Durability of geopolymer composites reinforced with vegetable fibers: Effects of alkaline activator, matrix dosage, and aging on the composite

Vegetable fibers have been identified as a potential solution to the brittle behavior of geopolymers. However, these binders are activated by alkaline solutions, which can degrade the fibers over time and compromise the performance of the composite. This study investigates the impact of different alkaline activators and matrix dosages (low vs. high free ion concentration) on the durability of sisal fiber-reinforced geopolymer composites. Aged composites were evaluated through three-point bending tests and water absorption measurements. The fibers removed from the matrix were evaluated by TGA and fibers immersed in the activating solution were analyzed by FTIR. The results show that sisal fibers can enhance the specific energy of the composites. However, mechanical tests revealed a significant reduction—up to 55.82 %—in specific energy for composites with high free ion concentration, which were exposed to outdoor conditions for 120 days. TG analysis highlighted the impact of free alkaline ions on the degradation of non-crystalline components and cellulose in the fibers. FTIR analysis indicates the occurrence of fiber degradation after immersion in alkaline solutions. SEM analysis further showed the presence of voids at the fiber-matrix interface in composites with high free ion content. These findings suggest that improper dosing of geopolymer matrices can lead to the degradation of vegetable fibers, regardless of environmental exposure or the type of alkaline activator used. Optimizing the free alkali ion content during matrix formulation can help mitigate, or even prevent, this degradation. A possible degradation mechanism is proposed.