Cementitious composites made with natural fibers: Investigation of uncoated and coated sisal fibers

Synthetic fibers commonly used in cementitious composites are known to directly contribute to the material cost, while causing concerns regarding their availability and carbon footprint. As a sustainable alternative to synthetic fibers, this study explored the use of sisal fiber. To address the potential degradability of this natural fiber, two different coatings, i.e., polyester resin and bio-based shellac, were examined. For this purpose, various mixtures made with a range of sisal fiber dosages were investigated, including both uncoated and coated sisal fibers. The study explored the mechanical properties of the developed mixtures through compressive, splitting tensile, and flexural strength tests. From the obtained results, notable improvements were observed in the main strength properties, compared to the mixtures that contained no fibers. Specifically, 20% and 42% increases were recorded on average for the splitting tensile and flexural strengths, respectively. The investigations also included weathering tests to evaluate the durability of sisal fibers under wet/dry cycles. This was accompanied by the scanning electron microscope (SEM) image analyses to provide a fundamental understanding of the interfacial properties between the fiber surface and the cementitious matrix. In addition, plastic shrinkage tests were conducted to evaluate how sisal fibers help with limiting early-age cracks. While the mixtures with no fiber fully cracked due to plastic shrinkage within two hours, an addition of 1.4% sisal fibers was enough to entirely avoid such cracks. The outcome of this study provided original information on how sisal fibers can be considered as an alternative to synthetic fibers for achieving high-performance cementitious composites, especially where the price and availability issues limit the access to synthetic fibers.