Geopolymerization And Ceramic Formation Mechanism Of The Graphene Oxide Reinforced Geopolymer

In this paper, nature metakaolin, alkalin solution and graphene oxide, etc, were used to prepare in-situ partially reduced graphene oxide/geopolymer composites(r GO/KGP). Completely reduced graphene oxide/leucite nanocomposites(r GO/leucite) were prepared from r GO/KGP by in situ reduction of r GO and post high temprature treatment process. The in situ reduction degree of GO in the composites during the synthesis and high temprature treatment were characterized. The geopolymerization mechanism was studied based on the r GO/ KGP. The effects of r GO addition and heat treatment temperature on the microstructure, crystallization mechanism and the mechanical properties of the r GO/ KGP composites were investigated systematically.The results show that the GO can be in situ reduced in the alkalin geopolymeric solutions and post high temperature treatment. The GO can be homogeneously dispersed and partially reduced in alkaline geopolymeric solutions. The reduction, wrinkled and folded degree of the r GO sheets were increased gradually with the temperature and time increased. The reduction is mainly attributed to decarboxylic reaction under the alkaline conditions. Other residual oxygen-function groups of r GO in the r GO/KGP can be in situ reduced after the high temperature treatment.The geopolymerization mechanism of the geopolymer based on nature metakaolin were: metakaolin dissolved from its surface after mixing metakaolin particles with alkaline geopolymeric solutions, the Si-O bond and Al-O bond bound hydrolyzed, Si and Al monomers diffused and polycondensed, five and six coordinates of Al-O sites converted into four coordinates, resulting the network structure which Si species condense with Al species with Si mainly in the form of Q4(3Al) and Al in four coordinate. Particles for the reaction produc ts became densification with the reaction time, and the amorphous degree decreased. The addition of GO has no obvious effects on the geopolymerization products. The GO bonded well with the matrix during the geopolymerization.Geopolymer nanocomposites could be synthesized by in situ reduction of graphene oxide under alkaline solutions. RGO was evenly dispersed in the final amorphous composite structures with improved mechanical properties. The flexural strength of the r GO/KGP reached a maximum value of 17.9 MPa, increased 45% with increasing GO content to 0.3wt.%. Crack deflection and propagation, r GO pull-out, wrapping and anchoring around geopolymer particles contribute to the improved fracture toughness and flexural strength.With proper treatment temperature, K-based r GO/KGP could convert into r GO/tetragonal-leucite ceramic composites. The partially reduced r GO in the composites completely in-situ reduced under the post high temperature treatment. The addition of r GO refined the leucite grains.After treated at 1000°C, the flexural strength of r GO/KGP reached 91.1 MPa, increased 120% compared with the KGP. The thermal shrinkage of the r GO/KGP could be divided into four stages, i.e. free water loss, capillary strain, physical contraction during polymerizati on of-OH groups, and viscous sintering of the composite, respectively. The crystallization of the composite was controlled by a three-dimensional growth mechanism. The scrolled and fold degree of r GO sheets in the r GO/tetragonal-leucite composites increased with the isothermal soaking times, resulting the decrease of the mechanical properties. RGO/cubic-leucite composites were prepared by in situ crystallization process with simultaneously thermal reduction of GO from 30 at.% Cs+ substitution of K+ based r GO/Cs KGP composites above 1000°C. Improvements in mechanical properties of r GO/cubic-leucite were also achieved through r GO reinforcement and grain refinement. Compared with pure cubic-leucite matrix, the flexural strength and fracture toughness of r GO/cubic-leucite increased by 37% and 14%, respectively.So the synthese of in situ reduced r GO/KGP provided a new and green way to prepare the composites.The addition of r GO improves the mechanical properties of composites, which might be attractive for applications in the future.