# Non-Destructive Study of the Microstructural Effects of Sodium and Magnesium Sulphate Attack on
## 4 Discussion  
### Microstructure Results  
With respect to the non-destructive microstructure characterization of silica fume mortars, firstly the increase in the short-term of the impedance spectroscopy resistances \(R_{1}\) and \(R_{2}\) in the mortars exposed to the 15% Na\({}_{2}\)SO\({}_{4}\) aggressive medium (Figures 9 and 11) could be related to the fact that the pozzolanic reaction of silica fume consumes the portlandite present in the mortars before it can react with the sulphate ions in the solution. Other authors [10; 45] have observed that the pozzolanic hydration of cement pastes containing other pozzolanic additives such as fly ash continues well after 28 days. The increase in \(R_{1}\) appeared to tail off earlier in the 5% silica fume mortars, which may be due to the reduced amount of silica fume present. It may also be possible that Na\({}_{2}\)SO\({}_{4}\) acted as an activator for the pozzolanic reactions [46]. As such, in the samples containing silica fume these reactions would develop faster and their products would increase the tortuosity and the refinement of the mortars' microstructure, that is, an increase of volume of the finer pores of the material and a reduction of the volume of pores with relatively high sizes, as observed by other authors in samples between 28 and 90 days after casting [30]. Furthermore, in mortars without silica fume, the formation  
Figure 16: Variation in compressive strength in mortars containing 10% silica fume. Figure 15: Variation in compressive strength in mortars containing 5% silica fume. of expansive products at the initial stages of sulphate attack could also have an influence on the observed increase in impedance spectroscopy resistances. This may be because after initially filling the pores [27], the confinement of expansive reaction products would produce residual stresses, leading to a reduction in compressive strength but not the destruction of the microstructure. As such, it appears that for reasons that probably differ in each of the mortar types, the effect of sulphate ion exposure on the microstructure did not result in a deterioration of the microstructure in any of the samples, as the values of the resistances \(R_{1}\) and \(R_{2}\) rose. In the long-term, a reduction of impedance spectroscopy resistances was observed. For 5% silica fume mortars, this fall was small and the lowest values of these parameters at 90 days corresponded to samples exposed to mixed sulphate solution, which agreed with the electrical resistivity measurements. The capacitance \(C_{1}\), which is related to the solid fraction of the sample, did not differ too much for all of the 5% silica fume mortars, although a slight decrease was also observed at higher ages for those kept in contact with the mixed sulphate solution. These results observed for both resistances and capacitance \(C_{1}\) suggests that the principal effect of the decalcification observed was a reduction in the pore network tortuosity as opposed to an increase in the total porosity for the 5% silica fume mortars. This decrease in tortuosity could be confirmed by the decrease shown by the capacitance \(C_{2}\), meaning a smaller contact surface between the electrolyte and solid, possibly resulting from a slight dissolution of hydration products. For 10% silica fume samples, the decrease with the exposure period of resistance \(R_{1}\) was higher in the samples exposed to the mixed sulphate solution, and it was also considerable for those immersed in the 15% MgSO\({}_{4}\) solution. This suggests that the reduction in the tortuosity of the pore network was more significant in the 10% silica fume samples. Furthermore, as has been previously described, the reduction in the long-term values of the \(R_{2}\) and \(C_{1}\) parameters for samples in contact with the aforementioned solutions was much more significant than the fall in resistance \(R_{1}\). This could mean an important reduction in the solid volume of these samples, and an increase in porosity.