## 3 Results and Discussion  
Table 2 reports the composition of the different fibers, as derived from the TGA reported in Figure 2. The results enable the investigation of the fibers' compositions. After a first weight loss deriving from the loss of water molecules, two overlapping peaks appear from 180 \({}^{\circ}\)C to 350 \({}^{\circ}\)C, deriving from hemicellulose and cellulose decomposition. Afterwards, at higher temperatures, the degradation of the lignin fraction takes place [29; 30; 31; 32; 33]. Figure 3 reports the relative workability of the fresh composites after the mixing stage, i.e., the ratio between the slump of the composite and the slump of the bare matrix. Since the water amount was unchanged, the addition of fibers reduced the slump values. As can be seen, the behavior of bamboo fibers is different from those of hemp and kenaf. A limited decrease in workability takes place as the amount of fibers increases, and consequently, a higher amount of added fiber was achievable (4 wt%). In kenaf and hemp modified materials, after a slight decrease in workability at the lowest amount (1 wt%), a large drop takes place at 2 wt% loading. This may be related to the formation, at this amount, of a 3D structure, i.e., a connected medium among the single fibers that constrains the matrix \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Fibers**} & \(\mathbf{T_{max}}\) & **Moisture** & **Pectin and/or** & **Cellulose** & **Lignin** & **Residue** \\  & (\({}^{\circ}\)C) & **(\%)** & **Hemicellulose(\%)** & **(\%)** & **(\%)** & **(\%)** \\ \hline Kenaf & 388 & \(\approx\)4 & \(\approx\)10 & \(\approx\)74 & / & 12 \\ Hemp & 363 & \(\approx\)3 & \(\approx\)12 & \(\approx\)48 & \(\approx\)14 & 23 \\ Bamboo & 339 & \(\approx\)3 & / & \(\approx\)60 & \(\approx\)10 & 26 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: TGA data of the fibers. Figure 2: TGA (**a**) and dTGA (**b**) profiles of the fibers. flow induced by the slightly different morphology of the fibers. Nevertheless, it should be underlined that despite the reduced workability of the composites, it was still possible to cast all of the specimens without the visible creation of macro-porosities. This last macroscopic evidence was further supported by the results of the water absorption test reported in Figure 4.  
Water absorption is generally accepted as an indication of the overall open porosity of materials. Indeed, all samples, except Ba4, show a comparable or even lower amount of absorbed water (and, therefore, of the overall porosity) as compared to the metakaolin matrix. It should be considered that the matrix is a porous medium, as determined by mercury intrusion porosimetry [7]. The substitution of a porous phase, i.e., the matrix, with a less porous one, the fiber, decreases the overall porosity. In theory, a contribution to the overall porosity could derive from the scarce adhesion of the organic fibers to the inorganic matrix. However, this effect, if present, should be limited. Consequently, a positive interaction of the fibers with the matrix, leading to the creation of a quite  
Figure 4: Absorbed water as a function of the fiber amount. Figure 3: Relative workability of the investigated composites. continuous transition zone between the two phases, can be proposed based on the mutual presence of hydroxyl groups in both phases. The progressive increase may derive from air bubble entrapment. Figure 5 reports the flexural strength of all investigated materials. A linear increase is found for bamboo and kenaf samples, the positive effect being stronger for the bamboo samples (about 80% for bamboo and about 20% for kenaf at the highest amount). Hemp samples show, instead, an almost constant value with a slight positive effect at the highest content (5%). Although it is difficult to compare these data with the existing literature, _ceteris paribus_, in similar metakaolin systems, [34] a comparable increase was obtained by the addition of sisal and pineapple fibers.