# Non-Destructive Study of the Microstructural Effects of Sodium and Magnesium Sulphate Attack on
## 3 Results
### 5% and 10% Silica Fume Mortars  
The variation in impedance spectroscopy parameters for the mortars containing 5% silica fume is shown in Figure 9. The resistance \(R_{1}\) showed a similar tendency (see Figure 9a) for the different aggressive media; this parameter increased at early ages and decreased later. However, the greatest  
Figure 8: Variation in compressive strength in mortars containing 0% silica fume exposed to the three different sulphate solutions. Figure 7: (**a**) Volume of pores per decade in samples containing 0% silica fume at 21, 60 and 90 days exposure to the three solutions; (**b**) Percentage of mercury retained in samples containing 0% silica fume at 21, 60 and 90 days exposure to the three solutions. decrease was observed in the samples exposed to the mixed sulphate solution, whereas the samples exposed to the 15% MgSO\({}_{4}\) solution only experienced a slight reduction. This reduction was not observed in the \(R_{2}\) and \(C_{1}\) parameters in the same samples (see Figure 9b,c) and in those exposed to the mixed solution, it was much less pronounced than the reduction in the \(R_{1}\) resistance. The \(C_{2}\) capacitance results (see Figure 9d) fell progressively for all the aggressive solutions studied, although the greatest decrease corresponded to the samples exposed to the mixed sulphate solution. The resistivity results for the 5% silica fume mortars correlated well with the \(R_{1}\) parameter (see Figure 10), although the reduction in the samples exposed to the 15% MgSO\({}_{4}\) solution was somewhat greater. Figure 9: (**a**) Variation in impedance spectroscopy resistance \(R_{1}\) in mortars with 5% silica fume; (**b**) Variation in impedance spectroscopy \(R_{2}\) in mortars with 5% silica fume; (**c**) Variation in impedance spectroscopy capacitance \(C_{1}\) in mortars with 5% silica fume; (**d**) Variation in impedance spectroscopy capacitance \(C_{2}\) in mortars with 5% silica fume. The impedance spectroscopy results for the 10% silica fume mortars, presented in Figure 11, differed from those measured for the 5% silica fume ones in several aspects. Firstly, the reduction in the \(R_{1}\) parameter (see Figure 11a) was much greater in the samples exposed to the mixed sulphate solution, and by the end of the test period the resistance values were in fact lower than at the beginning. The samples exposed to the 15% MgSO\({}_{4}\) solution also showed a more noticeable decrease in \(R_{1}\) resistance with the exposure period. However, the greatest differences were observed in the \(R_{2}\) and \(C_{1}\) parameters (see Figure 11b,c). In the samples exposed to the mixed sulphate solution, a clear decrease may be observed in these values, which was especially noticeable in the \(C_{1}\) capacitance (see Figure 11c). The 10% silica fume samples exposed to the 15% Na\({}_{2}\)SO\({}_{4}\) solution showed a constant increase in both \(R_{1}\) and \(R_{2}\). The capacitance \(C_{2}\) for these samples (see Figure 11d) was also lowest in the samples exposed to the mixed sulphate solution, and remained relatively constant in the samples exposed to the 15% Na\({}_{2}\)SO\({}_{4}\) solution. The resistivity results of 10% silica fume mortars (see Figure 12) also correlate well with the \(R_{1}\) resistance values obtained, as noted in the 5% silica fume mortars. Figure 11: _Cont._  
Figure 10: Variation in resistivity in mortars containing 5% silica fume. Regarding the mercury intrusion porosimetry results for the 5% silica fume mortars (see Figure 13), the samples exposed to the mixed solution showed a reduction in intrusion volume, which would suggest a decrease in porosity, and for the other solutions this parameter hardly changed. For the 10% silica fume mortars (see Figure 14), it was observed that only a slight increase in intruded mercury was recorded in the samples exposed to solutions containing magnesium, and in the samples exposed to the 15% Na\({}_{2}\)SO\({}_{4}\) solution, a slight reduction in the total volume of mercury intruded was observed.