Study On Microscopic Characteristics And Pore Evolution Mechanism Of Cement-based Materials Based On Micro-CT

The mechanical properties and frost resistance of cement-based materials are closely related to their microstructure,which includes pore structure in cement matrix,spatial distribution of reinforcing material,bonding between matrix materials and reinforcing materials and so on.Therefore,the analysis of the internal structure of cement-based materials from a microscopic scale is of great significance for understanding the macroscopic properties of materials.In this paper,the pore structure of cement stone and hardened cement mortar are taken as the research object.With the help of Micro-CT,it is respectively the changes of pore structure of cement stone during curing as well as pore structure of hardened cement mortar under freeze-thaw cycles that are studied.The strength strengthening mechanism of cement stone during curing and freeze-thaw damage mechanism of hardened cement mortar are explored from the pore scale.The main research contents are as follows:(1)The CT images of cement stone at submicron scale(0.697?m)were obtained,and the microstructure characteristics of cement stone as well as its evolution process during curing were described in detail.The representative elementary volume(REV)was taken from the 3D image,and pore space was extracted by the tangent-slope gray-level thresholding method while representativeness of the selected REV was verified by taking porosity as the index.Porosity,pore volume and pore surface area were calculated by counting the number of pore voxel,pore diameter and average pore diameter were calculated by local thickness method,pore connectivity was calculated by flood fill algorithm.Hydration degree of cement was determined by the ratio of the volume of hydrated cement particles to the total cement volume.The influence of water-cement ratio on pore structure and hydration degree of cement paste was studied.Combined with the results of compressive strength test,the strengthening mechanism of cement paste strength and the influence mechanism of water-cement ratio during curing were discussed.(2)The CT images of hardened cement mortar with micron scale(3.344 m)were obtained,and pore distribution characteristics and development characteristics of micro-cracks of hardened cement mortar under freeze-thaw cycles were described in detail.The representative elementary volume(REV)was taken from the 3D image,and pore space was extracted by the tangent-slope gray-level thresholding method while representativeness of the selected REV was verified by taking fine aggregate volume fraction as the index.Various pore parameters,such as porosity,pore volume,pore diameter,pore surface area,average pore diameter and pore connectivity,were calculated,and the evolution law of pore parameters of hardened cement mortar under freeze-thaw cycles and the influence of water-cement ratio on pore structure were studied.With fractal dimension as the statistical damage value,the damage degree of hardened mortar samples under different freeze-thaw cycles was measured using the box counting method based on Matlab,and the influence mechanism of water-cement ratio on the degree of freeze-thaw damage was analyzed.(3)The interfacial zone between cement matrix and fine aggregate was accurately identified,and the CT slice of hardened cement mortar was composed of four-value pixels by using digital image processing technology and self-programming based on Matlab.Two-dimensional finite element model was generated from the four-valued image based on the method of mesh mapping.Numerical simulation of hardened cement mortar under freeze-thaw cycles was carried out based on the microscopic model,and the stress and strain distribution as well as variation characteristics inside the material during freeze-thaw were obtained.