Characterization of Nano-Silica Local Metakaolin Based-Geopolymer: Microstructure and Mechanical Properties

The current study focused on the utilization of local clay for synthesis and characterization of meta-kaolin based geopolymers with and without nano-silica. The control geopolymers, for a compressive strength of 30 MPa, were optimized by using Liquid/Solid ratio of 0.55, NaOH concentration of 10 M and curing at 80 ̊C. The nano silica was added in an extended range of 1%, 2%, 3%, 5%, 7% and 10%. The synthesized nano-silica metakaolin based geopolymers was investigated by using compressive strength, XRD, XRF, FTIR, SEM, MIP, TG, UV/VIS spectroscopy, in addition to density, water absorption and initial setting times. The results indicated an increase in the compressive strength value with the incorporation of nano-silica in geopolymer mixes until the optimum percentage of 5%, while the 10% addition of nano-silica decreased the compressive strength by 5% as compared to the control geopolymer. The increase in the compressive strength was accredited to the increase in the content of N-A-S-H gel and the amorphous structure as shown by XRD and FTIR analysis. In addition, the optical transmittance analysis, MIP and SEM scans along with the results of density and water absorption have clearly shown the densification of the matrix formed for the optimal percentage of nano-silica. However, the initial setting time was found to reduce substantially with increase of nano-silica content. Moreover, the TG results have shown the 5% nano-added geopolymers to have greater thermal stability as compared to reference geopolymers. Finally, the adopted methodology in this research has shown that 5% nano-silica, is the optimal result for the synthesis and the production of local meta kaolin based geopolymer, How to cite this paper: Zidi, Z., Ltifi, M. and Zafar, I. (2020) Characterization of Nano-Silica Local Metakaolin Based-Geopolymer: Microstructure and Mechanical Properties. Open Journal of Civil Engineering, 10, 143-161. https://doi.org/10.4236/ojce.2020.102013 Received: May 19, 2020 Accepted: June 7, 2020 Published: June 10, 2020 Copyright © 2020 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access


Introduction
Geopolymers, in the last few decades, have shown a significant potential to be used as a cementitious material. Along with other properties, the contribution of geopolymers in reducing the environmental footprints of carbon dioxide by replacing cement appears quite appealing and significant. The basic elements required to produce the geopolymer include the source material containing the aluminum and silicon, and an alkaline solution to start the polymerization. Various source materials such as fly ash, biomass bottom ash, slag, silica fume and natural clays have been used for the synthesis of geopolymers. Metakaolin based-geo-polymers have recently gained attention of many researchers due to their unique properties such as rapid compressive strength development, lower thermal conductivity and better resistance to acid as compared to normal cement concrete [1]. However, the properties of the synthesized geopolymer depend significantly on the synthesis parameters: solvent type, source materials, mixing proportions and curing conditions [2] [3] [4] [5]. In this regard, the optimization of the synthesis parameters in relation to the material used appears to be one of the essential steps. In addition, it has been shown that the use of nanotechnologies is very beneficial in various applications due to their excellent physicochemical properties. Several researchers have studied the effect of different types of nano-oxides (nano-kaolin, nano-silica, nano-alumina, carbon nanotubes) on fresh and hardened geopolymer properties and reported that the introduction of nanoparticles in the geopolymer mix improves the mechanical and the microstructure properties [6] [7]. Nano-silica being one of the most commonly used nano material has shown high potential to enhance the properties of different materials [8] [9]. Indeed, the amorphous structure and the high specific surface of nano-silica are expected to produce denser geopolymer than those with traditional additives. Due to their size, nanoparticles can be easily dispersed in the alkaline solution, generating a large number of nucleation sites and resulting in a more homogeneous and dense network. Moreover, the particles size and the properties of the nano-silica strongly depend on the synthesis method [10] [11]. The sol-gel method has been used in many applications due to its advantage of providing different size particles [12] [13]. Various attempts have been made in the past few years to use the nano-silica for synthesis of geopolymers [14] [15]. Khater [14] has shown that the incorporation of nano-silica im-  [16]. Moreover, Phoo-Negernham et al. [17] have noted an improvement of about 31% in the compressive strength, 44% in flexure strength and more than 70% in elasticity, for a geopolymer reinforced by 2% of nano-silica. In addition, Gao et al. [15] have obtained an increase in the flexure strength from 12 to 15 MPa for geopolymer mixed with 1% of nano-silica. It was also noticed that the viscosity of the fresh paste of geopolymer decreased with the increase of the nano-silica amount [15]. While most of the cited works agreed that the addition of nano-silica particles generally improved the properties of synthesized geopolymers, however, the optimized percentage of nano-silica differed due to various experimental factors i.e. the method of dispersion of nano-silica, source of local meta-kaolin, the liquid/solid ratio, and the molarity of alkaline solution along with curing temperature.
In the light of above literature review, the present work was carried out to optimize the synthesis of geopolymer using local (Tunisian) metakaolin with and without nano-silica addition. The optimization of control geopolymer was done with regard to liquid/solid ratio, the molarity of alkaline solution and curing temperature. In addition, to determine the optimized percentage of nano-silica, the effect of various percentage of nano-silica addition on mechanical and microstructural properties for the local meta kaolin based-geopolymer were studied using X-ray Fluorescence (XRF), X-ray diffraction (XRD), Fourier Transform Infrared spectroscopy (FTIR), Scanning Electron Microscope (SEM), compressive strength, setting time, density, water absorption and UV/VIS/NIR spectroscopy. In addition, the Mercury Intrusion Porosimetry (MIP) and Thermogravimetry (TG) analysis was also done for the control and optimized nano-silica geopolymer. Nano-silica was synthesized by sol-gel method according to El Mir [18] protocol. The TEM image ( Figure 1) exhibits a uniform spherical particle with sizes ranging between 50 and 100 nm. The physical properties of nano-silica are summarized in Table 1. Open Journal of Civil Engineering

Optimization Approach for the Synthesis of the Control Geopolymer
The optimization for the synthesis of geopolymer was based on the effect of three critical synthesis parameters: liquid to solid ratio L/S, concentration of sodium hydroxide and curing temperature, on the compressive strength of the hardened geopolymer. The details of the test matrix for the synthesis of geopolymer are given in Table 2. In addition, the flowchart describing the optimizing of the geopolymer synthesis under different experimental conditions is shown in Figure 2. Compressive strength was considered as the control parameter for selection of the optimized test parameters. For each formulation, the compressive strength was determined after 7, 14 and 28 days using a Tonic-Technik Testing Machine moving at a constant cross-head displacement of 0.24 N/S according to the standard procedure of ASTM D1633-00 [19]. The compressive tests were performed on three samples for each composition and the average value of three compressive strengths was considered.

Activator Liquid to Source Metakaolin Ratio
The 28 days' compressive strength of hardened geopolymer paste with various L/S ratios is presented in Figure 6. The values of compressive strength were found to be 19 MPa, 30 MPa, 15 MPa and 7 MPa for L/S ratio of 0.5, 0.55, 0.6, and 0.65 respectively. It was observed that the optimum value of L/S ratio was found to be 0.55 with regard to the highest value of 28-day compressive strength.

NaOH Concentration
The results showed that the compressive strength increased as the concentration of Na(OH) is increased from 6 M to 10 M.   i.e. L/S ratio, NaOH concentration and curing temperature were found to be 0.55 M, 10 M and 80˚C respectively. All the geopolymers with or without nano-silica addition in the current were synthesized using these optimized values along with some considerations during the synthesis procedures.

Mix Proportion for Geopolymers Mixes
Firstly, the alkaline activator solution was prepared by mixing sodium hydroxide solution (SS) (

Characterization of Geopolymer Mixes
The setting time measurement of fresh mortars was performed by Vicat needle apparatus according to EN 196-3 2008. For all samples, the compressive strength test was determined after 28 days according to the standard procedure of ASTM D1633-00 [19]. The values of compressive strength correspond to the average of three measurements for each mixture. After the compression tests, the samples were crushed, washed with acetone to stop the reaction and dried at 60˚C and then, used for the microstructure analysis. X-ray diffraction (XRD) (Bruker D5005) was used to determine the crystalline phase of the ground powders using  (1) and (2) respectively: where: m = Weight of the sample (g) e = Thickness of the sample (cm) d = Diameter of the sample (cm) In addition, the mercury intrusion porosimetry was carried out by AUTOPORE IV 9500 from Micromeritics for the reference and optimized nano-silica added geopolymer. Thermo-gravimetric analysis is normally used to monitor the mass of the sample over time with change in temperature at constant rate. The TG analysis for the reference and optimized nano-silica added geopolymers was carried out by a STA6000 thermal analyzer with a temperature range of 25˚C to 450˚C and a constant rate of 5˚C/min.

Setting Times
The initial setting time of geopolymer pastes having different amounts of nano-SiO 2 are plotted in Figure 3. When the nano SiO 2 content is increased up to 5% in the fresh geopolymer pastes, the setting time is decreased gradually from 360 min to 45 min, but it can still be practically acceptable. This reduction is attributed to the acceleration of the chemical reaction at early ages which is related to the easy connection between the unsaturated Si-O and Si bonds of nano-silica atom located in the surface and the free ions of Metakaolin. The decrease can also be attributed to the increase of the viscosity of the paste, due to acceleration of water adsorption by the high specific surface of nano-silica. Beyond the 5% addition of nano-silica, it was almost impossible to measure the value of the initial setting time of pastes. Indeed, the increase in the percentage of nanoparticles Open Journal of Civil Engineering in mixtures (>5%, more particularly that of 10%), makes the mix very firm. It seems that the high reactivity of the nano-silica, by the surface effect, immobilizes the water which affects its viscosity [20].

Mechanical Properties
The effects of nano-silica addition on 28 days' compressive strength of geopolymer mortar are presented in Figure 4. It was observed that the compressive strength increased as the percentage of nano-silica increased until 5%. This is can be attributed to the enhancement of the geopolymer microstructure. However, it was also noticed that the further increase in the nano-silica percentage decreased the compressive strength values. Nevertheless, it must be stated that while the resistance decreased at 7% as compared to the optimum value i.e. 42 MPa at 5%, it still remained higher than that of control geopolymer. Addition of 10% nano-silica had a negative effect on the development of compressive strength, where it decreased by 5% when compared to that of the control geopolymer. This decrease is strongly related to the observations noted above concerning the significant reduction in setting time. Indeed, the increase in the percentage of nanoparticles can accelerate the kinetics of the geo-polymerization reaction, but it is likely to increase the agglomeration of these nanoparticles, which can disturb the microstructural organization of the geopolymer.
Several researchers have studied the effect of nano-silica on the compressive strength of synthesized geopolymers. Lo et al. have reported that the best mechanical resistance of geopolymer was observed in mortars containing 2% nano-silica [20]. Also, Phoo-Ngernkham et al. [17] showed that the incorporation of 1% to 2% NS was favorable to improve the mechanical resistance of geopolymer as compared to higher percentages. Deb et al. also studied the effect of nano-silica variation on the strength of alkali activated geoplymer and found the optimal percentage to be around 2% for the studied conditions [21]. It was observed that although the optimal percentage of the nano-silica for most of the past researchers was found to be around 2% while, the highest compressive strength among all the nano-silica percentages used for the current study was at 5% addition of nano-silica. 5% nano-silica addition has shown an improvement of 40% in compressive strength as compared to the control geopolymer while retaining the other geo-polymerization reaction parameters as constant.

Structural Characterization
XRD pattern of 28-day hardened geo-polymer samples admixed with various amounts of nano-silica is shown in Figure 5.
From XRD patterns, it is evident that some dissolvable minerals such as quartz and Mullite have remained in all the products. In general, XRD patterns of geopolymers have a diffuse hump at about 26˚ -30˚ with relatively smaller intensities indicates that the geopolymer material has almost complete amorphous structure. According to Figure 5 the intense peak is at 26.5˚ in 2θ and corresponds to quartz while at 28˚ it could be considered a broad hump that would correspond to the vitreous phase of the geopolymer. It was also noted that an increase in the percentage of nano-silica for the geo-polymer mixes induced a gradual increase in this XRD peak intensity. In addition, a slight shift of this peak was also seen when the nano-silica content increased from 1% to 5%. This can be explained by the improvement of the content of N-A-S-H gel, which is probably, attributed to the enhancement of the interaction of the dissolved species. On the other hand, the incorporation of 10% nanoparticles presents a negative effect on microstructural properties of the geo-polymer, indicated by a shift to lower angle due to the agglomeration, which hinders the reaction.
Furthermore, from these diffractograms a clear increase in the intensity of the SiO 2 peak with addition of nano-silica is visible, which indicates the existence of non-reacted nano-silica in geo-polymerization reaction. These observations confirm the same tendencies noted above regarding setting time and compressive strength. Therefore, the positive effect of nano-silica addition on the metakaolin based geo-polymer seems to be around 5%. Beyond this value, and particularly for a percentage of 10%, the negative effect is highlighted, by the decreasing of compression strength and illustrated by XRD analysis. These results are in agreement with those reported by Assaedi et al. [22] and Khater [14].    [24]. It should be noted that in various studies the values of the optimum ratios had varied with the experimental conditions used for optimizing the synthesis of geopolymers in addition to the chemical composition of the source metakaolin. Furthermore, the pH of all prepared samples was also presented in Table 4. Chindaprasirt et al. [25] have reported that the N-A-S-H gel became stable at pH ranging from 9 to 12. In this regard, it can be inferred that all the gels for the prepared mixes were stable; however, depending upon the local conditions available for the geo-polymerization, the further reaction will precede leading to different mechanical and physical properties with varied percentage of nano-silica.

Optical Transmission
Transmission spectroscopy is an effective technique to investigate the optical properties of geopolymers. By using the Dahm model [26], for any wavelength and thickness, the absorption, the transmission and the reflection are related as shown in Equation (3): For opaque specimen, the transmission is negligible therefore, Equation (4) governs: The transmission spectra of geopolymers with various amounts of nano SiO 2 (from 1% to 10%) in the UV/VIS/NIR range are presented in Figure 7. In this figure, three principal zones of transmission are depicted. The first zone, from 200 to 530 nm, presents an intense transmission, about 77% for pure geopolymer, while the increase in nano-silica content up to 5% caused the optical transmission to decrease by 17%. In the second zone, between 530 and 1000 nm, the curves show a decrease of 50%, this can be due to the fundamental absorption. On the other hand, the average transmission fractions are reduced by 11% compared to control sample. In both zones, the sample with 5% nano-silica possessed the lowest transmission fraction compared to the control mixes. Finally, for wavelengths beyond 1000 nm the transmission fraction vanished, it can attribute to the highest reflectance measured. The variation was due to the pore fraction generated during the synthesizing process. The experimental results indicated that the geopolymer having nano-silica had a dense network. Gasca-Tirado et al. [27] show that the porosity is the principal factor to modify the

Water Absorption
The water absorption and the density of the hardened geopolymer paste are shown in Figure 8. The curve shows that the density increased gradually when the nano-silica incorporation increased up to 5% and then decreased for 10% addition. While the opposite behavior was measured for the water absorption capacity of the synthesized geopolymer i.e. the water absorption curve gradually

Scanning Electron Microscopy (SEM)
The SEM scans of geo-polymers having different amounts of nano-SiO 2 are shown in Figure 9. It was observed that the incorporation of nano-silica improved the structural homogeneity of the geopolymer. Figure 9(b) and Figure   9(c) showed the high density of the matrix representing the geopolymers mixed with 2% and 5% of nano-silica respectively. Although the quantitative analysis for measuring the exact chemical ratio of the products formed by the incorporation of nano-silica was not done but the uniformity of the matrix and presence of the cracks were observed through SEM scans. Finally, as shown in Figure   9(d), the matrix presents micro-cracks probably not only due to the agglomeration of the nanoparticles but also because of the non-reactivity of a large part of nano-silica which might have caused the decrease in the mechanical properties of geoplomers at higher addition of nano-silica (10%). Similar findings were reported by other researchers as well Adak et al. [30] and Deb et al. [21].

Mercury Intrusion Porosimetry (MIP)
The pore size distribution for the control and optimized nano-added geopolymers i.e. 5% of nano silica addition is shown in Figure 10. It was observed that addition of 5% nano silica has resulted in two different effects on the pore size distribution of control geopolymers. Firstly, a clear reduction in the small sized  pores was observed for nano-silica added geopolymers and secondly, it was noticed that there is an increase in the pore size when 5% nano silica was added.
However, the final porosity estimated by the MIP indicated a clear reduction in the porosity values of 5% nano-added geopolymers as compared to the control sample. The values of porosity for the control and nano-silica added geopolymer were found to be 32.6% and 22.8% respectively. Open Journal of Civil Engineering These results are in line with the results of XRD, FTIR and compressive strength, water absorption and density, which have shown the 5% nano-silica geopolymers to have the better mechanical and micro-structural properties as compared to other series.

Thermogravimetry (TG) Analysis
The thermo-gravimetric and differential thermo-gravimetric variation of control and 5% nano-silica geopolymers with temperature is shown in Figure 11. It was shown in Figure 11 that the weight loss between 25˚C to 70˚C was on the higher side for the control geopolymer as compared to nano-silica geopolymer while after 70˚C, the slope of the TG curve for 5% nano-silica geopolymer was steeper as compared to control sample.
At 200˚C, the weight loss was found to be 5.5% and 8% for the control and 5% nano-silica geopolymer respectively. In Figure 11, a significant reduction in the mass was observed in a temperature range of 25˚C to 200˚C. This decrease can be mainly because of the evaporation of physically adsorbed water. For the same temperature range as shown in Figure 11, the slope of the differential thermo-gravimetric curve for control geopolymer is on the higher side causing the peak to move towards the lower temperature as compared to the peak of nano-silica added geopolymer. The shifting of peak to higher temperature for nano-added silica geopolymers indicates greater thermal stability as compared to control geopolymer. The increase in the thermal stability can be because of the improvement in the pore structure as indicated by the results of MIP and SEM. After 200˚C, the weight loss percentage for the control and nano-silica geopolymers begin to retard because the evaporation of free water had finished and the decomposition of chemically bound water had initiated [31].

Conclusion
The current study was done to optimize the synthesis of geoplomyer with and without incorporation of nano-silica using a local metakaolin as an aluminosilicate source. Firstly, the control geopolymers were synthesized and optimized based on the experimental parameters i.e. Liquid/Solid ratio, NaOH concentration and curing temperature, depicting the optimal values to be 0.55, 10 M and Figure 11. TG/DTG curves for geo-polymers with 0% and 5% nano-silica. Open Journal of Civil Engineering 80˚C respectively leading to a compressive strength of 30 MPa. To optimize the nano-silica percentage for synthesized geopolymer, the fresh, mechanical, optical, structural characteristics along with physical properties of the synthesized geopolymers at various percentages of nano-silica were investigated. The compressive strength of the synthesized geopolymer displayed positive impact with addition of nano-silica until the optimal percentage of 5%. The increase of 16.7% and 40% as compared to the control geopolymers was noticed with addition of 2% and 5% of nano-silica respectively, while the 10% nano-silica decreased the compressive strength by 5%. The increase in the compressive strength was mainly attributed to the increase of N-A-S-H gel as indicated by the increase of the FTIR intensity of T-O-Si band and the increase of the broadness of the XRD peak. Furthermore, the improvement of the microstructure for the optimum percentage of 5% was also confirmed by the optical transmittance analysis, MIP and SEM scans showing the increase in the homogeneity and the densification of the matrix. However, it was also noticed that the smaller specific surface of nano-silica has not only increased the density and reduced the water absorption but also has caused the acceleration of the geo-polymerization reaction as the setting time of the synthesized geopolymers substantial decrease by 8 times with 5% of nano-silica. Moreover, the TG analysis has shown the higher thermal stability for 5% nano-silica added geopolymers as compared to the control geopolymer. Finally, based on the results of current study, 5% nano-silica was found to be the optimum percentage for synthesis and the production of local metakaolin based geopolymer showing acceptable mechanical resistance and satisfactory physical and structural properties. Though the study has presented the experimental guideline for synthesis of geopolymers using local clay, however, detailed insights into chemical transformations linked to the geopolymer manufacturing process to develop practical recommendations are to be considered.