A method for preparing shell-modified biochar cement-based materials

Abstract

A method for preparing shell-modified biochar cementitious materials is disclosed. This method aims to address the technical problems of low biochar content, decreased compressive strength at high biochar content, and poor durability in existing traditional biochar cementitious materials. The method comprises: 1. Pretreatment of agricultural and forestry waste and shell waste; 2. Preparation of shell-modified biochar; 3. Mixing 5-30 parts by weight of shell-modified biochar, 55-80 parts by weight of ASTM Type I ordinary Portland cement, and 15 parts by weight of metakaolin, followed by mixing with 40 parts by weight of water to obtain a slurry; then molding and curing to obtain the final product. The compressive strength of this cementitious material is 39-60 MPa, which is 8%-58.5% higher than that of traditional biochar cementitious materials; the water absorption rate is 17%-18.5%, which is 12.5%-15% lower than that of traditional biochar cementitious materials, making it suitable for use in road and building materials.

Description

Technical Field

[0001] This invention relates to a method for preparing biochar cement-based materials, which belongs to the field of road and building materials. Background Technology

[0002] With the accelerating pace of urbanization, the increasing infrastructure construction has led to a continuous rise in the consumption of building materials. The production of building materials such as cement is often accompanied by high CO2 emissions.

[0003] Biochar can be prepared by pyrolysis of widely available biomass waste under limited oxygen conditions. Replacing part of the cement in the preparation of building materials with biochar can effectively reduce the carbon footprint of building materials. On the one hand, it can reduce the use of high-carbon-emission cement; on the other hand, biochar itself has a strong carbon sequestration capacity, which can solve the greenhouse gas emission problem of biomass waste. Currently, there are methods for replacing cement or coarse and fine aggregates with biochar in the preparation of cement-based materials and other building materials. However, existing methods have problems such as limited biochar addition, decreased compressive strength of materials at high addition levels, and poor material durability. The addition of large amounts of biochar with rich pore structure can introduce voids into cement-based materials, which are detrimental to the overall strength of the material. In addition, there are weak areas at the interface between biochar and cement. The weak bonding force between biochar and cement reduces the compressive strength of the material and makes it susceptible to external ion penetration and erosion. Summary of the Invention

[0004] This invention aims to address the technical problems of existing biochar-added cementitious materials, such as low biochar content, decreased compressive strength at high biochar content, and poor material durability. It provides a method for preparing high-compressive-strength, high-durability biochar-based cementitious materials modified with shell waste. This invention uses shell waste as a biochar modifier and employs a mechanochemical method to modify the biochar. The resulting biochar-based cementitious material exhibits high biochar content, high compressive strength, high interfacial strength, and high chloride ion immobilization capacity.

[0005] The preparation method of the shell-modified biochar cement-based material of the present invention is carried out according to the following steps:

[0006] Step 1: Pre-treatment of agricultural and forestry waste and shell waste: Dry the agricultural and forestry waste at a temperature of 75-85℃, and then crush it into fragments of less than 5mm to obtain agricultural and forestry waste powder; Wash the shell waste, dry it at a temperature of 75-85℃, and then crush it into powder to obtain shell waste powder.

[0007] Step 2: Preparation of shell-modified biochar: Mix shell waste powder with agricultural and forestry waste powder at a mass ratio of 1:(4-9) and place them in a ball mill jar. Ball mill the mixture at 200-800 rpm for 1-6 hours to obtain mixed powder. Then place the mixed powder in a high-temperature furnace and pyrolyze it at 800-900℃ under a nitrogen atmosphere for 2-6 hours. After natural cooling, shell-modified biochar is obtained.

[0008] Step 3: Molding and curing of shell-modified biochar cement-based material: Mix 5-30 parts of shell-modified biochar, 55-80 parts of ASTM Type I ordinary Portland cement, and 15 parts of metakaolin evenly according to the mass ratio, then add 40 parts of tap water and stir evenly to obtain a slurry; place the slurry in a mold for pre-curing, demold, and then cure normally to obtain shell-modified biochar cement-based material.

[0009] Furthermore, the agricultural and forestry waste mentioned in step one is one or a combination of several of waste wood chips, waste rice husks, and straw.

[0010] Furthermore, the shell waste mentioned in step one refers to one or more of the following: discarded oyster shells, clam shells, scallop shells, and mussel shells.

[0011] Furthermore, the pre-curing described in step three involves curing for 20 to 24 hours in an environment with a temperature of 20±5℃ and a humidity of 60±5%.

[0012] Furthermore, the normal maintenance described in step three involves maintaining the environment at a temperature of 20±2℃ and a humidity of 60% to 95% for 28 to 35 days.

[0013] The beneficial effects of this invention are:

[0014] 1. The cement-based material prepared by this patent can make extensive use of biochar prepared from waste biomass and shell waste that urgently needs to be properly disposed of in the aquatic industry, realizing the green disposal and high-value recycling of the two types of waste, reducing the use of high-carbon-emission cement materials, and reducing CO2 emissions from the production of building materials.

[0015] 2. Modified biochar from seashell waste promotes the hydration rate and degree of cement, fosters the development of highly bonded long-chain CSH gel networks, and enhances the compressive strength of cement-based materials with high biochar content.

[0016] 3. Modified biochar from seashell waste can enhance the micromechanical properties of the biochar-cement interface zone. By increasing the surface roughness of biochar, it strengthens the "interlocking" mechanism and bonding with cement. At the same time, high-calcium minerals can regulate the Zeta potential of the biochar surface, reduce the negative charge density, and promote the bonding with CSH gel.

[0017] 4. Cement-based materials reinforced with modified biochar from seashell waste have higher density and effectively inhibit the erosion of external solutions.

[0018] 5. Cement-based materials reinforced with modified biochar from seashell waste have a strong chloride ion fixation capacity, which can protect the internal steel bars from chloride ion corrosion, reduce the risk of steel bar corrosion, and increase the durability of building materials.

[0019] The compressive strength of the shell-modified biochar cement-based material of this invention reaches 39-60 MPa, which is 8%-58.5% higher than that of biochar cement-based materials with the same proportion of traditional biochar. The water absorption rate of this cement-based material is 17%-18.5%, which is 12.5%-15% lower than that of biochar cement-based materials with the same proportion of traditional biochar. It can be used in the fields of road and building materials. Attached Figure Description

[0020] Figure 1 These are the XRD patterns of the shell-modified biochar prepared in step two of Examples 1-4;

[0021] Figure 2 These are thermogravimetric analysis curves of the shell-modified biochar prepared in step two of Examples 1-4;

[0022] Figure 3 This is a diagram showing the mineral composition of the biochar surface in the shell-modified biochar prepared in step two of Example 1;

[0023] Figure 4 This is a correlation analysis diagram of the micromechanical properties at the transition zone between biochar and cement in the shell-modified biochar cement-based materials prepared in Examples 1-4.

[0024] Figure 5 These are scanning electron microscope images of the shell-modified biochar prepared in step two of Example 1 and the conventional biochar prepared in Comparative Example 1.

[0025] Figure 6 This is a graph showing the relationship between the compressive strength of the shell-modified biochar cement-based materials prepared in Examples 1, 5-8 and the traditional biochar cement-based materials in Comparative Examples 1-5 and the amount of biochar added.

[0026] Figure 7 These are hydration heat test diagrams of the shell-modified biochar cement-based material prepared in Example 1 and the conventional biochar cement-based material prepared in Comparative Example 1.

[0027] Figure 8 This is a comparison diagram of the micromechanical properties of the shell waste-modified biochar cement-based material prepared in Example 1 and the traditional biochar cement-based material prepared in Comparative Example 1 at the biochar-cement interface. Detailed Implementation

[0028] The embodiments of the present invention will be described in detail below. It should be emphasized that the following description is merely exemplary and is not intended to limit the scope and application of the present invention.

[0029] Example 1: The preparation method of the high compressive strength and high durability biochar cement-based material modified from seashell waste in this example is carried out according to the following steps:

[0030] Step 1: Pre-treatment of agricultural and forestry waste and shell waste: The recycled waste locust branches are dried at 80°C and then crushed into fragments smaller than 5mm to obtain locust branch powder; the recycled waste oyster shells from oyster farming farms are cleaned, dried at 80°C, and then crushed into powder to obtain oyster shell powder.

[0031] Step 2: Preparation of shell-modified biochar: 720g of acacia branch powder and 80g of oyster shell powder were mixed and divided equally into four 500mL nylon ball mill jars containing 900g of 95% zirconia beads. The mixture was ball-milled at 400rpm for 1 hour using a planetary ball mill to ensure that the oyster shell powder and acacia branch powder were fully combined to obtain a mixed powder. The mixed powder was then placed in a high-temperature furnace and pyrolyzed at 800℃ for 2 hours under a nitrogen atmosphere. After natural cooling, the shell-modified biochar was obtained, denoted as 10SBC.

[0032] Step 3: Molding and curing of shell-modified biochar cement-based material: Add 0.1 kg of shell-modified biochar, 1.6 kg of ASTM Type I ordinary Portland cement purchased from Green Island Cement Company in Hong Kong, and 0.3 kg of metakaolin to a mixer, mix evenly, then add 800 mL of tap water and stir for 5 minutes to obtain a slurry; place the slurry in a 50×50×50 cm mold, pre-cur it for 24 hours in an environment with a temperature of 20±5℃ and a humidity of 60±5%, and after demolding, transfer it to an environment with a temperature of 20±2℃ and a humidity of 95% for 28 days to obtain shell-modified biochar cement-based material.

[0033] Example 2: This example differs from Example 1 in that step two is replaced with the following: 760g of acacia branch powder and 40g of oyster shell powder are mixed and divided equally into four 500mL nylon ball mill jars containing 900g of 95% zirconia beads. The mixture is then ball-milled at 400rpm for 1 hour using a planetary ball mill to ensure that the oyster shell powder and acacia branch powder are fully combined to obtain a mixed powder. The mixed powder is then placed in a high-temperature furnace and pyrolyzed at 800℃ for 2 hours under a nitrogen atmosphere. After natural cooling, shell-modified biochar is obtained, denoted as 5SBC. The remaining steps are the same as in Example 1.

[0034] Example 3: This example differs from Example 1 in that step two is replaced with the following: 640g of acacia branch powder and 160g of oyster shell powder are mixed and divided equally into four 500mL nylon ball mill jars containing 900g of 95% zirconia beads. The mixture is then ball-milled at 400rpm for 1 hour using a planetary ball mill to ensure thorough bonding between the oyster shell powder and the acacia branch powder, resulting in a mixed powder. The mixed powder is then placed in a high-temperature furnace and pyrolyzed at 800℃ for 2 hours under a nitrogen atmosphere. After natural cooling, the shell-modified biochar is obtained, designated as 20SBC. The remaining steps are the same as in Example 1.

[0035] Example 4: This example differs from Example 1 in that step two is replaced with the following: 400g of acacia branch powder and 400g of oyster shell powder are mixed and divided equally into four 500mL nylon ball mill jars containing 900g of 95% zirconia beads. The mixture is then ball-milled at 400rpm for 1 hour using a planetary ball mill to ensure thorough bonding between the oyster shell powder and the acacia branch powder, resulting in a mixed powder. The mixed powder is then placed in a high-temperature furnace and pyrolyzed at 800℃ for 2 hours under a nitrogen atmosphere. After natural cooling, the shell-modified biochar is obtained, designated as 50SBC. The remaining steps are the same as in Example 1.

[0036] Control group: Traditional biochar was used as a control. The preparation method of traditional biochar was carried out according to the following steps.

[0037] Step 1: Pre-treatment of agricultural and forestry waste: The recycled waste locust branches are dried at a temperature of 80℃, and then crushed into fragments of less than 5mm to obtain locust branch powder;

[0038] Step 2: Preparation of traditional biochar: 720g of locust branch powder was divided into four 500mL nylon ball mill jars containing 900g of 95% zirconia beads. The mixture was ball milled at 400rpm for 1 hour using a planetary ball mill to obtain fine locust branch powder. The fine locust branch powder was then placed in a high-temperature furnace and pyrolyzed at 800℃ for 2 hours under a nitrogen atmosphere. After natural cooling, traditional biochar was obtained.

[0039] The XRD patterns obtained by X-ray diffraction (XRD) spectroscopy of the shell-modified biochar prepared in step two of Examples 1-4 are shown below. Figure 1 As shown, the shell-modified biochar prepared in step two of Examples 1-4 and the conventional biochar as a control were subjected to thermogravimetric analysis. The thermogravimetric analysis curves are shown below. Figure 2 As shown, from Figure 1 It can be seen that the main crystalline minerals on the shell-modified biochar are CaCO3, Ca(OH)2, CaO, and SiO2 crystals. From... Figure 2It can be seen that, in addition to calcium carbonate crystals, amorphous calcium carbonate also exists on the surface of the shell-modified biochar. The mineral composition of the shell-modified biochar prepared in step two of Example 1 and the biochar surface of traditional biochar are as follows: Figure 3 As shown in Table 1, the specific data is as follows.

[0040] Table 1. Mineral composition of shell-modified biochar surface

[0041]

[0042] Correlation analysis was performed on the micromechanical properties of the biochar-cement interface transition zone of the shell-modified biochar cement-based materials prepared in Examples 1-4 using mineral composition and nanoindentation tests on the biochar surface. The obtained correlation analysis curves are shown below. Figure 4 As shown, from Figure 4 It can be seen that the content of amorphous calcium carbonate is positively correlated with the content of high-density CSH gel and negatively correlated with the content of low-density CSH gel. This indicates that amorphous calcium carbonate is beneficial to the formation of a denser CSH structure in the relatively weak interfacial transition zone of biochar cement composites, thereby enhancing the mechanical properties of the material.

[0043] Example 5: This example differs from Example 1 in that step three is replaced by the following steps: 0.2 kg of shell-modified biochar, 1.5 kg of ASTM Type I ordinary Portland cement purchased from Green Island Cement Company in Hong Kong, and 0.3 kg of metakaolin are added to a mixer and mixed evenly. Then, 800 mL of tap water is added and stirred for 5 minutes to obtain a slurry. The slurry is placed in a 50×50×50 cm mold and pre-cured for 24 hours in an environment with a temperature of 20±5℃ and a humidity of 60±5%. After demolding, it is transferred to an environment with a temperature of 20±2℃ and a humidity of 95% for 28 days. Everything else is the same as in Example 1 to obtain a shell-modified biochar cement-based material.

[0044] Example 6: This example differs from Example 1 in that step three is replaced by the following steps: 0.3 kg of shell-modified biochar, 1.4 kg of ASTM Type I ordinary Portland cement purchased from Green Island Cement Company in Hong Kong, and 0.3 kg of metakaolin are added to a mixer and mixed evenly. Then, 800 mL of tap water is added and stirred for 5 minutes to obtain a slurry. The slurry is placed in a 50×50×50 cm mold and pre-cured for 24 hours in an environment with a temperature of 20±5℃ and a humidity of 60±5%. After demolding, it is transferred to an environment with a temperature of 20±2℃ and a humidity of 95% for 28 days. Everything else is the same as in Example 1 to obtain a shell-modified biochar cement-based material.

[0045] Example 7: This example differs from Example 1 in that step three is replaced by the following steps: Add 0.4 kg of shell-modified biochar, 1.3 kg of ASTM Type I ordinary Portland cement purchased from Green Island Cement Company in Hong Kong, and 0.3 kg of metakaolin to a mixer, mix evenly, then add 800 mL of tap water and stir for 5 minutes to obtain a slurry; place the slurry in a 50×50×50 cm mold, pre-cur it for 24 hours in an environment with a temperature of 20±5℃ and a humidity of 60±5%, and after demolding, transfer it to an environment with a temperature of 20±2℃ and a humidity of 95% for 28 days; everything else is the same as in Example 1 to obtain shell-modified biochar cement-based material.

[0046] Example 8: This example differs from Example 1 in that step three is replaced by the following steps: 0.6 kg of shell-modified biochar, 1.1 kg of ASTM Type I ordinary Portland cement purchased from Green Island Cement Company in Hong Kong, and 0.3 kg of metakaolin are added to a mixer and mixed evenly. Then, 800 mL of tap water is added and stirred for 5 minutes to obtain a slurry. The slurry is placed in a 50×50×50 cm mold and pre-cured for 24 hours in an environment with a temperature of 20±5℃ and a humidity of 60±5%. After demolding, it is transferred to an environment with a temperature of 20±2℃ and a humidity of 95% for 28 days. The rest is the same as in Example 1 to obtain shell-modified biochar cement-based material.

[0047] Comparative Example 1: This comparative example demonstrates a method for preparing biochar cement-based materials using traditional biochar methods, specifically following these steps:

[0048] Step 1: Pre-treatment of agricultural and forestry waste: The recycled waste locust branches are dried at a temperature of 80℃, and then crushed into fragments of less than 5mm to obtain locust branch powder;

[0049] Step 2: Preparation of traditional biochar: 720g of locust branch powder was divided into four 500mL nylon ball mill jars containing 900g of 95% zirconium oxide beads, and ball milled at 400rpm for 1 hour using a planetary ball mill to obtain fine locust branch powder; then the fine locust branch powder was placed in a high-temperature furnace and pyrolyzed at 800℃ for 2 hours under a nitrogen atmosphere, and then naturally cooled to obtain traditional biochar;

[0050] Step 3: Molding and curing of biochar cement-based material: Add 0.1 kg of traditional biochar, 1.6 kg of ASTM Type I ordinary Portland cement purchased from Green Island Cement Company in Hong Kong, and 0.3 kg of metakaolin to a mixer, mix evenly, then add 800 mL of tap water and stir for 5 minutes to obtain a slurry; place the slurry in a 50×50×50 cm mold, pre-cur it for 24 hours in an environment with a temperature of 20±5℃ and a humidity of 60±5%, and after demolding, transfer it to an environment with a temperature of 20±2℃ and a humidity of 95% for 28 days to obtain traditional biochar cement-based material.

[0051] Comparative Example 2: This comparative example differs from Comparative Example 1 in that the operation in step three is replaced by the following steps: Add 0.2 kg of traditional biochar, 1.5 kg of ASTM Type I ordinary Portland cement purchased from Green Island Cement Company in Hong Kong, and 0.3 kg of metakaolin to a mixer, mix evenly, then add 800 mL of tap water and stir for 5 minutes to obtain a slurry; place the slurry in a 50×50×50 cm mold, pre-cur it for 24 hours in an environment with a temperature of 20±5℃ and a humidity of 60±5%, and after demolding, transfer it to an environment with a temperature of 20±2℃ and a humidity of 95% for 28 days to obtain a traditional biochar cement-based material.

[0052] Comparative Example 3: This comparative example differs from Comparative Example 1 in that the operation in step three is replaced by the following steps: Add 0.3 kg of traditional biochar, 1.4 kg of ASTM Type I ordinary Portland cement purchased from Green Island Cement Company in Hong Kong, and 0.3 kg of metakaolin to a mixer, mix evenly, then add 800 mL of tap water and stir for 5 minutes to obtain a slurry; place the slurry in a 50×50×50 cm mold, pre-cur it for 24 hours in an environment with a temperature of 20±5℃ and a humidity of 60±5%, and after demolding, transfer it to an environment with a temperature of 20±2℃ and a humidity of 95% for 28 days to obtain a traditional biochar cement-based material.

[0053] Comparative Example 4: This comparative example differs from Comparative Example 1 in that the operation in step three is replaced by the following steps: Add 0.4 kg of traditional biochar, 1.6 kg of ASTM Type I ordinary Portland cement purchased from Green Island Cement Company in Hong Kong, and 0.3 kg of metakaolin to a mixer, mix evenly, then add 800 mL of tap water and stir for 5 minutes to obtain a slurry; place the slurry in a 50×50×50 cm mold, pre-cur it for 24 hours in an environment with a temperature of 20±5℃ and a humidity of 60±5%, and after demolding, transfer it to an environment with a temperature of 20±2℃ and a humidity of 95% for 28 days to obtain a traditional biochar cement-based material.

[0054] Comparative Example 5: This comparative example differs from Comparative Example 1 in that the operation in step three is replaced by the following steps: Add 0.6 kg of traditional biochar, 1.1 kg of ASTM Type I ordinary Portland cement purchased from Green Island Cement Company in Hong Kong, and 0.3 kg of metakaolin to a mixer, mix evenly, then add 800 mL of tap water and stir for 5 minutes to obtain a slurry; place the slurry in a 50×50×50 cm mold, pre-cur it for 24 hours in an environment with a temperature of 20±5℃ and a humidity of 60±5%, and after demolding, transfer it to an environment with a temperature of 20±2℃ and a humidity of 95% for 28 days to obtain a traditional biochar cement-based material.

[0055] The shell-modified biochar prepared in step two of Example 1 and the traditional biochar prepared in Comparative Example 1 were subjected to scanning electron microscopy (SEM) tests. The obtained SEM images are shown below. Figure 5 As shown, from Figure 5 It can be seen that, compared with traditional biochar, shell-modified biochar has a rougher surface and incorporates granular calcium-rich minerals.

[0056] The compressive strength of the shell-modified biochar cement-based materials prepared in Examples 1, 5-8 and the conventional biochar cement-based materials in Comparative Examples 1-5 were tested using a concrete compression testing machine. The compressive strength values ​​of the specimens are shown in Table 2, and the relationship between the compressive strength value and the amount of biochar added is shown in Figure 6.

[0057] Table 2 Comparison of compressive strength between shell-modified biochar cement-based materials and traditional biochar cement-based materials

[0058]

[0059] From Table 2 and Figure 6 The data shows that, under the same biochar addition, the compressive strength of the shell waste-modified biochar cement-based material is higher than that of the traditional biochar cement-based material. Under the conditions of biochar addition of 5%, 10%, 15%, 20%, and 30%, the compressive strength of the shell waste-modified biochar cement-based material is increased by 17.1%, 8.0%, 9.7%, 42.7%, and 58.5% respectively compared with the traditional biochar cement-based material. This indicates that shell-modified biochar strengthens the compressive strength of the cement-based material, and the strengthening effect is particularly prominent at high addition levels.

[0060] To compare the hydration degree of cement based on shell waste modified biochar with that of traditional biochar cement, the heat of hydration of the two materials prepared in Example 1 and Comparative Example 1 with a 5% biochar addition was measured. The results are as follows: Figure 7 As shown, the biochar cement-based material modified from seashell waste has a higher cumulative heat of hydration, indicating a higher degree of hydration.

[0061] To compare the durability of shell waste-modified biochar cement-based materials with traditional biochar cement-based materials, the water absorption rate of the two materials prepared in Example 1 and Comparative Example 1 with a 5% biochar content was measured. Before measuring the water absorption rate, the materials were dried at 60°C for more than 14 days, and then immersed in water for 3 days. The water absorption rate was determined by gravimetric method. The results showed that the water absorption rate of the shell waste-modified biochar cement-based material prepared in Example 1 was 17.5%, while the water absorption rate of the traditional biochar cement-based material prepared in Comparative Example 1 was 22.2%. The water absorption rate of the shell waste-modified biochar cement-based material was 4.7% lower than that of the traditional biochar cement-based material, indicating that it has fewer pores suitable for water permeation, thus reducing external ion penetration and erosion.

[0062] To compare the micromechanical properties of the shell waste-modified biochar cement-based material prepared in Example 1 with the traditional biochar cement-based material prepared in Comparative Example 1 at the biochar-cement interface, nanoindentation technology was used to perform indentation tests on multiple regions at the biochar-cement interface. The magnitude and distribution of the material's elastic modulus are shown in the figure below. Figure 8 As shown, from Figure 8 It can be seen that, compared with traditional biochar cement-based materials, the volume ratio of high-density CSH at the biochar-cement interface in shell waste modified biochar cement-based materials increased by 29% and the volume ratio of low-density CSH decreased by 32%, effectively enhancing the micromechanical properties at the interface.

[0063] Comparative Example 6: To compare the compressive strength and durability of the shell waste-modified biochar cement-based material prepared by the mechanochemical method with that of the physically mixed shell biochar cement-based material, this comparative example prepared a physically mixed shell biochar cement-based material. The specific preparation method is as follows:

[0064] 1. The recycled waste locust branches are dried at 80℃ and then crushed into fragments smaller than 5mm to obtain locust branch powder; the recycled waste oyster shells from oyster shell farms are cleaned, dried at 80℃, and then crushed into powder to obtain oyster shell powder.

[0065] 2. Pyrolysis of 720g of locust branch powder and 80g of oyster shell powder was carried out separately at 800℃ under nitrogen atmosphere for 2 hours. After natural cooling, they were thoroughly mixed to obtain a shell biochar physical mixture powder.

[0066] 3. Add 0.1 kg of shell biochar physical mixing powder, 1.6 kg of ASTM Type I ordinary Portland cement purchased from Hong Kong Green Island Cement Company, and 0.3 kg of metakaolin to the mixer, mix evenly, then add 800 mL of tap water and stir for 5 minutes to obtain a slurry; place the slurry in a 50×50×50 cm mold, pre-cur it for 24 hours in an environment with a temperature of 20±5℃ and a humidity of 60±5%, and after demolding, transfer it to an environment with a temperature of 20±2℃ and a humidity of 95% for 28 days to obtain a physically mixed shell biochar cement-based material.

[0067] The compressive strength and water absorption of the shell waste-modified biochar cement-based material prepared in Example 1 and the physically mixed shell biochar cement-based material prepared in Comparative Example 6 were tested. The test results are shown in Table 3.

[0068] Table 3 shows the compressive strength and water absorption of the materials prepared in Example 1 and Comparative Example 6.

[0069] Example 1 60 17.5 Comparative Example 6 56 20.0

[0070] As can be seen from the data in Table 3, the shell biochar prepared by the mechanochemical method in Example 1 has higher compressive strength and lower water absorption, which can reduce ion penetration and erosion. Therefore, the mechanochemical preparation method used in this invention is more efficient.

[0071] To compare the chloride ion fixation capabilities of the shell waste-modified biochar cement-based material and the traditional biochar cement-based material, the shell waste-modified biochar cement-based material prepared in Example 1 and the traditional biochar cement-based material prepared in Comparative Example 1 were crushed into particles that could pass through a 200-mesh sieve. The particles were then soaked in water for 60 days to ensure full hydration. Subsequently, 1 g of each material was soaked in 20 mL of a 420 mM NaCl solution at room temperature for 60 days, and the chloride ion fixation amount was measured. The results showed that the chloride ion fixation amount of the shell waste-modified biochar cement-based material prepared in Example 1 was 13.9%, while that of the traditional biochar cement-based material prepared in Comparative Example 1 was 10.5%. The chloride ion fixation amount of the shell waste-modified biochar cement-based material was 32.4% higher than that of the traditional biochar cement-based material. Therefore, the shell waste-modified biochar cement-based material prepared in Example 1 can effectively fix chloride ions and protect reinforcing steel bars from chloride ion corrosion.

[0072] This invention utilizes shells, a common waste material in the aquaculture and seafood processing industries, as raw materials. Through mechanochemical modification, biochar is obtained by enhancing the interfacial bonding between biochar and cementitious matrix, resulting in a novel shell-modified biochar. This leads to the preparation of biochar-based cementitious materials with high compressive strength and durability. This invention improves the compressive strength of the material by enhancing the high compatibility between biochar and cementitious materials and improving the interfacial bonding performance, while increasing the biochar content. Furthermore, the addition of metakaolin to activate the pozzolanic reaction with the surface-active Ca(OH)2 of the shell-modified biochar further strengthens the micromechanical properties of the interface. Simultaneously, the shell-modified biochar fills and improves the voids in cementitious materials and regulates hydration products, reducing water absorption and enhancing the fixation capacity for chloride ions, thus reducing the risk of steel reinforcement corrosion and increasing the durability of building materials.

Claims

1. A method for preparing a seashell-modified biochar cement-based material, characterized in that... This method is performed in the following steps: Step 1: Pre-treatment of agricultural and forestry waste and shell waste: Dry the agricultural and forestry waste at a temperature of 75-85℃, and then crush it into fragments of less than 5mm to obtain agricultural and forestry waste powder; Wash the shell waste, dry it at a temperature of 75-85℃, and then crush it into powder to obtain shell waste powder. Step 2: Preparation of shell-modified biochar: Mix shell waste powder with agricultural and forestry waste powder at a mass ratio of 1:(4-9) and place them in a ball mill jar. Ball mill the mixture at 200-800 rpm for 1-6 hours to obtain mixed powder. Then place the mixed powder in a high-temperature furnace and pyrolyze it at 800-900℃ under a nitrogen atmosphere for 2-6 hours. After natural cooling, shell-modified biochar is obtained. Step 3: Molding and curing of shell-modified biochar cement-based material: Mix 5-30 parts of shell-modified biochar, 55-80 parts of ASTM Type I ordinary Portland cement, and 15 parts of metakaolin evenly according to the mass ratio, then add 40 parts of tap water and stir evenly to obtain a slurry; place the slurry in a mold for pre-curing, demold, and then cure normally to obtain shell-modified biochar cement-based material.

2. The method for preparing a seashell-modified biochar cement-based material according to claim 1, characterized in that, The agricultural and forestry waste mentioned in step one is one or a combination of several of the following: waste wood chips, waste rice husks, and straw.

3. A method for preparing a seashell-modified biochar cement-based material according to claim 1 or 2, characterized in that, The shell waste mentioned in step one refers to one or a combination of several types of waste oyster shells, clam shells, scallop shells, and mussel shells.

4. A method for preparing a shell-modified biochar cement-based material according to claim 1 or 2, characterized in that, The pre-curing described in step three involves curing for 20 to 24 hours in an environment with a temperature of 20±5℃ and a humidity of 60±5%.

5. A method for preparing a seashell-modified biochar cement-based material according to claim 1 or 2, characterized in that, The normal maintenance described in step three involves maintaining the environment at a temperature of 20±2℃ and a humidity of 60% to 95% for 28 to 35 days.

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