*2.2. Sample Preparation*

In this test, CGFB samples were prepared using cement, fly ash, and gangue in the ratio of 1:4:6; the solid mass fraction was 78%; no additions were added, and the amounts of kaolin replacing cement were 0, 10, 20, 30, 40, and 50%. During the preparation of CGFB samples, an NJ-160 agitator was used for stirring for approximately 8 min. After the slurry was evenly mixed, it was poured into a *Φ*50 mm × 100 mm mould. The bubbles in the samples were removed by manual vibration and tamping. CGFB samples were removed from moulds after 24 h and cured for 28 days in a curing box at a temperature of 25 ◦C and relative humidity of 80%. Before the test, the two ends of the samples were smoothed with a grinding machine: the flatness tolerance of the end face was less than 0.05 mm, and unevenness was less than 0.002 mm [23]. Evident cracks on the surface of the CGFB samples were removed. A total of 18 samples, as shown in Figure 3, were prepared for this test. Based on the amounts of cement replaced with kaolin, they were divided into six groups: A, B, C, D, E, and F, corresponding to cement replacement of 0, 10, 20, 30, 40, and 50% chemically pure (CP), respectively, and each group contained three samples.

**Figure 3.** CGFB samples.

#### *2.3. Test Method*

The experimental test setup, including the loading, AE, DVC, SEM, XRD, and FTIR systems, is shown in Figure 4. During each test, loading, AE, and DVC systems were synchronized to have the same timestamps to facilitate analysis of the experimental results.

#### 2.3.1. Uniaxial Compression Tests

A Shimadzu AG-X250 electronic universal testing machine was used to conduct the uniaxial compression tests on the CGFB samples. This machine can perform uniaxial compression, tensile, and other mechanical tests using a maximum load of 250 kN [24–27]. When performing uniaxial compression tests, a preload pressure of 0.1 kN was first applied to the test sample, so that the indenter was in close contact with the test piece, and displacement loading control at a loading rate of 0.0005 mm/s was conducted.

**Figure 4.** Experimental testing system. (**a**) Uniaxial compression tests; (**b**) SEM experiment; (**c**) XRD experiment; (**d**) FTIR experiment.

#### 2.3.2. Acoustic Emission Experiment

The failure under uniaxial compression was monitored in real-time using the MIS-TRAS series PCI-2 AE system. An R3α type of AE sensor, a main amplifier of 40 dB, threshold of 45 dB, floating threshold of 6 dB, probe harmonic frequency of 100–600 kHz, and sampling frequency of 106 times/s were used [28]. Petroleum jelly (Vaseline) was applied between the sensor and samples for coupling them and reducing the acoustic impedance difference and reflection loss of energy at the interface. This ensured that the sensor received the AE signal with minimal loss. The sensor was fixed with adhesive tape, and the pencil lead fracture method proposed by American Society for Testing and Materials (ASTM) was used to calibrate the AE system for ensuring that the signal amplitude of each sensor was above 90 dB [29]. During the test, a video camera (Sony DVC) was used to record the failure under uniaxial loading.
