Simulation study on basic road performance and modi ﬁ cation mechanism of red mud modi ﬁ ed asphalt mixture

: In order to study the basic road performance and the mechanism of red mud modi ﬁ ed asphalt, Marshall test, rut test, freeze-thaw splitting test, and immersion Marshall test were carried out for matrix asphalt mixture and red mud modi ﬁ ed asphalt mixture, and molecular simulation technology was introduced to study the mechanism of red mud modi ﬁ ed asphalt. The results showed that the penetration rate of red mud modi ﬁ ed asphalt was lower than that of the matrix asphalt, and the softening point of asphalt was signi ﬁ cantly improved after the addition of red mud, and the mixing of red mud reduced the asphalt ductility greatly. The intensity ratio (test strength ratio; TSR) of the freeze-thaw test of red mud modi ﬁ ed asphalt under 3, 5, 7, 9, and 11% was increased by 13.18, 11.22, 13.54, 12.01, and 8.45%, respectively, compared with the matrix asphalt mixture, and the dynamic stability value increased by 127.23, 176.87, 200.99, 96.23, and 12.95%; the residual stability value of the immersion Marshall test increased obviously. Generally, the research of red mud modi ﬁ ed asphalt is quite worthy, it can solve the problem of aluminum ore slag and reuse of resources, and provide a new way to extend the industrial chain.


Introduction
Red mud is an industrial waste produced by alumina extraction in aluminum industry.With the vigorous development of aluminum industry, the discharge of red mud in China has been continuously increasing, causing serious environmental disasters.Therefore, how to minimize the harm of red mud and maximize the efficiency of red mud treatment and utilization has become extremely urgent [1].At present, the organic polymer modified asphalt material and SBS modified asphalt are widely used at home and abroad [2].However, the macromolecule polymer modified technology is not mature in China, and the cost is high, which greatly limits the development of modified asphalt in China.In recent years, domestic researchers have found that inorganic material modifiers [3][4][5][6] can not only improve the interface between asphalt and mineral materials, but also have the advantages of simple production process, low price, excellent performance, and abundant reserves.Sun et al. [4] selected seven kinds of inorganic nano-powders to modify asphalt, and found that silica powder had the best modification performance on asphalt.And through carrying out test on prepared modified asphalt mixture with silica, silane coupling agent, and stearic acid, it was found that nano-silica could significantly improve the high temperature stability and water stability of asphalt mixture.Zhang et al. [6] prepared unsaturated polyester resin (UPR)/bismaleimi-dodiphenyl methane (BDM) modified asphalt.Through tensile, viscosity, and fluorescence microscope tests, the best formula of BDM/UPR modified asphalt was determined.The results showed that compared with pure UPR modified asphalt, the tensile strength of BDM/UPR modified asphalt was 27.2% higher, the high temperature performance was significantly improved, and the low temperature performance was slightly poor but still better than epoxy asphalt.Zhao et al. [7] investigated the influence of warm mixing additives Sasobit and Deurex on amorphous poly alpha olefin modified asphalt mixture.Through mechanical property tests and long-term aging tests on the mixture, they found that an appropriate amount of Sasobit and Deurex had a significant impact on the viscosity and temperature sensitivity of asphalt, and also had a positive impact on its long-term aging performance.
Molecular simulation technology is a new technology for computer simulation test of model molecules, which has been widely used in asphalt field.Yao et al. [8] discussed the sensitivity of water in nano-hydrated lime modified asphalt mixture, and found that the polar groups of carbonyl groups indicated the degree of oxidation of asphalt binder.At the same time, the results of FTIR test show that carboxylic acid and ketone were the main aging products in asphalt.These two carbonyl groups were related to the rutting resistance and water sensitivity of asphalt mixture.The results of molecular simulation show that asphalt aging was helpful to reduce the water damage in asphalt mixture.Moreover, Yao et al. [9] used molecular simulation technology to verify the asphalt model.By calculating the density and viscosity of asphalt molecules and comparing with laboratory data, they finally obtained the volume modulus of the asphalt molecule model.In addition, many scholars at home and abroad have widely applied molecular simulation technology to asphalt materials [10][11][12][13][14].
Members of our research group have found through previous studies [15][16][17][18] that the addition of red mud could improve the consistency of matrix asphalt, foster the ability of asphalt to resist deformation, and enhance the high-temperature performance of matrix asphalt, but the road performance of red mud modified asphalt mixture has not been studied.Therefore, in the present study, Marshall test, rut test, freeze-thaw splitting test, and immersion Marshall test were conducted on red mud modified asphalt mixture with different dosages, and then molecular simulation technology was applied to conduct in-depth analysis of the mechanism of red mud modified asphalt.

Test raw material 2.1 Properties of materials
Asphalt: The matrix asphalt used in this experiment was "Donghai Brand" 70 # Class A road petroleum asphalt produced by Sinopec Maoming Branch.All indicators met the requirements of the Specification Test Procedures for Asphalt and Asphalt Mixture of Highway Engineering (JTG E20-2011), and the basic performance indicators are shown in Table 1.
Red mud: The industrial waste residue produced from Bayer process industrial alumina production of Guangxi Pingguo Aluminum Plant was used to conduct the test, as shown in Figure 1.Table 2 shows the main chemical composition content of red mud.

Preparation of red mud modified asphalt
Red mud: The red mud raw materials taken from the red mud storage yard were put into the oven and dried at 80°C for 24 h; after simple grinding and passing through 200 mesh sieve, they were bagged for standby.
The base asphalt was heated to 140°C, and the treated red mud was added into the matrix asphalt.The external adding method was adopted (3, 5, 7, 9, and 11%) and the high-speed shear machine was used to prepare the red

Mineral powder
The limestone powder produced by a mineral powder plant in Guangxi was adopted in this test and the corresponding index tests were carried out on the limestone powder.All test results were in line with the requirements of Test Specification for Aggregates of Highway Engineering (JTG E42-2005) as shown in Table 5.

Mineral aggregate gradation design
AC-13C continuous dense graded asphalt mixture was selected, with the critical sieve size of 2.36 mm and a pass rate of less than 40%.In combination with the comparison table of AC-13C grading control of surface asphalt mixture inside and outside Guangxi region, AC-13C asphalt concrete mineral aggregate gradation was determined as shown in Table 6.
3 Basic road performance of red mud modified asphalt mixture

Basic properties of red mud modified asphalt
The index of the dose of 3, 5, 7, 9, and 11% red mud modified asphalt (the red mud and asphalt were fully mixed) were tested and compared with the matrix asphalt (for the matrix asphalt, the same treatment was conducted to get a blank sample), the experimental results are shown in Table 7.As can be seen from Table 7: (1) At the same temperature, the penetration of red mud modified asphalt with different contents was lower than that of matrix asphalt, indicating that the addition of red mud could improve the consistency of base asphalt and the ability of asphalt to resist deformation; (2) the addition of red mud could significantly increase the softening point of asphalt, which reached the maximum value of 59.5°C when the content was 9%; and (3) the asphalt ductility was substantially reduced by adding red mud.It can be concluded that the asphalt modified with red mud was hardened, the resistance to deformation was enhanced, the temperature sensitivity was notably reduced, and the high temperature performance was also remarkably improved.

Determination of optimum asphalt content
The Marshall test mix proportion was designed in accordance with the Test Code for Asphalt and Asphalt Mixture of Highway Engineering (JTG E20-2011).First, the oil ratio of an asphalt mixture was estimated as the median oil ratio, and five different oil ratios were selected according to a certain interval between symmetric forward and backward to prepare standard Marshall specimens.Second, the density of the mixture of the prepared specimen was measured according to the specification requirements.Third, the mixture voidage ratio (VV), mineral aggregate clearance ratio (VMA), and asphalt saturation (VFA) were calculated by the density data.Fourth, the Marshall test of the mixture was conducted to determine the stability and flow value of the mixture under each oil ratio.Finally, the optimal oil-stone ratio of the AC-13 asphalt mixture was determined.The test results are shown in Table 8.
According to the test index curve and the specification requirements, the optimal asphalt content for this test was finally determined as optimum asphalt content (OAC) = (OAC 1 + OAC 2 )/2 = 3.99%.And the optimal asphalt content for each group of samples was determined by the same method.The results are shown in Table 9 below.As can be seen from Table 10, compared with the base asphalt mixture, the dynamic stability value of red mud modified asphalt mixture with five different contents increased by 127.23, 176.87, 200.99, 96.23, and 12.95%, respectively, indicating that the addition of modifier red mud could enhance the high temperature stability of asphalt mixture to some extent.When the red mud content was within the range of 0-7%, the dynamic stability value increased continuously.When the red mud content was 7%, the dynamic stability of the mixture reached the maximum.This was due to the characteristics of the red mud itself, which enabled it to be dispersed in the mixture to form a mortar by absorbing asphalt, so that the modified asphalt mixture can form a more stable cement, thus increasing the high temperature resistance of the mixture.While when the content of red mud was within the range of 7-11%, the dynamic stability decreased significantly.This was because the solubility of asphalt mixture to red mud was limited.When the red mud adhesion reached saturation in the asphalt, the excess red mud would agglomerate, causing uneven dispersion of red mud in asphalt, making the skeleton of the mixture form a certain strength weakness, thus resulting in the dynamic stability value to decline.eight specimens in each group, which were numbered and divided into two groups.The first group was taken out after soaking in 25°C water for 2 h.After vacuum saturation, the second group was placed in the refrigerator at −18°C for more than 16 h, and then placed in the constant temperature water tank at 60°C for more than 24 h.Next the two groups of specimens were placed in a constant temperature tank at 25°C for no less than 2 h.Then, the two groups of specimens were subjected to the splitting test with the asphalt mixture stability meter at the loading rate of 50 mm/min.The freeze-thaw splitting test results of asphalt mixture are shown in Table 11.

Water
As can be seen from Table 11, compared with the base asphalt mixture, the freeze-thaw splitting test strength ratio (TSR) increased by 13.18, 11.22, 13.54, 12.01, and 8.45%, respectively.With the increase in red mud content, the strength ratio of freeze-thaw splitting test fluctuated, but on the whole, it showed a trend of increasing first and then slowly decreasing.When the red mud content was 3%, the first maximum value of TSR appeared; then, when the content of red mud was 3-7%, the strength ratio of freeze-thaw splitting test decreased first and then increased.When the content of red mud was 7%, the second maximum value appeared, which was the maximum value, indicating that the addition of red mud modified asphalt could improve the water damage resistance of the mixture, that is, improve the water stability of asphalt mixture.

Immersion Marshall test of asphalt mixture
In this test, standard cylindrical forming specimens made by the same standard compaction method as the standard Marshall test were adopted.The difference was that the immersion Marshall specimens were held for 48 h in a constant temperature water tank at 60°C, and the other steps were consistent with the Marshall test.The immersion Marshall test results of six groups of asphalt mixture samples are shown in Table 12.
As can be seen from the test results in Table 12, the residual stability value of the five groups of mixture mixed with red mud modified asphalt in immersion Marshall test increased, respectively, by 2.51, 2.90, 4.26, 4.50, and 3.91% compared with the matrix asphalt mixture.The residual stability value of immersion Marshall test increased first and then decreased with the increase in red mud content.When the red mud content was 0-9%, the residual stability of the immersion Marshall test increased constantly, and reached the inflection point when it reached 9%.Then, the residual stability of immersion Marshall test decreased when the red mud content ranged from 9 to 11%.The change in the overall curve indicated that when the red mud content was 9%, the residual stability of asphalt mixture under immersion Marshall test reached the maximum.Therefore, it can be drawn that the addition of red mud modifier could improve the water stability of asphalt mixture to a certain extent, and the optimal red mud content was 9%.
4 Molecular simulation of red mud modified asphalt mechanism based on Monte Carlo method

Model construction
Based on the previous research results on the composition of red mud, hematite with the highest content was selected for the study [19,20].
The key to the success of molecular simulation is the effective establishment of molecular model.There are two main methods about asphalt modeling: one is the average molecular method, and the other is the assembly method.Asphalt contains about 105-106 chemical components, it is difficult to construct its complete structure by the current technical means.In this study, asphalt was divided into three groups: asphaltene, oil, and gum, and the most representative molecular model of three-component asphalt was constructed.
The asphaltene molecular model was selected from the average molecular structure obtained by Artok et al. [21] through NMR analysis; the colloidal molecular model was represented by 1,7-dimethylnaphthalene molecules with similar molecular structure to the early stage of asphaltene formation [22]; the oil molecular model was selected from C22H46 [23,24], which was consistent with the softening point and boiling point of most asphalt oil.The three-component structure of asphalt [25] is shown in Figure 2.
Sorption module in MS software is a tool to build amorphous model by Monte Carlo method, which can be used to build composite material model, small molecular solution model, pore filling model, and polymer blending model with various components at different mix ratios.The hematite structure was directly imported from the Import file.Before the surface adsorption simulation, the optimized hematite structure was cut along the (0 0 1) surface by using the command under the build menu in MS software, and the cut model was placed in a box with periodic structure to establish a vacuum layer.The threecomponent structure of asphalt was used separately, and the adsorption calculation was performed under the sorption module, with the task selected as Fixed loading, the temperature set to 298 K, the calculation method selected as Metropolis, the force field selected as Universal force field, the electrostatic interaction selected as Ewald & Group for processing, and the van der Waals interaction selected as Atom Based, while the calculation is carried out for 3,000,000 Monte Carlo steps for equilibrium and analysis.After completing the simulation based on the above Performance and modification mechanism of red mud modified asphalt mixture  7 Figure 3: Hematite adsorption asphalt model and adsorption energy map (followed by hematite adsorption glial, asphaltene, and oil).calculation details, the equilibrium structure and relevant adsorption information of asphalt adsorbed on two crystalline surfaces can be obtained.

MC simulation results and discussion
The adsorption sites of different components on the red mud are shown in Figure 3.A small amount of asphaltene molecules were gathered between the red mud vacuum layers, and the colloid almost barely adhered to the red mud surface mainly by adhesion, while oil was most concentrated in the vacuum layer.It can be seen from Figure 3 that the structure of red mud was not damaged due to the addition of asphalt components, which implied that red mud modified asphalt belonged to physical modification.From the energy distribution diagram of asphaltene, after adsorption of colloid and oil in red mud, it can be concluded that the adsorption site distribution of colloid was narrower than that of asphaltene and oil, which indicated that the adsorption of red mud on oil was relatively average, followed by asphaltene, and the colloid was only adsorbed at a specific location, but its adsorption energy was the largest.This might be related to the polarity of red mud and asphalt components, similar to the principle of similar phase dissolution in chemistry.The simulation results also confirmed the conclusion drawn in the test that red mud could absorb the oil in the asphalt, thus enhancing the adhesion between the two and further improving the performance of the asphalt.

Conclusion
By conducting Marshall tests, rutting tests, freeze-thaw splitting tests, and immersion Marshall tests on modified asphalt mixtures with different red mud content, and introducing molecular simulation technology, the mechanism of red mud modified asphalt was studied.The following conclusions have been drawn: (1) The penetration of red mud modified asphalt with different contents was lower than that of matrix asphalt, indicating that the addition of red mud could improve the consistency of base asphalt and the ability of asphalt to resist deformation; the addition of red mud could significantly increase the softening point of asphalt and the asphalt ductility was substantially reduced; (2) the high temperature stability test results of red mud modified asphalt shows that when the content of red mud was 7%, the dynamic stability value of the modified asphalt mixture would be 200.99%higher than that of the base asphalt mixture, which indicated that the addition of the modifier red mud could enhance the high temperature stability of the asphalt mixture to a certain extent; (3) the results of freeze-thaw splitting test show that when the content of red mud was 7%, the freeze-thaw splitting TSR of modified asphalt mixture reached 90.42%, which indicated that the addition of red mud could improve the water damage resistance of asphalt mixture; (4) immersion Marshall test results of red mud modified asphalt show that when the red mud content was 7%, the residual stability of modified asphalt mixture immersed in water reached 93.74%, indicating that the addition of modifier red mud could foster the water stability of asphalt mixture; and (5) Molecular simulation technology was used to analyze the adsorption of asphalt on red mud.The results showed that the hematite composition in red mud had the most adsorption sites for oil in asphalt, followed by asphaltene, and colloid had the least adsorption site but had the highest adsorption capacity.Specific adsorption capacity of asphalt molecular components in red mud needs further study.
Performance and modification mechanism of red mud modified asphalt mixture  3

Table 1 :
Basic technical properties of matrix asphalt

Table 2 :
Main chemical composition contents of red mud

Table 3 :
Limestone aggregate sieving results

Table 4 :
Limestone coarse aggregate technical indicators and requirements

Table 5 :
Mineral powder technical indicators test results

Table 6 :
AC-13 asphalt mixture gradation designAccording to the test method of T0719-2011 in the Test Code for Asphalt and Asphalt Mixture of Highway Engineering (JTG E20-2011), 300 mm × 300 mm × 50 mm rut specimens were prepared with the mixture by the wheel milling method; after rut specimens were rolled and formed, they were cooled at room temperature for more than 12 h, and then kept in oven at 60°C for 5 h; the rutting test was carried out at the test temperature of 60°C and the wheel pressure of 0.7 MPa.The rutting test results of modified asphalt mixtures with different red mud contents are shown in Table10.

Table 7 :
Test results of performance index of modified asphalt with different contents of red mud

Table 8 :
Matrix asphalt mixture Marshall test results

Table 9 :
OAC results of the asphalt mixture

Table 10 :
Rutting test results for different asphalt mixtures

Table 11 :
Results of freeze-thaw splitting test of red mud modified asphalt mixture

Table 12 :
Immersion Marshall test results of red mud modified asphalt mixture