## 2 Sulfate Corrosion--Basic Knowledge of the Process
It is a reaction, among other things, to the non-conceptual nature of research in this field and to the situation where some scientists have turned into participants in a competition for the best SEM image of concrete minerals. There is agreement, on the other hand, that sulfate corrosion can be classified as corrosive damage caused by the expansion of corrosion products. According to commonly used classifications, it is a type III corrosion--a type designating corrosive actions where the final cause of the destruction of a material is the internal expansion stress in concrete [16]. It is also crucial to be aware of the nature of the process of a given type or kind of corrosion, especially prior to formulating general and subsequently specific answers to the question of how to prevent that corrosion. We, therefore, follow the above-discussed analysis of the properties of newly formed products with a simplified description of the corrosion process itself. During sulfate corrosion, SO4\({}^{2-}\) ions first penetrate the original pore structure of concrete and react with those components of cement or concrete which have the highest affinity to them, i.e., C3A, Al2O3 and C4 AF. The pore structure of concrete is thus filled with the products of the corrosion reaction. The rate of filling depends primarily on the time, temperature and concentration of the attacking sulfates and, logically, also on the properties of the given concrete. Simultaneously with the filling up, i.e., in fact reinforcing of the pore structure, most mechanical properties of the given concrete are consequently temporarily improved. The word \"temporarily\" designates a paradox of this phenomenon. Less resistant concrete (C1 in Figure 2) reacts with sulfates more intensively, therefore the whole corrosion process is faster. When compared to the resistant concrete C2, the partial improvement of the properties of the non-resistant concrete C1 is more significant, but also significantly shorter. As a result, when comparing resistance of several concretes, it is not adequate to monitor only their compressive strength (especially for short time experiments), but it is necessary to monitor other parameters that are not deformed by the described phenomenon (mass, length changes, cracking, etc.). However, it is advisable to wait until the samples have disintegrated. Due to their increase in volume, the products of the corrosion reaction (ettringite, etc.) create gradually increasing stresses in the concrete structure. If these stresses exceed the tensile strength of the concrete, the concrete starts to disintegrate--not into dust, but into typical fragments cracked in a direction perpendicular  
Figure 1: (**a**) Crystals of ettringite [14], (**b**) crystals of thuamisite. to the direction of the main tensile stresses. Due to the above-mentioned reasons, the work described below does not discuss compressive strength of concrete as an indicator of durability of the tested materials when exposed to sulfates. Similar experience has been gained by other authors as well [17; 18; 19; 20]. Interestingly, this possible problem has not been discussed at all in the world's most published monographs [5; 6]. On the other hand, [PERSON] [11] approaches this possible problem very seriously. Simply \"The confused world of sulphate attack on concrete\". The explanation of the sulfate corrosion process described above also implies a way to prevent sulfate corrosion. Basically, there are the following options:  
1. To include materials in the binder/cement which do not react with sulfate ions or react only to a very limited degree;
2. To produce a very dense, diffusion-limiting matrix of cement paste and aggregate that would make it more difficult or even impossible for sulfates to penetrate the concrete mass;
3. In contrast to option 2, to produce a matrix with increased pore volume, preferably of not directly communicating pores.