## 3 Results
### Geopolymer Characterization
#### 3.2.4 Physical Analysis  
Microstructural observations defined that samples of the first series (Metakaolin (MK) + biochar) were characterized by a specific surface which increased as the percentage of biochar increased, as shown by BET analysis (Table 4). BET analysis shows an increase in specific surface with increasing biochar content, which is a porous material. Samples containing metakaolin and metakaolin + biochar were characterized by high open porosity. When biochar was introduced into the geopolymers, open porosity reduced: the sample with 100% metakaolin without biochar showed open porosity around 47%, while the sample containing 70% biochar had 42%. This result was due to biochar acting as a filler going into the pores, while the biochar itself is porous. The high values of open porosity were confirmed by measurements of apparent density and high values of water absorption. Apparent density remained constant as the percentage of biochar increased because the biochar influenced only the open porosity. The sample characterized by 70% biochar showed a decrease in open porosity but a high value of water absorption due to the significant capability of biochar to adsorb water (about six times its weight), according to [PERSON] [45], where the absorption capacity of \begin{table}
\begin{tabular}{c c} \hline
**SAMPLE** & **BET (m\({}^{2}\) g\({}^{-1}\))** \\ \hline MK100 & 23.81 \(\pm\) 0.1848 \\ \hline MK + 50\%Biochar & 39.21 \(\pm\) 0.3538 \\ \hline MK + 70\%Biochar & 46.45 \(\pm\) 0.7282 \\ \hline \end{tabular}
\end{table} 
Table 4: BET of MK100%, MK + 50%Biochar and MK + 70%Biochar samples at 90 curing days. cement pastes was reported. Similar findings can be found from other authors [14], with the addition of pyrolyzed agro-food biochar to magnesium phosphate cement. They show that biochar causes a decrease in porosity, confirming the filler effect. Figure 8 shows the same behaviour of all the geopolymers independently of the proportion of biochar, with an increase in water absorption for longer curing times. Furthermore, increasing the biochar content increases the extent of the phenomenon going from 46% for the MK100 geopolymer to 77% for the 70% of biochar geopolymer. This behaviour occurs because biochar is characterized by high absorbtion capacity (biochar can adsorb a quantity of water about six times its weight). From intrusion porosimetry analysis it is possible to follow the evolution of the curing process of the prepared geopolymers. The data reported in Table 5 show the decrease in total Hg intrusion volume, pore area and total porosity when comparing 28- and 90-day specimens without biochar due to the geopolymerization. On the other hand, specimens containing 70% biochar after 28 and 90 days showed an increase in total Hg intrusion volume, pore area and total porosity but a decrease in the dimensions of pores. This characteristic derives from the presence of biochar which is itself a porous material and also fills the open pores, reducing the dimensions. The effect of the biochar addition (20, 70 vol%) on the geopolymer microstructure is displayed in Figure 9 for samples after 90 days of curing. It is possible to note for all specimens analysed, the sigmoidal trend indicating spherical pore shape [53]. The cumulative intrusion curves are overlapped until the region where majority of pores are present for these materials in the range (6-100 nm). The plots show an increase in the total Hg volume intruded, synonymous with total pore volume, when the amount of biochar present in the composition increased. These results indicate that the presence of biochar plays a crucial role in the pore formation (dimensions, pore area and total volume) in the structure. Another interesting findings regarding the role played by biochar when added as filler or in substitution of metakaolin is reported in Table 5. The specimen containing 50% biochar as filler showed a lower total porosity with respect to those in substitution which display high pore volume, area and dimensions of pores.