## 2 Experimental Results  
The experimental results presented below have already been partially described in the works of one of the authors [21] (for concretes denoted further as C1 and C3-C5), but without a detailed analysis of the diffusion of free chlorides and their binding by the concrete skeleton in the context of the \"skin effect\" assessment, which was carried out only for the purposes of this article. Due to the importance of the sample preparation and testing methods (influencing the formulation of the mathematical model--see Section 3), the authors decided to briefly describe the experimental work used in the study as bellow. In this work, the test results of testing 5 types of normal concrete were used marked as C1-C5. Three types of natural rounded aggregate from Lafarge Basalt Mine in Lubier (Poland) compliant with the standard PN-EN 12620 [24], i.e., sand (0-2 mm) and gravel (2-8 mm and 8-16 mm), and five types of cement produced according to PN-EN 197-1 [25]at the Lafarge cement plant in Malogoszcz (Poland) were used for the concrete mixes. The following types of cements (with the standard denotations according to [25]) were used: Portland cement of high early strength (CEM I 42.5 R) in concrete C1, ash Portland cement (CEM II/B-V 32.5 R) in concrete C2, blast furnace cement (CEM III/A 32.5 N-LH/HSR/NA) in concrete C3, low-alkali Portland cement of normal early strength (CEM I 42.5 N-SR3/NA) in concrete C4, and pozzolanic cement (CEM IV/B (V) 32.5 R-LH/NA) in concrete C5. Their detailed chemical composition and basic properties (according to the producer's specification) are given in Table 1, and the detailed compositions of concrete mixes C1-C5 are presented in Table 2.  
In order to establish the basic hardened concrete properties, the tests of concretes C1-C5 were performed at the Laboratory of Civil Engineering of the Silesian University of Technology. The compressive strength was tested on 6 samples of 150 mm cubes for every type of concrete after 28 days moist curing according to PN-EN 12390-3 [26]. In order to determine the open porosity of hardened concrete, the following scheme was used: 1 sample for every type of concrete with a volume of about 10 cm\({}^{3}\), after 90 days \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Parameters** & **CEM I 42.5 R** & **CEM II/B-V** & **CEM III/A 42.5** & **CEM I 42.5** & **CEM IV/B (V)** \\  & & **32.5 R** & **N-LH/HSR/NA** & **N/SR3/NA** & **32.5 R-LH/NA** \\ \hline Constituents & SiO\({}_{2}\) & 19.38 & 26.77 & 29.08 & 21.15 & 30.18 \\ in weight (\%) & Al\({}_{2}\)O\({}_{3}\) & 4.57 & 10.82 & 6.30 & 3.93 & 11.92 \\  & Fe\({}_{2}\)O\({}_{3}\) & 3.59 & 3.97 & 1.37 & 5.14 & 4.72 \\  & CaO & 63.78 & 45.91 & 48.82 & 63.34 & 41.95 \\  & MgO & 1.38 & 1.62 & 4.36 & 1.28 & 1.72 \\  & K\({}_{2}\)O & 0.58 & 1.42 & 0.73 & 0.39 & 1.43 \\  & Na\({}_{2}\)O & 0.21 & 1.30 & 0.34 & 0.21 & 0.39 \\  & Eq. Na\({}_{2}\)O & 0.59 & 1.25 & 0.82 & 0.47 & 1.33 \\  & SO\({}_{3}\) & 3.26 & 2.77 & 2.74 & 2.61 & 2.65 \\  & Cl & 0.069 & 0.08 & 0.066 & 0.058 & 0.057 \\ Surface (cm\({}^{2}\)/g) & & 3535 & 3500 & 4760 & 3067 & 4457 \\ Density (g/cm\({}^{3}\)) & & 3.09 & 2.78 & 2.97 & 3.18 & 2.65 \\ Setting time (min) & beginning & 160 & 262 & 238 & 210 & 200 \\  & end & 200 & 382 & 293 & 255 & 290 \\ Compression & 2 days & 30.3 & 18.0 & 14.9 & 24.7 & 20.9 \\ strength (MPa) & 28 days & 56.9 & 45.2 & 57.6 & 54.1 & 43.2 \\ Water demand (\%) & & 27.6 & 29.8 & 31.6 & 27.8 & 29.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition and selected properties of cements according to the producer’s specification.