2.3.4. X-ray Diffraction Experiment

The crystal phase of the CGFB samples was analyzed using Rigaku Ultima IV XRD operating at a voltage of 40 kV and an emission current of 40 mA. The dried samples were ground using a mortar and screened using a 200-mesh sieve. During this, fragments with relatively few aggregates (coal gangue) were retained, and large particles such as aggregates (coal gangue) were removed. After the powder sample was tiled on the groove of the glass slide, it was placed into the instrument for testing.

#### 2.3.5. Fourier-Transform Infrared Spectroscopy Experiment

The functional groups on the surface of the CGFB samples were measured using Nicolet iS5 FTIR. When testing samples, the samples were first ground with a mortar and then dried. During this, fragments with relatively few aggregates (coal gangue) were retained, and large particles such as aggregates (coal gangue) were removed. After mixing the samples with pure potassium bromide at a ratio of 1:10, grinding, and pressing, they were placed in the spectrometer and scanned 50 times to obtain the infrared spectrum.

#### **3. Results**

#### *3.1. Uniaxial Compression Test Results*

The strength of CGFB is an important index for evaluating coal mine safety conditions and backfill effects. In this test, the data are collected synchronously through a computer using a sampling interval of 10 ms. Table 2 shows the uniaxial compressive strength (UCS), peak strain, and elastic modulus of the samples. Figure 5 shows the uniaxial compressive stress–strain curves, while Figure 6 shows comparisons of the UCS, peak strain, and elastic modulus of CGFB samples under different amounts of kaolin.


**Table 2.** Uniaxial test results of CGFB samples.

**Figure 5.** Stress–strain curves of CGFB samples. (**a**) A-3; (**b**) B-3; (**c**) C-3; (**d**) D-3; (**e**) E-2; (**f**) F-3.

**Figure 6.** Comparisons of (**a**) UCS, (**b**) elastic modulus, and (**c**) peak strain of CGFB samples under different amounts of kaolin.

As seen from Figure 5, the stress–strain curves of the CGFB samples have the same shape, and they all pass through initial compaction, elastic deformation, plastic yield, and post-peak strain softening stages. However, the values of UCS, peak strain, and elastic modulus are different, illustrating that the kaolin affects the mechanical properties of CGFB samples. As shown in Figure 6, the UCS, peak strain, and elastic modulus of CGFB samples are affected by the amounts of kaolin replacing cement. CGFB samples with 0% kaolin instead of cement exhibit highest average UCS (0.73 MPa), elastic modulus (215.29 MPa), and peak strain (0.0064), while those with 50% kaolin exhibit lower UCS (0.36 MPa), elastic modulus (75.04 MPa), and peak strain (0.0030). The mechanical properties decrease with the increase of amounts of kaolin instead of cement. The higher the content of kaolin, the better the elasticity, and easier the deformation of CGFB. In addition, the amounts of kaolin instead of cement affect the post-peak strain softening stage of CGFB. The post-peak stress of CGFB decreases with time. The higher the content of kaolin, the smaller the slope of the CGFB curve, and stronger the plastic deformation ability.

In addition, in the Figure 6, it is found that the average values of UCS for CGFB samples decrease integrally with the amounts of kaolin instead of cement. Meanwhile, the values of UCS for the D-3 sample (30% CP) and group E samples (40% CP) show a slight increase. Previous investigations have shown that the strength of the CGFB samples is mainly determined by the cement hydration [30–34]. Generally, the stronger the cement hydration is, the larger the corresponding strength of the CGFB sample is. Normally, the activity of kaolin is lower than that of cement, and the incorporated kaolin only partially replaces cement for hydration reaction. The hydration caused by cement is lower than that of cement. With the increase of kaolin proportion, the amount of cement involved in the hydration reaction decreases. Therefore, the cement hydration effect is weakened, and the UCS of CGFB samples decreases. At the same time, the kaolin, which is not involved in hydration, mainly plays a role in filling the pores of CGFB samples. The average porosities of the failure or fracture surface of CGFB samples decrease with the kaolin content, which are analyzed in Section 3.3. Thus, the integrity of the CGFB samples is enhanced. Based on the above analyses, the strength of the CGFB samples containing kaolin may increase under these two mechanisms of the kaolin on mechanical properties for CGFB samples, but which is lower than that of the CGFB samples without the kaolin.
