Innovation, Sustainability and Resilience of Asphalt Pavement MaterialsView this Special Issue
Electrolytic Manganese Residue-Modified Asphalt Performance Test and Micromechanism Analysis
The aim of this paper is to study the feasibility of using an electrolytic manganese residue (EMR) as modified asphalt. In this paper, after grinding the electrolytic manganese residue (EMR) into asphalt, the electrolytic manganese residue- (EMR-) modified asphalt was prepared with different mix ratios. The three major indicators of the modified asphalt were studied, and its modification mechanism was studied by differential scanning calorimetry, infrared spectroscopy, and atomic force microscopy. The adhesion force, surface energy, and dissipation energy of the asphalt before and after modification were analyzed by a force curve. The results show that the surface energy of the electrolytic manganese residue (EMR) is increased after grinding, the high temperature performance of the asphalt is improved, and the temperature sensitivity of the asphalt is decreased; however, the low-temperature performance is not improved obviously. When the powder oil ratio is 9%, the comprehensive performance of the asphalt is the best. The results of the infrared spectrum analysis show that the mixture of the electrolytic manganese residue (EMR) and asphalt does not produce new functional groups, and thus, the preparation method is a physical modification method. The differential scanning calorimetry (DSC) results show that the electrolytic manganese residue (EMR) can enhance the high-temperature stability of asphalt. It is found that the stability and antideformation ability of the modified asphalt improved.
Manganese is widely used in steel, nonferrous alloys, battery materials, the chemical industry, and agriculture [1–4]. China is the world’s leading producer of manganese. Manganese products mainly include electrolytic metal manganese, electrolytic manganese dioxide, manganese ferroalloy, and manganese ore. In 2018, China’s production capacity of electrolytic manganese residue (EMR) was 2.26 million tons, and the actual output was 1.4 million tons, accounting for 97% of the total output of the production capacity of electrolytic manganese in the world. Electrolytic manganese residue (EMR) is acid leaching slag produced in the process of electrolyte preparation, and it is the key pollutant generated by the electrolytic manganese industry. The grade of manganese ore in China is relatively low, and 8–10 tons of electrolytic manganese residues (EMRs) will be produced for every 1 ton of manganese produced. At present, the amount of electrolytic manganese residue (EMR) in China has exceeded 100 million tons, with an annual increase of more than 10 million tons. Electrolytic manganese residue (EMR) also has the characteristics of a high moisture content (22%∼28%), fine particles (up to 70% of particles have sizes <30 μm), a high sulfate content (20%∼30%, as calculated by SO3), and good thermal stability. The apparent density is 2039 kg/m3, and the packing density of wet slag and dry slag is 1785 kg/m3 and 1982 kg/m3, respectively. By improving the technology level of a harmless treatment and utilizing electrolytic manganese residue (EMR), reducing the storage and risk of electrolytic manganese residue (EMR) by scientific and reasonable means is being widely studied [5–8]. Therefore, the effective utilization of electrolytic manganese residue (EMR) resources can not only produce good environmental and social benefits but also bring good economic benefits to electrolytic manganese residue (EMR) plants [9–11].
At present, many studies have been carried out on the utilization of electrolytic manganese residue (EMR). Wang et al.  aimed to prepare nonsintered permeable bricks with a large amount of electrolytic manganese residue (EMR), which is discharged from the electrolytic manganese industry as raw materials. The mechanical properties and environmental properties are studied so that EMR can be successfully applied to pavement materials. Tang’s articles  also presented relevant research. Zhang et al.  studied the chemical composition, mechanical properties, hydration behavior, pore structure, and environmentally friendly properties of materials prepared with different calcium silicon ratios by using EMR and red mud as road base materials. The research results provide a direction for the large-scale and effective utilization of electrolytic manganese residue (EMR). Hou et al.  prepared Q-SACs by adding different amounts of electrolytic manganese residue (EMR) (10–45%) to study the chemical composition, basicity, setting time, and compressive strength of the mineral. The results show that the Q-SACs with electrolytic manganese residue (EMR) have a good strength, a low firing temperature, and good mechanical properties. Qin et al.  discussed the influence of electrolytic manganese slag on the road performance of an asphalt mixture. The results show that the high-temperature stability performance and corrosion resistance began to significantly increase by adding electrolytic manganese residue (EMR).
The objective of this study was to investigate electrolytic manganese residue- (EMR-) modified asphalt. Additionally, penetration tests, ductility tests, and softening point tests are important tests for future research and practical applications. Otherwise, XRD, IR, DSC, and AFM testing methods are applied to discuss the properties of the EMR-modified asphalt.
2. Experimental Design
In the test, the EMR is the water-quenched slag obtained from the ferromanganese blast furnace of the Bayi ferroalloy plant in Laibin, Guangxi. The EMR contains gray-green loose particles with an average particle size of ≤3 mm and has a density of approximately 2.89 g/cm3 and a loose bulk density of approximately 723 kg/m3. The chemical composition of the EMR is shown in Table 1.
The original EMR particles are grounded by a high-energy ball mill. After grinding for 25 min and 50 min, the diffraction analysis is carried out on the ground EMR by a Panak X’Pert Pro diffractometer in the Netherlands. The results are shown in Figure 1. In Figure 1(a), the peak intensity of the SiO2 diffraction peak obtained for the powder after grinding for 25 min is 630 a.u; in Figure 1(b), the peak intensity of the SiO2 diffraction peak obtained for the powder after grinding for 50 min is 496 a.u. With the increase in the mechanical grinding time, the intensity of the SiO2 diffraction peak obtained for the powder decreases, the crystallinity of the crystal decreases, and amorphous substances are produced at the same time. The activity of the EMR increases with the increase in the grinding time.
According to molecular dynamics and the dispersion theory , the smaller the particle size is, the larger the specific surface area of the EMR is and the larger the contact area with asphalt is; the longer the grinding time of the EMR is, the smaller the particle size is and the smaller the crystallinity is, thus enhancing the activity. The smaller the particle size is, the more uniform the distribution is and the more stable the blending performance with asphalt is. Therefore, the particle size is approximately 35 μm after ball milling for 50 min, and the EMR is mixed with an asphalt matrix.
2.1.2. Asphalt Matrix
The matrix asphalt used in this experiment is the “Donghai brand” road petroleum asphalt produced by Maoming Branch of Sinopec. The corresponding technical indicators of the matrix asphalt are in strict accordance with the technical requirements of the road petroleum asphalt (JTG F40-2004) standard. The penetration, softening point, and ductility indicators of the asphalt matrix are shown in Table 2.
2.2. Test Method
2.2.1. Physical Property Test
The major physical properties of asphalt, including penetration (temperature susceptibility, ASTM D5), softening point (high-temperature properties, ASTM D36), and ductility (low-temperature properties, ASTM D2801-95), were tested.
2.2.2. Differential Scanning Calorimetry
A DSC spectrometer (Zetzseh DSC204), which was produced in Germany, was used to determine the functional characteristics of asphalt in the experiment, in which the asphalt was heated at a rate of 10°C/min, and the nitrogen flow rate was 30 ml/min. The starting temperature was −10°C, and the ending temperature was 200°C.
2.2.3. Atomic Force Microscopy
AFM (Bruke Dimension ICON) was used to test the MA, RMMA, and RMMAFC. In the AFM experiment, a tiny and spiky tip (length: 125 μm; natural frequency: 70 kHz; spring constant: 3 N/m) was attached at the unsupported end of the cantilever and remained close to the asphalt film. Due to the close contact between the tips and the surface of the film, a repulsive or attractive force between the atoms of the top layer of asphalt and the AFM tip is created, which deflects the cantilever. The extent of the deflection absolutely depends on the force developed between the molecules of the tips and the film.
2.2.4. Fourier Transform Infrared Spectroscopy
The instrument used in the FTIR experiment is a Nicote 7T40FTIR Fourier transform infrared spectrometer (made in the United States), which was used to determine the functional characteristics of asphalt, and data was collected for the wavenumber range of 400–4000 cm−1.
2.3. Preparation of EMR Asphalt
The EMR milled for 50 min was dried to a constant mass at 105°C. An appropriate amount of asphalt was obtained and melted to the flowing state in an oven at 135°C. The modified asphalt was prepared by a high-speed shear dispersion emulsifier and heated at 160–170°C for 30 min, and the asphalt was prepared with a powder:oil ratio of 0%, 3%, 6%, 9%, 12%, and 15%.
3. Results and Discussion
3.1. Performance Test of the EMR Asphalt Mortar
3.1.1. Penetration Test
The penetration test was carried out at a series of temperatures (10°C, 15°C, 20°C, 25°C, and 30°C). The needle penetration value is shown in Table 3. With the increase in the powder oil ratio, the needle penetration value at high temperatures decreases continuously, but the change is not obvious at low temperatures. When the powder:oil ratio reaches 9%, the decrease in the amplitude of the high temperature value is larger than that obtained for the other powder:oil ratios, and then, the effect of increasing the powder:oil ratio on the needle penetration value is not obvious. The penetration index PI, the equivalent softening point , the equivalent brittle point , and the plastic temperature range are calculated by linear regression of the one-dimensional first-order equation, which was combined with the penetration values for a series of temperatures. The penetration index PI reflects the degree of penetration changing with temperature. The higher the PI, the lower the temperature sensitivity of asphalt. As shown in the table, when the PI value of the asphalt matrix is −0.9 and when the powder:oil ratio of the EMR micropowder asphalt reaches 9%, the PI first shows a peak value of −0.769; then, when the powder:oil ratio reaches 15%, the PI value again shows a peak value of −0.741 and continues to increase the powder:oil ratio, but the PI value decreases. It can be seen that when the powder:oil ratio is 15%, the temperature sensitivity of the asphalt is lower than that of the asphalt prepared with the other powder:oil ratios. The equivalent softening point reflects the high-temperature performance of asphalt. When the powder:oil ratio is 9%, the reaches the peak value of 51.1°C, which is approximately 2°C higher than that of the asphalt matrix at 49.4°C, and then slightly decreases. When the powder:oil ratio is 15%, the peak value of 51.3°C appears again. The equivalent brittle point reflects the low-temperature crack resistance of asphalt. first increased with the increase in the powder:oil ratio and then decreased to -11.8°C when the powder:oil ratio was 9%. The plastic temperature range represents the difference between and . The larger the temperature range is, the wider the temperature range is. first decreased and then increased with the addition of EMR, reaching the peak value when the powder:oil ratio was 9%, and then, ( reached the peak value again when the powder:oil ratio was 15%. The comprehensive analysis shows that the addition of EMR can improve the penetration resistance to a certain extent and can improve the high-temperature mechanical properties of asphalt; the best content is a 9% powder:oil ratio, but this ratio has no obvious effect on the low-temperature crack resistance of asphal
3.1.2. Ductility Test
The ductility reflects the extension length of asphalt at a specific temperature and tensile rate and reflects the pavement performance and low-temperature crack resistance. When the powder:oil ratio is less than or equal to 6%, the ductility of the modified asphalt and asphalt matrix is 150 cm. With the increase in the powder:oil ratio, the ductility of asphalt decreased; when the powder:oil ratio reached 15%, the ductility decreased to 87.3 cm. The results show that, with the addition of the inorganic EMR to the organic colloidal material asphalt, a whole blended structure is formed; however with the increase in the ratio of powder to oil, too much EMR lacks enough asphalt colloid to generate adhesion, so the ductility value under the external tensile action is lower than that of the original asphalt matrix.
3.1.3. Softening Point Test
The softening point reflects the high-temperature deformation resistance of asphalt. With the addition of EMR, the softening point of asphalt increased. When the powder:oil ratio is 9%, the softening point reaches the peak value of 57.9°C, which is 5.6°C higher than that of 52.3°C when the powder oil ratio is 3%; then, when the powder:oil ratio is increased, the temperature of the softening point decreases close to the performance of the matrix asphalt. The results show that the addition of EMR can reduce the temperature sensitivity of asphalt and improve its high temperature stability, which shows that the softening point temperature of asphalt increases.
3.2. Infrared Spectrum Analysis
A Thermo Nexus 470 FT-IR spectrometer was used to test the EMR and asphalt. The test range was 400–4000 cm−1, and the test was repeated 5 times. As shown in Figure 2, the peak at 1029 cm−1 in the spectrum of the EMR is the vibration absorption peak of Si-O in the EMR, and the peak at 468 cm−1 is the absorption band position of the alumina-like oxide in the corundum structure; the peaks are 2923 cm−1 and 2852 cm−1 in the spectrum of the matrix asphalt are the C-H (CH3-, –CH2-) stretching vibration bands of asphalt, the peaks at 1460 cm−1 and 1376 cm−1 are the C-H bending vibration bands of asphalt, and the peak at 1602 cm−1 is the stretching vibration band of the carbonyl (C=O) group in asphalt; the spectrum of the asphalt with a 6% powder:oil ratio is basically the same as that of the matrix asphalt, and only the stretching vibration band of the carbonyl (C=O) group shifts to 1608 cm−1; in the spectrum of the asphalt with a 9% powder:oil ratio, the stretching vibration band of C-H shifts to 2921 cm−1 and 2851 cm−1, that is to say, it moves toward the low-frequency direction, and the bending vibration band shifts to 1455 cm−1 and 1382 cm−1, moving toward the high-frequency region. According to the change in the functional group position in Figure 2 and the factors affecting the band position, it is found that the atoms of N, P, and S in the EMR form hydrogen bonds with the atoms of asphalt that do not shared electron pairs, but the polarity of the hydrogen bond that formed is also weak due to the weak polarity of the atoms of N, P, and S; the stretching vibration frequency of the weak hydrogen bond of the original bond will move toward the low-frequency direction, and the displacement is small. Meanwhile, the bending vibration of asphalt will move toward the high-frequency region, and the band is narrow, which will increase the activity of asphalt. The penetration and softening point performance indexes of asphalt are related to the relative proportion of high and low molecular weight hydrocarbons in the asphalt. After the EMR is addedto the asphalt during modification, the Al2O3 absorption band does not appear in the spectra obtained for the asphalt with 6% and 9% powder:oil ratios. According to the catalytic oxidation mechanism of petroleum asphalt, Al2O3 adsorbs the small and medium molecular weight components of the aliphatic hydrocarbons and aromatic hydrocarbons under high temperature conditions for oxidation. In the cross-linking and condensation reaction, when the high molecular weight increases, the unreacted part of Al2O3 will be replaced by a low molecular weight component and will continue to play the role of the catalytic oxidant so that the content of the high molecular weight hydrocarbon in the asphalt will increase. In addition, the distribution of the penetration value and penetration index PI will appear as a hump with the increase in the powder:oil ratio, which is consistent with the other results. In addition, as shown in the spectra of the asphalt with a 9% powder:oil ratio, the carbonyl group of the asphalt will be replaced. The shift in the (C=O) stretching vibration band to 1621 cm−1 in the high-frequency direction also indicates that the oxidation of asphalt occurs after the addition of EMR; meanwhile, the intermolecular forces increase, and the polarity increases.
The uniformity of the distribution of the modifier has a great influence on the performance and stability of the asphalt. Because of the complexity of the chemical composition of EMR, a series of physical and chemical reactions may take place after the asphalt is mixed. For the dispersion of the inorganic modifier mixed with asphalt, according to the intensity of the infrared absorption peak at the wavenumber of a specific functional group in the infrared spectra, the literature characterizes the relative amount of the inorganic modifier and asphalt and describes the uniform and stable state of the inorganic modifier mixed with asphalt by comparing whether the relative amount of the two changes linearly as the amount of the modifier changes. In this paper, the infrared absorption strength at the wavenumber 1602 cm−1 (denoted as H1602 cm−1) is used to characterize the relative amount of EMR, and the infrared absorption strength at the wavenumber 1455 cm−1 (denoted as H1455 cm−1)is used to characterize the relative amount of asphalt. The ratio of the two is used to characterize the relative content of EMR and asphalt in the modified asphalt. The results are shown in Table 4 below. At wavenumber 1602 cm−1, the infrared absorption strength of the EMR is 0.078; at 6%, the infrared absorption strength of the asphalt is 0.068, which is less than that of the EMR; at 9%, the infrared absorption strength of the asphalt is 0.083, which is greater than that of the EMR; these results are different from those of other modifiers because of the catalytic oxidation of the Al2O3 component in EMR. When the content of Al2O3 is small, the crosslinking and condensation reaction occurring between Al2O3 and asphalt takes place first to generate high molecular weight hydrocarbon. As the content of Al2O3 continues to increase, the reaction continues to generate high molecular weight material, and the infrared absorption strength increases accordingly. In addition, the unreacted surplus Al2O3 is also replaced by low molecular weight components. Therefore, although the infrared absorption strength increases, it is close to the infrared absorption strength of EMR. In addition, the linear change in the absorption strength of the two shows that the EMR can be mixed evenly in asphalt, which proves that the addition of EMR can improve the performance of asphalt.
3.3. Differential Scanning Calorimetry
The asphalt matrix, asphalt with a 9% powder:oil ratio, and asphalt with a 12% powder:oil ratio were tested by a German DSC 204 differential scanning calorimeter, as shown in the DSC curve obtained for the asphalt samples with different powder:oil ratios in Figure 3.
The base peak area of the DSC curve corresponds to the thermal effect. The DSC curve of the asphalt is shown in Table 5. The energy value of the endothermic peak is characterized by the peak area of the DSC curve. Under the same test conditions, the endothermic peak area of the EMR micropowder series asphalt is lower than that of the asphalt matrix, which indicates that the matrix asphalt changes more than the EMR-modified asphalt in the range of test temperatures. According to the peak temperature of the endothermic peak, the thermal stability of the asphalt with the 9% powder:oil ratio is the best, which is approximately 49% higher than that of the matrix asphalt; the stability of the asphalt with a 12% powder:oil ratio is the second best and is still higher than that of the matrix asphalt. The peak temperature of the endothermic peak of the asphalt matrix increases by approximately 28%, which is consistent with the three major indexes of the EMR micropowder asphalt, the penetration index PI, equivalent softening point , equivalent brittle point , and plastic temperature range . The performance of the asphalt with the 9% powder:oil ratio is better than that of the asphalt with the 12% powder:oil ratio. The results show that the heat absorption of the asphalt decreases, the high-temperature stability of the asphalt is obviously improved, and the temperature sensitivity of the asphalt is significantly improved after the addition of the EMR micropowder during the DSC test used to study the performance degradation.
3.4. AFM Analysis
3.4.1. AFM Image Analysis
Two groups of experiments were carried out on the matrix asphalt (MA) and the 9% EMR-modified asphalt (MRMA). The two-dimensional and three-dimensional microscopic phase diagram of the asphalt under AFM is shown in Figures 4 and 5. Prabir named three parts with different morphologies as the continuous phase, periphase, and bee phase. Image Pro Plus is used to process the image to highlight the contour of the bee structure; the bee structure was marked, and the area was counted, as shown in Figure 6. The statistical results of the area are shown in Table 6. The results and causes are as follows:(1)Before and after EMR modification, the microsurface of the asphalt has a bee structure. Under the influence of the EMR, the inclusion phase disappears, and the continuous phase is relatively flatter on the surface of the MRMA that it is on the surface of the MA; the number of bee structures increases significantly, the size of the bee structures increases, the difference between the sizes of the bee structures increases greatly, and some bee structure distortion and asphalt surface height differences are significant; there is a cross phenomenon between some of the bee structures, and the bee structures are still independent of each other. The bee structure accounts for more than 24% of the total surface area, which is approximately 3 times the contribution of the MA bee structure. The honeycomb structure on the surface of the MA is rare, isolated, and regular in shape. With a three-phase morphology, obvious inclusions can be observed. The inclusions occupy 52.5% of the surface area of the MA, and the continuous phase is relatively rough, with many needle-like crystals densely distributed.(2)The change in the bee-like structure of the MRMA showed that EMR could promote the formation and development of the bee-like structure. The asphaltene gum is wrapped by asphaltene to form micelles, which are suspended in the oil; the chemical stability of the gum is poor, and it easily oxidizes and condenses into asphaltene. When the slurry is cooled, the wax components gather in the gum and asphaltene and grow up gradually by using the wax center as the core; when the content of asphaltene and gum in the slurry increases, the size of the wax nodule center increases at the same time, resulting in only small wax crystals. The surface acicular crystal of the MA is a wax crystal. The extremely polar colloid can wrap around the wax crystal and prevent the growth of the wax crystal. The reduction state of the EMR makes the resin oxidize and condense into asphaltene with a high molecular weight (1 000–100 000), and the wax with a low molecular weight (below 1000) decreases its saturation concentration during cooling. After the wax crystal is unbound, the amount of wax precipitates increases under the effect of cooling crystallization and temperature stress. As a result, the number of MRMA wasps increased. Some studies have shown that the texture of the inclusion phase is hard and that of the continuous phase is soft. Therefore, it is speculated that after the disappearance of the inclusion phase in the EMR; due to the lack of binding of the inclusion phase, the bee-shaped structure develops along the long and short axes, so the size of the bee-shaped structure becomes larger.(3)The scale of the EMR particles is in the order of micrometers, and no obvious EMR particles are observed in the scale of nanometers, which indicates that the high-speed shear of the emulsifier makes the EMR evenly spread in the asphalt mortar at high temperatures. In addition, the EMR can react with the asphalt components to form a polymer with stable properties; the inclusion phase of the MRMA disappears, and the asphaltene and long-chain alkane in the mortar disappeared. The surface of the wax crystal adsorbs the polar groups in the asphaltene and the slurry, and the polar groups continue to form a double electric layer with the nearby asphaltene and the slurry. The directional dipole molecules of the double electric layer make the solvation layer wrap around the wax crystal. The inclusion phase consists of eutectic, polar micelle, and solvate layers. When the solvate layers are close to each other, there will be repulsive forces, so the inclusions do not cross each other. The EMR increases the concentration of the ionic compounds in asphalt mortar, and the ionic bond destroys the eutectic effect of the long-chain alkanes and wax crystal, causing the inclusion phase to disappear.
3.4.2. Force Displacement Curve
The microcosmic adhesion force (Fad) is composed of van der Waals force (Fvdw), capillary force (Fc), electrostatic force (FE), and chemical bond (FB) and is characterized by the lowest point of the withdrawal curve. According to the AFM test process, the asphalt sample and tip are all exposed to normal temperature air for quite a long time, so FE is zero; the silicon nitride probe does not react with asphalt, so FB is zero; the capillary force plays a role in promoting contact during the contact process and hindering separation during the withdrawal process, and the main source of microadhesion between the objects in the air is the capillary force, which is composed of hair fine action and liquid bridges. The force displacement curves of the MRMA and MA are shown in Figure 7.
3.4.3. Surface Energy and Dissipation Energy
The surface energy and dissipation energy of the MRMA and MA were calculated by the adhesion contact theory combined with the force displacement curve produced by AFM to explore the micromodification mechanism of the EMR from the point of view of work and energy [16, 17].
After the force displacement curve is obtained, we need to analyze the data, so we need to choose an appropriate theoretical model. There are many commonly used theories, but the JKR theory is suitable for high adhesion systems; in addition, the rigid requirement of the object is low. Johnson, Kendall, and Roberts use the surface energy theory to modify the Hertz theory and obtain the JKR contact mechanics model . Based on this model, the relationship between adhesion and the adhesion work is as follows:where R is the equivalent radius of the curvature of the probe tip and W is the adhesion work.
The JKR model realizes the transformation from adhesion to adhesion work, and then, according to the principle of work energy transformation [19, 20], the transformation from adhesion work to surface energy is based on the Fowkes model. The transformation relationship is shown in formulae (2) and (3):where is interface energy; is dispersion component of the surface energy; and is adhesion work between interfaces.
For materials composed of nonpolar hydrocarbons, the dispersion component plays an absolutely dominant role in the surface energy component, so . The expression of adhesion work can be approximately treated as follows:
It can be seen from formula (4) that, in the case of obtaining the probe surface energy , combined with the adhesion work between the probe and asphalt surface, the surface energy of asphalt can be calculated. According to the parameters provided by the manufacturer, the equivalent radius of the curvature of the probe is 20 nm, and the surface energy parameter is 1389.99 mJ/m2. The calculation results of the asphalt adhesion work and surface energy are shown in Table 7.
According to the data in Table 7, the following analysis can be performed:(1)The surface microadhesion of the MRMA is only 62% that of the MA. At the same time and at the same temperature, the wax is more stable than the oil, and the wax crystal causes the free radicals of the molecular chain to cross each other and form clusters; meanwhile, the long chain molecules of the oil can produce displacement and adhesion under the external load; at the same time and at the position where maximum adhesion occurs, the viscous resistance of the membrane prevents the tip from leaving the asphalt surface. Therefore, the MA is more sensitive than the MRMA to the contact of the needle tip, has stronger attraction forces, and shows greater surface adhesion. It is proved that the bee structure is the structure formed by the development of a wax nodule center, and its component is a wax component. Waxy crystallization can increase the brittleness of asphalt and lead to the decrease in adhesion. The waxy crystallization of the MRMA to the surface of asphalt results in a slight decrease in the surface low-temperature crack resistance and an increase in the internal adhesion.(2)The surface energy of the MRMA is 45% that of the MA. The increase in the asphaltene content will lead to the decrease in the surface energy; in addition, the oxidation condensation of the EMR will lead to the increase in the asphaltene content in the MRMA system, so the surface energy of the MRMA will decrease but will decrease to a lower energy than that of the MA. The decrease in the surface energy of the MRMA shows that compared with the MA, the MRMA and other substances consume less energy to form surfaces, and it is easier to form a new surface; the decrease in the surface energy of the MRMA also shows that the MRMA is easier than the MA to disperse, which is conducive to the emulsification of asphalt. The theory of solid physics points out that the surface of an object with a high surface energy is in a high energy state, and the surface of the system is unstable; thus, the surface energy is reduced through the adsorption of other substances and surface polymerization to ensure the thermodynamic stability of the system. A high surface energy is one of the reasons why the adhesion force of the MA is greater than that of the MRMA. To reduce the surface energy of the MA system, the adsorption of the probe tip is more obvious, which shows that the MA system is more unstable than the MRMA system. Therefore, the stability of the MRMA is better than that of the MA.(3)The results show that the adhesive work of the MRMA is 62% that of the MA. The MA has a higher surface energy than the MRMA, and a surface with a high surface energy tends to reduce its surface energy. High energy surfaces will absorb more dissipative energy to reduce the surface energy, causing more surface damage and plastic deformation. The MRMA has a higher surface stiffness than the MA, which will convert more external force into elastic potential energy and will release it during the withdrawal stage so that the part of external force that is converted into dissipation energy will be reduced; meanwhile, it is more difficult for a wax surface than an oil surface to produce interfacial damage and plastic deformation, so the MRMA has a lower sensitivity than the MA to external force energy. Therefore, the dissipation energy of the MRMA is lower than that of the MA. During a contact withdrawal cycle, the MA receives more dissipated energy than the MRMA. Under cyclic loading, more damage is accumulated quickly in the microstructure of the MA than in that of the MRMA; in addition, damage develops more rapidly toward the direction of structural failure, so it is easier to produce fatigue damage in the MA than in the MRMA.
Powder:oil ratios of 0%, 3%, 6%, 9%, 12%, and 15% of the EMR with a particle size of approximately 35 μm were mixed into the matrix asphalt. To compare the modification effect of the different powder:oil ratios, the penetration test, softening point test, and ductility test were carried out. At the same time, by also performing an X-ray diffraction test, an infrared spectrum analysis, an atomic force microscope analysis, and a differential scanning calorimetry analysis, the modification effect and mechanism of the EMR micropowder asphalt were explored:(1)After grinding, the crystallinity of the EMR particles decreased, and at the same time, amorphous substances were produced, the specific surface area of the powder increased, and the activity of the powder increased.(2)The high-temperature performance of the asphalt improved, and the temperature sensitivity of the asphalt reduced after the EMR was added. However, the low-temperature performance of the asphalt was not significantly improved. The comprehensive performance of the asphalt was the best when the powder:oil ratio was 9%.(3)The N, P, S, and other atoms in the EMR formed weak hydrogen bond with the atoms in the asphalt, which did not share the electron pair, so the activity of the asphalt was enhanced. At the same time, the Al2O3 in the EMR adsorbed the small and medium molecular components of aliphatic hydrocarbons and aromatic hydrocarbons for oxidation, cross-linking, and condensation reactions at high temperatures. When the high molecular weight increased, the unreacted part of Al2O3 was replaced by the low molecular weight part and continued to play the role of the catalytic oxidant and increased the content of the high molecular weight hydrocarbon in the asphalt component to improve the performance of the matrix asphalt.(4)The compatibility between the EMR and asphalt is good, so it is more suitable to describe the microadhesion contact of asphalt materials with the JKR theory; a high surface energy will make asphalt absorb more dissipation energy, so the surface energy will decrease, resulting in the instability of the system; EMR can reduce the surface energy of the MRMA by increasing the asphaltene component, so the stability of the MRMA is better than that of the MA.
The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.
Conflicts of Interest
The authors declare no conflicts of interest.
The research was supported by National Natural Science Foundation of China. (Grant no. 51768016) and Innovation Project of Guangxi Graduate Education, China (Grant no. YCSW2020043).
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