# Pressure Study on Pipe Transportation Associated with Cement Coal Gangue Fly-Ash Backfill Slurry
## 2 Materials and Methods  
The seize analysis was employed to classify the dimensions category of coal gangue while the standard hydrometer technology was use to obtain the particle size distribution (PSD) on fly-ash and cement by the MALVERN HYDRO 2000 MU (Malvern Panalytical company, Malvern, UK) (Figure 1). The Scanning Electron Microscopy (SEM) test of CGFB was done to describe the internal fiber structures on the purpose to obtain the morphology of the sample under the help of HITACHI S-3500N machine (Hitachi Group, Tokyo, Japan). The rheometer (Figure 2) was used in the test to exam the rheological behavior of the CGFB. Figure 3 provides the rheological properties of CGFB on 76.05%, 78.05% and 80.05% solid concentration. The data showed the variation of the shear stress with shear rate at all concentration follows a straight-line behavior. It verifies the CGFB slurry presents a non-Newtonian behavior and can be represented by a Bingham plastic fluid. In the tests, around 500 g slurry was prepared by mixing the required quantity of solid materials with tap water to obtain the desired mass concentration (Cm), an electronic balance with a deviation of \(+/-10^{-4}\) g was used for weighting the solid materials accurately. Shear rate under the controlled rate ranges 0 to 120 r\(\cdot\)s\({}^{-1}\) was employed to measure the corresponding slurry yield stress and viscosity as the 10 r\(\cdot\)s\({}^{-1}\) increasing measuring intervals. All the measurement was repeated three times to minimize errors occurred in the tests. Further, at each concentration, measurements were done at three samples on the same solid concentration slurry to assess the extent of repeatability. The test was conducted at the constant temperature of 19.6 \({}^{\circ}\)C. The representative samples of CGFB were figure out at backfilling laboratory in China University of Mining and Technology, Beijing. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Element** & **D10 (\(\mu\)m)** & **D30 (\(\mu\)m)** & **D50 (\(\mu\)m)** & **D60 (\(\mu\)m)** & **D100 (\(\mu\)m)** \\ \hline Coal gangue & 204.722 & 856.589 & 1952.857 & 2736.752 & 6634.312 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Particle size composition of the coal gangue use in the coal gangue-fly ash backfill (CGFB) slurry. Figure 1: The machine that used in the tests to exam the particle size and particle size distribution of fly ash and cement used in the CGFB sample configuration: (**a**) MALVERN HYDRO 2000 MU machine examine the particle size; (**b**) Particle size distribution on the fly ash and cement. From Figure 3, it is observed that for all CGFB coal gangue suspensions, the shear stress value increase with the increase in value of strain rate. The CGFB slurry can be idealized as a homogeneous fluid of a single phase with modified properties because of the Bingham plastic body characteristics [21; 22; 23]. In addition, the chemical reaction products (C-S-H) among the cement, fly ash and water provide the cohesive materials. The fine particles including cement, fly ash and the C-S-H present homogeneous characteristics, which was reported by many researchers. The coal gangue narrows the gap among the particles in the slurry. In the process of the CGFB pipe transportation, the coal gangue particles dispersed and were suspended in the slurry under the pump pressure, which was observed in the experimental test. Thus, on the CFD simulation, the assumption that the CGFB slurry is a homogeneous fluid of a single phase, ignoring the coal gangue coarse particle size, was made. Commonly, the Reynolds number was employed to classify the flow layer category to determine the laminar layer and turbulence layer. Typically, the figure of 2100 was used to watershed the laminar or turbulence. Turbulence is a characteristic state of the flow where Reynolds numbers are sufficiently large. Regarding the Bingham flow, the relative viscosity was introduced to calculate the Reynolds number [24].