Non-Destructive Study of the Microstructural Effects of Sodium and Magnesium Sulphate Attack on Mortars Containing Silica Fume Using Impedance Spectroscopy
Abstract
:1. Introduction
2. Materials and Methods
2.1. Sample Preparation and Curing
2.2. Sulphate Solutions
2.3. Impedance Spectroscopy
2.4. Resistivity
2.5. Mercury Intrusion Porosimetry
2.6. Compressive Strength
3. Results
3.1. 0% Silica Fume Mortars
3.2. 5% and 10% Silica Fume Mortars
4. Discussion
4.1. Microstructure Results
4.2. Compressive Strength Results
5. Conclusions
- The silica fume mortars immersed in the 15% Na2SO4 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% Na2SO4 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% Na2SO4 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.
- 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.
Acknowledgments
Author Contributions
Conflicts of Interest
References
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Compound | CEM I 42.5R (%) | Silica Fume (%) |
---|---|---|
CaO | 64.5 | 0.4 |
SiO2 | 20.0 | 94.1 |
Al2O3 | 5.7 | 0.3 |
Fe2O3 | 2.5 | 0.1 |
SO3 | 3.2 | 0.1 |
MgO | 0.9 | 0.3 |
Na2O | 0.1 | 0.6 |
K2O | 1.0 | 0.6 |
Solution Name | Mg2+ (g/L) | SO42− (g/L) | Mass Ratio Mg2+/SO42− | Mg2+ (mol/L) | SO42− (mol/L) | Molar Ratio Mg2+/SO42− |
---|---|---|---|---|---|---|
15% MgSO4 | 30.290 | 119.710 | 0.253 | 1.246 | 1.246 | 1.000 |
15% Na2SO4 | 0 | 101.442 | 0 | 0 | 1.056 | 0 |
7.5% MgSO4 + 7.5% Na2SO4 | 15.145 | 110.576 | 0.136 | 0.621 | 1.151 | 0.539 |
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Williams, M.; Ortega, J.M.; Sánchez, I.; Cabeza, M.; Climent, M.Á. Non-Destructive Study of the Microstructural Effects of Sodium and Magnesium Sulphate Attack on Mortars Containing Silica Fume Using Impedance Spectroscopy. Appl. Sci. 2017, 7, 648. https://doi.org/10.3390/app7070648
Williams M, Ortega JM, Sánchez I, Cabeza M, Climent MÁ. Non-Destructive Study of the Microstructural Effects of Sodium and Magnesium Sulphate Attack on Mortars Containing Silica Fume Using Impedance Spectroscopy. Applied Sciences. 2017; 7(7):648. https://doi.org/10.3390/app7070648
Chicago/Turabian StyleWilliams, Mark, José Marcos Ortega, Isidro Sánchez, Marta Cabeza, and Miguel Ángel Climent. 2017. "Non-Destructive Study of the Microstructural Effects of Sodium and Magnesium Sulphate Attack on Mortars Containing Silica Fume Using Impedance Spectroscopy" Applied Sciences 7, no. 7: 648. https://doi.org/10.3390/app7070648