## 1 Introduction
### The Effect of Silica Modulus, Alkali, and Aluminium on Si-O-T and T-O Vibrations of FTIR
This assertion is very important when analyzing the GP or AAB by using the FTIR technique. The frequency of vibration of Si-O-Al is lower when an activator containing a higher concentration of NaOH or a substance with a higher pH is used. [PERSON] et al. [36] reported that the presence of a silicate monomer could affect the Si/Al ratio and stretching vibration of the Si-O-Al bond in infrared. In addition, they identified Al-rich gel as the intermediate formation preceding the final gel deposition. However, the duration depends on the concentration and speciation of the silicate. [PERSON] et al. [84] support this claim in the classification and identification of GP. Concerning the reactivity of Al, [PERSON] et al. [30; 31] infused Al(OH)\({}_{3}\) into alkaline-activated ground blast furnace slag in synergy with ultrafine palm oil fuel ash (AAGU) (Figure 9) and observed an improvement in the amorphosity of the product, as indicated  
Figure 8: FTIR spectra in ordinary Portlandite cement (From IntechOpen Ref. [58] and used under Creative Commons CC-BY license). by the Si-O-T (T=Si or Al) band elongation. The hydroxyl component was observed to reduce the degree of polymerization. [PERSON] et al. [85] demonstrated via FTIR spectroscopy that the rate of release of Al to form C/N-A-S-H in the Si-O-T vibration band is dependent on the source of alumina, with a slower rate observed in an inorganic source from sodium aluminate (952 cm\({}^{-1}\)) compared to amorphous Al (964 cm\({}^{-1}\)) synthesized via the Sims and Bingham methods. Dealumination (the removal of Al from Si-O-T) by acidic attack or other processes could also increase this specific vibration band, as reported by [PERSON] et al. [86]. [PERSON] et al. [30] in their study (Figure 9) showed that the introduction of Al(OH)\({}_{3}\) powder into the AAB causes changes in the asymmetric stretching of Si-O-Al from 1023 cm\({}^{-1}\) to 1016.3 cm\({}^{-1}\). The important bands used to study the performance of materials are observed in Si-O and Al-O vibrations, such that the incorporation of Al causes asymmetric stretching to reduce the frequency from 1023 to 1016.3 cm\({}^{-1}\)[87]. [PERSON] et al. [87] also identified spectra band 600-800 cm\({}^{-1}\) to Al-O in IV-fold coordination with a SiO\({}_{4}\) atom in the bending mode. [PERSON] 1990 [88] also suggested the assignment of band 700 cm\({}^{-1}\) to indicate the substitution of more reactive tetrahedra alumina (AlO\({}_{4}\)) than its octahedral linkage (AlO\({}_{6}\)) within the silica network at a spectrum band of 620 cm\({}^{-1}\). [PERSON] et al. [89] asserted that the preponderance of AlO\({}_{4}\) (IV) in metakaolin (MK) remarkably differentiates it from kaolinite minerals, which have excess Al (VI). This implies that aluminate in IV-fold coordination, such as that found in metakaolin and class F flyash, is more reactive and beneficial to cementitious binders when used as base materials for GP synthesis [90; 91]. [PERSON] et al. [32] also assigned wavenumber 452 cm\({}^{-1}\) with the T-O bending vibration of AlO\({}_{4}\). [PERSON] et al. [69] validated the role played by Al in the process of GP polymerization by adding aluminum nitrate (Al(NO\({}_{3}\))\({}_{3}\)) to C-S-H, which increased the vibration of the Si-O-T bond. This signals an improvement in the amorphosity of the product (Figure 10) [45; 74]. NA2 and NA4 that contained Al(NO\({}_{3}\))\({}_{3}\) showed a broader peak between 900-1100 cm\({}^{-1}\) compared to the lesser frequency noted in the aluminate-free control sample (NC control). The amorphousness of the product increased due to the penetration of Al within Si-O-Si, thereby making the band  
Figure 9: The effect of Al on Si-O-T band asymmetric stretching in an alkaline binder (From Elsevier Ref. [30] and used with permission). 1000-1200 cm\({}^{-1}\) deeper and broader as the SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratio becomes lower.