Carbon nanofibers (CNF) are promising additives for the reinforcement of cementitious materials. However, due to their high specific surface and hydrophobic nature, CNF are difficult to disperse in ordinary Portland cement (OPC) mortars when using conventional plasticizers. In this work, we evaluate the production of mortars with CNF encapsulated by a thin layer of polyethylene glycol (PEG) covalently grafted at their surface (CNF@PEG). CNF@PEGs were chemically prepared using two methods with PEG of various molecular weights. In all cases, the dispersion of the CNF@PEG in water as well as in a simulated cement pore solution was improved. Remarkably, mortars fabricated with CNF@PEG4k (PEG of molecular weight 4000 g/mol) exhibited a 20 % increase in compressive strength, 10 % increase in flexural strength and 25 % increase of the Young's modulus measured after 7 days, in comparison to a reference mortar. Finally, electrochemical impedance spectroscopy (EIS) was used to assess the ionic resistivity of the mortar. Mortars containing CNF@PEG4K had an ionic resistivity 43 % higher than the reference sample at 28 days leading to better durability. These results demonstrate that encapsulation of CNF by PEG is a successful strategy to improve the mechanical performance and durability of CNF-containing OPC mortars.