# Non-Destructive Study of the Microstructural Effects of Sodium and Magnesium Sulphate Attack on
## 5 Conclusions  
The main conclusions that can be drawn from the results previously discussed can be summarized as follows:  
* The silica fume mortars immersed in the 15% Na\({}_{2}\)SO\({}_{4}\) solution performed well, and showed the most refined microstructure and highest compressive strengths compared to the samples exposed to the other aggressive media studied. This could be related to the beneficial effect of pozzolanic hydration of the silica fume additive, and also thepossible effect of 15% Na\({}_{2}\)SO\({}_{4}\) as an activator of pozzolanic reactions in silica fume. * In all samples containing silica fume, the deterioration observed was greatest in samples exposed to the magnesium sulphate solutions, especially for those exposed to the mixed one, despite the concentration of magnesium ions being lower. This could be due to the formation of non-conglomerate magnesium silica hydrates (as shown by the XRD analysis). However, the mixed solution results appear to suggest that the deterioration mechanism taking place in samples with silica fume blended cement exposed simultaneously to magnesium and sulphate ions is complex, and does not solely depend on reactions involving the magnesium ion. * In samples without silica fume, the microstructural effects resulting from exposure to each of the aggressive media did not vary greatly. However, regarding the compressive strength, the presence of magnesium appears to have a positive effect on this property, whereas the exposure to the 15% Na\({}_{2}\)SO\({}_{4}\) produced a high loss of strength in these samples. The beneficial effect of magnesium could probably be a result of the formation of a Brucite layer, and the negative effect of sodium sulphate on the compressive strength would be a consequence of internal stresses caused by the formation of reaction products. Figure 20: **(a)** Visual condition of the compressive strength samples containing 0% silica fume after 90 days exposure to the 15% sodium sulphate solution; **(b)** Visual condition of the compressive strength samples containing 0% silica fume after 90 days exposure to the mixed sulphate solution; **(c)** Visual condition of the compressive strength samples containing 0% silica fume after 90 days exposure to the 15% magnesium sulphate solution. * Mercury intrusion porosimetry appears to have limitations when used to monitor changes in cement based samples resulting from magnesium and sodium sulphate attack, possibly due to the influence of pores larger than the maximum diameter measured by the apparatus. * The results of this study indicate that the addition of silica fume brings a more refined microstructure and a higher compressive strength to mortars exposed to aggressive sodium and magnesium sulphate solutions, compared to mortars with no additions, during the study period. * According to the microstructural characterization results of this study, it appears that non-destructive impedance spectroscopy is the most sensitive technique for detecting the changes in the porous network of mortars with and without silica fume exposed to aggressive sodium and magnesium sulphate solutions. The research work included in the paper has been financially supported by the \"Ministerio de Economia y Competitividad\" (formerly \"Ministerio de Ciencia e Innovacion\") of Spain and FEDER through projects BIA2010-20548 and BIA2011-25721, and by the University of Alicante through project GRE13-25. The authors also wish to thank Cementos Portland Valderrivas S.A. and Grupo Ferroatlantica S.A. for providing the cement and silica fume (respectively) used in this study. The results included in this paper were obtained in the Master's final project carried out by [PERSON], under the supervision of [PERSON], [PERSON], and [PERSON], to obtain the Materials, Water, and Soil Engineering Master's degree at the University of Alicante (Spain). [PERSON] and [PERSON] wrote the paper and performed the experiments. [PERSON], [PERSON] and [PERSON] supervised the research work and revised the paper.