The surface modification of cellulose fibers was studied, and its influence on the initial hydration of fiber-cement mixtures was evaluated. The fiber modification was conducted through deposition of nano-silica on their surface via the sol-gel method. The fibers were characterized by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, and the Brunauer-Emmett-Teller method to determine their porosity and specific surface area. The inhibition index (II) was

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... in order to evaluate the effect of fibers at the initial hydration of the Portland cement. Modified fibers (MF) presented a homogeneous surface coating formed by spherical nano-silica. The crystallinity index of the fibers was reduced 15% and the specific surface area, volume, and pore diameter increased 209%, 134%, and 10%, respectively, after modification. Regarding the initial hydration, the results showed that the nano-silica present on the surface of the MF slightly accelerated the process of hardening and did not inhibit the hydration of the cement paste (without limestone and additives). The inhibition index of the composites was impaired when limestone (30%) and additives (2%) were added as partial replacement of cement, as well as when the water:cement ratio increased, retarding the initial hydration of the cement.