Ion transmission method for hydraulic concrete based on real pore structure and solution composition

Abstract

The invention discloses a hydraulic concrete ion transmission method based on a real pore structure and solution composition, and relates to the technical field of ion transmission. Comprising the following steps: constructing a multi-scale diffusion model based on fractal dimensions; obtaining a pore network based on a multi-scale diffusion model, and forming an electrochemical potential gradient through a local micro-region of the pore network; constructing an ion transmission model based on the electrochemical potential gradient, the capillary action parameter and the convection parameter; and evaluating an ion transmission result based on the ion transmission model. According to the method, the electrochemical potential gradient, capillary action parameters and convection parameters are considered, the constructed ion transmission model can well reproduce the ion distribution in the test, and the ion distribution in the concrete at different stages can be accurately predicted.

Description

Technical Field

[0001] The invention relates to the technical field of ion transmission, and in particular to a hydraulic concrete ion transmission method based on real pore structure and solution composition. Background Art

[0002] In recent years, in addition to the strength and mechanical properties of concrete structures, durability has gradually received widespread attention from academia and engineering circles. Especially for hydraulic concrete structures that are exposed to moisture and water for a long time, water migration and erosion by harmful substances are common forms of environmental degradation, which often seriously threaten the safety of the structure and cause a lot of economic losses. The erosion of harmful substances such as chlorides and sulfates seriously endangers the durability of concrete. Among them, chlorides mainly reduce the bearing capacity of the structure by corroding the embedded steel bars, which brings risks; sulfates are widely distributed in rainwater and industrial wastewater. Under the combined influence of physical and chemical effects, expansion products are generated, which produce expansion stress, causing the concrete to expand, crack, peel off and finally completely deteriorate.

[0003] At present, a large number of numerical studies have been conducted to simulate multi-scale diffusion behavior and sulfate attack under dry-wet cycles. Sukrit Kumar De et al. proposed a multi-scale diffusion model that includes pore structure, aggregate and interface transition zone, and explained the correlation between pores and diffusion at the microscale. Jian Gong et al. combined damage mechanics with the creep B3 model and introduced dry-wet cycle influencing factors to predict creep under dry-wet cycles and sulfate attack. Suining Zheng et al. reconstructed the three-dimensional structure of cement aggregate and ITZ through CEMHYD3D and cellular automata to study the difference in diffusion performance of sulfate ions in cement matrix and ITZ under dry-wet cycles. Samson tried to describe the degradation of unsaturated concrete under sulfate attack by establishing a multi-ion transport model, but it could not characterize the drying process. Although a large number of experimental and theoretical studies have been carried out, the macroscopic diffusion behavior of sulfate under dry-wet cycles has not been fully determined, and few studies have considered the effect of pore structure on diffusion.

[0004] Therefore, it is an urgent problem for those skilled in the art to propose a hydraulic concrete ion transport method based on real pore structure and solution composition to solve the difficulties existing in the prior art. Summary of the invention

[0005] In view of this, the present invention provides a hydraulic concrete ion transport method based on real pore structure and solution composition, which has certain significance for studying the ion transport and diffusion of hydraulic concrete with real pore structure and solution composition.

[0006] In order to achieve the above object, the present invention adopts the following technical solution:

[0007] A hydraulic concrete ion transport method based on real pore structure and solution composition comprises the following steps:

[0008] Constructing a multi-scale diffusion model based on fractal dimension;

[0009] Based on the multi-scale diffusion model, the pore network is obtained, and the electrochemical potential gradient is formed through the local micro-region of the pore network;

[0010] Construct an ion transport model based on electrochemical potential gradient, capillary action parameters, and convection parameters;

[0011] Based on the ion transport model, the ion transport results were evaluated.

[0012] Optionally, the specific contents of constructing a multi-scale diffusion model based on fractal dimension are:

[0013] A square with a size of 2000 nm was selected as the representative volume unit, and circular pores were selected for two-dimensional modeling, with a pore size distribution of 10–300 nm;

[0014] According to the porosity occupied by each pore size, the number of each type of pores within the RE range is calculated, and the corresponding circular pores are generated;

[0015] The generated circular pores are placed within the RE range and the invasion judgment between pores and boundaries, and between pores and pores is performed. If the circular pores placed each time are within the boundary range and do not invade each other with other circular pores, a two-dimensional model is generated. Otherwise, the pore placement information is regenerated and the invasion judgment is performed again until the non-invasion condition is met.

[0016] The generated two-dimensional model is meshed by using a direct partitioning method to obtain two-dimensional model image data;

[0017] Binarization is performed on the generated two-dimensional model image data, and the fractal dimension of the two-dimensional model is calculated by the box counting dimension method to obtain the fractal dimension;

[0018] The diffusion equation based on Fick's second law is described as:

[0019]

[0020] Where w is the water content in kg / m 3 , c is the sulfate concentration in mol / m 3 , D i is the diffusion coefficient, in m 2 / s, V is the average velocity of liquid flowing through the pore system under the action of capillary suction, and its unit is m / s.

[0021] Optionally, the specific content of forming the electrochemical potential gradient through the local micro-region of the pore network is:

[0022] The Nernst-Planck equation is used to describe the situation in which charged ions diffuse in a porous medium and move relative to the medium due to the electrical coupling effect. The electrical migration flux of the ions is:

[0023]

[0024] Among them, s i is the charge number of the i-th ion, F is the Faraday constant, which is 9.649×104C / mol, T is the absolute temperature, which is 298K, and Φ is the electrostatic potential.

[0025] Optionally, the Poisson equation based on Gaussian electrostatic theory is introduced to characterize the relationship between the charge imbalance and the potential gradient of the liquid pore solution system. Considering the unsaturated state of hydraulic concrete, the saturation parameter is used to correct formula (3), and the following is obtained:

[0026]

[0027] Among them, ε 0 is the dielectric constant of vacuum, 8.854×10-12C / (V·m), ε r is the relative dielectric constant of liquid water, which is 80.

[0028] Optionally, based on the electrochemical potential gradient, capillary action parameters and convection parameters, the specific contents of constructing the ion transport model are:

[0029] According to Fick's second law and the law of conservation of mass, the spatial and temporal distribution of ions is:

[0030]

[0031] Among them, c bi The concentration of ions solidified in the pore wall structure due to reaction or adsorption, in mol / m 3 , c fi is the concentration of ions free in the pore solution, in mol / m 3 , J i is the transport flow rate of the i-th ion, in mol / (m 2 s);

[0032] The migration of ions under unsaturated conditions is driven by diffusion and convection. The Nernst-Planck equation is used to expand Fick's theorem. Then the flux of ions under these two driving forces can be expressed as:

[0033]

[0034] in, is the porosity of concrete;

[0035]

[0036] The Langmuir isotherm is used to describe the relationship between solidified ions and free ions, and the saturation parameter correction formula (8) is introduced as follows:

[0037]

[0038] Among them, α and β are modified Langmuir adsorption constants, ω is a dimensionless constant and is 0.3;

[0039] Combining equations (8) and (9), we can get

[0040]

[0041] It can be seen from the above technical solution that, compared with the prior art, the present invention provides a hydraulic concrete ion transport method based on real pore structure and solution composition, which has the following beneficial effects:

[0042] (1) The ion transport model established by the present invention taking into account the electrochemical potential gradient, capillary action parameters and convection parameters can well reproduce the distribution of ions in the experiment and more accurately predict the ion distribution in concrete at different stages;

[0043] (2) The pore structure of the concrete of the present invention is not only curved and interlaced, but also multi-scale. The pore size distribution observed at different scales is also different. However, according to the fractal theory, the pore structure of concrete can be regarded as a fractal. Therefore, according to the self-similarity of fractals, it can be proved that the pore structures of concrete at different scales are similar.

[0044] (3) The method proposed in the present invention is of certain significance for the study of ion transport and diffusion in hydraulic concrete with real pore structure and solution composition. It simplifies the model to a certain extent and can make certain predictions on the experimental trends. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0046] Figure 1 A flow chart of a hydraulic concrete ion transport method based on real pore structure and solution composition provided by the present invention;

[0047] Figure 2 This is a block diagram of the ion transport model and boundary layer grid structure under the dry-wet cycle system provided by the present invention. DETAILED DESCRIPTION

[0048] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0049] Reference Figure 1 As shown, the present invention discloses a hydraulic concrete ion transmission method based on real pore structure and solution composition, comprising the following steps:

[0050] Constructing a multi-scale diffusion model based on fractal dimension;

[0051] Based on the multi-scale diffusion model, the pore network is obtained, and the electrochemical potential gradient is formed through the local micro-region of the pore network;

[0052] Construct an ion transport model based on electrochemical potential gradient, capillary action parameters, and convection parameters;

[0053] Based on the ion transport model, the ion transport results were evaluated.

[0054] Furthermore, the specific contents of constructing a multi-scale diffusion model based on fractal dimension are as follows:

[0055] A square with a size of 2000 nm was selected as the representative volume unit, and circular pores were selected for two-dimensional modeling, with a pore size distribution of 10–300 nm;

[0056] According to the porosity occupied by each pore size, the number of each type of pores within the RE range is calculated, and the corresponding circular pores are generated;

[0057] The generated circular pores are placed within the RE range and the invasion judgment between pores and boundaries, and between pores and pores is performed. If the circular pores placed each time are within the boundary range and do not invade each other with other circular pores, a two-dimensional model is generated. Otherwise, the pore placement information is regenerated and the invasion judgment is performed again until the non-invasion condition is met.

[0058] The generated two-dimensional model is meshed by using a direct partitioning method to obtain two-dimensional model image data;

[0059] Binarization is performed on the generated two-dimensional model image data, and the fractal dimension of the two-dimensional model is calculated by the box counting dimension method to obtain the fractal dimension;

[0060] The diffusion equation based on Fick's second law is described as:

[0061]

[0062] Where w is the water content in kg / m 3 , c is the sulfate concentration in mol / m 3 , D i is the diffusion coefficient, in m 2 / s, V is the average velocity of liquid flowing through the pore system under the action of capillary suction, and its unit is m / s.

[0063] Specifically, the representative volume element is abbreviated as RVE, and the RE range is the representative unit range;

[0064] Fractal geometry is an emerging method that studies irregular geometric forms. It can describe the morphological characteristics of non-integer-dimensional forms filling space. Its basic characteristic is the self-similarity of the material being described, that is, there is a certain similarity between the local and the whole of the fractal. Many studies have found that fractal geometry can better characterize the pore characteristics of cement-based materials through fractal dimensions, thereby linking the microstructure with the macroscopic properties of the material. The box counting method is used to check whether an object has fractal characteristics, that is, the geometry is placed on a uniformly divided grid, the number of grids required to cover the fractal is calculated, and the number of boxes used to cover the object is recorded by changing the grid accuracy. Through the fitting analysis of the entire fractal set, the relationship between the number of boxes required for coverage and the size is obtained.

[0065] Furthermore, the specific content of the electrochemical potential gradient formed by the local micro-region of the pore network is:

[0066] The Nernst-Planck equation is used to describe the situation in which charged ions diffuse in a porous medium and move relative to the medium due to the electrical coupling effect. The electrical migration flux of the ions is:

[0067]

[0068] Among them, s i is the charge number of the i-th ion, F is the Faraday constant, which is 9.649×104C / mol, T is the absolute temperature, which is 298K, and Φ is the electrostatic potential.

[0069] Specifically, under saturated and pressure gradient-free conditions, the migration of ions is mainly driven by the electrochemical potential gradient of the solution, where the chemical potential gradient is also called diffusion, which can cause charged solutes to move from areas of high concentration to areas of low concentration; the other driving force is controlled by the electric potential gradient, and its formation mechanism is due to the different movement speeds of heterogeneous charged ions in the solution. Generally speaking, faster-moving charged solutes carrying excess charges can form an electric field in the micro-region of the solution. In this process, the existence of the electric potential gradient accelerates the migration rate of slower-moving ions and reduces the migration rate of faster-moving ions. In view of the fact that after the dry-wet process hydraulic concrete comes into contact with the environmental water medium rich in multiple ions, the concrete pore solution is actually an environment where multiple ions coexist. Therefore, the electrical coupling effect formed by multiple ions is bound to have an important impact on the transmission of ions inside it.

[0070] Furthermore, the Poisson equation based on Gaussian electrostatic theory is introduced to characterize the relationship between the charge imbalance and the potential gradient of the liquid pore solution system. Considering the unsaturated state of hydraulic concrete, the saturation parameter is used to correct the formula (3), and the following is obtained:

[0071]

[0072] Among them, ε 0 is the dielectric constant of vacuum, 8.854×10-12C / (V·m), ε r is the relative dielectric constant of liquid water, which is 80.

[0073] Further, such as Figure 2 As shown in the figure, based on the electrochemical potential gradient, capillary action parameters and convection parameters, the specific contents of constructing the ion transport model are as follows:

[0074] According to Fick's second law and the law of conservation of mass, the spatial and temporal distribution of ions is:

[0075]

[0076] Among them, c bi The concentration of ions solidified in the pore wall structure due to reaction or adsorption, in mol / m 3 , c fi is the concentration of ions free in the pore solution, in mol / m 3 , J i is the transport flow rate of the i-th ion, in mol / (m 2 s);

[0077] The migration of ions under unsaturated conditions is driven by diffusion and convection. The Nernst–Planck equation is used to expand Fick's theorem. Then the flux of ions under these two driving forces can be expressed as:

[0078]

[0079] in, is the porosity of concrete;

[0080]

[0081] The Langmuir isotherm is used to describe the relationship between solidified ions and free ions, and the saturation parameter correction formula (8) is introduced as follows:

[0082]

[0083] Among them, α and β are modified Langmuir adsorption constants, ω is a dimensionless constant and is 0.3;

[0084] Combining equations (8) and (9), we can get

[0085]

[0086] Specifically, according to capillary dynamics, after the external environmental water intrudes into the pores of the concrete matrix, an adsorption layer is formed (water and the pore wall are wettable), and capillary negative pressure is generated under the action of surface tension. Therefore, the water transfer of unsaturated concrete is dominated by the capillary action caused by the surface tension of the adsorbent in the liquid pore structure; when the water level rises, the hydraulic structure concrete contacts the environmental water source, and the diffusion of water vapor in the concrete pore structure is much more difficult than in the atmospheric environment. Based on the classic theory of soil science Darcy's law, the relationship between the water liquid phase transfer flow rate and the hydraulic gradient in saturated porous media can be expressed as:

[0087]

[0088] Where q is the vector flow velocity, Ψ is the capillary potential, K(θ) is the hydraulic conductivity, and θ is the relative water content.

[0089] In 1931, Richards proposed an extended Darcy equation that can describe the water liquid phase transport in unsaturated porous media:

[0090]

[0091] Its standard form with capillary potential energy as the independent variable can be expressed by equation (14):

[0092]

[0093] Where C(Ψ) = dΘ / dΨ, K(ψ) is the hydraulic conductivity, which can be expressed as:

[0094]

[0095]

[0096] Among them, K s is the saturated permeability coefficient, θ is the saturation, K r (Ψ) and K r (Θ) are the relative permeabilities that vary with ψ and Θ, respectively, and can be expressed by formula (17):

[0097] K r (Θ) = Θ 1 / 2 [1-(1-θ 1 / m ) m ] 2 (17)

[0098] The saturated permeability coefficient reflects the difficulty of water transmission in porous media. It is defined as the amount of water that vertically passes through a unit area of ​​saturated concrete per unit time under a unit hydraulic gradient. Formally, the saturated permeability coefficient is expressed as the proportional coefficient of the relationship between the water movement rate and the hydraulic gradient in Darcy's law. The formula is as follows:

[0099]

[0100] The saturated permeability coefficient is 3×10 -11 m 2 / s.

[0101] In fact, during the liquid water migration process caused by dry-wet conditions, the entire system is in a non-equilibrium state. Here, the local equilibrium assumption of irreversible processes in the classical non-equilibrium thermodynamics theory is borrowed to approximately assume that the system is in equilibrium within the local volume element. Therefore, the equilibrium relationship between liquid water content and energy in hydraulic concrete (i.e., the moisture characteristic curve) can be applied to the non-equilibrium system.

[0102] The calculation formula proposed by Van Genuchten is used to describe the main curve of the isothermal adsorption curve (or moisture characteristic curve), and the formula is as follows:

[0103]

[0104] Wherein, m=1-1 / n, n and α are empirical parameters.

[0105] Convection parameters, i.e., convection peaks, are a special aggregation mechanism of ions formed under a specific dry-wet alternation environment. The ion concentration taken in by concrete increases first and then decreases with the increase of penetration depth. The peak ion concentration appears within 1 mm to 3 mm from the concrete surface. This is because sulfate ions produce a convection effect on the surface of concrete under dry-wet alternation conditions. By considering the convection peak, the ion diffusion rate is accelerated. For dry-wet alternation under different conditions, the larger the dry-wet ratio, the more it can promote the invasion of ions. The present invention selects the ion peak concentration under the T1_R7 system.

[0106] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part.

[0107] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A hydraulic concrete ion transport method based on real pore structure and solution composition, characterized in that: The following steps are involved: Constructing a multi-scale diffusion model based on fractal dimension; Based on the multi-scale diffusion model, the pore network is obtained, and the electrochemical potential gradient is formed through the local micro-region of the pore network; Construct an ion transport model based on electrochemical potential gradient, capillary action parameters, and convection parameters; Based on the ion transport model, the ion transport results were evaluated.

2. The method for ion transport in hydraulic concrete based on real pore structure and solution composition according to claim 1, characterized in that: The specific contents of constructing a multi-scale diffusion model based on fractal dimension are as follows: A square with a size of 2000 nm was selected as the representative volume unit, and circular pores were selected for two-dimensional modeling, with a pore size distribution of 10–300 nm; According to the porosity occupied by each pore size, the number of each type of pores within the RE range is calculated, and the corresponding circular pores are generated; The generated circular pores are placed within the RE range and the invasion judgment between pores and boundaries, and between pores and pores is performed. If the circular pores placed each time are within the boundary range and do not invade each other with other circular pores, a two-dimensional model is generated. Otherwise, the pore placement information is regenerated and the invasion judgment is performed again until the non-invasion condition is met. The generated two-dimensional model is meshed by using a direct partitioning method to obtain two-dimensional model image data; Binarization is performed on the generated two-dimensional model image data, and the fractal dimension of the two-dimensional model is calculated by the box counting dimension method to obtain the fractal dimension; The diffusion equation based on Fick's second law is described as: Where w is the water content in kg / m 3 , c is the sulfate concentration in mol / m 3 , D i is the diffusion coefficient, in m 2 / s, V is the average velocity of liquid flowing through the pore system under the action of capillary suction, and its unit is m / s.

3. The method for ion transmission in hydraulic concrete based on real pore structure and solution composition according to claim 1, characterized in that: The specific content of the electrochemical potential gradient formed by the local micro-region of the pore network is: The Nernst-Planck equation is used to describe the situation in which charged ions diffuse in a porous medium and move relative to the medium due to the electrical coupling effect. The electrical migration flux of the ions is: Among them, s i is the charge number of the i-th ion, F is the Faraday constant, which is 9.649×104C / mol, T is the absolute temperature, which is 298K, and Φ is the electrostatic potential.

4. The method for ion transmission in hydraulic concrete based on real pore structure and solution composition according to claim 3, characterized in that: The Poisson equation based on Gaussian electrostatic theory is introduced to characterize the relationship between the charge imbalance and the potential gradient of the liquid pore solution system. Considering the unsaturated state of hydraulic concrete, the saturation parameter is used to correct the formula (3), and the following is obtained: Among them, ε0 is the dielectric constant of vacuum, which is 8.854×10-12C / (V·m), ε r is the relative dielectric constant of liquid water, which is 80.

5. The method for ion transmission in hydraulic concrete based on real pore structure and solution composition according to claim 1, characterized in that: Based on the electrochemical potential gradient, capillary action parameters and convection parameters, the specific contents of constructing the ion transport model are as follows: According to Fick's second law and the law of conservation of mass, the spatial and temporal distribution of ions is: Among them, c bi The concentration of ions solidified in the pore wall structure due to reaction or adsorption, in mol / m 3 , c fi is the concentration of ions free in the pore solution, in mol / m 3 , J i is the transport flow rate of the i-th ion, in mol / (m 2 s); The migration of ions under unsaturated conditions is driven by diffusion and convection. The Nernst-Planck equation is used to expand Fick's theorem. Then the flux of ions under these two driving forces can be expressed as: in, is the porosity of concrete; The Langmuir isotherm is used to describe the relationship between solidified ions and free ions, and the saturation parameter correction formula (8) is introduced as follows: Among them, α and β are modified Langmuir adsorption constants, ω is a dimensionless constant and is 0.3; Combining equations (8) and (9), we can get

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