CO2 Mineralized Full Solid Waste Cementitious Material for Coal Mine Goaf Filling and Carbon Sequestration Potential Assessment

Bo Wang; Huaigang Cheng; Xiong Liu; Zichen Di; Huiping Song; Dongke Zhang; Fangqin Cheng

doi:10.1016/j.eng.2025.02.017

Engineering ›› 2025, Vol. 48 ›› Issue (5) :70 -80. DOI: 10.1016/j.eng.2025.02.017

Research Carbon Capture, Utilization, and Storage—Article

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CO₂ Mineralized Full Solid Waste Cementitious Material for Coal Mine Goaf Filling and Carbon Sequestration Potential Assessment

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Abstract

Coal is an essential component of global energy; however, the processes of coal mining and utilization produce significant amounts of coal mine goafs, accompanied by coal-based solid wastes and emitted CO₂, resulting in severe ecological and environmental challenges. In response to this issue, this study proposes a novel approach for filling coal mine goafs using cementitious materials prepared by coal-based solid wastes mineralized with CO₂ (15% in concentration). The CO₂ sequestration capacities of individual solid wastes are ranked as follows: carbide slag (CS) > red mud (RM) > fly ash (FA). The performance of filling material prepared from composite solid waste (FA–CS–RM) mineralized with CO₂ meets the filling requirements of goaf. The filling material (F60C20R20) obtained by CO₂ mineralization was 14.9 MPa in maximum compressive strength, increasing by 32.2% compared to the non-mineralized material. The prepared filling material exhibits excellent CO₂ sequestration capacity (i.e., 14.4 kg·t⁻¹ in maximum amount of CO₂ sequestration). According to the analysis of carbon sequestration potential, in China, the annual production of FA, CS, and RM is approximately 899, 30, and 107 Mt, respectively in the year of 2023. The utilization of FA, CS, and RM individually can achieve carbon emission reductions of 3.42, 10.78, and 0.61 Mt, respectively. The composite solid waste (FA–CS–RM) mineralized with CO₂ can achieve 1.23 Mt in carbon emissions reduction. Additionally, taking Yellow River Basin of China as a case study, the total volume of underground space in coal mine goafs from 2016 to 2030 is estimated at 8.16 Gm³, indicating that this technology can sequester 0.18 Gt of CO₂. This approach offers a promising solution for large-scale flue gas CO₂ sequestration, recycling coal-based solid wastes, and remediating coal mine goafs, contributing to green utilization of coal and the emission reduction of carbon.

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Keywords

CO₂ mineralization / Solid waste / Coal mine goaf / Filling material / Carbon sequestration potential assessment

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Bo Wang, Huaigang Cheng, Xiong Liu, Zichen Di, Huiping Song, Dongke Zhang, Fangqin Cheng. CO₂ Mineralized Full Solid Waste Cementitious Material for Coal Mine Goaf Filling and Carbon Sequestration Potential Assessment. Engineering, 2025, 48 (5) : 70-80 DOI:10.1016/j.eng.2025.02.017

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1. Introduction

Fossil fuels are a significant portion of global energy, but their CO₂ emissions are one of the primary causes of global warming, causing to major ecological and environmental challenges worldwide [1]. Fossil fuels primarily include oil, natural gas, and coal. According to the Statistical Review of World Energy, coal accounted for 26% of the total global energy consumption in 2023, presenting its importance in the energy mix [2]. However, coal mining and utilization generate large amounts of coal mine goafs [3], coal-based solid wastes (such as fly ash (FA) [4]), and substantial CO₂ emissions [5], leading to severe environmental challenges such as surface subsidence, ecological damage, and the greenhouse effect. There is an urgent need to focus on the research of coal mine goaf management, coal-based solid waste utilization, and CO₂ reduction to support global ecological construction and achieve global carbon neutrality goals.

In response to these challenges, extensive research has been conducted on coal mine goaf management and coal-based solid waste utilization. Regarding the coal mine goaf management, filling technology is widely used, where filling materials is a critical factor [6]. Traditional filling materials are mainly cement-based materials, using cement as a binder mixed with FA, steel slag, blast furnace slag, and bentonite [7]. Recently, with the advancements in coal mining technologies, utilizing coal-based solid waste for integration of filling and mining has become an important method, aiding in the comprehensive utilization of solid waste and coal mine goaf management [8],[9].

Coal-based solid waste is rich in valuable chemical components, including silicon oxide and aluminum oxide, which can also be used to produce geopolymers through alkali activation. These geopolymers represent a promising type of green, low-carbon cementitious materials, offering potential as a sustainable alternative to traditional cement [10],[11]. In recent years, the preparation of geopolymer as filling cementitious materials from coal-based solid waste has gained significant attention as filling materials [12], [13], [14]. In our previous work [13], the circulating fluidized bed FA, carbide slag (CS), and desulfurized gypsum were used as raw materials to prepare solid waste-based geopolymer as filling cementitious materials through the alkali activation of sodium hydroxide and sodium silicate activators. These materials exhibited excellent filling properties, with a 28-day compressive strength of 20.5 MPa, a setting ratio of 99.8%, and a fluidity of 250 mm.

CO₂ mineralization technology offers a promising solution for CO₂ reduction, by imitating the weathering process of natural silicate rocks [15]. In this process, CO₂ dissolves in water to produce carbonic acid, and then neutralizes with alkaline minerals to obtain stable solid carbonate. The standard Gibbs free energy of the formed carbonate salt is 60–180 kJ∙mol⁻¹ lower than that of CO₂ (400 kJ∙mol⁻¹) [16], which implies that the change in Gibbs free energy of CO₂ mineralization is less than zero (ΔG < 0). Thus, the mineralization reaction is thermodynamically favorable and occurs spontaneously without the need of external energy. Therefore, CO₂ mineralization enables the permanent storage of CO₂ without the risk of leakage [17], and is a spontaneous and exothermic process [18],[19], reducing the energy consumption of CO₂ sequestration. Recently, CO₂ mineralization using coal-based solid waste has received attention worldwide because they contain reactive components that can undergo mineralization reactions with CO₂ [20],[21].

Based on these, our research group previously proposed a new approach for preparing cementitious materials by mineralizing pure CO₂ in a slurry made from a blend of FA and CS [22]. The results indicated that with a CS to FA ratio of 3:7 and a curing temperature of 60 °C, the maximum three days compressive strength of cementitious blocks was 7 MPa, and the corresponding CO₂ sequestration amount was 13 kg∙t⁻¹. Subsequently, Wang et al. [23] proposed a strategy and technology for underground CO₂ sequestration in disturbed spaces from coal mining, involving the construction of functional filling spaces followed by the injection of captured CO₂ for sequestration. Xie et al. [24] conceived a novel idea for high-efficiency carbon-negative filling mining technology, developing and evaluating a carbon-negative high-porosity filling material structure composed of CO₂, gangue, and rapid binders for CO₂ sequestration capacity in coal mines. Ngo et al. [25] prepared filling materials by mineralizing CO₂ (14% vol) in a slurry made from cement, FA, and silicate activators. With 80% FA content, the 28-day compressive strength of the blocks was 5.12 MPa and a maximum CO₂ sequestration amount was 1.3 kg∙t⁻¹. Li et al. [26] prepared filling materials by mineralizing CO₂ in a slurry made from cement and tailings, and its 28-day compressive strength was about 2.61 MPa. Due to the relatively high cost of cement-based filling materials, to reduce the cost of filling material, Liu et al. [27] and Xia et al. [28] prepared a wholly solid waste slurry using modified magnesium slag, FA, and coal gangue, and then passing pure CO₂ gas for mineralization. This filling material was prepared by utilizing the synergistic cementitious properties of theses solid wastes. The mechanical properties of the blocks were significantly improved after CO₂ mineralization, and a maximum 28-day compressive strength was approximately 5 MPa. In summary, although preliminary research has been conducted on the preparation of filling materials by mineralizing CO₂ in slurries made entirely from coal-based solid waste, there is still significant room for improvement in the mechanical properties of these materials, and existing research primarily focuses on the mineralization process using pure CO₂ gas. Therefore, there is an urgent need to develop high-performance filling materials prepared by directly mineralizing low-concentration CO₂ (such as coal-fired power plant flue gas) in wholly solid waste for coal goaf management in coal mines, supporting the green and low-carbon transformation and development of the coal-based industry.

Herein, a new approach that directly uses industrial flue gas in the carbonation of three industrial wastes (FA, CS, and red mud (RM)) and procures filling materials for the coal mine goaf was proposed. The feasibility of preparing filling materials by mineralizing CO₂ simulated coal-fired power plant flue gas in a slurry made entirely from solid waste (FA–CS–RM) was investigated. The conceptual diagram of CO₂ mineralized full solid waste material for filling coal mine goaf is shown in Fig. 1. Firstly, the CO₂ mineralization process and carbon sequestration capacity of individual solid wastes were explored. Secondly, the performance, carbon sequestration amount, and mineralization enhancement mechanism of filling materials prepared by composite wholly solid waste mineralized with CO₂ were investigated. Finally, the material, energy, carbon stream, and carbon sequestration potential of the filling materials prepared by wholly solid waste mineralized with CO₂ were analyzed. This study aims to provide basic data and technical support for large-scale carbon sequestration, the utilization of coal-based solid waste, and coal goafs filling, ultimately, contributing to global ecological and environmental protection efforts and the early achievement of carbon neutrality goals.

2. Materials and methods

2.1. Materials

The raw materials mainly include FA, CS, and RM. FA are sourced from the Zhangshan Power Plant in Changzhi City, China. CS is obtained from Ruiheng Chemical Co., Ltd. in Changzhi City, China. RM is supplied by Shanxi Huarun Chinalco Co., Ltd. in Lvliang City, China. The phase composition of the raw materials was analyzed using an X-ray diffractometer (XRD; D2 PHASER, Bruker, Germany) (Fig. 2). The main phases of FA are mullite and quartz; the main phase of CS is calcium hydroxide (Ca(OH)₂), with minor amounts of calcium carbonate; the main phase of RM is hematite (Fe₂O₃), gibbsite, and cancrinite. The chemical composition of the raw materials was analyzed using an X-ray fluorescence spectrometer (XRF; PW4400) (Table 1). As shown in Table 1, the main components of FA (in terms of oxides) are Al₂O₃ (33.136%), SiO₂ (48.151%), Fe₂O₃ (6.187%), CaO (4.611%), and so forth. The main components of CS are CaO (95.330%), SiO₂ (2.337%), and so forth. The main components of RM are Fe₂O₃ (52.352%), Al₂O₃ (15.870%), SiO₂ (9.185%), TiO₂ (8.902%), Na₂O (6.532%), CaO (5.258%), and so forth. The particle size distribution of the samples was measured using a laser particle size analyzer (Malvern 3000, Malvern Panalytical, UK). The median particle sizes of FA, CS, and RM are approximately 20.70, 31.44, and 5.30 μm, respectively (Fig. 3).

2.2. CO₂ mineralization procedure of solid waste

The CO₂ mineralization reaction set-up is shown in Fig. 4, which mainly includes a CO₂/N₂ mixed gas cylinder, a gas mass flow meter, a mineralization reactor, a magnetic stirrer, an online pH meter, and a low-temperature constant temperature bath. Before starting the experiment, specific amounts of raw materials (FA, CS, and RM) were added to the reactor, respectively, along with a specific amount of water, controlling the solid–liquid ratio to 10 wt%. Then, the magnetic stirring and heating were initiated, and the pH of the slurry was recorded online. After that, CO₂/N₂ mixed gas (15% CO₂ and 85% N₂) was introduced into the mineralization reactor and the gas flow rate (400 mL∙min⁻¹) was controlled by the mass flow meter, while the pH of the slurry continues to be recorded online. When the pH of the reaction system remained constant, the CO₂/N₂ mixed gas was stopped, indicating the end of the mineralization reaction. After the reaction was completed, the mineralized suspension was filtered and dried. The mineralization products were collected for further characterization.

2.3. Preparation of CO₂ mineralized solid waste backfill material

FA, CS, and RM were mixed by a specific ratio in a mixing container and stirred evenly (Table 2). The item of F60C20R20 means it was prepared by 60 wt% FA, 20 wt% CS, and 20 wt% RM. A certain amount of water was added according to the water-to-solid ratio (0.5), and the mixture was stirred thoroughly for 10 min before introducing CO₂/N₂ mixed gas (400 mL∙min⁻¹). After 20 min the mixed gas was stopped. As control samples, the slurry was prepared at the same conditions without CO₂/N₂ mixed gas. Subsequently, the resulting slurry was placed into silicone molds (20 mm × 20 mm × 20 mm) and three samples were prepared for each group. After 24 h, the samples were demolded and placed in the room temperature for curing 3, 7, and 28 d.

2.4. Characterization

2.4.1. Characterization of CO₂ mineralized products from single solid waste

The phase of CO₂ mineralization product from the single solid waste (FA, CS, and RM) was characterized using an XRD with a Cu Kα radiation under a tube voltage of 30 kV and a tube current of 10 mA. The scanning range was 5°–80° with a scanning speed of 0.02 degree per second. The morphology of CO₂ mineralization product from the single solid waste was analyzed using a scanning electron microscope (SEM; JSM-IT 500 HR, JEOL Ltd., Japan). Before testing, the samples were treated with a platinum ion sputter coater (JFC-3000 FC, JEOL Ltd.) to enhance conductivity, thereby obtaining clearer images.

2.4.2. Characterization of composite fully solid waste filling materials

The samples were subjected to uniaxial compressive strength (UCS) tests after reaching the corresponding curing time. The UCS test was performed using a DYE-300S automatic computer cement folding and compression testing machine. The loading rate of 1 kN∙s⁻¹ was applied to the samples. The UCS of sample obtained after 3, 7, and 28 d curing were determined. All tests were repeated three times and the average UCS values were used to analyze further. The mini-slump test was carried out by a mini-slump cylinder with a diameter of 30 mm and a height of 50 mm. The mini-slump test was performed following the JC/T 1023–2007 standard from the Chinese National Standardization Commission. All mini-slump tests were repeated three times, and the average mini-slump values were used for further analysis. The phase and morphology of filling material after curing 28-day were analyzed using XRD and SEM. The internal defects of the test blocks were measured using concrete ultrasonic detector (HC-F900, Beijing Hichance Technology Co., Ltd., China). Industrial computed tomography (CT; GE V tome x L 300, Waygate Technologies, USA) was used to analyze the pore structure of test blocks at a scanning voltage of 70 kV, a current of 80 μA, and a resolution of 2 μm. The pore distribution of the sample was conducted by low-field nuclear magnetic resonance (MesoMR23-060-I, Suzhou NiuMag Analytical Instrument Cooperation, China).

2.4.3. Calculation method of CO₂ uptake

To calculate the CO₂ uptake, thermogravimetric analysis (TGA/DSC 3⁺, Mettler Toledo, Switzerland) was used for weight loss analysis of the sample with a heating rate of 10 °C∙min⁻¹ and a temperature range of 25–900 °C under N₂ atmosphere. The amount of fixed CO₂ was calculated based on the weight loss curve. The specific calculation Eqs. (1), (2) are as follows [25],[29].

(1) $S_{\mathrm{CO}_{2}}=\frac{P_{\mathrm{f}, \mathrm{CaCO}_{3}}-P_{\mathrm{i}, \mathrm{CaCO}_{3}}}{100} \times m_{\text {solid }} \times \frac{M_{\mathrm{CO}_{2}}}{M_{\mathrm{CaCO}_{3}}}$

(2) $P_{\mathrm{CaCO}_{3}}=\Delta W \times \frac{M_{\mathrm{CaCO}_{3}}}{M_{\mathrm{CO}_{2}}}$

where

S_{{C O}_{2}}

is the amount of fixed CO₂, mg;

P_{f, {C a C O}_{3}}

is the content of calcium carbonate in the post-mineralization sample, %;

P_{i, {C a C O}_{3}}

is the content of calcium carbonate in the pre-mineralization sample, %;

m_{s o l i d}

is the mass of the thermogravimetric sample, mg;

M_{{C O}_{2}}

is the relative molecular mass of CO₂, g∙mol⁻¹;

M_{{C a C O}_{3}}

is the relative molecular mass of calcium carbonate, g∙mol⁻¹;

P_{{C a C O}_{3}}

is the content of calcium carbonate in the sample, %; ΔW is the weight loss rate of the sample between 500–850 °C, %.

3. Results and discussion

3.1. CO₂ mineralization of single solid waste

3.1.1. CO₂ mineralization of FA

In this work, the FA, CS, and RM was used to prepare the CO₂ mineralized solid waste filling material. Hence, the research on CO₂ mineralization using single solid was an important step. Here, the CO₂ mineralization of FA was studied firstly. Generally, the pH change of the reaction system can indirectly reflect the process of mineralization. Therefore, the pH value of the CO₂ mineralization of FA was monitored with reaction time (Fig. 5(a)). It can be observed that the pH decreased from 9.3 to 6.1 and then remained a constant value, indicating the end of the reaction. Subsequently, the phase analysis of the CO₂ mineralization products from FA was conducted (Fig. 5(b)). The results indicate that calcium carbonate was hardly detected in the mineralization products after FA reacted with CO₂. Only mullite and quartz were found, which was same as the original phases of FA. A possible reason for this is that the CaO content (4.61%) in the selected FA is relatively low, resulting in an insufficient amount of calcium carbonate. Additionally, the microscopic morphology of the mineralization products was analyzed (Fig. 5(c)), where it can be seen that the surface of the FA particles was covered with blocky substances after reacting with CO₂. The blocky substances were may CaCO₃ [30]. The pH decreased when CO₂ was flowed into the reaction system, implying that the CO₂ was consumed. In order to obtain the amount of CO₂ sequestration, the TGA analysis of mineralization product was conducted. Fig. 5(d) shows the TGA curve of the mineralization products of FA, where it can be seen that the weight loss between 500–850 °C is primarily due to the decomposition of CaCO₃, resulting in calcium oxide and CO₂. According to Eqs. (1), (2). The amount of CO₂ uptake by FA is approximately 3.8 kg∙t⁻¹.

3.1.2. CO₂ mineralization of CS

CS, as a critical component of filling material, it is necessary to study the process of CO₂ mineralization. Fig. 6(a) shows the relationship between the pH of the CO₂ mineralization of CS and time. It can be seen that the pH decreased from 12.58 to 6.86 and then kept constant. Subsequently, the phase analysis of CO₂ mineralization products from CS was conducted (Fig. 6(b)). The results indicated that the mineralization product was calcite-type CaCO₃. Additionally, the microscopic morphology of products was analyzed (Fig. 6(c)), showing that the product is spindle-shaped calcite. Also, the CO₂ uptake was calculated based on the TGA curve of mineralization products in Fig. 6(d). The amount of CO₂ fixed by CS was approximately 359.3 kg∙t⁻¹.

3.1.3. CO₂ mineralization of RM

Similarly, the process of CO₂ mineralization of RM was investigated. Fig. 7(a) illustrates the relationship between the pH of the CO₂ mineralization of RM and time. The pH decreased from 10.3 to 7.25 and then remained constant, indicating the end of the reaction. Subsequently, the phase analysis of the CO₂ mineralization products from RM was performed (Fig. 7(b)), and the results indicated that RM reacted with CO₂ to produce calcite-type CaCO₃, but a significant amount of unreacted hematite, cancrinite, and others still remained. Additionally, the microscopic morphology of the CO₂ mineralization products of RM was analyzed (Fig. 7(c)), which shows that the surface of the RM particles was covered with blocky substances after reacting with CO₂. This blocky substance may be calcite-type CaCO₃ [31]. At the same time, the amount of CO₂ fixation was calculated by TGA of mineralization products of RM (Fig. 7(d)). The amount of CO₂ fixed by RM is approximately 5.7 kg∙t⁻¹.

Based on the above results, the FA, CS, and RM can sequester CO₂ through mineralization reaction, respectively. Moreover, their reaction process was different. In order to visually compare the CO₂ fixation capacity of FA, CS, and RM, their CO₂ sequestration capacity are listed in Table 3. The CO₂ sequestration capacity of individual solid wastes follows the order of CS > RM > FA.

3.2. Mechanical and flow properties of filling materials

Although a single solid waste can mineralize CO₂, its product basically has no mechanical properties and cannot meet the requirements for filling materials in coal mine goaf. Based on the synergistic effect between solid wastes, a fully solid waste cementitious filling material was prepared using FA, CS, and RM. Generally, the compressive strength of the filling material is a key parameter for its mechanical properties, and the strength change with curing age. Hence, it is necessary to test the UCS of the filling material at various curing ages. Fig. 8(a) illustrates the compressive strength of the filling material constituted by FA–CS–RM solid waste at the age of 3, 7, and 28 d before and after CO₂ mineralization. It can be seen that the strength of the test block cured at 28 d increased firstly and then decreased with the increasement of RM content. Moreover, the compressive strength of the test block after CO₂ mineralization was higher than the non-mineralized sample. For example, the compressive strength of test block (F60C35R05) increased from 7.5 to 10.5 MPa after CO₂ mineralization. The maximum strength of non-mineralized sample (F60C20R20) cured for 28 day was 11.27 MPa, while it increased up to 14.9 MPa after CO₂ mineralization indicating that the compressive strength increased by 32.2%.

In addition, fluidity is also a key indicator of filling materials. Fig. 8(b) shows the slump values of the all-solid waste backfill material before and after CO₂ mineralization. It can be seen that the slump values increased gradually as the amount of RM increases, and the slump values of the CO₂ mineralized samples is better than that of the non-mineralized samples. The slump values of the non-mineralized samples increased from 203 to 462 mm, while those of the CO₂-mineralized samples increased from 247 to 500 mm. The above results indicate that the all-solid waste filling material has a good fluidity.

3.3. Mechanisms of enhancing the mechanical properties of CO₂ mineralized filling material

3.3.1. Phase composition of filling materials

Generally, the structure of a material dictates its properties. To further elucidate the effects of material ratios and CO₂ mineralization on the structure of filling materials, XRD analysis was performed on test blocks cured for 28 d (Fig. 9). The results indicate that the diffraction peak of calcium hydroxide in the product gradually diminishes as the proportion of RM increases. One reason is that the content CS was decreased from 40% to 5%, the other reason is that the CS was consumed during the reaction. Following CO₂ mineralization, calcite-type CaCO₃ emerges in the product.

3.3.2. Microstructure analysis of filling materials

Furthermore, the microscopic morphology of the samples after CO₂ mineralization was analyzed (Fig. 10). SEM images reveal that needle-like and blocky substances appear in the test block in the absence of RM in the system (F60C40R00). As the RM content increases, a porous and flocculent structure was observed. For test block (F60C20R20), a structure characterized by interwoven with porous networks and blocky substances is observed, which may enhance the compressive strength. However, with a further increase in RM content, the test block structure reverts to a needle-like and blocky appearance.

To further elucidate the enhancement mechanism of CO₂ mineralization on the mechanical properties of filling materials, the internal defects of the test blocks (F60C20R20) before and after mineralization was analyzed using concrete ultrasonic detector. As shown in Fig. 11, the wave amplitude of block before and after CO₂ mineralization was 143.62 and 145.34 dB, respectively. The larger amplitude indicated that the less ultrasound loss when it passes through the medium. Hence, the internal defects of the test block after CO₂ mineralization were less than that of before mineralization. Meanwhile, the corresponding ultrasonic image show significant discontinuities for test block before CO₂ mineralization. However, a uniform distribution of ultrasonic image was obtained in the filling material after CO₂ mineralization. These results indicated that CO₂ mineralization can improve the structure of the filling material and increase its compressive strength. The possible reason was that the CaCO₃ was generated in the CO₂ mineralization system, which filled a large number of holes.

To further illustrate the impact of CO₂ mineralization on the microstructure of the test blocks, the three-dimensional structure of the test blocks (F60C20R20) before and after mineralization was analyzed using CT scanning (Fig. 12). In the CT images, the gray areas represent the material, while the colored areas indicate pores. By comparing the CT scan results before and after mineralization, it can be observed that the porosity of the mineralized material is lower than that of the unmineralized material, with a reduction in porosity of approximately 3.5%. The large pores are also significantly reduced, transforming into smaller pores. This indicates that mineralization helps to improve the structure of the material, reducing the occurrence of large pores, making the structure more uniform, and thus enhancing its mechanical properties.

In addition, the pore structure of the test blocks was analyzed using low-field nuclear magnetic resonance (Fig. 13). Before CO₂ mineralization, the test blocks had a relatively large number of pores, with a significant proportion of large pores (Fig. 13(a)). After CO₂ mineralization, the number of pores in the test blocks decreased, and the size of the pores became smaller (Fig. 13(b)), which may also be one of the reasons for the increase in their mechanical properties.

3.4. Assessment of CO₂ fixation potential of filling materials

Firstly, the CO₂ sequestration of filling materials prepared with different ratios after CO₂ mineralization was calculated using the TGA curves. Fig. 14(a) illustrates the TGA curves of CO₂ mineralized filling material cured for 28 d under varying ratios. The CO₂ sequestration can be calculated based on Eqs. (1), (2). As shown in the Fig. 14(b), the CO₂ sequestration first increased and then decreased with the increase in RM content. This trend may be attributed to the reduction in CS content, leading to a decrease in CO₂ sequestration. However, it is not the case that the highest CS content yields the maximum CO₂ sequestration. This phenomenon may occur because the addition of RM enhances the fluidity of the system (Fig. 8(b)), thereby facilitating the mass transfer reaction process and subsequently increasing CO₂ sequestration. The maximum CO₂ sequestration is approximately 14.4 kg∙t⁻¹.

Afterwords, the material, energy and carbon stream of CO₂-mineralized fully solid waste filling material was analyzed (Fig. 15(a)). The detail calculation process was shown in Appendix A. When FA, CS, and RM are compounded in proportions of 60, 35, and 5 wt%, respectively, 100 tons of mixed slag can prepare 149.44 tons cementitious material slurry and sequester 1.44 tons of CO₂. The energy consumption of pretreatment, stirring and grouting was analyzed in Table S1 in Appendix A. Additionally, the energy-related CO₂ emissions is 398.04 kg during the whole process. Therefore, a net reduction of 1.04 tons of CO₂ per 100 tons mixed slag can be achieved through this technology. In China, the annual production of FA, CS, and RM is approximately 899, 30, and 107 Mt, respectively in 2023. The utilization of FA, CS, and RM alone can achieve carbon emission reductions of 3.42, 10.78, and 0.61 Mt, respectively. The synergistic mineralization of FA–CS–RM can prepare 85.71 Mt of cementitious filling material, achieving a carbon emission reduction of 1.23 Mt (Fig. 15(b)). If the historical stockpiles of the three solid wastes (2500 Mt FA, 400 Mt CS, and 1400 Mt RM, respectively) are taken into account, the carbon emission reduction could reach 16.46 Mt (Fig. 15(b)).

If these cementitious materials prepared by solid waste mineralized CO₂ used to fill the underground space of coal mine, it not only achieves simultaneously large-scale carbon sequestration and utilizing coal-based solid waste, but also realizes the utilization of underground space of coal mine. Here, we take Yellow River Basin of China as an example because it is a major coal-producing region, accounting for about 70% of the nation's total coal production [32]. It is widely distributed across the regions of Shanxi, Inner Mongolia, Shaanxi, Henan, Shandong, Ningxia, Gansu, Qinghai, and so forth. During 2016–2020 period, according to data released by the Chinese National Bureau of Statistics, the total production of raw coal in major provinces of the Yellow River Basin from 2016 to 2020 was 14.094 Gt (Table S2 in Appendix A). Therefore, the volume of the coal mine goaf formed by coal mining from 2016 to 2020 was 2.59 Gm³ (Table S2). Here, if the average coal production is about 3 Gt per year in the major provinces along the Yellow River Basin from 2020 to 2030, a total of 30 Gt coal will be produced. The volume of the coal mine goaf from 2020 to 2030 was approximately 5.57 Gm³. The total underground space volume of the coal mine goaf from 2016 to 2030 was approximately 8.16 Gm³. These spaces can accommodate about 12.24 Gt filing materials (calculated at a density of 1.5 t∙m⁻³), thereby the potential of CO₂ sequestering (with a carbon sequestration rate of 1.44%) was 0.18 Gt.

4. Conclusions

This study proposes a new approach that integrates the treatment of coal mine goafs, the utilization of coal-based solid waste, and the mineralization and sequestration of CO₂. The main conclusions are summarized as follows.

(1) The technology for mineralizing and sequestering low-concentration CO₂ (15% vol) using single coal-based solid wastes such as FA, CS, and RM is feasible. The carbon sequestration capacities are 3.8, 359.3, and 5.7 kg∙t⁻¹ at room temperature and pressure, respectively.

(2) The performance of filling material prepared by the composite fully solid waste mineralized CO₂ meets the filling requirements. The mechanical properties first increase and then decrease as the RM content increases, while the fluidity gradually increases. CO₂ mineralization notably enhances both the mechanical properties and fluidity of the material. The maximum compressive strength of the mineralized filling material (F60C20R20) is 14.9 MPa, representing a 32.2% increase in mechanical properties and a 10.8% increase in fluidity compared to the non-mineralized filling material. The CO₂-mineralized material also exhibits a more uniform internal structure, and a reduction in the number of large pores.

(3) The composite fully solid waste mineralized CO₂ filling material demonstrates significant carbon sequestration potential. The maximum amount of CO₂ sequestration was approximately 14.4 kg∙t⁻¹. In China, utilization the annual production of FA, CS, and RM to prepare the composite solid waste (FA–CS–RM) mineralized CO₂ can reduce 1.23 Mt in carbon emission. Furthermore, the total underground space volume of the coal mine goafs in major provinces of the Yellow River Basin from 2016 to 2030 in China was approximately 8.16 Gm³, which can sequester 0.18 Gt of CO₂.

In conclusion, this technology offers a promising solution for the remediation of coal mine goafs, the comprehensive utilization of coal-based solid waste, and the mineralization and sequestration of CO₂ from flue gas. This holds significant importance for the green and low-carbon transformation, as well as the carbon neutrality goals of the global coal.

CRediT authorship contribution statement

Bo Wang: Writing – review & editing, Writing – original draft, Validation, Methodology, Investigation, Funding acquisition, Formal analysis. Huaigang Cheng: Writing – review & editing, Methodology, Funding acquisition, Conceptualization. Xiong Liu: Validation, Investigation, Formal analysis. Zichen Di: Validation, Investigation, Formal analysis. Huiping Song: Resources, Methodology, Investigation. Dongke Zhang: Writing – review & editing, Methodology. Fangqin Cheng: Writing – review & editing, Supervision, Methodology, Funding acquisition, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (U21A20321 and 22378241) and Research Project Supported by Shanxi Scholarship Council of China (2024-015).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.eng.2025.02.017.

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Abstract

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1. Introduction

2. Materials and methods

2.1. Materials

2.2. CO2 mineralization procedure of solid waste

2.3. Preparation of CO2 mineralized solid waste backfill material

2.4. Characterization

2.4.1. Characterization of CO2 mineralized products from single solid waste

2.4.2. Characterization of composite fully solid waste filling materials

2.4.3. Calculation method of CO2 uptake

3. Results and discussion

3.1. CO2 mineralization of single solid waste

3.1.1. CO2 mineralization of FA

3.1.2. CO2 mineralization of CS

3.1.3. CO2 mineralization of RM

3.2. Mechanical and flow properties of filling materials

3.3. Mechanisms of enhancing the mechanical properties of CO2 mineralized filling material

3.3.1. Phase composition of filling materials

3.3.2. Microstructure analysis of filling materials

3.4. Assessment of CO2 fixation potential of filling materials

4. Conclusions

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgments

Appendix A. Supplementary data

References

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2.2. CO₂ mineralization procedure of solid waste

2.3. Preparation of CO₂ mineralized solid waste backfill material

2.4.1. Characterization of CO₂ mineralized products from single solid waste

2.4.3. Calculation method of CO₂ uptake

3.1. CO₂ mineralization of single solid waste

3.1.1. CO₂ mineralization of FA

3.1.2. CO₂ mineralization of CS

3.1.3. CO₂ mineralization of RM

3.3. Mechanisms of enhancing the mechanical properties of CO₂ mineralized filling material

3.4. Assessment of CO₂ fixation potential of filling materials