## 3 Results and Discussion
### Microstructural Analysis
#### 3.4.3 FTIR Analyses
A similar pattern is observed at 518 cm\({}^{-1}\), suggesting a more developed silicate network and possible contributions from hydrated aluminates. Following MgSO\({}_{4}\) exposure (Figure 14), the O-H band intensity within the same region significantly decreases in Ref. Mg, while for 15% CS Mg, a significant change is not observed. This reduction for Ref. Mg reflects the consumption of Ca(OH)\({}_{2}\) due to brucite formation (Mg(OH)\({}_{2}\)), as a result of magnesium sulfate attack (see Equation (1)), according to the report by [PERSON] et al. [97], as well as a reduction in the chemically bound water in C-S-H, due to the destabilization of C-S-H gel due to sulfate salt that reacts with the C-S-H phase, leading to loss of calcium that may transform into the magnesium silicate hydrate (M-S-H) phase [97; 98]. \[\text{MgSO}_{4}+\text{Ca(OH)}_{2}\rightarrow\text{Mg(OH)}_{2}\text{ (brucite)}+\text{CaSO}_{4}\text{ (gypsum)} \tag{1}\]  
For 15% CS Mg, which does not exhibit a significant difference in the O-H stretching region compared to its counterpart, this behavior can be attributed to a reduced availability of Ca(OH)\({}_{2}\) resulting from both the lower clinker content and the pozzolanic reactions of CS at later ages [28]. Consequently, the potential for brucite formation is already limited due to the prior consumption of Ca(OH)\({}_{2}\), as previously demonstrated in studies involving slag (40% vol.) [99] and silica fume (6% vol.) [100], both of which were successfully incorporated to enhance sulfate resistance. Nonetheless, magnesium ions can still destabilize the C-S-H, converting it into M-S-H, a fibrous and non-cemetitious product, which may also contribute to chemically bound water, and has been particularly observed in cementitious systems that contain SCMs, such us blast furnace slag [101] and silica fume [95]. It is important to note that while M-S-H does not contribute appreciably to the early mass loss observed in TGA (80-200 \({}^{\circ}\)C), it can nevertheless influence the O-H stretching region in FTIR due to the presence of chemically bound hydroxyl groups. Therefore,the absence of a clear reduction in this band (region 3560-3435 cm\({}^{-1}\)) for 15% CS Mg does not contradict the TG/DTG findings. Subsequently, the intensity of the H-O-H bending vibration band (around 1650 cm\({}^{-1}\)) is noticeably higher for 15% CS than for Ref. Mg. While this increase could be attributed to greater gypsum formation, such an explanation is less likely in this case due to the reduced availability of Ca\({}^{2+}\) in CS-blended cements. As it was prior mentioned, the incorporation of CS lowers Ca(OH)\({}_{2}\) content through pozzolanic reactions, thereby limiting the formation of brucite and gypsum [95]. Instead, the higher intensity observed in this region can be more feasible associated with increased water retention linked to the formation of M-S-H phases [102]. Thus, beyond reducing the generation of expansive degradation products, the presence of CS appears to modify the nature of the hydration products in a way that promotes higher physically and chemically bound water retention, as it was shown for the M-S-H assessed in this study [103]. In the case of the Si-O stretching band (\(\sim\)930-950 cm\({}^{-1}\)) all samples exhibit peak broadening and slight intensity loss, especially in Ref. Mg. This broadening reflects progressive loss of silicate chain order consistent with decalcification of C-S-H and its transformation into more disordered M-S-H. Similar spectral changes have been reported by [PERSON] et al. [103], who observed peak broadening in FTIR spectra of C-S-H subjected to leaching in NH\({}_{4}\)Cl, attributed to silicate chain cross-linking and structural degradation. This effect is less pronounced in 15% CS Mg, supporting the idea that CS mitigates deterioration by promoting denser C-S-H or the formation of more chemically stable M-S-H non-cementitious gels.