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The compatibility of cold-mixed epoxy asphalt (CEA) significantly influences its mechanical properties, primarily mediated by the epoxy group content within CEA. However, the feasibility and mechanisms for regulating the external epoxy group content to achieve optimal performance have yet to be explored thoroughly. Herein, soybean oil, epoxidized soybean oil, and epoxy-terminated hyperbranched epoxy resin (EHER) were used to construct CEA with different external epoxy group contents, and their effects and mechanisms on the compatibility and mechanical properties of CEA were investigated based on experiments and molecular dynamics simulation. Our findings suggest that maintaining an external epoxy group content between 8.4%∼11.4% in the CEA blend and below 12.17% in the cured CEA is crucial. These criteria can be met by adding no more than 5 wt% EHER. CEA with 3 wt% EHER exhibited the most balanced compatibility and mechanical properties, characterized by a pot life of 91 min, a tensile strength of 5.02 MPa, an elongation at break of 61.3%, and a tensile toughness of 1.58 MJ/m3. The presence of external epoxy groups increased the cohesive energy density and the solubility parameter difference between epoxy resin and asphalt prior to curing, forming a "sea-island" structure with varying particle sizes, primarily comprising small-sized particles. This structure was preserved post-curing, establishing a stable system with minor energy alterations. Simultaneously, EHER enhanced the molecular rigidity and uniformity of CEA, thereby improving its mechanical properties. It is recommended to relate epoxy group content in CEA blends with indicators of compatibility and mechanical properties, enabling better prediction. • Desirable performance is exhibited by CEA with no more than 5 wt% EHER. • ER provides strength and asphalt acts as a toughener in CEA. • The "sea-island" structure is preserved post-curing with minor energy variations. • Epoxy group content in blends has a high grey correlation with performance. [ABSTRACT FROM AUTHOR]

• ESO made asphalt disperse uniformly and increased elongation at break of the CEA. • ESO did not react chemically with asphalt and epoxy monomer. • ESO changed the cohesion energy densities and solubility parameters. Cold-mixed epoxy asphalt (CEA) has attracted increasing attention as a superior steel bridge pavement material for its convenient construction and low energy consumption. In this paper, the compatibility of CEA was improved with incorporating biobased epoxidized soybean oil (ESO), and the micromorphology, dynamic mechanical properties, and mechanical properties were evaluated. The results showed that ESO made asphalt particles disperse uniformly, and decreased the number of the glass transition temperature of the CEA. ESO also increased elongation at break of the CEA. The compatibility mechanism was analyzed according to Fourier transform infrared spectrum and molecular dynamics simulation. The results showed that ESO did not react chemically with asphalt and epoxy monomer. Due to incorporating ESO into CEA, the difference values of cohesion energy densities and solubility parameters between asphalt and epoxy monomer decreased, while the interaction energy of the whole system strengthened. [ABSTRACT FROM AUTHOR]