## 3 Results and Discussion
### Phase Analysis
The images presented in Figure 9a-c demonstrate the geopolymerization product and clearly show the strong interfacial adhesion between the FA-based geopolymer paste and the OPC matrix. The presence of a high source of calcium and alumina-silicate in the main mixture contributed to the formation of C-A-S-H gel, which concentrates at the interfacial zone, where free ions of Ca\({}^{2+}\) from the OPC were involved in the geopolymerization, then led to improvements of strength and bond strength. The EDX analysis (to be shown in the next section) shows that the selected zones from FA-based geopolymer paste, OPC substrate, and the interfacial zone have major elements of calcium, silicate, and aluminium, and these results are in good agreements with that of XRD. The micro-cracks were observed in some of the SEM images that probably occurred due to (a) the SEM samples were selected from the samples after compression tests, (b) the development of micro-reaction in which internal stress occurred between particles [PERSON] et al. (2019). Figure 9: SEM images at the bonding zone (BZ) with; (**a**) 8 M, (**b**) 10 M, (**c**) 12 M, (**d**) 14 M, (**e**) energy-dispersive X-ray (EDX) analysis shows the contribution of iron oxide in the geopolymerization. To understand more thoroughly the mechanism occurred at the bonding zone between the OPC substrate and FA-based geopolymer paste, EDX analysis was performed, hence approximate elemental percentages were determined. Figure 10 shows the EDX analysis of various selected areas, which were chosen from different morphological images. It is obvious from Figure 10 that the selected area at the FA-based geopolymer paste interface has major elements with approximate Si/Al and Ca/Si ratios of 2.71 and 0.7, respectively. Nonetheless, those ratios were found to be 5.43 and 4.49 at the OPC substrate area, respectively. Furthermore, at the interfacial bonding zone area, those ratios were given as 2.36 and 4.10, respectively. The EDX results were in good agreement with the XRD results, which strongly demonstrate the contribution of the free ions Ca\({}^{2+}\) located at the interfacial zone in the formation of C-A-S-H gel. These ions are believed to be generated from the decomposition of calcium hydroxide (Ca(OH)\({}_{2}\)) produced during the hydration process of OPC concrete. Therefore, the formation of C-A-S-H gel with a high proportion at the interfacial zone led to the improvement of both mechanical and bonding strengths of the repaired material. Figure 10: Elemental composition in various areas as given by EDX analysis. The following equations express deeply the mechanism at the interfacial bonding zone between FA-based geopolymer paste and OPC substrate. The formation of (C\({}_{3}\)S and C\({}_{2}\)S) with water via a hydration process of calcium silicate is shown in Equations (4) and (5):  
\[2(3\text{CaO}.\text{SiO}_{2})+6\text{H}_{2}\text{O}\ \rightarrow 3\text{CaO}. 2\text{SiO}.3\text{H}_{2}\text{O}+3\text{Ca}(\text{OH})_{2} \tag{4}\]  
\[2(2\text{CaO}.\text{SiO}_{2})+4\text{H}_{2}\text{O}\ \rightarrow\text{CaO}. 2\text{SiO}.3\text{H}_{2}\text{O}+\text{Ca}(\text{OH})_{2} \tag{5}\]  
The resulting calcium hydroxide from the hydration process of OPC, in particular, the Ca\({}^{2+}\) ions, latter contributes with the aluminosilicate from the fresh FA-based geopolymer paste in the formation of C-A-S-H gel is expressed by Equation (6):  
\[\text{SiO}_{2}(\text{OH})_{4}+2\text{Al}(\text{OH})_{4}^{-1}+\text{Ca}^{2+}( \text{from OPC})\rightarrow[\text{C}-\text{A}-\text{S}-\text{H}]\ \text{gel} \tag{6}\]