Experimental Study On Relationship Between Micro-porosity Fractal Characteristics And Compressive Strength Of Cement-based Materials

Pore structure is the main factor affecting the strength of cement mortar and concrete.Initially,porosity was the only consideration for the strength uniformity of cement-based materials identified by many researchers,but as the study progressed,it was found that the pore size distribution also had a significant effect on the strength.Although researchers have conducted a large number of experimental studies on the relationship between different characterization parameters of pore structure and compressive strength,these parameter models that represent pore size distribution are still unable to accurately simulate the relationship between strength and pore structure.Therefore,many researchers focus on exploring the relationship between strength and pore distribution through different methods.In order to improve the reliability of microscopic pore structure and strength model,this research topic starts from cement paste,analyzes the evolution law of compressive strength and the characteristics of microscopic pore structure change,and explains the time evolution of compressive strength from microscopic pore structure characteristics.Then,establish the relationship between microscopic pore structure characterization parameters(porosity,average radius,fractal dimension)and compressive strength and fit the regression equation.What's more,Analyze and compare the optimal pore structure characterization parameters to describe more effectively and accurately.The distribution of the size distribution between microscopic pore structure and compressive strength in cement-based materials.In addition,through the analysis of the microscopic pore structure characteristics and compressive strength of 10different strength grades of concrete,the conclusions in the cement paste were further analyzed and verified.Through comparative analysis of concrete and cement paste,a more accurate and powerful relationship model of the pore structure distribution characteristics and compressive strength of cement-based materials were obtained.The result of research are as following:(1)In cement paste and concrete,both porosity and average pore size show a good negative correlation with fractal dimension.The larger the fractal dimension,the larger the number of micropores in space.Therefore,fractal dimension of pore volume can be regarded as a comprehensive parameter of pore shape and spatial distribution of pore structure.For different parameters of pore structure,fractal dimension of pore volume can more accurately reflect pore structure characteristics.(2)According to the regression relationship between porosity,average pore diameter,fractal dimension and compressive strength,it is found that porosity,average pore diameter and fractal dimension all have a power exponential function with compressive strength.But fractal dimension has better correlation with compressive strength than the former two.Therefore,fractal dimension can more accurately characterize the relationship between pore structure characteristics and compressive strength.(3)It is found that the fractal dimension of the volume of cement paste and concrete has a positive correlation with the compressive strength.The fractal dimension increases with the improvement of compressive strength.The fractal dimension becomes larger with the increase of compressive strength,which is consistent with the power function relation?=aD~b,where?is compressive strength,D is the fractal dimension,a,b are regression parameters,which are0.0548,6.3847 in cement paste and 6.9153,5.1554 in concrete,respectively.Therefore,the fractal dimension of mesoporous volume of cement-based materials in practical projects can be used to predict the compressive strength by calculating the fractal dimension of mesoporous volume of materials.