Quantitative Characterization And Numerical Simulation Of Micro-and Meso-structure Of Cement-based Materials

The cement-based material,a typical composite material,is the most widely used construction material worldwide,currently.At mesoscale level,it is often viewed as a three-phase composite composed of cement paste matrix,aggregate and the interfacial transition zone(ITZ)existing between them.It is accepted that,the macro mechanical and transport performance of the materials is closed related and determined by the formation and volume fraction of the compositions.To precisely predict the macro behaviors of the cement based materials at macroscopic level,it is for sure that the microstructure of the phases should be carefully characterized.Thus,this thesis started from the aspects of the cement matrix,aggregate,and ITZ,using Scanning electron microscopy(SEM)and X-ray computed tomography(XCT),these two kinds of direct testing techniques,to quantitatively determine the microstructure of the three phases.By coupling using analytical and numerical method,the forming mechanisms and interactions between different phases were also investigated.Following contents are included in the thesis:(1)Sectional plane analysis based on backscattered electron scanning electron microscope(SEM-BSE)images was used to provide insights into the nature of the pore structure.The effects of the w/c ratio on the total porosity and heterogeneity of the local porosity were analyzed.By coupling using Digital Image Processing(DIP)method,each single pore displayed on BSE images was captured,and the equivalent circular diameter,hydraulic radius,and roundness of each single pore were calculated to describe its size and shape characteristics.The relationship between the pore size and pore shape was also established by comparing the pore size distribution and the shape parameter distributions.Results indicates that pores become less spherical with increasing pore sizes.(2)Mortars were prepared with varying sizes and blended proportions with their air void system quantitatively characterized via X-CT.The role of sizes and blended proportions were combined via calculation of the fine aggregate specific surface area(SSA),and a roughly linear relationship between SSA and global void content was observed.Based on digital processing methods and Spherical Harmonic(SH)functions,the 2D circle diameter distribution and 3D sphere diameter distribution were derived from 2D analysis of each slice and 3D analysis of the stacked 3D microstructure,respectively.Further,the two kinds of distributions were calculated from each other using stereological methods and compared with the originally experimental results.The void size distributions for several of the mortars were obtained via high and low resolution scans,which were compared to verify the reliability of the results obtained.(3)Based on the reconstructed structure,the spatial distribution of air voids was characterized by various spacing factor indices.The effect of sand size and blended proportions were combined into sand specific surface area(SSA)in order to understand the role of SSA in spacing distribution.The spacing factor was defined and evaluated based on void – void proximity and paste – void proximity.For void – void proximity,the distribution was composed of the distances between each void to its nearest neighbor surface or centroid.The count dilation and random points methods were used to compute the paste – void proximity based on the reconstructed microstructure.The distribution obtained from different methods were compared with each other,as well as with analytical results,to verify their reliability.Characteristic parameters were quantitatively derived from the distribution curves for comparison and the relationship between average spacing factor and specific surface area of sand grains was established.The Protected Paste Volume(PPV)concept was used to understand the role of aggregate in determining the thickness of the Sphere of Influence(So I)zone.(4)A model concrete specimen with a single spherical ceramic particle acting as an aggregate is used to study the properties of the ITZ and its uneven distribution around the aggregate based on quantitative analysis of SEM-BSE images.A careful treatment of the statistics of the ITZ was employed.The average porosity of the ITZ at a smooth part of the aggregate surface was smaller than that found at a rough part of the aggregate surface.The effect of fine aggregate size on the formation of ITZ was investigated as well as the overlapping of ITZ between adjacent aggregate particles in real mortars.Via quantitative analysis on the backscattered electron image(BSE),the porosity and thickness of the ITZ was experimentally determined.Nearest surface function was further introduced to calculate the volume fraction of the ITZ based on the obtained thickness.The spacing factor between adjacent aggregate particles was defined and numerically calculated,which was further compared with value of ITZ thickness and inflection point for overlapping to understand the role of aggregate size on the overlapping phenomenon of ITZ.(5)Based on experimental results,a method is proposed to describe the local surface roughness(SR)at the pixel level and relate this quantity to other ITZ properties.K-means clustering method was further introduced to subdivide the surfaces into rough and smooth clusters.The microstructure of the ITZ in smooth and rough surfaces were further compared,and results indicate a more porous microstructure tends to form around the rough surfaces.Besides,a numerical method was introduced to create surfaces with varying surface roughness,and the packing density of the cement particle in the near-wall region was determined to illustrate the role of surface roughness in the formation of the ITZ.