# Reutilization of Recycled CDW Sand in Mortars, Paving Blocks, and Structural Concrete
## 3 Results and Discussion
### Properties of the Developed Paving Blocks
#### 3.3.3 Splitting Tensile Strength of Paving Blocks  
Figures 14-16 illustrate the splitting tensile strength of the developed paving blocks. Figure 14 presents the splitting tensile strength (Figure 14a) and breaking load (Figure 14b) of the paving blocks produced with CEM I 52.5R, incorporating increasing amounts of CDW. Figure 15 depicts these properties for the paving blocks made with CEM II/B-M(Q-L) 42.5R. According to Section 5.3.3.2 of Standard UNE EN 1338:2004 [43], the splitting tensile strength of paving blocks must exceed 3.6 MPa, with no individual values falling below 2.9 MPa. Additionally, breaking load must be above 250 N/mm of breaking length (184.1 \(\pm\) 0.9 mm). These minimum thresholds are indicated in the figures. The paving blocks produced with CEM I 52.5R met these specifications after only 3 curing days, regardless of the percentage of CDW recycled sand used. However, those made with CEM II/B-M(Q-L) 42.5R generally met these requirements only after 28 curing days. The wide variability in the results is primarily attributed to the sample preparation process and the testing methodology.  
Figure 14: Splitting tensile strength of the paving blocks developed with CEM I 52.5R and increasing amounts of CDW: (**a**) splitting tensile strength; (**b**) breaking load. Limits established by Standard UNE EN 1338:2004 [43] indicated with striped lines.  
Figure 13: Properties of the paving blocks made with the NA and CDW aggregates: (**a**) density; (**b**) water absorption.  
Figure 16 illustrates the RS (RS = R\({}_{\rm CDW}\)/R\({}_{\rm REF}\)) between the paving blocks that incorporated CDW aggregates and the REF samples, made with 100% natural calcareous aggregates. Although no significant variation was observed with increasing CDW aggregate content, the RS values generally ranged between 80 and 100%, which denotes a maximum 20% strength reduction compared to the REF samples.  
Both the mortars and the paving blocks generally exhibited a progressive strength reduction with increasing CSW content. This trend was more pronounced in mortars than in paving blocks (see Figure 11, Figure 14 and Figure 15), with mortars displaying greater homogeneity and less variation in the strength results. This is attributed to the higher uniformity and standardization of the mortar preparation process, and also to the more precise testing methodology. Figure 17 illustrates the correlation between the strength of the paving blocks and mortars, which enables paving block splitting tensile strength based on mortar compressive strength, and vice versa, to be estimated. The relatively wide data dispersion (R\({}^{2}\) = 0.74) is due primarily to the variability in the paving blocks results, which  
Figure 16: Relative splitting tensile strength between the paving blocks incorporating CDW RA and the REF paving blocks: (**a**) CEM 152.5R; (**b**) CEM II/B-M(Q-L) 42.5R.  
Figure 15: Splitting tensile strength of the paving blocks developed with CEM II/B-M(Q-L) 42.5R and increasing amounts of CDW: (**a**) splitting tensile strength; (**b**) breaking load. Limits established by Standard UNE EN 1338:2004 [43] indicated with striped lines.  
can be attributed to their manual preparation process and the challenges associated with adjusting the splitting tensile strength testing method.  
The results differ from previous research on concrete blocks [17], where the replacement of natural sand with recycled aggregates (RAs) led to only slight reductions in compressive strength, particularly at replacement rates of 25% or 50%. However, at higher replacement rates, an increase in transverse strength was observed. These discrepancies may be attributed to the higher strength of the reference mixes used in the present study, as well as variations in the type and particle size distribution of the RAs employed in both studies.