Study of electric-arc furnace dust (EAFD) in fly ash and rice husk ash-based geopolymers

Study of electric-arc furnace dust (EAFD) in fly ash and rice husk ash-based geopolymers

APT 1611 No. of Pages 12, Model 5G 25 May 2017 Advanced Powder Technology xxx (2017) xxx–xxx 1 Contents lists available at ScienceDirect Advanced ...

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APT 1611

No. of Pages 12, Model 5G

25 May 2017 Advanced Powder Technology xxx (2017) xxx–xxx 1

Contents lists available at ScienceDirect

Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt

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Original Research Paper

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Study of electric-arc furnace dust (EAFD) in fly ash and rice husk ash-based geopolymers

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Morgana Cristina Arnold a, Alexandre S. de Vargas b,⇑, Liane Bianchini c

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a

University Feevale, RS 239 n_2755, Novo Hamburgo, RS, Brazil Department of Materials Technology and Industrial Processes, University Feevale, RS 239 n_2755, Novo Hamburgo, RS, Brazil c Department of Quimical, University Feevale, RS 239 n_2755, Novo Hamburgo, RS, Brazil b

a r t i c l e

i n f o

Article history: Received 30 October 2016 Received in revised form 1 May 2017 Accepted 15 May 2017 Available online xxxx Keywords: Electric-arc furnace dust Wastes Geopolymer Solidification/stabilization

a b s t r a c t Electric-arc furnace dust (EAFD) is an industrial waste produced by the volatilization of metals during scrap melting in electric arc furnaces. This waste is classified as Class I – hazardous, because lead and cadmium concentrations are above the limits set in the leaching test. Processes are carried out in many countries to recover the metals contained in EAFD. In Brazil, these processes are usually not conducted in the industry because the low percentage of commercially valuable metals makes it economically unfeasible to recover them. One of the study alternatives is the use of EAFD in civil construction. Studies have shown that EAFD increases the mechanical strength of mortars and Portland cement-based concretes. However, EAFD delayed cement setting time, which can jeopardize its use in construction. Thus, this study aims to evaluate the effect of EAFD when added to fly ash (FA) and rice husk ash (RHA) based geopolymers. Geopolymer mortars were prepared at a ratio of 1:3 (FA + RHA: sand, particle size 4) and added with 0, 10, 15, and 20% EAFD in relation to the mass of FA + RHA. Compressive mechanical strength and leaching tests were carried out at the ages of 7, 28, and 91 days. Microstructural analyses were performed using XRD, FTIR, and SEM/EDS. EAFD did not negatively influence the geopolymerization process. The highest compressive strength results for the mortars containing the waste were found for 20% of EAFD. All mortars, regardless of EAFD content, were classified as non- hazardous Class II at the age of 91 days. Ó 2017 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.

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1. Introduction Electric-arc furnace dust (EAFD) is a steel waste produced by the volatilization of metals during scrap melting in electric arc furnaces. Volatilized metals, including Zn, Pb, Cd, Ni, and Cr, are oxidized and subsequently solidified in the form of a fine dust with a particle size of less than 10 lm. The amount of EAFD generated is approximately 1% of the steel produced, but in practice this amount may vary according to the scrap used and its previous processing. Worldwide, approximately 70% of the EAFD generated is disposed of in landfills, while the other 30% is processed mainly to recover metals [1]. Countries that use EAFD usually collect these oxides through a pyrometallurgical or hydrometallurgical process, or both. However, in Brazil these processes are still not in use since the percentages of heavy metals with commercial value are not economically feasible. The chemical characterization of this waste has been reported in several studies. The composition of EAFD varies depending on the raw materials used and the produc⇑ Corresponding author. E-mail address: [email protected] (A.S. de Vargas).

tion process, but its main components are as follows: Fe (37–49%), Zn (4–37%), Ca (2–5%), Pb (1–2%), and small amounts (less than 1%) of other metals such as Cr, Cd, Cu, and Al. Because it has Pb and Cd concentrations above the limits established by the standard in leaching tests, EAFD is classified as a hazardous waste - Class I [2–5]. Therefore, one of the study alternatives is to use EAFD in civil construction based on solidification/stabilization technology. Studies using EAFD were evaluated as a mixture in the production of the Portland cement clinker [6,7]. Other studies have evaluated the mechanical behavior of the cementitious matrix containing EAFD [8–12]. In these studies, it was observed that the samples containing EAFD exhibited superior mechanical behavior when compared to the reference samples. However, EAFD retarded the cement hydration reactions. The main element thought to be responsible for this phenomenon is zinc [13–18]. Some authors investigated the possibility of stabilizing EAFD waste in ceramic materials. The addition of this waste, at levels of 2.5 and 5% in relation to the mass, to mixtures with industrial clays resulted in ceramic structures with a higher mechanical strength and lower temperatures required to produce the material. However, metals such as Zn and Pb may volatilize during the

http://dx.doi.org/10.1016/j.apt.2017.05.007 0921-8831/Ó 2017 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.

Please cite this article in press as: M.C. Arnold et al., Study of electric-arc furnace dust (EAFD) in fly ash and rice husk ash-based geopolymers, Advanced Powder Technology (2017), http://dx.doi.org/10.1016/j.apt.2017.05.007

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ceramic manufacturing process at high temperatures, leading to concern about the presence of these volatile metals in the atmospheric emissions of the process [19]. Geopolymerization technology has attracted attention in several applications, including solidification/stabilization of these wastes. Geopolymers result from the polycondensation reaction of silica aluminum material by alkaline activation [20–24]. Its structure consists of AlO4 and SiO4 tetrahydra alternately bonded by sharing oxygen atoms [25]. Diverse industrial by-products and aluminosilicate source materials were proven to be suitable for producing geopolymer materials [26]. In most cases only a small amount of silica and alumina on the particle surface is sufficient to dissolve and take part in the reaction, solidifying the entire mixture and immobilizing any heavy metal contained in it [27]. Simple [28,29] or combined [30,31] fly ashes have been studied as precursor geopolymeric materials. Rice husk ash (RHA), produced through the combustion of rice husk, has been studied as a source of silica (SiO2) for the obtaining of geopolymers [32–36]. Shalini et al. [32] have verified that the increase in the replacement of FA by RHA in FA based geopolymers, and the use of a ground blast furnace slag (GBFS) have increased the compression strength from 15.80 MPa (10% RHA) to 18,80 MPa (30% RHA). Pieper [37] has verified that the partial replacement of metakaolin by RHA in geopolymers increased the compression strength from 5.90 MPa (0% RHA) to 11.40 MPa (30% RHA) at the age of 91 days. Detphan et al. [36] have also noticed increased compression geopolymer strength when using RHA. This may also be related to the SiO2/ Al2O3 (Si/Al) geopolymer molar relation. According to Davidovits et al. [38] the Si/Al molar relation is an important variable of geopolymers precursor materials. Since geopolymers are Si-O-Al bonds, the arrangement of these bonds will depend fundamentally on the Si/Al relations found in precursor materials (raw materials and alkaline activators). Therefore, the use of RHA may be an important source of SiO2 for geopolymers. The immobilization of toxic metals by geopolymerization was studied by Xu [39]. Geopolymer matrices based on FA and metakaolin containing Cd(II), Cu(II), Pb(II), and Cr(III) have been investigated. The authors identified a number of factors that may influence the immobilization of these metals such as the concentration of the activating alkaline solution. The stabilization of EAFD through geopolymerization technology was analyzed by Fernández Pereira [40] using various raw materials such as kaolin, metakaolin, and blast furnace slag. In general, geopolymers with stabilized metals showed high compressive strength and small percentages of Zn and Cr extraction in leaching tests, demonstrating that these metals were practically immobilized. The fractions of immobilized Pb and Cd, on the other hand, were highly variable according to geopolymer composition. Experimental investigations of geopolymerization in the presence of EAFD were also carried out by Nikolic et al. [41]. The chemical immobilization of zinc among aluminosilicate bonds was indicated by the presence of Zn in the amorphous phase of the geopolymer, but a certain amount of this metal was also observed in particles of raw material that did not react, which means that immobilization took place partially through physical encapsulation. Thus, this study aims to evaluate the effect of EAFD when added to fly ash (FA) and rice husk ash (RHA)-based geopolymers using solidification/stabilization technology.

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2. Materials and methods

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2.1. Materials

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The EAFD used was generated by a semi-integrated steel plant and collected by means of a socket filter. The chemical composition

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of the EAFD used in this research is shown in Table 1. The total amount of the EAFD sample used was obtained by mixing and homogenizing five 10 kg samples of dust collected over a period of 2 months, totaling 50 kg. No milling was performed. The EAFD grading curve was carried out using a 1064 Cilas laser diffraction grain meter. The mean diameter was 0.81 lm and 90% of the particles were smaller than 3.27 lm. Fig. 1 shows a granulometric distribution of EAFD. ABNT NBR Standard NM23/2001 procedures [42] were used to determine specific gravity. The result was 4.33 g/cm3. Table 2 shows EAFD leached extract – ABNT NBR standard 10,005/2004 procedures [43]. Based on the results of Table 2, EAFD is classified as Class I – Hazardous Waste because Pb and Cd concentrations are above the maximum limits allowed by ABNT NBR standard 10004/2004 [44]. Scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDS) was carried out to evaluate the particle morphology of EAFD. Fig. 2 shows an EAFD image. As seen in Fig. 2, EAFD morphology shows a predominance of agglomerated spherical particles, where the smaller particles form aggregates or cover larger particles, which is in agreement with the EAFD observed by Škavara et al. [45]. Semiquantitative analyses by EDS were carried out at points 1 to 16. The EDS results in Table 3 also confirm the findings obtained through the X-ray Fluorescence (XRF) (Table 1) and X-ray diffraction (XRD) analysis (Fig. 3) of EAFD composition, which show the predominant presence of Zn and Fe with lesser amounts of Pb and Cr. Oustadakis et al. [46] found that Zn is mainly associated with smaller and agglomerated particles, probably as zincite. In larger spherical-shaped particles it is possible to confirm the presence of franklinite. Magnetite, however, represents the smaller spheres distributed homogeneously in the EAFD. A diffraction spectrum of the EAFD X-ray is shown in Fig. 3. Crystalline phases of franklinite (ZnFe2O4), zincite (ZnO), hematite (Fe2O3), and quartz (SiO2) were identified. Such phases were also found in the EAFD in other works [5,47,48]. The Fourier Transform Infrared Spectroscopy (FTIR) spectrum of the EAFD is shown in Fig. 4a. Table 6 shows the vibration frequencies and assignments for EAFD. The RHA used was an ash sold in 20 kg packages. The chemical composition of RHA is shown in Table 1. The specific gravity of the RHA was 2.05 g/cm3 and the average particle diameter was 9.23 lm. The RHA spectra of FTIR and XRD are shown in Figs. 4b and 5a, respectively. Table 6 shows the vibration frequencies and assignments for RHA. The FA utilized for this study was obtained from a coal-fired thermoelectric power plant located in Southern Brazil. According

Table 1 Chemical composition of EAFD, FA and RHA determined by X-ray Fluorescence (XRF) analysis (wt.%). Elements

EAFD

FA

RHA

Fe2O3 ZnO SiO2 MnO CaO PbO K2O SO3 Cr2O3 MgO Al2O3 CuO P2O5 NiO

43.45 31.89 3.71 1.96 1.83 1.48 1.23 1.06 0.74 0.56 0.35 0.30 0.26 0.05

4.92 0.05 64.07 0.02 1.47 2.63 0.35 0.04 0.21 22.95 0.01 0.05 0.01

0.09 0.01 91.96 0.45 0.70 2.12 0.09 0.14 0.30 0.54 -

Please cite this article in press as: M.C. Arnold et al., Study of electric-arc furnace dust (EAFD) in fly ash and rice husk ash-based geopolymers, Advanced Powder Technology (2017), http://dx.doi.org/10.1016/j.apt.2017.05.007

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Fig. 1. Granulometric distribution of EAFD.

Table 2 EAFD leached extract. Elements (mg L EAFDa NBR 10004b a b

198 199 200 201

1

)

Cd

Pb

Cr Total

3.29 0.50

39.56 1.00

0.22 5.00

Electric-arc furnace dust. ABNT NBR 10004 [44] – Brazilian Standard-various limits permitted.

to XRD analysis (Fig. 5b) the FA is predominantly in the vitreous phase, including a certain amount of crystalline mullite, hematite, and quartz. Based on its chemical composition, FA is classified as Class F [49] with a low calcium content. The chemical composition

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of the FA employed was determined using an X-ray fluorescence spectrometer, as shown in Table 1. The specific gravity of the FA was 2.13 g/cm3 and the average particle diameter was 19.93 lm. The FA spectra of FTIR is shown in Fig. 4c. The alkaline solution was produced using NaOH (97% purity). The water used to prepare the alkaline solution was obtained from the local water supply company (tap water). The molar concentration of the alkaline solution (11.61 mol/L) was found in previous laboratory assays. For the production of alkali-activated mortars, natural quartz sand was employed as an aggregate, with four different standardized granular dimensions – 1.2, 0.6, 0.30, and 0.15 mm and a specific gravity of 2.62 g/cm3.

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15 14

9

10

5

1

11

12

7 6

2

SEI 20kV MATERIALS LABORATORY - FEEVALE

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3

4

X5,000

Fig. 2. Scanning electron microscopy (SEM) -magnification 5000- of EAFD particles. The numbers on the image refer to the energy-dispersive X-ray spectroscopy (EDS) analysis.

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Table 3 Results of semi-quantitative analysis with the aid of energy-dispersive X-ray spectroscopy (EDS) for the points identified in Fig. 2. Point

Cr

Fe

Zn

Pb

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

0.57 0.47 0.46 0.41 1.84 0.99 0.70 0.74 1.79 0.43 0.54 0.54 9.24 0.46 0.35 0.22

45.37 61.49 70.45 40.96 39.07 51.84 53.41 57.34 52.35 61.99 72.69 61.29 56.22 69.81 69.76 92.03

51.74 38.04 29.09 58.63 56.73 45.26 45.89 40.80 42.76 35.00 25.98 35.26 33.84 27.44 29.30 5.75

2.32 0.00 0.00 0.00 2.36 1.91 0.00 1.13 3.11 2.58 0.79 2.91 0.70 2.30 0.60 2.00

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2.2. Methods

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The research method consisted of evaluating the effect of different levels of EAFD on mechanical (compressive strength), environmental (leaching), and microstructural (XRD, FTIR, and SEM/EDS) properties of alkali-activated fly ash and rice husk ash-based geopolymer mortars with a NaOH solution, denominated geopolymers in this study. The compositions of the control geopolymer (0%) and geopolymers containing 10%, 15%, and 20% of EAFD (in relation to the FA + RHA mass) are presented in Table 4. In this table the molar relations of Na2O/SiO2 (N/S) and of SiO2/Al2O3 (S/ A) are also shown. N2O was calculated based on NaOH; SiO2, and the calculation of Al2O3 was based on FA + RHA. The NaOH solution was prepared 2 h before each geopolymer molding. This procedure was adopted to allow the solution to be at room temperature at the time of mixing with other materials. The activator solution was mixed with the FA and RHA for 3 min in a vertical axis mechanical mixer. Next, the sands were added from larger to smaller particle size and mixed for another 2 min. For geopolymers containing EAFD, the procedures were similar to those previously described, except that the EAFD was added immediately after smaller particle sand was added to the mechanical mixer. It is important to stress that EAFD is being used as a fine material filler so then it can be added to the sand. The geopolymers were poured into cylindrical molds with a 50 mm diameter and height of 100 mm, according to ABNT NBR

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Fig. 4. Fourier Transform Infrared Spectroscopy (FTIR) spectra for: (a) EAFD, (b) RHA, (c) FA.

standard 7215 [50]: 4 layers and 30 pestle strokes were applied to each layer. In order to accelerate the polymerization reactions of the aluminosilicates, the curing temperature was set at 50 °C in the first 24 h and 70 °C for another 24 h, totaling 48 h of thermal curing. Unlike previous studies [51,52], where the curing temperature of the FA-based geopolymers was 70 °C/24 h and 80 °C/24 h respectively, it was observed that a curing temperature of 70 °C/24 h with the FA + RHA-based geopolymers led to the expansion of the samples while still in the oven, reaching an expansion level of up to 1/3 the height of the metal mold (100 mm tall mold

Fig. 3. X-ray diffraction (XRD) spectra of Electric Arc Furnace Dust (EAFD).

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M.C. Arnold et al. / Advanced Powder Technology xxx (2017) xxx–xxx Table 4 Compositions of the control geopolymer (0%) and geopolymers containing 10%, 15%, and 20% of EAFD. EAFD rate (%)a

Amount (g) EAFD

0 10 15 20 a b c

Molar compositionc FA

0 45.9 68.85 91.80

300

RHA 159

NaOH 117.81

Waterb

Sand (mm)

246

1.2 344.25

0.6 344.25

0.3 344.25

0.15 344.25

N/S

S/A

0.25

8.28

% in mass of FA + RHA. Molar concentration of NaOH solution = 11.61 mol/L. Molar composition to obtain in fly ash and rice husk ash-based geopolymers using NaOH solution. N = Na2O (NaOH); S = SiO2 (FA + RHA); A = Al2O3 (FA + RHA).

Table 5 Results of the analysis of leached extracts in geopolymers containing different levels of EAFD. Geopolymer (% EAFD)

Age (days)

Cadmium mg L

7a 7 28 91 7 28 91 7 28 91

0 10

15

20

0.005 0.031 0.045 0.029 0.061 0.056 0.062 0.071 0.013 0.006 0.5

Limit according to ABNT NBR 10004/2004 [44]

Lead

Total chrome

0.048 0.677 1.115 0.805 2.120 1.356 0.629 2.267 0.547 0.435 1.0

ND 0.046 0.120 0.215 0.102 0.194 0.209 0.091 0.037 0.038 5.0

1

ND – Not Detected by flame atomic absorption spectrophotometry (chrome detection limit is 0.0106 mg/L). a The leaching test of the reference sample (0%) was conducted only at the age of 7 days.

Table 6 EAFD, RHA, FA, EAFD of 0%, 10% 15% and 15% geopolymer band assignments (ages 7 and 28 days). Band

EAFD

RHA

Wavenumber (cm 1 1a 2 2a 3 4 5 6

FA

EAFD 0%

EAFD 10%

EAFD 15%

EAFD 20%

1000

1006

1008

1000

Assigned to

1

)

1063

1059

a

985

794

796 694

870 796 692

861 796 698

864 798 697

866 798 697

451

443

464

460

460

460

532b

d Si–O–Si m Si–O–Al m Si-O d C–O (CO3 2-) m Si–O d Si–O–Si m Zn-O d Si–O (SiO4 Td)

m: Stretching vibrations; d: deformation vibrations. Adapted from Garcia-Lodeiro [56]. a Martins [57]. b Lungu et al. [58].

250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266

with the sample expanding to a height of 130 mm). Thus, a temperature of 50 °C was adopted in the first 24 h and 70 °C for another 24 h, which prevented expansion of the samples. It should be noted that the thermal curing of fly ash-based geopolymers is important to accelerate polycondensation reactions of aluminosilicates [24,51,53]. A polyethylene film was placed on the surface of the mold to avoid excessive water evaporation from the geopolymers. After thermal curing, the samples were removed from the metal molds and stored in the laboratory at room temperature until compressive strength tests and other characterizations. It is important to note that expansion of the samples was also verified with the control geopolymer (0%). This means that using RHA as a source of SiO2 in the geopolymer may be an important variable in the expansion of the material in the oven. This phenomenon should be investigated in more depth in future studies. Compressive mechanical strength tests were carried out with the geopolymers at ages of 7, 28, and 91 days. For each age, four

samples were subjected to a compressive strength test and the result was the average of the results. A hydraulic press with a capacity of 200 kN was used for compressive strength testing. The samples for the leaching tests and microstructural characterizations (XRD, FTIR, and SEM/EDS) were obtained from geopolymers subjected to compressive strength tests. This methodology was adopted in order to be able to relate compressive strength with the results of the leaching tests and microstructural properties. To evaluate the hazard characteristics of EAFD present in geopolymers, leaching tests were performed at the ages of 7, 28, and 91 days following the procedures described in ABNT NBR standard 10005/2004 [43]. This standard establishes that the sample must have particles smaller than 9.5 mm. Therefore, the geopolymers subjected to compressive strength tests were comminuted and 100 g of the material that passed through a 9.5 mm mesh sieve was subjected to the leaching test. This geopolymer was placed in a plastic bottle and filled with 2 liters of extract solution.

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Q

Counts/s

Q - Quartz (SiO2) M - Mulite (3Al2O3.2SiO2) C - Cristobalite (SiO2) H - Hematite (Fe2O3)

Q M Q Q Q M HM Q Q Q

M

Q Q

b C

a

2Ѳ Fig. 5. X-ray diffraction (XRD) spectra for: (a) RHA, (b) FA.

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This solution was prepared with 11.4 ml of glacial acetic acid (0.1 mol/L) and then topped up to a volume of 2 liters with deionized water. The pH of this solution should be 2.88 ± 0.05. The bottle was placed in a mechanical rotary shaker and remained there for 18 h. The liquid was filtered and subjected to analysis, whose results are shown in Table 5. ABNT NBR standard 10004/2004 [44] establishes maximum levels for the concentration of toxic metals in the leached extract (as shown in Table 5). Based on the residue leaching test or on the material containing the residue (such as the geopolymers studied in this work), and on the concentration value of at least one toxic metal that exceeds the maximum concentration established by the standard, this material is classified as Class I – hazardous. If the concentration of the studied metals is lower than the maximum amount established by the standard, this material is classified as Class II – non hazardous. Morphological characterization, using a scanning electron microscope (Model JSM-6510LV) equipped with a Panalytical energy dispersive analyzer (EDS; Minipal 4 model, (SEM/EDS) was carried out on geopolymers containing 10% EAFD subjected to compressive strength tests at ages 7, 28, and 91 days. After the compressive strength test, small fragments (about 0.5 cm2) of the geopolymer were collected and gold-coated for SEM/EDS analysis. The XRD and FTIR analyses were conducted on finely-ground powder (passed through a 200-µm sieve) obtained by crushing and milling geopolymer samples subjected to compressive strength tests at the age of 7 and 28 days. X-ray diffractograms of powder samples were obtained with a Philips diffractometer X’Pert, 40 kV, 40 mA, using Cu Ka radiation. Specimens were step-scanned at a rate of 0.05°/s in the 2h range of 5–75°. The FTIR spectra were obtained using a Perkin Elmer Spectrum Two, within the range of 400–1400 cm 1 at a resolution of 1 cm 1. Pellets for FTIR were prepared from specimens homogenized in KBr (dried in an oven at 105 °C). The specimens were prepared with 1% dried and ground KBr.

318

3. Results and discussion

284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316

319

3.1. Compression strength

320

Fig. 6 presents compression strength results of the control geopolymer (0%), and in geopolymers containing EAFD with additions of 10%, 15%, and 20%. It shows that at the age of 7 days the geopolymers with 0% (2.95 MPa) and 20% of EAFD (3.17 MPa) have greater compression strength when compared to the strength of geopolymers with 10%

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(1.40 MPa) and 15% of EAFD (2.10 MPa). It can be observed that geopolymers containing 20% of EAFD showed the highest strength to compression, regardless of age, when compared to the strength of geopolymers containing 10% and 15% of EAFD. Therefore, among the EAFD contents studied in this work (10%, 15%, and 20%), the ideal amount found was of 20%. However, geopolymers containing 20% of EAFD have reached lower strength to compression than the reference control geopolymers at 28 days of age (5.90 MPa) and at 91 days of age (6.80 MPa) when compared to the strength of the reference geopolymers (7.87 MPa and 9.30 MPa, respectively). That is to say, although having a tendency of increasing the strength along with the increasing of EAFD in the geopolymer matrix from 10% to 20%, the strength was lower in the 28 and 91 days old when compared to the reference geopolymer. Thus, the contents of EAFD studied in this work were not the ideal amounts to contribute to the increasing of the compression strength in relation to the reference sample. Pavão et al. [5] have identified that geopolymers containing 15% of EAFD reached higher strength to compression in all different ages of the samples studied (1, 7, 28, 91, and 180 days) when compared to the strength reached by the reference and the 5% and 25% EAFD geopolymers. The authors have shown that all geopolymers gained strength until the age of 180 days, with the exception of the geopolymers that contained 25%, which stabilized strength after the age of 28 days. Until this age, 25% EAFD geopolymers presented higher strength to compression if compared to the reference. At the ages of 91 and 180 days the reference geopolymer started showing higher strength when compared with the 25% EAFD samples. In relation to the geopolymers that contained 5%, there was no significant difference between them and the reference sample. Thus, in the present work, although the increase of the EAFD content from 10% to 20% has contributed to the increase in the compressive strength (Fig. 6), these contents did not contribute for the geopolymers which contained residues to present higher strength in relation to the reference geopolymer. That is, new studies concerning EAFD should be carried out with FA and RHA based geopolymers with the goal of identifying the ideal content of EAFD that may provide greater mechanical performance when compared to the reference geopolymer. According to Van Jaarsveld et al. [53], there is a limited amount of contaminant that any geopolymer matrix may tolerate without affecting its structural integrity, above which the presence of metallic ions weakens the geopolymer structure. Such limit is a function of each ion specific physical and chemical properties, so it varies according to the different elements. Nikolic et al. [41] have

Please cite this article in press as: M.C. Arnold et al., Study of electric-arc furnace dust (EAFD) in fly ash and rice husk ash-based geopolymers, Advanced Powder Technology (2017), http://dx.doi.org/10.1016/j.apt.2017.05.007

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Fig. 6. Development over time of the compression strength of geopolymers.

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observed a slight decrease in strength with the addition of EAFD in fly ash based geopolymer. Geopolymers which contained 0% (reference sample), 10%, 20%, and 30% of EAFD have reached the compression strength of 28 MPa, 25 MPa, 22 MPa, and 20 MPa, respectively. In other words, the increase in the content of EAFD has led to the decrease in the compression strength. Van Jaarsveld et al. [53] explained that other factors, such as raw material and the activator, have also influenced the limit of the use of residues or contaminants. That is, even though the ideal content of 15% of EAFD has contributed to higher strength in the geopolymers studied by Pavão et al. [5], for Nikolic et al. [41] the ideal content of EAFD found was different, as well as for the present work, since the reference geopolymer samples presented higher strength if compared to the samples which contained residues (Fig. 6). Van Jaarsveld et al. [53] have also explained that the contaminant metals affect the chemical and morphologic structure of the geopolymers without affecting the tetrahedral bonds of Al and Si seen in this material. Apparently, most of the structural effects are related to the ion size and to the valence of the metal. The bigger the ion, the greater is its tendency to immobilizing. In certain cases, it was also seen, that the geopolymers strength to compression may be greater with the increase in the concentration of heavy metals. That is to say, the presence of metals in the EAFD may contribute to enhancing the geopolymer mechanical performance when compared to the reference geopolymer. This will depend on which residue is used, on the concentration of the metals in this residue, on the precursor material, and on the activator used to obtain the geopolymer. Therefore, there are ideal contents of EAFD that could contribute to greater strength when compared to the reference geopolymer.

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3.2. Leaching

404

Table 5 shows the results of analyses of extracts leached from the geopolymers according to the concentration of EAFD at ages 7, 28, and 91 days, along with the maximum concentration limits determined by ABNT NBR standard 10004/2004 [44]. The concentration of lead in the leached extract was above the limit set by ABNT NBR standard 10004/2004 [44] for samples with 10% of EAFD at the age of 28 days, 15% at the ages of 7 and 28 days, and 20% at the age of 7 days. Nikolic et al. [41] also found that Pb is a constituent of major concern in the immobilization of EAFD in geopolymer matrices, while Cr and Cd were within the established limits. This behavior was also observed in the present study

371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401

405 406 407 408 409 410 411 412 413 414

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(Table 5), given that Cd was below the maximum concentration permitted by ABNT NBR standard 10004/2004 [44] (0.5 mg/L) in all the geopolymers containing EFAD, regardless of the amount added. In pure EAFD (Table 2), the concentration of Cd (3.29 mg/L) was above the level permitted by ABNT NBR standard 10004/2004 [44]. According to Fernández Pereira et al. [29], the results showed that the fractions of immobilized Pb and Cd are quite variable, while Cr is practically all immobilized in the geopolymer matrix. At the age of 91 days, however, a Pb concentration below the ceiling set by ABNT NBR standard 10004/2004 [44] was observed for all geopolymers. Therefore, irrespective of EAFD content, the geopolymers containing additions of EAFD were classified as non-hazardous materials Class II at the age of 91 days. This means that from being a hazardous waste class I, the geopolymers containing up to 20% EAFD were classified as non-hazardous materials Class II, which shows the efficiency of solidification/stabilization technology of EAFD in the geopolymer. It is important to note that the increase in the geopolymer age (until 91 days of age) was very significant so that the Pb concentration could be below the limit established by ABNT NBR standard 10004/2004 [44] for geopolymers which contain EAFD. This is related to the increase in the compression strength and to a higher densification of the geopolymer matrix, thus, benefiting the stabilization on the toxic metal in the matrix.

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3.3. X-ray diffraction

439

In general, the diffraction patterns show that the geopolymerization reaction products are predominantly amorphous compounds. Unlike zeolites, there is not enough time and space during the formation of geopolymers for the gel or paste to turn into a well-crystallized structure, which provides these materials with better mechanical properties [54]. X-ray diffraction spectra for geopolymers containing 0%, 10%, 15%, and 20% of EAFD at the ages of 7 and 28 days are show in Fig. 7 and Fig. 8, respectively. Based on the XRD spectra shown in Fig. 7 (7 days of age) and in Fig. 8 (28 days of age) a characteristic peak of the zincite phase (35.31°) can be seen in the spectra of the geopolymers containing EAFD (b, c, d), regardless of age or of EAFD content. This phase present in the EAFD can be seen in the XRD spectrum in Fig. 3. It is interesting to note that the intensity of the crystalline zincite peak (35.31°) rises with the increase of the geopolymer EAFD content from 10% (Fig 7b and Fig. 8) to 20% (Fig. 7d and Fig. 8d). This behavior is more evident when the geopolymer is at the age of 7 days (Fig. 7). It is also interesting to note that the crystalline phases identified in the EAFD (Fig. 3) –franklinite (ZnFe2O4) and magnetite (Fe2O3) were not identified in the spectra of geopolymers with 10% (b), 15% (c), and 20% (d) of EAFD, as shown in the Fig. 7 and Fig. 8. Quartz (SiO2) crystalline phases were identified both in the XRD spectrum of the fly ash sample (Fig. 5b) and in the geopolymer spectra, regardless of the EAFD content (0%, 10%, 15% and 20%), as shown in Fig. 7 and Fig. 8. These results are in accordance with Ryu et al. [55]. This shows that these phases are stable in an alkaline environment after the geopolymerization process. Furthermore, there is quartz peaks also related to the sand used for preparing the geopolymer mortars. However, mullite (16.55°, 31.03° and 35.23°) and hematite (33.23°) phases were identified in the fly ash spectrum (Fig. 5b) but were not identified in the spectra of the geopolymers regardless of age or EAFD content. As a result, both crystalline phases seen in the EAFD – franklinite (ZnFe2O4) and magnetite (Fe2O3) – and the crystalline phases found in FA – mullite (3Al2O32SiO2) and hematite (Fe2O3) – were not identified in the geopolymers spectra (Fig. 7 and Fig. 8) what demonstrates that some reactions which occur in the alkaline

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Q Q

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Fig. 7. X-ray diffraction (XRD) spectra for geopolymers containing (a) 0%, (b) 10%, (c) 15%, and (d) 20% of EAFD at the age of 7 days.

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2Ѳ Fig. 8. X-ray diffraction (XRD) spectra for geopolymers containing (a) 0%, (b) 10%, (c) 15%, and (d) 20% of EAFD at the age of 28 days.

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environment of the geopolymer may promote a structural disorganization of these crystalline phases. This is in accordance with Vargas et al. [51] who have studied the effect of the N/S (0.2 and 0.4) molar ratio in the mineralogy of fly ashes based geopolymers. The XRD spectra showed that FA original crystalline phases – quartz, mullite and hematite – were seen in all of the geopolymer spectra, regardless of any variable (N/S ratio, temperature or age). However, a trend towards intensity reduction of the crystalline peaks of quartz, mullite and hematite was observed at later ages for the N/S 0.40 samples. Apparently, some reactions promote a structural disorganization of the crystalline phases of the FA particles in consequence of the alkaline medium produced by the NaOH activator. This supports Fig. 11b and the semiquantitative EDS analysis of the geopolymer containing 10% of EAFD (Table 7), thus indicating that the contaminant metals present in the EAFD are spread in the geopolymer matrix. This behavior is in agreement with that reported by Nikolic et al. [41] and by Van Jaarsveld et al. [53]. Nikolic et al. [41] identified the crystalline phases of magnetite (Fe2O3) as the dominant crystalline phase and zincite (ZnO), which was present in smaller

amounts in the spectrum of EAFD. Only the magnetite phase was identified in geopolymers containing EAFD, but the intensity of the peaks was much smaller than that of the magnetite peaks identified in the spectrum of EAFD. No zincite phases were identified in geopolymers containing EAFD. According to Nikolic et al. [41], the absence of zincite in the spectrum of the geopolymer containing EAFD may indicate its participation in the amorphous aluminosilicate gel. Van Jaarsveld et al. [53] have evaluated the use of copper (Cu) and lead (Pb) oxides in fly ashes geopolymers. Also found no significant differences in the crystalline phases of XRD spectra, or in the formation of new phases after adding the contaminating metals to geopolymers. According to the authors, it appears that the metal is chemically bonded to the structure, although the bonding does not affect the basic Al and Si tetrahedral building blocks that make up the bulk of the geopolymer phases.

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3.4. Fourier Transform Infrared Spectroscopy (FTIR)

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Fig. 9 and Fig. 10 show the FTIR spectra of geopolymers containing 0%, 10%, 15 and 20% of EAFD at the age of 7 days and 28 days,

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Fig. 9. Fourier Transform Infrared Spectroscopy (FTIR) spectra of geopolymers containing 0%, 10%, 15% and 20% of EAFD at the age of 7 days.

d

c

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a

1a

2a 3 4

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Fig. 10. Fourier Transform Infrared Spectroscopy (FTIR) spectra of geopolymers containing 0%, 10%, 15% and 20% of EAFD at the age of 28 days.

9

respectively. Table 6 shows the vibration frequencies and assignments for all the samples studied. The vibration band in the original RHA (Fig. 4b) and FA (Fig. 4c) was at 1063 cm 1 and 1059 cm 1, respectively, which corresponds to the SiAOASi bonds. These bands were displaced to lower frequencies in the geopolymers containing of EAFD 1000 cm 1 (mas T-O (T: Si or Al)). This shows that EAFD does not interfere in the geopolymerization reactions of the alkali-activated samples, because the bands close to 1000 cm 1 are related to the effect of the alkali activator in the structure of geopolymer precursors (RHA and FA), which leads to the formation of amorphous alkaline aluminosilicates with three-dimensional structural chains. GarcíaLodeiro et al. [56] and Vásquez et al. [59] have shown that a band close to 1000 cm 1 is more typical of the main band in N–A–S–H gels. This demonstrates that the polycondensation reactions of the aluminosilicates typically occur in the presence of EAFD, thus contributing to the increase in the geopolymers strength in the long term, as shown in Fig. 6. The bands at 796 cm 1, 694 cm 1, 560 cm 1 and 443 cm 1 were identified in FA samples before and after alkali-activation (Fig. 4c). The band at 796 cm 1 is attributed to the a-quartz as reported in a previous study [60]. Bakharev [61] related the band close to 800 cm 1 to AlO4 vibrations and the band close to 688 cm 1 to symmetrical stretching vibrations of SiAOASi in FA matrices. Finally the bands 560 cm 1 and 460 cm 1 were also identified in the FA samples before and after the alkali activation. Such results are similar to the ones obtained by Palomo et al. [62], in which the same bands were identified before and after the alkali activation. To these authors, the 560 cm 1 band is a characteristic of the presence of mullite (Al in octahedral coordination). However, the main mullite peak (16.51°) was not identified in the geopolymers XRD spectra. This fact may be related to the low mullite concentration that was not found in the spectra. Another hypothesis is that the alkaline environment of the geopolymer may have contributed to the destabilization of the mullite particles, hence not presenting concentration enough to be identified by the XRD. Regions close to 460 cm 1 are attributed to angular deformations of SiO4 tetrahedrons (OASiAO), probably associated with quartz [60,63,64]. Similarly, Barbosa et al. [65] have also related the band 456 cm 1 to SiAO angular deformities. These results are in agreement with the XRD spectra, where crystalline quartz peaks were identified both before and after alkali-activation of the FA (Fig. 5c, Fig. 7 and Fig. 8). This demonstrates the chemical stability of the crystalline compounds in the presence of the strongly alkaline medium. This behavior was observed in a previous study with the activation of fly ash [52]. It is interesting to note that that these bands are present in geopolymers containing EAFD, which shows that EAFD did not interfere in geopolymerization or in the crystalline phases present in the FA-based geopolymer. As for the FTIR spectrum of the EAFD (Fig. 4a), two bands were identified: the 985 cm 1 band, which is related to the SiAO [57,66], and the 532 cm 1 band, which is related to the ZnAO [58,67]. These bands do not present themselves in a clear way in the spectra of the EAFD geopolymers. Considering the 985 cm 1 band, it can be superimposed on the 1000 cm 1 band of the geopolymers; Taking under consideration the 532 cm 1 band of EAFD geopolymers at the ages of 7 days (Fig. 9) and 28 days (Fig. 10), a wider valley was seen near 532 cm 1. This fact may be related to the influence of FA that, when near 552 cm 1 presented stretching vibrations related to the SiAO bonds. The appearance of bands in the regions of 867 cm 1 (d CAO) are typical of CO23 vibrational groups, present in inorganic carbonates [56]. This is related to free Na+ ions in the geopolymer matrix that reacted with the CO2 of the atmosphere. Characteristic peaks of carbonate compounds were not identified in the XRD spectra of

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B

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C

Fig. 11. SEM micrographs of the geopolymers containing 10% EAFD at: (a) 7 days, (b) 28 days, and (c) 91 days. The numbers on the images refer to EDS analyses. Magnification 1000X.

Table 7 EDS semi-quantitative analysis, indicated in Fig. 11 by points 1, 2, 3, 4, and 5 from 28-day old geopolymers with 10% EAFD. Points

1 2 3 4 5 a

582 583 584 585 586 587 588 589

590 591 592 593 594 595 596 597 598 599 600 601 602

Elementa Ratio (%) Na/Zna

Al

Si

Cr

Fe

Pb

19.35 11.68 11.14 17.40 16.26

12.38 26.59 7.32 7.03 5.97

64.09 59.50 63.34 22.26 38.86

0.21 0.00 0.94 0.60 0.45

3.89 2.16 17.15 52.70 37.77

0.09 0.08 0.10 0.00 0.69

Overlapping of Zn peaks with Na peaks.

the geopolymers (Figs. 7 and 8). This may be related to the low concentration of these compounds that were not detected in the XRD equipment. Therefore, even though the FTIR spectra of the EAFD geopolymers did not present well-defined bands of ZnAO bonds, crystalline peaks of zincite were identified in the XRD spectra demonstrating that this phase is stable in the geopolymer matrix (Fig. 7 and Fig. 8). 3.5. Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) SEM analyses of the geopolymers investigated revealed a dense and compact matrix with a high level of geopolymerization, whose main constituents are Na, Si, and Al. It is observed that FA particles are not identified, illustrating that the chemical fusion process of the particles (dissolution) in the alkaline environment was efficient, especially in geopolymers from 28 days. The evolution of the morphological aspect of mortars with an addition of 10% of EAFD at the ages of 7, 28, and 91 days can be seen in Fig. 11. At the age of 7 days, as shown in Fig. 11a, it is seen that the matrix has still some insolubilized particles, but at the ages of 28 days (Fig. 11b) and of 91 days (Fig. 11c) the morphology of the matrix

is considerably dense without any particles of the geopolymer matrix. That is to say polymerization reactions of the geopolymer matrix occur more efficiently at older ages (28 and 91 days), what supports the result of increased strength to compression of geopolymers over time, as shown in Fig. 6. Vargas et al. [51] presented images obtained by SEM of fly ash geopolymers. The authors have also observed that aging significantly influenced the geopolymer densification. Table 7 shows EDS analysis results of the points highlighted in Fig. 11b, which also indicate that the contaminating metals present in the EAFD are dispersed without much uniformity in the geopolymer mortar. The dispersion of metals through the system is directly related to the solubility of the metal salts: soluble salts in alkaline environments disperse readily while poorly soluble salts remain intact or significantly retain their particulate nature [68].

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4. Conclusion

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Even though the increasing of EAFD from 10% to 20% contributed to the increase of geopolymer compression strength, these levels did not contribute to an increased strength when compared to the reference geopolymer (0%). That is, new studies with EAFD

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should be carried out with FA and RHA based geopolymers with the main goal of identifying the ideal amount of EAFD that can provide greater mechanical performance when compared to the reference geopolymer. Geopolymers that contain 5% of EAFD have not presented any significant difference considering the compression strength when compared to the reference geopolymer. The FTIR spectra of the reference geopolymer (0%) and of the geopolymers which contained different amounts of EAFD (10, 15, and 20%) presented band close to 1000 cm 1, a band that is characteristic of the N–A–S–H geopolymer gel. This demonstrates that the polycondensation reactions of the aluminosilicates typically occur in the presence of EAFD, what contributed to the increase in the geopolymer strength over time. Crystalline phases of quartz (SiO2) and of zincite (ZnO) were identified in the XRD spectra of the FA and of the EAFD respectively, as well as in the XRD spectra of different levels of EAFD in geopolymers. That is, these phases are stable after the aluminosilicates polycondensation in the geopolymers. However, the crystalline phases present in the EAFD – franklinite (ZnFe2O4) and magnetite (Fe2O3) – and the crystalline phases present in FA – mullite (3Al2O32SiO2) and hematite (Fe2O3) – were not seen in the geopolymers XRD spectra, which demonstrates that some reactions that occur in the alkaline environment of the geopolymer could promote a structural disorganization of these phases. Regardless of EAFD content, all geopolymers were classified as non-hazardous materials Class II at the age of 91 days, since the concentration of metals in the leached extracts was below the maximum limits established by ABNT NBR standard 10004/2004. This means that the metals present in the EAFD were encapsulated inside the geopolymer matrix, thereby eliminating their hazardous nature. It is essential to point out that the aging of the geopolymer (until 91 days) was very important so that the Cd, Cr, and mainly Pb concentrations in the leached extract of the geopolymers that contained EAFD could be below what was established by NBR 10.004 (2004). This is related to the increase in the compressive strength and to a higher densification of the geopolymer matrix, thus benefiting the stabilization of the toxic metal in the matrix.

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