Study On Hydration And Permeability Of Cement-Based Materials

With the wide use of high performance concrete in bridges,high-rise buildings and ocean engineering,the damage to the construction structure caused by the durability failurs of concrete occurs frequently,which causes huge economic losses and brings a great potential safety risks.Among the various factors causing the failure of the durability of concrete,the corrosion of steel bars caused by the erosion of the chloride is the main factor that caused the destruction of most concrete structures.In this paper,based on the electrical resistivity method,the relationship between hydration and permeability of the cement-based materials was analyzed,and the advantages and disadvantages of different research methods were discussed.The main contents and conclusions can be summarized as follows.(1)The heat of hydration and electrical resistivity of the cement pastes with W/C of0.30,0.35,0.40,0.45 and 0.50(P0.30,P0.35,P0.40,P0.45 and P0.50)within 3d were measured,and the chloride ion diffusion coefficient of the pastes and concretes cured at 3d,7d,14d,28d,56d and 80d were tested.The results showed that with the increase of W/C,the heat of hydration and the chloride ion diffusion coefficient increased,and the electrical resistivity decreased at 3d.The samples with high W/C contained more water per unit volume so that average distance between cement particles was larger and more mineral ions dissolved,which made the hydration reaction more completely and heat of hydration was higher.The time for sample P0.50 to reach the peak of heat release rate of hydration was longest,which was 17.7h,while sample P0.30 reached the peak of rate at 14.5h.As for electrical resistivity results,because of the highest initial porosity and the most liquid space,the electrical resistivity of sample P0.50 developed most slowly.Based on the chloride ion diffusion coefficient of sample P0.30,the value of chloride diffusion coefficient of sample P0.35 increased 28%at 3d,while the chloride diffusion coefficient of sample P0.40 increased 140%,which indicated that it was not enough to make up for the high permeability caused by the high initial porosity,although its higher W/C and the degree of hydration.(2)The heat of hydration and electrical resistivity of the cement-based materials at hydration temperature of 10?,20?.30? and 40? within 3d were measured.The results showed that with the increase of the temperature,the heat of hydration and electrical resistivity increased.When the hydration temperature of sample P0.30 increased from 10? to 40?,the time to the rate peak of heat of hydration was shortened from 28.5h to 5.1 h and the value of the peak increased from 1.105×10-3W/g to 8.622×10-3W/g.According to Arrhenius's law,the activation energy of the hydration reaction within 3days was calculated by the results of heat of hydration and electrical resistivity at different temperatures.And the results were 30.18kJ/mol and 27.2kJ/mol respectively,which indicated that the results of two methods were reliable.(3)According to the characteristic points in the hydration reaction,the setting time of cement-based materials was predicted.The appearance of the characteristic points in the hydration reaction indicated that a new physical or chemical reaction had occurred.And there was the relation between the time to reach the peak of hydration and the setting time:tinitial=L33tpeak-798(R2-982)and tfinal=1.45tpeak-997(R2=0.968),and the relation between the characteristic points of electrical resistivity and the setting time was tinitial=2.48t?min-170(R2=0.9998)and tfinal=0.82tmax-401(R2=0.9413).Therefore,there was a good linear relationship between the setting time and the time of characteristic points on the heat development or the electrical resistivity response during hydration process Based on this conclusion,the setting time could be predicted by testing the heat of hydration or electrical resistivity.(4)The chloride ion diffusion coefficient of cement-based materials with different dosages of aggregate was measured by rapid chloride method(RCM)and non-contact electrical resistivity method(NCER)respectively.The results showed that for the paste samples,the values of DRcm were 1.22.4 times of DNCER at curing ages of 3d,7d,14d,28d,56d and 80d,and the relation between two methods was DRCM?2.625DNCER-0.604.For concrete samples with 50%of aggregates by volume,the values of DRCM were 2.8-5.2 times of DNCER,and the relationship between two methods was DRCM=3.772DNCER+0.057.It showed that there was a good correlation between the DRCM and the DNCER results.The chloride ion diffusion of concrete was by COMSOL Multiphysics software based on the test results of chloride ion diffusion of concrete.(5)The relationship between porosity and permeability was studied by Powers model and the porosity test.The results showed that there was a good linear relation between the capillary porosity and the chloride ion diffusion coefficient of the cement pastes with various W/Cs at the same age,but there was not an obvious correlation at different ages.The permeability of cement-based materials was not only related to the porosity,but also related to the microstructure and distribution of pore structure etc.