Investigation Of The Mesco-Mechanical Properties Of Calcium-Silicate-Hydrate

Calcium Silicate Hydrate(C-S-H)gel(C: CaO,S: SiO2,and H: H2O),the main binding phase,forms up to about 60-70% of volume of the paste and is responsible for most of the properties of cement-based materials.However,C-S-H gel is not a stable phase.One can realize it is a continuous layer which can bind the cement particles together at the nanoscale level.The nano-scale structures of C-S-H gel plays a significant relationship with the hydration degree,macro-mechanical and durability properties of cement-based materials.However,the relationship between composition,nano-structure and mechanical properties is still enigmatic.Furthemore,cement science cannot go far without quantitatively understanding the relationship between structure of C-S-H and macro mechanical and durability properties of cement-based materials at the nanoscale level.the effect of changes in the mescostructure of the C-S-H gel still requires further study.We need to get detailed insights into how the transformation between LD C-S-H and HD C-S-H gel,the changes of nanostructure characteristics of C-S-H gel and the influence between C-S-H gel and macroscopic properties of cement-based materials during hydration process.Experimental and theoretical methods are playing an important role in advancing understanding of C-S-H gel.Here,I employ mesco-computational modeling and mesco-theoretical mechanism to analyze the role of the pH value,combined water,transformation between LD and HD C-S-H gel on the physical and mechanical behavior of C-S-H gel and to get a mescoscopic understanding of the relationship between C-S-H gel and macro mechanical and durability properties of cement-based materials.The aim of this graduation thesis is to address the pores and transformation of different density of C-S-H gel effects on the cohesion within and between cement grains and to predict any changes in the structural properties of C-S-H gel,hydration degree and macro-characteristics of cement-based materials.This thesis provides a relationship between the nanostructure,the transformation factor,freeze-thaw damage factor and the cohesion of C–S–H gel with the simulation of hydration degree,mechanical and durability characteristics of cement mortar sample within the standard curing of 56 days.The mainly includes of this thesis is as the following:1.Meso-transformation mechanism of different density of C-S-H gelsBased on the macro-and mesco-experiments,the pH value,surface water content and morphology of crystal were observed.The formulas of combined water and pH value during the transformation of differnet desity of C-S-H gels were determined.Secondly,the determination equations were corrected through the TG-DTA test results.Then,based on the theory of chemical thermodynamics,the mechanism of the conversion between C-S-H(I)and C-S-H(II)gels was established and the thermodynamic equilibrium constants for different types of C-S-H gel transitions were obtained.Finally,the relationship between the mechanical parameters of C-S-H gel and cement mortar was established based on the conversion equation of C-S-H gel.2.Meso-kinetic mechanism of cement hydrationBased on the changes of liquid phase,gas phase,solid phase and pore structure during the hydration process,the mesoscopic kinetic equation of cement hydration reaction and hydration degree were established.Secondly,the dynamic conservation equations and the mesoscopic kinetic equations of the hydrate phases were proposed based on the hydration reaction.Simultaneously,a mesoscopic calculation equation for the degree of hydration was deduced considering the influence of the dynamics of water binding and the redistribution of pores.Finally,a nonlinear relationship between the degree of hydration and the macroscopic peak stress of cement mortar was established.3.Hydration Cohesion Equilibrium Equation of Cement-based materialsBased on the theory of meso-mechanical homogenization,the hydration cohesion equation of cement mortar was established.Then,the macroscopic compressive strength and flexural strength of the mortar are calculated by the hydration cohesion equation combined with the influence of the effects of stress gradient,loading time scale,relaxation time scale,pore structure expansion and peak stress of cement mortar.4.Mescoscopic damage mechanism of cement-based materials subjected to freeze-thaw cycelsThe new meso-damage model of cement-based materials subjected to 30 freeze-thaw cycls was determined based on the changes of the characteristics of pore structures.Then,the inner stress and strain of the pore structure of cement specimens sbujected to 30 freeze-thaw cycles were obtained combinong with the changes of hydrostatic pressure,crystallization pressure,low temperature adsorption pressure and hydration cohesion equation of cement mortar.Finally,based on the viscoelastic constitutive relation of cement mortar,the one-dimensional strain calculation equation of cement mortar after freeze-thaw cycles was was simulated with the results of the strain of pore structure.In summary,the aim of the current thesis is to address the effect of mesoscopic characteristics such as molar concentration,combined water rate,pore structure distribution and other characteristics of the cement mortar specimens after 56 cyring days to obtain the conversion equation between different types of C-S-H gels.Simultaneously,based on the conversion equation between different types of C-S-H gels,the mesoscopic hydration kinetics equation,mesoscopic calculation equation of cement hydration degree and the macro peak stress calculation equation of cement mortar twere obtained.Finally,based on the homogenization theory in mesoscopic mechanics,a mesomecular calculation equation for the hydration cohesion of cement mortar.Simultaneously,a multi-scale relationship was established between the hydration cohesion of cement mortar and the macromechanical properties or freeze-thaw damage of cement mortar.These new mesoscopic models or equcations provide a theoretical and experimental combination with multi-disciplinary integration methods or references for the further study of the macroscopic properties of cement-based materials based on the meso-structure evolution which is also the essence of cross-scale research on cement-based materials.