Cement-free magnesium slag-based ternary solid waste mine filling dry powder and its preparation and pumping construction method

Abstract

The application discloses a cement-free magnesium slag-based ternary solid waste filling dry powder for mine, and a preparation and pumping construction method thereof. The filling dry powder is prepared by mixing the following components in mass percentage: 70-90% of dry magnesium smelting slag powder, 8-15% of dry fly ash, 3-8% of flue gas desulfurization gypsum, 0.3-1.5% of quicklime, and 0.05-0.20% of polycarboxylate superplasticizer. No Portland cement is added in the application. The quicklime and the desulfurization gypsum are used to construct a double synergistic hydration system of alkali activation and sulfate activation, and the latent cementitious activity of the magnesium slag and the fly ash is activated. The preparation method is physical mixing of dry powder at room temperature, and no three wastes are discharged. When the application is constructed, water is added and stirred to form a slurry according to a water-cement ratio of 0.42-0.58, the slurry can be pumped through a long pipeline, and the compressive strength of the slurry after 28 days of room temperature curing underground is not less than 15 MPa. The application realizes the large-scale synergistic consumption of the three industrial solid wastes of magnesium slag, fly ash and desulfurization gypsum, the cost is greatly reduced compared with traditional cement-based filling materials, and the solidified body has no heavy metal leaching risk.

Description

Technical Field

[0001] This invention belongs to the field of industrial solid waste resource utilization and mine backfilling technology. Specifically, it relates to a cement-free magnesium slag-based ternary solid waste mine backfill dry powder material and its preparation and pumping construction method, which is applicable to coal mine, metal mine and non-metal mine goaf backfilling, roadway surrounding rock reinforcement and sealing and leak sealing projects. Background Technology

[0002] The management of goaf areas in underground mines is a core aspect of ensuring mining safety. Backfilling mining methods are widely used due to their effectiveness in controlling ground pressure and improving resource recovery rates. Backfilling materials, as the core technology of backfilling mining, directly determine the quality and cost of the backfilling project. Currently, cement-based concrete backfilling materials are commonly used in mine backfilling projects. These materials use silicate cement as the binder, combined with sand and gravel aggregates and water to prepare the backfilling slurry. While this type of material has advantages in strength and reliability, it has three inherent drawbacks: First, the high cost of purchasing and transporting cement and sand and gravel aggregates keeps the cost of backfilling projects high, especially in deep mines where economic constraints are particularly pronounced. Second, the energy consumption of cement clinker calcination is enormous, with approximately 0.8 to 0.9 tons of carbon dioxide emitted per ton of cement produced, which is unsustainable given increasing environmental pressures. Third, cement-based slurry has poor pumpability, making it prone to bleeding, segregation, and pipe blockage during long-distance pipeline transport. The hardened slurry also has a high shrinkage rate, making it susceptible to cracking and leakage in the later stages, affecting the long-term service performance of the backfill.

[0003] Meanwhile, the large-scale disposal of industrial solid waste has become a pressing practical problem. Magnesium smelting uses the silicothermic process (Pidgeon process), producing approximately 6-10 tons of magnesium reduction slag for every ton of magnesium produced. my country accounts for over 80% of global magnesium production capacity, resulting in annual magnesium slag emissions exceeding ten million tons. Coal-fired power plants emit approximately 600 million tons of fly ash annually; although the comprehensive utilization rate has improved, the stockpiled volume remains enormous. The annual output of desulfurization gypsum produced by wet flue gas desulfurization processes exceeds 100 million tons, with a comprehensive utilization rate of less than 40%. The long-term open-air storage of these three types of solid waste not only occupies vast amounts of land resources but also causes air pollution from dust, and rainwater leaching leads to the leaching of heavy metal ions, polluting surface and groundwater. Environmental disposal costs are extremely high, making it a key challenge for environmental governance in mines and surrounding areas.

[0004] Extensive research has been conducted both domestically and internationally on solid waste-based backfill materials. Chinese invention patent CN112919874A discloses a cement-based backfill material containing various solid wastes, composed of 5%–14% desulfurized gypsum, 22%–35% fly ash, 24%–35% water-quenched slag, 5%–6% cement, and 25%–35% tap water. Although this scheme utilizes three types of solid waste—fly ash, desulfurized gypsum, and water-quenched slag—it is essentially still a cement-based system, requiring the addition of 5%–6% ordinary Portland cement as a cementing core. Therefore, it cannot achieve completely cement-free utilization of all solid waste. Furthermore, the 7-day strength of the filling material in its embodiment is only 2.11 MPa, and the 14-day strength is only 2.60 MPa, far from meeting the engineering requirements for C15 strength grade (28-day compressive strength ≥ 15 MPa) for mine goaf filling. In addition, the material contains a large amount of tap water, making it a wet slurry system, which is inconvenient for long-distance transportation and immediate underground preparation. Moreover, it does not contain any rheology control components, making it impossible to guarantee the stability of long-distance pipeline pumping in deep wells.

[0005] Chinese invention patent CN115745486A discloses a two-component dry powder geopolymer grouting material based on solid waste, consisting of two components, A and B. Component A includes slag, composite activator (quick-dissolving sodium silicate and sodium hydroxide), water-reducing agent, and quick-setting agent, while component B includes desulfurized gypsum, fly ash, metakaolin, quicklime, retarder, and water-reducing agent. While this solution achieves complete solid waste treatment (excluding cement), it suffers from the following significant shortcomings: First, the material is designed as a two-component grouting system (A and B), requiring separate slurry preparation and mixing on-site. This results in complex construction processes, high equipment investment, and the fact that readily soluble sodium silicate and sodium hydroxide in component A are highly alkaline industrial raw materials, not solid waste sources, essentially still relying on external chemical activators. Second, the setting time control after mixing components A and B depends on the large-scale addition of retarders and accelerators, leading to poor operability and increased material costs. Third, this invention uses slag (blast furnace water-quenched slag) as the main precursor, completely neglecting the activation and utilization of magnesium smelting slag, fundamentally different from the technical route of this invention, which uses magnesium slag as the main cementitious phase. Fourth, this material is primarily intended for grouting and leak sealing projects, rather than large-scale goaf filling, resulting in fundamental differences in material usage, construction methods, and performance requirements.

[0006] Existing alkali-activated filling materials generally suffer from the following technical defects: (1) It still adds some cement, which cannot achieve 100% utilization of solid waste. Its ability to dispose of industrial waste is limited, and it has not been able to completely get rid of its dependence on cement clinker. (2) There is a lack of synergistic activation formulas for magnesium slag activity. The calcium-silicon ratio and glass structure of magnesium slag are different from those of blast furnace slag and water-quenched slag. Existing alkaline activation systems based on slag are not suitable for magnesium slag systems. It is difficult to make magnesium slag-based materials stably reach the C15 strength grade by alkaline activation alone. (3) The slurry is prone to bleeding and segregation. Under the conditions of deep well long-distance pipeline pumping, the solid particles of traditional solid waste-based slurry settle seriously, and the probability of pipe blockage is high, which cannot meet the requirements of continuous, stable and efficient underground transportation construction. (4) Over-reliance on industrial raw material activators, such as sodium silicate and sodium hydroxide, which are expensive and not solid waste, weakens the economic advantage of all-solid waste materials. (5) With water-quenched slag and ore slag as the main cementing components, my country’s annual discharge of magnesium smelting slag is huge but the resource utilization rate is extremely low. At present, there is no mature filling material technology solution specifically for magnesium slag-based ternary solid waste system.

[0007] In summary, there is currently no formulation system for dry powder backfill material for mining that simultaneously meets all the technical indicators of "100% cement-free, synergistic hydration of magnesium slag-based ternary solid waste, high rheological stability for deep well pumping, stable achievement of C15 strength in 28 days, and significantly reduced overall cost." Magnesium slag, as a calcium-silica industrial waste with potential hydraulic activity, has a chemical composition mainly composed of γ-C2S, Ca2SiO4, and a small amount of MgO, which is fundamentally different from the silica-alumina glassy structure of blast furnace slag. Its activation mechanism is unique, and existing alkali activation systems based on slag or fly ash cannot be easily transplanted. This invention addresses the above-mentioned technological gap by using magnesium slag as the main cementitious phase, compounded with fly ash as micro-aggregate, desulfurized gypsum as the sulfate activation source, and supplemented with a trace amount of quicklime to construct a dual synergistic hydration system of "alkali activation + sulfate activation." Furthermore, polycarboxylate superplasticizers are used to precisely control the rheological properties, thereby achieving a technological breakthrough in a mine backfill material that is entirely solid waste-based, low-cost, high-performance, and easy to pump. Summary of the Invention

[0008] To overcome the shortcomings of existing cement-based filling materials, such as high cost and large carbon emissions, and the unstable strength, easy bleeding, and difficulty in long-distance pumping of ordinary solid waste cementitious materials, this invention discloses a cementless magnesium slag-based ternary solid waste mining filling dry powder material and its preparation and pumping construction method. This material possesses high fluidity and long-distance pumpability, and can stably reach the C15 concrete strength grade after normal temperature underground curing, while simultaneously achieving efficient and coordinated disposal of large quantities of industrial waste.

[0009] This invention includes the following technical solutions: Formula for filling dry powder materials.

[0010] The cement-free magnesium slag-based ternary solid waste mine backfill dry powder material of the present invention is composed of the following dry powder components by mass percentage, and the total mass of each component is 100%: Magnesium smelting slag dry powder: 70%–90% — As the main cementing phase, it provides active calcium and silicon components, which are the main source of the strength of the solidified body. Magnesium slag is rich in active mineral phases such as γ-C2S and Ca2SiO4, which can be hydrated under activating conditions to form hydrated calcium silicate gel.

[0011] Dry fly ash: 8%~15% — As a micro-aggregate filling material in the cementitious system, it optimizes the rheological properties of the slurry, reduces the shrinkage deformation rate of the hardened body, and at the same time, the active Al2O3 it contains participates in the hydration reaction to generate calcium aluminate hydrate.

[0012] Power plant flue gas desulfurization gypsum: 3%–8% — as a sulfate activator, dissociating SO4. 2- It reacts with the active aluminum component in the system to form ettringite (AFt), which promotes the continuous hydration process.

[0013] Quicklime: 0.3%–1.5% — As an alkaline activator, it provides OH- after dissolving in water. - This disrupts the dense structure of the aluminosilicate glass in magnesium slag and fly ash, activating their potential cementitious activity. Quicklime and desulfurized gypsum together constitute a dual synergistic activation system of "alkali + salt".

[0014] Polycarboxylate superplasticizer: 0.05%~0.20% — Dispersed powder particles, which impart excellent fluidity to the slurry under low water-cement ratio conditions, fundamentally solving the problems of slurry bleeding and segregation.

[0015] In the dry powder filling material of the present invention, the magnesium smelting slag, dry fly ash, and power plant flue gas desulfurization gypsum are all general industrial solid wastes. No silicate cement or natural sand and gravel aggregates are added to the dry powder filling material.

[0016] Among the above components, the preferred calcium oxide content in the dry magnesium smelting slag powder is 40%–55%, the preferred silicon dioxide content is 20%–35%, and the fineness is passing through a 150-325 mesh sieve; the fineness of the dry fly ash meets the Class II fly ash standard (45μm square hole sieve residue ≤25%); the calcium sulfate dihydrate content in the power plant flue gas desulfurization gypsum is ≥75%, and the free water content is ≤3%; the effective calcium oxide content in the quicklime is ≥80%, and the fineness is passing through a 180-325 mesh sieve.

[0017] The method for preparing the dry powder filling material according to the present invention includes the following steps: (1) Raw material pretreatment: The dry powder of magnesium smelting slag, dry fly ash and power plant flue gas desulfurization gypsum are screened to remove impurities, and coarse particles and agglomerated materials are removed to obtain pretreated powder with uniform fineness; quicklime and polycarboxylate superplasticizer are directly used as commercially available finished powder raw materials without further grinding.

[0018] (2) Precise metering: An automated batching device is used to weigh each pre-treated powder and finished powder raw material according to the above mass ratio.

[0019] (3) Closed mixing: All the weighed powder is fed into a closed dry powder mixer and stirred continuously for 10 to 15 minutes at room temperature until it is uniformly mixed to obtain a homogeneous dry powder mixture. The stirring speed is preferably 20 to 50 r / min and the stirring time is preferably 10 to 12 minutes. The closed dry powder mixer is equipped with a dust removal device to collect the dust generated during the mixing process and reuse it in the mixing process.

[0020] (4) Sealed storage: Pack the homogeneous dry powder mixture into a sealed silo or woven bag for moisture protection and prevent the components from becoming ineffective due to moisture absorption.

[0021] The entire preparation process described above involves only physical mixing of dry powders at room temperature, without the need for high-temperature calcination or chemical reactions. The entire process generates no waste gas, wastewater, or waste residue, making it a green and environmentally friendly production process.

[0022] The downhole pumping construction method for filling dry powder material according to the present invention includes the following steps: The dry powder filling material is transported to the underground working face. Water is added to the dry powder and stirred, controlling the water-cement ratio to be 0.42–0.58 (preferably 0.45–0.52) to prepare a homogeneous, pumpable grout. The grout is then pumped into the goaf or surrounding rock fissures via pipeline using a grouting pump. It is cured under normal temperature and natural conditions underground. The grout spread is ≥500mm, and after 2 hours of standing, there is no bleeding or stratification. The solidified body has a 7-day compressive strength ≥8MPa and a 28-day compressive strength ≥15MPa, meeting the C15 concrete filling support standard. The pipeline has a horizontal transport distance ≥1000m or a vertical pumping depth ≥800m. The solidified body's heavy metal leaching concentration meets the requirements of GB / T14848 groundwater quality standards, ensuring no mine water and soil pollution.

[0023] This invention constructs a dual synergistic hydration system of "alkaline activation + sulfate activation", and its hydration and solidification mechanism is as follows: Step 1 – Alkaline activation and depolymerization of the vitreous body: Quicklime reacts with water to produce Ca(OH)2, releasing a large amount of OH-. - This causes the pH value of the slurry system to rise above 12. Under highly alkaline conditions, the potentially active silica-alumina glass structure in magnesium slag and fly ash is broken, Si-O bonds and Al-O bonds are broken, releasing active SiO2 and Al2O3.

[0024] Step 2 – Sulfate activation and secondary hydration: Desulfurized gypsum (CaSO4·2H2O) dissociates into Ca in water. 2+ and SO4 2- SO42- With active Al2O3 and Ca in the system 2+ The reaction produces ettringite (3CaO·Al2O3·3CaSO4·32H2O). The formation of ettringite consumes the active aluminum component in the system, pushing the equilibrium to the right and causing more glass to continue to depolymerize; on the other hand, the growth of ettringite crystals fills the pores, making the slurry denser.

[0025] Step 3 – Continuous formation and strength development of the cementitious phase: Released active SiO2 and Ca in the system 2+ The reaction produces hydrated calcium silicate gel (CSH), while active Al2O3 participates in the formation of hydrated calcium aluminate gel (CAH). CSH and CAH are the main sources of strength in the cured body. As hydration products continue to form, the porosity of the slurry gradually decreases, the microstructure becomes increasingly dense, and the strength of the cured body continues to increase.

[0026] Step 4 – Rheological Control: Polycarboxylate superplasticizer molecules adsorb onto the surface of powder particles, effectively breaking up powder agglomerates through electrostatic repulsion and steric hindrance, releasing free water trapped in the agglomerates. Under low water-cement ratio (0.42–0.58) conditions, the slurry maintains excellent fluidity, while avoiding the bleeding and segregation problems that are prone to occur under conventional high water-cement ratio conditions, ensuring the stability of long-distance pipeline pumping.

[0027] The entire hydration process can proceed spontaneously under normal temperature conditions (natural downhole temperature, usually 15-25℃), without the need for heating and curing, and without the need to add cement as a cementitious substrate.

[0028] Compared with the prior art, the present invention has the following beneficial effects: (1) The total proportion of ternary industrial solid waste reaches 96.5% to 99.7%, which can dispose of magnesium smelting slag, fly ash and desulfurization gypsum in large quantities, reducing land occupation and environmental pollution caused by waste slag storage.

[0029] (2) This invention does not add silicate cement. It relies on the alkaline-sulfate dual synergistic activation system composed of quicklime and desulfurized gypsum to activate the potential activity of magnesium slag. The compressive strength is stable at 15.3 to 18.2 MPa after 28 days. The solidified body has high density and low shrinkage, which can meet the strength requirements for filling and permanent support of goaf areas.

[0030] (3) Polycarboxylate superplasticizer improves the rheological properties of slurry, the slurry spread is not less than 508 mm, and there is no bleeding or only slight stratification after standing for 2 hours. It can achieve safe transportation of pipelines up to 867 meters long, and the probability of pipe blockage is lower than that of traditional cement-based materials.

[0031] (4) The dry powder filling material is mainly made of industrial waste residue, eliminating the purchase of cement and sand and gravel aggregates. The comprehensive material cost is about RMB 80 per ton, which is about 60% lower than that of traditional cement-based filling concrete, and has obvious economic advantages.

[0032] (5) The dry powder preparation process is only physical mixing at room temperature, with no waste gas, wastewater and waste residue emissions; the leaching concentration of heavy metals in the underground solidified material meets the groundwater quality standards and has little impact on the mine water environment.

[0033] (6) The preparation process is simple, requiring no high-temperature calcination and grinding equipment, only screening, batching and mixing equipment are needed for production, which is convenient for mines to build their own production lines or for external sales and promotion.

[0034] (7) The proportions of magnesium slag, fly ash, desulfurized gypsum and quicklime can be adjusted within a wide range, and have a certain adaptability to raw materials of different qualities and underground construction conditions. Attached Figure Description

[0035] Figure 1 This is a process flow diagram of the method for preparing cement-free magnesium slag-based ternary solid waste mine filling dry powder material according to the present invention. Figure 2 The graph shows a comparison of the compressive strength of the cured bodies of each embodiment after 7 days and 28 days. Figure 3 A comparison chart of slurry spread and maximum safe pumping distance for each embodiment and comparative example; Figure 4 A comparison chart of the 28-day compressive strength of cured bodies under different excitation systems. Detailed Implementation

[0036] To make the technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to embodiments and comparative examples. The embodiments described below are only some preferred embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art can make several adjustments and modifications without departing from the concept of the present invention, and these modifications all fall within the protection scope of the present invention.

[0037] The raw materials used in the various embodiments of this invention and their specifications are as follows: The dry magnesium smelting slag powder is taken from the magnesium reduction workshop of Fugu County Teda Coal Chemical Co., Ltd., and is magnesium reduction slag produced by the silicothermic magnesium smelting process. After crushing and grinding, it is passed through a 200-mesh sieve for later use. The dry fly ash is taken from a coal-fired power plant in Shaanxi Province, with a fineness of 22% residue on a 45-micron square-hole sieve, meeting the Class II fly ash standard. The flue gas desulfurization gypsum is taken from the wet desulfurization system of the same coal-fired power plant, with a calcium sulfate dihydrate content of 78% and a free water content of 2%, and is passed through a 200-mesh sieve for later use. The quicklime is commercially available construction quicklime powder with an effective calcium oxide content of 82% and a fineness passing through a 200-mesh sieve. The polycarboxylate superplasticizer is a commercially available powdered polycarboxylate superplasticizer with a water reduction rate of 28%. All proportions in each embodiment are by mass percentage, and the total mass of all components is 100%. The water-cement ratio refers to the ratio of the mass of added water to the total mass of the dry powder.

[0038] Figure 1 This is a process flow diagram of the dry powder filling method of the present invention.

[0039] Example 1 This embodiment represents the preferred formulation of the present invention.

[0040] The composition of each component of the backfill dry powder is as follows: 83.9% dry magnesium smelting slag powder, 10.5% dry fly ash, 5.0% power plant flue gas desulfurization gypsum, 0.5% quicklime, and 0.1% polycarboxylate superplasticizer.

[0041] Preparation method: Magnesium slag powder, fly ash, and desulfurized gypsum are sieved through a 200-mesh sieve to remove impurities, discarding coarse particles and agglomerated materials. Each component is weighed separately using an automatic batching system according to the above proportions. All weighed powders are then added to a closed dry powder mixer and stirred continuously at 35 rpm for 12 minutes at room temperature to ensure thorough mixing and a homogeneous dry powder mixture. The resulting dry powder mixture is then stored in sealed woven bags for moisture protection and later use.

[0042] During underground construction, the dry powder filling material is transported to the underground working face, water is added and stirred, and the water-cement ratio is controlled at 0.50. The mixture is stirred until the slurry is uniform and free of lumps. The slurry is then pumped to the goaf through pipeline using a grouting pump and cured under normal temperature and natural conditions underground.

[0043] Performance tests were conducted on the filling slurry and solidified body of this embodiment. The spread test was performed according to the cement paste fluidity test method, and the initial spread was measured to be 582 mm. After the slurry was allowed to stand for 2 hours, no bleeding or segregation was observed. Core samples were taken at 7 days of curing to test compressive strength; the average value of three parallel samples was 9.7 MPa. At 28 days of curing, the average compressive strength was 16.3 MPa, meeting the C15 strength grade requirement. In an 850-meter deep well pipeline pumping test conducted at the mine site, the entire pumping process was continuous and smooth, the pipeline pressure was stable, and no blockage residue was found on the inner wall of the pipeline after filling was completed.

[0044] Example 2 This embodiment is a high magnesium slag ratio scheme, which is suitable for mining areas where magnesium slag supply is sufficient and fly ash source is limited.

[0045] The composition of each component of the backfill dry powder is as follows: 88.0% dry magnesium smelting slag powder, 8.5% dry fly ash, 3.0% power plant flue gas desulfurization gypsum, 0.4% quicklime, and 0.1% polycarboxylate superplasticizer.

[0046] The preparation method is the same as in Example 1, with a sealed mixing time of 15 minutes. The water-cement ratio for construction is 0.52.

[0047] Performance test results: The initial expansion was 558 mm. After standing for 2 hours, there was no bleeding, but very slight stratification occurred, with a stratification degree of 1.8%, which did not affect pumping operations. The 7-day compressive strength was 8.6 MPa, and the 28-day compressive strength was 15.3 MPa, meeting the C15 strength standard. Pumping through the 736-meter deep well pipeline was successfully completed without pipe blockage. This example verifies that when the magnesium slag ratio is increased to 88%, and the gypsum and lime activators are relatively reduced, the dual-activation system can still effectively initiate magnesium slag hydration, achieve the required strength, and realize a large-scale utilization of magnesium slag.

[0048] Example 3 This embodiment is an activator-enhanced formulation, suitable for engineering scenarios where the activity of magnesium slag fluctuates greatly or requires high early strength.

[0049] The composition of each component of the backfill dry powder is as follows: 76.0% dry magnesium smelting slag powder, 10.0% dry fly ash, 8.0% power plant flue gas desulfurization gypsum, 1.5% quicklime, and 0.12% polycarboxylate superplasticizer.

[0050] The preparation method is the same as in Example 1, with a stirring time of 12 minutes. The water-cement ratio for construction is 0.48.

[0051] Performance test results: The initial expansion was 572 mm, and no bleeding occurred after 2 hours of standing. The initial setting time, measured by Vicat apparatus, was 92 minutes, meeting the time window requirements for downhole pumping operations. The 7-day compressive strength reached 11.3 MPa, and the 28-day compressive strength was 17.6 MPa. Pumping into the 850-meter deep well was successfully completed. In this embodiment, both gypsum and lime were present in high proportions, resulting in the strongest dual-stimulation effect and significantly accelerated early hydration. This method is suitable for engineering scenarios involving rapid filling and solidification of large-section goaf areas, where early formation of self-supporting strength is required.

[0052] Example 4 This embodiment is a low water-ash ratio, high-strength formula, with the proportion of magnesium slag slightly reduced and the proportions of fly ash and activator adjusted accordingly.

[0053] The composition of each component of the dry powder filling material is as follows: 81.5% dry magnesium smelting slag powder, 11.2% dry fly ash, 5.8% power plant flue gas desulfurization gypsum, 1.0% quicklime, and 0.12% polycarboxylate superplasticizer.

[0054] The preparation method is the same as in Example 1. The water-cement ratio is adjusted to 0.42 during construction.

[0055] Performance test results: Initial expansion was 512 mm, and no water seepage occurred after 2 hours of settling. The pumping pressure increased by approximately 15% compared to Example 1, but the 650-meter deep well pumping was still successfully completed. The 7-day compressive strength reached 12.4 MPa, and the 28-day compressive strength was 18.2 MPa.

[0056] Comparative Example 1 This comparative example uses traditional cement-based C15 filling concrete, prepared according to the conventional mix proportions for mine filling projects.

[0057] Material composition: P.O42.5 ordinary Portland cement, natural river sand, crushed stone, and water. After trial mixing and adjustment, the material usage per cubic meter of filling concrete is: 280 kg cement, 780 kg sand, 1020 kg crushed stone, and 175 kg water, with a water-cement ratio of 0.625. After mixing all materials evenly using conventional mixing processes, the mixture is pumped to the goaf through pipelines and naturally cured at ambient temperature underground.

[0058] Performance test results: The initial spread of the slurry was 418 mm. Significant bleeding began after 30 minutes of standing, reaching 8.2% after 1 hour, with obvious sedimentation and stratification of the solid particles. The 7-day compressive strength was 11.0 MPa, and the 28-day compressive strength was 16.4 MPa. During deep well pumping tests, pipe blockage occurred frequently, requiring pump shutdown and cleaning after approximately 200 meters of pumping. Fine shrinkage cracks were visible on the surface of the solidified body after curing to the designated age, and these cracks continued to propagate after 28 days. Considering the overall raw material cost, the cost per ton of filling material was 1.45 times that of Example 1 of this invention.

[0059] Comparative Example 2 This comparative example does not add water-reducing agent, and the rest of the formulation is the same as in Example 1, in order to verify the key role of water-reducing agent in the rheological properties of slurry.

[0060] The dry powder filling material formula is as follows: 83.95% magnesium slag, 10.5% fly ash, 5.0% desulfurized gypsum, and 0.55% quicklime, without adding polycarboxylate superplasticizer. The total content of all components is 100%. The preparation method is the same as in Example 1. To achieve pumpable fluidity during construction, the water-cement ratio needs to be increased to 0.67.

[0061] Performance test results: The initial spread was 432 mm. After increasing the water-cement ratio, significant bleeding occurred in the slurry after 20 minutes of settling, reaching 7.8% after 30 minutes, with substantial sedimentation of solid particles. The 28-day compressive strength was only 9.5 MPa, far below the C15 standard. This comparative example illustrates that without the rheological regulation effect of a water-reducing agent, the forced increase in water consumption to ensure pumpability leads to significant strength degradation. Furthermore, the bleeding stratification problem remains unresolved, making deep well pumping difficult.

[0062] Comparative Example 3 This comparative example does not add quicklime, and the rest of the formulation is the same as in Example 1, in order to verify the necessity of the alkaline activator.

[0063] The formula for the dry powder filling material is as follows: 84.4% magnesium slag, 10.5% fly ash, 5.0% desulfurized gypsum, 0% quicklime, and 0.1% water-reducing agent. The preparation method is the same as in Example 1, and the water-cement ratio during construction is 0.50.

[0064] Performance test results: Initial expansion was 576 mm, and no water seepage occurred after 2 hours of standing. However, the 7-day compressive strength was only 4.2 MPa, and the 28-day compressive strength was only 8.6 MPa, far below the C15 standard. Observation of the solidified body revealed internal porosity and severely insufficient strength development. This comparative example illustrates that without the alkaline environment provided by quicklime, the glassy structure of magnesium slag cannot effectively depolymerize and release active components, the sulfate activation effect of desulfurized gypsum loses its prerequisite, and the ternary synergistic hydration system cannot start normally.

[0065] Comparative Example 4 This comparative example does not add desulfurized gypsum, but the rest of the formulation is the same as in Example 1, in order to verify the necessity of sulfate activator.

[0066] The formula for the dry powder filling material is as follows: 88.9% magnesium slag, 10.5% fly ash, 0% desulfurized gypsum, 0.5% quicklime, and 0.1% water-reducing agent. The preparation method is the same as in Example 1, and the water-cement ratio during construction is 0.50.

[0067] Performance test results: Initial expansion was 568 mm, and no water seepage occurred after 2 hours of standing. The 7-day compressive strength was 5.6 MPa, and the 28-day compressive strength was 10.1 MPa, failing to meet the C15 standard. The strength of the solidified body was improved compared to Comparative Example 3 but still did not meet the standard. This comparative example demonstrates that while alkaline activation alone can hydrate some magnesium slag, the lack of sulfate activation conditions hinders the formation of ettringite, resulting in insufficient hydration and an inability to consistently achieve the required strength.

[0068] Test Example 1 Compressive strength tests for each embodiment.

[0069] The cured specimens from Examples 1 to 4 were subjected to uniaxial compressive strength tests according to standard methods. The specimen dimensions were 70.7 mm × 70.7 mm × 70.7 mm cubes. The curing conditions were ambient temperature curing at the well site, with a temperature range of 18 to 25 degrees Celsius. Six parallel specimens were prepared for each formulation, and three specimens were taken at 7 days and 28 days of curing for testing. The average value was taken as the compressive strength value at that age. The test results for each example are summarized in Table 1.

[0070] Table 1. Compressive strength test results for each embodiment ; The compressive strength test results of each embodiment are as follows: Figure 2 As shown.

[0071] Test Example 2 Rheological properties of slurry and pumping tests in each embodiment.

[0072] Rheological properties of the slurries from each embodiment and comparative example were tested, including initial spread, 2-hour bleeding rate, segregation, and pumpability. Spreadability testing was performed according to the cement paste fluidity test method. Bleeding rate was determined by filling a 1000 ml graduated cylinder with the well-mixed slurry, allowing it to stand for 2 hours, and then measuring the ratio of the upper bleeding volume to the total volume. Segregation was determined by dividing the slurry after 2 hours of standing into three equal parts from top to bottom, measuring the solids content of each part, and characterizing the degree of segregation by the ratio of the maximum solids content difference to the average solids content. Pumping tests were conducted on the mine's on-site pipeline, recording the maximum safe pumping distance and pipe blockage. The test results are summarized in Table 2.

[0073] Table 2. Test results of slurry rheological properties in each embodiment and comparative example. ; The above maximum safe pumping distance data comes from actual pumping test records at the mine site. The test pipeline was horizontally arranged along the underground mine roadway, and the pumping equipment was an explosion-proof grouting pump for coal mines, with pressure monitoring points set at 50-meter intervals. When the pumping distance exceeded the values ​​in the table and continued to extend, a sudden change in pipeline pressure or a significant decrease in pumping capacity was determined as the maximum safe pumping distance under that condition. Each data point is a measured value from three repeated tests, not a theoretical calculation result. The slurry spread and maximum safe pumping distance for each embodiment and comparative example are shown below. Figure 3 As shown.

[0074] Test Example 3 Verification of the synergistic effect of the system.

[0075] To verify the superiority of the dual synergistic activation system of this invention over single activation, the following comparative experiment was conducted. Each experimental group maintained a fixed composition of 84% magnesia slag powder, 10.5% fly ash, and 0.1% water-reducing agent, only changing the composition and ratio of the activator. A slurry was prepared with a water-ash ratio of 0.50, and the compressive strength was tested after 28 days of natural curing at room temperature.

[0076] Group 1: Only 0.5% quicklime was added, no desulfurization gypsum was added, and the remainder was supplemented with magnesium slag to 84%, resulting in a 28-day strength of 9.8 MPa.

[0077] Group 2: Only 5.0% desulfurized gypsum was added, without quicklime, and the remainder was supplemented with magnesium slag to reach 84%. The 28-day strength was 4.3 MPa, and the solidified body was loose.

[0078] Group 3: quicklime 1.5%, desulfurized gypsum 8.0%, 28-day strength 17.5 MPa.

[0079] Group 4: quicklime 0.3%, desulfurized gypsum 3.8%, 28-day strength 15.2 MPa.

[0080] Comparison of 28-day compressive strength of cured bodies under various activation systems (e.g.) Figure 4 As shown.

[0081] Test results show that: under single alkaline activation, the active components of magnesium slag are released, but the lack of sustained sulfate driving force results in insufficient hydration; under single sulfate activation without an alkaline environment, the glassy structure of magnesium slag cannot depolymerize, and the hydration reaction essentially stops; however, when both are present, even at low dosage levels, the required strength can be achieved, and with sufficient dosage, the strength is significantly improved. The synergistic effect of alkali and sulfate is far greater than the simple sum of their individual effects.

[0082] Test Example 4 A comprehensive economic comparison.

[0083] A comprehensive economic calculation was performed on Example 1 of the present invention and Comparative Example 1 of traditional cement-based filling materials, with the calculation scope being the raw material procurement cost. The unit price of each material was based on the actual procurement price of mining enterprises in Northwest China: cement 450 yuan per ton, natural sand 80 yuan per ton, crushed stone 70 yuan per ton, magnesium slag (self-produced waste residue, no cost included), fly ash (self-produced waste residue, no cost included), desulfurization gypsum (self-produced waste residue, no cost included), quicklime 300 yuan per ton, and polycarboxylate superplasticizer 6000 yuan per ton. The calculation results are shown in Table 3.

[0084] Table 3 Comprehensive Economic Comparison ; Calculations show that the comprehensive production cost of the dry powder filling material in Example 1 of this invention is approximately RMB 80 per ton, and the cost of the dry powder material in each cubic meter of filling slurry prepared with a water-cement ratio of 0.50 is approximately RMB 120. In Comparative Example 1, the material cost of traditional cement-based filling concrete is approximately RMB 310 per cubic meter. This invention reduces the material cost by approximately RMB 190 per cubic meter compared to traditional cement-based filling materials, a reduction of over 60%. Considering the savings in solid waste disposal costs, the overall economic benefits are even more significant.

[0085] The above performance test results show that the 28-day compressive strength of each embodiment of the present invention is stable between 15.3 and 18.2 MPa, meeting the C15 filling support strength requirements; the grout expansion is not less than 508 mm, the maximum safe pumping distance can reach 867 meters, the bleeding rate is low, and there is no stratification or only slight stratification; the 28-day strength under the dual-activation system is 75% to more than 4 times higher than that under the single activation system; the comprehensive material cost is reduced by about 60% compared with traditional cement-based materials, and the leaching concentration of heavy metals in the solidified material meets the groundwater quality standards.

Claims

1. A cement-free magnesium slag-based ternary solid waste mine backfill dry powder material, characterized in that, It is composed of the following dry powder components by mass percentage, with the total mass of all components being 100%: Dry powder of magnesium smelting slag: 70%–90% Dry fly ash: 8%–15%, Power plant flue gas desulfurization gypsum: 3%–8%, Quicklime: 0.3%–1.5%, Polycarboxylate superplasticizer: 0.05%~0.20%.

2. The cement-free magnesium slag-based ternary solid waste mine backfill dry powder material according to claim 1, characterized in that: The magnesium smelting slag powder, dry fly ash, and power plant flue gas desulfurization gypsum are all general industrial solid wastes; the filling dry powder material does not contain silicate cement or natural sand and gravel aggregate.

3. The cement-free magnesium slag-based ternary solid waste mine backfill dry powder material according to claim 1, characterized in that: The dry magnesium smelting slag powder contains 40%–55% calcium oxide and 20%–35% silicon dioxide by mass, and has a fineness that passes through a 150-325 mesh sieve; the fineness of the dry fly ash meets the Class II fly ash standard (≤25% residue on a 45μm square hole sieve); and the gypsum for desulfurization of power plant flue gas contains ≥75% calcium sulfate dihydrate and ≤3% free water by mass.

4. The cement-free magnesium slag-based ternary solid waste mine filling dry powder material according to claim 1, characterized in that: The quicklime contains ≥80% effective calcium oxide by mass and has a fineness of passing through a 180-325 mesh sieve. The quicklime and the power plant flue gas desulfurization gypsum together constitute a dual synergistic hydration system activated by alkaline activation and sulfate activation. The polycarboxylate high-efficiency water-reducing agent is a powdered polycarboxylate high-performance water-reducing agent with a water reduction rate ≥25%, which is used to regulate the rheological properties of the slurry to achieve low water-cement ratio pumping under a non-bleeding stratification state.

5. The method for preparing cement-free magnesium slag-based ternary solid waste mine backfill dry powder according to any one of claims 1 to 4, characterized in that, Includes the following steps: (1) Raw material pretreatment: The dry powder of magnesium smelting slag, dry fly ash and power plant flue gas desulfurization gypsum are screened to remove impurities, and coarse particles and agglomerated materials are removed to obtain pretreated powder; quicklime and polycarboxylate superplasticizer are directly used as finished powder raw materials without further grinding. (2) Precise measurement: Weigh each pre-treated powder and finished powder raw material according to the mass percentage range described in claim 1; (3) Closed mixing: Put all the weighed powder into a closed dry powder mixer and stir continuously for 10 to 15 minutes at room temperature until it is evenly mixed to obtain a homogeneous dry powder mixture. (4) Sealed storage: The homogeneous dry powder mixture is packed into a sealed silo or woven bag and sealed for moisture protection to prevent moisture absorption and failure.

6. The preparation method according to claim 5, characterized in that: The entire preparation process involves only physical mixing of dry powders at room temperature, without the need for high-temperature calcination or chemical reactions, and there is no discharge of waste gas, wastewater, or waste residue.

7. The preparation method according to claim 5, characterized in that: The closed dry powder mixer has a stirring speed of 20-50 r / min and a stirring time of 10-12 minutes. The closed dry powder mixer is equipped with a dust removal device to collect the dust generated during the mixing process and reuse it in the mixing process.

8. The method for underground pumping of cement-free magnesium slag-based ternary solid waste mine backfill dry powder as described in any one of claims 1 to 4, characterized in that, Includes the following steps: The dry powder filling material is transported to the underground working face. Water is added to the dry powder filling material and stirred. The water-cement ratio is controlled at 0.42-0.58 to form a homogeneous pumpable slurry. The slurry is injected into the goaf or surrounding rock fissures through pipelines using a grouting pump. Under normal temperature and natural conditions underground, the cured body has a 7-day compressive strength ≥8 MPa and a 28-day compressive strength ≥15 MPa, meeting the C15 concrete filling support standard.

9. The downhole pumping construction method according to claim 8, characterized in that: The water-cement ratio is 0.45 to 0.52; the slurry has no bleeding or stratification after standing for 2 hours, and its spread is ≥500 mm, meeting the requirements for deep well long-distance pipeline pumping construction; the pipeline has a horizontal conveying distance ≥1000 m or a vertical pumping depth ≥800 m.

10. The downhole pumping construction method according to claim 8, characterized in that: The solidified body is a goaf filling body or a permanent support structure for the surrounding rock. It does not crack or leak in the humid underground environment for a long time. The leaching concentration of heavy metals in the solidified body meets the requirements of GB / T 14848 groundwater quality standard and does not cause mine water and soil pollution.

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