## 3 Results and Discussion
### Characterization of the Cement Pastes
#### 3.1.2 Printability and Setting Properties
The optimal printing speed for extruding the investigated MgO/SiO\({}_{2}\) pastes was found to be 5 mm/s, which achieved the best results with the lowest Rp values (also see Table 1), associated with better shape fidelity. A lower printing velocity of 2.5 mm/s could also be used to achieve extrudable pastes, but this would lead to poorer printability (higher Rp values) and would be more time-consuming. Higher printing velocities were unsuitable for these samples as they led to the formation of voids and incomplete shapes, thus being inappropriate for the preparation of 3D structures. For example, Figure 3B shows images of two tridimensional prints: one obtained under the optimal selected printing conditions (Figure 3B, top), allowing for the preparation of customized ceramics, and another using non-optimized printing parameters (Figure 3B, bottom), inappropriate for the preparation of forsterite scaffolds. When comparing different formulations, it is clear that cellulose derivatives, which influence the plasticity of the formulations, can affect the printability quality of the samples. Specifically, samples containing HPMC exhibited a higher roughness Rp than those using CMC (see Table 1), indicating that CMC should be preferred when designing 3D printable M-S-H.  
To understand if the use of different cellulose derivatives would also influence the phases forming during the hardening of the cements, printed samples were investigated using XRD after 2 days of hydration, as reported in Figure 4. All cement-based samples display a similar bulk composition, confirming the formation of an abundant amorphous \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Printing Speed** & \multirow{2}{*}{**Rp [mm]**} & \multirow{2}{*}{**Extrudability**} \\  & **[mm/s]** & & \\ \hline HPMC\_5 & 2.5 & 0.44 & Continuous and controllable \\  & 5 & 0.35 & Continuous and controllable \\  & 7.5 & 0.43 & Continuous and controllable \\  & 10 & 0.49 & Discontinuous \\ \hline HPMC\_10 & 2.5 & 0.37 & Continuous and controllable \\  & 5 & 0.36 & Discontinuous \\  & 7.5 & 0.47 & Discontinuous \\  & 10 & 0.36 & Discontinuous \\ \hline CMC\_5 & 2.5 & 0.19 & Discontinuous \\  & 5 & 0.18 & Continuous and controllable \\  & 7.5 & 0.47 & Discontinuous \\  & 10 & 0.23 & Discontinuous \\ \hline CMC\_10 & 2.5 & 0.18 & Continuous and controllable \\  & 5 & 0.15 & Continuous and controllable \\  & 7.5 & 0.50 & Discontinuous \\  & 10 & 0.29 & Discontinuous \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Rp roughness parameter of the 2D printed cements extruded at different velocities, together with an evaluation of their extrudability. binder phase, M-S-H, upon the hydration of MgO/SiO\({}_{2}\). The presence of some unreacted Mg(OH)\({}_{2}\) is reasonably ascribed to the use of abundant MgO, since all samples were prepared using MgO/SiO\({}_{2}\) in a 2/1 molar ratio to favor the formation of the Mg\({}_{2}\)SiO\({}_{4}\) ceramic phase upon heating. Thus, set 3D-printed cements were employed to prepare the biocercamics of interest, as described in Section 2.4.