Physical-mechanical properties and microstructure changes in mortars with chemically treated coir fibers

Due to their high tensile strength and toughness, natural fibers have been proposed as reinforcing material in mortars to reduce cracking. Coir fiber has excellent potential for reuse in cementitious matrices among the various fiber types. However, this material is hydrophilic and has a high extractive content, including wax, oils, and other components in the structure. Different chemical treatments are used to remove contaminants from the fiber surface of the coir to minimize the presence of these elements. This study evaluated chemically treated coir fibers in mortars based on cement and lime. The effects of two levels of coir addition (1% and 2% relative to cement mass) treated with oxalic acid at a concentration of 5% w/w for 2 hours and sodium bicarbonate at 10% w/w for 120 hours were evaluated. The effect of these variables on compressive and tensile strength, capillary water absorption, accelerated carbonation, and microstructure analysis using X-ray microtomography and scanning electron microscopy were investigated. The results showed that with the addition of 2% coir treated with oxalic acid, mortars increased compressive strength by 8% and 22% in the tensile strength. Moreover, it was found that the treatment minimizes the carbonation rate in mortars with coir fibers, whose pore size distribution is also affected by the presence of the fibers and the treatment type. It can be concluded that the fiber's chemical treatment improves its properties for use in mortars, considering a synergistic analysis, and can be used as an alternative for composite material production.