# Reutilization of Recycled CDW Sand in Mortars, Paving Blocks, and Structural Concrete
## 2 Material and Methods
### Design, Preparation, and Characterization of Structural Concrete  
The mix proportions for the concrete samples designed to evaluate the influence of CDW recycled sand on the performance of structural concrete are summarized in Table 3. Mixes were prepared using the CEM II/B-M(Q-L) 42.5 cement. Consistently with the preparation of the mortars and paving blocks, the amount of PC (370 kg/m\({}^{3}\)) and superplasticizer (SKP380 provided by Sika, 0.7% by weight of cement, equivalent to 2.59 kg/m\({}^{3}\)) and the effective w/c ratio (0.50) were left constant across all the concrete mixes. The CDW volume used to replace NAs varied. The total amount of water in the mix was adjusted according to the water absorption and humidity of aggregates in order to have constant effective water. A reference mix with no CDW sand (labelled \"REF\") was also prepared, and 50 vol.% and 100 vol.% of natural sand were replaced with recycled CDW. Figure 3: Splitting tensile strength testing of the paving blocks made with NA and CDW aggregates: (**a**) equipment used; (**b**) fragments after the test. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Aggregate**} & **Subs.** & **Design.** & **Cement kg/m\({}^{3}\)** & **Effective Water, L** & **Effective w/c kg/m\({}^{3}\)** & **Gravel NA** & **SAD DW kg/m\({}^{3}\)** & **CDW kg/m\({}^{3}\)** & **kg/m\({}^{3}\)** \\ \hline Limestone & 0 & REF & & & & 890 & 0 \\ CDW & 50 & CDW50 & & & & & 445 & 386.7 \\ CDW & 100 & CDW100 & & & & & 0 & 773.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mix proportions for structural concrete. Concrete mixes were prepared using a concrete pan mixer. The workability, density, compressive strength, shrinkage, and permeability of the developed concrete were evaluated. Workability was determined using the Abrams cone test in accordance with Standard UNE-EN 12350-2 [44], with measurements taken 10 and 30 min after mixing started. For compressive strength testing, eight cubic specimens with dimensions of \(100\times 100\times 100\) mm\({}^{3}\) were used. Two prismatic specimens measuring \(100\times 100\times 300\) mm\({}^{3}\) were employed for shrinkage testing, while three cylindrical specimens, each 150 mm in diameter and 300 mm high, were employed for permeability testing. The utilized molds met Standard UNE-EN 12390-1 [45]. The concrete specimens were cured for up to 28 days in a temperature and humidity-controlled chamber at 20 \({}^{\circ}\)C and 100% relative humidity. Density was determined by weighing the cylindrical specimens after 28 curing days. Compressive strength was evaluated using the cubic specimens, cured for 7 and 28 days, in accordance with Standard UNE-EN 12390-3 [46]. Testing was conducted with a hydraulic press by applying a load rate of 0.6 MPa/s until failure. Four specimens per concrete mix and curing period were tested, and the mean compressive strength values, along with their corresponding standard deviations, were calculated. The relative strength (RS) of the concrete containing CDW was calculated as the ratio of the strength of the RAC to that of the REF mix. The shrinkage of the concrete samples was measured according to Standard UNE-EN 12390-16 [47], using prismatic specimens (dimensions of 100 \(\times\) 100 \(\times\) 200 mm\({}^{3}\)). Measurements were taken at 1, 7, 14, 18, and 28 curing days, using reference points placed on two faces of samples, as illustrated in Figure 4. In accordance with Standard UNE-EN 12390-16 [47], which specifies a relative humidity range of 50-70% \(\pm\) 5%, these concrete specimens were cured in a temperature and humidity-controlled chamber, at 20 \({}^{\circ}\)C and 70% relative humidity. To assess the durability of the concrete mixes, water penetration tests were performed in accordance with Standard UNE-EN 12390-8 [48].