# Non-Destructive Study of the Microstructural Effects of Sodium and Magnesium Sulphate Attack on
## 4 Discussion
### Microstructure Results
This indicates that the deleterious reactions were especially severe, and is also corroborated by the electrical resistivity results (Figure 12). This also reinforces the idea that the deterioration mechanism is not proportional to the concentration of magnesium present, as the samples exposed to the 15% MgSO\({}_{4}\) solution only experienced a reduction in pore network tortuosity (related to \(R_{1}\)). Regarding capacitance \(C_{2}\), this parameter is associated with the pore surface in contact with the electrolyte present in the material and it is related to the amount of wet pore surface. The \(C_{2}\) parameter increased at early ages for silica fume samples exposed to 15% Na\({}_{2}\)SO\({}_{4}\). This result is in agreement with the results observed in the resistances \(R_{1}\) and \(R_{2}\) results already discussed. The rise of capacitance \(C_{2}\) could indicate the formation of solids on the pore walls, probably due to continued pozzolanic hydration, and which may be accelerated by the activation effect of Na\({}_{2}\)SO\({}_{4}\)[46], or the formation of products of the reaction with sulphate ions [27]. Nevertheless, at higher ages the \(C_{2}\) capacitances measured in silica fume mortars in contact with the Na\({}_{2}\)SO\({}_{4}\) solution decreased. This could suggest that the negative effects of exposure to the solution in the mortar's microstructure began to be noticeable, producing a progressive destruction of irregularities on the pore surface. The silica fume mortars immersed in 15% MgSO\({}_{4}\) and in mixed sulphate solution showed a small increase in capacitance \(C_{2}\) at very early ages, although this parameter started to fall quickly. The lowest values were observed for the mixed sulphate solution. This result would suggest that there was a loss of pore surface area, which was greatest in the samples exposed to the mixed sulphate solution. This could mean that the deleterious reactions affected the surface area of the pore network in the samples exposed to solutions containing magnesium, probably due to the dissolution of the most exposed hydrated cement structures within the pore network as a result of decalcification, resulting in a \"smoothing\" of the pore walls. The more severe deterioration in the microstructure of silica fume mortars exposed to mixed solution, indicated by impedance spectroscopy parameters, was almost certainly caused by the reaction described in Equation (4), as an X-ray diffraction analysis of the white precipitate which formed on the samples (whose presence was also noted by other authors [19]) showed the presence of the magnesium silica hydrate Sepiolite (see Figure 17). It should be noted that although the peaks are relatively small, only crystalline compounds are detected by X-ray diffraction, and many magnesium silica hydrates are amorphous. Regarding the mercury intrusion porosimetry results, the higher volume of smaller pores observed for the silica fume mortars exposed to 15% Na\({}_{2}\)SO\({}_{4}\) could indicate that continued pozzolanic hydration results in a refinement of the mortars' microstructure. These results are in keeping with those obtained using impedance spectroscopy, and they would corroborate the aforementioned possible influence of Na\({}_{2}\)SO\({}_{4}\), which could play a role as an activator of pozzolanic reactions of silica fume [46]. However, the rest of the results of this technique did not correlate well with the other microstructure results observed, for example the previously mentioned decrease of intrusion volume in the 5% silica fume mortars immersed in the mixed solution, and also the fact that the Hg retained remained practically constant for all silica fume samples during the research (Figures 13 and 14), which would appear to imply that the pore network tortuosity and porosity were unaffected by the aggressive media. These results may possibly be explained by the fact that all pores greater than 900 \(\upmu\)m were not registered by the apparatus.