Carbonized basic magnesium sulfate cement and preparation method thereof

Abstract

The invention belongs to the technical field of basic magnesium sulfate cement, and particularly relates to carbonized basic magnesium sulfate cement and a preparation method thereof. According to the invention, BMSC is used as a raw material, carbon dioxide carbonization treatment is carried out, carbon dioxide and BMSC cement are subjected to carbonation reaction, insoluble substances such as magnesium carbonate and calcium carbonate are generated, pores of the BMSC cement are filled with the insoluble substances, pore size distribution is refined, and rapid carbon sequestration enhancement of cement is realized without adding other additives; therefore, the density and compressive strength of the BMSC cement material are improved. Meanwhile, carbon dioxide is absorbed and fixed, and carbon emission is reduced. The preparation method provided by the invention is simple, efficient and low in cost, the comprehensive performance of the BMSC material is expected to be greatly improved, and the application of the BMSC material in green buildings is promoted.

Description

Technical Field

[0001] The invention belongs to the technical field of basic magnesium sulfate cement, and particularly relates to carbonized basic magnesium sulfate cement and a preparation method thereof. Background Art

[0002] Basic magnesium sulfate cement (BMSC) is a new type of cementitious material prepared through a hydration reaction using magnesium oxide and magnesium sulfate as its primary raw materials. Compared to traditional cement, BMSC offers advantages such as rapid hardening, early strength, low alkali content, corrosion resistance, and freeze-thaw resistance. It holds broad application prospects in areas such as rapid repair, decoration, and green building.

[0003] However, existing BMSC materials still suffer from insufficient mechanical properties, with products prone to cracking and peeling, limiting their widespread application. This is primarily due to the large crystal size and loose needle-like structure of BMSC hydration products. Therefore, optimizing the microstructure of BMSC and improving its density and strength remains a key technical challenge.

[0004] In recent years, researchers have conducted extensive research attempting to improve the performance of BMSCs through methods such as admixture and composite modification. For example, one technique employs a combination of Portland cement and BMSC, increasing 28-day compressive strength by 15-20%. Another technique, by reinforcing BMSCs with carbon fibers, has boosted flexural strength by over 30%. However, these methods generally suffer from complex processes and high costs.

[0005] On the other hand, cement releases large amounts of CO2 during its production and use, making it a major source of industrial carbon emissions. Reducing cement's carbon emissions and achieving both CO2 reduction and resource utilization are pressing challenges in the building materials industry. Summary of the Invention

[0006] The present invention aims to provide a carbonized basic magnesium sulfate cement and a preparation method thereof. The preparation method provided by the present invention significantly improves the mechanical properties of BMSC materials, resulting in a carbonized basic magnesium sulfate cement with higher compressive strength and lower porosity. Furthermore, the preparation method provided by the present invention does not require complex synthesis steps or additives, is low-cost, and is easily scalable for industrial-scale production. It also achieves carbon dioxide fixation, reducing carbon emissions during the BMSC production process and significantly improving the sustainability of BMSC materials.

[0007] In order to achieve the above object, the present invention provides the following technical solutions:

[0008] The present invention provides a method for preparing carbonized basic magnesium sulfate cement, comprising the following steps:

[0009] Magnesium oxide, magnesium sulfate, a composite modifier and water are mixed to obtain magnesium sulfate cement paste; the magnesium oxide contains α-MgO;

[0010] placing the magnesium sulfate cement slurry in a mold for hardening and then demoulding to obtain a molded product;

[0011] subjecting the molded product to a first stage of standard air curing to obtain a first air-cured product;

[0012] placing the first air-cured product in a carbon dioxide atmosphere for carbonization to obtain a carbonized product;

[0013] The carbonized product is subjected to a second stage of standard air curing to obtain the carbonized basic magnesium sulfate cement.

[0014] Preferably, the magnesium oxide is light-burned magnesium oxide, and the mass percentage of α-MgO in the magnesium oxide is 60-65%.

[0015] Preferably, the composite modifier comprises organic acid, sulfate and phosphate; the mass ratio of organic acid, sulfate and phosphate in the composite modifier is 1-2:2-3:4-5.

[0016] Preferably, the mass percentage of the composite modifier to the mass percentage of the magnesium oxide is 0.1-3%.

[0017] Preferably, the hardening conditions include: an ambient temperature of 20°C ± 5°C, an ambient relative humidity of 60 ± 5%, and a time of 12 to 24 hours;

[0018] The conditions of the first stage standard air curing include: ambient temperature of 20°C ± 5°C, ambient relative humidity of 60±5%, and duration of 10 to 13 days.

[0019] Preferably, the carbonization conditions include: an initial relative pressure of carbon dioxide of 0 to 0.5 MPa, a temperature of 25 to 60° C., and a holding time of 2 to 48 hours.

[0020] Preferably, after obtaining the first air-cured product, before performing the carbonization, the process further includes: subjecting the first air-cured product to carbonization pre-curing to obtain a carbonized pre-cured product; and subjecting the carbonized pre-cured product to the carbonization; the carbonization pre-curing conditions include: an ambient temperature of 20°C ± 5°C, an ambient relative humidity of > 95%, and a time of 12 to 24 hours.

[0021] Preferably, the conditions of the second stage standard air curing include: an ambient temperature of 20°C ± 5°C, an ambient relative humidity of 60 ± 5%, and a duration of 10 to 13 days.

[0022] Preferably, the mixing comprises the following steps:

[0023] dissolving the magnesium sulfate in water to obtain a magnesium sulfate aqueous solution;

[0024] The magnesium sulfate aqueous solution and the composite modifier are first mixed to obtain a first mixture;

[0025] The first mixture and the magnesium oxide are mixed for the second time to obtain magnesium sulfate cement slurry.

[0026] The present invention provides carbonized basic magnesium sulfate cement prepared by the preparation method described in the above technical solution.

[0027] The present invention provides a method for preparing carbonized basic magnesium sulfate cement, comprising the following steps: mixing magnesium oxide, magnesium sulfate, a composite modifier, and water to obtain a magnesium sulfate cement slurry; the magnesium oxide containing α-MgO; placing the magnesium sulfate cement slurry in a mold for hardening and then demolding to obtain a molded product; subjecting the molded product to a first stage of standard air curing to obtain a first air-cured product; subjecting the first air-cured product to carbonization in a carbon dioxide pressure atmosphere to obtain a carbonized product; and subjecting the carbonized product to a second stage of standard air curing to obtain the carbonized basic magnesium sulfate cement. The present invention utilizes BMSC as a raw material and undergoes a carbonization treatment with carbon dioxide. The carbon dioxide reacts with the BMSC cement to form insoluble substances such as magnesium carbonate and calcium carbonate (wherein calcium carbonate is an impurity in the raw material MgO and the magnesium oxide contains approximately 2 wt% CaO). These substances fill the pores of the BMSC cement, refine the pore size distribution, and achieve rapid carbon fixation and enhancement of the cement without the addition of other admixtures, thereby improving the density and compressive strength of the BMSC cement material. Furthermore, the carbon dioxide is absorbed and fixed, reducing carbon emissions.

[0028] Compared with existing technologies, the preparation method provided by this invention is simple, efficient, and low-cost, and is expected to significantly improve the overall performance of BMSC materials and promote their application in green buildings. Furthermore, the preparation method provided by this invention, combined with CO2 capture and utilization (CCU) technology, provides a new approach to alleviating the pressure on carbon emissions reduction in the construction industry, and is of great significance for the development of low-carbon cement and the promotion of sustainable development in the construction industry.

[0029] The results of the examples demonstrate that the preparation method provided by the present invention significantly improves the mechanical properties of BMSC materials. Compared to the uncarbonized control sample, the compressive strength of the carbonized BMSC specimens produced by the present invention was significantly improved at 14 days (the product after carbonization) and at 28 days (the product after the second stage of standard air curing). This increase in compressive strength is primarily due to the reaction of CO2 with the cement matrix during the carbonization process to form strong phases such as magnesium carbonate, which optimizes the pore structure of the BMSC, resulting in a denser matrix and more closely packed crystals, forming a continuous three-dimensional network.

[0030] The preparation method provided by the present invention reduces the porosity of BMSC materials and improves their density. Scanning electron microscopy results confirm that after carbonization treatment, carbonate products such as magnesium carbonate fill the pores within the BMSC, significantly reducing porosity and increasing the density of the matrix. The reduction in porosity is primarily due to the carbonation reaction between CO2 and cement hydration products, resulting in the deposition of carbonate products within the pores. MIP and X-CT test results confirm the improved microstructure and pore distribution of the BMSC.

[0031] The preparation method provided by the present invention is simple to operate and the process parameters are easy to control. The present invention uses conventional BMSC as raw material and adopts CO2 gas for carbonization, does not require complicated synthesis steps and special equipment, and is easy to achieve industrial-scale production.

[0032] The preparation method provided by this invention achieves significant carbon sequestration, significantly improving the sustainability of BMSC materials. CO2 sequestration not only reduces carbon emissions during BMSC production but also enhances material strength and extends its service life through the formation of carbonates. This reduces carbon emissions throughout the BMSC lifecycle, resulting in significant environmental and social benefits.

[0033] In summary, the present invention effectively addresses the technical challenges of BMSC applications by improving material properties and promoting CO2 sequestration. The preparation method provided by the present invention offers a simple carbonization process, requiring no major changes to existing production processes, and exhibits significant results, promising broad application prospects in engineering fields such as construction and road construction.

[0034] Furthermore, in the present invention, the carbonization conditions include: a relative carbon dioxide pressure of 0-0.5 MPa, a temperature of 25-60°C, and a holding time of 2-48 hours. By strictly controlling the CO₂ pressure, temperature, and time during the carbonization process, the present invention can increase the carbonate content of BMSC by more than 2 times and significantly improve the CO₂ carbon fixation rate.

[0035] Furthermore, in the present invention, after obtaining the first air-cured product and before performing the carbonization, the process further includes: subjecting the first air-cured product to carbonization pre-curing to obtain a carbonized pre-cured product; and subjecting the carbonized pre-cured product to the carbonization process; wherein the carbonization pre-curing conditions include: an ambient temperature of 20°C ± 5°C, an ambient relative humidity greater than 95%, and a duration of 12 to 24 hours. By performing the carbonization pre-curing under relatively high relative humidity conditions before carbonization, the present invention can ensure a better carbonization effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 SEM microstructure observation and EDS characterization (5,000×) of the main products in the products prepared in Examples and Comparative Examples. Figure 1 Figure a is the AC-14 test result. Figure 1 Figure b is the CP5-14 test result. Figure 1 Figure c shows the test results of CP5-28;

[0037] Figure 2 Raman spectra of AC-14 and CP5-14;

[0038] Figure 3 Phase distribution of AC-14 and CP5-14 (Raman surface analysis cloud map);

[0039] Figure 4 XRD patterns of AC-14, CP0-14 and CP5-14 embodiments;

[0040] Figure 5 XRD patterns of AC-28, CP0-28, and CP5-28;

[0041] Figure 6 The main phase contents of AC, CP0 and CP5 at 14-day and 28-day ages were obtained by XRD.

[0042] Figure 7 The results of thermogravimetric infrared spectroscopy (TG-IR) and mid-infrared spectroscopy (FTIR) analysis of AC, CP0, and CP5 at 14 and 28 days of age are shown.

[0043] Figure 8 MIP pore size distribution curves of AC-14, CP0-14, CP5-14, and CP5-28;

[0044] Figure 9 is the MIP pore volume fraction of AC-14, CP0-14, CP5-14, and CP5-28;

[0045] Figure 10X-CT three-dimensional pore distribution of AC-14, CP0-14, CP5-14 and CP5-28;

[0046] Figure 11 DTG differential thermogravimetric curves in TG-IR of AC-14, CP0-14, and CP5-14;

[0047] Figure 12 DTG differential thermogravimetric curves in TG-IR of AC-28, CP0-28, and CP5-28;

[0048] Figure 13 Compressive strength of AC, CP0, CP1, CP3 and CP5 at 14 days and 28 days. DETAILED DESCRIPTION

[0049] The present invention provides a method for preparing carbonized basic magnesium sulfate cement, comprising the following steps:

[0050] Magnesium oxide, magnesium sulfate, a composite modifier and water are mixed to obtain magnesium sulfate cement paste; the magnesium oxide contains α-MgO;

[0051] placing the magnesium sulfate cement slurry in a mold for hardening and then demoulding to obtain a molded product;

[0052] subjecting the molded product to a first stage of standard air curing to obtain a first air-cured product;

[0053] placing the first air-cured product in a carbon dioxide atmosphere for carbonization to obtain a carbonized product;

[0054] The carbonized product is subjected to a second stage of standard air curing to obtain the carbonized basic magnesium sulfate cement.

[0055] In the present invention, unless otherwise specified, all preparation raw materials / components are commercially available products well known to those skilled in the art.

[0056] The present invention mixes magnesium oxide, magnesium sulfate, a composite modifier, and water to produce a magnesium sulfate cement paste; the magnesium oxide contains α-MgO. In the present invention, the magnesium oxide is preferably light-burned magnesium oxide, and the mass percentage of α-MgO (i.e., active MgO) in the magnesium oxide is preferably 60-65%. The magnesium oxide is specifically magnesium oxide powder. The magnesium sulfate is specifically MgSO4·7H2O. The molar ratio of α-MgO, magnesium sulfate, and water in the magnesium oxide is preferably 7:1:16 to promote the formation of the 5·1·7 phase.

[0057] In the present invention, the composition of the light-burned magnesium oxide is shown in Table 1:

[0058] Table 1 X-ray analysis results of light-burned magnesium oxide used in the examples of the present invention

[0059]

[0060] In the present invention, the composite modifier preferably includes an organic acid, a sulfate, and a phosphate. The organic acid is preferably citric acid, the sulfate is preferably aluminum sulfate, and the phosphate is preferably sodium metaphosphate. The mass ratio of the organic acid, sulfate, and phosphate in the composite modifier is preferably 1-2:2-3:4-5, and in embodiments, it can be 2:3:5. The mass of the composite modifier preferably accounts for 0.1-3% of the mass of the magnesium oxide, and in embodiments, it can be 1.3%.

[0061] In the present invention, the organic acid (preferably citric acid) in the composite modifier has a chelating effect and can adjust the pH value of the cement system; the sulfate can promote the hydration reaction process; and the phosphate can improve the rheological properties of the material and regulate the setting time.

[0062] In the present invention, the mixing preferably comprises the following steps:

[0063] dissolving the magnesium sulfate in water to obtain a magnesium sulfate aqueous solution;

[0064] The magnesium sulfate aqueous solution and the composite modifier are first mixed to obtain a first mixture;

[0065] The first mixture and the magnesium oxide are mixed for the second time to obtain magnesium sulfate cement slurry.

[0066] The present invention dissolves the magnesium sulfate in water to obtain a magnesium sulfate aqueous solution. The magnesium sulfate is specifically MgSO4·7H2O. The present invention has no special requirements for the specific implementation of the dissolution.

[0067] After obtaining the magnesium sulfate aqueous solution, the present invention first mixes the magnesium sulfate aqueous solution and the composite modifier to obtain a first mixture. The temperature of the first mixing is room temperature, the time is preferably 1 minute, and the first mixing is performed under stirring.

[0068] After obtaining the first mixed material, the present invention performs a second mixing of the first mixed material and magnesium oxide to obtain a magnesium sulfate cement slurry. The temperature of the second mixing is preferably room temperature, and the second mixing can be performed in a planetary mixer. The second mixing is preferably performed sequentially by low-speed stirring and mixing and high-speed stirring and mixing, and the rotation speed of the low-speed stirring and mixing is preferably 60 r / min, and the time is preferably 2 min; the rotation speed of the high-speed stirring and mixing is preferably 200 r / min, and the time is preferably 2 min.

[0069] After obtaining the magnesium sulfide cement slurry, the present invention places the magnesium sulfide cement slurry into a mold for hardening and then demolding to obtain a molded product. The present invention preferably pours the magnesium sulfide cement slurry into a mold, then vibrates the mold for 1 minute and smoothes it. The hardening can be carried out indoors. The hardening conditions preferably include: an ambient temperature of preferably 20°C ± 5°C, more preferably 20°C ± 2°C, an ambient relative humidity of preferably 60 ± 5%, and in the embodiment, it can be 60%, and a time of preferably 12 to 24 hours, and in the embodiment, it can be 24 hours.

[0070] After obtaining the molded product, the present invention performs a first-stage standard air curing on the molded product to obtain a first air-cured product. In the present invention, the conditions for the first-stage standard air curing preferably include: an ambient temperature of preferably 20°C ± 5°C, more preferably 20°C ± 2°C, an ambient relative humidity of preferably 60 ± 5%, and in the embodiment, it can be 60%, and a duration of preferably 10 to 13 days, and in the embodiment, it can be 13 days. In the present invention, the first-stage standard air curing is preferably performed in a standard air curing box.

[0071] After obtaining the first air-cured product, and before performing the carbonization, the present invention preferably further comprises: subjecting the first air-cured product to carbonization pre-curing to obtain a carbonized pre-cured product; and subjecting the carbonized pre-cured product to the carbonization. The carbonization pre-curing preferably occurs under the following conditions: an ambient temperature of preferably 20°C ± 5°C, more preferably 20°C ± 2°C, an ambient relative humidity of preferably > 95%, and a duration of preferably 12 to 24 hours, and in embodiments, 24 hours. The present invention preferably adjusts the moisture content in the pores and channels within the material through the carbonization pre-curing to create a carbonization environment.

[0072] After obtaining the first air-cured product or the carbonized pre-cured product, the present invention places the first air-cured product or the carbonized pre-cured product in a carbon dioxide atmosphere for carbonization to obtain a carbonized product. In the present invention, the carbonization is preferably carried out in a closed stainless steel reactor. The carbonization conditions preferably include: the initial relative pressure of carbon dioxide is preferably 0-0.5MPa, more preferably 0.1-0.3MPa, and most preferably 0.2MPa; the temperature is preferably 25-60°C, more preferably 30-50°C, and most preferably 40°C; the insulation time is preferably 2-48h, more preferably 4-36h, and most preferably 24h. During the carbonization process of the present invention, the CO2 pressure in the reactor is kept constant by adjusting the valve. In the present invention, the carbon dioxide is preferably industrial grade carbon dioxide, and the purity of the carbon dioxide is ≥99%.

[0073] In the present invention, the method for obtaining the initial relative pressure of carbon dioxide preferably includes: using a vacuum pump to evacuate the pressure of the closed carbonization environment to -0.1 MPa, and then introducing carbon dioxide into the closed carbonization environment until the concentration of carbon dioxide preferably reaches 0-0.5 MPa, more preferably reaches 0.1-0.3 MPa, and most preferably reaches 0.2 MPa.

[0074] After obtaining the carbonized product, the present invention subjects the carbonized product to a second stage of standard air curing to obtain the carbonized basic magnesium sulfate cement. In the present invention, the conditions for the second stage of standard air curing preferably include: an ambient temperature of preferably 20°C ± 5°C, more preferably 20°C ± 2°C, an ambient relative humidity of preferably 60 ± 5%, and in an embodiment, 60%, and a curing time of preferably 10 to 13 days, and in an embodiment, 13 days.

[0075] The present invention provides carbonized basic magnesium sulfate cement prepared by the preparation method described in the above technical solution.

[0076] The structure of the carbonized basic magnesium sulfate cement provided by the present invention is as follows:

[0077] (1) Crystal structure: In the XRD pattern of carbonized BMSC, obvious diffraction peaks of magnesium carbonate are observed at 2θ=32.6°, 43.1°, and 54.1°, and characteristic diffraction peaks of calcium carbonate are observed at 2θ=29.4°, 39.4°, and 47.5°, indicating the formation of carbonates such as magnesium carbonate and calcium carbonate.

[0078] (2) Micromorphology: Raman results show that typical carbonization products (such as magnesite and magnesite hydrate) are generated on the surface of BMSC after carbonization. -1 , 1095cm -1 and 330cm -1 The characteristic peaks appear, indicating that carbonate ions (CO3 2- ) symmetrical stretching vibrations. SEM results show that after carbonization, a large number of carbonate particles are generated within the BMSC, filling the pores. The carbonate particles are approximately 5 μm in size and exhibit a regular hexagonal or rhombic morphology. Furthermore, carbonization causes the needle-shaped hydration product crystals in the BMSC matrix to become tightly interwoven, reducing porosity and forming a dense three-dimensional network structure.

[0079] (3) Pore structure: Characterization by BET, MIP, and XCT methods showed that the porosity of BMSC decreased significantly after carbonization, while the average pore size increased significantly. The number of gel pores smaller than 10 nm decreased significantly, with the volume percentage decreasing from 25% to 4%. This indicates that the carbonization process refined the pore structure of BMSC and improved the density of the material.

[0080] The physical properties of the carbonized basic magnesium sulfate cement provided by the present invention are as follows:

[0081] (1) Compressive strength:

[0082] The compressive strength at 14 days is ≥35MPa (maximum 56.1MPa), which is 15-79% higher than that of ordinary magnesium sulfate cement;

[0083] The compressive strength at 28 days is ≥54.7MPa (maximum 58.3MPa), which is 20-28% higher than that of ordinary magnesium sulfate cement;

[0084] (2) Pore structure:

[0085] The porosity at 28 days (MIP) is ≤30% (the lowest is 23.6%), which is 0.6-21% lower than that of ordinary magnesium sulfide cement;

[0086] Specific surface area (BET) at 14 days of age ≤ 30m 2 / g(minimum 16.0m 2 / g), which is 14-44% lower than that of ordinary magnesium sulfate cement;

[0087] The mean pore diameter (MIP) at 28 days of age is ≥29nm (the highest reaches 40.3nm), which is 26-72% higher than that of ordinary magnesium sulfate cement;

[0088] (3) CO2 carbon sequestration:

[0089] Quantitative analysis by TG-IR, TGA and other methods showed that the CO2 carbon fixation rate of BMSC reached 27-43% (up to 42.6%) 14 days after carbonization, which is 0.5-2.5 times that of ordinary magnesium sulfate cement.

[0090] The carbonized basic magnesium sulfate cement provided by the present invention is pure BMSC (without any added fiber). The present invention utilizes a carbon fixation process to improve the mechanical properties of basic magnesium sulfate cement, reduce porosity, and simultaneously improve the carbon fixation efficiency (which can be quantified), making the carbonized material extremely sustainable.

[0091] In order to further illustrate the present invention, the technical solutions provided by the present invention are described in detail below in conjunction with the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0092] The preparation method of magnesium sulfate cement slurry used in the following examples is as follows:

[0093] (1) MgSO4·7H2O was prepared with water to obtain a uniform solution, and then the composite modifier was added and stirred for 1 min to obtain a mixed solution.

[0094] (2) Add the composite modifier to the mixed solution obtained in step (1) and stir for 1 minute, then add MgO powder, use a planetary mixer to stir at a low speed of 60 r / min for 2 minutes, and then stir at a medium speed of 200 r / min for 2 minutes to obtain a mixed solution.

[0095] (3) Add MgO powder to the mixed solution obtained in step (2) and mix for 2 minutes to obtain a uniform magnesium sulfate cement slurry.

[0096] Wherein: MgO powder is light-burnt magnesia (Light-burnt MgO): the mass content of active MgO (α-MgO) in the light-burnt magnesia is 60% to 65%; the molar ratio of α-MgO, MgSO4·7H2O and H2O in the light-burnt magnesia is 7:1:16, which promotes the formation of 5·1·7 phase.

[0097] The composite modifier comprises citric acid, a sulfate compound and a phosphate compound, wherein the mass ratio of the citric acid, the sulfate compound and the phosphate compound is 2:3:5. The mass of the composite modifier is 1.3% of the mass of the MgO powder.

[0098] In the following examples and comparative examples: (1) Mechanical properties: The compressive strength and flexural strength of BMSC samples before and after carbonization were determined with reference to GB / T 17671-2020 "Test method for strength of cement mortar"; (2) Microstructure: SEM and TEM were used to observe the micromorphology of BMSC before and after carbonization, XRD, FTIR, and TG-IR were used to test the crystal phase composition and functional group changes, and BET, MIP, and XCT were used to characterize the pore structure parameters; (3) Carbon fixation performance: TGA-IR (TG-IR and TGA combined analysis method), XRD, XRF and other methods were used to quantitatively analyze the amount of CO2 absorbed and fixed by BMSC during the carbonization process.

[0099] Comparative Example 1AC product

[0100] 1. The prepared magnesium sulfate cement slurry was poured into a 70.7 mm × 70.7 mm × 70.7 mm cubic mold and a 40 mm × 40 mm × 160 mm prism mold, respectively. The molds were vibrated for 1 minute and smoothed. After curing for 24 hours, the molds were demolded. The curing conditions were an ambient temperature of 20°C ± 2°C and a relative humidity of 60%. After demolding, the molds were vibrated and compacted (to avoid excessive porosity) and then placed in a standard curing room at an ambient temperature of 20°C ± 2°C and a relative humidity of 60% for 13 days (i.e., the first stage of standard air curing). BMSC specimens after the first stage of standard air curing were obtained.

[0101] 2. The BMSC sample obtained in step 1, after the first stage of standard air curing, was pre-cured at an ambient temperature of 20°C ± 2°C and a relative humidity of >95% for 24 hours. The resulting pre-cured product, designated as 14-day-old AC (AC-14) in Comparative Example 1, was tested, with all tests completed within 2 hours.

[0102] AC-14 was then placed in a standard air curing box and air-cured for 13 days at a temperature of 20±2°C and a relative humidity of 60% (second stage standard air curing). The BMSC sample after the second stage standard air curing was obtained, i.e., 28-day-old AC (denoted as AC-28).

[0103] Comparative Example 1 product performance test results:

[0104] (1) The compressive strength of AC-14 is 31.41 MPa;

[0105] (2) The compressive strength of AC-28 is 45.67 MPa;

[0106] (3) The CO2 carbon fixation rate of AC-14 is: 17.36%.

[0107] Example 1CPO

[0108] 1. The prepared magnesium sulfate cement slurry was poured into a 70.7 mm × 70.7 mm × 70.7 mm cubic mold and a 40 mm × 40 mm × 160 mm prism mold, respectively. The molds were vibrated for 1 minute and smoothed. After curing for 24 hours, the slurry was demolded. The curing conditions were an ambient temperature of 20°C ± 2°C and a relative humidity of 60%. After demolding, the slurry was vibrated to compact the slurry (to avoid excessive porosity) and then placed in a standard curing room at an ambient temperature of 20°C ± 2°C and a relative humidity of 60% for 13 days (i.e., the first standard air curing). BMSC specimens were obtained after the first stage of standard air curing.

[0109] 2. The BMSC obtained in step 1 after the first stage of standard air curing is placed in an ambient temperature of 20℃±2℃, an ambient relative humidity of >95%, and a placement time of 24h for carbonization pre-curing. The sample is planned to be obtained by carbonization treatment, and it needs to be placed in a carbon environment for carbonization immediately after being taken out. Put it into a stainless steel reactor, seal the container, turn on the vacuum pump to reduce the pressure in the container to -0.1MPa and stabilize it, then open the CO2 gas valve, adjust the initial relative pressure of CO2 to 0MPa, and after stabilization, the carbonization temperature is 25±2℃, and carbonization is continued for 24h to obtain a carbonized product. The carbonized product in this embodiment is defined as 14-day-old CP0 (denoted as CP0-14);

[0110] 3. After carbonization treatment, the sample was removed from the reactor to obtain CP0-14 and all tests were completed within 2 hours after leaving the carbonization environment.

[0111] 4. After the carbonization treatment, the obtained CP0-14 was placed in a standard air curing box and continued to be air-cured for 13 days at a temperature of 20±2°C and a relative humidity of 60% (second stage standard air curing) to obtain a carbonized basic magnesium sulfate cement product (28-day-old CP0, recorded as CP0-28).

[0112] Comparative Example 1 and Example 1 product performance test results:

[0113] (1) The compressive strength of the carbonized product (CP0-14) of Example 1 is 36.3 MPa. The compressive strength of the pre-cured product (AC-14) of Comparative Example 1 is 31.41 MPa. The compressive strength of the carbonized CP0-14 of Example 1 is 15.6% higher than that of the non-carbonized AC-14 of Comparative Example 1.

[0114] (2) The compressive strength of the carbonized basic magnesium sulfate cement product (CP0-28) obtained after the second stage standard air curing in Example 1 is 54.7 MPa, and the compressive strength of the non-carbonized basic magnesium sulfate cement product (AC-28) obtained after the second stage standard air curing in Comparative Example 1 is 45.67 MPa. The compressive strength of CP0-28 obtained in Example 1 is 19.8% higher than that of AC-28 obtained in Comparative Example 1.

[0115] (3) The CO2 carbon fixation rate of the product CP0-14 obtained after carbonization in Example 1 is 27.4%, while the carbon fixation rate of AC-14 obtained in Comparative Example 1 is 17.36%. The carbon fixation rate of CP0-14 obtained in Example 1 is 57.8% higher than that of AC-14 obtained in Comparative Example 1.

[0116] (4) The MIP porosity of the product CP0-14 prepared by carbonization in Example 1 is 29.7%, and the MIP porosity of AC-14 prepared in Comparative Example 1 is 29.83%. The MIP porosity of CP0-14 obtained in Example 1 is 0.4% lower than that of AC-14 obtained in Comparative Example 1.

[0117] The BET specific surface area of the product CP0-14 prepared by carbonization in Example 1 is 16.6 m 2 / g, and the BET specific surface area of AC-14 prepared in Comparative Example 1 is 25.759 m 2 / g, the BET specific surface area of CP0-14 obtained in Example 1 is 35.6% lower than that of AC-14 obtained in Comparative Example 1.

[0118] The average MIP pore size of the product CP0-14 prepared by carbonization in Example 1 is 29.4 nm, and the average MIP pore size of AC-14 prepared in Comparative Example 1 is 23.4 nm. The average MIP pore size of CP0-14 obtained in Example 1 is 25.6% higher than that of AC-14 obtained in Comparative Example 1.

[0119] Example 2CP1

[0120] 1. Pour the prepared magnesium sulfate cement slurry into a 70.7mm×70.7mm×70.7mm cubic mold and a 40mm×40mm×160mm prism mold. Vibrate the mold for 1 minute and smooth it. Harden for 24 hours before demolding. Hardening conditions are an ambient temperature of 20°C±2°C and a relative humidity of 60%. After demolding, vibrate and compact (to avoid excessive porosity) and place in a standard curing room at an ambient temperature of 20°C±2°C and a relative humidity of 60% for 13 days (i.e., the first standard air curing). BMSC specimens after the first stage of standard air curing are obtained.

[0121] 2. The BMSC sample obtained in step 1 after the first stage of standard air curing is placed in an ambient temperature of 20℃±2℃, an ambient relative humidity of >95%, and a placement time of 24h for carbonization pre-curing. The sample is planned to be obtained by carbonization treatment, and it needs to be placed in a carbon environment for carbonization immediately after being taken out. Put it into a stainless steel reactor, seal the container, turn on the vacuum pump to reduce the pressure in the container to -0.1MPa and stabilize it, then open the CO2 gas valve, adjust the initial relative pressure of CO2 to 0.1MPa, and after stabilization, the carbonization temperature is 25±2℃, and the carbonization is continued for 24h to obtain the carbonized product. The carbonized product in this embodiment is defined as CP1 at 14 days of age, recorded as CP1-14;

[0122] 3. After carbonization treatment, the sample was removed from the reactor to obtain CP1-14. All tests were completed within 2 hours after leaving the carbonization environment.

[0123] 4. After the carbonization treatment, the obtained CP1-14 was placed in a standard air curing box and continued to be air-cured for 13 days at a temperature of 20±2°C and a relative humidity of 60% (second stage standard air curing) to obtain a carbonized basic magnesium sulfate cement product (CP1 at 28 days of age, recorded as CP1-28).

[0124] Example 2 product performance test results:

[0125] (1) The compressive strength of the product (CP1-14) obtained after carbonization in Example 2 is 54.9 MPa, while the compressive strength of the product (AC-14) obtained after pre-curing in Comparative Example 1 is 31.41 MPa. The compressive strength of CP1-14 obtained after carbonization in Example 2 is 74.8% higher than that of AC-14 prepared in Comparative Example 1.

[0126] (2) The compressive strength of the carbonized basic magnesium sulfate cement product (CP1-28) obtained after the second stage of standard air curing in Example 2 is 55.9 MPa, while the compressive strength of the non-carbonized basic magnesium sulfate cement product (AC-28) obtained after the second stage of standard air curing in Comparative Example 1 is 45.67 MPa. The compressive strength of CP1-28 obtained in Example 2 is 22.4% higher than that of AC-28 obtained in Comparative Example 1.

[0127] Example 3CP3

[0128] 1. Pour the prepared magnesium sulfate cement slurry into a 70.7mm×70.7mm×70.7mm cubic mold and a 40mm×40mm×160mm prism mold. Vibrate the mold for 1 minute and smooth it. Harden for 24 hours before demolding. Hardening conditions are an ambient temperature of 20°C±2°C and a relative humidity of 60%. After demolding, vibrate and compact (to avoid excessive porosity) and place in a standard curing room at an ambient temperature of 20°C±2°C and a relative humidity of 60% for 13 days (i.e., the first standard air curing). BMSC specimens after the first stage of standard air curing are obtained.

[0129] 2. The BMSC sample obtained in step 1 after the first stage of standard air curing is placed in an ambient temperature of 20℃±2℃, an ambient relative humidity of >95%, and a placement time of 24h for carbonization pre-curing. The sample is planned to be obtained by carbonization treatment, and it needs to be placed in a carbon environment for carbonization immediately after being taken out. Put it into a stainless steel reactor, seal the container, turn on the vacuum pump to reduce the pressure in the container to -0.1MPa and stabilize it, then open the CO2 gas valve, adjust the initial relative pressure of CO2 to 0.3MPa, and after stabilization, the carbonization temperature is 25±2℃, and carbonization is continued for 24h to obtain a carbonized product. The carbonized product in this embodiment is defined as CP3 at 14 days of age, recorded as CP3-14;

[0130] 3. After carbonization treatment, the sample is removed from the reactor to obtain CP3-14. All tests will be completed within 2 hours after leaving the carbonization environment.

[0131] 4. After the carbonization treatment, the obtained CP3-14 was placed in a standard air curing box and continued to be air-cured for 13 days at a temperature of 20±2°C and a relative humidity of 60% (second stage standard air curing) to obtain a carbonized basic magnesium sulfate cement product (CP3 at 28 days of age, recorded as CP3-28).

[0132] Example 3 product performance test results:

[0133] (1) The compressive strength of the product (CP3-14) obtained after carbonization in Example 3 is 55.5 MPa, and the compressive strength of the product (14-day-old AC) obtained after carbonization pre-curing in Comparative Example 1 is 31.41 MPa. The compressive strength of the product CP3-14 obtained after carbonization in Example 3 is 76.7% higher than that of the non-carbonized BMSC (14-day-old AC) in Comparative Example 1.

[0134] (2) The compressive strength of the carbonized basic magnesium sulfate cement product (CP3-28) obtained after the second stage of standard air curing in Example 3 is 56.8 MPa, while the compressive strength of the non-carbonized basic magnesium sulfate cement product (AC-28) obtained after the second stage of standard air curing in Comparative Example 1 is 45.67 MPa. The compressive strength of CP3-28 obtained in Example 3 is 24.4% higher than that of AC-28 obtained in Comparative Example 1.

[0135] Example 4CP5

[0136] 1. Pour the prepared magnesium sulfate cement slurry into a 70.7mm×70.7mm×70.7mm cubic mold and a 40mm×40mm×160mm prism mold. Vibrate the mold for 1 minute and smooth it. Harden for 24 hours before demolding. Hardening conditions are an ambient temperature of 20°C±2°C and a relative humidity of 60%. After demolding, vibrate and compact (to avoid excessive porosity) and place in a standard curing room at an ambient temperature of 20°C±2°C and a relative humidity of 60% for 13 days (i.e., the first standard air curing). BMSC specimens after the first stage of standard air curing are obtained.

[0137] 2. The BMSC sample obtained in step 1 after the first stage of standard air curing is placed in an ambient temperature of 20℃±2℃, an ambient relative humidity of >95%, and a placement time of 24h for carbonization pre-curing. The sample is planned to be obtained by carbonization treatment, and it needs to be placed in a carbon environment for carbonization immediately after being taken out. Put it into a stainless steel reactor, seal the container, turn on the vacuum pump to reduce the pressure in the container to -0.1MPa and stabilize it, then open the CO2 gas valve, adjust the initial relative pressure of CO2 to 0.5MPa, and after stabilization, the carbonization temperature is 25±2℃, and carbonization is continued for 24h to obtain a carbonized product. The carbonized product in this embodiment is defined as CP5 at 14 days of age, recorded as CP5-14;

[0138] 3. After carbonization treatment, the sample is removed from the reactor to obtain CP5-14. All tests will be completed within 2 hours after leaving the carbonization environment.

[0139] 4. After the carbonization treatment, the obtained CP5-14 was placed in a standard air curing box and continued to be air-cured for 13 days at a temperature of 20±2°C and a relative humidity of 60% (second stage standard air curing) to obtain a carbonized basic magnesium sulfate cement product (CP5 at 28 days of age, recorded as CP5-28).

[0140] Example 4 Product performance test results:

[0141] (1) The compressive strength of the product (CP5-14) obtained after carbonization in Example 4 is 56.1 MPa, while the compressive strength of the product (AC-14) obtained after carbonization pre-curing in Comparative Example 1 is 31.41 MPa. The compressive strength of the product CP5-14 obtained after carbonization in Example 3 is 78.6% higher than that of the non-carbonized BMSC (AC-14) in Comparative Example 1.

[0142] (2) The compressive strength of the carbonized basic magnesium sulfate cement product (CP5-28) obtained after the second stage of standard air curing in Example 4 is 58.3 MPa, while the compressive strength of the non-carbonized basic magnesium sulfate cement product (AC-28) obtained after the second stage of standard air curing in Comparative Example 1 is 45.67 MPa. The compressive strength of CP5-28 obtained in Example 4 is 27.7% higher than that of AC-28 obtained in Comparative Example 1.

[0143] (3) The CO2 carbon fixation rate of the carbonized product CP5-14 in Example 4 is 42.6%, while the carbon fixation rate of the product AC-14 prepared in Comparative Example 1 is 17.36%. The carbon fixation rate of CP5-14 obtained in Example 4 is 145.4% higher than that of AC-14 obtained in Comparative Example 1.

[0144] (4) The MIP porosity of the carbonized product CP5-14 in Example 4 is 25.3%, while the MIP porosity of AC-14 prepared in Comparative Example 1 is 29.83%. The MIP porosity of CP5-14 obtained in Example 4 is 15.2% lower than that of AC-14 obtained in Comparative Example 1.

[0145] The BET specific surface area of the carbonized product CP5-14 in Example 4 is 16.0 m 2 / g, and the BET specific surface area of AC-14 prepared in Comparative Example 1 is 25.759 m 2 / g, the BET specific surface area of CP5-14 obtained in Example 4 is 37.9% lower than that of AC-14 obtained in Comparative Example 1.

[0146] The average MIP pore size of the carbonized product CP5-14 in Example 4 is 33.9 nm, and the average MIP pore size of the AC-14 prepared in Comparative Example 1 is 23.4 nm. The average MIP pore size of the CP5-14 obtained in Example 4 is 44.9% higher than that of the AC-14 obtained in Comparative Example 1.

[0147] Test results:

[0148] Figure 1 SEM microstructure observation and EDS characterization of the main products (5,000×), Figure 1 Figure a is the test result of AC-14 prepared in Comparative Example 1. Figure 1 Figure b shows the test results of CP5-14 prepared in Example 4. Figure 1 Figure c shows the test results of CP5-28 prepared in Example 4. AC-14 presents a dense needle-shaped crystal structure. The EDS test results show that the molar ratio of Mg, S and O in the crystal structure is 6.7:1:24.3, which is close to the theoretical value (6:1:21) of the main strength phase 5Mg(OH)2·MgSO4·7H2O (5·1·7 phase) of BMSC. This microscopic three-dimensional network structure is conducive to the formation of better material mechanical properties. After carbonization curing, the micromorphology of CP5-14 prepared in Example 4 changed significantly, and a large number of flake or scaly crystals appeared, replacing the needle-shaped crystals under non-carbonized conditions. EDS analysis shows that these flake crystals are mainly composed of Mg, C and O elements, with a molar ratio of 1.8:1:4.1, which is close to the theoretical value of MgCO3 (2:1:4). This shows that in the carbonization environment, the BMSC-based material undergoes a carbonation reaction, and the main carbonation product is MgCO3. In Example 4, when the curing was extended to 28 days after carbonization curing, the length and number of the needle-shaped 5·1·7 phase crystals in CP5-28 were further reduced.

[0149] Figure 2 Raman spectra of AC-14 and CP5-14. Figure 2 AC in the figure represents the Raman test result of “AC-14” product. The data analysis shows that 979cm -1 ,983cm -1 ,984cm -1 The peak is attributed to SO4 in MgSO4 or 5Mg(OH)2·MgSO4·7H2O 2- Symmetric stretching vibration mode (v1 mode). The corresponding standard spectrum of magnesium sulfate heptahydrate (RIX219) is found in the Raman database. Figure 2 The “CP5” in the figure indicates the Raman test results of the “CP-5” product. The data analysis shows that 1097 cm -1 , 1095cm-1 and 330cm -1 The peaks are derived from the CO3 of magnesite and magnesitehydrate 2- The symmetrical stretching vibration mode (v1 mode) of MgCl2 is shown in the Raman database, which corresponds to the standard spectra of magnesite (RHX149, RTX174) and hydromagnesite (RMX314, RMX315).

[0150] Depend on Figure 2 Comparing the Raman spectra of non-carbonized and carbonized BMSC samples (AC-14 and CP5-14) reveals that the characteristic peak intensity of the carbonation product gradually increases as the curing conditions transition to carbonation. This suggests that increasing the CO2 concentration accelerates the carbonation process, promoting the conversion of more magnesium sulfate and its hydrates to stable magnesium carbonate hydrates while suppressing the formation of some intermediate phases.

[0151] Figure 3 The phase distribution (Raman surface analysis cloud diagram) of AC-14 and CP5-14 is shown. Comparing the Raman surface scan results of non-carbonized and carbonized BMSC samples (AC-14 and CP5-14), it can be observed from the normalized color intensity that carbonization significantly changes the product composition of the BMSC surface, but the composition of the carbonation products is essentially the same. It is worth noting that in the CP5-14 sample, carbonization products and hydration products do not coexist in the same area.

[0152] Figure 4 XRD patterns of AC-14, CP0-14, and CP5-14; Figure 5 XRD patterns of AC-28, CP0-28 embodiment and CP5-28; Figure 6The XRD patterns of the main phases of AC-14, AC-28, CP0-14, CP0-28, CP5-14, and CP5-28 are shown. By comparing the phase transitions at different carbonization conditions at 14 days and after extended curing to 28 days, the main carbonization products were found to include periclase (MgO), brucite (Mg(OH)2), magnetite (MgCO3), quartz (SiO2), and 5·1·7 phase (5Mg(OH)2·MgSO4·7H2O). Among the 14-day-old BMSC samples, the magnetite peak intensity of the carbonization-cured samples (CP0-14 and CP5-14) was significantly greater than that of the standard air-cured sample (AC-14). The magnetite peak of CP5-14, carbonized at a pressure of 0.5 MPa, was the highest, followed by the atmospheric pressure carbonized sample (Example 1, CP0-14). This indicates that the total amount of carbonized products increases with increasing CO2 pressure. Furthermore, for samples cured to 28 days, the peak magnetite intensity of carbonized samples is generally lower than that of standard air-cured samples. This may be due to the impact of carbonization on the hydration development of the samples: the acidic effect of carbonization may lead to unstable development of various products in a non-carbonized environment.

[0153] Figure 7 The results of TG-IR and FTIR analysis of AC-14, AC-28, CP0-14, CP0-28, CP5-14, and CP5-28 are shown. Thermogravimetric (TG) curves show the percentage of mass loss corresponding to the change in sample decomposition temperature. TG results indicate that the mass loss of the carbonized samples (CP0-14, CP5-14, CP0-28, and CP5-28) is much greater than that of the samples cured under standard air conditions (AC-14 and AC-28). This additional mass loss may be due to the formation of more carbonation products in the samples. Regarding mass loss due to the release of H2O and CO2, infrared (IR) curves reveal differences in the infrared absorption patterns of H2O and CO2 for carbonized CP0-28 and CP5-28 compared to the uncarbonized AC control.

[0154] Figure 8 MIP pore size distribution curves of AC-14, CP0-14, CP5-14, and CP5-28; Figure 9is the MIP pore volume fraction of AC-14, CP0-14, CP5-14, and CP5-28. The AC-14 obtained by air curing showed strong peak signals in the gel pore area of 2-10nm and the capillary pore area of 10-100nm, indicating that the pore size distribution of the material is relatively uniform. After the introduction of CO2, the peak area of the small mesopore area of CP0-14 and CP5-14 gradually decreased, while the peak area of the large mesopore area first decreased and then increased, and finally formed a sharp main peak at 50-100nm. This shows that the carbonation effect of CO2 preferentially fills and blocks the smaller gel pores, resulting in a decrease in pore volume; then further erodes the hydration products, making the macropores more developed, and the pore size distribution curve shifts to the right side of the macropore area. This is because CO2 reacts with the BMSC matrix to form a carbonation reaction, and the precipitation of carbonate fills and blocks some micropores, resulting in an increase in pore size. Combined with the pore volume distribution of different pore sizes ( Figure 9 ), the present invention found that the <10 nm gel pore region of BMSC was preferentially filled during carbonation. After carbonation, the pore volume of CP5-14 in the gel pore region was significantly reduced by 56.8% compared to AC-14. With extended curing to 28 days, the porosity of CP5-28 continued to decrease. This indicates that the carbonation reaction did not prevent the continued hydration process and the formation of hydration products in the sample.

[0155] Figure 10 The X-CT three-dimensional pore distribution of AC-14, CP0-14, CP5-14, and CP5-28. A large number of pores with uniform distribution, large size and good connectivity were observed in the AC control example (AC-14) cured in air. When CO2 was introduced, the number of pores in CP0-14 and CP5-14 decreased significantly, the size became smaller, and they showed a discretely distributed island feature in three-dimensional space, and became more significant with the increase of CO2 pressure. This shows that the carbonation effect of CO2 significantly changed the pore structure of BMSC, resulting in a large number of pores being filled with carbonate crystals. As the curing time was extended to 28 days, the porosity of the CP5-28 embodiment further decreased. This is because the hydration reaction of the sample continued after the carbonization was terminated.

[0156] Figure 11 DTG differential thermogravimetric curves in TG-IR of AC-14, CP0-14, and CP5-14; Figure 12The DTG differential thermogravimetric curves in TG-IR for AC-28, CP0-28, and CP5-28 are shown. The thermal decomposition process of all samples can be divided into four main stages: dehydration (0-200°C), dehydroxylation (200-480°C), decarbonization (480-850°C), and desulfurization (850-1000°C). Overall, as the curing conditions change to carbonization curing, the weight loss in the decarbonization stage of the CP0 and CP5 examples increases compared to the AC control example. This indicates that forced carbonation significantly improves the carbon sequestration capacity of BMSC-based materials.

[0157] Figure 13 The compressive strength of AC-14, AC-28, CP0-14, CP0-28, CP1-14, CP1-28, CP3-14, CP3-28, CP5-14 and CP5-28. At the age of 14 days, the compressive strength of all proportioned samples showed a significant positive correlation with the carbonation pressure. Specifically, compared with conventional curing (AC control), carbonation curing increased the compressive strength of CP0, CP1, CP3 and CP5 embodiments by up to 15.4%, 74.6%, 76.6% and 78.5%, respectively. This significant strength improvement can be attributed to the rapid reaction of CO2 with the active components in the material to form carbonate minerals to fill the pores of the material, thereby densifying the matrix structure. It is worth noting that even under lower CO2 pressures, carbonation can still effectively improve the mechanical properties of magnesia cementitious materials, which indicates that the material has good responsiveness to CO2. When the curing period was extended to 28 days, all the ratios showed a trend of continuous growth, indicating that the early carbonization curing did not hinder its subsequent development. Compared with the conventional curing (AC control), the carbonization curing increased the compressive strength of the CP0, CP1, CP3 and CP5 examples at 28 days of age by 19.8%, 22.4%, 24.4% and 27.7%, respectively. All the results show that increasing the CO2 pressure can further increase the compressive strength, which shows that reasonable carbonization pressure control is of great significance for the continuous improvement of the later performance of BMSC.

[0158] Table 2 shows the porosity (including MIP porosity and X-CT porosity), specific surface area (including BET and MIP specific surface area), and pore size (including MIP pore size and X-CT pore size) of AC-14, CP0-14, CP5-14, and CP5-28. Table 3 shows the mass loss at different stages of the 14-day and 28-day-old cycles of AC, CP0, and CP5. Table 4 shows the CO2 storage performance indicators of AC, CP0, and CP5 at 14-day and 28-day-old cycles.

[0159] Table 2 Multi-scale pore structure parameters

[0160]

[0161] Table 3 Mass loss values after TG-IR combined analysis (%)

[0162] Object Dehydration stage Dehydroxylation stage Decarbonization stage Desulfurization stage total AC-14 17.92 7.19 10.05 6.94 42.1 CP0-14 18.03 6.72 14.82 6.32 45.89 CP5-14 17.87 2.37 21.63 7.3 49.17 AC-28 15.79 6.64 13.64 7.02 43.09 CP0-28 18.62 5.9 17.67 6.62 48.81 CP5-28 18.76 8.28 16.52 6.31 49.87

[0163] Table 4 CO2 storage performance indicators calculated based on quantitative analysis of various characterizations

[0164] Object <![CDATA[CO2 carbon sequestration rate [r c (%)]]]> AC-14 17.36 CP0-14 27.39 CP5-14 42.55 AC-28 23.97 CP0-28 34.52 CP5-28 32.95

[0165] The above test results demonstrate that the present invention accurately quantifies the carbonization effect of BMSC materials on CO2 fixation. Using a combined TG-IR and TGA analysis method, the amount of CO2 absorbed and fixed by BMSC under different carbonization conditions can be accurately determined. While conventional BMSC partially participates in the carbonation reaction with CO2 in high-humidity environments, the carbon fixation is very low, resulting in limited strength gains. However, the present invention utilizes a specific concentration of CO2 to carbonize BMSC, significantly improving their mechanical properties and carbon fixation efficiency. Calculations show that the method provided by the present invention can increase the carbonate content of BMSC by more than 2-fold, achieving a CO2 fixation rate of 28-43%. For example, the modified BMSC prepared in Example 4 exhibits a 78.6% increase in 14-day compressive strength and a 27.7% increase in 28-day compressive strength compared to conventional BMSC, while also increasing the CO2 fixation rate by 145.4%. Furthermore, the porosity is reduced by 15.2%, the specific surface area by 37.9%, and the average pore size by 44.9%. Therefore, the present invention effectively addresses the issues of insufficient mechanical properties and low CO2 utilization in existing BMSC materials.

[0166] It can be seen from the above examples that compared with the standard concrete carbonization method (standard carbonization, CO2 concentration is generally 20%, temperature 20°C, humidity 70%), the present invention provides a carbon fixation method that improves the mechanical properties and sustainability of BMSC. The method provided by the present invention uses BMSC as raw material, and through CO2 carbonization treatment (increasing CO2 concentration (above 99.9%) and pressurization (0.1MPa, 0.3MPa and 0.5MPa)), it can achieve rapid carbon fixation enhancement of BMSC cement at room temperature and pressure without adding other admixtures. CO2 reacts with BMSC to form insoluble substances such as magnesium carbonate and calcium carbonate, which fill the pores, refine the pore size distribution, and improve the density and strength of the material. At the same time, it realizes the absorption and fixation of CO2 and reduces carbon emissions. Compared with the existing technology, the process of the present invention is simple, efficient, and low-cost, and is expected to greatly improve the comprehensive performance of BMSC materials and promote their application in green buildings. In addition, the method provided by the present invention is combined with CO2 capture and utilization (CCU) technology, which provides a new idea for alleviating the pressure of carbon emission reduction in the construction industry, and is of great significance for the development of low-carbon cement and the promotion of sustainable development of the construction industry.

[0167] Although the above embodiment provides a detailed description of the present invention, it is only a part of the embodiments of the present invention, not all of the embodiments. Other embodiments can be obtained based on this embodiment without creativity, and these embodiments all fall within the scope of protection of the present invention.

Claims

1. A method for preparing carbonized basic magnesium sulfate cement, characterized in that: The following steps are involved: Mixing magnesium oxide, magnesium sulfate, a composite modifier and water to obtain magnesium sulfate cement slurry; the magnesium oxide contains α-MgO; placing the magnesium sulfate cement slurry in a mold for hardening and then demoulding to obtain a molded product; subjecting the molded product to a first stage of standard air curing to obtain a first air-cured product; placing the first air-cured product in a carbon dioxide atmosphere for carbonization to obtain a carbonized product; The carbonized product is subjected to a second stage of standard air curing to obtain the carbonized basic magnesium sulfate cement.

2. The preparation method according to claim 1, characterized in that The magnesium oxide is light-burned magnesium oxide, and the mass percentage of α-MgO in the magnesium oxide is 60-65%.

3. The preparation method according to claim 1, characterized in that The composite modifier comprises organic acid, sulfate and phosphate; the mass ratio of the organic acid, sulfate and phosphate in the composite modifier is 1-2:2-3:4-5.

4. The preparation method according to claim 1 or 3, characterized in that The mass percentage of the composite modifier to the mass percentage of the magnesium oxide is 0.1-3%.

5. The preparation method according to claim 1, characterized in that The curing conditions include: an ambient temperature of 20°C ± 5°C, an ambient relative humidity of 60 ± 5%, and a time of 12 to 24 hours; The conditions of the first stage standard air curing include: ambient temperature of 20°C ± 5°C, ambient relative humidity of 60±5%, and duration of 10 to 13 days.

6. The preparation method according to claim 1, characterized in that The carbonization conditions include: an initial relative pressure of carbon dioxide of 0 to 0.5 MPa, a temperature of 25 to 60° C., and a heat preservation time of 2 to 48 hours.

7. The preparation method according to claim 1, 5 or 6, characterized in that: After obtaining the first air-cured product, before performing the carbonization, the method further includes: subjecting the first air-cured product to carbonization pre-curing to obtain a carbonized pre-cured product; and subjecting the carbonized pre-cured product to the carbonization; the carbonization pre-curing conditions include: an ambient temperature of 20°C ± 2°C, an ambient relative humidity greater than 95%, and a time of 12 to 24 hours.

8. The preparation method according to claim 1 or 6, characterized in that The conditions of the second stage standard air curing include: ambient temperature of 20°C ± 5°C, ambient relative humidity of 60±5%, and duration of 10 to 13 days.

9. The preparation method according to claim 1, characterized in that The mixing comprises the following steps: dissolving the magnesium sulfate in water to obtain a magnesium sulfate aqueous solution; The magnesium sulfate aqueous solution and the composite modifier are first mixed to obtain a first mixture; The first mixture and the magnesium oxide are mixed for the second time to obtain magnesium sulfate cement slurry.

10. Carbonated basic magnesium sulfate cement prepared by the preparation method according to any one of claims 1 to 9.

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