# Reutilization of Recycled CDW Sand in Mortars, Paving Blocks, and Structural Concrete
## 3 Results and Discussion  
### Properties of Aggregates  
#### 3.1.1 Morphology and Chemical and Mineralogical Composition of Aggregates  
The morphology of the natural calcareous sand and recycled CDW sand used in the study is illustrated in Figure 6. Both sand types exhibited irregular particles characterized by rough surfaces and sharp edges. The chemical composition of the NA and CDW aggregates is listed in Table 4. The CDW sand primarily consisted of SiO\({}_{2}\) and CaO, with relatively large amounts of Al\({}_{2}\)O\({}_{3}\), and low contents of Fe\({}_{2}\)O\({}_{3}\), Na\({}_{2}\)O, MgO, and K\({}_{2}\)O. The total amount of SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), and Fe\({}_{2}\)O\({}_{3}\) was 41.3%, which is less than 70%, the value established by Standard UNE EN 450-1:2013 [49] for other pozzolanic materials commonly used in concrete, such as fly ashes. Additionally, to present pozzolanic activity, Standard UNE EN 450-1:2013 [49] establishes that loss on ignition (LOI) must be below 5% of the total composition. Therefore, the CDW sand was not expected to exhibit pozzolanic activity, or at least not significantly, and behaved more like an inert aggregate. Although this aggregate size is too large to exhibit pozzolanic activity with PC, pozzolanic behavior could enhance the aggregate-matrix interface. The X-ray fluorescence tests (XRF) did not detect significant amounts of sulfur. Figure 5: Concrete samples after water penetration depth measurements. Figure 6: Aggregates used in this study: (**a**) natural calcareous; (**b**) CDW. The LOI was determined at 1050 \({}^{\circ}\)C for 45 min and closely matched the obtained thermogravimetry results, as shown in Figure 7 (total weight losses expressed as a percentage). The calcareous aggregate exhibited significant weight loss between 630 \({}^{\circ}\)C and 920 \({}^{\circ}\)C, and reached approximately 45%, which is attributed to the decomposition of CaCO\({}_{3}\) and MgCO\({}_{3}\). Although this mass loss generates several overlapping peaks that cannot be separately evaluated, previous studies [50] indicate that the first band is associated with the decomposition of magnesium carbonate, and the second with calcium. The mass loss observed in the CDW aggregate (23.4%) is primarily attributed to the decomposition of limestone (CaCO\({}_{3}\)), as it predominantly occurred between 650 \({}^{\circ}\)C and 900 \({}^{\circ}\)C. The lower mass loss observed in the CDW waste, compared to the natural dolomitic limestone, is attributed to its lower limestone content, as the CDW waste was mixed with other materials, primarily ceramics. Notably, no mass loss related to the decomposition of cement hydrates was observed. The dehydration of calcium silicates (CSH) or ettrinite (AfT) would generate a band between 100 \({}^{\circ}\)C and 180 \({}^{\circ}\)C, while calcium aluminates (CAH) or calcium aluminosilicates (CASH) would generate a band between 180 \({}^{\circ}\)C and 240 \({}^{\circ}\)C [51]. Similarly, no mass loss associated with portlandite dehydration, which typically occurs between 520 \({}^{\circ}\)C and 600 \({}^{\circ}\)C, was observed. These results indicate that the quantity of the concrete waste in the CDW recycled sand was minimal. Thermogravimetric results indicate that the recycled CDW aggregate was expected to show improved stability at high temperatures. Finally, although ceramic residues typically experience minimal mass losses up to 1000 \({}^{\circ}\)C, their presence in the sample was confirmed by the X-ray diffraction analysis, as depicted in Figure 8. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline  & **Al2O3** & **SiO2** & **CaO** & **MgO** & **K2O** & **Na2O** & **Fe2O3** & **SO3** & **Other** & **LOI * \\ \hline CDW & 8.2 & 32.1 & 30.5 & 1.8 & 1.1 & 1.1 & 1.0 & 0.5 & 0.4 & 23.4 \\ \hline \end{tabular}  
* Determined at 1000 \({}^{\circ}\)C.  
\end{table} 
Table 4: Chemical composition of the recycled CDW aggregate, weight % (wt.%). Figure 7: DTG curves for the NA and CDW aggregates.