## 2 Materials and Methods
### Materials
#### 2.1.3 Aggregates  
Two different types of aggregates have been used for the development of this research: natural sand (NA) standardized for reference tests distributed by the Eduardo Torroja Institute of Construction Sciences (Madrid, Spain), and recycled sand from concrete waste (RCA) supplied by the TEC-REC plant (Madrid, Spain). First, the physical properties of these raw materials were analyzed in Table 4, following the recommendations of the reference standards.  
As can be seen in Table 4, the fine particle content of the recycled aggregates was higher than that obtained for the standard sand. This is due to the process of crushing and obtaining these secondary raw materials, and if their particle size is not controlled by sieving before use, they can cause a decrease in the mechanical strength of masonry mortars [64]. Furthermore, these recycled sands have a higher friability coefficient and a lower density, which implies a decrease in the mechanical properties of the mortars, as has been demonstrated in another research [65]. It should also be noted that these RCA had a higher water absorption coefficient, which results in a higher water demand during mortar mixing [66].  
Figure 1 shows the granulometric curve obtained for applying these aggregates in the manufacture of cement mortar. The aim was to obtain a continuous particle size that favors mixture workability, using a back-and-forth sieve shaker and a series of standardized sieves according to UNE-EN 933-2 [67].  
Finally, an elemental analysis was carried out to determine the chemical composition of the recycled concrete aggregates. The results obtained using the X-ray fluorescence technique are shown in Table 5.  
\begin{table}
\begin{tabular}{c c c} \hline
**Properties** & **NA** & **RCA** & **Normative** \\ \hline Fine Content (\%) & 1.23 \(\pm\) 0.06 & 3.87 \(\pm\) 0.10 & UNE–EN 933-1 [59] \\ Particle Form & Not relevant & Not relevant & UNE–EN 13139 [60] \\ Fineness Modulus (\%) & 2.56 \(\pm\) 0.14 & 4.11 \(\pm\) 0.08 & UNE–EN 13139 [60] \\ Friability Coefficient (\%) & 14.11 \(\pm\) 0.25 & 24.15 \(\pm\) 0.23 & UNE–EN 146404 [61] \\ Bulk Density (kg/m\({}^{3}\)) & 1560 \(\pm\) 17 & 1387 \(\pm\) 14 & UNE–EN 1097-3 [62] \\ Dry Density (kg/m\({}^{3}\)) & 2569 \(\pm\) 3 & 2398 \(\pm\) 16 & UNE–EN 1097-6 [63] \\ Water Absorption (\%) & 0.54 \(\pm\) 0.04 & 5.87 \(\pm\) 0.09 & UNE–EN 1097-6 [63] \\ \hline \end{tabular}
\end{table}
Table 4: Aggregates’ physical characterization.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline
**Aggregate** & **Al\({}_{2}\)O\({}_{3}\)** & **CaO** & **Fe\({}_{2}\)O\({}_{3}\)** & **K\({}_{2}\)O** & **MgO** & **SiO\({}_{2}\)** & **MnO** & **TiO\({}_{2}\)** & **SO\({}_{3}\)** & **P\({}_{2}\)O\({}_{5}\)** & **NaO\({}_{2}\)** & **I. Loss** \\ \hline RCA & 10.81 & 12.03 & 1.56 & 2.41 & 0.98 & 57.92 & 0.43 & 0.65 & 1.03 & 0.09 & 0.41 & 11.68 \\ \hline \end{tabular}
\end{table}
Table 5: Elemental composition of recycled concrete aggregates. Results in percentage by mass.  
As can be seen in Table 5, RCA used have a majority composition of SiO\({}_{2}\) (57.92%) as a consequence of the natural aggregate used in the manufacture of concrete. In addition, CaO and SO\({}_{3}\) impurities are observed, generally coming from gypsum waste mixed in the building demolition process [69]. High content of sulfur impurities can be detrimental to the durability of reinforcement or other steel elements that come into direct contact with these mortars. Therefore, it is important to insist on the need for selective demolition processes at the end of the life of the building [70]. Finally, ignition losses (11.68%) are a consequence of the organic matter content of these secondary raw materials.