**1. Introduction**

China is one of the large energy producer and consumer countries in the world, mainly relying on coal. Long-term and large-scale development of coal resources lead to massive accumulation of coal gangue [1,2]. In a coal mining environment, the large amount of coal gangue piled up on the ground encroaches the land, and pollutes both the air and water environments [3–5], seriously threatening the ecological security of the mining area and the sustainable development of the coal mining industry. Therefore, resource utilization of coal gangue is urgently required. Coal gangue are mainly reused as supplementary materials in backfilling technology, building construction, energy fuel, soil improvement, water purification, special cementitious materials, etc. [6–8]. Geopolymer is a kind of environmentally friendly mineral polymer material, which has an excellent cementitious property and is a hot topic in the field of industrial solid waste recycling. Geopolymer is an inorganic

polymer mainly produced by the polycondensation of aluminosilicate mineral materials with alkaline solution at room temperature, generally below 100 ◦C [9,10]. Geopolymer has the characteristics of high strength, fast hardness, high toughness, acid and alkali resistance, high temperature resistance, and other excellent properties. At present, geopolymers are widely used in the fields of green building materials, fire-resistant and high temperature resistant materials, sealing of toxic and dangerous wastes, aerospace materials, etc. [11,12]. The raw materials for preparing geopolymer are mainly composed of silica and alumina, including the natural clay mineral kaolinite, metakaolinite [13,14] and various industrial solid wastes, such as slag, fly ash, red mud, waste glass, and coal gangue [15–18]. The silica and alumina components account for 60–95% of coal gangue so that coal gangue has potential characteristics as a raw material to prepare geopolymer.

However, the coal gangue structure is stable. Its reactivity needs to be improved before use by activation methods, such as mechanically grinding, thermal and chemical activation [19,20]. Different activation methods have different activation effects because of the diversity and complexity of coal gangue. The activated coal gangue can be used alone or combined with other industrial solid wastes for the geopolymer reaction. The calcined coal gangue was mixed with alkaline solutions to prepare amorphous alkali-aluminosilicate cementitious materials, in which the cementitious materials prepared by modified water glass had higher compressive strength [21]. The impacts of sodium hydroxide modulus, alkali lye amount, and liquid–solid ratio on the strength and microstructure of coal gangue geopolymer were explored by Yi, et al [22]. With the increase of sodium hydroxide solution concentration, the strength of geopolymer materials increased, while the mass ratios and liquid–solid ratios did not increase linearly. The spontaneous coal gangue mixed with slag, fly ash, and alkaline solutions was used to prepare the spontaneous combustion gangue-slag-fly ash geopolymer [23,24]. The geopolymer's performance met the standard of Portland cement, and the modulus of water glass; the amount and type of activator can influence its mechanical strength. For coal gangue polymer mortars, the addition of slag and slaked lime improved the compressive strength of geopolymer [25]. The geopolymer recycled concrete was produced by hypergolic or calcined coal gangue, slag, fly ash, and recycled aggregate as raw materials. Its compressive and splitting tensile strengths were high and in line with the engineering requirements of concrete [12]. Other geopolymers with excellent properties were also prepared by coal gangue and thermal activated sludge [26,27] or red mud [28,29]. After adding fly ash, the geopolymer prepared by coal gangue, red mud, and fly ash with sodium silicate solution can reach a compressive strength of 7.3 MPa [30]. In addition, coal gangue can be used as aggregate to be cemented or compounded by other geopolymer materials. A paste filling material was prepared by cementing spontaneous combustion coal gangue with geopolymer, which was prepared by fly ash, cement, slag, and modified sodium silicate [31–33].

As a solid waste resource, coal gangue has potential value in geopolymer research. Recently, most of the researches have focused on experimental research of different material ratios, but the role of each component in the geopolymer preparation process was not completely revealed. This is of great significance for the theoretical guidance of geopolymer preparation using coal gangue. Besides, the activation effect changes with the diversity and complexity of coal gangue. Therefore, this paper mainly studied the microstructure change of coal gangue under different mechanical and thermal activation conditions. The active structural characteristics of activated coal gangue as raw material for preparing geopolymer were summarized. The macro mechanical property and micro structural characteristics of coal gangue-based geopolymer (CGGP) were measured and analyzed for in depth understanding of the polymerization of CGGP.

#### **2. Materials and Methods**

#### *2.1. Coal Gangue Sample*

Raw coal gangue samples were obtained after washing raw coal from a coal mine in mid-eastern China. The raw coal gangue was broken and screened to 80–100 mesh and dried for 6 h at 30 ◦C air environment to obtain the experimental sample, which was named as-received coal gangue, as shown in Figure 1.

**Figure 1.** Coal gangue samples. (**a**) Raw coal gangue; (**b**) As-received coal gangue.

The chemical composition of as-received coal gangue was measured by Bruker S8 Tiger X-Ray Fluorite Spectroscopy (XRF) at a voltage of 50 kV and current of 50 mA with the no-standard quantitative analysis. The loss on ignition was tested according to the international standard ASTM D7348-13. The main chemical compositions and loss on ignition of as-received coal gangue are shown in Table 1. The proximate and elemental analysis (air-dried base) are shown in Table 2.




**Table 2.** Proximate and elemental analysis of as-received coal gangue (wt%).

Table 1 shows that the main chemical components of as-received coal gangue were SiO2 and Al2O3, accounting for 66.88%, according to the quantitative analysis results of XRF standard-free samples. Table 2 shows that the ash content of as-received coal gangue was the highest, and the proportions of volatile and fixed carbon were relatively higher as per proximate analysis proportions. On the other hand, the elemental composition analysis showed that the content of carbon element was the highest, followed by that of oxygen.

#### *2.2. Methods and Processes*

#### 2.2.1. Activation of As-Received Coal Gangue

The as-received coal gangue was ground by a planetary mill (Instrument type: BM6Pro) continuously for 2 h, 10 h, and 20 h at 400 rpm to obtain mechanical activation coal gangue (named as 2 h-CG, 10 h-CG, and 20 h-CG respectively). Each grinding was carried out in a 250 ml stainless steel pot using 300 g zirconia balls (ball gradation was 116:2198 and weights were 0.86 g and 0.091 g respectively). The three grinding times were chosen according to the reference [19].

The as-received coal gangue after being ground for 2 h was calcined for 2 h at 700 ◦C, 800 ◦C, and 900 ◦C in a tube furnace (Instrument type: MTF 12/38/250/301) to obtain thermal activation coal gangue (named as 700 ◦C-CG, 800 ◦C-CG and 900 ◦C-CG respectively). The three calcination temperatures were determined according to the transformation temperature of the kaolinite structure to active metakaolinite [10].
