Geopolymerization and Ceramization Mechanism of Geopolymers (GPs) Activated by Mixed Ions and Properties of Cf/Cs1-xLixGPs Composites

Author:Yuan Jing Kun

Supervisor:jia de chang

Database:Doctor

Degree Year:2019

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Pages:159

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In this paper,two kinds of geopolymers system(GPs)activated by mixed alkali metal ions,Cs1-x-x Lix GPs and Cs1-x-x Nax GPs,were designed and prepared.Take these two geopolymer systems as the object of study,the whole process of geopolymerization reaction was disclosed,and the effects of alkali-activated ions during geopolymerization process were evaluated.The thermal evolution process of these two geopolymer systems were investigated and compared in order to insight the thermal evolution mechanism in the process of high temperature treatment.Besides,the differences of thermal evolution caused by the different kinds of alkali metal ions were also studied synchronously.In addition,the geopolymer composites reinforced by unidirectional continuously carbon fibers were fabricated,and the microstructure evolution and mechanical proporites were studied systematically to assessed the effects arouse by the high temperature treatement.As the research on the geopolymerization mechanism of geopolymers,it found that the geopolymerization process of geopolymers involved multiple reaction processes,such as dissolution,diffusion,polymerization-condensation,and structural rearrangement.According to the different of the reaction conditions or environment,the geopolymerization process of geopolymer could be divided into two stages,inoculation stage(Stage I)and reacting stage(Stage II),respectively.In the inoculation stage,the dissolution and diffusion processes were mainly involved,while the polymerization and structural rearrangement were dominant in the reacting stage.During the whole geopolymerization reaction process,Al atoms with penta-coordinated and hexa-coordinated were gradually converted into Al atoms with tetra-coordinated mode,and the conversion rate in reacting stage was much higher than that in the inoculation stage.Meanwhile,the basicity of the alkali metal ions or the reaction system could affect the overall polymerization reaction process of geopolymer system,and as the alkalinity of the system environment increased,the overall reaction rate and polymerization degree of the polymerization reaction were enhanced.Moreover,the influence of the electron effects of different excited ions on the polymerization process were highlighted in geopolymer system activated by the mixed alkali metal ions.By comparison,the alkali metal ions with a small atomic number in the geopolymer system could accelerate the progress of the polymerization reaction due to its electronegativity or strong ion pair effect.Through the study on the thermal evolution process of the geopolymer system during the high temperature treatment process,the results indicated that two kinds of geopolymer systems activated by the mixed alkali metal ions(substituted Cs+by Li+/Na+)exhibited similar thermal evolution process,and the introduction of Li+/Na+could promote the process of sintering and densification in the high temperature treatment process.However,the substitution Li+to Cs+did not change the phase composition of geopolymer system at room temperature;There was no atomic substitution between Li+and Cs+at the lattice level,and the corresponding ceramic products derived from Cs1-x-x Lix GPs should be a multi-phase system,consisting of crystalline pollucite,spodumene and amorphous glass phase.In contrast,an entirely different phenomenon was observed in Cs1-x-x Nax GPs system.The phase composites of Cs1-x-x Nax GPs system at room temperature was no longer completely amorphous after Cs+partical substituted by Na+,in which crystalline pollucite was observed.And the introduction of Na+could refine the pollucite grains gradually during the high temperature treatment process;In addition,the decrease in lattice constant of pollucite derived from Cs1-x-x Nax GPs indicated that the substitution and replacement between Na+and Cs+at the lattice level should have occurred,and the corresponding ceramic products only contained crystalline pollucite and amorphous glass phase.After the introduction of Li+in the CsGP system,the thermal expansion coefficient of the corresponding products gradually decreases with the increasing of Li+substitution,and the reason was mainly because the thermal expansion coefficient of spodumene was much smaller than that of pollucite.However,in case of Na+substitution,the thermal expansion coefficient of the corresponding products gradually increased as the increasing of Na+content in Cs1-x-x Nax GPs system,which was mainly because the thermal expansion coefficient of the amorphous glass phase was higher than that of garnet.The design and preparation of Cf/Cs1-x-x Lix GPs composites were conducted and the evolution of microstructure and mechanical properties of Cf/Cs1-x-x Lix GPs composites were compared and characterize.Although the amorphous structure of Cf was not suitable as a nucleation substrate,the crystallization temperature of Cf/CsGP composites were delayed significantly compared with CsGP matrix.Due to the reduction of the CsGP matrix content inside the Cf/CsGP composite and the"rigidity"of Cf itself,a large number of cracks were pregnant in the perpendicular to direction of the fiber arrangement in the corresponding product after high-temperature treatment,which would demage the integrity of the composite material.The mechanical properties of Cf/CsGP composites rised firstly and then decreased with the increased of treated temperature,and the peak vaule of the mechanical properties correspond to the products obtained after treated at 1200oC.However,the mechanical properties of the resulting composites showed a gradual decline due to the interfacial reaction between the matrix and the fibers.Compared with Cf/CsGP composites,the introduction of Li+could improve the mechanical properties of resulting products derived from Cf/Cs1-x-x Lix GPs composites after high-temperature treatment.In addition,the presence of Li+could optimize the structural configuration and mechanical properties of resulting products.The reason was mainly because that the introduction of Li+would reduce the thermal expansion coefficient of the matrix itself,which improved the thermodynamic compatibility between the matrix and the fiber.At the same time,it also could effectively inhibit the interfacial reaction between the fiber and the matrix in the high temperature environment,which also contributed to the maintenance and improvement of composites’ properties.