## 1 Introduction  
Advances in research and technology require improved building materials that perform better in service. This requirement often necessitates the use of fibers to improve composite properties [1]. Carbon, metallic, polymer, Kevlar, and E-glass fibers have been used as reinforcement materials in the production of fiber-reinforced composites for building construction applications around the globe [2, 3, 4]. Research findings have revealed that although these fibers significantly improve the mechanical properties of the composites, the high cost of the fibers limits their use in the production of composites [5]. Current materials and engineering research are increasingly showing concerns about developing energy-efficient and eco-friendly materials that will sustain the building industry [6, 7, 8, 9]. Thus, researchers in the building industry are exploring the use of natural fibers such as coir, sisal, palm, hemp, kenaf, wool, banana, bamboo, cotton, bagasse, jute, and flax. These fibers are cheaper substitutes for man-made fibers as reinforcement materials in composite production [10; 11]. The factors that guide the selection of these fibers for composite production include the type of matrix, fiber compatibility to the matrix, fiber dispersion, and the composite manufacturing process. Other factors include fiber type, extraction and treatment method, fiber aspect ratio, fiber content, fiber/matrix interface strength, hydrophobicity, and hydrophilicity of the fibers. One of such composite material matrices currently being explored in research is geopolymer [12; 13]. Geopolymers are eco-friendly and emerging sustainable alternative materials to ordinary Portland cement (OPC) in order to improve the environment's carbon footprint [14]. Geopolymers are a group of materials formed when aluminosilicate source materials are mixed with an alkaline reactant solution [15]. In addition to the eco-friendliness of this material, geopolymers are excellent fireproofing materials, resistive to corrosive environments, and have good thermal properties and compressive strength [16]. Interestingly, they also have low water permeability properties [17]. With these advantages, the manufacturing of geopolymers requires special handling due to the different chemicals used in the production process. The technology is more suitable in precast applications where the control of sensitive materials can be ensured. Moreover, like many other cement-based materials, geopolymers have low tensile strength and exhibit quasi-brittle behavior under low mechanical loading, limiting their widespread use in many applications [18]. Thus, to improve geopolymer toughness, fibers are added to the geopolymer matrix [19; 20]. The addition of natural fibers changes the brittle fracture nature of the geopolymer to a more ductile fracture [16]. The use of natural fibers in the production of geopolymers offers a myriad of advantages as these fibers are lightweight, resistant to a high alkaline environment, biodegradable, readily available, renewable, and non-hazardous [21; 22]. For instance, using cotton fibers up to 0.5 wt % improved the flexural strength and fracture toughness of fly ash geopolymer composite [23]. Further improvement in the performance (fiber and matrix adhesion) of these fibers is achieved through different forms of fiber treatment. These treatment forms include mercerization, hornification, and silane treatment [24; 25]. Extensive work has been carried out to investigate the performance of natural fibers such as coconut, sisal, flax, cotton, raffia, sweet sorghum, wood, bamboo, and wool in the production of geopolymer composites [26; 27; 28; 29; 30; 31]. The results of these studies prove promising. Hitherto, the utilization of coir fiber in the fabrication of eco-friendly geopolymer composites with improved properties is still limited. Coir fibers extracted from coconut hucks (an agro-waste) are most times incinerated or used in landfills, constituting environmental pollution and sanitation problems [32].