Investigation On Early Hydration Process And Pore Structure Evolution Of Cement-Based Materials By Impedance Technique

Until now,cement-based materials are still used widely in construction of infrastructures in hydraulic engineering,civil engineering and national defence.These infrastructures from different fields will confront different severe environment in our country,and thus,the macroscopic physical properties of cementitious materials,such as strength,permeability and durability,should be strengthen to meet the higher requirement.The hydration process and pore evolution of cementitious materials always influence their macroscopic physical properties.Therefore,to improve the macroscopic physical properties of cementitious materials,the hydration mechanism and pore evolution rule should be clarified.For this purpose,based on the impedance technique,the hydration process is studied through electrical viewpoint,and the pore structure evolution is investigated tentatively by fractal theory.The main contents of investigation and corresponding findings are as follows:1.At the initial stage of hydration,the cement particles are dissolved and release the charged ions into paste solution.The impedance modulus of pure cement paste decreases with the hydration and pure cement paste can be regared as a pure resistance approximately at this moment.When the impedance modulus decreases to the minimal value,the solution of pure cement paste reaches the saturation.Subsequently,the impedance modulus of pure cement paste grows up rapidly,and the impedance curves with different frequencies separate increasingly for each other.This phenomenon points out that a mass of solid phase hydrate is generated in the pure cement paste,and pore structure of pure cement paste is formed fundamentally.The early hydration of metakaolin blended cement paste can be divided into four stages:first dissolution stage,accelerative stage,second dissolution stage and hardened stage.Comparing with pure cement paste,the second dissolution stage is the characteristic hydration stage of matekaolin blended cement paste.This hydation stage is dominated primarily by pozzolanic reaction of metakaolin in the suitable alkaline paste soluion.The early hydation of high content limestone powder blended cement paste can also be divided into four stages:dissolution stage,accelerative stage,diffusion-dominating stage and hardened stage.By comparison with pure cement paste,dynamic balance stage is the characteristic hydration of high content limestone powder blended cement paste.This hydration stage is present only when the content of limestone powder is higher than 50%.2.Due to the fractal characteristic of cement-based materials pore structure and high ratio of small pore quantity in pore structure,the fractal tree-like model is adopted to model cement-based materials pore structure.Based on fractal tree-like model,pore structure network and relevant fractal electrical network are constructed,and thus,the pore structure of cement-based materials can be related to their impedance responses.Accounting to the multi-level and multi-branch of these two networks and fractal scaling law,the formulas of pore structure parameters including porosity,pore tortuosity and pore size distribution,are deduced systematically.The evolution of different pore structure parameters with hydration process is studied,and the results from modelling are comparing with those from Mercury Intrusion Porosimetry(MIP)testing,which highlights that impedance method is rapid and nondestructive.Additionally,by fractal model of the cement-based materials pore structure based on thermodynamic relation,it can be found that the pore surface fractal dimension of metakaolin blended cement paste grows increasingly as the hydration development.And 10%content of metakaolin is optimal for refining the pore structure of blended paste.3.From the research results of impedance method,both slag powder and polycarboxylate superplasticizer can retard the early hydration of cement pastes,and the effect of slag powder on cement early hydration is better than that of superplasticizer.In addition,the results from testing of cumulative hydration heat evolution,hydration heat releasing rate,compressive strength and setting time of cement pastes,verify the conclutions derived from impedance method.The retard effect of slag powder on cement hydration results from the dilution effect of slag powder on cement which leads to the insufficience of calcium hydroxide in cement pastes and incomplete activation of pozzolanic reaction of slag powder.The retard effect of polycarboxylate superplasticizer on cement hydration is due to the adsorption of polycarboxylate superplasticizer on surface of cement particles which generates the site-possessing in the space and hinders the precipitation of ions in the cement pastes.Furthormove,the quantitative relationship between cumulative hydration heat and pore volume is established preliminarily,and their relationship seems to be good linearly.4.According to fractal leaf vein network proposed,the effects of pore structure parameters and electrical conductivity of solution on the electrical properties of fractal leaf vein network are investigated,which is expected to give some suggestions for studying the pore structures of cementitious materials.Both the increase of initial pore diameter and the decrease of pore length of fractal leaf vein model means that the difficulty of ions migration is lowered,resulting in the reduction of total equivalent electrical resistance.The effects of scaling factors of pore diameter and pore length on total equivalent electrical resistance are similar with initial pore diameter and pore length,respectively.With the increment of level of fractal leaf vein network and the decrement of porosity of fractal leaf vein model,pore structure will become dense,which leads to the growth of total equivalent electrical resistance.Moreover,when the fracal dimension of pore size distribution and the conductivity of electrolyte solution increase,the total equivalent electrical resistance will decrease increasingly.The effects of concentration and temperature of electrolyte solution and applied frequency on the real part and imaginary part of electrical conductivity are studied.The rise of electrolyte temperature results in the increase of activation energy of ions in electrolyte solution and the acceleration of ions migration,which is beneficial to increase both the real part and imaginary part of electrical conductivity.The effects of electrolyte concentration on the real part electrical conductivity are the same with those of electrolyte temperature,while the relationship between electrolyte concentration and its imaginary part is irregular,which maybe associate with the relative small value of conductivity imaginary part and its high sensibility to factors.In addition,the imaginary part of electrolyte solution grows with the increment of applied frequency,however,the real part of electrolyte solution can not be influented by applied frequency.