# One-Part Alkali-Activated Pastes and Mortars Prepared with Metakaolin and Biomass Ash
## 3 Results and Discussion
### Characterization of Mortars
Moreover, this trend was found both with a maize cob ash and a maize stalk ash, even though the compressive strength is higher when maize cob ash is used, because of its higher reactivity. [PERSON] et al. [25] manufactured one-part mortars at the same w/s ratio of the present study and equal to 0.56. They reported that when cotton shell ash is used an increase in compressive strength is found up to a maximum value (at K/Al molar ratio of 4) and then a decrease is registered for higher K/Al molar ratios. One-part alkali-activated mortars manufactured with the heated biomass ash, instead, were prepared at the same w/s ratio of 0.65 by increasing the K/Al molar ratio. Unfortunately, it was not possible to go beyond a 1.5 molar ratio since the use of the heated biomass ash caused shorter setting time the higher the heated biomass ash content. Comparing the results obtained by mortars with the heated and unheated biomass ash, at the same w/s ratio of 0.65 and the same K/Al molar ratios, it was found that at 1.0 molar ratio, the heating of the material did not modify the mortar's compressive strength, whereas at a 1.5 molar ratio, the compressive strength of HASH_MK_1.5 mortar (2.3 MPa) was 28% higher than that of ASH_MK_1.5 mortar. This means that the thermal treatment of the biomass ash contributed also to an increase in their compressive strength. Therefore, the decomposition of calcium carbonate was beneficial for increasing the reactivity of the biomass ash used in this study. The higher compressive strength of one-part alkali-activated mixtures manufactured with calcined ash compared to those using a not-calcined one is reported also by [PERSON] et al. [25]. Here, it is shown that calcined cotton shell ash provides to mortars a slightly higher compressive strength than cotton shell ash as it is. As in the present work, the authors reported that the calcination of ash increases its \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Paste**} & \(\rho\) & **R\({}_{\rm c}\)** \\  & **(g/cm\({}^{3}\))** & **(N/mm\({}^{2}\))** \\ \hline ASH\_MK\_0.5 & 1.57 & 1.6 \\ ASH\_MK\_1.0 & 1.57 & 1.7 \\ ASH\_MK\_1.5 & 1.48 & 1.8 \\ ASH\_MK\_1.5b & 1.58 & 3.4 \\ ASH\_MK\_2.5b & 1.63 & 3.7 \\ HASH\_MK\_1.0 & 1.59 & 1.7 \\ HASH\_MK\_1.5 & 1.62 & 2.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Hardened density (\(\rho\)) and compressive strength (R\({}_{\rm c}\)) of one-part alkali-activated mortars. reactivity which in turn increases the compressive strength of mortars manufactured with it. However, compressive strength values obtained by authors in [25] were higher than those obtained in the present work, this means that biomass ash obtained by wood combustion is less reactive than cotton shell ash whether it is calcined or not. The morphological analysis of some selected one-part alkali-activated mortars investigated by SEM are reported in Figure 7. In the first three rows, mortars manufactured with unheated biomass ash and metakaolin with different K/Al molar ratios and w/s ratios are reported, whereas in the last row, the SEM images of one mortar prepared with the heated biomass ash are reported. Figure 7: SEM images of one-part alkali activated mortars: (**a**) and (**b**) ASH_MK_0.5; (**c**) and (**d**) ASH_MK_1.5; (**e**) and (**f**) ASH_MK_2.5b; (**g**) and (**h**) HASH_MK_1.5. Microstructures of mortars prepared with both heated and unheated ash at different ash/metakaolin ratio, hence K/Al molar ratio, seem not to differ very much. In all of them, metakaolin particles and biomass ash are visible, the former is characterized by plate-like shaped particles, whereas the latter is represented by cenospheres. Considering one-part alkali-activated mortars manufactured with the unheated biomass ash, a slight densification of the matrix is visible moving from ASH_MK_0.5 mortar (Figure 7a,b) and ASH_MK_1.5 (Figure 7c,d) to ASH_MK_2.5b (Figure 7e,f), since, in this last mortar, less metakaolin flakes are detectable and the surface appears smoother.