## 3 Results and Discussion
### Compressive Strength  
The compressive strength was evaluated for concrete mixtures cured in water and exposed to a MgSO\({}_{4}\) solution at selected ages, as shown in Figure 4 (where bars indicate mean values, while error bars indicate one standard deviation above and below mean values). Figure 4a shows the results obtained for concretes cured for 28, 56, 128, 128, 208 and 388 days. As expected, an increase in curing age leads to an improvement in the compressive strength of all mixtures. However, this mechanical property decreased monotonically as the RCA content increases, at all curing ages. Specifically, at early curing ages such as 28 and 56 days, when comparing the Ref. mixture (100% virgin materials) with M2 (containing 15% CS as SCM), the compressive strength decreased by 8.44% and 7.78%, respectively. At later curing ages, such as 208 and 388 days, mixture M2 achieved average compressive strengths of 70.62 MPa and 73.62 MPa, respectively. These values were only 5.9% and 4.0% lower than those of the Ref. mixture, indicating a reduced difference in mechanical performance over time. This behavior is attributed to the pozzolanic effect of the CS, which is known to be only moderately reactive and requires extended curing periods for its beneficial effects to become more evident [28; 41]. Similar findings were reported by [PERSON] et al. [74], who incorporated CS as SCM in proportions ranging from 10 to 20 wt% for mortar production. Their study showed a reduction in compressive strength at early ages (3, 7, and 28 days), which was attributed to the poor hydrophilicity of the material. This condition increases the amount of free water in the mixture and reduces the formation of hydration products. Despite its lower reactivity compared to OPC, mixtures incorporating up to 15% CS exhibit improved mechanical performance at extended ages [75]. In the case of concretes incorporating RCA (mixtures M3, M4, and M5), at 28 and 56 days of curing, the composite material with the highest content of recycled material, M5 (15% CS and 100% RCA), exhibited average reductions in compressive strength of 32.5% and 27.8%, respectively, compared to the reference mixture, as seen in Figure 4a. The trend remains at 388 days of curing, with reductions of 12.6%, 16.71%, and 22.0% observed in mixtures M3, M4, and M5, respectively, in comparison to the Ref. mixture. The reduction in the mechanical performance of concretes containing recycled concrete aggregates (RCA) has been reported by [PERSON] et al. [12], [PERSON] et al. [76], [PERSON] et al. [77] who attribute this behavior to the presence of adhered mortar. This residual mortar  
Figure 4: Compressive strength of concretes at different ages: (**a**) cured in water and (**b**) exposed to the 5% MgSO\({}_{4}\) solution. increases the porosity of the RCA, leading to higher water content in the new interfacial transition zone, which promotes the formation of larger calcium hydroxide (portlandite) crystals, thereby weakening the composite material. Additionally, RCAs may contain microcracks generated during demolition and subsequent crushing and processing stages. It is worth noting that the mechanical performance of the mixture including RCA could be further improved in terms of strength by the use of treated RCA. This inclusion was beyond the scope of this research, and it is recommended as future research. Figure 4b presents the compressive strength of the mixtures before exposure (i.e., after 28 days of water curing) and after 28, 100, 180, and 360 days of MgSO\({}_{4}\) exposure. All mixtures exhibited an initial increase in compressive strength at early exposure ages, followed by a gradual decline in this mechanical property as the exposure time increased. At 28 and 100 days of exposure, all mixtures showed higher compressive strength compared to the strength developed prior to the onset of exposure.