A molecular dynamics-based geopolymer-RAP interface model and its application

Abstract

The present invention discloses a geopolymer-RAP interface model and application based on molecular dynamics. The model is based on the molecular dynamics geopolymer-asphalt-aggregate composite interface model, and calculates the interface interaction energy, interface diffusion characteristics and interface adhesion mechanism according to the interface model. Starting from the atomic level, the model calculates the interface characteristics between geopolymer-aged asphalt and aged asphalt-aggregate, and simulates the influence of RAP with different aging degrees on the interface bonding performance. The obtained model has a simple structure, requires less computing power, and the obtained simulation data has an excellent fit with the test results. Based on the interface model established by the present invention, the interaction, diffusion and mechanical properties between geopolymer and recycled aggregate can be analyzed in detail from an atomic perspective. After comparison, the results obtained by model simulation are highly consistent with the sample test results, which has an important guiding role in the improvement of geopolymer-recycled aggregate.

Description

Technical Field

[0001] The present invention relates to a geopolymer-RAP mixture model, in particular to a geopolymer-RAP interface model constructed based on molecular dynamics and its application, belonging to the technical field of solid waste utilization. Background Art

[0002] With the rapid development of road construction, previously constructed roads are gradually entering the maintenance and repair phase. To fully utilize the solid waste generated by road maintenance and implement the concept of green maintenance, some scholars have proposed mechanically crushing waste asphalt pavement panels to produce recycled aggregate (RAP). This material, combined with cementitious materials, can be used in road base layers to recycle solid waste resources and save production resources and costs. Currently, cement is widely used as a cementitious material to stabilize RAP. Cement production consumes a large amount of resources and contributes 6%-8% of global CO2 emissions, causing significant environmental damage. Therefore, the development of cement faces significant challenges. To achieve sustainable development, the search for new green cementitious materials to replace cement is becoming a trend. Geopolymers are inorganic cementitious materials that utilize siliceous and aluminous raw materials such as metakaolin, slag, and fly ash, which are activated by alkali (typically NaOH, KOH, or Na2SiO3) to form a three-dimensional network structure. Geopolymers offer advantages such as high early strength, excellent high-temperature resistance, corrosion resistance, impermeability, and frost resistance. Furthermore, geopolymer production consumes very little energy, accounting for only 30% of the energy consumed in ordinary cement production. Furthermore, compared to cement, geopolymer production reduces CO2 emissions by 80%. Therefore, geopolymer-stabilized RAP is a promising road base construction material.

[0003] The interfacial bonding between geopolymer and RAP is a key factor affecting the mechanical properties of geopolymer-stabilized RAP substrates. In recent years, relevant researchers have conducted a number of experimental studies on the geopolymer-aggregate interface. (Edyta Pawluczuk (Geopolymer Concrete with Treated Recycled Aggregates: Macro and Microstructural Behavior. Journal of Building Engineering 2021, 44, 103317.) et al. used SEM-EDS to study the interface transition zone between recycled aggregate and geopolymer and found that after thermomechanical treatment, the coarse recycled aggregate formed aluminosilicate hydrated gel (CSH) and calcium aluminate silicate hydrated gel (CASH), which had a very favorable effect on the mechanical properties of geopolymer concrete. Sireesh Saride (Micro-Mechanical Interaction of Activated Fly Ash Mortar and Reclaimed Asphalt Pavement Materials. Construction and Building Materials 2016, 123 , 424–435.) proposed that since RAP is covered by an amorphous asphalt coating, the number of RAP particles on the exposed surface plays a major role in the formation of pozzolanic compounds with fly ash, affecting the interaction between RAP and other components in the composite.

[0004] However, existing research has primarily focused on exploring the mechanical mechanisms of geopolymers and recycled aggregates. The impact of geopolymer composition on chemical bonding has not been investigated, and there is a lack of a clear understanding of the interactions between organic and inorganic composites. Molecular dynamics simulations, which analyze and calculate interfaces at the atomic scale, are widely used to study the interfacial properties of various composite materials. Zhang (Molecular Dynamics and Experimental Study on the Adhesion Mechanism of Polyvinyl Alcohol (PVA) Fiber in Alkali-Activated Slag / Fly Ash. Cement and Concrete Research 2021, 145 , 106452.) et al. used MD simulation to simulate the adhesion of PVA with different Ca / (Si+Al) and Al / Si ratios to C(N-)ASH gels to study the adhesion mechanism of polyvinyl alcohol (PVA) fibers in alkali-activated slag / fly ash (AASF) matrix. Guan (Molecular Dynamic Simulations of InterfacialInteraction Mechanism between the NASH Gels and the Polyethene Fiber. Construction and Building Materials 2022, 349, 128769.) et al. calculated the atomic interactions between NASH gel and polyethylene chains, revealing hydrogen bonding, van der Waals forces, and mechanical interlocking at the interface. Wang (Molecular Dynamics Simulation Study on Interfacial Shear Strength between Calcium-Silicate-Hydrate and Polymer Fibers. Construction and Building Materials 2020, 257 , 119557.) et al. studied the shear strength, structure, and dynamics of the interface between calcium silicate hydrate (CSH) and polymer fibers, finding that Ca atoms at the interface play a significant role in interfacial bonding interactions. Currently, there is no atomic-level study of the interfacial properties of geopolymer-stabilized RAP systems, with only limited exploration of polymer composites, such as geopolymer-aggregate and asphalt-aggregate interfacial interactions. Kai (Understanding Geopolymer Binder-Aggregate Interfacial Characteristics at Molecular Level) Cement and Concrete Research 2021) et al. established a molecular model to study the form of geopolymer-aggregate interface bonds and interface fracture mode, and proposed that increasing the Si / Al ratio of geopolymer can reduce ITZ density, and reducing the Si / Al ratio can increase interface strength.

[0005] In addition, nanoindentation technology is an important means to quantitatively evaluate ITZ properties and has been used to study hardness, elastic modulus, material stiffness, etc. Li (Nanoscratch on Mechanical Properties of Interfacial TransitionZones (ITZs) in Fly Ash-Based Geopolymer Composites. Composites Science and Technology2021) et al. used nanoindentation technology to study the mechanical properties of the ITZ of geopolymer concrete and found that the hardness of the ITZ of geopolymer concrete with an Si modulus of 1.0 was significantly higher than that of the cementitious material. However, nanoindentation tests are affected by errors such as rough sample surface, misjudgment of the contact zero point, and indenter blunting. Molecular dynamics provides indentation simulation at the molecular scale, eliminating the need for extensive testing time and expense and eliminating experimental errors. Mohammad Rafat Sadat (Atomic-Scale Dynamics and Mechanical Response of Geopolymer Binder under Nanoindentation. Computational Materials Science 2018, 142 , 227–236) et al. used molecular dynamics simulation to investigate the mechanical response of geopolymers under nanoindentation with a spherical indenter. They found that the hardness of geopolymers increased with decreasing indenter size and increasing loading rate. Increasing the Si / Al ratio increased the hardness and Young's modulus of geopolymers. Du (Molecular Investigation on the Adhesion and Deformation Behaviors of AsphaltBinder under Nanoindentation. Construction and Building Materials 2021, 295 ,123683.) Nanoindentation of asphalt using a quartz indenter was performed on asphalt using molecular dynamics simulation to study the adhesion of asphalt at the nanoscale.

[0006] The degree of RAP aging affects the adhesion between geopolymers and recycled aggregates. Aged asphalt forms a thin film at the geopolymer-aggregate interface, and changes in asphalt aging cause changes in its viscosity and stiffness. Therefore, studying the effect of asphalt aging on geopolymer-stabilized RAP interfaces is of great significance. Several studies have examined the effects of aging on asphalt-aggregate interfaces, such as Hu (Atomic Mechanisms of Separation Failure at the Asphalt–Aggregate Interface and Its Dependence on Aging and Rejuvenation: Insights from Molecular Dynamics Simulations and DFT Calculations). Applied Surface Science 2022, 598, 153775.) et al. studied the fracture properties of asphalt-aggregate interfaces and the effects of aging on them. Severe aging caused asphalt-aggregate debonding. The aged interface had higher fracture resistance than the original interface, but its fatigue resistance decreased. Wang (Nanomechanical Characteristics of Interfacial Transition Zone in Nano-Engineered Concrete. Engineering 2021) et al. used atomic force microscopy to study the microscopic characteristics and mechanical properties of asphalt mixtures at different degrees of aging. Asphalt aging significantly increases the spatial variability of asphalt materials at the nanoscale, and mild aging increases asphalt cohesion and adhesion at the asphalt-aggregate interface. However, the effect of RAP aging on the bond between geopolymers and RAP has not been studied, and the bonding mechanism remains unclear. This makes it difficult to conduct targeted performance analysis of geopolymer-RAP mixtures in engineering projects using existing models, and extensive experiments and testing are still required to determine the mechanical properties of the mixture. Summary of the Invention

[0007] In response to the problems existing in the prior art, the first purpose of the present invention is to provide a geopolymer-RAP interface model based on molecular dynamics. Starting from the atomic level, it calculates the interface characteristics between geopolymer-aged asphalt and aged asphalt-aggregate, and simulates the influence of RAP with different aging degrees on the interfacial bonding performance. The resulting model has a simple structure, requires less computing power, and the obtained simulation data has an excellent fit with the experimental results.

[0008] A second objective of the present invention is to provide an application for constructing a molecular dynamics-based geopolymer-RAP interface model for calculating the interfacial bonding and nanomechanical properties of asphalt mixtures. This interface model allows for detailed atomic-level analysis of the interaction, diffusion, and mechanical bonding between geopolymer and recycled aggregate. Comparisons show that the model simulation results closely match those of sample tests, fully reflecting the nanomechanical properties of the samples and providing valuable guidance for improving geopolymer-recycled aggregate mixtures.

[0009] To achieve the above technical objectives, a geopolymer-RAP interface model based on molecular dynamics is constructed, characterized by comprising:

[0010] 1) Optimize the molecular model of geopolymer based on NASH gel structural components;

[0011] 2) Develop and optimize the component aggregate model based on the main components of the aggregate;

[0012] 3) Construct and optimize asphalt models of asphalts with different aging degrees based on the AAA-1 asphalt model;

[0013] 4) constructing and optimizing a composite interface model based on the geopolymer molecular model, aggregate model, and asphalt model obtained in 1), 2), and 3), wherein the composite interface model includes a geopolymer-asphalt interface model and an asphalt-aggregate interface model;

[0014] 5) The interfacial interaction energy, interfacial diffusion characteristics, and interfacial adhesion are calculated based on the composite interface model, and the calculated results are fitted and optimized with the experimental test results.

[0015] As a preferred solution, the Si / Al ratio in the geopolymer molecular model is 1.5-2, the number of geopolymer molecules in the simulation box is 35-45, and the density is set to 1.5-2 g / cm 3 .

[0016] As a preferred solution, the geopolymer molecular model is optimized by geometric optimization, and after optimization, a 500 ps dynamic simulation is performed under an NVT ensemble.

[0017] As a preferred solution, the aggregate model is a surface hydroxylated silica model.

[0018] As a preferred solution, the geopolymer molecular model and aggregate model are fitted using the clay force field established by Cygan.

[0019] As a preferred solution, the force field of the asphalt model adopts the CVFF force field.

[0020] As a preferred solution, the asphalt model includes: an unaged asphalt model, a short-term aged asphalt model and a long-term aged asphalt model; the asphalt model construction process is:

[0021] i) Introduce asphalt component molecules into the simulation box and set the initial density to 0.08~0.12g / cm 3 , get the initial asphalt model;

[0022] ii) The initial asphalt model is energy minimized using the steepest descent method and the conjugate gradient method, and is relaxed through the NVT ensemble and the NPT ensemble in turn.

[0023] As a preferred solution, the construction process of the composite interface model is: importing the geopolymer model-asphalt model-aggregate model into LAMMPS for simulation, and then fusing them for 1ns under the NPT ensemble and then performing relaxation and data analysis under the NVT ensemble in turn.

[0024] As a preferred solution, the interfacial interaction energy includes aggregate-asphalt interfacial interaction energy and geopolymer-asphalt interfacial interaction energy.

[0025] As a preferred solution, the calculation formula for the aggregate-asphalt interface interaction energy is:

[0026] Formula 1: ;

[0027] The calculation formula of the geopolymer-asphalt interface interaction energy is:

[0028] Formula 2: ;

[0029] In formulas 1 and 2: is the aggregate-asphalt interface interaction energy in equilibrium state, with the dimension of mJ / m 2 ; is the surface energy of aggregate, with the unit of mJ / m 2 ; is the surface energy of asphalt, with the unit of mJ / m 2 ; is the geopolymer-asphalt interfacial interaction energy in equilibrium state, with the dimension of mJ / m 2 ; is the surface energy of the geopolymer, with the dimension of mJ / m 2 ; is the surface energy of asphalt, with the unit of mJ / m 2 .

[0030] As a preferred solution, the interfacial diffusion characteristics are expressed by calculating the diffusion coefficients of asphalt on the aggregate surface, inside the asphalt molecules, and on the geopolymer surface.

[0031] As a preferred solution, the calculation process of the mean square displacement is as follows:

[0032] Formula 3: ;

[0033] Formula 4: ;

[0034] The interfacial adhesion properties are calculated by the following formula:

[0035] Formula 5: ;

[0036] Formula 6: ;

[0037] In formulas 3 to 6: is the position vector of particle i at time t, dimensionless; is the position vector of particle i at the initial moment, dimensionless; MSD is the mean square displacement, dimensionless; N is the total number of particles to be averaged, dimensionless; a is the slope of the MSD linear fitting line, dimensionless; D is the diffusion coefficient, dimensionless; is a constant that takes the value 1 in the interval [- w, w) (where w is the bin width) and 0 otherwise; g(r) is the radial distribution function, which is dimensionless; and n(r) is the coordination number, which is dimensionless.

[0038] As a preferred solution, the geopolymer-RAP interface model also includes constructing a nanoindentation model, the process of which is: using a rigid indenter to perform nanoindentation treatment on different positions of the composite interface model in the longitudinal direction at 0.1~0.5Å / Ps, and the indentation depth is 20~30Å.

[0039] As a preferred solution, the rigid indenter radius is 30-35 Å.

[0040] As a preferred solution, a fixed layer for fixing the model is provided at the bottom of the composite interface model, and the thickness is 8 to 12 Å.

[0041] As a preferred solution, the calculation process of the nanoindentation model is:

[0042] Formula 7: ;

[0043] Formula 8: ;

[0044] Formula 9: ;

[0045] Formula 10: ;

[0046] Formula 11: ;

[0047] Formula 12: ;

[0048] In formulas 7 to 12: H is the hardness, the dimension is GPa; is the maximum load, the dimension is nN; A is the contact area, the dimension is Å 2 ; r is the indenter radius, dimension is Å; is the actual indentation depth, with the dimension of Å; is the maximum indentation depth, with the dimension of Å; is the residual indentation depth, with the dimension of Å; E is the modulus of the sample, with the dimension of GPa; v is the Poisson's ratio of the sample, dimensionless; is the modulus of the rigid indenter, in GPa; is the Poisson's ratio of the rigid indenter, dimensionless; is a dimensionless parameter; S is the slope of the initial segment of the unloading curve, which is dimensionless.

[0049] As a preferred solution, the process of the test is:

[0050] Metakaolin and slag were mixed at low speed for 2 minutes to form a homogeneous solid mixture. Then, an alkali activator was added and stirred at low speed for 2 minutes and high speed for 2 minutes to prepare a fresh geopolymer slurry. Two types of samples were then prepared using this geopolymer slurry:

[0051] i) Pour the geopolymer slurry into a cubic mold, add natural aggregate or recycled aggregate when the amount is half, and then inject the remaining geopolymer slurry. After vibrating and curing in a curing box, a geopolymer-aggregate sample is obtained;

[0052] ii) cutting the interfaces between the geopolymer-natural aggregate sample and the geopolymer-recycled aggregate sample, and performing electron microscopy and energy dispersive spectroscopy analysis on the interfaces;

[0053] iii) The geopolymer is mixed with aggregate to prepare cylindrical samples of geopolymer-stabilized natural aggregate or recycled aggregate. After sealing and curing, the samples are subjected to a 7-day unconfined compressive strength test.

[0054] As a preferred solution, the proportions of the components in the geopolymer are: slag ratio is 0.5-0.6, alkali equivalent in the alkali solution is 6-10%, water glass modulus ratio is 1.2-1.5, and water-solid ratio is 0.3-0.5.

[0055] As a preferred solution, the mass ratio of the geopolymer to the aggregate is 1:15-20.

[0056] As a preferred solution, the mass percentage of the recycled aggregate in the total aggregate is 25-35%.

[0057] The present invention also provides an application of a polymer-RAP interface model based on molecular dynamics, which is used to calculate the interface properties and nanomechanical properties of asphalt mixtures.

[0058] Compared with the prior art, the beneficial technical effects of the technical solution of the present invention are:

[0059] 1) The geopolymer-RAP interface model provided by the present invention constructs a geopolymer-aged asphalt-aggregate composite interface model. Starting from the atomic level, it calculates the interfacial properties between geopolymer-aged asphalt and aged asphalt-aggregate, and simulates the effect of RAP with different degrees of aging on the interfacial bonding performance. The resulting model has a simple structure, requires less computing power, and the obtained simulation data has an excellent fit with the experimental results.

[0060] 2) In the technical solution provided by the present invention, based on the interface model established by the present invention, the interaction, diffusion, and mechanical bonding between geopolymer and recycled aggregate can be analyzed in detail from an atomic perspective. Upon comparison, the results obtained by model simulation are highly consistent with the sample test results, which can fully reflect the nanomechanical properties of the samples and play an important guiding role in the improvement of geopolymer-recycled aggregate. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 The geopolymer and aggregate model in Example 1 of the present invention;

[0062] in, Figure 1 (a) is the molecular model of the geopolymer in Example 1 of the present invention, Figure 1 (b) is the aggregate model in Example 1 of the present invention;

[0063] Figure 2 The molecular structure models of asphalts of different aging degrees in Example 1 of the present invention;

[0064] in, Figure 2 (a) is the molecular structure model of unaged asphalt. Figure 2 (b) is the molecular structure model of short-term aged asphalt. Figure 2 (c) is the molecular structure model of 20h long-term aging asphalt. Figure 2 (d) is the molecular structure model of asphalt aged for 40 hours;

[0065] Figure 3 The composite interface model in Example 1 of the present invention;

[0066] Figure 4 The nanoindentation model in Example 1 of the present invention;

[0067] Figure 5 This is a nanoindentation test diagram in Example 1 of the present invention;

[0068] in, Figure 5 (a) is a schematic diagram of the nanoindentation test process. Figure 5 (b) Nanoindentation depth load-displacement curve;

[0069] Figure 6 is the aggregate-aged asphalt interface interaction energy and the geopolymer-aged asphalt interface interaction energy in Example 1 of the present invention;

[0070] Figure 7 is the MSD of asphalt at different aging degrees in Example 1 of the present invention;

[0071] in, Figure 7 (a) is the MSD of unaged asphalt, Figure 7(b) MSD of short-term aged asphalt, Figure 7 (c) is the MSD of 20h long-term aged asphalt, Figure 7 (d) is the MSD of asphalt aged for 40 h;

[0072] Figure 8 is the diffusion coefficient of aged asphalt in different regions in Example 1 of the present invention;

[0073] Figure 9 is the density distribution of the geopolymer-asphalt interface in the Z direction in Example 1 of the present invention;

[0074] in, Figure 9 (a) is the density distribution of geopolymer-unaged asphalt in the Z direction, Figure 9 (b) is the density distribution of geopolymer-short-term aged asphalt in the Z direction, Figure 9 (c) is the density distribution of geopolymer-20h long-term aging asphalt in the Z direction, Figure 9 (d) Density distribution of geopolymer-40h long-term aged asphalt in the Z direction;

[0075] Figure 10 is Na in Example 1 of the present invention + The radial distribution function and coordination number of oxygen atoms in the stabilized material;

[0076] Figure 10 (a) is the Na content in unaged asphalt + The radial distribution function and coordination number of oxygen atoms in the stabilized material, Figure 10 (b) Na in short-term aged asphalt + The radial distribution function and coordination number of oxygen atoms in the stabilized material, Figure 10 (c) Na in 20h long-term aging asphalt + The radial distribution function and coordination number of oxygen atoms in the stabilized material, Figure 10 (d) is the Na content in asphalt aged for 40 hours + The radial distribution function and coordination number of oxygen atoms in the stabilized material;

[0077] Figure 11 This is a snapshot of the cross-linking of the geopolymer-aged asphalt molecule interface in Example 1 of the present invention;

[0078] Figure 12 This is the hardness-distance relationship curve in Example 1 of the present invention;

[0079] Figure 12 (a) is the hardness-distance relationship curve of unaged asphalt. Figure 12 (b) is the hardness-distance relationship curve of short-term aged asphalt. Figure 12(c) is the hardness-distance relationship curve of 20h long-term aging asphalt. Figure 12 (d) is the hardness-distance relationship curve of asphalt after 40h long-term aging;

[0080] Figure 13 is the elastic modulus-distance relationship curve in Example 1 of the present invention;

[0081] Figure 13 (a) is the elastic modulus-distance relationship curve of unaged asphalt, Figure 13 (b) is the elastic modulus-distance relationship curve of short-term aged asphalt. Figure 13 (c) is the elastic modulus-distance relationship curve of 20h long-term aging asphalt. Figure 13 (d) is the elastic modulus-distance relationship curve of asphalt after 40h long-term aging;

[0082] Figure 14 is the gradation curve of natural aggregate and recycled aggregate in the aggregate in Example 1 of the present invention;

[0083] Figure 15 This is a SEM image of the interface in Example 1 of the present invention;

[0084] in, Figure 15 (a) is the SEM image of the geopolymer-stabilized RAP interface. Figure 15 (b) SEM image of the geopolymer-stabilized gravel interface;

[0085] Figure 16 The chemical element distribution in the interface transition zone in Example 1 of the present invention;

[0086] in, Figure 16 (a) Chemical element distribution of geopolymer-stabilized RAP interface. Figure 16 (b) Chemical element distribution at the geopolymer-stabilized gravel interface;

[0087] Figure 17 This is a 7d unconfined compressive strength test of the sample obtained in Example 1 of the present invention. Implementation Method

[0088] The following comparative examples and embodiments are intended to further illustrate the present invention rather than to limit the present invention.

[0089] All models in this paper were constructed using Materials Studio 7.0. In the following simulations, periodic boundary conditions were used to minimize boundary effects. A Nose-Hoover thermostat and a Berendsen barometer were used to control temperature and standard atmospheric pressure. Electrostatic interactions were calculated using the Ewald summation method, and van der Waals interactions were calculated using the atom-based method, with a cutoff distance of 12 Å. MD simulations in the following examples were performed at 298 K, with a simulation time step of 1 fs.

[0090] The geopolymer and aggregate models were simulated using the clay force field (clayFF) developed by Cygan. The CVFF force field is a type I force field suitable for asphalt molecular dynamics simulations. The LJ potentials in the CVFF and ClayFF force fields are highly compatible and can successfully simulate the interaction between polymer and cementitious materials. The combined force field parameters are shown in Table 2. The first five energy terms in Table 2 can be used to characterize geopolymers and aggregates, while all energy terms can be used to characterize asphalt molecules. Example

[0091] A molecular dynamics-based geopolymer-RAP interface model is constructed, and the process is as follows:

[0092] 1) Constructing a geopolymer molecular model: Place 40 polymer structures into a simulation box and set the model density to 1.7 g / cm 3 After the model geometry was optimized, a 500 ps molecular dynamics simulation was performed under the NVT ensemble, and the obtained structure was as follows: Figure 1 (a) shown.

[0093] 2) Constructing the aggregate model: The main component of the aggregate is silica crystals. After the silica crystals are expanded, the surface of the crystals is hydroxylated. The resulting aggregate model is as follows: Figure 1 (b)

[0094] 3) Constructing the asphalt model: The asphalt molecular composition is shown in Table 1. At a constant temperature of 298 K and standard atmospheric pressure, a certain number of each component molecule of asphalt is randomly placed into the simulation box, and the initial density is set to 0.1 g / cm 3 , and the initial asphalt model was obtained. To prevent atomic overlap, the asphalt model was first energy minimized using the steepest descent method and the conjugate gradient method. This allowed the asphalt molecules to reach a relatively stable structure. The model was then subjected to a 300 ps NVT molecular dynamics simulation to fully mix the model, and then relaxed for 500 ps under the NPT ensemble to reduce the system size and obtain a stable equilibrium model. The resulting asphalt model is shown in the figure below. Figure 2 shown.

[0095] 4) Constructing composite interface model: Select the established asphalt model, aggregate model, and geopolymer molecular model to construct the geopolymer stabilized recycled aggregate composite model. The composite interface model is as follows: Figure 3 As shown in Figure 2, the interface model was imported into LAMMPS for simulation. The interface model was first fused in the NPT ensemble for 1 ns, then relaxed in the NVT ensemble for 3 ns, and data analysis was performed for the final 2 ns.

[0096] 5) Constructing nanoindentation model: In the present invention, a rigid spherical indenter is used to perform molecular dynamics simulation of nanoindentation at different positions of the composite material. The simulation model is as follows: Figure 4 As shown. The size of the composite material model is 80 mm×120 mm×300 mm, the radius of the rigid indenter is 35 Å, the initial distance from the model surface is 30 Å, and a fixed layer is set at the bottom of the composite material model to fix the position of the model, with a thickness of 10 Å. Periodic boundary conditions are set in the x and z directions of the model, and fixed boundary conditions are set in the y direction. After the model is established, the system is relaxed under the NVT ensemble to allow the system to reach equilibrium. After the relaxation is completed, the sample is loaded at a loading rate of 0.1 Å / Ps and pressed into the composite material interface model along the negative direction of the y-axis with a depth of 30 Å. After plastic deformation occurs, the indenter is unloaded and returns until it leaves the interface. The schematic diagram of the nanoindentation test procedure is shown in Figure 5 shown.

[0097] In order to understand the interaction characteristics of composite materials, the geopolymer-RAP interface model was divided, and the interaction energy of the aggregate-aged asphalt interface (Old-Interface) and the geopolymer-aged asphalt interface (New-Interface) was calculated respectively. The results are as follows: Figure 6 As shown in the results, all interaction energies are negative, indicating mutual adsorption between the composite materials. Simulation results show that the geopolymer-asphalt interfacial interaction energy is greater than the asphalt-aggregate interfacial interaction energy. Furthermore, with increasing asphalt aging, the interaction energy between asphalt and aggregate decreases significantly, while the interaction energy between asphalt and geopolymer increases first and then decreases.

[0098] In order to understand the diffusion characteristics between different parts of the composite material, the mean square displacement of asphalt on the aggregate surface (A1), inside the asphalt molecules (A2), and on the geopolymer surface (A3) were calculated respectively. The results are shown in Figure 2. Figure 7 The figure compares the mean square displacement changes of composite materials under different aging degrees RAP. Figure 7As can be seen in the figure, the mean square displacement curves for the four interface models increase with simulation time. Asphalt molecules diffuse at different speeds in different regions, with the diffusion order being A3 > A1 > A2. This is consistent with the finding that the geopolymer-asphalt interaction is stronger than the asphalt-aggregate interaction. Figure 8 Figure 2 shows the diffusion coefficients of aged asphalt in different regions. The diffusion rate of asphalt molecules in each region is affected by the degree of asphalt aging, with the order of diffusion being RAP0 > RAP1 > RAP2 > RAP3.

[0099] In order to better verify the interaction between geopolymer and recycled aggregate, the present invention also calculated the density distribution of different atoms along the z axis of the new geopolymer-RAP interface, such as Figure 9 As shown. The left side is geopolymer, the right side is recycled aggregate, and the middle is the interface transition zone. It can be observed that the density distribution of geopolymer at the interface is consistent with its diffusion rate. + The farthest diffusion is closer to the RAP surface, which indicates that Na + It is easier to establish a connection with asphalt molecules. At the interface, Na + The peak corresponds to the peak of oxygen atoms in asphalt, indicating that Na + With the increase of RAP aging degree, the double bond oxygen atoms in RAP increase, Na + The density at the interface also increases. At RAP2, the Na + The capture ability of the geopolymer is the strongest, and there is a strong interaction between the geopolymer and RAP. + The density actually decreases. This is because, in addition to the number of oxygen atoms, the interaction is also affected by the diffusion rate. At the highest degree of aging, the diffusion rate slows, resulting in a decrease in interaction. The density distribution curve shows that the presence of aged functional groups affects the interaction between the geopolymer and RAP. To further analyze the interaction between the geopolymer and recycled aggregate at the interface, the radial distribution function g(r) and coordination number n of the composite were calculated. This was calculated using the following formula:

[0100] ;

[0101] ;

[0102] Na + The radial distribution function and coordination number of oxygen atoms in the stabilized material are as follows: Figure 10 shown. Figure 10 At the distance of 2.4Å-2.79Å, the radial distribution function has the first peak, corresponding to Na + and oxygen atoms, which indicates that Na+ It forms a stable structure with the oxygen in the asphalt molecular structure. + Migrate from the geopolymer to the interface area and form electrostatic interactions with the double bond oxygen atoms in the asphalt. The source of adhesion energy is electrostatic interaction. + There are many weak peaks of oxygen atoms in the long range, which are highly correlated in space. As the aging degree of asphalt in RAP increases, the peak of radial distribution function increases first. This is because the increase of aging functional groups leads to the increase of double bond oxygen atoms in asphalt, and more oxygen atoms are combined with Na + The formation of coordination leads to a stronger electrostatic interaction between the geopolymer and the asphalt molecules with a higher degree of aging. According to the calculation results of the coordination number, when the aging degree is the highest (RAP3), the Na2O3 coordinated with the oxygen atom is affected by the diffusion rate. + The number of interfaces decreased from 2.39 to 2.0, which is consistent with the results of interface interaction energy and density distribution.

[0103] Figure 11 Snapshot of the cross-linking interface between NASH and aged asphalt molecules, with Na + The structures with electrostatic interactions are represented by ball-and-stick models to highlight the connections between them. This interface snapshot shows the Na + Coordination with double bond oxygen atoms in asphalt, Na + At the interface, it interacts with oxygen in asphalt molecules to form ion pairs.

[0104] Nanoindentation simulation is used to verify the ITZ distribution, and the Oliver and Pharr method is used to determine the hardness and modulus of the material. The hardness calculation formula is:

[0105] ;

[0106] P max is the maximum load; A is the contact area, which is related to the depth of the indenter:

[0107] ;

[0108] For spherical indenter:

[0109] ;

[0110] r is the radius of the indenter; h is the c is the actual pressing depth:

[0111] ;

[0112] h max is the maximum indentation depth; h f is the residual indentation depth.

[0113] The Young's modulus of the material is calculated as follows:

[0114] ;

[0115] E, ν are the modulus and Poisson's ratio of the test sample (the Poisson's ratio of the test sample is 0.35); E i , ν i are the modulus and Poisson's ratio of the rigid indenter (the modulus and Poisson's ratio of the rigid indenter are E i =1140GPa,ν i =0.07)

[0116] According to the displacement curve of the unloading part, the material reduced modulus is determined:

[0117] ;

[0118] The choice of parameter β is related to the shape of the indenter. For a spherical indenter, β=1.000; the contact stiffness S is the slope of the initial segment of the unloading curve.

[0119] The hardness and elastic modulus distribution of composite materials are as follows Figure 12 、 Figure 13 shown.

[0120] Figure 12 and 13 The figure shows the evolution of nanomechanical properties from geopolymer to aggregate through the two interface transition zones. The data in the figure indicate that the interface hardness and elastic modulus change with RAP aging. For the old interface transition zone, the hardness and elastic modulus at ITZ1 gradually decrease with increasing RAP aging. The weaker the asphalt aging, the more pronounced the weak interface becomes. In the new interface transition zone, RAP2 exhibits a significant increase in hardness and elastic modulus relative to RAP1 and RAP0, which to some extent improves the nanomechanical properties of the interface. After 40 hours of long-term aging, its nanomechanical properties decrease, consistent with the interfacial bond strength between geopolymer and RAP: RAP2 > RAP3. This phenomenon may be related to the mechanical interlocking between the geopolymer and recycled aggregate. After long-term aging, the diffusion rate of the aged asphalt slows significantly. Once the structure reaches stability, cracks form between the geopolymer and the aged asphalt, weakening the mechanical interlocking and resulting in a decrease in nanomechanical properties at the interface.

[0121] In order to verify the above dynamic simulation results, the present invention also carried out relevant experimental characterization, the specific process of which is as follows:

[0122] In this experiment, 1250 mesh metakaolin (MK) and 95 grade blast furnace slag (GGBS) were selected as precursor materials for the geopolymer. The chemical compositions of the precursor materials are shown in Table 3. The powders were activated with an alkaline solution of NaOH and Na2SiO3 to prepare the geopolymer. A slag / metakaolin ratio of 0.6, an alkali solution alkali equivalent of 8%, a water glass modulus of 1.5, and a water-solid ratio of 0.4 were selected. Recycled aggregate was obtained by crushing old pavement. Natural aggregate (NA) was also selected as a control. The physical properties of the natural and recycled aggregates used in this study are listed in Table 4.

[0123] MK and GGBS were mixed at low speed for 2 minutes to form a homogeneous solid mixture. Then, an alkali activator was added and stirred at low speed for 2 minutes and high speed for 2 minutes to prepare a fresh geopolymer slurry. Two types of samples were then prepared using this geopolymer slurry:

[0124] Fresh geopolymer slurry was slowly poured into a 10 × 10 × 10 mm cubic mold. When the slurry reached halfway up the mold, aggregate was added, and the geopolymer pouring continued. After pouring, the sample was vibrated, sealed with plastic film, and subsequently cured in a standard curing chamber for 28 days. After demolding, the sample was sectioned at the mid-interface. The sectioned surface was polished with 1200-grade sandpaper and then further polished with aluminum oxide. The exposed geopolymer-recycled aggregate interface was used for testing. A FEI QUANTAFEG250 field emission scanning electron microscope (SEM) and EDAX Element energy dispersive spectroscopy (EDS) were used for microscopic experimental investigations of geopolymer-stabilized RAP. The micromorphological features of the interfacial transition zone between the geopolymer and RAP were characterized, and line scans were performed across the interfacial transition zone to obtain elemental concentration distributions. Prior to SEM and EDS analysis, the observation surface was platinum-sprayed to eliminate charging effects that could affect image contrast.

[0125] In addition, the geopolymer stabilized RAP was prepared by replacing natural aggregate with 5% cementitious material content and 30% RAP content. The corresponding gradation curve is shown in Figure 14 Cylindrical specimens measuring 150 mm x 150 mm were formed using the static compression method specified in JTG E51-2009. The specimens were sealed in plastic bags and stored in a standard cement curing chamber (i.e., curing temperature 20°C ± 2°C, humidity > 95%) until mechanical testing. The specimens were subjected to 7-day unconfined compressive strength testing using a compression testing machine at a speed of 1 mm / min, in accordance with JTG E51-2009 T0805-1994.

[0126] The microstructure of the composite material was observed by SEM. The SEM image of the transition zone between the new and old interfaces is shown in Figure 2. Figure 15As shown, the geopolymer-recycled aggregate interface exhibits a loose structure and microcracks. This confirms the presence of an interstitial zone (ITZ) between the geopolymer and recycled aggregate, indicating that the ITZ is the weakest component of the geopolymer-stabilized recycled aggregate. Furthermore, microcracks can be observed within the geopolymer, likely formed during the precision cutting or polishing of the sample. Significant cracks, approximately 5 μm in width, are observed between the geopolymer and natural aggregate, indicating incompatibility between the geopolymer and natural aggregate.

[0127] Figure 16 (a) shows the linear distribution of reaction products formed around RAP in ITZ observed by energy dispersive spectroscopy. The change in counts along the EDS scanning line shows that the geopolymer is rich in Ca, Si, Al, and Na elements, which are the main components of NASH gel (hydration product of MK) and C-(A)-SH gel (hydration product of GGBFS). The Ca content increases significantly near the geopolymer interface, indicating that a Ca-rich reaction product is formed near the interface. At the same time, the Si content decreases and the Al content increases, which increases the Ca / Si ratio. This may be due to the formation of a C(N)-ASH gel with a certain degree of density and stability in the interface transition zone. The presence of RAP increases the Ca content at the interface, indicating that the aged asphalt molecules on the RAP surface have an effect on the Ca in the geopolymer. 2+ Isocations are attractive, Ca 2+ Migrate to the interface area and interact with the surface of recycled aggregate. The EDS analysis is consistent with the radial distribution function results in MD simulation, which shows that the C(N)-ASH gel in the interface transition area has a weak electrostatic interaction with the RAP surface and forms a chemical bond. + There is no strong affinity on the RAP surface, which may be due to the high content of slag in the geopolymer, most of the product is C-(A)-SH gel, and Ca 2+ (divalent cations) can replace Na + (monovalent cations), which generate stronger electrostatic interactions with the RAP surface. Figure 16 In (b), the content of chemical elements at the interface of geopolymer-stabilized natural aggregate decreases sharply, indicating that there is no chemical bonding at the interface. The interfacial bonding strength is mainly provided by mechanical biting, which proves that the presence of aged asphalt on the RAP surface affects the bonding performance between geopolymer and aggregate.

[0128] Furthermore, the present invention also tested the macro-scale mechanical properties of the samples and conducted a 7d unconfined compressive strength test on the obtained samples. The test results are as follows: Figure 16As shown in the figure, the unconfined compressive strength of geopolymer-stabilized crushed stone is consistently greater than that of geopolymer-stabilized RAP, which is consistent with the finding that the interfacial interaction energy of geopolymer-stabilized crushed stone is much higher than that of geopolymer-stabilized RAP. The degree of asphalt aging in RAP affects the mechanical properties of the geopolymer-stabilized RAP base. With increasing aging, the macroscopic mechanical properties first increase and then decrease, reaching their optimal macroscopic mechanical properties at RAP2. Unconfined compressive strength tests confirm the influence of RAP aging on interfacial bonding properties.

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[0130]

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[0132]

Claims

1. A geopolymer-RAP interface model based on molecular dynamics, characterized in that: include: 1) Construct and optimize the geopolymer molecular model based on the NASH gel structure; 2) Construct and optimize the aggregate model based on the main components of the aggregate; 3) Construct and optimize asphalt models of asphalts with different aging degrees based on the AAA-1 asphalt model; 4) constructing and optimizing a composite interface model based on the geopolymer molecular model, aggregate model, and asphalt model obtained in 1), 2), and 3), wherein the composite interface model includes a geopolymer-asphalt interface model and an asphalt-aggregate interface model; 5) Calculate the interfacial interaction energy, interfacial diffusion characteristics, and interfacial adhesion characteristics based on the composite interface model, and fit and optimize the calculated results with the experimental test results; The geopolymer molecular model is optimized by geometric optimization; the aggregate model is a surface hydroxylated silica model; the geopolymer molecular model and the aggregate model are fitted using the clay force field established by Cygan; the force field of the asphalt model is the CVFF force field; The asphalt model includes: an unaged asphalt model, a short-term aged asphalt model and a long-term aged asphalt model; the asphalt model construction process is as follows: i) Introduce asphalt component molecules into the simulation box and set the initial density to 0.08~0.12g / cm 3 , get the initial asphalt model; ii) The initial asphalt model is energy minimized using the steepest descent method and the conjugate gradient method, and is relaxed sequentially through the NVT ensemble and the NPT ensemble to obtain the following: The construction process of the composite interface model is as follows: the geopolymer model, asphalt model and aggregate model are imported into LAMMPS for simulation, and then fused for 1 ns under the NPT ensemble and then relaxed and analyzed in the NVT ensemble.

2. The molecular dynamics-based geopolymer-RAP interface model according to claim 1, characterized in that: The Si / Al ratio in the geopolymer molecular model is 1.5-2, the number of geopolymer molecules in the simulation box is 35-45, and the density is set to 1.5-2 g / cm 3 ; After the geometry optimization, a 500ps dynamic simulation was performed under the NVT ensemble.

3. The molecular dynamics-based geopolymer-RAP interface model according to claim 1, characterized in that: The interfacial interaction energy includes aggregate-asphalt interfacial interaction energy and geopolymer-asphalt interfacial interaction energy. The calculation formula of the aggregate-asphalt interfacial interaction energy is: Formula 1: ; The calculation formula of the geopolymer-asphalt interface interaction energy is: Formula 2: ; In formula 1: is the aggregate-asphalt interface interaction energy in equilibrium state, with the dimension of mJ / m 2 ; is the surface energy of aggregate, with the unit of mJ / m 2 ; is the surface energy of asphalt, with the unit of mJ / m 2 ; In formula 2: is the geopolymer-asphalt interfacial interaction energy in equilibrium state, with the dimension of mJ / m 2 ; is the surface energy of the geopolymer, with the dimension of mJ / m 2 ; is the surface energy of asphalt, with the unit of mJ / m 2 .

4. The molecular dynamics-based geopolymer-RAP interface model according to claim 1, characterized in that: The interfacial diffusion characteristics are represented by calculating the diffusion coefficients of asphalt on the aggregate surface, inside the asphalt molecules, and on the geopolymer surface. The calculation process of the mean square displacement is as follows: Formula 3: ; Formula 4: ; The interfacial adhesion was calculated by the following formula: Formula 5: ; Formula 6: ; In formulas 3 to 6: is the position vector of particle i at time t, dimensionless; is the position vector of particle i at the initial moment, dimensionless; MSD is the mean square displacement, dimensionless; N is the total number of particles to be averaged, dimensionless; a is the slope of the MSD linear fit line, dimensionless; D is the diffusion coefficient, dimensionless; δ is a constant, which takes the value of 1 in the interval [- w, w) and 0 otherwise; g(r) is the radial distribution function, dimensionless; n(r) is the coordination number, dimensionless.

5. The molecular dynamics-based geopolymer-RAP interface model according to claim 1, characterized in that: It also includes constructing a nanoindentation model, the process of which is: using a rigid indenter to perform nanoindentation treatment on different positions of the composite interface model in the height direction at 0.1~0.5Å / Ps, with an indentation depth of 20~30Å; the radius of the rigid indenter is 30~35Å; a fixed layer for fixing the model is provided at the bottom of the composite interface model, with a thickness of 8~12Å.

6. The molecular dynamics-based geopolymer-RAP interface model according to claim 5, characterized in that: The calculation process of the nanoindentation model is: Formula 7: ; Formula 8: ; Formula 9: ; Formula 10: ; Formula 11: ; Formula 12: ; In formulas 7 to 12: is the hardness, the dimension is GPa; is the maximum load, the dimension is nN; is the contact area, dimension is Å 2 ; is the indenter radius, dimension is Å; is the actual indentation depth, with the dimension of Å; is the maximum indentation depth, with the dimension of Å; is the residual indentation depth, with the dimension of Å; E is the modulus of the sample, with the dimension of GPa; v is the Poisson's ratio of the sample, dimensionless; is the modulus of the rigid indenter, in GPa; is the Poisson's ratio of the rigid indenter, dimensionless; is a dimensionless parameter; S is the slope of the initial segment of the unloading curve, which is dimensionless.

7. The polymer-RAP interface model based on molecular dynamics according to claim 1, characterized in that: The process of the test is as follows: Metakaolin and slag were mixed at low speed for 2 minutes to form a homogeneous solid mixture. Then, an alkali activator was added and stirred at low speed for 2 minutes and high speed for 2 minutes to prepare a fresh geopolymer slurry. Then, two types of samples were prepared using this geopolymer slurry: i) Pour the geopolymer slurry into a cubic mold, add natural aggregate or recycled aggregate when the amount is half, and then inject the remaining geopolymer slurry. After vibrating and curing in a curing box, a geopolymer-aggregate sample is obtained; ii) cutting the interfaces between the geopolymer-natural aggregate sample and the geopolymer-recycled aggregate sample, and performing electron microscopy and energy dispersive spectroscopy analysis on the interfaces; iii) Mixing geopolymer with aggregate to prepare cylindrical samples of geopolymer-stabilized natural aggregate or recycled aggregate. After sealing and curing, the samples are subjected to a 7-day unconfined compressive strength test; The proportions of the components in the geopolymer are as follows: slag accounts for 0.5-0.6, alkali equivalent in the alkali solution is 6-10%, water glass modulus ratio is 1.2-1.5, and water-solid ratio is 0.3-0.5; The mass ratio of the geopolymer to the aggregate is 1:15-20; The mass percentage of the recycled aggregate in the total aggregate is 25-35%.

8. The use of a molecular dynamics-based polymer-RAP interface model according to any one of claims 1 to 7, characterized in that: Used to calculate the interfacial properties and nanomechanical properties of asphalt mixtures.

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