Modified magnesium phosphate cement grouting material as well as preparation method and application thereof

Abstract

The invention discloses a modified magnesium phosphate cement grouting material as well as a preparation method and application thereof. The modified magnesium phosphate cement grouting material comprises magnesium phosphate cement prepared from dead burned magnesium oxide, ammonium dihydrogen phosphate, borax, a polycarboxylate superplasticizer and water; and a water repellent agent added to the magnesium phosphate cement. Further, glacial acetic acid is added during preparation of the magnesium phosphate cement. Furthermore, the modified magnesium phosphate cement grouting material further comprises silane coupling agent modified PVA (polyvinyl alcohol) fibers. The MPC is endowed with excellent compressive and breaking strength, low-temperature toughness, water resistance and controllable setting time by virtue of the synergistic effect of delayed coagulation of glacial acetic acid and the silane coupling agent modified PVA fiber and the effect of the water repellent agent, so that the MPC has high durability and construction feasibility in prestressed duct grouting engineering in severe cold areas; and a reliable solution is provided for engineering application in an extreme environment.

Description

Technical Field

[0001] The invention belongs to the field of building materials, and in particular relates to a modified magnesium phosphate cement grouting material and a preparation method and application thereof. Background Art

[0002] Grouting material (grouting agent) is a high-performance building material specially used for grouting of post-tensioned prestressed ducts. It is mainly composed of cement-based materials and a variety of admixtures. It is widely used in duct filling and equipment foundation reinforcement in prestressed structures such as bridges and buildings.

[0003] In the early 21st century, with global climate change and the rapid advancement of infrastructure construction in cold regions, the quality and durability of projects constructed in low-temperature environments have attracted considerable attention. Traditional grouting agents, primarily designed for ambient or high-temperature environments, often exhibit high water exudation, poor frost resistance, and insufficient early strength under low-temperature conditions. Magnesium phosphate cement (MPC) systems, for example, while exhibiting early-strength properties, generally suffer from inherent drawbacks such as low flexural strength, insufficient crack resistance, and poor water resistance. In particular, in low-temperature environments, the setting time is difficult to effectively control, and the rapid hydration reaction rate can lead to a limited construction window. This not only reduces the anchoring performance of prestressed tendons but also potentially weakens the stability of bridge structures. Furthermore, frost heave and freeze-thaw damage can easily lead to structural defects in the grouting, further compromising the safety of bridges in low-temperature environments. Ambient-temperature grouting materials, on the other hand, struggle to hydrate properly in low-temperature environments and are susceptible to frost damage, resulting in slow strength development. Developing grouting agents specifically designed for low-temperature environments has become a key technical focus for improving project quality, shortening construction periods, and ensuring long-term performance.

[0004] Bridges in high-altitude and seasonally frozen regions are often affected by freeze-thaw cycles, rainwater erosion, or groundwater infiltration. Therefore, in the engineering application of low-temperature grouting agents, water resistance is one of the key indicators that determine their long-term service performance. Due to the chemical instability of hydration products, traditional MPC systems are prone to microstructural degradation in humid environments, resulting in significantly weaker water resistance than silicate cement systems. If the grouting agent lacks water resistance, its internal pore structure is easily degraded by water intrusion, causing strength loss, volume expansion, and even affecting the bond with steel bars. This shows a direct correlation between insufficient water resistance of the grouting agent and steel corrosion.

[0005] Prestressed pipe grouting is the process of injecting grouting material into the reserved prestressed channels so that the grouting material fully wraps the prestressed tendons. Its function is to protect the prestressed steel bars from rusting and to make them well bonded with the concrete, thus ensuring the effective transmission of prestress. However, due to its insufficient crack resistance, traditional MPC materials are prone to microcracks under the action of freeze-thaw cycles, which intensifies the migration of moisture and chloride ions to the surface of the steel bars. At the same time, its poor water resistance will accelerate the electrochemical corrosion process in the grouting body-steel bar interface area. For the prestressed tendons in the channel grouting, their large specific surface area further amplifies the risk of corrosion, and in low temperature environments, it may induce the formation of through-cracks in the prestressed beam body. Therefore, the development of grouting materials that have both adaptability to negative temperature environments, excellent crack resistance and long-term water-resistant protection has become a key technical requirement to ensure the durability of the prestressed system. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the deficiencies and defects mentioned in the above background technology and provide a -25°C low-temperature resistant slow-setting enhanced water-resistant combined modified magnesium phosphate cement grouting material and its preparation method and application.

[0007] In order to solve the above technical problems, the technical solution proposed by the present invention is: A modified magnesium phosphate cement grouting material, comprising: magnesium phosphate cement made from dead-burned magnesium oxide, monoammonium phosphate, borax, polycarboxylate superplasticizer, and water; and hydrophobic agents added to magnesium phosphate cement; The hydrophobic agent is prepared by the following method: stearic acid powder, nano-silica and silane coupling agent KH-570 are dispersed in ethanol, and the mass ratio of nano-silica, ethanol, KH-570 and stearic acid is (4-5): (84-87): (2-3): (7-9); Based on the total mass of dead-burned magnesium oxide and ammonium dihydrogen phosphate as 100%, the amount of the hydrophobic agent is 0.75% to 1%.

[0008] As a further improvement, glacial acetic acid is further added during the preparation of the magnesium phosphate cement; based on the total mass of dead-burned magnesium oxide and ammonium dihydrogen phosphate being 100%, the added amount of glacial acetic acid is 0.08% to 0.12%.

[0009] As a further improvement, the modified magnesium phosphate cement grouting material also includes modified PVA fiber; the modified PVA fiber is obtained by soaking PVA fiber in an ethanol solution of silane coupling agent KH550; based on the total mass of dead-burned magnesium oxide and ammonium dihydrogen phosphate as 100%, the added amount of the modified PVA fiber is 0.2%~0.4%.

[0010] As a further improvement, in the magnesium phosphate cement, the mass ratio of dead-burned magnesium oxide to ammonium dihydrogen phosphate is (3.5-4.5):1, and the water-binder ratio is 0.16-0.18; based on the total mass of dead-burned magnesium oxide and ammonium dihydrogen phosphate as 100%, the amount of borax added is 10-12%, and the amount of polycarboxylate water reducer added is 0.5-0.75%.

[0011] As a further improvement, the mass fraction of KH550 in the ethanol solution of the silane coupling agent KH550 is 0.8-1.0%, and the immersion treatment time is 40 minutes.

[0012] As a further improvement, the hydrophobic agent is prepared by the following method: adding stearic acid powder to ethanol, heating and stirring until completely dissolved, then adding nano-silicon dioxide and KH-570 in sequence, and ultrasonically dispersing and filtering.

[0013] The present invention provides a method for preparing the modified magnesium phosphate cement grouting material, comprising the following steps: Dead-burned magnesium oxide, ammonium dihydrogen phosphate, and borax are dry-mixed and stirred, and then a polycarboxylate water reducer and water, or a mixed solution of a polycarboxylate water reducer, glacial acetic acid, and water is added, and stirred to obtain magnesium phosphate cement; The modified PVA fiber and / or the hydrophobic agent are added to magnesium phosphate cement and stirred, and then formed and cured.

[0014] The present invention provides an application of the modified magnesium phosphate cement grouting material. The modified magnesium phosphate cement grouting material is cured at -25°C and is suitable for a construction environment of 0°C to -25°C.

[0015] Magnesium phosphate cement (MPC) has been selected as the core material for prestressed duct grouting in extremely cold regions due to its unique performance advantages. Firstly, MPC is significantly environmentally friendly. Its hardening process releases little heat, minimizing temperature disturbances in low-temperature construction environments. It also has low carbon emissions and complies with green building standards. Even at extremely low temperatures (e.g., -25°C), MPC maintains stable setting properties and exhibits excellent frost resistance, resisting damage from freeze-thaw cycles and ice crystal expansion. Its high initial compressive strength makes it suitable for prestressed construction projects. However, conventional MPC suffers from insufficient flexural strength, poor crack resistance, difficulty in controlling low-temperature setting times, and poor water resistance. In particular, rapid hydration in extremely cold environments can lead to insufficient construction time. To address this, the present invention specifically incorporates retarding technology, fiber reinforcement, and water resistance optimization strategies to enhance its overall performance and ensure stability and reliability under various environmental conditions.

[0016] MPC is prone to shortening the setting time at low temperatures due to the rapid neutralization reaction between phosphate and magnesium oxide, affecting construction operability. To this end, the present invention achieves slow setting by adding a certain amount of glacial acetic acid during the stirring stage. The weak acidity of glacial acetic acid can inhibit the rapid reaction of phosphate and magnesium oxide. At the same time, acetate ions reduce the concentration of active magnesium ions through complexation, delaying the crystallization of hydration products; the dual role of glacial acetic acid in low-temperature environments is particularly critical: on the one hand, it inhibits ice crystal formation by lowering the freezing point of the slurry liquid phase; on the other hand, it hinders the contact of reactants through the surface adsorption effect. The synergistic effect extends the setting time by 70 minutes, successfully solving the pain point of insufficient construction window period in extreme low temperatures and ensuring the operability of the grouting process.

[0017] Leveraging the unique bifunctional properties of the silane coupling agent KH550, this invention achieves multi-dimensional interface reinforcement between PVA (polyvinyl alcohol) fibers and an MPC matrix. Silanol groups, generated by hydrolysis of the silane groups, form a dense siloxane network on the fiber surface through a polycondensation reaction, forming stable chemical bonds with the hydroxyl groups of the PVA molecular chains and simultaneously forming a physical coating. The amino groups on the organic end, due to their high polarity, physically adsorb to the fiber surface through hydrogen bonding and chemically bond with active components in the MPC hydration products, forming a gradient interface structure of "fiber-siloxane layer-cement." This dual bonding mechanism not only significantly enhances the anchoring strength between the fiber and the matrix but also blocks water penetration through the hydrophobic effect of the siloxane layer. Under low-temperature stress, the modified interface absorbs crack propagation energy through elastic deformation of chemical bonds and dynamic reconstruction of hydrogen bonds, enabling efficient stress transfer from the brittle matrix to the high-tenacity fiber. Ultimately, this achieves a synergistic improvement in the material's crack resistance, deformation capacity, and durability under extreme conditions.

[0018] First, the mechanical properties have been significantly enhanced. The modified fibers form a microscopic reinforcement network in the MPC matrix, effectively inhibiting the expansion of cracks and significantly improving the flexural strength. The toughness of the matrix shows a significant improvement at low temperatures (-25°C), and the crack morphology tends to be smooth, showing better crack resistance. Secondly, durability has also been optimized. The uniform dispersion of the fibers promotes the formation of hydration products, reduces the number of pores and connectivity in the matrix, making the matrix denser, reducing water absorption, and effectively improving water resistance and frost resistance. Finally, construction adaptability has also been improved. The evenly distributed fibers optimize the viscosity and workability of the slurry. Combined with retarding technology, the balance between the fluidity and strength development of the slurry is maintained under low-temperature construction conditions, ensuring the smooth progress of construction.

[0019] Hydrophobic agents improve the water resistance of magnesium phosphate cement through multiple mechanisms: Nanosilica, evenly dispersed in anhydrous ethanol, fills the microscopic pores of the cement matrix, reducing capillary water absorption channels. KH-570 silane coupling agent undergoes hydrolysis and condensation in the aqueous system, with its silanol groups chemically bonding with the Mg-O bonds in the cement hydration products and simultaneously grafting hydrophobic methacryloyloxy groups onto the surface of the material. Stearic acid molecules bind to the magnesium oxide hydration products through their carboxyl groups, with their long-chain alkyl groups oriented to form a dense hydrophobic layer. These three agents synergistically form a composite hydrophobic system of "pore filling, chemical bonding, and molecular directional arrangement" during the cement paste curing process, effectively blocking water penetration pathways.

[0020] By utilizing the above-mentioned modification technology, the present invention imparts MPC with excellent compressive and flexural strength, low-temperature toughness, water resistance, and controllable setting time through the synergistic effects of glacial acetic acid retarding and silane coupling agent-modified PVA fibers, as well as the effect of a hydrophobic agent. This enables it to have both high durability and construction feasibility in prestressed duct grouting projects in severely cold regions, providing a reliable solution for engineering applications in extreme environments.

[0021] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention is based on a mass ratio of dead-burned magnesium oxide to ammonium dihydrogen phosphate of 4:1 (hereinafter referred to as M / P=4, or a magnesium-phosphorus ratio of 4:1) in magnesium phosphate cement, and a curing condition of -25°C. Through tests on water-binder ratio, borax, and polycarboxylic acid-based water reducer, the optimal mix ratio is obtained: water-binder ratio 0.17, borax 12%, water reducer 0.6%, and glacial acetic acid 0.1%. At this time, the 30s fluidity, setting time, compressive strength, and flexural strength of MPC show the best balance.

[0022] (2) The modified PVA fibers added to the cement matrix not only provide more microscopic interfaces, promote the formation of hydration products, and make the contact between cement particles closer, but also improve the dispersion of the fibers in the cement paste, change the viscosity of the cement paste, and thus accelerate the hydration reaction. The silane coupling agent reacts with the Mg-Si-O- in the MPC through hydrolysis. 2+ The coordination significantly improved the interfacial bonding strength, and the modified PVA fibers were evenly distributed in the MPC, the cracks were smoother overall, and the toughness of the matrix was significantly enhanced.

[0023] (3) In the present invention, silane coupling agent KH550 is added to the PVA fiber for modification. KH550 can form a modified layer on the surface of the PVA fiber. The modified fiber surface is rougher, which helps to enhance the interfacial bonding force between the PVA fiber and the MPC hydration product. In addition, the silane molecules grafted onto the fiber surface improve its hydrophilicity. The surface microstructure can help the fiber to be more easily dispersed in the matrix, inhibit the expansion of cracks, and thus improve the toughness of the material.

[0024] (4) The present invention utilizes silane coupling agent modification technology to enhance the interfacial adhesion between the fiber and the MPC matrix, and forms a hydrophobic film on the fiber surface, which makes the fiber bond more tightly with the cement particles and reduces the water adsorption capacity of the pore surface. As a result, the MPC matrix becomes denser, reducing the number and connectivity of pores, thereby effectively reducing water absorption and improving the water resistance of the matrix.

[0025] (5) The present invention significantly improves the water resistance of magnesium phosphate cement through the combined action of hydrophobic agents: nano-silica uniformly fills the matrix pores in the dispersion medium, blocking capillary water absorption channels; silane coupling agents bond with cement products after hydrolysis, grafting hydrophobic groups on the surface to form a chemical barrier; and stearic acid molecules are oriented to form a dense hydrophobic layer. The three together form a multi-level protection system of "physical filling-chemical bonding-molecular arrangement" on the microstructure, synergistically inhibiting water penetration, allowing the material to maintain stable impermeability in extremely low-temperature environments and effectively extending the durability of the structure.

[0026] (6) The present invention adds 0.1% glacial acetic acid during the stirring process of magnesium phosphate cement. Its weak acidity can inhibit the rapid neutralization reaction of phosphate and magnesium oxide. At the same time, acetate ions reduce the concentration of active magnesium ions through complexation, delaying the crystallization of hydration products. Under low temperature conditions, glacial acetic acid further synergistically delays the hydration process by lowering the freezing point of the slurry and the surface adsorption effect, which can greatly extend the setting time of magnesium phosphate cement and improve the construction time in extreme low temperature environments.

[0027] Therefore, by modifying PVA fibers with a silane coupling agent, the present invention not only significantly improves their interfacial bonding with MPC but also more evenly disperses the PVA fibers in the slurry, thereby optimizing the grouting material's workability and mechanical properties. This provides a feasible technical path for the application of MPC grouting materials in extreme environments. These performance indicators already meet the requirements of duct grouting construction in low-temperature regions. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0029] Figure 1 The effect of modified PVA fiber on MPC fluidity and MPC coagulation time is shown in the figure: Figure 1 (a) is the effect of modified PVA fiber on MPC fluidity. Figure 1(b) Graph showing the effect of modified PVA fiber on the coagulation time of MPC.

[0030] Figure 2 This is a diagram showing the effect of modified PVA fiber on MPC toughness.

[0031] Figure 3 The SEM comparison of PVA fiber and modified PVA fiber: Figure 3 (a) is a 5 μm SEM image of PVA fiber. Figure 3 (b) 10 μm SEM image of PVA fibers, Figure 3 (c) is a 5 μm SEM image of the modified PVA fiber. Figure 3 (d) 10 μm SEM image of modified PVA fibers.

[0032] Figure 4 The effect diagram of water absorption was obtained by soaking MPC in water at 4~6℃ for 28 days after curing unmodified and modified PVA fibers at different dosages at -25℃ for 3 days.

[0033] Figure 5 This is a diagram showing the effect of different amounts of modified PVA fiber on the softening coefficient of MPC under 28-day immersion curing.

[0034] Figure 6 This is a diagram showing the effect of different dosages of modified PVA fiber on the strength retention rate of MPC under 28-day immersion curing.

[0035] Figure 7 SEM comparison of modified / unmodified MPC samples under water curing: Figure 7 (a) is a 1 μm SEM image of the unmodified MPC sample under water immersion curing. Figure 7 (b) is a 10 μm SEM image of the unmodified MPC sample under water immersion curing. Figure 7 (c) is a 1 μm SEM image of the modified MPC sample under water immersion curing. Figure 7 (d) 5 μm SEM image of the modified MPC sample under water immersion curing.

[0036] Figure 8 This is a diagram showing the effect of hydrophobic agent on the water absorption rate of MPC at -25℃.

[0037] Figure 9 This is a diagram showing the effect of hydrophobic agent on the MPC softening coefficient under 28-day immersion curing.

[0038] Figure 10 This is a diagram showing the effect of hydrophobic agent on the strength retention rate of MPC under 28-day immersion curing.

[0039] Figure 11 This is a flow chart of the grouting agent test of the present invention. DETAILED DESCRIPTION

[0040] To facilitate understanding of the present invention, the present invention will be described in more comprehensive and detailed form below in conjunction with the accompanying drawings and preferred embodiments. However, the protection scope of the present invention is not limited to the following specific embodiments.

[0041] Unless otherwise defined, all technical terms used hereinafter have the same meanings as those generally understood by those skilled in the art. The technical terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0042] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0043] This invention proposes a modified magnesium phosphate cement grouting material suitable for use in extreme low-temperature environments, designed to meet both strength and water resistance requirements. To this end, the material is optimized from three perspectives: a basic performance group, strength-enhancing functional groups, and water-resistance-enhancing functional groups. By flexibly combining different functional groups during application, the modified magnesium phosphate cement can meet construction requirements in extreme low-temperature environments.

[0044] The basic performance group consists of a base group and a retarding performance group. In some embodiments, the base group is composed of dead-burned magnesium oxide, ammonium dihydrogen phosphate, and water at a magnesium-to-phosphorus ratio of 4:1, plus borax, water, and a water reducer; the retarding performance group is prepared using glacial acetic acid.

[0045] In some embodiments, the basic performance group raw materials include dead-burned magnesium oxide, ammonium dihydrogen phosphate, borax, a polycarboxylate superplasticizer, glacial acetic acid, and water, with a mix ratio of 4:1 magnesium-phosphorus ratio, 0.17 water-binder ratio, 12% boron base ratio, 0.6% superplasticizer, and 0.1% glacial base ratio. The magnesium-phosphorus ratio is the mass ratio of dead-burned magnesium oxide to ammonium dihydrogen phosphate; the water-binder ratio is the mass ratio of water / (dead-burned magnesium oxide + ammonium dihydrogen phosphate); the boron base ratio is the mass ratio of borax / (dead-burned magnesium oxide + ammonium dihydrogen phosphate); the percentage of superplasticizer is based on the total mass of dead-burned magnesium oxide and ammonium dihydrogen phosphate; and the glacial base ratio is the mass ratio of glacial acetic acid / (dead-burned magnesium oxide + ammonium dihydrogen phosphate).

[0046] The material's performance is optimized by flexibly combining different functional groups, including dead-burned magnesium oxide, ammonium dihydrogen phosphate, borax, a mixture of glacial acetic acid and water, and a water reducer, as its basic performance components. PVA fiber and silane coupling agent KH550 serve as strength-enhancing functional groups, while a hydrophobic agent enhances water resistance. The combination of these three functional groups effectively improves the material's strength and water resistance.

[0047] In some embodiments, the strength-enhancing functional group is a modified PVA fiber made by combining the silane coupling agent KH550 and PVA fiber. Specifically, the silane coupling agent KH550 is slowly added to anhydrous ethanol and mixed evenly to obtain a 1 wt% coupling agent solution. The PVA fiber is then immersed in the coupling agent solution at room temperature and ultrasonically dispersed for 40 minutes. Finally, the fiber is washed and dried. The modified functional group is incorporated into the system at a modified functional group ratio of 0.2% to 0.4%. The modified functional group ratio is calculated as the mass ratio of modified PVA fiber to (dead-burned magnesium oxide + ammonium dihydrogen phosphate).

[0048] In some embodiments, the raw materials for the water resistance enhancing functional group include nano-silica, anhydrous ethanol, KH-570 silane coupling agent, and stearic acid, prepared in a mass ratio of 4:86:2:8 to form a hydrophobic agent. The hydrophobic agent dosage is 0.75% based on the total mass of dead-burned magnesium oxide and ammonium dihydrogen phosphate.

[0049] The base group, retarding performance group, strength-enhancing functional group, and water-resistance-enhancing functional group can be flexibly combined to meet different needs. For example: base group + glacial acetic acid; base group + modified PVA; base group + hydrophobic agent; base group + modified PVA + glacial acetic acid; base group + modified PVA + glacial acetic acid + hydrophobic agent; base group + modified PVA + hydrophobic agent, etc., with their performances superimposed.

[0050] In some embodiments, a base group is prepared using dead-burned magnesium oxide, ammonium dihydrogen phosphate, and water at a magnesium-phosphorus ratio of 4:1, and borax (boron base ratio of 12%), and a water ratio (i.e., a water-binder ratio of 0.17); secondly, a slow-setting performance group is prepared using glacial acetic acid at a glacial base ratio of 0.1%; finally, a modified PVA fiber is prepared using silane coupling agent KH550 and PVA fiber and is prepared as a strength enhancement functional group at a modified base ratio of 0.2% to 0.4%, and a water-resistant performance group is prepared using a hydrophobic agent at a dosage of 0.75%.

[0051] In some embodiments, a method for preparing a modified magnesium phosphate cement grouting material comprises the following steps: S1. Preparation of modified PVA fiber: Silane coupling agent KH550 was slowly added to anhydrous ethanol and mixed evenly to obtain a 1wt% coupling agent solution. PVA fibers were then immersed in the coupling agent solution at room temperature for ultrasonic dispersion for 40 minutes. Finally, the fibers were rinsed with clean water and placed in a hot air drying oven (90°C) for drying.

[0052] Preferably, the soaking time is 40 minutes and the drying time is 3 hours.

[0053] S2. Preparation of hydrophobic agent: First, stearic acid powder was added to ethanol and heated in a 60°C water bath with stirring until completely dissolved (approximately 30 minutes). Next, nanosilica and KH-570 were added sequentially and dispersed using a 500W ultrasonic wave until no agglomerates remained in the solution. Finally, the solution was filtered through a 200-mesh filter to remove any undispersed particles.

[0054] Preferably, the mixture is placed in a 70° C. water bath and heated with stirring for 30 minutes; when a 500W ultrasonic wave is used for dispersion treatment, the duration is 20 minutes.

[0055] S3. Preparation of modified magnesium phosphate cement: (1) Add dead-burned magnesium oxide, ammonium dihydrogen phosphate and borax into a cement paste mixer for dry mixing, then add a mixed solution of polycarboxylic acid water reducer, glacial acetic acid and water and stir to obtain magnesium phosphate cement.

[0056] Preferably, after adding the mixed solution of polycarboxylate water reducer, glacial acetic acid and water, it is necessary to stir at a low speed for 120 seconds, stir evenly, and then stir at a high speed for 90 seconds.

[0057] (2) When the matrix has shown good fluidity, the modified PVA fiber prepared in S1 (and / or the hydrophobic agent prepared in S2) is slowly and evenly added to the mixer and stirred with the magnesium phosphate cement prepared in step (1). The slurry is poured into a mold for molding, demoulded after 2 hours, and finally placed in a -25°C refrigerator for curing.

[0058] Preferably, the modified PVA fiber (or the hydrophobic agent prepared in S2) is added to the magnesium phosphate cement and stirred at a low speed for 120 seconds.

[0059] In the application of the modified magnesium phosphate cement grouting material of the present invention, glacial acetic acid is introduced during the stirring process and its property of delaying the low-temperature coagulation of magnesium phosphate cement is utilized, so that the material can still maintain controllable construction operability in an environment of 0°C to -25°C, meeting the engineering grouting needs in cold areas.

[0060] The present invention provides a cement-based composite grouting material with compressive and flexural strength, toughness, water resistance, and controllable setting time at -25°C. Composite modification technology optimizes the material ratio, introduces an organic-inorganic composite reinforcement phase to improve toughness, and uses a retarder to regulate the hydration process, thereby enhancing overall performance. Through the coordinated optimization of the basic performance group, the strength-enhancing functional group, and the water-resistance-enhancing functional group, the material's overall performance is significantly improved.

[0061] The base group is based on dead-burned magnesium oxide and ammonium dihydrogen phosphate at a magnesium-to-phosphorus ratio of 4:1. The retarding performance group significantly extends the setting time to 70 minutes in extremely low temperatures by incorporating glacial acetic acid (0.1% glacial to base ratio). A water-binder ratio of 0.17 and borax (12% borax to base ratio) are used to adjust the hydration reaction process. A polycarboxylate superplasticizer (0.6%) is added to optimize the slurry fluidity, ensuring a 30-second fluidity that meets construction controllability requirements. The strength-enhancing functional group uses silane coupling agent KH550 to modify PVA fibers, effectively improving fiber dispersion and interfacial bonding with the matrix.

[0062] During preparation, dead-burned magnesium oxide, ammonium dihydrogen phosphate, and borax are dry-mixed. A solution containing a water reducer, glacial acetic acid, and water is then added and stirred to form a slurry. Once the slurry reaches the required fluidity, modified PVA fibers with strength-enhancing functional groups are slowly added, stirred at low speed, and then injection molded. The test pieces are demolded after two hours and then cured at -25°C. Test results show that the modified material's flexural strength increases by 18.4% at room temperature and 22.2% at -25°C, while its compressive strength decreases only slightly, achieving an optimized balance of mechanical properties. Regarding water resistance, water absorption is reduced to 3.05%, the softening coefficient reaches 0.86, and the room-temperature and low-temperature strength retention rates increase to 86.7% and 81.2%, respectively. Microscopic analysis shows that the unmodified hydration product exhibits a loose, needle-like structure, while the addition of the modified fibers transforms it into a dense, spherical structure. The fibers are tightly bonded to the matrix, inhibiting water penetration.

[0063] To verify the mechanism of enhanced water resistance, a hydrophobic agent modification experiment was conducted, adding 0.75% hydrophobic agent to the base and retarding performance groups. The results showed that the hydrophobic agent further reduced hydration channels by filling pores and reducing surface energy, resulting in a synergistic improvement in water resistance.

[0064] This invention addresses the shortcomings of traditional MPC, such as brittleness, rapid initial setting, and susceptibility to hydrolysis, by providing core strength through a base group, regulating the construction window through a retarding set group, enhancing durability through a strength-enhancing functional group, and improving water resistance through a water-resistance-enhancing functional group. Its preparation process balances low-temperature adaptability, construction controllability, and cost-effectiveness, making it particularly suitable for prestressed duct grouting projects in extremely cold regions, providing a high-strength, durable material solution for infrastructure construction in extreme environments.

[0065] In the following example, dead-burned magnesium oxide, ammonium dihydrogen phosphate, and borax are first placed in a -25°C environment for freezing, and then water and glacial acetic acid are placed in a 0°C environment to prepare for the preparation of modified magnesium phosphate cement.

[0066] Example 1 Silane coupling agent KH550 was slowly added to anhydrous ethanol and mixed uniformly to obtain a 1wt% coupling agent solution. PVA fibers were then immersed in the coupling agent solution at room temperature and ultrasonically dispersed for 40 minutes. Finally, they were rinsed with clean water and dried in a hot air drying oven (90°C) for 3 hours. Dead-burned magnesium oxide, ammonium dihydrogen phosphate, and borax were added to a cement slurry mixer and dry-mixed. Polycarboxylate superplasticizer and water were then added and stirred at low speed for 120 seconds to obtain a uniform slurry. This was then stirred at high speed for 90 seconds to obtain magnesium phosphate cement. When the matrix exhibited good fluidity, the prepared modified PVA fibers were slowly and evenly added to the mixer and stirred at low speed with the prepared magnesium phosphate cement for 120 seconds. The resulting slurry (grouting material) was first poured into a Vicat apparatus and a slurry fluidity test mold to test the setting time and fluidity. The slurry was then poured into a mold, demolded after 2 hours, and finally placed in a -25°C refrigerator for curing. Two groups of single-addition tests were conducted, and the specific proportions are shown in Table 1 (wherein, % is based on the total mass of dead-burned magnesium oxide + ammonium dihydrogen phosphate).

[0067] Table 1 Design of the ratio of modified PVA fiber mixed with MPC grouting material

[0068] The test results of the modified PVA fiber on the working performance of MPC are as follows Figure 1 shown.

[0069] like Figure 1 As shown in Figure (a), without modified PVA fibers, the initial fluidity of the MPC slurry is 225 mm, demonstrating optimal fluidity. When the fiber content increases to 0.4%, the fluidity drops significantly to 186 mm, a decrease of 17.3%. This indicates that when the fiber content exceeds the critical value, the spatial network structure formed by the fibers in the slurry significantly inhibits the fluidity, causing a sharp increase in slurry viscosity and an intensified deterioration in fluidity.

[0070] like Figure 1 As shown in (b), the setting time of the unfibered baseline sample is 35 minutes, but after adding 0.4% modified PVA fiber, the setting time is shortened to 28 minutes, a 20% reduction. The introduction of modified fibers provides additional microscopic nucleation sites in the paste, accelerating the formation rate of hydration products. At the same time, the fiber surface properties enhance the interaction between cement particles, significantly shortening the setting and hardening process.

[0071] The test results of the mechanical properties of modified PVA fiber on MPC are as follows Figure 2 shown.

[0072] Figure 2In the absence of modified PVA fibers, the flexural compressive ratio of MPC at -25°C showed a natural downward trend with increasing curing time (3, 7, and 28 days). The incorporation of 0.4% modified PVA fibers significantly enhanced the toughness of the material through the effective bonding between the fibers and the matrix. The modified fibers optimized the microstructure of the MPC through stress transfer and crack isolation, and the fiber-matrix interfacial bonding gradually strengthened with curing time, ultimately achieving superior toughening.

[0073] The present invention conducted SEM (scanning electron microscope) tests on the unmodified (a, b) and silane coupling agent KH550 modified (c, d) samples, respectively. Figure 3 As shown in the figure, the difference between the two surfaces can be clearly seen: the surface of the unmodified fiber (a: 5μm, b: 10μm) is relatively smooth and free of particles. Its diameter distribution is uniform, and there are almost no particles attached, resulting in a large gap between it and the cement mortar, which affects the interfacial bonding performance and thus has poor crack resistance. After modification with KH550, the surface is rougher and the number of active sites is significantly increased. From (c: 5μm), it is found that its surface has obvious uneven attachments and there are signs of local erosion. It is observed that the diameter of multiple fibers (d: 10μm) remains stable, indicating that the modification process has not destroyed the original fiber structure.

[0074] The above results show that KH550 can form a modified layer on the surface of PVA fibers. The modified fiber surface is rougher, which helps to enhance the interfacial bonding between PVA fibers and MPC hydration products. In addition, the silane molecules grafted onto the fiber surface improve its hydrophilicity. The surface microstructure can help the fibers disperse more easily in the matrix, inhibit crack expansion, and thus improve the toughness of the material.

[0075] Example 2 Bridges in high-altitude and seasonally frozen regions are often affected by freeze-thaw cycles, rainwater erosion, or groundwater infiltration. Therefore, in the engineering application of low-temperature grouting agents, water resistance is a key indicator of their long-term service performance. Therefore, this example incorporates a water resistance test. Compression and flexural test specimens were prepared according to the designed mix ratio. The test considered water resistance performance in negative temperature environments. The specimens were placed in a freezer (-25°C) and immersed in water for curing. Under simulated cold conditions, indicators such as water absorption, softening coefficient, and strength retention were analyzed. Finally, SEM characterization was performed to illustrate the effect of modification on the water resistance of MPC.

[0076] The water resistance tests used a water-to-binder ratio of 0.17, M / P = 4, a borax dosage of 12%, and a water-reducing agent dosage of 0.6%. All water resistance tests were conducted at -25°C. The specific mix design is shown in the table (preparation method is the same as in Example 1), where UC-PVA represents unmodified PVA fiber and PVA represents surface-modified PVA fiber. Three sets of single-mixing tests were conducted, and the specific mix ratios are shown in Table 2 (wherein, % is based on the total mass of dead-burned magnesium oxide + ammonium dihydrogen phosphate).

[0077] Table 2 Design of modified PVA fiber composite-MPC grouting material ratio

[0078] The comparison results of water absorption of unmodified and modified magnesium phosphate cement are as follows: Figure 4 shown.

[0079] like Figure 4 As shown, the water absorption rate of MPC increases with age (1 day, 7 days, 14 days, 21 days, and 28 days). Without fiber addition, the water absorption rate is 6%-7.5%. Further observations revealed that the addition of unmodified PVA fibers resulted in a water absorption rate of 4%-5.2%, indicating that PVA fibers slightly improve the water resistance of MPC. This is likely due to the network structure created by the PVA fibers, which effectively reduces pore connectivity and hinders water penetration. The addition of modified PVA fibers further reduces the water absorption rate of MPC, to a range of 3%-3.6%. This is likely due to the modification effect of the silane coupling agent, which forms a hydrophobic film on the fiber surface and enhances the bonding between the fibers and cement particles, thereby improving the water resistance of MPC.

[0080] Effect of modified PVA fiber on the softening coefficient of MPC Figure 5 shown.

[0081] Figure 5 In the data, CS-D is the strength before immersion in water, CS-S is the strength after 28 days of immersion in water, and SC is the softening coefficient. At 0% dosage (control group), the softening coefficient of MPC is 0.73; when the dosage of modified PVA fiber increases to 0.4%, the softening coefficient increases to 0.81, which is 10.96% higher than that of the control group. The modified PVA fiber has a dual regulatory effect: when the dosage is 0.4%, the fibers will aggregate when forming the structure, resulting in an increase in porosity. At the same time, a disordered and defective network is formed between the fibers, and the effective force-bearing area is reduced. After modification by the silane coupling agent, the amino groups grafted onto the fiber surface also react with some ions in MPC (such as Mg 2+ ) forms coordination bonds, which can have higher bonding strength than traditional physical adsorption interfaces, effectively hindering water molecules from entering the matrix and improving the water resistance of MPC.

[0082] Effect of modified PVA fiber on MPC strength retention rate Figure 6 shown.

[0083] Figure 6 The effect of modified PVA fiber on the compressive strength of MPC at different dosages. Among them, I-CS is the initial strength before immersion in water, 28d-CS is the compressive strength after immersion in water for 28 days, and SRR represents the strength retention rate. As can be seen from the figure, when no modified PVA fiber is added, the initial compressive strength of MPC is 44.7 MPa. After immersion in water for 28 days, the strength drops to 27.6 MPa, and the strength retention rate is 61.74%, indicating that long-term immersion causes the matrix to deteriorate significantly due to the lack of fiber crack resistance and water-resistant enhancement. When the dosage increases to 0.4%, the initial strength drops to 40.2 MPa, which is attributed to the increase in porosity caused by fiber aggregation; but after immersion in water, the strength increases to 30.3 MPa, and the strength retention rate reaches 75.37%. The modified fiber reacts with the surface amino group Chemical bonding hinders water penetration, while fiber bridging slows crack propagation, significantly mitigating freeze-thaw damage and improving durability. Although a 0.4% addition sacrifices some initial strength, its compressive stability in long-term water immersion is superior to that of the undoped fiber group, making it suitable for projects requiring a balance between construction feasibility and water resistance.

[0084] The present invention conducted SEM (scanning electron microscope) tests on the unmodified MPC samples (a, b) and the modified MPC samples (c, d) under immersion curing, respectively. Figure 7 shown.

[0085] Figure 7 These are the SEM images of the unmodified / modified MPC in each group after immersion in water for 28 days. Figure 7 (a) and Figure 7 (b) shows the micromorphology of unmodified MPC at different magnifications. Numerous struvite crystals are clearly visible, with needle-shaped or columnar forms. This indicates that water curing provides sufficient water for the hydration reaction, significantly contributing to the growth of struvite crystals. Further observation revealed that excessive water causes disordered crystal growth, with numerous pores and cracks surrounding the crystals, which negatively impacts the bonding between the crystals and the overall structural density. Water then readily enters the matrix through these pores. Consequently, the strength and water resistance of the control matrix were average. Figure 7 (c) is the morphology of MPC after adding modified PVA fibers. It is found that the hydration products are generated in a partially spherical form. This may be because the chemical active sites on the PVA fiber interface accelerate the hydration reaction nearby, and a large number of crystal products are enriched in the interface layer. Figure 7 As shown in the SEM of (d), the fiber surface shows a significant accumulation of hydration products, which may be due to the chemical bonding between the hydroxyl groups on the fiber surface and the phosphate ions in the hydration products, thereby enhancing the interfacial adhesion between the fiber and the matrix.

[0086] The micromorphology of unmodified and modified MPC after 28 days of water curing was characterized by SEM. The hydration products of unmodified MPC were needle-rod-shaped, with disordered crystal growth, which reduced the strength and water resistance of the matrix. The modified MPC particles were tightly bound, with smooth crystal surfaces, which improved the water resistance of the matrix. The hydration products of MPC in the modified PVA fiber group were spherical, and the modified fibers showed accumulation of hydration products, which enhanced the interfacial bonding between the fiber and the matrix.

[0087] Example 3 Stearic acid powder was added to anhydrous ethanol, heated in a 70°C water bath and stirred for 30 minutes until completely dissolved. Next, nanosilica and KH-570 were added in sequence, and dispersed using 500W ultrasound for 20 minutes until there was no agglomeration in the solution. Finally, the solution was filtered using a 200-mesh filter to remove undispersed particles to obtain a hydrophobic agent. Among them, nanosilica, anhydrous ethanol, KH-570 silane coupling agent and stearic acid were in a mass ratio of 4:86:2:8.

[0088] Dead-burned magnesium oxide, ammonium dihydrogen phosphate, and borax are added to a cement slurry mixer for dry mixing, and then polycarboxylate water reducer and water are added. After stirring at a low speed for 120 seconds, a uniform slurry is obtained, and then stirred at a high speed for 90 seconds to obtain magnesium phosphate cement. When the matrix has shown good fluidity, the prepared hydrophobic agent is slowly and evenly added to the mixer and stirred with the prepared magnesium phosphate cement to obtain a grouting material.

[0089] To verify the effect of hydrophobic agents on water resistance, we conducted experiments combining magnesium phosphate cement with hydrophobic agents. The water-cement ratio for the water resistance tests was 0.17, M / P = 4, borax content was 12%, and superplasticizer content was 0.6%. The specific mix ratios are shown in Table 3, where H represents the hydrophobic agent (% is based on the total mass of dead-burned magnesium oxide + ammonium dihydrogen phosphate).

[0090] Table 3 Hydrophobic agent-MPC grouting material ratio design

[0091] The effect of hydrophobic agent on the water absorption rate of MPC at -25℃ Figure 8 shown.

[0092] like Figure 8As shown in the figure, when no hydrophobic agent was added (0%), the water absorption rate of MPC increased significantly with curing time, rising continuously from 6.19% at 1 day to 7.44% at 28 days, indicating that the matrix pores were not effectively sealed and the rate of water intrusion accelerated with age. After adding 0.75% hydrophobic agent, the water absorption rate decreased significantly and stabilized, reaching only 3.83% at 28 days, a decrease of 48.5% compared to the unadded group. During the short-term curing stage (1-7 days), the water absorption rate of the 0.75% admixture only increased slightly from 3.62% to 3.68%, while that of the control group increased from 6.19% to 6.99%. The hydrophobic agent inhibited the initial moisture penetration by quickly forming a dense barrier layer. During the long-term curing (28 days), the water absorption rate of the 0.75% admixture almost stagnated, while that of the control group continued to rise to 7.44%, indicating that the chemical modification of the hydrophobic agent significantly improved the long-term water resistance stability of MPC. Its water absorption control ability is significantly better than that of the unadmixed group, making it suitable for engineering scenarios with high requirements for water resistance and the need to control material costs.

[0093] The effect of hydrophobic agent on the softening coefficient of MPC under 28-day immersion curing Figure 9 shown.

[0094] like Figure 9 As shown, it can be clearly seen that the softening coefficient curve of the hydrophobic agent shows a slow upward trend, where CS-D is the strength before immersion in water, CS-S is the strength after immersion for 28 days, and SC is the softening coefficient. When no hydrophobic agent is added (0%), the initial compressive strength of MPC is 27.6 MPa. After immersion and curing for 28 days, the strength drops to 20.1 MPa, and the softening coefficient is 0.73, indicating that long-term immersion in water causes the matrix to lack hydrophobic protection, the dissolution of hydration products is accelerated, and the water resistance is poor. After the addition of 0.75% hydrophobic agent, the initial strength is increased to 32.1 MPa, and the strength after immersion in water is significantly increased to 26.2 MPa, and the softening coefficient reaches 0.82. The hydrophobic agent forms coordination bonds with the MPC matrix through hydrophobic molecules, thereby enhancing the bonding strength of the interface transition zone and effectively delaying the hydration products (such as ) dissolution, thereby inhibiting water penetration and reducing strength loss. Its comprehensive compressive strength and water resistance performance are optimal, and it is suitable for engineering scenarios that need to improve both short-term mechanical properties and long-term water resistance stability.

[0095] Effect of hydrophobic agent on MPC strength retention rate under 28d immersion curing Figure 10 shown.

[0096] like Figure 10The figure shows the effect of different hydrophobic agent dosages on the compressive strength of MPC. Here, I-CS represents the initial strength before immersion, 28d-CS represents the compressive strength after 28 days of immersion curing, and SRR represents the strength retention rate. At -25°C, the initial compressive strength of MPC without a hydrophobic agent (0%) was 44.7 MPa. After 28 days of immersion curing, the strength dropped to 27.6 MPa, with a strength retention rate of 61.74%. This indicates that low temperatures inhibit the full hydration reaction, resulting in a loose matrix structure and a significant decrease in strength after long-term immersion. With the addition of 0.75% hydrophobic agent, the initial strength decreased slightly to 39.2 MPa, but the strength increased to 32.1 MPa after immersion, with a strength retention rate of 81.89%. By inhibiting water penetration and optimizing the structure of the interfacial transition zone, the hydrophobic agent effectively mitigates the dissolution of hydration products and matrix degradation at low temperatures. Although the 0.75% addition sacrifices some initial strength, its compressive stability under long-term water immersion conditions is significantly better than that of the unadulterated group. It is suitable for engineering scenarios that require a balance between short-term construction performance and long-term durability in low-temperature environments.

[0097] Example 4 Silane coupling agent KH550 was slowly added to anhydrous ethanol and mixed uniformly to obtain a 1 wt% coupling agent solution. PVA fibers were then immersed in the coupling agent solution at room temperature and ultrasonically dispersed for 40 minutes. Finally, the fibers were rinsed with clean water and dried in a hot air drying oven (90°C) for 3 hours. Dead-burned magnesium oxide, ammonium dihydrogen phosphate, and borax were added to a cement slurry mixer and dry-mixed. A mixture of a polycarboxylate superplasticizer, water, and glacial acetic acid was then added. The mixture was stirred at low speed for 120 seconds to obtain a uniform slurry, which was then stirred at high speed for 90 seconds to obtain magnesium phosphate cement. Once the matrix exhibited good fluidity, the modified PVA fibers were slowly and evenly added to the mixer and stirred at low speed for 120 seconds. The resulting slurry (grouting material) was poured into a Vicat test mold and placed at -25°C for setting time testing. Two sets of single-mixing tests were conducted. The specific proportions are shown in Table 4 (where % is based on the total mass of dead-burned magnesium oxide and ammonium dihydrogen phosphate).

[0098] Table 4 Modified PVA fiber composite-MPC grouting material ratio design

[0099] This study found that under a -25°C environment, the initial setting time of the modified magnesium phosphate cement with the addition of 0.1% glacial acetic acid was significantly extended by 70 minutes compared with the group without addition. Specifically, the control group material without the addition of glacial acetic acid immediately started a rapid hydration reaction after grouting into the pipe, and the slurry formed initial strength within 28 minutes, resulting in a too short effective construction window; while the modified group slurry showed obvious two-stage reaction characteristics after injection - the first 98 minutes was a reaction latent period, at which time the slurry maintained good fluidity and no strength development, until the end of the latent period, the hydration reaction suddenly accelerated, and its strength growth rate was basically synchronized with that of the unmodified group. This regulatory effect is mainly due to the triple synergistic effect of glacial acetic acid: weak acidity inhibits the initial reaction activity of MgO and phosphate, acetate and phosphate compete for complexation, and the hydration reaction of MgO and phosphate is accelerated. By delaying the nucleation process and lowering the freezing point of the liquid phase by glacial acetic acid, the reaction medium maintains stability. This result shows that glacial acetic acid can precisely control the reaction process, creating a controllable operating time window for pipeline grouting construction in extremely low temperature environments, while ensuring normal development of strength in the later stages.

[0100] Figure 11 The following is a flow chart of the experimental process of the present invention. Dead-burned magnesium oxide, ammonium dihydrogen phosphate, and borax were added to a cement slurry mixer and dry-mixed. Subsequently, a mixed solution of a polycarboxylate superplasticizer and water was added and stirred evenly to obtain magnesium phosphate cement. The optimal water-cement ratio, borax, and superplasticizer dosages were determined through performance and mechanical property tests. Based on this optimal ratio, modified magnesium phosphate cement was prepared by incorporating modified PVA fibers. The modified magnesium phosphate cement was tested for properties such as compression ratio, fluidity, and setting time, and the specimen surface was carefully observed using a scanning electron microscope (SEM). Finally, a water resistance test was conducted, and the superior performance of the magnesium phosphate cement incorporating a hydrophobic agent / glacial acetic acid was verified using an unmodified magnesium phosphate cement as a control group.

[0101] Example 5 Silane coupling agent KH550 was slowly added to anhydrous ethanol and mixed evenly to obtain a 1wt% coupling agent solution. The PVA fiber was then immersed in the coupling agent solution at room temperature for ultrasonic dispersion for 40 minutes. Finally, it was rinsed with clean water and placed in a hot air drying oven (90°C) for drying for 3 hours.

[0102] Stearic acid powder was added to anhydrous ethanol, heated in a 70°C water bath and stirred for 30 minutes until completely dissolved. Next, nanosilica and KH-570 were added in sequence, and dispersed using 500W ultrasound for 20 minutes until there was no agglomeration in the solution. Finally, the solution was filtered using a 200-mesh filter to remove undispersed particles to obtain a hydrophobic agent. Among them, nanosilica, anhydrous ethanol, KH-570 silane coupling agent and stearic acid were in a mass ratio of 4:86:2:8.

[0103] Dead-burned magnesium oxide, ammonium dihydrogen phosphate, and borax were dry-mixed in a cement slurry mixer. A mixture of polycarboxylate superplasticizer, water, and glacial acetic acid was then added. After stirring at low speed for 120 seconds, a uniform slurry was obtained. This was then stirred at high speed for 90 seconds to form magnesium phosphate cement. Once the matrix exhibited good fluidity, the prepared modified PVA fiber and hydrophobic agent were slowly and evenly added to the mixer and stirred at low speed for 120 seconds with the prepared magnesium phosphate cement. The resulting slurry (grouting material) was poured into a Vicat test mold and placed at -25°C for setting time testing. Two sets of single-mixing tests were conducted. The specific mix ratios are shown in Table 5 (where % is based on the total mass of dead-burned magnesium oxide + ammonium dihydrogen phosphate).

[0104] Table 5

[0105] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any form. Therefore, any simple modifications, equivalent variations, and modifications to the above embodiments that do not depart from the technical solution of the present invention and are based on the technical essence of the present invention shall fall within the scope of protection of the technical solution of the present invention.

Claims

1. A modified magnesium phosphate cement grouting material, characterized in that: The modified magnesium phosphate cement grouting material comprises: magnesium phosphate cement made from dead-burned magnesium oxide, monoammonium phosphate, borax, polycarboxylate superplasticizer, and water; and hydrophobic agents added to magnesium phosphate cement; The hydrophobic agent is prepared by the following method: stearic acid powder, nano-silica and silane coupling agent KH-570 are dispersed in ethanol, and the mass ratio of nano-silica, ethanol, KH-570 and stearic acid is (4-5): (84-87): (2-3): (7-9); Based on the total mass of dead-burned magnesium oxide and ammonium dihydrogen phosphate as 100%, the amount of the hydrophobic agent is 0.75% to 1%.

2. The modified magnesium phosphate cement grouting material according to claim 1, characterized in that: Glacial acetic acid is also added to the magnesium phosphate cement during preparation; Based on the total mass of dead-burned magnesium oxide and ammonium dihydrogen phosphate as 100%, the added amount of glacial acetic acid is 0.08%~0.12%.

3. The modified magnesium phosphate cement grouting material according to claim 1 or 2, characterized in that: The modified magnesium phosphate cement grouting material further comprises modified PVA fiber; The modified PVA fiber is obtained by soaking the PVA fiber in an ethanol solution of a silane coupling agent KH550; Based on the total mass of dead-burned magnesium oxide and ammonium dihydrogen phosphate as 100%, the added amount of the modified PVA fiber is 0.2% to 0.4%.

4. The modified magnesium phosphate cement grouting material according to claim 1, characterized in that: In the magnesium phosphate cement, the mass ratio of dead-burned magnesium oxide to ammonium dihydrogen phosphate is (3.5-4.5):1, and the water-binder ratio is 0.16-0.

18. Based on the total mass of the dead-burned magnesium oxide and ammonium dihydrogen phosphate being 100%, the amount of borax added is 10-12%, and the amount of polycarboxylate superplasticizer added is 0.5-0.75%.

5. The modified magnesium phosphate cement grouting material according to claim 3, characterized in that: The mass fraction of KH550 in the ethanol solution of the silane coupling agent KH550 is 0.8-1.0%, and the soaking treatment time is 40 minutes.

6. The modified magnesium phosphate cement grouting material according to claim 1, 2 or 4, characterized in that: The hydrophobic agent is prepared by the following method: adding stearic acid powder into ethanol, heating and stirring until completely dissolved, then sequentially adding nano-silicon dioxide and KH-570, and ultrasonically dispersing and filtering.

7. A method for preparing the modified magnesium phosphate cement grouting material according to claim 3, characterized in that: The following steps are included: Dead-burned magnesium oxide, ammonium dihydrogen phosphate, and borax are dry-mixed and stirred, and then a polycarboxylate water reducer and water, or a mixed solution of a polycarboxylate water reducer, glacial acetic acid, and water is added, and stirred to obtain magnesium phosphate cement; The modified PVA fiber and / or the hydrophobic agent are added to magnesium phosphate cement and stirred, and then formed and cured.

8. An application of the modified magnesium phosphate cement grouting material according to claim 3, characterized in that: The modified magnesium phosphate cement grouting material solidifies at -25°C and is suitable for a construction environment of 0°C to -25°C.

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