Modified red mud rapid-hardening filling support material and preparation method thereof

Abstract

The invention provides a modified red mud rapid-hardening filling support material and a preparation method, and relates to the technical field of building materials, the material is composed of an inner-layer bearing filling material and an outer-layer alkali-resistant compact filling material. The inner layer comprises the following components in parts by weight: 60-80 parts of red mud, 15-30 parts of a mineralized modified cementing material and 2-8 parts of an early-strength setting-adjusting component; the outer layer is prepared from the following components in parts by weight: 50 to 75 parts of red mud, 15 to 35 parts of mineralized modified cementing material, 3 to 12 parts of Si-Al-Mg solid alkali composite additive and 0.5 to 5 parts of fine compact filler. The mineralized modified cementing material is prepared from industrial gypsum, fly ash, a carbon-containing substance and red mud through microwave segmented mineralization and activation; and the solid alkali composite additive promotes the formation of an aluminosilicate network solid sodium phase and a hydrotalcite-like solid anion phase. The method comprises the steps of red mud crushing and premixing, layered pulping and double-cavity filling bag sequential grouting, and CO2 controlled carbonization curing is carried out within 0.5-6 h after outer layer grouting. The method is suitable for rapid wall forming and long-term alkali resistance supporting beside gob-side entry retaining.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, specifically to a modified red mud quick-setting filling and support material and its preparation method. Background Technology

[0002] Red mud (or bauxite residue) is a solid waste generated during the production of alumina from bauxite. my country's alumina industry has developed rapidly, with domestic alumina production reaching 73.132 million tons in 2020, making it the world's largest alumina producer. Alongside this development, the discharge and stockpiling of red mud have increased year by year, with a particularly pronounced trend in recent years. Typically, producing 1 ton of alumina generates approximately 1.0-2.0 tons of red mud. Currently, the cumulative stockpile of red mud in China is around 500 million tons, with an estimated annual discharge of 90 million tons. The complex composition, small particle size, large discharge volume, and high alkalinity of red mud make its comprehensive utilization difficult. Internationally, red mud is mainly disposed of through dry stockpiling, dumping into the sea, and filling depressions. Some countries with limited land area, such as France, Germany, and Japan, directly discharge red mud into the ocean or lakes, where harmful substances pollute water bodies and harm marine life. Most countries, including my country, dispose of large quantities of red mud by stockpiling it, which occupies land, pollutes water systems, and the fine red mud particles, once dried, are blown by the wind, polluting the atmosphere. The alkaline waste liquid in red mud can pollute surface water and groundwater sources, and red mud storage sites must use expensive geomembrane materials. Currently, large-scale comprehensive utilization of red mud has not been achieved worldwide.

[0003] Industrial gypsum, primarily composed of CaSO4·2H2O (accounting for over 80%), is an industrial waste residue produced as a byproduct of industries such as phosphate chemicals, fluorochemicals, titanium chemicals, and flue gas desulfurization. It typically also contains various impurities including silica, iron and aluminum compounds, acid-insoluble substances, and organic matter. Currently, phosphogypsum, a byproduct of the phosphate chemical industry, constitutes the largest proportion of industrial gypsum in my country. In 2020 alone, Chinese phosphate chemical companies produced 94 million tons of phosphogypsum. While the application of industrial gypsum has reached a certain scale, complete utilization is still a long way off.

[0004] To reduce the loss of protective pillars in mining sections, my country has been continuously promoting pillarless mining methods in recent years. Gob-side roadway retention is one of the most commonly used methods in pillarless mining. Gob-side roadway retention refers to maintaining the original mining roadway along the edge of the goaf after the mining face has been mined. This technology effectively reduces roadway excavation, minimizes the loss of protective pillars, and increases resource recovery. For coal mining, it can also solve the problem of gas accumulation in the upper corner and prevent spontaneous combustion of coal pillars in the goaf. Gob-side roadway retention utilizes a single roadway twice, greatly improving resource recovery and alleviating the tension in mining succession. It represents a major reform in coal mining and goaf roadway layout technology. However, in deeper mining, the mine pressure intensifies, roadway diameter narrows significantly, and large deformation and load-bearing failure of the gob-side roadway backfill occur. Especially during the current gob-side roadway retention period, the roof support effect of the backfill beside the roadway is extremely poor, further exacerbating its deformation and damage. Therefore, the effectiveness of the backfilling and support along the roadway in gob-side retention is crucial for maintaining roadway stability. Currently, the main backfilling support methods for gob-side retention include cement-based backfilling and water-retaining material backfilling. Cement-based backfilling has high strength but is difficult to construct and consumes a lot of labor; water-retaining material backfilling is efficient but costly, has low strength, and suffers significant strength loss over time. Summary of the Invention

[0005] Technical problems to be solved

[0006] To address the shortcomings of existing technologies, this invention provides a modified red mud rapid-setting backfill support material and its preparation method, thus solving the problems of existing technologies.

[0007] Technical solution

[0008] To achieve the above objectives, the present invention provides the following technical solution: a modified red mud rapid-setting backfill support material and its preparation method, wherein the material comprises an inner layer of load-bearing backfill material and an outer layer of alkali-resistant dense backfill material, and is composed of the following components in parts by weight:

[0009] Inner layer load-bearing filling material: 60-80 parts red mud, 15-30 parts mineralized modified cementitious material, and 2-8 parts early-strength coagulating component;

[0010] Outer layer alkali-resistant dense filling material: 50-75 parts red mud, 15-35 parts mineralized modified cementitious material, 3-12 parts Si-Al-Mg solid alkali composite additive, and 0.5-5 parts micro-fine dense filler;

[0011] The mineralized modified cementitious material is a hydraulic cementitious material prepared by microwave segmented mineralization activation of industrial gypsum, fly ash, carbonaceous materials and red mud; the Si-Al-Mg solid-alkali composite additive is used to promote the formation of a synergistic solid-alkali structure of aluminosilicate network solid sodium phase and hydrotalcite-like solid anionic phase during the curing process; the early-setting component is selected from sulfoaluminate cement and high-alumina cement.

[0012] Preferably, the red mud is a highly alkaline red mud solid waste produced by the Bayer process in the alumina industry.

[0013] Preferably, the raw materials for preparing the mineralized modified cementitious material include, by weight, 55-75 parts of industrial gypsum, 15-30 parts of fly ash, 2-8 parts of carbonaceous material, and 5-20 parts of red mud.

[0014] Preferably, the preparation method of the mineralized modified cementitious material includes: mixing and grinding industrial gypsum, fly ash, carbonaceous material and red mud, followed by microwave segmented heating for mineralization activation, so that the material temperature meets the following requirements: first stage 450-650℃ for 5-20 min; second stage 650-900℃ for 10-30 min; then cooling and grinding to below 200 mesh to obtain the mineralized modified cementitious material.

[0015] Preferably, the iron phase component in the red mud of the mineralized modified cementitious material has a mass fraction of 10-40% as the sum of Fe2O3 and Fe3O4.

[0016] Preferably, the Si-Al-Mg solid alkali composite additive is composed of the following components by weight: 1-6 parts of active Si source, 1-8 parts of active Al source, and 0.5-4 parts of magnesium source; wherein the active Si source is selected from one or two of silica fume and nano silica, the active Al source is selected from one or two of metakaolin and high alumina cement, and the magnesium source is selected from one or two of lightly calcined magnesia and calcined dolomite powder.

[0017] Preferably, the micro-fine filler is selected from one, two or three of bentonite, slag powder and ultrafine quartz powder, and the amount added is 0.5 to 3 parts.

[0018] Preferably, the preparation method includes the following steps:

[0019] Sp1. Break the red mud into smaller than 3mm;

[0020] Sp2, the solid components of the inner layer load-bearing filling material and the outer layer alkali-resistant dense filling material are premixed to obtain the inner layer solid mixture and the outer layer solid mixture;

[0021] Sp3, add water separately and stir vigorously to prepare inner layer paste slurry and outer layer paste slurry, wherein the water-solid mass ratio of the inner layer paste slurry is 0.40-0.90:1 and the water-solid mass ratio of the outer layer paste slurry is 0.35-0.75:1;

[0022] Sp4. Inject the outer layer of paste slurry into the outer cavity of the double-cavity filling bag or the inner layer of bagged filling bag, and inject the inner layer of paste slurry into the inner cavity of the filling bag, so that the outer layer covers the inner layer to form a gradient wall structure and solidifies.

[0023] Preferably, Sp4 employs sequential grouting, first injecting an outer layer of paste slurry into the outer cavity to form an outer coating zone, and then injecting an inner layer of paste slurry into the inner cavity to form an inner bearing zone; and within 0.5 to 6 hours after the injection of the outer layer of paste slurry, the outer coating zone is subjected to controlled carbonization curing, with carbonization conditions of 5 to 20% CO2 volume fraction, curing time of 2 to 12 hours, and curing absolute pressure of 0.09 to 0.11 MPa or 0.10 to 0.15 MPa.

[0024] Preferably, the obtained alleyway support wall meets the following indicators: initial setting within 1 hour after filling; 7-day compressive strength not less than 12 MPa; 28-day compressive strength not less than 15 MPa; and the pH of the leachate is not greater than 11.5 as measured by the following leaching test conditions: the sample after 28 days of curing is crushed to a particle size not greater than 5 mm, deionized water is added at a liquid-to-solid mass ratio of 10:1, the mixture is shaken at (25±2)℃ for (18±2) h, filtered, and the pH of the filtrate is measured.

[0025] Beneficial effects

[0026] This invention provides a modified red mud rapid-setting backfill support material and its preparation method. It has the following beneficial effects:

[0027] 1. This invention utilizes the advantages of red mud's small particle size and high mineral content, employing composite additives to modify it through "alkali reduction and sodium fixation." This modified red mud is then compounded with modified industrial gypsum to prepare a paste-like filling material for roadway side support construction, achieving paste filling while ensuring strength. This not only utilizes red mud waste resources but also reduces the use of natural sand, gravel, and cement, resulting in significant social benefits.

[0028] 2. In this invention, the inner load-bearing filling material is a combination of "red mud (60-80 parts) + mineralized modified cementitious material (15-30 parts) + sulfoaluminate cement or high-alumina cement (2-8 parts)". The mineralized modified cementitious material hydrates rapidly after adding water at room temperature and generates cementitious products. The sulfoaluminate cement or high-alumina cement provides a highly active aluminate phase, which promotes the rapid formation of a three-dimensional skeleton structure in the system. This allows the slurry to change from a pumpable state to a solidified state with self-bearing capacity in a short time. As a result, initial setting can be achieved within 1 hour after filling, and the initial deformation resistance of the roadway side support wall can be quickly established. Simultaneously, by controlling the water-to-solid mass ratio of the inner layer at 0.40–0.90:1 and using double-cavity or bagged filling bags for grouting, the inner grout is constrained and solidified within the bag, which can significantly reduce the probability of segregation settlement and void formation, ensuring the wall section is dense and continuous. As a result, it achieves a compressive strength of not less than 12 MPa in 7 days and not less than 15 MPa in 28 days, meeting the actual needs of early resistance enhancement and mid-term stable bearing for roadway side support along the goaf. Moreover, this effect can be replicated and controlled through on-site parameters such as "mixing ratio—water-to-solid ratio—mixing energy—grouting sequence".

[0029] 3. The outer layer of the alkali-resistant dense filler of this invention, under the combined action of "Si-Al-Mg solid alkali composite additive (3-12 parts) + micro-fine dense filler (0.5-5 parts) + low water-to-solid ratio (0.35-0.75:1)," firstly forms an aluminosilicate network structure through the dissolution and condensation of active Si and active Al sources in a high-alkali porous solution, thereby preventing Na from entering the alkali-resistant dense filler. + The structure is fixed in the form of structural compensation ions, reducing the content of migratable bases; secondly, the formation of hydrotalcite-like structures is promoted through magnesium sources, and CO3 is also affected. 2- SO4 2- Anions are stabilized and fixed, suppressing secondary alkali migration caused by ion fluctuations in the pore solution. Simultaneously, micro-fine fillers fill the pores and reduce the proportion of interconnected pores, forming a low-permeability coating shell on the outer layer, further weakening the alkaline erosion and leaching migration channels of leachate on the inner layer and surrounding rock. On this basis, controlled carbonization with CO2 volume fraction of 5-20% is carried out within 0.5-6 hours after the outer layer is grouted, and cured for 2-12 hours. This allows CO2 to react with the outer pore solution and Ca source to generate carbonate precipitate and secondary densification, while lowering the pH of the pore solution and stabilizing the solid alkali structure. As a result, the pH of the leachate of the 28-day cured sample is no greater than 11.5 under the specified leaching conditions, significantly reducing the risk of alkali return and alkali leakage. This four-fold mechanism of "structure-based sodium fixation + LDH-based anion fixation + carbonization densification + low-permeability coating" gives the wall better water resistance stability and long-term service reliability in water-bearing surrounding rock environments, and is especially suitable for mine working conditions with high risks of seepage, long-term damp heat, or alkali migration. Attached Figure Description

[0030] Figure 1This is a process flow diagram of the present invention;

[0031] Figure 2 This is a diagram illustrating the material preparation process of the present invention;

[0032] Figure 3 This is a diagram illustrating the product manufacturing process of the present invention;

[0033] Figure 4 This is a structural diagram of the gradient wall structure of the dual-cavity filling bag / sleeve filling bag of the present invention. Detailed Implementation

[0034] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Specific Implementation Example 1:

[0036] like Figures 1 to 4 As shown, a modified red mud rapid-setting backfill support material and its preparation method are disclosed. The material comprises an inner layer of load-bearing backfill and an outer layer of alkali-resistant dense backfill, employing a gradient functional design of "inner load-bearing, outer alkali-resistant." The inner layer aims for rapid forming and early load-bearing capacity, while the outer layer aims to reduce permeability, inhibit alkali migration, and improve durability. The two layers form a continuous covering relationship within the same wall cross-section, achieving structural zoning through double-cavity backfill bags or bagged backfill bags. This couples material performance with construction technology into a repeatable and engineerable wall-forming method. To ensure feasibility, it is recommended to incorporate raw materials, microwave mineralization, slurry preparation, pump grouting, bag forming, controlled carbonization, and quality inspection into the same construction control chain, forming a closed-loop control from raw material arrival to wall acceptance. The material consists of the following components by weight:

[0037] The inner load-bearing filling material consists of 60-80 parts red mud, 15-30 parts mineralized modified cementitious material, and 2-8 parts early-setting component. The inner load-bearing filling material follows the implementation logic of "rapid setting—rapid thickening—rapid formation of a load-bearing skeleton." Red mud, as the main fine-grained solid phase, provides the slurry yield stress and anti-segregation foundation. The mineralized modified cementitious material provides hydration reactivity and early cementation structure. The early-setting component is used to lock the initial setting window within 1 hour and improve the strength growth slope from 1 day to 7 days. During engineering implementation, the red mud can be controlled to have a moisture content of no more than [missing value]. 25% of the material should be mineralized and have a particle size of no more than 3mm to reduce pumping fluctuations and lower the risk of settling inside the bag. It is recommended to use powder with a particle size of less than 200 mesh for the mineralized modified cementitious material to improve the reaction rate. It is recommended to uniformly disperse the early-setting component during the premixing stage of the inner layer solid mixture to avoid local instantaneous setting that could cause pipeline blockage. When the surrounding rock pressure is high or faster wall formation is required, the ratio of the inner layer can be adjusted to the upper limit of the mineralized modified cementitious material and the early-setting component. At the same time, the water-solid ratio of the inner layer should be controlled at 0.40 to 0.60:1 to improve the early structural density and shorten the molding time.

[0038] The outer layer of alkali-baric dense filling material consists of 50-75 parts red mud, 15-35 parts mineralized modified cementitious material, 3-12 parts Si-Al-Mg solid-alkali composite additive, and 0.5-5 parts micro-fine filler. The goal of this outer layer is to form a composite barrier of "low-permeability shell + solid-alkali reaction layer." Red mud and mineralized modified cementitious material constitute the outer basic cementing system. The Si-Al-Mg solid-alkali composite additive provides the active Si, active Al, and Mg sources required for the directional mineralization reaction. The micro-fine filler is used to fill pores and optimize particle size distribution. Reduce the proportion of interconnected pores; it is recommended to use a lower water-to-solid ratio (preferably 0.35-0.55:1) for the outer layer to obtain lower porosity and improve the barrier capacity of the outer layer against alkaline leachate. The bleeding rate and segregation rate of the outer slurry should be controlled. Stable grouting can be achieved on site through online density meter and slump expansion monitoring. When there is long-term seepage next to the tunnel or higher durability is required, the dosage of Si-Al-Mg solid-alkali composite additive can be adjusted to the upper limit range, and the micro-fine dense filler can be selected as a combination of bentonite and slag powder to simultaneously improve the water barrier and subsequent cementing and densification effect.

[0039] The mineralized modified cementitious material is a hydraulic cementitious material prepared by microwave segmented mineralization activation of industrial gypsum, fly ash, carbonaceous materials, and red mud. The core principle of this cementitious material is to utilize industrial gypsum to provide Ca and S sources, fly ash to provide an active Al-Si framework, carbonaceous materials to provide reduction and atmosphere control, and red mud to provide Al-Fe-Na multiphase minerals coupled with microwave absorption. This allows for the simultaneous dehydration, phase transformation, mineralization reaction, and amorphous activation during segmented heating in a microwave field, forming a mineralized phase and an active glassy phase with hydraulic reactivity. The resulting powder undergoes a rapid hydration reaction under room temperature and water conditions to generate cementing products. The cementing products and the surface of red mud particles undergo interfacial bonding, forming an early load-bearing framework and a later densified structure. The key to the feasibility of the project lies in controlling the segmented temperature, holding time, and grinding fineness to ensure that the product has sufficient activity without causing low-reactivity agglomeration due to excessive sintering.

[0040] The Si-Al-Mg solid-alkali composite additive is used to promote the formation of a synergistic solid-alkali structure during the curing process, consisting of an aluminosilicate network solid sodium phase and a hydrotalcite-like solid anionic phase. The solid-alkali mechanism adopts a "dual-channel synergistic" pathway. The first channel involves a dissolution-condensation reaction between an active Si source and an active Al source under alkaline conditions, forming an aluminosilicate network structure and converting Na+ into sodium. + The structure is fixed in the network framework in the form of structurally compensating cations, thereby reducing the content of migratable alkali and the alkalinity of the leachate; the second channel forms a hydrotalcite-like structure with dissolved Al species in the system through the Mg source, and its interlayer can adsorb and fix CO3. 2- SO4 2- The presence of anions stabilizes the ionic environment of the system and inhibits the continuous migration of basic ions; the two channels promote each other, and the aluminosilicate network reduces Na+. + Migration, the hydrotalcite-like structure reduces anion migration and fluctuations in pore solution ionic strength, jointly improving the long-term stability and water resistance of the outer alkali-resistant dense zone; to ensure that this mechanism is feasible in engineering, it is necessary to ensure that the active Si source, active Al source and Mg source are uniformly dispersed in the outer layer, and to control the water-to-solid ratio and curing humidity of the outer layer to maintain the continuous dissolution-precipitation reaction.

[0041] The early-setting component is selected from either sulfoaluminate cement or high-alumina cement. This component provides a highly reactive aluminate phase within a short time, promoting the rapid formation of needle-like or plate-like hydration products and a three-dimensional framework, thereby increasing yield stress and shortening initial setting time. When using sulfoaluminate cement, the focus can be on early strength enhancement and rapid molding; when using high-alumina cement, the focus can be on rapid setting and stability in high-alkali environments. In engineering implementation, it is recommended to premix the early-setting component with mineralized modified cementitious materials to ensure rapid dispersion and reaction after entering the slurry. Simultaneously, adjusting the water-to-solid ratio and stirring energy can prevent instantaneous agglomeration that could lead to localized false setting.

[0042] Red mud is a highly alkaline solid waste produced by the Bayer process in the alumina industry. Bayer process red mud typically has high Na2O content and high pH pore solution characteristics. Before project implementation, raw material entry indicators should be established. It is recommended to test at least the moisture content, particle size distribution, loss on ignition, Na2O (or soluble alkali), major oxides (Al2O3, Fe2O3, SiO2, CaO), and chloride ion content. To stabilize pumping and solidification reactions, the red mud should be crushed and homogenized upon arrival and stored. During storage, rain protection measures should be taken to avoid moisture content fluctuations that could cause uncontrolled proportioning. When the initial soluble alkali of the red mud is high, engineering compensation for the solidification effect can be achieved by increasing the dosage of the outer solidification composite additive and extending the controlled carbonization window, while ensuring that the inner early-strength system is not over-carbonized and affects the early reaction rate.

[0043] The raw materials for preparing mineralized modified cementitious materials include, by weight, 55-75 parts of industrial gypsum, 15-30 parts of fly ash, 2-8 parts of carbonaceous material, and 5-20 parts of red mud. The raw material selection and proportioning logic is as follows: industrial gypsum provides the CaSO4 source and undergoes dehydration and phase transformation during segmented heating; fly ash provides the Al-Si active framework and participates in mineralization to generate reactive phases; carbonaceous materials are used to regulate the local redox environment and promote the formation of certain phases, while also synergistically improving heating efficiency in the microwave field; red mud provides Al and Fe phases and acts as a microwave absorption coupler to improve the efficiency of segmented mineralization. During project implementation, it is recommended to control the free water content of industrial gypsum and perform pre-drying treatment to reduce energy consumption fluctuations in the first stage; for fly ash, it is recommended to use Grade II fly ash and control the unburned carbon content to avoid excessive reduction due to superposition with added carbonaceous materials; for carbonaceous materials, it is recommended to use powder with a particle size of less than 200 mesh to ensure uniform mixing; for red mud, it is recommended to crush it to less than 3 mm before addition and further grind it to less than 200 mesh to improve the uniformity of the mineralization reaction; to ensure batch stability, it is recommended to use a metering screw feeder and an online weighing system for continuous batching.

[0044] The preparation method of mineralized modified cementitious materials includes: mixing and homogenizing industrial gypsum, fly ash, carbonaceous materials, and red mud, followed by microwave segmented heating for mineralization activation. The material temperature is adjusted as follows: first stage: 450–650℃, held for 5–20 min; second stage: 650–900℃, held for 10–30 min; subsequently cooled and ground to below 200 mesh to obtain the mineralized modified cementitious material. For process implementation, a continuous or intermittent microwave high-temperature furnace is recommended. The microwave power should be matched with the material thickness to ensure uniform material layer temperature. Temperature measurement is recommended to use infrared thermography and thermocouple verification, with the center temperature of the material layer as the control benchmark. For mixing and grinding, a closed-circuit ball mill or vertical mill system is recommended to achieve uniform mixing and preliminary refinement of the raw materials. After homogenization, the moisture content should be controlled below 2% to reduce moisture absorption fluctuations during the microwave heating stage. The first stage at 450–650℃ mainly completes the gypsum dehydration phase change and partial pre-reaction, and rapid gas release is avoided through temperature control. The first stage, at 650–900℃, primarily involves mineralization and active phase formation, causing material agglomeration. The reaction completion rate is controlled by the holding time to prevent over-burning and sintering. During the cooling stage, natural or forced air cooling is recommended to quickly pass through the sensitive phase transition temperature zone and reduce agglomeration. After cooling, secondary grinding is performed, and the fineness is controlled by the residue on a 200-mesh sieve. For the finished product, specific surface area and initial setting time are recommended as factory inspection items. To facilitate on-site application, ton bags with moisture-proof linings can be used to prevent moisture absorption and subsequent activity degradation during storage.

[0045] The iron phase component in the red mud of the mineralized modified cementitious material, calculated as the sum of Fe2O3 and Fe3O4, has a mass fraction of 10–40%. This limitation is used to ensure that the red mud has sufficient microwave absorption coupling capability, enabling stable volumetric heating and local self-heating effects during segmented heating, thereby reducing heating time and improving mineralization reaction efficiency. In engineering implementation, the Fe2O3 and Fe3O4 content should be confirmed through red mud chemical analysis. If the iron phase content is too low, the microwave absorption capability can be compensated by increasing the dosage of red mud in the raw materials of the mineralized modified cementitious material to the upper limit range. At the same time, uniform heating should be ensured by controlling the loading thickness and microwave power density. If the iron phase content is too high, the second-stage holding time should be shortened and the powder dispersion should be strengthened to avoid local over-burning and sintering, which would lead to a decrease in activity. Verifiable means of this coupling mechanism include the stability of the heating curve, the comparison of energy consumption per unit output, and the consistency of the activity index of the finished product.

[0046] The Si-Al-Mg solid alkali composite additive consists of the following components by weight: 1-6 parts of active Si source, 1-8 parts of active Al source, and 0.5-4 parts of magnesium source; wherein the active Si source is selected from one or two of silica fume and nano-silica, the active Al source is selected from one or two of metakaolin and high-alumina cement, and the magnesium source is selected from one or two of light-burned magnesia and calcined dolomite powder. The formulation logic is as follows: the active Si source is used to provide soluble Si species to promote the formation of aluminosilicate networks and realize Na+. + For structural solidification, the active Al source provides soluble Al species to co-construct the framework with Si species, while also participating in the formation of early cementation products. The magnesium source forms a hydrotalcite-like structure and improves the ionic environment of the pore solution. For the outer alkali-resistant dense zone, it is recommended to dry-mix the composite additives and micro-fine fillers for 3-5 minutes before combining and pre-mixing them with the remaining powders to ensure sufficient dispersion of nano- or ultrafine components. To suppress agglomeration, staged water addition and increased shear strength can be adopted during the mixing and water addition stage. It is recommended to use a twin-shaft forced mixer or a high-speed colloid mixer for mixing. The mixing time is recommended to be controlled at 2-6 minutes, with the uniformity of the slurry and the absence of dry lumps as the criteria. When the salinity of the on-site water is high, deionized water or low-mineralized water should be used preferentially for the preparation of the outer layer to avoid interference of external ions with the solid-alkali reaction pathway and affect the pH control effect.

[0047] The fine micro-filler is selected from one, two, or three of bentonite, slag powder, and ultrafine quartz powder, with an addition amount of 0.5–3 parts. The mechanism of action of the fine micro-filler is the synergistic effect of particle filling and subsequent secondary reaction. Bentonite provides water absorption and swelling, micropore sealing effect, and improves the outer layer's impermeability. Slag powder can undergo a potential hydration reaction in an alkaline environment to form a secondary gel for continuous densification. Ultrafine quartz powder is used for gradation optimization and reducing pore connectivity. During implementation, the water-to-solid ratio of the outer layer and the pumping pressure should be balanced. Excessive use of fine micro-filler will significantly increase viscosity and pumping resistance. It is recommended to determine the pumpable viscosity window through on-site pumping tests and use pipeline pressure drop and filling speed as the basis for parameter adjustment. To ensure the continuity of the outer layer coating, the outer cavity of the bag can be pre-arranged and venting channels can be set before grouting to prevent air pockets and discontinuities during the injection of the outer grout.

[0048] The preparation method includes the following steps: The overall implementation logic of the preparation method is "raw material pretreatment - layered premixing - layered slurry preparation - bag cavity grouting - curing and maintenance - controlled carbonization - quality acceptance". Among them, layered premixing and layered slurry preparation are used to ensure that the reaction path and rheological properties of the inner and outer layers are controllable. Bag cavity grouting is used to ensure that the structural forming and functional zoning are repeatable. Controlled carbonization is used to reduce the pH of the outer layer pore solution and promote carbonate densification and solid alkali stabilization. To achieve engineering, it is recommended to configure a fixed or mobile mixing station, metering system, slurry delivery pump, pressure and flow monitoring device, bag mounting bracket and CO2 supply and pressure regulation device, and establish on-site operating procedures and emergency pipe blockage disposal procedures.

[0049] Sp1. Break the red mud into smaller particles (less than 3mm). This step is used to eliminate red mud agglomeration and hardening and to ensure the uniformity of the slurry. Jaw crushing + screening or impact crushing + screening can be used to ensure that the maximum particle size is no greater than 3mm. Particles larger than 3mm should be recycled and crushed again. It is also recommended to homogenize and stockpile the mud to reduce batch fluctuations. If the red mud has a high moisture content, it can be dried at low temperature or allowed to air dry naturally to stabilize the moisture content. After crushing, it is recommended to take samples and screen them to confirm the particle size distribution and record it as a basis for proportioning correction to ensure the stability of subsequent water-solid ratio control and pumping.

[0050] Sp2. Premix the solid components of the inner layer load-bearing filler and the outer layer alkali-resistant dense filler separately to obtain inner and outer solid mixtures. It is recommended to use a forced dry mixer for premixing and set the dry mixing process parameters for the inner and outer layers separately. The focus of the inner layer premixing is to ensure the uniform dispersion of the early-setting coagulating components and avoid local instantaneous coagulation. The focus of the outer layer premixing is to ensure the full deagglomeration and dispersion of the Si-Al-Mg solid alkali composite additive and the micro-fine dense filler. In practice, it is recommended to add materials in the order of "mixing the ultrafine components first - adding the main powder - and finally adding the red mud" to reduce the agglomeration of ultrafine powder and improve uniformity. The premixing time is recommended to be controlled within 3 to 8 minutes, and the uniformity should be confirmed by random sampling of component fluctuations. It is recommended to use the premixed material within a short period of time to avoid moisture absorption and clumping, which may affect the accuracy of metering and the rheology of the slurry.

[0051] Sp3. Instructions are to separately add water and vigorously stir to prepare inner and outer layer paste slurries. The water-to-solid mass ratio of the inner layer paste slurry is 0.40–0.90:1, and that of the outer layer paste slurry is 0.35–0.75:1. The core control points in the slurry preparation stage are the water-to-solid ratio, stirring energy, and feeding sequence. It is recommended to use a metering pump and flow meter to achieve closed-loop control of water addition, and to adjust the water addition temperature according to the ambient temperature to stabilize the setting time. The wider water-to-solid ratio range of the inner layer is suitable for adapting to different pumping distances and early strength requirements. For faster wall formation and higher early strength, [further details are needed]. For strength, the water-to-solid ratio of the inner layer should be 0.40 to 0.60:1. If the pumping distance is long or the pipe diameter is small, the water-to-solid ratio of the inner layer can be appropriately increased, but segregation should be prevented by increasing the stirring shear and controlling the solid content. The water-to-solid ratio of the outer layer should be lower to form a dense coating layer and reduce the permeation channels. It is recommended to use higher shear for stirring the outer layer to ensure that the solid-alkali composite additive and micro filler are fully dispersed. On-site, slurry density, spread, and standing water bleeding rate can be used as process indicators. Density deviation is used to correct the water-to-solid ratio, spread is used to determine pumpability, and water bleeding rate is used to determine the risk of outer layer densification.

[0052] Sp4. Inject the outer layer of paste slurry into the outer cavity of the double-cavity filling bag or the inner layer of the bag-like filling bag, and inject the inner layer of paste slurry into the inner cavity of the filling bag. This allows the outer layer to cover the inner layer, forming a gradient wall structure that then solidifies. Before installing the bag, the alleyway positioning, bottom leveling, and lateral constraint arrangement should be completed to ensure the cross-section stability of the bag after it becomes a wall. It is recommended to use a process where the outer layer is injected first to form a continuous covering shell, and then the inner layer is injected to form a load-bearing core. This avoids the inner layer of slurry expanding and squeezing first, which would cause discontinuity in the outer layer covering. During the grouting process, vents should be set up and the grouting pressure should be controlled. Excessive pressure will cause local bulging or cracking of the seams in the bag, while insufficient pressure will cause incomplete filling and air trapping. Stable control can be achieved on-site through pressure sensors and grout volume measurement. During the curing stage, the external humidity and temperature conditions of the bag should be maintained to avoid rapid water loss that could lead to surface cracking. If necessary, surface coating or spray curing can be carried out to ensure the continuous solidification reaction of the outer layer.

[0053] Sp4 employs sequential grouting, first injecting an outer layer of paste slurry into the outer cavity to form an outer coating zone, then injecting an inner layer of paste slurry into the inner cavity to form an inner bearing zone; and then subjecting the outer coating zone to controlled carbonization curing within 0.5 to 6 hours after the injection of the outer layer of paste slurry, with carbonization conditions of 5 to 20% CO2 volume fraction, curing time of 2 to 12 hours, and curing absolute pressure of 0.09 to 0.11 MPa or 0.10 to 0.15 MPa. The technical logic of sequential grouting is to first establish a low-permeability outer shell to lock the shape and block seepage, and then establish a high early-strength core to provide load-bearing and deformation resistance. The core principle of controlled carbonization is that CO2 dissolves in the outer pore solution and reacts with Ca source and alkaline ions to form carbonate precipitates that fill the pores and lower the pH of the pore solution. At the same time, it promotes the stabilization of anions in the hydrotalcite-like interlayer and the densification of the outer microstructure, thereby improving the alkali-resistant durability of the outer layer without significantly affecting the early strength of the inner layer. In engineering implementation, it is recommended to set up an independent CO2 injection point in the outer coating area. The CO2 flow rate is controlled by pressure regulating valves and mass flow meters at the inlet and outlet. During curing, the temperature fluctuation of the outer surface of the bag and the internal pressure fluctuations are monitored to avoid damage to the bag due to pressure peaks. For the two pressure windows of "0.09~0.11MPa" and "0.10~0.15MPa", it is recommended to select one on site according to the bag strength grade and sealing performance and implement it in a fixed manner to ensure process consistency and quality traceability. After carbonization, conventional wet curing should be restored to prevent the outer layer from drying too early and affecting the subsequent solidification reaction and the potential reaction of slag powder.

[0054] The resulting alleyway support wall meets the following indicators: initial setting within 1 hour after filling; 7-day compressive strength not less than 12 MPa; 28-day compressive strength not less than 15 MPa; and the pH of the leachate is not greater than 11.5 as measured by the following leaching test conditions: the sample after 28 days of curing is crushed to a particle size not greater than 5 mm, deionized water is added at a liquid-to-solid mass ratio of 10:1, shaken at (25±2)℃ for (18±2) h, filtered, and the pH of the filtrate is measured. The above indicator system corresponds to three types of engineering objectives: "construction window—bearing capacity—environmental constraints." Initial setting within 1 hour is used to ensure rapid wall formation along the roadway and reduce the risk of cumulative surrounding rock deformation. The 7-day and 28-day strength measurements are used to match the short-term resistance enhancement and medium-term stable bearing requirements of the roadway support wall. A leachate pH of no more than 11.5 is used to verify the alkalinity control effect after the synergistic effect of outer layer alkali resistance and controlled carbonization. To ensure the repeatability of compressive strength evaluation, it is recommended to prepare strength specimens on-site or in the laboratory using a unified method, controlling the molding density and curing conditions. The preferred method is standard wet curing conditions of 20–25℃ and relative humidity of no less than 95%, with strength tests conducted at 7 and 28 days. For leaching tests, it is recommended to simultaneously measure the Na+ in the filtrate. + Concentration and conductivity are used to reflect the content of migratable bases and ionic strength, if Na +Both concentration and pH meet the target, which can be used as indirect engineering criteria for the formation of the outer solid alkali structure and the carbonization and densification effect. In practical applications, "initial setting time, pumping pressure drop, outer layer water seepage, 7-day strength, 28-day strength, and leaching pH" can be used as acceptance data packages to form an auditable quality record system to support large-scale promotion and parameter reuse under different mine conditions.

[0055] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising a reference structure" does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes the element.

[0056] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A modified red mud rapid-setting backfill support material and its preparation method, characterized in that: The material comprises an inner layer of load-bearing filler and an outer layer of alkali-resistant dense filler, and is composed of the following components by weight: Inner layer load-bearing filling material: 60-80 parts red mud, 15-30 parts mineralized modified cementitious material, and 2-8 parts early-strength coagulating component; Outer layer alkali-resistant dense filling material: 50-75 parts red mud, 15-35 parts mineralized modified cementitious material, 3-12 parts Si-Al-Mg solid alkali composite additive, and 0.5-5 parts micro-fine dense filler; The mineralized modified cementitious material is a hydraulic cementitious material prepared by microwave segmented mineralization activation of industrial gypsum, fly ash, carbonaceous materials and red mud; the Si-Al-Mg solid-alkali composite additive is used to promote the formation of a synergistic solid-alkali structure of aluminosilicate network solid sodium phase and hydrotalcite-like solid anionic phase during the curing process; the early-setting component is selected from sulfoaluminate cement and high-alumina cement.

2. The modified red mud rapid-setting backfill support material according to claim 1, characterized in that: The red mud is a highly alkaline solid waste produced by the Bayer process in the alumina industry.

3. The modified red mud rapid-setting backfill support material according to claim 1, characterized in that: The raw materials for preparing the mineralized modified cementitious material include, by weight, 55-75 parts of industrial gypsum, 15-30 parts of fly ash, 2-8 parts of carbonaceous material, and 5-20 parts of red mud.

4. The modified red mud rapid-setting backfill support material according to claim 1, characterized in that: The preparation method of the mineralized modified cementitious material includes: mixing industrial gypsum, fly ash, carbonaceous materials and red mud, grinding and homogenizing them, and then performing microwave segmented heating for mineralization activation, so that the material temperature meets the following requirements: first stage 450-650℃ for 5-20 min; second stage 650-900℃ for 10-30 min; then cooling and grinding to below 200 mesh to obtain the mineralized modified cementitious material.

5. The modified red mud rapid-setting backfill support material according to claim 1, characterized in that: The iron phase component in the red mud of the mineralized modified cementitious material has a mass fraction of 10-40% as the sum of Fe2O3 and Fe3O4.

6. The modified red mud rapid-setting backfill support material according to claim 1, characterized in that: The Si-Al-Mg solid alkali composite additive is composed of the following components by weight: 1-6 parts of active Si source, 1-8 parts of active Al source, and 0.5-4 parts of magnesium source; wherein, the active Si source is selected from one or two of silica fume and nano silica, the active Al source is selected from one or two of metakaolin and high alumina cement, and the magnesium source is selected from one or two of lightly calcined magnesia and calcined dolomite powder.

7. The modified red mud rapid-setting backfill support material according to claim 1, characterized in that: The micro-fine filler is selected from one, two, or three of bentonite, slag powder, and ultrafine quartz powder, and the amount added is 0.5 to 3 parts.

8. A method for preparing a modified red mud rapid-setting backfill support material according to any one of claims 1-7, characterized in that: The preparation method includes the following steps: Sp1. Break the red mud into smaller than 3mm; Sp2, the solid components of the inner layer load-bearing filling material and the outer layer alkali-resistant dense filling material are premixed to obtain the inner layer solid mixture and the outer layer solid mixture; Sp3, add water separately and stir vigorously to prepare inner layer paste slurry and outer layer paste slurry, wherein the water-solid mass ratio of the inner layer paste slurry is 0.40-0.90:1 and the water-solid mass ratio of the outer layer paste slurry is 0.35-0.75:1; Sp4. Inject the outer layer of paste slurry into the outer cavity of the double-cavity filling bag or the inner layer of bagged filling bag, and inject the inner layer of paste slurry into the inner cavity of the filling bag, so that the outer layer covers the inner layer to form a gradient wall structure and solidifies.

9. The preparation method of a modified red mud rapid-setting backfill support material according to claim 8, characterized in that: Sp4 employs sequential grouting, first injecting an outer layer of paste slurry into the outer cavity to form an outer coating zone, then injecting an inner layer of paste slurry into the inner cavity to form an inner bearing zone; and then subjecting the outer coating zone to controlled carbonization curing within 0.5 to 6 hours after the injection of the outer layer of paste slurry, with carbonization conditions of 5 to 20% CO2 volume fraction, curing time of 2 to 12 hours, and curing absolute pressure of 0.09 to 0.11 MPa or 0.10 to 0.15 MPa.

10. The preparation method of a modified red mud rapid-setting backfill support material according to claim 9, characterized in that: The resulting alleyway support wall meets the following indicators: initial setting within 1 hour after filling; 7-day compressive strength not less than 12 MPa; 28-day compressive strength not less than 15 MPa; and the pH of the leachate is not greater than 11.5 as measured by the following leaching test conditions: the sample after 28 days of curing is crushed to a particle size not greater than 5 mm, deionized water is added at a liquid-to-solid mass ratio of 10:1, shaken at (25±2)℃ for (18±2) h, filtered, and the pH of the filtrate is measured.

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