Numerical Modelling On Diffusivity And Microstructures Of Cementitious Composites Via Nonspherical Particle Packing Model

It is a huge challenge to quantitatively characterize the influence of the microstructures of concrete on its diffusion behaviour through conventional experimental methods,because of the complexity of components and the diversity of factors affecting microstructures in concrete.Currently,it has been expecting a critical study how to develop effective characterizing strategies to quantitatively illuminate the dependence of the diffusivity of concrete on its microstructures.As a typical class of particle-reinforced composites,concrete can be regarded as random packings of hard anisotropic-shaped particles(e.g.,cement particles,aggregates,supplementary admixtures,etc.)from the microscopic view.The evolution of microstructures of concrete is,to some extent,dependent on its initial random packing structures,which is further determining the progression of its diffusion property.In preliminary studies,the random packing of spherical particles has been broadly investigated.However,realistic grains in practical concrete are normally nonspherical shape,rather than the simple spherical shape.Therefore,it is very crucial to explore and characterize the effect of particle shape on microstructure and the diffusivity of concrete.In this thesis,we firstly develop a novel overlapping detection algorithm for inter-elliptical/ellipsoidal particles,and then simulate random packing behaviors of mono-/multi-sized elliptical and ellipsoidal particles.Afterwards,the spherical harmonic function is used to construct a complex non-convex-shaped particle,namely,the star-shaped particle.The overlap detection for such non-convex particles is also explored by using a numerical strategy.Moreover,by the well-known random sequential packing scheme,we present the random packing of non-convex-shaped particles.In order to obtain the two-dimensional cross-section information from the three-dimentional packing structure,a sectional plane analysis algorithm suitable for arbitrary shaped particle is proposed.On the other hand,based on the method of Minkowski sum,the topological geometry of an interfacial transition zone(ITZ)with a constant thickness around regular polygon,ellipse,Platonic and ellipsoid particle are perfectly constructed.Based on this,we propose two algorithms to obtain the apparent ITZ thikcness:systematic line sampling(SLS)and normal line sampling(NLS).The results show that aggregate shape has a significant impact on the apparent ITZ thickness through the study of mono-sized regular polygon and Platonic particles,which mainly reflects in the circularity or sphericity of the aggregate shape.The overestimation degree of the ITZ thickness reduces with increasing circularity(or sphericity).This raises the question of whether circularity(or sphericity)is the unique parameter which can characterize the influence of aggregate shape on the overestimation degree of the ITZ thickness.Thus,different particle shape with the same circularity(or sphericity)needs to be considered.This paper intends to investigate the influence of the particle shape for ellipse and regular polygon with matching circularity,spheroidal and Platonic particles with matching sphericity.Results by using SLS algorithm indicate that the ranking of the overestimation degree of ITZ thickness follow:regular polygon>ellipse,Platonic particle>Oblate ellipsoid>Prolate ellipsoid,while the result by using NLS algorithm is:Oblate ellipsoid>Sphere>Prolate ellipsoid.However,there are no obvious rule between Platonic and spheroidal particle in NLS algorithm.This means the sphericity of a particle is an important parameter to quantitatively generalize the effect of particle shape,but it is not a unique one.Nevertheless,for the convenience of application,It is worth mentioning that the unified formulas for the overestimation degrees of ITZ thickness and fraction are given(based on the parameters of particle:circularity or sphericity),respectively.In addition,for multi-sized aggregate particle system,the overestimation degree of the statistical average of the apparent ITZ thickness decrease with increasing aggregate fineness.Subsequently,we propose and verify the normalized formula of ITZ area fraction and ITZ volume fraction.Meanwhile,it reveals that the overestimation degree of the ITZ area or volume fraction decrease with increasing aggregate volume fraction,aggregate fineness and the actual ITZ thickness.Furthermore,incorperated into the ITZ microstructural characteristics described above,the overestimation degree of the diffusivity of concrete is evaluated based on differential effective medium model.Such the theoretical model illustrates the effects of ITZ thickness and ITZ volume fraction,aggregate shape,fineness and fraction,and the diffusivity ratio of ITZ to cement paste on the overall diffusivity of concrete.Results show that the overestimation degree of the diffusivity of concrete increases at first and subsequently drop down with increasing aggregate volume fraction.The overestimation degree of the diffusivity decrease with increasing sphericity When ?agg<0.74.Results by using SLS algorithm indicate that the ranking of the overestimation degree of the diffusivity follow:Platonic particle>Oblate ellipsoid>Prolate ellipsoid.If the sphericity of the particle is 0.671 or 0.806,the ranking is:Oblate ellipsoid>Tetrahedron or cube>Prolate ellipsoid;if the sphericity of the particle is 0.939,the ranking is:Oblate ellipsoid>Prolate ellipsoid>Icosahedron.Finally,a hydration model of cement particle is established to simulate the hydration reaction behavior of Platonic particle via the inward shrinkage and outward expansion methods(erosion and dilation based on Minkowski sum and substract approach).Then,the hydration modeling is developed based on the combination of the above morphological approach and the hydration modeling framework of HYMOSTRUC.It is found that the cement particle shape has a significant impact on the hydration of cement and microstructure and transport of cement paste,mainly reflect on the degree of hydration increase with decreasing of sphericity of cement particle under the same w/c ratio and hydration time conditions.It also indicates that the porosity of the cement paste increase with increasing sphericity of the particle and w/c ratio,and relative diffusivity of hardened cement paste reduces with increasing degree of hydration,with decreasing sphericity of the particle andw/c ratio.The results of this thesis indicate that particle shape has a significant effect on both the microstructure and macroscopic properties.So,it is necessary to consider the possible influence of the particle shape on the quantitative relationship of composition-microstructure-properties of cementitious composites.