## 4 Discussion  
Differences of the gas permeability of investigated materials is caused by the differences in structure of tested materials, which lead to the different mechanisms of gas migration. Multi-grain materials, such as concrete or mudstone, have many pores and voids within the structure. Fine grains can decrease the gas permeability because the intergranular porosity is lower while the compaction of the material is higher; this phenomenon  
Figure 4: SEM imaging of the surface structure of investigated samples: (**a**) concrete \"2\" (year 2016); (**b**) mudstone (Carbon); (**c**) salt rock (Permanian); (**d**) epoxy resin; (**e**) epoxy resin + graphite (5% vol.); (**f**) stainless-steel.  
was also observed in other research [22]. However, gas flow through the sample is still noticeable and is the main mechanism of gas migration in that kind of material. However, it is not essential data for the purpose of this paper. Because of the low gas sealing properties of the investigated concretes, this type of material is not suitable for sealing purposes of underground excavations. However, it can still be a reinforcement and base liner.  
Plain materials, like salt rock, epoxy, or steel, do not have pores and voids in their structure. Gas is not flowing through the sample (there is no pressure gradient along the sample). Hydrogen particles can diffuse through the material by dislocations in crystal structure. It is a linear defect that induces the tensile stress. Gas elements can diffuse much easier through those kinds of zones [23]. Lattice diffusion is also possible, but in much higher temperatures, transcending Tamman temperature (in this temperature atoms in solid material acquire enough energy to make bulk reactivity and mobility significant, usually approximately half of the melting temperature) [24]. In this case, research temperature was much below the Tamman temperature.  
Interesting results were obtained with the fly ash epoxy sample. Graphite and haloysite are slightly increasing hydrogen permeability of the samples, while fly ash do not affect the sealing properties. Fly ash used for this research was not grinded but only sieved. Since fly ash from power plants is easily available, it could be a cost-effective filler by reducing the amount of epoxy resin in the sealing liner.  
Obtained results of epoxy resin hydrogen permeability is slightly lower than presented in literature. Other research, presented collectively in [16], showed the hydrogen permeability of epoxy resin of approximately 1.0 Barrer, while performed research of investigated samples gave the permeability coefficient of approximately 0.2 Barrer. Results of hydrogen diffusion coefficient presented in [17], which was \(6.9\times 10^{-11}\) m\({}^{2}\)/s, was also higher than the obtained \(1.5\times 10^{-12}\) from this research. Lower results may be caused by numerous factors, influencing on the gas permeability in polymeric materials like polymer chemistry (epoxide number), free volumes (crystallinity, orientation of molecules), porosity or voids (air inclusions), and fillers presence [16].  
Permeability coefficient of 316 SS steel was calculated using the Equations (3)-(6), based on the literature data [25]. Permeability of 316 L steel were taken from [26; 27]. Because of the sensitivity and construction of the setup, investigation of the steel samples was not possible. Permeability of steel is orders of magnitude lower than materials the setup is meant for. To investigate the steel or alloys, much thinner samples need to be used. The Setup is not designed for that kind of samples, but only for a core-shape samples. Calculated hydrogen permeability \(P_{\rm H2}\) of 316 SS steel was \(4.6\times 10^{-17}\) cm\({}^{3}\)STP-cm\({}^{-2}\).s\({}^{-1}\).cmHg\({}^{-1}\) (\(4.6\times 10^{-7}\) Barrer), and given [26; 27], hydrogen diffusion coefficient D of 316L steel was in range from \(10^{-13}\) to \(10^{-15}\) m\({}^{2}\)/s.