Performance and Degradation Behavior of Coconut and Sisal Fibers Exposed to Alkaline Solutions and Embedded in Concrete Matrices

Abstract

The use of natural lignocellulosic fibers as reinforcement in cementitious materials has attracted increasing interest because of their low environmental impact, renewability, and wide availability. However, the long-term durability of these fibers under highly alkaline conditions remains a major limitation for their application in cement-based composites. This study investigates the degradation behavior and performance of coconut and sisal fibers, under untreated and alkali-treated conditions, exposed to alkaline solutions and incorporated into concretes produced with two Portland cement types (PC V and PC IV). The experimental program included total organic carbon (TOC) analysis, gravimetric degradation tests under cyclic immersion conditions, Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), and compressive strength tests of fiber-reinforced concretes. The results demonstrated that alkalinity is the main factor governing fiber degradation, promoting the dissolution of hemicellulose and lignin, cellulose depolymerization, and progressive microstructural deterioration. TOC analysis revealed substantially higher organic matter release under alkaline exposure compared with thermal treatment alone, confirming the strong susceptibility of lignocellulosic components to alkaline attack. Sisal fibers exhibited higher initial degradation due to their elevated cellulose content and lower lignin fraction, whereas coconut fibers showed a greater chemical stability. Alkali treatment reduced the mass loss during degradation cycles, particularly for sisal fibers, indicating partial stabilization through the removal of alkali-sensitive amorphous constituents. FT-IR analyses showed that cellulose-related bands near 1400 cm–1 and 1028–1052 cm–1 are sensitive indicators of alkaline degradation, especially for sisal fibers exposed to the simulated pore solution. SEM observations confirmed significant surface deterioration, microfibril separation, and increased roughness after alkaline exposure with treated fibers generally exhibiting lower structural damage than untreated fibers. In the concrete mixtures, coconut fibers demonstrated superior compatibility with both cement matrices, maintaining compressive strength values close to those of the reference concrete. Conversely, sisal fibers caused significant reductions in compressive strength, despite their intrinsically higher tensile properties, indicating that chemical durability and fiber stability govern composite performance more strongly than the isolated fiber strength. The results further indicate that the moderate reduction in alkalinity provided by PC IV cement is insufficient to prevent the degradation of cellulose-rich fibers. Overall, the study demonstrates that fiber composition and chemical stability are critical factors controlling the performance of natural fiber-reinforced cementitious composites, highlighting coconut fibers as a more promising alternative for applications in alkaline environments.