# Experimental Research into the Ultrasonic P-Wave Velocity of Coal Slime Based Backfill Material
## 2 Experimental Procedures
### Characteristics of Experimental Raw Materials  
#### 2.1.1 Coal Slime  
Coal slime [24] is a by-product in the coal washing process, and its main constituents are coal and chemical materials retained in the coal-purification process, including silicon oxide (SiO\({}_{2}\)), aluminum oxide (Al\({}_{2}\)O\({}_{3}\)), sulphates, ammonium, and heavy metals. Coal slime, which generally contains fine particles and has a large specific surface area, is easily oxidized in air. With strong water retention capability and a high water-content, coal slime generally contains more than 20% water after dehydration. The ash content is commonly high, so coal slime is likely to be clustered and has a low calorific value; because coal slime generally contains many clay minerals, along with its high water content and fine particle size, most coal slime is highly viscous and has certain flowability. When stacking coal slime, the shape of coal slime is extremely unstable and the coal slime is washed away when it comes into contact with water while it flies upwards when it is dried, so coal slime is apt to pollute the environment. Considering this, the stacking, storage, and transportation of coal slime face some difficulties, and the direct use of coal slime is of low economic benefit. The main chemical constituents of coal slime are listed in Tables 1 and 2.  
Data in Table 2 are the relative contents of various main minerals in coal slime. Analysis of these constituents is of importance for understanding the material properties of coal slime and its applications in industrial production.  
SiO\({}_{2}\), as the primary component of coal slime, accounts for almost 60% of the total mass. SiO\({}_{2}\) is generally present in many minerals and rocks and exerts significant influences on the viscosity and structural stability of coal slime. The high-content SiO\({}_{2}\) imparts high hardness and compressive strength to coal slime.  
The Al\({}_{2}\)O\({}_{3}\) content in coal slime is 17.58%, so Al\({}_{2}\)O\({}_{3}\) is also an important component. Al\({}_{2}\)O\({}_{3}\) is commonly seen in many rocks and minerals and may affect the structural strength and fire-resistance of coal slime.  
1  
1  
1  
1  
1  
1  
1  
1  
1  
1  
1  
1  
1  
1  
1  
1  
1  
1  
1  
1  
1  
sulfoaluminate in the coal slime based cemented backfill materials can accelerate the rate of solidification of materials and also provide a certain strength to the cementing materials. The main chemical constituents of the high-water-content materials [27; 28; 29] A and B are listed in Tables 3 and 4.  
As shown in Figures 2 and 3, the particle size of the high-water-content materials is mainly in the range of 1 to 100 \(\upmu\)m. Within that range, the materials are likely to have the following physical properties:  
\(\dagger\) Ultrafine particles: the particles of high-water-content materials are generally at the micron scale. Therefore, the interaction between particles is significantly affected by surface effects;  
\(\ddagger\) Large surface area: due to the fine particles, the high-water-content materials have a very large specific surface area, facilitating their participation in chemical reactions and improving the activity of materials;  
\(\ddagger\) High hygroscopicity: due to the fine particles and large specific surface area, the high-water-content materials show favorable hygroscopicity and can adsorb and retain a relatively large content of water, thus reducing the rate of bleeding from filling bodies.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**CaO** & **Al2O3** & **Fe2O3** & **SO3** & **SiO2** & **MgO** & **K2O** & **Na2O** \\ \hline
31.32 & 1.34 & 0.50 & 37.05 & 4.42 & 2.97 & 0.26 & 0.08 \\ \hline \end{tabular}
\end{table}
Table 4: Chemical composition of high-water-content material B (plaster) (mass fraction) %.  
Figure 3: The particle size distribution of material B (plaster).  
Figure 2: The particle size distribution of material A (sulfoaluminate).