Experimental study of the mechanical stabilization of electric arc furnace dust using fluid cement mortars

Journal of Hazardous Materials 326 (2017) 26–35

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Experimental study of the mechanical stabilization of electric arc furnace dust using fluid cement mortars E.F. Ledesma a , J.R. Jiménez b,∗ , J. Ayuso b , J.M. Fernández c , J.de Brito d a

Área de Mecánica de los Medios Continuos y Teoría de Estructuras, Universidad de Córdoba, Espa˜ na Área de Ingeniería de la Construcción, Universidad de Córdoba, Espa˜ na Área de Química Inorgánica, Universidad de Córdoba Espa˜ na d CERIS-ICIST, DECivil, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal b c

h i g h l i g h t s • • • • •

This article optimizes the mechanical stabilization capacity of EAFD using currently available mortars. The EAFD hinders the hydration process of tricalcium silicate in cement mortars. Mortars with EAFD have a double hydrated hydroxide of Ca and Zn instead of portlandite. A maximum mass ratio of 6.67 kg EAFD per kg of cement was mechanically stabilized. This is the highest EAFD/cement ratio ever used to mechanically stabilize this type of hazardous waste.

a r t i c l e

i n f o

Article history: Received 8 August 2016 Received in revised form 17 November 2016 Accepted 18 November 2016 Available online 10 December 2016 Keywords: Electric arc furnace dust Mechanical stabilization Hazardous waste Leaching behaviour

a b s t r a c t This article shows the results of an experimental study carried out in order to determine the maximum amount of electric arc furnace dust (EAFD) that can be incorporated into fluid cement-based mortars to produce mechanically stable monolithic blocks. The leaching performance of all mixes was studied in order to classify them according to the EU Council Decision 2003/33/EC. Two mortars were used as reference and three levels of EAFD incorporation were tested in each of the reference mortars. As the incorporation ratio of EAFD/cement increases, the mechanical strength decreases. This is due to the greater EAFD/cement and water/cement ratios, besides the presence of a double-hydrated hydroxide of Ca and Zn (CaZn2 (OH)6 ·2H2 O) instead of the portlandite phase (Ca(OH)2 ) in the mixes made with EAFD, as well as non-hydrated tricalcium silicate. A mass ratio of 2:1 (EAFD: cement-based mortar) can be added maintaining a stable mechanical strength. The mechanical stabilization process also reduced the leaching of metals, although it was not able to reduce the Pb concentration below the limit for hazardous waste. The high amount of EAFD mechanically stabilized in this experimental study can be useful to reduce the storage volume required in hazardous waste landfills. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Steel can be 100% recycled [1,2]. According to the World Steel Association [3], the world’s steel production in 2014 was 1670 million tons, of which around 28% were obtained from electric arc furnaces (EAF). In Spain, there are 21 EAF steel mills that produce more than 75% of the steel consumed in Spain [4].

∗ Corresponding author at: Universidad de Córdoba, E.P.S. de Belmez, Avda. de la ˜ Universidad s/n, CP 14240, Córdoba, Espana. E-mail addresses: [email protected], [email protected] (J.R. Jiménez). http://dx.doi.org/10.1016/j.jhazmat.2016.11.051 0304-3894/© 2016 Elsevier B.V. All rights reserved.

EAF use scrap metal as raw material. During the production process, three categories of waste are generated: electrodes and refractory material, slag (EAFS) and electric arc furnace dust (EAFD) produced by purifying the gases generated. EAFD is highly toxic due to its zinc, lead and cadmium content [5]. According to European Union Decision 2014/955/UE [6], gaseous effluents containing hazardous substances are classified as hazardous waste. The composition and physicochemical properties of EAFS and EAFD may vary greatly from one steel mill to another, since they depend on the composition and quality of the scrap metal being melted. The main chemical constituents of EAFS are FeO, CaO, SiO2 , Al2 O3 , and MgO. The contents vary within the 10–40%, 22–60%, 6–34%, 3–14%, and 3–13% ranges, respectively [7]. Sofilic´ et al. [5]

27

0.0045 0.0040 0.0035 0.0030 0.0025 0.0020 0.0015 0.0010 0.0005 0.0000 10 Average diameter (nm)

1

100

Fig. 1. Size of pores of the EAFD sample.

3.0 2.5 Volume (%)

found that the main elements in EAFD are Fe, Zn, Mn, Ca, Mg, Si, Pb, S, Cr, Cu, Al, C, Ni, Cd, As and Hg. The authors further indicate that most of the mass of EAFD are metal oxides, silicates and sulphates. When the scrap metal is galvanised steel, the zinc content of EAFD increases, so that the mineralogical composition of the dust consists mainly of ZnO [8]. Sofilic´ et al. [9] studied the mineralogical composition of EAFD from three steel mills in Croatia, concluding that the composition of the dust generated during the processing of steel in EAFs must be studied individually. De Vargas et al. [10] detected via XRD that the majority of compounds were zinc and iron oxides. EAFD is included in the EU catalogue of hazardous waste materials due to the high content of heavy metals in its composition. The most common research areas and methods used to manage EAFD are solidification/stabilization to minimize the toxicity and leachability of heavy metals [5,11], to explore the recovery of zinc and lead from EAFD [12–15], the potential use of EAFD in ceramic matrices [16], cement and concrete matrices [17–20]. Cement is the best binder currently available for stabilization/solidification (S/S) of heavy metals, although their presence has a complex influence on the hydration reactions of cement [21–25]. De Vargas et al. [10] evaluated the incorporation of 5%, 15% and 25% wt. of EAFD in CEM-I Portland cement. These authors found a reduction of the initial mechanical strength of those cement pastes with large amounts of EAFD, although over long periods the mechanical strength recovered to 80% of the reference strength. However, for a 5% of EAFD incorporation ratio, the 28-day mechanical strength was highly affected. These authors also observed that EAFD slows the hydration reactions of Portland cement. The setting time of pastes made with 25% of EAFD was 36 h. Fares et al. [20] also found a direct relationship between the EAFD content and the setting time. These authors showed that replacing 3% of cement with EAFD increased the setting time from 6 to 33 h. Chen et al. [22] attributed the increases in the setting time to the presence of Zn2+ , which reacts with calcium ions of the clinker and inhibits the hydration of tricalcium silicate. Shi and Fernandez-Jiménez [26] studied the immobilization of hazardous and radioactive waste using alkaline cements. The results were encouraging for the use of this type of binder, provided that the cements are designed correctly. These types of cement have already been investigated in a previous work [27] and resulted in mortars and concrete with high durability. Pereira et al. [28] worked on the immobilization of EAFD using Portland cement CEM-I and CEM-II and adding fly ash type F. Better results were observed in the cement-only mix. They also studied two curing conditions, one in a laboratory environment and the other in saturated atmosphere achieved by using hermeticallysealed plastic bags. Better results were obtained when curing in a saturated atmosphere. Pereira et al. [29] used geopolymerization techniques with reactants such as sodium hydroxide, potassium hydroxide, sodium silicate, potassium silicate, kaolinite, metakaolinite and blast furnace slag for immobilization of EAFD waste. The use of potassium silicates and blast furnace slag improved the mechanical strength, and curing at 60 ◦ C increased the strength. The objective of this work is to maximize the loading of EAFD per kg of cement to produce mechanically stable monolith blocks using fluid cement-based mortars, in order to minimize the storage volume of landfill disposal. The leaching performance of all monolithic blocks was studied in order to determine its acceptability in landfills. To the best of the authors’ knowledge, this study tested the highest EAFD/cement ratio ever used to mechanically stabilize this type of hazardous waste. This allows a significant reduction of the storage volume required in hazardous waste landfills.

Incremental Pore Volume (cm3/g)

E.F. Ledesma et al. / Journal of Hazardous Materials 326 (2017) 26–35

2.0 1.5 1.0 0.5 0.0 0.01

0.1

1 10 Average diameter (µm)

100

Fig. 2. Particle size distribution of the EAFD sample.

2. Characterization of the EAFD The study was carried out on a sample of EAFD from a steel mill situated in the north of Spain. The specific surface was calculated using the Brunauer-Emmett-Teller (BET) method, determined by the absorption of N2 with Micromeritics ASAP 2010 equipment, obtaining a value of 4.6 m2 /g. The real particle density calculated in accordance with NLT 179:1995 [30] was 3.809 g/cm3 . Fig. 1 shows the distribution of the pores; there are more mesopores (2–50 nm) – 72% – than macropores (>50 nm) – 28%. Furthermore, the majority of the mesopores (57%) fall within a large-size range (20–50 nm). The particle size distribution was determined by laser diffraction in a “Beckman-Coulter LS-230” equipment, with a measurement range of 0.04–2000 ␮m. The size distribution of particles ranges from 0.04 ␮m to 200 ␮m (Fig. 2). The majority of the particles are below 10 ␮m (70%), the average particle size being 7 ␮m. These results have been confirmed via scanning electron microscopy (SEM) (Fig. 3), showing sizes lower than 7 ␮m. Sofilic´ et al. [5] found that the range of the majority of particles fell in the range 100–125 ␮m, much higher than that of this work. The chemical analysis has been made by wavelength dispersive X-ray fluorescence (WDXRF), using a S4 Pioneer apparatus of BRUKER, with a potential of 4 kV. The SEM micrograph was obtained with a Jeol scanning electron microscope: JSM-6300 model, with acceleration potential of 20 kV and work distance of 15 mm. Table 1 shows the percentage in weight of each compound/element present in the EAFD sample, the main ones being ZnO and Fe2 O3 , and to a lesser extent CaO, PbO and MnO. Other authors obtained Fe2 O3 as the main compound, and to a lesser extent ZnO, CaO and MnO [5].

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E.F. Ledesma et al. / Journal of Hazardous Materials 326 (2017) 26–35

Fig. 4. XRD of the EAFD sample.

Fig. 3. SEM micrograph of the EAFD sample. Table 1 Chemical characterization of the EAFD sample by WD-XRF. Compound/element

EAFD content (% wt.)

ZnO Fe2 O3 Cl CaO SiO2 Na2 O MnO PbO SO3 K2 O MgO Al2 O3 Cr2 O3 F P2 O5 SnO2 Br TiO2 CdO BaO NiO MoO3 ZrO2 Total

32.770 30.480 11.370 2.930 2.649 2.590 2.510 2.226 2.210 1.960 1.670 0.942 0.500 0.433 0.200 0.076 0.071 0.067 0.037 0.032 0.017 0.009 0.003 95.75

The crystalline mineral phases were determined by X-Ray diffraction (XRD). The EAFD sample, crushed with an agate mortar (<0.125 mm), was analysed in a Siemens D5000 diffractometer with monochromatic radiation of Cu K␣ (␭ = 1.5405 Å; 40 kV; 30 mA). A scanning speed of 2◦ /min, using a step of 0.02◦ every 0.6 s and scanning angles between 3 and 80◦ in units of 2␪, was used. The EAFD raw sample was analysed in a Bruker D8 Discover A25 diffractometer with monochromatic radiation of Cu K␣ (␭ = 1.5405 Å; 40 kV; 30 mA). A scanning speed of 0.00625◦ /min, using a step of 0.016◦ every 153.6 s and scanning angles between 10◦ and 70◦ in units of 2␪, was used. The identification of the main minerals was done by comparison with the JCPDS Powder Diffraction File database [31]. The EAFD waste is a polymetallic mixture of different oxides (Fig. 4), of which the majority of components are franklinite (ZnFe2O4), a spinel of Fe and Zn, d311 = 2.5426, (22-1012, d311 = 2.5430) [31] and zincite (ZnO), d101 = 2.4730, (36-1451, d101 = 2.4764) [31]. The presence of metallic Mn, d221 = 2.1057, (330887, d221 = 2.1040) [31] and other minority phases such as: MnO, d200 = 2.2221, (07-0230, d200 = 2.2230) [31], Quartz, d101 = 3.3414, (33-1161, d101 = 3.3420) [31], calcite (CaCO3), d104 = 3.0324, (050586, d104 = 3.0350) [31] and PbO2 , d111 = 3.1311, (37-0517,

d111 = 3.1324) [31], was also detected. The presence of franklinite and zincite agrees with the Zn content, higher than the stoichiometric ratio of Zn and Fe in the spinel. These results agree with those obtained by Suetens et al. [8]. The EAFD composition may vary greatly depending on the scrap metal [5,9]. Other authors found that the main phase was a spinel of zinc, iron, zinc and manganese ((ZnxMnyFe1-x-y) Fe2 O4 ), together with zincite [32], franklinite (ZnFe2 O4 ) and other spinels without discerning which is the main and zincite (ZnO) and quartz [33], Wustite, metallic Fe, Larnite and spinel of iron, zinc and chromium and sulphides of iron, zinc and copper [34], franklinite (ZnFe2O4), zincite (ZnO) and tricalcium silicate [35].

3. Environmental evaluation of the EAFD A compliance test was carried out according to standard UNEEN-12457-3:2003 [36] in order to determine the concentration of heavy metals and anions in the leached of the waste. The EAFD was classified according to the criteria established by EU Council Decision 2003/33/EC [37]. The dry sample’s mass was 0.175 kg. In the test two steps were followed to simulate short- and long-term exposure scenarios. In step 1 an amount of deionized water was added in order to establish a liquid/solid ratio (L/S) equal to 2 l/kg. The sample was then shaken in a tumbler for 6 h and passed through 0.45 ␮m filters. In step 2 more deionized water was added to the sample to reach an L/S ratio of 10 l/kg. The sample was shaken again for 18 h and then filtered. The eluate from the filtering process was analysed in an ICP-MS (Perkin Elmer ELAN DRC-e). This analysis quantified the 12 heavy metals specified by the European Landfill Directive: Ni, Cr, Sb, Se, Mn, Hg, As, Pb, Cd, Cu, Ba and Zn. In addition, the sulphate, fluoride and chloride anion contents were obtained by ion chromatography according to standard UNE-EN-ISO-10304-1: 2009 [38]. Table 2 shows the results obtained in the leaching test. The value limits are also shown for the classification of the waste according to EU Council Decision 2003/33/EC [37]. Cu, Cr, Zn, Cd, Hg and the fluorides exceeded the limit for inert material. Se, Mo and the sulphates exceeded the non-hazardous limit, while the elements that leached the most and failed to comply with the limits allowed by EU Council Decision 2003/33/EC [37] to be classified as hazardous material were Pb, which exceeded 110 and 18 times the limit for L/S = 10 and L/S = 2 respectively, and chlorides, which exceeded 1.6 times the limit for L/S = 2. Pereira et al. [28] compared the concentration of Cd, Cr and Pb in the leaching test with the limits established by the English Environmental Agency (EEA) [39], which were in accordance with those established by the European Directive [37]. The results obtained by these authors were 0.5 mg/kg for Cd, 5 mg/kg for Cr and 5 mg/kg for Pb, and therefore their material was classified as non-hazardous.

E.F. Ledesma et al. / Journal of Hazardous Materials 326 (2017) 26–35

29

Volume (%)

6 5

CEM

4

LF

3 2 1 0 0.01

0.1

1 Average diameter (µm)

10

100

Fig. 5. Particle size distribution of LF and cement.

Table 2 Leached concentrations of EAFD (mg/kg) and acceptance criteria (WAC, EU Council Decision 2003/33/EC). Element (mg/kg)

EAFD

Criteria EU Landfill Directive 2003/33/EC Inert

Non-hazardous

Hazardous

L/S = 2

L/S = 10

L/S = 2

L/S = 10

L/S = 2

L/S = 10

L/S = 2

L/S = 10

Cr Ni Cu Zn As Se Mo Cd Sb Ba Hg Pb Fluoride Chloride Sulphate

1.592a 0.046 1.686a 13.203a <0.01 3.072b 15.706b 0.119a 0.004 <0.01 0.033a 474.960c 8.4a 26900c 12000b

1.970a 0.053 2.17a 24.047a <0.05 2.762b 20.494b 0.138a 0.001 6.935 0.180a 5483.866c 65.8a 24100b 16300a

0.2 0.2 0.9 2 0.1 0.06 0.3 0.03 0.02 7 0.003 0.2 4 550 560

0.5 0.4 2 4 0.5 0.1 0.5 0.04 0.06 20 0.01 0.5 10 800 1000

4 5 25 25 0.4 0.3 5 0.6 0.2 30 0.05 5 60 10000 10000

10 10 50 50 2 0.5 10 1 0.7 100 0.2 10 150 15000 20000

25 20 50 90 6 4 20 3 2 100 0.5 25 200 17000 25000

70 40 100 200 25 7 30 5 5 300 2 50 500 25000 50000

Conditions of the test sample Conductivity (␮S/cm) Temperature (◦ C) pH

24800 20.8 13.25

8560 19.8 13.28

a b c

Exceeds the inert waste limit. Exceeds the non-hazardous waste limit. Exceeds the hazardous waste limit.

4. Mechanical stabilization of EAFD 4.1. Materials Two commercial mortars were used, hereon named M1 and M2. Both mortars were made with CEM-I 52.5 R-SR cement and a limestone filler. The M1 mortar contained 60% cement and 40% limestone filler (LF), while the M2 mortar contained 30% cement, 30% limestone filler and 40% siliceous natural sand (NS). The particle size distribution of NS and LF according to standard UNE-EN-933-1:2006 [40] is shown in Table 3. The highest percentage of sand particle size was 0.25–2 mm, while the limestone filler particles were below 0.25 mm. In accordance with standard UNE-EN-1097-6:2001 [41] the dry density of sand particles was 2.614 g/cm3 and the water absorption after 24 h was 0.26%. The result from the equivalent sand test measured according to UNEEN-933-8:2000 [42] was 99. The particle size distribution of limestone filler particles (LF) and cement (CEM) measured by laser diffraction is shown in Fig. 5. Most

Table 3 Particle size distribution. Sieve size (mm)

4 2 1 0.5 0.25 0.125 0.063

Percent passing (%) Natural sand

Limestone filler

100 99.93 99.02 81.81 4.89 0.21 0.17

100 74.33

of the particles of CEM (68%) and LF (51%) fall within the 3–32 ␮m range (Fig. 2SC). These results are important due to the fact that above 32 ␮m the particles are too large to hydrate completely and below 3 ␮m they contribute very little to the strength but require more water [43–46]. Fig. 6 shows the XRD patterns of the components of mortars M1 and M2, in which it can be seen that the natural sand (NS) is silica

30

E.F. Ledesma et al. / Journal of Hazardous Materials 326 (2017) 26–35

Fig. 6. XRD components of mortars M1 and M2.

(SiO2 ) (33-1161) [31] and the limestone filler (LF) is fundamentally calcite (CaCO3 ) (05-0586) [31]. The main phase of the cement is tricalcium silicate (Ca3 SiO5 ) (42-0551) [31]. 4.2. Experimental program Mortars M1 and M2 were tested without incorporating EAFD as reference mortars. Each of these reference mortars was mixed with EAFD according to the following ratios in mortar/waste weight: 2/1, 1/1 and 1/2. Table 4 shows the contents in weight of the materials and the loading of EAFD per kg of cement added (EAFD/cement), used in each of the mixes tested. Such high rates of EAFD/cement have not been previously used in the literature. The amount of water used was adjusted experimentally to have a fluid consistency according to standard UNE-EN-1015-3:2000 [47] within the 220–240 mm range, which allowed the mortar to be pumped. No admixtures were used. The water/cement (w/c) ratio increased as the amount of EAFD increased. In the group of mixes made with M2 the w/c ratio was greater than in that of M1, which was attributed to the angular shape of the natural sand particles used in mortar M2 (Fig. 1SC). The crystalline phases formed in the hardened mortar were identified by XRD (Fig. 7). The monolithic character of the mortars was determined following the procedure described in French standard NF-XP X31-212:2011 [48]. The tests required were compressive and splitting tensile strength before and after the test samples were immersed in water. Furthermore, the rate of leakage or rate of delitescence was determined after immersion in deionized water. For each mortar type, twelve cylindrical test samples 80 mm high and 40 mm in diameter were made. Six samples were randomly selected to determine the compressive and splitting tensile strength after 28 days of curing. The remaining six samples were immersed in deionized water 96 ± 4 h, with a ratio of L/S = 10, according to the conditions stated in Table 5. After immersion, the compressive and splitting tensile strengths were determined. Once the test samples were broken in the compressive strength test, before and after immersion, their dry bulk density was determined.

Fig. 7. XRD of the reference mortars before and after hydration of cement.

To determine the rate of delitescence, three of the six test samples immersed in water were randomly chosen. The water in contact with the test samples was passed through a 0.45 ␮m filter to determine the rate of delitescence (NF-XP X31-212:2011) [48]. The material remaining in the filter was heat-dried at a temperature of 105 ± 5 ◦ C. The rate of delitescence was obtained by means of the following equation (Eq. (1)): td = 100 ×

md m0s

(1)

- td : rate of delitescence (%); - md : dry mass of the solid particles remaining in the 0.45 ␮m filter (gr); - m0s : dry mass of the test sample (r). The leaching behaviour of the stabilized mix was checked according to standard NF-XP X31-211:2012 [49]. Two cylindrical specimens for each type of mortar were immersed for 24 h in deionized water in continuous motion by a magnetic stirrer. A ratio of L/S = 10 was used. The water in contact with the specimens was passed through 0.45 ␮m filters and the eluate from the filtering process was analysed in an ICP-MS following the procedure described in the previous section. 5. Results 5.1. XRD of the hardened mortars The presence of Zn and Pb in the EAFD slowed down the hydration of the cement. Vargas et al. [10] found delays of the setting time of up to 33 h, with incorporation of 25% of EAFD. Likewise, incorporations of up to 3% in mass of metals, such as Zn, Ni and Pb

E.F. Ledesma et al. / Journal of Hazardous Materials 326 (2017) 26–35

31

Table 4 Mortars mix design. Mortar type

CEMa (g/l)

NSb (g/l)

LFc (g/l)

EAFDd (g/l)

Water (g/l)

w/c

Consistency (mm)

EAFD/CEM

M1 M1FD/2:1 M1FD/1:1 M1FD/1:2 M2 M2FD/2:1 M2FD/1:1 M2FD/1:2

648.6 373.7 242.5 148.9 376.6 213.2 141.6 83.1

– – – – 502.1 284.3 188.9 110.9

432.4 249.1 161.7 99.3 376.6 213.2 141.6 83.1

– 311.3 404.2 496.6 – 355.2 472.2 554.5

373.0 391.0 384.5 395.2 332.7 368.1 383.7 395.3

0.58 1.05 1.59 2.65 0.88 1.73 2.71 4.75

230 226 233 227 236 223 226 222

0.00 0.83 1.67 3.33 0.00 1.67 3.33 6.67

a b c d

CEM-I 52.5 R-SR. Siliceous natural sand. Limestone filler. Electric arc furnace dust.

Table 5 Conditions/limitations of the tests. Test

Specimens

Climatic chambera

Curing time

Before immersion Dry bulk density Compressive strength Splitting tensile strength

6 Cylindrical 40 mm diameter × 80 mm height 3 Cylindrical 40 mm diameter × 80 mm height 3 Cylindrical 40 mm diameter × 80 mm height

Chamber-1 (7 days) Chamber-2 (21 days) Chamber-1 (7 days) Chamber-2 (21 days) Chamber-1 (7 days) Chamber-2 (21 days)

28 days 28 days 28 days

After immersion Compressive strength Splitting tensile strength Delitescence

3 Cylindrical 40 mm diameter × 80 mm height 3 Cylindrical 40 mm diameter × 80 mm height 3 Cylindrical 40 mm diameter × 80 mm height

Chamber-1 (7 days) Chamber-2 (21 days) Immersed in water (96 ± 4 h) Chamber-1 (7 days) Chamber-2 (21 days) Immersed in water (96 ± 4 h) Chamber-1 (7 days) Chamber-2 (21 days) Immersed in water (96 ± 4 h)

32 days 32 days 32 days

in cement caused decreases in the mechanical strength of up to 99% [50]. In this case, in order to guarantee full hydration of the cement, the mortar samples made with EAFD were demoulded after 7 days, due to the fact that the cement hydration time increased with the increase of EAFD content. Fig. 7 shows the XRD pattern and crystalline mineral phases of the hardened reference mortar before hydration (M1, M2) and after the mortars hydrated and hardened (M1H, M2H). In mortars M1 and M2 the main phase was that of calcite (CaCO3 ) (44-0551) [31] and tricalcium silicate (Ca3 SiO5 ) (42-0551) [31] was also detected in both mortars. Furthermore, quartz (SiO2 ) (33–1161) [31] was observed in M2, which is in accordance with the composition of this mortar, commented on in section 4.1. In both hardened mortars the presence of portlandite (Ca(OH)2 ) (04-0733) [31] as a consequence of the hydration of Portland cement, as well as very small amounts of ettringite (Ca6 Al2 (SO4 )3 (OH)12 ·26H2 O) (41-1451) [31], were detected. The XRD diagrams allow concluding that all the cement had set, since no lines of diffraction corresponding to dry cement were observed. In the hardened mortars the main phase was calcite (CaCO3 ) (44-0551) [31]. Figs. 8 and 9 show the effect of the incorporation of EAFD on the mineralogical composition of the hardened mortars. No portlandite phase was observed (Ca(OH)2 ) in the hardened mortar with the incorporation of EAFD waste in mortars M1 and M2. This happened in all the tested ratios: 2:1, 1:1 and 1:2, which is different from what occurred in the hardened reference mortars (Fig. 7). However, the presence of a hydrated oxide of Ca and Zn (CaZn2 (OH)6 ·2H2 O) (25-1449) [31] was observed. Furthermore, the main phases were calcite (CaCO3 ) and the spinel of iron and zinc (ZnFe2 O4 ) (22-1012) [31], unlike in the reference mortars where the main phase was calcite. In both mortars it was observed that increasing the EAFD content in the mix increased the content of ZnFe2 O4 compared to calcite, which is fully attributed to the composition of the EAFD used, which was fundamentally ZnO and ZnFe2 O4 (Fig. 4). It should be pointed out that the amount of calcite in M2 is less than that in M1, since M2 contains siliceous sand in its initial composition. In addition, in the mixes with M1 no zincite was observed, unlike in the mixes with M2. The highest content of calcite in the mortar M1 has favoured

Fig. 8. XRD of the set mixes of reference mortar M1 with different EAFD ratios.

the reaction with zincite (ZnO) to form CaZn2 (OH)6 ·2H2 O and for this reason the presence of zincite in the hardened mortar M1 is not observed. In short, on the one hand the presence of the phase CaZn2 (OH)6 ·2H2 O hindered the hydration process of tricalcium sil-

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5.3. Mechanical properties

Fig. 9. XRD of the set mixes of reference mortar M2 with different EAFD ratios.

Table 6 Dry bulk density of the hardened mortar. Mortar

Dry bulk density (kg/m3 )

M1 M1FD/2:1 M1FD/1:1 M1FD/1:2 M2 M2FD/2:1 M2FD/1:1 M2FD/1:2

1454 ± 0.005 1325 ± 0.009 1193 ± 0.016 1140 ± 0.006 1588 ± 0.006 1434 ± 0.008 1328 ± 0.009 1227 ± 0.009

icate, since in all the samples the presence of unhydrated tricalcium silicate (Ca3 SiO5 ) was observed (Figs. 8 and 9) and on the other hand the higher content of calcite in mortar M1 caused the presence of zincite (ZnO) in the hardened mortar M1 not to be observed and this is agreement with the loss of compressive strength being slightly lower in mortar M1 than in mortar M2.

5.2. Dry bulk density The dry bulk density of the hardened mortars decreases with the incorporation of EAFD (Table 6). The percent loss of dry bulk density varied linearly with the EAFD incorporation ratio. This loss in density is attributed to the lower apparent density of the waste and the higher content of water in the mix; the w/c ratio varied from 0.58 in M1 to 2.65 in M1FD/1:2 and from 0.88 in M2 to 4.75 in M2FD/1:2. Mortar M2 showed higher density than M1, despite its higher w/c ratio, which can be attributed to the better particle size distribution of the aggregates used in M2.

The following conditions must be met by the mixes described in Table 4 to be considered mechanically stable monolithic blocks: 28-day compression strength over 1 MPa, splitting tensile strength over 0.1 MPa and rate of delitescence after immersion lower than 10%. These criteria are in accordance with those established by the English Environmental Agency (EEA) [39]. Lampris et al. [51] and Fernández et al. [52] also considered that a mortar constitutes a stable solid when its 28-day compression strength is greater than 1 MPa. Table 7 shows the mean values and standard deviation of the compressive strength and splitting tensile strength before and after immersion of the test tubes. It also shows the rate of delitescence. As the incorporation ratio of EAFD increases, the mechanical strength of the mortar decrease. The decrease of mechanical strengths can be attributed to three factors: i) the higher content of Zn and Pb with the increases of EAFD/cement ratio ii) the greater water/cement ratio in the mixes that include EAFD (Table 4); iii) the formation during setting of a double hydrated oxide of Ca and Zn (CaZn2 (OH)6 ·2H2 O) in place of the formation of the portlandite phase (Ca(OH)2 ) in the mortars with EAFD. This is accompanied by unhydrated tricalcium silicate (demonstrated by XRD as mentioned), meaning the hydration of the cement was hindered, although there was more water than in the reference mortar. The high content of zinc and fluorides contributes to the non-hydration of cement and, therefore, to the losses in strength [21]. Likewise, all tested mortars have a compressive strength before and after immersion above 1 MPa at 28 days. However, in M1FD/1:2 and M2FD/1:2, the indirect tensile strength after immersion at 28 days does not reach 0.3 MPa. Therefore the maximum EAFD/cement ratio in mortars mechanically stable is 3.33 and 6.67 for M1 and M2 mortars respectively. It is pointed out that mortar M2 doubles the ratio of mortar M1, which is attributed to the presence of siliceous sand in mortar M2 and to its higher density. In this study the incorporation ratio of EAFD ranged from 33% and 67%, much higher than that used by other authors. De Vargas et al. [10] obtained a decrease in early strength (3 days) in cement pastes with EAFD. However, after 28 days they observed a significant increase in strength, i.e. for cement pastes with 5% of EAFD similar strengths to those of the reference paste were obtained. These authors, with 15% and 25% substitution ratio of cement with EAFD, obtained a 20% decrease in mechanical strength. Hekal et al. [53] tested substitution ratios of cement with EAFD of 0%, 1%, 3% and 5%. Only the cement paste with 1% of EAFD showed higher values than the reference paste. The mixes with 3% and 5% of EAFD lost strength by 45% and 52% respectively. These authors showed that the hydration time had a significant influence on mechanical strength. Balderas et al. [54] for cement substitutions for EAFD of 2%, 5%, 8% and 10%, pre-treated for 24 h in a solution of H2 SO4 (pH5), obtained higher strength than that of the reference cement paste, including up to 72 MPa after 42 days with 10% of EAFD. All types of mortar studied showed a rate of delitescence well below 10%. There was a slight increase in the rate of delitescence with the increase of EAFD in the mortars. Likewise, it can be said that the behaviour is similar in the M1 and M2 mortars studied. 5.4. Leaching behaviour of monolithic samples of mortars Table 8 shows the results obtained in the leaching test of monolithic samples of mortars. Only the elements that exceeded the limits of inert, non-hazardous or hazardous waste in the compliance test of the EAFD (Table 2) were considered in this study. Diffusion through the solid matrix of mortars is the main mechanism of release in the specimens tested according to standard NF-XP X31-211:2012 [49]. Hence the concentration of heavy metals

E.F. Ledesma et al. / Journal of Hazardous Materials 326 (2017) 26–35

33

Table 7 Results (mean value and standard deviation) of the mechanical properties. Mortar

M1 M1FD/2:1 M1FD/1:1 M1FD/1:2 M2 M2FD/2:1 M2FD/1:1 M2FD/1:2

Before immersion

After immersion

Rate of delitescence (%)

Compressive strength (MPa)

Splitting tensile strength (MPa)

Compressive strength (MPa)

Splitting tensile strength (MPa)

17.66 ± 1.02 7.93 ± 0.74 4.88 ± 0.84 2.17 ± 0.09 18.52 ± 0.38 8.47 ± 0.98 4.58 ± 0.25 1.51 ± 0.15

2.55 ± 0.15 1.21 ± 0.14 0.70 ± 0.01 0.34 ± 0.01 2.63 ± 0.09 0.96 ± 0.21 0.64 ± 0.22 0.24 ± 0.05

16.97 ± 2.36 8.79 ± 2.36 5.60 ± 0.73 2.22 ± 0.26 19.58 ± 1.56 8.81 ± 0.98 4.52 ± 0.46 1.69 ± 0.09

2.65 ± 0.06 0.75 ± 0.26 0.49 ± 0.19 0.25 ± 0.06 2.47 ± 0.43 0.65 ± 0.09 0.45 ± 0.11 0.24 ± 0.05

0.0115 0.0139 0.0156 0.0223 0.0064 0.0137 0.0144 0.0309

Table 8 Leaching behaviour of monolithic samples of mortar (NF-XP X31-211). Element (mg/kg) L/S = 10

Cr Cu Zn Se Mo Cd Hg Pb Fluoride Chloride Sulphate

EAFD

M1FD/2:1

(mg/kg)

(mg/kg)

(%)

(mg/kg)

(%)

(mg/kg)

(%)

(mg/kg)

(%)

(mg/kg)

(%)

(mg/kg)

(%)

1.970a 2.157a 24.047a 2.762b 20.494b 0.138a 0.180a 5483.866c 65.8a 24100b 16300a

0.029 0.012 7.930a <0.1 0.050 0.001 0.001 14.050b <10 3250a 754.525

98.55 99.42 67.02 96.38 97.56 99.57 99.38 99.74 84.80 86.51 95.37

0.038 0.015 11.350a <0.1 0.870a 0.001 0.002 16.700b <10 3800a 93.425

98.08 99.31 52.80 96.38 95.75 99.32 98.82 99.70 84.80 84.23 99.43

0.064 0.038 12.401a 0.815b 2.023a 0.002 0.024a 48.808b <10 6375a 58.65

96.74 98.24 48.43 70.48 90.13 98.31 86.39 99.11 84.80 73.55 99.64

0.039 0.023 10.002a 0.760b 2.120a 0.002 0.019a 35.821b <10 6100a 186.175

98.04 98.95 58.41 72.50 89.66 98.31 89.54 99.35 84.80 74.69 98.86

0.071 0.058 13.600a 1.557b 5.000a 0.010 0.083a 99.643c 17.95a 14600a 852.55

96.42 97.33 43.44 43.62 75.60 92.48 53.54 98.18 72.72 39.42 94.77

0.051 0.037 12.886a 1.352b 5.192a 0.009 0.065a 73.130c 15.6a 14300a 986

97.43 98.29 46.41 51.05 74.67 93.31 63.91 98.67 76.29 40.66 93.71

Conditions of the test sample 8560 Conductivity (␮S/cm) Temperature (◦ C) 19.8 13.28 pH a b c

2850 19.2 12.57

M2FD/2:1

M1FD/1:1

2280 21.4 12.38

3615 22.25 12.84

M2FD/1:1

3160 19.65 12.85

M1FD/1:2

4460 25.1 12.8

M2FD/1:2

4070 24.6 12.71

Exceeds the inert waste limit. Exceeds the non-hazardous waste limit. Exceeds the hazardous waste limit.

and ions decreased considerably in the eluate obtained according to this standard with respect to the compliance test [36], where water is in direct contact with the EAFD particles. Mortar type M2, made with sand and cement, showed a lower release of hazardous elements due to its higher density (Table 6). These results agree with those from Belebchouche et al. [55], who obtained better results concerning the leaching behaviour of Ni, Pb and Cr during the immobilization of hazardous waste rejection from the cutlery unit in mortar than cement pastes. The concentrations of Cr, Cu, Cd and sulphates did not exceed the inert limits established by the European Directive. Zn, Mo, Hg, fluorides and chlorides exceeded the inert limit. With incorporation ratios of EAFD up to 33%, mortars were able to immobilize Se without exceeding the hazardous waste limit. Although the Pb concentration in the eluate was reduced by over 98%, the mortars were not able to immobilize Pb so that it complied with the hazardous waste limit. The alkaline pH of cement based monolithic samples favours solubility of Pb, which is higher under alkaline leaching environment [56]. New techniques to improve the mechanical stabilization of Pb should be the focus of the next research. A compliance test for granular samples was also carried out according to standard UNE-EN-12457-3:2003 [36]. The results are shown as supplementary content (Table 1SC). 6. Conclusions This study shows the results of the mechanical stabilization of Electric Arc Furnace Dust (EAFD) using fluid cement-based mortars. The main metals identified in EAFD were Zn and Fe, and to a lesser extent Ca and Pb. The EAFD was a polymetallic mix of different oxides, the main components being franklinite (ZnFe2 O4 ) and

zincite (ZnO). The EAFD is classified as hazardous waste according to criteria established by EU Council Decision 2003/33/EC. Two type of mortars were used as reference. The incorporation of EAFD in the mortars linearly decreased the dry bulk density and the mechanical strength of the mixes, which is attributed to three factors: i) the higher content of Zn and Pb with the increases of EAFD/cement ratio; ii) the greater water/cement ratio of the mixes incorporating EAFD; iii) the formation of double hydrated hydroxide of Ca and Zn (CaZn2 (OH)6 ·2H2 O) instead of the formation of the portlandite phase (Ca(OH)2 ) in the mortars with EAFD, complemented by the presence of unhydrated tricalcium silicate. It was possible to make stable monolithic blocks with a compressive strength greater than 1 MPa using weight ratios of up to 2 parts of EAFD for each cement-based mortar part, which represents percentages of metal relative to cement content of: 6.39% of Pb and 87.79% of Zn for mortar M1FD1:2 and 12.78% of Pb and 175.59% of Zn for mortar M2FD1:2. The concentration of heavy metals and ions decreased considerably in the eluate obtained in the leaching test of monolithic samples of mortars, although the mortars tested were not able to immobilize Pb so that it complied with of the hazardous waste limit. New techniques to improve the mechanical stabilization of Pb should be the focus of new research. Mortar made with siliceous sand and limestone filler (M2) allowed mechanically stabilizing greater amounts of EAFD per kg of cement due to the mineral skeleton provided by sand and the higher density achieved in this mortar. Mortar M1FD/1:2 has reached a ratio of 3.33 kg EAFD per kg of cement, while for mortar M2FD/1:2 a ratio of 6.67 kg of EAFD was mechanically stabilized, which implies greater efficiency of the M2 mortar as explained above. This will allow significantly reducing the volume of hazardous waste landfill required.

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Acknowledgments The authors would like to thank the spanish public company ENRESA (Empresa Nacional de Residuos Radiactivos S.A.) for financial support this research via project (035-ES-IN-0140). They also ˜ thank Manuel Ordónez-Álvarez and all ENRESA staff for their dedication and professionalism. This work was partly supported by the Andalusian Regional Government (Research Groups FQM-214 and TEP-227) and XXI Own Program for the Promotion of Research at the University of Córdoba – Modality 4.2. The support of the CERIS-ICIST Research Institute, IST, University of Lisbon, and of FCT (Foundation for Science and Technology) is also acknowledged.

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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2016.11. 051.

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