The current study focused on the utilization of local Microstructure clay for synthesis and characterization of meta-kaolin based geopolymers with and without nano-silica. The control geopolymers, Microstructure 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, with regard to the improvement of physical properties, micro structure and compressive strength.

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 improves the geo-polymerization reaction, by the increase of the nucleation sites of N-A-S-H and C-S-H. An improvement of about 32% was reported in the compressive strength for a mortar containing 2.5% nano-silica [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.