# Non-Destructive Study of the Microstructural Effects of Sodium and Magnesium Sulphate Attack on
## 4 Discussion
### Microstructure Results
Visible examination of the photographs presented in Figures 18 and 19 suggests that pores of this size may have been formed. As such, the apparent illogical reduction may have been due to the formation of these larger pores, due to an extremely aggressive attack. Additionally, the technique has some intrinsic limitations, and some authors allege that it overestimates the volume of the smaller pores present [47]. Also, the mechanical strength results suggest a loss of strength, while the pore network does not change significantly or even gets more refined. This result adds value to this paper, because the most used (destructive) technique for the pore network characterization of cement based materials seems not to be useful in this case and there  
Figure 17: Results of the X-ray diffraction analysis performed on the white precipitate which formed on samples exposed to magnesium sulphate solutions. The formulations of the phases are sepiolite, Mg\({}_{4}\)Si\({}_{6}\)O\({}_{15}\)(OH)\({}_{2}\cdot\)6H\({}_{2}\)O; gypsum, CaSO\({}_{4}\cdot\)2H\({}_{2}\)O; posnjaklite, Cu\({}_{4}\)SO\({}_{4}\)(OH)\({}_{6}\cdot\)H\({}_{2}\)O; epsomite, MgSO\({}_{4}\cdot\)7H\({}_{2}\)O; magnesium sulphate hydrate, M–S–H; thenadite, Na\({}_{2}\)SO\({}_{4}\). is a need to investigate new (non-destructive) techniques to study microstructural changes due to sulphate attack. Regarding the microstructure of the 0% silica fume mortars, the impedance spectroscopy results were similar for all samples (see Figure 5). The initial relatively high values of the \(R_{1}\), \(R_{2}\), and \(C_{1}\) parameters could be a consequence of cement hydration. In the long-term, the resistances \(R_{1}\) and \(R_{2}\) continued increasing, and the capacitance \(C_{1}\) remained constant or even decreased. As \(C_{1}\) is most closely associated with the total porosity and solid volume of the samples, and resistances have been reported to be more greatly affected by the pore network tortuosity [12], it may be inferred that the formation of expansion products led to an increase in tortuosity in all samples. This was likely due to the formation of reaction products and not cement hydration, as a study on the hydration of paste made using the same cement found that the parameter \(R_{1}\) remained constant [48]. Additionally, the lack of an increase in the \(C_{1}\) parameter suggests that the total volume of the reaction products is insignificant. A possible interpretation of the progressive fall of the \(C_{2}\) capacitance (Figure 5d) in all samples (parameter related to the total pore surface area), could be that the reaction products formed were crystalline in nature, as observed in the Scanning Electron Microscope photographs presented by other authors [19]. As such, their formation may have covered the irregular and rough hydrated cement pore walls with regular forms, leading to an overall reduction in surface area. As discussed previously for the silica fume mortars, only minor changes in the microstructure were observed using mercury intrusion porosimetry, and this result could also be due to the previously mentioned limitations of this technique for registering pores greater than 900 \(\upmu\)m.  
Figure 19: **(a)** Visual condition of the compressive strength samples containing 10% silica fume after 90 days exposure to the 15% sodium sulphate solution; **(b)** Visual condition of the compressive strength samples containing 10% silica fume after 90 days exposure to the mixed sulphate solution; **(c)** Visual condition of the compressive strength samples containing 10% silica fume after 90 days exposure to the 15% magnesium sulphate solution.