Preparation method of magnesium carbonate trihydrate pellets as a cement early strength agent

Abstract

The application provides a preparation method of magnesium carbonate trihydrate pellets as a cement early strength agent, and relates to the technical field of cement-based material preparation, and comprises the following steps: magnesium carbonate trihydrate powder is mixed with water or an alkaline solution and then is rolled and pressed; the rolled and pressed material is placed in a disc granulator to obtain near-spherical pellets; the near-spherical pellets are cured under a sealed condition, and after screening, magnesium carbonate trihydrate pellets are obtained. Magnesium carbonate trihydrate is a magnesium solid carbon fixation product that can be formed at normal temperature and pressure, and when it is applied to a silicate cement-based material as a solid carrier of CO2, it has the functions of inducing cement hydration and promoting the development of the early strength of the silicate cement. In the application, a thin layer of a phase is generated on the surface of the magnesium carbonate trihydrate crystal through in-situ reaction, the phase not only combines the magnesium carbonate trihydrate particles into near-spherical secondary particles with certain strength, but also inhibits the flash setting phenomenon caused by the rapid dissolution of the magnesium carbonate trihydrate in the cement slurry.

Description

Technical Field

[0001] This invention relates to the field of cement-based material preparation technology, and in particular to a method for preparing magnesium carbonate trihydrate pellets as a cement early strength agent. Background Technology

[0002] Carbon dioxide gas is a byproduct of silicate cement production and a greenhouse gas; approximately one ton of carbon dioxide is released for every ton of cement clinker produced. Currently, carbon dioxide gas is used in the production of precast cement components or products in factories, representing an effective way to recover and utilize carbon dioxide. Carbon dioxide gas promotes the hydration of silicate cement; not only does the hydration of clinker mineral C3S accelerate under the influence of carbon dioxide, but C2S also exhibits a similar hydration-promoting effect. This significantly improves the early strength of cement, which is highly beneficial for the strength development of road cement concrete. However, using carbon dioxide atmospheres to cure cement concrete in road engineering presents several difficulties. It not only increases curing costs but also poses safety hazards due to the use of carbon dioxide cylinders. Furthermore, the carbonation process of carbon dioxide promotes curing by gradually penetrating from the concrete surface into the interior, making this process relatively slow.

[0003] Based on the applicant's preliminary research, carbon dioxide gas was solidified within the crystal structure of magnesium carbonate trihydrate (MgCO3·3H2O), using magnesium carbonate trihydrate as a solid carrier for carbon dioxide. It was found that magnesium carbonate trihydrate has a setting-accelerating and early-strength-promoting effect on silicate cement. A small amount of magnesium carbonate trihydrate can shorten the final setting time of reference cement to less than 10 minutes. Invention patent CN110981257B, "An Alkali-Free and Chlorine-Free Concrete Accelerator Based on Magnesium Carbonate Trihydrate," applies magnesium carbonate trihydrate to concrete accelerators. Magnesium carbonate trihydrate dissolves in alkaline cement solutions and releases CO3. 2- Ions. When magnesium carbonate trihydrate is mixed evenly with cement powder and water is added, CO3 will quickly appear throughout the slurry inside the cement mixture. 2- A rapid increase in ion concentration is observed. A 1% addition of magnesium carbonate trihydrate can achieve a significant accelerating effect on setting. Additions exceeding 3% result in flash setting, causing the cement paste to quickly lose its workability.

[0004] To address the aforementioned issues, and in order to increase the content of magnesium carbonate trihydrate, prevent excessively rapid setting, introduce more carbon dioxide, and promote cement hydration, the applicant performed a surface inertization treatment on magnesium carbonate trihydrate and prepared it into spherical secondary particles. Research showed that incorporating magnesium carbonate trihydrate spherical particles into silicate cement does not cause rapid setting of the cement, thus not affecting the workability of the cement paste. After pouring, the magnesium carbonate trihydrate spherical particles release CO3 into the cement paste. 2- Ions can promote cement hydration. Summary of the Invention

[0005] This invention provides a method for preparing magnesium carbonate trihydrate granules as a cement accelerator. Through in-situ reaction, a thin phase is generated on the surface of magnesium carbonate trihydrate crystals. This phase not only binds the magnesium carbonate trihydrate particles into near-spherical secondary particles with a certain strength, but also inhibits the flash setting phenomenon caused by the rapid dissolution of magnesium carbonate trihydrate in cement slurry. When these magnesium carbonate trihydrate granules are mixed with cement slurry, they are uniformly dispersed in the slurry and do not affect the workability of the slurry. When magnesium carbonate trihydrate begins to dissolve in the cement slurry, it releases Mg... 2+ and CO3 2- Ions cause Ca to occur simultaneously inside the slurry. 2+ The decrease in concentration and pH value promotes the hydration of C3S and C2S minerals in cement clinker.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] This invention provides a method for preparing magnesium carbonate trihydrate pellets as a cement early strength agent, comprising forming magnesium carbonate trihydrate pellets and then generating a thin layer phase on the surface of the pellets, the specific steps of which are as follows:

[0008] S1. Place magnesium carbonate trihydrate powder and water or alkaline solution in a mixer, mix and then crush and filter out excess water.

[0009] S2. Place the crushed material into a disc granulator to obtain near-spherical particles;

[0010] S3. The near-spherical particles are cured under sealed conditions. After curing, a thin layer of phase with basic magnesium carbonate as the main component is formed on the surface of the magnesium carbonate trihydrate crystal.

[0011] S4. After drying the granules obtained from curing, they are sieved to obtain magnesium carbonate trihydrate granules.

[0012] Preferably, in step S1, the magnesium carbonate trihydrate can be columnar magnesium carbonate trihydrate crystals obtained by reacting a suspension of magnesium chloride, magnesium oxide, or magnesium hydroxide with water and CO2 at a temperature of 15°C to 40°C and at normal pressure. Alternatively, it can be columnar magnesium carbonate trihydrate crystals obtained by reacting soluble carbonates, such as ammonium carbonate, sodium carbonate, and potassium carbonate, with soluble magnesium salts, such as magnesium sulfate and magnesium chloride, at a temperature of 15°C to 40°C and at normal pressure. The magnesium carbonate trihydrate powder is the columnar magnesium carbonate trihydrate crystals prepared above and their crushed products.

[0013] Preferably, in step S1, the water is ordinary drinking water, distilled water, or deionized water; the alkaline solution is one of the following: a saturated or unsaturated solution of magnesium hydroxide, a saturated or unsaturated solution of calcium hydroxide, or a filtrate with a pH value ≥10 from the reaction of silicate cement with water.

[0014] Preferably, in step S1, the mass ratio of magnesium carbonate trihydrate powder to water or alkaline solution is 1:(0.2~1.0).

[0015] Preferably, in step S1, after mixing the magnesium carbonate trihydrate powder with water or an alkaline solution, the mixture is first stirred at a speed of 50-200 rpm for 0.5-5 min, and then stirred at a speed of 500-2000 rpm for 0.5-2 min, with a rolling pressure of 5-20 MPa.

[0016] Preferably, in step S2, the rotation speed of the disc granulator is 600~800 rpm. After granulation, near-spherical particles with a particle size of φ0.18~2.36mm are screened out, and particles with a particle size not within this range are returned to the granulator for granulation.

[0017] Preferably, in step S3, if the added medium is water, the curing conditions are moist heat curing at 60~80℃ for 10min~2d under sealed conditions; if the added medium is an alkaline solution, the curing conditions are curing at 20~80℃ for 10min~7d under sealed conditions.

[0018] Preferably, in step S3, the phases formed on the surface of the magnesium carbonate trihydrate crystals are mainly spheroidal magnesium carbonate, hydromagnesia, fibrous hydromagnesia, etc., and the above phases all belong to basic magnesium carbonate. The mass percentage of the above phases in the magnesium carbonate trihydrate spheroids is ≤1.0%.

[0019] Preferably, in step S4, the drying temperature is 50~60℃, and the dried particles are sieved through 80 mesh, 32 mesh, 16 mesh, and 8 mesh sieves to form magnesium carbonate trihydrate granules with three particle size ranges: 0.18~0.50, 0.50~1.00, and 1.00~2.36 mm. These three particle size ranges correspond to the early strength development requirements at different ages.

[0020] The present invention also provides magnesium carbonate trihydrate granules prepared by the method.

[0021] The present invention also provides the application of the aforementioned magnesium carbonate trihydrate pellets as a cement early strength agent. After magnesium carbonate trihydrate is made into pellets, a thin layer of phase is formed on the surface, which can make its incorporation amount in cement ≥5% (by mass).

[0022] The beneficial effects of the technical solution of this invention are as follows:

[0023] This invention uses magnesium trihydrate, a magnesium-based carbon fixation product, as a solid carrier for carbon dioxide, overcoming many difficulties associated with carbon dioxide atmosphere curing of cement concrete. It also solves the problem of low magnesium trihydrate content in cement. Because magnesium trihydrate granules have good dispersibility in cement paste, the hydration reaction process is accelerated throughout the cement paste, inducing crystallization of hydration products and greatly promoting the early strength development of cement.

[0024] Specifically, the rotation speed gradually increases in step S1 to ensure thorough mixing of the solution with magnesium carbonate trihydrate. Different rotation speeds should be used when mixing different media with magnesium carbonate trihydrate to make the needle-like crystals more resilient after stirring. Different pressure conditions in step S1 allow for the formation of more tightly bound magnesium carbonate trihydrate granules in step S2 using a disc granulator. Finally, in step S3, the type and quantity of the thin-layer phase formed on the surface of the magnesium carbonate trihydrate granules are controlled by the curing temperature and time. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 Photographs of magnesium carbonate trihydrate granules prepared in Example 1 of this invention.

[0027] Figure 2 This is the SEM morphology of the internal crystals of magnesium carbonate trihydrate spheroids in Example 1 of the present invention.

[0028] Figure 3 Phase analysis of the composition of magnesium carbonate trihydrate spheroids in this invention. Detailed Implementation

[0029] To make the technical problems, technical solutions and advantages of the present invention clearer, a detailed description will be given below in conjunction with the accompanying drawings and specific embodiments.

[0030] This invention provides a method for preparing magnesium carbonate trihydrate granules as a cement accelerator. Through in-situ reaction, a thin phase is generated on the surface of magnesium carbonate trihydrate crystals. This phase not only binds the magnesium carbonate trihydrate particles into near-spherical secondary particles with a certain strength, but also inhibits the flash setting phenomenon caused by the rapid dissolution of magnesium carbonate trihydrate in cement slurry. The addition of magnesium carbonate trihydrate granules to cement does not cause rapid setting of the cement, nor does it affect the workability of the cement slurry. After pouring, the magnesium carbonate trihydrate granules release CO3 into the cement slurry. 2-Ions can promote cement hydration.

[0031] Magnesium carbonate trihydrate is a magnesium-based carbon-fixing product that can be formed at room temperature and pressure. It can be obtained by directly reacting magnesium salts, magnesium oxide, or magnesium hydroxide with carbon dioxide, or indirectly through the reaction of soluble carbonates. Magnesium carbonate trihydrate crystals are columnar and can be used directly or after being crushed.

[0032] Magnesium carbonate trihydrate powder is mixed with 0.2 to 1.0 times its mass of water or alkaline solution in a mixer. This mass ratio theoretically produces the maximum amount of basic magnesium carbonate. The mixture is first slowly stirred at 50 to 200 rpm for 0.5 to 5 minutes, then rapidly stirred at 500 to 2000 rpm for 0.5 to 2 minutes. The smaller the ratio of water or solution to magnesium carbonate trihydrate, the lower the mixing speed should be. Depending on the medium ratio, the mixture exists in three states: granular, agglomerated, and paste-like. The mixture is then compacted in a mill with an adjustable pressure of 5 MPa to 20 MPa. Granular mixtures can aggregate into larger granules after compaction, while paste-like mixtures may exude liquid after compaction, which needs to be filtered out. After compaction, the magnesium carbonate trihydrate in the mixture is more densely distributed. The compacted material is then granulated into small particles in a granulator, followed by rotary granulation to obtain near-spherical composite particles.

[0033] The water mentioned above refers to ordinary drinking water, distilled water, deionized water, etc. The alkaline solutions are solutions with a pH value of not less than 10, such as saturated or unsaturated magnesium hydroxide solutions, saturated or unsaturated calcium hydroxide solutions, and filtrates from the reaction of silicate cement with water. The silicate cements include general-purpose silicate cements such as PI and PII type silicate cement, PO ordinary silicate cement, PS slag silicate cement, PF fly ash silicate cement, PP pozzolanic silicate cement, and PC composite silicate cement, as well as PR road silicate cement, etc. The filtrate from the reaction of silicate cement with water is the slurry filtrate from cement reacting with more than 10 times its weight of water for more than 2 hours.

[0034] The above-mentioned particles, using water as a medium, are cured under sealed conditions at a temperature of 60℃~80℃ for 10 minutes to 2 days with humid heat; the higher the temperature, the shorter the curing time. When the temperature exceeds 70℃, the curing time is controlled below 30 minutes. Particles using an alkaline solution as a medium are cured under sealed conditions at a temperature of 20℃~80℃ for 10 minutes to 7 days; when the temperature exceeds 70℃, the curing time is controlled below 30 minutes. This invention selects different curing conditions according to different media to generate basic magnesium carbonate on the surface of trihydrate magnesium carbonate, thereby obtaining spherical particles.

[0035] When water is used as the medium, due to kinetic factors, magnesium carbonate trihydrate will only transform into spheroidal magnesia [4MgCO3·Mg(OH)2·5H2O] and hydromagnesite [4MgCO3·Mg(OH)2·4H2O] according to the reactions shown in formulas (1) and (2) after being cured in a humid environment at a temperature above 60℃. These two reactions are accelerated after 70℃, and the hydromagnesite is more easily formed as the curing temperature increases.

[0036] 5 (MgCO3·3H2O) → 4MgCO3·Mg(OH)2·5H2O + CO2+9 H2O (1)

[0037] 5 (MgCO3·3H2O) → 4MgCO3·Mg(OH)2·4H2O + CO2+10 H2O (2)

[0038] When Mg(OH)₂ solution is used as the medium, the phases of magnesium carbonate trihydrate change according to equations (3) and (4). Both reactions can be carried out at room temperature, and the reaction accelerates with increasing temperature.

[0039] 4 (MgCO3·3H2O) + Mg(OH)2 → 4MgCO3·Mg(OH)2·5H2O +7 H2O (3)

[0040] 4 (MgCO3·3H2O) + Mg(OH)2 → 4MgCO3·Mg(OH)2·4H2O + 8 H2O (4)

[0041] When Ca(OH)2 solution (including cement hydration slurry) is used as the medium, the transformation of magnesium carbonate trihydrate follows equations (5) and (6). These two reactions can be carried out at room temperature, and the reaction accelerates with increasing temperature.

[0042] 5 (MgCO3·3H2O) + Ca(OH)2 → 4MgCO3·Mg(OH)2·5H2O + CaCO3 + 10 H2O (5)

[0043] 5 (MgCO3·3H2O) + Ca(OH)2 → 4MgCO3·Mg(OH)2·4H2O + CaCO3 + 11 H2O (6)

[0044] As can be seen from the above process, the reaction of magnesium carbonate trihydrate to basic magnesium carbonate releases CO2 when water is used as the medium. When an alkaline solution is used as the medium, the reaction does not release CO2, making it more environmentally friendly.

[0045] The granules obtained after curing are dried in a drying oven at a temperature not exceeding 60℃ for 24 hours. The dried granules are then sieved through 80-mesh, 32-mesh, 16-mesh and 8-mesh sieves to form magnesium carbonate trihydrate granules with three particle size ranges: 0.18~0.50, 0.50~1.00 and 1.00~2.36 mm.

[0046] This invention generates a thin-layer phase on the surface of magnesium carbonate trihydrate crystals through in-situ reaction. The higher the curing temperature, the shorter the time required for the thin-layer phase to form. When the mixing medium is an alkaline solution, even at a low curing temperature, a sufficiently long curing time will still result in the formation of the thin-layer phase. The alkaline solution combines with the magnesium carbonate trihydrate particles to form near-spherical secondary particles with a certain strength, while also inhibiting the flash setting phenomenon caused by the rapid dissolution of magnesium carbonate trihydrate in cement slurry. When these magnesium carbonate trihydrate spherical particles are mixed with cement slurry, they are uniformly dispersed in the cement slurry without affecting the workability of the slurry. When magnesium carbonate trihydrate begins to dissolve in the cement slurry, it releases Mg... 2+ and CO3 2- Ions cause Ca to occur simultaneously inside the slurry. 2+ The decrease in concentration and pH value promotes the hydration of C3S and C2S minerals in cement clinker. Applying magnesium carbonate trihydrate as a solid carrier of CO2 in cement-based materials can induce cement hydration and promote the early strength development of cement.

[0047] The following description, in conjunction with specific embodiments, illustrates this point.

[0048] Example 1

[0049] Magnesium carbonate trihydrate powder was placed in a mixer with 0.3 times its weight of water. The mixture was first slowly stirred at 100 rpm for 1 minute, then rapidly stirred at 1000 rpm for 1 minute. The homogeneous mixture was removed from the mixer and placed in a roller press, where it was first rolled clockwise 5 times and then counterclockwise 5 times at 5 MPa. The rolled material was then placed in a disc granulator, with the speed adjusted to 700 rpm. After the granules were prepared, near-spherical particles with a diameter of φ0.18~2.36 mm were sieved, and particles outside this range were returned to the granulator for further granulation. The granules were then cured under sealed conditions at 60℃ for 48 hours. After curing, they were dried in a drying oven at 60℃ for 24 hours and cooled to room temperature to obtain magnesium carbonate trihydrate granules with good strength. The parameters are shown in Table 1.

[0050] The morphology of the magnesium carbonate trihydrate spherical particles prepared in this embodiment is as follows: Figure 1 As shown, the particle size range is φ0.18~2.36mm, with good sphericity and high strength. Figure 2The morphology of magnesium carbonate trihydrate particles inside the spheroids after curing is shown. Magnesium carbonate trihydrate is initially a smooth, needle-like crystal. When basic magnesium carbonate plate-like crystals form on the surface, a change in crystal surface morphology is observed. The thin phase layer formed on the surface of the magnesium carbonate trihydrate particles is less than 100 nm thick. The XRD pattern shows the characteristic peak of basic magnesium carbonate, which was absent in the original magnesium carbonate trihydrate, thus proving the formation of basic magnesium carbonate.

[0051] Figure 3 For the phase analysis of the composition of magnesium carbonate trihydrate spheroids of the present invention, wherein 1 # XRD patterns of magnesium carbonate trihydrate raw material, 2 # The XRD pattern of the product from Example 1 shows that the surface phase of the spherulites is mainly basic magnesium carbonate, while the interior remains magnesium carbonate trihydrate. Furthermore, the thin layer of phase on the surface of the magnesium carbonate trihydrate crystals is spheroidal magnesia, and combined with thermogravimetric analysis, the spheroidal magnesia content is less than 1%, with a spherulite density of approximately 1.0 g / cm³. 3 .

[0052] When using PO 42.5 ordinary Portland cement, and the amount of magnesium carbonate trihydrate pellets added is 5% of the cement mass, the mortar mixture has normal fluidity, and the compressive strength of the mortar samples after 1 day and 3 days is increased by 26.4% and 21.5% respectively compared with the sample without the additive.

[0053] Example 2

[0054] Other conditions were the same as in Example 1, except that the medium mixed with magnesium carbonate trihydrate was a saturated magnesium hydroxide solution, as shown in Table 1. The spherical magnesium carbonate trihydrate particles prepared in this example were tested and found to have good sphericity and high strength; the surface-forming phase was spheroidal magnesium carbonate, with a content of less than 1% and a bulk density of approximately 1.2 g / cm³. 3 .

[0055] When using PO 42.5 ordinary Portland cement, and the amount of magnesium carbonate trihydrate pellets added is 10% of the cement mass, the mortar mixture has normal fluidity, and the compressive strength of the mortar samples after 1 day and 3 days is increased by 33.2% and 28.0% respectively compared with the sample without the addition.

[0056] Example 3

[0057] Other conditions were the same as in Example 1, except that the medium used for mixing with magnesium carbonate trihydrate was a slurry filtrate of P.O42.5 cement and 10 times its volume of water reacted for 6 hours, with a pH of 12.67, and the amount added was 0.3 times the mass of the magnesium carbonate trihydrate powder. During mixing, the mixture was first slowly stirred at 80 rpm for 2 minutes, followed by rapid stirring at 800 rpm for 0.5 minutes. The compaction pressure was adjusted to 5 MPa, and the curing temperature was reduced from 60℃ to 40℃, as shown in Table 1. The spherical magnesium carbonate trihydrate particles prepared in this example showed good sphericity and high strength; the surface phase was mainly spheroidal magnesium carbonate, with a small amount of calcite also found, the total content of which was less than 1%, and the particle density was approximately 0.9 g / cm³. 3 .

[0058] When using PO 42.5 ordinary Portland cement, and the amount of magnesium carbonate trihydrate pellets added is 8% of the cement mass, the mortar mixture has normal fluidity, and the compressive strength of the mortar samples after 1 day and 3 days is increased by 28.3% and 30.5% respectively compared with the sample without the addition.

[0059] Comparative Example 1

[0060] Other conditions were the same as in Example 1, except that the damp heat curing temperature was 100°C and the curing time was 0.5 h. The spherical particles prepared in this comparative example were tested and found to have good sphericity and high strength. Figure 3 Middle 3 # To compare the XRD pattern of the product from Example 1, it is evident that a large amount of spheroidized magnesia and hydromagnesia are formed, accounting for over 70% of the total content, while residual magnesium carbonate trihydrate is less than 30%. The prepared non-trihydrate magnesium carbonate spheroids have a spheroid density of approximately 0.6 g / cm³. 3 .

[0061] When using PO 42.5 ordinary Portland cement, with magnesium carbonate trihydrate pellets added at 5% of the cement mass, the mortar mixture exhibited normal fluidity. However, the compressive strength of the mortar samples after 1 day and 3 days increased by only 0.5% and 2.1% respectively compared to the sample without the additive. This indicates that the curing temperature was too high, and since the main phase of the pellets was not magnesium carbonate trihydrate, the strength could not be improved.

[0062] Comparative Example 2

[0063] Other conditions were the same as in Example 2, except that the rolling pressure was changed to 0.5 MPa. The magnesium carbonate trihydrate particles prepared in this comparative example were tested and found to have poor sphericity and low strength; the thin-layer phase on the surface of the magnesium carbonate trihydrate crystals was spheroidal magnesia, with a content of less than 1%, and a spheroidal density of approximately 0.6 g / cm³. 3 .

[0064] When the amount of magnesium carbonate trihydrate pellets added is 5% of the cement mass, the pellets break during the mixing process due to the excessively low compaction pressure, resulting in poor fluidity of the mortar mixture and difficulty in casting. The compressive strength of the mortar samples after 1 day and 3 days increased by only 1.5% and 6.4% respectively compared to the samples without the addition.

[0065] Table 1 Comparison of Embodiments and Comparative Examples of the Invention

[0066]

[0067] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing magnesium carbonate trihydrate pellets as a cement early-strength agent, characterized in that, Includes the following steps: S1. Place magnesium carbonate trihydrate powder and water or alkaline solution in a mixer, mix and then roll it with a rolling pressure of 5-20 MPa, and filter out excess water. S2. Place the crushed material into a disc granulator to obtain near-spherical particles; S3. The near-spherical particles are cured under sealed conditions; S4. After drying the granules obtained from curing, they are sieved to obtain magnesium carbonate trihydrate granules. In step S3, the phase formed on the surface of magnesium carbonate trihydrate crystals after curing is basic magnesium carbonate, and the mass percentage of basic magnesium carbonate in the magnesium carbonate trihydrate spheres is ≤1.0%.

2. The method for preparing magnesium carbonate trihydrate pellets as a cement early-strength agent according to claim 1, characterized in that, In step S1, the magnesium carbonate trihydrate is obtained by reacting a suspension of magnesium chloride, magnesium oxide, or magnesium hydroxide with water and CO2 at a temperature of 15°C to 40°C and at normal pressure to obtain columnar magnesium carbonate trihydrate crystals; or by reacting soluble carbonates with soluble magnesium salts at a temperature of 15°C to 40°C and at normal pressure to obtain columnar magnesium carbonate trihydrate crystals.

3. The method for preparing magnesium carbonate trihydrate pellets as a cement early-strength agent according to claim 2, characterized in that, In step S1, the water is drinking water, distilled water, or deionized water; the alkaline solution is one of the following: a saturated or unsaturated solution of magnesium hydroxide, a saturated or unsaturated solution of calcium hydroxide, or a filtrate with a pH value ≥10 obtained from the reaction of silicate cement with water.

4. The method for preparing magnesium carbonate trihydrate pellets as a cement early-strength agent according to claim 3, characterized in that, In step S1, the mass ratio of magnesium carbonate trihydrate powder to water or alkaline solution is 1:(0.2-1.0).

5. The method for preparing magnesium carbonate trihydrate pellets as a cement early-strength agent according to claim 4, characterized in that, In step S1, after mixing magnesium carbonate trihydrate powder with water or alkaline solution, the mixture is first stirred at 50-200 rpm for 0.5-5 min, and then stirred at 500-2000 rpm for 0.5-2 min.

6. The method for preparing magnesium carbonate trihydrate pellets as a cement early-strength agent according to claim 1, characterized in that, In step S2, the rotation speed of the disc granulator is 600-800 rpm, and after granulation, near-spherical particles with a particle size of φ0.18-2.36 mm are screened out.

7. The method for preparing magnesium carbonate trihydrate pellets as a cement early-strength agent according to claim 1, characterized in that, In step S3, if the added medium is water, the curing conditions are: under sealed conditions, moist heat curing at 60-80℃ for 10 min to 2 days; if the added medium is an alkaline solution, the curing conditions are: under sealed conditions, curing at 20-80℃ for 10 min to 7 days.

8. Magnesium carbonate trihydrate pellets prepared by the method according to any one of claims 1 to 7.

9. The application of the magnesium carbonate trihydrate granules as a cement early-strength agent according to claim 8, characterized in that, The amount of the magnesium carbonate trihydrate pellets incorporated into the cement is ≥5% (by mass).

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