2.4.2. Effect of Different Ratios of Sodium Silicate/Sodium Hydroxide and Fly Ash/Alkaline Activators on the Mechanical Properties of Geopolymer

The current investigation looks into the influence of sodium silicate/sodium hydroxide ratios on geopolymer feasibility. Different studies showing the various proportions of sodium silicate/sodium hydroxide and fly ash/alkaline activator to improve geopolymer properties are listed in Table 8 [27,111]. The ideal preparation of fly ash can be determined by testing varying concentrations of sodium silicate, sodium hydroxide, fly ash, and alkaline activator.

Morsy et al. [136] evaluated the influence of sodium silicate/sodium hydroxide ratios on the viability of fly ash-based geopolymer synthesis at 80 ◦C. In this study, 10 M NaOH was combined with Na2SiO3 and alkaline activator ratios of 0.5, 1.0, 1.5, 2.0, and 2.5. The compressive strength of fly ash geopolymer mortars increased with age at 3, 7, 28, and 60 days. The compressive strengths of fly ash geopolymer mortars M1, M2, M3, M4, and M5 after three days were 34.7, 61.6, 40.4, 40.5, and 22.3 MPa, respectively. The S/N ratio of alkali activator had a significant impact on the strength of low-calcium fly ash geopolymer cured at 80 ◦C. Maximum strength was achieved when the ratio of sodium silicate to sodium hydroxide (S/N) was equal to 1. Other than that, future research should investigate preparation methods for mixtures and ensuring homogeneity so that they are comparable with other geopolymers.

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According to Liyana et al. [137], in their study, the proportions of Na2SiO3/NaOH solution and fly ash to alkaline activator were synthesised in four different ratios: 1.0, 1.5, 2.0, and 2.5, in a 24 h period during which curing was carried out at room temperature. According to the results, the fly ash/alkaline activator ratio of 2.0 had the highest results compared to other ratios, and the sodium silicate/sodium hydroxide ratio of 2.5 had the highest results compared to other ratios. The best mechanical properties can be obtained through research using various molarities and curing temperatures.

The study by Bakri et al. [138] used a 12 M concentration of NaOH and fly ash to alkaline activator ratios of 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0. Only the three ratios of 1.5, 2.0, and 2.5 were employed. Due to the geopolymer paste's high workability, which makes it challenging to handle, the ratios of 0.5 and 1.0 could not be used, and the ratio 3.0 could not be used due to the paste's low workability. Five different ratios of Na2SiO3/NaOH were used: 0.5, 1.0, 1.5, 2.0, and 2.5. The sample was cured for 24 h at 70 ◦C before being tested for compressive strength on the seventh day. The fly ash/alkaline activator ratio of 2.0 and the sodium silicate/sodium hydroxide ratio of 2.5 had the maximum compressive strength. Future studies could examine various curing temperatures to achieve the best compressive strength.

Nis [139] investigated geopolymer content using various NaOH concentrations and sodium silicate to sodium hydroxide ratios. The sodium silicate (Na2SiO3) and sodium hydroxide (NaOH) solutions were used with four different sodium silicate to sodium hydroxide ratios (1, 1.5, 2, and 2.5) and three different molarities (6 M, 10 M, and 14 M) for alkali activation to evaluate the impact of these parameters on the compressive strength of the alkali-activated fly ash/slag concrete under ambient-curing (AC) and delayed ovencuring (OC) conditions. The specimens' compressive strengths varied greatly with molarity concentration; those with the greatest NaOH molarity (14 M) concentration had the greatest compressive strength, as depicted in Figure 10. Other than that, more research can consider investigating the impact of oven-curing conditions on compressive strength.

**Figure 10.** Compressive strengths of alkali-activated fly ash/slag specimens based on alkaline activator ratio type [139].

Abdullah et al. [140] investigated several curing temperatures with a constant NaOH concentration of 12 M using different fly ash/alkaline activator ratios and Na2SiO3/NaOH ratios. The samples were cured at different temperatures from 40 ◦C to 80 ◦C for 24 h and compressive strength was tested on the seventh day. The fly ash/alkaline activator ratio of 2.0, sodium silicate/sodium hydroxide ratio of 2.5, and curing temperature of 60 ◦C resulted in the maximum compressive strength. Different curing days may be investigated in order to enhance compressive strength.

Based on various studies [137–141], the mechanical characteristics may be affected by the use of different ratios of sodium silicate, sodium hydroxide, and fly ash/alkaline activator. The strongest strength resulted from the fly ash/alkaline activator ratio of 2.0 and the sodium silicate/sodium hydroxide ratio of 2.5, which were used as the ideal ratio for sample preparation. However, from the previous investigation, more improvements can be made, which are optimising the curing temperature and curing durations as using various curing durations can potentially increase the geopolymer's mechanical qualities.

Although the mechanical characteristics are influenced by the ratio of sodium silicate/sodium hydroxide and fly ash/alkaline activator, the preparation of different molarities of sodium hydroxide is another key aspect that influences overall mechanical characteristics of geopolymer.

## 2.4.3. Effect of Sodium Hydroxide Molarity on the Mechanical Properties of Geopolymer

The molarity of the alkali activator, the curing temperature, the number of days, and other parameters all have an impact on the sample's characteristics during the creation of the geopolymer [27,111–113]. The different molarities of sodium hydroxide, that acts as an alkali activator and affects the mechanical properties of geopolymer, are listed in Table 9.

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Gum et al. [143] studied the impact of making geopolymer concrete with an alkaline activator on the compressive strength of mortars using fly ash as a binder and different curing temperatures and moles of sodium hydroxide. Fly ash was combined with a mixture of 6, 9, and 12 M NaOH, and the curing conditions were 60 ◦C in the oven and 20 ◦C outside for 7 classes of curing days. After the chemicals were mixed, it was poured into moulds with dimensions of 50 mm × 50 mm × 50 mm and measured for compressive strength according to ASTM C 109. An alkaline activator that used NaOH at a higher molarity demonstrated increased compressive strength. The compressive strength decreased as the SiO2/Na2O and Al2O3/Na2O ratios increased. When the SiO2/Na2O ratio exceeded 8.01 and the Al2O3/Na2O ratio exceeded 1.94, the strength decrease rate appeared to accelerate sharply at 28 days. Based on these findings, the strength at 28 days for series 1 appeared to have increased by more than 1.7 times at a NaOH molarity of 9 M when compared to a molarity of 6 M. However, the 9 M and 12 M results showed nearly identical strengths. This highlights the significance of the SiO2/Al2O3, SiO2/Na2O, and Al2O3/Na2O ratios in alkali-activated geopolymer based on fly ash. As SiO2/Al2O3 was constant in this investigation, the values of 8.01 and 1.94 for SiO2/Na2O and Al2O2/Na2O ratios yielded the best strength development. The use of NaOH and sodium silicate (SiO2/Na2O = 8) in a 1:1 ratio demonstrated that it is possible to activate the geopolymerisation of fly ash and create a significant increase in strength, with a compressive strength of around 47 MPa. The evaluations of the impacts of the SiO2/Na2O and Al2O3/Na2O ratios on strength under equal SiO2/Al2O3 ratios are illustrated in Figure 11. The requirement for high-strength concrete is over 40 MPa, demonstrating the possibility of employing fly ash as a cement substitute. Future research can evaluate whether increasing the molarity and pH of NaOH during the curing process will increase compressive strength, including multiple curing temperatures.

**Figure 11.** Compressive strength versus ages for molarity of NaOH [143].

Lee et al. [144] analysed the effects of increasing amounts of slag, water glass, and varying curing temperatures and NaOH molarities on curing time reduction. In the preparation, the alkali activators were water glass (Korean Industrial Standards, KS 3 grade; SiO2 (29%), Na2O (10%), H2O (61%, specific gravity 1.38 g/mL), and 98% pure NaOH. The room temperature for the combined alkali-activated fly ash/slag paste was

between 17 ◦C and 28 ◦C. For setting time tests, the molarity of NaOH was 4 M and 6 M, and the mass ratio of NaOH was 0.5, 1.0, and 1.5. Then, 8 M NaOH was used to accelerate the setting of alkali-activated fly ash/slag paste. For each mixed sample, a 100 mm × 200 mm cylinder mould was employed. The compressive strength and setting times of ASTM C 191-08 [139] were evaluated at 3, 7, 14, 28, and 56 days of curing. At 17 ◦C, the alkali-activated fly ash/slag paste took 55 min to start and 160 min to finish when the NaOH solution was 4 M and the water glass to NaOH solution by weight ratio was 0.5, as illustrated in Figure 12. Due to the presence of slag and water glass, the molarity of NaOH rose while the alkali activator's duration shortened. The quantity of slag grew by 25% and 30% after 28 days, respectively, but reduced after 56 days due to crack growth. Future research can examine different NaOH molarities to determine whether they can boost compressive strength.

**Figure 12.** Time versus NaOH solution molarity [144].

Khan et al. [146] examined the material properties of fly ash, copper slag, and crusher dust at different curing temperatures and NaOH concentrations. There were 16 different mix designs that used varying curing temperatures and NaOH concentrations. The design was cured for 28 days before testing, and the analysis revealed that the sodium silicate/sodium hydroxide (SS/SH) ratio should be maintained at 2.4. The molarity of NaOH should be kept at 14 M to produce maximum strength and dotted line was an average region, as shown in Figure 13. The setting time was found to decrease from 449.8 min to 340.8 min. There are some limitations, such as the fact that the greater the molarity, the greater the compressive strength, and this could be due to secondary parameters that may affect the performance of geopolymer, including mixing time and other parameters that can influence the complexity of the mix design; therefore, additional research is required to determine their characteristics.

Rathanasalam et al. [145] investigated different sodium hydroxide (NaOH) molarities of 10 M, 12 M, and 14 M and developed a mixture utilising 5%, 10%, and 15% ultrafine ground granulated blast furnace slag (UFGGBFS) replacing fly ash, with crushed stone or copper slag. After curing for 3, 7, and 28 days at 60 ◦C, the compressive strength was evaluated. The compressive strength of all mix designs was tested using a 150 mm × 150 mm × 150 mm cube. From the different types of design with different curing days depicted in Figures 14–16, it can be concluded that all the mixtures with 14 M NaOH concentration have the maximum compressive strength. Future studies can look into using higher NaOH molarities to determine the ideal NaOH molarity to make the mix design with the maximum compressive strength.

**Figure 13.** Main effects plot for S/N ratios based on compressive strength [146].

**Figure 14.** Different molarities in the mix design of GPC and copper slag at 3 days [145].

Bakri et al. [142] investigated the compressive strength of fly ash at various sodium hydroxide molarities. The sodium hydroxide molarities of 6, 8, 10, 12, 14, and 16 M and 1, 2, 3, and 7 curing days were used for the mix design samples. The proportion of fly ash to alkali activator was maintained constant at 2.50, as was the proportion of sodium silicate to sodium hydroxide. Prior to testing, all mixtures were cured at 70 ◦C, and the results indicated that for sodium hydroxide with molarity of 12 M, the compressive strength result was the highest among the other molarities on the third day, and on the seventh day, it demonstrated the highest compressive strength, as illustrated in Figure 17. Future research can examine whether increasing the curing temperature will increase compressive strength.

**Figure 15.** Different molarities in the mix design of GPC and copper slag at 7 days [145].

**Figure 16.** Different molarities in the mix design of GPC and copper slag at 28 days [145].

Previous studies have investigated the effect of molarity of sodium hydroxide on mechanical properties. According to the majority of the studies, the higher the sodium hydroxide molarity, the higher the mechanical characteristics of the geopolymer. Although lower compressive strength is seen in mix designs when sodium hydroxide molarity is 15 M, according to research by Khan et al. [146], this may not be due to the influence of sodium hydroxide. First, it might be affected by the addition of other materials such as copper slag and crusher dust, as well as other aspects that lower compressive strength such as the SS/SH ratio and curing temperature. Although increasing the molarity improves the mechanical properties of the geopolymer, research by Fakhrabadi et al. [147] shows that when the sodium hydroxide molarity is 15 M, unconfined compressive strength is lower than when the sodium hydroxide molarity is 11 M, while research by Bakri et al. [142] suggested that molarity of 14 M is optimal for improving the mechanical properties of fly ash. The development of sodium aluminate silicate hydrate was caused by an increase in the molarity of sodium hydroxide (NASH) [148]. The use of sodium hydroxide with a high molarity may enhance the geopolymerisation reaction and the dissolution of initially solid materials, leading to better compressive strength [149].

**Figure 17.** Compressive strength of all mixtures with different molarities and curing days [142].

The success of the geopolymer preparation demonstrates that it is possible to increase the material's strength through geopolymer preparation. However, the low thermal conductivity of geopolymer can be improved by adding metal filler to the mould insert.
