## 2 Basic Threats to RC Structure Durability--Short Literature Review
The authors undermine, to a certain extent, the classical idea of a sulfate attack caused by the migration of sulfate ions from soil or water and highlight the detrimental influence of gypsum. Another widespread corrosion agent is sodium chloride, found naturally in seawater and a part of the groundwater. Its significant concentrations are also transferred to the environment during the deicing of communication surfaces. Chlorides react with cement hydration products (calcium hydroxide i.e., portlandite, C-S-H phase), destroy the passivation layer and directly cause steel corrosion [12; 14; 15]. A specific source of chlorides is also coal mine water, a huge volume of which is discharged to surface waters. In the years 1995-1998, the daily discharge of Cl\({}^{-}\) and SO\({}_{4}\)\({}^{2-}\) salts to the upper drainage basin of the Odra and Vistula Rivers was approximately 7000 tons [16]. For example, water from the level of 665 m in the \"Wesola\" coal mine contains more than 35 g/dm\({}^{3}\) of chloride ions [17]; in extreme cases, that content reaches 200 g/dm\({}^{3}\)[16]. That water often comes into contact with industrial structures, introducing chloride ions into the concrete. Such a situation was described in the paper [18] where, among others, the corrosion of the RC silo caused by coal mine water led to extensive structural failure. Certain effects of chemical aggression also exert a mechanical impact. This happens in three ways:  
* New chemical compounds forming in concrete increase their volume and burst the surrounding material (ettringite, gypsum, thaumazite, Candlot salt, calcium carbonate, expansive calcium chloride, and expansive magnesium chloride);
* Similarly, swelling products of reinforcing steel corrosion burst the concrete cover (ion oxides and hydroxides);
* Soluble compounds forming in concrete discharge a portion of the matter beyond the concrete element (acid calcium carbonate). The abovementioned factors are not an exhaustive list of the enormous number of chemical substances aggressive for concrete, but they are the most common and relatively easy to examine ones. The latter part of the paper describes the results of basic laboratory tests concerning the assessment of concrete pollution with the typical chemical factors as well as relates the obtained results to the possibility and justifiability of industrial support structure repairs. Due to the nature of the paper, a chemical description of corrosive processes is not included herein because the content and aim of the paper are different. Such descriptions can be found in a number of references [7; 19; 20; 21]. The article also omits issues related to the use of typical non-destructive tests [22] and non-destructive electrochemical techniques to evaluate the corrosion of steel reinforcement--including the examination of the corrosion potential of steel and the corrosion rate of steel [23; 24; 25]. Such tests are extremely helpful in assessing the risk of steel corrosion in RC; however, their main advantage is determining the level of steel corrosion without disturbing the continuity of concrete cover. Meanwhile, the article describes the cases of constructions in which the degree of corrosion damage was so significant that the use of advanced non-destructive methods was unnecessary--most of the corrosion damage was clearly visible.