## 1 Introduction
### FTIR Spectra of Si-O-, T-O, Si-O-T and the Degree of Crystallization and Polymerization
A crystalline binder could be identified by the narrowness and depth of the Si-O-T band (where T is either Si or Al) in the wavenumber range 850-950 cm\({}^{-1}\), whereas the broader band around 3300-3650 cm\({}^{-1}\) indicates the concentrations of hydroxyl ion and water molecules. In addition, [PERSON] and [PERSON] [77] found that the concentration of hydroxyl ions and the silica modulus (SiO\({}_{2}\)/Na\({}_{2}\)O) ratio could affect the degree of polymerization and crystallinity of the binder. Figure 5: Vibration bands in OPC, AAS, and geopolymer binders (From Elsevier Ref. [63] and used with copyright permission). The polymerized GP product has a deeper and wider band trough than Portland cement (PC) and AAS binders [63]. The substitution of Si in Si-O-Si with the more electronegative Al\({}^{3+}\) led to a longer polymerized chain (Si-O-Al). FTIR could detect an increase in silicate reorganization as a result of the formation of longer chains, as observed in the system (Si/Al = 2) and in geopolymer synthesis [6]. Therefore, the process of product formation in Al-rich precursors, such as metakaolin, may differ from that in calcium silicate (Si-rich gel) dominant types, such as AAS. According to Figure 6 [77], an excess of alkali in the mixture increases the dissolution, causing the silica bond to break. This results in the replacement of the terminal hydroxyl ion with cations, which inhibits the polymerization of the binder. Therefore, the lower the concentration of alkaline, the lower the proliferation of terminal hydroxyls that result in chain discontinuity (lower polymerization). Due to the presence of a lower terminal hydroxyl, it appears that the Si-O-Si bond polymerizes more at lower alkaline concentrations. A lower concentration of alkali (hydroxyl ions) results in a lower Na/Si or Ca/Si ratio, which increases the amorphousness of the products. The Si-O-Si band stretches at 1080 cm\({}^{-1}\) in a sample of mild alkali (1.0 M NaOH) compared to 996 cm\({}^{-1}\) in a solution with a higher concentration (4 M NaOH) [77]. As evidenced by the disparity between a and h, increasing the Ca/Si ratio and hydrogen bonding from adjacent hydroxyl will increase the crystallinity of the product. The crystallinity of hydrogen bonds distinguishes jennite from tobermorite. These results are consistent with [PERSON] and [PERSON]'s [71] findings, as well as the correlation between the degree of geopolymerization and the frequency of vibration, which decreases from 1078 to 1015 cm\({}^{-1}\) as the amount of Al-O incorporated into the Si-O bond increases. They also assigned the wavenumber 1624 cm\({}^{-1}\) to the water H-O-H or -OH molecule's bending vibrations. Other researchers [78] indicated that the peaks and vibration  
Figure 6: FTIR spectra of zeolite **A** and associated phases obtained by hydrothermal synthesis: (a) unreacted metakaolin, (b) 1.0M NaOH, (c) 1.5M NaOH, (d) 2.0M NaOH, (e) 2.5M NaOH, (f) 3.0M NaOH, (g) 3.5M NaOH, (h) 4.0M NaOH, and (i) commercial zeolite A (Fluka No. 69836) (From JAUBA Ref. [77] and used under Creative Commons CC-BY license). modes for hydroxyl bonds attached to silica (Si-OH) would have peaks between 3164-3745 and 1610-1630 cm\({}^{-1}\). It is evident that FTIR could be used to distinguish amorphous and semi-crystalline samples through observation of Si-O-T bond vibrations.