# Concrete Gas Permeability: Implications for Hydrogen Storage Applications
## 1 Introduction
### Concrete Permeability  
Permeability is the capacity of a material to allow a fluid to pass through it without altering its composition. A material is considered impermeable or non-permeable when the amount of fluid passing through it is negligible [12,13]. The rate at which a fluid passes through the material depends on three fundamental factors:  
1. The porosity of the material and its structure: the volume, size, and distribution of the pores regulate the rate at which it absorbs liquids, mainly water containing certain aggressors and gases. 2. The viscosity of the considered fluid, which is affected by its temperature. 3. The pressure to which the fluid is subjected. Concerning the material and its structure, concrete is a heterogeneous material, dense but porous, as it allows a considerable amount of fluid to pass through it within a given time. It consists of a solid phase and a porous space or void volume, representing between 8% and 25% of the total volume [14]. The concrete presents a network of pores of different sizes which can reach the surface [15]. To be permeable, it must not only present pores but also have interconnected void spaces that allow for fluid passage [16,17]. In terms of durability, the most critical pores are those between \(10^{-2}\) and \(10^{-7}\) m (compaction pores, entrapped air, and capillary pores), while micro pores or gel pores have no incidence due to their size. Capillary pores, with a radius between 10 and 1000 nm, result from spaces initially occupied by mixing water and subsequently partially filled with hydration products. The most common hydration products include the following:  
1. Calcium silicate hydrate (C-S-H): It is the main hydration product of Portland cement and forms through the reaction of cement with water. This gel is amorphous and extends throughout the concrete matrix, filling the pores and voids of the material. 2. Calcium hydroxide (CH): This is another common hydration product formed through the reaction of cement with water. CH is crystalline and is deposited in the pores and voids of concrete, where it can react with other compounds to form additional products. It gives the concrete a highly alkaline character, ranging from pH 12 to 13 (protective for reinforcement). 3. Ettringite: This is a compound of calcium sulfate hydrate and aluminum that is formed when sulfate-containing cement is used. This product can be deposited in the pores and voids of concrete, contributing to the formation of the material's crystalline structure. 4. Hydrated silica: This is a compound that forms through the reaction of cement with silica. This hydration product is deposited in the pores and voids of concrete, contributing to its strength and durability. The hydration products formed during cement hydration in concrete play pivotal roles in reducing permeability [18]. They fill voids and pores, hindering the movement of gases and liquids. The C-S-H gel acts as a binding agent, forming a cohesive structure that resists fluid and gas flow. This process modifies the pore structure, creating smaller, interconnected pores that further impede permeability. Additionally, hydration products enhance chemical resistance, protecting concrete from deterioration and maintaining its long-term performance. Understanding these roles is essential for optimizing concrete mix designs, particularly in applications like hydrogen storage, where gas containment is critical. Besides hydration products, aggregate size significantly influences concrete porosity. Well-graded aggregates optimize packing and reduce permeability, while larger sizes increase the spacing between particles, potentially increasing permeability [19]. Concrete permeability is crucial for achieving greater durability and strength. Low-permeability concrete has a reduced water-cement (w/c) ratio and satisfactory moisture absorption capacity, allowing fewer aggressive agents to enter and making it more durable [20; 21; 22].