High-activity metakaolin-based composite mineral admixture as well as preparation method and application thereof

Abstract

The invention discloses a high-activity metakaolin-based composite mineral admixture as well as a preparation method and application thereof. The preparation method of the high-activity metakaolin-based composite mineral admixture comprises the following steps: adding a grinding aid into a kaolin-based material, and grinding to obtain kaolin-based material powder; carrying out ball-milling mixing on the kaolin-based tailing powder and an aluminum-based eutectic agent to obtain a raw material, drying the raw material, and carrying out stepped calcination to obtain a metakaolin-based material; carrying out ultrasonic dispersion mixing on the composite chemical admixture solution and the metakaolin-based material, and drying to constant weight to obtain an admixture-loaded high-activity metakaolin-based material; uniformly mixing the high-activity metakaolin-based material and the vaterite-based material to obtain a high-activity metakaolin-based composite mineral admixture; the admixture is mixed with a vaterite stabilizer to be applied to a cement-based material. The admixture disclosed by the invention has high activity, and is beneficial to improving the working performance of a cement-based material, and improving the strength and durability of the cement-based material at the same time.

Description

Technical Field

[0001] This invention relates to the field of cement concrete admixture technology, and in particular to a highly active metakaolin-based composite mineral admixture, its preparation method and application. Specifically, it addresses the problem of insufficient activity of kaolin solid waste raw materials by proposing a solution of step-by-step precise calcination and efficient synergistic activation. Background Technology

[0002] Cement concrete is the material foundation of the national economy, but its production process generates high carbon emissions. Utilizing mineral admixtures such as fly ash and mineral powder is an important technical approach to reduce cement carbon emissions. However, my country's energy structure transformation means that traditional mineral admixtures based on coal-fired power will be unable to meet the growing demand for cement concrete.

[0003] The LC was proposed in 2013 by the Swiss Federal Institute of Technology in Lausanne (EPFL) in collaboration with partners from Cuba and India. 3 The LC system has become a research hotspot in the international low-carbon cement field. 3 LC is a novel low-carbon cementitious system that adds calcined clay and limestone to the existing silicate cement system. The synergistic reaction of these two materials improves the cementitious properties of the material. The calcination temperature of clay is lower than that of cement clinker, and the calcination process does not decompose and release CO2. Compared to silicate cement, LC... 3 It can reduce CO2 emissions by up to 40%. However, currently LC 3 It suffers from drawbacks such as slow early strength development and insufficient fluidity, and consumes a large amount of high-quality clay resources, which is not in line with my country's national conditions for sustainable development. my country has a large amount of aluminum-rich solid waste, such as coal gangue and kaolin tailings, which can be used to prepare highly active metakaolin-based aluminosilicate materials after calcination. In addition, solid waste raw materials rich in calcium oxide, such as steel slag, carbide slag, and recycled micronized powder, can be carbonized to prepare active calcium carbonate with gelling properties, further reducing the carbon emissions of the gelling system.

[0004] Currently, the calcination of kaolin-based solid waste mainly employs suspension kilns, rotary kilns, and tunnel kilns. Existing technologies suffer from uneven heat transfer and slow cooling rates. Excessively high temperatures lead to silicon-oxygen network reconstruction and a decrease in aluminum coordination number, resulting in the formation of a mixture of crystalline and amorphous phases during calcination, leading to significant differences in the activity of the calcined metakaolin-based materials. Furthermore, the large temperature deviations in calcination due to the differences in kaolin mineral phases hinder the efficient preparation of highly active metakaolin-based materials. Simultaneously, the kaolin minerals in aluminum-rich solid waste exhibit a significant layered structure, adsorbing some water during mixing, resulting in a low free water content in the system and affecting the fluidity of cement concrete. Besides metakaolin, the minor components of the calcined aluminum-rich solid waste, such as quartz, iron oxide, and other clay minerals, alter the packing state and density of the slurry, also affecting its rheological properties.

[0005] Patent CN119306417A discloses a method for activating coal gangue, which involves treating coal gangue particles in a eutectic solvent, the eutectic solvent including HBA and HBD components. The HBA component is a mixture of choline chloride and amide compounds, and the HBD component is a mixture of carboxylic acid compounds and polyols. The pretreated coal gangue is then calcined at 400-450°C, achieving coal gangue activation treatment with lower energy consumption.

[0006] Patent CN118637866A discloses a graphene oxide-reinforced limestone-coal-based metakaolin cement and its preparation method. By modifying metakaolin with graphene oxide, a novel limestone-coal-based metakaolin cement is prepared as the calcined clay in limestone-calcined clay cement. The modified metakaolin can improve the dispersibility of graphene oxide, and graphene oxide can provide nucleation sites for hydration products to promote hydration, effectively improving the mechanical properties and durability of limestone-coal-based metakaolin cement.

[0007] Patent CN116143435A discloses an activated coal gangue early-strength mixture and flowability modifier, and their preparation method. The mixture involves crushing coal gangue and then calcining it at high temperature to form a calcined activated material. This calcined activated material is then further finely crushed and activated to form a mixture with a specific surface area of ​​600–1000 m². 2 / kg of activated coal gangue; further, add 5-50% of the activated coal gangue mass of high-calcium active material, then mix and homogenize. After aging, an early-strength activated coal gangue mixture and a flowability modifier are formed. The activity reaches 95% in 3 days and 96% in 28 days, with significantly improved flowability. It can replace clinker as a cement admixture at a dosage of 30-50%. Summary of the Invention

[0008] To address the problems existing in the above-mentioned background technology, the present invention provides a highly active metakaolin-based composite mineral admixture, its preparation method and application. The admixture has high activity, which is beneficial to improving the workability of concrete and increasing the strength and durability of concrete.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a highly active metakaolin-based composite mineral admixture includes the following steps: (1) The kaolin-based material is ground, and a grinding aid is added during the grinding process to obtain kaolin-based material powder; The grinding aid is selected from any one or a mixture of two of triethanolamine and ethylene glycol, and its dosage accounts for 0.5% to 5% of the mass of the kaolin-based material; The kaolin-based tailings powder has a particle size of less than 0.075 mm and a specific surface area of ​​≥600 m². 2 / kg; Furthermore, the kaolin-based material is hard kaolin, soft kaolin, sandy kaolin, coal-series kaolin, silt, sludge, or kaolin tailings containing kaolin minerals, preferably kaolin tailings, coal gangue, silt, or sludge, with a kaolin content greater than 40%; (2) The kaolin-based material powder obtained in step (1) is ball-milled and mixed with aluminum-based eutectic flux to form raw material with a residue of less than 15% on a 0.075mm sieve. Then it is dried at 100±5℃ to constant weight and then calcined in stages to obtain metakaolin-based material. The solid-liquid ratio of the kaolin-based material powder to the aluminum-based eutectic flux is 1:0.05 to 1:0.3; The aluminum-based eutectic flux is composed of AlCl3·6H2O and urea in a molar ratio of 1:1 to 1:2, and it is used to penetrate the interlayer of kaolin-based materials in the liquid phase. Furthermore, the stepped calcination procedure is as follows: first, the temperature is increased to 300 °C at 5 °C / min and held for 30 min; then, the temperature is increased to 600~650 °C at 5 °C / min and held for 60 min; then, the temperature is increased to 700~750 °C at 5 °C / min and held for 30 min; finally, the temperature is increased to 800~850 °C at 2 °C / min and held for 10 min; after calcination, the product is quenched with liquid nitrogen and washed with supercritical CO2 (40 °C, 15 MPa) for 30 min to remove residues; Furthermore, 0.5%~1% of NH4F by mass of the kaolin-based material powder is added during calcination, and the calcination atmosphere is a N2-O2 mixture (O2≤5vol%). (3) The composite chemical admixture solution and the metakaolin-based material obtained in step (2) are ultrasonically dispersed and mixed, and then further dried to constant weight to obtain a highly active metakaolin-based material loaded with admixture; The composite chemical admixture solution is obtained by mixing an organic dispersant and a nano-inorganic reinforcing material in an ethanol solution. The organic dispersant is preferably any one or a mixture of polycarboxylic acid water-reducing agents, phosphoric acid water-reducing agents, or naphthalene sulfonate water-reducing agents. The nano-inorganic reinforcing material is preferably any one of nano-calcium carbonate and nano-silica. The dispersant can inhibit the agglomeration phenomenon caused by the rapid water absorption of the layered structure of the metakaolin-based material and achieve gradient release of the dispersant in a slurry environment. The nano-inorganic material acts as a crystal nucleus, promoting its hydration reaction. Furthermore, the mass ratio of organic dispersant to nano-inorganic reinforcing material in the composite chemical admixture solution is 1:1 to 5:1; the solvent is a mixture of ethanol and water at a mass ratio of 4:1, and the mass concentration of organic dispersant in the solution is 1%-15%, while the mass concentration of nano-inorganic reinforcing material is 1%-10%. Furthermore, the volume ratio of the composite chemical admixture solution to the metakaolin-based material is 1.5~2:1; Furthermore, the ultrasonic frequency is 15~40kHz, and the ultrasonic dispersion and mixing time is 10~30min; Furthermore, the ultrasonically dispersed mixture was placed in a vacuum environment below 10 mbar for 30 min to 2 h, then centrifuged at 5000 to 10000 r / min for 2 to 10 min to separate excess water. Then, it was dried to constant weight under vacuum at 45±2℃ to remove the moisture from the admixture, while retaining the loaded admixture. Vacuum adsorption can improve the loading capacity and loading efficiency of the admixture on the metakaolin-based material, and promote the intercalation of the admixture into the layered structure. (4) Mix the highly active metakaolin-based material and the aragonite-based material obtained in step (3) at a mass ratio of 1:1 to 2:1 to obtain a highly active metakaolin-based composite mineral admixture; The aragonite-based material is a powder containing calcium carbonate or calcium oxide, with a residue of less than 10% on a 0.075mm sieve. Specifically, it is selected from any one or a mixture of more than one of limestone powder, recycled micro powder, waste cement slurry, steel slag, and carbide slag.

[0010] The present invention also provides a highly active metakaolin-based composite mineral admixture prepared by the above method.

[0011] The present invention also proposes an application of the above-mentioned highly active metakaolin-based composite mineral admixture in cement-based materials, specifically comprising: mixing the above-mentioned highly active metakaolin-based composite mineral admixture, clinker, and gypsum in a mass ratio of 15~45:80~50:5 to prepare cement-based materials.

[0012] A method for preparing a cement-based material based on the above-mentioned highly active metakaolin-based composite mineral admixture includes: mixing water with aragonite stabilizer and highly active metakaolin-based composite mineral admixture according to a set water-cement ratio, introducing industrial waste gas containing carbon dioxide, stirring for 10-60 minutes under sealed conditions for pre-carbonation, and then adding clinker and gypsum to the slurry and stirring until uniform to prepare the cement-based material.

[0013] Furthermore, the aragonite stabilizer is selected from any one or more of ammonium citrate, magnesium salt, phosphate and macromolecular polyacrylic acid, and its dosage is 0.5% to 5% of the mass of the aragonite-based material in the highly active metakaolin-based composite mineral admixture.

[0014] Furthermore, the industrial waste gas has a carbon dioxide concentration of 20% to 80% and a flow rate of 0.1 to 0.5 L / min / g, and can come from the tail gas of calcining kaolin-based materials, the tail gas of cement industry production, and the tail gas of steel slag or carbide slag production.

[0015] Compared with the prior art, the present invention has the following beneficial effects: (1) The aluminum-based eutectic flux of the present invention forms a liquid phase at the calcination temperature, which encapsulates the kaolin particles, lowers the melting point during the calcination process, hinders the atomic rearrangement required for ion diffusion and crystal nucleation and growth, and helps to improve the activation effect of metakaolin; combined with the synergistic effect of inorganic salts and organic matter, it can achieve highly amorphous active metakaolin at ≤850℃, while avoiding the residual side effects of traditional fluxes.

[0016] (2) Multi-temperature zone temperature control is adopted to achieve segmented dehydroxylation and crystal transformation. The structural water is fully removed by extending the heat preservation time in the low temperature zone (600-650℃), the amorphization is promoted in the medium temperature zone (700-750℃), and the residual crystalline phase (such as quartz) is eliminated by short-term treatment in the high temperature zone (800-850℃). Liquid nitrogen quenching locks in the highly active disordered structure.

[0017] (3) The dispersant and nanomaterials are preloaded into the layered structure of metakaolin. On the one hand, this reduces the performance decline caused by the rapid adsorption of free water due to its large specific surface area. On the other hand, the nanomaterials can also play a filling effect and a crystal nucleation effect, promoting the hydration reaction of the nanomaterials with calcium carbonate in the later stage. The clay adsorption group and electrostatic repulsion group are introduced into the dispersant molecule to generate a strong steric hindrance effect and electrostatic repulsion force.

[0018] (4) The composite effect of amorphous metakaolin and aragonite can improve the overall performance of cement-based materials. Metakaolin has strong volcanic ash activity. The main component of aragonite, calcium carbonate, reacts with the active alumina of metakaolin to form a stable hydration product, calcium monocarbonate. Amorphous aragonite obtained through crystal form regulation can further improve the degree and rate of reaction.

[0019] (5) Introducing industrial waste gas into the cement-based material preparation process can achieve carbon capture on the one hand, and improve the high-value resource utilization of industrial solid waste on the other hand, and reduce the consumption of cement clinker. Therefore, this technical solution meets the "dual carbon" requirements of the cement industry. Detailed Implementation

[0020] The present invention will be further described below with reference to specific embodiments.

[0021] In Examples 1-4, 6 and 8, a metakaolin-based composite mineral admixture was prepared by co-processing coal gangue and recycled micro powder, and cement mortar was prepared. The metakaolin-based composite mineral admixture accounted for 45% of the mass of the cementitious material. Its activity index was evaluated by the strength development at 7 days and 28 days.

[0022] Example 5 uses a type of kaolin tailings to prepare a metakaolin-based composite mineral admixture.

[0023] Example 7 uses steel slag to prepare metakaolin-based composite mineral admixture.

[0024] Blank example A cementitious material is formed by mixing 95% cement clinker and 5% gypsum.

[0025] Example 1 A method for preparing a highly active metakaolin-based composite mineral admixture includes the following steps: S1: A type of coal gangue was selected as the kaolin-based material, with a kaolin content of 45.6%. The specific chemical composition is shown in Table 1. 1 kg of coal gangue was added to a vertical roller mill, along with 1.5 wt% triethanolamine grinding aid, and ground until the specific surface area was 620 m². 2 / kg yields kaolin-based material powder; S2: Kaolin-based material powder and aluminum-based eutectic flux (AlCl3·6H2O : urea = 1:2 (molar ratio)) are added to a ball mill jar as raw material at a solid-liquid ratio of 1:0.3 (1kg powder + 0.3kg flux). The raw material is then placed in an atmosphere furnace and a N2-3%O2 mixed gas (flow rate 0.2 L / min) is introduced. Based on the powder mass, 0.8 wt% NH4F was added, and the temperature was increased according to the following procedure: first, the temperature was increased to 300 ℃ at 5 ℃ / min and held for 30 min, then increased to 650 ℃ at 5 ℃ / min and held for 60 min; then the temperature was increased to 720 ℃ at 5 ℃ / min and held for 30 min, then increased to 820 ℃ at 2 ℃ / min and held for 10 min; the temperature was then quenched with liquid nitrogen immediately after calcination. The calcined material was placed in a supercritical CO2 reactor (40℃, 15MPa) for 30 min to remove residual chlorides and organic matter, yielding a highly active metakaolin-based material. XRD analysis showed that the amorphous content was 49.5%. S3: (1) Preparation of admixture solution: Mix 10wt% phosphate water-reducing agent and 10wt% nano calcium carbonate solution in ethanol-water solution (4:1 volume ratio) at a mass ratio of 3:1. The particle size D90 of nano calcium carbonate is 40nm; (2) Ultrasonic-vacuum loading: Take 500g of metakaolin-based material, add admixture solution (volume ratio 1.8:1), ultrasonically disperse at 40kHz for 30 min, then transfer to a vacuum tank and pump to 8mbar, hold pressure for 2 h to allow admixture intercalation; (3) Separation and drying: Centrifuge at 10000 r / min for 2 min to remove free liquid, vacuum dry at 45℃ to constant weight to obtain metakaolin-based material loaded with admixture; S4: Mix 135 g of metakaolin-based material with added additives and 67.5 g of recycled micro powder in a three-dimensional mixer at a mass ratio of 2:1 for 20 min to obtain a highly active metakaolin-based composite mineral admixture. S5: Weigh 225g of mixing water at a water-cement ratio of 0.50, including 2g of ammonium citrate as a spheroidal aragonite stabilizer (3% of the mass of the recycled micro powder), add 202.5g of highly active metakaolin-based composite mineral admixture (accounting for 45% of the total cementitious material), then introduce the cement plant kiln tail exhaust gas (CO2 concentration 40%, flow rate 0.3 L / min / g), seal and stir for 30 min; weigh 225g of cement clinker and 22.5g of gypsum dihydrate (5%) and add them to the pre-carbonated slurry, stir for 30s, then add 1350g of standard sand and stir for 2 min, pour into a mold, and demold after standing in the laboratory for 24 h according to the national standard "Test Method for Strength of Cement Mortar (ISO Method)", and test the activity index at 7d and 28d.

[0026] Example 2 A method for preparing a highly active metakaolin-based composite mineral admixture is carried out according to the method of Example 1, except that the solid-liquid ratio of the kaolin-based tailings powder and the aluminum-based eutectic flux (AlCl3·6H2O : urea = 1:2 (molar ratio)) is 1:0.1 (1kg powder + 0.1kg flux).

[0027] Example 3 A method for preparing a highly active metakaolin-based composite mineral admixture is carried out according to the method of Example 1, except that the proportion of admixtures in S3 is: 10wt% polycarboxylate superplasticizer and 10wt% nano calcium carbonate solution are mixed in an ethanol-water solution (4:1 volume ratio) at a mass ratio of 1:1.

[0028] Example 4 A method for preparing a highly active metakaolin-based composite mineral admixture is carried out according to the method of Example 1, except that in S3, ultrasonic-vacuum loading is performed: 500g of metakaolin-based material is taken, an admixture solution (volume ratio 1.8:1) is added, ultrasonically dispersed at 40kHz for 10 min, and then transferred to a vacuum tank and evacuated to 8mbar, and held under pressure for 30 min to allow the admixture to intercalate.

[0029] Example 5 A method for preparing a highly active metakaolin-based composite mineral admixture is carried out according to the method of Example 1, except that S1 uses kaolin tailings as the kaolin-based material, with a kaolin content of 56.3% and a specific surface area of ​​450 m² after grinding. 2 / kg.

[0030] The heating regime for S2 was as follows: first, the temperature was increased to 300 °C at a rate of 5 °C / min and held for 30 min; then, the temperature was increased to 650 °C at a rate of 5 °C / min and held for 60 min; next, the temperature was increased to 750 °C at a rate of 5 °C / min and held for 30 min; then, the temperature was increased to 850 °C at a rate of 2 °C / min and held for 10 min; immediately after calcination, the temperature was quenched with liquid nitrogen. After cleaning with supercritical CO2 (40 °C, 15 MPa) for 30 min, XRD analysis showed that the amorphous content was 65.4%.

[0031] Example 6 A method for preparing a highly active metakaolin-based composite mineral admixture is carried out according to the method of Example 1, except that the admixture solution ratio in S3 is as follows: 10wt% polycarboxylate superplasticizer and 10wt% nano silica solution are mixed in an ethanol-water solution (4:1 volume ratio) at a mass ratio of 3:1, and the nano silica particle size D90 is 25nm.

[0032] Example 7 A method for preparing a highly active metakaolin-based composite mineral admixture is carried out according to the method of Example 1, except that in S4, 135 g of metakaolin-based material loaded with admixture and 67.5 g of steel slag are mixed in a three-dimensional mixer at a mass ratio of 2:1 for 20 min to obtain the metakaolin-based composite mineral admixture.

[0033] Example 8 A method for preparing a highly active metakaolin-based composite mineral admixture is carried out according to the method of Example 1, except that the aragonite stabilizer in S5 is a mixture of magnesium salt and phosphate, with a mass ratio of 1:1 and a total mass of 2g (3% of the regenerated micro powder).

[0034] Comparative Example 1 The method was carried out according to Example 1, except that the calcination step of kaolin was omitted, and the kaolin material was directly used in the preparation of cement-based materials.

[0035] S1: A type of coal gangue was selected as the kaolin-based material, and its specific chemical composition is shown in Table 1. 1 kg of coal gangue was added to a vertical roller mill, along with 1.5 wt% triethanolamine grinding aid, and ground until the specific surface area was 620 m². 2 / kg yields kaolin-based material powder.

[0036] S2: (1) Preparation of admixture solution: Mix 10wt% phosphate water-reducing agent and 10wt% nano calcium carbonate solution in ethanol-water solution (4:1 volume ratio) at a mass ratio of 3:1; (2) Ultrasonic-vacuum loading: Take 500g of kaolin-based tailings powder, add admixture solution (volume ratio 1.8:1), ultrasonically disperse at 40KHz for 30 min, then transfer to a vacuum tank and pump to 8mbar, hold pressure for 2 h to allow admixture intercalation; (3) Separation and drying: Centrifuge at 10000 r / min for 2 min to remove free liquid, vacuum dry at 45℃ to constant weight to obtain kaolin-based material loaded with admixture.

[0037] S3: Mix 135 g of kaolin-based material with added additives and 67.5 g of recycled micro powder in a three-dimensional mixer at a mass ratio of 2:1 for 20 min to obtain kaolin-based composite mineral admixture.

[0038] S4: Weigh 225g of mixing water at a water-cement ratio of 0.50, including 2g of ammonium citrate as a spheroidal aragonite stabilizer (3% of the mass of the recycled micro powder). Add 202.5g of kaolin-based composite admixture (45% of the total cementitious material). Then, introduce cement plant kiln tail gas (CO2 concentration 40%, flow rate 0.3 L / min / g) and seal and stir for 30 min. Weigh 225g of cement clinker and 22.5g of gypsum dihydrate (5%) and add them to the pre-carbonated slurry. Stir for 30s, then add 1350g of standard sand and stir for 2 min. Pour into a mold and, according to the national standard "Test Method for Strength of Cement Mortar (ISO Method)," let it stand in the laboratory for 24 h before demolding. Standard cure for 7 days and 28 days to test the activity index.

[0039] Comparative Example 2 The procedure was carried out according to the method of Example 1, except that S1 to S3 were the same as in Example 1, and S4 and S5 were combined into the following steps: Weigh 225g of mixing water, 202.5g of metakaolin-based material with added admixture (accounting for 45% of the total cementitious material), 225g of cement clinker, and 22.5g of gypsum dihydrate (5%) according to a water-cement ratio of 0.50, stir for 30s, then add 1350g of standard sand and stir for 2min, pour into a mold, and let stand in the laboratory for 24h according to the national standard "Test Method for Strength of Cement Mortar (ISO Method)" before demolding. Standard curing was performed for 7d and 28d to test the activity index.

[0040] Comparative Example 3 The process was carried out according to the method of Example 1, except that the additive solution in S3 was only an organic dispersant, and the other process flow and parameters were the same as in Example 1.

[0041] S3: (1) Preparation of admixture solution: Take 500g of metakaolin-based material and add it to a 10wt% phosphoric acid water-reducing agent solution (volume ratio 1.8:1). Disperse it by ultrasonication at 40KHz for 30 min, then transfer it to a vacuum tank and pump it to 8mbar. Hold the pressure for 2 h to allow the admixture to intercalate. (3) Separation and drying: Centrifuge at 10000 r / min for 2 min to remove the free liquid, and vacuum dry at 45℃ to constant weight to obtain metakaolin-based material loaded with admixture.

[0042] Comparative Example 4 The process was carried out according to the method of Example 1, except that in S3, mechanical stirring was used to mix the metakaolin-based material and the composite chemical additives. Other process flows and parameters were the same as in Example 1.

[0043] S3: (1) Preparation of admixture solution: Mix 10wt% phosphate water-reducing agent and nano calcium carbonate (10wt% nano liquid) in ethanol-water solution (4:1 volume ratio) at a mass ratio of 3:1; (2) Loading of admixture: Take 500g of metakaolin-based material, add admixture solution (volume ratio 1.8:1), and use an electric stirrer to mechanically stir at 500 r / min at 20℃ for 2h; (3) Separation and drying: Centrifuge at 10000 r / min for 2 min to remove free liquid, and vacuum dry at 45℃ to constant weight to obtain metakaolin-based material loaded with admixture.

[0044] Comparative Example 5 The method was carried out according to Example 1, except that aluminum-based eutectic agent and NH4F were not added during the calcination stage.

[0045] Comparative Example 6 The method was carried out according to Example 1, except that a constant-temperature calcination process was used for the coal gangue. During the calcination stage, the temperature was directly increased to 720 °C at a rate of 5 °C / min, held at that temperature for 90 min, and then quenched with liquid nitrogen immediately after the calcination was completed. All other process flows and parameters were the same as in Example 1.

[0046] Comparative Example 7 The process was carried out according to the method of Example 1, except that a slow cooling method was used in the cooling stage after calcination, gradually reducing the temperature to room temperature at a rate of 10 °C / min. Other process flows and parameters were the same as in Example 1.

[0047] Comparative Example 8 The procedure was carried out according to Example 1, except that the ultrasonic-vacuum loading and separation-drying steps were removed in step S3. In step S5, the composite chemical additive prepared in step S3 was added to the mixing water for subsequent processing. All other process flows and parameters were the same as in Example 1.

[0048] Comparative Example 9 The method was followed according to Example 1, except that S1-S4 were the same as in Example 1. In S5: 225g of mixing water was weighed at a water-cement ratio of 0.50, and 202.5g of metakaolin-based composite admixture (45% of the total cementitious material) was added. Then, cement plant kiln tail gas (40% CO2 concentration, flow rate 0.3 L / min / g) was introduced and stirred in a sealed container for 30 min. 225g of cement clinker and 22.5g of dihydrate gypsum (5%) were weighed and added to the pre-carbonated slurry, stirred for 30 s, and then 1350g of standard sand was added and stirred for 2 min. The mixture was then poured into a mold and, according to the national standard "Test Method for Strength of Cement Mortar (ISO Method)," it was allowed to stand in the laboratory for 24 h before demolding. The activity index was tested after standard curing for 7 days and 28 days. Compared to Example 1, the recycled micropowder in this example produced calcium carbonate with a calcite crystal form.

[0049] Table 1 Chemical composition of each raw material Table 2. Examples and Comparative Examples Table 3. Material ratio for the activity index test (kg / m³) 3 ) Table 4 Comparison of performance test results of metakaolin-based composite mineral admixtures prepared in each embodiment From the data in Table 4, we can obtain: Compared to the blank example, Example 1 showed a slight decrease in fluidity, a 4% increase in water-reducing agent dosage, a 10% decrease in 7-day strength, but a 6% increase in 28-day strength.

[0050] Compared to Example 1, Example 2 has a lower proportion of aluminum-based eutectic agent, resulting in reduced activity of the obtained metakaolin-based powder; Example 4 has shorter ultrasonic dispersion and holding time in step S3, and relatively lower additive loading, leading to a decrease in both workability and activity; Example 5 uses kaolin tailings with a slightly higher kaolin content, resulting in slightly higher activity of the composite mineral admixture prepared with the same mass of high-activity metakaolin-based material; Example 6 has different types of organic dispersant and nano-inorganic reinforcing material, with smaller nano-silica particles and slightly higher activity; Example 7 uses steel slag with a slightly higher calcium oxide content, resulting in slightly higher activity of the composite mineral admixture prepared with the same mass of aragonite-based material; Example 8 has different types of aragonite stabilizer used in step S5, but the activity of the prepared composite mineral admixture is not significantly different.

[0051] In Comparative Example 1, uncalcined kaolin solid waste was directly mixed with cement clinker. The uncalcined coal gangue had almost no activity, with an activity index of only 62% after 28 days. Furthermore, its high water absorption properties led to deterioration in fluidity.

[0052] Comparative Example 2 directly mixed metakaolin-based materials with cement clinker, lacking the reaction between calcium carbonate and activated alumina. Compared with the blank example, the strength decreased by 32% and 20% at 7 days and 28 days, respectively. Compared with Example 1, the activity was significantly reduced at 28 days due to insufficient early hydration products caused by the lack of calcium carbonate.

[0053] In Comparative Example 3, no nano-reinforcing additives were added. Compared to Example 1, its flowability was the same, but the activity index at 7d and 28d decreased by 7% and 10%, respectively. This was because the CASH gel formation rate of the hydration product was reduced under the condition of no reinforcing additives.

[0054] In Comparative Example 4, the composite chemical additive was loaded by mechanical stirring. The additive only adhered to the particle surface and failed to enter the interlayer structure. Compared with Example 1, the 28-day activity index decreased by 3%, but compared with Comparative Example 8, the 28-day activity index increased by 3%.

[0055] In Comparative Example 5, no eutectic agent was added during the calcination process. The lack of eutectic agent led to insufficient dehydroxylation and an increase in pseudo-amorphous phase. The activity index at 7d and 28d decreased by 19% and 21% respectively compared to Example 1, and by 9% and 7% respectively compared to Comparative Example 9.

[0056] Comparative Example 6 adopted a constant temperature calcination-induced sintering process, which resulted in a 40% decrease in the specific surface area of ​​the product, loss of active sites, and a 12% and 16% decrease in the activity index at 7d and 28d compared to Example 1.

[0057] Comparative Example 7 adopted a step-by-step cooling method, and the activity index was reduced by 14% compared with Example 1 after 28 days. This is because the slow cooling process inhibited the formation of amorphous γ-Al2O3.

[0058] The comparison of the above examples illustrates the importance of eutectic agents and stepped calcination regimes for the formation of amorphous metakaolin.

[0059] In Comparative Example 8, the composite chemical admixture was added during the cement mortar mixing process. The admixture adsorption was delayed, and particle agglomeration was aggravated. The 28-day activity index decreased by 6% compared with Example 1, but it led to a 14% decrease in initial fluidity and doubled the demand for water-reducing agent.

[0060] By comparing Example 1 with Comparative Examples 8 and 4, it was confirmed that ultrasonic loading is the key to reducing the demand for water-reducing agents and improving fluidity, because it enables the gradient release of dispersants between layers and the uniform dispersion of nano-reinforcing materials in the slurry.

[0061] In Comparative Example 9, no aragonite crystal form modifier was added during the precarbonation reaction, resulting in the formation of calcium carbonate with calcite crystals as the main form. The activity index at 7d and 28d decreased by 8% and 9% respectively compared to Example 1. This is because the metastable structure of aragonite is more conducive to early CASH nucleation than calcite.

[0062] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method of producing a high-activity, metakaolin-based composite mineral admixture, characterized by, The method comprises the following steps: (1) The kaolin-based material is ground, and a grinding aid is added during the grinding process to obtain a kaolin-based material powder; the kaolin-based tailing powder has a particle size of less than 0.075 mm and a specific surface area of ≥600 m 2 / kg. (2) The kaolin-based material is ground, and a grinding aid is added during the grinding process to obtain a kaolin-based material powder; the kaolin-based tailing powder has a particle size of less than 0.075 mm and a specific surface area of ≥600 m 2 / kg. (3) The The kaolin-based material is hard kaolin, soft kaolin, sandy kaolin, coal-based kaolin, silt, sludge or kaolin tailings containing kaolin minerals; The kaolin-based material powder obtained in step (1) is mixed with an aluminum-based eutectic agent by ball milling to obtain a raw material with a 0.075 mm residue of less than 15%, and then the raw material is dried at 100±5 ℃ to a constant weight, and then subjected to a step calcination to obtain a metakaolin-based material; The step calcination procedure is as follows: first, the temperature is raised to 300 ℃ at a rate of 5 ℃ / min, and then kept for 30 min; then the temperature is raised to 600-650 ℃ at a rate of 5 ℃ / min, and then kept for 60 min; then the temperature is raised to 700-750 ℃ at a rate of 5 ℃ / min, and then kept for 30 min; finally, the temperature is raised to 800-850 ℃ at a rate of 2 ℃ / min, and then kept for 10 min; The composite chemical admixture solution is obtained by mixing an organic dispersant and a nano-inorganic reinforcing material in an ethanol solution; The composite chemical admixture solution is obtained by mixing an organic dispersant and a nano-inorganic reinforcing material in an ethanol solution; The high-activity metakaolin-based composite mineral admixture is obtained by uniformly mixing the high-activity metakaolin-based material obtained in step (3) and the vaterite-based material. The vaterite-based material is a powder containing calcium carbonate or calcium oxide components, and the 0.075 mm residue is less than 10%.

2. The method of claim 1, wherein the high-activity, high- lewelling, composite mineral admixture is characterized by: The grinding aid in step (1) is selected from any one or a mixture of the two of triethanolamine and ethylene glycol, and the amount is 0.5%-5% of the mass of the kaolin-based material; The kaolin-based material is kaolin tailings, coal gangue, silt or sludge, and the kaolin content is greater than 40%; The kaolin content is greater than 40%; The solid-liquid ratio of the kaolin-based material powder to the aluminum-based eutectic agent in step (2) is 1:0.05-1:0.

3. The aluminum-based eutectic agent is composed of AlCl3·6H2O and urea at a molar ratio of 1:1-1:

2.

3. The method for preparing a highly active metakaolin-based composite mineral admixture according to claim 2, characterized in that, In step (2), 0.5%-1% of NH4F based on the mass of the kaolin-based material powder is added during calcination, and the calcination atmosphere is N2-O2 mixed gas with O2≤5 vol%; the product after calcination is quenched by liquid nitrogen, and the residual substances are removed by supercritical CO2 cleaning at 40 ℃ and 15 MPa for 30 min.

4. The method of claim 1, wherein the high-reactivity pozzolan-based composite mineral admixture is characterized by, The organic dispersant in step (3) is selected from any one or more than one mixture of polycarboxylic acid-based water reducing agent, phosphoric acid-based water reducing agent or naphthalene sulfonate-based water reducing agent; The nano-inorganic reinforcing material is selected from any one of nano-calcium carbonate and nano-silicon dioxide; The mass ratio of the organic dispersant to the nano-inorganic reinforcing material in the composite chemical admixture solution is 1:1-5:1; the solvent is a mixture of ethanol and water at a mass ratio of 4:1, the mass concentration of the organic dispersant in the solution is 1%-15%, and the mass concentration of the nano-inorganic reinforcing material is 1-10%; The volume ratio of the composite chemical admixture solution to the metakaolin-based material is 1.5-2:

1.

5. The method for preparing a highly active metakaolin-based composite mineral admixture according to claim 4, characterized in that, The ultrasonic frequency in step (3) is 15-40 kHz, and the ultrasonic dispersion mixing time is 10-30 min; the mixture after ultrasonic dispersion mixing is placed in an environment with a vacuum of 10 mbar or less, and maintained for 30 min-2 h, then centrifuged at a speed of 5000-10000 r / min for 2-10 min to separate excess water, and then dried in a vacuum environment at a temperature of 45±2℃ until constant weight, to remove the water of the admixture and retain the loaded admixture.

6. The method of claim 1, wherein the high-reactivity pozzolan-based composite mineral admixture is characterized by, The mass ratio of the high-activity metakaolin-based material to the vaterite-based material in step (4) is 1:1-2:

1. The vaterite-based material is selected from any one or more of limestone powder, recycled micro powder, waste cement paste, steel slag, and carbide slag.

7. A high-activity metakaolin-based composite mineral admixture prepared by the method of any one of claims 1-6.

8. Use of the high-reactivity, meta-kaolin-based composite mineral admixture according to claim 7 in cement-based materials, characterized in that, The cement-based material is prepared by mixing the high-activity metakaolin-based composite mineral admixture, clinker, and gypsum in a mass ratio of 15-45:80-50:

5.

9. A method of producing a cementitious material according to claim 8, characterised in that, The method comprises: The mixing water, vaterite stabilizer, and high-activity metakaolin-based composite mineral admixture are mixed in a set water-binder ratio, industrial waste gas containing carbon dioxide is introduced, and the mixture is pre-carbonated under stirring for 10-60 min in a sealed condition, then the clinker and gypsum are added to the slurry and stirred to mix uniformly to prepare the cement-based material.

10. The method of preparing a cementitious material according to claim 9, wherein, The vaterite stabilizer is selected from any one or more of ammonium citrate, magnesium salt, phosphate, and macromolecular polyacrylic acid, and the amount is 0.5%-5% of the mass of the vaterite-based material in the high-activity metakaolin-based composite mineral admixture. The carbon dioxide concentration in the industrial waste gas is 20%-80%, and the flow rate is 0.1-0.5 L / min / g, which can be the tail gas from calcination of the metakaolin-based material, tail gas from cement production, tail gas from steel slag or carbide slag production.

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