A low-carbon cement-based composite material and a preparation method thereof

Abstract

The application belongs to the technical field of building materials, and particularly relates to a low-carbon cement-based composite material and a preparation method thereof. Components of the low-carbon cement-based composite material of the application include: Portland cement, fly ash, alkali-modified biochar and mixing water; the alkali-modified biochar is prepared by a method comprising the following steps: A1, mixing and stirring biochar with alkali liquor; A2, solid-liquid separation, to obtain the alkali-modified biochar; the alkali liquor contained in the alkali-modified biochar has a mass of 60%-70% of the mass of the biochar; the mixing water is the filtrate obtained by solid-liquid separation in step A2. After mixing and stirring biochar with alkali liquor, solid-liquid separation to obtain alkali-modified biochar, and without any drying or heat treatment of the biochar obtained by solid-liquid separation, the active functional groups on the surface of the alkali-modified biochar can be effectively maintained, which helps to better improve the mechanical properties of the low-carbon cement-based composite material.

Description

Technical Field

[0001] This invention belongs to the field of building materials technology, specifically relating to a low-carbon cement-based composite material and its preparation method. Background Technology

[0002] Cement-based composite materials are among the most widely used materials in construction engineering. However, traditional cement production is energy-intensive and emits a large amount of carbon. Producing one ton of ordinary Portland cement emits approximately 0.8-0.9 tons of carbon dioxide, accounting for about 8% of global anthropogenic CO2 emissions. Incorporating industrial waste (such as fly ash) and agricultural waste (such as biochar) into cement can not only reduce production costs but also achieve resource utilization of solid waste, which is of significant environmental importance. Biochar, as a carbon sequestration material, can achieve carbon fixation and emission reduction during its production and use. However, the incorporation of fly ash and biochar often has an adverse effect on the later compressive strength of cement-based composite materials. This may be because: fly ash, as an auxiliary cementitious material, has low pozzolanic reactivity and slow early strength development; biochar, as a porous carbon material, has weak interfacial bonding with the cement matrix, which may lead to a decrease in strength.

[0003] Therefore, there is a need to provide an improved technical solution that addresses the shortcomings of the existing technology. Summary of the Invention

[0004] The purpose of this invention is to provide a low-carbon cement-based composite material and its preparation method, so as to help solve or improve the problem that adding fly ash or biochar to cement-based materials in the prior art will have an adverse effect on the performance of cement-based materials.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a low-carbon cement-based composite material, wherein the components of the low-carbon cement-based composite material include: silicate cement, fly ash, alkali-modified biochar, and mixing water; the alkali-modified biochar is prepared by a method comprising the following steps: A1, mixing and stirring the biochar with an alkali solution; A2, separating the solid and liquid to obtain the alkali-modified biochar; the mass of the alkali solution contained in the alkali-modified biochar is 60%-70% of the mass of the biochar; the mixing water is the filtrate obtained by solid-liquid separation in step A2.

[0006] Preferably, the raw materials of the low-carbon cement-based composite material, by mass percentage, include: 50%-60% silicate cement, 20%-30% fly ash, and 20%-30% biochar.

[0007] Preferably, in step A1, the ratio of biochar to alkaline solution is 1g:2.4mL; the mixing and stirring are carried out at room temperature for 10-15min; the alkaline solution is a KOH solution with a concentration of 0.1mol / L.

[0008] Preferably, in step A1, the biochar has a particle size of 0.5-75 μm; and the fly ash is Grade I fly ash that meets the requirements of GB / T 1596-2017 "Fly Ash for Cement and Concrete".

[0009] Preferably, the fly ash is alkali-activated fly ash or alkali-activated dried fly ash; the alkali-activated fly ash is obtained by the following steps: B1, mixing and stirring fly ash with alkali solution; B2, separating solid and liquid to obtain the alkali-activated fly ash; the mass of alkali solution contained in the alkali-activated fly ash is 25%-30% of the mass of fly ash; the alkali-activated dried fly ash is obtained by drying the alkali-activated fly ash.

[0010] Preferably, the mass ratio of the mixing water to the silicate cement is 0.3-0.6.

[0011] Preferably, the components of the low-carbon cement-based composite material further include a water-reducing agent and a defoamer; the amount of the water-reducing agent is 1%-2% of the total mass of silicate cement and fly ash; the amount of the defoamer is 0.02%-1% of the total mass of silicate cement and fly ash.

[0012] The present invention also provides a method for preparing a low-carbon cement-based composite material, which adopts the following technical solution: the method for preparing a low-carbon cement-based composite material as described above includes the following steps: (1) mixing the mixing water and the silicate cement evenly; (2) adding the alkali-modified biochar and stirring evenly; (3) adding the fly ash and mixing evenly to obtain a mixture; (4) injecting the mixture into a mold to form a green body, and curing the green body after demolding.

[0013] Preferably, in step (1), the water-reducing agent is first added to the mixing water and stirred evenly, and then the mixture of silicate cement and defoamer is added; in step (4), the curing method is standard curing.

[0014] Beneficial effects: This invention obtains alkali-modified biochar by mixing and stirring biochar with alkaline solution and then separating the solid and liquid phases. No drying or heat treatment is performed on the biochar obtained after solid-liquid separation, which can effectively maintain the active functional groups on the surface of the alkali-modified biochar. This invention uses the filtrate obtained from solid-liquid separation during the preparation of alkali-modified biochar as mixing water, which can effectively realize the recycling of alkaline solution.

[0015] In the low-carbon cement-based composite material of the present invention, fly ash retains the activity of natural volcanic ash, providing a basis for continuous hydration in the later stage, and alkali-modified biochar surface activation enhances interfacial bonding; the synergistic effect of the two ensures early strength, and the strength can continue to grow steadily during the curing process without shrinkage (the 28-day compressive strength can even reach more than 45 MPa); in addition, the present invention uses fly ash and biochar to replace part of the cement, which helps to reduce carbon emissions. Attached Figure Description

[0016] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. Wherein: Figure 1 This is a comparison chart of the 28-day compressive strength of samples S1-S12.

[0017] Figure 2 This is a comparison chart of the 28-day flexural strength of samples S1-S12.

[0018] Figure 3 Comparison of XRD phase composition of samples S1, S2, S4 and S7 after curing for 28 days.

[0019] Figure 4 Comparison of FITR characteristic peak absorbance measured after 28 days of curing for samples S1, S2, S4 and S7.

[0020] Figure 5 SEM images of samples S1, S2, S4 and S7 after 28 days of curing are shown below. Among them, (a1) is the 28-day SEM image of sample S1, (a2) is the SEM image of sample S1 at a different location than (a1), (b1) is the 28-day SEM image of sample S2, (b2) is the SEM image of sample S2 at a different location than (b1), (c1) is the 28-day SEM image of sample S4, (c2) is the SEM image of sample S4 at a different location than (c1), (d1) is the 28-day SEM image of sample S7, and (d2) is the SEM image of sample S7 at a different location than (d1).

[0021] Figure 6 SEM images of samples S1 and S2 after curing for 3 days, 14 days, and 28 days, respectively (and...) Figure 5 (Compared to SEM images of the same sample but at different locations); where (a1) is the 3d SEM image of sample S1, (a2) is the 3d SEM image of sample S2, (b1) is the 14d SEM image of sample S1, (b2) is the 14d SEM image of sample S2, (c1) is the 28d SEM image of sample S1, and (c2) is the 28d SEM image of sample S2.

[0022] Figure 7 This is a comparison chart of the compressive strength of samples S1, S2, S4 and S7 at different ages.

[0023] Figure 8 This is a comparison chart of carbon emissions.

[0024] Figure 9 This is a comparison chart of carbon efficiency. Detailed Implementation

[0025] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention are within the scope of protection of the present invention.

[0026] The present invention will now be described in detail with reference to embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in the embodiments of the present invention can be combined with each other.

[0027] This invention addresses the problem that adding fly ash or biochar to cement-based materials in the prior art can adversely affect the performance of cement-based materials, and provides a low-carbon cement-based composite material.

[0028] The low-carbon cement-based composite material of this invention comprises: silicate cement, fly ash, alkali-modified biochar, and mixing water. The alkali-modified biochar is prepared by a method including the following steps: A1, mixing and stirring the biochar with an alkali solution; A2, solid-liquid separation to obtain the alkali-modified biochar; the mass of the alkali solution in the alkali-modified biochar is 60%-70% of the mass of the biochar (e.g., 60%, 62%, 64%, 66%, 68%, or 70%); the mixing water is the filtrate obtained from the solid-liquid separation in step A2. The fly ash is raw fly ash that has not undergone alkali activation, acid activation, mechanical grinding, heat treatment, pre-soaking in water, or vacuum filtration (e.g., Buchner funnel vacuum filtration), retaining its natural pozzolanic activity.

[0029] This invention obtains alkali-modified biochar by mixing and stirring biochar with alkaline solution and then separating the solid and liquid phases. No drying or heat treatment is performed on the biochar obtained after solid-liquid separation, which can effectively maintain the active functional groups on the surface of the alkali-modified biochar. This invention uses the filtrate obtained from solid-liquid separation during the preparation of alkali-modified biochar as mixing water, which can effectively realize the recycling of alkaline solution.

[0030] In the low-carbon cement-based composite material of the present invention, fly ash retains the activity of natural volcanic ash, providing a basis for continuous hydration in the later stage, and the surface of alkali-modified biochar is activated, which helps to enhance the interfacial bonding between biochar and cement matrix; the two work synergistically to ensure early strength, and the strength can continue to grow steadily without shrinkage during the curing process; in addition, the present invention uses fly ash and biochar to replace part of the cement, which helps to reduce carbon emissions.

[0031] In a preferred embodiment of the low-carbon cement-based composite material of the present invention, the raw materials of the low-carbon cement-based composite material, by mass percentage, include: 50%-60% silicate cement (e.g., 50%, 52%, 54%, 56%, 58%, or 60%), 20%-30% fly ash (e.g., 20%, 22%, 24%, 26%, 28%, or 30%), and 20%-30% biochar (e.g., 20%, 22%, 24%, 26%, 28%, or 30%). At this ratio, the cement substitution rate reaches over 40%, and the 28-day compressive strength of this low-carbon cement-based composite material is even higher than that of pure cement (reaching over 45 MPa), with continuous and stable strength growth and no shrinkage.

[0032] In a preferred embodiment of the low-carbon cement-based composite material of the present invention, in step A1, the ratio of biochar to alkali solution is 1g:2.4mL; the mixing and stirring are carried out at room temperature for 10-15min (e.g., 10min, 11min, 12min, 13min, 14min or 15min).

[0033] In a preferred embodiment of the low-carbon cement-based composite material of the present invention, the alkaline solution is a KOH solution with a concentration of 0.1 mol / L. If the KOH concentration is too low, the activation effect on the aluminosilicate material will be insufficient; if the KOH concentration is too high, it may lead to excessive consumption of active components, triggering an alkali-silica reaction or microstructure deterioration, which is detrimental to long-term mechanical properties. Previous experimental studies have shown that over-activation occurs when the KOH concentration is 0.2 mol / L. The present invention uses a 0.1 mol / L KOH solution to perform alkali modification treatment on biochar, which can introduce oxygen-containing functional groups (such as -OH, -COOH) on the surface of biochar, enhancing the interfacial bonding between biochar and the cement matrix.

[0034] In a preferred embodiment of the low-carbon cement-based composite material of the present invention, in step A1, the particle size of the biochar is 0.5-75 μm; the fly ash is Grade I fly ash that meets the requirements of GB / T 1596-2017 "Fly Ash for Cement and Concrete".

[0035] In a preferred embodiment of the low-carbon cement-based composite material of the present invention, the fly ash is alkali-activated fly ash or alkali-activated dried fly ash; the alkali-activated fly ash is prepared by the following steps: B1, mixing and stirring fly ash with alkali solution; B2, separating the solid and liquid to obtain alkali-activated fly ash; the mass of alkali solution contained in the alkali-activated fly ash is 25%-30% of the mass of fly ash (e.g., 25%, 26%, 27%, 28%, 29%, or 30%); the alkali-activated dried fly ash is obtained by drying the alkali-activated fly ash. The alkali-activated dried fly ash helps improve the flexural strength of the prepared low-carbon cement-based composite material; the alkali-activated fly ash used in combination with alkali-modified biochar, compared to the case where no pretreatment of fly ash and biochar is performed, helps to improve the compressive strength and flexural strength of the prepared cement-based composite material to a certain extent.

[0036] In a preferred embodiment of the low-carbon cement-based composite material of the present invention, the mass ratio of mixing water to silicate cement is 0.3-0.6 (e.g., 0.3, 0.4, 0.5 or 0.6).

[0037] In a preferred embodiment of the low-carbon cement-based composite material of the present invention, the components of the low-carbon cement-based composite material further include a water-reducing agent and / or a defoamer; the amount of water-reducing agent is 1%-2% of the total mass of silicate cement and fly ash (e.g., 1%, 1.2%, 1.4%, 1.6%, 1.8% or 2%); the amount of defoamer is 0.02%-1% of the total mass of silicate cement and fly ash (e.g., 0.02%, 0.05%, 0.1%, 0.3%, 0.5%, 0.8% or 1%).

[0038] The present invention also proposes a method for preparing a low-carbon cement-based composite material. The method for preparing the low-carbon cement-based composite material in the embodiment of the present invention includes the following steps: (1) mixing water and silicate cement evenly; (2) adding alkali-modified biochar and stirring evenly; (3) adding fly ash and mixing evenly to obtain a mixture; (4) injecting the mixture into a mold to form a green body, and curing the green body after demolding.

[0039] In a preferred embodiment of the preparation method of the low-carbon cement-based composite material of the present invention, in step (1), the water-reducing agent is first added to the mixing water and stirred evenly, and then a mixture of silicate cement and defoamer is added.

[0040] In a preferred embodiment of the preparation method of the low-carbon cement-based composite material of the present invention, in step (4), the curing method is standard curing (20±2℃, relative humidity ≥95%; curing time is 28 days).

[0041] The low-carbon cement-based composite material and its preparation method of the present invention will be described in detail below through specific embodiments.

[0042] In the following examples: the fly ash used is Grade I fly ash (meeting the requirements of GB / T 1596-2017 "Fly Ash for Cement and Concrete"); the biochar used is straw biochar produced by Henan Housen Environmental Protection Technology Co., Ltd., which is pyrolyzed at 400-600℃ under anaerobic conditions, model swtt001, and is in the form of blackish-gray powder (particle size 0.5-75μm; i.e., can pass through a 250-mesh sieve), without caking; the water-reducing agent used is high-performance standard polycarboxylate water-reducing agent purchased from Hunan Zhongyan Building Materials Technology Co., Ltd., model ZY-HPWR-S; the defoamer used is polyether mortar defoamer produced by Shanxi Yuncheng Weike Building Materials Sales Center, model XP-2.

[0043] In the following experiments, "D", "AF", "AH", and "WF" refer to the corresponding treatments of fly ash (and / or biochar); specifically: “D” refers to the use of the corresponding dry sample (i.e., without pretreatment of fly ash or biochar, the corresponding raw material is used directly), denoted as sample-D; "AF" refers to a sample treated with alkali solution and then filtered to obtain a water-containing sample (without drying after filtration). Specifically, it includes the following steps: A1. Mix the sample (biochar or fly ash) with alkali solution (0.1 mol / L KOH solution) at room temperature (20-25℃) using a magnetic stirrer (using a magnetic stirrer manufactured by Xiniu Technology, with a maximum stirring capacity of 5L, a speed range of 0-2600r / min, model DF-101T) for 15 min (the ratio of alkali solution to sample is 2.4mL:1g); A2. Filter (using a combination of vacuum pump, Buchner funnel, and volumetric flask) to separate the solid and liquid, obtaining alkali-modified biochar or alkali-activated fly ash, denoted as sample-AF; the filtrate obtained by filtration can be used as mixing water.

[0044] "AH" refers to a sample that has been treated with an alkaline solution, filtered to obtain a water-containing sample, and then dried to obtain a dried sample (dried after filtration). Specifically, it includes the following steps: A1. Mix the sample (biochar or fly ash) with an alkaline solution (0.1 mol / L KOH solution) at room temperature (20-25℃) using a magnetic stirrer (the ratio of alkaline solution to sample is 2.4 mL: 1 g) for 15 min; A2. Filter the sample (using a combination of a vacuum pump, Buchner funnel, and volumetric flask) to separate the solid and liquid phases, obtaining alkali-modified biochar or alkali-activated fly ash. Dry the alkali-modified biochar or alkali-activated fly ash sample (dry at 80℃ to constant weight and then grind into powder for later use) to obtain the corresponding dried sample, denoted as sample-AH; the filtrate obtained from filtration can be used as mixing water.

[0045] The difference between “WF” and “AF” is that the alkaline solution is replaced with pure water; the steps include: A1, mixing biochar and water in a beaker at room temperature (20-25℃) (the ratio of water to sample is 2.4mL:1g) and soaking for 12h; A2, filtration (using a combination of vacuum pump, Buchner funnel and volumetric flask) to separate the solid and liquid, obtaining a solid with a water content of 60%-70%, which is denoted as sample-WF; the filtrate obtained by filtration is denoted as WF filtrate.

[0046] Note: In the following examples and comparative examples, the amounts of biochar and fly ash raw materials used are the same; that is, in the following examples and comparative examples, after equal amounts of biochar-D (or fly ash-D) are treated with AH, AF or WF, all the raw materials obtained from the treatment are used for subsequent mixing with cement.

[0047] Example 1 The components of the low-carbon cement-based composite material in this embodiment include: fly ash (fly ash-D), silicate cement and alkali-modified biochar (biochar-AF), mixing water, defoamer and water-reducing agent; by mass percentage, the raw materials of the low-carbon cement-based composite material include: 60% silicate cement, 20% fly ash (fly ash-D) and 20% biochar (biochar-D); the amount of defoamer is 0.5% of the total mass of silicate cement and fly ash; the amount of water-reducing agent is 1% of the total mass of silicate cement and fly ash; the mass ratio of mixing water to silicate cement (water-cement ratio) is 0.3:1.

[0048] Alkali-modified biochar (Biochar-AF) is prepared by the following steps: A1. Biochar and alkali solution (0.1 mol / L KOH solution) are mixed and stirred at room temperature using a magnetic stirrer (the ratio of alkali solution to biochar is 2.4 mL: 1 g) for 15 min; A2. Solid-liquid separation is achieved by vacuum filtration, yielding a solid with a water content of 67% ± 3% (the water content measured in multiple experiments; the water content of the sample obtained in each experiment may vary slightly; the water content refers to the mass of water in the filtered sample being 67% ± 3% of the biochar in step A1), which is alkali-modified biochar (all of the obtained modified biochar is used in the subsequent preparation of low-carbon cement-based composite materials); the filtrate obtained by vacuum filtration is the mixing water.

[0049] The preparation method of the low-carbon cement-based composite material in this embodiment includes the following steps: (1) Mix the defoamer with the silicate cement evenly; (2) Add the water-reducing agent to the mixing water, and then add the mixture of defoamer and silicate cement obtained in step (1). First, stir at a low speed of 140±5r / min for 3-4 min, and then stir at a high speed of 280±10r / min for 1 min until the mixture is uniform. (3) Add alkali-modified biochar to the mixture obtained by step (2), stir at a low speed of 140±5r / min for 0.5min, and then stir at a high speed of 280±10r / min for 1min to make it uniform. (4) Add fly ash to the mixture obtained in step (3) and stir at a low speed of 140±5r / min for 5min to mix evenly to obtain a mixture; (5) The mixture obtained in step (4) is injected into a 40×40×160mm mold to form a blank. The blank is then cured (cured for 24 hours at a temperature of 20±2℃ and relative humidity of ≥95%, and then demolded and cured under the same conditions for 28 days).

[0050] The sample obtained in this embodiment is denoted as S2.

[0051] Example 2 The only difference between this embodiment and Embodiment 1 is that in the composition of the low-carbon cement-based composite material in this embodiment, fly ash-AF is used instead of fly ash-D; all other components are the same as in Embodiment 1.

[0052] The sample obtained in this embodiment is denoted as S6.

[0053] Example 3 The only difference between this embodiment and Embodiment 1 is that in the composition of the low-carbon cement-based composite material in this embodiment, fly ash-AH is used instead of fly ash-D; all other components are the same as in Embodiment 1.

[0054] The sample obtained in this embodiment is denoted as S10.

[0055] Comparative Example 1 The only difference between this comparative example and Example 1 is that in the composition of the low-carbon cement-based composite material of this comparative example, biochar-D is used instead of biochar-AF (i.e., the step of alkali modification treatment of biochar is omitted), pure water is used for mixing, and the mass ratio of mixing water to silicate cement is 0.6; all other aspects are the same as in Example 1.

[0056] The sample obtained in this comparative example is denoted as S1.

[0057] Comparative Example 2 The only difference between this comparative example and Example 1 is that in the composition of the low-carbon cement-based composite material of this comparative example, biochar-AH is used instead of biochar-AF, and the mass ratio of mixing water to silicate cement is 0.6; all other aspects are the same as in Example 1.

[0058] The sample obtained in this comparative example is denoted as S3.

[0059] Comparative Example 3 The only difference between this comparative example and Example 1 is that in the composition of the low-carbon cement-based composite material of this comparative example, biochar-WF is used instead of biochar-AF, and the filtrate obtained by WF treatment of biochar (WF filtrate) is used as mixing water; the rest are the same as in Example 1.

[0060] The sample obtained in this comparative example is denoted as S4.

[0061] Comparative Example 4 The only difference between this comparative example and Example 1 is that in the composition of the low-carbon cement-based composite material of this comparative example, biochar-D is used instead of biochar-AF, fly ash-AF is used instead of fly ash-D, and the mass ratio of mixing water (the filtrate obtained by filtration after AF treatment of fly ash) to silicate cement is 0.5; all other components are the same as in Example 1.

[0062] The sample obtained in this comparative example is denoted as S5.

[0063] Comparative Example 5 The only difference between this comparative example and Example 1 is that in the composition of the low-carbon cement-based composite material of this comparative example, fly ash-AF is used instead of fly ash-D, biochar-AH is used instead of biochar-AF, and the mass ratio of mixing water to silicate cement is 0.5; all other components are the same as in Example 1.

[0064] The sample obtained in this comparative example is denoted as S7.

[0065] Comparative Example 6 The only difference between this comparative example and Example 1 is that in the composition of the low-carbon cement-based composite material of this comparative example, fly ash-AF is used instead of fly ash-D, biochar-WF is used instead of biochar-AF, and the mixing water is the filtrate obtained by filtration after AF treatment of fly ash; all other components are consistent with Example 1.

[0066] The sample obtained in this comparative example is denoted as S8.

[0067] Comparative Example 7 The only difference between this comparative example and Example 1 is that in the composition of the low-carbon cement-based composite material of this comparative example, fly ash-AH is used instead of fly ash-D, biochar-D is used instead of biochar-AF, the mixing water is the filtrate obtained by filtration after AF treatment of fly ash, and the mass ratio of mixing water to silicate cement is 0.6; all other components are the same as in Example 1.

[0068] The sample obtained in this comparative example is denoted as S9.

[0069] Comparative Example 8 The only difference between this comparative example and Example 1 is that in the composition of the low-carbon cement-based composite material of this comparative example, fly ash-AH is used instead of fly ash-D, biochar-AH is used instead of biochar-AF, and the mass ratio of mixing water to silicate cement is 0.6; all other components are the same as in Example 1.

[0070] The sample obtained in this comparative example is denoted as S11.

[0071] Comparative Example 9 The only difference between this comparative example and Example 1 is that in the composition of the low-carbon cement-based composite material of this comparative example, fly ash-AH is used instead of fly ash-D, biochar-WF is used instead of biochar-AF, and the mixing water is the filtrate obtained by filtration during the AH treatment of fly ash; all other components are consistent with Example 1.

[0072] The sample obtained in this comparative example is denoted as S12.

[0073] Experimental Example 1. Compressive strength and flexural strength tests: Test Method: According to the national standard GB / T17671-2021, "Test Method for Strength of Cement Mortar (ISO Method)," specimens with dimensions of 160mm × 40mm × 40mm were prepared. Flexural and compressive strength tests were conducted at curing ages of 3d, 7d, 14d, and 28d. The instrument used for strength testing was a universal testing machine (manufactured by Shenzhen Xin Sansi Materials Testing Co., Ltd., model CMT5205). The test parameters were pre-set on a microcomputer, and all parameters were required to comply with the relevant provisions of Section 10—Test Procedures in GB / T17671-2021. The instrument's test force accuracy was better than 0.5% of the indicated value, and the maximum test force was 200kN.

[0074] Test results: (1) The results of the compressive strength test are shown in Table 1 below. Figure 1 As shown: Table 1 Compressive strength test results (MPa)

[0075] Note: Each of the above samples underwent six parallel compressive strength tests; the water-cement ratio refers to the mass ratio of mixing water to silicate cement; the water-cement ratio of each sample was obtained through repeated preliminary experiments based on the principle of achieving uniform mixing of all raw materials and minimizing the amount of mixing water.

[0076] It can be seen from the above table: Sample S2 from Example 1 achieved a 28-day compressive strength of 47.08 MPa. Compared to sample S1, the compressive strength increased by 25.5%, demonstrating a significant improvement.

[0077] The strength development of the low-carbon cement-based composite material (sample S2) in Example 1 is as follows: 3-day compressive strength 27.68 MPa, 7-day compressive strength 31.09 MPa (12.3% increase compared to 3-day), 14-day compressive strength 34.94 MPa (12.4% increase compared to 7-day), and 28-day compressive strength 47.08 MPa (34.7% increase compared to 14-day). The compressive strength continuously increases from 3 days to 28 days without any shrinkage, meeting the long-term stability requirements of load-bearing structural engineering.

[0078] Sample S4 exhibited the highest compressive strength at 3d (31.12 MPa), but showed a shrinkage at 7d (a decrease of 22.7%), indicating that while biochar-WF helps achieve high early strength, its weak interfacial bonding results in poor long-term performance. Sample S4 is suitable for projects requiring tight deadlines, but attention should be paid to the stability of its strength in later stages.

[0079] Sample S7: The 28-day compressive strength was lower than the 14-day compressive strength, and a later shrinkage occurred, proving that the overtreatment of fly ash by AF treatment and biochar by AH treatment will lead to premature consumption of activity.

[0080] Sample S8: The 14-day compressive strength is lower than the 7-day compressive strength, indicating mid-term shrinkage.

[0081] Sample S10 (Example 3): It had the lowest 3-day compressive strength (15.60 MPa), but its 28-day compressive strength reached 38.85 MPa, showing potential for future growth.

[0082] The effect of fly ash pretreatment process on the 28-day compressive strength of cement-based materials: Samples S1, S2, S3, and S4 all used fly ash-D, with an average 28-day compressive strength of 37.78 MPa; samples S5, S6, S7, and S8 all used fly ash-AF, with an average 28-day compressive strength of 32.00 MPa; samples S9, S10, S11, and S12 all used fly ash-AH, with an average 28-day compressive strength of 35.35 MPa. That is, fly ash-D, compared to fly ash-AF, can increase the average 28-day compressive strength by 18.1%. This indicates that fly ash without alkali activation treatment retains its natural pozzolanic activity, avoiding the damage to later strength caused by over-activation.

[0083] The effect of biochar treatment process on the 28-day compressive strength of cement-based materials: Samples S2, S6, and S10, all treated with biochar-AF, had an average 28-day compressive strength of 41.73 MPa; samples S4, S8, and S12, all treated with biochar-WF, had an average 28-day compressive strength of 36.00 MPa; samples S1, S5, and S9, all treated with biochar-D, had an average 28-day compressive strength of 33.64 MPa; and samples S3, S7, and S11, all treated with biochar-AH, had an average 28-day compressive strength of 28.81 MPa. In other words, compared with the three groups of samples treated with biochar-D, the three groups of samples treated with biochar-AF showed an average increase in compressive strength of 24.1%, and compared with the three groups of samples treated with biochar-AH, the three groups of samples treated with biochar-AF showed an average increase in compressive strength of 44.9%. This indicates that alkali modification (AF) can effectively activate the functional groups on the surface of biochar, while further drying (AH) will destroy the active functional groups.

[0084] A comparison of samples S2 (fly ash-D + biochar-AF), S6 (fly ash-AF + biochar-AF), and S1 (fly ash-D + biochar-D) shows that the 28-day compressive strength of sample S2 increased by 19.9% ​​and 25.5% respectively compared to S6 and S1. This indicates that fly ash-D and biochar-AF have a significant synergistic effect, achieving a synergistic enhancement of compressive strength.

[0085] (2) The flexural strength test results are shown in Table 2 below. Figure 2 As shown: Table 2. Flexural strength test results (MPa)

[0086] Note: Each of the above samples underwent three parallel flexural strength tests.

[0087] Among them, the 14-day flexural strength of sample S10 can reach 9.89 MPa, which is much greater than that of other groups, making it suitable for projects with high bending resistance requirements (such as road surfaces, bridge decks, etc.).

[0088] Analysis of the high flexural strength mechanism of sample S10: S10 exhibits unique flexural strength development characteristics, with a 14-day flexural strength as high as 9.89 MPa, far exceeding that of other groups. This phenomenon reveals the difference in the impact of pretreatment methods on flexural and compressive properties.

[0089] The unique role of fly ash-AH: After AH treatment, the drying process (drying temperature can be 80℃, drying time can be 12h) causes a microcrack network and active sites to form on the particle surface of fly ash. These microcracks provide attachment sites for hydration products in the early hydration stage, promoting the directional growth of CSH gel. Unlike compressive strength, flexural strength is more sensitive to the quality of the interfacial transition zone (ITZ). The microcrack network formed by fly ash-AH forms a good mechanical bond with the cement matrix at 14 days, effectively resisting the propagation of microcracks under flexural stress.

[0090] Synergistic effect of biochar-AF: After AF treatment, the surface of biochar is rich in oxygen-containing functional groups (-OH, -COOH), which form chemical bonds with cement hydration products. At 14 days, the mechanical interlocking sites provided by fly ash-AH and the chemical bonding sites provided by biochar-AF work synergistically to form a dense ITZ, enabling the flexural strength of S10 to reach its peak.

[0091] 28-day performance degradation mechanism: The 28-day flexural strength of S10 decreased to 8.23 ​​MPa, but remained at a relatively high level. This degradation may be related to the following factors: the fly ash underwent AH treatment, which prematurely consumed some of the active components, leading to a decrease in pozzolanic reactivity in the later stages; the microcrack network formed during drying may become stress concentration points during the later hydration process. It is recommended to pay attention to long-term performance monitoring in practical applications.

[0092] (3) SPSS 26.0 statistical software was used to conduct one-way ANOVA and Tukey's HSD post-hoc test to systematically evaluate each factor.

[0093] The results of the analysis of variance are shown in Table 3 below: Table 3 Summary of Statistical Analysis Results

[0094] Note: *** indicates p<0.001, * indicates p<0.05.

[0095] One-way ANOVA showed that the biochar pretreatment method had a highly significant effect on the 28-day compressive strength (F=11.09, p=0.000005<0.001), while the fly ash pretreatment method had a significant effect (F=4.13, p=0.020<0.05).

[0096] 2. Flowability test: According to GB / T 2419-2005 "Method for Determination of Flowability of Cement Mortar", the flowability of cement mortar is measured by determining the range of expansion under specified vibration conditions. The mixed mortar is poured into a mold in two layers and compacted. After compaction, the mold is removed, and the mortar is vibrated 25 times using a vibration table. Then, the diameter of the mortar bottom surface in two mutually perpendicular directions is measured using vernier calipers; the average value is the mortar flowability.

[0097] The test results are shown in the table below: Table 4. Flowability test results (mm)

[0098] Flowability Analysis: Flowability is an important indicator reflecting the workability of cement-based composite materials. As shown in the table above, flowability is mainly affected by both the water-cement ratio and the pretreatment method. Notably, the low water-cement ratio group (0.3) exhibits higher flowability, which is closely related to the pretreatment method. Samples using fly ash-AF (S5-S8) generally have higher flowability (148.0-247.5 mm), possibly related to the change in surface charge distribution and improved dispersibility of fly ash particles after alkali activation treatment.

[0099] Engineering Applicability Evaluation: Based on the experimental results in Tables 1-4 above, the optimal combination S2 has a flowability of 152.5 mm, which is at a medium level and meets general construction requirements (usually requiring a flowability ≥ 140 mm). More importantly, S2 achieves the highest 28-day compressive strength while ensuring good workability, proving the feasibility of S2 in practical engineering. Although the S6 group (flowability 247.5 mm) has the highest flowability and excellent workability, its 28-day compressive strength is lower than that of S2, indicating that the balance between flowability and strength needs to be optimized according to engineering requirements.

[0100] 3. Microstructure analysis: I. To further reveal the influence mechanism of pretreatment methods on the microstructure of materials, XRD and FTIR analyses were performed on representative samples.

[0101] Test method: The X-ray diffraction experiment used a Bruker D8 ADVANCE X-ray diffractometer (XRD) from Germany. This instrument utilizes the diffraction signals generated when X-rays interact with materials to determine the crystal structure, composition, and physical properties of substances. The sample preparation procedure for scanning X-ray diffraction experiments is as follows: After the compressive strength test is completed, a portion of the sample is retained and soaked in anhydrous ethanol to terminate the hydration reaction. In the sample pretreatment stage, the selected sample is placed in a constant temperature drying oven and dried at 60°C for 15 hours. Then, it is ground into powder, and samples are sieved through a 200-mesh sieve for further experiments.

[0102] The Fourier Transform Infrared (FTIR) instrument used in this experiment is the Nicolet Apex, a Nicolet series Fourier Transform Infrared Spectrometer from Thermo Fisher Scientific. Its core principle is to utilize the interaction between infrared light and molecular vibrational energy levels, recording absorption spectra to infer the functional groups, chemical bond types, and structural differences within molecules. The sample preparation process is identical to that used in X-ray diffraction experiments and can be performed concurrently.

[0103] The intensities of key peaks in the 28-day XRD patterns of samples S1, S2, S4, and S7 are compared, as shown in Table 5 below. Figure 3 As shown: Table 5. Comparison of key peak intensities in XRD at 28 days (after ALS background subtraction)

[0104] Note: Peak intensity is the maximum intensity value within the window after ALS (Asymmetric Least Squares) background subtraction. Qz = Quartz (SiO2); CH = Portlandite (Ca(OH)2); Cc = Calcite (CaCO3). A lower CH peak indicates a more complete volcanic ash reaction, while a higher Qz peak indicates better ordered deposition of hydration products.

[0105] The absorbance of the key peaks of FTIR at 28 days of age for samples S1, S2, S4, and S7 was compared, as shown in Table 6 below. Figure 4 As shown: Table 6 Comparison of absorbance of key peaks in FTIR at 28 days of age

[0106] Note: A higher absorbance of the OH peak indicates a richer content of bound water and a more complete degree of hydration; a lower absorbance of the Si-O peak indicates a more complete substitution of Si-OT by CSH and a higher CSH content; CO3 2-The lower the peak absorbance, the more effectively carbonates in the biochar have participated in the hydration reaction. The independent contribution of CH to Ca(OH)2 was quantitatively separated by Gaussian deconvolution peak area, which is consistent with the CH peak intensity trend in Table 5.

[0107] For sample S2: XRD analysis (Table 5) shows that, after removing the ALS background, sample S2 of Example 1 had the lowest intensity of the calcium hydroxide (CH) characteristic peak, significantly lower than that of sample S1 of Comparative Example 1 and sample S7 of Comparative Example 5. A lower CH peak intensity indicates a more complete pozzolanic reaction of the fly ash, with Ca(OH)2 being continuously consumed and converted into CSH gel. Meanwhile, S2 exhibited the highest intensity of the quartz (Qz, 26.6°) diffraction peak, indicating orderly deposition of hydration products on the surface of quartz particles and a more complete crystal structure. Furthermore, S2 had the lowest intensity of the calcite (Cc, 29.4°) peak, indicating the lowest degree of carbonization, consistent with densification under a low water-ash ratio. This microstructural characteristic directly corresponds to the highest 28-day compressive strength observed in S2.

[0108] FTIR functional group analysis: FTIR spectroscopy further revealed the changes in chemical bonds on the material surface. The OH stretching vibration peak (3440 cm⁻¹) of sample (S2) in Example 1 was observed. -1 The highest absorbance (10.92) indicates that the hydration products contain abundant bound water and are fully hydrated. In contrast, the sample of Comparative Example 5 (sample S7) has the lowest absorbance for all characteristic peaks except for HOH and Ca(OH)2, indicating the lowest degree of hydration and reflecting the inhibitory effect of excessive pretreatment on the hydration reaction. The 3640 peak area of ​​S2 is the lowest, indicating that it has the least amount of CH residue, consistent with the lowest CH peak intensity in Table 5. The absorbance of the AFt peak of S2 is lower than that of S1 and S4, indicating that the low water-cement ratio dense system of Example 1 reduces the risk of delayed ettringite expansion.

[0109] Microstructure-performance correlation analysis: The excellent mechanical properties of sample S2 in Example 1 of this invention stem from its unique microstructure characteristics. Specifically: the lowest CH peak intensity indicates that the volcanic ash reaction is complete, and Ca(OH)2 is continuously consumed and transformed into CSH gel; the highest Qz peak intensity indicates that the hydration products are deposited in an orderly manner on the quartz surface and the crystal structure is perfect; the lowest Cc peak intensity indicates that the degree of carbonization is effectively suppressed by the low water-ash ratio. XRD phase analysis and the high absorbance of the OH peak in FTIR jointly confirm the high density, high strength, and stable growth mechanical properties.

[0110] The superior performance of sample S2 stems from the organic unity of a triple synergistic effect: I. Mechanism of Fly Ash Retaining Pozzolanic Activity: Fly ash-D retains its original vitreous structure and pozzolanic activity. The active SiO2 and Al2O3 in fly ash undergo a slow pozzolanic reaction under the action of Ca(OH)2 produced during cement hydration, generating hydration products such as CSH gel. XRD analysis shows that the intensity of the CH characteristic peaks in S2, after subtracting the ALS background, is significantly lower than that in S1, with a 32.2% decrease in the CH 34° peak. This indicates that the pozzolanic reaction of fly ash continues throughout the curing period, continuously consuming Ca(OH)2 and generating CSH gel. Although this process is slower in the early stages, it continues in the later stages, corresponding to the later strength increase of S2.

[0111] II. Mechanism of Biochar Surface Activation and Interface Enhancement: The modification effect of alkali treatment on biochar is mainly reflected in three aspects: KOH treatment generates more oxygen-containing functional groups (such as -OH, -COOH, -C=O) on the surface of biochar, enhancing the chemical bonding ability between biochar and cement matrix; filtration removes soluble impurities, avoiding the negative impact of impurities on cement hydration; alkali treatment exposes some pores in the biochar, increasing the specific surface area and thus providing more attachment sites for hydration products. FTIR analysis shows the OH stretching vibration peak of S2 (3440 cm⁻¹). -1 The absorbance of the product was the highest (10.92), indicating that its hydration products contained abundant bound water and were fully hydrated.

[0112] III. Densification Mechanism of Low Water-to-Cement Ratio: An extremely low water-to-cement ratio (0.3) significantly reduces the number of capillary pores and improves the material density. XRD analysis shows that S2 has the lowest calcite peak intensity, indicating that the extremely low water-to-cement ratio (0.3) effectively inhibits the carbonization reaction and facilitates the continuous development of the fly ash pozzolanic reaction. With proper pretreatment of fly ash and biochar, the low water-to-cement ratio achieves denser particle packing, less free water evaporation, and more sufficient space for hydration products to fill, thus improving the later-stage strength.

[0113] The aforementioned triple effect forms a positive feedback loop: biochar surface activation provides more attachment sites for hydration products, fly ash pozzolanic reaction continuously fills the pores, and the low water-ash ratio limits pore formation, ultimately achieving high density, high strength, and stable growth. This discovery reveals a threshold effect in the degree of pretreatment: moderate activation (AF treatment) can enhance interfacial bonding, but excessive treatment (AH treatment) can destroy active components, leading to a decline in performance.

[0114] For sample S4 (fly ash-D + biochar-WF): the 3-day compressive strength reached a high of 31.12 MPa (the highest among samples S1-S12), but the 7-day compressive strength dropped to 24.04 MPa, showing a significant decline. Microscopic analysis revealed that the FTIR spectrum showed the highest absorbance of the Si-O peak (38.17), indicating a rich CSH gel content, which is consistent with the early high strength; however, its CO3... 2- The characteristic peak absorbance (33.62) of sample S4 was lower than that of sample S1 (34.06), indicating that the biochar-WF failed to effectively activate the surface functional groups, resulting in poor interfacial bonding quality. Meanwhile, the CH 34° peak intensity of sample S4 indicated insufficient volcanic ash reaction. These microscopic characteristics corroborate the ITZ microcracks observed by SEM.

[0115] For sample S7 (fly ash-AF + biochar-AH), its 28-day compressive strength (21.60 MPa) was lower than its 14-day compressive strength (26.18 MPa), indicating a later-stage shrinkage. Microscopic analysis provided direct evidence: XRD patterns showed the highest intensity of the calcite peak and the highest intensity of the CH 18° peak, indicating that excessive pretreatment led to severe carbonization and destruction of the alkaline environment; the active components of fly ash were prematurely consumed during alkali activation, resulting in the loss of pozzolanic reactivity in the later stages, with a large amount of unconsumed CH remaining. FTIR spectra showed that, except for the characteristic peaks of HOH and Ca(OH)2, the absorbance of other characteristic peaks was the lowest, indicating the lowest degree of hydration. These results demonstrate the negative impact of excessive pretreatment: the active components of fly ash-AF were prematurely consumed, resulting in the loss of pozzolanic reactivity in the later stages; the surface-active functional groups of biochar-AH were destroyed, leading to weak interfacial bonding.

[0116] II. SEM Microscopic Morphology Analysis: To further reveal the influence of the interfacial transition zone (ITZ) on macroscopic mechanical properties, SEM micromorphology analysis was performed on representative samples (S1, S2, S4, S7).

[0117] The instrument used for electron microscopy microstructure observation was a COXEMEM-30AXPlus desktop scanning electron microscope from the School of Civil Engineering and Geographical Environment, Ningbo University. Through a cycle of "electron beam scanning - signal excitation - acquisition and reconstruction," the microstructure (morphology or composition) of the sample surface was converted into a visual image. The specific procedure for sample preparation for scanning electron microscopy (SEM) observation is as follows: After the compressive strength test was completed, representative samples were cut from different regions of the specimen and stored. In the sample pretreatment stage, the selected samples were placed in an electrically heated constant-temperature drying oven and dried at a constant temperature of 50℃. After drying, a plasma spraying technique was used to coat the sample surface with a uniform gold film to enhance its conductivity. Then, the instrument was used for targeted microstructure observation.

[0118] The analysis results are shown in Table 7 below. Figures 5-7 As shown: Table 7 Comparison of SEM microstructure features

[0119] Note: Table 7 and Figure 5 Correspondingly; the compressive strength of each sample is detailed in [reference needed]. Figure 7 .

[0120] for Figure 5 The specific analysis is as follows: (1) Sample S2 of Example 1 (e.g.) Figure 5 As shown in (b1) and (b2): The biochar particles exhibit a dense and uniform microstructure, with the surface tightly coated by CSH gel. The interfacial transition zone (ITZ) shows no obvious pores or cracks. The oxygen-containing functional groups (-OH, -COOH) on the biochar surface form chemical bonds with cement hydration products, while the porous structure provides growth sites for the CSH gel, achieving a dual enhancement mechanism of mechanical interlocking and chemical bonding. This is the microscopic basis for S2 achieving its highest 28-day compressive strength (47.08 MPa).

[0121] (2) Sample S1 (e.g.) Figure 5 As shown in (a1) and (a2): the microstructure is between S2 and S7, the biochar is generally bonded to the matrix, and there are a few interfacial pores. Untreated biochar has fewer functional groups on its surface and mainly relies on mechanical interlocking to bond with the matrix, with moderate interfacial bonding quality.

[0122] (3) Inverted sample S4 (e.g.) Figure 5 As shown in (c1) and (c2): Although the overall structure is relatively dense, obvious microcracks (approximately 1-2 μm wide) can be observed at the biochar-matrix interface. These interfacial defects stem from the failure of pre-soaking water treatment of biochar to effectively activate surface functional groups, resulting in weak chemical bonding. Shrinkage stress formed during early rapid hydration leads to interfacial cracking. As hydration progresses, the microcracks expand, leading to a reduction in strength.

[0123] (4) Over-treated sample S7 (fly ash-AF + biochar-AH; such as Figure 5 As shown in (d1) and (d2): the microstructure is the most porous, with numerous pores and microcracks. The surface of the biochar particles is smooth, with weak bonding to the matrix, and obvious peeling occurs at the interface. Excessive pretreatment destroys the active functional groups on the surface of the biochar, resulting in a loss of its bonding ability with the matrix, ultimately leading to the lowest strength and subsequent shrinkage.

[0124] Interfacial bonding quality-performance correlation analysis: FTIR and SEM analyses corroborated each other, quantifying the relationship between interfacial bonding quality and performance. The OH peak absorbance of sample S2 (10.92) was significantly higher than that of sample S7 (8.77), corresponding to a 118% difference in 28-day compressive strength (47.08 vs 21.60 MPa). This difference stems from the combined contribution of chemically bound water in the CSH gel and structural water in Ca(OH)2. The higher OH signal reflects a denser hydration product network. This finding indicates that interfacial bonding quality is a key factor determining long-term stability, and the choice of pretreatment method directly affects the formation quality of ITZ.

[0125] for Figure 6 The specific analysis is as follows: (1) When the plant reaches 3 days of age, it is then... Figure 6 (a1) It can be seen that sample S1 contains many smooth-surfaced spherical fly ash particles, the bonding between the particles and the matrix is ​​relatively loose, local cracks and pores are more obvious, and the hydration products are mainly scattered. In contrast, Figure 6 (a2) The surface of sample S2 particles and the surrounding matrix have more fine particulate and flocculent hydration products attached, and the degree of filling of the interface region is improved. This indicates that the alkali activation treatment enhances the surface activity of biochar, enabling it to provide more nucleation sites for hydration products and promote the deposition of hydration products on the particle surface and pore interface, thereby improving the compactness of the sample's microstructure.

[0126] (2) When the chicks reach 14 days of age, they should be... Figure 6 (b1) It can be seen that although sample S1 has a distinct honeycomb-like porous structure, the pore walls are generally smooth, and the interior of the pores is mainly open voids. No large amount of continuously attached or filled hydration products were observed, indicating that the induced deposition of hydration products by untreated biochar is limited. In contrast, Figure 6 (b2) The edges of the pore walls in sample S2 are rougher, and more flocculent gels and fine crystals are visible around the pores, indicating that the surface activity of biochar is enhanced after alkali modification, which can provide more nucleation and attachment sites for hydration products.

[0127] (3) When the animal reaches 28 days of age, it should be... Figure 6 (c1) As can be seen, although a small amount of needle-like hydration products were formed near the pores of the untreated biochar in sample S1, their distribution was relatively localized and did not form a continuous and dense crystal network. In contrast, Figure 6 In (c2), a large number of needle-like crystals are distributed around the inner wall of the pores of sample S2, and they show obvious radial growth characteristics. This indicates that the surface activity of biochar is enhanced after alkali modification treatment, which can provide more nucleation sites for hydration products, thereby promoting the deposition and growth of hydration products on the pore wall and in the pore cavity.

[0128] The comparison of electron microscopy images of samples S1 and S2 shows that after alkali modification, the silicon and aluminum components in biochar can be activated, thus achieving the activation of biochar volcanic ash activity.

[0129] 4. Carbon emissions: (1) Comparison of carbon emissions: The cementitious materials of sample S2, by mass percentage, include: 60% silicate cement (OCP), 20% fly ash (FA) and 20% biochar (BC); the total mass of fly ash and biochar accounts for 40% of the total mass of cementitious materials, which replaces 40% of ordinary silicate cement.

[0130] In terms of carbon emissions, straw biochar exhibits significant negative carbon emission characteristics. During its growth, straw absorbs carbon dioxide through photosynthesis, and after pyrolysis to produce biochar, the carbon is fixed in a stable aromatic ring structure, enabling long-term carbon sequestration. According to Life Cycle Assessment (LCA) research (Anand et al., Resources, Conservation and Recycling, 2025), the carbon emission factor of straw biochar is approximately -1.0 t CO2 eq / t, meaning that every ton of biochar used can fix approximately 1 ton of carbon dioxide equivalent. According to GB / T 51366-2019 "Standard for Calculating Carbon Emissions from Buildings," the carbon emission factor of ordinary silicate cement is 0.90 t CO2 eq / t, and that of fly ash is 0.03 t CO2 eq / t.

[0131] Calculated: Producing 1 ton of pure cementitious material: carbon emissions = 1 × 0.90 = 0.90 t CO2; Producing 1 ton of the cementitious material of Example 1 (OPC:FA:BC=0.6:0.2:0.2): OPC portion: 0.6 × 0.90 = 0.54 t CO2; FA portion: 0.2 × 0.03 = 0.006t CO2; BC section: 0.2 × (-1.0) = -0.20t CO2 (carbon sequestration); Total carbon emissions = 0.54 + 0.006 - 0.20 = 0.346 t CO2; Compared to the same weight of pure cement, each ton of cementitious material from Example 1 reduces carbon emissions by approximately 0.554 tons, a reduction rate of approximately 61.6%. If the long-term carbon sequestration effect of biochar in the material is considered, the actual carbon reduction effect is even more significant (e.g., Figure 8 As shown; Figure 8A comparison chart of carbon emissions from pure cement, sample S2, and the ordinary Portland cement, fly ash, and biochar used in sample S2.

[0132] (2) Carbon efficiency analysis: Since the cementitious materials of groups S1-S12 have the same composition, their carbon emissions are completely consistent, all at 0.346t CO2 / t. The core contribution of this invention is that, under the premise of achieving the same low-carbon goal (replacing 40% of cement), by selecting the pretreatment method, the 28-day compressive strength can be increased from 21.60MPa (group S7) to 47.08MPa (group S2), an increase of 118%.

[0133] Carbon efficiency is defined as the intensity generated per unit of carbon emissions, and the calculation formula is: , where η CE For carbon efficiency ( ), σ 28d The 28-day compressive strength (MPa) is given by E, where E is the carbon emission per unit of cementitious material (MPa). ).

[0134] The calculation results of carbon efficiency are as follows: Figure 9 As shown.

[0135] The calculation results show that the carbon efficiency of sample S2 (Example 1) is 136.07 MPa / (t CO2-eq), while the carbon efficiency of overtreated sample S7 is 62.43 MPa / (t CO2-eq). The carbon efficiency of S2 is 2.2 times that of S7, indicating that pretreatment optimization can significantly improve the carbon efficiency of the material and achieve a synergistic balance between low carbon and high strength.

[0136] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A low-carbon cement-based composite material, characterized in that, The low-carbon cement-based composite material is composed of silicate cement, fly ash, alkali-modified biochar and mixing water; the fly ash is Class I fly ash that meets the requirements of GB / T 1596-2017 "Fly Ash for Cement and Concrete". The alkali-modified biochar was prepared by a method consisting of the following steps: A1. Mix and stir the biochar with the alkaline solution; the alkaline solution is a KOH solution with a concentration of 0.1 mol / L; A2. Solid-liquid separation, without any drying or heat treatment of the biochar obtained from the solid-liquid separation, to obtain the alkali-modified biochar; the alkali solution contained in the alkali-modified biochar accounts for 60%-70% of the mass of the biochar; The mixing water is the filtrate obtained from solid-liquid separation in step A2; The raw materials of the low-carbon cement-based composite material, by mass percentage, include: 50%-60% silicate cement, 20%-30% fly ash, and 20%-30% biochar.

2. The low-carbon cement-based composite material as described in claim 1, characterized in that, In step A1, the ratio of biochar to alkaline solution is 1g:2.4mL; Mixing and stirring should be carried out at room temperature for 10-15 minutes.

3. The low-carbon cement-based composite material as described in claim 1, characterized in that, In step A1, the biochar has a particle size of 0.5-75 μm.

4. The low-carbon cement-based composite material as described in claim 1, characterized in that, The fly ash is alkali-activated fly ash or alkali-activated dried fly ash. The alkaline activated fly ash is produced by the following steps: B1. Mix fly ash with alkaline solution and stir. B2. Solid-liquid separation to obtain the alkali-activated fly ash; the mass of the alkali solution contained in the alkali-activated fly ash is 25%-30% of the mass of the fly ash; The alkali-activated dry fly ash is obtained by drying the alkali-activated fly ash.

5. The low-carbon cement-based composite material as described in claim 1, characterized in that, The mass ratio of mixing water to silicate cement is 0.3-0.

6.

6. The low-carbon cement-based composite material as described in claim 1, characterized in that, The components of the low-carbon cement-based composite material also include water-reducing agents and defoamers; The amount of the water-reducing agent is 1%-2% of the total mass of silicate cement and fly ash; The amount of defoamer used is 0.02%-1% of the total mass of silicate cement and fly ash.

7. The method for preparing the low-carbon cement-based composite material according to any one of claims 1-6, characterized in that, Includes the following steps: (1) Mix the mixing water and the silicate cement evenly; (2) Add the alkali-modified biochar and stir until homogeneous; (3) Add the fly ash and mix evenly to obtain a mixture; (4) The mixture is injected into the mold to form a blank, and the blank is cured after demolding.

8. The method for preparing the low-carbon cement-based composite material as described in claim 7, characterized in that, In step (1), the water-reducing agent is first added to the mixing water and stirred evenly, and then the mixture of silicate cement and defoamer is added; In step (4), the maintenance method is standard maintenance.

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