Numerical Simulation Of Transport Properties And Hydration Process Of Cement-Based Materials Reconstructed By Irregular Particles

The macroscopic properties of the specimen are determined by phase composition and spatial distribution in hydration microstructure of the cement-based composite. The hydration process is a multi-phase, multi-component process couple with complicated physical and chemical reactions at different scales. The chemical and mineral admixtures used in modern high-performance cement-based materials make the hydration process and microstructure evolution process more complicated. In recent decades, it is possible to simulate the hydration process and microstructure evolution of cement materials with the rapid development of computer technology. Several mature paste hydration models at home and abroad, such as HymoStruc model, ?ic model, HydratiCA model CEMHYD3D model, make mock object into spherical particles. The approximation restrict their hydration precision and limit application scope. Actually, the cement based composite material is a packing result by irregular particles in microscopic and mesoscopic scale. This paper aims to develop a 2D and 3D reconstruction method of irregular particles and introduces the method into CEMHYD3D model, then establishs a more perfect paste hydration model. Based on the multi-scale transition theory, this paper aims to divide the cement based composite material into four scales and establish corresponding scale microstructure model. The steady and unsteady chlorine ion transport process in saturated various scale microstructure are further simulated by numerical method.First of all, based on cellular automata theory, this paper developed a 2D and 3D reconstruction method of irregular particles controlled by appropriate eigenvalue vector, with a good repeatability. The paste initial microstructure reconstructed by this method is more close to the real system, in cement particle interval and integral curve, specific surface area, differential and integral pore size distribution curve and sinuosity of interconnected pores. This paper introduces this method into CEMHYD3D model and reconstruct paste initial microstructure combined with particle size distribution curve and mixture ratio. In addition, this paper developed a "simplified Bentz phases split rule", not need to do a long time, high precision experiment, four kinds of clinker mineral phases can be divided with a stereology principles and image processing technology. Compared with the experimental data of components (each component content, porosity, hydration heat release information), the perfected paste hydration model was verified own higher precision.Based on the multi-scale theory, this paper divided the cement based composite material into four scales and established corresponding scale microstructure model. This paper build "state orientation reconstruction model", which combined the advantage of "Hard core soft shell model" and "Fiber branching growth model". Low density C-S-H gel structure reconstructed by this model and the atomic force microscope real image almost entirely consistent. Different ages'hydration microstructure can be obtained by perfected paste hydration model, whose hydration products is further divided into high density and low density of C-S-H gel in this paper. This paper established mortar microstructure model with two phase composites, a fine aggregate (sand) and paste; fine aggregate (sand) in this model completely accords with the actual screening curve. In addition, established concrete microstructure model with three phase composites, coarse aggregate (gravel), mortar and interface transition zone; coarse aggregate (gravel) in this model are reconstructed by irregular particle algorithm and completely accords with the actual screening curve.Finally, based on the four scales microstructure model, the steady and unsteady chlorine ion transport process in saturated various scale microstructure are further simulated by numerical method. The "lattice grid transmission method" is developed to solve chloride ion concentration of each location in specimen at the steady-state internal transport conditions, then calculate chloride ions steady-state diffusion coefficient of each scale microstructure. By the numerical simulation results show:fine aggregates can reduce the steady state diffusion coefficient of mortar, the volume is the key factors while the influence of fineness modulus is very small; Interface transition zone can enlarge the diffusion coefficient of concrete while the aggregate has the opposite effect, and the influence of the latter is more obvious; With the same volume content of coarse aggregate, the smaller of the average particle size the greater of the diffusion coefficient. Using the "classical explicit central difference method" to simulate the unsteady transmission process of chlorine ion in saturated microstructure. By the numerical simulation results show:The early transmission depth of chloride ions in cement based material is larger, late the transfer rate slow down; The existence of aggregate make chloride ion transport channel narrow and make transmission path twists, leading to the chloride ion is very difficult to internal transfer; The interface transition zone can accelerate the chloride ion unsteady transmission process;Crack (especially connected to surface) will make the chloride ion diffusion depth increased significantly.