## 3 Results and Discussion
### Micro Hardness Analysis  
Hardness values were obtained from Vickers micro hardness tests on polished cross sections of the unsoaked samples. These values are depicted in Table 6 and compared with the hardness of different biocercamics and bone tissues. As adjacent MgO phases dissolve, an open porosity forms, and this provides transport channels for the migration of protons and Mg\({}^{2+}\) ions. In this case, the contiguity and volume fraction of the MgO phases are important determining factors in the penetration depth of the dissolution reaction. No external HA layer that would block the dissolution of the MgO was formed. For this reason, the dissolution of the MgO phase could continue, which gave rise to a porous scaffold with  
Figure 6: SEM micrographs of a directionally solidified rod grown at 50 mm/h after soaking in the simulated body fluid (SBF) solution for 28 days: longitudinal cross section (**a**) and an extended area of the sample surface in contact with the fluid (**b**). channels between 5 and 10 \(\upmu\)m in thickness. MgO dissolution is controlled by chemical reactions (Equations (1)-(3)) that involve the dissolution of MgO in a liquid medium to produce Mg\({}^{2+}\) and OH\({}^{-}\). As a consequence of water molecules, an intermediate brucite product was generated and subsequently dissociated into Mg\({}^{2+}\) and hydroxyl ions that form water by protonation. \[\text{MgO(s)}+\text{H}_{2}\text{O(l)}\rightarrow\text{MgOH}^{+}\text{(aq)}+ \text{OH}^{-}\text{(aq)} \tag{1}\]  
\[\text{MgOH}^{+}\text{(aq)}+2\text{H}_{2}\text{O(l)}\rightarrow\text{Mg(OH)}_{ 2}\text{(l)}+\text{H}_{3}\text{O}^{+}\text{(aq)} \tag{2}\]  
\[\text{Mg(OH)}_{2}\text{(l)}\rightarrow\text{Mg(aq)}^{2+}+2\text{OH}^{-}\text{(aq)} \tag{3}\]  
In the zone where MgO dissolution occurred, the monticellite phase (1) remained unchanged, while the composition of the other phase (2) changed, losing silicon and incorporating phosphorus to a composition similar to that of the montetite (CaHPO\({}_{4}\)), as indicated in Table 7.  
These compositional changes of the phases occurred by interaction of the sample surface with the liquid medium and have an influence on the final composition of the fluid. In Table 8, we present the ICP results of the SBF analysis after being in contact with a directionally solidified rod compared with the concentration values in the SBF before the bioactivity test. As can be seen in Table 8, during the reaction of the ceramic composite in SBF, the calcium and phosphorus ion concentrations did not increase during exposure, although an increase in the magnesium and silicon ion concentrations can be observed. This fact indicates the surface dissolution of the MgO primary phase and release to the fluid of Si\({}^{4+}\) from the soluble amorphous silicate phase (Table 3, Phase 3). Both the increase in the concentration of Mg ion in SBF after the bioactivity test and the porous structure formation evidence the dissolution of the MgO primary phase. This mechanism led to the \"in situ\" formation of a porous structure that could reproduce the bone structure and its mechanical properties. The increase in Mg and Si ion concentration is attributed to the exchange of Mg and Si ions from the sample with H\({}^{+}\) ions from the fluid. The MgO dissolution could proceed by the breaking of the relatively weak ionic divalent metal-oxygen bonds, which liberated the Mg ions directly into solution. This fast dissolution of magnesia contrasts to the behavior of other multi-oxide phases that require the breaking of more than one type of metal-oxygen bond. In these cases, the dissolution mechanism involves the sequential breaking of the bonds, which follows an order according to their reactivity. Some studies of the forsterite-rich olivine dissolution mechanisms have been reported by [PERSON] et al. [19]. They concluded that olivine dissolution rates are strongly influenced by pH, water activity, and mineral-fluid interfacial surface area.