High-strength non-burned brick prepared from construction waste and preparation method thereof

Abstract

The application relates to the field of building waste resource utilization, and discloses a high-strength baking-free brick prepared from building waste and a preparation method thereof, wherein the raw materials include, in parts by weight, building waste aggregate 60-70 parts, cement 20-30 parts, polyurea resin 6-10 parts, water reducing agent 1-5 parts, and water 5-10 parts; the polyurea resin comprises A component and B component in a mass ratio of 1-3:1, the A component is a secondary amine component, and the B component is an isocyanate curing agent. The baking-free brick prepared by taking the building waste as the aggregate, the cement as the cementing material and adding the polyurea resin as the reinforcing agent has excellent mechanical properties and water resistance, and the building waste can be reduced and utilized as resources.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of resource utilization of construction waste, in particular to a high-strength baking-free brick prepared from construction waste and a preparation method thereof. BACKGROUND

[0002] At present, the amount of construction waste in China accounts for 30-40% of the total amount of municipal waste, and the treatment of construction waste has become a major problem in urban construction. The main treatment method of construction waste is to fill the field, which not only occupies a large amount of land, but also consumes manpower and material resources, and at the same time, it hides the hidden danger of secondary pollution. The construction waste is made into baking-free brick products, which can solve the environmental load problem caused by construction waste and reduce the environmental damage and pollution caused by the production of building materials, and can realize the reduction and resource utilization of construction waste.

[0003] In the existing production process of construction waste baking-free brick, the construction waste is generally made into aggregate, then mixed with cement, sand, gravel and the like, and pressed into baking-free brick by vibration. For example, the "construction waste baking-free brick or block and preparation method thereof" disclosed in the Chinese patent document, with publication number CN102765914B, is made from the following raw materials by weight fraction: construction waste broken material 30-57 parts, coal gangue particles 25-50 parts, cement 8-15 parts, fly ash 3-18 parts, strong silicon sealing curing agent 0.01-0.025 parts and water 8-12 parts.

[0004] However, the construction waste baking-free brick prepared in the prior art generally has the disadvantages of relatively small structural strength, easy to crack, and low impact resistance, and its own structure is relatively loose, and the performance in moisture and water resistance is also relatively low, which affects its use performance. SUMMARY

[0005] The present application is to overcome the above-mentioned problems in the prior art of preparing baking-free brick from construction waste, and provides a high-strength baking-free brick prepared from construction waste and a preparation method thereof. The baking-free brick prepared by using construction waste as aggregate, cement as cementing material and adding polyurea resin as reinforcing agent has excellent mechanical properties and water resistance, and can realize the reduction and resource utilization of construction waste.

[0006] In order to achieve the above-mentioned purpose, the present application adopts the following technical scheme:

[0007] The high-strength baking-free brick prepared from construction waste comprises, by weight, 60-70 parts of construction waste aggregate, 20-30 parts of cement, 6-10 parts of polyurea resin, 1-5 parts of water reducing agent and 5-10 parts of water; the polyurea resin comprises A component and B component in a mass ratio of 1-3:1, the A component is a secondary amine component, and the B component is an isocyanate curing agent;

[0008] The preparation method of the secondary amine component comprises the following steps:

[0009] A) performing substitution reaction on 1,3-dichloro-1,3-dimethyl-1,3-diphenyl disiloxane with ethylenediamine to obtain a silicon-containing aromatic diamine; B) performing Michael addition reaction on the obtained silicon-containing aromatic diamine with 2-hydroxyethyl methacrylate phosphate to obtain the secondary amine component.

[0010] The baking-free brick of the present application uses construction waste aggregate as the main raw material, cement as the cementing material, and adds polyurea resin as the reinforcing agent; compared with traditional epoxy resin, the polyurea resin has the characteristics of good toughness and strong impact resistance and is not easy to crack under external impact; meanwhile, the polyurea resin has good weather resistance and long service life; when the polyurea resin is added to the baking-free brick as a reinforcing agent, a large organic molecular chain can be generated through the reaction of the amino-containing component and the isocyanate curing agent to cement the aggregate, thereby effectively improving the compressive strength and water resistance of the baking-free brick. However, the existing polyurea resin generally has the problem of short gel time, which will lead to short construction time and uneven stirring of materials when used for preparing baking-free bricks.

[0011] In order to prolong the gel time of the polyurea resin, the polyurea resin of the present application reacts with a secondary amine component and an isocyanate curing agent, and the reactivity of the secondary amine with the isocyanate curing agent is lower than that of a primary amine, so that the polyurea resin has a longer gel time, facilitating the mixing of materials and the preparation of the no-burning brick. Meanwhile, the present application further prolongs the gel time of the polyurea resin and improves the mechanical properties and water resistance of the polyurea resin by designing the molecular structure of the secondary amine component. In the preparation process of the secondary amine component of the present application, the substitution reaction of the halogen in 1,3-dichloro-1,3-dimethyl-1,3-diphenyl disiloxane with the amino group in ethylenediamine is carried out first to obtain a binary primary amine containing a silicon-containing segment and a benzene ring in the molecular chain; then the Michael addition reaction of the binary primary amine with 2-hydroxyethyl methacrylate phosphate is carried out to obtain a secondary amine component with a secondary amine structure. The introduction of the silicon-containing segment can reduce the viscosity of the polyurea resin, improve its permeability in the construction waste aggregate and cement mixture, and facilitate the mixing of materials, so that the polyurea resin can uniformly cement the particulate matter in the mixture; meanwhile, the introduction of the silicon-containing segment can improve the hydrophobicity of the polyurea resin and improve the permeability resistance and water resistance of the no-burning brick. However, the introduction of the silicon-containing segment can reduce the strength of the polyurea resin, so the present application also introduces a benzene ring into the secondary amine component. The introduction of the double benzene ring structure on the one hand improves the strength of the molecular chain and is beneficial to the improvement of the mechanical properties of the no-burning brick, and on the other hand increases the reaction steric hindrance of the secondary amine component with the isocyanate curing agent, further prolongs the gel time of the polyurea resin, and is beneficial to the preparation operation of the no-burning brick. Moreover, the present application uses 2-hydroxyethyl methacrylate phosphate to react with the binary primary amine to prepare the secondary amine, which can introduce phosphate groups into the secondary amine component, so that the no-burning brick has good flame retardant performance and improves the use performance of the no-burning brick.

[0012] As preferred, the molar ratio of 1,3-dichloro-1,3-dimethyl-1,3-diphenyl disiloxane to ethylenediamine in step A) is 1:2-2.5.

[0013] As preferred, the reaction conditions of step A) are as follows: 1,3-dichloro-1,3-dimethyl-1,3-diphenyl disiloxane and ethylenediamine are dissolved in toluene, and the reaction is carried out under nitrogen protection at 150-170℃ for 12-24h with stirring, and then the solvent is removed by reduced pressure distillation to obtain the silicon-containing aromatic diamine.

[0014] As preferred, the molar ratio of the silicon-containing aromatic diamine to 2-hydroxyethyl methacrylate phosphate in step B) is 1-1.2:2.

[0015] As preferred, the reaction conditions of step B) are as follows: the silicon-containing aromatic diamine and 2-hydroxyethyl methacrylate phosphate are mixed, and a catalyst is added, and the reaction is carried out under nitrogen protection at 80-100℃ for 12-24h with stirring to obtain the secondary amine component; the addition amount of the catalyst is 0.05-0.1% of the total mass of the reactants.

[0016] Preferably, the cement is Portland cement; and the water reducing agent is a polycarboxylate type water reducing agent.

[0017] The application further discloses a preparation method of the high-strength baking-free brick prepared from construction waste.

[0018] (1) construction waste sorting and crushing: the construction waste is sorted, cleaned and crushed after the recyclable components are removed, to obtain construction waste aggregates;

[0019] (2) mixing raw materials: the construction waste aggregates, cement, water reducing agent and water are mixed and stirred uniformly according to proportions, and then the A component and the B component in the polyurea resin are added in sequence, and the mixing and stirring are continued to obtain a mixture;

[0020] (3) pressing and forming: the mixture is pressed and formed to obtain a baking-free brick blank;

[0021] (4) curing: the baking-free brick blank is naturally cured to obtain the high-strength baking-free brick.

[0022] The application removes the recyclable components such as plastic, wood and metal from the construction waste, and crushes the construction waste into construction waste aggregates, and uses the construction waste aggregates as main raw materials, cement as cementing material and polyurea resin as reinforcing agent, and adopts a baking-free and steaming-free process to press and form the baking-free brick, and then the baking-free brick is fully cured, so that the prepared baking-free brick has excellent compressive strength and water resistance, becomes an environment-friendly building material, and realizes the reduction and resource utilization of the construction waste.

[0023] Preferably, the particle size of the construction waste aggregates crushed in step (1) is less than or equal to 5 mm.

[0024] Preferably, the temperature during the pressing and forming in step (3) is 80-100 DEG C, and the pressure is 15-25 MPa.

[0025] Preferably, the curing time in step (4) is 15-30 days.

[0026] Therefore, the application has the following beneficial effects:

[0027] (1) the construction waste aggregates are used as main raw materials, and the cement is used as cementing material to prepare the baking-free brick, which can solve the environmental load problem caused by the construction waste, reduce the environmental damage and pollution caused by the production of building materials, and realize the reduction and resource utilization of the construction waste;

[0028] (2) the polyurea resin is added as a reinforcing agent in the baking-free brick, which can effectively improve the compressive strength and water resistance of the baking-free brick;

[0029] (3) By designing the molecular structure of the secondary amine component in the polyurea resin, the permeability of the polyurea resin is improved, the gel time of the polyurea resin is prolonged, the preparation of the non-burning brick is facilitated, the hydrophobicity, the flame retardancy and the molecular chain strength of the polyurea resin are improved, the mechanical properties and the water resistance of the non-burning brick are improved, and the non-burning brick is endowed with flame retardancy. DETAILED DESCRIPTION

[0030] The application will be further described below in combination with specific embodiments.

[0031] In the application, all raw materials can be purchased from the market or commonly used in the industry, and the methods in the following examples are conventional methods in the field unless otherwise specified.

[0032] The construction waste used in the embodiments of the application is waste floor concrete (C18), the cement is P.O 42.5 ordinary portland cement, the water reducing agent is a polycarboxylate-based water reducing agent purchased from Wuhan Runxingyuan Technology Co., Ltd., and the isocyanate curing agent is Wanhua HT300.

[0033] Overall example:

[0034] A method for preparing high-strength non-burning bricks from construction waste, comprising the following steps:

[0035] (1) Sorting and crushing the construction waste: after sorting the construction waste and removing recyclable components such as plastics and metals, the construction waste is washed and crushed, and the crushed construction waste is sieved to obtain construction waste aggregates with a particle size of ≤5 mm;

[0036] (2) Preparing the secondary amine component in the polyurea resin:

[0037] A) 1,3-dichloro-1,3-dimethyl-1,3-diphenyl disiloxane and ethylenediamine with a molar ratio of 1:2-2.5 are dissolved in toluene, and stirred at 150-170℃ under nitrogen protection for 12-24h, and then the solvent is removed by reduced pressure distillation to obtain a silicon-containing aromatic diamine;

[0038] B) 2-hydroxyethyl methacrylate phosphate is added to the obtained silicon-containing aromatic diamine, the molar ratio of the silicon-containing aromatic diamine to the 2-hydroxyethyl methacrylate phosphate is 1-1.2:2, and sodium methoxide catalyst is added, the addition amount of the sodium methoxide is 0.05-0.1% of the total mass of the silicon-containing aromatic diamine and the 2-hydroxyethyl methacrylate phosphate; the mixture is stirred at 80-100℃ under nitrogen protection for 12-24h to obtain the secondary amine component;

[0039] (3) raw material mixing: 60-70 parts by weight of construction waste aggregate, 20-30 parts of cement, 1-5 parts of water reducing agent and 5-10 parts of water are added into a mixer and stirred for 5-10 min, then a secondary amine component and an isocyanate curing agent with a mass ratio of 1-3:1 (the total amount of the secondary amine component and the isocyanate curing agent is 6-10 parts) are added in turn, and stirring is continued for 3-5 min to obtain a mixture;

[0040] (4) compression molding: the mixture is compression molded to obtain a non-burned brick blank; the temperature during compression molding is 80-100℃, and the pressure is 15-25 MPa;

[0041] (5) curing: the non-burned brick blank is naturally cured for 15-30 days to obtain the high-strength non-burned brick.

[0042] Example 1:

[0043] A method for preparing a high-strength non-burned brick from construction waste, comprising the following steps:

[0044] (1) construction waste sorting and crushing: the construction waste is sorted, the recyclable components such as plastic and metal are removed, then washed and crushed, the crushed construction waste is sieved (pore size is 4.75 mm), and the undersize material is taken to obtain construction waste aggregate;

[0045] A) 1,3-dichloro-1,3-dimethyl-1,3-diphenyl disiloxane and ethylenediamine with a molar ratio of 1:2.2 are dissolved in toluene, stirred at 160℃ under nitrogen protection for 18h, and then the solvent is removed by reduced pressure distillation to obtain a silicon-containing aromatic diamine;

[0046] B) 2-hydroxyethyl methacrylate phosphate is added to the obtained silicon-containing aromatic diamine, the molar ratio of the silicon-containing aromatic diamine to 2-hydroxyethyl methacrylate phosphate is 1:2, and sodium methoxide catalyst is added, the amount of sodium methoxide added is 0.06% of the total mass of the silicon-containing aromatic diamine and 2-hydroxyethyl methacrylate phosphate; stirred at 90℃ under nitrogen protection for 18h to obtain the secondary amine component;

[0047] (3) raw material mixing: 60-70 parts by weight of construction waste aggregate, 20-30 parts of cement, 1-5 parts of water reducing agent and 5-10 parts of water are added into a mixer and stirred for 5-10 min, then a secondary amine component and an isocyanate curing agent with a mass ratio of 1-3:1 (the total amount of the secondary amine component and the isocyanate curing agent is 6-10 parts) are added in turn, and stirring is continued for 3-5 min to obtain a mixture;

[0048] (4) compression molding: the mixture is compression molded to obtain a non-burned brick blank; the temperature during compression molding is 80-100℃, and the pressure is 15-25 MPa;

[0049] (5)Curing: the unfired brick blank is naturally cured for 28 days to obtain the high-strength unfired brick.

[0050] Example 2:

[0051] A method for preparing a high-strength unfired brick from construction waste, comprising the following steps:

[0052] (1) Construction waste sorting and crushing: the construction waste is sorted, and the recyclable components such as plastics and metals are removed, and then washed and crushed. The crushed construction waste is sieved (pore size is 4.75 mm), and the undersize material is taken to obtain construction waste aggregate; (2) Preparation of the secondary amine component in the polyurea resin: the preparation method is the same as that in Example 1;

[0053] (3) Raw material mixing: 60 parts of construction waste aggregate, 30 parts of cement, 5 parts of water reducing agent and 10 parts of water are added into a mixer and stirred for 5 minutes, and then 3 parts of the secondary amine component and 3 parts of the isocyanate curing agent are added in turn, and stirring is continued for 5 minutes to obtain a mixture;

[0054] (4) Pressing and forming: the mixture is pressed and formed to obtain an unfired brick blank with a size of 240 mm x 115 mm x 125 mm; the temperature during pressing and forming is 90°C, and the pressure is 20 MPa;

[0055] (5) Curing: the unfired brick blank is naturally cured for 28 days to obtain the high-strength unfired brick.

[0056] Example 3:

[0057] A method for preparing a high-strength unfired brick from construction waste, comprising the following steps:

[0058] (1) Construction waste sorting and crushing: the construction waste is sorted, and the recyclable components such as plastics and metals are removed, and then washed and crushed. The crushed construction waste is sieved (pore size is 4.75 mm), and the undersize material is taken to obtain construction waste aggregate; (2) Preparation of the secondary amine component in the polyurea resin: the preparation method is the same as that in Example 1;

[0059] (3) Raw material mixing: 60 parts of construction waste aggregate, 30 parts of cement, 5 parts of water reducing agent and 10 parts of water are added into a mixer and stirred for 5 minutes, and then 3 parts of the secondary amine component and 3 parts of the isocyanate curing agent are added in turn, and stirring is continued for 5 minutes to obtain a mixture;

[0060] (4) Pressing and forming: the mixture is pressed and formed to obtain an unfired brick blank with a size of 240 mm x 115 mm x 125 mm; the temperature during pressing and forming is 90°C, and the pressure is 20 MPa;

[0061] (5) Curing: the unfired brick blank is naturally cured for 28 days to obtain the high-strength unfired brick.

[0062] Comparative Example 1 (without adding polyurea resin):

[0063] A method for preparing a non-burned brick by using construction waste, comprising the following steps:

[0064] (1) Construction waste sorting and crushing: the construction waste is sorted, and the recyclable components such as plastics and metals are removed, and then washed and crushed. The crushed construction waste is sieved (pore size is 4.75 mm), and the undersize material is taken to obtain construction waste aggregate;

[0065] (2) Raw material mixing: 65 parts of construction waste aggregate, 25 parts of cement, 3 parts of water reducing agent and 7 parts of water are added into a mixer and stirred for 10 min to obtain a mixture;

[0066] (3) Pressing forming: the mixture is pressed to form a 240 mm x 115 mm x 125 mm non-burned brick blank; the temperature during pressing forming is 90℃, and the pressure is 20 MPa;

[0067] (4) Curing: the non-burned brick blank is naturally cured for 28 days to obtain the non-burned brick.

[0068] Comparative Example 2 (using primary amine reaction):

[0069] Comparative Example 2 and Example 1 differ in that the polyurea resin comprises a primary amine component and an isocyanate curing agent, and the preparation method of the primary amine component is as follows: 1,3-dichloro-1,3-dimethyl-1,3-diphenyl disiloxane and ethylenediamine with a molar ratio of 1:2-2.5 are dissolved in toluene, and stirred at 150-170℃ for 12-24h under nitrogen protection, and then the solvent is removed by reduced pressure distillation to obtain the primary amine component; the rest are the same as in Example 1.

[0070] Comparative Example 3 (without introducing a silicon-containing segment in the secondary amine component):

[0071] Comparative Example 3 and Example 1 differ in that the preparation method of the secondary amine component in the polyurea resin is as follows: 2-hydroxyethyl methacrylate phosphate is added to ethylenediamine, the molar ratio of ethylenediamine to 2-hydroxyethyl methacrylate phosphate is 1:2, and sodium methoxide catalyst is added, and the addition amount of sodium methoxide is 0.06% of the total mass of ethylenediamine and 2-hydroxyethyl methacrylate phosphate; stirred at 90℃ for 18h under nitrogen protection to obtain the secondary amine component; the rest are the same as in Example 1.

[0072] Comparative Example 4 (without introducing a benzene ring in the secondary amine component):

[0073] Comparative Example 4 and Example 1 differ in that the preparation method of the secondary amine component in the polyurea resin is as follows:

[0074] A) 1-3-dichlorotetramethyldisiloxane and ethylenediamine in a molar ratio of 1:2.2 were dissolved in toluene, and stirred at 160°C for 18h under nitrogen protection. After removing the solvent by distillation under reduced pressure, the silicic diamine was obtained;

[0075] B) 2-hydroxyethyl methacrylate phosphate was added to the obtained silicic diamine, and the molar ratio of the silicic diamine to 2-hydroxyethyl methacrylate phosphate was 1:2. Sodium methoxide was added as a catalyst, and the amount of sodium methoxide added was 0.06% of the total mass of the silicic diamine and 2-hydroxyethyl methacrylate phosphate. The mixture was stirred at 90°C for 18h under nitrogen protection to obtain the secondary amine component. The rest was the same as in Example 1.

[0076] The properties of the unfired bricks prepared in the above examples and comparative examples were tested, and the results are shown in Table 1. The flame retardant property was tested according to the method in GB8624-2012, and the rest of the properties were tested according to the method in GB / T 2542-2012.

[0077] Table 1: Test results of the properties of the unfired bricks.

[0078]

[0079] As can be seen from Table 1, the unfired bricks prepared in Examples 1-3 using the method of the present application have good mechanical properties, water resistance, frost resistance and flame retardant properties. The properties of the unfired bricks prepared in Comparative Example 1 without adding polyurea resin to the raw materials are significantly lower than those in Example 1, indicating that the addition of polyurea resin can significantly improve the performance of the unfired bricks. In Comparative Example 2, the primary amine in the polyurea resin is used instead of the secondary amine component in the present application to react with the isocyanate curing agent. Since the reaction rate of the primary amine with the isocyanate is faster than that of the secondary amine, the gelation speed of the polyurea resin is too fast, making it difficult to mix uniformly with the raw materials, and the performance of the unfired bricks cannot be effectively improved. Moreover, the polyurea resin does not undergo addition with 2-hydroxyethyl methacrylate phosphate, lacking the effect of the phosphate group, and the flame retardant properties of the unfired bricks will also decrease. In Comparative Example 3, the polyurea resin does not have a silicon-containing segment and a benzene ring in the secondary amine component. The viscosity of the polyurea resin is large, and the penetration is poor. Moreover, the molecular chain is short and lacks the effect of the benzene ring, the reaction steric hindrance of the secondary amine component with the isocyanate is reduced, the gelation time is fast, and it is also difficult to mix uniformly with the raw materials, and the performance of the unfired bricks cannot be effectively improved. In Comparative Example 4, the polyurea resin does not have a benzene ring in the secondary amine component, and the mechanical properties of the polyurea resin decrease. Moreover, the gelation time of the polyurea resin is shortened due to the lack of the effect of the benzene ring, resulting in a decrease in the mechanical properties of the unfired bricks compared to Example 1.

Claims

1. A high-strength, non-fired brick made from construction waste, characterized in that, By weight, the raw materials include: 60-70 parts of construction waste aggregate, 20-30 parts of cement, 6-10 parts of polyurea resin, 1-5 parts of water-reducing agent, and 5-10 parts of water; the polyurea resin includes component A and component B in a mass ratio of 1-3:1, component A is a secondary amine component, and component B is an isocyanate curing agent. The preparation method of the secondary amine component includes the following steps: A) 1,3-Dichloro-1,3-dimethyl-1,3-diphenyldisiloxane is subjected to a substitution reaction with ethylenediamine to obtain a silicon-containing aromatic diamine; B) The obtained silane-containing aromatic diamine is subjected to a Michael addition reaction with 2-hydroxyethyl methacrylate phosphate to obtain the secondary amine component.

2. The high-strength, non-fired brick prepared from construction waste according to claim 1, characterized in that, In step A), the molar ratio of 1,3-dichloro-1,3-dimethyl-1,3-diphenyldisiloxane to ethylenediamine is 1:2 to 2.

5.

3. The high-strength, non-fired brick prepared from construction waste according to claim 1 or 2, characterized in that, The reaction conditions for step A) are as follows: 1,3-dichloro-1,3-dimethyl-1,3-diphenyldisiloxane and ethylenediamine are dissolved in toluene, and the mixture is stirred at 150-170°C for 12-24 hours under nitrogen protection. The solvent is removed by vacuum distillation to obtain the silicon-containing aromatic diamine.

4. The high-strength, non-fired brick prepared from construction waste according to claim 1, characterized in that, In step B), the molar ratio of silanol diamine to 2-hydroxyethyl methacrylate phosphate is 1 to 1.2:

2.

5. The high-strength, non-fired brick prepared from construction waste according to claim 1 or 4, characterized in that, The reaction conditions for step B) are as follows: a silane-containing aromatic diamine and 2-hydroxyethyl methacrylate phosphate are mixed and a catalyst is added. The mixture is stirred and reacted at 80-100°C for 12-24 hours under nitrogen protection to obtain the secondary amine component. The amount of catalyst added is 0.05-0.1% of the total mass of the reactants.

6. The high-strength, non-fired brick prepared from construction waste according to claim 1, characterized in that, The cement is silicate cement; the water-reducing agent is a polycarboxylate-based water-reducing agent.

7. A method for preparing high-strength, non-fired bricks using construction waste as described in any one of claims 1 to 6, characterized in that, Includes the following steps: (1) Construction waste sorting and crushing: Construction waste is sorted, and after removing the recyclable components, it is washed and crushed to obtain construction waste aggregate; (2) Raw material mixing: Mix construction waste aggregate, cement, water-reducing agent and water in proportion and stir evenly. Then add component A and component B of polyurea resin in sequence and continue to stir and mix evenly to obtain the mixture. (3) Pressing and molding: Press the mixture into shape to obtain non-fired brick blanks; (4) Curing: The high-strength unfired bricks are obtained by naturally curing the unfired brick blanks.

8. The preparation method according to claim 7, characterized in that, The particle size of the construction waste aggregate obtained by crushing in step (1) is ≤5mm.

9. The preparation method according to claim 7, characterized in that, In step (3), the temperature during pressing is 80-100℃ and the pressure is 15-25MPa.

10. The preparation method according to claim 7, characterized in that, The curing time in step (4) is 15 to 30 days.

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