# One-Part Alkali-Activated Pastes and Mortars Prepared with Metakaolin and Biomass Ash
## 3 Results and Discussion  
### Characterization of Pastes  
Results obtained by isothermal micro-calorimetry are reported in Figure 5.  
All the samples studied in this experimentation show that the reaction kinetics involves two reaction stages, which are found also in common geopolymers: dissolution and polymerization [32; 33]. These two stages were also found in [24; 25] for one-part inorganic polymers. The first one, related to the exothermic dissolution of alkali salts and breakdown of metakaolinite [25], immediately starts when the sample is mixed with water. In the present mixtures, the dissolution stage is represented by the initial decreasing part of the curves. On the other hand, the peak attributed to the polymerization reactions occurred at different times depending on the sample composition. When the pure biomass  
Figure 4: Water stability of one-part alkali activated mortars after 3 months of immersion. ash was put in water (ASH_H2O) a second exotherm was observed around 3-4 h, but the exact origin was not studied. In order to compare the reaction to a better-known system, KOH was selected as a simple \"model\" activator. Upon dissolving the ash in water, K+ and OH- ions were formed which justifies the choice. When metakaolin was mixed with the KOH solution (MK_KOH_1.00) the peak maximum was reached after 20 h (Figure 5). In one-part alkali-activated pastes, however, the maximum of the polymerization occurred after 10 h (Figure 5). In these pastes, by increasing the K/Al molar ratio, the rate of heat released also increased and the reaction continued for a longer period. The two-part alkali-activated paste MK_KOH_1.00 shows a slower reaction, which continued even after 5 days (130 h--end of test), with respect to the other pastes for which the main part of the reaction lasted around 60 h (Figure 5). By integrating the heat flow during the first 60 h of test, the reaction enthalpy as a function of K/Al molar ratio can be calculated (Figure 6). The sample prepared with pure biomass ash and water released a total enthalpy of \(-45.9\) J/g, whereas the two-part paste MK_KOH_1.00 developed a total enthalpy equal to \(-90.2\) J/g, after 60 h. On the other hand, analyzing the results obtained by one-part alkali-activated pastes it was found that by increasing the amount of biomass ash, hence the K/Al ratio, the reaction enthalpy also increased up to a maximum value obtained by the paste with K/Al = 2.50, which was equal to \(-63.5\) J/g. This result means that the stoichiometric value was reached, close to a K/Al molar ratio equal to 2.50. The maximum reaction enthalpy obtained by ASH_MK_2.50 could also mean that this paste is the one with the highest mechanical properties. It is worthy to note that the initial line did not go through zero because of the dissolution enthalpy of the ash in water, which might be different in presence of metakaolinite. Moreover, the samples were not prepared inside the calorimeter, but materials were mixed outside and then put in the instrument; therefore, the heat flow registered at the beginning of the test and thus the calculated reaction enthalpy were slightly different from the real ones. At the same K/Al molar ratio equal to 1.00, the two-part paste MK_KOH_1.00 had a reaction enthalpy 30% higher than that of the one-part alkali-activated paste ASH_MK_1.00. This result means that at the same K/Al molar ratio, a much lower amount of metakaolin reacted in the one-part system compared to that of the two-part system. It is probable that not all the K in the ash was available for the reaction, or that some side reactions occurred. This needs further investigation. The relationship between heat flow and reaction enthalpy of one-part alkali-activated pastes and potassium-rich biomass ash content was also studied in other papers [24; 25], where two different potassium-rich biomass ashes coming from agricultural wastes were used. [PERSON] et al. [24] found that  
Figure 5: Heat flow development of pastes up to 60 h at T = 25 °C.