A modified magnesium phosphate cement grouting material and its preparation method and application

Abstract

This invention discloses a modified magnesium phosphate cement grouting material, its preparation method, and its application. The modified magnesium phosphate cement grouting material comprises: magnesium phosphate cement made from recalcined magnesium oxide, ammonium dihydrogen phosphate, borax, polycarboxylate superplasticizer, and water; and a hydrophobic agent added to the magnesium phosphate cement. Further, glacial acetic acid is added during the preparation of the magnesium phosphate cement. Further still, the modified magnesium phosphate cement grouting material also includes silane coupling agent-modified PVA fibers. This invention, through the synergistic effect of glacial acetic acid retarding and silane coupling agent-modified PVA fibers, as well as the effect of the hydrophobic agent, endows MPC with excellent compressive and flexural strength, low-temperature toughness, water resistance, and controllable setting time, making it both highly durable and feasible for construction in prestressed duct grouting projects in frigid regions, providing a reliable solution for engineering applications in extreme environments.

Description

Technical Field

[0001] This invention belongs to the field of building materials, and particularly relates to a modified magnesium phosphate cement grouting material, its preparation method, and its application. Background Art

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

[0003] In the early 21st century, with the rapid advancement of global climate change and infrastructure construction in cold regions, the quality and durability of engineering projects under low-temperature construction environments have become a major concern. Traditional grouting agents are primarily designed for normal or high-temperature environments, often exhibiting problems such as high bleeding, poor frost resistance, and insufficient early strength under low-temperature conditions. Taking magnesium phosphate cement (MPC) as an example, while it possesses early strength characteristics, it generally suffers from inherent defects such as low flexural strength, insufficient crack resistance, and poor water resistance. In particular, the setting time is difficult to control effectively in low-temperature environments, and the excessively rapid hydration reaction rate can lead to insufficient workable window for construction. This not only reduces the anchorage performance of prestressed tendons but may also weaken the stability of bridge structures. Furthermore, freeze-thaw cycles can easily cause structural defects in the grout, further reducing the safety of bridges in low-temperature environments. Meanwhile, grouting materials designed for normal temperatures struggle to hydrate properly in low-temperature environments and are easily susceptible to frost damage, resulting in slow strength development. Developing grouting agents specifically designed for low-temperature environments has become an important technological direction for improving engineering quality, shortening construction time, and ensuring long-term performance.

[0004] Bridges in high-altitude and seasonally frozen regions are frequently 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 determining their long-term service performance. Traditional MPC systems, due to the chemical instability of hydration products, are prone to microstructural degradation in humid environments, resulting in significantly weaker water resistance compared to silicate cement systems. If the grouting agent's water resistance is insufficient, its internal pore structure is easily degraded by moisture intrusion, leading to strength loss, volume expansion, and even affecting the bond with reinforcing steel. This indicates a direct correlation between insufficient water resistance of the grouting agent and steel corrosion.

[0005] Prestressed duct grouting involves injecting grouting material into pre-reserved prestressed ducts, ensuring the grout fully encapsulates the prestressing tendons. Its function is to protect the prestressed tendons from corrosion and ensure a good bond between them and the concrete, guaranteeing effective prestress transfer. However, traditional MPC materials, due to insufficient crack resistance, are prone to microcracks under freeze-thaw cycles, exacerbating the migration of moisture and chloride ions to the rebar surface. Simultaneously, their poor water resistance accelerates the electrochemical corrosion process at the grout-rebar interface. For prestressed tendons within duct grouting, their large specific surface area further amplifies the risk of corrosion, potentially inducing through-cracks in the prestressed beam under low-temperature conditions. Therefore, developing grouting materials that combine adaptability to negative temperature environments, excellent crack resistance, and long-term water-resistant protection has become a key technological requirement for ensuring the durability of prestressed systems. 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 background art above, and to provide a low-temperature retarding and water-resistant composite modified magnesium phosphate cement grouting material with a setting resistance of -25℃ and its preparation method and application.

[0007] To solve the above-mentioned technical problems, the technical solution proposed by this invention is as follows:

[0008] A modified magnesium phosphate cement grouting material, the modified magnesium phosphate cement grouting material comprising:

[0009] Magnesium phosphate cement is made from calcined magnesium oxide, ammonium dihydrogen phosphate, borax, polycarboxylate superplasticizer, and water.

[0010] and the hydrophobic agent added to magnesium phosphate cement;

[0011] 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);

[0012] The total mass of recalcined magnesium oxide and ammonium dihydrogen phosphate is 100%, and the hydrophobic agent content is 0.75%~1%.

[0013] As a further improvement, glacial acetic acid is added during the preparation of the magnesium phosphate cement; the amount of glacial acetic acid added is 0.08%~0.12% based on the total mass of reburned magnesium oxide and ammonium dihydrogen phosphate of 100%.

[0014] As a further improvement, the modified magnesium phosphate cement grouting material also includes modified PVA fibers; the modified PVA fibers are obtained by soaking PVA fibers in an ethanol solution of silane coupling agent KH550; the amount of modified PVA fibers added is 0.2%~0.4% based on the total mass of recalcined magnesium oxide and ammonium dihydrogen phosphate of 100%.

[0015] As a further improvement, in the magnesium phosphate cement, the mass ratio of deburned magnesium oxide to ammonium dihydrogen phosphate is (3.5~4.5):1, the water-binder ratio is 0.16~0.18; the total mass of deburned magnesium oxide and ammonium dihydrogen phosphate is 100%, the amount of borax added is 10~12%, and the amount of polycarboxylate superplasticizer added is 0.5~0.75%.

[0016] 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 soaking treatment time is 40 minutes.

[0017] 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-silica and KH-570 in sequence, and ultrasonically dispersing and filtering.

[0018] This invention provides a method for preparing the modified magnesium phosphate cement grouting material, comprising the following steps:

[0019] Magnesium phosphate cement is obtained by dry mixing of calcined magnesium oxide, ammonium dihydrogen phosphate, and borax, followed by the addition of polycarboxylate superplasticizer and water, or by adding a mixed solution of polycarboxylate superplasticizer, glacial acetic acid, and water.

[0020] The modified PVA fiber and / or the hydrophobic agent are added to magnesium phosphate cement and stirred, then molded and cured.

[0021] This invention provides an application of the modified magnesium phosphate cement grouting material, which is cured at -25°C and is suitable for construction environments from 0°C to -25°C.

[0022] Magnesium phosphate cement (MPC) has been selected as the core material for prestressed duct grouting in frigid regions due to its unique performance advantages. Firstly, MPC exhibits significant environmental benefits; its hardening process releases little heat, reducing temperature interference in low-temperature construction environments, while also having low carbon emissions, meeting green building standards. Even at extreme low temperatures (e.g., -25°C), MPC maintains stable setting properties, exhibits excellent frost resistance, and can withstand freeze-thaw cycles and ice crystal expansion damage. Furthermore, it possesses high initial compressive strength, making it suitable for prestressed engineering requirements. However, traditional MPC suffers from insufficient flexural strength, poor crack resistance, difficulty in controlling low-temperature setting time, and poor water resistance, especially in frigid environments where rapid hydration can lead to insufficient construction time. Therefore, this invention specifically introduces retarding technology, fiber reinforcement, and water resistance optimization strategies to improve its overall performance and ensure stability and reliability under various environmental conditions.

[0023] At low temperatures, MPC (Medium-Polymer Grouting) is prone to a shortened setting time due to the rapid neutralization reaction between phosphate and magnesium oxide, affecting the operability of construction. To address this, this invention achieves delayed setting by adding a certain amount of glacial acetic acid during the stirring stage. The weak acidity of glacial acetic acid inhibits the rapid reaction between phosphate and magnesium oxide, while acetate ions reduce the concentration of active magnesium ions through complexation, delaying the crystallization of hydration products. The dual effect of glacial acetic acid at low temperatures is particularly crucial: on the one hand, it inhibits ice crystal formation by lowering the freezing point of the grout liquid phase; on the other hand, it hinders the contact of reactants through surface adsorption. This synergistic effect extends the setting time by 70 minutes, successfully solving the problem of insufficient construction window at extreme low temperatures and ensuring the operability of the grouting process.

[0024] Leveraging the unique dual-functionality of the silane coupling agent KH550, this invention achieves multi-dimensional interfacial reinforcement between PVA (polyvinyl alcohol) fibers and the MPC matrix. The silanol groups generated by the hydrolysis of its silicon groups construct 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 film. Meanwhile, the amino functional groups at the organic end, due to their high polarity, physically adsorb onto the fiber surface through hydrogen bonding and chemically bond with the 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 moisture penetration pathways through the hydrophobic effect of the siloxane layer. During 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-toughness fiber, ultimately achieving a synergistic improvement in the material's crack resistance, deformation capacity, and durability under extreme environments.

[0025] First, the mechanical properties are significantly enhanced. The modified fibers form a micro-reinforcing network in the MPC matrix, effectively inhibiting crack propagation and significantly improving flexural strength. Furthermore, the matrix toughness shows considerable improvement at low temperatures (-25℃), with smoother crack morphology and better crack resistance. Second, durability is also optimized. The uniform dispersion of the fibers promotes the formation of hydration products, reducing the number and connectivity of pores in the matrix, resulting in a denser matrix, lower water absorption, and effectively improved water resistance and freeze-thaw resistance. Finally, construction adaptability is improved. The uniformly distributed fibers optimize the viscosity and workability of the slurry, and combined with retarding technology, a balance between slurry fluidity and strength development is maintained under low-temperature construction conditions, ensuring smooth construction.

[0026] Hydrophobic agents improve the water resistance of magnesium phosphate cement through multiple mechanisms: Nano-silica, uniformly dispersed in anhydrous ethanol, reduces capillary water absorption channels by filling the micropores of the cement matrix; KH-570 silane coupling agent undergoes hydrolysis-condensation in the aqueous system, where its silanol groups form chemical bonds with the Mg-O bonds in the cement hydration products, while simultaneously grafting hydrophobic methacryloyloxy groups onto the material surface; stearic acid molecules bind to magnesium oxide hydration products through their carboxyl groups, and their long-chain alkyl groups are oriented to form a dense hydrophobic layer. These three agents synergistically construct a composite hydrophobic system of "pore filling-chemical bonding-directional molecular arrangement" during the cement paste curing process, effectively blocking water penetration pathways.

[0027] By utilizing the above-mentioned modification technology, through the synergistic effect of glacial acetic acid retarding and silane coupling agent modification of PVA fibers, as well as the effect of hydrophobic agents, this invention endows MPC with excellent compressive and flexural strength, low-temperature toughness, water resistance, and controllable setting time. This makes it both durable and feasible for construction in prestressed duct grouting projects in frigid regions, providing a reliable solution for engineering applications in extreme environments.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] (1) This invention is based on a mass ratio of 4:1 (hereinafter referred to as M / P=4, or magnesium-phosphorus ratio 4:1) of reburned magnesium oxide to ammonium dihydrogen phosphate in magnesium phosphate cement, with a curing condition of -25℃. Through tests on water-cement ratio, borax and polycarboxylate superplasticizer, the optimal mix proportion was obtained: water-cement ratio 0.17, borax 12%, superplasticizer 0.6%, and glacial acetic acid 0.1%. Under this condition, the 30s fluidity, setting time, compressive strength and flexural strength of MPC showed the best balance.

[0030] (2) The modified PVA fibers incorporated into the cement matrix not only provide more micro-interfaces, promoting the formation of hydration products and making the contact between cement particles closer, but may also accelerate the hydration reaction by improving the dispersion of fibers in the cement paste and changing the viscosity of the cement paste. Silane coupling agents combine –Si –O – with Mg in MPC through hydrolysis. 2+ The coordination significantly improved the interfacial bonding strength, and the modified PVA fibers were evenly distributed in the MPC, resulting in a smoother overall crack and significantly enhanced matrix toughness.

[0031] (3) In this 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 products. In addition, the grafting of silane molecules onto the fiber surface improves its hydrophilicity. The microstructure of the surface can help the fiber to be more easily dispersed in the matrix, inhibit crack propagation, and thus improve the toughness of the material.

[0032] (4) This 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, making its bond with cement particles tighter and reducing the adsorption capacity of the pore surface for water. Therefore, the MPC matrix becomes denser, reducing the number and connectivity of pores, thereby effectively reducing the water absorption rate and improving the water resistance of the matrix.

[0033] (5) This invention significantly improves the water resistance of magnesium phosphate cement through the combined effect of hydrophobic agents: nano-silica uniformly fills the pores of the matrix in the dispersion medium, blocking capillary water absorption channels; after hydrolysis, the silane coupling agent bonds with the cement products, and hydrophobic groups are grafted onto the surface to form a chemical barrier; stearic acid molecules are oriented to construct a dense hydrophobic layer. The three form a multi-level protection system of "physical filling-chemical bonding-molecular arrangement" in the microstructure, which synergistically inhibits water penetration, so that the material maintains stable impermeability in extreme low temperature environments and effectively extends the durability period of the structure.

[0034] (6) In this invention, 0.1% glacial acetic acid is added during the mixing process of magnesium phosphate cement. Its weak acidity can inhibit the rapid neutralization reaction between phosphate and magnesium oxide. At the same time, acetate ions reduce the concentration of active magnesium ions through complexation, thus delaying the crystallization of hydration products. Under low temperature conditions, glacial acetic acid further delays the hydration process by lowering the freezing point of the slurry and through surface adsorption effects, which can effectively extend the setting time of magnesium phosphate cement and improve the workability time under extreme low temperature conditions.

[0035] Therefore, by modifying PVA fibers with a silane coupling agent, this invention not only significantly improves their interfacial bonding performance with MPC but also more uniformly disperses the PVA fibers in the grout, thereby optimizing the workability and mechanical properties of the grout. This provides a feasible technical path for the application of MPC grout in extreme environments. These performance indicators already meet the requirements for grouting construction in low-temperature regions. Attached Figure Description

[0036] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0037] Figure 1 The effect of modified PVA fibers on the flowability and setting time of MPC is shown in the figure: Figure 1 (a) is a graph showing the effect of modified PVA fibers on the flowability of MPC. Figure 1 (b) is a graph showing the effect of modified PVA fiber on the coagulation time of MPC.

[0038] Figure 2 The figure shows the effect of modified PVA fibers on the toughness of MPC.

[0039] Figure 3 SEM comparison images of PVA fiber and modified PVA fiber: Figure 3 (a) is a 5μm SEM image of PVA fibers. Figure 3 (b) 10μm SEM image of PVA fiber, Figure 3 (c) is a 5μm SEM image of the modified PVA fiber. Figure 3 (d) 10 μm SEM image of modified PVA fiber.

[0040] Figure 4 The effect of different PVA fiber dosages on water absorption rate was obtained by immersing MPC in water at 4~6℃ for 28 days after curing at -25℃ for 3 days.

[0041] Figure 5 The figure shows the effect of different dosages of modified PVA fibers on the softening coefficient of MPC under 28 days of immersion curing.

[0042] Figure 6 The figure shows the effect of different dosages of modified PVA fibers on the strength retention rate of MPC under 28-day water immersion curing.

[0043] Figure 7SEM comparison images of modified / unmodified MPC samples under water immersion curing: Figure 7 (a) is a 1μm 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 1μm SEM image of the modified MPC sample under water immersion curing. Figure 7 (d) is a 5μm SEM image of the modified MPC sample under water curing.

[0044] Figure 8 The graph shows the effect of hydrophobic agents on the water absorption rate of MPC at -25℃.

[0045] Figure 9 The graph shows the effect of hydrophobic agents on the softening coefficient of MPC under 28 days of immersion curing.

[0046] Figure 10 The figure shows the effect of hydrophobic agent on the strength retention rate of MPC under 28-day water immersion curing.

[0047] Figure 11 This is a flowchart of the grouting agent test of the present invention. Detailed Implementation

[0048] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0049] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0050] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0051] This invention proposes a modified magnesium phosphate cement grouting material suitable for extreme low-temperature environments, aiming to meet both strength and water resistance requirements. To this end, the material is optimized from three aspects: basic performance groups, strength-enhancing functional groups, and water resistance-enhancing functional groups. In application, different functional groups can be flexibly combined to ensure that the modified magnesium phosphate cement meets construction requirements in extreme low-temperature environments.

[0052] The basic performance group consists of a base group and a retarding performance group. In some embodiments, the base group is based on calcined magnesium oxide, ammonium dihydrogen phosphate, and water in a magnesium-to-phosphorus ratio of 4:1, with the addition of borax, water, and a water-reducing agent; the retarding performance group is prepared using glacial acetic acid.

[0053] In some embodiments, the basic performance group raw materials include: calcined magnesium oxide, ammonium dihydrogen phosphate, borax, polycarboxylate superplasticizer, glacial acetic acid, and water, wherein the mixing ratio is as follows: magnesium-to-phosphorus ratio 4:1, water-to-binder ratio 0.17, boron-based ratio 12%, superplasticizer 0.6%, and glacial acetic acid-based ratio 0.1%. Wherein: the magnesium-to-phosphorus ratio is the mass ratio of calcined magnesium oxide to ammonium dihydrogen phosphate; the water-to-binder ratio = water / (calcined magnesium oxide + ammonium dihydrogen phosphate) mass ratio; the boron-based ratio = borax / (calcined magnesium oxide + ammonium dihydrogen phosphate) mass ratio; the percentage of superplasticizer is based on the total mass of calcined magnesium oxide + ammonium dihydrogen phosphate; and the glacial acetic acid-based ratio = glacial acetic acid / (calcined magnesium oxide + ammonium dihydrogen phosphate) mass ratio.

[0054] Using a mixture of recalcined magnesium oxide, ammonium dihydrogen phosphate, borax, glacial acetic acid, and water, along with a water-reducing agent, as the basic performance group, the material's properties are optimized by flexibly combining different functional groups. PVA fiber and silane coupling agent KH550 serve as strength-enhancing functional groups, while a hydrophobic agent enhances water resistance. This combination of three functional groups effectively improves both the material's strength and water resistance.

[0055] In some embodiments, the strength-enhancing functional group is made by modifying PVA fibers with silane coupling agent KH550 and PVA fibers. Specifically, silane coupling agent KH550 is slowly added to anhydrous ethanol and mixed evenly to obtain a 1 wt% coupling agent solution. Then, PVA fibers are immersed in the coupling agent solution at room temperature and ultrasonically dispersed for 40 minutes. Finally, they are washed and dried. The modified group is incorporated into the system at a ratio of 0.2% to 0.4%. Wherein: Modified group ratio = modified PVA fiber / (mass ratio of recalcined magnesium oxide + ammonium dihydrogen phosphate).

[0056] In some embodiments, the raw materials for the water-resistant enhancement functional group include: nano-silica, anhydrous ethanol, KH-570 silane coupling agent, and stearic acid, prepared into a hydrophobic agent in a mass ratio of 4:86:2:8. The hydrophobic agent dosage is 0.75%, based on the total mass of recalcined magnesium oxide + ammonium dihydrogen phosphate.

[0057] The base group, retarding performance group, strength-enhancing functional group, and water resistance-enhancing functional group can be flexibly combined according to 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., whose properties are superimposed.

[0058] In some embodiments, a base group is prepared by mixing calcined magnesium oxide, ammonium dihydrogen phosphate, and water at a magnesium-to-phosphorus ratio of 4:1, and borax (boron-based ratio of 12%) with water (i.e., a water-to-binder ratio of 0.17); secondly, a retarding performance group is prepared using glacial acetic acid at a glacial-based ratio of 0.1%; finally, a water-resistant performance group is prepared by using silane coupling agent KH550 and PVA fiber to prepare modified PVA fiber and preparing a strength-enhancing functional group at a modified base ratio of 0.2% to 0.4%, and adding a hydrophobic agent at a dosage of 0.75%.

[0059] In some embodiments, the preparation method of modified magnesium phosphate cement grouting material includes the following steps:

[0060] S1. Preparation of modified PVA fibers:

[0061] Slowly add silane coupling agent KH550 to anhydrous ethanol and mix evenly to obtain a 1wt% coupling agent solution. Then, immerse PVA fibers in the coupling agent solution at room temperature and ultrasonically disperse them for 40 minutes. Finally, rinse with water and dry in a hot air drying oven (90℃).

[0062] Preferably, the soaking time is 40 minutes and the drying time is 3 hours.

[0063] S2, Preparation of hydrophobic agents:

[0064] First, stearic acid powder was added to ethanol and heated in a 60°C water bath while stirring until completely dissolved (approximately 30 minutes). Next, nano-silica and KH-570 were added sequentially, and the mixture was dispersed using 500W ultrasound until no agglomeration occurred in the solution. Finally, the solution was filtered through a 200-mesh filter to remove undispersed particles.

[0065] Preferably, the mixture is placed in a 70°C water bath and heated and stirred for 30 minutes; when using 500W ultrasonic waves for dispersion, the duration is 20 minutes.

[0066] S3. Preparation of modified magnesium phosphate cement:

[0067] (1) Add calcined magnesium oxide, ammonium dihydrogen phosphate and borax to a cement paste mixer for dry mixing, then add a mixed solution of polycarboxylate superplasticizer, glacial acetic acid and water, and stir to obtain magnesium phosphate cement.

[0068] Preferably, after adding the mixed solution of polycarboxylate superplasticizer, glacial acetic acid and water, it is necessary to stir at low speed for 120 seconds, stir evenly, and then stir at high speed for 90 seconds.

[0069] (2) When the matrix has shown good fluidity, slowly and evenly add the modified PVA fiber prepared in S1 (and / or the hydrophobic agent prepared in S2) into the mixer and stir it with the magnesium phosphate cement prepared in step (1). Pour the slurry into the mold to form it. Demold it after 2 hours and finally put it in a -25℃ refrigerator to cure it.

[0070] Preferably, the modified PVA fiber (or the hydrophobic agent prepared by S2) should be added to the magnesium phosphate cement and stirred at low speed for 120 seconds.

[0071] In the application of the modified magnesium phosphate cement grouting material of the present invention, due to the introduction of glacial acetic acid during the mixing process, the material can maintain controllable workability in an environment of 0℃ to -25℃, thus meeting the grouting needs of cold regions.

[0072] This invention provides a cement-based composite grouting material that exhibits compressive and flexural strength, toughness, water resistance, and controllable setting time at a low temperature of -25℃. The material composition is optimized through composite modification technology, an organic-inorganic composite reinforcing phase is introduced to improve toughness, and a retarder is used to regulate the hydration process, thereby enhancing overall performance. Through synergistic optimization of three parts—basic performance group, strength-enhancing functional group, and water resistance-enhancing functional group—the overall performance of the material is significantly improved.

[0073] The base group is based on reburned 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 at extreme low temperatures by incorporating glacial acetic acid (0.1% glacial-based ratio). Simultaneously, a water-to-binder ratio of 0.17 and borax (12% boron-based ratio) are used to regulate the hydration reaction process, and polycarboxylate superplasticizer (0.6% admixture) are added to combine the base group and the retarding performance group into a basic performance group to optimize slurry fluidity, ensuring that the 30-second fluidity meets the requirements for construction controllability. The strength-enhancing functional base uses silane coupling agent KH550 to modify PVA fibers, effectively improving fiber dispersibility and interfacial bonding with the matrix.

[0074] In the preparation process, calcined magnesium oxide, ammonium dihydrogen phosphate, and borax were first dry-mixed, and then a solution containing water-reducing agent, glacial acetic acid, and water was added and stirred into a slurry. After the slurry reached the required fluidity, modified PVA fibers with strength-enhancing functional groups were slowly added, stirred at low speed, and then molded. After demolding for 2 hours, the test blocks were cured at -25℃. The test results showed that the flexural strength of the modified material increased by 18.4% at room temperature and 22.2% at -25℃, while the compressive strength decreased only slightly, achieving a balanced optimization of mechanical properties. In terms of water resistance, the water absorption rate decreased to 3.05%, the softening coefficient reached 0.86, and the strength retention rates at room temperature and low temperature increased to 86.7% and 81.2%, respectively. Microscopic analysis showed that the unmodified hydration products were loose needle-like, while the products after adding modified fibers transformed into a dense spherical structure, and the fibers were tightly bonded to the matrix, inhibiting water penetration.

[0075] To verify the mechanism of enhanced water resistance, a hydrophobic agent modification experimental group was added, in which 0.75% hydrophobic agent was simultaneously incorporated into the basic group and the retarding performance group. The results showed that the hydrophobic agent further reduced hydration channels by filling pores and lowering surface energy, thereby synergistically improving water resistance.

[0076] This invention addresses the shortcomings of traditional MPC, such as high brittleness, rapid initial setting, and easy hydrolysis, by using a base group to provide core strength, a retarding performance group to regulate the construction window, a strength-enhancing functional group to improve durability, and a water-resistant enhancement functional group to improve water resistance. Its preparation process balances low-temperature adaptability, construction controllability, and cost-effectiveness, making it particularly suitable for prestressed duct grouting projects in frigid regions. It provides a high-strength, tough, and durable material solution for infrastructure construction in extreme environments.

[0077] In the following examples, calcined magnesium oxide, ammonium dihydrogen phosphate, and borax were first frozen at -25°C, and then water and glacial acetic acid were placed at 0°C to prepare for the preparation of modified magnesium phosphate cement.

[0078] Example 1

[0079] 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 and ultrasonically dispersed for 40 minutes. Finally, the fibers were rinsed with water and dried in a hot air drying oven (90℃) for 3 hours. Burnt magnesium oxide, ammonium dihydrogen phosphate, and borax were added to a cement paste mixer for dry mixing. Then, polycarboxylate superplasticizer and water were added, and the mixture was stirred at low speed for 120 seconds to obtain a homogeneous paste. This paste 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 for 120 seconds with the prepared magnesium phosphate cement. The resulting paste (grouting material) was first poured into a Vicat apparatus and a cement paste fluidity test mold to test the setting time and fluidity. The paste was then poured into a mold for molding, demolded after 2 hours, and finally placed in a -25℃ refrigerator for curing. Two sets of single-doping experiments were conducted, and the specific ratios are shown in Table 1 (where % is based on the total mass of recalcined magnesium oxide + ammonium dihydrogen phosphate).

[0080] Table 1. Design of grouting material composition for modified PVA fiber with added MPC

[0081]

[0082] The test results of modified PVA fiber on the working performance of MPC are as follows: Figure 1 As shown.

[0083] like Figure 1 As shown in (a), the initial flowability of the MPC slurry without modified PVA fibers was 225 mm, exhibiting optimal flowability. When the fiber content increased to 0.4%, the flowability decreased 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 has a significant inhibitory effect on the flowability, leading to a sharp increase in slurry viscosity and an exacerbated trend of flowability deterioration.

[0084] like Figure 1 As shown in (b), the setting time of the reference sample without fiber was 35 minutes, while the setting time was shortened to 28 minutes after incorporating 0.4% modified PVA fiber, a reduction of 20%. The introduction of modified fiber provided additional microscopic nucleation sites for the slurry, accelerated the formation rate of hydration products, and enhanced the interaction between cement particles due to the enhanced surface properties of the fiber, thereby significantly shortening the setting and hardening process.

[0085] The test results of the mechanical properties of modified PVA fiber on MPC are as follows: Figure 2 As shown.

[0086] Figure 2In the case of MPC without modified PVA fibers, the flexural-to-compression ratio at -25℃ showed a natural decreasing trend with increasing curing time (3d, 7d, 28d). After incorporating 0.4% modified PVA fibers, the effective bonding between the fibers and the matrix significantly improved the material's toughness. The modified fibers optimized the microstructure of MPC through stress transfer and crack isolation, and the fiber-matrix interface bonding gradually strengthened with curing time, ultimately achieving a superior toughening effect.

[0087] This invention conducted SEM (scanning electron microscopy) tests on unmodified samples (a, b) and samples modified with silane coupling agent KH550 (c, d), respectively. Figure 3 As shown, the differences between the two surfaces are clearly visible: the unmodified fiber (a: 5μm, b: 10μm) has a smoother surface with no particles attached, and its diameter is evenly distributed. The lack of particles results in a large gap between the fiber and the cement mortar, which affects the interfacial bonding performance and thus leads to poor crack resistance. After modification with KH550, the surface is rougher, and the number of active sites increases significantly. Obvious uneven deposits and signs of local erosion are visible on the surface of (c: 5μm). The diameter of many fibers in (d: 10μm) remains stable, indicating that the modification process did not destroy the original fiber structure.

[0088] 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 force between PVA fibers and MPC hydration products. In addition, the grafting of silane molecules onto the fiber surface improves its hydrophilicity, and the surface microstructure can help the fibers disperse more easily in the matrix, inhibit crack propagation, and thus improve the toughness of the material.

[0089] Example 2

[0090] 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 determining their long-term service performance. This embodiment therefore includes a water resistance test. Compressive and flexural specimens were prepared according to the designed formula. The test considered water resistance performance under negative temperature conditions. The specimens were placed in a freezer (-25℃) for immersion curing. By simulating cold conditions, the water absorption rate, softening coefficient, and strength retention rate were analyzed. Finally, SEM characterization was used to explain the effect of modification on the water resistance of MPC.

[0091] The water resistance test used a water-cement ratio of 0.17, M / P=4, borax content of 12%, and water-reducing agent content of 0.6%. All water resistance tests were conducted at -25℃. The specific formulation design is shown in the table (the preparation method is the same as in Example 1), where UC-PVA represents unmodified PVA fiber, and PVA here represents surface-modified PVA fiber. Three groups of single-component tests were conducted, and the specific formulation is shown in Table 2 (where % is based on the total mass of calcined magnesium oxide + ammonium dihydrogen phosphate).

[0092] Table 2. Mix design of modified PVA fiber composite-MPC grouting material

[0093]

[0094] The comparison results of water absorption rates between unmodified and modified magnesium phosphate cement are as follows: Figure 4 As shown.

[0095] like Figure 4 As shown, the water absorption rate of MPC increases with age (1d, 7d, 14d, 21d, 28d), with a water absorption rate of 6%-7.5% without fiber addition. Further observation revealed that the water absorption rate was 4%-5.2% with the addition of unmodified PVA fibers, indicating that PVA fibers can slightly improve the water resistance of MPC. This is likely due to the network structure constructed by the PVA fibers, which effectively reduces pore connectivity and hinders water penetration. With the addition of modified PVA fibers, the water absorption rate of MPC decreased further, ranging from 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 after modification and enhances the bonding between the fibers and cement particles, thus improving the water resistance of MPC.

[0096] The effect of modified PVA fibers on the softening coefficient of MPC is as follows: Figure 5 As shown.

[0097] Figure 5 In the figure, CS-D represents the strength before immersion in water, CS-S represents the strength after 28 days of immersion in water, and SC represents the softening coefficient. At 0% dosing (control group), the softening coefficient of MPC was 0.73; when the modified PVA fiber dosing increased to 0.4%, the softening coefficient increased to 0.81, a 10.96% improvement over the control group. Modified PVA fibers exhibit a dual regulatory effect: at a dosing of 0.4%, the fibers aggregate during structure formation, leading to increased porosity. Simultaneously, a disordered and defective network forms between the fibers, reducing the effective stress-bearing area. The amino groups grafted onto the fiber surface after silane coupling agent modification also interact with some ions in MPC (such as Mg²⁺). 2+ It forms coordination bonds, which can achieve higher interfacial bonding strength than traditional physical adsorption, effectively preventing water molecules from entering the matrix and improving the water resistance of MPC.

[0098] The effect of modified PVA fibers on the strength retention rate of MPC is as follows: Figure 6 As shown.

[0099] Figure 6 The effect of modified PVA fibers on the compressive strength of MPC under different dosages is shown in the figure. I-CS represents the initial strength before immersion in water, 28d-CS represents the compressive strength after 28 days of immersion curing, and SRR represents the strength retention rate. As shown in the figure, without modified PVA fibers, the initial compressive strength of MPC was 44.7 MPa, which decreased to 27.6 MPa after 28 days of immersion curing, with a strength retention rate of 61.74%. This indicates that long-term immersion significantly deteriorated the matrix due to the lack of fiber-based crack resistance and water-resistant reinforcement. When the dosage increased to 0.4%, the initial strength decreased to 40.2 MPa, attributed to the increased porosity caused by fiber aggregation; however, the strength increased to 30.3 MPa after immersion, with a strength retention rate of 75.37%. The modified fibers, through surface amino groups... The chemical bonds between the fibers hinder water molecule penetration, while the fiber bridging delays crack propagation, thus significantly mitigating freeze-thaw damage and improving durability. Although the 0.4% admixture sacrifices some initial strength, its compressive stability in long-term immersion environments is superior to the unadmixed group, making it suitable for engineering scenarios that require a balance between construction feasibility and water resistance.

[0100] This invention conducted SEM (scanning electron microscopy) tests on unmodified MPC samples (a, b) and modified MPC samples (c, d) under water immersion curing conditions, as follows: Figure 7 As shown.

[0101] Figure 7 SEM images of unmodified / modified MPCs in each group after 28 days of water immersion curing. Figure 7 (a) and Figure 7 (b) shows the microstructure of unmodified MPC at different magnifications. Numerous struvite crystals are clearly observed, exhibiting needle-like or columnar morphology. This indicates that water curing provided sufficient moisture during the hydration reaction, significantly contributing to the growth of struvite crystals. Further observation revealed that excessive moisture caused disordered crystal growth, resulting in numerous pores and cracks around the crystals. This negatively impacted the bonding strength between crystals and the overall structural density, allowing water to more easily penetrate the matrix through these pores. Consequently, the control group matrix generally exhibited lower strength and water resistance. Figure 7 (c) The morphology of MPC after the addition of modified PVA fibers shows that the hydration products are formed in a partially spherical morphology. This may be because the chemically active sites at the PVA fiber interface accelerate the hydration reaction in their vicinity, resulting in a large amount of crystalline products filling the interface layer. Figure 7As shown in SEM (d), the fiber surface exhibits 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.

[0102] The microstructure of unmodified / modified MPC after 28 days of water curing was characterized by SEM. The hydration products of unmodified MPC were needle-like and rod-shaped, with disordered crystal growth, reducing the strength and water resistance of the matrix. Modified MPC particles were tightly bonded, with smooth crystal surfaces, improving the water resistance of the matrix. In the modified PVA fiber group, the hydration products of the MPC were spherical, and the modified fibers showed accumulation of hydration products, enhancing the interfacial bonding between the fiber and the matrix.

[0103] Example 3

[0104] Stearic acid powder was added to anhydrous ethanol and heated in a 70°C water bath with stirring for 30 minutes until completely dissolved. Next, nano-silica and KH-570 were added sequentially, and the mixture was dispersed using 500W ultrasound for 20 minutes until no agglomeration occurred in the solution. Finally, the solution was filtered through a 200-mesh filter to remove undispersed particles, yielding a hydrophobic agent. The mass ratio of nano-silica, anhydrous ethanol, KH-570 silane coupling agent, and stearic acid was 4:86:2:8.

[0105] Burnt magnesium oxide, ammonium dihydrogen phosphate, and borax are added to a cement paste mixer for dry mixing. Then, polycarboxylate superplasticizer and water are added. After mixing at low speed for 120 seconds, a uniform paste is obtained. Then, it is mixed at 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 mixed with the prepared magnesium phosphate cement to obtain the grouting material.

[0106] To verify the effect of the hydrophobic agent on water resistance, this invention also conducted a series of experiments on the combination of magnesium phosphate cement and the hydrophobic agent. The water-cement ratio for the water resistance test was 0.17, M / P=4, borax content was 12%, and water-reducing agent content was 0.6%. The specific proportion design is shown in Table 3, where H represents the hydrophobic agent (where % is based on the total mass of reburned magnesium oxide + ammonium dihydrogen phosphate).

[0107] Table 3. Mix Design of Hydrophobic Agent-MPC Grouting Material

[0108]

[0109] The effect of hydrophobic agents on the water absorption rate of MPC at -25℃ is as follows: Figure 8 As shown.

[0110] like Figure 8As shown, without the addition of a hydrophobic agent (0%), the water absorption rate of MPC increased significantly with curing time, rising from 6.19% at day 1 to 7.44% at day 28, 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 tended to stabilize, reaching only 3.83% at day 28, a decrease of 48.5% compared to the untreated group. During the short-term curing period (1-7 days), the water absorption rate of the 0.75% dosage only increased slightly from 3.62% to 3.68%, while the control group increased from 6.19% to 6.99%. The hydrophobic agent inhibited the initial water penetration by rapidly forming a dense barrier layer. During the long-term curing period (28 days), the increase in water absorption rate of the 0.75% dosage approached stagnation, while the control group continued to rise to 7.44%. This indicates that the chemical modification effect of the hydrophobic agent significantly improved the long-term water resistance stability of MPC, and its water absorption rate control ability was significantly better than that of the undoped group. It is suitable for engineering scenarios with high requirements for water resistance and the need to control material costs.

[0111] The effect of hydrophobic agents on the softening coefficient of MPC under 28-day immersion curing is as follows: Figure 9 As shown.

[0112] like Figure 9 As shown, the softening coefficient curve of the hydrophobic agent clearly shows a slow upward trend. Here, CS-D represents the strength before immersion in water, CS-S represents the strength after 28 days of immersion, and SC represents the softening coefficient. Without the hydrophobic agent (0%), the initial compressive strength of MPC was 27.6 MPa. After 28 days of immersion, the strength decreased to 20.1 MPa, and the softening coefficient was 0.73. This indicates that long-term immersion leads to a lack of hydrophobic protection in the matrix, accelerating the dissolution of hydration products and resulting in poor water resistance. After adding 0.75% hydrophobic agent, the initial strength increased to 32.1 MPa, and the strength after immersion significantly increased to 26.2 MPa, with a softening coefficient reaching 0.82. The hydrophobic agent forms coordination bonds with the MPC matrix through hydrophobic molecules, enhancing the bonding strength of the interfacial transition zone and effectively delaying the dissolution of hydration products (such as...). It dissolves, thereby inhibiting water penetration and reducing strength loss. Its overall compressive strength and water resistance are optimal, making it suitable for engineering scenarios that require simultaneous improvement of short-term mechanical properties and long-term water resistance stability.

[0113] The effect of hydrophobic agent on MPC strength retention rate under 28-day immersion curing is as follows: Figure 10 As shown.

[0114] like Figure 10The figure shows the effect of different amounts of hydrophobic agent on the compressive strength of MPC. I-CS represents the initial strength before immersion in water, 28d-CS represents the compressive strength after 28 days of immersion curing, and SRR represents the strength retention rate. At -25℃, the initial compressive strength of MPC without hydrophobic agent (0%) was 44.7 MPa. After 28 days of immersion curing, the strength decreased to 27.6 MPa, with a strength retention rate of 61.74%. This indicates that low temperature inhibits the full hydration reaction, and the loose matrix structure leads to a significant decrease in strength after long-term immersion. After adding 0.75% hydrophobic agent, the initial strength slightly decreased to 39.2 MPa, but the strength increased to 32.1 MPa after immersion, with a strength retention rate of 81.89%. The hydrophobic agent effectively slows down the dissolution of hydration products and matrix deterioration under low-temperature conditions by inhibiting water penetration and optimizing the interfacial transition zone structure. Although the 0.75% admixture sacrifices some initial strength, its compressive stability under long-term immersion conditions is significantly better than that of the unadmixed group, making it suitable for engineering scenarios that require balancing short-term construction performance and long-term durability in low-temperature environments.

[0115] Example 4

[0116] 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 and ultrasonically dispersed for 40 minutes. Finally, after rinsing with water, the fibers were dried in a hot air drying oven (90℃) for 3 hours. Reburned magnesium oxide, ammonium dihydrogen phosphate, and borax were added to a cement paste mixer for dry mixing. A mixture of polycarboxylate superplasticizer, water, and glacial acetic acid was then added and stirred at low speed for 120 seconds to obtain a homogeneous paste. This was followed by high-speed mixing 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 paste (grouting material) was poured into a Vicat apparatus mold and placed at -25℃ for setting time testing. Two sets of single-component experiments were conducted; the specific proportions are shown in Table 4 (where % is based on the total mass of reburned magnesium oxide + ammonium dihydrogen phosphate).

[0117] Table 4. Proportioning Design of Modified PVA Fiber Composite-MPC Grouting Material

[0118]

[0119] This study found that at -25℃, the initial setting time of modified magnesium phosphate cement with added 0.1% glacial acetic acid was significantly prolonged by 70 minutes compared to the unmodified group. Specifically, the control group without glacial acetic acid initiated a rapid hydration reaction immediately after grouting into the pipe, achieving initial strength within 28 minutes, resulting in a short effective construction window. In contrast, the modified group exhibited a distinct two-stage reaction characteristic after injection—a 98-minute latency period during which the grout maintained good fluidity and showed no strength development, until the hydration reaction suddenly accelerated after the latency period, with its strength growth rate essentially synchronized with the unmodified group. This regulatory effect mainly stems from the triple synergistic effect of glacial acetic acid: its weak acidity inhibits the initial reactivity of MgO and phosphate, and acetate and phosphate ions compete for complexation. The reaction process is slowed down by glacial acetic acid, which lowers the freezing point of the liquid phase and maintains the stability of the reaction medium. These results indicate that glacial acetic acid can create a controllable operating time window for pipeline grouting construction in extreme low-temperature environments by precisely controlling the reaction process, while ensuring normal strength development in the later stages.

[0120] Figure 11 This is a flowchart of the experimental process for this invention. Burnt magnesium oxide, ammonium dihydrogen phosphate, and borax were added to a cement paste mixer for dry mixing. Then, a mixture of polycarboxylate superplasticizer and water was added and stirred until homogeneous to obtain magnesium phosphate cement. The optimal water-cement ratio, borax dosage, and superplasticizer dosage were determined through workability and mechanical property tests. Based on this optimal ratio, modified PVA fibers were incorporated to prepare modified magnesium phosphate cement. The modified magnesium phosphate cement was tested for properties such as flexural strength, fluidity, and setting time. The surface of the specimens was examined using a scanning electron microscope (SEM). Finally, a water resistance test was conducted, using unmodified magnesium phosphate cement as a control group, to verify the advantages of magnesium phosphate cement with added hydrophobic agent / glacial acetic acid in various performance aspects.

[0121] Example 5

[0122] Slowly add silane coupling agent KH550 to anhydrous ethanol and mix evenly to obtain a 1wt% coupling agent solution. Then, immerse PVA fibers in the coupling agent solution at room temperature and ultrasonically disperse them for 40 minutes. Finally, rinse with water and dry in a hot air drying oven (90℃) for 3 hours.

[0123] Stearic acid powder was added to anhydrous ethanol and heated in a 70°C water bath with stirring for 30 minutes until completely dissolved. Next, nano-silica and KH-570 were added sequentially, and the mixture was dispersed using 500W ultrasound for 20 minutes until no agglomeration occurred in the solution. Finally, the solution was filtered through a 200-mesh filter to remove undispersed particles, yielding a hydrophobic agent. The mass ratio of nano-silica, anhydrous ethanol, KH-570 silane coupling agent, and stearic acid was 4:86:2:8.

[0124] Burned magnesium oxide, ammonium dihydrogen phosphate, and borax were added to a cement paste mixer for dry mixing. Then, a mixture of polycarboxylate superplasticizer, water, and glacial acetic acid was added and mixed at low speed for 120 seconds to obtain a homogeneous paste. This paste was then mixed at high speed for 90 seconds to obtain magnesium phosphate cement. When the matrix exhibited good fluidity, the prepared modified PVA fiber and hydrophobic agent were slowly and evenly added to the mixer and mixed at low speed with the prepared magnesium phosphate cement for 120 seconds. The resulting paste (grouting material) was poured into a Vicat apparatus mold and placed at -25℃ for setting time testing. Two sets of single-component experiments were conducted; the specific proportions are shown in Table 5 (where % is based on the total mass of burned magnesium oxide + ammonium dihydrogen phosphate).

[0125] Table 5

[0126]

[0127] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Therefore, any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention should fall within the protection scope of the present invention.

Claims

1. A modified magnesium phosphate cement grouting material, characterized in that, The modified magnesium phosphate cement grouting material includes: Magnesium phosphate cement is made from calcined magnesium oxide, ammonium dihydrogen phosphate, borax, polycarboxylate superplasticizer, and water. and the hydrophobic agent 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); The total mass of recalcined magnesium oxide and ammonium dihydrogen phosphate is 100%, and the hydrophobic agent content is 0.75%~1%.

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

3. The modified magnesium phosphate cement grouting material according to claim 2, characterized in that, The modified magnesium phosphate cement grouting material also includes modified PVA fibers; The modified PVA fiber is obtained by soaking PVA fiber in an ethanol solution of silane coupling agent KH550. The modified PVA fiber is added at a rate of 0.2% to 0.4% based on the total mass of recalcined magnesium oxide and ammonium dihydrogen phosphate, which is 100%.

4. The modified magnesium phosphate cement grouting material according to claim 1, characterized in that, In the magnesium phosphate cement, the mass ratio of deburned magnesium oxide to ammonium dihydrogen phosphate is (3.5~4.5):1, and the water-binder ratio is 0.16~0.18; the total mass of deburned magnesium oxide and ammonium dihydrogen phosphate is 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 ethanol solution of the silane coupling agent KH550 contains 0.8-1.0% KH550 by mass, and the soaking 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 to ethanol, heating and stirring until completely dissolved, then adding nano-silica and KH-570 in sequence, and ultrasonically dispersing and filtering.

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

8. The application of the modified magnesium phosphate cement grouting material according to claim 3, characterized in that, The modified magnesium phosphate cement grouting material is cured at -25°C and is suitable for construction environments from 0°C to -25°C.

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