A modified potassium magnesium phosphate cement and its application

Abstract

The present invention belongs to the field of building materials technology, and discloses a modified potassium magnesium phosphate cement and its application. The present invention uses a mixture of carbide slag, fly ash and sodium fluorosilicate to replace part of overburned magnesium oxide to prepare modified potassium magnesium phosphate cement. Fly ash, carbide slag and sodium fluorosilicate are mixed in a mass ratio of 6.5:3:0.5. Studies have shown that the compressive strength of modified potassium magnesium phosphate cement increases first and then decreases with the increase of the modified raw material dosage. When the dosage is 20%, the compressive strength reaches a maximum, and its 28d compressive strength increase reaches 22%. At this time, the compressive strength is 45.82MPa. The modified potassium magnesium phosphate cement of the present invention also shows good water resistance, wherein the compressive strength retention coefficients of the two groups of specimens M1 and M2 under the 7+60d curing system are 0.9443 and 1.0141 respectively. The present invention provides a modified potassium magnesium phosphate cement with better performance, which is of great significance to the promotion and application of potassium magnesium phosphate cement.

Description

Technical Field

[0001] The invention belongs to the technical field of building materials and relates to modified potassium magnesium phosphate cement and application thereof. Background Art

[0002] Magnesium phosphate cement (MPC) is a cementitious material formed through an acid-base neutralization reaction between soluble phosphates and high-temperature calcined magnesium oxide. It combines the properties of inorganic ceramics and cement, offering significant advantages in areas such as rapid repair and special protection. Magnesium phosphate cement can be divided into two categories based on the phosphate raw material: magnesium ammonium phosphate cement (MAPC) and magnesium potassium phosphate cement (MKPC). Magnesium ammonium phosphate cement is produced by the reaction of magnesium oxide with ammonium dihydrogen phosphate. Its hydration product is MgNH₄PO₄·6H₂O (struvite), which exhibits superior strength and stability. Magnesium potassium phosphate cement is produced by the reaction of magnesium oxide with potassium dihydrogen phosphate. Its hydration product is MgKPO₄·6H₂O (K-struvite). Compared to MAPC, MKPC does not produce ammonia during hydration, making the reaction more controllable and offering significant advantages in engineering applications.

[0003] The MKPC system also exhibits remarkable rapid hardening properties, while also possessing excellent interfacial bonding and volume stability. Based on these properties, the material is widely used in the field of rapid repairs in civil engineering. However, since K-struvite, the main hydration product of MKPC, is easily dissolved in water, its water resistance is poor; and the magnesium oxide required to prepare MKPC requires high-temperature reburning, which consumes a lot of energy and results in high preparation costs. These issues limit the widespread application of MKPC in practical engineering. Therefore, modifying MKPC to improve its performance is of great theoretical and practical significance. Summary of the Invention

[0004] To solve the above-mentioned technical problems, the present invention provides a kind of modified potassium magnesium phosphate cement and application thereof. The present invention uses fly ash, carbide slag and sodium fluorosilicate to replace part of overburned magnesium oxide to prepare modified potassium magnesium phosphate cement. Fly ash, carbide slag and sodium fluorosilicate are mixed according to the mass ratio of 6.5:3:0.5. Studies have shown that the compressive strength of modified potassium magnesium phosphate cement increases first and then decreases with the increase of fly ash, carbide slag and sodium fluorosilicate dosage. When the dosage is 20%, the compressive strength reaches the maximum, and its 28d compressive strength increase reaches 22%. At this time, the compressive strength is 45.82MPa. The modified potassium magnesium phosphate cement of the present invention also shows good water resistance, wherein M1 and M2 two groups of test pieces reach the best compressive strength retention coefficient under 7+60d curing system, which is 0.9443 and 1.0141. The present invention provides a kind of modified potassium magnesium phosphate cement with better performance, which is of great significance to the promotion and application of potassium magnesium phosphate cement.

[0005] On the one hand, the present invention provides a modified potassium magnesium phosphate cement, the raw materials of which are composed of potassium dihydrogen phosphate, overburned magnesium oxide, fly ash, carbide slag, sodium fluorosilicate, borax, silicon dioxide and water. Fly ash, carbide slag and sodium fluorosilicate are used as the modified raw materials of the modified potassium magnesium phosphate cement, and the mass ratio of the fly ash, carbide slag and sodium fluorosilicate is 8.5:1:0.5 to 6.5:3:0.5. The ratio of the added mass of the modified raw materials to the added mass of the overburned magnesium oxide is 1:9 to 2:8. Preferably, the mass ratio of the fly ash, carbide slag and sodium fluorosilicate is 6.5:3:0.5; and the ratio of the added mass of the modified raw materials to the added mass of the overburned magnesium oxide is 2:8.

[0006] Furthermore, the ratio of the added mass of potassium dihydrogen phosphate to the total mass of fly ash, carbide slag, and overburned magnesium oxide is 1:3, the ratio of the added mass of borax to the total mass of fly ash, carbide slag, sodium fluorosilicate, overburned magnesium oxide, potassium dihydrogen phosphate, and borax is 0.05:1. The ratio of the added mass of silicon dioxide to the added mass of borax is 1:1. The ratio of the added mass of water to the total mass of fly ash, carbide slag, sodium fluorosilicate, overburned magnesium oxide, potassium dihydrogen phosphate, and borax is 0.2:1.

[0007] Specifically, the present invention prepares modified potassium magnesium phosphate cement using the above-mentioned raw materials, and finds that after the introduction of a mixture of fly ash, carbide slag and sodium fluorosilicate for modification, when the dosage is not higher than 20%, the early strength of the composite system is basically the same as that of the control group, but it has a significant improvement effect on the long-term mechanical properties. As the dosage increases to 20%, the 28d compressive strength shows a monotonically increasing trend, with the maximum increase reaching 22%. At this time, the compressive strength is 45.82MPa.

[0008] In another aspect, the present invention claims protection for the use of the modified potassium magnesium phosphate cement in building materials, wherein the modified potassium magnesium phosphate cement has a compressive strength of not less than 41 MPa after curing for 28 days, and the modified potassium magnesium phosphate cement is water resistant.

[0009] Specifically, the present invention prepared modified potassium magnesium phosphate cement using the above raw materials and conducted water resistance tests. It was found that the compressive strength of MKPC-FA-CS showed a nonlinear change pattern of first increasing and then decreasing with the addition of fly ash, calcium carbide slag, and sodium fluorosilicate. When the addition amount was controlled in the range of 10% to 20%, the samples showed good water resistance under the three curing systems of 7+7d, 7+28d, and 7+60d. Among them, the water resistance of the two groups of specimens M1 and M2 was more outstanding than that of the other groups of specimens. The compressive strength retention coefficients were 0.8321 and 0.8541 respectively under the 7+7d curing system, increased to 0.9677 and 0.9481 under the 7+28d curing system, and reached the best under the 7+60d curing system, which were 0.9443 and 1.0141. In the range of 10% to 20% dosage, the compressive strength of the composite system is positively correlated with the dosage of the modified raw material, showing a monotonically increasing trend with the increase of dosage, and continues to increase with the extension of age.

[0010] Compared with the prior art, the technical solution provided by the present invention has at least the following beneficial effects or advantages:

[0011] (1) The present invention uses fly ash, carbide slag and sodium fluorosilicate to replace part of the overburned magnesium oxide to prepare modified potassium magnesium phosphate cement. Studies have shown that the compressive strength of the modified potassium magnesium phosphate cement first increases and then decreases with the increase of the fly ash, carbide slag and sodium fluorosilicate content, reaching a maximum value when the content is 20%. Through experiments, the present invention found that after the introduction of fly ash, carbide slag and sodium fluorosilicate for modification, when the content of the modified raw materials is not higher than 20%, the early strength (within 7 days) of the composite system is basically the same as that of the control group, but it has a significant improvement effect on the long-term mechanical properties. As the content increases to 20%, the compressive strength at 28 days shows a monotonically increasing trend, with the maximum increase reaching 22%. At this time, the compressive strength is 45.82MPa.

[0012] (2) By comparing the flexural and compressive strengths of potassium magnesium phosphate cement before and after immersion in water, the present invention found that the addition of fly ash, carbide slag and sodium fluorosilicate can improve the water resistance of magnesium phosphate cement and increase the strength retention rate of potassium magnesium phosphate cement. When the addition amount is controlled in the range of 10% to 20%, the samples show good water resistance under the three curing systems of 7+7d, 7+28d and 7+60d. Among them, the water resistance of the two groups of specimens M1 and M2 is more outstanding than that of the other groups of specimens. The compressive strength retention coefficients are 0.8321 and 0.8541 respectively under the 7+7d curing system, and increase to 0.9677 and 0.9481 under the 7+28d curing system, and reach the best of 0.9443 and 1.0141 under the 7+60d curing system. In the range of 10% to 20% dosage, the compressive strength of the composite system is positively correlated with the dosage of the modified raw material, showing a monotonically increasing trend with the increase of dosage, and continues to increase with the extension of age.

[0013] (3) The present invention observes the microstructure of potassium magnesium phosphate cement mortar specimens through SEM-EDS testing. When the content of fly ash, carbide slag and sodium fluorosilicate reaches 20%, the microstructure of potassium magnesium phosphate cement reaches the densest and the performance is the best. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0015] Figure 1 The effect of different modified raw material dosages on the compressive strength of potassium magnesium phosphate cement.

[0016] Figure 2 This paper shows the effect of different modified raw material dosages on the flexural strength of potassium magnesium phosphate cement.

[0017] Figure 3 The effect of different modified raw material dosages on the setting time of potassium magnesium phosphate cement.

[0018] Figure 4 These are SEM images of potassium magnesium phosphate cement prepared under different modified raw material dosage conditions. Figure 4 A is the SEM image of potassium magnesium phosphate cement with 0% modified raw material content after curing for 7 days; Figure 4 Figure B is the SEM image of potassium magnesium phosphate cement with 20% modified raw material content after curing for 7 days; Figure 4 Figure C is the SEM image of potassium magnesium phosphate cement with 30% modified raw material content after curing for 7 days; Figure 4 D is the SEM image of potassium magnesium phosphate cement with 0% modified raw material content after curing for 7 days and then soaking in water for 7 days; Figure 4 E in the figure is the SEM image of potassium magnesium phosphate cement with 20% modified raw material content after curing for 7 days and then soaking in water for 7 days; Figure 4 Figure F is the SEM image of potassium magnesium phosphate cement with 30% modified raw material content after curing for 7 days and then soaking in water for 7 days.

[0019] Figure 5 This paper shows the effects of different modified raw material dosages and immersion ages on the compressive strength retention coefficient of potassium magnesium phosphate cement.

[0020] Figure 6 The effects of different modified raw material dosages and immersion ages on the compressive strength of potassium magnesium phosphate cement.

[0021] Figure 7 This paper shows the effects of different modified raw material dosages and immersion ages on the flexural strength retention coefficient of potassium magnesium phosphate cement.

[0022] Figure 8 This paper studies the effects of different modified raw material dosages and immersion ages on the flexural strength of potassium magnesium phosphate cement. DETAILED DESCRIPTION

[0023] The technical solutions of the present invention are described below with reference to the following examples. However, the present invention is not limited to the following examples. The experimental methods and detection methods described in each example are conventional methods unless otherwise specified. The reagents and materials described are commercially available unless otherwise specified. The percentages in the following examples are percentages by mass unless otherwise specified.

[0024] Burnt magnesia (MgO) was purchased from Dashiqiao Tianyi Refractory Co., Ltd., and its main components are MgO2 content ≥92%, SiO2 content ≤4.5%, CaO content ≤2.0%, Fe2O3 content ≤1.20%, and IL content ≤0.3%.

[0025] Potassium dihydrogen phosphate (KH2PO4) was purchased from Guangzhou Suixin Chemical Co., Ltd.

[0026] Borax (Na2B4O7·10H2O, abbreviated as NBO) was purchased from Guangzhou Suixin Chemical Co., Ltd.

[0027] Quartz sand (SiO2) was purchased from Xiamen ASI Standard Sand Co., Ltd.

[0028] Fly ash (FA) was purchased from Henan Borun Casting Materials Co., Ltd. Its main chemical composition is SiO2 content ≤ 50%, Al2O3 content ≥ 30%, Cl - Content ≤0.02%, SO3 content ≤3%, CaO content ≤10%, ALK content ≤1.5%, Fe content 0.8~1.0%, fCaO content ≤1.0%; its main parameters are fineness of 5μm square hole sieve residue ≤18%, ignition vector (ignition difference subtraction method) ≤5%, water content ≤1%, density ≤2.6 g / cm 3 , bulk density 0.63~1.38 g / cm 3 .

[0029] Calcium carbide slag (CS) was purchased from Shandong Jinjier New Materials Co., Ltd., with an active ingredient content of 70%.

[0030] Sodium fluorosilicate was purchased from Shandong Yukang Chemical Co., Ltd. with a purity of 99%.

[0031] Example 1

[0032] This example provides the preparation, curing and performance testing of potassium magnesium phosphate cement.

[0033] 1. Preparation and curing of potassium magnesium phosphate cement

[0034] Based on optimization from previous research, the water-binder ratio was set at 0.20, the mortar-sand ratio was set at 1, and the mass ratio of fly ash, carbide slag, and sodium fluorosilicate was set at 6.5:3:0.5. The composite ratios of FA and CS in potassium magnesium phosphate cement (MKPC) were divided into five groups. The mass ratio of FA and CS replacing MgO in MKPC (FA+CS / MgO) is shown in Table 1. A control group was also established, with only fly ash added (the mass ratio of fly ash to overburned magnesia was 2:8). The specific steps involved:

[0035] Table 1 Experimental design of modified raw materials instead of magnesium oxide

[0036]

[0037] Note: In Table 1, W is water, and B is MgO+FA+CS+Na2SiF6+KH2PO4+NBO.

[0038] Weigh the raw materials according to the proportions in Table 1. Place the weighed magnesium oxide, fly ash, carbide slag, sodium fluorosilicate, KH2PO4, and borax into a cement mortar mixer. Slowly stir at 60 r / min for 30 seconds until the powder is uniform. Then add standard sand (quartz sand) and slowly stir for 60 seconds. After thoroughly mixing, add water. After adding water, stir slowly for 30 seconds, then rapidly stir at 125 r / min for 60 seconds. Then, pour into a 160mm×40mm×40mm mold and vibrate to form. Since potassium magnesium phosphate cement sets quickly, it can be demolded after standing for 30 minutes. Specimens were placed in a standard curing room at a temperature of (20±2)°C and a humidity of ≥90% and cured to the specified ages (including 1 day, 3 days, 7 days, 28 days, and immersion ages of 7+7 days, 7+28 days, and 7+60 days). Potassium magnesium phosphate cement (MKPC-FA-CS) incorporating fly ash, carbide slag, and sodium fluorosilicate was obtained. Potassium magnesium phosphate cement (MKPC) without the addition of modified raw materials and potassium magnesium phosphate cement (MKPC-FA) with only fly ash were also prepared according to the above method. The tests showed that the MKPC-FA with only fly ash had a setting time of 19 minutes, a compressive strength of 34.86 MPa at 28 days, and a flexural strength of 4.23 MPa at 28 days. These compressive and flexural strengths were significantly lower than those of the MKPC-FA-CS modified with carbide slag, fly ash, and sodium fluorosilicate.

[0039] 2. Compressive strength and flexural strength test of potassium magnesium phosphate cement

[0040] Based on the technical requirements of GB / T 1346-2011 "Test Method for Water Consumption, Setting Time and Soundness of Cement of Standard Consistency", the present invention measures the setting characteristics of potassium magnesium phosphate cement (MKPC and MKPC-FA-CS). During the test, in order to accurately characterize the setting behavior of the material, a Vicat instrument was used to regularly measure the slurry hardening characteristics, where the data collection interval was set to 30 seconds. When the specimen approached the initial setting stage, it was adjusted to 15 seconds for intensive observation. Given that the potassium magnesium phosphate cement system has significant rapid setting characteristics, there is a significant temporal coupling phenomenon between its initial and final setting processes. To avoid test blind spots and ensure data reliability, this test focuses on monitoring the final setting time as a key performance indicator. The entire test strictly adhered to the needle penetration judgment criteria required by the specification. By real-time recording the changes in the probe penetration resistance, the critical point where the slurry transitions from the plastic state to the rigid state was accurately captured. This optimized testing scheme not only meets the core requirements of the standard method, but also effectively adapts to the special testing needs of rapid-setting cementitious materials. The compressive strength test results are as follows: Figure 1 The flexural strength test results are shown in Figure 2 shown.

[0041] Depend on Figure 1The compressive strengths of the M0 group at 1, 3, 7, and 28 days were 25.21 MPa, 28.33 MPa, 29.55 MPa, and 35.56 MPa, respectively. The control MKPC specimens exhibited significant early strength development, with their 1-day compressive strength reaching approximately 70% of their 28-day compressive strength, fully demonstrating the performance advantages of the potassium magnesium phosphate cement system as a rapid repair material. Notably, the material exhibited significantly weaker strength growth after 3 days, reflecting the lack of late-stage strength development inherent in conventional MKPC materials. Modification with fly ash, carbide slag, and sodium fluorosilicate, at a content of up to 20%, maintained the early strength (within 3 days) of the composite system roughly comparable to that of the control group. However, the long-term mechanical properties were significantly improved. Increasing the content of the modified raw materials to 20% resulted in a monotonic increase in the 28-day compressive strength, reaching 45.82 MPa by 22%. The 1-day strengths of the M1 and M2 specimens were 22.34 MPa and 19.55 MPa, respectively, reaching 60% and 43% of their 28-day strengths. This indicates that when the modified raw material content ranges from 10% to 20%, the MKPC-FA-CS composite maintains excellent early performance while also achieving improved late-stage strength. However, above 20%, the early-stage strength of the composite system decreases compared to the control. When the content exceeds 20%, the decrease in strength of the MKPC-FA-CS system stems from the physical adhesion of FA particles to the MgO surface, which inhibits the full reaction between the phosphate and the magnesium source, hindering the crystallization of the key strength phase, potassium magnesium phosphate (MKP). Furthermore, excess FA acts as an inert filler, embedding itself into the intercrystalline spaces. Excessive carbide slag leads to the enrichment of calcium hydroxide within the system, disrupting the continuous skeletal structure of MKP and weakening the overall load-bearing capacity of the material. After the addition of modified raw materials, the compressive strength first increases and then decreases. The compressive strength is the highest when the addition amount is 20%. The reason is that FA has a micro-aggregate effect. FA fills the capillary pores of the cement paste, making the capillary pores in the cement paste finer, which can optimize the mortar particle grading and pore structure, and improve the density of the mortar. The addition of carbide slag provides a strong alkaline environment, which can activate the potential activity of the reaction material and promote the hydration reaction to generate more gel products, thereby improving the early density and strength. Sodium fluorosilicate can be hydrolyzed to generate active silicic acid (H4SiO4) and fluoride ions (F in the alkaline environment formed by carbide slag. - ), silicic acid and Mg in potassium magnesium phosphate 2+The reaction produces magnesium silicate gel, which fills pores and interweaves with potassium magnesium phosphate crystals to form a dense network structure, reducing internal defects. Fluoride ions also promote the rapid crystallization of hydration products (MgKPO4·6H2O), improving early strength. However, when the addition level exceeds 20%, the compressive strength begins to decrease. This is because a considerable number of loose, porous particles in FA and the porous phase (platy-shaped calcium hydroxide) formed by carbide slag in an alkaline environment absorb some water, reducing the water required for the reaction of the potassium magnesium phosphate cement-based material, inhibiting the development of potassium magnesium phosphate crystals and thus reducing the compressive strength of the material. Furthermore, FA has an adsorption effect. After addition, FA absorbs some potassium dihydrogen phosphate, affecting the hydration reaction of potassium magnesium phosphate cement and reducing the number of hydration products. This effect primarily affects the later strength.

[0042] Depend on Figure 2 As the proportion of fly ash, carbide slag, and sodium fluorosilicate increases, the flexural strength of the system shows a significant decreasing trend. After a 28-day curing period, the flexural strength of the control (M0) reached 7.4 MPa, while the strength of specimens in groups M1 to M4, which incorporated fly ash, carbide slag, and sodium fluorosilicate, decreased to 6.5, 6.3, 5.9, and 4.1 MPa, respectively, representing decreases of 12%, 15%, 20%, and 45%, respectively. The mechanism of this decrease in flexural strength is attributed to the lack of effective chemical bonding between the fly ash and carbide slag particles, as well as the sodium fluorosilicate, and the potassium magnesium phosphate cement matrix, resulting in weak interfacial bonding. With increasing fly ash, carbide slag, and sodium fluorosilicate content, the destructive effects of fly ash, carbide slag, and sodium fluorosilicate on the matrix's crystalline network intensify, significantly affecting the integrity of the hydrated product's crystal structure. Under external shear stress, this weak interfacial effect, combined with structural defects, ultimately leads to a step-wise reduction in the material's bearing capacity.

[0043] 3. Potassium magnesium phosphate cement setting time test

[0044] The setting time test of MKPC and MKPC-FA-CS paste samples was carried out in accordance with GB / T 1346-2011 "Test Method for Water Consumption, Setting Time and Stability of Cement Standard Consistency". The pure paste after mixing the mixture powder with water was placed in a container and allowed to stand for a few minutes. The initial setting time was tested using a Vicat apparatus according to the method specified in the national standard. Considering the fast setting and hardening characteristics of potassium magnesium phosphate cement, the sample setting time was recorded to an accuracy of 30s, and the recording interval was shortened to 15s when approaching the initial setting. Due to the short time interval between the initial setting and final setting of potassium magnesium phosphate cement, this test only measured the final setting time as its setting time. The test results are as follows: Figure 3 shown.

[0045] Depend on Figure 3It can be seen that when no modifying raw materials are added, the setting time of MKPC is 16.39 minutes. As the amount of modifying raw materials increases, the setting time increases. When the modifying raw materials are added at 40%, the setting time is the longest, reaching 25.94 minutes. This indicates that the addition of fly ash, carbide slag, and sodium fluorosilicate can extend the setting time of MKPC. This is primarily due to the following factors: First, the addition of modifying raw materials dilutes the entire reaction system. In the hydration reaction of potassium magnesium phosphate cement, magnesium oxide (MgO) and soluble phosphates require contact and reaction for setting. The addition of modifying raw materials reduces the concentration of the substances involved in the primary reaction, slowing the reaction rate. Second, some components in fly ash and carbide slag (such as activated calcium oxide (CaO) and calcium hydroxide) may react with phosphates in the system, consuming some of the phosphates. Third, fly ash particles fill pores, hindering contact between the reactants (MgO and phosphates). However, the added sodium fluorosilicate can be hydrolyzed in the alkaline environment formed by carbide slag to generate active silicic acid (H4SiO4) and fluoride ions (F - ), fluoride ions promote the rapid crystallization of hydration products (MgKPO4·6H2O), which can shorten the coagulation time.‌

[0046] 4. SEM-EDS test of potassium magnesium phosphate cement

[0047] After curing for 7 days, the M0, M2, and M3 samples were immersed in water for 7 days and not immersed in water for 7 days, and then the bonding samples were prepared. The bonding interface was cut to obtain a 1 cm 3 The sample is a mortar specimen containing fine aggregate. After the sampling is completed as required, the sample used for SEM-EDS testing must be immediately immersed in anhydrous alcohol for 24 hours to terminate the hydration reaction. Subsequently, the sample is placed in a vacuum furnace for drying. In view of the poor conductivity of cement, in order to avoid adverse effects on the test results, the sample needs to be vacuumed and gold-sprayed. Finally, with the help of Model JSM-5600LV scanning electron microscope, the micromorphology and element distribution of the interface area were observed, and the results are as follows Figure 4 shown.

[0048] Depend on Figure 4It can be seen that MKPC without modified raw materials contains a large amount of unreacted MgO and various blocky crystals. As the content of modified raw materials increases, the number of crystals decreases, and the voids and cracks inside MKPC gradually decrease. This is because the modified raw material particles fill the voids inside MKPC, making the structure denser. When the modified raw material content is 20%, the internal structure of MKPC-FA-CS is neatly arranged, the crystals are arranged in the same direction, and the structure is more compact. Small spherical fly ash particles and irregular carbide slag particles fill the voids and cracks inside MKPC, improving the compressive strength of MKPC. However, when the modified raw material content exceeds 30%, the excessive fly ash and carbide slag particles in MKPC destroy the connection between the crystals, resulting in a decrease in performance. When immersed in water, MKPC develops more tiny cracks, and the hydration products have poor crystallinity and a loose structure, which leads to a decrease in the strength of MKPC. At 20% modified raw material content, the MKPC-FA-CS microstructure becomes denser, with reduced porosity and a higher structural density. Fly ash and carbide slag promote the formation of flaky and massive hydration products. Fluorosilicates form Si-O-Si bonds, enhancing cement adhesion and reducing interfacial cracks, thereby increasing MKPC strength. At 30% content, the presence of massive hydration products increases, leading to increased voids and cracks within the MKPC and a decrease in performance. Therefore, the performance of MKPC-FA-CS reaches its optimum when the modified raw material content is 20%.

[0049] 5. Water resistance test of potassium magnesium phosphate cement

[0050] In order to systematically evaluate the water erosion resistance of potassium magnesium phosphate cement mortar, this study introduces the strength retention coefficient (W n ) as a quantitative characterization indicator. The test process strictly follows the requirements for water stability testing in GB / T 50082-2009, "Standard for Test Methods for Long-term Performance and Durability of Ordinary Concrete," and multiple groups of samples are tested for performance under water conditions. This coefficient is obtained by calculating the ratio of the mechanical properties of the samples before and after immersion in water. The specific calculation formula is: W n =f cn / f c , where f cn Represents the compressive (flexural) strength of the specimen after water immersion, f c is the original strength value of the specimen under the reference state (unit: MPa). Figures 5 to 8 This evaluation system effectively reveals the performance evolution of materials under the action of water media and provides data support for durability research.

[0051] Depend on Figures 5 to 8The compressive and flexural strengths of the MKPC and MKPC-FA-CS mortars after immersion in water were lower than those before immersion. The strength of the MKPC mortar decreased significantly after immersion in water. This was attributed to the decrease in pH within the MKPC specimens after immersion, which led to the gradual dissolution and precipitation of the primary hydration product, potassium magnesium phosphate (MKP) gel, and unreacted phosphates. This reduced the amount of MKP produced, increasing the internal porosity of the MKPC mortar and resulting in decreased material strength and water resistance. Different cementitious systems exhibited significant differences in their performance under water. Compared with unimmersed specimens, the compressive and flexural properties of the MKPC matrix decreased after immersion in water. After 7, 28, and 60 days of water curing, the compressive strength retention coefficients decreased to 0.7916, 0.8349, and 0.7414, respectively, while the flexural strength retention coefficients decreased to 0.8942, 0.8511, and 0.7424, respectively. In contrast, although the mechanical properties of MKPC-FA-CS decreased after immersion, the retention rate showed that the strength retention rate after adding the modified raw materials was higher than that without them, indicating that the addition of fly ash, carbide slag, and sodium fluorosilicate can significantly improve the water resistance of MKPC cement mortar. The compressive strength of MKPC-FA-CS showed a nonlinear change pattern, first increasing and then decreasing with the addition of the modified raw materials. When the addition content was controlled between 10% and 20%, the sample exhibited good water resistance under the three curing systems of 7+7d, 7+28d, and 7+60d. Among them, the water resistance of the specimens in groups M1 and M2 is more outstanding than that of the specimens in other groups. The compressive strength retention coefficients are 0.8321 and 0.8541 respectively under the 7+7d curing system, which are increased to 0.9677 and 0.9481 under the 7+28d curing system, and reach the best of 0.9443 and 1.0141 under the 7+60d curing system. Figure 6 The data show that in the range of 10% to 20% of the dosage, the compressive strength of the composite system is positively correlated with the dosage of the modified raw material, presents a monotonically increasing trend with the increase of the dosage, and continues to increase with the extension of age.

[0052] Research has confirmed that the appropriate incorporation of fly ash, carbide slag, and sodium fluorosilicate significantly improves the durability of potassium magnesium phosphate cement (MKPC). The compressive and flexural strength retention coefficients of the composite system with the modified raw materials are both higher than those of the composite system without the modified raw materials. This mechanism of action stems primarily from a dual synergistic effect: physically, fly ash, carbide slag, and sodium fluorosilicate exert a physical filling effect, optimizing the matrix pore structure and reducing defect density; chemically, the unhydrated components in the system undergo a secondary reaction with the fly ash, carbide slag, and sodium fluorosilicate active substances in a liquid phase, forming a network of hydration products with cementitious properties. This microstructural reconstruction not only increases the material's compressive strength development rate but also enhances its impermeability and resistance to water erosion. Improving the water resistance of MKPC mortar mainly improves its density. The addition of carbide slag provides a strong alkaline environment, which can activate the potential activity of the reaction materials and promote the hydration reaction to generate more gel products, thereby improving the early density and strength. Sodium fluorosilicate can be hydrolyzed in the alkaline environment formed by carbide slag to generate active silicic acid (H4SiO4) and fluoride ions (F - ), silicic acid and Mg in potassium magnesium phosphate 2+ The reaction generates magnesium silicate gel, which fills pores and interweaves with magnesium potassium phosphate crystals to form a dense network structure, reducing internal defects. Fluoride ions also promote the rapid crystallization of the hydration product (MgKPO4·6H2O), improving early strength. The micro-aggregate effect of fly ash refines the capillary pores in the cement paste. The cross-linking between fly ash particles and hydration products improves the density of the mortar, reduces the precipitation of unreacted phosphate, and increases the formation of its hydration product, MKP. MKP is the main factor affecting the strength of MKPC, thereby improving the water resistance of the MKPC-FA-CS composite system.

[0053] As described above, the basic principles, main features and advantages of the present invention are well described. The above embodiments and descriptions are merely descriptions of preferred embodiments of the present invention, and the present invention is not limited to the above embodiments. Various changes and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the spirit and scope of the present invention should fall within the scope of protection determined by the present invention.

Claims

1. A modified potassium magnesium phosphate cement, characterized in that: The raw materials are composed of potassium dihydrogen phosphate, overburned magnesium oxide, fly ash, carbide slag, sodium fluorosilicate, borax, silicon dioxide and water; Fly ash, carbide slag and sodium fluorosilicate are used as modified raw materials for the modified potassium magnesium phosphate cement; The mass ratio of the fly ash, carbide slag and sodium fluorosilicate is 8.5:1:0.5 to 6.5:3:0.5; The ratio of the added mass of the modified raw material to the added mass of the overburned magnesium oxide is 1:9 to 2:

8.

2. The modified potassium magnesium phosphate cement according to claim 1, characterized in that The mass ratio of the fly ash, carbide slag and sodium fluorosilicate is 6.5:3:0.5; The ratio of the added mass of the modified raw material to the added mass of the overburned magnesium oxide is 2:

8.

3. The modified potassium magnesium phosphate cement according to claim 1, characterized in that The ratio of the added mass of the potassium dihydrogen phosphate to the total added mass of the modified raw materials and overburned magnesium oxide is 1:

3.

4. The modified potassium magnesium phosphate cement according to claim 1, characterized in that The ratio of the added mass of the borax to the total added mass of the modified raw material, overburned magnesium oxide, potassium dihydrogen phosphate and borax is 0.05:

1.

5. The modified potassium magnesium phosphate cement according to claim 1, characterized in that The ratio of the added mass of silicon dioxide to the added mass of borax is 1:

1.

6. The modified potassium magnesium phosphate cement according to claim 1, characterized in that The ratio of the added mass of the water to the total added mass of the modified raw materials, overburned magnesium oxide, potassium dihydrogen phosphate and borax is 0.2:

1.

7. Use of the modified potassium magnesium phosphate cement according to any one of claims 1 to 6 in building materials.

8. The use according to claim 7, characterized in that The compressive strength of the modified potassium magnesium phosphate cement after curing for 28 days is not less than 41 MPa.

9. The use according to claim 7, characterized in that The modified potassium magnesium phosphate cement has water resistance.

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