Mineral Sequestration of CO₂ Using Metallurgical Slags: A Brief Literature Review

Abstract

The metallurgical industry is a significant contributor to CO₂ emissions. One approach to mitigating these emissions is the implementation of carbon capture, storage, and utilisation (CCSU) technology. In this paper, we present a literature review on the mineral sequestration of CO₂ using metallurgical slags within the CCSU framework. Both direct and indirect carbonation processes are examined, with a focus on key process parameters. Additionally, the utilisation of by-products of the process, a crucial aspect of CCSU technology, is also discussed.

1. Introduction

In recent decades, climate change has become one of the most serious global challenges. According to the IPCC report Climate Change 2022: Mitigation of Climate Change [1], anthropogenic global net greenhouse gas (GHG) emissions reached their highest level in history between 2010 and 2019, with the fastest growth recorded in the transport and industry sectors. The steel industry is a particularly problematic area, as it remains one of the largest sources of CO₂ emissions due to its main use of coal as a reducing agent [2,3].

The steel industry is responsible for approximately 2.8 gigatons of CO₂ emissions per year, accounting for about 8% of global energy-related emissions, or up to 10% when indirect emissions from electricity generation are included [4]. The International Energy Agency (IEA) predicts that the iron and steel sector will continue to contribute a significant proportion of emissions until 2050 [5], indicating an urgent need to implement emission reduction solutions in this industry.

In response to this challenge, technologies for carbon capture, storage, and utilisation (CCUS—carbon capture, utilisation, and storage) are receiving increasing interest, in particular mineral sequestration, which involves permanently binding CO₂ in the form of stable carbonates. Mineral carbonation is defined as a process based on the reaction of CO₂ with metal oxides MO in Equation (1), resulting in the formation of insoluble carbonates and the release of heat in varying amounts [6]:

MO + CO₂ ⟶ MCO₃ + heat

(1)

For the two most common oxides, CaO and MgO, these reactions can be described by Equations (2) and (3) [7,8]:

CaO + CO₂ ⟶ CaCO₃ + 179 kJ/mol

(2)

MgO + CO₂ ⟶ MgCO₃ + 118 kJ/mol

(3)

Using mineral carbonation as a CO₂ utilisation method is advantageous since it is a naturally occurring process forming thermodynamically stable [9], environmentally neutral products that can be used economically.

This method was first proposed by Seifritz in 1990 [10]. Since then, research on the process has been continuously performed, using natural raw materials and, in recent years, primarily waste [7,11].

This process can be carried out using waste materials containing calcium and magnesium oxides [12], such as metallurgical slags [13], fly ash [14,15,16], cement waste [17], cement kiln dust [18], and other industrial residues [19]. The use of these secondary raw materials not only enables CO₂ emissions to be reduced but also provides an effective method for managing difficult-to-recycle waste, thereby supporting the concept of a circular economy [11].

In this review article, we provide an innovative, comprehensive analysis of the potential of metallurgical slags for mineral CO₂ sequestration. These waste materials are especially suitable due to their high reactive oxide content [20,21] and their theoretical potential for CO₂ sequestration being 325.2–407.1 g/kg [22]. The average theoretical CO₂ absorption for metallurgical slags has also been determined: for blast furnace slags, it is 41.3 ± 1.3 g/kg; for oxygen furnace slags, it is 40.2 ± 1.7 g/kg; and for electric arc furnace slag, it is 36.8 ± 1.0 g/kg [23].

We discuss direct (single-stage) and indirect (two-stage) mineral carbonation technologies, in which Ca²⁺ and Mg²⁺ ions are first extracted using reagents such as CH₃COOH, HNO₃, NaOH, NH₄Cl, or HCl, and then reacted with CO₂ [24,25].

An important contribution of this review is the presentation of selected examples of pilot and demonstration installations in which mineral CO₂ sequestration technology has been applied under conditions close to those seen in real life. This analysis allows for an assessment of the practical possibilities for implementing this technology on a larger scale and is expected to help in identifying the technological and economic barriers that need to be overcome on the road to commercialisation.

The concept presented in the article—combining CO₂ emission reduction with metallurgical waste utilisation—is an example of an innovative approach to redirecting industry towards a low-carbon and sustainable future. This also emphasises the need for continued research and technological development to support climate goals.

The analysis highlights the significant potential of this technology while emphasising the need for optimisation to ensure its efficiency and economic feasibility.

The approach presented in this study is also an example of synergy between environmental protection and material management. As an innovative concept, it requires further research, yet it already offers a promising direction for sustainable industrial transformation and climate change mitigation.

2. Direct Mineral Sequestration—Process Characteristics

Direct mineral sequestration (also known as in situ or ex situ carbonation) involves reacting CO₂ with alkaline materials to form thermodynamically stable carbonates.

In the case of metallurgical slags, which are by-products of iron and steel production, there is potential to exploit their chemical composition—they contain significant amounts of CaO and MgO, as well as silica (SiO₂) and aluminium oxides (Al₂O₃). Slags, particularly those from basic oxygen furnaces (BOFs) and electric arc furnaces (EAFs), are rich in calcium and magnesium oxides (CaO, MgO), which are key reagents in the carbonation process. This process can be carried out under conditions of elevated pressure and temperature or in an aqueous environment, which affects its efficiency.

Gas (CO₂, exhaust gases)—metallurgical slags

In this method, gas (CO₂ or exhaust gases) is subjected to a direct reaction with finely ground solid slag [26]. The gas–solid process is a two-phase reaction that does not involve a liquid phase, and this method is considered the simplest [13,27]:

(CaO, MgO) × (SiO₂)_y(s) + xCO₂(g) → x(Ca, Mg)CO₃(s) + ySiO₂(s)

(4)

Gas (CO₂, exhaust gases)—aqueous suspension of metallurgical slags

Carbonation using a water suspension of slag improves the reaction kinetics by dispersing the slag particles in water. Gas (CO₂ or exhaust gases) is passed through the suspension or introduced under pressure, leading to the formation of carbonic acid, which reacts with the Ca²⁺ and Mg²⁺ ions leached from the slag. The slag carbonation process in this method consists of four main stages [28,29]:

• Slag dissolution:

(Ca, Mg)₂SiO₄ + 4H⁺ + 2(Ca²⁺, Mg²⁺) + SiO₂(aq) + H₂O

(5)

• Carbon dioxide solubility:

CO₂ ↔ CO₂ (aq)

(6)

• Carbonation reactions:

HCO₃⁻ ↔ CO₃²⁻ + H⁺

(7)

(Ca²⁺, Mg²⁺) + H₂O ↔ (CaOH⁺, MgOH⁺) + H⁺

(8)

(Ca²⁺, Mg²⁺) + H⁺ + CO₃²⁻ ↔ (CaHCO₃⁺, MgHCO₃⁺) + H⁺
(Ca²⁺, Mg²⁺) + CO₃²⁻ ↔ (CaCO₃, MgCO₃)(aq)

(9)

SiO₂(aq) + H₂O ↔ HSiO₃⁻ + H⁺

(10)

SiO₂(aq) + (Ca²⁺, Mg²⁺) + H₂O ↔ (CaHCO₃⁺, MgHCO₃⁺) + H⁺

(11)

• Precipitation reaction:

(CaCO₃, MgCO₃) (aq) → (CaCO₃, MgCO₃) (s)

(12)

SiO₂(aq) → SiO₂(s)

(13)

In an aqueous environment, the basic oxides involved in the carbonation process (CaO and MgO) in metallurgical slags react to form hydroxides [13]:

CaO + H₂O→Ca(OH)₂

(14)

MgO + H₂O→Mg(OH)₂

(15)

Slag contains a certain amount of hydraulic materials (mainly C₃S and β-C₂S) which, when in contact with water, undergo hydration and form C–S–H and Ca(OH)₂. The phases that primarily undergo carbonation are C₃S, C₂S, C–S–H, and Ca(OH)₂ [13,30,31]:

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

(16)

CaO∙nSiO₂∙mH₂O (C−S−H) + CO₂ → CaCO₃ + SiO₂ (gel) + mH₂O

(17)

3CaO⋅SiO₂ + yH₂O + (3 − x)CO₂ → xCaO⋅SiO₂⋅yH₂O + (3 − x)CaCO₃

(18)

2CaO⋅SiO₂ + yH₂O + (2 − x)CO₂ → xCaO⋅2SiO₂⋅yH₂O + (2 − x)CaCO₃

(19)

The rate of this process is controlled by the diffusion of carbon dioxide into the pore space and chemically by the availability of dissolved calcium ions in the liquid phase. After an initial period of the rapid carbonation of Ca(OH)₂ present in fresh slag, Ca²⁺ ions from calcium silicates diffuse towards the surface of the particles, from which they are then leached out. Calcium from the solution precipitates as calcite on the outer surface of the slag particles, slowing down the carbonation process, most likely as a result of the pores being blocked by calcite precipitating on the surface of the slag particles, thus hindering CO₂ diffusion [32,33,34].

The carbonation process can be carried out using various types of metallurgical slags, such as converter slags, electric furnace slags, arc furnace slags, and steel slags (

Table 1. Parameters of direct mineral CO₂ sequestration using metallurgical slags.

Slag Type	CaO Content	Process Characterisation				Reference
		Type of Process, L/S, Duration	Temperature	Pressure	Results
Basic oxygen furnace (BOF) steel slags	CaO: 42.5 ± 1.9%	CO2—slurry duration: 16 h	60–140 °C	0.5–10 MPa	~160–190 g CO2/kg slag	[35]
		gas (CO2 concentration 20%), relative humidity: 65%	20 °C		calcium conversion–0.73%	[36]
	CaO: 41.5%	slag ratios: 0 (without water), 2, 5, 10 duration: 0.5, 1, 6, 12, 24, 48, 96 min.	50, 75, 100 °C	1, 100, 200, 250, 300 kg/cm2	carbonation degree: min. 2.74%, max. 71.1%	[37]
	free CaO: 6.28; 4.90; 3.94% CaO: 42.28; 41.71; 41.11%	CO2 (g), H2O (g)	from ambient temperature to ~450 °C		transformation from CaO to CaCO3	[38]
	CaO: 34.287; 45.761	duration: up to 240 h	25, 90 °C	0.1 MPa	calcite and aragonite	[28]
	CaO: 51%	CO2—compacts duration: from 15 min to 4 h	50 °C	0.13, 0.10 MPa	CO2 uptake: 18, 21%	[39]
	Ca: 212.42, 319.08 g/kg	40–47% CO2 flow L/S: 0.17 duration: 30 min.	25–37 °C 45–53 °C	atmospheric	CO2 uptake: 4–6%	[40]
	CaO: 54.59%	CO2—10% water in the wet powder sample max. duration: 1, 2, 4 h	30, 60 °C		max. CO2 sequestration: ~7%	[41]
	CaO: 46.70%	CO2—slurry L/S: 10:1 duration: 0 3, 5, 8, 10, 15, 20, 30 min	25 °C		carbonation degree: 9.54–38.28%	[42]
Blast furnace slag (BFS)	CaO: 42.81%	CO2—slurry L/S: 3:1, 4:1 duration: 2, 4 days	20 ± 2 °C 90 ± 2 °C	1.0, 3.0, 5.0 MPa	sequestration: 3.45% (20 ± 2 °C) 13.21% (90 ± 2 °C)	[43]
Ground granulated blast furnace slag (GGBFS)	CaO: 39.80%	CO2—slurry L/S: 0.2; 0.3 humidity: 65 ± 5% CO2 concentration: 10 ± 0.5% duration: 7, 14, 21, 28 days	40 ± 1 °C		CO2 sequestration: 1.32, 1.83%	[44]
	CaO: 42.5%	L/S: from 0.25:1 to 3:1	20–80 °C	1 to 6 MPa	sequestration potential: 29.47 kg CO2/Mg slag (29.47 g CO2/kg slag)	[45]
	CaO: 44.0%	direct gas–solid duration: 30 d	ambient		degree of carbonation: 39%	[46]
Electric arc furnace (EAF) slag	CaO: 20.91%	CO2—slurry L/S: 2:1, 5:1, 10:1 duration: 0.5, 1, 2, 3, 4 h	ambient	0.5 MPa	sequestration capacity: 58.36 g CO2/kg slag	[47]
	CaO: 432,333.3, 402,000, 445,333.3, 500,333.3 mg/kg	L/S: 0 to 0.6 L/kg duration: from 0.5 to 24 h	30 to 50 °C	0.3 MPa	max. CO2 uptakes: 130 g CO2/kg	[32]
		suspension gas mixture (15% CO2, 85% N2) L/S = 10 kg/kg			CO2 sequestration capacity: 7.66 g CO2/100 g slag (76.6 g CO2/kg slag)	[48]
	CaO: 45%	CO2—compacts duration: from 15 min to 4 h	50 °C	0.13, 1 MPa	CO2 uptake: 7.3; 11.2%	[39]
	CaO: 23.42%	100% CO2, gas mixture (40% CO2, 60% air) water: 1.25–1.5 kg duration: 4, 4.5 h	70, 100 °C		states of carbonation (values in wt. % as CaCO3): initial: 1.2 1 h–2.2%, 2 h–2.3%, 3 h–2.4%, 4 h–2.3%	[49]
	CaO: 27.10%	CO2 slurry duration: from 20 to 60 min	20, 40, 60 °C		CO2 uptake of 7.7% carbonation degree: 30.2%	[50]
Electric arc furnace reducing slag (EAFRS)		L/S: 25 mL/g	25–80 °C		max. conversion: 0.394 kg CO2/kg slag (394 g CO2/kg slag)	[51]
Electric arc furnace slag (EAF), ladle furnace slag (LF)		accelerated carbonation	20–40 °C		the lowest pH decreased leaching: Ca, Cu, Ba, Fe, Mn, Pb	[52]
Ladle furnace (LF) slag	CaO: 51.32%	10% CO2, balanced with air (fluidised bed reactor)	up to 200 °C	up to 0.6 MPa	1 kg of LF slag could capture 0.262 kg of CO2 (262 g CO2/kg slag)	[53]
		gas mixture (15% CO2, 85%N2)—suspension L/S = 10 kg/kg			CO2 sequestration capacity: 24.7 g CO2/100 g slag (247 g CO2/kg slag)	[48]
	CaO: 50.06%		20 ± 2 °C 90 ± 2 °C		sequestration: 27.72% 29.90%	[43]
Ladle (white) slag (LS)		gas mixture (10% CO2, 90% N2)—slurry CO2—slurry L/S = 5, 10, 20, 30, 35 mL/g duration: 12 h	25, 50 °C	0.2, 0.4, 0.6 MPa	sequestration capacity: 276.65 g CO2/kg slag (10% CO2), 359.79 g CO2/kg slag (pure CO2)	[54]
Fine-grained stainless steel slag		CO2—mixture with 10% water	80 °C	2 MPa	CO2 binding: 59 g/kg slag	[55]
Linz–Donawitz steel slag	CaO: 35.5%	CO2—suspension L/S = 2–20 kg/kg	25–225 °C	0.1–3.0 MPa	max. carbonation degree of Ca content: 74%	[33]
Low Carbon steel LF slag	CaO: 48.4%	100% CO2, gas mixture (40% CO2, 60% air) water: 1.25–1.5 kg duration: 4, 4.5 h	70, 100 °C		states of carbonation (values in wt. % as CaCO3): initial: 2.1 1 h–10.8%, 2 h–16.6%, 3 h–21.4%, 4 h–24%	[49]
Ultra-fine steelmaking slag		aqueous carbonation	40–160 °C	4.83 MPa	CO2 capture capacities per gram of dry solid slag: 0.127 kg CO2	[56]
Stainless steel slags: argon oxygen decarburisation (AOD) slag	CaO: 54.8%	slurry carbonation duration: 5–120 min	30–180 °C	0.2–3 MPa	CO2 uptake: 0.26 g CO2/g slag (260 g CO2/kg slag)	[57]
Stainless steel slag	CaO: 42.59%	100% CO2, gas mixture (40% CO2, 60% air) water: 1.25–1.5 kg duration: 4 h	100 °C		states of carbonation (values in wt. % as CaCO3): initial: 1.5 1 h–2.2%, 2 h–2.5%, 3 h–3.6%, 4 h–3.9%	[49]
Stainless steel slag—continuous casting (CC) slag	CaO: 50.0%				0.31 g CO2/g slag (310 g CO2/kg slag)	[58]
Stainless steel slag		gas−slurry	90 °C	0.8 MPa	CO2 uptake: 9.35, 11.49, 13.37, 15.56% CO2	[31]
	CaO: 46.4%	5–100% CO2, duration: 160 h	10–60 °C	0.15 MPa	total CO2 uptakes: 95 g CO2/kg; 106 g CO2/kg slag	[58]
	CaO: 41.51%	gas mixture (CO2–20–25%)-slag duration: 2 h		0.1, 0.3, 0.5, 1.0 MPa	carbon fixation rate: 6.4%	[59]
Steelmaking slag	CaO: 28.27%	aqueous route L/S: 10 duration: 3 h	room	0.6 MPa	max. sequestration: 82 g CO2/kg slag	[60]
	CaO: 28.27%	gas–solid route duration: 3 h	room	0.3 MPa	Max. sequestration: 11.1 g CO2/kg slag	[60]
Steel slag	CaO: 64.73± 2.14% 51.79 ± 2.31% 39.10 ± 1.78%	aqueous route L/S: 1, 5, 10, 15, 20 mL/g	25, 45, 65, 85, 105 °C	0.1, 0.5, 1.0, 1.5, 2 MPa	max. sequestration capacity: 283.5 g CO2/kg slag max. carbonation efficiency: 51.61%	[61]
	CaO: 45.3 ± 0.5%	CO2—slurry duration: 16 h	20–140 °C	up to 2.0 MPa	~180–160 g CO2/kg slag	[35]
	CaO: 37.8%	gas mixture (CO2–20%) L/S = 0, 10, 20, 30, 50, 100% duration: 12 h, 1, 3, 7, 14 days alkali activators: NaOH, Na2SiO3·9H2O	20 °C		carbonation degree: max. 49.4% CO2 uptake: 20.1%	[62]

). In the literature, tests have been conducted under various experimental conditions, including variable temperature, pressure, process duration, and liquid-to-slag ratio (L/S).

Research on the degree of carbonation of metallurgical slags is also conducted using simple methods. For example, Johnson [63] determined the degree of carbonation of stainless steel slags based on the weight gain in the samples. The highest increase, approx. 20%, and thus the highest amount of bound CO₂ was observed for slag–water suspensions with a water-to-slag ratio of 0.125, and the lowest mass increase, approx. 7.5%, was found at a water-to-slag ratio of 0.2. In the case of blast furnace slags, a relatively low weight gain of only about 3% was reported [63].

2.1. The Effect of Temperature on the Direct Carbonation Process

Temperature is one of the key factors determining the efficiency of slag carbonation [64], but its impact is strongly modulated by the CO₂ partial pressure, liquid-to-solid ratio (L/S), reaction time, and chemical composition of the material (Figure 1).

In the case of BOF converter slags, it has been shown that, at ambient temperature, the degree of Ca conversion is limited to less than 1% [36], while, in the range of 50–100 °C, absorption can reach from a few percentage points to over 70%, depending on the pressure and process conditions [37]. Research by Quaghebeur et al. [35] has shown that, in the range of 60–140 °C, it is possible to bind about 160–190 g CO₂/kg slag, while Librandi et al. [39] obtained 18–21% at 50 °C. Su et al. [37] found that temperature significantly impacted the degree of BOF slag carbonation. Increasing the temperature from 50 °C to 100 °C resulted in a corresponding increase in the degree of carbonation, from 11.2% to 14.9%.

Similar relationships are observed in the case of blast furnace slag. For example, Elyasi Gomari et al. (2024) [43] demonstrated an increase in sequestration from 3.45% at 20 °C to 13.21% at 90 °C, while Ukwattage et al. [45] reported that the CO₂ binding potential at 20–80 °C and elevated pressure can reach 29.5 kg/t. Ukwattage et al. [45] found that, of the three reaction temperatures tested (20 °C, 50 °C, and 80 °C), 50 °C proved to be optimal in terms of CO₂ sequestration efficiency.

For EAF slag, the absorption at ambient temperature has been reported to be 58 g/kg [47].

In the case of LF and LS slags, a clear effect of temperature up to 200 °C is observed: Mohamed et al. [53] determined the absorption capacity to be 0.262 kg CO₂/kg, and Elyasi Gomari et al. [43] reported an increase in sequestration from 27.7% at 20 °C to 29.9% at 90 °C. The negative impact of temperature on the carbonation of LS slags becomes apparent as the temperature increases from 25 °C to 80 °C, as reported by Capelo et al. [54]. At higher temperatures, the nucleation and growth of CaCO₃ may be hindered due to the reduced solubility of CO₂, which adversely affects the carbonation process [54].

Stainless steel slags also show strong temperature dependence. Boone et al. [55] documented a binding of 59 g/kg at 80 °C, and Nielsen et al. [58] reported 95–106 g/kg at 60 °C. Huijgen et al. [33] achieved a maximum Ca carbonation degree of 74% at 100 °C.

Studies conducted by Zhu et al. [61] on steel slag showed that carbonation efficiency initially decreases with increasing temperature from 25 °C to 45 °C. Above 45 °C, however, a significant increase in process efficiency is observed. When the temperature was increased to 85 °C, the CO₂ sequestration capacity increased from 157.9 to 260.7 g of CO₂ per kg of slag. This indicates that temperature significantly influences the kinetics of the carbonation reaction.

Research conducted by Quaghebeur et al. [35] showed that an increase in CO₂ capture efficiency was observed for steel slag with increasing temperature, while a decrease in sorption capacity was observed for BOF slag at higher temperatures. The authors attribute these differences to the distinct phase compositions of the materials. BOF slag contains a significant amount of Ca(OH)₂, which undergoes carbonation easily at lower temperatures. In contrast, SS slag consists mainly of calcium and magnesium silicates (Ca–Mg), which require higher temperatures for an effective reaction with CO₂.

Studies conducted by Huijgen et al. [33] demonstrated that reaction temperature exerts a bidirectional effect on the rate of the carbonation process. At higher temperatures, the leaching of calcium ions from the slag matrix proceeds more rapidly, which promotes the reaction; however, at the same time, the solubility of carbon dioxide in the solution decreases, potentially limiting the further progress of carbonation. The authors [33] observed that the degree of carbonation increases with temperature in the range of 25–175 °C, whereas above 200 °C, the conversion declines. This decrease is attributed to the reduced solubility of CO₂, which becomes the limiting factor at elevated temperatures. Furthermore, Huijgen et al. [33] indicated that the calcium leaching step represents the rate-controlling stage in the range of 25–150 °C, while above 200 °C, the CO₂ dissolution and diffusion step becomes the rate-limiting factor governing the overall reaction kinetics.

In summary, temperature has a positive effect on the metallurgical slag carbonation efficiency, especially in the range of 50–100 °C, where a significant increase in conversion is observed [43,61]. At higher temperatures (>100 °C), very high absorption rates can be achieved, although the process may be limited [33].

2.2. The Influence of Pressure on the Efficiency of Carbonation of Metallurgical Slags

An analysis of the literature indicates that the CO₂ partial pressure is one of the parameters determining the efficiency of the mineral carbonation of metallurgical slags.

In the case of BOF slags, Su et al. [37] confirmed that pressure impacts the degree of carbonation. Increasing the CO₂ pressure within a broad range of 1–300 kg/cm² led to a conversion increase of around 20%.

The results of BFS and LS slag tests conducted by Elyasi Gomari et al. [43] indicate a negative correlation between pressure and CO₂ sequestration, as the amount of CO₂ absorbed decreases with increasing pressure. The highest sequestration levels, 14.75% for BFS and 30.04% for LS, respectively, were obtained at a pressure of 1 MPa, while these values decreased to 13.16% and 26.69% at a pressure of 5 MPa.

Librandi et al. [39] presented results indicating a slight increase in CO₂ absorption during EAF slag carbonation. This averaged 10–20%, with an increase in pressure from 0.13 to 1.0 MPa.

Zhu et al. [61] observed that, at pressures below 0.5 MPa, both the sequestration capacity of slags and the carbonation efficiency of steelmaking increase significantly with pressure, rising from 166.9 g(CO₂)/kg to 214.8 g(CO₂)/kg and from 30.88% to 39.42%, respectively. Above the threshold of 0.5 MPa, however, both the sequestration capacity and the carbonation efficiency stabilise up to a pressure of 2.0 MPa. The authors assumed that the CO₂ pressure should be 0.5 MPa.

Ukwattage et al. [45] found that the initial rate of CO₂ sequestration (after 10 h) increased gradually with pressure; at 6 MPa, it was more than twice as high as at 1 MPa. The authors explain this behaviour with Henry’s law, according to which, at a given temperature, the concentration of a gas dissolved in a liquid is directly proportional to its partial pressure above the solution. A higher partial pressure of CO₂, therefore, leads to a greater amount of CO₂ dissolved in water, as well as more intense forcing of gas molecules into the liquid phase, which further facilitates dissolution. They also cite studies [65] showing that increasing pressure raises the frequency of energy-transferring collisions between gas molecules and reactant molecules, which helps maintain a Boltzmann distribution of energy and momentum and thus accelerates the reaction. In theory, this means that higher gas pressure should shorten the time needed to complete sequestration. However, analysis of the final pressure drop in their experiments showed that the overall extent of sequestration did not differ significantly across the initial pressure range of 1–6 MPa. On this basis, the authors conclude that the effect of CO₂ pressure on the total degree of carbonation is limited, because CO₂ transfer to the solid is not the rate-controlling step. Given sufficiently long reaction times, the system can approach near-equilibrium, and the most likely rate-controlling step is the diffusion of calcium ions from the particle interior toward its surface [33].

However, the effect of pressure on the carbonation process is closely related to other process parameters [13]. Increased pressure can lead to an increase in the rate of carbonation, which results from a higher concentration of CO₂ molecules in the reaction system. However, above a certain pressure threshold, the efficiency of the process may be reduced, possibly as a result of rapid carbonate formation, which may block the pores of the slag material, limiting the active surface area available for carbonation. In addition, the formation of protective carbonate layers on the surface of the slag may hinder the diffusion of CO₂ into its interior and limit contact with reactive mineral components. As a result, the efficiency of the carbonation process is reduced, leading to a reduction in the amount of CO₂ sequestered [13,43,45].

2.3. The Influence of the Liquid-to-Solid Ratio (L/S) on the Efficiency of Carbonation of Metallurgical Slags

An analysis of data from the literature indicates that the liquid-to-solid ratio (L/S) is one of the most important process parameters determining the efficiency of the mineral carbonation of metallurgical slags [33,54,61]. A change in L/S affects both the reaction kinetics and the total CO₂ binding capacity of the material.

The liquid-to-solid ratio (L/S) is a key parameter in the carbonation process, determining both the availability of Ca²⁺ ions in the solution and the efficiency of mass transport. Too low an L/S ratio limits the dissolution of reactive phases, while too high a ratio leads to system dilution and reduced CO₂ concentration in the liquid, which hence reduces the reaction rate.

Research by Su et al. [37] showed that the presence of water has a significant impact on the carbonation degree of BOF slag: in its absence, the carbonation degree decreased from 17% to 6.2%, indicating that water is an essential medium for effective carbonation.

Elyasi Gomari et al. [43] analysed the effect of the liquid-to-solid ratio (L/S) on the amount of CO₂ sequestered during the carbonation of BFS and LS slags. Experiments were carried out at L/S ratios of 90 mL/30 g (3:1), 120 mL/30 g (4:1) and 150 mL/30 g (5:1). For BFS (calcimeter results), the degrees of carbonation were 13.21% (3:1), 14.75% (4:1) and 10.92% (5:1), respectively, while for LS slag, they were 29.9% (3:1), 30.04% (4:1) and 28.23% (5:1). The authors [43] showed that CaCO₃ formation in both BFS and LS increased as the L/S ratio rose from 3:1 to 4:1, whereas a further increase to 5:1 led to a reduction in the degree of carbonation. At L/S = 5:1, the larger volume of liquid hindered mass transfer between the solid and liquid phases, resulting in incomplete particle dispersion and a lower rate of Ca²⁺ leaching. In addition, the formation of an outer CaCO₃ layer on the grain surface impeded the extraction of calcium from the particle interior, leading to less effective carbonation. Conversely, reducing the L/S ratio to 3:1 decreased the concentration of leached Ca²⁺ in the solution due to the smaller liquid volume per unit mass of solid, which impaired both CO₂ absorption efficiency and the rate of CaCO₃ formation. On this basis, the authors concluded that an L/S ratio of 4:1 is the most suitable for this system, as it provides a balance between effective liquid–solid interaction and high CO₂ absorption efficiency, resulting in improved overall carbonation performance [43].

Similar conclusions can be drawn from studies on EAF slag. For example, Omale et al. [47] showed that, at L/S ratios of 2:1, 5:1, and 10:1, under conditions of 0.5 MPa pressure and ambient temperature, the sequestration efficiency reached up to 58 g CO₂/kg. In the present study, for an L/S ratio of 2:1, the sequestration capacity was 41.47 ± 0.85 g CO₂/kg of slag after 2 h and 49.21 ± 1.00 g CO₂/kg after 3 h of the process. For an L/S ratio of 5:1, these values were slightly higher − 48.11 ± 4.81 g CO₂/kg after 2 h and 58.36 ± 5.84 g CO₂/kg after 3 h—while for the highest L/S ratio of 10:1, the sequestration capacity reached 39.37 ± 0.24 g CO₂/kg after 2 h and 42.33 ± 0.86 g CO₂/kg after 3 h, respectively. These results confirm that a high L/S ratio, e.g., 10:1, leads to reduced CO₂ sequestration due to the excessive amount of water, which hinders the diffusion of CO₂ molecules and their interaction with Ca²⁺ ions. The authors indicate that an L/S ratio of 5:1 is the most favourable [47].

Research conducted by Zhu et al. [61] confirms that the process of the direct carbonation of steel slag is closely linked to the diffusion and dissolution of CO₂ in water, as well as the dissolution of alkaline species present in the slag. At liquid-to-solid ratios below 5 mL/g, water acts as a medium that facilitates the reaction between the solid phase and gaseous CO₂; however, in this range, the leaching of Ca²⁺ ions from the solid matrix is insufficient, which limits the overall extent of the reaction. When the liquid-to-solid ratio increases to 5 mL/g, both the sequestration capacity (K) and the carbonation rate increase significantly—from 98.2 to 217.6 g CO₂/kg and from 10.73% to 39.92%, respectively. However, as the L/S ratio is increased further, both the sequestration capacity and the carbonation rate decrease.

As the liquid-to-solid ratio increases, excess water in the reaction system hinders CO₂ penetration due to capillary effects. This barrier suppresses contact between CO₂ and active reaction sites, leading to incomplete carbonation. Consequently, the overall carbonation efficiency of steel slag is reduced [61].

In summary, the optimal L/S ratio depends on the slag type, pressure, and process temperature. Literature data indicate that medium L/S ratios ensure the most favourable balance between the dissolution of reactive components and the CO₂ concentration in the liquid phase [43,47]. Too low L/S values limit Ca²⁺ mobilisation, while excessive dilution at high L/S ratios slows down carbonation kinetics [61]. Therefore, the highest sequestration efficiencies are generally achieved within an intermediate L/S range, although for highly reactive slags, comparable results can also be obtained at lower ratios [43,47,61].

2.4. The Influence of Reaction Time on the Carbonation Process of Metallurgical Slags

The reaction time plays an important role in the carbonation process, influencing the degree of conversion of free CaO and MgO to carbonates. Most studies indicate a rapid increase in the degree of carbonation in the initial phase of the reaction, followed by a gradual slowdown, which results from the growth of a carbonate layer on the surface of the grains and, thus, the limitation of CO₂ diffusion.

The sequestration capacity increases with reaction time [47]. However, after a certain period, the carbonation process reaches a maximum, beyond which it proceeds very slowly due to the formation of a layer of carbonation products on the slag surface. This layer hinders gas penetration and further leaching of Ca ions from the slag structure, leading to passivation. In the study by Omale et al. [47], such a state was reached after 3–4 h of the process, and the subsequent increase in sequestration capacity was already minor, from 58.36 to 5.95 g CO₂/kg of slag.

Huijgen et al. [33] found that, during the first 2 min of the process, when using steelmaking slags, more than 40% of calcium is carbonated, whereas only an additional ~13% reacts when the reaction time is extended to 30 min, which confirms the rapid depletion of the most reactive fractions of the material.

When analysing the effect of reaction time on the degree of carbonation, it is crucial to account for its dependence on temperature. As shown by the results of Elyasi Gomari et al. [43], an increase in temperature from 20 °C to 90 °C combined with an extension of the reaction time from one to four days led to an increase in CO₂ sequestration in ladle slag from 101.23 to 149.32 kg CO₂/t, corresponding to an increase of approximately 48%.

In summary, the reaction time strongly influences the degree of carbonation, but its importance decreases as the process progresses. Most CO₂ absorption occurs in the initial phase, lasting from several minutes to several hours, but prolonging the reaction past this point results in an increasingly slower increase in efficiency [33,47]. The optimal reaction times vary depending on the slag type and reaction conditions (temperature, pressure, L/S), but, in most studies, prolonging the process beyond several hours does not bring proportional results [47].

2.5. Effect of the Carbonation Process on the Phase Composition of Metallurgical Slags

The main product of the carbonation process is calcium carbonate (

Table 2. The influence of the carbonation process on the phase composition of metallurgical slags.

Slag Type	Phase Composition of Slags	Phase Composition of Slags After the Carbonation	Reference
Basic oxygen furnace (BOF) slags	Slag 1: CaO, MgO, FeO, Fe2O3, Ca(OH)2, Mg(OH)2, SiO2, Al2O3, Fe3O4, Ca2SiO4, Ca2MgSi2O7, Mg2SiO4, Ca2FeAlO5 Slag 2: CaO, MgO, Fe2O3, Ca(OH)2, Mg(OH)2, Ca2SiO4, CaCO3, SiO2, Al2O3, Ca2Fe2O5, CaAl2Si2O8, Ca2Fe2O5, MgCr2O4	CaCO3 in the form of calcite and aragonite is the main product of carbonation. In addition, small amounts of MgCO3 were found.	[28]
Basic-oxygen furnace slag	~40% dicalcium silicate (Ca2SiO4–C2S), ~30% calcium ferrite (Ca2Fe2O5), ~20% Mg-wüstite ((Mg, Fe)O), 5–10% free lime (free-CaO) and <5% metal iron	Ca(OH)2 formed from free lime hydration was the most reactive phase, followed by C2S. Carbonation of Ca2Fe2O and Mg-wüstite also contributed to the overall slag carbonation.	[37]
Basic oxygen furnace (BOF) slags	hatrurite (C3S), larnite (C2S), grossular (C3A), mayenite (C12A7), srebrodolskite(C2F), C2S, RO phase, f-CaO	After carbonation, a gradual increase in the intensity of calcium carbonate peaks and a decrease in the intensity of calcium silicate peaks were observed, with a complete disappearance of the calcium oxide peak. The RO and calcium ferrite peaks remain relatively unchanged, indicating that mainly calcium silicate and CaO undergo carbonation, while the iron-bearing phases do not react significantly. Initially, amorphous calcium carbonate is formed, which over time transforms into calcite.	[42]
Ground granulated blast furnace slag (GGBFS)	amorphous glass phase	The carbonation process resulted in the formation of a significant amount of calcium carbonate (calcite), amounting to 9.32%.	[46]
Oxygen decarburization slag (AOD slag)	~81.5% dicalcium silicate (Ca2SiO4), ~10% magnesite (MgO), ~8.4% fluorite (CaF2), magnesium–calcium silicate (Ca3MgSi2O8), magnesium–aluminium spinel (MgO∙Al2O3)	83.4% aragonite (CaCO3(A)), 10.2% dicalcium silicate (Ca2SiO4), 6.2% silica (SiO2), hydrated calcium silicate (Ca5(OH)2Si6O16∙4H2O)—trace phase	[66]
Electric arc furnace (EAF) slag	mayenite (Ca12Al14O33), forsterite (Mg2SiO4), calcite (CaCO3), larnite (Ca2SiO4), merwinite (Ca3Mg(SiO4)2), magnesite (MgCO3), hematite (Fe2O4), monohydrocalcite (CaCO3. H2O), portlandite (Ca(OH)2), wustite (FeO), chromite (FeCr2O4), quartz (SiO2), angenite (CaCO3)2	After carbonation, calcite becomes the dominant phase, with aragonite (CaCO3) and dolomite (CaMg(CO3)2) also present. The intensity of the mayenite, larnite, and merwinite peaks decreases, and some of these peaks disappear completely.	[47]
Electric arc furnace (EAF) slag	amorphous 15.6%, wüstite (FeO) 34.9%, magnetite (Fe2+Fe23+O4) 10%, hematite (Fe2O3) 1.5%, calcium ferrite (Fe2O5Ca2) 1.3%, β-C2S (β-Ca2SiO4) 18%, α-C2S (α-Ca2SiO4) 3.5%, gehlenite (Ca2Al(AlSi)O7) 8.7%, mayenite (Ca12Al14O33) 3.8%	amorphous 4%, wüstite (FeO) 9%, magnetite (Fe2+Fe23+O4) 4%, hematite (Fe2O3) 5%, calcium ferrite (Fe2O5Ca2) 7%, β-C2S (β-Ca2SiO4) 1%, α-C2S (α-Ca2SiO4) 9%, gehlenite (Ca2Al(AlSi)O7) 2%, mayenite (Ca12Al14O33) 7%	[49]
Electric arc furnace (EAF) slag	oxide (FeO), aluminosilicates (i.e., akermanite, Ca4MgAl3SiO14, gehlenite, Ca2Al2SiO7), larnite (Ca2SiO4) and merwinite (Ca3MgSi2O8), (CaCO3), iron manganese oxide (Fe2MnO4)	After carbonation, the peaks corresponding to calcium carbonate show increased intensity, larnite is no longer detectable, and the merwinite peaks become less intense. The remaining phases do not exhibit significant reactivity in the carbonation process.	[50]
Stainless steel slag	amorphous 22%, magnetite (Fe2+Fe23+O4) 3%, akermanite (Ca2MgSi2O7) 23%, bredigite (Ca7Mg(SiO4)4) 36%, merwinite (Ca3Mg(SiO4)2) 2.1%, enstatite (Mg2Si2O6) 3.3%, rankinite (Ca3Si2O7) 2%, quartz-crist-trid. (SiO2) 4.4%, gibbsite (γ-Al(OH)3) 1.3%, periclase (MgO) 1.9%, magnesite (MgCO3) 5.9%, calcite (CaCO3) 1.3%	amorphous 8.5%, hematite (Fe2O3) 2.7%, akermanite (Ca2MgSi2O7) 24.7%, rankinite (Ca3Si2O7) 45.8%, gibbsite (γ-Al(OH)3) 1.3%, periclase (MgO) 6.4%, calcite (CaCO3) 3.4%	[49]
Stainless steel slag	merwinite [Ca3Mg(SiO4)2], bredigite [Ca14Mg2(SiO4)8], portlandite [Ca(OH)2], cuspidine (Ca4Si2O7F2), akermanite−gehlenite [Ca2Mg-(Si2O7)−Ca2Al(AlSiO7)], periclase (MgO), magnesium−iron−chromium oxide [(Mg,Fe)Cr2O4]	After carbonation, the portlandite phase is no longer present, and the intensity of diffraction peaks of mineral phases such as merwinite, cuspidine, bredygite, periclase, and akermanite–gehlenite has decreased significantly. At the same time, a new phase has appeared—calcite.	[55]
Stainless steel slag	calcite (CaCO3), gehlenite (Ca2Al(AlSiO7), akermanite (Ca2Mg(Si2O7)), merwinite (Ca3Mg (SiO4)2), bredigite (Ca7Mg(SiO4)4), portlandite (Ca (OH)2), periclase (MgO), free lime (CaO), donathite ((Fe,Mg)(Cr,Fe)2O4), cuspidine (Ca4(Si2O7)(F,OH)2), Calcio-olivine (γ-Ca2SiO4)	Carbonation products: calcite and Mg-calcite. Portlandite undergoes carbonation first. Carbonation of merwinite and bredygite leads to the formation of calcite and Mg-calcite.	[58]
Steelmaking slag	mayenite (Ca12Al14O33), forsterite (Mg2SiO4), calcite (CaCO3), larnite or dicalcium silicate (Ca2SiO4), merwinite (Ca3Mg(SiO4)2) wustite (FeO), magnetite (Fe3O4), monohydrocalcite (CaCO3∙H2O), portlandite (Ca(OH)2), chromite (FeCr2O4), quartz (SiO2)	After carbonation, the portlandite peaks disappeared, and the intensity of the calcite peaks increased slightly. Peaks of aragonite (CaCO3) and dolomite (CaMg(CO3)2) appeared. A decrease in intensity was observed, and in some places the peaks of mayenite, larnite, and merwinite disappeared, while as a result of the dissolution of these minerals, peaks of corundum (Al2O3) and magnetite appeared.	[60]
Low Carbon steel LF slag	amorphous 19.2%, SC2—γ (MUMME) (γ-Ca2SiO4) 49.8%, gehlenite (Ca2Al(AlSi)O7) 11.84%, mayenite (Ca12Al14O33) 5.8%, alpha iron (Fe) 1.07%, magnetite (Fe3O4) 1.3%, wüstite (FeO) 0.95%, quartz-crist-trid. (SiO2) 1.16%, periclase (MgO) 4.21%, magnesite (MgCO3) 1.55%, calcite (CaCO3) 1.2%	amorphous 11.1%, SC2—γ (MUMME) (γ-Ca2SiO4) 43.06%, gehlenite (Ca2Al(AlSi)O7) 9.46%, mayenite (Ca12Al14O33) 5.92%, alpha iron (Fe) 0.44%, magnetite (Fe3O4) 1.7%, wüstite (FeO) 0.15%, quartz-crist-trid. (SiO2) 3.1%, periclase (MgO) 1.38%, magnesite (MgCO3) 2.24%, calcite (CaCO3) 19.72%	[49]

), most commonly occurring in the form of calcite [28,42,47,49,55,58,60], less frequently as aragonite [28,47,60,66], or as a mixture of both polymorphs [28]. Carbonation can also yield dolomite, reported in processes involving electric arc furnace (EAF) slag [47] and steelmaking slags [60], as well as MgCO₃ [28,49]. In some studies, the formation of Mg-calcite has also been observed [58]. Ca(OH)₂ formed as a result of hydration is the most reactive phase [47,55,58,60]. Carbonation mainly affects calcium silicates and CaO, whereas iron-bearing phases do not react to any significant extent [42].

3. Indirect Mineral Sequestration—Process Characteristics

Indirect slag carbonation has been tested using reagents such as CH₃COOH, NaOH, NH₄Cl, HCl, (NH₄)₂SO₄, and NH₄OH (Table 3). However, NH₄Cl is most commonly used, the reaction of which is as follows [67,68]:

4NH₄Cl + 2CaO∙SiO₂ → 2CaCl₂ + SiO₂↓ + 4NH₃ + 2H₂O

(20)

4NH₃ + 2CO₂ + 2H₂O + 2 CaCl₂ → 2CaCO₃↓ + 4NH₄Cl

(21)

The use of acetic acid as an activator of the slag carbonation process has also been proposed [25,69]. Research on this is based on the results of Kakizawa et al. [70], in which acetic acid was used to extract calcium ions from wollastonite:

CaSiO₃ + 2CH₃COOH → Ca²⁺ + 2CH₃COO⁻ + H₂O + SiO₂

(22)

Ca²⁺ + 2CH₃COO⁻ + CO₂ + H₂O → CaCO₃↓ + 2CH₃COOH

(23)

However, in the case of steel slags, the authors also took into account the magnesium ion content, the reaction of which with acetic acid may proceed as follows:

MgSiO₃ + 2CH₃COOH → Mg²⁺ + 2CH₃COO⁻ + H₂O + SiO₂

(24)

Mg²⁺ + 2CH₃COO⁻ + CO₂ + H₂O → MgCO₃↓ + 2CH₃COOH

(25)

This process is studied with regard to the production of relatively pure carbonate in particular [25,69].

Table 3. Parameters of indirect mineral CO₂ sequestration using metallurgical slags.

Slag Type	CaO Content	Process Characterisation					Reference
		Type of Process, L/S, Duration	Reagent	Temperature	Pressure	Results
Blast furnace slag	CaO: 40.6% (±0.1)		CH3COOH NaOH	30–70 °C	0.1–3 MPa	binding of 1 kg of CO2 requires 4.4 kg slag, 3.6 dm3 CH3COOH, and 3.5 kg NaOH, resulting in 2.5 kg of 90% CaCO3	[25]
Converter slag	CaO: 44.5%	duration: 2 h	HCl, CH3COOH; NH4Cl	80 °C		CO2 absorbed: 22.9 Mg/h slag required: 141.4 Mg/h CaCO3 generated: 52.3 Mg/h	[67]
Blast furnace slag		(NH4)2SO4-to-slag mass ratio of 3:1 duration: 1 h	(NH4)2SO4	370 °C		total CO2 sequestration capacity: 316 kg/Mg	[71]
Steelmaking slag	CaO: 38.7%	acid-to-solid ratio: 0.5; 1.0 g/g duration: 1 h	TBP, CH3COOH, ultrapure water	40–94 °C		max. leached ratio of CaO: 75%	[72]
Basic oxygen furnace slag	CaO: 40%	duration: 10, 15, 60 min	NH4Cl, NH4OH	10, 80 °C		99.5% carbonation ratio; CO2 bound by calcium: 30.52%; vaterite	[73]
Basic oxygen furnace slag	CaO: 47.74%	50 g/L duration: 24 h	HCl, CH3COOH	25–900 °C		CO2 absorption: 2.6–6.9 mmol/g	[74]
Steel slag	Ca: 35.79%	NH4Cl, S/L ratio: 200 g/L duration: 15, 30, 45, 60 min	NH4Cl	30, 40, 60, 80 °C		CO2 sequestration capacity: 46–178 g/kg	[75]
Steel slag	Ca: 15.58%	HCl L/S ratio: 50 kg/kg solution concentration: 1.2 mol/L duration: 0.25 min		25 °C		max. Ca extraction: 87%	[24]
Blast furnace slag	Ca (30.4 wt. %)	pressure-swing carbonation duration: 5, 10, 30, 40, 50, 60, 120, 180, 240, 300, 360 min		20 °C	0.1 MPa, under vacuum	max. CO2 uptake capacities: 21.1 and 24.5 g CO2/kg, respectively, corresponding to 48.0 CaCO3/g and 55.7 CaCO3/g; max. CO2 uptake: 267.1 g CO2/kg, corresponding to 607.0 g CaCO3/g	[76]
Electric arc furnace slag	CaO: 39.04%	carbonation under microwave irradiation	2.4 mol/L NH4Cl—0.8 mol/L CaCl2—1.6 mol/L NH3·H2O—400 mL	12, 25, 35 °C		vaterite	[77]
Steel converter slag	CaO: 51.4%		NH4Cl	20, 45, 50, 60 °C		70% of the CO2 was utilised and converted into PCC	[68]
Steel converter slag	CaO: 44.9%		NH4NO3, CH3COONH4 NH4Cl	30 °C		max. Ca extraction efficiency: 73%	[78]

Table 3. Parameters of indirect mineral CO₂ sequestration using metallurgical slags.

Slag Type	CaO Content	Process Characterisation					Reference
		Type of Process, L/S, Duration	Reagent	Temperature	Pressure	Results
Blast furnace slag	CaO: 40.6% (±0.1)		CH3COOH NaOH	30–70 °C	0.1–3 MPa	binding of 1 kg of CO2 requires 4.4 kg slag, 3.6 dm3 CH3COOH, and 3.5 kg NaOH, resulting in 2.5 kg of 90% CaCO3	[25]
Converter slag	CaO: 44.5%	duration: 2 h	HCl, CH3COOH; NH4Cl	80 °C		CO2 absorbed: 22.9 Mg/h slag required: 141.4 Mg/h CaCO3 generated: 52.3 Mg/h	[67]
Blast furnace slag		(NH4)2SO4-to-slag mass ratio of 3:1 duration: 1 h	(NH4)2SO4	370 °C		total CO2 sequestration capacity: 316 kg/Mg	[71]
Steelmaking slag	CaO: 38.7%	acid-to-solid ratio: 0.5; 1.0 g/g duration: 1 h	TBP, CH3COOH, ultrapure water	40–94 °C		max. leached ratio of CaO: 75%	[72]
Basic oxygen furnace slag	CaO: 40%	duration: 10, 15, 60 min	NH4Cl, NH4OH	10, 80 °C		99.5% carbonation ratio; CO2 bound by calcium: 30.52%; vaterite	[73]
Basic oxygen furnace slag	CaO: 47.74%	50 g/L duration: 24 h	HCl, CH3COOH	25–900 °C		CO2 absorption: 2.6–6.9 mmol/g	[74]
Steel slag	Ca: 35.79%	NH4Cl, S/L ratio: 200 g/L duration: 15, 30, 45, 60 min	NH4Cl	30, 40, 60, 80 °C		CO2 sequestration capacity: 46–178 g/kg	[75]
Steel slag	Ca: 15.58%	HCl L/S ratio: 50 kg/kg solution concentration: 1.2 mol/L duration: 0.25 min		25 °C		max. Ca extraction: 87%	[24]
Blast furnace slag	Ca (30.4 wt. %)	pressure-swing carbonation duration: 5, 10, 30, 40, 50, 60, 120, 180, 240, 300, 360 min		20 °C	0.1 MPa, under vacuum	max. CO2 uptake capacities: 21.1 and 24.5 g CO2/kg, respectively, corresponding to 48.0 CaCO3/g and 55.7 CaCO3/g; max. CO2 uptake: 267.1 g CO2/kg, corresponding to 607.0 g CaCO3/g	[76]
Electric arc furnace slag	CaO: 39.04%	carbonation under microwave irradiation	2.4 mol/L NH4Cl—0.8 mol/L CaCl2—1.6 mol/L NH3·H2O—400 mL	12, 25, 35 °C		vaterite	[77]
Steel converter slag	CaO: 51.4%		NH4Cl	20, 45, 50, 60 °C		70% of the CO2 was utilised and converted into PCC	[68]
Steel converter slag	CaO: 44.9%		NH4NO3, CH3COONH4 NH4Cl	30 °C		max. Ca extraction efficiency: 73%	[78]

The indirect carbonation process is characterised by a high carbonation efficiency and the generation of high-purity by-products. The principal parameters determining its efficiency include the slag particle size, reaction temperature, residence time, extractant concentration, and solid-to-liquid ratio [24,78,79]:

• A decrease in particle size leads to an increase in the specific surface area, thereby enhancing the mass transfer rate. Consequently, the comminution of slag particles significantly improves both the kinetics and efficiency of calcium extraction.
• Higher extractant concentrations favour calcium dissolution; however, excessively elevated concentrations may induce the co-dissolution of impurities from the slag matrix, ultimately lowering the purity of the precipitated CaCO₃.
• The effect of temperature on calcium extraction is strongly dependent on the chemical characteristics of the extractant applied, indicating that its optimisation requires case-specific evaluation.
• A reduction in the solid-to-liquid ratio generally promotes calcium extraction; nonetheless, this improvement occurs at the expense of the overall process throughput, thus necessitating a balance between extraction efficiency and production capacity.
• Despite its high efficiency, indirect carbonation entails higher process costs due to the use of chemical reagents [79,80,81].

4. Application of the Process in Practice

Research is being conducted on the application of metallurgical slag carbonation in practice, and into the use of carbonated slags in building material production in particular. The mineral carbonation process using metallurgical slag is slowly being implemented on an industrial scale.

Stolaroff et al. [34] proposed an alternative method of CO₂ binding using metallurgical slag (steel slag) in a system consisting of two parallel layers: one is drained and discharged at a given time, while the other binds carbon dioxide. Water is sprayed from a height of 10 m above the slag layer, which then infiltrates the bed and is drained into wells. The Ca(OH)₂-saturated solution is pumped back through the sprayer, absorbing CO₂ from the air as it falls, leading to the precipitation of CaCO₃. According to the authors, a slag layer with an area of one hectare and a height of 10 m could bind approximately 32,000 Mg of CO₂, leading to the formation of 73,000 Mg of CaCO₃. The implementation of such a system would require the use of approximately 140,000 Mg of slag.

Research has been conducted on the use of carbonated metallurgical slag as an aggregate and as a supplementary cementitious material (SCM) [66,82,83,84,85]. Carbonation may be a method for treating slag used as road aggregate due to its ability to reduce volume instability [86]. Autoclaved carbonation reduced the pulverisation rate of steel slag by 56.24%, decreased free-CaO from 5.46% to 3.27%, and lowered the crushing value to 14.61%, confirming improved volume stability and mechanical performance of the slag [86]. Studies on the use of carbonated slag as an aggregate have shown that a concrete mix containing carbon steel slag as a 50% substitute for natural fine-grained aggregates had a higher compressive strength, lower water absorption, and reduced shrinkage compared to the control concrete [87].

The incorporation of carbonated steel slag into cementitious systems significantly influences their mechanical performance by enhancing their resistance to aggressive chemical environments, enabling the precise modulation of setting times to achieve optimal hydration conditions and improved flexural strength [80,88,89,90,91,92,93]. Furthermore, the utilisation of steel slag contributes to an increased CO₂ sequestration capacity, thereby offering both mechanical and environmental benefits.

Research has also been conducted on the use of carbonated steel slag as a mineral additive in concrete. The results showed that the addition of ground slag improved the mechanical properties and reduced the pore diameter in hardened mortar [62].

The use of metallurgical slags for mineral CO₂ sequestration has also been the subject of national and international projects. A major project analysing this topic was “Carbon8—CO₂ntainerᵀᴹ—Capturing and adding value to CO₂ & hazardous waste to produce valuable aggregates for construction” [94].

ENEA’s pilot-scale tests

Pilot-scale tests of the wet carbonisation process for steel slag have also been carried out in a rotary kiln. The product of the process was hardened under controlled conditions for 28 days, and then its physical and chemical properties were tested. The absorption results obtained were significantly higher than those obtained in laboratory tests [40]. According to Librandi et al. (2017) [40], in lab-scale dynamic tests, the CO₂ uptake of BOF slag granules reached ~5.5% after 28 days of curing, whereas in pilot-scale tests, the CO₂ uptake increased to up to 10.4% (BOF1) and 7% (BOF2). These results confirm that the pilot-scale carbonation process achieved approximately twofold higher CO₂ absorption than the laboratory-scale tests under comparable conditions [40].

The Slag2PCC

The Slag2PCC process has been implemented in a pilot plant. The Energy Engineering and Environmental Protection research group at Aalto University in Finland has launched the world’s first pilot plant for the indirect carbonation of steel slags. The process can process up to 20 kg of solid steel slag and produce approximately 10 kg of calcium carbonate [68].

The reagent ammonium salt solvent (NH₄Cl, NH₄NO₃, or CH₃COONH₄) can be regenerated and reused in the calcium extraction stage, making the process more economically viable [68].

Future technological development will need to focus on minimising NH₃ losses and enhancing CO₂ solubility [95,96].

Carbonisation Process for the Comprehensive Utilisation of Steel Slag Project—China

According to the authors [97], in 2021, the second stage of the Baogang Group’s.

The “Carbonisation process for the comprehensive utilisation of steel slag” demonstration project was launched in China [97].

Carbstone technology

Carbstone technology is based on the reaction of calcium- and magnesium-containing minerals with CO₂, which results in the formation of carbonate binders. Research has shown that these can be mixtures of coal slag and steel slag, which are compacted using a press and then carbonised in an autoclave to produce paving stones made of Carbstone [98]. After forming, the Carbstone blocks undergo CO₂ curing, which is essential for their strength and durability. The average tensile strength of Carbstone paving stones when split was 3.6 MPa [99].

Mineral Carbonation Plant

Experimental trials at a mineral carbonisation facility situated at the Seongam Incineration Plant in Ulsan, Republic of Korea, have demonstrated a reduction in CO₂ concentration to 1.1%, corresponding to an average sequestration efficiency of 89.7%. The CaO conversion rate was >90%. The carbonated product exhibits long-term stability and presents potential for application in the construction sector [100].

This process is also used in Carbon8 and Blue Planet technologies.

Carbon8 technology

CO₂ntainerᵀᴹ, developed by Carbon8, binds CO₂ using accelerated carbonation technology through direct capture from the chimney. It is able to capture between 1500 and 4000 Mg of CO₂ per year [101]. This technology involves the production of aggregates through the direct carbonation of alkaline waste such as steel slag or dust from cement kilns, and safely captures and binds CO₂ within 20 min. The amount of CO₂ captured depends on the reactivity of the waste used and ranges from 10 to 30% of the waste mass. In this technology, the slurry passes through a granulator, into which CO₂ is introduced. CO₂ absorption ranges from 100 to 200 kg CO₂/Mg of aggregate. The product—CircaBuild—can be used for the production of concrete and for road construction [101].

Blue Planet technology

Blue Planet, which has launched a pilot programme in Pittsburgh at the San Francisco Bay Aggregates plant, produces fine and coarse artificial aggregates manufactured through a mineral carbonation process. Waste concrete, cement kiln dust, steel slag, fly ash, bauxite residues, and silicate rocks are calcium-rich sources of so-called geomass, which produce a CO₂-binding aggregate coating. All Blue Planet aggregates are thoroughly tested in accordance with ASTM specifications [102,103]. Every 45,359 kg of Blue Planet CaCO₃ Aggregate contains 19,958 kg of sequestered CO₂ [102].

CarbiCrete technology

The CarbiCrete process involves the production of cement-free concrete that undergoes carbonation for 24 h, after which the cement is completely replaced by steel slag. Products containing CarbiCrete exhibit equivalent or improved mechanical properties and durability compared to traditional concrete. Quebec and Canal Block in Port Colborne, Ontario, manufacture cement-free concrete products using CarbiCrete technology [104]. Cement-Free Decarbonised CMUs meet CSA A165.1-14 (R2019) and ASTM C90 requirements [104].

5. Conclusions

A review of the literature confirms that mineral CO₂ sequestration using metallurgical slags is a promising method supporting the decarbonisation of the steel industry. Owing to their high content of reactive calcium oxide, slags can be effectively applied in direct and indirect carbonation processes, leading to the formation of stable carbonates and the permanent fixation of CO₂.

The effectiveness of carbonation depends mainly on process parameters such as temperature, pressure, liquid-to-solid ratio, and, in the case of indirect processes, the type of extraction reagent. Differences in slag composition and measurement approaches make direct quantitative comparison of results between studies difficult, so the reported data should be interpreted as indicative.

The technologies analysed in this review share a common operational framework based on CO₂ absorption through the aqueous or gas−solid carbonation of steelmaking slags. The approach discussed here focuses on the use of existing metallurgical by-products as reactive materials for CO₂ binding and on integrating carbonation with industrial waste management in line with the circular economy.

Beyond reducing CO₂ emissions, this strategy supports the valorisation of secondary resources and enables the production of useful materials, such as aggregates.

In conclusion, the mineral carbonation of metallurgical slags represents a technologically mature and environmentally beneficial pathway within the broader CCUS framework. With continued optimisation and integration into industrial practice, it could become an important component of the transition toward a low- or zero-emission economy.

6. Directions for Future Research

Despite its great potential, the commercialisation of mineral CO₂ sequestration technology using metallurgical slags requires further research and optimisation. In particular, the following targets are recommended:

• Optimisation of process parameters: the determination of conditions allowing for maximum efficiency with minimum energy input, taking into account the mechanical activation of the material and the influence of the varied chemical composition of slags.
• Development of hybrid technologies: combining direct and indirect methods to improve the efficiency and quality of the carbonates obtained.
• Life cycle assessment (LCA): an assessment of the environmental and economic balance of carbonation processes under industrial conditions, taking into account the energy consumption and by-products generated.
• Research on the stability and application of products: a long-term assessment of the durability of carbonate compounds and development of new ways to use mineralised slags in a circular economy.
• Process scaling and integration with industry: the design of pilot and industrial installations, including the integration of technology with existing steelworks and cement plants.

Mineral CO₂ sequestration using metallurgical slags is a technology with great environmental and economic potential, which, with sufficient research and development, could ultimately become a key element of the industry’s climate neutrality strategy.