## 3 The Use of FTIR Spectra for Binder Durability
### The Effect of a Sulfate Attack on the H-O-H Bond and Si-O-T Vibrations in FTIR Spectra  
The type of cation in the binder is a significant factor that can affect the position of the band. On this note, [PERSON] et al. [66] exposed a fly ash GP sample to MgSO\({}_{4}\) and Na\({}_{2}\)SO\({}_{4}\) solutions, and [PERSON] et al. [28] exposed an alkaline-activated palm oil fuel ash/slag to the same solution. FTIR spectra have shown a different pattern of spectra due to sulfate exposures in GP [66] and AAB [28] systems with lower and higher Si/Al ratios in the precursors used for the synthesis, respectively. Regardless of the binder types, FTIR observed the same characteristics when H-O-H/OH and S-O-T were observed in both systems (Figures 13 and 14) with reference to the control samples. It was reported that exposing the samples to a sulfate attack could cause ion exchange due to decalcification in AAB systems and the dealumination in GP systems. The skeletal frameworks were dominated by CASH and NASH, respectively. Figure 12: Sulfuric acid attack of palm oil fuel ash/slag alkaline binders (From JMCE Ref. [100] and used with permission). The spectra band (Si-O-T) of the samples and the control (Figure 13) were observed and it was discovered that ion exchanges occurred. The presence of gypsum led to the vibration of S-O bonds at 602 and 670 cm\({}^{-1}\) in the MgSO\({}_{4}\)-exposed fly ash/slag geopolymer, but it was absent in Na\({}_{2}\)SO\({}_{4}\) exposure. However, in the AAB system, there was a remarkable shift in S-O bands from 690 to 691 cm\({}^{-1}\) in MgSO\({}_{4}\) and Na\({}_{2}\)SO\({}_{4}\), respectively (Figure 14). This indicates the disintegration and depolymerization of Si-O-Si(Al) due to the removal of Al\({}^{3+}\) and Ca\({}^{2+}\) in GP and AAB systems, respectively, to form gypsum (CaSO\({}_{4}\)). [PERSON] [23] found sulfate (S-O) vibrations from a gypsum or sulfate attack with their water of crystallization at 658-668 cm\({}^{-1}\) (triplet band). In Figure 13, the asymmetric stretching of the Si-O-Si(Al) band at 970 cm\({}^{-1}\) in the control shifted to 1018 and 1115 cm\({}^{-1}\), and its bending vibration shifted from 450 cm\({}^{-1}\) to 454 and 455 cm\({}^{-1}\), respectively, in Na and Mg sulfates, respectively, due to dealumination. A similar effect was observed in the AAB system (Figure 14) [28] due to decalcification at 1021 cm\({}^{-1}\) in the control became 1027 and 1033 cm\({}^{-1}\) in Mg sulfate and Na sulfate, respectively. Similarly, the Si-O-T bending vibration increased from 449 to 457 and 459 cm\({}^{-1}\), respectively. Figure 13: Effect of a sulfate attack on a Si-O-T band (From RILEM Ref. [66] and used with permission). The water/cement (w/c) ratio in the binder could also affect the crystallization of the sample (Figure 13). Sulfate has a greater impact on the binder because a higher water/binder ratio promotes crystallization and the proliferation of hydroxyl ions due to hydrogen bonding. In the GP system, the MgSO\({}_{4}\)-exposed sample prepared at w/b = 0.6 had greater HOH/OH vibration (3460 cm\({}^{-1}\)) than the control (3450 cm\({}^{-1}\)) and became 3410 cm\({}^{-1}\) at a lower w/b ratio = 0.5, but no changes were observed in the spectra of Na\({}_{2}\)SO\({}_{4}\)-exposed regardless of w/b ratios [70]. The MgSO\({}_{4}\)-exposed sample had more C-O vibration (CO\({}_{2}\)) vibrating at 2350 cm\({}^{-1}\), indicating a higher w/b ratio of 0.5 and 0.6, but this was not present in the Na\({}_{2}\)SO\({}_{4}\)-exposed sample (SES). The C-O vibration caused by calcite (CO\({}_{3}\)\({}^{2-}\)) is more pronounced in the SES, but it is absent in the MgSO\({}_{4}\)-exposed sample (MES) [94,96]. In the AAB system (Figure 14), the SES has more C-O stretching vibration (CO\({}_{2}\)) at 2362 cm\({}^{-1}\), while the C-O from CO\({}_{3}\)\({}^{2-}\) vibrated at a lower frequency of 1428 cm\({}^{-1}\) in the SES compared to the MES (1443 cm\({}^{-1}\)) and the control sample (1430 cm\({}^{-1}\)).