Lightweight concrete for mine filling and method for preparing the same

Abstract

The application discloses a kind of light weight concrete for mine filling and a preparation method thereof, and relates to the technical field of light weight building material.The method comprises sequentially performing chrome acid oxidation etching, nano-aluminum oxide coating deposition and amino silane grafting on short-cut polypropylene fibers to obtain modified polypropylene fibers; sequentially performing methyl triethoxysilane hydrophobic grafting, hexadecyl trimethoxysilane secondary enhancement and stearic acid co-assembly treatment on fumed nano-silica to obtain a composite hydrophobic modifier; mine tailings, cement, fly ash, slag powder, the modified fiber, the composite hydrophobic modifier, polyvinyl alcohol and nano-calcium carbonate are mixed to prepare pulp, and then poured into a mold after adding foam for curing to obtain the light weight concrete.The application realizes the synchronous optimal improvement of the mechanical strength and impermeability of the concrete by the synergistic effect of fiber multistage modification and nano-particle super-hydrophobic structure.

Description

Technical Field

[0001] This invention relates to the field of lightweight building materials technology, specifically to a lightweight concrete for mine filling and its preparation method. Background Technology

[0002] Timely backfilling of underground goaf areas in mines is a key technical measure to ensure safe mine production and control surface subsidence. Lightweight concrete has attracted widespread attention in the field of mine goaf backfilling due to its advantages such as light weight, good fluidity, and pumpability. However, the large number of pores introduced into lightweight concrete to achieve low density inevitably leads to two aspects of performance degradation while reducing the material's bulk density: firstly, high porosity significantly weakens the material's mechanical load-bearing capacity, making it difficult to meet the engineering requirements of supporting the surrounding rock of the goaf and bearing the overlying strata; secondly, the interconnected pore network forms a rapid seepage channel for groundwater and acidic seepage water in the mining area, causing the backfill to suffer continuous erosion and dissolution during long-term service, seriously threatening the long-term stability and service life of the backfill. How to improve the mechanical strength and impermeability of lightweight concrete while maintaining its lightweight characteristics is the core technical challenge restricting the large-scale promotion and application of this type of material in the field of mine backfilling. Summary of the Invention

[0003] The purpose of this invention is to provide a lightweight concrete for mine filling and its preparation method, so as to solve the technical problems mentioned in the background art.

[0004] To achieve the above objectives, the present invention provides the following technical solution:

[0005] A method for preparing lightweight concrete for mine filling includes the following steps:

[0006] 1) Short polypropylene fibers were subjected to chromic acid mixed solution oxidation etching treatment, aluminum isopropoxide sol-gel method nano-alumina coating deposition and γ-aminopropyltriethoxysilane grafting treatment in sequence to obtain modified polypropylene fibers.

[0007] 2) The fumed silica nano-silica was subjected to hydrophobic grafting treatment with methyltriethoxysilane, secondary hydrophobic enhancement treatment with hexadecyltrimethoxysilane, and superhydrophobic structure treatment with stearic acid co-assembly to obtain organosilicon / nano-SiO2 composite hydrophobic modifier.

[0008] 3) Mine tailings, ordinary silicate cement, fly ash, slag powder, the modified polypropylene fiber, the organosilicon / nano-SiO2 composite hydrophobic modifier, polyvinyl alcohol and nano-calcium carbonate are mixed to form a slurry, pre-made foam is added, mixed and poured into molds, and cured to obtain the lightweight concrete for mine filling.

[0009] This invention constructs a micro-reinforcing skeleton network by performing multi-level surface modification on polypropylene fibers. The mechanism by which this network improves the mechanical strength of lightweight concrete lies in the following: chromic acid oxidation etching treatment forms a dense micron-sized pit and groove structure on the fiber surface, and introduces oxygen-containing polar functional groups (hydroxyl, carbonyl, carboxyl, etc.) at the molecular level. This transforms the originally non-polar, low-surface-energy polypropylene fiber surface into a high-roughness, high-polarity active surface. This not only significantly increases the effective contact area, providing abundant physical anchoring points for subsequent coatings, but also significantly improves the wetting affinity between the fiber and inorganic components. The nano-alumina transition coating, with its high hardness and high modulus, acts as a buffer for the elastic modulus gradient transition between the flexible polypropylene fiber and the rigid cement matrix, effectively alleviating the stress concentration at the interface caused by the significant difference in stiffness. Simultaneously, the porous structure of the nano-Al2O3 coating increases the specific surface area, further strengthening the micro-mechanical interlocking effect between the fiber and the matrix. The outermost grafted aminosilane coupling agent functional layer establishes a continuous and robust stress transfer channel between the fiber and the matrix through hydrogen and chemical bonding between its amino groups and the hydroxyl groups on the surface of the hydrated calcium silicate gel in the cement hydration products. This three-layer structure—the etched and roughened substrate, the nano-ceramic transition coating, and the chemically bonded interface layer—enables the load to be efficiently transferred from the cement matrix to the three-dimensionally distributed fiber skeleton network via a step-by-step transfer path. This fully leverages the bridging, deflection, and pull-out energy dissipation effects of the fibers on microcracks, significantly improving the compressive and flexural strength of the lightweight concrete.

[0010] This invention improves the impermeability of lightweight concrete by constructing an organosilicon / nano-SiO2 composite system with a multi-level hydrophobic structure. The mechanism lies in the primary hydrophobication treatment of methyltriethoxysilane, which grafts a short-chain methyl organosilicon molecular layer onto the surface of nano-SiO2 particles, transforming the particle surface from hydrophilic to hydrophobic and establishing a preliminary hydrophobic barrier. The secondary grafting treatment of hexadecyltrimethoxysilane further assembles a dense long-chain alkyl organosilicon molecular layer outside the primary hydrophobic layer. 16 Long-chain molecules, arranged in an ordered manner, construct a high-density nonpolar alkyl molecular fence on the particle surface. Their spatial coverage effect is far superior to that of short-chain molecules, significantly reducing the surface free energy of the particles. Stearic acid co-assembly treatment, on the other hand, superimposes carbon molecules with C... on the bilayer silane-grafted surface. 18A long-chain fatty acid hydrophobic reinforcement layer, with stearic acid molecules anchored to residual silanol sites via hydrogen bonds and forming an alternating superhydrophobic film with long-chain alkylsilane molecules, constructs a rough hydrophobic structure on the surface of nanoparticles at both the micro and nano scales. After this composite hydrophobic modifier is dispersed in the concrete matrix, the high specific surface area of ​​the nanoparticles effectively fills the capillary pores and microcracks in the interface transition zone within the cement paste, physically cutting off the pathways for water penetration. Simultaneously, the multi-level organosilicon hydrophobic film on the particle surface gives the pore walls a superhydrophobic characteristic, significantly increasing the water contact angle and changing the capillary attraction from positive to negative, fundamentally reversing the direction of capillary action driving water in the pores, preventing water from penetrating and migrating into the concrete interior under capillary action. The synergistic effect of nanofilling densification and the multi-level hydrophobic barrier mechanisms simultaneously blocks the intrusion path of permeable water from both the pore structure and pore wall properties dimensions, significantly improving the impermeability and long-term durability of lightweight concrete.

[0011] Preferably, in step 1), the length of the chopped polypropylene fiber is 6-12 mm and the diameter is 18-30 μm.

[0012] Preferably, in step 1), the concentration of potassium dichromate in the chromic acid mixture is 0.05–0.15 mol / L; and the concentration of aluminum isopropoxide in the nano-alumina coating deposition is 0.3–0.6 mol / L.

[0013] Preferably, in step 1), the amount of γ-aminopropyltriethoxysilane used is 5% to 10% of the mass of polypropylene fiber; the grafting reaction temperature is 65 to 80°C, and the reaction time is 2 to 4 hours.

[0014] Preferably, in step 2), the amount of methyltriethoxysilane used is 20% to 40% of the mass of nano-SiO2, the reaction temperature is 65 to 80°C, and the reaction time is 4 to 6 hours.

[0015] Preferably, in step 2), the amount of hexadecyltrimethoxysilane used is 15% to 30% of the mass of nano-SiO2; the reaction conditions for the secondary hydrophobic enhancement treatment with hexadecyltrimethoxysilane are: reflux reaction at 70 to 85°C for 6 to 10 hours under nitrogen protection.

[0016] Preferably, in step 2), the mass ratio of nano-SiO2 to stearic acid in the stearic acid co-assembly superhydrophobic structure treatment is 1:(0.3~0.8). The stearic acid self-assembly is completed by stirring at 50~60℃ for 2~4h, and the temperature is raised to 70~80℃ and stirred continuously for 1~2h to complete the co-assembly curing.

[0017] Preferably, in step 3), based on 100 parts of the dry weight of the mine tailings, the following components are used: 60-80 parts of ordinary silicate cement, 25-40 parts of fly ash, 10-20 parts of slag powder, 4-8 parts of modified polypropylene fiber, 3-6 parts of organosilicon / nano-SiO2 composite hydrophobic modifier, 0.8-2.0 parts of polyvinyl alcohol, and 1.0-3.0 parts of nano-calcium carbonate.

[0018] This invention reveals in experiments that while the organosilicon / nano-SiO2 composite hydrophobic modifier imparts excellent impermeability to concrete, it significantly negatively interferes with the reinforcing effect of modified polypropylene fibers. After the composite hydrophobic modifier is dispersed in the concrete slurry, its multi-level organosilicon hydrophobic film inevitably undergoes non-directional physical adsorption and deposition on the surface of the modified polypropylene fibers during slurry mixing. This results in the outermost aminosilane coupling agent functional layer of the fiber being covered and encapsulated by hydrophobic nanoparticles, shielding the active amino groups on the fiber surface and preventing them from establishing effective chemical bonds with cement hydration products. This is equivalent to artificially introducing a low-surface-energy hydrophobic barrier between the fiber and the cement matrix, leading to a sharp decrease in the fiber-matrix interfacial bond strength. Load cannot be efficiently transferred from the matrix to the fiber network, and the fiber's bridging, crack-prevention, and pull-out energy dissipation mechanisms cannot be fully utilized, thus significantly weakening the mechanical reinforcing effect of the modified polypropylene fibers. To address this technical problem, this invention introduces polyvinyl alcohol (PVA) and nano-calcium carbonate during the concrete slurry preparation process. PVA, as a water-soluble polymer, has abundant hydroxyl groups on its molecular chain that preferentially form hydrogen bonds with residual silanol and stearic acid carboxyl groups on the surface of the composite hydrophobic modifier. This coats the hydrophobic nanoparticles with a hydrophilic PVA protective film, effectively reducing the free surface activity of the hydrophobic particles in the slurry and inhibiting their non-directional migration and deposition onto the fiber surface. This allows the aminosilane active functional layer on the fiber surface to remain exposed without being shielded. Nano-calcium carbonate, with its high specific surface area and alkaline active sites, acts as a nucleation site for the cement hydration reaction, accelerating the early formation and densification of hydrated calcium silicate gel in the fiber-matrix interface transition zone. It constructs a dense inorganic cementitious transition zone between the aminosilane layer on the fiber surface and the cement hydration products, significantly enhancing the interfacial bonding strength. The selective adsorption shielding mechanism of PVA and the interfacial nucleation-cementation reinforcement mechanism of nano-calcium carbonate work synergistically to simultaneously solve the negative impact of organosilicon / nano-SiO2 composite hydrophobic modifier on the reinforcement effect of modified polypropylene fibers from two dimensions: eliminating interference sources and strengthening interfacial bonding. This allows the two modification effects of mechanical reinforcement and anti-permeability improvement to be fully exerted in the same system without interference, achieving synergistic optimization of the mechanical strength and anti-permeability performance of lightweight concrete.

[0019] Preferably, in step 3), the pre-made foam is prepared by foaming an animal protein-based foaming agent diluted with water.

[0020] A lightweight concrete for mine filling is prepared by the method described above.

[0021] Compared with the prior art, the beneficial effects of the present invention are:

[0022] 1. Through multi-level surface modification, a gradient stress transmission channel and mechanical interlocking network are constructed between polypropylene fibers and cement matrix, which effectively alleviates the stress concentration at the interface, gives full play to the bridging and pull-out energy dissipation effect of fibers on microcracks, and significantly improves the compressive and flexural strength of lightweight concrete.

[0023] 2. Construct a multi-level organosilicon / nano-SiO2 composite hydrophobic system, using nanoparticles to fill capillary pores and form a micro-nano dual-scale superhydrophobic film layer, blocking the water penetration path from two dimensions: physical density and reversing the capillary driving force, significantly improving the impermeability and long-term durability of lightweight concrete.

[0024] 3. By introducing polyvinyl alcohol for selective adsorption to shield hydrophobic interference and combining it with nano-calcium carbonate to strengthen interface nucleation and bonding, the negative impact of hydrophobic modifiers on the active interface of fibers is effectively eliminated, ensuring that the dual functions of reinforcement and impermeability do not interfere with each other and work together efficiently in the same system. Attached Figure Description

[0025] Figure 1 This is a low-magnification SEM image of the lightweight concrete surface prepared in Example 4 of the present invention.

[0026] Figure 2 This is a medium-magnification SEM image of the lightweight concrete surface prepared in Example 4 of the present invention.

[0027] Figure 3 This is a high-magnification SEM image of the lightweight concrete surface prepared in Example 4 of the present invention. Detailed Implementation

[0028] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0029] Example 1

[0030] A method for preparing lightweight concrete for mine filling includes the following steps:

[0031] Step 1) Add potassium dichromate to a pre-mixed and cooled dilute sulfuric acid solution of 375 mL concentrated sulfuric acid and 875 mL deionized water, and stir until completely dissolved to prepare a potassium dichromate-chromic acid mixture with a concentration of 0.12 mol / L. Add 50 g of chopped polypropylene fibers (6-12 mm in length and 18-30 μm in diameter) to the above chromic acid mixture, stir at 60 °C for 30 min, remove and neutralize with 5 wt% sodium carbonate solution, then rinse repeatedly with deionized water until the pH of the washing solution is 7.0 ± 0.5, and vacuum dry at 65 °C for 5 h to obtain oxidized etched polypropylene fibers.

[0032] Aluminum isopropoxide was dissolved in 400 mL of anhydrous ethanol by stirring to prepare a 0.45 mol / L aluminum isopropoxide solution. 5 mL of 1 mol / L hydrochloric acid solution was slowly added dropwise as an acid catalyst under vigorous stirring. The hydrolysis reaction was carried out at 65 °C for 4 h to obtain aluminum sol. The above-mentioned oxidized and etched polypropylene fibers were immersed in the aluminum sol for 20 min, then removed and dried at 65 °C for 0.5 h. This immersion-drying process was repeated three times. The fibers were then heat-treated in a tube furnace under a nitrogen atmosphere at a rate of 1.5 °C / min to 130 °C, held for 4 h, and then naturally cooled to room temperature to obtain nano-Al₂O₃ coated polypropylene fibers.

[0033] 4.5 g of γ-aminopropyltriethoxysilane was dissolved in a mixed solvent of 90 mL anhydrous ethanol and 10 mL deionized water. The pH was adjusted to 4.5 with glacial acetic acid, and the mixture was stirred and hydrolyzed at 30 °C for 1 h to obtain a coupling agent hydrolysate. The above-mentioned nano-Al2O3 coated polypropylene fibers were immersed in the coupling agent hydrolysate and stirred at 70 °C for 3 h. After being removed, the fibers were washed three times with anhydrous ethanol and cured at 115 °C for 2 h to obtain modified polypropylene fibers.

[0034] Step 2) Add 20g of fumed silica nanoparticles to 800mL of anhydrous ethanol and ultrasonically disperse for 25min. Add 7g of methyltriethoxysilane, adjust the pH to 4.0 with glacial acetic acid, and reflux and stir at 70℃ for 3h. Remove the solvent by vacuum distillation, dry under vacuum at 85℃ for 10h, and grind through a 400-mesh sieve to obtain primary hydrophobic nano-SiO2 powder.

[0035] 16g of the above-mentioned primary hydrophobic nano-SiO2 powder was added to 400mL of cyclohexane and ultrasonically dispersed for 18min. Then, 5g of hexadecyltrimethoxysilane was added, and the mixture was refluxed and stirred at 80℃ for 8h under nitrogen protection. The mixture was filtered while hot, washed three times with cyclohexane, and dried under vacuum at 80℃ for 12h to obtain bilayer silane-grafted nano-SiO2.

[0036] 14g of stearic acid was added to 120mL of anhydrous ethanol and stirred at 50℃ until completely dissolved. 12g of the above-mentioned bilayer silane-grafted nano-SiO2 was added in batches to the stearic acid-ethanol solution, and stirred at 55℃ for 3h to complete self-assembly. The temperature was then raised to 75℃ and stirring continued for 1.5h to complete co-assembly and curing. Ethanol was removed by vacuum distillation, and the mixture was vacuum dried at 70℃ for 8h. The mixture was then ground through a 300-mesh sieve to obtain the organosilicon / nano-SiO2 composite hydrophobic modifier.

[0037] Step 3) Add 100 parts by weight of mine tailings, 75 parts by weight of ordinary Portland cement (PO 42.5 grade), 35 parts by weight of Grade I fly ash, and 18 parts by weight of S95 grade slag powder to a mixer and dry mix at low speed for 2 minutes. Add 3g of polycarboxylate superplasticizer to 180g of deionized water and mix evenly. Add this mixture to the dry-mixed powder and stir at 500rpm for 5 minutes to prepare a base slurry. Add 7 parts by weight of modified polypropylene fiber, 5 parts by weight of organosilicon / nano-SiO2 composite hydrophobic modifier, 1.8 parts by weight of polyvinyl alcohol (degree of polymerization 1700, degree of alcoholysis ≥98%), and 2.5 parts by weight of nano-calcium carbonate (particle size 40-80nm) to the slurry and stir for 3 minutes to mix evenly. Dilute the animal protein-based foaming agent with 40 times the amount of water to prepare a density of approximately 50kg / m³. 3 The foam was added to the slurry at a volume ratio of approximately 2.5:1 and stirred at low speed for 2 minutes to mix evenly. The foam slurry was poured into a mold for molding, steam-cured at 60℃ and relative humidity ≥95% for 24 hours, then demolded and cured under standard conditions at 20℃ and relative humidity ≥95% for 28 days to obtain lightweight concrete for mine filling.

[0038] Example 2

[0039] A method for preparing lightweight concrete for mine filling includes the following steps:

[0040] Step 1) Add potassium dichromate to a pre-mixed and cooled dilute sulfuric acid solution of 375 mL concentrated sulfuric acid and 875 mL deionized water, and stir until completely dissolved to prepare a potassium dichromate-chromic acid mixture with a concentration of 0.08 mol / L. Add 50 g of chopped polypropylene fibers (6-12 mm in length and 18-30 μm in diameter) to the above chromic acid mixture, stir at 60 °C for 30 min, remove and neutralize with 5 wt% sodium carbonate solution, then rinse repeatedly with deionized water until the pH of the washing solution is 7.0 ± 0.5, and vacuum dry at 65 °C for 5 h to obtain oxidized etched polypropylene fibers.

[0041] Aluminum isopropoxide was dissolved in 400 mL of anhydrous ethanol by stirring to prepare a 0.4 mol / L aluminum isopropoxide solution. 5 mL of 1 mol / L hydrochloric acid solution was slowly added dropwise as an acid catalyst under vigorous stirring. The hydrolysis reaction was carried out at 65 °C for 4 h to obtain aluminum sol. The above-mentioned oxidized and etched polypropylene fibers were immersed in the aluminum sol for 20 min, then removed and dried at 65 °C for 0.5 h. This immersion-drying process was repeated three times. The fibers were then heat-treated in a tube furnace under a nitrogen atmosphere at a rate of 1.5 °C / min to 130 °C, held for 4 h, and then naturally cooled to room temperature to obtain nano-Al₂O₃ coated polypropylene fibers.

[0042] 3.0 g of γ-aminopropyltriethoxysilane was dissolved in a mixed solvent of 90 mL anhydrous ethanol and 10 mL deionized water. The pH was adjusted to 4.5 with glacial acetic acid, and the mixture was stirred and hydrolyzed at 30 °C for 1 h to obtain a coupling agent hydrolysate. The above-mentioned nano-Al2O3 coated polypropylene fibers were immersed in the coupling agent hydrolysate and stirred at 70 °C for 3 h. After being removed, the fibers were washed three times with anhydrous ethanol and cured at 115 °C for 2 h to obtain modified polypropylene fibers.

[0043] Step 2) Add 20g of fumed silica nanoparticles to 800mL of anhydrous ethanol and ultrasonically disperse for 25min. Add 5g of methyltriethoxysilane, adjust the pH to 4.0 with glacial acetic acid, and reflux and stir at 70℃ for 3h. Remove the solvent by vacuum distillation, dry under vacuum at 85℃ for 10h, and grind through a 400-mesh sieve to obtain primary hydrophobic nano-SiO2 powder.

[0044] 16g of the above-mentioned primary hydrophobic nano-SiO2 powder was added to 400mL of cyclohexane and ultrasonically dispersed for 18min. Then, 4g of hexadecyltrimethoxysilane was added, and the mixture was refluxed and stirred at 75℃ for 8h under nitrogen protection. The mixture was filtered while hot, washed three times with cyclohexane, and dried under vacuum at 80℃ for 12h to obtain bilayer silane-grafted nano-SiO2.

[0045] 8g of stearic acid was added to 120mL of anhydrous ethanol and stirred at 50℃ until completely dissolved. 12g of the above-mentioned bilayer silane-grafted nano-SiO2 was added in batches to the stearic acid-ethanol solution, and stirred at 55℃ for 3h to complete self-assembly. The temperature was then raised to 75℃ and stirring continued for 1.5h to complete co-assembly and curing. Ethanol was removed by vacuum distillation, and the mixture was vacuum dried at 70℃ for 8h. The mixture was then ground through a 300-mesh sieve to obtain the organosilicon / nano-SiO2 composite hydrophobic modifier.

[0046] Step 3) Add 100 parts by weight of mine tailings, 65 parts by weight of ordinary Portland cement (PO 42.5 grade), 30 parts by weight of Grade I fly ash, and 12 parts by weight of S95 grade slag powder to a mixer and dry mix at low speed for 2 minutes. Add 3g of polycarboxylate superplasticizer to 180g of deionized water and mix evenly. Add this mixture to the dry-mixed powder and stir at 500rpm for 5 minutes to prepare a base slurry. Add 5 parts by weight of modified polypropylene fiber, 4 parts by weight of organosilicon / nano-SiO2 composite hydrophobic modifier, 1.0 part by weight of polyvinyl alcohol (degree of polymerization 1700, degree of alcoholysis ≥98%), and 1.5 parts by weight of nano-calcium carbonate (particle size 40-80nm) to the slurry and stir for 3 minutes to mix evenly. Dilute the animal protein-based foaming agent with 40 times the amount of water to prepare a density of approximately 50kg / m³. 3 The foam was added to the slurry at a volume ratio of approximately 2.5:1 and stirred at low speed for 2 minutes to mix evenly. The foam slurry was poured into a mold for molding, steam-cured at 60℃ and relative humidity ≥95% for 24 hours, then demolded and cured under standard conditions at 20℃ and relative humidity ≥95% for 28 days to obtain lightweight concrete for mine filling.

[0047] Example 3

[0048] A method for preparing lightweight concrete for mine filling includes the following steps:

[0049] Step 1) Add potassium dichromate to a pre-mixed and cooled dilute sulfuric acid solution of 375 mL concentrated sulfuric acid and 875 mL deionized water, and stir until completely dissolved to prepare a potassium dichromate-chromic acid mixture with a concentration of 0.1 mol / L. Add 50 g of chopped polypropylene fibers (6-12 mm in length and 18-30 μm in diameter) to the above chromic acid mixture, stir at 60 °C for 30 min, remove and neutralize with 5 wt% sodium carbonate solution, then rinse repeatedly with deionized water until the pH of the washing solution is 7.0 ± 0.5, and vacuum dry at 65 °C for 5 h to obtain oxidized etched polypropylene fibers.

[0050] Aluminum isopropoxide was dissolved in 400 mL of anhydrous ethanol by stirring to prepare a 0.45 mol / L aluminum isopropoxide solution. 5 mL of 1 mol / L hydrochloric acid solution was slowly added dropwise as an acid catalyst under vigorous stirring. The hydrolysis reaction was carried out at 65 °C for 4 h to obtain aluminum sol. The above-mentioned oxidized and etched polypropylene fibers were immersed in the aluminum sol for 20 min, then removed and dried at 65 °C for 0.5 h. This immersion-drying process was repeated three times. The fibers were then heat-treated in a tube furnace under a nitrogen atmosphere at a rate of 1.5 °C / min to 130 °C, held for 4 h, and then naturally cooled to room temperature to obtain nano-Al₂O₃ coated polypropylene fibers.

[0051] 3.5 g of γ-aminopropyltriethoxysilane was dissolved in a mixed solvent of 90 mL anhydrous ethanol and 10 mL deionized water. The pH was adjusted to 4.5 with glacial acetic acid, and the mixture was stirred and hydrolyzed at 30 °C for 1 h to obtain a coupling agent hydrolysate. The above-mentioned nano-Al2O3 coated polypropylene fibers were immersed in the coupling agent hydrolysate and stirred at 70 °C for 3 h. After being removed, the fibers were washed three times with anhydrous ethanol and cured at 115 °C for 2 h to obtain modified polypropylene fibers.

[0052] Step 2) Add 20g of fumed silica nanoparticles to 800mL of anhydrous ethanol and ultrasonically disperse for 25min. Add 6g of methyltriethoxysilane, adjust the pH to 4.0 with glacial acetic acid, and reflux and stir at 70℃ for 3h. Remove the solvent by vacuum distillation, dry under vacuum at 85℃ for 10h, and grind through a 400-mesh sieve to obtain primary hydrophobic nano-SiO2 powder.

[0053] 16g of the above-mentioned primary hydrophobic nano-SiO2 powder was added to 400mL of cyclohexane and ultrasonically dispersed for 18min. Then, 4.5g of hexadecyltrimethoxysilane was added, and the mixture was refluxed and stirred at 75℃ for 8h under nitrogen protection. The mixture was filtered while hot, washed three times with cyclohexane, and dried under vacuum at 80℃ for 12h to obtain bilayer silane-grafted nano-SiO2.

[0054] 12g of stearic acid was added to 120mL of anhydrous ethanol and stirred at 50℃ until completely dissolved. 12g of the above-mentioned bilayer silane-grafted nano-SiO2 was added in batches to the stearic acid-ethanol solution, and stirred at 55℃ for 3h to complete self-assembly. The temperature was then raised to 75℃ and stirring continued for 1.5h to complete co-assembly and curing. Ethanol was removed by vacuum distillation, and the mixture was vacuum dried at 70℃ for 8h. The mixture was then ground through a 300-mesh sieve to obtain the organosilicon / nano-SiO2 composite hydrophobic modifier.

[0055] Step 3) Add 100 parts by weight of mine tailings, 70 parts by weight of ordinary Portland cement (PO 42.5 grade), 30 parts by weight of Grade I fly ash, and 15 parts by weight of S95 grade slag powder to a mixer and dry mix at low speed for 2 minutes. Add 3g of polycarboxylate superplasticizer to 180g of deionized water and mix evenly. Add this mixture to the dry-mixed powder and stir at 500rpm for 5 minutes to prepare a basic slurry. Add 6 parts by weight of modified polypropylene fiber, 4.5 parts by weight of organosilicon / nano-SiO2 composite hydrophobic modifier, 1.5 parts by weight of polyvinyl alcohol (degree of polymerization 1700, degree of alcoholysis ≥98%), and 2 parts by weight of nano-calcium carbonate (particle size 40-80nm) to the slurry and stir for 3 minutes to mix evenly. Dilute the animal protein-based foaming agent with 40 times the amount of water to prepare a density of approximately 50kg / m³. 3The foam was added to the slurry at a volume ratio of approximately 2.5:1 and stirred at low speed for 2 minutes to mix evenly. The foam slurry was poured into a mold for molding, steam-cured at 60℃ and relative humidity ≥95% for 24 hours, then demolded and cured under standard conditions at 20℃ and relative humidity ≥95% for 28 days to obtain lightweight concrete for mine filling.

[0056] Example 4

[0057] A method for preparing lightweight concrete for mine filling includes the following steps:

[0058] Step 1) Add potassium dichromate to a pre-mixed and cooled dilute sulfuric acid solution of 375 mL concentrated sulfuric acid and 875 mL deionized water, and stir until completely dissolved to prepare a potassium dichromate-chromic acid mixture with a concentration of 0.15 mol / L. Add 50 g of chopped polypropylene fibers (6-12 mm in length and 18-30 μm in diameter) to the above chromic acid mixture, stir at 60 °C for 30 min, remove and neutralize with 5 wt% sodium carbonate solution, then rinse repeatedly with deionized water until the pH of the washing solution is 7.0 ± 0.5, and vacuum dry at 65 °C for 5 h to obtain oxidized etched polypropylene fibers.

[0059] Aluminum isopropoxide was dissolved in 400 mL of anhydrous ethanol by stirring to prepare a 0.6 mol / L aluminum isopropoxide solution. 5 mL of 1 mol / L hydrochloric acid solution was slowly added dropwise as an acid catalyst under vigorous stirring. The hydrolysis reaction was carried out at 65 °C for 4 h to obtain aluminum sol. The above-mentioned oxidized and etched polypropylene fibers were immersed in the aluminum sol for 20 min, then removed and dried at 65 °C for 0.5 h. This immersion-drying process was repeated three times. The fibers were then heat-treated in a tube furnace under a nitrogen atmosphere at a rate of 1.5 °C / min to 130 °C, held for 4 h, and then naturally cooled to room temperature to obtain nano-Al₂O₃ coated polypropylene fibers.

[0060] 5.0 g of γ-aminopropyltriethoxysilane was dissolved in a mixed solvent of 90 mL anhydrous ethanol and 10 mL deionized water. The pH was adjusted to 4.5 with glacial acetic acid, and the mixture was stirred and hydrolyzed at 30 °C for 1 h to obtain a coupling agent hydrolysate. The above-mentioned nano-Al2O3 coated polypropylene fibers were immersed in the coupling agent hydrolysate and stirred at 80 °C for 4 h. After being removed, the fibers were washed three times with anhydrous ethanol and cured at 115 °C for 2 h to obtain modified polypropylene fibers.

[0061] Step 2) Add 20g of fumed silica nanoparticles to 800mL of anhydrous ethanol and ultrasonically disperse for 25min. Add 8g of methyltriethoxysilane, adjust the pH to 4.0 with glacial acetic acid, and reflux and stir at 80℃ for 4h. Remove the solvent by vacuum distillation, dry under vacuum at 85℃ for 10h, and grind through a 400-mesh sieve to obtain primary hydrophobic nano-SiO2 powder.

[0062] 16g of the above-mentioned primary hydrophobic nano-SiO2 powder was added to 400mL of cyclohexane and ultrasonically dispersed for 18min. Then, 6g of hexadecyltrimethoxysilane was added, and the mixture was refluxed and stirred at 85℃ for 10h under nitrogen protection. The mixture was filtered while hot, washed three times with cyclohexane, and dried under vacuum at 80℃ for 12h to obtain bilayer silane-grafted nano-SiO2.

[0063] 16g of stearic acid was added to 120mL of anhydrous ethanol and stirred at 50℃ until completely dissolved. 12g of the above-mentioned bilayer silane-grafted nano-SiO2 was added in batches to the stearic acid-ethanol solution, and the mixture was stirred at 60℃ for 4h to complete self-assembly. The temperature was then raised to 80℃ and stirring continued for 2h to complete co-assembly and curing. Ethanol was removed by vacuum distillation, and the mixture was vacuum dried at 70℃ for 8h. The mixture was then ground through a 300-mesh sieve to obtain the organosilicon / nano-SiO2 composite hydrophobic modifier.

[0064] Step 3) Add 100 parts by weight of mine tailings, 80 parts by weight of ordinary Portland cement (PO 42.5 grade), 40 parts by weight of Grade I fly ash, and 20 parts by weight of S95 grade slag powder to a mixer and dry mix at low speed for 2 minutes. Add 3g of polycarboxylate superplasticizer to 180g of deionized water and mix evenly. Add this mixture to the dry-mixed powder and stir at 500rpm for 5 minutes to prepare a base slurry. Add 8 parts by weight of modified polypropylene fiber, 6 parts by weight of organosilicon / nano-SiO2 composite hydrophobic modifier, 2.0 parts by weight of polyvinyl alcohol (degree of polymerization 1700, degree of alcoholysis ≥98%), and 3 parts by weight of nano-calcium carbonate (particle size 40-80nm) to the slurry and stir for 3 minutes to mix evenly. Dilute the animal protein-based foaming agent with 40 times the amount of water to prepare a density of approximately 50kg / m³. 3 The foam was added to the slurry at a volume ratio of approximately 2.5:1 and stirred at low speed for 2 minutes to mix evenly. The foam slurry was poured into a mold for molding, steam-cured at 60℃ and relative humidity ≥95% for 24 hours, then demolded and cured under standard conditions at 20℃ and relative humidity ≥95% for 28 days to obtain lightweight concrete for mine filling.

[0065] Example 5

[0066] A method for preparing lightweight concrete for mine filling includes the following steps:

[0067] Step 1) Add potassium dichromate to a pre-mixed and cooled dilute sulfuric acid solution of 375 mL concentrated sulfuric acid and 875 mL deionized water, and stir until completely dissolved to prepare a potassium dichromate-chromic acid mixture with a concentration of 0.05 mol / L. Add 50 g of chopped polypropylene fibers (6-12 mm in length and 18-30 μm in diameter) to the above chromic acid mixture, stir at 60 °C for 30 min, remove and neutralize with 5 wt% sodium carbonate solution, then rinse repeatedly with deionized water until the pH of the washing solution is 7.0 ± 0.5, and vacuum dry at 65 °C for 5 h to obtain oxidized etched polypropylene fibers.

[0068] Aluminum isopropoxide was dissolved in 400 mL of anhydrous ethanol by stirring to prepare a 0.3 mol / L aluminum isopropoxide solution. 5 mL of 1 mol / L hydrochloric acid solution was slowly added dropwise as an acid catalyst under vigorous stirring. The hydrolysis reaction was carried out at 65 °C for 4 h to obtain aluminum sol. The above-mentioned oxidized and etched polypropylene fibers were immersed in the aluminum sol for 20 min, then removed and dried at 65 °C for 0.5 h. This immersion-drying process was repeated three times. The fibers were then heat-treated in a tube furnace under a nitrogen atmosphere at a rate of 1.5 °C / min to 130 °C, held for 4 h, and then naturally cooled to room temperature to obtain nano-Al₂O₃ coated polypropylene fibers.

[0069] 2.5 g of γ-aminopropyltriethoxysilane was dissolved in a mixed solvent of 90 mL anhydrous ethanol and 10 mL deionized water. The pH was adjusted to 4.5 with glacial acetic acid, and the mixture was stirred and hydrolyzed at 30 °C for 1 h to obtain a coupling agent hydrolysate. The above-mentioned nano-Al2O3 coated polypropylene fibers were immersed in the coupling agent hydrolysate and stirred at 65 °C for 2 h. After being removed, the fibers were washed three times with anhydrous ethanol and cured at 115 °C for 2 h to obtain modified polypropylene fibers.

[0070] Step 2) Add 20g of fumed silica nanoparticles to 800mL of anhydrous ethanol and ultrasonically disperse for 25min. Add 4g of methyltriethoxysilane, adjust the pH to 4.0 with glacial acetic acid, and reflux and stir at 65℃ for 2h. Remove the solvent by vacuum distillation, dry under vacuum at 85℃ for 10h, and grind through a 400-mesh sieve to obtain primary hydrophobic nano-SiO2 powder.

[0071] 16g of the above-mentioned primary hydrophobic nano-SiO2 powder was added to 400mL of cyclohexane and ultrasonically dispersed for 18min. Then, 3g of hexadecyltrimethoxysilane was added, and the mixture was refluxed and stirred at 70℃ for 6h under nitrogen protection. The mixture was filtered while hot, washed three times with cyclohexane, and dried under vacuum at 80℃ for 12h to obtain bilayer silane-grafted nano-SiO2.

[0072] 6g of stearic acid was added to 120mL of anhydrous ethanol and stirred at 50℃ until completely dissolved. 12g of the above-mentioned bilayer silane-grafted nano-SiO2 was added in batches to the stearic acid-ethanol solution, and stirred at 50℃ for 2h to complete self-assembly. The temperature was then raised to 70℃ and stirring continued for 1h to complete co-assembly and curing. Ethanol was removed by vacuum distillation, and the mixture was vacuum dried at 70℃ for 8h. The mixture was then ground through a 300-mesh sieve to obtain the organosilicon / nano-SiO2 composite hydrophobic modifier.

[0073] Step 3) Add 100 parts by weight of mine tailings, 60 parts by weight of ordinary Portland cement (PO 42.5 grade), 25 parts by weight of Grade I fly ash, and 10 parts by weight of S95 grade slag powder to a mixer and dry mix at low speed for 2 minutes. Add 3g of polycarboxylate superplasticizer to 180g of deionized water and mix evenly. Add this mixture to the dry-mixed powder and stir at 500rpm for 5 minutes to prepare a base slurry. Add 4 parts by weight of modified polypropylene fiber, 3 parts by weight of organosilicon / nano-SiO2 composite hydrophobic modifier, 0.8 parts by weight of polyvinyl alcohol (degree of polymerization 1700, degree of alcoholysis ≥98%), and 1 part by weight of nano-calcium carbonate (particle size 40-80nm) to the slurry and stir for 3 minutes to mix evenly. Dilute the animal protein-based foaming agent with 40 times the amount of water to prepare a density of approximately 50kg / m³. 3 The foam was added to the slurry at a volume ratio of approximately 2.5:1 and stirred at low speed for 2 minutes to mix evenly. The foam slurry was poured into a mold for molding, steam-cured at 60℃ and relative humidity ≥95% for 24 hours, then demolded and cured under standard conditions at 20℃ and relative humidity ≥95% for 28 days to obtain lightweight concrete for mine filling.

[0074] Comparative Example 1: The difference from Example 4 is that modified polypropylene fiber, organosilicon / nano SiO2 composite hydrophobic modifier, polyvinyl alcohol and nano calcium carbonate are not added in step 3), while the remaining steps and conditions are exactly the same as in Example 4.

[0075] Comparative Example 2: The difference from Example 4 is that in step 3), no organosilicon / nano SiO2 composite hydrophobic modifier, polyvinyl alcohol and nano calcium carbonate are added, only 8 parts by weight of modified polypropylene fiber are added, and the remaining steps and conditions are exactly the same as in Example 4.

[0076] Comparative Example 3: The difference from Example 4 is that in step 3), modified polypropylene fiber, polyvinyl alcohol and nano calcium carbonate are not added, but only 6 parts by weight of organosilicon / nano SiO2 composite hydrophobic modifier is added. The remaining steps and conditions are exactly the same as in Example 4.

[0077] Comparative Example 4: The difference from Example 4 is that polyvinyl alcohol and nano calcium carbonate are not added in step 3), but 8 parts by weight of modified polypropylene fiber and 6 parts by weight of organosilicon / nano SiO2 composite hydrophobic modifier are added. The remaining steps and conditions are exactly the same as in Example 4.

[0078] Comparative Example 5: The difference from Example 4 is that polyvinyl alcohol is not added in step 3), while the remaining steps and conditions are exactly the same as in Example 4.

[0079] Comparative Example 6: The difference from Example 4 is that nano-calcium carbonate is not added in step 3), while the remaining steps and conditions are exactly the same as in Example 4.

[0080] Comparative Example 7: The difference from Example 4 is that in step 1), the chopped polypropylene fibers are not subjected to chromic acid oxidation etching, nano-alumina coating deposition, and γ-aminopropyltriethoxysilane grafting treatment. Instead, 8 parts by weight of unmodified chopped polypropylene fibers (6-12 mm in length and 18-30 μm in diameter) are directly used to replace the modified polypropylene fibers in the slurry of step 3). The remaining steps and conditions are exactly the same as in Example 4.

[0081] Comparative Example 8: The difference from Example 4 is that in step 2), the methyltriethoxysilane hydrophobic grafting treatment, hexadecyltrimethoxysilane secondary hydrophobic enhancement treatment, and stearic acid co-assembly superhydrophobic structure treatment of fumed nano-silica are not performed. Instead, 6 parts by weight of unmodified fumed nano-silica (particle size 15nm) are directly used to replace the organosilicon / nano-SiO2 composite hydrophobic modifier in the slurry of step 3). The remaining steps and conditions are exactly the same as in Example 4.

[0082] Performance testing:

[0083] 1. Dry density test

[0084] The test was conducted according to GB / T 11969—2020 "Test Methods for Performance of Autoclaved Aerated Concrete". 100mm×100mm×100mm cubic specimens cured for 28 days were dried to constant weight in a forced-air drying oven at 60±5℃ (the interval between two consecutive weighings should not be less than 24 hours, and the mass difference should not exceed 0.1%). After cooling to room temperature, the length, width, and height of the specimens were measured three times with vernier calipers, and the average value was used to calculate the volume. The dried mass was weighed using an electronic balance with an accuracy of 0.1g, and the dry density was calculated using the formula ρ=m / V. The arithmetic mean of three specimens in each group was taken as the test result.

[0085] 2. 28-day compressive strength test

[0086] The test was conducted according to GB / T 11969—2020 "Test Methods for Performance of Autoclaved Aerated Concrete". 100mm×100mm×100mm cube specimens, cured to 28 days of standard age, were placed on a universal testing machine with the side surface as it was during molding. A uniform loading rate of 2000 N / s was applied until the specimen failed. The maximum failure load F was recorded. The compressive strength was calculated using the formula σc=F / A, where A is the area under pressure (100mm×100mm=10000mm²). 2 The arithmetic mean of three specimens in each group was taken as the test result.

[0087] 3. 28-day flexural strength test

[0088] Tests were conducted according to GB / T 11969—2020 "Test Methods for Performance of Autoclaved Aerated Concrete". A 100mm×100mm×400mm prism specimen, cured to 28 days of standard age, was placed on a flexural testing apparatus and subjected to three-point bending loading with a span of 300mm. The load was applied uniformly at a rate of 50 N / s until the specimen fractured. The maximum failure load F was recorded, and the result was calculated using the formula σf = 3FL / (2bh). 2 Calculate the flexural strength, where L is the span of 300 mm, b is the specimen width of 100 mm, and h is the specimen height of 100 mm. Take the arithmetic mean of 3 specimens in each group as the test result.

[0089] 4. Impermeability test

[0090] The test was conducted using the stepwise pressure method as specified in GB / T 50082—2009, "Standard for Test Methods of Long-Term Performance and Durability of Ordinary Concrete". A frustum-shaped specimen, with an upper diameter of 175 mm, a lower diameter of 185 mm, and a height of 150 mm, and cured to 28 days of standard age, was mounted on a permeameter. After sealing the edges, water pressure was applied from the bottom of the specimen, initially at 0.1 MPa and maintained for 8 hours. The pressure was then increased by 0.1 MPa every 8 hours until water seepage appeared on the top surface of the specimen. The water pressure value P (MPa) was recorded at this point. The permeability grade was expressed as 10P (e.g., P=1.2 MPa is the permeability grade P12). Six specimens were taken from each group for testing.

[0091] 5. 24-hour water absorption rate test

[0092] The test was conducted according to GB / T 11969—2020 "Test Methods for Performance of Autoclaved Aerated Concrete". 100mm×100mm×100mm cubic specimens cured to 28 days were dried to constant weight at 60±5℃, and the dried mass m0 was measured. The dried specimens were then completely immersed in deionized water at 20±2℃ for 24 hours. After soaking, the specimens were removed, and the surface water was wiped off with a damp cloth. The saturated mass m1 was immediately measured. The 24-hour water absorption rate was calculated using the formula W=(m1−m0) / m0×100%. The arithmetic mean of three specimens from each group was taken as the test result.

[0093] 6. Softening coefficient test

[0094] Tests were conducted according to GB / T 11969—2020. Specimens from the same batch, cured to 28 days of standard age, were divided into two groups. One group was dried to constant weight at 60±5℃, and the dry compressive strength σdry was measured using the method described above. The other group was completely immersed in deionized water at 20±2℃ for 48 hours, then removed, wiped dry, and immediately measured for wet compressive strength σwet. The softening coefficient was calculated using the formula K=σwet / σdry. A higher softening coefficient indicates better water resistance of the material. Three specimens from each group were measured separately, and the arithmetic mean was taken.

[0095] 7. Drying shrinkage rate test

[0096] Tests were conducted according to GB / T 11969—2020, "Test Methods for Performance of Autoclaved Aerated Concrete". Stainless steel probes were attached to both ends of 100mm×100mm×400mm prism specimens cured to 28 days of standard age. The initial length L0 was measured using a length comparator. The specimens were then placed in a constant temperature and humidity drying environment at 20±2℃ and 60±5% relative humidity. The specimens were removed after 7, 14, and 28 days of drying, and the length L0 was measured again using a length comparator. t According to the formula ε=(L0−L t ) / L0×10 3 Calculate the drying shrinkage value (mm / m), and use the 28-day drying shrinkage value as the final result. Take the arithmetic mean of 3 specimens in each group.

[0097] Table 1: Performance Test Results of Examples and Comparative Examples

[0098]

[0099] Comparative Example 1 is a blank reference group without any added modified components. Its 28-day compressive strength is only 1.8 MPa, flexural strength is only 0.35 MPa, impermeability grade is only P2, 24-hour water absorption rate is as high as 35.6%, softening coefficient is only 0.52, and drying shrinkage rate is 0.68 mm / m. All properties are poor and cannot meet the basic requirements of mine backfilling projects.

[0100] Comparative Example 2, which only added modified polypropylene fibers without adding hydrophobic modifiers, had its compressive strength increased to 3.8 MPa and its flexural strength increased to 0.90 MPa, which were 111% and 157% higher than those of Comparative Example 1, respectively, indicating that the fiber-reinforced skeleton network can significantly improve mechanical properties; however, its impermeability grade was only P3 and its 24-hour water absorption rate was still as high as 28.2%, indicating that fiber reinforcement made almost no contribution to impermeability.

[0101] Comparative Example 3, which only added an organosilicon / nano-SiO2 composite hydrophobic modifier without adding modified fibers, improved its impermeability grade to P8, reduced its 24-hour water absorption rate to 15.5%, and increased its softening coefficient to 0.73, indicating that the multi-level hydrophobic barrier can effectively improve impermeability performance; however, its compressive strength was only 2.0 MPa, which was only a limited improvement compared to Comparative Example 1, indicating that the hydrophobic modifier contributed almost nothing to the mechanical strength.

[0102] Comparative Example 4, which included modified polypropylene fiber and a silicone / nano-SiO2 composite hydrophobic modifier but omitted polyvinyl alcohol and nano-calcium carbonate, achieved a permeability rating of P8, comparable to Comparative Example 3. However, its compressive strength was only 2.8 MPa, significantly lower than the 3.8 MPa of Comparative Example 2 using modified fiber alone, and its flexural strength was only 0.60 MPa, also significantly lower than the 0.90 MPa of Comparative Example 2. This result directly confirms the negative interference of the hydrophobic modifier on the fiber reinforcement effect during slurry mixing: the adsorption and deposition of hydrophobic nanoparticles on the fiber surface shields the aminosilane active functional layer, leading to a decrease in the fiber-matrix interfacial bonding strength.

[0103] The compressive strength of Comparative Example 5 (without polyvinyl alcohol) was 3.5 MPa, which was improved from 2.8 MPa in Comparative Example 4, but still significantly lower than 6.1 MPa in Example 4. The compressive strength of Comparative Example 6 (without nano-calcium carbonate) was 5.0 MPa, which was significantly better than that of Comparative Example 5. This indicates that the selective adsorption and shielding effect of polyvinyl alcohol is the primary factor in eliminating hydrophobic interference, while the interfacial nucleation and cementation effect of nano-calcium carbonate is an auxiliary factor in further strengthening interfacial adhesion. Only through the synergistic effect of both can the optimal mechanical properties (6.1 MPa in Example 4) be achieved.

[0104] Comparative Example 7 used unmodified polypropylene fibers instead of modified polypropylene fibers, and its compressive strength was only 3.2 MPa and its flexural strength was only 0.70 MPa, which were only 52% and 47% of those in Example 4, respectively. This indicates that ordinary polypropylene fibers that have not undergone the three-step surface engineering treatment of chromic acid etching, nano-alumina coating, and aminosilane grafting have extremely weak interfacial bonding with the cement matrix due to their non-polar surface, smooth surface, and lack of anchoring points, making it difficult to fully exert the reinforcing effect of the fibers.

[0105] Comparative Example 8 used unmodified nano-silica instead of the organosilicon / nano-SiO2 composite hydrophobic modifier. Its compressive strength was 5.5 MPa and flexural strength was 1.30 MPa, similar to Example 4. However, its impermeability rating was only P4, its 24-hour water absorption rate was as high as 25.0%, and its softening coefficient was only 0.60, far lower than Example 4's P12, 10.2%, and 0.86. This indicates that although ordinary hydrophilic nano-SiO2 without three-step hydrophobic modification can fill pores to some extent, the surface of the particles remains hydrophilic, making it impossible to reverse the direction of capillary driving force, resulting in very limited improvement in impermeability.

[0106] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the essence and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing lightweight concrete for mine filling, characterized in that, Includes the following steps: 1) Short polypropylene fibers were subjected to chromic acid mixed solution oxidation etching treatment, aluminum isopropoxide sol-gel method nano-alumina coating deposition and γ-aminopropyltriethoxysilane grafting treatment in sequence to obtain modified polypropylene fibers. 2) The fumed silica nano-silica was subjected to hydrophobic grafting treatment with methyltriethoxysilane, secondary hydrophobic enhancement treatment with hexadecyltrimethoxysilane, and superhydrophobic structure treatment with stearic acid co-assembly to obtain organosilicon / nano-SiO2 composite hydrophobic modifier. 3) Mine tailings, ordinary silicate cement, fly ash, slag powder, the modified polypropylene fiber, the organosilicon / nano-SiO2 composite hydrophobic modifier, polyvinyl alcohol and nano-calcium carbonate are mixed to form a slurry, pre-made foam is added, mixed and poured into molds, and cured to obtain the lightweight concrete for mine filling.

2. The method for preparing lightweight concrete for mine filling according to claim 1, characterized in that, In step 1), the length of the chopped polypropylene fiber is 6-12 mm and the diameter is 18-30 μm.

3. The method for preparing lightweight concrete for mine filling according to claim 1, characterized in that, In step 1), the concentration of potassium dichromate in the chromic acid mixture is 0.05–0.15 mol / L; In the deposition of the nano-alumina coating, the concentration of aluminum isopropoxide is 0.3–0.6 mol / L.

4. The method for preparing lightweight concrete for mine filling according to claim 1, characterized in that, In step 1), the amount of γ-aminopropyltriethoxysilane used is 5% to 10% of the mass of polypropylene fiber; the grafting reaction temperature is 65 to 80°C, and the reaction time is 2 to 4 hours.

5. The method for preparing lightweight concrete for mine filling according to claim 1, characterized in that, In step 2), the amount of methyltriethoxysilane used is 20% to 40% of the mass of nano-SiO2, the reaction temperature is 65 to 80°C, and the reaction time is 4 to 6 hours.

6. The method for preparing lightweight concrete for mine filling according to claim 1, characterized in that, In step 2), the amount of hexadecyltrimethoxysilane used is 15% to 30% of the mass of nano-SiO2; the reaction conditions for the secondary hydrophobic enhancement treatment with hexadecyltrimethoxysilane are: reflux reaction at 70 to 85°C for 6 to 10 hours under nitrogen protection.

7. The method for preparing lightweight concrete for mine filling according to claim 1, characterized in that, In step 2), the mass ratio of nano-SiO2 to stearic acid in the stearic acid co-assembly superhydrophobic structure treatment is 1:(0.3~0.8). Stirring at 50~60℃ for 2~4h completes the self-assembly of stearic acid, and then heating to 70~80℃ and stirring continuously for 1~2h completes the co-assembly curing.

8. A method for preparing lightweight concrete for mine filling according to claim 1, characterized in that, In step 3), based on 100 parts of dry tailings from the mine, the following components are used: 60-80 parts of ordinary silicate cement, 25-40 parts of fly ash, 10-20 parts of slag powder, 4-8 parts of modified polypropylene fiber, 3-6 parts of organosilicon / nano-SiO2 composite hydrophobic modifier, 0.8-2.0 parts of polyvinyl alcohol, and 1.0-3.0 parts of nano-calcium carbonate.

9. A method for preparing lightweight concrete for mine filling according to claim 1, characterized in that, In step 3), the pre-made foam is obtained by foaming an animal protein-based foaming agent diluted with water.

10. A lightweight concrete for mine filling, characterized in that, It is prepared by the method described in any one of claims 1 to 9 above.

Images