Effect of Carbon Fixation Time on the Properties of Gangue–Fly Ash Composite Filling Materials: Carbon Fixation Amount and Rheological Properties

Abstract

Coal-based solid wastes are used for carbon fixation, which can achieve the dual purpose of resource utilization of coal-based solid wastes and CO₂ storage, but carbon fixation has a negative impact on the rheological properties of filling slurry. This paper explores the effect of carbon fixation time on the carbon fixation performance and rheological properties of coal gangue (CG)–fly ash (FA) composite filling materials (CFS) through rheometer, X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and other testing methods. The results show that, with an increase in the carbon fixation time, the carbon fixation amount of the CFS shows a trend of increasing first and then stabilizing. Considering the carbon fixation amount and rheological properties of the CFS together, the optimal carbon fixation time is 2 h. At this time, the carbon fixation amount of the CFS is 1.18%, and the yield stress and plastic viscosity are 155.93 Pa and 0.17 Pa·s, respectively. However, with a further increase in the carbon fixation time, the carbon fixation amount basically tends to be stable, mainly because the calcium ions in the CFS are gradually consumed by the reaction as the carbon fixation time increases. The research results are of great significance for improving the utilization of coal-based solid waste and CO₂ storage.

1. Introduction

A large amount of solid waste is discharged during the development and utilization of coal resources, mainly including CG and FA. The large amount of coal-based solid waste discharged and the low utilization rate pose the risks of resource waste and environmental pollution [1,2,3]. Globally, coal mining generates approximately 800 million tonnes of CG annually, representing 15% to 20% of the total coal output. Concurrently, annual FA production has surpassed 600 million tonnes [4,5]. At present, the amount of coal-based solid waste is large, and the discharge occupies a large area, which not only brings safety problems such as landslides but also causes a series of ecological and environmental problems such as air pollution and water environment damage [6,7,8]. At the same time, the development and utilization of coal also emit a large amount of CO₂. In 2023, emissions from energy consumption reached 37.4 billion tons of CO₂, an increase of 410 million tons compared to the previous year [9]. CO₂ emissions can lead to global warming and trigger a series of environmental problems, such as melting glaciers, rising sea levels, and more extreme weather events [10,11,12]. Therefore, in the face of ecological and environmental pressure, how to safely, efficiently, and greenly dispose of bulk solid waste and mineralize and store CO₂ are some of the urgent problems that need to be solved at present.

Developing a functional carbon-fixing slurry from CG and injecting it into goafs offers a dual benefit: it controls rock strata movement and mitigates surface subsidence while simultaneously addressing solid waste disposal and enabling large-scale CO₂ sequestration [13,14,15]. However, extensive research indicates that CO₂ absorption significantly degrades the rheological performance of filling slurry [16,17,18,19]. Guo et al. [20] investigated the influence mechanism of carbonization temperature on the rheological properties and CO₂ storage performance of filling slurry. The research results showed that the rheological properties of fresh AGS conformed to the Herschel–Bulkley model. When the temperature rose, its yield stress, plastic viscosity, and thixotropy increased first and then decreased. At 50 °C, the CO₂ sealing amount reached the maximum of 2.55%. Bo et al. [21] proposed a new method to prepare cementitious materials for filling goaf using CO₂ from coal-based solid waste. The results showed that the mechanical properties of the mineralized filling material met the requirements for filling goaf, the maximum compressive strength was 32.2% higher than that of the unmineralized material, and the flowability decreased. Zhu et al. [22] investigated how carbonation pressure influences CO₂ sequestration efficiency and rheological behavior in backfill slurries. Their research demonstrated that elevated carbonation pressures enhanced the slurry’s CO₂ uptake capacity but concurrently degraded its flow characteristics. An optimal pressure of 0.7 MPa was identified, where the CSBS exhibited a yield stress of 171.66 Pa, plastic viscosity of 0.0998 Pa·s, and hysteresis loop area measuring 1376 Pa/s. Liu et al. [23] examined how agitation speed and gas injection rate influence the fluid behavior of CO₂-activated cemented backfill (CO₂-MCSB). Their findings revealed that calcium carbonate formed during carbonation occupied interstitial spaces within the mixture. This pore-filling effect enhanced interparticle contacts, consequently elevating the slurry’s yield stress. Current investigations into the rheological behavior of carbonated backfill primarily address variables such as mixture composition, carbonation pressure levels, and CO₂ introduction rates. However, research examining the impact of carbonation duration on CFS rheology remains limited, and the fundamental mechanisms governing its temporal evolution are not yet fully understood.

Based on the above analysis, this paper studies the effect of carbon fixation time on the carbon fixation amount and rheological properties of CFS, and it characterizes it through X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and other means to reveal its carbon fixation and rheological change mechanism. These findings establish a theoretical and experimental basis for utilizing gangue-derived backfill materials to achieve efficient CO₂ mineralization and valorize the resulting products. This advancement holds significant implications for advancing carbon neutrality objectives and promoting the sustainable management of solid waste streams.

2. Materials and Methods

2.1. Raw Materials

The raw materials used are coal gangue (CG) and fly ash (FA). Among them, the CG is provided by Northwest Mining Co., Ltd., Xi’an, China. The original CG has a large particle size and low activity, and it needs to be crushed and milled by a crusher and a ball mill. The FA is used conventional low-calcium FA (the CaO mass fraction is less than 10%), and the particle size distribution of the CG and FA is tested by a laser particle size instrument (Figure 1). The D₅₀ of the CG and FA is 62.54 μm and 13.95 μm, respectively, and the D₉₀ is 151.86 μm and 65.79 μm, respectively. The micromorphology of the CG and FA is analyzed by a scanning electron microscope analyzer (Figure 2). The surface of the CG is relatively rough, and the distribution of large and small particles is uneven. The FA contains more spherical glass beads. The chemical composition of the CG and FA is analyzed by an X-ray fluorescence spectrometer (XRF) (

Table 1. Chemical composition of raw material, wt%.

No.	SiO2	Al2O3	Fe2O3	CaO	K2O	MgO	Na2O	TiO2	SO3
CG	54.45	22.88	9.64	0.98	3.64	2.71	1.92	1.08	1.46
FA	56.79	27.25	4.61	3.28	2.45	1.21	0.63	1.38	1.79

). The CG and FA mainly contain SiO₂, Al₂O₃, CaO, and K₂O. CO₂ gas is provided by Qingdao Antaike Gas Co., Ltd., Qingdao, China, with a concentration greater than 99%, and it is food grade.

2.2. Test Methods

2.2.1. Slurry Preparation and Carbon Fixation Process

According to the ratio in

Table 2. Slurry mixing ratio.

Croup	Cementitious Materials		Water–Binder Ratio	Carbon Sequestration Time/h
	CG	FA
CFS0	80%	20%	0.5	0
CFS0.5				0.5
CFS1				1
CFS2				1.5
CFS3				2

, the CG and FA were first mixed evenly, and then tap water was added and stirred for 5 min. After the slurry was prepared, it was poured into a carbon fixation reactor. The operation process of the carbon fixation reactor is detailed in the literature [20]. The reactor conditions were maintained at 25 °C with 500 r/min agitation under 0.1 MPa gas pressure [24]. Following mixing, one slurry aliquot was transferred to a 500 mL beaker for rheological characterization, while another portion was cast into 30 mm cubical molds for subsequent microstructural analysis.

2.2.2. Carbon Sequestration Calculation

Thermogravimetric analysis of samples after curing for 3 days was performed using a TG 209 F3 Tarsus thermal analyzer from Netzsch, Germany. About 10 mg of the sample was placed in a crucible clamp for testing, with a sensitivity of 0.1 μg, an accuracy of 0.01%, a heating rate of 10 °C/min, and a temperature range of 30–1000 °C. CO₂ absorption was calculated as follows:

$$W_{{\mathrm{CO}}_2}(\%)={\displaystyle \frac{W_{{\mathrm{CaCO}}_3,\mathrm{f}}-W_{{\mathrm{CaCO}}_3,\mathrm{i}}}{M_{{\mathrm{CaCO}}_3}}}\times {\displaystyle \frac{M_{{\mathrm{CO}}_2}}{W_{\mathrm{solid}}}}\times 100\%$$

(1)

where $W_{{\mathrm{CO}}_2}$ is the amount of carbon fixed in the CFS; $W_{{\mathrm{CaCO}}_3,\mathrm{f}}$ is the amount of CaCO₃ in the CFS; $W_{{\mathrm{CaCO}}_3,\mathrm{i}}$ is the initial content of CaCO₃ in the unfixed carbon sample; $M_{{\mathrm{CO}}_2}$ and $M_{{\mathrm{CaCO}}_3}$ are the molar masses of CO₂ and CaCO₃, respectively; and $W_{\mathrm{solid}}$ is the mass of the sample.

2.2.3. Rheological Test

A rheometer (Anton Paar 92, Graz, Austria) was used to perform rheological tests on the CFS. The rheological test scheme was based on that in [25].

2.2.4. XRD Test

Some samples were taken and soaked in anhydrous ethanol for 24 h, and the anhydrous ethanol was changed every 12 h. Then, the samples were dried in a vacuum oven at 45 °C for 24 h and broken to 200 mesh. A D2 Phaser X-ray diffractometer from Bruker Company in Germany was used for the test, with a scanning speed of 5°/min and a scanning range of 5°~70°.

2.2.5. FTIR Test

The same hydration and crushing methods as in Section 2.2.4 were used for sample testing. A Nicolet iS20 spectrometer, produced by Thermo Scientific, Waltham, MA, USA, was used for sample testing. The sample was mixed and ground with KBr at a mass fraction of 1:100–1:200 and pressed at 30 MPa, and the scanning wavelength range was 400–4000 cm⁻¹.

2.2.6. SEM Test

Sample morphology was examined using a ZEISS GeminiSEM 300 field-emission environmental scanning electron microscope (Oberkochen, Germany). The analysis was conducted at an accelerating voltage of 20 kV, with specimens prepared by gold sputter coating prior to imaging.

3. Results and Analysis

3.1. Effect of Carbon Fixation Time on Carbon Fixation Amount and Rheological Properties of CFS

3.1.1. Effect of Carbon Fixation Time on Carbon Fixation Amount of CFS

A TG curve of the CFS sample is shown in Figure 3. The mass loss from ambient temperature to 100 °C corresponds to the mass loss of water and hydrated gel (C-S-H) in the sample [26]. Under high-temperature conditions of 300~540 °C, the mass loss is due to the decomposition of calcium hydroxide [27]. The mass loss at 540~950 °C is due to the decomposition of calcium carbonate [28]. According to the calculation method of carbon fixation in Section 2.2.2, the carbon fixation amount of the CFS samples at different carbon fixation times is obtained, as shown in Figure 4. With an increase in the carbon fixation time, the CO₂ storage capacity of the CFS samples shows a trend of increasing first and then tending to be flat. When the carbon fixation time is 2 h, the carbon fixation amount is 1.13%, and 1 ton of CFS can achieve a CO₂ reduction of 11.3 kg compared to traditional filling after carbon fixation. When the carbon fixation time is greater than 2 h, the CO₂ storage capacity remains basically unchanged. There are two main reasons for this: ① As the reaction proceeds, the active calcium ions in the filling slurry are gradually consumed, and the concentration of the reactants decreases. ② The calcium carbonate generated by the reaction is deposited on the surface of the particles to form a dense calcification layer, which hinders the diffusion of CO₂ into the interior of the particles and inhibits the migration of internal calcium and magnesium ions to the outside [29].

3.1.2. Effect of Carbon Fixation Time on CFS Rheological Parameters

Figure 5a shows the relationship between the shear rate and shear stress of the CFS. As can be seen in the figure, the shear stress of the slurry increases with the increase in the carbon fixation time. This is mainly because, after the introduction of CO₂, it will react chemically with the alkali metal ions (Ca²⁺ and Mg²⁺) in the slurry to generate carbonates, making the flocculation network structure inside the slurry complex, resulting in poor flow properties of the slurry. When the carbon fixation time is greater than 1 h, the shear stress increasing trend gradually slows down at the same shear rate. This is mainly because, as the carbon fixation time increases, the carbonates produced adhere to the surface of the particles, hindering their further reaction [30]. As shown in Figure 5b, at the same shear rate, the apparent viscosity change trend is the same as the shear stress change trend, but the reasons for the change are not introduced here [31].

By using the H-B model to fit the shear stress–shear rate curve, the plastic viscosity and yield stress of the slurry can be obtained, and the rheological equation is as follows [20]:

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; and when n < 1, it is a pseudoplastic fluid. The fitting results are shown in

Table 3. Fitting rheological model of CFS at different carbon fixation times.

No.	Rheological Model	Fitting Results	τ0/Pa	η/Pa·s	R2
CFS0	H-B	τ = 124.89 + 0.08γ0.86	124.89	0.08	0.9987
CFS0.5		τ = 131.27 + 0.12γ0.84	131.27	0.12	0.9989
CFS1		τ = 150.34 + 0.16γ0.92	150.34	0.16	0.9995
CFS2		τ = 155.93 + 0.17γ0.97	155.93	0.17	0.9992
CFS3		τ = 157.97 + 0.17γ0.98	157.97	0.17	0.9994

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

Figure 6 illustrates the dependence of the CFS yield stress and plastic viscosity on carbonation duration. The data indicate that both rheological parameters increased progressively with longer carbonation times. Specifically, yield stress rose from 124.89 Pa to 157.97 Pa (representing a 26% increase), while plastic viscosity increased from 0.08 Pa·s to 0.17 Pa·s (a 113% increase). Furthermore, the figure demonstrates that, beyond 1 h of carbonation, both yield stress and plastic viscosity stabilized, a phenomenon whose underlying mechanism was addressed in the preceding discussion.

Under the action of shear force, the upward and downward lines of the rheological curve of the slurry do not coincide, forming a hysteresis loop whose integral area can represent the thixotropic characteristics of the slurry. The hysteresis loop area is shown in Figure 7. As the carbon fixation time increases, the hysteresis loop area of the CFS gradually increases and then tends to stabilize, which is consistent with the trend of yield stress and plastic viscosity.

3.2. Effect of Carbon Fixation Time on the Composition and Microstructure of CFS

3.2.1. Morphology and Microstructure

Figure 8 shows the SEM-EDS images of the CFS at different carbon fixation times. When carbon is not fixed, it can be observed that there are a large number of pores in the CFS, the particle distribution is relatively loose, and only a small amount of flocculent substances are observed on the particle surface. The friction between the particles is small, which helps the flow of the slurry. After carbon fixation for 3 h, it can be seen that the microstructure of the CFS sample becomes denser, and the pore structure decreases. This is because the accumulation of carbonation products fills the sample pores, resulting in the deterioration of the rheological properties of the slurry. Through the EDS elemental analysis, it can be seen that, compared with the unfixed carbon, the distribution of C elements in the CFS after carbon fixation increases, mainly from CO₂, which further verifies that CO₂ reacts with Ca ions to generate calcium carbonate.

3.2.2. XRD

Figure 9 presents the XRD patterns of the CFS under varying carbonation conditions. These patterns reveal the emergence of a calcium carbonate (CaCO₃) diffraction peak at 2θ ≈ 29.5° following carbonation treatment. This characteristic peak intensifies progressively with an extended carbonation duration, initially increasing before stabilizing—a trend aligning with the rheological observations detailed in Section 3.1. This is mainly because, as the carbon fixation time increases, the calcium ions in the filling slurry are gradually consumed by the reaction to generate carbonate products, which deteriorate the rheological properties of the slurry. After further increasing the carbon fixation time, the amount of calcium carbonate generated remains basically unchanged, and the yield stress and plastic viscosity of the slurry also tend to stabilize [32].

3.2.3. FTIR

Figure 10 shows the FTIR spectra of the CFS at different carbon fixation times. The main infrared characteristic peaks of the CFS are as follows: (1) The stretching vibration peak at 1420 cm⁻¹ and the bending vibration peak at 875 cm⁻¹ correspond to the CO₃²⁻ in carbonates such as CaCO₃ in the sample. (2) The asymmetric stretching vibration peak at 1030~1050 cm⁻¹ and the bending vibration peak at 460~465 cm⁻¹ correspond to Si-O-T(Al Si) in CaO·Al₂O₃·2SiO₂ [33,34]. By comparing the vibration peaks of CO₃²⁻, it can be seen that more CO₃²⁻ is generated after CO₂ is sealed compared with that before carbon fixation, resulting in an increase in the peak intensity, yield stress, and plastic viscosity of the slurry. As the carbon fixation time increases, the peak intensity variation decreases, which is consistent with the results of the XRD and thermogravimetric tests.

4. Conclusions

(1)

The carbon fixation amount of the CFS increases first and then tends to be stable with an increase in the carbonization time. When the carbon fixation time is 2 h, the carbon fixation amount is 1.13%.

(2)

After further increasing the carbon fixation time, the carbon fixation amount gradually tends to be stable. On the one hand, this is because the calcium ions in the solution are gradually consumed, and, on the other hand, it is because the generated calcium carbonate is deposited on the surface of the particles, thereby forming a calcification layer, which hinders the diffusion of CO₂ into the particles.

(3)

Prolonged carbonation promotes calcium carbonate formation, which increases the packing density within the slurry microstructure. This microstructural evolution adversely impacts the rheological performance of the CFS. Combined with the CO₂ storage capacity, 2 h can be used as the optimal carbonization time. At this time, the yield stress and plastic viscosity of the CFS are 155.93 Pa and 0.17 Pa·s, respectively.

(4)

After the CFS fixes carbon, the generated calcium carbonate and hydration products fill the pores of the sample, making the microstructure of the CFS denser and the pore structure reduced.

This study delves into the influence of carbon fixation time on the carbon sequestration amount and rheological properties of CFS, providing valuable guidance for improving the carbon reduction and processability of CFS in practical engineering applications. However, the economy, carbon sequestration effect, and feasibility of CFS in practical engineering applications are also crucial and worthy of further research.