REUSE OF WAELZ SLAG AS RECYCLED AGGREGATE FOR STRUCTURAL CONCRETE Sabrina Sorlini(1), Carlo Collivignarelli(1), Giovanni Plizzari(2), Michele Delle Foglie(3) (1) Department of Civil Engineering, University of Brescia, Italy (2) Department of Technologies, University of Bergamo, Italy (3) Pontenossa S.p.a., Italy Abstract ID Number: 299 Author contacts Authors
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Postal address
Sabrina Sorlini
sabrina.sorlini@ing.unibs.it
+39-0303715503
Department of Civil Engineering, University o Brescia, via Branze 38, 25123 Brescia
Carlo Collivignarelli
+39-030carlo.collivignarelli@ing.unibs.it 3715503
Department of Civil Engineering, University o Brescia, via Branze 38, 25123 Brescia
Giovanni Plizzari
giovanni.plizzari@ing.unibs.it
+39-0352053351
Dept. of Engineering Design and Technologies, University of Bergamo, viale Marconi 5/A, 24044 Dalmine (BG)
Michele Foglie
info@pontenossa-spa.it
+39-035701006
Pontenossa s.p.a., Via Prealpina Orobica, 60, 24028 Ponte Nossa (BG)
Delle
Contact person for the paper: Sabrina Sorlini Presenter of the paper during the Conference: Sabrina Sorlini Total number of pages of the paper (this one excluded):
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REUSE OF WAELZ SLAG AS RECYCLED AGGREGATE FOR STRUCTURAL CONCRETE Sabrina Sorlini (1), Carlo Collivignarelli (1), Giovanni Plizzari (2), Michele Delle Foglie (3) (1) Department of Civil Engineering, University of Brescia, Italy (2) Department of Engineering Design and Technologies, University of Bergamo, Italy (3) Pontenossa S.p.A., Italy
Abstract The Waelz slag was characterized for the main physical (grain size distribution, water absorption and density) and chemical (chemical composition, alkali reactivity, leaching behaviour) properties to evaluate its suitability for final reuse in concrete. Eventually, different concrete mixtures were produced with the following characteristics: 7 mixes with a cement content of 300 kg/m3 (Type II/A-LL, CEM 32.5R according to EN 197-1), water/cement=0,54, 200-400-600 kg/m3 of Waelz slag; 7 mixtures with 250-300-350-400 kg/m3 of cement (Type II/A-LL CEM 42.5R), water/cement=0,52, 200-400-600 kg/m3 Waelz slag. The concrete mixes were characterized by conventional physical-mechanical (slump, setting time, compression, tensile and flexural strength, as well as modulus of elasticity) and leaching tests (following the Italian Standard for waste reuse). A very good compressive strength (similar to conventional concrete) was achieved after 28 days of curing with values of about 25 MPa and 35-40 MPa, for 32.5R and 42.5R cement respectively (independently from waste dosage). As far as tensile and flexural strength are concerned, concrete with Waelz slag is similar or even better than the conventional one, while the modulus of elasticity is reduced from 10 to 40%. Finally, the environmental acceptability is complied in all concrete specimens since metals leachability is lower than the Italian Standards for wastes reuse. Key words: Concrete, Recycled Aggregates, Waelz Slag, Structural Applications.
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1.
INTRODUCTION
Concrete technology has quickly developed during the last two decades and material performance has been significantly enhanced so that High Performance Concrete (HPC) is now a reality. HPC is not a commodity but a range of products, each specifically designed to satisfy the performance requirements for different specific applications [1]. Beside this research activity, a growing interest on the possibility of using recycling wastes in concrete was observed [19] since this allows to save natural resources as conventional aggregates and to avoid to increase the dimensions of the landfill. In the European Union, the steel industry produces about 700,000 tons/year of Electric Arc Furnace (EAF) dusts [7]. The Waelz process can be applied to convert EAF dusts (with zinc concentration of 18-35%) into an impure zinc oxide, called Waelz oxide (with zinc concentration of 55-65%), that can be reprocessed in metallurgical plants. During the Waelz process, EAF dust and pellets are mixed with coke breeze (reducing agent) and other additives (lime or gypsum) and are continuously fed into the rotary kiln. The furnace temperature, 700-800 째C, allows a reduction and vaporization of zinc (and other volatile metals like lead and cadmium), and a consequent oxidation/condensation that generates an impure zinc oxide, called Waelz oxide. The final residue generated during this process is the Waelz slag that is classified as dangerous waste according to Italian Ministerial Decree 05/02/97 [13] and can be reused for metal recovery in secondary melting processes, according to Ministerial Decree 2002/161 [14]. Today, in Italy, this waste is disposed into IIB landfill (that in the future will be classified as dangerous waste landfill, according to Legislative Decree 2003/36 [15]). The interest of this work is to investigate the potential reuse of Waelz slag in partial substitution of the natural aggregate in concrete production. 2.
WAELZ PLANT
The Waelz slag studied in this experimentation was produced from the Waelz plant of Pontenossa (Bergamo), represented in Figure 1. This plant treats about 500-550 ton/day of EAF dust and produces about 120-130 ton/year of Waelz oxide (that is recovered in zinc processing industry) and 230-260 ton/year of Waelz slag (that is disposed into landfill). EAF dust and Waelz oxide chemical composition are represented in Table 1. The molten residue, containing low zinc and lead content, is water cooled at the outlet of the kiln to form the Waelz slag. Table 1: EAF dust and Waelz oxide metals composition (%) Parameter
EAF dust
Waelz oxide
Zinc
24.5
64.4
Lead
3.6
7.8
Cadmium
0.04
0.08
Iron
24.8
2.07
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Figure 1: Waelz plant of Pontenossa (Bergamo)
3.
MATERIALS AND METHODS
4.1 Raw wastes Two different types of Waelz slag were analysed in this study: the fresh slag, after water cooling, and the cured slag after a storage of about six months in landfill. These wastes were characterized according to the technical norms for natural aggregates (UNI 8520) in order to determine the granular distribution, specific weight and water absorption. The following chemical properties were also determined: chemical composition, leaching test, organic matter content and alkali reactivity. 4.2 Concrete mixtures The experimentation was developed in two phases, as indicated in Table 1: in the first one different concrete mixtures were prepared with a partial substitution of natural aggregate with fresh and cured Waelz slag with dosages of 200-400 kg/m3 and 300 kg/m3 of cement 32.5 R (type II/A-LL); in the second phase, only fresh slag-concrete was produced. 4.3 Concrete characterization Concrete specimens were characterized with physical (slump, specific weight, swelling, setting time), mechanical tests (according to the Italian Standard 91.100.30) and chemical tests (chemical composition and leaching behaviour) according to the Italian Ministerial Decree 98/72 [13]. Concrete mechanical characterization during the experimental phase 1 was performed by the determination of compressive strength after 7, 28 and 60 days of curing; in the second experimental phase the compressive strength was performed after 7, 28, 60, 90, 120 and 300 days; the traction and flexural strength and the modulus of elasticity were analysed after 28 days of curing.
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Some of the concrete mixtures, after 28 days of curing, were tested with a standardized leaching procedure for monolithic sample (Ministerial Decree n. 98/72 [13]): a cubic sample (side=150 mm) was dipped in the extractive solution (with liquid/solid ratio equal to 5 by volume), that was regularly renewed at 8 steps for 16 days. The final leaching concentration for each metal is calculated as the sum of the concentrations leached at each step. All the leachate solutions were analysed with ICP-plasma technique. Table 1: Composition of the adopted concrete mixtures Mixture
Recycled aggregate Type
[kg/m3]
Natural aggregate [kg/m3]
Cement Type
Superplasti Water/ Slump cizer cement
[kg/m3] [l/m3]
[mm]
st
1 experimental phase 1 Natural
-
-
1930
32.5
300
0.54
180
2
Fresh slag
200
1730
32.5
300
0.54
190
3
Fresh slag
400
1530
32.5
300
0.54
180
4
Cured slag
200
1730
32.5
300
0.54
180
5
Cured slag
400
1530
32.5
300
0.54
190
nd
2 experimental phase 6 Natural
-
-
1930
42.5
300
3
0.52
200
7
Fresh slag
200
1730
42.5
300
3
0.48
190
8
Fresh slag
400
1530
42.5
300
3
0.50
180
9
Fresh slag
600
1330
42.5
300
3
0.51
200
10
Fresh slag
400
1530
42.5
250
3
0.60
170
11
Fresh slag
400
1530
42.5
350
3
0.54
180
12
Fresh slag
400
1530
42.5
400
3
0.50
160
-
1930
32.5
300
3
0.49
18
400
1530
32.5
300
3
0.49
18
13 Natural 14 4.
Fresh slag
EXPERIMENTAL RESULTS
4.1 Raw wastes Wastes chemical composition, represented in Table 2, shows that both fresh and cured slag have a very high concentrations of iron, that is an important element of foundry fly ashes, and calcium, that is added as lime during the Waelz process. Zinc and lead present lower
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concentrations because they volatilises during the Waelz process; other metals, like manganese, cadmium, copper and arsenic, are present in low concentrations. Table 2: Chemical composition of the raw wastes Parameter
Fresh slag Cured slag Parameter
Fresh slag Cured slag
H2O (%)
7.00
5.72
Mn (%)
2.9
2.6
Zn (%)
5.75
6.30
Ni (%)
0.12
0.05
Pb (%)
1.2
1.3
Cd (ppm)
0.44
0.12
Fe (%)
43.6
42.4
Cr tot. (ppm)
6950
3940
Ca (%)
11.35
12.35
Cr VI (ppm)
<1.00
<1.00
Mg (%)
2.05
1.92
As (ppm)
59.3
49.9
Na (%)
0.78
0.25
Se (ppm)
11.2
11.0
K (%)
0.10
0.04
Hg (ppm)
0.75
0.53
-
Si (%)
3.67
3.20
Cl (ppm)
3545
4255
Al (%)
1.94
1.13
F- (ppm)
5930
4420
0.56
--
45600
35400
Cu (%)
0.60
SO4 (ppm)
4.2 Concrete mixtures The mean values of the compressive strength of concrete mixtures prepared for the initial phase of the research (phase 1) are represented in Figure 2. It can be noticed that the higher amount of waste reused in concrete mixture does not decrease concrete mechanical properties. Another important result is that all the concrete mixtures, after 28 days of curing, reach the minimum compressive strength of 15 MPa that is the minimum value for structural concrete in many building codes [EC2]. As no differences were observed between the two types of slag, during the second experimental phase only fresh slag was studied. The results represented in Figure 3a show that the mechanical properties of natural and wastecontaining concrete are very similar and that higher waste contents do not produce negative effects on concrete mechanical properties, as shown by comparing mixtures 6, 7, 8 and 9. The compressive strength increases with higher cement contents as shown by mixtures 10, 8, 11 and 12 (only mixture 12 shows an anomalous result). In all concrete mixtures with 300 kg/m3 of cement 42.5R (6, 7, 8 and 9), the compressive strength is higher than 35 MPa; in all the mixtures with 300 kg/m3 of cement 32.5R (13 and 14) the compressive strength ranges between 30 and 35 MPa. All the concrete mixtures increase their compressive strength with time, as shown for mixtures 6, 7, 8 and 9 in Figure 3b: the strength increase from 28 to 300 days of curing is about 30% for natural concrete and 20-35 and 45% respectively for mixtures 7, 8 and 9 (characterised by an increasing amount of Waelz slag.
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Figure 2: Compressive strength of concrete mixtures after 28 days of curing (1st Experim. Phase) 200 kg/m3 slag
30
400 kg/m3 slag 25
[MPa]
20 15 10 5 0 Natural concrete
Fresh slag
Cured slag
45
60 Compressive strength [MPa]
Compressive strength [MPa]
Figure 3: Compressive strength of concrete mixtures after 28 days of curing (a) and with time (b) (2nd Experimental Phase) 40 35 30 25 20 15 10 5
Concrete mixture
14
13 Natural
12
11
10
9
8
7
6 Natural
0
6 Natural
7
8
9
55 50 45 40 35 30 7
a
28
60
90
120
300
b
Curing time [day]
Figure 4: Tensile and flexural strength (a) and modulus of elasticity (b) of concrete mixtures after 28 days of curing (2nd Experimental Phase) Tensile strength Flexure strength
40000 Modulus of elasticity [MPa]
Tensile strength [MPa]
7 6 5 4 3 2 1
30000 20000 10000 0
0 6 Natural
7
8 9 Concrete mixture
13
14
a
6 Natural
7
8
9
Concrete mixture
13
14
b
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Also the tensile and flexural strength are not significantly influenced by the addition of waste that, in some cases, improves the mechanical properties of concrete (see the tensile strength in Figure 4a). As far as the modulus of elasticity is concerned, the waste addition produces a reduction ranging from 10 to 40% with respect to conventional concrete (with the exception of the anomalous value of the modulus of mixture 7), particularly for mixtures 13 and 14 (Figure 4b). The leaching test results on concrete cubic samples after 28 days of curing (Table 3) show that metal leaching is always lower than limit values of Ministerial Decree 98/72 [13]. In the leachate of natural concrete an higher concentration of arsenic is observed while the leachability for sulphate, chlorides and lead is higher for waste-containing concrete. For other elements, like copper, zinc, nickel and cadmium, the instrument detection limit is not so low to have a significant comparison among the different concrete mixtures. Table 3: Main experimental results from leaching tests Concrete Mixtures Cement
6 Natural 7
8
9
13 Natural 14
Limits
42.5
42.5
42.5
42.5
32.5
32.5
D.M 98
Fluoride
[mg/l] < 0.10
< 0.10
< 0.10
< 0.10
< 0.10
< 0.10
1.5
Sulphate
[mg/l] 1.2
1.6
1.6
1.6
1.2
1.6
250
Chloride
[mg/l] 3.5
3.5
5.3
5.3
3.5
5.3
200
Copper
[mg/l] < 0.005
< 0.005
< 0.005
< 0.005
< 0.005
< 0.005 0.05
Zinc
[mg/l] < 0.01
< 0.01
< 0.01
< 0.01
< 0.01
< 0.01
3
Nickel
[µg/l] < 1.00
< 1.00
< 1.00
< 1.00
< 1.00
< 1.00
10
Arsenic
[µg/l] 0.7
0.6
0.3
0.17
0.26
0.11
50
Cadmium
[µg/l] < 0.10
< 0.10
< 0.10
< 0.10
< 0.10
< 0.10
5
Chromium [µg/l] <1.00
<1.00
<1.00
<1.00
10
9.4
50
Lead
[µg/l] 0.4
0.3
0.8
1.1
1.3
1.3
50
Selenium
[µg/l] < 0.10
< 0.10
< 0.10
< 0.10
< 0.10
< 0.10
10
Mercury
[µg/l] 0.23
0.29
0.17
0.42
0.9
0.5
1
pH
-
5.
10.5-11.4 10.6-11.5 10.7-11.4 11.1-11.8 10.2-11.2 10.5-11.5 5.5<>12
CONCLUDING REMARKS
An experimental program was carried out on concrete specimens with the Waelz slag in partial substitution of natural aggregates. Specimens of a reference concrete with natural aggregates
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were also made. Mechanical tests were performed to determine the compressive and the tensile strength as well as the elastic modulus of concrete with Waelz slag. Leaching tests were also carried out on cubic samples after 28 days of curing. The experimental results show that Waelz slag can be successfully reused as aggregate for concrete mixtures. From the physical-mechanical characterization of concrete (compression, traction, flexure, modulus of elasticity) carried out after different curing times, it can be seen that the Waelz slag, also with a dosage of 600 kg/m3, does not influence the mechanical properties with respect to the reference concrete. The environmental compatibility of this “recycled” concrete is confirmed by leaching tests, which show a similar behaviour between the reference and the waste- containing concrete. In summary, this “recycled concrete” could be used for both structural and “non-structural” applications such as foundations, beams and columns (cast-in-place or prefabricated), manufacturing of manholes, curbs and pipes. Another important remark is that both natural and waste containing concrete improve their mechanical properties with time, till 300 days of curing. This demonstrates that concrete performance is not decreased by the use of Waelz slag. ACKOWLEDGEMENTS The Authors are grateful to engineer Rossana Stagnoli, for carrying out the experimental work, and to the technicians of the laboratory for testing materials of the University of Brescia for their assistance. A special acknowledgement goes to Dr Fabio Corazza of CTG Italcementi Group (Bergamo) and to the staff of Pontenossa Company for their collaboration in materials characterization. REFERENCES [1] Shah, S.P., and Ahmad, S.H., High Performance Concrete: Properties and Applications, Mc Graw Hill, 1994. [2] Artioli, A., ‘Recupero di materiali da rifiuti dell’industria siderurgica. I “fumi” d’acciaieria elettrica. L’esperienza CONSIDER’, Seminario ‘Il sistema produttivo e il recupero dei rifiuti’, Torino, 28-29 ottobre, 1998. [3] Associazione Italiana di Metallurgia, ‘Corso itinerante forno elettrico’, Milano, 22-23 febbraio, 2001. [4] Atzeni, C., Massida, L. and Sanna, U., ‘Use of granulated slag from lead and zinc processing in concrete tecnology’, Cement and Concrete Research (1996). [5] Al-Zaid, R. and Al-Sugair, F., ‘Investigation of potential uses of electric-arc furnace dust (EAFD) in concrete’, Cement and Concrete Research, 27, (1997), 267-278. [6] Burdese, A., ‘Metallurgia e tecnologia dei materiali metallici’, UTET Libreria (1992). [7] Barna, R., Ryong Bae, H., Mehu, J., ‘Assessment of chemical sensitivity of Waelz slag’, Waste Managemement, 20, (2000), 115-124. [8] Busè, R. and Ligabue, M., ‘Il recupero di piombo e zinco dai fumi d’acciaieria’, Convegno ‘Riutilizzo delle polveri d’acciaieria’, Brescia (1997). [9] Collepardi, M., ‘Scienza e tecnologia del calcestruzzo’, Editore Ulrico Hoepli Milano (1991).
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[10] Collivignarelli, C., Riganti, V., Sorlini, S. and Bertanza, G., ‘Reuse of stabilized wastes as “recycled aggregate” in concrete production’, Proceedings of the 12th IGWT Symposium Poznan-Gdynia, Poland, 5- 11 september (1999). [11] Fleischanderl, M., Sauert, F., Gebert, W. and Milani, F., ‘Zinc management and recycling within the iron and steel industy’, Seminario ‘Acciaierie ad impatto ambientale zero’, Brescia, 18-19 novembre (1999). [12] Italian Ministerial Decree 05/02/97 (1997) ‘Accomplishment of CEE directives n. 91/156/CEE concerning wastes, n. 91/689/CEE concerning hazardous wastes, and 94/62/CE concerning packaging wastes’. [13] Italian Ministerial Decree 98/72 (1998) for ‘Not dangerous wastes that can be reused through the simplified procedures as defined in n. 31 and 33 articles of Legislative Decree n. 22 of 5/2/97’. [14] Italian Ministerial Decree 2002/161 (2002) for ‘Dangerous wastes that can be reused through the simplified procedures as defined in n. 31 and 33 articles of Legislative Decree n. 22 of 5/2/97’. [15] Italian Legislative Decree 2003/36 (2003) for ‘Accomplishment of 1999/31/EC Directive for solid waste landfill’. [16] Luxan, M.P., Sotolongo, R., Dorremo, F., ‘Characteristics of the slags produced in the fusion of scrape steel by electric arc furnace’, Cement and Concrete Research, 30 (2000), 517-519. [17] Murat, M., ‘Effect of large additions of Cd, Pb, Cr, Zn, to cement raw meal on the composition and the properties of the clinker and the cement’, Cement and Concrete Research, 26 (1996), 377-385. [18] Olmo, I., Chachon, E. and Irabien, A., ‘Influence of lead, zinc, iron(III) and chromium (III) oxides on the setting time and strenght development of Portland cement’, Cement and Concrete Research, 31 (2001), 1213-1219. [19] Pedersini, L. and De Mirando, U., ‘Riutilizzo della scoria d’acciaieria da forno elettrico’, Seminario ‘Il sistema produttivo e il recupero dei rifiuti’, Torino, 28-29 ottobre (1998). [20] prEN 1992-1-1, “Eurocode 2 - Design of Concrete Structures - Part 1-1: General rules and rules for buildings”, draft, April 2003. [21] Wascon 2003 ‘Fifth International Conference on the Environmental and Technical Implication of Construction with Alternative Materials’, 4-5 June (2003), San Sebastiàn, Spain.
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