Surface-modified coal gangue aggregate concrete, its preparation method and application

Abstract

This invention discloses surface-modified coal gangue aggregate concrete, its preparation method, and its application, belonging to the fields of solid waste resource utilization and building materials. The concrete includes cement, fine aggregate, surface-modified coal gangue coarse aggregate, water-reducing agent, and water. The surface-modified coal gangue coarse aggregate has an aluminum-substituted hydrated calcium silicate gel layer with a gradient transition from the bulk to the outside. It is prepared by coating the coal gangue coarse aggregate with a modified slurry, reacting it in situ, and curing it. The modified slurry includes, by weight, 100 parts of silicate cementitious material, 5-25 parts of pozzolanic active silica powder, 1-8 parts of alkaline etching component without soluble silicate, and 40-120 parts of water. Without adding soluble silicate, the surface clay minerals of the coal gangue are transformed in situ into a gradient interface bonding phase, eliminating the discrete interface and simultaneously improving mechanical properties and freeze-thaw resistance.

Description

Technical Field

[0001] This invention relates to the fields of solid waste resource utilization and building materials technology, specifically to surface-modified coal gangue aggregate concrete, its preparation method, and its application. Background Technology

[0002] Coal gangue is a large-scale industrial solid waste generated during coal mining and washing. Long-term stockpiling not only occupies land but also poses environmental risks such as spontaneous combustion and leaching pollution. Its large-scale, high-value utilization is an important way to achieve solid waste management and carbon peaking / neutralization goals. The main mineral components of coal gangue are kaolinite, illite, and other clay minerals, as well as quartz. After crushing and screening, it can be used as coarse aggregate in concrete, partially or completely replacing natural crushed stone, thereby alleviating the shortage of natural aggregate resources while disposing of solid waste.

[0003] However, the surface layer of coal gangue coarse aggregate is rich in open pores, bedding fissures, and loosely bound clay minerals, resulting in high water absorption and low surface strength. This creates a wide and loose interfacial transition zone between the coal gangue and cement paste. This interfacial transition zone is the weakest link in cement-based materials, rich in platy calcium hydroxide crystals and pores, making it easy for cracks to initiate and propagate. Therefore, concrete prepared directly using coal gangue coarse aggregate generally has lower compressive strength, splitting tensile strength, and durability indicators such as freeze-thaw resistance and impermeability compared to concrete made with natural aggregates, limiting the application of coal gangue coarse aggregate in load-bearing and cold-region structures.

[0004] To improve the interfacial properties between coarse coal gangue aggregate and cement paste, various aggregate surface modification methods have been developed in this field, but all have objective shortcomings. Firstly, cement paste or silica fume-cement paste is used to coat the aggregate, relying on the paste filling and pozzolanic reaction to improve the interface. However, this method consumes cement and only introduces a modified layer-mortar interface on top of the original aggregate-mortar interface. A discrete internal interface remains between the aggregate and the modified layer, increasing the number of interfaces rather than decreasing them, and the clay minerals on the aggregate surface are not eliminated. Secondly, water glass or sodium silicate solution is used for modification. The sodium silicate remaining on the aggregate surface reacts with the calcium hydroxide generated from cement hydration to form hydrated calcium silicate gel, filling the pores and forming a film on the surface, which improves the interface to some extent. However, the addition of soluble silicates introduces free soluble silica into the system, posing a risk of alkali-silica reaction and efflorescence on the concrete surface. Related studies also indicate that water glass soaking reduces concrete durability and requires strict concentration control. Third, modification with nanomaterials such as nano-silica sol is used. Although active silica can react with calcium hydroxide to form hydrated calcium silicate, its strong cohesiveness easily forms a continuous film on the aggregate surface, hindering cement hydration; therefore, its dosage should not be too high. Fourth, acid etching pretreatment is used to remove weak phases on the surface, but the acid concentration must be strictly controlled, resulting in heavy pollution. Furthermore, its essence is to remove material weakness rather than densify it. In addition, while the alkaline activation to form hydrated calcium silicate gel from coal gangue is a known chemical reaction in this field, it is usually used for overall geological polymerization of coal gangue powder, not for the directional construction of an interface layer on the surface of coarse aggregate.

[0005] In summary, existing technologies either leave discrete internal interfaces between the aggregate and the modified layer, or cause alkali-silica reactions, efflorescence, and reduced durability due to the addition of soluble silicates, or weaken the aggregate by removing material, or hinder hydration due to film formation. All of these methods fail to transform the originally harmful clay minerals on the surface of coal gangue into a beneficial interfacial bonding phase that transitions gradient with the aggregate bulk and eliminates the discrete interface without the addition of soluble silicates. Therefore, how to transform the clay minerals on the surface of coal gangue in situ into an interfacial bonding phase that transitions gradient from the aggregate bulk outwards, thereby eliminating the discrete interface between the aggregate and the modified layer and simultaneously improving the mechanical properties and freeze-thaw resistance of coal gangue aggregate concrete, is a technical problem that urgently needs to be solved in this field.

[0006] The formation of the interface transition zone originates from a localized increase in the water-cement ratio of the aggregate surface boundary layer in fresh concrete: free water accumulates on the aggregate surface, causing the hydration products in this area to become loose, and calcium hydroxide to become directionally enriched and coarsened, forming a porous, low-strength weak zone. For coarse coal gangue aggregate, the high water absorption of its loose surface clay minerals further exacerbates water migration and porosity development in the boundary layer, making the interface transition zone wider and weaker, thus becoming a preferred pathway for crack initiation and propagation. Therefore, simply adding a layer of cementitious coating to the outer surface of the aggregate cannot eliminate the inherent physical interface between the aggregate body and the coating layer; cracks can still propagate along this discrete interface, which is the fundamental reason why the improvement effect of grouting methods is limited.

[0007] On the other hand, the dissolution and reprecipitation behavior of aluminosilicate solid waste under alkaline conditions has been reported: alkali can depolymerize the silicon-oxygen tetrahedron and aluminum-oxygen octahedron networks of clay minerals such as kaolinite and illite, releasing soluble silicate and aluminate ions; in calcium-rich environments, the dissolved silicon and aluminum can combine with calcium to form hydrated calcium silicate or aluminum-substituted hydrated calcium silicate. However, existing studies have mostly focused on increasing the porosity of solid waste powders and preparing adsorbent materials or geopolymer cementitious materials. Their dissolution-reprecipitation occurs within the slurry bulk and is randomly oriented, failing to directionally confine the reaction to the surface of coarse aggregates to construct an interfacial transformation layer that is continuous with the aggregate bulk and exhibits a gradient transition in composition. The key difference between this invention and existing technologies lies in how to reverse this chemical process, typically used for pore enhancement or bulk polymerization, to directionally construct a dense gradient interfacial binder phase on the aggregate surface.

[0008] Furthermore, the large-scale utilization of coal gangue coarse aggregate must balance engineering performance and long-term durability stability, avoiding the introduction of new deteriorating factors due to modification. While adding soluble silicates can improve the interface in the short term, it introduces free soluble silicates that can trigger alkali-silicic acid reactions and efflorescence, posing a potential hazard to long-term service environments such as cold regions and hydraulic engineering projects. Therefore, without adding soluble silicates, transforming the harmful clay minerals on the surface of coal gangue into a favorable gradient interface bonding phase in situ is of great significance for realizing the large-scale application of coal gangue coarse aggregate in load-bearing and harsh environmental structures.

[0009] Coal gangue emissions account for over 10% of raw coal production, making it one of the largest industrial solid wastes in my country in terms of both emissions and stockpiles. Long-term open-air storage not only occupies vast amounts of land but also poses environmental risks such as spontaneous combustion releasing harmful gases and rainwater leaching polluting soil and groundwater. Improving its comprehensive utilization rate is of significant practical importance for saving land, preventing pollution, and achieving resource recycling. Using crushed coal gangue to prepare coarse aggregate for concrete is one of the important directions for its large-scale, high-value utilization. However, due to the aforementioned interface defects, coal gangue coarse aggregate is currently mostly used in low-grade, non-load-bearing subgrades and filling projects, making it difficult to enter the field of structural concrete, which requires higher mechanical properties and durability. Therefore, developing surface modification methods that can fundamentally improve the interface properties of coal gangue coarse aggregate without introducing new durability risks is of great value in broadening the application range of coal gangue coarse aggregate. Summary of the Invention

[0010] To address the problems existing in the prior art, the present invention aims to provide a surface-modified coal gangue aggregate concrete, its preparation method, and its application. Without adding soluble silicates, the clay minerals on the surface of the coal gangue are transformed in situ into an aluminum-substituted hydrated calcium silicate gel layer that transitions from the aggregate body to the outside, eliminating the discrete interface between the aggregate and the modified layer. This simultaneously improves the compressive strength, splitting tensile strength, and freeze-thaw durability of the concrete, and achieves high-value utilization of coal gangue solid waste and reduction of cement usage.

[0011] To achieve the above objectives, the present invention provides a surface-modified coal gangue aggregate concrete, comprising cement, fine aggregate, surface-modified coal gangue coarse aggregate, water-reducing agent, and water; the surface-modified coal gangue coarse aggregate is coal gangue coarse aggregate with an aluminum-substituted hydrated calcium silicate gel layer on the surface, the composition of the aluminum-substituted hydrated calcium silicate gel layer transitioning from the coal gangue body outwards in a gradient manner; the surface-modified coal gangue coarse aggregate is obtained by coating coal gangue coarse aggregate with a modified slurry, reacting in situ, and curing; by mass, the modified slurry comprises 100 parts of silicate cementitious material, 5-25 parts of pozzolanic active silica powder, 1-8 parts of alkaline etching component without soluble silicates, and 40-120 parts of water.

[0012] The core of this invention lies in the following: the alkaline etching component, which does not contain soluble silicates, dissolves the aluminosilicate clay minerals on the surface of coal gangue in situ within the strongly alkaline environment of the modified slurry, causing them to depolymerize and release active aluminum and active silicon. The released active aluminum and active silicon, along with active silicon provided by volcanic ash active silica powder and calcium ions provided by the hydration of silicate cementing materials, are in situ reprecipitated on the coal gangue surface as aluminum-substituted hydrated calcium silicate gel, forming a transformation layer with a gradient transition from the coal gangue bulk to the outside. Thus, the originally harmful, removed, or sealed surface clay minerals of the coal gangue are transformed in situ into an interfacial bonding phase, eliminating the discrete internal interface between the aggregate and the modified layer. Etching, a conventional method of material removal and weakening, achieves net densification of the interface through in-situ reprecipitation.

[0013] Furthermore, the alkaline etching component is selected from at least one of sodium hydroxide, potassium hydroxide, and sodium carbonate, and the pH of the modified slurry is 11.5 to 13.5, which causes the aluminosilicate clay minerals on the surface of the coal gangue to depolymerize and dissolve without significantly eroding the coal gangue body. Since the etching component does not contain soluble silicates, the active silicon in the conversion layer comes only from the silicon dissolved from the surface of the coal gangue and the active silica powder in volcanic ash. No external soluble silicates are introduced into the system, thereby fundamentally avoiding the alkaline silicate reaction and surface alkali bloom caused by the addition of soluble silicon.

[0014] Furthermore, the volcanic ash activated silica micropowder has a silica content of not less than 90% and a specific surface area of ​​not less than 15,000 m². 2 / kg of silica fume; the silica fume supplements active silicon, making the molar ratio of calcium to silicon and aluminum in the aluminum-substituted calcium silicate hydrate gel in the conversion layer 1.2 to 1.7. This aluminum-rich aluminum-substituted calcium silicate hydrate gel consumes free silicon in the system and absorbs alkali metal ions during its formation, giving the conversion layer the ability to inhibit both alkali-silicic acid reactions and efflorescence, thereby significantly improving the long-term freeze-thaw resistance and chemical corrosion resistance of concrete.

[0015] Furthermore, the alkaline etching component accounts for 1% to 8% of the silicate cementitious material by mass, and the thickness of the formed aluminum-substituted hydrated calcium silicate gel layer is 5 μm to 50 μm. On the one hand, the alkaline etching component provides a limited hydroxyl budget, and the dissolution terminates automatically as the hydroxyl groups are consumed. The total amount of surface clay minerals available for dissolution and depolymerization is constrained by the stoichiometry of this hydroxyl budget. Therefore, the average depth that can be achieved by dissolution is mainly determined by the amount of etching component and is basically unrelated to the reaction rate. This is the dominant factor limiting the depth. On the other hand, the in-situ redeposited aluminum-substituted hydrated calcium silicate gel forms a dense shell at the dissolution front, blocking the transport channels for further dissolution. Its main function is to inhibit local over-dissolution at highly reactive areas, make the dissolution front more uniform, and provide additional transport barriers when approaching the upper limit of depth. This is a secondary factor limiting the depth. Both mechanisms work together, with hydroxide ion budgeting as the primary mechanism and gel self-sealing as a secondary mechanism, to achieve dual self-limitation of the dissolution depth to the surface of coal gangue without damaging the aggregate itself, thus enabling depth control without the need for the addition of soluble calcium salts.

[0016] Furthermore, the modified slurry also includes triisopropanolamine, which accounts for 0.02% to 0.10% of the silicate cementitious material by mass. Triisopropanolamine selectively coordinates aluminum and iron ions dissolved from the surface of coal gangue without capturing calcium ions, inhibiting the precipitation of dissolved aluminum into hydrated calcium aluminate or aluminum hydroxide at locations far from the aggregate surface. It also directionally transports dissolved aluminum to the conversion layer and increases its degree of aluminum substitution, thereby enhancing the conversion layer's resistance to alkali-silicic acid reactions without weakening calcium ion supply or inhibiting the formation of hydrated calcium silicate.

[0017] Furthermore, the modified slurry also includes sodium gluconate, which accounts for 0.05% to 0.5% of the mass of the silicate cementitious material. Sodium gluconate extends the reaction window of the modified slurry on the surface of coal gangue and improves its wetting of the porous surface of coal gangue, enabling the transformation layer to grow uniformly and continuously in a gradient. It should be noted that the dosage of sodium gluconate is 0.05% to 0.5% of the mass of silicate cementitious material. It only moderately delays the reaction in time, but does not prevent the formation and self-sealing of gel. The gel will still form a dense shell as expected at the dissolution front, and does not change the hydroxide budget and reaction stoichiometry of the system. Therefore, its role is to distribute the equal amount of dissolution constrained by the hydroxide budget more uniformly in time and space. What is improved is the continuity and uniformity of the transformation layer, while the average depth reached by dissolution is still dominated by the hydroxide budget and remains basically unchanged. Sodium gluconate does not contain phosphonic acid groups and does not form stable insoluble chelates with calcium ions. Therefore, it does not inhibit the formation of hydrated calcium silicate, unlike organophosphonic acid components that strongly deprive calcium ions and hinder the formation of hydrated calcium silicate.

[0018] Furthermore, the modified slurry also includes N,N-bis(2-hydroxypropyl)glycine, which accounts for 0.1% to 0.8% of the silicate cementitious material by mass. The aminodiol groups of this component selectively coordinate to dissolve aluminum without abstracting calcium ions, while its carboxyl groups anchor to the etched coal gangue surface, providing retarding and wetting. This allows a single component to simultaneously achieve directional aluminum coordination, surface anchoring, and reaction window extension. It also bridges the aluminum-substituted hydrated calcium silicate gel in the conversion layer with a multi-toothed structure, thereby improving the interfacial toughness of the conversion layer.

[0019] Further, the particle size of the coal gangue coarse aggregate is 5 mm to 26.5 mm, of which the 5 mm to 10 mm particle size range accounts for 15 wt% to 30 wt%, and the 10 mm to 26.5 mm particle size range accounts for 70 wt% to 85 wt%; the mass ratio of silicate cementitious material to coal gangue coarse aggregate in the modified slurry is 0.05 to 0.15. By mass, the surface-modified coal gangue aggregate concrete comprises 330 to 380 parts cement, 680 to 740 parts fine aggregate, 980 to 1060 parts surface-modified coal gangue coarse aggregate, 1.5 to 2.5 parts water-reducing agent, and 150 to 180 parts water.

[0020] This invention also provides a method for preparing the above-mentioned surface-modified coal gangue aggregate concrete, including steps such as preparing a modified slurry, coating and etching coal gangue coarse aggregate, allowing it to stand until initial setting, curing to obtain surface-modified coal gangue coarse aggregate, and mixing concrete. This invention also provides applications of the above-mentioned surface-modified coal gangue aggregate concrete in road engineering, cold-region concrete engineering, hydraulic concrete engineering, or building materials for the resource utilization of mining solid waste.

[0021] Compared with existing technologies, the beneficial effects of this invention are as follows: First, it transforms the surface clay minerals of coal gangue, which are usually considered harmful and need to be removed or sealed, into an aluminum-substituted hydrated calcium silicate interface bonding phase that forms a gradient transition from the aggregate bulk to the outside. This eliminates the discrete internal interface between the aggregate and the modified layer, allowing the etching method, which weakens the material removal process, to achieve net densification of the interface through in-situ re-precipitation. Second, no soluble silicates are added throughout the process; the silicon source of the conversion layer comes from the self-dissolving silica and silica fume of the coal gangue surface, fundamentally avoiding the need for external addition. The alkali-silicic acid reaction and efflorescence induced by soluble silicon have better long-term durability compared with the modification route of adding sodium silicate; thirdly, the etching depth is self-limited to the surface by the dosage of alkaline etching components and the self-sealing of redeposition gel, and the modification depth can be controlled without adding soluble calcium salts, without damaging the aggregate body; fourthly, it simultaneously improves the compressive strength, splitting tensile strength and freeze-thaw durability of coal gangue aggregate concrete, and realizes the high-value utilization of coal gangue solid waste and the reduction of cement usage, which is in line with the development direction of green and low-carbon building materials.

[0022] From the perspective of reaction thermodynamics and kinetics, the construction of the conversion layer in this invention comprises three interconnected processes. First, there is the dissolution process, where the hydroxyl groups provided by the alkaline etching components depolymerize the aluminosilicate network of clay minerals on the surface of coal gangue. Aluminum enters the liquid phase as tetrahydroxyaluminate and silicon as silicate. The rate of this process increases with increasing hydroxyl concentration and decreases as hydroxyl groups are consumed. Second, there is the reprecipitation process, where the hydration of silicate cementitious materials continuously releases calcium ions and maintains high alkalinity in the liquid phase. The dissolved aluminum, silicon, and active silicon provided by silica fume reach supersaturation in the high-calcium microregions on the aggregate surface, resulting in in-situ precipitation of aluminum-substituted hydrated calcium silicate gel. Since the calcium ions mainly originate from cement hydration near the aggregate surface, precipitation preferentially occurs on the aggregate surface rather than in the slurry bulk, thus forming a gradient conversion layer on the aggregate surface with gradually changing aluminum content, continuous with the aggregate bulk. Thirdly, there is the self-sealing process. The dense gel that precipitates in the early stage covers the dissolution front, blocking the further transport of hydroxide ions and dissolved ions. Since the total amount of clay minerals available for dissolution is already constrained by the stoichiometry of the limited hydroxide ion budget, the upper limit of the dissolution depth is mainly set by this budget. On this basis, the gel self-sealing further inhibits local over-dissolution and makes the dissolution front converge evenly. Both are dominated by the hydroxide ion budget and secondary by the gel self-sealing, which together make the dissolution depth converge to the surface layer.

[0023] It is particularly important to note that the calcium ions in this invention originate solely from the hydration of silicate cementitious materials; no additional soluble calcium salts are introduced into the system. This is because, in a strongly alkaline environment, added soluble calcium salts immediately react with hydroxide ions to form calcium hydroxide precipitate and with carbonate ions to form calcium carbonate precipitate, failing to stably provide free calcium ions and disrupting the dissolution-reprecipitation sequence. In contrast, the calcium hydroxide provided by cement hydration coexists with the alkaline etching components in the same strongly alkaline system; both are alkaline and do not precipitate each other, thus maintaining a high-alkalinity dissolution environment while continuously providing a calcium source for reprecipitation. Therefore, this invention uses cement hydration calcium as the sole calcium source, combined with the limited hydroxide budget of the etching components and the self-sealing of the reprecipitated gel, achieving a dual self-limitation of dissolution depth, with a self-consistent mechanism that does not rely on added calcium salts.

[0024] The molecular components selected in this invention are all stable in a strongly alkaline environment, and their functions are limited to coordinating aluminum without depriving calcium, regulating the reaction window and wetting, and anchoring and bridging. They only act on the modified paste thin layer of aggregate pretreatment and do not enter the large-volume concrete. The amino group and three hydroxyl groups of triisopropanolamine have selective coordination ability for trivalent aluminum and iron ions, but very weak coordination for divalent calcium ions. Therefore, without reducing the calcium source or inhibiting the formation of hydrated calcium silicate, it inhibits the random precipitation of dissolved aluminum in the paste body and directs its transport to the aggregate surface conversion layer. Sodium gluconate is a polyhydroxycarboxylate that gently delays the reaction and improves wetting by adsorbing on the surface of hydration products, and does not contain phosphonic acid groups that form stable insoluble chelates with calcium ions. N,N-bis(2-hydroxypropyl)glycine coordinates aluminum with its aminodiol and anchors the etched surface with its carboxyl group. A single molecule has the above-mentioned directional coordination and retarding wetting functions, and forms a bridge in the conversion layer with a multidentate structure. The aforementioned selectivity is the fundamental difference between the molecular components of this invention and organophosphonic acid retarders that strongly scavenge calcium ions and inhibit the formation of hydrated calcium silicate.

[0025] In controlling the modification depth, the dosage of the alkaline etching component determines the total amount of hydroxide ions available for etching. Therefore, the upper limit of the average achievable etching depth is determined stoichiometrically. This upper limit mainly depends on the dosage and is largely independent of the reaction rate. When the dosage is 1% to 8% of the silicate cementitious material mass, the thickness of the resulting conversion layer is 5 μm to 50 μm. If the dosage is too low, the etching is insufficient, and the conversion layer is too thin and discontinuous. If the dosage is too high, the etching extends beyond the surface and penetrates into the aggregate bulk, weakening the aggregate strength. The limited hydroxide budget acts as the dominant factor in setting the upper limit of the depth, while the self-sealing effect of the reprecipitated gel at the etching front acts as a secondary factor in inhibiting local over-etching and making the front uniform. The combination of these two factors constitutes a dual self-limiting mechanism for depth, allowing the modification to precisely converge to the aggregate surface without the addition of soluble calcium salts. Therefore, it can be seen that components that extend the reaction window in time (such as sodium gluconate) do not change the hydroxide budget, and thus do not significantly increase the average dissolution depth. Instead, they make the distribution of equal amounts of dissolution constrained by the budget more uniform. This is consistent with the results in Example 4 described later, where the average thickness of the conversion layer remains basically unchanged (from about 27 μm to about 28 μm) while the continuity and freeze-thaw resistance are improved.

[0026] The essential difference between the conversion layer constructed in this invention and existing external coating layers lies in the gradient transition characteristics of its composition. Because the conversion layer is formed by in-situ dissolution and reprecipitation of the coal gangue surface, its inner side is continuously connected to the aluminosilicate of the coal gangue matrix, while its outer side is homogeneously connected to the hydrated calcium silicate of the cement mortar. The aluminum and calcium contents change continuously along the thickness direction, eliminating abrupt compositional interfaces and mechanical property abrupt changes, thus eliminating discrete interfaces where stress concentration and preferential crack propagation occur. Based on the synergistic effect of the aforementioned gradient interface and resistance to alkali-silicic acid reaction and efflorescence, the surface-modified coal gangue aggregate concrete of this invention can be applied to road engineering, cold-region concrete engineering, hydraulic concrete engineering, and building materials for the resource utilization of mining solid waste, fields requiring high interfacial strength, freeze-thaw resistance, and erosion resistance.

[0027] This invention exhibits good adaptability to different sources of coal gangue. While the types and contents of clay minerals, as well as the ratio of silica to alumina, vary among different mining areas, their surface layers are all rich in aluminosilicate clay minerals that can depolymerize under alkaline conditions. Therefore, by adjusting the type and dosage of the alkaline etching component, the dosage of silica fume, and the reaction window, an aluminum-substituted calcium silicate hydrate gradient conversion layer can be constructed in situ on the aggregate surface. For coal gangue with a high clay mineral content and a relatively loose surface layer, the dosage of the alkaline etching component and the dosage of silica fume can be appropriately increased; for denser coal gangue, these dosages can be reduced accordingly, ensuring that the conversion layer thickness and gel composition fall within the range defined by this invention.

[0028] This invention also has positive implications for reducing cement usage and carbon emissions. On the one hand, coal gangue coarse aggregate replaces natural crushed stone, thus disposing of large quantities of industrial solid waste and reducing the mining of natural aggregates. On the other hand, silica fume, as a pozzolanic active silica powder, participates in the construction of the conversion layer, partially replacing the cementitious function of cement. Furthermore, the improved interfacial properties reduce the amount of cementitious materials required to achieve the same strength. These factors collectively reduce the overall carbon emissions of concrete, aligning with the development direction of green and low-carbon building materials.

[0029] The preparation method of the present invention is simple and easy to operate. The coating and etching can be completed in conventional stirring equipment, and the curing can be carried out at room temperature and pressure. No special equipment or high temperature and high pressure conditions are required, which facilitates industrial implementation. Attached Figure Description

[0030] Figure 1 This is a process flow diagram of the method for preparing surface-modified coal gangue aggregate concrete according to the present invention;

[0031] Figure 2 This is a schematic diagram of the surface structure of the surface-modified coal gangue coarse aggregate of the present invention;

[0032] Figure 3This is a graph showing the relationship between the thickness of the conversion layer and the 28-day compressive strength of concrete under different alkaline etching component dosages according to the present invention.

[0033] Figure 4 This is a comparison chart showing the freeze-thaw resistance of concrete in the embodiments and comparative examples after 200 freeze-thaw cycles. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention. Unless otherwise specified, conditions in the following embodiments are performed under conventional conditions or conditions recommended by the manufacturer; all raw materials used are commercially available unless otherwise specified. The mechanical properties and durability data in the following embodiments and comparative examples are obtained under the same molding, standard curing, and testing conditions, and are comparable to each other; the compressive strength and splitting tensile strength are data at 28 days of age, the freeze-thaw resistance is the result of 200 freeze-thaw cycles using the rapid freezing method, and the water absorption rate and crushing index are the corresponding indicators of concrete made from modified aggregates.

[0035] The preparation process of the surface-modified coal gangue aggregate concrete is as follows: Figure 1 As shown, the process includes, in sequence, the preparation of modified slurry, coating and etching of coal gangue coarse aggregate, standing until the modified slurry initially sets, curing to obtain surface-modified coal gangue coarse aggregate, and mixing of concrete.

[0036] The raw materials used in the following examples and comparative examples are as follows. Coal gangue coarse aggregate: taken from washed coal gangue in coal mines, the main minerals are kaolinite, illite and quartz. After jaw crushing and screening, coarse aggregate with a particle size of 5mm to 26.5mm and an apparent density of about 2350kg / m³ is obtained. 3 The water absorption rate is approximately 4.8%, and the crushing index is approximately 18.5%. It should be dried to constant weight at 105℃ before use. Silicate cementitious material: P·O42.5 ordinary Portland cement, with a specific surface area of ​​approximately 360 m². 2 / kg. Volcanic ash activated silica micropowder: silica ash, silica mass content approximately 94%, specific surface area approximately 18000 m². 2 / kg. Alkaline etching components: sodium hydroxide, potassium hydroxide, or sodium carbonate, all analytical grade, free of soluble silicates. Triisopropanolamine, sodium gluconate, and N,N-bis(2-hydroxypropyl)glycine: all analytical grade, wherein N,N-bis(2-hydroxypropyl)glycine is prepared by N-alkylation of diisopropanolamine and chloroacetic acid in the presence of sodium hydroxide. The specific steps are as follows: Dissolve 0.50 mol diisopropanolamine in 250 mL of deionized water and cool in an ice bath; separately dissolve 0.50 mol chloroacetic acid in 100 mL of water and neutralize with 30% NaOH to pH 7-8 (to produce sodium chloroacetate); slowly add the sodium chloroacetate solution dropwise to the DIPA solution while simultaneously adding 30% NaOH to maintain the pH of the system at 9-11; heat to 70-80℃, stir and reflux for 5-6 hours, and monitor until the reaction is complete; if free acid is required, cool and adjust the pH to 2-3 with hydrochloric acid, concentrate under reduced pressure, and recrystallize from water-ethanol to obtain the product; when used in the alkaline modified slurry of this invention, it can be directly added in the form of sodium salt, and filtration is required or trace amounts of NaCl are permissible. Water-reducing agent: Polycarboxylate-based high-performance water-reducing agent, with a solid content of approximately 40% and a water reduction rate of approximately 28%. Fine aggregate: Natural medium sand, with a fineness modulus of approximately 2.6 and a mud content of less than 1%. Mixing water is tap water. Various mechanical and durability tests were conducted according to GB / T 50081 and GB / T 50082, respectively. The thickness of the aluminum-substituted hydrated calcium silicate gel layer was determined by scanning electron microscopy, the molar ratio of calcium to the sum of silicon and aluminum was determined by energy dispersive spectroscopy, and the alkali-silicic acid reaction expansion rate was determined by the rapid mortar bar method. This invention employs a two-stage process: first, coating and etching the coarse aggregate of coal gangue, then curing it, and finally mixing it into concrete. The purpose is to ensure that the in-situ dissolution and reprecipitation of the surface clay minerals of coal gangue by the alkaline etching components are fully completed within the modified slurry thin layer during the aggregate pretreatment stage. After the conversion layer is formed and cured to a stable state, it is then used in concrete. This ensures the gradient construction of the conversion layer, avoids the alkaline etching components from entering the large-volume concrete and affecting its setting and hydration, and also prevents the migration of soluble alkali in the hardened concrete. In the following examples, the time for standing until the modified slurry initially sets and the subsequent curing are all controlled according to the temperature and humidity conditions described in each example; the mass fractions are all calculated based on the same standard.

[0037] Example 1. The modified slurry for surface-modified coal gangue coarse aggregate was composed of 100 parts by weight of silicate cementitious material, 15 parts by weight of silica fume, 4.5 parts by weight of sodium hydroxide, 0.06 parts by weight of triisopropanolamine, and 90 parts by weight of water. Sodium hydroxide accounted for 4.5% of the mass of silicate cementitious material, and triisopropanolamine accounted for 0.06% of the mass of silicate cementitious material. In preparation, the above components were first stirred for 150 seconds to obtain the modified slurry. Then, dried coal gangue coarse aggregate was added to the modified slurry at a mass ratio of silicate cementitious material to coal gangue coarse aggregate of 0.10 and stirred for 200 seconds at a stirring temperature of 25°C, so that the modified slurry coated the surface of the coal gangue. Subsequently, the slurry was allowed to stand at 20±2°C and 60% relative humidity until initial setting, and then cured for 7 days at 20±2°C and 95% relative humidity to obtain surface-modified coal gangue coarse aggregate. Measurements showed that the thickness of the obtained aluminum-substituted hydrated calcium silicate gel conversion layer was approximately 27 μm, with a composition that gradients outward from the coal gangue bulk, and a molar ratio of calcium to the sum of silicon and aluminum of approximately 1.45. Figure 2 As shown, scanning electron microscopy revealed that no loose interface transition zone characteristic of the original coal gangue coarse aggregate was observed between the modified coal gangue and the cement mortar, and no discernible discrete interface was found between the conversion layer and the coal gangue matrix. Energy dispersive spectroscopy (EDS) showed that the aluminum-silicon ratio continuously decreased and the calcium-silicon ratio continuously increased from the coal gangue matrix outwards, confirming the gradient transition characteristics of the conversion layer composition. X-ray diffraction showed that the characteristic diffraction peaks of clay minerals in the conversion layer weakened, while the diffuse diffraction characteristics of hydrated calcium silicate increased. Concrete was obtained by mixing 165 parts by weight of water, 355 parts by weight of P·O42.5 cement, 710 parts by weight of medium sand, 1020 parts by weight of the above-mentioned surface-modified coal gangue coarse aggregate, and 2.0 parts by weight of polycarboxylate superplasticizer. The concrete was measured to have a 28-day compressive strength of 41.2 MPa, a 28-day splitting tensile strength of 3.15 MPa, a water absorption rate of 2.0%, and a crushing index of 12.5%. After 200 freeze-thaw cycles, the mass loss was 1.86%, and the relative dynamic modulus of elasticity was 90%.

[0038] Example 2. The difference between this example and Example 1 is that the modified slurry contains 5 parts silica fume and 1 part sodium hydroxide (1% of the silicate cementitious material mass), while the remaining components and process are the same. The resulting conversion layer thickness is approximately 5 μm, and the molar ratio of calcium to the sum of silicon and aluminum is approximately 1.20. The resulting concrete has a 28-day compressive strength of 37.6 MPa, a 28-day splitting tensile strength of 2.82 MPa, a water absorption rate of 2.9%, and a crushing index of 14.8%. After 200 freeze-thaw cycles, the mass loss is 2.80%, and the relative dynamic modulus of elasticity is 84%. This example corresponds to the lower limits of the alkaline etching component dosage, silica fume dosage, molar ratio, and conversion layer thickness; the performance is still significantly better than unmodified coal gangue concrete.

[0039] Example 3. The difference between this example and Example 1 is that the modified slurry contains 25 parts silica fume and 8 parts sodium hydroxide (accounting for 8% of the mass of silicate cementitious material), while the remaining components and processes are the same. The resulting conversion layer thickness is approximately 50 μm, and the molar ratio of calcium to the sum of silicon and aluminum is approximately 1.70. The resulting concrete has a 28-day compressive strength of 39.8 MPa, a 28-day splitting tensile strength of 3.02 MPa, a water absorption rate of 2.2%, and a crushing index of 13.0%. After 200 freeze-thaw cycles, the mass loss is 2.20%, and the relative dynamic modulus of elasticity is 87%. This example corresponds to the upper limit of the above parameters. Compared with Examples 1 and 2, it can be seen that there is an intermediate value within the range of 1% to 8% for the alkaline etching component to achieve the optimal overall performance.

[0040] Example 4. Based on Example 1, this example further adds 0.25 parts of sodium gluconate (0.25% of the silicate cementitious material mass) to the modified slurry, with the remaining components and process remaining the same. The resulting conversion layer thickness is approximately 28 μm, and the molar ratio of calcium to the sum of silicon and aluminum is approximately 1.46. The resulting concrete has a 28-day compressive strength of 41.4 MPa, a 28-day splitting tensile strength of 3.18 MPa, a water absorption rate of 1.9%, and a crushing index of 12.2%. After 200 freeze-thaw cycles, the mass loss is 1.70%, and the relative dynamic modulus of elasticity is 91%. Comparing Example 4 with Example 1, the average thickness of the conversion layer after adding sodium gluconate only changed from about 27 μm to about 28 μm, remaining essentially unchanged. This is because the average depth achievable through dissolution is mainly determined by the hydroxide budget provided by the alkaline etching component in a stoichiometric manner. Sodium gluconate only moderately delays the reaction in time and does not change the hydroxide budget, thus not significantly increasing the average dissolution depth. The role of sodium gluconate is to extend the reaction window and improve coating wetting, making the budget-constrained equal amount of dissolution more uniformly distributed on the porous surface of coal gangue, thereby making the conversion layer more continuous and dense. This is confirmed by the improvement in its water absorption rate, crushing index, and freeze-thaw resistance (mass loss decreased from 1.86% in Example 1 to 1.70%, and relative dynamic elastic modulus increased from 90% to 91%) compared to Example 1. It can be seen that the average thickness of the conversion layer remains basically unchanged while the continuity and durability are improved, which is completely consistent with the aforementioned dual self-limiting mechanism dominated by the hydroxide budget and secondary to gel self-sealing.

[0041] Example 5. This example differs from Example 1 in that 0.45 parts of N,N-bis(2-hydroxypropyl)glycine (0.45% of the silicate cementitious material mass) are used instead of triisopropanolamine in Example 1, and sodium gluconate is not added. The remaining components and process are the same. The resulting conversion layer thickness is approximately 27 μm, and the molar ratio of calcium to the sum of silicon and aluminum is approximately 1.45. The resulting concrete has a 28-day compressive strength of 41.3 MPa, a 28-day splitting tensile strength of 3.17 MPa, a water absorption rate of 1.9%, and a crushing index of 12.3%. After 200 freeze-thaw cycles, the mass loss is 1.80%, and the relative dynamic modulus of elasticity is 90%. It is evident that N,N-bis(2-hydroxypropyl)glycine alone can simultaneously perform the functions of aluminum directional coordination and retarding wetting, achieving an effect comparable to that of a two-component combination of triisopropanolamine and sodium gluconate.

[0042] Example 6. The difference between this example and Example 1 is that the alkaline etching component is replaced with an equal mass of potassium hydroxide instead of sodium hydroxide; the remaining components and process are the same. The resulting conversion layer thickness is approximately 26 μm, and the molar ratio of calcium to the sum of silicon and aluminum is approximately 1.44. The resulting concrete has a 28-day compressive strength of 40.8 MPa, a 28-day splitting tensile strength of 3.12 MPa, a water absorption rate of 2.1%, and a crushing index of 12.7%. After 200 freeze-thaw cycles, the mass loss is 2.00%, and the relative dynamic modulus of elasticity is 89%. It is evident that using potassium hydroxide as an alkaline etching component without soluble silicates can also achieve in-situ dissolution of surface clay minerals in coal gangue and the gradient construction of the conversion layer.

[0043] Example 7. The difference between this example and Example 1 is that the alkaline etching component is replaced with sodium carbonate of equal alkali equivalent, while the other components and processes are the same. The resulting conversion layer thickness is approximately 24 μm, and the molar ratio of calcium to the sum of silicon and aluminum is approximately 1.42. The resulting concrete has a 28-day compressive strength of 40.2 MPa, a 28-day splitting tensile strength of 3.08 MPa, a water absorption rate of 2.2%, and a crushing index of 13.0%. After 200 freeze-thaw cycles, the mass loss is 2.10%, and the relative dynamic modulus of elasticity is 88%. It can be seen that using sodium carbonate as an alkaline etching component without soluble silicates can also achieve in-situ dissolution of surface clay minerals in coal gangue and the construction of a gradient conversion layer. Sodium carbonate has a milder alkalinity, and the resulting conversion layer thickness is slightly less than that obtained by etching with sodium hydroxide.

[0044] Example 8. Based on Example 1, this example adds 0.30 parts of N,N-bis(2-hydroxypropyl)glycine to the modified slurry, i.e., triisopropanolamine and N,N-bis(2-hydroxypropyl)glycine are used together. The remaining components and process are the same. The resulting conversion layer thickness is approximately 29 μm, the molar ratio of calcium to the sum of silicon and aluminum is approximately 1.47, and the conversion layer is continuous and dense. The resulting concrete has a 28-day compressive strength of 41.3 MPa, a 28-day splitting tensile strength of 3.19 MPa, a water absorption rate of 1.8%, and a crushing index of 12.1%. After 200 freeze-thaw cycles, the mass loss is 1.68%, and the relative dynamic modulus of elasticity is 91%. It is evident that when aluminum-oriented coordination components are used together, the continuity and freeze-thaw resistance of the conversion layer can be maintained at a superior level.

[0045] Example 9. The difference between this example and Example 1 is that the mass ratio of silicate cementitious material to coal gangue coarse aggregate in the modified slurry is increased from 0.10 to 0.15, i.e., the amount of modified slurry is increased. The remaining components are the same as the process. The resulting conversion layer is approximately 30 μm thick, with a molar ratio of calcium to the sum of silicon and aluminum of approximately 1.45, and the conversion layer is continuous and dense. The resulting concrete has a 28-day compressive strength of 41.0 MPa, a 28-day splitting tensile strength of 3.16 MPa, a water absorption rate of 1.9%, and a crushing index of 12.3%. After 200 freeze-thaw cycles, the mass loss is 1.75%, and the relative dynamic modulus of elasticity is 90%. It can be seen that increasing the amount of modified slurry within the mass ratio of silicate cementitious material to coal gangue coarse aggregate of 0.05 to 0.15 can make the conversion layer more uniform and dense, and maintain excellent concrete performance.

[0046] Comparative Example 1: The sodium hydroxide in the modified slurry of Example 1, which does not contain soluble silicates, was replaced with sodium silicate of equal alkali equivalent. That is, the modification route of adding soluble silicates was adopted. The remaining components and processes were the same.

[0047] Comparative Example 2 omits sodium hydroxide in the modified slurry, i.e., only silicate cementitious materials, silica fume and water are used to prepare a coating slurry to coat the coarse aggregate of coal gangue, and the remaining components are the same as the process.

[0048] Comparative Example 3 omits triisopropanolamine from the modified slurry of Example 1, retaining only etching and silica fume replenishment, with the remaining components and processes being the same.

[0049] Comparative Example 4: Sodium hydroxide was omitted from the modified slurry, while triisopropanolamine was retained. That is, triisopropanolamine was added to the coating slurry as an additive without in-situ etching, and the remaining components were the same as the process.

[0050] Comparative Example 5: The sodium hydroxide content in the modified slurry of Example 1 was increased to 12 parts (12% of the mass of silicate cementitious material), exceeding the upper limit specified in this invention. The remaining components were the same as the process.

[0051] Comparative Example 6 omits the silica fume in the modified slurry of Example 1, using only the silicon dissolved from the etching components as the silicon source, with the remaining components and processes being the same.

[0052] The concrete properties of each comparative example are as follows. The concrete obtained in Comparative Example 1 has a 28-day compressive strength of 40.5 MPa and a 28-day splitting tensile strength of 3.20 MPa. However, the conversion layer is a product of residual sodium silicate film formation and filling, rather than a gradient transition layer from the bulk to the outside. Its thickness is approximately 18 μm, and the molar ratio of calcium to the sum of silicon and aluminum is approximately 1.10. After 200 freeze-thaw cycles, the mass loss is 3.50%, the relative dynamic modulus of elasticity is only 80%, and significant efflorescence appears on the surface. The rapid mortar bar method shows a 14-day expansion rate of 0.12%, exceeding the limit of 0.10%. The concrete obtained in Comparative Example 2 has a 28-day compressive strength of 36.5 MPa and a 28-day splitting tensile strength of 2.75 MPa. A discrete interface still exists between the aggregate and the mortar coating layer, and no conversion layer has formed. After 200 freeze-thaw cycles, the mass loss is 3.20%, and the relative dynamic modulus of elasticity is 79%. The concrete obtained in Comparative Example 3 had a 28-day compressive strength of 38.5 MPa and a 28-day splitting tensile strength of 2.92 MPa. The conversion layer thickness was approximately 20 μm, but its continuity and degree of aluminum substitution were low. The molar ratio of calcium to the sum of silicon and aluminum was approximately 1.30. After 200 freeze-thaw cycles, the mass loss was 2.90%, and the relative dynamic modulus of elasticity was 83%. The concrete obtained in Comparative Example 4 had a 28-day compressive strength of 31.5 MPa and a 28-day splitting tensile strength of 2.15 MPa. No conversion layer was formed, which was comparable to that of unmodified coal gangue concrete. This indicates that without in-situ etching, the addition of triisopropanolamine alone cannot improve the interface. The concrete obtained in Comparative Example 5 had a 28-day compressive strength of 33.0 MPa and a 28-day splitting tensile strength of 2.30 MPa. Excessive alkaline etching components caused the etching to penetrate deep into the aggregate matrix, increasing the conversion layer thickness to approximately 70 μm, but damaging the aggregate matrix. The molar ratio of calcium to the sum of silicon and aluminum was approximately 1.55. After 200 freeze-thaw cycles, the mass loss was 4.00%, and the relative dynamic modulus of elasticity was 75%. The concrete obtained in Comparative Example 6 had a 28-day compressive strength of 34.5 MPa and a 28-day splitting tensile strength of 2.50 MPa. Due to the lack of silica fume for silicon replenishment, the gel composition was unbalanced, the molar ratio of calcium to the sum of silicon and aluminum was approximately 1.00, and the conversion layer was unstable.

[0053] As a reference, natural aggregate concrete was prepared using the same mix proportion but replacing surface-modified coal gangue coarse aggregate with an equal volume of natural crushed stone. Its 28-day compressive strength was 42.0 MPa, its 28-day splitting tensile strength was 3.10 MPa, its mass loss after 200 freeze-thaw cycles was 1.0%, and its relative dynamic modulus of elasticity was 92%. Unmodified coal gangue concrete was prepared using unmodified coal gangue coarse aggregate. Its 28-day compressive strength was 30.5 MPa, its 28-day splitting tensile strength was 2.05 MPa, its water absorption rate was 4.8%, its crushing index was 18.5%, it failed after 150 freeze-thaw cycles, its mass loss after 200 freeze-thaw cycles was 4.6%, and its relative dynamic modulus of elasticity was 74%. The test data of the above examples and comparative examples are summarized in Table 1.

[0054] Table 1 Summary of performance test data for concrete in the examples and comparative examples

[0055] Sample 28-day compressive strength / MPa 28 d splitting tensile strength / MPa Water absorption rate / % Crushing index / % Conversion layer thickness / μm Ca / (Si+Al) Freeze-thaw mass loss / % Relative dynamic modulus / % Natural aggregate concrete 42.0 3.10 1.2 8.5 — — 1.0 92 Unmodified coal gangue concrete 30.5 2.05 4.8 18.5 0 — 4.6 74 Example 1 41.2 3.15 2.0 12.5 27 1.45 1.86 90 Example 2 37.6 2.82 2.9 14.8 5 1.20 2.80 84 Example 3 39.8 3.02 2.2 13.0 50 1.70 2.20 87 Example 4 41.4 3.18 1.9 12.2 28 1.46 1.70 91 Example 5 41.3 3.17 1.9 12.3 27 1.45 1.80 90 Example 6 40.8 3.12 2.1 12.7 26 1.44 2.00 89 Example 7 40.2 3.08 2.2 13.0 24 1.42 2.10 88 Example 8 41.3 3.19 1.8 12.1 29 1.47 1.68 91 Example 9 41.0 3.16 1.9 12.3 30 1.45 1.75 90 Comparative Example 1 40.5 3.20 2.2 13.0 18 1.10 3.50 80 Comparative Example 2 36.5 2.75 2.8 14.5 0 — 3.20 79 Comparative Example 3 38.5 2.92 2.4 13.8 20 1.30 2.90 83 Comparative Example 4 31.5 2.15 4.5 18.0 0 — 4.40 73 Comparative Example 5 33.0 2.30 3.5 16.5 70 1.55 4.00 75 Comparative Example 6 34.5 2.50 3.0 15.5 22 1.00 3.60 78

[0056] The necessity and synergistic effect of the various technical features of this invention are evident from the test data in Table 1. Comparing Example 1 and Comparative Example 1, their compressive strength and splitting tensile strength are similar. However, Comparative Example 1, due to the addition of soluble silicate, has residual free soluble silica in its system. After 200 freeze-thaw cycles, its relative dynamic elastic modulus is only 80%, with a mass loss of 3.50%, and it exhibits significant efflorescence and a high mortar rod expansion rate. In contrast, Example 1, without the addition of soluble silicate, uses an aluminum-rich aluminum-substituted hydrated calcium silicate gel that consumes free silica and absorbs alkali metal ions. Its relative dynamic elastic modulus is 90%, and its mass loss is only 1.86%, demonstrating significantly better long-term durability than Comparative Example 1. This indicates that the core advantage of this invention compared to the sodium silicate-added route lies in the alkali-resistant silicate reaction and efflorescence-resistant long-term durability achieved through alkaline etching without soluble silicate, rather than simply an increase in strength.

[0057] Comparing Example 1 with Comparative Example 2, omitting the alkaline etching component resulted in only a coating layer. A discrete interface remained between the aggregate and the coating layer, and no in-situ dissolution and reprecipitation of the surface clay minerals occurred. The compressive strength decreased from 41.2 MPa to 36.5 MPa, and the splitting tensile strength decreased from 3.15 MPa to 2.75 MPa. This indicates that the in-situ dissolution of the surface clay by the alkaline etching component and its subsequent reprecipitation into a gradient conversion layer is key to eliminating the discrete interface and improving interface strength. Comparing Example 1 with Comparative Example 3, omitting triisopropanolamine reduced the continuity and aluminum substitution degree of the conversion layer, and the relative dynamic modulus decreased from 90% to 83%. This indicates that triisopropanolamine strengthened the conversion layer by directional coordination dissolution of aluminum and increasing the aluminum substitution degree. Comparing Comparative Example 3 and Comparative Example 4, Comparative Example 3, by retaining the etching as the underlying method but omitting the molecular component of oriented aluminum, still achieved a compressive strength of 38.5 MPa. In contrast, Comparative Example 4, by omitting etching and retaining only triisopropanolamine, achieved a compressive strength of only 31.5 MPa, comparable to unmodified coal gangue concrete. Both examples demonstrate that alkaline etching as the underlying method is fundamental and a prerequisite, and that the molecular components can only function within the in-situ dissolution-reprecipitation framework established by etching. The performance improvement resulting from the combination of the two exceeds the sum of their individual contributions, exhibiting a significant nonlinear synergistic effect.

[0058] Comparing Example 1 with Comparative Example 5, when the alkaline etching component exceeded the upper limit of 12%, the etching penetrated deep into the aggregate matrix. Although the conversion layer thickened to approximately 70 μm, the aggregate matrix was damaged, and the compressive strength decreased from 41.2 MPa to 33.0 MPa. This indicates that the alkaline etching component dosage must be limited to 1%–8%, and the conversion layer thickness must be limited to 5 μm–50 μm to ensure that the etching depth is self-limited to the surface without damaging the aggregate matrix. Comparing Example 1 with Comparative Example 6, after omitting silica fume, the molar ratio of calcium gel to the sum of silicon and aluminum decreased to approximately 1.00. The gel composition became unbalanced, the conversion layer became unstable, and the compressive strength decreased to 34.5 MPa. This indicates that supplementing active silica with volcanic ash active silica powder and balancing the gel to an aluminum-substituted hydrated calcium silicate with a molar ratio of calcium to the sum of silicon and aluminum of 1.2–1.7 are necessary conditions for forming a stable gradient conversion layer. Furthermore, as can be seen from Examples 1, 2, and 3, as the alkaline etching component doping concentration increased from 1% to 4.5% to 8%, the conversion layer thickness increased from 5 μm to 27 μm to 50 μm, while the 28-day compressive strength first increased and then stabilized, achieving the optimal strength at the intermediate doping concentration. Figure 3 As shown in Examples 1, 4, and 5, the addition of sodium gluconate or N,N-bis(2-hydroxypropyl)glycine as a single component can further improve the continuity and freeze-thaw resistance of the conversion layer while maintaining mechanical properties. Figure 4As shown. Sodium gluconate only extends the reaction window without changing the hydroxide budget; therefore, the average thickness of the conversion layer in Example 4 remains essentially unchanged from Example 1 (approximately 27 μm becomes approximately 28 μm). The improvement lies in the continuity and uniformity of the conversion layer, consistent with the dual self-limiting mechanism dominated by the hydroxide budget and secondary by gel self-sealing. In summary, this invention, through alkaline etching without soluble silicates, silica fume-based silica-balanced gel composition, dual depth limiting through dosage and reprecipitation self-sealing, and the synergistic effect of directionally coordinated aluminum molecular components, transforms harmful clay minerals on the surface of coal gangue into a gradient-transition interfacial binder phase in situ. This simultaneously improves the mechanical properties and freeze-thaw resistance of coal gangue aggregate concrete without the addition of soluble silicates.

[0059] To quantitatively illustrate the contribution and synergy of each feature, a comparison was made using unmodified coal gangue concrete as the baseline. The 28-day compressive strength of unmodified coal gangue concrete was 30.5 MPa. Comparative Example 2, which only underwent slurry coating, had a strength of 36.5 MPa, an increase of 5.99 MPa. Comparative Example 4, which used only triisopropanolamine as an admixture without etching, had a strength of 31.5 MPa, an increase of 1.0 MPa. Example 1, which simultaneously incorporated etching, silica fume supplementation, and directional coordination of triisopropanolamine, had a strength of 41.2 MPa, an increase of 10.7 MPa compared to the unmodified baseline. It is evident that the strength increase of Example 1 is significantly greater than the sum of the individual contributions of slurry coating and triisopropanolamine admixture. The combination of the two resulted in an excess increase of approximately 3.71 MPa, indicating a significant nonlinear synergistic effect between the in-situ dissolution-reprecipitation framework established by etching and the molecular components of directionally coordinated aluminum, rather than a simple additive effect.

[0060] Regarding freeze-thaw durability, Example 1 maintained a relative dynamic modulus of elasticity of 90% and a mass loss of 1.86% after 200 freeze-thaw cycles, while the unmodified coal gangue concrete failed after only 150 freeze-thaw cycles, and its relative dynamic modulus of elasticity dropped to 74% and its mass loss reached 4.6% after 200 cycles. In terms of relative dynamic modulus of elasticity, Example 1 improved by approximately 21.6% compared to the unmodified baseline. The mechanism is that the conversion layer eliminates the loose interface transition zone on the surface of the coal gangue, which is a preferential area for freeze-thaw damage. The dense aluminum-substituted hydrated calcium silicate reduces the content of freezeable water and permeability at the interface. At the same time, the aluminum-rich gel absorbs alkali metal ions and inhibits alkali-silicic acid reaction, avoiding the exacerbation of damage by expansion products during freeze-thaw cycles. Compared with Comparative Example 1, which uses added soluble silicate, Example 1 has a relatively higher dynamic elastic modulus of about 10 percentage points and a lower mass loss of about 1.64 percentage points at similar strength, and there is no efflorescence or excessive expansion of mortar rods. This further confirms the decisive contribution of the technical feature of not adding soluble silicate to long-term durability.

[0061] As can be seen from the comparison of Examples 1, 6, and 7, the three alkaline etching components—sodium hydroxide, potassium hydroxide, and sodium carbonate—which do not contain soluble silicates, can all achieve in-situ dissolution of surface clay minerals and the construction of a gradient conversion layer. The resulting concrete properties are similar, with sodium carbonate having a slightly thinner conversion layer and slightly lower performance due to its milder alkalinity. As can be seen from the comparison of Examples 1, 4, 5, and 8, the molecular components such as triisopropanolamine, sodium gluconate, and N,N-bis(2-hydroxypropyl)glycine, which are stable in strong alkali and only coordinate aluminum without abstracting calcium, can improve the continuity of the conversion layer and freeze-thaw resistance while maintaining mechanical properties, whether used alone or in combination. This contrasts sharply with the use of organophosphonic acid retarders, which strongly abstract calcium ions and inhibit the formation of hydrated calcium silicate, thus hindering redeposition and weakening the conversion layer.

[0062] Regarding the aggregate's inherent properties, the unmodified coal gangue coarse aggregate has a water absorption rate of approximately 4.8% and a crushing index of approximately 18.5%. However, after modification according to this invention, taking Example 1 as an example, the corresponding concrete properties are improved, with the water absorption rate decreasing to 2.0% and the crushing index decreasing to 12.5%. This is because the dense aluminum-substituted calcium silicate hydrate conversion layer seals the open pores and microcracks on the surface of the coal gangue and enhances the mechanical integrity of the surface. In contrast, Comparative Examples 2 and 4, which did not undergo in-situ etching, did not have their surface pores effectively sealed, resulting in limited improvement in water absorption rate and crushing index. Based on the above embodiments and comparative examples, this invention, through the synergistic effect of four factors—alkaline etching without soluble silicates, silica fume-based silica-balancing gel composition, dual depth limits of admixture and reprecipitation self-sealing, and molecular components that coordinate aluminum without calcium depletion—transforms harmful clay minerals on the surface of coal gangue into an interfacial bonding phase with a compositional gradient transition in situ, without the addition of soluble silicates and soluble calcium salts. This simultaneously and stably improves the mechanical properties and freeze-thaw resistance of coal gangue aggregate concrete.

[0063] Furthermore, comparing Example 8 with Examples 1 and 5 reveals that when triisopropanolamine and N,N-bis(2-hydroxypropyl)glycine are used together, the former focuses on selectively coordinating and dissolving aluminum in the liquid phase and inhibiting its random precipitation, while the latter focuses on anchoring to the etched surface and providing retarded wetting and bridging. Their complementary effects result in a superior conversion layer in terms of continuity, density, and freeze-thaw resistance. After 200 freeze-thaw cycles, the mass loss is as low as 1.68%, and the relative dynamic elastic modulus reaches 91%, which is superior to the single-component examples 1 and 5. This indicates that the molecular component system of the present invention can be flexibly configured according to different requirements for the continuity and toughness of the conversion layer, and there is no mutual inhibition between the molecular components; none of them abstract calcium ions or hinder the redeposition of hydrated calcium silicate. Therefore, the beneficial effects of the various technical features and their combinations in the present invention are consistently verified in multiple examples, and a systematic comparison with the comparative examples fully supports the defined technical solution.

[0064] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. Surface-modified coal gangue aggregate concrete, characterized in that, The mixture includes cement, fine aggregate, surface-modified coal gangue coarse aggregate, water-reducing agent, and water. The surface-modified coal gangue coarse aggregate is coal gangue coarse aggregate with an aluminum-substituted hydrated calcium silicate gel layer on the surface. The composition of the aluminum-substituted hydrated calcium silicate gel layer transitions from the coal gangue body to the outside. The surface-modified coal gangue coarse aggregate is prepared by coating coal gangue coarse aggregate with a modified slurry, reacting in situ, and curing. By mass, the modified slurry includes 100 parts of silicate cementitious material, 5-25 parts of pozzolanic active silica powder, 1-8 parts of alkaline etching component without soluble silicate, and 40-120 parts of water.

2. The surface-modified coal gangue aggregate concrete according to claim 1, characterized in that, The alkaline etching component that does not contain soluble silicates is selected from at least one of sodium hydroxide, potassium hydroxide, and sodium carbonate; the pH of the modified slurry is 11.5 to 13.

5.

3. The surface-modified coal gangue aggregate concrete according to claim 2, characterized in that, The volcanic ash activated silica micropowder has a silica content of not less than 90% and a specific surface area of ​​not less than 15,000 m². 2 / kg of silica fume; the molar ratio of calcium to the sum of silicon and aluminum in the aluminum-substituted hydrated calcium silicate gel layer is 1.2 to 1.

7.

4. The surface-modified coal gangue aggregate concrete according to claim 3, characterized in that, The alkaline etching component accounts for 1% to 8% of the mass of the silicate cementitious material; the thickness of the aluminum-substituted hydrated calcium silicate gel layer is 5 μm to 50 μm.

5. The surface-modified coal gangue aggregate concrete according to claim 4, characterized in that, The modified slurry also includes triisopropanolamine, which accounts for 0.02% to 0.10% of the mass of the silicate cementitious material.

6. The surface-modified coal gangue aggregate concrete according to claim 5, characterized in that, The modified slurry also includes sodium gluconate, which accounts for 0.05% to 0.5% of the mass of the silicate cementitious material.

7. The surface-modified coal gangue aggregate concrete according to claim 4, characterized in that, The modified slurry also includes N,N-bis(2-hydroxypropyl)glycine, wherein the N,N-bis(2-hydroxypropyl)glycine accounts for 0.1% to 0.8% of the mass of the silicate cementitious material.

8. The surface-modified coal gangue aggregate concrete according to claim 1, characterized in that, The coarse aggregate of the coal gangue has a particle size of 5mm to 26.5mm, of which the 5mm to 10mm particle size range accounts for 15wt% to 30wt% and the 10mm to 26.5mm particle size range accounts for 70wt% to 85wt%; the mass ratio of silicate cementitious material to coarse aggregate of coal gangue in the modified slurry is 0.05 to 0.15; by mass parts, the surface-modified coal gangue aggregate concrete includes 330 to 380 parts of cement, 680 to 740 parts of fine aggregate, 980 to 1060 parts of surface-modified coal gangue coarse aggregate, 1.5 to 2.5 parts of water-reducing agent and 150 to 180 parts of water.

9. The method for preparing surface-modified coal gangue aggregate concrete according to any one of claims 1 to 8, characterized in that, The process includes the following steps: S1, weighing silicate cementitious material, volcanic ash active silica powder, an alkaline etching component free of soluble silicates, and water according to the stated mass proportions, and stirring to obtain a modified slurry; S2, adding dried coal gangue coarse aggregate to the modified slurry and stirring for 120s to 300s at a temperature of 15℃ to 40℃ to obtain coal gangue coarse aggregate coated with the modified slurry; S3, adding the coal gangue obtained in step S2... The coarse aggregate is left to stand at a temperature of 20±2℃ and a relative humidity of not less than 50% until the modified paste has initially set; S4, the coal gangue coarse aggregate obtained in step S3 is cured at a temperature of 20±2℃ and a relative humidity of not less than 95% for 3 to 14 days to obtain surface-modified coal gangue coarse aggregate; S5, water, cement, fine aggregate, the surface-modified coal gangue coarse aggregate and water-reducing agent are mixed to obtain surface-modified coal gangue aggregate concrete.

10. The application of the surface-modified coal gangue aggregate concrete according to any one of claims 1 to 8 in road engineering, cold region concrete engineering, hydraulic concrete engineering or building materials for the resource utilization of mining solid waste.

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