Study On The Early Reaction Mechanism Of Metakaolin-based Geopolymers

Geopolymer is a new type of aluminosilicate construction material that is eco-friendly and energy-saving.The early geopolymerization process is the important stage in determining the structure and properties of geopolymers.It is of great significance to study the mechanism and regulation of the reaction process to promote the wide application of geopolymer materials.In order to strengthen the research on the stages and characteristics of early geopolymerization,this thesis focuses on metakaolin-based geopolymer,and conducts research on the dissolution characteristics of precursor,chemical polymerization mode,spatial evolution of phases,and reaction characteristics of each stage.The study revealed two chemical polymerization modes,three spontaneous dehydration mechanisms,and the formation mechanisms of three types of pores in the early geopolymerization.The spatial construction rules are proposed for the generation and accumulation of gels.The research about physicochemical change mechanisms and key influencing factors of six stages in early geopolymerization is improved.The major contributions of the work presented in this thsis are listed as follows:(1)Through the dissolution rates and correlations of silicon and aluminum ions in metakaolin activated by different activators,this thesis analyzes the dissolution characteristics of metakaolin and the adjustment effect of each ion in alkali activator on the dissolution effect.It is found that the dissolution rate of aluminum ions in metakaolin is faster than silicon ions and the dissolution rates of silicon and aluminum ions maintain fixed ratios.Abundant silicate ions provided by alkali-activator can advance the chemical polymerization process,and can inhibit and delay the dissolution of silicon element through dissolution equilibrium.(2)The evolution processes of the phase,characteristic functional groups and tetrahedral units in the reaction system are studied,to explore the mechanism of chemical polymerization and structural damage.It is found that dehydration will reduce the crystallinity,and cause the chemical polymerization mode to chang from free and ordered links to nearby disordered links,to repair the local defects and damages of framework.Those two polymerization modes are both accompanied by chemical dehydration respectively,and the loss of chemically combined water is completed earlier than the dehydroxylation.(3)The volume stability of matrix and the spatial evolution of solid and liquid phases are characterized,to analyze the process of gel growth and the pore structure formation.It is found that the growth of gel particles to form a solid matrix follows the spatial law that gels fix residues and solidify to form a framework,and then new gels densely fill the framework.As the gels grow,the non-uniformity of capillary pressure and matrix strength produces non-uniform stress and structural damage,shown as the evolution from gel pore,collapsed pore to torn pore.(4)Through the characteristics and indicators of solidification and dehydration in early geopolymerization,this thesis analyzes the distinction basis and staged characteristics of each reaction stage.It is found that the early gepolymerization can be divided into six chemical stages: dissolution and dismantling,nucleation,free ordered polymerization in shortrange,restricted ordered polymerization in shortrange,second dissolution equilibrium,disordered polymerization in long range.The characteristics of those six stages are respectively thick paste,initial setting and solidifaction,bleeding pores on the surface,autogenous shrinkage of matrix,low water loss rate,and secondary increase in strength.