The influence of calcium oxide addition on properties of fiber reinforced cement-based composites

In this project several fiber reinforced cement-based composites (FRCCs) were studied, in which a CaO-based expansive agent was added in order to help in reducing the cracking induced by drying shrinkage. Two different kinds of fibers were tested: brass-coated hooked steel fibers and flat and flexible amorphous metallic fibers. All the mixtures were characterized for fresh and hardened state, by measuring the consistency of fresh mixtures, compressive and flexural strength, as well as length changes under drying shrinkage test condition for the hardened state. Their microstructures were also investigated by mercury intrusion porosimeter and SEM observations. The effect on mechanical performance of thermal pre-treatment at 80 °C was also evaluated. Results obtained confirmed the effectiveness of CaO addition (even at a low-dosage) on the stability of mixtures under drying shrinkage. It also proved to be effective in terms of flexural strength improvement when used with brass-coated fibers. The reason of this synergy probably lies in the formation of calcium hydroxyzincate (CHZ) crystals at the interface between the fibers and surrounding cement paste. These CHZ crystals, as observed by SEM, are likely able to noticeably improve the quality of the interface of the fiber-matrix by increasing adhesion.