## 3 Results and Discussion
### Mechanical Properties  
For the hardened-state properties, the results of the mechanical compressive strength test are discussed. The concretes were subjected to axial compressive strength tests at 7 and 28 days. Figure 12 presents the average compressive strength values and the respective standard deviation ranges for each mix. As shown in Figure 12, the SCLC-EC sample exhibited greater variation in compressive strength values compared to the other mixtures studied. This behaviour can be explained by the intrinsic nature of the commercial lightweight aggregates used (expanded clay), which have greater porosity and can present variable mechanical properties, such as crushing resistance, depending on their microstructure and production process. Previous studies [2; 13] have highlighted that this variability is a characteristic challenge of the use of lightweight aggregates in structural concretes. Figure 11: Average fresh bulk density of the SCLCs. Figure 12: Compressive strength values at 7 and 28 days for each mix. Another potential factor is the influence of the surface texture of commercial aggregates. Aggregates with lower uniformity may affect the homogeneity of the cement matrix during mixing, creating localized zones of lower or higher density, which directly reflects on the dispersion of the strength results. In addition, the rheological behaviour of SCLC-EC, combined with the lower water absorption of expanded clay (14%, according to Table 2), may have reduced the efficiency of the interaction between the aggregate and the cement paste, impacting the paste-aggregate transition. Despite the greater variability, SCLC-EC presented average strength values that exceeded the minimum limits established by ACI 213 R-03 for structural lightweight concrete. This result reinforces its technical feasibility and highlights the importance of considering the variability of commercial aggregates when designing lightweight concretes. Future studies could focus on the detailed analysis of the aggregate microstructure and the refinement of the mixing process to mitigate these effects. Based on the results obtained, all concretes, even at 7 days, met the minimum compressive strength requirements for structural concrete as per [36; 37; 38], which sets a minimum strength of 20 MPa, and for lightweight structural concrete as per [35], which requires a minimum strength of 17 MPa. At 28 days, the strength values significantly exceeded the minimum required for lightweight concretes, showing considerable strength gains. In the work of [PERSON] et al. [31], it was found that as the amount of larger lightweight aggregate (D\({}_{\text{max}}\) > 10 mm) increased, the compressive strength decreased, supporting a previous study [28]. Therefore, a solution to mitigate the influence of larger aggregate diameter is to partially replace the larger lightweight aggregate with a smaller lightweight aggregate (less than 10 mm). However, this may increase the concrete bulk density in most cases [2; 32]. Since the aggregates used in the mixes had a standardized maximum diameter of 9.5 mm, this behaviour was not observed in the results of the present study. However, the influence of these factors became noticeable only after 7 days. According to [PERSON] et al. [31], lightweight aggregates do not affect compressive strength at early ages, as the cementitious matrix is responsible for withstanding mechanical stresses. In this regard, the volume paste-to-aggregate (Vp/Vs) ratio directly affects compressive strength, as a lower ratio (<0.60) results in a larger volume of mortar and a smaller volume of coarse aggregate [13]. Figure 13 presents a relationship between the volume of lightweight aggregate per cubic meter, the mechanical strength from 7 to 28 days, and the water absorption of the lightweight aggregates. It can be noticed that the highest compressive strength gains corresponded to mixes with larger volumes of aggregate per cubic meter and lower aggregate water absorption values, as seen in the SCLC-EC and SCLC-SMR37 mixes.