Comparative Analysis of Sisal–Cement Composite Properties After Chemical and Thermal Fiber Treatments

The use of sisal fibers to reinforce concrete and mortar enables the development of sustainable cement-based materials suitable for various construction elements. However, the high-water absorption of natural fibers can cause dimensional instability and poor fiber–matrix bonding, which reduces strength over time. Physical and chemical treatments can decrease water absorption and enhance the dimensional stability and bonding properties of fibers, but their effects on composite performance require further clarification. This study produced composites with 2%, 3%, and 4% by mass of sisal fibers subjected to different treatments, including hornification, washed alkaline treatment, and unwashed alkaline treatment. Fibers were characterized through water absorption, dimensional variation, scanning electron microscopy (SEM), X-ray diffraction, thermogravimetric analysis and direct tensile testing. Composites were evaluated by water absorption, capillarity, drying shrinkage, direct tensile and four-point bending tests to assess the influence of fiber treatment and content. Results showed that alkaline treatment significantly improved the physical and mechanical properties of sisal fibers. Consequently, composites reinforced with alkaline-treated fibers achieved superior performance compared to those reinforced with hornified fibers, with the best results observed at the highest fiber mass fraction (4%). These findings demonstrate the potential of treated sisal fibers to enhance the durability and mechanical behavior of natural fiber-reinforced cementitious composites.