## 3 Results and Discussion
### Compressive Strength of FA-Based Geopolymer Repair Material  
Figure 5 exhibits the compressive strength of FA-based geopolymer paste in the function of NaOH concentrations at different curing ages. It can be observed from the same figure and in respect of NaOH molarity that the compressive strength has a significant improvement during the curing periods of 3, 14, 28, and 60 days. Furthermore, the FA-based geopolymer paste has developed a noticeable strength at different ages where the NaOH molarity has been varied from 8 to 12 M. Nevertheless, at 14 M of NaOH solution, the compressive strength slightly decreased by 13.8% and 18.9% after 60 days when compared to that of 10 and 12 M, respectively. The increase in compressive strength at different NaOH molarity with time could be due to the high dissolution of FA content in the mixture, in which the geopolymerization process became faster, then led to an earlier formation of geopolymer gels, which in turn causes of a reduction of setting time.  
The compressive strength was found to decrease when 14 M of NaOH solution was used. This indicates that an excess of alkaline solution negatively affects the strength development. This is believed to be due to the excess regeneration amounts of OH- and Na+ ions, which in turn causes a geopolymeric congestion system, then inhibits the continuity of the formation of the geopolymeric gels. The optimal compressive strengths at 3, 14, 28, and 60 days were observed to be 32.8, 47.4, 76.5, and 92.5 MPa, respectively, which correspond to the optimal NaOH concentration of 12 M with a significant improvement of 95.1% after 60 days of curing. Similar studies have shown the effect of NaOH molarity on geopolymerization, where the results revealed that 12 M is a sufficient concentration to obtain high strengths [26]. Furthermore, the increase of sodium hydroxide concentration leads to a high dissolution of the silica and alumina in which more cations (Si4+ and Al3+) will be generated, which in turn contributes to the creation of new bonds during the gel-building phase [27]. This improvement in strength is mainly due to the presence of  
Figure 4: Setting time of FA-based geopolymer paste vs. NaOH molarity.  
quasi-high amounts of CaO, which is 18.4% of the total amount of FA mass. Furthermore, the dissolution of CaO components leads to the formation of C-A-S-H gel, besides the formation of N-A-S-H gel as a result of the incorporation of Si\({}^{4+}\) and Al\({}^{3+}\) with the activator solution, and this leads to the improvement of strength [1; 23]. To explain the main chemical mechanism occurring during the geopolymerization process, Equation (1) shows the chemical reaction which converts the silica and aluminium crystals into aqueous phases in the presence of sodium hydroxide solution.  
\[(\mathrm{SiO_{2},\ Al_{2}O_{3}from\ fly\ ash})+2\mathrm{NaOH}+5\mathrm{H_{2}O} \rightarrow\mathrm{Si(OH)_{4}+2 Al(OH)_{4}^{-}+2 Na^{+}} \tag{1}\]  
It should be noted that the presence of a sufficient amount of iron oxide (Fe\({}_{2}\)O\({}_{3}\)) of 19.57% of the total mass of FA has contributed to the strength enhancement of the final geopolymer product. This is due to a typical formation of a new gel phase known as ferroisalate gel as a result of the incorporation of Fe\({}^{+2}\) with the activator solution and other ions from the geopolymeric system via a similar chemical process as the Si\({}^{4+}\), Al\({}^{3+}\)- and Ca\({}^{2+}\). This was stated by [PERSON] et al. (2018) [28] where the ferroisalate has been identified in a geopolymer mix.