## 1 Introduction  
Studies conducted worldwide [1; 2; 3; 4] have shown that one of the most important and common causes of structure degradation is the accumulation of exogenous/external, polluting, organic or inorganic materials on surfaces. The accumulation of exogenous/external materials initially leads to loss of aesthetic appearance, but in time causes the degradation of the building material, which implicitly determines maintenance works and repairing costs, as well as indirect costs (for example, the reduction, the hindrance or stopping the activity in the building during the repair works, etc.). One solution to improve this situation is the use of nanotechnology by creating materials with a self-cleaning capacity in the construction field. The photocatalytic oxidation-reduction reaction under the influence of ultraviolet (UV) radiation of titanium dioxide (TiO\({}_{2}\)) was reported as the Honda-Fujishma effect in 1972 [5]. It is known that cementitious composites containing TiO\({}_{2}\) nanoparticles, under UV rays, have a specific behavior: on one hand, they catalyze the decomposition of organic molecules, which are generally large, into compounds with a simpler and smaller molecule, and on the other hand they increase the hydrophilicity of their surface. The two mechanisms, namely the photocatalytic oxidation of the chemical pollutants and increasing the surface hydrophilicity under UV action, determine the self-cleaning effect of cementitious composites containing TiO\({}_{2}\) nanoparticles. When this stage is achieved, exogenous/external substances or particles can be more easily removed by rainwater [1; 5; 6; 7; 8; 9; 10; 11; 12], as schematically shown in Figure 1. Moreover, the literature shows that catalytic photoactivation capacity contributes to the reduction of NO\({}_{\text{x}}\) pollutants in the air and surface decontamination of viruses, bacteria and sponges [13; 14; 15; 16; 17]. On the other hand, the literature [5] indicates that TiO\({}_{2}\) nanoparticles alone lose their hydrophilicity property as soon as the UV action ceases, but in combination with the amount of SiO\({}_{2}\) contained by cement, the UV-activated effect extends even in days of complete darkness (no UV radiation). The literature [18; 19; 20; 21; 22] indicates an influence of the addition of TiO\({}_{2}\) nanoparticles on the structure of the cementitious composite, at the microstructural level, by modifying the distribution and size of the pores. Reduction of the general porosity by a decrease of the total volume of the pores simultaneously with the reduction of their dimensions will cause a local densification and, consequently, a modification of both the physico-mechanical characteristics, especially the mechanical strength, as well as the water absorption capacity and the permeability to liquids or gaseous substances. [PERSON] et al. [19] reports an improvement in the mechanical strength of high-performance concrete and a reduction in capillary porosity of concrete, in general, by the addition of 1% TiO\({}_{2}\) nanoparticles. [PERSON] et al. [18] confirms the possibility of reducing the total porosity by changes occurring at the microstructural level as a result of the addition of TiO\({}_{2}\) nanoparticles. [PERSON] et al. [23] confirms this hypothesis as a consequence of an inhomogeneous dispersion of nanoparticles in the cementitious matrix, recommending an incorporation method with ultrasonic dispersion. On the other hand, in terms of the capacity for the generation of the hydroxyl radical, respectively, of a modification of the chemical bonds, [PERSON] and [PERSON] [24] confirm, in accordance with other reports in the literature [25; 26; 27], that the specific surface, i.e., the total porosity, has no influence on the performance of photo-catalytic characteristics of the materials with the addition of TiO\({}_{2}\) nanoparticles. [PERSON] and [PERSON] [28], consistent with [PERSON] et al.