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Evaluation of the steel slag incorporation as coarse aggregate for road construction: technical requirements and environmental impact assessment
Journal of Cleaner Production xxx (2015) 1e12
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Evaluation of the steel slag incorporation as coarse aggregate for road construction: technical requirements and environmental impact assessment ez-De-Guinoa Vilaplana a, Tatiana García-Armingol b, Victor J. Ferreira a, Aitana Sa c lez d, Ana M. Lo pez-Sabiro n a, n , Cristina Lausín-Gonza Alfonso Aranda-Uso a , * n Ferreira Germa mez, 15, 50018 Zaragoza, Spain Research Centre for Energy Resources and Consumption (CIRCE), CIRCE Building, Campus Río Ebro, Mariano Esquillor Go mez, Research Centre for Energy Resources and Consumption (CIRCE) e Universidad de Zaragoza, CIRCE Building, Campus Río Ebro, Mariano Esquillor Go 15, 50018 Zaragoza, Spain c University of Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain d ArcelorMittal Global R&D, Avil es 3340, Spain a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 March 2015 Received in revised form 21 July 2015 Accepted 23 August 2015 Available online xxx
This work analyses the environmental impact of using electric arc furnace slags as secondary raw material in pavement and its comparison with the traditional materials used in road construction. Chemical and technical evaluations of the main characteristics of the black slags as coarse aggregate were carefully developed. The environmental analysis was carried out by using the Life Cycle Assessment methodology. The Life Cycle Inventory data was processed to obtain emissions grouped in terms of impact categories based on the Centre of Environmental Science of Leiden University baseline method at midpoint level. The results obtained revealed that some of the most relevant environmental impacts, such as carbon footprint, abiotic depletion, ozone layer depletion and photochemical oxidation, depend highly on the road construction processes, although, in the two scenarios analysed, the bitumen production was demonstrated to be the most contributing stage. These indicators also concluded that important environmental benefits could be obtained from the use of black slag as course aggregate in road construction. Consequently, the results shown here could be added to the list of technical criteria for their inclusion into a multi-objective optimisation methodology. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Life cycle assessment (LCA) Steel slag Electric arc furnace (EAF) Hot bituminous mixture Road construction
1. Introduction Steel industry is one of the major responsible of anthropogenic CO2 emissions. But, in order to mitigate the climate change, the reduction of these emissions is not easy due to the high dependence on electricity and coal (Eloneva et al., 2010). Nowadays, most steel is produced either via two main industrial routes: the integrated process, which uses coal as reducing agent and iron as raw material or the Electric Arc Furnace (EAF) route, where electricity is used to melt steel into the end product. The second route, also named secondary production since it uses steel
scrap, is a less energy intensive process and it is shown as a promising alternative to be close to a theoretical zero CO2 emission (Morfeldt et al., 2014). Apart from the energy consumption associated to the production processes, the increase of steel consumption has also yield to an important growth of the volume of residues. Steelmaking residues are defined as by-products obtained from the conversion process of iron to steel. In particular, steel slag represents a significant proportion of these by-products (Proctor et al., 2000); for instance, the production of 3 tonnes of stainless steel is estimated to generate about 1 tonne of stainless steel slag. Consequently, a large amount of steel slag is yearly produced in the world (around fifty million tonnes per year).
* Corresponding author. Tel.: þ34 976761863; fax: þ34 976732078. E-mail address: [email protected] (G. Ferreira). http://dx.doi.org/10.1016/j.jclepro.2015.08.094 0959-6526/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Ferreira, V.J., et al., Evaluation of the steel slag incorporation as coarse aggregate for road construction: technical requirements and environmental impact assessment, Journal of Cleaner Production (2015), http://dx.doi.org/10.1016/ j.jclepro.2015.08.094
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V.J. Ferreira et al. / Journal of Cleaner Production xxx (2015) 1e12
Nomenclature ADTHv CML EAF HF JCPDS LCA LCI LF LOI MB ZA
average daily traffic of heavy vehicles Centre of Environmental Science of Leiden University electric arc furnace concrete road surface Joint Committee on Powder Diffraction Standards life cycle assessment life cycle inventory ladle furnace loss on ignition bituminous mixtures artificial gravel
The huge amount of generated slags and their environmental impact have prompted scientists and engineers to work on novel solutions, based on industrial more friendly concepts that allow using these residues as raw material for pavements (RaCenovic et al., 2013). Steel slags can be used in road construction in order to replace natural aggregates, which reduces the environmental impact by reducing the consumption of natural and non-renewable aggregates and the quantity of slag deposited on landfill sites, both in asphalt (Mladenovi c et al., 2015) and cement based roads (Saezde-Guinoa Vilaplana et al., 2015). However, technical and environmental studies are required to determine the technical feasibility and the potential environmental benefits. Two types of slag are produced in the EAF steelmaking process: EAF slag, also called black slag, and Ladle Furnace (LF) slag, also known as secondary refining slag or white slag (RaCenovi c et al., 2013). In this work, the scenarios analysed are focused on the slags obtained in EAF, where about 40% of the global steel production takes place (Muhmood et al., 2009). Although, different kind of recycled materials can be used as aggregates in road construction, it is crucial to know the properties that such aggregates have in order to ensure that they meet the same quality standards as natural aggregates (Mladenovic et al., 2015). In particular, EAF slag has been frequently used as pavement aggregate due to their excellent mechanical properties, which make themselves suitable for asphalt layers with any kind of traffic load. For example, EAF slags-derived aggregates improve the skid resistance of the pavement and they also reduce the risk of aquaplaning due to their higher permeability (Liapis and Likoydis, 2012). To address the mentioned above strategy, this work has been focused on technical analysis and environmental evaluations. On the one hand, in order to analyse if black slags meet the technical requirements to be introduced as coarse aggregate in hot bituminous mixture for different course layers associated to the heavy traffic categories, the results of a comprehensive chemical analysis were compared with the typical certain oxide ranges found in the literature (Sofili c et al., 2011), which have been reported as consequence of the quality of steel produced. On the other hand, in order to estimate the environmental performance of the steelmaking black slag as secondary raw material for the road construction, the Life Cycle Assessment (LCA) methodology was applied. The LCA is a methodology widely accepted to be used for analysing environmental impacts of products and processes (Society of Environmental Toxicology and Chemistry (SETAC), 1993). Previous works, such as Rebitzer et al. (2004), have technically and scientifically demonstrated the viability of using LCA to characterise the environmental implications of a wide range of industrial activities.
In addition, its use is also strongly encouraged by European Union policies and regulation, i.e., the European Action Plan on Sustainable Consumption and Production and Sustainable Industrial Policy (COM(2008) 397), the ETAP action Plan (COM(2004) 38 final), etc. Previously, the impacts of the road construction have been assessed using LCA methods but most of these studies have only analysed the environmental impact considering the use of waste ttir, 2005) and crushed concrete associated such as fly ash (Birgisdo with traditional reinforced concrete and asphalt pavements (Rajendran and Gambatese, 2007). However, there is a lack of studies about steelmaking black slag as a coarse aggregate in hot bituminous mixture quantifying the environmental performance using the LCA methodology. Only Mladenovi c et al. (2015) address the environmental analysis of a road aggregate based on EAF slags and its comparison with natural aggregates but, in contrast to the analysis shown in the research, they do not take into account the chemical and physical characterisation of the slags. Furthermore, to the authors' knowledge, there is no scientific literature focused on a particular environmental evaluation using Spanish conditions, consequently, this study represents a relevant contribution. The main objective is, therefore, to characterise the Spanish steelmaking black slag to study its environmental performance taking into account the technical specifications for road and bridge works, focussing on heavy vehicles requirements, according to technical specifications (UNE-EN 933-2, 1996). 2. Methodology 2.1. Steel black slag characterisation for road construction 2.1.1. Chemical characterisation of the steel black slag The studied steel slag was characterised using X-ray fluorescence spectrometry to determine the chemical composition, while X-ray diffraction technique (XRD) was used to identify crystalline phases in it. To this end, diffraction patterns were measured in the 2Ɵ range of 5e80 using CuKa radiation. 2.1.2. Technical specifications of steel slag as coarse aggregate in hot bituminous mixture The hot bituminous mixture is defined as a type of asphalt in which hydrocarbon ligand and aggregates (including mineral powders) are combined. The aggregate acts as the structural skeleton of the pavement whereas the hydrocarbon ligand covers homogeneously all particles of aggregates, acting as the glue of the mixture. The use of the steel slag as coarse aggregate in the hot bituminous mixture was evaluated in this study by comparing the results obtained from tests with the specific technical requirements of coarse aggregate for road and bridge works in Spain (PG-3). According to the standard classification of the geometrical properties of aggregates (UNE-EN 933-2, 1996), a coarse aggregate is defined as the fraction of the total amount of aggregate retained on the sieve of 2 mm. The specific requirements of the aggregates depend on the category of the road and the Average Daily Traffic of Heavy Vehicles (ADTHv), classified according Table 1. In particular, in base course layers for categories of heavy traffic denoted as T00 and T0, the coarse aggregate cannot be obtained by crushing gravel from granular deposits of limestone quarries and nature. By the contrary, in the case of base course layers corresponding to the T1 and T2 heavy traffic, material from crushing natural gravel can be used providing the particle size of the used coarse aggregate before its crushing was 6 times greater than the maximum size of the final aggregate.
Please cite this article in press as: Ferreira, V.J., et al., Evaluation of the steel slag incorporation as coarse aggregate for road construction: technical requirements and environmental impact assessment, Journal of Cleaner Production (2015), http://dx.doi.org/10.1016/ j.jclepro.2015.08.094
V.J. Ferreira et al. / Journal of Cleaner Production xxx (2015) 1e12
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Table 1 Heavy traffic categories. Categories of heavy traffic
T00
T0
T1
T2
T31
T32
T41
T42
ADTHv (heavy vehicles/day)
4000
<4000 2000
<2000 800
<800 200
<200 100
<100 50
<50 25
<25
Therefore, the coarse aggregate displays typical characteristics related to: the differentiated size fractions for collecting and management of aggregates separately for their introduction in the cold hoppers, the physicochemical weathering appreciable alteration under the most unfavourable conditions that could occur in the working area. Thus, the coarse aggregate characteristics must meet different requirements depending on its final use. In order to examine some of these characteristics, the following tests were carried out: a) Angularity of coarse aggregate (for particles partially or completely crushed), b) Shape of the coarse aggregate (flakiness index test procedure), c) Resistance to fragmentation (Los Angeles coefficient), d) Resistance to polishing of the coarse aggregate in base course layers (Accelerated polished coefficient); and, e) Coarse aggregate washing (content of impurities). 2.2. Environmental analysis of steelmaking slag as coarse aggregate in road construction The environmental analysis of steelmaking slag as a raw material in hot bituminous mixture manufacturing was carried out by means of the LCA methodology. This methodology is defined as pez-Sabiro n et al., 2014) where the iterative among its phases (Lo old data will be replaced with new information leading to a more realistic evaluation. The most up-to-date structure of the LCA is proposed by the standard 14040 (ISO 14040, 2006). As can be observed in Fig. 1, this methodology can be synthesised in four main phases as is also reported by Khasreen et al. (2009) or Udo de Haes and Heijungs (2007). The first of them is focused on defining the objective of the study, the functional unit and the limits of the system involved (scope). After the first step, Life Cycle Inventory (LCI) is devoted to gather all the required data and the calculation procedures aimed
Inventory Analysis (LCI) Datagathering & Modelling
Goal & scope definition
Interpretation
Environmental Impact assessment And results
Fig. 1. Main phases of an LCA study.
to quantify the relevant inputs and outputs of the production system. This process is iterative and may be repeated if further information is required during its implementation. The system under study must be modelled as a complex sequence of unitary operations that share information among themselves and with the environment through inputs and outputs. During the third phase of the study, the evaluation of the significant potential environmental impacts, using the data from the inventory phase, is carried out. Here, the level of detail, the selection of the impacts to evaluate and the methods of evaluation depend on the objectives and scope of the study. Finally, the last phase is focused on the life cycle interpretation. This phase consists in the interpretation of the results from the inventory phase and from the evaluation of impacts, as well as the eventual compiling of conclusions and recommendations for the improvement of the environmental performance of the system under study.
2.2.1. Scope of the analysis and functional unit In this study, the identification of an alternative raw material to be used as coarse aggregated (black slag), including a thorough analysis of technical specifications, is a potential goal. In this case, the product to be analysed is a new type of pavement which contains black slag as coarse aggregate in asphalt mixture. The life cycle of a product can generally be subdivided into three phases of ‘production’, ‘use’ and ‘disposal’, where final energies are involved. This work is only focused on the construction stage of the pavement. Therefore, the use and disposal stages are excluded from the LCI. The environmental impact assessment was carried out by midpoint approach. For this purpose, CML method was used. Ac n et al. (2013), midpoint cording to Guinee (2001) and Aranda-Uso indicators are considered to be points in the causeeeffect chain (environmental mechanism) of a particular impact category somewhere between stressor and endpoints. For such midpoint indicators, the characterisation factors can, therefore, be calculated to reflect on the importance related to an emission or extraction in a LCI by environmental metrics. Thus, 9 category metrics, namely abiotic depletion, acidification, eutrophication, global warming (considering global warming potential to 100 years), ozone layer depletion, human toxicity, aquatic toxicity, territorial ecotoxicity and photochemical oxidation have been emphasised and fully analysed. In addition, a method to calculate the cumulative energy demand has been used. This method provides the total energy demand, expressed in terms of primary energy which is associated with the energy expenditure for the energy supply in the entire life cycle of a product or service. Finally, the functional unit was defined then as 1 m2 of pavement.
2.2.2. System description and boundaries The study is mainly focused on the road construction stage (bounded by the dotted lines in Fig. 2), where large amounts of aggregates, representing more than 90 wt.% of the bituminous mixture, are required. Its implementation includes the following operation phases:
Please cite this article in press as: Ferreira, V.J., et al., Evaluation of the steel slag incorporation as coarse aggregate for road construction: technical requirements and environmental impact assessment, Journal of Cleaner Production (2015), http://dx.doi.org/10.1016/ j.jclepro.2015.08.094
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V.J. Ferreira et al. / Journal of Cleaner Production xxx (2015) 1e12
Fig. 2. Description of the system boundaries.
Preparation and testing of the surface settlement, Material input, Extension, wetting, if applicable, and compaction of each pouring section, and Refining of the surface of the last poured section. Several categories of heavy traffic are considered in the study. Based on the categories shown in Table 1, the study is focused on the range of the categories of heavy traffic from T2 to T4, which can use roads made from recycled granular materials, steel materials, by-products and inert waste products, as long as the required technical requirements are met and the origin of the materials is declared. In addition, the thickness of each layer should be taken into account depending on the esplanade (E1 e acceptable esplanade, E2 e good esplanade and E3 e very good esplanade). These thicknesses are shown in Table 2. The layers are denoted according to the following references: MB e bituminous mixtures HF e concrete road surface ZA e artificial gravel
Additionally, there are several types of road surfaces for each heavy traffic category and esplanade. They are classified according to the last number of their code: 1: Bituminous mixtures on granular layer, 2: Bituminous mixtures on soilecement, 3: Bituminous mixtures on gravelecement soilecement, 4: Concrete paving.
built
over
Table 2 shows the profiles of required thicknesses for each base layer depending on the esplanade quality, the heavy traffic, the kind of pavement, and the bituminous mixtures or concrete. A profile 3221 (from Table 2), i.e. 35 cm of artificial gravel and 15 cm of bituminous mixture, corresponding with the thickness of good esplanade (E2) and the heavy traffic category T32 (between 50 and 100 vehicles per day) paved with bituminous mixtures, was selected as the reference configuration for both study cases. 2.2.3. Life cycle inventory (LCI) As mentioned above, the stages of maintenance and disposal have been excluded from the environmental analysis and only the
Please cite this article in press as: Ferreira, V.J., et al., Evaluation of the steel slag incorporation as coarse aggregate for road construction: technical requirements and environmental impact assessment, Journal of Cleaner Production (2015), http://dx.doi.org/10.1016/ j.jclepro.2015.08.094
V.J. Ferreira et al. / Journal of Cleaner Production xxx (2015) 1e12
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Table 2 Thicknesses for each layer according to the esplanade and heavy traffic category. Heavy traffic category T2 Esplanade thicknesses
T31
T32
T41
T42
E1
E2
E3
transparent and made in house data base, for creating specific modules for this LCA study.
stage of the road construction has been considered. Thus, the environmental evaluation is focused on the phases of raw material supplied from natural resources, transport and processing up to the road construction, such as shown in Fig. 2. Firstly, a base case, where natural aggregates are used (Case A), is analysed. Then, the substitution of 75 wt.% of natural aggregates by black steel slag is studied (Case B). Additionally, both the direct energy input for the road construction and the energy involved in the processes of raw materials production (including mining and transformation) and transport process are considered. Table 3 shows the amount of the main materials considered in the LCI. The LCA methodology is then performed through the LCI to define and quantify energy, resources and material flows within the road construction process and the application of the case studies under review to be converted into the environmental metrics categories shown in Table 4. These categories were chosen as environmental burdens associated with the life cycle of the system considered in the context of this study. Finally, the environmental analysis by means of the LCA methodology application is performed using SIMAPRO v7.3 software containing data bases, which were carefully selected and contrasted to own LCI information to generate a relevant, reliable,
2.2.4. Cut-off criteria This research follows the cut-off criterion used by authors in previous studies (Ferreira et al., 2015) to ensure that all relevant environmental impacts were represented in the study. In particular, flows of materials less than 1% of the cumulative mass of all the inputs and outputs depending on the type of flow of the LCI model have been excluded their environmental relevance is not a concern. However, if the sum of the neglected material flows exceeded 5% of mass, energy or environmental relevance, it should be taken into account in the analysis. 3. Results 3.1. Evaluation of steel slag for road construction 3.1.1. General characterisation of black slag The X-ray fluorescence spectrometry was used to investigate the chemical composition of steel slag. The results, shown in Table 5, demonstrate that the typical oxides, SiO2, Al2O3, Fe2O3 and CaO, are
Table 3 Main inputs and outputs for Life cycle inventory. H
Case A Thickness cm
Case B Aggregate kg
Water L
MB e 15 325 e ZA e 35 470 100 Road transport to move raw materials* Distances 40 km 10 km
Bitumen kg
Thickness cm
Aggregate kg
Water L
Bitumen kg
17 e
MB e 15 ZA e 35
81 470
316 100
21 e
500 km
Distances
40 km
10 km
500 km
*Truck mileages > 16t with typical average of the European fleet have been assumed for transportation evaluation.
Please cite this article in press as: Ferreira, V.J., et al., Evaluation of the steel slag incorporation as coarse aggregate for road construction: technical requirements and environmental impact assessment, Journal of Cleaner Production (2015), http://dx.doi.org/10.1016/ j.jclepro.2015.08.094
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V.J. Ferreira et al. / Journal of Cleaner Production xxx (2015) 1e12
Gehlenite: Ca2Al2SiO7 (JCPDS 00-020-0199) Periclase: MgO (JCPDS 00-001-1235)
Table 4 Impact category metrics. Impact category
Unit
Abiotic depletion Acidification Eutrophication Global warming Ozone layer depletion Human toxicity Aquatic toxicity Territorial ecotoxicity Photochemical oxidation
kg kg kg kg kg kg kg kg kg
Sb eq SO2 eq PO3 4 eq CO2 eq CFC-11 eq 1,4-DB eq 1,4-DB eq 1,4-DB eq C2H4 eq
present in the chemical composition. As can be observed, the Fetotal and CaO are 25 and 22 wt.% respectively, whereas the contents of Si, Al and Mg are in smaller proportion, which is in agreement with the results reported in the specialised technical reports (BSE: Badische stahlengineering GMBH, 2015) and previous works focused on the use of electric arc furnace steel slag as raw material (Iacobescu et al., 2013). In addition, the composition of the slag studied are in the ranges of the typical values of black slag considering the same quality of steel produced (Sofili c et al., 2011), namely, SiO2 (6.5e35%), Al2O3 (1e13.44%), Fe2O3 (1e31.2%) and CaO (18.4e60.0%). Additionally, the XRD was used as a technique to complement the X-ray fluorescence spectrometry for the identification of the present mineral phases. Diffraction data were processed by specialised software, and specific recorded relative intensities of standard X-ray diffraction lines (according to the Joint Committee on Powder Diffraction Standards (JCPDS)) were marked on the XRD pattern of black slag (see Fig. 3). Thus, the following components were found by the XRD analysis of the black slag:
Calcium aluminate: CaAl2O4 (JCPDS 00-034-0440) Wuestite: FeO (JCPDS 01-089-7100) Silicate of Magnesium/Iron: (Mg,Fe)2SiO4 (JCPDS 00-021-1258) Calcium and Aluminium Silicate: Ca2AlSiO5.5 (JCPDS 00-0470699) Calcium ferrate: Ca2Fe2O5.12 (JCPDS 00-049-1555) Table 5 Chemical composition (dry matter %) of steelmaking black slag. Compounds
Black slag
Fe total SiO2 Mn CaO MgO Al2O3 K2O Na2O S P Zn Pb TiO2 Cl Cr2O3 F LOIb
25.32 15.64 4.60 22.39 7.96 12.14 0.02 0.22 0.07 0.10 0.75 0.01 0.75 0.03a 0.81a e 3.06
a
Semi-quantitative results. Loss-on-ignition. It can be noted that the Loss on Ignition (LOI) parameter exhibited a negative value because the method of calculation used is very susceptible to atmospheric moisture content-negative weight losses (i.e. weight grains). b
The XRD analysis confirmed that the slag mainly consisted of metal oxide in various complex oxides, silicate forms and calcium aluminate. These results of the XRD analysis could be comparable with the results published by several authors (Iacobescu et al., 2013; Kavussi and Qazizadeh, 2014; Waligora et al., 2010). The mineralogical composition of steel slags and X-ray diffraction analysis of slag samples was performed by these authors. Their results, including the ones of this study, reveal to have usually complex structure, which lead many overlapping peaks reflected from crystalline phases present in steel slags. These phases are formed mainly due to the chemical composition of slag and the slow cooling rate applied during processing (Tossavainen et al., 2007). 3.1.2. Technical specifications of steel slag as coarse aggregate in hot bituminous mixture According to the methodology described above, the test results associated with the physicochemical weathering, summarised in Table 6, revealed that the values are below maximum values established by the respective norm. These results suggest that the soluble components, which can leach by the black slag, are not a potential risk to the environment or to the building elements placed in surrounding areas. Table 7 summarises the test results of technical specifications associated to the typical characteristics of coarse aggregates to be used in different course layers. These results show that coarse aggregates meet the technical specifications required. In particular, the results show that the black slag can be recommended to be used as a coarse aggregate in hot bituminous mixtures in the type of base layer for the T2 and T3 heavy traffic categories, since most of the tests were positive for those categories. Moreover, the slag could be also used in the type of surface course layer in the T3 and T4 categories. Thus, these latter results ensures that the main characteristics of the coarse aggregate are maintained during the collecting and management of the aggregate, when they are produced or supplied separately in differentiated size fractions up to their introduction in the cold hoppers. Nonetheless, the content of impurities of the black slag samples was 1.2 wt.%, higher than the maximum value established in the norm (Zheng et al., 2013) (<0.5 wt.%). It is recommended to perform a washing, aspiration or another cleaning method prior to the use of black slag as coarse aggregate in hot bituminous mixtures. Slag as raw material in asphalt mixture for road construction shows some disadvantages associated to the some free lime (fCaO), which can be hydrated causing volume instability and porosity of slags, leading an excessive asphalt absorption caused by pores, and therefore, high cost. Nonetheless, it has been largely reported in literature that the introduction of steel slag, as coarse aggregate in asphalt mixture, can improve the mechanical characteristics such as the stability and the deformation resistance (Ahmedzade and Sengoz, 2009) or increase the fatigue life of the asphalt mixtures due to the need of using a higher binder content (Arabani and Azarhoosh, 2012). In addition, the slag use also enhances other pavement performances, such as the drainage with permeability (Shen et al., 2009) and the moisture stability (Xie et al., 2012) among other important properties, namely, skid resistance (Shen et al., 2009), deformation at high temperatures resistance (Wu et al., 2007) and cracking and stripping resistance, avoiding early the damage of the pavement (Xue et al., 2006). Therefore, all these results and those obtained in this study show a clear feasibility of using steel slag coarse aggregate in asphalt mixture.
Please cite this article in press as: Ferreira, V.J., et al., Evaluation of the steel slag incorporation as coarse aggregate for road construction: technical requirements and environmental impact assessment, Journal of Cleaner Production (2015), http://dx.doi.org/10.1016/ j.jclepro.2015.08.094
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Fig. 3. XRD pattern of black slag.
Table 6 Results obtained for some of the characteristic parameters of black slag samples associated to the physicochemical weathering. Parameters Sand equivalent pH COD Phenols Sulphates Chlorides Fluorides Antimony Molybdenum Arsenic Barium Cadmium Zinc Cupper Chrome Mercury Nickel Lead Selenium
Results 90 8.35 14.2 <0.1 588 10 7.7 <0.004 <0.1 0.011 8.2 <0.04 <0.1 <0.1 <0.1 <0.002 <0.1 <0.1 <0.01
Units
Norm
e
UNE-EN 933-8 (2012) UNE-EN 1744-3 (2003)
Uds. pH mg/kg mg/kg SO2 4 mg/kg Cl mg/kg F mg/kg Sb mg/kg Mo mg/kg As mg/kg Ba mg/kg Cd mg/kg Mg Zn/kg Cu mg/kg Cr mg/kg Hg mg/kg Ni mg/kg Pb mg/kg Se mg/kg
Legal limits <50 e <500 <1 <1000 <800 <10 <0.06 <0.5 <0.5 <20 <0.04 <4 <2 <0.5 <0.01 <0.4 <0.5 <0.1
3.2. Environmental analysis As mentioned above, from the technical point of view, the use of black slag as coarse aggregate concluded to be workable in asphalt mixture for road construction. However, this study has an additional goal compared to most studies found in literature, which consists in the development of a comprehensive environmental assessment using the methodology described in previous sections. To this end, the study is focused in two cases. The first one, named case A, using natural coarse aggregate and case B incorporating 75 wt.% of black slag in the asphalt mixture. In both cases, the coarse aggregate meets the technical specifications summarised in the UNE-EN 933-5 (1999) and UNE-EN 933-8 (2012) and described in Tables 6 and 7. 3.2.1. Evaluation of case study A Initially, a thorough analysis of the main contributions to the total environmental impact was performed. In this sense, the transport stage and the consumption of raw materials as bitumen, aggregates, water or diesel to produce energy were considered and
evaluated through the LCA methodology. The total environmental impact associated to case A for the different environmental indicators considered in this research is depicted in Table 8. Additionally, Fig. 4 shows the main results obtained related to the relative impacts between the previous parameters associated to this case study. On the one hand, as can be seen in Fig. 4 and in agreement with Mladenovi c et al. (2015), the highest impact is allocated in the bitumen consumption, which is close to 86% of the total impact in case of abiotic depletion or ozone layer depletion indicators and from 44% to 76% in case of the rest of impact categories. This result can be attributed to the relative impact associated to the bitumen production, since it is based on the refining process of heavy crude oils with high sulphur content. On the other hand, the transport and the aggregate consumption stages are the second highest contribution depending on the indicator considered. Whereas the aggregates are higher than transport in human toxicity and ecotoxicity categories and the transport stage shows a greater impact in case of abiotic depletion, acidification and global warming; both display a quite similar impact for eutrophication and photochemical oxidation. Finally, the water consumption exhibits the lowest environmental impact in this case study. Regarding the global warming indicator, measured as 18.2 kg CO2 eq, the highest impacts are reflected again by the bitumen consumption, which can be associated to the crude oil production as well as the gas burned during the refinery process. In case of the transport, the most relevant contribution to global warming is related to the operation stage due to the fossil fuel consumption. Finally, concerning the aggregate consumption, the most important stage is the energy consumption during the crushing process. Taking into account the previous results where the bitumen consumption was concluded to have the highest impact, the construction of the surface layer, mainly composed by bitumen, is therefore, the most relevant phase in the road construction, as it can be seen in Fig. 5. In addition, the surface layer construction for the T3 and T4 heavy traffic categories shows environmental impacts between 66% and 92% of the total environmental impact, in comparison to the base layer construction. Additionally, Table 9 shows the main results obtained for the evaluation of the accumulative energy demand along the life
Please cite this article in press as: Ferreira, V.J., et al., Evaluation of the steel slag incorporation as coarse aggregate for road construction: technical requirements and environmental impact assessment, Journal of Cleaner Production (2015), http://dx.doi.org/10.1016/ j.jclepro.2015.08.094
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V.J. Ferreira et al. / Journal of Cleaner Production xxx (2015) 1e12
Table 7 The technical specifications associated to the typical characteristics of the coarse aggregate to be used in layers of different base course layers (Note that (*) means “Service road”). Angularity of coarse aggregate (total or partially crushed particles) UNE-EN 933-5 (1999) Type of layer
Heavy traffic category
Surface course Binder course Base
100
T00
T0 and T1
T2
90
100
Black slag sample (result)
T3 and berms
T4
90
75 75 (*)
75
Angularity of coarse aggregate (rounded particles) UNE-EN 933-5 (1999) Type of layer
Heavy traffic category
Surface course Binder course Base
0
T00
T0 and T1
Black slag sample (result)
T2
1
0
99
T3 and berms
T4
1
10 10 (*)
1
10
Flakiness index of coarse aggregate UNE-EN 933-8 (2012)
Black slag sample (result)
Heavy traffic category T00
T0 to T31
T3 and berms
20
25
30
T4 2
Los Angeles coefficient UNE-EN 1097-2 (2010) Type of layer
T00 and T0 Surface course Binder course Base
Black slag sample (result)
Heavy traffic category T1
20 25 25
T2
T3 and berms
T4
25
27 25 (*)
30
Accelerated polished coefficient UNE-EN 1097-8 (2000)
Black slag sample (result)
Heavy traffic category T00 and T0
T0 to T31
T3, T4 and berms
56
50
44
cycle of the road construction. Again, as it happened during the environmental evaluation, the bitumen consumption in the surface layer construction reveals the highest energy demand values. 3.2.2. Evaluation of case study B As mentioned before, the best option to introduce the black slag into the road construction seemed to be using it as an arid component in the base and surface layer from T2 to T4 heavy traffic categories; since the final mixture enhances the wear resistance properties in comparison to natural aggregates, according to the technical characterisation results obtained. However, some works, such as those included in the review made by Huang et al. (2007) have reported the successful Table 8 Total environmental impact associated to case A. Impact category
Unit
Total
Abiotic depletion Acidification Freshwater eutrophication Global warming Ozone layer depletion Human toxicity Freshwater ecotoxicity Terrestrial ecotoxicity Photochemical oxidation
kg kg kg kg kg kg kg kg kg
0.47 0.156 0.032 18.2 1$105 12.4 3.22 0.072 0.008
Sb eq SO2 eq PO4 eq CO2 eq CFC-11 eq 1,4-DB eq 1,4-DB eq 1,4-DB eq C2H4 eq
1
application of black slag in road construction as aggregate in bituminous surface course mixtures. In addition, it must be noted that the basic character of the black slag also improves the adhesion with conventional bitumen. Thus, only the application of black slag as aggregate in bituminous surface course mixtures in road construction is considered in this case study B. This is the reason why in LCI (Table 3) the incorporation of black slag (75 wt%.) as aggregate is just considered in the surface layer (MB-15). Similar to case A, the case study B was analysed to determinate the main contributions of the different parameters considered in the total environmental impact. Besides the parameters analysed in case A related to the transport stage and the consumption of raw materials, other relevant parameters were also included in the evaluation. In particular, the use of black slag requires a previous conditioning process but it also implies a saving of raw materials at the same time that it avoids the use of landfills due to recovering of the black slag. The total environmental impact associated to case B for the different environmental indicators considered in this research can be observed in Table 10. Additionally, Fig. 6 shows the main results related to the relative impacts associated to each stage considered in the analysis. On the one hand, the results reveal that environmental benefits can be obtained by avoiding the landfill use and raw material consumption for all the impact categories, the highest impact is due to, again, the bitumen consumption, especially in case of abiotic depletion or ozone layer depletion indicators. Therefore, as
Please cite this article in press as: Ferreira, V.J., et al., Evaluation of the steel slag incorporation as coarse aggregate for road construction: technical requirements and environmental impact assessment, Journal of Cleaner Production (2015), http://dx.doi.org/10.1016/ j.jclepro.2015.08.094
V.J. Ferreira et al. / Journal of Cleaner Production xxx (2015) 1e12
Fig. 4. Environmental evaluation of case A. Relative contribution of the different stages.
Fig. 5. Environmental evaluation of case A. Relative contribution of the different road layer construction.
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Table 9 Accumulative energy demand of case A. Energy
Total
Bitumen
Aggregates
Water
Machinery
Transport
MJ %
1122 e
890 79
109 8
0.6 0.1
25 2.2
97 8.7
Table 10 Total environmental impact associated to case B. Impact category
Unit
Total
Abiotic depletion Acidification Freshwater eutrophication Global warming Ozone layer depletion Human toxicity Freshwater ecotoxicity Terrestrial ecotoxicity Photochemical oxidation
kg kg kg kg kg kg kg kg kg
0.521 0.151 0.029 16.6 z0 11.3 2.88 0.066 0.008
Sb eq SO2 eq PO4 eq CO2 eq CFC-11 eq 1,4-DB eq 1,4-DB eq 1,4-DB eq C2H4 eq
explained in case A, the energy consumed and the different processes involved in the production of bitumen are the main contributor to the environmental impact associated to the road construction, which, again, is in qualitative agreement with the results previously reported (Mladenovi c et al., 2015). In addition, the transport and the aggregate consumption stages remain as the second highest contribution depending on the indicator analysed. On the other hand the relative impact of a new stage should be carefully considered. This stage is related to the conditioning of the black slag which, although it had not been assigned
an environmental impact since it was a residue, must be crushed before being used in road construction causing some environmental affection. In this case B, the carbon footprint was quantified as 16.6 kg CO2 eq, maintaining the most relevant contributions for each singular stage similar to those obtained for the case A. Furthermore, regarding the black slag consumption, the most relevant contribution is the energy consumption associated to the crushing process. Finally, the accumulative energy demand evaluation for the road construction was also performed in this case and the main results are shown in Table 11 expressed in terms of primary energy. As in the environmental evaluation, the bitumen consumption used in the surface layer construction reveals the highest energy demand values. 3.2.3. Comparison of case A and case B After analysing individually both case studies, a comparative evaluation was carried out. For this purpose, Fig. 7 depicts the relative variations obtained between case A (reference) and case B, where negative variations means a reduction of the environmental impact in case B respected to case A, whereas, positive variations indicate an increment of the environmental impact when black slag was incorporated as raw material. The results conclude that most of the indicators, except abiotic depletion, ozone layer depletion and photochemical oxidation, exhibited a reduction when case B was considered. This fact can be mainly attributed to the bitumen quantities used in each situation, which are higher in case B than in case A. This result is in partial agreement with a previous work (Mladenovi c et al., 2015), where only the abiotic depletion displayed this behaviour.
Fig. 6. Environmental evaluation of case B. Relative contribution of the different stages.
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V.J. Ferreira et al. / Journal of Cleaner Production xxx (2015) 1e12
11
Table 11 Accumulative energy demand to case B. Energy
Total
Bitumen
Aggregates
Water
Machinery
Transport
Slag treatment
Landfill avoided
Raw materials avoided
MJ %
1208
1092.9 90.5%
76.1 6.3%
0.6 0.0%
27.3 2.3%
106.3 8.8%
29.3 2.4%
33.6 2.8%
90.9 7.5%
Fig. 7. Comparison of the environmental performance of case A (taken as a reference) and case B.
Nevertheless, it should be noted that, since the main contributor to these three indicators is the bitumen production, the results are highly sensible to the amount of bituminous mixture required to meet the technical requirements of each particular situation. As mentioned above, the use of higher amount of asphalt in Case B could be due to the porosity of slags, which leads excessive asphalt absorption. Finally, regarding the carbon footprint indicator, considering the results shown in Tables 8 and 10, a reduction around 10% approximately can be achieved when the black slag was introduced in the surface layer. This latter corresponds to 1.6 kg CO2 eq, which, despite the differences between both studies, is in total agreement with the previous work (Mladenovi c et al., 2015). 4. Conclusions A chemical characterisation of the black slag was performed in this study by using the X-ray fluorescence spectrometry technique. The analysis showed an acceptable content of lime (around 22%) which according to the XRD pattern was revealed to be bounded to iron and silicon in the phases of calcium ferrate and calciumaluminium silicate (Ca2AlSiO5.5). This observation suggested good technical properties to use the black slags as a material road
construction, but an accurate quantification of the free lime content is strongly recommended. According to technical specifications for road and bridge works in Spain, black slag can be used as coarse aggregate in hot bituminous mixtures in the type of base layer for the T2 and T3 heavy traffic categories. Moreover, the black slag can be also used in the type of surface course layer in the T3 and T4 categories. The environmental evaluation of two cases studies was performed: (i) with natural aggregates (Case A) and (ii) with the substitution of 75 wt.% of natural aggregates by black steel slag in the hot bituminous mixtures (Case B). For both cases the bitumen consumption had the highest impact. In particular, in the depletion or ozone layer depletion indicators, the high impact is expected to be due to the refining process of heavy crude oils. This result was also reflected in the evaluation of the accumulative energy demand along the life cycle of the road construction. However, additional parameters were also included in the evaluation of the use of black slag as material, such as the avoided use of landfills. In the alternative scenario (case B), some environmental benefits were obtained by avoiding the landfill use and raw material consumption for all the impact categories. Finally, the comparison between two cases showed negative variations (around 10%) of most of the impact categories,
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indicating a reduction of the environmental impact in case B (recycling of black slag) respected to case A (traditional road construction). However, the characteristics of the selected black slags should be carefully analysed for each particular situation because the high porosity of the slag may lead to an increment in the asphalt consumption which is expected to have a deleterious effect in some of the environmental indicators. Consequently, it is highly advisable to establish a compromise between the technical requirements, the amount of aggregates substituted by black slags and the environmental implications. Acknowledgements The research leading to these results has received funding from the Spanish Ministry for Science and Innovation, INNPACTO Programme 2010, under grant agreement IPT-310000-2010-027 e VALOR project. Authors thanks to the involved project partners for providing support to this research. The authors want also to acknowledge the support of the project LCE4Roads: “Life Cycle Engineering approach to develop a novel EU-harmonized sustainability certification system for cost-effective, safer and greener road infrastructures”, a FP7 Project supported by the European Commission under grant agreement No. 605748. References Ahmedzade, P., Sengoz, B., 2009. Evaluation of steel slag coarse aggregate in hot mix asphalt concrete. J. Hazard. Mater. 165, 300e305. Arabani, M., Azarhoosh, A.R., 2012. The effect of recycled concrete aggregate and steel slag on the dynamic properties of asphalt mixtures. Constr. Build. Mater. 35, 1e7. n, A., Ferreira, G., Lo pez-Sabiro n, A.M., Mainar-Toledo, M.D., Zabalza Aranda-Uso Bribi an, I., 2013. Phase change material applications in buildings: an environmental assessment for some Spanish climate severities. Sci. Total Environ. 444, 16e25. ttir, H., 2005. Life Cycle Assessment Model for Road Construction and Use of Birgisdo Residues from Waste Incineration (Ph.D. thesis). Institute of Environment & Resources, Technical University of Denmark. BSE: Badische stahlengineering GMBH, 2015. Electric arc furnace slag e a product not wate: saving money by using slag as a building material. Available in. http:// www.bse-kehl.de/. Eloneva, S., Puheloinen, E.-M., Kanerva, J., Ekroos, A., Zevenhoven, R., Fogelholm, C.J., 2010. Co-utilisation of CO2 and steelmaking slags for production of pure CaCO3 e legislative issues. J. Clean. Prod. 18, 1833e1839. pez-Sabiro n, A.M., Aranda, J., Mainar-Toledo, M.D., Aranda-Uso n, A., Ferreira, G., Lo 2015. Environmental analysis for identifying challenges to recover used reinforced refractories in industrial furnaces. J. Clean. Prod. 88, 242e253. Guinee, J.e.a., 2001. Life Cycle Assessment d an Operational Guide to the ISO Standards. Centre of Environmental Sciences (CML): Leiden University. Huang, Y., Bird, R.N., Heidrich, O., 2007. A review of the use of recycled solid waste materials in asphalt pavements. Resour. Conserv. Recycl. 52, 58e73. Iacobescu, R.I., Pontikes, Y., Koumpouri, D., Angelopoulos, G.N., 2013. Synthesis, characterization and properties of calcium ferroaluminate belite cements produced with electric arc furnace steel slag as raw material. Cem. Concr. Compos. 44, 1e8. ISO 14040, 2006. Environmental Management e Life Cycle Assessment e Principles and Framework. Kavussi, A., Qazizadeh, M.J., 2014. Fatigue characterization of asphalt mixes containing electric arc furnace (EAF) steel slag subjected to long term aging. Constr. Build. Mater. 72, 158e166. Khasreen, M., Banfill, P.F., Menzies, G., 2009. Life-cycle assessment and the environmental impact of buildings: a review. Sustainability 1, 674e701.
Liapis, I., Likoydis, S., 2012. Use of electric arc furnace slag in thin skideresistant surfacing. Procedia Soc. Behav. Sci. 48, 907e918. pez-Sabiro n, A.M., Royo, P., Ferreira, V.J., Aranda-Uso n, A., Ferreira, G., 2014. Lo Carbon footprint of a thermal energy storage system using phase change materials for industrial energy recovery to reduce the fossil fuel consumption. Appl. Energy 135, 616e624. Mladenovi c, A., Turk, J., Kova c, J., Mauko, A., Coti c, Z., 2015. Environmental evaluation of two scenarios for the selection of materials for asphalt wearing courses. J. Clean. Prod. 87, 683e691. Morfeldt, J., Nijs, W., Silveira, S., 2014. The impact of climate targets on future steel production e an analysis based on a global energy system model. J. Clean. Prod. 103, 469e482. Muhmood, L., Vitta, S., Venkateswaran, D., 2009. Cementitious and pozzolanic behavior of electric arc furnace steel slags. Cem. Concr. Res. 39, 102e109. Proctor, D.M., Fehling, K.A., Shay, E.C., Wittenborn, J.L., Green, J.J., Avent, C., Bigham, R.D., Connolly, M., Lee, B., Shepker, T.O., Zak, M.A., 2000. Physical and chemical characteristics of blast furnace, basic oxygen furnace, and electric arc furnace steel industry slags. Environ. Sci. Technol. 34, 1576e1582. RaCenovi c, A., Malina, J., Sofili c, T., 2013. Characterization of ladle furnace slag from carbon steel production as a potential adsorbent. Adv. Mater. Sci. Eng. 2013. Article ID 198240. Rajendran, S., Gambatese, J.A., 2007. Solid waste generation in asphalt and reinforced concrete roadway life cycles. J. Infrastruct. Syst. 13, 88e96. Rebitzer, G., Ekvall, T., Frischknecht, R., Hunkeler, D., Norris, G., Rydberg, T., Schmidt, W.P., Suh, S., Weidema, B.P., Pennington, D.W., 2004. Life cycle assessment: part 1: framework, goal and scope definition, inventory analysis, and applications. Environ. Int. 30, 701e720. ez-De-Guinoa Vilaplana, A., Ferreira, V., Lo pez Sabiro n, A.M., Ferreira, G., 2015. Sa Utilization of Ladle Furnace slag from a steelwork for laboratory scale production of Portland cement. Constr. Build. Mater. 94, 837e843. Shen, D.-H., Wu, C.-M., Du, J.-C., 2009. Laboratory investigation of basic oxygen furnace slag for substitution of aggregate in porous asphalt mixture. Constr. Build. Mater. 23, 453e461. Society of Environmental Toxicology and Chemistry (SETAC), 1993. Guidelines for Life Cycle Assessment. A Code of Practice. Sofili c, T., Mladenovi c, A., Sofili c, U., 2011. Defining of EAF steel slag application possibilities in asphalt mixture production. J. Environ. Eng. Landsc. Manag. 19, 148e157. Tossavainen, M., Engstrom, F., Yang, Q., Menad, N., Lidstrom Larsson, M., Bjorkman, B., 2007. Characteristics of steel slag under different cooling conditions. Waste Manag. 27, 1335e1344. Udo de Haes, H.A., Heijungs, R., 2007. Life-cycle assessment for energy analysis and management. Appl. Energy 84, 817e827. UNE-EN 933-2, 1996. Test for Geometrical Properties of Aggregates. Part 2: Determination of Particle Size Distribution. Test Sieves, Nominal Size of Apertures. UNE-EN 933-5, 1999. Test for Geometrical Properties of Aggregates. Part 5: Determination of Percentage of Crushed and Broken Surfaces in Coarse Aggregate Particles. UNE-EN 933-8, 2012. Tests for Geometrical Properties of Aggregates e Part 8: Assessment of Fines e Sand Equivalent Test. UNE-EN 1097-2, 2010. Tests for Mechanical and Physical Properties of Aggregates e Part 2: Methods for the Determination of Resistance to Fragmentation. UNE-EN 1097-8, 2000. Tests for Mechanical and Physical Properties of Aggregates. Part 8: Determination of the Polished Stone Value. UNE-EN 1744-3, 2003. Tests for Chemical Properties of Aggregates e Part 3: Preparation of Eluates by Leaching of Aggregates. Waligora, J., Bulteel, D., Degrugilliers, P., Damidot, D., Potdevin, J.L., Measson, M., 2010. Chemical and mineralogical characterizations of LD converter steel slags: a multi-analytical techniques approach. Mater. Charact. 61, 39e48. Wu, S., Xue, Y., Ye, Q., Chen, Y., 2007. Utilization of steel slag as aggregates for stone mastic asphalt (SMA) mixtures. Build. Environ. 42, 2580e2585. Xie, J., Wu, S., Lin, J., Cai, J., Chen, Z., Wei, W., 2012. Recycling of basic oxygen furnace slag in asphalt mixture: material characterization & moisture damage investigation. Constr. Build. Mater. 36, 467e474. Xue, Y., Wu, S., Hou, H., Zha, J., 2006. Experimental investigation of basic oxygen furnace slag used as aggregate in asphalt mixture. J. Hazard. Mater. 138, 261e268. Zheng, H., Kaliyan, N., Morey, R.V., 2013. Aspen Plus simulation of biomass integrated gasification combined cycle systems at corn ethanol plants. Biomass Bioenergy 56, 197e210.
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Evaluation of the steel slag incorporation as coarse aggregate for road construction: technical requirements and environmental impact assessment ez-De-Guinoa Vilaplana a, Tatiana García-Armingol b, Victor J. Ferreira a, Aitana Sa c lez d, Ana M. Lo pez-Sabiro n a, n , Cristina Lausín-Gonza Alfonso Aranda-Uso a , * n Ferreira Germa mez, 15, 50018 Zaragoza, Spain Research Centre for Energy Resources and Consumption (CIRCE), CIRCE Building, Campus Río Ebro, Mariano Esquillor Go mez, Research Centre for Energy Resources and Consumption (CIRCE) e Universidad de Zaragoza, CIRCE Building, Campus Río Ebro, Mariano Esquillor Go 15, 50018 Zaragoza, Spain c University of Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain d ArcelorMittal Global R&D, Avil es 3340, Spain a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 March 2015 Received in revised form 21 July 2015 Accepted 23 August 2015 Available online xxx
This work analyses the environmental impact of using electric arc furnace slags as secondary raw material in pavement and its comparison with the traditional materials used in road construction. Chemical and technical evaluations of the main characteristics of the black slags as coarse aggregate were carefully developed. The environmental analysis was carried out by using the Life Cycle Assessment methodology. The Life Cycle Inventory data was processed to obtain emissions grouped in terms of impact categories based on the Centre of Environmental Science of Leiden University baseline method at midpoint level. The results obtained revealed that some of the most relevant environmental impacts, such as carbon footprint, abiotic depletion, ozone layer depletion and photochemical oxidation, depend highly on the road construction processes, although, in the two scenarios analysed, the bitumen production was demonstrated to be the most contributing stage. These indicators also concluded that important environmental benefits could be obtained from the use of black slag as course aggregate in road construction. Consequently, the results shown here could be added to the list of technical criteria for their inclusion into a multi-objective optimisation methodology. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Life cycle assessment (LCA) Steel slag Electric arc furnace (EAF) Hot bituminous mixture Road construction
1. Introduction Steel industry is one of the major responsible of anthropogenic CO2 emissions. But, in order to mitigate the climate change, the reduction of these emissions is not easy due to the high dependence on electricity and coal (Eloneva et al., 2010). Nowadays, most steel is produced either via two main industrial routes: the integrated process, which uses coal as reducing agent and iron as raw material or the Electric Arc Furnace (EAF) route, where electricity is used to melt steel into the end product. The second route, also named secondary production since it uses steel
scrap, is a less energy intensive process and it is shown as a promising alternative to be close to a theoretical zero CO2 emission (Morfeldt et al., 2014). Apart from the energy consumption associated to the production processes, the increase of steel consumption has also yield to an important growth of the volume of residues. Steelmaking residues are defined as by-products obtained from the conversion process of iron to steel. In particular, steel slag represents a significant proportion of these by-products (Proctor et al., 2000); for instance, the production of 3 tonnes of stainless steel is estimated to generate about 1 tonne of stainless steel slag. Consequently, a large amount of steel slag is yearly produced in the world (around fifty million tonnes per year).
* Corresponding author. Tel.: þ34 976761863; fax: þ34 976732078. E-mail address: [email protected] (G. Ferreira). http://dx.doi.org/10.1016/j.jclepro.2015.08.094 0959-6526/© 2015 Elsevier Ltd. All rights reserved.
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V.J. Ferreira et al. / Journal of Cleaner Production xxx (2015) 1e12
Nomenclature ADTHv CML EAF HF JCPDS LCA LCI LF LOI MB ZA
average daily traffic of heavy vehicles Centre of Environmental Science of Leiden University electric arc furnace concrete road surface Joint Committee on Powder Diffraction Standards life cycle assessment life cycle inventory ladle furnace loss on ignition bituminous mixtures artificial gravel
The huge amount of generated slags and their environmental impact have prompted scientists and engineers to work on novel solutions, based on industrial more friendly concepts that allow using these residues as raw material for pavements (RaCenovic et al., 2013). Steel slags can be used in road construction in order to replace natural aggregates, which reduces the environmental impact by reducing the consumption of natural and non-renewable aggregates and the quantity of slag deposited on landfill sites, both in asphalt (Mladenovi c et al., 2015) and cement based roads (Saezde-Guinoa Vilaplana et al., 2015). However, technical and environmental studies are required to determine the technical feasibility and the potential environmental benefits. Two types of slag are produced in the EAF steelmaking process: EAF slag, also called black slag, and Ladle Furnace (LF) slag, also known as secondary refining slag or white slag (RaCenovi c et al., 2013). In this work, the scenarios analysed are focused on the slags obtained in EAF, where about 40% of the global steel production takes place (Muhmood et al., 2009). Although, different kind of recycled materials can be used as aggregates in road construction, it is crucial to know the properties that such aggregates have in order to ensure that they meet the same quality standards as natural aggregates (Mladenovic et al., 2015). In particular, EAF slag has been frequently used as pavement aggregate due to their excellent mechanical properties, which make themselves suitable for asphalt layers with any kind of traffic load. For example, EAF slags-derived aggregates improve the skid resistance of the pavement and they also reduce the risk of aquaplaning due to their higher permeability (Liapis and Likoydis, 2012). To address the mentioned above strategy, this work has been focused on technical analysis and environmental evaluations. On the one hand, in order to analyse if black slags meet the technical requirements to be introduced as coarse aggregate in hot bituminous mixture for different course layers associated to the heavy traffic categories, the results of a comprehensive chemical analysis were compared with the typical certain oxide ranges found in the literature (Sofili c et al., 2011), which have been reported as consequence of the quality of steel produced. On the other hand, in order to estimate the environmental performance of the steelmaking black slag as secondary raw material for the road construction, the Life Cycle Assessment (LCA) methodology was applied. The LCA is a methodology widely accepted to be used for analysing environmental impacts of products and processes (Society of Environmental Toxicology and Chemistry (SETAC), 1993). Previous works, such as Rebitzer et al. (2004), have technically and scientifically demonstrated the viability of using LCA to characterise the environmental implications of a wide range of industrial activities.
In addition, its use is also strongly encouraged by European Union policies and regulation, i.e., the European Action Plan on Sustainable Consumption and Production and Sustainable Industrial Policy (COM(2008) 397), the ETAP action Plan (COM(2004) 38 final), etc. Previously, the impacts of the road construction have been assessed using LCA methods but most of these studies have only analysed the environmental impact considering the use of waste ttir, 2005) and crushed concrete associated such as fly ash (Birgisdo with traditional reinforced concrete and asphalt pavements (Rajendran and Gambatese, 2007). However, there is a lack of studies about steelmaking black slag as a coarse aggregate in hot bituminous mixture quantifying the environmental performance using the LCA methodology. Only Mladenovi c et al. (2015) address the environmental analysis of a road aggregate based on EAF slags and its comparison with natural aggregates but, in contrast to the analysis shown in the research, they do not take into account the chemical and physical characterisation of the slags. Furthermore, to the authors' knowledge, there is no scientific literature focused on a particular environmental evaluation using Spanish conditions, consequently, this study represents a relevant contribution. The main objective is, therefore, to characterise the Spanish steelmaking black slag to study its environmental performance taking into account the technical specifications for road and bridge works, focussing on heavy vehicles requirements, according to technical specifications (UNE-EN 933-2, 1996). 2. Methodology 2.1. Steel black slag characterisation for road construction 2.1.1. Chemical characterisation of the steel black slag The studied steel slag was characterised using X-ray fluorescence spectrometry to determine the chemical composition, while X-ray diffraction technique (XRD) was used to identify crystalline phases in it. To this end, diffraction patterns were measured in the 2Ɵ range of 5e80 using CuKa radiation. 2.1.2. Technical specifications of steel slag as coarse aggregate in hot bituminous mixture The hot bituminous mixture is defined as a type of asphalt in which hydrocarbon ligand and aggregates (including mineral powders) are combined. The aggregate acts as the structural skeleton of the pavement whereas the hydrocarbon ligand covers homogeneously all particles of aggregates, acting as the glue of the mixture. The use of the steel slag as coarse aggregate in the hot bituminous mixture was evaluated in this study by comparing the results obtained from tests with the specific technical requirements of coarse aggregate for road and bridge works in Spain (PG-3). According to the standard classification of the geometrical properties of aggregates (UNE-EN 933-2, 1996), a coarse aggregate is defined as the fraction of the total amount of aggregate retained on the sieve of 2 mm. The specific requirements of the aggregates depend on the category of the road and the Average Daily Traffic of Heavy Vehicles (ADTHv), classified according Table 1. In particular, in base course layers for categories of heavy traffic denoted as T00 and T0, the coarse aggregate cannot be obtained by crushing gravel from granular deposits of limestone quarries and nature. By the contrary, in the case of base course layers corresponding to the T1 and T2 heavy traffic, material from crushing natural gravel can be used providing the particle size of the used coarse aggregate before its crushing was 6 times greater than the maximum size of the final aggregate.
Please cite this article in press as: Ferreira, V.J., et al., Evaluation of the steel slag incorporation as coarse aggregate for road construction: technical requirements and environmental impact assessment, Journal of Cleaner Production (2015), http://dx.doi.org/10.1016/ j.jclepro.2015.08.094
V.J. Ferreira et al. / Journal of Cleaner Production xxx (2015) 1e12
3
Table 1 Heavy traffic categories. Categories of heavy traffic
T00
T0
T1
T2
T31
T32
T41
T42
ADTHv (heavy vehicles/day)
4000
<4000 2000
<2000 800
<800 200
<200 100
<100 50
<50 25
<25
Therefore, the coarse aggregate displays typical characteristics related to: the differentiated size fractions for collecting and management of aggregates separately for their introduction in the cold hoppers, the physicochemical weathering appreciable alteration under the most unfavourable conditions that could occur in the working area. Thus, the coarse aggregate characteristics must meet different requirements depending on its final use. In order to examine some of these characteristics, the following tests were carried out: a) Angularity of coarse aggregate (for particles partially or completely crushed), b) Shape of the coarse aggregate (flakiness index test procedure), c) Resistance to fragmentation (Los Angeles coefficient), d) Resistance to polishing of the coarse aggregate in base course layers (Accelerated polished coefficient); and, e) Coarse aggregate washing (content of impurities). 2.2. Environmental analysis of steelmaking slag as coarse aggregate in road construction The environmental analysis of steelmaking slag as a raw material in hot bituminous mixture manufacturing was carried out by means of the LCA methodology. This methodology is defined as pez-Sabiro n et al., 2014) where the iterative among its phases (Lo old data will be replaced with new information leading to a more realistic evaluation. The most up-to-date structure of the LCA is proposed by the standard 14040 (ISO 14040, 2006). As can be observed in Fig. 1, this methodology can be synthesised in four main phases as is also reported by Khasreen et al. (2009) or Udo de Haes and Heijungs (2007). The first of them is focused on defining the objective of the study, the functional unit and the limits of the system involved (scope). After the first step, Life Cycle Inventory (LCI) is devoted to gather all the required data and the calculation procedures aimed
Inventory Analysis (LCI) Datagathering & Modelling
Goal & scope definition
Interpretation
Environmental Impact assessment And results
Fig. 1. Main phases of an LCA study.
to quantify the relevant inputs and outputs of the production system. This process is iterative and may be repeated if further information is required during its implementation. The system under study must be modelled as a complex sequence of unitary operations that share information among themselves and with the environment through inputs and outputs. During the third phase of the study, the evaluation of the significant potential environmental impacts, using the data from the inventory phase, is carried out. Here, the level of detail, the selection of the impacts to evaluate and the methods of evaluation depend on the objectives and scope of the study. Finally, the last phase is focused on the life cycle interpretation. This phase consists in the interpretation of the results from the inventory phase and from the evaluation of impacts, as well as the eventual compiling of conclusions and recommendations for the improvement of the environmental performance of the system under study.
2.2.1. Scope of the analysis and functional unit In this study, the identification of an alternative raw material to be used as coarse aggregated (black slag), including a thorough analysis of technical specifications, is a potential goal. In this case, the product to be analysed is a new type of pavement which contains black slag as coarse aggregate in asphalt mixture. The life cycle of a product can generally be subdivided into three phases of ‘production’, ‘use’ and ‘disposal’, where final energies are involved. This work is only focused on the construction stage of the pavement. Therefore, the use and disposal stages are excluded from the LCI. The environmental impact assessment was carried out by midpoint approach. For this purpose, CML method was used. Ac n et al. (2013), midpoint cording to Guinee (2001) and Aranda-Uso indicators are considered to be points in the causeeeffect chain (environmental mechanism) of a particular impact category somewhere between stressor and endpoints. For such midpoint indicators, the characterisation factors can, therefore, be calculated to reflect on the importance related to an emission or extraction in a LCI by environmental metrics. Thus, 9 category metrics, namely abiotic depletion, acidification, eutrophication, global warming (considering global warming potential to 100 years), ozone layer depletion, human toxicity, aquatic toxicity, territorial ecotoxicity and photochemical oxidation have been emphasised and fully analysed. In addition, a method to calculate the cumulative energy demand has been used. This method provides the total energy demand, expressed in terms of primary energy which is associated with the energy expenditure for the energy supply in the entire life cycle of a product or service. Finally, the functional unit was defined then as 1 m2 of pavement.
2.2.2. System description and boundaries The study is mainly focused on the road construction stage (bounded by the dotted lines in Fig. 2), where large amounts of aggregates, representing more than 90 wt.% of the bituminous mixture, are required. Its implementation includes the following operation phases:
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Fig. 2. Description of the system boundaries.
Preparation and testing of the surface settlement, Material input, Extension, wetting, if applicable, and compaction of each pouring section, and Refining of the surface of the last poured section. Several categories of heavy traffic are considered in the study. Based on the categories shown in Table 1, the study is focused on the range of the categories of heavy traffic from T2 to T4, which can use roads made from recycled granular materials, steel materials, by-products and inert waste products, as long as the required technical requirements are met and the origin of the materials is declared. In addition, the thickness of each layer should be taken into account depending on the esplanade (E1 e acceptable esplanade, E2 e good esplanade and E3 e very good esplanade). These thicknesses are shown in Table 2. The layers are denoted according to the following references: MB e bituminous mixtures HF e concrete road surface ZA e artificial gravel
Additionally, there are several types of road surfaces for each heavy traffic category and esplanade. They are classified according to the last number of their code: 1: Bituminous mixtures on granular layer, 2: Bituminous mixtures on soilecement, 3: Bituminous mixtures on gravelecement soilecement, 4: Concrete paving.
built
over
Table 2 shows the profiles of required thicknesses for each base layer depending on the esplanade quality, the heavy traffic, the kind of pavement, and the bituminous mixtures or concrete. A profile 3221 (from Table 2), i.e. 35 cm of artificial gravel and 15 cm of bituminous mixture, corresponding with the thickness of good esplanade (E2) and the heavy traffic category T32 (between 50 and 100 vehicles per day) paved with bituminous mixtures, was selected as the reference configuration for both study cases. 2.2.3. Life cycle inventory (LCI) As mentioned above, the stages of maintenance and disposal have been excluded from the environmental analysis and only the
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Table 2 Thicknesses for each layer according to the esplanade and heavy traffic category. Heavy traffic category T2 Esplanade thicknesses
T31
T32
T41
T42
E1
E2
E3
transparent and made in house data base, for creating specific modules for this LCA study.
stage of the road construction has been considered. Thus, the environmental evaluation is focused on the phases of raw material supplied from natural resources, transport and processing up to the road construction, such as shown in Fig. 2. Firstly, a base case, where natural aggregates are used (Case A), is analysed. Then, the substitution of 75 wt.% of natural aggregates by black steel slag is studied (Case B). Additionally, both the direct energy input for the road construction and the energy involved in the processes of raw materials production (including mining and transformation) and transport process are considered. Table 3 shows the amount of the main materials considered in the LCI. The LCA methodology is then performed through the LCI to define and quantify energy, resources and material flows within the road construction process and the application of the case studies under review to be converted into the environmental metrics categories shown in Table 4. These categories were chosen as environmental burdens associated with the life cycle of the system considered in the context of this study. Finally, the environmental analysis by means of the LCA methodology application is performed using SIMAPRO v7.3 software containing data bases, which were carefully selected and contrasted to own LCI information to generate a relevant, reliable,
2.2.4. Cut-off criteria This research follows the cut-off criterion used by authors in previous studies (Ferreira et al., 2015) to ensure that all relevant environmental impacts were represented in the study. In particular, flows of materials less than 1% of the cumulative mass of all the inputs and outputs depending on the type of flow of the LCI model have been excluded their environmental relevance is not a concern. However, if the sum of the neglected material flows exceeded 5% of mass, energy or environmental relevance, it should be taken into account in the analysis. 3. Results 3.1. Evaluation of steel slag for road construction 3.1.1. General characterisation of black slag The X-ray fluorescence spectrometry was used to investigate the chemical composition of steel slag. The results, shown in Table 5, demonstrate that the typical oxides, SiO2, Al2O3, Fe2O3 and CaO, are
Table 3 Main inputs and outputs for Life cycle inventory. H
Case A Thickness cm
Case B Aggregate kg
Water L
MB e 15 325 e ZA e 35 470 100 Road transport to move raw materials* Distances 40 km 10 km
Bitumen kg
Thickness cm
Aggregate kg
Water L
Bitumen kg
17 e
MB e 15 ZA e 35
81 470
316 100
21 e
500 km
Distances
40 km
10 km
500 km
*Truck mileages > 16t with typical average of the European fleet have been assumed for transportation evaluation.
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Gehlenite: Ca2Al2SiO7 (JCPDS 00-020-0199) Periclase: MgO (JCPDS 00-001-1235)
Table 4 Impact category metrics. Impact category
Unit
Abiotic depletion Acidification Eutrophication Global warming Ozone layer depletion Human toxicity Aquatic toxicity Territorial ecotoxicity Photochemical oxidation
kg kg kg kg kg kg kg kg kg
Sb eq SO2 eq PO3 4 eq CO2 eq CFC-11 eq 1,4-DB eq 1,4-DB eq 1,4-DB eq C2H4 eq
present in the chemical composition. As can be observed, the Fetotal and CaO are 25 and 22 wt.% respectively, whereas the contents of Si, Al and Mg are in smaller proportion, which is in agreement with the results reported in the specialised technical reports (BSE: Badische stahlengineering GMBH, 2015) and previous works focused on the use of electric arc furnace steel slag as raw material (Iacobescu et al., 2013). In addition, the composition of the slag studied are in the ranges of the typical values of black slag considering the same quality of steel produced (Sofili c et al., 2011), namely, SiO2 (6.5e35%), Al2O3 (1e13.44%), Fe2O3 (1e31.2%) and CaO (18.4e60.0%). Additionally, the XRD was used as a technique to complement the X-ray fluorescence spectrometry for the identification of the present mineral phases. Diffraction data were processed by specialised software, and specific recorded relative intensities of standard X-ray diffraction lines (according to the Joint Committee on Powder Diffraction Standards (JCPDS)) were marked on the XRD pattern of black slag (see Fig. 3). Thus, the following components were found by the XRD analysis of the black slag:
Calcium aluminate: CaAl2O4 (JCPDS 00-034-0440) Wuestite: FeO (JCPDS 01-089-7100) Silicate of Magnesium/Iron: (Mg,Fe)2SiO4 (JCPDS 00-021-1258) Calcium and Aluminium Silicate: Ca2AlSiO5.5 (JCPDS 00-0470699) Calcium ferrate: Ca2Fe2O5.12 (JCPDS 00-049-1555) Table 5 Chemical composition (dry matter %) of steelmaking black slag. Compounds
Black slag
Fe total SiO2 Mn CaO MgO Al2O3 K2O Na2O S P Zn Pb TiO2 Cl Cr2O3 F LOIb
25.32 15.64 4.60 22.39 7.96 12.14 0.02 0.22 0.07 0.10 0.75 0.01 0.75 0.03a 0.81a e 3.06
a
Semi-quantitative results. Loss-on-ignition. It can be noted that the Loss on Ignition (LOI) parameter exhibited a negative value because the method of calculation used is very susceptible to atmospheric moisture content-negative weight losses (i.e. weight grains). b
The XRD analysis confirmed that the slag mainly consisted of metal oxide in various complex oxides, silicate forms and calcium aluminate. These results of the XRD analysis could be comparable with the results published by several authors (Iacobescu et al., 2013; Kavussi and Qazizadeh, 2014; Waligora et al., 2010). The mineralogical composition of steel slags and X-ray diffraction analysis of slag samples was performed by these authors. Their results, including the ones of this study, reveal to have usually complex structure, which lead many overlapping peaks reflected from crystalline phases present in steel slags. These phases are formed mainly due to the chemical composition of slag and the slow cooling rate applied during processing (Tossavainen et al., 2007). 3.1.2. Technical specifications of steel slag as coarse aggregate in hot bituminous mixture According to the methodology described above, the test results associated with the physicochemical weathering, summarised in Table 6, revealed that the values are below maximum values established by the respective norm. These results suggest that the soluble components, which can leach by the black slag, are not a potential risk to the environment or to the building elements placed in surrounding areas. Table 7 summarises the test results of technical specifications associated to the typical characteristics of coarse aggregates to be used in different course layers. These results show that coarse aggregates meet the technical specifications required. In particular, the results show that the black slag can be recommended to be used as a coarse aggregate in hot bituminous mixtures in the type of base layer for the T2 and T3 heavy traffic categories, since most of the tests were positive for those categories. Moreover, the slag could be also used in the type of surface course layer in the T3 and T4 categories. Thus, these latter results ensures that the main characteristics of the coarse aggregate are maintained during the collecting and management of the aggregate, when they are produced or supplied separately in differentiated size fractions up to their introduction in the cold hoppers. Nonetheless, the content of impurities of the black slag samples was 1.2 wt.%, higher than the maximum value established in the norm (Zheng et al., 2013) (<0.5 wt.%). It is recommended to perform a washing, aspiration or another cleaning method prior to the use of black slag as coarse aggregate in hot bituminous mixtures. Slag as raw material in asphalt mixture for road construction shows some disadvantages associated to the some free lime (fCaO), which can be hydrated causing volume instability and porosity of slags, leading an excessive asphalt absorption caused by pores, and therefore, high cost. Nonetheless, it has been largely reported in literature that the introduction of steel slag, as coarse aggregate in asphalt mixture, can improve the mechanical characteristics such as the stability and the deformation resistance (Ahmedzade and Sengoz, 2009) or increase the fatigue life of the asphalt mixtures due to the need of using a higher binder content (Arabani and Azarhoosh, 2012). In addition, the slag use also enhances other pavement performances, such as the drainage with permeability (Shen et al., 2009) and the moisture stability (Xie et al., 2012) among other important properties, namely, skid resistance (Shen et al., 2009), deformation at high temperatures resistance (Wu et al., 2007) and cracking and stripping resistance, avoiding early the damage of the pavement (Xue et al., 2006). Therefore, all these results and those obtained in this study show a clear feasibility of using steel slag coarse aggregate in asphalt mixture.
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Fig. 3. XRD pattern of black slag.
Table 6 Results obtained for some of the characteristic parameters of black slag samples associated to the physicochemical weathering. Parameters Sand equivalent pH COD Phenols Sulphates Chlorides Fluorides Antimony Molybdenum Arsenic Barium Cadmium Zinc Cupper Chrome Mercury Nickel Lead Selenium
Results 90 8.35 14.2 <0.1 588 10 7.7 <0.004 <0.1 0.011 8.2 <0.04 <0.1 <0.1 <0.1 <0.002 <0.1 <0.1 <0.01
Units
Norm
e
UNE-EN 933-8 (2012) UNE-EN 1744-3 (2003)
Uds. pH mg/kg mg/kg SO2 4 mg/kg Cl mg/kg F mg/kg Sb mg/kg Mo mg/kg As mg/kg Ba mg/kg Cd mg/kg Mg Zn/kg Cu mg/kg Cr mg/kg Hg mg/kg Ni mg/kg Pb mg/kg Se mg/kg
Legal limits <50 e <500 <1 <1000 <800 <10 <0.06 <0.5 <0.5 <20 <0.04 <4 <2 <0.5 <0.01 <0.4 <0.5 <0.1
3.2. Environmental analysis As mentioned above, from the technical point of view, the use of black slag as coarse aggregate concluded to be workable in asphalt mixture for road construction. However, this study has an additional goal compared to most studies found in literature, which consists in the development of a comprehensive environmental assessment using the methodology described in previous sections. To this end, the study is focused in two cases. The first one, named case A, using natural coarse aggregate and case B incorporating 75 wt.% of black slag in the asphalt mixture. In both cases, the coarse aggregate meets the technical specifications summarised in the UNE-EN 933-5 (1999) and UNE-EN 933-8 (2012) and described in Tables 6 and 7. 3.2.1. Evaluation of case study A Initially, a thorough analysis of the main contributions to the total environmental impact was performed. In this sense, the transport stage and the consumption of raw materials as bitumen, aggregates, water or diesel to produce energy were considered and
evaluated through the LCA methodology. The total environmental impact associated to case A for the different environmental indicators considered in this research is depicted in Table 8. Additionally, Fig. 4 shows the main results obtained related to the relative impacts between the previous parameters associated to this case study. On the one hand, as can be seen in Fig. 4 and in agreement with Mladenovi c et al. (2015), the highest impact is allocated in the bitumen consumption, which is close to 86% of the total impact in case of abiotic depletion or ozone layer depletion indicators and from 44% to 76% in case of the rest of impact categories. This result can be attributed to the relative impact associated to the bitumen production, since it is based on the refining process of heavy crude oils with high sulphur content. On the other hand, the transport and the aggregate consumption stages are the second highest contribution depending on the indicator considered. Whereas the aggregates are higher than transport in human toxicity and ecotoxicity categories and the transport stage shows a greater impact in case of abiotic depletion, acidification and global warming; both display a quite similar impact for eutrophication and photochemical oxidation. Finally, the water consumption exhibits the lowest environmental impact in this case study. Regarding the global warming indicator, measured as 18.2 kg CO2 eq, the highest impacts are reflected again by the bitumen consumption, which can be associated to the crude oil production as well as the gas burned during the refinery process. In case of the transport, the most relevant contribution to global warming is related to the operation stage due to the fossil fuel consumption. Finally, concerning the aggregate consumption, the most important stage is the energy consumption during the crushing process. Taking into account the previous results where the bitumen consumption was concluded to have the highest impact, the construction of the surface layer, mainly composed by bitumen, is therefore, the most relevant phase in the road construction, as it can be seen in Fig. 5. In addition, the surface layer construction for the T3 and T4 heavy traffic categories shows environmental impacts between 66% and 92% of the total environmental impact, in comparison to the base layer construction. Additionally, Table 9 shows the main results obtained for the evaluation of the accumulative energy demand along the life
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Table 7 The technical specifications associated to the typical characteristics of the coarse aggregate to be used in layers of different base course layers (Note that (*) means “Service road”). Angularity of coarse aggregate (total or partially crushed particles) UNE-EN 933-5 (1999) Type of layer
Heavy traffic category
Surface course Binder course Base
100
T00
T0 and T1
T2
90
100
Black slag sample (result)
T3 and berms
T4
90
75 75 (*)
75
Angularity of coarse aggregate (rounded particles) UNE-EN 933-5 (1999) Type of layer
Heavy traffic category
Surface course Binder course Base
0
T00
T0 and T1
Black slag sample (result)
T2
1
0
99
T3 and berms
T4
1
10 10 (*)
1
10
Flakiness index of coarse aggregate UNE-EN 933-8 (2012)
Black slag sample (result)
Heavy traffic category T00
T0 to T31
T3 and berms
20
25
30
T4 2
Los Angeles coefficient UNE-EN 1097-2 (2010) Type of layer
T00 and T0 Surface course Binder course Base
Black slag sample (result)
Heavy traffic category T1
20 25 25
T2
T3 and berms
T4
25
27 25 (*)
30
Accelerated polished coefficient UNE-EN 1097-8 (2000)
Black slag sample (result)
Heavy traffic category T00 and T0
T0 to T31
T3, T4 and berms
56
50
44
cycle of the road construction. Again, as it happened during the environmental evaluation, the bitumen consumption in the surface layer construction reveals the highest energy demand values. 3.2.2. Evaluation of case study B As mentioned before, the best option to introduce the black slag into the road construction seemed to be using it as an arid component in the base and surface layer from T2 to T4 heavy traffic categories; since the final mixture enhances the wear resistance properties in comparison to natural aggregates, according to the technical characterisation results obtained. However, some works, such as those included in the review made by Huang et al. (2007) have reported the successful Table 8 Total environmental impact associated to case A. Impact category
Unit
Total
Abiotic depletion Acidification Freshwater eutrophication Global warming Ozone layer depletion Human toxicity Freshwater ecotoxicity Terrestrial ecotoxicity Photochemical oxidation
kg kg kg kg kg kg kg kg kg
0.47 0.156 0.032 18.2 1$105 12.4 3.22 0.072 0.008
Sb eq SO2 eq PO4 eq CO2 eq CFC-11 eq 1,4-DB eq 1,4-DB eq 1,4-DB eq C2H4 eq
1
application of black slag in road construction as aggregate in bituminous surface course mixtures. In addition, it must be noted that the basic character of the black slag also improves the adhesion with conventional bitumen. Thus, only the application of black slag as aggregate in bituminous surface course mixtures in road construction is considered in this case study B. This is the reason why in LCI (Table 3) the incorporation of black slag (75 wt%.) as aggregate is just considered in the surface layer (MB-15). Similar to case A, the case study B was analysed to determinate the main contributions of the different parameters considered in the total environmental impact. Besides the parameters analysed in case A related to the transport stage and the consumption of raw materials, other relevant parameters were also included in the evaluation. In particular, the use of black slag requires a previous conditioning process but it also implies a saving of raw materials at the same time that it avoids the use of landfills due to recovering of the black slag. The total environmental impact associated to case B for the different environmental indicators considered in this research can be observed in Table 10. Additionally, Fig. 6 shows the main results related to the relative impacts associated to each stage considered in the analysis. On the one hand, the results reveal that environmental benefits can be obtained by avoiding the landfill use and raw material consumption for all the impact categories, the highest impact is due to, again, the bitumen consumption, especially in case of abiotic depletion or ozone layer depletion indicators. Therefore, as
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Fig. 4. Environmental evaluation of case A. Relative contribution of the different stages.
Fig. 5. Environmental evaluation of case A. Relative contribution of the different road layer construction.
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Table 9 Accumulative energy demand of case A. Energy
Total
Bitumen
Aggregates
Water
Machinery
Transport
MJ %
1122 e
890 79
109 8
0.6 0.1
25 2.2
97 8.7
Table 10 Total environmental impact associated to case B. Impact category
Unit
Total
Abiotic depletion Acidification Freshwater eutrophication Global warming Ozone layer depletion Human toxicity Freshwater ecotoxicity Terrestrial ecotoxicity Photochemical oxidation
kg kg kg kg kg kg kg kg kg
0.521 0.151 0.029 16.6 z0 11.3 2.88 0.066 0.008
Sb eq SO2 eq PO4 eq CO2 eq CFC-11 eq 1,4-DB eq 1,4-DB eq 1,4-DB eq C2H4 eq
explained in case A, the energy consumed and the different processes involved in the production of bitumen are the main contributor to the environmental impact associated to the road construction, which, again, is in qualitative agreement with the results previously reported (Mladenovi c et al., 2015). In addition, the transport and the aggregate consumption stages remain as the second highest contribution depending on the indicator analysed. On the other hand the relative impact of a new stage should be carefully considered. This stage is related to the conditioning of the black slag which, although it had not been assigned
an environmental impact since it was a residue, must be crushed before being used in road construction causing some environmental affection. In this case B, the carbon footprint was quantified as 16.6 kg CO2 eq, maintaining the most relevant contributions for each singular stage similar to those obtained for the case A. Furthermore, regarding the black slag consumption, the most relevant contribution is the energy consumption associated to the crushing process. Finally, the accumulative energy demand evaluation for the road construction was also performed in this case and the main results are shown in Table 11 expressed in terms of primary energy. As in the environmental evaluation, the bitumen consumption used in the surface layer construction reveals the highest energy demand values. 3.2.3. Comparison of case A and case B After analysing individually both case studies, a comparative evaluation was carried out. For this purpose, Fig. 7 depicts the relative variations obtained between case A (reference) and case B, where negative variations means a reduction of the environmental impact in case B respected to case A, whereas, positive variations indicate an increment of the environmental impact when black slag was incorporated as raw material. The results conclude that most of the indicators, except abiotic depletion, ozone layer depletion and photochemical oxidation, exhibited a reduction when case B was considered. This fact can be mainly attributed to the bitumen quantities used in each situation, which are higher in case B than in case A. This result is in partial agreement with a previous work (Mladenovi c et al., 2015), where only the abiotic depletion displayed this behaviour.
Fig. 6. Environmental evaluation of case B. Relative contribution of the different stages.
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Table 11 Accumulative energy demand to case B. Energy
Total
Bitumen
Aggregates
Water
Machinery
Transport
Slag treatment
Landfill avoided
Raw materials avoided
MJ %
1208
1092.9 90.5%
76.1 6.3%
0.6 0.0%
27.3 2.3%
106.3 8.8%
29.3 2.4%
33.6 2.8%
90.9 7.5%
Fig. 7. Comparison of the environmental performance of case A (taken as a reference) and case B.
Nevertheless, it should be noted that, since the main contributor to these three indicators is the bitumen production, the results are highly sensible to the amount of bituminous mixture required to meet the technical requirements of each particular situation. As mentioned above, the use of higher amount of asphalt in Case B could be due to the porosity of slags, which leads excessive asphalt absorption. Finally, regarding the carbon footprint indicator, considering the results shown in Tables 8 and 10, a reduction around 10% approximately can be achieved when the black slag was introduced in the surface layer. This latter corresponds to 1.6 kg CO2 eq, which, despite the differences between both studies, is in total agreement with the previous work (Mladenovi c et al., 2015). 4. Conclusions A chemical characterisation of the black slag was performed in this study by using the X-ray fluorescence spectrometry technique. The analysis showed an acceptable content of lime (around 22%) which according to the XRD pattern was revealed to be bounded to iron and silicon in the phases of calcium ferrate and calciumaluminium silicate (Ca2AlSiO5.5). This observation suggested good technical properties to use the black slags as a material road
construction, but an accurate quantification of the free lime content is strongly recommended. According to technical specifications for road and bridge works in Spain, black slag can be used as coarse aggregate in hot bituminous mixtures in the type of base layer for the T2 and T3 heavy traffic categories. Moreover, the black slag can be also used in the type of surface course layer in the T3 and T4 categories. The environmental evaluation of two cases studies was performed: (i) with natural aggregates (Case A) and (ii) with the substitution of 75 wt.% of natural aggregates by black steel slag in the hot bituminous mixtures (Case B). For both cases the bitumen consumption had the highest impact. In particular, in the depletion or ozone layer depletion indicators, the high impact is expected to be due to the refining process of heavy crude oils. This result was also reflected in the evaluation of the accumulative energy demand along the life cycle of the road construction. However, additional parameters were also included in the evaluation of the use of black slag as material, such as the avoided use of landfills. In the alternative scenario (case B), some environmental benefits were obtained by avoiding the landfill use and raw material consumption for all the impact categories. Finally, the comparison between two cases showed negative variations (around 10%) of most of the impact categories,
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Please cite this article in press as: Ferreira, V.J., et al., Evaluation of the steel slag incorporation as coarse aggregate for road construction: technical requirements and environmental impact assessment, Journal of Cleaner Production (2015), http://dx.doi.org/10.1016/ j.jclepro.2015.08.094