Physico-mechanical and morphological behavior of hydrothermally treated plant fibers in cementitious composites

Abstract

There is an increasing interest in using natural fibers to replace synthetic fibers in fiber cement composites. However, the hydrophilicity and geometric instability of natural fibers undermines their suitability for this purpose. A potential solution is to apply hydrothermal treatment to modify the natural fibers before use. However, the efficacy of this treatment on all natural fibers is still unclear. This research investigated the influence of hydrothermal treatment cycles on three natural (plant) fibers and the resulting performance when used in fiber cement composites. African oil palm (Elaeis guineensis) mesocarp fibers and bast fibers sourced from hemp (Cannabis sativa) and flax (Linum usitatissimum) plants were used. These fibers were subjected to six hydrothermal treatment cycles and assessed for tensile strength, vapor sorptivity, chemical and morphological differences. Both untreated and hydrothermally treated fibers were incorporated in fiber cement composites of varying compositions and characterized for flexural strength, porosity and mineralogical phases. The treatment affected the fibers differently depending on their composition. It enhanced fibers with high cellulose, low ketones and organic matter contents and was found to be milder on flax fibers than hemp and palm fibers. Dynamic vapor sorption time on treated fibers decreased by 1 hour 11 minutes, 4 hours 15 minutes and 5 hours 55 minutes in flax, hemp and palm fibers respectively. The cellulose decomposition peak was maintained at 375ºC in flax fibers but shifted from 330 to 375ºC in hemp fibers and 350 – 375ºC in palm fibers. FTIR peak ratios revealed that flax fibers lost lignin and hemicellulose proportionally while lignin degraded the most in palm fibers and hemicellulose in hemp fibers. The treatment enhanced the crystallinity of flax fibers better than hemp fibers but depleted the crystallinity of palm fibers. Flax and hemp fibers had increased tensile strength and improved the performance of fiber cement composites with hydrothermal treatment while palm fibers did not. Generally, hydrothermally treated flax and hemp fibers increased the specific energy (SE) by 6 – 24% and 2 – 19% respectively while palm fibers showed a decrease in SE by 20%. An enhanced bridging ability was exhibited by the treated hemp and flax fibers compared to their untreated fibers. The amorphous matrices of fiber cement composites with fly ash and crystalline matrices of fiber cement composites with palm oil fuel ash performed comparably well with flax and hemp fibers as reinforcement. The use of palm oil fuel ash in fiber cement composites offered greater resistance to fiber pull out and had a greater modulus of rupture (MOR) than those with fly ash. This resulted in untreated fibers performing better in fiber cement composites with palm oil fuel ash while treated fibers performed best in those with fly ash. Overall, hydrothermal treatment was an efficient means of enhancing the performances of fibers with high cellulose, low ketones and organic matter as reinforcement for fiber cement composites.