# New Methods for Assessing External Sulfate Attack on Cement-Based Specimens
## 3 Results  
### Visual Inspections  
At different time intervals (up to 24 weeks), the macroscopic degradation of cement paste specimens was observed during the ESA. The main stages of this degradation are as follows:  
* After 12 weeks of exposure, a circumferential crack appeared at the exposed edge of the immersed base of the specimens (see Figure 5a).
* After 20 weeks, the cracks grew wider and larger, moving from the edges towards the center. Then the cylinders started to exhibit clearly visible crack patterns and began to lose their cohesion (Figure 5b).
* After 24 weeks, there was a significant deterioration of the cylinder at its immersed edge (see Figure 5c). At the periphery of the bottom base (immersed and not protected by epoxy), there was substantial damage and loss of cohesion, while at the center of this base, there were no visible signs of damage.  
It can be concluded that the expansion is more significant in the saturated portion of the semi-immersed sample (in contact with the Na\({}_{2}\)SO\({}_{4}\) solution), which results in significant tensile stresses near the periphery (see Figure 5). These observations are comparable to those obtained by [10] from the side and front. During ESA, significant tensile stresses developed near the surface, resulting in this particular cracking pattern characterized by the appearance of orthoradial macroracks and the deterioration of peripheral areas. The entire degradation process can be explained by the crystallization pressure theory, which states that the crystals must form from an oversaturated solution in order to generate the energy necessary for expansion. It is also crucial that the crystals precipitate in small pore spaces so that the stress generated by expansion is greater than the tensile strength of the material [10].  
Figure 5: Degradation of cement paste due to ESA: (**a**) after 12 weeks; (**b**) after 20 weeks; (**c**) after 24 weeks.  
During this experimental work, the ESA-induced expansion was not monitored. However, the visual macroscopic deformations suffered by the cement paste samples confirm the primary role of expansion in the degradation process (cracks appeared and propagated where the specimen deformed the most). It is notable that the observed deformation shape was consistent with the results of a calculation based on the model developed at Gustave Eiffel University, based on a thermodynamic approach to describe the formation of crystals inside porous medium [42; 43] to re-evaluate the structural damage caused by internal sulfate attack (ISA). Despite the fact that each type of sulfate attack has a specific physicochemical mechanism, [PERSON]'s model helped to predict qualitatively the deformation of the studied specimens subjected to ESA.  
As the modeling was not intended to be quantitative, it is not described here. Nevertheless, it is worth pointing out that the considered geometry of the samples and boundary conditions were all consistent with the experiments. Details of the modeling can be found in [44].  
In line with the numerical prediction of swelling (Figure 6a), circumferential cracks were experimentally observed at the surface of the portion exposed to the attacking solution (see Figure 6b,c).