A method for preparing a carbon-fixing carbide slag cement-based material based on a three-stage curing method

Abstract

The present invention relates to a method for preparing a carbonated carbide slag cement-based material by means of a three-stage curing method. Using the pre-treated carbide slag, by controlling the mixing order of the carbide slag, cement and water, adopting a three-stage curing method of "pre-curing + carbon curing + standard curing", and optimizing factors such as the carbide slag content, water-binder ratio, curing time, etc., a carbonated carbide slag cement-based material with good carbon sequestration efficiency and excellent mechanical properties is obtained. Among them, the carbide slag, as a carbon sequestration enhancing material, not only solves the problems of low diffusion rate of CO2 in cement particles and low content of carbon-curable substances in the matrix, improves the carbon sequestration efficiency of the CO2-cured cement-based material, but also the CaCO3 generated by the reaction greatly improves the mechanical properties of the product. The compressive strength of the obtained material can reach 45.1 MPa, the flexural strength can reach 4.5 MPa, the degree of carbonation can reach 84.57%, and the carbon sequestration rate is 62.06%.

Description

Technical Field

[0001] The present invention relates to the technical field of building material preparation, and particularly to a method for preparing a carbon-fixing carbide slag cement-based material based on a three-stage curing method. Background Art

[0002] With the gradual increase in the emission of greenhouse gas CO2, carbon emission reduction strategies have been put on the agenda. Most research scholars use building materials for carbon curing to achieve the purpose of carbon emission reduction. The principle is that CO2 reacts with pore water to form carbonic acid, and then carbonic acid reacts with Portland cement clinker minerals and their hydration products to produce carbon mineralization reaction, generating stable CaCO3, making the material more dense, improving product performance and reducing the carbon footprint of the product. However, the direct CO2 curing technology for cement-based materials still has the problem of low diffusion rate of CO2 inside cement particles, resulting in relatively low overall carbon fixation efficiency of this technology.

[0003] Carbide slag is a kind of industrial waste residue, which is a by-product of hydrolyzing calcium carbide (CaC2) to produce acetylene. Its main component is Ca(OH)2 (content > 80%), and it has great potential for mineralizing and fixing CO2. The high-content Ca in carbide slag can be used 2+ to cooperate with the Ca in cement 2+ for collaborative carbon fixation. Therefore, the harmless treatment and high-efficient resource utilization of carbide slag are the only way to solve the environmental pollution problem in the carbide industry.

[0004] However, incorporating carbide slag has a great negative impact on the mechanical properties of cement-based materials. Incorporating carbide slag into cement paste will reduce the compressive strength of cement-based materials at each age. Therefore, for carbide slag cement-based materials, how to improve the carbon fixation efficiency while reducing the strength loss caused by incorporating carbide slag and further improve the mechanical properties of cement-based materials still requires further research. Summary of the Invention

[0005] Aiming at the deficiencies of the prior art, the technical problem to be solved by the present invention is how to improve the carbon fixation efficiency and mechanical properties of carbon-fixing carbide slag cement-based materials. In this regard, the present invention provides a method for preparing a carbon-fixing carbide slag cement-based material based on a three-stage curing method.

[0006] To solve the above technical problem, the present invention provides the following technical solutions:

[0007] The present invention provides a method for preparing a carbon-fixing carbide slag cement-based material based on a three-stage curing method, including the following steps:

[0008] (1) Pretreatment of carbide slag;

[0009] (2) Preparation of carbide slag cement-based specimens:

[0010] (2-1)Weigh the raw materials according to the ratio: carbide slag, cement and water, and set aside.

[0011] (2-2)First, mix the carbide slag with half of the mass of water and conduct the first stirring; then, while stirring, add the cement and the other half of the mass of water and conduct the second stirring; after the raw materials are mixed, conduct the third stirring to obtain the carbide slag cement paste mixture.

[0012] (2-3)Inject the carbide slag cement paste mixture into the mold, and after demolding, obtain the carbide slag cement-based specimen.

[0013] (3)Prepare the carbonated carbide slag cement-based material based on three-stage curing:

[0014] (3-1)Put the carbide slag cement-based specimen into a constant temperature and humidity curing box for pre-curing.

[0015] (3-2)After pre-curing, put the carbide slag cement-based specimen into a carbonation box for carbon curing.

[0016] (3-3)After carbon curing, put the carbide slag cement-based specimen into a standard curing room for standard curing to obtain the carbonated carbide slag cement-based material.

[0017] When the carbide slag is first mixed with water to form a calcium hydroxide suspension, and then when the cement and the other half of the mass of water are added, calcium hydroxide crystals can grow layer by layer on the surfaces of dicalcium silicate hydrate and tricalcium silicate hydrate particles, enabling a saturated calcium hydroxide solution to be stored between the layers. During carbon curing, relatively uniform internal and external carbonation degrees can be achieved; and as the calcium hydroxide crystals continue to grow, layered microcracks parallel to the cleavage plane can be generated in the calcium oxide crystals. These layered microcracks are beneficial for the uniform discharge of pore water from the inside to the outside of the carbide slag cement-based specimen during the pre-curing stage, maintaining relatively consistent moisture contents inside and outside the specimen, and can guide carbon dioxide in the environment to smoothly enter the interior of the specimen for carbonation during the carbon curing stage.

[0018] The particle fineness of the pretreated carbide slag is relatively high. When directly mixed with cement, it is prone to agglomeration due to electrostatic adsorption. The step-by-step stirring process avoids the rapid setting caused by the simultaneous contact of carbide slag and cement with excessive moisture. At the same time, adding water for stirring in batches can reduce the initial viscosity of the paste, gradually disperse the particles through the interval between the two water additions, reduce local water accumulation, ensure uniform humidity distribution inside the material during the pre-curing stage, and inhibit the generation of shrinkage cracks.

[0019] When mixing carbide slag, cement and water, first mixing the carbide slag with water and then adding the cement also has the following advantages: ① Optimize the release of the chemical activity of carbide slag: The main component of carbide slag is Ca(OH)2, its particles are fine and it has strong water retention. Mixing with water first can fully dissolve Ca(OH)2 and release OH -Ions form an alkaline environment, promoting the subsequent hydration reaction of silicate minerals in cement (such as the hydration of C3S), and at the same time providing more Ca sources for the absorption of CO2 in carbonation curing, making full preparations for the subsequent carbon solidification reaction. 2+ ② Avoid the agglomeration of carbide slag and improve the dispersion uniformity: Carbide slag is prone to forming aggregates due to its strong water retention. Mixing it with half of the water alone first can break the electrostatic adsorption between particles through shear force to ensure uniform dispersion. This pretreatment can prevent microstructural defects (such as coarsening of pores) caused by too high local carbide slag concentration when mixed with cement later, and such microdefects will greatly reduce the strength of the specimen. ③ Promote the stage control of the carbonation reaction: Ca(OH)2 in carbide slag reacts with CO2 preferentially in carbonation curing to form CaCO3 (carbon fixation reaction), and the carbonation reaction of cement hydration products (such as C-S-H gel) follows. Stirring step by step can ensure the full progress of the carbonation reaction of carbide slag, avoid competing with the cement hydration reaction, and thus improve the overall carbon fixation rate.

[0020] The present invention uses the method of mineralizing carbide slag cement-based materials with CO2 for the absorption and fixation of CO2. It can improve the mechanical properties and durability of concrete to a certain extent, effectively capture CO2 without affecting the performance of cement-based materials, and convert CO2 into stable carbonate products to achieve the permanent sequestration of CO2.

[0021] The present invention uses carbide slag as a reinforcing carbon fixation material to improve the carbon fixation efficiency of CO2-cured cement-based materials, which can solve the problems of low diffusion rate of CO2 in cement particles and low content of carbon-curable substances in the matrix. The CaCO3 generated by the carbon solidification reaction greatly improves the mechanical properties of the product (wherein, the principle of the carbon solidification reaction of carbide slag is as shown). Figure 1 shown).

[0022] Preferably, in the step (1), the carbide slag is crushed and passed through a 200-mesh sieve;

[0023] Among them, the smaller the particle size of the carbide slag, the larger its specific surface area, that is, it is more likely to contact with CO2, so the reaction rate is accelerated, which is more conducive to the progress of the carbon solidification reaction.

[0024] Preferably, in the step (2), the carbide slag cement-based specimen is a carbide slag cement paste specimen, a carbide slag cement mortar specimen or a carbide slag concrete specimen;

[0025] Preferably, in the step (2-1), by weight, the raw material composition includes: 100-500 parts of carbide slag, 200-600 parts of cement and 205 parts of water;

[0026] Preferably, the ratio of the mass of the water to the total mass of the cement and the carbide slag is 0.3-0.5; the mass of the carbide slag accounts for 15%-60% of the total mass of the cement and the carbide slag;

[0027] More preferably, in the step (2-1), the ratio of the mass of the water to the total mass of the cement and the carbide slag is 0.5; the mass of the carbide slag accounts for 15% of the total mass of the cement and the carbide slag;

[0028] Among them, the present invention regulates the ratio of the mass of water to the total mass of cement and carbide slag (i.e., water-binder ratio). The increase of water-binder ratio usually leads to an increase in the carbon fixation rate of the product and a decrease in strength. However, considering that pre-curing is required to adjust the pore water content, the water-binder ratio of the specimen after pre-curing is too low, resulting in too little water content in the pores, and the CO3 generated during the carbon curing process 2- Less, less CaCO3 content, the strength decreases accordingly, so it is not appropriate to select too low a water-binder ratio for regulation. At the same time, considering that the added calcium carbide slag has a large specific surface area, that is, the specific surface area of ​​calcium carbide slag is S / V=0.937m 2 / g, the specific surface area of ​​cement is S / V=0.862 m 2 / g, which makes the carbide slag more water-absorbent than cement, and will consume part of the water, taking away the water for cement hydration. In order to ensure the full dissolution of the carbide slag and ensure good strength of the product while maintaining appropriate fluidity and workability, a water-cement ratio of 0.5 is preferred.

[0029] The present invention regulates the percentage of the mass of carbide slag in the total mass of cement and carbide slag (i.e., the amount of carbide slag added). The increase in the amount of carbide slag added usually leads to an increase in the carbon fixation rate of the product and a decrease in strength. This is because the increase in the amount of carbide slag added reduces the amount of cement used, resulting in a decrease in cement hydration products, an increase in porosity, and a significant decrease in strength. Therefore, it is not appropriate to select an excessively high amount of carbide slag for regulation. At the same time, considering that the addition of carbide slag increases the content of Ca(OH)2, it will absorb CO2, generate CaCO3, fill the pores of the product, and improve the CO2 curing degree, so that the net slurry strength of the carbide slag after carbon curing is improved. In order to improve the carbon fixation efficiency while reducing the strength loss caused by the addition of carbide slag, a carbide slag addition of 15% is preferred.

[0030] Preferably, in step (2-1), the cement is 42.5 ordinary Portland cement;

[0031] Preferably, in the step (2-2), for the JJ-5 type cement mortar mixer (designed by the Cement Research Institute of China Building Materials Academy, which is the unified equipment specified in the national standard "Test Method for Strength of Cement Mortar (ISO Method) GB / T 17671-1999"), the revolution speed of the mixing blades during the first mixing is 62±5 r / min, the rotation speed of the mixing blades is 140±5 r / min, and the time is 3-5 min; the revolution speed of the mixing blades during the second mixing is 62±5 r / min, the rotation speed of the mixing blades is 140±5 r / min, and the time is 3-5 min; the revolution speed of the mixing blades during the third mixing is 125±10 r / min, the rotation speed of the mixing blades is 285±10 r / min, and the time is 3-5 min;

[0032] The purpose of slow mixing first and then fast mixing is to ensure the uniformity of mixing and improve the performance of the cement slurry. The specific purposes can be divided into the following points: ① Uniform mixing: Slow mixing can effectively preliminarily mix raw materials such as carbide slag, cement, and water, making them fully contact and avoiding segregation, ensuring the uniform distribution of each component. ② Reducing bubble generation: Slow mixing can reduce the generation of bubbles during mixing, avoiding the influence of bubbles on the density and strength of the mixture. ③ Sufficient wetting: During the slow mixing stage, the materials can be gradually wetted, ensuring that the moisture can fully penetrate into the carbide slag and cement materials, promoting the hydration reaction. ④ Improving mixing efficiency: After the preliminary mixing is completed, switching to fast mixing can quickly improve the overall uniformity of the mixture, ensuring the complete fusion of all components and improving the mixing efficiency. ⑤ Improving the performance of the slurry: Through such a mixing method, the fluidity and workability of the cement slurry can be enhanced, improving the convenience and quality of subsequent construction.

[0033] Preferably, in the step (2-3), after injecting into the mold, cover the mold with plastic wrap and demold after 24-36 h;

[0034] Preferably, in the step (3-1), during pre-curing, the air circulation on the surface of the carbide slag cement-based specimen needs to be maintained; the pre-curing temperature is (20±3) °C, the relative humidity is (60±10) %RH, and the curing time is 0.5-4 d;

[0035] More preferably, use a fan to blow the carbide slag cement-based specimen, the rotation speed of the fan is 1000±50 r / min, the air volume is 30 m 3 / h, the distance between the fan and the carbide slag cement-based specimen is 1-2 m, and the placement direction of the carbide slag cement-based specimen is the same as the wind direction of the fan;

[0036] More preferably, after pre-curing, the water loss rate of the carbide slag cement-based specimen is 7-50%;

[0037] Among them, pre-curing is carried out before carbon curing to adjust the pore water content in the specimen to an appropriate state, promoting the subsequent carbon curing. This is beneficial for CO2 to enter the material interior and there is more H2O to react with CO2 to form carbonic acid. This is because too high water content will hinder the subsequent entry of CO2 into the cement-based specimen interior, affecting the transmission of CO2 gas in the capillary pores, resulting in a lower carbonic acid concentration and making it difficult for carbon solidification to occur. Too low water content will cause insufficient water, making it difficult to form carbonic acid and weakening the reaction degree with the calcium-containing substances in the specimen. Therefore, the pore water content is related to the generation amounts of HCO3 - and CO3 2- , and further affects the subsequent generation amount of CaCO3, that is, pre-curing is related to the carbon sequestration efficiency of CO2 and the strength of the carbon sequestered carbide slag cement-based material. In summary, the water content of the specimen after pre-curing is a key factor in the CO2 curing process. The pre-curing process is essentially a water loss process. Therefore, different pre-curing times are designed to detect the change in the moisture content of the specimen during the pre-curing process.

[0038] The water loss rate β of the specimen is: β = ( m f - m 0) / m h , where, m f is the mass of the specimen before pre-curing; m 0 is the mass after pre-curing (before CO2 curing); m h is the theoretical moisture content of the specimen, m h = m f ×α%, where α% is the percentage of water in the total mass of the carbide slag cement paste in the raw material mix ratio.

[0039] Among them, during the pre-curing process, factors such as the rotation speed, air volume, distance from the specimen, and orientation of the machine fan need to be controlled to ensure the best pre-curing effect and the overall performance of the specimen. The specific control requirements are as follows: ① Fan rotation speed: The rotation speed of the fan directly affects the air flow speed, and then affects the water evaporation rate. Too high a rotation speed causes the specimen water to evaporate too quickly, resulting in too fast drying of the material surface and insufficient internal water, increasing the risk of specimen cracking. Therefore, the machine fan should be maintained at a medium rotation speed (1000 ± 50 r / min) for pre-curing. ② Air volume: The air volume determines the amount of air passing through the specimen per unit time. Too large an air volume leads to rapid surface drying, while the internal water cannot be replenished in time, resulting in uneven water loss of the specimen. Therefore, appropriately adjust the air volume (30 m 3 / h) to ensure that the test piece is evenly exposed to the wind and to avoid excessive local water loss. ③ Distance from the test piece: The distance between the fan and the test piece affects the uniformity of the wind speed and air volume. If the distance is too close, the local wind speed will be too high; if the distance is too far, the wind speed will be insufficient, affecting the pre-curing effect. Therefore, a distance of 1 - 2 meters is maintained for pre-curing. ④ Orientation: The orientation of the fan affects the air flow direction, which in turn affects the uniformity of water loss of the test piece. If the wind direction of the fan is unreasonable, some areas will experience excessive water loss while other areas will have uneven water loss, resulting in a poor pre-curing effect. Therefore, the test piece is adjusted to be placed along the wind direction of the fan to ensure that the air can evenly flow through all surfaces of the test piece, avoiding local water loss and poor pre-curing effect.

[0040] Preferably, in the step (3 - 2), the temperature of carbon curing is (20 ± 3) °C, the relative humidity is (70 ± 5) %RH, the curing time is 0.5 - 14 d, and the volume concentration of CO2 in the curing environment is ≥ 20%;

[0041] Preferably, the CO2 gas used for carbon curing is: liquefied and purified carbon dioxide gas with a purity of 99.99%; or a mixed gas of liquefied and purified carbon dioxide gas with a purity of 99.99% and industrially configured air, where the volume concentration of carbon dioxide in the mixed gas is ≥ 20%; or flue gas recovered from a cement plant and / or a steel plant, where the volume concentration of carbon dioxide in the flue gas is ≥ 20%.

[0042] Preferably, in the step (3 - 3), the temperature of standard curing is (20 ± 3) °C, the relative humidity is ≥ 95 %RH, and the curing time is 0.5 - 14 d.

[0043] Among them, standard curing is carried out after carbon curing. The product after carbon curing is neutral due to the formation of a large amount of CaCO3, which is not conducive to protecting the passivation film of the steel bar. The subsequent standard curing can restore the alkalinity, making the concrete cover layer alkaline, which is not only beneficial to the later placement of the steel bar but also can further improve the product strength.

[0044] More preferably, in the step (3 - 1), the pre-curing time is 4 d; after pre-curing, the water loss rate of the carbide slag cement-based test piece is 20 - 50%; in the step (3 - 2), the carbon curing time is 14 d; in the step (3 - 3), the standard curing time is 14 d.

[0045] The technical solution of the present invention has achieved the following beneficial technical effects:

[0046] The present invention provides a method for preparing a carbon-fixing carbide slag cement-based material by means of a three-stage curing method. After the carbide slag is pretreated, it is incorporated into the cement specimens. By regulating the mixing order of the carbide slag, cement and water and the sub-division stirring speed, a three-stage curing method of "pre-curing + carbon curing + standard curing" is adopted, and factors such as the carbide slag content, water-binder ratio, and curing time are optimized, so as to obtain a carbon-fixing carbide slag cement-based material with good carbon-fixing efficiency and excellent mechanical properties. Among them, the compressive strength of the carbon-fixing carbide slag cement-based material obtained by the present invention can reach 45.1 MPa, the flexural strength can reach 4.5 MPa, the carbon curing degree can reach 84.57%, and the carbon-fixing rate can reach 62.06%.

[0047] The present invention adopts the mixing order of first adding carbide slag and then adding cement. By optimizing the material dispersibility and reaction order, the carbide slag is fully dissolved and dispersed, which is beneficial to the preferential progress of the subsequent carbide slag carbonization reaction, avoids the competition with the carbonization reaction of cement hydration products, and the full dissolution and dispersion of the carbide slag make the carbonization reaction more uniform, significantly improving the comprehensive performance of carbon curing and avoiding problems such as micro-defects and performance deterioration caused by uneven dispersion.

[0048] The present invention adopts a three-stage curing method of "pre-curing + carbon curing + standard curing", and selects five factors, namely water-binder ratio, pre-curing time, carbide slag content, carbon curing time, and standard curing time, for optimization, aiming to obtain a carbon-fixing carbide slag cement-based material with good carbon-fixing efficiency and excellent mechanical properties. Among them, the water-binder ratio and pre-curing time will affect the pore water content. If the water-binder ratio is too low and the pre-curing time is too long, the water loss rate of the specimen will be too large and the water content in the pores will be too small (see the experimental data corresponding to Table 3), and the generated HCO3 - 、CO3 2- content in the carbon curing process is less, ultimately resulting in a reduction in the amount of CaCO3 generated, affecting the strength and carbon-fixing efficiency of the carbon-fixing carbide slag cement-based material. The carbide slag content will affect the content of Ca(OH)2, further affecting the amount of CaCO3 generated by reacting with CO2, and ultimately affecting the strength and carbon-fixing efficiency of the material. The carbon curing time will affect the content of CO2. If the CO2 concentration is too low, it will be unfavorable for the formation of CaCO3, ultimately resulting in a reduction in the strength and carbon-fixing efficiency of the material. The standard curing time will affect the generation amount of hydrated cementitious materials (Ca(OH)2, C-S-H). If the standard curing time is too short, it will be unfavorable for the further improvement of the subsequent strength. Among them, the preparation process and principle of the carbon-fixing carbide slag cement-based material of the present invention are as Figure 2 shown.

[0049] In the present invention, carbide slag is incorporated into cement specimens. The incorporated carbide slag can promote the diffusion of CO2 in cement stone, facilitate the absorption and fixation of CO2, and improve the carbon curing efficiency of carbon-cured cement. At the same time, carbon curing can compensate for the strength loss caused by the incorporation of carbide slag, and the generated CaCO3 can increase the density of the material, comprehensively improving the mechanical properties of cement-based building materials. In addition, the incorporation of carbide slag also reduces the cement consumption, which can not only achieve carbon emission reduction of greenhouse gases, but also effectively utilize industrial solid waste, realizing the green concept of solid waste carbon fixation, and having the characteristics of alleviating the greenhouse effect and improving economic benefits.

[0050] The present invention adopts the concept of synergism between the preparation of low-carbon cement-based materials and the resource utilization of carbide slag to solve the problems of low carbon fixation efficiency of CO2 carbon-cured cement-based materials technology and the difficult problem of resource utilization of carbide slag. Using carbide slag for carbon fixation not only solves the problem of carbon emission reduction, but also effectively utilizes industrial solid waste. The method of the present invention provides a theoretical basis and technical guidance for the performance regulation of carbon-fixing solid waste concrete, reduces unnecessary trial-and-error costs, can realize the resource utilization of carbide slag, and improves the performance of cement-based materials and captures and fixes CO2. At the same time, the method of the present invention is applicable to carbon fixation by the CO2 mineralization method and can be used for the production of batch precast components. The industrial process and application of CO2 mineralization production of precast components are as Figure 3 shown. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 Schematic diagram of the carbide slag carbon curing reaction of the present invention;

[0052] Figure 2 Preparation process and schematic diagram of the carbon-fixing carbide slag cement-based material of the present invention;

[0053] Figure 3 Industrial process and application diagram of CO2 mineralization production of precast components;

[0054] Figure 4 Results diagram of the carbon curing degree corresponding to different experimental groups in Example 1 of the present invention;

[0055] Figure 5 Relationship diagram of compressive and flexural strengths and various factors at different levels in Example 1 of the present invention;

[0056] Figure 6 Relationship diagram of carbon curing degree, carbon fixation rate and various factors at different levels in Example 1 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0057] Example 1

[0058] This example provides a method for preparing a carbon-fixing carbide slag cement-based material by using a staged curing method, including the following steps:

[0059] (1) Pretreatment of carbide slag: Crush the carbide slag, sieve it through a 200-mesh sieve, and reserve it for use.

[0060] (2) Preparation of carbide slag cement-based specimens:

[0061] (2-1) Weigh the raw materials according to the ratio: By weight, the raw material composition includes: 100-500 parts of carbide slag, 200-600 parts of ordinary Portland cement with a strength grade of 42.5, and 205 parts of water (see Table 1-3 for details), and reserve them for use; among them, the ratio of the mass of water to the total mass of cement and carbide slag (i.e., water-binder ratio) is shown in Table 1-3; the dosage of carbide slag: the ratio of the mass of carbide slag to the total mass of cement and carbide slag is shown in Table 1-3.

[0062] (2-2) First, mix the carbide slag with half of the mass of water and conduct the first stirring. The revolution speed of the stirring blades for the first stirring is 62 r / min, the rotation speed of the stirring blades is 140 r / min, and the time is 3 min.

[0063] Subsequently, add cement and the other half of the mass of water while stirring to conduct the second stirring. The revolution speed of the stirring blades for the second stirring is 62 r / min, the rotation speed of the stirring blades is 140 r / min, and the time is 3 min.

[0064] After the raw materials are mixed, conduct the third stirring. The revolution speed of the stirring blades for the third stirring is 125 r / min, the rotation speed of the stirring blades is 285 r / min, and the time is 3 min; obtain the carbide slag cement paste mixture.

[0065] (2-3) Inject the carbide slag cement paste mixture into a 40×40×160 mm mold, cover the mold with plastic wrap to prevent water evaporation, and demold after 24 h to obtain the carbide slag cement-based specimens.

[0066] (3) Prepare the carbonated carbide slag cement-based material based on three-stage curing:

[0067] (3-1) Place the carbide slag cement-based specimens in a thermo-hygrostat for pre-curing.

[0068] During pre-curing, it is necessary to keep the air circulation on the surface of the carbide slag cement-based specimens, and use the fan of the machine blower to blow the specimens; the temperature for pre-curing is (20±3) °C, the relative humidity is (60±10) %RH, and the curing time is 0.5-4 d (see Table 1-3 for details).

[0069] Among them, the rotation speed of the fan is 1000 r / min, the air volume is 30 m 3 / h, the distance between the fan and the carbide slag cement-based specimens is 1.5 m, and the placement direction of the carbide slag cement-based specimens is the same as the wind direction of the fan.

[0070] After pre-curing, the water loss rate of the carbide slag cement-based specimens was 7.59% - 48.24%;

[0071] (3 - 2) After pre-curing, the carbide slag cement-based specimens were placed in a carbonation chamber for carbon curing;

[0072] The temperature for carbon curing was (20 ± 3) °C, the relative humidity was (70 ± 5) %RH, the curing time was 0.5 - 14 d (see Table 1 - 3 for details), and the volume concentration of CO2 in the carbonation chamber was 20% (the used CO2 was a 20% volume concentration obtained by mixing liquefied and purified CO2 gas with a purity of 99.99% and industrial air);

[0073] (3 - 3) After carbon curing, the carbide slag cement-based specimens were placed in a standard curing room for standard curing to obtain the carbonated carbide slag cement-based material;

[0074] The temperature for standard curing was (20 ± 3) °C, the relative humidity was ≥95%RH, and the curing time was 0.5 - 14 d (see Table 1 - 3 for details).

[0075] Table 1 Factor level table

[0076]

[0077] Table 2 Mix proportion of carbide slag cement-based materials with different water-binder ratios and carbide slag dosages

[0078]

[0079] In the table: "kg / m 3 " means: the mass of a certain raw material contained in each cubic meter of the specimen; for example, "102.5 kg / m 3 carbide slag" means: the mass of carbide slag contained in each cubic meter of the specimen.

[0080] Performance determination: According to the standards of "Test Method for Strength of Cement Mortar" GB / T 17671 - 2021 and "Standard Test Method for Long-Term Performance and Durability of Ordinary Concrete" GB / T 50082 - 2009, compressive and flexural strength tests, carbon curing tests, phenolphthalein tests (carbon curing degree), and muffle furnace tests (carbon fixation rate) were carried out on the obtained carbonated carbide slag cement-based materials (experimental numbers corresponding to the conditions in Table 3), and the grouping and experimental results are shown in Table 3.

[0081] ①Compressive and flexural strength tests: The compressive and flexural strength tests were carried out in accordance with the "Test Method for the Strength of Cement Mortar" (GB / T 17671-2021). An automatic cement strength testing machine was used for the compressive and flexural strength tests, with a maximum load of 300 kN. The loading process was controlled by load, and the loading rates for compressive and flexural strength were 2.4 kN / s and 50 N / s respectively.

[0082] ②Carbon curing test: The carbon curing test (i.e., step (3-2), the carbon curing step in the three-stage curing) was carried out in accordance with the rapid carbonation test method in the "Standard Test Method for Long-Term Performance and Durability of Ordinary Concrete" (GB / T 50082-2009). A carbonation chamber was used for the carbon curing test. The CO2 concentration in the carbonation chamber was maintained at 20%, the relative humidity inside the chamber was controlled within (70±5)%, and the temperature was controlled within the range of (20±3)°C.

[0083] ③Degree of carbon curing: After carbon curing in step (3-2) and standard curing in step (3-3), the carbonated carbide slag paste specimens were taken out and broken respectively. A 1% phenolphthalein alcohol solution (the alcohol solution contains 20% distilled water) was sprayed to obtain phenolphthalein pictures (such as Figure 4 ). The phenolphthalein pictures were imported into Image J software for threshold division, and the proportion of the area of the carbon-cured (colorless) region was extracted, which is the degree of carbon curing.

[0084] ④Carbon fixation rate: Before calcining the cement-based material with a muffle furnace testing machine, sheet samples (30±5 g) need to be cut from the specimens. The samples need to be placed in an anhydrous ethanol solution for pretreatment to prevent their hydration reaction, and then placed in an oven at 65°C until constant weight and then put into the muffle furnace testing machine and heated to 520°C and 950°C for calcination. The temperature range of 520°C - 950°C is mainly for the decomposition of CaCO3. The carbon fixation rate calculation formula is as follows:

[0085] α=(m 520°C -m 950°C ) / (m c ×M max )×100%

[0086] In the formula, α is the carbon fixation rate based on the mass loss between 520°C and 950°C, m 520°C is the mass (g) of the sample at 520°C, m 950°C is the mass (g) at 950°C, m c is the mass of cement in the material (g), and M max is the maximum theoretical CO2 absorption amount of cement, and its calculation formula is as follows:

[0087] M max =0.785(CaO - 0.7SO3)+1.09MgO+1.42Na2O+0.93K2O

[0088] In the formula, the chemical formula represents the percentage of the substance in the cement by mass.

[0089] Table 3 Test results of compressive strength, flexural strength, carbon curing degree, and carbon sequestration rate

[0090] Number A Water-cement ratio B Dosage of carbide slag / % C Pre-curing time / d D Carbon-curing time / d E Standard-curing time / d Flexural strength / MPa Compressive strength / MPa Degree of carbon curing / % Carbon sequestration rate / % Water loss rate after pre-curing / % 1 0.3 0 0.5 0.5 0.5 2.7 19.5 10.8 22.13 7.59 2 0.3 15 1 1 1 2.6 24.5 19.9 27.10 10.56 3 0.3 30 2 3 3 2.0 27.8 18 33.14 22.64 4 0.3 45 3 7 7 2.1 20.3 19 64.27 26.92 5 0.3 60 4 14 14 4.6 37.6 85.8 68.19 35.06 6 0.35 0 1 3 7 2.3 37.27 7 26.07 6.90 7 0.35 15 2 7 14 3 40.8 2.3 33.0 11.02 8 0.35 30 3 14 0.5 2.35 28.97 24.2 34.56 18.06 9 0.35 45 4 0.5 1 1.38 10.57 0.7 34.51 25.46 10 0.35 60 0.5 1 3 1.7 5.97 2 30.86 9.83 11 0.4 0 2 14 1 5.25 38.3 57.1 53.78 11.95 12 0.4 15 3 0.5 3 2.7 16.7 26.3 31.76 16.80 13 0.4 30 4 1 7 1.2 20.7 23.3 36.56 25.33 14 0.4 45 0.5 3 14 4.15 15.57 0 34.49 9.61 15 0.4 60 1 7 0.5 4.7 5.93 40.6 43.43 21.95 16 0.45 0 3 1 14 3.2 26.5 13.3 16.07 18.41 17 0.45 15 4 3 0.5 0.91 25.03 28.9 34.37 24.70 18 0.45 30 0.5 7 1 7.8 20.4 6.3 33.64 8.47 19 0.45 45 1 14 3 4.6 7.8 0.9 34.05 13.35 20 0.45 60 2 0.5 7 3.3 5.7 1.1 30.46 17.05 21 0.5 0 4 7 3 1.84 23.17 67 57.14 24.53 22 0.5 15 0.5 14 7 9.0 38.9 54.6 49.49 8.57 23 0.5 30 1 0.5 14 4.55 14.13 0 31.79 13.08 24 0.5 45 2 1 0.5 2.65 11.83 6.8 38.43 16.62 25 0.5 60 3 3 1 3.7 10.73 31.2 22.13 22.24

[0091] The control test adopted non-three-stage curing, that is, pre-curing and carbon curing were not involved, only standard curing for 28 d was carried out, and other steps and parameters were the same as those in the above method. Multifactor experiments were also carried out on the water-binder ratio and the dosage of carbide slag. The control group data after standard curing for 28 d under different water-binder ratios and dosages of carbide slag are shown in Table 4.

[0092] Table 4 Test results of compressive strength and flexural strength after 28 d standard curing of the control group

[0093] Number A Water-cement ratio B Dosage of carbide slag / % Flexural strength / MPa Compressive strength / MPa 1 0.3 0 9 34.725 2 0.3 15 6.2 41.2 3 0.3 30 6.7 29.25 4 0.3 45 5.35 24.175 5 0.3 60 3.23 12.47 6 0.35 0 6.6 41.7 7 0.35 15 5.35 33.5 8 0.35 30 6.45 25.6 9 0.35 45 5.45 29.35 10 0.35 60 3.73 12.75 11 0.4 0 4.35 38.425 12 0.4 15 6.1 25.8 13 0.4 30 4.55 24.1 14 0.4 45 4.7 19.2 15 0.4 60 2.6 11.7 16 0.45 0 3.75 29.53 17 0.45 15 5.45 36.4 18 0.45 30 4.4 30.5 19 0.45 45 4.6 20.5 20 0.45 60 3.7 12.43 21 0.5 0 4.8 33.92 22 0.5 15 4.0 28.64 23 0.5 30 5.03 28.28 24 0.5 45 4.3 18.72 25 0.5 60 2.47 9.84

[0094] Among them, phenolphthalein tests were carried out on the specimens before and after standard curing (that is, after carbon curing in step (3-2) and after standard curing in step (3-3)), and the carbon curing degree was analyzed. The results are shown in Figure 4 (The first row in the figure corresponds to experiment numbers 1-5 in Table 3, the second row corresponds to experiment numbers 6-10 in Table 3, the third row corresponds to experiment numbers 11-15 in Table 3, the fourth row corresponds to experiment numbers 16-20 in Table 3, and the fifth row corresponds to experiment numbers 21-25 in Table 3; "PC" in the figure represents the experimental group with a carbide slag dosage of 0, "15" represents the experimental group with a carbide slag dosage of 15%, "C" represents after carbon curing, and "B" represents after standard curing).

[0095] From Figure 4 it can be seen that after standard curing, the unhydrated cement particles further hydrate to produce Ca(OH)2 and C-S-H gel substances, resulting in further strength improvement. At the same time, the Ca(OH)2 and C-S-H generated during subsequent standard curing are alkaline, while the CaCO3 generated during carbon curing is neutral. Therefore, the alkalinity of the specimens after subsequent standard curing is restored, and the alkaline area (pink) of the specimens increases, while the neutral area (colorless) decreases. Taking 60-C as an example, after carbon curing, all the samples are carbon-cured, fixing the CO2 gas. After subsequent standard curing, the surrounding of the alkaline area (pink) begins to recover, providing favorable conditions for placing steel bars. It shows that subsequent standard curing not only fixes the CO2 gas but also restores the alkalinity. The concrete protective layer thickness is alkaline, which is beneficial to the later placement of steel bars and can further improve the strength of the product.

[0096] Analysis of extreme variance:

[0097] Perform extreme variance analysis on the data obtained in Table 3 to determine the influence of factors on the compressive strength, flexural strength, carbon curing degree, and carbon sequestration rate. Among them, for the compressive strength, flexural strength, carbon curing degree, and carbon sequestration rate of each factor listed in each experimental group in Table 3 at different levels, conduct orthogonal test analysis. After weighted averaging of different experimental groups under the same factor and the same conditions, the relationships between the compressive strength, flexural strength and each factor at different levels are as Figure 5 shown, and the relationships between the carbon curing degree, carbon sequestration rate and each factor at different levels are as Figure 6 shown.

[0098] As Figure 5 shown, the water-binder ratio of 0.30 in the figure corresponds to the weighted average result of the experimental groups numbered 1-5 in Table 3; the dosage of 0% (i.e., the dosage of carbide slag is 0%) in the figure corresponds to the weighted average result of the experimental groups numbered 1, 6, 11, 16, and 21 in Table 3; Figure 5 , Figure 6 The data corresponding to other factor levels in are obtained in the same way.

[0099] From Figure 5It can be seen that: ① As the water-binder ratio increases, the compressive strength decreases while the flexural strength increases. This is because as the water-binder ratio increases, the microstructure inside the specimen changes. Specifically, excessive water leads to an increase in porosity, which is not conducive to the improvement of compressive strength. However, the increase in water causes an increase in the hydrated gel products generated by cement hydration, increasing the toughness of the material and helping to improve the flexural capacity of the material. ② As the content of carbide slag increases, the compressive strength decreases, and the change in flexural strength is not obvious. This is because as the content of carbide slag increases, the amount of cement used decreases, the hydration products decrease, and the porosity increases, resulting in a decrease in compressive strength. Secondly, the flaky structure of Ca(OH)₂ in carbide slag will cause slippage under pressure, leading to a decrease in compressive strength. At the same time, carbide slag has high hydrophilicity and absorbs the water used for cement hydration, reducing the strength of the carbonated carbide slag paste. However, it should be noted that the carbonated carbide slag with a content of 60% (the weighted average results of the experimental groups numbered 5, 10, 15, 20, and 25 in Table 3) has a compressive strength and a flexural strength that are 11.4% and 14.3% higher respectively than those of the standard-cured control specimens (the weighted average results of the experimental groups numbered 5, 10, 15, 20, and 25 in Table 4), indicating that carbon curing maintenance compensates for the strength loss caused by the incorporation of carbide slag. ③ As the pre-curing time increases, the overall change in compressive strength is not significant, while the flexural strength decreases, indicating that if the specimen loses too much water, it will be unfavorable for the formation of hydrated gel materials, the toughness of the material decreases, and the flexural strength decreases. ④ As the carbon curing time increases, the compressive and flexural strengths generally show an upward trend. This is because as the carbon curing time increases, the Ca(OH)₂ in the incorporated carbide slag provides nucleation sites for the reaction products, enabling them to absorb more CO₂, accelerating the precipitation of reaction products, improving the degree of CO₂ curing, and thus increasing the strength. At the same time, as the carbon curing time extends, the silica gel (SiO₂·H₂O) and CaCO₃ generated by carbon curing can fill the pores of the gel material, making it more dense and increasing the strength. In addition, a small amount of aragonite and vaterite generated in the early stage (3d) will transform into calcite after subsequent carbon curing (7d). Calcite has a higher strength, resulting in an increase in strength. ⑤ As the standard curing time increases, the compressive and flexural strengths as a whole show an upward trend. This is because as the standard curing time increases, the unhydrated cement particles further hydrate, generating Ca(OH)₂ and C-S-H gel substances, causing the strength to gradually increase.

[0100] It can be seen from Figure 6 that: ① As the water-binder ratio increases, the degree of carbon curing and the carbon fixation rate as a whole show a trend of first decreasing and then increasing. This is because as the water-binder ratio increases, in the initial stage, excessive water will hinder the entry of CO₂ into the matrix, resulting in a decrease in the carbon curing ability. However, after reaching a certain water-binder ratio, the role of water begins to dominate, and HCO₃ - and CO₃ 2-, which promoted the progress of the carbon solidification reaction, resulting in a gradual increase in the degree of carbon solidification and the carbon sequestration rate. This phenomenon reflects the dual effects of moisture in the carbon solidification process of the mixture, with both inhibitory and promoting effects. ② With the increase in the content of carbide slag, the degree of carbon solidification and the carbon sequestration rate also showed a trend of first decreasing and then increasing. This is because at the beginning, the particle size of the added carbide slag is relatively fine, which plays a role in filling pores and hindering the diffusion channel of CO2. When the added carbide slag increases to a certain extent, the possibility of contact between the Ca(OH)2 component and CO2 increases, and it will absorb CO2, increasing the carbon sequestration rate until the reactive Ca(OH)2 is completely carbon solidified; at the same time, the added Ca(OH)2 will react with SiO2 and Al2O3 in the cement phase through the pozzolanic reaction, and the generated C-S-H will react with CO2, thereby improving the degree of CO2 curing. ③ With the increase in the pre-curing time, the degree of carbon solidification and the carbon sequestration rate gradually increased. This is because the loss of moisture provided a convenient channel for CO2 gas to enter the interior of the neat paste, resulting in an increase in the degree of carbon solidification and the carbon sequestration rate. ④ With the increase in the carbon curing time, both of them gradually increased. This is because in the initial stage, CO2 gas diffused inside the matrix, dissolved in water to form H2CO3, and then reacted with the alkaline substance Ca(OH)2 in the carbide slag particles to form stable CaCO3 precipitates. The longer the carbon curing time, the continuous progress of the carbon solidification reaction, and the generated CaCO3 will provide nucleation sites, and the van der Waals forces between molecules begin to attract each other, making more CaCO3 crystals grow continuously here, accelerating the reaction progress and improving the degree of CO2 curing. ⑤ With the increase in the standard curing time, both of them showed no obvious change, indicating that the standard curing time has no effect on the carbon solidification degree and the carbon sequestration rate test, and the result of variance analysis is the same.

[0101] Furthermore, it can be seen from the range analysis that the primary and secondary order of the factors affecting the compressive strength of the obtained materials is: D > B > E > A > C, that is: carbon curing time > carbide slag content > standard curing time > water-binder ratio > pre-curing time. The results show that the carbon curing time has a greater impact on the compressive strength of the obtained materials. The optimal combination of the corresponding compressive strength of the obtained materials is: A1B2C3D5E5, that is: water-binder ratio 0.30, carbide slag content 15%, pre-curing time 2d, carbon curing time 14d, standard curing time 14d.

[0102] The primary and secondary order of the factors affecting the flexural strength of the obtained materials is: C > D > A > E > B, that is: pre-curing time > carbon curing time > water-binder ratio > standard curing time > carbide slag content. The results show that the pre-curing time has a greater impact on the flexural strength of the obtained materials. The optimal combination of the corresponding flexural strength of the obtained materials is: A5B2C1D5E5, that is: water-binder ratio 0.50, carbide slag content 15%, pre-curing time 0.5d, carbon curing time 14d, standard curing time 14d.

[0103] The primary and secondary order of each factor affecting the carbon curing degree of the obtained material is: D > C > B > A > E, that is: carbon curing time > pre-curing time > carbide slag content > water-binder ratio > standard curing time. The results show that the carbon curing time has a greater impact on the carbon curing degree of the obtained material. The optimal combination of the carbon curing degree corresponding to the obtained material is: A5B5C5D5E2, that is: water-binder ratio 0.50, carbide slag content 60%, pre-curing time 4d, carbon curing time 14d, standard curing time 1d.

[0104] The primary and secondary order of each factor affecting the carbon fixation rate of the obtained material is: D > B > E > A > C, that is: carbon curing time > carbide slag content > standard curing time > water-binder ratio > pre-curing time. The results show that the carbon curing time has a greater impact on the carbon fixation rate of the obtained material. The optimal combination of the carbon fixation rate corresponding to the obtained material is: A5B5C5D5E4, that is: water-binder ratio 0.5, carbide slag content 60%, pre-curing time 4d, carbon curing time 14d, standard curing time 7d.

[0105] Combined with the results of variance analysis, in the range analysis, when the water-binder ratio is 0.3, the compressive strength of the corresponding material is the largest; when the water-binder ratio is 0.5, the flexural strength, carbon curing degree, and carbon fixation rate of the corresponding material are the largest; in the variance analysis, the water-binder ratio has a certain impact on the test results of compressive strength, flexural strength, and carbon fixation rate, and has a more significant impact on the carbon curing degree test. Therefore, the water-binder ratio of 0.5 is selected.

[0106] In the range analysis, when the carbide slag content is 15%, the corresponding compressive strength and flexural strength are the largest; when the carbide slag content is 60%, the carbon curing degree and carbon fixation rate are the largest; in the variance analysis, the carbide slag content has a significant impact on the compressive strength test, has no impact on the flexural strength and carbon fixation rate, and has a more significant impact on the carbon curing degree. Therefore, the content of 15% is selected.

[0107] In the range analysis, when the pre-curing time is 2d, the corresponding compressive strength is the largest; when the pre-curing time is 0.5d, the corresponding flexural strength is the largest; when the pre-curing time is 4d, the corresponding carbon curing degree and carbon fixation rate are the largest; in the variance analysis, the pre-curing time has no impact on the compressive strength, has a certain impact on the flexural strength and carbon fixation rate, and has a more significant impact on the carbon curing degree. Therefore, the pre-curing time of 4d is selected.

[0108] In the range analysis, when the carbon curing time is 14d, the impact is the largest. Therefore, the carbon curing time of 14d is selected;

[0109] In the range analysis, when the standard curing time is 14d, the impact on the compressive strength and flexural strength is the largest; when the standard curing time is 1d, the corresponding carbon curing degree is the largest; when the standard curing time is 7d, the corresponding carbon fixation rate is the largest; in the variance analysis, the standard curing time has a more significant impact on the compressive strength test and has no impact on others. Therefore, the standard curing time of 14d is selected.

[0110] Finally, in the preparation of the carbon-fixing carbide slag cement-based material of the present invention, preferably: the water-binder ratio is 0.5, the carbide slag content is 15%, the pre-curing time is 4 days, the carbon curing time is 14 days, and the standard curing time is 14 days.

[0111] Example 2

[0112] Using the optimal preparation conditions obtained in Example 1 to prepare the carbon-fixing carbide slag cement-based material, this example provides a method for preparing the carbon-fixing carbide slag cement-based material by a staged curing method, which includes the following steps:

[0113] (1) Pretreatment of carbide slag: Crush the carbide slag, pass it through a 200-mesh sieve, and set aside;

[0114] (2) Preparation of carbide slag cement-based specimens:

[0115] (2-1) Weigh the raw materials according to the ratio: by weight, the raw material composition includes: 61.5 parts of carbide slag, 348.5 parts of ordinary Portland cement with a strength grade of 42.5, and 205 parts of water, and set aside;

[0116] Among them, the ratio of the mass of water to the total mass of cement and carbide slag (i.e., the water-binder ratio) is 0.5; the carbide slag content: the mass of carbide slag accounts for 15% of the total mass of cement and carbide slag;

[0117] (2-2) First, mix the carbide slag and half of the mass of water, and conduct the first stirring. The revolution speed of the stirring blades for the first stirring is 62 r / min, the rotation speed of the stirring blades is 140 r / min, and the time is 3 min; then, while stirring, add cement and the other half of the mass of water, and conduct the second stirring. The revolution speed of the stirring blades for the second stirring is 62 r / min, the rotation speed of the stirring blades is 140 r / min, and the time is 3 min; after the raw materials are mixed, conduct the third stirring. The revolution speed of the stirring blades for the third stirring is 125 r / min, the rotation speed of the stirring blades is 285 r / min, and the time is 3 min; obtain the carbide slag cement paste mixture;

[0118] (2-3) Inject the carbide slag cement paste mixture into a 40×40×160 mm mold, cover the mold with plastic wrap to prevent water evaporation, and demold after 24 h to obtain the carbide slag cement-based specimen;

[0119] (3) Prepare the carbon-fixing carbide slag cement-based material based on three-stage curing:

[0120] (3-1) Place the carbide slag cement-based specimens in a constant temperature and humidity curing box for pre-curing;

[0121] During pre-curing, the air circulation on the surface of the carbide slag cement-based specimens should be maintained, and a machine fan is used to blow the specimens; the pre-curing temperature is (20±3)°C, the relative humidity is (60±10)%RH, and the curing time is 4 days; among them, the fan speed is 1000 r / min, the air volume is 30 m 3 / h, the distance between the fan and the carbide slag cement-based specimens is 1.5 m, and the placement direction of the carbide slag cement-based specimens is the same as the wind direction of the fan;

[0122] After pre-curing, the water loss rate of the carbide slag cement-based specimens is 42.55%;

[0123] After (3-2) pre-curing, the carbide slag cement-based specimens are put into a carbonation chamber for carbon curing;

[0124] The carbon curing temperature is (20±3)°C, the relative humidity is (70±5)%RH, the curing time is 14 days, and the CO2 concentration in the carbonation chamber is 20%;

[0125] After (3-3) carbon curing, the carbide slag cement-based specimens are put into a standard curing room for standard curing to obtain the carbon-fixed carbide slag cement-based material;

[0126] The standard curing temperature is (20±3)°C, the relative humidity ≥95%RH, and the curing time is 14 days.

[0127] According to the measurement method of Example 1, the compressive strength, flexural strength, carbon curing degree, and carbon fixation rate of the obtained material are measured. The results show that the compressive strength of the obtained material is 45.1 MPa, the flexural strength is 4.5 MPa, the carbon curing degree is 84.57%, and the carbon fixation rate is 62.06%.

[0128] Comparative Example 1

[0129] The difference between this comparative example and Example 2 is that when mixing carbide slag, water and cement in step (2-2), first mix cement and half of the mass of water for the first stirring, and then add carbide slag and the other half of the mass of water while stirring for the second stirring; other steps and parameters are the same as those in Example 2.

[0130] According to the measurement method of Example 1, the compressive strength, flexural strength, carbon curing degree, and carbon fixation rate of the obtained material are measured. The results show that the compressive strength of the obtained material is 34.2 MPa, the flexural strength is 3.2 MPa, the carbon curing degree is 59.35%, and the carbon fixation rate is 47.90%.

[0131] From the comparison between Comparative Example 1 and Example 2, it can be seen that when the stirring order of cement first and then carbide slag is adopted, the carbide slag and cement cannot be fully dissolved, and segregation occurs; after demolding, it is found that there are many pores on the surface of the test piece; such a stirring method makes the working performance and compressive and flexural strength of the material poor; secondly, since the carbide slag cannot be dissolved well, the carbon solidification degree and carbon fixation rate are also lower than those in Example 2. The specific analysis is as follows.

[0132] Disadvantages of mixing cement first and carbide slag later: ① Inhibit the dispersion of carbide slag: If cement and part of the water are mixed first, the cement particles will quickly form hydration products (such as CSH gel) to wrap the unreacted clinker particles, making it difficult to disperse the carbide slag when it is added later, forming local agglomerations. This will cause microstructural defects inside the material, resulting in reduced time strength. ② Reduce the activity of carbide slag: The water retention of carbide slag requires a large amount of water to fully dissolve. If cement and part of the water are mixed first, the cement hydration will deprive the carbide slag of water for dissolution, hindering the carbide slag Ca 2+ Precipitation, Ca 2+ ③ Impact on mechanical properties of materials: Uneven dispersion of carbide slag will lead to coarsening of pore structure, which will aggravate the strength loss caused by the addition of carbide slag. At the same time, uneven dispersion will also make carbide slag unable to dissolve well, resulting in uneven carbonization, affecting the carbonization effect, and further affecting the carbon solidification degree and carbon fixation rate compared with Example 2.

[0133] Comparative Example 2

[0134] The difference between this comparative example and Example 2 is that the pre-curing treatment of step (3-1) is not involved, that is, in this comparative example, the carbide slag cement-based specimen obtained in step (2) is directly subjected to carbon curing and standard curing treatment; the other steps and parameters are the same as those in Example 2.

[0135] The compressive strength, flexural strength, carbon curing degree, and carbon fixation rate of the obtained material were measured according to the measurement method of Example 1. The results show that the compressive strength of the obtained material is 30.5 MPa, the flexural strength is 6.6 MPa, the carbon curing degree is 24.14%, and the carbon fixation rate is 33.13%.

[0136] From the comparison between Comparative Example 2 and Example 2, it can be seen that the lack of pre-curing treatment results in more pore water in the specimen, which hinders the entry of CO2 gas into the material interior, leading to a decrease in the degree of carbon solidification and the carbon sequestration rate. At the same time, the content of the entered CO2 gas decreases, resulting in a reduction in the content of CaCO3 generated by the carbon solidification reaction. The generated calcite CaCO3 is beneficial to the improvement of the plastic strength of the material, thus leading to a decrease in the compressive strength compared to Example 2. In addition, it is found that the flexural strength of the specimen without pre-curing increases because the content of the generated CaCO3 decreases, and only the hydrated cementitious material C-S-H in the material interior provides the toughness strength, resulting in an increase in the flexural strength.

[0137] Comparative Example 3

[0138] The difference between this comparative example and Example 2 lies in that the carbon curing time in step (3-2) is different, that is, the carbon curing time in step (3-2) of this comparative example is 7d; other steps and parameters are the same as those in Example 2.

[0139] According to the measurement method of Example 1, the compressive strength, flexural strength, degree of carbon solidification, and carbon sequestration rate of the obtained material were measured. The results show that the compressive strength of the obtained material is 38.3 MPa, the flexural strength is 3.7 MPa, the degree of carbon solidification is 72.20%, and the carbon sequestration rate is 56.06%.

[0140] From the comparison between Comparative Example 3 and Example 2, it can be seen that the reduction of the carbon curing time will lead to a decrease in the compressive strength, flexural strength, degree of carbon solidification, and carbon sequestration rate. This is because the reduction of the carbon curing time results in a decrease in the CO2 concentration, which is not sufficient to carry out the carbon solidification reaction with the added carbide slag Ca(OH)2. The carbon solidification reaction is not completely reacted, and the contents of the generated silica gel (SiO2·H2O) and CaCO3 also decrease. The pores of the specimen are not filled, thus leading to a decrease in the compressive strength, flexural strength, degree of carbon solidification, and carbon sequestration rate compared to Example 2. In addition, a small amount of aragonite and vaterite generated in the early stage (3d) will transform into calcite after the later carbon curing (7d). The calcite has not been completely transformed after 7d of carbon curing time, thus leading to a decrease in the compressive strength and flexural strength compared to those in Example 2 with 14d of carbon curing.

[0141] Comparative Example 4

[0142] The difference between this comparative example and Example 2 lies in that the standard curing in step (3-3) is not involved, that is, after the carbon curing in step (3-2) of this comparative example, the carbonated carbide slag cement-based material is obtained; other steps and parameters are the same as those in Example 2.

[0143] According to the measurement method of Example 1, the compressive strength, flexural strength, carbon curing degree, and carbon sequestration rate of the obtained material were measured. The results show that the compressive strength of the obtained material is 40.5 MPa, the flexural strength is 3.9 MPa, the carbon curing degree is 86.42%, and the carbon sequestration rate is 62.78%.

[0144] It can be seen from the comparison between Comparative Example 4 and Example 2 that the lack of subsequent standard curing will hinder the further improvement of strength. This is because the unhydrated cement particles will not be able to hydrate further, hindering the formation of Ca(OH)2 and C-S-H gel substances, resulting in a decrease in strength compared to that of Example 2 with 14 days of standard curing.

[0145] It should be noted that there is a slight decrease in the carbon curing degree and carbon sequestration rate after standard curing. This is because the carbon curing product CaCO3 does not exist as an inert material during the subsequent hydration process, but can react with the aluminum in the cement matrix to form monocarboaluminate hydrate, promoting the hydration reaction. The formed more stable and less soluble carboaluminate hydrate can delay the transformation of ettringite to monosulfate type, playing a role in stabilizing ettringite. The test results are similar to those of Figure 4 the orthogonal test results, and the carbon curing degree decreases to varying degrees after standard curing.

[0146] Comparative Example 5

[0147] The difference between this comparative example and Example 2 is that it does not involve the pre-curing in step (3-1) and the carbon curing in step (3-2). That is, the carbide slag cement-based specimens obtained in step (2) of this comparative example are directly subjected to standard curing for 28 days; other steps and parameters are the same as those in Example 2 (i.e., the experimental group numbered 22 in Table 4).

[0148] According to the measurement method of Example 1, the compressive strength, flexural strength, carbon curing degree, and carbon sequestration rate of the obtained material were measured. The results show that the compressive strength of the obtained material is 28.64 MPa, the flexural strength is 4.0 MPa, the carbon curing degree is 0%, and the carbon sequestration rate is 0%.

[0149] From the comparison between Comparative Example 5 and Example 2, it can be seen that the compressive strength and flexural strength of the specimens without pre-curing and carbon curing both decrease, and the degree of carbon solidification and the carbon sequestration rate are 0%. This indicates that adding 15% of carbide slag will cause a decrease in strength. Without the three-stage curing, there will be no increase in strength and no economic and environmental benefits. The product benefits of directly conducting the 28-day standard curing will be greatly discounted. On the contrary, if the three-stage curing is carried out, it can compensate for the strength loss caused by the addition of carbide slag, and the generated CaCO3 can increase the density of the material, comprehensively improving the mechanical properties of the cement-based building materials. In addition, the addition of carbide slag also reduces the amount of cement used, which can not only achieve carbon emission reduction of greenhouse gases, but also effectively utilize industrial solid waste, realizing the green concept of solid waste carbon sequestration, and having the characteristics of alleviating the greenhouse effect and improving economic benefits.

Claims

1. A method for preparing a carbon-fixing carbide slag cement-based material based on a three-stage curing method, characterized in that, It includes the following steps: (1) Pretreatment of carbide slag; (2) Preparation of carbide slag cement-based specimens: (2-1) Weigh the raw materials according to the ratio: carbide slag, cement and water, and reserve them; The raw material composition includes: 100-500 parts of carbide slag, 200-600 parts of cement and 205 parts of water; (2-2) First, mix the carbide slag with half of the mass of water and conduct the first stirring; then, add the cement and the other half of the mass of water while stirring to conduct the second stirring; after the raw materials are mixed, conduct the third stirring to obtain the carbide slag cement paste mixture; the revolution speed of the stirring blades for the first stirring is 62±5 r / min, the rotation speed of the stirring blades is 140±5 r / min, and the time is 3-5 min; the revolution speed of the stirring blades for the second stirring is 62±5 r / min, the rotation speed of the stirring blades is 140±5 r / min, and the time is 3-5 min; the revolution speed of the stirring blades for the third stirring is 125±10 r / min, the rotation speed of the stirring blades is 285±10 r / min, and the time is 3-5 min; (2-3) Inject the carbide slag cement paste mixture into the mold, and after demolding, obtain the carbide slag cement-based specimen; (3) Prepare the carbonated carbide slag cement-based material based on three-stage curing: (3-1) Put the carbide slag cement-based specimen into a constant temperature and humidity curing box for pre-curing; keep the air circulation on the surface of the carbide slag cement-based specimen during pre-curing; the pre-curing temperature is (20±3)°C, the relative humidity is (60±10)%RH, and the curing time is 0.5-4 d; after pre-curing, the water loss rate of the carbide slag cement-based specimen is 7-50%; (3-2) After pre-curing, transfer it to a carbonation box for carbon curing; the carbon curing temperature is (20±3)°C, the relative humidity is (70±5)%RH, the curing time is 0.5-14 d, and the volume concentration of CO2 in the curing environment is ≥20%; (3-3) After carbon curing, transfer it to a standard curing room for standard curing, and that's it; the standard curing temperature is (20±3)°C, the relative humidity is ≥95%RH, and the curing time is 0.5-14 d.

2. The method according to claim 1, characterized in that In the step (1), the pretreatment method of the carbide slag is: crush the carbide slag and pass it through a 200-mesh sieve.

3. The method according to claim 1, characterized in that In the step (2-1), the ratio of the mass of water to the total mass of cement and carbide slag is 0.5; the mass of the carbide slag accounts for 15% of the total mass of cement and carbide slag.

4. The method according to claim 1, wherein In the step (2-1), the cement is ordinary Portland cement; in the step (2-3), after injecting into the mold, cover the mold with plastic wrap and demold after 24-36 h.

5. The method according to claim 1, characterized in that, In the step (3-1), the pre-curing time is 4 d; after pre-curing, the water loss rate of the carbide slag cement-based specimen is 20-50%; In the step (3-2), the carbon curing time is 14 d; in the step (3-3), the standard curing time is 14 d.

Images