*3.3. Microstructure Characteristics*

The microstructure and morphology of the fracture surface of CGFB samples are analyzed using SEM and are shown in Figure 9. It can be seen that the microstructure and morphology of fracture surfaces of different CGFB samples are different. Because of the disappearance of the coating layer on the surface of CGFB samples, the rate of hydration reaction increases, and a large amount of flocculated calcium-silicate-hydrate (C-S-H) gel forms in the main body. Ettringite (AFt) having a needle-like structure is mostly distributed in pores. These crystals are closely connected, which improves the strength of the sample. When the amount of kaolin is 10%, more hydration products (AFt and C-S-H gel) are observed. The internal structure of the samples is compact, and the number of pores is small. When the amount of kaolin is more than 10%, the number of hydration products decreases, and that of irregular particles increases. At 50% kaolin content, fewer hydration products are observed, but the internal structure is relatively dense, which indicates that kaolin mainly fills the pores in higher quantities.

**Figure 9.** SEM image of fracture surface of CGFB samples. (**a**) 0% CP; (**b**) 10% CP; (**c**) 20% CP; (**d**) 30% CP; (**e**) 40% CP; (**f**) 50% CP.

PCAS is a professional software tool for the identification and quantitative analysis of pore systems and fracture systems [36]. It can automatically identify all types of pores and fractures in images and obtain all geometric and statistical parameters. Concerning pore recognition, it can import various pore images, remove clutter automatically, segment and recognize pores automatically through binarization, output the geometric and statistical parameters, and display the result vector image and rose diagram. It can also display various geometric parameters of all pores in the data table, including the number of pores, area, length, width, directivity, and shape coefficient, and obtain statistical parameters such as region percentage (porosity), average form factor, probability entropy, fractal dimension, and sorting coefficient [37]. In this study, PCAS is used to quantitatively analyze the micropore structure of CGFB samples. The pore map after PCAS processing is shown in Figure 10, where the black zone represents the non-porous area, and the colored zone represents pores where the software automatically recognizes and artificially corrects some recognition errors and defects. For different pores (unconnected pores), the PCAS uses a different color mark, while the same color mark is used for connected pores [38]. All CGFB samples are observed and analyzed in this manner. The minimum diameter of the closed pore in PCAS is 2, and the minimum pore area is 50. The region percentages (porosities) of the samples are listed in Table 3.

**Figure 10.** *Cont*.

**Figure 10.** Binarization of CGFB microscopic pores. (**a**) A-1; (**b**) A-2; (**c**) A-3; (**d**) B-1; (**e**) B-2; (**f**) B-3; (**g**) C-1; (**h**) C-2; (**i**) C-3; (**j**) D-1; (**k**) D-2; (**l**) D-3; (**m**) E-1; (**n**) E-2; (**o**) E-3; (**p**) F-1; (**q**) F-2; (**r**) F-3.


**Table 3.** Region percentage (porosity) of CGFB samples.

Many studies [39–42] have shown that porosity has a significant relationship with UCS. Table 3 shows the porosity of CGFB samples with different amounts of kaolin instead of cement. The average porosity of each group of CGFB is compared with the average UCS, and their relationship under different amounts of kaolin instead of cement is shown in Figure 11.

Generally speaking, when the porosity of CGFB samples decreases, the UCS also decreases. However, in Figure 11, an opposite trend is seen. The reason is that the hydration is the main factor of kaolin affecting the strength of CGFB samples. The activity of kaolin is lower than that of cement, but with the increase of kaolin content, the hydration of cement decreases, so the UCS of CGFB samples decreases. However, kaolin also fills the internal pores of CGFB samples, which improves their integrity and reduces the corresponding porosity. In summary, the porosity of CGFB samples decreases with the increase of kaolin instead of cement, and the UCS gradually decreases.

**Figure 11.** Comparison of the relationship between UCS and porosity of CGFB with different amount of kaolin instead of cement.

#### **4. Discussion**

The goal of coal mine filling is to meet the needs of coal mine production safety at the lowest possible cost. Therefore, the selection of appropriate CGFB samples material is related not only to the filling cost but also the safety and stability of the goal. After adding kaolin into CGFB samples, the sample will have effects of morphology, micro-aggregate, and activity of the kaolin admixture. The morphological effects of kaolin addition are mainly reflected in the influence of kaolin particle size, shape, and other factors on the performance of CGFB samples. The micro-aggregate effects are that the particles are relatively fine and can be evenly dispersed in the gangue aggregate and flocculation structure, filling the internal pores, which helps improve the internal uniformity of CGFB samples. Kaolin is rich in active SiO2 and Al2O3. The essence of its pozzolanic activity is that SiO2 and Al2O3 are excited in alkaline environments. The content of soluble active components in kaolin is very low; therefore, the degree of pozzolanic reaction of kaolin is low initially.

To study the influence of different amounts of kaolin content on the hydration products of CGFB samples, phases of CGFB samples cured for 28 days are analyzed via XRD, and the diffraction patterns are shown in Figure 12. Similar diffraction patterns of CGFB samples are seen, but the intensity of the peaks is different from that of the hydrated products, which form Ca(OH)2, C-S-H gel, AFt, etc. When the amount of kaolin is less than 10%, the peak of Ca(OH)2 becomes weaker, the SiO2 peak becomes stronger, and the C-S-H gel diffraction peak becomes stronger with an increase in the amount of kaolin instead of cement. This indicates that some active substances in kaolin consume Ca(OH)2 to participate in the two hydration reactions. When the amounts of kaolin instead of cement are greater than 10%, the peak value of SiO2 continues to increase, and the diffraction peaks of Ca(OH)2, AFt, and C-S-H become weak with an increase in the amount of kaolin content. This shows that after a certain point of kaolin addition, reduction in cement content per unit volume will negatively affect the secondary hydration of kaolin.

To further investigate the effects of kaolin on CGFB samples, the chemical structures of the prepared samples are characterized via FTIR spectroscopy. Figure 13 shows the infrared spectrum of CGFB samples, which shows that FTIR spectra of CGFB samples are similar. The broad absorption band at wavenumbers of 2976.99–3592.77 cm−<sup>1</sup> characterizes the stretching vibration of Al-OH in the [AlO4] tetrahedron, and the absorption peak of 1588.25 to 1784.38 cm−<sup>1</sup> represents the bending vibration of H2O in C-S-H [43]. Absorption peaks at 1349.18–1588.25 cm−<sup>1</sup> represent the O-C-O asymmetric stretching vibration of carbonate, indicating that the CGFB samples experienced slight carbonization

during the characterization process [44]. Absorption peaks of 936.78–1349.18 cm−<sup>1</sup> corresponds to the asymmetric stretching vibration of Si-O in tetrahedron [45], while those at 820.31–886.33 cm−<sup>1</sup> reflects the existence of [Al(Fe)-O], which means that some Al-OH in AFt is replaced by Fe-OH [46].

**Figure 12.** XRD patterns of CGFB samples.

**Figure 13.** FTIR spectrum of CGFB samples.

Figure 14 gives the pozzolanic reaction of kaolin in the CGFB sample. Pore water rich in Ca2+, AlO2 −, and SiO3 <sup>2</sup><sup>−</sup> first infiltrate CGFB, and the cement particles in CGFB samples hydrate to form Ca(OH)2 and other products in the liquid phase. The hydration reaction of cement is as follows:

$$\text{C}\_{3}\text{S} + \text{nH} \rightarrow \text{C-S-H} + \text{(3-x)}\text{CH}\tag{1}$$

$$\text{C}\_2\text{S} + \text{mH} \rightarrow \text{C-S-H} + \text{(2-x)}\text{ CH}\tag{2}$$

**Figure 14.** Pozzolanic reaction process of kaolin in the CGFB sample.

The hydration product will infiltrate the kaolin particles. During slurry condensation, the hydration products in the matrix crystallize, and the CGFB samples material have a certain strength through ionic bonds and intermolecular forces. Now, alkaline inclusions gradually form on the surface of the kaolin particles. With an increase in the curing time from 0 to 28 days, kaolin particles are gradually eroded by alkaline inclusions, resulting in chemical changes and development of the active state. Because of the different ion concentrations inside and outside the coating, ion penetration expands the coating. When the pressure of the coating reaches its limit, the active component reacts with the ions to form C-S-H and other products. The formula for secondary hydration of kaolin is as follows:

$$\text{CaSiO}\_2 + \text{m}\_1\text{Ca(OH)}\_2 + \text{xH2O} \rightarrow \text{m}\_1\text{CaO} \cdot \text{SiO}\_2 \cdot \text{xH}\_2\text{O} \tag{3}$$

$$\text{Al}\_2\text{O}\_3 + \text{m}\_2\text{Ca(OH)}\_2 + \text{yH2O} \rightarrow \text{m}\_2\text{CaO} \cdot \text{Al}\_2\text{O}\_3 \cdot \text{yH}\_2\text{O} \tag{4}$$

The test results show that the strength of CGFB samples decreases with an increase in kaolin content. When the amount of kaolin instead of cement is 10%, the contribution of secondary hydration of some active components in kaolin to CGFB strength is lower than that of normal hydration of cement, but kaolin has a certain filling effect. Therefore, in the study, the strength of CGFB samples with kaolin sample decreased, but there is no significant difference compared with those of CGFB samples without kaolin, which is approximately 0.7 MPa. When the amount of kaolin is greater than 10%, the activity of kaolin is lower than that of cement, which mainly plays the role of filling. The decrease in cement content directly leads to a decrease in the hydration products in CGFB samples, a decrease in the cohesive force, and a consequent decrease in CGFB strength.

Therefore, considering requirements of CGFB strength for coal mine production safety, this study recommends replacing 10% cement in CGFB samples by kaolin as the most suitable option among those tested. That is to say, when the kaolin content is 10%, it can play a similar role as cement in CGFB samples and meet the mine-filling strength requirements. This paper mainly discusses the influence of kaolin on mechanical properties and microstructure of CGFB; however, carrying out systematic research on the durability and engineering applications of CGFB samples with kaolin is also necessary.

#### **5. Conclusions**

(1) The stress–strain curves of the CGFB samples are essentially the same, and they all pass through initial compaction, elastic deformation, plastic yield, and post-peak strain softening stages. The uniaxial compressive strength, peak strain, and elastic modulus decrease with the kaolin content. The average uniaxial compressive strength, elastic modulus, and peak strain of CGFB samples with 10% amount of kaolin are 0.68, 192.37, and 0.0053 MPa, respectively, which are close to those of CGFB samples with no kaolin.

(2) The kaolin content affects the failure characteristics of CGFB samples, which show tensile failure accompanied by local shear failure, and the failure degree increases with the kaolin content. The fluctuation of Ra value of CGFB samples increases, and the difference of its cumulative Ra value decreases with the kaolin content, which increases tensile cracks in CGFB samples. The porosity of the fracture surface shows a decreasing trend as a whole. The reason is that the hydration is the main factor of kaolin affects the strength of CGFB samples, and the activity of kaolin is lower than that of cement. With the increase of kaolin content, the hydration of cement decreases, so the UCS of CGFB samples decreases. However, kaolin also fills the internal pores of CGFB samples, which improves their integrity and reduces the corresponding porosity. In summary, the porosity of CGFB samples decreases with the increase of kaolin, and the UCS gradually decreases.

(3) When the amount of kaolin is 10%, the internal structure of the CGFB sample is more compact, and the number of pores is less. When it is more than 10%, with an increase in the kaolin content, the decrease in cement content per unit volume leads to a decrease in the number of AFt and C-S-H gel, the peak of the SiO2 diffraction peak becomes stronger, the C-S-H diffraction peak becomes weaker, and the number of irregular particles increases. As the average uniaxial compressive strength, elastic modulus, and peak strain of CGFB samples with 10% amount of kaolin are close to those of CGFB samples with no kaolin, replacing 10% cement in CGFB samples by kaolin is the most suitable option recommended.

**Author Contributions:** Conceptualization, D.Y.; methodology, D.Y.; software, F.L.; validation, C.Z., F.W. and N.J.; data curation, Z.Z.; writing-original draft preparation, F.L.; writing-review and editing, F.L. and D.Y.; funding acquisition, D.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Science Foundation of China, grant number 51904167 and 52074169; the Taishan Scholars Project; the Taishan Scholar Talent Team Support Plan for Advantaged and Unique Discipline Areas; the SDUST Research Fund; and the Open Research Fund for the Key Laboratory of Safety and High-efficiency Coal Mining, grant number JYBSYS2019201.

**Data Availability Statement:** The data are available and explained in this article; readers can access the data supporting the conclusions of this study.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

