SC has the opposite effect. Although various NOX emission trends are observed under different conditions, the results are dominated by a common mechanism − the competition between NOX reduction caused by HHO gas and the increase in co-firing temperature on NOX generation. The NOX of SC is all lower than that of PC. The SO2 emission from the LC co-firing decreases with the increase of HHO, while the SO2 from lignite increases. The SO2 of LC-SC can be reduced from ∼ 142 ppm to 0 ppm. HHO gas injection inhibits the release and oxidation of sulfur in lean coal. The differences in coal types lead to different changes in SO2 of co-firing. HHO gas reduced the SO3 content of lignite fly ash by 38.01 %. The high-temperature reduction atmosphere promotes the decomposition of sulfates and increases SO2 emissions. HHO co-firing refines the particle size of fly ash. The ultra-fine and micron particle matter (PM) increases, and the content of PM>100μm in Lignite-SC decreased by 10.32 %.

Photodegradation of 2-chlorodibenzo-p-dioxin (2-CDD) on the surface of municipal solid waste incineration fly ash: Kinetics and product analysis

Considering that waste incineration fly ash is the main carrier of dioxins and can migrate over long distances in the atmosphere, it is of great significance to study the photochemical transformation behavior of dioxins on the surface of fly ash. In this work, 2-chlorodibenzo-p-dioxin (2-CDD) was selected to conduct a systematic photochemical study. The influence of various factors on the photodegradation of 2-CDD were first explored, and the results showed that small particle size of fly ash, low concentration of 2-CDD and appropriate level of humidity were more conducive to photodegradation, with the highest degradation percentage reaching 76%–84%. The components of fly ash (Zn (Ⅱ), Al (Ⅲ), Cu (Ⅱ) and SiO2) also had a certain promoting effect on the degradation of 2-CDD, which increases the degradation efficiency by 10%–20%, because they could act as effective photocatalysts to produce free radicals for reaction. With a higher total light exposure intensity, natural light environments led to a more complete degradation of 2-CDD than laboratory Xe lamp irradiation (90% degradation Vs. 79% degradation). Based on chemical probe and radical quenching experiment, hydroxyl radical also contributed to 2-CDD photodegradation on fly ash. A total of 16 intermediate products were detected by mass spectrometry analysis, and four initial reaction pathways of 2-CDD were speculated in the process, including dechlorination, ether bond cleavage, hydroxyl substitution, and hydroxyl addition. According to the results of density functional theory calculation, the reaction channels of ether bond cleavage and •OH attack were determined. The toxicity assessment software tool (TEST) was used to assess the toxicity and bioconcentration coefficient of reaction products, and it was found that the overall toxicity of the photodegradation products was reduced. This study would provide new insights into the environmental fate of dioxins during long-range atmospheric migration process.

Electrical Discharge Coating of Mild Steel with Fly Ash Suspension

Mild steel is a soft substance that rusts easily. It is easy to wear and corrode when exposed to harsh circumstances, such as in manufacturing sector. As a result, the mild steel surface must be modified to boost its durability and wearability. There are various techniques for coating mild steel and one of them is electrical discharge coating (EDC). In this study, the surface of mild steel was modified using EDC with fly ash suspension. Prior to the experiment, the element content, particle size, and surface morphology of fly ash were investigated. The EDC process was carried out using a Sodick AQ35L die-sinker EDM machine. The effects of varied amounts of fly ash on the features of the coated surface were also investigated. The experimental results revealed that raising the fly ash content increases the micro-hardness of the coated surface. Furthermore, utilising 20 g/l of fly ash powder resulted in the best coated surface finish with the least surface roughness. The concentration of fly ash has a significant impact on the thickness of the coating layer. These findings substantiate the viability of recycling fly ash as an environmentally sustainable coating material for modifying mild steel surfaces through the EDC method.

Measurements and Prediction of Ash Deposition in a Cyclone-Fired Boiler Operating under Variable Load Conditions

Measurements of ash deposition rates were made between the secondary superheater and reheater sections of a 450 MW cyclone-fired lignite boiler as the operational load varied from 33 to 100%. Significant reductions in deposition rates with a decrease in operational load were observed. To uncover the causative mechanisms behind these observations, operational data from the power plant were used to carry out computational fluid dynamic (CFD) simulations of the boiler. After ascertaining that the gas temperatures and velocities at various sections within the boiler were being represented adequately, decoupled simulations of the ash deposition process on the deposit probe were carried out using a finely resolved boundary layer mesh. Fly ash particle size distribution (PSD) and its concentration for the decoupled calculations were determined from stand-alone cyclone barrel simulations. The ash partitioning (mass %) between the fly ash and slag was found to be ~50:50, which was in line with previous field observations, and it did not vary significantly across different cyclone loads. The predicted PSD of the deposit ash was concentrated in the size range 10–30 microns, which was in agreement with cross-sectional images of the deposit obtained from the measurements. At lower loads, sharp variations in the deposition rates were predicted in the gas temperature range 950–1150 K. The particle kinetic energy—particle viscosity-based capture methodology utilized in this study in conjunction with appropriate ash compositions, ash viscosity models and gas temperature estimates can help estimate slagging propensities at different loads reasonably well in these systems.

Removal characteristics of SO3 in the low-low temperature electrostatic precipitator

ABSTRACT As SO3 endangered the industrial equipment and environment in the coal-fired power plants, it was necessary to give full play to the SO3 removal effect in a low-low-temperature electrostatic precipitator (LLT-ESP). The relationship between SO3 and fly ash particles was investigated during the cooling process. In addition, the SO3 removal performance in the LLT-ESP was further analyzed. The migration and deposition of SO3 into fly ash particles were promoted with the decrease in the gas temperature and the fly ash particle size, as well as the increase in the SO3 concentration. As the temperature of the gas decreased below the acid dew point (ADP), the H2SO4 vapor tended to migrate into fly ash particles with smaller sizes. The SO3 mass concentration increased significantly in fly ash particles whose sizes ranged from 0.03 to 0.26 μm. When the operating voltage turned up, the SO3 removal efficiency was increased from 43.2% to 60.1%. The increase in the inlet fly ash concentration promoted the SO3 removal, and the removal efficiency increased by nearly 20%. When the gas temperature was below the ADP, it had little influence on the SO3 removal efficiency.

A study of the association involving pore characteristics and compressive strength of cement pastes incorporating fine fly ash

The evaluation of mechanical properties in materials derived from cements incorporating ultrafine fly ash was performed. An examination of the influences of curing time, fly ash concentration, and particle size on the compressive strength of materials derived from cements was executed. Upon the integration of ultrafine fly ash into materials derived from cements, the pore structure was scrutinized using the mercury intrusion technique, and the impact of fly ash concentration and particle size on diverse pore structure parameters was investigated. According to the findings, the quantity of macropores in materials derived from cements progressively diminished with the rise in fly ash concentration, and the porosity consistently decreased with the extension of curing time, culminating in an elevation of compressive strength. The proportion of gel pores in materials derived from cements contracted with the increment in fly ash concentration, resulting in a decline in compressive strength. Fly ash particles of reduced dimensions exhibit heightened compressive strength due to the diminution in particle size. Employing fractal theory, the fractal dimension of the pore surface in materials derived from cements was ascertained, and an outstanding correlation between the fractal dimension and the total pore surface area was discovered. Grounded in the connection concerning the compressive strength of materials derived from cements and assorted pore structure parameters, a suitable relationship model has been devised between the total pore volume, transitional pore volume, and compressive strength. At the same time, a fitting scope of the model had been determined.

Enhancement of strength of blast furnace flue dust – iron oxide – fly ash composite briquette using ANOVA-based mathematical model

ABSTRACT Utilisation of by-products of an integrated steel plant improves productivity with diverse mix of raw materials used for iron making. The strength of a briquette made with Blast furnace flue dust and iron oxide with fly ash as a binder depends upon the briquetting pressure, particle size of fly ash, fly ash quantity, and curing temperature. In this study, the effect of process variables on the strength was investigated with ANOVA. The correlation between the process parameters and the strength was examined experimentally and RSM-based optimisation was performed during the preparation of briquettes. The RSM provided optimisation with a confidence level of 95% using the CCD design. This paper demonstrates the correlation between applied pressure, particle size, fly ash quantity, and curing temperature. The developed mathematical model would help in predicting the variation in the compressive strength with the change in the level of different parameters. This model can be useful for setting the optimum value of the parameters for achieving the target compressive strength.

Preparation of mullite whiskers from high alumina fly ash and its reinforced porous structure

Porous has been formed in samples when mullite whiskers are prepared by firing compacts of high-aluminum fly ash and ammonium aluminum sulfate hydrate with a small amount of sodium dihydrogen phosphate and SiO2 at high temperature for 4 h. The effects of the particle size of fly ash, the firing temperature, and the content of sodium dihydrogen phosphate on the porous structure and the formation of whiskers are discussed. The results show that the smaller the particle size of fly ash, the more uniform porous in sample after fired, which is more conducive to the nucleation of mullite whiskers. Additionally, the amount of whiskers formation increased significantly with the firing temperature increasing, nonetheless, the pore size of porous reduced and the aspect ratio of whiskers gradually increased by reducing the content of sodium dihydrogen phosphate; owing to the reduction of glass melt in the matrix, the Al2O3/SiO2 ratio of the formed mullite whiskers gradually decreased. Thus, in order to obtain mullite whiskers self-reinforcement porous ceramics with the relatively low cost and no pollution to the environment, through the above preparation process, the addition amount of sodium dihydrogen phosphate and calcination temperature should be properly controlled.

Numerical simulation on the deposition characteristics of MSWI fly ash particles in a cyclone furnace

In melting municipal solid waste incineration (MSWI) fly ash by cyclone furnace, the deposition characteristics of particles affect the slag flow and the secondary MSWI fly ash formation. In this study, the composition mechanism based on critical viscosity is selected as the particle deposition model to predict the deposition and rebound of particles on the furnace wall. The Riboud model with an accurate viscosity prediction performance is selected, then the particle deposition model is integrated into a commercial computational fluid dynamics (CFD) solver through the user-defined function (UDF) to realize the coupling of particle motion and deposition process. The results show that under the same case, the deposition rate decreases obviously with the increase of MSWI fly ash particle size. And the escape rate reaches a maximum at particle size 120 μm. Controlling the particle size of fly ash particles within 60 μm can effectively reduce the generation of secondary MSWI fly ash. During the forward movement of the fly ash inlet position, the escape of MSWI fly ash particles with large particle sizes has been significantly weakened. This measure not only lowers the post-treatment cost but also dramatically reduces the pretreatment step of MSWI fly ash before the melting and solidification process. In addition, the deposition rate and quality will reach the maximum values, respectively, along with gradually increasing MSWI fly ash input flow. Overall, this study has the guiding significance for reducing the pretreatment steps and post-treatment costs of MSWI fly ash by melting in the cyclone furnace.

PVA fiber-reinforced ultrafine fly ash concrete: Engineering properties, resistance to chloride ion penetration, and microstructure

The aim of the study was to systematically investigate effect of the particle size of fly ash on the resistance to chloride ion penetration of concrete as well as the workability and mechanical properties. Two different particle sizes, i.e., D50, particle size <15.22 μm (FA) and D50, particle size <3.12 μm (ultrafine fly ash, UFA) were used for the investigation. The use of PVA fibers helps minimize issues such as brittleness. The mechanical properties of polyvinyl alcohol fiber-reinforced ultrafine fly ash (PVA-UFA) composites as well as the resistance to chloride ion penetration were investigated. The results showed that UFA has a higher engineering value in terms of work performance improvement. UFA is useful in improving the mechanical properties of concrete and the resistance to chloride ion penetration at all ages. The optimum content of UFA is 25%. The 28 d flexural strengths of PVA-UFA specimens were 11.01% higher than those of control group. Moreover, the electric flux and DRCM decreased by 10.96% and 42.58%, respectively. The flexural strength and chloride ion penetration resistance of the composite material with PVA fibers were improved compared with those of the single UFA material owing to their better microstructure. Therefore, PVA-UFA can be applied in coastal highways, bridges, and other structures.

Green activating silica-alumina insoluble phase of fly ash to synthesize zeolite P with high adsorption capacity for Pb(II) in solution

Using fly ash as a raw material to synthesize zeolite is an effective way to obtain cheap adsorbent. However, the high energy consumption of activating the insoluble silica-alumina phase of fly ash limits its practical application. In this study, a low-energy, wet-grinding hydrothermal method was used to activate the insoluble phase of fly ash to synthesize zeolite P, and the adsorption capacity of zeolite for lead ion in solution was studied. Wet-grinding can reduce fly ash particle size to nanometer scale and break long-chain Q4(Al) into short-chain low-polymer. Without aging, the activated material could directly synthesize zeolite P with a relative crystallinity of 89.37 %. Compared with the common alkali-fusion method, the wet-grinding method reduced energy consumption by 45.93 % and increased synthetic zeolite relative crystallinity by 7.51 %. The synthesized zeolite P could effectively remove Pb2+ ions from solution through ion exchange and hydroxyl reaction, and the adsorption capacity reached 497.01 mg/g.

Analysis of the synergistic effect of particle size of compound mineral admixtures on the hydration properties and porosity of Portland cement under low-temperature conditions

The mechanical properties of cement matrix composites can be severely affected at low temperatures, limiting the environment in which they can be used. To ameliorate this problem, the addition of silica fume and fly ash to the matrix is an effective method. This study used silica fume and flew ash as composite admixtures mixed with Portland cement to characterize the heat of hydration, mechanical properties, and microstructure of the composite cementitious materials. The test results show that the particle size distribution of fly ash and silica fume can influence the hydration reaction rate and the whole hydration process. By adjusting the particle size of the admixtures, the structural loosening and strength attenuation of the concrete caused by low temperatures can be improved. At low temperatures, when cement hydration is low, the coarser the particle size, the better the gradation between silica fume and fly ash and the better the filling of the pores. In addition, the specific surface area of silica fume was fixed at 19.1m 2 /g, and the correlation between fly ash particle size intervals in mineral admixtures was strongest in (30~40μm), (40~50μm) on the total porosity. Silica fume in the (>0.4μm) and (0.25~0.4μm) particle size intervals had the strongest correlation with porosity. Finally, a prediction model between the particle size interval of the mineral admixture and the total porosity of the concrete under standard and low temperature curing conditions was established. • The admixture effectively fills micro-pores • Methods for improving the strength of cementitious materials at low temperatures • The particle size distribution affects the rate of hydration and the hydration process • The hydration rate can be increased by improving the particle size of the admixture.

CFD Modeling for the Influence of Fly Ash Particle Size on the Different Properties of High Concentrated Slurry Transportation in Horizontal Pipe

The disposal of fly ash from thermal power plants is becoming a serious concern for environmental engineers. This article attempts to study the influence of particle size of the fly ash on the various properties of high concentrated slurry (50 - 70 % by weight) flow through a horizontal pipe with the help of a commercial CFD code ANSYS FLUENT. Modified Herschel-Bulkley model and SST k-ω turbulence model is used for computational analysis, and the computational outcomes are validated with the available literature. In the present study, the fly ash samples were procured from different coal-based thermal power plants. Various properties: physical and rheological are obtained experimentally. The specific gravity of particles and the static settling concentration (SSC) of the slurry increases with the decrease in the particle size. The rheological properties increase as the concentration increases, and the slurry behaves like a non-Newtonian fluid at a high concentration. In addition, the particle size and concentration intensively impact on the skin friction coefficient and velocity distribution. However, no significant impact on the velocity profile at 70 % concentration (by weight).&#x0D; HIGHLIGHTS&#x0D; &#x0D; The rheological properties increase as the concentration increases, and the slurry behaves like a non-Newtonian fluid at a high concentration&#x0D; CFD modelling of the slurry flow at high concentration with modified Herschel-Bulkley model and SST k-ω turbulence model&#x0D; The particle size and concentration intensively impact on the skin friction coefficient&#x0D; &#x0D; GRAPHICAL ABSTRACT

Migration and control of mercury in hazardous chemical waste incineration

This study evaluated the emission and control characteristics of mercury during hazardous chemical waste incineration. Meanwhile, for the complex flue gas conditions generated by hazardous chemical waste incineration, the commercial activated carbon used in the incineration system was modified with CuBr2, and its Hg0 removal performance was tested. The results showed that the major proportion of Hg was in the fly ash, and the content accounted for 74.53 % of the total Hg. The Hg content in the fly ash increased with decreasing fly ash particle size. In the exhaust flue gas, the Hg content was 11.05 % of the total Hg, in which the mass percentages of Hg2+ and Hg0 were 59.08 % and 40.92 %, respectively. The flue gas purification equipment of the incineration system had a removal efficiency of 92.40 % for HgT. The saturated Hg0 removal capacity of the modified activated carbon was improved by 33.51 times. Under simulated incineration flue gas conditions, the modified activated carbon was able to achieve an Hg0 removal efficiency of 92.85 %. The results can contribute to the understanding of Hg emissions from hazardous chemical incineration systems and provide guidance for efficient Hg removal from activated carbon.

Speciation and leaching characteristics of heavy metals from municipal solid waste incineration fly ash

• Characteristics of heavy metals in MSWI fly ash were investigated. • Coarse and fine particles MSWI fly ash can enrich more heavy metals. • Long-term environmental risk was low for As and Cr, but high for Pb and Hg. • CaO can effectively inhibit the leaching of As and Pb from MSWI fly ash. Fly ash from municipal solid waste incineration (MSWI) is classified as hazardous waste because it is rich in heavy metals (HMs), dioxins and other hazardous substances. In this paper, four kinds of MSWI fly ash were selected and tested to understand their physical and chemical properties, HM enrichment pattern, HM fugacity pattern and leaching toxicity. The results show that the main mineral components of MSWI fly ash are chlorine minerals and calcium minerals (NaCl, NaSO 4 , KCl, CaClOH, CaCO 3 , CaSO 4 , etc.), while only NaSO 4 and NaCl exist in MSWI fly ash without CaO injection. The HM contents in MSWI fly ash were As (25.8–922.0 μg/g), Cr (26.9–173.3 μg/g), Pb (988.8–1259.4 μg/g) and Hg (7.29–8.47 μg/g) and showed a trend of an initial decrease and later increase as the MSWI fly ash particle size decreased. The major species of As and Cr in the four MSWI fly ash samples were As(V) and Cr(III); Pb mainly existed as PbCl 2 and PbSO 4 . The injection of CaO could make the fly ash alkaline and promote the formation of Ca 3 (AsO 4 ) 2 , CaCrO 4 and PbCl 2 , which reduced the leaching rate of As and Pb to<10 % and 15 %, respectively. Hg exists mainly in the species of Hg 0 , Hg-OM, HgCl 2 , HgO and HgSO 4 , and its distribution is mainly influenced by the flue gas components. These results can provide a basis for studying the escape mode and leaching toxicity of HMs in MSWI fly ash.

Heat flow modelling of the alkaline activation of fly ash with sodium hydroxide in the presence of portlandite

Alkali-activated cement is a promising alternative to replace totally or partially Portland cement. The heat flow and the degree of reaction are widely analyzed variables on the alkali-activation process. In this paper, a phenomenological-based semi-physical model of the alkali-activation process was developed. The model predicts the heat flow and the degree of reaction, by correlating the composition of the fly ash, the NaOH concentration (alkaline activator), the concentration of portlandite and curing temperature. The model considers balanced chemical reaction and the process diffusive phenomena. For validation microcalorimetric measurements at 25 °C and 35 °C was made, and secondly Freisleben-Hansen model was fitted with the experimental data. The results indicate a reaction acceleration for concentration of NaOH in (6–10) M and a reaction deceleration due to portlandite. The fly ash particle size and diffusivity-associated factors have an incidence of more than 5 % on the heat flow and degree of reaction.