Effect of Carbonization Pressure on CO₂ Sequestration and Rheological Properties of Coal Gangue-Based Backfilling Slurry

Abstract

The wet carbonation of coal gangue-based backfilling slurry (CGBS) is considered to be an effective method for the resource utilization of coal gangue solid waste and CO₂ sequestration, but CO₂ sequestration has a negative impact on the rheological properties of CGBS. This investigation explores the effect of carbonization pressure on the rheological properties and CO₂ sequestration properties of CGBS by using a carbonization reactor, a rheometer, X-ray diffraction, a nitrogen adsorption–desorption instrument, a scanning electron microscope and other testing methods. The results show that increasing the carbonization pressure can increase the CO₂ sequestration capacity of CGBS, and the carbonization products produced make the pores of CGBS smaller and the structure more compact; however, increasing the carbonization pressure will reduce the rheological properties of the slurry, and the optimal carbonization pressure is 0.7 MPa. At this time, the yield stress, plastic viscosity and hysteresis loop area of CGBS are 171.66 Pa, 0.0998 Pa·s and 1376 Pa/s, respectively. However, when the carbonization pressure is further increased, the CO₂ sequestration capacity tends to remain unchanged. This is mainly because the carbonization pressure causes the carbonization reaction to intensify, forming a calcified layer on the particle surface, which hinders the penetration of CO₂ into the particles. This study is of great significance for improving the utilization rate of gangue solid waste and CO₂ sequestration.

1. Introduction

During the development and utilization of coal resources, a large amount of solid waste will be discharged, mainly including coal gangue and fly ash. Among them, the annual discharge of coal gangue is about 800 million tons, accounting for 15~20% [1,2] of the coal resource output, and the annual output of fly ash has also exceeded 600 million tons [3]. Due to the large discharge of coal gangue and fly ash and the low comprehensive utilization rate, there are a series of ecological and environmental problems, such as resource waste and environmental pollution risks [4,5,6]. At the same time, facing the increasingly severe global climate situation, the Chinese government has proposed the dual carbon strategic goals of “carbon peak in 2030 and carbon neutrality in 2060” [7,8]. In the context of the national dual carbon goals, how to reduce carbon emissions in the coal industry and achieve carbon sequestration has become a difficult problem that needs to be solved urgently. Due to the complex composition of coal gangue and fly ash, which contain a large amount of calcium and magnesium crystalline phases, they can react with CO₂ in solution to produce carbonates [9,10,11]. Therefore, coal gangue and fly ash can be used as raw materials for CO₂ mineralization, and their mineralization products can be comprehensively utilized, which can achieve the dual goals of large-scale disposal of coal gangue and fly ash and long-term sequestration of CO₂.

Using coal gangue and fly ash to prepare coal gangue-based backfilling slurry (CGBS) and fill it into goaf to store CO₂ can not only control rock movement and reduce surface subsidence but also achieve large-scale sequestration of CO₂ and disposal of solid waste [12,13,14]. However, a large number of studies have found that the rheological properties of backfilling slurry will significantly deteriorate after absorbing CO₂. Nego et al. [9] found that after CO₂ gas was introduced into fly ash slurry, the slurry yield stress increased from 2.97 Pa to 36.01 Pa, and the plastic viscosity increased from 0.84 Pa·s to 57.29 Pa·s; Sun et al. [15] analyzed the effects of CO₂ flow rate and aeration stirring time on the rheological properties of magnesium slag-based backfilling slurry. The results showed that the yield stress and plastic viscosity of the backfilling slurry were positively correlated with the CO₂ flow rate and aeration stirring time. Park et al. [16] studied the reaction of silicate cement with supercritical CO₂ at different temperatures. The results showed that after 24 h of reaction between silicate cement and supercritical CO₂, the carbonization of the cement slurry was basically completed, and the rheological properties deteriorated. Liu et al. [17] studied the effects of stirring rate, stirring time and aeration rate on the fluidity of the magnesium-coal-based solid waste CO₂ backfill material. The results showed that the calcium carbonate produced by the carbonation reaction filled the voids of the slurry, increased the collision between the internal particles, and thus increased the yield stress of the slurry. In summary, research on the flow properties of backfilling slurry after carbon fixation has so far focused on factors such as material ratio, stirring time and CO₂ flow rate. There are few studies on the effect of carbon fixation pressure on the rheological properties of CGBS, and its change mechanism has not yet been clarified.

Based on the above analysis, the effect of carbonization pressure on the rheological properties of CGBS was studied in this paper, and its rheological change mechanism was revealed by characterization by means of X-ray diffraction (XRD), a nitrogen adsorption–desorption instrument (Brunauer–Emmett–Teller Method, BET), a scanning electron microscope (SEM), etc. This study lays a theoretical and experimental foundation for the efficient mineralization of CO₂ and its comprehensive utilization of products by gangue-based filling materials. The research results are of great significance for the realization of dual carbon goals and the comprehensive utilization of solid waste.

2. Materials and Methods

2.1. Raw Materials

The coal gangue (CG) and fly ash (FA) used in the experiment were provided by Jinjitan Coal Mine of Shaanxi Future Energy Chemical Co., Ltd., and Shaanxi Binzhou Electric Power Co., Ltd., (Shaanxi, China), respectively. The particle size distribution of CG and FA was tested by a laser particle size analyzer (Figure 1), and the average particle sizes of CG and FA were 3.10 μm and 18.81 μm, respectively. The chemical composition and mineral composition of CG and FA were analyzed by an X-ray fluorescence spectrometer (XRF) and X-ray diffraction (XRD) (

Table 1. Chemical composition of CG and FA, wt%.

No.	SiO2	Al2O3	Fe2O3	CaO	K2O	MgO	Na2O	TiO2	SO3
CG	55.87	22.46	10.15	1.08	4.56	2.02	1.45	1.20	1.03
FA	54.04	31.44	4.84	3.08	2.18	1.07	0.74	1.26	1.35

and Figure 2). The alkaline activator was analytically pure sodium hydroxide with a mass fraction of 97%, produced by Sinopharm Chemical Reagent Co., Ltd (Xuzhou, China).

2.2. Experimental Method

2.2.1. Slurry Preparation and CO₂ Sequestration

The experimental setup is shown in Figure 3. According to the ratio in

Table 2. Slurry mixing ratio.

Croup	CG, g	FA, g	Water, g	Carbonization Pressure, MPa
CGBS1	70	30	60	0.1
CGBS2				0.3
CGBS3				0.5
CGBS4				0.7
CGBS5				0.9
CGBS6				1.1

, CG, FA and water were stirred evenly to form a slurry and then poured into the reactor. First, CO₂ gas was introduced to expel the air in the reactor; then, the outlet valve was closed. The equipment was then started, and the pressure, temperature and motor speed in the reactor could be adjusted through the control panel. The temperature in the reactor was set to 30 °C, the speed was 200 r/min, and stirring was carried out for 30 min [17]. Then, part of the slurry was poured into a 500 mL beaker for testing the rheological properties, and the other part was poured into a 30 mm × 30 mm × 30 mm mold for the subsequent testing of its microscopic properties.

2.2.2. CO₂ Sequestration Capacity

The CO₂ sequestration capacity of CGBS was measured by using an analytical balance with a weighing accuracy of 0.0001 g. The average value was taken after three tests, and the mass of CGBS before carbonization was recorded as m₀; after CO₂ sequestration, the balance was used to measure again and recorded as m₁. The CO₂ sequestration capacity is shown in Equation (1):

$${\mathrm{C}\mathrm{O}}_{2 \mathrm{u}\mathrm{p}\mathrm{t}\mathrm{a}\mathrm{k}\mathrm{e}}\left(\mathrm{\%}\right)={\displaystyle \frac{{\mathrm{m}}_1-{\mathrm{m}}_0}{{\mathrm{m}}_0}},$$

(1)

where ${\mathrm{C}\mathrm{O}}_{2 \mathrm{u}\mathrm{p}\mathrm{t}\mathrm{a}\mathrm{k}\mathrm{e}}$ is the CO₂ sequestration amount, m₀ is the sample mass before carbonization and m₁ is the sample mass after carbonization.

2.2.3. Rheological Test

The rheological properties of the CGBS were tested by using a rheometer (Anton Paar 72, Graz, Austria) equipped with a four-bladed rotor (ST22-4V-40) featuring a blade length of 40 mm and a diameter of 22 mm (Figure 4). Prior to testing, the CGBS was placed in a cylindrical container with a diameter of 110 mm and a height of 120 mm. The rotor position was adjusted by using the control program to ensure it was centered in the container cross-section, with a height difference of 20 mm from the CGBS level to minimize edge effects on the test results [18].

The program for measuring the rheological parameters of the CGBS is shown in Figure 4. Pre-shearing was performed for 60 s at a shear rate of 60 s⁻¹, and the formal rheological test was performed after standing for 30 s. The up and down lines of data acquisition were step-type, and the shear rate range was 0~120 s⁻¹. One data point was taken every 3 s, and a total of 120 data points were taken during the test phase.

2.2.4. XRD Test

The samples were tested by a D2 Phaser X-ray diffractometer from Bruker, Berlin, Germany. The scanning speed was 5°/min, and the scanning range was 5~70°.

2.2.5. BET Test

The ASAP 2460 nitrogen adsorption–desorption instrument (Micromeritics, GA, USA) was used to characterize the pores and specific surface area of the samples. During the sample test, the nitrogen adsorption–desorption pressure (P/P0) ranged from 0.05 to 0.995, and the temperature was 77 K.

2.2.6. SEM Test

The sample morphology was observed by a Gemini SEM 300 field emission environmental scanning electron microscope (ZEISS, Oberkochen, Germany). The instrument operating voltage was 20 KV, and the sample was treated with gold spraying.

3. Results and Discussion

3.1. Effect of Carbonization Pressure on CO₂ Sequestration and Rheological Properties of CGBS

3.1.1. Effect of Carbonization Pressure on CO₂ Sequestration Performance of CGBS

The effect of carbonization pressure on the CO₂ sequestration performance of CGBS is shown in Figure 5. As the carbonization pressure increases, the CO₂ sequestration capacity of CGBS increases first and then stabilizes. When the carbonization pressure is 0.7 MPa, the CO₂ sequestration capacity is 0.38%, which is 1.92 times higher than that at 0.1 MPa. When the carbonization pressure is further increased, the CO₂ sequestration capacity changes slightly. There may be two main reasons: ① When the carbonization pressure exceeds 0.7 MPa, the carbonation reaction intensifies. A dense calcium carbonate shell is generated on the surface of the particles, which hinders the penetration of CO₂ into the interior of the particles, slowing down the carbonation reaction [19]. ② The increase in pressure also intensifies the generation of hydration products. The rapid increase in hydration products will make the slurry structure dense [20], resulting in basically unchanged CO₂ sequestration capacity.

3.1.2. Effect of Carbonization Pressure on Rheological Parameters of CGBS

The rheological properties of a slurry are important indicators for measuring its flow and workability in engineering applications [21]. The relationship between shear stress and shear rate in the slurry system is explored; then, the rheological model is fitted to determine the yield stress and plastic viscosity of the slurry, which can provide a theoretical basis for determining the slurry ratio and selecting parameters in engineering [22,23,24]. Figure 6 shows the shear rate–shear stress and apparent viscosity–shear stress curves of CGBS. It can be found that under the same shear stress, the shear stress and apparent viscosity of the slurry increase after CO₂ is stored at different pressures. The reason is that the calcium carbonate and hydration products produced after CO₂ is stored adhere to the surface of the particles, resulting in poor fluidity of the slurry [25]. Figure 6b shows the curve of apparent viscosity changing with the shear rate. It can be found that with the increase in the shear rate, the apparent viscosity first gradually decreases and then tends to stabilize, indicating that the slurry has shear thinning behavior [26].

Repeated measures ANOVA was conducted on the shear stress of CGBS obtained at different carbonization pressures, and the results of the within-subjects effect tests are shown in

Table 3. Within-subjects effect tests.

Source	Test Methods	F	p
Carbonization pressure	Mauchly’s Test for Sphericity	19,385.7	***
	Greenhouse–Geisser	19,385.7	***
	Huynh–Feldt	19,385.7	***

*** p < 0.001

. From Table 3, the F-value of the carbonization pressure factor is 19,385.7, and the p-value less than 0.001, which shows that there is a significant difference in the shear stress of CGBS corresponding to the different carbonization pressures.

The H-B model is used to fit the experimental data, and the rheological equation is [27]

τ = τ₀ + ηγⁿ

(2)

When n = 1 and τ₀ = 0, it is a Newtonian fluid; when n = 1 and τ₀ > 0, it is a Bingham fluid; when n > 1, it is an expansion fluid; when n < 1, it is a pseudo-plastic fluid. The fitting results are shown in

Table 4. Fitting rheological models of CGBS at different carbonization pressures.

No.	Rheological Model	Fitting Results	τ0/Pa	η/Pa·s	n	R2
CGBS1	H-B	τ = 127.16 + 0.0399γ1.32	127.16	0.0399	1.32	0.9995
CGBS2		τ = 138.95 = 0.0596γ1.26	138.95	0.0596	1.26	0.9990
CGBS3		τ = 146.88 = 0.0685γ1.23	146.88	0.0685	1.23	0.9992
CGBS4		τ = 171.66 = 0.0998γ1.54	171.66	0.0998	1.54	0.9994
CGBS5		τ = 192.24 = 0.1028γ1.70	192.24	0.1028	1.70	0.9995
CGBS6		τ = 212.49 = 0.1145γ1.54	212.49	0.1145	1.54	0.9996

, and R² is greater than 0.99, which indicates a good fitting effect.

Yield stress is the maximum stress that prevents the slurry from plastic deformation, and plastic viscosity is a property that hinders the flow of the slurry, reflecting the deformation rate in the slurry system [28,29]. The relationship between the yield stress and plastic viscosity of CGBS with carbonization pressure is shown in Figure 7. It can be seen that with the increase in carbonization pressure, the yield stress of the slurry increased from 127.16 Pa to 189.49 Pa, an increase of 0.49 times; the plastic viscosity increased from 0.04 Pa·s to 0.11Pa·s, an increase of 1.75 times. It can be seen from the figure that when the carbonization pressure is 0.7 MPa, the yield stress of the slurry increases sharply. This may be caused by the production of a large amount of calcium carbonate, which corresponds to the results of Section 3.1.1 above.

As shown in Figure 8, the thixotropy of the slurry is usually represented by the hysteresis loop formed by the shear stress rising and falling cycles, and its integrated area can represent the thixotropic characteristics of the slurry [6,30]. The magnitude of thixotropy depends on the number of flocculation structures that hinder the flow inside the slurry, which is an important indicator for measuring rheological properties [31]. The hysteresis loop area is shown in Figure 9. When the carbonization pressure increases from 0.1 MPa to 1.1 MPa, the hysteresis loop area increases from 895 Pa/s to 1730 Pa/s. This is because the carbonates and hydration products produced during the carbon fixation process make the slurry system complex and the thixotropic characteristics of the slurry increase [32].

3.2. Effect of Carbonization Pressure on Composition and Microstructure of CGBS

3.2.1. XRD

The XRD patterns of CGBS at different carbonization pressures are shown in Figure 10. As shown in Figure 10, due to carbonization, the diffraction peak of calcium carbonate crystals (CaCO₃, 2ϴ ≈ 29.5°) appears in the diffraction pattern of CGBS and as the carbonization pressure increases. The diffraction peak of calcium carbonate crystals increases first and then remains basically unchanged. When the carbonization pressure is 0.7 MPa, the diffraction peak of calcium carbonate crystals is the strongest. After further increasing the carbonization pressure, the diffraction peak of calcium carbonate tends to remain unchanged. This may be because when the carbonization pressure is too high, the carbonization reaction intensifies, and a dense calcium carbonate shell is formed on the surface of the particles. This hinders the penetration of CO₂ into the interior of the particles, slowing down the carbonization reaction rate [11].

3.2.2. BET

Figure 11 shows the BET results of CGBS at different carbonization pressures. The specific surface area of CGBS1 is 45.86 m²/g, and the pore volume is 0.0745 cm³/g, while the specific surface area and pore volume of CGBS2 are 20.65 m²/g and 0.0605 cm³/g, respectively. The results show that with the increase in carbonization pressure, the specific surface area and pore volume of CGBS gradually decrease. On the one hand, the calcium carbonate produced by carbon fixation makes the slurry system structure more compact [33]. On the other hand, it may be due to the increase in carbonization pressure, the increase in hydration reaction rate, and the increase in hydration products, which leads to the decrease in pores in the slurry and the decrease in specific surface area [34].

3.2.3. Morphology and Microstructure

Figure 12 shows the SEM images of CGBS at different carbonization pressures. At 0.1 MPa, it can be observed that the CGBS slurry contains a large number of pores, the particle distribution is relatively loose and only a small amount of flocculent material is observed on the particle surface. This is because the carbonization pressure is low, which has an inhibitory effect on the dissolution and hydration of CG and FA. Most particles still maintain their initial shape, the pores between particles are larger, the lubricating water film is thicker, and the force is smaller [35]. This phenomenon is consistent with the yield stress and plastic viscosity obtained from the rheological curve under this carbonization pressure, which is conducive to the flow of the slurry. As the carbonization pressure increases, hydration products of different shapes appear in the CGBS. The hydration and carbonation products accumulate and fill the sample pores, making the CGBS microstructure more compact and the pore structure reduced, which is consistent with the BET test results.

4. Carbon Footprint Calculation and Analysis

The life cycle approach (LCA) was used to initially study the carbon emissions of samples CGBS1 and CGBS4. The system boundary of CGBS in this study is shown in Figure 13. The LCA includes the entire life cycle of raw material extraction and transportation, product production, product transportation, solid waste recycling and reuse [36]. Based on the above life cycle system, the calculation model of CGBS carbon emissions is established, as shown in Equations (3)–(8):

$${\mathrm{C}}_{\mathrm{e}}={\mathrm{M}}_1\times {\mathrm{C}\mathrm{E}\mathrm{F}}_{\mathrm{e}1}+{\mathrm{M}}_2\times {\mathrm{C}\mathrm{E}\mathrm{F}}_{\mathrm{e}2}+{\mathrm{M}}_3\times {\mathrm{C}\mathrm{E}\mathrm{F}}_{\mathrm{e}3}+{\mathrm{M}}_4\times {\mathrm{C}\mathrm{E}\mathrm{F}}_{\mathrm{e}4}$$

(3)

$${\mathrm{C}}_{\mathrm{t}}=\left({\mathrm{M}}_1+{\mathrm{M}}_2+{\mathrm{M}}_3+{\mathrm{M}}_4\right)\times {\mathrm{S}}_{\mathrm{t}1}\times {\mathrm{C}\mathrm{E}\mathrm{F}}_{\mathrm{t}1}+{\mathrm{M}}_{\mathrm{s}}\times {\mathrm{S}}_{\mathrm{t}2}\times {\mathrm{C}\mathrm{E}\mathrm{F}}_{\mathrm{t}2}$$

(4)

$${\mathrm{C}}_{\mathrm{p}}={\mathrm{M}}_{\mathrm{s}}\times {\mathrm{C}\mathrm{E}\mathrm{F}}_{\mathrm{p}}$$

(5)

$${\mathrm{C}}_{\mathrm{r}}={\mathrm{M}}_1\times {\mathrm{C}\mathrm{E}\mathrm{F}}_{\mathrm{r}1}+{\mathrm{M}}_2\times {\mathrm{C}\mathrm{E}\mathrm{F}}_{\mathrm{r}2}$$

(6)

$${\mathrm{C}}_{\mathrm{a}}={\mathrm{M}}_{\mathrm{s}}\times {\mathrm{C}\mathrm{O}}_{2 \mathrm{u}\mathrm{p}\mathrm{t}\mathrm{a}\mathrm{k}\mathrm{e}}\left(\mathrm{\%}\right)$$

(7)

$$\mathrm{C}={\mathrm{C}}_{\mathrm{e}}+{\mathrm{C}}_{\mathrm{t}}+{\mathrm{C}}_{\mathrm{p}}+{\mathrm{C}}_{\mathrm{r}}$$

(8)

where C_e is the carbon emission during the introduction of raw materials, CO₂ equivalence; M₁, M₂, M₃ and M₄ are the masses of FA, CG, NaOH and water in the functional unit AGBS, t; CEF_e1, CEF_e2, CEF_e3 and CEF_e4 are the carbon emission factors of FA, CG, NaOH and water extraction stages, KgCO₂eq·t⁻¹; C_t is the total carbon emission of various raw materials, products and energy transportation, CO₂ equivalence; CEF_t1 is the carbon emission factor of the raw material transportation stage, KgCO₂eq·t⁻¹; CEF_t2 is the carbon emission factor in the CGBS transportation stage, KgCO₂eq·t⁻¹; S_t1 and S_t2 are the raw material transportation distance and CGBS transportation distance, respectively, km; M_s is the mass of the functional unit CGBS, t; C_p denotes the total carbon emissions generated during the product production process, CO₂ equivalence; CEF_p is the carbon emission factor in the CGBS production stage, KgCO₂eq·t⁻¹; C_r is the carbon emission reduced by recycling industrial solid waste, and its value is negative, CO₂ equivalence; CEF_r1 and CEF_r2 are the carbon emission factors of CG and FA recycling, and their values are negative, KgCO₂eq·t⁻¹; and C_a denotes the carbon emissions reduced by functional unit CO₂ sequestration, and its value is negative, CO₂ equivalence. C is the total carbon emission of functional unit CGBS, CO₂ equivalence.

Table 5. Calculation results of carbon emissions, kg/t.

NO.	Ce	Ct	Cp	Cr	Ca	C
CGBS1	38.72	85.40	1.26	−92.17	0	33.21
CGBS4	38.72	85.40	1.26	−92.17	−33.85	−0.64

shows the results of the carbon emission calculation.

As shown in Table 5, the CGBS’s primary source of carbon emissions is the transportation of raw materials and products. Consequently, it is essential to enhance transportation efficiency, prioritize the local sourcing of materials, and minimize long-distance transport. The total carbon emissions of CGBS1 and CGBS4 are 33.21 kg and −0.64 kg, respectively. It can be seen that after adding carbonization pressure, negative carbon filling can be achieved throughout the life cycle. In 2023, China’s carbon dioxide emissions amounted 12.6 billion tons, and the goaf space generated by coal mining is 3.3 billion m³. If CGBS is used to store CO₂ in the goaf, carbon emissions reductions of 2.0588 million tons can be achieved throughout the life cycle. This approach is beneficial for the implementation of Carbon Capture, Utilization and Storage technologies in coal mines.

5. Conclusions

The effect of carbonization pressure on the rheological properties of CGBS was studied in this paper, and the evolution mechanism of the CGBS microstructure was explored from a microscopic perspective. The main conclusions are as follows:

(1)

Increasing the carbonization pressure in the reactor can increase the CO₂ sequestration capacity of CGBS. When the carbonization pressure is 0.7 MPa, the CO₂ sequestration capacity is 0.38%. When the carbonization pressure is further increased, the CO₂ sequestration capacity tends to be stable.

(2)

After increasing the carbonization pressure, the calcium carbonate produced will make the slurry structure dense, resulting in the deterioration of the rheological properties of CGBS. Combined with the CO₂ sequestration capacity, 0.7 MPa can be used as the optimal carbonization pressure.

(3)

When the carbonization pressure is 0.7 MPa, the yield stress, plastic viscosity and hysteresis loop area of CGBS are 181.66 Pa, 0.1024 Pa·s and 1376 Pa/s, respectively.

(4)

After increasing the carbonization pressure, the calcium carbonate and hydration products produced fill the pores of the sample, which reduces the specific surface area and pore volume of CGBS and makes the microstructure denser.