Role of polyacrylonitrile fiber diameter and aspect ratio on mechanical performance and pore structure of cementitious mortars

This study investigates how the geometric parameters of synthetic microfibers—specifically fiber diameter and resulting aspect ratio—govern the transition between crack-bridging–dominated reinforcement and matrix-densification–driven strengthening in cementitious mortars. Polyacrylonitrile (PAN) fibers with identical length (6 mm) but three distinct diameters (≈ 15, 35, and 40 μm) are incorporated at volume fractions ranging from 0.2% to 0.6%, enabling isolation of aspect ratio effects under controlled conditions. Rather than treating fiber reinforcement as a purely mechanical addition, the results demonstrate that below a critical fiber diameter, PAN fibers act as an active microstructural modifier. Finer fibers with higher aspect ratios promote a shift in the governing strengthening mechanism—from conventional crack-bridging to simultaneous pore refinement and enhanced stress transfer at the fiber–matrix interface. This transition is evidenced by a coupled improvement in compressive and flexural strength, accompanied by a pronounced reduction in harmful capillary and macropores, as confirmed by mercury intrusion porosimetry (MIP)and scanning electron microscopy (SEM). In contrast, mortars reinforced with coarser PAN fibers exhibit behavior consistent with classical fiber reinforcement models, where mechanical performance remains largely insensitive to fiber content and microstructural refinement is limited. These findings indicate that fiber geometry defines not only reinforcement efficiency but also the dominant physical mechanism by which fibers interact with the cementitious matrix.