## 4 Mechanical Properties and Erosion Resistance of Coal Gangue Fly Ash Blocks  
### Study on the Mechanical Properties of Coal Gangue Fly Ash Blocks  
Mechanical properties were evaluated using a NYL-2000 microcomputer-controlled electronic universal testing machine produced by China Nanjing Speiker Test Instrument Co., Ltd. Two types of specimens were prepared: standard cylindrical specimens (\(\upphi\)50 mm \(\times\) 100 mm) and cubic specimens with a side length of 100 mm. After demolding, all specimens were numbered and labeled. The cylindrical specimens were subsequently sectioned into two sizes: \(\upphi\)50 mm \(\times\) 25 mm for the Brazilian splitting test, and \(\upphi\)50 mm \(\times\) 50 mm for the shear test. The cubic specimens were used for compressive strength tests. (1) Through rock mechanics tests, mechanical properties data such as compressive strength, tensile strength, shear strength, and elastic modulus of coal gangue concrete specimens were obtained. The specific results are shown in Table 19.  
As shown in Table 19, the compressive strength of coal gangue block specimens generally exceeds 30 MPa, with an average value of 32.1 MPa. The average tensile strength is 2.93 MPa, and the average shear strength is 4.00 MPa. The calculated cohesion is 2.70 MPa, the internal friction angle is 31\({}^{\circ}\), and the tensile-to-compressive strength ratio is 0.08. The average elastic modulus is 3.5 GPa, and the Poisson's ratio is 0.23. At peak stress, the average strain is 2.57%. Three specimens with identical specifications were placed on a compression testing machine and loaded at a constant displacement rate of 0.25 mm/min. Strain data were  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Category**} & **Experimental** & **Diameter** & **Height** & **Peak Strain \(\upvarepsilon_{\mathrm{c}}\)** & **Modulus of** & **Strength** \\  & **Group Number** & **(mm)** & **(mm)** & **(10\({}^{-3}\))** & **Elasticity \(E\) (GPa)** & **(MPa)** \\ \hline \multirow{4}{*}{Compressive strength} & 1-1-1 & 50.34 & 100.39 & 2.45 & 3.43 & 31.94 \\  & 1-1-2 & 50.25 & 100.57 & 2.65 & 3.59 & 32.12 \\  & 1-1-3 & 50.45 & 100.21 & 2.61 & 3.48 & 32.24 \\  & mean & 50.3  
recorded under varying stress levels until specimen failure. Based on the measured data, the stress-strain curve of coal gangue concrete was plotted, as shown in Figure 7.  
The stress-strain curve of coal gangue concrete reflects its deformation characteristics and failure mode under compression. As shown in the figure, the curve can be divided into four typical stages:  
1. Compaction stage: Pre-existing structural voids and microcracks in the specimen gradually close, resulting in densification and early-stage nonlinear deformation. The volume of the specimen decreases as the load increases. 2. Elastic stage: During this stage, the stress-strain curve is approximately linear. When concrete is subjected to stress, it undergoes elastic deformation, meaning that stress and strain exhibit a linear relationship. When the external force is removed, the concrete returns to its original shape. The characteristic of this stage is that stress increase is directly proportional to strain increase, and the elastic modulus of concrete remains constant. 3. Elasto-plastic stage: As stress continues to increase, concrete gradually enters the elasto-plastic stage, where the relationship between stress and strain is no longer linear, and the elastic modulus gradually decreases. During this stage, micro-cracks begin to develop and propagate within the material. When stress reaches the yield point (peak point), concrete begins to exhibit plastic deformation. 4. Failure stage: Beyond the peak stress, concrete undergoes rapid failure, characterized by a sharp strain increase accompanied by stress reduction. Despite structural collapse, the material retains a degree of residual strength at this stage. 5.