# Reutilization of Recycled CDW Sand in Mortars, Paving Blocks, and Structural Concrete
## 1 Introduction
Additionally, the potential for contamination in CDW-derived RAs is significant because materials like concrete, rubble, bricks, tiles, and asphalt, which are common components of CDW, can be processed into RAs. Consequently, the variability in the composition of CDW presents the challenges of ensuring the consistency and reliability of RAs, particularly in structural applications where material performance is critical [13]. To address these challenges, research has focused on optimizing processing techniques to enhance the quality and performance of RAs. Advanced crushing, sieving, and washing processes have been developed to remove contaminants, to reduce variability in particle size distribution, and to improve overall RA quality. In the present study, CDW-derived sand was subjected to multiple washing cycles to reduce its sulphate content, along with pre-separation, controlled crushing, and sieving. Investigating the properties of this specific CDW and its behavior as raw material in the development of more sustainable building materials will contribute to expanding the current knowledge base and to advancing toward a more sustainable and circular construction industry. Despite advancements made in processing and treatment, adopting RAs in the construction industry remains limited. One of the main obstacles for the widespread RA use is the perception that they are of lower quality than Nas, and they are, therefore, mainly used in landfills [27]. This perception is often reinforced by the absence of standardized guidelines and regulations that govern RA use in construction [28]. For instance, the Spanish Structural Concrete Code restricts RA use to those that exclusively derive from structural concrete, limiting their use to a maximum of 20% by weight of natural gravel. However, other European standards, such as DS 2426 from Denmark [29], NEN 8005 from the Netherlands [30], and NB 26 from Norway [31], allow the use of moderate and high-quality fine RAs. Additionally, Standard NF EN 206-1/CN from France [32] allows high-quality fine RA use, albeit with specific limitations [27]. To overcome these obstacles, it is important to conduct more studies that generate new knowledge and contribute to establishing consistent standards that guarantee the quality and safety of RAs. This will help to build trust and will encourage their use in a wider variety of applications because, although many studies have investigated CDW aggregate use in concrete [6], very few studies have examined their reutilization in non-structural elements, such as paving blocks or mortars. Previous studies by [PERSON] et al. [33] investigated mortars incorporating recycled sand with a particle size of less than 8 mm, primarily composed of calcite and quartz. The results indicated a slight reduction in both compressive and flexural strength. [PERSON] et al. [16] developed mortars that replaced natural sand with RAs from CDW at rates of up to 100%. Although the results showed that higher RA content resulted in higher primary energy demand and greater water consumption, mortars containing 50% RA exhibited the best overall performance. This was characterized by higher carbon capture potential, improved compressive strength, and acceptable tensile adhesion strength. The mortars developed by [PERSON] and [PERSON] [34] reused recycled sand consisting of 88.5% concrete- or mortar-based materials, 4.7% natural stones and aggregates, and approximately 6.5% other compounds. These mortars also showed a positive environmental impact, particularly when replacing 40% to 60% of natural sand. Although a slight reduction in strength was noted, this was balanced by a significant conservation of natural resources. [PERSON] et al. [17] investigated the use of RAs from CDW as replacements for coarse and fine aggregates in concrete bricks and blocks. The two types of RAs used consisted primarily of old concrete rubble, along with small amounts of natural stones, clay bricks, and other impurities such as wood, tiles, and metals.