Magnesium phosphate cement prepared from calcined brucite tailings

Abstract

The invention discloses magnesium phosphate cement prepared from calcined brucite tailings. The magnesium phosphate cement is prepared from calcined brucite tailing powder and phosphate, wherein in parts by weight, the use amount of the calcined brucite tailing powder is 40-80 parts, and the use amount of the phosphate is 20-45 parts; the calcined brucite tailing powder is obtained by calcining tailings generated by exploiting brucite ores; the content of magnesium oxide in the brucite tailings is 20%-55% according to the mass percent. According to the method, the problems of high carbon emission and high cost caused by preparation of magnesium phosphate cement from magnesite are solved, the selection range of alkaline components of the magnesium phosphate cement is expanded, the brucite tailings can be effectively utilized, and the adverse effect on the environment is reduced.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, specifically to a magnesium phosphate cement prepared using calcined brucite tailings. Background Technology

[0002] Magnesium phosphate cement is an inorganic cementitious material that rapidly sets and hardens after being mixed with reburned magnesium oxide, soluble phosphates, and a retarder and water. It combines the properties of cement and ceramics and is also known as a chemically bonded ceramic. Furthermore, magnesium phosphate cement exhibits rapid setting and hardening, high early strength, excellent bonding properties, and good volume stability, making it promising for applications in repair and reinforcement, structural protection, and heavy metal ion curing.

[0003] Burned magnesium oxide, the alkaline component (comprising 50%–80%) of magnesium phosphate cement, is typically obtained by calcining magnesite at temperatures above 1600℃. However, magnesite resources in my country are unevenly distributed, mainly concentrated in Liaoning and Shandong provinces. Furthermore, in recent years, due to the protection of mineral resources and the environment, the mining of magnesite has been gradually restricted, leading to significant price fluctuations for magnesium oxide. In addition, carbon-containing minerals such as magnesite release CO2 during calcination; theoretically, producing 1 ton of magnesium oxide generates 1.1 tons of CO2. Combined with fuel emissions from industrial production, the actual carbon emissions generated during magnesium oxide production are already higher than those of ordinary silicate cement, further hindering the expansion of magnesium phosphate cement into a wider market.

[0004] Natural brucite is primarily composed of Mg(OH)₂ and often occurs in association with minerals such as dolomite, calcite, lizardite, and magnesite. It offers a cost advantage over synthetic Mg(OH)₂. Industrially, brucite can be used as a precursor for magnesium oxide production, a flame retardant, for wastewater neutralization, and in papermaking. However, limitations in brucite sorting technology result in the accumulation of large quantities of brucite tailings or low-grade brucite that go unused, leading to resource waste and increased environmental burden. The theoretical magnesium oxide content in natural brucite can reach up to 69%. If brucite tailings could be used in the preparation of magnesium phosphate cement, the range of alkaline components for magnesium phosphate cement could be expanded.

[0005] The existing technology for preparing magnesium phosphate cement directly from uncalcined brucite and tailings faces a fundamental technical obstacle: because brucite has a higher reactivity than recalcined magnesium oxide, the hydration reaction rate with phosphate is too fast, resulting in a short initial setting time under conventional mix proportions, and a strength of less than 15 MPa after 3 hours, which cannot meet the basic requirements of rapid hardening and early strength for engineering applications. Summary of the Invention

[0006] In view of the above-mentioned shortcomings of the existing technology, the purpose of this invention is to provide a magnesium phosphate cement prepared by calcining brucite tailings, so as to solve the problem of high carbon emissions of magnesium phosphate cement in the existing technology, improve the performance of magnesium phosphate cement, and reduce the environmental impact of brucite tailings accumulation.

[0007] This invention provides a magnesium phosphate cement prepared using calcined brucite tailings, wherein the magnesium phosphate cement comprises calcined brucite tailings powder and phosphate; wherein, by weight, the amount of calcined brucite tailings powder is 40-80 parts and the amount of phosphate is 20-45 parts; the calcined brucite tailings powder is obtained by calcining brucite tailings; and by mass percentage, the magnesium oxide content in the brucite tailings is 20%-55%.

[0008] Preferably, the calcined brucite tailings powder is obtained by the following method:

[0009] The brucite tailings or low-grade brucite are crushed and then calcined at 800℃~1250℃ for 30min~120min. The calcined product is then ground to obtain the calcined brucite tailings powder.

[0010] Preferably, the maximum particle size of the brucite tailings after crushing before calcination is less than 5 mm, and borax is added at the same time. The amount of borax added shall not exceed 3 wt% of the mass of the brucite tailings.

[0011] Preferably, the particle size of the calcined brucite tailings powder is less than 150 μm.

[0012] Preferably, the magnesium phosphate cement further includes a retarder; the amount of retarder is 0 to 10 parts by weight.

[0013] Preferably, the retarder is borax.

[0014] Preferably, the phosphate is one or more of ammonium dihydrogen phosphate, potassium dihydrogen phosphate, and sodium dihydrogen phosphate.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] 1. This invention employs a low-temperature calcination process (800℃~1250℃) of magnesia tailings to prepare magnesium phosphate cement. Compared to the traditional method of high-temperature calcination of magnesite above 1600℃ to prepare recalcined magnesium oxide, this invention finds that the main reaction during calcination is dehydration (Mg(OH)2→MgO+H2O). Even if the tailings contain small amounts of carbonaceous impurities such as magnesite, dolomite, and calcite, the carbon emissions are far lower than those from magnesite calcination (theoretical value of 1.1 tons of CO2 / ton of product from magnesite calcination). This temperature range can effectively remove crystal water to form low-activity periclase (MgO) and transform silicate minerals (serpentine) in the tailings into inert phases such as forsterite (Mg2SiO4) and calcium magnesium olivine (CaMgSiO4), which do not participate in the hydration reaction. More importantly, adding no more than 3% borax to the brucite tailings during calcination can significantly reduce the calcination temperature and the amount of calcium magnesium olivine phase formed, while retaining more periclase phase. Experiments show that the magnesium phosphate cement prepared by this invention not only has an excellent early strength development rate (significant strength improvement after 3 hours), but also meets the required setting time without the use of a retarder. By adjusting the setting time with a retarder, it can be extended to more than 2 hours, providing a new path for the green preparation of high-performance magnesium phosphate cement.

[0017] 2. This invention innovatively uses brucite tailings with a magnesium oxide content of 20%~55% as raw material, effectively solving the dual problems of resource waste and environmental pollution. Large amounts of tailings generated during traditional brucite sorting processes are stockpiled for extended periods due to their low grade and high processing costs, not only occupying land resources but also potentially posing environmental risks. This invention, through a rational proportioning and precise calcination process, transforms minerals such as serpentine, calcite, magnesite, and dolomite, which are originally considered impurities in low-grade brucite or tailings, into useful inert components during calcination. Adding a small amount of borax during calcination lowers the calcination temperature while reducing the formation of calcium magnesium olivine, retaining more magnesium oxide and turning waste into treasure. This not only provides a stable and low-cost raw material source for the alkaline components of magnesium phosphate cement but also significantly reduces raw material costs. Simultaneously, this invention provides brucite mining enterprises with a new way to utilize tailings for high-value purposes; each ton of tailings can produce approximately 0.4~0.6 tons of high-performance magnesium phosphate cement, significantly improving resource utilization efficiency. Attached Figure Description

[0018] Figure 1 The XRD pattern of the original brucite tailings.

[0019] Figure 2 The XRD pattern of the calcined brucite tailings powder prepared in Example 1 is shown.

[0020] Figure 3 The XRD pattern of the calcined brucite tailings powder prepared in Example 2 is shown.

[0021] Figure 4 The XRD pattern of the calcined brucite tailings powder prepared in Example 3 is shown.

[0022] Figure 5 The XRD pattern of the calcined brucite tailings powder prepared in Example 4 is shown.

[0023] Figure 6 The XRD pattern of the calcined brucite tailings powder prepared in Example 5 is shown.

[0024] Figure 7 The XRD pattern of the calcined brucite tailings powder prepared in Example 6 is shown.

[0025] Figure 8 The XRD pattern of the calcined brucite tailings powder prepared in Example 7 is shown.

[0026] Figure 9 The XRD pattern of reburned magnesium oxide obtained from calcined magnesite in Comparative Example 1 is shown. Detailed Implementation

[0027] The technical solutions of the present invention will be clearly and completely described in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the present invention are within the scope of protection of the present invention.

[0028] Unless otherwise specified in the specific circumstances, the numerical ranges listed herein include upper and lower limits, as well as all integers and fractions within that range, but are not limited to the specific values ​​listed when the range is defined.

[0029] I. A type of magnesium phosphate cement prepared using calcined brucite tailings

[0030] The magnesium phosphate cement of the present invention comprises calcined brucite tailings powder and phosphate; wherein, by weight, the amount of calcined brucite tailings powder is 40-80 parts and the amount of phosphate is 20-45 parts; the calcined brucite tailings powder is obtained by calcining brucite tailings or low-grade natural brucite; by mass percentage, the magnesium oxide content in the brucite tailings or low-grade natural brucite is 20%-55%.

[0031] This invention, through a systematic study of the bottlenecks in existing magnesium phosphate cement technology, discovered that traditional processes rely on the high-temperature calcination of magnesite above 1600℃ (typically 1700℃ for 4 hours) to prepare recalcined magnesium oxide. This process not only consumes enormous amounts of energy and has extremely high carbon emission intensity, but also limits the source of raw materials to regions such as Liaoning and Shandong. Simultaneously, large quantities of brucite tailings or some low-grade brucite contain impurities such as serpentine, dolomite, magnesite, calcite, and quartz, leading to resource waste and environmental pollution. However, existing technologies attempting to directly utilize brucite to prepare magnesium phosphate have found that even brucite tailings or low-grade brucite react too quickly, resulting in slow early strength development. Therefore, this invention aims to solve the triple technical problems of "high carbon emissions - resource waste - performance limitations," abandoning the traditional approach of pursuing a single high-purity periclase phase and instead focusing on the reconstruction and functional utilization of the mineral composition of brucite tailings. Through in-depth analysis, this invention has discovered that the low-temperature calcination range of 800℃ to 1250℃ has unique advantages: this temperature allows for sufficient dehydration of brucite to form a low-activity periclase phase, while also minimizing the conversion of associated silicate minerals into inert phases such as forsterite and calcium magnesium olivine. Even if some of these minerals are converted into inert phases and do not participate in the hydration reaction, they can still optimize the rheology of the slurry and the microstructure of the hardened body. Adding a small amount of borax during calcination can further lower the calcination temperature, significantly reducing the formation of calcium magnesium olivine and retaining more magnesium oxide. Therefore, this invention designs a process route for brucite tailings with a magnesium oxide content of 20%~55% or low-grade brucite through moderate crushing and medium-low temperature calcination, and constructs an optimized ratio system of calcined brucite tailings powder (40~80 parts), phosphate (20~45 parts), and retarder (0~10 parts). This design avoids the carbon emission defects of calcining magnesite at 1700℃, and solves the problems of excessive impurities in brucite tailings and excessively high activity of brucite when used directly.

[0032] After implementation, this invention not only successfully achieved low-carbon substitution of raw materials, but also achieved unexpected technical results during the experiment: In terms of setting time control, the setting time of magnesium phosphate cement prepared by traditional recalcined magnesium oxide is short and difficult to control, making it difficult to meet the needs of complex engineering projects, which greatly limits the application scenarios of magnesium phosphate cement. However, the setting time of the samples prepared by this invention can be precisely controlled within the range of 15.0~157.0 min (the longest can reach 157.0 min, and the shortest can reach 15.0 min). This is due to the synergistic effect of the silicate phase and the addition of borax during calcination, which not only retains more periclase, but also reduces the burden of periclase reactivity; In terms of strength development, the performance of multiple samples even surpasses that of traditional processes, with better compressive strength development at 3h and 1d. Surprisingly, XRD analysis revealed a coexisting structure of forsterite and periclase, which formed a unique "active-inert" dual-phase synergistic mechanism during hydration. The inert phase not only did not hinder hydration but also optimized particle packing density, resulting in more stable early strength development. Meanwhile, carbonaceous minerals such as dolomite and calcite, previously considered defects, showed significantly reduced carbon emissions during calcination at 800℃~1250℃ because the temperature did not reach the threshold for violent decomposition. Furthermore, the calcium magnesium olivine phase, formed by the reaction of its decomposition products with silicates, unexpectedly increased the microstructure density of the hardened body, thus enhancing the early strength of magnesium phosphate cement. Further verification revealed multiple superior advantages of this system: the calcination temperature is 350℃~800℃ lower than that of magnesite, resulting in a significant reduction in energy consumption; the complex mineral composition in the tailings, after calcination reconstruction, provides the cement with a wider performance adjustment window; and even more surprisingly, through proportion optimization, it is entirely possible to achieve a comprehensive improvement in strength indicators while maintaining rapid hardening characteristics, demonstrating the scientific feasibility of the waste-to-treasure technology path.

[0033] In some embodiments of this invention, the natural brucite used is composed of low-grade brucite and brucite tailings. In natural brucite resources, the grade is mainly determined by the magnesium oxide content and the types and proportions of impurity minerals. High-grade brucite typically refers to high-quality ore with a magnesium oxide content of over 60%, whose main mineral phase is pure brucite, and associated impurities (such as serpentine, calcite, magnesite, dolomite, and quartz) content is less than 10%. This type of ore, due to its high activity and stable composition, can be directly used in the preparation of high-end flame retardants, special ceramics, and high-purity magnesium oxide. Brushcite tailings or low-grade brucite refer to ore with a magnesium oxide content of less than 55%. In this type of low-grade brucite, the brucite phase accounts for approximately 50% to 70%, and it is also enriched with a large amount of impurity phases such as serpentine, calcite, magnesite, dolomite, and quartz. These brucite tailings are difficult to utilize economically and efficiently using traditional technologies and are thus stockpiled for a long time, not only occupying land resources but also potentially causing environmental pollution. Directly using brucite tailings to prepare magnesium phosphate cement presents challenges such as short setting time and low early strength. Therefore, this invention uses brucite tailings as raw material and processes it to obtain calcined brucite tailings powder. The calcined brucite tailings powder is obtained through the following method:

[0034] The brucite tailings are calcined at 800℃~1250℃ for 30min~120min, and the calcined product is then ground to obtain the calcined brucite tailings powder. Excessive calcination temperature leads to low magnesium oxide activity, resulting in a slow reaction during magnesium phosphate cement preparation; excessively low calcination temperature leads to the formation of highly active magnesium oxide, causing an overly rapid and uncontrollable reaction during magnesium phosphate cement preparation. Therefore, the calcination temperature can be 800℃, 950℃, 1100℃, 1250℃, and all ranges and sub-ranges therein. Excessive calcination time affects the activity of magnesium oxide, making subsequent reactions uncontrollable and increasing energy consumption; short calcination time leads to incomplete decomposition or the formation of highly active magnesium oxide, also resulting in uncontrollable reactions during use. Therefore, the calcination time can be 30min, 60min, 90min, 120min, and all ranges and sub-ranges therein. It should be understood that, in the implementation scheme, any of the above ranges can be combined with any other range.

[0035] In some embodiments of the present invention, it has been further found that borax is added to the brucite tailings during calcination; the amount of borax added, calculated by mass percentage, does not exceed 3 wt% of the mass of the brucite tailings. This not only reduces the calcination temperature but also reduces the formation of the calcium magnesium olivine inert phase while retaining more magnesium oxide, which is beneficial for the preparation of magnesium phosphate cement.

[0036] In some embodiments of the present invention, the particle size of the calcined brucite tailings powder is less than 150 μm. In actual use, coarse-grained calcined brucite tailings powder is not conducive to the strength development of magnesium phosphate cement. In this case, it is usually necessary to control the particle size or specific surface area of ​​the raw material within a suitable range.

[0037] In some embodiments of the present invention, the magnesium phosphate cement further includes a retarder; the amount used is 0 to 10 parts by weight. The retarder is borax. In the present invention, the addition of a retarder is to obtain a longer setting time and facilitate operation. The amount of retarder controlled within the range described in the present invention has little impact on the strength of the magnesium phosphate cement, but the amount of retarder should not be too high, otherwise it will affect the performance of the magnesium phosphate cement. Therefore, the amount of retarder can be 0, 1 part, 2 parts, 3 parts, 4 parts, 5 parts, 6 parts, 7 parts, 8 parts, 9 parts, 10 parts, etc., and all ranges and sub-ranges therebetween; it should be understood that in the embodiments, any of the above ranges can be combined with any other range.

[0038] In some embodiments of the present invention, the phosphate is one or more of ammonium dihydrogen phosphate, potassium dihydrogen phosphate, and sodium dihydrogen phosphate. In practical applications, using one or more of the phosphates described in the present invention in combination can achieve the technical effects described in the present invention.

[0039] II. Examples and Comparative Examples

[0040] (1) The relevant test methods and regulations are as follows:

[0041] 1. The test was conducted at a temperature of 20 ± 2℃ and a humidity of 65 ± 5℃.

[0042] 2. The setting time was tested using a Vicat apparatus in accordance with JC / T 2857-2024 "Magnesium Phosphate Cementitious Materials". Timing was started from the moment the magnesium phosphate cementitious material sample came into contact with water.

[0043] 3. The compressive strength test shall be conducted using a universal testing machine in accordance with the description in JC / T 2857-2024 "Magnesium Phosphate Cementitious Materials".

[0044] (2) Examples and Comparative Examples

[0045] Example 1

[0046] The method for preparing calcined brucite tailings powder for preparing magnesium phosphate cement in this embodiment includes the following steps:

[0047] (1) Crush the brucite tailings.

[0048] (2) Mix the above-mentioned magnesia tailings with 1.5wt% borax, then calcine at 1000℃ for 2h, and then remove and cool naturally.

[0049] (3) Grind all of the above calcined products through a square hole sieve with a pore size of 75 μm to obtain the calcined magnesia tailings powder.

[0050] The calcined brucite tailings powder was used to prepare magnesium phosphate cement samples. The specific proportions are shown in Table 1. During preparation, the components were mixed evenly.

[0051] Example 2

[0052] The method for preparing calcined brucite tailings powder for preparing magnesium phosphate cement in this embodiment includes the following steps:

[0053] (1) Crush the brucite tailings.

[0054] (2) The above-mentioned magnesia tailings and 1wt% borax mixture were calcined at 1100℃ for 1.5h, and then taken out and cooled naturally.

[0055] (3) Grind all of the above calcined products through a square hole sieve with a pore size of 75 μm to obtain the calcined magnesia tailings powder.

[0056] The calcined brucite tailings powder was used to prepare magnesium phosphate cement samples. The specific proportions are shown in Table 1. During preparation, the components were mixed evenly.

[0057] Example 3

[0058] The method for preparing calcined brucite tailings powder for preparing magnesium phosphate cement in this embodiment includes the following steps:

[0059] (1) Crush the brucite tailings.

[0060] (2) Mix the above-mentioned magnesia tailings with 3wt% borax, then calcine at 800℃ for 2h, and then remove and cool naturally.

[0061] (3) Grind all of the above calcined products through a square hole sieve with a pore size of 75 μm to obtain the calcined magnesia tailings powder.

[0062] The calcined brucite tailings powder was used to prepare magnesium phosphate cement samples. The specific proportions are shown in Table 1. During preparation, the components were mixed evenly.

[0063] Example 4

[0064] The method for preparing calcined brucite tailings powder for preparing magnesium phosphate cement in this embodiment includes the following steps:

[0065] (1) Crush the brucite tailings.

[0066] (2) Mix the above-mentioned magnesia tailings with 0.5wt% borax, then calcine at 1250℃ for 30min, and then remove and cool naturally.

[0067] (3) Grind all of the above calcined products through a square hole sieve with a pore size of 75 μm to obtain the calcined magnesia tailings powder.

[0068] The calcined brucite tailings powder was used to prepare magnesium phosphate cement samples. The specific proportions are shown in Table 1. During preparation, the components were mixed evenly.

[0069] Example 5

[0070] The method for preparing calcined brucite tailings powder for preparing magnesium phosphate cement in this embodiment includes the following steps:

[0071] (1) Crush the brucite tailings.

[0072] (2) The above-mentioned magnesia tailings and 1.5wt% borax mixture were calcined at 1150℃ for 1h, and then taken out and cooled naturally.

[0073] (3) Grind all of the above calcined products through a square hole sieve with a pore size of 75 μm to obtain the calcined magnesia tailings powder.

[0074] The calcined brucite tailings powder was used to prepare magnesium phosphate cement samples. The specific proportions are shown in Table 1. During preparation, the components were mixed evenly.

[0075] Example 6

[0076] The method for preparing calcined brucite tailings powder for preparing magnesium phosphate cement in this embodiment includes the following steps:

[0077] (1) Crush the brucite tailings.

[0078] (2) Mix the above-mentioned magnesia tailings with 1 wt% borax, then calcine at 1100℃ for 1 h, and then remove and cool naturally.

[0079] (3) Grind all of the above calcined products through a square hole sieve with a pore size of 75 μm to obtain the calcined magnesia tailings powder.

[0080] The calcined brucite tailings powder was used to prepare magnesium phosphate cement samples. The specific proportions are shown in Table 1. During preparation, the components were mixed evenly.

[0081] Example 7

[0082] The method for preparing calcined brucite tailings powder for preparing magnesium phosphate cement in this embodiment includes the following steps:

[0083] (1) Crush the brucite tailings.

[0084] (2) Mix the above-mentioned magnesia tailings with 2wt% borax, then calcine at 1050℃ for 1h, and then remove and cool naturally.

[0085] (3) Grind all of the above calcined products through a square hole sieve with a pore size of 75 μm to obtain the calcined magnesia tailings powder.

[0086] The calcined brucite tailings powder was used to prepare magnesium phosphate cement samples. The specific proportions are shown in Table 1. During preparation, the components were mixed evenly.

[0087] Comparative Example 1

[0088] The preparation method for magnesium phosphate cement using calcined magnesite includes the following steps:

[0089] (1) Crush the magnesite.

[0090] (2) The above magnesite powder was calcined at 1700℃ for 4 hours and then cooled naturally.

[0091] (3) After cooling the above calcined product, grind it all through a square hole sieve with a pore size of 75 μm to obtain recalcined magnesium oxide.

[0092] The calcined magnesium oxide prepared in Comparative Example 1 was used to prepare magnesium phosphate cement samples. The specific composition ratio is shown in Table 1. When preparing the samples, the components should be mixed evenly.

[0093] Comparative Example 2

[0094] A method for preparing magnesium phosphate cement using uncalcined brucite or tailings includes the following steps:

[0095] (1) Crush the brucite tailings to below 50 μm.

[0096] (2) Grind all the above-mentioned magnesia tailings particles through a square hole sieve with a pore size of 75 μm.

[0097] The brucite tailings powder prepared in Comparative Example 2 was used to prepare magnesium phosphate cement samples. The specific composition ratio is shown in Table 1. When preparing the samples, the components should be mixed evenly.

[0098] Table 1. Composition of magnesium phosphate cement samples (parts by mass)

[0099]

[0100] III. Application Test

[0101] Figure 1 The XRD pattern of the original brucite tailings is shown, which shows that in addition to the brucite phase, it also contains serpentine phase, dolomite phase, magnesite phase, calcite phase and quartz phase. Figure 2 , Figure 3 , Figure 4 , Figure 5 , Figure 6 , Figure 7 , Figure 8 and Figure 9 The XRD patterns are those of Examples 1, 2, 3, 4, 5, 6, 7, and Comparative Example 1, respectively. Figure 2 , Figure 3 , Figure 4 , Figure 5 , Figure 6 , Figure 7 and Figure 8 As can be seen, the calcined brucite tailings powder prepared by this invention, in addition to containing periclase phase and a small amount of quartz phase, also forms a large amount of forsterite phase, some calcium magnesium olivine phase, and enstatite phase. The impurity content and calcination regime affect the content of forsterite and calcium magnesium olivine phases. These two minerals are essentially inert and do not participate in hydration reactions. More importantly, after introducing no more than 3% borax during the calcination process, the formation of calcium magnesium olivine phase is significantly reduced, and more periclase phase is retained. Therefore, the calcined brucite tailings powder can be directly used to prepare magnesium phosphate cement. Figure 9 The image shows the XRD pattern of recalcined magnesium oxide obtained from calcined magnesite. The main phase composition is periclase, with no other impurities.

[0102] To verify the performance of magnesium phosphate cement prepared from calcined brucite tailings powder, the compressive strength of all the magnesium phosphate cement samples prepared above was determined according to the "Test Method for Strength of Cement Mortar" (GB / T 17671-2021), with a fixed water-cement ratio of 0.2. The test results are shown in Table 2.

[0103] Table 2 Performance test results of magnesium phosphate cement samples

[0104]

[0105] Combine Table 1-2 and Appendix Figure 1-8 Analysis shows that:

[0106] (1) Figure 1The complex mineral composition of untreated brucite tailings is clearly shown. Besides the main phase brucite, it also contains significant amounts of impurities such as serpentine, dolomite, magnesite, calcite, and quartz. The brucite phase exhibits excessively high hydration activity, and the presence of impurity minerals further exacerbates the uncontrollability of the reaction. Based on the results of Comparative Example 2, the direct use of uncalcined brucite to prepare magnesium phosphate cement presents a problem of difficulty in coordinating setting time and strength development. Although the spectrum shows the presence of carbonaceous minerals (magnesite, dolomite, and calcite), the CO2 emissions generated by the high-temperature calcination process are relatively low. Calcination treatment allows for a wider range of setting times (15.0–157.0 min), increases 3-hour strength to 20.4–44.9 MPa, 1-day strength to 32.5–66.8 MPa, and 7-day strength to 42.3–89.2 MPa. The core performance indicators fully meet engineering application standards, successfully solving the technical problem of the inability to directly apply untreated waste.

[0107] (2) Figure 2-8 The phase composition of calcined brucite tailings powder obtained under different calcination regimes in Examples 1-7 is shown. It can be seen that after calcination, the brucite phase completely disappears and is transformed into the periclase main phase. At the same time, some silicate minerals such as serpentine are transformed into the forsterite phase. Furthermore, with changes in calcination temperature and holding time, calcium magnesium olivine phase and enstatite phase appear. Figure 9 The phase composition of calcined magnesite powder is shown. Under traditional processes, calcined magnesite at 1700℃ for 4 hours yields a single-phase magnesium oxide, dominated by periclase, with no other impurity phases. The sharp and intense diffraction peaks of periclase indicate high crystallinity and low activity, resulting in limited performance adjustment. In contrast, the multiphase coexistence structure of this invention allows periclase to provide hydration activity to ensure strength, while inert phases (magnesia olivine and calcium magnesium olivine) regulate the reaction rate, forming a "activity-inertness" synergistic performance regulation system. This allows for flexible adjustment of the setting time and strength matching relationship according to requirements, overcoming the performance limitations of traditional single-phase structures.

[0108] (3) According to Figure 2-8The results clearly show that the content of magnesia-olivine and calcium magnesia-olivine phases increases with increasing calcination temperature. Furthermore, the increased calcination temperature further increases the crystallinity of the periclase phase, thus reducing its activity. This gradient change in phase composition allows for precise control of performance: for example, calcination at 1000℃ results in higher periclase activity; combined with a small amount of inert phase, a fast-hardening, high-strength product with a setting time of 15.0 min and a 3-hour strength of 32.6 MPa can be prepared (e.g., sample 2). Calcination at 1250℃ increases the inert phase content and reduces periclase activity, yielding long-term construction products with a setting time of 27.5–157.0 min and stable strength (e.g., samples 1 and 6), thus meeting the performance requirements of different construction scenarios. Compared to the single periclase phase structure of calcined magnesite, the presence of impurity phases endows the material with the potential for full-range performance regulation, ranging from "fast-hardening and high-strength" to "long-term slow-setting," thereby better adapting to different construction rhythm requirements.

[0109] (4) Due to the different mineral compositions of brucite and magnesite, the phase composition of the calcined powder also leads to significant differences in the performance of magnesium phosphate cement prepared using the calcined powder. As can be observed from Table 2, the setting time of magnesium phosphate cement prepared by traditional recalcined magnesium oxide is only 11.0 min (narrow control range, which is not conducive to the long-term operation requirements of complex projects), the 3-hour strength is 31.1 MPa, the 1-day strength is 48.3 MPa, and the 7-day strength is 63.5 MPa. The setting time of magnesium phosphate cement prepared from calcined brucite tailings powder can be precisely adjusted from 15.0 to 157.0 min (a range of 142.0 min, with a controllability far exceeding that of traditional processes). The 3-hour strength range is 20.1 to 44.9 MPa (with a maximum value of 44.9 MPa, an improvement of 44.4% compared to Comparative Example 1), and the 1-day strength range is 32.5 to 66.8 MPa (with a maximum value of 66.8 MPa, an improvement of 38.3% compared to Comparative Example 1). This allows for adjustable setting time while meeting strength requirements.

[0110] (5) According to the test results in Table 2, increasing the calcination temperature and holding time is beneficial to obtaining low-activity magnesium oxide, which is beneficial to the control of the performance of magnesium phosphate cement. For example, Sample 6 was prepared from the calcined powder obtained in Example 4 (calcined at 1250℃ for 30 min). Although the strength result was not as good as Comparative Example 1, the setting time could reach up to 157.0 min, and the 3-hour strength could reach 20.4 MPa, the 1-day strength 32.5 MPa, and the 7-day strength 42.3 MPa, which fully met the strength requirements of long-term construction scenarios. Sample 5 was prepared from the calcined powder of Example 5 (calcined at 1150℃ for 1 h). The setting time could also reach 78.5 min, the 3-hour strength 26.2 MPa, the 1-day strength 40.5 MPa, and the 7-day strength 54.9 MPa. The silicate phase reacts fully at high temperature to form a dense inert skeleton, which not only significantly delays the hydration reaction, but also ensures the stability of strength development under long setting time, avoiding the problem of "prolonged setting time inevitably leads to strength reduction" in traditional processes.

[0111] (6) Due to the large number of impurities in brucite tailings, inert phases such as magnesium olivine and calcium magnesium olivine will gradually form after calcination. Although this will not affect the hydration reaction of magnesium phosphate cement, it will consume some magnesium oxide to participate in the reaction. As described in Examples 2, 3, 4 and 7, adding no more than 3 wt% of borax by mass of brucite tailings during calcination can significantly reduce the amount of calcium magnesium olivine formed in the calcined brucite tailings powder, while retaining more magnesium oxide, which is beneficial to the performance development of magnesium phosphate cement. According to the results of sample 8 in Table 2, the prepared magnesium phosphate cement achieved a setting time of 30.5 min, a 3-hour strength of 44.9 MPa, a 1-day strength of 66.8 MPa, and a 7-day strength of 89.2 MPa, which is significantly better than Comparative Example 1.

[0112] (7) Retarder is an important factor in controlling the setting time of magnesium phosphate cement. In traditional processes, excessive retarder dosage can lead to a significant decrease in strength. However, as shown in Table 2, due to the inert regulatory effect of impurity phases in calcined brucite tailings powder, some samples can achieve better performance without the addition of retarder. For example, Sample 1, prepared by Example 4 (calcined at 1250℃ for 30 min), has a setting time of 27.5 min, and its early and late strengths are comparable to those of Comparative Example 1. Sample 9 (without retarder) has a setting time of 19.5 min, a 3-hour strength of 35.5 MPa, a 1-day strength of 50.2 MPa, and a 7-day strength of 69.4 MPa, achieving the optimal combination of "no retarder + high performance". However, by adjusting the amount of retarder, the setting time of Samples 6 and 10 can be extended to 40.0~157.0 min, but the strength is lower than that of the samples without retarder. Although sample 5 has a lower strength (26.2 MPa at 3 hours), its setting time reaches 78.5 minutes, and its 1-day and 7-day strengths can meet the needs of special engineering applications. This significantly reduces the dependence on retarder and improves the convenience of construction and cost advantages.

[0113] (8) This invention achieves directional reconstruction of the mineral phase structure by calcining brucite tailings and combining it with thermodynamic control, thus solving the technical defects of direct application of brucite and the environmental problems caused by the accumulation of brucite tailings. From a mineralogical perspective, the serpentine, dolomite and other impurity minerals coexisting in brucite ore undergo solid-phase reactions at high temperatures to generate silicate inert phases such as forsterite and calcium magnesium olivine. These inert phases remain chemically inert during hydration, do not consume effective hydration components, and can fill micropores by optimizing particle size distribution, thereby reducing the porosity of the hardened body and making the microstructure more compact. This compacted structure not only makes the early strength development more stable, but also improves the long-term durability of the material.

[0114] (9) This invention achieves the transformation of "waste" into high-performance materials through controlled calcination treatment of brucite tailings. Adding no more than 3% borax during calcination lowers the calcination temperature and reduces the formation of the calcium magnesium olivine phase, while retaining more magnesium oxide. The magnesium phosphate cement prepared from calcined brucite tailings powder not only achieves a wide adjustable setting time range of 15.0~157.0 min, but also ensures that the compressive strength meets requirements, realizing a leap from "low-grade raw materials to high-performance products," greatly expanding the application scenarios of brucite tailings.

[0115] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the technical solutions. Those skilled in the art should understand that any modifications or equivalent substitutions to the technical solutions of the present invention without departing from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A magnesium phosphate cement prepared using calcined brucite tailings, characterized in that, The magnesium phosphate cement comprises calcined brucite tailings powder and phosphate; wherein, by weight, the amount of calcined brucite tailings powder is 40-80 parts and the amount of phosphate is 20-45 parts. The calcined brucite tailings powder is obtained by calcining brucite tailings; the magnesium oxide content in the brucite tailings is 20%~55% by mass percentage.

2. The magnesium phosphate cement according to claim 1, characterized in that, The calcined brucite tailings powder is obtained by the following method: The calcined brucite tailings were calcined at 800℃~1250℃ for 30min~120min, and the calcined product was then ground to obtain the calcined brucite tailings powder.

3. The magnesium phosphate cement according to claim 2, characterized in that, Before calcination, the brucite tailings are crushed to a maximum particle size of less than 5 mm. Borax is added at the same time, and the amount of borax added shall not exceed 3 wt% of the mass of the brucite tailings.

4. The magnesium phosphate cement according to claim 2, characterized in that, The particle size of the calcined magnesia tailings powder is less than 150 μm.

5. The magnesium phosphate cement according to claim 1, characterized in that, The magnesium phosphate cement also includes a retarder; the amount of retarder is 0 to 10 parts by weight.

6. The magnesium phosphate cement according to claim 5, characterized in that, The retarder is borax.

7. The magnesium phosphate cement according to claim 1, characterized in that, The phosphate is one or more of ammonium dihydrogen phosphate, potassium dihydrogen phosphate, and sodium dihydrogen phosphate.

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