## 3 Results
### Photoactivity  
Table 5 shows the NOx percentages removed after 5 h of radiation for the photocatalytic cements with P25 under different humidity conditions (0% and 65% RH). Both mortars without TiO\({}_{2}\) show a little photocatalytic activity. The mortars are able to achieve small photocatalytic efficiencies in the absence of TiO\({}_{2}\) as a result of the hydroxides present in their matrices [27]. A slight increase in NOx removal is observed as the amount of photocatalyst present in the mortar samples increases. This trend has also been observed by other authors who used different photocatalysts but with a similar proportion of additive to that used in the present study [23; 27; 28]. The photoactivities were slightly worse in the presence of humidity due to the competition of water and NOx molecules for active centres on the TiO\({}_{2}\) surface. Higher photoactivity was obtained with cement G, with NOx removal values for G-10 of 23.34% and 20.90% for 0% and 65% RH, respectively. In contrast, the corresponding values for W-10 were only 12.53% and 12.98%. The main compositional difference between the two cements is the higher percentage of Fe\({}_{2}\)O\({}_{3}\) in cement G, which could be favouring this higher photocatalytic activity. In photocatalysis, the effective surface of the photocatalyst is the surface which is in contact with the pollutant as well as that which is exposed to radiation [29]. When incorporating the photocatalyst in the mortar bulk, most of it is inactive for NO oxidation given that photons cannot fall on them. With this in mind, the catalyst was only incorporated into a 1 cm thick surface layer on the specimen to ensure a higher efficiency process. In this way, material requirements can be reduced, making the final product a little more cost effective [30]. This layer was made with and without sand and with catalyst additions of 10%wt. and more, which meant the proportion of water had to be increased to 1.3 due to its adsorption by the catalyst. Nonetheless, the presence of this layer with its different composition to the rest of the specimen did not affect its properties in terms of flexo-traction and compression up to the tested catalyst proportion of 10%wt. For higher catalyst loads, especially in the presence of sand, the thickness of this layer and/or the water addition would need to be optimized [26] (Table 1). Figure 5 shows the percentage of NOx removed for 0% RH and 65% RH with the incorporation of this layer. When comparing these results with those shown in Table 5, it can be seen that the photocatalytic efficiencies obtained with the photocatalyst incorporated in the surface layer are much higher. This suggests that slightly increasing the water content and working with smaller amounts of mixture allows better dispersion of the TiO\({}_{2}\) in the mixture [26]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Samples** & **0\% RH** & **65\% RH** \\ \hline
**G** & 5.61 & 11.84 \\ \hline
**G-5** & 15.02 & 14.24 \\ \hline
**G-10** & 23.34 & 20.90 \\ \hline
**W** & 0.00 & 1.53 \\ \hline
**W-5** & 9.34 & 3.57 \\ \hline
**W-10** & 12.53 & 12.98 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: NOx removed (%) according to different study conditions. Figure 5: _Cont._  
In addition, significant differences can be seen between the two types of surface layer, with and without sand. Photoactivity is higher for the layers which contain sand, regardless of the amount of catalyst. In the absence of humidity, NOx removal rates of 67-68% were obtained, values which remained stable for the 10-50% range of catalyst added to the surface layer. However, without sand, although the amount of removed NOx rose as the amount of catalyst increased, with 50% TiO\({}_{2}\) the elimination rate of NOx was just 44%. This difference may be attributable to the presence of sand allowing greater availability of the TiO\({}_{2}\) and favouring maintenance of its photocatalytic function.