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Concretes made of EAF slag and AOD slag aggregates from stainless steel process: Mechanical properties and durability
Construction and Building Materials 76 (2015) 313–321
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Concretes made of EAF slag and AOD slag aggregates from stainless steel process: Mechanical properties and durability G. Adegoloye, A.-L. Beaucour ⇑, S. Ortola, A. Noumowé University of Cergy-Pontoise, Laboratoire de Mécanique & Matériaux du Génie Civil, EA 4114, F-95000 Cergy-Pontoise, France
h i g h l i g h t s EAF and AOD slag aggregates are more dense but more porous than natural aggregates. Very few mineral phases can show expansive reaction in EAF and AOD slag aggregates. The use of EAF and AOD slag aggregates improves concrete mechanical properties. Concretes made of EAF and AOD slag aggregates show a slightly but limited expansion.
a r t i c l e
i n f o
Article history: Received 28 April 2014 Received in revised form 17 October 2014 Accepted 5 December 2014
Keywords: Stainless steel slag EAF slag Stabilised AOD slag Coarse aggregates Concrete Mechanical properties Expansion Durability
a b s t r a c t The aim of this study is to investigate the opportunity using EAF and AOD slags aggregates in concrete. First, physicochemical and mineralogical properties of these stainless steel slag aggregates are determined. Second, the silico-calcareous aggregates of reference concretes are replaced by each of these steel slag aggregates in different proportions. The results show a slight improvement of the mechanical properties for concretes made of stainless steel slag aggregates. The use of EAF and AOD slag aggregates can slightly decrease concrete durability-related properties and increase linear expansion. But these characteristics fit the standards requested for construction use. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Currently, stainless steel is essentially manufactured from recycled scrap in electric arc furnace and refined in an Argon Oxygen Decarbonisation vessel. The produced steel slags contain EAF (Electric Arc Furnace) slag, AOD (Argon Oxygen Decarburisation) slag and LM (Ladle Metallurgy) slag. The use of these stainless steel slags was very limited because of their high chromium content [1,2]. However, recent research studies have shown that it is possible to reduce the leachable chromium of these slags and make them usable [3]. Stainless steel slags are nowadays only used as aggregates in road construction and their future recovery in concrete could be interesting. This new way of valorisation would help to achieve several aims: conservation of the natural aggregate resources and reduction of the high costs of slag’s processing and ⇑ Corresponding author. Tel.: +33 (0)1 34 25 69 75; fax: +33 (0)1 34 25 69 41. E-mail address: [email protected] (A.-L. Beaucour). http://dx.doi.org/10.1016/j.conbuildmat.2014.12.007 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
storage. To date, very few studies have been conducted on the use of slags from stainless steel process as aggregate in concrete. Published studies exclusively deal with carbon steel slag [4–14]. This work aims to study possibility of using slags from stainless steel process (EAF slag and stabilised AOD slag) as aggregates in concrete. According to published studies, EAF slag aggregates from carbon steel manufacture have a good mechanical strength and a high specific density (3.6 on average) mainly due to their high content in iron oxide. Despite their good mechanical behaviour, carbon steel slag can show sometimes a dimensional instability due to the free lime and magnesia hydration [5,6,9,10,15–17]. Indeed these chemical reactions result in an increased volume of oxides: about 5–10% for Ca(OH)2 and more than 100% for Mg(OH)2. Free lime hydrates quickly but free MgO hydrates at a much lower rate, causing significant volume change after months or even years [18]. To limit these expansive effects, steel slag is left outdoor and exposed to weather during several months. Published studies
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[4–8] and [13] also indicate that EAF slag aggregates from carbon steel manufacture have a water absorption coefficient which may strongly vary (from 0.2% to 10.5%) and an intrinsic porosity which is sometimes important. These both parameters can affect the durability of concrete made of steel slag aggregates. In stainless steel process, EAF slag is a similar crystalline rock but its chemical composition is different from that of EAF slag from carbon steel process [19]. Its content in iron oxide is lower and may explain its lower specific density. This could lead to a lower strength noticed in concrete made of this type of EAF slag aggregates. The AOD slag naturally falls to a powder on cooling due to its mineralogy. The dicalcium silicate (Ca2SiO4) phase is commonly present in this slag and the b ? c dicalcium silicate transition which occurs during cooling causes its disintegration [20–21]. Here, the Ca2SiO2 is stabilised under beta form, producing a solid crystalline slag in order to study their use as aggregates in concrete, that possibility has not yet been studied in the literature. The resulting steel slag has almost the same physical and mechanical characteristics as EAF slag but a different mineralogical composition [1,20]. Compared to BOF slag [22–23], stabilised AOD slag has a lower density and a much lower content in iron oxides. The results of an experimental study about two types of slags from stainless steel process, EAF slag and stabilised AOD slag, are presented. Firstly, physicochemical and mechanical properties of stainless steel slag aggregates are determined and compared to EN 12620 requirements for their use as aggregates in concrete. The mineralogical composition of these stainless steel slag aggregates is also analysed. Secondly, the natural aggregates (silico-calcareous) of reference concretes are substituted, partially to totally, by stainless steel slag aggregates (EAF slag and stabilised AOD slag) and several properties are measured on concrete samples. In this study, high-strength concretes are designed in order to highlight the influence of slag aggregate strength on the concrete behaviour. The measurement of the apparent density, compressive strength, tensile strength and elasticity modulus is useful to assess the impact of stainless steel slag aggregate strength on concrete’s mechanical properties. The evolution of concrete mechanical properties has been followed over the time up to 365 days. Similarly, to assess the impact of stainless steel slag aggregates on concrete’s durability, porosity and permeability are measured. Because of the possible expansion of stainless steel slag aggregates, the measurement of the longitudinal strain of prismatic specimens stored in water provides comparison of dimensional variations between concretes made of EAF or AOD aggregates and those made of silico-calcareous aggregates. SEM observations were also made on interface between stainless steel slag aggregates and cement paste.
2. Study of EAF slag and stabilised AOD slag aggregates In this study, the granular size of EAF slag and stabilised AOD slag aggregates is (4–20 mm). In order to limit the risk of expansion, these steel slags have been subjected to weathering in outdoor conditions during several months. Different tests on aggregates are made to determine their physico-chemical and mineralogical characteristics. Tests are also made according to EN 12620 standard in order to make sure they can be used as aggregates in concrete.
2.1. Tests To know the chemical composition of the aggregates, a study was performed through X-ray fluorescence analysis on the two types of slag aggregates. To complete this aggregates chemical analysis, other tests were made according to EN 1744-1 in order to measure the contents of total sulphur, acid-soluble sulphates, water-soluble chlorides and free lime. For these tests, samples are taken in accordance with the EN 932-1 standard procedures. The intrinsic characteristics of aggregates also depend on their mineralogical nature. To obtain more representative results of the mineralogical variability within a slag aggregates sample, the mineralogical study was conducted here in two steps. Firstly, an identification of aggregate’s groups under UV lamps is made with two wavelengths (254 and 365 nm) [24]. For each type of stainless steel slag aggregates, two groups are identified depending on their fluorescence. This first stage of mineralogical study provides a sampling of slag aggregates at a larger scale. Secondly, for each identified group, X-ray diffraction analyses and SEM observations with mapping by EDX are performed on five samples. For X-ray diffraction analysis, samples were scanned with a Philips diffractometer using copper Ka radiation, with 1130/00 generator and 1050/30 goniometer. The wavelength of the incident X-ray is equal to 1.54 Â. The diffractometer was running at 40 kV and 20 mA. The step widths was 0.025° from 6° to 66° 2-theta and 0.0025° from 64° to 70° 2-theta. The counting time was 1.5 s per step. For SEM observations, the used device is a LEICA S340i model with a 20 kV accelerating voltage. A Bruker’s Quantax X-ray energy dispersive (EDX) detector was also used to determine the mineralogical composition. Polished samples were evaluated using a backscatter detector. The apparent specific density and the water absorption are determined according to the EN 1097-6 standard. Pycnometer method is used. The Los Angeles coefficient is tested according to EN 1097-2 standard. All these tests define the physical and mechanical properties of the aggregates.
2.2. Chemical compositions of EAF slag and stabilised AOD slag aggregates The chemical composition of slag aggregates studied (EAF slag and stabilised AOD slag) is shown in Table 1. The SiO2 and the CaO are the main components of these steel slag aggregates: about 80% of the total chemical composition. The iron oxides content is very low for both slags. The stabilised AOD slag basicity index is 2.3 while that of EAF slag is around 1.3. EN 12620 standard indicates the maximum total sulphur content (Table 2) and acid-soluble sulphates content (Table 3). The maximum allowable values of these contents depend on the type of the aggregates. There is no specification about stainless steel slag aggregates. Only blast furnace slag aggregates are mentioned in the standard. For water-soluble chlorides, there is no maximum value for aggregates but its content is required in the calculation of the chloride content of the concrete. These minerals contents in EAF and AOD slag aggregates are reported in Table 4. All the measured values comply with the requirements of EN 12620 standard for the use as aggregates in concrete. Stainless steel slag aggregates contents in total sulphur and in acid-soluble sulphates are far lower than the required maximum
Table 1 Chemical composition of EAF slag and stabilised AOD slag aggregates (percent per mass). Contents (%)
CaO
SiO2
MgO
Al2O3
Cr2O3
MnO
TiO2
FeO
CaF2
Total
EAF slag Stabilised AOD slag
41.7 58.4
34.7 26.4
9.1 2.1
6.3 2.1
3.5 0.3
2.1 0.1
2.2 0.2
0.5 0.2
– 9.4
100.1 99.2
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Table 2 Maximum allowable values of total sulphur content [EN 12620]. Aggregates
Total sulphur content (percentage by mass)
Aggregates other than air-cooled blast furnace slag Air-cooled blast furnace slag
61 62
Table 3 Categories for maximum values of acid-soluble sulphate content [EN 12620]. Aggregates
Acid soluble sulphate content (percentage by mass)
Category AS
Aggregates other than aircooled blast furnace slag
60.2 60.8 >0.8 No requirement
AS0.2 AS0.8 ASDeclared ASNR
Air-cooled blast furnace slag
61.0 >1.0 No requirement
AS1.0 ASDeclared ASNR
values. This limits the risk of sulphate expansion. As stainless steel slag contents in water-soluble chlorides are very low, the risks of damage due to a bad concrete setting and to the steels corrosion are also reduced. Free lime content in EAF slag and stabilised AOD slag aggregates is very low, below 1% by weight. Thus expansion due to its hydration could be neglected [9]. Free MgO content in EAF slag and stabilised AOD slag aggregates could not have been determined by chemical analyses. However the presence of periclase (MgO) in stabilised AOD slag aggregates were detected through SEM and EDX analyses. 2.3. Mineralogical composition of EAF slag and AOD slag aggregates The mineralogical properties of EAF and AOD slags aggregates are determined through XRD analyses and SEM–EDX studies. The mineralogical composition of EAF slag aggregates is different from that of stabilised AOD slag aggregates. XRD patterns of EAF slag aggregates are very complex, with several overlapping peaks resulting from the main present minerals. X-ray diffraction analyses show that EAF slag aggregates contain: some silicates (akermanite Ca2Mg(Si2O7), merwinite Ca3Mg(SiO4)2, andratite Ca3Al2FeSi3O12, cuspidine Ca4Si2O7F2, rankinite Ca3Si2O7) and some metallic oxides (chromium spinels MgCr2O4 and perovskite CaTiO3). XRD patterns of AOD slags aggregates show less variety of minerals. The main identified mineral phases are dicalcium silicates ß-Ca2SiO4, fluorite CaF2, calcium sulphide CaS and periclase MgO. SEM-BSE observations with Energy-Dispersive X-ray (EDX) analyses are performed in order to study the distribution of the mineral phases identified by XRD analyses (Figs. 1–4). For EAF slag, they indicate that akermanite and merwinite are the main mineral phases. Those minerals are stable. For stabilised AOD slag, the mineralogical composition is more homogeneous, as they essentially contain the stabilised beta-dicalcium silicates. Some other secondary phases
Fig. 1. Mineralogical composition of EAF slag aggregates group 1.
are present like fluorite CaF2 and periclase MgO (in a very small amount). This mineralogical study shows that MgO is not found in the EAF slag aggregates, neither through X-ray diffraction, nor by SEM–EDX analyses. Mg is mainly within akermanite, merwinite or spinels. XRD analyses and MEB-EDX analyses do not show the presence of free lime. Only XRD spectrums of CaCO3 and (CaMg)CO3 were observed. It is likely that carbonation of CaO is done when slag aggregates are subjected to weathering in outdoor conditions. In stabilised AOD slag aggregates very small amount of MgO was observed. In conclusion, in the stainless steel slag aggregates, the content in mineral phases likely to present an expansive reaction (free CaO or free MgO) is very low. 2.4. Physical and mechanical characteristics of EAF slag and AOD slag aggregates EAF slag aggregates and stabilised AOD slag aggregates have the same physical characteristics even if they have different colours: dark grey or black for EAF slag aggregates and light green for stabilised AOD slag aggregates. These both stainless steel slag aggregates have generally a rough surface and a heterogeneous appearance; from compact to cavernous (Figs. 5 and 6). The apparent specific density and the water absorption coefficient of aggregates are indicated in Table 5. Three measurements are done for each type of aggregate. The EAF slag and stabilised AOD slag have an apparent specific density higher than that of the silico-calcareous aggregates: 2.8 against 2.5. Nevertheless these values remain lower than the average amount of 3.6 for
Table 4 Measured contents in stainless steel slag aggregates. Contents (percent by mass)
EAF slag
Stabilised AOD slag
Total sulphur (%) Acid-soluble sulphates (%) Water-soluble chlorides (%) Free lime (%)
0.26 0.02 0.0005 0.07
0.18 0.21 0.0020 0.07
Fig. 2. Mineralogical composition of EAF slag aggregates group 2.
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Fig. 3. Mineralogical composition of stabilised AOD slag aggregates group 1.
Fig. 5. EAF slag aggregates appearance.
Fig. 4. Mineralogical composition of stabilised AOD slag aggregates group 2.
the carbon steel slag aggregates [4–8]. Indeed these latter have a higher content of metal oxides. The water-absorption coefficients of stainless steel slag aggregates are slightly higher than that of the silico-calcareous aggregates. The stainless steel slag aggregates also have a very good resistance to fragmentation: their Los Angeles coefficients range from 16 to 23. According to EN 12620 standard, EAF slag aggregates category’s is LA25 and stabilised AOD slag aggregates’ one is LA20. LA20 and LA25 are respectively the second and third best categories of the nine proposed by EN 12620 standard. LA coefficient shows that the slag aggregates from stainless steel process have good mechanical characteristics. As specific feature, they are at the same time more dense but paradoxically more porous than silico-calcareous aggregates. This could be related to the presence of metal oxides in the solid phase of aggregates.
3. Study of concrete made of EAF slag and stabilised AOD slag aggregates 3.1. Mix proportion and specimen preparation In this study, all or part of the silico-calcareous aggregates of a reference concrete has been replaced by stainless steel slag aggregates. As aggregate strength plays a more important role in high-strength concrete than in ordinary one, seven types of highstrength concrete (ratio W/C = 0.3) are designed. They differ in their composition in coarse aggregates. One concrete contains only silico-calcareous aggregates (SC, reference concrete), concretes are composed of 50%, in volume percent of silico-calcareous aggregates
Fig. 6. Stabilised AOD slag aggregates appearance.
and 50% of stainless steel slag aggregates (EAF slag or stabilised AOD slag); the last two concretes are made of 100% of stainless steel slag aggregates (EAF slag or stabilised AOD slag). The volume of aggregates remains the same from one concrete to another. The mixing water is obtained by considering the water content in aggregates and the water provided by the superplasticiser. The different concretes have S4 consistency class (EN 206 standard) and the amount of superplasticiser varies from one mix concrete to another in order to keep the same slump. The modified polycarboxylatebased superplasticiser (cimfluid 3002) is used. The mix compositions of these concretes are presented in Table 6. For fine aggregates only silico-calcareous aggregates are used. Their water absorption is lower than that of silico-calcareous coarse aggregates and is estimated at 1%. The binder is Portland cement CEM I 52.5 R. 3.2. Methods Seven mixtures are tested. For each test and each mixture, three concrete samples stored in water at 20 °C are used. Mechanical properties of concretes are investigated. The measurements of compressive and tensile strengths were done on all studied concretes. Compressive and splitting tests were
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G. Adegoloye et al. / Construction and Building Materials 76 (2015) 313–321 Table 5 Physical properties of EAF slag and stabilised AOD slag aggregates. Aggregates
Apparent specific density Water absorption (mass%) Porosity (volume%) Los Angeles coefficient (LA)
EAF slag (4/20)
Stabilised AOD slag (4/20)
Silico-calcareous (4/20)
Average
Standard deviation
Average
Standard deviation
Average
2.8 2.6 7.3 23
±0.2 ±0.2 ±0.3 –
2.8 3.0 8.4 16
±0.3 ±0.2 ±0.3 –
2.5 1.7 4.2 30
carried out on 16 32 cm specimens according to EN 12390 standard. The loading rate is 0.5 MPa per second. The dynamic Young’s modulus is determined by acoustic test according to EN 12504-4 standard. The equipment used is the ultrasonic device ‘‘Pundit’’. It generates low-frequency ultrasonic pulses and measures the time to pass from one transducer to the other. The pulse velocity is calculated and the dynamic modulus is determined from its relation with the Poisson’s ratio and the concrete oven-dried density. The good acoustical contact between the sample and the face of each transducer is ensured by the use of petroleum jelly. Before the test, the 15 7 cm concrete samples are drying at 60 °C until constant weight in laboratory oven. The measurement of the mechanical properties is performed at various ages (28, 90 and 365 days) in order to identify possible damage with time, due to the forming of expansive compound. To facilitate the possible hydration reactions, the samples have been stored in water at 20 °C. To determine whether any expansion occurred, length measurement of prisms is made at 28, 90 and 365 days, following NF P 18454 standard (Fig. 7). Our tests consist in measuring longitudinal deformations of three concrete prismatic samples (7 7 28 cm) stored in water at 20 °C. The first value is taken on 24-h-aged concrete samples. Reference stainless steel studs are casted into the mid-points of the top and bottom faces of the prisms. An invar rod is used to calibrate the length of the measurement apparatus. For each measurement, the prism is kept in the same position (top and bottom position, the same prism face towards the apparatus). Some measurements of gas permeability and water porosity were also performed to obtain some durability indicators of concretes made of steel slag aggregates. The gas permeability test is made with a constant load device ‘‘CEMBUREAU’’ with nitrogen as neutral percolating gas. According to AFPC-AFREM protocol, concrete cylinders are cut by using a diamond blade saw, to obtain 15 5 cm concrete discs. Three discs are tested for each concrete mix. Only discs located in central portion of the cylinder are retained; discs at the both extremities are rejected. The samples are drying at 60 °C in laboratory oven. The density and water porosity of the concretes are determined by hydrostatic weighing according to NF EN 12390-7 standard. SEM observations were also carried out on concrete samples stored in water for 365 days to detect possible cracking (longitudinal or radial) at the interface between the stainless steel
Fig. 7. Measurement of concrete expansion.
slag aggregates and the cement paste. These possible cracking could be caused by the expansion of EAF or stabilised AOD slag aggregates.
4. Influence of EAF slag and stabilised AOD slag aggregates on concrete mechanical properties High-strength concretes made of EAF slag and stabilised AOD slag aggregates are slightly denser than the only natural aggregates-composed ones (silico-calcareous): apparent density of the hardened concrete made of silico-calcareous aggregates is 2.42 while the hardened concrete made of slag aggregates vary from 2.60 to 2.64, depending on the proportion of the slag aggregates in the concrete mixes. So the use of EAF slag or stabilised AOD slag aggregates increases the concrete’s density from 7% to 9%. That increase occurs with a slight rise of the concrete’s mechanical strengths. Fig. 8 shows the evolution of the concrete 28-days compressive and tensile strengths with the proportion of EAF slag and stabilised AOD slag coarse aggregates. Each value is the average of three measurements. Standard deviation of the data is ±3 MPa. The 28-days compressive strength of concretes made of natural aggregate is 67 MPa. When EAF slag or stabilised AOD slag are used as
Table 6 Mix composition of concrete. Contents for m3 of concrete
Silico-calcareous
EAF slag
Volume percent of slag in coarse aggregate Cement content CEM1 52.5 (kg) Silico-calcareous coarse aggregates 4/20 (kg) EAF slag coarse aggregates 4/20 (kg) AOD slag coarse aggregates 4/20 (kg) SC fine aggregates 0/4 (kg) Effective water (kg) Superplasticiser (kg) Slump (cm)
0% 500 1052 0 0 649 150 3.3 19
50% 500 526 592 0 649 150 3.6 18
Stabilised AOD slag 100% 500 0 1183 0 649 150 3.8 16
50% 500 526 0 600 649 150 3.6 19
100% 500 0 0 1200 649 150 3.7 17
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Mechanical strengths (MPa)
80
73 73
71 72
67
60 40 20 6.1
6.2 6.2
6.4 6.4
0 0 50 1 00 Volume percent of SC aggregate substitued by slag aggregates (%) Silico-calcareous compressive strength stabilised AOD compressive strength Stabilised AOD tensile strength
EAF compressive strength EAF tensile strength silico-calcareous tensile strength
Fig. 8. 28-Days compressive and splitting tensile strengths of concretes made of EAF slag and stabilised AOD slag aggregates.
Dynamic modulus (GPa)
60
46
50
53
51
55
EAF slag
40
stabilised AOD slag 20
Silico-calcareous 0 0
50
100
Volume percent of SC aggregate substituted by slag aggregates Fig. 9. 28-Days dynamic Young modulus of concretes made of EAF slag and stabilised AOD slag aggregates.
coarse aggregates, the mean value of the compressive strength is 73 MPa, showing an increase of 9%. Tensile strength also increases when the natural aggregates are replaced by stainless steel slag aggregates. The results show a small tensile strength variation between traditional and steel slag aggregate concretes. This difference would be more evident for normal strength concrete. Indeed, the improvement of the Interfacial Transition Zone (ITZ) due to the highest surface porosity of stainless steel slag aggregates is more pronounced for high w/c ratio concretes. Fig. 9 shows that the dynamic Young’s modulus also increases when natural aggregates are replaced by EAF slag or stabilised AOD slag aggregates. The increase in dynamic modulus is 10% with EAF slag coarse aggregates and 20% with stabilised AOD slag coarse aggregates. The higher dynamic modulus is due to the higher density of EAF slag and stabilised AOD slag aggregates compared to that of the silico-calcareous aggregates. The good mechanical performances certainly result from the higher strength of stainless steel slag aggregates (Los Angeles coefficient LA = 23 for the EAF slag and LA = 16 for the stabilised AOD slag). It could also result from the crushed shape, the rough surface and the higher porosity of the used stainless steel slag aggregates, which provide a better adhesion with the cement paste. These results are consistent with the literature [4,8,12]. The analysis of failure planes show transgranular fractures and confirm the good matrix-aggregate adhesion for slag aggregate and for natural aggregate (Fig. 10). This is linked to the low water to cement ratio of high-strength concrete. Compressive strengths over the time of concrete made of slag aggregates are presented on Fig. 11. Over the time, no drop has
been noticed regarding the strength of concrete made of EAF and stabilised AOD slag aggregates.
5. Durability indicators and volume stability over the time of concretes made of EAF slag and stabilised AOD slag aggregates 5.1. Porosity and permeability Porosity and gas permeability are durability indicators. In this study, the 28-days water porosity of high-strength concretes made of steel slag aggregates varies from 12.1% to 12.4% (Table 7) and the 28-days gas permeability from 2.0 to 2.3 10 16 m2 (Fig. 12). Those values are slightly higher than those of high-strength concretes made of silico-calcareous aggregates. This is mainly due to the higher porosity of EAF slag and stabilised AOD slag aggregates. For a same concrete mixture, concrete made of steel slag aggregates are thereby slightly more sensitive to aggressive agents and to freeze–thaw cycles. According to the approach developed in the AFGC guide (2004), concretes made of steel slag aggregates belong to the average category of durability (porosity between 12% and 14% and permeability between 1 and 3 10 16 m2). Values of Table 7 and Fig. 12 match with the recommended limits for building constructions of 30–50 years service-life [25]. However, the porosity and gas permeability of concretes made of stainless steel slag aggregates may be improved by adding fillers which could fill the voids between the aggregate particles and the cement particles. Such as in [4], the use of air-entrainer admixture could
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Table 7 28-Days water porosity of concretes made of EAF slag and stabilised AOD slag aggregates. Silico-calcareous aggregates
EAF slag aggregates
Stabilised AOD slag aggregates
Volume percent of slag in coarse aggregate (%) Porosity (%)
0
50
100
50
100
9.9
10.9
12.4
10.8
12.1
Permeability 10-16m2
Concrete with
2.3 2.2
1.9 1.6 0.8 0 0.9
50 EAF slag
100 stabilised AOD slag
Volume percent of silico-calcareous aggregates substituted by slag aggregates Fig. 12. 28-Days permeability of concretes made of EAF slag and stabilised AOD slag aggregates. Fig. 10. Failure plane after compressive strength test on concrete made of steel slag aggregate and silico-calcareous aggregate. Table 8 Evolution over the time of water porosity of concretes.
compressive strength MPa
90 100% silico-calcareous 100% EAF slag 100% stabilised AOD slag
28 days
90 days
365 days
9.9 12.4 12.1
9.9 12.4 12.1
9.6 12.0 12.0
60
30
0 0
100
EAF slag
200 Time (days) Stabilised AOD slag
300
400
Silico-calcareous
Fig. 11. Compressive strength over the time of concretes made of EAF slag and stabilised AOD slag aggregates.
help to improve the durability-related properties of concrete made of stainless steel slag aggregates. The water porosity over the time (28, 90 and 365 days) regarding concretes made of 100% stainless steel slag coarse aggregates is presented in Table 8. It shows that for stabilised AOD and EAF slag aggregates concretes the water porosity has slightly reduced over the time, like that of natural aggregate concrete. That porosity reduction is consistent with the increase of the compressive strength over the time. 5.2. Volume stability and microscopic observations about concretes made of EAF slag and stabilised AOD slag aggregates Fig. 13 shows the expansion curve of EAF slag and stabilised AOD slag aggregates concretes during 365 days. Each value is the average of three measurements on three different prism samples. The results indicate shrinkage at 28 days for concretes with EAF
slag aggregates and concretes with silico-calcareous aggregates. This shrinkage results from the chemical reaction of the cement with water. It is not observed for concrete made of stabilised AOD slag aggregates in which it could have been offset by swelling phenomena. After one year, the concretes made of stainless steel slag aggregates show a higher expansion than that of concretes made of silico-calcareous aggregates: 0.017% for concretes with EAF slag aggregates and 0.026% for concretes made of stabilised AOD slag aggregates, against 0.008% for concrete made of silicocalcareous aggregates. These values are consistent with the expansion of concretes made of nonreactive alkali aggregates (from 0.01% to 0.03% at one year) [26]. The expansion increases with the volume fraction of stainless steel slag coarse aggregates. XRD analyses did not show the presence of glassy phases in slags so that the expansion cannot result from alkali-siliceous reaction. No ettringite is determined in SEM observations either. Consequently, additional dimensional change could be related to some hydration reactions with free lime or free magnesium oxides. XRD and SEM–EDS analyses only show a few proportion of MgO in stabilised AOD slag. Only few content of free lime was also detected according to EN 1744-1 standard in both aggregates. However, according to FD P 18-456 standard, the maximum limit acceptable for the longitudinal strains (0.03% at 12 months for concrete stored in water at 60 °C) has not been exceeded. SEM observations of crack network at the paste-aggregate interface and in concrete samples were carried out (Figs. 14 and 15). For these observations, samples are taken in 365-days concretes made of EAF slag, stabilised AOD slag and silico-calcareous aggregates. SEM observations show intact coarse aggregates (EAF slag, stabilised AOD slag and silico-calcareous) but cracking through the cement paste and in some areas at the interfaces between the paste and EAF slag or stabilised AOD aggregates. No intergranular
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0.04
ΔL/L0 (%)
0.03 Silico-calcareous
0.02
50% EAF
0.01
100% EAF
0 -0.01
0
100
200
300
400
50% AOD 100% AOD
-0.02 -0.03 Time (days) Fig. 13. Longitudinal strain of high-strength concretes made of EAF slag and stabilised AOD slag aggregates.
Fig. 14. SEM observations of an interface between EAF slag aggregates and cement paste in 365-days concrete. Slight cracking in some areas.
substitute of the natural aggregates (silico-calcareous). Stainless steel slag aggregates have an apparent specific density higher than that of the silico-calcareous aggregates. They also show a very good resistance to fragmentation. Physical properties and mechanical strengths of concrete made of stainless steel slag aggregates are slightly higher than those of concrete made of silico-calcareous aggregates. Compressive strength increases by 9% when the natural aggregates are replaced by stainless steel slag aggregates. Tensile strength increases by 3%. The increase in dynamic modulus is 10% with EAF slag coarse aggregates and 20% with stabilised AOD slag coarse aggregates. Although durability indicators for concrete made of stainless steel slag aggregates (water porosity and gas permeability) show values slightly higher than those of traditional concrete. The expansion of 365-days water-cured concrete specimen increases with the volume fraction of stainless steel slag coarse aggregates. All the measured values match with the recommended limits for building constructions. The mechanical strengths evolution over the time and the conservation of the Young’s modulus indicate that there is no damage due to possible chemical reactions or expansive reactions of stainless steel slag aggregates. From the results obtained on the tested mixtures, this study reveals a new type of concretes with specific properties; as they are denser and more porous than traditional concrete. Furthermore, stainless steel slag aggregate concretes provide an environmental solution: sustainable recovery of industrial byproducts and conservation of natural resources.
Acknowledgements The authors thank UGITECH SA for financial contribution and scientific collaboration.
References
Fig. 15. SEM observations of an interface between stabilised AOD slag aggregates and cement paste in 365-days concrete. Slight cracking in some areas.
cracking, which could indicate a potential expansive reaction, has been observed. 6. Conclusion According to the criteria of EN 12620 Aggregates for concrete standard, the studied stainless steel slag aggregates (EAF slag and stabilised AOD slag) may be used in concrete as a
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Concretes made of EAF slag and AOD slag aggregates from stainless steel process: Mechanical properties and durability G. Adegoloye, A.-L. Beaucour ⇑, S. Ortola, A. Noumowé University of Cergy-Pontoise, Laboratoire de Mécanique & Matériaux du Génie Civil, EA 4114, F-95000 Cergy-Pontoise, France
h i g h l i g h t s EAF and AOD slag aggregates are more dense but more porous than natural aggregates. Very few mineral phases can show expansive reaction in EAF and AOD slag aggregates. The use of EAF and AOD slag aggregates improves concrete mechanical properties. Concretes made of EAF and AOD slag aggregates show a slightly but limited expansion.
a r t i c l e
i n f o
Article history: Received 28 April 2014 Received in revised form 17 October 2014 Accepted 5 December 2014
Keywords: Stainless steel slag EAF slag Stabilised AOD slag Coarse aggregates Concrete Mechanical properties Expansion Durability
a b s t r a c t The aim of this study is to investigate the opportunity using EAF and AOD slags aggregates in concrete. First, physicochemical and mineralogical properties of these stainless steel slag aggregates are determined. Second, the silico-calcareous aggregates of reference concretes are replaced by each of these steel slag aggregates in different proportions. The results show a slight improvement of the mechanical properties for concretes made of stainless steel slag aggregates. The use of EAF and AOD slag aggregates can slightly decrease concrete durability-related properties and increase linear expansion. But these characteristics fit the standards requested for construction use. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Currently, stainless steel is essentially manufactured from recycled scrap in electric arc furnace and refined in an Argon Oxygen Decarbonisation vessel. The produced steel slags contain EAF (Electric Arc Furnace) slag, AOD (Argon Oxygen Decarburisation) slag and LM (Ladle Metallurgy) slag. The use of these stainless steel slags was very limited because of their high chromium content [1,2]. However, recent research studies have shown that it is possible to reduce the leachable chromium of these slags and make them usable [3]. Stainless steel slags are nowadays only used as aggregates in road construction and their future recovery in concrete could be interesting. This new way of valorisation would help to achieve several aims: conservation of the natural aggregate resources and reduction of the high costs of slag’s processing and ⇑ Corresponding author. Tel.: +33 (0)1 34 25 69 75; fax: +33 (0)1 34 25 69 41. E-mail address: [email protected] (A.-L. Beaucour). http://dx.doi.org/10.1016/j.conbuildmat.2014.12.007 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
storage. To date, very few studies have been conducted on the use of slags from stainless steel process as aggregate in concrete. Published studies exclusively deal with carbon steel slag [4–14]. This work aims to study possibility of using slags from stainless steel process (EAF slag and stabilised AOD slag) as aggregates in concrete. According to published studies, EAF slag aggregates from carbon steel manufacture have a good mechanical strength and a high specific density (3.6 on average) mainly due to their high content in iron oxide. Despite their good mechanical behaviour, carbon steel slag can show sometimes a dimensional instability due to the free lime and magnesia hydration [5,6,9,10,15–17]. Indeed these chemical reactions result in an increased volume of oxides: about 5–10% for Ca(OH)2 and more than 100% for Mg(OH)2. Free lime hydrates quickly but free MgO hydrates at a much lower rate, causing significant volume change after months or even years [18]. To limit these expansive effects, steel slag is left outdoor and exposed to weather during several months. Published studies
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[4–8] and [13] also indicate that EAF slag aggregates from carbon steel manufacture have a water absorption coefficient which may strongly vary (from 0.2% to 10.5%) and an intrinsic porosity which is sometimes important. These both parameters can affect the durability of concrete made of steel slag aggregates. In stainless steel process, EAF slag is a similar crystalline rock but its chemical composition is different from that of EAF slag from carbon steel process [19]. Its content in iron oxide is lower and may explain its lower specific density. This could lead to a lower strength noticed in concrete made of this type of EAF slag aggregates. The AOD slag naturally falls to a powder on cooling due to its mineralogy. The dicalcium silicate (Ca2SiO4) phase is commonly present in this slag and the b ? c dicalcium silicate transition which occurs during cooling causes its disintegration [20–21]. Here, the Ca2SiO2 is stabilised under beta form, producing a solid crystalline slag in order to study their use as aggregates in concrete, that possibility has not yet been studied in the literature. The resulting steel slag has almost the same physical and mechanical characteristics as EAF slag but a different mineralogical composition [1,20]. Compared to BOF slag [22–23], stabilised AOD slag has a lower density and a much lower content in iron oxides. The results of an experimental study about two types of slags from stainless steel process, EAF slag and stabilised AOD slag, are presented. Firstly, physicochemical and mechanical properties of stainless steel slag aggregates are determined and compared to EN 12620 requirements for their use as aggregates in concrete. The mineralogical composition of these stainless steel slag aggregates is also analysed. Secondly, the natural aggregates (silico-calcareous) of reference concretes are substituted, partially to totally, by stainless steel slag aggregates (EAF slag and stabilised AOD slag) and several properties are measured on concrete samples. In this study, high-strength concretes are designed in order to highlight the influence of slag aggregate strength on the concrete behaviour. The measurement of the apparent density, compressive strength, tensile strength and elasticity modulus is useful to assess the impact of stainless steel slag aggregate strength on concrete’s mechanical properties. The evolution of concrete mechanical properties has been followed over the time up to 365 days. Similarly, to assess the impact of stainless steel slag aggregates on concrete’s durability, porosity and permeability are measured. Because of the possible expansion of stainless steel slag aggregates, the measurement of the longitudinal strain of prismatic specimens stored in water provides comparison of dimensional variations between concretes made of EAF or AOD aggregates and those made of silico-calcareous aggregates. SEM observations were also made on interface between stainless steel slag aggregates and cement paste.
2. Study of EAF slag and stabilised AOD slag aggregates In this study, the granular size of EAF slag and stabilised AOD slag aggregates is (4–20 mm). In order to limit the risk of expansion, these steel slags have been subjected to weathering in outdoor conditions during several months. Different tests on aggregates are made to determine their physico-chemical and mineralogical characteristics. Tests are also made according to EN 12620 standard in order to make sure they can be used as aggregates in concrete.
2.1. Tests To know the chemical composition of the aggregates, a study was performed through X-ray fluorescence analysis on the two types of slag aggregates. To complete this aggregates chemical analysis, other tests were made according to EN 1744-1 in order to measure the contents of total sulphur, acid-soluble sulphates, water-soluble chlorides and free lime. For these tests, samples are taken in accordance with the EN 932-1 standard procedures. The intrinsic characteristics of aggregates also depend on their mineralogical nature. To obtain more representative results of the mineralogical variability within a slag aggregates sample, the mineralogical study was conducted here in two steps. Firstly, an identification of aggregate’s groups under UV lamps is made with two wavelengths (254 and 365 nm) [24]. For each type of stainless steel slag aggregates, two groups are identified depending on their fluorescence. This first stage of mineralogical study provides a sampling of slag aggregates at a larger scale. Secondly, for each identified group, X-ray diffraction analyses and SEM observations with mapping by EDX are performed on five samples. For X-ray diffraction analysis, samples were scanned with a Philips diffractometer using copper Ka radiation, with 1130/00 generator and 1050/30 goniometer. The wavelength of the incident X-ray is equal to 1.54 Â. The diffractometer was running at 40 kV and 20 mA. The step widths was 0.025° from 6° to 66° 2-theta and 0.0025° from 64° to 70° 2-theta. The counting time was 1.5 s per step. For SEM observations, the used device is a LEICA S340i model with a 20 kV accelerating voltage. A Bruker’s Quantax X-ray energy dispersive (EDX) detector was also used to determine the mineralogical composition. Polished samples were evaluated using a backscatter detector. The apparent specific density and the water absorption are determined according to the EN 1097-6 standard. Pycnometer method is used. The Los Angeles coefficient is tested according to EN 1097-2 standard. All these tests define the physical and mechanical properties of the aggregates.
2.2. Chemical compositions of EAF slag and stabilised AOD slag aggregates The chemical composition of slag aggregates studied (EAF slag and stabilised AOD slag) is shown in Table 1. The SiO2 and the CaO are the main components of these steel slag aggregates: about 80% of the total chemical composition. The iron oxides content is very low for both slags. The stabilised AOD slag basicity index is 2.3 while that of EAF slag is around 1.3. EN 12620 standard indicates the maximum total sulphur content (Table 2) and acid-soluble sulphates content (Table 3). The maximum allowable values of these contents depend on the type of the aggregates. There is no specification about stainless steel slag aggregates. Only blast furnace slag aggregates are mentioned in the standard. For water-soluble chlorides, there is no maximum value for aggregates but its content is required in the calculation of the chloride content of the concrete. These minerals contents in EAF and AOD slag aggregates are reported in Table 4. All the measured values comply with the requirements of EN 12620 standard for the use as aggregates in concrete. Stainless steel slag aggregates contents in total sulphur and in acid-soluble sulphates are far lower than the required maximum
Table 1 Chemical composition of EAF slag and stabilised AOD slag aggregates (percent per mass). Contents (%)
CaO
SiO2
MgO
Al2O3
Cr2O3
MnO
TiO2
FeO
CaF2
Total
EAF slag Stabilised AOD slag
41.7 58.4
34.7 26.4
9.1 2.1
6.3 2.1
3.5 0.3
2.1 0.1
2.2 0.2
0.5 0.2
– 9.4
100.1 99.2
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Table 2 Maximum allowable values of total sulphur content [EN 12620]. Aggregates
Total sulphur content (percentage by mass)
Aggregates other than air-cooled blast furnace slag Air-cooled blast furnace slag
61 62
Table 3 Categories for maximum values of acid-soluble sulphate content [EN 12620]. Aggregates
Acid soluble sulphate content (percentage by mass)
Category AS
Aggregates other than aircooled blast furnace slag
60.2 60.8 >0.8 No requirement
AS0.2 AS0.8 ASDeclared ASNR
Air-cooled blast furnace slag
61.0 >1.0 No requirement
AS1.0 ASDeclared ASNR
values. This limits the risk of sulphate expansion. As stainless steel slag contents in water-soluble chlorides are very low, the risks of damage due to a bad concrete setting and to the steels corrosion are also reduced. Free lime content in EAF slag and stabilised AOD slag aggregates is very low, below 1% by weight. Thus expansion due to its hydration could be neglected [9]. Free MgO content in EAF slag and stabilised AOD slag aggregates could not have been determined by chemical analyses. However the presence of periclase (MgO) in stabilised AOD slag aggregates were detected through SEM and EDX analyses. 2.3. Mineralogical composition of EAF slag and AOD slag aggregates The mineralogical properties of EAF and AOD slags aggregates are determined through XRD analyses and SEM–EDX studies. The mineralogical composition of EAF slag aggregates is different from that of stabilised AOD slag aggregates. XRD patterns of EAF slag aggregates are very complex, with several overlapping peaks resulting from the main present minerals. X-ray diffraction analyses show that EAF slag aggregates contain: some silicates (akermanite Ca2Mg(Si2O7), merwinite Ca3Mg(SiO4)2, andratite Ca3Al2FeSi3O12, cuspidine Ca4Si2O7F2, rankinite Ca3Si2O7) and some metallic oxides (chromium spinels MgCr2O4 and perovskite CaTiO3). XRD patterns of AOD slags aggregates show less variety of minerals. The main identified mineral phases are dicalcium silicates ß-Ca2SiO4, fluorite CaF2, calcium sulphide CaS and periclase MgO. SEM-BSE observations with Energy-Dispersive X-ray (EDX) analyses are performed in order to study the distribution of the mineral phases identified by XRD analyses (Figs. 1–4). For EAF slag, they indicate that akermanite and merwinite are the main mineral phases. Those minerals are stable. For stabilised AOD slag, the mineralogical composition is more homogeneous, as they essentially contain the stabilised beta-dicalcium silicates. Some other secondary phases
Fig. 1. Mineralogical composition of EAF slag aggregates group 1.
are present like fluorite CaF2 and periclase MgO (in a very small amount). This mineralogical study shows that MgO is not found in the EAF slag aggregates, neither through X-ray diffraction, nor by SEM–EDX analyses. Mg is mainly within akermanite, merwinite or spinels. XRD analyses and MEB-EDX analyses do not show the presence of free lime. Only XRD spectrums of CaCO3 and (CaMg)CO3 were observed. It is likely that carbonation of CaO is done when slag aggregates are subjected to weathering in outdoor conditions. In stabilised AOD slag aggregates very small amount of MgO was observed. In conclusion, in the stainless steel slag aggregates, the content in mineral phases likely to present an expansive reaction (free CaO or free MgO) is very low. 2.4. Physical and mechanical characteristics of EAF slag and AOD slag aggregates EAF slag aggregates and stabilised AOD slag aggregates have the same physical characteristics even if they have different colours: dark grey or black for EAF slag aggregates and light green for stabilised AOD slag aggregates. These both stainless steel slag aggregates have generally a rough surface and a heterogeneous appearance; from compact to cavernous (Figs. 5 and 6). The apparent specific density and the water absorption coefficient of aggregates are indicated in Table 5. Three measurements are done for each type of aggregate. The EAF slag and stabilised AOD slag have an apparent specific density higher than that of the silico-calcareous aggregates: 2.8 against 2.5. Nevertheless these values remain lower than the average amount of 3.6 for
Table 4 Measured contents in stainless steel slag aggregates. Contents (percent by mass)
EAF slag
Stabilised AOD slag
Total sulphur (%) Acid-soluble sulphates (%) Water-soluble chlorides (%) Free lime (%)
0.26 0.02 0.0005 0.07
0.18 0.21 0.0020 0.07
Fig. 2. Mineralogical composition of EAF slag aggregates group 2.
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Fig. 3. Mineralogical composition of stabilised AOD slag aggregates group 1.
Fig. 5. EAF slag aggregates appearance.
Fig. 4. Mineralogical composition of stabilised AOD slag aggregates group 2.
the carbon steel slag aggregates [4–8]. Indeed these latter have a higher content of metal oxides. The water-absorption coefficients of stainless steel slag aggregates are slightly higher than that of the silico-calcareous aggregates. The stainless steel slag aggregates also have a very good resistance to fragmentation: their Los Angeles coefficients range from 16 to 23. According to EN 12620 standard, EAF slag aggregates category’s is LA25 and stabilised AOD slag aggregates’ one is LA20. LA20 and LA25 are respectively the second and third best categories of the nine proposed by EN 12620 standard. LA coefficient shows that the slag aggregates from stainless steel process have good mechanical characteristics. As specific feature, they are at the same time more dense but paradoxically more porous than silico-calcareous aggregates. This could be related to the presence of metal oxides in the solid phase of aggregates.
3. Study of concrete made of EAF slag and stabilised AOD slag aggregates 3.1. Mix proportion and specimen preparation In this study, all or part of the silico-calcareous aggregates of a reference concrete has been replaced by stainless steel slag aggregates. As aggregate strength plays a more important role in high-strength concrete than in ordinary one, seven types of highstrength concrete (ratio W/C = 0.3) are designed. They differ in their composition in coarse aggregates. One concrete contains only silico-calcareous aggregates (SC, reference concrete), concretes are composed of 50%, in volume percent of silico-calcareous aggregates
Fig. 6. Stabilised AOD slag aggregates appearance.
and 50% of stainless steel slag aggregates (EAF slag or stabilised AOD slag); the last two concretes are made of 100% of stainless steel slag aggregates (EAF slag or stabilised AOD slag). The volume of aggregates remains the same from one concrete to another. The mixing water is obtained by considering the water content in aggregates and the water provided by the superplasticiser. The different concretes have S4 consistency class (EN 206 standard) and the amount of superplasticiser varies from one mix concrete to another in order to keep the same slump. The modified polycarboxylatebased superplasticiser (cimfluid 3002) is used. The mix compositions of these concretes are presented in Table 6. For fine aggregates only silico-calcareous aggregates are used. Their water absorption is lower than that of silico-calcareous coarse aggregates and is estimated at 1%. The binder is Portland cement CEM I 52.5 R. 3.2. Methods Seven mixtures are tested. For each test and each mixture, three concrete samples stored in water at 20 °C are used. Mechanical properties of concretes are investigated. The measurements of compressive and tensile strengths were done on all studied concretes. Compressive and splitting tests were
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Apparent specific density Water absorption (mass%) Porosity (volume%) Los Angeles coefficient (LA)
EAF slag (4/20)
Stabilised AOD slag (4/20)
Silico-calcareous (4/20)
Average
Standard deviation
Average
Standard deviation
Average
2.8 2.6 7.3 23
±0.2 ±0.2 ±0.3 –
2.8 3.0 8.4 16
±0.3 ±0.2 ±0.3 –
2.5 1.7 4.2 30
carried out on 16 32 cm specimens according to EN 12390 standard. The loading rate is 0.5 MPa per second. The dynamic Young’s modulus is determined by acoustic test according to EN 12504-4 standard. The equipment used is the ultrasonic device ‘‘Pundit’’. It generates low-frequency ultrasonic pulses and measures the time to pass from one transducer to the other. The pulse velocity is calculated and the dynamic modulus is determined from its relation with the Poisson’s ratio and the concrete oven-dried density. The good acoustical contact between the sample and the face of each transducer is ensured by the use of petroleum jelly. Before the test, the 15 7 cm concrete samples are drying at 60 °C until constant weight in laboratory oven. The measurement of the mechanical properties is performed at various ages (28, 90 and 365 days) in order to identify possible damage with time, due to the forming of expansive compound. To facilitate the possible hydration reactions, the samples have been stored in water at 20 °C. To determine whether any expansion occurred, length measurement of prisms is made at 28, 90 and 365 days, following NF P 18454 standard (Fig. 7). Our tests consist in measuring longitudinal deformations of three concrete prismatic samples (7 7 28 cm) stored in water at 20 °C. The first value is taken on 24-h-aged concrete samples. Reference stainless steel studs are casted into the mid-points of the top and bottom faces of the prisms. An invar rod is used to calibrate the length of the measurement apparatus. For each measurement, the prism is kept in the same position (top and bottom position, the same prism face towards the apparatus). Some measurements of gas permeability and water porosity were also performed to obtain some durability indicators of concretes made of steel slag aggregates. The gas permeability test is made with a constant load device ‘‘CEMBUREAU’’ with nitrogen as neutral percolating gas. According to AFPC-AFREM protocol, concrete cylinders are cut by using a diamond blade saw, to obtain 15 5 cm concrete discs. Three discs are tested for each concrete mix. Only discs located in central portion of the cylinder are retained; discs at the both extremities are rejected. The samples are drying at 60 °C in laboratory oven. The density and water porosity of the concretes are determined by hydrostatic weighing according to NF EN 12390-7 standard. SEM observations were also carried out on concrete samples stored in water for 365 days to detect possible cracking (longitudinal or radial) at the interface between the stainless steel
Fig. 7. Measurement of concrete expansion.
slag aggregates and the cement paste. These possible cracking could be caused by the expansion of EAF or stabilised AOD slag aggregates.
4. Influence of EAF slag and stabilised AOD slag aggregates on concrete mechanical properties High-strength concretes made of EAF slag and stabilised AOD slag aggregates are slightly denser than the only natural aggregates-composed ones (silico-calcareous): apparent density of the hardened concrete made of silico-calcareous aggregates is 2.42 while the hardened concrete made of slag aggregates vary from 2.60 to 2.64, depending on the proportion of the slag aggregates in the concrete mixes. So the use of EAF slag or stabilised AOD slag aggregates increases the concrete’s density from 7% to 9%. That increase occurs with a slight rise of the concrete’s mechanical strengths. Fig. 8 shows the evolution of the concrete 28-days compressive and tensile strengths with the proportion of EAF slag and stabilised AOD slag coarse aggregates. Each value is the average of three measurements. Standard deviation of the data is ±3 MPa. The 28-days compressive strength of concretes made of natural aggregate is 67 MPa. When EAF slag or stabilised AOD slag are used as
Table 6 Mix composition of concrete. Contents for m3 of concrete
Silico-calcareous
EAF slag
Volume percent of slag in coarse aggregate Cement content CEM1 52.5 (kg) Silico-calcareous coarse aggregates 4/20 (kg) EAF slag coarse aggregates 4/20 (kg) AOD slag coarse aggregates 4/20 (kg) SC fine aggregates 0/4 (kg) Effective water (kg) Superplasticiser (kg) Slump (cm)
0% 500 1052 0 0 649 150 3.3 19
50% 500 526 592 0 649 150 3.6 18
Stabilised AOD slag 100% 500 0 1183 0 649 150 3.8 16
50% 500 526 0 600 649 150 3.6 19
100% 500 0 0 1200 649 150 3.7 17
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Mechanical strengths (MPa)
80
73 73
71 72
67
60 40 20 6.1
6.2 6.2
6.4 6.4
0 0 50 1 00 Volume percent of SC aggregate substitued by slag aggregates (%) Silico-calcareous compressive strength stabilised AOD compressive strength Stabilised AOD tensile strength
EAF compressive strength EAF tensile strength silico-calcareous tensile strength
Fig. 8. 28-Days compressive and splitting tensile strengths of concretes made of EAF slag and stabilised AOD slag aggregates.
Dynamic modulus (GPa)
60
46
50
53
51
55
EAF slag
40
stabilised AOD slag 20
Silico-calcareous 0 0
50
100
Volume percent of SC aggregate substituted by slag aggregates Fig. 9. 28-Days dynamic Young modulus of concretes made of EAF slag and stabilised AOD slag aggregates.
coarse aggregates, the mean value of the compressive strength is 73 MPa, showing an increase of 9%. Tensile strength also increases when the natural aggregates are replaced by stainless steel slag aggregates. The results show a small tensile strength variation between traditional and steel slag aggregate concretes. This difference would be more evident for normal strength concrete. Indeed, the improvement of the Interfacial Transition Zone (ITZ) due to the highest surface porosity of stainless steel slag aggregates is more pronounced for high w/c ratio concretes. Fig. 9 shows that the dynamic Young’s modulus also increases when natural aggregates are replaced by EAF slag or stabilised AOD slag aggregates. The increase in dynamic modulus is 10% with EAF slag coarse aggregates and 20% with stabilised AOD slag coarse aggregates. The higher dynamic modulus is due to the higher density of EAF slag and stabilised AOD slag aggregates compared to that of the silico-calcareous aggregates. The good mechanical performances certainly result from the higher strength of stainless steel slag aggregates (Los Angeles coefficient LA = 23 for the EAF slag and LA = 16 for the stabilised AOD slag). It could also result from the crushed shape, the rough surface and the higher porosity of the used stainless steel slag aggregates, which provide a better adhesion with the cement paste. These results are consistent with the literature [4,8,12]. The analysis of failure planes show transgranular fractures and confirm the good matrix-aggregate adhesion for slag aggregate and for natural aggregate (Fig. 10). This is linked to the low water to cement ratio of high-strength concrete. Compressive strengths over the time of concrete made of slag aggregates are presented on Fig. 11. Over the time, no drop has
been noticed regarding the strength of concrete made of EAF and stabilised AOD slag aggregates.
5. Durability indicators and volume stability over the time of concretes made of EAF slag and stabilised AOD slag aggregates 5.1. Porosity and permeability Porosity and gas permeability are durability indicators. In this study, the 28-days water porosity of high-strength concretes made of steel slag aggregates varies from 12.1% to 12.4% (Table 7) and the 28-days gas permeability from 2.0 to 2.3 10 16 m2 (Fig. 12). Those values are slightly higher than those of high-strength concretes made of silico-calcareous aggregates. This is mainly due to the higher porosity of EAF slag and stabilised AOD slag aggregates. For a same concrete mixture, concrete made of steel slag aggregates are thereby slightly more sensitive to aggressive agents and to freeze–thaw cycles. According to the approach developed in the AFGC guide (2004), concretes made of steel slag aggregates belong to the average category of durability (porosity between 12% and 14% and permeability between 1 and 3 10 16 m2). Values of Table 7 and Fig. 12 match with the recommended limits for building constructions of 30–50 years service-life [25]. However, the porosity and gas permeability of concretes made of stainless steel slag aggregates may be improved by adding fillers which could fill the voids between the aggregate particles and the cement particles. Such as in [4], the use of air-entrainer admixture could
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Table 7 28-Days water porosity of concretes made of EAF slag and stabilised AOD slag aggregates. Silico-calcareous aggregates
EAF slag aggregates
Stabilised AOD slag aggregates
Volume percent of slag in coarse aggregate (%) Porosity (%)
0
50
100
50
100
9.9
10.9
12.4
10.8
12.1
Permeability 10-16m2
Concrete with
2.3 2.2
1.9 1.6 0.8 0 0.9
50 EAF slag
100 stabilised AOD slag
Volume percent of silico-calcareous aggregates substituted by slag aggregates Fig. 12. 28-Days permeability of concretes made of EAF slag and stabilised AOD slag aggregates. Fig. 10. Failure plane after compressive strength test on concrete made of steel slag aggregate and silico-calcareous aggregate. Table 8 Evolution over the time of water porosity of concretes.
compressive strength MPa
90 100% silico-calcareous 100% EAF slag 100% stabilised AOD slag
28 days
90 days
365 days
9.9 12.4 12.1
9.9 12.4 12.1
9.6 12.0 12.0
60
30
0 0
100
EAF slag
200 Time (days) Stabilised AOD slag
300
400
Silico-calcareous
Fig. 11. Compressive strength over the time of concretes made of EAF slag and stabilised AOD slag aggregates.
help to improve the durability-related properties of concrete made of stainless steel slag aggregates. The water porosity over the time (28, 90 and 365 days) regarding concretes made of 100% stainless steel slag coarse aggregates is presented in Table 8. It shows that for stabilised AOD and EAF slag aggregates concretes the water porosity has slightly reduced over the time, like that of natural aggregate concrete. That porosity reduction is consistent with the increase of the compressive strength over the time. 5.2. Volume stability and microscopic observations about concretes made of EAF slag and stabilised AOD slag aggregates Fig. 13 shows the expansion curve of EAF slag and stabilised AOD slag aggregates concretes during 365 days. Each value is the average of three measurements on three different prism samples. The results indicate shrinkage at 28 days for concretes with EAF
slag aggregates and concretes with silico-calcareous aggregates. This shrinkage results from the chemical reaction of the cement with water. It is not observed for concrete made of stabilised AOD slag aggregates in which it could have been offset by swelling phenomena. After one year, the concretes made of stainless steel slag aggregates show a higher expansion than that of concretes made of silico-calcareous aggregates: 0.017% for concretes with EAF slag aggregates and 0.026% for concretes made of stabilised AOD slag aggregates, against 0.008% for concrete made of silicocalcareous aggregates. These values are consistent with the expansion of concretes made of nonreactive alkali aggregates (from 0.01% to 0.03% at one year) [26]. The expansion increases with the volume fraction of stainless steel slag coarse aggregates. XRD analyses did not show the presence of glassy phases in slags so that the expansion cannot result from alkali-siliceous reaction. No ettringite is determined in SEM observations either. Consequently, additional dimensional change could be related to some hydration reactions with free lime or free magnesium oxides. XRD and SEM–EDS analyses only show a few proportion of MgO in stabilised AOD slag. Only few content of free lime was also detected according to EN 1744-1 standard in both aggregates. However, according to FD P 18-456 standard, the maximum limit acceptable for the longitudinal strains (0.03% at 12 months for concrete stored in water at 60 °C) has not been exceeded. SEM observations of crack network at the paste-aggregate interface and in concrete samples were carried out (Figs. 14 and 15). For these observations, samples are taken in 365-days concretes made of EAF slag, stabilised AOD slag and silico-calcareous aggregates. SEM observations show intact coarse aggregates (EAF slag, stabilised AOD slag and silico-calcareous) but cracking through the cement paste and in some areas at the interfaces between the paste and EAF slag or stabilised AOD aggregates. No intergranular
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0.04
ΔL/L0 (%)
0.03 Silico-calcareous
0.02
50% EAF
0.01
100% EAF
0 -0.01
0
100
200
300
400
50% AOD 100% AOD
-0.02 -0.03 Time (days) Fig. 13. Longitudinal strain of high-strength concretes made of EAF slag and stabilised AOD slag aggregates.
Fig. 14. SEM observations of an interface between EAF slag aggregates and cement paste in 365-days concrete. Slight cracking in some areas.
substitute of the natural aggregates (silico-calcareous). Stainless steel slag aggregates have an apparent specific density higher than that of the silico-calcareous aggregates. They also show a very good resistance to fragmentation. Physical properties and mechanical strengths of concrete made of stainless steel slag aggregates are slightly higher than those of concrete made of silico-calcareous aggregates. Compressive strength increases by 9% when the natural aggregates are replaced by stainless steel slag aggregates. Tensile strength increases by 3%. The increase in dynamic modulus is 10% with EAF slag coarse aggregates and 20% with stabilised AOD slag coarse aggregates. Although durability indicators for concrete made of stainless steel slag aggregates (water porosity and gas permeability) show values slightly higher than those of traditional concrete. The expansion of 365-days water-cured concrete specimen increases with the volume fraction of stainless steel slag coarse aggregates. All the measured values match with the recommended limits for building constructions. The mechanical strengths evolution over the time and the conservation of the Young’s modulus indicate that there is no damage due to possible chemical reactions or expansive reactions of stainless steel slag aggregates. From the results obtained on the tested mixtures, this study reveals a new type of concretes with specific properties; as they are denser and more porous than traditional concrete. Furthermore, stainless steel slag aggregate concretes provide an environmental solution: sustainable recovery of industrial byproducts and conservation of natural resources.
Acknowledgements The authors thank UGITECH SA for financial contribution and scientific collaboration.
References
Fig. 15. SEM observations of an interface between stabilised AOD slag aggregates and cement paste in 365-days concrete. Slight cracking in some areas.
cracking, which could indicate a potential expansive reaction, has been observed. 6. Conclusion According to the criteria of EN 12620 Aggregates for concrete standard, the studied stainless steel slag aggregates (EAF slag and stabilised AOD slag) may be used in concrete as a
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