A method for determining a phase diagram of a MgO-MgSO4-MgCl2-H2O quaternary system

Abstract

The application discloses a method for determining a cement phase diagram of a MgO-MgSO4-MgCl2-H2O quaternary system, which comprises the following steps: obtaining phase data of the MgO-MgSO4-MgCl2-H2O quaternary system; establishing a thermodynamic database and verifying the reliability of the data; establishing a cement phase diagram of a MgO-MgSO4-MgCl2 ternary system according to the thermodynamic database; and constructing a cement phase diagram of the MgO-MgSO4-MgCl2-H2O quaternary system according to the ternary system phase diagram. The application solves the problem that the cement phase diagram research in the prior art still stays at the ternary system phase diagram stage, and also solves the problems that the workload of determining the cement phase diagram by using a large number of phase-only experimental point methods is large, the accuracy is low, and the phase behavior of unknown components cannot be predicted.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of cement materials, in particular to a method for determining a cement phase diagram of an MgO-MgSO4-MgCl2-H2O quaternary system. BACKGROUND

[0002] In the field of cement materials, a phase diagram can intuitively reveal the influence of multiple factors such as mixing ratio, processing technology, temperature and water-cement ratio on the hydration phase composition of a product, and can even predict the level of macroscopic performance of the product and guide actual production to obtain a product with better use performance. Compared with a ternary phase diagram, a quaternary phase diagram can more vividly show the evolution law of the phase composition and comprehensively study the influence of more independent variables.

[0003] At present, the research on phase diagrams is still in the stage of ternary system phase diagrams, and the main method for determining cement phase diagrams is to arrange a large number of phase-only experiments, but this method is greatly affected by the source and activity of the raw materials used, and the boundary conditions determined may be different in different experiments, and are also affected by the molding process and testing method used when determining, and for some unstable or harshly formed substances, the phase diagram determined by the experimental method may not realize the existence of this region; meanwhile, determining a phase diagram needs to consume a lot of time and effort, and human errors are easy to occur in the experimental process, and the phase behavior of unknown components cannot be predicted. SUMMARY

[0004] The purpose of the application is to provide a method for quickly and effectively determining a MgO-MgSO4-MgCl2-H2O quaternary system cement phase diagram.

[0005] Technical scheme: In order to achieve the above purpose, the method for determining a MgO-MgSO4-MgCl2-H2O quaternary system cement phase diagram comprises the following steps:

[0006] Step S1: obtaining the phase data of the MgO-MgSO4-MgCl2-H2O quaternary system;

[0007] Step S2: establishing a thermodynamic database and verifying the data reliability;

[0008] Step S3: establishing a MgO-MgSO4-MgCl2 ternary system cement phase diagram according to the thermodynamic database;

[0009] Step S4: constructing a MgO-MgSO4-MgCl2-H2O quaternary system cement phase diagram according to the ternary system cement phase diagram.

[0010] The phase parameters in step S1 include solid phase data and liquid phase data, wherein the solid phase data includes molar Gibbs free energy, molar enthalpy of formation, reaction equilibrium constant and constant pressure heat capacity, and the liquid phase data includes ions appearing in the liquid phase and ion interaction parameters, wherein the ion interaction parameters include parameters of different groups of heteroionic double ions, homionic double ions, neutral particles and charged ions.

[0011] The Gibbs free energy of any magnesium salt i in the MgO-MgSO4-MgCl2-H2O quaternary system is:

[0012]

[0013] G i (T) is the Gibbs free energy of the magnesium salt i at any Kelvin temperature T;

[0014] The heat capacity of the magnesium salt i at different temperatures is obtained and the function relationship between the temperature t is:

[0015]

[0016] Wherein, A, B, C, D and E are fitting parameters; t=T / 1000, T is the Kelvin temperature K;

[0017] The standard entropy of the magnesium salt i The standard enthalpy And the standard Gibbs free energy Is:

[0018]

[0019]

[0020] The formula (2)-(4) is brought into the formula (1) to obtain the Gibbs free energy of the magnesium salt i related to the fitting parameters and the temperature t:

[0021] G i (T) = A (t-ln t) -Bt 2 / 2-Ct 3 / 6+Dt 4 / 12-E / (2t)+F-Gt (5),

[0022] Wherein,

[0023]

[0024]

[0025] Wherein, S0i represents the free entropy of i at the temperature of 298.15 K, represents the free enthalpy of i at a temperature of 298.15 K.

[0026] wherein according to the formulae (1)-(6) the chemical structure of the magnesium salt i is assumed to be MX-nH20, wherein n represents the number of bound water molecules contained in the magnesium salt, M represents a cation and X represents an anion, the heat capacity of the MX-nH20 magnesium salt is:

[0027]

[0028] wherein, and are the heat capacities of the salt with and without crystal water at the Kelvin temperature T, respectively; is the contribution value of one crystal water molecule in the salt with crystal water;

[0029] the free entropy and the free enthalpy are:

[0030]

[0031]

[0032] wherein, is the standard entropy of the salt without crystal water at the Kelvin temperature T; is the contribution value of one crystal water molecule in the salt with crystal water at a temperature of 298.15 K; is the standard enthalpy of the salt without crystal water at the Kelvin temperature T at a temperature of 298.15 K; is the contribution value of one crystal water molecule in the salt with crystal water.

[0033] wherein, for any reaction A + +B - = AB, the reaction equilibrium constant log K AB is:

[0034] Δ r G AB θ = Δ f G AB - Δ f G A- - Δ f G B+ (11),

[0035] Δ r G AB θ = - RT log K AB * ln(10) (12);

[0036] where Δ r G AB θ is the Gibbs free energy of the chemical reaction, Δ f G is the Gibbs formation energy of the substance, R is the gas constant, and T is the temperature in Kelvin.

[0037] where the heteroionic ion pair is replaced by a homologous element ion pair with similar properties to the target ion, or the average value of the interaction parameters of the equivalent state substance in the system is calculated to replace the interaction parameters of the heteroionic ion pair. The interaction parameters for any cation M and anion X are:

[0038]

[0039] where P M-X is the interaction parameter of ion M and X, y represents the number of heteroionic ion pairs similar to the M and X ion pair in the system containing the same ions, and P y represents the interaction parameter of the heteroionic ion pair.

[0040] where the establishment of the thermodynamic database in step S2 refers to classifying the data of each phase according to elements, ions, and mineral phases, respectively corresponding to the three inherent system classification modules of SOLUTION_MASTER_SPECIES, SOLUTION_SPECIES, and PHASES in the PHREEQC software. The ion interaction parameters are classified into different groups according to heteroionic ion pairs, homionic ion pairs, neutral particles, and charged ions, and input into different ion pair groups in the PITZER module, finally forming the thermodynamic database of the MgO-MgSO4-MgCl2-H2O quaternary system.

[0041] To confirm the reliability of the data in the thermodynamic database, the solubility, pH value, and ion concentration of each phase need to be calculated, and it needs to be verified whether the substance can precipitate from the solution. Finally, the calculation results are compared with the experimental results for verification.

[0042] where the solubility of the substance in the solution is also related to the ion activity. For any phase D d E e , there is an electrolytic reaction in the solution The solubility product IAP DE of this reaction is:

[0043] LAP DE = {D +} d · {E -} e = γ d D+ · [D+ ] d ·γ e E+ ·[E - ] e (14),

[0044] wherein, {D +} d , [D + ] d and γ d D+ respectively refer to the ion activity, concentration and activity coefficient of any ion D + ; {E -} e , [E - ] e and γ e E+ respectively refer to the ion activity, concentration and activity coefficient of any ion E - ;

[0045] The saturation index of the reaction is:

[0046]

[0047] wherein, K is the solubility product constant, which is the ion activity product corresponding to the dissolution equilibrium of the phase, and for solid substances

[0048] is equivalent to the equilibrium constant;

[0049] When the saturation index SI of the target phase in the liquid phase system is greater than 0, it indicates that it is in a supersaturated state, and the corresponding solid phase will precipitate, and when SI is less than 0, the system is unsaturated, and will maintain the current state.

[0050] wherein, the MgO-MgSO4-MgCl2 ternary system cement phase diagram established according to the thermodynamic database in step S3 is specifically: a three-layer iterative calculation is performed, the uppermost layer variable is H, which represents the molar ratio of water to salt, in order to form a cement slurry with practical value, the water consumption range [H1, H2] meeting the requirements of fluidity and setting time is selected as the value range of H, H increases with H1 as the starting point, H2 as the terminal point, and H step as the step size;

[0051] The second layer variable is the amount of MgSO4, keeping the sum of the moles of MgSO4 and MgCl2 as N1 unchanged, the amount of MgSO4 increases with 0 as the starting point, L1 as the terminal point, and L as the step size, and L1 is any natural number in the range of [0, N1];

[0052] The bottom layer variable is the molar ratio of MgO to salt, which increases with 0 as the starting point, L2 as the terminal point, and L2 is increased for the step size, L2 [10, 30];

[0053] The hydration product composition of different mix proportions corresponding to each point traversed is calculated, and in the calculation process, each H point is traversed to obtain a ternary phase diagram of MgO-MgSO4-MgCl2, and each H point in the range is traversed to obtain a series of ternary system cement phase diagrams under different H.

[0054] In the step S4, the MgO-MgSO4-MgCl2-H2O quaternary system cement phase diagram is constructed according to the ternary system cement phase diagram, and specifically, a new axis about the variable H is added according to the boundary line information of each region in the ternary phase diagram, and the synchronous change of the boundary line of different regions with the change of H is confirmed, and the boundary line is obtained. The interface formed by the intersection of the blocks represents different phase composition parts in the quaternary system cement phase diagram; in order to better show the difference between different blocks, each block is filled with different colors, and a complete MgO-MgSO4-MgCl2-H2O quaternary system cement phase diagram can be obtained.

[0055] Beneficial effects: the present application has the following advantages: 1. Compared with the traditional experimental method, the present method is based on the thermodynamic data of each phase, uses the PHREEQC software to establish the ternary system cement phase diagram, does not need to prepare and test experiments, reduces the error caused by human operation, reduces the workload, and has high accuracy and can predict the phase behavior of unknown components;

[0056] 2. The present method is based on the ternary system cement phase diagram to construct the MgO-MgSO4-MgCl2-H2O quaternary system cement phase diagram, which can more accurately describe the composition and properties of the cement material. BRIEF DESCRIPTION OF DRAWINGS

[0057] Figure 1 The present application is a method flow chart;

[0058] Figure 2 The horizontal section of the MgO-MgSO4-MgCl2-H2O system phase diagram at the water and salt molar ratio of 18 and the verification point;

[0059] Figure 3 The horizontal section of the MgO-MgSO4-MgCl2-H2O system phase diagram at the water and salt molar ratio of 22;

[0060] Figure 4 The horizontal section of the MgO-MgSO4-MgCl2-H2O system phase diagram at the water and salt molar ratio of 26;

[0061] Figure 5The phase diagram of the MgO-MgSO4-MgCl2-H2O system at the side view and verification point when the MgCl2 content is 0;

[0062] Figure 6 The phase diagram of the MgO-MgSO4-MgCl2-H2O system at the side view and verification point when the MgSO4 content is 0;

[0063] Figure 7 This is the vertical section of the phase diagram of the MgO-MgSO4-MgCl2-H2O system at the molar ratio MgSO4 / MgCl2 = 1. Detailed Implementation

[0064] The technical solution of the present invention will be described in detail below with reference to the embodiments and accompanying drawings.

[0065] like Figure 1 As shown, the method for determining the phase diagram of a quaternary cement system of MgO-MgSO4-MgCl2-H2O according to the present invention includes the following steps:

[0066] Step S1: Obtain the phase data of each phase in the MgO-MgSO4-MgCl2-H2O quaternary system; the phase parameters include solid phase data and liquid phase data. The solid phase data includes molar Gibbs free energy of formation, molar enthalpy of formation, reaction equilibrium constant and isobaric heat capacity. The liquid phase data includes the ions that will appear in the liquid phase and the ion interaction parameters. The ion interaction parameters include parameters of different groups of oppositely charged ions, like-charged ions, neutral particles and charged ions.

[0067] In the MgO-MgSO4-MgCl2-H2O quaternary system, the main hydration products are various magnesium salts. The Gibbs free energy for any magnesium salt i is:

[0068]

[0069] Among them, G i (T) is the Gibbs free energy of magnesium salt i at any Kelvin temperature T;

[0070] Obtain the heat capacity of magnesium salt i at different temperatures Functional relationship with temperature t:

[0071]

[0072] Where A, B, C, D and E are all fitting parameters; t = T / 1000, where T is the Kelvin temperature in K;

[0073] Standard entropy of magnesium salt i Standard enthalpy and standard Gibbs free energy for:

[0074]

[0075]

[0076] Substituting formula (2)-(4) into formula (1) to obtain the Gibbs free energy of magnesium salt i related to fitting parameters and temperature t is:

[0077] G i (T) = A(t - In t) - Bt 2 / 2 - Ct 3 / 6 + Dt 4 / 12 - E / (2t) + F - Gt (5),

[0078] wherein,

[0079]

[0080]

[0081] wherein, represents the free entropy of i at a temperature of 298.15 K, represents the free enthalpy of i at a temperature of 298.15 K.

[0082] wherein, according to formula (1)-(6), the chemical structure of magnesium salt i is MX·nH2O, wherein n represents the number of bound water contained in the magnesium salt, M represents a cation, and X represents an anion, and the heat capacity of the MX·nH2O magnesium salt is:

[0083]

[0084] wherein, and are the heat capacities of the salt containing crystal water and the salt not containing crystal water at Kelvin temperature T, respectively; is the contribution value of one crystal water molecule in the salt containing crystal water;

[0085] the free entropy and the free enthalpy are:

[0086]

[0087]

[0088] wherein, is the standard entropy of the salt not containing crystal water at Kelvin temperature T; is the contribution value of one crystal water molecule in the salt containing crystal water at a temperature of 298.15 K; The standard enthalpy of a salt without crystal water at a temperature T in Kelvin at a temperature of 298.15 K; The contribution value of one crystal water molecule in a salt with crystal water at a temperature of 298.15 K.

[0089] where, for any reaction A + +B - = AB, the reaction equilibrium constant logK AB is:

[0090] Δ r G AB θ = Δ f G AB - Δ f G A- - Δ f G B+ (11),

[0091] Δ r G AB θ = - RTlogK AB * ln(10) (12);

[0092] where, Δ r G AB θ is the Gibbs free energy of the chemical reaction, Δ f G is the Gibbs formation energy of the substance, R is the gas constant, and T is the Kelvin temperature K.

[0093] where the heteroionic ion is replaced by a homologous element ion similar to the target ion, or the average value of the interaction parameter of the equivalent state substance in the system is calculated to replace the interaction parameter of the heteroionic ion pair, and the interaction parameter of any cation M and anion X is:

[0094]

[0095] where, P M-X is the interaction parameter of ion M and X, y represents the number of heteroionic ion pairs similar to the M and X ion pair in the system containing the same ion, and P y represents the interaction parameter of the heteroionic ion pair.

[0096] Step S2: establishing a thermodynamic database and verifying the reliability of the data; wherein establishing a thermodynamic database refers to classifying each phase data according to elements, ions and mineral phases, and respectively corresponding to the three inherent system classification modules of SOLUTION_MASTER_SPECIES, SOLUTION_SPECIES and PHASES in the PHREEQC software, and the ion interaction parameters are classified into different ion pair groups in the PITZER module according to different groups of heteroelectricity double ions, homoelectricity double ions, neutral particles and charged ions, and finally forming a thermodynamic database of the MgO-MgSO4-MgCl2-H2O quaternary system;

[0097] In order to confirm the reliability of the data in the thermodynamic database, the solubility, pH value and ion concentration of each phase need to be calculated, and it is verified whether the substance can be precipitated from the solution, and finally the calculation results are compared with the experimental results for inspection.

[0098] Wherein whether the substance can be precipitated from the solution is also related to the ion activity, for any phase D d E e , there is an electrolytic reaction in the solution The solubility product IAP DE of the reaction is:

[0099] LAP DE = {D +} d · {E -} e = γ d D+ · [D + ] d · γ e E+ · [E - ] e (14),

[0100] Wherein, {D +} d , [D + ] d and γ d D+ respectively refer to the ion activity, concentration and activity coefficient of any ion D + , {E -} e , [E - ] e and γ e E+ respectively refer to the ion activity, concentration and activity coefficient of any ion E - ;

[0101] The saturation index of the reaction is:

[0102]

[0103] wherein K is the solubility product constant, which is the ion activity product corresponding to the solubility equilibrium of the phase, and for solid material

[0104] is equivalent to the equilibrium constant;

[0105] When the saturation index SI of the target phase in the liquid phase system is greater than 0, it indicates that it is in a supersaturated state, and the corresponding solid phase will precipitate, and when SI is less than 0, the system is unsaturated, and the current state will be maintained.

[0106] Step S3: Establishing the MgO-MgSO4-MgCl2 ternary system cement phase diagram according to the thermodynamic database; specifically: performing a three-layer iterative calculation, the uppermost layer variable is H, representing the molar ratio of water to salt, in order to form a cement slurry with practical value, the water consumption range [H1, H2] meeting the requirements of fluidity and setting time is selected as the value range of H, H increases with H1 as the starting point, H2 as the terminal point, and H step = H1+ i, i = 0, 1, 2, …, 1000;

[0107] The second layer variable is the amount of MgSO4, keeping the sum of the moles of MgSO4 and MgCl2 as N1 unchanged, the amount of MgSO4 increases with 0 as the starting point, L1 as the terminal point, L1 = 0 + i, i = 0, 1, 2, …, N1;

[0108] The bottom layer variable is the molar ratio of MgO to salt, which increases with 0 as the starting point, L2 as the terminal point, L2 = 0 + i, i = 0, 1, 2, …, 30;

[0109] For each point traversed, the composition of the hydration product is calculated, and in the calculation process, each H point is traversed to obtain a MgO-MgSO4-MgCl2 ternary phase diagram, and after traversing each H point in the range, a series of ternary system cement phase diagrams under different H are obtained. By comparing Figures 2-4 , with the increase of water content in the system, part of the area gradually disappears or expands in the ternary system cement phase diagram, resulting in the displacement of the boundary line between the regions, which further expands the research on the influence of water content on the mixing ratio of MgO-MgSO4-MgCl2 three raw materials.

[0110] Step S4: Constructing the MgO-MgSO4-MgCl2-H2O quaternary system cement phase diagram according to the ternary system phase diagram; Specifically: according to the boundary line information of each region in the ternary phase diagram, add a new axis about variable H, and confirm the synchronous change of the boundary line of different regions with the change of H, get the three-dimensional interface composed of the boundary line, and each block surrounded by the interface represents different phase composition parts in the quaternary system cement phase diagram; In order to better show the difference between different blocks, each block is filled with different colors, that is, the complete MgO-MgSO4-MgCl2-H2O quaternary system cement phase diagram can be obtained.

[0111] The phase diagram contains 5 independent solid phases, which will be transformed with each other with the change of different mixing ratio, and there are 11 different block combinations in the phase diagram, the structure of these combinations is presented by the longitudinal section view of Figure 5 、 Figure 6 and Figure 7 . In addition to containing the characteristic hydration products of magnesium oxychloride cement and magnesium oxysulfide cement system respectively, it also has the composite structure of the best strength phase of the two systems, which provides guidance for further exploring the mixing ratio corresponding to the macroscopic performance such as fold pressure ratio and water resistance of MgO-MgSO4-MgCl2-H2O system.

Claims

1. A method for determining the phase diagram of a quaternary cement system of MgO-MgSO4-MgCl2-H2O, characterized in that, Includes the following steps: Step S1: Obtain the phase data of each phase in the MgO-MgSO4-MgCl2-H2O quaternary system; Step S2: Establish a thermodynamic database and verify the reliability of the data; Step S3: Establish a ternary cement phase diagram for the MgO-MgSO4-MgCl2 system based on a thermodynamic database. Specifically, perform a three-layer iterative calculation. The top-level variable is H, representing the molar ratio of water to salt. To form a cement paste with practical value, the water content range [H1, H2] that meets the requirements of fluidity and setting time is selected as the range of H values. H starts from H1 and ends at H2. step Increase the step size; The second layer of variables is the amount of MgSO4 used. Keeping the sum of the molar numbers of MgSO4 and MgCl2 constant at N1, the amount of MgSO4 used starts at 0 and ends at L1. The increment is determined by the step size, where L1 is any natural number in the range [0, N1]. The most fundamental variable is the molar ratio of MgO to salt, which starts at 0 and ends at L2. The step size is increased, L2∈[10,30]; For each point traversed, the composition of hydration products is calculated for different mix proportions. During the calculation process, a ternary phase diagram of MgO-MgSO4-MgCl2 is obtained for each H point traversed. After traversing every H point within the range, a series of ternary cement phase diagrams under different H are obtained. Step S4: Construct a quaternary cement phase diagram of MgO-MgSO4-MgCl2-H2O based on the ternary cement phase diagram. Specifically, based on the boundary line information of each region in the ternary phase diagram, add a new axis about the variable H, and confirm that the boundary lines of different regions change synchronously with the change of H, to obtain a three-dimensional interface formed by the boundary lines. The blocks enclosed by this interface represent the different phase components in the quaternary cement phase diagram. In order to better show the differences between the different blocks, fill each block with a different color to obtain the complete MgO-MgSO4-MgCl2-H2O quaternary cement phase diagram.

2. The method for determining the phase diagram of a quaternary cement system of MgO-MgSO4-MgCl2-H2O according to claim 1, characterized in that, The phase data mentioned in step S1 include solid phase data and liquid phase data. The solid phase data includes molar Gibbs free energy of formation, molar enthalpy of formation, reaction equilibrium constant, and constant pressure heat capacity. The liquid phase data includes ions that will appear in the liquid phase and ion interaction parameters. The ion interaction parameters include parameters for different groups of oppositely charged ions, like-charged ions, neutral particles, and charged ions.

3. The method for determining the phase diagram of a quaternary cement system of MgO-MgSO4-MgCl2-H2O according to claim 2, characterized in that, In the MgO-MgSO4-MgCl2-H2O quaternary system, for any magnesium salt The Gibbs free energy is: （1）， in, Magnesium salts at any Kelvin temperature T Gibbs free energy; Obtaining magnesium salts heat capacity at different temperatures With temperature Functional relationship between them: （2）， Among them, A, B, C, D and E are all fitting parameters; T is the Kelvin temperature in Kelvin; Magnesium salts Standard entropy Standard enthalpy and standard Gibbs free energy for: （3）， （4）； Substituting equations (2)-(4) into equation (1) yields magnesium salts. With fitting parameters and temperature The relevant Gibbs free energy is: （5）， in, （6）， （7）， in, express The free entropy at 298.15 K. express Free enthalpy at 298.15 K.

4. The method for determining the phase diagram of a quaternary cement system of MgO-MgSO4-MgCl2-H2O according to claim 3, characterized in that, According to formulas (1)-(6), let magnesium salts be... The chemical formula of magnesium salt is MX·nH2O, where n represents the number of bound water molecules contained within the magnesium salt, M represents a cation, and X represents an anion. Therefore, the heat capacity of MX·nH2O magnesium salt is: （8）， in, and The heat capacities of salts containing and without crystallization at Kelvin temperature T are respectively. The contribution value of one water of crystallization molecule in a salt containing water of crystallization; Free entropy and free enthalpy for: (9)， (10)， in, is the standard entropy of an anhydrous salt at Kelvin temperature T; The contribution of one water of crystallization molecule in a salt containing water of crystallization at a temperature of 298.15 K; The standard enthalpy of an anhydrous salt at 298.15 K at Kelvin temperature T; This represents the contribution of one water of crystallization molecule in a salt containing water of crystallization at a temperature of 298.15 K.

5. The method for determining the phase diagram of a quaternary cement system of MgO-MgSO4-MgCl2-H2O according to claim 2, characterized in that, For any reaction A + + B - = AB, reaction equilibrium constant logK AB for: (11)， (12)； in, It is the Gibbs free energy of a chemical reaction. R is the Gibbs formation energy of the substance, R is the gas constant, and T is the Kelvin temperature in K.

6. The method for determining the phase diagram of a quaternary cement system of MgO-MgSO4-MgCl2-H2O according to claim 2, characterized in that, The oppositely charged ions are replaced by ions of the same group with similar properties to the target ion, or by calculating the average value of the interaction parameters of equivalent substances within the system to replace the interaction parameters of the oppositely charged ions. The interaction parameters for any cation M and anion X are: (13)， in, y represents the interaction parameter between ions M and X, and y represents the number of oppositely charged ion pairs in a system containing the same ions, similar to the relationship between M and X ions. The interaction parameter represents the interaction between oppositely charged ion pairs.

7. The method for determining the phase diagram of a quaternary cement system of MgO-MgSO4-MgCl2-H2O according to claim 2, characterized in that, Step S2, establishing a thermodynamic database, refers to classifying the phase data according to elements, ions, and mineral phases, corresponding to the three inherent system classification modules SOLUTION_MASTER_SPECIES, SOLUTION_SPECIES, and PHASES in the PHREEQC software. Ion interaction parameters are then classified and input into different ion pair groups in the PITZER module according to different groups of oppositely charged ions, like-charged ions, and neutral particles and charged ions, ultimately forming a thermodynamic database for the MgO-MgSO4-MgCl2-H2O quaternary system. To confirm the reliability of the data in the thermodynamic database, it is necessary to calculate the solubility, pH value, and ion concentration of each phase, verify whether the substances can precipitate from the solution, and finally compare the calculation results with the experimental results.

8. The method for determining the phase diagram of a quaternary cement system of MgO-MgSO4-MgCl2-H2O according to claim 7, characterized in that, Whether a substance can precipitate from the solution also depends on the ion activity, for any phase. An electrolytic reaction occurs in the solution. The solubility product of this reaction for: (14)， in, , and Each refers to any ion D + The ion activity, concentration, and activity coefficient. , and Each refers to any ion E - The ion activity, concentration, and activity coefficient; The saturation index of this reaction is: (15)， in, is the solubility product constant, which is the ion activity product corresponding to the dissolution equilibrium of a phase, and is equivalent to the equilibrium constant for solid substances; In a liquid system, when the saturation index of the target phase is When the value is greater than 0, it indicates a supersaturated state, and corresponding solid phase precipitates will form. When... When the value is less than 0, the system is unsaturated and will maintain its current state.

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