Experimental Study on the Suspending Mechanism of Suspending Agent in Coal-Based Solid Waste Slurry for Long-Distance Pipeline Transportation

Abstract

The transportation of coal-based solid waste filling slurry (CSWFS) through pipelines for underground goaf injection is essential for enhancing mine safety and promoting green, low-carbon coal mining. To address the issue of pipeline blockage caused by the suspension sensitivity of CSWFS during long-distance transportation, this study proposes the addition of the suspending agent hydroxypropyl methyl cellulose (HPMC) to transform the filling slurry into a stable suspending slurry. The mechanism by which the suspending agent modifies the rheological property of CSWFS was elucidated and verified. Firstly, an evaluation index system for the suspending state of CSWFS based on the “experimental test and theoretical calculation” was established. The values for layering degree, bleeding rate time-loss, and the corresponding average time-loss rate over 0 to 120 min of A1–A5 CSWFS were recorded as 24 mm–2 mm, 3.0–0.2%, 252.4–54.2%, and 149.6–14.6%, respectively. The concentration gradient evaluation result, C/C_A = 0.91 (≥0.8), confirmed that the suspending agent maintained a stable suspending state over time for CSWFS. Secondly, it was demonstrated that the suspending agent HPMC modified the rheological property of A1–A5 CSWFS by increasing its plastic viscosity, which strengthened the viscous resistance to particle settling, thereby transforming a semi-stable slurry into a stable one. Additionally, the formation of a spatial suspending network by the suspending agent ensures that no pipeline blockage accidents occured in practical engineering applications. Furthermore, the XRD and SEM tests were utilized to verify the microstructure of the top (T) and bottom (B) samples in A4 block. It was concluded that the type of hydration products, occurrence forms, lapping compactness, and microstructural development were consistent, ultimately forming a high-strength, dense, hardened filling block. Finally, numerical simulation confirmed that the addition of suspending agent in A4 slurry formed a comprehensive spatial suspending network and a well-structured, unified system. This is one effective approach which could contribute to addressing the technical issue of pipeline blockage during long-distance pipeline transportation.

1. Introduction

The harmonious development of resource extraction and environment protection is the central theme of China’s coal industry, with the technical advantages of green filling mining gaining increasing recognition [1,2,3,4]. CSWFS is a composite mixture comprising coal gangue, cement, fly ash, and water, which is subsequently transported through pipelines and injected into underground voids (goaf). This technology is crucial for enhancing mine safety, promoting green and low-carbon coal mining, and preventing spontaneous coal combustion and fire accidents. However, some technical challenges in practical implementation have hindered the development of this technique. As identified by Academician Yu [5,6], a major technical challenge in filling mining is maintaining the stability of CSWFS concentration. This challenge is most evident in the frequent pipeline blockages encountered during projects, where concentration stability mainly refers to the layered deposition within the CSWFS. The maintenance of stable CSWFS concentration is essential for pipeline transportation and is usually attempted by increasing the slurry concentration to a critical value. However, the plastic viscosity and yield stress of CSWFS are highly sensitive to its concentration, and any increase significantly escalates the resistance per unit length during pipeline transportation [7,8]. Given the presence of layered deposits and the flat extension of coal seams, the transportation of CSWFS to goaf is characterized by long distance, extended duration, and large length-to-height ratio, typical of coal mine filling operations. This is particularly evident in large-scale coal mines when mining boundary regions. Even minor increases in resistance per unit length can cause the total resistance increase significantly during long-distance pipeline transportation, potentially resulting in blockage and bursting accidents. Consequently, CSWFS is highly sensitive to resistance during long-distance pipeline transportation. The resistance encountered during the pipeline transportation of CSWFS is highly sensitive to slurry concentration. Thus, increasing the concentration of CSWFS to ensure stability during long-distance filling operations may introduce risks. Addressing this sensitivity to prevent pipeline blockage is crucial for the advancement of CSWFS filling mining techniques. To address this challenge, various methods have been explored, such as inducing turbulence within the flow and incorporating concrete additive agents. The authors’ team has considered the inherent rheological properties of CSWFS and introduced hydroxypropyl methyl cellulose (HPMC), as a suspending agent, into the slurry. This modification enables the slurry to achieve a uniform suspending state, reducing pipeline transportation resistance to approximately half of that associated with increased concentration methods. This is one of the breakthroughs for the long-distance pipeline transport of CSWFS. Since the implementation of this technique, no pipeline blockages or explosions have been reported [9,10,11,12,13]. Therefore, elucidating the suspension modification mechanism of the suspending agent in CSWFS is of great importance, as it provides more precise guidance for engineering practices.

The action mechanism of suspending agents in civil engineering has been extensively investigated by several researchers. Wang et al. [14] demonstrated that cellulose ether enhances water retention and thickening properties in cement mortar. From a rheological perspective, Zhu et al. [15] observed that increasing the content of HPMC elevates the plastic viscosity and yield stress of the slurry, thereby improving its stackability. Xu et al. [16] noted that an optimal addition of HPMC, ranging from 0.4‰ to 0.5‰, significantly enhances the water retention and workability of mortar. Brachaczek [17] reported that even the minimal additions of cellulose ether significantly increase the water retention rate of slurry, with HPMC showing superior performance compared to other cellulose ethers under similar conditions. Batista et al. [18] highlighted the potential of HPMC as an effective additive for improving the thermal and mechanical performance of rendering mortars, thereby contributing to enhanced energy efficiency in building construction. These findings have played a critical role in advancing the concrete industry. However, the use of suspending agents in the pipeline transportation of CSWFS has not been well explored. Thereby, the primary novelty and aim of this study lie in the establishment of an evaluation index system for the suspending state of CSWFS, as well as the investigation and verification of the effects and suspension modification mechanism of the suspending agent HPMC in CSWFS. This study proposes an approach that effectively suspends the slurry while minimizing pipeline transport resistance and preventing pipe blockages.

1.1. Evaluation of Macroscopic Effect of Suspending Agent

In this study, HPMC with a viscosity of 200,000 cps was employed as the suspending agent for CSWFS in a coal mine located in Hebei Province. HPMC is synthesized through an etherification reaction of cellulose following alkalization with the etherifying agents chloromethane and propylene oxide [19]. It presents as a white, odorless, and tasteless powder, with a particle size allowing over 100% to pass through a 100-mesh sieve (pore size of 0.15 mm), and a specific gravity typically ranging from 1.26 to 1.31 g/cm³. It is soluble in water and most polar organic solvents, with its solubility in water being unaffected by the PH value. The addition of HPMC is calculated as a percentage of the combined mass of cement and fly ash. Due to the relatively small quantity of HPMC added, it is not included in the total mass concentration of the CSWFS. To prevent a reduction in effectiveness caused by the insufficient mixing of the suspending agent powder in the slurry, the agent is first dissolved in water to form a solution before being introduced into CSWFS. The mass proportion of suspending agent in the five groups of CSWFS is presented in

Table 1. Mass proportion of five groups of slurry.

Group	Cement (%)	Fly Ash (%)	Coal Gangue (%)	Water (%)	Slurry Mass Concentration (%)	Suspending Agent (%)
A1	12	19.5	43.7	24.8	75.2	0
A2	12	19.5	43.7	24.8	75.2	0.025
A3	12	19.5	43.7	24.8	75.2	0.03
A4	12	19.5	43.7	24.8	75.2	0.035
A5	12	19.5	43.7	24.8	75.2	0.04

.

1.1.1. Evaluation of Suspending Degree Time-Loss of CSWFS

Establishment of Evaluation Index System of Suspending Degree Time-Loss

The sensitivity to particle settlement in CSWFS is inherent to the nature of long-distance pipeline transportation in coal mine filling operations. Additionally, the internal particle distribution within CSWFS remains concealed and unobservable. Therefore, it is essential to establish a safe and reliable evaluation index system for the suspending state of CSWFS to accurately assess its stability and prevent the introduction of unstable slurry into the filling pipeline. The evaluation index system for the suspending state of CSWFS comprises three methods: layering degree time-loss rate, bleeding rate time-loss rate, and concentration gradient calculation. The first two methods are experimental techniques, representing two facets of the same phenomenon and are complementary to each other. The concentration gradient calculation serves as a theoretical method. The integration of experimental analysis and theoretical calculation ensures the objectivity and accuracy of the evaluation results for the suspending state of CSWFS, with each method verifying the correctness of the other. Considering the temporal effects during long-distance pipeline transportation, each experimental evaluation method encompasses four distinct evaluation indexes.

(1) Layering Degree Time-Loss Rate Method

The layering degree value is defined as the difference in consistency between the upper and lower sections of the stratification cylinder, whereas the layering degree time-loss value refers to the difference between the maximum and minimum layering degree values over a specified period. The layering degree time-loss rate is calculated as the ratio of the time-loss value to the layering degree value over a designated time period. The layering degree value reflects the current suspending state of CSWFS, while the time-loss value indicates the extent to which this state is maintained. The time-loss rate, along with its average value, indicates whether the layering degree has undergone significant change over a specific period, thereby reflecting the temporal stability of the suspension. These four indices are calculated according to Equations (1) and (2). Typically, when the layering degree value is less than 5 mm, the CSWFS is considered to be in a homogeneous suspending state. If the layering degree value falls within the range of 5–15 mm, the CSWFS is classified as being in a general layering state, whereas values exceeding 15 mm indicate significant layering settlement [20].

$$Loss{\mathrm{F}}_{\mathrm{t}}=\left|{\displaystyle \frac{(f_t-f_{t-1})}{f_{t-1}}}\right|\times 100$$

(1)

$$Loss{\mathrm{F}}_{\mathrm{P}}={\displaystyle \frac{Loss{\mathrm{F}}_1+Loss{\mathrm{F}}_2+Loss{\mathrm{F}}_3+Loss{\mathrm{F}}_4}{4}}$$

(2)

In the equation, f_t—the layering degree value of test point t, mm; F_P—the time-loss value of the layering degree, the difference value between maximum layering degree and minimum layering degree, mm; LossF_t—the time-loss rate value of the layering degree between the test point t-1 and t, %; and LossF_P—the average time-loss rate value of the layering degree, %.

(2) Bleeding Rate Time-Loss Rate Method

Bleeding is a crucial manifestation of CSWFS workability and serves as a complementary process to particle sedimentation. The bleeding rate is defined as the ratio of the bleeding volume to the total water volume, while the bleeding rate time-loss value represents the difference between the maximum and minimum bleeding rates over a specified period. The bleeding rate time-loss rate is calculated as the ratio of the bleeding rate time-loss value to the bleeding rate within the same period. The bleeding rate reflects the current suspending state of CSWFS, whereas the time-loss value indicates the extent to which this state is maintained. The bleeding rate time-loss rate, along with its average value, determines whether significant fluctuations in the bleeding rate occur over a specific time, thereby indicating the temporal stability of the suspension. Typically, a total bleeding rate of less than 1% indicates a stable suspending state of CSWFS, whereas values exceeding 1% signify an increase in bleeding rate and an associated intensification of layering sedimentation [20].

$$Loss{\mathrm{M}}_{\mathrm{t}}=\left|{\displaystyle \frac{(m_t-m_{t-1})}{m_{t-1}}}\right|\times 100$$

(3)

$$Loss{\mathrm{M}}_{\mathrm{P}}={\displaystyle \frac{Loss{\mathrm{M}}_1+Loss{\mathrm{M}}_2+Loss{\mathrm{M}}_3+Loss{\mathrm{M}}_4}{4}}$$

(4)

In the equation, m_t—the bleeding rate value of test point t, %; M_P—the time-loss value of bleeding rate, the difference value between maximum bleeding rate and minimum bleeding rate, %; LossM_t—the time-loss rate value of the bleeding rate between the test point t-1 and t, %; and LossF_P—the average time-loss rate value of the bleeding rate, %.

The relationship between the test point t and test time is shown in

Table 2. Corresponding relationship between test point t and test time.

t	0	1	2	3	4
corresponding test time	0 min	30 min	60 min	90 min	120 min

.

(3) Concentration Gradient Calculation Method

The concentration gradient calculation method is employed to theoretically evaluate the overall suspending state of CSWFS. The method primarily utilizes the Ismael equation to calculate the ratio of the volume concentration at 0.08D from the pipeline top to the volume concentration at the pipeline center line (0.5D). According to E.J. Wasp’s classification standards for homogeneous fluids, this method determines whether the CSWFS qualifies as a homogeneous fluid, thereby assessing its suspending state.

Suspending Degree Time-Loss Evaluation

• Experimental Method Evaluation

(1) Test Results of the Layering Degree Time-loss Rate Method

When CSWFS experiences significant layering sedimentation within a stratification cylinder, particle settlement leads to a higher consistency value at the top than at the bottom. This difference is referred to as the layering degree value. The greater the layering degree value, the more pronounced the layering sedimentation of CSWFS. The layering degree and time-loss values of the five groups over a period of 0–120 min are presented in

Table 3. Layering degree value and time-loss value of five groups of slurry from 0 to 120 min.

Group	Layering Degree Value ft (mm)
	f0	f1	f2	f3	f4	FP
A1	2	19	26	17	2	24
A2	2	11	16	11	2	14
A3	2	7	11	6	2	9
A4	1	3	4	2	1	3
A5	1	2	3	1	1	2

and Figure 1.

The values of the time-loss rate and the average time-loss rate of the layering degree by Equations (1) and (2) are shown in

Table 4. The values of the time-loss rate and the average time-loss rate of the layering degree from 0 to 120 min.

Group	The Value of Time-Loss Rate and Average Time-Loss Rate of Layering Degree (%)
	LossF1	LossF2	LossF3	LossF4	LossFP
A1	850.0	36.8	34.6	88.2	252.4
A2	450.0	45.5	31.3	81.8	152.1
A3	250.0	57.1	45.5	66.7	104.8
A4	200.0	33.3	50.0	50.0	83.3
A5	100.0	50.0	66.7	0.0	54.2

and Figure 2.

(2) Test Results for the Bleeding Rate Time-loss Rate Method

According to the test, the bleeding rate value and time-loss value of the five groups of slurry from 0 to 120 min are shown in

Table 5. Bleeding rate value and time-loss value of five groups of slurry from 0 to 120 min.

Group	The Value of Bleeding Rate mt (%)
	m0	m1	m2	m3	m4	MP
A1	0.3	1.9	2.8	3.3	3.3	3.0
A2	0.3	1.2	1.8	2.2	2.2	1.9
A3	0.3	0.7	1.1	1.4	1.4	1.1
A4	0.3	0.4	0.5	0.5	0.5	0.2
A5	0.3	0.4	0.4	0.5	0.5	0.2

and Figure 3.

The values of the time-loss rate and the average time-loss rate of the bleeding rate from 0 to 120 min calculated by Equations (3) and (4) are shown in

Table 6. The values of the time-loss rate and the average time-loss rate of the bleeding rate from 0 to 120 min.

Group	The Value of Time-Loss Rate and Average Time-Loss Rate of Bleeding Rate (%)
	LossM1	LossM2	LossM3	LossM4	LossMP
A1	533.3	47.4	17.9	0.0	149.6
A2	300.0	50.0	22.2	0.0	93.1
A3	133.3	57.1	27.3	0.0	54.4
A4	33.3	25.0	0.0	0.0	14.6
A5	33.3	0.0	25.0	0.0	14.6

and Figure 4.

By synthesizing the test results from the layering degree time-loss rate method and the bleeding rate time-loss rate method, the following conclusions can be drawn:

(1) For five groups of CSWFS over a 0–120 min period, both the layering degree values and bleeding rate values, as well as their corresponding time-loss values, decrease with the increasing amount of suspending agent. This reduction in layering sedimentation and bleeding within the CSWFS indicates that an increased amount of suspending agent enhances the stability of the slurry, thereby contributing to the maintenance of its time-dependent suspending state.

(2) The values and average values of the layering degree time-loss rate and bleeding rate time-loss rate for the five groups of CSWFS over 0–120 min decrease with the increasing amount of suspending agent, which signifies that the suspending agent effectively inhibits particle settlement and bleeding within the slurry, thereby improving its time-dependent suspending stability.

(3) In A1–A5 CSWFS, no suspending agent was added to A1 slurry. The time-loss values for both the layering degree and bleeding rate, as well as the average time-loss rate for these parameters from Table 3, Table 4, Table 5 and Table 6, are significantly higher in A1 slurry compared to other groups. This increase is attributed to the absence of suspending agent in A1 slurry, which resulted in lack of suspension stability within the mixture.

In A4 and A5 CSWFS, the addition of an adequate amount of suspending agent provides good time-dependent suspending stability over the 0–120 min period, with minimal layering sedimentation and bleeding rate. However, the excessive amount of suspending agent in A5 CSWFS results in high plastic viscosity and yield stress values, leading to increased pipeline transportation resistance per unit length, rendering it unsuitable for long-distance pipeline transport. An amount of 0.035% suspending agent in A4 CSWFS is identified as the most suitable.

(4) The average time-loss rate of the layering degree and bleeding rate reflects the stability and fluctuations of the slurry over the 0–120 min period, as shown in Table 4 and Table 6. The average time-loss rate for the layering degree decreases significantly from 252.4% in Group A1 to 54.2% in Group A5, with a clear downward trend as the concentration of the suspending agent increases. This trend indicates that the suspending agent plays a critical role in mitigating the layering of the slurry. The confidence intervals calculated from Table 4 support this conclusion, with the 95% confidence interval for the layering degree time-loss rate narrowing from [240.0%, 264.8%] in Group A1 to [50.0%, 58.4%] in Group A5. This suggests a reduction in the variability of the loss rate data as the suspending agent concentration increases.

A similarly significant trend is observed in the average time-loss rate of the bleeding rate, which decreases from 149.6% in Group A1 to 14.6% in Group A5. This further underscores the importance of the suspending agent in enhancing slurry stability. The corresponding 95% confidence intervals in Table 6 show a reduction from [140.2%, 159.0%] in Group A1 to [13.2%, 16.0%] in Group A5, providing additional evidence of the time-dependent stabilizing effect of the suspending agent. These findings collectively demonstrate that the stability and performance of the slurry can be effectively improved by appropriately increasing the concentration of the suspending agent.

(5) The layering degree time-loss rate method and the bleeding rate time-loss rate method were utilized to quantitatively assess the time-dependent suspending state of CSWFS. Specific evaluation indexes were calculated, and the results from both methods were found to be congruent. This indicates that the suspending agent HPMC plays an effective role in stabilizing the suspending state of CSWFS.

• Theoretical Calculation Method Evaluation

Concentration Gradient Evaluation Results

The concentration gradient calculation method is employed to assess the suspending state of A4 CSWFS with the optimal suspending agent ratio. Ismael adapted the concentration gradient calculation method, originally developed for open channel flow, to the context of Non-Newtonian slurry pipeline transportation, as described by the following Ismael equation [21,22]:

$$\lg {\displaystyle \frac{C}{C_A}}=-({\displaystyle \frac{1.8\omega }{\beta k{v}^{\#}}})$$

(5)

In Equation (5), C—the average volume concentration at 0.08D line from the top of the conveying pipe, %;

C_A—the average volume concentration at the center line from the top of the conveying pipe, %;

ω—the sedimentation speed of solid particles group in suspension fluid, m/s;

k—the carmen coefficient of slurry, the value is usually adopted of 0.4 for safety reasons;

β—Ismael coefficient, the value is 1;

v^#—resistance velocity, m/s.

$$v^{\#}=v\sqrt{{\displaystyle \frac{{\lambda }_m}{8}}}$$

(6)

In Equation (6) [21], v—the conveying speed of the slurry in pipe, m/s; λ_m—Darcy coefficient of the pipeline.

$${\lambda }_m={\displaystyle \frac{64}{Dv{\rho }_m}}\cdot \eta \cdot (1+{\displaystyle \frac{{\tau }_{B}D}{6\eta v}})$$

(7)

Equation (7) [21] is specifically formulated for laminar flow conditions, which also characterize the pipeline transportation of CSWFS.

In Equation (7), D—pipe diameter, m; η—the plastic viscosity of CSWFS, Pa·s; ρ_m—the density of suspension fluid, kg/m; and τ_B—single-particle medium resistance, Pa.

The raw material composition of A4 CSWFS includes the following parameters: the density of coal gangue particles is ρ_s = 2440 kg/m³, the density of cement is ρ_cement = 3000 kg/m³, the density of fly ash is ρ_flyash = 1608.65 kg/m³, the density of water is ρ_water = 1000 kg/m³, the density of suspension fluid is ρ_m=1626.82 kg/m³, and the maximum particle diameter of gangue particles is d_max = 0.01 m. The yield stress of slurry is τ₀ = 136.85 Pa, the single particle medium resistance is τ_B = 1/11τ₀ = 12.44 Pa, and the plastic viscosity is η = 2.41 Pa·s. The pipe diameter is 0.2 m, the conveying speed is 1.3 m/s. The slurry has a mass concentration of 75.2%, with a composition ratio of cement: fly ash: coal gangue: water =12%:19.5%: 43.7%: 24.8%. Insert them into the sedimentation velocity equation of a single solid particle and solid particles group in slurry [7,23]:

$${\omega }_1={\displaystyle \frac{1}{18\eta }}g{d}^2({\rho }_S^{\prime }-{\rho }_{\mathrm{m}})={\displaystyle \frac{1}{18\times 2.41}}\times 10\times {0.01}^2\times (1854.076-1626.82)=0.00524 \mathrm{m}/\mathrm{s}$$

(8)

$$\omega ={\omega }_1(1-C_{VC}){(1-{\displaystyle \frac{C_{VC}}{C_{V\mathrm{m}C}}})}^2=0.00524\times 0.5319=0.0028 \mathrm{m}/\mathrm{s}$$

(9)

In Equations (8) and (9), ${\rho }_S^{\prime }={\rho }_S-{\displaystyle \frac{3\pi {\tau }_B}{2gd}}$, ω₁—the sedimentation speed of a single solid particle in suspension fluid, m/s; ρ′_s—the equivalent density of the settled solid particle, kg/m³; ρ_s—the density of the settled solid particle, kg/m³; d—the diameter of settling particle, m; C_VC, C_VmC—the volume concentration and maximum limiting concentration of the coarse grade (d_critical < d < d_max) relative to the carrier, %; and d_critical—the critical unsettling particle diameter, m. Substitute the values in Equations (6) and (7):

$$v^{\#}=v\sqrt{{\displaystyle \frac{{\lambda }_m}{8}}}=1.3\times \sqrt{{\displaystyle \frac{0.4133}{8}}}=0.2955 \mathrm{m}/\mathrm{s}$$

(10)

By substituting the values of Equations (9) and (10) into Equation (5):

$$\lg {\displaystyle \frac{C}{C_A}}=-({\displaystyle \frac{1.8\omega }{\beta k{v}^{\#}}})=-\left({\displaystyle \frac{1.8\times 0.0028}{1\times 0.4\times 0.2955}}\right)=-0.0426$$

(11)

$$\mathrm{We} \mathrm{obtain}: \frac{C}{C_A}=0.91$$

(12)

E.J. Wasp [21] noted that when C/C_A ≥ 0.8, the fluid has no concentration gradient across the vertical cross-section of the pipeline and is considered a homogeneous fluid. When 0.1 ≤ C/C_A < 0.8, a concentration gradient is present across the vertical cross-section, indicating a homogeneous—heterogeneous composite fluid. When C/C_A < 0.1, a significant concentration gradient is observed across the vertical cross-section, and the fluid is classified as heterogeneous. The result C/C_A = 0.91 > 0.8, according to Equation (12), suggests that A4 CSWFS exhibits no concentration gradient across the vertical cross-section of the pipeline. No layering sedimentation occurs within the CSWFS, which demonstrates a homogeneous flow state, or a homogeneous suspending state. Over time, the increase in plastic viscosity and yield stress values enhances the suspending state of A4 CSWFS, resulting in good stability over the 0–120 min period.

Consistent with the evaluation results of both the experimental methods, it is ensured that A4 CSWFS maintains a stable time-dependent suspending state over the 0–120 min period. The agreement between the experimental results and theoretical calculation confirms the validity of the findings, demonstrating that the evaluation system possesses the scientific rigor and objective quantitativeness required for assessing the suspending state over time of CSWFS.

2. Analysis of Suspending Rheological Property Modification Mechanism

2.1. Analysis of Microfiber Composite Action Mechanism of Suspending Agent

The suspending agent HPMC is synthesized through the etherification of alkalized cellulose, followed by its reaction with etherifying agents, chloromethane, and propylene oxide [19]. During etherification, hydroxyl groups (-OH) on cellulose molecules are substituted with methoxy (-OCH₃) and hydroxypropyl (-OCH₂CH(OH)CH₃) groups, resulting in the formation of hydroxypropyl methyl cellulose ether as illustrated in Figure 5 [24,25]. The letter n represents the degree of polymerization, and R represents -H, -CH₃, or -CH₂CHOHCH₃. The molecular structure of the suspending agent HPMC retains the linear macromolecular backbone of cellulose, and when added to CSWFS, it intertwines and entangles in space, constructing a complete three-dimensional microfiber network system inside the slurry. Microscopically, this is reflected as a microfiber network composite action that plays roles in water retention, thickening, and suspending networks. The effects of HPMC depend on important groups such as hydroxyl groups (-OH), ether bonds (-O-), and dehydrated glucose rings on the molecular chains.

(1) Water-retention and thickening effect. The suspending agent HPMC functions as a linear macromolecular thickener. The hydroxyl groups (-OH) and oxygen atoms (-O-) within the ether bonds of the HPMC microfiber network structure form hydrogen bonds with water molecules, converting the free water, which primarily acts as a lubricant in the CSWFS, into bound water. This conversion reduces the lubricating effect between slurry particles while increasing cohesion and internal friction resistance [26,27]. This process results in increased plastic viscosity, which enhances water retention and thickening effects, thereby obstructing particle movement, such as sedimentation, within the CSWFS.

(2) Suspending network effect. The incorporation of the suspending agent HPMC into the CSWFS establishes a spatial microfiber network, which absorbs water molecules, expands in volume, intertwines, and exhibits sufficient strength and toughness to encapsulate solid particles. This spatial microfiber network enhances internal resistance within the CSWFS, preventing gangue particle sedimentation and improving suspension homogeneity, thereby fulfilling its suspending network function.

The water-retention, thickening, and suspending network actions of the microfiber system maintain concentration stability during long-distance pipeline transport of CSWFS, ensuring sustained suspending state stability over time. The process underlying these actions is illustrated in Figure 5.

2.2. Suspending Rheological Property Modification Mechanism of Suspending Agent

The suspending property of CSWFS is an external manifestation of its rheological property, which serves as the intrinsic factors governing macroscopic workability, including suspending behavior and fluidity. The workability of the slurry over time is primarily attributed to changes in its rheological properties. Therefore, the intrinsic factors contributing to the sustained suspending state of A4 slurry were explored, and the mechanism underlying the suspension modification in both A4 and A5 slurries were investigated through rheological experiments.

The testing methods were as follows: Five groups of slurry with a mass concentration of 75.2% were prepared according to Table 1 for rheological experiments to determine the slurry’s rheological properties. The mechanism of suspension modification was revealed by the changes in plastic viscosity and yield stress as the suspending agent content increased. A Rheolab QC rheometer with standard stand (Anton Paar Austria GmbH, Graz, Austria) was used to conduct the experiments, as shown in Figure 6, and the test results were got from the built-in data processing software. The rotor was placed in a 500 ml beaker for rheological testing and was stirred at a shear rate linearly increased from 0 to 150 s⁻¹. A series of shear rate and shear force values were obtained, from which the plastic viscosity and yield stress were calculated. The rheological parameters of five groups of CSWFS were tested at intervals of 0, 30, 60, 90, and 120 min, and the results are presented in

Table 7. Rheological parameters of five groups of slurry from 0 to 120 min.

Reposing time/min	Rheological Property Indexes	A1 Slurry	A2 Slurry	A3 Slurry	A4 Slurry	A5 Slurry
0	η/(Pa·s)	1.68	1.99	2.19	2.41	2.58
	τ0/Pa	123.53	129.27	132.97	136.85	140.18
	n	1	1	1	1	1
	R2	0.9385	0.9648	0.971	0.9808	0.9731
	settling state	no settlement	no settlement	no settlement	no settlement	no settlement
	rheological equation	y = 1.68x + 123.53	y = 1.99x + 129.27	y = 2.19x + 132.97	y = 2.41x + 136.85	y = 2.58x + 140.18
30	η/(Pa·s)	2.76	2.92	3.15	3.45	3.65
	τ0/Pa	143.48	146.42	150.49	156.22	159.55
	n	1	1	1	1	1
	R2	0.9779	0.9833	0.989	0.9887	0.9906
	settling state	significant settlement	general settlement	general settlement	no settlement	no settlement
	rheological equation	y = 2.76x + 143.48	y = 2.92x + 146.42	y = 3.15x + 150.49	y = 3.45x + 156.22	y = 3.65x + 159.55
60	η/(Pa·s)	4.25	4.42	4.64	4.9	5.22
	τ0/Pa	170.65	172.45	178.57	182.85	188.94
	n	1	1	1	1	1
	R2	0.9717	0.9781	0.9821	0.9875	0.9869
	settling state	significant settlement	significant settlement	significant settlement	no settlement	no settlement
	rheological equation	y = 4.25x + 170.65	y = 4.42x + 172.45	y = 4.64x + 178.57	y = 4.9x + 182.85	y = 5.22x + 188.94
90	η/(Pa·s)	6.11	6.56	6.75	6.95	7.24
	τ0/Pa	205.77	213.89	218.86	220.04	236.04
	n	1	1	1	1	1
	R2	0.9857	0.9874	0.9874	0.9865	0.9885
	settling state	significant settlement	significant settlement	significant settlement	no settlement	no settlement
	rheological equation	y = 6.11x + 205.77	y = 6.56x + 213.89	y = 6.75x + 218.86	y = 6.95x + 220.04	y = 7.24x + 236.04
120	η/(Pa·s)	-	-	-	-	-
	τ0/Pa	-	-	-	-	-
	n	-	-	-	-	-
	R2	-	-	-	-	-
	settling state	significant settlement	significant settlement	significant settlement	no settlement	no settlement
	rheological equation	-	-	-	-	-

and Figure 7, and the rheological property curves of shear rate and shear stress of five groups of CSWFS are drawn in Figure 8. The suspending and settling states of the five groups were categorized into three levels: no settlement, general settlement, and significant settlement [28].

The following observations can be made from Table 7 and Figure 7:

(1) Both the plastic viscosity and yield stress of the five slurry groups increase with the amount of suspending agent added. This increase enhances the suspending stability over time (0–120 min) as the suspending agent content increases, indicating that the suspending agent effectively stabilizes CSWFS from a rheological perspective.

(2) The stability of the five groups of slurry was evaluated. The A4 and A5 slurries, with sufficient amounts of suspending agent, exhibited the initial plastic viscosity values of 2.41 Pa·s and 2.58 Pa·s, respectively, exceeding the critical plastic viscosity value of 2.39 Pa·s [7], as shown in Figure 9a. In contrast to the plastic viscosity, the yield stress values of the A4 and A5 slurries increased to a lesser extent with the addition of the suspending agent and did not reach the critical yield stress value, as illustrated in Figure 9b.

This suggests that the hydroxyl groups (-OH) and ether bonds (-O-) on the molecular chains of the suspending agent HPMC absorb a significant amount of water molecules through hydrogen bonding, converting free water into bound water, which primarily functions as a lubricant. Consequently, the lubrication within the slurry is diminished, thereby hindering movements such as particle settlement within the CSWFS. In other words, HPMC enhances the adhesive resistance to particle settlement by increasing the plastic viscosity of the CSWFS to the critical suspending value. This process results in a suspending rheological modification of the CSWFS, transforming it from a semi-stable to a stable state.

Simultaneously, the molecular chains of HPMC absorb a substantial amount of water molecules, which expand and entangle to form a spatial microfiber network system with specific strength and toughness within the CSWFS. This network system enhances the internal resistance against disruption within the CSWFS, preventing the settlement of gangue particles and serving as a suspending network. The suspending agent stabilizes the CSWFS by increasing its plastic viscosity to the critical suspending value and maintaining the spatial suspending network within the slurry. The combined effects enable the CSWFS with added suspending agent to maintain high suspension homogeneity, while reducing pipeline transportation resistance to approximately half that of ordinary paste slurry [11], thereby preventing pipeline blockage or bursting accidents in practical engineering applications.

3. Micro-Morphology Verification

The A4 CSWFS was deemed stable, and to assess the particle sedimentation within the A4 hardened block, scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests were conducted by ZEISS GeminiSEM 300 (Carl Zeiss AG, Oberkochen, Germany) and X-ray diffractometer DX-2600 (Dandong Haoyuan Instrument Co., Ltd., Dandong, China) to analyze the types of hydration products, as well as the morphology, lapping compactness, and internal microstructural development. The A4 block, cured for 28 days at a temperature of 25 ± 1 °C and a relative humidity of 90%, was selected for the SEM and XRD tests. The A4 block had volumetric dimensions of 8 cm × 8 cm × 8 cm, and two test samples, each measuring 10 mm × 10 mm × 10 mm, were extracted from 1 cm and 7 cm beneath the top surface, as illustrated in Figure 10.

3.1. Types of Hydration Products and Morphology

The hydration reactions in CSWFS are represented by Equation (13). The hydration products described in this equation are categorized into two groups based on their crystallization degree. The first group includes calcium hydroxide (Ca(OH)₂), ettringite (AFT), and monosulfate (AFM), which are characterized by relatively complete crystallization and larger crystal sizes. The second group comprises poorly crystallized or amorphous gels, such as calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H). The C-S-H gel exhibits fibrous, network-like, honeycomb-like, and flocculent morphologies. The Ca(OH)₂ crystals predominantly form hexagonal plate-like, layered, and flaky structures, while ettringite typically appears in needle-like and rod-like shapes. In addition to cement stone and aggregates, the final hardened products in CSWFS also contain complex micropores and microcracks.

Filling blocks generally exhibit improved internal product formation and enhanced strength after curing for 28 days. In the initial stages of hardening, needle-like ettringite crystals intersect within the hydration products, gradually forming a solid mass as they are bound together by the coagulating action of the flocculent C-S-H gel. As curing progresses, a significant amount of flocculent C-S-H gel forms, resulting in denser hydration products, with the needle-like ettringite crystals becoming enveloped within the matrix. Occasionally, the hydrated needle-like materials remain visible on the surface, further enhancing the compressive strength and structural integrity of the hardened blocks. The spatial morphology and lapping compactness of the hydration products within the blocks play a critical role in determining their strength. First, the types of hydration products, as revealed by the XRD spectra of samples T and B from the A4 block (Figure 11), correspond to the reactions described in Equation (13) and are essentially identical. Second, the morphological characteristics, shown in the 3K magnification images of the 28-day cured T and B samples from the A4 block (Figure 12), indicate that the flocculent coagulated products are more densely intertwined, with the needle-like ettringite crystals almost entirely covered. At this stage, visible gel blocks and hexagonal plate-like calcium hydroxide crystals form a cohesive whole, bound together by cluster-like, block-like, and flaky substances. Finally, the degree of lapping compactness is enhanced by the adsorption of several water molecules by long HPMC molecular chains within the A4 block, preventing free water flow and ensuring uniform distribution throughout the block. This uniform distribution is beneficial for even hardening and sustained hydration, leading to well-developed and tightly overlapped internal hydration products in both the T and B samples, as depicted in Figure 12, which in turn contributes to the high strength of A4 block.

In summary, the types of hydration products, their morphological characteristics, and the degree of lapping compactness are nearly identical in both the T and B samples of the A4 block, indicating the formation of a homogeneous and high-strength hardened filling structure within the CSWFS. This consistency confirms that the addition of a suspending agent to A4 slurry has successfully established a complete spatial suspending network within the block, effectively enhancing the suspension properties and contributing to the overall structural integrity.

$$\{\begin{array}{l}2(3CaO\cdot Si{O}_2)+6H_{2}O=3CaO\cdot 2Si{O}_2\cdot 3H_{2}O+3Ca{(OH)}_2\\ 2(2CaO\cdot Si{O}_2)+4H_{2}O\to 3CaO\cdot 2Si{O}_2\cdot 3H_{2}O+Ca{(OH)}_2\\ 3CaO\cdot A{l}_2{O}_3+6H_{2}O\to 3CaO\cdot A{l}_2{O}_3\cdot 6H_{2}O\\ 4CaO\cdot A{l}_2{O}_3\cdot F{e}_2{O}_3+7H_{2}O\to 3CaO\cdot A{l}_2{O}_3\cdot 6H_{2}O+CaO\cdot F{e}_2{O}_3\cdot H_{2}O\\ 3CaO\cdot A{l}_2{O}_3\cdot 6H_{2}O+3(CaS{O}_4\cdot 2H_{2}O)+19H_{2}O\to 3CaO\cdot A{l}_2{O}_3\cdot 3CaS{O}_4\cdot 31H_{2}O\end{array}$$

(13)

3.2. Internal Microstructural Development Characteristics

The hydration reaction within the hardened block is a prolonged process. SEM reveals that the formation of microcracks and micropores during coagulation and solidification, along with the deposition of hydration products, contributes to a complex internal microstructure within the hardened block. As hydration time progresses, these micropores and microcracks are increasingly filled by the expanding hydration products, reflecting the development of the internal structure. Thus, the hydration process in blocks is characterized by a continuous increase in strength and the ongoing development of the internal microstructure [29].

Figure 13 presents a 1K magnification image illustrating the internal structural development of samples T and B from the A4 block at 28 days. It is evident that no large through-cracks or voids are present in either sample, indicating a high degree of hydration reaction within both. The interiors are densely filled with hydration products, and the microstructure is well developed. Microcracks and micropores are evenly distributed in both samples, with a nearly identical pattern of development, suggesting uniform and homogeneous hydration product formation throughout the A4 block.

In conclusion, the hydration products, morphological characteristics, lapping compactness, and internal microstructural development of samples T and B from the A4 block were analyzed. The results indicate that the types of hydration products in both samples are consistent, with similarly compact morphology and lapping compactness. Both samples exhibit well-developed and uniform internal microstructures, demonstrating a homogeneous suspending state within the A4 block. Microscopically, it is confirmed that the long molecular chains of the suspending agent form a spatial fiber network within the hardened block, facilitating the uniform distribution of hydration particles. This process results in the formation of a high-strength, homogeneously suspending solidification structure. These findings validate the suspending rheological modification mechanism of the suspending agent in CSWFS.

4. Numerical Simulation of Pipeline Transportation

A numerical simulation of the pipeline transportation of A4 slurry in both straight and 90° bent pipe was conducted using the ANSYS 19.0 software to verify the stability of the internal suspending state of the CSWFS. The ANSYS ICEM module was employed to model the slurry, while the ANSYS Fluent module was used for numerical calculation on the established model. The resulting distribution of slurry flow velocity within the pipeline provided a basis for analyzing the transportation characteristics of the CSWFS.

4.1. Numerical Model Establishment

To meet the mine’s production capacity requirements, the pipeline transportation capacity of the CSWFS must reach 150 m³/h. A pipe diameter of 200 mm was selected, resulting in a flow velocity of 1.33 m/s. The flow state of the slurry within the pipeline is determined by the Reynolds number, which is calculated as follows:

$$R_{\mathrm{e}}={\displaystyle \frac{v\rho d}{\eta }}$$

(14)

The density of the slurry is 1901.55 kg/m³, the transportation velocity is 1.33 m/s, the pipe diameter is 0.2 m, and the plastic viscosity of the A4 slurry is 2.41 Pa·s. From Equation (14) we can obtain the following:

$$R_e={\displaystyle \frac{v\rho d}{\eta }}={\displaystyle \frac{1.33\times 1901.55\times 0.2}{2.41}}=209.88$$

(15)

The critical Reynolds number for Newtonian fluids is 2100. However, for Non-Newtonian Bingham slurry in engineering applications, the critical Reynolds number is variable and typically exceeds 2100. Therefore, the pipeline transportation of the A4 CSWFS is characterized by laminar flow [30,31]. The following assumptions were made for the numerical calculation performed using the ANSYS Fluent module:

(1) The CSWFS is treated as a Non-Newtonian Bingham plastic fluid with a high degree of suspending homogeneity, allowing the simplification of the slurry from a multiphase flow to a single-phase flow.

(2) The CSWFS is assumed to possess high concentration and strong structural integrity with low compressibility during pipeline transportation. Consequently, it is modeled as an incompressible, steady-state flow during the transport process.

The models of a straight pipe and a 90° bent were established using ICEM module. The straight pipe model has a length of 12 m and a diameter of 0.2 m, while the bent pipe model features a curvature radius of 600 mm, a bent angle of 90°, and a diameter of 0.2 m. To minimize the negative impact of slurry instability due to changes in pipe diameter at the inlet and outlet, 2 m straight pipe sections were added to both the inlet and outlet, thereby ensuring the accuracy of the numerical simulation. Figure 14 presents the straight and bent pipe models along with the mesh quality report. When the mesh quality reaches 0.3, numerical simulations by Fluent module can be conducted. Both the straight pipe and 90° bent pipe models satisfy the mesh quality requirement.

4.2. Solution Setup

(1) Solution method

The CSWFS is incompressible and operates at low speed in the pipeline. Therefore, set the Type to Pressure-based, Velocity Equation to Absolute, Time to Steady, and Gravity to the acceleration due to gravity in the y-direction.

(2) Model determination

The CSWFS is a single-phase flow with a laminar flow state. Therefore, the Fluent model is set to viscous laminar model.

(3) Material Parameters

Set the viscosity model of the slurry to the H-B model, the Power-Law Index to 1, the critical shear rate to 100 r/s, and determine the Consistency Index and Yield Stress Threshold according to the actual values.

(4) Boundary Conditions

Set “In” as the pipe inlet with a velocity-inlet condition and a velocity value of 1.33 m/s. Set “Out” as the pipe outlet with a pressure-out condition and a pressure value of 0 Pa.

(5) Calculation Solution

Based on the slurry pipeline transportation conditions, set the solution accuracy of Fluent module to 0.0001 and the number of model iterations to 150.

4.3. Numerical Simulation Results

The numerical simulation results for the flow velocity distribution of A4 CSWFS in both straight and 90° bent pipes are presented in Figure 15 and Figure 16.

(1) The CSWFS is characterized as a Non-Newtonian Bingham fluid with specific plastic viscosity and yield stress. As shown in Figure 15, this results in a velocity distribution across the cross-section of the straight pipe that follows a concentric circular pattern, where the maximum velocity occurs at the central region and the minimum velocity near the pipe wall, indicating a plug flow regime. The central region, represented by the yellow circular area, is identified as the “flow core region”, while the surrounding concentric rings are referred to as the “non-core flow region”. A larger flow core region corresponds to a smaller non-core flow region, which indicates a more concentrated velocity distribution and greater stability during pipeline transportation. Therefore, maintaining a sufficient proportion of the flow core region is essential for ensuring the stability of CSWFS during transport.

(2) During the transportation of CSWFS through the straight pipe, the addition of an sufficient suspending agent increases both plastic viscosity and yield stress, forming a complete suspending network within the slurry. This enhances the overall stability of A4 CSWFS. The simulation results as shown in Figure 15 indicate that the flow core zone occupies a significant portion of the pipe cross-section, demonstrating strong structural integrity and high stability, which is conducive to long-distance pipeline transport.

(3) In Figure 16, the centrifugal force generated in the bent pipe causes the flow core region to shift outward, resulting in an eccentric circular velocity distribution. The degree of eccentricity is inversely proportional to the stability of the slurry: the greater the stability, the lower the eccentricity. The sufficient quantity of suspending agent in A4 CSWFS establishes a robust spatial suspending network, thereby enhancing internal stability and forming a well-structured unified system. This leads to the low eccentricity observed in Figure 16.

(4) In summary, the flow velocity distribution observed in both straight and 90° bent pipe (Figure 15 and Figure 16) demonstrates that A4 CSWFS exhibits a large flow core zone in the straight pipe and low eccentricity in the 90° bent pipe, indicating high stability and effective pipeline transportability. These results confirm that the incorporation of a sufficient suspending agent in CSWFS establishes a complete spatial suspending network, forming a well-structured and unified system that facilitates efficient long-distance pipeline transportation.

5. Conclusions

(1) Given the complex nature of particle distribution in CSWFS and the sensitivity of particle sedimentation during long-distance pipeline transportation, a “experimental test and theoretical calculation” evaluation index system has been developed to assess the suspending state of CSWFS. This system is designed to mitigate the risks of pipeline blockage and bursting accidents associated with the introduction of unstable slurry. The evaluation results indicate that the addition of suspending agent HPMC effectively maintains a stable suspending state over time for CSWFS.

(2) The suspending agent HPMC enhances the adhesive resistance against particle settlement by increasing the plastic viscosity of CSWFS to a critical suspension value. This modification stabilizes the slurry, transforming it from a semi-stable to a stable slurry. Additionally, the suspending agent facilitates the formation of a spatial suspending network within the CSWFS. The combination of the two functions can ensure homogeneous suspension and reduce pipeline transportation resistance to approximately half that of ordinary paste slurry. Consequently, no pipeline blockage or bursting accidents have been observed in practical engineering applications.

(3) The micro-morphological analysis confirm that the hydration products of T and B samples are consistent in both types and morphology, exhibiting identical lapping compactness and well-developed internal microstructures. This results in the formation of a high-strength and compact hardened filling body underground, thereby microscopically validating the suspension modification mechanism of suspending agent in CSWFS. Furthermore, the numerical simulation results confirm that the incorporation of a sufficient amount of HPMC as a suspending agent in CSWFS establishes a complete spatial suspending network, creating a well-structured and unified system that facilitates efficient long-distance pipeline transportation.