**1. Introduction**

Nanotechnology has become a very important research topic in recent years for the scope and variety of applications in almost all fields of engineering. Mainly though, it has been important in the development of new materials. For example, nanotechnology has been implemented in the study of building materials in order to improve the mechanical properties and give the materials other additional properties. In these studies, the addition of carbon nanotubes to reduce fissures and improve mechanical properties [1], as well as the insertion of nanoparticles of silica (*n*-SiO2) [2] and iron oxide (Fe2O3) for the sealing of nanopores [3] has been highlighted. Other types of features have been given to cement, when broadband semiconductor nanoparticles are added, such as the decontaminating of air, and the self-cleaning in buildings, through photocatalytic activity [4]. These properties are obtained, for example, when photocatalytic reactions are performed in the nanoparticles interface incorporated in the cementitious matrices. These reactions are defined as electrochemical processes by absorption of radiant energy (UV light) within the photocatalyst, which is usually a semiconductor with broadband [5]. When photons of a required wavelength (photons with energy higher than the band gap of the photocatalyst), are absorbed by the photocatalyst, electrons from the valence band are promoted to the conduction band crossing through the band gap (forbidden region for electronic states), and electron-hole pairs are generated. These pairings carry opposed free charges in the absence of an electric field, and recombine rapidly (in approximately of 30 ns), releasing excess energy, mainly as heat [6]. If there are previously adsorbed chemical species in the catalyst surface, recombination is prevented and redox reactions occur between these species and the photogenerated pairs. In this case, the electrons (í) react with oxidizing agents, and the holes (+) react with reducing agents. Photocatalysis is normally effected under aerobic conditions, (when oxygen acts as an acceptor species) and will react with the electrons (e<sup>í</sup>) to form a superoxide radicals (O2 •í ). In turn, the water is used as a reducing agent, and it will react with the holes (h+) to form hydroxyl radicals (OH• ) [5]. The following Equations (1)–(3) correspond to the reactions carried out in the photocatalyst interface:

$$\text{TiO}\_2 + h\nu \rightarrow \text{TiO}\_2\text{(e}^- + \text{h}^+\text{)}\tag{1}$$

$$\rm{TiO\_2(h^+) + H\_2O \text{ ad.} \to TiO\_2 + OH^\* \text{ad} + H^+ \text{} \tag{2}$$

$$\text{TiO}\_2\text{ (e}^-\text{)} + \text{O}\_2 \rightarrow \text{TiO}\_2 + \text{O}\_2^{\text{-}} \tag{3}$$

Studies with photocatalytic cements have been performed mostly by adding TiO2 nanoparticles, and UV radiation as a photon source. The use of TiO2 is due to its low toxicity and high stability compared to other semiconductors studied for this application (ZnO, CdS, WO3) [6]. The NO*<sup>x</sup>* abatement has been evaluated for cement pastes added with TiO2 nanoparticles at different ratios of rutile and anatase [7]. In this same way, there has been reported the mineralization of different VOCs (Volatile Organic Compounds) by means of building materials added with TiO2 [8]. The self-cleaning ability in these kind of cements is evaluated by standard UNI 11259, using Rhodamine B as an organic dye. In this case, important results have been obtained with mortars and cement pastes, using TiO2 as addition [9–11]. The Rhodamine B is selected for evaluating this property in cements, mainly because it is very soluble in water, its discoloration can be followed by colorimetry and it has little sensitivity to the alkalinity of cementitious materials [10]. Additionally, the rhodamine B has polycyclic aromatic hydrocarbons in its chemical structure, compounds that are usually found as pollutants in urban environments. On the other hand, it is known that the main weakness of these cements is given by the nature of TiO2, which is only activated in the presence of UV light. The wavelengths in the UV region of the solar spectrum correspond to a low percentage and, thus, there is minimal use of stimulus of light when implementing these cements into a real life scenario. An alternative though, according to the research performed by Asahi, is to modify the photocatalyst with impurities to improve the efficiency of the process. This research found that the presence of nitrogen in the structure of TiO2 as substitution by oxygen creates Ti–N bonds and electronic states in forbidden energy levels [12,13]. This anionic partial substitution increases the range of the solar spectrum absorbed [14]. In this case, nanoparticles of TiO*x*N*y* can absorb wavelengths between 365 nm and 500 nm [15,16]. An approach of the behavior of these nanoparticles in building materials was done by mixing nano-TiO2í*x*N*y* with calcium carbonate (CaCO3). This blend reduced the concentration of NO*x* and 2-propanol between 360 and 436 nm [17]. The addition of nano-TiO2í*x*N*<sup>y</sup>* to the cement is entirely unknown; therefore, the main objective of this research is to discuss the behavior of these nanoparticles in the cement and obtain a construction material with a self-cleaning property in the presence of UV and visible light.
