## 4 Mechanical Properties and Erosion Resistance of Coal Gangue Fly Ash Blocks
### Erosion Resistance Test of Coal Gangue Concrete Blocks  
There is serious chemical erosion in the underground environment of coal mines. Considering the different mine environments in multiple mining areas, there are many sulfur-containing coal seams in Guizhou, Shanxi and other mining areas in China, and the sulfuric acid produced by its oxidation can cause the pH of mine water to be as low as 2-4; the pH value of common groundwater in the North China mining area and the Shendong mining area is between 7.5 and 8.5. If there is water seepage in sandstone and siltstone surrounding rock, the pH value can be as high as 10. Most of China's mining areas adopt roadway shotcrete technology. After the carbonation of roadway shotcrete, there are Ca(OH)\({}_{2}\) solution and grouting material (such as sodium silicate solution ) leakages, which can lead to the pH value of concrete environment reaching 12-13. Therefore, in this experiment, tap water solution, HCI solution with pH value of 1-3, alkaline NaOH solution with pH value of 9-11 and Na\({}_{2}\)SO\({}_{4}\) solution with pH value of 7-8 were selected to carry out anti-erosion experiments on coal gangue concrete blocks. Twelve cylindrical coal gangue concrete specimens (\(\phi\)50 mm \(\times\) 100 mm) were immersed in each solution, totaling 48 specimens. After standard curing, the specimens were immersed in the four solutions and tested for compressive strength at different exposure durations (10, 20, 30, and 40 days). The resulting data illustrate the variation in compressive strength under different corrosive environments and immersion durations, as presented in Table 20.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{ \begin{tabular}{c} **Soaking** \\ **Days/d** \\ **MPa** \\ \end{tabular}  } & \multicolumn{2}{c}{**Tap Water Solution**} & \multicolumn{2}{c}{**HCI Solution**} & \multicolumn{2}{c}{**NaOH Solution**} & \multicolumn{2}{c}{**Na\({}_{2}\)SO\({}_{4}\) Solution**} \\ \cline{2-9}  & \begin{tabular}{c} **Compressive** \\ **Strength/** \\ **MPa** \\ \end{tabular}  & \begin{tabular}{c} **Hydrolysis** \\ **Coefficient** \\ \end{tabular}  & \begin{tabular}{c} **Compressive** \\ **Strength/** \\ **MPa** \\ \end{tabular}  & \begin{tabular}{c} **Erosion** \\ **Resistance** \\ **Coefficient** \\ \end{tabular}  & \begin{tabular}{c} **Compressive** \\ **Strength/MPa** \\ \end{tabular}  & \begin{tabular}{c} **Erosion** \\ **Resistance** \\ **Coefficient** \\ \end{tabular}  & \ \begin{tabular}{c} **Compressive** \\ **Strength/MPa** \\ \end{tabular}  & \begin{tabular}{c} **Erosion** \\ **Resistance** \\ **Coefficient** \\ \end{tabular}  \\ \hline \(10\) & 32.01 & 1.02 & 28.11 & 0.87 & 32.1 & 1.00 & 30.82 & 0.96 \\ \(20\) & 31.87 & 1.01 & 26.23 & 0.83 & 32.74 & 1.02 & 29.85 & 0.93 \\ \(30\) & 31.34 & 1.00 & 25.18 & 0.78 & 33.75 & 1.05 & 29.53 & 0.92 \\ \(40\) & 31.12 & 0.99 & 24.56 & 0.73 & 34.35 & 1. As shown in Table 20, the compressive strength of specimens immersed in tap water remained essentially stable over time, and the hydrolysis coefficient exhibited no significant change, indicating that a neutral environment exerts minimal influence on the specimens' mechanical properties. Specimens immersed in HCl and Na\({}_{2}\)SO\({}_{4}\) solutions exhibited a continuous decline in compressive strength with immersion time, with the most pronounced reduction observed during the first 10 days. After 40 days of immersion in Na\({}_{2}\)SO\({}_{4}\) solution, the compressive strength decreased by approximately 10%, whereas immersion in HCl solution for the same duration led to a 30.7% reduction. This indicates that acidic environments have a significantly greater deteriorative effect on coal gangue concrete than sulfate environments, with hydrochloric acid exhibiting notably higher corrosivity. In contrast, specimens immersed in alkaline NaOH solution exhibited a different trend. Their compressive strength gradually increased over time, reaching a 7% gain by day 40.