# Reutilization of Recycled CDW Sand in Mortars, Paving Blocks, and Structural Concrete
## 2 Material and Methods
### Design, Preparation, and Characterization of Mortars and Paving Blocks  
After characterizing the natural and recycled sand, mortars and paving blocks were prepared to assess the suitability of the CDW aggregate as a partial replacement for natural sand in these applications. The applied mix proportions are summarized in Table 2. Both mortars and paving blocks were produced using two PC types: CEM I 52.5R and CEM II/B-M(Q-L) 42.5R. A reference mix (labeled \"REF\") containing only natural limestone aggregates was prepared, while recycled CDW replaced up to 100% of the NA sand volume (vol.%). In addition to the reference mix, four natural sand replacement levels were investigated. Given the two analyzed cement types, there was a total of 10 series: 5 using CEM I 52.5R and 5 using CEM II/B-M(Q-L) 42.5R. For clarity, they are referred to as CEM I and CEM II, respectively, throughout this work. All the mortar and paving block mixes were prepared at a constant cement-to-sand-to-water (cs:w) weight ratio of 1:3:0.45, along with a constant amount of fluidizing agent SKP380, provided by Sika (1.46 kg/m\({}^{3}\), equivalent to 0.25% by weight of cement). The total amount of water was adjusted to maintain consistent effective water content, which, as defined by UNE EN 206:2013 [32], is the water available for the cement hydration process, excluding the water absorbed by aggregates. In this study, the effective water-to-cement ratio (w/c) was kept constant in all the prepared samples (mortars, paving blocks, and concrete). To achieve this, all the mixes were designed by considering the weight of aggregates saturated with a dry surface, and the water absorption coefficient and humidity content of the aggregates were contemplated to calculate the total amount of water required for each mix. To simulate real fabrication conditions, the packing block aggregates were used with their natural moisture content. In contrast, to minimize the influence of aggregate moisture and to achieve better control over the effective w/c ratio, the aggregates used for mortar production were pre-dried. The mortar samples were mixed using an automatic mortar mixer, following the specifications of Section 6.2 of Standard UNE EN 196-1 [41]. Each designed mix was cast into molds with three cavities, each measuring 40 \(\times\) 40 \(\times\) 160 mm\({}^{3}\). The molds were filled in two layers, with each layer compacted using an automatic horizontal jolting table, applying \begin{table}
\begin{tabular}{c c c} \hline \hline
**Property** & **Test** & **Standard** \\ \hline Particle size distribution & Sieve analysis & UNE-EN 933-1 [38] \\ Percentage of fines (\(<\)0.063 mm) & Sieve analysis & UNE-EN 933-1 [38] \\ Assessment of fines & Sand equivalent & UNE-EN 933-8 [39] \\ Specific weight & Pycometer & UNE-EN 1097-6 [40] \\ Water absorption & Sand absorption cone & UNE-EN 1097-6 [40] \\ Resistance to wear & Micro-Deval * & UNE 146404 [37] \\ \hline \hline \end{tabular}  
* 500 g material, 2.5 kg stainless steel balls, 2.5 kg water, 1500 revolutions at 100 \(\pm\) 5 rpm.  
\end{table} 
Table 1: Properties analyzed, procedures, and standards used to characterize the aggregates. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Aggregate** & **Substitution** & **Designation** & **Cement (*)** & **Effective Water,** & **Effective** & **NA** & **CDW** \\  & **Vol.\%** & & **kg/m\({}^{3}\)** & **L** & **w/c** & **kg/m\({}^{3}\)** & **kg/m\({}^{3}\)** \\ \hline Limestone & 0 & REF & & & & 1757.8 & 0 \\  & 25 & CDW25 & & & & 1318.4 & 381.9 \\ CDW & 50 & CDW50 & 586 & 264 & 0.45 & 878.9 & 763.8 \\  & 75 & CDW75 & & & & 439.5 & 1145.6 \\  & 100 & CDW100 & & & & 0 & 1527.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mix proportions for mortars and paving blocks. (*) Note: this table applies to the series produced using both cement types (CEM I and CEM II). 60 drops at a rate of one per second.