## 2 Materials and Methods
### Sample Preparation  
Young and mature coconut fibers replaced asbestos to produce fiber-cement flat sheets using a water-to-binder ratio of 0.4. The ratio for the total mix of Portland cement-sand-crushed rock dust-water was 1:1:1:0.4. The addition of fibers ranged from 5% to 9.5% in increments of 0.5% by weight of binder. The mixing procedure shown in Figure 3. Firstly, the young and mature fibers were shredded for use as fiber reinforcement in the mixture (Figure 3a). Secondly, the coconut fiber was added and mixed along with binder and fine aggregate using a pan mixer with a capacity of 0.3 m\({}^{3}\) (Figure 3b). Next, the fresh mixture of fiber-cement flat sheets was poured into the mold and compressed (Figure 3c). Finally, the flat fiber-cement sheets were placed on a flip platform and removed from the molds to cure the specimens (Figure 3d). The experimental tests included bulk density, modulus of rupture, thermal conductivity, and microstructure to compare the influence of using young and mature coconut fibers in fiber-cement flat sheet applications. For bulk density and modulus-of-rupture tests, specimens with a dimension of 250 \(\times\) 250 \(\times\) 15 mm were cast. Thermal conductivity was determined using specimens with dimensions of 300 \(\times\) 300 \(\times\) 15 mm. The mix proportions of fiber-cement flat sheets are listed in Table 4.  
Three types of chemical accelerators--sodium silicate (Na\({}_{2}\)SiO\({}_{3}\)), designated SS mortar, aluminum sulfate (Al\({}_{2}\)(SO\({}_{4}\))\({}_{3}\)) designated AS mortar, and calcium chloride (CaCl\({}_{2}\)), designated CC mortar--were used to support for the hardening of the specimens. These chemical compounds can significantly improve the hydration kinetics of cement, leading to faster setting and increased early strength. Applying ASTM C807-21 [12], this study determined the best accelerator for fiber-cement flat sheets. The proportion of accelerators used was approximately 0.05 wt% by weight of binder. As shown in Figure 4, for all  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Mix Proportion** & **OPC (g)** & **FNA (g)** & **CRD (g)** & **YCF/MCF (g)** & **Water (g)** \\ \hline CF-50 & 1000 & 1000 & 1000 & 50 & 400 \\ CF-55 & 1000 & 1000 & 1000 & 55 & 400 \\ CF-60 & 1000 & 1000 & 1000 & 60 & 400 \\ CF-65 & 1000 & 1000 & 1000 & 65 & 400 \\ CF-70 & 1000 & 1000 & 1000 & 70 & 400 \\ CF-75 & 1000 & 1000 & 1000 & 75 & 400 \\ CF-80 & 1000 & 1000 & 1000 & 80 & 400 \\ CF-85 & 1000 & 1000 & 1000 & 85 & 400 \\ CF-90 & 1000 & 1000 & 1000 & 90 & 400 \\ CF-95 & 1000 & 1000 & 1000 & 95 & 400 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Mix proportion of fiber–cement flat sheets.  
Figure 3: The preparation of fiber–cement flat sheets specimens.  
mortar mixes at 120 min, significant differences in the depth of the Vicat needle were found between the CT (mortar without accelerators) and the mixes of SS mortar (with Na\({}_{2}\)SiO\({}_{3}\)), AS mortar (with Al\({}_{2}\)(SO4)\({}_{3}\)), and CC mortar (with CaCl\({}_{2}\)). It is also worth noting that the hardening period of the SS mortar was faster than that of the AS and CC mortars. Thus, sodium silicate (Na\({}_{2}\)SiO\({}_{3}\)) was the optimum material for use as an accelerator for the fiber-cement flat sheets. Similar observation was found in the study of [PERSON], which confirmed that the CaCl\({}_{2}\) is good chemical accelerators to support for the hardening in concrete [13].