## 3 Results and Discussion
### Mechanical Properties
The other mixtures had lower strength gains, particularly SCLC-SBA50, which, despite having a higher volume of lightweight aggregate compared to SCLC-SBA90, had a higher water absorption value than SCLC-SBA90. High water absorption within the aggregate can directly influence its crushing strength, making the aggregate more brittle and contributing to the lower performance of the concrete in terms of compressive strength. Thus, in agreement with [PERSON] et al. [2], in addition to the proportion of lightweight aggregates in the mix and their water absorption, other specific characteristics of the aggregates, such as crushing strength, can influence the performance of SCLC. A directly proportional relationship between the compressive strength of the studied concretes and the crushing strength of each aggregate was observed, where the mix with the strongest aggregate (SCLC-EC) showed the highest strength gain, while the mix with the weakest aggregate (SCLC-SBA50) showed the lowest strength gain at 28 days. Conventionally, in normal-density concrete, compressive strength is primarily related to the water-to-cement ratio (w/c) and the amount of binder in the mix [27; 33]. However, this relationship cannot be applied effectively to concretes with lightweight aggregates due to the difficulty of determining how much of the mixing water will be absorbed by the aggregates. In this study, the water-to-cement ratio (w/c) was not a significant variable, as the same ratio was maintained in all mixtures. Furthermore, the internal water absorbed by each aggregate during the pre-saturation process was not considered in the calculations, following the premise that this water remains retained within the aggregates and is not released during mixing, as supported by [35]. Therefore, the lightweight aggregate becomes the main factor influencing not only the strength of the concrete, but also all its other properties [27]. Supporting the compressive strength results, Figure 14 shows the self-compacting lightweight concrete specimens after failure, allowing for observation of the aggregates' behaviour within the cement matrix in terms of dispersion and homogeneity. It is evident in all the concretes that, unlike conventional concrete, the fracture surface passed through the aggregates, reaffirming the lightweight aggregates' influence as the  
Figure 14: SCLC specimens after axial compression testing. Figure 13: Relationship between strength gain, aggregate volume, and aggregate absorption. limiting factor in compressive strength. According to previous studies [34; 35], this occurs due to the improvement of the paste-aggregate transition zone and because there is no significant difference between the elastic moduli of the paste and the lightweight aggregate, leaving the lightweight aggregate as the weakest element in the mixture. Parallel to this, dry bulk density was measured at 7 and 28 days of curing, along with the compressive strength test. Figure 15 presents the dry bulk density values at 7 and 28 days of curing and the average dry bulk density for each studied mix. From the results, it is noted that only SCLC-SBA90 showed a bulk density above the limit defined by [38], which is 2.00 g/cm\({}^{3}\). However, this behaviour can be explained by the higher amount of lightweight aggregate in the SCLC-SBA90 composition, as seen in Table 5, which led to greater water absorption. One possible solution would be to increase the aggregate size [23], as larger lightweight aggregates have lower bulk density and lower water absorption [18]. Moreover, considering that the variation is not significant, previous studies [20; 23; 31; 36] have found that bulk densities greater than 2.03 g/cm\({}^{3}\) still met the classification for lightweight self-compacting concrete, as the other requirements were fulfilled. For instance, [PERSON] et al. [2] reported compressive strength values between 22.5 MPa and 30 MPa at 28 days for expanded clay-based self-compacting lightweight concrete, with densities ranging from 1.80 to 1.95 g/cm\({}^{3}\).