Bayer process red mud-based unfired ceramsite as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of solid waste ceramsite, and particularly relates to Bayer process red mud-based unfired ceramsite as well as a preparation method and application thereof. The unfired ceramsite is of a gradient structure composed of a core part and a shell part. The core part comprises Bayer process red mud, fly ash, steel slag micro powder and gypsum; the shell part comprises Bayer process red mud, slag cement, silica fume and a composite activator; the composite exciting agent is composed of sodium hydroxide and water glass; the cylinder compressive strength of the unfired ceramsite is greater than or equal to 3.5 MPa, and the bulk density is less than or equal to 1000kg / m < 3 >. The obtained gradient ceramsite bears a main load by the high-strength shell layer in a cylinder pressure test, and the porous structure of the core layer inhibits the increase of the overall density, so that the cylinder pressure strength is greater than or equal to 3.5 MPa, the stacking density is less than or equal to 1000kg / m < 3 >, sintering and additional lightweight aggregate are avoided, and the traditional path of strength-density synchronous increase is jumped out; and a creative thought of constructing a functional gradient according to the self activity difference of the industrial solid waste is formed.

Description

Technical Field

[0001] This application belongs to the field of solid waste ceramsite technology, and particularly relates to a Bayer process red mud-based non-fired ceramsite, its preparation method, and its uses. Background Technology

[0002] Red mud is a major solid waste generated by the aluminum industry. Approximately 0.8–1.5 tons of red mud are produced for every ton of alumina produced, resulting in annual emissions exceeding ten million tons. With a pH of 10–13, long-term stockpiling of red mud occupies land and causes soil alkalization and heavy metal pollution. Traditional ceramsite production involves high-temperature firing of clay, consuming high-quality minerals and resulting in high energy consumption and carbon emissions. Existing red mud ceramsite, due to its homogeneous formulation, has a bulk density exceeding 1100 kg / m³ when the compressive strength is greater than 3 MPa. 3 The hydration reaction at room temperature requires 7–14 days of curing, and the red mud content is usually less than 30 wt%, resulting in low solid waste utilization. High-alkali red mud is easily soluble in acidic environments, which limits its application in water treatment. Summary of the Invention

[0003] This application provides a Bayer process red mud-based non-fired ceramsite, its preparation method, and its uses, to solve the following technical problem: how to obtain gradient ceramsite with increased compressive strength but without increased bulk density using Bayer process red mud without firing.

[0004] In a first aspect, embodiments of this application provide a Bayer process red mud-based non-fired ceramsite, wherein the non-fired ceramsite has a gradient structure consisting of a core and a shell. The core comprises Bayer red mud, fly ash, steel slag powder, and gypsum. The shell contains Bayer red mud, slag cement, silica fume, and a composite activator; The composite activator is composed of sodium hydroxide and water glass; The non-fired ceramsite has a compressive strength ≥ 3.5 MPa and a bulk density ≤ 1000 kg / m³. 3 .

[0005] Optionally, based on the total mass of the unfired ceramsite, the total amount of Bayer red mud is 50wt%–70wt%.

[0006] Optionally, the core and the shell are bonded together by an interfacial hydration reaction, and the thickness of the shell is 0.5 mm–2 mm.

[0007] Optionally, the mass ratio of sodium hydroxide to water glass in the composite activator is 1:1–1:3.

[0008] Optionally, the specific surface area of ​​the Bayer process red mud is 30 m². 2 / g–50m 2 / g and moisture content ≤10wt%.

[0009] Secondly, embodiments of this application provide a method for preparing Bayer process red mud-based non-fired ceramsite according to any one of the first aspects, characterized in that the method includes: Bayer red mud was dried and sieved to obtain pretreated red mud; The pretreated red mud, fly ash, steel slag powder and gypsum are mixed and water is added and stirred to obtain a plastic core material; The plastic core material is granulated to obtain wet core balls; The pretreated red mud, slag cement, silica fume and composite activator are mixed and water is added and stirred to obtain the shell slurry. The shell slurry is coated onto the surface of the wet core ball to obtain a gradient wet ball; The gradient wet-bulb was subjected to room temperature static curing and steam curing in sequence to obtain Bayer process red mud-based non-fired ceramsite.

[0010] Optionally, the steam curing temperature is 60℃–80℃ and the time is 12h–24h; the steam curing heating rate is ≤20℃ / h.

[0011] Optionally, the particle size of the wet core spheres is 4mm–18mm.

[0012] Optionally, the plasticity index of the plastic core material is 10–15; and the fluidity of the shell slurry is 180 mm–220 mm.

[0013] Thirdly, embodiments of this application provide the use of non-fired ceramsite as building insulation aggregate or water treatment filter material as described in any of the first aspects, wherein, when the non-fired ceramsite is used as water treatment filter material, the non-fired ceramsite is soaked in a 5wt% hydrochloric acid solution for 2 hours and then washed with water until neutral.

[0014] The technical solutions provided in this application have the following advantages compared with the prior art: The low cementitious activity and narrow particle size distribution of Bayer process red mud are key factors leading to the contradiction that "increasing the compressive strength of non-fired ceramsite necessarily requires increasing the bulk density." This application's embodiments construct a "core-shell gradient structure" within the particles: 1. The core is mainly composed of a quaternary system of red mud-fly ash-steel slag-gypsum, with active Al2O3 and SiO2 in fly ash and steel slag and SO4 provided by gypsum. 2- In an alkaline environment, an interwoven network of ettringite and CASH is generated, thereby solidifying a "low-density but highly porous" framework to provide internal support for the subsequent shell without increasing the overall density. 2. The shell consists of a highly active outer layer composed of red mud, slag cement, silica fume, and a composite activator (NaOH + water glass). The slag cement and silica fume react in the OH... - and SiO3 2-Under dual excitation, high calcium-to-silicon ratio CSH gel is rapidly generated and densely wrapped in situ along the core surface, thus forming a high-strength shell without significantly increasing the diameter of the ceramic particles; 3. The core and shell layers share red mud as a continuous phase, and the interface is chemically compatible. During the curing process, the pores in the core provide growth space for hydration products, while the hydration products in the shell anchor inward, thereby locking the contribution of "high strength" to the shell layer and the contribution of "lightweight" to the core layer, achieving mechanical-density decoupling. In the final graded ceramsite test, the high-strength shell layer bore the main load, while the porous structure of the core layer suppressed the overall density increase, thus simultaneously satisfying the requirements of a compressive strength ≥ 3.5 MPa and a bulk density ≤ 1000 kg / m³. 3 It eliminates the need for sintering and additional lightweight aggregates, breaking away from the traditional path of "simultaneous growth of strength and density" and forming a creative approach that constructs functional gradients based on the inherent activity differences of industrial solid waste. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0016] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1~6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "contains" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0017] In a first aspect, embodiments of this application provide a Bayer process red mud-based non-fired ceramsite, wherein the non-fired ceramsite has a gradient structure consisting of a core and a shell. The core comprises Bayer red mud, fly ash, steel slag powder, and gypsum. The shell contains Bayer red mud, slag cement, silica fume, and a composite activator; The composite activator is composed of sodium hydroxide and water glass; The non-fired ceramsite has a compressive strength ≥ 3.5 MPa and a bulk density ≤ 1000 kg / m³. 3 .

[0018] "Bayer process red mud": This patent application refers to the tailings from the Bayer process of alumina. "Gradient structure": This patent application refers to the presence of two layers, a core and a shell, within the non-fired ceramsite. These core and shell layers exhibit continuous variations in chemical composition, porosity, and density, and are bonded together through interfacial hydration. "Cylinder compressive strength": This patent application refers to the strength value, measured in MPa, of non-fired ceramsite when it is packed into a specified steel cylinder and subjected to axial pressure at a specified loading rate, before the entire ceramsite mass fails. "Bulk density": This patent application refers to the mass per unit volume of non-fired ceramsite after it has freely fallen into a standard container. The unit is kg / m³. 3 .

[0019] The high alkalinity of Bayer process red mud inhibits the gelation reaction, leading to a "strength-density" contradiction in existing homogeneous formulations. Claim 1 of this invention divides Bayer process red mud into a core and a shell. The core uses Bayer process red mud, fly ash, steel slag powder, and gypsum to form a porous framework, thereby reducing the bulk density. The shell uses a composite activator of Bayer process red mud, slag cement, silica fume, sodium hydroxide, and water glass to rapidly generate CSH gel and a dense layer of ettringite, providing a cylinder compressive strength of ≥3.5MPa within a shell thickness range of 0.5mm–2mm. The core-shell interface is bonded through a synchronous hydration reaction, thus avoiding delamination and offsetting shrinkage stress, resulting in a cylinder compressive strength ≥3.5MPa and a bulk density ≤1000kg / m³. 3 This invention addresses the technical challenge of "how to produce graded ceramsite with increased compressive strength but without increased bulk density using Bayer process red mud without firing." The innovative approach involves, for the first time, utilizing the Bayer process red mud itself to create a functional division of "lightweight core + high-strength shell," rather than simple homogeneous blending. Compressive strengths include, but are not limited to, 3.5 MPa, 4.0 MPa, and 4.2 MPa; bulk density includes, but is not limited to, 1000 kg / m³. 3 950kg / m 3 864kg / m 3 .

[0020] In some embodiments, the total amount of Bayer red mud is 50wt%–70wt%, based on the total mass of the unfired ceramsite.

[0021] "Total admixture": In this patent application, it refers to the percentage of Bayer process red mud in the total mass of non-fired ceramsite.

[0022] When the total content of Bayer red mud is increased to 50wt%–70wt%, both the core and shell layers obtain a high proportion of solid waste; the composite activator in the shell layer can still ensure sufficient cementitious products, thus maintaining the high strength of the shell layer; the core layer, due to the synergistic effect of gypsum, fly ash, and steel slag powder, forms a porous structure, thereby offsetting the density increase brought about by the high content; finally, the non-fired ceramsite maintains a compressive strength ≥3.5MPa while the bulk density does not increase. Creative idea: Under the premise that the total content of a single solid waste is ≥50wt%, strength-density decoupling is still achieved through gradient division of labor. Total content: 50wt%, 55wt%, 60wt%, 65wt%, 70wt%, etc. In some embodiments, the core layer and the shell layer are combined through an interfacial hydration reaction, and the thickness of the shell layer is 0.5mm–2mm.

[0023] "Interfacial hydration reaction": This patent application refers to the Ca on the contact zone between the core and the shell. 2+ Al 3+ Si 4+ SO4 2- OH - Ions interdiffusion generate CSH gel, ettringite, and hydrotalcite-like products, thereby chemically bonding the core and shell. "Shell thickness": In this patent application, this refers to the uniform thickness of the shell slurry along the radial direction of the unfired ceramic particles after curing.

[0024] The shell thickness is limited to 0.5mm–2mm. When the shell thickness is ≥0.5mm, the dense shell layer is sufficient to isolate the porous core structure from external loads, thus ensuring a cylinder compressive strength ≥3.5MPa. When the shell thickness is ≤2mm, the shell volume ratio is controlled, thus avoiding an increase in overall bulk density. The interfacial hydration reaction creates a continuous transition between the core and shell, preventing shell delamination. Ultimately, the non-fired ceramsite improves cylinder compressive strength without increasing bulk density. Innovative approach: For the first time, a millimeter-level thickness window is used to balance strength contribution and weight penalty. Shell thicknesses: 0.5mm, 1.0mm, 1.5mm, 2.0mm, etc.

[0025] In some embodiments, the mass ratio of sodium hydroxide to water glass in the composite activator is 1:1–1:3.

[0026] The mass ratio of sodium hydroxide to water glass is 1:1–1:3, thus providing a gradient of alkalinity and soluble silica; the rapid dissolution of sodium hydroxide provides OH-. - This allows for early activation of slag cement and Bayer process red mud; water glass continuously provides SiO3. 2-This allows for the subsequent addition of CSH gel; increased shell densification leads to improved cylinder compressive strength; and shell densification does not significantly increase weight in the thickness direction, thus preventing an increase in bulk density. Creative approach: Using a two-component alkali-silicon composite excitation instead of a single strong alkali achieves simultaneous strength growth and shrinkage compensation. Mass ratio of sodium hydroxide to water glass: 1:1, 1:2, 1:3, etc.

[0027] In some embodiments, the specific surface area of ​​the Bayer process red mud is 30 m². 2 / g–50m 2 / g and moisture content ≤10wt%.

[0028] "Specific surface area": ​​In this patent application, it refers to the specific surface area value of Bayer process red mud after drying and sieving, measured by the BET method. "Moisture content": In this patent application, it refers to the percentage of water mass in the Bayer process red mud relative to the total mass of the Bayer process red mud.

[0029] Bayer process red mud specific surface area 30m² 2 / g–50m 2 The Bayer red mud particles are granulated at a density of / g and a moisture content of ≤10wt%, ensuring that the surface of the particles is fully exposed and that free water does not occupy the pores. During core granulation, gypsum, fly ash, and steel slag powder can be uniformly adhered to the surface of the Bayer red mud, thereby promoting early micro-expansion and porous structure. The shell of the Bayer red mud also has a high specific surface area, allowing it to be rapidly eroded by the composite activator and release Al. 3+ Si 4+ Ultimately, a lightweight core and a high-strength shell were achieved simultaneously. The innovative approach involved using the specific surface area of ​​Bayer red mud as the physical entry point for gradient activation for the first time. Specific surface area: 30 m². 2 / g、35m 2 / g、40m 2 / g、45m 2 / g, 50m 2 / g, etc.; moisture content: 10wt%, 8wt%, 6wt%, 4wt%, 2wt%, etc.

[0030] Secondly, embodiments of this application provide a method for preparing Bayer process red mud-based non-fired ceramsite according to any one of the first aspects, characterized in that the method includes: Bayer red mud was dried and sieved to obtain pretreated red mud; The pretreated red mud, fly ash, steel slag powder and gypsum are mixed and water is added and stirred to obtain a plastic core material; The plastic core material is granulated to obtain wet core balls; The pretreated red mud, slag cement, silica fume and composite activator are mixed and water is added and stirred to obtain the shell slurry. The shell slurry is coated onto the surface of the wet core ball to obtain a gradient wet ball; The gradient wet-bulb was subjected to room temperature static curing and steam curing in sequence to obtain Bayer process red mud-based non-fired ceramsite.

[0031] "Pretreated Red Mud": This patent application refers to the powdered form of Bayer process red mud after drying and sieving. "Plastic Core Material": This patent application refers to a plastic mixture of pretreated red mud, fly ash, steel slag powder, gypsum, and water. "Wet Core Ball": This patent application refers to spherical wet particles of the plastic core material after granulation. "Shell Slurry": This patent application refers to a slurry of pretreated red mud, slag cement, silica fume, composite activator, and water. "Gradient Wet Ball": This patent application refers to a wet composite ball with the shell slurry coating the surface of the wet core ball.

[0032] Drying and sieving reduce the moisture content and increase the specific surface area of ​​Bayer process red mud, thereby enhancing its subsequent reaction activity. The plastic core material exhibits plasticity and binding properties within a plasticity index of 10–15, allowing for the formation of regularly shaped wet core spheres through granulation. The wet core spheres have a particle size of 4 mm–18 mm, ensuring uniform coating thickness of the subsequent shell slurry. The shell slurry has a flowability of 180 mm–220 mm, enabling self-leveling and the formation of a dense shell layer during the coating process. Gradient wet spheres undergo initial coagulation of the shell slurry through ambient temperature curing, followed by accelerated hydration through steam curing at 60℃–80℃, resulting in a high-strength, dense shell structure within 12 h–24 h. The final non-fired ceramsite exhibits a compressive strength ≥3.5 MPa and a bulk density ≤1000 kg / m³. 3 Creative approach: For the first time, a three-step sequence of "stepwise granulation - gradient coating - steam curing" was proposed to achieve strength-density decoupling under the non-fired process.

[0033] In some embodiments, the steam curing temperature is 60℃–80℃ and the time is 12h–24h; the steam curing heating rate is ≤20℃ / h.

[0034] "Heating rate": This patent application refers to the average rate at which the temperature of the steam curing chamber rises from room temperature to the target temperature, in °C / h. The heating rate is ≤20 °C / h, thus avoiding temperature stress caused by the temperature difference between the inside and outside of the wet-bulb tube; temperature stress is suppressed, thus preventing micro-cracks in the shell; the shell remains intact and dense, resulting in a cylinder compressive strength ≥3.5 MPa; the absence of cracks in the shell prevents an additional increase in density, resulting in a bulk density ≤1000 kg / m³. 3 Creative concept: For the first time, a heating rate window is included in the patent, ensuring that steam curing without heating does not cause cracking. Heating rates: 5℃ / h, 10℃ / h, 15℃ / h, 20℃ / h, etc.

[0035] In some embodiments, the wet core spheres have a particle size of 4 mm–18 mm.

[0036] "Particle size": In this patent application, "particle size" refers to the equivalent sphere diameter determined by sieving after wet core sphere molding. The wet core sphere particle size is 4mm–18mm, thus ensuring a stable shell layer thickness ratio of 0.5mm–2mm after the shell slurry coating. If the particle size is too small, the shell layer will be relatively thicker, leading to an increase in bulk density; if the particle size is too large, the shell layer will be thinner, leading to a decrease in cylinder compressive strength. The 4mm–18mm range balances strength and density, thus solving this technical problem. Particle sizes: 4mm, 6mm, 8mm, 10mm, 12mm, 14mm, 16mm, 18mm, etc.

[0037] In some embodiments, the plastic core material has a plasticity index of 10–15; and the shell slurry has a flowability of 180 mm–220 mm.

[0038] "Plasticity Index": In this patent application, it refers to the difference between the liquid limit and the plastic limit of the plastic core material as measured by geotechnical testing standards. "Flowability": In this patent application, it refers to the expanded diameter of the shell slurry as measured by cement mortar flowability testing, in mm.

[0039] With a plasticity index of 10–15, the wet core spheres maintain their shape and do not collapse during granulation; with a flowability of 180 mm–220 mm, the shell slurry self-levels and does not segregate during coating; the shape retention of the wet core spheres and the uniformity of the shell layer together ensure the integrity of the gradient structure, resulting in a cylinder compressive strength ≥3.5 MPa; and with a uniform shell layer that does not thicken, the bulk density is ≤1000 kg / m³. 3 Plasticity index: 10, 11, 12, 13, 14, 15, etc.; Flowability: 180mm, 190mm, 200mm, 210mm, 220mm, etc.

[0040] Thirdly, embodiments of this application provide the use of non-fired ceramsite as building insulation aggregate or water treatment filter material as described in any of the first aspects, wherein, when the non-fired ceramsite is used as water treatment filter material, the non-fired ceramsite is soaked in a 5wt% hydrochloric acid solution for 2 hours and then washed with water until neutral.

[0041] "Building Insulation Aggregate": This patent application refers to non-fired ceramsite used in lightweight aggregate concrete or insulation mortar, with a bulk density ≤1000kg / m³. 3 "Water treatment filter media": This patent application refers to non-fired ceramsite used for filtering wastewater, which is soaked in 5wt% hydrochloric acid solution for 2 hours and then washed with water until neutral, with an acid solubility of ≤3wt%.

[0042] The compressive strength of the unfired ceramsite is ≥3.5MPa and the bulk density is ≤1000kg / m³. 3This directly meets the mechanical and thermal requirements of building insulation aggregates. When used as a water treatment filter material, it is soaked in a 5wt% hydrochloric acid solution for 2 hours and then washed with water until neutral to remove residual alkali on the surface and seal soluble alkali metal channels. The acid solubility rate is ≤3wt%, ensuring that the carrier does not disintegrate in acidic wastewater environments. Ultimately, the same non-fired ceramsite can be expanded into various application scenarios through simple acid washing, solving the problem of expanding applications after "increasing cylinder compressive strength without increasing bulk density". Creative idea: The first acid washing pretreatment is used as an application, realizing the dual-scenario switching of the same grade of ceramsite.

[0043] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to industry standards. If there is no corresponding industry standard, then generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0044] Example 1 A method for preparing Bayer process red mud-based non-fired ceramsite, the method comprising the following steps in sequence: (1) The Bayer process red mud was dried and sieved to obtain pretreated red mud with a moisture content of 8wt% and a specific surface area of ​​40m² / g. (2) The pretreated red mud, fly ash, steel slag powder and gypsum are mixed according to the core ratio, and water of 18wt% of the total mass of the core is added and stirred to obtain a plastic core material with a plasticity index of 12. (3) The plastic core material is granulated by a disc granulator at a rotation speed of 40 r / min and an inclination angle of 45° to obtain wet core balls with a particle size of 10 mm; (4) Mix the pretreated red mud, slag cement, silica fume and composite activator according to the shell ratio, add 22wt% water of the total mass of the shell, and stir to obtain a shell slurry with a fluidity of 200mm. (5) Spray the shell slurry onto the surface of the wet core ball, controlling the shell thickness to 1 mm, to obtain a gradient wet ball; (6) The gradient wet bulbs were allowed to stand at room temperature for 24 hours, and then heated to 70°C at a heating rate of 15°C / h and steam-cured for 18 hours to obtain Bayer process red mud-based non-fired ceramsite.

[0045] Example 2 A method for preparing Bayer process red mud-based non-fired ceramsite, the method comprising the following steps in sequence: (1) The Bayer red mud was dried and sieved to obtain pretreated red mud with a moisture content of 6wt% and a specific surface area of ​​35m² / g. (2) The pretreated red mud, fly ash, steel slag powder and gypsum are mixed according to the core ratio, and water of 16wt% of the total mass of the core is added and stirred to obtain a plastic core material with a plasticity index of 11. (3) The plastic core material is granulated by a disc granulator at a rotation speed of 35 r / min and an inclination angle of 42° to obtain wet core balls with a particle size of 8 mm; (4) The pretreated red mud, slag cement, silica fume and composite activator are mixed according to the shell ratio, and 23wt% of water is added to the total mass of the shell. The mixture is stirred to obtain a shell slurry with a fluidity of 190mm. (5) Spray the shell slurry onto the surface of the wet core ball, controlling the shell thickness to 0.8 mm, to obtain a gradient wet ball; (6) The gradient wet bulbs were allowed to stand at room temperature for 24 hours, and then heated to 65°C at a heating rate of 10°C / h and steam-cured for 20 hours to obtain Bayer process red mud-based non-fired ceramsite.

[0046] Example 3 A method for preparing Bayer process red mud-based non-fired ceramsite, the method comprising the following steps in sequence: (1) The Bayer red mud was dried and sieved to obtain pretreated red mud with a moisture content of 4wt% and a specific surface area of ​​45m² / g. (2) The pretreated red mud, fly ash, steel slag powder and gypsum are mixed according to the core ratio, and water of 19wt% of the total mass of the core is added and stirred to obtain a plastic core material with a plasticity index of 14. (3) The plastic core material is granulated by a disc granulator at a rotation speed of 45 r / min and an inclination angle of 48° to obtain wet core balls with a particle size of 15 mm; (4) Mix the pretreated red mud, slag cement, silica fume and composite activator according to the shell ratio, add 24wt% water of the total mass of the shell, and stir to obtain a shell slurry with a fluidity of 210mm. (5) Spray the shell slurry onto the surface of the wet core ball, controlling the shell thickness to 1.5 mm, to obtain a gradient wet ball; (6) The gradient wet bulbs were allowed to stand at room temperature for 24 hours, and then heated to 75°C at a heating rate of 12°C / h and steam-cured for 14 hours to obtain Bayer process red mud-based non-fired ceramsite.

[0047] Example 4 A method for preparing Bayer process red mud-based non-fired ceramsite, the method comprising the following steps in sequence: (1) The Bayer red mud was dried and sieved to obtain pretreated red mud with a moisture content of 7wt% and a specific surface area of ​​38m² / g. (2) The pretreated red mud, fly ash, steel slag powder and gypsum are mixed according to the core ratio, and water of 17wt% of the total mass of the core is added and stirred to obtain a plastic core material with a plasticity index of 13. (3) The plastic core material is granulated by a disc granulator at a rotation speed of 38 r / min and an inclination angle of 46° to obtain wet core balls with a particle size of 12 mm; (4) Mix the pretreated red mud, slag cement, silica fume and composite activator according to the shell ratio, add 21wt% water of the total mass of the shell, and stir to obtain a shell slurry with a fluidity of 185mm. (5) Spray the shell slurry onto the surface of the wet core ball, controlling the shell thickness to 1.2 mm, to obtain a gradient wet ball; (6) The gradient wet bulbs were allowed to stand at room temperature for 24 hours, and then heated to 68°C at a heating rate of 8°C / h and steam-cured for 22 hours to obtain Bayer process red mud-based non-fired ceramsite.

[0048] Comparative Example 1 A method for preparing homogeneous, non-fired ceramsite, the method comprising the following steps in sequence: (1) The Bayer process red mud was dried and sieved to obtain pretreated red mud with a moisture content of 8wt% and a specific surface area of ​​40m² / g. (2) The pretreated red mud, cement and fly ash are mixed in a mass ratio of 30wt%:30wt%:40wt%, and 15wt% of water is added to the total mass. The mixture is stirred to obtain a homogeneous slurry. (3) The homogeneous slurry is granulated by a disc granulator to obtain wet pellets with a particle size of 10 mm; (4) The wet pellets are cured at room temperature for 14 days to obtain homogeneous non-fired ceramsite.

[0049] Comparative Example 2 A method for preparing Bayer process red mud-based non-fired ceramsite, the method comprising the following steps in sequence: (1) The Bayer process red mud was dried and sieved to obtain pretreated red mud with a moisture content of 8wt% and a specific surface area of ​​40m² / g. (2) The pretreated red mud, fly ash, steel slag powder and gypsum are mixed according to the core ratio, and water of 18wt% of the total mass of the core is added and stirred to obtain a plastic core material with a plasticity index of 12. (3) The plastic core material is granulated by a disc granulator at a rotation speed of 40 r / min and an inclination angle of 45° to obtain wet core balls with a particle size of 10 mm; (4) Mix the pretreated red mud, slag cement, silica fume and composite activator according to the shell ratio, add 22wt% water of the total mass of the shell, and stir to obtain a shell slurry with a fluidity of 200mm. (5) Spray the shell slurry onto the surface of the wet core ball, controlling the shell thickness to 1 mm, to obtain a gradient wet ball; (6) The gradient wet bulbs were allowed to stand at room temperature for 24 hours, and then continued to be cured at room temperature for 13 days to obtain Bayer process red mud-based non-fired ceramsite.

[0050] Experimental methods for evaluating results: Cylinder compressive strength: Performed according to GB / T17431.2-2010 "Lightweight aggregates and their test methods Part 2: Cylinder compressive strength test", with a loading rate of 1kN / s, record the maximum load and calculate the cylinder compressive strength.

[0051] Bulk density: Performed according to GB / T17431.1-2010 "Lightweight aggregates and their test methods Part 1: Bulk density test", the aggregates are dropped freely into a standard volumetric cylinder, the mass is weighed and the bulk density is calculated.

[0052] Water absorption rate: Soak the non-fired ceramsite in water at (20±2)℃ for 24 hours, wipe the surface dry, weigh it, and calculate the percentage increase in mass.

[0053] Acid solubility: Soak the non-fired ceramsite in a 5wt% hydrochloric acid solution for 2 hours, rinse with deionized water until neutral, dry and weigh, and calculate the percentage of mass loss.

[0054] Maintenance cycle: Record the total time from the start of molding to the performance test, in days.

[0055]

[0056] As shown in Table 1, the technological advancements of this application's technical solution include: 1. Significantly improved compressive strength: The compressive strength of Examples 1–4 is ≥3.9 MPa, with a maximum of 4.2 MPa, which is 50% higher than Comparative Example 1 (2.8 MPa) and 37% higher than Comparative Example 2 (3.0 MPa), thus meeting the strength requirements of lightweight aggregates for structures.

[0057] 2. Bulk density decreased synchronously: the bulk density of Examples 1–4 was ≤945 kg / m³. 3 The lowest is 920 kg / m 3 Comparison Example 1 (1150 kg / m 3 The concentration was reduced by 20%, compared to control ratio 2 (1050 kg / m³). 3 It reduces the weight by 11%, thus achieving high strength without increasing its own weight.

[0058] 3. Reduced water absorption: The water absorption of Examples 1–4 was 17–19%, which was lower than that of Comparative Example 1 (22%) and Comparative Example 2 (20%), thereby improving frost resistance and durability.

[0059] 4. Significantly reduced acid solubility: The acid solubility of Examples 1–4 was 2.1–2.8%, which was much lower than that of Comparative Example 1 (8.5%) and Comparative Example 2 (7.9%), thus meeting the corrosion resistance requirements of filter media for acidic wastewater treatment.

[0060] 5. The maintenance cycle is shortened from 14 days to 1 day, and production efficiency is increased by 14 times, thereby significantly reducing energy consumption and manufacturing costs.

[0061] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A Bayer process red mud-based non-fired ceramsite, characterized in that, The non-fired ceramsite has a gradient structure consisting of a core and a shell. The core comprises Bayer red mud, fly ash, steel slag powder, and gypsum. The shell contains Bayer red mud, slag cement, silica fume, and a composite activator; The composite activator is composed of sodium hydroxide and water glass; The cylinder compressive strength of the non-burned ceramic aggregate is ≥ 3.5 MPa and the bulk density is ≤ 1000 kg / m 3 .

2. The non-fired ceramsite according to claim 1, characterized in that, Based on the total mass of the non-fired ceramsite, the total amount of Bayer red mud is 50wt%–70wt%.

3. The non-fired ceramsite according to claim 1, characterized in that, The core and the shell are bonded together through an interfacial hydration reaction, and the thickness of the shell is 0.5mm–2mm.

4. The non-fired ceramsite according to claim 1, characterized in that, The mass ratio of sodium hydroxide to water glass in the composite activator is 1:1–1:

3.

5. The non-fired ceramsite according to claim 1, characterized in that, The specific surface area of ​​the Bayer process red mud is 30 m². 2 / g–50m 2 / g and moisture content ≤10wt%.

6. A method for preparing Bayer process red mud-based non-fired ceramsite according to any one of claims 1 to 5, characterized in that, The method includes: Bayer red mud was dried and sieved to obtain pretreated red mud; The pretreated red mud, fly ash, steel slag powder and gypsum are mixed and water is added and stirred to obtain a plastic core material; The plastic core material is granulated to obtain wet core balls; The pretreated red mud, slag cement, silica fume and composite activator are mixed and water is added and stirred to obtain the shell slurry. The shell slurry is coated onto the surface of the wet core ball to obtain a gradient wet ball; The gradient wet-bulb was subjected to room temperature static curing and steam curing in sequence to obtain Bayer process red mud-based non-fired ceramsite.

7. The preparation method according to claim 6, characterized in that, The steam curing temperature is 60℃–80℃ and the time is 12h–24h; the steam curing heating rate is ≤20℃ / h.

8. The preparation method according to claim 6, characterized in that, The wet core spheres have a particle size of 4mm–18mm.

9. The preparation method according to claim 6, characterized in that, The plastic core material has a plasticity index of 10–15; the shell slurry has a flowability of 180 mm–220 mm.

10. The use of the non-fired ceramsite as described in any one of claims 1 to 5 as a building insulation aggregate or a water treatment filter material, wherein, When the non-fired ceramsite is used as a water treatment filter material, the non-fired ceramsite is soaked in a 5wt% hydrochloric acid solution for 2 hours and then washed with water until neutral.

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