Utilization of Natural Soils as a Remediation Method for Electric Arc Furnace and Ladle Slags

Abstract

Steel slags are solid residual materials formed as by-products throughout the process of steel production within the steelmaking industry. These wastes have good physical properties such as high stiffness and friction angle for use as road fill materials or in geotechnical applications. However, the presence of heavy metals and high alkalinity levels constitute significant environmental hazards and set limitations on using slags in engineering applications. While there have been investigations into the mechanical characteristics of steel slags, research on assessing potential harm when utilizing the materials in engineering applications is rare. This study examines the mitigation methods to address the environmental problems associated with steel slags. To do this, two different steel slags with different production techniques were treated with soils of different properties such as fine and coarse sand, bentonite, kaolin, and natural clay. The pH and electrical conductivity (EC) values of pure steel slags were determined using the water leach test (WLT). Variations in pH and EC values of steel slags subjected to treatment were evaluated through both WLT and sequential water leach (SWLT) tests. As a result, the high strength, stiffness, and drainage capability of EAF and LS steel slags make these materials suitable for road filling. This is further backed by their soaked and unsoaked CBR values. During the water leach tests, notable decreases in pH were observed with a 60% natural clay (NC) solution, resulting in a decrease of 1.2 and 0.7 in EAF and LS, respectively. The addition of sand had a negligible impact on pH due to its inert characteristics. Moreover, in sequential water leach tests, the most significant decrease in pH was observed with NC (with a reduction of 2.0 points for EAF and 0.9 points for LS) through enhanced ion exchange and extended periods of dilution and buffering. Also, the use of NC resulted in substantial decreases in EC for EAF and LS, with reductions of 77% and 81%, respectively. Moreover, heavy metal concentrations in leachate waters from pure steel slags have been detected, and the effect of treatment on aluminum and iron concentrations has been determined. The results indicate that the use of natural soil significantly drops the pH and lowers the trace metal concentrations within the leachate.

1. Introduction

In general, slag is a solid residue generated in the metal, iron, and steel-making industries, composed primarily of light oxides, silicates, and borates, as well as accumulates on the surface of the molten product as a result of low density itself [1,2,3]. Molten steel is obtained by processing peak iron, derived from raw iron ore, in basic oxygen and electric arc furnaces along with alloys, scraps, and fluxes for the removal of impurities [4]. As a result of steel production, steel slag is a by-product from either the conversion of iron to steel in a basic oxygen furnace (BOF) or the melting of scrap to make steel in an electric arc furnace (EAF). The steel slags from these two furnaces are very similar [5]. However, ladle steel slag, resulting from further refining in the ladle, is different from furnace steel slag [1,5]. A schematic illustration of the steps involved in steel fabrication is depicted in Figure 1. The majority of these wastes are typically stored in landfills, which can result in environmental harm such as the contamination of soil and water, as well as air pollution [6]. Over the past two decades, there has been a substantial increase in the utilization of steel slag. Steel slag is mainly consumed worldwide in road construction, cement manufacturing, soil stabilization, hydraulic works, generating bio-cement, and engineering applications, as well as being stored on-site for disposal [7,8,9,10].

As of 2022, worldwide steel production was reported as 1.885 billion tons annually. Based on data from the production stages, steel slag waste is generally produced at 15–20% for 1 ton of raw steel [11,12]. When considering the amount of steel production worldwide, the resulting wastes cause storage problems. Storage in facilities or stockpiling and using wastes in the constructions without precautions cause environmental damage, water pollution, and public health problems since the steel slags have extreme alkalinity levels. Furthermore, the calcium-rich steel slag increases pH values and may significantly affect the trace metal concentrations within the steel slag leachate [11,13,14,15,16].

In terms of sustainability, minimum consumption and protection of useful natural or artificial resources take precedence, as well as the reuse or recycling of harmful wastes resulting from industrial processes in a way that causes the least environmental damage [17,18]. In this regard, geotechnical engineering applications greatly contribute to reusing or recycling steel slag waste by taking precautions against harmful effects like alkalinity and trace metal leaching. Mitigation methods such as bitumen coating, blending with water treatment residuals (WTR) [19] and the synergistic effect of red soil and rice straw [20], the immersion of slags in anions or cations (passivation), and buffering with subbases were employed by several researchers [11,14,16,21], as well as humidification, alkaline pretreatment [15], and microbially induced carbonate precipitation [22] to reduce high alkalinity and trace metal concentrations.

There are numerous studies available regarding the mechanical properties of basic oxygen furnace (BOF), electric arc furnace (EAF), and ladle furnace slag (LS), which aim to characterize and remediate the material [21,23,24,25,26]. Even though the high pH and the consequent potential for trace metal leaching were mentioned, so far, no study has been encountered that evaluated the suitable remediation techniques for environmental concerns related to EAF and LS. The present study examined the mechanical and environmental characteristics of electric arc (EAF) and ladle furnace (LS) steel slags. The pH and electrical conductivity (EC) measurements were conducted on steel slags that had undergone pretreatment using fine and coarse sand, kaolin, bentonite, and natural clay. The analyses were conducted using water leach tests (WLTs) and sequential water leach tests (SWLTs). Furthermore, an analysis has been conducted to identify the levels of heavy metal concentrations in leachate derived from pure steel slags. Additionally, the impact of treatment methods on aluminum and iron concentrations has been assessed.

2. Materials and Methods

EAF and LS steel slags sieved #10 were used in the experiments. EAF is obtained from the electric arc furnace, while LS is the slag derived from the secondary treatment of the molten steel from the same furnace. The natural soils utilized to treat the EAF and LS are bentonite (B), kaolin (K), natural clay (NC), coarse sand (CS), and fine sand (FS). The physical properties of the materials are presented in

Table 1. Physical properties of materials.

Material	$\mathbf{Specific} \mathbf{Gravity}, {\mathbf{G}}_{\mathbf{s}}$	Plasticity $\mathbf{Index}, {\mathbf{I}}_{\mathbf{p}}$ (%)	$\mathbf{Optimum} \mathbf{Water} \mathbf{Content}, {\mathbf{w}}_{\mathbf{o}\mathbf{p}\mathbf{t}}$ (%)	$\mathbf{Maximum} \mathbf{Dry} \mathbf{Unit}, {{\mathsf{\gamma }}_{\mathbf{d}\mathbf{r}\mathbf{y}}}_{\mathbf{m}\mathbf{a}\mathbf{x}}$ (kN/m3)	pH	USCS	AASHTO	Color
EAF	3.62	Non-Plastic	8.98	24.38	12.4	GP	A-1-a	Black/Gray
LS	2.73	Non-Plastic	17.83	17.12	12.5	GP	A-1-a	Gray
Bentonite	2.65	250	28	12.35	10.8	CH	A-6	Yellow
Kaolin	2.63	28	-	-	8.7	CH	A-6	White
Natural Clay	2.61	53	26	13.10	7.8	CH	A-6b	Beige/Gray
Coarse Sand	2.71	Non-Plastic	7.72	16.28	10.9	SP	A-2-7	Light Gray
Fine Sand	2.65	Non-Plastic	15.20	15.35	8.9	SP	A-2-7	Beige

.

EAF and LS steel slags were supplied just after being solidified in the stock site and thus assumed as unaged steel slags. According to pycnometer [27] tests, EAF slag has a higher specific gravity than other materials due to its iron content. In contrast, LS showed a lower specific gravity value than EAF since it underwent secondary processing. A similar situation exists in the maximum dry unit weight values obtained from compaction tests. The classification of materials pertaining to the Unified Soil Classification System (USCS) and the American Association of State Highway and Transportation Officials (AASHTO) was determined through the conduction of sieve analysis [28] and Atterberg limits [29] tests. Accordingly, EAF and LS poorly graded gravel (GP), bentonite, kaolin, natural clay and high-plasticity clay (CH), and sands were classified as poorly graded (SP). Figure 2 displays the materials, while Figure 3 shows the grain size distribution based on sieve analysis. The sieve analysis constitutes the passing of the sample material through a series of sieves with various sizes of mesh to determine the percentage of particles in each size fraction.

Additionally, the compaction parameters of the steel slags were determined using standard energy [30], and the experiments were concluded upon the observation of free water at the base of the mold. EAF, with the compaction parameters of ${{\gamma }_{dry}}_{max}=24.38 \mathrm{k}\mathrm{N}/{\mathrm{m}}^3$, has the highest dry unit weight compared to other materials, and the obtained values are consistent with previous studies [23,24]. As a treatment method, natural soil amendments with the steel slags of EAF and LS were applied to decrease the high pH of wastewater. In order to assess the impact of soil additives, initial pH measurements were taken for pure steel slags, after which the soils were mixed with the slags at weights of 20%, 40%, and 60%, and the pH was subsequently measured. The suspensions were prepared with a solid/liquid (S/L) ratio of 1:20 (2.5 g/50 mL) via distilled water [31]. Each suspension was placed in 50 mL tubes on the rotator and agitated at 30 rpm for 18 h [32]. Following 18 h of agitation, the suspension was directly transferred to the centrifuge and allowed to settle for 5 min. The pH measurement was conducted quickly upon completion of the process. The process for the water leach tests (WLTs) is illustrated in Figure 4.

In addition, sequential water leach tests (SWLTs) were performed on pure and treated EAF and LS following ASTM-D3987-12. Similar to the WLTs, each suspension was prepared at a ratio of S/L:1/20. Then, pure or treated steel slag and liquid mixtures were subjected to filtration employing syringe filters and filter sheets to extract leachates for the purpose of pH assessment. Following the completion of the extraction process, the soils were thoroughly mixed, and the aforementioned procedure was then repeated. Thus, the effect of the SWLT approach was observed via the variation in pH. Figure 5 depicts the SWLT schema.

Additionally, a comprehensive evaluation was conducted to analyze the mechanical characteristics of steel slags from the perspective of geotechnical engineering, including several tests such as direct shear [33], constant head permeability [34], the Los Angeles Abrasion test [35], water absorption [36], bulk density [37], the crushing coefficient [38], the impact coefficient [39], and unsoaked–soaked California Bearing Ratio (CBR) testing [40].

3. Results and Discussion

3.1. The Mechanical Properties of EAF and LS

Based on the conducted experiments, the hydraulic conductivity of EAF and LS was calculated as $1.11\times {10}^{-2} \mathrm{cm}/\mathrm{s}$ and $3.23\times {10}^{-4} \mathrm{cm}/\mathrm{s}$, respectively, as seen in

Table 2. The mechanical properties of steel slags.

Physical Properties	Unit	EAF	LS
Coefficient of permeability, k	(cm/s)	38	29
CBRunsoaked	(%)	30.2	26.4
CBRsoaked	(%)	53.6	46.3
Los Angeles Value	(%)	18.58	28.22
Water Absorption	(%)	3.40	4.70
Bulk Density	(kg/m3)	2973	2206
Crushing Coefficient	(%)	17.10	26.86
Impact Coefficient	(%)	5.46	3.80

. The LS has a lower permeability coefficient compared to EAF due to its larger number of fine grains.

EAF steel slag typically demonstrates superior physical characteristics compared to LS. These properties refer to higher permeability, strength, durability, a lower water absorption rate, and greater resistance to mechanical forces. The variations can be linked to changes in their chemical composition and formation mechanisms, as well as the decrease in the percentage of iron oxide during the secondary treatment of ladle steel slag. Based on the mechanical test results, EAF slag typically has a more robust structure and is more appropriate for challenging engineering applications.

Soaked–unsoaked CBR tests were performed with consideration for the optimal water content and maximum dry unit volume predetermined by the proctor tests. In the case of unsoaked CBR, EAF had better performances compared to the LS. It may be attributed to the reduction in the iron concentration of the LS subsequent to secondary treatment as well as the rigidity, rougher surface, and grain size distribution of EAF. Likewise, the soaked CBR results have been higher than those in the unsoaked condition. This can be clarified by the process of hydration under appropriate temperature and curing factors [41,42]. According to the corrected curves of CBR tests, both LS and EAF have sufficient performance in terms of the potential strength of subbase and base materials. As is known, steel slags contain oxides such as silica, alumina, lime, magnesia, and iron. Under suitable temperature and curing conditions, free lime and free magnesium oxides in contact with water form hydroxides, causing delayed expansion or swelling [21,43,44,45]. In particular, it is known that free lime will be greatly hydrated, and this will eventually cause expansions throughout its service life and cause stability problems. Free magnesia has a slower hydration rate than free lime, so the above-mentioned delayed expansion is experienced [25]. In order to examine the expansion potential of the existing EAF and LS, the samples were kept in the water tank after being compressed into CBR molds and the expansion in vertical axes was reported, and as a result, expansion occurred due to hydration reactions, as seen in Figure 6.

3.2. X-ray Diffraction of EAF and LS (XRD)

When an X-ray beam interacts with an atom, one of two outcomes occurs: (1) the beam gets absorbed, leading to the ejection of electrons from the atom, or (2) the beam undergoes scattering [46]. This method is utilized to examine crystal structures and interatomic spacing by considering the scattering phenomenon. Consequently, it is a widely employed non-destructive technique that furnishes valuable insights into the characteristics of samples, including structure, texture, phase composition, grain size, crystallinity, and strain [47]. For this study, samples EAF and LS were sieved through the #40 sieve, followed by X-ray diffraction analysis, as seen in Figure 7 and Figure 8. The chemical composition of steel slags may be complex due to the gradual production phases and the incorporation of additives throughout the production procedure. Indeed, this complexity is observed in the X-ray Diffraction (XRD) results. The composition of EAF slag is mostly comprised of silicates and light oxides. The LS mostly comprises silicate-oxide compounds and mayenite. The XRD patterns of relevant slags exhibit conformity with the chemical compositions as reported in the literature [42,48,49].

3.3. X-ray Fluorescence of EAF and LS (XRF)

XRF analyses were conducted to simplify and provide detailed information about the intricate composition observed in the XRD results, as seen in Figure 9. X-ray fluorescence (XRF) is a rapid and precise analytical technique used to determine the chemical composition of a wide range of materials. It can analyze solids, liquids, powders, and other forms with minimal sample preparation. XRF finds applications in diverse industries such as metal, cement, petroleum, polymer, plastic, food, mining, and environmental analysis. It delivers accurate and reliable results, detecting elements from (sub)ppm levels up to 100%. XRF employs energy-dispersed (EDXRF) or wavelength-dispersed (WDXRF) systems to identify factors present in samples. The analysis time varies depending on the desired accuracy and the number of elements to be analyzed. Overall, XRF provides valuable insights through spectral analysis, facilitating the identification and quantifying of elemental concentrations [50,51,52]. It can be clearly seen in

Table 3. XRF of EAF and LS.

Composition		EAF (%)	LS (%)
Na2O	Sodium Oxide	0.13	0.18
MgO	Magnesium Oxide	2.50	7.70
Al2O3	Aluminium Oxide	2.18	8.86
SiO2	Silicon Dioxide	5.26	9.15
P2O5	Phosphorus Pentoxide	0.14	0.04
K2O	Potassium Oxide	0.12	0.11
CaO	Calcium Oxide	33.34	44.03
TiO2	Titanium Dioxide	0.17	0.12
MnO	Manganese Oxide	3.51	0.89
Fe2O3	Iron (III) Oxide	34.54	21.19
Cr2O3	Chromium Oxide	0.82	0.29
SO3	Sulphur Trioxide	1.50	3.57
Cl	Chloride	0.05	0.05
Ba	Barium	<0.01	0.68
Cu	Copper	0.02	0.03
Nb	Niobium	0.06	<0.01
Sr	Strontium	0.02	0.07
V	Vanadium	0.07	0.02
Zn	Zinc	1.10	0.90
Zr	Zirconium	<0.01	<0.01
LOI	Loss on Ignition	14.47	2.11
Total		99.99	99.99

that EAF mostly consists of CaO and Fe₂O₃ in a similar manner to how LS consists of CaO and Fe₂O₃ as well. The XRF results are consistent with the findings reported in the existing literature [53,54,55].

3.4. The pH and EC Results of Pure EAF and LS

The concentration of hydrogen ions in solution plays a significant role in determining the concentration of most chemical species. As a result, the concentration of hydrogen ions serves as an essential criterion for both natural waters and wastewater. The representation of the concentration of hydrogen ions is often denoted as the power of hydrogen (pH), which is defined as the negative logarithm of the hydrogen ion concentration [56] as seen in the equation below.

$$\mathrm{p}\mathrm{H}=-\mathrm{l}\mathrm{o}\mathrm{g}\left[{\mathrm{H}}^{+}\right]$$

(1)

Moreover, pH serves as the conventional method for quantifying the acidity or alkalinity of a solution, making it one of the prevailing techniques used in assessing soil and water quality. The pH scale encompasses a range of values from 0 to 14, with a pH of 7 considered neutral. Substances with a pH below 7 are classified as acidic, while those with a pH over 7 are categorized as basic. As the acidity of a substance increases, its pH value approaches 1, whereas conversely, as the alkalinity of a substance increases, its pH value approaches 14 [57]. The term basic side is sometimes used interchangeably with alkaline; however, it is important to note that alkalinity and basicity are separate concepts. Alkalinity generally refers to the capacity of a liquid medium to resist changes in pH via its buffering capabilities. The evaluation aims to determine the capacity of aquatic systems to neutralize the effects of acidic and basic substances, hence maintaining a generally stable pH level [57,58,59].

The steel slags, EAF and LS, are composed of light oxides, silicates, and borates as previously stated. Hydration processes are performed by these steel slags as a consequence of their chemical composition, leading to their alkaline nature. The possible hydration reactions can be seen as follows [48]:

$$\mathrm{C}\mathrm{a}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{a}{\left(\mathrm{O}\mathrm{H}\right)}_2$$

(2)

$$\mathrm{M}\mathrm{g}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{M}\mathrm{g}{\left(\mathrm{O}\mathrm{H}\right)}_2$$

(3)

$$2{\mathrm{C}\mathrm{a}}_2\mathrm{S}\mathrm{i}{\mathrm{O}}_4+{4\mathrm{H}}_2\mathrm{O}\to 3\mathrm{C}\mathrm{a}\mathrm{O}·2\mathrm{S}\mathrm{i}{\mathrm{O}}_2·{3\mathrm{H}}_2\mathrm{O}+\mathrm{C}\mathrm{a}{\left(\mathrm{O}\mathrm{H}\right)}_2$$

(4)

$$2{\mathrm{C}\mathrm{a}}_2\mathrm{S}\mathrm{i}{\mathrm{O}}_5+{6\mathrm{H}}_2\mathrm{O}\to 3\mathrm{C}\mathrm{a}\mathrm{O}·2\mathrm{S}\mathrm{i}{\mathrm{O}}_2·{3\mathrm{H}}_2\mathrm{O}+3\mathrm{C}\mathrm{a}{\left(\mathrm{O}\mathrm{H}\right)}_2$$

(5)

$$\mathrm{C}\mathrm{a}\mathrm{O}·7{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3+{12\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{a}\mathrm{O}·7{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3·{6\mathrm{H}}_2\mathrm{O}+{6\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3·{\mathrm{H}}_2\mathrm{O}$$

(6)

After the hydration reactions, when light oxides are exposed to water, they undergo dissolution, resulting in a rise in concentrations of hydroxide ions $\left[{\mathrm{O}\mathrm{H}}^{-}\right]$, hence elevating the pH level [8,16]. In order to determine the pH and EC of the pure steel slags, 12 suspensions were prepared with a ratio of S/L:1/20 after being sieved from #10. The WLTs process, as seen in Figure 3, was applied to measure the pH and EC with the calibrated pH/EC-meter device. The phases for WLTs are shown in Figure 10.

As seen in Figure 11, the average pH for EAF and LS was measured as 12.1 and 12.5, respectively. Based on the pH measurements, both steel slags have an extremely basic nature with average pH. Moreover, upon contact with water, the resultant leachate of these steel slags is expected to possess a considerable alkaline nature. Based on the specifications provided by the U.S. Environmental Protection Agency [58], it is recommended that the pH level of aquatic environments should fall within the range of 6.5 to 8. In accordance with the regulations set up by the Ministry of Environment, clean water is required to preserve a pH level within a range of 6.5 to 8.5. Additionally, the discharge of industrial metal wastewater into the environment has to conform to pH requirements falling between 6 and 9, as stated by the water pollution regulations of the Ministry of Environment [60]. Accordingly, the pH level of the leachate resulting from pure steel slags of EAF and LS surpasses the specified limitations.

The EC values of pure steel slags were obtained in the study through the utilization of leachate samples acquired from WLTs, which were also used for pH measurements. Basically, the electrical conductivity (EC) and total dissolved solids (TDS) are utilized as water quality indicators to quantify the level of salinity [61]. The electrical conductivity (EC) of water refers to its ability to facilitate the transmission of an electrical current within a liquid medium. The electrical current within the solution is made more accessible by the presence of ions, leading to an increase in conductivity as the concentration of ions rises [56]. The results depicted in Figure 12 indicate that the light oxides and silicates present in the current wastes dissolve in the liquid medium, release a significant number of ions, and contribute to higher EC values as the ion concentration increases. Moreover, the reported EC values exceed the threshold specified by the EPA.

The pH and EC results, as reported, hold significance in relation to environmental concerns and the management of industrial waste. Using pure steel slag in geotechnical or engineering applications has resulted in releasing leachate into nature, which can potentially lead to adverse impacts on both water quality and soil fertility. Hence, it is essential to carefully consider the utilization and disposal of steel slag within the concept of waste management procedures.

3.5. The pH Results of Treated EAF and LS

As a way to mitigate the high pH levels of the pure EAF and LS, additives including kaolin (K), bentonite (B), natural clay (NC), and coarse sand (CS) were employed. The mixtures were prepared with weight percentages of 20%, 40%, and 60%. The pH of the resulting leachates was measured using WLTs, with a solid-to-liquid ratio (S/L) of 1/20, as seen in Figure 3. Accordingly, all additives lowered the pH. The most significant reductions were observed in EAF and LS with the addition of 60% NC, resulting in decreases of 1.2 and 0.7 points, respectively, as shown in Figure 13. In general, sands, being inert substances without a negative surface [62,63], do not discharge ions that could potentially alter the pH level of the leachate. Therefore, the addition of sand resulted in the lowest reduction. The negligible 0.1-point decline in pH values that was noted can be accounted for by the dilution effect of the addition of sand.

In turn, adding the clay samples to steel slags at different weight percentages resulted in varying pH reductions in leachates as the amount of clay increased. The observed reduction in pH after the combining of steel slag with clay and the ensuing repetition of the water leach test can be ascribed to a combination of factors. The clay’s buffering capacity [64,65,66], which enables it to resist changes in pH, likely contributed to the neutralization of the alkalinity caused by the steel slag. The interaction of the steel slag and clay components through acid–base reactions, ion exchange, and adsorption by clay particles may have resulted in the generation of acidic substances and a decrease in alkaline concentrations in the leachate. Given that the pH of NC is close to neutral, the impact of dilution was also apparent in the outcomes. However, the pH measurements recorded after treatment failed to comply with the established standards set by the U.S. Environmental Protection Agency (EPA) and the Ministry of Environment.

3.6. The pH/EC Results of EAF and LS by SWLTs

Experimental studies have indicated that steel slags have the potential to be a suitable replacement for natural aggregates typically used in road filling [5,11,26,67,68,69] and have sufficient drainage capability [70], resulting in improved performance as the ratio of fine content decreases [71]. However, toxic trace metals released due to high alkalinity cause environmental damage. In this regard, a series of sequential leachate tests (SWLTs), as seen in Figure 4, were conducted to assess the environmental impact of utilizing steel slag as a base or subbase layer in the road embankment section. These experiments were conducted in a controlled scenario where layers of kaolin (K), bentonite (B), natural clay (NC), and coarse (CS) and fine sand (FS) were positioned beneath the steel slag. The purpose is to evaluate the environmental impacts of the leachate that exits the system by replicating the process of leachate interacting with steel slag and passing through the layers beneath.

Based on the findings, the leachate derived from EAF and LS experienced the greatest impact from the buffering and dilution effect in the NC contribution, resulting in a decrease of 2.0 points in EAF and 0.9 points in LS, respectively, as seen in Figure 14. Here, SWLTs enable improved ion exchange, extended dilution, and buffering time. Despite the SWLT process prolonging the dilution and buffering times of kaolin and bentonite clays, the pH values remained similar to those obtained from WLTs. Moreover, the results of the EC showed a significant reduction of 77% and 81% in EAF and LS, respectively, following the process of SWLTs with the NC additive, as seen in Figure 15. The phenomenon in question can be elucidated by the process of dilution, which is affected by the inherent neutrality of the NC. Also, an overall reduction in EC was observed for all additives as a result of the increasing dilution time and buffering duration via the SWLT procedure.

In addition, fine sand was added to the steel slag, which was assumed to be used as a base or subbase in road filling at rates of 20%, 40%, 60%, and 80% by weight to treat or reduce the amount of waste. For this case, SWLTs were performed to evaluate and model the environmental impact of the leachate that exits the system after passing through the layers of K, B, NC, and CS located beneath the steel slags. The changes in the ratio of suspension with the addition of sand in LS, where the ratio decreases, were examined by focusing on the pH variations in LS steel slag after the SWLT process. As a result, the most significant decrease was measured at 1.1 points with the addition of 80% sand, as seen in Figure 16. However, although the pH value was lower than the untreated SWLT’s pH, it was noted that the overall treatment did not result in a substantial decrease in pH. This is also valid for other clay additives. This situation implies that, even though the proportion of LS waste in suspensions has decreased, the pollution level remains high since the waste is a byproduct of secondary processing and contains a higher percentage of fine particles compared to the EAF. Moreover, agitating the leachate of treated LS with the CS did not affect the pH.

In the case of EAF, the final leachate pH value obtained after the SWLT process from the combination of %80FS+%20EAF-NC decreased by 2.4 points, reaching a value of 9, as seen in Figure 17. Thus, the most significant decrease was achieved when the 80% enhanced sample was agitated with NC. The pre-treatment process did not have an impact on the presence of bentonite; however, the pH level decreased as the ratio of EAF in kaolin clay reduced, gradually reaching a decrease of 1.1 points. Similar to the case in LS, the pH of the treated EAF was not impacted by CS agitation.

3.7. Toxic Trace Element Results of Pure and Treated EAF and LS

EAF and LS steel slags are extremely basic, as determined by the pH measurements, and the leachate that forms when they come into contact with water will also be extremely alkaline, hence resulting in an increase in the concentration of trace elements. Environmental issues such as toxic element release and the contamination of aquatic environments can result from the careless use of steel slags in geotechnical applications, owing to their high alkalinity and chemical composition. XRF and XRD results were considered for the measurement of Al, Fe, Cr, V, Zn, and Ba concentrations in leachate obtained from WLTs of pure steel slags.

Table 4. Toxic metal concentrations of EAF and LS.

Samples	EC (mS/cm)	TDS (ppt)	pH	Al (mg/L)	Fe (mg/L)	Cr (mg/L)	V (mg/L)	Zn (mg/L)	Ba (mg/L)
EAF:	2.16	1.1	12.1	9.2	2.3	0.04	-	1.2	-
LS:	7.22	3.62	12.6	1.5	0.16	<0.02	-	0.35	-
EPA SMCL (mg/L):		6.5~8		0.2	0.3	0.1	NS	5	2

MCL: Maximum contaminant levels; NS: Not specified.

shows the metal analysis results as well as the pH/EC/TDS of the leachates that were measured. The results indicate that alkalinity has an effect on metal concentrations, as both EAF and LS leakage waters contain Al concentrations that surpass the specified limit. The decrease in raw iron in the production due to secondary processing in the ladle furnace results in a lower Fe concentration in LS compared to EAF. Also, the Fe concentration in the EAF leachate exceeds the EPA-specified regulation.

Previous tests indicated that the addition of NC resulted in the lowest pH/EC/TDS values. Leachates from LS and EAF were passed through NC by the SWLTs in order to track alterations in the concentrations of trace metals. Consequently, the concentrations in the case of %100SteelSlag+NC dropped below the lower limits, as seen in

Table 5. Toxic metal concentrations of steel slags after SWLTs.

Samples	EC (mS/cm)	TDS (ppt)	pH	Al (mg/L)	Fe (mg/L)	Zn (mg/L)
EAF:	0.62	0.31	9.3	<0.005	<0.25	<0.1
LS:	0.68	0.39	9.8	<0.5	NS	NS
EPA SMCL (mg/L):		6.5~8		0.2	0.3	5

MCL: Maximum contaminant levels; NS: Not specified.

.

Then, the effect of improvement with FS on concentrations in the sequential system was also discussed, specifically in the combination of %80FS+%20SteelSlags-NC. The effect of treatment on metal concentrations has thus been reported, as seen in

Table 6. Toxic metal concentrations of treated steel slags after SWLTs.

Samples	EC (mS/cm)	TDS (ppt)	pH	Al (mg/L)	Fe (mg/L)	Zn (mg/L)
EAF:	0.57	0.28	8.6	<0.005	<0.25	<0.1
LS:	0.54	0.27	9.7	<0.5	NS	NS
Sand	0.09	0.05	9.1	NS	<0.25	NS
EPA SMCL (mg/L):		6.5~8		0.2	0.3	5

MCL: Maximum contaminant levels; NS: Not specified.

. Accordingly, the levels of trace metals have once again decreased below the minimum thresholds.

4. Conclusions

The mechanical and environmental characteristics of EAF and LS steel slags were investigated in this study, obtaining the following results.

EAF and LS are suitable choices for road filling because of the strength parameters $({\varphi °}_{EAF}=38, {\varphi °}_{LS}=29)$ and stiffness values generated by their chemical composition. Particularly, both soaked and unsoaked CBR values indicate whether these materials can be employed as subbases or bases. Furthermore, samples have sufficient drainage capability when considering the permeability test results. However, in the case of steel slag as a filling, it is necessary to assess the long-term swelling potentials to mitigate stability issues that may arise during the lifespan of the materials.

The leachate extracted from pure EAF and LS utilizing WLTs exhibited notably high pH, EC, and TDS levels. Hence, it can be inferred that using EAF and LS in engineering applications should be avoided unless appropriate measures are taken to address environmental considerations. In the study, the treatment methods were applied by mixing natural clay, bentonite, kaolin, and coarse sand soils with EAF and LS at the percentages of 20%, 40%, 60%, and 80% by weight. According to the WLT results, the most significant reductions were observed in EAF and LS with the addition of 60% NC, resulting in decreases of 1.2 and 0.7 points, respectively. The pH value in the sand plugin remained largely unchanged due to the inert nature of the sand.

Experimental studies have shown that steel slags could replace natural aggregates in road filling with sufficient drainage capability; however, the release of toxic trace metals due to high alkalinity poses environmental risks. Sequential leachate tests (SWLTs) were conducted to assess the environmental impact of using steel slag in road embankments, with layers of kaolin (K), bentonite (B), natural clay (NC), and coarse (CS) and fine sand (FS) positioned beneath the slag. Leachate derived from the electric arc furnace (EAF) and ladle furnace slag (LS) experienced the greatest impact from buffering and dilution effects in NC, resulting in pH decreases of 2.0 points for EAF and 0.9 points for LS. SWLTs improved ion exchange and prolonged dilution and buffering times, with similar pH values to standard leachate tests (WLTs) for kaolin and bentonite. Additionally, SWLTs with the NC additive led to significant reductions of 77% and 81% in electrical conductivity (EC) for EAF and LS, respectively, attributed to dilution affected by NC’s neutrality. Overall, SWLTs resulted in decreased EC for all additives due to increased dilution time and buffering duration.

Fine sand was incorporated into steel slag, intended for road filling as a base or subbase, at rates of 20%, 40%, 60%, and 80% by weight to mitigate waste. Sequential leachate tests (SWLTs) were conducted to assess the environmental impact, passing through layers of K, B, NC, and CS beneath the slag. In LS, adding sand led to a decreased pH, with the most significant decrease of 1.1 points observed at 80% sand addition. However, despite the reduction, pH values remained relatively high compared to untreated SWLTs, indicating persistent pollution levels. Agitation with CS did not affect pH in LS. For EAF, the most substantial pH decrease of 2.4 points occurred when 80% sand-enhanced samples were agitated with NC. Bentonite presence was unaffected by pre-treatment, but the pH decreased gradually as the EAF ratio in kaolin clay decreased, reaching a decrease of 1.1 points. CS agitation had no impact on the treated EAF pH.

The high alkalinity of EAF and LS steel slags contributes to the formation of extremely alkaline leachate upon contact with water, leading to increased trace element concentrations. Environmental concerns, such as the release of toxic elements and aquatic contamination, arise from the careless use of steel slags in geotechnical applications due to their alkalinity and chemical composition. Analysis of leachates from pure steel slags revealed elevated metal concentrations, with both EAF and LS leakage waters exceeding specified limits for Al and Fe. The addition of NC resulted in the lowest pH/EC/TDS values, and %100SteelSlag+NC combinations led to concentrations falling below minimum limits. Improvement with FS, especially in %80FS+%20SteelSlags-NC combinations, further decreased trace metal levels below regulatory thresholds. These findings emphasize the importance of considering additives with a negative charge and buffering capacity like NC to mitigate environmental risks associated with steel slag usage.

In conclusion, EAF and LS steel slags can be used as a base or subbase in road filling with appropriate measures. Among these measures, the use of negatively charged and high-buffering-capacity clay minerals, which have the potential to lower pH and alkalinity as well as reduce heavy metal concentrations, is recommended.