Multi-element solid waste-based auxiliary cementing material and preparation method thereof

Abstract

The invention discloses a multi-element solid waste-based auxiliary cementing material and a preparation method thereof, the cementing material is prepared by taking steel slag, carbide slag and sintered red mud which are acidified and modified by microbial bacteria as raw materials, adding water, mixing and carbonizing; according to the invention, mineral components in the steel slag are effectively utilized, and the steel slag is modified through acid production of solid waste and microorganisms, so that a new mode of resource utilization of wastes is successfully established, and conversion from wastes to resources is realized; the carbon immobilization capacity of the modified steel slag is obviously enhanced through microbial modification, full-amount carbonization of the steel slag is achieved, compared with a traditional acid leaching carbonization method, the method is not limited to partial carbonization through filtrate any more, the preparation method is green and safe, environmental pollution and potential safety hazards caused by adopting strong acid in chemical acidification are avoided, and the method has great application prospects.

Description

Technical Field

[0001] The invention relates to an auxiliary gelling material, in particular to an auxiliary gelling material based on multiple solid wastes, and also to a preparation method of the above-mentioned gelling material. Background Art

[0002] With the continuous increase of industrial waste, the effective resource utilization of steel slag has become one of the research focuses. Steel slag is a by-product of the steel smelting process. It is rich in silicon, calcium, iron and other components, which make steel slag have potential hydraulic activity. Under certain conditions, through activation treatment and doping, steel slag can generate materials with good cementitious properties. The research on steel slag-based auxiliary cementitious materials not only helps to alleviate the environmental pressure caused by steel slag stacking, but also provides new environmentally friendly alternatives for construction and road materials. Compared with traditional cement, the preparation of steel slag-based cementitious materials has lower energy consumption and higher resource utilization, and it exhibits good compressive strength, wear resistance and impermeability in application. This material has broad application prospects in road base, roadbed filling, building foundation and other aspects.

[0003] Among them, chemical acid modification is one of the conventional improvement methods for steel slag-based auxiliary cementitious materials. Strong acids (such as sulfuric acid and hydrochloric acid) are usually used to treat steel slag. Although it can effectively improve its activity, there are problems that are difficult to solve, such as the strong corrosiveness of the acid, high energy consumption in the treatment process, high operational risks, and difficulty in treating waste acid. Summary of the invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a multi-solid waste-based auxiliary cementitious material, and also to provide a method for preparing the above-mentioned cementitious material.

[0005] Technical solution: The multi-solid waste-based auxiliary cementitious material of the present invention is obtained by using steel slag, carbide slag and sintered red mud modified by microbial acidification as raw materials, adding water and mixing and carbonizing.

[0006] The steel slag has a mineral composition of olivine, calcium feldspar, limestone, periclase, alumina, calcium silicate and iron-containing minerals such as magnetite and hematite; the mass ratio of the steel slag, carbide slag, sintered red mud and water modified by microbial acidification is 0.7-0.8:0.05-0.2:0.05-0.2:1-10.

[0007] Wherein, the microorganisms are one or a combination of Lactobacillus acidophilus, Lactobacillus casei and Lactobacillus fermentum, and the three are combined in a mass ratio of 3:0 to 3:0 to 3.

[0008] The acidification step is to activate microbial powder and then mix it with steel slag for modification.

[0009] The method for preparing the above-mentioned multi-solid waste-based auxiliary gelling material comprises the following steps:

[0010] (1) The microbial powder was placed in water and stirred thoroughly. After it was fully precipitated, the supernatant was added to LB medium and placed in an oscillating incubator for activation. The growth morphology of the microorganisms at different activation times and the changes in OD600 and pH were as follows: Figure 3 as well as Figure 4 As shown; the supernatant of the cultured bacterial solution is added to steel slag powder for acidification modification, and after the modification is completed, the steel slag modified by microbial acidification is obtained by drying and grinding;

[0011] (2) Steel slag modified by microbial acidification, pure water, carbide slag and sintered red mud are mixed and carbonized. After the carbonization is completed, the steel slag is dried and ground to obtain a multi-solid waste-based auxiliary cementitious material.

[0012] Wherein, in step (1), the LB medium is composed of water, peptone, yeast extract, sodium chloride, sodium hydroxide and ampicillin, and the mass ratio of water, peptone, yeast extract and sodium chloride in the LB medium is 90-110:0.8-1.2:0.4-0.6:0.8-1.2, preferably 100:1:0.5:1; the concentration of ampicillin is 80-120 mg / ml, and the added volume is 0.5-1.5 ml.

[0013] Wherein, in step (1), the temperature of the shaking incubator is 20-30°C, the shaking rate is 150-190 r / min, and the shaking time is 12-32 h.

[0014] Wherein, in step (1), the solid-liquid ratio of the steel slag powder to the cultured bacterial solution is 1:1-10, and the acidification modification time is 8-12 hours.

[0015] Wherein, in step (2), the carbonization temperature is 20-30°C, the humidity is 60-80%, the CO2 concentration is 20-30%, and the carbonization time is 8-12h, preferably the temperature is 20°C, the humidity is 80%, the CO2 concentration is 20%, and the carbonization time is 12h.

[0016] Among them, steel slag, steel slag modified by microbial acidification, calcium carbide slag or sintered red mud is ball milled for 20 to 40 minutes and sieved with a 100 to 200 mesh sieve, and the specific surface area is not less than 350m 2 / kg, preferably using a ball mill for 20 minutes and sieving with a 200-mesh sieve; the drying temperature is 60-80°C, the time is 8-12h, preferably the temperature is 60°C, the time is 12h.

[0017] Principle of the invention: The multi-solid waste-based auxiliary cementitious material of the present invention adopts steel slag modified by microbial acidification, mixed with calcium carbide slag and sintered red mud for carbonization, wherein the steel slag is modified by utilizing the acid-producing characteristics of microorganisms, and the organic acids produced by specific microorganisms during metabolism can react with the active components (such as CaO, C2S) in the steel slag under mild conditions to release more soluble calcium ions. In addition, organic acids such as lactic acid generated by microbial metabolism have a multi-functional group structure, which can not only effectively dissolve the mineral components in the steel slag, but also regulate the product morphology through chelation, thereby enhancing the cementitious properties of the steel slag. Moreover, due to the small size of lactic acid bacteria, after modifying the steel slag, they provide nucleation sites for the carbonization reaction, which significantly promotes the occurrence of the carbonization reaction and can achieve the full carbonization of the steel slag.

[0018] However, the pH of steel slag after acidification modification is significantly reduced. The acidic environment will inhibit the dissolution of alkaline substances (such as calcium hydroxide or calcium oxide), resulting in a decrease in the rate at which carbon dioxide reacts with these substances to generate carbonates, which is not conducive to the occurrence of carbonization reaction; and carbonates (such as calcium carbonate) generated under low pH conditions are more easily dissolved, and the reaction tends to proceed in reverse, making it difficult to form a stable solid precipitate. The acidic environment will also consume the alkaline substances required for the reaction, and may aggravate the corrosion effect, further interfering with the normal progress of the carbonization process. Therefore, the present invention uses carbide slag and sintered red mud from bulk solid waste to optimize the environment of the carbonization reaction, and uses carbide slag, sintered red mud and modified steel slag to form a multi-solid waste, which solves the problems caused by the above-mentioned acidified modified steel slag, and uses solid waste to assist microorganisms to produce acid to modify steel slag, thereby further improving the utilization rate of steel slag and the strength, durability and carbon sequestration performance of the resulting multi-solid waste-based auxiliary cementitious material.

[0019] Since the auxiliary cementitious material of the present invention exhibits good mechanical properties and durability after hardening, the cementitious activity of the steel slag enables it to solidify and form during the reaction process, has a stable structure, and can improve the strength and durability of the composite material. Therefore, it has broad prospects for application in road base, roadbed filling, building foundation, etc., can effectively reduce the use of traditional cementitious materials, reduce carbon emissions in the construction industry, and has positive significance for promoting sustainable development and green buildings.

[0020] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) The present invention effectively utilizes the mineral components in steel slag, and modifies the steel slag through solid waste-assisted synergistic microbial acid production, successfully establishing a new model of waste resource utilization, realizing the transformation of waste into resources, and making the steel slag show a significant improvement in carbon sequestration; (2) The carbon sequestration capacity of steel slag is further enhanced through microbial modification. Compared with unmodified steel slag under the same conditions, the carbon sequestration amount is increased by about 1 times, achieving full carbonization of steel slag, and significantly improving the utilization rate of steel slag; (3) The preparation method is green and safe, avoiding environmental pollution and safety hazards caused by the use of strong acid for chemical acidification. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Scanning electron microscope image of steel slag modified by microbial acidification;

[0022] Figure 2 This is a scanning electron microscope image of the multi-solid waste-based auxiliary gelling material of the present invention;

[0023] Figure 3 The microbial growth morphology at different incubation times, including (a) 0 h, (b) 8 h, (c) 16 h, and (d) 32 h incubation time;

[0024] Figure 4 Optical density and pH diagram of microorganisms at different growth stages. DETAILED DESCRIPTION

[0025] The technical scheme of the present invention is further described below in conjunction with the examples. The test materials used in the examples can all be purchased through conventional channels.

[0026] Example 1

[0027] The preparation method of the multi-solid waste-based auxiliary gelling material of the present invention comprises the following steps:

[0028] (1) Take 5g of Lactobacillus acidophilus powder and put it into 500mL of pure water. Use a magnetic stirrer set to a speed of 500r / min and stir for 10min. After stirring, wait for 10min and take 10ml of the supernatant and put it into 500ml LB culture medium for microbial activation culture; set the temperature of the shaking incubator to 25℃ and the shaking rate to 170r / min. After 32h of cultivation, take out the bacterial solution and measure its pH and optical density (OD600); take 10g of steel slag and 100g of the cultivated bacterial solution and mix them evenly. After sealing and fully reacting for 12h, take it out and put it into an oven. Set the temperature to 60℃ and dry it for 12h. Then grind and sieve (200 mesh) to obtain the microbial acidified steel slag; The scanning electron microscope image of the microbial acidified steel slag is as follows Figure 1As shown in the figure, obvious structural damage occurs on the surface of the modified slag, such as the appearance of holes.

[0029] (2) 8g of steel slag modified by microbial acidification was mixed evenly with 1g of sintered red mud and 1g of calcium carbide slag, added into 10g of pure water, stirred evenly and placed in a carbonization box for carbonization treatment. The temperature of the carbonization box was set at 20°C, the humidity was set at 80%, and the carbonization time was set at 24h; the carbonized multi-solid waste was placed in an oven for drying and grinding to obtain a multi-solid waste-based auxiliary cementitious material.

[0030] The morphology of the prepared multi-solid waste-based auxiliary cementitious material is as follows: Figure 2 As shown, its surface is relatively rough and there are obvious products generated, which fill the pores generated by the modification of microbial acid and enhance its mechanical properties. The carbon fixation capacity of the multi-solid waste-based auxiliary cementitious material prepared in Example 1 was determined by thermogravimetric analysis to be 169.87 kg / t.

[0031] Example 2

[0032] The preparation method of the multi-solid waste-based auxiliary gelling material of the present invention comprises the following steps:

[0033] (1) Take 10g of Lactobacillus casei powder and put it into 500mL of pure water, and stir it for 10min using a magnetic stirrer set to a speed of 500r / min. After stirring, wait for 10min, take 10ml of the supernatant and put it into 400ml LB culture medium for activation culture of microorganisms; set the temperature of the shaking incubator to 30°C and the shaking rate to 150r / min, take out the bacterial solution after 32h of cultivation, and measure its pH and optical density (OD600); take 10g of steel slag and 30g of the cultivated bacterial solution and mix them evenly, seal it, wait for full reaction for 12h, take it out and put it into an oven, set the temperature to 60°C, dry it for 12h, and then grind and sieve (200 mesh) to obtain the steel slag modified by microbial acidification;

[0034] (2) 8g of steel slag modified by microbial acidification was mixed evenly with 1.5g of sintered red mud and 0.5g of calcium carbide slag, added into 50g of pure water, stirred evenly and placed in a carbonization box for carbonization treatment. The temperature of the carbonization box was set at 20°C, the humidity was set at 80%, and the carbonization time was set at 24h; the carbonized multi-solid waste was placed in an oven for drying and grinding to obtain a multi-solid waste-based auxiliary cementitious material.

[0035] The carbon fixation capacity of the multi-solid waste-based auxiliary cementitious material prepared in Example 2 was determined by thermogravimetric analysis to be 189.43 kg / t.

[0036] Example 3

[0037] The preparation method of the multi-solid waste-based auxiliary gelling material of the present invention comprises the following steps:

[0038] (1) Take 10g of fermented lactobacillus powder and put it into 300mL of pure water. Use a magnetic stirrer and set the speed to 500r / min to stir for 10min. After stirring, wait for 10min and take 10ml of the supernatant and put it into 200ml LB culture medium for activation culture of microorganisms; the temperature of the oscillating incubator is 30℃ and the oscillation rate is 150r / min. After 32h of cultivation, take out the bacterial solution and measure its pH and optical density (OD600); take 10g of steel slag and 50g of the cultivated bacterial solution and mix them evenly. After sealing and fully reacting for 12h, take it out and put it into an oven. Set the temperature to 60℃ and dry it for 12h. Then grind and sieve (200 mesh) to obtain the steel slag modified by microbial acidification;

[0039] (2) 8g of steel slag modified by microbial acidification was mixed evenly with 0.5g of sintered red mud and 1.5g of calcium carbide slag, added into 50g of pure water, stirred evenly and placed in a carbonization box for carbonization treatment. The temperature of the carbonization box was set at 20°C, the humidity was set at 80%, and the carbonization time was set at 24h; the carbonized multi-solid waste was placed in an oven for drying and grinding to obtain a multi-solid waste-based auxiliary cementitious material.

[0040] The carbon fixation capacity of the multi-solid waste-based auxiliary cementitious material prepared in Example 3 was determined by thermogravimetric analysis to be 204.56 kg / t.

[0041] Example 4

[0042] The preparation method of the multi-solid waste-based auxiliary gelling material of the present invention comprises the following steps:

[0043] (1) Take 5g of Lactobacillus acidophilus and 5g of Lactobacillus casei powder and put them into 300mL of pure water, and stir them for 10min using a magnetic stirrer set to a speed of 500r / min. After stirring, wait for 10min and take 10ml of the supernatant and put it into 100ml of LB culture medium for activation culture of microorganisms; set the temperature of the shaking incubator to 30°C and the shaking rate to 180r / min, take out the bacterial solution after 32h of cultivation, and measure its pH and optical density (OD600); take 10g of steel slag and 100g of the cultivated bacterial solution and mix them evenly, seal it and wait for full reaction for 12h, take it out and put it into an oven, set the temperature to 60°C and dry it for 12h, then grind and sieve (200 mesh) to obtain the steel slag modified by microbial acidification;

[0044] (2) 7.5 g of steel slag modified by microbial acidification was mixed evenly with 0.5 g of sintered red mud and 2 g of calcium carbide slag, added into 50 g of pure water, stirred evenly and placed in a carbonization box for carbonization treatment. The temperature of the carbonization box was set at 20°C, the humidity was set at 80%, and the carbonization time was set at 24 h; the carbonized multi-solid waste was placed in an oven for drying and grinding to obtain a multi-solid waste-based auxiliary cementitious material.

[0045] The carbon fixation capacity of the multi-solid waste-based auxiliary cementitious material prepared in Example 4 was determined by thermogravimetric analysis to be 236.76 kg / t.

[0046] Example 5

[0047] The preparation method of the multi-solid waste-based auxiliary gelling material of the present invention comprises the following steps:

[0048] (1) Take 6g of Lactobacillus acidophilus, 2g of Lactobacillus casei, and 2g of fermented Lactobacillus powder and put them into 300mL of pure water. Use a magnetic stirrer to set the speed to 500r / min and stir for 10min. After stirring, wait for 10min and take 10ml of the supernatant and put it into 100ml of LB culture medium for activation culture of microorganisms; set the temperature of the shaking incubator to 30°C and the shaking rate to 180r / min. After 32h of cultivation, take out the bacterial solution and measure its pH and optical density (OD600); take 10g of steel slag and 100g of the cultivated bacterial solution and mix them evenly. After sealing, take out and put it into an oven after sufficient reaction for 12h. Set the temperature to 60°C and dry it for 12h. Grind and sieve (200 mesh) to obtain the steel slag modified by microbial acidification;

[0049] (2) 8.5 g of steel slag modified by microbial acidification was mixed evenly with 0.5 g of sintered red mud and 1 g of calcium carbide slag, added into 100 g of pure water, stirred evenly and placed in a carbonization box for carbonization treatment. The temperature of the carbonization box was set at 20°C, the humidity was set at 80%, and the carbonization time was set at 24 h; the carbonized multi-solid waste was placed in an oven for drying and grinding to obtain a multi-solid waste-based auxiliary cementitious material.

[0050] The carbon fixation capacity of the multi-solid waste-based auxiliary cementitious material prepared in Example 5 was determined by thermogravimetric analysis to be 265.24 kg / t.

[0051] Comparative Example 1

[0052] A gelling material, compared with the embodiment, adopts hydrochloric acid to acidify steel slag, and the preparation method comprises the following steps:

[0053] (1) Add 5 g of hydrochloric acid to 100 g of pure water, stir evenly to prepare a slag modified solution, take 10 g of slag and mix evenly with 100 g of the prepared solution, seal the container, wait for a full reaction for 12 hours, take it out and put it in an oven, set the temperature to 60°C, dry it for 12 hours, then grind and sieve (200 mesh) for use;

[0054] (2) 8.5g of modified steel slag was mixed with 0.5g of sintered red mud and 1g of carbide slag, added to 10g of pure water, stirred evenly and placed in a carbonization box for carbonization treatment. The temperature of the carbonization box was set to 20°C, the humidity was set to 80%, and the carbonization time was set to 24h; the carbonized steel slag was placed in an oven for drying and ground for later use. The carbon fixation amount was determined by thermogravimetric analysis to be 103.43kg / t.

[0055] Comparative Example 2

[0056] A gelling material, compared with the embodiment, adopts acetic acid acidified steel slag, and the preparation method comprises the following steps:

[0057] (1) Add 5 g of acetic acid to 100 g of pure water and stir to prepare a slag modified solution; mix 10 g of slag with 30 g of the prepared solution, seal the container, wait for a full reaction for 12 h, take it out and put it into an oven, set the temperature to 60 ° C, dry it for 12 h, and then grind and sieve (200 mesh) for use;

[0058] (2) 8g of modified steel slag was mixed with 1.5g of sintered red mud and 0.5g of carbide slag, added to 50g of pure water, stirred evenly and placed in a carbonization box for carbonization treatment. The temperature of the carbonization box was set to 20°C, the humidity was set to 80%, and the carbonization time was set to 24h; the carbonized steel slag was placed in an oven for drying and ground for later use. The carbon fixation amount was determined by thermogravimetric analysis to be 117.43kg / t.

[0059] Comparative Example 3

[0060] A cementitious material, compared with the embodiment, adopts formic acid acidified steel slag, and the preparation method comprises the following steps:

[0061] (1) Add 5g of formic acid to 100g of pure water and stir to prepare a slag modified solution; take 10g of slag and mix it with 50g of the prepared solution, seal it, wait for it to react for 12h, take it out and put it into an oven, set the temperature to 60°C, dry it for 12h, and then grind and sieve it (200 mesh) for use;

[0062] (2) 8g of modified steel slag was mixed with 0.5g of sintered red mud and 1.5g of carbide slag, added to 50g of pure water, stirred evenly and placed in a carbonization box for carbonization treatment. The temperature of the carbonization box was set to 20°C, the humidity was set to 80%, and the carbonization time was set to 24h; the carbonized steel slag was placed in an oven for drying and ground for later use. The carbon fixation amount was determined by thermogravimetric analysis to be 124.43kg / t.

[0063] The properties of the multi-solid waste-based auxiliary gelling material of the present invention are as follows:

[0064] The multi-solid waste-based auxiliary cementitious material prepared in Example 5 and the unmodified steel slag were used as auxiliary cementitious materials to replace part of the cement casting test blocks, with the replacement amounts being 10%, 15%, 20%, 25%, and 30%. The dosage of various materials in the test pieces is as follows: The dosage of cementitious material is 373kg / m 3 The amount of coarse aggregate is 1072kg / m 3 , the amount of fine aggregate is 758kg / m 3 , water consumption is 164kg / m 3 The dosage of water reducing agent is 5.6kg / m 3 In order to characterize the performance of the auxiliary cementitious material, the compressive and splitting strengths of the specimens at 28 days were measured.

[0065] Table 1. 28d compressive and splitting strength of cement and steel slag specimens

[0066]

[0067] Table 2. 28d compressive and splitting strength of cement and multi-solid waste-based auxiliary cementitious materials

[0068]

[0069] As the dosage of steel slag and multi-solid waste-based auxiliary cementitious materials increases, the compressive and splitting strengths show a gradual downward trend. It can be seen from Tables 1 and 2 that when the dosage is 20%, the compressive and splitting strengths of the pure cement group are 43.54 and 2.61MPa, respectively, the compressive and splitting strengths of the multi-solid waste-based auxiliary cementitious materials group are 40.34 and 2.73MPa, respectively, and the steel slag group is 37.54 and 2.56MPa, respectively. Compared with the steel slag group, the compressive and splitting strengths of the multi-solid waste-based auxiliary cementitious materials group are larger, and there is no obvious decrease compared with the pure cement group, and its splitting strength is also improved to a certain extent. In addition, according to the specific needs of actual applications, the parameters can be flexibly adjusted to selectively achieve technical goals such as higher carbon fixation rate, higher compressive strength, and faster carbonization rate, indicating that multi-solid waste-based auxiliary cementitious materials can be used as active admixtures for cement-based cementitious materials.

[0070] Therefore, the multi-solid waste-based auxiliary cementitious material of the present invention enhances the carbon sequestration capacity of steel slag through microbial modification, and assists the solid waste to further complete the full carbonization, successfully establishing a new model for waste resource utilization, realizing the transformation of waste into resources, and has great application prospects.

Claims

1. A multi-solid waste-based auxiliary cementitious material, characterized in that: The gelling material is obtained by using steel slag, carbide slag and sintered red mud modified by microbial acidification as raw materials, adding water, mixing and carbonizing.

2. The auxiliary cementitious material according to claim 1, characterized in that: The mass ratio of the steel slag, carbide slag, sintered red mud and water modified by the microbial acidification is 0.7-0.8: 0.05-0.2: 0.05-0.2: 1-10.

3. The auxiliary gelling material according to claim 1, characterized in that: The microorganisms are one or a combination of Lactobacillus acidophilus, Lactobacillus casei and Lactobacillus fermentum.

4. The auxiliary cementitious material according to claim 1, characterized in that: The acidification is to activate microbial powder and then mix it with steel slag for modification.

5. A method for preparing the multi-solid waste-based auxiliary gelling material according to claim 1, characterized in that: The following steps are involved: (1) putting microbial powder into water and stirring it thoroughly, taking the supernatant after it is fully precipitated, adding LB culture medium and putting it into an oscillating incubator for activation; taking the supernatant of the cultured bacterial solution and adding steel slag powder for acidification modification, drying and grinding after the modification is completed to obtain steel slag acidified and modified by microorganisms; (2) Steel slag modified by microbial acidification, pure water, carbide slag and sintered red mud are mixed and carbonized. After the carbonization is completed, the steel slag is dried and ground to obtain a multi-solid waste-based auxiliary cementitious material.

6. The preparation method according to claim 5, characterized in that: In step (1), the LB medium is composed of water, peptone, yeast extract, sodium chloride, sodium hydroxide and ampicillin, the mass ratio of water, peptone, yeast extract and sodium chloride in the LB medium is 90-110:0.8-1.2:0.4-0.6:0.8-1.2, the concentration of ampicillin is 80-120 mg / ml, and the added volume is 0.5-1.5 ml.

7. The preparation method according to claim 5, characterized in that: In step (1), the temperature of the shaking incubator is 20-30° C., the shaking rate is 150-190 r / min, and the shaking time is 12-32 h.

8. The preparation method according to claim 5, characterized in that: In step (1), the solid-liquid ratio of the steel slag powder to the cultured bacterial solution is 1:1-10, and the acidification modification time is 8-12 hours.

9. The preparation method according to claim 5, characterized in that: In step (2), the carbonization temperature is 20-30° C., the humidity is 60-80%, the CO2 concentration is 20-30%, and the carbonization time is 8-12 hours.

10. The preparation method according to claim 5, characterized in that: Steel slag, steel slag modified by microbial acidification, calcium carbide slag or sintered red mud are ball milled for 20 to 40 minutes and sieved with a 100 to 200 mesh sieve, with a specific surface area of ​​not less than 350m 2 / kg; the drying temperature is 60-80°C and the time is 8-12h.

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