A cementitious material for mine filling and a method of making the same

Abstract

The application discloses a kind of cementitious materials for mine filling and its preparation method, it is related to light building material technical field.The method is by alkali thermal activation and organic silane grafting composite modification to red mud, form hydrophobic coupling layer and release active component;Simultaneously with slag, metakaolin constructs two-component gel system, and introduces nano magnesium oxide and calcium stearate and carries out co-milling interface control.Nano magnesium oxide promotes gel to be uniformly deposited and wrapped on the surface of hydrophobic red mud, and calcium stearate is matched with the surface energy of organic silane coupling layer, to eliminate weak area of gel distribution.The method is by micro-interface coupling and synergistic anti-shrinkage mechanism, significantly reduces the shrinkage of filling body, improves volume stability and material density.

Description

Technical Field

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

[0002] Red mud is a highly alkaline industrial solid waste discharged during the production of alumina from bauxite using the Bayer process or sintering process. Large-scale stockpiling of red mud not only occupies significant land resources, but its high alkalinity (pH typically 10-13) also easily leads to soil alkalization and groundwater pollution, posing a serious threat to the ecological environment. Mine backfilling is one of the important ways to dispose of large quantities of industrial solid waste such as red mud. Using red mud as the matrix of backfill cementitious materials can achieve large-scale resource utilization of red mud and provide effective support for goaf areas. However, existing red mud-based backfill cementitious materials generally suffer from high shrinkage rates. Significant shrinkage occurs during the hydration and hardening process of the backfill, easily leading to cracks and voids within the backfill and at the interface between the backfill and the surrounding rock. This results in a decreased roof contact rate between the backfill and the surrounding rock, severely weakening the backfill's load-bearing capacity for the goaf roof and affecting mine safety and surface subsidence control. Summary of the Invention

[0003] The purpose of this invention is to provide a cementitious material 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: A method for preparing a cementitious material for mine filling includes the following steps: (1) The red mud is crushed and activated by alkaline heat in an alkaline solution. After the reaction is completed, solid-liquid separation is performed and the solid obtained is washed and dried to obtain alkaline heat activated red mud. (2) The alkaline-heat activated red mud is grafted with a silane coupling agent under acidic conditions. After the reaction is completed, solid-liquid separation is performed, and the separated solid is washed, dried and ground to obtain organosilane grafted modified red mud. (3) Mix metakaolin and slag powder, add water and stir evenly to obtain a two-component slurry; (4) Add nano-magnesium oxide and calcium stearate to the two-component slurry and perform co-milling to obtain a modified two-component gel slurry; (5) The organosilane grafted modified red mud, cement, the modified two-component gel slurry and mixing water are mixed to obtain filling slurry; (6) The filling slurry is molded and cured to obtain the cementitious material for mine filling.

[0005] In the technical solution of this invention, the shrinkage resistance of red mud cementitious materials is improved synergistically from the following two aspects: (1) By modifying red mud particles through alkaline thermal activation-organosilane grafting composite modification, an organosilane coupling layer with hydrophobic characteristics is formed on the particle surface. This modified layer regulates the shrinkage behavior of the filling cementitious material from two levels. On the one hand, the organosilane coupling layer changes the surface energy state of the red mud particles, reduces the adsorption affinity of the particle surface for free water, and makes the distribution of water participating in the hydration reaction in the filling slurry system more uniform, reducing uneven drying shrinkage caused by excessive local water accumulation or rapid evaporation; at the same time, the presence of the coupling layer improves the interfacial bonding quality between red mud particles and cement hydration products, enhances the density of the interfacial transition zone, effectively inhibits the migration and evaporation of water along the interfacial transition zone, thereby reducing the driving force of drying shrinkage. On the other hand, alkaline-thermal activation treatment releases the dormant active silica-alumina components in the red mud. These active components can participate in secondary hydration reactions during subsequent hydration processes. The resulting hydration products act as space fillers in the microporous structure of the filling body, refining the capillary pore size distribution, reducing the self-shrinkage effect caused by capillary negative pressure, and further improving the volume stability of the filling body. It should be noted that while the organosilane coupling layer effectively regulates water migration behavior, it also objectively makes the surface of the red mud particles exhibit low surface energy hydrophobic characteristics. This characteristic places higher demands on the interfacial wetting matching between the red mud particles and other gel components.

[0006] (2) By introducing a slag-metakaolin dual-component synergistic gel system, the shrinkage behavior was deeply regulated from two dimensions: the composition of hydration products and the micropore structure of the filling cementitious material. Metakaolin has a highly disordered layered aluminosilicate structure, which can rapidly dissolve and release a large amount of active silica-alumina components in an alkaline environment. It works synergistically with the calcium ions released by slag hydration to form a denser gel network skeleton. This interlocking gel network structure establishes an effective rigid support inside the filling body, constraining the volume shrinkage trend caused by water consumption during the hydration process. At the same time, the dual-component combination of metakaolin and slag makes the types of hydration products more abundant. The generated CASH and NASH multi-element gel phases intertwine and fill the micropores of the filling body, effectively refining the pore size distribution and reducing the proportion of macropores and interconnected pores, thereby reducing the shrinkage driving effect of capillary negative pressure on the filling body. In addition, the hydration reaction of slag in the two-component gel system releases heat relatively slowly, which complements the rapid dissolution and release of metakaolin in time. This makes the generation of hydration products more uniform and continuous in time, avoiding the internal stress concentration and microcrack initiation caused by concentrated heat release during hydration and sudden generation of hydration products in the single-component system. This further ensures the volume stability of the filling material throughout the hardening process.

[0007] Preferably, the alkaline solution in step (1) is a sodium hydroxide solution with a concentration of 2 to 4 mol / L.

[0008] Preferably, the solid-liquid ratio of the red mud to the sodium hydroxide solution is 1 g:(3-5) mL; the temperature of the alkaline thermal activation reaction is 80-95°C, and the time is 2-4 h.

[0009] Preferably, the solid-liquid separation method in step (1) is vacuum filtration; the washing uses deionized water and is carried out until the pH of the washing wastewater is 9-10; the drying is carried out at 60°C until constant weight.

[0010] Preferably, the silane coupling agent in step (2) is vinyltriethoxysilane; the grafting reaction is carried out in anhydrous ethanol.

[0011] Preferably, the amount of vinyltriethoxysilane added is 3-6 wt% of the mass of the alkaline-thermally activated red mud; the acidic conditions are achieved by adjusting the pH of the system to 4.0-5.0 using acetic acid solution; and the grafting reaction is carried out at a temperature of 60-70°C for 3-5 hours.

[0012] Preferably, the mass ratio of metakaolin to slag powder in step (3) is 32:(45-50).

[0013] Preferably, the mass of the nano-magnesium oxide in step (4) is 1.5 to 3.0% of the total mass of metakaolin and slag powder, and the mass of the added calcium stearate is 0.8 to 1.5% of the total mass of metakaolin and slag powder.

[0014] In experiments, this invention found that the alkaline-thermal activation-organosilane grafting composite modification of red mud particles resulted in the organosilane coupling layer effectively regulating water migration behavior while giving the red mud particle surface a low surface energy hydrophobic characteristic. At the same time, it hindered the sufficient wetting contact between the modified red mud particles and the two-component gel slurry to a certain extent, leading to an insufficiently continuous deposition and distribution of the gel phase around the modified red mud particles, resulting in localized weak gel coverage areas. These weak areas became the preferred starting points for shrinkage deformation and microcrack initiation during the curing process, preventing the synergistic anti-shrinkage effect of the above two aspects from being fully realized. To address this technical problem, this invention introduces nano-magnesium oxide and calcium stearate into a two-component gel system. The nano-magnesium oxide, after ball milling and dispersion, is anchored to the surface of slag and metakaolin particles. When the modified two-component gel slurry is mixed with organosilane-grafted modified red mud, these nano-magnesium oxide particles anchored to the gel phase particles act as bridging phases, establishing physical overlap channels between the hydrophobic surface of the red mud particles and the hydrophilic matrix of the gel slurry. This promotes the uniform deposition and encapsulation of the gel phase onto the modified red mud particle surface. Simultaneously, the hydrophobic calcium stearate film coating the slag and metakaolin particles has similar surface energy characteristics to the organosilane coupling layer on the red mud surface, resulting in good interfacial affinity. This allows the solid phase particles in the gel slurry to spread more smoothly on the modified red mud surface and form a uniform and continuous transition layer, effectively eliminating weak areas in the gel distribution. By synergistically regulating the interface between nano-magnesium oxide and calcium stearate, efficient coupling and synergy at the microscopic level are achieved, thereby minimizing the shrinkage rate of the filling material.

[0015] Preferably, the mass ratio of the organosilane-grafted modified red mud, cement, and the modified two-component gel slurry in step (5) is 20:(2-2.5):(12-13).

[0016] A cementitious material for mine filling is prepared by the method described above.

[0017] Compared with the prior art, the beneficial effects of the present invention are: 1. By alkaline thermal activation of red mud and grafting with organosilane, a hydrophobic coupling layer is formed, which reduces water evaporation and uneven drying shrinkage; at the same time, active silicon-aluminum components are released to participate in secondary hydration, refine capillary pore size, and reduce self-shrinkage. The two work together to improve volume stability.

[0018] 2. The slag-metakaolin bicomponent system generates an interlocking dense gel network and a multi-component gel phase, which refines the pores, homogenizes the hydration and exothermic process, effectively constrains shrinkage, avoids stress concentration and microcracks, and maintains volume stability throughout the hardening process.

[0019] 3. By introducing nano-magnesium oxide and calcium stearate for synergistic interface regulation, nano-magnesium oxide bridging promotes uniform deposition and encapsulation of gel on the hydrophobic red mud surface, and calcium stearate and organosilane coupling layer surface energy match, eliminating local weak areas of gel, and minimizing the shrinkage rate of the filling body through micro-coupling. Attached Figure Description

[0020] Figure 1 This is a low-magnification SEM image of the surface of the cementitious material prepared in Example 4 of the present invention.

[0021] Figure 2 This is a high-magnification SEM image of the surface of the cementitious material prepared in Example 4 of the present invention.

[0022] Figure 3 This is a high-magnification SEM image of the surface of the cementitious material prepared in Comparative Example 6 of this invention.

[0023] Figure 4 This is a low-magnification SEM image of the surface of the cementitious material prepared in Comparative Example 7 of this invention. Detailed Implementation

[0024] 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.

[0025] Example 1 A cementitious material for mine filling, comprising the following steps: Step (1) Alkali-thermal activation treatment of red mud: After crushing the raw red mud ore, pass it through a 200-mesh standard sieve (pore size 75μm), and weigh 500g of red mud fine powder for later use. Dissolve sodium hydroxide (analytical grade) in deionized water to prepare a 2000mL sodium hydroxide solution with a concentration of 3mol / L. Add 500g of red mud fine powder to the above solution and stir at a constant temperature of 90℃ for 3h. After the reaction is completed, vacuum filter and wash the filter cake repeatedly with deionized water until the pH value of the washing liquid drops to 9.5. Dry the washed solid in a 60℃ forced-air drying oven to constant weight, take it out, grind it, seal and store it to obtain alkali-thermal activated red mud.

[0026] Step (2) Organosilane grafting modification of alkali-thermal activated red mud: Weigh 200g of alkali-thermal activated red mud obtained in step (1), disperse it in 1200mL of anhydrous ethanol, add 10g of vinyltriethoxysilane (purity ≥97%), adjust the pH of the system to 4.5 dropwise with 10% acetic acid solution, stir the reaction at 65℃ for 4h, after the reaction is completed, vacuum filter, wash 3 times with anhydrous ethanol and 3 times with deionized water, dry the washed solid in a 105℃ forced-air drying oven for 5h, grind and pass through a 200-mesh standard sieve (pore size 75μm) to obtain organosilane grafted modified red mud.

[0027] Step (3) Preparation of two-component slurry: Weigh 32g of metakaolin (specific surface area ≥15m²) 2 / g) and 48g S95 grade granulated blast furnace slag powder (specific surface area ≥400m²) 2 / kg), dry mix the two evenly, add 40g of deionized water, and stir until the slurry is uniform and free of dry powder lumps to obtain a two-component slurry.

[0028] Step (4) Preparation of modified two-component gel slurry: Add nano-magnesium oxide (particle size 30nm, purity ≥99%, 2.5% of the total mass of metakaolin and slag powder) and calcium stearate (industrial grade, 1.3% of the total mass of metakaolin and slag powder) to the two-component slurry obtained in step (3). After preliminary stirring and dispersion, transfer to a planetary ball mill and ball mill at a speed of 400r / min for 1.5h. The ball-to-material mass ratio is 4:1. After ball milling, separate the slurry from the grinding balls to obtain the modified two-component gel slurry.

[0029] Step (5) Preparation of filling slurry: Weigh 200g of organosilane grafted modified red mud obtained in step (2) and 24g of composite silicate cement (P·C 42.5 grade). Dry mix the two for 4min to make them evenly mixed. Then add 128g of modified two-component gel slurry obtained in step (4) and 100g of deionized water as mixing water. Stir in a mixer at 600r / min for 10min until the slurry is uniform to obtain filling slurry.

[0030] Step (6) Molding and curing: The filling slurry obtained in step (5) is poured into a 40mm×40mm×160mm stainless steel triple mold that has been pre-coated with release oil. It is vibrated on a vibrating table for 5 minutes to remove internal air bubbles. After being covered with plastic film, it is placed in a standard curing room with a temperature of 20±2℃ and a relative humidity of ≥95% for static curing. After 24 hours, it is demolded and cured under standard curing conditions for 28 days to obtain a cementitious material for mine filling.

[0031] Example 2 A cementitious material for mine filling, comprising the following steps: Step (1) Alkali-thermal activation treatment of red mud: After crushing the raw red mud ore, pass it through a 200-mesh standard sieve (pore size 75μm), and weigh 500g of red mud fine powder for later use. Dissolve sodium hydroxide (analytical grade) in deionized water to prepare a 2000mL sodium hydroxide solution with a concentration of 3mol / L. Add 500g of red mud fine powder to the above solution and stir at a constant temperature of 90℃ for 3h. After the reaction is completed, vacuum filter and wash the filter cake repeatedly with deionized water until the pH value of the washing liquid drops to 9.5. Dry the washed solid in a 60℃ forced-air drying oven to constant weight, take it out, grind it, seal and store it to obtain alkali-thermal activated red mud.

[0032] Step (2) Organosilane grafting modification of alkali-thermal activated red mud: Weigh 200g of alkali-thermal activated red mud obtained in step (1), disperse it in 1200mL of anhydrous ethanol, add 7g of vinyltriethoxysilane (purity ≥97%), adjust the pH of the system to 4.5 dropwise with 10% acetic acid solution, stir the reaction at 65℃ for 4h, after the reaction is completed, vacuum filter, wash 3 times with anhydrous ethanol and 3 times with deionized water, dry the washed solid in a 105℃ forced-air drying oven for 5h, grind and pass through a 200-mesh standard sieve (pore size 75μm) to obtain organosilane grafted modified red mud.

[0033] Step (3) Preparation of two-component slurry: Weigh 32g of metakaolin (specific surface area ≥15m²) 2 / g) and 46g S95 grade granulated blast furnace slag powder (specific surface area ≥400m²) 2 / kg), dry mix the two evenly, add 40g of deionized water, and stir until the slurry is uniform and free of dry powder lumps to obtain a two-component slurry.

[0034] Step (4) Preparation of modified two-component gel slurry: Add nano-magnesium oxide (particle size 30nm, purity ≥99%, 1.8% of the total mass of metakaolin and slag powder) and calcium stearate (industrial grade, 0.9% of the total mass of metakaolin and slag powder) to the two-component slurry obtained in step (3). After preliminary stirring and dispersion, transfer to a planetary ball mill and ball mill at a speed of 400r / min for 1.5h. The ball-to-material mass ratio is 4:1. After ball milling, separate the slurry from the grinding balls to obtain the modified two-component gel slurry.

[0035] Step (5) Preparation of filling slurry: Weigh 200g of organosilane grafted modified red mud obtained in step (2) and 21g of composite silicate cement (P·C 42.5 grade). Dry mix the two for 4min to make them evenly mixed. Then add 123g of modified two-component gel slurry obtained in step (4) and 100g of deionized water as mixing water. Stir in a mixer at 600r / min for 10min until the slurry is uniform to obtain filling slurry.

[0036] Step (6) Molding and curing: The filling slurry obtained in step (5) is poured into a 40mm×40mm×160mm stainless steel triple mold that has been pre-coated with release oil. It is vibrated on a vibrating table for 5 minutes to remove internal air bubbles. After being covered with plastic film, it is placed in a standard curing room with a temperature of 20±2℃ and a relative humidity of ≥95% for static curing. After 24 hours, it is demolded and cured under standard curing conditions for 28 days to obtain a cementitious material for mine filling.

[0037] Example 3 A cementitious material for mine filling, comprising the following steps: Step (1) Alkali-thermal activation treatment of red mud: After crushing the raw red mud ore, pass it through a 200-mesh standard sieve (pore size 75μm), and weigh 500g of red mud fine powder for later use. Dissolve sodium hydroxide (analytical grade) in deionized water to prepare a 2000mL sodium hydroxide solution with a concentration of 3mol / L. Add 500g of red mud fine powder to the above solution and stir at a constant temperature of 90℃ for 3h. After the reaction is completed, vacuum filter and wash the filter cake repeatedly with deionized water until the pH value of the washing liquid drops to 9.5. Dry the washed solid in a 60℃ forced-air drying oven to constant weight, take it out, grind it, seal and store it to obtain alkali-thermal activated red mud.

[0038] Step (2) Organosilane grafting modification of alkali-thermal activated red mud: Weigh 200g of alkali-thermal activated red mud obtained in step (1), disperse it in 1200mL of anhydrous ethanol, add 8g of vinyltriethoxysilane (purity ≥97%), adjust the pH of the system to 4.5 dropwise with 10% acetic acid solution, stir the reaction at 65℃ for 4h, after the reaction is completed, vacuum filter, wash with anhydrous ethanol 3 times and deionized water 3 times in sequence, dry the washed solid in a 105℃ forced-air drying oven for 5h, grind and pass through a 200-mesh standard sieve (pore size 75μm) to obtain organosilane grafted modified red mud.

[0039] Step (3) Preparation of two-component slurry: Weigh 32g of metakaolin (specific surface area ≥15m²) 2 / g) and 47g S95 grade granulated blast furnace slag powder (specific surface area ≥400m²) 2 / kg), dry mix the two evenly, add 40g of deionized water, and stir until the slurry is uniform and free of dry powder lumps to obtain a two-component slurry.

[0040] Step (4) Preparation of modified two-component gel slurry: Add nano magnesium oxide (particle size 30nm, purity ≥99%, 2.0% of the total mass of metakaolin and slag powder) and calcium stearate (industrial grade, 1.2% of the total mass of metakaolin and slag powder) to the two-component slurry obtained in step (3). After preliminary stirring and dispersion, transfer to a planetary ball mill and ball mill at a speed of 400r / min for 1.5h. The ball-to-material mass ratio is 4:1. After ball milling, separate the slurry from the grinding balls to obtain the modified two-component gel slurry.

[0041] Step (5) Preparation of filling slurry: Weigh 200g of organosilane grafted modified red mud obtained in step (2) and 23g of composite silicate cement (P·C 42.5 grade). Dry mix the two for 4min to make them evenly mixed. Then add 125g of modified two-component gel slurry obtained in step (4) and 100g of deionized water as mixing water. Stir in a mixer at 600r / min for 10min until the slurry is uniform to obtain filling slurry.

[0042] Step (6) Molding and curing: The filling slurry obtained in step (5) is poured into a 40mm×40mm×160mm stainless steel triple mold that has been pre-coated with release oil. It is vibrated on a vibrating table for 5 minutes to remove internal air bubbles. After being covered with plastic film, it is placed in a standard curing room with a temperature of 20±2℃ and a relative humidity of ≥95% for static curing. After 24 hours, it is demolded and cured under standard curing conditions for 28 days to obtain a cementitious material for mine filling.

[0043] Example 4 A cementitious material for mine filling, comprising the following steps: Step (1) Alkali-thermal activation treatment of red mud: After crushing the raw red mud ore, pass it through a 200-mesh standard sieve (pore size 75μm), and weigh 500g of red mud fine powder for later use. Dissolve sodium hydroxide (analytical grade) in deionized water to prepare a 2500mL sodium hydroxide solution with a concentration of 4mol / L. Add 500g of red mud fine powder to the above solution and stir at a constant temperature of 95℃ for 4h. After the reaction is completed, vacuum filter, and wash the filter cake repeatedly with deionized water until the pH value of the washing liquid drops to 10. Dry the washed solid in a 60℃ forced-air drying oven to constant weight, take it out, grind it, seal and store it to obtain alkali-thermal activated red mud.

[0044] Step (2) Organosilane grafting modification of alkali-thermal activated red mud: Weigh 200g of alkali-thermal activated red mud obtained in step (1), disperse it in 1200mL of anhydrous ethanol, add 12g of vinyltriethoxysilane (purity ≥97%), adjust the pH of the system to 5 dropwise with 10% acetic acid solution, stir the reaction at 70℃ for 5h, after the reaction is completed, vacuum filter, wash 3 times with anhydrous ethanol and 3 times with deionized water, dry the washed solid in a 105℃ forced-air drying oven for 5h, grind and pass through a 200-mesh standard sieve (pore size 75μm) to obtain organosilane grafted modified red mud.

[0045] Step (3) Preparation of two-component slurry: Weigh 32g of metakaolin (specific surface area ≥15m²) 2 / g) and 50g S95 grade granulated blast furnace slag powder (specific surface area ≥400m²) 2 / kg), dry mix the two evenly, add 40g of deionized water, and stir until the slurry is uniform and free of dry powder lumps to obtain a two-component slurry.

[0046] Step (4) Preparation of modified two-component gel slurry: Add nano-magnesium oxide (particle size 30nm, purity ≥99%, 3.0% of the total mass of metakaolin and slag powder) and calcium stearate (industrial grade, 1.5% of the total mass of metakaolin and slag powder) to the two-component slurry obtained in step (3). After preliminary stirring and dispersion, transfer to a planetary ball mill and ball mill at a speed of 400r / min for 1.5h. The ball-to-material mass ratio is 4:1. After ball milling, separate the slurry from the grinding balls to obtain the modified two-component gel slurry.

[0047] Step (5) Preparation of filling slurry: Weigh 200g of organosilane grafted modified red mud obtained in step (2) and 25g of composite silicate cement (P·C 42.5 grade). Dry mix the two for 4min to make them evenly mixed. Then add 130g of modified two-component gel slurry obtained in step (4) and 100g of deionized water as mixing water. Stir in a mixer at 600r / min for 10min until the slurry is uniform to obtain filling slurry.

[0048] Step (6) Molding and curing: The filling slurry obtained in step (5) is poured into a 40mm×40mm×160mm stainless steel triple mold that has been pre-coated with release oil. It is vibrated on a vibrating table for 5 minutes to remove internal air bubbles. After being covered with plastic film, it is placed in a standard curing room with a temperature of 20±2℃ and a relative humidity of ≥95% for static curing. After 24 hours, it is demolded and cured under standard curing conditions for 28 days to obtain a cementitious material for mine filling.

[0049] Example 5 A cementitious material for mine filling, comprising the following steps: Step (1) Alkali-thermal activation treatment of red mud: After crushing the raw red mud ore, pass it through a 200-mesh standard sieve (pore size 75μm), and weigh 500g of red mud fine powder for later use. Dissolve sodium hydroxide (analytical grade) in deionized water to prepare a 1500mL sodium hydroxide solution with a concentration of 2mol / L. Add 500g of red mud fine powder to the above solution and stir at a constant temperature of 80℃ for 2h. After the reaction is completed, vacuum filter and wash the filter cake repeatedly with deionized water until the pH value of the washing liquid drops to 9. Dry the washed solid in a 60℃ forced-air drying oven to constant weight, take it out, grind it, seal and store it to obtain alkali-thermal activated red mud.

[0050] Step (2) Organosilane grafting modification of alkali-thermal activated red mud: Weigh 200g of alkali-thermal activated red mud obtained in step (1), disperse it in 1200mL of anhydrous ethanol, add 6g of vinyltriethoxysilane (purity ≥97%), adjust the pH of the system to 4 dropwise with 10% acetic acid solution, stir the reaction at 60℃ for 3h, after the reaction is completed, vacuum filter, wash 3 times with anhydrous ethanol and 3 times with deionized water, dry the washed solid in a 105℃ forced-air drying oven for 5h, grind and pass through a 200-mesh standard sieve (pore size 75μm) to obtain organosilane grafted modified red mud.

[0051] Step (3) Preparation of two-component slurry: Weigh 32g of metakaolin (specific surface area ≥15m²) 2 / g) and 45g S95 grade granulated blast furnace slag powder (specific surface area ≥400m²) 2 / kg), dry mix the two evenly, add 40g of deionized water, and stir until the slurry is uniform and free of dry powder lumps to obtain a two-component slurry.

[0052] Step (4) Preparation of modified two-component gel slurry: Add nano-magnesium oxide (particle size 30nm, purity ≥99%, 1.5% of the total mass of metakaolin and slag powder) and calcium stearate (industrial grade, 0.8% of the total mass of metakaolin and slag powder) to the two-component slurry obtained in step (3). After preliminary stirring and dispersion, transfer to a planetary ball mill and ball mill at a speed of 400r / min for 1.5h. The ball-to-material mass ratio is 4:1. After ball milling, separate the slurry from the grinding balls to obtain the modified two-component gel slurry.

[0053] Step (5) Preparation of filling slurry: Weigh 200g of organosilane grafted modified red mud obtained in step (2) and 20g of composite silicate cement (P·C 42.5 grade). Dry mix the two for 4min to make them evenly mixed. Then add 120g of modified two-component gel slurry obtained in step (4) and 100g of deionized water as mixing water. Stir in a mixer at 600r / min for 10min until the slurry is uniform to obtain filling slurry.

[0054] Step (6) Molding and curing: The filling slurry obtained in step (5) is poured into a 40mm×40mm×160mm stainless steel triple mold that has been pre-coated with release oil. It is vibrated on a vibrating table for 5 minutes to remove internal air bubbles. After being covered with plastic film, it is placed in a standard curing room with a temperature of 20±2℃ and a relative humidity of ≥95% for static curing. After 24 hours, it is demolded and cured under standard curing conditions for 28 days to obtain a cementitious material for mine filling.

[0055] Comparative Example 1 (without alkaline thermal activation): The difference from Example 4 is that the red mud is not subjected to alkaline thermal activation in step (1). Instead, 500g of the original red mud after being crushed and sieved is directly used for organosilane grafting modification in step (2). The remaining steps are the same as in Example 4.

[0056] Comparative Example 2 (without organosilane grafting modification): The difference from Example 4 is that step (2) is omitted, and the alkaline-heat activated red mud obtained in step (1) is used directly to replace the organosilane grafted modified red mud for the composite preparation in step (5). The remaining steps are the same as in Example 4.

[0057] Comparative Example 3 (without alkaline thermal activation and organosilane grafting modification): The difference from Example 4 is that steps (1) and (2) are omitted, and the original red mud after crushing and sieving is directly used to replace the organosilane grafted modified red mud for the composite preparation in step (5). The remaining steps are the same as in Example 4.

[0058] Comparative Example 4 (without adding metakaolin, only using slag powder): The difference from Example 4 is that metakaolin is not added in step (3), and 32g of metakaolin is replaced with an equal mass of S95 grade slag powder (i.e., the total amount of slag powder used is 82g). The remaining steps are the same as in Example 4.

[0059] Comparative Example 5 (without adding slag powder, only using metakaolin): The difference from Example 4 is that slag powder is not added in step (3), and 50g of slag powder is replaced with an equal mass of metakaolin (i.e., the total amount of metakaolin is 82g). The remaining steps are the same as in Example 4.

[0060] Comparative Example 6 (without adding metakaolin and slag powder): The difference from Example 4 is that steps (3) and (4) are omitted, and no modified two-component gel slurry is added in step (5). Only 200g of organosilane grafted modified red mud is mixed with 25g of cement, and the mixing water is adjusted to 140g of deionized water. The remaining steps are the same as in Example 4.

[0061] Comparative Example 7 (without nano magnesium oxide and calcium stearate): The difference from Example 4 is that nano magnesium oxide and calcium stearate are not added in step (4), and the two-component slurry obtained in step (3) is directly ball-milled. The remaining steps are the same as in Example 4.

[0062] Comparative Example 8 (only nano-magnesium oxide is added, without calcium stearate): The difference from Example 4 is that calcium stearate is not added in step (4), only nano-magnesium oxide of 3.0% of the total mass of metakaolin and slag powder is added for ball milling treatment, and the remaining steps are the same as in Example 4.

[0063] Comparative Example 9 (calcium stearate only, no nano magnesium oxide): The difference from Example 4 is that nano magnesium oxide is not added in step (4), only 1.5% of the total mass of metakaolin and slag powder calcium stearate is added for ball milling, and the remaining steps are the same as in Example 4.

[0064] Performance testing: 1. Compressive strength test: Referring to the "Test Method for Strength of Cement Mortar (ISO Method)" (GB / T 17671-2021), the 40mm×40mm×160mm standard specimens prepared in each example and comparative example were cured for 7 days and 28 days respectively, and then the compressive strength was tested on a universal testing machine. The loading rate was set to 2.4kN / s. Three specimens were taken from each group for parallel testing, and the average value was taken as the final result.

[0065] 2. Drying shrinkage rate test: Referring to the "Test Method for Drying Shrinkage of Cement Mortar" (JC / T 603-2004), stainless steel measuring heads were pre-embedded at both ends of the specimen during molding. Immediately after demolding, the initial length of the specimen was measured with a length comparator as the reference value. Then, the specimen was placed in a drying shrinkage curing chamber at a temperature of 20±2℃ and a relative humidity of 50±5%. The change in specimen length was measured with a length comparator at 7d and 28d. The drying shrinkage rate at each age was calculated according to the formula "drying shrinkage rate = (initial length - measured length) / measuring gauge length × 100%". Three specimens were taken from each group and the average value was taken.

[0066] 3. Water absorption rate test: Take out the specimens after curing for 28 days, dry them in a 60℃ forced-air drying oven to constant weight, and weigh the dried mass m0. Then, completely immerse the specimens in deionized water at 20±2℃ for 48 hours. After taking them out, quickly wipe off the excess water on the surface with a damp cloth and weigh the saturated surface dry mass m1. Calculate the 28-day water absorption rate according to the formula water absorption rate = (m1-m0) / m0×100%. Take 3 specimens from each group and take the average value.

[0067] Table 1: Performance Test Results of Examples and Comparative Samples

[0068] From the above test results, we can conclude that: (1) The 28-day drying shrinkage rate of Examples 1 to 5 is between 0.021% and 0.033%, and the 28-day compressive strength is between 5.9 and 7.5 MPa. Among them, Example 4 has the best performance (28-day shrinkage rate of only 0.021% and compressive strength of 7.5 MPa), indicating that excellent low shrinkage and mechanical properties can be obtained within the parameter range of the present invention.

[0069] (2) Comparative Examples 1-3 illustrate that both alkaline heat activation and organosilane grafting are indispensable: the 28-day shrinkage rate of Comparative Example 3 (both omitted) is as high as 0.075%, which is 3.6 times that of Example 4; the 28-day shrinkage rates of Comparative Examples 1 and 2 are 0.058% and 0.052%, respectively, both much higher than that of Example 4.

[0070] (3) Comparative Examples 4 to 6 illustrate the necessity of the metakaolin-slag two-component system: the shrinkage rate of Comparative Example 6 (completely gel-free system) reached 0.068% after 28 days; the shrinkage rates of Comparative Example 4 (slag only) and Comparative Example 5 (metakaolin only) were 0.045% and 0.048% respectively, indicating that the synergistic effect of the two is better than either single component.

[0071] (4) Comparative Examples 7-9 illustrate the necessity of the synergistic combination of nano-magnesium oxide and calcium stearate: The shrinkage rate of Comparative Example 7 (without either) was 0.038% after 28 days; the shrinkage rates of Comparative Examples 8 and 9 when either was added alone were 0.032% and 0.035%, respectively. Although they were better than Comparative Example 7, they were still significantly higher than 0.021% in Example 4, which confirms that the synergistic effect of the two is indispensable.

[0072] 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 a cementitious material for mine filling, characterized in that, Includes the following steps: (1) The red mud is crushed and activated by alkaline heat in an alkaline solution. After the reaction is completed, solid-liquid separation is performed and the solid obtained is washed and dried to obtain alkaline heat activated red mud. (2) The alkaline-heat activated red mud is grafted with a silane coupling agent under acidic conditions. After the reaction is completed, solid-liquid separation is performed, and the separated solid is washed, dried and ground to obtain organosilane grafted modified red mud. (3) Mix metakaolin and slag powder, add water and stir evenly to obtain a two-component slurry; the mass ratio of metakaolin to slag powder is 32:(45-50); (4) Add nano-magnesium oxide and calcium stearate to the two-component slurry and co-mill to obtain a modified two-component gel slurry; the mass of the nano-magnesium oxide is 1.5-3.0% of the total mass of metakaolin and slag powder, and the mass of the added calcium stearate is 0.8-1.5% of the total mass of metakaolin and slag powder; (5) The organosilane-grafted modified red mud, cement, and the modified two-component gel slurry are mixed with mixing water to obtain a filling slurry; the mass ratio of the organosilane-grafted modified red mud, cement, and the modified two-component gel slurry is 20:(2-2.5):(12-13); (6) The filling slurry is molded and cured to obtain the cementitious material for mine filling.

2. The method for preparing a cementitious material for mine filling according to claim 1, characterized in that, The alkaline solution mentioned in step (1) is a sodium hydroxide solution with a concentration of 2 to 4 mol / L.

3. The method for preparing a cementitious material for mine filling according to claim 2, characterized in that, The solid-liquid ratio of the red mud to the sodium hydroxide solution is 1 g:(3-5) mL; the temperature of the alkaline thermal activation reaction is 80-95℃, and the time is 2-4 h.

4. The method for preparing a cementitious material for mine filling according to claim 1, characterized in that, The solid-liquid separation method in step (1) is vacuum filtration; the washing uses deionized water and is carried out until the pH of the washing wastewater is 9-10; the drying is carried out at 60°C until constant weight.

5. The method for preparing a cementitious material for mine filling according to claim 1, characterized in that, The silane coupling agent in step (2) is vinyltriethoxysilane; the grafting reaction is carried out in anhydrous ethanol.

6. A method for preparing a cementitious material for mine filling according to claim 5, characterized in that, The amount of vinyltriethoxysilane added is 3-6 wt% of the mass of the alkaline-thermally activated red mud; the acidic conditions are achieved by adjusting the pH of the system to 4.0-5.0 using acetic acid solution; the grafting reaction is carried out at a temperature of 60-70°C for 3-5 hours.

7. A cementitious material for mine filling, characterized in that, It is prepared by the method described in any one of claims 1 to 6 above.

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