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Review

Recycling of Coal Fly Ash in Building Materials: A Review

by
Xuhang Lu
1,
Bo Liu
1,2,*,
Qian Zhang
1,
Quan Wen
1,
Shuying Wang
1,
Kui Xiao
1 and
Shengen Zhang
1,*
1
Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China
2
Xinjiang Research Institute for Nonferrous Metals, Urumqi 830009, China
*
Authors to whom correspondence should be addressed.
Minerals 2023, 13(1), 25; https://doi.org/10.3390/min13010025
Submission received: 14 November 2022 / Revised: 15 December 2022 / Accepted: 20 December 2022 / Published: 23 December 2022
(This article belongs to the Special Issue Fly Ashes: Characterization, Processing and Utilization)
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Abstract

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

1. Introduction

Coal fly ash (CFA) is the dust collected from flue gas after coal combustion, which is one of the main solid wastes generated in the coal burning industry. CFA particles are mostly spherical with a smooth surface [1]. CFA is primarily composed of SiO2, Al2O3, CaO, Na2O, Fe2O3, MgO, K2O and other metal oxides. CFA also contains trace elements such as Cd, Cr, Pb and Hg [2] and small amounts of radioactive elements such as 226Ra, 232Th and 40K [3]. In terms of phase composition, in addition to a large amount of the amorphous glass phase, CFA also contains mullite, quartz, calcite, magnetite and hematite. The world generates more than 800 million tons of CFA every year [4]. However, a large amount of CFA is discharged, which causes serious damage to the environment and threatens human health [5].
The recycling of CFA is a good alternative disposal method, which can obtain significant economic and environmental benefits [6]. Therefore, the resource utilization of CFA has become a hotspot. According to the core collection database of the Web of Science (Figure 1), the number of published papers related to CFA utilization has increased significantly in the past decade. The fields involved in these studies mainly included chemical fertilizers, soil conditioners, mine backfilling and building materials. Phosphorus, potassium, calcium, magnesium and other elements in CFA can help improve soil texture, water-holding capacity, pH and nutrient content [7,8,9]. In addition, phosphorus, potassium, calcium, and other elements in CFA are the nutrient elements needed by plants. Therefore, the use of CFA as a soil conditioner and fertilizer can help increase crop yields [10,11]. Due to the low content of beneficial elements such as phosphorus and calcium in CFA, more CFA needs to be used in actual production. The use of more CFA will increase the content of toxic elements in CFA and increase the leaching risk of these toxic elements. Therefore, the heavy use of CFA may pollute soil and groundwater [12]. Taking advantage of the pozzolanic activity of CFA, some studies have focused on the development of low-cost mine backfill materials [13,14]. Limited by the uneven regional distribution of CFA, its consumption in the field of mine backfilling is low. Compared with other utilization methods, the use of CFA to produce building materials has obvious advantages in terms of CFA consumption and the added value of products. Therefore, more than 70% of reports focus on the preparation of building materials, mainly cement, concrete, ceramics, and geopolymers (Figure 1). For example, the studies [15,16,17] focus on the utilization of CFA in the production of building materials.
The resource utilization ratio and mode of CFA vary greatly in different countries, as shown in Figure 2 [18]. Obviously, the utilization ratio of CFA has approached or even reached 100% in some countries, while it is still low in many other countries. Specifically, in EU and Japan, CFA is almost completely utilized; in China, America and India, the utilization ratios of CFA are 70.1%, 60.1% and 67%, respectively; in Australia, the utilization ratio of CFA is only 43.9%. Besides, CFA is mainly used as the raw material of building materials in major countries in the world, while CFA is more widely used to produce cement products (cement and concrete). For example, in the EU and China, most CFA is used to produce cement and concrete; in the United States and Australia, CFA is mainly used for mine backfilling, soil improvement and production of building materials, among others; in Japan and India, CFA is mainly used to produce building materials and soil improvers.
With the increase of CFA production and the increasingly severe environmental situation, the resource utilization of CFA has attracted extensive attention. In view of the remarkable advantages of CFA’s building materials utilization that other utilization methods can not compare, that is, it is more helpful to solve the problem of CFA’s massive stockpiling. Therefore, in this study, we focus on the latest research progress of CFA in building materials such as cement, concrete, ceramics and geopolymers. In addition, we also analyzed the impact of CFA management policies on the use of CFA in the construction industry and the impact on solving the problem of CFA accumulation.

2. Properties, Classification and Hazard Assessment of CFA

2.1. Properties of CFA

CFA may contain mullite, quartz, calcite, magnetite, hematite and other crystalline phases, as well as an amorphous glass phase. The CFA particles are mostly spherical with a smooth surface and few micropores. In addition, CFA particles have an average diameter of less than 10μm, a bulk density of 1–1.8 g·cm−3 and a specific surface area of 2500–4000 cm2·g−1 [19].
CFA has pozzolanic activity and can generate hydration products dominated by N-A-S-H gel via pozzolanic reactions [20,21]. The hydration process of CFA is mainly divided into three parts [22]. First, H+ in the solution exchanges with Ca2+ and Na+ on the surface of the CFA particles. Second, OH− ions attack the Si-O-T (T=Si or Al) bond to break it. Subsequently, Al3+ and Si4+ are released with the destruction of the CFA glassy network structure. Finally, a gel was formed by polymerization. In addition, compared to CFA with a low Ca/Si ratio, CFA with a high Ca/Si ratio was found to have higher activity [23,24].

2.2. Classification of CFA

The classification standards for CFA are different in different countries. According to the difference in composition, the American ASTM C618–19 standard [25] classifies CFA into classes N, F and C, as shown in Table 1. It can be seen from Table 1 that the classification standards based on ASTM C618–19 mainly refer to the content of some chemical components in CFA. For example, CFA with a total SiO2 + Al2O3 + Fe2O3 content not less than 70% and not more than 4% SO3 content was defined as class N. For CFA with total content of SiO2 + Al2O3 + Fe2O3 not less than 50%, when CaO content was not more than 18%, it was defined as Class F (with pozzolanic properties), whereas when CaO content was more than 18%, it was defined as Class C (with pozzolanic properties and gelling properties). In addition, the chemical composition of CFA varies with the type of coal burned. CFA produced by burning lignite and sub bituminous coal has higher CaO content and lower ignition loss, while CFA produced by burning bituminous coal and anthracite has lower CaO content and higher ignition loss [26].
In China, CFA is classified into Class C and Class F according to the type of coal (or CaO content). The former is produced by calcining lignite or sub bituminous coal and its CaO content is not less than 10%. The latter is produced by calcining bituminous coal and anthracite and its CaO content is lower than 10%. In addition, CFA in China is divided into CFA for mortar and concrete mixing, and CFA for cement active mixing materials (specified in GB/T 1596–2017 [27]).

2.3. Hazard Assessment of CFA

CFA contains heavy metals, such as Cd, Cr, Pb and Hg [2], which may pollute the environment. Leaching results of heavy metals from CFA in some literatures are shown in Table 2. Zhang et al. [28] conducted a TCLP test on two types of CFA in the United States and found that the leaching concentration of Ba in all CFA samples was lower than 0.03 mg/L, whereas Cd, Cr, Pb, Hg and As heavy metals were not detected. Zhao et al. [29] conducted a TCLP test on CFA from five regions in China. The results showed that the leaching concentration of heavy metals such as Cd, Cr, Ba, Hg and As in all CFA was lower than 0.1 mg/L, which was below the threshold specified by TLCP. Gupta et al. [30] conducted a TCLP test on CFA from two different regions in India, and also found that the leaching concentrations of heavy metals such as Cd, Cr, Pb and As in all CFA were lower than 0.1 mg/L and lower than the threshold value. Longos et al. [31] conducted TCLP tests on CFA from the Philippines. The results showed that the leaching concentrations of heavy metals such as Cd, Cr, Pb, Ba, Hg and As in CFA were below the threshold value. Therefore, the leaching concentrations of heavy metals in CFA are within a safe range.
CFA contains 226Ra, 232Th, 40K and other radioactive elements [3], which may cause radiation hazards to buildings and human bodies. Lu et al. [32] evaluated the radioactivity of CFA in two different regions of China and found that the annual effective dose of radioactive elements in CFA was 0.078–0.216, which is lower than the threshold value (1 mSv y−1) specified by the International Commission on Radiation Protection (ICRP) [33]. Singh et al. [34] evaluated the radioactivity of CFA in eight different regions of India. The results showed that the annual effective dose of the sample was within the range of 0.2761–0.4667 mSv y−1, which was below the threshold. Mahura et al. [35] evaluated the radioactivity of CFA from two different thermal power plants in India and found that the annual effective dose of radioactive elements in the sample was 0.061–0.166 mSv·y−1. Therefore, the content of radioactive elements in CFA is within the safe range.

3. Application of CFA in Building Materials

3.1. CFA Cement

Cement produces a great deal of greenhouse gas emissions of approximately 0.85–0.92 tons of CO2 for every ton of cement [37,38]. Accordingly, the global cement industry’s CO2 emissions will exceed 3.4 billion tons by 2020 [39]. Reducing the proportion of clinker in cement products is key to achieving CO2 emission reduction in the cement industry. Therefore, research on CFA as cement admixtures has become a hot topic in recent years.
In addition to being directly used as building materials, CFA cement is often used to improve subgrade materials and encapsulate heavy metals. Li et al. [40] prepared CFA-based cementitious material with cement, CFA and other solid wastes, and studied the feasibility of this CFA-based cementitious material as road foundation material. The results show that the 7d compressive strength of CFA-based cementitious materials meets the Chinese highway standards, and the durability can also meet the relevant standards. Besides, ettringite and C-A-S-H gel in the CFA-based cementitious material can solidify heavy metals, which leads to the leaching concentration of heavy metals such as Hg and Cu in the sample being lower than the threshold value specified in the Chinese drinking water standard. In addition, the research of Chethan et al. [41] shows that the volume stability and durability of black cotton soil stabilized with CFA cement can be improved.
However, the application of CFA in cement is still facing challenges due to the continuous improvement of cement strength requirements of building materials. For example, adding an appropriate amount of CFA can improve the long-term strength of cement but reduce its early strength [42]. The early strength of CFA cement can be improved by increasing its curing temperature [43,44]. This is because the increase of temperature is conducive to stimulating the hydration potential of CFA cement, thus accelerating the formation of hydration products such as C-S-H gel and ettringite, ultimately improving its strength. However, traditional curing methods, such as steam curing and oven curing, can easily cause sample cracking. To solve these problems, the microwave curing of CFA cement has been developed. According to Kong et al. [45], the 1-day compressive strength of CFA cement after 45 minutes of microwave curing is equivalent to 4 times normal curing at 20 °C and 1.3 times steam curing at 80 °C. Wang et al. [46] found 1-day compressive strength of CFA cement after 5.29 h microwave curing can reach 4.7 times of the samples cured at room temperature and 1.15 times of the samples cured at 60 °C. The improvement of mechanical properties of CFA cement by microwave curing is better than that by traditional hot curing, which can be explained as that microwave curing has higher energy transfer efficiency than traditional hot curing, thus forming a denser structure.
In addition, the strength of high-volume CFA cement also needs to be improved. From Wang et al. [47], adding 3% NS can increase the 28d compressive strength of CFA cement by 20%. Huang et al. [48] also found that adding 2.5% NS could increase the compressive strength of CFA cement mortar by more than 50%. The improvement of cement strength is due to the reaction of NS and calcium hydroxide, which provides a nucleation site for the formation of C-S-H gel, while the remaining calcium hydroxide provides a highly alkaline environment for the hydration of CFA. Furthermore, the addition of other cementitious materials was found to be helpful to improve the mechanical properties of CFA cement. Compared with the cement containing 60% CFA without metakaolin (MK), the compressive strength of cement can be increased by 216% and the initial setting time can be shortened by 11% when MK (5wt.%) when a particle size of 5.6 μm is added [49]. The improvement of mechanical properties of CFA cement is considered that the addition of MK refines the pore structure of cement matrix. In short, the modification method and related mechanism of CFA cement can be shown in Figure 3. No matter which method is used, it can ultimately be attributed to the densification of the matrix. However, when selecting additives that can improve the performance of CFA cement, we suggest selecting solid wastes rich in aluminum silicate, such as MK, because the cost of such additives is lower than that of nanoparticles, and it is conducive to the collaborative disposal and utilization of CFA and other solid wastes.

3.2. CFA Concrete

In 1935, CFA was used as a substitute for Portland cement in concrete [50]. The proper addition of CFA can improve the workability [51] and durability [52,53] of concrete, reduce the hydration heat [54], and enhance the strength of concrete [55,56]. The preparation process and performance of CFA concrete in some studies are summarized in Table 3. According to Kumar et al. [55], the concrete with 25% CFA has the best 28d compressive strength of 48.69 MPa. Similar to this result, Nagrockienė et al. [56] found that the concrete with 35% CFA has the best compressive strength of 57.5 MPa. The addition of CFA was found to promote the formation of Friedel salt (C3A·CaCl2·10H2O) from chloride ions in concrete [57,58]. Therefore, CFA is often used as an additive to improve the durability of concrete. Previous studies have shown that adding 30%–50% CFA can increase the chloride ion penetration-resistant ability of concrete by 31–55% [58,59], while 20%–50% CFA can reduce the water absorption of concrete by 20.9%–33% [59,60].
The pozzolanic reaction rate of CFA is low [61]. Excess CFA deteriorates the combination between the gel matrix and aggregate, widening the cracks in the interface transition zone (ITZ) (Figure 4) [62] and leading to a decline in the concrete performance, particularly the early performance. In this way, the performance of concrete, especially its early strength, is significantly reduced. According to Kumar et al. [63], adding 40% CFA will reduce the 14d compressive strength of concrete by approximately 26% and the 180d strength by 10%. Herath et al. [62] found that the 7d compressive strength of concrete with 80% fly ash was only 22.4 MPa.
To improve the strength of concrete with a high CFA content, additives were introduced into the concrete. Shaikh et al. [64] found that adding 2% NS could increase the 3d compressive strength of concrete with 60% CFA by approximately 75%. The performance improvement was attributed to the reaction of NS with calcium hydroxide generated by cement hydration to generate additional C-S-H gel, which optimizes the microstructure by filling the concrete matrix. Besides, Zhang et al. [65] found that adding 2% NS can increase the 28d compressive strength of CFA concrete by 11%. In addition, the amount of NS added is not the more the better. When the amount of NS added exceeds 3%, it may adversely affect the durability of concrete. They think that the performance improvement is due to the filling effect of NS. In addition, some cementitious materials can also be used as additives to improve the performance of CFA concrete. According to Nie et al. [66], adding 20% MK can increase the 7d and 90d compressive strength of concrete with 60% CFA by 63% and 50%, respectively. The improvement in performance is believed to be caused by the addition of MK to optimize the ITZ of concrete.
However, ordinary concrete contains a lot of cement, which leads to high energy consumption and does not conform to the low-carbon theme pursued by the construction industry. Alkali-activated CFA concrete (AAFC) is considered to be a low carbon building material that can replace ordinary Portland cement (OPC) concrete because it does not contain cement. According to Ghafoor et al. [67], the 28d compressive strength of AAFC can reach 21.5 MPa under environmental curing conditions. Huseien et al. [68] found that the addition of granular blast furnace slag (GBFS) can improve the strength of AAFC, but will increase the carbon footprint of AAFC. When CFA: GBFS=7:3, the 7d compressive strength of AAFC exceeds 30 MPa, while the 28d compressive strength is close to 50 MPa. In addition, each cubic meter of AAFC produced only emits 25kg CO2. Although AAFC can meet the strength requirements of most structural engineering. However, with the continuous improvement of social requirements for the strength of building materials, it is necessary to further improve the strength of AAFC.
Table
Table 3. CFA-based materials studies reported in the literature: raw materials; activators; L/S, curing conditions and sample properties.
Table 3. CFA-based materials studies reported in the literature: raw materials; activators; L/S, curing conditions and sample properties.
Class of
Samples		Raw Materials
and Activators		CuringL/SaCompressive Strength [MPa]Ref.
T[°C]RH[%]7-d14-d28-d
CementCFA, PC, SlagStr.0.5~28n.r.~45[47]
CFA, OPCStr.n.r.n.r.n.r.~45[48]
ConcreteCFA, OPC, Fine and
coarse aggregate, 		n.r.0.430.23n.r.48.69[55]
CFA, PC, SandStr.0.4754.2n.r.57.5[56]
CFA, OPC, SandRoom.n.r.0.4~21n.r.~33[64]
CFA, PC, Sand, Coarse aggregate Room.n.r.n.r.n.r.n.r.52.3[65]
CFA, OPC, Sand, Coarse aggregateStr.0.32~31n.r.~49[66]
GeopolymersCFA, CCR801; 602n.r.0.49~10.318[69]
CFA, NaOH, Na2SiO3Room.n.r.n.r.~11.7n.r.~13.7[70]
CFA, Clay, NaOH,
Na2SiO3		2001; Room.2n.r.0.2534n.r.n.r.[71]
CFA, GBFS, NaOH,
Na2SiO3		Room.1; 652; Room.3950.4~31n.r.~32.5[72]
CFA, MK, Sand, NaOH, Na2SiO320 ± 2950.65n.r.n.r.~65[73]
CFA: coal fly ash, PC: Portland cement; OPC: ordinary Portland cement; CCR: Calcium Carbide Residue; GBFS: Granulated blast furnace slag. Str.: strand curing. n.r.: not reported. Room.: room temperature. 801; 602: step 1: 80 °C; step 2: 60 °C. 2001; Room2: step 1: 200 °C; step 2: room temperature. Room1; 652; Room3: step 1: room temperature; step 2: 65 °C; step 3: room temperature.

3.3. Ceramics from CFA

CFA is rich in the silica and alumina components required for the preparation of ceramics. In addition, the glass phase in the CFA promotes the liquid-phase sintering of ceramics. Thus, CFA is considered to be a low-cost raw material for ceramics preparation. The principal crystalline phases of ceramics from CFA include mullite, anorthite, cordierite and forsterite. The process parameters and performance of the ceramics from the CFA are summarized in Table 4. At present, CFA-based ceramics are mainly used as building materials (such as floor materials, refractory/thermal insulation materials), ceramic membranes, ceramic membrane supports and so on.
Jiang et al. [74] synthesized ceramics that can be used as industrial structural materials and building flooring materials from CFA. They found that pretreatment of CFA with NaOH solution was beneficial to the desilication reaction of CFA, which could further promote the formation of anorthite phase during sintering. Thus, the bending strength and apparent porosity of CFA ceramics are 77.6 ± 2.31 MPa and 0.42 ± 0.09, respectively. Luo et al. [75] used mechanochemical methods (grinding the mixture of CFA and NaOH solution with a ball mill) to pre-activate CFA to prepare architectural ceramics with better performance. The results show that the flexural strength of the ceramics prepared by mechanochemically treated CFA is 23.5% higher than that of the ceramics prepared by untreated CFA at the sintering temperature of 1100 °C, and the porosity is 30.3% lower. The improvement of the sample performance is attributed to the mechanochemical activation, which can convert part of Al (VI) into Al (IV), and Al (IV) can replace Si4+ in the tetrahedron, leading to the more unstable silicate structure of activated CFA than untreated CFA. Similarly, Jagadeep et al. [83] used CFA to prepare ceramic tiles, and the results show that the prepared CFA-based ceramic tiles have good ceramic matrix and glaze.
In addition, ceramics often have good fire resistance/heat insulation performance, but its high cost makes it difficult to be widely used. For this reason, some researchers use low-cost solid wastes such as CFA as raw materials to prepare refractory ceramics [76]. For example, Nguyen et al. [77] prepared refractory ceramics with forsterite and spinel phases using CFA and caustic calcined magnesite as the main raw materials. The results showed that the shrinkage of the ceramics was less than 11.3% and the refractoriness was higher than 1655 °C. Krasnyi et al. [84] prepared light refractory ceramics with a density of 0.5g·cm−3 using CFA as raw material, and its thermal conductivity is only 0.23–0.48 W/(m·K). In addition, the research of Wan et al. [85] shows that the use of CFA is conducive to improving the strength of ceramic samples and reducing their thermal conductivity. The results show that CFA-based ceramics with low thermal conductivity and high strength can be prepared by using 65.04 CFA. At 1600 °C, the thermal conductivity of the ceramic is 0.732w/(m·k), and the bending strength is 47.0 ± 4.1MPa.
Traditional ceramic membrane is difficult to be widely used because of its high price. Therefore, the use of low-cost CFA and other solid wastes to synthesize ceramic membranes has attracted extensive attention. Huang et al. [78] prepared ceramic membranes with high porosity from CFA and other raw materials. They found that when the sintering temperature exceeded 1200 °C, part of mullite phase changed into anorthite phase. Under the above conditions, the porosity of CFA-based ceramic membrane can reach 44.76%, and the bending strength is close to 30MPa. Similarly, Wang et al. [79] also synthesized low-cost porous ceramics from CFA. In order to improve the porosity of CFA-based ceramics. Fu et al. [80] introduced MoO3 into the preparation of ceramics, they found that the addition of 10% MoO3 significantly increased the porosity of the ceramic membrane by 34% and reduced the flexural strength by only 9%. The addition of MoO3 is believed to promote the formation of a fine-rod mullite phase and increase the interlocking degree of the mullite phase. In addition, Li et al. [81] found that the addition of AlF3 can not only improve the porosity of CFA-based ceramics, but also help to improve their strength. Adding 6% AlF3 was found to increase the flexural strength of ceramics from CFA by approximately 146% and the porosity by 37%. In this study, AlF3 is believed to contribute to the formation of the secondary mullite phase and promote the transformation of the mullite phase from rods to whiskers.
Since the mullite phase can provide high strength for ceramics, the preparation of CFA-based ceramic supports with mullite as the main crystal phase has aroused the interest of researchers. For example, Fan et al. [82] found that when CFA and bauxite are used to prepare refractory ceramics, the flexural strength and other properties of ceramics are closely related to CFA content and sintering temperature. This is because CFA particles melt at 1170 °C, which in turn promotes the connection between CFA particles and increases the bending strength between particles. When the sintering temperature is 1300 °C and the ratio of CFA to bauxite is 3:2, the flexural strength and porosity of the ceramics are 69.6 MPa and 27% respectively. In addition, Li et al. [81] prepared the ceramic membrane support with CFA and alumina as raw materials, and found that its strength was as high as 59.1 MPa. Using solid wastes such as CFA as raw materials to prepare ceramics can save non-renewable clay and other minerals, but the high sintering temperature in the ceramic preparation process still leads to the high cost of ceramics, which hinders its application to a certain extent. In addition, the color of CFA-based ceramics is mostly black or gray, which also hinders its application to some extent.

3.4. Geopolymers from CFA

Geopolymer is a new type of inorganic cementitious material formed by the reaction of aluminum rich silicate materials in alkaline environment. CFA is often used as a precursor of geopolymerization due to its high content of silica and alumina. Compared with OPC, CFA-based geopolymers have obvious advantages in high-temperature resistance and heavy metal solidification [86,87,88]. According to the results of Payakanitiab et al. [86], calcium silicate hydrate in cement dehydrate at 95–105 °C and peel off at 800–1000 °C, while the 7 d compressive strength of CFA-based geopolymer is still more than 30 MPa after insulation at 1000 °C for 1 h. Liu et al. [87] compared the mechanical properties and lead solidification effects of CFA-based geopolymer containing 2% Pb2+ with Portland cement containing 2% Pb2+. It was found that the 7d compressive strength of CFA geopolymer was 87% higher than that of cement, and the leaching concentration of Pb2+ in geopolymer was more than 60% lower than that in cement. Therefore, the research of CFA-based geopolymers has attracted extensive attention.
The reaction mechanism of CFA-based geopolymers is still controversial, but the process is often considered to include three steps: first, [SiO4] and [AlO4] in the CFA are dissolved, and [Si(OH)4] and [Al(OH)4]− monomers are formed through depolymerization; next, oligomer gel is formed by polymerization between monomers; finally, the oligomer gel is further polymerized to form an aluminosilicate geopolymer with a three-dimensional network structure. Geopolymerization starts from the surface of the CFA and gradually extends to the interior (Figure 5 [89]).
At present, CFA-based geopolymers are not only directly used as building materials, but also often used for solidification/stabilization of hazardous wastes and soil stabilization. Li et al. [90] found that the geopolymer prepared by CFA and lead zinc slag has a certain strength, and successfully disposed of hazardous waste lead zinc slag. When the mass ratio of CFA and lead zinc slag is 9:1, the compressive strength of CFA-based geopolymer reaches 28.96 MPa, and the leaching concentration of Ni and Zn (0.184 and 93.17 mg/L respectively) is lower than the maximum value specified in GB 16889–2008, while Pb is not detected. Murmu et al. [91] tried to stabilize black cotton soil with CFA-based geopolymer. Under the condition that the modulus of alkali activator is 5 M, the 7d compressive strength of black cotton soil stabilized by CFA-based geopolymer (CFA content 5–20%) can reach 907–1106 kPa, and meet the relevant standard values. In addition, the research of Odeh et al. also shows that CFA-based geopolymers have a good effect in stabilizing subgrade materials.
To solve the problems of high production cost and high environmental pollution risk caused by highly alkaline activators, researchers have developed new activators for CFA-based geopolymers, including carbide slag [69] and wood fly ash [92]. Calcium carbide slag is a solid waste with high alkalinity that is produced during acetylene production process. Using Calcium Carbide Residue (CCR) as an activator, Wang et al. [69] prepared CFA-based geopolymer. The results showed that the 28-d compressive strength of the geopolymer could reach 18 MPa at a mass ratio of CFA to CCR of 1:2. Wood fly ash is a type of solid waste with high potassium content and alkalinity, generated from wood and wood product incineration [92]. Lin et al. [92] prepared a CFA-based geopolymer using wood fly ash and sodium silicate solution as activators. When the wood fly ash content was 15%, the geopolymer sample exhibited the best performance. However, the strength of geopolymer prepared with low-cost activator is low, and it is difficult to meet the use requirements. In future research, new composite activators (such as a mixture of sodium hydroxide and low-cost activators) can be used to reduce the cost and environmental impact of activators, while ensuring that CFA-based geopolymers have considerable mechanical properties.
Before curing, applying a certain pressure to the slurry helps improve the performance of the geopolymer and reduce the amount of activator [93]. According to Ahmad et al. [70], compared with a sample without pressure, the 7-d compressive strength of CFA-based geopolymer can be increased by 185% and the porosity can be reduced by 23.6% when a molding pressure of 20 MPa is applied. They also found that when the molding pressure is increased from 20 MPa to 40 MPa, the 7d compressive strength of CFA-based geopolymer can be increased by 47.8% [71]. Similar to the above results, Ranjbar et al. [94] found that when the forming pressure is increased from 13.8 to 41.4 MPa, the compressive strength of CFA-based geopolymer can be increased by 59.5%. The improvement in geopolymer performance is attributed to the fact that the applied pressure promotes the dissolution of CFA and the formation of more bridging oxygen, which is conducive to the formation of longer molecular chains, and ultimately leads to the formation of more dense aluminosilicate structure.
Researchers have also studied the influence of additives, such as fibers and nanoparticles, on the properties of CFA-based geopolymers. According to the research results of Cai et al. [72], adding 3% polyvinyl alcohol (PVA) fiber can increase the 28d compressive strength and flexural strength of CFA-based geopolymers by more than 20% and 40%, respectively. Using 1.5% NS and 0.6% PVA fibers as additives, Zhang et al. [73] increased the 28d compressive strength and flexural strength of CFA-based geopolymer by nearly 40%. The improvement of performance is attributed to the bridge formed by fiber in the CFA-based geopolymer matrix. When the sample is subjected to external forces, the bridge consumes this energy through fiber stretching and fracture, thereby inhibiting crack growth, and ultimately improving the mechanical properties of the sample. In addition, nanoparticles can fill the space between CFA particles, thereby improving the microstructure and mechanical properties of CFA-based geopolymers. Therefore, some researchers also used nano CaCO3 and nano calcium oxide (NCO) to improve the mechanical properties of CFA-based geopolymers [95,96].

4. Management Policy of CFA

Great differences were observed in the classification and management policies of CFA among countries. In 2014, the United States Environmental Protection Agency classified CFA as ordinary solid waste from hazardous solid waste, which led to the utilization ratio of CFA in the United States exceeding 60% in 2017, increasing annually [97]. On the contrary, the Indonesian government issued regulations in 2014 to classify CFA as hazardous waste [98].
In 1994, the Chinese government promulgated the Administrative Measures for the Comprehensive Utilization of coal fly ash [99], which stated that the main disposal mode of CFA in China is resource utilization rather than stockpiling and landfilling. Under the influence of this policy, the comprehensive utilization ratio of CFA in China increased from 35% in 1994 to 68% in 2011. In 2013, the Chinese government revised the Administrative Measures for the Comprehensive Utilization of CFA. The policy further determines the direction of resource utilization of CFA, mainly based on building materials, including cement mixtures, concrete admixtures, and wall materials. To encourage the building materials utilization of coal fly ash, the announcement on improving the value added tax policy [100] for comprehensive utilization of resources issued in 2021 will refund 70% of the tax on some CFA-based building materials. The main products enjoying tax refund include concrete and ceramic products with more than 70% CFA, cement products of P.O. 42.5 and above with more than 20% CFA, and cement clinker products with more than 40% CFA.
As one of the countries with the highest utilization ratio of CFA in the world, Japan issued the Air Pollution Prevention Law in 1968 and classified CFA [101]. The Basic Environmental Law promulgated in 1993 established strict detection standards for mercury, organic phosphorus compounds, lead and other pollutants in CFA [102]. In 1991, the “Resource Active Utilization Promotion Law” was issued [103], after the revision in 2001, the CFA extended producer responsibility system was established and the legal constraints on improving CFA utilization rate were strengthened. To promote the resource utilization of CFA, the “Tax System for Promoting Fiscal Investment through the Reform of Energy Supply Structure” provides for tax reduction and rebate for the investment in the equipment for the resource utilization of CFA. The Development Bank of Japan also offers high financing rate (40%) and low interest rate (1.9%) for financing related to CFA treatment equipment. Therefore, the management policies of CFA have a great impact on the use of CFA in the construction industry, as well as on solving the problem of CFA accumulation.

5. Conclusions and Prospects

(1) The CFA contains heavy metals such as Cd, Cr, Pb, and As, and radioactive elements such as 40K, 226Ra, and 232Th. The references available suggest that the leaching concentration of heavy metals and the annual effective dose of radioactive elements in CFA are within the safe range.
(2) Adding the proper amount of CFA can improve the performance of cement and concrete, but excessive CFA will affect their strength, especially the early strength. The strength of CFA cement and concrete, especially the early strength, can be improved by optimizing the curing system and adding nano particles or other supplementary cementitious materials. Alkali-activated CFA concrete is a low-carbon green building material that can replace ordinary Portland cement concrete. Cement and concrete are still the main building materials for the resource utilization of CFA. The use of CFA as a single admixture can no longer meet the requirements of the cement and concrete industries. In the future, composite mixing technologies with CFA as the main material will be an inevitable demand for the development of cement and concrete. Modification of CFA should be a focus of attention.
(3) Ceramics with mullite, anorthite, forsterite and spinel crystal phases produced from CFA have broad prospects for application. At present, the research focus of CFA-based ceramics is on the preparation and optimization of floor materials, refractory materials, filter membranes and its supports. However, the high sintering temperature of CFA-based ceramics leads to high energy consumption and high cost, which makes it mainly used as high-end products. In addition, the color of CFA-based ceramics is mostly black or gray, which also hinders its application to some extent. In the future, researchers should actively explore methods to solve these problems.
(4) CFA can be used as the raw material for geopolymers alone, and can also be used with metakaolin, granulated blast furnace slag, and other substances to prepare geopolymers. CFA-based geopolymers have shown great potential for replacing ordinary Portland cement. In future, the development of low-cost and environment-friendly excipients should become a research priority. The formation mechanism of CFA-based geopolymers remains controversial, and further research is required. In addition, the life cycle assessment of CFA-based geopolymers should also be paid sufficient attention to analyze their long-term economic and environmental impacts.
(5) The management policies of CFA have a great impact on the resource utilization of CFA, especially the utilization of building materials. To a large extent, the more comprehensive and reasonable CFA management policies are, the more widely CFA is applied in the construction industry, and the more conducive it is to improving the comprehensive utilization ratio of CFA.

Author Contributions

X.L., writing—original draft preparation; B.L., writing—review and editing; Q.Z., Q.W., S.W., K.X. and S.Z.—review and offering suggestions. All authors have read and agreed to the published version of the manuscript.

Funding

This work was sponsored by the National Key R&D Program of China (Grants No. 2019YFC1907101, 2021YFC1910504), Key R&D Program of Ningxia Hui Autonomous Region (Grants No. 2021BEG01003), the National Natural Science Foundation of China (Grants No. U2002212), Xijiang Innovation and Entrepreneurship Team (Grants No. 2017A0109004), Macao Young Scholars Program (Grants No. AM2022024), Beijing Natural Science Foundation (Grants No. L212020), Guangdong Basic and Applied Basic Research Foundation (Grants No. 2021A1515110998, 2020A1515110408), China Postdoctoral Science Foundation (Grants No. 2022BG019), the Fundamental Research Funds for the Central Universities (Grants No. FRF-BD-20–24A, FRF-TP-20–031A1, FRF-IC-19–017Z, 06500141), Integration of Green Key Process Systems MIIT and Scientific and Technological Innovation Foundation of Foshan (Grants No. BK22BE001, BK21BE002).

Data Availability Statement

Not applicable.

Acknowledgments

Thanks very much to the academic editors and reviewers for their earnest and valuable comments, which improved the quality of this manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Minerals 13 00025 g001 550
Figure 1. Statistics of relevant literatures on the CFA utilization from 2012 to 2021 (Data come from the core collection of Web of science).
Figure 1. Statistics of relevant literatures on the CFA utilization from 2012 to 2021 (Data come from the core collection of Web of science).
Minerals 13 00025 g001
Minerals 13 00025 g002 550
Figure 2. Utilization ratio and utilization mode of CFA in major countries in the world [18].
Figure 2. Utilization ratio and utilization mode of CFA in major countries in the world [18].
Minerals 13 00025 g002
Minerals 13 00025 g003 550
Figure 3. Modification methods and related mechanisms of CFA cement.
Figure 3. Modification methods and related mechanisms of CFA cement.
Minerals 13 00025 g003
Minerals 13 00025 g004 550
Figure 4. SEM of interface transition zone cracks, the red arrows are the size of them: (a) concrete with 65% CFA; (b) concrete with 80% CFA [62].
Figure 4. SEM of interface transition zone cracks, the red arrows are the size of them: (a) concrete with 65% CFA; (b) concrete with 80% CFA [62].
Minerals 13 00025 g004
Minerals 13 00025 g005 550
Figure 5. Reaction mechanism of CFA-based geopolymers [89]. (a) Macro mechanism model of CFA alkali activation reaction; (b) descriptive model of the alkali activation of CFA according to one or several particle levels.
Figure 5. Reaction mechanism of CFA-based geopolymers [89]. (a) Macro mechanism model of CFA alkali activation reaction; (b) descriptive model of the alkali activation of CFA according to one or several particle levels.
Minerals 13 00025 g005
Table
Table 1. Classification of CFA based on chemical composition.
Table 1. Classification of CFA based on chemical composition.
Chemical Compound (%)Class (America)Class (China)
NFCFC
SiO2 + Al2O3 + Fe2O3, min705050--
CaONegligible (report only)18 (maximum)>18<10≥10
SO3, max455--
Moisture, max333--
LOI, max106A6--
A: The use of Class F pozzolan containing up to 12.0 % loss on ignition may be approved by the user if either acceptable performance records or laboratory test results are made available. LOI: Loss of index.
Table
Table 2. Leaching results of heavy metals from CFA in some literatures (mg/L).
Table 2. Leaching results of heavy metals from CFA in some literatures (mg/L).
No.CdCrPbBaHgAsRef.
1n.d.n.d.n.d.0.01n.d.n.d.[28]
2n.d.n.d.n.d.0.03n.d.n.d.
3<0.01<0.01n.d.3.8–4.7<0.00001<0.01[29]
4<0.01<0.001n.d.0.5–0.9<0.000001<0.1
5<0.01<0.001n.d.0.7–0.9<0.1<0.00001
6<0.01<0.01n.d.0.2–0.3<0.1<0.00001
7<0.01<0.01n.d.3.3–3.9<0.00001<0.01
8<0.0001<0.1<0.01-n.d.<0.1[30]
9<0.0001<0.1<0.011.5–1.6n.d.<0.1
100.000420.0350.0272.5440.000850.069[31]
TCLP limit1551000.25[36]
n.d.: not detected; TCLP limit: U.S. EPA Regulatory levels for classifying a waste as hazardous based on TCLP results; -: no regulatory limit.
Table
Table 4. CFA-based ceramics studies reported in the literature: raw materials, sintering condition and sample properties.
Table 4. CFA-based ceramics studies reported in the literature: raw materials, sintering condition and sample properties.
Raw MaterialsSintering ConditionMain PhaseSample PropertiesRef.
T[°C]Time[h][MPa] 1[%] 2[g·cm−3] 3[%] 4
CFA11752anorthite, albite77.6 ± 2.31~18n.r.0.42 ±
0.09		[74]
>CFA, Feldspar, High plastic clay>11001mullite32.5811.252.260.89[75]
CFA, ladle
furnace slag		1188-anorthite--2.49-[76]
CFA, CCM,
Olivine, Talc,
Kaolin		15502Forsterite, Spinel15.5–17.35.9–
11.3		2.365–2.51016.3–
24.2		[77]
CFA, dextrin,
carboxymethy cellulose		12002mullite,
anorthite		29.05-1.4244.76[78]
CFA, Quartz,
Magnesite		13002cordierite23.92-1.6133.16[79]
CFA, Al(OH)313002mullite40.8 ±
1.9		~1.6~1.455.71 ± 0.42[80]
CFA, Al2O312002mullite59.1n.r.1.3128.05[81]
CFA, Bauxite13002mullite69.6~6.5n.r.~29[82]
1: Flexural strength; 2: Shrinkage; 3: Bulk density; 4: Porosity. CCM: Calcined caustic magnesite.
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Lu, X.; Liu, B.; Zhang, Q.; Wen, Q.; Wang, S.; Xiao, K.; Zhang, S. Recycling of Coal Fly Ash in Building Materials: A Review. Minerals 2023, 13, 25. https://doi.org/10.3390/min13010025

AMA Style

Lu X, Liu B, Zhang Q, Wen Q, Wang S, Xiao K, Zhang S. Recycling of Coal Fly Ash in Building Materials: A Review. Minerals. 2023; 13(1):25. https://doi.org/10.3390/min13010025

Chicago/Turabian Style

Lu, Xuhang, Bo Liu, Qian Zhang, Quan Wen, Shuying Wang, Kui Xiao, and Shengen Zhang. 2023. "Recycling of Coal Fly Ash in Building Materials: A Review" Minerals 13, no. 1: 25. https://doi.org/10.3390/min13010025

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