<p>Carbon fiber reinforced concrete (CFRC) often suffers from weak bonding between its cement matrix and carbon fibers (CF) because the fibers have an inert surface.To overcome this limitation, we introduce phenylpropyl emulsion (SAE) as an interfacial modifier and examine its strengthening mechanism through macroscopic and microscopic experiments and multiscale simulations using discrete element and molecular dynamics methods. Macroscopic experiments show that both CF and SAE significantly improve the mechanical properties of CFRC, although the effectiveness of SAE depends on its dosage. Specifically, a 4% SAE dosage increases the compressive and shear strengths of CFRC (with 1.5% CF) by 16.5% and 12.1%, respectively, while a 2% dosage boosts early flexural strength by 24.4%. At the microscopic level, SAE enhances the bond at the interface by providing physical anchorage through benzene rings and by forming chemical interactions via COO⁻-Ca²⁺complexes. Discrete element simulations confirm that the modified material has higher ultimate strength and toughness, and molecular dynamics simulations show that SAE lowers the interfacial energy and strengthens the bond between CF and the matrix through ionic and hydrogen bonding. This work offers a solid theoretical basis for improving CFRC interfaces and informs the design of innovative polymer modifiers.</p> Graphical abstract <p></p>