## 3 Results and Discussion
### SEM Analysis  
SEM images of the printed specimen with the different NaOH solutions are shown in Figure 8.  
Different microstructures were observed, regardless of the NaOH concentrations used, including dense areas, grainy areas, and needles of different lengths. These microstructures were associated with the compounds identified by XRD and thermal analysis (i.e., thermostatrite, pirssonite, portlandite, and calcite--Figures 7 and 8). To link a microstructure to a compound, a point-counting procedure was conducted. EDX analysis was performed in selected areas, identifying and quantifying the elements present. The quantification was conducted in atomic percentages using the standardless PB-ZAF method in EDX software ([[https://edx.com/](https://edx.com/)]([https://edx.com/](https://edx.com/)), accessed on 16 April 2025). Then, the Na:Ca ratios were calculated and compared to the theoretical ratios of the identified compounds. They were: 1.15 for pirssonite and 0.00 for calcite and/or portlandite. Figure 9 shows the selected areas for the EDX analysis.  
The Na:Ca ratios were 0.1, 1.6, 2.3, and 3.1 for areas I, II, III, and IV, respectively. The shape and the Na:Ca ratio of area I were very close to the shape and values reported in the literature [20], which confirms unreacted or partially reac  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**NaOH** **Molarity (mol/l)** & **12.9** & **15.6** & **19.0** \\ \hline \multirow{8}{*}{**Related products and temperature range (\({}^{\circ}\)C)**} & **Thermonatric** & \multirow{2}{*}{1.8} & \multirow{2}{*}{1.1} & \multirow{2}{*}{1.3} \\  & **(25–140)** & & & \\ \cline{1-1} \cline{5-5}  & **Pirssonite (140–335)** & & & \\ \cline{1-1} \cline{5-5}  & **Portlandite (335–560)** & & & \\ \cline{1-1} \cline{5-5}  & **Carbonated phases (560–1100)** & & & \\ \hline \multicolumn{2}{c}{**Weight loss from curing (\%)**} & 8.2 & 6.7 & 5.5 \\ \hline \multicolumn{2}{c}{**Total water (\%)**} & 12.2 & 12.4 & 10.2 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Weight loss and water balance of prismatic specimens under different concentrations of NaOH.  
Figure 8: SEM images of the 3D printed sample with different NaOH concentrations: (**a**) 12.9 mol/L; (**b**) 15.6 mol/L; and (**c**) 19 mol/L.  
areas II and IV were similar, but their Na:Ca ratio is different. This suggests that their degree of crystallization might be different or that these crystals may have different natures. They could be related to pirsonite and/or thermonatric. Area III presents a different shape and an intermediate Na:Ca ratio, indicating a possible mix of the reaction products (i.e., portlandite, pirsonite, and thermonatric). These microstructural differences are important because they can affect the overall performance of the material. For instance, areas containing unreacted calcite could result in localized weak areas that are detrimental to mechanical strength, while mixed reaction product regions could be a sign of continued hydration or phase stability variations. Heterogeneity in this way could affect durability through its influence on the material's response to environmental conditions, such as moisture or carbonation.  
Different microstructures were observed, regardless of the NaOH concentration used, including dense areas, grainy areas, and needles of different lengths.