## 3 Results and Discussion
### Bonding Strength between FA-Based Geopolymer Paste and OPC Concrete Substrate  
It is obvious from Figure 6 that the bonding strength obtained between FA-based geopolymer paste and the OPC substrate exhibits behaviour similar to the compressive strength of the FA-based geopolymer. From the same figure, the shear bonding strength between both OPC substrate and the FA-based geopolymer repair material strongly depends on the NaOH concentration, which increased with the increase of the NaOH molarity. However, at a certain concentration of 14 M, the shear bonding strength slightly decreased by 16.36% from that value at 12 M. The decrease which is shown at 14 M can be explained by the non-complete dissolution process of FA, which has been affected by the excess amounts of ions in the geopolymeric system, this leads to chemical congestion of those excess ions which in turn inhibits if not delays the dissolution process of the precursor (FA) hence, contribute in the formation of effloresces gel as will be explained later in this section.  
The increase of slant shear bond strength may be due to the presence of a sufficient amount of CaO in the prime material. During the geopolymerization process, the formation of C-A-S-H gel occurred faster due to the high reactivity and selectivity of Ca\({}^{2+}\) cations generated from the dissolution of CaO, hence, this gel can also be formed by the incorporation of free cations of Ca\({}^{2+}\) from the OPC substrate. In other words, new cross-link bonds were built between the OPC substrate and the fresh FA-based geopolymer paste via a physical reaction between the free Ca\({}^{2+}\) cations from the surface of the OPC substrate and the fresh FA-based geopolymer paste. This interaction occurs at the interfacial transition zone and leads to a huge enhancement of the bonding zone (BZ), thus increasing the strength.  
Figure 5: Compressive strengths of FA-based geopolymer paste with various NaOH molarity.  
The relationship between FA-based geopolymer paste repair material and slant shear bond strength in the function of NaOH concentration is shown in Figure 7. It is clear that the NaOH concentration has a similar effect on both compressive strength and slant shear bond strength. Both trends exhibit an obvious increase from 8 to 12 M, then slightly decreased at 14 M. From the same figure, it is clear that the maximum strength which corresponds to the strong bond strength was obtained at an optimal NaOH concentration of 12 M. It is evident from these mechanical tests besides the morphological ones that the use of an alkaline activator with 12 M of NaOH can optimize the final geopolymer mix design, in which the mechanical and microstructural properties can be improved. In other words, the presence of NaOH in a sufficient amount led to the achievement of a high strength of the repair material, which in fact, higher than the substrate concrete. Therefore, in this study, the designed mix of OPC concrete was C30/37 strength, where the geopolymer repair material has a maximum strength of 92.5 MPa at 60 days. This clearly explains the strong evaluation of the bonding strength between both materials and, hence, it showed the efficiency of the produced geopolymer as a repair material.  
It is worth mentioning that a high concentration of NaOH leads to an excess of free ions in the mixture, subsequently contributing to the formation of what is called efflorescence. This latter is a type of crystalline deposit of salts in a white or greyish tint, which is formed due to the evaporation of water in the presence of a high amount of ions such as Na\({}^{+}\) which in turn are introduced in the formation of salts. As a result of that, a reduction of strength will occur, hence a weak bonding strength will take place between the OPC substrate and FA-based geopolymer repair material [17; 29; 30].  
Figure 6: Bond strength of FA-based geopolymer paste with various NaOH molarity.  
Figure 7: The relationship between overlay FA-based geopolymer paste compressive strength and slant shear bond strength.