*3.2. Compaction Test*

According to JTG E51-2009 "Test methods of materials stabilized with inorganic binders for highway engineering" [36], heavy compaction tests were carried out on cement stabilized gangue with a cement content of 3%, 4%, 5%, 6%, and 7% and cement stabilized macadam with a cement content of 4%, 5% and 6% (Figure 7). *Crystals* **2021**, *11*, 993 8 of 18

(Figure 8).

(**a**) Material Preparation (**b**) Electric Compaction Instrument

**Figure 7.** Compaction test. **Figure 7.** Compaction test.

### *3.3. Unlateral Limited Compressive Strength Test 3.3. Unlateral Limited Compressive Strength Test*

According to the compaction test results, the maximum dry density and optimum moisture content of cement stabilized gangue and cement stabilized macadam with different cement contents were determined. The specimens were formed according to the compaction degree of 96%. According to JTG E51-2009 "Test methods of materials stabilized with inorganic binders for highway engineering" [36], the unconfined compressive strength test of cement stabilized coal gangue and cement stabilized macadam was car-According to the compaction test results, the maximum dry density and optimum moisture content of cement stabilized gangue and cement stabilized macadam with different cement contents were determined. The specimens were formed according to the compaction degree of 96%. According to JTG E51-2009 "Test methods of materials stabilized with inorganic binders for highway engineering" [36], the unconfined compressive strength test of cement stabilized coal gangue and cement stabilized macadam was carried

ried out for seven days. The static pressure was applied by the press, and 13 specimens

curing period, the specimens were soaked in water for 24 h, and then the surface moisture was removed for subsequent testing of 7-day unconfined compressive strength

According to JTG E51-2009 "Test methods of materials stabilized with inorganic binders for highway engineering" [36], the splitting tensile test was carried out on cement

**Figure 8.** Unconfined compressive strength test.

*3.4. Splitting Tensile Strength Test* 

(**a**) Specimen (**b**) Experimental instruments

out for seven days. The static pressure was applied by the press, and 13 specimens (of size 150 mm × Φ150 mm) for each group were formed. After demolding, the specimens were cured in the standard curing room for seven days. On the last day of the curing period, the specimens were soaked in water for 24 h, and then the surface moisture was removed for subsequent testing of 7-day unconfined compressive strength (Figure 8). (of size 150 mm × Φ150 mm) for each group were formed. After demolding, the specimens were cured in the standard curing room for seven days. On the last day of the curing period, the specimens were soaked in water for 24 h, and then the surface moisture was removed for subsequent testing of 7-day unconfined compressive strength (Figure 8).

According to the compaction test results, the maximum dry density and optimum moisture content of cement stabilized gangue and cement stabilized macadam with different cement contents were determined. The specimens were formed according to the compaction degree of 96%. According to JTG E51-2009 "Test methods of materials stabilized with inorganic binders for highway engineering" [36], the unconfined compressive strength test of cement stabilized coal gangue and cement stabilized macadam was carried out for seven days. The static pressure was applied by the press, and 13 specimens

(**a**) Material Preparation (**b**) Electric Compaction Instrument

**Figure 7.** Compaction test.

*3.3. Unlateral Limited Compressive Strength Test* 

**Figure 8.** Unconfined compressive strength test. **Figure 8.** Unconfined compressive strength test.

*Crystals* **2021**, *11*, 993 8 of 18

### *3.4. Splitting Tensile Strength Test 3.4. Splitting Tensile Strength Test Crystals* **2021**, *11*, 993 9 of 18

According to JTG E51-2009 "Test methods of materials stabilized with inorganic binders for highway engineering" [36], the splitting tensile test was carried out on cement According to JTG E51-2009 "Test methods of materials stabilized with inorganic binders for highway engineering" [36], the splitting tensile test was carried out on cement stabilized gangue with a cement content of 3%, 4%, and 5%. Thirteen specimens (of size 150 mm × Φ150 mm) in total were prepared. After demolding, the standard curing was carried out for 90 days according to the method of T0845-2009 [36]. During the test, the loading rate of the press was controlled at 1 mm/min. The splitting tensile test process is shown in Figure 9. stabilized gangue with a cement content of 3%, 4%, and 5%. Thirteen specimens (of size 150 mm × Φ150 mm) in total were prepared. After demolding, the standard curing was carried out for 90 days according to the method of T0845-2009 [36]. During the test, the loading rate of the press was controlled at 1 mm/min. The splitting tensile test process is shown in Figure 9.

**Figure 9.** Splitting tensile test. **Figure 9.** Splitting tensile test.

### *3.5. Freeze–Thaw Resistance Test 3.5. Freeze–Thaw Resistance Test*

According to JTG E51-2009 "Test methods of materials stabilized with inorganic binders for highway engineering" [36], the freeze–thaw tests of cement stabilized gangue and cement stabilized macadam with a cement content of 4%, 5%, 6%, and 7% were carried out, respectively. Each group was statically pressed with a press to form 13 specimens (of size 150 mm × Φ150 mm). After demolding and marking, the specimens were put into a low-temperature chamber and frozen at −18 °C for 16 h. After freezing for one According to JTG E51-2009 "Test methods of materials stabilized with inorganic binders for highway engineering" [36], the freeze–thaw tests of cement stabilized gangue and cement stabilized macadam with a cement content of 4%, 5%, 6%, and 7% were carried out, respectively. Each group was statically pressed with a press to form 13 specimens (of size 150 mm × Φ150 mm). After demolding and marking, the specimens were put into a low-temperature chamber and frozen at −18 ◦C for 16 h.

cycle, they were weighed, and each was melted in a 20 °C water tank for 8 h. After

completed, and the unconfined compressive strength was measured. The process of the freeze–thaw test and the appearance of specimens before and after freeze–thaw are

8h

16h

8h

5cycles

16h

8h

16h

8h

16h

8h

Time

Thaw

20℃

Freeze


(**a**) Low temperature test chamber (**b**) Freeze–thaw cycles

16h

shown in Figure 10 and Figure 11, respectively.

After freezing for one cycle, they were weighed, and each was melted in a 20 ◦C water tank for 8 h. After melting, the surface of the specimen was dried, and the sample was weighed again. The above freezing and thawing cycles were repeated until five freeze–thaw cycles were completed, and the unconfined compressive strength was measured. The process of the freeze–thaw test and the appearance of specimens before and after freeze–thaw are shown in Figures 10 and 11, respectively. cycle, they were weighed, and each was melted in a 20 °C water tank for 8 h. After melting, the surface of the specimen was dried, and the sample was weighed again. The above freezing and thawing cycles were repeated until five freeze–thaw cycles were completed, and the unconfined compressive strength was measured. The process of the freeze–thaw test and the appearance of specimens before and after freeze–thaw are shown in Figure 10 and Figure 11, respectively.

According to JTG E51-2009 "Test methods of materials stabilized with inorganic binders for highway engineering" [36], the freeze–thaw tests of cement stabilized gangue and cement stabilized macadam with a cement content of 4%, 5%, 6%, and 7% were carried out, respectively. Each group was statically pressed with a press to form 13 specimens (of size 150 mm × Φ150 mm). After demolding and marking, the specimens were put into a low-temperature chamber and frozen at −18 °C for 16 h. After freezing for one

*Crystals* **2021**, *11*, 993 9 of 18

shown in Figure 9.

**Figure 9.** Splitting tensile test.

*3.5. Freeze–Thaw Resistance Test* 

stabilized gangue with a cement content of 3%, 4%, and 5%. Thirteen specimens (of size 150 mm × Φ150 mm) in total were prepared. After demolding, the standard curing was carried out for 90 days according to the method of T0845-2009 [36]. During the test, the loading rate of the press was controlled at 1 mm/min. The splitting tensile test process is

(**a**) Low temperature test chamber (**b**) Freeze–thaw cycles

**Figure 10.** Freeze–thaw test process. **Figure 10.** Freeze–thaw test process.

(**a**) Before freezing and thawing (**b**) After freezing and thawing

**Figure 11.** Freeze–thaw test. **Figure 11.** Freeze–thaw test.

### *3.6. Drying Shrinkage Test 3.6. Drying Shrinkage Test*

According to JTG E51-2009 "Test methods of materials stabilized with inorganic binders for highway engineering" [36], drying shrinkage tests were carried out on cement stabilized gangue with a cement content of 4%, 5%, and 6% (Figure 12). The mixture was prepared with the best moisture content obtained in the experiment, and the specimen was formed according to the compaction degree of 96%. The static pressure method was used to form the mixture, the speed was 2 kN/s, and the specimen size was 100 mm × 100 mm × 400 mm per beam. Each group had six specimens, out of which three specimens were used to measure the shrinkage deformation while the other three specimens were used to measure the drying shrinkage water loss ratio. The data was recorded every day during the first seven days and after every two days after 7-day age for 31 days. According to JTG E51-2009 "Test methods of materials stabilized with inorganic binders for highway engineering" [36], drying shrinkage tests were carried out on cement stabilized gangue with a cement content of 4%, 5%, and 6% (Figure 12). The mixture was prepared with the best moisture content obtained in the experiment, and the specimen was formed according to the compaction degree of 96%. The static pressure method was used to form the mixture, the speed was 2 kN/s, and the specimen size was 100 mm × 100 mm × 400 mm per beam. Each group had six specimens, out of which three specimens were used to measure the shrinkage deformation while the other three specimens were used to measure the drying shrinkage water loss ratio. The data was recorded every day during the first seven days and after every two days after 7-day age for 31 days.

**Figure 12.** Dry shrinkage test.

**4. Test Results and Data Discussion**  *4.1. Gradation Optimization Design* 

(**a**) Specimens (**b**) Conservation environment

First, the raw materials of coal gangue with different particle sizes were sieved. The sieving/screening results are shown in Table 8. According to Appendix A of JTG/TF20-2015 "Technical guidelines for construction of highway roadbases" [37], the gradation design of inorganic binder stabilized material was optimized. Three control points were selected: a nominal maximum particle size of 31.5 mm with a passing rate of 100%, a particle size of 4.75 mm with a passing rate of 40%, and a particle size of 0.075

 (**a**) Before freezing and thawing (**b**) After freezing and thawing

(**a**) Specimens (**b**) Conservation environment

According to JTG E51-2009 "Test methods of materials stabilized with inorganic binders for highway engineering" [36], drying shrinkage tests were carried out on cement stabilized gangue with a cement content of 4%, 5%, and 6% (Figure 12). The mixture was prepared with the best moisture content obtained in the experiment, and the specimen was formed according to the compaction degree of 96%. The static pressure method was used to form the mixture, the speed was 2 kN/s, and the specimen size was 100 mm × 100 mm × 400 mm per beam. Each group had six specimens, out of which three specimens were used to measure the shrinkage deformation while the other three specimens were used to measure the drying shrinkage water loss ratio. The data was recorded every day during the first seven days and after every two days after 7-day age for 31 days.

**Figure 12.** Dry shrinkage test. **Figure 12.** Dry shrinkage test.

### **4. Test Results and Data Discussion 4. Test Results and Data Discussion**

### *4.1. Gradation Optimization Design 4.1. Gradation Optimization Design*

**Figure 10.** Freeze–thaw test process.

**Figure 11.** Freeze–thaw test.

*3.6. Drying Shrinkage Test* 

First, the raw materials of coal gangue with different particle sizes were sieved. The sieving/screening results are shown in Table 8. According to Appendix A of JTG/TF20-2015 "Technical guidelines for construction of highway roadbases" [37], the gradation design of inorganic binder stabilized material was optimized. Three control points were selected: a nominal maximum particle size of 31.5 mm with a passing rate of 100%, a particle size of 4.75 mm with a passing rate of 40%, and a particle size of 0.075 First, the raw materials of coal gangue with different particle sizes were sieved. The sieving/screening results are shown in Table 8. According to Appendix A of JTG/TF20- 2015 "Technical guidelines for construction of highway roadbases" [37], the gradation design of inorganic binder stabilized material was optimized. Three control points were selected: a nominal maximum particle size of 31.5 mm with a passing rate of 100%, a particle size of 4.75 mm with a passing rate of 40%, and a particle size of 0.075 mm with a passing rate of 2%. The power function model *y* = *ax<sup>b</sup>* was used to construct the gradation curve of coal gangue. Three control points were brought into the formula of power function model as follows:

$$y = a\mathbf{x}^b\tag{1}$$

where: *x* is particle size (mm), *y* is the passing rate (%), and '*a*' and '*b*' are the coefficients (*a* = 18.807, *b* = 0.4843).

Therefore, the particle gradation of coal gangue after adjustment is shown in Table 9 and Figure 13 (c-c-2 refers to grade c-c-2 recommended in JTG/TF20-2015 "Technical guidelines for construction of highway roadbases" [37]). Therefore, the particle size distribution of coal gangue designed in this paper was well within the recommended c-c-2 grading range, which meets the gradation requirements for a secondary highway base and subbase [37].The particle gradation of crushed stone is given in Table 10 and plotted in Figure 14.

**Table 8.** Screening results of coal gangue aggregates with different particle sizes.




mm with a passing rate of 2%. The power function model *y* = *ax*<sup>b</sup> was used to construct the gradation curve of coal gangue. Three control points were brought into the formula of

*y* = *ax<sup>b</sup>*

Therefore, the particle gradation of coal gangue after adjustment is shown in Table 9 and Figure 13 (c-c-2 refers to grade c-c-2 recommended in JTG/TF20-2015 "Technical guidelines for construction of highway roadbases" [37]). Therefore, the particle size distribution of coal gangue designed in this paper was well within the recommended c-c-2 grading range, which meets the gradation requirements for a secondary highway base and subbase [37].The particle gradation of crushed stone is given in Table 10 and plotted

where: *x* is particle size (mm), *y* is the passing rate (%), and '*a*' and '*b*' are the coefficients

(1)

power function model as follows:

**Table 9.** Coal gangue particle grading.

**Table 8.** Screening results of coal gangue aggregates with different particle sizes. **Sieve Size/mm 31.5 26.5 19 16 13.2 9.5 4.75 2.36 1.18 0.6 0.3 0.15 0.075** 0–4.75 mm - - - - - - 100 62 45.9 31.7 24.8 15.2 9.2 4.75–9.5 mm - - - - 100 96.8 0 - - - - - - 9.5–19 mm - - 100 75.9 40 7.9 0.6 - - - - - - 19–31.5 mm 100 71.9 6.1 3.1 2 - - - - - - - -

**Sieve Size/mm 31.5 26.5 19 16 13.2 9.5 4.75 2.36 1.18 0.6 0.3 0.15 0.075** Passing (%) 100 92 78.3 72 65.6 56 40 24.1 14.6 9.0 5.4 3.3 2.0 Upper of C-C-2 100 100 87 82 75 66 50 36 26 19 14 10 7 Lower of C-C-2 100 90 73 65 58 47 30 19 12 8 5 3 2

**Table 10.** Crushed stone particle grading.

C-C-2 100 100–90 87–73 82–65 75–58 66–47 50–30 36–19 19–8 7–2

Composite gradation

(*a* = 18.807, *b* = 0.4843).

in Figure 14.

**Figure 13.** Grading curve of coal gangue.

**Table 10.** Crushed stone particle grading.


**Figure 14.** Grading curve of crushed stone. **Figure 14.** Grading curve of crushed stone.

### *4.2. Compaction Test 4.2. Compaction Test*

cement stabilized gangue.

5.2 5.2

Optimum moisture content

Maximum dry density

2.32 2.31 2.3

Cement content (%) <sup>3</sup> <sup>4</sup> <sup>5</sup> <sup>6</sup> <sup>7</sup>

4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0

Optimum moisture content *ω*opt/%

4.9

2.3

5.3

The maximum dry density and optimum moisture content of cement stabilized gangue cement stabilized macadam with different cement contents are shown in Figure 15. It can be seen from Figure 15a that the maximum dry density and optimal moisture content of cement stabilized coal gangue increased with the increase in cement content, and the change range is small. When the cement content was 7%, the maximum dry The maximum dry density and optimum moisture content of cement stabilized gangue cement stabilized macadam with different cement contents are shown in Figure 15. It can be seen from Figure 15a that the maximum dry density and optimal moisture content of cement stabilized coal gangue increased with the increase in cement content, and the change range is small. When the cement content was 7%, the maximum dry density

density and optimum moisture content of cement stabilized gangue were the highest,

stabilized macadam was higher. The optimum moisture content was smaller than that of

4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0

The representative value of 7-day unconfined compressive strength of cement stabilized macadam with 4% cement content was *R*<sup>d</sup> = 4.0 MPa. The representative values of

5

2.35

5.2

Optimum moisture content

Maximum dry density

2.37

Cement content (%) <sup>4</sup> <sup>5</sup> <sup>6</sup>

5.1

Maximum dry density *ρ*

dmax/(g.cm-3

)

2.4

2.30

2.35

2.40

2.45

2.50

2.55

2.60

5.5

Maximum dry density *ρ*

2.33

*4.3. 7-Day Unconfined Compressive Strength*

dmax/(g.cm-3

)

2.20 2.25 2.30 2.35 2.40 2.45 2.50 2.55 2.60

(**a**) Coal gangue (**b**) Crushed stone **Figure 15.** Test results of compaction test of mixture.

Optimum moisture content *ω*opt/%

and optimum moisture content of cement stabilized gangue were the highest, while with 3% cement content, these were the minimum. Compared with Figure 15a,b, under the condition of the same cement content, the maximum dry density of cement stabilized macadam was higher. The optimum moisture content was smaller than that of cement stabilized gangue. density and optimum moisture content of cement stabilized gangue were the highest, while with 3% cement content, these were the minimum. Compared with Figure 15a,b, under the condition of the same cement content, the maximum dry density of cement stabilized macadam was higher. The optimum moisture content was smaller than that of cement stabilized gangue.

The maximum dry density and optimum moisture content of cement stabilized gangue cement stabilized macadam with different cement contents are shown in Figure 15. It can be seen from Figure 15a that the maximum dry density and optimal moisture content of cement stabilized coal gangue increased with the increase in cement content, and the change range is small. When the cement content was 7%, the maximum dry

100 10 1 0.1 0.0

Aggregate size (mm)

Composite gradation Upper limit of gradation Lower limit of gradation

*Crystals* **2021**, *11*, 993 12 of 18

**Figure 13.** Grading curve of coal gangue.

**Figure 14.** Grading curve of crushed stone.

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

*4.2. Compaction Test* 

Passing Aggregate %

**Figure 15.** Test results of compaction test of mixture. **Figure 15.** Test results of compaction test of mixture.

### *4.3. 7-Day Unconfined Compressive Strength 4.3. 7-Day Unconfined Compressive Strength*

The representative value of 7-day unconfined compressive strength of cement stabilized macadam with 4% cement content was *R*d = 4.0 MPa. The representative values of The representative value of 7-day unconfined compressive strength of cement stabilized macadam with 4% cement content was *R*<sup>d</sup> = 4.0 MPa. The representative values of the 7-day unconfined compressive strength of cement stabilized gangue with different cement contents are shown in Figure 16. It is clear from Figure 16 that the representative value of unconfined compressive strength of cement stabilized gangue with 4% cement content was about 57.5% of the cement stabilized macadam with the same cement content. Compared with the 7-day unconfined compressive strength of 3% cement stabilized gangue, the 7-day unconfined compressive strength of cement stabilized gangue with 4%, 5%, 6%, and 7% cement increased by 0.2 MPa, 1.7 MPa, 1.9 MPa, and 2.1 MPa, respectively. Reasons may be that with the increase in cement content, the pores around the specimen are correspondingly reduced, which can make the structural shape more dense, so as to increase the strength of cement stabilized coal gangue [38]. The 7-day unconfined compressive strength of 7% cement stabilized gangue was higher than that of the 4% cement stabilized macadam. It can be seen from Figure 16 and Table 11 that when the cement content is the same, the 7-day test results of cement stabilized coal gangue in this paper were higher than those of other researchers in Table 11. In this paper, the 7-day unconfined compressive strength of cement stabilized coal gangue with 3% and 4% cement content can meet the requirements for a medium and light traffic base (2.0–4.0 MPa) and a heavy traffic subbase (2.0–4.0 MPa) of class II and below highways, respectively; other researchers needed the cement content of cement stabilized coal gangue to reach 5% or 6%, respectively.

Compared with the 7-day unconfined compressive strength requirements of cement stabilized material base and subbase for different highway grades in JTG/TF20-2015 ("Technical guidelines for construction of highway roadbases" [37]), in this paper, the 7-day unconfined compressive strength of cement stabilized coal gangue with cement content of 5% met the requirements for a heavy traffic base (3.0–5.0 MPa) and an extremely heavy and extra heavy traffic subbase (2.5–4.5 MPa) of class II and below highways. The 7-day unconfined compressive strength of cement stabilized coal gangue with a cement content of 6% and 7% met the requirements of an extremely heavy traffic base (3.0–5.0 MPa) and an extremely heavy traffic subbase of class II and below highways requirements for an extra heavy traffic base (4.0–6.0 MPa) and subbase (2.5–4.5 MPa), respectively.

5% or 6%, respectively.

respectively.



the 7-day unconfined compressive strength of cement stabilized gangue with different cement contents are shown in Figure 16. It is clear from Figure 16 that the representative value of unconfined compressive strength of cement stabilized gangue with 4% cement content was about 57.5% of the cement stabilized macadam with the same cement content. Compared with the 7-day unconfined compressive strength of 3% cement stabilized gangue, the 7-day unconfined compressive strength of cement stabilized gangue with 4%, 5%, 6%, and 7% cement increased by 0.2 MPa, 1.7 MPa, 1.9 MPa, and 2.1 MPa, respectively. Reasons may be that with the increase in cement content, the pores around the specimen are correspondingly reduced, which can make the structural shape more dense, so as to increase the strength of cement stabilized coal gangue [38]. The 7-day unconfined compressive strength of 7% cement stabilized gangue was higher than that of the 4% cement stabilized macadam. It can be seen from Figure 16 and Table 11 that when the cement content is the same, the 7-day test results of cement stabilized coal gangue in this paper were higher than those of other researchers in Table 11. In this paper, the 7-day unconfined compressive strength of cement stabilized coal gangue with 3% and 4% cement content can meet the requirements for a medium and light traffic base (2.0–4.0 MPa) and a heavy traffic subbase (2.0–4.0 MPa) of class II and below highways, respectively; other researchers needed the cement content of cement stabilized coal gangue to reach

Compared with the 7-day unconfined compressive strength requirements of cement stabilized material base and subbase for different highway grades in JTG/TF20-2015 ("Technical guidelines for construction of highway roadbases" [37]), in this paper, the 7-day unconfined compressive strength of cement stabilized coal gangue with cement content of 5% met the requirements for a heavy traffic base (3.0–5.0 MPa) and an extremely heavy and extra heavy traffic subbase (2.5–4.5 MPa) of class II and below highways. The 7-day unconfined compressive strength of cement stabilized coal gangue with a cement content of 6% and 7% met the requirements of an extremely heavy traffic base (3.0–5.0 MPa) and an extremely heavy traffic subbase of class II and below highways requirements for an extra heavy traffic base (4.0–6.0 MPa) and subbase (2.5–4.5 MPa),

### *4.4. Splitting Tensile Strength of Cement Stabilized Coal Gangue* respectively, and the splitting tensile strength of the water cement stabilized macadam

The representative values of the 90-day splitting tensile strength of cement stabilized gangue with different cement contents are shown in Figure 17. It can be seen that the 90-day splitting tensile strength values of cement stabilized coal gangue with 3%, 4%, and 5% content were 0.65 MPa, 0.87 MPa, and 1.06 MPa. Compared with the 90-day splitting tensile strength of 3% cement stabilized gangue, the 7-day unconfined compressive strength of cement stabilized gangue with 4% and 5% cement increased by 0.22 MPa and 0.41 MPa, respectively. The results also indicated that the 90-day splitting tensile strength of cement stabilized coal gangue increased with cement content, and showed the same change law as the unrestricted compressive strength. The splitting tensile strength mainly reflects the cementing ability of cement stabilized aggregate inside aggregate. It mainly depends on the cohesion between aggregates. The C–S–H gel after hydration of cement is the source of this cohesive force [40]. Therefore, an increase in cement content can improve the splitting tensile strength of cement stabilized coal gangue. gangue mixture with different gradations of 5.5% cement content in [43] were 0.749 MPa, 0.652 MPa, and 0.731 MPa. It was found that the splitting tensile strengths of cement stabilized coal gangue in this paper were higher than those of [42,43]. Reasons may be that the different gradations will lead to different maximum dry density and optimal water content of the sample. In this paper, the power function model *y* = *axb* [36] was used. The corresponding maximum dry density of the optimized gradation sample was greater than the maximum dry density of [42,43], and the optimal water content was also less than the maximum dry density of [42,43] resulting in a more compact structure of cement stabilized coal gangue optimized in this paper. The recommended range of values for the splitting tensile strength of cement stabilized macadam is 0.4–0.6 MPa in the specifications [36]. Clearly, the 90-day splitting tensile strength of cement stabilized coal gangue with a cement content of 3% and 4% can meet the requirements of the specification.

**Figure 17.** Test results of splitting tensile. pressive strength loss BDR was greater than the frost resistance requirements of lime fly **Figure 17.** Test results of splitting tensile.

strength loss diagram of cement stabilized gangue with 4%, 5%, 6%, and 7% cement content. It can be seen from these results that the compressive strength loss BDR of the cement stabilized coal gangue with cement contents of 4%, 5%, 6% and 7% were greater than 75% after five freeze-thaw cycles, and the mass loss rate was less than 1%. Its com-

The splitting tensile strength of the cement stabilized macadam gangue mixture with different gradations of 5.5% cement content in [42] was 0.648 MPa and 0.723 MPa, respectively, and the splitting tensile strength of the water cement stabilized macadam gangue mixture with different gradations of 5.5% cement content in [43] were 0.749 MPa, 0.652 MPa, and 0.731 MPa. It was found that the splitting tensile strengths of cement stabilized coal gangue in this paper were higher than those of [42,43]. Reasons may be that the different gradations will lead to different maximum dry density and optimal water content of the sample. In this paper, the power function model *y* = *ax<sup>b</sup>* [36] was used. The corresponding maximum dry density of the optimized gradation sample was greater than the maximum dry density of [42,43], and the optimal water content was also less than the maximum dry density of [42,43] resulting in a more compact structure of cement stabilized coal gangue optimized in this paper. The recommended range of values for the splitting tensile strength of cement stabilized macadam is 0.4–0.6 MPa in the specifications [36]. Clearly, the 90-day splitting tensile strength of cement stabilized coal gangue with a cement content of 3% and 4% can meet the requirements of the specification.

### *4.5. Freeze–Thaw Resistance Test of Cement Stabilized Coal Gangue*

Table 12 and Figure 18 show the frost resistance index and freeze-thaw compressive strength loss diagram of cement stabilized gangue with 4%, 5%, 6%, and 7% cement content. It can be seen from these results that the compressive strength loss BDR of the cement stabilized coal gangue with cement contents of 4%, 5%, 6% and 7% were greater than 75% after five freeze-thaw cycles, and the mass loss rate was less than 1%. Its compressive strength loss BDR was greater than the frost resistance requirements of lime fly ash stabilized materials in the heavy freezing area of Expressway and class I Highway (70%) [44], and the mass loss rate was less than the specification requirements (5%) [36]. The compressive strength loss of the cement stabilized gangue with 5% and 6% cement was the least, followed by the cement stabilized gangue with 7% cement which had a higher loss in compressive strength after freeze-thaw cycles. The compressive strength loss with 4% cement content was the highest. Reasons may be that the cement stabilized coal gangue with a cement content of 4% has a large number of pores due to the small cement content, and due to its high water content, the volume expansion of the water in the pores during the freezing process is large, which destroys the original pore structure and produces microcracks, resulting in greater strength loss than the other three cement contents [38]. Therefore, the cement content and moisture content are the main factors affecting the compressive strength loss of the cement stabilized coal gangue after freeze-thaw cycles.

**Table 12.** Frost resistance index.


### *4.6. Drying Shrinkage Test*

It can be seen from Table 13 and Figure 19 that with the increase in cement content, the drying shrinkage strain of the cement stabilized gangue increased. At the same time, the hydration and setting of cement will cause volume shrinkage, which is positively correlated with cement content. Therefore, the larger the cement content, the larger the deformation of cement stabilized gangue, and correspondingly the larger the dry shrinkage strain. The changing trend was drastic at first, and then slowed down with the increase in age [38].

freeze-thaw cycles.

**Table 12.** Frost resistance index.

5

10

15

20

**Figure 18.** Freeze–thaw compressive strength loss diagram. **Figure 18.** Freeze–thaw compressive strength loss diagram.



ash stabilized materials in the heavy freezing area of Expressway and class I Highway (70%) [44], and the mass loss rate was less than the specification requirements (5%) [36]. The compressive strength loss of the cement stabilized gangue with 5% and 6% cement was the least, followed by the cement stabilized gangue with 7% cement which had a higher loss in compressive strength after freeze-thaw cycles. The compressive strength loss with 4% cement content was the highest. Reasons may be that the cement stabilized coal gangue with a cement content of 4% has a large number of pores due to the small cement content, and due to its high water content, the volume expansion of the water in the pores during the freezing process is large, which destroys the original pore structure and produces microcracks, resulting in greater strength loss than the other three cement contents [38]. Therefore, the cement content and moisture content are the main factors affecting the compressive strength loss of the cement stabilized coal gangue after

**Cement Content/% 4% 5% 6% 7%**

BDR/% 75.16 94.33 92.26 89.29 Mass loss/% 0.07 0.26 0.81 0.24

**Figure 19.** Relationship between dry shrinkage strain and age of cement stabilized gangue. **Figure 19.** Relationship between dry shrinkage strain and age of cement stabilized gangue.

### **5. Conclusions 5. Conclusions**

highways.

the published version of the manuscript.

and Electric Power (YK2020-11).

In this paper, according to JTG/TF20-2015, "Technical guidelines for construction of highway roadbases" [37], the gradation design of inorganic binder stabilized materials was optimized, and the power function model *y* = *ax<sup>b</sup>* was used to optimize the gradation of coal gangue. The optimized coal gangue gradation curve met the gradation requirements for the secondary highway base and subbase. It provides a basis, guide, and reference for the application of coal gangue materials in a high-grade highway base. In this paper, according to JTG/TF20-2015, "Technical guidelines for construction of highway roadbases" [37], the gradation design of inorganic binder stabilized materials was optimized, and the power function model *y* = *ax<sup>b</sup>* was used to optimize the gradation of coal gangue. The optimized coal gangue gradation curve met the gradation requirements for the secondary highway base and subbase. It provides a basis, guide, and reference for the application of coal gangue materials in a high-grade highway base.

(1) Coal gangue was prepared by a sorting and crushing process, and the grading of coal gangue raw materials was optimized by a power function model. The prepared cement stabilized coal gangue pavement base material with 4% cement content had a 7-day unconfined compressive strength of 2.3 MPa, a 90-day splitting tensile strength of 0.87 MPa, a frost resistance index BDR of 75.16%, a mass loss rate of 0.07%, and a 31-day dry shrinkage strain of 627.5 × 10−<sup>6</sup> , which can be used for medium and light traffic bases and the heavy traffic subbase of class II and below (1) Coal gangue was prepared by a sorting and crushing process, and the grading of coal gangue raw materials was optimized by a power function model. The prepared cement stabilized coal gangue pavement base material with 4% cement content had a 7-day unconfined compressive strength of 2.3 MPa, a 90-day splitting tensile strength of 0.87 MPa, a frost resistance index BDR of 75.16%, a mass loss rate of 0.07%, and a 31-day dry shrinkage strain of 627.5 <sup>×</sup> <sup>10</sup>−<sup>6</sup> , which can be used for medium and light traffic bases and the heavy traffic subbase of class II and below highways.

strength of the cement stabilized coal gangue. An increase in cement content can improve the 90-day splitting tensile strength of cement stabilized coal gangue. Cement content and moisture content are the key factors affecting the frost resistance index of cement stabilized coal gangue. The larger the cement content is, the larger the dry shrinkage strain of cement stabilized coal gangue is, and the shrinkage

(3) In view of the possible problems of cement stabilized coal gangue as a pavement base, the comprehensive experimental analyses were carried out to verify the feasi-

**Author Contributions:** J.G. and X.Y. designed the experiments; M.L., Q.W., D.W., B.Y. and H.L. carried out the experiments; X.Y. and M.L. analyzed the experimental results; J.G. and M.L. reviewed, and edited the manuscript; J.G. received the funding. All authors have read and agreed to

**Funding:** This project was sponsored by the National Natural Science Foundation of China (51779095), the Program for Science & Technology Innovation Talents in Universities of Henan Province (20HASTIT013), Sichuan Univ, the State Key Lab Hydraul & Mt River Engn (SKHL2007), and the Innovation project of the 12th postgraduate of North China University of Water Resources

**Data Availability Statement:** All the relevant data and models used in the study have been pro-

vided in the form of figures and tables in the published article.

strain decreases first and then increases with an increase in age.

bility of cement stabilized coal gangue as a highway base and subbase.


**Author Contributions:** J.G. and X.Y. designed the experiments; M.L., Q.W., D.W., B.Y. and H.L. carried out the experiments; X.Y. and M.L. analyzed the experimental results; J.G. and M.L. reviewed, and edited the manuscript; J.G. received the funding. All authors have read and agreed to the published version of the manuscript.

**Funding:** This project was sponsored by the National Natural Science Foundation of China (51779095), the Program for Science & Technology Innovation Talents in Universities of Henan Province (20HASTIT013), Sichuan Univ, the State Key Lab Hydraul & Mt River Engn (SKHL2007), and the Innovation project of the 12th postgraduate of North China University of Water Resources and Electric Power (YK2020-11).

**Data Availability Statement:** All the relevant data and models used in the study have been provided in the form of figures and tables in the published article.

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

## **References**

