## 4 Conclusions  
This study evaluated the performance of concrete mixtures containing a 15 vol% partial replacement of ordinary Portland cement (OPC) with copper slag (CS) as supplementary cementitious material (SCM), along with 0%, 20%, 50%, and 100% replacement of natural coarse aggregate by recycled coarse aggregate (RCA) sourced from construction and demolition concrete waste. Replacing 15% of cement with CS, a non-carbon-emitting byproduct, is estimated to reduce binder-related CO\({}_{2}\) emissions by approximately 5.7% [4]. Similarly, RCA use avoids primary aggregate extraction and landfill disposal, advancing circular material flows in construction. The mixtures were evaluated based on their compressive strength under water curing conditions and exposure to a 5% MgSO\({}_{4}\) solution, as well as through structural analysis. The main findings of this study are outlined below:  
1. The visual degradation of the mixtures intensifies as the exposure period extended from 180 to 360 days, with more pronounced deterioration observed along specimen edges. Additionally, the white layer on the surface became more clearly defined and exhibited some cracking, indicating a more severe sulfate attack over time. However, no significant visual differences were observed between the mixture M2 (15% CS and 0% RCA) and those containing increasing dosages of RCA (M3, M4, and M5, with 20%, 50%, and 100% of RCA, respectively). 2. The incorporation of 15% CS as SCM for OPC reduced compressive strength at early ages (e.g., 28 and 56 days). However, this effect diminishes over time, and at later curing ages (e.g., 388 days), the strength becomes comparable to that of the reference mixture. Conversely, the use of recycled concrete aggregate (RCA) results in a monotonic decrease in compressive strength as the RCA content increases, regardless of the curing age. 3. At 28 and 100 days of exposure, an increase in compressive strength was observed in cementitious mixtures, whether or not they contained CS and/or RCA, when added to a 5% MgSO\({}_{4}\) solution. This short-term gain is attributed to pore filling by expansive phases such as gypsum and ettrinigite. However, at longer exposure durations (180 and 360 days), all mixtures showed a loss in mechanical strength. This long-term loss is associated with the progressive degradation of the binding matrix, including C-S-H decalcification, M-S-H formation, and internal cracking. 4. Fourier-transform infrared spectroscopy (FTIR) analyses revealed that the presence of 15% CS modifies the sulfate magnesium attack mechanism by partially converting C-S-H into M-S-H. Compared to the Ref., the CS-blended paste exhibited reduced formation of brucite and gypsum, indicating a less expansive and less damaging degradation mechanism. This behavior suggests that CS contributes to stabilizing the microstructure under MgSO\({}_{4}\) exposure by promoting the formation of more stable, but non-cementitious hydrates. 5. TGA/DTG results demonstrate that incorporating 15 vol% CS modifies the thermogravimetric performance of the cement paste under MgSO\({}_{4}\) exposure. The intensity reduction of the C-S-H peak and the distinct peak intensity between 390 and 500 \({}^{\circ}\)C suggests the formation of M-S-H resulting from magnesium-induced decalcification. Compared to the Ref., CS-blended cement exhibited lower brucite and gypsum-related mass losses, indicating a less aggressive degradation process. Additionally, the re duced portlandite content in CS-blended cement samples reflect the simultaneous effect of pozzolanic reaction and sulfate attack, highlighting a degradation mechanism distinct from pure OPC. As a future study, higher CS dosages and hybrid SCMs should be tested for optimal strength/densification, as well as examining the impact of using RCA surface pre-treatments--such as surface treatments, pozzolanic coatings, or pre-saturation techniques--and using optimized mixing protocols that employ stabilizers, water-reducing admixtures, or graded RCA blending.