Effect of treated Alfa fibers on hydration kinetics, mechanical, and adhesion properties of fiber cement composite

Abstract

The addition of Alfa fibers into a cementitious matrix leads to a composite with improved mechanical properties – in particular, its flexural strength. However, Alfa fibers have been found to degrade over time, due to the highly alkaline environment of the cementitious medium. This study examined the effect of treatments applied to the Alfa fibers on the mechanical performances and adhesion of the cementitious composite. Treatments of the surface of the Alfa fibers (alkaline and hydrothermal treatments, coating with sulfoaluminate cement, ultrasound and enzymatic treatments) were applied, with the aim of improving the behavior in the long term. The raw and treated Alfa fibers were incorporated into cement mortars, at different lengths (10 and 20 mm), at an addition rate of 1% vol. After 90 days, an improvement in flexural strength was observed in the mortars with 1% vol. of treated fibers; contrary to untreated fibers. The alkaline and hydrothermal treatments are the most effective of the bunch. An increase in the strengths of 44% and 32% respectively was noted, in comparison to untreated fiber. This finding was confirmed by a modification in the hydration reactions of the composites. A compatibility factor between the fiber and mortar was determined, showing that composites with Alfa fibers having undergone alkaline and hydrothermal treatments are more compatible with the mortar. To measure the interactions (shear stress) between the Alfa fiber and the cement mortar, a specific measuring device, based on a fiber immersed in mortar, was put in place. An increase in shear stress with fibers after treatment, suggesting that the fiber/mortar adherence properties are improved.