## 3 Results  
### Mechanical properties  
Though the mortars in our study are not specifically for masonry work, a study was nevertheless made of their consistency and the effect on consistency of adding the catalyst. No changes were observed in this respect with the addition of P25 to cements G and W, though it was noted that the mixtures were slightly drier, especially those of cement W. This is due to the specific surface area and shape index of the particles of the photocatalytic addition which have a higher water adsorption capacity in the open time of the mixture in workable state. The water vapour adsorption capacity for the P25 was found to be 0.35 mg\(\cdot\)m\({}^{-3}\), which explains this phenomenon.  
Table 1 and Figure 1 show the results of the flexural and compressive strength tests, respectively. As can be seen, all specimens display the same strength versus time pattern. The presence of the photocatalyst does not seem to produce any important alterations in terms of flexural strength in either cement W or G. Though there is an overall trend for flexural strength to decrease as the amount of catalyst is increased, this decrease is not significant. As for compression strength, a decrease can be seen with the incorporation of the TiO\({}_{2}\) catalyst in both the W and G cements, though without falling below the minimum value of 52.5 MPa established in the corresponding standards. The compression strength in the W series is slightly higher, both with and without the addition of the photocatalyst, than the corresponding values in the G series. The differences in compression strength between the cements without and with added catalyst are higher from 90 days onwards. Loss of resistance has also been observed by other authors with the incorporation of more than 1%wt. TiO\({}_{2}\)[17]. The decrease in strength is normally caused by an increase in porosity [18; 19; 20]. In our samples, an increase in porosity was also observed caused by the addition of TiO\({}_{2}\) nanoparticles, as will be shown later.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**days** & **G** & **G-5** & **G-10** & **W** & **W-5** & **W-10** \\ \hline
**2** & 6.33 & 6.13 & 5.61 & 6.42 & 6.08 & 6.06 \\ \hline
**28** & 9.05 & 8.21 & 8.29 & 8.65 & 8.70 & 7.53 \\ \hline
**90** & 10.40 & 9.41 & 9.41 & 9.95 & 9.61 & 8.88 \\ \hline
**200** & 11.20 & 9.86 & 9.81 & 10.32 & 10.18 & 10.28 \\ \hline
**365** & 12.26 & 11.20 & 10.91 & 12.01 & 10.69 & 10.32 \\ \hline \end{tabular}
\end{table}
Table 1: Results of flexural strength tests (MPa).  
Table 2 shows the initial and final setting times for the two untreated cements and for the same two cements with the addition of the photocatalyst. The starting times for the setting of the two cement types were similar and the addition of the photocatalyst had the same effect on both types, namely a significant lowering of this starting time. With the incorporation of 5%wt. of P25, setting commenced approximately 1 h sooner. However, the incorporation of larger quantities of P25 did not result in further significant changes in this respect, with a difference of just 10 minutes between G-5 and G-10. It was not possible to establish a correct study for the W-10, which displayed a high degree of dryness as observed in the consistency study. The mixtures studied all comply with the legislation in force which establishes an initial setting time of \(\geq 45\) min. The results show that the photocatalyst acts as a setting accelerator. This same behaviour has been observed by other authors [21; 22] who incorporated a SiO\({}_{2}\)-based photocatalyst in Portland type I cement (manufactured in Taiwan), though with higher proportions of photocatalyst. The size, specific surface area and shape index of the particles of the photocatalyst, whether TiO\({}_{2}\) or SiO\({}_{2}\), contribute to this setting acceleration and to a higher porosity which, as previously mentioned, also results in a reduction in compression strength.