## 3 Results and Discussion
### Phase Analysis  
The XRD patterns of raw FA and FA-based geopolymer paste with various NaOH molarities are shown in Figure 8. It can be observed from the pattern in Figure 8 that the main characterized crystalline peaks in FA were quartz (SiO\({}_{2}\)), hematite (Fe\({}_{2}\)O\({}_{3}\)), and mullite, which is well supported by X-ray fluorescence (XRF) analysis [1; 31]. Beidellite ((Na, Ca)Al\({}_{2}\)Si\({}_{4}\)O\({}_{10}\)(OH)\({}_{2}\) was another important peak detected in all FA-based geopolymer gates with different NaOH concentrations at 20\(\sim\)27\({}^{\circ}\). When comparing the latter with that of FA at the same range of 20, it is evident that Beidellite was formed during geopolymerization, where the Na\({}^{+}\) and Ca\({}^{2+}\) cations have taken a place in the reaction. This also can be demonstrated by the increase of the intensity of different peaks at various NaOH concentrations (see Figure 8). It can be also seen from the XRD results that new crystalline peaks were identified in the pattern of all FA-based geopolymer paste, which correspond to fayalite (Fe\({}_{2}\)SiO\({}_{4}\)) at 20\(\sim\)37\({}^{\circ}\). These new crystalline phases are believed to be formed due to the attribution of iron oxide in the geopolymeric system, in which the Fe\({}^{+}\) cation taken a place into the geopolymerization in two ways; i) firstly, a dissolution process of the hematite (Fe\({}_{2}\)O\({}_{3}\)) from the raw material in the presence of alkaline activator, where various Fe\({}^{+}\) cations are generated into the mixture; then, ii) secondly, those generated Fe\({}^{+}\) cations partially band in a chemical reaction with Si\({}^{+4}\) by sharing the same oxygen atoms, subsequently, three-dimensional crystals of Fe\({}_{2}\)SiO\({}_{4}\) are formed. The appearance of beidellite ((Na, Ca)Al\({}_{2}\)Si\({}_{4}\)O\({}_{10}\)(OH)\({}_{2}\) and fayalite (Fe\({}_{2}\)SiO\({}_{4}\)) crystalline peaks, demonstrate that after the dissolution of different element oxides in the presence of a sufficient concentration of the activator solution, new phases were built, which explains the improvement of strength, thus a great bonding strength obtained. During the geopolymerisation, the Ca\({}^{2+}\) and Na\({}^{+}\) ions were attributed to the geopolymeric network and led to the formation of gels as given by Equation (2). \[\text{Na}^{+}+\text{Ca}^{2+}+\text{Si}(\text{OH})_{4}^{-}+2\text{Al}(\text{OH} )_{4}^{-}\ \rightarrow\ (\text{Na},\text{Ca})\text{Al}_{2}\text{Si}_{4}\text{O}_{10}(\text{OH})_{2}( \text{Beidellite}) \tag{2}\]  
Similarly, [PERSON] et al. (2018) [28] suggested that the formation of fayalite can occur through a chemical reaction of hydrated ferric oxyhydroxide with silica as shown in Equation (3). \[\text{OFeOH}.\text{H}_{2}\text{O}+\text{Si}(\text{OH})_{4}\ \rightarrow\ \text{Fe}(\text{OH})_{3}.\text{SiO}_{2}(\text{fayalite})+2\text{H}_{2} \text{O} \tag{3}\]  
Figure 8: X-ray diffraction (XRD) analysis of precursor and FA-based geopolymer paste with various NaOH polarity. Q = quartz (SiO\({}_{2}\): PDF = 01-079-1911), B = beidellite ((Na\({}_{2}\)Ca)Al\({}_{2}\)Si\({}_{4}\)O\({}_{10}\)(OH)\({}_{2}\); PDF = 00-058 2019); F = fayalite (Fe\({}_{2}\)SiO\({}_{4}\): PDF = 00-034-0178), H = hematite (Fe\({}_{2}\)O\({}_{3}\): PDF = 00-033-0664). Scanning Electron Microscope (SEM) Images at the Bonding Zone (BZ) between Geopolymer and Concrete Substrate  
Figure 9 displays the images of the interfacial transition zone (ITZ) between the FA-based geopolymer paste and OPC substrate as given by the SEM analysis. It is evident from the same figure that both geopolymer and OPC matrices can be clearly distinguished. It can also be seen from the images in Figure 9 that the FA-based geopolymer paste exhibits a denser and compact matrix with almost fully reacted FA particles. [PERSON] et al. (2019) [PERSON] et al. (2019) stated that the unreacted FA particles play a significant role in the development of strength.