Micro- and nanoparticle mineral coating for enhanced properties of carbon multifilament yarn cement-based composites

The reinforcing efficiency of Textile-Reinforced Concrete (TRC) is often limited by the low penetrability of the cement products into the inner fibers of the multifilament, leading to premature failure of the composite due to low bonding strength. Usually, polymer coatings are applied to carbon yarns to improve their bond properties. However, such coatings are sensitive to elevated temperatures and, in many cases, also to moist and alkaline environments (ageing) and, consequently, the composite material does not perform according to expectations. This paper presents an investigation aimed at improving control of the bonding of carbon multifilament yarn to a cement-based matrix by means of inorganic particle coatings. A carbon multifilament yarn was impregnated by micro- and nano-silica fillers and compared to conventional epoxy-polymer-coated yarns in terms of pullout behavior and microstructure morphology. The micro-silica filler coating proved to be the most efficient, enhancing both pullout strength and toughness, well beyond the effects induced by the nano-silica and epoxy coatings. The superior performance of the micro-silica coating is attributed to the high degree of impregnation of the inter-fiber space by the particles and to the pozzolanic reactions of the silica particles, promoting the formation of beneficial nanocrystalline products within the multifilament yarn. The nano-silica did not show the same effect due to extensive agglomeration and subsequent coating segmentation during the coating's drying process. Although the epoxy polymer effectively coated the fibers, the low penetration of cement hydration products into the yarn resulted in inferior bonding strength compared to that obtained by using the micro-silica coating. It may be concluded that the coating of multifilament yarns with mineral fillers offers an efficient option for improving the performance of TRC materials. As such, it is recommended to use pozzolan particles that disperse well and effectively impregnate the yarns, filling in the bundle spaces.