## 1 Introduction  
As population levels have grown worldwide, so new problems have arisen for mankind to deal with, including high levels of pollutant emissions as the result of fossil fuel combustion. These harmful agents not only endanger human health and the environment but also lower the durability of concrete structures through acidification and degradation and consequently increase maintenance and restoration costs. According to European Commission data, atmospheric contamination causes 370,000 premature deaths each year in the European Union [1]. High levels of pollution in the air we breathe provoke cardio-respiratory problems (arrhythmias, asthma, bronchitis, allergies) [2]. Vehicular traffic is the main source of atmospheric emissions. The highest concentrations of air pollution tend to be found in urban environments, especially densely populated urban centres. The flat external surfaces of construction elements found in these urban centres are exposed to solar radiation and air contamination. Measures that can be adopted to improve air quality include greater use of public transport and the lowering of speed limits. One very recent method for air quality enhancement involves the use of photocatalytic products in elements used for urban construction. Previous studies have shown that these photocatalytic products are able to degrade a wide range of organic compounds (e.g., volatile organic compounds) and some inorganic compounds (NOx and SO2), which are aggressive for both the materials and the environment [3; 4; 5; 6].  
The incorporation of photocatalysts in different construction materials was first conceived in Japan at the start of the 1990s [7]. Its use has extended to the rest of the world, notably accelerating in the last decade [8]. Photocatalytic performance in urban environments can be affected by environmental factors such as incident radiation intensity, relative humidity, temperature and wind, in addition to other intrinsic factors of the photocatalyst when this is associated to a cement-based support [9; 10].  
Though the effect of the photocatalytic decontamination of construction materials is clear, there remain a number of unresolved problems when using these materials in real applications [11]. There are presently various companies manufacturing photocatalytic cements and other materials for their application on facades, streets, pavements, roofs and so forth, with multiple examples of their application in Japan and Europe [12]. Though photocatalytic materials are now being marketed, many of the products have not been competitive or commercially successful. There is therefore a need to synthesise new photocatalytic materials and design strategies or methods for their incorporation in the binders and additives which form the basis of these materials. It is vital not only that the photocatalysts remain active despite the strongly alkaline matrix they are incorporated in but also that the construction materials continue to meet the legally required specifications in terms of mechanical properties.  
An analysis is undertaken in this work of the incorporation of photocatalysts in mortars, studying the structural changes that take place in the photocatalytic mortars and their capacity for NOx elimination. The selected photocatalysts (Aeroxide TiO\({}_{2}\) P25 and Kronos vlp7000) are two of the commercial photocatalysts that have displayed the highest efficiency in the oxidation of NOx in published studies [13].