Degradation kinetics and aging mechanisms on sisal fiber cement composite systems

The kinetics of vegetable (sisal) fiber degradation and the mechanisms responsible for deterioration of continuous sisal fiber cement composites are presented in this paper. Two matrices were used: one with 50% partial cement replacement by metakaolin (PC–MK) and a reference matrix having as binder only Portland cement (PC). The durability performance of the composite systems is examined and the mechanisms for the significant delay in the fiber degradation when the total amount of calcium hydroxide is reduced from the matrix discussed. The composites were subjected to 5, 10, 15, 20 and 25 cycles of wetting and drying and then tested under a four point bending load configuration in order to determine the flexural behavior and cracking mechanisms with progressive aging. Furthermore, composites stored under controlled lab conditions were tested under bending load at ages ranging from 28 days to 5 years. Fibers extracted from the aged composites were subjected to thermal analysis, Fourier transform infrared spectroscopy and microscopical observations in order to evaluate the changes in chemical composition and microstructure. Two fiber degradation mechanisms were observed in the PC composites: fiber mineralization due to the precipitation of calcium hydroxide in the fiber cell and surface and degradation of cellulose, hemicellulose and lignin due to the adsorption of calcium and hydroxyl ions. The degradation process occurs rapidly and after 10 cycles of wetting/drying a quite expressive modification in the flexural behavior is observed. The residual mechanical parameters after 25 cycles were the same as those observed in the unreinforced matrix. For the PC–MK composite fiber mineralization was not observed due to the low content of CH in the matrix.