Mechanical properties of alkali activated blast furnace slag pastes reinforced with carbon fibers

Alkaline activation has become a useful procedure to obtain high strength cement composites without Portland cement. Different industrial byproducts can be activated using this technique, e.g. blast furnace slag, fly ash or metakaolin. Thus a new generation of more sustainable composites is being developed. However, the main disadvantage of these composites is their high drying shrinkage strain. Among the different approaches to address this problem, shrinkage reducing admixtures are the most used, but the addition of fibers can be useful for this purpose. In this work, alkali activated slag (AAS) pastes have been reinforced with carbon fibers (CF). The effect of the concentration of alkaline activator (Na2O% and silica modulus) and the fiber aspect ratio (using different length fibers with the same diameter) on the mechanical properties has been assessed. Mechanical characterization comprised bending and compressive strength tests, ultrasonic pulse velocity and density measurements, and drying shrinkage tests, in which CF were capable of improving the mechanical strengths of AAS pastes while controlling the specimens strain due to shrinkage. CF additions increased bending strength up to five times, and increases up to 20% in compressive strength were observed. Furthermore, CF can be a convenient addition to control the drying shrinkage of AAS composites. Even in adverse conditions, such us 50% RH, where unreinforced specimens actually broke, CF could guarantee the stability of the specimen.