Construction process of red mud-based concrete retaining wall

Abstract

This invention discloses a red mud-based concrete retaining wall and its construction process, comprising the following steps: S1, pretreatment and performance analysis of red mud materials; S2, composite modification treatment of red mud materials, employing a synergistic modification technology of curing agent doping, mineral admixture, and surface-active treatment to optimize its mechanical properties and durability; S3, mix design and preparation of red mud-based concrete, determining a mix proportion that meets the strength requirement of C20 or higher based on modified red mud, cementitious materials, aggregates, admixtures, and water; S4, foundation trench treatment and formwork support for the retaining wall; S5, mixing, transportation, and pouring of red mud-based concrete; S6, concrete vibration, finishing, and curing; S7, formwork removal and backfilling behind the wall. This invention also provides a red mud-based concrete retaining wall produced by this process. This invention effectively solves the environmental pressure of red mud storage, reduces engineering costs, improves resource utilization, and has significant economic, social, and environmental benefits.

Description

Technical Field

[0001] This invention relates to the field of road engineering technology, and in particular to a construction process for a red mud-based concrete retaining wall. Specifically, it relates to an environmentally friendly building material prepared from industrial solid waste and its construction method, and especially to a red mud-based concrete retaining wall and its complete construction process. Background Technology

[0002] With the rapid development of infrastructure construction, the demand for traditional building materials such as sand, stone, and cement is enormous, leading to increased resource consumption and environmental pressure. At the same time, the alumina industry discharges massive amounts of red mud solid waste annually. Red mud is a highly alkaline solid residue produced after extracting alumina from bauxite; approximately 1.0 to 2.0 tons of red mud are generated for every ton of alumina produced. Statistics show that the amount of red mud stockpiled exceeds hundreds of millions of tons and continues to increase by tens of millions of tons annually. The large-scale stockpiling of red mud not only occupies land, but its high alkalinity (pH 10-13) and potential presence of trace heavy metal ions also pose a risk of polluting groundwater and soil. Furthermore, dam failures could seriously threaten the ecological environment and the safety of people's lives and property. Therefore, realizing the large-scale, high-value utilization of red mud has become an urgent industrial and social need.

[0003] Applying red mud to building materials, especially civil engineering materials, is one of the important ways to utilize red mud. While there are some existing studies on the resource utilization of red mud, these studies and application attempts often have limitations:

[0004] Applications tend to be limited to low-end or non-load-bearing structures: Most studies focus on using red mud as roadbed filler, pavement base stabilizing material, or to prepare non-load-bearing blocks and bricks. Although recent studies have reported that red mud can be made into high-strength materials (such as compressive strength exceeding 60 MPa) under laboratory conditions, or has been used to replace cement in road bases, directly applying these achievements to highway retaining walls (especially gravity retaining walls), which have extremely high requirements for structural integrity, long-term durability, and safety, lacks technical feasibility and engineering reliability verification.

[0005] Limited by technical approaches, large-scale engineering applications are difficult: Some patented technologies rely on special processing techniques (such as microwave hydrothermal curing), which require sophisticated equipment and consume a lot of energy, making them unsuitable for the dispersed, large-volume pouring characteristics of highway engineering projects. More commonly, existing technologies often focus only on the material formulation itself or only provide laboratory preparation methods, severely lacking systematic process parameters and quality control standards that connect with on-site construction processes (such as batching, transportation, pouring, vibration, and curing).

[0006] The improvement of inherent defects of red mud is not systematic: the high alkalinity and compositional fluctuations of red mud can affect the workability, long-term volume stability and durability of concrete. Existing modification methods may be relatively simple (such as using only cement for curing) and have failed to form a comprehensive solution for all the challenges it faces when used in structural concrete (such as the risk of alkali-aggregate reaction, tendency for early plastic cracking, and differences in strength development patterns).

[0007] Therefore, a long-standing and unresolved technical problem in this field exists: how to develop a high-value utilization technology for red mud that can not only be realized in the laboratory but also directly applied to highway engineering sites, ensuring structural safety and reliability, and possessing complete construction processes and quality control standards, especially for critical structures such as retaining walls. The lack of existing technology has led to a cautious attitude in the engineering community towards the large-scale, high-value-added application of red mud, hindering the resource utilization process of this solid waste. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the purpose of this invention is to provide a construction process for red mud-based concrete retaining walls. This process is the first to systematically apply composite-modified red mud on a large scale to highway gravity retaining walls, forming a complete technical system from raw material processing, modification, concrete preparation to on-site construction and maintenance. This realizes the high-value and large-scale utilization of red mud, while ensuring the safety, durability and economy of the retaining wall structure.

[0009] The above-mentioned objective of this invention is achieved through the following technical solutions:

[0010] This invention provides a construction process for a red mud-based concrete retaining wall, comprising the following steps: S1, Red mud material pretreatment and performance analysis: The raw red mud is dried, crushed, and sieved, and its physicochemical composition and heavy metal leaching characteristics are determined; S2, Red mud material composite modification treatment: The pretreated red mud is uniformly mixed with a curing agent, mineral admixtures, and surfactants to perform composite modification, thereby obtaining modified red mud material; the curing agent includes at least one of cement, lime, and carbide slag; the mineral admixture includes at least one of fly ash, slag powder, and metakaolin; S3, Red mud-based concrete mix design and preparation: Using the modified red mud material as a key component, it is mixed with cementitious materials, coarse aggregate, fine aggregate, admixtures, and water to design and determine the concrete mix proportion that meets the target strength grade; S4, Retaining wall foundation trench treatment and formwork support: According to... The design drawings are used for measurement and layout. The foundation trench is excavated to the design elevation. The foundation is compacted and leveled. Then, a formwork system that meets the design dimensions and stability is erected. S5. Mixing, transportation and pouring of red mud-based concrete: The components determined in step S3 are put into the concrete mixing plant for centralized mixing to obtain red mud-based concrete mixture with qualified workability. The mixture is transported to the site by concrete truck and poured into the formwork supported in step S4 in layers and continuously. S6. Concrete vibration, finishing and curing: The poured concrete is fully vibrated with an immersion vibrator to remove air bubbles and ensure compaction. The surface is smoothed before the concrete initially sets. After pouring, water-retaining material is covered in time and moisturizing curing is carried out for no less than 7 days. S7. Formwork removal and backfilling: After the concrete strength reaches more than 75% of the design strength, the formwork is removed. Then, the backfill material is backfilled symmetrically in layers and compacted.

[0011] According to one embodiment of the present invention, in step S2, the specific process of the composite modification is as follows: first, the red mud is premixed with a surfactant accounting for 1% to 5% of the dry weight of the red mud, and allowed to stand for aging for 2 to 4 hours; then, a curing agent accounting for 8% to 20% of the dry weight of the red mud and a mineral admixture accounting for 10% to 30% of the dry weight of the red mud are added, and the mixture is mechanically mixed in a stirring device for 15 to 30 minutes to ensure uniform mixing; after mixing, the mixture is piled up and cured for 3 to 7 days to obtain the modified red mud material.

[0012] According to one embodiment of the present invention, the surfactant is sodium dodecylbenzenesulfonate, lignin sulfonate, polycarboxylate superplasticizer, or one or more thereof; the curing agent is a composite of ordinary silicate cement with a strength grade of 42.5 and quicklime in a mass ratio of (3-5):1; the mineral admixture is a composite of grade II fly ash and grade S95 slag powder in a mass ratio of 1:(1-2).

[0013] According to one embodiment of the present invention, in step S3, the mix proportion of the red mud-based concrete includes the following components per cubic meter: modified red mud material: 180-250 kg; cementitious material: 280-350 kg, wherein the cementitious material is P·O42.5 cement; coarse aggregate: 1000-1200 kg, wherein the coarse aggregate is continuously graded crushed stone with a maximum particle size not exceeding 31.5 mm; fine aggregate: 650-750 kg, wherein the fine aggregate is medium sand with a fineness modulus of 2.3-3.0; admixture: 4-8 kg, wherein the admixture is polycarboxylate high-performance water-reducing agent; water: 150-180 kg; the 28-day cubic compressive strength of the red mud-based concrete is not less than 30 MPa, and the workability meets the requirement of a slump of 150 mm ± 30 mm.

[0014] According to one embodiment of the present invention, in step S4, after the foundation trench is excavated, if the foundation is a soft soil layer, it needs to be replaced. The replacement material is graded gravel or modified red mud stabilized soil, which is compacted in layers with a compaction degree of not less than 93%. The template system adopts steel templates. Before installation, a release agent is applied. After installation, it is ensured that the joints are tight, the support is firm, the verticality deviation of the template is less than 1%, and the plane position deviation is less than ±20mm.

[0015] According to one embodiment of the present invention, in step S5, the mixing is carried out using a forced mixer, and the total mixing time is not less than 120 seconds; the transportation is carried out using a mixer transport vehicle, and the mixing is maintained during transportation to prevent segregation, and the time from discharge to placement into the mold does not exceed 90 minutes; during the pouring, a layered progressive method is adopted, with each layer having a thickness of not more than 50cm, and the interval between two adjacent layers does not exceed the initial setting time of the lower layer of concrete.

[0016] According to one embodiment of the present invention, in step S6, the vibration is performed using an immersion vibrator. The vibrator rod should be inserted vertically into the concrete, inserted quickly and withdrawn slowly, with the moving distance not exceeding 1.5 times the effective radius of the vibrator rod. The insertion depth into the lower layer of concrete is 5-10 cm, and the vibration duration at each vibration point is 20-30 seconds, until the concrete surface shows a layer of slurry, no longer sinks significantly, and no air bubbles emerge. The curing is performed by covering with geotextile or plastic film and sprinkling water to keep it moist. During the curing period, the temperature difference between the concrete surface and the outside temperature should not exceed 25°C, and the pH value of the curing water should be neutral.

[0017] According to one embodiment of the present invention, in step S7, the backfill material behind the wall is preferably permeable gravel or modified red mud stabilized soil made by mixing modified red mud obtained in step S2 with soil; the backfill must be carried out in layers, with each layer having a loose thickness of no more than 30cm, and is symmetrically rolled or compacted using a small road roller or plate compactor, with the compaction degree meeting the design requirements and not less than 95%.

[0018] The present invention also provides a red mud-based concrete retaining wall constructed using the construction process described above. The retaining wall is a gravity or semi-gravity structure, and its main structure is made of red mud-based concrete. In the red mud-based concrete, the amount of modified red mud material accounts for 50% to 70% of the total mass of cementitious materials. After 28 days of curing, the compressive strength of the concrete core samples of the key parts of the retaining wall is not lower than the design strength grade, the impermeability grade is not lower than P6, and the frost resistance grade is not lower than F50.

[0019] According to one embodiment of the present invention, a monitoring element is pre-embedded in the wall body of the retaining wall for long-term monitoring of the stress, strain and displacement of the wall body; the top, back and drainage hole parts of the retaining wall are protected by an alkali-resistant coating or surface layer.

[0020] In summary, compared with the prior art, the present invention has at least one of the following beneficial technical effects:

[0021] High-value and large-scale disposal of red mud: This invention, through innovative composite modification technology, significantly improves the effective utilization rate of red mud in concrete (accounting for more than 50% of cementitious materials), breaking through the limitations of traditional low-dosage applications, and providing a practical and feasible engineering path for the disposal of million-ton-level red mud solid waste, with extremely significant environmental benefits.

[0022] Reliable performance and guaranteed structural safety: Through systematic material design and process control, the red mud-based concrete produced not only meets the strength requirements of retaining walls (28-day strength ≥30MPa), but also its durability indicators such as impermeability and frost resistance reach or even exceed those of conventional concrete, ensuring the safety and stability of the retaining wall structure in long-term complex environments.

[0023] The invention provides a complete set of construction techniques with strong operability: For the first time, it provides a complete process chain from "red mud pretreatment → composite modification → concrete preparation → on-site construction (layout, formwork, pouring, vibration, curing, backfilling)". Each step has specific technical parameters and quality control points, forming a standardized, replicable and scalable set of construction methods, solving the "last mile" problem in engineering applications.

[0024] Significant economic benefits: Red mud, as an industrial waste residue, has extremely low cost. Using this invention can significantly reduce the cost of concrete materials. According to calculations, compared to traditional C20 concrete retaining walls, the unit cost of red mud-based concrete can be reduced by more than 5%. At the same time, it reduces the costs of red mud storage and disposal, as well as the environmental costs of traditional building material production, resulting in significant overall economic benefits.

[0025] Promoting technological progress in the industry: This invention closely integrates the resource utilization of solid waste with the construction of major transportation infrastructure, which is in line with the national green development and "zero-waste city" construction direction. It has a demonstrative and leading role in promoting technological innovation and industrial upgrading in the building materials and infrastructure industries. Attached Figure Description

[0026] Figure 1 This is a flowchart of the present invention. Detailed Implementation

[0027] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0028] In the description of this application, it should be noted that the terms "upper," "lower," "inner," "outer," "top / bottom," etc., indicating the orientation or positional relationship are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0029] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installed," "equipped with," "sleeved / connected," "connected," etc., should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; it can be a connection within two components. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0030] Firstly, referring to Figure 1 This invention provides a construction process for a red mud-based concrete retaining wall, comprising the following steps:

[0031] S1. Pretreatment and performance analysis of red mud materials: The raw red mud is dried, crushed, and sieved, and its physicochemical composition and heavy metal leaching characteristics are determined.

[0032] S2. Composite modification treatment of red mud materials: The pretreated red mud is uniformly mixed with a curing agent, mineral admixtures and surfactants to carry out composite modification and obtain modified red mud materials; the curing agent includes at least one of cement, lime and carbide slag; the mineral admixtures include at least one of fly ash, slag powder and metakaolin.

[0033] S3. Red mud-based concrete mix design and preparation: Using modified red mud as the key component, it is mixed with cementitious materials, coarse aggregate, fine aggregate, admixtures and water to design and determine the concrete mix proportion that meets the target strength grade.

[0034] S4. Retaining wall foundation trench treatment and formwork support: Measure and set out according to the design drawings, excavate the foundation trench to the design elevation, compact and level the foundation, and then erect a formwork system that meets the design dimensions and stability.

[0035] S5. Mixing, transporting and pouring of red mud-based concrete: The components determined in step S3 are put into the concrete mixing plant for centralized mixing to obtain red mud-based concrete mixture with qualified workability; the mixture is transported to the site by concrete truck and poured into the formwork supported in step S4 in layers and continuously.

[0036] S6. Concrete Vibration, Finishing and Curing: Use an immersion vibrator to fully vibrate the poured concrete to remove air bubbles and ensure compaction; smooth the surface before the concrete initially sets; cover with water-retaining material in a timely manner after pouring and perform moist curing for no less than 7 days.

[0037] S7. Formwork Removal and Backfilling: After the concrete strength reaches more than 75% of the design strength, remove the formwork; then backfill the wall material in layers symmetrically and compact it.

[0038] Preferably, in step S2, the specific process of composite modification is as follows: First, red mud is premixed with a surfactant accounting for 1% to 5% of the dry weight of the red mud, and allowed to stand for aging for 2 to 4 hours; then, a curing agent accounting for 8% to 20% of the dry weight of the red mud and a mineral admixture are added, and mechanically mixed in a stirring device for 15 to 30 minutes to ensure uniform mixing; after mixing, the mixture is piled up and cured for 3 to 7 days to obtain the modified red mud material. This stepwise composite modification process first uses surfactants to improve the surface properties of red mud particles and reduce their hydrophilicity, which is beneficial to the uniform mixing and reaction with other components in the later stage; then, the curing agent and mineral admixture are introduced, and through the synergistic effects of physical filling and chemical activation (such as the reaction of Ca(OH)2 with active SiO2 and Al2O3 in red mud), the alkalinity of the system is effectively reduced, the gelation activity is improved, and the particle size distribution is improved, thereby comprehensively improving the performance of the modified product.

[0039] Preferably, the surfactant is one or more of sodium dodecylbenzene sulfonate, lignin sulfonate, and polycarboxylate superplasticizer. The curing agent is a composite of ordinary Portland cement with a strength grade of 42.5 and quicklime at a mass ratio of (3-5):1. The mineral admixture is a composite of Grade II fly ash and Grade S95 slag powder at a mass ratio of 1:(1-2). This preferred combination leverages the synergistic effect between materials: cement provides early strength, lime provides a highly alkaline environment and activates the activity of red mud and mineral admixtures; fly ash and slag powder have pozzolanic and micro-aggregate effects, which can further consume calcium hydroxide, refine the pore structure, and improve later strength and durability.

[0040] Preferably, in step S3, the mix proportion of the red mud-based concrete includes the following components per cubic meter: modified red mud material: 180–250 kg; cementitious material: 280–350 kg (preferably P·O 42.5 cement); coarse aggregate: 1000–1200 kg (continuously graded crushed stone, maximum particle size ≤31.5 mm); fine aggregate: 650–750 kg (medium sand with a fineness modulus of 2.3–3.0); admixture: 4–8 kg (polycarboxylate superplasticizer); water: 150–180 kg. This mix proportion design ensures that the modified red mud material (considered as an active functional admixture) accounts for more than 50% of the total cementitious material, achieving a high proportion of red mud utilization. Through precise control of admixtures, the concrete mixture was guaranteed to have excellent workability (slump 150mm±30mm) and a 28-day compressive strength that remained stable above 30MPa, meeting the conventional strength design requirements of C20~C25 for highway retaining walls.

[0041] Preferably, in step S4, after the foundation trench is excavated, if the foundation is a soft soil layer, replacement treatment is required. The replacement material is graded gravel or modified red mud stabilized soil, which is compacted in layers with a compaction degree of not less than 93%. The formwork system uses steel formwork. Before installation, a release agent is applied, and after installation, it is ensured that the joints are tight, the supports are firm, the verticality deviation of the formwork is less than 1%, and the planar position deviation is less than ±20mm. Strict foundation trench treatment is fundamental to ensuring the bearing capacity and overall stability of the retaining wall base, while high-precision formwork support is key to ensuring the geometric dimensions and appearance quality of the wall.

[0042] Preferably, in step S5, a forced mixer is used for mixing, and the total mixing time is not less than 120 seconds to ensure that all components, especially the modified red mud material, are evenly dispersed. A mixer truck is used for transportation, and the mixture is kept stirred during transport to prevent segregation. The time from discharge to placement in the mold does not exceed 90 minutes to prevent loss of workability. During pouring, a layered approach is adopted, with each layer not exceeding 50cm in thickness. The interval between two adjacent layers should not exceed the initial setting time of the lower layer of concrete to ensure good interlayer bonding and avoid cold joints.

[0043] Preferably, in step S6, an immersion vibrator is used for compaction. The vibrator rod should be inserted vertically into the concrete, inserted quickly and withdrawn slowly, with a moving distance not exceeding 1.5 times the effective radius of the vibrator rod. The insertion depth into the lower layer of concrete is 5-10 cm, and the vibration duration at each point is 20-30 seconds, until the concrete surface shows a layer of slurry, no longer significantly sinks, and no air bubbles emerge. Sufficient compaction is a necessary step to ensure the internal density and strength development of the red mud-based concrete. Curing is carried out by covering with geotextile or plastic film and sprinkling water to maintain moisture. During curing, the temperature difference between the concrete surface and the outside temperature should not exceed 25°C, and the pH value of the curing water should be neutral. To address the potential early hydration heat and alkalinity issues in red mud-based concrete, strengthening moisture retention and temperature control during curing is crucial, effectively preventing shrinkage cracks and promoting long-term performance development.

[0044] Preferably, in step S7, the backfill material behind the wall should preferably be permeable gravel or modified red mud stabilized soil prepared by mixing modified red mud obtained in step S2 with the soil. This facilitates drainage behind the wall and reduces hydrostatic pressure. Backfilling must be carried out in layers, with each layer having a loose thickness of no more than 30 cm. A small road roller or plate compactor should be used for symmetrical rolling or tamping, ensuring the compaction degree meets design requirements and is no less than 95%. Symmetrical layered compaction is a key measure to prevent eccentric pressure and displacement of the retaining wall during backfilling.

[0045] Secondly, this invention provides a red mud-based concrete retaining wall constructed using any of the aforementioned construction techniques. The retaining wall is a gravity or semi-gravity structure, with its main structure made of red mud-based concrete. In the red mud-based concrete, the amount of modified red mud material accounts for 50% to 70% of the total mass of the cementitious materials, achieving efficient utilization of red mud. Experience has verified that after 28 days of curing, the compressive strength of concrete core samples from key parts of the retaining wall is not lower than the design strength grade, the impermeability grade is not lower than P6, and the frost resistance grade is not lower than F50, meeting the durability requirements of highway engineering for retaining wall structures.

[0046] Furthermore, monitoring elements such as stress gauges, strain gauges, or inclinometers can be pre-embedded within the retaining wall to monitor the stress distribution, strain state, and horizontal displacement of the wall over the long term, providing data support for the safe operation of the project and the design of similar projects. In addition, protective treatments such as alkali-resistant polymer mortar coatings or facings can be applied to the top, back, and drainage holes of the retaining wall, which are susceptible to rainwater erosion or the potential release of alkaline substances, further enhancing its long-term service performance.

[0047] Example 1: Mix design and performance verification of red mud-based concrete

[0048] This embodiment mainly illustrates the modification of red mud and the preparation and performance testing process of red mud-based concrete.

[0049] Raw materials and pretreatment:

[0050] Red mud: Taken from an alumina plant stockpile, it is Bayer process red mud. After drying at 105℃ and crushing, it was sieved through a 0.075mm square-hole sieve. Its chemical composition (mass fraction) was mainly: SiO2 18.5%, Al2O3 22.1%, Fe2O3 30.8%, CaO 4.2%, Na2O 6.5%, and loss on ignition 10.2%. Leaching toxicity testing was conducted according to the "Solid Waste Leaching Toxicity Leaching Method - Sulfuric Acid and Nitric Acid Method" (HJ / T 299). The heavy metal content was all below the limits of the "Identification Standard for Hazardous Waste" (GB 5085.3), and it was determined to be general industrial solid waste. The initial pH value was 11.8.

[0051] Other materials: P·O 42.5 ordinary Portland cement (compliant with GB 175); quicklime (effective CaO content ≥85%); Grade II fly ash (compliant with GB / T 1596); S95 grade granulated blast furnace slag powder (compliant with GB / T 18046); industrial grade sodium dodecylbenzene sulfonate (SDBS); polycarboxylate high-performance water-reducing agent (solid content 40%); tap water; 5~31.5mm continuously graded crushed stone; Zone II medium sand with a fineness modulus of 2.7.

[0052] Optimization of red mud composite modification process:

[0053] This invention reveals that simple mechanical mixing is insufficient to ensure sufficient interaction between the modifier and red mud particles. A stepwise mechanical activation modification process is employed:

[0054] a. Pre-dispersion and surface modification: Take 1000 kg of dry red mud powder, add 20 kg of SDBS (accounting for 2% of the dry weight of red mud), and mix for 5 minutes using a high-speed vortex mixer (speed 1500 rpm) to allow the surfactant to initially coat the red mud particles and improve their surface hydrophilicity. After mixing, let it stand and age in a sealed container for 3 hours.

[0055] b. Construction of the composite cementitious system: Transfer the aged material into a horizontal twin-shaft forced mixer (capacity 2m³). Add 120kg of P·O 42.5 cement and 30kg of quicklime (total weight of curing agent 150kg, accounting for 15% of the dry weight of red mud), then add 200kg of Grade II fly ash and 100kg of S95 slag powder (total weight of mineral admixtures 300kg, accounting for 30% of the dry weight of red mud). Stir at 45 rpm for 25 minutes until the material is uniform in color and free of visible lumps or color differences.

[0056] c. Curing and Activation: The mixture was transferred to an airtight curing chamber, piled to a height of approximately 1.5m, and covered with a plastic film to maintain humidity. It was cured for 5 days at an ambient temperature of (25±5)℃. During curing, the temperature at the center of the pile was monitored daily. Records showed a temperature peak on days 2-3, reaching a maximum of approximately 45℃, followed by a slow decline, indicating continuous hydration and pozzolanic reaction in the system. At the end of curing, the pH value of the modified red mud material was measured to have decreased to approximately 10.2, with a moisture content of approximately 8%.

[0057] Concrete mix design and performance testing:

[0058] Design objective: C25 red mud-based concrete with a slump of 160mm±20mm to meet the construction requirements of highway gravity retaining walls.

[0059] Benchmark mix proportion: After systematic trial mixing, the material usage per cubic meter of concrete was determined as follows: modified red mud: 220 kg; P·O 42.5 cement: 320 kg; crushed stone: 1100 kg; sand: 700 kg; water: 165 kg; polycarboxylate superplasticizer: 6.5 kg (1.2% of the total cementitious materials). Under this mix proportion, modified red mud accounts for 40.7% of the total cementitious materials (cement + modified red mud). If the 150 kg of cement and lime contained in the modified red mud itself are included, the active components from red mud contribute more than 55% to the total cementitious system, achieving a high proportion of red mud utilization.

[0060] Fresh concrete performance testing: Tested according to the "Standard for Test Methods of Performance of Ordinary Concrete Mixtures" (GB / T 50080). In the example, the initial slump of the concrete was 170 mm, the spread was 450 mm, and the slump loss after 1 hour was 25 mm. The mixture exhibited good cohesion, with no segregation or bleeding.

[0061] Hardened mechanical properties: Under standard curing conditions, the test results are shown in Table 1 below. Two comparative examples were also set up: Comparative example A (ordinary C25 concrete): 380 kg of cement, no red mud, and the absolute volume of other materials remained unchanged from this example; Comparative example B (unmodified red mud concrete): an equal amount (220 kg) of dried unmodified undisturbed red mud was used to directly replace the modified red mud material in this example, and the amount of other materials remained unchanged.

[0062] Table 1 Comparison of the Development of Mechanical Properties of Concrete

[0063] Group 3D compressive strength (MPa) 7-day compressive strength (MPa) 28-day compressive strength (MPa) 28-day flexural strength (MPa) Embodiments of the present invention 15.8 24.3 36.7 5.2 Comparative Example A (Ordinary Concrete) 16.5 25.1 38.2 5.4 Comparative Example B (Unmodified Red Mud) 6.2 10.5 18.5 2.8

[0064] Analysis: The concrete in this embodiment of the invention achieves a 28-day compressive strength of 36.7 MPa, fully meeting the C25 design grade requirements, and its performance difference from ordinary concrete (38.2 MPa) is minimal (relative strength 96%). In contrast, Comparative Example B (using unmodified red mud) exhibits a severely insufficient strength of only 18.5 MPa, fully demonstrating the necessity and crucial role of the composite modification technology in this invention. The strength development pattern of the concrete in this embodiment (3-day strength approximately 43% of 28-day strength) is similar to that of ordinary concrete, facilitating project progress control.

[0065] Long-term durability performance verification:

[0066] Permeability resistance: Permeability tests were conducted according to the "Standard for Test Methods of Long-Term Performance and Durability of Ordinary Concrete" (GB / T 50082). After 24 hours of constant pressure at 1.2 MPa, the average water penetration height of the specimens in the example was 35 mm, and their permeability resistance grade was calculated to be greater than P10. Comparative Example A was P12, and the specimen in Comparative Example B had already permeated under a pressure of 0.8 MPa.

[0067] Freeze-thaw resistance: A rapid freeze-thaw test (F100) was conducted. After 100 freeze-thaw cycles, the concrete in the example showed a mass loss rate of 2.1% and a relative dynamic modulus of elasticity retention of 89%, meeting the F100 requirements (mass loss ≤5%, dynamic modulus of elasticity ≥60%). Comparative Example A showed 1.8% and 91% respectively, while Comparative Example B failed after 50 cycles.

[0068] Drying shrinkage: The drying shrinkage value was tested after 180 days. The example sample was 420 × 10⁻⁻⁴. 6 Compared with Comparative Example A (400×10⁻ 6 It is comparable to, but far lower than, comparative example B (580×10⁻). 6 This indicates that the modification effectively suppressed the shrinkage increase problem that might be caused by the high alkalinity and particle characteristics of red mud.

[0069] Resistance to carbonation and steel corrosion: 28-day rapid carbonation depth was 4.5 mm in the example and 3.8 mm in Comparative Example A. In the accelerated salt spray test of reinforced concrete specimens (25 mm protective layer), the initial rusting time of the steel in the example specimen was delayed by approximately 15% compared to Comparative Example A, indicating that the highly alkaline environment still has a positive effect on steel protection after modification and did not induce harmful alkali-aggregate reaction (assessed according to the "Standard for Alkali Content Limits in Concrete" CECS53).

[0070] Example 2: On-site construction process of red mud-based concrete retaining wall

[0071] Reference Figure 1 This embodiment is based on a shoulder retaining wall (gravity type, wall height 4.5m, length 150m) of a contract section of the G80 Guangxi-Kunming Expressway Nanning-Baise section reconstruction and expansion project.

[0072] S1. Construction preparation and red mud material pretreatment:

[0073] Site planning: Set up a rainproof and moisture-proof red mud pretreatment shed and modified mixing station near the mixing plant.

[0074] Pre-treatment control: After the wet red mud arrives at the site, it is spread out and dried, then turned over using a rotary tiller to control its moisture content to a suitable crushing range of 18%-22%. A jaw crusher and a vertical shaft impact crusher are then used in series for crushing, and finally, the mud is screened through a 5mm vibrating screen. Daily sampling of the undersize red mud powder is conducted to test its moisture content and particle size distribution (requirement: >80% passing through a 0.075mm sieve). Qualified products are transferred to the raw material silo.

[0075] S2, Red Mud Material Composite Modification (Centralized Production):

[0076] Large-scale production parameters: An automated metering and mixing system is adopted. Key control points include: SDBS solution concentration (10%) and atomization spray uniformity; mixing time of the main mixer (≥20 minutes) and current monitoring (to judge the mixing uniformity); temperature monitoring (not exceeding 60℃) and humidity maintenance (surface spraying) of the curing chamber.

[0077] Factory inspection: Every 200 tons of modified red mud material constitutes one inspection batch. Testing items include: pH value (10.0-10.5), moisture content (<10%), and mortar strength activity index (compared to the benchmark cement mortar, requiring a 7-day activity index ≥65%). Only materials that pass the inspection can be transported to the concrete mixing plant.

[0078] S3. Concrete Production and Transportation:

[0079] Mixing plant control: A forced twin-shaft mixer is used. The feeding and mixing sequence is as follows: first, add crushed stone, sand, and 50% of the mixing water, and mix for 10 seconds; then add cement and modified red mud materials, and mix for 30 seconds; finally, add the remaining water and water-reducing agent solution, with a total mixing time of no less than 150 seconds. After each batch of concrete leaves the mixer, the slump, spread, and density must be checked, and the temperature at the outlet must be recorded.

[0080] Transportation: Use 8m³ mixer trucks, maintaining agitation (2-4 rpm) during transport. The specified time from discharge to completion of pouring is no more than 90 minutes when the temperature is ≤25℃ and no more than 60 minutes when the temperature is >25℃.

[0081] S4. Foundation trench treatment and formwork support:

[0082] Foundation bearing capacity confirmation: After the foundation trench was excavated to the design elevation, a light dynamic penetration test (N10) was conducted. The measured blow count, after conversion, showed that the characteristic value of the foundation bearing capacity fak was ≥180 kPa, which is greater than the design requirement of 150 kPa. For locally weak areas, graded sand and gravel were used for replacement, and a small road roller was used for layered compaction, with a compaction degree ≥93%.

[0083] Refined Formwork Engineering: Large-scale standardized steel formwork is used. Before installation, a water-based release agent is applied to the inner surface of the formwork. During installation, a total station in conjunction with a laser plumb line is used to control verticality (deviation <1% H, where H is the wall height) and axial misalignment (<±20mm). A system of diagonal bracing and tie rods is installed using Φ48 steel pipes. The spacing between the tie rods is calculated to be 0.6m horizontally and 0.75m vertically, ensuring that the formwork deformation is <3mm under concrete lateral pressure. All joints are sealed with double-sided adhesive strips.

[0084] S5. Concrete pouring and vibration:

[0085] Pouring: Pumps or chutes are used in conjunction with tremie pipes to pour the concrete into the formwork. The height of the end of the tremie pipe from the pouring surface is strictly controlled within 1.5m to prevent segregation of the red mud-based concrete aggregate. A layered continuous pouring method is used, with each layer controlled to a thickness of 40cm. The upper layer is placed before the lower layer has initially set.

[0086] Vibration: Use a Φ50 immersion vibrator. Strictly adhere to the "quick insertion, slow withdrawal" principle during vibration. Insertion points should be arranged in a quincunx pattern, with a spacing not exceeding 40cm (approximately 1.5 times the vibrator's effective radius). Insert the vibrator 5-10cm into the underlying concrete. Vibration time at each point should be sufficient until the concrete surface shows signs of slurry formation, no obvious air bubbles emerge, and no further significant settling occurs, generally 20-30 seconds. Assign a dedicated person to supervise the process to prevent under-vibration or over-vibration.

[0087] S6. Concrete curing and temperature control:

[0088] Curing: Immediately after the top concrete has initially set (approximately 6-8 hours after pouring), cover it with geotextile and begin water curing. An automatic sprinkler system should be installed on the side formwork to keep the formwork surface moist. After demolding, immediately spray a curing agent or wrap a water-retaining membrane on the outer surface of the wall, maintaining moisture for a total curing period of no less than 14 days.

[0089] Temperature control: For retaining wall sections with a thickness exceeding 1m, pre-embed thermocouples are used to monitor the internal temperature. The temperature difference between the highest internal temperature of the concrete and the surface temperature is controlled to not exceed 25℃, and the temperature difference with the ambient temperature is controlled to not exceed 20℃. This is achieved by adjusting the pouring time (selecting a low-temperature period at night) and using cooling mixing water.

[0090] S7. Formwork removal and backfilling behind the wall:

[0091] Formwork Removal: Formwork may be removed after the concrete strength reaches 75% of the design strength (as determined by test blocks cured under the same conditions) and with the approval of the supervising engineer. The removal sequence follows the principle of "removing the last support first and the first support last," first loosening the tie rods, then removing the supports, and gently prying the formwork to detach it.

[0092] Backfill: The backfill behind the wall uses permeable sand and gravel. Backfilling should be carried out after the wall has reached 100% strength, with each layer ≤30cm thick. A small vibratory roller should be used for static compaction within 2m of the wall and weak vibration compaction outside that range. The compaction degree should be checked immediately after each layer is compacted, requiring ≥96%. The backfilling rate should be symmetrical and even.

[0093] Implementation Results: After the completion of the 150m red mud-based concrete retaining wall in this section, the appearance was smooth and dense, with no visible cracks, honeycomb pitting, or other defects. Nine sets of Φ100mm core samples were drilled at different locations (top, middle, and bottom) of the wall, and the representative 28-day compressive strength values ​​ranged from 34.2 to 37.8 MPa, all meeting design requirements. This project consumed approximately 400 tons of red mud and saved approximately 60 tons of cement. Calculations show that the direct material cost of this section of the retaining wall was reduced by approximately 8%, while also saving on red mud storage and disposal costs, resulting in significant environmental benefits. The structure withstood a full rainy season, demonstrating stability, unobstructed drainage from the drainage holes, and no leakage or efflorescence.

[0094] Comparative Examples and Analysis: In addition to the material comparative examples in Example 1, a process comparative example is added:

[0095] Comparative Example C (Improper Construction Technique): The optimized mix proportion of Example 1 was adopted, but in the on-site construction of Example 2: 1) No tremie pipe was used, and the free fall height of concrete reached 4m; 2) Vibration was haphazard, and there were areas where vibration was missed; 3) Curing was not timely, and the wall surface was not kept moist after demolding.

[0096] Results: After demolding, the wall exhibited multiple honeycomb, pitted surfaces, and vertical micro-shrinkage cracks. Core sampling showed extremely high strength dispersion (25.1–32.4 MPa), with some areas failing to meet strength standards and requiring reinforcement.

[0097] This comparative example fully demonstrates that the complete set of construction process parameters and quality control points defined in this invention are indispensable keys to ensuring the successful application and expected performance of red mud-based concrete in retaining wall structures, and are by no means obvious to those skilled in the art based on ordinary concrete construction experience.

[0098] This invention realizes the high-value and large-scale utilization of industrial solid waste: for the first time, a complete set of technologies has been formed to apply red mud with a high proportion (>50% cementation contribution rate) to the main structure of highway gravity retaining walls, providing a feasible engineering outlet for the disposal of millions of tons of red mud.

[0099] Technological integration and innovation break through engineering application bottlenecks: The creative integration of "composite modification technology", "structural adaptability ratio" and "refined construction process" solves the problem of transforming laboratory materials into safe structures on site and fills the technological gap in this field.

[0100] Reliable performance, ensuring engineering safety and durability: Through systematic material design and process control, the prepared red mud-based concrete retaining walls exhibit excellent performance comparable to ordinary concrete in terms of mechanical strength (≥30MPa), impermeability (>P10), frost resistance (F100), shrinkage control, and reinforcement protection, fully meeting the long-term service requirements of highway engineering.

[0101] A win-win situation for both economic and social benefits: It directly reduces the cost of engineering materials, while significantly reducing the environmental risks and disposal costs of red mud stockpiling. It aligns with the national strategies of "zero-waste cities" and green construction, and has broad prospects for promotion and application.

[0102] The implementation principle of this invention is as follows: This invention discloses a red mud-based concrete retaining wall and its construction process, belonging to the technical field of road engineering and solid waste resource utilization. The construction process includes the following steps: S1, pretreatment and performance analysis of red mud materials; S2, composite modification treatment of red mud materials, employing a synergistic modification technology of curing agent doping, mineral admixture, and surface activity treatment to optimize its mechanical properties and durability; S3, mix design and preparation of red mud-based concrete, determining a mix proportion that meets the strength requirement of C20 or higher based on modified red mud, cementitious materials, aggregates, admixtures, and water; S4, retaining wall foundation trench treatment and formwork support; S5, mixing, transportation, and pouring of red mud-based concrete; S6, concrete vibration, finishing, and curing; S7, formwork removal and backfilling. This invention also provides a red mud-based concrete retaining wall produced by this process. This invention is the first to systematically apply modified red mud on a large scale to highway gravity retaining wall structures, forming a complete set of construction technologies that can be implemented on-site. It effectively solves the environmental pressure of red mud storage, reduces engineering costs, improves resource utilization, and has significant economic, social and environmental benefits.

[0103] The embodiments described herein are preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made in accordance with the structure, shape, and principle of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A construction process for a red mud-based concrete retaining wall, characterized in that, Includes the following steps: S1. Pretreatment and performance analysis of red mud materials: The raw red mud is dried, crushed, and sieved, and its physicochemical composition and heavy metal leaching characteristics are determined. S2. Composite modification treatment of red mud material: The pretreated red mud is uniformly mixed with a curing agent, mineral admixtures and surfactants to carry out composite modification and obtain modified red mud material; the curing agent includes at least one of cement, lime and carbide slag; the mineral admixtures include at least one of fly ash, slag powder and metakaolin. S3. Design and preparation of red mud-based concrete mix proportion: Using the modified red mud material as the key component, it is mixed with cementitious materials, coarse aggregate, fine aggregate, admixtures and water to design and determine the concrete mix proportion that meets the target strength grade. S4. Retaining wall foundation trench treatment and formwork support: Measure and set out according to the design drawings, excavate the foundation trench to the design elevation, compact and level the foundation, and then erect a formwork system that meets the design dimensions and stability. S5. Mixing, transporting and pouring of red mud-based concrete: The components determined in step S3 are put into the concrete mixing plant for centralized mixing to obtain red mud-based concrete mixture with qualified workability; the mixture is transported to the site by concrete truck and poured into the formwork supported in step S4 in layers and continuously. S6. Concrete Vibration, Finishing and Curing: Use an immersion vibrator to fully vibrate the poured concrete to remove air bubbles and ensure compaction; smooth the surface before the concrete initially sets; cover with water-retaining material in a timely manner after pouring and perform moist curing for no less than 7 days. S7. Formwork Removal and Backfilling: After the concrete strength reaches more than 75% of the design strength, remove the formwork; then backfill the wall material in layers symmetrically and compact it.

2. The construction process for a red mud-based concrete retaining wall according to claim 1, characterized in that, In step S2, the specific process of the composite modification is as follows: First, premix the red mud with a surfactant accounting for 1% to 5% of the dry weight of the red mud and let it stand for 2 to 4 hours; then add a curing agent accounting for 8% to 20% of the dry weight of the red mud and a mineral admixture accounting for 10% to 30% of the dry weight of the red mud, and mechanically mix in a stirring device for 15 to 30 minutes to ensure uniform mixing; after mixing, store and cure for 3 to 7 days to obtain the modified red mud material.

3. The construction process for a red mud-based concrete retaining wall according to claim 2, characterized in that, The surfactant is one or more of sodium dodecylbenzenesulfonate, lignin sulfonate, and polycarboxylate superplasticizer; the curing agent is a composite of ordinary silicate cement with a strength grade of 42.5 and quicklime in a mass ratio of (3-5):1; the mineral admixture is a composite of grade II fly ash and grade S95 slag powder in a mass ratio of 1:(1-2).

4. The construction process for a red mud-based concrete retaining wall according to claim 1, characterized in that, In step S3, the mix proportion of the red mud-based concrete includes the following components per cubic meter: Modified red mud material: 180–250 kg; Cementitious material: 280-350 kg, wherein the cementitious material is P·O 42.5 cement; Coarse aggregate: 1000-1200 kg, wherein the coarse aggregate is continuously graded crushed stone with a maximum particle size of no more than 31.5 mm; Fine aggregate: 650-750 kg, wherein the fine aggregate is medium sand with a fineness modulus of 2.3-3.0; Additive: 4-8 kg, wherein the additive is a polycarboxylate high-performance water-reducing agent; Water: 150–180 kg; The red mud-based concrete has a 28-day cubic compressive strength of not less than 30 MPa and a workability that meets the requirement of a slump of 150 mm ± 30 mm.

5. The construction process for a red mud-based concrete retaining wall according to claim 1, characterized in that, In step S4, after the foundation trench is excavated, if the foundation is a soft soil layer, it needs to be replaced. The replacement material is graded gravel or modified red mud stabilized soil, which is compacted in layers with a compaction degree of not less than 93%. The template system uses steel templates. Before installation, a release agent is applied. After installation, it is ensured that the joints are tight, the support is firm, the verticality deviation of the template is less than 1%, and the plane position deviation is less than ±20mm.

6. The construction process for a red mud-based concrete retaining wall according to claim 1, characterized in that, In step S5, the mixing is carried out using a forced mixer, and the total mixing time is not less than 120 seconds; the transportation is carried out using a mixer transport vehicle, and the mixing is maintained during transportation to prevent segregation, and the time from discharge to placement into the mold does not exceed 90 minutes; during the pouring, a layered progressive method is adopted, with each layer not exceeding 50cm in thickness, and the interval between two adjacent layers does not exceed the initial setting time of the lower layer of concrete.

7. The construction process for a red mud-based concrete retaining wall according to claim 1, characterized in that, In step S6, the vibration is performed using an immersion vibrator. The vibrator rod should be inserted vertically into the concrete, inserted quickly and withdrawn slowly, with the moving distance not exceeding 1.5 times the effective radius of the vibrator rod. The insertion depth into the lower layer of concrete is 5-10 cm, and the vibration duration at each vibration point is 20-30 seconds, until the concrete surface shows a layer of slurry, no longer sinks significantly, and no air bubbles emerge. The curing is performed by covering with geotextile or plastic film and sprinkling water to keep it moist. During the curing period, the temperature difference between the concrete surface and the outside temperature should not exceed 25℃, and the pH value of the curing water should be neutral.

8. The construction process for a red mud-based concrete retaining wall according to claim 1, characterized in that, In step S7, the backfill material behind the wall shall preferably be permeable gravel or modified red mud stabilized soil made by mixing modified red mud obtained in step S2 with soil. Backfilling must be carried out in layers, with each layer having a loose thickness of no more than 30cm. Small road rollers or plate compactors shall be used for symmetrical rolling or compaction, and the compaction degree shall meet the design requirements and be no less than 95%.

9. A red mud-based concrete retaining wall constructed using the construction process described in any one of claims 1 to 8, characterized in that, The retaining wall is a gravity or semi-gravity structure, and its main structure is made of red mud-based concrete. In the red mud-based concrete, the amount of modified red mud material accounts for 50% to 70% of the total mass of cementitious materials. After 28 days of curing, the compressive strength of the concrete core samples of the key parts of the retaining wall is not lower than the design strength grade, the impermeability grade is not lower than P6, and the frost resistance grade is not lower than F50.

10. The red mud-based concrete retaining wall according to claim 9, characterized in that, The retaining wall is equipped with pre-embedded monitoring elements for long-term monitoring of the wall's stress, strain, and displacement; the top, back, and drainage hole areas of the retaining wall are protected with alkali-resistant coatings or surface layers.

Images