1. Introduction
As one of the most significant fossil energy sources in industry, coal is crucial to the social and economic advancement of people in the majority of the world’s nations [
1]. With the development of the economy, the energy consumption of human beings has increased, and more and more coal needs to be mined [
2]. Coal mining serves as one of the major basic industries. Compared with other countries in the world, coal accounts for more than 50% or more of China’s energy supply, but more than ninety percent of China’s coal comes from underground mining [
3,
4,
5]. The underground mining of coal causes surface subsidence and also produces a large amount of waste such as coal gangue, which causes serious pollution to the ecological environment [
6]. At the same time, the more constrained space such as the working face roadway in the mining airspace is prone to accidents due to air leakage [
7], so it is necessary to fill and seal the areas with higher constraint requirements such as the airspace or the working face roadway in the production process of coal mines to ensure that the coal mining is carried out safely [
8,
9].
In recent years, CGPB mixtures have been used for ground control and gangue waste disposal in underground coal mining operations [
10]. CGFB, prepared using a mixture of cement, gangue, fly ash, and water, is transported through boreholes and pipelines to underground mining areas [
11,
12,
13]. This method not only controls subsidence but also reduces the emission accumulation of hazardous wastes (coal gangue) on the surface, thus solving the environmental safety problems associated with it (spontaneous burning of stacks of coal gangue and pollution by fly ash dust) to a certain extent [
14]. During the filling process of CGFB, it is necessary to ensure that the CGFB slurry is efficiently transported to the designated area without pipeline clogging, which requires the CGFB slurry to have smooth transportability [
15]. The rheological parameters of the slurry are influenced by its chemistry (binder hydration), hydraulics (suction development), and other factors [
16,
17]. Due to the makeup of CGFB, the binders used (cement) react chemically with water, also known as binder hydration, producing hydration products and consuming water [
18]. The consumption of water leads to a reduction in pore water pressure or the development of suction inside the CGFB, which is macroscopically manifested by the slurry reflecting different visco-plasticity during the flow process. The traditional filling slurry test mostly focuses on the steady-state visco-plasticity test of the slurry, but there are interactions between the slurry and the pipe wall during the flow process, and the slurry also shows contact during the internal flow process. Moreover, the material will show viscoelasticity when encountering the unsteady flow state, which is specifically expressed as the impact on the pipe and the particle collision between the slurry, so it is necessary to assess the fresh slurry’s viscoelastic plasticity shown in the flow process, Therefore, it is necessary to study the viscoelasticity of fresh slurry in the flow process.
Electrical resistivity (ER) is an important means of studying the properties of cementitious materials, which represents the ability of a material to resist the flow of current per unit length in a given cross-section [
19,
20,
21]. This method can reflect changes in the internal structure of the material (number of pore structures and tortuosity, pore solution saturation, etc.) [
22], is widely used in the study of cement–concrete materials and filled materials in terms of the coagulation process, monitoring of destabilization and damage processes, etc., and has the advantages of simplicity, rapidity, low cost, and non-destructive nature [
23,
24]. Resistivity testing, as a non-destructive measurement condition, can be a good test for internal reactions between materials. Hong Jae Yim et al. investigated non-destructive measurements such as resistivity to evaluate the microstructural evolution of fresh cement paste for 24 h and compared it with the setting time of the Vicat pin test. Observable non-destructive parameters, including setting time, can reflect the generation of cement hydrate and change the bending path of porous materials [
25]. Xu Wenbin et al. proposed a non-destructive testing method to determine the mechanical properties of CPB using electrical resistivity (ER). Based on the logarithmic relationship with ER obtained at 90 days, the strength of CPB was developed to be evaluated using the resistivity testing method [
26,
27]. However, fewer studies have been conducted to characterize the viscoelasticity and plasticity of gangue fly ash slurries using resistivity.
This paper presents an experimental investigation into the fundamental aspects of CGFB and its rheological behavior, exhibiting viscoelasticity and plasticity. The resistivity of the material was also tested for different solids, cement contents, and times. The CGFB’s characteristic flow curves and the fluctuating law of viscoelastic parameters were introduced and analyzed. The changing pattern of resistivity of the material was also analyzed to investigate the effect of concentration and cement content on the rheological properties of CGFB. The findings this paper presents are aimed at investigating the characterization of CGFB slurry rheological parameters to guide slurry pipeline transportation.
4. Conclusions
In this paper, the viscoelasticity-plasticity of CGFB slurries with varying solid contents and cement dosages was examined, and the yield stress, viscosity, modulus, and resistivity of the materials were tested experimentally. The findings lead to the following conclusions:
1. The concentration of solids has an important effect on the rheological properties of the material. When the solid phase content increases from 72% to 76%, the viscosity and yield stress of the material both increase, and the increase reaches 32.77% and 51.22%, respectively. The reason is that the increase in concentration leads to a decrease in the free water content, which in turn increases the contact area between the particles. With the increase in cement content from 8% to 16%, the yield stress increased by 53.07% and the viscosity increased by 79.78%, which is due to the flocculation and hydration of the cement.
2. The viscoelasticity of the material under the dynamic shear action at the frequency of 1 Hz, the energy storage modulus is greater than the loss modulus in the low-strain region, and the material shows a greater proportion of the elastic nature of the solid, and with the increase in the strain, the energy storage modulus and the dissipation modulus both show a decreasing trend. This is due to the shear damage suffered by the structure inside the pulp. In the high-strain region, the dissipation modulus and energy storage modulus are the same, and the tangent of the phase angle also increases with the increase in strain, indicating that the elasticity of the material gradually decreases and the viscosity increases. With the increase in cement content, the energy storage modulus and dissipation modulus of the material show an increasing trend, which is related to the filling effect of cement, the hydration reaction of cement, as well as the change in the microstructure of the material.
3. Compared to gangue and fly ash, cement has a substantially lower electrical resistance, while the electrical resistivity of CGFB materials shows an increasing trend with increasing solid concentration, with an increase of 21.05% for SC72-76. This is due to the mixing process of ions in the full reaction and binding, resulting in a reduction in ionic pathways in the slurry, which leads to an increase in resistivity. The resistivity is determined by the ion-conducting mineral composition of cement, fly ash, and gangue in the fine gangue slurry, whose charge carriers are ions. As the concentration increases, the decrease in the conductive path leads to an increase in resistivity.
4. In CGFB filling slurry, with the increase in cement content, the resistivity generally shows a downward trend, which is caused by the increase in the electrical resistivity of cement and changes in the physical and chemical properties of the filling slurry. It should be noted that when the cement content exceeds 12%, the resistivity trend may tend to stabilize because the cement filling effect has reached saturation.