*3.2. Macro Failure Characteristics*

Figure 7 shows the macro-failure patterns of CGFB samples, which can be divided into tensile and shear failures, under different amounts of kaolin. The amounts of kaolin indeed affect the failure characteristics of CGFB samples as the failure degree increases with the increase of kaolin instead of cement. When the amount of kaolin is 50%, the highest amount of tensile cracks in CGFB samples is seen. The CGFB samples were gradually fractured along the diagonal direction with the increase of kaolin instead of cement. After the CGFB samples are fractured, the large tensile cracks on their surface increase, and most of them are distributed near the gangue particles. This shows that the decrease of cement content leads to the decrease of internal bonding degree. The ultimate failure characteristics of CGFB samples show tensile failure accompanied by local shear failure.

**Figure 7.** Failure characteristics of CGFB samples. (**a**) A-1; (**b**) B-2; (**c**) C-2; (**d**) D-3; (**e**) E-3; (**f**) F-3.

A single stress–strain curve cannot reflect the development process, but the failure degree of CGFB samples with different amounts of kaolin addition can be revealed in detail by analyzing the AE Ra and cumulative Ra value. The different characteristics of tensile failure and shear failure can also be understood in detail. The Ra value of the AE is the ratio of the rise time to amplitude, which is an important index for determining the fracture mode. Shiotani et al. [35] calculated the Ra value of rock under bending and shear tests and concluded that a low Ra value corresponds to a shear crack, while a high Ra value corresponds to a tensile crack.

In Figure 8, the relationship between axial stress and cumulative RA value over time during failure of CGFB samples with different kaolin contents is shown to reveal the failure characteristics. It can be seen that when the CGFB samples without kaolin fail, the initial Ra value is at a low level, and the cumulative Ra value increases slowly, which indicates the occurrence of shear failure. Near peak stress, the Ra value suddenly increases, and the cumulative Ra value increases rapidly. This phenomenon can be understood as the point when the CGFB samples without kaolin start getting compacted under the action of the vertical load, resulting in transverse tensile stress. As alluded to earlier, due to existence of gangue particles and a large number of holes, microcracks, and other defects in CGFB samples, their internal structure is under uneven stress, and the effective bearing area reduces, resulting in shear failure of mutual dislocation initially. With the gradual increase in vertical load, friction between a large number of holes and microcracks also increases, which inhibits mutual dislocation, and CGFB samples attain a stable state. As the load continues increasing and becomes higher than the compressive strength of CGFB samples, a crack begins to develop, and the sample finally fractures.

**Figure 8.** Acoustic emission (AE) characteristics of CGFB samples. (**a**) A-1; (**b**) B-2; (**c**) C-2; (**d**) D-3; (**e**) E-3; (**f**) F-3.

With the increase of the amount of kaolin instead of cement, the fluctuation of the Ra value of CGFB samples increases, and the difference of its cumulative Ra value decreases, which increases the number of tensile cracks in CGFB samples. This shows that a decrease in the cement content leads to a decrease in the degree of internal bonding. During the compression process, a large amount of elastic energy is stored in the gangue particles. With the increase in sample deformation, energy is released near the gangue particles and causes chain failure of the surrounding structure, resulting in tensile failure accompanied by local shear failure of the sample.
