## 1 Introduction
### The Effect of Silica Modulus, Alkali, and Aluminium on Si-O-T and T-O Vibrations of FTIR  
In cementitious binders, the vibrations of T-O and Si-O-T are crucial. [PERSON] et al. [63] found that a Si-O-T bending vibration around 450-500 cm\({}^{-1}\) is more pronounced in GP-A than in the 1.4 nm tobermorite of the shorter chain in (OPC-D) (Figure 5). As shown in Figure 6, as the concentrations of alkalis (OH\({}^{-}\) and Na\({}_{2}\)O) increase, the Si-O-T bands become narrower. The formation of zeolites or aluminosilicate chains increases as the silica modulus (SiO\({}_{2}\)/Na\({}_{2}\)O) increases to 0.69 [79]. Spectra (N) were activated by NaOH, whereas W15 required a combination of Na\({}_{2}\)SiO\({}_{4}\) and NaOH (Figure 7). The higher Na\({}_{2}\)O content narrows the Si-O-T band, whereas the addition of silica increases its degree of polymerization, as indicated by W50 (1020 cm\({}^{-1}\)) in comparison with N (1000 cm\({}^{-1}\)). [PERSON] et al. [80] reported a spectrum band at 988 cm\({}^{-1}\) assigned to the orthosilicate unit [Si\({}_{2}\)O]\({}^{6-}\), in which partial substitution of Al\({}^{3+}\) for Si\({}^{4+}\) is possible. [PERSON] and [PERSON] [77], along with [PERSON] and [PERSON] [53], assigned the bending vibration of Si-O-T to the range 400-520 cm\({}^{-1}\), whereas [PERSON] et al. [81] reported the distinction in the Si-O-T band in fly ash GP due to the varying proportion of silica modulus (SiO\({}_{2}\)/Na\({}_{2}\)O) in the alkaline activators. [PERSON] et al. [23] also reported that the Si-O-Si stretching vibration within the layers of CSH gels overlaps with the gypsum S-O vibration within the bands 890-955 cm\({}^{-1}\). Figure 7: Si-O-T asymmetric vibration band with silica modulus, N-NaOH activator, and both (NaOH+Na\({}_{2}\)SiO\({}_{3}\)) in W50 such that Na\({}_{2}\)O/SiO\({}_{2}\) = 0.69 (From Elsevier Ref. [79] and used with copyright permission). In addition, symmetric stretching of the tetrahedral unit of Si-O and Al-O was centered between 890 and 955 cm\({}^{-1}\), whereas asymmetric stretching was observed between 870 and 940 cm\({}^{-1}\) (Figure 8) [58]. The O-Si-O bonds stretch symmetrically and asymmetrically at 450-525 cm\({}^{-1}\) for antisymmetric and symmetric bending, respectively, while the Si-O-Si bonds stretch between 800 and 10 cm\({}^{-1}\). Si-O elongation oscillates between 840-990 and 840 cm\({}^{-1}\), whereas bending vibrates between 520-538 cm\({}^{-1}\). The vibrations of tetrahedral Al-O (AlO\({}_{4}\)) induced by ettringite (Aft) formation were detected at wavenumbers of 705-820 cm\({}^{-1}\), while octahedral (AlO\({}_{6}\)) was at 510-520 cm\({}^{-1}\) and 414-460 cm\({}^{-1}\)[58]. The Si-O-T asymmetric stretching vibration can also be detected in FTIR spectra at the wavenumber range of 1116-1148 cm\({}^{-1}\) for an unreacted phase in rice husk ash (RHA) and metakaolin (MK) precursor [32], whereas the Si-O-T bending vibration was noted within 520-550 cm\({}^{-1}\) for an unreacted phase in OPC. [PERSON] et al. [80] attributed these asymmetric stretching and bending bands of Si-O-T to gehlenites (CASH-(Ca\({}_{2}\)AlAlSiO\({}_{7}\))) and akermanite (CMSH-Ca\({}_{2}\)Mg[Si\({}_{2}\)O\({}_{7}\)]) within the mellinite framework of blast furnace slag. [PERSON] et al. [60] also detected symmetric stretching of Si-O-T bridges between 671 and 717 cm\({}^{-1}\). [PERSON] et al. [65] also identified symmetric stretching of silica gel (Q\({}^{2}\) unit) with a wavenumber of 815 cm\({}^{-1}\) and asymmetric stretching with a lower degree of polymerization at 970 cm\({}^{-1}\). Similarly, the concentration of NaOH could affect aluminum (Al) substitution in alkaline binders due to more dissolution that causes freer Si-O and Al-O monomers to be available prior to the condensation to form oligomers [31; 81; 82; 83]. The presence of a higher concentration of alkali in a geopolymer will reduce the amorphousity of the sample and the frequency of vibration increases as the polymerization of Si-O-Si increases [37].