Firing-Associated Recycling of Coal-Fired Power Plant Fly Ash

Coal-fired power plant fly ash is a global environmental concern due to its small particle size, heavy metal content, and increased emissions. Although widely used in concrete, geopolymer, and fly ash brick production, a large amount of fly ash remains in storage sites or is used in landfills due to inadequate raw material quality, resulting in a waste of a recoverable resource. Therefore, the ongoing need is to develop new methods for recycling fly ash. The present review differentiates the physiochemical properties of fly ash from two coal combustion processes: fluidized bed combustion and pulverized coal combustion. It then discusses applications that can consume fly ash without strict chemical requirements, focusing on firing-associated methods. Finally, the challenges and opportunities of fly ash recycling are discussed.


Introduction
Since the late 19th, coal-fred power has continuously grown and has been an essential source of global electricity. As of 2019, coal-fred power accounts for 36.7% of the world's total electricity production and 22.4% of the total electricity of countries in the organization for economic cooperation and development (OECD) [1]. Due to environmental concerns and the need for sustainable development, coal-fred power has gradually been replaced by other energy sources, such as wind, solar, natural gas, biomass, and combustible waste power. However, under the pressure of economic development, despite the decrease in proportion, the total capacity of coal-fred power continues to grow, especially in Asian countries such as China, India, and Southeast Asia, threatening net-zero targets by 2050. Te IEA predicts that coal-fred power generation from 2021 to 2024 will increase by 4.1% in China, 11% in India, and 12% in Southeast Asia [2].
Environmental concerns arise from fuel extraction operations, the coal power generation process, and emission treatment. Te primary discharge of coal combustion includes acidic gases such as SO 2 , NO x , and CO 2 ; heavy metal vapors such as mercury; and solid combustion residues such as dust, fy ash, and bottom ash. Strongly acidic gases, including NO x and SO 2 , go through chemical processes depending on the combustion system. Conventional boilers use low-ash coal grades and the combustion chamber temperature is usually above 1400°C. At this temperature, nitrogen gas oxidizes, forming NO x in the fue gas. Te fue gas is directed to an adsorption tower that performs selective catalytic reduction (SCR) to convert NO x into nitrogen gas and water. Te SO 2 gas formed by the oxidation of sulfurcontaining substances is absorbed in a wet fue gas desulfurization (WFGD) system with lime/limestone slurry to produce calcium sulfate. Fluidized bed combustion boilers often use high-ash coal grades and the temperature of the combustion chamber usually does not exceed 900°C to limit the generation of NO x . In this system, limestone can be fed into the combustion chamber to absorb SO 2 gas [3].
Coal fy ash is also a primary environmental concern due to its small particle size and heavy metal contents. Continued increases in coal-fred power generation have resulted in a continued increase in global coal ash emissions. While a large amount of coal fy ash is used in nonfring construction materials such as concrete [4][5][6], fy ash bricks [7,8], and geopolymer [9,10], a signifcant amount remains at the storage site or is used for landflls. A recent review of fy ash usage in China shows that 56% of fy ash is used in construction, 35% is used in landflls, and 9% is used in other applications [11].
For safety reasons, fy ash used as pozzolanic additives in concrete and cement has stringent quality requirements. Te American standard ASTM C618-19 classifes fy ash used for concrete into classes F and C by chemical composition, loss-on-ignition, physical properties, and pozzolanic activity [12]. Chemically, the most signifcant diference between Class C and Class F is in the CaO content and the total SiO 2 + Al 2 O 3 + Fe 2 O 3 content. Due to its high CaO content, often associated with lime and calcium sulfate, Class C fy ash features self-adhesive properties [13]. Class F fy ash contains a higher SiO 2 content than Class C and shows pozzolanic activity in mortar and concrete. Ones that do not belong to Class C and Class F are not considered pozzolanic additives. Due to the quality requirements mentioned above, a large portion of fy ash piles up in storage sites by the power plant or is used for landflls and mine backflls [14,15].
Under environmental stress, intensive studies have focused on thoroughly treating and utilizing fy ash in more value-added applications. In India, 32.87% of fy ash was left untreated during 2017-2018 [16], which decreased to 7.59% by 2020-2021 [17]. During the year 2020-2021, ten modes of fy ash utilization were efective: cement (25.81%), mine flling (6.20%), bricks and tiles (12.98%), reclamation of lowlying area (15.59%), ash dyke raising (7.94%), roads and fyovers (15.04%), agriculture (0.03%), concrete (0.83%), hydropower sector (0.03%), and others (7.97%) [17]. In this example, the main distribution of fy ash is in construction, of which the portion used for landfll and foundation reclamation accounts for a greater proportion than that used in cement, concrete, brick, and tiles. Te environmental concerns associated with such applications include wind-blown dust [18] and the dissolution of toxic metals into groundwater [19,20]. Terefore, methods of using fy ash to mitigate these risks are still under study.
Due to its porous structure, fy ash has a large specifc surface area, which is suitable as a substrate in waste gas and wastewater treatment. In gas treatment, fy ash is mixed with calcium hydroxide to adsorb SO 2 in fue gas [21,22]. Tis mixture can also absorb toxic organic vapors such as toluene [23] and m-xylene [24]. In wastewater treatment, fy ash improves the precipitation of heavy metals [25], boron [26], and phosphate [27] in the presence of calcium hydroxide. In addition, fy ash can adsorb phenolic compounds [28,29], dyes [30,31], and pesticides [32]. Although economically and technically efcient to some extent, these applications accumulate toxic substances on the fy ash, making postadsorption treatment of fy ash even more complicated.
Other applications of fy ash include raw materials for glass [33,34], glass ceramics [35,36], sintered bricks [37,38], and ceramic tiles [39,40]. Tis review covers the physical and chemical properties of coal fy ash and focuses on the fring-associated recycling of this material. Te main objective of the review is to provide evidence that coal fy ash can be used as ceramic raw material without violating regulations on the release of heavy metals into the environment during use. Te review also proves that this method can recycle a large amount of fy ash remaining in the storage sites because it does not place any limit on the origin and quality of fy ash.

Methodology
Most of the documents cited in this review paper are publications from Google Scholar and Web of Science databases. Te statistical data from the International Energy Agency (IEA) and the India Central Electricity Authority (CEA) and test methods of the American Society for Testing and Materials (ASTM) and the International Standards Organization (ISO) are also cited.
As a supplement to the review, coal fy ash samples were collected and characterized. Te samples were collected from four coal-fred power plants in Vietnam: Dong Trieu, Mong Duong I, Uong Bi, and Ninh Bình. Dong Trieu and Mong Duong I power plants apply the fuidized bed combustion (FBC) technique, whereas Uong Bi and Ninh Binh power plants employ the pulverized coal combustion (PCC) technique. Te fy ash was dried overnight at 115°C before being characterized by X-ray difractometry, scanning electron microscopy, and laser scattering particle size analysis.
A Bruker X-ray Difractometer model D2 Phaser was used for phase analysis. Care was taken to keep the top surface of the sample mass fat and on the plane of the top of the sample holder to avoid possible displacement errors. Te copper radiation source of the instrument was operated at 30 kV and 20 mA while scanning between 10 and 60 degrees 2-θ. Te data collected from the difractometer were analyzed using Crystal Impact's Match software using the COD database.
Te microstructural properties of fy ash were investigated by a JOEL 6360LV scanning electron microscope. Fly ash samples were sprinkled onto PELCO Tabs ™ carbon conductive tabs adhered to the SEM sample holders, which were then gently tapped to remove nonadherent particles. Before being placed in the SEM instrument, the sample holders were placed in a sputter coater to coat the fy ash particles with a thin layer of platinum for conductive purposes. Te SEM ran under a vacuum.
Te fy ash particle size distribution was analyzed using a HORIBA Partica Mini LA350 instrument, which used a laser source with a wavelength of 650 nm. Te instrument works on the principle of laser difraction and Mie's light scattering theory.

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Journal of Analytical Methods in Chemistry

Properties of Coal-Fired Power Plant Fly Ash
Coal combustion techniques signifcantly infuence the morphology, particle shape, and particle size distribution of fy ash. Figure 1 presents the scanning electron microscopy image of fy ash from four coal-fred thermal power plants.
Due to high-temperature (>1400°C) combustion, the fux agents in the PCC fy ash melted, stuck to unmelted particles, and rounded under surface tension, forming spherical particles upon cooling. On the other hand, FBC fy ash particles have irregular shapes as they were created from a low-temperature (<900°C) combustion process and thus did not have sufcient liquid phase to form spherical particles. Fly ash is a collection of fne particles that have harmful efects on human health and ecology. Signifcantly, the wind can blow fne fy ash particles long distances, making fy ash a global health and environmental concern [41,42]. Most coal fy ash has particle sizes under 100 micrometers [43][44][45]. Figure 2 shows the particle size distribution curves of four types of fy ash. Tey present the frequency distribution (q in %) of particles within a specifc size range. Although the particle size distribution curves are diferent and the combustion methods show no trend in these curves, they all have one thing in common: 90% of each sample (D90) is smaller than 100 micrometers and the median particle size (D50) is in the range of 12.4-17.6 micrometers.
Te chemical composition of fy ash is quite complicated, depending on the coal origin and the combustion technique. By weight percentage, dominant in fy ash is SiO 2 , followed by Al 2 O 3 , and other oxides such as Fe 2 O 3 , CaO, K 2 O, Na 2 O, and TiO 2 ( Table 1). Te introduction of limestone to the FBC boilers adds CaO to the solid combustion residues. Fly ash is also a source of heavy metals, including but not limited to V, Cr, Mn, Co, Ni, As, and Hg [46,48,49]. In addition, there are distinct diferences between the chemical composition of diferent size fractions of fy ash from a boiler. Signifcantly higher concentrations of total carbon and heavy metals are found in smaller particles. Slightly higher concentrations of major elements, Si, Al, Fe, Ca, K Na, Mg, Mn, and Ti, are present in coarser ones [46,48]. Fortunately, most heavy metals are locked in the glass phase of fy ash [50]. However, the ability to release heavy metals and rare Earth elements from fy ash in landflls is always a community concern. Te allowable limits for the disposal of coal combustion residuals vary between countries and regions but may be tightened in the future [51,52].
Te mineral composition of fy ash depends on the coal type and combustion techniques. X-ray difraction (XRD) analysis of a series of fy ash showed a signifcant variation in the glass phase content, from 45 to 80% [53]. Besides the glass phase, quartz is the most popular crystalline phase. Its difraction peaks stand out in the XRD patterns of all types of coal fy ash. Since PCC boilers operate at temperatures above 1400°C, high enough for mullite crystallization, its fy ash often contains mullite (3Al 2 O 3 .2SiO 2 ) [54,55]. Te mineral composition of mullite is heterogeneous due to the substitution of impurities at the aluminum sites, forming solid solutions. XRD and nuclear magnetic resonance analysis on a bituminous coal fy ash showed a mullite average chemical composition of Al 4.61 Fe 0.05 Ti 0.02 Si 1.32 O 9.66 [56]. FBC boilers operate at approximately 850°C, which is not high enough for mullite formation; thus, no difraction peaks of mullite can be observed. Instead, hematite, phengite and anhydrite gehlenite might be present in the XRD patterns of FBC fy ash [4,55,57]. Figure 3 presents the XRD patterns of four types of fy ash. PCC fy ash shows only two crystal phases, quartz and mullite, while FBC fy ash shows three crystalline phases, including quartz, phengite, and hematite.
In addition to oxide solids, fy ash may contain significant amounts of unburnt carbon (UC), a result of incomplete combustion. Te UC content depends on the combustion technology, operating conditions, and coal type. It is assessed by the loss-on-ignition (LOI) test [58]. Termal analysis of fy ash confrms the presence of UC with a large exothermic peak incorporated with weight loss. Te starting point of UC combustion in TG-DTA analysis varies with the type of coal fy ash [55,59,60]. Typically, the amount of UC in fy ash is higher than that in bottom ash [61].
Fly ash UC is an inexpensive source of activated carbon for adsorbing harmful components of fue gas such as NO x [62], mercury vapor [63,64], and organic substances [65]. However, UC is a factor hindering the use of fy ash in many construction applications. From the concrete perspective, UC adsorbs air-entraining admixture, reducing the physical properties of concrete [66]. As specifed by ASTM C618 -19 standard, the LOI content of fy ash used as pozzolanic additives in concrete should be under 6% with some tolerance to the Class F fy ash [12].
Te leaching test performed on fy ash by the EN 12457-4 test method shows that the heavy metal concentrations in the leachate are below the inert waste limits specifed by the European Landfll Directive [67]. However, the long-term leaching test indicates a potential risk to soil and groundwater [68]. As has been demonstrated in various studies, it is possible to immobilize heavy metals of solid wastes from wastewater treatment plants [4,69], red sludge from bauxite processing plants [70], and blast furnace slag [71][72][73] in the structure of glass [74], glass ceramics [75], and ceramic materials [76]. Likewise, introducing fy ash to ceramic production is a potential solution to lock the heavy metals of fy ash in the ceramic network.
Due to high SiO 2 and Al 2 O 3 contents, fy ash is suitable as a precursor for aluminosilicate crystalline phases, such as mullite [77,78] and cordierite [79][80][81][82]. In addition, the high CaO content in certain types of fy ash, especially those from FBC boilers using limestone to absorb SO 2 , makes them suitable as a raw material for glass ceramics that contain calcium silicate and calcium aluminosilicate crystalline phases [83][84][85][86]. Furthermore, due to its high glass phase content, fy ash is also considered a fuxing agent that promotes sintering in ceramics.

Bricks.
Fly ash can replace up to 80% of clay in clay bricks, but some adjustments should be made. Te presence of a large amount of fy ash signifcantly reduces the raw mix plasticity index. Terefore, it is necessary to add plastic additives to facilitate extrusion. In addition, the porous structure of fy ash particles impedes the sintering process, increasing the porosity of the sintered product even at low degrees of replacement [87]. Hence, it may be necessary to grind the fy ash to break down its porous structure and increase the contact surface area to speed up the sintering process [37]. Along with the high porosity, the high refractoriness of fy ash requires an increase of the fring temperature by 50 to 100°C so that the physical properties of the sintered products meet those of conventional clay bricks. Most reports indicate that when using large amounts of fy ash, the fring temperature is 1050°C or higher [37,88]. A test at the industrial scale reveals a number of obstacles that should be overcome: brick swelling and deformation due to local melting and the occurrence of black core and cracks [89].
It is possible to manufacture bricks with 100% of the solid material being fy ash. In this case, shaping requires a temporary binder because most fy ash lacks adhesive capacity. Kayali reported that although the apparent density of the fred product is 28% lower than that of conventional clay bricks, its compressive strength is 24% higher, and brick-tomortar bond strength is 44% higher than those of the best local clay bricks [8].
It is noteworthy that heavy metals are immobilized in the ceramic structure of the brick. Sutcu et al. [88] showed that the release of heavy metals, including Cr, Mn, Ni, Cu, Zn, As, Cd, Ba, Hg, and Pb, was substantially lower than the maximum allowable limits regulated by U.S. Environmental Protection Agency (EPA) for hazardous solid waste and the threshold limits of EPA Victoria (Australia) for industrial solid waste. A study by Leiva et al. also shows that the leaching contents are below the limits of the European Landfll Directive for granular waste, the Italian national regulation for the reuse of waste in construction material, and the Dutch Soil Quality Decree for bound or shaped materials [47].

Ceramic Tiles.
Fly ash with high fux (Fe 2 O 3 + Na 2 O + K 2 O) contents can be used to replace feldspar in ceramic tile production. Olgun et al. were able to improve the modulus of rupture of wall tiles by replacing potassium feldspar in the raw mix with fy ash and borax solid waste without having to change the sintering temperature [90]. As the porous structure of fy ash hinders sintering, resulting in high porosity, it is mainly used as a raw material for wall tiles, equivalent to Group BIII as classifed by EN 14411. Tis type of ceramic tile is not subject to the loading force in use but requires high water absorption for good mortar adhesion, allowing higher water absorption and lower fexural strength than those of the foor tiles [91].
Similar to that observed in clay bricks, the disruption of porous fy ash particles also improves the sintering of the ceramic tiles [92]. When high alumina fy ash is used and a foor tile is the desired product, the fring temperature must be increased to achieve the required physical properties. In particular, the sintering process becomes more difcult when the Al 2 O 3 content is up to 40%, equivalent to freclay refractory bricks. Wang et al. showed that for producing  ceramic tiles with water absorption below 0.5% from a raw mix with 70% high alumina fy ash, the sintering temperature is as high as 1300°C [93]. Solid wastes with high fux contents, such as glass waste [94,95], boron waste [90,96], or lithium mine tailings, are viable additives that help lower the sintering temperature [97]. Wang et al. used high alumina content fy ash (40% Al 2 O 3 ) and waste glass to make insulating ceramic tiles, consisting of a layer of foam and a layer of dense material. Te raw mix of the foam layer consists of waste glass (50%), fy ash (30%), clay (15%), and feldspar (5%). Te raw mix of the dense layer consists of fy ash (60-80%), clay (15%), and quartz (5-25%). With single-loading pressing and calcination at 1200°C, the foam layer performs insulation capacity with an average pore diameter of 300-500 micrometers. In addition, the bi-layer ceramic shows a fexural strength of 31.5 MPa. Tis way, over 70 weight percent of the tile originates from industrial waste [94].
Most fy ash has a Fe 2 O 3 content of 4-10% (Table 1). Te presence of a remarkably high Fe 2 O 3 content gives a clay brick color to fy ash-based ceramic bodies [8]. On the other hand, the combination of Fe 2 O 3 and UC causes the black core in bricks [98]. Terefore, if masking the ceramic body color is desired, an engobe glaze with high opacity and whiteness should be applied on the surface of the tiles before the decoration is fnished.
Although a test method for determining lead and cadmium leaching from ceramic tiles is available [99], there is no harmonized requirement for hazardous element concentrations in the leachate of ceramic tiles. However, one can infer from the studies on bricks that ceramic tiles are also capable of immobilizing hazardous elements, even better than bricks because of their denser structure formed by higher sintering temperatures.

Insulating Materials and Lightweight Concrete Aggregates.
Insulating materials and lightweight concrete aggregates feature porous structures constructed by closed pores. Te sintering of these materials requires a simultaneous generation of gas and molten phases with sufcient quantity and viscosity. Te molten phase entraps the gas, forming closed pores upon cooling. Fly ash is an excellent candidate for this application due to its UC and high glass phase contents. Te UC oxidizes at elevated temperatures releasing carbon dioxide gas while the glass phase is ready to melt. However, if fy ash is the only raw material, the fring temperature should be up to 1300-1400°C for bloating [100]. Furthermore, UC might be low in certain fy ash sources (Table 1). Terefore, fuxes and gas-forming agents are required for (1) generating gases, (2) reducing the fring temperature, and (3) adjusting the quantity and viscosity of the liquid phase for entrapping the generated gases. Te fuxes of interest include waste glass [101,102], limestone [103,104], and synthetic chemicals such as sodium salts [105,106]. Some fuxes, including limestone and soda, also play the role of gas-forming agents.
Since most fy ash is not self-adhesive, a temporary binder is required to form the green pellets. On a laboratory scale, organic binders such as polyvinyl alcohol have been tested [100]. On a larger scale, clay is a popular binder due to its availability and low cost [103,106]. Besides, bentonite [102,104] and ordinary Portland cement [104] have been used.

Mullite and Cordierite Ceramics.
Fly ash is also a raw material for the production of mullite ceramics due to its high SiO 2 and Al 2 O 3 contents. Low alumina fy ash often requires an alumina augmenting agent such as aluminum oxide [107], bauxite [77,108], or high alumina industrial wastes [78,109]. Unlike FBC fy ash, mullite is usually available in PPC fy ash. Te presence of mullite makes the mullite crystallization in PCC fy ash-based ceramics easier than in those using FBC fy ash. Although fuxes are available in fy ash and start forming a liquid phase at relatively low temperatures, fy ash-based mullite ceramics are often sintered at high temperatures, usually above 1400°C, because mullite is a refractory phase. Making dense mullite ceramics from PCC fy ash and bauxite, Dong et al. showed that the solid-state reaction between cristobalite and corundum occurs at temperatures below 1300°C, followed by the dissolution of the corundum into the liquid phase at higher temperatures where secondary crystallization occurs. Te formation and recrystallization of mullite lead to volume expansion which is slightly dominant over the shrinkage of the sintering process. At 1600°C, the material achieves a relative density of 93.94% with spherical pores and fracture strength of 186.19 MPa [77]. Jung et al. combined aluminum oxide with PCC fy ash, from which UC was removed, to produce mullite ceramics at temperatures of 1400, 1500, and 1600°C. Although the pellet is cold isostatic pressing, the density of the fred product was relatively low (63%). Tis low density is due to the exaggerated grain growth of needle-shaped mullite crystals incorporated with voids formation [107].
Te introduction of the supplements alters the sintering mechanism either by preventing excessive grain growth or by creating new phases, thereby improving the physical properties of the sintered products. Te presence of 3Y-PSZ inhibits the crystal growth of mullite, leading to an improved fracture strength [107]. Magnesia efectively promotes sintering, signifcantly above 1450°C. It slightly reduces the linear thermal expansion coefcient (LTEC) at 1300°C by forming low thermal expansion α-cordierite. However, it slightly increases the LTEC above 1400°C due to the formation of high expansion corundum and the spinel (MgAl 2 O 4 ) [108]. SiC enables the growth of mullite needleshaped crystals out of the saturated glass phase, resulting in better bonding of the crystals, reducing the true porosity but increasing the closed-pore porosity. As a result, thermal conductivity and cold crushing strength are improved [110]. TiO 2 hinders the sintering process at low temperatures but promotes sintering above 1300°C. Tis phenomenon is useful for the unsaturated sintering of porous mullite ceramic membrane supports [111].
Te porous structure of fy ash particles and the recrystallization of mullite favor the fabrication of porous mullite ceramics for insulation or fltration purposes. Chen et al. fabricated porous materials from coal mine waste kaolin and spherical hollow fy ash, using bentonite as the binder and calcium iodate (Ca(IO 3 ) 2 .6H 2 O) as a focculant. After fring at 1550°C, the resulting product has a porosity of 44.73-46.12%, with 99% of the porosity being closed pores, and mullite is the main crystalline phase. Te freeze-gel casting/polymer sponge technique can produce a porous mullite ceramic with a porosity of 66.1% and an average compressive strength of 45 MPa [112].
Fly ash is also suitable as a raw material for cordierite ceramics, a material well known for its low LTEC. Cordierite crystallizes in the orthorhombic system. Its dimorphism is hexagonal indialite which is more refractory than cordierite [113][114][115]. Although the IMA formula of cordierite is 2MgO.2Al 2 O 3 .5SiO 2 , it appears as a solid solution in practice [116,117]. Tanks to its low LTEC, cordierite ceramic is highly resistant to thermal shock. Tis material also possesses high chemical resistance and a low dielectric constant. Te synthesis of cordierite from fy ash requires MgO supplementing materials such as talc [118], magnesia [119], magnesite [120], or dolomite [121]. Unlike mullite, the sintering temperature of cordierite ceramics is usually below 1200°C. In addition to indialite, fy ash-based cordierite ceramics may contain mullite, cristobalite, periclase, and spinel [118,120]. Porous cordierite ceramics from fy ash can be used for microfltration membranes [82], catalytic substrates [118], and membrane supports [121].

Foam Glass and Glass
Ceramics. Tanks to its high silicate glass content, fy ash can be used as a raw material for glass and glass-ceramic synthesis. Erol et al. produce glass by melting fy ash at 1500°C, glass ceramics by annealing the obtained glass at 1150°C, and ceramics by fring green pellets at 1200°C. Te only phase in the glass products is amorphous, the crystalline phase in the glass-ceramic products is augite (Ca(Mg, Fe 3+ , Al) (Si, Al) 2 O 6 ), and in the ceramic is quartz, mullite, and enstatite ((Mg, Fe)SiO 3 ) [34]. However, making glass and glass ceramics products from fy ash can be difcult due to high SiO 2 and Al 2 O 3 (network formers) contents and insufcient fuxes (network modifers). Terefore, it is necessary to add fux ingredients to lower the melting temperature.
One of the most attentive applications of fy ash-based glass is glass foam, which is considered a low-cost insulation material in construction. Besides fy ash, glass foam raw mixes often have fux and foaming agents. Most of them are industrial wastes. Te commonly used fux is waste glass. It is used in large quantities comparable to fy ash [33,122]. Alternatively, borax [122] and soda [123] can be added to the raw mix as network former and modifer, respectively. Foaming agents can be obtained from a variety of sources, including calcium carbonate [122], soda [123], and SiC [33,124]. For instance, Bai et al. produced glass foam by melting a raw mix of fy ash, waste glass, and SiC waste at 950°C. Te resulting product expanded 5.81 times compared to the green body [33].
Another fascinating application of fy ash-based glass is glass ceramics because of their high mechanical strength and thermal shock resistance. Te crystalline phase in fy ash-Journal of Analytical Methods in Chemistry based glass ceramics is controlled by adjusting the composition of the raw mix. Te two glass ceramic systems of great interest are SiO2-Al2O3-CaO [125,126] and SiO 2 -Al 2 O 3 -MgO [79,127]. When a high amount of Fe 2 O 3 is available in fy ash, the system SiO 2 -Al 2 O 3 -Fe 2 O 3 -CaO is of interest [128]. Depending on the chemical composition of the mix, the crystalline phases in the SiO 2 -Al 2 O 3 -CaO glassceramics include diopside (Ca(Mg, Al) (Si, Al) 2 O 6 ) [125], augite (Ca(Mg, Fe)Si 2 O 6 ) [34,125], and wollastonite (CaSiO 3 ) [129]. SiO 2 -Al 2 O 3 -MgO glass ceramics are of the most interest because they contain cordierite, a mineral with low LTEC, and are suitable for thermal shock applications [79,127].

Portland Cement Clinker.
In cement production, fy ash often serves as a pozzolanic additive, but high UC fy ash is not suitable for this application. Another possibility of using fy ash in cement production is to replace clay in the raw mixes of Portland cement clinker [130][131][132][133], belite cement clinker [134], and belite-sulfoaluminate cement clinker [135]. Tis perspective has been investigated from the laboratory to the industrial scale. Laboratory studies showed that the use of fy ash reduced the sintering temperature of Portland cement clinker [130,131]. Commercial demonstration on high carbon fy ash showed that the obtained fy ash-based Portland cement performs a higher compressive strength than the normal ones et al.l ages, despite a lower fneness [132]. Te use of high carbon high fy ash has the additional beneft of fuelsaving [133].
Unlike ceramic fring, SO 2 formed in the fring process of clinker can be reabsorbed by calcium oxide. Te resulting calcium sulfate helps reduce the gypsum amount required for cement setting control [136]. Terefore, using fy ash as raw material for cement clinker brings valuable economic, technical, and environmental benefts.

Challenges and Opportunities
Although it is demonstrated that fy ash-based glass and ceramic products can immobilize the heavy metals and rare elements of fy ash in their oxide networks, they are not suitable as food and drug containers. Tis restriction is mainly due to the stringent regulations applied to these products [137][138][139]. On the other hand, SO x emissions from burning fy ash are also of concern because the SO 3 content in certain types of fy ash may be high. High sulfur fy ash is often associated with FBC boilers that introduce limestone into the furnace to absorb SO x gas. Te SO 3 content in fy ash can be up to 14% [140]. A dramatic increase in the sulfur dioxide content in fue gases during the fring of FBC fy ash-based ceramic tiles was observed. [92]. Unlike ceramic fring, SO 2 formed in the fring process of cement clinker can be reabsorbed and becomes a valuable source of sulfate for cement setting control. Terefore, characterizing fy ash before recycling in these applications is critical in preventing the redistribution of toxic SO 2 into the environment.

Conclusion
Coal fy ash is a global concern due to its small particle size and heavy metal contents with increasing emissions. Faced with concerns about wind-blowing dust and heavy metal leakage from fy ash landflls and storage sites, eforts to use fy ash for safe and value-added applications are underway. Notable among them are fring-associated measures, including the production of ceramics, lightweight concrete aggregates, glass, and glass ceramics. Tere is almost no restriction for fy ash in these applications. Most notably, the recycling of fy ash and other industrial solid wastes can be combined in these applications without violating regulations on heavy metals released during use. Besides, the unburnt carbon content can alleviate the heat required for fring. Te fring temperature, product phase composition, and physical properties of the products can be controlled by supplementing agents. However, the sulfur content might be high in certain types of fy ash and go into the fue gas as sulfur dioxide. Tis problem can be solved in cement kilns but should be addressed in other applications.

Data Availability
All data generated and analyzed to support the fndings of this study are available within the article.

Conflicts of Interest
Te authors declare that they have no conficts of interest.

Authors' Contributions
Conceptualization and supervision were done by Vu Ti Ngoc Minh. Vu Ti Ngoc Minh reviewed the article. Data accuracy was tested by Pham Hung Vuong. Pham Hung Vuong reviewed the article. Conceptualization and data accuracy were done by Vu Hoang Tung. Data analysis was performed by Cao To Tung. Cao To Tung reviewed the article. Nguyen Ti Hong Phuong performed data analysis.