## 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
As a result of the presence of alkali present in the activator, calcite formation is more likely in the SES or MES, according to Equation (9) [5]. \[2\text{NaOH}+\text{CO}_{2}\rightarrow\text{Na}_{2}\text{CO}_{3}+\text{H}_{2}\text {O} \tag{9}\]  
Furthermore, as shown in Figure 14, the frequency of vibration of H-O-H and OH-bonds in Na\({}_{2}\)SO\({}_{4}\) (3564 cm\({}^{-1}\)) and MgSO\({}_{4}\) (3476 cm\({}^{-1}\)) was higher than in the control sample (3459 cm\({}^{-1}\)). This could be due to the formation of additional water, as shown in Equation (9). [PERSON] [86] also established that an increase in vibration by molecular water at 1600 cm\({}^{-1}\) could be due to a sulfate attack, as was clearly observed through H-O-H vibration bands, as shown in Figure 15. A sulfate attack reduces the molecular water vibration bands, as shown in Figs. 13 and 14, in the MgSO\({}_{4}\)-exposed GP (1640 to 1630 cm\({}^{-1}\)) and AAB (1647 to 1629 cm\({}^{-1}\)) [28]. Figure 14: The effect of a sulfate attack on a Si-O-T band (From Elsevier Ref. [28] and used with permission). Therefore, the destruction of OPC or GP binders by acid or a sulfate attack is caused by the destruction of the Si-O-T structural framework as a result of the removal of calcium and the formation of sulfate-based expansive structures, such as ettringite and gypsum. Due to the reaction of Portlandite with sulfate compounds, OPC will sustain more damage than GP or AAB. Finally, Table 1 shows the summary of the major bands essential to the characterization of cementitious binders by using the FTIR technique. \begin{table}
\begin{tabular}{c c c c} \hline
**Bonds** & **Wavenumber** & **Assignment** & **References** \\ \hline Si-O-T & 950–1200 cm\({}^{-1}\) & Very strong band of asymmetric stretching & [22; 23; 65; 112; 113] \\ \hline Si-O-T & 1116–1148 cm\({}^{-1}\) & \begin{tabular}{c} Unreacted sample of metakaolin and rice husk \\ ash \\ \end{tabular} & [32; 113; 114] \\ \hline Si-O-T & 671–717 cm\({}^{-1}\) & symmetric stretching of Si-O-T & [60; 63; 80] \\ \hline Si-O-T & 435–540 cm\({}^{-1}\) & Bending & [22; 23; 63; 65; 77] \\ \hline -Si-OH & 815–988 cm\({}^{-1}\) (Q\({}^{2}\)Al) & Symmetric stretching & [63; 69] \\ \hline -C-O & 1421–1458 cm\({}^{-1}\) & \
\begin{tabular}{c} Very strong stretching vibration \\ (CO\({}_{3}\)\({}^{2}\)-) \\ \end{tabular} & [14; 24; 27; 44; 69; 113; 115; 116] \\ \hline -C-O & 2350-2355 cm\({}^{-1}\) & Asymmetric stretching (CO\({}_{2}\)) & [94; 95; 96] \\ \hline -C-O & 875–877 cm\({}^{-1}\) & Strong vibration (CO\({}_{2}\)) & [10; 95; 113] \\ \hline S-O (SO\({}_{4}\)) & 1115–1200 cm\({}^{-1}\) & Very strong sulfate asymmetrical vibration & [10; 14; 23] \\ \hline S-O (SO4) & 660–667 cm\({}^{-1}\) &
\begin{tabular}{c} Bending vibration strong in CaSO\({}_{4}\) and weak in \\ Na\({}_{2}\)SO\({}_{4}\) \\ \end{tabular} & [13; 61; 66; 95] \\ \hline \end{tabular}
\end{table} 
Table 1: Some of the significant bands in the FTIR spectra of cementitious materials. Figure 15: Variation in the vibration of molecular water due to a sulfate attack (a) control, (b) 5% sodium sulfate exposure, (c) 5% magnesium sulfate exposure, and (d) combination (From Elsevier Ref. [86] and used with permission).