Research

Due to its unique qualities, concrete is the most extensively utilized substance in the construction sector after water. However, because one ton of Portland cement produces about one ton of CO2, the Portland cement manufacturing process has considerable disadvantages. As a result, an alternative to Portland cement appears to be required. Geopolymer is a new and environmentally friendly cementitious material that can be used in place of Portland cement. Chemically and mechanically, geopolymer concrete outperforms traditional concrete. The weight ratio of water to dry matter used in polymerization, as well as the weight ratio of sodium silicate to NaOH solution, have an impact on the compressive strength of geopolymer concrete. As a result, additional research into these variables appeared to be necessary. This paper specifically looked at how nanosilica and zeolite affected the mechanical strength of metakaolin-based geopolymer concrete. Nanosilica was added to metakaolin-based geopolymer concrete to improve mechanical characteristics. Furthermore, using zeolite in a metakaolin-based aluminosilicate source lowers the mechanical strength of geopolymer concrete while also lowering the cost. The optimal weight ratios for polymerization water to dry matter and sodium silicate solution to NaOH solution were 0.4741 and 1.5, respectively, resulting in maximum compressive pressures of 3, 7, and 28 days.


Introduction
Due to the unique qualities of concrete, it is the most extensively utilized substance in the construction sector after water. The demand for concrete is predicted to rise in the future, resulting in a significant increase in the production of Portland cement as a concrete raw material (Malhotra, 1999). The Portland cement manufacturing process, on the other hand, has drawbacks. The manufacture of Portland cement emits a substantial quantity of CO2 into the environment (Davidovits, 1994), with one ton of Portland cement producing close to one ton of CO2 (Malhotra, 2006). Climate change, on the other hand, as a result of global warming, has emerged as one of the most pressing environmental concerns worldwide (Mccaffrey, 2002;. As a result, it appears that a Portland cement substitute is required. Geopolymer has recently been proposed as a new, environmentally friendly cement component to replace Portland cement. Davidovits (1988), a famous French scientist, was the first to develop geopolymers from the family of mineral polymers. Poly is a polymeric prefix and sialate is a suggested name for poly (sialate) for the chemical detection of geopolymers. Silicon-oxo-aluminate is an abbreviated term for the silicon-oxo-aluminate chain. Figure 1 shows many types of poly (sialate) (Davidovits, 1988;Davidovits, 1991;Van Jaarsveld et al., 2002).

Fig. 1. Chemical structure of poly (sialates)
Inorganic alumina silicates called geopolymers are formed by mixing the primary sources of advanced polymerization of silica and alumina (Al2O3) with an alkaline activating solution . Raw materials for polymerization include natural sources such as zeolites, industrial sources such as metakaolin, and wastes such as fly ash or blast furnace slag (Xu & Van Deventer, 2000). Rapid chemical interactions between Si and Al occur during the polymerization process and form three-dimensional Si-O-Al polymer chains . Geopolymer concretes have very high compressive, flexural and tensile strength (Hardjito et al., 2004;Amnadnua et al., 2013;Duxson et al., 2007), fast hardening (Lee & Van Deventer, 2002) and high fire and heat resistance (Hosseini et al., 2020;Sakkas et al., 2014;Sarker et al., 2014), low permeability, very low creep, and resistance to salt attacks And acids (Palomo et al., 1999;Zhang et al., 2013). Metakaolin is one of the first resources used in polymerization. One of the first materials used in polymerization was metakaolin. Metakaolin is formed by calcining kaolin at 750 ° C. In this study, metakaolin was used as a source of polymerization.
The most common alkaline solution used in polymerization is a mixture of NaOH or KOH solutions with sodium or potassium silicate solutions (Hardjito et al., 2004;Nouri et al., 2019). Alkaline solution parameters such as NaOH solution concentration, sodium silicate solution concentration, Na2O to SiO2 ratio in sodium silicate solution, and the weight ratio of sodium silicate solution to NaOH solution play a key role in geopolymer concrete, optimization of these parameters seems critical.
Researchers have conducted several studies in this field, which will be briefly discussed below. In his early geopolymer research, Davidovits used NaOH or KOH solutions without silicate solutions as activating alkaline solutions (Davidovits, 1987;Davidovits, 1988). In addition, Xu & Van Deventer., (2000) Used nonsilicate NaOH and KOH solutions as activating alkaline solutions in their research on geopolymers and found that KOH provides better results in this type of activating alkaline solution.
In their research, Palomo et al., (1999) Showed that activating alkaline solution plays an important role in the polymerization reaction, and adding silicate solution to NaOH and KOH can increase the reaction rate and improve Na 2 O to SiO 2 ratio of 2 and 45 percent concentration with NaOH solution 40 percent concentration. They reported the results (Palomo et al., 1999;Nozari et al., 2021). Furthermore, they discovered that utilizing NaOH and sodium silicate produces better results than using KOH and potassium silicate solutions. In their study on fireproof geo-polymers, Cheng et al., (2003) found comparable results . In their research based on fly ash, Hardjito et al., (2004) used a mixture of sodium silicate solution with utilizing sodium silicate solution with a ratio of 2 and a 40% concentration, the best NaOH weight ratio is 1.5.
Nanosilica is a group of microscopic SiO 2 particles that are chemically bonded together to produce larger particles. The advantage of primary nano-silica over silica is its very large surface area, which allows it to respond faster to the substrate. Nanosilica has a wide range of industrial applications, including pozzolans and fillers. In this article, the authors attempted to investigate the alkaline solution's relevant parameters and its effect on the compressive strength of metakaolin-based geo-polymeric concrete. Furthermore, the effect of varying the ratios of zeolite to metakaolin and increasing nano-silica on the compressive strength of geopolymeric concrete was investigated.

Material
Metakaolin is the primary source of polymerization in this research. Metakaolin was purchased from the knowledge-based company Bana Bonyan Zist Fanavar, and its XRF analysis is reported in Table (1). Natural zeolite from the Clinoptilolite classification is employed. This zeolite was obtained from the mine of After Shargh-e-Semnan, which was purchased from Afrandtooska Co. Table (2) shows the XRF analysis of the zeolite used in this study. Figure (2) also shows the metakaolin and zeolite images that were used. Shimi Pars Isfahan Co. provided the industrial liquid NaOH, which had a purity of 98 percent and a concentration of 50 percent. The liquid sodium silicate solution with a Na 2 O to SiO 2 ratio of 3 was purchased from the Nafis Silicate Sepahan Company, whose analysis is depicted in Figure (2). The consumed sand and gravel in this test were prepared from Tirazheh Beton Co. around Isfahan. The consumed sand was crushed gravel, then it was granulated by ASTM standard sieve, and a gravel mixture of 10 and 7 mm mesh was used. The coefficient of fineness of gravel was measured at 3.2913. In addition, the sand equivalent (SE) of the consumed sand was measured at 73. The weight ratio of sand to gravel was selected 1. The consumed water was the piped water of Isfahan city. Polycarboxylate superplasticizer was bought from expertized concrete clinic to prevent adding water to the mixture and increase concrete workability. The used nano-silica in this research was bought from Iranian nano knowledge-based Co.

Experimental method
To commence, preliminary tests were carried out on a variety of mixes to determine the metakaolin criteria and alkaline level of solutions. The main concrete composition was then chosen following this phase. It was necessary to make three concrete samples with different amounts of added water in the mixture: 0, 40, and 80 kg/m3. First, preliminary tests on mixes were carried out, with many combinations being mixed up and tested to identify the metakaolin criteria and alkaline level of solutions. After that, the main concrete composition was chosen. It was necessary to make three concrete samples for the excess water in the combination, each with a mixture design of 0, 40, and 80 kg/m3. Samples were subjected to a 7-day compressive strength test. Despite the fact that the sample without the extra water had a higher compressive strength in the results, this mixture had an executive problem in the mixer and execution. It was unusable in practice. As a result, in this study, 40 g of extra water in the mixture was chosen.
The weight ratio of sodium silicate to the optimum NaOH solution was determined using four mixtures in the first part. In all of these mixtures, a 50 percent concentration of NaOH solution was used, with a weight ratio of 1.5, 2, 2.5, and 3 for sodium silicate solution to NaOH solution. Furthermore, in each of the three mixtures, the weight ratio of alkaline solution to metakaolin was 1. The superplasticizer was 4 wt% of metakaolin. Mixtures of the first part are shown in Table (5). In these mixtures:  NaOH solution has been used with 50% concentration.  Weight ratios of sodium silicate to NaOH solution are 1.5, 2, 2.5, and 3, respectively.  Weight ratio of alkaline solution to metakaolin is 1.  Metakaolin scale is 350.  Superplasticizer is 4 wt% of metakaolin  Gravel to sand ratio is 1.  The ratio of 7 to 10 mm gravel is 50%.  The ratio of metakaolin to granule is 21%.  The ratio of granule to total concrete weight is 75%.
First, the alkaline solution containing NaOH and sodium silicate solution was mixed based on the previous day's combinations, and the resultant solutions were cooled down for one day. Subsequently, the dry components, such as metakaolin, gravel, and sand, were combined for 2 minutes in the concrete launch mixer. Upon completion, samples were cast and densified according to ASTM specifications. To densify the samples, they were vibrated for 10 minutes on a vibrator table. In this step, the dry curing mixture in the oven was selected at 80 °C and 24 h by the initial tests and results of the other articles about curing mixtures. Afterward, the samples were taken out of the oven at the conclusion of curing and tested for compressive strength for 3, 7, and 28 days to determine the parameter of sodium silicate to NaOH weight ratio. In the analysis section, the results of this test will be displayed.
The impact of utilizing zeolite as a substitute for a part of metakaolin in conjunction with geo-polymeric concrete was investigated in the second phase of the test. In the second phase of the test, the impact of using zeolite as a substitute for a portion of metakaolin in combination with geo-polymeric concrete was investigated.
This portion utilized the MR1.5 mixture, which had a sodium silicate to NaOH weight ratio of 1.5, based on the previous part's results. Metakaolin was alternated with zeolite in ratios of 10, 20, 30, 40, and 50 percent. Table (6) depicts these mixtures. In these compositions, the weight ratio of sodium silicate to NaOH is 1.5, while the other ratios are the same as in the predecessors. The experimental approach of preparation and construction was identical to that utilized in the previous phase. The samples were then submitted to compressive strength tests for 3, 7, and 28 days to figure out the best alternative percent, the results of which are shown in the analytical section. In the third section, three different combinations of nano-silica with 2, 3, and 4 wt percent were alternated in metakaolin mixes to determine whether it had a consequence on concrete compressive strength. The mixes for this component are included in Table (7). In this subsection, samples were prepared and constructed in about the same method as in the first, and 3, 7, and 28-day compressive strength tests were performed on samples whose findings would be displayed in the results analysis section. After revealing the results of the previous section, MR1.5, MN2, and MZ50 ratios of flexural and tensile samples were used to investigate the mechanical strengths of geo-polymeric concretes based on metakaolin, such as flexural and tensile strengths.
Samples were subjected to flexural and tensile strength was measured, the results of which will be provided in the results analysis sections later. Figure (3) illustrates the prefabricated concrete samples.

Fig. 4. The effect of extra water on the compressive strength
According to the results, adding extra water to the concrete greatly diminishes its strength, as the 7-day compressive strength in 0,40, and 80 g, extra water combinations was 31.1, 25.6, and 13.8 MPa, respectively. The effect of the important parameter of adding extra water to the participated dry materials in polymerization, such as metakaolin, dry NaOH in NaOH solution, and dry sodium silicate in sodium silicate solution, on the compressive strengths of geopolymer concrete is demonstrated in this matter. Expanding the water-to-dry-materials ratio in the concrete mixture to 40 and 80 kg/m3 will enhance the water-to-dry-materials ratio. As a result, the 7-day compressive strength has decreased to 18 and 55 percent, respectively. When the extra water is not added to the mixture, it has higher compressive strength, but it has execution issues and the concrete has limited workability. As a consequence, a 40 kg/m3 additional water mixture was chosen to ensure that the concrete would have enough workability, at least by lowering the compressive strength. Figure 5 depicts the acquired data from the first portion, which were analyzed in order to optimize the wt percent of sodium silicate than NaOH and analyze this parameter.  (5) indicates that the maximum 3, 7, and 28-day compressive strengths relevant to MR1.5 mixture are 21.5, 23.8, and 25.6 MPa, respectively, while the lowest 3, 7, and 28-day compressive strengths relevant to MR3 mixture are 12, 13.5, and 15 MPa. In which the sodium silicate to NaOH weight ratio is 1.5. According to the results, the wt percent parameter of sodium silicate solution to NaOH solution is an effective parameter on the 3, 7, and 28-day compressive strength, and the optimal ratio of this parameter is 1.5, which results in the greatest 3, 7, and 28-day compressive strength. Increasing this ratio lessened the 3, 7, and 28-day compressive strengths to 44, 40, and 41 percent, respectively. In their investigation of geopolymer concrete, Esparham, and Moradikhou proposed 1.5 as even the best ratio of sodium silicate to NaOH . KOH and sodium silicate were utilized as alkaline solutions in prior research that examined the impact of various alkaline solutions on the compressive strength of geopolymer concrete, and the optimal ratio of sodium silicate to KOH was measured at 1.5, which is analogous to this research. This is due to the importance of water to the dry substances being used in polymerization, which includes metakaolin, dry NaOH in NaOH solution, and dry sodium silicate in sodium silicate solution, on the compressive strength of geopolymer concrete (Esparham, 2020;.

Fig. 6. The role of water to dry materials ratio on 3, 7, and 28-day compressive strength
The water to dry materials ratio in MR1.5, MR2, MR2.5, and MR3 combinations was 0.4741, 0.4804, 0.4872, and 0.4941, respectively. The results demonstrate that the parameter of water to dry participating materials ratio in polymerization has no significant influence on the compressive strength of geopolymer concrete. In this paper, the optimal ratio of this parameter was measured as 0.4741, resulting in the greatest 3, 7, and 28day compressive strength. Increasing this ratio decreases compressive strength as if the compressive strength was greatly lowered in 0.4911. This problem was also discovered in prior studies (Esparham, 2020;. Furthermore, Hardjito et al discovered comparable results in their investigation (Hardjito et al., 2004). Figure (7) indicates the conclusions of the second portion of the tests that are important to the zeolite change to metakaolin. As can be seen in Figure (7), alternating zeolite lowered the 3, 7, and 28day compressive strength of metakaolin-based geopolymer concrete by 10%, as compared to 33, 21, and 20% for the control sample. This is due to the crystal structure of zeolite (Ortega et al., 2000), which lowers polymerization significantly. Increasing the alternating percent of zeolite from 10% to 50% enhances the compressive strength as though lessening the 3, 7, and 28-day compressive strength in MZ50 mixtures with 50% zeolite alternation altered to 14, 8, and 5% than the control sample. This increase in MZ10 and MZ50 strength can be attributed to the pyramidal structure of clinoptilolite, which strengthens the metakaolin geopolymer matrix. Because of execution issues, the compressive strength of the MZ40 combination was lowered by 40% compared to the MZ30 mixture. Andrejkovičová et alfound similar results in their research on the geopolymer cement paste based on metakaolin and natural zeolite from the clinoptilolite classification. They reported the optimum zeolite alternation of 50% (Andrejkovičová et al., 2016).

Fig. 7. The role of zeolite alternation to metakaolin on 3, 7, and 28-day compressive strength
Furthermore, these researchers discovered a greater rise in strength than the control sample in this percentage. These findings suggest that, while alternating 50 percent zeolite to metakaolin lowered the 28-day compressive strength of geopolymer concrete by 5%, the target price of the geopolymer concrete in this mode is reduced by one-third. Figure 8 presents the associated results to the third section of the tests and investigation of the influence of nano-silica on 3, 7, and 28-day compressive strength.

Fig. 8. the role of increasing nano-silica on 3, 7, and 28-day compressive strength
As shown in Figure 8, adding nano-silica can boost the geopolymer concrete's 3, 7, and 28-day compressive strength. Furthermore, by increasing 2 wt percent metakaolin to 3 and 4 wt percent, the highest 3, 7, and 28day compressive strength may be observed in the MNS4 mixture. The analysis of the results will be given later, in the last section.
To investigate the other mechanical strengths of the geo-polymeric concrete based on metakaolin, MR1.5, MN2, and MZ50 combinations of the tensile and flexural samples were tested, the results of which are given in Figures (9) and (10). As can be seen in Figures (9) and (10), the geo-polymeric concrete samples based on metakaolin have 7 and 28-day tensile strengths of 1.44 and 1.72 MPa without additives, respectively, which increase to 1.68 and 1.88 MPa when 2 percent nano-silica is added. This suggests that the sample size increased by 17 and 10%, respectively, over the control sample. Furthermore, substituting 50% zeolite for metakaolin diminishes the 28-day tensile strength by around 20%. Concerning flexural strength, similar results were observed. The control sample had flexural strengths of 2.62 and 3.02 MPa after 7 and 28 days. By alternating 50 percent zeolite with 2 percent nano-silica, they were modified to 3.6 and 4.05 MPa, respectively. Nano-silica actually improves the concrete's strength. According to the conclusions, increasing nano-silica increased the compressive, flexural, and tensile strengths of geo-polymeric concrete based on metakaolin, with flexural strength being the most significant. This issue can be linked to nano-chemical silicas and physical effects in filling the micro-pores of concrete fractures and reinforcing the transmission zone, which increases the flexural and tensile strengths of samples compared to the control sample (Qing et al., 2007;Li, 2004;Collepardi et al., 2004). In their study, Esparham et al revealed that adopting nano-silica increased compressive, tensile, and flexural strengths by up to 4% . Compressive, tensile, and flexural strength are all diminished when zeolite was incorporated into geopolymer concrete. However, it cuts the cost of concrete.

Results
It was tried in this research to study some effective parameters of alkaline solutions and their effect on the compressive strength of geo-polymeric concrete based on metakaolin. In addition, it was tried in this research to examine the effect of adding nano-silica and zeolite on the compressive, tensile, and flexural strengths of geo-polymeric concrete based on metakaolin. In this regard, the following results can be stated based on the obtained results from the results of the previous part.
• The weight ratio of sodium silicate to NaOH solution is one of the effective parameters of the compressive strength of geo-polymeric concrete. This parameter is 1.5 in the optimum condition which reaches the maximum 3, 7, and 28-day compressive strength. Increasing this ratio to 3, reduces 3, 7, and 28-day compressive strength to 44, 40, and 41%.
• The optimum extra water amount in this research was measured at 40 kg/m 3 , which provides the proper workability for the geo-polymeric concrete based on metakaolin with the minimum compressive strength by increasing the extra water.
• The weight ratio of water to the dry participated materials in geo-polymerization is another effective parameter of the compressive strength of geo-polymeric concrete. The optimum value of this parameter was measured at 0.4741 in this research which obtains the maximum 3, 7, and 28-day compressive strength. In addition, a significant reduction was observed in the compressive strength by increasing this ratio.
• Increasing nano-silica increases the 3, 7, and 28-day compressive strength of geo-polymeric concrete based on metakaolin, and the optimum value of increasing silica nanoparticles was measured at 4 wt% of metakaolin as an alternative in this research.
• Adding nano-silica to 2wt% of metakaolin significantly increases the flexural and tensile strength of geopolymeric concrete based on metakaolin.
• Alternation of zeolite reduces 3, 7, and 28-day compressive strength of geo-polymeric concrete based on metakaolin. The optimum amount of this alternation was measured by 50% as the 28-day compressive strength reduction to 5% made this concrete affordable, and reduced the target price to one-third.
• Alternating 50% of metakaolin to zeolite reduced the flexural and tensile strengths in geo-polymeric concrete based on metakaolin to 23 and 20%, respectively. However, this makes the concrete affordable.