## 2 Materials and Methods
### Coir Fiber  
Coir fiber was selected for the present study as it is considered by the Food and Agriculture Organization (FAO) of the United Nations as one of the future fibers. Coir fiber is available in large quantities with a global annual production of about 650,000 tonnes. Its cross-section is multicellular, with an average of 30-300 or more cells. Each cell has a diameter of 12-14 \(\upmu\)m and a length-to-diameter ratio of the order 35 [49]. The tensile strength, Young's modulus, and total elongation of bulk coin fibers vary from 95 to 230 MPa, 2.2 to 6 GPa, and 15 to 51.4%, respectively [1]. This fiber is covered with a waxy-like \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Mass Ratio (\%)** & **SiO\({}_{2}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **Fe\({}_{2}\)O\({}_{3}\)** & **TiO\({}_{2}\)** & **K\({}_{2}\)O** & **MnO** & **ZrO\({}_{2}\)** & **CaO** & **BaO** & **LOI** \\ \hline
**Kaolin** & 60.09 & 26.15 & 1.46 & 0.27 & 0.70 & 0.20 & 0.01 & 0.06 & 0.21 & 10.85 \\ \hline
**MK** & 60.50 & 34.30 & 2.41 & 0.15 & 0.20 & 0.15 & 0.09 & 0.04 & 0.01 & 2.15 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of kaolin and metakaolin (MK). Figure 1: (**a**) Raw kankara kaolin clay. (**b**) Calcined raw kankara kaolin clay (metakaolin). (**c**) XRD diffractogram of raw kaolin and metakaolin. material \"cuticle\". Studies have shown that pure coir fiber has cellulose (32.86-43.44%), lignin (40.52-45.84%), hemicellulose (0.15-0.28%), pectin, and water-soluble as its main chemical constituents [49; 50; 51]. These relative percentages vary with the maturity age of the nuts from which the fiber has been extracted [50]. The coir fibers used in this study were extracted from the outer shell of coconut husks purchased locally and supplied by a coconut vendor in a central market in Kaduna State, Nigeria. Figure 2a-c shows the coir fiber's processing flow from de-husking to air-drying. De-husking of the coconut was the first step taken to process the coir fibers from the coconut husks. The coir fibers were observed to be brown, indicating that the fibers were extracted from matured coconuts. The de-husked coconut fibers were then soaked in distilled water for about 30 min to make the coir fibers soft and facilitate ease of combing, stretching, and separating fibers into individual single filaments. The fibers were then pre-treated in 5% NaOH solution (mercerization) for 24 h to remove the wax, oil, and different impurities present in the coir fibers [32; 52]. According to [PERSON] et al. [32], 5% NaOH concentration is the optimum alkaline concentration to treat coir fibers using the traditional mercerization technique. A higher NaOH concentration could degrade the coir fibers' cellulosic structure. The treatment of the coir fibers improves the fibers' durability in service [53; 54; 55]. After the pre-treatment process, the coir fibers were washed in distilled water until a pH value of \(\approx\)8.0 was achieved. Air drying of the pre-treated coir fibers (PCFs) for 24 h to remove free water followed. The PCFs were observed to change color from light brown to dark brown. The air-dried PCFs were then combed, stretched, cut into lengths of \(\approx\)30 mm, and subsequently stored in an airtight plastic bag. Table 2 shows the index properties of the pre-treated coir fiber used in this study. Table 3 presents the chemical composition of the untreated and pre-treated coir fiber. The result shows an increase in the cellulose content of the pre-treated coir fibers against their untreated counterpart. The increase in cellulose content is attributed to the removal of some amount of lignin and hemicellulose from the fiber [49]. The longitudinal morphology of the untreated and pre-treated coir fibers from scanning electron microscopy (SEM) is shown in Figure 3a,b. The micrograph of the coir fibers revealed that the fibers are solid though irregular in size. The pre-treatment also modified the surface texture of the coir fibers.