## 3 Results
### Geopolymer Characterization
#### 3.2.3 Chemical Stability  
In order to assess the geopolymerisation process and the three-dimensional reticulation, pH and ionic conductivity were determined at different times (5, 30, 60, 120, 240, 480, 1440 min) to evaluate the release of ions.  
The first set of samples (Metakaolin + biochar) (Figure 7a) showed constant values of pH (from 10.5 to 11) as the biochar content increased, with no significant variations with respect to the geopolymer constituted by metakaolin only. The pH values were alkaline because of the natural alkaline nature of the geopolymers [27]. Moreover, the pH of alkali activators is very high, at around 14, but the geopolymers immersed in water develop a less alkaline pH due to the geopolymerization phenomenon because the activators (NaOH + Na-SiL) are not free, having reacted with the SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\) available from the metakaolin.  
Figure 7: pH and conductivity trend of (**a**) first series samples and (**b**) second series samples at 90 days of curing.  
Both the 28 and 90 (Figures 6a and 7a) days-of-curing samples were characterized by conductivity values which increased with added biochar (from 410 to 604 m Sm\({}^{-1}\)). The chemical analysis of biochar shows the presence of Ca, and K, and the increase in conductivity is related to the release of these elements, indeed the pH characteristic of biochar is alkaline, as reported by [PERSON] et al. [22].  
This means that biochar didn't hinder the geopolymerization process which improved with time. Samples characterized by 90 days of curing were characterised by an increase in conductivity during the 24 h due to the number of ions released in the aqueous environment (from 445 to 604 m Sm\({}^{-1}\) for the sample with metakaolin and 70% biochar). An important aspect is the trend of conductivity depending on curing time. The increase in curing time involved a higher stability material and a higher geopolymerization degree confirmed by a reduction in released ions in the solution. This meant that the conductivity values decreased from 785 to 604 m Sm\({}^{-1}\), for example, for the same sample with metakaolin and 70% of biochar at 28 and 90 curing days.  
The second series samples (Metakaolin + IBA + biochar) (Figure 7b) confirmed the results of the first series, showing constant values of pH around 10.5-11.0. The ionic conductivity increased as the percentage of biochar increased (from 646 to 801 m Sm\({}^{-1}\)) for the sample with 90 days of maturation, due to the saline content of biochar and IBA. For example, the sample containing 20% of biochar had a conductivity value around 500 m Sm\({}^{-1}\) while the sample with 70% of biochar content had a value around 700 m Sm\({}^{-1}\). This means that the samples with higher percentage of biochar, for example 70%, showed higher release of ions in solution due to their microstructure and ions released related to the biochar.  
The ionic conductivity of the second series samples showed higher values than samples of the first series (from 604 to 801 m Sm\({}^{-1}\)) because the interaction with the external environment was higher and the presence of IBA in the formulation led to less stability and compactness of the material.  
The third series' composition was characterized by constant values of pH around 11 and conductivity around 800 m Sm\({}^{-1}\).