Science Direct was the principal database from which the papers were downloaded as a result of its structured approach for searching articles via keywords and filters. Google scholar and Semantic scholar augmented the search with the same keywords. The combinations of keywords used to find literature included “solid waste and grout”, “carbon dioxide and grout”, “carbon mineralization and solid waste” and “carbon sequestration and grout”. The articles were initially filtered on the databases on the basis of the year of publication, i.e., from 2014-2024, the accessibility of the article (open access), and the type of article (restricting it to research and review articles), and 250 articles were ultimately selected on the basis of their titles in relation to the review theme. The articles were then imported into Mendeley and Zotero and further sorted after the titles and abstracts were read, which led to 158 articles being more relevant to the subject. Furthermore, a detailed reading of the 158 articles for relevant information related to the research questions led to 79 most relevant papers that also satisfied the inclusion criteria:

a. CO₂ mineralization and sequestration are related to industrial wastes, natural minerals and synthetic adsorbers made from these wastes and minerals.

b. Research and review articles

c. Publications in the last 10 years (2014-2024)

d. English Language articles

On the basis of the topic under consideration, papers that typically consider grout and its related materials such as concrete and mortar preparation and their subsequent carbonation during or after mixing and, to a lesser extent, use carbonated solid waste-based materials and carbon dioxide adsorbers in grout, concrete and mortar preparation were considered. Among these papers, those published in English and that doubled as research and review articles were included. Figure 1 shows the trend of publication volume reporting on solid waste-based materials applied in carbonation and grout formation. In the year 2024 alone, as many as 25 articles were published as at the time of this review was conducted, demonstrating the relevance of this research area. The sharp drop in research output recorded in 2021 could be attributed to the global shutdown during the pandemic, which affected not only trade but also educational and research institutions observing lockdown protocols, thereby limiting research output, especially for laboratory-based experiments.

A critical look into the article types revealed that many publications were devoted to reviewing previous research. Consequently, the data were broadly classified into review and research articles to provide clarity for analysis. The original and review articles totaled 47 and 32 publications, respectively, each addressing specific materials and/or CO₂ mineralization technologies.

The collected data revealed that a number of original research articles deployed solid waste-based materials in single and mixtures to either become carbonated in dry or slurry form. Currently, there seems not to be a standardized classification for these feedstocks. Researchers often report the names of the materials, describe the source and provide information on their physical properties and chemical or mineralogical composition in the literature, except for review papers, which do not strictly follow that reporting style. In Figure 2 , the solid waste-based materials and other raw materials applied in the forty-seven (47) original research articles are categorized as the main carbonation materials or sources of Ca/Mg ions that react with CO₂.

Twenty-four (24) research studies exclusively employed a singular source or a combination of industrial wastes in carbonation tests. and carbonated ladle refining slag, fly ash, and red mud from bauxite residue, respectively, achieving commendable outcomes. In the carbonation experiment conducted by Lin et al. (2024) , the ladle refining slag exhibited a CaO content of 71.13%, resulting in a carbonation of 217.2 kg CO₂ equivalent per tonne of slag. Additionally, around nineteen (19) of the remaining twenty-four (24) articles combined various industrial wastes, such as BFS, BOFS, EAFS, MSW, GGBFS or GGBS, LFG, SS, CG, ASR, NHL, SF, CFA, CFD fly ash, FGD gypsum, DG, raw and recycled glass, and soda lime, to constitute the primary feedstock for the carbonation experiments. examined the potential for causticity reduction in fresh red mud, which was partially combined with gypsum, through both active and passive carbonation methods. In the active scenario, CO₂ gas was introduced at a flow rate of 0.5 ml/min, while the passive scenario involved slurry carbonation under ambient conditions. In a separate experiment, a composite of cork fly ash (CFA), municipal solid waste fly ash and bottom ash (MSW-FA and MSW-BA) underwent accelerated carbonation in a chamber with 99.99% pure 1 g of CO₂ at 15 bar pressure and ambient temperature for a duration of 72 hours, yielding 0.82 g of CO₂ sequestration. Samples of the identical feedstock exposed to natural carbonation for one month sequestered 2.6 g of CO₂ ().

Concrete or cement-related materials, including demolition waste, followed purely industrial waste materials, and their mixtures were the second most commonly used raw material in the original research, which focused on carbon mineralization. performed accelerated carbonation in a climatic chamber with 5% CO₂ on a mixture of natural aggregates (NA), recycled masonry aggregate (RMA) and OPC and achieved sequestration of 27 kg CO₂/t of mix. The researchers further highlighted the decrease in the porosity, increase in bulk density, enhanced mechanical properties and shortened curing time of the mixture through the accelerated carbonation method using these raw materials. Similarly, achieved 20.4% CO₂ uptake under optimal conditions for a 7-day period when concrete slurry waste with a water/solid ratio in the range of 0 - 0.70 was carbonated via the accelerated carbonation approach.

The remaining material combinations have not received much attention, as the original research samples for this work all fall below five (5) publication articles. Nevertheless, Yuen et al. ( Yuen et al., 2016 ) and Martins et al. ( Martins et al., 2018 ) demonstrated that natural mineral-based wastes or tailings and mixtures of cement-based wastes, industrial wastes and natural minerals have good carbon mineralization potential. Table 1 summarizes the raw materials, their major carbonation compositions and methods of carbonation for industrial wastes, concrete-based wastes and their mixtures.

In the literature, many experiments have sourced natural minerals as feedstocks for carbon mineralization. Table 2 presents a list of carbon mineralization experiments in which mixtures of natural minerals and industrial wastes containing alkali and alkaline silicate minerals were utilized as feedstock raw materials. Magnesium-rich mafic and ultramafic minerals present favorable mineralogy for carbonation reactions because of the presence of Mg, Ca and Fe ions, whose physically, chemically or thermally activated oxides and hydroxides have the potential to undergo spontaneous reactions in the presence of CO₂ under ambient conditions. Brucite is a mineral with excellent reactivity for mineral carbonation ( Stokreef et al., 2022 ). From their literature review, Stokreef et al. (2022) highlighted the influence of the degree of serpentinization on the amount of brucite generated in ultramafic mine tailings, which could serve as raw materials for mineral carbonation. Xie et al. (2016) studied the carbon mineralization of wollastonite [Ca₃(Si₃O₉)], brucite [Mg(OH)₂], manganite [MnO(OH)], olivine [(Mg,Ca,Fe)₂SiO₄], etc., together with coal combustion fly ash, red mud, lime mud, municipal solid waste and some other industrial wastes. The advantages associated with carbonating naturally occurring ultramafic silicate minerals transcend beyond atmospheric CO₂ reduction. Theoretically, the Gibbs free energy released from the spontaneous reaction between carbon dioxide and ultramafic silicate minerals is considered a potential source for electricity generation (). The chemical equations (3) and (4) demonstrate the case of brucite and wollastonite reacting with CO₂ to form MgCO₃ and CaCO₃ with free energy released.

$\text{Mg}{\left(\text{OH}\right)}_2+{\text{CO}}_2\to {\text{MgCO}}_3+{\text{H}}_2\text{O}213.54\text{KWh}/\text{t}$ (3)

${\text{CaSiO}}_3+{\text{CO}}_2\to {\text{CaCO}}_3+{\text{SiO}}_{2}255.6\text{KWh}/\text{t}$ (4)

In practice, natural minerals are not essentially mined and crushed for carbonation experiments; however, the discussions in the literature demonstrate that the tailings emanating from mining operations where the target mineral exists in mafic and ultramafic rock deposits are used in experiments for ex situ or in situ carbonation in tailings storage facilities (TSFs) (). One important consideration when using some of these raw materials for carbonation experiments is pretreatment ( Li et al., 2018 ; Stokreef et al., 2022 ). Broadly, Li et al. (2018) classified pretreatment methods into three categories (3): thermal, chemical and mechanical methods. Subjecting ultramafic rock tailings and industrial wastes to these pretreatment processes enhances reactivity and accelerates carbonation under specified test conditions by liberating Ca/Mg/Fe ions through heating, leaching and increasing the surface area and promoting their bonding with carbonate ions.

Different terms have been used to describe carbon dioxide sequestration or utilization mineralization processes in the literature. Analysis of the literature compiled for this review reveals that there is probably no unified or standardized nomenclature system for the classification of CO₂ mineralization reactions, leading to different names and descriptions for the process. Some of the reaction pathways and sequestration mechanisms are depicted in Figure 3 .

From the literature compiled, two-way classification terms include direct and indirect carbonation; single-stage and two-stage carbonation; ex situ and in situ carbonation; natural carbonation and accelerated carbonation; and active and passive carbonation. There are sub-types for direct, indirect, single-stage,

two-stage, ex situ and in situ carbonation types, and myriads of carbonation types are classified in Figure 4 . Mineral direct air carbon capture (MACC), mineral carbon capture (MCC), the rolling carbonation method, CO₂ bubbling in slurry, CO₂ curing, microwave roasting technology, high gravity carbonation, flue gas absorption liquid reactions and calcium looping cycle technology are the remaining carbonation methods reported in the literature.

Many studies have used these terms to describe mineral carbonation. According to Pan et al. (2015) , in situ carbonation involves the direct injection of CO₂ into underground geologic formations, such as basalt or other igneous rocks. When CO₂ is injected into these formations, it reacts with minerals present in the rock (such as calcium or magnesium silicates), forming stable carbonate minerals and effectively locking away the CO₂ ( Stokreef et al., 2022 ). This process occurs in a deep underground environment, ensuring long-term storage. In situ carbonation benefits from leveraging existing geological formations to permanently fix CO₂ as solid carbonates within the host rock, making it a promising solution for large-scale CO₂ sequestration. However, factors such as the capacity of geological formations, the availability of reactive minerals, and the cost of injecting and monitoring CO₂ storage can pose challenges (; Stokreef et al., 2022 ).

Ex situ carbonation occurs above ground in regulated industrial environments. It employs calcium or magnesium silicate feedstocks, comprising natural ores or industrial byproducts, to react with CO₂ ( Pan et al., 2015 ). Ex situ carbonation may be categorised by two methodologies ( Stokreef et al., 2022 ): direct carbonation and indirect carbonation. The specifics of the sub-types are addressed in the next section. Ex situ carbonation provides versatility in feedstock selection, and the reaction parameters may be refined to enhance efficiency. Nonetheless, it is presently less economically feasible because to the substantial energy requirements for processing and managing materials. Nonetheless, ex situ carbonation presents opportunities in industry, particularly in contexts where alkaline wastes are abundant and may be converted into valuable products like green cementitious materials ( Pan et al., 2015 ).

Direct and indirect carbonation are two primary methods for mineral carbonation, each with distinct mechanisms and efficiencies. It is the most widely used term to describe carbonation according to the analysis of the literature used in this review, as visualized in Figure 3 . Direct carbonation is a simpler, one-step process in which CO₂ reacts directly with solid materials, often in either a gas‒solid or aqueous phase, which are the two main sub-types of direct carbonation ( Wicaksono & Triwigati, 2023 ). In gas‒solid carbonation, minerals such as calcium oxide (CaO) react with CO₂ to form stable carbonates such as calcium carbonate (CaCO₃). In aqueous carbonation, CO₂ dissolves in water to form carbonic acid, which dissociates into bicarbonate and carbonate ions that combine with metal ions such as calcium. The main advantages of direct carbonation include its simplicity and ability to encapsulate heavy metals. However, it suffers from slow reaction kinetics, low carbonation efficiency, and the need for high energy to optimize reactions ( Zhang et al., 2023 ).

Indirect carbonation, by contrast, is a multistep process that first extracts metal ions (e.g., calcium and magnesium) from solid feedstocks via a leaching agent and then reacts these extracted ions with CO₂ to form carbonates. This method allows for more control over the reaction conditions and typically results in higher carbonation efficiency and purer products. However, the use of chemicals such as acids or ammonium salts increases the complexity and cost of operation, and scaling up the process for industrial use poses challenges. Despite the higher operational complexity, indirect carbonation shows greater promise for high-efficiency carbon sequestration ( Wang et al., 2024 ). Table 3 summarizes the main concepts of direct and indirect carbonation with some authoritative literature on the concepts.

In other studies, natural and accelerated carbonation have been used to describe mineral carbonation. Natural carbonation is simply the carbonation of materials under atmospheric conditions of temperature, pressure, humidity and CO₂ concentration. For example, prepared solid waste-based slurry of cork fly ash and municipal solid waste fly ash with ultrapure deionized water with a solid‒liquid ratio of 0.91 and exposed samples of the slurry to atmospheric conditions (natural carbonation) for one month. In the same experiment, samples prepared in the same way as those used for natural carbonation were carbonated in a reactor containing pure pumped gas of 99.99% CO₂ at 15 bar pressure corresponding to 1 g of CO₂; this was the case for accelerated carbonation. The duration of the experiment was 72 hours, and samples were taken at designated times for analysis. In the AC experiments, 0.82 g of CO₂ was captured from an initial mixture weighing 33.6 g. In the NC tests, the amount of CO₂ sequestered increased to 2.6 g, which was attributed to a greater availability of CO₂ and an extended test duration. Owing to the fact that accelerated carbonation is designed to enhance quick carbonation, factors such as temperature, pressure, the concentration of CO₂ gas, the flow rate of slurry and gas, and the rotating speed of the reactor (given that the reaction occurs in a rotating pack bed reactor) must be critically considered in such experiments.

To investigate the quick carbonation of electric arc furnace slag (EAFRS) ( Pan, Chung et al., 2017 ) applied accelerated carbonation, also referred to as high-gravity carbonation (), to an EAFRS slurry in a rotating pack bed reactor, taking into consideration the variations in the pH of the reaction with time. At a solid‒liquid ratio of 25 mL/g, a gas flow rate of 2.5 L/min, a slurry flow rate of 0.6 - 1.5 L/min and other preset conditions, the process achieved a total carbonation of 0.38 kg CO₂ per kg of EAFRS in a duration of 40 minutes.

Qin et al. (2023) applied these terms to describe their carbonation experiments with red mud and gypsum slurry. The slurry was prepared by mixing 50 g of red mud together with 0, 4, 6, 8, or 10% gypsum replacement with 100 mL of deionized water. In the passive carbonation experiment, samples were exposed to atmospheric conditions for a total of 168 days, which, in essence, is no different from the natural carbonation (NC) explained by Sorrentino et al. (2024) . The underlying reaction mechanism involves atmospheric CO₂ in air dissolving in the aqueous component to form bicarbonate and carbonate ions, which subsequently react with the divalent metallic ions leached into the aqueous component from the solid waste materials to form carbonates. Active carbonation, which is also not different from accelerated carbonation, uses the same samples, but the samples are carbonated in a closed system where captured air or CO₂ is injected at a predetermined flow rate into the reaction chamber for carbonation to occur. In the work of Qin et al. (2023) , the prepared slurry was fed into a bottle supplied with gas at 0.5 mL/min flow rate for 8 hours. The samples were collected at specified time intervals for drying and further analyses to determine the degree of carbonation.

In the introduction, the activation mechanisms commonly deployed to induce faster reaction kinetics during mineral carbonation are thermal, physical or mechanical and chemical activation techniques. A commonly deployed mechanical activation method involves grinding samples to reduce the particle size for large surface areas, which in effect increases the rate of carbonation.

Whereas grinding can take place separately before the ground samples are exposed to carbonation reactions ( Rashid et al., 2023 ), researchers have also explored the technique of concurrently grinding and carbonating samples ( Benhelal et al., 2018 ; Julcour-Lebigue et al., 2015 ). Julcour-Lebigue et al. (2015) explained that the inefficiency of direct aqueous carbonation is partly due to the formation of a passivation layer during carbonation, which shields the inner unreacted raw materials from becoming carbonated. The process of grinding while carbonating (concurrent carbonation) tends to break the passivation layer and exposes new surfaces for mineralization to occur. Grinding solid waste materials and conducting carbonation in tandem constitute single-stage carbonation. In a study conducted by Rashid et al. (2023) , olivine, dunite and lizardite feedstocks concurrently ground and carbonized achieved six times the degree of carbonation in a reference experiment without grinding media. Zirconia, alumina and stainless steel were some of the grinding media that were initially used to study optimum grinding conditions while considering the media particle size, temperature, time and blade length of the turbine. Furthermore, they explain the two-stage carbonation as the initial dissolution step followed by the carbonate precipitation step, in which the grinding aspect is still integral. However, the two-stage approach can occur either in an open (bubbling) system or a closed system with controlled pressure and temperature conditions for a specified reaction time. A major challenge associated with concurrent grinding and carbonation is the wear and tear of the reaction chamber caused by the grinding media and reacting components ( Rashid et al., 2023 ). Therefore, a proposal is made for either periodically changing the lining of the chamber or preprocessing the grinding before carbonation or reducing the quantity of grinding media ( Rashid et al., 2023 ).

Under this method of carbonation, different approaches have been discussed in the literature. Carbonated products can be generated via freshly demolished concrete waste and activated natural minerals, as is the case in (), in which they deployed the mechanism of bubbling CO₂ gas into deionozed water (DI) to form carbonic acid, which subsequently dissolves and leaches Ca/Mg/Si ions in freshly demolished concrete waste with some degree of hydration, including C-S-H gels, Ca(OH)₂ and ettringite, and heat-activated serpentine. To induce precipitation, the pH swinging through the initially bubbled CO₂ was undertaken by injecting air into the setup until the pH increased to an alkaline regime favorable for leached divalent metals to react with the carbonate ions initially formed during CO₂ bubbling. In separate experiments, strong 3 M HCl acid was used as the lixiviant to leach the metal ions in the concrete waste and serpentine, while CO₂ bubbling at a flow rate of 30 mL/min was performed in the same system. The pH of the system was maintained at 9 by adding measured NaOH (50 wt%) to ensure the precipitation of carbonates. Whereas the calcium carbonate precipitated from the two-phase pH swing proved to be excellent in replacing cement in concrete, they also concluded that Mg-silicate minerals proved more satisfactory in the pH swing process than did the calcium ions released from hydrated cement waste because of the low solubility of CaCO₃ in the pH regime created by CO₂ bubbling ( Rim et al., 2021 ). The pH swing, in essence, has been mostly categorized as indirect carbonation. However, Rim et al. (2021) reported that a different approach, i.e., CO₂ bubbling and air degassing, was used to achieve the swing instead of the well-established procedure described in the second case of using strong acid and sodium hydroxide to control the pH. Notably, the slurry density (solid‒liquid ratio) and temperature are critical factors that Rim and colleagues considered in their study.

In addition to the CO₂ bubbling approach, other studies have investigated the carbonation of solid waste in its concrete forms after slurry formation. Red mud artificial aggregates (RMAAs) prepared from sintering red mud (SRA) at different solid-liquid ratios before pelletization were subjected to CO₂ curing in a simulated environment with 20% CO₂ and a 60˚C temperature, mimicking the coal power plant gas environment ( Chen et al., 2023 ). Standard curing of the samples was continued for seven (7) days after CO₂ curing for a 3-day period. Among other results, XRD analysis revealed that the initial high peaks of dicalcium silicate (C₂S) substantially decreased after one (1) day of CO₂ curing, whereas high peaks of calcium carbonate were observed, possibly because of its reaction with CO₂ to form calcium carbonate (CaCO₃). Equation (5) is the reaction pathway ( Chen et al., 2023 ).

$2\text{CaO}\cdot {\text{SiO}}_2+{\text{CO}}_2+{\text{H}}_2\text{O}\to {\text{CaCO}}_3+{\text{SiO}}_{2\left(\text{gel}\right)}$ (5)

Moreover, thermogravimetric analysis (TGA) was performed on the original and carbonated samples to reveal the effects of the 0%, 10%, 20% and 30% water‒solid (w/s) ratios on the carbon dioxide uptake and degree of calcium conversion, and the results revealed that the samples with 20% and 30% w/s ratios presented the highest CO₂ uptake rates of approximately 18.5% and 59.3%, respectively, with a calcium conversion degree (CCD), particularly up to the third (3^rd) day of carbon curing ( Chen et al., 2023 ). The CO₂ uptake estimates were performed via the expressions in equations (6) and (7). Water is essential for the hydration reaction to occur to make the hydration products, which are carbonable to be present for the dissolved CO₂ in the same available moisture for the carbon mineralization reaction and sequestration to complete, available. As a result, the samples prepared at 0 and 10% w/s did not have adequate hydration products or sufficient water to dissolve the CO₂ for the carbonation reaction to occur at a faster rate. However, researchers have reported that in the presence of high w/s, the rate of carbonation is initially impeded or slowed because excess water shields red mud particles from CO₂ gas as well as slows holistic diffusion of the gas.

${\text{CO}}_2\left(\text{wt\%}\right)=\frac{\Delta m_{{\text{CO}}_{\text{2}}}}{m_{150˚\text{C}}}\times 100$ (6)

${\text{CO}}_2\text{update}\left(\text{wt\%}\right)=\frac{{\text{CO}}_{2\text{carbonated}\left(\text{wt\%}\right)}-{\text{CO}}_{2\text{initial}\left(\text{wt\%}\right)}}{100-{\text{CO}}_{2\text{carbonated}\left(\text{wt\%}\right)}}$ (7)

In equation one, $\Delta m_{{\text{CO}}_{\text{2}}}$ is the mass loss between 500˚C to 850˚C due to the decomposition of CaCO₃ resulting from carbonation, and $m_{150˚\text{C}}$ is the mass of the dry sample at 105˚C. In equation (2), ${\text{CO}}_{2\text{carbonated}\left(\text{wt\%}\right)}$ is the mass loss rate of the carbonated sample owing to CaCO₃ decomposition, and ${\text{CO}}_{2\text{initial}\left(\text{wt\%}\right)}$ represents the mass loss rate of the original sample due to CaCO₃ decomposition. In carbonation experiments, another established formula for estimating the degree of carbonation is the use of the degree of calcium conversion computed via equation (8) ( Chen et al., 2023 ).

${\delta }_{\text{Ca}}\left(\%\right)=\frac{\frac{\frac{\Delta m_{{\text{CO}}_{\text{2}}}}{m_{150˚\text{C}}}}{100-\frac{\Delta m_{{\text{CO}}_{\text{2}}}}{m_{150˚\text{C}}}}\times \frac{1}{M_{{\text{CO}}_{\text{2}}}}}{\frac{{\text{CaO}}_{\text{total}}\text{wt}\%}{M_{\text{CaO}}}}\times 100$ (8)

where ${\delta }_{\text{Ca}}\left(\%\right)$ represents the degree of calcium conversion (CCD), $M_{{\text{CO}}_{\text{2}}}$ represents the molar mass of carbon dioxide, $M_{\text{CaO}}$ represents the molar mass of CaO, and ${\text{CaO}}_{\text{total}}\text{wt}\%$ represents the total percentage of calcium oxide in the raw material. The remaining parameters are the same as those explained in the previous equations. Theoretically, the quantity of absorbable CO₂ during the carbonation reaction can be estimated via Huntzinger’s formula or Steinour’s formula, as shown in equations (9) and (10) (; Huntzinger et al., 2009 ; Steinour, 1959 ).

$\begin{array}{c}{M}_{th}\left(\%\right)=0.785\left(CaO-0.56CaC{O}_3-0.7S{O}_3\right)+1.091MgO\\ +0.71N{a}_{2}O+0.468\left(K_{2}O-0.632KCl\right)\end{array}$ (9)

$M_{th}\left(\%\right)=0.785\left(CaO-0.7S{O}_3\right)+1.091MgO+1.42N{a}_{2}O+0.935K_{2}O$ (10)

where $M_{th}\left(\%\right)$ represents the hypothetical CO₂ sequestration potential. The higher the sequestration potential is, the higher the carbonation efficiency ( Ahmed et al., 2024 ). However, underscored the point that these theoretical estimates assume idealized conditions and should not be applied in actual estimates where many variables are at play. A carbon curing study was conducted on concrete prepared with OPC and varying amounts of wood-based biochar (WBC) by Roychand et al. (2023) . In addition to investigating the optimum mixture for suitable mechanical properties, the optimum mixture samples were cured in a chamber containing 10% CO₂, 23˚C temperature and 70% relative humidity for 7 and 28 days. The carbonation depth after the test duration was ascertained via phenolphthalein solution (0.5% in 50% ethanol) such that regions on the cured concrete block with hydration products such as Ca(OH)₂ turned pink, whereas the carbonated regions showed no coloration due to the presence of carbonated compounds. The compressive strength test revealed greater strength in the samples cured with carbon for 28 days than in the samples cured for 7 days and the uncarbonated control samples ( Roychand et al., 2023 ).

The calcium looping cycle comprises two main steps: carbonation and calcining (). These two steps work through interconnected circulating reactors in which CaO sorbents capture flue gas CO₂ to form CaCO₃ in the carbonator between 600˚C and 700˚C, and the pure CO₂ generated from calcining the CaCO₃ in the calciner yields back the CaO, which is recirculated for continuation of the reactions shown below in equations 11 and 12 ( Afandi et al., 2024 ).

$\text{CaO}+{\text{CO}}_2\to {\text{CaCO}}_3\text{ΔH}\circ 298\text{K}=-178\text{kJ}/\text{mol}$ (11)

${\text{CaCO}}_3\to \text{CaO}+{\text{CO}}_2\text{ΔH}\circ 298\text{K}=+178\text{kJ}/\text{mol}$ (12)

Sorbents are generally synthesized from industrial wastes, natural minerals and synthetic materials. Regardless of the benefits, high energy consumption is associated with this reversible reaction as well as the sintering and attrition effects resulting from the mechanical movement, pressure and temperature/heat under which the sorbents are subjected ( Erans et al., 2016 ), thereby affecting the sorbent’s stability and number of carbonation cycles ( Afandi et al., 2024 ).

The microwave roasting method applied by ( Han et al., 2020 ) was not specific to the carbonation step but rather focused on the roasting of blast furnace slag and its subsequent leaching. Instead of traditional roasting or heating of the samples to activate particles, they employed the microwave roasting technique, in which ground BF slag uniformly mixed with ammonium sulfate (NH₄)₂SO₄ at a 1:2 mass ratio was roasted in a microwave supplied with 1200 W of power, a 2.45 GHz frequency and a temperature of 340˚C for several minutes. The roasted samples were then subjected to leaching and carbonation tests, and the results were satisfactory, with low energy consumption, a reaction time of 2 minutes, a high sulfation rate and the potential to absorb 261.1 kg CO₂/ton BF slag ( Han et al., 2020 ). Carbonation types classified and unclassified on the basis of the literature used in this review are depicted in Figure 5 .

A careful analysis of the reaction pathways leading to carbon mineralization and sequestration in direct and indirect carbonation approaches demonstrates reaction pathways either by direct reactions between divalent metallic ions in the solid component and CO₂ gas or the leaching of metallic ions and their subsequent carbonation from the dissolved CO₂ gas in the aqueous medium.

In the direct gas‒solid reaction pathway, challenges have been raised with respect to slow reaction kinetics, which is partly due to the stable form in which solid waste or natural minerals may exist. Henceforth, activation mechanisms can excite ions in both the solid and gaseous phases, increase the surface area or leach ions to increase the reaction rates and achieve adequate carbonation in ex situ carbonation. Comparatively, it is easier to mechanically, thermally or chemically activate samples in ex situ carbonation studies or projects than in situ carbonation in geosequestration projects. Researchers have considered the energy demands associated with some activation mechanisms and how they affect the overall aim of sequestering carbon dioxide when it is released in such processes. Life cycle assessment (LCA) and technoeconomic assessment (TEA) have been conducted to determine the energy and cost balances of some mineralization techniques (). Direct aqueous carbonation offers an enhanced reaction pathway by first dissolving and leaching metals from solid waste and simultaneously hydrolyzing CO₂ gas to form carbonate and bicarbonate ions to aid in faster reactions and high degrees of carbonation. Typical reaction pathways for this carbonation mechanism are presented below ( AzdarPour et al., 2018 ; Benhelal et al., 2019 ; Farhang et al., 2019 ; Sanna et al., 2014 ).

$\left(\text{Ca/Mg}\right){\text{SiO}}_2+2{\text{H}}^{+}\to \left({\text{Ca}}^{2+}{\text{/Mg}}^{2+}\right)+{\text{SiO}}_2+{\text{H}}_2\text{O}\left(\text{leaching stage}\right)$ (13)

${\text{CO}}_2\left(\text{g}\right)+{\text{H}}_2\text{O}\left(\text{l}\right)\to {\text{H}}_2{\text{CO}}_3\left(\text{aq}\right)\left(\text{carbon dioxide dissolution}\right)$ (14)

${\text{H}}_2{\text{CO}}_3\left(\text{aq}\right)\to {\text{H}}^{+}+{\text{HCO}}_3^{-}\left(\text{bicarbonate ion formation}\right)$ (15)

${\text{HCO}}_3^{-}\to {\text{H}}^{+}+{\text{CO}}_3^{2-}\left(\text{carbonate ion formation}\right)$ (16)

$\begin{array}{l}\left({\text{Ca}}^{2+}{\text{/Mg}}^{2+}\right)+{\text{CO}}_3^{2-}\\ \to \left(\text{Ca/Mg}\right){\text{CO}}_3\left(\text{nucleation and precipitation of carbonates}\right)\end{array}$ (17)

Indirect carbonation also uses similar reaction pathways in separate steps. Additionally, the acids, bases and ammonium salts used in the leaching process present faster reaction rates in the dissolution stage than water in direct aqueous carbonation technology does. The carbonation step occurs immediately after or simultaneously with the dissolution phase, thereby precipitating carbonates. These experiments are often undertaken at various temperatures, pressures, humidities, CO₂ concentrations, particle sizes, grain sizes and water‒solid (w/s) or solid‒liquid (s/l) ratios and in reactors that synergistically contribute to optimum carbonation reactions and products.

Compared with solution-based methods, carbon curing has a slightly different reaction pathway. During carbonation during CO₂ curing, the gas reacts with the hydration products developed after mixing and curing the samples (standard curing) for 24 hours. Although the samples may contain hydrated water depending on the amount of water used and the nature of the solid components, they are not inundated, as in the case of direct aqueous and indirect techniques. As such, researchers have reported the reaction pathway between the hydration products of dicalcium or tricalcium silicates and aluminates (C₂S/C₃S/C₃A) hydrolyzing to form C-S-H or C-A-S-H gels, Ca(OH)₂ and ettringite, which subsequently enter the carbonation phase to form various carbonate polymorphs, such as calcite, aragonite and vaterite. Ahmed et al. (2024) presented a series of generalized reaction pathways leading to the carbonation of cement-based slurry hydration products in equations 18 to 24 ( Digulla & Bringezu, 2023 ; ; Pan, Shi et al., 2017 ; Zhan et al., 2019 ). Reactions 18 to 20 correspond to the hydration stage, and reactions 21 to 24 involve the carbonation reaction.

$\begin{array}{l}3\text{CaO}\cdot {\text{SiO}}_2+\left(3+y-x\right){\text{H}}_2\text{O}\\ \to x\text{CaO}\cdot {\text{SiO}}_2\cdot y{\text{H}}_2\text{O}+\left(3-x\right)\text{Ca}{\left(\text{OH}\right)}_2\end{array}$ (18)

$\begin{array}{l}3\text{CaO}\cdot {\text{Al}}_2{\text{O}}_3+3\left({\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}\right)+26{\text{H}}_2\text{O}\\ \to 3\text{CaO}\cdot {\text{Al}}_2{\text{O}}_3\cdot 3{\text{CaSO}}_4\cdot 32{\text{H}}_2\text{O}\end{array}$ (19)

$\begin{array}{l}2\text{CaO}\cdot {\text{SiO}}_2+\left(2+y-x\right){\text{H}}_2\text{O}\\ \to x\text{CaO}\cdot {\text{SiO}}_2\cdot y{\text{H}}_2\text{O}+\left(2-x\right)\text{Ca}{\left(\text{OH}\right)}_2\end{array}$ (20)

$\begin{array}{l}3\left(3\text{CaO}\cdot {\text{SiO}}_2\right)+\left(3-x\right){\text{CO}}_2+y{\text{H}}_2\text{O}\\ \to x\text{CaO}\cdot {\text{SiO}}_2\cdot y{\text{H}}_2\text{O}+\left(3-x\right){\text{CaCO}}_3\end{array}$ (21)

$\begin{array}{l}2\left(2\text{CaO}\cdot {\text{SiO}}_2\right)+\left(2-x\right){\text{CO}}_2+y{\text{H}}_2\text{O}\\ \to x\text{CaO}\cdot {\text{SiO}}_2\cdot y{\text{H}}_2\text{O}+\left(2-x\right){\text{CaCO}}_3\end{array}$ (22)

$\begin{array}{l}3\text{CaO}\cdot {\text{Al}}_2{\text{O}}_3\cdot 3{\text{CaSO}}_4\cdot 32{\text{H}}_2\text{O}+3{\text{CO}}_2\\ \to {\text{Al}}_2{\text{O}}_3\cdot x{\text{H}}_2\text{O}+3{\text{CaCO}}_3+3\left({\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}\right)+\left(26-x\right){\text{H}}_2\text{O}\end{array}$ (23)

$x\text{CaO}\cdot {\text{SiO}}_2\cdot y{\text{H}}_2\text{O}+x{\text{CO}}_2\to x{\text{CaCO}}_3+{\text{SiO}}_2\cdot z{\text{H}}_2\text{O}+\left(y-z\right){\text{H}}_2\text{O}$ (24)

In the course of mineral carbonation, researchers have established several relevant factors that impede reaction rates and general reaction kinetics. Mathematical models have therefore been devised to model and study the interplay of these factors slowing reactions. The shrinking core model and surface coverage model are the two most common mathematical models discussed in the literature for investigating the reaction kinetics of CO₂ mineralization.

The shrinking core model assumes that reaction particles are spherical in shape and nonporous and react isothermally with the surrounding fluid to yield a nonreactive outer layer surrounding a reactive inner core ( Digulla & Bringezu, 2023 ; Myers et al., 2019 ). Equation (25) is the general mathematical law governing reaction rates according to the shrinking core model parameters ( Myers et al., 2019 ).

$t=\frac{\rho r^2}{6D{C}_g}\left\{1-3{\left(\frac{r-\delta }{r}\right)}^2+2{\left(\frac{r-\delta }{r}\right)}^3\right\}$ (25)

where the time required to achieve a given reaction depth (δ) as a function of diffusivity (D) is (t), the unreacted material density is (ρ), the particle radius is (r), and the gas phase CO₂ concentration is (C_g). noted that the general equation does not consider the surface roughness factor (RF) of the grains and particles,which could impact the initial reaction rates and reaction depth (δ). Considering this factor, the new equation for estimating the accelerated reaction time (t’) is as follows:

$t^{\prime }=\frac{\frac{\rho r^2}{6D{C}_g}\left\{1-3{\left(\frac{r-\delta }{r}\right)}^2+2{\left(\frac{r-\delta }{r}\right)}^3\right\}}{\left(H/\delta \right)RF}$ (26)

H is the largest scale at which the fractal dimension of a sample is constant, as determined by the box-counting method ( Myers et al., 2019 ). They further suggested replacing the diffusivity (D) with bulk diffusivity (D_bulk) when studying heterogeneous samples.

Four underlying assumptions are considered for the surface coverage model in the mineral carbonation reaction according to ( Chen et al., 2020 ).

The general equation is shown below:

$r_s=k_s\phi =\frac{1}{S_{g}M}\left(\frac{\text{d}{\delta }_{\text{CaO}}}{\text{d}t}\right)$ (27)

where $k_s$ is the rate constant (mole.min⁻¹ m⁻²), $\phi$ represents the fraction of active sites uncovered by the reacted products, $M$ (g/mol) is the molecular weight of the reactant solid particle, $S_g$ (m²g⁻¹) is the initial specific surface area of the solid particle, and $\frac{\text{d}{\delta }_{\text{CaO}}}{\text{d}t}$ is the rate of carbonation conversion. However, these models have rarely been applied in studies of the reaction kinetics of solid waste-based slurry or grouts carbonated for geotechnical support.

Mineral carbonation reactions not only target reducing the global emissions of carbon dioxide gas but also generate useful products for industrial applications. Largely, the building and construction industry has emerged as having the potential to be an enormous beneficiary of mineral carbonation according to the literature. The demand for cement and the continuous expansion of cities and municipalities in terms of building construction, bridges, dams and roads influence the copious release of CO₂ gas into the atmosphere and, at the same time, cement waste and construction demolition waste generation. The mining and metallurgical industries, on the other hand, also contribute to waste generation in the form of waste rocks, tailings and slags. In a similar vein, power generation sectors, which rely on burning coal, leave behind fly ash and other wastes, including the flue gas released into the atmosphere. Through continuous research, researchers have found that the reactivity of these waste streams, which was previously discussed, is favorable for mineral carbonation to produce cement-based materials, thus recarbonating ( Ho et al., 2024 ) and precipitating calcium carbonate (PCC) ( Meijssen et al., 2023 ) for concrete, mortar and grout, which is useful in the construction industry. Therefore, Table 4 shows a number of engineering construction applications in which carbonated solid waste materials in solid or slurry forms were applied.

In investigations of carbonated solid waste applied in concrete or mortar slurry, researchers commonly study engineering strength by testing the uniaxial compressive strength and tensile strength (; Zhang et al., 2016 ; Assaggaf et al., 2019 ) to compare the strength of control samples, which are practically verifiable, as they conform to national and international standardized tests such as the Chinese GB/T and ASTM standards. The strength test is the only requirement to meet for safe practical use of a material, and if the material meets the set specification, its usage is approved. However, this is not always the case. Further tests for environmental safety have been conducted, as was the case in some of the literature ( Chen et al., 2023 ). The heavy metal leaching test is a ubiquitous test for solid waste-based concrete and slurry products with the potential to leach lead, arsenic, zinc, chromium and a number of other heavy metals or potentially toxic elements that could be associated with the raw materials before the carbonation reaction is carried out. Hydration products such as C-A-S-H gels have been demonstrated to stabilize heavy metal ions ( Liu et al., 2018 ). Considering that carbon is sequestered through CO₂ bubbling into the slurry and injecting into underground voids for ground stability and CO₂ fixation and storage, leaching tests are relevant to ensure groundwater safety prior to injecting the slurry. In such cases, both mechanical and leachability tests should be undertaken for ground stability and safety assurance. The paper and plastics industries are also reported to utilize PCC as a raw material for production ().