# Reutilization of Recycled CDW Sand in Mortars, Paving Blocks, and Structural Concrete
## 4 Conclusions  
This study evaluates using CDW-derived sand as RA in mortars, paving blocks, and concrete. The key findings are as follows:  
* The CDW aggregate was composed of mainly limestone and ceramics, with low sulfur content, lower fine content, and no pozzolanic activity. It was slightly harder than natural limestone, and had lower density and higher water absorption. The similar properties of NA and RA suggest that the CDW sand can be effectively reused in sustainable construction materials.
* The workability of paving blocks slightly improved with 20 vol.% CDW, but decreased with higher content, while mortars and concrete showed improved workability with the CDW sand.
* The density of all the samples decreased as CDW content increased, and ranged from 2.35 g/cm\({}^{3}\) in the REF sample to 2.10 g/cm\({}^{3}\) with 100% CDW recycled sand.
* The water absorption in the paving blocks increased with the CDW content, limiting their suitability to non-frost areas. Water permeability was higher in RAC, but the water penetration values met the Structural Code limits for all the environmental exposure classes.
* The paving blocks made with CEM I 52.5R met splitting tensile strength requirements after 3 curing days, while those with CEM II/B-M(Q-L) 42.5R met the standard after 28 curing days.
* Compressive strength generally decreased with higher CDW content. The RS of the mortars with up to 50 vol.% CDW remained above 80% at all curing ages, and the RS of all the mortars cured for 28 days exceeded 70%, regardless of the CDW content. The compressive strength of the RAC samples met the minimum requirements for reinforced concrete.  
The use of RAs is a key strategy for sustainable construction because it reduces environmental impacts and improves resource efficiency. This study confirms that the CDW-derived sand used can effectively replace natural limestone sand in both structural and non-structural applications. Although regulatory restrictions limit the use of RAs in structural concrete, the study highlights opportunities for reusing this significant waste material, with the optimal replacement percentage determined by specific application requirements. The results support the development of standardized guidelines for RAs in non-structural applications, encouraging broader industry adoption and providing environmental benefits. Further research, along with supportive policies and economic incentives, is essential to overcome the current challenges of using RAs and to promote circular construction practices.  
Conceptualization, L.R., A.M.P., A.P., M.R.-F. and D.H.-F.; data curation, L.R., V.A., D.H.-F. and A.M.-E.; formal analysis, M.R.-F., V.A. and A.M.-E.; funding acquisition, A.P., M.R.-F. and D.H.-F.; investigation, L.R., M.R.-F., V.A. and A.M.-E.; methodology, L.R., A.M.P. and A.P.; project administration, A.M.P., A.P., M.R.-F. and D.H.-F.; resources, A.P. and D.H.-F.; supervision, L.R., A.M.P. and V.A.; writing--original draft, V.A., D.H.-F. and A.M.-E.; writing--review and editing, L.R., A.M.P., A.P. and M.R.-F. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Generalitat Valenciana, the regional government of Valencia, through project INVEST/2022/243, and by Universitat Jaume I (UJI) through project UJI-B2021-34.  
The raw data supporting the conclusions of this article will be made available by the authors on request.  
The authors are grateful to HOPE & EFFORT S.L. for supplying the CDW recycled sand and to Elite Cementos for providing the cement used in this research. They would alsolike to express their gratitude to the Universitat Jaume I (UJI) for providing the means to conduct this research and the Scientific Instrumentation Centre of the UJI for conducting the microstructural characterization tests.  
The authors declare no conflict of interest.