Hydrophobic chlorine-fixing ca-al layered double hydroxide material and preparation method and application thereof

Abstract

The application discloses a hydrophobic chlorine-fixing Ca-Al layered double hydroxide material and a preparation method and application thereof, and belongs to the technical field of admixtures for building materials. The method comprises the following steps: dissolving Ca(NO3)2*4H2O and Al(NO3)3*9H2O in water according to a ratio of (1.5-4.5):1, adding lye to adjust the pH to 10.5-12, reacting for 0.5-5 hours, filtering and washing to obtain a precursor; adding the precursor into a carboxylate solution, and hydrothermally reacting at 60-120 DEG C for 2-12 hours; filtering and freeze-drying to obtain a carboxylate intercalated C-LDH; dispersing the C-LDH in an ethanol-water mixed solvent, adding an organic silane hydrophobic modifier, and reacting for 2-10 hours; and washing and drying to obtain the material. The material prepared by the method has the dual functions of fixing endogenous chlorine ions and blocking exogenous chlorine ions.

Description

Technical Field

[0001] This invention relates to the field of building material admixtures, and particularly to hydrophobic, chlorine-fixing Ca-Al layered bimetallic hydroxide materials, their preparation methods, and applications. Background Technology

[0002] Cement-based materials are the most commonly used cementitious materials in marine structures, underground structures, and transportation infrastructure, and their service performance directly affects the durability of engineering structures. In chloride-containing environments (seawater, sea fog, de-icing salt, underground brine, etc.), chloride ions can penetrate into the pore system of the hardened cement body with moisture and gradually migrate to the surface of the reinforcing steel. When the chloride ion concentration exceeds the critical value for the destruction of the passivation film on the reinforcing steel, it will induce pitting corrosion, expansion, and crack propagation in the steel, ultimately leading to a decrease in the structural load-bearing capacity or even failure. Therefore, improving the ability of cement-based materials to resist chloride ion corrosion is an important research direction in the field of engineering materials.

[0003] The susceptibility of existing cement-based materials to chloride ion attack stems primarily from the following two inherent defects: (1) Hardened cement is highly hydrophilic, which easily forms channels for chloride ion migration. Cement-based materials are inherently highly hydrophilic with a well-developed pore system, allowing water to rapidly penetrate the interior through capillary action. Since chloride ions mainly migrate through the liquid phase, water penetration directly determines the rate of chloride ion intrusion. Although existing technologies have attempted to block water migration by improving the pore structure or adding hydrophobic agents, the hydrophobic effect is unstable and easily decays, making it difficult to achieve effective protection against long-term chloride ion intrusion.

[0004] (2) The cement system has limited ability to solidify chloride ions. C3A in cement can react with chloride ions to form Friedel's salt, but this reaction is affected by the sulfate content, aluminate content and alkalinity of the system, and the chloride solidification ability is unstable, resulting in insufficient protection of the material against endogenous chloride ions.

[0005] To enhance the resistance of cement-based materials to chloride ion corrosion, researchers have explored various technical approaches. Layered bimetallic hydroxides (LDHs), due to their unique layered structure and anion exchange capacity, have been widely studied for their application in chloride ion curing. LDHs consist of positively charged metal hydroxide layers, interlayer anions, and water molecules. Their interlayer anions can react with chloride ions in the environment. - Ion exchange occurs, transferring Cl... - It is cured within the interlayer structure. For example, CN110316990A discloses a calcium-aluminum base layered double hydroxide chloride ion curing agent, comprising CaAl-NO3-LDHs, the surface of which is coated with stearic acid. Stearic acid is used as a carrier to achieve the slow-release of CaAl-NO3-LDHs in cement-based materials, thereby achieving the curing of chloride ions. -Effective curing. CN113929344A discloses a Ca-Al-polycarboxylate superplasticizer with early strength and slow-release curing chloride ion function, wherein the polycarboxylate superplasticizer is bonded between the layered calcium-aluminum compound skeleton.

[0006] However, the above technologies still have the following shortcomings: (1) Focusing only on chloride ion curing while neglecting the barrier against external chloride ion intrusion. Existing LDH-based chloride ion curing agents only achieve curing of chloride ions that have entered the material through interlayer anion exchange, but cannot prevent chloride ions from the external environment from continuously penetrating with moisture. During long-term service, the continuous intrusion of external chloride ions will exceed the curing capacity of the material, leading to protection failure.

[0007] (2) Poor LDH particle dispersibility affects curing efficiency. The LDH surface contains a large number of -OH groups, and strong hydrogen bonding makes the particles prone to agglomeration. Significant agglomeration may occur when the LDH content reaches 3 wt%, leading to a decrease in specific surface area and affecting the curing efficiency of Cl. - The binding ability is weakened. Although existing technologies attempt to improve the dispersibility of LDH in cementitious matrices through modification with silane coupling agents, the purpose of modification is mainly to enhance interfacial compatibility rather than to endow the material with hydrophobic properties.

[0008] (3) Lack of synergistic protection measures against both endogenous and exogenous chloride ions. Existing materials either only have chloride-fixing capabilities or only have hydrophobic barrier effects, and cannot achieve synergistic protection against both endogenous chloride ion solidification and exogenous chloride ion barrier, making it difficult to meet the long-term durability requirements of cement-based materials in complex environments such as sea sand, seawater, and underground saltwater.

[0009] Therefore, there is an urgent need for a new type of inorganic functional material that can efficiently solidify endogenous chloride ions through interlayer anion exchange and effectively block exogenous chloride ions from entering with moisture through surface hydrophobic modification, thereby fundamentally improving the durability and long-term service performance of cement-based materials in chloride salt environments. Summary of the Invention

[0010] The technical problem to be solved and the technical task proposed by this invention is to improve and refine existing technical solutions, and to provide a hydrophobic, chloride-fixing Ca-Al layered bimetallic hydroxide material, its preparation method, and its application, so as to achieve synergistic and efficient protection against endogenous and exogenous chloride ions in cement-based materials. To this end, this invention adopts the following technical solution.

[0011] In a first aspect, a method for preparing a hydrophobic, chlorine-fixing Ca-Al layered bimetallic hydroxide material is provided, comprising the following steps: S1: Dissolve Ca(NO3)2·4H2O and Al(NO3)3·9H2O in water at a molar ratio of (1.5~4.5):1 to prepare a mixed metal salt solution. Add alkali solution under stirring to adjust the pH of the solution to 10.5~12. React for 0.5~5 hours. After the reaction is complete, filter and wash the suspension to obtain the Ca-Al LDH precursor. S2: Prepare a carboxylate solution with a mass fraction of 5% to 60%, add the Ca-Al LDH precursor to the carboxylate solution, control the mass ratio of Ca-Al LDH precursor to carboxylate to be (1 to 4): 1, and perform hydrothermal reaction at 60 to 120°C for 2 to 12 hours. After filtration and freeze-drying, carboxylate-intercalated C-LDH is obtained. S3: The C-LDH is dispersed in an ethanol-water mixed solvent at a solid-liquid ratio of 1:(3-40). An organosilane hydrophobic modifier is added under stirring conditions so that the mass ratio of LDH to hydrophobic modifier is (2-12):1. After reacting for 2-10 hours, the mixture is washed and dried to obtain a hydrophobic solid chlorine type Ca-Al layered bimetallic hydroxide material.

[0012] This technical solution employs a stepwise preparation process of "precursor synthesis through co-precipitation → intercalation reaction → final surface hydrophobic modification," achieving stepwise controllable LDH layer construction, interlayer anion intercalation, and surface hydrophobic modification. Step S1 yields an LDH precursor with good crystallinity and nitrate as the dominant interlayer component; the binding force between nitrate and the interlayer is weak, making it easily replaced by subsequent polycarboxylate anions. Step S2 utilizes a higher reaction temperature and a longer reaction time to facilitate the full diffusion of polycarboxylate anions into the LDH interlayer, replacing the original nitrate ions and achieving efficient intercalation. Freeze-drying preserves the interlayer structure and dispersion state of the intercalation product. Step S3 uses an ethanol-water mixed solvent to provide a suitable medium environment for silane hydrolysis and condensation. The organosilane forms a hydrophobic layer on the LDH surface through hydrolysis and condensation reactions, achieving strong bonding through Si-O-Al and Si-O-Ca chemical bonds.

[0013] As a preferred technical means: the alkaline solution in step S1 is a NaOH solution or a KOH solution with a molar concentration of 0.5 to 10 mol / L.

[0014] NaOH and KOH are the most commonly used alkali sources for the co-precipitation synthesis of LDH. A molar concentration of 0.5–10 mol / L can ensure that the pH value is stably controlled within the range of 10.5–12 during the dropwise addition process, avoiding the formation of amorphous phases caused by excessively concentrated alkali solutions in some areas.

[0015] As a preferred technical means: the volume fraction of ethanol in the ethanol-water mixed solvent in step S3 is 30% to 80%.

[0016] An ethanol volume fraction of 30%–80% can provide a suitable medium environment for the hydrolysis and condensation reaction of organosilanes. Too low an ethanol content (<30%) will lead to excessively rapid hydrolysis of silanes and uncontrolled condensation polymerization; too high an ethanol content (>80%) will result in insufficient hydrolysis of silanes, affecting the quality of hydrophobic layer formation.

[0017] As a preferred technical means: the organosilane hydrophobic modifier in step S3 is added by dropping, with a dropping rate of 0.1 to 6 mL / min, and the reaction time of 2 to 10 hours is the time to continue the reaction after the dropping is completed.

[0018] Adding organosilane by dropwise avoids LDH agglomeration or uneven coating caused by excessively high local concentrations, ensuring uniform distribution of silane on the surface of each LDH particle. A drop rate of 0.1–6 mL / min allows for controlled addition of silane. After addition, the reaction continues for 2–10 hours, providing sufficient time for chemical bonding between silane and the LDH surface.

[0019] As a preferred technical means, the carboxylate is selected from one or more of citrate, tartrate, and maleate.

[0020] Polycarboxylic acid anions such as citrate, tartrate, and maleate have multiple carboxyl functional groups, which can provide stronger interlayer electrostatic interactions and more ion exchange sites. Compared with monocarboxylic acid or inorganic anions such as nitrate and nitrite, they have higher anion exchange capacity and more stable interlayer structure.

[0021] As a preferred technical means, the organosilane hydrophobic modifier is selected from one or more of hexadecyltrimethoxysilane, octadecyltriethoxysilane, octyltrimethoxysilane, and octyltriethoxysilane.

[0022] Long-chain alkylsilanes contain hydrophobic alkyl long chains, which can form a stable hydrophobic layer on the LDH surface through hydrolysis and condensation reactions, effectively reducing the surface energy of the material and giving it excellent hydrophobic properties.

[0023] Secondly, a hydrophobic, chlorine-fixing type Ca-Al layered bimetallic hydroxide material is provided, which is prepared by the aforementioned preparation method, and the material comprises: Ca-Al layered bimetallic hydroxide host plate; Polycarboxylate anions intercalated between the layers of the main body plate; and An organosilane hydrophobic modified layer bonded to the surface of the main body plate; The polycarboxylic acid anions are used to undergo ion exchange with chloride ions, thereby solidifying the chloride ions between the layers; the organosilane hydrophobic modification layer is used to make the surface of the material hydrophobic, so as to block the penetration of water and chloride ions.

[0024] This technical solution achieves both interlayer polycarboxylate intercalation modification and surface organosilane hydrophobic modification on the same Ca-Al LDH material, forming a synergistic protective structure of "internal solid chloride + external barrier". The interlayer polycarboxylate anions possess high anion exchange activity and can react with free Cl- in the pore fluid. - Ion exchange occurs, transferring Cl... - The curing process, occurring between LDH layers, effectively reduces the concentration of free chloride ions in the pore fluid. The surface organosilane hydrophobic modification layer significantly reduces the material's surface energy, making the material overall hydrophobic and preventing external moisture and dissolved chloride ions from penetrating into the cement-based material. This synergistic effect fundamentally solves the technical challenge of simultaneously addressing both endogenous chloride ion curing and exogenous chloride ion blocking in existing technologies.

[0025] As a preferred technical means, the molar ratio of calcium to aluminum in the Ca-Al layered bimetallic hydroxide main plate is 1.5 to 4.5:1.

[0026] A Ca / Al molar ratio in the range of 1.5 to 4.5 can ensure the integrity of the LDH main layer structure, while providing a suitable interlayer positive charge density, which is conducive to the stable intercalation of polycarboxylate anions and subsequent chloride ion exchange reactions.

[0027] Thirdly, an application of a hydrophobic, chlorine-fixing type Ca-Al layered bimetallic hydroxide material is provided, which is a hydrophobic, chlorine-fixing type Ca-Al layered bimetallic hydroxide material obtained by the aforementioned preparation method, or the aforementioned hydrophobic, chlorine-fixing type Ca-Al layered bimetallic hydroxide material; the material is added as an admixture to cement-based materials at a dosage of 1% to 8% of the mass of the cementitious material.

[0028] This technical solution incorporates the hydrophobic, chloride-fixing Ca-Al LDH material directly into cementitious materials as an admixture, rather than through surface coating. This allows the material to be uniformly dispersed within the cement matrix, providing dual protection through both endogenous chloride ion solidification and exogenous chloride ion blocking. A dosage of 1%–8% ensures the protective effect while avoiding the adverse effects of excessive admixture on cement hydration and mechanical properties.

[0029] Beneficial effects: 1. A stable "intercalation-hydrophobic" dual-functional synergistic structure is formed. By introducing polycarboxylate anions into the Ca-Al LDH interlayer and constructing an organosilane hydrophobic modification layer on the surface, synergistic protection of endogenous chloride ion solidification and exogenous chloride ion blocking is achieved on the same material. The interlayer polycarboxylate anions efficiently solidify chloride ions through an ion exchange mechanism. - The surface organosilane hydrophobic layer blocks external moisture and Cl by reducing surface energy. -The synergistic effect of penetration and penetration fundamentally solves the technical problem that existing technologies cannot simultaneously address the issue of endogenous chloride ion solidification and exogenous chloride ion barrier.

[0030] 2. Significantly enhances chloride ion curing capacity. The polycarboxylate anion intercalation provides a high density of exchangeable anion sites, resulting in higher ion exchange capacity and faster exchange kinetics. Tests show that the chloride ion binding rate of hardened cement paste incorporating the material of this invention is 10%–60% higher after 28 days compared to the unincorporated sample.

[0031] 3. Achieve overall hydrophobic modification of the material. By constructing an organosilane hydrophobic modification layer on the LDH surface, the material is transformed from hydrophilic to hydrophobic (contact angle ≥90°), effectively blocking the penetration channels of external moisture and chloride ions.

[0032] 4. It has virtually no impact on the mechanical properties of cement-based materials. Tests showed that the hardened cement paste incorporating the material of this invention maintained a compressive strength of 54.4–57.0 MPa after 28 days, showing no significant adverse changes compared to the unadulterated sample.

[0033] 5. The preparation process is simple and environmentally friendly. It employs a stepwise preparation process of co-precipitation, hydrothermal intercalation, and surface modification. The raw materials are readily available, the operation is simple, and high temperature and pressure are not required, making it suitable for industrial production. Furthermore, all raw materials used are non-toxic components, aligning with the development direction of green building materials. Attached Figure Description

[0034] Figure 1 This is a process flow diagram of the preparation method of the hydrophobic solid chlorine type Ca-Al layered bimetallic hydroxide material of the present invention. Detailed Implementation

[0035] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings.

[0036] In the following examples, the cementing material used is P·I 42.5 silicate cement, the chemical composition of which is shown in Table 1. Table 1 Chemical composition of silicate cement

[0037] Example 1 A method for preparing a hydrophobic, chlorine-solidifying Ca-Al layered bimetallic hydroxide material, such as... Figure 1 As shown, it includes the following steps: S1: Dissolve Ca(NO3)2·4H2O and Al(NO3)·9H2O (Ca / Al molar ratio 1.5:1) in 500 mL of deionized water. Stir at 600 rpm while adding 2 mol / L NaOH solution to maintain the pH at 10.5. Continue the reaction for 2 h. After the reaction is complete, filter and wash to obtain the Ca-Al LDH precursor.

[0038] S2: Prepare 200 mL of sodium citrate solution with a mass fraction of 10 wt%, add 20 g of LDH precursor to the solution, react in a hydrothermal reactor at 100 ℃ for 8 h, filter, freeze dry for 24 h to obtain C-LDH.

[0039] S3: 10 g of C-LDH was added to 100 mL of an ethanol-water mixed solvent (70% ethanol by volume) and dispersed evenly. 1 mL of hexadecyltrimethoxysilane was added dropwise at a rate of 0.1 mL / min while stirring at 600 rpm. After the addition was complete, the reaction continued for 6 h. The suspension was filtered, washed, and dried at 60 ℃ for 12 h to obtain the final hydrophobic solid-chlorine type Ca-Al LDH.

[0040] Example 2 A method for preparing a hydrophobic, chlorine-fixing Ca-Al layered bimetallic hydroxide material includes the following steps: S1: Dissolve Ca(NO3)2·4H2O and Al(NO3)·9H2O (Ca / Al molar ratio 2.5:1) in 600 mL of deionized water. Stir at 700 rpm while adding 5 mol / L KOH solution to maintain the pH at 11.0. Continue the reaction for 3 h. After the reaction is complete, filter and wash to obtain the Ca-Al LDH precursor.

[0041] S2: Prepare 250 mL of potassium tartrate solution with a mass fraction of 15 wt%, add 40 g of LDH precursor to the solution, react in a hydrothermal reactor at 60 ℃ for 2 h, filter, freeze dry for 36 h to obtain C-LDH.

[0042] S3: 12 g of C-LDH was added to 250 mL of an ethanol-water mixture (60% ethanol by volume) and dispersed evenly. 1 mL of hexadecyltrimethoxysilane was added dropwise at a rate of 1 mL / min while stirring at 800 rpm. After the addition was complete, the reaction continued for 5 h. The suspension was filtered, washed, and dried at 60 ℃ for 18 h to obtain the final hydrophobic solid-chlorine type Ca-Al LDH.

[0043] Example 3 A method for preparing a hydrophobic, chlorine-fixing Ca-Al layered bimetallic hydroxide material includes the following steps: S1: Dissolve Ca(NO3)2·4H2O and Al(NO3)·9H2O (Ca / Al molar ratio 3:1) in 700 mL of deionized water. Stir at 800 rpm while adding 8 mol / L KOH solution to maintain the pH at 12.0. Continue the reaction for 4 h. After the reaction is complete, filter and wash to obtain the Ca-Al LDH precursor.

[0044] S2: Prepare 300 mL of a 30 wt% disodium maleate solution, and add 180 g of the LDH precursor to the solution. React in a hydrothermal reactor at 120 °C for 10 h, filter, and freeze-dry for 24 h to obtain C-LDH.

[0045] S3: 100 g of C-LDH was added to 4 L of ethanol-water mixed solvent (ethanol volume fraction 30%) and dispersed evenly. 50 mL of hexadecyltrimethoxysilane was added dropwise at a rate of 6 mL / min while stirring at 700 rpm. After the addition was complete, the reaction continued for 10 h. The suspension was filtered, washed, and dried at 60 ℃ for 20 h to obtain the final hydrophobic solid-chlorine type Ca-Al LDH.

[0046] Example 4 A method for preparing a hydrophobic, chlorine-fixing Ca-Al layered bimetallic hydroxide material includes the following steps: S1: Dissolve Ca(NO3)2·4H2O and Al(NO3)·9H2O (Ca / Al molar ratio 4.5:1) in 900 mL of deionized water. Stir at 500 rpm while adding 10 mol / L KOH solution to maintain the pH of the solution at 11.5. Continue the reaction for 5 h. After the reaction is complete, filter and wash to obtain the Ca-Al LDH precursor.

[0047] S2: Prepare 100 mL of a 60 wt% trisodium citrate solution, add 120 g of LDH precursor to the solution, react in a hydrothermal reactor at 110 ℃ for 12 h, filter, freeze dry for 24 h to obtain C-LDH.

[0048] S3: 100 g of C-LDH was added to 1 L of ethanol-water mixed solvent (50% ethanol by volume) and dispersed evenly. 8.3 mL of hexadecyltrimethoxysilane was added dropwise at a rate of 1.5 mL / min while stirring at 400 rpm. After the addition was complete, the reaction continued for 10 h. The suspension was filtered, washed, and dried at 60 ℃ for 24 h to obtain the final hydrophobic solid-chlorine type Ca-Al LDH.

[0049] Application examples The hydrophobic, chloride-fixing Ca-Al layered bimetallic hydroxide from Examples 1-4 was used as an admixture to prepare silicate cement paste. The specific application method was as follows: to simulate an endogenous chloride ion environment, sodium chloride was added directly. Before sample preparation, the mixing water was divided into two parts: one part was used to dissolve sodium chloride, and the other part was mixed evenly with the admixture of this invention. Then, the two parts of mixing water were added to the silicate cement in batches, thoroughly stirred, and then sand was added and mixed evenly. The mixture was then poured into a 40×40×40 mm mold, compacted, sealed, and placed in a standard curing room for curing. The amount of admixture used was 1%–8% of the total mass of the cementitious material. During the preparation process, the water-cement ratio of the cementitious material was 0.4, and a commercially available polycarboxylate superplasticizer (40% solids content) was used to adjust the fluidity of the paste.

[0050] Comparative Example 1 The cementitious material in Comparative Example 1 contained only silicate cement and did not contain the hydrophobic, chloride-fixing admixture described in this invention. During preparation, the water-cement ratio of the cementitious material was 0.4.

[0051] Performance testing The mechanical properties, hydrophobic properties, and resistance to chloride ion attack of the samples from Comparative Example 1 and Examples 1-4 were determined. Among them: The compressive strength of the samples was tested according to GB / T 17671-2021 "Test Method for Strength of Cement Mortar (ISO Method)".

[0052] The water-soluble chloride ion content was measured 28 days before and after the addition of the admixture, according to SL / T 352-2020 "Test Procedure for Hydraulic Concrete". The chloride ion curing rate was the ratio of the difference between the initial chloride ion content and the remaining chloride ion content to the initial chloride ion content.

[0053] The hydrophobicity test method is as follows: After molding, the sample is cured under standard curing conditions for 28 days, then removed and dried at 40–60 °C to constant weight to eliminate the influence of free water on the sample surface. The dried sample is placed on a horizontal test platform, and a certain volume (2–5 μL) of deionized water droplets is slowly added to the sample surface. The morphological image of the droplets on the sample surface is acquired by a contact angle measuring device, and the static contact angle formed between the droplets and the sample surface is calculated.

[0054] The water absorption rate is tested as follows: the mass of the sample is weighed before immersion in water and after 24 hours of immersion, and the water absorption rate is calculated using the following formula:

[0055] The dosage of hydrophobic chlorine-fixing admixtures in Comparative Example 1 and Examples 1-4, as well as the performance test results of each group of samples, are shown in Table 2.

[0056] Table 2 Test results of Comparative Example 1 and Examples 1-4

[0057] As can be seen from the test results in Table 2, after incorporating the hydrophobic chloride-fixing admixture in Examples 1–4, the 28-day chloride ion curing rate increased from 43.2% in the comparative example to 48.0%–68.0%, an increase of approximately 10%–60%, significantly enhancing the material's ability to cure chloride ions; the material surface contact angle increased from 54° to 90°–108°, changing from a hydrophilic state to a stable hydrophobic state; and the 28-day water absorption rate decreased from 6.8% to 3.0%–4.9%, a decrease of approximately 28%–56%. Furthermore, the 28-day compressive strength of the cement-based material remained within the range of 54.4–57.0 MPa, showing no significant adverse changes compared to Comparative Example 1 (56.2 MPa), indicating that the addition of the admixture had a relatively small impact on the mechanical properties of the cement-based material. The above results show that the hydrophobic chloride-fixing admixture of the present invention, without significantly affecting the mechanical properties of cement-based materials, achieves effective protection against endogenous and exogenous chloride ions through the synergistic effect of chloride ion solidification and hydrophobic barrier, thus significantly improving the durability of cement-based materials.

[0058] The above embodiments are merely examples for clear illustration and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations, and any obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing a hydrophobic, chlorine-fixing type Ca-Al layered bimetallic hydroxide material, characterized in that, Includes the following steps: S1: Dissolve Ca(NO3)2·4H2O and Al(NO3)3·9H2O in water at a molar ratio of (1.5~4.5):1 to prepare a mixed metal salt solution. Add alkali solution under stirring to adjust the pH of the solution to 10.5~12. React for 0.5~5 hours. After the reaction is complete, filter and wash the suspension to obtain the Ca-Al LDH precursor. S2: Prepare a carboxylate solution with a mass fraction of 5% to 60%, add the Ca-Al LDH precursor to the carboxylate solution, control the mass ratio of Ca-Al LDH precursor to carboxylate to be (1 to 4): 1, and perform hydrothermal reaction at 60 to 120°C for 2 to 12 hours. After filtration and freeze-drying, carboxylate-intercalated C-LDH is obtained. S3: The C-LDH is dispersed in an ethanol-water mixed solvent at a solid-liquid ratio of 1:(3-40). An organosilane hydrophobic modifier is added under stirring conditions so that the mass ratio of LDH to hydrophobic modifier is (2-12):

1. After reacting for 2-10 hours, the mixture is washed and dried to obtain a hydrophobic solid chlorine type Ca-Al layered bimetallic hydroxide material.

2. The method for preparing a hydrophobic, chlorine-fixing Ca-Al layered bimetallic hydroxide material according to claim 1, characterized in that: The alkaline solution mentioned in step S1 is a NaOH solution or a KOH solution with a molar concentration of 0.5–10 mol / L.

3. The method for preparing a hydrophobic, chlorine-fixing Ca-Al layered bimetallic hydroxide material according to claim 1, characterized in that: The volume fraction of ethanol in the ethanol-water mixed solvent in step S3 is 30% to 80%.

4. The method for preparing a hydrophobic, chlorine-fixing Ca-Al layered bimetallic hydroxide material according to claim 1, characterized in that: The organosilane hydrophobic modifier mentioned in step S3 is added by dropping, with a dropping rate of 0.1 to 6 mL / min, and the reaction time of 2 to 10 hours is the time to continue the reaction after the dropping is completed.

5. The method for preparing a hydrophobic, chlorine-fixing type Ca-Al layered bimetallic hydroxide material according to claim 1, characterized in that: The carboxylate is selected from one or more of citrate, tartrate, and maleate.

6. The method for preparing a hydrophobic, chlorine-fixing type Ca-Al layered bimetallic hydroxide material according to claim 1, characterized in that: The organosilane hydrophobic modifier is selected from one or more of hexadecyltrimethoxysilane, octadecyltriethoxysilane, octyltrimethoxysilane, and octyltriethoxysilane.

7. A hydrophobic, chlorine-fixing type Ca-Al layered bimetallic hydroxide material, characterized in that, The material is prepared by the method for preparing a hydrophobic, chlorine-fixing Ca-Al layered bimetallic hydroxide material according to any one of claims 1 to 6, comprising: Ca-Al layered bimetallic hydroxide host plate; Polycarboxylate anions intercalated between the layers of the main body plate; and An organosilane hydrophobic modified layer bonded to the surface of the main body plate; The polycarboxylic acid anions are used to undergo ion exchange with chloride ions, thereby solidifying the chloride ions between the layers; the organosilane hydrophobic modification layer is used to make the surface of the material hydrophobic, so as to block the penetration of water and chloride ions.

8. The hydrophobic, chlorine-fixing type Ca-Al layered bimetallic hydroxide material according to claim 7, characterized in that: The molar ratio of calcium to aluminum in the Ca-Al layered bimetallic hydroxide matrix is ​​1.5 to 4.5:

1.

9. An application of a hydrophobic, chlorine-fixing type Ca-Al layered bimetallic hydroxide material, characterized in that: The hydrophobic, chlorine-fixing Ca-Al layered bimetallic hydroxide material obtained by the preparation method according to any one of claims 1-6, or the hydrophobic, chlorine-fixing Ca-Al layered bimetallic hydroxide material according to claim 7 or 8; the material is added as an admixture to cement-based materials at a dosage of 1% to 8% of the mass of the cementitious material.

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