Low-temperature high-durability magnesium phosphate cement and preparation method thereof

Abstract

According to the low-temperature high-durability magnesium phosphate cement and the preparation method thereof, dead burned magnesia powder serves as a core gelling base material and can provide a large amount of active Mg < 2 + >, ammonium dihydrogen phosphate can provide an acid environment and PO4 < 3->, the dead burned magnesia powder and the ammonium dihydrogen phosphate are subjected to an acid-base neutralization reaction in the presence of water to generate ammonium magnesium phosphate hydration products, and the magnesium phosphate hydration products are subjected to a low-temperature high-durability magnesium phosphate cement. The products are interwoven to form a hardened body framework, and a strength foundation is laid for cement; borax is adopted as a key retarder, borate ions generated after the borax is dissolved in water can be adsorbed on the surfaces of dead burned magnesia powder particles, a magnesium borate complex film is formed, the dissolution of Mg < 2 + > and the reaction of PO4 < 3-> are hindered, the cement setting time is effectively delayed, and the construction requirements are met; and the borax can also refine the crystal structure of a hydration product, reduce the porosity, improve the impermeability and the volume stability of a hardened body, and avoid shrinkage cracking.

Description

Technical Field

[0001] This invention belongs to the field of road construction materials technology, specifically relating to a low-temperature high-durability magnesium phosphate cement and its preparation method. Background Technology

[0002] Magnesium phosphate cement, as a novel inorganic cementitious material, exhibits broad application prospects in fields such as rapid road repair, emergency airport runway repair, and prefabricated building component connection due to its outstanding advantages, including rapid setting and early strength, good volume stability, and high interfacial bond strength with old concrete. Compared to traditional Portland cement, its early strength development is particularly excellent, typically reaching over 70% of the design strength within 1-3 hours after pouring. This significantly shortens the engineering curing cycle and meets the needs of emergency repairs and rapid construction, thus becoming one of the research hotspots in the field of civil engineering materials in recent years.

[0003] However, in practical engineering applications, the insufficient low-temperature adaptability and long-term durability of magnesium phosphate cement have become increasingly prominent, severely restricting its widespread application in cold regions, high-humidity environments, and corrosive media. Low-temperature environments significantly reduce its hydration reaction rate, leading not only to slow early strength development but also potentially incomplete hydration, increasing the porosity and loosening the structure of the hardened body, ultimately affecting its mechanical properties and service safety. Furthermore, magnesium phosphate cement is susceptible to corrosion from water, carbon dioxide, chloride ions, and other media during service, resulting in durability failures such as strength reduction, surface powdering, and microcrack propagation. Especially under freeze-thaw cycles in cold regions, the expansion of internal moisture upon freezing accelerates structural damage, further shortening its service life.

[0004] To address the aforementioned issues, existing technologies primarily improve the performance of magnesium phosphate cement by adjusting the raw material ratios (e.g., optimizing the molar ratio of magnesium oxide to phosphate, changing the type and dosage of retarders) or introducing organic modifiers (e.g., polymer emulsions, nanoparticles). However, adjusting the raw material ratios has limited effect on improving low-temperature performance and durability, and can easily lead to increased costs or deterioration in workability. While organic modifiers can improve the density and impermeability of cement paste to some extent, they suffer from poor compatibility with inorganic cementitious materials and are prone to aging at high temperatures, making it difficult to meet the long-term use requirements in complex engineering environments.

[0005] Mineral admixtures, as widely available, low-cost, and environmentally friendly modifiers, have been widely used in the field of cement-based materials. Through physical filling, pozzolanic reactivity, and interface optimization effects, they can effectively improve the microstructure of cement paste, enhancing its strength, impermeability, and frost resistance. However, current research largely focuses on the impact of single mineral admixtures on their room-temperature performance, lacking exploration of performance regulation mechanisms for low-temperature environments. Furthermore, the synergistic optimization effect of different types and dosages of mineral admixtures on the long-term durability of magnesium phosphate cement remains unclear, and the supporting system preparation process and performance evaluation system need further improvement.

[0006] Therefore, developing a magnesium phosphate cement and its preparation method that can simultaneously achieve rapid hydration and hardening at low temperatures and high durability during long-term service by rationally selecting and compounding mineral admixtures has become an urgent technical problem to be solved.

[0007] Therefore, developing a magnesium phosphate cement and its preparation method that can simultaneously achieve rapid hydration and hardening at low temperatures and high durability during long-term service by rationally selecting and compounding mineral admixtures has become an urgent technical problem to be solved. Summary of the Invention

[0008] The purpose of this invention is to provide a low-temperature high-durability magnesium phosphate cement and its preparation method, so as to overcome the problem that the existing technology cannot meet the requirements of rapid hydration hardening under low-temperature environment and high durability during long-term service.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A low-temperature, high-durability magnesium phosphate cement comprises, by weight, the following raw materials: 90 parts of reburned magnesium oxide powder, 30 parts of ammonium dihydrogen phosphate, 4.5 parts of borax, 12-36 parts of mineral admixtures, 125-151.5 parts of sand, and 20-26.5 parts of water.

[0010] Preferably, the admixture is a mixture of one or more of fly ash, silica fume, or metakaolin.

[0011] Preferably, the magnesium oxide content in the recalcined magnesium oxide powder is ≥97%.

[0012] Preferably, the relative density of the ammonium dihydrogen phosphate is 1.803 g / cm³. 3 It has a solubility of 37.4g in water at 20℃, a pH value of 4.3, and a content of ≥98% ammonium dihydrogen phosphate.

[0013] Preferably, the borax has a Na2B4O7·10H2O content ≥99.5%, a molecular weight of 381.37, and a clarity test result ≤4.

[0014] Preferably, the fly ash density is 2.55 g / cm³. 3 Bulk density 1.12 g / cm³ 3 The residue on a 5μm square-hole sieve was 16%, and the loss on ignition was 2.8%.

[0015] Preferably, the silica fume contains ≥98.1% SiO2 and ≥98.1% Cl. - ≤0.01%, loss on ignition ≤1.48%.

[0016] Preferably, the river sand has a particle size of 0–2.5 mm. Standard sand has a particle size of 0.08–2 mm, a mud content of <0.20%, a loss on ignition of <0.4%, and a SiO2 content of >96%.

[0017] A method for preparing low-temperature, high-durability magnesium phosphate cement includes the following steps: The calcined magnesium oxide powder, ammonium dihydrogen phosphate, borax, mineral admixtures, and sand are mixed evenly to obtain a mixture. Low-temperature, high-durability magnesium phosphate cement can be obtained by gradually adding water to the mixture, mixing it evenly, and then pouring and curing it.

[0018] Preferably, by mass, the composition comprises 90 parts of calcined magnesium oxide powder, 30 parts of ammonium dihydrogen phosphate, 4.5 parts of borax, 12-36 parts of mineral admixtures, 125-151.5 parts of sand, and 20-26.5 parts of water.

[0019] Compared with the prior art, the present invention has the following beneficial technical effects: This invention provides a low-temperature, high-durability magnesium phosphate cement, using recalcined magnesium oxide powder as the core cementitious material, which can provide a large amount of active magnesium. 2+ Ammonium dihydrogen phosphate can provide an acidic environment and react with PO4. 3- Under aqueous conditions, the two undergo an acid-base neutralization reaction to produce magnesium ammonium phosphate hydration products. These products intertwine to form a hardened skeleton, laying the foundation for the strength of cement. By using borax as a key retarder, the borate ions generated after dissolving in water adsorb onto the surface of reburned magnesium oxide powder particles, forming a magnesium borate complex film, which hinders the setting of magnesium oxide. 2+ Dissolution with PO4 3- The reaction effectively delays the cement setting time to meet construction requirements; moreover, borax can refine the crystal structure of hydration products, reduce porosity, improve the impermeability and volume stability of the hardened body, and avoid drying shrinkage cracking. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the mixed powder in Embodiment 5 of the present invention.

[0021] Figure 2 These are schematic diagrams of the test specimens in Examples 1 to 7 and Comparative Example 1 of the present invention.

[0022] Figure 3 The graphs show the flowability test data from Examples 1 to 7 and Comparative Example 1 of this invention.

[0023] Figure 4 The diagrams show the low-temperature flexural strength of Examples 1 to 7 and Comparative Example 1 of the present invention.

[0024] Figure 5 The diagrams show the low-temperature compressive strength of Examples 1 to 7 and Comparative Example 1 of the present invention.

[0025] Figure 6 The diagrams show the flexural strength after corrosion in Examples 1 to 7 and Comparative Example 1 of this invention.

[0026] Figure 7 These are the compressive strength diagrams after corrosion in Examples 1 to 7 and Comparative Example 1 of the present invention. Detailed Implementation

[0027] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0028] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0029] This invention provides a low-temperature, high-durability magnesium phosphate cement, which, by weight, comprises the following raw materials: 90 parts of reburned magnesium oxide powder, 30 parts of ammonium dihydrogen phosphate, 4.5 parts of borax, 12-36 parts of mineral admixtures, 125-151.5 parts of sand, and 20-26.5 parts of water.

[0030] In a specific embodiment of the present invention, the mineral admixture is one or a mixture of fly ash, silica fume or metakaolin.

[0031] The recalcined magnesium oxide powder is made by calcining magnesite at a high temperature of over 1600℃ in an industrial kiln, followed by crushing and grinding into 180-220 mesh powder. The powder is light yellow in color and contains ≥97% magnesium oxide.

[0032] The ammonium dihydrogen phosphate is of industrial purity grade, a white crystalline solid with a relative density of 1.803 g / cm³. 3 It has a solubility of 37.4g in water at 20℃, a pH value of 4.3, and a content of ≥98% ammonium dihydrogen phosphate.

[0033] The borax is of analytical grade, white crystals, with a Na2B4O7·10H2O content ≥99.5%, a molecular weight of 381.37, and a clarity test result ≤4.

[0034] The fly ash is Class I ash, a gray powder with a density of 2.55 g / cm³. 3 Bulk density 1.12 g / cm³ 3 The residue on a 5μm square-hole sieve was 16%, and the loss on ignition was 2.8%.

[0035] The silica fume is high-quality silica fume with an SiO2 content ≥98.1% and a Cl content ≥98.1%. - ≤0.01%, loss on ignition ≤1.48%.

[0036] The metakaolin is white powder with a particle size of 1250 mesh and a whiteness of ≥92%.

[0037] The sand is river sand or standard sand.

[0038] The river sand has a particle size of 0–2.5 mm. Standard sand has a particle size of 0.08–2 mm, a mud content of <0.20%, a loss on ignition of <0.4%, and a SiO2 content of >96%.

[0039] This invention provides a method for preparing low-temperature high-durability magnesium phosphate cement, comprising the following steps: The calcined magnesium oxide powder, ammonium dihydrogen phosphate, borax, mineral admixtures, and sand are mixed evenly to obtain a mixture. Low-temperature, high-durability magnesium phosphate cement can be obtained by gradually adding water to the mixture, mixing it evenly, and then pouring and curing it.

[0040] This invention provides a low-temperature, high-durability magnesium phosphate cement, comprising 90 parts by weight of recalcined magnesium oxide powder. In this invention, the recalcined magnesium oxide powder is ground to a 200-mesh powder after high-temperature calcination; preferably, the magnesium oxide content in the recalcined magnesium oxide powder is ≥97%. This invention uses recalcined magnesium oxide powder as the main raw material for magnesium phosphate cement, which is not only widely available but also has a high magnesium oxide content, ensuring excellent durability of the prepared magnesium phosphate cement. Furthermore, the low reactivity of recalcined magnesium oxide mitigates the adverse effects of excessively vigorous reaction between magnesium oxide and ammonium dihydrogen phosphate on material strength and construction.

[0041] In a specific embodiment of the present invention, based on 90 parts by weight of reburned magnesium oxide powder, the low-temperature high-durability magnesium phosphate cement provided by the present invention includes 30 parts by weight of phosphate. By adding ammonium dihydrogen phosphate as a matrix component of the magnesium phosphate cement, the present invention provides an acidic environment and anions for the hydration reaction, thereby reacting with the reburned magnesium oxide powder to generate hydration products that improve the strength of the magnesium phosphate cement, thus obtaining a high-durability magnesium phosphate cement.

[0042] In a specific embodiment of the present invention, based on 90 parts by weight of reburned magnesium oxide powder, the low-temperature high-durability magnesium phosphate cement provided by the present invention includes 4.5 parts of borax. In this invention, boron is of analytical grade, a white crystalline solid, with a Na₂B₄O₇·10H₂O content ≥99.5%, a molecular weight of 381.37, and a clarity test result ≤4. The present invention, by using borax as a retarder, can react with the Mg in the magnesium phosphate cement paste. 2+ It has a strong complexing ability, which reduces the concentration of magnesium ions and thus delays the formation of hydration products. By controlling the amount of retarder, the setting time of magnesium phosphate cement can be controlled, so that it can be adjusted according to the actual working conditions and greatly expand the application scenarios of magnesium phosphate cement.

[0043] Based on 90 parts by weight of reburned magnesium oxide powder, the high-strength retarded magnesium phosphate cement provided by this invention includes 12-36 parts of mineral admixtures. In this invention, the admixtures include fly ash, metakaolin, and silica fume. In this invention, the fly ash is Grade I fly ash, a gray powder with a density of 2.55 g / cm³. 3 Bulk density 1.12 g / cm³ 3 The kaolin has a 16% residue on a 5μm square-hole sieve, a loss on ignition of 2.8%, and a SiO2 content ≥45%. The metakaolin is a white powder with a particle size of 1250 mesh, a whiteness ≥92%, and a SiO2 content ≥55%. The silica fume has a SiO2 content ≥98.1% and a Cl content ≥98.1%. -≤0.01%, loss on ignition ≤1.48%. This invention uses fly ash, metakaolin, and silica fume as admixtures. Fly ash improves the workability of the slurry through the "ball effect," and its fine particles can fill the pores of hydration products, reducing drying shrinkage and temperature shrinkage. Later, its active substances will also undergo a secondary reaction with unreacted magnesium oxide, slightly increasing strength. Metakaolin is rich in highly reactive SiO2 and Al2O3, which can react with excess Mg in the system. 2+ A pozzolanic reaction occurs, generating stable hydrated magnesium silicate and hydrated magnesium aluminate, significantly improving the early and later strength of cement and enhancing its resistance to chemical attack. The extremely fine silica fume particles enable "multi-stage filling," deeply filling micropores and reducing the porosity of the hardened body from 15%-20% to below 5%, greatly improving compressive strength, impermeability, and frost resistance. In one embodiment of the invention, the admixture can be in the form of 12, 24, or 36 parts by mass.

[0044] Based on 90 parts by weight of reburned magnesium oxide powder, the low-temperature high-durability magnesium phosphate cement provided by this invention includes 20-25.7 parts by weight of water. By controlling the amount of water, this invention can, on the one hand, ensure good fluidity of the slurry, and on the other hand, ensure the hydration reaction of the magnesium phosphate cement proceeds.

[0045] Based on 90 parts by weight of reburned magnesium oxide powder, the low-temperature high-durability magnesium phosphate cement provided by this invention preferably further includes 124.5, 136.5, 148.5, and 165.5 parts by weight of aggregate. In this invention, the aggregate is preferably river sand, with a particle size preferably 0–2.5 mm, a mud content <0.20%, a loss on ignition <0.4%, and a SiO2 content >96%. This invention reduces the amount of magnesium phosphate cement used and lowers costs by adding river sand as aggregate.

[0046] This invention provides a low-temperature, high-durability magnesium phosphate cement, comprising the following raw materials by weight: 90 parts of recalcined magnesium oxide powder, 30 parts of ammonium dihydrogen phosphate, 4.5 parts of borax, 12-36 parts of mineral admixtures, 124.5-151.5 parts of sand, and 25.7 parts of water; the mineral admixtures include fly ash, silica fume, and metakaolin. In this invention, recalcined magnesium oxide powder serves as the core cementitious material, providing a large amount of active magnesium. 2+ Ammonium dihydrogen phosphate can provide an acidic environment and react with PO4. 3-- Under aqueous conditions, the two undergo an acid-base neutralization reaction to produce magnesium ammonium phosphate hydration products. These products intertwine to form a hardened skeleton, laying the foundation for the strength of cement. By using borax as a key retarder, the borate ions generated after dissolving in water adsorb onto the surface of reburned magnesium oxide powder particles, forming a magnesium borate complex film, which hinders the setting of magnesium oxide. 2+ Dissolution with PO4 3-The reaction effectively delays cement setting time, meeting construction requirements; borax can also refine the crystal structure of hydration products, reduce porosity, improve the impermeability and volume stability of the hardened body, and avoid drying shrinkage cracking; the use of mineral admixtures can optimize cement performance from different dimensions; fly ash particles are spherical, which can improve the workability of the paste through the "ball effect," and its fine particles can also fill the pores of hydration products, reducing drying shrinkage and temperature shrinkage. In the later stage, its active substances will also undergo a secondary reaction with unreacted magnesium oxide, slightly improving the strength; metakaolin is rich in highly active SiO2 and Al2O3, which can react with excess Mg in the system. 2+ A volcanic ash reaction occurs, generating stable hydrated magnesium silicate and hydrated magnesium aluminate, significantly improving the early and later strength of cement and enhancing its resistance to chemical attack. The extremely fine silica fume particles enable "multi-level filling," deeply filling micropores and reducing the porosity of the hardened body from 15%-20% to below 5%, greatly improving compressive strength, impermeability, and frost resistance. Sand, as the main aggregate, uses its high strength to construct a rigid skeleton, bearing and transferring external loads, preventing the cement paste from cracking due to insufficient strength. Simultaneously, its excellent volume stability inhibits shrinkage during cement hydration, reducing early plastic cracks. This invention, through optimization of the composition and dosage of phosphate cement, not only improves the workable mechanical properties of magnesium phosphate cement but also significantly enhances its low-temperature durability, greatly expanding its application range in rapid repair scenarios.

[0047] This invention provides a method for preparing the low-temperature high-durability magnesium phosphate cement described in the above technical solution, comprising the following steps: (1) Add the calcined magnesium oxide powder, ammonium dihydrogen phosphate, borax, mineral admixtures and sand to a mixing pot and stir at low speed for 60 seconds to obtain a mixed dry material; (2) Add water to the mixed dry material, stir at low speed for 30s, then stir at high speed for 30s, then stop stirring for 30s, then continue stirring at high speed for 90s, and finally cure to obtain low temperature high durability magnesium phosphate cement.

[0048] In this invention, the mixing of the recalcined magnesium oxide powder, ammonium dihydrogen phosphate, borax, mineral admixtures, and sand is preferably carried out in a cement mortar mixer.

[0049] In this invention, the preferred method for mixing the recalcined magnesium oxide powder, ammonium dihydrogen phosphate, borax, and mineral admixtures is as follows: first, the recalcined magnesium oxide powder, ammonium dihydrogen phosphate, and borax are mixed; then, the mineral admixtures are added in 2-3 batches to obtain a mixed dry material. In this invention, the mixing is preferably carried out under stirring conditions. Adding the mineral admixtures in 2-3 batches in this invention facilitates their uniform dispersion in the mixed dry material.

[0050] In this invention, when the raw materials of the low-temperature high-durability magnesium phosphate cement also include aggregates, the aggregates are preferably mixed together with calcined magnesium oxide powder, ammonium dihydrogen phosphate, borax, and mineral admixtures.

[0051] After obtaining the mixture, the present invention adds water to the mixed dry material and stirs it, and finally cures it to obtain low-temperature high-durability magnesium phosphate cement.

[0052] In this invention, the preferred stirring time is to stir at low speed for 60 seconds to obtain a mixed dry material, add water and stir at low speed for 30 seconds, then stir at high speed for 30 seconds, then stop stirring for 30 seconds, and then continue stirring at high speed for 90 seconds.

[0053] After mixing, the present invention preferably pours the mixed slurry into a mold and then cures it. The present invention does not have any special limitations on the material and size of the mold. The present invention also does not have any special limitations on the specific operation of pouring the slurry into the mold; operations well known to those skilled in the art can be used.

[0054] In embodiments of the present invention, the curing temperature can be room temperature, and the curing method is natural air curing. Preferably, the mold is removed after 2-3 hours of curing.

[0055] The preparation process proposed in this invention is simple and easy to implement, requiring no additional purchase of new equipment; it can be completed using existing production facilities. Furthermore, this process effectively avoids the construction difficulties caused by the rapid setting speed and large heat release during hydration of magnesium phosphate cement, laying a favorable foundation for the large-scale promotion and application of phosphate cement.

[0056] This invention provides the application of the low-temperature high-durability magnesium phosphate cement described in the above-described technical solution or the low-temperature high-durability magnesium phosphate cement prepared by the preparation method described in the above-described technical solution in emergency repair of airport road surfaces in high-altitude environments.

[0057] The present invention does not impose any special limitations on the specific operation of the application; conventional application operations familiar to those skilled in the art can be used.

[0058] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0059] Example 1 A low-temperature, high-durability magnesium phosphate cement comprises, by weight, the following raw materials: 90 parts of reburned magnesium oxide powder, 30 parts of ammonium dihydrogen phosphate, 4.5 parts of borax, 12 parts of fly ash, 136.5 parts of sand, and 21.9 parts of water. The preparation method of the low-temperature high-durability magnesium phosphate cement is as follows: (1) Weigh out 800g of magnesium oxide, 267g of ammonium dihydrogen phosphate, 40g of borax, 107g of fly ash, 1214g of sand, and 194.2g of water respectively. (2) Add the calcined magnesium oxide powder, ammonium dihydrogen phosphate, borax, fly ash and sand into a JC / T 681 planetary cement mortar mixer and mix at low speed for 60 seconds to obtain a mixed dry material; (3) Add water to the dry mixture obtained in step (2), stir at low speed for 30 seconds, then stir at high speed for 30 seconds, then stop stirring for 30 seconds, and then continue stirring at high speed for 90 seconds to obtain low temperature high durability magnesium phosphate cement.

[0060] Example 2 The difference from Example 1 is that fly ash is replaced with metakaolin. All other conditions are the same as in Example 1, specifically: A low-temperature, high-durability magnesium phosphate cement is composed of the following components: 800g magnesium oxide, 267g ammonium dihydrogen phosphate, 40g borax, 107g metakaolin, 1214g sand, and 194.2g water. Example 3 The difference from Example 1 is that fly ash is replaced with silica fume. All other conditions are the same as in Example 1, specifically: A low-temperature, high-durability magnesium phosphate cement is composed of the following components: 800g magnesium oxide, 267g ammonium dihydrogen phosphate, 40g borax, 107g silica fume, 1214g sand, and 194.2g water. Example 4 A low-temperature high-durability magnesium phosphate cement, by weight, comprises the following raw materials: 90 parts of reburned magnesium oxide powder, 30 parts of ammonium dihydrogen phosphate, 4.5 parts of borax, 12 parts of silica fume, 12 parts of fly ash, 148.5 parts of sand, and 23.8 parts of water. Other conditions are the same as in Example 1, specifically: A low-temperature, high-durability magnesium phosphate cement is composed of the following components: 800g magnesium oxide, 267g ammonium dihydrogen phosphate, 40g borax, 107g fly ash, 107g silica fume, 1321g sand, and 211.4g water. Example 5 The difference from Example 4 is that fly ash is replaced with metakaolin. All other conditions are the same as in Example 4, specifically: A low-temperature, high-durability magnesium phosphate cement is composed of the following components: 800g magnesium oxide, 267g ammonium dihydrogen phosphate, 40g borax, 107g metakaolin, 107g silica fume, 1321g sand, and 211.4g water. The mixed powder obtained according to this embodiment is as follows: Figure 1 As shown.

[0061] Example 6 The difference from Example 4 is that silica fume is replaced with metakaolin. All other conditions are the same as in Example 4, specifically: A low-temperature, high-durability magnesium phosphate cement is composed of the following components: 800g magnesium oxide, 267g ammonium dihydrogen phosphate, 40g borax, 107g fly ash, 107g metakaolin, 1321g sand, and 211.4g water. Example 7 A low-temperature high-durability magnesium phosphate cement, by weight, comprises the following raw materials: 90 parts of reburned magnesium oxide powder, 30 parts of ammonium dihydrogen phosphate, 4.5 parts of borax, 12 parts of fly ash, 12 parts of metakaolin, 12 parts of silica fume, 165.5 parts of sand, and 26.5 parts of water. Other conditions are the same as in Example 1. Specifically: A low-temperature, high-durability magnesium phosphate cement is composed of the following components: 800g magnesium oxide, 267g ammonium dihydrogen phosphate, 40g borax, 107g fly ash, 107g metakaolin, 1321g sand, and 211.4g water. Comparative Example 1 A low-temperature high-durability magnesium phosphate cement, by weight, comprises the following raw materials: 90 parts of reburned magnesium oxide powder, 30 parts of ammonium dihydrogen phosphate, 4.5 parts of borax, 124.5 parts of sand, and 20 parts of water. Other conditions are the same as in Example 1. Specifically: A low-temperature, high-durability magnesium phosphate cement is composed of the following components: 800g magnesium oxide, 267g ammonium dihydrogen phosphate, 40g borax, 1107g sand, and 177.1g water. Example Flow effect verification The fluidity test was conducted according to the "Method for Determination of Flowability of Cement Mortar" (GB / T 2419-2005). The fluidity of magnesium phosphate cement mortar was determined using a tumbler, a truncated cone mold, a tamping rod, and a ruler. The specific testing steps are as follows: S1. Moisten the surfaces of the jumping table and the truncated cone mold with a damp cloth, place the truncated cone mold in the center of the jumping table, and put on the mold sleeve. S2. Fill the truncated cone mold with magnesium phosphate cement mortar in two layers. Use a tamping rod to tamp each layer 15 times evenly. After tamping, remove the mold sleeve and use a scraper to level the mortar that is higher than the truncated cone mold.

[0062] S3. Gently lift the truncated cone mold vertically upwards, start the jumping table to vibrate 25 times, measure the diameter of the mortar spreading at the bottom and the two relatively vertical diameters, and take the average value of the two to characterize the fluidity.

[0063] Verification of the flow effect of the scale: The procedure for determining the fluidity of the magnesium phosphate cement mortar prepared in the comparative example is the same as that in the example. Table 1 Flowability of Magnesium Phosphate Cement Mortar

[0064] Note: wt% indicates the mass fraction of this material component relative to the mass of calcined magnesium oxide plus ammonium dihydrogen phosphate.

[0065] like Figure 3 As shown, compared with the comparative examples, the flowability of Examples 2, 3, 4, and 5 of this invention is significantly improved, with an increase of over 10%, achieving a self-leveling effect. Both silica fume and fly ash can effectively improve the flowability of MPC mortar, with silica fume showing the best improvement effect, achieving a flowability of up to 185 mm. This is mainly attributed to the spherical shape of silica fume and fly ash particles, which can exert a "ball bearing" effect, thereby significantly improving the flowability of the slurry. Since the particle size of silica fume is smaller than that of fly ash, its effect on improving the flowability of MPC mortar is better than that of fly ash. In contrast, the addition of metakaolin leads to a decrease in the flowability of MPC mortar, only 145 mm. This may be because metakaolin is an ultrafine mineral material, with a fineness much higher than that of magnesium oxide powder. As its dosage increases, the specific surface area of ​​the mixed system increases significantly, resulting in a relative decrease in the free water content of the slurry. At the same time, the irregular flaky particle shape of metakaolin increases the interparticle friction, further weakening the flowability. In addition, metakaolin, as a highly active mineral admixture, may have active components (such as SiO2 and Al2O3) that react with the MPC system, consuming some water and thus reducing its fluidity.

[0066] In the co-blending test, when silica fume and metakaolin were co-blended, the MPC mortar could still achieve self-leveling, indicating that silica fume could effectively offset the negative impact of metakaolin on fluidity. However, when fly ash and metakaolin were co-blended, the fluidity was lower than that of metakaolin alone, ranking lowest among all groups, indicating that the compensation effect of fly ash on the fluidity loss caused by metakaolin was limited.

[0067] Low-temperature mechanical property performance verification According to the "Test Method for Strength of Cement Mortar (ISO Method)" (GB / T 17671-2021), the flexural strength and compressive strength of the magnesium phosphate cement specimens provided in Examples 1 to 7 and Comparative Example 1 were tested after curing at -30℃ for 3h, 1d, 3d, and 7d.

[0068] Table 2 Flexural Strength of MPC Mortar under Low-Temperature Curing

[0069] Table 3 Compressive Strength of MPC Mortar under Low-Temperature Curing

[0070] like Figure 4 , Figure 5 As shown, under curing conditions at -30℃, the strength indices of all specimens met the repair requirements. The low-temperature mechanical strength of the magnesium phosphate cement with added mineral admixtures was superior to that of the traditional magnesium phosphate cement in Comparative Example 1. The embodiments described in this invention, while maintaining the compressive strength without decrease, achieved a 7-day flexural strength of 15.1 MPa, an increase of 16.1% compared to Comparative Example 1, and a 7-day compressive strength of 50.3 MPa, an increase of 9.6% compared to Comparative Example 1. The mineral admixture-modified magnesium phosphate cement of this invention exhibits excellent low-temperature mechanical properties while improving fluidity, thus facilitating the use of magnesium phosphate cement in low-temperature environments.

[0071] Corrosion resistance verification: According to the "Test Method for Strength of Cement Mortar (ISO Method)" (GB / T 17671-2021), the compressive strength and flexural strength of the magnesium phosphate cement specimens provided in Examples 1-7 and Comparative Example 1 were tested after being soaked in water, sulfate solution, and snow-melting salt solution for 90 days. The specific test steps are as follows: S1. Prepare three portions of water of equal mass, and use two of them to prepare a 5% sodium sulfate solution and a 5% snow-melting salt (sodium acetate) solution.

[0072] S2. Magnesium phosphate cement test blocks prepared based on Examples 1-7 and Comparative Example 1 were cured in air and immersed in water, 5% sodium sulfate solution, and 5% de-icing salt (sodium acetate solution), respectively. The solutions were changed every 30 days. S3. After soaking for 90 days, remove the specimen and test all flexural and compressive strengths.

[0073] The results are shown in Tables 4 and 5: Table 4 Flexural strength of magnesium phosphate cement after corrosion

[0074] Table 5 Compressive strength of magnesium phosphate cement after corrosion

[0075] like Figure 6 , Figure 7As shown in Examples 1-7 and Comparative Example 1, the strength changes indicate that the addition of mineral admixtures can effectively improve the chemical corrosion resistance of magnesium phosphate mortar. Among the three groups of single mineral admixture groups, the flexural strength of the single silica fume MPC mortar specimens, after soaking in sulfate solution and de-icing salt solution for 90 days, increased by 19% and 14% respectively compared to ordinary mortar after soaking for the same time, and the compressive strength increased by 31% and 35% respectively. As shown in Examples 1-3 and Comparative Example 1, the compressive strength of the single silica fume magnesium phosphate mortar specimens, after soaking in de-icing salt solution for 90 days, actually increased by 12.3% compared to that cured at room temperature. The flexural strength of the MPC mortar specimen with single-admixture metakaolin decreased significantly after 90 days of corrosion compared to that cured at room temperature, but the value was still the largest among the three groups. The flexural strength decreased by 24% after 90 days of sulfate corrosion, which was the largest decrease among the three groups. As can be seen from Examples 1, 5, and 7, the compressive strength test results after corrosion in different solutions are similar, indicating that the magnesium phosphate mortar specimen with fly ash has good and stable resistance to common corrosion, which is beneficial to the long-term service of magnesium phosphate.

[0076] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A low-temperature, high-durability magnesium phosphate cement, characterized in that, By weight, it includes the following raw materials: 90 parts of calcined magnesium oxide powder, 30 parts of ammonium dihydrogen phosphate, 4.5 parts of borax, 12-36 parts of mineral admixtures, 125-151.5 parts of sand, and 20-26.5 parts of water.

2. The low-temperature high-durability magnesium phosphate cement according to claim 1, characterized in that, The admixture is a mixture of one or more of fly ash, silica fume, or metakaolin.

3. The low-temperature high-durability magnesium phosphate cement according to claim 1, characterized in that, The magnesium oxide content in the recalcined magnesium oxide powder is ≥97%.

4. The low-temperature high-durability magnesium phosphate cement according to claim 1, characterized in that, The relative density of the ammonium dihydrogen phosphate is 1.803 g / cm³. 3 It has a solubility of 37.4g in water at 20℃, a pH value of 4.3, and a content of ≥98% ammonium dihydrogen phosphate.

5. The low-temperature high-durability magnesium phosphate cement according to claim 1, characterized in that, The borax has a Na2B4O7·10H2O content ≥99.5%, a molecular weight of 381.37, and a clarity test result ≤4.

6. The low-temperature high-durability magnesium phosphate cement according to claim 2, characterized in that, The density of the fly ash is 2.55 g / cm³. 3 Bulk density 1.12 g / cm³ 3 The residue on a 5μm square-hole sieve was 16%, and the loss on ignition was 2.8%.

7. The low-temperature high-durability magnesium phosphate cement according to claim 2, characterized in that, The silica fume contains ≥98.1% SiO2 and ≥98.1% Cl. - ≤0.01%, loss on ignition ≤1.48%.

8. The low-temperature high-durability magnesium phosphate cement according to claim 1, characterized in that, The river sand has a particle size of 0–2.5 mm. Standard sand has a particle size of 0.08–2 mm, a mud content of <0.20%, a loss on ignition of <0.4%, and a SiO2 content of >96%.

9. A method for preparing low-temperature, high-durability magnesium phosphate cement, characterized in that, Includes the following steps: The calcined magnesium oxide powder, ammonium dihydrogen phosphate, borax, mineral admixtures, and sand are mixed evenly to obtain a mixture. Low-temperature, high-durability magnesium phosphate cement can be obtained by gradually adding water to the mixture, mixing it evenly, and then pouring and curing it.

10. The method for preparing low-temperature high-durability magnesium phosphate cement according to claim 9, characterized in that, By weight, the composition is: 90 parts of recalcined magnesium oxide powder, 30 parts of ammonium dihydrogen phosphate, 4.5 parts of borax, 12-36 parts of mineral admixtures, 125-151.5 parts of sand, and 20-26.5 parts of water.

Images