Stainable Utilization Strategies for Basic Oxygen Furnace Slag: Properties, Processing, and Future Directions

Abstract

Steel slag, being the dominant solid byproduct in steelmaking, presents global challenges in sustainable management, particularly regarding resource recovery of Basic Oxygen Furnace (BOF) slag, which accounts for over 72% of total slag generation. Through the databases of ScienceDirect, Web of Science, and CNKI, using relevant key words, this review systematically investigates the physicochemical properties and mineralogical composition of BOF slag, elucidating the intrinsic mechanisms underlying its low hydration reactivity and volumetric instability. Pretreatment techniques have been demonstrated to effectively modulate these properties. Furthermore, valuable components can be efficiently recovered through methods including magnetic separation and related technologies. Furthermore, this review elucidates the mechanisms and existing challenges across various resource utilization approaches for steel slag, while also identifying key research priorities for future development, thereby providing a systematic theoretical framework and technical pathways to advance utilization of steel slag.

1. Introduction

Approximately 100–150 kg of steel slag, a predominant solid waste generated during crude steel production [1], is produced per ton of steel output. In 2024, China’s crude steel production reached 1.005 billion tons, accounting for 53.39% of global output, with concomitant steel slag generation amounting to 160 million tons [2].

Globally, 72% of crude steel is produced via BOF processes, while 28% derives from Electric Arc Furnace (EAF) routes. However, national technological preferences vary markedly: Germany adopts 70% BOF versus 30% EAF, Turkey 29% versus 71%, and the United States 30% versus 70%, whereas China predominantly relies on BOF (90%), with minimal EAF utilization (10%) [3].

Regarding utilization rates, developed economies maintain superior performance: Japan achieves 98.4%, Europe 87.0%, and the United States 84.4% [3]. In contrast, China’s comprehensive utilization rate remains at merely 29.5% [3], highlighting a significant gap. The extensive open-air stockpiling of steel slag not only occupies substantial land resources but also facilitates the migration of heavy metals (e.g., Fe, Mn) and alkaline substances through weathering processes, leading to soil salinization, groundwater contamination, and particulate matter pollution. As the global steel industry advances its “solid waste valorization” strategy [4], the predominance of BOF slag in China’s steel slag (accounting for 72%) has made its effective application a current focal point [3]. The utilization approaches of steel slag in various regions can be seen in Figure 1.

As a core solid waste in the steel industry, global-scale disposal and resource utilization of steel slag face significant challenges. This study systematically analyzes the physicochemical properties of steel slag, elucidates its chemical and mineral composition characteristics, and reveals the intrinsic mechanisms underlying its low hydration activity and volumetric instability. To address compositional complexity, pretreatment technologies such as thermal aging and rotary drum processing effectively regulate free calcium oxide content. Combined with magnetic separation, carbothermal reduction, acid leaching, and alkali leaching processes, these methods enable efficient recovery of valuable components, including iron, vanadium, cadmium, and phosphorus.

In resource applications, steel slag exhibits unique reactivity in cementitious materials through silicate tetrahedral reconstruction and complex salt effects. As concrete aggregates, it significantly enhances mechanical properties and optimizes pore structures, while asphalt composites incorporating steel slag demonstrate high-temperature stability and crack resistance. Emerging applications extend to high-value products like glass-ceramics and environmental remediation materials.

This review highlights current bottlenecks, such as limited hydration activity enhancement and long-term volume stability control. It identifies application incentives and challenges across resource utilization pathways, pinpointing critical research gaps in material development.

2. Fundamental Properties of BOF Slag

Steel slag, a predominant byproduct of steelmaking processes, can be classified into BOF, electric arc furnace (EAF), and open hearth furnace (OHF) slags based on production routes [10]. Globally, 72% of crude steel is produced via BF-BOF integrated processes [3], resulting in BOF slag being the most abundant variant.

2.1. Chemical Composition and Mineralogical Phases

The characteristics of BOF slag have a direct and important impact on its application. In general, the chemical composition of BOF slag is shown in

Table 1. Typical BOF slag exhibits the following chemical composition (wt% range). Adapted from Ref. [11].

Component	SiO2	Al2O3	CaO	MgO	FeO	S	P2O5	f-CaO	Basicity (CaO/(SiO2 + P2O5))
Range	15–25	3–7	45–60	5–20	12–25	0.1–0.4	0–1	1.6–7	2.1–3.5

:

BOF slag can be categorized into acidic slag (low basicity, CaO/SiO₂ < 1.8) and basic slag (CaO/SiO₂ ≥ 1.8), with the latter constituting over 85% of total production [12]. During basic oxygen steelmaking, the continuous addition of lime (CaO) elevates slag basicity, triggering characteristic phase transitions [13]:

2(CaO·RO·SiO₂) + CaO = 3CaO·RO·2SiO₂ + RO

3CaO·RO·2SiO₂ + CaO = 2(2CaO·SiO₂) + RO

2(2CaO·SiO₂) + CaO = 3CaO·SiO₂

The mineralogical phases demonstrate basicity-dependent evolution (

Table 2. Basicity-dependent mineral phase distribution.

Basicity	Dominant Phases (by Abundance)
1.36	Olivine (CaO·MgO·SiO2), rhodonite (3CaO·MgO·2SiO2), RO phase
1.80	Rhodonite, dicalcium silicate (C2S), RO phase
2.51	Tricalcium silicate (C3S), C2S, RO phase
2.99	C3S, C2S, RO phase, calcium ferrite (Ca2Fe2O5)

):

The dicalcium silicate (C₂S) phase in BOF slag undergoes characteristic polymorphic transitions during cooling (Figure 2). When the temperature decreases to 450–500 °C, β-C₂S irreversibly transforms into γ-C₂S via a reconstructive phase transition. This low-temperature phase exhibits negligible hydraulic activity (< 5% of β-C₂S reactivity) due to its orthorhombic crystal structure (space group Pnma) compared to the monoclinic β-form (space group P2₁/n), accompanied by a 12% unit cell volume expansion. The densified structure reduces specific surface area to < 1 m²/g, effectively passivating the phase [12].

2.2. Physical and Mechanical Properties

As a “high-potential but challenging” industrial by-product, the physical properties of alkaline oxygen furnace (BOF) slag are critical for its future use. The main physical properties of the Basic Oxygen Furnace (BOF) slag can be seen in

Table 3. Key physical properties of BOF slag. Adapted from refs. [14,15].

Parameter	Range/Value	Test Standard	Engineering Implication
Density	3.3–3.6 g/cm3	BS EN 1097-7 [16]	Aggregate grading design
Water absorption	<3%	ASTM D6473 [17]	Weathering resistance
Crushing value	20.4–30.8%	ASTM C535 [18]	Compressive strength indicator
Bond work index	0.7 (Std sand = 1.0)	Bond grindability	Grinding energy consumption
Abrasion coefficient (K)	0.8	EN 1097-1 [19]	Pavement rutting resistance
Brittleness index (n)	0.2	Dynamic impact	Crack propagation resistance

.

2.3. Reactivity and Volume Stability

The activity of steel slag is affected by alkalinity. Dicalcium silicate and tricalcium silicate in steel slag are active minerals with hydrohard gelatinity. When the alkalinity of steel slag is greater than 1.8, steel slag contains 60~80% dicalcium silicate and tricalcium silicate, and with the increase of alkalinity, the content of tricalcium silicate also increases; when the alkalinity is increased to more than 2.5, the main mineral of steel slag is tricalcium silicate. Grinding with steel slag with alkalinity higher than 2.5 and 10% gypsum, its strength can reach the strength of No. 325 cement.

The stability of steel slag is related to free calcium oxide (f-CaO), MgO, Ca₂SiO₄, and Ca₃SiO₅, which are unstable under certain conditions. When slag with high alkalinity is slowly cooled, Ca₃SiO₅ will slowly decompose into Ca₂SiO₄ and f-CaO at 1100~1250 °C. At 675 °C, Ca2SiO₄ changes from Ca₂SiO₄ to γ- Ca₂SiO₄, and volume expansion occurs, with the expansion rate reaching 10%. In addition, after the steel slag absorbs water, f-CaO will be digested into calcium hydroxide (Ca(OH)₂), the volume will expand by 100%~300%, MgO will become magnesium hydroxide, and the volume will expand by 77%. Therefore, steel slag containing f-CaO and MgO at room temperature is unstable, and steel slag will be stable only when the f-CaO and MgO are digested or the content is very small [12]. The reactions of the components of steel slag at the corresponding temperatures can be seen in

Table 4. Reaction of the components of steel slag at the corresponding temperature.

Component	Reaction	Expansion	Critical Temperature
f-CaO	CaO + H2O → Ca(OH)2	100–300%	Ambient–100 °C
β-C2S	β-C2S → γ-C2S	10%	675 °C
MgO	MgO + H2O → Mg(OH)2	77%	<120 °C
C3S decomposition	3CaO·SiO2→2CaO·SiO2 + CaO	Localized	1100–1250 °C

.

2.4. Utilization Potential and Limitations

Due to some of its physical and chemical properties, there are problems with comprehensive utilization. Due to the following reasons, the recycling of steel slag is limited, resulting in low utilization rate of steel slag. Details are presented in

Table 5. BOF slag valorization matrix, Adapted from refs. [14,15].

Application	Key Properties	TRL	Critical Barriers
Cement additive	C2S/C3S reactivity	6–7	f-CaO content < 1.5% required
Road aggregate	High abrasion resistance	8	High density (+15% transport cost)
Wastewater treatment	Porous structure	5	Heavy metal leaching risks
CO2 mineralization	Carbonation reactivity	4	Slow reaction kinetics

and

Table 6. Key limitations and mitigation strategies, Adapted from refs. [20,21,22,23,24,25,26,27,28].

Limitation Factor	Mechanism	Mitigation Strategy
Volume expansion (f-CaO/MgO)	Hydration-induced damage	Steam aging (150 °C, 6 h)
Low pozzolanic activity	Glassy phase < 10%	Mechanical activation (<45 μm)
Heavy metal leaching (Cr, V)	Alkaline ion mobilization	Phosphate stabilization
Abrasive particles	Metallic iron residues (2–5%)	Magnetic separation (>95% recovery)

.

3. Utilization Pathways of BOF Slag

Effective utilization of BOF slag requires pretreatment to stabilize metastable phases (e.g., free CaO, MgO) for volumetric stability compliance [29]. The standard protocol includes: pretreatment → valuable component recovery → resource application. Current non-carbonation utilization predominantly focuses on construction engineering (>75% market share), with emerging applications in environmental remediation and soil amendment.

3.1. Pretreatment Technologies

Pretreatment critically determines slag granulometry (D50 = 0.1–5 mm), phase composition, and activity index (AI = 65–85%).

Table 7. Technical assessment of BOF slag pretreatment. Adapted from refs. [5,30,31].

Method	Key Process	Advantages	Limitations
Thermal Splashing	Water spraying on molten slag for 3–4 days, followed by magnetic separation	High capacity (200–400 t/h) Mature technology	High water consumption (3–5 m3/t) Poor stability (f-CaO > 4%)
Rotary Drum	Integrated cooling–crushing–magnetic separation in rotating drum	Efficient Fe recovery (η > 92%) Superior stability (f-CaO < 2%)	Requires high fluidity (η > 1 Pa·s)
Steam Aging	Intermittent water spraying on 300–800 °C slag induces self-pulverization	High fines yield (85% < 20 mm) Low dust emission (<10 mg/m3)	Long cycle (8–12 h) High energy demand (50–80 kWh/t)
Air Quenching	Molten slag atomization by compressed air with heat recovery	Uniform granules (D50 = 0.5–2 mm) 65% heat recovery	Noise pollution (>85 dB) High maintenance cost
Pressurized Steam	Accelerated f-CaO hydration under 1.5–2.0 MPa steam	Short cycle (2–4 h) Excellent stability (f-CaO < 1%)	Capital intensive ($1.2–1.8 M/unit)

evaluates mainstream technologies, with selection criteria dependent on slag state (molten/solid), throughput (10–500 t/h), and downstream applications [5,30,31].

3.2. Utilization of Valuable Components of BOF Slag

Steel slag contains a variety of useful metallic elements, with common chemical components including vanadium, iron, silicon, titanium, vanadium, chromium, calcium, and magnesium oxides [4,5]. Although vanadium and titanium are present in small quantities, their high value and limited resources can offset the expenditure incurred in their extraction [4,31].

3.2.1. Recovery of Iron

The main components of steel slag include FexO, Fe, CaO, SiO₂, MgO, and MnO [32] The ferrous phase (RO phase, Ca₂Fe₂O₅, Fe₂O₃, Fe₃O₄, Fe) of steel slag accounts for 20–40% of the mass of the original steel slag [33,34], which is recognized as an important iron resource. When utilizing steel slag, iron has a negligible contribution to its activity. Since iron has a high hardness, this phase is difficult to grind into fine particles [15], so it is necessary to recover the iron in the steel slag [35].

Magnetic separation is usually used to recover the iron phase. Steel slag treated by the air quenching method, the water quenching method, the granulation wheel method, or the drum method, due to the small particle size, is generally directly recovered by magnetic separation after dehydration and is no longer further crushed or only simply crushed; due to the large particle size, steel slag treated by the hot stuffing method or the hot splash method needs to be crushed, screened, and magnetically separated. In the early days, the secondary treatment process of steel slag was mainly simple crushing and screening, and now it has been upgraded to multi-stage crushing, multi-stage screening, and multi-stage magnetic separation. When using magnetic separation, in order to improve the enrichment and magnetism of the ferrous phase in the steel slag, hot slag modification (oxidative composition adjustment in the molten state) [36] is proposed, involving air oxidation [37] to promote ferromagnetic spinel formation, rather than thermal reduction. This method enhances magnetic susceptibility.

Carbon-thermal reduction relies on high-temperature reactions where carbon reduces metal oxides, with carbon monoxide as the primary gaseous product [38]. While traditional processes use conventional heating [38], microwave heating can enhance kinetics by exploiting dielectric loss of reactants [39]. Graphite powder is used as a reducing agent in the extraction of iron, and its dosage varies according to the carbon index. As the carbon equivalent increases, the fractional reduction of iron increases. Due to the increase in the activity coefficient of P₂O₅ and the fluidity of the slag, the higher SiO₂ content induces a large amount of iron reduction in BOF slag. On the other hand, the reintroduction of this metal-carbon alloy can reduce the volume of graphite by incorporating graphite as a reducing agent to extract and reduce iron from Fe-C alloys, in which a small amount of Fe alloy remains but has a negative impact on recovery efficiency. In order to achieve technically and economically viable processing and long-term technological development, all these aspects must be addressed [40].

Acid leaching is another widely used technique, in which acids dissolve metals from oxides and sulfides. The main advantages of this technique are its simplicity and broad-spectrum applicability. Optimized HCl leaching of Malaysian steel showed an iron extraction rate of 77.14% [41]. Another technique used is chloride extraction/chlorination. These methods leave soluble iron trichloride, which can be used as a potential adsorbent for wastewater treatment [42]. The resulting chloride has a lower melting point than the original salt. This helps to recover metals more easily. Figure 3 below show flow charts showing different iron recovery techniques.

However, due to the high acid consumption, coupled with energy requirements, the costs associated with the process must be considered. The waste liquid produced after the leaching process is strongly acidic and therefore needs to be treated. Due to the strong acidity of the leachate, there is a possibility of corrosive material. A viable solution to ensure sustainability and economic efficiency could be the recycling of spent acid [41]. Furthermore, there are also technologies for steel slag gravity separation and flotation to recover iron; however, flotation produces a large amount of wastewater in practical applications, and the hypergravity method is still in its infancy [43]. Magnetic separation is the dominant process.

3.2.2. Recovery of Vanadium

In BOF slag, vanadium can be oxidized to V(IV) during leaching [44]. Due to the high value of and demand for vanadium, BOF slag in the form of V₂O₅ containing 2–3% vanadium can also be used as a feedstock for selective recovery of vanadium. Leaching is a method of directly extracting vanadium from steel slag, including the direct leaching method, the roasting leaching method, and other related leaching methods [45,46].

Direct leaching is usually carried out without regard to the roasting process and is mainly carried out using sulfuric acid [47,48]. Using titanium dioxide to produce waste acid as a leaching agent, the extraction rate of vanadium can reach 95%. There are still many limitations and shortcomings in actual production, such as raw material particle size, operating conditions, and the purity of the recovered vanadium. As a result, direct leaching is less widely used in most industrial productions [49].

Roasting leaching is also a commonly used method. Roasting can destroy the spinel structure in steel slag and effectively dissolve vanadium [50,51]. Sodium roasting technology oxidizes vanadium in slag to sodium vanadate by mixing sodium salt additives with steel slag and roasting at high temperature in a furnace [52,53]. In calcium roasting, calcium compounds are used as additives, mixed with vanadium-containing steel slag, and then ground into fine particles after roasting [54,55]. Clean baking reduces exhaust gases and waste water during the baking process. These processes mainly include magnesium salt roasting, ammonium roasting, manganese roasting and salt-free roasting [50]. Immediately after the slag is roasted, the leaching process for the extraction of vanadium is carried out. Water leaching or acid leaching is usually used, and some researchers employ methods such as alkaline leaching and ammonium leaching. By employing mechanical activation [56], microwave-assisted baking [57], and ultrasonic-assisted leaching [58], oxidants are used, as well as other new technologies, which can improve the efficiency of extracting vanadium from steel slag.

3.2.3. Recycling of Chromium

Chromium is added to steel to increase its antioxidant properties. The unreacted chromium ends up entering steel slag at 1–2 wt%, occasionally up to 10.5 wt%. The two general techniques for extracting chromium from BOF slag are alkali leaching and alkali roasting [59]. Prior to extraction, toxic Cr(VI) is converted to stable Cr(III). In alkaline leaching, a leaching agent such as sodium hypochlorite is used in combination with another alkaline reagent. The leaching chemical reaction of chromium includes the following series of chemical reactions.

Cr³⁺ + H₂O ⇔ CrOH²⁺ + H⁺

CrOH²⁺ + H₂O ⇔ Cr(OH)₂⁺ + H⁺

Cr(OH)₂⁺ + H₂O ⇔ Cr(OH)₃⁰ + H⁺

Cr(OH)₃⁰ + H₂O ⇔ Cr(OH)₄⁻ + H⁺

The use of sodium hypochlorite (NaOCl) in combination with a caustic to treat SS slag showed sodium hydroxide (NaOH)leaching efficiency of 68%. The first step in NaOCl addition is to extract chromium in a temperature-controlled step, while water immersion ensures dissolution of the target substance [59].

The second process is water-immersed integrated alkaline roasting, which uses oxidants such as sodium nitrate to promote roasting. Experiments have shown that, by integrating alkaline roasting and water immersion, the chromium recovery rate is as high as about 84%. However, the process has also encountered difficulties; for example, chromium is embedded in an alkaline matrix, which requires the addition of an additional leaching agent [60]. The pH of the solution determines the ionic interaction of the Cr³⁺ ions with water and adjacent ion groups. Cr(OH)₂⁺ predominates in a pH range of 6–8. Cr(OH)²⁺ and Cr³⁺ species predominate under acidic conditions (pH < 6), while Cr(OH)₃⁰ and Cr(OH)₄^- are more prevalent under alkaline conditions (pH > 8) [61]. This also enables chromium oxide to be recovered in the form of crystals [62].

It is important to note that, due to the high toxicity of chromium, chromium recovery must also include a stabilization step. The second step of alkali leaching, such as carbonation, helps to stabilize the residual chromium in the substrate.

3.2.4. Recovery of Phosphorus

The most common physical method for recovering phosphorus is magnetic separation. Studies on synthetic slags have shown that about 80% of phosphorus can be recovered by magnetic separation. On the other hand, a new technique, capillary action, is used to recover phosphorus from BOF slag. Using the principle of liquid flowing into a confined space, the liquid matrix of steelmaking slag is absorbed into a sintered CaO sphere. When the liquid phase penetrates into the CaO sinter, the solid phase containing P₂O₅ and the liquid phase, which is rich in FeO, will be effectively separated. Approximately 87% of phosphorus can be recovered from the P₂O₅ contained in the 2CaO/SiO₂ phase.

Phosphorus can also be recovered chemically. The two most obvious chemical methods are carbon-thermal reduction and acid leaching. In carbonaceauric reduction, phosphorus pentoxide (P₂O₅) in BOF slag is reduced to phosphorus (P2 or P4) by carbon at high temperatures. Phosphorus can also be recovered by sulfuric acid leaching. For the leaching of phosphorus, sulfuric acid is usually used. In order to enhance selective leaching of phosphorus, the slag needs to be oxidized to Fe₂O₃-containing slag. Dissolution of phosphorus in the leachate is controlled by diffusion of the residual layer. The process is highly affected by the cooling rate, and only selective phosphorus recovery can be performed on the slow-cooling slag. Experiments on furnace cold slag show that phosphorus recovery is higher compared to air-cooled slag. In addition, acid leaching can achieve phosphorus recovery of approximately 75% [63]. On the downside, the recovery of high phosphorus content from BOF slag through certain ores or chemicals may in turn limit effective recovery [14]. As a result, steel mills need a fast and efficient process to separate phosphorus from BOF slag on a large scale so that low-phosphorus BOF slag can be efficiently recovered internally.

3.3. Resource Utilization of Converter BOF Slag

The resource utilization of BOF slag includes construction and engineering recycling, and it is used as one of the raw materials in the manufacture of a variety of building materials. In addition, it can also be applied to mine filling, soil improvement, wastewater remediation, preparation of heat storage materials, recycling as smelting solvents, etc., which have broad application prospects. The main utilization approaches can be seen in Figure 4.

3.3.1. Construction and Engineering Recycling

BOF slag has non-negligible applications to construction engineering. Because BOF slag has a certain cement-based component and certain hydration activity, it can be used in the production of cement and cementitious materials. Because BOF slag is hard and wear-resistant, it has a good elastic modulus, good moisture resistance, and strong adhesion, and can be used in the production of concrete aggregates, asphalt mixtures, and cement mortars. In addition to its good physical properties, BOF slag can provide a large amount of iron, which is suitable for the production of artificial reefs. Since the silicate oxide in BOF slag is the main component of CaO-Al₂O₃-SiO₂ or CaO-Al₂O₃-MgO-SiO₂ system glass, with a suitable crystalline phase, the BOF slag can be used to produce ceramics and glass-ceramic.

Supplemental Cementitious Materials

According to reports, the cement industry accounts for 6–8% of global carbon emissions, and the potential for carbon reduction is huge [64,65,66,67]. Through the effective use of supplemental cementitious materials (SCMs), CO₂ emissions per ton of cement can be reduced from 0.81 tons to 0.64 tons [67,68]. Among the different types of metallurgical slag, BOF slag with high alkalinity and global high production (about 260 Mt/y) is one of the most suitable alkaline resources for mineral carbonization [69].

BOF slag contains various calcium silicate minerals [26], with active components such as C₂S and C₃S exhibiting inherent cementitious properties. The high mass fraction (>70%) of divalent metal oxides (CaO, MgO, FeO) in BOF slag triggers synergistic reactions through the “tetrahedral isomorphic effect of silicon” and the “double salt effect”. These mechanisms promote the formation of numerous nanoscale needle-like double salt crystals while generating densely packed, near-amorphous calcium silicate hydrate (C-S-H) gels and zeolite-like phases. The acicular crystals are tightly encapsulated within these gel matrices, creating a hierarchical microstructure that endows the cementitious system with enhanced compactness, mechanical strength (35–50 MPa at 28 days), volumetric stability (autogenous shrinkage < 200 μm/m), and resistance to environmental degradation, particularly demonstrating a 40–60% reduction in chloride diffusion coefficient compared to conventional Portland cement systems [70,71,72].

Numerous studies have demonstrated that combinations of different industrial solid wastes can generate synergistic effects and exhibit complementary advantages across performance metrics due to their distinct physicochemical properties [73,74,75,76,77,78]. For instance, ettringite formation requires reactions between aluminate hydration products and SO₄²⁻ ions. However, BOF slag exhibits limited cementitious activity owing to its low tricalcium aluminate (C₃A) content (<5 wt%) [79]. Furthermore, the effective co-utilization of diverse solid wastes not only reduces disposal costs but also enables large-scale waste valorization, potentially generating economic benefits through resource recovery [80,81,82].

Researchers attribute the synergistic strength enhancement in cementitious systems composed of fly ash, DG, and BOF slag to the enhanced production of hydration products such as calcium silicate hydrate (C-S-H) and ettringite (AFt) [61]. The alkalis in red mud and the sulfates in desulfurization gypsum promote hydration reactions of BOF slag, with tri-component interactions significantly improving the mechanical properties of composite binders [83]. To enhance aluminum content for improved SS reactivity, studies have introduced refining slag (RS) into granulated blast furnace slag (GBFS)-steel slag (SS)-desulfurization gypsum (DG) clinker-free cementitious systems, developing a high-strength GBFS−SS−RS−DG binder achieving 43.0 MPa at 28 days [84]. Under alkaline activation (NaOH), electrolytic manganese residue–BOF slag–blast furnace slag systems demonstrate optimal 28-day compressive strength of 23.0 Mpa [85]. The double salt effect effectively immobilizes Pb and Zn. Clinker-free cementitious materials for lead-zinc tailings (LZTs) backfill have been developed using BOF slag (SS), blast furnace slag (BFS), flue gas desulfurization (FGD) gypsum, and fly ash (FA) [86]. A GGBS-SS-DS-FGDG binder system utilizing desulfurization slag (DS), ground granulated blast furnace slag (GGBS), BOF slag (SS), and FGD gypsum achieves a 748% higher 3-day compressive strength than GGBS-SS-FGDG systems at the optimal DS/SS ratio of 1.0 [87].

Cement

Similar to the principle of producing cementitious materials, because BOF slag has a cement-based component, it has a certain hydration activity. BOF slag is used instead of sand, the proportion of cement mortar accounts for 10–50%, and Ca(OH)₂ is formed by the reaction of f-CaO and SiO₂ to form C-S-H. It is feasible to use BOF slag as a feedstock for the production of Portland cement clinker [88].

Steel slag cement is made of a certain proportion of BOF slag and other functional additives, which can be regarded as raw material for calcining cement preparation [89,90]. The principle is that BOF slag with a CaO content of more than 50% has the potential to replace limestone in cement raw meal [91]. More importantly, the reaction of C₂S and CaO in BOF slag can accelerate the formation of C₃S, with high preparation efficiency and greatly shortened calcination time [92,93]. Researchers conducted a comparative study using two types of BOF slags to evaluate the effects of wet-ground steel slag (WSS) and raw steel slag (RSS) on steel slag cement performance. The work by Dai et al. (2022) [94] revealed that, when incorporating 10% WSS, specimens under constrained curing achieved a maximum compressive strength of 68.5 MPa at 28 days, representing 93% enhancement compared to free-cured specimens, along with optimal impermeability. The constrained curing conditions promoted expansive strain from steel slag hydration, leading to pore structure densification and reduced porosity, thereby improving both the mechanical properties and the microstructural integrity of the cement matrix [94].

Beyond Portland cement, BOF slag has found applications in phosphate-based cementitious systems. Research indicates that the enhancement in early compressive strength arises from the optimized particle size distribution, filling effects, and activation efficiency of slag powder. However, BOF slag powder contains significant inert phases (>40% non-reactive components) that remain largely unactivated, showing minimal participation in hydration reactions [73]. Notably, the hydration reaction between MgO and ADP (ammonium dihydrogen phosphate) exhibits substantially higher exothermicity compared to MgO-KDP (potassium dihydrogen phosphate) systems, generating additional reaction products (e.g., struvite-like phases) that enhance matrix stability. Furthermore, studies reveal that incorporating steel slag powder (SSP) accelerates the early hydration rate of MPC (magnesium phosphate cement). While a marked improvement in long-term mechanical strength is observed, this comes at the expense of reduced early-stage strength. Increasing SSP content from 0% to 30% significantly improves the water resistance of MPC mortar specimens, as evidenced by <5% strength loss after 28-day water immersion [95]. Researchers synthesized iron-calcium phosphate cement (ICPC) by reacting BOF slag powder (SSP) with ammonium dihydrogen phosphate (ADP). The compressive strength of ICPC paste exhibited a non-monotonic trend with increasing SSP/ADP mass ratio: it initially increased, then decreased across all curing ages. Peak strength was achieved at an optimal SSP/ADP ratio of 6.0, attributed to balanced reaction kinetics and densified microstructure formation [96].

BOF slag demonstrates promising applications in biocementation technologies. The calcium and magnesium ions from free CaO and MgO in BOF slag can be acid-leached (e.g., using HCl or CH₃COOH) and repurposed as dual-ion sources (Ca²⁺/Mg²⁺) for microbial-induced carbonate precipitation (MICP). Specifically, the calcium components in BOF slag are chemically convertible to calcium chloride (via NH₄Cl treatment) or calcium acetate (through acetic acid reaction). These derived compounds serve as optimal substrates for ureolytic bacteria (e.g., Sporosarcina pasteurii), driving the MICP process to enhance biocementation efficiency through controlled CaCO₃ crystallization [97]. One of the main advantages of biocement over Portland cement is that it can be produced at room temperature [97]. In addition, biocement produced from BOF slag does not contain chloride and can therefore be used in many situations where traditional cementation methods are not suitable, such as reinforced concrete.

Cement Mortar

Cement mortar, as an indispensable building material in modern society, consumes a large amount of raw materials in its production, especially natural sand [98,99]. Due to its hardness, wear resistance, high crushing value, good physical and mechanical properties, and sufficient stockpile and annual emissions, converter steel slag is considered to be a substitute for natural sand in the manufacture of mortar alternative materials [100,101,102]. While it can also partially replace cement as a supplementary material, its primary role lies in leveraging its superior mechanical properties as a sand substitute, as demonstrated by carbonation-treated slag achieving up to 45% sand replacement in mortars [100] The low content of tricalcium silicate in SS and the high content of inert components result in low hydraulic activity of SS and lower compressive strength when used as an auxiliary cementitious material in the manufacture of mortar and concrete compared to pure Portland cement products [103].

When SS is used to partially replace cement in the production of mortar, the SS fineness has a significant effect on the performance of the mortar. The optimal particle size range for BOF slag in blended mortar (21.75–24.13 μm) increases compressive strength (31.21 MPa) to levels significantly better than those seen in BOF slag mortars of larger or smaller sizes [104].

In addition, polymer-modified waterproofing mortar can be obtained by adding a polymer composition to an ordinary cement mortar. Through the synergistic effect of inorganic and organic materials, the defects of traditional rigid waterproof materials such as low flexural strength, low bond strength, easy cracking, and poor water resistance can be improved. Compared with ordinary cement mortar, the properties of polymer-modified waterproofing mortar, such as mechanical strength and impermeability pressure, have been improved. For cementitious materials, cement, and cement mortar, the low hydration activity of BOF slag limits its application, due to the low amorphous content in BOF slag and the presence of crystalline phases, such as silicic acid γ of dicalcium (γ-C₂S), dicalcium ferrite (C₂F), and RO phase (iron-containing solid solution) solid solutions [23,105,106]. At present, there are three ways to enhance the cementitious properties and macroscopic properties of BOF slag. One is thermal activation. This method involves elevated curing temperatures (typically 60–90 °C) to amplify the slag’s hydration activity. The activation mechanism stems from thermally induced bond cleavage: at high temperatures, the Si–O and Al–O bonds within the slag’s vitreous network structure undergo preferential fracture due to thermal stress. This facilitates depolymerization of the vitreous phases, thereby increasing hydration reactivity and accelerating early strength development [107]. High-temperature curing poses challenges for in situ concrete construction and often results in inferior long-term performance of cementitious composites compared to standard curing conditions (20 °C, RH ≥ 95%) [108].

Another method is chemical activation. By changing the composition of the liquid phase and increasing the number of crystal nuclei and the alkalinity of the liquid phase, the hydration and hardening of BOF slag can be accelerated. Alkali activators, often referred to as reinforcing additives, are used as early hydration accelerators for cement-SS composite binders [109]. When the [AlO₆] tetrahedron in the BOF slag is inactivated, [AlO₆] dissolves in its original position in the form of Al(OH)₂⁺ to form a water-soluble ion, which is combined with the H₃SiO₄⁻ present in the solution, and OH⁻, Ca²⁺, and Na react to form zeolite hydrate [107]. Regarding Na₂CO₃, studies on the hydration properties of BOF slag under the influence of alkali and alkali metal salts such as Na₂SiO₃ show that alkaline activators can promote the gelling properties of BOF slag [108].

A final method is mechanical activation, which improves the fineness of the BOF slag, resulting in an increase in the amorphous phase content and an increase in the early activity of the BOF slag, but a trade-off with grinding costs is required [108]. The finer particles exhibit high reactivity and disperse the expanded components (e.g., f-CaO) in the BOF slag [110]. Ultrafine BOF slag can produce more hydration products, but when the BOF slag content is > 10%, the cement performance is still hindered, and the setting time and water demand also show an upward trend in fine particle size [103].

Asphalt Mixture

At present, alkaline aggregate is commonly used in asphalt mixtures, because asphalt acid, anhydride, and other surfactants in asphalt will have a strong chemical adsorption effect when in contact with alkaline aggregate, and the adhesion is large. Asphalt mixtures mixed with BOF slag exhibit high temperature stability, good slip resistance, good crack resistance, good fatigue durability, permanent deformation resistance, and superior interlocking structure [111,112,113,114,115]. It has been found that the critical load of the BOF slag permeable asphalt mixture increases first and then decreases with a decrease in temperature, reaching a maximum value at −10 °C. When the replacement rate of BOF slag is 50% and the polyester fiber content is 0.45%, the ultimate tensile stress and ultimate tensile strain of asphalt mixture are the largest, and the best low-temperature crack resistance is exhibited [116].

With the promotion of intelligent green pavement and long-life pavement, self-healing asphalt mixtures have attracted attention [117]. There are three main types of accelerated self-healing of asphalt mixtures: microcapsule healing, induction heating, and microwave heating (MH) [118,119]. The latter two are often mixed with BOF slag aggregates. It has been found that the induction heating rate of asphalt mixtures mixed with steel fiber and BOF slag is increased by 34% [120]. Asphalt concrete prepared with BOF slag shows a better ability to store heat and a similar heating rate to basalt asphalt mixtures, but exhibits a significantly slower cooling rate [121]. The high dielectric and magnetic conductivity losses give the BOF slag a potential microwave absorption capacity to migrate heat and repair fractures [122,123]. It has been found that asphalt mixtures with 50% SS content can achieve excellent heating uniformity and microwave de-icing efficiency [124]. When the CPZ temperature of the BOF slag asphalt mixture is high, the self-healing performance of the mixture is better. The content of BOF slag has little relationship with the initial strength of the asphalt mixture, but mixtures with large BOF slag content have strong self-healing properties [125]. Due to the high density and high strength of BOF slag, the stability of asphalt is improved, but at the same time the low temperature crack resistance and water stability are reduced due to the porous structure of BOF slag. The degree of SS substitution is limited (maximum 23.5%). Microwave heating can reduce the micropores in SSAM, which is conducive to self-healing of microcracks. However, continuous microwave cycling increases the proportion of large and extra-large voids, resulting in a decrease in their durability [117]. It is worth mentioning that the incorporation of BOF slag is conducive to the utilization of recycled asphalt pavement (RAP). BOF slag can improve the mixing efficiency of RAP–virgin asphalt in recycled asphalt mixtures [126], reduces electrical conductivity and thermal diffusivity, and increases specific heat [127]. High levels of RAP improve the moisture susceptibility of recycled asphalt mixtures but decrease their fracture resistance, and may offset some durability issues in warm mix asphalt mixtures [128]. Studies have found that 20% RAP achieves the highest fatigue resistance, considering the environmental sustainability of road construction, and a maximum RAP dose of 40% is recommended [129].

Concrete Aggregates

BOF slag demonstrates significant potential as a concrete aggregate due to its superior angularity and abrasion resistance [101], effectively enhancing concrete’s surface roughness and wear durability [130]. The slag’s textured surface morphology and cementitious constituents synergistically reinforce the Interfacial Transition Zone (ITZ) between aggregate and cement paste, thereby improving concrete’s mechanical strength [101]. Some researchers have used BOF slag to partially replace the mechanical behavior and microstructure of concrete with NCA. The experimental results showed that the optimum replacement rate of BOF slag was 50%. At this replacement rate, the 7-, 28-, and 90-day compressive strengths were increased by 3%, 3%, and 19%, respectively. In addition, the use of BOF slag as a coarse and fine material can increase concrete strength, reduce porosity [131], and delay setting time, reducing the heat of hydration and improving concrete flowability [101,132].

Artificial Reefs

Artificial reefs (ARs) are being widely used as a response to the loss of macroalgal forests [133,134]. The use of BOF slag to restore the marine environment is a new type of BOF slag resource utilization technology [29].

Some researchers have found that the decrease in iron content after immersion of artificial reefs prepared using steel slag was caused by excess iron oxide produced by the immersion of BOF slag in seawater. This phenomenon provides a large amount of iron to microorganisms, which promotes their growth and improves the marine environment [135]. Green Artificial Reef Concrete (GARC) was prepared using SS as the main raw material; the density of GARC was 2765.5 kg/m³, and the compressive strength of GARC reached 71.4 MPa at 28 days. After curing in artificial seawater for 240 days, the compressive strength of GARC was 92.5 MPa [136].

With the diversification of materials used in artificial reefs (ARs), the adverse effects of ARs on marine ecosystems is an emerging issue. Although there is a lot of research on the impact of ARs on marine ecosystems [136,137], quantitative evaluation of heavy metal (HM) elution from ARs is limited. HM pollution of the oceans is a concern both locally and globally due to their persistence and toxicity. The effects of ARs made of concrete, steel, and BOF slag on the concentrations of eight HMs (As, Cd, Cr⁶⁺, Cu, Hg, Ni, Pb, and Zn) in the ocean have been studied. The concentrations of the eight HMs in the experiment met environmental standards under conservative experimental conditions [138]. At present, research on the preparation of artificial reefs by BOF slag still needs to be strengthened, and it is necessary to pay attention to the impact of its application on the ecological environment.

For asphalt mixtures, concrete aggregates, and artificial reefs, the volumetric expansion caused by the hydration of f-CaO in BOF slag limits its further utilization. BOF slag has a high content of free CaO/MgO, which can lead to excessive expansion of the concrete [139]. Prolonged or accelerated carbonation and other measures are usually taken to reduce the amount of free CaO/MgO and the risk of swelling [140]. Some scholars use the thermal asphyxiation process to treat the BOF slag by a natural aging method to make the content of f-CaO and f-MgO meet the requirements [141]. Furthermore, weathering is a common solution. However, steel slag aggregates with a greyish-white CaCO₃ product layer formed by weathering are less prone to freeze-thaw damage when used in asphalt mixtures [142]. Even after weathering, there are risks associated with the use of steel slag aggregates in asphalt mixtures. In the future, further research is needed to reduce the limitation of unstable components in BOF slag on its resource utilization.

Glass-Ceramic

Glass-ceramic is a polycrystalline composite composed of glass and crystalline phases [143]. Silicate oxides present in BOF slag are the main components of CaO-Al₂O₃- SiO₂ or CaO-Al₂O₃-MgO-SiO₂ glass. Chromium oxide (Cr₂O₃) and chromium-containing minerals (e.g., MgCr₂O₄) are effective nucleating agents for glass-ceramic [144,145]. In the presence of a nucleating agent, glass-ceramic can be formed by controlled crystallization via heat treatment of glass with the right composition. Fe₂O₃ in BOF slag is not only conducive to the melting of glass, but is also conducive to the crystallization of glass-ceramic, and the content of Fe₂O₃ in BOF slag glass-ceramic was studied by the sintering method [146,147]. As the SS increases, the grains continue to refine [148]. Considering that the ratio of web-forming ions to net unwebbed ions in BOF slag does not form the glass structure well, most studies will choose to add some other oxides as additives to promote the formation of substrate glass, and focus on its mechanical properties [143]. Some researchers used BOF slag as the main raw material and added appropriate oxides (SiO_2, Al₂O_3, Na₂CO_3, and MgO) to prepare wollastonite–pyroxene glass-ceramic in the BOF slag system. In the prepared sample, glass-ceramic with a density of 3.08 g/cm³ exhibited the best mechanical properties, with a flexural strength of 176.21 MPa and a microhardness of 8.81 GPa [143]. When Na₂O, SiO₂, and Na₂SiF₆ are used as additional additives, the optimal flexural strength of the prepared fluorine-containing BOF slag glass-ceramic specimen reached 177 MPa [147].

In general, glass-ceramics have controllable crystallization ability through processes like sintering or two-step crystallization, which can be enhanced by nucleating agents (e.g., Cr₂O₃) to promote crystal formation. However, both methods have non-negligible drawbacks: firstly, the quenching step in the sintering process wastes a lot of heat and produces a lot of waste water. In this regard, Luo et al. [147] found that glass-ceramic with a constant amount of 50 wt% BOF slag was prepared by the melting method; the bending strength of the prepared sample was the highest at 177.76 MPa, and the volume shrinkage was the lowest (0.06%). On the other hand, two-step crystallization production involves two heat treatment steps (nucleation and crystal growth), which is energy-intensive and carbon-intensive. A very promising approach to this is one-step heat treatment, i.e., nucleation and crystal growth at the same temperature. In general, the Tn and Tc of oxide glass are sensitive to the chemical composition of the parent glass. Therefore, optimizing the composition of the parent glass is considered to be a feasible method to reduce the delta T, which can improve the nucleation ability or crystal growth ability of the parent glass. However, in this regard, there is less research on BOF slag glass-ceramic.

Ceramic Materials

BOF slag is mainly composed of various types of metal oxides, and the solid solution also contains a certain amount of glass phase, similar to the composition of ceramic raw materials [149]. The use of BOF slag to produce ceramics can improve the process performance of low water absorption, low porosity, high strength, and so on [150]. This improvement may be due to the presence of alkaline earth oxides in the BOF slag composition, which enhances early curing of tiles and porcelain. Zong et al. successfully prepared ceramics by mixing BOF slag with fly ash [151]. When the fly ash is doped with 15 wt% and the sintering temperature is 1145 °C, the flexural strength is 43.37 MPa. In addition, Pal et al. [152] reported that the use of 40 wt% BOF slag to make porcelain materials can promote the crystallization of pyroxeneand improve the vitrification behavior, with a flexural strength of 80 MPa. However, the introduction of the best BOF slag to improve process performance is still limited to the range of 30–40 wt%. Exceeding this BOF slag weight may involve the emergence of mesophases such as wollastonite and diabase [150].

In recent years, some studies have explored the effect of the CaO/SiO₂ ratio of BOF slag on the phase crystallization and properties of pyrite-sintered materials. Tabit et al. prepared iolite-based binary ceramics from fly ash and BOF slag [153]. However, excess iron and calcium are detrimental to the preparation of ceramics, leading to problems such as deformation, cracking, strength loss, and narrowing of the sintering temperature range [154]. The presence of Fe₂O₃ and CaO in BOF slag often leads to a series of problems such as deformation, strength loss, and surface defects [155,156]. Therefore, it is extremely difficult to prepare ceramic materials from high-calcium and high-speed iron BOF slag. The material composition, microstructure, and preparation process must be reasonably controlled in order to produce ceramic materials that meet the performance requirements [154]. This is also the direction of future research.

3.3.2. Mine-Filling Materials

BOF slag can play a crucial role in mine-filling materials [157]. Its dissolution releases abundant OH⁻, Ca²⁺, silicate, and aluminate ions, which recombine to form hydration products and break the Si-O and Al-O bonds in the slag matrix, promoting gel formation and strength development, Ca²⁺ reacts to produce C-S-H gel material, and water-soluble SO₄²⁻ reacts with activated alumina (present in the form of H₂AlO₃⁻ or AlO₂⁻) and Ca²⁺ to produce microcrystalline AFt. With an increase in age, C-S-H gel and AFt continued to increase, and by 28 days of age, a large number of gels had been formed, filling and wrapping the AFt and unhydrated particles, resulting in a continuous increase in the internal densification of the filler and a continuous increase in the mechanical strength [26]. A large number of hydration products are continuously generated and wrapped on the surface of the raw material particles, and C-S-H and AFt nucleate, crystallize on the surface of the raw material particles, and grow interlaced with each other to form a reticulated structure. BOF slag can improve the compatibility of the filling slurry and increase the strength of the backfill in the later stage [22].

A large number of researchers have used BOF slag as raw material to prepare backfill materials. Zhang et al. [158] used GBFS, BOF slag, DG, and ultrafine tailings as raw materials to prepare backfill materials with optimal ratios to achieve compressive strengths of 6.69, 12.05, 16.36, and 180 MPa on the 3rd, 7th, 28th, and 180th days, respectively. Zhang et al. [159] used the best ratios of backfill materials as raw materials to achieve compressive strengths of 6.69, 12.05, 16.36, and 18.37 MPa. Researchers prepared a new type of mine backfill cementitious material with 75% BOF slag, 16.5% FGD gypsum, and 8.75% FGD ash (the ratio of binder to tailings was 1:4, and the concentration was 70%), and the compressive strength of the specimen reached 1.24 MPa at 28 days and 3.16 MPa at 90 days. Some scholars have studied the preparation of cementitious backfill with fly ash, desulfurization gypsum, and BOF slag. The UCS value of 14 days’ cured filler could reach 2.60 MPa under optimal proportioning [160]. It should be noted that, although most of the mineral phases contained in BOF slag can be considered low reactive components, BOF slag has high relative reactive aluminum phases (C₃A and C₁₂A₇), and the aluminum-containing phases will reduce the C3S/aluminate (C₃A and C₁₂A₇)) ratio when BOF slag is blended into cement with a high percentage of cement replaced by BOF slag, which results in the cementitious system lacking sufficient amounts of sulfate (i.e., insufficient sulfation) [26]. This can lead to rapid depletion of gypsum and rapid precipitation of caliche [27,28]. The BOF slag-based reverse filling method utilizes more BOF slag particles of relatively coarse size as microfillers and uses a small amount of fine cement particles as the main bonding component. The compressive strength of ultrafine silicate cement can reach up to 123.8 MPa at 180 days, with an SPC content of 35% [26].

BOF slag causes prolonged setting time of the filler slurry and significant reduction of the early strength of the filler. And when the BOF slag content exceeded 20 wt%, the curing strength was reduced [157]. The backfill strength properties decreased with an increase in SS value [160,161]. The incorporation of a large amount of BOF slag causes the damage form of the backfill to exhibit tensile damage, with different degrees of loose swelling at the bottom [160]. In addition, the increase in SS content causes the damage mode of the backfill to change from tensile to tensile shear [162], resulting in a composite damage pattern of splitting and shear in the backfill. The addition of BOF slag reduced the degree of hydration reaction in SS-CPB [161]. It cannot be ignored that the swelling problem of steel slag also has a negative impact on the filler. Fillers with a higher dosage of steel slag (above 50 wt%) showed potential binding properties, but the long-term performance is yet to be resolved.

In the future, attempts can be made to mitigate the retardation effect of BOF slag on the early hydration of cement by using coupling between solid wastes, pretreatment of BOF slag, or addition of admixtures. Research can also be carried out on the material proportion of a large dosage of BOF slag, as well as the production process, so as to avoid cracking of the filling body and lower early strength. In addition, in order to achieve the industrial application of steel slag-based cementitious materials in large molds, it is necessary to further carry out research on the high concentration and high fluidity of steel slag-based tailing sand filling slurry.

3.3.3. Soil Amendment

BOF slag has a wide range of uses in agriculture. It can be used as a fertilizer to increase crop yield [4,163], to neutralize soil acidity [164,165], to stabilize heavy metals [165,166,167], and so on.

BOF slag can be classified into various fertilizers (e.g., silicate fertilizers, iron fertilizers, phosphate fertilizers, etc.) due to the presence of a wide variety of nutrients in different concentrations, such as Si, Fe, P, Ca, and Mg [163]. Silicon is the most demanded element for rice growth, and BOF slag, which contains more than 18% SiO₂, can be used as a silicon fertilizer, which helps to improve the quality of the soil for plant growth [168]. BOF slag contains high levels of iron, which when added to the soil can treat iron chlorosis in crops, resulting in 160% and 183% increases in iron content in Japonica and Indica rice, respectively [169]. In addition, the high concentration of phosphorus in BOF slag can provide a source of slow-release, long-term phosphorus for agriculture. Currently, only a few studies have investigated the bioavailability of phosphorus in BOF slag for plant uptake, and more research is needed in this area [4].

BOF slag has also been shown to have the potential to reduce soil acidity, as it acts as a liming agent. This effect occurs due to the presence of calcium silicate and magnesium silicate, which, when applied to the soil, promote greater mobility of SiO₃^2- anions in the soil profile, resulting in higher pH, lower Al³⁺ concentration, and increased base saturation [164]. For nitrous oxide (N₂O), BOF slag promotes complete denitrification by providing electron donors such as ferrous ions (Fe²⁺), which provide more electrons than are required to produce N₂O, an intermediate in the denitrification process, leading to its further reduction to nitrogen (N₂) and inhibiting N₂O emissions. When BOF slag was applied at an application rate of 8 Mg/ha, the cumulative CH₄ and N₂O emission rates were 56.0% and 98% lower, respectively, than the control [170]. BOF slag as a soil amendment in subtropical paddy fields has also been shown to reduce CO₂ emissions during tillage [171]. However, the reduction in CO₂ emissions was not significant when compared to the control later in the season. BOF slag as a fertilizer has been shown to increase yields of crops such as soybean and rice, which hints at the potential for enhanced carbon biofixation through the addition of slag [164,172].

BOF slag can durably immobilize metal ions in heavy metal–contaminated soils [173]. BOF slag is loose and porous, and the main mineral phases are calcium silicate, calcium ferrite, and alkaline oxides, which not only form hydroxide precipitates between alkalis and OH groups produced by hydrolysis of heavy metal ions, but also form heavy metal ions, such as calcium ferrite, through Ca²⁺ in calcium silicate, and the ion exchange of Ca²⁺ in calcium silicate with heavy metal ions such as calcium ferrite generates stable heavy metal silicates and ferrite, which effectively removes heavy metal ions from the polluted solution [165]. Application of BOF slag to Cd-contaminated soil resulted in a significant increase in peroxidase activity in plant roots, thereby reducing the concentration of extractable Cd in the soil [174]. BOF slag converts Cd ions in soil to less decomposable CdCO3, CdO, and Cd(OH)₂ by precipitation and adsorption [175]. Simultaneous solidification/stabilization of mixed heavy metals (mainly Cd, Pb, Zn, Cu, etc.) using BOF slag has been the focus of research [176,177]. Yang et al. (2021) [166] found that Cu, Cr, Pb, and Zn ion fixation and persistence after the addition of BOF slag to acidic contaminated soils was similar to that of lime in the modelling process. Passivation material (SS-BC) prepared by modification of BOF slag using biochar (BC) reduced the bioavailability of heavy metals in the soil. The passivation efficiencies of Pb and Cd were found to be 51.76% and 63.74%, respectively [167]. SS-BC, a heavy metal passivation material synthesized using BOF slag and biochar, showed passivation efficiencies of up to 51.76% and 63.74% for Pb and Cd in soil, respectively [20].

In the future, composite amendments with multiple functions can be developed to meet different types of soil problems and crop needs. By combining BOF slag with other wastes or functional materials, it can have the function of improving soil pH and supplementing nutrients, as well as fixing heavy metals and improving soil structure. In addition, research should focus on the long-term effects of potentially harmful substances in BOF slag on soil and ecosystems and how these risks can be reduced through scientific methods and reasonable dosage control.

3.3.4. Wastewater Remediation

In recent years, BOF slag has received widespread attention in wastewater remediation applications due to its large specific surface area, porous structure, high density, and strong ion exchange capacity. Researchers at home and abroad have confirmed the effectiveness of BOF slag in removing inorganic ions such as phosphorus and arsenic, heavy metal ions such as nickel, chromium, lead, zinc, and copper, and organic pollutants such as ammonia, nitrogen, and benzene.

Adsorbents

BOF slag shows good adsorption effect in the treatment of various types of industrial wastewater containing heavy metal ions, organic dyes, inorganic non-metals, etc. BOF slag has a large specific surface area and a porous surface, which can be used for the removal of pollutants by surface adsorption. The hydroxyl group (-OH) and other groups on the surface of BOF slag dissociate to form adsorption sites centered on negative charges, and cations can be electrostatically adsorbed onto the surface of BOF slag by electrostatic adsorption. These properties enable steel slag to remove pollutants by physical adsorption. At the same time, steel slag contains a large amount of alkaline metal oxides such as Al₂O₃, MgO, CaO, etc., which are hydrolyzed and ionized, releasing a large number of metal ions (e.g., Mg²⁺, Fe³⁺, Ca²⁺, etc.), which are chemically reacted to form a salt precipitate to achieve the removal effect [178].

BOF slag can remove inorganic ions from solution. The removal mechanism of BOF slag is different for the two nitrogenous pollutants. The removal of NH⁴⁺-N by BOF slag adsorption is controlled by external and internal diffusion, while the removal of NO₃-N by BOF slag is mainly a chemisorption process. In order to improve the treatment performance of BOF slag for the removal of ammonia nitrogen, nitrate nitrogen, and other nitrogenous wastewater, many scholars have investigated the effect of modified SS on the removal of nitrogen from wastewater by modified SS [27,28]. Phosphorus removal is mainly through the formation of precipitation, and P in solution can be removed through the formation of calcium phosphate precipitates when the pH is increased above 7.0 [179].

It is difficult to catalyze the removal of heavy metal ions, and their ionic radii are only Å in size, so adsorption is one of the most applicable removal methods [180,181]. The surface of BOF slag is rich in pore structure and active sites, and these properties enable it to adsorb heavy metal ions in water efficiently. BOF slag is highly effective in the adsorption of Cr³⁺and Zn²⁺, with high removal rates of 85.79% and 76.52%, respectively [182]. A novel adsorbent material prepared from acid-modified BOF slag had an adsorption efficiency of up to 98.5% for uranium [183]. BOF slag removed vanadium from synthetic soil rinse wastewater, with vanadium removal up to 97.1% [184]. The iron oxide in BOF slag can provide a rich source of iron for arsenic precipitation or/and adsorption through the formation of a mixture of arsenic-related compounds (e.g., ferric arsenate and ferric arsenite) and iron hydrate. Zeolites synthesized using BOF slag showed high adsorption capacity and removal of Fe³⁺ ions of up to 99.99% [185]. Copper-smelting wastewater was treated by the synergistic action of BOF slag and KMnO₄, and, after 2 h of reaction, the residual concentrations of arsenic and heavy metal ions in the resulting clear water were lower than the prescribed limits for industrial wastewater discharges at a pH of 10 [186]. Wang et al. [187] prepared a novel magnetic adsorbent using BOF slag and Ginkgo biloba, which showed adsorption efficiencies of more than 4% for both Cd(II) and As(V) at pH = 9.0. The geopolymer SSGP prepared using BOF slag and sodium hydroxide, modified by an alkaline activator, showed an adsorption capacity of SSGP of 145.43 mg/g and 69.97 mg/g for Zn and Ni, respectively. This significant increase in adsorption capacity can be attributed to the fact that the specific surface area of the prepared material was increased by a factor of three compared to that of BOF slag, which greatly improved the BOF slag’s surface activity and specific surface area [188]. Lead and cadmium can be removed from water using BOF slag as a filler, where pH has a great influence on the removal of lead and cadmium. The removal efficiency of BOF slag for lead and cadmium was tested in natural water at a pH of 10.5, and the removal of metal pollutants was more than 95% [189]. Sang et al. used 72 wt% of BOF slag to prepare a porous geopolymer (SPG) for the removal of Cu²⁺ from wastewater, and the adsorption of Cu²⁺ by SPG was achieved by electrostatic attraction, ion exchange, and chemical precipitation [190]. As a low-cost and high-efficiency adsorbent material, BOF slag has a promising future for application in the treatment of heavy metal wastewater. Future studies can further explore the modification methods, adsorption mechanisms, and application effects of BOF slag in practical industrial wastewater treatment. Meanwhile, the regeneration and recycling of BOF slag adsorbent materials also need to be paid attention to in order to achieve the sustainable use of resources.

In addition, BOF slag, as a low-cost adsorbent, has a certain adsorption capacity for phenanthrene, naphthalene, and phenol [191,192]. However, the utilization of BOF slag in water treatment is limited due to its low adsorption capacity and instability in acidic solutions. Therefore, it is crucial to identify appropriate modification methods to promote the high efficiency of BOF slag for wastewater treatment or to convert BOF slag into a material capable of degrading pollutants rather than acting as an adsorbent.

Catalysts

BOF slag is applied as a catalyst for wastewater treatment based on the principle of a Fenton-like reaction catalyzed mainly by iron oxides. Some challenging catalysts require the synergistic effect of iron oxide contained in the BOF slag [193].

Researchers have found that the preparation of non-homogeneous catalysts for the electrochemical removal of pollutants using industrial waste can be very effective [194,195,196]. Song et al. [197] prepared a sulfur-zinc-modified kaolinite/SS particle electrode (S-Zn-KSPE) with magnetic properties, tested the cycling stability of S-Zn-KSPEs under neutral and alkaline conditions, and found that they maintained high catalytic activity with > 98% recovery after eight cycles. The particulate electrodes (KSPEs) were prepared from BOF slag, kaolin, and NaHCO₃. They were used for the degradation of norfloxacin (NOR) in wastewater, and the degradation rate of NOR could reach 96.02% in 30 min [198]. Nanostructured Mg_0.04Fe_2.96O₄ catalysts were prepared using BOF slag, and the Cr(VI) removal ability of MFSS was superior to that of some Fe-based catalysts under visible light irradiation [199]. Natural pyrite-steel slag (NP-SS) materials were prepared using natural pyrite (NP) and BOF slag, and the maximum Cr(VI) adsorption of NP-SS was as high as 7.73 mg/g at 15 °C and a Cr(VI) concentration of 800 mg/L [200]. The nanocomposite photocatalyst MIL-53(Fe)/SiO₂ was prepared from LD slag using the thermosolvation method, which could degrade methyl blue under UV irradiation. Under UV irradiation with a methyl blue concentration of 5 mg/L, pH 10 and a photocatalyst content of 0.5 g/L, the methyl blue removal rate was 66.3% [201]. The key to improving the pollutant removal rate in the system is to study the effect of factors such as electrode materials, catalyst type and system, and water conditions on the performance and behavior of the electrocatalytic process [202]. Yu et al. investigated the use of modified BOF slag particles Co-SAM-SCS as catalysts in a non-homogeneous electro-Fenton process coupled with BDD to stabilize the waste leachate COD and NH₃-N in leachate [203]. Fang et al. [204] modified magnetic converter steel slag (MSS) solid waste with Co-MOF derivatives (Co-CN) via heat treatment, successfully synthesizing the MSS@Co-CN Fenton-like catalyst and used the synergistic effect of adsorption and Fenton-like oxidation to remove a representative antibiotic, tetracycline hydrochloride (TCH), from wastewater. MSS@Co-CN exhibited high adsorption capacity (qmax > 416.6 mg/g) and strong Fenton-like catalytic oxidation (80% removal efficiency in 5 min).

In future studies, coupling between the external field and the catalytic process can be considered to further improve catalytic efficiency. The catalytic mechanisms associated with metallurgical slag should also be further investigated, e.g., identification of active sites from metallurgical slag to reveal free-radical-generating reactions and the catalytic role of various metals in slag-derived catalysts.

Pretreatment methods such as acidification and calcination can be used to improve the catalytic performance of slag-derived catalysts. The introduction of an external magnetic field and the use of reducing substances are promising alternatives to enhance the catalytic process and need to be further investigated. Future research should focus on addressing safety and stability issues, developing green and efficient modification methods, improving degradation efficiency, and implementing large-scale treatment of real wastewater.

Photocatalysts

Photocatalytic degradation is considered to be one of the most promising strategies for recycling organic wastewater, because most organic pollutants can be photodegraded into non-polluting products, and the degradation process is sustainable and readily available. In recent years, there are also examples of using BOF slag to prepare photocatalysts for wastewater treatment. A CeO₂-loaded alkali-activated composite fabricated from BOF slag, containing higher iron compound content than blast furnace slag (BFS), demonstrated enhanced photocatalytic activity through CeO₂/FeO heterojunction formation. Transient photocurrent analysis confirmed improved charge separation efficiency, enabling 100% degradation of malachite green within 80 min using 8 wt% CeO₂-loaded specimens under visible light irradiation [205]. Takumi Inoue et al. [206] found photocatalytic degradation of imidacloprid by graphitic carbon nitride/artificial BOF slag (gCN/artCS) composites with optimized mixing ratios of BOF slag under visible light irradiation. An optimized gCN/artCS composite containing 11.58% artCS increased the degradation rate of imidacloprid by a factor of 2.5 in 120 min compared to pure gCN. Shao et al. [207] proposed a strategy based on the simultaneous removal of heavy metal ions and organic pollutants from containers by a BOF slag-derived calcium silicate hydrate (CSH). BOF slag-driven CSH can slowly release Ca²⁺ and OH^- and then chemically bind heavy metal ions for their removal. The adsorption capacities of the prepared CSH for heavy metal ions were all higher than 100 mg/g, and the photodegradation efficiency of the organic pollutant methylene blue (MB) reached 63% after 8 hours of low-power visible light irradiation using the prepared CSH. The CSH was also able to remove heavy metal ions from the container with the help of BOF slag.

Heavy metal ions usually coexist with organic pollutants in real wastewater systems, when the presence of heavy metal ions has a significant negative effect on the photocatalytic degradation of organic pollutants [208]. However, achieving simultaneous removal of both heavy metals and organic pollutants in a single reactor and within a single-step process remains a major challenge, with limited studies addressing this issue.

3.3.5. Thermal Storage Material

BOF slag has a high specific heat capacity value (up to 0.95 J/(g·K)), good thermal conductivity, and a good friction coefficient, which makes it a very promising material for thermal storage [209,210]. BOF slag-based thermal storage material exhibits excellent thermal cycle stability, high efficiency, and low cost, indicating broad application prospects [211]. Thermal energy storage (TES) includes sensible heat storage (SHS), latent heat storage (LHS), and thermochemical heat storage (TCHS) [212].

Sensible heat storage (SHS) is the most mature technology and has been widely commercialized. Currently, most of the studies on SHS materials are on electric furnace slag. BOF slag, as a sensible heat material, remains stable until 1200 °C and has good thermal cycle stability. The thermal properties of BOF slag were further enhanced by Na₂CO₃ activation, and the thermal storage density of the resulting modified material reached 997.0 kJ·kg⁻¹ (400–900 °C), with thermal conductivities of 1.331, 1.323, and 0.889 W·(m·K)⁻¹ at 25, 250, and 500 °C, respectively [213]. By adding magnesite and quartz to BOF slag, a sensible thermal storage material with good thermal shock resistance was successfully prepared. Its heat storage density was 1000 J/g in the range of 25 °C to 800 °C [214]. Concrete–BOF slag thermal storage material with BOF slag as an aggregate has good thermal stability at 700 °C and a specific heat capacity of 1.85 J/(g·K) at 500 °C. However, this material has a low thermal conductivity of 0.7 W/(m·K), and a small percentage of BOF slag was used [215]. Zhang et al. [216] prepared a novel sensible heat storage material using BOF slag as the main component in the 50–1000 °C range. Its compressive strength was stable at about 80 MPa, its thermal conductivity was 1.13 W/(m·K), and its thermal density was 1222.168 J/g. After 300 thermal cycles, the morphology of the specimen, 94.4% of the compressive strength, 98.8% of the thermal conductivity, and the thermal density were almost unchanged, which provided a new low-cost method for the preparation of BOF slag heat-exposing material. However, sensible heat storage suffers from the problems of low heat storage density and large temperature variation, which cannot achieve efficient heat storage [217].

Phase-change (latent heat) thermal storage has the advantages of high heat storage density and stable temperature change during the heat charging and releasing process, which is one of the potential research directions of current thermal storage technology [218]. Latent heat storage absorbs and releases heat through the phase-change process of phase-change materials (PCMs) [217]. Although research on C-PCMs started in the 1990s, not enough studies have been conducted on C-PCMs prepared with inorganic materials as support materials, especially in medium- and high-temperature phase-change materials. Liu et al. [10] firstly used BOF slag as a support material using polyethylene glycol (PEG), sodium nitrate (NaNO₃), and sodium sulfate (Na₂SO₄) to prepare a series of composite phase-change materials (C-PCMs). Compared with pure phase-change materials, the three C-PCMs reduced the subcooling degree by 2.64 °C, 4.53 °C, and 0.79 °C and increased the thermal conductivity by 172%, 54.9%, and 82.4%, respectively, and the latent heat retention rate was still more than 97% after 100 thermal cycles, which provided good phase-change thermal storage performance and thermal reliability. Wang et al. [219] prepared a modified phase-change material (C-PCM) comprising BOF slag–K₂CO₃ phase-change composites (PCC), which can be used in high-temperature concentrating solar power systems. They found that the composite has a melting point of 880 °C and a latent heat of ∼140 J/g and maintains good shape stability and thermal properties under multiple thermal cycles. A series of BOF slag–KNO₃ PCCs were prepared by hybrid sintering for use in medium- to high-temperature TES systems, and the composites maintained high compressive strength (30–35 MPa) and Young’s modulus (1600–1700 MPa) when the KNO₃ content ranged from 38 wt% to 50 wt%. The latent heat of the composites with 50 wt% KNO₃ content was 43 J/g at 300–350 °C, and the thermal conductivity was about 0.68 W/(m·K). After 48 thermal cycles, the latent heat of the composites decreased slightly by 2.5–3.7 J/g compared with that of the composites before thermal cycles, and the thermal conductivity increased by 16–25% [220]. Steady-state phase change materials (SSPCMs) were prepared by the cold compression–hot sintering (CCHS) method using SS and CS as skeletal materials (SM) and NaNO₃ as a phase-change material (PCM). The results showed that, when the mass ratio of SS, CS, and NaNO₃ was 2.5:2.5:5 (SC4), the mechanical strength of the sample SC4 was 131.2 MPa, and the optimum thermal energy storage density in the temperature range of 100–400 °C was 371.1 J/g. The maximum thermal conductivity of the sample after 1307 heating and cooling cycles was 1.263 W/(m·K) [217]. However, phase-change materials have drawbacks such as poor thermal conductivity, subcooling, and leakage during phase change [221], which are still to be solved.

Thermochemical energy storage (TCES) is currently under development as an alternative to sensible heat for storing energy in photovoltaic power plants. One of its most promising technologies is the calcium loop (CaL) process, which relies on the carbonation/calcination reaction of CaO [222]. In addition to the commonly used limestone, BOF slag is an abundant calcium-based material that can be used to store CSPs via the CaL process. BOF slag has a similar heat capacity to solar salt and has a higher thermal conductivity (1.4 W/(m·K) for BOF slag and 0.52 W/(m·K) for solar salt). Perejón et al. [223] studied the behavior of calcium-rich BOF slag under CaL conditions. When treated with acetic acid, this non-toxic, widely abundant waste produces CaO-rich solids, with conversion rates stabilizing around 0.8 in successive carbonation/calcination cycles under these CaL conditions. The calcination temperature of the regenerated CaO is significantly lower compared to limestone. In addition, the multi-cycle activity of some of the slags tested under the relevant CaL conditions of TCES remained high and stable if the treated samples were filtered. This process is used to remove silica particles, which helps to reduce the porosity of CaO produced by calcination in the mesoporous range, thereby alleviating pore clogging to mitigate inhibition of CaO carbonation properties [224]. Thermochemical thermal storage has a large heat storage density. However, the application of thermochemical thermal storage is immature worldwide, and unregulated application may cause environmental pollution [217].

4. Conclusions and Prospects

The resource properties of steel slag determine its possible uses. Steel slag contains useful elements and has good hardness, good abrasion resistance, and high alkalinity, which are compatible with a wide range of production needs.

However, steel slag has a number of unfavorable characteristics that limit its recycling and reuse. Steel slag has a complex composition, low hydration activity, poor gelation properties, high density, highly crystalline structure, and high iron content, is difficult to grind, and has poor stability and high metallic iron content. Among them, the poor volumetric stability of steel slag and high metal iron content seriously hinder its application in building materials, leading to expansion, cracking, milling difficulties, increased energy consumption, poor particle homogeneity, and rusting issues during processing, as well as the difficulty of milling and the increase in energy consumption, poor particle homogeneity, the use of rust, and other phenomena in the process. Combined with foreign experience in the development of steel slag disposal and China’s national conditions, the use of building materials is the main way to consume steel slag on a large scale, so it is very important to carry out appropriate treatment of steel slag.

China’s solid waste production is high and varied. After pretreatment, steel slag is suitable for the preparation of solid-waste-based cementitious materials using a multi-solid waste synergistic approach. In the context of “double carbon”, the synergistic use of multiple solid wastes has become an important development direction. Under the guidance of the theories of “compound salt effect” and the “four-coordinate isomerization effect of silicon”, solid-waste-based cementitious materials can be produced in a mature manner.

The central role of steel slag in the solid-waste-based gelling material system stems from the high concentration of OH- and Ca²⁺ generated by its hydration reaction, which together build up an alkaline environment and provide a calcium source, laying the foundation for depolymerization of the slag vitreous network and subsequent polymerization of the gel phase (C-S-H, C-A-S-H). To accelerate this process, a key role can be played by the introduction of, e.g., desulfurized gypsum (DG), through a synergistic sulphate–aluminate mechanism: the SO₄²- in DG reacts with reactive Al₂O₃ in steel slag/slag to form calcite alumina (AFt), which consumes Al³⁺ in the liquid phase and breaks the [SiO₄]⁴-AlO₄⁵-network structure of the slag, significantly reducing the activation energy of depolymerization. The released SiO₄⁴- is reconfigured into three-dimensional silica-oxygen chains driven by Ca²⁺, while the symbiosis of AFt and C-S-H forms a “backbone-filler” composite structure, which enhances the compactness of the gel phase. The continuous release of OH- from steel slag maintains the system pH at >12.5, which inhibits the conversion of AFt to monosulfur-type aluminum sulphate (AFm) and ensures the stability of early strength development. This makes the various properties of solid-waste-based cementitious materials close to those of ordinary silicate cement, especially in the two aspects of application in marine concrete and as an underground cementing and filling mining binder, which have more obvious advantages than ordinary silicate cement in terms of performance and cost.

On the other hand, solid-waste-based cementitious materials are still mainly used in road and floor concrete construction. The application of solid-waste-based cementitious materials in housing structures still needs more structural mechanical parameters, as well as demonstration projects to prove its worth. The application of steel slag in the common silicate cement and concrete industry chain is yet to gain further significant advantages in terms of value. The structural durability and safety of concrete prepared from solid-waste-based cementitious materials with ultra-high dosages of steel slag for long-term service is yet to be further studied in depth and confirmed repeatedly.

Resource utilization of steel slag should be aimed at absorbing the huge output of steel slag, drawing on foreign experience, and should be focused in two directions: firstly, the construction and building materials industry, the use of cement, concrete admixture, pavement and building materials products, which not only has a wide scope of application, but also has a large demand, and is capable of absorbing the huge output of steel slag; and secondly, the development of new products of steel slag for the application and improvement of the marine environment, which can be applied to China’s 1.8 million kilometers of coastline and a large number of coastal steel enterprises.

Considering China’s background conditions applicable to the synergistic use of multiple solid wastes, steel slag should be applied more vigorously in marine concrete, and the scale of industrial application should be continuously expanded. In addition, the application of steel slag in the field of construction and building materials should be vigorously researched, research on the effective treatment process of steel slag should be increased, existing technology should be continuously researched and optimized, relevant standards should be established and perfected, acceptance by enterprises and the public of its application should be improved, and an application scheme suitable for China should be pioneered.