Abstract
The primary objectives of this study are to investigate the feasibility of a heavy media separation process with magnetite (Fe3O4) suspension for upgrading the quality of mixed recycled coarse aggregates from construction and demolition waste (CDW) in Korea and to determine a range of effective operating density of Fe3O4 suspension for producing high-quality RCA acceptable to structural concrete applications. For the purposes, six 200 kg recycled coarse aggregates (RCAs) samples were collected from a conventional recycling plant in Korea. Subsequently, the samples were processed by a heavy media separation process using Fe3O4 suspensions with various densities from 2.65 g/cm3 to 2.40 g/cm3 with an interval of 0.05 g/cm3. Next, a series of tests was performed in the laboratory to evaluate properties of finished sink and float products from the HMS processes, including oven-dry density, absorption capacity, and physical durability. Furthermore, compressive strength of concrete cylinders (150 mm by 300 mm) made of the finished sink products from Fe3O4 suspensions with three different densities (2.4, 2.5, and 2.6 g/cm3) was tested in this study. As a result, it was demonstrated that the HMS process using Fe3O4 suspension with a density ranging between 2.40 and 2.65 g/cm3 was effective for upgrading mixed CDW RCAs in Korea to high-quality RCAs acceptable for structural concrete applications.
1. Introduction
It has been demonstrated that quality of the recycled concrete aggregate depends on the content of the porous and cement mortar attached to the recycled coarse aggregate (RCA) [1]. The presence of the weak phase results in lower density and higher water absorption of RCA than natural coarse aggregate (NCA), which unfavorably affects the properties of fresh and hardened concrete [2]. For example, concrete made of RCA may delay setting time and need more water for the same workability as normal concrete. Furthermore, concrete made of RCA has relatively low mechanical properties (e.g., density, compressive strength, and modulus of elasticity) and higher permeability and higher carbonation rate and greater reinforced steel corrosion characteristics compared to normal concrete. Therefore, many national standards and international recommendations [3–9] limit the use of RCA in concrete structures according to the density and absorption capacity of RCA (see Table 1).
A number of researchers have proposed various mineral dressing operations for improving the quality of RCA by removing the attached cement paste. Nagataki et al. [10] observed that increasing the number of crushing stages is effective for reducing the attached mortar content and damage in RCA. Tam et al. [11] demonstrated that attached cement paste can be significantly removed from RCA through acid treatment. Shima et al. [12] studied thermal treatments combined with abrasion for improving the purity of RCA. An electrical impulse method [13, 14] was studied as an innovative tool for improving the quality of RCA. Bru et al. developed a microwave-assisted recycling process for the recovery of high-quality aggregates from concrete waste [15]. Recently, a near-infrared sorting technology was constructed for upgrading the quality of mixed recycled aggregates from CDW [16]. However, so far, there has been no widely accepted method in the market, especially for improving RCA in mixed CDW which are composed of concrete, asphalt concrete, ceramic tile, red brick, wood, and plastic [17, 18].
Heavy media separation (HMS) method has been demonstrated to be effective for separating high-quality RCA from mixed CDW by means of sink-float separation in a heavy liquid. The heavy liquid recommended in ASTM C123 [19] includes zinc chloride (ZnCl2) in water, 1,1,1,2-tetrabromoethane (C2H2Br4), and zinc bromide (ZnBr2) in water. Augulo et al. [20] used ZnCl2 water solution with a density of 1.9 g/cm3 and bromoform (CHBr3) ethylic alcohol solution with densities of 2.2 and 2.5 g/cm3 for categorizing Brazilian CDW in the order of density in the laboratory. Lee et al. [21] sorted RCA in the order of the density from 2.16 to 2.69 g/cm3 through a HMS process using C2H2Br4 methyl alcohol solution with varying densities in the laboratory. According to ASTM C123 [19], ZnCl2 water solution and ZnBr2 water solution are only recommended for specific gravity up to about 2.0 and 2.4, respectively; thus, they may be inappropriate for separating high-quality RCA for structural concrete, such as Grade I in KS F 2573 [5], Class H in JIS A 5021 [7], and Type 3 in RILEM [3]. In contrast, CHBr3 and C2H2Br4 have specific gravities of 2.89 and 2.95, respectively. Consequently, those are useful for separating high-quality RCA with a density equal to or higher than 2.5 g/cm3. However, there are several practical limitations to using CHBr3 and C2H2Br4 solution as follows: prohibitively high cost, flammability, toxicity, and solubility of organic materials such as asphalt concrete. In contrast, Angulo et al. [17] used a pseudoheavy medium using ferrosilicon-water suspension to classify mixed CDW aggregate by a HMS process. In the study, they investigated the effect of the medium density on the porosity of the resulting RCA and its impact on the mechanical properties of concrete in the laboratory. Ferrosilicon power with a gravity specific of 6.7~7.1 containing 13~16% silicon is commonly used for the heavy media separation process in the mine industry [22]. However, it is still expensive to be used for producing recycled coarse aggregates for the construction industry.
In this study, the authors developed a prototype HMS system with Fe3O4-water suspension as a cost-effective pseudoheavy medium in practice. A Fe3O4 suspension is commonly used as a medium solid in coal and iron industries due to its cost-effectiveness, high density, resistance to abrasion and corrosion, and recoverability for reuse. The main objectives of this study are to investigate the feasibility of a heavy media separation process with Fe3O4 suspension for upgrading the quality of mixed recycled coarse aggregates from construction and demolition waste (CDW) in Korea and to determine a range of effective operating density of Fe3O4 suspension in the HMS process for producing high-quality RCA acceptable to structural concrete applications.
2. Heavy Media Separation (HMS) Using Magnetite (Fe3O4) Suspension
2.1. Mixed Recycled Coarse Aggregates from CDW in Korea: Input Material in HMS
The raw material input to the HMS process in this study is mixed recycled coarse aggregates from CDW (or referred to as CDW RCA) in Korea, which were produced by a series of crushing and screening processes in a conventional recycling plant in Korea (see Figure 1). The CDW RCAs in Korea are generally composed of aggregates with various amounts of attached mortar, occupying about 90% of the volume, and foreign substances such as ceramic tile, asphalt concrete, red brick, mortar, wood chip, and plastic, occupying about 10% of the volume (see Figure 2). Those foreign substances have lower density and higher absorption capacity than natural aggregates for concrete. Average density and absorption capacity of various foreign substances in common Korean CDW RCA is summarized in Table 2. It has been known that the oven-dry (OD) density of typical Korean CDW RCAs ranges between 1.95 g/cm3 and 2.23 g/cm3, with an average of about 2.15 g/cm3, and the absorption capacity ranges between 6.5% and 10.3%, with an average of about 7.75%. Obviously, the values do not satisfy the requirement for high-quality recycled aggregates for structural concrete, such as Grade I in KS F 2573 [5], Class H in JIS A 5021 [7], and Type 3 in RILEM [3].
2.2. Magnetite (Fe3O4) Suspension in HMS
Magnetite concentrate powders produced by a beneficiation plant in China were used as the medium solid for the HMS process. The Chinese magnetite concentrate has a dry density of 4.75 g/cm3, and the particle size is mostly between 45 and 75 μm with an average particle size of 54.9 μm. However, it contains some forms of impurities such as phosphorus, aluminum, titanium, and silica, which may lower dry density compared to that of pure magnetite (5.18 g/cm3).
The Fe3O4 suspension with a specific density was prepared by diluting the magnetite concentrate powers with water. The mass of magnetite required to produce a heavy medium with the specified density and mass was determined using the following equation: where is the density of magnetite concentrate powder and is the target density of Fe3O4 suspension. For example, the Fe3O4 suspension with a target density of 2.6 g/cm3 needed 78.63 g of the magnetite concentrate in a 100 g dense medium, which occupies approximately 43% of the total volume of the dense medium.
In this study the target density of the magnetite suspension ranges between 2.40 g/cm3 and 2.65 g/cm3 with an interval of 0.05 g/cm3. The upper limit of target density (2.65 g/cm3) was determined from an observation in the laboratory that the viscosity of the Fe3O4 suspension suddenly increased when the density of the magnetite suspension exceeded the threshold value and significantly decreased workability and efficiency of the HMS process.
2.3. Prototype System for HMS Using Fe3O4 Suspension
A prototype HMS system was developed by a research team in Dong-A University for producing high-quality RCA from mixed CDW RCAs. A flow chart of the prototype system is illustrated in Figure 3. In the HMS process, mixed CDW RCAs in Korea were first loaded to a vibration screen, and only particles smaller than 50 mm were put into a separating machine (drum separator in this study; see Figure 4(a)) that contained Fe3O4 suspension with a certain target density. The drum separator is a heart of the HMS process, which automatically divides the input material into two groups. Consequently, particles with a higher density than Fe3O4 suspension sank, whereas those with a lower density floated (see Figure 4(b)). The sink particles were discharged at the bottom of the drum separator (see Figures 5(c) and 5(d)), and the float particles were transferred to the upper part of the rotating cylinder (see Figure 5(a)) and then emitted (see Figure 5(b)). Note that the drum separator was designed to revolve 5 cycles per a minute to maintain appropriate stability of medium in the HMS process. Both discharged particles became black due to the magnetite medium adhering to the surface of the products. The medium on the surface of the sink and float was rinsed away with distributed water spraying systems on vibration screens. Then, finished sink and float products are separately stacked in the yard.
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In addition, the prototype HMS system includes treatment and circulation systems for recycling magnetite medium and water as to minimize the environmental impacts of using recycled aggregates. The magnetic medium circulates in the circuit to keep the medium density in the drum separator constant in a certain limit. Some magnetite media from the drum separator, attaching to the sink or float particles, were collected to the supply tank A (see Figure 6(b)). In contrast, the magnetite medium in rinsing water was recovered by two-stage magnetic separation process (see Figure 6). Nonmagnetic tailings and condensed magnetite are stored in tanks B and C, respectively. The density of the medium in tank A was kept constant within certain limits by monitoring the medium density using a nuclear density gauge and automatically supplying the required amount of water and concentrated medium. Then, the magnetite medium with constant density is continuously supplied to the drum separator. It was observed that the two-stage magnetic separation process is very effective for recycling most of the magnetite with a magnetite loss of about 100 g per 1 m3 of input materials. In addition, an overflow from the heavy media slurry feed in tank C is directed to the water tank after a treatment process in tank C. The heavy media slurry is condensed in a filter press, and clean water is supplied to rinsing water recirculating in the circuit (see Figure 6).
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The investment cost for construction of the prototype HES system was approximately 1.5 million US $, which includes construction of steel frames (237K US $), heavy medium separation system (382K US $), magnetite recycling system (276K US $), water disposal plant (216K US $), and thickening tank and dewatering facility (245K US $). Based on these investment costs, the annual capital costs are about 225K US $ based a depreciation time of ten years and 5% interest rate. Moreover, annual operation costs are estimated to be about 130K US $ on the assumption of an operating time of 2400 hr/year, medium wear expenses of 16K US $, energy costs of 38K US $, and maintenance and insurance costs of 2.5% and 1.0%, respectively. Assuming a treatment capacity of 150 ton/hr, the resulting processing cost is about 0.98 US $/ton of processed material.
3. Test Program
3.1. Density and Absorption Capacity of RCAs
Test samples used in this study were five 200 kg RCAs with a size distribution of 13~45 mm, which were collected from a conventional CDW recycling plant in Korea (see Figure 2). The six RCA samples were processed by using the HMS system with six different densities of Fe3O4 suspension, ranging between 2.65 g/cm3 and 2.40 g/cm3 with an interval of 0.05 g/cm3.
After each separation process, a series of tests were performed to evaluate the properties of the finished sink and float product such as OD density and absorption capacity. Then, the values were correlated with the density of Fe3O4 suspension in the HMS process. The test results were used to investigate the effects of the medium density in the HMS process on the properties of the resulting product and to determine an optimum medium density in the HMS process.
The average density and absorption capacity of the RCAs in the OD condition were determined according to ASTM C128 [23]. where is the weight of the recycled aggregate samples in the OD condition and is the weight of water displacement measured from the RCA samples in the SSD condition (, where and are the weight of RCA in the SSD condition measured in air and in water, resp.). In addition, the absorption capacity of the RCA samples was determined as follows:
3.2. Measurements of Physical Durability of RCAs
In addition, a series of tests was performed to investigate the physical durability and the impurity contents of the sink and float produced by the HMS process using the six different densities of Fe3O4 suspension. For evaluating the physical durability of RCAs, the soundness test and wear resistance test were performed according to ASTM C88 and ASTM C535, respectively. In the soundness test, the resistance to disintegration by sulfates was evaluated by repeating immersion in saturated solution of sodium sulfate followed by oven drying. The weight loss after five cycles of tests was used as a means of the soundness of the RCAs subjected to weathering action in concrete. Furthermore, the wear resistance of processed RCAs was evaluated by using the Los Angeles test. Note that only large-size RCAs larger than 19 mm were considered in this study. The Los Angeles test involves ball milling the aggregate with steel balls for about 33.3 minutes with a rotation velocity of 30 r/min, sieving the fine materials over a 1.7 mm (number 12) sieve, and measuring the mass loss. The weight loss after the Los Angeles test is used as a means of wear resistance.
3.3. Compressive Strength of Concrete Made of RCAs
Compressive strength of concrete made of RCAs was performed to investigate the effect of processed RCAs on mechanical properties of hardened concrete. The concrete for compressive tests consists of type I Portland cement, river sand, and coarse aggregates obtained from five different sources. The concrete mixture proportions are summarized in Table 3. Coarse aggregates in concrete mix 1 used unprocessed RCA, and mixes 2, 3, and 4 used the sink product in the HMS process with the medium density of 2.45, 2.55, and 2.65 kg/cm3, respectively, which is called RCA-2.45, RCA-2.55, and RCA-2.65. In addition, NCAs were used for mix 5. Total seventy-five concrete cylinders (fifteen for each concrete mix) were cast in 150 by 300 mm plastic molds according to ASTM C31 [24]. The cylinders were water-cured after being demolded on the next day. Uniaxial compressive test was conducted at 7, 14, and 28 days. The cylinders were ground at both ends before testing to remove any surface irregularities and to ensure that the ends were perpendicular to the sides of the specimen. Compressive strength of the cylinders was measured using a Universal Testing Machine (UTM) with a capacity of 1000 kN according to ASTM C39-14 [25]. Tests were performed at a loading rate of approximately 0.25 MPa/s.
4. Results and Discussion
4.1. OD Density and Absorption Capacity of Improved RCAs
Figure 7 is a plot representing the variations of OD density of finished float and sink products in the HMS processes as a density of Fe3O4 suspension ranging between 2.4 g/cm3 and 2.65 g/cm3. As expected, the density of the finished sink product is slightly higher than the target density of Fe3O4 suspension in a drum separator. The use of Fe3O4 suspension with a density of 2.65 g/cm3 produced sink particles with an average OD density of 2.69 g/cm3, which is comparable to those of natural coarse aggregate (NCA). As the medium density decreases from 2.65 g/cm3 to 2.4 g/cm3, the OD density of sink particles decreases from 2.69 g/cm3 to 2.51 g/cm3, which are greater than the minimum criteria (i.e., 2.5 g/cm3) for high-quality recycled aggregates for structural concrete applications (i.e., Class H in JIS [7], Type 3 in RILEM [3], and Grade I in KS [5]). An approximate equation that relates the density of the OD density of sink products in the HMS process () and the density of the Fe3O4 suspension in the drum separator () is established by a linear regression analysis as follows:From (4), the minimum density of Fe3O4 suspension to produce RCA with a density of 2.5 g/cm3 is calculated to be 2.369.
In addition, the variations of absorption capacity of float and sink particles with Fe3O4 suspension in a drum separator during the HMS process are shown in Figure 8. Similar to OD density, the absorption capacity of sink particles tends to increase as the medium density increases from 2.4 g/cm3 to 2.65 g/cm3. Magnetite suspension with a density of 2.65 g/cm3 resulted in finished sink product with an average absorption capacity of 1.88%, which is as low as those of NCAs. As the density decreases from 2.65 g/cm3 to 2.4 g/cm3, the absorption capacity increases from 1.88% to 2.69%, all of which are less than the maximum value for the high-quality recycled aggregates (i.e., 3%) for structural concrete. It was observed that the decreasing rate of the absorption capacity of sink products gradually decreases as the medium density increases, and the absorption capacity of sink products tends to converge into a certain values around 2.0%. In this study, a quadratic equation was found to best express the relationship between the absorption capacity of sink products () and the density of the Fe3O4 suspension () as follows: From (5), the minimum value of for complying with the minimum requirement of absorption capacity of RCA is 2.37, which is equivalent to the value obtained from the minimum criterion for the OD density of RCA.
In the national standards and international recommendations, the quality of RCAs is evaluated based on the dual-performance criteria on oven-dry density and absorption capacity of RCAs. Therefore, it is of interest to investigate the relationship between the density and absorption capacity of finished sink particles from the HMS process in this study. Figure 9 shows the relationship between the OD density and the absorption capacity of sink and float particles in the HMS processes with various densities of Fe3O4 suspension. The dataset for the sink and float particles after the HEM processes is presented as solid and open squares, respectively, and a test result for the mixed CDW RCA before HMS is shown as solid circle in the figure. Consistent with the observation from other researchers [1], there is a close correlation between OD density and absorption capacity of RCAs: higher OD density results in lower absorption capacity. The close correlation could be explained by the fact that both density and absorption capacity are affected by the amount of mortar attached to the surface of the RCAs [1, 17, 26, 27]. It was observed that impurities in the mixed CDW RCA (e.g., plastic, asphalt concrete, ceramic, and red brick) are clearly separated from the high-quality RAC in the density and absorption curve. Therefore, it can be said that the prototype HMS system in this study is effective for classifying high-quality RCAs complying with the dual-performance criteria for structural concrete applications [3–9], even though the HMS process uses a single property (density) of input materials.
4.2. Physical Durability of Improved RCAs
Figure 10 is a plot representing the variations of mass losses of finished sink products after the Los Angeles test and sulfate ion test with a density of Fe3O4 suspension in the HMS process. Magnetite suspension with a density of 2.4 g/cm3 resulted in the finished sink product with mass losses of 18.9% and 9.34% after the Los Angeles test and the sulfate ion test, respectively. The values are lower than the minimum criteria of RCAs for structural concrete applications [5, 7]. Furthermore, as medium density increases to 2.65 g/cm3, the mass loss of the sink products gradually decreases to 12.1% and 4.33% after the Los Angeles test and sulfate ion test, respectively. The mass losses after both tests are as low as the values of natural granite in Korea. It is known that mass loss of aggregates from the Los Angeles test indicates the abrasion resistance of aggregates [28]; the aggregates with lower mass loss are interpreted as having better resistance to breaking or damage during transportation and handling and as having improved mechanical properties (e.g., hardness, compressive strength, and tensile strength) [29]. In addition, the lower mass losses in the sulfate ion tests indicate sound aggregates that have good resistance to freeze-thaw weathering. Therefore, the HMS process is effective for improving the physical durability (i.e., abrasion resistance and soundness to weathering) of RCA to acceptable level for structural concrete applications.
4.3. Compressive Strength
Compressive strength of concrete is one of the most critical properties of structural concrete in construction applications. Figure 11 shows the compressive strengths of concrete cylinders made of RCAs before and after the HMS process with varying the density of magnetite suspension.
For the concrete cylinders made with the unprocessed RCA (mix 1), the compressive strength was reduced by about 30% compared to that of the reference cylinders at the same testing day. The 7-, 14-, and 28-day compressive strengths of concrete mix 1 were 17.1, 22.3, and 25.2 MPa, respectively. The reduced compressive strength is mainly due to the weak residual mortar layer and higher water absorption capacity of the aggregate (7.74% in this study) [30, 31]. Moreover, the 28-day compressive strength of 25 MPa is lower than the design compressive strength of 30 MPa. Therefore, the unprocessed RCA is not appropriate for structural concrete applications without special cares such as lowering the water-to-cement ratio and/or limiting the RCA fraction (the percentage of coarse aggregate replaced with unprocessed RCA) in a concrete mixture design. Note that the RCA fraction is limited to 20~45% of coarse aggregate for using the RCA to structural concrete applications in some national standards or international recommendations.
In contrast, the compressive strengths of concrete cylinders made with processed RCAs (RCA-2.45, RCA-2.55, and RCA-2.66) using the HMS system in this study are equivalent to those of NCA concrete. The 7-, 14-, and 28-day compressive strengths of concrete cylinders made with the RCA-2.45 were 22.7, 27.3, and 32.5 MPa, respectively, which are 90~95% of the reference cylinders. Furthermore, compressive strengths tend to increase as the magnetite medium density increases. For the concrete mix with the RCA-2.55, the 7-, 14-, and 28-day compressive strengths were 23.1, 28.2, and 33.7 MPa, respectively, which are about 95% of the reference cylinders. The compressive strengths further increases up to 96~99% of the reference cylinders when using the RCA-2.65. The 28-day compressive strengths of the concrete cylinders with the three processed RCAs exceed the design compressive strength (30 MPa). Therefore, it can be seen that the processed RCAs from the HMS systems are appropriate for structural concrete applications, if necessary with only minor alteration of water-to-cement ratio and/or higher RCA fraction than using the unprocessed RCA.
4.4. Production Rate of High-Quality RCA
Figure 12 shows the relationship between medium density in the HMS process and the production rate, which is defined in this study as the weight of the finished sink products over the total weight of input materials into the HMS process. The production rate gradually increases from 14% to 42% as the medium density decreases from 2.65 g/cm3 to 2.4 g/cm3. Among critical properties of RCA, it has been observed that density and absorption capacity are the most influential factors for the production of the high-quality RCAs in the construction applications. The minimum operating density in the HMS process that produces high-quality RCA complying with the dual-performance criteria [3, 5, 7] on the density and absorption capacity is about 2.37 g/cm3 based on (4) and (5). However, the actual operating medium density was decided to be about 2.45 g/cm3 by taking into account some uncertainties during the HMS operation. The corresponding production rate to the medium density of 2.45 g/cm3 is about 40%, which simultaneously results in about 55% of tailings (i.e., float particles in the HMS process). The tailings could be used as subbase materials on the pavement construction without further recycling process.
5. Conclusions
In this study, a prototype heavy media separation (HMS) system that uses magnetite (Fe3O4) suspension as pseudoheavy media was developed to upgrade the quality of mixed recycled coarse aggregates from construction and demolition waste (CDW RCA) in Korea. To verify the feasibility of the prototype system, a series of tests was performed to evaluate the properties of finished sink and float products from the HMS process with varying a density of Fe3O4 suspension from 2.65 g/cm3 to 2.4 g/cm3. Conclusions based on experiments in this study are drawn as follows:(1)The prototype HMS system is composed of three main systems for heavy media separation process, magnetite recovering process, and waste water recycling process. Magnetite medium in rinsing water is recovered by two-stage magnetic separation process, which was effective for recycling most of the magnetite with a magnetite loss of about 100 g per 1 m3 of input materials. In addition, waste water was treated by a series of settling and filtering processes, and clean water is supplied to rinsing water recirculating in the circuit. As such, the prototype systems could effectively reduce the environmental impact of using recycled aggregates. A simple cost analysis estimates a processing cost of about 0.98 US $/ton to produce high-quality recycled coarse aggregates.(2)As density of Fe3O4 suspension in a drum separator ranges between 2.65 g/cm3 and 2.4 g/cm3, critical properties of the sink products (OD density, absorption capacity, and physics durability) remain to have acceptable values for high-quality RCAs in structural concrete applications. With decreasing the medium density from 2.65 g/cm3 and 2.4 g/cm3, the OD density decreases from 2.69 g/cm3 to 2.51 g/cm3; the absorption capacity increases from 1.88% to 2.69%; the mass losses from the Los Angeles test (or the sulfate ion test) increase from 9.34% to 18.9% (or from 4.33% to 12.1%).(3)The compressive strengths of concrete cylinders made with processed RCAs (RCA-2.45, RCA-2.55, and RCA-2.66) using the prototype HMS system in this study are equivalent to those of NCA concrete. The 7-, 14-, and 28-day compressive strengths of concrete cylinders made with the RCA-2.45 were 22.7, 27.3, and 32.5 MPa, respectively, which are 90~95% of the reference cylinders.(4)The production rate gradually increases from 14% to 42% as the medium density decreases from 2.65 g/cm3 to 2.4 g/cm3. In this study, the actual operating medium density was decided to be 2.45 g/cm3 by taking into account some uncertainties during the HMS operation. The corresponding production rate is about 40%, which simultaneously results in about 55% of tailings (i.e., float particles in the HMS process). The tailings could be used as subbase materials on the pavement construction without further recycling process.
Competing Interests
The authors declare that there is no conflict of interests regarding the publication of this manuscript.
Acknowledgments
This work was supported by Infrastructure and Transportation Technology Promotion Research Program funded by Ministry of Land, Infrastructure and Transport (Grant no. 15DRP-B066470-03) and supported by Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (Grant no. 20151520100990).