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The industrial sector is increasingly interested in the production of yarns based on renewable, efficient, innovative, and ecological resources. Among these precursors, cellulose nanofibrils (NFC) have gained attention due to their interesting characteristics for the development of new products. However, their dimensions pose challenges in their use. Therefore, this study aimed to analyze the influence of NFC concentrations and dimensions on the quality of bicomponent yarns. The bicomponent yarns were produced using a coaxial needle and the dry–wet-spinning method. The outer layer was made of guar gum (3 % w/v) and the inner layer of NFC bleached at two different degrees of refining (NFC A and NFC B). Yarn production involved varying concentrations of NFC (4.5 %, 3.5 % and 2.5 % w/v) NFC, which were characterized by real density analyses, tensile strength, water absorption, thermogravimetry, and scanning electron microscopy. Guar gum showed more hydrophilic characteristics, and its delay in hardening caused a less circular section and flaps on the side of the yarn. NFC B exhibited the smallest average diameter, and higher concentrations resulted in yarns with greater mechanical strength, more circular cross-sections, and fewer microstructural defects. [ABSTRACT FROM AUTHOR]

• 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]