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• Cellulose fibers modified with nanosilica presented lower real density. • Fiber-cement with modified fibers led to improved mechanical properties. • Modified fibers led to lower bulk density of the composites. • The CH and CC content in the fiber-cement were changed according to the types of fibers used. • This study contributed to the improvement of technologies for fiber modification. The present study investigated the effect of incorporating different contents of modified and unmodified cellulose pulp fibers on the mechanical, physical and mineralogical properties of fiber-cement composites. The cellulose pulp fibers were modified using the sol–gel process, which resulted in the deposition of nanosilica on their surface. The raw materials used to form the composites included: Portland cement, calcium carbonate, additives and cellulose pulp fibers. Two types of fibers (modified and unmodified) and five fiber contents (1%, 3%, 5%, 7% and 9%) were evaluated. The results revealed that the presence of nanosilica on the surface of the fibers of the modified cellulose pulp increased the resistance to axial compression and decreased the density of the fiber-cement composites, although such properties have been reduced proportionally with the increase of fiber content. Nanosilica increased interaction of the fibers with the cement matrix hydrates, decreasing the presence of calcium hydroxide and increasing the presence of calcium silicate hydrates. The interacting influence of the decrease of hydrophilicity, increase of surface area and roughness of the modified fibers allowed a better fiber/matrix interaction and fiber-cement performance. Further studies are necessary to realize which of the aforementioned factors is dominant for the fiber-cement performance. Modification of the pulp fibers with nanosilica are presented as potential fiber treatment to be used in the production of cement reinforced composites. [ABSTRACT FROM AUTHOR]