Method for preparing composite cementitious material by incorporating biochar into red mud-phosphogypsum- blast furnace slag and application thereof

Abstract

The application discloses a method for preparing a composite cementing material by mixing biochar into red mud-phosphogypsum-blast furnace slag and application, and the biochar obtained through pyrolysis is pre-soaked in water; the red mud, the phosphogypsum and the blast furnace slag are dried, ground and sieved respectively to obtain red mud particles, phosphogypsum particles and blast furnace slag powder; the biochar, the red mud particles, the phosphogypsum particles, the blast furnace slag powder, water and an activator are fully stirred to obtain the composite cementing material. The biochar, the red mud, the phosphogypsum and the blast furnace slag can be converted into the composite cementing material with good durability and mechanical properties, and the maximum consumption and resource utilization of the red mud and the phosphogypsum bulk solid wastes are realized. Compared with the prior art, the application has the advantages of low cost, simple operation and the ability to meet the road traffic requirements, and the red mud and the phosphogypsum can be recycled, harmless and reduced, the goal of synergistic effect of pollution reduction and carbon reduction is achieved, and good economic benefits, social benefits and environmental benefits are obtained.

Description

Technical Field

[0001] The invention relates to a method for preparing a composite cementitious material by adding biochar into red mud-phosphogypsum-blast furnace slag and application thereof, belonging to the technical field of environmentally friendly cementitious materials. Background Art

[0002] The construction industry, a major resource consumer, is a major contributor to natural resource depletion, energy consumption, and high carbon emissions. Therefore, the production of low-carbon, environmentally friendly building materials is becoming increasingly important. With the advancement of waste resource utilization, research on the use of solid waste to prepare cementitious materials is increasing. Red mud and phosphogypsum are produced in large quantities and are insoluble in water. The high content of SiO₂, Al₂O₃, and CaO in these materials facilitates the formation of hydration products. However, due to the high levels of free and dissolved alkali in red mud, the preparation of cementitious materials often requires high pretreatment costs. The low activity of red mud leads to low utilization rates (generally less than 20%), and the soluble phosphorus and fluorine in phosphogypsum result in a long setting time for the cementitious materials. Therefore, the addition of blast furnace slag, biochar, and an activator, which have similar compositions to cement, to red mud and phosphogypsum can provide sufficient cementitious activity, fully decompose the aluminosilicates in the red mud, release large amounts of bound alkali, aluminum, and silicon, promote hydration reactions, mitigate the negative effects of phosphogypsum, and fix sodium to alleviate the efflorescence problem.

[0003] For pavement construction materials, their mechanical properties and stability directly impact their service life and quality. Currently, commonly used pavement materials are primarily non-renewable resources such as asphalt, lime, cement, and sand and gravel. With the expansion of urbanization, the artificial use of surfaces (such as paving) is also increasing. This leads to an imbalance in the transfer of moisture and heat between the ground and the atmosphere, creating a heat island effect.

[0004] Currently, cementitious materials are typically prepared in China using a combination of red mud and phosphogypsum. The combined amount of red mud and phosphogypsum used in this process generally does not exceed 50%. The resulting cementitious materials take a long time to set, and their performance is not as good as expected. Summary of the Invention

[0005] In order to alleviate the problems of high stockpiles, low utilization rates and poor cementing activity of red mud and phosphogypsum, the present invention provides a method and application of preparing a composite cementitious material by adding biochar to red mud-phosphogypsum-blast furnace slag. Blast furnace slag, biochar and an activator are used to optimize red mud and phosphogypsum to improve various performance indicators of the composite cementitious material, providing a new method for the resource utilization of red mud and phosphogypsum, while achieving a certain mechanical strength and removal effect while reducing production costs.

[0006] The technical solutions of the present invention are as follows:

[0007] (1) Tobacco waste is dried, pyrolyzed, ground, and sieved to obtain biochar particles.

[0008] (2) Red mud, phosphogypsum and blast furnace slag are dried, ground and sieved to obtain red mud particles, phosphogypsum particles and blast furnace slag powder.

[0009] (3) Pre-soaking the biochar particles obtained in step (1) to a saturated state.

[0010] (4) Mixing the red mud particles, phosphogypsum particles, blast furnace slag powder, and saturated biochar particles obtained in step (2), adding water and an activator, and stirring thoroughly to obtain a composite cementitious material.

[0011] Preferably, the drying condition in step (1) is drying at 105-120°C for 24-48 hours; the pyrolysis is heating the tobacco waste to 300-600°C at a heating rate of 5-10°C / min in a nitrogen atmosphere and then pyrolyzing at a constant temperature for 2-4 hours; the sieve is 100-300 mesh.

[0012] Preferably, the drying in step (2) is to dry the red mud, phosphogypsum and blast furnace slag in an electric constant temperature blast drying oven, wherein the drying temperature of the red mud is 105-120°C, the drying temperature of the phosphogypsum is 100-170°C, and the drying temperature of the blast furnace slag is 105-120°C, and the drying is carried out for 24-48 hours; the sieve is 60-100 mesh.

[0013] Preferably, the pre-soaking in step (3) is to stir the biochar and water at a mass ratio of 1:100 at 20±2° C. for 6 to 8 hours.

[0014] Preferably, the mass ratio of the red mud particles, phosphogypsum particles, blast furnace slag powder and saturated biochar in step (4) is 1-3:1-3:1-2:0.1-1.

[0015] Preferably, the amount of water used in step (4) is 40-60% of the mass of the solid mixture.

[0016] Preferably, the activator in step (4) is water glass with a modulus of 1.0 to 1.5.

[0017] Preferably, the amount of the activator in step (4) is 5-10% of the mass of the solid mixture.

[0018] Preferably, the stirring process in step (4) is carried out at room temperature, with a rotation speed of 300 to 450 rpm and a stirring time of 10 to 20 min.

[0019] The composite gelling material prepared by the method is used in public road traffic construction.

[0020] The principle of the invention is as follows: red mud is an alkaline solid waste, phosphogypsum is an acidic solid waste, the composition of blast furnace slag is similar to that of cement, and biochar above 300°C is weakly alkaline and has excellent water retention and adsorption properties. Combining the four to prepare a cementitious material can not only ensure that the product is a neutral product, but also meet environmental standards.

[0021] First, red mud and phosphogypsum will undergo acid-base neutralization reaction during the mixing process to reduce the alkalinity; secondly, since the active silicon and aluminum content in red mud is low, blast furnace slag can provide sufficient active silicon and aluminum elements to promote the formation of hydration products; finally, since the composition of red mud, phosphogypsum and blast furnace slag is relatively complex and contains certain polluting elements (heavy metals, fluorine, soluble phosphorus), in order to prevent the release of pollutants and pollute the environment, adding a certain amount of biochar can not only prevent the release of pollutants but also have an internal solidification effect.

[0022] Beneficial effects of the present invention

[0023] (1) The present invention uses water glass as an activator, which can not only activate the aluminum silicate in red mud, but also has better stability in water, making it more feasible in industrial production.

[0024] (2) The present invention pre-treats red mud, phosphogypsum and blast furnace slag to form a fine particle mixture, which is beneficial to increasing the nucleation sites of the hydration reaction of the cementitious material and promoting the formation of hydration products.

[0025] (3) The cementitious material obtained by the present invention has an initial setting time of ≥10 h, a final setting time of ≤21 h, an unconfined compressive strength of 3 to 5 MPa at 7 days, an unconfined compressive strength of 3.7 to 5.3 MPa at 14 days, and an unconfined compressive strength of 4 to 5.2 MPa at 28 days, indicating that the prepared cementitious material meets the strength requirements of the pavement base and subbase.

[0026] (4) The present invention can dispose of red mud and phosphogypsum to the greatest extent, providing a new approach for the resource utilization of solid waste and the reduction of environmental pollution caused by the storage of pollutants.

[0027] (5) Biochar prepared from tobacco waste can promote hydration reaction in the process of preparing cementitious materials, improve the workability of cementitious materials and enhance the mechanical properties of cementitious materials.

[0028] (6) The combination of red mud, phosphogypsum, blast furnace slag, and biochar can not only promote the formation of hydration products and improve the mechanical properties of cementitious materials, but also meet environmental requirements at a low cost.

[0029] (7) Saturated biochar reduces the absorption of water in the early stage. Sufficient water can promote the early hydration reaction. Biochar soaked to saturation can release water in the later stage of hydration, so that the hydration reaction proceeds from the inside to the outside and has the potential for internal solidification. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 A flow chart for the implementation of the present invention;

[0031] Figure 2 XRD pattern of red mud particles;

[0032] Figure 3 XRD pattern of phosphogypsum particles;

[0033] Figure 4 XRD pattern of composite cementitious materials;

[0034] Figure 5 Comparison of setting time of Examples 1-4;

[0035] Figure 6 Unconfined compressive strength of Examples 1-4. DETAILED DESCRIPTION

[0036] The present invention will be further described in detail with reference to the following examples. It should be understood that the specific examples described herein are intended only to illustrate the present invention and are not intended to limit the present invention. Where specific conditions are not specified in the examples, the experiments were carried out under conventional conditions or conditions recommended by the manufacturer. Where the manufacturer of the reagents or instruments is not specified, all are conventional products that can be purchased commercially.

[0037] Example 1

[0038] A method for preparing a composite cementitious material by adding biochar to red mud-phosphogypsum-blast furnace slag, such as Figure 1 As shown, the following steps are included:

[0039] (1) Tobacco waste from a tobacco company in Kunming, Yunnan Province was dried in an electric constant temperature forced air drying oven at 105°C for 24 h, then placed in a tube furnace and heated to 300°C at a heating rate of 5°C / min in a nitrogen atmosphere for constant temperature pyrolysis for 2 h, and ground in an agate mortar through a 100-mesh sieve to obtain biochar for use;

[0040] (2) The Bayer red mud produced by an aluminum company in Wenshan Prefecture, Yunnan Province was dried in an electric constant temperature blast drying oven at 105°C for 24 h, and then ground with an agate mortar and passed through a 60-mesh sieve to obtain red mud particles for later use;

[0041] (3) Phosphogypsum produced by a phosphogypsum company in Kunming, Yunnan Province was dried in an electric constant temperature blast drying oven at 100°C for 24 h, and ground with an agate mortar to obtain 60-mesh phosphogypsum particles for later use;

[0042] (4) Blast furnace slag produced by a pig iron smelting company in Yunnan Province was dried in an electric constant temperature blast drying oven at 105°C for 24 h, ground with an agate mortar and passed through a 60-mesh sieve to obtain blast furnace slag powder for later use;

[0043] (5) The biochar obtained in step (1) and water were placed in a beaker at a ratio of 1:100 and stirred on a magnetic stirrer at 20±2°C for 6 h. The mixture was filtered using a vacuum pump under a 0.45 μm microporous filter membrane until all the soaked biochar was collected on wet filter paper to obtain saturated biochar.

[0044] (6) The red mud particles obtained in step (2), the phosphogypsum particles obtained in step (3), the blast furnace slag powder obtained in step (4), and the saturated biochar in step (5) are mixed in a mass ratio of 1:1:1:0.1, the mass of water used is 40% of the total mass of the solid mixture, the mass of water glass (1.0 modulus) used is 5% of the total mass of the solid mixture, and the solid mixture, water and water glass are placed in a stirrer at room temperature and a rotation speed of 300 rpm and stirred for 10 minutes to obtain a composite cementitious material.

[0045] Example 2

[0046] A method for preparing a composite cementitious material by adding biochar to red mud-phosphogypsum-blast furnace slag, such as Figure 1 As shown, the following steps are included:

[0047] (1) Tobacco waste from a tobacco company in Kunming, Yunnan Province was dried in an electric constant temperature forced air drying oven at 110°C for 36 h, then placed in a tube furnace and heated to 400°C at a heating rate of 7°C / min in a nitrogen atmosphere for constant temperature pyrolysis for 3 h. The biochar was ground in an agate mortar and passed through a 200-mesh sieve to obtain biochar for later use.

[0048] (2) The Bayer red mud produced by an aluminum company in Wenshan Prefecture, Yunnan Province was dried in an electric constant temperature blast drying oven at 110°C for 36 h, and ground with an agate mortar and passed through an 80-mesh sieve to obtain red mud particles for later use;

[0049] (3) Phosphogypsum produced by a phosphogypsum company in Kunming, Yunnan Province was dried in an electric constant temperature blast drying oven at 120°C for 36 h, and ground with an agate mortar to 80 mesh to obtain phosphogypsum particles for later use;

[0050] (4) Blast furnace slag from a pig iron smelting company in Yunnan Province was dried in an electric constant temperature blast drying oven at 110°C for 36 h, and ground with an agate mortar and passed through an 80-mesh sieve to obtain blast furnace slag powder for later use;

[0051] (5) The biochar obtained in step (1) and water were placed in a beaker at a ratio of 1:100 and stirred on a magnetic stirrer at 20±2°C for 7 h. The mixture was filtered using a vacuum pump under a 0.45 μm microporous filter membrane until all the soaked biochar was collected on the wet filter paper to obtain saturated biochar;

[0052] (6) The red mud particles obtained in step (2), the phosphogypsum particles obtained in step (3), the blast furnace slag powder obtained in step (4), and the saturated biochar obtained in step (5) are mixed in a ratio of 2:2:1.5:0.5, the mass of water used is 50% of the total mass of the solid mixture, and the mass of water glass (1.25 modulus) used is 7.5t% of the total mass of the solid mixture. The solid mixture, water and water glass are placed in a stirrer at room temperature and a speed of 375 rpm and stirred for 15 minutes to obtain a composite cementitious material.

[0053] Example 3

[0054] A method for preparing a composite cementitious material by adding biochar to red mud-phosphogypsum-blast furnace slag, such as Figure 1 As shown, the following steps are included:

[0055] (1) Tobacco waste from a tobacco company in Kunming, Yunnan Province was dried in an electric constant temperature forced air drying oven at 120°C for 48 h, then placed in a tube furnace and heated to 500°C at a heating rate of 10°C / min in a nitrogen atmosphere for 4 h. The waste was ground in an agate mortar and passed through a 300-mesh sieve to obtain biochar for later use.

[0056] (2) The Bayer red mud produced by an aluminum company in Wenshan Prefecture, Yunnan Province was dried in an electric constant temperature blast drying oven at 120°C for 48 h, and ground with an agate mortar and passed through a 100-mesh sieve to obtain red mud particles for later use;

[0057] (3) Phosphogypsum produced by a phosphogypsum company in Kunming, Yunnan Province was dried in an electric constant temperature blast drying oven at 140°C for 48 h, and ground with an agate mortar to obtain 100 mesh phosphogypsum particles for later use;

[0058] (4) Blast furnace slag from a pig iron smelting company in Yunnan Province was dried in an electric constant temperature blast drying oven at 120°C for 48 h, and ground with an agate mortar and passed through a 100 mesh sieve to obtain blast furnace slag powder for later use;

[0059] (5) The biochar obtained in step (1) and water were placed in a beaker at a mass ratio of 1:100 and stirred on a magnetic stirrer at 20±2°C for 8 h. The mixture was filtered using a vacuum pump under a 0.45 μm microporous filter membrane until all the soaked biochar was collected on the wet filter paper to obtain saturated biochar;

[0060] (6) The red mud particles obtained in step (2), the phosphogypsum particles obtained in step (3), the blast furnace slag powder obtained in step (4), and the saturated biochar obtained in step (5) are mixed in a mass ratio of 3:3:2:1, wherein the mass of water used is 60% of the total mass of the solid mixture, and the mass of water glass (1.25 modulus) used is 10% of the total mass of the solid mixture. The solid mixture, water, and water glass are placed in a stirrer at room temperature and a speed of 450 rpm and stirred for 20 minutes to obtain a composite cementitious material.

[0061] Example 4

[0062] A method for preparing a composite cementitious material by adding biochar to red mud-phosphogypsum-blast furnace slag, such as Figure 1 As shown, the following steps are included:

[0063] (1) Tobacco waste from a tobacco company in Kunming, Yunnan Province was dried in an electric constant temperature forced air drying oven at 105°C for 48 h, then placed in a tube furnace and heated to 600°C at a heating rate of 5°C / min in a nitrogen atmosphere for constant temperature pyrolysis for 2 h, and ground in an agate mortar through a 200-mesh sieve to obtain biochar for use;

[0064] (2) The Bayer red mud produced by an aluminum company in Wenshan Prefecture, Yunnan Province was dried in an electric constant temperature blast drying oven at 105°C for 48 h, and ground with an agate mortar and passed through a 60-mesh sieve to obtain red mud particles for later use;

[0065] (3) Phosphogypsum produced by a phosphogypsum company in Kunming, Yunnan Province was dried in an electric constant temperature blast drying oven at 170°C for 24 h, and ground with an agate mortar to obtain 60-mesh phosphogypsum particles for later use;

[0066] (4) Blast furnace slag from a pig iron smelting company in Yunnan Province was dried in an electric constant temperature blast drying oven at 105°C for 48 hours, and ground with an agate mortar and passed through a 60-mesh sieve to obtain blast furnace slag powder for later use.

[0067] (5) The biochar obtained in step (1) and water were placed in a beaker at a mass ratio of 1:100 and stirred on a magnetic stirrer at 20±2°C for 6 h. The mixture was filtered using a vacuum pump under a 0.45 μm microporous filter membrane until all the soaked biochar was collected on the wet filter paper to obtain saturated biochar;

[0068] (6) The red mud particles obtained in step (2), the phosphogypsum particles obtained in step (3), the blast furnace slag powder obtained in step (4), and the saturated biochar obtained in step (5) are mixed in a mass ratio of 2:2:1.5:0.5, the mass of water used is 40% of the total mass of the solid mixture, and the mass of water glass (1.5 modulus) used is 5% of the total mass of the solid mixture. The solid mixture, water and water glass are placed in a stirrer at room temperature and a speed of 350 rpm and stirred for 10 minutes to obtain a composite cementitious material.

[0069] Comparative Example 1

[0070] The raw materials and conditions in the preparation process of this embodiment are the same as those in Example 1, except that no biomass carbon is added. Specifically,

[0071] (1) The Bayer red mud produced by an aluminum company in Wenshan Prefecture, Yunnan Province was dried in an electric constant temperature blast drying oven at 105°C for 24 h, and then ground with an agate mortar and passed through a 60-mesh sieve to obtain red mud particles for later use;

[0072] (2) Phosphogypsum produced by a phosphogypsum company in Kunming, Yunnan Province was dried in an electric constant temperature blast drying oven at 100°C for 24 h, and ground with an agate mortar to obtain 60-mesh phosphogypsum particles for later use;

[0073] (3) Blast furnace slag produced by a pig iron smelting company in Yunnan Province was dried at 105°C in an electric constant temperature blast drying oven for 24 hours and ground with an agate mortar through a 60-mesh sieve to obtain blast furnace slag powder for use.

[0074] (4) The red mud particles obtained in step (1), the phosphogypsum particles obtained in step (2), and the blast furnace slag powder obtained in step (3) are mixed in a mass ratio of 1:1:1, the mass of water used is 40% of the total mass of the solid mixture, and the mass of water glass (1.0 modulus) used is 5% of the total mass of the solid mixture. The solid mixture, water and water glass are placed in a stirrer at room temperature and a rotation speed of 300 rpm and stirred for 10 minutes to obtain a composite cementitious material.

[0075] Comparative Example 2

[0076] The conditions and raw materials of this embodiment are the same as those of Example 1, except that tobacco waste is replaced with sawdust, specifically:

[0077] (1) Sawdust from a timber company in Kunming, Yunnan Province was dried in an electric constant temperature forced air drying oven at 105°C for 24 h, then placed in a tube furnace and heated to 300°C at a heating rate of 5°C / min in a nitrogen atmosphere for 2 h. The sawdust was ground in an agate mortar and passed through a 100-mesh sieve to obtain biochar for later use.

[0078] (2) The Bayer red mud produced by an aluminum company in Wenshan Prefecture, Yunnan Province was dried in an electric constant temperature blast drying oven at 105°C for 24 h, and then ground with an agate mortar and passed through a 60-mesh sieve to obtain red mud particles for later use;

[0079] (3) Phosphogypsum produced by a phosphogypsum company in Kunming, Yunnan Province was dried in an electric constant temperature blast drying oven at 100°C for 24 h, and ground with an agate mortar to obtain 60-mesh phosphogypsum particles for later use;

[0080] (4) Blast furnace slag produced by a pig iron smelting company in Yunnan Province was dried in an electric constant temperature blast drying oven at 105°C for 24 h, ground with an agate mortar and passed through a 60-mesh sieve to obtain blast furnace slag powder for later use;

[0081] (5) The biochar obtained in step (1) and water were placed in a beaker at a mass ratio of 1:100 and stirred on a magnetic stirrer at 20±2°C for 6 h. The mixture was filtered using a vacuum pump under a 0.45 μm microporous filter membrane until all the soaked biochar was collected on the wet filter paper to obtain saturated biochar;

[0082] (6) The red mud particles obtained in step (2), the phosphogypsum particles obtained in step (3), the blast furnace slag powder obtained in step (4), and the saturated biochar obtained in step (5) are mixed in a ratio of 1:1:1:0.1, the mass of water used is 40% of the total mass of the solid mixture, the mass of water glass (1.0 modulus) used is 5% of the total mass of the solid mixture, and the solid mixture, water and water glass are placed in a stirrer at room temperature and a rotation speed of 300 rpm and stirred for 10 minutes to obtain a composite cementitious material.

[0083] Comparative Example 3

[0084] The conditions and raw materials of this embodiment are the same as those of Example 1, except that the activator glass water is replaced with a sodium carbonate standard solution with a purity of 5%, specifically:

[0085] (1) Tobacco waste from a tobacco company in Kunming, Yunnan Province was dried in an electric constant temperature forced air drying oven at 105°C for 24 h, then placed in a tube furnace and heated to 300°C at a heating rate of 5°C / min in a nitrogen atmosphere for constant temperature pyrolysis for 2 h, and ground in an agate mortar through a 100-mesh sieve to obtain biochar for use;

[0086] (2) The Bayer red mud produced by an aluminum company in Wenshan Prefecture, Yunnan Province was dried in an electric constant temperature blast drying oven at 105°C for 24 h, and then ground with an agate mortar and passed through a 60-mesh sieve to obtain red mud particles for later use;

[0087] (3) Phosphogypsum produced by a phosphogypsum company in Kunming, Yunnan Province was dried in an electric constant temperature blast drying oven at 100°C for 24 h, and ground with an agate mortar to obtain 60-mesh phosphogypsum particles for later use;

[0088] (4) Blast furnace slag produced by a pig iron smelting company in Yunnan Province was dried in an electric constant temperature blast drying oven at 105°C for 24 h, ground with an agate mortar and passed through a 60-mesh sieve to obtain blast furnace slag powder for later use;

[0089] (5) The biochar obtained in step (1) and water were placed in a beaker at a ratio of 1:100 and stirred on a magnetic stirrer at 20±2°C for 6 h. The mixture was filtered using a vacuum pump under a 0.45 μm microporous filter membrane until all the soaked biochar was collected on wet filter paper to obtain saturated biochar.

[0090] (6) The red mud particles obtained in step (2), the phosphogypsum particles obtained in step (3), the blast furnace slag powder obtained in step (4), and the saturated biochar in step (5) are mixed in a mass ratio of 1:1:1:0.1, the mass of water used is 40% of the total mass of the solid mixture, the mass of the sodium carbonate standard solution used is 5% of the total mass of the solid mixture, and the solid mixture, water, and the sodium carbonate standard solution are placed in a stirrer at room temperature and a rotation speed of 300 rpm and stirred for 10 minutes to obtain a composite cementitious material.

[0091] Comparative Example 4

[0092] The conditions and raw materials of this embodiment are the same as those of Example 1, except that the biochar is not soaked to form saturated biomass. Specifically,

[0093] (1) Tobacco waste from a tobacco company in Kunming, Yunnan Province was dried in an electric constant temperature forced air drying oven at 105°C for 24 h, then placed in a tube furnace and heated to 300°C at a heating rate of 5°C / min in a nitrogen atmosphere for constant temperature pyrolysis for 2 h, and ground in an agate mortar through a 100-mesh sieve to obtain biochar for use;

[0094] (2) The Bayer red mud produced by an aluminum company in Wenshan Prefecture, Yunnan Province was dried in an electric constant temperature blast drying oven at 105°C for 24 h, and then ground with an agate mortar and passed through a 60-mesh sieve to obtain red mud particles for later use;

[0095] (3) Phosphogypsum produced by a phosphogypsum company in Kunming, Yunnan Province was dried in an electric constant temperature blast drying oven at 100°C for 24 h, and ground with an agate mortar to obtain 60-mesh phosphogypsum particles for later use;

[0096] (4) Blast furnace slag produced by a pig iron smelting company in Yunnan Province was dried in an electric constant temperature blast drying oven at 105°C for 24 h, ground with an agate mortar and passed through a 60-mesh sieve to obtain blast furnace slag powder for later use;

[0097] (5) The red mud particles obtained in step (2), the phosphogypsum particles obtained in step (3), the blast furnace slag powder obtained in step (4), and the biochar obtained in step (1) are mixed in a mass ratio of 1:1:1:0.1, the mass of water used is 40% of the total mass of the solid mixture, the mass of water glass (1.0 modulus) used is 5% of the total mass of the solid mixture, and the solid mixture, water and water glass are placed in a stirrer at room temperature and a rotation speed of 300 rpm and stirred for 10 minutes to obtain a composite cementitious material.

[0098] The performance results of the composite paste materials prepared in Examples 1 to 4 and Comparative Examples 1 to 4 are shown in Table 1

[0099] Table 1 Performance results of composite cementitious materials prepared in Examples 1 to 4 and Comparative Examples 1 to 4

[0100]

[0101] As can be seen from Table 1, by comparing Example 1 with Comparative Example 1, it can be seen that the workability and mechanical properties of the composite cementitious material prepared by adding biochar to red mud-phosphogypsum-blast furnace slag under the activation of an activator are improved, meeting the requirements of road construction. The reason is that biochar has a rich pore structure and excellent water retention capacity. It can release water in the late hydration stage and has the ability to internally solidify, which can not only improve the workability of the cementitious material but also improve the mechanical properties of the cementitious material. The hydroxyl (-OH), carbonyl (-C=O), and carboxylic acid (-COOH) contained in biochar promote the adsorption and aggregation of water molecules, have an internal solidification effect in the late hydration stage, and can also react with Ca 2+ Combine to form a stable complex, thereby promoting hydration reaction and improving mechanical properties.

[0102] Comparing Example 1 with Example 2, it can be seen that the workability and mechanical properties of the composite cementitious material prepared using tobacco waste as biochar raw material are improved. The reason is that compared with other biomass, the biochar prepared from tobacco waste contains higher total carbon and organic carbon content, and has higher porosity and specific surface area, so it has higher activity, which can promote the hydration reaction in the process of preparing the cementitious material and has higher mechanical properties.

[0103] Comparing Example 1 with Comparative Example 3 shows that using water glass as an activator can promote strength development and achieve unconfined compressive strength that meets road construction requirements. This is because water glass not only activates the aluminosilicates in red mud, releasing more active Si and Al to promote hydration, but also its own silicon participates in the hydration reaction and promotes the formation of hydration products. Compared with sodium hydroxide and potassium hydroxide, water glass is more stable in water. This makes the performance of the cementitious material more stable in humid environments. Compared with sodium hydroxide and potassium hydroxide, water glass has a lower preparation cost, making it more feasible for industrial production. In addition, since red mud itself contains a large amount of Na, the addition of sodium hydroxide will cause sodium salts to precipitate, resulting in whitening. Potassium hydroxide has a high alkalinity and can be used as a red mud activator, causing alkalinity in the cementitious material, which is not conducive to strength development.

[0104] Comparison of Example 1 with Comparative Example 4 shows that the use of saturated biochar can improve the unconfined compressive strength of cementitious materials. The reason is that saturated biochar reduces the early absorption of water, and sufficient water can promote the early hydration reaction. The saturated biochar can release water in the later stage of hydration, so that the hydration reaction proceeds from the inside out, and has the potential for internal solidification.

[0105] Figures 2-3 The XRD diagram of red mud and phosphogypsum shows that red mud and phosphogypsum contain certain active components that can promote hydration reaction (SiO2, Ca3Al2[SiO4]3, Na2O·Al2O3·2SiO2, CaSO4·2H2O and CaSO4·0.5H2O).

[0106] Figure 4 Figure 2 is the XRD diagram of the composite cementitious material. It can be seen from the figure that red mud, phosphogypsum, blast furnace slag and biochar can react to generate hydration products, indicating that the experiment is feasible.

[0107] Figures 5-6 This graph compares the setting time and unconfined compressive strength of a composite cementitious material. This comparison reveals that the changes in setting time and mechanical strength are related to the amount and source of biochar added. This is because excessive amounts of biochar and biochar from different sources increase the capillary structure of the cementitious material, increasing water retention and pore structure, leading to reduced workability and mechanical properties. The optimal conditions were ultimately determined: a 3:3:2:1 ratio of red mud particles, phosphogypsum particles, blast furnace slag powder, and biochar, with water and water glass used at 40% and 5% of the solid mixture's mass.

[0108] In summary, the composite cementitious materials prepared in Examples 1 to 4 can be used in public road traffic construction.

Claims

1. A method for preparing a composite cementitious material by adding biochar to red mud-phosphogypsum-blast furnace slag, characterized in that: The specific steps are as follows: (1) Drying tobacco waste, pyrolyzing it, grinding it, and sieving it to obtain biochar particles; (2) drying, grinding and sieving red mud, phosphogypsum and blast furnace slag respectively to obtain red mud particles, phosphogypsum particles and blast furnace slag powder; (3) pre-soaking the biochar particles obtained in step (1) to a saturated state; (4) mixing the red mud particles, phosphogypsum particles, blast furnace slag powder obtained in step (2), and the saturated biochar particles obtained in step (3), adding water and an activator, and stirring thoroughly to obtain a composite cementitious material; The activator in step (4) is water glass with a modulus of 1.0 to 1.5; The amount of the activator in step (4) is 5 to 10% of the total mass of the solid mixture.

2. The method for preparing a composite cementitious material by adding biochar to red mud-phosphogypsum-blast furnace slag according to claim 1, characterized in that: The drying condition in step (1) is drying at 105-120° C. for 24-48 hours; the pyrolysis is heating the tobacco waste to 300-600° C. at a heating rate of 5-10° C. / min in a nitrogen atmosphere and then pyrolyzing at a constant temperature for 2-4 hours; and the tobacco waste is ground and sieved through a 100-300 mesh sieve.

3. The method for preparing a composite cementitious material by adding biochar to red mud-phosphogypsum-blast furnace slag according to claim 1, characterized in that: The drying in step (2) is to dry the red mud, phosphogypsum and blast furnace slag in an electric constant temperature blast drying oven, wherein the drying temperature of the red mud is 105-120°C, the drying temperature of the phosphogypsum is 100-170°C, and the drying temperature of the blast furnace slag is 105-120°C, and the drying time is 24-48h; after grinding, the mixture is sieved through a 60-100 mesh sieve.

4. The method for preparing a composite cementitious material by adding biochar to red mud-phosphogypsum-blast furnace slag according to claim 1, characterized in that: The pre-soaking in step (3) is to stir the biochar and water at a mass ratio of 1:100 at 20±2°C for 6 to 8 hours.

5. The method for preparing a composite cementitious material by adding biochar to red mud-phosphogypsum-blast furnace slag according to claim 1, characterized in that: The mass ratio of the red mud particles, phosphogypsum particles, blast furnace slag powder and saturated biochar in step (4) is red mud particles: phosphogypsum particles: blast furnace slag powder: saturated biochar = (1-3): (1-3): (1-2): (0.1-1).

6. The method for preparing a composite cementitious material by adding biochar to red mud-phosphogypsum-blast furnace slag according to claim 1, characterized in that: The amount of water used in step (4) is 40-60% of the total mass of the solid mixture.

7. The method for preparing a composite cementitious material by adding biochar to red mud-phosphogypsum-blast furnace slag according to claim 1, characterized in that: The stirring process in step (4) is at room temperature, the rotation speed is 300-450 rpm, and the stirring time is 10-20 min.

8. Use of the composite gelling material prepared by the method according to any one of claims 1 to 7 in public road traffic construction.

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