## 3 Results and Discussion
### Identification of the Reaction Product  
The XRD spectra of the 3D-printed specimens are shown in Figure 6.  
All of the XRD patterns show the peaks of quartz related to the presence of standardized sand and calcite from unreacted limestone in the 3D-printed samples. Irrespective of the NaOH molarity, the reaction products are thermonatrite (Na\({}_{2}\)CO\({}_{3}\), H\({}_{2}\)O), portlandite (Ca(OH)\({}_{2}\)), and prissonite (Na\({}_{2}\)Ca(CO\({}_{3}\))\({}_{2}\), 2H\({}_{2}\)O). Moreover, calcite peaks are visible. They could be associated with unreacted limestone.  
Figure 5: Weight loss of printed samples after printing (24 h curing on the powder bed, 48 h curing at ambient conditions and then drying at 45 \({}^{\circ}\)C).  
Figure 6: XRD analysis patterns and identified crystalline phases.  
TGA\(-\)DTA diagrams of samples printed with different NaOH water solutions are shown in Figure 7. They present five endothermic peaks with weight loss and two endothermic peaks without weight loss. Below 200 \({}^{\circ}\)C, the observed peaks were attributed to dissociation reaction (water release) in thermonatrite (around 112 \({}^{\circ}\)C) and pirssonite (around 170 \({}^{\circ}\)C) [24] according to the following reactions, respectively.  
\[\text{Na}_{2}\text{CO}_{3},\text{H}_{2}\text{O}\rightarrow\text{Na}_{2}\text{ CO}_{3}+\text{H}_{2}\text{O} \tag{1}\]  
\[\text{Na}_{2}\text{Ca}(\text{CO}_{3})_{2},\text{2H}_{2}\text{O}\rightarrow\text{ Na}_{2}\text{Ca}(\text{CO}_{3})_{2}+\text{2H}_{2}\text{O} \tag{2}\]  
The peak around 440 \({}^{\circ}\)C was related to the dehydration of portlandite (Equation (3)) and was consistent with the literature [25].  
\[\text{Ca}(\text{OH})_{2}\rightarrow\text{Ca}\text{O}+\text{H}_{2}\text{O} \tag{3}\]  
Above 600 \({}^{\circ}\)C, the peaks were linked to the decarbonisation of calcareous compounds: according to previous literature [24], nyerecite (Na\({}_{2}\)Ca(CO\({}_{3}\))\({}_{2}\) from dehydration of pirssonite) and limestone, according to the following reactions.  
\[\text{Na}_{2}\text{Ca}(\text{CO}_{3})_{2}\rightarrow\text{Na}_{2}\text{CO}_{3 }+\text{Ca}\text{O}+\text{CO}_{2} \tag{4}\]  
\[\text{Ca}\text{CO}_{3}\rightarrow\text{Ca}\text{O}+\text{CO}_{2} \tag{5}\]  
The peaks without weight loss, at 386 \({}^{\circ}\)C and around 568 \({}^{\circ}\)C, were associated with crystallographic transformations in the nyerecite structure (low-temperature form to high-temperature form) [26] resulting from dehydration of pirssonite and to allotropic transformation of quartz (\(\alpha\)\(\leftrightarrow\)\(\beta\)), reported previously [27].  
The identified compounds (i.e., thermonatrite, pirssonite, portlandite, calcite, and quartz) were previously identified through XRD analysis. This indicates that all the reaction products were crystallized, whatever the NaOH concentration. The main differences between samples came from the TGA curves and, so, from the quantities of reaction products. Table 1 summarizes all the weight loss measured on TGA curves and the final weight loss after curing (i.e., free water evaporation).  
Figure 7: TGA/DTA analysis diagrams.  
The amount of portlandite appears to remain stable, regardless of the NaOH solution's molarity, while the amount of the other reaction products varies with the molarity.