**2. Raw Materials**

### *2.1. Coal Gangue*

The undisturbed coal gangue used in the test was provided by Hongchang Building Materials Co., Ltd. in Hebi City, Henan Province. The reserves of coal gangue in Hebi City are more than 20 million tons, and these are persistently accumulating. The undisturbed coal gangue was screened in the mining area, and the coal gangue with a particle size less than 30 mm was separated (Figure 1). The coal gangue with a particle size greater than 30 mm was selected for crushing and classification. In order to ensure the particle size of coal gangue, an impact crusher was used for crushing (Figure 2), and the particles greater than 30 mm were crushed and sieved. According to the particle size distribution of coal gangue, it was divided into four grades: machine-made sand, 4.75–9.5 mm, 9.5–19 mm, and 19–31.5 mm, as shown in Figure 3. *Crystals* **2021**, *11*, 993 3 of 18 gangue. Subsequently, through a compaction test, an unconfined compressive strength test, a freeze–thaw test, a splitting test, and a drying shrinkage test, the mechanical properties and durability of cement stabilized gangue were analyzed. The objective was to provide a basis, guide, and reference for the application of coal gangue materials in a high-grade highway base. **2. Raw Materials**  *Crystals* **2021**, *11*, 993 3 of 18 gangue. Subsequently, through a compaction test, an unconfined compressive strength test, a freeze–thaw test, a splitting test, and a drying shrinkage test, the mechanical properties and durability of cement stabilized gangue were analyzed. The objective was to provide a basis, guide, and reference for the application of coal gangue materials in a high-grade highway base. **2. Raw Materials** 

Gemini300 field emission scanning electron microscope (manufacturer: Carl Zeiss, Germany) was used to inspect the surface structure of concrete samples; Bruker D8 Advance X-ray diffractometer (manufacturer: Brooke Company, Germany) was used for X-ray diffraction analysis, and XRD data were analyzed with jade 6 software; The element distribution in the micro region of the material was analyzed qualitatively and quantitatively by SYMMETRYS EBSD energy chromatograph (manufacturer: Oxford Company, UK).The results of the scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS) analysis, and X-ray diffraction (XRD) analysis are shown in Figure 4. The chemical composition and other physical and mechanical properties are shown in Tables 1–3. It can be seen that the basic physical and mechanical properties met the requirements for the class II highway base and subbase in the Chinese standard JTG/TF20-2015 "Technical guidelines for construction of highway roadbases" [37]. *2.1. Coal Gangue* The undisturbed coal gangue used in the test was provided by Hongchang Building Materials Co., Ltd. in Hebi City, Henan Province. The reserves of coal gangue in Hebi City are more than 20 million tons, and these are persistently accumulating. The undisturbed coal gangue was screened in the mining area, and the coal gangue with a particle size less than 30 mm was separated (Figure 1). The coal gangue with a particle size greater than 30 mm was selected for crushing and classification. In order to ensure the particle size of coal gangue, an impact crusher was used for crushing (Figure 2), and the particles greater than 30 mm were crushed and sieved. According to the particle size distribution of coal gangue, it was divided into four grades: machine-made sand, 4.75–9.5 mm, 9.5–19 mm, and 19–31.5 mm, as shown in Figure 3. *2.1. Coal Gangue* The undisturbed coal gangue used in the test was provided by Hongchang Building Materials Co., Ltd. in Hebi City, Henan Province. The reserves of coal gangue in Hebi City are more than 20 million tons, and these are persistently accumulating. The undisturbed coal gangue was screened in the mining area, and the coal gangue with a particle size less than 30 mm was separated (Figure 1). The coal gangue with a particle size greater than 30 mm was selected for crushing and classification. In order to ensure the particle size of coal gangue, an impact crusher was used for crushing (Figure 2), and the particles greater than 30 mm were crushed and sieved. According to the particle size distribution of coal gangue, it was divided into four grades: machine-made sand, 4.75–9.5 mm, 9.5–19 mm, and 19–31.5 mm, as shown in Figure 3.

**Figure 1.** Separation of coal gangue. **Figure 1.** Separation of coal gangue. **Figure 1.** Separation of coal gangue.

**Figure 2.** Coal gangue crushing. **Figure 2. Figure 2.**  Coal gangue crushing. Coal gangue crushing.

(**a**) Machine-made sand (0–4.75 mm) (**b**) 4.75–9.5 (mm) (**c**) 9.5–19 (mm) (**d**) 19–31.5 (mm)

**Figure 3.** Sampling of coal gangue. **Figure 3.** Sampling of coal gangue. *Crystals* **2021**, *11*, 993 5 of 19

the requirements for the class II highway base and subbase in the Chinese standard JTG/TF20-2015 "Technical guidelines for construction of highway roadbases" [37]. (**a**) SEM microscopy of coal gangue. 012345678 Mn Ca O Ca Si C Fe Mg Mn Al Fe Energy/keV (**a**) SEM microscopy of coal gangue. 0 1 2 3 4 5 6 7 8 0 5000 10000 15000 20000 Mn Ca O Ca Si <sup>C</sup> Fe Mg Mn Al Fe Energy/keV counts (**b**) EDS analysis of coal gangue. 10 20 30 40 50 60 70 80 0 500 1000 1500 2000 Intensity(a.u.) 2θ/° A SiO<sup>2</sup> B CaCO<sup>3</sup> C FeAl3Si<sup>2</sup> D Al2(Si2O<sup>5</sup> )(OH)<sup>4</sup> E KAl<sup>2</sup> ( Si3AlO10 ) ( OH) <sup>2</sup> <sup>A</sup> A A <sup>A</sup> <sup>A</sup> <sup>A</sup> <sup>A</sup> <sup>A</sup> B C D E A D (**c**) XRD spectrums of coal gangue.

(**b**) EDS analysis of coal gangue. **Figure 4.** Microstructure of coal gangue. **Figure 4.** Microstructure of coal gangue.

**Apparent Density/g.cm−<sup>3</sup>**

**Apparent Density/g.cm−<sup>3</sup>**

0

5000

10000

counts

15000

20000

**Bulk Density/g.cm−<sup>3</sup>**

**Table 1.** Chemical elements of coal gangue (atomic fraction)/wt.%.

**O C Fe Mg Ca Al Si Mn**

2.618 1.473 43.8 27.8 0.85 1.2

**Crushing Value/%**

**Bulk Density/g.cm−<sup>3</sup> Porosity/% Fineness Modulus Water Absorption** 

**Dust Content below 0.075 mm/%**

**Soft Rock Content/%**

**Rate/%**

**Table 2.** Physical and mechanical properties of coal gangue coarse aggregate (wt.%).

**Porosity/%**

**Table 3.** Physical and mechanical properties of coal gangue fine aggregate (wt.%).

**Table 1.** Chemical elements of coal gangue (atomic fraction)/wt.%. (**c**) XRD spectrums of coal gangue. **Figure 4.** Microstructure of coal gangue.

10 20 30 40 50 60 70 80

<sup>A</sup> <sup>A</sup> <sup>A</sup> <sup>A</sup> <sup>A</sup> <sup>A</sup>

C

2θ/°

*Crystals* **2021**, *11*, 993 5 of 18

A

B

<sup>D</sup> E A D

A

A SiO2 B CaCO3 C FeAl3Si2 D Al2(Si2O5)(OH)4 E KAl2( Si3AlO10 ) ( OH) 2


56.6 24.2 15.2 2.2 0.9 0.4 0.3 0.2

**Table 2.** Physical and mechanical properties of coal gangue coarse aggregate (wt.%).


**Table 3.** Physical and mechanical properties of coal gangue fine aggregate (wt.%). **Table 3.** Physical and mechanical properties of coal gangue fine aggregate (wt.%).


### *2.2. Crushed Stone 2.2. Crushed Stone*  The gravel and machine-made sand used in the test were provided by Hongchang

0

500

1000

Intensity(a.u.)

1500

2000

The gravel and machine-made sand used in the test were provided by Hongchang Building Materials Co., Ltd. in Hebi City, Henan Province. Based on the gradation/particle size distribution, the gravel and sand were classified into four grades (machine-made sand, 4.75–9.5 mm gravel, 9.5–19 mm gravel, and 19–1.5 mm gravel), as shown in Figure 5. Building Materials Co., Ltd. in Hebi City, Henan Province. Based on the gradation/particle size distribution, the gravel and sand were classified into four grades (machine-made sand, 4.75–9.5 mm gravel, 9.5–19 mm gravel, and 19–1.5 mm gravel), as shown in Figure 5.

**Figure 5.** Sampling of crushed stone. **Figure 5.** Sampling of crushed stone. *Crystals* **2021**, *11*, 993 6 of 18

The results of the scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) analysis, and X-ray diffraction (XRD) analysis are shown in Figure 6. The results of the scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) analysis, and X-ray diffraction (XRD) analysis are shown in Figure 6. Its chemical composition and physical and mechanical properties are shown in Tables 4–6. It can be seen that its basic physical and mechanical properties can meet the requirements for the class II highway base and subbase in the Chinese standard JTG/TF20-2015 "Technical guidelines for construction of highway roadbases" [37]. Its chemical composition and physical and mechanical properties are shown in Tables 4– 6. It can be seen that its basic physical and mechanical properties can meet the requirements for the class II highway base and subbase in the Chinese standard JTG/TF20-2015 "Technical guidelines for construction of highway roadbases" [37].

**(a)** SEM microscopy of crushed stone

**Figure 6.** *Cont*.

Ca

012345678

A

Energy/keV **(b)** EDS analysis of crushed stone

A CaCO3

10 20 30 40 50 60 70 80

<sup>A</sup> <sup>A</sup> <sup>A</sup> <sup>A</sup> AA <sup>A</sup> <sup>A</sup> <sup>A</sup>

2θ/°

**(c)** XRD spectrums of crushed stone

**Figure 6.** Microstructure of crushed stone.

0

5000

10000

15000

O

Ca C

0

1000

2000

3000

Intensity(a.u.)

4000

5000

6000

**(c)** XRD spectrums of crushed stone

**Figure 6. Figure 6.** Microstructure of crushed stone. Microstructure of crushed stone.

**Table 4.** Chemical elements of crushed stone (atomic fraction)/wt.%.


Its chemical composition and physical and mechanical properties are shown in Tables 4– 6. It can be seen that its basic physical and mechanical properties can meet the requirements for the class II highway base and subbase in the Chinese standard JTG/TF20-2015

"Technical guidelines for construction of highway roadbases" [37].

**(a)** SEM microscopy of crushed stone

**Table 5.** Physical and mechanical properties of crushed stone coarse aggregate (wt.%).


**Table 6.** Physical and mechanical properties of crushed stone fine aggregate (wt.%).


### *2.3. Cement*

Ordinary Portland cement (P·O·42.5) with a density of 3150 kg/m<sup>3</sup> was selected as cement. The properties of cement are shown in Table 7. It can be seen that the cement used in this test met the requirements of TG/TF30 "Technical guidelines for the construction of highway cement concrete pavements". It can be used as the admixture in cement stabilized gangue and cement stabilized macadam.

**Table 7.** Technical properties of cement.


## **3. Test Method**

### *3.1. Gradation Optimization*

Firstly, the raw materials of coal gangue and crushed stone with different particle sizes were sieved to determine the gradation curve. Further, the gradation of coal gangue was optimized according to JTG/TF20-2015 "Technical guidelines for construction of highway roadbases" [37] to meet the gradation requirements for a secondary highway base and subbase. The gravel was screened and synthesized to meet the standard gradation based on the original four particle size ranges.
