3D printing fly ash-slag-steel slag geopolymer concrete capable of reducing plastic shrinkage and improvement method

Abstract

The invention relates to the technical field of 3D printing building materials, in particular to 3D printing fly ash-slag-steel slag geopolymer concrete capable of reducing plastic shrinkage and an improvement method, and the geopolymer concrete is prepared from, by mass, 300 parts of fly ash, 120-300 parts of slag powder, 0-180 parts of steel slag powder, 900 parts of quartz sand, 11.5 parts of sodium hydroxide, 42.5 parts of sodium silicate and 200-300 parts of water. The improvement method comprises the steps that steel slag is activated, steel slag powder is subjected to mechanical ball-milling treatment, the reaction activity of the steel slag powder is improved, activated steel slag powder is obtained, and the ball-milling treatment time of the steel slag powder is 30-90 minutes; dry material mixing: uniformly mixing fly ash, slag powder, the activated steel slag powder and quartz sand according to a preset mass ratio to obtain a basic powder mixture; exciting and pulping: adding an alkaline exciting agent into the basic powder mixture, uniformly mixing, adding water, and stirring, so as to obtain the geopolymer concrete with extrudability. The early-stage plastic shrinkage can be reduced by mechanically treating the activated components.

Description

Technical Field

[0001] This invention relates to the field of 3D printed building materials technology, and in particular to a 3D printed fly ash-slag-steel slag geopolymer concrete with reduced plastic shrinkage and an improvement method thereof. Background Technology

[0002] With the rapid development of intelligent construction technologies, 3D-printed concrete technology has become a global hot topic in research and engineering applications. This technology, relying on digital design and layer-by-layer molding, enables formwork-free construction, demonstrating significant advantages in saving material and labor costs, improving construction efficiency, and enhancing structural and aesthetic freedom. Compared to traditional construction methods, 3D-printed concrete can effectively reduce resource consumption and environmental impact, and provides a new technological path for the high-precision manufacturing of complex structures, offering crucial support for the construction industry's development towards higher efficiency, greener practices, and greater intelligence.

[0003] In the current development of 3D printing building technology, cement-based materials still dominate. However, the calcination and production of cement clinker are accompanied by a large amount of carbon dioxide emissions, with carbon emissions per unit output typically reaching 800-900 kg·t. -1 This has made the cement industry one of the most significant sources of carbon emissions in the construction sector, posing a significant constraint on the ecological environment and carbon reduction targets. Therefore, developing and applying low-carbon, green alternative materials to reduce the environmental impact of cement-based materials is widely considered a key path to promoting the sustainable transformation of 3D printing building technology.

[0004] Against this backdrop, 3D-printed geopolymer concrete, a cementitious system using industrial solid waste or aluminum-silicon-rich materials as primary precursors, is considered a potential low-carbon solution to replace traditional cement-based materials due to its significantly reduced clinker usage and carbon dioxide emissions during preparation. Related research indicates that geopolymer concrete has met or even nearly met the application requirements of conventional cement-based materials in terms of mechanical properties and durability, demonstrating promising engineering application prospects.

[0005] However, along with its environmental advantages, geopolymer concrete is prone to shrinkage deformation in its early stages. Due to its rapid reaction rate, refined pore structure, and significant differences in internal moisture migration behavior compared to cement systems, geopolymer concrete often exhibits large volume shrinkage in its early stages, which can easily induce plastic shrinkage cracking, thus adversely affecting the molding stability and structural integrity of 3D printed components.

[0006] Currently, the above-mentioned plastic shrinkage problem is usually addressed by adjusting the raw material ratio, introducing additives, or changing the environmental curing conditions. However, in practical applications, these methods are still difficult to effectively suppress plastic shrinkage while ensuring the printability of the material, and their effect on controlling early shrinkage is limited.

[0007] Therefore, there is an urgent need for a technical solution that can effectively reduce the plastic shrinkage of fly ash-slag-steel slag geopolymer materials without significantly affecting 3D printing performance, so as to meet the requirements of 3D printed building components for molding stability and early crack resistance.

[0008] Chinese Patent Application Publication No. CN117143514A discloses a protective coating material for inhibiting plastic shrinkage of 3D-printed concrete, and its preparation and molding methods. The protective coating material for inhibiting plastic shrinkage of 3D-printed concrete of this invention, by weight, is mainly composed of the following components mixed by stirring in a light-protected environment: UV resin photosensitive prepolymer: 30-55 parts, UV resin reactive diluent: 25-40 parts, UV resin viscosity modifier: 1.0-2.0 parts, UV resin photoinitiator: 1.0-2.0 parts, and functionalized modifying agent: 0.05-1.0 parts. A high-pressure uniform spraying and 3D printing integrated process is adopted. While the 3D-printed concrete component is being formed, the protective coating material is uniformly sprayed onto its surface. After irradiation with ultraviolet light, a high-performance protective coating is rapidly formed. This coating significantly reduces the early plastic shrinkage value of the 3D-printed concrete component, inhibits the formation of micro-cracks on the surface of the 3D-printed concrete component, and significantly improves the overall durability of the 3D-printed component. Therefore, the existing technologies mentioned above cannot solve the problems of large early volume shrinkage and cracking caused by plastic shrinkage in 3D printed concrete prepared by alkali-activated geopolymer cementitious material system by addressing the material itself. Summary of the Invention

[0009] Therefore, the present invention provides a method for improving 3D printed fly ash-slag-steel slag geopolymer concrete with reduced plastic shrinkage, in order to overcome the problems of large early volume shrinkage and cracking caused by plastic shrinkage in 3D printed concrete prepared by alkali-activated geopolymer cementitious material system in the prior art.

[0010] To achieve the above objectives, on the one hand, the present invention provides a 3D-printed fly ash-slag-steel slag geopolymer concrete with reduced plastic shrinkage, wherein the geopolymer concrete is composed of the following components in the following mass proportions: 300 parts fly ash, 120-300 parts slag powder, 0-180 parts steel slag powder, 900 parts quartz sand, 11.5 parts sodium hydroxide, 42.5 parts sodium silicate, and 200-300 parts water.

[0011] Furthermore, the fly ash composition includes 30%–50% SiO2, 15%–30% Al2O3, and 10%–15% CaO; The slag powder composition includes 20%–35% SiO2, 10%–15% Al2O3, and 45%–55% CaO; The steel slag powder composition includes 15%–20% SiO2, 10%–20% Al2O3, and 40%–50% CaO.

[0012] Furthermore, the slag powder has a particle size of 0.5μm to 60μm, the fly ash has a particle size of 0.5μm to 250μm, the steel slag powder has a particle size of 0.5μm to 800μm, and the quartz sand has a particle size of 200μm to 400μm.

[0013] Furthermore, 120-300 parts of slag powder and 0-180 parts of steel slag powder are mixed in the geopolymer concrete; wherein the steel slag powder is mechanically ball-milled for 30-90 minutes.

[0014] On the other hand, the present invention also provides an improved method for 3D-printed fly ash-slag-steel slag geopolymer concrete with reduced plastic shrinkage, applied to the above-mentioned 3D-printed fly ash-slag-steel slag geopolymer concrete with reduced plastic shrinkage, comprising: Step S1: The steel slag powder is subjected to mechanical ball milling to improve its reactivity and obtain activated steel slag powder. The ball milling time of the steel slag powder is 30-90 min. Step S2: Mix fly ash, slag powder, activated steel slag powder and quartz sand evenly according to the preset mass ratio to obtain a basic powder mixture; Step S3: Add an alkaline activator to the base powder mixture and mix evenly. Then add water and stir to obtain 3D printed geopolymer concrete with low plastic shrinkage.

[0015] Further, step S2 includes: Step S21: Fly ash, granulated blast furnace slag, steel slag and quartz sand are crushed and ground separately, and the average particle size of granulated blast furnace slag and quartz sand is obtained respectively. Step S22: Determine the first stirring time and the first stirring speed based on the average particle size of the fly ash, granulated blast furnace slag, steel slag and quartz sand; Step S23: Prepare fly ash, granulated blast furnace slag, steel slag and quartz sand according to the preset ratio, and perform one mixing based on the first mixing time and the first mixing speed to obtain a uniform basic powder mixture.

[0016] Further, step S22 includes: Step S221: Determine the first difference based on the difference in the average particle size of the fly ash, granulated blast furnace slag, steel slag and quartz sand; Step S222: Compare the first difference with a preset difference, and determine the first stirring time and the first stirring speed based on the comparison result.

[0017] Further, step S3 includes: Step S31: Sodium hydroxide and sodium silicate are dissolved in water at a preset mass ratio and allowed to stand at room temperature for 12 to 24 hours to obtain a uniform and stable alkaline activator. Step S32: Add the alkaline activator to the base powder mixture and mix evenly to obtain a first mixture; Step S33: The first mixture is stirred with water at a first stirring speed for a first stirring time to make it initially mixed, and then stirred at a second stirring speed for a second stirring time to make it fully mixed into a uniform slurry, wherein the second stirring speed is greater than the first stirring speed.

[0018] Further, in step S2, the amount of slag powder is 180-300 parts and the amount of steel slag powder is 60-180 parts.

[0019] Furthermore, the second stirring speed is determined based on the fluidity of the first mixture after initial mixing with water.

[0020] Compared with existing technologies, the beneficial effects of this invention are that the geopolymer concrete of this invention uses slag powder, fly ash, steel slag powder, and quartz sand as its components, which enables the resource utilization of waste, reduces costs, and minimizes early plastic shrinkage. By adjusting the content of slag and steel slag, carbon emissions and energy consumption can be significantly reduced, and resource utilization can be improved. The mechanical ball milling method for steel slag can enhance its reactivity, further reducing early plastic shrinkage and improving the material's performance and stability.

[0021] Furthermore, by thoroughly mixing slag powder, fly ash, steel slag powder, and quartz sand according to a preset mass ratio, this invention can improve the uniformity of the basic powder mixture, thereby increasing the efficiency and quality of subsequent alkali activation. Using slag powder, fly ash, and steel slag powder as raw materials can also reduce costs. Adjusting the proportions of the basic powder mixture can reduce carbon emissions and energy consumption, as well as plastic shrinkage. Adding water to the first mixture at a preset water-cement ratio and stirring according to a preset stirring method can improve production efficiency and homogeneity, further enhancing the pore structure.

[0022] Furthermore, the present invention can improve the reactivity and microstructure of steel slag by mechanically ball milling it, thereby reducing its plastic shrinkage. Attached Figure Description

[0023] Figure 1 The flowchart shows the improved method for reducing plastic shrinkage in 3D-printed fly ash-slag-steel slag geopolymer concrete according to the present invention. Figure 2 This is a flowchart of step S2 of the improved method for reducing plastic shrinkage of 3D-printed fly ash-slag-steel slag geopolymer concrete according to the present invention. Figure 3 This is a flowchart of step S22 of the improved method for reducing plastic shrinkage of 3D-printed fly ash-slag-steel slag geopolymer concrete according to the present invention. Figure 4 This is a flowchart of step S3 of the improved method for reducing plastic shrinkage of 3D-printed fly ash-slag-steel slag geopolymer concrete according to the present invention. Figure 5 This is a comparison chart of the particle size of steel slag powder under different preset ball milling times in an embodiment of the present invention; Figure 6 Comparison of the microstructure of steel slag powder under different preset ball milling times in embodiments of the present invention; Figure 7 This is a comparison chart of plastic shrinkage of polymer concrete with different preset steel slag admixtures according to embodiments of the present invention; Figure 8 Comparison of plastic shrinkage of geopolymer concrete prepared from steel slag powder under different preset ball milling times in embodiments of the present invention. Detailed Implementation

[0024] To make the objectives and advantages of the present invention clearer, the present invention will be further described below with reference to embodiments; it should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention.

[0025] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0026] It should be noted that in the description of this invention, the terms "upper", "lower", "left", "right", "inner", "outer", etc., which indicate directions or positional relationships, are based on the directions or positional relationships shown in the accompanying drawings. This is only for the convenience of description and is not intended to indicate or imply that the device or element must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation of this invention.

[0027] Furthermore, it should be noted that, in the description of this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0028] This invention provides a 3D-printed fly ash-slag-steel slag geopolymer concrete with reduced plastic shrinkage. The geopolymer concrete is composed of the following components by mass ratio: 300 parts fly ash, 120-300 parts slag powder, 0-180 parts steel slag powder, 900 parts quartz sand, 11.5 parts sodium hydroxide, 42.5 parts sodium silicate, and 200-300 parts water.

[0029] Specifically, the fly ash composition includes 30%–50% SiO2, 15%–30% Al2O3, and 10%–15% CaO; the slag powder composition includes 20%–35% SiO2, 10%–15% Al2O3, and 45%–55% CaO; and the steel slag powder composition includes 15%–20% SiO2, 10%–20% Al2O3, and 40%–50% CaO.

[0030] Specifically, the slag powder has a particle size of 0.5μm to 60μm, the fly ash has a particle size of 0.5μm to 250μm, the steel slag powder has a particle size of 0.5μm to 800μm, and the quartz sand has a particle size of 200μm to 400μm.

[0031] Specifically, 120-300 parts of slag powder and 0-180 parts of steel slag powder are mixed in the geopolymer concrete; wherein the steel slag powder is mechanically ball-milled for 30-90 minutes.

[0032] The geopolymer concrete of this invention comprises slag powder, fly ash, steel slag powder, and quartz sand, enabling resource utilization of waste, reducing costs, and minimizing early plastic shrinkage. By adjusting the content of slag and steel slag, carbon emissions and energy consumption can be significantly reduced, while resource utilization efficiency can be improved. The mechanical ball milling method for steel slag enhances its reactivity, further reducing early plastic shrinkage and improving material performance and stability.

[0033] This invention also provides an improved method for reducing plastic shrinkage in 3D-printed fly ash-slag-steel slag geopolymer concrete. Please refer to [link to relevant documentation]. Figure 1 The flowchart shown is a method for improving 3D-printed fly ash-slag-steel slag geopolymer concrete to reduce plastic shrinkage according to the present invention, including: Step S1: The steel slag powder is subjected to mechanical ball milling to improve its reactivity and obtain activated steel slag powder. The ball milling time of the steel slag powder is 30-90 min. This invention can effectively reduce the particle size of steel slag and improve its activity by mechanical ball milling, thus significantly improving the plastic shrinkage of geopolymers.

[0034] Step S2: Mix fly ash, slag powder, activated steel slag powder and quartz sand evenly according to the preset mass ratio to obtain a basic powder mixture; Specifically, in step S2, the amount of slag powder is 180-300 parts and the amount of steel slag powder is 60-180 parts.

[0035] Please see Figure 2 The diagram shows a flowchart of step S2 of the improved method for reducing plastic shrinkage in 3D-printed fly ash-slag-steel slag geopolymer concrete according to the present invention. Specifically, step S2 includes: Step S21: Fly ash, granulated blast furnace slag, steel slag and quartz sand are crushed and ground separately, and the average particle size of granulated blast furnace slag and quartz sand is obtained respectively. It should be noted that those skilled in the art will understand that any existing equipment and method capable of obtaining the average particle size of fly ash, granulated blast furnace slag, steel slag, and quartz sand falls within the protection scope of this invention, and will not be elaborated further here.

[0036] Step S22: Determine the first stirring time and the first stirring speed based on the average particle size of the fly ash, granulated blast furnace slag, steel slag and quartz sand; Please see Figure 3 This is a flowchart of step S22 of the improved method for reducing plastic shrinkage in 3D-printed fly ash-slag-steel slag geopolymer concrete according to the present invention. Specifically, step S22 includes: Step S221: Determine the first difference based on the difference in the average particle size of the fly ash, granulated blast furnace slag, steel slag and quartz sand; Step S222: Compare the first difference with a preset difference, and determine the first stirring time and the first stirring speed based on the comparison result.

[0037] Step S23: Prepare fly ash, granulated blast furnace slag, steel slag and quartz sand according to the preset ratio, and perform one mixing based on the first mixing time and the first mixing speed to obtain a uniform basic powder mixture.

[0038] Step S3: Add an alkaline activator to the base powder mixture and mix evenly. Then add water and stir to obtain 3D printed geopolymer concrete with low plastic shrinkage.

[0039] Please see Figure 4 This is a flowchart of step S3 of the improved method for reducing plastic shrinkage in 3D-printed fly ash-slag-steel slag geopolymer concrete according to the present invention. Specifically, step S3 includes: Step S31: Sodium hydroxide and sodium silicate are dissolved in water at a preset mass ratio and allowed to stand at room temperature for 12 to 24 hours to obtain a uniform and stable alkaline activator. Step S32: Add the alkaline activator to the base powder mixture and mix evenly to obtain a first mixture; Step S33: The first mixture is stirred with water at a first stirring speed for a first stirring time to make it initially mixed, and then stirred at a second stirring speed for a second stirring time to make it fully mixed into a uniform slurry, wherein the second stirring speed is greater than the first stirring speed.

[0040] Specifically, the second stirring speed is determined based on the fluidity of the first mixture after initial mixing with water.

[0041] Specifically, water is added to the first mixture at a preset water-to-gel ratio. In an embodiment of the present invention, the preset water-to-gel ratio is 0.34.

[0042] Specifically, the amount of steel slag powder added and its particle size distribution after ball milling directly affect the initial consistency and water retention of the slurry, thus influencing early plastic shrinkage. Therefore, the first stirring speed and the first stirring time need to be dynamically compensated based on the physical characteristic parameters of the steel slag.

[0043] In a specific embodiment, the measured average particle size D50 of the steel slag powder in the basic powder mixture is obtained and recorded as the first particle size value. The preset reference particle size is 50 μm, and the preset difference value is 10 μm. The first particle size value is compared with the preset reference particle size to obtain the first difference value. When the first difference value is greater than the preset difference value, it indicates that the steel slag particles are relatively coarse (e.g., ball milling time is less than 30 min), the porosity between particles is large, and segregation is prone to occur. At this time, it is necessary to increase the stirring energy, increase the first stirring speed to 60 r / min, and extend the first stirring time to 5 min. When the first difference value is less than or equal to the preset difference value, the particle size distribution is better (e.g., ball milling time is between 60-90 min). At this time, the first stirring speed (slow speed) is set to 45 r / min, and the first stirring time is 3 min.

[0044] After initial mixing, based on the thixotropic requirements of the slurry, the second stirring speed (fast) is set to 150 r / min and the second stirring time is set to 5 min to ensure that the alkali activation reaction is sufficient and that the slurry has good printability.

[0045] Example 1: The effect of different ball milling times on the particle size distribution of steel slag was investigated. The particle size variation is shown in Table 1. Table 1. Particle size of steel slag at different ball milling times Ball milling time D10(μm) D50(μm) D90(μm) 0 minutes 32.31 171.42 361.45 30 minutes 3.61 59.45 192.66 60 minutes 2.89 50.06 170.69 90 minutes 2.74 57.46 245.64 Note: D10 represents the particle size corresponding to a cumulative distribution percentage of 10%; D50 represents the particle size corresponding to a cumulative distribution percentage of 50%; D90 represents the particle size corresponding to a cumulative distribution percentage of 90%.

[0046] Please see Figure 5 , Figure 6 As shown, it is a comparison diagram of the particle size of steel slag powder under different preset ball milling times in the embodiments of the present invention and a comparison diagram of the microstructure of steel slag powder under different preset ball milling times in the embodiments of the present invention.

[0047] Understandably, ball milling significantly refines the size of steel slag particles, thereby altering their cumulative particle size distribution. Compared to unmilled steel slag, the cumulative particle size curve after ball milling shifts noticeably towards smaller particle sizes, indicating effective particle breakage. The refining effect is most significant at 30 and 60 minutes of milling, with a marked decrease in both median particle size and the proportion of coarse particles, resulting in a more reasonable particle size distribution. This suggests that within this timeframe, the ball milling process primarily involves particle breakage and deagglomeration, which is beneficial for the thorough crushing and dispersion of steel slag particles. However, when the milling time is further extended to 90 minutes, the cumulative particle size curve shows a certain degree of reversal, manifested as an increase in characteristic particle size, and a weakening of the refining effect. This phenomenon may be related to the significant increase in the specific surface area and surface energy of the steel slag particles during prolonged ball milling, leading to agglomeration or cold welding between fine particles, causing some particles to exist as agglomerates and be measured as having a larger equivalent particle size. Therefore, in this example, moderate ball milling (30-60 min) effectively refines steel slag particles, while excessive ball milling may lead to particle agglomeration, hindering further particle size reduction. Scanning electron microscopy (SEM) results further illustrate the influence of ball milling time on the size and morphology of steel slag particles. Compared to unmilled steel slag, the particle size significantly decreased and the particle distribution became more uniform after ball milling for 30 and 60 min, indicating that the ball milling process at this stage primarily involved particle breakage, which is beneficial for refining the steel slag. This result is consistent with the overall trend of the cumulative particle size distribution curve shifting towards smaller particle sizes. However, when the ball milling time was extended to 90 min, significant particle agglomeration was observed in the SEM images, with some fine particles existing as agglomerates, leading to an increase in their equivalent particle size and thus weakening the refining effect of further ball milling. Therefore, in this embodiment, moderate ball milling (30-60 min) is more conducive to the effective refinement of steel slag particles, while excessively long ball milling times may cause particle agglomeration, hindering further particle size reduction.

[0048] Example 2 investigated the effect of steel slag content on plastic shrinkage. The total mass fraction of cementitious materials (fly ash, slag powder, and steel slag powder) was fixed at 600 parts, and the water-cement ratio was fixed at 0.34. The ratio of slag to steel slag powder (the steel slag powder was not ball-milled) was adjusted. The experimental results are shown in Table 2. Table 2 Plastic shrinkage with different steel slag contents fly ash content Slag powder dosage steel slag powder dosage Quartz sand content Sodium hydroxide Sodium silicate water Plastic shrinkage (%) 300 copies 300 copies 0 copies 900 copies 11.5 copies 42.5 copies 204 copies -2.15 300 copies 240 copies 60 copies 900 copies 11.5 copies 42.5 copies 204 copies -1.56 300 copies 180 copies 120 copies 900 copies 11.5 copies 42.5 copies 204 copies -1.40 300 copies 120 copies 180 copies 900 copies 11.5 copies 42.5 copies 204 copies -1.59 Example 3 investigated the effect of ball milling time on plastic shrinkage. The following ingredients were fixed by mass: 300 parts fly ash, 180 parts slag powder, 120 parts steel slag, 900 parts quartz sand, 11.5 parts sodium hydroxide, and 42.5 parts sodium silicate. The water-cement ratio was fixed at 0.34 (204 parts water). The ball milling time of the steel slag was adjusted. The experimental results are shown in Table 3. Table 3 Plastic shrinkage of steel slag at different ball milling times Ball milling time D50(μm) Plastic shrinkage (%) 0 minutes 171.42 -1.40 30 minutes 59.45 -0.81 60 minutes 50.06 -0.67 90 minutes 57.46 -1.17 Note: D50 represents the particle size corresponding to a cumulative distribution percentage of 50%.

[0049] Please see Figure 7 , Figure 8 The figures show a comparison of the plastic shrinkage of geopolymer concrete with different preset steel slag content and a comparison of the plastic shrinkage of geopolymer concrete prepared with steel slag powder under different preset ball milling times according to embodiments of the present invention.

[0050] Based on the experimental results, the preferred method is to use 120 parts of steel slag and to perform mechanical ball milling for 60 minutes.

[0051] The technical solution of the present invention has been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will all fall within the scope of protection of the present invention.

Claims

1. A 3D-printed fly ash-slag-steel slag geopolymer concrete with reduced plastic shrinkage, characterized in that, The geopolymer concrete is composed of the following components by mass proportions: 300 parts fly ash, 120-300 parts slag powder, 0-180 parts steel slag powder, 900 parts quartz sand, 11.5 parts sodium hydroxide, 42.5 parts sodium silicate, and 200-300 parts water.

2. The 3D-printed fly ash-slag-steel slag geopolymer concrete with reduced plastic shrinkage according to claim 1, characterized in that, The fly ash composition includes 30%–50% SiO2, 15%–30% Al2O3, and 10%–15% CaO; The slag powder composition includes 20%–35% SiO2, 10%–15% Al2O3, and 45%–55% CaO; The steel slag powder composition includes 15%–20% SiO2, 10%–20% Al2O3, and 40%–50% CaO.

3. The 3D-printed fly ash-slag-steel slag geopolymer concrete with reduced plastic shrinkage according to claim 1, characterized in that, The slag powder has a particle size of 0.5μm to 60μm, the fly ash has a particle size of 0.5μm to 250μm, the steel slag powder has a particle size of 0.5μm to 800μm, and the quartz sand has a particle size of 200μm to 400μm.

4. The 3D-printed fly ash-slag-steel slag geopolymer concrete with reduced plastic shrinkage according to claim 1, characterized in that, The geopolymer concrete contains 120-300 parts of slag powder and 0-180 parts of steel slag powder; wherein the steel slag powder is mechanically ball-milled for 30-90 minutes.

5. An improved method for 3D-printed fly ash-slag-steel slag geopolymer concrete with reduced plastic shrinkage, applied to the 3D-printed fly ash-slag-steel slag geopolymer concrete with reduced plastic shrinkage as described in any one of claims 1-4, characterized in that, include: Step S1: The steel slag powder is subjected to mechanical ball milling to improve its reactivity and obtain activated steel slag powder. The ball milling time of the steel slag powder is 30-90 min. Step S2: Mix fly ash, slag powder, activated steel slag powder and quartz sand evenly according to the preset mass ratio to obtain a basic powder mixture; Step S3: Add an alkaline activator to the base powder mixture and mix evenly. Then add water and stir to obtain 3D printed geopolymer concrete with low plastic shrinkage.

6. The improved method for reducing plastic shrinkage in 3D-printed fly ash-slag-steel slag geopolymer concrete according to claim 5, characterized in that, Step S2 includes: Step S21: Fly ash, granulated blast furnace slag, steel slag and quartz sand are crushed and ground separately, and the average particle size of granulated blast furnace slag and quartz sand is obtained respectively. Step S22: Determine the first stirring time and the first stirring speed based on the average particle size of the fly ash, granulated blast furnace slag, steel slag and quartz sand; Step S23: Prepare fly ash, granulated blast furnace slag, steel slag and quartz sand according to the preset ratio, and perform one mixing based on the first mixing time and the first mixing speed to obtain a uniform basic powder mixture.

7. The improved method for reducing plastic shrinkage in 3D-printed fly ash-slag-steel slag geopolymer concrete according to claim 6, characterized in that, Step S22 includes: Step S221: Determine the first difference based on the difference in the average particle size of the fly ash, granulated blast furnace slag, steel slag and quartz sand; Step S222: Compare the first difference with a preset difference, and determine the first stirring time and the first stirring speed based on the comparison result.

8. The improved method for reducing plastic shrinkage in 3D-printed fly ash-slag-steel slag geopolymer concrete according to claim 5, characterized in that, Step S3 includes: Step S31: Sodium hydroxide and sodium silicate are dissolved in water at a preset mass ratio and allowed to stand at room temperature for 12 to 24 hours to obtain a uniform and stable alkaline activator. Step S32: Add the alkaline activator to the base powder mixture and mix evenly to obtain a first mixture; Step S33: The first mixture is stirred with water at a first stirring speed for a first stirring time to make it initially mixed, and then stirred at a second stirring speed for a second stirring time to make it fully mixed into a uniform slurry, wherein the second stirring speed is greater than the first stirring speed.

9. The improved method for reducing plastic shrinkage in 3D-printed fly ash-slag-steel slag geopolymer concrete according to claim 5, characterized in that, In step S2, the amount of slag powder is 180-300 parts and the amount of steel slag powder is 60-180 parts.

10. The improved method for reducing plastic shrinkage in 3D-printed fly ash-slag-steel slag geopolymer concrete according to claim 8, characterized in that, The second stirring speed is determined based on the fluidity of the first mixture after initial mixing with water.

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