2.2.1. Design of Asphalt Mastics

In the study, asphalt mastic was defined and designed as a mixture of bitumen and calcareous or siliceous fillers. In order to minimize the effect of the mineral powder volume on the asphalt mastics, they were designed with a ratio of bitumen to filler = 1:1 by volume [21–23]. The filler was first dried at 150 ◦C for 3 h. Then, the bitumen was heated in a furnace at 150 ◦C for 1 h. After placing the filler in a vessel with a temperature of 150 ◦C, an enhanced stirring process was conducted to mix the bitumen and filler homogenously at a speed of 1000 rpm for 30 min. The three types of asphalt mastics (LM, BM, and GM) were then obtained in order to carry out the following tests. °C, an enhanced stirring process was conducted to mix the bitumen and filler homogenously at a speed of 1000 rpm for 30 min. The three types of asphalt mastics (LM, BM, and GM) were then obtained in order to carry out the following tests. 2.2.2. Design of the Asphalt Mastic–Aggregate Interface Specimens In order to quantitatively measure the adhesion of the asphalt mastic–aggregate interface, the asphalt mastic–aggregate interface specimens were prepared as shown in Fig-

#### 2.2.2. Design of the Asphalt Mastic–Aggregate Interface Specimens ure 2. The mechanical failure tests were conducted by controlling the asphalt mastic thick-

In order to quantitatively measure the adhesion of the asphalt mastic–aggregate interface, the asphalt mastic–aggregate interface specimens were prepared as shown in Figure 2. The mechanical failure tests were conducted by controlling the asphalt mastic thickness within 1 mm. The sample preparation processes included the following: (1) Installing a mold with the aggregates; (2) aligning the cylindrical aggregates (Φ20 × 20 mm) in self-made fixtures; (3) loading the asphalt mastic in between two pieces of aggregates; (4) controlling the thickness of the asphalt mastic within 1 mm; (5) demolding the asphalt–aggregate interface specimen; (6) removing the extra asphalt mastic around the interface. Then, the bond strength of the interface was tested using a testing machine. In this study, a 1 mm spacer was used to control the thickness of the asphalt mastic, and three types of mastics and three aggregates were used to prepare nine groups of asphalt mastic–aggregate interface specimens. The specific specimen designs and names are shown in Table 6. ness within 1 mm. The sample preparation processes included the following: (1) Installing a mold with the aggregates; (2) aligning the cylindrical aggregates (Φ20 × 20 mm) in selfmade fixtures; (3) loading the asphalt mastic in between two pieces of aggregates; (4) controlling the thickness of the asphalt mastic within 1 mm; (5) demolding the asphalt–aggregate interface specimen; (6) removing the extra asphalt mastic around the interface. Then, the bond strength of the interface was tested using a testing machine. In this study, a 1 mm spacer was used to control the thickness of the asphalt mastic, and three types of mastics and three aggregates were used to prepare nine groups of asphalt mastic–aggregate interface specimens. The specific specimen designs and names are shown in Table 6.

**Figure 2.** Schematic of the preparation of asphalt mastic–aggregate interface specimens. **Figure 2.** Schematic of the preparation of asphalt mastic–aggregate interface specimens.

**Table 6.** Specimen designs and names of asphalt mastic–aggregate interfaces.

**Type of Interface Name**

Basalt mastic–Basalt BM–B Basalt mastic–Granite BM–G

Limestone mastic–Limestone LM–L Limestone mastic–Basalt LM–B Limestone mastic–Granite LM–G


**Table 6.** Specimen designs and names of asphalt mastic–aggregate interfaces.

*2.3. Experimental Tests*

2.3.1. Rheological Testing of Asphalt Mastics


#### 2.3.2. Bond Strength Testing of the Asphalt Mastic–Aggregate Interface

As shown in Figure 3a–e, a universal testing machine with an accuracy of 1 N was employed to measure the maximum force using a deformation-controlled model. Its loading speed was 0.01 mm/s. The bond strength of the interface was then calculated by:

$$f\_t = \frac{F}{S} \tag{1}$$

where *F* is the maximum failure force; *S* is the interfacial area; *f<sup>t</sup>* is the bond strength of the interface.

**Figure 3. Figure 3.**Testing of asphalt mastic–aggr Testing of asphalt mastic–aggregate interface specimens: ( egate interface specimens. **a**) holding fixture; (**b**) specimen fixed by holding fixture; (**c**) specimen fixed into universal testing machine; (**d**) specimen failure surface; (**e**) loading curve.

Granite mastic–Limestone GM–L Granite mastic–Basalt GM–B Granite mastic–Granite GM–G

• A dynamic shear rheometer (DSR) was employed in this study to evaluate the rheological properties of the asphalt mastics, focusing on the influence of fillers. In this test, mastic specimens with a 2 mm thickness were prepared and sandwiched between two flat plates with a diameter of 8 mm. One of the two plates was fixed and the other one was oscillated back and forth around the central axis at a certain angular velocity. The mastic specimens were tested at a temperature interval of 10 °C in the range of 30–60 °C and the scanning rate was 10 rad/s. According to the curves, the complex shear modulus G\*, phase angle δ, and rutting factor (G\*/sin δ) of the asphalt mastics at different temperatures were obtained. The results were employed to evaluate high-tem-

perature performance of the asphalt mastics at moderate temperatures;

2.3.2. Bond Strength Testing of the Asphalt Mastic–Aggregate Interface

• The bending-beam rheometer (BBR) test was employed to characterize the low-temperature cracking resistance of the asphalt mastics. In this research, the BBR test was performed at −6 °C, −12 °C, and −18 °C. During testing, a constant load of 980 ± 50 mN was added in the middle of the mastic beam for 240 s. The deflection was automatically recorded in order to calculate the creep stiffness (S) and m-value (m). In order to resist thermal cracking at low temperatures, the creep stiffness (S) and the

As shown in Figure 3a–e, a universal testing machine with an accuracy of 1 N was employed to measure the maximum force using a deformation-controlled model. Its loading speed was 0.01 mm/s. The bond strength of the interface was then calculated by:

௧ <sup>=</sup>

where *F* is the maximum failure force; *S* is the interfacial area; *ft* is the bond strength of the

(1)

*2.3. Experimental Tests* 

interface.

2.3.1. Rheological Testing of Asphalt Mastics

m-value must meet certain requirements.

#### 2.3.3. Water Absorption of Asphalt Mastics

In order to understand the diffusion behavior of water in the asphalt mastics under normal temperature and pressure, the moisture absorption rates of the asphalt mastics subjected to different water immersion periods were measured using a gravimetric method, and the change principle of the water contents in the asphalt mastics was analyzed by measuring the change in its mass against time [24]. The specific testing processes were given as follows: (1) A customized aluminum plate mold (the mass is *m*0) was used to prepare an asphalt mastic film of 50 mm × 50 mm × 0.3 mm; (2) an analytical balance was used to weigh the aluminum plate and the asphalt mastic before water immersion (the mass is *m*1); (3) the samples were immersed in distilled water and removed at regular intervals; (4) the water was wiped off with filter paper and the mass m<sup>t</sup> of the asphalt mastic plus the aluminum plate was measured. The test results were recorded after soaking times of 1 h, 4 h, 12 h, and 24 h . . .

The moisture absorption rate (*Mt*) of the asphalt mastic at the time of immersion *t* was then calculated by:

$$M\_{\rm f} = \frac{m\_{\rm f} - m\_{\rm f}}{m\_{\rm 1} - m\_{\rm 0}} \times 100\% \tag{2}$$

### 2.3.4. Water Attack Testing of the Asphalt Mastic–Aggregate Interface

In order to investigate the influence of different water conditions on the bond strength of the asphalt mastic–aggregate interface, the temperature (10~40 ◦C), static water immersion time (7 d, 14 d), and water pressure action time (12 h, 24 h) were assessed to test their influence on the mechanical properties of the interface. As calculated by the effect of standard axle load and average velocity, the variation range of the pore water pressure of the asphalt pavement surface layer is generally 0.20~0.57 MPa [25,26]. In this study, a self-designed pressure device was used to simulate the water pressure. Its pressure was 0.5 MPa.

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

#### *3.1. Physical Features of Three Types of Filler*

The Brunauer–Emmett–Teller (BET) method was applied to calculate the specific surface area of the fillers according to nitrogen adsorption isotherm measurements. Using the Barrett–Joyner–Halenda (BJH) model, the pore size distributions of the fillers were derived from the adsorption branches of the isotherms. The pore volume distributions of the mineral fillers as assessed by physical adsorption under high vacuum conditions (measured using Micromeritics ASAP2020 PLUS) are shown in Figure 4. The pore size distribution ranges of LP and GP were roughly similar, ranging from 4 to 20 nm, while the pore size distribution of BP was relatively wider, ranging from 4 to 40 nm. The BJH pore volume, the average pore size, and BET specific surface area of the fillers are listed in Table 7. It can be observed that the pore volumes of LP and GP were 0.007 cm3/g and 0.009 cm3/g, respectively, which were obviously lower than that of BP with 0.034 cm3/g. The average pore sizes of BP and GP were similar, i.e., 3.810 nm and 3.819 nm, respectively, and the average pore size of LP was slightly smaller at 3.059 nm. According to the specific surface area calculated by BET, it can be seen that the specific surface area of LP was the smallest (1.899 m2/g), while the specific surface area of BP was the largest (9.008 m2/g). Moreover, the specific surface area of GP (3.42 m2/g) was between LP and BP. As previously reported, the differences in the pore volume and specific surface area of the mineral powders certainly affect the selective absorption of the asphalt component according to the filler, and then affect the performance of the asphalt mastic and mixture [27]. *Materials* **2023**, *16*, x FOR PEER REVIEW 8 of 18

**Figure 4.** Pore size distribution of mineral powders by BET. **Figure 4.** Pore size distribution of mineral powders by BET.



bitumen into the particles and this influences the rheological response.

Average pore size/nm 3.059 3.810 3.819

Surface area/m2/g 1.899 9.008 3.42

micromorphology of the fillers as assessed using SEM (Tescan, Brno, Czech Republic, Vega 3) is shown in Figure 5. It can be observed that the particle morphology and surface texture of the three mineral fillers were different. The LP particles were relatively smooth, without obvious edges and corners, and the particle size was relatively fine, with a small amount of fine flocculent particles attached to the surface of the coarse ones. BP had a relatively complex surface texture, a rough texture, a large number of holes, and small channels. The GP particles had a clear outline, more polygonal particles, obvious edges and corners, and no obvious holes in the particles. It can be hypothesized that BP absorbs light components of

The microscopic morphology of the mineral fillers directly affects the selective absorp-

(**a**) (**b**)

The microscopic morphology of the mineral fillers directly affects the selective absorption of asphalt, resulting in changes in the rheological properties of asphalt mastics. The micromorphology of the fillers as assessed using SEM (Tescan, Brno, Czech Republic, Vega 3) is shown in Figure 5. It can be observed that the particle morphology and surface texture of the three mineral fillers were different. The LP particles were relatively smooth, without obvious edges and corners, and the particle size was relatively fine, with a small amount of fine flocculent particles attached to the surface of the coarse ones. BP had a relatively complex surface texture, a rough texture, a large number of holes, and small channels. The GP particles had a clear outline, more polygonal particles, obvious edges and corners, and no obvious holes in the particles. It can be hypothesized that BP absorbs light components of bitumen into the particles and this influences the rheological response. micromorphology of the fillers as assessed using SEM (Tescan, Brno, Czech Republic, Vega 3) is shown in Figure 5. It can be observed that the particle morphology and surface texture of the three mineral fillers were different. The LP particles were relatively smooth, without obvious edges and corners, and the particle size was relatively fine, with a small amount of fine flocculent particles attached to the surface of the coarse ones. BP had a relatively complex surface texture, a rough texture, a large number of holes, and small channels. The GP particles had a clear outline, more polygonal particles, obvious edges and corners, and no obvious holes in the particles. It can be hypothesized that BP absorbs light components of bitumen into the particles and this influences the rheological response.

**Pore Features LP BP GP**  Pore volume/cm3/g 0.007 0.034 0.009 Average pore size/nm 3.059 3.810 3.819 Surface area/m2/g 1.899 9.008 3.42

The microscopic morphology of the mineral fillers directly affects the selective absorption of asphalt, resulting in changes in the rheological properties of asphalt mastics. The

*Materials* **2023**, *16*, x FOR PEER REVIEW 8 of 18

 LP BP GP

**Figure 4.** Pore size distribution of mineral powders by BET.

0 20 40 60 80 100 120

孔孔(nm)

Pore size (nm)

0.0

0.3

0.6

0.9

孔孔孔dV/dD(cc/nm/g)

1.2

1.5

1.8

**Table 7.** Physical parameters of mineral powders.

**Figure 5.** Morphology of mineral filler surface: (**a**) LP; (**b**) BP; and (**c**) GP. **Figure 5.** Morphology of mineral filler surface: (**a**) LP; (**b**) BP; and (**c**) GP.
