## 4 Results and Discussion
### Concrete Made with Recycled Products
#### 4.4.1 Compressive Strength  
Table 8 and Figure 6 show the comparison between the tested recycled concretes and the reference natural concretes for the considered resistance classes (C10/15, C25/30, and C30/37). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Mix**} & \multirow{2}{*}{**Curing Time**} & \multicolumn{2}{c}{**Control Mixture**} & \multicolumn{2}{c}{**50\% Recycled Mixture**} & \multicolumn{2}{c}{**Cement Sludge Mix**} \\ \cline{3-6}  & & **Density** & **Cmp.r.** & **Density** & **Compr.** & **Density** & **Compr.** \\  & **(kg/m\({}^{3}\))** & **Strength (MPa)** & **(kg/m\({}^{3}\))** & **Strength (MPa)** & **(kg/m\({}^{3}\))** & **Strength (MPa)** \\ \hline \multirow{5}{*}{**C10/15**} & 3 & 2345 & 15.13 & 2316 & 14.68 & 2301 & 13.50 \\  & 7 & 2309 & 18.26 & 2340 & 18.10 & 2296 & 15.48 \\ \cline{1-1}  & 14 & 2317 & 19.41 & 2352 & 18.81 & 2303 & 16.74 \\ \cline{1-1}  & 21 & 2294 & 19.67 & 2331 & 19.65 & 2308 & 17.40 \\ \cline{1-1}  & 28 & 2370 & 20.20 & 2379 & 19.80 (\(-2\%\)) & 2314 & 17.53 (\(-13\%\)) \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Compressive strength results at different curing time. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{1}{c}{**Mortar Sample**} & \multicolumn{2}{c}{**Failure Load**} & \multicolumn{2}{c}{**Compressive Strength**} & \multicolumn{1}{c}{**Rm,c**} & \multicolumn{1}{c}{**Ratio, \(\mathbf{Rm,c}\)} \\ \hline \multirow{2}{*}{Demineralized water samples} & 1 & 32.70 & 19.51 & & \\  & 2 & 31.03 & 19.39 & 19.42 & - \\  & 3 & 30.97 & 19.35 & & \\ \hline \multirow{2}{*}{Microfiltered water samples} & 1 & 30.56 & 19.10 & & \\  & 2 & 30.55 & 19.10 & 19.09 & 98\% \\ \cline{1-1}  & 3 & 30.52 & 10.07 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Compressive strength of mortar samples. The results show that, regardless of the concrete grade, the control mixture and the 50% recycled mixture reached similar compressive strength values at 28 curing days, with a slight decrease of 2% for all the concrete classes analyzed. This reduction can be associated with the presence of microfiltered water, which, compared to the control water, involves a slight decrease in terms of compressive strength of the mixtures, as previously demonstrated. In fact, Table 6 highlights that the use of microfiltered water allows to reach compressive strength values equal to 98% of the compressive strength provided by samples manufactured with control water. The authors [PERSON] et al. [27], obtained similar results, as concretes made with recycled water obtained through a sedimentation process showed a slight reduction in compressive strength. Despite this, the results confirm what was stated by [PERSON] et al., 2014 [22]. The authors highlighted that concrete made with recycled aggregates recovered from returned concrete in partial substitution for natural aggregates (30% by weight) has similar compressive strength to reference concrete. [PERSON] et al. [21] recorded a reduction in resistance in the range of 3-22%. However, this decrease is not substantial, and the strength drop can be compensated for by normal mixture adjustments to achieve the desired strength. At the same time, the mixture with cement sludge as filler generally shows a decrease in compressive strength of about 15% for C10/15 concrete grade and 7% for C25/30 and  
Figure 6: Compressive strength of concretes made with recycled and natural products.