## 3 Results and Discussion
### Water Absorption, Density and Voids  
Figure 6 presents the bulk density evolution of the concrete mixtures cured in water over hydration time. The Ref. mixture (M1) consistently showed the highest density values, indicating a compact and well-hydrated matrix. In the case of M2, containing 15 vol% CS, a slight reduction in density was observed at early ages (28 days), but the difference narrowed by 180 days and remained small at 360 days. This trend suggests the progressive contribution of the pozzolanic activity of CS to matrix densification, especially at extended curing ages, such as 180 and 360 days, which aligns with that reported in previous studies [4; 28; 41]. The implemented curing temperature (23 \(\pm\) 2 \({}^{\circ}\)C) was within the optimal range for the progression of pozzolanic reactions, including the slow-reacting phases in CS such as amorphous silicates and aluminates. Prior studies [85] confirm that prolonged curing under these conditions promotes C-S-H formation and densification of the microstructure. Mixtures M2, M3, and M4, combining CS and increasing amounts of RCAs, exhibited a monotonic reduction in density proportional to the RCA content. At 180 days, this reduction was already evident and reflects the influence of RCA's higher porosity and lower specific gravity. The results at 360 days confirm that later hydration stages only partially mitigate the effect of RCA on the bulk density of the concrete mixtures. Figure 5: Compressive strength ratio of concretes exposed to MgSO4 and cured in water (Red dashed line indicates equal compressive strength for water curing and sulfate exposure). Figure 6: Densities of concrete mixtures at different ages. Subsequently, Figure 7 shows the water absorption capacity of the mixtures as a function of curing age. Both the Ref. and M2 mixtures exhibit the lowest absorption levels, with very similar values from 180 days onward. This behavior confirms that the replacement of cement by 15% of CS does not negatively affect the permeability of the concrete matrix, and even improves it at extended curing ages, through refinement of the pore structure [28]. In mixtures with RCA and 15% CS (M2-M4), water absorption monotonically increases as the RCA replacement level increases. At 180 days, the differences between the Ref. and 15 CS-RCA-containing mixes were already pronounced, highlighting the persistent influence of RCA's internal porosity; a similar trend was obtained in a previous work [4]. Although some reduction in absorption occurs between 28, 180 days and 360 days, due to ongoing hydration, the improvements are insufficient to compensate for the inherently porous nature of the RCAs [12]. Finally, Figure 8 displays the void content evolution across curing ages. Ref. and M2 mixtures maintain the lowest void content values at all ages. Notably, at 180 days, the voids in M1 decreased further compared to 28 days, likely due to the delayed formation of additional C-S-H from the reaction of CS. This confirms the interpretation that CS promotes progressive pore filling in extended curing conditions, as reported in previous studies [28; 41]. In contrast, mixtures containing 15% CS-RCA (M3-M5) exhibit higher void contents overall, consistent with the increased porosity, microcracking, and heterogeneity of the interfacial transition zone (ITZ) introduced by the RCAs [12]. However, by 360 days, a noticeable reduction in void content (%) is observed in M2 and M3, suggesting that ongoing hydration partially compensates for the microstructural weaknesses introduced by RCAs. Despite this improvement, their void levels remain above those of Ref., though slightly lower than M2, indicating that the combined use of CS and RCAs may promote a viable densification mechanism over time, potentially offsetting some of the drawbacks associated with RCA alone. Although global voids and water absorption were measured in this study, the pore size distribution and connectivity--which directly influence sulfate ingress kinetics--were not characterized.