## 3 Results and Discussion
### Microstructural Analysis
#### 3.4.3 FTIR Analyses  
Figure 13 exhibits the FTIR spectra of OPC pastes and those incorporating 15 vol% of CS as cement replacement after 28 days of water curing, while Figure 14 shows the corresponding spectra after 180 days of exposure to either water or a 5% MgSO4 solution. In both Figures 13 and 14, the broad band between 3643 and 3446 cm\({}^{-1}\) region corresponds to the O-H stretching vibrations, related to chemically bound water in calcium hydroxide (portlandite) and C-S-H phases, as previously reported by [PERSON] et al. [90] in their study. At 28 days (Figure 13), the intensity of this band is slightly lower for the 15% CS W, likely due to the dilution effect associated with clinker replacement, which delays hydration reactions in early stages, as it has also been confirmed by [PERSON] et al. [40]. In the same period, the band at 1656 cm\({}^{-1}\), linked to H-O-H bending from absorbed water or hydration products shows a slightly higher intensity in Ref. W, which suggests a higher degree of hydration at this stage. The shoulder at 1421 cm\({}^{-1}\) and the region between 873 and 730 cm\({}^{-1}\) are associated with the stretching vibrations of CO3\({}^{2-}\) groups, resulting from carbonation reactions of Ca(OH)2 or unreacted Clinker particles, with atmospheric CO2, probably during the sample preparation. The 15% CS W sample exhibits slightly higher intensities at 1421 and 873 cm\({}^{-1}\), possibly due to the greater exposure of unreacted clinker  
Figure 14: FTIR results at the age of 180 days for the studied cement pastes exposed to magnesium sulphate (red dashed lines indicate main vibrational bands). Figure 13: FTIR results at the age of 28 days for the studied cement pastes (red dashed lines indicate main vibrational bands). particles to atmospheric CO\({}_{2}\) at early ages. The peaks near 1122 cm\({}^{-1}\)-990 cm\({}^{-1}\), and 518 cm\({}^{-1}\), are related to asymmetric stretching vibrations of Si-O-Si and Si-O-Al present in the C-S-H gel, as reported by [90]. Both Ref. W and 15% CS W show a similar and slightly higher intensity signal, respectively, in these regions, suggesting that the 15% CS replacement is able to compensate the cement reduction through a filler effect, as reported by [89], promoting C-S-H formation. After 180 days of water curing (Figure 14), the intensity of the O-H band (region 3560-3435 cm\({}^{-1}\)) is notably higher in 15% CS W compared to Ref. W. This enhancement is attributed to pozzolanic reactions between CS and portlandite at later ages, generating additional C-S-H and increasing the overall degree of hydration, as confirmed by [PERSON] et al. [28]. Likewise, the H-O-H bending vibration band (around 1650 cm\({}^{-1}\)) is noticeably higher in the 15% CS W sample compared to Ref. W. This increased intensity may be attributed to the greater amount of C-S-H formed at later ages, which is indicative of a higher degree of hydration [28]. Consequently, this leads to a greater amount of chemically bound water in the matrix, explaining the higher absorbance observed in this region. Consequently, the asymmetric stretching band of CO\({}_{3}\)\({}^{2-}\) (around 1440 cm\({}^{-1}\)) is associated with carbonates formation [90]. After 180 days of water curing, 15% CS W shows a slightly higher intensity than Ref. W, which may be attributed to early carbonation during sample preparation and the finer pore structure generated by the addition of CS. After MgSO\({}_{4}\) exposure, both samples show comparable trends, suggesting that carbonation proceeds proportionally across all systems, possibly coexisting with sulfate-induced transformations. Additionally, the bands observed near 1050 and 930 cm\({}^{-1}\) are associated with the asymmetric stretching of Si-O-Si and Si-O-Al bonds, characteristic of C-S-H gel [90]. After 180 days of water curing, 15% CS W shows broader and more intense peaks in this region, indicating the formation of additional C-S-H due to pozzolanic reactions [28].