Preparation method of high-carbon-sequestration shield muck baking-free brick

Abstract

The invention relates to the technical field of shield muck treatment, in particular to a preparation method of a high-carbon-sequestration shield muck baking-free brick, which comprises the following steps: mixing quick lime and steel slag micropowder in proportion, adding water, controlling the temperature, and standing to form an alkaline activated material; uniformly spreading the calcium-aluminum-iron-loaded red mud-based hydrotalcite on the surface of the alkaline activated material, and spraying slurry of carbide slag and water to form a hierarchical carbon sequestration precursor; after standing, adding water into the modified charcoal loaded with calcium magnesium phosphate and a water reducing agent, pre-stirring to form slurry, and adding the slurry into a hierarchical carbon sequestration precursor at a constant speed; adding the premixed original-state shield muck and a curing agent into the hierarchical carbon sequestration precursor, then adding high-calcium cement and fly ash, and stirring; after compression molding, curing and post-curing through a damp-heat curing and gradient pressurizing carbonization process to obtain the baking-free brick. A hierarchical precursor is constructed by quick lime, steel slag, red mud-based hydrotalcite and the like, and modified charcoal is combined, so that active ions are promoted to be dissolved out and uniformly distributed, the CO2 reaction efficiency is enhanced, and the carbon sequestration effect is improved.

Description

Technical Field

[0001] The invention relates to the technical field of shield slag treatment, and in particular to a method for preparing high-carbon-fixed shield slag unburned bricks. Background Art

[0002] In the field of building materials, using industrial solid waste (such as shield debris, steel slag, red mud, etc.) to prepare unburned bricks is an important direction for achieving low-carbon building materials.

[0003] Existing technologies have initially achieved solid waste resource utilization and CO2 sequestration by mixing solid wastes such as steel slag and fly ash with cementitious materials and combining them with carbonization and curing processes. For example, calcium oxides in steel slag can be converted into calcium carbonate through carbonization, red mud-based hydrotalcite can adsorb CO2 to form a stable mineral phase, and biochar, due to its high specific surface area, can strengthen brick structures.

[0004] However, the traditional process has the defect of low carbon fixation efficiency. The low carbon fixation efficiency leads to the failure of active components such as calcium, aluminum and iron in industrial solid waste to be fully converted into carbonate minerals. A large amount of unreacted solid waste particles remain inside the brick body, forcing the production to rely on adding traditional cementitious materials such as cement to maintain strength. This not only increases costs, but also weakens the environmental value of unfired bricks replacing waste materials. In addition, insufficient carbon fixation makes it impossible for carbonization products to form a continuous cementitious network. The internal structure of the brick body is loose, and the compressive strength is generally lower than 20MPa, which makes it difficult to meet the strength and durability requirements of load-bearing wall materials, limiting its large-scale application in the construction field.

[0005] The information disclosed in this background technology section is only intended to deepen the understanding of the overall background technology of the present invention and should not be regarded as an admission or any form of suggestion that the information constitutes the prior art known to those skilled in the art. Summary of the Invention

[0006] In order to solve the above technical problems, the present invention provides a method for preparing high-carbon-fixing shield slag unburned bricks. A hierarchical precursor is constructed by quicklime, steel slag, red mud-based hydrotalcite, etc., and modified biochar is combined to promote the dissolution and uniform distribution of active ions, enhance the CO2 reaction efficiency, and improve the carbon fixation effect.

[0007] The present invention provides a method for preparing high-carbon-fixed shield slag unburned bricks, comprising:

[0008] Quicklime and steel slag powder are mixed in proportion and then water is added. The mixture is placed under controlled temperature and allowed to stand to form an alkaline activated material. More specifically, quicklime and water undergo a violent hydration reaction to generate calcium hydroxide, releasing a large amount of OH. - , so that the pH of the system rises rapidly to 12-13, forming a strong alkaline environment. The calcium, aluminum and iron components contained in the steel slag powder undergo differential dissolution under high alkaline conditions:

[0009] Calcium component: Free CaO and Ca(OH)2 directly dissociate into Ca 2+ , some calcium silicate minerals are destroyed by alkaline erosion, releasing the encapsulated Ca 2+ ;

[0010] Aluminum component: Al2O3 as amphoteric oxide, with OH - The reaction generates soluble aluminate ions, which are further hydrolyzed into Al 3+ ;

[0011] Iron component: Although Fe2O3 is difficult to dissolve in strong alkali, its occurrence form in steel slag (such as calcium ferrite, fayalite) is due to Ca 2+ As the concentration increases, lattice distortion occurs, and Fe 3+ Gradual release into solution through ion exchange;

[0012] The temperature control and static process promotes the reaction balance, making the active ions (Ca 2+ 、Al 3+ 、Fe 3+ ) is fully dissolved and evenly distributed, providing sufficient carbon fixation substrate for the subsequent ion exchange and carbonization reaction of red mud-based hydrotalcite, solving the core problem of insufficient activation of active components in solid waste;

[0013] The red mud-based hydrotalcite loaded with calcium, aluminum and iron is evenly spread on the surface of the alkaline activated material, and a slurry of carbide slag and water is sprayed to form a hierarchical carbon fixation precursor;

[0014] More specifically, the red mud-based hydrotalcite loaded with calcium, aluminum and iron is evenly spread on the surface of the alkaline activated material. The layered double hydroxide structure of the red mud-based hydrotalcite is activated by OH in a high alkaline environment. - Effect, OH - Due to its high charge density and small radius, its binding force with the positive charge of the layer is stronger than the original CO3 between the layers. 2- , which leads to the expansion of interlayer spacing and increased electrostatic repulsion. At the same time, the high concentration of Ca dissolved in slag / quicklime 2+ The formation of ion concentration gradient drives the exchange, the electrostatic balance between the layers is broken, and the interlayer aluminum and iron ions and the Ca dissolved by the alkaline activation material are promoted. 2+ Cross-layer ion exchange occurs, and hydrotalcite releases Al 3+ 、Fe 3+ and adsorb Ca 2+ Forming calcium aluminum iron mixed metal sites, Ca 2+ As a core calcium source for carbon fixation, Al 3+ / Fe 3+By hydrolysis to generate colloids that adsorb CO2 and catalyze mineralization reactions, the density of mixed sites is greatly improved compared to that of a single metal, providing a multi-dimensional reaction center for carbonization. At the same time, spraying carbide slag slurry enriches the local calcium concentration, strengthens ion exchange and synergistic dissolution, and constructs a hierarchical carbon fixation precursor with high calcium activity and multiple metal sites, providing a high-density active interface for subsequent carbonization reactions, solving the problems of insufficient activation of active components in solid waste and insufficient internal carbonization in traditional processes.

[0015] After standing, the modified biochar loaded with magnesium calcium phosphate and the water reducer are pre-mixed into a slurry with water and added to the hierarchical carbon fixation precursor at a uniform speed; more specifically, the high specific surface area and mesoporous structure of biochar provide a loading platform for magnesium calcium phosphate, so that it is evenly dispersed in the system to avoid agglomeration. Magnesium calcium phosphate exists as a crystalline composite phosphate or an amorphous phosphate complex. Calcium ions can react with CO2 to form calcium carbonate through carbonization, which directly contributes to the carbon fixation efficiency. Magnesium ions can further broaden the carbon fixation path by forming magnesium carbonate or synergizing with aluminum and iron ions to form basic carbonates. In addition, phosphate can react with Ca in the system. 2 + Mg 2+ Combined to form calcium magnesium phosphate, this type of mineral is not only chemically stable and can fix CO2 for a long time, but can also fill the pores of the brick through chemical bonding, improve the structural density, and thus enhance the mechanical strength of the unfired brick;

[0016] The water reducer promotes the uniform dispersion of modified biochar and calcium magnesium phosphate by reducing the surface tension of the slurry;

[0017] Premixed undisturbed shield slag and a curing agent are added to the hierarchical carbon fixation precursor, and then high-calcium cement and fly ash are added for stirring. More specifically, the premixed undisturbed shield slag and the curing agent can improve the viscosity of the slag, activate the activity of aluminosilicate minerals, neutralize acidic substances and pre-generate CaCO3 crystal nuclei through the alkaline excitation and ion exchange of the curing agent, thereby creating a low-resistance and high-activity environment for subsequent reactions. The premixed system is added to the hierarchical carbon fixation precursor, whose high alkalinity can accelerate the capture of CO2. The slag skeleton and the curing agent product form dual sites, which not only provide a nucleation base for carbonation but also enhance mechanical support. After the high-calcium cement and fly ash are added, the high-calcium cement efficiently fixes carbon through carbonation, and the hydration products react with the active components of the fly ash to form a pozzolanic reaction, filling the pores and improving the density. The fly ash can also reduce the amount of cement and improve the fluidity, synergistically achieving the improvement of carbon fixation efficiency and mechanical properties, taking into account both environmental protection and cost advantages.

[0018] After pressing and forming, the unfired bricks are obtained after wet heat curing and gradient pressure carbonization curing and post-curing.

[0019] The alkaline activated material formed by quicklime and steel slag powder releases a large amount of Ca 2+Active ions such as calcium and iron form a three-dimensional nested reaction interface with red mud-based hydrotalcite. The calcium concentration is enriched by spraying calcium carbide slag slurry, which promotes the ion exchange between the aluminum and iron ions in the hydrotalcite layer and the calcium components of the steel slag, forming a hierarchical carbon fixation precursor with high calcium activity and multiple metal sites. The modified biochar loaded with magnesium calcium phosphate is dispersed by a water reducer and then integrated into the system. Its mesoporous structure and surface mineral phase not only provide adsorption sites for CO2, but also form a multiphase coupled reaction system with the calcium, aluminum and iron active ions in the precursor. The calcium and magnesium ions in magnesium calcium phosphate fix CO2 through carbonization and synergistic mineralization reactions, the phosphate group strengthens the pore chemical bonding and improves the density, and the biochar ensures the uniform distribution of active sites with its high dispersion. The three together promote the deep dissolution and uniform distribution of the active components of steel slag and red mud, significantly enhancing the contact efficiency and reaction depth between solid waste and CO2. While solving the problems of insufficient activation of active ingredients, insufficient carbonization penetration and single function of biochar in traditional processes, it also achieves the simultaneous improvement of high carbon fixation efficiency and mechanical properties of unfired bricks.

[0020] As a preferred embodiment of the present invention, the unburned bricks include, by mass: 50-65 parts of original shield slag, 15-20 parts of high calcium cement, 10-15 parts of steel slag powder, 8-12 parts of quicklime, 5-10 parts of fly ash, 5-8 parts of carbide slag, 1.4-3.5 parts of red mud-based hydrotalcite, 1-2 parts of water reducer, 0.6-1.5 parts of modified biochar and 0.5-1 part of curing agent.

[0021] As a preferred embodiment of the present invention, the free CaO content in the high calcium cement is 7-10%, the tricalcium silicate content is ≥62%, and the tricalcium aluminate content is ≤5.5%;

[0022] High free CaO can quickly hydrate to generate Ca(OH)2, providing a high alkaline environment to further activate the dissolution of calcium, aluminum and iron components in steel slag / red mud, and directly serve as a substrate for carbonization reaction, thereby improving carbon fixation efficiency; high content of tricalcium silicate can quickly hydrate to generate CSH gel, thereby enhancing the early strength of the brick body, and the Ca(OH)2 released by its hydration further supplements the calcium carbide source; low content of tricalcium aluminate can reduce shrinkage cracking caused by hydration heat, thereby avoiding excessive consumption of Ca 2+ , ensuring that more calcium components are used for carbonization reaction rather than hydration products.

[0023] As a preferred embodiment of the present invention, a method for preparing red mud-based hydrotalcite comprises:

[0024] The red mud is mixed with hydrochloric acid having a concentration of 30-37% in a mass ratio of 1:4-5, and acid washed with stirring at 60-80°C. After solid-liquid separation, the activated aluminum-iron slag is obtained. More specifically, the hydrochloric acid reacts with the aluminum and iron oxides in the red mud to dissolve and release aluminum and iron ions, thereby removing impurities and obtaining the activated aluminum-iron slag.

[0025] According to the Ca / (Al+Fe) molar ratio of 1.8-2.2, active aluminum iron slag, calcium carbonate and deionized water are mixed and ball-milled for 30-60 minutes, and then sodium aluminate solution is added. The pH of the system is controlled to 10-12, and the reaction is hydrothermally reacted at 120-150 ° C for 6-8 hours to generate calcium aluminum iron hydrotalcite precursor; More specifically, calcium carbonate is partially dissolved under ball milling and alkaline conditions, releasing Ca 2+ , together with the metal ions in the active aluminum iron slag, constitute the metal ion source of hydrotalcite. The sodium metaaluminate solution adjusts the pH of the system to 10-12, promoting the co-precipitation reaction of the metal ions. Under hydrothermal conditions of 120-150℃, Ca 2+ 、Al 3+ 、Fe 3+ With OH - Combined to form a layered double hydroxide (LDH) precursor, while the Ca / (Al+Fe) molar ratio is controlled at 1.8-2.2 to ensure moderate layer charge density and provide space for subsequent ion exchange;

[0026] The precursor is added to a Na2CO3 solution, stirred and replaced at 50-70°C, washed with deionized water until neutral, and then vacuum dried at 60-80°C until the moisture content is ≤5% to obtain red mud-based hydrotalcite; more specifically, the precursor is placed in a Na2CO3 solution, and the Cl - With CO3 2- Ion exchange reaction occurs to form more stable carbonate hydrotalcite, and the interlayer carbonate reacts through the ion exchange mechanism (such as CO3 2- +CO2+H2O→2HCO3 - ) rapidly adsorbs CO2 and acts as a weak base site to promote its hydration to carbonate ions.

[0027] As a preferred embodiment of the present invention, a method for preparing modified biochar comprises:

[0028] Lightly burned magnesium oxide, calcium carbonate, ammonium dihydrogen phosphate according to Mg 2+ :Ca 2+ :PO4 3- A 25-30 wt% suspension is prepared at a molar ratio of 1:0.5-1:1.8-2.2;

[0029] The plant-based materials are carbonized at 550-620°C in an oxygen-limited manner and then ground and sieved to obtain biochar. More specifically, the plant-based materials (such as rice husks) are converted into biochar through oxygen-limited pyrolysis, retaining the rich pore structure and surface functional groups, while the high temperature removes volatile impurities and enhances the material stability.

[0030] The biochar was immersed in the suspension at a solid-liquid ratio of 1:5-1:8 and vacuum loaded at 0.5-0.7 MPa. More specifically, the air in the biochar pores was expelled under vacuum, and the suspension was forced to penetrate into the micropores under negative pressure, significantly increasing the Mg content. 2 + , Ca 2+ PO4 3- It has excellent loading efficiency and distribution uniformity, avoiding the surface adsorption limitations of traditional immersion method.

[0031] After gradient drying, the modified biochar is calcined at 600-700°C under N2 protection. More specifically, the temperature is raised in a step-by-step manner to avoid pore collapse or load shedding caused by rapid evaporation of water, thereby ensuring the integrity of the biochar skeleton and the retention of loaded components. Calcination in an N2 atmosphere promotes a solid-phase reaction between the loaded metal ions and phosphates to generate crystalline composite phosphates or amorphous phosphate complexes with both high chemical stability and reactivity.

[0032] As a preferred solution of the present invention, the original shield slag is screened, crushed, acidified and washed, and dried before being added;

[0033] More specifically, it includes:

[0034] Use a vibrating screen to screen the original shield slag with a mesh size of 5mm to remove impurities such as gravel and metal debris with a particle size greater than 5mm, ensuring that the coarse particle content in the slag is ≤5%;

[0035] The sieved soil is fed into a jaw crusher or roller crusher to be crushed to a particle size of ≤2mm, with the proportion of fine particles ≤0.15mm being ≥40% to improve the uniformity of subsequent reactions;

[0036] Soak the crushed soil in 5% citric acid solution for 2 hours to remove surface metal oxide impurities, then rinse with clean water until neutral and dry;

[0037] The crushed soil is placed in a drying box and dried at 105°C to constant weight (time ≥ 4 hours), and the final moisture content is controlled to ≤ 8% to avoid excessive moisture affecting the hydration of the cementitious material.

[0038] The shield slag treated as described above can effectively remove impurities such as gravel and metal debris, optimize particle grading to improve reaction uniformity, remove surface metal oxides through acidification and active washing, activate active sites on the particle surface, and use low moisture content to avoid dilution of cementitious materials, ensuring sufficient hydration reaction, thereby significantly enhancing the interfacial bonding strength between the slag and components such as curing agents and high-calcium cement, and improving the molding quality and mechanical properties of unfired bricks.

[0039] As a preferred embodiment of the present invention, the following treatment is performed between the moist heat curing and the gradient pressure carbonization process:

[0040] A CO2 mixture containing 10-15% water vapor is introduced under normal pressure for 1-2 hours;

[0041] More specifically, water vapor condenses in the micropores of the green body to form a liquid film, in which CO2 dissolves to form a carbonic acid solution, which penetrates into the interior of the green body through capillary action, promoting deep carbonization reaction.

[0042] As a preferred embodiment of the present invention, the gradient pressure carbonization process includes:

[0043] The pressure is increased to 0.8-1 MPa at a rate of 0.15-0.25 MPa / min, and the CO2 concentration is controlled at 90-95% for 2-4 hours. More specifically, high-pressure CO2 quickly penetrates into the micropores of the green body and reacts with the pre-activated Ca 2+ The reaction generates calcium carbonate crystal nuclei, and the CO2 concentration of 90-95% ensures the carbonization rate, while moderate pressure prevents micropore blockage;

[0044] After a short period of pressure reduction to 0.3-0.6 MPa, the pressure is increased to 1-2 MPa at a rate of 0.05-0.15 MPa / min, and the CO2 concentration is increased to 95-100%, which is maintained for 4-8 hours. The intermediate pressure reduction can release the internal stress, causing micro-cracks in the calcium carbonate layer formed in the early stage, creating new channels for the subsequent CO2 diffusion, and promoting the unreacted Ca2CO3 between the layers. 2+ Migrate to the surface; higher pressure and pure CO2 environment drive carbonization deep into the material, and calcium carbonate continues to grow on the surface of the formed crystal nuclei, filling the pores and enhancing the structural density;

[0045] Reduce the pressure to normal pressure at a rate of 0.1-0.3 MPa / min.

[0046] As a preferred embodiment of the present invention, the water reducer is at least one of a polycarboxylic acid-based water reducer and a naphthalene-based water reducer; more specifically, the water reducing rate of the water reducer is ≥15%. The polycarboxylic acid-based water reducer has a high water reducing rate, can reduce the water-cement ratio, improve the carbonization gas diffusion efficiency, and delay cement hydration; the naphthalene-based water reducer enhances the cohesiveness of the slurry, facilitates molding, shortens the initial setting time, and increases the carbonization depth. The combination of the two can take into account both water reduction and strength improvement.

[0047] As a preferred embodiment of the present invention, the curing agent is at least one of phosphates and sulfides; more specifically, phosphate curing agents can improve early strength, optimize system pH to accelerate carbonization, fix heavy metals and enhance freeze-thaw resistance; sulfide curing agents can efficiently solidify heavy metals and promote Ca 2+ Release, improve gas diffusion, refine crystals and enhance material toughness.

[0048] Compared with the prior art, the present invention has the following advantages: the alkaline activation material formed by quicklime and steel slag powder not only provides a high alkaline environment for the dissolution of calcium, aluminum and iron components in the steel slag, but also releases a large amount of Ca 2+ Active ions such as magnesium phosphate and calcium phosphate are dispersed in the water-reducing agent, and the modified biochar loaded with magnesium phosphate is evenly integrated into the system after being dispersed with a water-reducing agent. Its mesoporous structure and the mineral phase loaded on the surface not only provide adsorption sites for CO2, but also form a multiphase coupled reaction system with the calcium, aluminum and iron active ions in the hierarchical carbon fixation precursor. This synergistic effect promotes the deep dissolution and uniform distribution of calcium, aluminum and iron components in steel slag and red mud. The modified biochar provides a large number of reaction sites for CO2, which significantly enhances the contact efficiency and reaction depth between the active components of solid waste and CO2. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 It is a schematic flow chart of a method for preparing high-carbon-fixed shield slag unfired bricks of the present invention. DETAILED DESCRIPTION

[0050] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0051] The materials in the specific implementation manner are all obtained through commercial means;

[0052] Example 1:

[0053] Raw materials preparation:

[0054] Weigh by mass: 60 parts of original shield slag, 18 parts of high calcium cement, 12 parts of steel slag powder, 10 parts of quicklime, 8 parts of fly ash, 6 parts of carbide slag, 2.5 parts of red mud-based hydrotalcite, 1.5 parts of polycarboxylic acid water reducer, 1 part of modified biochar, and 0.8 part of phosphate curing agent.

[0055] High calcium cement: free CaO content 8.5%, tricalcium silicate content 65%, tricalcium aluminate content 5%.

[0056] Preparation of red mud-based hydrotalcite: 100 g of red mud was mixed with 450 g of 35% hydrochloric acid, stirred and pickled at 70°C, and solid-liquid separation was performed to obtain active aluminum-iron slag; the active aluminum-iron slag, calcium carbonate and deionized water were mixed at a Ca / (Al+Fe) molar ratio of 2.0 and ball-milled for 45 minutes, sodium metaaluminate solution was added, the pH was controlled at 11, and a hydrothermal reaction was carried out at 135°C for 7 hours to generate a precursor; Na2CO3 solution was added to the precursor, stirred and replaced at 60°C, washed with deionized water until neutral, and vacuum dried at 70°C to a moisture content of ≤5%.

[0057] Preparation of modified biochar: Lightly burned magnesium oxide, calcium carbonate, and ammonium dihydrogen phosphate were mixed according to Mg 2+ :Ca 2+ :PO4 3- A 28 wt% suspension was prepared in a molar ratio of 1:0.8:2.0; the plant-based material was carbonized at 600°C with oxygen limitation, ground and sieved; the biochar and suspension were vacuum loaded at 0.6 MPa in a solid-liquid ratio of 1:6; after gradient drying, the mixture was calcined at 650°C under N2 protection.

[0058] Treatment of original shield slag: sieve with 5mm vibrating screen, crush with jaw crusher to particle size ≤2mm, soak in 5% citric acid solution for 2 hours, rinse with clean water until neutral, and dry at 105℃ to constant weight.

[0059] This embodiment provides a method for preparing high-carbon-fixed shield slag unburned bricks, comprising:

[0060] S1: Mix quicklime and steel slag powder, add water, control the temperature at 50℃ and let it stand for 1 hour to form an alkaline activated material;

[0061] S2: Spread the red mud-based hydrotalcite evenly on the surface of the alkaline activated material, spray a slurry of carbide slag mixed with an appropriate amount of water to form a hierarchical carbon fixation precursor, and let it stand for 30 minutes;

[0062] S3: pre-mix the modified biochar and polycarboxylate water reducer with water to form a slurry, and then add it to the hierarchical carbon fixation precursor at a uniform speed;

[0063] S4: Add the pre-mixed original shield slag and phosphate curing agent, add high calcium cement and fly ash and mix evenly;

[0064] S5: Press the stirred material into a shape, first perform wet heat curing at 50°C for 2 hours, then introduce a CO2 mixed gas containing 12% water vapor at normal pressure and maintain for 1.5 hours; increase the pressure to 0.9MPa at a rate of 0.2MPa / min, control the CO2 concentration to 93%, and maintain for 3 hours; briefly reduce the pressure to 0.5MPa, then increase the pressure to 1.5MPa at a rate of 0.1MPa / min, increase the CO2 concentration to 98%, and maintain for 6 hours; then reduce the pressure to normal pressure at a rate of 0.2MPa / min;

[0065] S6: The bricks after CO2 curing are moved to room temperature (20±5°C) and cured naturally for 7 days, during which time water is sprayed regularly to keep the surface moist, thereby obtaining unfired bricks.

[0066] Example 2:

[0067] The difference from Example 1 is that: the content of steel slag powder and modified biochar is increased, and the proportion of original shield slag is reduced;

[0068] 50 parts of original shield slag, 18 parts of high calcium cement, 15 parts of steel slag powder, 10 parts of quicklime, 8 parts of fly ash, 6 parts of carbide slag, 2.5 parts of red mud-based hydrotalcite, 1.5 parts of polycarboxylic acid water reducer, 1.5 parts of modified biochar, and 0.8 parts of phosphate curing agent;

[0069] The remaining aspects are the same as those in Example 1.

[0070] Example 3:

[0071] The difference from Example 1 is that the dosage of high calcium cement and steel slag powder is reduced, and the proportion of original shield slag is increased;

[0072] 65 parts of original shield slag, 15 parts of high calcium cement, 10 parts of steel slag powder, 10 parts of quicklime, 8 parts of fly ash, 6 parts of carbide slag, 2.5 parts of red mud-based hydrotalcite, 1.5 parts of polycarboxylic acid-based water reducer, 1 part of modified biochar, and 0.8 parts of phosphate curing agent;

[0073] The remaining aspects are the same as those in Example 1.

[0074] Example 4:

[0075] The difference from Example 1 is that: the amount of high calcium cement and steel slag powder is reduced, the proportion of original shield slag and carbide slag is increased, and the type of water reducer is changed;

[0076] 62 parts of original shield slag, 16 parts of high calcium cement, 11 parts of steel slag powder, 10 parts of quicklime, 8 parts of fly ash, 7 parts of carbide slag, 2.5 parts of red mud-based hydrotalcite, 1 part of naphthalene-based water reducer, 1 part of modified biochar, and 0.8 part of phosphate curing agent;

[0077] The remaining aspects are the same as those in Example 1.

[0078] Comparative Example 1: The difference from Example 1 is that commercially available magnesium-aluminum hydrotalcite is used instead of the red mud-based hydrotalcite of the present invention, and the rest is the same as Example 1;

[0079] Comparative Example 2: The difference from Example 1 is that commercially available unmodified biochar is used instead of the modified biochar of the present invention, and the rest is the same as Example 1;

[0080] Comparative Example 3: The difference from Example 1 is that step S5 of "then introducing a CO2 mixed gas containing 12% water vapor under normal pressure for 1.5 hours" is not performed. The rest is the same as Example 1;

[0081] Comparative Example 4: The raw material preparation steps are exactly the same as those in Example 1; when preparing unburned bricks, the steps of "putting quicklime, steel slag powder, red mud-based hydrotalcite, carbide slag, modified biochar, polycarboxylic acid-based water reducer, original shield slag, phosphate curing agent, high calcium cement, and fly ash into a mixer at one time, and then adding water and stirring directly" are used instead of S1-S4 in Example 1;

[0082] Examples 1-4 and Comparative Examples 1-4 were subjected to the following tests:

[0083] The compressive strength test method of the specimen is:

[0084] (1) Cut the unburned bricks after curing into standard test blocks (size: 50mm×50mm×50mm), with at least 5 specimens in each group. The surface of the test blocks must be flat and free of visible cracks or defects.

[0085] (2) Before testing, the specimens were placed in a room temperature (20 ± 2 °C) environment for 24 hours; then, they were pressurized in a universal material testing machine at a loading rate of 0.5-1.0 MPa / s until the specimens failed;

[0086] (3) Compressive strength (MPa) = compressive area of ​​test block (mm 2 ) / maximum breaking load (N)

[0087] (4) The final compressive strength result is the average value of 5 test blocks. If the deviation of a single value exceeds ±15%, it will be rejected.

[0088] The water absorption test method of the specimen is:

[0089] (1) Place the test piece in an oven and dry it at 105°C until it reaches a constant weight (the difference between two weighings is ≤ 0.1%), and record the dry mass m d ;

[0090] (2) Immerse the test piece completely in clean water at 20±5℃ for 24 hours. After taking it out, wipe off the surface moisture with a wet cloth and weigh the saturated mass m. s ;

[0091] (3) Water absorption rate = (m s -m d ) / m d

[0092] (4) The final water absorption result is the average value of three test blocks, with an allowable deviation of ≤2%.

[0093] The carbon fixation rate of the specimen is tested by thermogravimetric analysis (TGA), and the specific method is as follows:

[0094] (1) Take the brick powder after curing (particle size ≤ 75 μm), dry it and weigh 10 ± 0.5 mg;

[0095] (2) The test temperature range was 25°C to 1000°C, the heating rate was controlled at 10°C / min, and the atmosphere was nitrogen (flow rate 50 mL / min) or air (flow rate 50 mL / min);

[0096] (3) Record the mass loss in the range of 400-800 °C (corresponding to the decomposition of CaCO3 into CaO and CO2);

[0097] Carbon fixation rate calculation formula: Carbon fixation rate (%) = CO2 release amount (g) / initial mass of sample (g) × 100%;

[0098] After the above test, the results in Table 1 are obtained:

[0099] Table 1 Test results

[0100] Compressive strength (MPa) Water absorption (%) Carbon fixation rate (%) Example 1 22.1 9.8 24.5 Example 2 21.5 10.4 26.2 Example 3 20.9 11.5 24.8 Example 4 21.4 10.2 27.2 Comparative Example 1 10.3 14.2 15.6 Comparative Example 2 12.1 13.5 17.4 Comparative Example 3 12.8 12.7 18.3 Comparative Example 4 11.7 13.9 16.5

[0101] Comparative Example 1 uses commercially available magnesium-aluminum hydrotalcite instead of red mud-based hydrotalcite. Due to the lack of the synergistic effect of calcium, aluminum and iron elements in red mud, there is no Ca between the hydrotalcite layers. 2+ active sites, resulting in incomplete carbonization reaction, Ca 2+ The utilization rate is greatly reduced, and a stable gel structure cannot be formed inside the material, which leads to a decrease in compressive strength, an increase in water absorption rate, and a decrease in carbon fixation rate.

[0102] Comparative Example 2 uses unmodified biochar, which has no active mineral load of magnesium calcium phosphate in its pores, and cannot effectively adsorb CO2 and induce calcium carbonate crystallization, resulting in poor pore structure of the brick and disordered crystal growth, resulting in insufficient compressive strength, easy water penetration, and low carbon fixation efficiency.

[0103] Comparative Example 3 omitted the atmospheric pressure CO2 pre-activation step, the anions between the hydrotalcite layers were not fully replaced, the interlayer spacing was not effectively expanded, and the Ca 2+ The less exposed active sites increase the CO2 diffusion resistance during subsequent gradient pressure carbonization, limiting the reaction rate and depth, and ultimately leading to a decline in various material properties.

[0104] In Comparative Example 4, the layered activation was cancelled and the raw materials were directly mixed, which destroyed the synergistic mechanism of "Ca(OH)2 gradient diffusion-hydrotalcite directional adsorption" between the alkaline activation material and the hydrotalcite, hindered the ion migration and reaction process, and caused the amount of gelled product generated to be reduced and the structure to be loose, resulting in reduced compressive strength and carbon fixation rate and increased water absorption rate.

[0105] 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 the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for preparing high-carbon-fixed shield slag unburned bricks, characterized in that: include: Mix quicklime and steel slag powder in proportion, add water, control the temperature and let it stand to form alkaline activated material; Evenly spreading the red mud-based hydrotalcite loaded with calcium, aluminum and iron on the surface of the alkaline activation material, and spraying a slurry of carbide slag and water to form a hierarchical carbon fixation precursor; After standing, the modified biochar loaded with magnesium calcium phosphate and the water reducer are pre-mixed with water to form a slurry, and then added to the hierarchical carbon fixation precursor at a uniform speed; Adding premixed original shield slag and curing agent to the layered carbon fixation precursor, and then adding high calcium cement and fly ash for stirring; After pressing and forming, the unfired brick is obtained after being cured by a wet heat curing and gradient pressure carbonization process and post-curing.

2. The method for preparing high-carbon-fixed shield slag unburned bricks according to claim 1, characterized in that: Calculated by mass, the unburned bricks include: 50-65 parts of original shield slag, 15-20 parts of high calcium cement, 10-15 parts of steel slag powder, 8-12 parts of quicklime, 5-10 parts of fly ash, 5-8 parts of carbide slag, 1.4-3.5 parts of red mud-based hydrotalcite, 1-2 parts of water reducer, 0.6-1.5 parts of modified biochar and 0.5-1 part of curing agent.

3. The method for preparing high-carbon-fixed shield slag unburned bricks according to claim 2, characterized in that: The high calcium cement has a free CaO content of 7-10%, a tricalcium silicate content of ≥62%, and a tricalcium aluminate content of ≤5.5%.

4. The method for preparing high carbon-fixed shield slag unburned bricks according to claim 1, characterized in that: The preparation method of the red mud-based hydrotalcite comprises: The red mud is mixed with hydrochloric acid with a concentration of 30-37% in a mass ratio of 1:4-5, and pickled with stirring at 60-80°C, and active aluminum-iron slag is obtained after solid-liquid separation; According to the Ca / (Al+Fe) molar ratio of 1.8-2.2, active aluminum-iron slag, calcium carbonate and deionized water are mixed and ball-milled for 30-60 minutes, and then sodium metaaluminate solution is added. The pH of the system is controlled at 10-12, and the mixture is hydrothermally reacted at 120-150°C for 6-8 hours to generate a calcium-aluminum-iron hydrotalcite precursor; The precursor is added to a Na2CO3 solution, stirred and replaced at 50-70°C, washed with deionized water until neutral, and then vacuum dried at 60-80°C until the moisture content is ≤5% to obtain the red mud-based hydrotalcite.

5. The method for preparing high carbon-fixed shield slag unburned bricks according to claim 1, characterized in that: The method for preparing the modified biochar comprises: Lightly burned magnesium oxide, calcium carbonate, ammonium dihydrogen phosphate according to Mg 2+ :Ca 2+ :PO4 3- A 25-30 wt% suspension is prepared at a molar ratio of 1:0.5-1:1.8~2.2; The plant-based materials are subjected to oxygen-limited carbonization at 550-620°C and then ground and sieved to obtain biochar; The biochar is immersed in the suspension at a solid-liquid ratio of 1:5-1:8 and vacuum loaded at 0.5-0.7 MPa; After gradient drying, the modified biochar is calcined at 600-700° C. under N 2 protection to obtain the modified biochar.

6. The method for preparing high-carbon-fixed shield slag unburned bricks according to claim 1, characterized in that: The original shield slag is screened, crushed, acidified, washed and dried before being added.

7. The method for preparing high-carbon-fixed shield slag unburned bricks according to claim 1, characterized in that: The following treatments are performed between the moist heat curing and the gradient pressure carbonization process: A CO2 mixture containing 10-15% water vapor is introduced under normal pressure and maintained for 1-2 hours.

8. The method for preparing high-carbon-fixed shield slag unburned bricks according to claim 7, characterized in that: The gradient pressurization carbonization process comprises: Increase the pressure to 0.8-1 MPa at a rate of 0.15-0.25 MPa / min, control the CO2 concentration at 90-95%, and maintain it for 2-4 hours; After briefly reducing the pressure to 0.3-0.6 MPa, increase the pressure to 1-2 MPa at a rate of 0.05-0.15 MPa / min, increase the CO2 concentration to 95-100%, and maintain it for 4-8 hours; Reduce the pressure to normal pressure at a rate of 0.1-0.3 MPa / min.

9. The method for preparing high carbon-fixed shield slag unburned bricks according to claim 1, characterized in that: The water reducer is at least one of a polycarboxylic acid-based water reducer and a naphthalene-based water reducer.

10. The method for preparing high carbon-fixed shield slag unburned bricks according to claim 1, characterized in that: The curing agent is at least one of phosphates and sulfides.

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