# Photocatalytic Recycled Mortars: Circular Economy as a Solution for Decontamination
## 4 Results and Discussion
### Mechanical Strengths  
The values registered for mechanical strength at 28 days are shown in Figure 2, which illustrates the comparison of results between the two families.  
Figure 2a shows that for the family made with conventional cement and 40% of NS replaced by RS (M40), the compressive strength is equal to that corresponding to 20% replacement, approximately 46 MPa, which is only 10% less than the reference mortar (M0) at 52 MPa. For full replacement (M100), the drop in compressive strength increases to 16%, with an average value of 44 MPa. This agrees with [PERSON] et al. [51], suggesting that as the RS content increases, the compressive strength remains similar to or larger than that of the control mortar. This may be due to the reduction of effective water as the percentage of RS is increased, as explained by [PERSON] et al. [35], or because of the greater number of fine particles that can fill the gaps [49]. In this investigation, the content of fine RS was higher than NS (12.4% vs. 2.26%), favoring the filling of voids in the mortar matrix and diluting the loss of mechanical strength. However, in most mortar mixes, there is a loss of compressive strength as the replacement percentage increases [34]. In all mortar mixes, the compressive strength is greater than the value recommended by the GB 28635-2012 standard [55] (average strength \(\geq\) 30 MPa; any individual strength \(\geq\) 25 MPa) and values obtained in other studies [19; 56].  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Consistency** & **Consistency** \\ \hline M0 & 134 & PM0 & 129 \\ M20 & 131 & PM20 & 125 \\ M40 & 130 & PM40 & 113 \\ M100 & 111 & PM100 & 102 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Consistency values of mortar families (mm).  
Figure 1: Porous appearance of PM100 versus smooth appearance of other mixes.  
In all cases, the compressive strength obtained in the photocatalytic family is greater than that of their counterparts made with conventional cement. In this way, they start from an average resistance of 59 MPa for PM0, reaching 48 MPa for PM100.  
Flexural strength can be correlated with other characteristics such as susceptibility to cracking and adhesive strength of mortar [51]. In this test (Figure 2b), a similar value of flexural strength was obtained for the 0%, 20%, and 40% replacement, 15 MPa, except for the M100 and PM100 families, for which a value of 13 MPa was found. This agrees with values obtained by [PERSON] et al. [51], whose research used the same type and dosage of cement. Therefore, no improved behavior of the photocatalytic family was observed in this test. All values registered in the flexural strength test show very good performance of these mortars for their possible use in pavement, with values greater than 12 MPa in all mortars, exceeding the values obtained by other authors [56; 57].  
The differences between the strength of mixtures with 0% and 40% RS are practically insignificant, which confirms that increasing the percentage to 40% does not just mean decreased mechanical properties. This agrees with [PERSON] et al. [14], who established a maximum replacement ratio of up to 50% of natural sand by mixed recycled sand without significantly affecting the hardened mortar properties.  
Generally, a decrease in strength is observed as the percentage of recycled aggregate is increased, as reported in previous works. This decrease is more gradual and more noticeable in the photocatalytic family. However, the increase in RS up to 40% showed only a 7% decrease in strength with respect to 20% replacement, and up to 9% with respect to the reference mortar.