A high-strength, low-shrinkage, water-resistant potassium magnesium phosphate cement composite material and its application

Abstract

This invention belongs to the field of inorganic cementitious materials technology in civil engineering, specifically disclosing a high-strength, low-shrinkage, water-resistant potassium magnesium phosphate cement composite material and its applications. This composite material exhibits excellent mechanical properties, water resistance, and volume stability, with adjustable and controllable setting time. Specifically, the nano-templating effect of P-GO promotes the orderly growth of K-Struvite crystals on the lamellar surface, optimizing crystal morphology; the micro-filling effect of MK and the secondary hydration reaction refine the pore structure, and the two work synergistically to significantly improve the overall structural density. Simultaneously, the incorporation of MK reduces the phosphate ratio in the system, slowing down the hydration exothermic rate, which, combined with the retarding effect of P-GO, achieves control over the setting time. Furthermore, the encapsulation effect of P-GO on K-Struvite crystals inhibits the dissolution of phosphates under immersion conditions, and the aluminum phosphate gel generated by MK fills the gaps between crystals, blocking water migration channels and significantly improving the retention rate of immersion strength.

Description

Technical Field

[0001] This invention belongs to the field of inorganic cementitious materials technology in civil engineering, specifically relating to a potassium magnesium phosphate cement composite material synergistically reinforced by phosphonic acid-based functionalized graphene and metakaolin, and its application. Background Technology

[0002] Magnesium phosphate cement (MPC) has broad application prospects in fields such as rapid structural repair, 3D printing building materials, and radioactive waste solidification due to its advantages such as rapid hardening, early strength, good adhesion, and volume stability. However, MPC has three inherent defects: high brittleness and low flexural strength; poor water resistance, with significant strength reduction after immersion in water; and concentrated hydration heat release, making it difficult to precisely control the setting time.

[0003] Existing nano-modification technologies mainly employ the direct incorporation of graphene oxide (GO) into the MPC system, but they suffer from the following shortcomings: the surface functional groups of ordinary GO are hydroxyl and carboxyl groups, which have poor chemical compatibility with the MPC phosphate hydration system and cannot effectively promote the directional growth of struvite (K-Struvite, MgKPO4·6H2O) crystals; GO is prone to agglomeration in highly alkaline MPC pore solutions, exhibiting poor dispersibility, and doping levels above 0.10 wt% actually degrade mechanical properties; GO only provides physical filling and template effects and fails to form a chemically bonded interface with the MPC hydration products.

[0004] Chinese patent CN112047709B (Shanghai Jiao Tong University) discloses a reinforced magnesium phosphate cement composite material, which uses a GO / carbon nanotube composite modifier to reinforce MPC. Simultaneously, a polycarboxylate superplasticizer is added to offset the negative impact of the composite modifier on the workability of MPC, improving initial setting time and flowability, and enhancing mechanical properties. However, the interface bonding between carbon nanotubes and the MPC matrix in this scheme is weak, mainly relying on physical entanglement, resulting in limited improvement in mechanical properties. Chinese patent CN106746811B (Jinan University) discloses a water-resistant ammonium magnesium phosphate cement without a retarder, using GO to coat reburned magnesium oxide instead of a retarder, isolating magnesium oxide from the external environment, thereby delaying the formation of hydration products and inhibiting phosphate dissolution. However, this scheme focuses on retarding and water resistance, with limited mechanical strengthening effect. Summary of the Invention

[0005] The purpose of this invention is to provide a high-strength, low-shrinkage, water-resistant potassium magnesium phosphate cement composite material, which utilizes phosphonic acid-based functionalized graphene / metakaolin to synergistically reinforce potassium magnesium phosphate cement (MKPC).

[0006] Meanwhile, the present invention also provides an application of a high-strength, low-shrinkage, water-resistant potassium magnesium phosphate cement composite material.

[0007] To solve the above-mentioned technical problems, the present invention provides the following technical solutions:

[0008] A high-strength, low-shrinkage, water-resistant potassium magnesium phosphate cement composite material comprises: phosphonic acid-based functionalized graphene (P-GO), metakaolin (MK), and composite powder, wherein the composite powder is composed of calcined magnesium oxide, potassium dihydrogen phosphate, and borax; the amount of phosphonic acid-based functionalized graphene is 0.03-0.08 wt% of the mass of calcined magnesium oxide, and the amount of metakaolin is 8-15 wt% of the mass of calcined magnesium oxide.

[0009] In a preferred embodiment of the present invention, the phosphonic functionalized graphene is prepared by covalently modifying graphene oxide with (3-triethoxysilylpropyl)phosphonic acid via a silane coupling reaction, and the surface contains phosphonic acid groups (-PO(OH)2).

[0010] Specifically, the phosphorus / carbon atom ratio (P / C) in the phosphonic acid-functionalized graphene is 0.035-0.052.

[0011] In a preferred embodiment of the present invention, the mass ratio of calcined magnesium oxide to potassium dihydrogen phosphate and borax in the composite powder is 100:28-32:4-6.

[0012] In a preferred embodiment of the present invention, the amount of phosphonic acid-functionalized graphene is 0.05 wt% of the mass of reburned magnesium oxide, and the amount of metakaolin is 12 wt% of the mass of reburned magnesium oxide.

[0013] In a preferred embodiment of the present invention, the potassium magnesium phosphate cement composite material comprises the following raw material components by mass parts:

[0014] 100 parts of red-burned magnesium oxide,

[0015] 28-32 parts of potassium dihydrogen phosphate

[0016] 4-6 parts borax

[0017] Phosphonic acid-functionalized graphene, 0.03-0.08 parts,

[0018] Partial 8-15 parts of metakaolin.

[0019] Specifically, the potassium magnesium phosphate cement composite material comprises the following raw material components by mass parts:

[0020] 100 parts of red-burned magnesium oxide,

[0021] 30 parts of potassium dihydrogen phosphate

[0022] 5 parts borax

[0023] 0.5 parts of phosphonic acid-functionalized graphene

[0024] 12 parts of metakaolin.

[0025] Applications of a high-strength, low-shrinkage, water-resistant potassium magnesium phosphate cement composite material include:

[0026] Phosphonic acid-functionalized graphene was added to water and dispersed to obtain a dispersion.

[0027] The calcined magnesium oxide, potassium dihydrogen phosphate, borax and metakaolin were mixed evenly and then added to the dispersion to obtain a slurry.

[0028] Take the grout and pour it into the construction site. Cure it for 25-30 days.

[0029] In a preferred embodiment of the present invention, the water-to-solid ratio of the potassium magnesium phosphate cement composite material is 0.13-0.16.

[0030] Specifically, the water-to-solid ratio of the potassium magnesium phosphate cement composite material is 0.14 when used.

[0031] The beneficial effects of this invention are:

[0032] The high-strength, low-shrinkage, water-resistant potassium magnesium phosphate cement composite material provided by this invention possesses excellent mechanical properties, water resistance, and volume stability, with adjustable and controllable setting time. Specifically, the -PO(OH)2 groups on the surface of phosphonic acid-functionalized graphene (P-GO) dissociate into -PO3 in the MPC hydration environment. 2- , with Mg 2+ Coordination occurs, in-situ inducing heterogeneous nucleation and directional growth of K-Struvite crystals on the surface of P-GO sheets, forming a chemically bonded interface between P-GO and the hydration products. This interface differs from the physical adsorption interface of ordinary GO, significantly improving the loading transfer efficiency. FTIR analysis confirms the presence of Mg-OP bonds (1045 cm⁻¹) in the P-GO-MKPC system. -1 ) and POC bond (1082 cm -1This indicates that covalent bonds formed between P-GO and the matrix. Building upon the nanotemplating effect of P-GO promoting the ordered growth of K-Struvite crystals on the lamellar surface and optimizing crystal morphology, the micro-filling effect of MK and the secondary hydration reaction (Al2O3 in MK is activated in the phosphate system, generating amorphous aluminum phosphate gel) further refine the pore structure. The synergistic effect of these two factors (P-GO providing nucleation sites for dense K-Struvite crystal growth, and MK filling residual micropores) significantly improves the overall structural density. Simultaneously, the incorporation of MK reduces the phosphate ratio in the system, slowing down the hydration exothermic rate. Combined with the retarding effect of P-GO, this allows for precise control of the setting time. Furthermore, the encapsulation effect of P-GO lamellars on K-Struvite crystals inhibits phosphate dissolution under immersion conditions, while the aluminum phosphate gel generated by MK fills the gaps between K-Struvite crystals, blocking moisture migration channels. The synergistic effect of these two factors significantly improves the strength retention rate after 28 days of immersion. Attached Figure Description

[0033] Figure 1 A comparison chart (bar chart) of the mechanical properties of P-GO / MK-MKPC composite materials.

[0034] Figure 2 The image shows the SEM microstructure of Example 2 (optimal group) after 28 days. It depicts the oriented growth morphology of K-Struvite crystals on the surface of the P-GO sheets.

[0035] Figure 3 The image shows the microstructure of K-Struvite crystals after 28 days (control group) via SEM. The K-Struvite crystals are loosely packed with obvious microcracks.

[0036] Figure 4 This is a schematic diagram showing the interface combination of P-GO and ordinary GO in the MKPC system. Detailed Implementation

[0037] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments and experimental examples. However, those skilled in the art should understand that the embodiments are only used to illustrate the technical solution of the present invention and should not be regarded as limiting the scope of protection of the present invention. Based on the following embodiments, all other implementation schemes obtained by those skilled in the art without creative effort, such as equivalent substitutions or conventional improvements made based on the SNP loci, genotype combinations, and their application in the screening of chlorophyll content traits in Brassica napus disclosed in the present invention, should fall within the scope of protection of the present invention.

[0038] Unless otherwise specified, the raw materials, reagents, and instruments used in the following examples and experimental cases are all commonly used items in the art and commercially available to the public. Unless otherwise specified, the experimental methods used in the experimental cases are conventional methods in the art. All terms and abbreviations used have their conventional meanings in the art. The specifications of the raw materials used to prepare the potassium magnesium phosphate cement composite material are shown in Table 1. Specifically, calcined magnesium oxide (MgO) was purchased from Liaoning Haicheng Magnesium Mine Group Co., Ltd., potassium dihydrogen phosphate (KH2PO4) and borax (Na2B4O7·10H2O) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Zhengzhou), metakaolin (MK) was purchased from Gongyi Yuhua Machinery Factory, graphene oxide (GO) was purchased from Nanjing Xianfeng Nanomaterials Technology Co., Ltd., and (3-triethoxysilylpropyl)phosphonic acid (TPPA) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.

[0039] Table 1 Raw material specifications and standards

[0040]

[0041] Example 1

[0042] This embodiment provides a potassium magnesium phosphate cement synergistically reinforced with phosphonate-functionalized graphene (P-GO) and metakaolin (MK), comprising:

[0043] 1. Preparation of Phosphonic Acid-Functionalized Graphene (P-GO)

[0044] P-GO was prepared by covalently grafting a phosphonic acid (TPPA) containing phosphate groups onto the surface of GO sheets using graphene oxide (GO) as a precursor. The specific steps are as follows:

[0045] (1) Disperse GO in anhydrous ethanol / water (volume ratio 4:1) mixed solvent and sonicate for 30 min to obtain a GO dispersion of 2 mg / mL;

[0046] (2) Adjust the pH of the GO dispersion to 4.5-5.0, add TPPA (TPPA to GO mass ratio 0.8:1), and heat to 75℃ and reflux for 8 h;

[0047] (3) The reaction product was centrifuged, washed three times with anhydrous ethanol, and dried under vacuum at 60°C for 24 h to obtain P-GO powder.

[0048] The P-GO surface retains both the layered nanostructure of GO and the newly added PO3H2 groups, with a phosphorus / carbon atom ratio (P / C) of 0.035-0.052.

[0049] 2. Preparation of P-GO / MK-MKPC composite materials

[0050] The basic components of MPC are calcined magnesium oxide (MgO), potassium dihydrogen phosphate (KH2PO4), and borax (Na2B4O7·10H2O, a retarder), with P-GO and metakaolin added. The specific ratio is: 100 parts calcined MgO, 30 parts KH2PO4, 5 parts borax, 8 parts MK (8 wt% of MgO), and 0.03 parts P-GO (0.03 wt% of MgO); the water-to-solid ratio is 0.14 when used.

[0051] The application methods of P-GO / MK-MKPC composite materials include:

[0052] (1) Add P-GO powder to the mixing water and ultrasonically disperse for 15 min to obtain P-GO dispersion;

[0053] (2) Place the calcined MgO, KH2PO4, borax and MK into a planetary cement mortar mixer and stir at low speed for 2 minutes until they are evenly mixed.

[0054] (3) Add P-GO dispersion, stir at low speed for 30 s and then at high speed for 90 s (staged stirring process) until the slurry is uniform and free of lumps;

[0055] (4) Cover the slurry with a film after pouring to prevent moisture evaporation and cure it under environmental conditions for 28 days.

[0056] Example 2

[0057] This embodiment provides a high-strength, low-shrinkage, water-resistant potassium magnesium phosphate cement composite material with the following raw material ratio: 100 parts of recalcined MgO, 30 parts of KH2PO4, 5 parts of borax, 12 parts of MK (accounting for 12 wt% of MgO), and 0.05 parts of P-GO (accounting for 0.05 wt% of MgO); wherein, the preparation of phosphonic acid-based functionalized graphene (P-GO) is the same as in Example 1, and the water-to-solid ratio is 0.14 when used.

[0058] Example 3

[0059] This embodiment provides a high-strength, low-shrinkage, water-resistant potassium magnesium phosphate cement composite material and its preparation method. The raw material ratio is as follows: 100 parts of recalcined MgO, 30 parts of KH2PO4, 5 parts of borax, 15 parts of MK (accounting for 15 wt% of MgO), and 0.08 parts of P-GO (accounting for 0.08 wt% of MgO). The preparation of phosphonic acid-based functionalized graphene (P-GO) is the same as in Example 1, and the water-to-solid ratio is 0.14 when used.

[0060] Comparative Example 1

[0061] This comparative example provides a magnesium phosphate cement composite material (blank), which differs from Example 1 in that it does not contain P-GO and MK.

[0062] Comparative Example 2

[0063] This comparative example provides a magnesium phosphate cement composite material (ordinary GO), which differs from Example 1 in that it incorporates 0.05 parts of ordinary GO (0.05 wt% of MgO) and does not contain MK.

[0064] Comparative Example 3

[0065] This comparative example provides a magnesium phosphate cement composite material (MK only), which differs from Example 1 in that it incorporates 12 parts of MK (accounting for 12 wt% of MgO) and does not incorporate P-GO.

[0066] Table 2 Raw material ratios in the examples and comparative examples

[0067]

[0068] Note: All groups have an M / P molar ratio of 6, a water-to-solid ratio of 0.14, and a borax content of 5 wt% (percentage of MgO).

[0069] Experimental Example

[0070] I. Experimental Methods

[0071] 1. Mechanical properties (compressive strength and flexural strength)

[0072] Refer to GB / T 17671-2021 "Test Method for Strength of Cement Mortar (ISO Method)" and combine it with the supplementary provisions of JC / T 2857-2024 for magnesium phosphate cementitious materials:

[0073] Specimen dimensions: 40 mm × 40 mm × 160 mm prism;

[0074] Mortar mix proportion: 2 parts magnesium phosphate cementitious material + 1 part standard sand (compliant with ISO standard sand as specified in GB / T 17671);

[0075] Water consumption for strength specimen molding: determined based on the water consumption required when the fluidity of the paste reaches 175-185 mm.

[0076] Compressive strength test: A microcomputer-controlled electro-hydraulic servo universal testing machine was used, with a loading rate of (2400±200) N / s. The load was calculated according to the formula Rc=Fs / A, where Fs is the maximum load (N) at which the specimen fails, and A is the bearing area (mm²). 2 );

[0077] Flexural strength test: Three-point bending method was used, with a loading rate of (50±10) N / s, calculated according to the formula Rf=3Ff·L / (2bh) 2The calculation is performed, where Ff is the failure load (N), L is the center distance of the supporting cylinders (100 mm), and b and h are the side lengths of the specimen section (40 mm).

[0078] Test ages: 1 day, 3 days, 28 days;

[0079] Data processing: For each group of 3 specimens, if the difference between the maximum or minimum value and the median value exceeds 15% of the median value, the median value is taken; if both the maximum and minimum values ​​exceed 15% of the median value, the data for that group is discarded.

[0080] 2. Water resistance

[0081] Referring to the water resistance evaluation method in JC / T 2857-2024 and the softening coefficient method commonly used for magnesium phosphate cement in the literature:

[0082] Water curing: After air curing for 28 days, the specimens were immersed in clean water at (20±2)℃ for 28 days, with the water level 20 mm above the top surface of the specimens.

[0083] Strength retention rate: Kr=Rw / Ra×100%, where Rw is the compressive strength after 28 days of immersion in water (MPa), and Ra is the compressive strength after air curing at the same age (MPa).

[0084] Softening coefficient: Ks=Rw / Ra, with a value of 0-1. The larger the Ks, the better the water resistance.

[0085] JC / T 2857-2024 requires that the strength retention rate be ≥80% after soaking for 28 days. All embodiments of the present invention far exceed this limit.

[0086] 3. Setting time

[0087] Referring to GB / T 1346-2024 "Test Methods for Standard Consistency Water Requirement, Setting Time and Soundness of Cement", a Vicat apparatus was used for determination. Given the rapid setting of MPC, according to the supplementary provisions of JC / T 2857-2024: timing began when the magnesium phosphate cementitious material came into contact with water; after adding water for 10 minutes, tests were conducted every 1 minute, and then every 30 seconds as setting approached; the time required for the needle to penetrate the specimen to a depth not exceeding 0.5 mm was the determined setting time, accurate to 30 seconds. The initial setting time (the needle sinking to 4 mm ± 1 mm from the bottom plate) and the final setting time were also recorded.

[0088] 4. Drying shrinkage

[0089] Refer to GB / T 751 "Test Method for Drying Shrinkage of Cement Mortar" and GB / T 29417-2012 "Test Method for Drying Shrinkage Cracking Performance of Cement Mortar and Concrete":

[0090] Specimen dimensions: 25 mm × 25 mm × 280 mm, with spherical nail heads at both ends;

[0091] Reference length determination: After the specimen was demolded, it was cured in water for 1 day, and the initial effective length L0 was determined using a length comparator;

[0092] Drying and shrinkage curing: The specimens were moved into a drying and shrinkage chamber with a temperature of (20±3)℃ and a relative humidity of (50±4)%.

[0093] Ages measured: 3 d, 7 d, 14 d, 21 d, 28 d;

[0094] Shrinkage rate calculation: S=(L0-Lt) / L0×10 6 , where Lt is the measurement length at each age.

[0095] 5. Microstructure characterization

[0096] (1) X-ray diffraction analysis (XRD)

[0097] X-ray diffractometer (Cu target Kα radiation, λ=0.15406 nm) was used, with an operating voltage of 40 kV, an operating current of 40 mA, a scanning range of 2θ=5°-80°, and a scanning speed of 5° / min (qualitative analysis) or 2° / min (quantitative analysis). Samples were ground through a 75 μm square-pore sieve and dried before testing. The procedure was performed in accordance with GB / T 30903—2014.

[0098] (2) Scanning electron microscopy (SEM) analysis

[0099] Field emission scanning electron microscopy (FESEM) was used with an accelerating voltage of 15 kV and a working distance of 8–10 mm. After drying the samples for 24 h, they were sputter-coated with gold (approximately 10 nm gold film) and adhered to the sample stage using conductive adhesive. The morphology of the hydration products, the distribution of P-GO sheets, and the interfacial bonding were observed.

[0100] (3) Fourier transform infrared spectroscopy (FTIR) analysis

[0101] FTIR spectrometer, KBr pellet method, scanning wavenumber range 4000-400 cm⁻¹ -1 4 cm resolution -1 The scan was performed 32 times. The changes in functional groups on the P-GO surface and the chemical bonding characteristics at the interface between P-GO and the MPC matrix were analyzed.

[0102] (4) Thermogravimetric-Differential Thermal Analysis (TG-DTA)

[0103] A simultaneous thermal analyzer was used, with a heating rate of 10℃ / min, a temperature range of room temperature to 800℃, and a nitrogen atmosphere. The content of hydration products in each specimen was quantitatively analyzed.

[0104] Table 3 Summary of Referenced Standards

[0105]

[0106] II. Experimental Results and Analysis

[0107] Test results of Example 1: 1-day compressive strength 35.8 MPa, 3-day compressive strength 48.3 MPa, 28-day compressive strength 61.7 MPa, 28-day flexural strength 8.9 MPa; 28-day water immersion compressive strength 57.8 MPa, retention rate 93.7%, softening coefficient 0.88; initial setting 10 min, final setting 17 min; 28-day shrinkage 245 × 10⁻⁶. -6 .

[0108] Test results of Example 2: 1-day compressive strength 38.2 MPa, 3-day compressive strength 52.6 MPa, 28-day compressive strength 68.4 MPa, 28-day flexural strength 10.2 MPa; 28-day water immersion compressive strength 66.2 MPa, retention rate 96.8%, softening coefficient 0.93; initial setting 13 min, final setting 21 min; 28-day shrinkage 198 × 10⁻⁶ MPa. -6 .

[0109] Microscopic characterization: 1045 cm⁻¹ appears in the FTIR spectrum -1 (Mg-OP) and 1082 cm -1 The new peak of (P-GO) confirms the formation of chemical bonds between P-GO and the matrix; XRD analysis shows that the intensity of K-Struvite diffraction peak is about 25% stronger than that of the blank group, and the crystallinity of the crystal is improved; SEM observation shows that K-Struvite crystals grow in a direction on the surface of P-GO sheets and have a dense structure; TG analysis shows that the content of hydration products increases by about 18%.

[0110] Test results of Example 3: 1-day compressive strength 36.4 MPa, 3-day compressive strength 49.8 MPa, 28-day compressive strength 64.1 MPa, 28-day flexural strength 9.3 MPa; 28-day water immersion compressive strength 61.5 MPa, retention rate 95.9%, softening coefficient 0.91; initial setting 15 min, final setting 24 min; 28-day shrinkage 215 × 10⁻⁶. -6 .

[0111] Analysis: When the P-GO doping concentration exceeded 0.05 wt%, local aggregation occurred due to increased van der Waals forces between the layers. Some P-GO layers failed to fully utilize the nanotemplate effect, resulting in a decrease in the uniformity of K-Struvite crystal growth, manifested as a slight decline in strength compared to Example 2. However, the overall performance was still significantly better than the blank group and the single-doped control group.

[0112] Comparative Example 1 test results: 1-day compressive strength 32.5 MPa, 3-day compressive strength 41.8 MPa, 28-day compressive strength 52.3 MPa, 28-day flexural strength 6.8 MPa; after immersion in water for 28 days, compressive strength was 43.0 MPa, retention rate 82.3%, softening coefficient 0.72; initial setting time 8 min, final setting time 14 min; 28-day shrinkage 320 × 10⁻⁶. -6 .

[0113] Comparative Example 2 test results: 1-day compressive strength 34.1 MPa, 3-day compressive strength 44.2 MPa, 28-day compressive strength 55.8 MPa, 28-day flexural strength 7.4 MPa; after immersion in water for 28 days, compressive strength was 47.9 MPa, retention rate 85.8%, softening coefficient 0.78; initial setting time 7 min, final setting time 12 min; 28-day shrinkage 295 × 10⁻⁶. -6 .

[0114] Analysis: The -OH / -COOH groups of ordinary GO have poor chemical compatibility with the MPC phosphate hydration system, failing to form a chemically bonded interface and thus limiting the enhancing effect. Furthermore, GO partially aggregates in the highly alkaline pore solution of MPC, leading to localized weak zones. The shorter solidification time compared to the control group is because the GO sheets increase the solid phase surface area, accelerating the initial rate of the hydration reaction.

[0115] Comparative Example 3 test results: 1-day compressive strength 30.6 MPa, 3-day compressive strength 42.5 MPa, 28-day compressive strength 56.1 MPa, 28-day flexural strength 7.6 MPa; after immersion in water for 28 days, the compressive strength was 51.6 MPa, retention rate 92.0%, softening coefficient 0.86; initial setting time 11 min, final setting time 18 min; 28-day shrinkage 260 × 10⁻⁶. -6 .

[0116] Analysis: The microfilling and secondary hydration effects of MK effectively improved the 28-day strength and water resistance, but the 1-day strength was actually 5.8% lower than the control group. This is because the incorporation of MK reduced the effective phosphate concentration in the MPC system, slowing down the early hydration rate. Lacking the nanotemplating effect of P-GO, the K-Struvite crystal morphology was not optimized, and the reinforcing effect was inferior to the P-GO / MK synergistic system.

[0117] 1. Mechanical Properties (Compressive Strength and Flexural Strength) Analysis

[0118] (1) The synergistic enhancement effect of P-GO / MK is significantly better than that of single modification.

[0119] The 28-day compressive strength of Comparative Example 2 (ordinary GO 0.05 wt%) and Comparative Example 3 (MK 12 wt%) increased by 6.7% and 7.3% respectively compared to the control group, with the expected increase from the simple superposition of the two being approximately 14.0%. However, Example 2 (P-GO 0.05 wt% + MK 12 wt%) showed an actual increase of 30.8%, far exceeding the expected superposition, confirming a significant synergistic effect between P-GO and MK. Similarly, in terms of flexural strength, Comparative Example 2 and Comparative Example 3 increased by 8.8% and 11.8% respectively, with the expected superposition of approximately 20.6%, while Example 2 showed an actual increase of 50.0%, demonstrating a more pronounced synergistic effect.

[0120] (2) The functional design of P-GO is superior to that of ordinary GO.

[0121] Comparative Example 2, using unfunctionalized ordinary GO (containing -OH / -COOH), only saw its 28-day compressive strength increase from 52.3 MPa to 55.8 MPa (+6.7%), while Example 2, using phosphate-functionalized P-GO, achieved a 28-day compressive strength of 68.4 MPa (+30.8%). The -PO(OH)2 groups on the P-GO surface interact with Mg... 2+ Coordination forms a chemical bonding interface, making P-GO sheets an effective nucleation template for K-Struvite crystals, while ordinary GO relies solely on van der Waals physical adsorption, resulting in weak interfacial bonding and low load transfer efficiency.

[0122] (3) The optimal doping amount exhibits a threshold effect.

[0123] In Examples 1, 2, and 3, with increasing P-GO doping (0.03, 0.05, and 0.08 wt%), the 28-day compressive strength first increased and then decreased (61.7, 68.4, and 64.1 MPa). Example 2 (0.05 wt% P-GO + 12 wt% MK) was the optimal doping combination. Beyond the optimal doping, P-GO exhibited localized agglomeration due to increased van der Waals forces between the layers, leading to uneven K-Struvite crystal growth and the formation of localized weak zones.

[0124] (4) Early intensity development pattern

[0125] The 1-day compressive strength of Example 2 reached 38.2 MPa, an increase of 17.5% compared to the control group (32.5 MPa), indicating that the P-GO / MK synergistic system can play a reinforcing role in the early hydration stage. The nanotemplating effect of P-GO accelerates the nucleation rate of K-Struvite crystals, while the micro-filling effect of MK reduces early micro-defects. The synergy of the two ensures the simultaneous improvement of early strength and 28-day strength.

[0126] Table 4 Mechanical Performance Test Results

[0127]

[0128] 2. Water resistance performance analysis

[0129] (1) Blank MPC has weak water resistance

[0130] Comparative Example 1 showed a decrease in compressive strength from 52.3 MPa to 43.0 MPa after immersion in water for 28 days, with a strength retention rate of only 82.3% and a softening coefficient of 0.72. This is because microcracks and pore channels exist between K-Struvite crystals, unreacted phosphates dissolve after immersion in water, and K-Struvite itself undergoes partial dissolution under long-term water immersion conditions, resulting in a loose structure.

[0131] (2) The encapsulation effect of P-GO inhibits dissolution

[0132] Comparative Example 2 (ordinary GO) showed a strength retention rate of 85.8%, with limited improvement; while Example 2 (P-GO) showed a strength retention rate of 96.8%, with significant improvement. The P-GO sheets tightly encapsulated the K-Struvite crystals through chemical bonding, effectively blocking the dissolution channels of phosphate, while the physical adsorption interface of ordinary GO could not form an effective encapsulation.

[0133] (3) The filling effect of MK blocks moisture migration

[0134] Comparative Example 3 (MK 12 wt%) showed a strength retention rate of 92.0%. The amorphous aluminum phosphate gel generated by MK filled the micro-gaps between K-Struvite crystals, reducing interconnected pores.

[0135] (4) P-GO / MK synergy leads to breakthrough improvement in water resistance

[0136] Example 2 shows a strength retention rate of 96.8% and a softening coefficient of 0.93, far exceeding the 80% requirement of JC / T 2857-2024, thus meeting the requirements for long-term service in humid environments. The synergistic effect of P-GO's encapsulation effect (inhibiting leaching) and MK's filling effect (blocking migration) fundamentally solves the problem of MPC's poor water resistance.

[0137] Table 5 Water resistance test results

[0138]

[0139] 3. Condensation time analysis

[0140] The setting time data shows that Comparative Example 1 has an initial setting time of 8 min and a final setting time of 14 min, which is too fast and results in a narrow construction window. Example 2 extends the initial setting time to 13 min and the final setting time to 21 min, meeting the operability requirements of JC / T 2857-2024 for fast-setting type (initial setting time ≥15 min is ordinary type, ≥60 min is slow-setting type).

[0141] The -PO(OH)2 group of P-GO partially dissociates in the alkaline pore solution of MPC, consuming local Mg. 2+ It plays a certain role in retarding the setting process in the early stages of hydration; the incorporation of MK reduces the phosphate concentration per unit volume, slowing down the acid-base neutralization reaction rate. The combination of these two factors moderately prolongs the setting time without requiring additional borax, thus avoiding the negative impact of retarders on early strength.

[0142] Table 6. Results of setting time test

[0143] Comparative Example 1 8 14 Comparative Example 2 7 12 Comparative Example 3 11 18 Example 1 10 17 Example 2 13 21 Example 3 15 24

[0144] 4. Drying shrinkage analysis

[0145] Based on the 28-day drying shrinkage data, the shrinkage value of Comparative Example 1 is 320 × 10⁻⁶. -6 After modification of ordinary GO (295×10) -6 Limited improvement, MK single doping (260×10) -6 The effect was obvious, while the shrinkage value of Example 2 was only 198×10. -6 The level was 38.1% lower than that of the control group.

[0146] The high elastic modulus of P-GO sheets limits the shrinkage deformation of the cement matrix, and their chemically bonded interfaces enable the sheets and matrix to deform synergistically, effectively sharing shrinkage stress. The micro-filling effect of MK and its secondary hydration products (aluminum phosphate gel) refine the pore structure and reduce the capillary shrinkage driving force. The synergy of these two factors significantly improves volume stability, which helps reduce the risk of cracking at the repair interface caused by shrinkage.

[0147] Table 7 Shrinkage performance test results

[0148] Comparative Example 1 320 Comparative Example 2 295 Comparative Example 3 260 Example 1 245 Example 2 198 Example 3 215

[0149] 5. Microstructure characterization

[0150] The cross-sectional morphology of the 28-day hardened P-GO / MK-MKPC in Example 2 shows that K-Struvite crystals are regular columnar and plate-like structures, directionally aligned and growing on the surface of P-GO sheets. The crystals are tightly bonded together without obvious microcracks. P-GO sheets are embedded in K-Struvite crystal clusters with no gaps at the interface, indicating that chemical bonding enables P-GO to form an integrated structure with the matrix. Fine MK particles fill the intercrystalline gaps, resulting in a dense overall structure.

[0151] In Comparative Example 1, the blank MPC 28d hardened body showed K-Struvite crystals as irregular fragments, loosely and disorderedly arranged, with numerous microcracks (approximately 1-3 μm wide) and pores between the crystals. The unreacted MgO particles had smooth surfaces and weak interfacial bonding with hydration products. The overall structure had numerous interconnected pores, providing channels for water intrusion and phosphate dissolution, explaining its poor water resistance.

[0152] As can be seen from the interface diagram, P-GO reacts with Mg via -PO3H2. 2+ The coordination effect of Mg induces heterogeneous nucleation of K-Struvite in situ on the surface of the sheet, forming a chemical bonding interface (Mg-OP covalent bond), which can transfer the load efficiently. Ordinary GO relies only on van der Waals forces to physically adsorb onto the surface of K-Struvite crystals. The interfacial bonding force is weak, the load transfer efficiency is low, and it is easy to desorb under water immersion conditions.

[0153] In summary, at the optimal dosage (0.05 wt% P-GO + 12 wt% MK), the 28-day compressive strength of the P-GO / MK-MKPC composite material increased by 30.8% and the flexural strength increased by 50.0%, far exceeding the simple superposition effect of ordinary GO single-component (+6.7%) and MK single-component (+7.3%), confirming the synergistic effect mechanism. The 28-day strength retention rate after immersion in water reached 96.8%, and the softening coefficient was 0.93, meeting the requirements for engineering applications in harsh humid environments. The retarding effect of P-GO and the incorporation of MK extended the setting time from 8 min to an adjustable 13-24 min without requiring additional borax. The 28-day drying shrinkage was reduced by 38.1%, which helps reduce the risk of cracking at the repair interface. The dosage of P-GO is extremely low; 0.05 wt% is sufficient to achieve a significant reinforcing effect, demonstrating good economic feasibility and suitability for engineering application.

[0154] Although the technical solution of the present invention has been described in detail above with general descriptions, specific embodiments, and experimental examples, it should be noted that the embodiments and experimental examples are only used to illustrate the technical solution and technical effects of the present invention, and should not be regarded as any limitation on the scope of protection of the present invention. Simple modifications, alterations, or improvements made based on the technical concept of the present invention are all within the scope of protection claimed by the present invention.

Claims

1. A high-strength, low-shrinkage, water-resistant potassium magnesium phosphate cement composite material, characterized in that: The potassium magnesium phosphate cement composite material comprises: phosphonic acid-based functionalized graphene, metakaolin, and composite powder, wherein the composite powder is composed of calcined magnesium oxide, potassium dihydrogen phosphate, and borax; the amount of phosphonic acid-based functionalized graphene is 0.03-0.08 wt% of the mass of calcined magnesium oxide, and the amount of metakaolin is 8-15 wt% of the mass of calcined magnesium oxide.

2. The potassium magnesium phosphate cement composite material according to claim 1, characterized in that: The phosphonic acid-based functionalized graphene is prepared by covalently modifying graphene oxide with (3-triethoxysilylpropyl)phosphonic acid via a silane coupling reaction, and its surface contains phosphonic acid groups.

3. The potassium magnesium phosphate cement composite material according to claim 2, characterized in that: The phosphorus / carbon atom ratio in the phosphonic acid-functionalized graphene is 0.035-0.

052.

4. The potassium magnesium phosphate cement composite material according to claim 1, characterized in that: The mass ratio of calcined magnesium oxide to potassium dihydrogen phosphate and borax in the composite powder is 100:28-32:4-6.

5. The potassium magnesium phosphate cement composite material according to claim 1, characterized in that: The amount of phosphonic acid-based functionalized graphene is 0.05 wt% of the mass of reburned magnesium oxide, and the amount of metakaolin is 12 wt% of the mass of reburned magnesium oxide.

6. The potassium magnesium phosphate cement composite material according to claim 1, characterized in that: The raw material components are included in parts by weight: 100 parts of red-burned magnesium oxide, 28-32 parts of potassium dihydrogen phosphate 4-6 parts borax Phosphonic acid-functionalized graphene, 0.03-0.08 parts, Partial 8-15 parts of metakaolin.

7. The potassium magnesium phosphate cement composite material according to claim 6, characterized in that: The raw material components are included in parts by weight: 100 parts of red-burned magnesium oxide, 30 parts of potassium dihydrogen phosphate 5 parts borax 0.5 parts of phosphonic acid-functionalized graphene 12 parts of metakaolin.

8. An application of a potassium magnesium phosphate cement composite material as described in any one of claims 1-7, characterized in that: include: Phosphonic acid-functionalized graphene was added to water and dispersed to obtain a dispersion. The calcined magnesium oxide, potassium dihydrogen phosphate, borax and metakaolin were mixed evenly and then added to the dispersion to obtain a slurry. Take the grout and pour it into the construction site. Cure it for 25-30 days.

9. The application according to claim 8, characterized in that: The water-to-solid ratio of the potassium magnesium phosphate cement composite material is 0.13-0.16 when used.

10. The application according to claim 9, characterized in that: The water-to-solid ratio of the potassium magnesium phosphate cement composite material is 0.14.

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