Method for preparing cementitious material from red mud modified by thermal activation

Abstract

Disclosed in the present invention is a method for preparing a cementitious material from red mud modified by thermal activation. The method comprises the following steps: step 1, selecting red mud, drying the red mud to remove free moisture, and grinding same to a target particle size; placing the dried red mud in a high-temperature furnace for calcination; after the calcination is finished, cooling the red mud to room temperature by means of air cooling; and step 2, preparing a cementitious material from the pretreated red mud in cooperation with gypsum and carbide slag. In the present invention, by means of a calcination treatment, the silica-alumina mineral structure in red mud is destroyed and converted into a high-activity amorphous form, thereby significantly improving the reactivity of red mud. Moreover, the method makes full use of industrial solid waste red mud, reduces the pollution thereof to the environment, and realizes the recycling of resources. In addition, the red mud modified by thermal activation can efficiently react with an activator to prepare a cementitious material having good mechanical properties and good durability, thereby providing a reliable technical path for high value-added applications of red mud.

Description

A method for preparing cementitious materials from thermally activated and modified red mud. Technical Field

[0001] This invention belongs to the field of red mud activation technology, specifically relating to a method for preparing cementitious materials from thermally activated and modified red mud. Background Technology

[0002] The aluminosilicate structure in Bayer process red mud is stable but has low activity. While domestic and international scholars have made some progress in the thermal activation of red mud, the composition of red mud varies significantly depending on the bauxite source and process conditions, leading to considerable uncertainty regarding the activation methods and conditions.

[0003] Because Bayer process red mud generally has high Fe2O3 and Al2O3 content and low alkali and CaO content, its activity is insufficient. When directly applied to cement, building materials, and soil remediation, it often exhibits problems such as "difficulty in quickly forming stable cementitious properties with other materials" or "prone to efflorescence." Furthermore, different bauxite sources and refining processes result in variations in the particle size distribution and mineral phase composition of Bayer process red mud, further increasing the difficulties in large-scale industrial application.

[0004] Li Shaochun et al. investigated the effects of mechanical and thermal activation treatments on the properties of cement-based materials using Bayer process red mud. The results showed that thermal activation was more effective than mechanical activation. High-temperature treatment caused dehydroxylation reactions in some minerals within the red mud, disrupting its stable chemical structure. This loosened the silicon-oxygen tetrahedron and aluminum-oxygen tetrahedron structures, forming a metastable aluminosilicate structure. This structure significantly improved the activity of the red mud, providing an important foundation for the preparation of high-performance cementitious materials. In contrast, while mechanical activation could disrupt the surface structure of red mud particles to some extent, its effect on the internal stable aluminosilicate structure was limited, thus its overall effect was less significant than that of thermal activation.

[0005] Existing research indicates that the low activity of red mud has become a key bottleneck restricting its efficient utilization. Red mud, an industrial waste generated during alumina production, is characterized by its large quantity and complex composition. Due to its naturally low activity, red mud faces numerous difficulties in resource utilization, limiting its widespread application in building materials, environmental protection, and other fields. Therefore, improving the activity of red mud is the primary task for achieving its large-scale resource utilization.

[0006] In summary, the performance of Bayer process red mud with low activity is limited when directly applied to the preparation of cementitious materials. Currently, when using thermally activated modified red mud in conjunction with other solid wastes to prepare cementitious materials, the exploration of thermal activation conditions such as temperature and time for Bayer process red mud is not systematic. The effects of different combinations of conditions on the activation of red mud and other solid wastes have not been fully studied. The selection of solid waste types is limited, there is a lack of exploration of new or potential solid wastes, and the synergistic mechanism is unclear. Summary of the Invention

[0007] To overcome the shortcomings of existing technologies, the present invention aims to provide a method for preparing cementitious materials from thermally activated and modified red mud. This method is characterized by its simple process, low cost, environmental friendliness, high efficiency, and excellent performance. Through calcination, the silica-alumina mineral structure in the red mud is destroyed and transformed into a highly active amorphous form, significantly improving the reactivity of the red mud. Simultaneously, this method fully utilizes industrial solid waste red mud, reducing its environmental pollution and achieving resource recycling. Furthermore, the thermally activated and modified red mud can react efficiently with activators to prepare cementitious materials with excellent mechanical properties and good durability, providing a reliable technical path for the high-value-added application of red mud.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] A method for preparing cementitious materials from thermally activated and modified red mud includes the following steps;

[0010] Step 1: Select red mud, dry it to remove free water, and grind it to the target particle size; place the dried red mud in a high-temperature furnace for calcination; after calcination, cool the red mud to room temperature using air cooling.

[0011] Step 2: Prepare a cementitious material by co-processing the pretreated red mud with gypsum and carbide slag.

[0012] In step 1, the furnace is calcined at a set temperature of 800-1000℃ for a certain period of time, 2-4 hours, and the heating rate is controlled at 10-15℃ / min during the calcination process.

[0013] Furthermore, in step 1, the furnace is calcined at a set temperature of 800℃ for a certain period of time (2 hours), and the heating rate is controlled at 10℃ / min during the calcination process.

[0014] In step 1, the selected Bayer process red mud has a slightly higher calcium content, and the composition by mass ratio is as follows: Fe2O3: 10-30%; Al2O3: 20-30%; SiO2: 20-30%; CaO: 15-25%; TiO2: 2-10%; Na2O: 2-6%; Others: 1-5% (including MgO, K2O, etc.).

[0015] Step 1 specifically involves the following steps: Step (1): Spread the red mud sample evenly in a tray, with a thickness not exceeding 2 cm, to ensure uniform drying; preheat the oven:

[0016] Step (2): Preheat the oven to 105℃ and keep the temperature stable during the drying process;

[0017] Step (3): Place the selected red mud into the drying oven and start timing; during the drying process, the red mud can be turned over every once in a while (e.g., every 6 hours) to ensure even drying.

[0018] Step (4): End of drying: After 24 hours, turn off the oven and take out the red mud; put the dried red mud into a desiccator to cool to room temperature to avoid moisture absorption.

[0019] In step 1, the target particle size is 50-300 μm. This range strikes a balance between activity, cost, and process feasibility, making it suitable for most industrial applications.

[0020] The red mud pretreated in step 1 is characterized by the following: thermal activation disrupts the crystal structure of silicon and aluminum minerals in the red mud, transforming them from ordered crystals to amorphous forms, significantly improving the reactivity of the red mud. Calcination and subsequent grinding refine the red mud particles, significantly increasing the specific surface area and providing more active sites for subsequent chemical reactions. The dissolution rate of elements such as silicon and aluminum in the thermally activated red mud is increased, making it easier to react with activators to generate geopolymers or other cementing materials. During the thermal activation process, certain mineral phases in the red mud (such as nepheline and hematite) undergo phase transformation or decomposition, generating new active phases, further enhancing the chemical activity of the red mud. The thermally activated red mud exhibits higher stability and can adapt to different environmental conditions, providing a foundation for the preparation of high-performance all-solid-waste red mud-based cementing materials.

[0021] In step 2, the mass ratio of red mud to gypsum and carbide slag is gypsum:carbide slag:thermally activated red mud = 25:17:58. In step 2, the gypsum particle size is 10-100 μm, and the carbide slag particle size is 20-150 μm.

[0022] Finer gypsum particles (10-100 μm) have a larger specific surface area, enabling them to react more quickly with active components (such as Al2O3 and SiO2) in red mud and carbide slag, forming cementitious products such as ettringite (AFt). Fine-particle gypsum is also more easily and uniformly dispersed in mixtures, avoiding excessively high or low local concentrations, thus improving the overall performance of the cementitious material. Appropriately fine gypsum particles contribute to improved early and later strength of cementitious materials.

[0023] Ca(OH)₂ and CaO in carbide slag are key components in the cementitious reaction. Finer particles (20-150 μm) can release active ingredients more quickly, promoting the cementitious reaction. Carbide slag particles act as micro-aggregate fillers in cementitious materials; appropriately fine particles can fill the voids between red mud particles, improving the material's density and strength. Fine-particle carbide slag is also easier to mix uniformly with red mud and gypsum, ensuring a consistent reaction.

[0024] Synergistic chemical reactions among the cementitious material components obtained in step 2:

[0025] Red mud: After being thermally activated at 800℃, the silica and aluminum minerals in the red mud are transformed into highly active amorphous forms, providing abundant silicon and aluminum active components.

[0026] Calcium carbide slag: Calcium carbide slag is rich in CaO and can provide a high concentration of Ca in an alkaline environment. 2+ Ions promote the dissolution and polymerization of silicon-aluminum components.

[0027] Gypsum: The addition of gypsum (CaSO4·2H2O) not only adjusts the sulfur-aluminum ratio of the system, but also reacts with the aluminum phase in red mud to form ettringite (AFt), further enhancing the early strength and stability of the material.

[0028] The synergistic effect of the three is reflected in the fact that red mud provides a source of silicon and aluminum, calcium carbide slag provides a source of calcium and an alkaline environment, and gypsum regulates the reaction process and generates a reinforcing phase, which together promotes the hydration reaction and improves the mechanical properties of cementitious materials.

[0029] Gypsum can be natural gypsum or industrial by-product gypsum (such as desulfurization gypsum), which is widely available and inexpensive, meeting the requirements of resource utilization; carbide slag: its strong alkalinity (pH>12) can effectively activate the activity of red mud, while inhibiting the leaching of heavy metal ions in red mud, thus improving the environmental safety of the material.

[0030] In a cementitious material prepared from red mud, gypsum, and carbide slag, ettringite (AFt) crystals and CSH gel were obtained as cementing products; the ettringite (AFt) crystals and CSH gel were tightly bonded together by hydrogen bonds and chemical bonds.

[0031] Electrasite: Provides early strength and fills large pores, but may cause expansion.

[0032] CSH gel: Provides late-stage strength, fills small pores, and enhances durability.

[0033] Etnacite (AFt) crystals and CSH gels influence the properties of cementitious materials through synergistic and competitive reactions. By optimizing the raw material ratio and curing conditions, the formation of ettringite and CSH gels can be regulated, thereby obtaining cementitious materials with excellent performance.

[0034] The cementitious material exhibits a dense and uniform structure, with evenly distributed hydration products. Eettsonite (AFt) crystals are regularly needle-like or columnar, interwoven to form a robust framework. Extensive CSH gel generation fills the pores, resulting in a highly dense microstructure. The size range of AFt crystals is typically 1-10 μm (length) and 0.1-1 μm (diameter); crystal size is influenced by raw material ratios, curing conditions, and red mud activity, significantly affecting the material's early strength, durability, and microstructure. Pores in the cementitious material primarily form between red mud, gypsum, and carbide slag particles, between AFt crystals and CSH gel, within microchannels, and in bubble-like cavities. This cementitious material is used in the construction industry.

[0035] Building materials field

[0036] Cement substitutes: As a partial or complete replacement for ordinary silicate cement, they are used to prepare building materials such as concrete and mortar, reducing carbon emissions from cement production.

[0037] Geopolymer materials: used to prepare high-performance geopolymer concrete, suitable for building structures, road engineering and bridge construction, with high strength, corrosion resistance and high temperature resistance.

[0038] The beneficial effects of this invention are:

[0039] Excellent mechanical properties:

[0040] High strength: Cementitious materials have high compressive strength, which can meet the strength requirements of building structures, road engineering and other materials.

[0041] Good toughness: The interwoven structure of CSH gel and ettringite crystals gives the material a certain toughness, which improves its crack resistance.

[0042] Good microstructure:

[0043] Dense and uniform: SEM images show that the material has a dense and uniform microstructure, with reasonable distribution of ettringite crystals and CSH gel, and low porosity.

[0044] Excellent crystal growth: Eettsonite crystals are regular needle-like or columnar, interwoven to form a strong framework structure, which further enhances the mechanical properties of the material.

[0045] The present invention is rich in hydration products: XRD and FTIR analysis show that a large amount of ettringite (3CaO·Al2O3·3CaSO4·32H2O) and CSH gel are generated in the material. These hydration products are tightly bound by hydrogen bonds and chemical bonds, which significantly improves the mechanical properties and durability of the material.

[0046] The chemical bond composition is complex: FTIR spectroscopy shows that the material contains Si-O-Si, Al-O-Si, OH, HOH, and SO4. 2- The presence of various chemical bond vibration peaks indicates that components such as silicates, aluminates, and sulfates fully participate in the hydration reaction, forming a complex and stable chemical bond network.

[0047] Low porosity: Microstructural analysis shows that CSH gel and ettringite crystals almost completely fill the pores in the material, forming a dense structure with low porosity, thereby improving the material's impermeability and freeze resistance.

[0048] Well-grown crystals: SEM images show that the ettringite crystals grow well, are regular needle-like or columnar, and are evenly distributed. This crystal structure provides the material with excellent mechanical strength and stability.

[0049] Sufficient reaction: Microscopic analysis showed that thermal activation at 800℃ significantly improved the activity of red mud, making its reaction with carbide slag and gypsum more complete, generating a large number of hydration products that help improve material performance, such as CSH gel and ettringite.

[0050] High-temperature optimization effect: The thermal activation temperature of 800℃ avoids the problem of insufficient reaction at low temperature and prevents the destruction of mineral structure caused by excessively high temperature, thus achieving the best synergistic reaction effect of red mud, carbide slag and gypsum.

[0051] In summary, the cementitious materials prepared by thermal activation at 800℃ have the characteristics of dense structure, abundant hydration products, stable chemical bonds, low porosity, and good crystal growth. These microscopic properties together endow the materials with excellent mechanical properties, durability, and environmental adaptability. Attached Figure Description

[0052] Figure 1 shows the basic properties of Bayer process red mud: (a) XRD diagram; (b) particle size distribution diagram.

[0053] Figure 2 shows the basic properties of carbide slag: (a) XRD diagram; (b) particle size distribution diagram.

[0054] Figure 3 shows the basic properties of desulfurized gypsum: (a) XRD pattern; (b) particle size distribution.

[0055] Figure 4 shows the compressive strength of the cementitious material.

[0056] Figure 5 shows the XRD patterns of the hydration products of thermally activated red mud-based solid waste cementitious materials (a. 3d; b. 28d).

[0057] Figure 6 shows the FTIR spectra of the hydration products of thermally activated red mud-based multi-solid waste cementitious materials (a. 3d; b. 28d).

[0058] Figure 7 shows the SEM image of the cementitious material (28d). Detailed Implementation

[0059] The present invention will now be described in further detail with reference to the accompanying drawings.

[0060] Example 1: Preparation of cementitious materials by calcining red mud at 800℃

[0061] The red mud components selected in this embodiment are: Fe2O3: 19%; Al2O3: 25%; SiO2: 25%; CaO: 18%; TiO2: 5%; Na2O: 3%; others: 5% (including MgO, K2O, etc.).

[0062] 1. Calcination conditions

[0063] Temperature: 800℃

[0064] Time: 2 hours

[0065] Heating rate: 10℃ / min

[0066] Cooling method: Air cooling for 25 minutes

[0067] 2. Raw material ratio

[0068] Calcinated red mud: 58%

[0069] Gypsum: 25%

[0070] Calcium carbide slag: 17%

[0071] 3. Preparation process

[0072] Calcination of red mud:

[0073] The red mud was dried and then ground to 100-200 μm.

[0074] Place it in a muffle furnace and heat it to 800℃ at a rate of 10℃ / min, then hold it at that temperature for 2 hours.

[0075] After calcination, air cool for 25 minutes.

[0076] Raw material mixing:

[0077] The calcined red mud, gypsum, and carbide slag were mixed evenly in a mass ratio of 25:17:58.

[0078] Add water and stir:

[0079] Add an appropriate amount of water (water-to-binder ratio 0.6) and stir until a uniform slurry is formed.

[0080] Molding and maintenance:

[0081] Pour the slurry into the mold and vibrate it to compact it.

[0082] Cured for 28 days at 20℃ and humidity >90% to obtain gel material.

[0083] Example 2: Preparation of cementitious materials by calcining red mud at 900℃

[0084] The red mud components selected in this embodiment are: Fe2O3: 20%; Al2O3: 24%; SiO2: 26%; CaO: 17%; TiO2: 6%; Na2O: 2%; others: 5% (including MgO, K2O, etc.).

[0085] 1. Calcination conditions

[0086] Temperature: 900℃

[0087] Time: 3 hours

[0088] Heating rate: 12℃ / min

[0089] Cooling method: Air cooling for 30 minutes

[0090] 2. Raw material ratio

[0091] Calcinated red mud: 58%

[0092] Gypsum: 25%

[0093] Calcium carbide slag: 17%

[0094] 3. Preparation process

[0095] Calcination of red mud:

[0096] The red mud was dried and then ground to 50-100 μm.

[0097] Place it in a muffle furnace and heat it to 900℃ at a rate of 12℃ / min, then hold it at that temperature for 3 hours.

[0098] After calcination, air cool for 30 minutes.

[0099] Raw material mixing:

[0100] The calcined red mud, gypsum, and carbide slag were mixed evenly in a mass ratio of 25:17:58.

[0101] Add water and stir:

[0102] Add an appropriate amount of water (water-to-binder ratio 0.6) and stir until a uniform slurry is formed.

[0103] Molding and maintenance:

[0104] Pour the slurry into the mold and vibrate it to compact it.

[0105] Cured for 28 days at 25℃ and humidity >90%.

[0106] Example 3. Preparation of cementitious materials by calcining red mud at 1000℃

[0107] The red mud components selected in this embodiment are: Fe2O3: 21%; Al2O3: 25%; SiO2: 26%; CaO: 17%; TiO2: 6%; Na2O: 2%; others: 3% (including MgO, K2O, etc.).

[0108] 1. Calcination conditions

[0109] Temperature: 1000℃

[0110] Time: 4 hours

[0111] Heating rate: 15℃ / min

[0112] Cooling method: Air cooling for 20 minutes

[0113] 2. Raw material ratio

[0114] Calcinated red mud: 58%

[0115] Gypsum: 25%

[0116] Calcium carbide slag: 17%

[0117] 3. Preparation process

[0118] Calcination of red mud:

[0119] The red mud was dried and then ground to 200-300 μm.

[0120] Place it in a muffle furnace and heat it to 1000℃ at a rate of 15℃ / min, then hold it at that temperature for 4 hours.

[0121] After calcination, air cool for 20 minutes.

[0122] Raw material mixing:

[0123] The calcined red mud, gypsum, and carbide slag were mixed evenly in a mass ratio of 25:17:58.

[0124] Add water and stir:

[0125] Add an appropriate amount of water (water-to-binder ratio 0.3) and stir until a uniform slurry is formed.

[0126] Molding and maintenance:

[0127] Pour the slurry into the mold and vibrate it to compact it.

[0128] Curing was carried out for 28 days at 30℃ and humidity >90%. The cementitious material was then obtained.

[0129] Specific application examples:

[0130] The Bayer red mud (BRM) used in this application was sourced from an alumina company in Guizhou Province. X-ray fluorescence spectroscopy was used to analyze the composition of the red mud raw material, and the results are shown in Table 1. The main components of the red mud include SiO2, Al2O3, CaO, Fe2O3, and Na2O, which together account for more than 89% of the total mass of the red mud. The CaO content in the red mud reaches 19.52 wt%, which is relatively high compared to typical Bayer red mud, but still lower than the CaO content in sintered red mud. The Fe2O3 content is 18.11 wt%, falling into the category of low-iron red mud. The Na2O content is 4.74 wt%, indicating that this red mud has strong alkalinity. Based on these compositional characteristics, Bayer red mud possesses the potential to serve as a raw material for alkaline aluminosilicates and has potential feasibility in the preparation of cementitious materials. Through reasonable process development, its effective utilization in the field of cementitious materials can be achieved.

[0131] Table 1. Main chemical components of Bayer process red mud

[0132] Chemical composition: SiO2, Al2O3, Fe2O3, CaO, Na2O, O2O, OSO3, TiO2, other contents: 22.64, 24.18, 18.11, 19.5, 24.74, 2.0, 22.0, 24.49, 2.28

[0133] XRD analysis of Bayer process red mud revealed that its main mineral composition was chlorite ((Fe,Al,Mg)6(Si,Al)4O). 10 The red mud contains Al(OH)3, Katoite (Ca3Al2(SiO4)(OH)8), Hematite (Fe2O3), Boehmite (AlOOH)), calcite (Na6Ca2(AlSiO4)6(CO3SO4)(OH)2), Quartz (SiO2), Calcite (CaCO3), Kaolinite (Al2Si2O5(OH)4), and gibbsite (Al(OH)3). Calcite in red mud is a feldspar mineral, generally a sodium and calcium carbonate-aluminosilicate. Gibbsite is a small amount of Al(OH)3 remaining after Al2O3 extraction from bauxite. Laser particle size analysis was performed on the Bayer process red mud raw material to analyze its particle size distribution. As shown in Figure 1(b), the particle size distribution of the red mud is between 0.01 and 305 μm, with D50 = 8.967 μm, D10 = 1.217 μm, and D90 = 117.694 μm.

[0134] The carbide slag (CS) raw material used in this embodiment was obtained from a company in Guizhou. The composition of the carbide slag raw material was analyzed by X-ray fluorescence spectroscopy. The analysis results are shown in Table 2. The main component is CaO, reaching 91.12%. In addition, there are small amounts of SiO2 and Al2O3.

[0135] Phase analysis of the calcium carbide slag was performed, and the results are shown in Figure 2(a). The main minerals in the calcium carbide slag raw material are portlandite (Ca(OH)2) and calcite (CaCO3), with portlandite being more abundant and exhibiting stronger characteristic peaks. Laser particle size distribution analysis was performed on the calcium carbide slag raw material. As shown in Figure 2(b), the particle size distribution of the calcium carbide slag is between 0.01 and 3080 μm, with D50 = 17.807 μm, D10 = 1.523 μm, and D90 = 735.028 μm.

[0136] Table 2 Main Chemical Composition of Calcium Carbide Slag

[0137] Chemical composition: SiO2, Al2O3, Fe2O3, MgO, CaONa2, Oxide2O3, OSO3, P2O5, TiO2, other contents: 2.78, 2.08, 0.31, 0.119, 1.120, 0.02, 1.16, 0.01, 0.09, 2.32

[0138] The desulfurization gypsum (DG) used in this embodiment was sourced from a coal-fired power plant in Guizhou Province. Its chemical composition is dominated by SO3 and CaO, reaching 52.02% and 42.80% respectively, accounting for 94.82% of the total mass.

[0139] Phase analysis of the desulfurized gypsum raw material was performed, and the results are shown in Figure 3(a). The main mineral in the desulfurized gypsum raw material is hemihydrate gypsum (Gypsum, CaSO4·0.5H2O), and its characteristic peaks are relatively strong. Laser particle size analysis was performed on the desulfurized gypsum raw material to analyze its particle size distribution. As shown in Figure 3(b), the particle size distribution of the desulfurized gypsum is between 0.01-3080 μm, with D50=10.418 μm, D10=1.149 μm, and D90=660.989 μm.

[0140] Table 3 Main Chemical Composition of Desulfurized Gypsum

[0141] Chemical composition: SiO2, Al2O3, Fe2O3, MgO, CaONa2, K2O, OSO3, P2O5, TiO2, other contents: 2.26, 0.50, 0.42, 0.124, 2.80, 0.42, 0.135, 2.02, 1.02, 0.08, 0.23

[0142] The mixing ratios for preparing cementitious materials by synergistic use of thermally activated and modified red mud, gypsum, and carbide slag are shown in the table below. The experimental mixing ratios used in the experiment are also shown in the table below.

[0143] Table 4 Sample Mix Proportions

[0144] Group Number Gypsum Calcium Carbide Slag Red Mud Water-Cement Ratio 158 (100℃) 258 (700℃) 325 1758 (800℃) Neat Paste 0.6458 (900℃) 558 (1000℃)

[0145] 1. Compressive strength of thermally activated red mud-based solid waste cementitious materials

[0146] A high-volume thermally activated Bayer process solid waste cementitious material (58% red mud content) was prepared using industrial solid wastes such as thermally activated Bayer process red mud, desulfurized gypsum, and carbide slag. The compressive strength of the prepared cementitious material specimens was systematically tested. The test results show that the compressive strength of the system at different ages (3d, 7d, 28d) is shown in Figure 4. The figure clearly shows that the thermal activation temperature has a significant impact on the mechanical properties of the cementitious material. As the thermal activation temperature gradually increases, the compressive strength of the thermally activated red mud-based solid waste cementitious material shows a trend of first increasing and then decreasing.

[0147] The thermal activation temperature of Bayer process red mud has a decisive influence on the compressive strength of all-solid-waste cementitious materials. Unactivated red mud has extremely low cementitious activity and is difficult to form effective hydration products. When activated at 800℃, the red mud has a higher content of active alumina and silica, which can form more hydration products with desulfurized gypsum and carbide slag, significantly improving the compressive strength, especially reaching the optimal value at 800℃ (11.0 MPa after 28 days). 800℃ is the optimal activation temperature under current experimental conditions, which can maximize the cementitious activity of red mud, improve the mechanical properties of materials, and provide a scientific basis for the resource utilization of red mud. Therefore, the thermal activation temperature of 800℃ provides an important process basis for the preparation of high-performance red mud-gypsum-carbide slag ternary cementitious materials.

[0148] 2. XRD Analysis of Thermally Activated Red Mud-Based Multi-Solid Waste Cementitious Materials

[0149] XRD analysis was performed on the hydration products of cementitious material samples from the thermally activated red mud-gypsum-carbide slag ternary solid waste system cured for 3 days and 28 days, and the results are shown in Figure 5. Through systematic analysis of the hydration products and mineral composition of cementitious materials at different ages, it was found that the main hydration product was ettringite (Ca6Al2(SO4)3(OH)). 12 ·26H2O), CSH gel, calcite (CaCO3), gypsum dihydrate (CaSO4·2H2O), quartz (SiO2), hematite (Fe2O3), and calcium aluminum feldspar (Ca2Al2SiO7).

[0150] In summary, optimizing the hydration performance of red mud-based solid waste cementitious materials requires controlling the thermal activation temperature of red mud at around 800℃ to maximize the generation of hydration products, increase the degree of hydration in both early and late stages, and thus enhance the mechanical properties of the cementitious materials. This research provides important theoretical support for the resource utilization of red mud and the optimized design of cementitious materials.

[0151] FTIR Analysis of Thermally Activated Red Mud-Based Multi-Solid Waste Cementitious Materials

[0152] Infrared spectroscopy analysis (Figure 6) revealed the reaction mechanism and structural characteristics of the cementitious material by analyzing the changes in chemical bonds at different thermal activation temperatures. At 3643 cm⁻¹ -1 and 3410 cm -1 The OH vibration at 1625 cm⁻¹ indicates that, with increasing temperature, both free and hydrogen-bonded OH bonds gradually participate in the reaction, with the reaction being most complete at 800 °C, producing a large amount of CSH gel and hydrated products such as ettringite. -1 The HOH bending vibration at 1430 cm⁻¹ further confirmed the presence of bound water molecules in the hydration products, and the hydration reaction reached its optimal state at 800 °C. -1 and 875 cm -1 CO3 at the location 2- Vibration indicates that the carbonization reaction between the material and carbon dioxide is significantly enhanced at 800℃, producing a greater amount of calcium carbonate. 1120 cm -1 SO4 at the location 2- Vibration analysis showed that gypsum fully dissolved at 800℃, with sulfate ions participating extensively in the formation of ettringite. 990 cm -1 673 cm -1 and 605 cm -1 The Si-O and Al-O vibrations at 800℃ indicate that the silicon-oxygen and aluminum-oxygen bonds in the red mud fully participate in the reaction, generating a large number of silicate and aluminate structures. In summary, thermal activation at 800℃ can maximize the activity of red mud, promote its synergistic reaction with carbide slag and gypsum, and generate high-performance cementitious materials.

[0153] 4. SEM Analysis of Thermally Activated Red Mud-Based Multi-Solid Waste Cementitious Materials

[0154] Figure 7 shows SEM images of the Bayer process red mud-gypsum-carbide slag ternary cementitious materials after 28 days of thermal activation at different temperatures. Sample B-800, activated at 800℃, exhibited the best activity. Its microstructure was dense and uniform, with ettringite crystals arranged in regular needle-like or columnar shapes, interwoven to form a robust framework. Extensive CSH gel formation filled the pores, indicating that 800℃ thermal activation maximally stimulated the activity of the red mud, promoting its full reaction with gypsum and carbide slag. In conclusion, 800℃ thermal activation can optimize the activity of red mud, promote hydration reactions, and form a dense and uniform microstructure, thereby significantly improving the performance of the cementitious materials.

[0155] This invention employs a high-temperature thermal activation process to effectively break down the stable crystal structure within Bayer process red mud, releasing active silicon-aluminum components to significantly improve the overall performance of red mud-based materials. Furthermore, this invention achieves synergistic resource utilization of red mud by compounding auxiliary cementitious materials such as carbide slag and desulfurized gypsum into the thermally activated red mud. The work accomplished by this invention will provide a solid scientific basis and technical support for the large-scale resource utilization of red mud in the building materials field, while also pointing to future research directions for the efficient utilization of red mud and promoting the industrialization of Bayer process red mud-based cementitious materials.

Claims

1. A method for preparing cementitious materials from thermally activated and modified red mud, characterized in that, Includes the following steps; Step 1: Select red mud, dry it to remove free water, and grind it to the target particle size; place the dried red mud in a high-temperature furnace for calcination; after calcination, cool the red mud to room temperature using air cooling. Step 2: Prepare a cementitious material by co-preparing the pretreated red mud with gypsum and carbide slag.

2. The method for preparing cementitious materials from thermally activated and modified red mud according to claim 1, characterized in that, In step 1, the furnace is calcined at a set temperature of 800-1000℃ for a certain period of time, 2-4 hours, and the heating rate is controlled at 10-15℃ / min during the calcination process.

3. The method for preparing cementitious materials from thermally activated modified red mud according to claim 1, characterized in that, In step 1, the selected Bayer red mud components are as follows by mass ratio: Fe₂O₃: 10-30%; Al₂O₃: 20-30%; SiO₂: 20-30%; CaO: 15-25%; TiO₂: 2-10%; Na₂O: 2-6%; Others: 1-5% (including MgO, K₂O, etc.).

4. The method for preparing cementitious materials from thermally activated and modified red mud according to claim 1, characterized in that, Step 1 specifically involves the following steps: Step (1): Spread the red mud sample evenly in the tray, with a thickness not exceeding 2 cm, to ensure uniform drying; preheat the oven: Step (2): Preheat the oven to 105℃ and keep the temperature stable during the drying process; Step (3): Place the selected red mud into the drying oven and start timing; during the drying process, the red mud can be turned over every once in a while (e.g., every 6 hours) to ensure even drying; Step (4): End of drying: After 24 hours, turn off the oven and take out the red mud; put the dried red mud into a desiccator to cool to room temperature to avoid moisture absorption.

5. The method for preparing cementitious materials from thermally activated and modified red mud according to claim 1, characterized in that, In step 1, the target particle size is 50-300 μm.

6. The method for preparing cementitious materials from thermally activated and modified red mud according to claim 1, characterized in that, In step 2, the mass ratio of red mud to gypsum and carbide slag is gypsum:carbide slag:thermally activated red mud = 25:17:

58.

7. The method for preparing cementitious materials from thermally activated modified red mud according to claim 1, characterized in that, In step 2, the gypsum is 10-100 μm thick. Carbide slag: 20-150 μm.

8. The method for preparing cementitious materials from thermally activated modified red mud according to claim 1, characterized in that, Synergistic chemical reactions among the cementitious material components obtained in step 2: In a cementitious material prepared from red mud, gypsum, and carbide slag, ettringite (AFt) crystals and CSH gel were obtained as cementing products; the ettringite (AFt) crystals and CSH gel were tightly bonded together by hydrogen bonds and chemical bonds.

9. The method for preparing cementitious materials from thermally activated modified red mud according to claim 1, characterized in that, The cementitious material has a dense and uniform structure, with uniformly distributed hydration products. Eettsonite (AFt) crystals are regular needle-like or columnar, interwoven to form a robust framework structure. CSH gel is generated in large quantities and fills the pores, resulting in a highly dense microstructure overall. The size range of ettsonite (AFt) crystals is typically 1-10 μm (length) and 0.1-1 μm (diameter).

10. The application of the method for preparing cementitious materials from thermally activated modified red mud according to any one of claims 1-9, characterized in that, The cementitious material is used in the construction industry.

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