## 2 Materials and Methods  
### Theoretical Aspects  
The mechanism that underlies the increase in hydrophilicity of cementitious composites surfaces containing TiO\({}_{2}\) nanoparticles under exposure to UV rays can be explained on the basis of the increase in hydroxyl groups (OH\({}^{-}\)). This phenomenon, identified through X-ray photoemission spectroscopy (XPS), Fourier transformation infrared spectroscopy (FTIR) or nuclear magnetic resonance (NMR), is schematically shown in Figure 2[5; 36; 37; 38]. The transition of the surface, under the influence of UV radiation, to a metastatic thermodynamic state is the result of the coexistence of forms of molecular water and dissociated water. In general, under the action of UV rays, titanium dioxide is a semiconductor with a band gap of about 3.0 eV in the presence of UV light with a wavelength between 100 and 400 nm. By energy absorption, TiO\({}_{2}\) generates electrons (e\({}^{-}\)) and holes (h\({}^{+}\)) (Equation (1)). Electrons tend to reduce the Ti(IV) cation to the Ti(III) ion (Equation (2)) and the voids oxidize the O\({}_{2}\)\({}^{-}\) anions (Equation (3)). This process will release oxygen by creating gaps on the surface of titanium dioxide; that latter gives the possibility of binding water molecules to the release of hydroxyl radicals (OH\({}^{-}\)). \[\mathrm{TiO}_{2}+\mathrm{hv}\rightarrow\mathrm{e}^{-}+\mathrm{h}^{+} \tag{1}\]  
\[\mathrm{e}^{-}+\mathrm{Ti}^{4+}\rightarrow\mathrm{Ti}^{3+} \tag{2}\]  
\[4\mathrm{h}^{+}+4\mathrm{O}_{2}{}^{-}\to 2\mathrm{O}_{2} \tag{3}\]  
Since the oxidation capacity of the (h\({}^{+}\)) voids is higher than the reduction capacity of (e\({}^{-}\)) electrons, and on the surface of the photocatalyst there is a single layer of adsorbed H\({}_{2}\)O molecules, hydroxyl radicals (OH\({}^{-}\)) are formed. Being highly oxidizing, these hydroxyl groups react with organic molecules producing peroxyl free radicals, which will react with molecular oxygen in a chain of reactions up to the CO\({}_{2}\) and H\({}_{2}\)O end products. On the other hand, (e\({}^{-}\)) electrons reduce oxygen to the free O\({}_{2}\)\({}^{-}\) radical, which will react with the resulting peroxy molecules during the reaction between hydroxyl groups and organic molecules, ultimately leading to a chain of reactions up to the final CO\({}_{2}\) and H\({}_{2}\)O products [5; 39]. As a general reaction, this process can be explained according to Equation (4). \[\mathrm{Organic\ compound}+\mathrm{O}_{2} \tag{4}\]  
In the case of cementitious composite surfaces with TiO\({}_{2}\) content, the literature also indicates [5; 40] that photogenerated holes (h+) increase the length of ties within the TiO\({}_{2}\) network (Figure 2, left), bringing the surface to a metastable state that allows adsorption of molecular water simultaneously with the formation of new hydroxyl groups and the release of a proton. The research considers that these hydroxyl groups are less thermodynamically stable, therefore the surface will allow the water droplet to be flattened to cover a larger area in order to stabilize [5; 40; 41; 42]. Consequently, at the level of the cementitious surface, oxido-reduction reactions occur. This mechanism is shown in Figure 2, top right. Regarding the hydrophilicity, the literature [5] also specifies the influence that surface roughness and homogeneity have on the contact angle between the water drop and the surface and indicates the ways by which [PERSON]'s Equation (5) is modified by the introduction of the roughness correction factor by [PERSON] (Equation (6)) and [PERSON]'s correction for the situation of a heterogeneous two-phase surface (Phase 1 and Phase 2) (Equation (7)) for ensuring homogeneity.