2.5.2. Wheel Track Test of CEBM

The size of the rutting specimen of CEBM was 300 mm × 300 mm × 50 mm, and the specimens were cured at 25 ◦C for 24 h and 72 h (Complete curing). Before the test, the samples were placed in the rutting instrument for more than 5 h to maintain a constant temperature, and the test temperature, wheel loading, and rate were 60 ◦C, 0.7 MPa, and 42 times/min, respectively.

#### 2.5.3. Low-Temperature Bending Test of CEBM

The Universal testing machine (UTM-100) was used for testing the low-temperature crack resistance of CEBM. The specimen size was 250 mm × 30 mm × 35 mm. Each group had 3 specimens. The test temperature was −10 ◦C, the span of the beam was 200 mm, and the loading rate was 50 mm/min. The schematic diagram is shown in Figure 5. *Materials* **2022**, *15*, x FOR PEER REVIEW 9 of 22

**Figure 5.** Three-point bending test [42]. **Figure 5.** Three-point bending test [42].

#### 2.5.4. Water Sensitivity Test of CEBM

2.5.4. Water Sensitivity Test of CEBM The immersion Marshall stability test and the freeze-thaw splitting test were conducted according to the criteria of the "Standard Test Method of Bitumen and Bituminous Mixtures for Highway Engineering" JTG E20-2011. Each group had 8 specimens, 4 of which were conditional and 4 were non-conditional, which were used to test the water The immersion Marshall stability test and the freeze-thaw splitting test were conducted according to the criteria of the "Standard Test Method of Bitumen and Bituminous Mixtures for Highway Engineering" JTG E20-2011. Each group had 8 specimens, 4 of which were conditional and 4 were non-conditional, which were used to test the water stability of CEBM with different cement and emulsified bitumen contents.

#### stability of CEBM with different cement and emulsified bitumen contents. *2.6. Test Flow Chart*

**Figure 6.** Test flow chart.

*2.6. Test Flow Chart*  The test flow chart is shown in Figure 6.

The test flow chart is shown in Figure 6.

The immersion Marshall stability test and the freeze-thaw splitting test were conducted according to the criteria of the "Standard Test Method of Bitumen and Bituminous Mixtures for Highway Engineering" JTG E20-2011. Each group had 8 specimens, 4 of which were conditional and 4 were non-conditional, which were used to test the water

stability of CEBM with different cement and emulsified bitumen contents.

**Figure 6.** Test flow chart. **Figure 6.** Test flow chart.

#### **3. Experimental Results and Discussion**

**Figure 5.** Three-point bending test [42].

2.5.4. Water Sensitivity Test of CEBM

The test flow chart is shown in Figure 6.

*2.6. Test Flow Chart* 

#### *3.1. Analysis of Surface Morphology and Surface Energy*

3.1.1. Surface Micro-Morphology of CEBM

Cement and emulsified bitumen were used as the composite binders in the CEBM. A SEM was used to observe the micro-morphology of CEBM cured for 24 h. The 1500×, 3000×, 5000×, and 8000× magnified figures of the sample are shown in Figure 6. The hydration products of cement mainly include acicular and reticulated hydrated calcium silicate (C-S-H), flaky calcium hydroxide (C-H), and columnar ettringite (C-A-S-H) [43,44]. In Figure 7a,b, there are many micropores in the CEBM, which are the air voids where water evaporates from the demulsification of emulsified bitumen. In Figure 7c, the flake calcium hydroxide is well wrapped by the demulsified bitumen. From Figure 7c,d, the hydration products of cement that can be observed are uniformly distributed in the demulsified bitumen. And the circular hydrated calcium silicate, columnar ettringite, and flaky calcium hydroxide can be clearly observed in the CEBM [18,21]. In Figure 7d, the emulsified bitumen forms a film after demulsification and wraps the surface of the aggregate, mineral powder, and cement concrete, and the hydration products of cement are also evenly distributed in the CEBM. The hydration products can pierce the bitumen film and smooth areas, and form bonds with other hydration products or the aggregate surface. The bitumen and cement composite bind the aggregate and mineral power together [45].

On the one hand, water and liquid emulsified bitumen can significantly increase the flowability of the CEBM, so it can mix at room temperature and has the advantage of self-compaction. On the other hand, the cementitious phase in the CEBM was dispersed within the emulsified bitumen. The cement hydration consumed a portion of the water that occupies the micro air void spaces between the emulsified bitumen and aggregate, which had a stiffening effect on emulsified bitumen [43,46]. The hydration products of the aggregate and cement, the skeleton function of the cement, and the encapsulation and adhesion function of the emulsified bitumen complement each other, and together with the hydration products of cement interweave with demulsified bitumen to enhance the overall stability and form the strength of CEBM.

**Figure 7.** Microscopic morphology figures of CEBM ((**a**) 1500×; (**b**) 3000×; (**c**) 5000×; (**d**) 8000×). **Figure 7.** Microscopic morphology figures of CEBM ((**a**) 1500×; (**b**) 3000×; (**c**) 5000×; (**d**) 8000×).

3.1.2. Surface Free Energy Analysis of CEBM 3.1.2. Surface Free Energy Analysis of CEBM

In the SFE theory, the surface tension of matter is composed of the Lifshitz-Van Der Waals interaction (ௗ) and the Lewis acid-base interaction (ା/ି) [47,48]. The contact angles of distilled water, glycol, and glycerol titration on the surfaces of bitumen, aggregate, and cement blocks were measured to calculate their surface energy. The surface energy parameters of the bitumen and aggregate were calculated by Young's Equations (2)–(4) [47,49]. Via substitution into Young's equation, the three unknowns (ௗ, ା, ି ) were solved simultaneously. The results are shown in Table 9. In the SFE theory, the surface tension of matter is composed of the Lifshitz-Van Der Waals interaction (*γ d* ) and the Lewis acid-base interaction (*γ* <sup>+</sup>/−) [47,48]. The contact angles of distilled water, glycol, and glycerol titration on the surfaces of bitumen, aggregate, and cement blocks were measured to calculate their surface energy. The surface energy parameters of the bitumen and aggregate were calculated by Young's Equations (2)–(4) [47,49]. Via substitution into Young's equation, the three unknowns (*γ d* , *γ* <sup>+</sup>, *γ* −) were solved simultaneously. The results are shown in Table 9.

$$
\gamma\_L \frac{1 + \cos \theta}{2} = \sqrt{\gamma\_S^d \gamma\_L^d} + \sqrt{\gamma\_S^+ \gamma\_L^-} + \sqrt{\gamma\_S^- \gamma\_L^+} \tag{2}
$$

$$
\gamma\_L = \gamma^d + \gamma\_L^p \tag{3}
$$

$$
\gamma\_L^p = 2\sqrt{\gamma\_L^+ \gamma\_L^-} \tag{4}
$$

where ௌ ௗ, ௌ ା, ௌ ି represent the dispersion component, Lewis acid number, and Lewis base number of the tested solid, respectively; , ௗ, ା, ି express the surface free energy, dispersion component, Lewis acid number, and Lewis base number of the test liquor, respectively. where *γ d S* , *γ* + *S* , *γ* − *S* represent the dispersion component, Lewis acid number, and Lewis base number of the tested solid, respectively; *γL*, *γ d L* , *γ* + *L* , *γ* − *L* express the surface free energy, dispersion component, Lewis acid number, and Lewis base number of the test liquor, respectively.


**Table 9.** SFE parameters of bitumen and aggregate (25 ◦C, mJ/m<sup>2</sup> ).

Adhesion work refers to the work performed when separating the two phases that contact (adhere to) each other on two new surfaces. The adhesion work >0 indicates that the adhesion process can proceed spontaneously. The adhesion process between the residual bitumen of the emulsified bitumen and aggregate can be explained by SFE theory, and SFE can calculate the adhesion work between the bitumen and aggregate. The cohesive work of the same (single-phase) substance was computed using Equation (5); the interface between the two substances (two-phase) can be calculated according to Equation (6) [48,50]. However, The volatilization of water and hydration of cement are the main reasons for the rapid strength formation of CEBM. Therefore, it is necessary to study the adhesion work between the emulsified bitumen, the cement block, and the aggregate before and after the demulsification of the emulsified bitumen. According to the surface energy theory, before bitumen demulsification, water, bitumen, and aggregate coexist, and the adhesion work Wbsw of the three-phase system of water, bitumen, and aggregate is calculated by the Equation (7) [51].

$$\gamma\_{\text{ii}} = 2\gamma\_{\text{i}} = \Im\left(\gamma\_{\text{i}}^d + \gamma\_{\text{i}}^p\right) \tag{5}$$

$$\gamma\_{\vec{\imath}\vec{\jmath}} = 2(\sqrt{\gamma\_i^d \gamma\_j^d} + \sqrt{\gamma\_i^+ \gamma\_j^-} + \sqrt{\gamma\_i^- \gamma\_j^+}) \tag{6}$$

$$\mathcal{W}\_{bsw} = \gamma\_{bs} + \gamma\_{ww} - \gamma\_{bw} - \gamma\_{sw} \tag{7}$$

where *γii* is the cohesive work of the same substance; *γij* is the interface energy of two substances; *Wbsw* is the adhesion work of the water, bitumen, and aggregate three-phase system before demulsification; *γbs*, *γbw*, *γsw* represent the adhesion work between the bitumen and aggregate, bitumen and water, and aggregate and water, respectively; *γww* is the cohesive force between water and water; and *γ d i* , *γ* + *i* , *γ* − *i* represent the dispersion component, Lewis acid, and base number (*i* and *j* are the *b*, *s*, and *w*; the *b*, *s*, and *w* represent bitumen, aggregate, and water, respectively).

The adhesion work of bitumen towards the aggregate and cement mortar block was calculated by substituting the parameters in Table 9. The adhesion work >0 indicates that the adhesion process can proceed spontaneously. The results are shown in Figure 8. Before demulsification, the SFE of the bitumen–aggregate–water three-phase system was reduced due to the existence of water in the bitumen–aggregate interface. The adhesion work between the emulsified bitumen and aggregate is negative, and the adhesion between emulsified bitumen and aggregate may not happen spontaneously due to the existence of water. Therefore, the liquid emulsified bitumen can improve the workability of the mixture and ensures that the mixture can be evenly mixed and self-compacted. The adhesion work of the emulsified bitumen with the limestone aggregate and cement mortar before demulsification is <sup>−</sup>129.97 mJ/m<sup>2</sup> and <sup>−</sup>108.80 mJ/m<sup>2</sup> , that is, the surface free energy changes are 129.97 mJ/m<sup>2</sup> and 108.80 mJ/m<sup>2</sup> , which shows that the adhesion of the emulsified bitumen with limestone aggregate and cement mortar cannot occur without other physicochemical effects.

age resistance.

After demulsification, the adhesion work between the residual bitumen and aggregate is positive, and the residual bitumen and aggregate can bond spontaneously. The demulsification film of bitumen further wraps and adheres to the aggregate, and establishes a spatial network structure in the mixture, thus forming strength. After the demulsification of emulsified bitumen, the adhesion work between 70# bitumen and SBS modified bitumen with the aggregate is positive, which means the change of surface free energy is negative, indicating that their adhesion is spontaneous and can be bonded without external work. In addition, after the demulsification of the emulsified bitumen, the adhesion work of emulsified bitumen towards the limestone and cement mortar block is 75.09 mJ/m2 and 171.36 mJ/m2, respectively; the adhesion work of 70# bitumen towards the limestone and cement mortar block is 65.24 mJ/m2 and 144.61 mJ/m2, respectively; and the adhesion work of SBS modified bitumen towards the limestone and cement mortar block is 74.24 mJ/m2 and 122.73 mJ/m2, respectively. The adhesion work of emulsified bitumen after demulsification towards limestone is 15.1% and 1.1% higher than that of 70# bitumen and SBS modified bitumen, and the adhesion work of emulsified bitumen after demulsification towards cement mortar is 18.5% and 39.6% higher than that of 70# bitumen and SBS modified bitumen. The results show that the adhesion work of emulsified bitumen after demulsification with limestone and cement mortar is higher than that of 70# bitumen and SBS modified bitumen, which can ensure that the CEBM possesses good water dam-

**Figure 8.** Bitumen-aggregate interface adhesion work. **Figure 8.** Bitumen-aggregate interface adhesion work.

*3.2. Influence Factors of Mechanical Performance of CEBM*  3.2.1. Influence of Cement and Emulsified Bitumen Content on Mechanical Performance of CEBM The Marshall specimens are prepared with 8% and 10% emulsified bitumen and 8%, 10%, and 12% sulphoaluminate cement. The flowability of CEBM with different cement and emulsified bitumen contents is shown in Table 10, and the test results of its Marshall stability are shown in Figure 9. In Figure 9, the Marshall stability of CEBM increases with the increase of the emulsified bitumen (8–12%) and cement (8–12%) contents; when the cement content is 12%, no large difference is observed for the Marshall stabilities of CEBM with 8% and 10% emulsified bitumen content. The CEBM with 12% cement and 8% emulsified bitumen has the largest Marshall stability of 10.88 kN, while with 8% cement and After demulsification, the adhesion work between the residual bitumen and aggregate is positive, and the residual bitumen and aggregate can bond spontaneously. The demulsification film of bitumen further wraps and adheres to the aggregate, and establishes a spatial network structure in the mixture, thus forming strength. After the demulsification of emulsified bitumen, the adhesion work between 70# bitumen and SBS modified bitumen with the aggregate is positive, which means the change of surface free energy is negative, indicating that their adhesion is spontaneous and can be bonded without external work. In addition, after the demulsification of the emulsified bitumen, the adhesion work of emulsified bitumen towards the limestone and cement mortar block is 75.09 mJ/m<sup>2</sup> and 171.36 mJ/m<sup>2</sup> , respectively; the adhesion work of 70# bitumen towards the limestone and cement mortar block is 65.24 mJ/m<sup>2</sup> and 144.61 mJ/m<sup>2</sup> , respectively; and the adhesion work of SBS modified bitumen towards the limestone and cement mortar block is 74.24 mJ/m<sup>2</sup> and 122.73 mJ/m<sup>2</sup> , respectively. The adhesion work of emulsified bitumen after demulsification towards limestone is 15.1% and 1.1% higher than that of 70# bitumen and SBS modified bitumen, and the adhesion work of emulsified bitumen after demulsification towards cement mortar is 18.5% and 39.6% higher than that of 70# bitumen and SBS modified bitumen. The results show that the adhesion work of emulsified bitumen after demulsification with limestone and cement mortar is higher than that of 70# bitumen and SBS modified bitumen, which can ensure that the CEBM possesses good water damage resistance.

#### *3.2. Influence Factors of Mechanical Performance of CEBM*

3.2.1. Influence of Cement and Emulsified Bitumen Content on Mechanical Performance of CEBM

The Marshall specimens are prepared with 8% and 10% emulsified bitumen and 8%, 10%, and 12% sulphoaluminate cement. The flowability of CEBM with different cement and emulsified bitumen contents is shown in Table 10, and the test results of its Marshall stability are shown in Figure 9. In Figure 9, the Marshall stability of CEBM increases with the increase of the emulsified bitumen (8–12%) and cement (8–12%) contents; when the cement content is 12%, no large difference is observed for the Marshall stabilities of CEBM with 8% and 10% emulsified bitumen content. The CEBM with 12% cement and 8% emulsified bitumen has the largest Marshall stability of 10.88 kN, while with 8% cement

and 8% emulsified bitumen, it has the lowest stability (8.26 kN). However, it still satisfies the requirement of the Marshall stability of HMA (≥8 kN).

**Table 10.** Flowability of CEBM with different cement and emulsified bitumen contents.


**Figure 9.** Marshall stability of CEBM with different cement and emulsified bitumen contents. **Figure 9.** Marshall stability of CEBM with different cement and emulsified bitumen contents.

The hydration of cement makes use of the free water produced by the demulsification of emulsified bitumen, which accelerates the demulsification of bitumen [16,44]. Emulsified bitumen residues and cement hydration products are the composite binders of the CEBM, which explains why the strength of the mixture increases gradually with the increase of the cement content. For emulsified bitumen, the free water produced by demulsification not only promotes the hydration reaction of cement but also enables the mixture to have great flowability and self-compacting characteristics [45,47]. With the increase of the emulsified bitumen content, the flowability of the mixture is enhanced. However, with the further increase of the emulsified bitumen content, both the free bitumen and free water content will increase [52,53] such that the 12% cement content may increase the free water content and free bitumen content, and the free water will produce the air void in the CEBM after its evaporation; therefore, the free bitumen act as the lubricant between the aggregates and cement concrete, thereby reducing the mechanical performance of The hydration of cement makes use of the free water produced by the demulsification of emulsified bitumen, which accelerates the demulsification of bitumen [16,44]. Emulsified bitumen residues and cement hydration products are the composite binders of the CEBM, which explains why the strength of the mixture increases gradually with the increase of the cement content. For emulsified bitumen, the free water produced by demulsification not only promotes the hydration reaction of cement but also enables the mixture to have great flowability and self-compacting characteristics [45,47]. With the increase of the emulsified bitumen content, the flowability of the mixture is enhanced. However, with the further increase of the emulsified bitumen content, both the free bitumen and free water content will increase [52,53] such that the 12% cement content may increase the free water content and free bitumen content, and the free water will produce the air void in the CEBM after its evaporation; therefore, the free bitumen act as the lubricant between the aggregates and cement concrete, thereby reducing the mechanical performance of CEBM.

#### CEBM. 3.2.2. Curing Time Effect on the Mechanical Performance of CEBM

**Table 10.** Flowability of CEBM with different cement and emulsified bitumen contents. **Emulsified Bitumen Flowability of Different Cement Content (s) 8% 10% 12%**  8% BCR 18.2 19.6 22.1 10% BCR 16.4 18.6 20.2 3.2.2. Curing Time Effect on the Mechanical Performance of CEBM The strengths of the CEBM with 8% emulsified bitumen and 8% and 10% cement are The strengths of the CEBM with 8% emulsified bitumen and 8% and 10% cement are tested after 6 h, 12 h, and 24 h curing, respectively, and the test results are shown in Figure 10. Figure 10 shows that the Marshall stability of CEBM increases continuously with the increase of the curing time (0–24 h). In detail, the Marshall stability of CEBM with 8% BCR and 10% cement after 24 h curing is 68.1% and 167.7% higher than that of CEBM after 12 h and 6 h curing. When the curing time is 6 h, the Marshall stability of the CEBM with 8% emulsified bitumen and 10% cement is 3.44 kN, which satisfies the requirement of the Marshall stability CMA (≥3 kN). After 24 h curing, the Marshall stability of the CEBM meets the requirements of HMA (≥8 kN); therefore, it has a good early-strength property.

tested after 6 h, 12 h, and 24 h curing, respectively, and the test results are shown in Figure 10. Figure 10 shows that the Marshall stability of CEBM increases continuously with the

and 10% cement after 24 h curing is 68.1% and 167.7% higher than that of CEBM after 12 h and 6 h curing. When the curing time is 6 h, the Marshall stability of the CEBM with 8% emulsified bitumen and 10% cement is 3.44 kN, which satisfies the requirement of the

Marshall stability CMA (≥3 kN). After 24 h curing, the Marshall stability of the CEBM meets the requirements of HMA (≥8 kN); therefore, it has a good early-strength property.

Marshall stability CMA (≥3 kN). After 24 h curing, the Marshall stability of the CEBM meets the requirements of HMA (≥8 kN); therefore, it has a good early-strength property.

*Materials* **2022**, *15*, x FOR PEER REVIEW 15 of 22

**Figure 10.** Marshall stability of CEBM after 6 h, 12 h, and 24 h curing times. **Figure 10.** Marshall stability of CEBM after 6 h, 12 h, and 24 h curing times.

3.2.3. Test Conditions Effect on Mechanical Performance of CEBM 3.2.3. Test Conditions Effect on Mechanical Performance of CEBM 3.2.3. Test Conditions Effect on Mechanical Performance of CEBM

The mechanical performance of CEBM was tested under different test conditions, such as 25 °C drying, a 25 °C water bath, and a 60 °C water bath, to investigate the test condition's effect on the mechanical performance of CEBM. The results are similar to Figure 11. From Figure 11, the CEBM shows the highest Marshall stability under the 25 °C drying. After the 25 °C water bath, the stability decreased slightly, and the stability of the CEBM with 8% BCR + 8% cement and 8% BCR + 10% cement cured for 6 h decreased by 7.9% and 3.1%, respectively, while the stability of the CEBM with 8% BCR + 8% cement and 8% BCR + 10% cement cured for 24 h decreased by 2.4% and 9.6%, respectively. After the 60 °C water bath, the stability of CEBM decreases more obviously. The changing trend of the stability of CEBM under different test conditions is consistent. The mechanical performance of CEBM was tested under different test conditions, such as 25 ◦C drying, a 25 ◦C water bath, and a 60 ◦C water bath, to investigate the test condition's effect on the mechanical performance of CEBM. The results are similar to Figure 11. From Figure 11, the CEBM shows the highest Marshall stability under the 25 ◦C drying. After the 25 ◦C water bath, the stability decreased slightly, and the stability of the CEBM with 8% BCR + 8% cement and 8% BCR + 10% cement cured for 6 h decreased by 7.9% and 3.1%, respectively, while the stability of the CEBM with 8% BCR + 8% cement and 8% BCR + 10% cement cured for 24 h decreased by 2.4% and 9.6%, respectively. After the 60 ◦C water bath, the stability of CEBM decreases more obviously. The changing trend of the stability of CEBM under different test conditions is consistent. The mechanical performance of CEBM was tested under different test conditions, such as 25 °C drying, a 25 °C water bath, and a 60 °C water bath, to investigate the test condition's effect on the mechanical performance of CEBM. The results are similar to Figure 11. From Figure 11, the CEBM shows the highest Marshall stability under the 25 °C drying. After the 25 °C water bath, the stability decreased slightly, and the stability of the CEBM with 8% BCR + 8% cement and 8% BCR + 10% cement cured for 6 h decreased by 7.9% and 3.1%, respectively, while the stability of the CEBM with 8% BCR + 8% cement and 8% BCR + 10% cement cured for 24 h decreased by 2.4% and 9.6%, respectively. After the 60 °C water bath, the stability of CEBM decreases more obviously. The changing trend of the stability of CEBM under different test conditions is consistent.

**Figure 11.** Marshall stability of CEBM under different test conditions. **Figure 11.** Marshall stability of CEBM under different test conditions.

#### **Figure 11.** Marshall stability of CEBM under different test conditions. *3.3. Mixture Performance Test of CEBM*

3.3.1. High Temperature Stability of CEBM

The rutting resistance of the mixture cured for 24 h and 72 h was studied by a wheel track test. The results are shown in Figures 12 and 13. From Figure 12, with the increase of

25℃ dtying 25℃ water bath 60℃ water bath

the loading cycles, the cumulative deformation (rutting depth) of the mixture gradually increases, and the rutting depth growth rate of the CEBM cured for 72 h is lower than that of the CEBM cured for 24 h. Under conditions with the same number of loading cycles, the rutting depth of the CEBM with 72 h of curing time is much lower than that of the CEBM with 24 h of curing time. When the loading time is 45 min and 60 min, the rutting depth of the CEBM cured for 72 h is 0.167 mm and 0.173 mm, respectively, and for 24 h is 0.272 mm and 0.308 mm, respectively. The rutting depth of CEBM cured for 72 h is 62.87% and 78.03% less than that of maintenance for 24 h. The maximum rutting depth of CEBM is 0.3 mm, which is far less than the existing 1–2 mm rutting depth of the cement emulsified bitumen [27]. From Figure 13, after curing for 24 h, the dynamic stability (DS) of CEBM is 18,333 times/mm, and for 72 h, it is 63,000 times/mm, which is much higher than the requirement of HMA, indicating that due to the high strength offered by the cement, the high-temperature stability of CEBM is very good. increases, and the rutting depth growth rate of the CEBM cured for 72 h is lower than that of the CEBM cured for 24 h. Under conditions with the same number of loading cycles, the rutting depth of the CEBM with 72 h of curing time is much lower than that of the CEBM with 24 h of curing time. When the loading time is 45 min and 60 min, the rutting depth of the CEBM cured for 72 h is 0.167 mm and 0.173 mm, respectively, and for 24 h is 0.272 mm and 0.308 mm, respectively. The rutting depth of CEBM cured for 72 h is 62.87% and 78.03% less than that of maintenance for 24 h. The maximum rutting depth of CEBM is 0.3 mm, which is far less than the existing 1–2 mm rutting depth of the cement emulsified bitumen [27]. From Figure 13, after curing for 24 h, the dynamic stability (DS) of CEBM is 18,333 times/mm, and for 72 h, it is 63,000 times/mm, which is much higher than the requirement of HMA, indicating that due to the high strength offered by the cement, the high-temperature stability of CEBM is very good. increases, and the rutting depth growth rate of the CEBM cured for 72 h is lower than that of the CEBM cured for 24 h. Under conditions with the same number of loading cycles, the rutting depth of the CEBM with 72 h of curing time is much lower than that of the CEBM with 24 h of curing time. When the loading time is 45 min and 60 min, the rutting depth of the CEBM cured for 72 h is 0.167 mm and 0.173 mm, respectively, and for 24 h is 0.272 mm and 0.308 mm, respectively. The rutting depth of CEBM cured for 72 h is 62.87% and 78.03% less than that of maintenance for 24 h. The maximum rutting depth of CEBM is 0.3 mm, which is far less than the existing 1–2 mm rutting depth of the cement emulsified bitumen [27]. From Figure 13, after curing for 24 h, the dynamic stability (DS) of CEBM is 18,333 times/mm, and for 72 h, it is 63,000 times/mm, which is much higher than the requirement of HMA, indicating that due to the high strength offered by the cement, the high-temperature stability of CEBM is very good.

The rutting resistance of the mixture cured for 24 h and 72 h was studied by a wheel track test. The results are shown in Figures 12 and 13. From Figure 12, with the increase of the loading cycles, the cumulative deformation (rutting depth) of the mixture gradually

The rutting resistance of the mixture cured for 24 h and 72 h was studied by a wheel track test. The results are shown in Figures 12 and 13. From Figure 12, with the increase of the loading cycles, the cumulative deformation (rutting depth) of the mixture gradually

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*Materials* **2022**, *15*, x FOR PEER REVIEW 16 of 22

*3.3. Mixture Performance Test of CEBM* 

*3.3. Mixture Performance Test of CEBM* 

3.3.1. High Temperature Stability of CEBM

3.3.1. High Temperature Stability of CEBM

**Figure 12.** Rutting increasing curve of CEBM. **Figure 12.** Rutting increasing curve of CEBM. **Figure 12.** Rutting increasing curve of CEBM.

**Figure 13.** Dynamic stability of CEBM. **Figure 13.** Dynamic stability of CEBM. **Figure 13.** Dynamic stability of CEBM.

3.3.2. Low-Temperature Crack Resistance of CEBM

The low-temperature bending test was conducted to evaluate the low-temperature crack resistance of CEBM. The results are shown in Figure 14 and Table 11. The maximum bending tensile strain of ordinary cement emulsified bitumen mixture trabecula is about 2100 µε, and the bending stiffness modulus is about 1800 MPa [28]. According to Figure 14,

the maximum failure loading of CEBM is 0.46 kN, and the corresponding mid-span deflection is 0.52 mm. From Table 11, the bending tensile strength of the trabeculae is 4.28 MPa. Compared with ordinary cement emulsified bitumen mixture, the maximum bending tensile strain of CEBM increases by 19.19%., and the bending stiffness modulus decreases by 4.98%. The Low-temperature performance has been improved to some extent. Although there is no technical requirement for the failure strain in the low-temperature bending test of CMA, the maximum flexural tensile strain of CEBM still meets the requirement of China's criterion (JTG D50-2017) that the flexural strain of the HMA should be greater than 2000µε. This performance is improvement due to the existence of the emulsified bitumen, which can provide flexibility for CEBM and improve the Low-temperature crack resistance of CEBM. 14, the maximum failure loading of CEBM is 0.46 kN, and the corresponding mid-span deflection is 0.52 mm. From Table 11, the bending tensile strength of the trabeculae is 4.28 MPa. Compared with ordinary cement emulsified bitumen mixture, the maximum bending tensile strain of CEBM increases by 19.19%., and the bending stiffness modulus decreases by 4.98%. The Low-temperature performance has been improved to some extent. Although there is no technical requirement for the failure strain in the low-temperature bending test of CMA, the maximum flexural tensile strain of CEBM still meets the requirement of China's criterion (JTG D50-2017) that the flexural strain of the HMA should be greater than 2000με. This performance is improvement due to the existence of the emulsified bitumen, which can provide flexibility for CEBM and improve the Low-temperature crack resistance of CEBM.

The low-temperature bending test was conducted to evaluate the low-temperature crack resistance of CEBM. The results are shown in Figure 14 and Table 11. The maximum bending tensile strain of ordinary cement emulsified bitumen mixture trabecula is about 2100 με, and the bending stiffness modulus is about 1800 MPa [28]. According to Figure

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3.3.2. Low-Temperature Crack Resistance of CEBM

**Figure 14.** Loading-deformation curve of low-temperature bending test. **Figure 14.** Loading-deformation curve of low-temperature bending test.

**Table 11.** Low temperature bending test results. **Table 11.** Low temperature bending test results.


3.3.3. Water sensitivity of CEBM (1) Immersion Marshall test 3.3.3. Water sensitivity of CEBM

#### The results of the water immersion Marshall test of CEBM at 25 °C and 60 °C are (1) Immersion Marshall test

shown in Table 12. According to Table 12, the Marshall stabilities of CEBM both before and after water immersion are higher than 8 kN (the HMA requirement), and the residual Marshall stability of CEBM after water immersion increases at first and then decreases with the increase of the cement content. The water-immersed residual stability (IRS) of ordinary cement emulsified bitumen mixture is 85.3% [28]. The IRS of CEBM is shown in Figure 15. The 25 °C water immersion-conditioned CEBM with 8% BCR and 8% cement has the smallest IRS value of 92.4%, and the 60 °C IRS of CEBM with 8% BCR and 10% cement has the smallest IRS of 130.3%. The 25 °C IRS of CEBM increases with the increase of cement content, while the 60 °C IRS of CEBM initially increases and then decreases with The results of the water immersion Marshall test of CEBM at 25 ◦C and 60 ◦C are shown in Table 12. According to Table 12, the Marshall stabilities of CEBM both before and after water immersion are higher than 8 kN (the HMA requirement), and the residual Marshall stability of CEBM after water immersion increases at first and then decreases with the increase of the cement content. The water-immersed residual stability (IRS) of ordinary cement emulsified bitumen mixture is 85.3% [28]. The IRS of CEBM is shown in Figure 15. The 25 ◦C water immersion-conditioned CEBM with 8% BCR and 8% cement has the smallest IRS value of 92.4%, and the 60 ◦C IRS of CEBM with 8% BCR and 10% cement has the smallest IRS of 130.3%. The 25 ◦C IRS of CEBM increases with the increase of cement content, while the 60 ◦C IRS of CEBM initially increases and then decreases with the increase of the cement content, and reaches the maximum value at 10% cement content. All of the IRS values of CEBM are greater than ordinary cement emulsified bitumen mixture, which is obviously better than the requirements of higher than 80%.


**Table 12.** Marshall stabilities of CEBM with and without water immersion. **Marshall Stability (25 °C) Marshall Stability (60 °C)** 

**Table 12.** Marshall stabilities of CEBM with and without water immersion.

the increase of the cement content, and reaches the maximum value at 10% cement content. All of the IRS values of CEBM are greater than ordinary cement emulsified bitumen

mixture, which is obviously better than the requirements of higher than 80%.

*Materials* **2022**, *15*, x FOR PEER REVIEW 18 of 22

**Figure 15.** Water-immersed residual stability (IRS) of CEBM. **Figure 15.** Water-immersed residual stability (IRS) of CEBM.

#### (2) Freeze-thaw splitting test (2) Freeze-thaw splitting test

The results of the freeze-thaw indirect tensile strength test are shown in Figure 16. According to Figure 16, the freeze-thaw indirect tensile strength ratio (TSR) of the CEBM with 10% BCR and 8% cement content is the smallest value, which is 83.3%, and the TSR value of the CEBM with 8% BCR and 12% cement content is the highest, which is 105.3%. The TSR of the mixture increases with the increase of the cement content. Since the splitting strength of the conditional group in the freeze-thaw splitting test is about 0.4 MPa, the TSR increases with the decrease of the splitting strength of the unconditional group. This explains why with the increase in the cement content, although the TSR increases, its splitting strength also decreases. In addition, all of the TSR values of the above CEBMs The results of the freeze-thaw indirect tensile strength test are shown in Figure 16. According to Figure 16, the freeze-thaw indirect tensile strength ratio (TSR) of the CEBM with 10% BCR and 8% cement content is the smallest value, which is 83.3%, and the TSR value of the CEBM with 8% BCR and 12% cement content is the highest, which is 105.3%. The TSR of the mixture increases with the increase of the cement content. Since the splitting strength of the conditional group in the freeze-thaw splitting test is about 0.4 MPa, the TSR increases with the decrease of the splitting strength of the unconditional group. This explains why with the increase in the cement content, although the TSR increases, its splitting strength also decreases. In addition, all of the TSR values of the above CEBMs meet the requirements of higher than 75%. Therefore, the CEBM has good water stability.

meet the requirements of higher than 75%. Therefore, the CEBM has good water stability.

**Figure 16.** Splitting strength and freeze-thaw splitting ratio of CEBM. **Figure 16.** Splitting strength and freeze-thaw splitting ratio of CEBM.

#### **4. Conclusions**

**4. Conclusions**  The SEM and surface energy theory of the CEBM were studied using modern testing technology, and the strength formation mechanism of CEBM was revealed. The influence of the emulsified bitumen, the cement dosage, and the curing time on the strength of CEBM was studied. In addition, the road performance of CEBM is evaluated by road per-The SEM and surface energy theory of the CEBM were studied using modern testing technology, and the strength formation mechanism of CEBM was revealed. The influence of the emulsified bitumen, the cement dosage, and the curing time on the strength of CEBM was studied. In addition, the road performance of CEBM is evaluated by road performance tests. The following conclusions were obtained.

formance tests. The following conclusions were obtained. (1) Before demulsification, the SFE of the bitumen–aggregate–water three-phase system was reduced due to the replacement of the bitumen–aggregate interface with water. The adhesion work between the emulsified bitumen and aggregate is negative, and the adhesion between the emulsified bitumen and aggregate may not happen spontaneously due to the existence of water. Meanwhile, water exists in bitumen and aggregate, which improves the workability of CEBM and ensures its uniform mixing and self-compacting. After demulsification, the adhesion work between the residual bitumen and aggregate is positive, and the residual bitumen and aggregate can bond spontaneously. The free water produced by the demulsification of bitumen reacts with the cement, the hydration products of cement form a skeleton in aggregate, and the demulsified bitumen further encap-(1) Before demulsification, the SFE of the bitumen–aggregate–water three-phase system was reduced due to the replacement of the bitumen–aggregate interface with water. The adhesion work between the emulsified bitumen and aggregate is negative, and the adhesion between the emulsified bitumen and aggregate may not happen spontaneously due to the existence of water. Meanwhile, water exists in bitumen and aggregate, which improves the workability of CEBM and ensures its uniform mixing and self-compacting. After demulsification, the adhesion work between the residual bitumen and aggregate is positive, and the residual bitumen and aggregate can bond spontaneously. The free water produced by the demulsification of bitumen reacts with the cement, the hydration products of cement form a skeleton in aggregate, and the demulsified bitumen further encapsulates the aggregate and cement and bonds them together. The skeleton of the cement and the adhesion of bitumen complement each other, and establish a spatial network structure in the CEBM, thus forming high strength.

sulates the aggregate and cement and bonds them together. The skeleton of the cement and the adhesion of bitumen complement each other, and establish a spatial network structure in the CEBM, thus forming high strength. (2) The emulsified bitumen content, cement content, and curing conditions have significant effects on the mechanical stability of CEBM. When the cement content is 12% and the emulsified bitumen content is 8%, the CEBM has the maximum Marshall stability of 10.88 kN; when the cement content is 8% and the emulsified bitumen content is 8%, the CEBM has the maximum Marshall stability of 8.26 kN. All of these values are even higher (2) The emulsified bitumen content, cement content, and curing conditions have significant effects on the mechanical stability of CEBM. When the cement content is 12% and the emulsified bitumen content is 8%, the CEBM has the maximum Marshall stability of 10.88 kN; when the cement content is 8% and the emulsified bitumen content is 8%, the CEBM has the maximum Marshall stability of 8.26 kN. All of these values are even higher than the requirement for the hot mix bitumen mixture (≥8 kN). In addition, when the curing time is 6 h, all the Marshall stabilities of CEBM can reach the stability requirement of CMA (≥3 kN).

than the requirement for the hot mix bitumen mixture (≥8 kN). In addition, when the curing time is 6 h, all the Marshall stabilities of CEBM can reach the stability requirement of CMA (≥3 kN). (3) Due to the hardening effect of cement, the CEBM has an excellent rutting resistance at high temperatures, and the dynamic stability is 18,333 times/mm cured for 24 h. On the other hand, due to the viscoelasticity of bitumen, the maximum flexural-tensile strain at

(3) Due to the hardening effect of cement, the CEBM has an excellent rutting re-

h. On the other hand, due to the viscoelasticity of bitumen, the maximum flexural-tensile strain at low temperature is 2503 με, which even meets the requirement of the flexural-

low temperature is 2503 µε, which even meets the requirement of the flexural-tensile strain of hot mix bitumen mixture (≥2000 µε). The water immersion residue is higher than 110% with 10% cement, and the TSR is higher than 85%, indicating the CEBM has good water stability.

(4) CEBM has good working and mechanical properties; therefore, it is technically feasible to use CEBM as a new material for road construction and maintenance. For comprehensive economic considerations, the recommended dosage of CEBM emulsified bitumen is 8%, and that of cement is 8–10%. The cement dosage can be determined according to the relevant engineering requirements.

**Author Contributions:** Conceptualization, T.B., A.C. and Y.G.; Data curation, J.Y., J.F., Y.L., T.B., A.C., Y.G., F.W. and Q.L.; Funding acquisition, J.Y. and T.B.; Investigation, J.Y. and Y.L.; Methodology, J.Y., Y.L., A.C., S.W. and C.L.; Project administration, T.B.; Resources, F.W.; Software, Y.L., F.W. and Q.L.; Supervision, J.F.; Validation, Y.L. and F.W.; Writing—original draft, J.F. and A.C.; Writing—review & editing, J.Y., Y.L., Y.G., F.W., S.W., Q.L. and C.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 52108415), the National Key Research and Development Program of China (No. 2018YFB1600200), the Natural Science Foundation of China (No. 51778515), the key technical innovation projects of Hubei Province (No. 2019AEE023), the plan for outstanding young and middle-aged scientific and technological innovation team in universities of Hubei Province (No. T2020010), the scientific research fund of Hunan Provincial Education Department (No. 18A117), the Key R&D Program of Hubei Province (No. 2020BCB064), and the support for the project from the science and technology projects fund of Changsha city (No. kq2004065).

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no competing financial interest.

#### **References**

