## 1 Introduction  
The durability of reinforced concrete structures is highly related to the permeability properties of concrete, with the concrete cover over the steel reinforcement being the most critical zone in this regard [1]. The permeability properties depend mainly on the characteristics of the inner porous network of concrete (porosity, connectivity of the pores, and pore size distribution). The concrete cover can be permeated by water or aqueous solutions containing harmful ions, such as chloride (Cl\({}^{-}\)) ions, or by gaseous substances such as oxygen (O\({}_{2}\)) or carbon dioxide (CO\({}_{2}\)) from the atmosphere [2]. All these transport processes have a marked influence on steel reinforcement corrosion in concrete. Both the contamination of concrete by Cl\({}^{-}\) ions and the carbonation of the concrete cover due to CO\({}_{2}\) are known to be circumstances leading to the onset of steel corrosion [3]. Later, the intensity of the process (corrosion rate) is highly dependent on the moisture of concrete and on the chloride content. The availability of O\({}_{2}\) at the steel-concrete interface, mainly determined by the gas permeability of the concrete cover, may also play a significant role on the steel corrosion rate in certain cases or stages of the corrosion process [4, 5]. One of the most popular strategies to avoid durability problems of reinforced concrete structures is the use of permeability-reducing admixtures (PRAs) [6]. These products are classified as admixtures able to reduce the water permeability of concrete under nonhydrostatic conditions (PRAN), and those that can also function under hydrostatic pressures (PRAH) [7]. These products can be used as surface treatments of concrete structures, under the form of coatings [8], or as additives incorporated to the concrete mix in order to obtain integral waterproof concrete [9]. Crystalline admixtures (CA) are products with a twofold effect: reducing the permeability of concrete and self-healing cracks [10]. CAs are composed of particles of different size and chemical compositions, including cement, fillers, Pozzolans, slags, sands and \"active chemicals\" [10]. Most CAs are commercial products containing proprietary constituents and their formulations are kept confidential. It is important to consider that the constituents and chemical compositions of different CAs may be quite diverse; hence, different behaviors and properties can be found in practice. Certain components of CA show high hydrophilic character, they can react in presence of water to form insoluble pore/crack blocking precipitates [7], which contribute to reducing the permeability of concrete and provide some ability to self-seal cracks [10]. It is also possible that in some cases the newly formed precipitates can react with the surrounding products of cement hydration [11]. In this work, we will focus on the effects of using a commercial CA as an integral PRA for a conventional Portland cement concrete on several selected durability-related properties. There is a general consensus on the ability of CAs to significantly reduce the permeability of concrete and other cement-based materials to water under pressure [11; 12; 13; 14]. There is also some evidence that CAs can increase the resistance of concrete to water capillary absorption [15]. Hence, it seems that CAs can be considered as PRAH and PRAN [16]. Regarding the resistance of concrete to Cl\({}^{-}\) ion penetration, the reported results seem to be inconclusive: some authors found that the incorporation of an admixture characterized by crystallization activity had no detectable effect on the Cl\({}^{-}\) diffusion coefficient (natural diffusion test) of a Portland cement concrete incorporating fly ash [17], while other researchers detected some reduction in the Cl\({}^{-}\) diffusion coefficient (natural diffusion through ponding test) when incorporating a CA to a Portland cement and a Portland-Limestone cement [14].