Geopolymer-based sandy mudstone composite subsoil and preparation method and application thereof

Abstract

The present application relates to the technical field of mine ecological restoration, and discloses a sandy mudstone composite guest soil based on geopolymer and a preparation method and application thereof, aiming to solve the problems of pollution caused by accumulation of solid waste of sandy mudstone in open-pit coal mines, low resource utilization rate, lack of guest soil for ecological restoration in mining areas, poor water and fertilizer retention, and weak erosion resistance. The present application takes the sandy mudstone in open-pit coal mines as the core aggregate, and is compounded with a complex geopolymer cementing material, an alkaline activator, a modified functional agent, and a water and fertilizer retention component. After treatment, the compressive strength reaches about 2 MPa, and then the product is made into a finished composite guest soil by a breaker and a pulverizer. The finished guest soil has good bulkiness and excellent biocompatibility, and has the abilities of stable water and fertilizer retention and anti-loss performance. The raw materials are locally sourced, the preparation process is simple, and the finished product can be directly applied to ecological restoration in areas such as goaf, dump, and slope in open-pit coal mines, so as to achieve the dual goals of on-site disposal of solid waste and ecological restoration.

Description

Technical Field

[0001] This invention relates to the field of mine ecological restoration technology, specifically to a sandy mudstone composite topsoil based on geopolymers, its preparation method, and its application. Background Technology

[0002] During open-pit coal mining, materials such as overburden layers, roof and floor slabs, and interbedded gangue form large quantities of sandy mudstone solid waste. This type of solid waste is abundant and loosely textured. Direct dumping in spoil heaps or open-air stockpiles can easily cause continuous dust pollution, leading to the deterioration of the surrounding atmospheric environment. Furthermore, due to its loose structure and lack of cohesion, it accelerates soil erosion under rainwater runoff, causing surface soil erosion and river siltation, severely encroaching on valuable land resources. After mining disturbance, the original soil system in the goaf, spoil heaps, and slope areas is completely destroyed, exhibiting an extremely barren state with very low organic matter content, weak water retention capacity, and disrupted nutrient cycling mechanisms. This makes ecological restoration projects highly dependent on external soil imports to rebuild the vegetation base.

[0003] Traditional remediation methods commonly use purchased topsoil as topsoil, but such soil resources are scarce, extraction and transportation costs are high, long-distance transportation faces limitations due to road conditions and increased energy consumption, and the process of extracting soil from other locations damages the original farmland ecosystem, causing new environmental imbalances. Existing topsoil improvement technologies using mudstone as raw material attempt to utilize mine solid waste, but generally suffer from structural defects, such as insufficient cementation strength leading to low resistance to wind and water erosion, easy surface peeling and particle loss under natural rainfall or irrigation conditions, and poor pore structure resulting in a lack of water and fertilizer retention, failing to maintain the water and nutrient gradients required for plant growth, and poor biocompatibility, making it difficult to support the stable establishment and long-term development of herbaceous and shrub root systems. Geopolymers, as a novel green cementing material, can construct a three-dimensional aluminosilicate network structure through alkaline activation, theoretically enhancing the physicochemical stability of sandy mudstone. Furthermore, the raw materials can be derived from industrial byproducts such as slag and fly ash, conforming to the principle of resource recycling. However, current geopolymer-modified mudstone topsoil technology mostly adopts direct spreading process without systematically considering the matching relationship between the looseness of the topsoil and the needs of plant growth. It lacks scientific definition of the strength development law during the curing stage, especially ignoring the synergistic mechanism between the curing strength threshold and the subsequent crushing process. This results in modified materials that are either too dense to hinder root penetration or have insufficient strength to resist environmental erosion. It is impossible to balance structural stability and ecological functionality, which restricts the large-scale application of sandy mudstone solid waste in mine restoration. Summary of the Invention

[0004] To address the aforementioned technical shortcomings, the purpose of this invention is to provide a sandy mudstone composite topsoil based on geopolymers, its preparation method, and its application. The topsoil is cured to approximately 2 MPa and then pulverized, ensuring that the sandy mudstone, after modification with geopolymers, possesses sufficient erosion resistance, water retention, and fertilizer retention properties, while also meeting the requirements for loose topsoil to suit the needs of plant root growth and sowing, and maximizing the utilization of sandy mudstone solid waste.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: The geopolymer-based sandy mudstone composite topsoil comprises, by mass, 50-70 parts sandy mudstone, 15-25 parts geopolymer cementitious material, 1-2 parts alkaline activator, 1 part functional modifier, 1 part water-retaining and fertilizer-retaining component, with the remainder being deionized water. The sandy mudstone is obtained from overburden or spoil heaps in open-pit coal mines, crushed, screened to a particle size ≤2mm, and dried to a moisture content ≤5%. The geopolymer cementitious material is a mixture of slag, fly ash, and desulfurized gypsum in a mass ratio of 1:1:2, and the total content of active silica and alumina in the cementitious material is ≥70%.

[0006] Furthermore, the alkaline activator is a compound of water glass and sodium hydroxide, with a mass ratio of 4:1 and an activator modulus of 1.2 to 1.6, used to efficiently activate the active silica-alumina components in the geopolymer cementitious material and sandy mudstone.

[0007] Furthermore, the modified functional agent is a mixture of humic acid and bentonite in a mass ratio of 2:1; the water-retaining and fertilizer-retaining component is a mixture of biochar and polyacrylamide in a mass ratio of 3:1, which is suitable for the looseness and functional requirements of the composite topsoil.

[0008] This invention also relates to a method for preparing the above-mentioned sandy mudstone composite topsoil, comprising the following steps: Step 1, Raw material pretreatment: Crush the sandy mudstone, screen it to a particle size ≤2mm through a vibrating screen, remove impurities, place it in a drying equipment, dry it at 80~105℃ until the moisture content is ≤5%, and cool it for later use. Step 2, activation of the gelling system: Mix 15-25 parts of geopolymer gelling material with 1-2 parts of alkaline activator, add 8-10% of deionized water by total mass, stir at high speed of 800-1200 r / min for 10-15 min, let stand for 20-30 min to activate, and obtain a uniform activated gelling slurry. Step 3, Aggregate Mixing and Modification: The pretreated sandy mudstone is put into a mixer and stirred at 500-800 r / min for 5 min. Then, 1 part of the modified functional agent and 1 part of the water-retaining and fertilizer-retaining component are added in sequence, and the mixture is stirred for 10 min to obtain a homogeneous mixed aggregate. Step 4, Spreading and Curing: Pour the activated cementitious slurry into the mixed aggregate, add the remaining deionized water, and stir at 500-800 r / min for 20-30 min until the system is free of lumps to obtain the topsoil slurry; spread the topsoil slurry evenly and cure it at room temperature and pressure until its compressive strength reaches about 2 MPa. Step 5, Crushing and Molding: After the topsoil has been cured to the required standard, it is lifted up as a whole and sent into a soil crusher to be crushed, thus obtaining the finished sandy mudstone composite topsoil based on geopolymer.

[0009] Furthermore, in step 4, the paving thickness is 5-10cm, the curing period is 7-14 days, and the moisture content of the topsoil is maintained at 20-25% during the curing period; for arid areas, the curing period is extended to 14-21 days, and a moisturizing agent is sprayed every 3 days to ensure that the gelation reaction is fully carried out.

[0010] Furthermore, in step 5, after the soil is crushed by the pulverizer, the particle size of the finished topsoil is controlled to be 0.5-2mm to meet the sowing and transplanting needs of ecological restoration in different mining areas.

[0011] This invention also relates to the application of the above-mentioned sandy mudstone composite topsoil, which is used for the ecological restoration of open-pit coal mine goaf, spoil heaps and slopes, with a laying thickness of 15-30cm; for plots with excessive salinity, 0.3-0.5 parts of desulfurized gypsum are added to adjust the pH value of the topsoil to 6.5-7.8, making it suitable for the planting and growth of barren-tolerant herbs and shrubs.

[0012] The beneficial effects of this invention are as follows: This invention uses a high proportion of open-pit coal mine sandy mudstone as the core raw material to achieve on-site disposal of solid waste, solve the problems of sandy mudstone accumulation pollution and land waste, and at the same time significantly reduce the cost of imported soil raw materials, thus achieving the environmental protection goal of "treating waste with waste".

[0013] In the method of this invention, the soil is modified with a geopolymer and cured to 2MPa before being crushed. The finished topsoil has both looseness and erosion resistance, and will not compact and affect the growth of plant roots, while also resisting rainwater erosion. Combined with functional components, it has excellent water and fertilizer retention, suitable pH value, and strong biocompatibility, which can quickly help vegetation establishment.

[0014] The preparation process of this invention does not require large-scale special equipment and can be used in conjunction with existing crushing, mixing and drying equipment in mines. The spreading, curing and crushing processes are flexible and controllable, and can be prepared and used on-site in the mining area, which greatly reduces transportation and construction costs and is suitable for large-scale promotion.

[0015] This invention is applied to the ecological restoration of open-pit coal mines, which can quickly achieve vegetation coverage of mined-out areas and slopes, curbing dust and soil erosion. The raw materials are mostly industrial solid waste and mining solid waste, with low overall cost. Compared with purchasing soil from outside sources, it can save costs and has broad prospects for promotion. Detailed Implementation

[0016] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. 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.

[0017] Traditional open-pit coal mining operations generate large amounts of sandy mudstone solid waste. Direct accumulation of this waste easily leads to dust pollution, soil erosion, and the occupation of significant land resources. Its efficient resource utilization is a core unresolved issue in the field of mine environmental protection. Simultaneously, the original soil structure of mining subsidence areas, spoil heaps, and slopes is damaged, resulting in soil infertility and insufficient water and fertilizer retention capacity. Ecological restoration heavily relies on imported soil. Traditional imported soil methods suffer from high costs, transportation difficulties, and the potential to damage the local ecosystem. Existing imported soil based on mudstone, however, suffers from insufficient structural stability, poor resistance to erosion, and lack of biocompatibility, making it difficult to meet long-term restoration needs.

[0018] To address these issues, this invention proposes a composite topsoil based on geopolymers for sandy mudstone, aiming to solve the aforementioned problems. This composite topsoil, through specific component ratios and raw material pretreatment, synergistically achieves resource utilization of sandy mudstone solid waste and optimization of topsoil performance.

[0019] The composite topsoil made of sandy mudstone comprises, by mass, 50-70 parts sandy mudstone, 15-25 parts geopolymer cementitious material, 1-2 parts alkaline activator, 1 part modified functional agent, 1 part water-retaining and fertilizer-retaining component, with the remainder being deionized water.

[0020] The sandy mudstone was selected from overburden or solid waste from open-pit coal mining. To ensure its suitability as aggregate, the sandy mudstone underwent pretreatment. Specifically, it was crushed and screened through a vibrating screen to a particle size of 2 mm or less to remove impurities and obtain uniform particle size. Subsequently, the sandy mudstone was dried to a moisture content of 5% or less to avoid excessive moisture affecting subsequent cementation reactions.

[0021] The geopolymer cementitious material is a compound of various industrial solid wastes rich in silicon and aluminum, such as slag, fly ash, and desulfurized gypsum. As a preferred embodiment, the cementitious material can be compounded from slag, fly ash, and desulfurized gypsum in a mass ratio of 1:1:2. To ensure the activity of the cementitious reaction, the total content of active silica and aluminum oxide in the cementitious material is controlled to be greater than or equal to 70%.

[0022] Alkaline activators are used to activate the active silica-alumina components in geopolymer cementitious materials and sandy mudstones. Various alkaline substances can be selected as activators, such as sodium hydroxide solution, sodium silicate solution, sodium carbonate solution, or combinations thereof. Their concentration and dosage can be adjusted according to the type and reactivity of the geopolymer cementitious material to achieve activation.

[0023] Modifying functional agents are added to improve specific properties of the composite soil. These functional agents can be selected from various organic or inorganic materials, such as humic acid, bentonite, cellulose, lignin, or combinations thereof. Their function is to enhance the structural stability, biocompatibility, or stress resistance of the composite soil.

[0024] Water- and fertilizer-retaining components are added to enhance the composite topsoil's ability to retain water and nutrients. These components can be made from various materials with water- and fertilizer-retaining capabilities, such as biochar, polyacrylamide, superabsorbent polymers, vermiculite, or combinations thereof. The aim is to provide a stable water and nutrient environment for plant growth.

[0025] Deionized water is added as a margin to adjust the overall moisture content of the composite topsoil system. The amount of water is controlled to be sufficient to form a workable slurry, while avoiding excessive dilution that could affect the gel strength, ensuring that all components can be fully mixed and undergo hydration reactions.

[0026] This invention effectively solves the problem of resource utilization of solid waste in mining areas by using sandy mudstone solid waste from open-pit coal mines as the main aggregate and introducing geopolymer cementitious materials for modification. The resulting composite topsoil exhibits improved structural stability, erosion resistance, and water and fertilizer retention capacity. It can provide a suitable substrate for the ecological restoration of barren areas such as goafs, spoil heaps, and slopes, thereby supporting plant establishment and growth and promoting the restoration of the ecological environment in mining areas.

[0027] In some of the above-mentioned schemes, alkaline activators were proposed to activate the active silica-alumina components in the geopolymer cementitious material and sandy mudstone. However, in the process of implementation, the activation efficiency of the activator may be insufficient, resulting in incomplete cementation reaction, which affects the formation of the three-dimensional network structure of the geopolymer and the stability of the composite soil. In response, it was further proposed that the alkaline activator is a mixture of water glass and sodium hydroxide, with a mass ratio of 4:1 and an activator modulus of 1.2 to 1.6, which is used to efficiently activate the active silica-alumina components in geopolymer cementitious materials and sandy mudstone.

[0028] Specifically, the alkaline activator is a key component in the geopolymer reaction system. Its role is to provide a strongly alkaline environment, promoting the dissolution of active silica-alumina components in silica-alumina-rich raw materials (such as sandy mudstone, slag, fly ash, etc.) and further polymerizing them to form a stable three-dimensional network structure. A single activator may suffer from insufficient activation efficiency or incomplete reaction. In this invention, the alkaline activator is a combination of water glass and sodium hydroxide. Water glass (sodium silicate solution) provides soluble silicates, providing a silicon source for the geopolymer polymerization reaction, while its alkalinity aids in the dissolution of silica-alumina components. Sodium hydroxide provides a strongly alkaline environment, accelerating the dissolution and activation of silica-alumina components and adjusting the pH of the system. The combination of these two components achieves a synergistic effect, balancing silicon source supply and alkaline activation, thereby improving activation efficiency. Besides the combination of water glass and sodium hydroxide, alkaline activators can also be combinations of other alkaline substances, such as the combination of potassium hydroxide and potassium silicate, or the combination of sodium carbonate and sodium silicate. These combinations are also designed to provide a suitable alkaline environment and soluble silicon source to activate the activity of geopolymer cementitious materials.

[0029] Furthermore, the mass ratio of water glass to sodium hydroxide is 4:1. This mass ratio is a specific proportion of water glass and sodium hydroxide in the compound activator, designed to optimize the composition of the activator so that it provides sufficient alkalinity while also providing an appropriate amount of silicate components. The 4:1 mass ratio ensures that the system has moderate alkalinity, avoiding structural damage to the cementitious material or uncontrollable excessively rapid reactions due to excessive alkalinity, while preventing reduced reactivity due to excessively low alkalinity. This ratio helps balance the supply of silicates and the alkaline activation intensity, thereby promoting the full dissolution and polymerization of the active silica-alumina components. In other embodiments, this compound mass ratio can also be adjusted according to the specific geopolymer cementitious material and the component characteristics of sandy mudstone. For example, a ratio of 3:1 or 5:1 can be used to adapt to the activation requirements of different raw materials, but the activation effect and soil properties need to be verified experimentally.

[0030] Furthermore, the activator modulus is controlled within the range of 1.2 to 1.6. The activator modulus (usually referring to the molar ratio of SiO2 / Na2O or SiO2 / K2O) is a crucial parameter for measuring the ratio of silicon oxides to alkali metal oxides in alkaline activators, directly affecting the polymerization rate, product structure, and final properties of geopolymers. Controlling the activator modulus within this range ensures optimal polymerization of silicates. A lower modulus (e.g., close to 1.2) indicates higher alkalinity, facilitating rapid initial dissolution and activation; a higher modulus (e.g., close to 1.6) provides more silicon source, promoting the formation of a denser, more stable three-dimensional network structure. Modulus within this range efficiently activates the active silica-alumina components in the geopolymer cementitious material and sandy mudstone, promoting the formation of a stable aluminosilicate three-dimensional network structure, thereby enhancing the erosion resistance and overall performance of the composite soil. In some cases, the activator modulus can also be adjusted according to specific application requirements or raw material characteristics. For example, for some raw materials with low activity, a slightly lower modulus may be needed to provide stronger alkaline activation; while for clays that require higher strength and durability, a slightly higher modulus may be needed to promote more complete polymerization.

[0031] Through the above technical solution, the alkaline activator is optimized into a compound of water glass and sodium hydroxide, and the mass ratio of the two is precisely controlled at 4:1, with the activator modulus ranging from 1.2 to 1.6. This significantly improves the activation efficiency of the geopolymer cementitious material and the active silica-alumina components in sandy mudstone. Water glass provides the necessary silicon source, while sodium hydroxide provides a strongly alkaline environment. Their synergistic effect promotes the rapid dissolution of the silica-alumina components and their participation in the polymerization reaction. The specific compounding ratio and modulus ensure that the alkalinity of the activator is moderate and the degree of silica polymerization is within the optimal range, avoiding the problems of insufficient activation or reaction imbalance that may occur with a single activator. This allows the geopolymer to fully form a stable three-dimensional network structure, effectively improving the structural stability, erosion resistance, and water and fertilizer retention properties of the composite topsoil. This solves the problem of insufficient activation efficiency of the activator leading to incomplete cementation reaction and affecting the stability of the composite topsoil, providing a superior topsoil material for the ecological restoration of open-pit coal mines.

[0032] Some of the above-mentioned solutions propose using modified functional agents and water- and fertilizer-retaining components to improve the performance of composite topsoil. However, if their specific composition and ratio are not clearly defined in this process, it may lead to an inability to effectively adapt to the looseness and functional requirements of the composite topsoil, thereby affecting the structural stability and biocompatibility of the topsoil and reducing its applicability in the ecological restoration of mining areas.

[0033] In response, it was further proposed that the modified functional agent be a mixture of humic acid and bentonite in a mass ratio of 2:1; and the water-retaining and fertilizer-retaining component be a mixture of biochar and polyacrylamide in a mass ratio of 3:1, which is suitable for the looseness and functional requirements of the composite topsoil.

[0034] Specifically, the modified functional agent is composed of humic acid and bentonite in a mass ratio of 2:1. Humic acid, as a natural organic macromolecular compound, has multiple functions, including improving soil aggregate structure, enhancing soil fertility, and strengthening plant stress resistance. Its main role is to provide organic matter to the imported soil, promote the formation of aggregates, and chelate heavy metal ions. Besides being combined with bentonite, humic acid can also be used alone as a soil conditioner, or combined with other organic substances such as alginic acid and amino acids. Bentonite is a clay mineral with montmorillonite as its main mineral component. It is characterized by good adsorption, cation exchange capacity, swelling ability, and colloidal dispersibility, effectively improving the water and fertilizer retention capacity of the imported soil and improving its physical structure. Besides being combined with humic acid, bentonite can also be combined with other inorganic minerals such as zeolite and diatomaceous earth, or used directly as a soil conditioner. Humic acid and bentonite are compounded at a mass ratio of 2:1 to synergistically enhance the organic matter-improving effect of humic acid and the mineral adsorption and water-retention effect of bentonite, thereby achieving optimal structural stability and nutrient supply capacity.

[0035] Meanwhile, the water-retaining and fertilizer-retaining component is composed of biochar and polyacrylamide in a mass ratio of 3:1. Biochar is a carbon-rich solid material formed by the pyrolysis of biomass under anaerobic conditions. Its significant characteristics include a porous structure, high specific surface area, and abundant surface functional groups, which can significantly improve the porosity of the imported soil, adsorb nutrients, and improve the soil microbial environment. Besides being compounded with polyacrylamide, biochar can also be compounded with organic materials such as humus and peat, or applied alone. Polyacrylamide is a water-soluble polymer with good flocculation, thickening, and water-retaining properties. Its function is to enhance the stability of the imported soil aggregates, reduce soil erosion, and improve the water-holding capacity of the imported soil. Besides being compounded with biochar, polyacrylamide can also be compounded with natural polymers such as starch and cellulose, or used alone as a soil structure conditioner. Biochar and polyacrylamide were compounded at a mass ratio of 3:1 to optimize the balance between water retention and fertilizer retention. The porous structure of biochar was used to adsorb nutrients and retain moisture, while the cementing effect of polyacrylamide was used to enhance the aggregation of topsoil and moisture regulation, so as to meet the functional requirements of composite topsoil under different environments.

[0036] Through the above technical solution, the present invention clearly defines the specific composition and compounding ratio of the modified functional agent and the water-retaining and fertilizer-retaining components, thereby effectively solving the problem of the looseness and functional requirements of the composite topsoil.

[0037] Specifically, the modified functional agent is a mixture of humic acid and bentonite in a 2:1 mass ratio. Humic acid, as an organic matter source, effectively improves the aggregate structure of the imported soil and enhances its biological activity; bentonite provides high ion exchange capacity and excellent water retention performance. The synergistic effect of this specific ratio ensures that the composite imported soil maintains necessary looseness while preserving good structural stability, effectively preventing soil erosion and continuously supplying nutrients needed for plant growth. Simultaneously, the water- and fertilizer-retaining components are a mixture of biochar and polyacrylamide in a 3:1 mass ratio. The porous structure of biochar significantly improves the porosity and nutrient retention of the imported soil; polyacrylamide, as a high-molecular polymer, effectively enhances the aggregate capacity and water regulation performance of the imported soil. This optimized ratio achieves a good balance between water and fertilizer retention, enabling the composite imported soil to better meet the functional needs of different environments. Overall, this technical solution significantly improves the uniformity and applicability of composite topsoil by precisely controlling the composition and ratio of functional additives, thereby comprehensively optimizing its overall performance in ecological restoration of mining areas and providing a more favorable soil environment for plant establishment and growth.

[0038] Among the aforementioned solutions, a composite topsoil of sandy mudstone was proposed to efficiently utilize sandy mudstone solid waste and improve the ecological restoration effect of mining areas. However, in the preparation process, the existing methods directly spread and molded the topsoil without optimizing the curing strength threshold and crushing process, resulting in unstable topsoil structure, insufficient erosion resistance, poor water and fertilizer retention performance, and inability to meet the looseness requirements of plant root growth and sowing.

[0039] To address this issue, a method for preparing sandy mudstone composite topsoil based on geopolymers is proposed. This method effectively solves the aforementioned technical problems through a systematic sequence of steps. Specifically, the method includes the following steps: The first step is raw material pretreatment. In this step, the sandy mudstone is crushed and sieved through a vibrating screen to a particle size ≤2mm to ensure the uniformity of the sandy mudstone particles and increase its specific surface area, thus facilitating sufficient contact and reaction with the subsequent cementitious materials. Besides a vibrating screen, drum screens or air screens can also be used for sieving. Simultaneously, impurities such as organic matter or large stones are removed from the sandy mudstone to avoid adverse effects on the geopolymer cementation reaction and to ensure the purity of the topsoil. Impurity removal methods can include manual sorting, air separation, or washing. Subsequently, the treated sandy mudstone is placed in a drying device and dried at 80–105℃ until the moisture content is ≤5% to effectively remove moisture. Excessive moisture content will dilute the concentration of the alkaline activator, affecting the activation efficiency of the geopolymer and the cementation reaction process; therefore, controlling the moisture content is crucial to ensuring the smooth progress of the subsequent reaction. Besides oven drying, microwave drying or hot air circulation drying can also be used.

[0040] The next step is the activation of the gelling system. Mix 15-25 parts of geopolymer gelling material with 1-2 parts of alkaline activator, and add 8-10% deionized water by weight. The geopolymer gelling material (such as a mixture of slag, fly ash, and desulfurized gypsum) needs to be activated by an alkaline activator (such as a mixture of water glass and sodium hydroxide) to dissolve its internal silica-alumina components and form a stable three-dimensional network structure. Deionized water, as the reaction medium, not only participates in the activation reaction but also adjusts the consistency of the slurry. Subsequently, stir at a high speed of 800-1200 rpm for 10-15 minutes to ensure rapid and uniform mixing of all components and accelerate the dissolution of the gelling material by the activator. Besides a high-speed mixer, a planetary mixer or shear mixer can also be used. After stirring, allow it to stand for 20-30 minutes to allow the activator to fully act on the gelling material, promoting further dissolution of the silica-alumina components and initiating the formation of initial polymerization products, thereby obtaining a uniform activated gelling slurry.

[0041] The next step is aggregate mixing and modification. Pretreated sandy mudstone is added to a mixer and stirred at 500–800 rpm for 5 minutes to ensure uniform dispersion. Then, one part of a functional modifier and one part of a water- and fertilizer-retaining component are added sequentially, and stirring continues for 10 minutes to ensure these functional components are uniformly adhered to or dispersed on or between the sandy mudstone particles. The introduction of functional modifiers (such as humic acid and bentonite blends) and water- and fertilizer-retaining components (such as biochar and polyacrylamide blends) aims to impart additional functional properties to the sandy mudstone aggregate, such as improving soil structure, enhancing water and fertilizer retention capacity, and improving biocompatibility, thereby obtaining a homogeneous aggregate mixture. Besides a mixer, drum mixers or twin-screw mixers can also be used.

[0042] Next is the paving and curing step. The prepared activated gelatinous slurry is poured into the homogeneous aggregate mixture, and the remaining deionized water is added to adjust the slurry consistency. It is stirred at 500–800 rpm for 20–30 minutes until the system is free of lumps, forming a homogeneous topsoil slurry. Subsequently, the topsoil slurry is evenly spread into a layered structure of a certain thickness, which can be done manually or with a mechanical paver. The core of this step is to cure the topsoil under normal temperature and pressure until its compressive strength reaches approximately 2 MPa. This specific strength threshold is optimized to ensure that the topsoil possesses sufficient structural stability, erosion resistance, and water and fertilizer retention capacity before subsequent crushing, effectively resisting soil erosion, while avoiding excessive strength that would lead to crushing difficulties or excessive energy consumption, thus providing a suitable physical basis for subsequent crushing and molding. During the curing process, the stable three-dimensional network structure of the geopolymer is fully formed, locking in the water and fertilizer retention components.

[0043] Finally, there is the crushing and molding step. After the compressive strength of the topsoil has reached the required standard, it is lifted as a whole and sent to a soil pulverizer for crushing. Crushing is the key operation in this method to achieve the looseness of the topsoil. Through mechanical crushing, the cured topsoil is broken down into granular particles suitable for plant growth and sowing. The soil pulverizer can be a hammer mill, roller mill, or impact mill, etc., and its selection and operating parameters will affect the final particle size distribution and looseness of the topsoil. After crushing, the finished sandy mudstone composite topsoil based on geopolymer is obtained.

[0044] The above technical solution effectively solves the performance defects of the topsoil caused by the lack of optimized curing and crushing processes in existing technologies. First, the fine pretreatment of the sandy mudstone ensures the uniformity and reactivity of the aggregates, providing a stable foundation for the subsequent cementation reaction. Second, the activation step of the cementing system fully stimulates the reactivity of the geopolymer cementitious material and the alkaline activator, promoting the efficient dissolution and polymerization of the silica-alumina components, laying the foundation for the formation of a stable three-dimensional network structure. Third, the aggregate mixing and modification step, by introducing modifying functional agents and water- and fertilizer-retaining components, functionalizes the sandy mudstone aggregates before the cementation reaction, significantly improving the biocompatibility, water and fertilizer retention capacity, and structural stability of the topsoil. Most importantly, this method innovatively introduces a combined process of "spreading and curing until the compressive strength reaches approximately 2 MPa" and "crushing and molding." During the curing stage, by controlling the topsoil to reach a specific compressive strength threshold, sufficient structural strength and erosion resistance are ensured on a macroscopic level, effectively resisting soil erosion and providing a stable growth environment for plant roots. Simultaneously, the stable three-dimensional network structure of the geopolymer is fully formed during this stage, locking in water- and fertilizer-retaining components and improving the long-term performance of the topsoil. Subsequently, the cured topsoil is broken down into loose granules through crushing and molding. This not only endows the topsoil with good air and water permeability, greatly improving root penetration and growth conditions, but also makes it suitable for sowing and transplanting, solving the problem that traditional geopolymer-modified topsoil is too hard and unfavorable to plant growth after direct spreading. In summary, the composite topsoil prepared by this invention maintains excellent erosion resistance and water and fertilizer retention properties while achieving a looseness suitable for plant growth, achieving a balance between performance and application requirements. This efficiently utilizes sandy mudstone solid waste and significantly improves the ecological restoration effect of mining areas.

[0045] Some of the above-mentioned solutions propose spreading and curing to enable the topsoil to reach the target strength. However, the curing effect may be unstable due to differences in environmental conditions during this process. In particular, in arid areas, insufficient moisture can easily interrupt the cementation reaction, affecting the uniformity and reliability of the final performance of the topsoil.

[0046] In response, further optimization of step 4 in the above preparation method is proposed. Specifically, in step 4, the spreading thickness is 5-10 cm, the curing period is 7-14 days, and the moisture content of the topsoil is maintained at 20-25% during the curing period. For arid areas, the curing period is extended to 14-21 days, and a moisturizing agent is sprayed every 3 days to ensure that the gelation reaction is fully carried out.

[0047] Specifically, the topsoil slurry is spread to a thickness of 5–10 cm. This thickness helps ensure a uniform distribution of heat and moisture within the topsoil layer, preventing insufficient internal reaction due to excessive thickness or affecting structural strength due to insufficient thickness. For example, equipment such as scrapers or pavers can be used, and the thickness of the topsoil slurry can be precisely controlled by adjusting the scraper height or paver parameters; alternatively, pre-set molds or templates can be used to ensure that the thickness of each paving meets the requirements. Under normal temperature and pressure, the curing period for the topsoil is set at 7–14 days to ensure that the geopolymer cementitious material reacts fully with the sandy mudstone to form a stable aluminosilicate network structure, achieving the expected strength and performance. This curing period can be maintained under normal temperature and pressure conditions by avoiding external disturbance and providing appropriate covering or moisture retention to maintain a stable curing environment; alternatively, it can be dynamically adjusted by real-time monitoring of the compressive strength or cementitious reaction degree of the topsoil. Throughout the curing period, the moisture content of the topsoil should be maintained between 20% and 25% to ensure the necessary moisture is provided for the activation and hydration reactions of the geopolymers, promoting the dissolution, polymerization, and cross-linking of the silica-alumina components to form a dense structure. This can be achieved through regular spraying, covering with a moisturizing film, or using an automatic spraying system combined with real-time monitoring of the moisture content using a humidity sensor; alternatively, the initial moisture content can be precisely controlled during the preparation of the topsoil slurry, and after a thorough one-time water replenishment at the beginning of curing, moisture evaporation can be reduced through sealed curing. Considering the rapid evaporation of water and the potential reduction in the gelation reaction rate in arid regions, it is further proposed to extend the curing cycle to 14–21 days in these areas to compensate for the rapid evaporation and ensure sufficient time for the gelation reaction to proceed. Simultaneously, in arid regions, a moisturizing agent, such as water or a mixture containing a small amount of water-retaining components, should be sprayed every 3 days to actively replenish moisture and potentially reduce evaporation through the properties of the moisturizing agent, providing a continuously stable and humid environment for the geopolymer reaction. The spraying frequency and the type of moisturizing agent can also be adjusted according to actual environmental conditions and the water retention capacity of the topsoil.

[0048] The above technical solutions optimized the specific parameters of paving thickness, curing period, and moisture content maintenance, and introduced adaptive adjustment measures for arid regions. A paving thickness of 5–10 cm facilitates the uniform distribution of heat and moisture within the topsoil, preventing insufficient reaction or inadequate structural strength due to improper thickness. A standard curing period of 7–14 days provides ample time for the geopolymer cementitious material to fully react with the sandy mudstone, forming a stable network structure. Maintaining a moisture content of 20%–25% ensures sufficient moisture during the reaction process, promoting the activation and cross-linking of the silica-alumina components. Furthermore, for arid regions, extending the curing period to 14–21 days effectively compensates for the adverse effects of rapid moisture evaporation; simultaneously, spraying a moisturizing agent every 3 days actively replenishes moisture, maintaining a suitable reaction environment. These measures work together to ensure that the geopolymer gelation reaction can proceed fully under various environmental conditions, thereby solving the problems of unstable curing effect caused by differences in environmental conditions and the easy interruption of gelation reaction due to insufficient moisture. They significantly improve the uniformity and reliability of the final performance of the composite topsoil, ensuring its ecological restoration effect in complex environments such as open-pit coal mine goaf, spoil heaps, and slopes.

[0049] Some of the above-mentioned solutions propose a crushing and molding step to prepare the finished imported soil. However, in this process, the particle size of the crushed imported soil is not clearly controlled, which may lead to insufficient or excessive looseness of the imported soil, making it unable to meet the sowing and transplanting needs of ecological restoration in different mining areas, and affecting the growth of plant roots and the restoration effect.

[0050] In response, it was further proposed that after the soil is crushed by the pulverizer in step 5, the particle size of the finished topsoil should be controlled to be 0.5-2mm to meet the sowing and transplanting needs of ecological restoration in different mining areas.

[0051] Specifically, the pulverization of topsoil refers to the process of breaking up the cured topsoil into particles of the required size through mechanical action to prepare finished topsoil. This is a crucial step in transforming large, consolidated materials into loose particles. In practice, various types of pulverizing equipment can be used. For example, a hammer mill can be selected, which uses high-speed rotating hammers to impact, shear, and grind the topsoil, achieving efficient pulverization and is suitable for processing brittle materials. Alternatively, a roller mill can be used, which uses relatively rotating rollers to squeeze, shear, and grind the topsoil. Its advantage lies in the uniform particle size of the product and less over-pulverization, making it suitable for applications requiring finer particle sizes.

[0052] The finished topsoil particle size control of 0.5–2 mm refers to the precise control of the particle size of the crushed topsoil to between 0.5 mm and 2 mm through crushing equipment and / or subsequent screening equipment. This specific particle size range is determined comprehensively based on the soil structure requirements for plant growth in ecological restoration and the convenience of sowing and transplanting operations. In the process, the particle size distribution of the crushed product can be directly affected by adjusting operating parameters such as the rotation speed of the pulverizer, the gap between the hammer and the liner, and the roller spacing. Real-time monitoring and feedback adjustment can be performed using online screening equipment to ensure that most particles fall within the target range. Another method is to screen the crushed product through a multi-stage vibrating screen or drum screen after crushing, separating fine powder smaller than 0.5 mm and coarse particles larger than 2 mm, retaining only particles in the 0.5–2 mm range as the finished topsoil. The separated coarse particles can be returned to the pulverizer for secondary crushing, while the fine powder can be used for other purposes or mixed back in proportion as needed.

[0053] The ability to adapt to the sowing and transplanting needs of ecological restoration in different mining areas means that the prepared topsoil particle size range can meet the operational requirements of various ecological restoration scenarios in mining areas, whether directly sowing seeds (sowing) or transplanting seedlings (transplanting), and provide a suitable physical environment for plant growth. A particle size range of 0.5–2 mm ensures good air and water permeability of the topsoil, which is beneficial for seed germination and seedling root respiration, while avoiding compaction caused by excessive fine particles and poor water retention caused by excessive coarse particles. This structure has good compatibility for sowing and transplanting of different types of herbaceous plants (such as grasses and legumes) and shrubs (such as sea buckthorn and caragana). At the same time, this particle size distribution makes the topsoil easy to level during laying and has a certain degree of resistance to wind and water erosion, reducing the risk of topsoil loss. For ecological restoration in different mining areas (such as arid and semi-arid areas), topsoil with this particle size range can provide a stable substrate, supporting the establishment and growth of various tolerant plants.

[0054] Through the above technical solution, in step 5, the particle size of the finished topsoil after pulverization by the pulverizer is precisely controlled within a specific range of 0.5–2 mm. This effectively solves the problem of unsuitable topsoil looseness, which cannot meet the sowing and transplanting needs of ecological restoration in different mining areas. This particle size range ensures that the topsoil possesses both good air and water permeability, facilitating root penetration, respiration, and absorption of water and nutrients, while avoiding compaction due to excessively small particle size or insufficient water and fertilizer retention and the risk of loss due to excessively large particle size. By precisely controlling the particle size, the composite topsoil of this invention can flexibly adapt to the sowing and transplanting of various plants such as herbs and shrubs in ecological restoration of different mining areas, providing a stable and suitable physical environment for seed germination, seedling establishment, and subsequent growth, thereby significantly improving the success rate and long-term stability of ecological restoration.

[0055] Among the aforementioned solutions, composite topsoil was proposed for ecological restoration. However, in areas with excessive salinity, the pH value of the topsoil is unsuitable, leading to difficulties in plant establishment and growth. Therefore, a solution to optimize the application of composite topsoil is proposed.

[0056] Specifically, the composite topsoil is used for the ecological restoration of open-pit coal mine goafs, spoil heaps, and slopes. This composite topsoil is intended for use in severely damaged mining environments, such as goafs after open-pit coal mining, spoil heaps for accumulated waste, and slopes formed by mining activities. The composite topsoil is laid to a thickness of 15–30 cm. The laying thickness refers to the depth of the composite topsoil covering the surface of the restoration area. This thickness range is determined comprehensively based on factors such as plant root growth requirements, soil water and fertilizer retention capacity, and erosion resistance. In actual construction, precise control can be achieved using a laser rangefinder or ruler in conjunction with paving machinery to ensure uniform topsoil layer thickness that meets design requirements; alternatively, the required thickness can be gradually achieved through layered laying and compaction, simultaneously improving the density and stability of the topsoil layer.

[0057] Furthermore, for areas with excessive salinization, it is proposed to add 0.3–0.5 parts of desulfurized gypsum. Desulfurized gypsum is an industrial byproduct, mainly composed of calcium sulfate, which improves soil physicochemical properties and reduces soil pH and salinity. In areas with excessive salinization, the soil pH is too high or the salt content is too high, severely inhibiting plant growth. Desulfurized gypsum can be added and mixed with sandy mudstone, geopolymer cementitious materials, and other components during the preparation of composite topsoil to ensure its uniform dispersion in the topsoil system; alternatively, desulfurized gypsum can be pre-spread on the surface of the salinized area before laying the composite topsoil, allowing it to fully interact with the topsoil and underlying soil through subsequent water infiltration.

[0058] By adding desulfurized gypsum, the pH of the topsoil is adjusted to 6.5–7.8. pH is an indicator of soil acidity and alkalinity, and it has a decisive impact on plant nutrient absorption and microbial activity. Adjusting the pH of the topsoil to 6.5–7.8 aims to provide a suitable slightly acidic to neutral growing environment for most plants, especially tolerant herbs and shrubs. The optimal ratio can be determined through precise control of the amount of desulfurized gypsum added, combined with the acidity and alkalinity of the topsoil raw materials, and by conducting small-scale tests during the preparation process to achieve the target pH range. After the topsoil is laid, the pH of the topsoil layer can be monitored regularly. If the pH deviates from the target range, a small amount of acidic or alkaline regulator can be applied for fine-tuning. Ultimately, this topsoil environment is suitable for the planting and growth of tolerant herbs and shrubs.

[0059] The above-mentioned technical solutions utilize composite topsoil for the ecological restoration of open-pit coal mine goafs, spoil heaps, and slopes. A layer thickness of 15–30 cm is provided to offer ample growing space and stable physical support for plant roots, effectively preventing soil erosion. Furthermore, for areas with excessive salinity, adding 0.3–0.5 parts of desulfurized gypsum allows for precise pH adjustment of the topsoil to 6.5–7.8 through its acid-neutralizing properties. This pH range creates a suitable soil environment for the planting and growth of tolerant herbaceous plants and shrubs, effectively solving the problem of limited plant growth caused by pH mismatch in salinized areas. These technical solutions not only fully utilize the excellent properties of sandy mudstone composite topsoil based on geopolymers, maximizing its effectiveness in harsh mining environments, but also significantly improve the success rate and efficiency of ecological restoration in salinized areas through precise pH control, ensuring healthy plant growth and stable ecosystem recovery.

[0060] Application Examples Example 1 The raw material ratio of the sandy mudstone composite topsoil (parts by weight) is as follows: 60 parts sandy mudstone, 20 parts geopolymer cementitious material (5 parts slag, 5 parts fly ash, 10 parts desulfurized gypsum, active SiO2 + Al2O3 content 75%), 1.5 parts alkaline activator (1.2 parts water glass + 0.3 parts sodium hydroxide, modulus 1.4), 1 part modified functional agent (0.67 parts humic acid + 0.33 parts bentonite), 1 part water-retaining and fertilizer-retaining component (0.75 parts biochar + 0.25 parts polyacrylamide), and 7 parts deionized water.

[0061] Preparation steps: Follow the above preparation method, dry the sandy mudstone to a moisture content of 4%; let the cementitious slurry stand for 25 minutes to activate it; spread the topsoil slurry to a thickness of 8 cm, cure it at room temperature for 10 days, and the compressive strength reaches 2.1 MPa; after being crushed by a pulverizer, the particle size is adjusted to 1-2 mm to obtain the finished topsoil.

[0062] Performance indicators: The finished topsoil has a water retention rate of 68%, a pH value of 7.0, and good looseness with no compaction; when laid in open-pit coal mine spoil heaps with a thickness of 20cm, the seedling emergence rate is 94% after 28 days of sowing Bermuda grass, and there is no significant loss after continuous rainfall.

[0063] Example 2 The raw material ratio of the sandy mudstone composite topsoil (parts by weight) is as follows: 65 parts sandy mudstone, 18 parts geopolymer cementitious material (4.5 parts slag, 4.5 parts fly ash, 9 parts desulfurized gypsum, active SiO2+Al2O3 content 72%), 1 part alkaline activator (0.8 parts water glass + 0.2 parts sodium hydroxide, modulus 1.3), 1 part modified functional agent, 1 part water-retaining and fertilizer-retaining component, and 6 parts deionized water.

[0064] Preparation steps: Follow the above method, adapt to the working conditions of arid mining areas, spread the topsoil slurry to a thickness of 7cm, cure for 18 days, spray with a moisturizing agent every 3 days, and the compressive strength after curing is 2.0MPa; after the soil is crushed by a pulverizer, the particle size is adjusted to 0.5-1mm; lay it on the slope of an open coal mine to a thickness of 25cm, add an extra 0.4 parts of desulfurized gypsum to adjust the pH value, and transplant sea buckthorn seedlings.

[0065] Performance indicators: The finished topsoil has a water retention rate of 65%, a pH value of 6.9, and excellent erosion resistance; the survival rate of sea buckthorn seedlings after 45 days is 90%, the slope has no signs of landslides or erosion, and the ecological restoration effect is stable.

[0066] The sandy mudstone composite topsoil of this invention has widely available and readily available raw materials. The sandy mudstone is taken from solid waste produced by open-pit coal mines, and the slag, fly ash, and desulfurized gypsum used in the geopolymer cementitious materials are all bulk industrial solid wastes, resulting in low raw material costs. The preparation process does not involve harsh conditions such as high temperature and high pressure, and each step is compatible with existing mine production equipment, eliminating the need for significant additional equipment costs. The finished topsoil is suitable for various restoration scenarios in open-pit coal mines, with high vegetation survival rates and long-lasting restoration effects. It can effectively solve the core pain points of ecological restoration in mining areas and has significant industrial applicability and large-scale promotion value.

[0067] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A composite topsoil based on geopolymers in sandy mudstone, characterized in that, By mass fraction, the raw material components include: 50-70 parts sandy mudstone, 15-25 parts geopolymer cementitious material, 1-2 parts alkaline activator, 1 part modified functional agent, 1 part water-retaining and fertilizer-retaining component, and the remainder being deionized water; the sandy mudstone is taken from the overburden or solid waste of open-pit coal mining, crushed and screened to a particle size ≤2mm, and dried to a moisture content ≤5%; the geopolymer cementitious material is compounded from slag, fly ash, and desulfurized gypsum in a mass ratio of 1:1:2, and the total content of active silica and aluminum oxide in the cementitious material is ≥70%.

2. The sandy mudstone composite topsoil according to claim 1, characterized in that, The alkaline activator is a compound of water glass and sodium hydroxide, with a mass ratio of 4:1 and an activator modulus of 1.2 to 1.

6. It is used to efficiently activate the active silica-alumina components in geopolymer cementitious materials and sandy mudstone.

3. The sandy mudstone composite topsoil according to claim 1, characterized in that, The modified functional agent is a mixture of humic acid and bentonite in a mass ratio of 2:1; the water-retaining and fertilizer-retaining component is a mixture of biochar and polyacrylamide in a mass ratio of 3:1, which is suitable for the looseness and functional requirements of the composite topsoil.

4. A method for preparing sandy mudstone composite topsoil as described in any one of claims 1 to 3, characterized in that, Includes the following steps: Step 1, Raw material pretreatment: Crush the sandy mudstone, screen it to a particle size ≤2mm through a vibrating screen, remove impurities, place it in a drying equipment, dry it at 80~105℃ until the moisture content is ≤5%, and cool it for later use. Step 2, activation of the gelling system: Mix 15-25 parts of geopolymer gelling material with 1-2 parts of alkaline activator, add 8-10% of deionized water by total mass, stir at high speed of 800-1200 r / min for 10-15 min, let stand for 20-30 min to activate, and obtain a uniform activated gelling slurry. Step 3, Aggregate Mixing and Modification: The pretreated sandy mudstone is put into a mixer and stirred at 500-800 r / min for 5 min. Then, 1 part of the modified functional agent and 1 part of the water-retaining and fertilizer-retaining component are added in sequence, and the mixture is stirred for 10 min to obtain a homogeneous mixed aggregate. Step 4, Spreading and Curing: Pour the activated cementitious slurry into the mixed aggregate, add the remaining deionized water, and stir at 500-800 r / min for 20-30 min until the system is free of lumps to obtain the topsoil slurry; spread the topsoil slurry evenly and cure it at room temperature and pressure until its compressive strength reaches about 2 MPa. Step 5, Crushing and Molding: After the topsoil has been cured to the required standard, it is lifted up as a whole and sent into a soil crusher to be crushed, thus obtaining the finished sandy mudstone composite topsoil based on geopolymer.

5. The preparation method according to claim 4, characterized in that, In step 4, the paving thickness is 5-10cm, the curing period is 7-14 days, and the moisture content of the topsoil is maintained at 20-25% during the curing period. For arid areas, the curing period is extended to 14-21 days, and a moisturizing agent is sprayed every 3 days to ensure that the gelation reaction is fully carried out.

6. The preparation method according to claim 4, characterized in that, In step 5, after the soil is crushed by the pulverizer, the particle size of the finished topsoil is adjusted to 0.5-2mm to meet the sowing and transplanting needs of ecological restoration in different mining areas.

7. An application of the sandy mudstone composite topsoil as described in any one of claims 1 to 3, characterized in that, The composite topsoil is used for the ecological restoration of open-pit coal mine goaf, spoil heaps, and slopes, with a thickness of 15-30cm. For areas with excessive salinity, 0.3-0.5 parts of desulfurized gypsum are added to adjust the pH value of the topsoil to 6.5-7.8, making it suitable for the planting and growth of barren-tolerant herbs and shrubs.