(BOF) Steel Slag for Railway Ballast Material
sustainability
Article A Feasibility Study on the Application of Basic Oxygen Furnace (BOF) Steel Slag for Railway Ballast Material
Taehoon Koh 1,*, Sung-Woo Moon 2 ID , Hyuksang Jung 3, Yeonung Jeong 4 ID and Sukhoon Pyo 1 ID 1 New Transportation Systems Research Center, Korea Railroad Research Institute, 176 Railroad Museum Road, Uiwang-si, Gyeonggi-do 16105, Korea; [email protected] 2 Department of Civil Engineering, Nazarbayev University, 53 Qabanbay Batyr Ave, Astana 010000, Kazakhstan; [email protected] 3 Department of Railroad Construction & Safety Engineering, Dongyang University, 145 Dongyangdaero Punggi-eup, Yeongju-si, Gyeongsangbuk-do 36040, Korea; [email protected] 4 Department of Civil and Environmental Engineering, National University of Singapore, 1 Engineering Drive 2, Singapore 117576, Singapore; [email protected] * Correspondence: [email protected]; Tel.: +82-31-460-5661
Received: 8 December 2017; Accepted: 20 January 2018; Published: 23 January 2018
Abstract: Railway ballast, for which natural crushed stone aggregates have been generally used, is an essential track component for the distribution of train loads along the rails and sleepers to the roadbed. However, the use of natural crushed stone aggregate causes environmental destruction as well as dust production in train service. This paper evaluates the feasibility of using the basic oxygen furnace (BOF) steel slag as railway ballast material. A series of physical and chemical quality tests are performed to investigate the characteristics of the materials associated with the effect of aging period due to the remaining free CaO and MgO in the BOF steel slag. Three different aging periods (i.e., 0, 3, and 6 months) are used to compare with various standards and the properties of the crushed stone aggregates. It is demonstrated that the physical and chemical properties of the BOF steel slag with different aging periods satisfy all requirements of standards sufficiently. Especially, the BOF steel slag without aging (i.e., 0 month) provides the similar physical and chemical properties, when compared to the BOF steel slag with aging (i.e., 3 and 6 months). Thus, it is possible to apply the BOF steel slag regardless of aging periods to the railway ballast materials instead of natural crushed stone aggregates.
Keywords: basic oxygen furnace (BOF) steel slag; electric arc furnace (EAF) steel slag; blast furnace (BF) iron slag; railway ballast; aging; free lime
1. Introduction Generally, railway ballast helps distributing train loads along the rails and sleepers to the roadbed. Natural crushed stone aggregates which have similar engineering properties of concrete aggregates are widely employed as railway ballast materials. It is because the natural crushed stone aggregates reduce vibration as well as dust production in train service resulting from the cushioning effect of itself, and are easy to be installed. The performance of ballast layers essentially depends on the quality of the ballast materials, which is defined by the characteristics of individual particles (e.g., durability, shape, particle size distribution, weight of unit volume, absorption factor, and wear rate of particles). For the currently used natural ballast, there are three problematic issues: (1) it is continuously exposed to abrasion and crush due to running train loading; (2) crushed or fragmented particles clogging the voids of the roadbed cause some problems with respect to the function of roadbed
Sustainability 2018, 10, 284; doi:10.3390/su10020284 www.mdpi.com/journal/sustainability Sustainability 2018, 10, 284 2 of 12 drainage systems; (3) both Multiple Tie Tamper (MTT) track maintenance works for track deformation and ballast cleaning lead to accelerating sudden settlements of the ballast layer. Thus, it is necessary to develop the eco-friendly and geo-engineered alternative ballast materials to provide improvement in stability during the ballasted track maintenance. Based on this background, this study aims to evaluate the feasibility of the steel slag as the railway ballast material to substitute for the natural ballast. Steel industry slags, byproduct from iron and steel making, mainly consist of blast furnace (BF) iron slag, basic oxygen furnace (BOF) steel slag, and electric arc furnace (EAF) steel slag which have been actively used for various applications in construction. Namely, BF iron slag (air cooled BF slag and granulated BF slag) has long been utilized as a construction material (aggregate and concrete binder etc.), and steel slag (BOF slag and EAF oxidizing slag) has been the subject of numerous research efforts with the aim of used as aggregates for road, embankment, reinforced concrete elements, or high-performance concrete with good bond with reinforcements, eco-friendly prestressed concrete sleeper [1–4]. However, steel slag still has not been widely utilized as aggregate for concrete production limited by Construction Specification, due to its unqualified properties with the specification. Recently, it was reported that the steel slag has been effectively used as railway ballast which is the largest component of the railway track by weight and by volume due to its advantages in construction as well as maintenance [5]. The National Slag Association (NSA) explained that steel industry slag could be excellent ballast material in several aspects: (1) higher durability than the quality standards, (2) cost effectiveness of sustainability and maintenance, and (3) minimum occurrence of mud pumping. Steel slag was applied to ballast materials for railways in the western region adjoining the California-Nevada border in United States [5]. Jones [6] described that coarse water-granulated BF slag and steel slag could be utilized as ballast materials for all railwayse including even high-speed railways, and fine-grained slag could be also used as a supplementary material to prevent the ballast from penetrating the roadbed layers. In addition, it was reported that the slag characteristics of angular and rough shapes stabilizes the track structure due to mutually interlocking. Therefore, it leads to: (1) stabilizing the uneven surface, (2) increasing the lateral resistance of track, (3) outstanding drainage performance, and (4) a high resistance to freezing and thawing [6]. For this intention, a series of physical and chemical tests in this study were performed for estimating engineering properties of the BOF slag as one of steel slags with three different aging periods (i.e., 0, 3, and 6 months), since remaining free CaO and MgO in the BOF slag may have an influence on its dimensional stability; free CaO causes expansion at early ages with the formation of Ca(OH)2 while free MgO makes expansion in long term period with the formation of Mg(OH)2. In addition, the test results were verified by the comparison with various standards and the natural ballast. It is expected that over 20 million tons of the produced steel industry slag in Korea will be successfully recycled for the ballast materials annually. Thus, this study would contribute to: (1) environmental protection by substituting for the destruction of mountains to obtain the natural ballast, (2) enhancement of the track durability and drainage ability due to the outstanding wear resistance of the slag, and (3) increase of track stability due to the heavier weight of the track.
2. Theoretical Background
2.1. Current Status of Steel Industry Slag Generat Ion and Recycling In 2016, the total amount of world crude steel production was approximately 1630 GN, and the average growth rates between years 2010 and 2015 was 2.5% per annum [7]. Even though there is no exact data for world steel slag production, it is estimated that 1600 to 2400 GN of steel slag are produced in 2016 over the world based on typical ratios of slag to steel output [8]. Thus, it should be noted that a substantial amount of byproducts have inevitably been produced from steelmaking industry. However, it is also known that the steel industries have difficulty in coping with increasing costs for managing steel slag and securing the sites required for slag aging due to the lack of efficient Sustainability 2018, 10, x FOR PEER REVIEW 3 of 12
noted that a substantial amount of byproducts have inevitably been produced from steelmaking industry. SustainabilityHowever,2018, 10it ,is 284 also known that the steel industries have difficulty in coping with increasing 3costs of 12 for managing steel slag and securing the sites required for slag aging due to the lack of efficient processing methodsmethods and new demands. In addition, a large number of raw materials and energy are required and a varietyvariety ofof byproductsbyproducts (e.g., BF slag, BOF slag, and EAF slag) are also producedproduced in several processes suchsuch asas iron-making,iron-making, steelmaking,steelmaking, andand rolling.rolling. Figure1 1 shows the total total amount amount of of steel steel industry industry slag slag divided divided into into BOF BOF slag slagand BF and slag BF in slag major in majorcountries, countries, including including generated, generated, imported imported and andstored stored amounts amounts [9–12]. [9–12 ].The The total total amount amount of of steel industry slagslag generatedgenerated in in United United States States was was 153 153 GN GN in in 2011, 2011, consisting consisting of 80of GN80 GN of BF of slagBF slag and and 73 GN 73 ofGN BOF of BOF slag, slag, as reported as reported by the by United the United States GeologicalStates Geological Survey Survey (USGS). (USG The amountS). The ofamount steel industry of steel slagindustry produced slag produced in Japan wasin Japan 383 GN was in 383 2012, GN and in 2012, the amount and the of amount BF slag of (i.e., BF246 slag GN) (i.e., was 246 produced GN) was muchproduced more much than more that of than BOF that slag of (i.e., BOF 137 slag GN). (i.e., 137 GN). Korea Iron and Steel Association (KOSA) described that more than 99% of the produced slag is recycled inin variousvarious areas: areas: (1) (1) a a raw raw material material of of cement cement (75%), (75%), (2) (2) civil civil engineering engineering (15.9%), (15.9%), (3) sub-base (3) sub- coursebase course materials materials for roads for roads (5.2%), (5.2%), and (4) and fertilizers (4) fertilizers (2.7%) (2.7%) [10]. [10].
500 BOF steel slag BF iron slag 400
217 300 137
200 107
73 246 234 Production of Slag (GN) of Slag Production 100 137 80 11 0 23 Korea A & N USA Japan EU (2013) (2012) (2011) (2012) (2011)
FigureFigure 1. Generation 1. Generation of steel of steel industry industry slag inslag major in major countries countries (Note: (Note: A & NA means& N means Australia Australia & New & Zealand).New Zealand). 2.2. Standards for Railway Ballast Material 2.2. Standards for Railway Ballast Material In order to evaluate the test results conducted in this study, the following guidelines for ballast are consideredIn order to as evaluate suggested the bytest Korea results Railroad conducted Corporation in this study, Standard the following (KRCS) [13 guidelines]: (1) unit for weight ballast of aggregatesare considered and solidas suggested content inby aggregates; Korea Railroad (2) abrasion Corporation and hardness Standard of (KRCS) coarse aggregates [13]: (1) unit by weight use of the of Losaggregates Angeles and machine; solid content (3) compressive in aggregates; strength; (2) abrasion and (4) immersion and hardness expansion of coarse in water, aggregates as presented by use inof Tablethe Los1. Moreover, Angeles machine; ballast is required(3) compressive to have thestrength; shape ofand a cube (4) immersion or polyhedron expansion with rich in edges water, and as nearlypresented identical in Table facets, 1. Moreover, without harmful ballast shapesis required such to as have a thin the piece shape of stone.of a cube or polyhedron with rich edgesAs shown and innearly Table identical2, several facets, standards without for the ha evaluationrmful shapes of railwaysuch as ballasta thin materialspiece of stone. are employed in Australia,As shown the in United Table States, 2, several Canada, standards and the for United the evaluation Kingdom, commonlyof railway adoptingballast materials Los Angeles are abrasionemployed test. in Australia, Thus, it the indicates United that States, the Canada Los Angeles, and the abrasion Unitedis Kingdom, the most commonly critical element adopting for Los the evaluationAngeles abrasion of ballast test. material, Thus, it indicates although that there the are Lo differencess Angeles abrasion of standards is the (i.e., most 20–25% critical in element Australia, for Canada,the evaluation and the of United ballast Kingdom, material, 40% although in theUnited there States).are differences of standards (i.e., 20–25% in Australia, Canada, and the United Kingdom, 40% in the United States).
Sustainability 2018, 10, 284 4 of 12
Table 1. Quality standards for ballast [13].
Classification Criteria Test Methods Unit weight Higher than 14 kN/m3 ASTM C29 Abrasion Less than 25% ASTM C535-16 Hardness Higher than 20 ASTM D2240 Compressive strength Higher than 80 N/mm2 ASTM C170 Immersion expansion Less than 1.5% JIS A 5015 Contents of Clay ASTM C 142 harmful Coal and brown coalLess than 3.5% ASTM C123 materials Soft fragments AASHTO T 189-70
Table 2. Standards for physical properties of ballast material in major countries (Australia, the United States, Canada, and United Kingdom).
Ballast Property Australia USA Canada UK Korea Aggregate Crushing Value <25% - - <22% - Los Angeles Abrasion <25% <40% <20% <20% <25% Flakiness Index <30% - - <35% - Misshapen Particles <30% - <25% - - Sodium Sulphate - <10% <5% - - Soundness Magnesium Sulphate - N/A <10% - - Soft and Friable Pieces - <5% <5% - - Fines (
3. Physical and Chemical Quality Test Methods
3.1. Overivew Steel slag generally consists of free-CaO, free-MgO (periclase), various types of crystalline phases such as olivine, merwinite, γ-2CaO·SiO2, 2CaO·Fe2O3, and RO phases (CaO-MgO-SiO2-FeO solid solution) [14]. Depending on the steel manufacturing process (i.e., BOF and EAF) or cooling methods (i.e., natural drying and rapid cooling), it is proved that the chemical, physical and mineralogical properties are various. It is noted that free lime (CaO) in the steel slag is from two different sources: (1) residual free lime originally contained in the raw materials, and (2) precipitated lime from the molten slag. The remaining free lime (CaO) is known to react with water, transferring into Portlandite [Ca(OH)2][14–17]. This transformation induces 98% volumetric expansion of BOF slag [18]. In addition, free MgO in BOF slag also reacts with water, forming Mg(OH)2 with 148% of volumetric expansion. Moreover, Wang [17] and Wang et al. [19] illustrated that the volume of the BOF slag expands at least 7.5% with the expansive force (i.e., approximately 0.64–1.28 MPa). The treatment of the free CaO and MgO contained in the BOF slag is a critical issue to apply the slag to the concrete aggregate due to the expansion characteristic (i.e., at least 7.5% of its original volume). Therefore, and the several aging methods, such as air aging treatment and steam aging method, have been employed [20–22]. It is necessary to consider the effects of aging to treat the free CaO and MgO in terms of physical and chemical quality of the BOF slag. In this study, the BOF slag samples provided by a steelmaking company in Korea were used for the various tests up to six months to take into account the effects of aging. Typically, it is well-known that the recycled BOF slag requires at least six months for the aging process to reduce the expansion due to free CaO and MgO [22–24]. Therefore, the different periods of aging (i.e., 0, 3 and 6 months) were used for this study. The test results were evaluated by the comparison of the several standards. Sustainability 2018,, 10,, 284x FOR PEER REVIEW 5 of 12
The chemical compositions of BOF slag samples tested in this study are as follows: SiO2 14.3%, Al2O3 The chemical compositions of BOF slag samples tested in this study are as follows: SiO 14.3%, 1.6%, CaO 45.1%, Fe 16.8%, MgO 3.6%, MnO 4.6%, TiO2 1.3%, S 0.2%. 2 Al2O3 1.6%, CaO 45.1%, Fe 16.8%, MgO 3.6%, MnO 4.6%, TiO2 1.3%, S 0.2%. 3.2. Test Methods and Standards 3.2. Test Methods and Standards The basic physical properties as well as the quality of the BOF slag for railway ballast material The basic physical properties as well as the quality of the BOF slag for railway ballast material were evaluated in six test methods for determining: (1) unit weight of aggregate (ASTM C29 [25]), (2) were evaluated in six test methods for determining: (1) unit weight of aggregate (ASTM C29 [25]), resistance to abrasion of coarse aggregate (ASTM C535 [26]), (3) hardness (ASTM D2240 [27]), (4) (2) resistance to abrasion of coarse aggregate (ASTM C535 [26]), (3) hardness (ASTM D2240 [27]), compressive strength (ASTM C170 [28]), (5) immersion expansion (JIS A 5015 [29]), (6) contents of (4) compressive strength (ASTM C170 [28]), (5) immersion expansion (JIS A 5015 [29]), (6) contents of harmful materials (ASTM C142 [30], ASTM C123 [31], AASHTO T 189-70 [32]). harmful materials (ASTM C142 [30], ASTM C123 [31], AASHTO T 189-70 [32]). The test method for unit weight of aggregate (ASTM C29 [25]) was conducted to measure the The test method for unit weight of aggregate (ASTM C29 [25]) was conducted to measure the weight of a unit volume of the slag resulting from dividing the weight by the volume of the container. weight of a unit volume of the slag resulting from dividing the weight by the volume of the container. The degree of ballast abrasion, which is one of the most critical factors, was evaluated by the The degree of ballast abrasion, which is one of the most critical factors, was evaluated by the abrasion abrasion test for coarse aggregate via the Los Angeles machine (ASTM C535 [26]). The test was test for coarse aggregate via the Los Angeles machine (ASTM C535 [26]). The test was performed by performed by rotating the Los Angeles machine for each size of aggregate as a pre-defined number rotating the Los Angeles machine for each size of aggregate as a pre-defined number of rotations, and the of rotations, and the number of 12 sieve and aggregate size of below 1.7 mm were used. Percentage number of 12 sieve and aggregate size of below 1.7 mm were used. Percentage amounts passing the amounts passing the No. 12 (1.7 mm) sieve due to abrasion were expressed in terms of original weight No. 12 (1.7 mm) sieve due to abrasion were expressed in terms of original weight of the test sample before of the test sample before the test. the test. The hardness test (ASTM D2240 [27]) was performed by the shore hardness tester to measure The hardness test (ASTM D2240 [27]) was performed by the shore hardness tester to measure the rebounding height of the hammer dropped on the test sample from a constant height. The shore the rebounding height of the hammer dropped on the test sample from a constant height. The shore hardness tester of the D-type model was employed to allow measuring the rebounding height of a hardness tester of the D-type model was employed to allow measuring the rebounding height of diamond weight of 4 g dropped from the height of 25 cm with the test scale of 0 to 140. a diamond weight of 4 g dropped from the height of 25 cm with the test scale of 0 to 140. Unconfined compressive strength of the ballast is one of the most essential requirements because Unconfined compressive strength of the ballast is one of the most essential requirements because unqualified unconfined compressive strength causes crushed aggregate and excessive deformation unqualified unconfined compressive strength causes crushed aggregate and excessive deformation of the ballast. The test method for unconfined compressive strength (ASTM C170 [28]) was carried of the ballast. The test method for unconfined compressive strength (ASTM C170 [28]) was carried out to determine the strength of the material expressed in terms of the compressive load divided by out to determine the strength of the material expressed in terms of the compressive load divided by the cross-sectional area of the test sample. A test sample with dimensions of 5 × 5 × 5 cm3 made by the cross-sectional area of the test sample. A test sample with dimensions of 5 × 5 × 5 cm3 made by aggregates (below 1.7 mm) was placed under the displacement control test at a rate of 1 mm/min, as aggregates (below 1.7 mm) was placed under the displacement control test at a rate of 1 mm/min, shown in Figure 2. as shown in Figure2.
(a) (b)
Figure 2. The test for unconfined unconfined compressive strength: ( a) Before; (b) After.
Figure 3 presents the immersion expansion test (JIS A 5015 [26]) that was implemented to evaluate the immersion expansion associated with remaining free CaO in BOF slag reacting on water, as follows:
CaO + H2O → Ca(OH)2, (1) Sustainability 2018, 10, 284 6 of 12
Figure3 presents the immersion expansion test (JIS A 5015 [ 26]) that was implemented to evaluate the immersion expansion associated with remaining free CaO in BOF slag reacting on water, as follows: Sustainability 2018, 10, x FOR PEER REVIEW 6 of 12
CaO + H2O → Ca(OH)2, (1)
MgO + H2O → Mg(OH)2, (2) MgO + H2O → Mg(OH)2, (2) The test sample adjusted by particle size was placed in three separate layers with the optimal moistureThe content test sample based adjusted on the by compaction particle size te wasst methods placed in (ASTM three separate D4792 layers[33]), withand thethen optimal the measurementmoisture content device based was oninstalled. the compaction The test test sample methods was (ASTMmaintained D4792 at [ 33the]), constant and then temperature the measurement of ◦ ◦ 70device °C ± 3 was°C for installed. six hours, The and test the sample constant was maintainedtemperature at was the decreased constant temperature to room temperature of 70 C ± in3 theC for curingsix hours, equipment. and the The constant test was temperature conducted was on decreaseda daily basis to room for 10 temperature days. The inimmersion the curing expansion equipment. ratioThe was test estimated was conducted by the on comparison a daily basis of forthe 10final days. measurement The immersion and the expansion initial measurement. ratio was estimated by the comparison of the final measurement and the initial measurement.
(a) (b)
FigureFigure 3. Immersion 3. Immersion expansion expansion test: test: (a) (Preparinga) Preparing samples; samples; (b) ( bImmersion.) Immersion.
The harmful substances such as clay, coal and brown coal, and soft fragments, were evaluated The harmful substances such as clay, coal and brown coal, and soft fragments, were evaluated by three different test methods (i.e., ASTM C142 [30], ASTM C123 [31], AASHTO T 189-70 [32]), and by three different test methods (i.e., ASTM C142 [30], ASTM C123 [31], AASHTO T 189-70 [32]), they should be less than 3.5% of the ballast samples. First, the test method for clay content (ASTM and they should be less than 3.5% of the ballast samples. First, the test method for clay content (ASTM C142 [30]) was performed to determine the quantity of rock pieces which can be easily crushed due C142 [30]) was performed to determine the quantity of rock pieces which can be easily crushed due to to harmful substances (e.g., a lump of clay) contained in the slag aggregate. Second, the test method harmful substances (e.g., a lump of clay) contained in the slag aggregate. Second, the test method for for lightweight materials such as coal and brown coal included in the aggregate (ASTM C123 [31]) lightweight materials such as coal and brown coal included in the aggregate (ASTM C123 [31]) was was employed to estimate the number of the harmful materials in the aggregate with bulk density employed to estimate the number of the harmful materials in the aggregate with bulk density lower lower than 20 kN/m3. The aggregate including the large number of lightweight materials is than 20 kN/m3. The aggregate including the large number of lightweight materials is inapplicable to inapplicable to concrete structures and/or ballast requiring high strength. Third, the test method for concrete structures and/or ballast requiring high strength. Third, the test method for the soft fragments the soft fragments of coarse aggregate (AASHTO T 189-70 [32]) was conducted to examine the degree of coarse aggregate (AASHTO T 189-70 [32]) was conducted to examine the degree of softness in the of softness in the aggregate via scratch hardness. The use of coarse aggregates including a large aggregate via scratch hardness. The use of coarse aggregates including a large number of soft or number of soft or easily crushable particles leads to significant damages on the concrete properties, easily crushable particles leads to significant damages on the concrete properties, such as the hardness, such as the hardness, elastic characteristics, and durability. elastic characteristics, and durability. 4. Physical and Chemical Quality Test Results 4. Physical and Chemical Quality Test Results
4.1.4.1. Physical Physical Quality Quality Test Test Results Results FigureFigure 4 4presents presents the testtest resultsresults for for determining determining the the unit unit weight weight with with three three different different aging periods.aging periods.The unit The weight unit ofweight the BOF of the slag BOF aggregate slag aggregate of 19.2 to 20.3of 19.2 kN/m to 20.33 was kN/m calculated,3 was calculated, and the average and the value average value of 19.7 kN/m3 is much larger than the KRCS requirement of 14.0 kN/m3. Since the difference of the unit weight between three different aging periods are 0.9 kN/m3, the aging effect on the unit weight is ignorable. Compared to the natural aggregate of 15.5 to 16.5 kN/m3 (average value of 16.0 kN/m3), the BOF slag aggregate is approximately 23% heavier than the natural aggregate. Since it allows BOF slag Sustainability 2018, 10, 284 7 of 12 of 19.7 kN/m3 is much larger than the KRCS requirement of 14.0 kN/m3. Since the difference of the unit weight between three different aging periods are 0.9 kN/m3, the aging effect on the unit weight is ignorable. Compared to the natural aggregate of 15.5 to 16.5 kN/m3 (average value of 16.0 kN/m3), the BOF slag aggregate is approximately 23% heavier than the natural aggregate. Since it allows BOF slag railway ballast to get a higher resistance to the lateral pressure resulting from running trains, the BOF slag aggregateSustainability 2018 is more, 10, x FOR beneficial PEER REVIEW than the natural aggregate. As illustrated in Equation (3),7 of the 12 unit weight of sleeper has a significant influence on its lateral resistance, and the lateral resistance increases withrailway increasing ballast the to unit get a weight. higher resistance It is clearly to the appeared lateral pressure that the resulting improved from lateral running resistance trains, the contributes BOF to higherslag aggregate resistance is more against beneficial lateral than pressure the natural caused aggregate. by the As centrifugal illustrated force in Equation on curved (3), the tracks unit and a twistedweight railway of sleeper track has than a significant natural aggregate. influence on its lateral resistance, and the lateral resistance increases with increasing the unit weight. It is clearly appeared that the improved lateral resistance contributes to higher resistance against lateral pressure caused by the centrifugal force on curved F = a·W + b·γ·Se + c·γ·Ss, (3) tracks and a twisted railway track than natural aggregate.
F: Lateral resistance for each railway sleeperF = a (kg)∙W + b∙γ∙Se + c∙γ∙Ss, (3) W: WeightF: Lateral of railwayresistance sleeper for each (kg) railway sleeper (kg) 3 γ: UnitW: weightWeight of railway ballast (kg/cmsleeper (kg)) 3 3 Se: Firstγ: Unit moment weight on of theballast end (kg/cm face of) railway sleeper (cm ) S : First moment on the end face of railway sleeper (cm3)3 Ss: First moment on the side face of railway sleeper (cm ) S : First moment on the side face of railway sleeper (cm3) a, b, c: Coefficients for each railway type (See Table3). a, b, c: Coefficients for each railway type (See Table 3).
30 KRCS Standard = 14.0 kN/m3
) 25 3
20
15
10
Unit Weight (kN/m Unit Weight 5
0 036 Aging (month) FigureFigure 4. Unit4. Unit weight weight test test resultsresults with three three different different aging aging periods. periods.
Table 3. Coefficients of Equation (3) depending on track types. Table 3. Coefficients of Equation (3) depending on track types. Type a b c PSC railway sleeper—crushedType gravel 0.75 a 29 b c 1.8 WoodenPSC railway railway sleeper—crushed sleeper—crushed gravel gravel 0.75 0.75 29 29 1.8 1.3 WoodenWooden railway railway sleeper—worn sleeper—crushed gravel gravel 0.6 0.75 29 29 1.3 1.4 Wooden railway sleeper—worn gravel 0.6 29 1.4 Table 4 presents physical quality test results with different aging periods. Figure 5 shows the test results for abrasion with three different aging periods. The estimated wear rates of the BOF slag Table4 presents physical quality test results with different aging periods. Figure5 shows the aggregate of 18.2 to 19.5% (average value of 18.7%) satisfy the KRCS requirements which should be test resultsless than for 25%. abrasion Since the with difference three different of the wear aging rates periods. between The three estimated different aging wear periods rates of were the only BOF slag aggregate1.3%, the of 18.2aging to effect 19.5% on (averagethe abrasion value was of ignorable. 18.7%) satisfyIt is expected the KRCS that the requirements abrasion resistance which of should the be less thanBOF 25%.slag aggregate Since the is differencemuch higher of than the wearthe natura ratesl betweenaggregate, three showing different the wear aging rates periods of 21 to were27%. only 1.3%,Thus, the aging the use effect of BOF on slag the abrasionaggregate wasas the ignorable. ballast materials It is expected contributes that to the the abrasioneffective maintenance resistance of the BOFmanagement slag aggregate of railway is much ball higherast since than much the lower natural abrasion aggregate, and crush showing of ballast the material wear rates result of in 21 the to 27%. Thus,decrease the use of of mud BOF pumping slag aggregate and the clogging as the ballast of the materialsroadbed. contributes to the effective maintenance management of railway ballast since much lower abrasion and crush of ballast material result in the Table 4. Physical quality test results with different aging periods. decrease of mud pumping and the clogging of the roadbed. Aging Periods (Month) 0 3 6 Natural KRCS (2011) Type 1st 2nd 1st 2nd 1st 2nd Railway Ballast [10] Ave. Ave. Ave. Trial Trial Trial Trial Trial Trial 15.50 >14.0 Unit weight (kN/m3) 20.33 20.30 20.31 19.36 19.32 19.35 19.22 19.37 19.30 −16.50 (kN/m3) Sustainability 2018, 10, 284 8 of 12
Table 4. Physical quality test results with different aging periods.
Aging Periods (Month) 0 3 6 KRCS Type Natural Railway 1st 2nd 1st 2nd 1st 2nd Ballast (2011) [10] Ave. Ave. Ave. Trial Trial Trial Trial Trial Trial 15.50 >14.0 UnitSustainability weight (kN/m 20183, )10, x20.33 FOR PEER 20.30 REVIEW 20.31 19.36 19.32 19.35 19.22 19.37 19.30 8 of 12 −16.50 (kN/m3) Sustainability 2018, 10, x FOR PEER REVIEW 8 of 12 AbrasionAbrasion (%) (%) 18.2 18.2 18.318.3 18.218.2 19.3 19.5 19.5 19.4 19.4 18.4 18.4 18.6 18.6 18.5 18.5 21~27 21~27 <25 <25 (%) (%) Hardness (HS) 60 65 63 68 69 69 64 66 65 40–50 >20 (HS) HardnessAbrasion (HS)(%) 18.2 60 18.365 18.263 19.368 19.569 19.469 18.4 64 18.6 66 18.565 21~2740–50 >20 <25 (HS)(%) CompressiveCompressive strength >80 Hardness (HS) 185185 60 19219265 18818863 186 68 17469 174 18069 180 17064 170 180 18066 17565 175 100–140 40–50 100–140 >80 >20 (N/mm (HS) 2) strength(N/mm (N/mm2) 2) (N/mm2) Compressive 2 ImmersionImmersion expansion expansion2 185 192 188 186 174 180 170 180 175 100–140 >80 (N/mm ) strength (N/mm ) 0.840.84 0.830.83 0.840.84 0.86 0.85 0.85 0. 0.8686 0.56 0.56 0.60 0.60 0.58 0.58 - - <1.5 <1.5 (%) (%) in water (%) Immersionin water expansion (%) 0.84 0.83 0.84 0.86 0.85 0.86 0.56 0.60 0.58 - <1.5 (%) in water (%) 30 KRCS Standard = 25% 30 25 KRCS Standard = 25% 25 20 20 15 15 10 10 L.A Abrasion Loss L.A (%) 5
L.A Abrasion Loss L.A (%) 5 0 036 0 Aging (month) 036 Aging (month) FigureFigure 5. Abrasion5. Abrasion test test results results withwith three different different aging aging periods. periods. Figure 5. Abrasion test results with three different aging periods. Figure 6 presents the test results of hardness with three different aging periods. The examined Figure6 presents the test results of hardness with three different aging periods. The examined hardnessFigure (HS) 6 presents of the BOF the slagtest aggregateresults of hardnessof 63 to 69 wi HSth (averagethree different value agingof 66 HS) periods. sufficiently The examined satisfies hardnesshardnessthe KRCS (HS) (HS) requirements of the of the BOF BOF slag which slag aggregate aggregate should be of of larger 63 63 toto 69than HS 20%. (average (average The test value value results of of66 show 66HS) HS) sufficiently the sufficiently aging effect satisfiessatisfies on the KRCSthehardness KRCS requirements canrequirements be ignored which which and shouldhardness should be beof larger largerthe BOF thanthan slag 20%. 20%. aggregate The The test testis results30 results to 40% show showlarger the thethanaging aging that effect of effect theon on hardnesshardnessnatural can aggregate. becan ignored be ignored and and hardness hardness of of the the BOFBOF slag aggregate aggregate is is30 30 to to40% 40% larger larger than than that thatof the of the naturalnatural aggregate. aggregate. 100 100 80 KRCS Standard = 20 HS 80 KRCS Standard = 20 HS 60 60 40
Hardness (HS) 40
Hardness (HS) 20 20 0 036 0 036Aging (month)
Aging (month) Figure 6. Hardness test results with three different aging periods. FigureFigure 6. Hardness6. Hardness test test results results with three three different different aging aging periods. periods. Figure 7 shows the test results of the unconfined compressive strength with three different periods.Figure The 7 estimated shows the unconfined test results compressive of the unconfined strength compressiveof the BOF slag strength aggregate with was three in a different range of Figure7 shows the test results of the unconfined compressive strength with three different periods. periods.175 to 188 The N/mm estimated2 (average unconfined value compressiveof 183 N/mm streng2). Sinceth ofthe the test BOF results slag aggregate were approximately was in a range 220% of The estimated unconfined compressive strength of2 the BOF slag aggregate was in a range of 175 to 175higher to 188than N/mm the 2KRCS (average requirements value of 183 (80 N/mm N/mm2). Since) and the 125–188% test results higher were than approximately the unconfined 220% highercompressive than strengththe KRCS of therequirements natural aggregate, (80 N/mm the difference2) and 125–188% of the test higher results than (i.e., the13 N/mm unconfined2) with compressiverespect to the strength aging periods of the naturalillustrates aggregate, the aging th eeffect difference on the of unconfined the test results compressive (i.e., 13 N/mm strength2) with can respectbe ignored. to the aging periods illustrates the aging effect on the unconfined compressive strength can be ignored. Sustainability 2018, 10, 284 9 of 12
188 N/mm2 (average value of 183 N/mm2). Since the test results were approximately 220% higher than the KRCS requirements (80 N/mm2) and 125–188% higher than the unconfined compressive strength of the natural aggregate, the difference of the test results (i.e., 13 N/mm2) with respect to the agingSustainability periods illustrates2018, 10, x FOR the PEER aging REVIEW effect on the unconfined compressive strength can be ignored.9 of 12
200 KRCS Standard = 80 N/mm2
150 ) 2
100 (N/mm
50
Unconfined Compressive Strength Strength Compressive Unconfined 0 036 Aging (month)
FigureFigure 7. Unconfined 7. Unconfined compressive compressive strength strength test re resultssults with with three three diffe differentrent aging aging periods. periods.
The results of immersion expansion tests indicate that the immersion expansion ratio of the BOF Theslag resultswith range of immersion of to 0.58 to expansion 0.84% is much tests lower indicate thanthat the KRCS the immersion requirements expansion (i.e., less ratio than of1.5%). the BOF slag withHowever, range further of to expansion 0.58 to 0.84% from isthe much transformation lower than of free the MgO KRCS to requirementsMg(OH)2 may occur (i.e., at less long than ages 1.5%). However,over a further year because expansion this chemical from the reaction transformation slowly crops of up. free Thus, MgO identifying to Mg(OH) free2 mayMgOoccur content at in long the ages overBOF a year slag because is suggested this chemical before the reaction actual use slowly of the cropsslag as up. a ballast Thus, material identifying even freethough MgO BOF content slag in the BOFaggregate slag is constructed suggested upon before railway the actual track usehas been of the co slagntinuously as a ballast in the materialopen-air aging even state, though and BOF can slag aggregatebe more constructed stabilized. upon railway track has been continuously in the open-air aging state, and can be more stabilized.Table 5 illustrates the test results for harmful substances in aggregates. From the test results, it was noted that the BOF slag did not contain any harmful (e.g., lead, mercury, organophosphorus Table5 illustrates the test results for harmful substances in aggregates. From the test results, compounds, tetrachlorethylene, trichloroethylene, and cyanogen), and included few quantities much it was noted that the BOF slag did not contain any harmful (e.g., lead, mercury, organophosphorus less than criteria. It might be because such substances were burned by the high temperature during compounds,the steelmaking tetrachlorethylene, process. The trichloroethylene,absence of harmful andsubstances cyanogen), in the andBOF included slag ballast few makes quantities it to be much less thanmore criteria. stable than It might natural be ballast because including such substances harmful substances. were burned by the high temperature during the steelmaking process. The absence of harmful substances in the BOF slag ballast makes it to be more stable4.2. than Chemical natural Quality ballast Test including Results harmful substances. Generally, BOF slag is considered as one of the industrial by-products, and it is essential to measure the contentTable of 5. eachChemical harmful quality substance test results in the with BOF different slag to aging satisfy periods. the soil contamination standard for evaluating the suitability. The chemical quality test results showed that the slag was Aging Periods (Month) stable to be used for theItem ballast since the most Criteriaharmful substances were not detected and even the detected harmful substances were very small amounts which were0 within 3criteria as 6indicated in Table 5. Lead <3 mg/L N.D. N.D. N.D. Copper <3 mg/L N.D. 0.003 0.007 TableArsenic 5. Chemical quality test <1.5 results mg/L with different N.D. aging periods. N.D. 0.001 Cadmium <0.3 mg/L 0.007Aging Periods 0.009 (Month) 0.006 MercuryItem <0.005Criteria mg/L N.D. N.D. N.D. 036 Organophosphorus compounds <1 mg/L N.D. N.D. N.D. Lead <3 mg/L N.D. N.D. N.D. Tetrachlorethylene <0.1 mg/L N.D. N.D. N.D. TrichloroethyleneCopper <0.3<3 mg/L mg/L N.D. N.D. 0.003 N.D. 0.007 N.D. CyanogenArsenic <1.5 <1 mg/Lmg/L N.D. N.D. N.D. N.D. 0.001 0.35 HexavalentCadmium chromium <0.3<1.5 mg/L mg/L 0.007 N.D. 0.009 N.D. 0.006 0.03 MercuryOil <0.005 <5% mg/L N.D. N.D. N.D. N.D. N.D. N.D. Organophosphorus compounds <1 mg/L N.D. N.D. N.D. Tetrachlorethylene <0.1 mg/L N.D. N.D. N.D. Trichloroethylene <0.3 mg/L N.D. N.D. N.D. Cyanogen <1 mg/L N.D. N.D. 0.35 Hexavalent chromium <1.5 mg/L N.D. N.D. 0.03 Oil <5% N.D. N.D. N.D. Sustainability 2018, 10, 284 10 of 12
4.2. Chemical Quality Test Results Generally, BOF slag is considered as one of the industrial by-products, and it is essential to measure the content of each harmful substance in the BOF slag to satisfy the soil contamination standard for evaluating the suitability. The chemical quality test results showed that the slag was stable to be used for the ballast since the most harmful substances were not detected and even the detected harmful substances were very small amounts which were within criteria as indicated in Table5.
5. Conclusions This paper describes the feasibility of the application of BOF slag for railway ballast material by conducting physical and chemical quality tests based on standards. A variety of test results associated with the aging effect were evaluated by comparing with KRCS criteria and natural railway ballast. The following observations are made:
1. The unit weight of the BOF slag (i.e., average value of 19.7 kN/m3) was much higher than that of natural aggregate (i.e., average value of 16.0 kN/m3). Thus, it is expected that the BOF slag as the railway ballast material reveals higher resistance against lateral pressure caused by the centrifugal force on curved track than natural aggregate. 2. From abrasion and hardness tests, the wear rate of the BOF slag was approximately 25% lower than that of natural aggregate and hardness of the BOF slag was 30 to 40% larger than that of the natural aggregate. It appears that the BOF slag as the railway ballast material is beneficial in terms of the maintenance of a railway ballast due to a greater abrasion resistance and hardness under repeated train loading than that of currently used natural ballast. 3. From unconfined compressive tests, the test results presented that the unconfined compressive strength of the BOF slag was 220%, and 125 to 188% larger than that of KRSC standard and natural aggregate, respectively. It is expected that the BOF slag as the railway ballast material reduces the particle breakage of coarse aggregates and excessive deformation of the railway ballast. 4. From the immersion expansion test, the produced expansion ratios of 0.58 to 0.84% sufficiently met the KRSC standard (i.e., less than 1.5%). 5. For the aging effect, it was noted that three different aging periods had little effect on the changes of physical properties, and the aging effect can be ignored up to six months with the test sample. However, the steel slag may be expanded over long periods such as several years due to the expansion behavior of free MgO, which could be considered as a future research topic. 6. For chemical quality test and the test for harmful substances in aggregates, no particular problems were detected in the test sample while further specific leaching test for the BOF slag could be considered as a future research topic to comply with environmental requirements before actual application of the BOF slag as railway ballast material.
It is demonstrated that the BOF slag is successfully applicable to the railway ballast since the test results cover the physical and chemical quality requirements. Especially, the BOF slag without an aging period (i.e., zero months) provides the similar physical and chemical properties compared to the BOF slag with aging periods (i.e., three and six months). Thus, it is possible to apply the BOF slag without aging to the railway ballast material instead of natural crushed stone aggregate. Nevertheless, the investigation for longer period expansion behavior of the steel slag would be regarded as a follow-up study with chemistry-based analysis because free MgO expansion generally occurs at long ages. More precision quality tests with environmental requirements can also support actual utilization of the BOF slag. The substitution of the BOF slag as the railway ballast material for the natural ballast material contributes to improving the quality of the ballast as well as reducing costs for the ballast maintenance if proper follow-up studies are conducted.
Acknowledgments: The research described herein was sponsored by a grant from R&D Program of the Korea Railroad Research Institute, Republic of Korea. Sustainability 2018, 10, 284 11 of 12
Author Contributions: Taehoon Koh and Sung-Woo Moon conceived and designed the experiments; Hyuksang Jung performed the experiments; Yeonung Jeong and Sukhoon Pyo analyzed the data; Taehoon Koh. and Sung-Woo Moon wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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