Revealing compatibility mechanism of nanosilica in asphalt through molecular dynamics simulation

Preparation and modification mechanism study of microwave-treated crumb rubber and waste engine oil-modified asphalt

The objective of this study was to characterize the performance of waste engine oil (WEO) and microwave-treated crumb rubber (CR)-modified asphalt (WEO-MCRA) and analyze the modification mechanism. The viscosity and dynamic shear rheological (DSR) tests were carried out to evaluate the viscoelasticity property of WEO-MCRA. The storage stability and fluorescence microscope (FM) tests were used to characterize the compatibility of the components. The Fourier transform infrared spectroscopy (FTIR) and molecular dynamic simulation were introduced to analyze the change of function groups and modification mechanism. The results demonstrated that introducing Wt.20% CR treated with microwave and Wt.6% WEO obtained a lower viscosity, excellent storage stability, and satisfactory elasticity properties of asphalt. The morphology of modifiers presented a thread-like structure microscopic with the range of WEO content Wt.3%-Wt.6%. Molecular dynamic simulations revealed that the aromatic may be intensively absorbed by CR and increase the likelihood of phase separation. WEO reduced the binding energy of CR to aromatic from 178.0 to 151.5 kcal/mol, which will contribute to the disaggregation of CR clusters. The diffusion coefficient shows a more obvious decrease with the addition of WEO and microwave treatment, which will benefit the stability of the asphalt. This study can provide a reference for the recycling of CR and WEO.

Assessing the impact of ultra-thin diamond nanothreads on the glass transition temperature of a bituminous binder

Low-temperature cracking and rutting are the most destructive problems of bitumen that hinder the application of high-performance bitumen engineering, which is dependent on its glass transition temperature (Tg). Through in silico studies, this work has systematically investigated the Tg of a bituminous binder with the addition of diamond nanothread (DNT) fillers with varying filler content, alignment, distribution, and functional groups. In general, the glass transition phenomenon of the bitumen is determined by the mobility of its constituent molecules. Tg is found to increase gradually with the increase in the weight percentage of DNT and then decreases when the weight percentage exceeds 5.05 wt%. The enhancement effect on Tg is weakened when DNTs are distributed vertically or functionalized with functional groups. Specifically, DNT fillers induce inhomogeneity, which promotes the motion of small molecules while hindering the motion of large molecules. The aggregation of DNTs and the molecular environment in the vicinity of DNTs directly affect Tg. In summary, aggregation and adhesion are the dominant mechanisms affecting the mobility of the constituent molecules in the DNT/bitumen system and thus its glass transition temperature. This work provides in-depth insights into the underlying mechanisms for the glass transition of a bituminous binder, which could serve as theoretical guidance for tuning the low-temperature performance of the bituminous binder.

Molecular dynamics-based study of the modification mechanism of asphalt by graphene oxide

CONTEXT

Graphene oxide(GO) has been widely used in asphalt modification due to its excellent properties. To reveal the interaction effect between GO and asphalt materials, the microscopic behavior and molecular structure changes of asphalt and GO/asphalt were investigated by molecular dynamics (MD) simulations. Mean square displacement (MSD) results showed that GO significantly inhibited the diffusion of molecules of asphalt components. Radial distribution function (RDF) results that GO destroys the original sol-type structure of asphalt. Simultaneously, GO adsorbed resins at low-temperature, adsorbed asphaltenes at high-temperature, and dispersed as a dispersed phase in the light components. The concentration of the dispersed phase in the asphalt colloidal structure was increased and the mutual attraction was enhanced. This improves the deformation resistance at high temperature, but weakens the ductility at low temperatures.

METHODS

To investigate the mechanism of action of GO-modified asphalt, the asphalt model and the GO/asphalt composite system model were constructed using the Amorphous Cell module in Materials Studio 2020 software. Subsequently, molecular dynamics simulations of the GO/asphalt composite system were performed using the Forcite module, while the interactions between atoms and molecules were described using the COMPASS II force field.

Research on the synergistic modification effect and the interface mechanism of GO/SBS compound-modified asphalt based on experiments and molecular simulations

Although there have been reports showing the modification effect of carbon nanomaterials on asphalt, there are few studies on whether carbon nanomaterials and polymers can have synergistic modification effects on asphalt. At the same time, the complex composition of asphalt makes it difficult to determine the interface mechanism between the modifier and the asphalt. In this study, graphene oxide (GO) and styrene-butadiene-styrene block copolymer (SBS) were selected as modifiers. A combined experimental and molecular simulation research method was used to study the synergistic modification effect and the interface mechanism between the modifier and the asphalt. The results show that the modification effect of GO/SBS incorporated into asphalt is significantly superior to that of GO or SBS incorporated individually and GO/SBS has a synergistic modification effect. Although the binding strength between SBS and asphalt is weak, the GO surface (GO (0 0 1)) can simultaneously bind with SBS and asphalt, increasing the binding strength of SBS and asphalt as well as promoting the dispersion of SBS in asphalt, so that GO/SBS shows a synergistic modification effect and improves properties such as low-temperature ductility, rheology and storage stability at macroscopic level. Intercalated and exfoliated structure can be formed between GO side (GO (0 1 0)) and asphalt, which improves the anti-aging properties of the asphalt. Physical bonding is the main interface binding for GO/SBS compound-modified asphalt. GO bonds to asphalt or SBS by hydrogen bonds and there are only dispersion forces between SBS and asphalt, resulting in a higher binding strength between GO and asphalt or SBS than between SBS and asphalt.

aRDG Analysis of Asphaltene Molecular Viscosity and Molecular Interaction Based on Non-Equilibrium Molecular Dynamics Simulation

Understanding the noncovalent (weak) interactions between asphaltene molecules is crucial to further comprehending the viscosity and aggregation behavior of asphaltenes. In the past, intermolecular interactions were characterized indirectly by calculating the radial distribution function and the numerical distribution of distances/angles between atoms, which are far less intuitive than the average reduced density gradient (aRDG) method. This study selected three representative asphaltene molecules (AsphalteneO, AsphalteneT, and AsphalteneY) to investigate the relationship between viscosity and weak intermolecular interactions. Firstly, a non-equilibrium molecular dynamics (NEMD) simulation was employed to calculate the shear viscosities of these molecules and analyze their aggregation behaviors. In addition, the types of weak intermolecular interactions of asphaltene were visualized by the aRDG method. Finally, the stability of the weak intermolecular interactions was analyzed by the thermal fluctuation index (TFI). The results indicate that AsphalteneY has the highest viscosity. The aggregation behavior of AsphalteneO is mainly face-face stacking, while AsphalteneT and AsphalteneY associate mainly via offset stacking and T-shaped stacking. According to the aRDG analysis, the weak interactions between AshalteneT molecules are similar to those between AshalteneO molecules, mainly due to van der Waals interactions and steric hindrance effects. At the same time, there is a strong attraction between AsphalteneY molecules. Additionally, the results of the TFI analysis show that the weak intermolecular interactions of the three types of asphaltene molecules are relatively stable and not significantly affected by thermal motion. Our results provide a new method for better understanding asphaltene molecules' viscosity and aggregation behavior.

Multi-Scale Characterization of High-Temperature Properties and Thermal Storage Stability Performance of Discarded-Mask-Modified Asphalt

In the context of the global pandemic of COVID-19, the use and disposal of medical masks have created a series of ethical and environmental issues. The purpose of this paper is to study and evaluate the high temperature properties and thermal storage stability of discarded-mask (DM)-modified asphalt from a multi-scale perspective using molecular dynamics (MD) simulation and experimental methods. A series of tests was conducted to evaluate the physical, rheological, thermal storage stability and microscopic properties of the samples. These tests include softening point, rotational viscosity, dynamic shear rheology (DSR), Fourier transform infrared (FT-IR) spectroscopy and molecular dynamics simulation. The results showed that the DM modifier could improve the softening point, rotational viscosity and rutting factor of the asphalt. After thermal storage, the DM-modified asphalt produced segregation. The difference in the softening point between the top and bottom of the sample increased from 2.2 °C to 17.1 °C when the DM modifier admixture was increased from 1% to 4%. FT-IR test results showed that the main component of the DM modifier was polypropylene, and the DM-modified asphalt was mainly a physical co-blending process. MD simulation results show that the DM modifier can increase the cohesive energy density (CED) and reduce the fractional free volume (FFV) of asphalt and reduce the binding energy between base asphalt and DM modifier. Multi-scale characterization reveals that DM modifiers can improve the high temperature performance and reduce the thermal storage stability of asphalt. It is noteworthy that both macroscopic tests and microscopic simulations show that 1% is an acceptable dosage level.

Experimental Study on Durability Degradation of Geopolymer-Stabilized Soil under Sulfate Erosion

In this study, the potential application of slag-fly ash-based geopolymers as stabilizers for soft soil in sulfate erosion areas was investigated to promote environmental protection and waste residue recycling. The changes in the physical and mechanical properties and microstructure characteristics of cement-stabilized soil/geopolymer-stabilized soil under sulfate erosion were comparatively studied through tests such as appearance change, mass change, strength development, and microscopic examination. The results show that the sulfate resistance of stabilized soil is significantly affected by the stabilizer type. In the sulfate environment, the cement-stabilized soil significantly deteriorates with erosion age due to the expansion stress induced by AFt, while the geopolymer-stabilized soil exhibits excellent sulfate resistance. The slag-fly ash ratio (10:0, 9:1, 8:2 and 7:3) is an important factor affecting the sulfate resistance of geopolymer-stabilized soils, and the preferred value occurs at 9:1 (G-2). When immersed for 90 d, the unconfined compressive strength value of G-2 is 7.13 MPa, and its strength retention coefficient is 86.6%. The N-A-S-H gel formed by the polymerization in the geopolymer contributes to hindering the intrusion of sulfate ions, thereby improving the sulfate resistance of stabilized soil. The research results can provide a reference for technology that stabilizes soil with industrial waste in sulfate erosion areas.

Discussion on molecular dynamics (MD) simulations of the asphalt materials

The application of asphalt materials in pavement engineering has been increasingly widespread and sophisticated over the past several decades. Variations in the properties of asphalt binder during mixing, transportation, and paving can affect the performance of asphalt pavement. However, the asphalt material is a non-homogeneous and complex organic substance, consisting of various molecules with widely various molecular weights, elemental compositions, and structures. This complexity leads to difficulties for researchers to clearly and immediately understand the properties of asphalt materials and their variations. The multi-scale research approach combines macroscopic experimental data and microscopic simulation results from a practical engineering perspective. It helps to improve the understanding of asphalt materials. The molecular dynamics (MD) simulation proposes a corresponding molecular model of asphalt material based on experimental data, and the simulation algorithm is able to derive properties similar to those of real asphalt. This paper provides a comprehensive review of the current studies on MD simulation of asphalt materials, including modeling, properties, and multi-scale analysis. As a key part of the computational simulation, this paper discusses the typical asphalt binder and asphalt-aggregate interface models constructed by different groups, and also presents their differences from real samples and their feasibility based on fundamental properties. After the introduction of molecular models, the extensive work made by researchers based on molecular models is categorically reviewed and discussed. The strengths and weaknesses of MD simulation methods in the study of asphalt materials are also summarized in order to provide the reader with a more comprehensive understanding of the relevant contents and to guide subsequent research.

Multiscale mechanisms of asphalt performance enhancement by crumbed waste tire rubber: insight from molecular dynamics simulation

The recycling of waste tires is a major environmental problem facing mankind, and the addition of crumbed waste tire rubber (CWTB) to the base asphalt is an extremely promising recycling method. However, the modification mechanism of CWTB to asphalt is not well understood, which restricts the development of CWTB modified asphalt. In this study, the mechanism of CWTB modification of asphalt was explored by dynamic mechanical analysis (DMA), fluorescence microscopy, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and molecular dynamic (MD) simulations. After verifying the asphalt reasonableness using glass transition temperature, CWTB modified asphalt was simulated and experimented. The results showed that CWTB enhanced the high temperature performance of the base asphalt. The microscopic mechanism by which this phenomenon occurs is that CWTB has the largest binding energy with the aromatics (1150-1350 kcal/mol), followed by the saturates (740-830 kcal/mol), followed by the resins (90-330 kcal/mol), and the smallest binding energy with the asphaltenes (100-140 kcal/mol), which causes CWTB to absorb the light components of the asphalt (aromatics and saturates). In addition, the introduction of CWTB reduces the diffusion coefficient of asphalt. In this process, CWTB will gradually swell, which causes CWTB to bind more and more tightly with the base asphalt, and eventually the good high temperature performance of CWTB is transferred to the base asphalt. The macroscopic manifestation of this process is that the rutting factor of CWTB-modified asphalt is significantly higher than that of virgin asphalt. This study can provide basic theoretical support for the application of CWTB-modified asphalt.