Recycled micro-powder synergistically enhanced low-shrinkage concrete and preparation method thereof

Abstract

The present application belongs to the technical field of concrete preparation, and specifically relates to a recycled micro-powder synergistically enhanced low-shrinkage concrete and a preparation method thereof. After slurry preparation of recycled concrete micro-powder, carbon dioxide gas is introduced, and part of carbonation is controlled according to the pH and residual amount of calcium hydroxide after degassing; the obtained slurry is divided into two parts, one part is mixed into concrete, and the other part is mixed with slag powder, cement, silica fume and polyvinyl alcohol to prepare a mineral coating slurry, and the obtained slurry is used for rolling coating, curing and drying of biochar particles, and then the coated particles are soaked in a water solution containing a shrinkage reducing agent. The obtained coated biochar curing particles are added in the later stage of wet mixing of the concrete. The method can realize resource utilization of recycled concrete micro-powder and biomass materials, make part of carbon dioxide undergo mineralization reaction, has the characteristics of low-carbon utilization, and is beneficial to improving the integrity of the particles in the mixing process, reducing the autogenous shrinkage and drying shrinkage of the concrete.

Description

Technical Field

[0001] This invention belongs to the field of concrete preparation technology, specifically a recycled micro powder synergistic reinforcement low-shrinkage concrete and its preparation method. Background Technology

[0002] During the crushing and screening process of waste concrete to prepare recycled aggregate, a certain amount of recycled concrete powder is generated. This powder mainly contains hardened cement paste particles, fine aggregate powder, and a small amount of incompletely hydrated cement components. Due to differences in the source, service life, and crushing process of waste concrete, the composition, particle size, and water absorption properties of recycled concrete powder vary. When untreated recycled powder is directly added to concrete, it may absorb some of the mixing water, leading to a decrease in concrete fluidity and affecting setting time, strength, and volume stability.

[0003] To improve the utilization rate of recycled micro-powder and reduce resource consumption and carbon emissions during concrete production, carbon dioxide-containing gas can be used to carbonate the recycled micro-powder. Carbon dioxide can react with calcium hydroxide and other calcium-containing components in the micro-powder to form carbonates. This method utilizes construction solid waste and fixes some carbon dioxide, aligning with the development trend of low-carbon concrete. However, the reactive calcium-containing components in different batches of recycled micro-powder vary. If the carbonation process is controlled solely by a fixed ventilation time, insufficient or excessive carbonation may occur, thus affecting the activity of the micro-powder and the performance of the concrete.

[0004] Low water-cement ratio concrete may experience self-shrinkage during hardening due to decreased internal moisture, increasing the risk of early cracking. External curing primarily affects the surface of the component; for denser or larger cross-section concrete, its internal water replenishment effect may be limited. Biochar, with its porous structure, can absorb a certain amount of water or aqueous solutions containing shrinkage-reducing agents, and can be used as an internal curing material. However, untreated biochar particles may break, shed powder, or prematurely release absorbent liquid during concrete mixing, potentially affecting the workability of the concrete.

[0005] Therefore, it is necessary to provide a method for preparing low-carbon, low-shrinkage concrete suitable for actual concrete production, in order to improve the utilization of recycled micro powder, the integrity of biochar particles, and the shrinkage performance of concrete. Summary of the Invention

[0006] To address at least one of the aforementioned problems, this invention provides a method for preparing low-shrinkage concrete synergistically reinforced with recycled micropowder, characterized by comprising the following steps: S1. Mix recycled concrete powder with water to form a powder slurry. While stirring, introduce carbon dioxide gas into the slurry. After stopping the gas flow, continue stirring and remove free air bubbles to obtain a partially carbonized powder slurry. S2. Based on the mass of dry-based recycled concrete micro powder in the slurry, the partially carbonized micro powder slurry is divided into a first part and a second part, wherein the first part accounts for 70%-90% and the second part is the remainder, and the total of the two is 100%; S3. The second part is mixed with slag powder, silicate cement, silica fume, polyvinyl alcohol aqueous solution and water to form a mineral coating slurry. The mineral coating slurry is applied to the surface of dry biochar particles at least twice, and the particles are rolled into granules, wet cured and dried to obtain particles with a porous mineral coating layer. S4. Place the porous mineral coating particles in a negative pressure impregnation container, evacuate the container, add a curing agent, and continue to soak after restoring normal pressure to remove the free liquid on the particle surface, thereby obtaining coated biochar internal curing particles. S5. Add the first part to the concrete mixture, and after the main wet mixing of the concrete mixture is completed, add the coated biochar internal curing particles, and discharge the material after wet mixing.

[0007] Further, in step S1, the mass ratio of recycled concrete powder to water is 1:0.7-1.1, the D90 of the recycled concrete powder is not greater than 75μm, and the volume fraction of carbon dioxide in the carbon dioxide-containing gas is 10%-30%.

[0008] Furthermore, in step S1, after the recycled concrete powder is mixed with water to form a powder slurry, it is stirred and dispersed at 300-800 r / min at 15-35℃. After stopping the aeration, stirring is continued for 3-10 min to reduce the influence of free air bubbles on the pH measurement.

[0009] Furthermore, in step S1, the degree of carbonation is controlled by at least two of the following: the carbonate increment measured by thermogravimetric analysis, the inlet and outlet carbon dioxide mass balance, and the pH of the slurry after degassing; the pH of the slurry after degassing is 10.5-11.5, and the Ca(OH)2 content in the carbonized sample is 20%-70% of that before carbonation.

[0010] In step S1, after carbon dioxide is introduced into the micronized slurry, it exists in the form of dissolved carbon dioxide, bicarbonate ions, and carbonate ions, and reacts with calcium ions dissolved in the slurry to form calcium carbonate. In addition, by controlling the degree of partial carbonation, excessive consumption of Ca(OH)2 in the regenerated micronized powder can be reduced, and the alkaline environment required for the regenerated micronized powder to be used in subsequent gelling systems can be maintained.

[0011] In step S2, the second portion of the partially carbonized micro-powder slurry is filtered to obtain a concentrated filter cake. Preferably, the second portion of the partially carbonized micro-powder slurry is vacuum filtered to obtain a concentrated filter cake with a solid content of 80%.

[0012] In step S2, the partially carbonized micronized powder slurry is divided into a first part and a second part. The first part serves as the partially carbonized micronized powder slurry for concrete, and can be used in whole or in part depending on the mixing volume of a single batch of concrete. The second part serves as the partially carbonized micronized powder slurry for mineral coating. This method ensures that the coating layer and the micronized powder used in the concrete have the same source and similar mineral composition, resulting in a high degree of consistency in hydration shrinkage rate and coefficient of thermal expansion, which is beneficial for improving the material compatibility between the coating layer and the cementitious matrix.

[0013] Furthermore, in step S3, based on the dry solids mass of the mineral coating layer, the partial carbonized recycled concrete micro powder in the second part accounts for 60%-70%, slag powder accounts for 18%-25%, silicate cement accounts for 8%-15%, silica fume accounts for 2%-5%, and polyvinyl alcohol dry solids account for 0.2%-0.8%, with the total mass percentage of each component being 100%.

[0014] Furthermore, the biochar granules are made from lignocellulosic biomass raw materials through oxygen-limited pyrolysis carbonization at 500-700℃. The lignocellulosic agricultural and forestry biomass raw materials are selected from at least one of wood, bamboo, fruit shells and straw. The particle size is 2-5mm, the fixed carbon content is not less than 60%, the ash content is not more than 25%, and the moisture content is dried to not more than 5% before use.

[0015] Furthermore, the preparation method of biochar granules is as follows: the residue of bamboo processing is dried to constant weight, crushed and sieved to obtain 3-6 mm bamboo material, heated to 500-700℃ at a rate of 6-10℃ / min under an inert atmosphere and kept at that temperature for 1-4 hours, continuously cooled to below 80℃ by passing inert gas, lightly crushed and sieved to obtain 2-5 mm bamboo charcoal, washed with deionized water at a solid-liquid ratio of 1:8-12, and then dried to a moisture content of no more than 5%, thus obtaining dried biochar granules.

[0016] Further, in step S3, slag powder, cement and silica fume are dry-mixed to obtain dry-mixed powder. Then, the second part of partially carbonized micro powder slurry, polyvinyl alcohol aqueous solution and water are mixed and added to the dry-mixed powder, and stirred evenly to form mineral-coated slurry.

[0017] Furthermore, in step S3, the water-to-solid ratio of the mineral-coated slurry is 0.28-0.4, the ratio of the dry solids of the porous mineral coating layer to the dry mass of the biochar particles is 0.8-1.8:1, and the coated particles are cured for 12-24 hours at 20-30℃ and relative humidity not less than 90%.

[0018] Furthermore, in step S3, the particles are subjected to surface carbonization treatment after wet curing, before drying, or after drying; the volume fraction of carbon dioxide in the gas used is 10%-25%, the treatment temperature is 20-35℃, and the treatment time is 10-30 min. Surface carbonization can reduce surface free alkali and improve surface stability.

[0019] In step S3, a mineral coating layer is formed on the surface of the dried biochar and wet curing is completed, followed by negative pressure impregnation. This sequence avoids the need for repeated coating layer hydration, surface carbonization, or high-temperature drying after the biochar absorbs liquid, thus preventing the loss of internal curing agents in the post-treatment stage.

[0020] The mineral coating layer consists of partially carbonized recycled concrete micropowder, slag powder, cement, and silica fume. Cement provides early hydration bonding, while slag powder and silica fume participate in subsequent reactions and improve interparticle filling. The partially carbonized recycled micropowder serves as the main fine powder component. Polyvinyl alcohol is added to the coating slurry in the form of a low-concentration aqueous solution to improve wet bonding and dry granulation.

[0021] Further, in step S4, a vacuum is drawn to -0.095 to -0.075 MPa and maintained for 10-20 minutes. While maintaining negative pressure, a curing agent is added until the particles are completely submerged. Normal pressure is restored within 5-15 minutes, and soaking continues for 20-60 minutes. The curing agent is an aqueous solution containing 2%-6% by mass of a polyether-type shrinkage-reducing agent.

[0022] In step S4, the dried mineral coating layer still has interconnected pores. During negative pressure impregnation, air is first expelled from the pores of the coating layer and biochar. After restoring normal pressure, the liquid enters the pores under the pressure difference. After soaking, the surface free liquid is removed by draining, preferably on a sieve, more preferably on a 1mm sieve for 3 minutes. Repeated wiping with absorbent materials should be avoided to prevent inconsistent surface moisture content between different batches. The treated granules are stored in a sealed container and preferably used in concrete within 24 hours.

[0023] Further, in step S5, according to the mixing volume of a single batch of concrete, a portion or all of the first part is added to the concrete mixture, and after the main wet mixing of the concrete mixture is completed, the coated biochar internal curing particles are added, and the mixture is discharged after wet mixing.

[0024] Further, in step S5, coarse aggregate, fine aggregate, cement, slag powder and fly ash are first dry-mixed for 20-40 seconds, then the first part of partially carbonized micro powder slurry, part of water and part of polycarboxylate superplasticizer are added and wet-mixed for 45-75 seconds, then the remaining water and the remaining polycarboxylate superplasticizer are added and wet-mixed for 20-40 seconds, and finally the coated biochar internal curing granules are added and wet-mixed for 25-50 seconds before being discharged.

[0025] In step S5, the coated biochar internal curing particles are added after the main wet mixing is completed, so that the particles enter the mixing system which has formed a cement paste coating and lubrication effect. This helps to reduce the direct friction between the particles and coarse aggregate and improve the integrity of the particles during the mixing process.

[0026] A recycled micropowder synergistic reinforced low-shrinkage concrete is prepared by the preparation method of recycled micropowder synergistic reinforced low-shrinkage concrete described in any of the above technical solutions.

[0027] Furthermore, with 1m 3 The concrete comprises 240-310 kg of silicate cement, 70-130 kg of slag powder, 0-60 kg of fly ash, 40-80 kg of partially carbonized recycled concrete powder provided in the first part, 145-180 kg of water, 650-760 kg of fine aggregate, 900-1050 kg of coarse aggregate, 20-45 kg of coated biochar internal curing granules, and 4-8 kg of polycarboxylate superplasticizer. The effective water-cement ratio of the concrete is 0.3-0.42.

[0028] The present invention has the following beneficial effects: When preparing partially carbonized micropowder slurry, controlling the carbonization degree of recycled concrete micropowder, compared to simply using a fixed aeration time, can prevent excessive carbonization leading to loss of activity and ensure product performance stability. Using the same batch of partially carbonized micropowder separately for concrete and particle coating reduces the need for additional powder types and ensures consistent material sources for both components. The biochar undergoes molding, curing, and drying of the mineral coating before liquid absorption, avoiding internal water loss caused by post-absorption drying. The porous mineral coating transforms the direct contact between biochar and coarse aggregate into contact between the mineral layer and aggregate, exhibiting good material compatibility with the cement paste. Detailed Implementation

[0029] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0030] In this invention, the simulated gas containing 20% ​​carbon dioxide by volume is a mixture of 20% CO2, 5% O2, and 75% N2 by volume; the purity of CO2, O2, and N2 is not less than 99.9%. The preparation method of biochar particles is as follows: bamboo processing residue is dried at 105℃ to constant weight, crushed and sieved to obtain 3-6mm bamboo material, heated to 600℃ at a rate of 10℃ / min under nitrogen purging and held for 2h, continuously cooled to below 80℃ under nitrogen purging, lightly crushed and sieved to obtain 2-5mm bamboo charcoal, washed 3 times with deionized water at a solid-liquid ratio of 1:10, and then dried at 60℃ to a moisture content of not more than 5%, thus obtaining dried biochar particles.

[0031] The following materials are used: Recycled concrete powder: D90≤75μm, moisture content≤5%; Silica fume: SiO2 mass fraction preferably ≥85%; Polyvinyl alcohol: degree of alcoholysis 88-99%, added by aqueous solution, measured by dry solids; Cement: P·O 42.5 or equivalent general-purpose Portland cement; Slag powder: S95 or equivalent granulated blast furnace slag powder; Fly ash: Grade II low-calcium fly ash; Polycarboxylate superplasticizer: PCE-40 slow-release polycarboxylate superplasticizer; Coarse aggregate consists of 30% 5-10mm limestone crushed stone and 70% 10-20mm limestone crushed stone; Fine aggregate consists of 60% quartz natural medium sand and 40% limestone manufactured sand, with a fineness modulus of 2.6-2.9 after mixing.

[0032] Example 1 A method for preparing low-shrinkage concrete synergistically reinforced with recycled micropowder includes the following steps: S1. Dry the recycled concrete powder to constant weight at 40℃ and pass it through a 75μm sieve; weigh 10kg of dry-based recycled concrete powder and 8.5kg of water, add them to a 30L reaction vessel equipped with a stirrer and gas dispersion device, and stir at 600r / min for 5min at 25℃; introduce a simulated gas composed of 20%CO2, 5%O2 and 75%N2 at a total flow rate of 3L / min, and record the CO2 volume fraction at the inlet and outlet simultaneously; take a small amount of slurry every 10min, degas it and measure the pH; when the pH is 10.8-11.1, stop the gas supply and continue stirring for 5min; take a sample, dry it at 40℃ and perform thermogravimetric analysis; when the residual Ca(OH)2 is 40%-55% of that before carbonization, a qualified partially carbonized powder slurry is obtained; S2. Thoroughly mix the qualified slurry. Take 80% of the dry-based recycled concrete powder as the first part and the remainder as the second part. The first part contains 8 kg of dry-based powder and 6.8 kg of water. The second part contains 2 kg of dry-based powder and 1.7 kg of water. Vacuum filter the second part and adjust the solid content of the filter cake to 80% to obtain a concentrated filter cake containing 2 kg of dry powder and 0.5 kg of water. S3. Mix 0.65 kg of S95 slag powder, 0.34 kg of P·O 42.5 cement, and 0.09 kg of silica fume dry for 3 minutes to obtain a dry-mixed powder. Prepare a 5% (w / w) aqueous solution from 0.015 kg of polyvinyl alcohol solids. Mix the concentrated filter cake obtained in step S2, the 5% polyvinyl alcohol aqueous solution, and 0.267 kg of water for 2 minutes. Then add the dry-mixed powder and stir at 600 r / min for 5 minutes to form a uniform mineral-coated slurry (total dry solids 3.095 kg, water-to-solid ratio 0.34). Weigh 2.58 kg of biochar granules and add them to a disc granulator. Stir at 25 r / min for 5 minutes. Spray 60% of the total amount of mineral-coated slurry, continue rolling for 3 minutes, let stand for 5 minutes, then spray the remaining amount within 4 minutes and roll for 5 minutes; the dry solids of the mineral coating layer to the dry mass of biochar is 1.2:1; place the wet particles in an environment of 25℃ and relative humidity above 95% for 18 hours, then place them in an environment of 20% CO2, 5% O2 and 75% N2 atmosphere, 25℃ and relative humidity for 15 minutes, then dry them at 40℃ until the moisture content is not higher than 8%, and sieve out obvious agglomerates and broken particles to obtain particles with a porous mineral coating layer; S4. Weigh 2 kg of particles with a porous mineral coating (record mass M0), place them in a negative pressure impregnation tank, evacuate to -0.085 MPa and maintain for 15 min, add curing agent (aqueous solution containing 5% polyether shrinkage reducer by mass) from the bottom of the tank under negative pressure, so that the liquid level is 3 cm above the particle layer, restore normal pressure within 10 min, and continue soaking for 30 min, then transfer to a 1 mm sieve to drain the liquid for 3 min, to obtain coated biochar internally cured particles (weigh wet mass M1), with a liquid absorption rate of 36% (liquid absorption rate = (M1-M0) / M0×100%), seal the particles and use them within 24 h; S5, at 0.08m 3 The mixing quantities are as follows: 21.6 kg of P·O 42.5 cement, 8 kg of S95 slag powder, 3.2 kg of II fly ash, 7.4 kg of carbonized micro-powder slurry from the first part obtained in step S2 (4 kg of dry solids of carbonized micro-powder from the first part and 3.4 kg of water), plus 8.976 kg of water, 56 kg of fine aggregate (33.6 kg of natural medium quartz sand and 22.4 kg of manufactured limestone sand), 80.8 kg of coarse aggregate (24.24 kg of 5-10 mm limestone crushed stone and 56.56 kg of 10-20 mm limestone crushed stone), 2.4 kg of coated biochar internal curing granules, and 0.44 kg of PCE-40 polycarboxylate superplasticizer (containing 0.264 kg of water). Coarse aggregate, fine aggregate, cement, slag powder, and fly ash are added to a mixer and dry-mixed for 30 seconds. Then, the first part of the carbonized micro powder slurry, 70% additional water (6.283 kg), and 70% polycarboxylate superplasticizer (0.308 kg) are added and wet-mixed for 60 seconds. After that, the remaining additional water (2.693 kg) and the remaining polycarboxylate superplasticizer (0.132 kg) are added and wet-mixed for 30 seconds. Finally, the coated biochar internal curing granules are added and wet-mixed for another 35 seconds before being discharged.

[0033] Example 2 This embodiment differs from Embodiment 1 in the following ways: In step S1, 10 kg of dry-based recycled concrete powder and 7 kg of water are weighed and added to a reaction vessel. The mixture is stirred at 300 r / min for 5 min at 25°C. Simulated gas containing 20% ​​CO2 is introduced at a total flow rate of 3 L / min. When the pH of the degassed slurry is 11.3-11.5, the gas is stopped, and stirring is continued for 3 min. When the residual amount is measured to be 60%-70% of that before carbonization by thermogravimetric analysis, a qualified partially carbonized powder slurry is obtained. In step S2, 70% of the dry-based recycled concrete powder is taken as the first part, and the remainder is taken as the second part; the first part contains a total of 7 kg of dry-based powder and 4.9 kg of water; the second part contains a total of 3 kg of dry-based powder and 2.1 kg of water; the second part is filtered until the solid content is 80%, and a concentrated filter cake containing 3 kg of dry powder and 0.75 kg of water is obtained.

[0034] In step S3, weigh 0.975 kg of S95 slag powder, 0.51 kg of P·O 42.5 cement, and 0.135 kg of silica fume, and dry mix for 3 min. Prepare a 5% (w / w) aqueous solution from 0.023 kg of polyvinyl alcohol solids. Mix the concentrated filter cake, the 5% polyvinyl alcohol aqueous solution, and 0.122 kg of water for 2 min, then add the dry-mixed powder and stir at 600 r / min for 5 min to obtain a uniform mineral-coated slurry (total dry solids 4.643 kg, water-to-solid ratio 0.28). Weigh 5.803 kg of biochar granules and stir at 25 r / min. In a disc granulator with a rotation speed of 0 min, the entire coating slurry is sprayed twice, at 60% and 40% respectively. The specific process of spraying and rolling is as described in Example 1. The dry solids of the mineral coating layer and the dry mass ratio of biochar are 0.8:1. The wet granules are cured in an environment of 25°C and relative humidity of 90% or higher for 12 hours. Then, they are treated in an environment of 20% CO2, 5% O2 and 75% N2 atmosphere, 25°C and relative humidity of 75% for 10 minutes. Finally, they are dried at 40°C until the moisture content is no higher than 8%.

[0035] In step S4, 2 kg of particles with a porous mineral coating layer are weighed, vacuumed to -0.075 MPa and maintained for 10 min, and 2% by mass of polyether shrinkage-reducing agent aqueous solution is added under negative pressure until the liquid level is 30 mm above the particle layer. Normal pressure is restored within 15 min, and soaking continues for 20 min. After draining for 3 min, the liquid is weighed and the liquid absorption rate is calculated.

[0036] In step S5, at 0.08m 3 The mixing quantities are as follows: 19.2 kg of P·O 42.5 cement, 5.6 kg of S95 slag powder, 0 kg of II fly ash, 5.44 kg of carbonized micro powder slurry from the first part obtained in step S2 (3.2 kg of dry solids of carbonized micro powder from the first part, 2.24 kg of water), 9.328 kg of water, 52 kg of fine aggregate (31.2 kg of natural medium quartz sand, 20.8 kg of manufactured limestone sand), 84 kg of coarse aggregate (25.2 kg of 5-10 mm limestone crushed stone, 58.8 kg of 10-20 mm limestone crushed stone), 1.6 kg of coated biochar internal curing granules, and 0.32 kg of PCE-40 polycarboxylate superplasticizer (containing 0.192 kg of water). Add coarse aggregate, fine aggregate, cement, and slag powder to a mixer and dry mix for 30 seconds. Then add the first part of carbonized micro powder slurry, 70% additional water (6.53 kg), and 70% polycarboxylate superplasticizer (0.224 kg) and wet mix for 60 seconds. Then add the remaining additional water (2.798 kg) and the remaining polycarboxylate superplasticizer (0.096 kg) and wet mix for 30 seconds. Finally, add the coated biochar internal curing granules and continue wet mixing for 25 seconds before discharging to obtain the final product.

[0037] Example 3 This embodiment differs from Embodiment 1 in the following ways: In step S1, 10 kg of dry-based recycled concrete powder and 11 kg of water are weighed and added to a reaction vessel. The mixture is stirred at 800 r / min for 5 min at 25°C. Simulated gas containing 20% ​​CO2 is introduced at a total flow rate of 3 L / min. When the pH of the degassed slurry is 10.5-10.7, the gas is stopped, and stirring is continued for 10 min. When the residual amount is measured to be 25%-35% of that before carbonization by thermogravimetric analysis, a qualified partially carbonized powder slurry is obtained. In step S2, 90% of the dry-based recycled concrete powder is taken as the first part, and the remainder is taken as the second part; the first part contains a total of 9 kg of dry-based powder and 9.9 kg of water; the second part contains a total of 1 kg of dry-based powder and 1.1 kg of water; the second part is filtered until the solid content is 80%, and a concentrated filter cake containing 1 kg of dry powder and 0.25 kg of water is obtained.

[0038] In step S3, 0.325 kg of S95 slag powder, 0.17 kg of P·O 42.5 cement, and 0.045 kg of silica fume were weighed and dry-mixed for 3 min. 0.0075 kg of polyvinyl alcohol solids were prepared into a 5% aqueous solution. The concentrated filter cake, the 5% polyvinyl alcohol aqueous solution, and 0.227 kg of water were mixed and added to the dry-mixed powder. The mixture was stirred at 600 r / min for 5 min to obtain a uniform mineral-coated slurry (total dry solids 1.548 kg, water-to-solids ratio 0.4). 0.86 kg of biochar granules were weighed and rolled and coated using the two-stage spraying method described in Example 1. The dry solids to dry biochar mass ratio of the mineral coating layer was 1.8:1. The wet granules were cured in an environment of 25°C and 95% relative humidity for 24 h. Then, they were treated in an environment of 20% CO2, 5% O2, and 75% N2 atmosphere, 25°C, and 75% relative humidity for 30 min. Finally, they were dried at 40°C until the moisture content was no higher than 8%.

[0039] In step S4, 2 kg of particles with a porous mineral coating layer are weighed, vacuumed to -0.095 MPa and maintained for 20 min, and 6% by mass of polyether shrinkage-reducing agent aqueous solution is added under negative pressure until the liquid level is 30 mm above the particle layer. Normal pressure is restored within 5 min, and soaking continues for 60 min. After draining for 3 min, the liquid is weighed and the liquid absorption rate is calculated.

[0040] In step S5, at 0.08m 3 Weigh out 24.8 kg of P·O 42.5 cement, 10.4 kg of S95 slag powder, 3.2 kg of II fly ash, 13.44 kg of the carbonized micro powder slurry from the first part obtained in step S2 (6.4 kg of dry solids of the carbonized micro powder from the first part and 7.04 kg of water), add 6.016 kg of water, 60.8 kg of fine aggregate (36.48 kg of natural medium quartz sand and 24.32 kg of manufactured limestone sand), 72 kg of coarse aggregate (21.6 kg of 5-10 mm limestone crushed stone and 50.4 kg of 10-20 mm limestone crushed stone), 3.6 kg of coated biochar internal curing granules, and 0.64 kg of PCE-40 polycarboxylate superplasticizer (containing 0.384 kg of water). Coarse aggregate, fine aggregate, cement, slag powder, and fly ash are added to a mixer and dry-mixed for 30 seconds. Then, the first part of the carbonized micro powder slurry, 70% additional water (4.211 kg), and 70% polycarboxylate superplasticizer (0.448 kg) are added and wet-mixed for 60 seconds. After that, the remaining additional water (1.805 kg) and the remaining polycarboxylate superplasticizer (0.192 kg) are added and wet-mixed for 30 seconds. Finally, the coated biochar internal curing granules are added and wet-mixed for another 50 seconds before being discharged.

[0041] Example 4 Compared with Example 1, in step S3, the wet particles are cured in an environment of 25°C and relative humidity above 95% for 18 hours, and then dried directly at 40°C until the moisture content is not higher than 8% without CO2 surface treatment; the rest are the same as in Example 1.

[0042] Example 5 Compared with Example 1, in step S3 of this embodiment, the raw material dosage and two rolling coating conditions of step S3 of Example 1 are used to control the wet particles, which are cured in an environment of 25°C and relative humidity of not less than 95% for 18 hours; first, they are dried at 40°C to a moisture content of not more than 8%, and then placed in an environment of 20% CO2, 5% O2 and 75% N2 atmosphere, 25°C and relative humidity of 75% for 15 minutes to remove obvious agglomerates and broken particles, and particles with a porous mineral coating layer are obtained; the rest are the same as in Example 1.

[0043] Comparative Example 1 Compared with Example 1, this comparative example did not introduce recycled concrete powder and coated biochar internal curing particles.

[0044] This comparative example provides a method for preparing concrete, the specific steps of which are: according to 0.08m 3 Weigh out 25.6 kg of cement, 8 kg of slag powder, 3.2 kg of fly ash, 56 kg of fine aggregate (33.6 kg of natural medium quartz sand and 22.4 kg of manufactured limestone sand), 83.2 kg of coarse aggregate (24.96 kg of 5-10 mm limestone crushed stone and 58.24 kg of 10-20 mm limestone crushed stone), and add 12.376 kg of water and 0.44 kg of PCE-40 polycarboxylate superplasticizer (introducing 0.264 kg of water). Add coarse aggregate, fine aggregate, cement, slag powder and fly ash to a mixer and dry mix for 30 seconds. Then add 70% additional water (8.663 kg) and 70% polycarboxylate superplasticizer (0.308 kg) and wet mix for 60 seconds. Then add the remaining additional water (3.713 kg) and the remaining polycarboxylate superplasticizer (0.132 kg) and wet mix for 30 seconds before discharging to obtain the final product.

[0045] Comparative Example 2 Compared with Example 1, this comparative example does not include coated biochar internal maintenance particles.

[0046] This comparative example provides a method for preparing concrete, specifically: weighing 10 kg of dry-based recycled concrete powder and 8.5 kg of water, stirring at 25℃ and 600 r / min for 5 min, without introducing simulated CO2 gas, to obtain an uncarbonated powder slurry; according to 0.08 m 3Weigh out 21.6 kg of cement, 8 kg of slag powder, 3.2 kg of fly ash, 7.4 kg of uncarbonized micro powder slurry (4 kg of uncarbonized micro powder dry solids and 3.4 kg of water), 56 kg of fine aggregate (33.6 kg of natural medium quartz sand and 22.4 kg of manufactured limestone sand), 83.2 kg of coarse aggregate (24.96 kg of 5-10 mm limestone crushed stone and 58.24 kg of 10-20 mm limestone crushed stone), and add 8.976 kg of water and 0.44 kg of PCE-40 polycarboxylate superplasticizer (introducing 0.264 kg of water). Add coarse aggregate, fine aggregate, cement, slag powder and fly ash to a mixer and dry mix for 30 seconds. Then add uncarbonized micro powder slurry, 70% additional water (6.283 kg) and 70% polycarboxylate superplasticizer (0.308 kg) and wet mix for 60 seconds. Then add the remaining additional water (2.693 kg) and the remaining polycarboxylate superplasticizer (0.132 kg) and wet mix for 30 seconds before discharging to obtain the final product.

[0047] Comparative Example 3 Compared with Example 1, this comparative example includes a partially carbonized micronized slurry in the first part, but does not include coated biochar internal maintenance particles.

[0048] This comparative example provides a method for preparing concrete, specifically: S1. Refer to step S1 in Example 1; S2. Prepare according to step S2 of Example 1; the difference is that only the slurry of 4 kg of dry powder and 3.4 kg of water in the first part is used for concrete preparation, and the second part is not used in this comparative example. S3, according to 0.08m 3 The mixing quantities are as follows: 21.6 kg of cement, 8 kg of slag powder, 3.2 kg of fly ash, 7.4 kg of partially carbonized micro powder slurry (4 kg of partially carbonized micro powder dry solids and 3.4 kg of water), plus 8.976 kg of water, 56 kg of fine aggregate (33.6 kg of natural medium quartz sand and 22.4 kg of manufactured limestone sand), 83.2 kg of coarse aggregate (24.96 kg of 5-10 mm limestone crushed stone and 58.24 kg of 10-20 mm limestone crushed stone), and 0.44 kg of PCE-40 polycarboxylate superplasticizer (including 0.264 kg of water). The mixture is prepared by mixing according to the order of addition in step S5 of Example 1.

[0049] Comparative Example 4 Compared with Example 1, this comparative example does not coat the biochar particles with a mineral layer, but directly impregnates the biochar particles under negative pressure.

[0050] This comparative example provides a method for preparing concrete, specifically: S1. Refer to step S1 in Example 1; S2. Prepare according to step S2 of Example 1. Based on the concrete mixing quantity, take the corresponding mass from the first part for concrete preparation. The second part is not used for particle coating.

[0051] S3. Weigh 1.032 kg of dried biochar granules and place them in a negative pressure impregnation tank. Evacuate the tank to -0.085 MPa and maintain the vacuum for 15 min. While maintaining the negative pressure, add a curing agent (a 5% polyether shrinkage-reducing agent aqueous solution) from the bottom of the tank so that the liquid level is 3 cm above the granule layer. Restore normal pressure within 10 min and continue soaking for 30 min. Then transfer the solution to a 1 mm sieve and drain for 3 min to obtain impregnated biochar granules. Calculate the liquid absorption rate. S4. Following the raw materials and dosages in step S5 of Example 1, after completing the main wet mixing in the order of Example 1, add all the impregnated biochar particles prepared in step S3, wet mix for 35 seconds, and then discharge the material.

[0052] Comparative Example 5 Compared with Example 1, this comparative example uses atmospheric pressure immersion for the porous mineral coating particles instead of negative pressure immersion.

[0053] This comparative example provides a method for preparing concrete, specifically: S1, S2, and S3 all refer to steps S1, S2, and S3 of Example 1; S4. Weigh 1.765 kg of porous mineral-coated particles, immerse them directly in a 5% polyether shrinkage-reducing agent aqueous solution without vacuuming, soak them for 45 min under normal pressure, and then drain them on a 1 mm sieve for 3 min to obtain wet coated biochar particles soaked under normal pressure. S5. Except for replacing the inner curing particles of coated biochar with the wet coated biochar particles prepared in step S4 that have been soaked under normal pressure, all other materials and processes are the same as in step S5 of Example 1.

[0054] Comparative Example 6 Compared with Example 1, this comparative example does not add polyvinyl alcohol during the preparation of the mineral coating slurry.

[0055] This comparative example provides a method for preparing concrete, specifically: S1 and S2 are both based on steps S1 and S2 of Example 1; S3. Weigh 0.65 kg of slag powder, 0.34 kg of cement, and 0.09 kg of silica fume, and dry mix for 3 min to obtain dry-mixed powder; mix the concentrated filter cake obtained in step S2 with 0.547 kg of water for 2 min, then add the dry-mixed powder and stir at 600 r / min for 5 min; weigh 2.58 kg of biochar particles, and prepare porous mineral-coated particles according to the conditions of two sprayings, rolling, wet curing, surface carbonization, and drying in step S3 of Example 1; S4 and S5 refer to steps S4 and S5 of Example 1.

[0056] Comparative Example 7 Compared with Example 1, this comparative example uses over-carbonized micro powder in step S1, while the preparation of the remaining steps is the same as in Example 1. That is, during the preparation process in step S1, during the carbonization process, the gas is continuously ventilated until the pH is 8.5 after degassing, and the gas is stopped when the residual amount of Ca(OH)2 is found to be less than 10% of that before carbonization after drying at 40°C and thermogravimetric analysis. Stirring continues for 5 minutes.

[0057] Comparative Example 8 Compared with Example 1, the order of adding the coated biochar internal curing particles in step S5 of this comparative example is changed. The preparation of the remaining steps is the same as in Example 1. That is, in step S5, the amount of each raw material is the same as in Example 1. 80.8 kg of coarse aggregate, 56 kg of fine aggregate, 21.6 kg of cement, 8 kg of slag powder, 3.2 kg of fly ash and 2.4 kg of coated biochar internal curing particles are added to the mixer at the same time. After dry mixing for 30 seconds, the first part of partially carbonized micro powder slurry, 70% additional water (6.283 kg) and 70% polycarboxylate superplasticizer (0.308 kg) are added and wet mixed for 60 seconds. Then, the remaining additional water (2.693 kg) and the remaining polycarboxylate superplasticizer (0.132 kg) are added and wet mixed for 30 seconds before discharging.

[0058] Related tests: In this invention, the pH test standard is as follows: after stopping the aeration, continue stirring and degassing according to the regulations of each group, and measure using a high-alkali resistant composite electrode at 25°C. The difference between two consecutive readings should not exceed 0.1.

[0059] Thermogravimetric analysis: The slurry was dried to constant weight at 40℃, ground to a size not exceeding 75μm, and heated from 30℃ to 1000℃ at a rate of 10℃ / min under a nitrogen atmosphere. The relative changes of Ca(OH)2 and CaCO3 before and after carbonization were calculated.

[0060] The specific test results are shown in Table 1.

[0061] Table 1. Process control and particle performance test results of some carbonized micro powders

[0062] The liquid absorption rate is the liquid absorption rate of the coated biochar internal maintenance particles prepared in each embodiment and comparative example; among them, the liquid absorption rate of *Comparative Example 4 is the liquid absorption rate calculated after directly impregnating the biochar particles without coating them with a mineral layer.

[0063] Concrete mixture properties 1. Slump and retention over time: Refer to the relevant provisions in GB / T 50080-2016, measure the initial slump S0 within 5 minutes after discharge; let the mixture stand in a sealed container for 60 minutes, then remix at low speed for 30 seconds before measurement, measure S60, and calculate the slump retention rate over 60 minutes = S60 / S0 × 100%.

[0064] 2. Air content and water bleeding rate: In accordance with the relevant provisions in GB / T 50080-2016, the air content is determined by the pressure method; the water bleeding rate is calculated based on the cumulative mass of precipitated water and the amount of mixing water.

[0065] 3. Setting time: Mortar samples were sieved from the fresh concrete, and the initial and final setting times were determined using the penetration resistance method. Timing began when all materials were in contact with water.

[0066] The test results are shown in Table 2.

[0067] Table 2 Test results of relevant properties of concrete mixture

[0068] The test results above show that Example 2 has the highest initial slump and 60-minute retention rate, indicating good fluidity and retention over time, but its bleeding rate reaches 0.6%, indicating relatively weak stability. Example 3 has the lowest initial slump and the longest setting time, indicating that its low water-cement ratio, high cementitious material content, and high particle content increase the viscosity of the system. Example 1 has relatively balanced overall workability. Comparative Examples 4 and 8 have slump retention rates of only about 70% and high air content, indicating that pre-dry mixing of bare biochar or particles may cause water absorption and particle breakage. Comparative Example 7 has the longest initial and final setting times, indicating that excessive carbonization may have an adverse effect on the setting process.

[0069] Mechanical properties of concrete Compressive strength: In accordance with the relevant provisions in GB / T 50081-2019, 100mm cube specimens were used to determine the compressive strength at 3, 7, 28 and 90 days. Three specimens were used per batch for each age. The results were calculated by dividing the failure load by the bearing area, and dimensional conversions and outlier handling were performed according to the standard.

[0070] Splitting tensile strength: In accordance with the relevant provisions in GB / T 50081-2019, 100mm cube specimens were used to determine the 28-day splitting tensile strength, with at least 3 specimens per group.

[0071] The test results are shown in Table 3.

[0072] Table 3 Test results of mechanical properties of concrete

[0073] The test results above show that Example 3 exhibits the highest strength at all ages, indicating that a low water-cement ratio and a higher amount of cementitious material can effectively enhance the product's strength. Example 1 shows relatively stable overall strength. Example 2, due to its high water-cement ratio, has the lowest strength, but still shows a trend of continuous increase with age. Comparative Example 3, with partially carbonized micropowder, is generally superior to Comparative Example 2, with uncarbonized micropowder, indicating that moderate carbonization is beneficial for improving strength; the strength of Comparative Example 7, with excessive carbonization, decreases significantly.

[0074] Shrinkage properties Self-shrinkage: The specimens were prepared and the deformation was tested according to the corrugated pipe method in GB / T 50082-2024. The final setting time of each group of concrete was taken as the zero point for self-shrinkage test. The samples were continuously collected for 7 days under sealed conditions at 20±1℃. The self-shrinkage values ​​were extracted 1 day, 3 days and 7 days from the zero point. The shrinkage values ​​were uniformly expressed as positive numbers.

[0075] Drying shrinkage: Concrete shrinkage test method in GB / T 50082-2024 was used. 100mm×100mm×515mm prisms were used, 3 in each group. Demolding was performed after 24 hours. Standard curing was carried out until 7 days to determine the reference length. Then, the length changes were measured in an environment of 20±2℃ and 60%±5% relative humidity for 14, 28, 56 and 90 days. Table 4 lists the test results for 28 days and 90 days.

[0076] The test results are shown in Table 4.

[0077] Table 4 Shrinkage performance test results

[0078] As shown in Table 4, Example 2 exhibited the lowest self-shrinkage value, indicating that a higher effective water-cement ratio and the use of coated biochar internal maintenance particles effectively mitigated early self-drying. Example 3, while showing higher early self-shrinkage, had the lowest drying shrinkage value. Example 1 demonstrated superior performance in both self-shrinkage and drying shrinkage, significantly lower than Comparative Example 3, which only incorporated a portion of carbonized micropowder. Comparative Example 4 showed a significant inhibitory effect on early self-shrinkage, but its lower strength suggests that simply increasing water storage capacity does not guarantee overall performance. Comparative Example 2 (without carbonized micropowder) and Comparative Example 7 (with excessive carbonization) exhibited higher shrinkage values, with Comparative Example 7 showing the greatest shrinkage at all ages, indicating that excessive carbonization is detrimental to volume stability.

[0079] Unless otherwise stated, the amounts of raw materials and intermediate products recorded in steps S1-S4 of each embodiment are the amounts of intermediate products used in a single batch of preparation, and the amounts of raw materials and intermediate products recorded in step S5 are the actual amounts used in a single batch of concrete. In step S5, a specified mass can be taken from the corresponding intermediate products obtained in steps S1-S4 according to the concrete mixing volume. Unused intermediate products are not included in the mix proportions and performance tests of that batch of concrete. When the amount of intermediate products obtained in a single batch is insufficient to meet the requirements for subsequent concrete preparation and performance testing, multiple batches are prepared in parallel while maintaining the same raw material mass ratios and process conditions. The products obtained under the same process conditions are then combined, mixed evenly, and taken according to the mass specified in step S5.

[0080] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0081] Although embodiments of this application have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of this application, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing low-shrinkage concrete synergistically reinforced with recycled micropowder, characterized in that, Includes the following steps: S1. Mix recycled concrete powder with water to form a powder slurry. While stirring, introduce carbon dioxide gas into the slurry. After stopping the gas flow, continue stirring and remove free air bubbles to obtain a partially carbonized powder slurry. S2. Based on the mass of dry-based recycled concrete micro powder in the slurry, the partially carbonized micro powder slurry is divided into a first part and a second part, wherein the first part accounts for 70%-90% and the second part is the remainder, and the total of the two is 100%; S3. The second part is mixed with slag powder, silicate cement, silica fume, polyvinyl alcohol aqueous solution and water to form a mineral coating slurry. The mineral coating slurry is applied to the surface of dry biochar particles at least twice, and the particles are rolled into granules, wet cured and dried to obtain particles with a porous mineral coating layer. S4. Place the porous mineral coating particles in a negative pressure impregnation container, evacuate the container, add a curing agent, and continue to soak after restoring normal pressure to remove the free liquid on the particle surface, thereby obtaining coated biochar internal curing particles. S5. Add the first part to the concrete mixture, and after the main wet mixing of the concrete mixture is completed, add the coated biochar internal curing particles, and discharge the material after wet mixing.

2. The method for preparing low-shrinkage concrete synergistically reinforced with recycled micropowder according to claim 1, characterized in that, In step S1, the mass ratio of recycled concrete powder to water is 1:0.7-1.1, the D90 of the recycled concrete powder is not greater than 75μm, and the volume fraction of carbon dioxide in the carbon dioxide-containing gas is 10%-30%.

3. The method for preparing low-shrinkage concrete synergistically reinforced with recycled micropowder according to claim 1, characterized in that, In step S1, the pH of the degassed slurry is 10.5-11.5, and the Ca(OH)2 content in the carbonized sample is 20%-70% of that before carbonization.

4. The method for preparing low-shrinkage concrete synergistically reinforced with recycled micropowder according to claim 1, characterized in that, In step S2, the second part of the carbonized micro powder slurry is filtered to obtain a concentrated filter cake.

5. The method for preparing low-shrinkage concrete synergistically reinforced with recycled micropowder according to claim 1, characterized in that, In step S3, based on the dry solids mass of the mineral coating layer, the carbonized recycled concrete micro powder in the second part accounts for 60%-70%, slag powder accounts for 18%-25%, silicate cement accounts for 8%-15%, silica fume accounts for 2%-5%, and polyvinyl alcohol dry solids account for 0.2%-0.8%, with the total mass percentage of each component being 100%.

6. The method for preparing low-shrinkage concrete synergistically reinforced with recycled micropowder according to claim 1, characterized in that, The biochar granules are made from lignocellulosic biomass raw materials through oxygen-limited pyrolysis carbonization. The particle size is 2-5mm, the fixed carbon content is not less than 60%, the ash content is not more than 25%, and the moisture content is not more than 5% before use.

7. The method for preparing low-shrinkage concrete synergistically reinforced with recycled micropowder according to claim 1, characterized in that, In step S3, the water-to-solid ratio of the mineral-coated slurry is 0.28-0.4, the ratio of the dry solids of the porous mineral coating layer to the dry mass of the biochar particles is 0.8-1.8:1, and the coated particles are cured for 12-24 hours at 20-30℃ and relative humidity not less than 90%.

8. The method for preparing low-shrinkage concrete synergistically reinforced with recycled micropowder according to claim 1, characterized in that, In step S3, the particles are subjected to surface carbonization treatment after wet curing, before drying, or after drying; the volume fraction of carbon dioxide in the gas used is 10%-25%, the treatment temperature is 20-35℃, and the treatment time is 10-30min.

9. The method for preparing low-shrinkage concrete synergistically reinforced with recycled micropowder according to claim 1, characterized in that, In step S4, a vacuum is drawn to -0.095 to -0.075 MPa and maintained for 10-20 minutes. While maintaining negative pressure, a curing agent is added until the particles are completely submerged. Normal pressure is restored within 5-15 minutes and soaking continues for 20-60 minutes.

10. A low-shrinkage concrete synergistically reinforced with recycled micropowder, characterized in that, It was prepared using the method for preparing low-shrinkage concrete with synergistic reinforcement of recycled micropowder as described in any one of claims 1-9.

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