Comparative study on the enhancing effects of CaSO₄ and CaCl₂ supplementation on red mud carbonation

Abstract

Red mud (RM), a strongly alkaline solid waste generated during alumina production, can undergo carbonation with CO₂ for mineral sequestration. To investigate the promoting effect of external calcium sources on RM carbonation, desulfurization gypsum (CaSO₄) and calcium chloride (CaCl₂) were selected as supplements, and RM samples with/without these calcium sources were prepared. Experiments were conducted under various CO₂ concentrations (100, 15, 1%) and atmospheric conditions for both RM suspensions and solid-state RM (simulating open-air piles). The results showed that: (1) With increasing CO₂ concentration, the time for RM suspensions to reach pH equilibrium shortened (30 min for 100% CO₂ vs. 15 h for 1%), and the equilibrium pH decreased (to 6.8 for 100% CO₂ vs. 8.3 for 1%); (2) Under atmospheric conditions, the pH of RM suspensions supplemented with CaSO₄ and CaCl₂ decreased to 8.6 and 8.0, respectively, with CaCO₃ characteristic peak intensity increasing compared to pure RM; (3) For solid RM, the two calcium sources lowered the minimum pH to 8.8 (CaSO₄) and 8.4 (CaCl₂), ultimately stabilizing around 9.0, whereas pure RM remained at 10.1. The CO₂ sequestration capacities reached 45.3 g/kg and 47.2 g/kg, respectively, while forming a porous CaCO₃ coating on the RM particles. The calcium sources significantly enhanced the stability and durability of the carbonation reaction, providing a scientific basis for long-term CO₂ sequestration.

1 Introduction

Since the Industrial Revolution, carbon dioxide (CO₂) emissions have increased significantly. The widespread use of fossil fuels has led to numerous climate and environmental issues (Yu et al., 2022). Carbon capture, utilization, and storage technologies have garnered significant attention due to their substantial potential for reducing emissions. Among these, mineral carbonation storage technology is a research hotspot in the field of carbon sequestration due to its several benefits, including permanence, safety, and resource efficiency (Feng et al., 2024; Yin et al., 2025; Weng et al., 2024). Alkaline solid wastes, including steel slag, fly ash, calcium carbide residue, and RM, can all be safely and permanently sequestered as stable carbonates through mineralization and storage, according to numerous researchers who have since studied the carbonation of alkaline minerals (Wang et al., 2021; Altiner, 2019; Lin et al., 2017). Long-term CO₂ sequestration is made possible by this method, which also successfully mitigates environmental hazards associated with solid waste by converting it into potentially profitable building materials or chemical feedstocks (Zhang Q. et al., 2025; Zhang et al., 2022; Jiang et al., 2022). Due to its strong alkalinity, large production volume, comparatively low calcium content, and substantial carbon sequestration potential, RM has been a prominent topic in mineral carbonation research among various industrial solid wastes (Li et al., 2020; Clark et al., 2015; He et al., 2014).

Certain calcium-bearing minerals found in RM, such as calcium silicate and tricalcium aluminate, can release Ca²⁺ during carbonation. Stable CaCO₃ is created when this Ca²⁺ combines with dissolved CO₂ (as carbonate ions) (Khaitan et al., 2010; Revathy et al., 2017; Dilmore et al., 2007). However, the active calcium content in RM is limited, and its occurrence state is complex. This makes it challenging to accomplish effective and stable CO₂ sequestration due to low natural carbonation efficiency, sluggish response rates, and a propensity for pH rebound. As a result, improving RM’s carbonation efficiency and long-term stability has become a crucial issue in this industry that requires immediate attention. RM carbonation has been the subject of numerous studies in recent years, with a primary focus on the following areas: first, process parameter optimization; second, reaction mechanism investigation; third, carbonation product properties and applications; and fourth, the impact of external additives, specifically the role of calcium supplementation in promoting the reaction process.

Shen (2023) introduced pure CO₂ into sintered RM slurry and found that the specific surface area of the carbonated RM significantly increased, with the formation of fine calcite crystals. Su (2020a) investigated the carbonation dealkalization process, achieving a dealkalization rate of 30.3% under optimized conditions of temperature, pressure, and solid–liquid ratio. Studies by Zhang W. C. et al. (2025) and Duraisamy and Chaunsali (2025) have shown that post-mineralization materials exhibit improved compressive strength and lower carbon emission intensity. These studies validate the feasibility of RM carbonation for carbon sequestration and performance enhancement. The majority of studies, however, concentrate on high CO₂ concentrations or pressurized circumstances, which leads to a lack of alignment with real atmospheric storage or use scenarios. Research focuses on process effectiveness, but long-term stability, pH rebound problems, and the mechanisms controlling calcium ion migration and transformation during carbonation are not fully explained. Systematic comparative studies on the impacts of various calcium sources in both slurry and solid reaction systems are still noticeably lacking, despite the recognition of the significance of supplementing external calcium sources.

Smith et al. (2003) showed that even after a longer period of low pH, the pH rebounded through the slow dissolution of carbonate minerals in the form of tricalcium aluminate (3CaO·Al₂O₃). Rai et al. (2013) reported that after 15 min of contact between RM and high-concentration CO₂, the pH of the RM slurry dropped to 10.77–7.466. However, once the contact with CO₂ ceased, the pH rebounded, particularly when no additional Ca²⁺ was supplied, indicating that the pH decrease in this reaction is temporary. This result emphasizes how important it is to supplement calcium sources externally. Ca²⁺ and CO₂ are essential components for the carbonation process in RM. Ca²⁺ content is thought to be the carbonation reaction’s limiting factor as long as CO₂ is continually supplied, requiring Ca²⁺ addition to maintain the reaction (Liu, 2019). Han and Tokunaga (2014) investigated the promoting effect of CaSO₄ on soil carbonation, providing insights for utilizing industrial by-product gypsum to enhance carbon sequestration in RM. RM’s capacity to sequester CO₂ can be greatly increased by raising the concentration of Ca²⁺ (Maryol and Lin, 2015; Li, 2017). However, existing literature lacks comparisons of the effects of calcium sources with different properties on carbonation under simulated atmospheric conditions. Additionally, the majority of research uses expensive pure chemical calcium sources. Treating waste with waste and cutting expenses are two advantages of using large industrial solid wastes, such as desulfurization gypsum, as calcium supplies.

Therefore, using industrial by-product desulfurization gypsum and CaCl₂ solution as calcium sources, this study methodically examined the promotion effect and mechanism of exogenous calcium supplementation on the carbonation of RM under atmospheric and various CO₂ concentration conditions. Initially, the study investigated how the addition of calcium sources affected pH evolution and equilibrium during RM slurry carbonation at varying CO₂ concentrations. Second, under long-term simulated atmospheric circumstances for both RM slurry and solid-state carbonation, it compared the variations in calcium ion behavior, final pH, and product stability among various calcium sources. Lastly, the study clarified how additional calcium sources improve the long-term stability of CO₂ sequestration by changing the mineral composition and microstructure of carbonation products. This study provides theoretical foundations and data support for low-cost carbon sequestration and the resource utilization of RM.

2 Materials and methods

2.1 Materials

2.1.1 Red mud (RM)

The second RM pile at the Chalco (China Aluminum Corporation) Shandong Branch provided the RM utilized in this study. Table 1 displays the findings of a wet chemical analysis performed on a representative sample of the RM. According to the examination, the RM contains a considerable amount of Si, Al, and Fe, primarily in the form of stable oxides such as SiO₂, Al₂O₃, and Fe₂O₃. Heavy elements such as Mn, Zn, Cu, Cr, and Pb were also found in trace concentrations (Li et al., 2019). According to the RM sample’s wet sieving examination, 78% of the sample included particles smaller than 48 μm. The RM sample’s density and porosity were determined to be 2.0 g/cm³ and 45%, respectively. RM has diverse physical and chemical characteristics depending on the environment (Xu et al., 2013). Under normal circumstances, aragonite and calcite, which constitute the skeletal minerals of RM, serve as the source of the initial CaCO₃ in RM. Their total content, determined via XRD semi-quantitative analysis, is approximately 5.49 wt.%. These minerals exhibit limited hydrophilicity. Free SiO₂, Al₂O₃, and Fe₂O₃ are the compounds in RM that are actually hydrophilic. The amount of adsorbed water that forms increases with the concentration of these chemicals. Consequently, only a small quantity of gravitational water is released under vibration, if any, even though RM might have a moisture content of 40 to 70%. RM’s primary mineral constituents are Al₂Si₂O₅(OH)₄, Mg₂CO₃(OH)₂(H₂O)₃, and AlO(OH).

2.1.2 Desulfurization gypsum (CaSO₄)

The desulfurization gypsum used in this experiment was obtained from Zouping County, Shandong Province. It appears white, with fine particles and a powdery texture. The measured pH was 5.68. The chemical composition of the desulfurization gypsum is shown in Table 2. The main chemical components of the desulfurization gypsum are CaO, SO₃, SiO₂, and MgO. The mineral composition of the desulfurization gypsum used in this experiment mainly consists of hydrated calcium sulfate.

2.1.3 Calcium chloride (CaCl₂)

The anhydrous calcium chloride used in this experiment was obtained from Tianjin Juhengda Chemical Co., Ltd. It appears as white, porous particles that are hygroscopic and easily soluble in water, ethanol, acetone, and acetic acid. The pH of the substance ranges from 8.0 to 10.0, with a relative molecular mass of 110.98. It complies with the standard HG/T 5439-2018 [Ministry of Industry and Information Technology (MIIT), 2018].

2.1.4 High-purity CO₂ and N₂

The high-purity CO₂ used in this experiment was supplied by Yantai Deyi Gas Co., Ltd., with a concentration of 99.9%, and complies with the standard GB/T 23938–2009 (General Administration of Quality Supervision, Inspection and Quarantine of the People’s Republic of China, Standardization Administration of the People’s Republic of China, 2009). The high-purity N₂ was supplied by Qingdao Deyi Gas Co., Ltd., with a concentration of 99.999%, and complies with the standard GB/T 8979–2008 (General Administration of Quality Supervision, Inspection and Quarantine of the People’s Republic of China, Standardization Administration of the People’s Republic of China, 2008).

2.2 Specimens preparation

• Place the RM in an oven and dry it at 105 °C to a constant weight. Then, weigh 40 g of the RM, and measure 400 mL of distilled water. Prepare a fresh RM suspension with a concentration of 100 g/L in a 600 mL glass reactor. Divide the fresh RM suspension sample into three groups using the quartering method. Using the CO₂ and N₂ flow rate equilibrium method, introduce CO₂ at concentrations of 100, 15, and 1% into the suspension, and conduct experiments on the carbonation of the RM suspension at various CO₂ concentrations. To minimize experimental error, three parallel samples were set up for each test group, with the average of three measurement data points serving as the final result. The schematic diagram of the main research content is shown in Figure 1. The experimental gas passes through a hydrophobic filter to prevent air from the atmosphere from backflowing into the reactor. The purpose of this reactor design is to maintain a constant flow rate of gas in the top space of the reactor while simultaneously measuring the pH and electrical conductivity (EC) of the suspension, and ensuring that no gas leakage occurs within the reactor.

• Place the RM in an oven and dry it at 105 °C to a constant weight. Then, prepare three sets of 250 mL RM suspension samples using distilled water. Three parallel samples were set up for each test group, with the average of three measurement data points serving as the final result. The first group of samples is a pure RM suspension with a concentration of 50 g/L, used for the carbonation experiment of pure RM. The second group of samples uses desulfurization gypsum as an external calcium source. A 250 mL test solution (RM + CaSO₄) is prepared by mixing 50 g/L of RM with 50 g/L of gypsum, which is used for the carbonation experiment of RM supplemented with gypsum as the calcium source. The third group of samples uses calcium chloride (CaCl₂) as an external calcium source. RM is added to a 0.1 mol/L CaCl₂ solution to prepare a 250 mL test solution (RM + CaCl₂), which is used for the carbonation experiment of RM supplemented with CaCl₂ as the calcium source.

• To simulate the conditions under which RM, exposed in open-air piles, undergoes mineral carbonation reaction with atmospheric CO₂, 10 kg of fresh RM is dispersed over an area of 1 m², allowing the RM to fully contact and react with CO₂ in the atmosphere. Three parallel samples were set up for each test group, with the average of three measurement data points serving as the final result. After the initial mixing, three groups of RM samples are prepared, with and without various external calcium sources. Multiple studies in the field of industrial solid waste mineral carbonation have confirmed that a calcium source addition of 1–2 mol/kg represents a reasonable range balancing reaction efficiency and economic viability (Duraisamy and Chaunsali, 2025). Therefore, this study selected 1 mol/kg as the addition amount for both CaSO₄ and CaCl₂, with the core objective of ensuring sufficient Ca²⁺ supply in the system to clearly demonstrate the promoting effect of the calcium source on RM carbonation. The original RM that has not undergone carbonation is designated as RM₀. The first group of samples is pure RM without any added calcium source. The second group of samples is RM with the addition of 1 mol/kg of CaSO₄ (RM + CaSO₄). The third group of samples is RM with the addition of 1 mol/kg of CaCl₂ (RM + CaCl₂). These three sets of specimens were used to conduct carbonation tests on solid RM under atmospheric conditions.

Figure 1

2.3 Experimental methods

2.3.1 pH determination

The experiment utilized a Mettler Toledo bench-top pH meter, model FE28-Standard, equipped with a LE438 three-in-one glass pH composite electrode. The pH meter was calibrated before each test. After each measurement, the electrode was rinsed with deionized water and dried with filter paper to remove residual moisture, preventing interference with subsequent measurement accuracy.

2.3.2 X-ray diffraction test (XRD)

The X-ray diffractometer used in this experiment is a Bruker X-ray diffractometer, model D8 ADVANCE. The samples were placed in a 2 mm deep sample holder. Mineral composition was determined between 5° and 80° with a resolution of 0.02° and a dwell time interval of 2 s (Ji, 2021). After the experiment, phase identification was performed using Jade 6.5 software, and semi-quantitative phase analysis was conducted using the reference intensity ratios (RIR) method. The mineral compositions of the RM and modified RM composite materials were analyzed and compared based on the PDF card library.

2.3.3 Thermogravimetric analysis test (TGA)

The thermogravimetric analyzer used in the test is a Mettler-Toledo TGA2 model. Place 5–10 mg of sample into a crucible, position the crucible on the crucible holder, then close the instrument. Use high-purity nitrogen as the protective gas. Set the test temperature range to 30–1,000 °C with a heating rate of 10 °C/min.

2.3.4 Scanning electron microscopy-energy dispersive spectroscopy test (SEM-EDS)

The scanning electron microscope used in this experiment was an Apreo SHiVac field emission scanning electron microscope equipped with a secondary electron detector. It operated at a voltage of 10 kV and employed area scanning. A carbon conductive tape was applied to the specimen holder, and the sample was firmly attached to the tape to ensure a secure fixation on the holder. After fixation, the sample was gold-coated for conductivity. The sample was subsequently magnified at an appropriate magnification for detailed observation, enabling further analysis of the surface morphology, crystal formation, and hydration state of the RM-based composite material at the microscopic level.

3 Results and analysis

3.1 Analysis of the effect of various CO₂ concentrations on the carbonation of RM suspensions

3.1.1 Analysis of the effect of various CO₂ concentrations on the pH of the samples

Under the condition of no external calcium source, the CO₂ and N₂ flow rate balance method was employed to introduce CO₂ at concentrations of 100%, 15%, and 1% into the RM suspension for testing its neutralizing effect on the RM (Li et al., 2013). CO₂ was introduced at a flow rate of 1 L/min for each concentration, while the suspension was continuously stirred with a magnetic stirrer at 200 rpm. After 24 h of CO₂ aeration, the gas supply was stopped. The pH and electrical conductivity (EC) of the RM suspension were continuously measured to assess the pH rebound of the RM when exposed to atmospheric conditions.

The gas absorption equilibrium curves for various CO₂ concentrations are shown in Figure 2. The pH of the RM suspension gradually decreased to equilibrium as the aeration time increased. When CO₂ at 100% concentration was introduced, the pH of the RM suspension rapidly decreased to equilibrium within 30 min. When CO₂ at 15% concentration was introduced, the pH gradually decreased to equilibrium within 3 h. With CO₂ at 1% concentration, the pH reached equilibrium after 15 h. The time to reach equilibrium decreased with increasing CO₂ concentration. The RM suspension aerated with 100% CO₂ reached equilibrium almost 15 h faster than the suspension aerated with 1% CO₂. The equilibrium pH values for the CO₂ concentrations of 100, 15, and 1% were 6.8, 7.6, and 8.3, respectively. The equilibrium pH varied with the CO₂ concentration, with the pH of the RM suspension aerated with 100% CO₂ being 1.5 units lower than that aerated with 1% CO₂. This is because CO₂ dissolves in the pore water of the RM suspension, reacting with water molecules to form carbonic acid (H₂CO₃). Carbonic acid is a weak acid that exists in dynamic chemical equilibrium in solution, transitioning between carbonate ions (CO₃²⁻) and bicarbonate ions (HCO₃⁻). The bicarbonate ions further react with more water molecules to form carbonate ions, which are precursors to carbonate precipitation. The chemical reaction principle is presented as Equations (1–6) (Ilahi et al., 2024):

Figure 2

3.1.2 Analysis of pH rebound of the samples after exposure to the atmosphere following gas aeration equilibrium

Figure 3 shows the pH rebound of the RM suspension after exposure to the atmosphere following gas aeration equilibrium. Although the pH of the suspension rapidly decreased during CO₂ aeration, once the aeration was stopped and the reactor was opened to allow the RM suspension to fully interact with the atmosphere, the pH of the suspension quickly rebounded to between 9.3 and 9.4. Among them, the RM suspension aerated with 100% CO₂ showed a pH rebound of 2.6, the suspension aerated with 15% CO₂ showed a pH rebound of 1.77, and the suspension aerated with 1% CO₂ showed a pH rebound of 1.0. The RM suspension aerated with 100% CO₂ exhibited a greater pH rebound compared to the other two samples. This indicates that the pH decrease observed in Figure 2 is due to the dissolution of CO₂ gas in the aqueous phase in the form of H₂CO₃ or CO₂. In simulated experiments on RM systems, Smith et al. (2003) measured a dissolution rate of Ca₃Al₂O₆ at 1.2 × 10⁻⁸ mol/(m²·s) under conditions of pH 8.5 and 25 °C. The dissolution rate of Ca₃Al₂O₆ is significantly lower than the degassing rate of carbonates. Based on the pH rebound curve, the pH rapidly recovers within 4 h after gas cessation, consistent with the kinetics of gas degassing. Dissolved H₂CO₃ and HCO₃⁻ in the sealed system rapidly decompose into CO₂ upon exposure to the atmosphere, causing a decrease in H⁺ concentration and a rapid pH rebound. After 4 h of gas cessation, the pH of all samples increases slowly. This phenomenon stems from the gradual dissolution of Ca₃Al₂O₆. As the suspension is stirred under atmospheric conditions, the dissolved carbonates gradually degas. When considering only the influence of soluble alkalinity in the pore water, the pH equilibrium value of the RM is 10.0. The rapid pH decrease observed in the closed system is also the result of the depletion of alkalinity in the pore water. This finding highlights the importance of supplementing calcium sources, as the addition of calcium is more favorable for the formation of stable carbonate minerals, thereby achieving long-term CO₂ carbonation. Therefore, unless the original RM contains sufficient calcium, the addition of an external calcium source is considered a necessary condition for the carbonation of alkaline waste slag minerals.

Figure 3

3.2 Analysis of the effect of various calcium sources on the carbonation of RM suspension under atmospheric conditions

3.2.1 Analysis of the effect of various calcium sources on the pH of the samples

Under atmospheric conditions at room temperature and pressure, a long-term batch experiment was conducted on three groups of RM suspensions, including those with various external calcium sources and those without any external calcium sources. The three groups of samples (RM, RM + CaSO₄, and RM + CaCl₂) were continuously stirred at 400 rpm using a magnetic stirrer for 56 days. Distilled water was added to the reactor every day to maintain a constant solution volume and total weight. After each volume adjustment, the pH and electrical conductivity (EC) of the suspension were measured 30 min later. Every 5 days, an aliquot was collected from each sample to measure the calcium ion concentration.

The pH and Ca²⁺ concentration changes over time for the three groups of samples during the experiment are shown in Figure 4. The addition of external calcium sources significantly enhanced the carbonation efficiency of the RM. The carbonation test of pure RM successfully eliminated most of the alkalinity in the pores of the RM. Under atmospheric exposure, the pH of all three groups of samples decreased to below 10. In contrast, the RM suspension with added external calcium sources exhibited a lower pH value. When considering only the effect of pore water alkalinity, the equilibrium pH of RM is 9.9 (Khaitan et al., 2009a), and the pH of the sample without an added calcium source is 10, which is in close agreement with the measured equilibrium pH of the pore water. The pH of the sample with added CaSO₄ reached 8.6, which is 1.1 lower than the pH of the sample without an added calcium source. The pH of the sample with added CaCl₂ reached 8.0, which is 1.7 units lower than that of the sample without added calcium source. This indicates that the addition of external calcium sources led to higher mineral carbonation. As the experiment progressed, the change in Ca²⁺ concentration verified the presence of the mineral carbonation reaction, as seen in Figure 4b. CaCl₂ is a highly soluble salt with an extremely rapid dissolution rate, dissociating instantaneously into Ca²⁺ and Cl⁻ ions within the experimental system. The initial Ca²⁺ concentration in the RM + CaCl₂ sample was significantly higher than in other groups, and its initial pH decline rate was slightly faster than that of the RM + CaSO₄ group. This indirectly demonstrates that its dissolution rapidly supplies Ca²⁺, preventing it from becoming a rate-limiting factor. CaSO₄ exists in dissolution equilibrium, enabling stable maintenance of Ca²⁺ concentration in aqueous solutions. In this study, the Ca²⁺ concentration in the RM + CaSO₄ sample remained consistently stable at 450–500 mg/L over the long term, consistent with the Ca²⁺ concentration range of saturated gypsum solutions. This indicates that its dissolution rate can continuously match the reaction demand driven by CO₂ dissolution. Since Ca²⁺ was present in the form of CaCO₃ precipitation or other calcium-containing phases, the pure RM sample had the lowest Ca²⁺ content.

Figure 4

3.3 Analysis of the effect of various calcium sources on the carbonation of solid RM under atmospheric conditions

3.3.1 Analysis of the effect of various calcium sources on the pH of solid samples

Three sets of test samples (RM, RM + CaSO₄, and RM + CaCl₂) were placed in the atmosphere and mixed in the same manner to simulate the mineral carbonation reaction between RM, exposed to the atmosphere, and CO₂. The experiment lasted for 115 days. The RM layer in this study was laid at a thickness of 5 mm to ensure thorough contact between CO₂ and the RM. This experiment focused on the surface core region within the industrial pile where CO₂ can effectively diffuse, and reactions can fully occur, aiming to precisely elucidate the mechanism by which calcium sources promote the carbonation of RM. This experiment involved only one thorough mixing during sample preparation to ensure full contact between RM and the external calcium source. No additional mixing occurred during subsequent testing, fully simulating the static accumulation state of industrial piles and avoiding artificial mixing interference with the natural diffusion and reaction processes of CO₂. Samples were then taken for pH measurement and X-ray diffraction analysis to determine the main elemental composition of the RM.

The pH changes of the three test samples over time during the experiment are shown in Figure 5. On the 72nd day of the experiment, the pH of the pure RM sample decreased to 10.1. In the subsequent period of the experiment, the pure RM sample continued to be exposed to air, and its pH stabilized without further decline. This indicates that the alkalinity in the pore water of the RM was depleted due to the reaction with CO₂ in the atmosphere. Due to the rapid consumption of pore water alkalinity after the addition of external calcium sources, the two groups of samples with added CaSO₄ and CaCl₂ exhibited lower initial pH values. With the progression of the mineral carbonation reaction, the pH of these two groups of samples gradually decreased. The sample with added CaSO₄ showed a pH of 8.8 on the 56th day of the experiment, and then increased slightly to around 9.0 during the subsequent test. The pH of the CaCl₂-added specimens decreased to 8.4 on the 42nd day of the test and increased slightly during the subsequent tests, basically remaining at about 9.0. External calcium sources can influence the final pH equilibrium value. However, compared to the RM suspension test system, the rate of pH decrease in solid-state RM tests is less affected by the type of calcium source added.

Figure 5

3.3.2 Mineral composition analysis

The XRD patterns of the test samples before and after the experiment are shown in Figure 6. The main phases of Bayer-process RM are quartz, calcite, calcium zeolite, gibbsite, and boehmite. During the mineral carbonation process, carbonation reactions occur simultaneously, where Ca²⁺ in the sample reacts with CO₂ from the atmosphere to form CaCO₃ precipitation. To confirm the presence of CaCO₃, three sets of RM samples were measured by XRD tests after a period of exposure to the atmosphere. From the figure, after carbonation, the characteristic diffraction peak intensity of CaCO₃ (at 2θ = 29.4°) increased in all three sample groups. Among them, the sample containing added CaCl₂ exhibited the greatest increase in CaCO₃ diffraction peak intensity following carbonation. Using the RIR method for semi-quantitative analysis of CaCO₃ production, the uncertainty in the weight percentage of CaCO₃ is “±0.3 wt.%.” The weight percentage of CaCO₃ in pure RM samples increased by 2.8 wt.% after carbonation. In contrast, the weight percentage of CaCO₃ in samples with added CaSO₄ increased by 4.8 wt.% after carbonation, while that in samples with added CaCl₂ increased by 5.2 wt.%. The proportional increase in peak intensity directly reflects the trend of increased CaCO₃ phase content. Indicates that CO₂ reacts with alkaline substances in RM to form CaCO₃ with relatively complete crystallinity. It can be observed that compared to the samples with added CaSO₄ and CaCl₂, the pure RM sample shows the smallest change in the intensity of the diffraction peaks after the carbonation reaction, indicating that Bayer-process RM has a relatively low reactivity with CO₂. Specimens with added CaSO₄ and specimens with added CaCl₂ showed an increasing trend in CaCO₃ precipitation after 100 days of reaction in the atmosphere. This is a good indication that the addition of a calcium source facilitates the carbonation reaction of RM.

Figure 6

3.3.3 Carbon sequestration potential analysis

According to XRD test results, the primary change in RM after carbonation is the formation of CaCO₃ through the reaction of calcium-containing mineral phases. Therefore, for further investigation, thermogravimetric analysis (TGA) was conducted on the carbonated RM samples. Mass loss between 400 °C and 800 °C corresponds to the decomposition of various CaCO₃ phases (Wang et al., 2024). The mass loss at this temperature range was analyzed to quantify the proportion of CaCO₃ in the samples.

Calculate the actual CO₂ mineralization of the carbonized RM samples according to Equation 7 (Wang et al., 2024).

In the above equation:

• : actual CO₂ mineralization;

• : mass of the sample after calcination at 400 °C;

• : mass of the sample after calcination at 800 °C.

The weight loss curves, CaCO₃ content, and actual CO₂ mineralization of the samples are shown in Figure 7. The carbon sequestration calculations for each sample are shown in Table 3. The CaCO₃ content in original red mud (RM₀) is only about 5.5 wt.%, primarily consisting of aragonite and calcite, which form the skeletal minerals of RM. After carbonation treatment, the CaCO₃ content in the RM group increased to 8.33 wt.%, representing a 2.83 wt.% increase compared to RM₀, while the CO₂ sequestration capacity simultaneously rose to 36.6 g/kg. The introduction of an external calcium source significantly enhanced the carbonation effect on RM. In samples treated with CaSO₄, the CaCO₃ content rose to 10.3 wt.%, representing a 4.8 wt.% increase over RM₀, with CO₂ sequestration capacity rising to 45.3 g/kg. Samples treated with CaCl₂ demonstrated optimal performance, achieving a CaCO₃ content of 10.72 wt.%—a 5.22 wt.% increase over RM₀—and corresponding CO₂ sequestration capacity of 47.2 g/kg. This result clearly indicates that exogenous calcium sources can effectively promote the formation of stable CaCO₃, thereby significantly enhancing its CO₂ sequestration potential. Notably, CaCl₂ exhibited a stronger enhancement effect than CaSO₄, consistent with findings from pH and XRD experimental studies. However, the DTG curve indicates that the samples containing CaSO₄ exhibit superior CaCO₃ crystallinity.

Figure 7

3.3.4 Microstructural analysis

Figures 8a,b presents the SEM images of the sample before and after the carbonation test. The results show that during the CO₂ neutralization of the alkalinity in RM, a mineral carbonation reaction occurs. Initially, the alkalinity of the pore water solution in the RM is rapidly neutralized, resulting in the formation of soluble carbonate ions (CO₃²⁻). These ions then react with Ca²⁺ to form CaCO₃ precipitates. Subsequently, the alkalinity of the RM decreases gradually. Combining the results of mineral analysis tests with the principle of the RM carbonation reaction, it can be inferred that the composition formed after the RM carbonation reaction is CaCO₃. From the figure, it can be observed that after the carbonation reaction, the surface morphology of the RM particles has changed. Before carbonation, the surface of RM particles was relatively dense, exhibiting only a small number of primary pores. After carbonation, a porous CaCO₃ coating formed on the particle surface, accompanied by the emergence of new nanoscale pores within the particles. The encapsulation of the surface by fine particles is more evident. Spaces are present between the particles, and the structure of the sample is irregular. During the carbonation reaction, calcium ions leach out from the original RM particles, leaving behind some new nanoscale spaces. At the same time, the carbonates formed after carbonation fill the existing narrow spaces, leading to a reduction in the volume of large spaces and an increase in the number of nanoscale spaces (Yadav et al., 2010).

Figure 8

Surface scanning of the specimens before and after carbonation reaction was performed, with the mass percentage results for C/O/Ca elements and EDS images shown in Figures 8c,d. The mass percentages of all three elements in the post-carbonation sample increased to varying degrees compared to the pre-carbonation sample. Specifically, the mass percentage of C increased by 15.8%, while the Ca content rose from 0.88% to 3.17%, indicating that CO₂ was fixed in the form of carbonate. This result aligns with the enhanced characteristic peak of CaCO₃ observed in the XRD analysis, further confirming the occurrence of the carbonation reaction.

3.4 Discussion of results

As the CO₂ concentration increases, the RM suspension takes less time to reach pH equilibrium and has a lower pH equilibrium value. However, when the specimens were exposed to air after the carbonation reaction, the pH of the specimens increased again, and the pH of the specimens reacted with 100% CO₂ increased the most. This is because CO₂ gas is dissolved in the aqueous phase as H₂CO₃ or CO₂, and the dissolved carbonate is gradually degassed as the suspension is stirred under atmospheric conditions. In contrast, the specimens with added calcium sources had lower initial and equilibrium pH values and increased CaCO₃ precipitation. Therefore, it is necessary to supplement the calcium source into the carbonation reaction system to promote the reaction to produce stable CaCO₃ precipitation, to achieve the purpose of permanently sequestering CO₂ and reducing the alkalinity of RM.

External calcium sources can influence the final pH equilibrium value. However, compared to the RM suspension test system, the rate of pH decrease in solid-state RM tests is less affected by the type of calcium source added. This is because under the carbonation test conditions of RM suspensions, the CO₂ mineralization and carbonation kinetics are not controlled by the CO₂ dissolution rate. Adding external calcium sources significantly accelerates the carbonation reaction of RM. In contrast, RM reacts slowly in the solid state, where the reaction rate is governed by CO₂ dissolution rather than the rate at which calcium sources (added as solid CaSO₄ and CaCl₂) dissolve. The core difference between the “suspension system” and “solid system” experiments directly points to the controlling effect of CO₂ dissolution rate. In the RM suspension, CO₂ dissolves rapidly and mixes thoroughly with the solution. When CaSO₄ or CaCl₂ is added to these samples, the pH decrease rate is significantly faster than in pure RM samples, while Ca²⁺ concentration remains stable. This indicates that in systems where CO₂ dissolution is unrestricted, the reaction rate is governed by Ca²⁺ supply (calcium source dissolution). The addition of an external calcium source significantly accelerates the reaction. In solid RM experiments, the pH decline rates among the three sample groups showed minimal variation. This phenomenon stands in stark contrast to the suspension system: despite the addition of CaCl₂ or CaSO₄, the reaction rates did not significantly diverge due to differences in calcium source types. The core reason lies in the pore structure of solid RM, which restricts CO₂ mass transfer and dissolution. CO₂ must first slowly dissolve into pore water before reacting with Ca²⁺. When CO₂ undergoes mass transfer in porous media, its dissolution rate constant is significantly lower than the diffusion rate of Ca²⁺ in aqueous solutions (Ilahi et al., 2024). This indicates that CO₂ dissolution is more likely to become the rate-limiting factor under solid-state RM carbonation conditions. The comparison between these two sets of experiments directly demonstrates that “whether CO₂ dissolution is rate-limiting” is the key variable determining the reaction rate.

4 Conclusions and prospects

4.1 Conclusion

This study uses CaSO₄ and CaCl₂ as external calcium sources, in combination with Bayer-process RM, to prepare RM samples with various external calcium sources, as well as those without any external calcium source. Through laboratory experiments, the carbonation reactions of Bayer-process RM with CO₂ under various conditions were simulated. The RM carbonation experiments were conducted under scenarios such as CO₂ gas ventilation and exposure to atmospheric CO₂. The study investigated the promoting effect of external calcium sources on the carbonation process of RM and validated the effectiveness of external calcium sources in enhancing the carbonation of RM. The main research conclusions are as follows:

• Su et al. (2020b) enhanced the carbon sequestration capacity of RM under optimized temperature and pressure conditions, achieving a maximum sequestration capacity of 1.36 ± 0.02 times the initial carbon content. Khaitan et al. (2009b) investigated the relationship between CO₂ neutralization efficiency and CO₂ partial pressure, achieving CO₂ sequestration of 21 g/kg. In contrast, the RM samples in this study achieved a pure carbon sequestration of 22.8 g/kg after CaCl₂ addition—1.9 times the initial carbon content. This result surpasses the previous studies (direct comparison is complicated by uncertainties in carbonate content across different RM sources) and requires neither high pressure nor pure CO₂, making it more applicable to real-world scenarios. Through calcium supplementation, this study enhanced sequestration capacity by 86.9% compared to pure RM samples under ambient conditions, validating the superiority of the calcium-enhanced strategy.

• This study designed 5 mm thin layers of RM to eliminate CO₂ diffusion limitations and precisely capture the calcium source enhancement mechanism. In industrial RM piles, excessive thickness makes CO₂ diffusion the primary limiting factor. Based on this study’s finding that “calcium source addition accelerates surface carbonation,” an industrial application strategy can be proposed: First, layer the pile and periodically turn it to enhance CO₂ contact efficiency with the RM. Second, integrate a spray system to supplement the calcium source solution, intensifying carbonation reactions in deeper layers of RM.

4.2 Prospects and limitations

• The CO₂ mineralization utilization technology is permanently safe and has great application potential. The RM after the carbonation reaction is mainly used in the construction industry. CO₂ is permanently trapped by reacting with Ca²⁺ in the RM to form CaCO₃ particles. However, the Ca²⁺ content in RM is limited, so supplementing the calcium source is more conducive to the formation of stable carbonate minerals for long-term carbonation of CO₂. The technology’s carbon storage capacity can reach hundreds of millions of tons per year, and its application is promising.

• From a cost perspective, as shown in Table 4, CaCl₂ has a higher procurement cost (1,200 CNY/t) than CaSO₄ (20 CNY/t). Correspondingly, the cost of CO₂ sequestration using CaCl₂ (5.84 CNY/kg CO₂) is higher than that of CaSO₄ (0.13 CNY/kg CO₂). However, in terms of enhancing the carbonation reaction of RM, CaCl₂ demonstrates superior efficacy compared to CaSO₄. For modifying industrial RM stockpiles, CaSO₄ holds greater potential for large-scale application due to its low cost and wide availability. Conversely, CaCl₂ is better suited for scenarios demanding high carbonation efficiency. From an environmental impact perspective, the final concentrations of Cl⁻ and SO₄²⁻ both comply with environmental protection standards. CaSO₄ is recycled as solid waste, reducing landfill pollution and yielding superior synergistic environmental benefits. While the high solubility of CaCl₂ necessitates precautions against soil salinization risks, its application in road materials minimizes leaching hazards as Cl⁻ ions are encapsulated within aggregates.

• Laboratory experiments did not simulate the temperature and humidity fluctuations of industrial piles, which may affect calcium source dissolution and CO₂ diffusion efficiency. Subsequent studies should incorporate environmental simulations of complex climatic conditions. Current analysis primarily relies on kinetic inference and macroscopic characterization (XRD, TGA, and SEM-EDS), lacking in situ monitoring data of intermediate products. Future research should integrate in situ infrared spectroscopy, nuclear magnetic resonance, and other techniques to deepen mechanistic investigations.

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