Sustainable Alternatives to Cement: Synthesizing Metakaolin-Based Geopolymer Concrete Using Nano-Silica

: The emission of carbon dioxide gas from the cement manufacturing industry has raised concerns about global warming. Geopolymer concrete (GC) is gaining attention as a sustainable and environmentally friendly alternative to traditional cement concrete. The current study focused on using local clay to synthesize and characterize metakaolin-based GC with varying percentages of nanosilica (NS) (1.5%, 3.0%, 4.5%, 6.0%, and 7.5% by weight of MK content) using NaOH/sodium silicate. The geopolymer specimens were cured at room temperature for 28 days, and their workability, compressive, tensile, and ﬂexural strengths were measured to evaluate the inﬂuence of NS on the concrete’s mechanical properties. The study found that the compressive, tensile, and ﬂexural strengths of the GC increased gradually up to 6.0% NS, but any further increase in its ratio resulted in a reduction in mechanical characteristics. The study concludes that the addition of 6.0% NS in metakaolin (MK)-based GC produces the highest mechanical properties, improving the compressive strength of the GC mix by 34.3% compared to the control GC mix and improving the ﬂexural and split tensile strengths by 39% and 37%, respectively, compared to control GC strengths. Furthermore, the statistical analysis conﬁrms nano-silica’s signiﬁcant impact on geopolymer concrete’s mechanical properties, emphasizing its role in improving performance and sustainability as an alternative to cement-based materials.


Introduction
Cement-based concrete is one of the most widely used fundamental materials in the construction of civil infrastructure.On the other hand, a number of concerns have been expressed concerning its execution, and as a consequence, many in the construction industry and academics have begun to consider several new alternatives.In recent years, as the planet has become warmer, the production process of cement has become one of the most significant contributors to global warming.It has negative environmental effects and has contributed to a decline in the material's value.Extensive research studies have consistently demonstrated that the production of one tonne of cement necessitates the utilization of approximately 2.8 tonnes of raw materials and results in the emission of about one tonne of carbon dioxide (CO 2 ) [1,2].These findings highlight the significant resource utilization and environmental impact associated with cement manufacturing processes.
These issues have encouraged engineers and scientists to explore alternate construction materials that are more ecologically friendly than cement-based concrete.Geopolymer concrete as an alternative to cement-based concrete is becoming increasingly common in practice.
In recent years, geopolymers have shown great promise for use as a cementitious material.With these characteristics, geopolymers have the potential to significantly reduce the environmental footprint of CO 2 emissions by substituting cement usage, making them highly attractive and impactful.This is because geopolymers reduce the amount of cement used.In addition, permeability and resistance to corrosive agents are crucial properties of GC that contribute to its sustainability.Geopolymer concrete has demonstrated promising outcomes in reducing permeability and enhancing resistance against corrosive agents [3].Compared to conventional concrete, GC exhibits lower permeability and improved resistance to chloride penetration, mitigating the deterioration and corrosion of steel reinforcement [4].These properties enhance the durability of structures, reducing the need for frequent maintenance and aligning with sustainability objectives.The starting ingredient for the polymerization, which must be an alkaline solution, and the source material, which must include aluminum (Al) and silicon (Si), are both necessary components for the production of the geopolymer.The source material can be waste materials, for example, fly ash (FA) [5,6] or ground granulated blast furnace slag (GGBFs) [7,8], or it can be of natural origin, such as zeolite, or it can be synthetic, such as metakaolin (MK) [9,10].All of these are entirely dependent on the price, the existence of the material, and the desired characteristics.In contrast to OPC, geopolymer is produced through the polycondensation of (Si) and (Al) in the presence of a significant amount of alkali in order to achieve excellent inherent properties.These properties include, but are not limited to, excellent thermal stability, resistance to fire, mechanical properties, and better resistance to chemical degradation [11,12].According to Ma et al. [12] and Ng et al. [13], who conducted a study of the relevant literature, a great deal of studies have documented the structure and material performance of GC.These researchers found that GC may be made from any mineral or waste product from industry, such as amorphous Si and Al [12].Nowadays, MK, GGBS, and FA are the most common alkali activator binders used in GC production.MK-based geopolymers have lately attracted the interest of researchers owing to their unique features, such as early strength growth, decreased thermal conductivity, superior acid resistance, and the potential to provide superior performance in concrete compared to conventional concrete [12].Nevertheless, "the characteristics of the produced geopolymer depend considerably on the production parameters such as raw materials, ratios of mixing, and curing conditions" [14,15].In spite of the fact that it is beneficial, the most notable issue is the cost of the MK itself, as it is not a waste product and therefore can be expensive; in addition, the workability is not quite optimal [16].
In the past several decades, nanotechnology has emerged as a feasible option for dramatically enhancing the strength and durability of building materials.Due to their superior physicochemical qualities, nanotechnologies are outstanding in a variety of applications.Many studies have investigated the influence of various nano-oxides, including nano-silica (NS), nano-kaolin, carbon nanotubes, and nano-alumina, on newly synthesized materials.The hardened geopolymer was analyzed, and it was observed that the addition of nanoparticles to the geopolymer mixture enhances its microstructural and mechanical features [17,18].Nano-silica, among the most widely utilized nanomaterials, has shown the ability to improve the characteristics of several materials [19,20].Khater et al. [21] demonstrated that the addition of NS enhances the geo-polymerization process by increasing the number of N-A-S-H and C-S-H nucleation sites.Moreover, the inclusion of 2.5% NS in mortar resulted in a remarkable increase by 32% in compressive strength [22].In addition, Phoo-Negernham et al. [23] observed an increase of around 44% in flexure strength, 31% in compressive strength, and more than 70% in elasticity for a geopolymer cemented with 2% NS.Furthermore, Gao et al. [24] reported that the flexure strength of geopolymer including 1% NS increased from 12 to 15 MPa.Although the majority of published studies concluded that the inclusion of NS particles enhanced the characteristics of produced geopolymers, the optimal proportion of NS varied according to different experimental conditions, such as the source of binder, the liquid/solid ratio, the technique of NS distribution, and the temperature at which it cures, as well as the molarity of the alkaline solution.In addition, it is evident from the literature that the influence of NS on the mechanical characteristics of low-calcium FA or MK-based GC is relatively limited.Insufficient research was discovered to support the usage of metakaolin-based concrete in structural design codes and applications.For a deeper understanding of the mechanical performance of GC, more investigation is required.Consequently, the purpose of this research is to determine the influence of varying percentages of NS in MK-based GC to improve mechanical behavior.

Materials 2.1.1. Geopolymer Concrete Properties
The utilized GC was comprised of a geopolymer paste derived by chemically activating MK with an alkaline activating solution, along with the addition of a superplasticizer, water, and coarse and fine aggregates.The MK complied with the ASTM C618 requirements [25].The sodium silicate (Na 2 SiO 3 ) and sodium hydroxide solution (NaOH) were mixed to generate the activating solution, and they were obtained from ZAG, Bereket-Kimya.The NaOH solution was purchased as a flake and had a purity level of about 98%, while the Na 2 SiO 3 had a composition of 55.5% water, 29.42% SiO 2 , 13.7% sodium oxide, and a modulus of roughly 2.14 (where Ms = Si 2 O/Na 2 O).Both the coarse and fine aggregates were manufactured from locally sourced crushed limestone, with the biggest particles measuring 12 and 4 mm, respectively.Table 1 displays the results of an ASTM C127compliant sieve analysis of aggregates and other physical characteristics [26].Moreover, a novel production of "polycarboxylic-dependent superplasticizers" was used in a variety of mixes.The chemical and physical features of MK are detailed in Table 2.The study utilized CemSyn-XLP, a brand of NS sourced from Bee-chem Chemicals Ltd. in Kanpur.CemSyn-XLP is known for its unique properties; it is ideal for the intended application.Table 3 outlines the general characteristics of NS.

Mix Proportions of Materials
Several specimen batches of geopolymer were produced and evaluated.Table 4 provides the mix design information for GC mixes.Mixes of geopolymer were prepared with varying percentages of NS (1.5%, 3.0%, 4.5%, 6.0%, and 7.5% by weight of MK content).NaOH solutions and Na 2 SiO 3 were used to produce the alkaline activator used in GC synthesis.Before 24 h of casting, a 12 M NaOH solution was added to a solution of Na 2 SiO 3 [28].First, the widely available 98% pure NaOH pellets were dissolved in water to make the 12 M NaOH solution.For practical and economic considerations, the proportion of Na 2 SiO 3 to NaOH was around 1.5 to 2.5 [29].The research utilized a value of 2.5, whereas the proportion of solution to binder was just 0.5.Both coarse and fine aggregates were mixed for 2 min during the first mixing phase before binder components were added.In addition, the raw ingredients were mixed for an additional 2 min in an 80-L-capacity pan mixer.Moreover, the superplasticizer, alkali activator, and more water were slowly added to the mixture, and the process of mixing was continued for an additional 3 min.In order to attain homogeneity and also uniformity, the freshly formed concrete was mixed for an additional 2 min [27].Figure 1 shows the GC production procedure.

Mix Proportions of Materials
Several specimen batches of geopolymer were produced and evaluated.Tabl vides the mix design information for GC mixes.Mixes of geopolymer were prepar varying percentages of NS (1.5%, 3.0%, 4.5%, 6.0%, and 7.5% by weight of MK co NaOH solutions and Na2SiO3 were used to produce the alkaline activator used in G thesis.Before 24 h of casting, a 12 M NaOH solution was added to a solution of N [28].First, the widely available 98% pure NaOH pellets were dissolved in water t the 12 M NaOH solution.For practical and economic considerations, the propo Na2SiO3 to NaOH was around 1.5 to 2.5 [29].The research utilized a value of 2.5, w the proportion of solution to binder was just 0.5.Both coarse and fine aggregate mixed for 2 min during the first mixing phase before binder components were ad addition, the raw ingredients were mixed for an additional 2 min in an 80-L-capac mixer.Moreover, the superplasticizer, alkali activator, and more water were slowly to the mixture, and the process of mixing was continued for an additional 3 min.I to attain homogeneity and also uniformity, the freshly formed concrete was mixed additional 2 min [27].Figure 1 shows the GC production procedure.

Casting and Curing of Geopolymer Concrete
Six cylinder specimens 100 × 200 mm and three prisms 100 × 100 × 500 mm produced for each mix, as shown in Figure 2. The specimens were compacted mechanical vibrating machine, which decreased the amount of air voids within t After the casting process, the alkaline solution was prevented from evaporating by ing the specimens with nylon sheets.After that, the specimens were stored under tory conditions for about 24 h, following which they were taken from the casting and cured at room temperature (27 °C) until testing day.It is worth mentioning inclusion of NS in geopolymer concrete allows for geopolymerization to occur at a

Casting and Curing of Geopolymer Concrete
Six cylinder specimens 100 × 200 mm and three prisms 100 × 100 × 500 mm were produced for each mix, as shown in Figure 2. The specimens were compacted using a mechanical vibrating machine, which decreased the amount of air voids within the GC.After the casting process, the alkaline solution was prevented from evaporating by covering the specimens with nylon sheets.After that, the specimens were stored under laboratory conditions for about 24 h, following which they were taken from the casting molds and cured at room temperature (27 • C) until testing day.It is worth mentioning that the inclusion of NS in geopolymer concrete allows for geopolymerization to occur at ambient temperatures by providing a catalyst effect.Nevertheless, the NS-free specimens (MK-NS-0) were cured for 48 h at 60 • C in a hot air furnace, after that, the specimens were stored at ambient temperature for 28 days.
Constr.Mater.2023, 4, FOR PEER REVIEW 0) were cured for 48 h at 60 °C in a hot air furnace, after that, the specimens were stor at ambient temperature for 28 days.

Testing Procedures
Workability is an important property of fresh concrete that measures its ease of ha dling, placing, compacting, and finishing.According to ASTM: C 143 [30], the slump te was conducted to determine the workability of the geopolymer concrete immediately a ter mixing.Figure 3 illustrates the testing of specimens following ASTM C39 [31] an ASTM C09 [32] for compression and splitting tensile strength tests, respectively, whi were conducted on cylindrical specimens measuring 100 × 200 mm.Additionally, thre point bending tests based on the RILEM 50-FMC/198 [33] were performed on notch prismatic specimens with dimensions of 100 × 100 × 500 mm."A linear variable displac ment transducer (LVDT) was utilized to measure the vertical deflection at the center of t notched prismatic samples".In the bottom middle of the specimens, notches of 40 mm height and 3 mm in width were cut.The specimens were loaded at a displacement-co trolled speed of 0.. 02 mm/min.Equation (1) [34] was applied to determine the flexur strength of the specimens.

Testing Procedures
Workability is an important property of fresh concrete that measures its ease of handling, placing, compacting, and finishing.According to ASTM: C 143 [30], the slump test was conducted to determine the workability of the geopolymer concrete immediately after mixing.Figure 3 illustrates the testing of specimens following ASTM C39 [31] and ASTM C09 [32] for compression and splitting tensile strength tests, respectively, which were conducted on cylindrical specimens measuring 100 × 200 mm.Additionally, three-point bending tests based on the RILEM 50-FMC/198 [33] were performed on notched prismatic specimens with dimensions of 100 × 100 × 500 mm."A linear variable displacement transducer (LVDT) was utilized to measure the vertical deflection at the center of the notched prismatic samples".In the bottom middle of the specimens, notches of 40 mm in height and 3 mm in width were cut.The specimens were loaded at a displacement-controlled speed of 0.02 mm/min.Equation (1) [34] was applied to determine the flexural strength of the specimens.
0) were cured for 48 h at 60 °C in a hot air furnace, after that, the specimens were store at ambient temperature for 28 days.

Testing Procedures
Workability is an important property of fresh concrete that measures its ease of ha dling, placing, compacting, and finishing.According to ASTM: C 143 [30], the slump te was conducted to determine the workability of the geopolymer concrete immediately a ter mixing.Figure 3 illustrates the testing of specimens following ASTM C39 [31] an ASTM C09 [32] for compression and splitting tensile strength tests, respectively, whic were conducted on cylindrical specimens measuring 100 × 200 mm.Additionally, thre point bending tests based on the RILEM 50-FMC/198 [33] were performed on notche prismatic specimens with dimensions of 100 × 100 × 500 mm."A linear variable displac ment transducer (LVDT) was utilized to measure the vertical deflection at the center of th notched prismatic samples".In the bottom middle of the specimens, notches of 40 mm height and 3 mm in width were cut.The specimens were loaded at a displacement-co trolled speed of 0.. 02 mm/min.Equation (1) [34] was applied to determine the flexur strength of the specimens.

Slump Test
Figure 4 illustrates the results of slump tests performed to determine the influence of incorporating NS on the workability of GC mixes based on MK.The slump test was conducted in accordance with the ASTM C143 [30] standard.The findings indicated that NS might decrease the slump in GC.As NS particles were smaller than metakaolin particles, their aspect ratio was greater, leading to more water absorption and hence a decrease in concrete slump.The slumping was further reduced when NS inclusion increased (The term "aspect ratio" refers to the ratio of the length or height of a particle or object to its width or diameter).Moreover, NS may react quickly with alkaline solutions and water to produce a thick and viscous liquid.This was likely the reason why concrete including NS, had a lower slump.

Slump Test
Figure 4 illustrates the results of slump tests performed to determine the influence of incorporating NS on the workability of GC mixes based on MK.The slump test was conducted in accordance with the ASTM C143 [30] standard.The findings indicated that NS might decrease the slump in GC.As NS particles were smaller than metakaolin particles, their aspect ratio was greater, leading to more water absorption and hence a decrease in concrete slump.The slumping was further reduced when NS inclusion increased (The term "aspect ratio" refers to the ratio of the length or height of a particle or object to its width or diameter).Moreover, NS may react quickly with alkaline solutions and water to produce a thick and viscous liquid.This was likely the reason why concrete including NS, had a lower slump.

Compressive Strength
At 7 and 28 days of age, the compressive strength of MK-based GC was studied and evaluated.In Figure 5, the compressive strengths of GC with additions ranging from 0 to 7.5% NS are displayed.The compressive strength of the control mix at 28 days was 39.1 MPa, but the addition of NS raised it to 52.5 MPa.The compressive strengths of the mixes with 1.5-7.5% NS were much greater than those of the control mix, as demonstrated by the figure.The mixes MK-NS-1.5, MK-NS-3, MK-NS-4.5, MK-NS-6.0, and MK-NS-7.5 had greater compressive strength values after 28 days compared to the mix MK-NS-0 by 15.6%, 23.8%, 31.2%,34.3%, and 25.6%, respectively.This demonstrates that there was a correlation between the increases in the amount of NS in the mixes and the increase in compressive strength.Nevertheless, adding more than 4.5% NS to GC had very little impact on the improvement of the material's compressive strength.

Compressive Strength
At 7 and 28 days of age, the compressive strength of MK-based GC was studied and evaluated.In Figure 5, the compressive strengths of GC with additions ranging from 0 to 7.5% NS are displayed.The compressive strength of the control mix at 28 days was 39.1 MPa, but the addition of NS raised it to 52.5 MPa.The compressive strengths of the mixes with 1.5-7.5% NS were much greater than those of the control mix, as demonstrated by the figure.The mixes MK-NS-1.5, MK-NS-3, MK-NS-4.5, MK-NS-6.0, and MK-NS-7.5 had greater compressive strength values after 28 days compared to the mix MK-NS-0 by 15.6%, 23.8%, 31.2%,34.3%, and 25.6%, respectively.This demonstrates that there was a correlation between the increases in the amount of NS in the mixes and the increase in compressive strength.Nevertheless, adding more than 4.5% NS to GC had very little impact on the improvement of the material's compressive strength.MK-NS-6.0 is the mix with the greatest 28-day compressive strength in the series.The micropropagation capabilities of nanoparticles may account for the increased compressive strength of GC caused by NS.Additionally, the inclusion of more than 99% silica (SiO2) in NS resulted in the creation of larger silicate chains when added to concrete.Increased silica concentration resulted in denser, stronger gels with enhanced mechanical characteristics [35].With NS amounts in excess of 6.0%, the strength improvement decreased.This is mostly owing to the fact that the 6.0% NS percentage was adequate for a complete reaction as a result of its large surface area.The addition of NS in excess of 6.0% did not contribute to the reaction and therefore did not improve the material's strength.Despite the  MK-NS-6.0 is the mix with the greatest 28-day compressive strength in the series.The micropropagation capabilities of nanoparticles may account for the increased compressive strength of GC caused by NS.Additionally, the inclusion of more than 99% silica (SiO 2 ) in NS resulted in the creation of larger silicate chains when added to concrete.Increased silica concentration resulted in denser, stronger gels with enhanced mechanical characteris-tics [35].With NS amounts in excess of 6.0%, the strength improvement decreased.This is mostly owing to the fact that the 6.0% NS percentage was adequate for a complete reaction as a result of its large surface area.The addition of NS in excess of 6.0% did not contribute to the reaction and therefore did not improve the material's strength.Despite the fact that the resistance reduced at 7.5% compared to the optimal value of 52.5 MPa at 6.0%, it was still more than that of the control geopolymer.This was comparable with the findings of the study conducted by Zeineb Zidi et al. [36].As demonstrated in Figure 5, the impact of NS on compressive strengths at 7 and 28 days of age follows the same pattern.
The present study showed that a percentage of 6.0% NS added to the mix yielded the maximum compressive strength, and Zeineb Zidi et al. [36] reported an almost similar trend.While some previous studies have revealed that a percentage of NS between 2% and 4% is optimum [24,37], it is crucial to acknowledge that the effectiveness of NS in enhancing compressive strength may depend on various parameters, such as the qualities of the concrete mix, the amount of additional material, and the types and properties of nanoparticles.

Tensile Strength
Figure 6 illustrates the influence of NS addition on the 7-and 28-day tensile strengths of MK-based GC with 0-7.5%.The tensile strength improved as the ratio of NS increased till it reached 6.0%.The split tensile strength of MK-based GC improved by 37% when NS was added at a ratio of 6.0%.This may be related to the geopolymer's improved microstructure.In addition to this, it was observed that the tensile strength values reduced when the ratio of NS was increased further.Nevertheless, it must be noted that, despite the fact that the strength reduced by 7.5% compared to the optimal value of 3.93 MPa at 6.0%, it endured more than that of the control geopolymer.Adding 7.5% NS negatively impacted the development of tensile strength, which is directly correlated with a substantial reduction in setting time.Generally, when the proportion of nanoparticles is increased in the geopolymer mixture, it can accelerate the geopolymerization process.However, this increase in nanoparticles also promotes their aggregation, causing them to clump together.These aggregates can disrupt the organized structure of the geopolymer, potentially compromising its strength properties, including tensile strength.So, while increasing the nanoparticle content can speed up geopolymerization, it can also lead to nanoparticle agglomeration, which may have detrimental effects on the overall strength of the geopolymer when the NS content exceeds a certain threshold.A. Ravitheja et al. [38] and Zeineb Zidi et al. [36] also investigated the influence of NS variation on the tensile strength of alkali-activated geopolymer and determined that the best percentages for the examination circumstances were around 6.0% and 5.0%, respectively.

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Zidi et al. [36] also investigated the influence of NS variation on the tensile strength of alkali-activated geopolymer and determined that the best percentages for the examination circumstances were around 6.0% and 5.0%, respectively.

Flexural Strength
Figure 7 demonstrates the flexural strength values of MK-based GC without and with NS at 7 and 28 days.As shown in Figure 7, the influence of NS on flexural strengths at 7, and 28 days of age follows the same pattern.The outcomes after 28 days are greater than those at 7 days because of the continual geo-polymerization process.The addition of NS at (1.5, 3.0, 4.5, 6.0, and 7.5%) contents improved the flexural strength of prepared geopol-

Flexural Strength
Figure 7 demonstrates the flexural strength values of MK-based GC without and with NS at 7 and 28 days.As shown in Figure 7, the influence of NS on flexural strengths at 7, and 28 days of age follows the same pattern.The outcomes after 28 days are greater than those at 7 days because of the continual geo-polymerization process.The addition of NS at (1.5, 3.0, 4.5, 6.0, and 7.5%) contents improved the flexural strength of prepared geopolymer by 23%, 30%, 36.8%,39.4%, and 32%, respectively.This increase might be due to the small size of the NS particles, which leads to a higher surface area and improved distribution in the GC matrix, resulting in stronger chemical bonds and reduced spacing between the particles.This resulted in an overall stronger structure with improved flexural strength with considering sustainable behaviors [39,40].Additionally, the results showed that the addition of 6.0% NS resulted in the highest magnitude in flexural strength.It was also determined that there was not a significant difference between incorporating 4.5% and 6.0% of NS in terms of their effect on enhancing the flexural strength of GC.These improvements were found to be 36.8%and 39.4%, respectively.

Flexural Strength
Figure 7 demonstrates the flexural strength values of MK-based GC without and with NS at 7 and 28 days.As shown in Figure 7, the influence of NS on flexural strengths at 7, and 28 days of age follows the same pattern.The outcomes after 28 days are greater than those at 7 days because of the continual geo-polymerization process.The addition of NS at (1.5, 3.0, 4.5, 6.0, and 7.5%) contents improved the flexural strength of prepared geopolymer by 23%, 30%, 36.8%,39.4%, and 32%, respectively.This increase might be due to the small size of the NS particles, which leads to a higher surface area and improved distribution in the GC matrix, resulting in stronger chemical bonds and reduced spacing between the particles.This resulted in an overall stronger structure with improved flexural strength with considering sustainable behaviors [39,40].Additionally, the results showed that the addition of 6.0% NS resulted in the highest magnitude in flexural strength.It was also determined that there was not a significant difference between incorporating 4.5% and 6.0% of NS in terms of their effect on enhancing the flexural strength of GC.These improvements were found to be 36.8%and 39.4%, respectively.It was also noted that the improved strength started to decrease when the NS content exceeded 6.0%.This is mostly due to the formation of NS layers, which dramatically decrease the nanoparticle dispersion in the polymer matrix.This decrease in strength can be  It was also noted that the improved strength started to decrease when the NS content exceeded 6.0%.This is mostly due to the formation of NS layers, which dramatically decrease the nanoparticle dispersion in the polymer matrix.This decrease in strength can be attributed to multiple factors.Firstly, when the NS content exceeds a certain threshold, the concentration of nanoparticles increases.This higher concentration can lead to the aggregation of particles, causing them to clump together and form layers.As a result, the nanoparticles are not evenly dispersed throughout the polymer matrix, leading to uneven distribution and reduced dispersion.Secondly, the formation of NS layers creates physical barriers within the matrix, hindering the mobility and movement of nanoparticles.This causes them to disperse uniformly, resulting in localization in specific regions rather than a uniform distribution.Both aggregation and immobility contribute to the decreased dispersion of nanoparticles.This negatively affects the mechanical properties and performance of nanocomposite materials, as the effective dispersion is crucial for achieving desired enhancements and interactions between nanoparticles and the matrix.

Statistical Analysis
This section presents a statistical analysis conducted to investigate the effects of different percentages of NS on the mechanical properties of MK-based GC.The study employed regression analysis, correlation evaluation, and the determination of p-values to examine the relationship between the NS content and its impact on the mechanical characteristics of GC.The significance of differences was also assessed to understand the significance of the observed variations.Excel software was utilized for efficient data processing and analysis.
As shown in Table 5, the correlation coefficients revealed a strong positive correlation between the percentage of NS and the compressive strength (0.82), tensile strength (0.84), and flexural strength (0.81).These high correlation values indicate a robust relationship between the amount of NS and the mechanical properties.Moreover, the low p-values (0.045, 0.033, and 0.048) indicate that these relationships are statistically significant, with a significance level lower than 0.05.Additionally, the adjusted R-square values (0.59, 0.65, and 0.58) indicate that 59%, 65%, and 58% of the variations in compressive, tensile, and flexural strength, respectively, can be explained by the percentage of NS.It can be noted that the p-value and significance are exactly the same, since there are only two variables (NS and strength).These statistical findings provide strong evidence for the significance of the relationship between NS and the mechanical properties of geopolymer concrete, contributing to the development of sustainable alternatives to conventional cement-based concrete.

Conclusions
This investigation studied the impact of various NS percentages on the slump, compressive strength, tensile strength, and flexural strength of GC based on MK.The findings of this research may be stated as follows: • The addition of NS to MK-based GC can result in a reduction in slumps.This is because NS has a smaller size and a higher specific surface area than other materials, and it increases the aspect ratio in the concrete.

•
The inclusion of NS in MK-based GC improved the mechanical properties to a percentage of 6.0%.Adding more than 6.0% of NS decreased the mechanical strength of the geopolymer compared to the optimal value of 6.0%.

•
The current study revealed that 6.0% NS was the optimal percentage for producing a local MK-based GC with acceptable mechanical properties.

•
It was discovered that adding 6% NS to MK-based GC and curing it at ambient temperature resulted in a 34.4% increase in its compressive strength compared to the control mix.Furthermore, the split tensile strength and flexural strength showed an improvement of 37% and 39.4%, respectively, compared to their respective control geopolymer strengths.

•
The findings of this study highlight the promising mechanical characteristics of GC synthesized from MK and NS, positioning it as a viable option for concrete construction applications.The versatility of GC makes it suitable for use in diverse sectors.However, to fully exploit its potential, there is a need for further research in several key areas.Future investigations should focus on analyzing the microstructure of GC, evaluating long-term durability properties, and studying time-dependent deformations such as creep and shrinkage.By addressing these aspects, we can enhance our understanding of GC and contribute a successful implementation in the construction industry.
In light of future research opportunities, it is highly recommended to establish a comprehensive relationship between MK-based GCs and their influence on key properties such as compressive strength, tensile strength, and flexural strength.Additionally, proposing a relationship that links the tensile and flexural strength of the concrete to the compressive strength would greatly benefit the engineering community.To achieve this, further investigations and experimental studies are crucial to derive a reliable and practical relationship.This advancement will pave the way for broader adoption and implementation of MK-based geopolymer concrete in various construction projects, contributing to the overall sustainability and performance of concrete structures.

Figure 4 .
Figure 4.The influence of NS on the slump of MK-based GC.

Figure 4 .
Figure 4.The influence of NS on the slump of MK-based GC.

Figure 5 .
Figure 5. Compressive strength changes in MK-based GC with NS.

Figure 5 .
Figure 5. Compressive strength changes in MK-based GC with NS.

Figure 6 .
Figure 6.Tensile strength changes in MK-based GC with NS.

Figure 6 .
Figure 6.Tensile strength changes in MK-based GC with NS.

Figure 6 .
Figure 6.Tensile strength changes in MK-based GC with NS.

Figure 7 .
Figure 7. Flexural strength changes in MK-based GC with NS.

Figure 7 .
Figure 7. Flexural strength changes in MK-based GC with NS.

Table 1 .
[27]e analysis and physical properties of course and fine aggregates with permission from[27].

Table 3 .
The properties of NS.

Table 4 .
Proportional mix of the geopolymer concrete.

Table 4 .
Proportional mix of the geopolymer concrete.

Table 5 .
Statistical analysis results between nano-silica % and mechanical properties.