*Article* **Influence of Filler Type and Rheological Properties of Asphalt Mastic on the Asphalt Mastic–Aggregate Interaction**

**Guangxun E <sup>1</sup> , Jizhe Zhang 2,\*, Quanjun Shen <sup>1</sup> , Ping Ji <sup>3</sup> , Jing Wang <sup>2</sup> and Yushuai Xiao <sup>2</sup>**


**Abstract:** The asphalt mastic–aggregate interaction plays an important role in the overall properties of asphalt mixtures and their durability in service in flexible pavements. This paper aims to study the influence of the physico-chemical features of fillers and the rheological properties of asphalt mastics on the bonding behavior between asphalt and aggregate, and the interfacial deterioration mechanism when subjected to static water immersion and pressured water immersion. It was found that the filler type (limestone powder, basalt powder, and granite powder) had a certain influence on the complex modulus of asphalt mastics, and its pore volume and specific surface area had significant effects on the phase angles and permeability of asphalt mastics. The effect of water pressure can accelerate the deterioration of bond strength of the asphalt mastic–aggregate interface in the short term, indicating that the dynamic water pressure generated by the driving load promotes the water damage process in asphalt pavements. In comparison, the residual bond strength ratio of the granite–asphalt mastic aggregate was the highest, while its bond strength was lower than that of the interface between limestone–asphalt mastics and limestone aggregate. This demonstrated that a low asphalt mastic complex modulus and a high phase angle are helpful in improving the durability of asphalt mixtures subjected to static and pressured water immersion conditions.

**Keywords:** filler; asphalt mastic; interfacial bond strength; asphalt–aggregate interaction; moisture damage

#### **1. Introduction**

Owing to the advantages of smooth surfaces, driving comfort, low noise, and easy maintenance, flexible asphalt pavements have become the main pavement type for highgrade highways in most countries [1,2]. The asphalt mixture used in flexible asphalt pavements is a multiphase composite, which is composed of asphalt binder, filler, voids, aggregates of different sizes, and the asphalt–aggregate interface [3]. Therefore, it is normally considered to be a heterogeneous material and its properties can be strongly influenced by different material variables and their proportions.

In previous research, pavement engineers and researchers recognized that the asphalt mastic and the asphalt mastic–aggregate interface strongly determine the overall performance of the asphalt mixtures and their durability in service life [4]. The asphalt mastic in the asphalt mixture is the cementing component and consists of bitumen and fine aggregate, and its composition and properties directly affect the road performance of asphalt mixtures. The physico-chemical properties of the mineral fillers incorporated have a significant impact on the performance of asphalt mastics. Zhang et al. used oxide analytical reagents to represent mineral aggregate fillers and to study their effects on the properties of asphalt mastics and the interfaces. It was found that the effects of calcium oxide (CaO) were greater than those of silicon dioxide (SiO2) due to the stronger interaction

**Citation:** E, G.; Zhang, J.; Shen, Q.; Ji, P.; Wang, J.; Xiao, Y. Influence of Filler Type and Rheological Properties of Asphalt Mastic on the Asphalt Mastic–Aggregate Interaction. *Materials* **2023**, *16*, 574. https:// doi.org/10.3390/ma16020574

Academic Editor: Simon Hesp

Received: 18 November 2022 Revised: 19 December 2022 Accepted: 28 December 2022 Published: 6 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

<sup>1</sup> Shandong Key Laboratory of Highway Technology and Safety Assessment, Shandong Hi-Speed Group Co., Ltd., Jinan 250101, China

between asphalt binder and CaO [5]. Based on the research on the mesoscopic properties of mineral fillers, Lv et al. found that physical properties such as mineral filler fineness and mesoscopic gradation had a certain correlation with the mesoscopic strength of asphalt mastics [6]. Barra et al. analyzed the influence of the properties of limestone mineral powder and granite mineral powder on the softening point, penetration, and adhesion of asphalt mastic [7]. White studied the shear creep response of an airport asphalt mastic and the results indicated that two types of asphalt mastics had different properties due to different dusts [8,9]. Compared with the size of the filler, its shape, surface texture, specific surface area, and mineral components had more significant effects on the properties. Based on the microstructure of fillers, Geber et al. analyzed the effects of the particle size distribution, microscopic morphology, mercury intrusion porosity, specific surface area, and hydrophobicity of limestone powder and dolomite powder on the rheological properties of asphalt mastics [10]. The particle size, hydrophobicity, and content of fillers have significant effects on the rheological properties of asphalt mastics.

In recent years, more studies have also been carried out to investigate the adhesion of the asphalt mastic–aggregate interface. In general, four mechanisms have been adopted to explain the adhesion between asphalt mastic and mineral aggregate: surface energy, chemical reaction, molecular orientation, and mechanical contact [11]. Yi et al. studied the influence of different factors on the adhesion of the asphalt–aggregate interaction based on the surface energy theory and showed that the measured surface energy using an AFM method can represent the surface characteristics of materials [12]. Based on the molecular orientation theory, Huang et al. found that the fundamental cause of adsorption of asphalt on the aggregate surface was the polarity of the asphalt and aggregate and that the compositions of asphaltene and gelatine greatly determined adhesion [13]. Using digital imaging techniques and an abrasion method, Kuang et al. studied the macroscopic changes on the particle surface of limestone and granite before and after treatment and their influences on interfacial adhesion between asphalt and limestone/granite. It was found that the interfacial bonding between asphalt/mastic and aggregate in asphalt mixtures is mainly attributed to the micro/meso-physical and chemical actions in the vicinity of the interface [14].

Most importantly, factors such as water, temperature, environmental conditions, and other incorporated recycled waste, such as waste oil and industrial waste powder, also have important effects on the rheology and durability of asphalt mixtures [15–17]. Because, in practice, asphalt pavements have a long service life, they suffer from different degrees of defects. Water/moisture damage is a common early defect that can lead to the loosening and peeling of aggregates, which seriously affects the pavements service performance and shortens its service life [17–19]. Under the dual action of water and driving traffic load, the asphalt mastic–aggregate interface may become the weakest zone of the asphalt mixture, even after initially exhibiting a good interfacial bond strength.

The stripping potential of asphalt mixtures is usually evaluated based on the cohesion bond between the binder and aggregates. It is often caused by the loss of the mastic– aggregate bond and results in poor durability of asphalt mixtures. This study, therefore, aimed to experimentally assess the correlation between the physico-chemical features of fillers, the rheological properties of asphalt mastics, and the asphalt mastic–aggregate interaction before and after static and dynamic water attack. Three types of fillers and aggregates that are normally used in asphalt mixtures (limestone, basalt, and granite) were chosen and characterized. The influence of fillers on the rheological properties of asphalt mastics was characterized using a dynamic shear rheometer (DSR). The direct bond strength of the asphalt–aggregate interface as a key property indicator was measured and evaluated. The water attack testing was conducted by increasing the water pressure. Understanding the bond strength development and the deterioration of the asphalt mastic–aggregate interface will help to provide scientific guidelines for the design of asphalt mixtures.

#### **2. Materials and Experimental**

*2.1. Materials*

2.1.1. Bitumen

The bitumen used as a binder in the asphalt mixtures was a type of AH-70# road petroleum (produced by a local company in Shandong, China). It was an unmodified type with a penetration value of 60–70 (according to ASTM D5, it was 60/70 grade). Its properties, which were in accordance with Chinese test methods, are listed in Table 1.

**Table 1.** Properties of AH-70 bitumen.


#### 2.1.2. Filler

The mineral powders used as fillers in the asphalt mixtures in this study were limestone mineral powder (LP), basalt mineral powder (BP), and granite mineral powder (GP), which were sourced from these three types of minerals. These mineral particles, which were less than 4.75 mm in size, were ground in a laboratory mill for 2 min and then passed through a 0.075 mm sieve to obtain powder. The densities of the three kinds of mineral powder were measured using a pycnometer (Chinese standard, T0352) and the results are listed in Table 2.

**Table 2.** Relative densities of mineral powders.


The particle size distributions of these three fillers, measured using a laser diffraction technique (Tester of LS230), are shown in Table 3. *D*10, *D*50, and *D*<sup>90</sup> represent the minimum particle size with a pass rate of 10%, 50%, and 90%, respectively. It was found that the *D*10, *D*50, and *D*<sup>90</sup> of LP exhibited a relatively smaller size than those of the other BP and GP fillers. This indicates that LP was finer than the others, and BP and GP had similar sizes.

**Table 3.** Particle size distributions of three mineral powders.


The mineral composition of the mineral powder has an important influence on its chemical reaction with bitumen [20]. Table 4 shows the oxide chemical components of the three mineral fillers measured by X-ray fluorescence (XRF) (Rigaku, Tokyo, Japan, Supermini200). It can be seen that the chemical composition of the three mineral powders was significantly different. The main component of LP was CaO and its content was about 83% by mass, while the content of SiO<sup>2</sup> was the lowest (5.8%) compared to the others. The LP is, therefore, considered as an alkaline mineral. As regards the basalt powder, its main chemical components were SiO2, Al2O3, Fe2O3, and CaO, and the SiO<sup>2</sup> content was 46% (mass fraction), which makes it a neutral mineral powder. The main components of the

granite powder were SiO<sup>2</sup> and Al2O3, and its SiO<sup>2</sup> content was 64% (by mass). Thus, it is regarded as an acidic mineral powder. These chemical components cause the powders to have noticeably different chemical reactions with bitumen. **Table 4.** Oxide composition of mineral fillers (%). **Oxide Filler Type**

(mass fraction), which makes it a neutral mineral powder. The main components of the granite powder were SiO2 and Al2O3, and its SiO2 content was 64% (by mass). Thus, it is regarded as an acidic mineral powder. These chemical components cause the powders to


In order to detect the mineral crystals in the three kinds of fillers and then to identify

**Table 4.** Oxide composition of mineral fillers (%). **LP BP GP**

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have noticeably different chemical reactions with bitumen.

In order to detect the mineral crystals in the three kinds of fillers and then to identify their influence on the performance of asphalt mastics and interfaces, their diffraction patterns were obtained by XRD (Rigaku SmartLab), and the mineral crystal analysis was carried out using the HighScore Plus software. The results are shown in Figure 1. It was found that (1) the main mineral crystalline components of LP were calcite (CaCO3), dolomite (CaMg(CO3)2), and a small amount of quartz (SiO2). Its acidic polar group forms a stable double bond structure, and the chemical bond is not easily destroyed when exposed to water. (2) The main mineral components of BP were olivine (Mg2(SiO4)) and pyroxene (Ca(Mg,Fe)Si3O6). (3) The main mineral components of GP were quartz (SiO2), albite (Na[AlSi3O8]), and potassium feldspar (K[AlSi3O8]). The acidic quartz is normally weakly bonded with bitumen and does not easily form a strong bonding interface. their influence on the performance of asphalt mastics and interfaces, their diffraction patterns were obtained by XRD (Rigaku SmartLab), and the mineral crystal analysis was carried out using the HighScore Plus software. The results are shown in Figure 1. It was found that (1) the main mineral crystalline components of LP were calcite (CaCO3), dolomite (CaMg(CO3)2), and a small amount of quartz (SiO2). Its acidic polar group forms a stable double bond structure, and the chemical bond is not easily destroyed when exposed to water. (2) The main mineral components of BP were olivine (Mg2(SiO4)) and pyroxene (Ca(Mg,Fe)Si3O6). (3) The main mineral components of GP were quartz (SiO2), albite (Na[AlSi3O8]), and potassium feldspar (K[AlSi3O8]). The acidic quartz is normally weakly bonded with bitumen and does not easily form a strong bonding interface.

**Figure 1.** XRD diffractions of mineral fillers: (**a**) limestone powder; (**b**) basalt powder; (**c**) granite powder. **Figure 1.** XRD diffractions of mineral fillers: (**a**) limestone powder; (**b**) basalt powder; (**c**) granite powder.

#### 2.1.3. Aggregates 2.1.3. Aggregates

The cylindrical aggregate samples, made of limestone, basalt, and granite, were prepared to a size of Φ20 × 20 mm. First, a drill with an inner diameter of 20 mm was used to take a core of a large volume of the aggregate mineral, and then a high-precision doublesided cutting machine was used to cut the cylindrical core sample, ensuring a height of 20 mm. Thereafter, both surfaces of the sample were polished with sandpaper to ensure a similar texture. Finally, the aggregate samples were washed in boiling water at 100 °C and dried. Their physical properties are listed in Table 5. The technical indicators of the three aggregates met the requirements of the Chinese Specification JTG F40-2004 "Technical Specification for Highway Asphalt Pavement Construction". The cylindrical aggregate samples, made of limestone, basalt, and granite, were prepared to a size of Φ20 × 20 mm. First, a drill with an inner diameter of 20 mm was used to take a core of a large volume of the aggregate mineral, and then a high-precision double-sided cutting machine was used to cut the cylindrical core sample, ensuring a height of 20 mm. Thereafter, both surfaces of the sample were polished with sandpaper to ensure a similar texture. Finally, the aggregate samples were washed in boiling water at 100 ◦C and dried. Their physical properties are listed in Table 5. The technical indicators of the three aggregates met the requirements of the Chinese Specification JTG F40-2004 "Technical Specification for Highway Asphalt Pavement Construction".


**Properties Limestone Basalt Granite Requirement** Density/g/cm3 2.816 3.111 3.070 no less than 2.6 Crushing value/% 23.1 10.2 19.8 no less than 26 Abrasion value/% 20.2 16.6 14.3 no less than 28

Adhesion grade 5 5 3 -

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**Table 5.** Basic performance index of aggregates.

**Table 5.** Basic performance index of aggregates. *2.2. Preparation of the Specimens*

#### *2.2. Preparation of the Specimens* 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
