## 2 Sulfate Corrosion--Basic Knowledge of the Process  
Sulfate corrosion, i.e., by \(\text{SO}_{4}\)[2]- ions, was identified in concrete already in the second half of the 19 th century. Since then, hundreds of experiments have been conducted and hundreds of manuscripts have been published, e.g., [7; 8; 9; 10]. Nevertheless, the topic of sulfate corrosion is still relevant. At first glance, sulfate corrosion may resemble a simple process, but it is not [11; 12]. In order to select suitable parameters to monitor the degree of a corrosion attack and an accurate interpretation of the results, it is necessary to get acquainted with at least the basic facts and contexts. There are a number of marginal conditions, such as the cation type, concentration, temperature and pressure of the acting medium, immersion time, etc. Some need to be considered, others do not. The situation has also been complicated by the fact that various chemical and mineralogical substance forms that cause sulfate corrosion have only been discovered gradually. The biggest danger is the historically first discovered ettrinigite, whose complex formula 3 CaO\(\cdot\)Al\({}_{2}\)O\(\cdot\)3 CaSO\({}_{4}\cdot\)32H\({}_{2}\)O clearly indicates what constitutes its danger. If a substance with 32 molecules of water crystallizes, it is obvious that it will expand significantly. Ettrinigite does not merely expand, it expands very rapidly and substantially. The final volume expansion of the reaction product in regard to the starting components is up to 270%. Identically to ettrinigite, gypsum CaSO\({}_{4}\) 2H\({}_{2}\)O forms under certain conditions during the reaction of sulfates with the components of concrete (cement). Gypsum is also undesirable, but its expansion is lower than that of ettrinigite. The last element (for this summary) which has been identified as a possible cause of concrete corrosion by sulfates is the mineral thaumasite CaCO\({}_{3}\)-CaSO\({}_{4}\)-CaSiO\({}_{3}\cdot\)15H\({}_{2}\)O, which crystallizes equally to ettrinigite in a hexavalent system. Like in the case of ettrinigite, its crystalline structure contains silicon atoms which are surrounded by octahedrons of OH\({}^{-}\) ions and therefore forms very similar crystals. It should be noted that thaumasite forms in a specific narrow temperature range of 0-5 \({}^{\circ}\)C, only (sometimes at higher temperatures at a lower crystallization rate). At the given low temperature, the calcium salts that thaumasite needs for its crystallization are more easily dissolved. As a result, thaumasite is not found in all concretes subjected to sulfate attacks. A comparison of SEM images of ettrinigite and thaumasite is presented in Figure 1. The morphological similarity of the crystals is quite evident and it should come as no surprise then that these two minerals used to be confused in the past. Even today, it is not completely clear how and under what conditions thaumasite forms in concrete and whether it can exist without ettrinigite. In 2020, [PERSON] et al. [13] stated the following in response to a question about the origin of thaumasite crystals in concrete: \"_A certain degree of controversy still exists around the formation of thaumasite and its possible connection with ettrinigite. The current state of knowledge can roughly be grouped together around the three following theories_:  
1. _According to the first theory, thaumasite is formed through ettrinigite by substitution of aluminium for silicon_. 2. _In the second case, ettrinigite acts as a nucleator for the thaumasite formation.__According to the third theory, thuumasite could be formed directly without influence of ettringite in the process at all_[13]. If there are three theories striving to explain one process, then the process is clearly not sufficiently understood. Perhaps that could partially explain why one of the most renowned personalities in the field of concrete technology, [PERSON], published a critical paper in 2004 with the title _\"The confused world of sulphate attack on concrete\"_[15].