## 2 Materials and Methods
### Preparation of Samples  
Unreinforced (MGP control) and reinforced metakaolin-based geopolymer (MGP--0.5% PCF, to MGP--2% PCF) were prepared to study the effects of coir fiber reinforcement on the properties of the geopolymer. Table 4 summarizes the mix proportion used in the preparation of the samples. A total of five different sample formulations based on fiber composition that varied from 0% to 2% were investigated for compressive and flexural strength. For each sample formulation, three number samples were prepared at each fiber composition for the various mechanical strength tests performed.  
In the preparation of the specimen, for each formulation, an appropriate amount of metakaolin (MK), chopped pre-treated coir fibers (PCF), and alkaline solution (Na\({}_{2}\)SiO\({}_{3}\)/NaOH) were measured separately as per Table 4. The measured MK and PCF were poured into a mortar mixing machine and then dry-mixed for about 60 s to ensure the homogeneous distribution of the coir fibers within the MK powder. The addition of PCF varied from 0.5% to 2% of the mass of MK in the MGP control formulation. This step was followed by the gradual addition of alkaline solution into the mixer until a homogeneous paste was obtained. The MK powder to alkaline liquid that represented the solid to liquid (S/L) mass ratio used was fixed at 0.97 for all the formulations. The mixing time for each formulation was kept at 3 min. Mixing of the specimen was preceded by specimen filling in different molds specific for the test to be carried out. Different cubic (50 \(\times\) 50 \(\times\) 50 mm) and prismatic (40 \(\times\) 40 \(\times\) 160 mm) molds were filled with the geopolymer paste in two layers. Each was subjected to vibrations on a vibrating table for about 10 s to relieve the geopolymer mixture of air bubbles trapped within. The top of the molds were leveled with a smooth trovel, and various identification marks were inscribed on cast samples for ease of sampling.  
Furthermore, polythene sheets were used to cover the top surface of the molds to prevent efflorescence and excessive moisture loss. The sealed samples were ambient cured in the laboratory (26 \({}^{\circ}\)C \(\pm\) 1 \({}^{\circ}\)C, 60% \(\pm\) 5% humidity) for 24 h. Samples were de-molded (Figure 5a,b) and kept in sealed polythene bags at room temperature until mechanical tests were performed after 28 days. In our previous study [14], the early stage (7 days) and 28-day strength of this geopolymer formulation (without the inclusion of fibers) was reported. Since this geopolymer's early stage (7 days) and 28-day performance (without the inclusion of fibers) was already known, we explored only the effect of coir fiber addition at 28 days.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & \multirow{2}{*}{**MK (g)**} & **Pre-Treated COIR** & **Na\({}_{2}\)SiO\({}_{3}\)/NaOH** & **Solid/Liquid** \\
**Formulation** & & **Fiber (PCF) (g)** & **(by wt.)** & **(S/L) Ratio** \\ \hline MGP control & 1200 & 0 & 0.24 & 0.97 \\ MGP—0.5\% PCF & 1194 & 6 & 0.24 & 0.97 \\ MGP—1\% PCF & 1188 & 12 & 0.24 & 0.97 \\ MGP—1.5\% PCF & 1182 & 18 & 0.24 & 0.97 \\ MGP—2\% PCF & 1176 & 24 & 0.24 & 0.97 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Mix proportions of samples.