## 3 Results
### Geopolymer Characterization
#### 3.2.5 Microstructural Analysis  
SEM analysis confirmed the previous observations. In fact, the surface of the sample without biochar was characterized by smooth and compact aspect with isolated pores (Figure 10a). When the biochar was added to the matrix, the surface became porous, with tubular and hive structures typical of the biochar nature (see white marked area) (Figure 10b). Morphological characterization reported by other authors shows uneven ridges on the surface typical of biochar derived from woody biomass [18; 21]. However, as the percentage of biochar inside the matrix increased, the surface presented less porosity because biochar filled the pores: a compact and resistant material was obtained. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Total Intrusion Hg Volume** & \multirow{2}{*}{**Total Pore Area (m\({}^{2}\) g\({}^{-1}\))**} & **Average Pore Diameter** & **Total Porosity** \\  & **(m\({}^{3}\) g\({}^{-1}\))** & & **(\(\mu\)m)** & **(\%)** \\ \hline MK100 28 d & 399.5 \(\times\) 10\({}^{-6}\) \(\pm\) 1 \(\times\) 10\({}^{-6}\) & 41.552 & 0.0385 \(\pm\) 0.0001 & 45.2198 \\ MK100 90 d & 371.9 \(\times\) 10\({}^{-6}\) \(\pm\) 1 \(\times\) 10\({}^{-6}\) & 39.841 & 0.0373 \(\pm\) 0.0001 & 43.9117 \\ MK + 20\%biochar 90 d & 406.2 \(\times\) 10\({}^{-6}\) \(\pm\) 1 \(\times\) 10\({}^{-6}\) & 49.290 & 0.0330 \(\pm\) 0.00015 & 45.4361 \\ MK + 70\%biochar 90 d & 412.5 \(\times\) 10\({}^{-6}\) \(\pm\) 1 \(\times\) 10\({}^{-6}\) & 51.127 & 0.0323 \(\pm\) 0.0001 & 46.1124 \\
50 MK-50 BC 28 d & 459.5 \(\times\) 10\({}^{-6}\) \(\pm\) 1 \(\times\) 10\({}^{-6}\) & 58.196 & 0.0316 \(\pm\) 0.0001 & 49.7563 \\ MK + 50\%biochar 28 d & 371.6 \(\times\) 10\({}^{-6}\) \(\pm\) 1 \(\times\) 10\({}^{-6}\) & 53.040 & 0.0280 \(\pm\) 0.00007 & 43.9968 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: MIP parameters determined for the different compositions (MK: metakaolin; B: biochar). Figure 9: Cumulative intrusion curves for samples containing 0–20 and 70 vol% of biochar after 90 days of curing. When biochar replaced metakaolin, and was not added to it (series 3), total porosity increased (Figure 11). The open porosity increased because there was a higher content of biochar with respect to metakaolin responsible for densification. The increase in open porosity was confirmed by water absorption values. In fact, when biochar was added to metakaolin the value was around 36%, while when biochar replaced metakaolin, the absorption value was 54%. Sample characterization showed that samples of the second set containing aluminosilicate precursor (Metakaolin + IBA + Biochar) as the matrix, with 90 days of curing, were characterized by reduction in apparent density as the percentage of biochar increased. Biochar was inserted into matrix's pores and consequently produced a lightweight and porous material. The WA% values increased for several reasons, first, due to biochar content, with biochar being a lightweight material with high water absorption capacity and the microstructural properties of IBA. The trend in Figure 12 is different with respect to the set without a matrix. The addition of 20% of biochar leads to a decrease in WA because biochar particles can fill the porosity due to IBA. This behaviour was observed by other authors [19; 21] who also attribute the reduction in water absorption to the filler effect of biochar particles due to its finer particle size, and internal curing effect contributing to hydration and pore filling. Otherwise for higher additions of of biochar (70%) the lightweight characteristic and the adsorption capability of biochar leads to an increase in water absorption of geopolymers. Figure 11: External (**a**) and internal (**b**) surface of SERIES 3 where MK is replaced by biochar. Figure 10: Internal surface (**a**) of MK 100%; Internal surface (**b**) of MK + 70%biochar. The first context simulated in a climatic chamber an indoor environment (T = 20 \({}^{\circ}\)C, humidity = 50%).