Preparation of red mud-based geopolymer materials from MSWI fly ash and red mud by mechanical activation

Waste Management 83 (2019) 202–208

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Preparation of red mud-based geopolymer materials from MSWI fly ash and red mud by mechanical activation Yuancheng Li a, Xiaobo Min a,b,⇑, Yong Ke a,b,c, Degang Liu a, Chongjian Tang a,b a

School of Metallurgy and Environment, Central South University, Changsha, Hunan 410083, China Chinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, Changsha, Hunan 410083, China c Guangdong Jiana Energy Technology Co. Ltd., Qingyuan 513056, China b

a r t i c l e

i n f o

Article history: Received 7 June 2018 Revised 5 November 2018 Accepted 11 November 2018

Keywords: Mechanical activation MSWIFA Red mud Red mud-based geopolymer materials

a b s t r a c t A novel method to activate red mud was proposed in this study. Municipal solid waste incineration fly ash (MSWIFA) and red mud were utilized to prepare red mud-based geopolymer materials (RGM). The hydration characteristics of RGM were studied by X-ray diffraction, scanning electronic microscopy, and Fourier transform infrared spectroscopy. The long-term stability and physical properties of RGM were tested by freeze–thaw cycle, European Community Bureau reference (BCR) and unconfined compressive strength (UCS) tests. Results showed that mechanical activation can not only effectively activate red mud, but also effectively improve the reaction of MSWIFA and red mud. When 14% sodium silicate was added to the binder, the UCS reached 12.75 MPa at 28 days. In the RGM, aluminosilicate was effectively activated by mechanical activation and reacted with calcium ion to form complex hydration products. The activator reacts adequately with activated aluminum to form a high-strength geopolymer. The freeze–thaw cycles and BCR test results also showed that the RGM had long-term stability and the characteristics satisfied the requirements of MU10 fly ash bricks. This study demonstrated that RGM may be utilized in cement composites. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction The disposal of nonferrous industrial solid waste and municipal solid waste has always been an important environmental problem in China (Fei et al., 2017; Liu et al., 2018). At present, such industrial solid waste is used for safe landfill. The landfill occupies a vast amount of land, which has an impact on its sustainable use (Li et al., 2016, 2018). Red mud is a typical nonferrous industrial solid waste (Liu and Poon, 2016; Xue et al., 2016). The annual output of red mud worldwide has been estimated to reach more than 70 million tons (Geng et al., 2016; Zhu et al., 2016a). Municipal solid waste incineration fly ash (MSWIFA) is an industrial solid waste in waste-to-energy power plants and contains different types of soluble salts and heavy metals (Lancellotti et al., 2010; Liu et al., 2017). Approximately more than 3500 tons of MSWIFA is produced in China every day. In extreme conditions, these industrial solid wastes may contaminate soil or groundwater and cause serious environmental problems (Gao et al., 2017). Therefore, a method

⇑ Corresponding author at: School of Metallurgy and Environment, Central South University, Changsha, Hunan 410083, China. E-mail address: [email protected] (X. Min). https://doi.org/10.1016/j.wasman.2018.11.019 0956-053X/Ó 2018 Elsevier Ltd. All rights reserved.

that can attenuate the harm of and effectively utilize industrial waste is urgently needed. The geopolymer is an innovative solidification/stabilization technology. It utilizes industrial solid wastes such as fly ash and red mud as raw material to form a 3D network of cementitious materials under alkaline conditions through polymerization (Duxson et al., 2005; Lancellotti et al., 2010). Ye et al. indicated that alkaline heat treatment could be used to improve the solidification of red mud-based geopolymer (Ye et al., 2016). However, this kind of treatment method is costly, and the red mud-based geopolymer exhibits low compressive strength and poor durability (PachecoTorgal et al., 2012). Thus, the industrialization of this kind of treatment method is extremely difficult. At present, mechanical milling is widely used in the pretreatment of materials due to low carbon dioxide emission and flexible industrial operations (Li et al., 2016; Wei et al., 2017). At the same time, some researchers have indicated that the properties of metakaolin geopolymers could be improved by high-calcium fly ash (Chindaprasirt et al., 2007). Therefore, the effective activation of red mud and use of highcalcium MSWIFA to improve red mud-based geopolymer properties bear important research significance for resource utilization (Yang et al., 2018). In this paper, mechanical milling was applied to activate the aluminosilicate in red mud. Sodium silicate solution

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and dissolved aluminosilicate were applied to react with highcalcium MSWIFA to produce recycled geopolymers. This work mainly investigated the performance of red mudbased geopolymer materials (RGM) by using red mud and highcalcium MSWIFA as partial substitutes of silicon aluminum compounds under mechanical activation conditions. First, the feasibility of preparing the binder with MSWIFA and red mud was explored. Next, the effect of the activator and mechanical activation on the unconfined compressive strength (UCS) was investigated. Last, the durability of RGM was determined by the freeze–thaw cycle test, and the possible mechanical activation mechanism was proposed. This work will provide a new way for the disposal of industrial waste with high calcium and aluminum content.

constituents of MSWIFA (Power et al., 2011). Therefore, components of calcium mainly came from MSWIFA in the production process of RGM. The red mud used in this experiment was provided by a Bayer factory in southern China. As seen in Table 1 and Fig. 1b, Al, Si, and Fe elements in red mud were found in compounds Fe2O3, Ca3Al(SiO4)(OH)8, Al0.5Si0.75O2.25, and CaAl2Si6O16(H2O)4 (Zhu et al., 2016a). Therefore, components of aluminosilicate mainly came from red mud in the production process of RGM (Zhu et al., 2016b). The sodium silicate solution was made up of 7.91% sodium oxide, 23.72% silicon dioxide, and 66.0% water. The sodium hydroxide was analytical grade.

2. Materials and methods

2.2. Experimental procedures

2.1. Reagents and materials

The mixture sample was milled for 30 min at 500 rpm to less than 80 lm in size by a ball-milling machine. The activator was prepared from a sodium silicate solution and sodium hydroxide in a liquid–solid ratio of 10:1.1. The prepared activator was added to the ball-milled mixture sample according to the design proportion in Table 2. In addition, 50 ml of liquid to each 100 g mixture was added to ensure full reaction of red mud and MSWIFA. The volume of the liquid refers to the volume of both the activator solution and water. After mixing with the liquid, the sample was mixed for 3 min and then consolidated in a cubic mold of 20 mm to cure. The curing condition was standard curing for 3, 7, and 28 days.

The MSWIFA was collected from electrostatic precipitator silos in a waste incineration plant located in southeast China (Wu et al., 2011). MSWIFA and red mud samples should be more than 80% of particle size less than 80 lm and dried at 105 °C (Li et al., 2016). The pH of MSWIFA and red mud were 12.04 and 9.86, respectively. The main constituents of MSWIFA were examined by X-ray diffraction (XRD) and X-ray fluorescence (XRF) as shown in Fig. 1a and Table 1, respectively. CaClOH, NaCl, KCl, and CaCO3 were the main crystalline phases in MSWIFA (Fig. 1a) (Zheng et al., 2010). Table 1 shows that Ca, Cl, and Na were the main

Fig. 1. XRD diagrams of MSWIFA (a) and red mud (b) samples. 1, CaClOH (PDF# 73-1885); 2, NaCl (75-0306); 3, KCl (75-0298); 4, SiO2 (79-1906); 5, CaCO3 (72-1653); 6, Fe2O3 (79-1741); 7, Ca3Al2(SiO4)(OH)8 (38-0368); 8, CaAl2Si6O16(H2O)4 (70-1921); 9, Al0.5Si0.75O2.25 (37-1460).

Table 1 Characteristics of red mud and MSWIFA samples. Elements composition (%)

MSWIFA Red mud

Ca

Si

Al

Fe

Na

Cl

Mg

V

K

Sb

43.4 12.3

1.71 7.53

0.45 10.6

0.98 22.7

4.43 3.12

20.5 –

1.64 0.35

– –

4.14 0.09

– –

Zn

Pb

Cu

Cr

As

Cd

Ni

Se

Ba

LOI*

0.59 0.01

0.17 0.02

0.10 0.01

0.08 0.26

– 0.05

– –

– 0.02

– –

– –

17.9 11.0

Elements composition (%)

MSWIFA Red mud

LOI*: Loss on ignition, mass loss at 1200 °C.

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Table 2 Mix proportions of binder and activator. ID

Bindera (%)

Activatorb (%)

Mechanical activation (min)

A8-M0 A10-M0 A12-M0 A14-M0 A16-M0 A0-M0 A0-M30 A14-M30

100 100 100 100 100 100 100 100

8 10 12 14 16 0 0 14

0 0 0 0 0 0 30 30

80° (2h) at a rate of 10° min1. The microstructure was observed by scanning electron microscopic (SEM) at 20.0 kV at 6000 times. Fourier transform infrared (FTIR) spectroscopy of samples was accomplished with Nicolet IS10 using standard the KBr pellet method. The XRF spectra of samples were obtained with Japan Science ZSX100e using Rh anode radiation (Ke et al., 2018). 3. Results and discussion 3.1. UCS of the prepared geopolymer

a

The binder is composed of MSWIFA and red mud with a mass ratio of 30:70. b The content of this activator is evaluated by the mass of sodium silicate to binder.

In this paper, three groups of experiments were used to prove the feasibility of the production of synthetic RGM from red mud and MSWIFA. First, 20%, 30%, 40%, and 50% of the MSWIFA replaced red mud in the binder. Second, the effect of an alkali activator on the UCS of RGM was researched. The mass ratio of alkali activators–binder varied from 8:100 to 16:100. Finally, series 3 was designed to research the effect of mechanical activation on RGM, and the possible mechanical activation mechanism was proposed. Specific experiments are shown in Table 2. 2.3. Leaching test Toxicity Characteristic Leaching Procedures (TCLP) were used to test the leaching toxicity of all samples (Min et al., 2017). The buffer solution was a liquid with a pH of 2.88 constituted of acetic acid and deionized water. The samples to be tested were crushed to particles smaller than 9.5 mm in diameter. The crushed particles and the buffer solution were added to the bottle in a liquid–solid ratio of 20:1. The sample to be added to the buffer solution was oscillated for 18 h at 30 rpm on the flip oscillator. ICP-OES was used to analyze the filtered leaching solution (Li et al., 2016; Liang et al., 2017). At the same time, The European Community Bureau Reference (BCR) was used to detect the chemical speciation of Cu, Zn, Cr and Pb in samples. The detailed BCR method is in the supplementary material (Xie et al., 2013). 2.4. Performance test The UCS value was obtained according to Test Methods for Wall Bricks. A TYA-300B rigid hydraulic pressure servo machine (Xinluda, Wuxi, China) was used for UCS tests with a deformation rate of 2.4 kN/s (GB/T2542-2012, 2012). Water absorption rate experiment was obtained according to Fly Ash Brick. Five samples were dried at 105 °C, and then the mass of the sample was weighed (m0). The dried sample were placed in water for 24 h at 10 °C to 30 °C, and then the mass of the sample was weighed (m24). Water absorption rate of each sample was calculated using Eq. (1) as obtained (JC239-2001, 2001).

Water absorption rate ¼

m24  m0  100 m0

3.1.1. Effects of MSWIFA on the UCS of binder Fig. 2 shows the UCS of the binder after replacing red mud with 20%, 30%, 40% and 50% MSWIFA. Fig. 2 shows with the increase of the MSWIFA, the UCS of the binder increased. However, when the MSWIFA replacement exceeded 30%, the UCS of the binder decreased. By comparing the characteristics of MSWIFA and red mud, we found that the pH of MSWIFA was much higher than that of red mud. Therefore, the addition of MSWIFA will improve the alkalinity of the system (the pH from 10.54 to 11.67). The increase of alkalinity is not only beneficial to the dissolution of aluminosilicates, but also facilitates the polymerization reaction (Bakharev et al., 2003). In addition, calcium ions react with dissolved aluminosilicates to form complex hydration products (Tzanakos et al., 2014). These complex hydration products exhibit good strength as well as the ability to solidify heavy metals. Therefore, the addition of MSWIFA enhances the UCS of the binder. Meanwhile, MSWIFA contains large amounts of chlorine. Chlorine induces the structural discontinuity of the gel and affects the final performance (Lee and van Deventer, 2002). In addition, excess calcium will enter the geopolymeric aluminosilicate structure, which is not conducive to the formation of a 3D geopolymeric aluminosilicate network structure and consequently reduces UCS (MacKenzie et al., 2007). Therefore, the UCS of the binder began to decrease when the MSWIFA replacement exceeded 30%. Finally, 30% MSWIFA replacement of red mud was selected as the best mixture. 3.1.2. Effects of activator and mechanical activation on UCS Fig. 3 shows the UCS of geopolymer samples with various proportions of activator. In this article, sodium silicate solution was used as an activator. Sodium silicate plays a dual role: (1) it provides a high-alkalinity environment for the red mud and fly ash polymerization, and (2) it can provide silicon and increase the reac-

ð1Þ

Freeze–thaw cycles test was obtained according to Fly Ash Brick. Three samples were placed in the apparatus for 5 h at 15 °C to 20 °C and another 3 h at 10–20 °C. The above steps were performed 15 times and the mass loss rate was obtained (JC239-2001, 2001). 2.5. Analysis The phase composition was analyzed by XRD. All diffraction patterns were obtained by scanning the goniometer from 10° to

Fig. 2. UCS with variations of the MSWIFA content.

Y. Li et al. / Waste Management 83 (2019) 202–208

Fig. 3. UCS with variations of the sodium silicate content.

tion rate (Rowles and O’connor, 2003). Therefore, with the increase of the sodium silicate from 8% to 14%, the UCS of the geopolymer increased (Fig. 3). We found that the UCS of geopolymer samples was higher when the modulus was 2 (equal to a 0.54 M ratio of Na2O/SiO2). At the same time, the UCS was highest when 14% activator (equal to a 2.08 M ratio of SiO2/Al2O3) was added to geopolymer samples. When the molar ratio of SiO2/Al2O3 was higher than 2.2, the geopolymer samples will crack, and the lower molar ratio will cause the silicon aluminum polymerization to be incomplete. Previous researchers have shown that the UCS of the geopolymer is highest when the molar ratio of SiO2/Al2O3 is between 1.8 and 2.2 (Kaya and Soyer-Uzun, 2016; Zhang et al., 2014). As shown in Fig. 4, the UCS of the geopolymer (A14-M30) was greatly improved compared with the single mechanical activation (A0-M30). Mechanical activation helped change the lattice structure of aluminate to increase its dissolution efficiency. Compared with A0-M0 (3.77 mg/L), the dissolution concentration of Al in A0-M30 (13.1 mg/L) was vastly improved. However, the content of silica in red mud and MSWIFA was low, and the dissolved silicon was not enough to polymerize with the dissolved aluminum. The activator can provide silicon and increase the reaction rate of mechanically-activated samples and activator to form a highstrength geopolymer. Thus, the UCS of the A0-M30 was not obviously increased. This result explains how mechanical activation increases the polymerization of RGM.

Fig. 4. The UCS of the geopolymer under different conditions.

205

Fig. 5. XRD diagrams of the A0-M0 (a), A0-M30 (b) and A14-M30 (c). 1, NaCl (PDF# 88-2300); 2, Andradite (87-1971, 84-2015); 3, Katoite (86-1314); 4, CaCO3 (881807); 5, Fe2O3 (79-1741); 6, Aluminite (70-1103); 7, Ettringite (41-1451).

3.2. Cementitious mechanisms of the RGM 3.2.1. XRD analysis The XRD patterns of A0-M0, A0-M30, and A14-M30 are shown in Fig. 5. Compared with raw MSWIFA (Fig. 1a), CaClOH peaks decreased in Fig. 5b and c, indicating that the Eq. (2) may have occurred under the addition of the activator (Zheng et al., 2011):

CaClOH + NaOH ! Ca(OH)2 + NaCl

ð2Þ

In addition, we found peaks of amorphous calcium aluminosilicate hydrate phases (ettringite) in Fig. 5c. The peaks of amorphous gels were observed from 20° to 36° (2h) in Fig. 5c (Phair and Van Deventer, 2002; Wongsa et al., 2017). We speculate that mechanical activation improved the dissolution of the katoite and aluminite, and activators encouraged the formation of ettringite in A14-M30. Additionally, part of the calcium hydroxide reacted with sodium silicate or entered the new aluminosilicate structure to form ettringite (Feng et al., 2009). 3.2.2. FTIR spectroscopy analysis The infrared spectra of MSWIFA, red mud, A0-M0, A0-M30, A14-M30 and the mechanical activation powder (MA powder) are shown in Fig. 6. The vibrations of asymmetric OACAO in A0M0, A0-M30, and A14-M30 were at 1433 cm1 (Lee and van Deventer, 2002). This band implied the presence of carbonate. Combined with the XRD analysis, we speculated that calcium carbonate should be present in A14-M30. The bands at 562 cm1 and 997 cm1 were the vibrations of asymmetric AlAO (Palomo et al., 1999) and SiAOASi (Chindaprasirt et al., 2009; Onisei et al., 2012), respectively. The bands were slightly decreased in MA powder, indicating that mechanical activation changed the crystal structures by dynamic creation. The bands at 785 cm1 were the vibrations of asymmetric AlAO (Zhang et al., 2016). A band at 785 cm1 was found in A0-M30 and MA powder after mechanical activation, but no band at 785 cm1 was found in A14-M30 and A0-M0 (Abbasi et al., 2016). Zhang et al. indicated that the AlAO stretching vibration at 785 cm1 may indicate the presence of an aluminosilicate compound, which is easily dissolved in alkaline solution (Zhang et al., 2015). As the band at 785 cm1 increases in A0-M30, the dissolution concentration of Al increases (from 3.77 mg/L to 13.1 mg/L). These findings confirm that mechanical activation promotes the dissolution effi-

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Fig. 6. FTIR patterns of MSWIFA (a), red mud (b), A0-M0 (c), A0-M30 (d), A14-M30 (e) and MA powder (f).

ciency of aluminosilicates. When the activator was added to A14M30, the dissolution concentration of Al decreases (from 13.1 mg/L to 0.13 mg/L). These results suggest that the added sodium silicate is sufficient to react with the dissolved aluminate (Zheng et al., 2011). This means that the following reactions may have occurred under the addition of the activator (Rovnaník, 2010):

The above reaction also explains why the UCS of the A14-M30 was highest when the molar ratio of SiO2/Al2O3 was 2.0. The bands at 668 and 682 cm1 in the A14-M30 are vibrations of the asymmetric of SiAOASi (Al), meaning that the six-coordinated Al(VI) changed into a four-coordinated participant in the framework structure (Tchakoute Kouamo et al., 2012). Compared with raw red mud, the bands of A14-M30 at 873 cm1 [Al(Fe)AO] greatly increase. We speculate that some Fe-ettringite phase was formed (Li et al., 2018). The above chemical reactions indicate that mechanical activation and sodium silicate can effectively improve the activity of red mud and MSWIFA. 3.2.3. SEM In Fig. 7b, we find a substantial amount of red mud particles similar to those in Fig. 7a. In addition, a small amount of amorphous phase may be observed in Fig. 7b. Compared with Fig. 7b,

the microstructure of A14-M30 shown in Fig. 7c is denser, and a few needle-shaped particles are observed. The newly-formed needle-shaped hydration products of A14-M30 (RGM) can fill its pores (Lin and Lin, 2006), while fine particles and calcium carbonate can increase the density of RGM. Therefore, RGM possessed higher compressive strength. In order to study the relationship between the hydrated products and the ettringite-containing heavy metals, we designed a set of control experiments. Pb, Zn, and Fe were added to the ettringite to simulate the formation of RGM. The specific experimental method was the same as our previous experiment method (Li et al., 2018). Fig. 7d shows the microstructures of ettringite containing heavy metals. In general, ettringite is columnar in structure, but heavy metals may change its structure. As a result, needle-shaped particles may be observed in Fig. 7d. Compared with Fig. 7c, we also found that the characteristics of A14-M30 are similar to Fig. 7d. Combining our previous research, we speculate that mechanical activation and the activator causes calcium and aluminosilicate to form ettringite and other complex hydration products, increasing the UCS of RGM (A14-M30). 3.3. Leaching test Table 3 shows the amount of leaching heavy metals measured using the TCLP method. The heavy metal leaching concentration of RGM was much lower than that of raw materials and mixture materials, implying that the newly-formed hydration products in RGM played an important role in the solidification of heavy metals.

The leaching test showed that the leaching concentrations of most heavy metals were lower than the detection limit. At the same time, previous researchers have reported that chloride ions can affect the amount of leaching heavy metals. Table S1 shows that the soluble chloride ion concentration in RGM (827.6 mg/L) was less than in MSWIFA (8931.7 mg/L). The decrease in chloride ion concentration explains the decrease in the amount of leaching heavy metals (Zheng et al., 2011). To explore the stability of heavy metals and the safety of RGM as a cement composite, the modified BCR method was used to study the chemical speciation of heavy metals in RGM (Xie et al., 2013). Fig. S1a shows that most of the Pb, Zn and Cu in MSWIFA are in the weak acid soluble fraction and the reducible fraction, implying that the heavy metals in the MSWIFA can dissolve out in combination with acid rain, thereby causing severe environmental problems. Fig. S1b shows that some of the heavy-metal

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Fig. 7. SEM images of red mud (a) and A0-M0 (b); A14-M30 (c) and ettringite containing heavy metals (d).

Table 3 Metal concentrations in the TCLP leachates and USEPA limits (means under detection limit).

Pb(mg/L) As(mg/L) Zn(mg/L) Cr(mg/L) Cd(mg/L) Ni(mg/L) Hg(mg/L) Be(mg/L) Cu(mg/L) Se(mg/L) Ba(mg/L) Sb(mg/L) Co(mg/L) V(mg/L) a

Red mud

MSWIFA

Mixturea

RGM

USEPA limits

1.05 0.01 1.05 0.01 0.01 0.01 0.01 0.01 0.2 0.01 0.01 0.06 0.01 0.1

0.55 0.01 0.01 0.35 0.01 0.01 0.01 0.01 0.01 0.02 0.05 0.01 0.01 0.6

0.7 0.01 0.7 0.15 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.35

0.01 0.01 0.55 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.05

5 5 5 1 0.2

1 100

The Mixture is the mass ratio of red mud/MSWIFA (70:30) in the mixed.

chemical speciation in RGM was transformed into residual and oxidizable fractions, that is, Pb, Zn, Cr and Cu. In general, heavy metals in residual and oxidizable fractions are stable. It is inferred that Pb, Zn, Cr and Cu and hydration products display interaction behavior, such as ettringite and various double-layered hydroxides of calcium alumino/ferric hydrates (Choi et al., 2009). The chemical speciation transformations of Pb, Zn Cr and Cu indicate the outstanding encapsulation effect of the RGM. Thus, we speculate that RGM is safe as an alternative to cement composites. 3.4. Performance of RGM as substitute of fly ash brick The JC239-2001 standard was used to test whether RGM can be used as a substitute for fly ash brick (JC239-2001, 2001). In order to evaluate the durability of the sample, conducting the water absorption experiment was extremely necessary. In general, the water absorption of fly ash brick was from 14.29% to 16.70%. The water absorption of RGM was 16.02%.

Freeze–thaw cycles were used to verify the stability of the RGM. The mass loss rate of concrete from freeze–thaw cycles are generally less than 5.0% (Yang et al., 2009). The results showed that the mass loss rate of RGM from freeze–thaw cycles were 1.78%. The above experimental results imply that RGM has good durability and long-term stability. 4. Conclusions The optimum amount of MSWIFA used as a binder with red mud was 30%, the mass of sodium silicate relative to binder was 14%, the modules of sodium silicate were 2.0, and the RGM was left to rest for 28 days at ambient temperature. The UCS of RGM reached 12.75 MPa, and the other characteristics of RGM met the standard of MU10 fly ash bricks. In addition, the freeze–thaw cycles and BCR test results show that the RGM has good durability and long-term stability. Therefore, the RGM synthesized from red mud and MSWIFA not only solves the environmental pollution

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problem, but also provides a new method for recycling industrial solid wastes. This paper presents a new method for using red mud and MSWIFA to prepare RGM by mechanical activation. Mechanical activation can not only effectively activate red mud, but also effectively improve the reaction of MSWIFA and red mud. Nonetheless, further studies are needed to improve RGM performance.

Acknowledgements The authors gratefully acknowledge the key project of National Natural Science Foundation of China (51634010), the Natural Science Foundation of China (51474247 and 51474102), the China Postdoctoral Science Foundation (2017M612583), Key research and development project of Hunan Province, China (2015SK2084), and Science and Technology Project of Hunan, China (2017RS3010).

Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.wasman.2018.11.019.

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