Fiber-reinforced magnesium phosphate cement with setting time capable of being regulated and controlled in large range and preparation method of fiber-reinforced magnesium phosphate cement

Abstract

The invention relates to fiber-reinforced magnesium phosphate cement with the setting time capable of being regulated and controlled in a large range and a preparation method thereof, and belongs to the technical field of preparation of building raw materials. The cement comprises the following raw materials in parts by weight: 35-55 parts of modified magnesium oxide, 20-35 parts of acid phosphate, 5-10 parts of a retarder, 15-30 parts of a mineral admixture, 2-5 parts of a polymer water reducer, 0.5-2.5 parts of modified carbon fibers and 8-16 parts of water. The invention also provides a preparation method of the magnesium phosphate cement. According to the invention, magnesium oxide and aluminum oxide are subjected to a solid-phase reaction at a high temperature, a layer of magnesium aluminate spinel is grown on the surface of magnesium oxide as an isolating layer to modify the surface of magnesium oxide, and meanwhile, the temperature, time and raw material dosage in the solid-phase reaction process can be controlled, so that the solidification time of magnesium phosphate cement can be regulated and controlled between 20 minutes and 135 minutes. The cement has the advantages of high early strength, fast strength development, high bonding strength, good volume stability, convenience in construction and the like.

Description

[0001] This invention relates to a fiber-reinforced magnesium phosphate cement with adjustable setting time and its preparation method, belonging to the field of building raw material preparation technology. Background Technology

[0002] Magnesium phosphate cement is an inorganic cementitious material obtained by the acid-base neutralization reaction of magnesium oxide and soluble phosphate, also known as chemically bonded ceramic. It possesses advantages such as rapid hardening, early strength, and good fluidity. In particular, its high bond strength with the repaired material, good wear resistance, and good volume stability make magnesium phosphate cement a promising material for rapid repair of concrete pavements and structures. Furthermore, it holds significant military importance in the rapid construction and repair of national defense projects and military airports.

[0003] However, magnesium phosphate cement has also revealed some problems in practical applications, specifically: poor controllability of setting time, construction difficulties due to excessively rapid setting speed, and high heat release during hydration. Without any retarding measures, the hydration reaction of magnesium phosphate cement is rapid, with an extremely short setting time, often only a few minutes. This is completely insufficient to meet the required operating time for construction, leading to significant difficulties during the process. Furthermore, residual boron in the hydration products can diffuse into the environment and even dissolve into groundwater with rainwater, posing a significant threat to the environment and human health.

[0004] To facilitate construction, it is necessary to extend the setting time of magnesium phosphate cement. Currently, the mainstream method is to directly add borax, boric acid, or other boron-containing retarders to the magnesium phosphate cement. However, the direct addition of large amounts of boron-containing retarders results in excessively slow early strength development of the magnesium phosphate cement, significantly impacting its final strength. For example, patent CN108069692A describes a magnesium phosphate cement grout for rapid repair, using boric acid as the main retarder. While the setting time can be controlled within a small range, the excessive use of boric acid leads to extremely low early flexural and compressive strength, and significantly affects its later strength.

[0005] Therefore, a new method can be sought as the primary retarding mechanism, eliminating or reducing the use of boron-containing retarders. This ensures the early strength development of magnesium phosphate cement without affecting its final strength, while also guaranteeing sufficient construction time, which is of paramount importance for the application of magnesium phosphate cement. For example, patent CN115724643A introduces a high-refractory magnesium phosphate cement-based material with controllable hydration reaction. By carbonating sintered magnesium oxide powder, a dense layer containing hydration products such as basic magnesium carbonate is formed on the surface of the sintered magnesium oxide particles, solving the problem of excessively fast setting time in magnesium phosphate cement-based materials. However, its setting time is still relatively short, its early strength is too low, and the preparation process is relatively complex, limiting its application.

[0006] While the aforementioned methods can extend the setting time of magnesium phosphate cement to some extent, the extension is limited, and they have a significant negative impact on early-stage strength and later-stage strength. Most importantly, the setting time cannot be adjusted according to specific construction conditions. To address these problems, this invention provides a fiber-reinforced magnesium phosphate cement with a widely adjustable setting time and its preparation method. Summary of the Invention

[0007] To address the shortcomings and deficiencies of existing magnesium phosphate cements and considering the specific requirements of construction conditions, this invention provides a fiber-reinforced magnesium phosphate cement with a wide range of controllable setting time and its preparation method. The innovative method involves a solid-phase reaction between magnesium oxide and alumina at high temperature, resulting in the growth of a magnesium aluminate spinel layer on the surface of the magnesium oxide as an isolation layer. This isolation layer hinders the contact and reaction between magnesium oxide and acidic phosphate, slowing down the cement hydration rate and allowing hydration products to accumulate slowly. The accumulation of hydration products causes the magnesium oxide particles to expand continuously, eventually leading to the rupture of the isolation layer, accelerating the hydration reaction rate, and rapidly solidifying the system. Furthermore, this invention etches and modifies the surface of the carbon fibers, significantly strengthening their bond with the magnesium phosphate cement matrix and greatly improving flexural and compressive strength. This magnesium phosphate cement retains traditional advantages such as early strength, good fluidity, high bond strength, good wear resistance, and good volume stability, while its setting time can be controlled between 20 min and 135 min, greatly satisfying the operational time requirements for construction.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] In a first aspect, the present invention provides a fiber-reinforced magnesium phosphate cement with a wide range of adjustable setting time, comprising, by weight, the following raw materials: 35-55 parts modified magnesium oxide, 20-35 parts acid phosphate, 3-6 parts retarder, 15-30 parts mineral admixture, 2-5 parts polymer water-reducing agent, 0.5-2.5 parts modified carbon fiber, and 8-16 parts water.

[0010] Preferably, the modified magnesium oxide is prepared by the following method:

[0011] (1) Magnesium oxide, aluminum oxide and titanium dioxide are mixed by ball milling after being properly proportioned.

[0012] (2) The ball milled product is placed in a muffle furnace and heated and calcined, then kept warm and cooled.

[0013] (3) The calcined product is crushed, ball-milled and sieved to obtain modified magnesium oxide.

[0014] More preferably, the magnesium oxide in step (1) is one or more of light-burned magnesium, heavy-burned magnesium, and fused magnesium, with a fineness of 325-800 mesh.

[0015] Furthermore, the lightly calcined magnesium is of industrial grade with a purity greater than 92% (by mass).

[0016] Furthermore, the recalcined magnesium is of industrial grade with a purity greater than 92% (by mass).

[0017] Furthermore, the fused magnesium is of industrial grade with a purity greater than 92% (by mass).

[0018] More preferably, the alumina in step (1) is industrial-grade α-alumina with a purity greater than 98% (by mass) and a fineness of 1250-2500 mesh.

[0019] More preferably, the titanium dioxide in step (1) has a purity greater than 97% (by mass) and a fineness of 2500 mesh.

[0020] More preferably, in step (1), there are 92-96 parts of magnesium oxide, 3-6 parts of aluminum oxide, and 1-2 parts of titanium dioxide.

[0021] More preferably, in step (1), the ball milling speed is 320 r / min, the ball milling time is 0.5-1.5 h, and the powder-to-ball mass ratio is 1:15.

[0022] More preferably, the calcination temperature in step (2) is 1150-1550℃, and the holding time is 2-12h.

[0023] More preferably, the ball milling speed in step (3) is 320 r / min, the ball milling time is 2-3 h, and the powder-to-ball mass ratio is 1:15.

[0024] More preferably, the modified magnesium oxide in step (3) has a fineness of 200-325 mesh.

[0025] Preferably, the acid phosphate in the formulation is composed of one or both of potassium dihydrogen phosphate and ammonium dihydrogen phosphate.

[0026] More preferably, the potassium dihydrogen phosphate is industrial grade with a purity greater than 98% (by mass).

[0027] More preferably, the ammonium dihydrogen phosphate is industrial grade with a purity greater than 98% (by mass).

[0028] Preferably, the retarder in the formulation is composed of one or more of borax, anhydrous boric acid, and tartaric acid.

[0029] More preferably, the borax is industrial grade, with a purity greater than 99% (by mass) and a fineness of 65-200 mesh.

[0030] More preferably, the anhydrous boric acid is industrial grade with a purity greater than 99% (by mass).

[0031] More preferably, the tartaric acid is industrial grade with a purity greater than 99.5% (by mass).

[0032] Preferably, the mineral admixture in the formula is composed of one or more of fly ash, metakaolin, silica fume, and slag.

[0033] More preferably, the fly ash is Grade I fly ash emitted by a thermal power plant, with a fineness of 325-800 mesh.

[0034] Preferably, the kaolin is commercially available metakaolin with a fineness of 325 mesh.

[0035] More preferably, the silica fume is commercially available silica fume with a fineness of 325-2000 mesh.

[0036] More preferably, the slag is neutral slag produced by blast furnace iron smelting, which is crushed and screened to obtain slag powder with a fineness of 800-2000 mesh.

[0037] Preferably, the polymeric water-reducing agent is Melment F10 type sulfonated melamine condensate powder produced by BASF in Germany.

[0038] Preferably, the modified carbon fiber described in the formulation is prepared by the following method:

[0039] A certain concentration of nitric acid is placed in a fume hood, carbon fiber is added, and soaked for a certain period of time; the carbon fiber is poured out and rinsed repeatedly with clean water, and then placed in an oven to be heated and dried to obtain modified carbon fiber.

[0040] More preferably, the carbon fiber is 12k short-cut carbon fiber with a single filament diameter of 7.3μm and a length of 3-12mm.

[0041] More preferably, the mass fraction of the nitric acid is 34%-68%.

[0042] More preferably, the soaking time is 2-6 hours and the drying temperature is 50-60℃.

[0043] In a second aspect, to achieve the above-mentioned objective, the present invention provides a method for preparing fiber-reinforced magnesium phosphate cement with a widely adjustable setting time, comprising the following steps:

[0044] ① Preparation of modified magnesium oxide

[0045] A: Weigh 92-96 parts of magnesium oxide, 3-6 parts of aluminum oxide and 1-2 parts of titanium dioxide, place them in a planetary ball mill, and mix them at 320 r / min for 0.5-1.5 h.

[0046] B: The obtained ball-milled product was calcined in a muffle furnace at 1150-1550℃ with a heating rate of 5-8℃ / min, held at that temperature for 2-12 hours, and then naturally cooled to room temperature.

[0047] C: Crush the calcined product, ball mill it at 320 r / min for 2-3 h, sieve it, and take the 200-325 mesh sieve material to obtain modified magnesium oxide.

[0048] ② Preparation of modified carbon fibers

[0049] In a fume hood, carbon fibers are immersed in 34%-68% nitric acid for 2-6 hours, then rinsed repeatedly with clean water and dried in an oven at 50-60℃ to obtain modified carbon fibers.

[0050] ③ Take 35-55 parts of modified magnesium oxide, 20-35 parts of acidic phosphate, 3-6 parts of retarder, 15-30 parts of mineral admixture, 2-5 parts of polymer water-reducing agent, and 0.5-2.5 parts of modified carbon fiber, and mix them thoroughly with a dry powder mixer to obtain magnesium phosphate cement dry powder.

[0051] ④ Place the prepared magnesium phosphate cement powder in a cement paste mixer, add 8-16 parts of water, stir thoroughly, pour into a mold, place on a vibrating table to compact, wait for it to solidify naturally, and then demold to obtain the magnesium phosphate cement of the present invention.

[0052] The technical principle behind the fiber-reinforced magnesium phosphate cement with a widely adjustable setting time selected in this invention is as follows:

[0053] To facilitate understanding of this invention, it is necessary to explain the hydration process of traditional magnesium phosphate cement and the retarding mechanism of borax. After magnesium phosphate cement is mixed with water, the acidic phosphates dissolve, releasing H₂. + PO4 3- H2PO4 - NH4 + (K+ The plasma forms an acidic solution; magnesium oxide dissolves in the acidic aqueous solution, consuming H₂. + , releasing Mg 2+ The pH value of the system gradually increased, and Mg... 2+ With P04 3- H2 P0 4- NH4 + (K + The reaction produces a hydrated gel of ammonium (potassium) magnesium phosphate salt complex (MgNH4PO4·6H2O or MgKPO4·6H2O); finally, the hydration products bond with the unreacted magnesium oxide particles, and the system rapidly solidifies and hardens. Meanwhile, the borax in the system dissolves in water, forming B4O7. 2- It is rapidly adsorbed onto the surface of magnesium oxide particles, forming a layer of B4O7. 2- and Mg 2+ A hydration product barrier layer primarily slows down the dissolution of magnesium oxide and NH4+. 4+ and PO4 3- Contact with magnesium oxide is used to achieve the purpose of slowing down the setting process.

[0054] This invention utilizes a solid-state sintering method, where magnesium oxide and aluminum oxide undergo a solid-state reaction at high temperatures to grow a layer of magnesium aluminum spinel (MgAl2O4) on the surface of magnesium oxide as a separating layer. During hydration, the presence of this separating layer significantly slows down the dissolution rate of the modified magnesium oxide in acidic aqueous solutions. 2+ The release rate of NH4 will decrease. + (K + H + PO4 3- The hydration of magnesium phosphate cement slows down because the hydration products can only slowly penetrate the isolation layer through the gaps in the magnesium aluminum spinel crystals and come into contact with the surface of the modified magnesium oxide particles. The continuous accumulation of hydration products causes the modified magnesium oxide particles to expand, eventually leading to the rupture of the isolation layer and the release of large amounts of NH4. + (K + H + PO4 3- When ions come into contact with modified magnesium oxide particles, the hydration reaction rate accelerates, rapidly forming a large amount of phosphate ammonium (potassium) magnesium salt complex hydration gel, and the system quickly solidifies.

[0055] Controlling the growth of magnesium aluminate spinel by adjusting the temperature, holding time, and alumina dosage of the solid-phase reaction will result in magnesium aluminate spinel with inconsistent crystallinity. Higher temperatures, longer holding times, and larger alumina dosages lead to more complete crystallinity, thicker surfaces, and smaller gaps in the magnesium aluminate spinel. During the hydration of magnesium phosphate cement, the retarding effect of the isolation layer with different thicknesses and gap sizes varies significantly. A thicker isolation layer with smaller gaps results in a lower dissolution rate of modified magnesium oxide in acidic aqueous solutions, reducing the NH4+ content. + (K + H + PO4 3- The slower the penetration through the gaps in the isolation layer, the less likely the isolation layer will rupture as the modified magnesium oxide particles expand, resulting in a longer setting time for magnesium phosphate cement. Conversely, if the magnesium aluminum spinel crystallizes incompletely, leading to a thinner surface and larger gaps in the isolation layer, the setting time of the magnesium phosphate cement will be shorter. Therefore, this invention, by controlling the above reaction conditions, prepares modified magnesium oxide with an isolation layer of inconsistent thickness and gaps on its surface. Using this modified magnesium oxide to make magnesium phosphate cement allows for a wide range of controllable setting times.

[0056] Compared with the prior art, the beneficial effects achieved by the present invention are:

[0057] (1) The fiber-reinforced magnesium phosphate cement prepared by the present invention has a wide range of adjustable setting time. An isolation layer is formed on the surface of the raw material magnesium oxide by the solid-phase reaction of magnesium oxide and aluminum oxide under high temperature conditions, which reduces the amount of boron-containing retarder and achieves the purpose of retarding.

[0058] (2) The fiber-reinforced magnesium phosphate cement with adjustable setting time prepared by the present invention can achieve the purpose of adjustable setting time of magnesium phosphate cement by controlling the reaction conditions of the magnesium oxide surface modification stage and wrapping an isolation layer with inconsistent thickness and gaps on the magnesium oxide surface, which greatly satisfies the construction conditions.

[0059] (3) The fiber-reinforced magnesium phosphate cement prepared by the present invention has a wide range of adjustable setting time, which greatly reduces the amount of boron-containing retarder used. Under the premise of ensuring that it has a long setting time, it will not affect the development of its early strength, nor will it affect its later strength.

[0060] (4) The fiber-reinforced magnesium phosphate cement prepared by this invention has a low amount of boron-containing retarder, which reduces environmental pollution and harm to the human body. It is an environmentally friendly material that conforms to the concept of green development.

[0061] (5) The fiber-reinforced magnesium phosphate cement with adjustable setting time prepared by the present invention uses a large amount of mineral admixtures such as fly ash, silica fume, kaolin and slag, which not only greatly reduces the cost of raw materials, but also realizes the sustainable utilization of solid waste and saves energy.

[0062] (6) The fiber-reinforced magnesium phosphate cement with adjustable setting time prepared by the present invention uses nitric acid etching modified carbon fiber as reinforcement, which greatly improves the flexural and compressive strength of magnesium phosphate cement, and also enhances the volume stability of magnesium phosphate cement to a certain extent. Detailed Implementation

[0063] The specific embodiments described herein are merely illustrative examples intended to provide further explanation of this application and to make the objectives, technical solutions, and advantages of the present invention clearer.

[0064] The specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.

[0065] Unless otherwise specified, the specific experimental methods described herein are conventional methods; unless otherwise specified, the reagents and materials described herein are commercially available.

[0066] Example 1:

[0067] The preparation of a fiber-reinforced magnesium phosphate cement with adjustable setting time includes the following steps:

[0068] Weigh out 96 parts of magnesium oxide (lightly calcined magnesium: heavily calcined magnesium: fused magnesium = 1:5:2, mass ratio), 3 parts of alumina and 1 part of titanium dioxide in a muffle furnace. Raise the temperature to 1300℃ in a muffle furnace at a rate of 8℃ / min, hold for 6 hours, and allow to cool naturally to room temperature to prepare modified magnesium oxide. Prepare 34% nitric acid and soak 5 mm carbon fibers for 3 hours to prepare modified carbon fibers.

[0069] Weigh out 40 parts of modified magnesium oxide, 20 parts of acidic phosphate (potassium dihydrogen phosphate: ammonium dihydrogen phosphate = 1:1, mass ratio), 3 parts of retarder (borax: anhydrous boric acid: tartaric acid = 2:1:1, mass ratio), 20 parts of mineral admixture (fly ash: metakaolin: silica fume: slag = 8:3:3:2, mass ratio), 3 parts of polymer water-reducing agent, 1 part of modified carbon fiber, and 13 parts of water. Prepare magnesium phosphate cement powder according to the above steps. In accordance with the national standard GB / T17671-2021, mix the cement powder with water, pour the cement slurry into molds, vibrate, record the initial setting time, demold after solidification, cure under natural conditions, and test the flexural and compressive strength.

[0070] Example 2:

[0071] The preparation of a fiber-reinforced magnesium phosphate cement with adjustable setting time includes the following steps:

[0072] Weigh out 92 parts of magnesium oxide (lightly calcined magnesium: heavily calcined magnesium: fused magnesium = 1:5:2, mass ratio), 6 parts of alumina and 2 parts of titanium dioxide in a muffle furnace. Raise the temperature to 1300℃ in a muffle furnace at a rate of 8℃ / min, hold for 6 hours, and allow to cool naturally to room temperature to prepare modified magnesium oxide. Prepare 34% nitric acid and soak 5 mm carbon fibers for 3 hours to prepare modified carbon fibers.

[0073] Weigh out 40 parts of modified magnesium oxide, 20 parts of acidic phosphate (potassium dihydrogen phosphate: ammonium dihydrogen phosphate = 1:1, mass ratio), 3 parts of retarder (borax: anhydrous boric acid: tartaric acid = 2:1:1, mass ratio), 20 parts of mineral admixture (fly ash: metakaolin: silica fume: slag = 8:3:3:2, mass ratio), 3 parts of polymer water-reducing agent, 1 part of modified carbon fiber, and 13 parts of water. Prepare magnesium phosphate cement powder according to the above steps. In accordance with the national standard GB / T17671-2021, mix the cement powder with water, pour the cement slurry into molds, vibrate, record the initial setting time, demold after solidification, cure under natural conditions, and test the flexural and compressive strength.

[0074] Example 3:

[0075] The preparation of a fiber-reinforced magnesium phosphate cement with adjustable setting time includes the following steps:

[0076] Weigh out 92 parts of magnesium oxide (lightly calcined magnesium: heavily calcined magnesium: fused magnesium = 1:5:2, mass ratio), 6 parts of alumina, and 2 parts of titanium dioxide in a muffle furnace. Raise the temperature to 1500℃ in the muffle furnace at a rate of 8℃ / min, hold for 6 hours, and allow to cool naturally to room temperature to prepare modified magnesium oxide. Prepare 34% nitric acid and soak 5mm carbon fibers for 3 hours to prepare modified carbon fibers.

[0077] Weigh out 40 parts of modified magnesium oxide, 20 parts of acidic phosphate (potassium dihydrogen phosphate: ammonium dihydrogen phosphate = 1:1, mass ratio), 3 parts of retarder (borax: anhydrous boric acid: tartaric acid = 2:1:1, mass ratio), 20 parts of mineral admixture (fly ash: metakaolin: silica fume: slag = 8:3:3:2, mass ratio), 3 parts of polymer water-reducing agent, 1 part of modified carbon fiber, and 13 parts of water. Prepare magnesium phosphate cement powder according to the above steps. In accordance with the national standard GB / T17671-2021, mix the cement powder with water, pour the cement slurry into molds, vibrate, record the initial setting time, demold after solidification, cure under natural conditions, and test the flexural and compressive strength.

[0078] Example 4:

[0079] The preparation of a fiber-reinforced magnesium phosphate cement with adjustable setting time includes the following steps:

[0080] Weigh out 92 parts of magnesium oxide (lightly calcined magnesium: heavily calcined magnesium: fused magnesium = 1:5:2, mass ratio), 6 parts of alumina, and 2 parts of titanium dioxide in a muffle furnace. Raise the temperature to 1500℃ in the muffle furnace at a rate of 8℃ / min, hold for 12 hours, and allow to cool naturally to room temperature to prepare modified magnesium oxide. Prepare 34% nitric acid and soak 5mm carbon fibers for 3 hours to prepare modified carbon fibers.

[0081] Weigh out 40 parts of modified magnesium oxide, 20 parts of acidic phosphate (potassium dihydrogen phosphate: ammonium dihydrogen phosphate = 1:1, mass ratio), 3 parts of retarder (borax: anhydrous boric acid: tartaric acid = 2:1:1, mass ratio), 20 parts of mineral admixture (fly ash: metakaolin: silica fume: slag = 8:3:3:2, mass ratio), 3 parts of polymer water-reducing agent, 1 part of modified carbon fiber, and 13 parts of water. Prepare magnesium phosphate cement powder according to the above steps. In accordance with the national standard GB / T17671-2021, mix the cement powder with water, pour the cement slurry into molds, vibrate, record the initial setting time, demold after solidification, cure under natural conditions, and test the flexural and compressive strength.

[0082] Example 5:

[0083] The preparation of a fiber-reinforced magnesium phosphate cement with adjustable setting time includes the following steps:

[0084] 92 parts of magnesium oxide (lightly calcined magnesium: heavily calcined magnesium: fused magnesium = 1:5:2, mass ratio), 6 parts of alumina and 2 parts of titanium dioxide were weighed out in a muffle furnace at a rate of 8℃ / min to 1500℃, held for 12h, and then naturally cooled to room temperature to prepare modified magnesium oxide; 5mm carbon fibers were soaked in 68% nitric acid for 6h to prepare modified carbon fibers.

[0085] Weigh out 40 parts of modified magnesium oxide, 20 parts of acidic phosphate (potassium dihydrogen phosphate: ammonium dihydrogen phosphate = 1:1, mass ratio), 3 parts of retarder (borax: anhydrous boric acid: tartaric acid = 2:1:1, mass ratio), 20 parts of mineral admixture (fly ash: metakaolin: silica fume: slag = 8:3:3:2, mass ratio), 3 parts of polymer water-reducing agent, 1 part of modified carbon fiber, and 13 parts of water. Prepare magnesium phosphate cement powder according to the above steps. In accordance with the national standard GB / T17671-2021, mix the cement powder with water, pour the cement slurry into molds, vibrate, record the initial setting time, demold after solidification, cure under natural conditions, and test the flexural and compressive strength.

[0086] The specific properties of a fiber-reinforced magnesium phosphate cement with adjustable setting time are as follows:

[0087]

[0088] As shown in the table above, the setting time of the fiber-reinforced magnesium phosphate cement prepared in Examples 1-5 of this invention is between 20 min and 135 min, which is a relatively large time range. This time range can be freely adjusted according to specific construction conditions, solving the construction difficulties caused by the excessively short setting time of traditional magnesium phosphate cement and providing significant construction convenience. Furthermore, the fiber-reinforced magnesium phosphate cement prepared by this invention exhibits rapid early strength development and extremely high final strength: 3-hour flexural strength of 7.3-8.4 MPa, 3-hour compressive strength of 48.5-56.9 MPa; 7-day flexural strength of 12.9-15.7 MPa, and 7-day compressive strength of 78.2-92.9 MPa.

[0089] Comparing Examples 1, 2, 3, and 4, Example 2 increased the amount of alumina and titanium dioxide by twice that of Example 1, resulting in a 175% increase in setting time. In Example 3, the temperature was increased from 1300°C in Example 2 to 1500°C, extending the setting time from 55 min to 90 min, a 64% increase. Comparing Examples 3 and 4, with the same temperature, the holding time was extended from 6 h to 12 h, extending the setting time from 90 min to 132 min, a 47% increase. This indicates that increasing the amount of alumina and titanium dioxide, raising the temperature, and extending the holding time can all improve the crystallinity of magnesium aluminate spinel, meaning a thicker insulating layer and smaller gaps on the surface of modified magnesium oxide, effectively extending the setting time of the resulting fiber-reinforced magnesium phosphate cement.

[0090] Compared with Examples 4 and 5, the mass fraction of nitric acid used was increased from 35% to 68%, and the treatment time was increased from 3 hours to 6 hours. The setting time of the resulting magnesium phosphate cement remained essentially unchanged, but the flexural and compressive strengths showed significant increases, especially the 7-day strength, with a 22% increase in flexural strength and a 19% increase in compressive strength. This is because the higher mass fraction of nitric acid and the longer treatment time resulted in a rougher surface and more active groups on the modified carbon fibers, effectively increasing the bonding force between the modified carbon fibers and the magnesium phosphate cement matrix, thus significantly improving the strength of the magnesium phosphate cement.

[0091] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any modifications and equivalent changes made to the above embodiments based on the technical concept of the present invention, as well as various variations and improvements made by those skilled in the art to the technical solutions of the present invention, shall still fall within the scope of the technical solutions of the present invention.

[0092] The numerical range described in this invention includes all values ​​within this range, and also includes any range value composed of any two values ​​within this range. Different values ​​of the same indicator appearing in all embodiments of this invention can be arbitrarily combined to form a range value.

Claims

1. A fiber-reinforced magnesium phosphate cement with a widely adjustable setting time, characterized in that, The product is composed of the following raw materials by weight: 35-55 parts modified magnesium oxide, 20-35 parts acid phosphate, 3-6 parts retarder, 15-30 parts mineral admixture, 2-5 parts polymer water-reducing agent, 0.5-2.5 parts modified carbon fiber, and 8-16 parts water.

2. The fiber-reinforced magnesium phosphate cement with adjustable setting time according to claim 1, characterized in that, The modified magnesium oxide is made from the following raw materials by weight: 92-96 parts magnesium oxide, 3-6 parts aluminum oxide, and 1-2 parts titanium dioxide; the modified magnesium oxide is obtained by high-temperature calcination at 1150-1550℃ with a heating rate of 5-8℃ / min, followed by crushing, ball milling, and sieving; the magnesium oxide is one or more of light-calcined magnesium, heavy-calcined magnesium, and fused magnesium, with a fineness of 325-800 mesh.

3. The fiber-reinforced magnesium phosphate cement with adjustable setting time according to claim 1, characterized in that, The acidic phosphate is composed of one or both of potassium dihydrogen phosphate and ammonium dihydrogen phosphate.

4. The fiber-reinforced magnesium phosphate cement with adjustable setting time according to claim 1, characterized in that, The retarder is composed of one or more of borax, anhydrous boric acid, and tartaric acid.

5. The fiber-reinforced magnesium phosphate cement with adjustable setting time according to claim 1, characterized in that, The mineral admixture is composed of one or more of fly ash, metakaolin, silica fume, and slag.

6. The fiber-reinforced magnesium phosphate cement with adjustable setting time according to claim 1, characterized in that, The polymeric water-reducing agent is Melment F10 type sulfonated melamine condensate powder produced by BASF in Germany.

7. The fiber-reinforced magnesium phosphate cement with adjustable setting time according to claim 1, characterized in that, The modified carbon fiber is obtained by immersing and etching carbon fiber with nitric acid, followed by washing and drying; the mass fraction of the nitric acid is 34%-68%; and the length of the carbon fiber is 3-12 mm.

8. A fiber-reinforced magnesium phosphate cement with a widely adjustable setting time according to claim 1, characterized in that, The carbon fiber is a 12k short-cut carbon fiber with a single filament diameter of 7.3μm.

9. A method for preparing fiber-reinforced magnesium phosphate cement with adjustable setting time according to any one of claims 1-8, characterized in that, Including the following steps: ① Preparation of modified magnesium oxide: 92-96 parts of magnesium oxide, 3-6 parts of aluminum oxide and 1-2 parts of titanium dioxide are calcined at 1300℃~1500℃, and then crushed, ball-milled and sieved to obtain modified magnesium oxide; ② Preparation of modified carbon fiber: After carbon fiber is etched by soaking in nitric acid, it is washed with water and dried to obtain modified carbon fiber; ③ Take 35-55 parts of modified magnesium oxide, 20-35 parts of acid phosphate, 3-6 parts of retarder, 15-30 parts of mineral admixture, 2-5 parts of polymer water-reducing agent, and 0.5-2.5 parts of modified carbon fiber, and mix them to prepare magnesium phosphate cement dry powder. ④ Add 8-16 parts of water to the prepared magnesium phosphate cement powder, stir and pour into a mold, vibrate to compact, and demold after natural solidification to obtain fiber-reinforced magnesium phosphate cement.

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