## 1 Introduction
Moreover, only a small fraction of coir fiber is used in composite material applications from the more than 2 million tons of yields of fibers from the annual 42 million tons of coconuts produced worldwide [32; 33]. The use of coir fibers in geopolymer composite production may be a way to recycle and reuse this waste material more sustainably. Hence, there is a need to explore coir fiber as a potential reinforcement material in the production of geopolymer composites for sustainable building applications. Coir fiber is an abundant fiber obtained from the coconut palm (_Cocos nucifera_ Linn) with plantation and cultivation mainly in the tropics. This fiber is processed by de-husking coconut and separating it from its fibrous mesocarp. The average length of the fiber is in the range of 0.15-0.28 m, and the diameter varies from 0.1 to 0.5 mm [34]. Coir fibers are used to manufacture rubberized coir pads, ropes, mats, brushes, carpets, thermal insulation, and road construction materials. Coir fiber is non-toxic, chemically reliable, cheap, and abundant. Like any other plant fiber, coir fiber is biodegradable with a slow decomposition rate of 20 years [35]. Furthermore, it retains a high percentage of its original tensile strength in composites [34] and has recently been used as a reinforcing material in plastics, clay cement bricks, and geopolymers. However, an in-depth literature review has revealed a lack of published work on coir fiber as reinforcement in geopolymers. Although works were reported on reinforcing coir fiber in fly ash matrices [1; 19; 36], polymer matrices [37; 38; 39; 40; 41; 42; 43; 44], and other cement-fiber composites [45; 46], nothing is reported on the effect of coir fibers on the properties and microstructure of metakaolin-based geopolymer matrix. The study by [PERSON] et al. [1] shows that 1 weight percent 3 mm length, 0.5 mm diameter coir fiber reinforcement in fly ash geopolymer had good mechanical properties. [PERSON] et al. [36] also studied coir fiber as a reinforcing element in fly-ash-based geopolymer mortar composite. The weight percent of fibers used in the study was 0, 0.25, and 0.5 wt % of the geopolymer. For each fiber content, the length of coconut fibers was varied in the composite (10, 20, and 30 mm). The use of coconut fibers with up to 0.5% fiber content in the study improved the mechanical properties of fly-ash-based geopolymer mortar. [PERSON] et al. [19] also investigated the properties of high-calcium fly ash geopolymer mortar containing varying proportions of coir and sisal fibers at 0%, 0.5%, 0.75%, and 1% volume fractions. The study proved coir fiber to be a good reinforcement for improving the geopolymer's tensile and flexural strength performances, although the ultrasonic pulse velocity (UPV), dry density, workability, and compressive strength values tended to decrease with increasing fiber content. In the work of [PERSON] et al. [45], pre-treated coir fibers were used to reinforce cement-fiber composites as insulating material. Coir-cement composites demonstrated mechanical properties close to convectional wood-wood cement boards (WWCBs). The bending strength of 1.7 MPa in an acceptable density range of 500-600 Kg/m\({}^{3}\) was recorded. Pre-treated coir fibers also showed better cement/fiber interaction than untreated ones, with less slip softening and more slip hardening during fiber pull-out tests. In addition, the resultant composite material proved to be a good alternative where good thermal properties are desirous as the thermal conductivity is in the range of 0.08-0.11 W/mK. The mechanical and morphological properties of chemically treated coir-filled polypropylene (PP) composites were also investigated by [PERSON] et al. [38]. Treated coir fibers were mixed with PP granules at 10/90, 15/85, 20/80, and 25/75 weight percent mixing ratios. The mechanical properties of the composites fabricated from treated coirs were better than those of untreated ones.