# Reutilization of Recycled CDW Sand in Mortars, Paving Blocks, and Structural Concrete
## 3 Results and Discussion
### Properties of the Developed Concrete
#### 3.4.2 Density and Compressive Strength of the Developed Concrete Samples  
Table 8 summarizes the density and compressive strength of the developed concrete samples. RAC density decreased by 4.2% with the 50 vol.% substitution and by 6.8% with the 100 vol.% replacement of NA with CDW. This reduction is attributed to the lower density of the CDW particles (see Table 6), which led to a smaller RA mass for a given substitution percentage in volume. These findings are consistent with previous studies about RA use in concrete [23; 25; 52; 55], which report a gradual decline in the density of hardened concrete as the proportion of RAs rises. Our results also align with a previous study by [PERSON] et al. [52], in which natural limestone aggregates are replaced with various types of ceramic materials (bricks, tiles, and sanitaryware) in RAC. This substitution led to a reduction in density of up to 3.3% when incorporating up to 30% ceramic waste. The compressive strength of the concrete samples decreased as the percentage of CDW increased, Figure 18, with minimal variability in the results. The RS of the RAC cured for 28 days exceeded 70%, which indicates that strength loss was less than 30% when completely replacing natural sand with the CDW recycled sand. These findings, along with the higher workability observed in the CDW concrete samples, are consistent with the results obtained for mortars (Figures 10 and 11, mortars with CEM II/B-M(Q-L) 42.5R). Despite having a lower density, the CDW aggregate exhibited slightly higher wear resistance than NA (see Table 6). Consequently, the lower RAC strength cannot be attributed to lower RA particles quality, but may have resulted from the reduced fine content of the CDW aggregate, which might have led to a less compact matrix, along with a delay in PC hydration caused by the gradual release of water absorbed by the aggregates. The RS of the developed concrete improved with curing time. Previous studies using ceramic materials as RAs in concrete have attributed this improvement to a pozzolanic reaction between the fine ceramic particles and portlandite formed during PC hydration, which enhances the interface between the aggregate and the cementitious matrix [8; 52; 56]. However, the chemical composition and LOI of the CDW used in this study (see Section 3.1.1) suggest only limited pozzolanic activity. The improvement in RS with curing time herein observed is attributed mainly to greater CDW aggregate absorption. This likely facilitates a slower release of water, which results in more gradual PC hydration over time. Previous studies into RAC demonstrate that the compressive strength of concrete is significantly influenced by the type of aggregate used. Consistently with the results obtained in this study, several researchers [8; 19; 20; 55; 21] report decreased compressive strength when using RAs in comparison to samples with NAs. Conversely, some studies, like those \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & & \multicolumn{4}{c}{**Compressive Strength**} \\ \cline{2-7}  & & & **7 Days** & & & **28 Days** & \\ \cline{2-7}  & **Density, kg/m\({}^{3}\)** & **Mean** & **Std. Deviation** & **RS** & **Mean** & **Std. Deviation** & **RS** \\  & **MPa** & **MPa** & **MPa** & **MPa** & **MPa** & **MPa** & **\%** \\ \hline REF & 2343.4 & 37.2 & 0.8 & - & 45.7 & 1.8 & - \\ CDW50 & 2245.3 & 28.9 & 0.5 & 77.7 & 39.0 & 1.7 & 85.3 \\ CDW100 & 2184.9 & 24.9 & 1.5 & 66.9 & 33.7 & 2.1 & 73.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Density and compressive strength of the developed concrete samples. by [PERSON] [23], [PERSON] et al. [25], and [PERSON] et al. [57], observed improved compressive strength when incorporating ceramic RAs. [PERSON] et al. [58] investigated the use of RAs from stoneware ceramic tiles and report a slight reduction in strength when replacing natural limestone sand. However, when 20% to 100% (by volume) of natural coarse aggregates were substituted, strength was either maintained or slightly improved. Similar studies by [PERSON] et al.