MgSO4-based high-strength low-shrinkage grouting and pressing material and preparation method thereof

Abstract

The application discloses a magnesium oxysulfate-based high-strength low-shrinkage grouting and pressing material and a preparation method thereof, and belongs to the technical field of building materials. The grouting and pressing material is composed of light-burned magnesium oxide powder, magnesium sulfate heptahydrate, alkali magnesium sulfate nanosheets co-modified by phytic acid and dopamine, graded quartz sand, basalt fiber, citric acid, boric acid, polycarboxylate superplasticizer, defoaming agent and water. The phosphate and phenolic hydroxyl groups on the surface of the alkali magnesium sulfate nanosheets co-modified by phytic acid and dopamine can form interface bonding with 5.1.7 phase crystals, and the physical constraint of the basalt fiber can synergistically inhibit shrinkage; the hydration rate is regulated by the compounding of citric acid and boric acid, so that the structure of the hardened body is dense. The material has high strength, low shrinkage and good water resistance, and is suitable for engineering applications such as the sleeve grouting connection of fabricated buildings.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, and in particular to a sulfur-oxygen magnesium-based high-strength low-shrinkage grouting material and its preparation method. Background Technology

[0002] Magnesium oxysulfate cement is a magnesium-based cementitious material made primarily from lightly calcined magnesium oxide and magnesium sulfate solution. It boasts advantages such as lightweight hardened form, good fire resistance, strong adhesion, and excellent corrosion resistance. Compared to traditional silicate cement, magnesium oxysulfate cement has lower production energy consumption and carbon emissions, and it does not absorb moisture or exhibit efflorescence, and it is less corrosive to reinforcing steel. Therefore, it is considered a promising green and low-carbon cementitious material.

[0003] In recent years, research on the modification of magnesium sulfate cement has made some progress. In terms of chemical modification, citric acid can delay setting time, improve strength, and effectively inhibit shrinkage; admixtures such as phosphoric acid, tartaric acid, and boric acid can induce the formation of a large amount of the 5·1·7 phase (5Mg(OH)2·MgSO4·7H2O) in magnesium sulfate cement, optimizing the composition of hydration products and microstructure. Regarding mineral admixtures, fly ash and slag can optimize pore structure and reduce harmful porosity through the micro-aggregate effect. In terms of reinforcement and toughening, basalt fiber and polypropylene fiber have been used to improve the flexural strength and crack resistance of materials. In the field of grouting materials, studies have attempted to prepare grouting materials using magnesium sulfate cement as a matrix and investigated the effects of water-reducing agents and fibers on performance.

[0004] Despite these shortcomings, existing technologies still have the following limitations. First, there is often a trade-off between the strength and volume shrinkage of magnesium oxysulfate cement. High strength typically depends on a high MgO content and a low water-to-sulfur ratio, but excessive MgO hydration to form Mg(OH)₂ can lead to volume expansion and microcracks, resulting in decreased long-term volume stability. Second, existing modification methods mostly focus on single functions (such as improving strength or water resistance), lacking strategies for synergistic control of high strength and low shrinkage. Third, the application of nanomaterials in magnesium oxysulfate cement is still insufficient, and research on nanoscale reinforcement and chemical anchoring mechanisms is scarce. Furthermore, for the specific application scenario of grouting materials, existing magnesium oxysulfate-based materials still need improvement in terms of fluidity, adjustable setting time, and long-term volume stability. Therefore, developing a magnesium oxysulfate-based grouting material that combines high strength and low shrinkage is of significant practical importance. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by proposing a high-strength, low-shrinkage grouting material based on magnesium oxide and its preparation method.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A high-strength, low-shrinkage grouting material based on magnesium oxide sulfur oxide comprises the following components by weight: 280-320 parts of lightly calcined magnesium oxide powder, 120-150 parts of magnesium sulfate heptahydrate, 2-5 parts of phytic acid-dopamine co-modified basic magnesium sulfate nanosheets, 400-450 parts of graded quartz sand, 8-15 parts of basalt fiber, 1-2 parts of citric acid, 0.5-1.5 parts of boric acid, 2-4 parts of polycarboxylate superplasticizer, 0.3-0.8 parts of defoamer, and 130-160 parts of water.

[0007] The lightly calcined magnesium oxide powder has a total magnesium oxide content of ≥85%, an active magnesium oxide content of ≥60%, and a specific surface area of ​​≥20m². 2 / g; the graded quartz sand has a particle size of 20 mesh to 140 mesh in continuous gradation and a mud content of <0.5%; the basalt fiber has a length of 6 to 12 mm and a diameter of 13 to 15 μm; the defoamer is one of polyether defoamer or organosilicon defoamer.

[0008] The preparation method of the phytic acid-dopamine co-modified basic magnesium sulfate nanosheets includes the following steps: S1. Dissolve phytic acid aqueous solution and dopamine hydrochloride in deoxygenated and deionized water. Under inert gas protection, add sodium hydroxide solution dropwise to adjust the pH of the system to 7.5-8.5. Then, magnetically stir at 300-400 rpm for 25-35 min at 25-35℃ to obtain phytic acid-dopamine pre-assembled complex solution.

[0009] Under weakly alkaline conditions (pH 7.5–8.5), the catechol structure of dopamine is slowly oxidized, initiating a series of oxidative self-polymerization reactions to form polydopamine oligomers rich in catechol and amino functional groups. Inert gas protection can inhibit excessive self-polymerization of dopamine, maintaining it in a controllable oligomer state. Simultaneously, deprotonated phytic acid molecules, with their high-density negative charge, bind to the amino and phenolic hydroxyl groups of dopamine and its oligomers through electrostatic attraction and hydrogen bonding, forming a phytic acid-dopamine pre-assembled complex. This assembly retains the phytic acid's affinity for Mg... 2+ Its strong chelating ability, combined with the interfacial adhesion properties of polydopamine, provides a structure-guided basis for subsequent crystal growth.

[0010] S2. Dissolve magnesium sulfate monohydrate in deoxygenated and deionized water, add the phytic acid-dopamine pre-assembled complex solution and mix well. Add magnesium hydroxide under ultrasonic dispersion at 200-300W power, and continue ultrasonic dispersion for 5-10 min, followed by magnetic stirring for 15 min to form a suspension. Transfer the suspension to a polytetrafluoroethylene-lined hydrothermal reactor with a filling degree of 60-70%, and heat to 120-160℃ at a heating rate of 2-5℃ / min. Maintain the temperature for 6-10 h. After the reaction, allow it to cool naturally to room temperature, and centrifuge at 4000-6000 rpm to collect the precipitate. Wash the precipitate three times with deionized water and twice with anhydrous ethanol, and then freeze-dry it for 24-48 h at a temperature below -50℃ and a vacuum degree below 10 Pa to obtain phytic acid-dopamine co-modified basic magnesium sulfate nanosheets.

[0011] Under hydrothermal conditions, magnesium hydroxide partially dissolves and releases Mg. 2+ and OH - With SO4 in the liquid phase 2- 5.1.7 phase basic magnesium sulfate crystals are gradually generated via a dissolution-precipitation mechanism. In this process, the phytic acid-dopamine pre-assembly plays a dual regulatory role: it is speculated that the phosphate groups of phytic acid chelate Mg... 2+ They preferentially adsorb onto specific crystal faces, affecting the relative growth rate of each facet and thus limiting crystal growth along the thickness direction, inducing the formation of two-dimensional nanosheet morphology. This crystal facet regulation is similar to the template effect of organic molecules on the growth of inorganic crystals.

[0012] The hydrothermal temperature was controlled at 120–160℃, which provided the activation energy required for crystal nucleation and growth while preventing the decomposition of the organic modification layer. After the reaction, free impurities were removed by centrifugation and washing, and water was directly sublimated by freeze-drying, effectively avoiding hard agglomeration of the nanosheets due to capillary forces. Finally, basic magnesium sulfate nanosheets with surface covalently modified phytic acid and dopamine were obtained.

[0013] Preferably, in step S1, the molar ratio of phytic acid to dopamine hydrochloride is 1:1.5 to 2.5.

[0014] Preferably, in step S2, the molar ratio of magnesium hydroxide to magnesium sulfate monohydrate is 4 to 5:1.

[0015] A method for preparing a sulfur-oxygen-magnesium-based high-strength, low-shrinkage grouting material includes the following steps: T1. Phytic acid-dopamine co-modified basic magnesium sulfate nanosheets are dispersed in 40-60% of the total water content of the formulation and ultrasonically treated at 200-300W for 10-20 minutes to obtain a nanosheet dispersion.

[0016] Phytic acid-dopamine co-modified basic magnesium sulfate nanosheets are the core functional component in the formulation. The nanosheets are covalently modified with phytic acid and polydopamine: phytic acid provides phosphate groups, and polydopamine provides catechol and amino groups. These functional groups endow the nanosheets with two functions: firstly, as nanoscale fillers, they can effectively fill the micropores in cement paste, creating a dense microstructure; secondly, the surface of the phytic acid-dopamine co-modified basic magnesium sulfate nanosheets contains functional groups such as phosphate and phenolic hydroxyl groups. In the cement hydration environment, these functional groups can coordinate with Mg... 2+ This bonding allows the nanosheets to form an interfacial bond with the 5·1·7 phase crystal. This interfacial bond helps to enhance the interfacial bonding force between the nanosheets and the matrix, enabling the nanosheets to be more effectively anchored in the hydration product network. This, in turn, exerts a certain restraining effect on the relative displacement between the hydration product crystal networks, thus inhibiting volume shrinkage to some extent.

[0017] However, phytic acid-dopamine co-modified basic magnesium sulfate nanosheets have a large specific surface area and high surface energy, making them prone to aggregation when directly added to an aqueous phase. Therefore, in step T1, they are pre-dispersed in a portion of water, and the aggregates are opened by ultrasonic treatment (200–300 W, 10–20 min), allowing them to be uniformly dispersed in the liquid phase. The localized high temperature and pressure generated by the cavitation effect of ultrasound helps to open the nanosheet aggregates and allow them to be uniformly dispersed in the liquid phase.

[0018] T2. Dissolve magnesium sulfate heptahydrate in the remaining water, add citric acid and boric acid, and stir at 400-600 rpm for 5-10 minutes to mix evenly to obtain a modified magnesium sulfate solution.

[0019] Magnesium sulfate heptahydrate was dissolved in the remaining water, and citric acid and boric acid were added. Citric acid is the most classic modifier for magnesium oxysulfate cement; its mechanism of action is through the reaction of carboxyl groups with Mg. 2+ The complexation effect inhibits the excessive formation of Mg(OH)₂ and promotes the formation of 5.1.7 phase crystals. Boric acid and citric acid are used in combination. Citric acid reacts with Mg through its carboxyl group. 2+ The complexation effect regulates the early hydration process, inhibits the excessive generation of Mg(OH)2 and promotes the formation of 5·1·7 phase crystals; boric acid and citric acid participate in the coordination equilibrium in the solution. The two may delay the arrival of the hydration exothermic peak by forming boric acid-citric acid complex or gel precursor, providing a more suitable time window for the full nucleation and growth of 5·1·7 phase crystals, thereby reducing the microstructural defects caused by early rapid hydration and making the hardened body structure more compact.

[0020] T3. Mix the nanosheet dispersion with the modified magnesium sulfate solution until homogeneous to obtain a mixture.

[0021] T4. Mix lightly calcined magnesium oxide powder with graded quartz sand and dry mix for 2-4 minutes. Then, add basalt fiber in 2-3 batches during the dry mixing process. After each batch is added, continue to dry mix for 30-60 seconds. After all the fiber is added, continue to dry mix for 1-2 minutes until the fiber is completely dispersed to obtain a premixed dry material.

[0022] Lightly calcined magnesium oxide powder and graded quartz sand were pre-dry-mixed, and then basalt fibers were added in 2-3 batches. The basalt fibers were 6-12 mm in length and 13-15 μm in diameter, belonging to macroscopic short fibers. The purpose of adding them in batches was to avoid the clumping problem caused by adding the fibers all at once. After each batch was added, it was dry-mixed for 30-60 seconds to allow the fibers to be initially dispersed in the powder before adding the next batch. The basalt fibers formed a three-dimensional randomly distributed network structure in the matrix. When the matrix was subjected to load and microcracks were generated, the fibers crossed the crack surface through bridging, transmitting and dissipating stress, thereby inhibiting crack propagation. At the same time, the interfacial adhesion between the fibers and the matrix could resist the tensile stress caused by shrinkage, providing physical constraints on the shrinkage deformation of the matrix.

[0023] T5. Add the mixture to the premixed dry material, stir at a low speed of 135-145 rpm for 1-3 minutes, and then stir at a high speed of 280-300 rpm for 2-4 minutes. During the stirring process, add polycarboxylate superplasticizer and defoamer. The discharged material is the sulfur-oxygen magnesium-based high-strength low-shrinkage grouting material.

[0024] The mixture is added to the premixed dry material, and a staged stirring process is adopted, first at low speed (135-145 rpm) and then at high speed (280-300 rpm). The low-speed stirring stage ensures that the liquid fully wets the powder and fiber, avoiding dust and clumping; the high-speed stirring stage ensures that the components are evenly dispersed under shear force. Polycarboxylate superplasticizer and defoamer are added during this process to play a dispersing and defoaming role, respectively, ensuring the fluidity and density of the slurry.

[0025] The core design concept of this preparation method lies in "stepwise pre-dispersion and progressive mixing": phytic acid-dopamine co-modified basic magnesium sulfate nanosheets are pre-dispersed ultrasonically to avoid agglomeration; fibers are dry-mixed in batches to prevent clumping; the liquid and solid phases are prepared separately and then mixed to ensure uniform distribution of each component in the final slurry. The purpose of this process design is to allow each functional component to fully exert its role: phytic acid-dopamine co-modified basic magnesium sulfate nanosheets provide nanoscale filling and chemical anchoring; citric acid and boric acid regulate the composition of hydration products; and basalt fibers provide macroscale bridging reinforcement. These three components work synergistically to ultimately achieve a balance between high strength and low shrinkage.

[0026] Preferably, the process also includes molding and curing steps: after the slurry obtained in step T5 is poured into the mold, the surface is covered with a plastic film and cured for 20 to 28 hours in an environment with a temperature of 15 to 25°C and a relative humidity of ≥60%, the mold is removed, and then the mold is transferred to an environment with a temperature of 18 to 22°C and a relative humidity of 55 to 65% for curing until the specified age.

[0027] To avoid adverse transformation of hydration products under excessively humid conditions and to ensure early strength development, a two-stage process is adopted: first, high humidity retention (to promote hydration), followed by appropriate dry curing (to improve volume stability).

[0028] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention uses phytic acid-dopamine co-modified basic magnesium sulfate nanosheets as the core reinforcing component. Phytic acid molecules provide phosphate groups, and dopamine hydrochloride undergoes in-situ self-polymerization into polydopamine during preparation, thereby providing catechol and amino functional groups. These two components form an organic modification layer on the nanosheet surface. The phosphate and phenolic hydroxyl groups in this modification layer can react with the Mg on the surface of the 5.1.7 phase crystal of cement hydration products. 2+ Coordination bonds are formed, firmly anchoring the nanosheets within the crystal network. This interfacial bonding enhances the interfacial bond between the nanosheets and the matrix, thereby constraining the relative displacement between the hydrated product crystal networks. This effectively suppresses drying shrinkage without reducing strength, thus resolving the contradiction between high strength and low shrinkage in magnesium oxysulfate cement.

[0029] 2. This invention combines phytic acid-dopamine co-modified basic magnesium sulfate nanosheets with basalt fibers. The nanosheets fill micropores at the nanoscale and enhance the crystalline network through chemical bonding, while the basalt fibers form a three-dimensional randomly distributed network structure at the micrometer scale, bridging and dissipating stress across crack surfaces. Together, they form a cross-scale reinforcement mechanism of "nanofilling and chemical anchoring + micrometer fiber bridging and physical constraint," jointly improving the material's mechanical properties and volume stability.

[0030] 3. This invention uses a mixture of citric acid and boric acid as a coagulation regulator and phase accelerator. Citric acid reacts with Mg through its carboxyl group. 2+ The complexation effect of boric acid regulates the early hydration process and promotes the formation of 5·1·7 phase crystals; boric acid plays a retarding role, synergistically delaying the hydration exothermic peak with citric acid, thus avoiding microstructural defects caused by excessively rapid early hydration. The combined action of the two weak acids provides a suitable time window for the full crystallization of hydration products, making the hardened body structure more compact, thereby improving its water resistance and volume stability.

[0031] 4. The grouting material provided by this invention uses lightly calcined magnesium oxide and magnesium sulfate heptahydrate as the main cementing materials, combined with graded quartz sand and the aforementioned functional components, achieving high strength and low shrinkage performance while ensuring sufficient fluidity of the grout. The magnesium oxychloride cement system itself does not contain chloride ions, and its hydration products are nearly neutral, posing no risk of corrosion to reinforcing steel. It also overcomes the shortcomings of traditional magnesium oxychloride cement, such as easy moisture absorption and efflorescence, making it particularly suitable for engineering scenarios with high durability requirements, such as grouting connections for prefabricated building sleeves and grouting of prestressed ducts. Attached Figure Description

[0032] Figure 1 The present invention provides a process flow diagram for the preparation method of sulfur-oxygen magnesium-based high-strength low-shrinkage grouting material. Detailed Implementation

[0033] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with existing known technologies. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0034] Figure 1 This is a process flow diagram of the preparation method of magnesium sulfate-based high-strength and low-shrinkage grouting material of the present invention, which includes the preparation of nanosheet dispersion and modified magnesium sulfate solution, the mixing of the two and the addition of them to premixed dry material, the grouting material being obtained by segmented stirring, and the subsequent molding and curing process.

[0035] Example 1: A high-strength, low-shrinkage grouting material based on magnesium oxide, comprising the following components by weight: 280 parts of lightly calcined magnesium oxide powder, 120 parts of magnesium sulfate heptahydrate, 2 parts of phytic acid-dopamine co-modified basic magnesium sulfate nanosheets, 400 parts of graded quartz sand, 8 parts of basalt fiber, 1 part of citric acid, 0.5 parts of boric acid, 2 parts of polycarboxylate superplasticizer, 0.3 parts of defoamer, and 130 parts of water.

[0036] The lightly calcined magnesium oxide powder has a total magnesium oxide content of 85-92%, an active magnesium oxide content of 60-75%, and a specific surface area ≥20m². 2 The graded quartz sand has a particle size of 20-140 mesh with a mud content of <0.5%; the basalt fiber has a length of 6 mm, a diameter of 13 μm, and is graded CBF13-6, purchased from Zhejiang Shijin Basalt Fiber Co., Ltd.; the defoamer is a polyether defoamer, model SH-E325-B.

[0037] The preparation method of the sulfur-oxygen-magnesium-based high-strength, low-shrinkage grouting material includes the following steps: T1. Phytic acid-dopamine co-modified basic magnesium sulfate nanosheets were dispersed in 40% of the total water content of the formulation and ultrasonically treated at 200W for 10 min to obtain a nanosheet dispersion.

[0038] T2. Dissolve magnesium sulfate heptahydrate in the remaining water, add citric acid and boric acid, and stir at 400 rpm for 5 minutes to mix evenly to obtain a modified magnesium sulfate solution.

[0039] T3. Mix the nanosheet dispersion with the modified magnesium sulfate solution until homogeneous to obtain a mixture.

[0040] T4. Mix lightly calcined magnesium oxide powder with graded quartz sand and dry mix for 2 minutes. Then, add basalt fiber in two batches during the dry mixing process. After each batch is added, continue to dry mix for 30 seconds. After all the fiber is added, continue to dry mix for 1 minute until the fiber is completely dispersed to obtain the premixed dry material.

[0041] T5. Add the mixture to the premixed dry material, stir at a low speed of 135 rpm for 1 min, and then stir at a high speed of 280 rpm for 2 min. During the stirring process, add polycarboxylate superplasticizer and defoamer. The discharged material is the sulfur-oxygen magnesium-based high-strength low-shrinkage grouting material.

[0042] Molding and curing: After the slurry obtained in step T5 is poured into the mold, the surface is covered with plastic film and cured for 20 hours in an environment with a temperature of 15℃ and a relative humidity of ≥60% before demolding. Then, it is transferred to an environment with a temperature of 18℃ and a relative humidity of 55% for curing until the specified age.

[0043] The preparation method of the phytic acid-dopamine co-modified basic magnesium sulfate nanosheets includes the following steps: S1. Weigh 13.2 g of a 50% phytic acid aqueous solution (containing 0.01 mol of pure phytic acid) and 2.845 g of dopamine hydrochloride (0.015 mol, molar ratio 1:1.5), and dissolve them in 150 mL of deoxygenated deionized water. Under nitrogen protection, add sodium hydroxide solution dropwise to adjust the pH of the system to 7.5, and then magnetically stir at 300 rpm for 25 min at 25 °C to obtain a phytic acid-dopamine pre-assembled complex solution.

[0044] S2. Dissolve 14.24 g of magnesium sulfate monohydrate (0.1 mol) in 150 mL of deoxygenated deionized water, and add the above complexing solution and mix thoroughly. Under ultrasonic dispersion at 200 W, add 23.33 g of magnesium hydroxide (0.4 mol, molar ratio 4:1), and ultrasonically disperse for 5 min, then continue magnetic stirring for 15 min to form a suspension. Transfer the suspension to a 60% filled polytetrafluoroethylene-lined hydrothermal reactor (using a 500 mL reactor), heat to 120 °C at a heating rate of 2 °C / min, and maintain the temperature for 6 h. After the reaction, allow it to cool naturally to room temperature, and centrifuge at 4000 rpm to collect the precipitate. Wash the precipitate three times with deionized water and twice with anhydrous ethanol, and then freeze-dry it at -55 °C and a vacuum of 8 Pa for 24 h to obtain phytic acid-dopamine co-modified basic magnesium sulfate nanosheets. According to the measurements, the phytic acid-dopamine co-modified basic magnesium sulfate nanosheets prepared in this example have a phytic acid grafting amount of 8.2 wt% and a dopamine grafting amount of 0.9 wt%.

[0045] Example 2: A high-strength, low-shrinkage grouting material based on magnesium oxide, comprising the following components by weight: 300 parts of lightly calcined magnesium oxide powder, 135 parts of magnesium sulfate heptahydrate, 3.5 parts of phytic acid-dopamine co-modified basic magnesium sulfate nanosheets, 410 parts of graded quartz sand, 11.5 parts of basalt fiber, 1.5 parts of citric acid, 1.0 part of boric acid, 3 parts of polycarboxylate superplasticizer, 0.55 parts of defoamer, and 145 parts of water.

[0046] The lightly calcined magnesium oxide powder has a total magnesium oxide content of 85-92%, an active magnesium oxide content of 60-75%, and a specific surface area ≥20m². 2 The graded quartz sand has a particle size of 20-140 mesh with a mud content of <0.5%; the basalt fiber has a length of 12 mm and a diameter of 15 μm, is graded CBF15-12, and was purchased from Zhejiang Shijin Basalt Fiber Co., Ltd.; the defoamer is an organosilicon defoamer, model BS 1860, and was purchased from Shanghai Wenpu Chemical Technology Co., Ltd.

[0047] The preparation method of the sulfur-oxygen-magnesium-based high-strength, low-shrinkage grouting material includes the following steps: T1. Phytic acid-dopamine co-modified basic magnesium sulfate nanosheets were dispersed in 50% of the total water content of the formulation and ultrasonically treated at 250W for 15 minutes to obtain a nanosheet dispersion.

[0048] T2. Dissolve magnesium sulfate heptahydrate in the remaining water, add citric acid and boric acid, and stir at 500 rpm for 8 minutes to mix evenly to obtain a modified magnesium sulfate solution.

[0049] T3. Mix the nanosheet dispersion with the modified magnesium sulfate solution until homogeneous to obtain a mixture.

[0050] T4. Mix lightly calcined magnesium oxide powder with graded quartz sand and dry mix for 3 minutes. Then, add basalt fiber in two batches during the dry mixing process. After each batch is added, continue to dry mix for 40 seconds. After all the fiber is added, continue to dry mix for 1.5 minutes until the fiber is completely dispersed to obtain the premixed dry material.

[0051] T5. Add the mixture to the premixed dry material, stir at a low speed of 140 rpm for 2 minutes, and then stir at a high speed of 290 rpm for 3 minutes. During the stirring process, add polycarboxylate superplasticizer and defoamer. The discharged material is the sulfur-oxygen magnesium-based high-strength low-shrinkage grouting material.

[0052] Molding and curing: After the slurry obtained in step T5 is poured into the mold, the surface is covered with plastic film and cured for 25 hours in an environment with a temperature of 20℃ and a relative humidity of ≥60% before demolding. Then, it is transferred to an environment with a temperature of 20℃ and a relative humidity of 60% for curing until the specified age.

[0053] The preparation method of the phytic acid-dopamine co-modified basic magnesium sulfate nanosheets includes the following steps: S1. Weigh 13.2 g of 50% phytic acid aqueous solution (0.01 mol) and 3.793 g of dopamine hydrochloride (0.02 mol, molar ratio 1:2.0), and dissolve them in 150 mL of deoxygenated deionized water. Under nitrogen protection, add sodium hydroxide solution dropwise to adjust the pH of the system to 8.0, and then magnetically stir at 350 rpm for 30 min at 30 °C to obtain a phytic acid-dopamine pre-assembled complex solution.

[0054] S2. Dissolve 14.24 g of magnesium sulfate monohydrate (0.1 mol) in 150 mL of deoxygenated deionized water, and add the above complexing solution and mix thoroughly. Under ultrasonic dispersion at 250 W, add 26.24 g of magnesium hydroxide (0.45 mol, molar ratio 4.5:1), and ultrasonically disperse for 7.5 min, then continue magnetic stirring for 15 min to form a suspension. Transfer the suspension to a 60% filled polytetrafluoroethylene-lined hydrothermal reactor, heat to 140 °C at a heating rate of 3.5 °C / min, and maintain the temperature for 8 h. After the reaction, allow it to cool naturally to room temperature, and centrifuge at 5000 rpm to collect the precipitate. Wash the precipitate three times with deionized water and twice with anhydrous ethanol, and then freeze-dry it at -55 °C and a vacuum of 8 Pa for 36 h to obtain phytic acid-dopamine co-modified basic magnesium sulfate nanosheets. According to the measurements, the phytic acid-dopamine co-modified basic magnesium sulfate nanosheets prepared in this example have a phytic acid grafting amount of 10.5 wt% and a dopamine grafting amount of 1.2 wt%.

[0055] Example 3: A high-strength, low-shrinkage grouting material based on magnesium oxide, comprising the following components by weight: 320 parts of lightly calcined magnesium oxide powder, 150 parts of magnesium sulfate heptahydrate, 5 parts of phytic acid-dopamine co-modified basic magnesium sulfate nanosheets, 450 parts of graded quartz sand, 15 parts of basalt fiber, 2 parts of citric acid, 1.5 parts of boric acid, 4 parts of polycarboxylate superplasticizer, 0.8 parts of defoamer, and 160 parts of water.

[0056] The lightly calcined magnesium oxide powder has a total magnesium oxide content of 85-92%, an active magnesium oxide content of 60-75%, and a specific surface area ≥20m². 2 The graded quartz sand has a particle size of 20-140 mesh with a mud content of <0.5%; the basalt fiber has a length of 12 mm and a diameter of 15 μm, is graded CBF15-12, and was purchased from Zhejiang Shijin Basalt Fiber Co., Ltd.; the defoamer is an organosilicon defoamer, model BS 1860, and was purchased from Shanghai Wenpu Chemical Technology Co., Ltd.

[0057] The preparation method of the sulfur-oxygen-magnesium-based high-strength, low-shrinkage grouting material includes the following steps: T1. Phytic acid-dopamine co-modified basic magnesium sulfate nanosheets were dispersed in 60% of the total water content of the formulation and ultrasonically treated at 300W for 20 minutes to obtain a nanosheet dispersion.

[0058] T2. Dissolve magnesium sulfate heptahydrate in the remaining water, add citric acid and boric acid, and stir at 600 rpm for 10 minutes to mix evenly to obtain a modified magnesium sulfate solution.

[0059] T3. Mix the nanosheet dispersion with the modified magnesium sulfate solution until homogeneous to obtain a mixture.

[0060] T4. Mix lightly calcined magnesium oxide powder with graded quartz sand and dry mix for 4 minutes. Then, add basalt fiber in 3 batches during the dry mixing process. After each batch is added, continue to dry mix for 60 seconds. After all the fibers are added, continue to dry mix for 2 minutes until the fibers are completely dispersed to obtain the premixed dry material.

[0061] T5. Add the mixture to the premixed dry material, stir at a low speed of 145 rpm for 3 minutes, and then stir at a high speed of 300 rpm for 4 minutes. During the stirring process, add polycarboxylate superplasticizer and defoamer. The discharged material is the sulfur-oxygen magnesium-based high-strength low-shrinkage grouting material.

[0062] Molding and curing: After the slurry obtained in step T5 is poured into the mold, the surface is covered with plastic film and cured for 28 hours in an environment with a temperature of 25℃ and a relative humidity of ≥60% before demolding. Then, it is transferred to an environment with a temperature of 22℃ and a relative humidity of 65% for curing until the specified age.

[0063] The preparation method of the phytic acid-dopamine co-modified basic magnesium sulfate nanosheets includes the following steps: S1. Weigh 13.2 g of 50% phytic acid aqueous solution (0.01 mol) and 4.741 g of dopamine hydrochloride (0.025 mol, molar ratio 1:2.5), and dissolve them in 150 mL of deoxygenated deionized water. Under nitrogen protection, add sodium hydroxide solution dropwise to adjust the pH of the system to 8.5, and then magnetically stir at 400 rpm for 35 min at 35 °C to obtain a phytic acid-dopamine pre-assembled complex solution.

[0064] S2. Dissolve 14.24 g of magnesium sulfate monohydrate (0.1 mol) in 150 mL of deoxygenated deionized water, and add the above complexing solution and mix thoroughly. Under ultrasonic dispersion at 300 W, add 29.16 g of magnesium hydroxide (0.5 mol, molar ratio 5:1), and ultrasonically disperse for 10 min, followed by magnetic stirring for 15 min to form a suspension. Transfer the suspension to a 60% filled polytetrafluoroethylene-lined hydrothermal reactor, heat to 160 °C at a heating rate of 5 °C / min, and maintain the temperature for 10 h. After the reaction, allow it to cool naturally to room temperature, and centrifuge at 6000 rpm to collect the precipitate. Wash the precipitate three times with deionized water and twice with anhydrous ethanol, and then freeze-dry it at -55 °C and a vacuum of 8 Pa for 48 h to obtain phytic acid-dopamine co-modified basic magnesium sulfate nanosheets. According to the measurements, the phytic acid-dopamine co-modified basic magnesium sulfate nanosheets prepared in this example have a phytic acid grafting amount of 12.8 wt% and a dopamine grafting amount of 1.4 wt%.

[0065] Comparative Example 1: Based on Example 2, the difference is that: no phytic acid-dopamine co-modified basic magnesium sulfate nanosheets were added, step T1 was omitted (the original water volume of T1 was incorporated into T2 to keep the total water volume consistent with Example 2), and the rest was the same as Example 2.

[0066] Comparative Example 2: Based on Example 2, the difference is that the phytic acid-dopamine co-modified basic magnesium sulfate nanosheets are replaced with an equal amount of unmodified basic magnesium sulfate nanosheets (i.e., the addition of phytic acid aqueous solution and dopamine hydrochloride is omitted in step S1, and only deoxygenated deionized water is used instead; no phytic acid-dopamine pre-assembled complexing solution is added in S2), and the rest is the same as in Example 2.

[0067] Comparative Example 3: Based on Example 2, the difference is that boric acid is not added, the amount of citric acid is the same as in Example 2, and the rest is the same as in Example 2.

[0068] Comparative Example 4: Based on Example 2, the difference is that basalt fiber is not added, otherwise it is the same as Example 2.

[0069] Comparative Example 5: Based on Example 2, the difference is that the phytic acid-dopamine co-modified basic magnesium sulfate nanosheets are replaced with an equal amount of phytic acid-modified basic magnesium sulfate nanosheets (i.e., in S1, only the phytic acid aqueous solution is used instead of the mixed solution of phytic acid aqueous solution and dopamine hydrochloride; in S2, this is used instead of the phytic acid-dopamine pre-assembled complex solution), and the rest is the same as in Example 2.

[0070] Comparative Example 6: Based on Example 2, the difference is that the phytic acid-dopamine co-modified basic magnesium sulfate nanosheets were replaced with an equal amount of dopamine-modified basic magnesium sulfate nanosheets (i.e., no phytic acid aqueous solution was added in steps S1 and S2 during preparation), and the rest was the same as in Example 2.

[0071] Performance testing: The grouting materials prepared in Examples 1-3 and Comparative Examples 1-6 were subjected to performance testing according to the following standard methods: 1. Flowability test: The test was conducted in accordance with the standard GB / T 50448-2015 "Technical Specification for Application of Cement-based Grouting Materials".

[0072] 2. Setting time test: The test was conducted in accordance with the standard GB / T 1346-2011 "Test Methods for Standard Consistency Water Requirement, Setting Time and Soundness of Cement".

[0073] 3. Mechanical strength test: The test was conducted in accordance with standard GB / T 17671-2021 "Test Method for Strength of Cement Mortar (ISO Method)". The specimen size was 40mm×40mm×160mm, and the flexural strength and compressive strength were tested at 3d, 7d, 14d, and 28d.

[0074] 4. Drying shrinkage rate test: The test was conducted according to standard JC / T 603-2004 "Test Method for Drying Shrinkage of Cement Mortar". The specimen size was 25mm×25mm×280mm. The drying shrinkage rate at 28 days was tested and calculated.

[0075] 5. Water resistance test: The specimens cured for 28 days were completely immersed in water at (20±2)℃ for 28 days. The compressive strength was then measured, and the softening coefficient was calculated. Softening coefficient = Compressive strength after immersion in water for 28 days / Compressive strength after standard curing for 28 days.

[0076] The test results are as follows: Table 1. Test results for examples and comparative examples.

[0077] Data Analysis: Analysis of Table 1 shows that the sulfur-oxygen-magnesium-based high-strength, low-shrinkage grouting materials prepared in Examples 1-3 all have an initial flowability of 300 mm or higher, a 28-day compressive strength of 88.6-94.0 MPa, a flexural strength of 16.2-18.5 MPa, a drying shrinkage rate of 0.031-0.042%, and a softening coefficient of 0.91-0.93. Example 2 exhibits the best overall performance (28-day compressive strength 94.0 MPa, flexural strength 18.5 MPa, drying shrinkage rate 0.031%, softening coefficient 0.93).

[0078] In Comparative Example 1, the basic magnesium sulfate nanosheets without phytic acid-dopamine co-modification showed a decrease in compressive strength to 62.7 MPa after 28 days, an increase in shrinkage to 0.094%, and a decrease in softening coefficient to 0.80. This indicates that phytic acid-dopamine co-modified basic magnesium sulfate nanosheets are the core component for achieving high strength, low shrinkage, and good water resistance. Without phytic acid-dopamine co-modification, modification with citric acid and boric acid alone cannot sufficiently densify the microstructure, nor can it provide chemical anchoring constraints for shrinkage.

[0079] Comparative Example 2 replaced unmodified basic magnesium sulfate nanosheets with phytic acid-dopamine co-modified basic magnesium sulfate nanosheets. The 28-day compressive strength was 78.3 MPa, and the shrinkage rate was 0.058%. The unmodified nanosheets lacked phosphate and catechol groups on their surface, and could not form chemical bonds with the 5·1·7 phase crystals. They existed only as inert fillers, and their effect on inhibiting shrinkage was far less than that of the co-modified nanosheets. At the same time, the unmodified nanosheets were prone to agglomeration, and the flowability dropped to 290 mm, indicating that surface modification is crucial to the dispersibility of the nanosheets.

[0080] In Comparative Example 3, without the addition of boric acid, the initial setting time was shortened to 72 min, the final setting time to 152 min, the drying shrinkage rate increased to 0.072%, and the softening coefficient decreased to 0.79. The absence of boric acid resulted in an excessively rapid hydration reaction, insufficient development of the 5.1.7 phase crystal network, and a loose structure. This highlights the necessity of combining citric acid and boric acid, as boric acid provides a sufficient time window for the full growth of the 5.1.7 phase crystals.

[0081] In Comparative Example 4, without the addition of basalt fibers, the 28-day flexural strength decreased to 13.1 MPa, a 29.2% reduction compared to Example 2, while the shrinkage rate increased to 0.052%. The basalt fibers form a three-dimensional, randomly distributed network structure within the matrix, bridging crack surfaces to transfer and dissipate stress, while the interfacial adhesion resists shrinkage tensile stress. The absence of basalt fibers significantly reduces crack resistance and shrinkage constraint.

[0082] Comparative Example 5 replaced the phytic acid-dopamine co-modified basic magnesium sulfate nanosheets with phytic acid-modified basic magnesium sulfate nanosheets only. The 28-day compressive strength was 80.6 MPa, the flexural strength was 14.3 MPa, the shrinkage rate was 0.051%, and the softening coefficient was 0.84. All these indicators were lower than those of Example 2 but higher than those of Comparative Example 2. This indicates that although phytic acid modification alone can achieve the desired effect through the interaction of phosphate groups with Mg... 2+ Coordination occurs, but the lack of catechol groups for dopamine prevents in-situ anchoring modification and enhanced interfacial adhesion of polydopamine.

[0083] Comparative Example 6 replaced the phytic acid-dopamine co-modified nanosheets with dopamine-only modified nanosheets. Its 28-day compressive strength was 77.5 MPa, flexural strength was 13.2 MPa, shrinkage rate was 0.056%, softening coefficient was 0.78, and flowability was 288 mm. All these indicators were lower than those of Example 2 and Comparative Example 5. Comparing Comparative Example 5 and Comparative Example 6 reveals that the dopamine-only modified nanosheets lack the phosphate groups provided by phytic acid. The phosphate groups play at least two roles in this invention: one is through chelation of Mg... 2+ Firstly, it preferentially adsorbs onto specific crystal planes, thus regulating the growth of the 5·1·7 phase crystals. Secondly, it provides a high density of negative charges, imparting good dispersibility to the nanosheets through electrostatic repulsion. Without phytic acid, the dispersibility of the nanosheets significantly decreased, with the flowability dropping to 288 mm. Simultaneously, due to the lack of crystal plane regulation by phosphate groups, the nanosheets' intervention effect on the growth of hydration product crystals was weakened, with an intensity lower than that of the comparative example 5 modified only with phytic acid.

[0084] The performance comparison between Comparative Examples 5 and 6 shows that the enhancement and shrinkage inhibition effects of nanosheets modified with phytic acid or dopamine alone are inferior to those of nanosheets co-modified with both. This indicates that the co-modification of phytic acid and dopamine produced a significant synergistic effect in this invention, and the functional complementarity of the two is of great significance for fully leveraging the enhancing effect of the nanosheets.

[0085] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A sulfur-oxygen-magnesium based high-strength, low-shrinkage grouting material, characterized in that, By weight, it comprises the following components: 280-320 parts of lightly calcined magnesium oxide powder, 120-150 parts of magnesium sulfate heptahydrate, 2-5 parts of phytic acid-dopamine co-modified basic magnesium sulfate nanosheets, 400-450 parts of graded quartz sand, 8-15 parts of basalt fiber, 1-2 parts of citric acid, 0.5-1.5 parts of boric acid, 2-4 parts of polycarboxylate superplasticizer, 0.3-0.8 parts of defoamer, and 130-160 parts of water; The lightly calcined magnesium oxide powder has a total magnesium oxide content of ≥85%, an active magnesium oxide content of ≥60%, and a specific surface area of ​​≥20m². 2 / g; the graded quartz sand has a particle size of 20 mesh to 140 mesh with continuous gradation and a mud content of <0.5%; the basalt fiber has a length of 6 to 12 mm and a diameter of 13 to 15 μm; the defoamer is one of polyether defoamer or organosilicon defoamer; The preparation method of the phytic acid-dopamine co-modified basic magnesium sulfate nanosheets includes the following steps: S1. Dissolve phytic acid aqueous solution and dopamine hydrochloride in deoxygenated and deionized water. Under inert gas protection, add sodium hydroxide solution dropwise to adjust the pH of the system to 7.5-8.

5. Then, stir magnetically at 300-400 rpm for 25-35 min at 25-35℃ to obtain phytic acid-dopamine pre-assembled complex solution. S2. Dissolve magnesium sulfate monohydrate in deoxygenated and deionized water, add the phytic acid-dopamine pre-assembled complex solution and mix well. Add magnesium hydroxide under ultrasonic dispersion at 200-300W power, and continue ultrasonic dispersion for 5-10 min, followed by magnetic stirring for 15 min to form a suspension. Transfer the suspension to a polytetrafluoroethylene-lined hydrothermal reactor with a filling degree of 60-70%, and heat to 120-160℃ at a heating rate of 2-5℃ / min. Maintain the temperature for 6-10 h. After the reaction, allow it to cool naturally to room temperature, and centrifuge at 4000-6000 rpm to collect the precipitate. Wash the precipitate three times with deionized water and twice with anhydrous ethanol, and then freeze-dry it for 24-48 h at a temperature below -50℃ and a vacuum degree below 10 Pa to obtain phytic acid-dopamine co-modified basic magnesium sulfate nanosheets.

2. The sulfur-oxygen-magnesium based high-strength, low-shrinkage grouting material according to claim 1, characterized in that, In step S1, the molar ratio of phytic acid to dopamine hydrochloride is 1:1.5 to 2.

5.

3. The sulfur-oxygen-magnesium based high-strength, low-shrinkage grouting material according to claim 1, characterized in that, In step S2, the molar ratio of magnesium hydroxide to magnesium sulfate monohydrate is 4-5:

1.

4. A method for preparing a sulfur-oxygen-magnesium-based high-strength, low-shrinkage grouting material as described in any one of claims 1 to 3, characterized in that, Includes the following steps: T1. Phytic acid-dopamine co-modified basic magnesium sulfate nanosheets were dispersed in 40-60% of the total water content of the formulation and ultrasonically treated at 200-300W power for 10-20 min to obtain a nanosheet dispersion. T2. Dissolve magnesium sulfate heptahydrate in the remaining water, add citric acid and boric acid, and stir at 400-600 rpm for 5-10 minutes to mix evenly to obtain a modified magnesium sulfate solution. T3. Mix the nanosheet dispersion with the modified magnesium sulfate solution until homogeneous to obtain a mixture; T4. Mix lightly calcined magnesium oxide powder with graded quartz sand and dry mix for 2-4 minutes. Then, add basalt fiber in 2-3 batches during the dry mixing process. After each batch is added, continue to dry mix for 30-60 seconds. After all the fiber is added, continue to dry mix for 1-2 minutes until the fiber is completely dispersed to obtain a premixed dry material. T5. Add the mixture to the premixed dry material, stir at a low speed of 135-145 rpm for 1-3 minutes, and then stir at a high speed of 280-300 rpm for 2-4 minutes. During the stirring process, add polycarboxylate superplasticizer and defoamer. The discharged material is the sulfur-oxygen magnesium-based high-strength low-shrinkage grouting material.

5. The preparation method of the sulfur-oxygen-magnesium-based high-strength, low-shrinkage grouting material according to claim 4, characterized in that, It also includes molding and curing steps: After the slurry obtained in step T5 is poured into the mold, the surface is covered with a plastic film and cured for 20 to 28 hours in an environment with a temperature of 15 to 25°C and a relative humidity of ≥60% before demolding. Then it is transferred to an environment with a temperature of 18 to 22°C and a relative humidity of 55 to 65% for curing until the specified age.

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