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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.

Industrial solid waste M-resin (MR) contains high sulfur content which was difficult to be comprehensively utilized. According to the vulcanization reaction mechanism, MR was used as vulcanizing agent, nano zinc oxide (ZnO) as an activator, tetramethyl thiuram disulfide (TMTD) as an accelerator to produce M-resin-based stabilizer. MR (0.5, 1, 1.5, 2, 2.5), ZnO (0.1, 0.2, 0.3, 0.4, 0.5) and TMTD (0.06, 0.08, 0.1, 0.12, 0.14) were mixed with different contents, a three-factor and five-level orthogonal experiment was designed to test the penetration, ductility, softening point and 48 h softening point difference of SBS modified asphalt. Based on the four indexes, the basic formula of the stabilizer was obtained. The influence of kaolin, an auxiliary material, on the thermal stability was compared, and the formula were optimized, the appropriate dosage was determined. The microscopic morphology of the modified asphalt and SBS developmental fineness was analyzed with the fluorescent microscopy technology, and the SBS developmental fineness was studied with the generated binary graph and skeleton graph. DSR test was carried out to detect rheological properties of modified asphalt with different dosages of MR-based stabilizes. The results show that MR can significantly improve the thermal storage stability of modified asphalt. The ratio of MR:TMTD:ZnO:Kaolin is 1.5:0.5:0.08:12, and the appropriate dosage is 1.02 %. The stabilizer can reduce the 48 h softening point difference 33.6 ℃, increase the softening point 13.6 ℃, decrease the penetration degree 9.7 mm, but it has little effect on 5 ℃ ductility index. The microscopic images show that a cavity two-phase continuous network structure is formed inside the modified asphalt under the appropriate dosage of M-resin-based stabilizer. The SBS developmental fineness is 2.36 μm, and the uniformity is good. DSR test also shows that stabilizer can effectively improve the high temperature performance of SBS modified asphalt.

The thermal storage stability of styrene–butadiene–styrene tri-block copolymer modified bitumen (SBSPMB) is the key to avoid performance attenuation during storage and transportation in pavement engineering. However, existing evaluation index softening point difference within 48 h (ΔSP48) cannot effectively distinguish this attenuation of SBSPMB. Thus, conventional physical indexes, rheological properties, and micro-structure characteristics of SBSPMB during a 10-day storage were investigated in this research. Results showed that during long-term thermal storage under 163 °C for 10 days, penetration, ductility, softening point, recovery rate (R%), and anti-rutting factor (G*/sinδ) were decayed with storage time increasing. This outcome was ascribed to the phase separation of SBS, which mainly occurred after a 4-day storage. However, ΔSP48 after a 6-day storage met the specification requirements (i.e., below 2.5 °C). Thus, the attenuation degree of asphalt performance in field storage was not effectively characterized by ΔSP48 alone. Results from network strength (I) and SBS swelling degree tests revealed that the primary cause was SBS degradation and base asphalt aging. Moreover, conventional indexes, including penetration, ductility, and softening point, were used to build a prediction model for rheological properties after long-term storage using partial least squares regression model, which can effectively predict I, R, Jnr, G*/sinδ, and SBS amount. Correlation coefficient is above 0.8. G*/sinδ and I at the top and bottom storage locations had high coefficient with SBS amount. Thus, phase separation of SBSPMB should be evaluated during thermal storage.

In order to improve the problem of the bad thermal storage stability of Styrene-Butadiene-Styrene Block Copolymer Modified Bitumen (SBS-modified bitumen), the vulcanization reaction was selected as the basic mechanism for the research and development of the stabilizer. Sulfur, tetramethylthiuram disulfide (TMTD), zinc oxide, butylated hydroxytoluene (BHT), kaolin (Al2O3·2SiO2·2H2O) and carbon-white (SiO2) were used as raw materials, and 32 kinds of components with different contents of raw materials were designed. The 48-h segregation softening point and other modified bitumen technical indicators, such as ductility, penetration, penetration index, viscosity, and so on, were tested and analyzed. The fluorescence microscope test was also conducted to explore the crosslinking situation based on the vulcanization reaction. The results show that the component with good performance of the SBS-modified bitumen stabilizer powder was S:TMTD:ZnO:BHT: Al2O3·2SiO2·2H2O = 60:16:4:20:1250 or S:TMTD:ZnO:BHT:SiO2 = 60:16:4:20:625. The optimum content of stabilizer powder was 0.9% (S/modified bitumen). The stabilizer was dry powder and can be directly put into bitumen. It can cross-link SBS and especially has an effect on preventing SBS segregation. The technical performance of SBS-modified bitumen with stabilizer powder, such as hot temperature performance, low temperature performance, anti-aging performance and thermal storage stability, all meet the relevant specifications.