**2. Materials and Methods**

#### *2.1. Raw Materials*

As shown in Figure 1, the CGFB samples used in this test are composed of cement, fly ash, gangue, kaolin, and water. Their main components and contents are listed in Table 1.


**Table 1.** Main components and contents.

Here, 32.5 grade ordinary Portland cement (OPC) produced by Shandong Rizhao No. 3 cement plant was used. Grade II fly ash from the Huangdao Power Plant in Qingdao City, Shandong Province, which has a light grey appearance, was used. The gangue (particle size < 25 mm) was obtained from the solid waste produced in roadway excavation, coal mining, and separation in the Shandong Daizhuang coal mine. The kaolin was taken from the water-washed kaolin produced by a material factory in Guangdong Province. The 80-μm-square pore sieve residue is less than 8%, and the 45-μm-square pore sieve residue is less than 25%, which meets the technical requirements of general Portland cement (GB/T 175-2007) [21].

**Figure 1.** Raw materials. (**a**) Cement; (**b**) Fly ash; (**c**) Gangue; (**d**) Kaolin.

The microscopic morphology of kaolin was analyzed by a scanning electron microscopy (SEM) system. The powder sample was dipped on the conductive adhesive using a wooden stick, and the kaolin surface was plated with gold. The morphology of the sample was observed at 10,000× magnification, as shown in Figure 2a. The layered structure of kaolin particles is evident in the figure, and each block of kaolin particles is composed of many lamellar structures closely superimposed together, with clear edges and corners. The morphology of kaolin particles is irregular and the particle size is different, which is conducive to backfill the internal pores of CGFB samples, increasing its internal occlusal degree and improving its internal structure.

**Figure 2.** Microstructure analysis of kaolin. (**a**) SEM of kaolin; (**b**) XRD pattern of kaolin.

The crystal phase of the kaolin admixture was analyzed using XRD, as shown in Figure 2b [22]. The XRD patterns show multiple dispersion peaks of quartz, feldspar, muscovite, and kaolinite, among which quartz and feldspar are mainly composed of SiO2 and Al2O3, respectively. The kaolin admixture is rich in active SiO2 and Al2O3, which can replace a part of the cement for pozzolanic reaction and improve the internal bonding of CGFB samples.

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