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Building and Environment 42 (2007) 2580–2585

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Utilization of steel slag as aggregates for stonemastic asphalt (SMA) mixtures

Shaopeng Wu�, Yongjie Xue, Qunshan Ye, Yongchun Chen

Key Laboratory of Silicate Materials Science and Engineering of Ministry of Education, Wuhan University of Technology, Wuhan 430070, China

Received 17 January 2006; received in revised form 13 April 2006; accepted 13 June 2006

Abstract

Steel slag is a byproduct making up a portion of 15–20% of iron output in an integrated steel mill. Most of them are deposited in slag

storing yards and thus results in many serious environment problems in China. This paper aims to explore the feasibility of utilizing steel

slag as aggregates in stone mastic asphalt (SMA) mixtures, and properties of such asphalt mixtures are evaluated as well. X-ray

diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) were employed to study the

compositions, structure and morphology of aggregates. Volume properties and pavement performances of SMA mixture with steel slag

were also evaluated as compared to that with basalt as aggregates. Results indicated that volume properties of SMA mixture with steel

slag satisfied the related specifications and expansion rate was below 1% after 7 days. When compared with basalt, high temperature

property and the resistance to low temperature cracking of SMA mixture were improved by using steel slag as aggregate. In-service SMA

pavement with steel slag also presented excellent performance on roughness and British Pendulum Number (BPN) coefficient of surface.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Steel slag; Aggregate; Recycling and utilization; Asphalt mixture; Pavement performances

1. Introduction

Steel slag is produced during the oxidation of steel pelletsin an electric arc furnace, it makes up a portion of 15–20%of iron output [1]. As a kind of fact the utilization of thisby-product is relatively low in China. Most of them aredeposited in slag storing yards, and thus result in manyserious environment problems. On the other hand, there isa great demand for aggregate in civil engineering industry,such as highway paving, dam construction, and so on.China is in short of the construction materials as so manyconstruction projects going on at the same time, as well asin other countries. To meet the great demand onaggregates, many mountains and rivers have been exhaust-ingly exploited, which lead to the pollution and destroy ofenvironment. Therefore, it is vital for researchers to findout the novel aggregates substituting for conventional onessuch as basalt.

e front matter r 2006 Elsevier Ltd. All rights reserved.

ildenv.2006.06.008

ing author. Tel./fax: +86 27 8716 2595.

ess: wusp@mail.whut.edu.cn (S. Wu).

Researches regarding recycling and utilization of steelslag in different fields have been carried out in recent years[2–5]. It was used as mineral additive for cement-basedmaterials to improve mechanical properties of concrete[6,7]. Monshi and Asgarani [8] used steel slag to producePortland cement with iron slag and limestone, andconfirmed that the compressive strength of concrete wasabove standard values for type I Portland cement. Shih etal.[9] studied the characteristics of bricks made from steelslag and revealed that it reduced the required firingtemperature. Steel slag could also be used to remove somehazardous substances such as ionic copper and ionic leadfrom waste water [10,11]. In Maslehuddin’s research[12,13], steel slag was used as aggregate in concrete.Researches have confirmed that the durability of concretewith steel slag was improved, and the compressive strengthand split tensile strength were much higher than that withlimestone. However, few reports were found about utiliza-tion of steel slag as aggregate in asphalt mixture [14].This work intends to explore the feasibility of steel slag

aggregates for SMA mixture, and compare it with basalt

ARTICLE IN PRESSS. Wu et al. / Building and Environment 42 (2007) 2580–2585 2581

aggregate. The comparison of these two materials issubstantiated employing the X-ray diffraction (XRD),scanning electron microscopy (SEM), and mercury intru-sion porosimetry (MIP) from the aspects of compositions,structure and morphology of aggregates. Volume perfor-mances, expansion performance and pavement perfor-mances of SMA mixtures are also evaluated.

2. Experiments

2.1. Raw materials

The modified asphalt binder used was graded as PG76-22, with a penetration of 62 (0.1mm at 25 1C 100 g and 5 s),ductility of 47 cm (at 5 1C) and softening point of 82 1C.Steel slag was obtained from Wuhan Iron & steel (Group)Corporation by hot-sprinkling method and used assubstitute for basalt. After 3 years aging, the f-CaO contentof steel slag fell below 6%. Various properties of bothaggregate are shown in Table 1. Limestone powder smallerthan 75 mm and short-chorpped polyester fiber were used asmineral filler and drain down stabilizer, respectively. Thedosage of fiber filler was 3% by the total weight of the mix.

2.2. Experimental design and test methods

In this study, SMA with normal maximum aggregate size13mm shortened as SMA-13 was selected as the asphaltmixture and aggregate gradation hereafter. Due to thestrong absorption of fine steel slag particles towardsbitumen at liquid, it was not chosen to substitute for finebasalt aggregate smaller than 2.36mm. The percentages ofeach fraction used to meet the specification limits ofcombined gradation are presented as follows:

Steel slag (9.5–16mm in grain size): 48%Steel slag (4.75–9.5mm): 24%Basalt stone (2.36–4.75mm): 6%Fine basalt stone smaller than 2.36mm: 11%Mineral fillers smaller than 0.075mm: 11%It can be seen that approximately 80 percentages of the

whole blended aggregates were substituted by steel slag ofvarious sizes.

Pore structure analysis of different aggregates wasperformed by Carloerna-200 Mercury Intrusion porosi-meter and the sample particle was smaller than 6mm indiameter with the maximum pressure 374MPa.

To observe the morphology of steel slag and basaltaggregate, the fracture surfaces of the selected aggregate

Table 1

Physical properties of aggregate

Items Bulk density

(g/cm3)

Cumulus

density (g/cm3)

Water

absorption (%)

Steel slag 3.30 1.90 1.29

Basalt 2.85 1.70 0.68

particles were coated in vacuum with a thin layer of goldand observed with a SEM-model Hitachi S-2500, made inJapan.Mineral phase identification was conducted using D/

MAX-III X-ray diffractometer with CuKa1 radiation,made in Japan.Specimens for Marshall testing were prepared with an

automatic Marshall Compactor according to ASTMD1559. Specimens also were pretreated in a water bath at60 1C for 7 days for the expansion performance study.Volume changes were recorded with the Archimedesprinciple per 24 h.Rutting test were employed specimens of

300mm� 300mm� 50mm at 60 1C under dry conditions.A solid-rubber wheel traveling at a speed of 42 cycles/minand at a pressure of 0.7MPa was used to correlate withrutting. Rut depths were measured per 20 s and dynamicstability was calculated through the equation as follows:

DS ¼15N

d60 � d45,

where DS was dynamic stability (cycles/mm); N wastraveling speed of the wheel, 42 cycles/min; d45, d60 wererutting depths at the test time of 45 and 60min,respectively.A MTS810 electro-hydraulic servo testing system was

used to investigate mechanical properties of SMA mixtureat low temperature. The specimens prepared by Marshallcompactor were subjected to indirect tension test at 0 1C,and the strain and stress values were recorded automati-cally by the data acquisition system.

3. Results and discussion

3.1. Composition, structure and morphology of aggregates

XRD measurement result for characterizing the mineralcomposition of steel slag is shown in Fig. 1. It indicates thatthe main mineral components of steel slag are 3CaO � SiO2

(C3S), 2CaO � SiO2 (C2S) and RO (including FeO, MgOand MnO). The high temperature (1600 1C) of blast oxygenfurnace leads to the formation of all of those crystalsabove-mentioned and thus makes the slag stable at roomtemperature. The pore structures as well as porosity of steelslag and basalt determined by MIP are shown in Table 2.The porosity of steel slag is as large as 24 times of basalt(5.76% and 0.24%, respectively), which indicates that steelslag has a porous structure. Among all the pores of steelslag, half of them are at the range of 0.01–0.1 mm, while

Crushing value

(%)

Polishing stone

value (%)

Binder

adhesion (%)

LA abrasion

(%)

12.1 58 X95 13.2

12.7 48 X85 15.8

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most pores of basalt are larger than 0.1 mm. Fig. 2 alsodemonstrates that plenty of pores can be observed clearlyon the surface of steel slag by SEM image, which impliesthat steel slag is a kind of porous material compared withbasalt.

10

CP

S

1.00K

20

3.04C3S

2.76C2S

2.16R0

30

2 θ (°)40 60 7050

Fig. 1. XRD of steel slag.

Table 2

Pore structures of steel slag and basalt by method of MIP

Aggregate types Total porosity (%) Pore distrib

40.001mm

Steel slag 5.76 99.9

Basalt 0.24 100

Fig. 2. SEM images of s

Table 3

Marshall test results of SMA mixture

Items Optimal asphalt content (%) Density (g/cm3)

Steel slag 6.4 2.610

Basalt 6.2 2.520

Specificationsa — —

aTechnical specifications for construction of highway asphalt pavements, JT

3.2. Volumetric performances

Marshall Test results of SMA mixtures are shown inTable 3, wherein clearly shows that steel slag has morereliable consistency. The difference of voids in mineralaggregate (VMA) between the two aggregates is very small.As a kind of porous-structure material, steel slag absorbsmore asphalt during mix blending, so the optimal asphaltcontent of SMA containing steel slag is a little bit largerthan that of basalt. All the results listed in Table 3,including air-void content, VMA and Marshall stabilitymeet the requirements of Chinese technical specification interms of SMA mix design.

3.3. Expansion performance

SMA characterization in terms of expansion is essentialfor mixture design and evaluation for durability considera-tion. Fig. 3 illustrates the changes of expansion rate ofSMA mixture containing steel slag as aggregate over time.A marked increase in expansion rate is observed when

ution (%)

40.01mm 40.1mm 41mm

99.9 48.0 20.5

100 100 58.3

teel slag and basalt.

Air voids (%) VMA (%) Marshall stability (kN)

3.9 18.5 10.8

4.0 18.7 11.5

3–4 X17.0 X6.0

G F40-2004, China.

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Time / h

Exp

ansi

on r

ate

/ %

1.2

0.8

0.6

0.4

0.2

1

00 24 48 72 96 120 144 168

Fig. 3. Expansion rate of SMA mixture containing steel slag as aggregate.

9

8

Com

pres

sion

Str

ess

/MP

a

Compression Strain %

7

6

5

4

3

2

00 10 20 30

Steel slag Basalt

40 50

1

Fig. 4. Compression stress–strain curves of SMA mixture.

Table 4

Peak strain and critical value of dW=dV for SMA mixture

Aggregate types �0 (%) Critical value of dW=dV (kJ/m3)

Steel slag 13.5 73.3

Basalt 11.0 46.6

S. Wu et al. / Building and Environment 42 (2007) 2580–2585 2583

specimens are immersed in water, and further increasingthe immersed time results in increased expansion rate witha very small slope. The expansion rate is below 1% after 7days with no significant increase, which indicates that thestability of steel slag can be improved by suitablepreprocessing technology and long aging time.

3.4. High temperature property

Rutting is defined as the permanent deformation of aflexible pavement, dependent on the magnitude of the loadand the relative strength of the pavement layers. Hightemperature property of asphalt pavement is the ability ofresisting permanent deformation under repeated trafficloadings in summer. According to the rutting test results,the dynamic stability of SMA mixture containing steel slagis 6775 cycles/mm, which is much higher than that ofcontaining basalt (5890 cycles/mm). The stone-skeletonstructure formed by steel slag after compacting in SMAmixture can enhance significantly the shearing resistanceunder loading, compared to basalt aggregate. The porousstructure of steel slag could absorb the extensive oil, whichdecreased permanent deformation caused by bleeding athigh temperature. Besides, the higher alkali value of steelslag improves the adhesion performance between aggregateand bitumen, therefore, all of such advantages result in theenhancement of high temperature property for steel slagSMA mix.

3.5. Resistance to low temperature cracking

Materials damage can be usually represented by strainenergy density function with the following form [15]:

dW

dV¼

Z �0

0

sij d�ij, (1)

where sij and �ij are stress and strain, respectively. Thecritical value of dW=dV represents the area under the

stress–strain curve when the stress reaches the peak point,and �0 is the corresponding strain, as shown in Fig. 4. Thecritical value of dW=dV at 0 1C is employed to evaluate theresistance to low temperature cracking. Functions of twocompression stress–strain curves obtained from polynomialregression are as follows:

Containing steel slag:

s ¼ � 3� 10�5�4 þ 0:0029�3 � 0:1116�2

þ 1:5422�þ 0:5167, ð2Þ

Containing basalt:

s ¼ � 3� 10�5�4 þ 0:0026�3 � 0:0909�2

þ 1:184�þ 0:6129. ð3Þ

Critical compression strain energy density function isobtained after integral calculation, as follows:

Containing steel slag:

dW=dV ¼ � 3� 10�5�5=5þ 0:0029�4=4

� 0:1116�3=3þ 1:5422�2=2þ 0:5167�. ð4Þ

Containing basalt:

dW=dV ¼ � 3� 10�5�5=5þ 0:0026�4=4

� 0:0909�3=3þ 1:184�2=2þ 0:6129�. ð5Þ

The �0 value of SMA mixture containing steel slag is13.5%, higher than that of containing basalt (11.0%), seenin Fig. 4. When they are substituted into Eqs. (4) and (5),the critical value dW=dV is obtained. As Table 4 revealed,the critical value of dW=dV is 77.3 kJ/m3 for SMA mixture

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Table 5

Performances of test road

Test item Service time (months)

6 12 18 24

Bulk density of core (g/cm3) 2.511 2.520 2.524 2.525

Abrasion and friction coefficient (BPN) 62 60 56 55

Surface texture depth (mm) 1.2 0.9 0.8 0.8

S. Wu et al. / Building and Environment 42 (2007) 2580–25852584

with steel slag, but 46.6 kJ/m3 for basalt mixture, whichconfirms that the resistance to low temperature cracking ofSMA mixture with steel slag is better.

3.6. Test section

For the purpose of comparison between steel slag andbasalt SMA mix, two test sections of steel slag SMA mixhave been constructed as surface friction course in the samedriving direction on the asphalt overlay upon old cementconcrete pavement of Wuhan–Huangshi expressway, Hu-bei province, China. The test section for steel slag SMAmix is about 400m in length and two lanes of 11m in totalwidth, while the other section for basalt mixture isapproximately 2000m in length. The traffic volume isexpected to be more than 1� 107 ESALs per year for thesetwo test sections. Furthermore, weather condition isthought to be extremely severe with the annual rainfall of2000mm and the pavement surface temperature rang from�10 1C in winter to 70 1C in summer.

In order to get fundamental understanding of SMAmixture performances in field, tracking inspection andstudy was conducted after construction. Visible inspectionfound that no significant distresses, for example, rutting,bleeding, cracking and water damage-induced strippingoccurred on steel slag surface friction course during past 2years. At present, steel slag test section performs quite wellas the same to basalt test section visibly. Besides visibleinspection, some performances, particularly in terms ofsurface texture depth and abrasion and friction coefficient/British Pendulum Number (BPN) were tested in field andrecorded every 6 months (see in Table 5). It shows that theSMA pavement with steel slag present excellent perfor-mances. The BPN reduces only 11 percent and the texturedepth remains more than 65 percent after a 2-year service.The measured density of drilled cores also changes slightlyas the function of traffic post-compaction.

4. Conclusions

Based on the limited study of the utilization of steel slagin SMA mixtures, following conclusions can be obtained:

1.

Steel slag obtained by hot-sprinkling method is a verysuitable aggregate with porous structure for preparingSMA mixtures after 3 years aging.

2.

All the volume performances of SMA mixture contain-ing steel slag as aggregates can meet the relatedrequirements of specifications, though the substitutionof steel slag for basalt increases the optimal bitumencontent slightly.

3.

Expansion rate of SMA mixture with steel slag is below1% after 7 days, which ensures the stability of steel slagin SMA mixtures.

4.

Compared with SMA mixture with basalt, the hightemperature property of SMA mixture with steel slag isimproved, the better physical properties of steel slagenhances the ability of resisting permanent deformationat high temperature.

5.

The critical value of dW=dV for SMA mixture withsteel slag is also increased, which results in the betterresistance to low temperature cracking.

6.

The test roads shows excellent performances after 2-years service, with abrasion and friction coefficient of55BPN and surface texture depth of 0.8mm.

In short, the successful utilization of steel slag asaggregate in pavement construction can provide a newand more cost effective approach for aggregate resources,and decrease the threats of solid wastes to environment.However, more studies should be carried out on itsrecycling process and wide application in future.

Acknowledgements

The authors wish to express their thanks to the WuhanIron & Steel (Group) Corporation (China) for financialsupport of the research.

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