A modified yellow river sediment based fibrous calcium silicate thermal insulation material and a preparation method thereof

Abstract

The application belongs to the field of thermal insulation materials, and particularly discloses a fiber calcium silicate thermal insulation material based on modified Yellow River silt and a preparation method thereof. The Yellow River silt is modified first, so that the surface of the silt has adhesion to adsorb wood fibers to form a fiber reinforcement. Then, the modified Yellow River silt is mixed with NMMO solution of plant fibers, slaked lime, cement, fly ash and other substances to form a crosslinked structure, thereby preparing the fiber calcium silicate thermal insulation material. The fiber calcium silicate thermal insulation material prepared by the application has a bending strength of 0.72-0.86 MPa, a compressive strength of 1.74-1.85 MPa, a dry shrinkage of 0.01-0.02, a thermal conductivity of 0.078-0.089 w / m.k, and no cracks. The application not only improves the toughness of the calcium silicate thermal insulation material and reduces the occurrence of cracking, but also realizes the resource utilization of the Yellow River silt.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of thermal insulation materials, and particularly relates to a fiber calcium silicate thermal insulation material based on modified Yellow River sediment and a preparation method thereof. BACKGROUND

[0002] The calcium silicate thermal insulation material is mainly composed of calcium and silicon materials, and a proper amount of fiber is added, foaming and molding, and then steam curing to form a light thermal insulation material, which has the advantages of light weight, high strength, waterproof, moisture-proof, and no deformation, and has a wide application prospect in the field of building. At present, the calcium silicate thermal insulation material generally has the problems of poor toughness and cracking, which seriously limits the application of the calcium silicate thermal insulation material. At the same time, the silicon material used for producing the calcium silicate thermal insulation material is mainly derived from active silicon oxide compounds and quartz sand.

[0003] The Yellow River sediment mainly contains silicon dioxide, but there is no production and application of calcium silicate thermal insulation material based on the sediment. The Yellow River sediment has a large content and high processing cost, and how to utilize the sediment is also an urgent matter. SUMMARY

[0004] In view of the problems and deficiencies in the prior art, the purpose of the present application is to provide a fiber calcium silicate thermal insulation material based on modified Yellow River sediment and a preparation method thereof.

[0005] To achieve the purpose of the application, the technical scheme adopted by the present application is as follows:

[0006] The present application provides a preparation method of modified sediment, comprising the following steps:

[0007] (1) The sediment is soaked in acid solution for 20-30 min, and then dried to obtain sediment A;

[0008] (2) Urea, thiourea and lignocellulose are added to an alkali solution, and then stirred and reacted, and then the sediment A is added and stirred uniformly to obtain a mixture B;

[0009] (3) The mixture B is kept at 60-80℃ for 4-8 h, and then dried to obtain the modified sediment.

[0010] According to the preparation method of the modified sediment, preferably, the acid solution is hydrochloric acid.

[0011] According to the preparation method of the modified sediment, preferably, the mass ratio of the sediment A, urea, thiourea, lignocellulose and alkali solution is 100: (1-1.5): (1-1.5): (0.5-1): (10-16).

[0012] According to the preparation method of the modified sediment, preferably, the concentration of the alkali solution is 4 wt%-9 wt%, and the temperature of the alkali solution is 4-6 ℃. The low temperature of 4-6 ℃ is favorable for the rapid dissolution of the wood fibers and the uniform dispersion of the fibers on the surface of the sediment.

[0013] According to the preparation method of the modified sediment, preferably, the alkali solution is a sodium hydroxide solution.

[0014] According to the preparation method of the modified sediment, preferably, the sediment is Yellow River sediment.

[0015] According to the preparation method of the modified sediment, preferably, the sediment has a specific surface area of 300-600 m 2 / kg.

[0016] The second aspect of the present application provides a modified sediment product prepared by the preparation method of the first aspect.

[0017] The third aspect of the present application provides an application of the modified sediment product of the second aspect in the preparation of a calcium silicate thermal insulation material.

[0018] The fourth aspect of the present application provides a calcium silicate thermal insulation material. The raw materials of the calcium silicate thermal insulation material, in terms of weight parts, include 10-15 parts of NMMO, 5-8 parts of plant fibers, 100 parts of the modified sediment, 40-50 parts of slaked lime, 10-30 parts of cement, 5-15 parts of fly ash, 45-70 parts of water, 5-9 parts of calcium stearate, and 1-4 parts of a foaming agent. The modified sediment is the modified sediment product of the second aspect of the present application.

[0019] The fifth aspect of the present application provides a preparation method of the calcium silicate thermal insulation material of the fourth aspect, including the following steps.

[0020] Step 1: each raw material is weighed according to the raw material composition of the calcium silicate thermal insulation material of the fourth aspect of the present application.

[0021] Step 2: NMMO is dissolved in part of the water to obtain an NMMO aqueous solution; plant fibers are added to the NMMO aqueous solution, and the mixture is uniformly mixed at 100-150 ℃ to obtain a mixture C.

[0022] Step 3: a foaming agent and calcium stearate are dissolved in the remaining water to obtain a solution D; the modified sediment, slaked lime, cement, and fly ash are uniformly mixed to obtain a mixture E.

[0023] Step 4: the solution D is added to the mixture C and stirred to form a mixed solution F; the mixture E is added to the mixed solution F and uniformly stirred to obtain a foamed slurry.

[0024] Step 5: the foamed slurry is pressed into shape, and then curing treatment is performed, to obtain the calcium silicate thermal insulation material.

[0025] According to the preparation method of the calcium silicate thermal insulation material, preferably, the water content in the aqueous NMMO solution is 10-17 wt%.

[0026] According to the preparation method of the calcium silicate thermal insulation material, preferably, the specific operation of pressing the foamed slurry into shape and then performing curing treatment is as follows: the foamed slurry is pressed into shape to obtain a shaped green body, the shaped green body is first cured at 60-80℃ for 8-12 h, and then cured at 0.8-1.2 MPa and 160-180℃ for 12-16 h, and cooled to room temperature, to obtain the calcium silicate thermal insulation material.

[0027] According to the preparation method of the calcium silicate thermal insulation material, preferably, the pressure during the pressing into shape is 8-10 MPa, and the time for the pressing into shape is 5-8 min.

[0028] Compared with the prior art, the present application has the following beneficial effects:

[0029] (1) The present application modifies the Yellow River sediment, first soaks the Yellow River sediment in hydrochloric acid to modify and activate the surface of the sediment, then adds the sediment into a solution formed by urea, thiourea, wood fiber and alkali solution, forms a small amount of inconstant sodium silicate on the surface of the sediment under alkaline conditions, so that the sediment has adhesion, and the wood fiber can be attached to the surface of the sodium silicate to form a fiber reinforcement. By further curing in water bath at 60-80℃, the adhesion of the wood fiber on the surface of the Yellow River sediment and the activity of the Yellow River sediment itself are enhanced, to form modified Yellow River sediment. The modified Yellow River sediment can form a cross-linked structure with slaked lime, cement, fly ash and other substances, to further improve the crack resistance of the calcium silicate thermal insulation material.

[0030] (2) The modified Yellow River sediment is applied to prepare the calcium silicate thermal insulation material. In the preparation of the calcium silicate thermal insulation material, the plant fiber is dissolved in the NMMO solution, and then the mixture of the modified Yellow River sediment, slaked lime, cement and fly ash is added to the NMMO solution containing the plant fiber, so that the plant fiber is uniformly dispersed between the raw materials. After the foamed slurry is pressed into a shape and heated to 60-80℃, the solubility of the plant fiber in the green body decreases with the increase of the temperature, and then the plant fiber is re-precipitated in the green body, forming a reticular wrapping structure, which is beneficial to the structural stability of the green body. Then, the autoclaved curing (0.8-1.2 MPa, 160-180℃) is further carried out. Under the high temperature condition, the urea decomposes to generate ammonia gas, increasing the number of pores, so that the structure density of the green body is smaller and the thermal insulation property is better. At the same time, the plant fiber is dissolved again at 160-180℃, and in the dissolution process, the calcium silicate thermal insulation material gradually forms a high crystalline tobermorite structure, and the dissolved fiber becomes fluidized and gradually penetrates into the gaps and structures of the material. During the temperature reduction to room temperature, the fiber is re-precipitated and wrapped and adhered to other substances and structures in the calcium silicate thermal insulation material, thereby improving the combination density and strength of the plant fiber and the calcium silicate thermal insulation material, and effectively reducing the occurrence of cracks.

[0031] (3) In one embodiment of the present application, the prepared calcium silicate thermal insulation material has a bending strength of 0.72-0.86 MPa, a compressive strength of 1.74-1.85 MPa, a dry shrinkage of 0.01-0.02, a thermal conductivity of 0.078-0.089 w / m.k, and no cracks are generated.

[0032] In summary, the present application not only improves the toughness of the calcium silicate thermal insulation material and reduces the occurrence of cracking, but also realizes the resource utilization of the Yellow River sediment. DETAILED DESCRIPTION

[0033] The following examples are only used to further illustrate the present application. It should be noted that all the technical and scientific terms used in the present application have the same meaning as those in the technical field to which the present application belongs, unless otherwise specified. The experimental methods not specified in the following examples all use conventional techniques in the technical field, or follow the conditions recommended by the manufacturers; the reagents or instruments not specified by the manufacturers are all conventional products that can be obtained from the market.

[0034] In order to enable those skilled in the art to more clearly understand the technical solutions of the present application, the technical solutions of the present application will be described in detail below with specific examples.

[0035] Example 1: Preparation of modified Yellow River sediment

[0036] The present embodiment provides a preparation method of modified Yellow River sediment, comprising the following steps:

[0037] (1) Wash the Yellow River sediment clean, then grind it to a particle size specific surface area of 300 m 2 / kg, then immerse it in dilute hydrochloric acid for 30 min, take it out and dry it at 80°C to obtain sediment A;

[0038] (2) According to the weight fraction, mix 1 part of urea, 1.5 parts of thiourea and 0.5 parts of lignocellulose uniformly, then add them to 16 parts of a sodium hydroxide solution with a temperature of 6°C and a concentration of 4 wt%, then quickly add 100 parts of sediment A to it, stir for 5 min, heat it at 80°C in a water bath for 4 h, then dry it at 80°C to obtain modified Yellow River sediment.

[0039] Example 2: Preparation of modified Yellow River sediment

[0040] This example provides a method for preparing modified Yellow River sediment, which includes the following steps:

[0041] (1) Wash the Yellow River sediment clean, then grind it to a particle size specific surface area of 600 m 2 / kg, then immerse it in dilute hydrochloric acid for 30 min, take it out and dry it at 60°C to obtain sediment A;

[0042] (2) According to the weight fraction, mix 1.5 parts of urea, 1 part of thiourea and 1 part of lignocellulose uniformly, then add them to 10 parts of a sodium hydroxide solution with a temperature of 4°C and a concentration of 9 wt%, then quickly add 100 parts of sediment A to it, stir for 10 min, heat it at 60°C in a water bath for 8 h, then dry it at 80°C to obtain modified Yellow River sediment.

[0043] Example 3: Preparation of modified Yellow River sediment

[0044] This example provides a method for preparing modified Yellow River sediment, which includes the following steps:

[0045] (1) Wash the Yellow River sediment clean, then grind it to a particle size specific surface area of 450 m 2 / kg, then immerse it in dilute hydrochloric acid for 30 min, take it out and dry it at 70°C to obtain sediment A;

[0046] (2) According to the weight fraction, mix 1.2 parts of urea, 1.2 parts of thiourea and 1 part of lignocellulose uniformly, then add them to 12 parts of a sodium hydroxide solution with a temperature of 5°C and a concentration of 8%, then quickly add 100 parts of sediment A to it, stir for 8 min, heat it at 70°C in a water bath for 6 h, then dry it at 90°C to obtain modified Yellow River sediment.

[0047] Embodiments 1-3 of the present application modify the Yellow River sediment, first soak the Yellow River sediment in hydrochloric acid to modify the surface, then add the sediment to a solution formed by urea, thiourea, wood fiber and alkali solution, a small amount of sodium silicate is formed on the surface of the sediment under alkaline conditions, which makes it sticky, so that the wood fiber is adsorbed on the surface of the sodium silicate to form a fiber reinforced body, then water bath curing at 60-80℃, enhance the adhesion of wood fiber on the surface of the Yellow River sediment and the activity of the Yellow River sediment itself, and form modified Yellow River sediment.

[0048] Embodiment 4: Preparation of modified fiber calcium silicate thermal insulation material

[0049] The present embodiment provides a preparation method of a modified fiber calcium silicate thermal insulation material, wherein the raw materials for preparing the modified fiber calcium silicate thermal insulation material include, by weight fraction: 10 parts of NMMO, 5 parts of plant fiber, 100 parts of modified Yellow River sediment prepared in Embodiment 1, 40 parts of slaked lime, 30 parts of cement, 5 parts of fly ash, and 66.5 parts of water.

[0050] The specific preparation steps are as follows:

[0051] (1) Dissolve 10 parts of NMMO in water to obtain an NMMO solution with a water content of 17%; add 5 parts of plant fiber to the NMMO solution and heat to 150℃, stirring for 30 min to obtain a mixture A;

[0052] The plant fiber is selected from sawdust fiber with a mesh number of 20;

[0053] (2) Mix 100 parts of modified Yellow River sediment, 40 parts of slaked lime, 30 parts of cement and 5 parts of fly ash uniformly to obtain a mixture B;

[0054] (3) Boil the remaining water and add 1% of a foaming agent based on the total mass of the mixture B and 3% of calcium stearate based on the total mass of the mixture B to obtain a mixed solution C; pour the mixed solution C into the mixture A, stir quickly for 10 s, then quickly pour the mixture B into it and stir at 600 r / min for 120 s to obtain a foamed slurry;

[0055] (4) Pour the foamed slurry into a mold, press and form under 10 MPa for 5 min to obtain a formed green body; first cure the formed green body at 60℃ for 12 h, then perform steam pressure curing at 0.8 MPa and 180℃ for 12 h, take out the green body after curing and cool it to room temperature to obtain the modified fiber calcium silicate thermal insulation material.

[0056] Embodiment 5: Preparation of modified fiber calcium silicate thermal insulation material

[0057] The embodiment provides a preparation method of modified fibrous calcium silicate thermal insulation material, wherein raw materials for preparing the modified fibrous calcium silicate thermal insulation material include, in terms of weight fractions, 15 parts of NMMO, 8 parts of plant fiber, 100 parts of modified Yellow River silt prepared in the embodiment 2, 50 parts of slaked lime, 10 parts of cement, 15 parts of fly ash, and 49.5 parts of water.

[0058] Specific preparation steps are as follows:

[0059] (1) 15 parts of NMMO are dissolved in water to obtain an NMMO solution with a water content of 10%; 8 parts of plant fiber are added to the NMMO solution, heated to 100 DEG C, and stirred for 30 min to obtain a mixture A;

[0060] (2) 100 parts of modified Yellow River silt, 50 parts of slaked lime, 10 parts of cement and 15 parts of fly ash are uniformly mixed to obtain a mixture B;

[0061] (3) the remaining water is boiled, and 2% of a foaming agent based on the total mass of the mixture B and 5% of calcium stearate based on the total mass of the mixture B are added to obtain a mixed solution C; the mixed solution C is poured into the mixture A, stirred rapidly for 5 s, then the mixture B is rapidly poured into the mixture A and stirred at 700 r / min for 180 s to obtain a foamed slurry.

[0062] (4) the foamed slurry is poured into a mold, and is pressed and formed under 8 MPa for 5 min to obtain a formed green body; the formed green body is first cured at 80 DEG C for 8 h, then is subjected to autoclave curing at 1.2 MPa and 160 DEG C for 16 h, and after curing, the green body is taken out and cooled to room temperature to obtain the modified fibrous calcium silicate thermal insulation material.

[0063] Embodiment 6: preparation of modified fibrous calcium silicate thermal insulation material

[0064] The embodiment provides a preparation method of modified fibrous calcium silicate thermal insulation material, wherein raw materials for preparing the modified fibrous calcium silicate thermal insulation material include, in terms of weight fractions, 12 parts of NMMO, 6 parts of plant fiber, 100 parts of modified Yellow River silt prepared in the embodiment 3, 45 parts of slaked lime, 20 parts of cement, 10 parts of fly ash, and 58 parts of water.

[0065] Specific preparation steps are as follows:

[0066] (1) 12 parts of NMMO are dissolved in water to obtain an NMMO solution with a water content of 13%; 6 parts of plant fiber are added to the NMMO solution, heated to 120 DEG C, and stirred for 30 min to obtain a mixture A;

[0067] (2) 100 parts of modified Yellow River silt, 45 parts of slaked lime, 20 parts of cement and 10 parts of fly ash are uniformly mixed to obtain a mixture B;

[0068] (3) Boil the remaining water, and add 1.5% of the total mass of the mixture B of foaming agent and 4% of the total mass of the mixture B of calcium stearate to obtain a mixed solution C; pour the mixed solution C into the mixture A, after rapid stirring for 8 s, quickly pour the mixture B into it and stir at 650 r / min for 150 s to obtain a foamed slurry.

[0069] (4) Pour the foamed slurry into a mold, and press form under 9 MPa for 5 min to obtain a formed body; first cure the formed body at 70℃ for 10 h, then perform autoclave curing at 1.0 MPa and 170℃ for 14 h, after the curing is completed, take out the body and cool to room temperature to obtain the modified fibrous calcium silicate thermal insulation material.

[0070] Comparative Example 1

[0071] The content of Comparative Example 1 is basically the same as that of Example 6, except that the modified Yellow River sediment is not used in step (2).

[0072] Comparative Example 2

[0073] The content of Comparative Example 2 is basically the same as that of Example 6, except that the plant fiber is not dissolved by NMMO, that is, Comparative Example 2 does not perform step (1) in Example 6, and in step (3), the mixed solution C is directly poured into the plant fiber.

[0074] Comparative Example 3

[0075] The content of Comparative Example 3 is basically the same as that of Example 6, except that the modified Yellow River sediment is not used in step (2), and the plant fiber is not dissolved by NMMO, that is, Comparative Example 3 does not perform step (1) in Example 6, and in step (3), the mixed solution C is directly poured into the plant fiber.

[0076] Comparative Example 4

[0077] The content of Comparative Example 4 is basically the same as that of Example 6, except that the quartz sand powder is used instead of the modified Yellow River sediment in step (2).

[0078] The flexural strength and compressive strength of the calcium silicate thermal insulation materials prepared in Examples 4-6 and Comparative Examples 1-4 of the present application were tested according to the method specified in GB / T 50081-2019, the density was tested according to the provisions of GB / T 7019-2014, the dry shrinkage rate was tested according to the provisions of JC / T 564.1-2008, and the thermal conductivity of each sample was tested according to the method specified in GB / T 10294-2008. The test results are shown in Table 1.

[0079]

[0080] Results analysis:

[0081] As can be seen from the comparison of Example 6 and Comparative Example 1 in Table 1, after the Yellow River sediment is modified, the flexural strength and compressive strength of the calcium silicate thermal insulation material are improved, the dry shrinkage and thermal conductivity of the calcium silicate thermal insulation material are reduced, and no cracks are generated. This is because the activity of the Yellow River sediment is improved after modification, so it can more effectively participate in the hardening and cementation reaction of the calcium silicate thermal insulation material, making the inter-particle bonding more compact, and the support effect of the Yellow River sediment skeleton higher. Therefore, under the condition of lower density, the compressive strength and flexural strength of the calcium silicate thermal insulation material are improved, the dry shrinkage and cracks are greatly reduced and decreased, and the thermal conductivity is also significantly reduced.

[0082] As can be seen from the comparison of Example 6 and Comparative Example 2 in Table 1, compared with Comparative Example 2 which does not use NMMO to dissolve plant fibers, the NMMO solution containing dissolved plant fibers is mixed with other raw materials in the present application, so that the density of the prepared calcium silicate thermal insulation material is reduced, and the compressive strength and flexural strength of the calcium silicate thermal insulation material are improved, and the dry shrinkage and cracks of the material are greatly reduced and decreased. This is because the plant fibers are dissolved in the NMMO solution and then added to the mixture, which can make them uniformly dispersed among other raw materials. When cured at 60-80°C, the solubility decreases and the fibers are reprecipitated, forming a network structure around the green body, which is beneficial to the formation and structure stability of the green body. Further through autoclave curing, the fibers are dissolved again at 160-180°C, and in the process of dissolution, high crystalline tobermorite structure is gradually formed in the green body, and the re-dissolved fibers gradually penetrate into the gaps and structure of the material in a fluidized state. Finally, as the molding temperature decreases, the fibers are reprecipitated and form entanglement and adhesion with other substances and structures in the calcium silicate thermal insulation material, effectively reducing the generation of cracks, thereby improving the combination density and strength of the plant fibers and the calcium silicate thermal insulation material, and improving the stability of the material structure.

[0083] As can be seen from the comparison of Example 6 and Comparative Example 3 in Table 1, when the Yellow River sediment is not modified and the plant fibers are not dissolved in the NMMO solution, the obtained material not only has poor mechanical properties and high dry shrinkage, but also has many cracks, and cannot be prepared into a calcium silicate thermal insulation material that meets the requirements.

[0084] As can be seen from Comparative Example 1 and Comparative Example 4 in Table 1, the product prepared by using unmodified Yellow River sand has slightly poorer performance than the product prepared by using quartz sand. As can be seen from Example 6 compared with Comparative Example 4, after the modified Yellow River sand is used in the present application, the density, dry shrinkage, crack number and thermal conductivity of the product all decrease, but the bending strength and compressive strength are obviously improved. This shows that the calcium silicate thermal insulation material prepared by using the modified Yellow River sand is superior to the calcium silicate thermal insulation material prepared by using the traditional quartz sand.

[0085] In summary, the present application uses modified sand to prepare calcium silicate thermal insulation material, which not only improves the toughness of the calcium silicate thermal insulation material and reduces the occurrence of cracking, but also realizes the resource utilization of the Yellow River sand.

[0086] The above examples are specific embodiments of the present application, but the embodiments of the present application are not limited by the above examples, and any other combination, change, modification, replacement or simplification within the design idea of the present application also falls within the protection scope of the present application.

Claims

1. A modified silt product, characterized in that, It was prepared using the following method: (1) Soak the mud and sand in acid solution for 20-30 minutes, and then dry it to obtain mud and sand A; (2) Add urea, thiourea and lignocellulose to sodium hydroxide solution, stir to react, and then add mud and sand A to it and stir evenly to obtain mixture B; (3) The mixture B is kept at 60-80℃ for 4-8 h and dried to obtain modified mud and sand; The mass ratio of the silt A, urea, thiourea, lignocellulose, and sodium hydroxide solution is 100:(1-1.5):(1-1.5):(0.5-1):(10-16). The concentration of the sodium hydroxide solution is 4 wt% to 9 wt%, and the temperature of the sodium hydroxide solution is 4 to 6 °C.

2. The application of the modified silt product according to claim 1 in the preparation of calcium silicate thermal insulation materials.

3. A calcium silicate thermal insulation material, characterized in that, By weight, the raw materials of the calcium silicate insulation material include: 10-15 parts NMMO, 5-8 parts plant fiber, 100 parts modified silt, 40-50 parts quicklime, 10-30 parts cement, 5-15 parts fly ash, 45-70 parts water, 5-9 parts calcium stearate, and 1-4 parts foaming agent; the modified silt is the modified silt product as described in claim 1.

4. The method for preparing the calcium silicate thermal insulation material according to claim 3, characterized in that, Includes the following steps: Step 1: Weigh each raw material according to the raw material composition of the calcium silicate insulation material as described in claim 3; Step 2: Dissolve NMMO in a portion of water to obtain an NMMO aqueous solution. Add plant fiber to the NMMO aqueous solution and mix at 100-150°C to obtain mixture C. Step 3: Dissolve the foaming agent and calcium stearate in the remaining water to obtain solution D; mix the modified mud, quicklime, cement and fly ash evenly to obtain mixture E; Step 4: Add solution D to mixture C and stir to form mixture F; then add mixture E to mixture F and stir evenly to obtain foam slurry; Step 5: Press the foam slurry into shape, and then perform curing treatment to obtain calcium silicate insulation material.

5. The method for preparing the calcium silicate thermal insulation material according to claim 4, characterized in that, The water content in the NMMO aqueous solution is 10–17 wt%.

6. The method for preparing calcium silicate thermal insulation material according to claim 5, characterized in that, The specific operation of pressing the foam slurry into shape and then curing it is as follows: press the foam slurry into shape to obtain a molded blank, keep the molded blank at 60-80℃ for 8-12 h, and then keep it at 0.8-1.2 MPa and 160-180℃ for 12-16 h, and cool it to room temperature to obtain calcium silicate insulation material.

7. The method for preparing the calcium silicate thermal insulation material according to claim 5 or 6, characterized in that, The pressing pressure is 8–10 MPa, and the pressing time is 5–8 min.

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