## 3 Results and Discussion
### Microstructural Analysis
#### 3.4.2 TG-DTG Analyses
DTG peak: (1) Ettringite (E), Calcium aluminicate silicate hydrate (C-(A)-S-H), physically absorbed water; (2) Sulphate phases (Affm phase); (3) Brucite (Mg(OH)\({}_{2}\)), Magnesium Silicate Hydrate (M-S-H), Si-OH groups; (4) Portlandite (Ca(OH)\({}_{2}\)), Mg-OH groups in M-S-H; (5) Calcium Carbonate (CaCO\({}_{3}\)). Figure 12: (a) TG curves and (b) DTG curves of the Ref. W and 15% CS W for 360 days exposure to magnesium sulphate. DTG peak: (1) dehydration of the hydration products (e.g., C-S-H); (2) Ettringite (E), Calcium aluminicate silicate hydrate (C-(A)-S-H), physically absorbed water; (3) Sulpha phases (AFm phase); (4) Brucite (Mg(OH)\({}_{2}\)), Magnesium Silicate Hydrate (M-S-H), Si-OH groups; (5) Portlandite (Ca(OH)\({}_{2}\)), Mg-OH groups in M-S-H; (6) Calcium Carbonate (CaCO\({}_{3}\)). gradually from 390 to 500 due to its non-crystalline nature, according to the reported by [PERSON] et al. [91], it does not contribute significantly to the intensity observed in the 80-200 \({}^{\circ}\)C region. This transformation may be facilitated due to the lower alkalinity of the CS-blended cement, when portlandite is consumed by the pozzolanic reaction and further depleted by Mg\({}^{2+}\) and SO\({}_{4}\) ions. In contrast, Ref. Mg maintains a high peak intensity in the same range, similar to that of Ref. W. It may be explained due to the higher content of portlandite, that provides a chemical buffer, delaying the transformation of C-S-H into M-S-H. Additionally, the formation of expansive sulfate phases such as gypsum, thaumasite and etringite can retain physically bound water that also dehydrates within this temperature range (80-200 \({}^{\circ}\)C), thereby amplifying the apparent peak intensity [92]. Subsequently, a second DTG peak (labelled b) emerges between 100 and 150 \({}^{\circ}\)C is visible, and more pronounced in Ref. Mg than in 15% CS Mg. As reported by [PERSON] et al. [93], this region reflects the overlapping dehydration of sulphate phases, including etringite, thaumasite, and gypsum. The stronger intensity in Ref. Mg suggests a greater formation of gypsum, due to the higher availability of Ca\({}^{2+}\) from portlandite. Conversely, the lower intensity in 15% CS Mg supports the hypothesis that less gypsum precipitates and that the dominant degradation product in 15% CS Mg is M-S-H, consistent with a more advanced attack on the C-S-H structure. Following the sulfate-related dehydration peak between 100 and 145 \({}^{\circ}\)C, a third peak (labelled c) emerges between 300 and 410 \({}^{\circ}\)C, which is attributed to the dihydroxylation of brucite (Mg(OH)\({}_{2}\)) and early stages of M-S-H dehydration, mainly due to the dehydration of Si-OH groups in M-S-H, as identified by [PERSON] et al. [93] and [PERSON] et al. [94]. Both Ref. Mg and 15% CS Mg exhibit noticeable intensities in this region, confirming that magnesium ingress has resulted in the formation of brucite, a typical reaction product of Ca(OH)\({}_{2}\) with Mg\({}^{2+}\). Because 15% CS Mg contains less portlandite, it likely precipitates less brucite but more M-S-H, indicating that Mg\({}^{2+}\) ions in that system are more directly involved in the decalcification of C-S-H [95]. The subsequent peak (labelled d), located around 450-520 \({}^{\circ}\)C, is commonly associated with the dihydroxylation of portlandite [90], in this region 15% CS Mg shows greater intensity than Ref. Mg. It can be explained considering the overlapping thermal effects of M-S-H. As reported by [PERSON] et al. [91], water loss at 390-500 \({}^{\circ}\)C may also result from dihydroxylation of Si-OH and Mg-OH groups in M-S-H, since its non-crystalline structure and gradual water release [91; 94]. Given the assumption that 15% CS Mg has experienced more extensive C-S-H decalcification and likely formed more M-S-H, it is reasonable to attribute the increased intensity at this temperature to the combined contributions of residual portlandite and the dehydroxylation of Mg-OH groups in M-S-H [91].