Microstructural analysis, physical, chemical components and tensile properties of coir fibre long-term exposed in Ca(OH)2 as representative of concrete pore solution

In this study, the microstructure, chemical composition, linear density, tensile modulus of elasticity, tensile strength and strain at break of coir fibres exposed to saturated Ca(OH)2 solution (i.e., pH = 12.5) up to 24 weeks were investigated. Different fibre surface modification methods were applied on coir fibres to investigate their effectiveness on the long-term tensile performance of coir fibres. The surface modification methods included treating fibres in: (1) boiling water, (2) 0.01 g/ml NaOH solution, or (3) 5% H2O2 solutions for 2 h, respectively. Coir fibres after modifications were then immersed in Ca(OH)2 solution up to 12 weeks and tested in tension at time intervals of 0, 4 and 12 weeks, respectively. Coir fibres without surface treatment were also immersed in both Ca(OH)2 solution and distilled water up to 24 weeks and tested in tension at intervals of 0, 2, 4, 12 and 24 weeks, respectively. Statistical significance test was used to discuss the change in the fibre linear density and tensile properties. The results showed that all the surface modifications selected had no significant influence on the linear density of coir fibres. Thermogravimetric analysis and Fourier-transform infrared spectroscopy confirmed the decomposition of hemicellulose of coir fibres and the generation of CaCO3 on the fibre surface after exposure to Ca(OH)2 solution. The average tensile strength of the coir fibre reduced to 74% and 66% respectively after 12 and 24 weeks exposure in the Ca(OH)2 solution in comparison with that of fibre exposed in distilled water. The corresponding average ultimate strain also reduced to 53% and 27% at 12 and 24 weeks. No obvious tendency could be identified in the average tensile E-modulus of coir fibre after long-term exposure. Through scanning electron microscopy, it was found that failure along the microfibrils and tensile failure of the microfibrils were the two main failure modes of coir fibre under tension. Surface modification with NaOH changed the permeability of coir fibre surface. Tylosis decomposition occurred due to H2O2 solution modification. Neither H2O2 solutions nor boiling water modification had significant influence on the tensile behaviour of coir fibre.