Research On Finite Element Method Modeling Of Calcium Leaching Of The Cement Based Materials

Cement-based materials are widely used in various infrastructure construction projects,and their dissolution problems have always been a hot topic in material durability research.Under long-term dissolution,a large amount of cement hydrate in cement-based materials is hydrolyzed,dissolved,and lost,resulting in degradation of physical and mechanical properties of the material.Reasonable description of the spatial and temporal distribution of dissolution in the structures and the degradation of material properties caused by dissolution is of great significance for the study of engineering structure safety and material durability.Laboratory tests and numerical simulations are commonly used methods to study the dissolution of cement-based materials.In numerical simulation research,finite difference method and finite volume method are common methods to solve the dissolution model.However,there are relatively few systematic studies on solving the dissolution model with finite element method.Therefore,based on the establishment of a mathematical model of calcium dissolution,the finite element method of the model is studied in this paper.The main research contents and results are as follows:(1)Based on the mechanism of hydrodynamic dispersion and convection of calcium ions in the pore fluid of cement-based materials,a mathematical model of calcium leaching is established considering the seepage process of pore fluids.The model achieves the coupling of the calcium ion transport equation and the seepage control equation by porosity.Based on the Galerkin weighted residual method,the finite element calculation formula of the model is derived,and the main principles and special treatment methods involved in the finite element analysis process are systematically expounded.The algorithm is implemented by using the FORTRAN programming language to design a three-dimensional finite element calculation program.(2)A series of numerical simulation experiments are carried out for the contact dissolution and osmotic dissolution test process of cement-based materials.The applicability of the model and algorithm to the two types of calcium leaching problems is verified by comparison with the literature data.Based on the model,the temporal and spatial distribution of threedimensional contact dissolution and one-dimensional osmosis dissolution are studied.Meanwhile,The difference between the two types of calcium leaching problems are revealed by comparison.The results show that: the ion exchange boundary conditions can be used to simulate the dissolution of surface materials in contact with the dissolution medium.The threedimensional contact dissolution is similar to the one-dimensional and two-dimensional contact dissolution.The osmotic dissolution rate is faster than the contact dissolution,and osmotic dissolution is directional.The dissolution has directionality,the stronger the effect of seepage is,the faster of the cement based materials dissolves,and the more obvious the direction of dissolution is.(3)The sensitivities of the parameters of finite element model and material parameters are numerically studied.The results show that: the spatial step size,time step and capillary porosity contribution coefficient have little effect on the calculation accuracy.The mass transfer coefficient has no obvious influence on the spatiotemporal distribution of dissolution,but has a great influence on dissolution rate of the surface.The degree of dispersion has a great influence on the distribution of dissolution characteristics and the dissolution rate.(4)Based on the proposed model,the analysis of the osmotic dissolution of the gravity dam and the contact dissolution of the water pipeline are completed.The gravity dam results show that: The dissolution of the dam is mainly distributed on the upstream water-facing side,and the degree of dissolution is gradually weakened from the surface to the inside.As the service time increases,the dissolution continues to develop internally.The larger the permeability coefficient,the faster of the dam concrete dissolves.The amount of calcium dissolved is proportional to the permeability coefficient.The limit allowable permeability coefficient of the analyzed concrete is 3.11E-10 meters per second.The water pipeline results show that: The contact dissolution rate of the lining concrete is very slow.The leaching process of the inner surface layer of the lining concrete has little effect on the durability of the water pipeline structure during service life.