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Soils and Foundations 59 (2019) 1385–1398

Particle-size effect of basic oxygen furnace steel slag in stabilizationof dredged marine clay

Arlyn Aristo Cikmit a, Takashi Tsuchida a,⇑, Gyeongo Kang b,Ryota Hashimoto a, Hideki Honda c

aDepartment of Civil and Environmental Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1, Kagamiyama,

Higashi-Hiroshima, Hiroshima 739-8527, JapanbHonam Regional Infrastructure Technology Management Center, Chonnam National University, 50 Daehak-ro, Yeosu, Jeollanamdo 550-749,

Republic of KoreacSteel Research Laboratory, JFE Steel Corporation, 1-1, Minami Watarida-Cho, Kawasaki City, Kanagawa 210-0855, Japan

Received 22 August 2018; received in revised form 30 May 2019; accepted 24 June 2019Available online 3 September 2019

Abstract

The aim of this study is to provide a comprehensive analysis of the strength development of dredged soil stabilization with differentgrain sizes of basic oxygen furnace (BOF) slag. Laboratory vane shear (LVS) tests, unconfined compressive (UC) strength tests, and flowvalue tests were conducted to understand the time-strength behavior of stabilized dredged clay. The results show that the strength wassignificantly affected by the different maximum particle sizes, despite the same free lime content. An equation to predict the strengthdevelopment of a larger maximum grain size from a smaller maximum grain size was proposed using a modified BOF slag rate of addi-tion and its calculated specific surface area. The results indicate that the equation is feasible for predicting the strength development ofactual construction from laboratory test results.� 2019 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Stabilization of marine clay; Basic oxygen furnace steel slag; Chemical stabilization; Grain size effect; Unconfined compressive strength

1. Introduction

A large body of research shows that Portland cementcan assure the quality and durability of the stabilizationof dredged marine clay (Kang et al., 2016; Kitazume andTerashi, 2013; Seng and Tanaka, 2011; Tang et al., 2001;Tsuchida and Tang, 2015; Tsuchida et al., 2007; Watabeand Noguchi, 2011). However, Rubenstein, 2012; Worrellet al., 2001 claimed that not only does the manufactureof Portland cement consume energy and resources, but itsproduction and usage are responsible for five percent of

https://doi.org/10.1016/j.sandf.2019.06.013

0038-0806/� 2019 Production and hosting by Elsevier B.V. on behalf of The

This is an open access article under the CC BY-NC-ND license (http://creativec

Peer review under responsibility of The Japanese Geotechnical Society.⇑ Corresponding author.E-mail address: ttuchida@hiroshima-u.ac.jp (T. Tsuchida).

the total carbon monoxide emitted around the world.Therefore, a sustainable stabilizer, such as basic oxygenfurnace (BOF) slag, is proposed to replace Portland cementin treating dredged marine clay.

Steel slag is generally considered to be a by-product thatcan be classified into two types based on its processingflow. They are BOF slag, from iron-to-steel conversion,and electric arc furnace (EAC) slag, from the melting ofscrap in steel manufacturing (Horii et al., 2013; Lee,1974; Shi, 2004). Studies have shown that the constituentminerals of BOF slag, such as free lime (f-CaO) and freepericlase (f-MgO), have lower cementitious propertiesand could cause volume instability (Wachsmuth et al.,1981; Wang et al., 2010; Yildirim and Prezzi, 2011). Mostapplications of BOF slag in civil works have been done

Japanese Geotechnical Society.

ommons.org/licenses/by-nc-nd/4.0/).

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for the purpose of reducing the free lime content or addingan activator rather than benefitting from it.

BOF slag has limited cementitious properties due toboth a lack of tricalcium silicate (C3S) and the presenceof wustite solid solution (FeO) as the main mineral(Belhadj et al., 2012; Deng et al., 2017; Shi and Qian,2000; Wilkinson et al., 2010). Murphy et al. (1997) offeredmultiple ways of enhancing the cementitious product insteel slag, such as adjusting FeO to Fe2O3 by an oxidationprocess and water quenching in the cooling process of steelslag. Montgomery and Wang (1991) reported the effects ofadding instant-chilled slag (ICS) to concrete, namely, itgave greater indirect tensile strength and flexural strengththan a corresponding control concrete containing a lime-stone aggregate. Those treatments for enhancing thecementitious properties brought about the need for addi-tional procedures before the steel slag could be utilized.

Ahmedzade and Sengoz (2006) and (Asi, 2007) studiedthe influence of steel slag on the mechanical propertiesand electrical conductivity of asphalt concrete mixtures.They found that steel slag, as a road construction aggre-gate, produced a better resistance to permanent deforma-tion and greater stiffness in hot mixed asphalt (HMA)concrete than limestone. Kandhal and Hoffman (1997),however, suggested that steel slag could only replace eitherfine or coarse aggregates and that encouraging a 100%replacement would cause over-asphalting and in-servicetraffic compaction, resulting in flushing. Previous studies(Arribas et al., 2014; Kandhal and Hoffman, 1997; Wanget al., 2010; Xue et al., 2006) indicated that it is quite diffi-cult to use a BOF slag aggregate since various treatmentsare required prior to its usage to reduce the volume insta-bility (long-term expansion). For concreting purposes, thereplacement of limestone with steel slag resulted in the needfor a significantly longer time for the concrete to initiatereinforcement corrosion and to crack (Arribas et al.,2014; Liu et al., 2014). However, the steel slag could onlyreplace up to 50% of the function of limestone due to itslarge bulk density (1.4 times higher than limestone). Thus,the application of BOF slag as an aggregate in road con-struction and concrete still requires supplementary sourcessuch as limestone.

The use of steel slag for various purposes has been con-firmed as safe for humans and environmentally friendly.Through various tests, Proctor et al. (2000) showed thatBOF slag is a material that is non-hazardous to humanhealth and the environment. Nevertheless, the avoidanceof using various slag types and sources for a single purposewas suggested because it is easier to perform a risk assess-ment. There is the potential for metals to leach from slagand to migrate through the air because the larger particlesof steel slag would be exposed to weathering processes andbecome smaller particles. Such a phenomenon may onlyoccur in an air-exposed application, but for other purposes,such as fill, landscape, submerged fill, and reducing theaccessibility of slag for suspension, it will be sufficientlysafe for the environment. Motz and Geiseler (2001) also

highlighted the non-hazardous potential of BOF slag tothe environment because the results from a leachate testshowed no significant impact other than an increasing pHdue to the partial solution of free lime.

Little attention has been paid to the selection of BOFslag as a stabilizer for soils with a high moisture contentand without a prior reduction of the free lime. Otherresearch on BOF slag, such as for road material (Shenet al., 2009a; Shen et al., 2009b), concrete aggregate(Qiang et al., 2016; Rondi et al., 2016), sand-compaction-pile fill material (Kinoshita et al., 2012; Takahashi et al.,2011), and hydraulic structure ‘‘armourstones” (Motzand Geiseler, 2001) revealed that the recycling of steel slagprior to the weathering process is advantageous. Mahieuxet al. (2009) and Poh et al. (2006) focused on BOF slagin studies using compaction and the optimum water con-tent or a lower moisture content, which are inappropriatefor dredged marine clay stabilization purposes.

In Japan, the significant difference between laboratoryresults and actual field investigations has been one of themost common problems in soft soil stabilization usingBOF slag. There are no standards for correlating thestrength obtained from laboratory tests with that obtainedfrom field tests. Despite the same quantity of the addition,the same free lime content or the same characteristics of theBOF slag being used in both laboratory and field investiga-tions, the maximum grain size used in field constructionhas often been much larger than that in the laboratory.The reasons for this practice are the time-consuming natureof the tests and the limited resources, including the absenceof a large-scale strength apparatus and the limited clay andBOF slag samples for the large-size samples.

The present study provides a set of general characteris-tics for dredged marine clay, BOF slag, stabilized dredgedmarine clay, and the strength development of stabilizeddredged marine clay using various maximum grain sizesof BOF slag. The effects of different maximum grain sizesof BOF slag on the strength development of stabilizeddredged marine clay were also studied. The knowledgegained will be helpful for accurately predicting the strengthdevelopment of clay treated with BOF slag in field con-struction works using the results of laboratory strengthtests. Previous studies based on field construction investi-gations were also compared with this study to furtherunderstand the factors that significantly affect the strengthdevelopment of dredged marine clay stabilized with BOFslag.

2. Experimental design

The experiment was conducted in two stages. The phys-ical properties of Tokuyama Port dredged marine clay andBOF slag were initially characterized. The primary aim ofthe experiment was to measure the strength developmentof stabilized soils by conducting a number of shear strengthtests, namely, laboratory vane shear (LVS) tests andunconfined compressive (UC) tests. They were performed

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as described below. Flow value tests were also conductedfor each different mix proportion.

Fig. 1. BOF slag with maximum grain size <37.5 mm.

Table 2Physical properties of Basic Oxygen Furnace slag.

Property Value

Initial moisture content, % 8.4Average of specific gravity, g/cm3 3.15Specific gravity of coarse aggregate <4.75 mm 2.96Specific gravity of fine aggregate <4.75 mm 3.23Absolute dry density, g/cm3 3.02Water absorption rate of coarse aggregate, % 4.6Water absorption rate of fine aggregate, % 2.5Maximum size of aggregate, mm 37.5Coarse-grained size (>75 lm), % 89.5Fine-grained size (<75 lm), % 10.5Free CaO, (f-CaO), % 8.49

2.1. Materials

The dredged marine clay used for this investigation wasdrawn from the sea bottom of Tokuyama Port, YamaguchiPrefecture, Japan. The Tokuyama Port marine clay had aninitial moisture content, wn, equal to 0.9–1.0 of its liquidlimit, wL. Using a grab-type dredger, the moisture contentof the marine clay increased 1.2–1.3 times (Terashi andKatagiri, 2005). The dredged marine clay was highly plas-tic, with a liquid limit of 107.2% and a plastic limit of38.6%. It had a fines content of 90.0% and a specific gravity(Gs) of 2.65. The measured average organic matter was8.2% and the salinity was 3.5%. The laboratory resultsfor the Tokuyama Port marine clay are summarized inTable 1.

Fresh BOF slag was acquired from a single batch andsupplied by the Fukuyama Facility of JFE Steel Corpora-tion. The BOF slag was ground from its natural size to thegrain sizes shown in Fig. 1. No chemical pre-treatment wasapplied to the BOF slag. Its physical properties are listed inTable 2. Each sealed pack contained granular material witha maximum particle size of less than 37.5 mm and with anaverage initial moisture content of 8.4%. The grain-size dis-tribution of BOF slag consisted of 10.5% fine particles(Fig. 2). The BOF slag had a surface dry density of3.19 g/cm3 and an absolute dry density of 3.08 g/cm3.The water absorption rates of the BOF slag were 4.6%and 2.5% for the coarse aggregate and the fine aggregate,respectively. An ethylene glycol method was performed todetermine 8.49% free lime.

X-ray fluorescence and X-ray diffraction tests were con-ducted to acquire the elemental composition and crystallinecomponents of the BOF slag and to compare the resultswith other types of BOF slag from previous studies(Murphy et al., 1997; Shi, 2004) (Table 3). The totalamounts of the main elements determined in this study,such as calcium oxide, iron, and silicon oxide, were similarto those reported in previous studies. The crystalline com-ponents of the BOF slag (Fig. 3) also exhibited a similarcomposition, for example, Portlandite (Ca(OH)2), Calcite(CaCO3), Larnite (Ca2SiO4), Lime (CaO), Wustite (FeO),

Table 1Geotechnical and physical properties of Tokuyama Port marine clay.

Property Value

Liquid limit, (wL) % 107.2Plastic limit, (wP) % 38.6Plastic index, (PI) % 68.5Specific gravity, (Gs) 2.65Coarse-grained soil (>75 lm), % 9.9Fine-grained size (<75 lm), % 90.0Unified Soil Classification System (USCS) CH-OHpH 7.2Loss of ignition, (LOI) % 8.2

Srebrodolskite (Ca2FeO5), and Magnetite (Fe3O4). It isnotable that the chemical constituents of BOF slag maybe highly variable depending on the raw materials andthe manufacturing process.

2.2. Sample preparation and curing

To begin the sample preparation, the dredged marineclay was thoroughly separated from the coarse particles,such as coral reef and shells, using a 2-mm sieve. The fil-tered clay was then hermetically stored in a box to avoidredundant moisture changes prior to mixing the sample.Before it could be sifted using various sieves to acquire dif-ferent maximum grain sizes, the BOF slag was air dried for24 h in a 20 ± 2 �C room at 60% humidity. The designedgrain-size BOF slag was then wrapped in a plastic bag toavoid the loss of free lime due to contact with the air(Yildirim and Prezzi, 2015). Pre-cooled artificial seawater,with 3.5% salinity, was added to the dredged clay toachieve an initial setting water content of w0 = 1.5 wL.The dredged marine clay and BOF slag were then blendedfor five minutes at specific mix proportions (Table 4), usinga heavy-duty hand mixer, to produce a uniform soil mix-ture. Particle segregation was not seen after the mixing pro-

Table 3BOF slag composition (weight, %).

Chemical BOF slag in this study BOF slaga BOF slagb

SiO2 14 8–20 15Al2O3 2.6 1–6 5Fe2O3 33 10–35 29CaO 41 30–55 38MgO 2.3 5–15 6.5MnO 3 2–8 5TiO2 0.41 0.4–2 1S 0.031 0.05–0.15 n/aP 3 0.2–2 0.5Cr 0.12 0.1–0.5 n/a

a Shi (2004).b Murphy et al. (1997).

Fig. 3. X-ray diffraction results for BOF slag: 1Portlandite, 2Calcit

Fig. 2. Grain-size distribution of Tokuyama Port clay and BOF slag.

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cess. Katayama and Tsuchida (2015) reported that signifi-cant segregation does not occur in a soil mixture (soil slurryand BOF slag) if the grain size of the BOF slag is less than25.5 mm.

Immediately after mixing the sample, it was graduallypoured into the molds by applying light tapping every threelayers to eliminate the trapping of air inside the soil mix-ture. The BOF slag addition (RBOF) was determined by avolumetric ratio calculated with Eq. (1).

RBOF ¼ V BOF

V soil þ V water þ V BOF

� 100 ð%Þ ð1Þ

where VBOF is the solid volume of the BOF slag, Vsoil is thesolid volume of soil, and Vwater is the volume of water. To

e, 3Larnite, 4Lime, 5Wustite, 6Srebrodolskite, and 7Magnetite.

Table 4Mix proportions and curing times of soils stabilized with BOF slag.

Normalized clay water content (w/wL) RBOF (%) Maximum particle size Curing time

1.5 20, 30 <0.89 mm, <2mm, <4.75 mm, <9.5 mm 0.5 h, 2, 5, 7, 10, 15 (hours)(160.72%) 1, 2, 3, 7, 28, 90 (days)1.5 20, 30 <37.5 mm 0.5 h, 2, 5, 7, 10, 15 (hours)(160.72%) 1, 2, 3, 5, 7, 14, 28, 90 (days)1.5 30 0.85–9.5 mm, 2–9.5 mm, 4.5–9.5 mm 1, 3, 7, 28, 90 (days)(160.72%) 9.5–37.5 mm

Table 5Mold sizes of samples for LVS and UC tests, and vane blade size.

BOF slag addition Size of LVS sample Size of UC sample Size of vane blade

Maximum particle size (mm) Diameter (mm) � Height (mm) Diameter (mm) � Height (mm) Diameter (mm) � Height (mm)<0.89 60 � 60 50 � 100 20 � 20<2.00<4.75<9.5

<37.5 140 � 80 100 � 200 40 � 60

0.85–9.5 n/a 50 � 100 n/a2–9.54.75–9.59.5–37.5 n/a 100 � 200 n/a

Fig. 4. Laboratory vane shear test apparatus.

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verify a delicate mixture, the sample unit weight in eachmold was compared to its calculated unit weight.

For the strength tests in this study, two types of acryliccylindrical molds and two types of tinplate molds were uti-lized (Table 5). Two different sizes of acrylic molds, ø60mm � 60 mm for the BOF slag < 10 mm and ø140mm � 80 mm for the BOF slag < 37.5 mm, were adoptedfor the LVS tests and placed in a 20 ± 2 �C room at 60%relative humidity. Two different sizes of tinplate molds,ø50 mm � 100 mm for the BOF slag < 10 mm and ø100mm � 200 mm for the BOF slag < 37.5 mm, were adoptedfor the UC tests. The samples in the tinplate molds weresubmerged with both top and bottom ends exposed to dis-tilled water at 20 ± 2 �C.

2.3. Tests performed

2.3.1. Flow value test

The flow value test was originally a standard test used inJapan JHS A 313, 1992 to determine the workability ofconcrete, but recently it has also been utilized to under-stand the workability and flowability of stabilized materialfor pipe-transportation purposes in embankment construc-tion. The flow value test is a simple measurement of thespreading diameter of a slumped soil mixture. In this study,the soil mixture was poured into an acrylic mold (/80mm � 80 mm) that had been previously lubricated with athin layer of oil. Then, during the filling process, bubbleswere carefully removed by lightly tapping the side of themold. The excess mixture was trimmed off. Finally, thecylindrical mold was lifted and, after one minute, the aver-age diameters at both perpendicular axes were measured toobtain the flow value.

2.3.2. Laboratory vane shear test

The LVS test, a scaled-down version of the field vaneshear test (JGS 1441-2012), was adapted to measure thelow-strength samples (Fig. 4). Samples that had been pre-cured for 0.5–15 h in the acrylic molds (Fig. 5) were testedusing six degrees/minutes of rotation speed. A rectangularvane was attached where the height � diameter varied from20 � 20 mm to 40 � 60 mm with a thickness of 0.8 mm

Fig. 5. Mold dimensions of laboratory vane shear test.

15

16

17

18

19

15% 20% 25% 30% 35%

m/Nk(thgie

wtinU

3 )

BOF slag rate of addition RBOF (%)

BOF max particle <2mmBOF max particle <4.75mmBOF max particle <9.5mmBOF max particle <37.5mm

Initial water content = 1.5wL

Fig. 6. Measured unit weight of soils stabilized with BOF slag.

1390 A.A. Cikmit et al. / Soils and Foundations 59 (2019) 1385–1398

(Table 5). Tests were consistently terminated after the vanehad rotated 60�or when the sample had reached the peak ofthe measured torque. From the measured torque, theundrained shear strength, Su, was obtained using the fol-lowing equation:

SuVane ¼ M

p D3=6� �þ HD2=2

� �� d3=12� �þ d2La=2

� �� � ð2Þ

where M is the torque produced by the vane blade, D is thediameter of the vane, H is the height of the vane, d is thediameter of the shaft, L is the contacted length of the vaneshaft, and a is the friction coefficient of the shaft, the valueof which is 1. In this equation, it is assumed that the fric-tion between the vane shaft and the soil is equal to theshear strength, Suvane.

An oversized correction was applied to the specimenswith particle sizes of 9.5–37.5 mm to avoid any additionalstrength caused by the contact between large particlesand the vane blade because the diameter of the vane bladewas limited to 60 mm. The material retained in the 19-mmsieve was discarded and replaced with an equal amount ofmaterial that could pass through the 19-mm sieve andretained at 9.5 mm.

2.3.3. Unconfined compressive testThe UC test, which is a common test used to measure

the soil shear strength, was selected in this experimentdue to its features of being effortless, economical, and lesstime-consuming. The UC test was accurately performed asper the Japanese code (JGS 0511-2014). An automaticloading machine with load cell capacities of 2 kN, 5 kN,and 20 kN and a linear variable differential transformer(LVDT) were vertically attached to measure the stress-strain of the samples. Both ends of the sample surfaceswere carefully trimmed and aligned to a ±1 mm tolerancebefore the testing proceeded.

To retain the maximum ratio of particle size to diameterof 1/6 (ASTM D2166-16), a larger mold of /100mm � 200 mm was used for the sample with a maximumparticle size of BOF slag of less than 37.5 mm. A constantstrain rate of 0.5–1% per min was accounted for by per-forming the UC test on every three samples with curingtimes of 15 h to 90 d. For the acceptance criteria, the stan-dard deviation between two specimens with the same mixproportion should not exceed 10% of the mean strength.

3. Results and discussions

3.1. Unit weight

Fig. 6 provides an overview of the unit weight of the sta-bilized soils as a function of the 20–30% addition rate ofthe BOF slag at the identical initial water content of 1.5wL. In the soil mixtures with the grain size of 0–2 mm,the 20–30% addition rate increased the unit weight to16.36–18.04 kN/m3. For the soil mixtures with the maxi-mum grain sizes of 0–4.75 mm and 0–9.5 mm, the 20–30% addition rate increased the unit weight to 16.56–18.35 kN/m3 and 16.78–18.68 kN/m3, respectively. The soilmixtures with the grain size of 0–37.5 mm produced theunit weight of 16.84–18.72 kN/m3. From the results, it isclear that the 10% difference in the BOF slag addition rateproduced an approximately 1.1 times higher unit weight atany particle size.

Despite the same addition rate and initial water content,a larger maximum grain size of the BOF slag produced alarger unit weight value. This increased unit weight wasdue to the different initial dry densities of the BOF slag.A grain size of 0–4.75 mm had a dry density of 2.96 g/cm3, while a grain size of 4.75–37.5 mm had a dry densityof 3.23 g/cm3. The stabilized soils with large unit weightswould be advantageous as counterweight elements in fillingconstruction.

Fig. 7. Flow value of stabilized dredged marine clay with different BOFslag rates of addition and maximum particle sizes.

A.A. Cikmit et al. / Soils and Foundations 59 (2019) 1385–1398 1391

3.2. Flow value

Fig. 7 shows the results of the flow value test as a func-tion of the BOF slag addition at various maximum grainsizes with the same initial clay water content of wn = 1.5wL. The figure indicates that a larger maximum particle sizetends to produce a larger flow value of stabilized soils atthe same initial moisture content, except for the BOF slagwith a maximum grain size of 37.5 mm. The larger flowa-bility is related to a larger dry density of the BOF slag,which produces a higher self-weight of the mixtures. It isbelieved that the different particle sizes of the BOF slag willsignificantly affect the flow value. This causes the pipe dis-tribution to be more difficult due to a lower flowability.Therefore, in adopting a larger particle size of BOF slag,the appropriate amounts for the addition rate and the ini-tial water content should be taken into consideration inorder to reach a delicate flow value.

There is a significant relationship between the flow valueand the immediate shear strength of the stabilized soilsusing the BOF slag (Fig. 8). However, this result doesnot agree with the data on the BOF slag which included

Fig. 8. Normalized flow value to vane shear strength at immediate curingtime of clay stabilized with BOF slag.

particle sizes larger than 37.5 mm (Fig. 8). This is likelyto be due to the small cylinder of 80 mm � 80 mm usedin the flow test which has an unsuitable ratio due to thelarge particle size of 37.5 mm. As a result, the flow valuetest is not suitable for determining the flow value of gran-ular material with over-sized particles.

3.3. Stress-strain curve

The stress-strain curves of the stabilized soils using BOFslag with curing times from 1 to 90 d on samples with max-imum particle sizes of 2 mm and 37.5 mm, RBOF = 30%,and w0 = 1.5 wL, are shown in Fig. 9. The strengths pro-duced by these two maximum particle sizes were distinct,with the strength produced by 2 mm at 28 d being approx-imately 3.5 times higher than that produced by 37.5 mm.

The nature of the changes in the clay due to a longercuring time and becoming stronger but brittle as the BOFslag with smaller particle sizes is added, is indicated bythe stress-strain curve behavior (Fig. 9). The strain at fail-ure of the smaller maximum particle sizes changed due tothe curing time. This strain can be divided into two types:(1) for an early curing time of up to 3 d, the strain at failurewas 2% or more, which is similar to soft soil, and (2) for alater curing time, the strain at failure was 1.2%, which issimilar to stiff soil.

Unlike stabilized soils with a <2-mm particle size andgreatly varied stress-strain curve shapes, the stress-straincurve shape formed by the stabilized soils using a 37.5-mm particle size showed no significant alteration in shapewith the change in curing time. The strain at failure of lar-ger grain sizes showed greater values of 1.5–2.5%, which isgenerally similar to soft clay.

3.4. Time-strength mobilization

Basic oxygen furnace slag is a chemically-active mate-rial. Shi (2004) reported that the presence of C3S, C2S,and C4AF in steel slag endorses the cementitious propertiesof the slag. Based on the components of BOF slag, it hasbeen suggested that the strength mechanism of dredgedclay stabilized with BOF slag is a hydration reaction dueto the presence of mineral components such as C2S, C3S,and C4AF, and to the pozzolanic reaction obtainedbetween the portlandite (Ca(OH)2) and the silica and alu-mina minerals resolved in the clay.

Importantly, the free lime content in the stabilized soilsin this study was recorded to be very high, namely, 8.49%.According to Shi (2004) and Yildirim and Prezzi (2011),free lime is the undissolved lime in the steel-refining processcoming from two sources, precipitated lime and residuallime. Without access to water from fractures or pores, forexample, residual lime may not be able to hydrate.Wachsmuth et al. (1981) reported that precipitated lime islimited to 4%, meaning that approximately 5.49% or moreof the lime in this study was residual which acted similarlyto quicklime. Due to the grinding process, the residual lime

0

500

1,000

1,500

2,000

2,500

0 1 2 3 4 5Strain (%)

Curing time:1day2days3days7days28days90days

RBOF = 20%Grain size <37.5mm

0

500

1,000

1,500

2,000

2,500

0 1 2 3 4 5

m/Nk(ssertS

2 )

Strain (%)

Curing time:1day2days3days7days28days90days

RBOF = 30%Grain size <2mm

Fig. 9. UC test results of soils stabilized with BOF slag with maximum particle sizes of 2 mm and 37.5 mm.

Fig. 10. Unconfined compressive strength of soils stabilized with BOFslag with various free lime contents.

1392 A.A. Cikmit et al. / Soils and Foundations 59 (2019) 1385–1398

can be exposed to water. The existence of free lime will pro-duce portlandite (Ca(OH)2) which can lead to the solidifi-cation of soft soils.

CaO + H2O ! Ca(OH)2 + heat ð3ÞThe additional water in soft soil is driven out through

steam since the reaction produces heat. The hydration reac-tion substantially reduces the thickness of the absorbedwater layer, causing a reduced susceptibility to water addi-tion. Later, the pozzolanic reaction occurs; the portlandite,silica, and alumina minerals from the clay react to form acalcium silicate hydrate (C-S-H) gel to further bind the soilskeleton together (Rogers and Roff, 1997). The chemicalequation reported by Kiso et al. (2008) and further studiedby Toda et al. (2018) can be expressed by Eqs. (4) and (5).

Ca(OH)2 + SiO4 + nH2O ! C-S-H ð4ÞCa2þ+AlO2 + X� + nH2O ! Ca4Al2O6X2�nH2O ð5Þ

Toda et al. (2018) reported that there was a significantrise in the pH in the soils stabilized with BOF slag. Thedissolution of portlandite in BOF slag can produce ahyper-alkaline condition in the water that will enhancethe formation of the C-S-H gel. The effect of humic acidon the alkaline condition of water was fully considered.However, the 8.2% loss of ignition (LOI) in this studywas relatively low. Kang et al., 2017 studied the effect ofvarious humic acid contents on the strength developmentof cement-treated clay, and it was reported that less than10% humic acid barely has any significant effect on thestrength development of stabilized soils.

Fig. 10 illustrates the unconfined compressive strengthof soils stabilized with BOF slag and different free lime con-

tents from various studies (CDIT, 2017). It is clearly seenthat the strength of the stabilized soils was mainly affectedby the free lime content as well as by the grain sizes of theBOF slag.

Although the UC test is a beneficial test for measuringthe strength qu of most stabilized soils, this type of strengthtest is limited when measuring soil samples because of anundrained shear strength of less than 2.5 kN/m2. There-fore, another simple strength test, the LVS test, wasadopted to measure samples with low undrained shearstrength Su. In this study, the measured undrained shearstrength, qu, obtained from the UC test, was determinedas 2 times the value of the undrained shear strength, Su,obtained from the LVS test, namely, qu = 2Su.

Fig. 11. Time-strength mobilization of soils stabilized with BOF slag.

A.A. Cikmit et al. / Soils and Foundations 59 (2019) 1385–1398 1393

The results of the tests showed that the strength develop-ment of soils stabilized with BOF slag of different maxi-mum particle sizes varied considerably with the variouscuring times. Fig. 11 demonstrates the mean collected val-ues from the LVS and UC tests performed on samples sta-bilized with BOF slag with a 20–30% ratio of addition anddifferent maximum particle sizes. Generally, in soil stabi-lization using BOF slag, the strength development of stabi-lized soils shows three different stages which are an inactivezone, a high acceleration zone, and a moderate accelerationzone. These results are similar to those in previousresearches (Sato et al., 2016; Kang et al., 2019, Weerakonet al., 2018).

3.4.1. Strength at early-stage of curing time

The strength development of stabilized soils at the earlystage of curing time was seen to be significantly affected by

Fig. 12. Strength development of stabilized soils at early curing time.

the grain size of the BOF slag. The inactive zone or the no-strength gain period was characterized by relatively con-stant strength, until a certain curing time was clearly seenbetween the smaller maximum grain size (<9.5 mm) andthe larger grain size (<37.5 mm) (Fig. 12). The soil mixtureswith a maximum grain size of less than 9.5 mm exhibitedup to 5 h of dormancy, while those with a larger maximumgrain size of >37.5 mm had no significant strength gain forup to 10 h.

Although the strength increment of the larger particlesize was insignificant at the early stage of curing time, theimmediate strength (0.5 h of curing time) of the larger par-ticles showed a considerably higher value than the smallerparticles. The stabilized soils using BOF slag with a maxi-mum particle size of 37.5 mm produced approximately 2.5kN/m2 of unconfined compressive strength. This indicatesthat at the immediate curing time, the strength was largelyaffected by the particle size and that later, the size effect wasneutralized by the amount of free lime it contained.

3.4.2. Strength at later-stage of curing time

In the semi-log scale for the relationship between thestrength of stabilized soils and the curing time, it is clearthat the strength development changed to linear lines after7 d (Fig. 13). The later strength mobilization was also lar-gely affected by the particle size. With the same free limecontent and initial clay water content, the smaller particlesize produced a higher strength increment, which is likelyto be due to a larger contact surface between the free limeand the soil skeleton.

3.4.3. Strength increment of stabilized soilsThe different particle sizes also affected the strength

increment of the early strength of stabilized soils. The coef-ficient of strength increment of the stabilized soils was

Fig. 13. Time-strength mobilization of soils stabilized with BOF slag insemi-logarithmic scale.

Fig. 14. Strength increment coefficient of stabilized soils at: (a) 0.5–5 h, (b)5–72 h, and (c) 3–90 d.

Fig. 15. Relationship between compressive strength and secant modulusof stabilized soils.

1394 A.A. Cikmit et al. / Soils and Foundations 59 (2019) 1385–1398

divided into three parts based on the different strengthdevelopment that was previously explained. It thereforeranged from 0.5 to 5 h to 5–72 h for the early stage of cur-ing time and 3–90 d for the later stage of curing time.Fig. 14 illustrates the relationship between various maxi-mum grain sizes and the strength increment coefficients,a1, a2, and a3 (illustrated in Figs. 12 and 13), which are cal-culated as follows:

a1 ¼ Dðlog quÞDðlogtÞ for the inactive zone ð6aÞ

a2 ¼ Dðlog quÞDðlogtÞ for the high acceleration zone ð6bÞ

a3 ¼ DquDðlogtÞ for the moderate acceleration zone ð6cÞ

From Fig. 14a, it can be seen that the coefficient ofstrength increment shows a distinct increment with thesmaller maximum particle size. Moreover, the larger theaddition rate of BOF slag, the wider the gap in strengthincrement between the different maximum grain sizes.

Fig. 14b indicates the strength increment coefficient, a2,of the stabilized soils from 5 to 72 h of curing time. Thestrength increment shown by a 30% rate of addition pro-

duced a lower value than the 20% rate of addition. Thisis due to the different levels of initial strength exhibitedby the two different rates of addition. The strength incre-ment coefficient for this period of curing time ranged from1.35 to 1.75, which indicated a significant difference instrength increment from the previous curing time of 0.5–5 h, a1.

Fig. 14c indicates the strength increment coefficient ofthe stabilized soils at the later stage of curing time, rangingfrom 3 to 90 d (of curing time). It is assured that the rate ofaddition significantly affected the strength development ofthe stabilized soils in this period of time. Unlike a1 anda2, the strength increment at the later stage of curing time,a3, is significantly higher with the 30% addition rate thanthe 20% addition rate.

3.5. Secant modulus

Secant modulus, E50, of the stabilized soils was deter-mined from the slope of the stress-strain curve obtainedfrom the UC test. The secant modulus determined in thisstudy and that from a previous study conducted by Kanget al., 2019 are shown in Fig. 15, indicating that the secantmodulus of the stabilized soils can be estimated to be 88.5to 120 qu, where qu is the unconfined compressive strengthof the stabilized soils.

Based on the combined data, it appears that the correla-tion of the secant modulus from the unconfined stressstrength was not significantly affected by the different max-imum grain sizes. Despite the range in grain sizes used inthis study, the value of the secant modulus can be predictedusing a constant value obtained from the relationshipbetween the stress strength values of the stabilized soils.

3.6. Effect of BOF slag grain size on strength mobilization

To compare the difference between this study and theprevious study documented in a Japan steel slag manual,the levels of unconfined compressive strength of stabilizedsoils at 7, 28, and 90 d of curing time as a function of dif-

Fig. 16. Relationship between different maximum grain sizes of BOF slagand unconfined compressive strength at 7, 28, and 90 days.

0

100

200

300

400

500

600

700

800

900

0 200 400 600 800 1000

m/Nk(

htgnertseviss erp

mocdeni fnocn

U2 )

Curing time (hours)

0.85mm-9.5mm2mm-9.5mm4.75mm-9.5mm9.5mm-37.5mm

Tokuyama Port clayw0=1.5wL, wL=117.5%f-CaO=8.49%RBOF= 30%

Fig. 17. Strength development of stabilized soils with a lack of fineparticles.

A.A. Cikmit et al. / Soils and Foundations 59 (2019) 1385–1398 1395

ferent maximum particle sizes are plotted (Fig. 16). Theprevious study (JISF, 2008) also showed a similar effectof the different particle sizes on the strength with a largermaximum particle size of BOF slag. The strength was com-parably smaller than that of the stabilized soils with smallerparticle sizes. Furthermore, the strength ratio of the maxi-mum grain sizes of 4.75 mm and 20 mm from this studyand the previous study is Su(4.75/20) = 1.54 and 1.50, respec-tively, which indicates that despite the different free limecontents, the stabilized soils show a similar strength ratiobetween the different maximum particle sizes.

3.7. Strength estimation equation

Due to the large energy consumption required to grindBOF slag from the original size to a micro size, a large par-ticle (up to 37.5 mm) is generally adopted in actual fieldconstruction works. However, from a practical point ofview, it is difficult to measure the strength of stabilized soilsusing the actual grain size (37.5 mm) in the laboratory.Large volumes of soil and BOF slag are required to preparethe large sample to retain the adequate ratio between thelargest particle and the diameter of the sample. Therefore,predicting the actual field strength from a smaller particlesize in the laboratory is becoming a common practice.

Although the use of a smaller particle size in the labora-tory has been adopted to predict the actual strength inembankment construction, it has been implied in previousresearches that there were substantial differences betweenthe actual strength and the laboratory test results(Tanaka et al., 2014; Yuzoh et al., 2012). This was mainlydue to the different distributions of grain sizes adopted inthe laboratory and in the actual field construction. Conse-quently, a modification of this method is necessary for pre-dicting the actual strength.

Fig. 17 illustrates the strength of the stabilized soilsusing various ranges of grain size (0.85–9.5 mm, 2–9.5 mm, 4.75–9.5 mm, and 9.5–37.5 mm) as a function of

the curing time. With the absence of smaller particles, thestrength shows a lower increment than other ranges.Significant strength increments might still be affected bythe very small particles of BOF slag attached around thelarger particles since it was difficult to completely removethem using only the sieving method under atmosphericconditions. The results of the strength test fell in the rangeof 9.5–37.5 mm, lacking smaller grain sizes, and could notexhibit any strength mobilization within the elapsed curingtime. From these results, the granular BOF slag could bedivided into two types: (1) smaller particle sizes(<9.5 mm) that behaved like cement and (2) larger particlesizes (>9.5 mm) that behaved with less chemically activematerial and were similar to aggregates.

According to the results, the rate of addition in Eq. (1)was modified to define the mass which is highly active tohydrate, and is described in Eq. (7) as follows:

RBOFðhydrate-massÞ ¼ MBOFðGBÞM soil þMwater þMBOF

� 100 ð%Þð7Þ

where MBOF(GB) is the mass of chemically active BOF slag.This value is determined by the boundary of the grain sizewhere the larger size shows no gain in strength. The bound-ary was taken as 9.5 mm in this study, which may differfrom other types of BOF slag.

The contact area between the smaller grain size of theBOF slag and the clay particles which produce lime-hydrating minerals is closely related to the surface area ofthe binder. In a previous study by Zhang and Napier-Munn (1995), the different specific surface areas were foundto be the reason for the various strength increments ofPortland cement at the early and later stages, despite thecement having identical mineral constituents. The mea-sured specific surface area of Portland cement could beaccurately determined by their model using the particle sizedistribution.

Curing time: 28 daysqu = 166.5xR² = 0.87

Curing time: 7 daysqu = 61.2xR² = 0.72

Curing time: 90daysqu = 251.8xR² = 0.86

10

510

1,010

1,510

2,010

2,510

3,010

3,510

0 5 10 15 20

,htgnertsevisserp

mo cdenifno cn

Uq u

(kN

/m2 )

SSA*RBOF(hydrate-mass) (m2/kg)

RBOF = 30% (7days)RBOF = 20% (7days)RBOF = 30% (28days)RBOF = 20% (28days)RBOF = 30% (90days)RBOF = 20% (90days)

Fig. 18. Specific surface area multiplied by modified rate of additionversus unconfined compressive strength of soils stabilized with BOF slag.

1396 A.A. Cikmit et al. / Soils and Foundations 59 (2019) 1385–1398

Eq. (8) (Herdan, 1953) was adopted to calculate thespecific surface area of granular material with a ball-shape-like assumption.

SSA ¼ 6

q

Xn

i¼1

wi

xi

� �ð8Þ

and

xi ¼xh2 þ xj2� �

xh þ xj� �

4

� �1=3

ð9Þ

where Ss is the surface area (m2/kg), n is the number ofintervals in the grain size distribution, wi is the weight ofretained fraction size I, xi is the harmonic mean size deter-mined by Eq. (9), where xh and xj are the upper and lowergrain size intervals (mm), respectively, and q is the densityof the material. The interval of the grain size was dividedusing the ASTM E11 sieve designation.

The strength development of stabilized soils at a certaincuring time can be predicted using the multiplied value ofthe specific surface area with the additional rate of BOFslag. The specific surface SSA of the BOF slag, where thegrain size is less than 9.5 mm, was calculated with Eqs.(8) and (9). Fig. 18 shows the relationship between the

Table 6Calculated SSA, SSA * RBOF(hydrate-mass), and qu at curing times of 7, 28, and

RBOF Maximum particle size Calculated SSA RBOF(hydrate-mass)

(%) (mm) (m2/kg) (%)

30 37.5 6.22 40.030 10 8.84 50.030 4.75 13.55 50.030 2 16.83 50.030 0.89 21.37 50.020 37.5 6.22 29.020 10 8.84 36.920 4.75 13.55 36.920 2 16.83 36.9

strength and the total effective specific surface of theBOF slag, SSA * RBOF(hydrate-mass). It clarifies that thestrength development at a later curing time could be wellpredicted by using the value of SSA * RBOF(hydrate-mass) ofthe stabilized soils shown below:

quð90dÞ ¼ 251:8 SSA � RBOF ðhydrate�massÞ� � ð10Þ

quð28dÞ ¼ 166:5 SSA � RBOF ðhydrate�massÞ� � ð11Þ

quð7dÞ ¼ 61:2 SSA � RBOF ðhydrate�massÞ� � ð12Þ

where qu(d) is the unconfined compressive strength at curingtime d of the stabilized soils with BOF slag.

The relationship between the unconfinedcompressive strength of the stabilized soils and theSSA * RBOF(hydrate-mass) (Table 6 and Fig. 18) can be usedto estimate the strength of the stabilized soils with a variedgrain size of the BOF slag. Typically, the maximum grainsize of BOF slag in the Japanese construction market is37.5 mm, and this size is commonly adopted in manyconstruction projects. Using Eqs. (10), (11), and (12) for var-ious curing times, the strength of BOF slag using the com-mon larger size (37.5 mm) can be predicted using Eq. (13).

quð37:5 mmÞ ¼quð4:75 mmÞ

SSA � RBOF ðhydrate�massÞ� �

ð4:75mmÞ

� SSA � RBOF ðhydrate�massÞ� �

ð37:5mmÞ ð13Þwhere qu(37.5 mm) is the stress strength of the soils stabi-lized with BOF slag using the actual size, and qu(4.75 mm)is the stress strength of soils stabilized soils in the labora-tory using a grain size of 4.75 mm.

4. Conclusions

From the data presented in this paper, and consideringthe limitations of this study, the following conclusionscan be drawn:

� Under similar conditions of initial clay moisture andfree lime content, the different grain size distributionsof BOF slag were seen to have a significant effect onthe strength development of the stabilized marine-dredged clay.

90 days.

SSA * RBOF(hydrate-mass) qu(7 days) qu(28 days) qu(90 days)

(m2/kg) (kN/m2) (kN/m2) (kN/m2)

2.50 136.43 338.41 477.464.42 341.53 1083.22 1790.006.77 340.00 1093.70 1838.638.42 369.36 1163.70 2015.2210.68 761.57 1884.00 2479.171.80 136.26 238.00 327.243.26 322.71 691.43 930.685.00 370.55 810.09 1179.136.21 279.48 868.79 1414.90

A.A. Cikmit et al. / Soils and Foundations 59 (2019) 1385–1398 1397

� At an early stage of curing time, the addition of BOFslag with larger maximum particle sizes produced longerinactive or negligible gains in strength. In this study, theBOF slag with a grain size of 37.5 mm was inactive forup to 10 h, while the slag with a smaller maximum grainsize had an average of 5 h of inactivity.

� The correlation of the secant modulus of BOF slag was88.5 to 120 qu, where qu was the unconfined compressivestrength obtained from the UC test. In this study, thebest value for predicting the E50 was found to be 88.5qu. The correlation did not appear to be significantlyaffected by a different maximum grain size.

� Equations to estimate the strength of the soils stabilizedwith BOF slag were proposed using the specific surfacearea value obtained from the grain size distributionand the modified BOF rate of addition. They demon-strated a good agreement for predicting the actual sizeemployed in the field, using a smaller maximum grainsize in the laboratory.

� Applying the proposed Eq. (13), the actual strengthusing a maximum grain size equal to 37.5 mm can bepredicted using the results from the laboratory, with asmaller grain size of 4.75 mm.

Acknowledgments

The authors gratefully acknowledge the financial sup-port provided by the JFE Steel Corporation, Japan, theHiroshima Research and Engineering Office of Port andAirport, Chugoku Regional Development Bureau, Min-istry of Land, Infrastructure, Transport and Tourism andthe Monbugakusho (MEXT) scholarship. They would alsolike to thank Editage (www.editage.jp) for the English lan-guage editing.

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