Study On Diffusion And Mechanical Properties Of Cement Pastes With A Microstructure Evolution Model

Cement is the cementitious material of concrete,the microstructure of cement paste is closely related to the construction quality and durability of hydraulic concrete structures.In essence,it is necessary to grasp the relationship between characteristics of microstructure evolution and the macro and meso properties of cement pastes for solving many engineering problems related to hydraulic concrete structures.At present,effects of the real morphology of the microstructure are less considered in studies on properties of cement-based materials.By means of the rebuilt microstructure model,this paper chooses the topic about the microstructure of cement pastes and studies in aspect of the microstructure description and material properties of cement pastes.This paper focuses on influences of these main concerns about material properties and environmental conditions on the hydration kinetics,pore structure characteristics,diffusion and mechanical properties of cement pastes.The purpose of this paper is to understand the mechanism that influences the performance of cement paste on the microscale.The main research work and innovation achievements are as follows:(1).A three-dimensional microstructure model combined with the introduced“effective diffusivity” hydration model is built to describe the microstructure evolution of hydrated cement pastes.This model could reflect different hydration rates of cement particles with different particle size and consider effects of growing particles interactions.In reference to physical meanings of parameters in the introduced hydration model,this paper thoroughly analyzes these parameters related to the diffusion process and makes the modified hydration model compatible with a system with different particle sizes.Moreover,it should accurately grasp influences of cement fineness,water cement ratio and environmental temperature and humidity on the hydration rate in early and long age and correctly drive the formation of microstructures of cement pastes.The virtual three-dimensional microstructure of cement pastes generated by this model is characterized by methods such as the image processing technology and combustion algorithm in this paper.Effects of the volume fraction of hydrated crystals nucleus on virtual microstructures are discussed.By calibrating the volume fraction,the simulated pore size distribution could be compared well with the experimental observations.Based on the really growing form of hydration products,outer hydration products in this microstructure model are proposed to form like the “nucleation and growth” manner.The influence of such an algorithm modification on features of virtual microstructures is discussed.In this paper,influences of hydration degree,water cement ratio and cement fineness on the microstructure of cement paste are analyzed.The variations of products distribution and C-S-H density under varied curing temperatures are discussed and the simulated results reflect a coarsening(capillary)pore structure under high curing temperature.The microstructure under different humidity curing conditions is also simulated here.(2).Based on the microstructure model,internal structures of outer products and inner products are built.According to this two-scale cement pastes model,the change of capillary pores during the dissolution of CH and the decalcification of C-S-H is simulated.The characteristic parameters of leached pore structures are consistent with experimental results,which indicates that the proposed simulation framework could discribe the evolution of cappilaty pores with the size from tens of nanometers to several microns in leached cement pastes.Based on the simulation of leached structures,the random walk algorithm is adopted to simulate the ionic diffusivity of cement paste in the process of hydration and leaching,which considers the varied transport properties in capillary and gel pores.Influences of the modified pore structure on diffusivity properties of leached cement paste are thoroughly analyzed and the calculated diffusivity parameters of leached cement paste are in accordance with published experimental results.(3).This paper chooses appropriate geometric parameters of lattice grid and builds a two-dimensional microstructure lattice model to study the mechanical behavior of cement pastes under uniaxial tension.The two dimensional lattice model could reveal the theoretical fracture process of materials in essence and ensure the reliability of research results.By means of lattice model,influences of hydration degree,cement fineness and water cement ratio on the cracking process and mechanical parameters of cement paste under axial tensile are analyzed.The relationship between the microstructure characteristics and the mechanical behavior of cement paste is revealed.