## 2 Materials and Methods
### Raw Materials
#### 2.1.4 Biochar Absorption  
A test has been provided in order to assess biochar's water absorption and release capacity. The test was carried out following the tea-bag method [46; 47]: a tea filter, filled with about 5 g of biochar, was immerged in water, simultaneously a second empty filter was immersed in order to evaluate the amount of water absorbed by the biochar at different times. \[\text{Abs}\,\frac{\text{{g}}}{\text{{g}}}\text{{dried biochar}}=\frac{{P} \text{{fbiochar}}^{\prime}\text{{s}}\text{{s}}\text{{s}}\text{{c}}\text{{s}} \text{{c}}\text{{s}}\text{{c}}\text{{s}}\text{{c}}\text{{s}}\text{{c}}\text{{s}} \text{{c}}\text{{s}}\text{{c}}\text{{  
2. Addition of sodium hydroxide solution (8 M) and sodium silicate solution (Si/Na = 3) (Rm = 3). 3. Possible addition of few millilitres of water in order to obtain a liquid mixture. Water must be added if strictly necessary, otherwise it could slow down the subsequent hardening. 4. Intensive shaking until a homogeneous and fluid paste was formed, in order to avoid formation of bubbles. 5. Casting of the paste into a plastic mould. 6. Closing of the mould inside a plastic bag for 24 h in order to avoid cracking and breakage. 7. Maintaining the cast at room temperature. 8. The curing phase at room temperature lasted 7 days, 30 days and 90 days. In this study two different series of samples were prepared, both containing biochar, in order to estimate its possible interaction with the base matrix. Because the biochar is very lightweight, it was added to metakaolin in volume percentages. FIRST SERIES: metakaolin 100 wt%, and different percentages of biochar (20%, 30%, 40%, 50% and 70 vol%) were added to the base structure (Table 1 and Figure 2); \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**SAMPLE** & **MK (g)** & **IBA (g)** &
\begin{tabular}{c} **BIOCHAR** \\ **(vol\%)** \\ \end{tabular} & **NaOH (mL)** & **Na2 SiO (mL)** & **H2O** \\ \hline MK100 & 50 & \(\backslash\) & \(\backslash\) & 24 & 34 & \(\backslash\) \\ \hline MK + 20\%biochar & 50 & \(\backslash\) & (20\%) & 24 & 34 & \(\backslash\) \\ \hline MK + 30\%biochar & 50 & \(\backslash\) & (30\%) & 24 & 34 & \(\backslash\) \\ \hline MK + 40\%biochar & 50 & \(\backslash\) & (40\%) & 24 & 34 & \(\backslash\) \\ \hline MK + 50\%biochar & 50 & \(\backslash\) & (50\%) & 24 & 34 & \(\backslash\) \\ \hline MK + 70\%biochar & 50 & \(\backslash\) & (70\%) & 24 & 34 & 1 \\ \hline
50 MK-50 IBA & 25 & 25 & \(\backslash\) & 8 & 20 & \(\backslash\) \\ \hline
50 MK-50 IBA + 20\%biochar & 25 & 25 & (20\%) & 8 & 20 & 2 \\ \hline
50 MK-50 IBA + 30\%biochar & 25 & 25 & (30\%) & 8 & 20 & 5 \\ \hline
50 MK-50 IBA + 40\%biochar & 25 & 25 & (40\%) & 8 & 20 & 7 \\ \hline
50 MK-50 IBA + 50\%biochar & 25 & 25 & (50\%) & 8 & 20 & 9 \\ \hline
50 MK-50 IBA + 70\%biochar & 25 & 25 & (70\%) & 8 & 20 & 11 \\ \hline
50 MK-50 BC & 35.4 & \(\backslash\) & 14.60 g & 24 & 34 & \(\backslash\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Formulations of Series 1 and Series 2 samples (MK: Metakaolin; IBA: incinerator bottom ash; BC: Biochar in substitution). Figure 2: MK100 + 70%Biochar sample at 90 curing days. The samples were compact with a smooth and porous surface. The surface showed efflorescence which created white stains. The efflorescence was a result of the migration of salts to the surface. The bottom ash (IBA) used in the study has high electrical conductivity values (0.7 mS/cm) due to the presence of chloride (340 mg/L) and sulphate (570 mg/L) anions and calcium (123 mg/L) and sodium (230 mg/L) ions. Efflorescence can also be generated from the reaction of excess of Na, deriving from the activating solutions, with atmospheric CO\({}_{2}\) forming Na\({}_{2}\)CO\({}_{3}\), and can, therefore, be present in every geopolymer. The efflorescence increased as the percentage of biochar increased because the effect of the biochar content on the efflorescence is related to the increase in porosity that permits the migration of salts contained in the IBA.