# Non-Destructive Study of the Microstructural Effects of Sodium and Magnesium Sulphate Attack on
## 4 Discussion
### Compressive Strength Results  
The highest compressive strength results for silica fume mortars corresponded to those exposed to the 15% Na\({}_{2}\)SO\({}_{4}\) solution (Figures 15 and 16). This is in agreement with previously discussed microstructure results, and it shows the beneficial effect of silica fume on compressive strength, as well as the possible role of Na\({}_{2}\)SO\({}_{4}\) as an activator of pozzolanic reactions of silica fume.  
The lowest compressive strengths in the long-term for silica fume mortars were observed in magnesium sulphate solutions, especially in the mixed sulphate solution. This result could be expected from the microstructural characterization test results, and could be due to the conversion of conglomerate calcium silica hydrate gels into non-conglomerate magnesium silica hydrates (see Figure 17), which brings a severe loss of compressive strength, according to several authors [21; 22; 23]. Nevertheless, in the short-term the compressive strength was smaller for the samples in contact with the 15% MgSO\({}_{4}\) solution than for those exposed to the mixed solution. This result could suggest that the presence of the magnesium ion may have interfered with the hydration process where silica fume blended cements were used.  
In spite of that, in general the extent of deterioration in the samples exposed to the 15% MgSO\({}_{4}\) solution was lower than in the results presented by other authors [18; 19; 21; 22; 23; 25; 26], and it is important to emphasize that the most severe deterioration did not occur in the solution with the greatest concentration of magnesium, as discussed previously. This could mean that the deterioration mechanism in the samples containing silica fume blended cements is complex, and possibly that the deterioration does not depend solely on the concentration of magnesium ions present. Other authors have noted that in solutions with different ionic ratios, the effects are not necessarily in line with those that would be expected considering solely the concentrations of individual aggressive ions present [53].  
With respect to 0% silica fume mortars, the highest compressive strengths were observed for the 15% MgSO\({}_{4}\) solution, despite this solution having the greatest concentration of sulphate ions (see Table 2), whereas the loss in this parameter was greatest in the samples exposed to the 15% Na\({}_{2}\)SO\({}_{4}\) solution (Figure 8). Therefore, it appears that the presence of magnesium reduces the deleterious effect of sulphate exposure in terms of compressive strength for mortars without silica fume. In fact, there was a clear positive correlation between the ionic ratio of magnesium to sulphate and compressive strength. This appears to correlate well with the results obtained by other authors using less concentrated solutions [19], and strongly suggests that the formation of Brucite reduced the extent of attack by sulphate ions. The reduction in compressive strength observed in samples exposed to the sodium sulphate solution may have resulted from internal stresses caused by the formation of expansive reaction products within the pore network, in agreement with the attack mechanism recently proposed by [PERSON] et al. [28]. It may be observed from the photographs presented in Figure 20 that the external parts of the samples had broken away, as suggested by the aforementioned mechanism.  
Lastly, regarding the comparison between the mortars with and without silica fume, it is important to highlight that the compressive strength of the silica fume mortars was much more affected by exposure to the magnesium sulphate solutions, while the greatest deleterious effect on the strength for mortars without silica fume addition was due to exposure to the 15% Na\({}_{2}\)SO\({}_{4}\) solution. However, for all aggressive media, the compressive strengths of silica fume mortars were higher than those observed for mortars without silica fume during the exposure period studied.