Influence of surface-modification, length and volume fraction of carbon short fibers on the mechanical properties of calcium aluminate cement systems

In this study, we investigate the influence of short carbon fibers on the mechanical properties of calcium aluminate cement (CAC) based systems. The main focus of this study is on the bending tensile strength. When a compressive force acts upon a concrete/mortar beam resting freely (on two supports) it will bend and generate tensile stress at its bottom. This tensile stress generated by bending is called flexural tension - and the resistance to flexural tension is the bending tensile strength. In a first step, we determine the best surface treatment of fiber in relation to the fiber-matrix connection through bending tensile and ESEM measurements. Since the oxidation process of the carbon fibers promise to have a major influence, different oxidation temperatures were investigated. In order to determine suitable values of cement/filler and water/cement-value for the fiber-containing system, diverse parameter studies were carried out with a variation of these two values. As a result of the experiments the highest increase in bending tensile strength could be observed with 1 vol-% of 2.5 mm carbon short fibers (oxidized at 500 degrees C). The increase of the bending tensile strength for this kind of system is up to 200 % compared to the fiber-free one. In order to determine the post-curing effect of the system, we also record the 1 week values. In addition to the positive effect on the bending tensile strength, the admixture of fibers into calcium aluminate cement system results in a significant reduction in volume shrinkage during the hardening process.