Enhancing Mechanical Properties of Expansive Soil Through BOF Slag Stabilization: A Sustainable Alternative to Conventional Methods

Abstract

This study investigates the stabilization of expansive soil using basic oxygen furnace (BOF) slag, an eco-friendly steel by-product, as an alternative to conventional stabilizers like ordinary Portland cement. By evaluating varying concentrations of BOF slag and lime as an activator, the research aims to improve the soil’s mechanical properties, addressing issues like low bearing capacity and high shrink–swell potential. Bentonite clay was treated with different BOF slag ratios (10%, 20%, and 30%) and activated with lime (1%, 3%, and 5%). After mixing and compaction, samples were cured and tested for unconfined compressive strength (UCS), shear wave velocity (BE), and free swell. Microscopic analyses (SEM) provided insight into structural changes post-stabilization, revealing improved properties with increased BOF and lime concentrations. Notably, stabilization with 30% BOF slag and 5% lime achieves a compressive strength of 810 kPa, meeting the minimum subgrade soil stabilization requirement (700 kPa) set by the Federal Highway Administration. This research underscores the potential of BOF slag as a sustainable and practical material for bentonite clay stabilization, offering a promising solution for enhancing soil properties while contributing to environmental sustainability through industrial by-product repurposing.

1. Introduction

Expansive soils are among the most problematic soils due to their susceptibility to swelling and shrinking with changes in water content, resulting in vertical or horizontal shifts in terrain [1]. Swelling in expansive soils primarily concerns the type and quantity of pore spaces, along with how these spaces interact with water [2]. The behavior of expansive soils in terms of volume change is fundamentally influenced by the diffuse double layer, mostly due to expandable clay minerals like montmorillonite with their expanding lattice structures [3]. All this leads to structural damage, including cracks and instability in buildings and roads; foundation problems; damage to underground utilities and pipelines; adverse agricultural effects on root growth and water retention; and damage to landscaping and surface structures. Addressing these issues requires comprehensive soil assessments, the adoption of appropriate construction techniques, and the implementation of ongoing management strategies to mitigate expansive soil’s detrimental effects on both built and natural environments.

Soil stabilization enhances the physical and mechanical properties of soil, making it capable of bearing the weight of subgrades, pavements, and foundations. Often, natural engineering properties are not sufficient to support structural loads. Soil stabilization aims to reduce soil permeability and compressibility for earthworks while enhancing its shear resistance. Chemical stabilization by incorporating different amounts of commercially available materials and industrial wastes as additives is recognized as the most effective method nowadays. The chemical interaction between the additive and the soil leads to improvements in the soil’s strength, stiffness, gradation, workability, and plasticity, particularly in the subgrade layer.

Stabilizing high plasticity clays typically involves conventional stabilizers, such as cement and lime, which are often favored for their ability to quickly and substantially enhance soil strength [4]. However, cement use comes with significant drawbacks, including high energy demands and cost, along with its substantial environmental impact—contributing to approximately 7% of global CO₂ emissions. Lime has demonstrated effectiveness in stabilizing subgrade soils, though its performance varies with environmental conditions, as seen in Ethiopian road projects [5]. Lime stabilization, while also effective, encounters limitations, particularly during wetting and drying cycles, which can compromise its ability to control swelling in expansive soils [6].

In recent years, eco-friendly alternatives for soil stabilization have garnered increasing attention, driven by the need for sustainable construction practices. One such alternative is calcium sulfoaluminate (CSA) cement, an unconventional stabilizer that offers environmentally superior properties whilst achieving comparable soil stabilization results to ordinary Portland cement (OPC) [7,8,9,10,11,12,13]. Studies have shown that CSA cement and gypsum contribute to soil strength enhancement, particularly in combination with reactive clays and sands [14]. High confining pressures in triaxial shear tests have shown improved shear behavior in CSA-treated sands, supporting its use in geotechnical applications under load [12]. However, despite its advantages, CSA cement can be relatively expensive, prompting researchers to explore more cost-effective solutions for soil stabilization.

Another promising category of stabilizers is alkali-activated materials, often considered as alternatives to traditional cement. Alkali-activated materials encompass a broad range of binder systems formed through a chemical reaction between an alkali metal source and a solid silicate powder [15]. These materials are noted for their low carbon footprint [16], dense contact layer in the microstructure [17], high compressive strength [18,19], and rapid strength development [20]. However, their use presents certain challenges, including the need for elevated activation temperatures, controlled relative humidity, and precise mix designs [15,21]. Additionally, the cost and availability of alkaline activators can be limiting factors in some regions. In response, researchers have suggested the use of by-products, such as agricultural wastes, basic oxygen furnace (BOF) slag, and ground granulated blast-furnace slag (GGBFS), as cost-effective alternatives to create alkali-activated binders. Studies show that agricultural by-products, like sawdust ash, can significantly improve geotechnical properties, especially in organic-rich soils [22]. Agricultural waste additives have also demonstrated promising results in soil stabilization, paralleling the benefits of industrial by-products like BOF slag for sustainable engineering practices [23].

The global production of iron slag was estimated to range between 330 million and 390 million tons in 2023, while steel slag production varied between 190 million and 290 million tons [24]. Steel production generates substantial quantities of BOF slag, a by-product that has shown great potential for soil stabilization. Several studies have highlighted the use of steel waste materials, including BOF slag, as easily available and inexpensive alternatives for soil stabilization [25,26,27,28,29]. The incorporation of steel slag not only improves the quality of soil but also solves waste disposal issues, contributing to the development of sustainable and eco-friendly stabilization methods. Moreover, converting BOF slag into a powdered cementitious material requires only about 10% of the energy needed for manufacturing OPC, making it an energy-efficient and viable alternative for soil stabilization [30].

Dicalcium silicate (${\mathrm{C}}_2\mathrm{S}$), which is present in BOF slag, is a key distinction between the mineral composition of OPC and BOF slag, which hydrates more slowly than the tricalcium silicate (${\mathrm{C}}_3\mathrm{S}$) found in OPC [28,31]. The improvement in soil strength, when it is stabilized with BOF slag, can be attributed to several mechanisms. These include the hydration of ${\mathrm{C}}_2\mathrm{S}$, similar to OPC, the availability of free lime and the pozzolanic reaction that occurs between ${\mathrm{C}\mathrm{a}\left(\mathrm{O}\mathrm{H}\right)}_2$ and the silicon/aluminum present in clay minerals [25,26]. Notably, BOF slag fines have been shown to significantly enhance soil strength and durability, particularly when higher percentages of slag fines are used and an extended curing period is employed [28]. Additionally, the incorporation of activators such as CaO and MgO into BOF slag has demonstrated optimal strength gains after 90 days of curing [32]. Over time, microstructural changes in clay treated with BOF slag become increasingly pronounced as cementitious compounds develop [26].

Finer BOF slag particles tend to increase the strength and decrease the compressibility of clay more efficiently than coarser particles when examining the impact of different particle sizes on the strength development of marine clay and kaolin clay [25,27]. This effectiveness is attributed to the larger surface area of finer particles, which facilitates more efficient chemical reactions and stronger bonding within the clay matrix. These findings underscore the significant potential of BOF slag fines in civil engineering applications, particularly in improving the properties of fine-grained soils. However, it is important to note that there has not been much research on the application of BOF slag for stabilizing expansive soils. Therefore, there is a need to explore the behavior of BOF slag in stabilizing expansive soils both independently and in combination with conventional activators like lime. Such investigations would provide a foundation for future research into the use of non-conventional additives in soil stabilization.

This study aims to examine the effects of varying concentrations of BOF slag (10%, 20%, and 30%) and the role of lime as an activator (at ratios of 1%, 3%, and 5%) on the mechanical properties of bentonite clay. The stabilization techniques will be evaluated through a series of tests, including unconfined compressive strength (UCS), bender element (BE), free swell, and scanning electron microscopy (SEM), to gain a deeper understanding of the fundamental processes involved. Shear wave velocity (Vs), determined using BE tests, is a critical parameter in geotechnical engineering for estimating soil stiffness, determining subgrade and foundation stability, and mapping soil stratigraphy for seismic applications [33,34,35,36,37]. By correlating BE-derived Vs with UCS results, this study offers a comprehensive evaluation of soil stabilization techniques highlighting the potential of BOF slag as a sustainable and effective solution for expansive soil stabilization while enhancing reliable geotechnical design practices.

Previous studies have demonstrated the effectiveness of BOF slag in stabilizing kaolin and marine clays, but its application in expansive soils remains underexplored. Expansive soils present unique challenges due to their significant swelling and shrinking in response to moisture changes. Given its finer particle size and higher reactivity, BOF slag may achieve superior stabilization in expansive soils compared to conventional stabilizers like cement and lime, which are limited in cyclic wetting and drying conditions. By examining BOF slag alone and in combination with lime, this study provides new insights into its potential as a sustainable solution for improving the mechanical and swelling properties of expansive soils.

2. Materials and Methods

2.1. Materials

In this study, bentonite clay, classified as silt with high plasticity (MH) according to the Unified Soil Classification System (USCS), served as the expansive soil for stabilization. The primary binder used for stabilizing bentonite clay is BOF slag, with lime serving as an activator. The BOF slag, stockpiled for aging, was supplied by a steel company in Kazakhstan, near Karaganda. A visual representation of the materials investigated in this study is illustrated in Figure 1, while

Table 1. Basic properties of bentonite clay.

Property	Value	Standard
USCS Classification	MH	ASTM D1921 [38]
Plastic Limit, %	49	ASTM D4318 [39]
Liquid Limit, %	94.67	ASTM D4318 [39]
Plasticity Index, %	45.65	ASTM D4318 [39]
Fine, %	>50	QICPIC
Specific Gravity, ${\mathrm{G}}_{\mathrm{s}}$	2.68	ASTM D854 [40]
Optimum Moisture Content, %	32.4	ASTM D698 [41]
Maximum Dry Density, $\mathrm{k}\mathrm{g}/{\mathrm{m}}^3$	1364	ASTM D698 [41]

provides the fundamental characteristics of bentonite clay and

Table 2. Basic properties of BOF slag.

Property	Value	Standard
Specific gravity of coarse aggregate, ${\mathrm{G}}_{\mathrm{s}}$	3.08	ASTM C127 [42]
Specific gravity of fine aggregate, ${\mathrm{G}}_{\mathrm{s}}$	3.14	ASTM C128 [43]
Absorption rate of coarse aggregate, %	3.58	ASTM C127 [42]
Absorption rate of fine aggregate, %	3.05	ASTM C128 [43]
Maximum particle size, mm	19	ASTM C136 [44]
Coarse aggregate (>4.75 mm), %	62.5	ASTM C136 [44]
Fine aggregate (<4.75 mm), %	37.5	ASTM C136 [44]

presents the characterization of BOF slag.

To obtain the gradation curve of the bentonite clay, the QICPIC particle size and shape analyzer tool was used, which has a range of measurement of 4.2–8665 µm. The ASTM C136 [44] standard was followed in determining the particle size distribution of BOF slag. Figure 2 represents the gradation curves for both bentonite clay and BOF slag. The uniformity coefficient (Cu) and coefficient of gradation (Cc) of bentonite clay are 5.72 and 0.85, respectively, and for BOF slag, Cu = 18.33 and Cc = 0.87. Recognizing the advantage of fine particles in soil stabilization, previous studies have emphasized grinding BOF slag to obtain finer particles [25,27]. The mineralogical characteristics of the soil were identified through the application of X-ray diffraction (XRD) analysis (Figure 3a), and specific chemical bonds were identified using Fourier Transform Infrared Spectroscopy (FTIR) (Figure 3b). Parameters of XRD: Cu Kα radiation with λ = 1.5406 Å, step size = 0.02° 2θ, time per step = 1 s. Bentonite clay comprises minerals such as montmorillonite, quartz, illite, calcium sulfate hydrate, and clinochlore. Employed together, XRD and FTIR offer a comprehensive understanding of a material’s composition (Figure 3).

Characteristic absorption bands in the FTIR spectra include those at 3692 cm⁻¹ and 3623 cm⁻¹, corresponding to stretching vibrations of OH stretching of structural hydroxyl groups of Si-OH and Al-OH in clay minerals [45,46,47], typically found in minerals like kaolinite or montmorillonite, as confirmed by corresponding peaks in the XRD pattern. An absorption band at 1635 cm⁻¹ and 993 cm⁻¹ is attributed to a layered silicate montmorillonite mineral (Si-O stretching) [45,47]. The presence of an absorption band at 910 cm⁻¹ is often related to the bending vibration of Al-Al-OH, indicative of illite or other aluminum-rich phyllosilicates [48]. Absorption bands at 796 cm⁻¹, 689 cm⁻¹, and 522 cm⁻¹ indicate Si–O stretching vibrations, signatures of quartz [49], aligning with peaks observed in the XRD results, confirming the presence of crystalline minerals detected through their unique diffraction patterns.

The X-ray diffraction (XRD) analysis of the BOF slag and lime, presented in Figure 4, reveals a complex mineral composition, with each phase contributing to soil stabilization properties. Prominent peaks around 2θ ≈ 15–20° indicate the presence of pentahydrate and calcium hydroxide—hydrated phases that enhance pozzolanic reactions and support soil binding and moisture regulation. Kirschsteinite, a calcium iron silicate observed around 2θ ≈ 20–25°, is common in steel slags and is known for its durability and structural benefits. Calcite, identified near 2θ ≈ 25–30°, adds binding potential through carbonation reactions, which further aid stabilization. Srebrodolskite, a calcium iron oxide found across multiple peaks, reinforces the structural integrity of the slag. Additionally, minor components such as lithium silicon sodium and barium copper oxide contribute supplementary binding properties and chemical stability. Collectively, these mineral phases highlight BOF slag’s potential to enhance the mechanical and chemical stability of soils, establishing it as a valuable material for sustainable geotechnical applications (Figure 4a). Figure 4b represents minerals portlandite and calcite, present in lime.

The chemical compositions of the materials were determined via XRF analysis, as shown in

Table 3. XRF test results of the materials used in this study.

Compound	Bentonite Clay (%)	BOF Slag (%)	Lime (%)
MgO	-	8.8	0.1
${\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3$	8	1.2
${\mathrm{S}\mathrm{i}\mathrm{O}}_2$	26.4	10.3	0.2
CaO	1.7	37.5	80.3
MnO		3.3
${\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3$	9.6	27.4
${\mathrm{K}}_2\mathrm{O}$	1.9		-
${\mathrm{T}\mathrm{i}}_2\mathrm{O}$	-	0.6
${\mathrm{S}\mathrm{O}}_3$		0.4	-
LOI	47.6	89.5	80.6

. Major components of BOF slag include calcium oxide (CaO), iron oxide (${\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3$), silicon dioxide (${\mathrm{S}\mathrm{i}\mathrm{O}}_2$), and magnesium dioxide (MgO), while bentonite clay primarily consists of aluminum oxide (${\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3$), silicon dioxide (${\mathrm{S}\mathrm{i}\mathrm{O}}_2$), and iron oxide (${\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3$).

2.2. Design of Mixtures and Sample Preparation

In the mixture design, various amounts of BOF slag (10, 20, and 30% by soil mass) were initially added to the soil to determine the optimal proportion, resulting in the highest strength gain and shear wave velocity. Subsequently, 30% BOF slag was selected as the fixed value due to its superior strength compared to samples with 10% and 20% BOF slag addition. The mixture design is represented in

Table 4. Mixture design of stabilized bentonite clay.

Soil	Stabilizer	Activator	Curing Days
Bentonite clay	BOF 10%		3, 7, 14, 28
	BOF 20%		3, 7, 14, 28
	BOF 30%		3, 7, 14, 28
	BOF 30%	Lime 1%	3, 7, 14, 28
	BOF 30%	Lime 3%	3, 7, 14, 28
	BOF 30%	Lime 5%	3, 7, 14, 28

. As per the standards set by the Federal Highway Administration, subgrade soil stabilization requires a minimum strength of 700 kPa when stabilized with lime [50]. Since the addition of 30% BOF slag alone did not meet the minimum strength requirement, the effect of lime as an activator for BOF slag was investigated by adding lime in amounts of 1%, 3%, and 5% by soil mass. All the materials underwent initial drying in the oven before utilization. The optimum moisture content (OMC) and maximum dry density (${\mathsf{\gamma }}_{\mathrm{dmax}}$) were determined according to the ASTM D698 [41] through standard proctor compaction testing. The methodology of the mixture design and sample preparation is shown in Figure 5. First, dry ingredients were mixed together, and water was added according to the total mass of solids. Then, the mixture was mixed manually (due to high viscosity), followed by mixing with a Hobart mixer to obtain a uniform texture. Following the mixing of ingredients in the Hobart mixer, samples were prepared using 50 × 100 steel molds coated with oil for ease of removal. Additionally, the molds were coated with a plastic film and rubber band on both sides to avoid drying of the sample. Manual compaction, involving 25 strokes applied in three layers, was performed, with each layer being scratched with a spatula before the addition of subsequent layers to ensure better contact. Subsequently, the height and diameter of the extruded samples were measured, and to stop moisture loss, polyethylene was wrapped around them using rubber bands. Samples were cured at room temperature. After the designated curing period, they were subjected to testing using the bender element (BE) and unconfined compressive strength (UCS) tests. Additionally, free swell tests and scanning electronic microscope (SEM) examinations were performed.

2.3. Testing Methods

To assess the mechanical properties of the stabilized soil, the unconfined compressive strength (UCS) test and the bender element (BE) test were conducted. A crucial indicator of a soil’s load-bearing capacity is the UCS test, which determines the maximum axial compressive stress that a cylindrical soil sample can sustain before failure. This test was conducted as per ASTM D2166 [51] at a uniform compression rate of 1 mm/min (Figure 6) using an ELE International apparatus. The height and diameter of each sample were recorded, and the samples were placed into the cell by carefully fixing the loading axis. Then, each sample was subjected to compressive loading until it reached the failure point.

To evaluate the shear wave velocity (${\mathrm{V}}_{\mathrm{s}}$) within each specimen, the BE technique was employed, following ASTM D8295 [52], using a GDS BE apparatus. This method entails placing piezoelectric sensors at both the top and bottom of a soil sample, with the sample’s height inputted into the GDS software v 1.5.2 for analysis. Before placing the sample on the plane, two holes were made and then greased with a gel for accurate connection with sensors (Figure 7). ${\mathrm{V}}_{\mathrm{s}}$ is determined by measuring the time it takes for the wave to travel the length of the specimen, starting at the transmitter and ending at the receiver, using the first-arrival approach, as shown in Figure 8. Here, “first arrival” denotes the time from the start of the transmitted signal to the start of the received signal. Researchers identified the first shear wave’s arrival by selecting a point immediately following the initial bump where the signal returned to zero. As depicted in Figure 7, a near-field effect within the low-frequency range (2–12.5 kHz) prompted the selection of L/λ ratios greater than 4 for the samples. Additionally, our observations indicated that the near-field effect diminishes when L/λ > 4 [53]. Furthermore, it was observed that employing higher frequencies could mitigate the near-field effect in more rigid samples, which aligns with the findings in [54]. This approach is applied to improve the accuracy of ${\mathrm{V}}_{\mathrm{s}}$ estimations derived from BE tests.

The 1-D free swell test was carried out using an oedometer following the guidelines of ASTM Standard ASTM D4546 [55]. The test was performed at designated curing intervals (3, 7, 14, and 28 days). This laboratory procedure assesses soil swelling potential by measuring the volumetric increase of a soil sample as it absorbs water under unconfined swelling conditions. During the test, a soil specimen is placed in a cylindrical mold and fully saturated with water, with a porous disc positioned on top to ensure unrestricted swelling (Figure 9). The height increase of the soil sample is monitored over a specific period, and the swelling ratio is calculated by comparing the height increase to the original height of the soil sample. This method quantitatively measures the soil’s expansion in response to moisture. The free swell test continued until the soil specimen reached a stable condition.

The microscopic structure of the stabilized bentonite clay was examined using a scanning electron microscope (SEM) analysis. This technique involves directing a focused beam of high-energy electrons onto the sample’s surface, leading to the emission of secondary electrons captured by a detector. The signals gathered produce an image of the sample’s surface that is greatly enlarged. To improve the final quality, the stabilized soil powder was mounted and covered with a 10 nm layer of gold prior to the test. SEM examination was performed using a Zeiss Crossbeam 540 system, with images captured at magnifications of 5000× and 10,000×, enabling detailed observation of the clay’s microstructure.

3. Results

3.1. OMC-MDD

As illustrated in Figure 10a, a discernible trend emerges where the optimum moisture content (OMC) increases as the maximum dry density (MDD) decreases with an increasing proportion of basic oxygen furnace (BOF) slag. Incorporating more BOF slag fines into the soil composition potentially enhances its water absorption capacity, thereby increasing the OMC. Conversely, the improvement in MDD associated with the inclusion of BOF slag could stem from the enhanced packing density achieved through the integration of fine slag particles, effectively filling the gaps between clay particles.

Figure 10b shows that adding 1% and 3% lime causes OMC to rise and MDD to fall. Adding 5% lime, on the other hand, has the reverse effect, lowering OMC and raising MDD. This phenomenon can be explained by the immediate flocculation effects and the initiation of long-term pozzolanic reactions, diminishing the soil’s water demand for achieving peak compaction due to the gradual densification of the structure and the synthesis of cementitious compounds.

The findings imply that the initial incorporation of lime up to a specific limit (i.e., 3%) induces a decrease in MDD, promoting soil particle flocculation and agglomeration, resulting in a looser soil configuration. When the lime concentration surpasses this critical point, there is an observed increment in MDD, likely due to the onset of pozzolanic reactions that yield a more compacted and stable soil framework.

Regarding OMC, its initial rise with lime content up to 3% can be attributed to the increased hydration requirements for the lime–soil interaction and to accommodate the expanded volume within a less dense soil matrix. Beyond this lime percentage, the OMC experiences a decline, likely due to the soil framework beginning to compact due to the emergence of cementitious substances, thereby lowering the water requirement for optimal compaction. Similar lime effects on OMC and MDD were observed in a previous study [56].

3.2. The Effect of BOF Slag %

The study evaluated the influence of BOF slag on bentonite soil stabilization by integrating three distinct BOF slag ratios (10%, 20%, and 30%) into the soil. The results from the (a) UCS and (b) BE tests conducted over a curing period of 3 to 28 days are presented in Figure 11. Throughout the entire range of curing durations, the inclusion of 30% BOF slag consistently produced the highest UCS and ${\mathrm{V}}_{\mathrm{s}}$ values. A consistent trend of improvement was observed with the mixture containing 10% BOF slag, which exhibited the lowest value compared with the 20% and 30% BOF–clay mixtures, which presented slightly increasing results. The most noticeable changes in the results were observed after a 3-day curing period, with subsequent intervals (7, 14, and 28 days) not showing significant increases in strength or ${\mathrm{V}}_{\mathrm{s}}$. However, the highest results were achieved after 28 days of curing.

This investigation demonstrates a clear correlation between the BOF slag content, curing period, UCS, and ${\mathrm{V}}_{\mathrm{s}}$ of the soil stabilized by BOF slag. Figure 12 illustrates stress–strain curves for bentonite clay enhanced with 10%, 20%, and 30% BOF slag. Initially, during the early curing days, particularly the 3rd and 7th days, there is a noticeable tendency towards plastic deformation. On the fourteenth day of curing, however, a notable shift towards brittle characteristics can be noted, signifying the beginning of hydration processes in the mixture. The main cause of this early strength development is the free lime component in the BOF slag, which reacts with water to initiate a hydration process. This eventually causes pozzolanic reactions that significantly improve the mechanical qualities of the soil [25]. An important factor in raising the performance of stabilized soil is the creation of cementitious links and hydration products. Therefore, the stress–strain curves for different concentrations of BOF slag (10%, 20%, and 30%) offer important insights into the stabilizing process by showing the effects of the length of curing time as well as the percentage of BOF slag that is integrated into the expanding soil.

3.3. The Effect of Lime % on Soil Stabilized by BOF

After determining that 30% BOF slag was the most effective in terms of strength and ${\mathrm{V}}_{\mathrm{s}}$ for stabilizing bentonite clay, the subsequent research step focused on exploring the efficiency of hydrated lime in the strength optimization of the stabilized soil. The results, as depicted in Figure 13, illustrate the evolution of (a) UCS and (b) BE values for mixtures containing 30% BOF slag and varying lime ratios (1%, 3%, and 5%) at intervals of 3, 7, 14, and 28 days.

A consistent upward trend in UCS and ${\mathrm{V}}_{\mathrm{s}}$ values was observed throughout the curing duration, correlating with increased lime additions. This improvement is likely attributable to the enhanced presence of calcium oxide from the added lime, which plays a pivotal role in initiating and fostering the stabilization effect. The results underscore the critical roles played by both the proportion and the duration of curing time in enhancing the performance of BOF slag-stabilized soil. Incorporating 5% lime marked a significant enhancement, elevating the peak strength to 870 kPa and achieving a ${\mathrm{V}}_{\mathrm{s}}$ value of approximately 500 m/s after 28 days.

The relationship between stress and strain for the stabilized soil with additions of 1%, 3%, and 5% lime is shown in Figure 14. Extended plastic deformation can be prominently seen in the early stages of curing, particularly on the third day when 1% lime is added, and then it changes to a more brittle behavior in the following days. In contrast, samples with higher lime contents (3% and 5%) exhibit brittle characteristics throughout the curing process, with the brittleness intensifying as the curing duration extends. This pattern signifies the progression of hydration reactions within the sample, which depend on time. Because of the creation of cementitious bonds and hydration products, these reactions play a crucial role in improving the stabilized soil’s characteristics. One of the most important factors in improving the material’s overall effectiveness is the amount of lime applied and the curing time.

3.4. UCS and $V_s$ Correlation

Figure 15 illustrates the correlation between UCS and ${\mathrm{V}}_{\mathrm{s}}$ values, derived from the data of UCS and BE tests. Linear correlations were formulated, revealing a consistent alignment among all correlations. The determination coefficients for the impact of varying BOF contents (10%, 20%, and 30%) at 0.9598 and lime contents (1%, 3%, and 5%) at 0.9111 indicate a strong linkage between UCS and ${\mathrm{V}}_{\mathrm{s}}$ values, demonstrating a superior predictive capacity. These findings highlight significant correlations between UCS and V_s values, suggesting that rising ${\mathrm{V}}_{\mathrm{s}}$ values are correlated with rising UCS values. The identification of such a profound correlation is beneficial for the evaluation of soil treatment efficacy. Consequently, this methodology supports the testing procedure’s efficacy and financial viability, allowing engineers and researchers to accurately assess the results of soil stabilization procedures.

3.5. Free Swell Test

The free swell test constitutes a crucial laboratory evaluation designed to ascertain a soil’s swelling capacity and establish methods for subgrade stabilization. Figure 16 displays the soil samples’ free swell percentages across various curing days (3, 7, 14, and 28 days). For instance, the free swell percentage for pure bentonite clay samples cured for 3 days was reported as 20%. However, this percentage declined to 18%, 4%, and 2% with the addition of BOF slag contents of 10%, 20%, and 30%, respectively. This decreasing trend in swelling was similarly observed in samples cured over 7, 14, and 28 days. As the 20% BOF slag content was incorporated, the free swell percentage for samples cured for 7 days dropped from 4% to 3%, mirroring reductions in 14-day and 28-day cured samples. When 30% BOF slag was added, the free swell percentage for 7 days dropped from 2% to 1% and fell below 1% for samples cured for 14 and 28 days. Consequently, the free swell percentage was negligible as the BOF dosage increased. The findings imply the significant role of curing duration in mitigating swelling. Moreover, the 30% BOF slag content emerges as the most effective in reducing the swelling potential.

3.6. SEM

Figure 17 provides a detailed examination of the microstructural changes and visual characteristics observed in the bentonite clay samples after they were treated with varying concentrations of BOF slag over 28 days. The analysis indicates that incorporating higher concentrations of BOF slag, characterized by its fine particulate nature, led to a transformation in the surface texture of the clay samples, transitioning them towards a more flocculated appearance. This change highlights a notable alteration in the texture of the samples, suggesting a fundamental shift in their microstructural composition. Figure 17a precisely displays the microstructure of untreated bentonite clay, revealing a relatively uniform and homogeneous texture. This baseline texture serves as a contrast to the changes observed in subsequent samples. As shown in Figure 17b, adding 30% BOF slag to the bentonite clay resulted in a markedly different texture characterized by a pronounced angular and flocculated structure. This texture not only illustrates the physical changes but also signifies the mineralogical transformations occurring due to the interaction between the clay and the BOF slag. Further modifying the mixture by introducing a lime activator, in a proportion of 5% in combination with 30% BOF slag, yielded even more distinct changes, as depicted in Figure 17c. The resultant texture was noticeably flakier—an effect attributed to the lime’s ability to enhance the hydration process of the BOF slag. The microstructure became increasingly angular and fragmented as a result of the hydration interaction between the lime, the BOF slag, and the silica in the clay.

The evolution in texture from the homogenous state of pure bentonite clay to the flaky and angular structures observed in samples treated with BOF slag and the lime activator underscores a direct relationship between the increase in BOF slag concentration and the formation of a cementitious reaction within the soil matrix. This hydration contributes to significant densification of the material, playing a crucial role in developing a final product that exhibits enhanced strength and cohesion. This transformation is pivotal, illustrating the potential of BOF slag, especially when activated with lime, to substantially improve the mechanical properties of bentonite clay, making it a more viable and robust material for civil engineering applications.

4. Discussion

The findings from this study underscore the significant impact that BOF slag, in conjunction with lime as a catalyzing agent, has in terms of enhancing the soil stabilization process. An increase in BOF slag content directly correlates with improvements in both UCS and ${\mathrm{V}}_{\mathrm{s}}$, thus making the soil stronger and more stable. Furthermore, prolonging the curing time is advantageous for the formation of cementitious links, which in turn increases the material’s overall strength.

The chemical interaction between the silica oxide (${\mathrm{S}\mathrm{i}\mathrm{O}}_2$) in soil and calcium oxide (CaO) in BOF slag is the main mechanism responsible for this strength improvement. The soil particles create strong, cohesive cementitious connections as a result of this reaction. However, because of the slow hydration process caused by the presence of dicalcium silicate (${\mathrm{C}}_2\mathrm{S}$) in BOF slag, the rate of strength gain can slow down [28]. These results emphasize the importance of optimizing the lime content and curing duration when designing and implementing BOF slag-based soil stabilization strategies. By fine-tuning these variables, the stabilization process can be significantly improved, providing valuable insights for the effective use of BOF slag in geotechnical engineering applications.

Furthermore, the correlation between UCS and ${\mathrm{V}}_{\mathrm{s}}$ serves as a critical instrument for monitoring the success of soil stabilization. By measuring ${\mathrm{V}}_{\mathrm{s}}$ values, engineers, and scientists can accurately predict UCS, eliminating the need for further labor-intensive and expensive testing. The free swell test indicated a noticeable decrease in soil expansion with the addition of BOF slag. A 30% BOF addition rate significantly reduced the swelling ratio to close to 0, even after 3 days of curing. The strength enhancements observed in stabilized soils are closely linked to pozzolanic reactions, as similarly observed in lime-stabilized kaolinite, where pozzolanic interactions improve soil integrity [57]. Salimi and Ghorbani [32] found that a longer curing time increases pozzolanic activity in lime–slag mixes, further enhancing clay stability. The incremental UCS gains over longer curing times among the mixes under study are also consistent with their findings. Additionally, Poh et al. [28] observed that fine slag particles contribute to soil strength by forming denser microstructures, which is consistent with our SEM images showing flocculated, compacted structures at higher BOF slag contents. Furthermore, microstructural analyses provide a detailed comprehension of the processes of hydration and the creation of structures in the stabilized clay matrix.

The Federal Highway Administration has established minimum UCS requirements, and 750 kPa is the minimum required for lime-stabilized subgrade soil with identical sample dimensions. Thus, this threshold was adopted as the benchmark for this investigation. Utilizing BOF slag alone at a 30% substitution rate showed the highest efficiency in soil stabilization efforts. However, the UCS value for 30% BOF slag after 7 days (485 kPa) did not meet the Federal Highway Administration’s minimum standards [50]. Consequently, to improve the BOF slag’s stabilizing function, a lime activator was added to the 30% BOF slag substitution level. Among various lime concentrations tested (1%, 3%, and 5%), a 5% lime concentration was found to be the most effective throughout the curing period. A blend of 30% BOF slag and 5% lime achieved the benchmark UCS value of 700 kPa, with a recorded strength of 810 kPa, thus meeting the established strength criteria. This study emphasizes the critical role of selecting an optimal BOF slag and lime mix to meet specific strength requirements. Future studies need to focus on stabilized samples’ long-term stability and durability. Field-scale trials and real-world applications are also necessary to confirm the effectiveness of the combinations that have been recommended in this research in real engineering scenarios.

5. Conclusions

This research underscores the effectiveness of BOF slag as a stabilizing material for bentonite clay, offering a sustainable and eco-friendly approach to managing challenging soils in civil engineering contexts. The findings show that the proper ratio of lime to BOF slag can produce the required geotechnical properties, meeting the requirements for subgrade soil stabilization. This study provides valuable insights for engineers and researchers seeking to develop effective and environmentally conscious soil stabilization strategies for geotechnical applications. The study comes to the following important conclusions based on the results of the tests that were conducted:

• With a higher BOF content and a longer curing time, soil shows improved strength. After 3 days of curing, there is a notable increase in UCS and ${\mathrm{V}}_{\mathrm{s}}$, which is followed by a more moderate process.
• The results of the BE test show a strong correlation with those of the UCS test, suggesting that the BE test can reliably and non-destructively predict a soil–slag mixture’s UCS performance.
• It is recommended to incorporate 5% lime for bentonite clay stabilization with BOF slag, as this will result in the highest UCS and BE test results as well as the lowest free swell ratio.
• SEM pictures show that as the BOF content rises, especially when a lime activator is added, the microstructure of clay–BOF mixtures becomes more flocculated (less uniform and smooth), angular, and smaller in size.
• The Federal Highway Administration states that 700 kPa of subgrade soil stability must be achieved after 7 days of lime curing using φ50 mm × H100 mm molds [45]. Based on soil mass, this need can be satisfied for bentonite clay stabilization with BOF slag through the addition of 30% BOF slag and 5% lime.