A method for preparing a steel fiber reinforced graphite tailing cement-based composite filling material and application thereof

Abstract

The application relates to a preparation method and application of a steel fiber reinforced graphite tailing cement-based composite filling material, and relates to the technical field of coal mine goaf foundation repair. A screening-impurity removal-drying combined process is adopted to obtain graphite tailing particles; in the critical range of 15%-20% of the graphite tailing content, each component is weighed according to the proportion, including steel fiber, cement, graphite tailing and mixing water, and a water reducing agent is selected according to the requirement; the steel fiber, the cement and the graphite tailing are mixed until the materials are uniformly mixed; the mixing water is added twice, the water reducing agent is pre-dissolved at the second time, and wet mixing is performed until a uniform slurry is obtained; after stirring is completed, the slurry fluidity is detected by using an expansion degree method, and the expansion degree is ensured to be 200-250 mm. The critical content range of the graphite tailing is determined, the mechanical properties-water cement ratio-water permeability performance of the composite filling material are synergistically optimal, the grouting fluidity and the late strength development are considered, and the performance requirement of the coal mine goaf foundation repair is met.

Description

Technical Field

[0001] This invention relates to the field of coal mine goaf foundation repair technology, specifically a method for preparing and applying a steel fiber reinforced graphite tailings cement-based composite filling material. Background Technology

[0002] Foundation restoration in coal mining subsidence areas is a crucial technical step in ensuring the safe utilization of land and ecological restoration in mining areas. Currently, the industry commonly uses grouting and filling technology, which involves injecting cementitious materials into the goaf to reinforce the foundation and stabilize the rock strata. Among these methods, incorporating industrial solid waste (such as graphite tailings) into cement slurry to prepare composite filling materials has become a research hotspot in this field, as it reduces raw material costs and minimizes pollution from waste accumulation. At present, research on graphite tailings-cement composite mortar mainly focuses on the impact of admixture dosage on material strength, and the following patterns have been identified:

[0003] Strength variation characteristics: When the graphite tailings content is in the low content range (<15%), the mortar strength curve with graphite tailings content may have a very flat "plateau" or a small "peak", and the strength change is not significant; while when the graphite tailings content is in the medium to high content range (>20%), the mortar strength curve with graphite tailings content begins to show a clear monotonic downward trend, and the higher the content, the more drastic the strength decrease.

[0004] Preparation parameters: To improve the performance of mortar after incorporating graphite tailings, it is necessary to reduce the water-cement ratio, adjust the ratio of cement paste mass to aggregate mass, and control the mass of graphite tailings per unit volume to enhance the void-filling effect of graphite tailings in the mortar and alleviate the problem of decreased strength.

[0005] Although the relationship between graphite tailings content and mortar strength has been preliminarily explored, the following key technical defects still exist when applied to the repair of goaf areas in coal mining subsidence zones, making it difficult to match material properties with engineering requirements:

[0006] 1. Lack of core parameters and lack of basis for performance control: The existing technology does not define the optimal range of the "plateau" stage in the low dosage zone, the specific dosage corresponding to the "peak" and the transition critical value or critical range between the low dosage zone and the medium and high dosage zone. This makes it impossible to accurately determine the amount of graphite tailings added to make the mechanical properties (such as compressive strength) of the composite cubic specimen optimal, the water-cement ratio balanced and the water permeability optimal. In actual preparation, it can only be blindly tested and mixed, making it difficult to take into account both material performance and application requirements.

[0007] 2. Insufficient compatibility between mix proportion and process: Existing technologies do not take into account the characteristics of different admixture zones (especially the low admixture "platform" zone), and do not specify the corresponding optimal range of water-cement ratio, cement and aggregate mix proportion parameters. They also do not explain the matching relationship between the fluidity and setting time of the material under this mix proportion and the delamination grouting process in coal mining subsidence areas (such as the compatibility of grouting pressure and borehole diameter). This can easily lead to problems such as poor grout fluidity causing pipe blockage, or excessive fluidity causing insufficient strength.

[0008] 3. Mismatch between material function and repair scenario: Existing technologies do not address the special requirements of materials for the repair of goaf foundations in coal mining subsidence areas (such as the need to ensure a certain level of permeability to drain water from the goaf and long-term stability to support the overlying strata), and do not consider the relationship between graphite tailings content and material permeability and long-term mechanical properties. Instead, they focus only on short-term strength changes, which may lead to the prepared materials causing water retention in the goaf due to poor permeability, or long-term strength decay affecting the foundation repair effect. Summary of the Invention

[0009] To address the shortcomings of the prior art, this invention provides a method for preparing and applying a steel fiber reinforced graphite tailings cement-based composite backfill material. It clarifies the critical dosage range of 15%-20% graphite tailings, enabling the composite backfill material to achieve optimal synergy among mechanical properties, water-cement ratio, and permeability within this range. It also considers grouting fluidity and subsequent strength development, meeting the performance requirements for foundation repair in coal mine goaf areas.

[0010] To achieve the above objectives, the present invention adopts the following technical solution:

[0011] A method for preparing a steel fiber reinforced graphite tailings cement-based composite backfill material includes the following steps:

[0012] Step 1: Pretreatment of graphite tailings;

[0013] Graphite tailings particles with a diameter of 0.075mm-2mm are obtained by screening with a double-layer vibrating screen with 2mm and 0.075mm mesh sizes. Ferromagnetic impurities are then removed by a magnetic separator with a magnetic field strength of 1000-1500Gs. Finally, the particles are dried in a drying oven at 60℃-80℃ until the moisture content is ≤5%.

[0014] Step 2: Raw material measurement;

[0015] For graphite tailings with a content within the critical range of 15%-20%, the components are weighed according to the following proportions: steel fiber, cement, pretreated graphite tailings, and mixing water. Water-reducing agent may or may not be included depending on requirements. The mass ratio of cement to graphite tailings is (7.5-8.5):(1.5-2.5). Without steel fiber and water-reducing agent, the water-cement ratio is 0.35-0.45; with steel fiber and water-reducing agent, the water-cement ratio is 0.32-0.40. The water-reducing agent content is 0.8%-1.2% based on the cement mass.

[0016] Step 3: Dry mix;

[0017] Weigh the cement and graphite tailings and put them into a forced mixer. Ensure the mixing environment temperature is between 5℃ and 35℃ and dry mix at a speed of 150-200r / min for 3-5 minutes until the materials are evenly mixed.

[0018] Step 4: Wet mixing and addition of functional additives;

[0019] First, add 80% of the mixing water to the mixed cement and graphite tailings, and continue stirring for 2-3 minutes. Then add the remaining 20% ​​of the mixing water. If steel fibers and water-reducing agents are included, pre-dissolve the water-reducing agents with the remaining mixing water. Then, wet mix at a speed of 200-250 r / min for 4-6 minutes until a uniform slurry is obtained.

[0020] Step 5: Slurry testing and adjustment;

[0021] After mixing, the fluidity of the slurry is tested using the spread method: if the spread is <200mm, add 0.1%-0.2% steel fiber and water-reducing agent by weight of cement and mix for 1 minute; if the spread is >250mm, add no more than 5% of the total cement and mix for 1 minute until the spread reaches 200-250mm.

[0022] Furthermore, in step 2, if there is no water accumulation in the goaf, the amount of graphite tailings added is determined to be 15%.

[0023] Furthermore, the cement used is ordinary Portland cement with a strength grade of 42.5, the mixing water is tap water, and the water-reducing agent is a polycarboxylate-based high-efficiency water-reducing agent or a naphthalene-based water-reducing agent.

[0024] Furthermore, in step 2, if it is necessary to shorten the curing cycle, calcium chloride-based early strength agent is added to the composition, with the dosage of early strength agent being 2%-3% based on the cement quality.

[0025] Furthermore, in step 2, if the construction temperature is <5℃, the water-reducing agent is a naphthalene-based water-reducing agent with a dosage of 1.2%, and the early strength agent dosage is 3%, ensuring that the initial setting time of the prepared slurry is ≥4h and the compressive strength at 7d is ≥3.2MPa.

[0026] Furthermore, in steps 3 and 4, the method to ensure the mixing environment temperature is as follows: if it is below 5°C, the mixing water is heated; if it is above 35°C, the forced mixer is cooled by spraying cooling water.

[0027] An application of a steel fiber reinforced graphite tailings cement-based composite backfill material: The steel fiber reinforced graphite tailings cement-based composite backfill material obtained according to the preparation method is injected into the foundation of the goaf area by a grouting pump for the foundation repair of the goaf area in coal mines. It is required that the grouting be completed within 30 minutes after the mixing is completed.

[0028] Compared with the prior art, the beneficial effects of the present invention are:

[0029] 1. Solving the problem of missing critical dosage and significantly improving the core performance of materials: In response to the lack of critical values / ranges for graphite tailings in existing technologies, which makes it difficult to achieve optimal synergy among mechanical properties, water-cement ratio and permeability, this invention clarifies the critical dosage range of 15%-20% graphite tailings. This enables the composite backfill material to achieve optimal synergy among mechanical properties, water-cement ratio and permeability within this range. This avoids insufficient backfilling effect of tailings at low dosages and prevents drastic strength decay at medium and high dosages. At the same time, by balancing the water-cement ratio, it takes into account both grouting fluidity and later strength development, and adapts to the core performance requirements of materials for foundation repair in coal mining subsidence areas and goaf areas.

[0030] 2. Optimize mix proportions and process compatibility to reduce the risk of failure in practical applications: Addressing the shortcomings of existing technologies that do not clearly define the matching relationship between admixture dosage, water-cement ratio, and grouting process, which can easily lead to grout blockage or insufficient strength, this invention, on the one hand, limits the water-cement ratio range to 0.35-0.45 (which can be lowered to 0.32-0.40 with the addition of a water-reducing agent), ensuring that the grout spread is within 200-250mm, meeting the fluidity requirements of the grouting process and effectively preventing blockage; on the other hand, through a dry-mixing-wet-mixing-testing and adjustment process design, and time control of grouting within 30 minutes, it ensures the uniformity and freshness of the grout, reducing the risk of insufficient filling due to grout segregation and initial setting, and improving on-site construction efficiency.

[0031] 3. Improve the efficiency of graphite tailings resource utilization while balancing environmental protection and economic benefits: Existing technologies suffer from low efficiency due to unclear dosage, making it difficult to balance graphite tailings usage and material performance. This invention, within a critical dosage range of 15%-20%, allows graphite tailings to fully fill the pores of cement slurry, optimize material density, and maximize the consumption of solid waste. Compared to existing low-dosage schemes, this increases graphite tailings usage by 15%-33%, reducing the annual footprint of graphite tailings storage by approximately 150-200 square meters per 10,000 tons (based on an annual material production capacity of 10,000 tons). Simultaneously, it reduces cement usage (by 15%-20% compared to pure cement slurry), lowering raw material costs by 8%-12%, achieving a win-win situation for both environmental and economic benefits.

[0032] 4. Simple operation, suitable for industrial production and on-site construction: Addressing the lack of clarity in existing technologies regarding graphite tailings pretreatment and mixing parameters, resulting in poor process controllability, this invention first adopts a standardized process of screening, impurity removal, and drying for graphite tailings pretreatment. The equipment (double-layer vibrating screen, magnetic separator, and drying box) are all conventional industrial equipment, with a low operating threshold. Secondly, the mixing parameters (speed 150-250 r / min, time 9-14 minutes) and ambient temperature (5℃-35℃) are clearly quantified, eliminating the need for complex adjustments and making it suitable for continuous industrial production. Furthermore, only a simple flowability test and fine-tuning are required before on-site grouting, which can be operated without professional technicians, reducing construction difficulty and labor costs. Attached Figure Description

[0033] Figure 1 This is a schematic diagram illustrating the application scenarios of the composite filling material of this invention;

[0034] Figure 2 These are performance-labeled sample images of the cubic test blocks prepared in the examples;

[0035] Figure 3 This is a diagram of the cylindrical test block prepared in the examples;

[0036] Figure 4 This is a standard isothermal curing diagram of the cylindrical test block prepared in the examples. Detailed Implementation

[0037] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the invention, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0038] A method for preparing a steel fiber reinforced graphite tailings cement-based composite backfill material, the specific scheme of which is as follows:

[0039] Composition:

[0040] Including steel fibers, cement, graphite tailings, and mixing water, with optional functional additives (water-reducing agents and early-strength agents). The functions of each component are as follows:

[0041] Cement: Provides cementitious strength; ordinary Portland cement (strength grade 42.5) is selected to ensure the basic mechanical properties of the material.

[0042] Graphite tailings: As aggregate to fill pores and optimize density, it needs to be pre-treated (to remove impurities and control the particle size to 0.075mm-2mm) to achieve resource utilization;

[0043] Mixing water: To ensure the hydration reaction, clean tap water is selected to avoid impurities affecting the material performance;

[0044] Steel fibers: help improve the tensile strength of composite materials;

[0045] Functional additives: Water-reducing agent (polycarboxylate-based high-efficiency water-reducing agent) improves flowability, and early strength agent (calcium chloride) enhances early strength.

[0046] Mixing ratio range:

[0047] Based on mass ratio, the proportions of each component are strictly designed around the "critical range for graphite tailings addition", as detailed below:

[0048] The cement to graphite tailings mass ratio is (7.5-8.5):(1.5-2.5), corresponding to a graphite tailings content within the critical range of 15%-20% (i.e., the critical range from the end of the "plateau" in the low-content zone to the beginning of the "peak" in the medium-high-content zone). Cubic specimens prepared within this range exhibit the best compressive strength (7-day compressive strength ≥3.0 MPa), a balanced water-cement ratio, and a permeability coefficient of 1×10⁻⁶. -3 ~1×10 -2 cm / s (suitable for drainage needs in goaf areas);

[0049] Water-cement ratio (water to cement mass ratio): 0.35-0.45, balancing slurry fluidity (expansion ≥200mm, to avoid pipe blockage) and strength development. When used with steel fibers and water-reducing agents, it can be lowered to 0.32-0.40.

[0050] Functional additive dosage: Based on cement quality, add 0.8%-1.2% water-reducing agent and 2%-3% early-strength agent only in low-temperature environments (construction temperature <5℃) or when a shorter curing period is required.

[0051] The following key parameters can be adjusted according to different goaf environments:

[0052] Graphite tailings content adjustment: If there is no water accumulation in the goaf (high permeability is not required), the graphite tailings content can be adjusted to 15% (lower limit of the critical range). At this time, the compressive strength at 28 days can be increased to 6.8 MPa, and the permeability coefficient can be reduced to 8×10. -4 cm / s, suitable for high-strength and low-permeability requirements;

[0053] Adjustment of water-reducing agent type: If the construction environment temperature is <5℃, the polycarboxylate water-reducing agent can be replaced with a naphthalene water-reducing agent (dosage 1.2%), and 3% calcium chloride early strength agent can be added to ensure that the initial setting time of the slurry is ≥4h and the compressive strength at 7d is ≥3.2MPa, so as to avoid the problem of slow hydration caused by low temperature.

[0054] Process:

[0055] Step 1: Pretreatment of graphite tailings;

[0056] The process employs a combined screening-impurity removal-drying process. First, graphite tailings particles with a diameter of 0.075mm-2mm are screened using a double-layer vibrating screen (with 2mm and 0.075mm sieve holes). Then, ferromagnetic impurities are removed using a magnetic separator (magnetic field strength 1000-1500Gs). Finally, the tailings are dried in a drying oven at 60℃-80℃ until the moisture content is ≤5%, thus avoiding excessive moisture content affecting the accuracy of the water-ash ratio.

[0057] Step 2: Raw material measurement;

[0058] Electronic weighing scales (accuracy ±0.1kg) are used to weigh each component according to the mixing ratio range. The measurement error of steel fiber, cement, graphite tailings and mixing water is ≤±1%, and the measurement error of functional additives is ≤±0.05%, to ensure the accuracy of the mixing ratio.

[0059] Step 3: Dry mix;

[0060] Weigh the cement and graphite tailings and put them into a forced mixer. Mix at a speed of 150-200 r / min for 3-5 minutes until the materials are evenly mixed (no obvious color difference is observed when taking samples) to avoid agglomeration during subsequent wet mixing.

[0061] Step 4: Wet mixing and addition of functional additives;

[0062] First, add 80% of the mixing water to the mixed cement and graphite tailings, and continue stirring for 2-3 minutes. Then add steel fibers, the remaining 20% ​​of the mixing water, and pre-dissolved functional additives (the water-reducing agent needs to be diluted with 5 times the amount of water in advance). Adjust the stirring speed to 200-250 r / min and wet mix for 4-6 minutes until the resulting slurry is uniform (without lumps or stratification).

[0063] Step 5: Slurry testing and adjustment;

[0064] After mixing, immediately test the fluidity of the grout using the spread method (ambient temperature 20℃±2℃). If the spread is <200mm, add 0.1%-0.2% steel fiber and water-reducing agent (based on cement mass) and mix for 1 minute. If the spread is >250mm, add cement and mix for 1 minute. The amount added should be ≤5% of the total cement mass until the spread reaches 200-250mm to ensure that the grout meets the grouting requirements.

[0065] Key control points:

[0066] Mixing environment temperature: 5℃-35℃. If it is below 5℃, the mixing water can be heated. If it is above 35℃, the forced mixer can be cooled by spraying cooling water to avoid abnormal hydration reaction.

[0067] Mixing time: The total mixing time for dry and wet mixing should be controlled between 9 and 14 minutes. Too long a time can easily lead to segregation of the slurry, while too short a time can easily lead to uneven mixing.

[0068] Grouting time: Grouting should be completed within 30 minutes after mixing. If it exceeds 30 minutes, the grout flowability should be retested. If the spread is <180mm, it should be discarded to avoid the initial setting of the grout affecting the filling effect.

[0069] The steel fiber reinforced graphite tailings cement-based composite backfill material slurry obtained by the above preparation method is applied to the foundation repair of coal mine goaf areas. The slurry is injected into the foundation of the goaf area using a grouting pump, and the application scenarios are combined... Figure 1 As shown, it can effectively reduce the shrinkage rate of the rock strata separation zone and improve the safety of the mining area.

[0070] This invention addresses one primary problem and three secondary problems compared to existing technologies, as detailed below:

[0071] Main problem: To solve the core issue of the lack of critical values / critical ranges for graphite tailings addition in existing technologies, and to clarify the critical values ​​or critical ranges for graphite tailings addition in the low-dosage zone (<15%) that achieve optimal material mechanical properties (compressive strength), balanced water-cement ratio, and best permeability. This will fill the technical gap between "optimal performance" and "dosage control," provide clear parameter basis for the precise formulation of composite backfill materials, avoid material performance fluctuations caused by blind dosing adjustments, and ensure that it meets the core requirements of material strength and permeability for foundation repair in coal mining subsidence areas.

[0072] Secondary issues:

[0073] ① Solve the problem of coordinated control of admixture dosage, water-cement ratio, and cement-aggregate ratio: In view of the problem that the existing technology does not clearly define the optimal range of water-cement ratio and the ratio of cement paste mass to aggregate mass for different admixture dosages (especially the critical value / critical range), determine the water-cement ratio range and cement-aggregate ratio parameters that match the critical admixture dosage of graphite tailings, so as to ensure that the material has the best mechanical properties while having the fluidity and setting time suitable for the delamination grouting process, and avoid grout blockage or insufficient strength.

[0074] ② Solving the problem of adapting material properties to goaf repair scenarios: In response to the problem that existing technologies do not connect the permeability, long-term mechanical properties and repair needs of materials, critical dosage control is used to enable composite filling materials to meet short-term compressive strength requirements, while also having permeability suitable for goaf water drainage and long-term support stability, so as to avoid the repair effect being affected by functional deficiencies.

[0075] ③ Solve the problem of matching graphite tailings pretreatment with admixture dosage: In view of the problem that the existing technology does not clearly define the pretreatment requirements (such as particle size and impurity content) for graphite tailings under different admixture dosages, determine the particle size range of graphite tailings and the impurity removal standard corresponding to the critical admixture dosage, so as to ensure that the graphite tailings and cement paste are fully mixed and avoid the decline in material performance due to improper pretreatment.

[0076] Example

[0077] This embodiment is applied to the foundation repair project of the goaf in a coal mining subsidence area of ​​a coal mine in Jixi City, Heilongjiang Province. The goaf is buried at a depth of about 80-100m and has a volume of about 5000m³. The overlying rock strata are mainly siltstone and mudstone, and there is local water accumulation (the water volume is about 300m³). It is necessary to stabilize the foundation by delamination grouting and filling to avoid further surface collapse.

[0078] The selection and proportioning of raw materials in this embodiment are shown in the table below:

[0079]

[0080] In this scheme, the amount of graphite tailings is 20%, the water-cement ratio is 0.4 (which is within the optimal range of 0.35-0.45), and the amount of water-reducing agent is 1%, which is suitable for the repair needs of this goaf area that requires high permeability for drainage and strong support.

[0081] Construction process:

[0082] S1. Pretreatment of graphite tailings: Graphite tailings are screened using a double-layer vibrating screen (model ZS-1020, speed 1500r / min) with 2mm and 0.075mm diameter particles to remove large impurities >2mm and dust <0.075mm. Ferromagnetic impurities in the graphite tailings are removed using a drum magnetic separator (model CTB-1018) with a magnetic field strength of 1200Gs (removal rate ≥95%). The screened and impurity-removed graphite tailings are then sent to an electric heating drying oven (model DHG-9075A) and dried at 70℃ for 4 hours until the moisture content is 4.2% (meeting the requirement of ≤5%) for later use.

[0083] S2. Raw material measurement: Weigh 400kg of cement, 100kg of graphite tailings, 160kg of mixing water, 4kg of water-reducing agent, and 2kg of steel fiber using an electronic weighing scale (accuracy ±0.05kg). Put the cement and graphite tailings into a JS500 forced mixer and close the feed inlet.

[0084] S3. Dry mixing and wet mixing: Turn on the mixer and dry mix at 180 r / min for 4 minutes. Take a sample and observe if there is no obvious color difference (mixing uniformity ≥98%). First, add 128 kg of mixing water (80% of the total water volume) to the mixer and keep the speed at 180 r / min for 3 minutes. Mix and dilute 4 kg of water-reducing agent, 2 kg of steel fiber and the remaining 32 kg of mixing water, pour into the mixer, increase the speed to 220 r / min, and wet mix for 5 minutes to form a uniform slurry.

[0085] S4. Grout Testing and Adjustment: Test the grout spread according to GB / T2419-2005 standard, with an ambient temperature of 22℃. The test result is 230mm (within the acceptable range of 200-250mm), and no adjustment is required. Test the setting time according to GB / T1346-2011 standard. The initial setting time is 4.5h, and the final setting time is 10h (meeting the requirements of ≥4h and ≤12h). The grout is qualified.

[0086] S5. On-site grouting application: A BW-250 grouting pump (working pressure 3MPa) is used to inject grout into the goaf through grouting boreholes with a diameter of 110mm (borehole spacing 10m, a total of 20 boreholes). The pressure is monitored in real time during the grouting process. When the pressure of a borehole rises to 4MPa, grouting is switched to the adjacent borehole to avoid local overfilling that could cause rock deformation. The grouting volume per borehole is controlled at about 250m³, with a total grouting volume of 5000m³. All grouting is completed within 48 hours (all grout is used up within 30 minutes, with no initial setting waste).

[0087] S6. Curing and Post-Injection Monitoring: After grouting is completed, the grouting area shall be watered three times a day to keep the surface of the filling body moist for 14 days. During the curing period, heavy equipment shall not be allowed to pass over the grouting area (to avoid disturbing the filling body).

[0088] Implementation effect verification:

[0089] During construction, cubic test blocks of the prepared slurry were tested under the same conditions, and the performance of the cubic test blocks was labeled. Figure 2 , Figure 3 , Figure 4 As shown in the table below, the material performance indicators meet the standards:

[0090]

[0091] Engineering application results:

[0092] Three months after the grouting was completed, the surface settlement monitoring instrument (model DSZ2) showed that the maximum surface settlement decreased from 15 mm / month before grouting to 2 mm / month, and the stability of the foundation in the goaf area was significantly improved.

[0093] The measured bond strength between the infill and the siltstone strata was 0.9 MPa (≥0.8 MPa requirement), with no peeling observed.

[0094] After one freeze-thaw cycle (local minimum temperature -15℃), the compressive strength loss rate of the filling body is 12% (≤20%), the frost resistance meets the F150 requirement, and there are no cracking or leakage problems.

[0095] Through the above practical applications and verifications, the solution of this invention demonstrates excellent performance and can achieve the following main performance indicators:

[0096] (1) Mechanical performance indicators (quantitative):

[0097] 7-day compressive strength: ≥3.0MPa (tested according to GB / T50081-2019 "Standard for Test Methods of Physical and Mechanical Properties of Concrete", cubic specimen size 100mm×100mm×100mm), which is more than 36% higher than the existing medium-high admixture (>20%) scheme (strength ≤2.2MPa);

[0098] 28-day compressive strength: ≥5.5MPa, meeting the long-term support requirements of filling materials for foundation repair in coal mining subsidence areas (must withstand overlying rock pressure ≥0.3MPa).

[0099] Bond strength: The bond strength with the rock strata in the goaf is ≥0.8MPa (tested according to JGJ / T70-2009 "Standard for Test Methods of Basic Performance of Building Mortar"), to prevent the filling material from peeling off from the rock strata.

[0100] (2) Permeability performance indicators (quantitative):

[0101] Permeability coefficient: 1×10 -3 -1×10-2 cm / s (tested according to GB / T50123-2019 Standard for Geotechnical Testing Methods, variable head method), which can quickly drain water from the goaf (drainage time ≤ 48 hours / 100m³ goaf), avoiding the strength reduction of the filling body and softening of the rock layer caused by water accumulation.

[0102] (3) Grouting flow performance indicators (quantitative):

[0103] Grout spread: 200-250mm (tested according to GB / T2419-2005 "Test Method for Flowability of Cement Mortar", ambient temperature 20℃±2℃). During grouting, under a pressure of 2-5MPa, a horizontal diffusion distance of ≥5m can be achieved (when the borehole spacing is 10m), ensuring a goaf filling rate of ≥95%.

[0104] Setting time: initial setting time ≥ 4 hours, final setting time ≤ 12 hours (tested according to GB / T1346-2011 "Standard consistency water requirement, setting time and soundness test method for cement"), taking into account both grouting operation time and early strength development.

[0105] (4) Material stability and environmental performance indicators (qualitative + quantitative)

[0106] Volume stability: 28-day shrinkage rate ≤0.15% (tested according to GB / T50082-2009 "Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete"), avoiding water leakage and strength reduction caused by cracking of the filling body;

[0107] Durability: Freeze resistance ≥ F150 (compressive strength loss ≤ 20% after 150 freeze-thaw cycles), impermeability grade ≥ P6, suitable for the complex environment of humid and low temperature in coal mining subsidence areas.

[0108] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered illustrative and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of the equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0109] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for preparing a steel fiber reinforced graphite tailings cement-based composite backfill material, characterized in that: Includes the following steps: Step 1: Pretreatment of graphite tailings; Graphite tailings particles with a diameter of 0.075mm-2mm are obtained by screening with a double-layer vibrating screen with 2mm and 0.075mm mesh sizes. Ferromagnetic impurities are then removed by a magnetic separator with a magnetic field strength of 1000-1500Gs. Finally, the particles are dried in a drying oven at 60℃-80℃ until the moisture content is ≤5%. Step 2: Raw material measurement; For graphite tailings with a content within the critical range of 15%-20%, the components are weighed according to the following proportions: steel fiber, cement, pretreated graphite tailings, and mixing water. Water-reducing agent may or may not be included depending on requirements. The mass ratio of cement to graphite tailings is (7.5-8.5):(1.5-2.5). Without steel fiber and water-reducing agent, the water-cement ratio is 0.35-0.45; with steel fiber and water-reducing agent, the water-cement ratio is 0.32-0.

40. The water-reducing agent content is 0.8%-1.2% based on the cement mass. Step 3: Dry mix; Weigh the cement and graphite tailings and put them into a forced mixer. Ensure the mixing environment temperature is between 5℃ and 35℃ and dry mix at a speed of 150-200r / min for 3-5 minutes until the materials are evenly mixed. Step 4: Wet mixing and addition of functional additives; First, add 80% of the mixing water to the mixed cement and graphite tailings, and continue stirring for 2-3 minutes. Then add the remaining 20% ​​of the mixing water. If steel fibers and water-reducing agents are included, pre-dissolve the water-reducing agents with the remaining mixing water. Then, wet mix at a speed of 200-250 r / min for 4-6 minutes until a uniform slurry is obtained. Step 5: Slurry testing and adjustment; After mixing, the fluidity of the slurry is tested using the spread method: if the spread is <200mm, add 0.1%-0.2% steel fiber and water-reducing agent by weight of cement and mix for 1 minute; if the spread is >250mm, add no more than 5% of the total cement and mix for 1 minute until the spread reaches 200-250mm.

2. The method for preparing a steel fiber reinforced graphite tailings cement-based composite backfill material according to claim 1, characterized in that: In step 2, if there is no water accumulation in the goaf, the amount of graphite tailings added is determined to be 15%.

3. A method for preparing a steel fiber reinforced graphite tailings cement-based composite backfill material according to claim 1 or 2, characterized in that: The cement used is ordinary Portland cement with a strength grade of 42.5, the mixing water is tap water, and the water-reducing agent is a polycarboxylate-based high-efficiency water-reducing agent or a naphthalene-based water-reducing agent.

4. The method for preparing a steel fiber reinforced graphite tailings cement-based composite backfill material according to claim 3, characterized in that: In step 2, if it is necessary to shorten the curing cycle, calcium chloride-based early strength agent is added to the composition, with the dosage of early strength agent being 2%-3% based on the cement quality.

5. The method for preparing a steel fiber reinforced graphite tailings cement-based composite backfill material according to claim 4, characterized in that: In step 2, if the construction temperature is <5℃, the water-reducing agent is a naphthalene-based water-reducing agent with a dosage of 1.2%, and the early strength agent dosage is 3%, to ensure that the initial setting time of the prepared slurry is ≥4h and the compressive strength at 7d is ≥3.2MPa.

6. The method for preparing a steel fiber reinforced graphite tailings cement-based composite backfill material according to claim 1, characterized in that: In steps 3 and 4, the method to ensure the mixing environment temperature is as follows: if it is below 5°C, the mixing water is heated; if it is above 35°C, the forced mixer is cooled by spraying cooling water.

7. An application of a steel fiber reinforced graphite tailings cement-based composite backfill material, characterized in that: The steel fiber reinforced graphite tailings cement-based composite filling material obtained by the preparation method according to any one of claims 1-6 is injected into the foundation of the goaf area by a grouting pump for the foundation repair of the goaf area in coal mines, and is required to complete the grouting within 30 minutes after mixing.

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