# Non-Destructive Study of the Microstructural Effects of Sodium and Magnesium Sulphate Attack on
## 1 Introduction  
Currently, the use of non-destructive techniques for characterizing the microstructure of cement-based materials has become an important research field [1; 2; 3; 4; 5; 6; 7]. Among these techniques, impedance spectroscopy has been greatly developed during the last few years. Other authors have successfully studied cement-based materials using this non-destructive technique in order to observe the formation of Friedel's salt [8], study the effects of curing conditions [9] and pozzolanic additions [10], determine the non-steady state chloride diffusion coefficient [11], and the distribution of moisture [12]. However, the use of impedance spectroscopy to study the effects of sulphate attack on the microstructure of cement-based materials in general has hardly been considered [13]. The effect of sulphate attack is commonly studied in terms of expansion [14], mechanical strength loss [15] and mass variations [16], and the use of active additions to reduce sensitivity to sulphate attack. Microstructural changes are observed mainly by scanning electron microscopy (SEM) analysis [14], mercury intrusion porosimetry (MIP), thermogravimetry (TG), and X-ray diffraction (XRD). Little attention has been paid to the non-destructive study of the microstructural origins of changes in these properties, and especially in the case of the partial replacement of cement with silica fume, used as an addition to improve the sulphate resistance of cements [17; 18; 19] (among other positive effects). While both the deleterious effects of sulphate attack on Portland cement based materials, and the positive effect of silica fume addition on resistance to this phenomenon are well known [17; 19],relatively less attention has been paid to the effect of the associated cation. The classic deleterious effects of sulphate attack (ettringite and gypsum formation) are associated with the sulphate anion. It has been suggested that among the most typical associated cations, calcium and sodium are innocuous, while magnesium is aggressive [20]. On one hand, the chemical reactions produced by the sodium sulphate attack are shown in Equations (1) and (2). The reaction shown in Equation (2) produces the formation of expansive ettringite crystals. \[\mathrm{Ca(OH)_{2}+Na_{2}SO_{4}+2H_{2}O\to CaSO_{4}\cdot 2H_{2}O+2 NaOH} \tag{1}\]  
\[3 CaO\cdot Al_{2}O_{3}+3 CaSO_{4}\cdot 2H_{2}O+26H_{2}O\to 3 CaO\cdot Al_{2}O_{3}\cdot 3 CaSO_{4} \cdot 32H_{2}O \tag{2}\]  
On the other hand, the chemical reactions produced by the magnesium ion are shown in Equations (3) and (4). The reaction shown in Equation (3) results in the formation of magnesium hydroxide, or brucite, and is not greatly deleterious as calcium hydroxide does not contribute much to the strength of the hardened cement paste. The reaction shown in Equation (4) is referred to as decalcification henceforth, and results in a severe loss of compressive strength due to the conversion of conglomer calcium silica hydrate gels into non-conglomerate magnesium silica hydrates [21; 22; 23]. \[\mathrm{Mg^{2+}+Ca(OH)_{2}\to Mg(OH)_{2}+Ca^{2+}} \tag{3}\]  
\[\mathrm{Mg^{2+}+C-S-H\to M-S-H+Ca^{2+}} \tag{4}\]  
In addition, several authors have observed that the performance of mortars and concretes containing silica fume is worse than those without when exposed to magnesium sulphate solutions [18; 19; 22; 23; 24; 25; 26]. It has been suggested that due to the lower availability of calcium hydroxide, the reaction shown in Equation (3) does not take place to the same extent. It has been suggested that this reaction is preferential and leads to a lower availability of magnesium ions for the second, more damaging decalcification reaction shown in Equation (4) [23]. The formation of a double layer of brucite and gypsum has been observed [19], and it has been suggested that this forms a barrier, reducing the permeability of the cement paste [14; 27].