## 3 Results and Discussion
### Microstructural Analysis
#### 3.4.4 SEM Analysis  
Microstructural characterization by SEM was performed on both matrices (100% OPC and 85% OPC-15% CS) after curing in water and after exposure to the MgSO\({}_{4}\) solution. Figure 15 shows the SEM images of the matrices at 180 days. It can be observed that the mixtures cured in water exhibit a homogeneous structure. However, for the mixtures exposed to the MgSO\({}_{4}\) solution, the matrix with 100% OPC shows microcracks (highlighted in yellow boxes), which may be attributed to the accumulation of expansive products. In the image taken at 8000\(\times\) magnification for the OPC sample, the formation of ettringite (AFt) is observed, although in smaller amounts compared to the Na\({}_{2}\)SO\({}_{4}\) attack. This difference is consistent with the specific mechanism of MgSO\({}_{4}\) attack, in which it reacts with portlandite and C-S-H gel to form Mg(OH)\({}_{2}\) (brucite) and gypsum (CaSO\({}_{4}\) 2H\({}_{2}\)O), with minimal AFt production. In the image corresponding to the 15% CS matrix, slatted ettringite can be observed. Its morphology varies due to the pH of the mixture, which influences the shape and formation of the crystals [104].  
Although the transformation from C-S-H to M-S-H is inferred through converging evidence from FTIR and TGA, future work should include SEM-EDS or STEM-EDS mapping to directly confirm spatial gradients in decalcification and Mg uptake.  
While the present study evaluated magnesium sulfate attack through visual inspection, porosity, density, and strength loss, it did not include direct quantification of Mg\({}^{2+}\) or SO\({}_{4}\)\({}^{2-}\) ingress. Such measurements, whether via chemical profiling or modeling, could offer deeper insight into ion transport mechanisms and degradation front evolution. Future work should incorporate approaches such as those described by [PERSON] et al. [105] where advection-diffusion transport equations with time- and space-dependent boundary conditions are applied to simulate aggressive ion penetration in cementitious matrices.  
In summary, the results of this study demonstrate the potential of copper slag (CS) to mitigate sulfate-induced deterioration mechanisms, primarily by reducing portlandite content and promoting the formation of less expansive M-S-H phases. However, alternative SCM systems--such as ground granulated blast furnace slag, metakaolin, and alkali-activated binders--have shown strong sulfate resistance under similar exposure conditions, due to their refined pore structures and reduced CH content (e.g., [106; 107; 108]). While the present study focuses on CS due to its local abundance and dual functionality, future work should include head-to-head comparisons with these systems to assess performance and scalability under aggressive environments, such as MgSO\({}_{4}\) exposure. In this regard, [PERSON] et al. [109] recently demonstrated that optimizing the CaO/Al\({}_{2}\)O\({}_{3}\) and  
Figure 15: SEM of cement paste mixtures exposed to water and (**a**) OPC (100% OPC) mixture; (**b**) 15% CS (85% OPC and 15% CS).  
SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) molar ratios in slag/fly ash-based geopolymers can significantly improve the mechanical performance and water stability of stabilized clays. Their findings reveal that the balance between geopolymerization and hydration reactions plays a critical role in durability performance, offering valuable insight for future comparative studies involving SCM-based systems exposed to sulfate-rich environments.