Influence of type of fibers on the properties of high performance cement-based composites

• Cement-based composites with expansive agent reinforced by several kinds of fibers.
• Synergy between brass- and zinc-coated fibers with CaO improving flexural strength.
• PET fiber degradation due to alkaline hydrolysis promoted by expansive agent.

In this work twenty fiber reinforced cement-based composites (FRCCs) were studied, in which CaO-based expansive agent was used. Five different kinds of fibers were added, three hooked metallic: steel, zinc-coated and brass-coated; two plastic: corrugated polypropylene (PP) and hooked polyethylene terephthalate (PET) fibers. All the twenty mixtures, as well as a reference mixture without fibers, were characterized by recording fresh consistency, compressive and flexural strength, as well as drying shrinkage strains. Results obtained showed an improved flexural strength if high dosage of CaO-based expansive agent is used with either zinc or brass-coated fibers. The reason could be the formation of calcium hydroxide zincate (CHZ) crystals at the interface between fibers and cement paste promoted by alkaline environment. These CHZ crystals were observed by SEM, and they are likely able to enhance the quality of the interface fiber-matrix by increasing adhesion. On the other hand, the use of CaO seemed to accelerate PET fiber degradation due to alkaline hydrolysis leading to reduced FRCC mechanical performance. When used in the presence of either PP fibers or steel hooked fibers, CaO produced insignificant effects on mechanical performance. Finally, drying shrinkage was reduced due to the addition of CaO, whichever the type of fiber used.