Concrete　structures　are　susceptible　to　fracture　at　different　strain　rates.　The　aim　of　this　study　was　to　investigate　the　multi-scale　mechanism　and　effects　of　eco-friendly　cellulose　fibers　on　quasi-static　and　dynamic　mechanical　properties.　The　quasi-static　and　dynamic　mechanical　properties　of　concrete　were　investigated　using　a　hydraulic　servo　system,　pullout　test,　direct　tensile　test,　drop　hammer　test,　and　split-Hopkinson　pressure　bar　(SHPB).　The　effects　of　cellulose　fibers　on　the　hydration　and　microstructure　of　the　cementitious　composites　were　characterized　via　thermogravimetric　analysis　(TG),　X-ray　diffraction　(XRD),　Fourier　transform　infrared　spectroscopy　(FTIR),　scanning　electron　microscopy　(SEM),　and　mercury　intrusion　porosimetry　(MIP)　tests.　Through　molecular　dynamics　(MD)　investigations,　this　study　deciphered　the　atomistic　origins　of　mechanical　enhancement　in　cellulose-modified　cementitious　composites,　establishing　quantitative　correlations　between　macroscopic　performance　metrics　and　the　underlying　chemical-structural　determinants.　Finally,　the　reinforcement　mechanisms　of　the　cellulose　fibers　in　the　cementitious　composites　under　varying　strain　rates　were　systematically　investigated.　The　mechanism　of　cellulose　fiber　reinforcement　in　concrete　involves　regulated　moisture　release　from　fiber　cavities　during　hydration　equilibrium,　induction　of　calcium　hydroxide　precipitation　through　solution　dilution,　and　promotion　of　pore　structure　densification　via　controlled　humidity　gradient　attenuation.　This　self-limiting　process　enhances　the　mechanical　strength　by　optimizing　interfacial　transition　zone　integrity　through　hydrostatic　pressure-mediated　crack　deflection　and　energy　dissipation　mechanisms,　effectively　converting　the　impact　energy　into　elastic　strain　energy　through　strategically　modified　fracture　pathways.　©　2025　Elsevier　Ltd