Cement chromium-reducing microcapsules and methods for their preparation

Abstract

This invention belongs to the field of cement-based materials technology and discloses a cement-based chromium-reducing microcapsule and its preparation method. The microcapsule includes a capsule wall and a core. The capsule wall is a water-soluble outer membrane, and the core includes a porous water-absorbing framework, a reducing agent, and a soluble aluminum salt. The soluble aluminum salt is embedded inside the porous water-absorbing framework, and the reducing agent is attached to the outer pores of the porous water-absorbing framework. The core portion of this invention forms a porous water-absorbing framework, precisely controlling the release rate of the effective components of the core material. The reducing agent is released first, followed by the soluble aluminum salt. The preferential release of the reducing agent rapidly reduces the Cr(VI) concentration, while the subsequent release of the aluminum salt promotes the formation of the AFm phase. This ensures the orderly synergy of the two chromium-reducing mechanisms over time, avoiding mutual interference and optimizing the overall chromium-reducing efficiency.

Description

Technical Field

[0001] This invention belongs to the field of cement-based materials technology, specifically relating to a cement-based chromium-reducing microcapsule and its preparation method. Background Technology

[0002] Cement, as a core cementitious material in the field of civil engineering, contains water-soluble hexavalent chromium (Cr(VI)), which is highly toxic and mobile. The "GB 31893-2015 Limits and Determination Methods for Water-soluble Chromium(VI) in Cement" clearly limits the content of water-soluble hexavalent chromium in cement to no more than 10 mg / kg. Reducing and fixing hexavalent chromium is a key technical direction for the environmental modification of cement.

[0003] Currently, the industry generally uses ferrous salts as a reducing agent for hexavalent chromium. In order to overcome the defects of ferrous ions being easily oxidized and easily deactivated at high temperatures during cement grinding, existing technologies (such as the invention patent with publication number CN117401928A) have adopted microencapsulation technology to protect the ferrous-based reducing components, so as to improve the stability of the reducing agent and the effectiveness of chromium reduction.

[0004] However, existing microcapsule chromium reduction technologies rely solely on a single chemical reduction mechanism, lack an ordered controlled-release structure, and have a weak capsule protection system. The reducing agent is still prone to oxidation and deactivation during storage and cement processing, ultimately leading to insufficient actual chromium reduction effect in cement processing. This fails to meet the industry's demand for efficient chromium reduction, long-term fixation, and compatibility with cement performance. Summary of the Invention

[0005] The purpose of this invention is to provide a cement chromium-reducing microcapsule to solve the technical problems of existing technologies that rely solely on chemical reduction without ordered controlled release, and where the reducing agent is easily oxidized and deactivated due to weak protection, resulting in insufficient actual chromium reduction effect in cement processes.

[0006] The aforementioned cement-based chromium-reducing microcapsule includes a capsule wall and a capsule core. The capsule wall is a water-soluble outer membrane, and the capsule core includes a porous water-absorbing skeleton, a reducing agent, and a soluble aluminum salt. The soluble aluminum salt is embedded inside the porous water-absorbing skeleton, and the reducing agent is attached to the pores on the outer side of the porous water-absorbing skeleton.

[0007] Preferably, the reducing agent is any one or a mixture of two or more of ferrous nitrate, ferrous sulfate, stannous sulfate, and stannous nitrate.

[0008] Preferably, the soluble aluminum salt is any one or a mixture of two or more of aluminum nitrate, aluminum chloride, and aluminum acetate.

[0009] Preferably, the water-soluble outer membrane is formed of polyvinyl alcohol, and the porous absorbent skeleton is composed of calcium alginate.

[0010] Preferably, the particle size of the cement chromium-reducing microcapsules is ≤80μm.

[0011] This invention also provides a method for preparing cement chromium-reducing microcapsules as described above, comprising the following steps: S1. Under inert gas protection, the reducing agent is added to the sodium alginate solution and stirred, and then ultrasonically pulverized to obtain solution A; S2. A solution of soluble aluminum salt is added dropwise to solution A, and solution B is obtained by high-speed shearing and stirring. S3. Solution B is atomized into fine droplets using an atomizer and then dropped into a polyvinyl alcohol solution, and stirred at a constant temperature to obtain solution C; S4. Add solution C to calcium chloride solution and stir evenly. Then freeze-dry to obtain cement chromium-reducing microcapsules.

[0012] Preferably, the mass ratio of the reducing agent, the sodium alginate, and the soluble aluminum salt is 1:0.5~1.5:1~3.

[0013] Preferably, in step S3, the amount of polyvinyl alcohol added accounts for 1-3% of the mass of the solute in solution B, and the constant temperature stirring temperature is 60°C.

[0014] Preferably, in step S4, the freeze-drying temperature is -20°C, and the concentration of the calcium chloride solution is 1-3%.

[0015] The technical advantages of this invention are as follows: This invention simultaneously utilizes a reducing agent and a soluble aluminum salt. The former reduces water-soluble Cr(VI) to low-toxicity Cr(III) through chemical reduction, while the latter promotes the formation of the AFm phase (layered double hydroxide) in cement, which effectively adsorbs unreduced chromate ions, forming secondary fixation. Simultaneously, this invention forms a porous, water-absorbing framework in the microcapsule core, precisely controlling the release rate of the effective components of the core material. The reducing agent is released first, followed by the soluble aluminum salt. The preferential release of the reducing agent rapidly reduces the Cr(VI) concentration, and the subsequent release of the aluminum salt promotes the formation of the AFm phase. This ensures the orderly synergy of the two chromium-reducing mechanisms over time, avoiding mutual interference and optimizing the overall chromium-reducing efficiency. As a microcapsule structure, the water-soluble membrane layer forms the capsule wall, which, together with the internal network framework, constitutes a multi-layered protective system. The capsule wall effectively isolates water vapor and oxygen from the external environment, effectively solving the technical problems of easy oxidation, deliquescence, and deactivation of the active ingredients inside the microcapsules during storage, transportation, and stirring, thus significantly extending the product's shelf life. After using the chromium-reducing agent of the present invention, the water-soluble hexavalent chromium content in cement products is reduced by ≥95%, and the long-term hexavalent chromium leaching is reduced by ≥90%. Attached Figure Description

[0016] Figure 1This is a schematic diagram illustrating the technical principle of the dual chromium reduction mechanism achieved by cement chromium-reducing microcapsules according to the present invention. Figure 2 SEM image of the sample corresponding to the microstructure test in Example 2 of the present invention; Figure 3 SEM images of the samples corresponding to the microstructure test of Comparative Example 4. Figure 4 These are the original images of the water-soluble hexavalent chromium detection of the corresponding samples in Examples 1-3 and the reference group of the present invention; Figure 5 The original images are of the water-soluble hexavalent chromium detection results for the samples corresponding to Comparative Examples 1-5. Detailed Implementation

[0017] The following detailed description of the embodiments, with reference to the accompanying drawings, will further illustrate the specific implementation of the present invention, in order to help those skilled in the art to have a more complete, accurate, and in-depth understanding of the inventive concept and technical solution of the present invention.

[0018] like Figures 1-5 As shown, the present invention provides a cement-based chromium-reducing microcapsule, which includes a capsule wall and a core. The capsule wall is a water-soluble outer membrane formed of polyvinyl alcohol, which can dissolve or disintegrate rapidly in cement slurry. The core includes a porous water-absorbing skeleton, a reducing agent, and a soluble aluminum salt. The soluble aluminum salt is embedded inside the porous water-absorbing skeleton, and the reducing agent is attached to the pores on the outside of the porous water-absorbing skeleton.

[0019] The porous absorbent framework is composed of calcium alginate. The water-soluble outer membrane is formed of polyvinyl alcohol.

[0020] The reducing agent is any one of ferrous nitrate, ferrous sulfate, stannous sulfate, and stannous nitrate, or a mixture of any two or more of the aforementioned elemental reducing agents.

[0021] The soluble aluminum salt is any one of aluminum nitrate, aluminum chloride, and aluminum acetate, or a mixture of any two or more of the aforementioned soluble aluminum salts.

[0022] The particle size of the cement chromium-reducing microcapsules is ≤80μm.

[0023] The present invention also provides a method for preparing cement chromium-reducing microcapsules, comprising the following steps.

[0024] S1. Under inert gas protection, the reducing agent is added to the sodium alginate solution and stirred, and then ultrasonically pulverized to obtain solution A.

[0025] Inert gases can be nitrogen or rare gases, such as argon.

[0026] S2. A solution of soluble aluminum salt is added dropwise to solution A, and solution B is obtained by high-speed shearing and stirring.

[0027] The mass ratio of the reducing agent, the sodium alginate, and the soluble aluminum salt is 1:0.5~1.5:1~3.

[0028] In this step, the solute in the resulting solution B is the encapsulation core. At this time, sodium alginate serves as the substrate for the framework forming, constituting a porous framework prototype. Soluble aluminum salts participate in cross-linking and are embedded in the framework prototype, while the reducing agent is physically adsorbed into the outer pores of the framework prototype. Thus, sodium alginate, soluble aluminum salts, and reducing agent form an encapsulation core with a framework prototype.

[0029] S3. Solution B is atomized into fine droplets using an atomizer and then dropped into a polyvinyl alcohol solution. The solution is then stirred at a constant temperature to obtain solution C.

[0030] The amount of polyvinyl alcohol added is 1-3% of the mass of the solute in solution B, and the constant temperature stirring temperature is 60℃.

[0031] In this step, after constant temperature stirring, polyvinyl alcohol coats the surface of the tiny droplets formed in solution B, forming microcapsule prototypes.

[0032] S4. Add solution C to calcium chloride solution and stir evenly. Then freeze-dry to obtain cement chromium-reducing microcapsules.

[0033] The freeze-drying temperature is -20℃. The concentration of the calcium chloride solution is 1~3%.

[0034] In this step, calcium chloride undergoes ion exchange with the microcapsule prototype, and calcium ions replace sodium ions, causing the skeleton prototype to form a calcium alginate gel with a three-dimensional network structure. Then, water is removed by freeze drying, and the calcium alginate gel forms an interconnected porous structure, i.e., a porous water-absorbing skeleton.

[0035] The specific implementation method is as follows: Example 1 Example 1 provides a cement chromium-reducing microcapsule and its preparation method, including the following steps: S1: Under nitrogen protection, 10g of ferrous nitrate was added to 100ml of 5% sodium alginate solution and stirred. The solution was then ultrasonically pulverized to obtain solution A.

[0036] S2: Add 100 ml of 5% aluminum nitrate solution dropwise to solution A, and then prepare solution B by high-speed shearing and stirring.

[0037] S3: Atomize solution B into fine droplets using an atomizer and then drop them into 100 ml of a 1% polyvinyl alcohol solution. Stir at a constant temperature of 60°C to obtain solution C.

[0038] S4: Add solution C to a 2% calcium chloride solution and stir evenly. Then freeze-dry at -20℃ to obtain cement chromium-reducing microcapsules.

[0039] The cement-based chromium-reducing microcapsules prepared by the above method have a water-soluble outer membrane (polyvinyl alcohol) on the surface and a porous water-absorbing skeleton containing two effective components: a reducing agent and a soluble aluminum salt. The soluble aluminum salt is embedded inside the porous water-absorbing skeleton, and the reducing agent is attached to the outside of the porous water-absorbing skeleton.

[0040] Cement chromium-reducing microcapsules can be added at appropriate stages of the cement process, such as by uniformly blending with the cement after grinding.

[0041] Example 2 Example 2 provides a cement chromium-reducing microcapsule and its preparation method, including the following steps: S1: Under nitrogen protection, 10g of ferrous nitrate was added to 100ml of 10% sodium alginate solution and stirred. The solution was then ultrasonically pulverized to obtain solution A.

[0042] S2: Add 100 ml of 25% aluminum nitrate solution dropwise to solution A, and then prepare solution B by high-speed shearing and stirring.

[0043] S3: Atomize solution B into fine droplets using an atomizer and then drop them into 100 ml of a 1% polyvinyl alcohol solution. Stir at a constant temperature of 60°C to obtain solution C.

[0044] S4: Add solution C to a 2% calcium chloride solution and stir evenly. Then freeze-dry at -20℃ to obtain cement chromium-reducing microcapsules.

[0045] The cement-based chromium-reducing microcapsules prepared by the above method have a water-soluble outer membrane (polyvinyl alcohol) on the surface and a porous water-absorbing skeleton containing two effective components: a reducing agent and a soluble aluminum salt. The soluble aluminum salt is embedded inside the porous water-absorbing skeleton, and the reducing agent is attached to the outside of the porous water-absorbing skeleton.

[0046] Cement chromium-reducing microcapsules can be added at appropriate stages of the cement process, including during the cement grinding stage.

[0047] Example 3 Example 3 provides a cement chromium-reducing microcapsule and its preparation method, including the following steps: S1: Under nitrogen protection, 10g of ferrous nitrate was added to 100ml of 15% sodium alginate solution and stirred. The solution was then ultrasonically pulverized to obtain solution A.

[0048] S2: Add 100 ml of 30% aluminum nitrate solution dropwise to solution A, and then prepare solution B by high-speed shearing and stirring.

[0049] S3: Atomize solution B into fine droplets using an atomizer and then drop them into 100 ml of a 1% polyvinyl alcohol solution. Stir at a constant temperature of 60°C to obtain solution C.

[0050] S4: Add solution C to a 2% calcium chloride solution and stir evenly. Then freeze-dry at -20℃ to obtain cement chromium-reducing microcapsules.

[0051] The cement-based chromium-reducing microcapsules prepared by the above method have a water-soluble outer membrane (polyvinyl alcohol) on the surface and a porous water-absorbing skeleton containing two effective components: a reducing agent and a soluble aluminum salt. The soluble aluminum salt is embedded inside the porous water-absorbing skeleton, and the reducing agent is attached to the outside of the porous water-absorbing skeleton.

[0052] Cement chromium-reducing microcapsules can be added at appropriate stages of the cement process, including during the cement grinding stage.

[0053] The comparison is provided below, and the details are as follows.

[0054] Comparative Example 1 Compared with Example 1, Comparative Example 1 has the same technical solution for preparation method, but has the following technical differences: the mass fraction of 100ml sodium alginate solution in step S1 of the preparation method is 3%.

[0055] Comparative Example 2 Compared with Example 1, Comparative Example 2 has the same technical solution for preparation method, but has the following technical differences: the mass fraction of 100ml sodium alginate solution in step S1 of the preparation method is 25%.

[0056] Comparative Example 3 Compared with Example 1, Comparative Example 3 has the same technical solution for preparation method, but has the following technical differences: no ferrous nitrate or any other reducing agent is added in the preparation method.

[0057] Comparative Example 4 Compared with Example 1, Comparative Example 4 has the same technical solution for preparation method, but has the following technical differences: no aluminum nitrate or any other soluble aluminum salt is added in the preparation method.

[0058] Comparative Example 5 Compared with Example 1, Comparative Example 5 has the same technical solution for preparation method, but has the following technical differences: no polyvinyl alcohol or any other water-soluble substance is added in the preparation method to form a coating on the core, and the resulting product has no water-soluble outer membrane.

[0059] The products obtained in Examples 1-3 and Comparative Examples 1-5 were uniformly blended with the ground cement, and then the cement was used as a raw material to cast the corresponding samples. The products obtained by casting with cement without any added microcapsules were used as the reference group samples. The water-soluble hexavalent chromium content of each sample was measured (original images of water-soluble hexavalent chromium detection for the corresponding samples in Examples 1-3 and the reference group are shown below). Figure 4 As shown; the original images of the water-soluble hexavalent chromium detection of the corresponding samples in Comparative Examples 1-5 are as follows. Figure 5 The results are shown in Table 1. The four indicators tested were: chromium leaching at 28 days, compressive strength at 3 days, and compressive strength at 28 days.

[0060] Table 1. Performance Comparison of Products Obtained in Examples 1-3 and Comparative Examples 1-2

[0061] Table 1 shows the differences in water-soluble hexavalent chromium content and 28-day chromium leaching amount among the baseline group, examples, and comparative examples: Benchmark group: The background value of hexavalent chromium in blank cement was high, far exceeding the limit of 10 mg / kg in GB 31893-2015; Comparative Example 1: The concentration and actual dosage of sodium alginate solution were both lower than those in the examples, resulting in a significantly reduced chromium-reducing effect. The water-soluble hexavalent chromium content was 8 mg / kg, and the 28-day chromium leaching amount was 2.3 mg / kg, both significantly higher than the corresponding indicators in the examples. This indicates that insufficient sodium alginate may prevent the formation of a stable porous framework, leading to uncontrolled release of the reducing agent and aluminum salts and a greatly diminished effect on reducing hexavalent chromium. Furthermore, the sample strength was lower than that of the examples and the baseline group, suggesting that failure to control the orderly release of the reducing agent and soluble aluminum salts leads to a decrease in the strength of the cement product.

[0062] Comparative Example 2: In contrast to Comparative Example 1, the concentration and actual dosage of sodium alginate solution were higher than those in the examples. The results showed a water-soluble hexavalent chromium content of 28 mg / kg and a chromium leaching amount of 12 mg / kg after 28 days. The chromium-reducing effect was significantly lower than in the examples, even lower than in Comparative Example 1, indicating that excessive sodium alginate may lead to an overly dense skeleton, preventing component release and rendering the chromium-reducing mechanism ineffective. The strength of the cement product showed little difference from the examples and the baseline group, indirectly suggesting that the reducing agent may not have been released normally, resulting in ineffective removal of hexavalent chromium.

[0063] The corresponding embodiments can effectively reduce hexavalent chromium within a specific sodium alginate dosage range, and the sample strength index is basically consistent with that of the reference group sample. This indicates that the sodium alginate concentration range or dosage range is the key parameter for controlling the amount of porous water-absorbing skeleton generated. This enables the orderly release of reducing agent and soluble aluminum salt, thereby achieving the dual goals of reducing hexavalent chromium and ensuring product strength index at the same time.

[0064] Comparative Example 3: The water-soluble hexavalent chromium content was 42 mg / kg, and the chromium leaching amount after 28 days was 18 mg / kg, only slightly better than the baseline group, indicating that the core chromium reduction mechanism is completely absent without the reducing agent. The sample strength corresponding to Comparative Example 3 was lower than that of Example 1 and the baseline group, proving that the reducing agent is an indispensable core component of this scheme.

[0065] Comparative Example 4: The water-soluble hexavalent chromium content was 38 mg / kg, and the chromium leaching amount after 28 days was 13 mg / kg, which was significantly worse than that of the other examples. This indicates that without soluble aluminum salts, sufficient AFm phase layered double hydroxides cannot be generated, and relying solely on a single reducing agent will not result in secondary curing, leading to a large amount of unreduced chromate leaching later. Furthermore, the strength of Comparative Example 4 was lower than that of Example 1 and the baseline group, proving that soluble aluminum salts are also a necessary component in the dual mechanism.

[0066] Comparative Example 5: The water-soluble hexavalent chromium content was 57 mg / kg, and the chromium leaching amount after 28 days was 26 mg / kg, significantly worse than all the examples, only slightly better than the standard group. This indicates that the water-soluble outer membrane formed by polyvinyl alcohol coating is a necessary technical feature to solve the problems of easy oxidation, deliquescence, and deactivation of the reducing agent. Furthermore, its sample strength was not only worse than all the examples, but also worse than the reference group and all comparative examples, only better than Comparative Example 1. This also shows that the water-soluble outer membrane is the core structure for protecting the activity of the reducing agent and ensuring the chromium removal effect. Without the water-soluble outer membrane, the removal effect of hexavalent chromium is only better than Comparative Example 1, which did not form an effective porous water-absorbing framework.

[0067] On the other hand, the microstructure of the cured cement products corresponding to Example 2 and Comparative Example 4 was also tested. Specifically, the corresponding cement was mixed with water for 5 minutes (cement:water = 1:2), then immediately filtered. The solid that did not pass through the filter paper was collected, hydration was immediately terminated, and the product was vacuum dried. The microstructure of the dried product was then tested, and the corresponding SEM images were obtained. EDS micro-area elemental analysis was also performed on the corresponding products, and the corresponding analysis results are shown in the table. The SEM image of Example 2 is shown below. Figure 2 Analyze the measuring points as follows Figure 2 The elemental quantitative analysis results of the EDS micro-regions at the marked locations 1 and 2 are shown in Table 2; the SEM images of Comparative Example 4 are shown in Table 2. Figure 3 Analyze the measuring points as follows Figure 3 The results of the quantitative elemental analysis of the EDS micro-regions at the marked positions 1 and 2 are shown in Table 3.

[0068] Table 2. Results of EDS micro-area elemental quantitative analysis in Example 2

[0069] Table 3. Results of EDS micro-area elemental quantitative analysis of Comparative Example 4

[0070] pass Figure 2 , Figure 3 The differences between Tables 2 and 3 show that, for Example 2, Cr was consistently detected at both test points in the SEM image (Mass% were 5.35% and 6.40%, and Atom% were 2.46% and 3.41%, respectively). This directly proves that chromium was successfully embedded in the layered structure of the AFm phase, achieving secondary fixation of chromium. Combined with the previous reduction effect of the reducing agent on hexavalent chromium, the dual chromium reduction mechanism desired by this invention was realized. Furthermore, this table provides a supplementary information on the microscopic mechanism for macroscopic chromium reduction rate testing (such as the detection of water-soluble hexavalent chromium in the GB 31893-2015 standard).

[0071] On the other hand, the above charts also show that, for the test results of Example 2, the Ca / Al atomic ratios at the two test points are approximately 1.57 and 1.69, respectively, which are completely within the standard stoichiometric range of 1.5 to 2.0 for the AFm phase. The S element corresponds to the interlayer sulfate ion, which directly proves that the soluble aluminum salt added in this application successfully promoted the formation of the target AFm phase, and verifies the feasibility and rationality of the technical solution.

[0072] The present invention has been described above by way of example with reference to the accompanying drawings. Obviously, the specific implementation of the present invention is not limited to the above-described manner. Any non-substantial improvements made using the inventive concept and technical solution of the present invention, or the direct application of the inventive concept and technical solution of the present invention to other occasions without modification, are all within the protection scope of the present invention.

Claims

1. A cement-based chromium-reducing microcapsule, characterized in that, The cement-based chromium-reducing microcapsule comprises a capsule wall and a capsule core. The capsule wall is a water-soluble outer membrane, and the capsule core comprises a porous water-absorbing skeleton, a reducing agent, and a soluble aluminum salt. The soluble aluminum salt is embedded inside the porous water-absorbing skeleton, and the reducing agent is attached to the pores on the outer side of the porous water-absorbing skeleton.

2. The cement chromium-reducing microcapsule according to claim 1, characterized in that, The reducing agent is any one or a mixture of any two or more of ferrous nitrate, ferrous sulfate, stannous sulfate, and stannous nitrate.

3. The cement chromium-reducing microcapsule according to claim 1, characterized in that, The soluble aluminum salt is any one or a mixture of any two or more of aluminum nitrate, aluminum chloride, and aluminum acetate.

4. The cement chromium-reducing microcapsule according to claim 1, characterized in that, The water-soluble outer membrane is formed of polyvinyl alcohol, and the porous water-absorbing skeleton is composed of calcium alginate.

5. The cement chromium-reducing microcapsule according to claim 1, characterized in that, The particle size of the cement chromium-reducing microcapsules is ≤80μm.

6. A method for preparing cement chromium-reducing microcapsules according to any one of claims 1-5, characterized in that, Includes the following steps: S1. Under inert gas protection, the reducing agent is added to the sodium alginate solution and stirred, and then ultrasonically pulverized to obtain solution A; S2. A solution of soluble aluminum salt is added dropwise to solution A, and solution B is obtained by high-speed shearing and stirring. S3. Solution B is atomized into fine droplets using an atomizer and then dropped into a polyvinyl alcohol solution, and stirred at a constant temperature to obtain solution C; S4. Add solution C to calcium chloride solution and stir evenly. Then freeze-dry to obtain cement chromium-reducing microcapsules.

7. The method for preparing cement chromium-reducing microcapsules according to claim 6, characterized in that, The mass ratio of the reducing agent, the sodium alginate, and the soluble aluminum salt is 1:0.5~1.5:1~3.

8. The method for preparing cement chromium-reducing microcapsules according to claim 6, characterized in that, In step S3, the amount of polyvinyl alcohol added accounts for 1-3% of the mass of the solute in solution B, and the constant temperature stirring temperature is 60℃.

9. The method for preparing cement chromium-reducing microcapsules according to claim 6, characterized in that, In step S4, the freeze-drying temperature is -20°C, and the concentration of the calcium chloride solution is 1-3%.

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