基于孔隙形态学特征的水泥浆体渗流指数

Percolation Exponent of Cement Paste Based on Pore Morphology

渗流指数n是通用有效介质方程中的重要参数,其可以对水泥浆体中微观孔隙结构的扩散性能进行表征。为了明晰渗流指数的物理学含义并准确计算不同配合比的水泥浆体在不同水化程度时的渗流指数,对水灰比0.30、0.35、0.40、0.45、0.50水泥浆体试件在水化龄期28、60、90、120、180 d时进行NEL(Nernst-Einstein Lab)测试和核磁共振测试得到其对应的离子扩散系数和孔隙参数。结果表明:随着水化龄期的推进,水泥浆体的毛细孔隙率逐渐减小,2.5~10.0 nm孔径凝胶孔隙率先快速增大后缓慢减小,水泥浆体中的离子扩散为毛细孔中的自由扩散作用和凝胶孔中的辅助扩散作用的耦合;不同水灰比的水泥浆体渗流指数n随着水化程度的增加呈现不同的变化规律,水泥浆体材料渗流指数n与水泥浆体形态学参数具有强相关性;从水泥浆体孔隙结构体系的形态学层面对水泥浆体材料渗流指数n所呈现的变化规律进行了解释,并将实验数据进行拟合得到不同水灰比水泥浆体渗流指数n与水化程度的定量关系式。

Introduction Accurately and effectively predicting ionic diffusion coefficient of cement-based materials is one of the crucial issues in the study of concrete durability.Recently,many novel prediction methods based on General Effective Media(GEM)equation have been proposed and their applicability has been verified.As a significant parameter in diffusivity models based on GEM equation,percolation exponent n characterizes the diffusion properties of micro-pore structures in cement paste.The value of percolation exponent n is affected by the shape and direction of the medium,namely the morphological characteristics of micro-pore structures in cement paste.With the hydration process,the material composition and pore structure of cement change greatly,as well as the value of percolation exponent n.The value-taking of percolation exponent n has a great influence on the accuracy of the diffusion prediction model.However,a systematic study to reveal its physical meaning and determine its value is deficient.In this paper,the strong correlation between percolation exponent n and the morphological characteristic of cement paste was verified,and quantitative expressions of percolation exponent n was given by fitting experimental data.Methods Reference cement complying GB8076-2008 was used to prepare 25 groups of cylindrical cement paste specimens(Φ=100 mm,h=200 mm)with water-cement ratios(w/c ratio)of 0.30/0.35/0.40/0.45/0.50 and curing periods of 28/60/90/120/180 d,respectively.There were 3 specimens in each group,2 of which were cut in the center of specimen height(50-150 mm)to obtain 4standard test specimens(Φ=100 mm,h=50 mm)for NEL test,and 1 of which was drilled at the center of height and section of specimen to obtain a cylinder(Φ=32 mm,h=50 mm)for low-field NMR(Nuclear Magnetic Resonance)test.Saturated saline water was saturated into specimens for NEL test using the NELD-CCM vacuum water saturation instrument at vacuum degree of 87%.The NELD-CCM540 cement chloride ion effective diffusion coefficient tester was used in NEL test,4 specimens in a group were tested and the test data was averaged as the representative effective diffusion coefficient.To determine the pore characteristics of specimens in each group,NMR test was conducted for corresponding specimen twice.For the first time,deionized water was saturated into specimens for NMR test using NELD-CCM vacuum water saturation instrument at vacuum degree of 98%,and NMR test was performed for saturated specimen to obtain total transverse relaxation time spectra of water in pore and chemically combined water.The specimen was then dried in an oven at 110℃for 12 h and placed for 3 h to reduce the sample temperature to room temperature.NMR test was performed again for dry specimen to get transverse relaxation time spectra of chemically combined water.Combined with total spectra,pore size distribution of tested specimen was available.Moreover,the porosity of specimen was obtained by establishing the quantitative relation between water volume and nuclear magnetic signal.Results and discussion For all groups of specimens,the effective diffusion coefficient increases firstly and then decreases with hydration process.Considering the change of gel pores and capillary pores quantity and effective diffusion coefficient data simultaneously,it can be known that the auxiliary effect of gel pores on diffusion cannot be ignored.Capillary porosity of all specimens keeps decreasing,while the 2.5-10.0 nm gel porosity increases greatly in the first 60 d of curing and decreases at a relatively smaller rate after 60 d of curing.With the increase of w/c ratio of cement paste,the proportion of diffusion effect of capillary pores to that of gel pores increase.As the hydration process carries on,the pore characteristics of the specimens shows the following rules:the continuous pore size s_(c)of the pore structure system decreases for specimens with w/c ratio of0.30/0.35/0.40/0.45/0.50,and the continuous pore size s_(c)of the pore structure system changes to a relatively small extent for specimens with w/c ratio of 0.50;the tortuosity factorτof pore structure system decreases at first and then increases,which is influenced by coupling effect of hydration products formation and morphological difference between two kinds of pores.The sample correlation matrix between percolation exponent n,continuous pore size s_(c,)and tortuosity factorτreflects the strong correlation between these three parameters.Percolation exponent n is negatively correlated with continuous pore size s_c,and positively correlated with the tortuosity factorτ.The results prove the morphological meaning at physical level of percolation exponent n.With the increased hydration degree,the percolation exponent n of cement paste with different w/c ratio shows different variation:the percolation exponent n of cement paste with w/c ratio of 0.30 and 0.35 increases,the percolation exponent n of cement paste with w/c ratio of 0.40 and 0.45 decreases at the beginning and then increases,and the percolation exponent n of cement paste with w/c ratio of 0.50 decreases.The different trends are primarily due to different diffusion effect ratios between capillary pores and gel pores.Conclusions Based on the theory of micropore structure analysis,the relationship between percolation exponent n and diffusion property of cement paste is explained from the aspect of mechanism.For cement paste with different w/c,the difference in ratio between capillary porosity and gel porosity leads to the difference in change law of pore characteristic parameters and percolation exponent n.With the experimental results,the relational expression of function between percolation exponent n and hydration degree was obtained.