In recent decades, a critical challenge facing modern society is the surge in greenhouse gas (GHG) emissions, leading to climate change with a global temperature increase of 0.8°C over the last century. Anthropogenic activities, including deforestation, fossil fuel consumption, and the meat industry, primarily contribute to the rise in these gases, notably CO₂. With its concentration reaching 419 ppm in 2021 (compared to 360 ppm in the 19th century), urgent measures are required to address this environmental concern (Azadi et al., 2019).

Given this scenario, it becomes imperative to explore alternative strategies for mitigating CO₂ emissions and reducing dependence on fossil fuels. Carbon capture and storage (CSS) technology emerges as an economically feasible strategy, potentially preventing the emission of eight billion tonnes of CO₂ by 2050 (IEA50, 2021). This technology encompasses geological, oceanic, and mineral storage, with carbon mineralization being a key component.

Carbon mineralization involves a reaction between CO₂ and Ca and Mg-minerals, forming stable carbonated products (CaCO₃ and MgCO₃) and providing a permanent solution for CO₂ storage. This process accelerates the natural weathering of rocks, transforming it from a millennia-long process to mere minutes or hours. The resulting by-products are stable solids with long-term storage capacity (Sanna et al., 2014).

Direct carbonation (Lackner et al., 1995) occurs in one step through a direct gas-solid or gas-liquid-solid reaction. While effective, it requires high temperature and pressure for acceleration. In contrast, indirect carbonation involves two reactions: the extraction of metal ions (mainly Mg and Ca) from corresponding containing phases and subsequent carbonation between CO₂ and the resulting solution. This route is more viable owing to its gentle reaction conditions, elevated carbonation effectiveness, and the generation of high-value secondary materials (Lee et al., 2021).

Notably, industrial wastes like steel slag, with high alkalinity and reactivity, are suitable for this process. The carbonation potential of these wastes for CO₂ sequestration was evaluated suggesting that mineral carbonation could reduce global anthropogenic CO₂ emissions by 12.5% (Pan et al., 2020). Waste materials such as cement waste, red mud, municipal incinerator ash, and steel slag are viable for the carbonation reaction, addressing both environmental concerns and waste disposal challenges (Izumi et al., 2021).

Steel slag, a by-product of steel manufacturing, has garnered significant attention for its potential applications in various sectors, particularly in construction. The importance of steel slag lies in its ability to serve as an alternative to natural resources, which are becoming increasingly scarce due to overexploitation. Its utilization in construction offers a multifaceted approach to resource conservation and environmental sustainability (Sun and Wang, 2024). Substituting traditional aggregates can effectively conserve finite natural resources that are both unevenly distributed and rapidly diminishing (Zhang et al., 2024). This effort holds particular significance in the context of sustainable development, where the responsible stewardship of natural resources is paramount and practices to mitigating the environmental impact associated with conventional construction activities are fundamental. In addition, the extraction and processing of natural aggregate entails significant energy consumption and environmental disturbance, including habitat destruction and landscape alteration. By reducing reliance on these materials through the utilization of steel slag, construction projects can minimize their ecological footprint and lessen pressure on sensitive ecosystems. Finally, repurposing, and harnessing steel slag can yield significant economic advantages, curbing waste and diminishing expenses linked with industrial by-products management.

Steel slag is produced in large quantities, with availability depending on the production of steel in each region.

As per the U.S. Geological Survey, specific data regarding actual ferrous slag production in the U.S. is not available. However, it is estimated that domestic slag sales reached approximately 17 million tons, valued at around $460 million in 2021. This slag was processed by 28 companies serving operational iron and steel factories or reprocessing previously deposited slag wastes at approximately 124 dedicated plants across 33 states (Cris Candice Tuck, 2022). In the context of India, each tonne of steel production generates about 200 kg of steel slag (Ministry of India, 2023). Owing to the lack of efficient disposal methods for steel slag, substantial slag piles have emerged around steel plants, evolving into a significant source of water, air, and land pollution (Mayes et al., 2008).

In Europe, an annual generation of approximately 45 million tons of slag (originating from iron and steel waste) is reported (EUROSLAG, 2024). This manufacturing originates from the procedures of ore conversion into iron, the conversion of hot iron into steel, and the melting of scrap in an electric arc furnace, or from the subsequent refining of crude steel.

It is crucial to underline that the availability of steel slag can depend on various conditions, such as the rate of steel production, the type of steel being manufactured, and the methods employed for slag disposal. Consequently, the precise availability of steel slag may fluctuate over time and across different locations.

Calculations indicate that utilizing 4.7 tons of steel slag could result in the absorption of one ton of CO₂, producing 2.3 tons of CaCO₃. If the entirety of the slag were employed for CaCO₃ production, it could lead to the consumption of 53 million tons of CO₂ and the generation of 120 million tons of CaCO₃ (Eloneva et al., 2012). Other projected statistical estimates suggest that utilizing all steel slag for mineral carbonation could result in the potential sequestration of between 138 and 209 million tons of CO₂ annually, constituting approximately 9.1%–10.4% of the iron and steel industry’s total CO₂ emissions (Pan et al., 2017) and evaluate that the total CO₂ potential for mineralization from 2020 to 2,100 will fall within the range of 26–42 gigatons (Myers and Nakagaki, 2020).

Despite the promising potential of carbon mineralization, its industrial applications face obstacles related to energy and cost consumption. Overcoming these challenges requires a focus on recovering value-added by-products derived from carbonation and generating revenues. The carbonated products can find applications in construction materials, paper, and paint filler. This review focuses on offering a comprehensive overview of the scientific literature concerning steel slag carbonation. It delves into ex-situ mineral carbonation methods, analyses slag properties and examines their effects on dissolution and carbonation mechanisms. Additionally, it outlines the potential reuse of carbonated slag in the cement industry, emphasizing how the carbonation process can enhance the hydraulic properties of this by-product. Finally, it also highlights the advantages and challenges associated with the diffusion process. The work is not overly specialistic because it is directed towards all scientific communities engaged in exploring available circular economy strategies to mitigate greenhouse gas emissions.

To investigate the literature data about steel slag carbonation, text research was addressed to academic journals and scientific publications by Scopus.

Relevant keywords in the abstract and paper titles “steel slag” and “carbonation” were used. 437 papers were found on January 18, 2024. Among them, 25 works are review papers.

Following the bibliographic search, a cluster analysis was conducted by VOSviewer software ¹ (van Eck and Waltman, 2010). This enables the exploration of co-occurring textual data derived from the bibliometric database, facilitating a systematic analysis of the literature. The results are reported in Figure 1, showing a two-dimensional depiction of the research field, where the keywords are represented as circles, with closely correlated terms positioned near each other. The size of the bubbles indicates the number of publications where the specified term (or keyword) is mentioned.

Figure 1 shows that keywords have been separated into 6 clusters. The initial group, depicted by red bubbles and comprising 75 items, primarily centers on carbonation reactions, with a focus on the kinetics mechanism. The second group (68 items) highlighted by green circles, is dedicated to the process’s environmental effects and the possibilities of reusing the by-products, by addressing them to the cement industry. The third cluster (59 items) represented by blue bubbles contains keywords addressed to contaminations and pollution, such as emission control and metal leachability. The fourth group (49 items), highlighted by yellow bubbles, is addressed to reaction optimization, with terms such as activation energy, concentration, composition, and enzyme kinetics.

Clusters 5 (violet) and 6 (azure) are more devoted to metallurgical fields, with keywords related to steel production, such as arc furnace, emissions, and flue gas. These last two clusters highlighted correlations that are out of the scope of this work.

Drawing insights from the cluster analysis results (refer to Figure 1), an in-depth examination of the literature was conducted. The formulation of the work presented below emerged through a reading of the selected full-text articles.

Steel Slag Characterization section characterizes steel slag by mineralogical and chemical composition, Carbonation and Reaction Parameters section describes the carbonation reaction and the influence of reaction parameters, Kinetics Models section compares the two main kinetic models, proposed by literature data, Methodologies to Improve the Mineral Carbonation Efficiency section describes different methodologies to improve the carbonation degree and Carbonated Slag Application in the Cement Industry section takes into account the reuse of carbonated steel slag in the cement industry as aggregates and supplementary cementitious material and as soil stabiliser, Advantages and Limitations of Steel Slag Carbonation section resumes the advantages of carbonation process for steel slag and Conclusion and Perspectives section concludes the manuscript.

Steel slag is a waste of steel plants, comprising various types such as Electric Arc Furnace (EAF) slag, Argon Oxygen Decarburization (AOD) slag, Basic Oxygen Furnace (BOF) slag, Ladle Refining Furnace (LF) slag, and Blast Furnace (BF) slag (Zhang et al., 2023). The categorization is based on the diverse steelmaking procedures employed. Figure 2 shows what the steel slag looks like after its production. Indeed, the chemical and physical properties of steel slag are influenced by different production conditions and feedstock inputs. For example, in the process of EAF steelmaking and oxygen converter steelmaking, CaO and MgO are added to the smelting furnace to remove dangerous elements such as P and S in molten steel. Oxygen is used to remove impurities and carbon into Basic-Oxygen Furnaces and Electric Arc Furnaces. By this treatment, elements such as Al, Si, Mn, and P are oxidized to respective oxides. The last step of the steelmaking process is the refining by desulfurization process to remove impurities, oxygen, nitrogen, hydrogen, and carbon in the ladle furnace, where LF slag is a carbon by-product (Humbert and Castro-Gomes, 2019; Luo and He, 2021; Zhang et al., 2023).

Steel slag encompasses various valuable components, including 24%–65% CaO, 10%–32% SiO₂, 3%–20% Al₂O₃, 0%–37% Fe₂O₃, and 3%–20% MgO and the chemical composition, based on the type of slag, is reported in Table 1. As illustrated, the chemical composition is influenced by the origin of slag. EAF and AOD slag may have elevated levels of Cr, posing a risk of significant pollution if it is directly discharged into soil and water resources (Chen et al., 2021a; Zhang et al., 2023).

CO₂ sequestration is influenced by the mineralogical composition of steel by-products (see Table 2).

Except for BF slag, which is completely amorphous, other slags contain mainly Ca and Mg-phases as larnite, periclase and Ca-Mg-Al silicate. The mineral composition of steel slag primarily consists of 15%–25% dicalcium silicate (Ca₂SiO₄, C₂S), 20%–25% tricalcium silicate (Ca₃SiO₅, C₃S), 40%–45% RO (inert phases and combination between Mg, Fe and Ca), along with small quantities of free CaO and MgO (f-CaO and f-MgO). The primary constituents of Electric Arc Furnace slag (EAF) are CaO, SiO₂, and Fe₂O₃. In BOF and EAF slag Fe-phases are identified as FeO or Fe₂O₃ due to high FeO quantity. Argon Oxygen Decarburization slag (AOD) contains a substantial amount of Cr₂O₃, and its direct discharge poses a significant threat to water and soil resources (Chen et al., 2021b).

Mineral compositions of steel slag have a considerable influence on CO₂ sequestration and the main reactive species are periclase (MgO), calcium silicate (C₃S and C₂S) and merwinite. Thermodynamics calculations demonstrate that the carbonation of these phases is a spontaneous process (∆G is negative) at standard temperature and pressure as reported in the formula (1) (Ragipani et al., 2021; Zhang et al., 2023): 1 / 2 2 CaO • SiO 2 s + CO 2 g → CaCO 3   s + 1 / 2   SiO 2 s (1) ∆ H ° = − 116.2  kJ  mol − 1 ∆ G ° = − 90.1  kJ  mol − 1

Carbonation reaction occurs in nature but its kinetic is slow, also requiring millions of years. It is divided into two pathways, as shown in Figure 3. Direct carbonation happens when a step reaction can be highlighted, whereas, in the indirect process, two subsequent steps can be enhanced: extraction of alkaline metals by acidic solvents from steel slags and carbonation.

Generally, there are models used to depict the carbonation reaction, namely, the surface coverage model and the shrinking core model as shown in Figure 4 (Pan et al., 2014; Li et al., 2018; Chen et al., 2020b).

The coverage model proposes that the reaction rate is directly linked to the portion of the surface covered by the reactant. As the reaction progresses over time, the active surface area remains uncovered by reaction products until reaching maximum conversion.

The phase containing calcium reacts with CO₂, forming calcium carbonate that covers the active surface of basic oxygen furnaces, leading to surface coverage.

The model shows how ions compete when they dissolve or form solids, how the area that can react changes over time, and how the reactions on the surface affect the movement of matter between the liquid and the solid (Ragipani et al., 2021; Wang et al., 2023a). Additionally, it accounts for the influence of variability in size distribution and diverse morphology in shaping the changes in the active surface area over time.

On the other hand, the shrinking core model has been employed to determine the step that predominantly controls the rate of particle reactions. (Castellote et al., 2008). The primary premise of this model is that the reaction initiates on the outer surface of the solid material, advances through a narrow front, and eventually permeates the entire solid, forming a fully reacted product known as the “ash” layer.

Subsequently, the rate of the reaction is controlled by either the chemical reaction transpiring at the interface between the reacted and unreacted sections or the diffusion of reactants through the product layer.

Within the shrinking core model, an initially unreacted core progressively diminishes in size as the reaction proceeds (Tu et al., 2015). Furthermore, the creation of small calcite crystals on the surface of the solid forms a protective layer that envelops the reactive particles. This layer serves to shield the particles from further reactions (Wei et al., 2021). Initially, the chemical reaction governs the process, particularly in the initial phase. However, as the reaction progresses, it transitions into diffusion through the porous layer, becoming the rate-limiting step. Carbonate conversion occurs at a notably slower rate compared to leaching conversion and is found to be constrained by diffusion through the calcium carbonate layer that forms. The apparent activation energy for the aqueous carbonation of steel slag is determined to be 4.8 kJ mol⁻¹. This indicates the amount of energy required to initiate the reaction (Tu et al., 2015).

Moreover, both models are general models, then the actual kinetics may vary depending on the specific conditions and composition of the steel slag (Tu et al., 2015; Ragipani et al., 2021; Wang et al., 2023a).

The main methodologies to improve the efficiency of mineral carbonation are the use of additives (chelating agents, HCl, CH₃COOH, and HCOOH) and altering conditions such as change of solid-to-liquid ratio, carbon dioxide concentration, pressure, temperature, and particle size (Wu et al., 2017; Yadav and Mehra, 2021). These factors can accelerate the kinetic of mineral dissolution, which is the lowest step, favouring the silicate dissolution.

The cement industry is one of the major contributors to the environmental impact of energy consumption and emission of GHGs, including carbon dioxide (Mei et al., 2022). This industrial sector contributes 7.4% of global CO₂ emissions since production of 1 kg of cement releases 0.5–0.7 kg of CO₂ into the atmosphere (Kim et al., 2022; Younsi, 2022). Since cement is an essential component of concrete, its production is responsible for at least 70% of GHGs from the manufacture of concrete. To reduce the CO₂ emissions from concrete products, cement industries have implemented many solutions such as using supplementary cementitious materials (SCMs) as partial replacement for clinker, the main component of Portland cement, or cement during concrete production. Supplementary cementitious materials can be different by-products such as fly ash, steel slag and cement wastes (Younsi, 2022).

Steel slag has been proposed to be employed for road filler production. Nevertheless, the inclusion of f-CaO and f-MgO in steel slag and the low content of hydraulic components, such as C₂S and C₃S, can induce delayed expansion and cracking in cement-based materials, leading to prolonged volume instability (Li et al., 2022). Moreover, recent studies have indicated significant alterations in the mechanical properties of steel slag and volume instability and a decrease of heavy metals leaching, following carbonation treatment, making it a possible supplementary cementing material or aggregate in concrete and cement applications (Mo et al., 2017; Dong et al., 2021). The accelerated carbonation eliminates the expansive compounds like f-CaO/MgO in fresh slag enabling the potential of these by-products as building materials (Li and Wu, 2022; Wang et al., 2023).

Some opportunities for reuse in building applications are reported in the following.

Leveraging steel slag CO₂ sequestration can provide economic, environmental, and social advantages:

1. Economical carbon capture: the steel slag reuse obviates the necessity of establishing a new carbon capture facility, thereby reducing the costs of CO₂ sequestration (Sarperi et al., 2014).

Although promising advantages, some challenges necessitate careful consideration and innovative solutions:

1. Chemical heterogeneity: the chemical composition of steel slags is inherently diverse, introducing significant complexities in accurately modelling the dissolution kinetics. This heterogeneity poses a challenge to researchers seeking to understand and predict the behaviour of steel slag during carbonation, demanding sophisticated modelling approaches (Ragipani et al., 2021).

Addressing these challenges is critical for establishing steel slag carbonation as a viable and widely adopted industrial process.

To apply the carbonation process at an industrial scale, the sustainability and possible impacts of this technology must be studied focusing on the process’s energies. To quantify the impact a life cycle assessment is the instrument. However, only limited studies can be found (Suer et al., 2022; Watjanatepin et al., 2023) and generally, they are referred to as preliminary results.

The mineral carbonation process mimics and accelerates natural weathering, where rocks break down over an extended period due to weather conditions, including the reaction of CO₂ with specific rocks to form carbonates. However, the natural process is too slow to significantly impact atmospheric CO₂ levels in a human timescale. Additionally, the abundance of magnesium and calcium silicate deposits in the Earth’s crust makes mineral carbonation a potentially scalable solution for CO₂ sequestration.

This study provides insights into the steel slag carbonation process, highlighting its advantages and challenges. While it emphasizes the potential for both CO₂ sequestration and waste valorisation, it also underscores the need for additional research to address challenges and make the technology economically feasible.

Direct gas-solid carbonation is a simple process where CO₂ gas reacts directly with solid metal oxides to produce carbonates. The specific reactions vary depending on the feedstock.

In contrast, indirect carbonation offers benefits in terms of the superior purity of the resulting product. However, it is a more intricate process that may demand more energy due to the supplementary leaching step. Additionally, the challenges of recycling extractants contribute to increase costs in the carbon mineralization process.

Various approaches to accelerate carbonation, such as employing high pressure and/or high temperature, increased stirring speeds, and additives, have been explored. However, these techniques may lead to higher energy consumption and could affect the carbonation reaction. Hence, optimizing the steel slag carbonation process with minimal heat and power usage is essential but presents a significant challenge for direct carbonization.

While research on the reuse and recycle of steel slag after carbonation is not diffuse, the fundamental chemistry of mineral carbonation is well understood. Challenges, including the requirement for substantial starting material, slow reaction rates of natural minerals, and energy demands, need to be addressed.

Prospective research efforts could centre on enhancing the carbonation process for steel slag and optimizing parameters for scaling up applications.

Another big challenge of this topic is the reuse of carbonated steel slag in the cement industry. As different studies show, the introduction of steel slag as SCMs reduces the compressive strength of the cement and this reduction is proportional to the percentage of replacement (Zhang et al., 2011; Srivastava et al., 2023). Otherwise, the carbonation treatment improves the hydraulic properties of steel slag making them suitable for this use. It is mandatory to study deeply the mechanism of hydration of the slag in the matrix cement focusing on the possible leaching of heavy metals and the compressive strength.

In conclusion, mineral carbonation holds promise for mitigating climate change through CO₂ sequestration and adding an extra value to steel slag. Despite existing challenges, the potential benefits make it a compelling area for further research, and it is a way to favour a circular process.