## 1 Introduction  
The penetration of chloride ions into concrete structures is one of the main causes of their steel reinforcement's corrosion, especially in facilities located in the coastal zone, as well as those treated with de-icing salts. The ingress of chlorides into concrete is primarily a diffusion process taking place in the pore solution, but in general, it is more complex, and its course is also significantly influenced by the pore space structure, concentration and type of constituents dissolved in the pore liquid, interactions between chloride and other ions, age of concrete, temperature, and process of chloride binding by concrete skeleton [1]. The values of chloride diffusivity are the same influenced by the water-cement ratio, volume of aggregate, amount of cement and supplementary cementitious materials. All these factors affect the porosity of concrete, its pores' shape and tortuosity, and ability to diffuse and bind chloride ions [2]. Chloride ions present in the hardened concrete occur in the ionic form in the pore solution (free chlorides) and are bound to the pore walls of cement matrix by the physical adsorption forces or by the chemical forces with the cement gel [3]. Initially, bound chlorides consist of insoluble salts of the MgCl\({}_{x}\)OH\({}_{y}\) type which, in the presence of certain additives, form hydroxy salts of the Ca(OH)\({}_{2}\)-CaCl\({}_{2}\).2H\({}_{2}\)O type. In addition, calcium chloride CaCl\({}_{2}\) reacts with the aluminate constituents of the hardened cement paste to form the so-called Friedel's salt 3 CaO\(\cdot\)Al\({}_{2}\)O\({}_{3}\cdot\)CaCl\({}_{2}\cdot\)10H\({}_{2}\)O, which is poorly soluble in an alkaline environment [4; 5]. At normal temperature, [PERSON]'s salt is formed in solutions with a concentration of up to 1 g of chloride ion per 100 cm\({}^{3}\) of solution, while at a higher concentration, unstable chloroaluminate is formed. In the case of chlorides diffusing from the outside, their binding by the C-S-H gel is difficult. However, their adsorption on the C-S-H gel surface and the formation of chloroaluminates in accordance with the pore liquid equilibrium and hydrated cement minerals is possible [6]. In the published works, the relationship between the content of bound and free chlorides is expressed with the help of mathematical relationships called binding isotherms [7]. Some researchers report a linear relationship between the content of free and bound chlorides taking into account the type of solution and its concentration [8]. However, these dependencies did not fit well to the research results in a wide range of free chloride concentrations. Therefore, non-linear relationships between the content of free and bound chlorides have been proposed. The most common are the Langmuir [9] and Freundlich [10] isotherms. The first is recommended for use with a low content of free chlorides (<0.05 mol/L), and the second with a high content (>0.01 mol/L) [11]. The isotherm proposed by [PERSON], [PERSON], [PERSON] (BET) [12] is also used. It was found that this isotherm may fit the test results mainly when the concentration of free chloride ions is lower than 1 mol/L [13]. Some works propose completely new formulas in this regard, as in [14] where an exponential relation is introduced. The coefficients occurring in the above-mentioned isotherms are determined experimentally using various research methods discussed in detail in [13]. In the equilibrium method, hardened cement paste samples are placed in a solution of chloride ions of known concentration and allowed to reach equilibrium. For the 10 mm thick samples, a period of up to 1 year is required. After the steady state has been achieved, the concentration of the external solution is tested by potentiometric titration. By immersing the samples in solutions of various concentrations, a graph of the adsorption isotherm is obtained. The pore solution extraction method is another one, and a complete binding isotherm may be obtained on a single specimen.