Fly ash-based lightweight thermal insulation concrete and method for preparing the same

Abstract

The present application relates to the technical field of lightweight thermal insulation concrete, in particular to a lightweight thermal insulation concrete based on fly ash and a preparation method thereof, which is prepared from the following raw materials: cement clinker, modified fly ash microbeads, ceramsite, nano silica fume, redispersible emulsion powder, polypropylene fiber, polycarboxylate superplasticizer, composite foam stabilizer, synthetic foaming agent and deionized water; the present application sequentially sets gradient activation grinding, hydroxyl activation grafting, core-shell coating and low-temperature whisker growth modification processes for fly ash, gradient activation grinding is matched with triethanolamine grinding aid, fly ash original particle agglomerates are broken by means of segmented variable speed grinding, the dense glass body surface layer of fly ash is broken, and the silicon-aluminum active sites in the internal particles are exposed; the opening of the inert chemical bonds on the surface of fly ash by means of alkaline activation liquid, and the entry of organic functional groups on the surface of the powder by means of silane coupling agent improve the hydrophilic adaptability of fly ash particles from the molecular level.

Description

Technical Field

[0001] This invention relates to the field of lightweight thermal insulation concrete technology, specifically to a lightweight thermal insulation concrete based on fly ash and its preparation method. Background Technology

[0002] With the continuous advancement of the solid waste resource utilization industry in China, coal-fired power plants continuously generate massive amounts of fly ash solid waste. The traditional landfill disposal method not only occupies a large area of ​​land resources, but the soluble salts in fly ash can also easily cause soil and groundwater pollution due to rainwater seepage. Therefore, incorporating fly ash into building insulation concrete has become the mainstream disposal method in the industry. In conventional processes, most fly ash is directly ground and mixed into concrete. However, the surface of raw fly ash is dense and smooth with few active sites, resulting in low interfacial bonding strength with cement clinker hydration products. After being incorporated into concrete, interfacial gaps are prone to appear, ultimately leading to common problems such as decreased mechanical strength, excessive drying shrinkage, and disordered internal pore structure in the finished insulation concrete.

[0003] Existing commercially available lightweight insulating concrete formulation systems mostly rely on conventional foaming components to achieve pore formation in the slurry. They lack matching foam stabilizing components to coordinate and regulate the stability of bubbles. During the foaming and molding stage, bubbles are prone to merging and collapsing, resulting in a high proportion of interconnected pores inside the finished product. This directly increases the dry density of the concrete, leading to an increase in the thermal conductivity of the material. Consequently, the thermal insulation performance fails to meet the performance standards for external wall insulation projects.

[0004] Meanwhile, existing fly ash treatment methods are mostly limited to mechanical grinding or simple acid-base activation, which can only superficially break the glass phase shell of fly ash and cannot optimize the surface morphology of fly ash particles at the microscopic level. The lack of anchoring structure between the filler and the cementitious matrix makes it difficult to give full play to the advantages of fly ash admixture, which restricts the widespread application of fly ash-based lightweight thermal insulation concrete in thermal insulation components, roof insulation layers and other scenarios. Summary of the Invention

[0005] The purpose of this invention is to provide a lightweight thermal insulation concrete based on fly ash and its preparation method.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A lightweight thermal insulation concrete based on fly ash is prepared from the following raw materials in parts by weight: 100-120 parts cement clinker, 60-80 parts modified fly ash microspheres, 30-50 parts ceramsite, 10-15 parts nano silica fume, 5-8 parts redispersible latex powder, 2-4 parts polypropylene fiber, 1-3 parts polycarboxylate superplasticizer, 0.5-1.5 parts composite foam stabilizer, 0.8-1.2 parts synthetic foaming agent, and 40-55 parts deionized water. The modified fly ash microspheres were prepared using a modification process involving gradient activation grinding, surface hydroxyl activation-room-temperature covalent grafting, atmospheric pressure core-shell coating, and low-temperature directional whisker growth.

[0007] As a further technical solution, the ceramsite has a particle size of 5-10mm and a bulk density of ≤300kg / m³. 3 The nano-silica particles have a diameter of 20-50 nm and a silica purity of ≥98%.

[0008] As a further technical solution, the polypropylene fiber has a length of 6-12 mm and a diameter of 20-40 μm.

[0009] As a further technical solution, the alkali-resistant composite foam stabilizer is prepared by compounding stearic acid and potassium silicate in a mass ratio of 1:4; the alkali-resistant synthetic foaming agent is a fatty alcohol polyoxyethylene ether sodium sulfate anionic foaming agent.

[0010] As a further technical solution, the gradient activation grinding process of the modified fly ash microspheres is as follows: select Grade I raw fly ash, add 0.5-1% of triethanolamine grinding aid by weight of fly ash, grind in stages under high-purity nitrogen protection, pre-grind at low speed of 200r / min for 2min to break up powder agglomeration, and finely grind at high speed of 600r / min for 8-10min to obtain ultrafine fly ash powder with activated surface micro-defects.

[0011] As a further technical solution, the surface hydroxyl activation and silane covalent grafting process is as follows: First, prepare a weakly alkaline sodium hydroxide activation solution with pH 11-12. Mix ultrafine fly ash with the activation solution at a solid-liquid ratio of 1:3-5 g / mL, and ultrasonically disperse at 300W for 30 minutes to activate the inert glassy body on the fly ash surface and generate a large number of active silanol groups. Filter and drain at room temperature for later use. Then, prepare a silane hydrolysis modification solution, which consists of anhydrous ethanol, deionized water, glacial acetic acid, and KH-570 silane coupling agent. The volume ratio of ethanol to water is 3:1. Adjust the pH to 3.5-4.0 with glacial acetic acid. Add 3-5% KH-570 by weight of ultrafine fly ash and stir at low speed at room temperature for 20 minutes to complete the controlled hydrolysis. The activated fly ash powder was immersed in the hydrolysis modification solution. The system temperature was controlled at 40-50℃ and the mixture was stirred at 200-300r / min for 2 hours to allow the hydrolyzed hydroxyl groups of silane to undergo dehydration and covalent bonding with the active hydroxyl groups on the fly ash surface. After the reaction was completed, the mixture was filtered and dried at 105℃ to constant weight to obtain highly stable silane-grafted fly ash.

[0012] As a further technical solution, the ambient temperature and pressure core-shell coating process is as follows: prepare a 5-6% sodium alginate aqueous solution, and mix silane-grafted fly ash with the solution at a solid-liquid ratio of 1:4-5 g / mL, stirring for 20 minutes to ensure uniform mixing; add an 8% calcium chloride crosslinking solution, with the volume of the calcium chloride crosslinking solution being 25%-30% of the volume of the sodium alginate aqueous solution, adding it at a uniform rate for 15 minutes, and then crosslinking and curing at ambient temperature and pressure for 40 minutes; and then gradually air-dry at 25℃ and 55% relative humidity for 8 hours to form a dense and continuous flexible calcium alginate gel buffer layer on the surface of the fly ash, eliminating the defect of abrupt changes in the interfacial modulus of the multilayer structure.

[0013] As a further technical solution, the low-temperature directional whisker growth process is as follows: a total ion concentration of 0.8 mol / L and Mg... 2+ SiO3 2- A magnesium silicate precursor solution with a molar ratio of 1:1.2, containing magnesium chloride hexahydrate and anhydrous sodium silicate, with 0.1 mol / L sodium bicarbonate added as a pH buffer and ion penetration aid; fly ash microspheres coated with the gel layer with a solid-liquid ratio of 1:6-8 g / mL to the precursor solution, maintaining a stable pH of 9-10, and undergoing a low-temperature, normal-pressure hydrothermal reaction at 120-130℃ for 6-8 hours; sodium bicarbonate weakens the ion barrier properties of calcium alginate gel, promotes uniform ion penetration, and allows magnesium silicate whiskers to grow in situ on the gel layer surface, forming a gradient composite interface that is soft inside and rigid outside.

[0014] A method for preparing lightweight thermal insulation concrete based on fly ash includes the following steps: S1. Powder premixing: Weigh cement clinker, modified fly ash microspheres, and nano silica fume according to the proportion, put them into the mixing equipment, dry mix at 350-550r / min for 4min to eliminate powder agglomeration and obtain a uniform dry powder mixture. S2. Aggregate fiber blending: Sprinkle the ceramsite and polypropylene fiber evenly into the dry powder mixture, and stir at a low speed of 200r / min for 3min to avoid fiber agglomeration and damage to the ceramsite pores. S3. Functional liquid foaming modification: Polycarboxylate superplasticizer, composite foam stabilizer and synthetic foaming agent are dissolved in deionized water in sequence and stirred at room temperature for 2 minutes to obtain functional mixture; the mixture is poured into the mixture and stirred at high speed of 900 r / min for 6 minutes, the ambient temperature is controlled at 20-25℃ and the relative humidity is ≥60%, and the final pH of the slurry is adjusted to be stable at 10.5-11.5 to achieve uniform foaming; S4. Gradient curing during pouring: The foamed mixture is poured into the mold at a uniform speed and left to stand for 2 hours to set; a film covering for moisture retention is adopted for curing, with a total curing period of 28 days, to obtain finished concrete.

[0015] As a further technical solution, the moisturizing and maintenance is as follows: in the early stage, maintain at room temperature of 20-25℃ and relative humidity of ≥90% for 7 days, and in the later stage, maintain at constant temperature of 20-25℃ and relative humidity of 95%-98% for 21 days.

[0016] Compared with the prior art, the beneficial effects of the present invention are: This invention employs a four-stage modification process for fly ash, consisting of gradient activation grinding, hydroxyl activation grafting, core-shell coating, and low-temperature whisker growth. Gradient activation grinding, combined with a small amount of triethanolamine grinding aid, utilizes segmented variable-speed grinding to break down the original fly ash particle agglomerates, shattering the dense glassy surface and exposing the latent silica-alumina active sites within the particles. Hydroxyl activation and silane covalent grafting, aided by an alkaline activating solution, open the inert chemical bonds on the fly ash surface. Then, an organic functional group is introduced onto the powder surface using a silane coupling agent, improving the hydrophilic compatibility of the fly ash particles at the molecular level. Sodium alginate core-shell coating forms a flexible organic gel layer on the outer layer of the fly ash, buffering the volumetric stress during cement hydration and preventing microcracks at the filler-cement interface. Low-temperature in-situ grown magnesium silicate whiskers protrude from the fly ash particle surface, forming a microscopic anchor structure that effectively enhances the intercalation and adhesion between fly ash and cement hydration products. In the formulation, polypropylene fibers are randomly and interwoven within the concrete matrix. Individual fibers can pull and impede the micro-cracks in the hardened matrix, limiting crack extension and expansion. The composite foam stabilizer and synthetic foaming agent each play their respective roles. The foam stabilizer is adsorbed on the surface of the bubble liquid film to improve the toughness of the liquid film, while the foaming component decomposes stably to generate closed micropores. The nano-silica fume particles are small in size and can fill the fine gaps generated by cement hydration, refining the pore structure inside the matrix. As a lightweight aggregate, ceramsite reduces the overall self-weight of concrete by relying on its hollow structure. The polycarboxylate superplasticizer improves the fluidity of the slurry under low water-cement ratio conditions and reduces the interconnected large pores left by the evaporation of excess free water.

[0017] 2. The modification processes are seamlessly integrated to form a complete modification scheme. The preceding grinding and alkali activation processes open up the surface active structure of fly ash, providing reaction attachment sites for subsequent silane grafting, gel coating, and in-situ whisker growth. The four modification processes are progressively implemented to transform fly ash from an inert filler to an active functional filler. Modified fly ash microspheres are used in combination with nano-silica fume. The coarse and fine particles form a multi-level gradation filling system. The nano-silica fume fills the gaps between the modified fly ash particles, further densifying the microstructure of the concrete matrix. The two work together to reduce the amount of cement clinker used while ensuring the density of the matrix. A composite foam stabilizer and a synthetic foaming agent form a foaming compound system. The foam stabilizer locks in the tiny pores generated by the foaming agent. Together, they significantly reduce the probability of bubble breakage and merging, resulting in a significant increase in the proportion of closed-cell insulation pores inside the concrete. This optimizes insulation performance while ensuring lightweighting. Polypropylene fibers are uniformly dispersed in a cementitious matrix composed of modified fly ash and cement clinker. The modified fly ash anchors the fiber ends firmly with surface whiskers, and the fibers, in turn, constrain the volume deformation caused by the hydration shrinkage of the fly ash. Together, they reduce the drying shrinkage rate of the concrete. A polycarboxylate superplasticizer optimizes the amount of water used in the slurry mixing. The reduction of excess free water prevents the formation of interconnected pores due to subsequent water evaporation, and works synergistically with the foaming compound system to stabilize the cell morphology. The combination of modified fly ash, lightweight ceramsite, fibers, and foaming additives forms a composite synergistic system of "lightweight aggregate weight reduction, active filler reinforcement, fiber crack prevention, and additive foam stabilization," achieving easy foaming of the slurry, stable molding dimensions, and closed-cell structure.

[0018] 3. With the synergistic effect of the modified process and formulation components, the final fly ash-based lightweight insulating concrete possesses the comprehensive advantages of low density, low thermal conductivity, high compressive strength, and low drying shrinkage. From a product performance perspective, the finished dry density can be controlled at 630 kg / m³. 3 Within this range, the thermal conductivity is significantly better than that of traditional fly ash-blended insulating concrete, meeting the requirements for lightweight building envelope construction while reducing building heating energy consumption through superior thermal insulation performance. The material's 28-day compressive strength remains stable above 3.8 MPa, overcoming the shortcomings of traditional lightweight insulation materials, such as low strength and inability to be used as a standalone integrated structural insulation component. Simultaneously, the drying shrinkage rate is significantly reduced, making it less prone to cracking and delamination over long-term use, thus extending the service life of the insulation layer. From a resource utilization and industrial perspective, this invention enables the large-scale disposal of waste fly ash industrial solid waste, significantly reducing the amount of cement clinker used. This solves the environmental problem of fly ash pollution from stockpiling and reduces the production cost of concrete raw materials. The entire preparation process is mild and easy to implement for mass production, requiring no stringent high-temperature and high-pressure equipment. It is compatible with the modification of existing concrete mixing plant production lines, broadening the resource utilization path of fly ash in the field of building insulation and demonstrating outstanding economic and environmental benefits. Attached Figure Description

[0019] Figure 1 This is a comparison chart of the 28-day compressive strength of the example and the comparative example. Detailed Implementation

[0020] 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.

[0021] This invention provides a lightweight thermal insulation concrete based on fly ash, which is prepared by mixing cement clinker, modified fly ash microspheres, ceramsite, nano silica fume, redispersible latex powder, polypropylene fiber, polycarboxylate superplasticizer, composite foam stabilizer, synthetic foaming agent, and deionized water in a specified ratio; this invention also provides a corresponding method for preparing the lightweight thermal insulation concrete.

[0022] In this invention, the raw materials preferably consist of the following parts by weight: 100-120 parts cement clinker, 60-80 parts modified fly ash microspheres, 30-50 parts ceramsite, 10-15 parts nano silica fume, 5-8 parts redispersible latex powder, 2-4 parts polypropylene fiber, 1-3 parts polycarboxylate superplasticizer, 0.5-1.5 parts composite foam stabilizer, 0.8-1.2 parts synthetic foaming agent, and 40-55 parts deionized water. The cement clinker is preferably ordinary silicate cement clinker, which can be obtained from conventional building material manufacturers. The modified fly ash microspheres are preferably prepared through four modification processes: gradient activation grinding, surface hydroxyl activation-room temperature covalent grafting, atmospheric pressure core-shell coating, and low-temperature directional whisker growth. The raw material is Class I unprocessed fly ash from power plants. The ceramsite preferably has a particle size of 5-10 mm and a bulk density ≤300 kg / m³. 3 Lightweight shale ceramsite, preferably with a particle size of 6-9 mm and a bulk density of 220-280 kg / m³. 3 The preferred nano-silica fume is high-purity nano-silica fume with a particle size of 20-50 nm and a silica purity of ≥98%, with a more preferred particle size of 25-45 nm and a silica purity of ≥99%. The preferred polypropylene fiber is engineering-grade short polypropylene fiber with a length of 6-12 mm and a diameter of 20-40 μm, with a more preferred fiber length of 7-11 mm and a more preferred fiber diameter of 25-35 μm. The preferred composite foam stabilizer is prepared by compounding stearic acid and potassium silicate in a mass ratio of 1:4; the preferred synthetic foaming agent is sodium fatty alcohol polyoxyethylene ether sulfate anionic foaming agent. Both additives can be directly purchased from chemical raw material manufacturers.

[0023] The preferred gradient activation grinding process for the modified fly ash microspheres of the present invention is as follows: select Grade I raw fly ash, add 0.5-1% of triethanolamine grinding aid by weight of fly ash, perform segmented grinding under high-purity nitrogen protection, pre-grind at low speed of 200 r / min for 2 min to break up powder agglomeration, and finely grind at high speed of 600 r / min for 8-10 min to obtain ultrafine fly ash powder; the amount of triethanolamine added is more preferably 0.6%-0.9% of fly ash weight, and the high-speed fine grinding time is more preferably 8.5-9.5 min.

[0024] The preferred method for surface hydroxyl activation and silane covalent grafting of the present invention is as follows: First, prepare a weakly alkaline sodium hydroxide activation solution with a pH of 11-12. Mix ultrafine fly ash with the activation solution at a solid-liquid ratio of 1:3-5 g / mL, ultrasonically disperse at 300W for 30 min, filter, and drain at room temperature for later use. Next, prepare a silane hydrolysis modification solution, which consists of anhydrous ethanol, deionized water, glacial acetic acid, and KH-570 silane coupling agent. The volume ratio of ethanol to water is 3:1. Adjust the pH to 3.5-4.0 with glacial acetic acid. Add 3-5% (by weight of ultrafine fly ash) of KH-570 and perform low-speed hydrolysis at room temperature. Controlled hydrolysis was completed by stirring for 20 minutes; the activated fly ash powder was immersed in the hydrolysis modification solution, and the system temperature was controlled at 40-50℃ and the reaction was carried out at a constant temperature of 200-300 r / min for 2 hours; after the reaction was completed, the mixture was filtered and dried at 105℃ to constant weight to obtain silane-grafted fly ash; the pH was preferably 11.2-11.8, the solid-liquid ratio was preferably 1:3.5-4.5 g / mL, the KH-570 addition amount was preferably 3.5-4.5% of the mass of ultrafine fly ash, the reaction temperature was preferably 42-48℃, and the stirring speed was preferably 220-280 r / min.

[0025] The preferred method for the ambient temperature and pressure core-shell coating process of this invention is as follows: prepare a 5-6% (w / w) sodium alginate aqueous solution, with a solid-liquid ratio of silane-grafted fly ash to the solution of 1:4-5 g / mL, and stir for 20 min to mix evenly; add an 8% (w / w) calcium chloride crosslinking solution, with the volume of the calcium chloride crosslinking solution being 25%-30% of the volume of the sodium alginate aqueous solution, and add it dropwise at a uniform rate for 15 min; crosslink and cure at ambient temperature and pressure for 40 min; and then air dry at 25℃ and 55% relative humidity for 8 h. The preferred concentration of the sodium alginate aqueous solution is 5.2-5.8%, the preferred solid-liquid ratio is 1:4.2-4.8 g / mL, and the preferred volume percentage of the calcium chloride solution is 26%-29%.

[0026] The preferred low-temperature directional whisker growth process of this invention involves preparing a total ion concentration of 0.8 mol / L and Mg... 2+ SiO3 2-A magnesium silicate precursor solution with a molar ratio of 1:1.2, containing magnesium chloride hexahydrate and anhydrous sodium silicate, with 0.1 mol / L sodium bicarbonate added as a pH buffer and ion penetration aid; fly ash microspheres coated with a gel layer with a solid-liquid ratio of 1:6-8 g / mL to the precursor solution, a stable pH of 9-10, and a hydrothermal reaction at 120-130℃ and normal pressure for 6-8 h; magnesium silicate whiskers are grown in situ on the surface of the gel layer; the solid-liquid ratio is more preferably 1:6.5-7.5 g / mL, the system pH is more preferably 9.2-9.8, the hydrothermal temperature is more preferably 122-128℃, the hydrothermal duration is more preferably 6.5-7.5 h, and the whisker size is more preferably 3.5-4.5 μm in length and 0.25-0.45 μm in diameter.

[0027] The present invention also provides a method for preparing the above-mentioned lightweight thermal insulation concrete based on fly ash, preferably comprising the following steps: S1. Powder premixing: Weigh cement clinker, modified fly ash microspheres, and nano silica fume according to the proportion, put them into the mixing equipment, and dry mix at 350-550 r / min for 4 min to obtain a uniform dry powder mixture; the mixing speed is more preferably 400-500 r / min.

[0028] S2. Aggregate and fiber blending: Sprinkle the ceramsite and polypropylene fiber evenly into the dry powder mixture and stir at a low speed of 200r / min for 3min.

[0029] S3. Functional liquid foaming modification: Polycarboxylate superplasticizer, composite foam stabilizer and synthetic foaming agent are dissolved in deionized water in sequence and stirred at room temperature for 2 min to obtain functional mixture; the mixture is poured into the mixture and stirred at high speed of 900 r / min for 6 min, the ambient temperature is controlled at 20-25℃ and the relative humidity is ≥60%, and the final pH of the slurry is adjusted to be stable at 10.5-11.5; the ambient temperature is more preferably 21-24℃, the relative humidity is more preferably ≥65%, and the final pH of the slurry is more preferably 10.8-11.2.

[0030] S4. Gradual Curing During Pouring: The foamed mixture is poured into the mold at a uniform speed and allowed to stand for 2 hours to set. A film covering for moisture retention curing is adopted, with a total curing cycle of 28 days. The preferred moisture retention curing method is: 7 days of room temperature curing at 20-25℃ and relative humidity ≥90% in the early stage, and 21 days of constant temperature and humidity curing at 20-25℃ and relative humidity 95%-98% in the later stage. The preferred curing temperature is 21-24℃, the preferred relative humidity in the early stage is ≥92%, and the preferred relative humidity in the later stage is 96%-97%.

[0031] This invention uses waste fly ash as the main modified filler, optimizing the surface structure of the fly ash through a four-stage modification process. Combined with specific proportions of raw materials and a step-by-step preparation process, the resulting concrete is lightweight, high-strength, and exhibits excellent thermal insulation properties, achieving the resource utilization of solid waste. The entire preparation process is simple to operate, has good molding stability, and utilizes widely available raw materials, effectively reducing concrete production costs. Compared to existing conventional lightweight insulating concrete, this invention features in-situ growth of magnesium silicate whiskers on the surface of modified fly ash microspheres, enhancing the interfacial bonding between the powder and the cement matrix. Combined with a composite foam stabilizer and a dedicated anionic foaming agent, the concrete exhibits fine and uniform internal pores, balancing compressive strength and thermal insulation coefficient, overcoming the shortcomings of traditional fly ash-added concrete, such as poor thermal insulation, low strength, and large volume shrinkage.

[0032] To further illustrate the technical effects of the present invention, the following experiments were conducted through examples and comparative examples. The raw materials used in each example and comparative example were of the same source: P.O52.5 silicate cement clinker was selected; Grade I unprocessed fly ash was taken from dry fly ash from a thermal power plant; shale ceramsite, nano silica fume, polypropylene fiber, and various chemical additives were all commercially available industrial products.

[0033] Example 1: Raw material weight ratio: The composition includes 100 parts cement clinker, 60 parts modified fly ash microspheres, 30 parts ceramsite, 10 parts nano silica fume, 5 parts redispersible latex powder, 2 parts polypropylene fiber, 1 part polycarboxylate superplasticizer, 0.5 parts composite foam stabilizer, 0.8 parts synthetic foaming agent, and 40 parts deionized water. The composite foam stabilizer is a mixture of stearic acid and potassium silicate in a 1:4 mass ratio; the synthetic foaming agent is sodium fatty alcohol polyoxyethylene ether sulfate anionic foaming agent; the ceramsite has a particle size of 5mm and a bulk density of 300kg / m³. 3 The nano-silica particles are 20nm in diameter and have a silica purity of 98%; the polypropylene fibers are 6mm in length and 20μm in diameter.

[0034] Preparation of modified fly ash microspheres: (1) Gradient activation grinding: Select Grade I raw fly ash, add 0.5% triethanolamine grinding aid by weight of fly ash, grind in stages under high-purity nitrogen protection, pre-grind at low speed of 200r / min for 2min to break up powder agglomeration, and fine grind at high speed of 600r / min for 8min to obtain ultrafine fly ash powder.

[0035] (2) Surface hydroxyl activation-room temperature covalent grafting: Prepare a weakly alkaline activation solution of sodium hydroxide with pH 11, and the solid-liquid ratio of ultrafine fly ash to activation solution is 1:3 g / mL. Disperse the solution by ultrasonication at 300W for 30 min, filter and drain at room temperature; prepare a silane hydrolysis modification solution with anhydrous ethanol to deionized water volume ratio of 3:1, adjust the pH to 3.5 with glacial acetic acid, add KH-570 silane coupling agent accounting for 3% of the mass of ultrafine fly ash, and stir at low speed at room temperature for 20 min to complete the hydrolysis; immerse the activated fly ash powder in the modification solution, and stir the system at a constant temperature of 40℃ and 200r / min for 2 h. After the reaction, filter and dry at 105℃ to constant weight to obtain silane-grafted fly ash.

[0036] (3) Core-shell coating at room temperature and pressure: Prepare a 5% sodium alginate aqueous solution, and mix silane-grafted fly ash with the solution at a solid-liquid ratio of 1:4 g / mL. Stir for 20 min to mix evenly. Add an 8% calcium chloride crosslinking solution at a volume of 25% of the sodium alginate solution volume. Add the solution at a uniform rate for 15 min. Crosslink and cure at room temperature and pressure for 40 min. Gradient air dry for 8 h at 25℃ and 55% relative humidity.

[0037] (4) Low-temperature directional whisker growth: Prepare a solution with a total ion concentration of 0.8 mol / L and Mg 2+ SiO3 2- A magnesium silicate precursor solution with a molar ratio of 1:1.2 was prepared, and 0.1 mol / L sodium bicarbonate was added. After coating, the solid-liquid ratio of fly ash microspheres to the precursor solution was 1:6 g / mL. The pH of the system was stabilized at 9, and the system was subjected to hydrothermal reaction at 120℃ and normal pressure for 6 hours. Magnesium silicate whiskers grew in situ on the surface of the microspheres, thus completing the preparation of modified fly ash microspheres.

[0038] Preparation of lightweight thermal insulation concrete: S1 Powder Premix: Weigh the corresponding proportions of cement clinker, modified fly ash microspheres, and nano silica fume and feed them into the mixing equipment. Dry mix at 350 r / min for 4 min to obtain a dry powder mixture.

[0039] S2 Aggregate Fiber Mixing: Evenly sprinkle metered ceramsite and polypropylene fiber into the dry powder mixture, and stir at a low speed of 200r / min for 3min.

[0040] S3 Functional Liquid Foaming Modification: Polycarboxylate superplasticizer, composite foam stabilizer, and synthetic foaming agent are added to deionized water in sequence and stirred at room temperature for 2 minutes to prepare a functional mixture. The mixture is then poured into the mixture and stirred at 900 r / min for 6 minutes. The ambient temperature is controlled at 20℃ and the relative humidity is 60%. The final pH of the slurry is adjusted to 10.5.

[0041] S4 Gradient Curing: Foamed slurry is poured into the mold at a uniform speed and left to stand for 2 hours to set; the film is used for staged moisturizing curing, with the initial curing at 20℃ and 90% relative humidity for 7 days, and the later curing at 20℃ and 95% relative humidity for 21 days, for a total curing period of 28 days to obtain finished concrete.

[0042] Example 2: Raw material weight ratio: The composition includes 120 parts cement clinker, 80 parts modified fly ash microspheres, 50 parts ceramsite, 15 parts nano silica fume, 8 parts redispersible latex powder, 4 parts polypropylene fiber, 3 parts polycarboxylate superplasticizer, 1.5 parts composite foam stabilizer, 1.2 parts synthetic foaming agent, and 55 parts deionized water. The composite foam stabilizer is a mixture of stearic acid and potassium silicate in a 1:4 mass ratio; the synthetic foaming agent is sodium sulfate anionic fatty alcohol polyoxyethylene ether foaming agent; the ceramsite has a particle size of 10 mm and a bulk density of 300 kg / m³. 3 The nano-silica particles are 50nm in diameter and 98% pure; the polypropylene fibers are 12mm in length and 40μm in diameter.

[0043] Preparation of modified fly ash microspheres: (1) Gradient activation grinding: Select Grade I raw fly ash, add 1% triethanolamine grinding aid by weight of fly ash, grind in stages under high-purity nitrogen protection, pre-grind at low speed of 200r / min for 2min to break up powder agglomeration, and fine grind at high speed of 600r / min for 10min to obtain ultrafine fly ash powder.

[0044] (2) Surface hydroxyl activation-room temperature covalent grafting: Prepare a weakly alkaline activation solution of sodium hydroxide with pH 12, and the solid-liquid ratio of ultrafine fly ash to activation solution is 1:5 g / mL. Disperse the solution by ultrasonication at 300W for 30 min, filter and drain at room temperature; prepare a silane hydrolysis modification solution, with anhydrous ethanol and deionized water in a volume ratio of 3:1, adjust the pH to 4.0 with glacial acetic acid, add KH-570 silane coupling agent accounting for 5% of the mass of ultrafine fly ash, and stir at low speed at room temperature for 20 min to complete the hydrolysis; immerse the activated fly ash powder in the modification solution, and stir the system at a constant temperature of 50℃ and 300r / min for 2 h. After the reaction, filter and dry at 105℃ to constant weight to obtain silane-grafted fly ash.

[0045] (3) Core-shell coating at room temperature and pressure: Prepare a 6% sodium alginate aqueous solution, and mix silane-grafted fly ash with the solution at a solid-liquid ratio of 1:5 g / mL. Stir for 20 min to mix evenly. Add an 8% calcium chloride crosslinking solution at a volume of 30% of the sodium alginate solution volume. Add the solution at a uniform rate for 15 min. Crosslink and cure at room temperature and pressure for 40 min. Then, air dry at 25℃ and 55% relative humidity for 8 h.

[0046] (4) Low-temperature directional whisker growth: Prepare a solution with a total ion concentration of 0.8 mol / L and Mg 2+SiO3 2- A magnesium silicate precursor solution with a molar ratio of 1:1.2 was prepared, and 0.1 mol / L sodium bicarbonate was added. After coating, the solid-liquid ratio of fly ash microspheres to the precursor solution was 1:8 g / mL. The pH of the system was stabilized at 10, and the system was subjected to hydrothermal reaction at 130℃ and normal pressure for 8 hours. Magnesium silicate whiskers grew in situ on the surface of the microspheres, thus completing the preparation of modified fly ash microspheres.

[0047] Preparation of lightweight thermal insulation concrete: S1 Powder Premix: Weigh the corresponding proportions of cement clinker, modified fly ash microspheres, and nano silica fume and feed them into the mixing equipment. Dry mix at 550 r / min for 4 min to obtain a dry powder mixture.

[0048] S2 Aggregate Fiber Mixing: Evenly sprinkle metered ceramsite and polypropylene fiber into the dry powder mixture, and stir at a low speed of 200r / min for 3min.

[0049] S3 Functional Liquid Foaming Modification: Polycarboxylate superplasticizer, composite foam stabilizer, and synthetic foaming agent are added to deionized water in sequence and stirred at room temperature for 2 minutes to prepare a functional mixture. The mixture is then poured into the mixture and stirred at 900 r / min for 6 minutes. The ambient temperature is controlled at 25℃ and the relative humidity is 60%. The final pH of the slurry is adjusted to 11.5.

[0050] S4 Gradient Curing: Foamed slurry is poured into the mold at a uniform speed and left to stand for 2 hours to set; the film is used for staged moisturizing curing, with the initial curing at 25℃ and 90% relative humidity for 7 days, and the later curing at 25℃ and 98% relative humidity for 21 days, for a total curing period of 28 days to obtain finished concrete.

[0051] Example 3: Raw material weight ratio: The composition includes 110 parts cement clinker, 70 parts modified fly ash microspheres, 40 parts ceramsite, 12.5 parts nano silica fume, 6.5 parts redispersible latex powder, 3 parts polypropylene fiber, 2 parts polycarboxylate superplasticizer, 1 part composite foam stabilizer, 1 part synthetic foaming agent, and 47.5 parts deionized water. The composite foam stabilizer is a mixture of stearic acid and potassium silicate in a 1:4 mass ratio; the synthetic foaming agent is sodium lauryl alcohol polyoxyethylene ether sulfate anionic foaming agent; the ceramsite has a particle size of 7.5 mm and a bulk density of 300 kg / m³. 3 The nano-silica particles are 35nm in diameter and 98.5% pure; the polypropylene fibers are 9mm long and 30μm in diameter.

[0052] Preparation of modified fly ash microspheres: (1) Gradient activation grinding: Select Grade I raw fly ash, add 0.75% triethanolamine grinding aid by weight of fly ash, grind in stages under high-purity nitrogen protection, pre-grind at low speed of 200r / min for 2min to break up powder agglomeration, and fine grind at high speed of 600r / min for 9min to obtain ultrafine fly ash powder.

[0053] (2) Surface hydroxyl activation-room temperature covalent grafting: Prepare a weakly alkaline activation solution of sodium hydroxide with pH 11.5, with a solid-liquid ratio of ultrafine fly ash to activation solution of 1:4 g / mL, ultrasonically dispersed at 300W for 30 min, filtered and drained at room temperature; prepare a silane hydrolysis modification solution with anhydrous ethanol to deionized water volume ratio of 3:1, adjust the pH to 3.75 with glacial acetic acid, add KH-570 silane coupling agent accounting for 4% of the mass of ultrafine fly ash, and stir at low speed at room temperature for 20 min to complete hydrolysis; immerse the activated fly ash powder in the modification solution, and stir at a constant temperature of 45℃ and 250r / min for 2 h, filter after reaction, and dry at 105℃ to constant weight to obtain silane-grafted fly ash.

[0054] (3) Core-shell coating at room temperature and pressure: Prepare a 5.5% sodium alginate aqueous solution, and mix silane-grafted fly ash with the solution at a solid-liquid ratio of 1:4.5 g / mL. Stir for 20 min to mix evenly. Add an 8% calcium chloride crosslinking solution at a volume of 27.5% of the sodium alginate solution volume. Add the solution at a uniform rate for 15 min. Crosslink and cure at room temperature and pressure for 40 min. Then, air dry at 25℃ and 55% relative humidity for 8 h.

[0055] (4) Low-temperature directional whisker growth: Prepare a solution with a total ion concentration of 0.8 mol / L and Mg 2+ SiO3 2- A magnesium silicate precursor solution with a molar ratio of 1:1.2 was prepared, and 0.1 mol / L sodium bicarbonate was added. After coating, the solid-liquid ratio of fly ash microspheres to the precursor solution was 1:7 g / mL. The pH of the stabilized system was 9.5. The system was subjected to hydrothermal reaction at 125℃ and normal pressure for 7 h. Magnesium silicate whiskers grew in situ on the surface of the microspheres, thus completing the preparation of modified fly ash microspheres.

[0056] Preparation of lightweight thermal insulation concrete: S1 Powder Premix: Weigh the corresponding proportions of cement clinker, modified fly ash microspheres, and nano silica fume and feed them into the mixing equipment. Dry mix at 450 r / min for 4 min to obtain a dry powder mixture.

[0057] S2 Aggregate Fiber Mixing: Evenly sprinkle metered ceramsite and polypropylene fiber into the dry powder mixture, and stir at a low speed of 200r / min for 3min.

[0058] S3 Functional Liquid Foaming Modification: Polycarboxylate superplasticizer, composite foam stabilizer, and synthetic foaming agent are sequentially added to deionized water and stirred at room temperature for 2 minutes to prepare a functional mixture. The mixture is then poured into the mixture and stirred at 900 r / min for 6 minutes. The ambient temperature is controlled at 22.5℃ and the relative humidity is 65%. The final pH of the slurry is adjusted to 11.

[0059] S4 Gradient Curing: Foamed slurry is poured into the mold at a uniform speed and left to stand for 2 hours to set; the film is used for staged moisturizing curing, with the initial curing at 22.5℃ and 92% relative humidity for 7 days, and the later curing at 22.5℃ and 96.5% relative humidity for 21 days, for a total curing period of 28 days to obtain the finished concrete.

[0060] Comparative Example 1: The raw material weight ratio, specifications of various auxiliary materials, and parameters of the entire concrete preparation process were completely consistent with those in Example 3. Only four modification processes, namely gradient activation grinding, hydroxyl grafting, core-shell coating, and whisker growth, were omitted. Original Grade I fly ash was directly used as filler and incorporated into the formula. The remaining steps remained unchanged, and the comparative sample concrete was finally prepared.

[0061] Comparative Example 2: The composite foam stabilizer was removed from the formula, and the weight proportions of all other raw materials, the preparation process of modified fly ash, and the step-by-step preparation parameters of concrete were all the same as those in Example 3. In the functional liquid preparation stage, only polycarboxylate superplasticizer and synthetic foaming agent were added, and the other operation steps remained unchanged to obtain the comparative sample concrete.

[0062] Comparative Example 3: The addition of polypropylene fiber was cancelled, and the weight proportions of other raw materials, the complete preparation process of modified fly ash, the mixing and foaming, and the pouring and curing parameters were all the same as in Example 3. In the aggregate mixing step, only ceramsite was added, and no fiber material was added. The other steps remained unchanged, and the comparative sample concrete was prepared.

[0063] Comparative Example 4: The modified fly ash preparation only involves three steps: gradient activation grinding, hydroxyl-activated silane grafting, and sodium alginate core-shell coating. The low-temperature hydrothermal whisker growth step of magnesium-silicon precursor liquid is omitted. The remaining formula parameters and concrete preparation process are completely in accordance with Example 3 to obtain the comparative sample concrete.

[0064] test: After uniform curing for 28 days, three independent performance tests were carried out on samples from Examples 1-3 and Comparative Examples 1-4 respectively. Six parallel test blocks were prepared for each group of samples, and the arithmetic mean was taken after removing the extreme values ​​as the final test data.

[0065] Experiment 1: Dry density and thermal conductivity test: Test method: 1. Dry density: Place 100mm×100mm×100mm cubic specimens in a constant temperature oven and dry them at 105℃ until constant weight. Weigh the specimens and calculate the dry density based on the measured volume and shape. 2. Thermal conductivity: Use a hot-wire thermal conductivity meter with specimens of 300mm×300mm×30mm at room temperature (23℃). Perform the test three times in parallel for each group and take the average value. The lower the dry density and the smaller the thermal conductivity value, the better the lightweight thermal insulation performance of the concrete.

[0066] Table 1. Test results of dry density and thermal conductivity

[0067] The dry density of Examples 1 to 3 is concentrated in the range of 612-628 kg / m³. 3 Within the specified range, the thermal conductivity is below 0.072 W / (m·K), exhibiting excellent overall lightweight and thermal insulation performance. Comparative Example 1 directly uses undisturbed fly ash. The fly ash particles have low activity and poor workability, resulting in large, interconnected pores in the foamed slurry. This increased internal density leads to a significant rise in dry density, and the interconnected pores accelerate heat conduction, significantly increasing the thermal conductivity. Comparative Example 2 lacks the synergistic effect of a composite foam stabilizer, resulting in poor pore stability in the foamed agent. During the foaming process, bubbles easily break and merge, reducing the number of closed insulation pores, and causing a simultaneous increase in density and thermal conductivity. Comparative Example 3 lacks polypropylene fibers, resulting in slight density increase due to pore shrinkage and deformation, with a relatively small impact on thermal conductivity. Comparative Example 4 lacks a surface whisker structure, leading to poor interfacial bonding between fly ash and the cement matrix, increased internal micropores, and higher density and thermal conductivity than the three examples of this invention.

[0068] Experiment 2: Compressive strength test: Test method: Standard cubic specimens of 100mm×100mm×100mm were prepared and subjected to compressive strength tests using a universal pressure testing machine at a loading rate of 0.5MPa / s. The 28-day compressive strength was determined according to the static compression test method. The higher the strength value, the better the mechanical properties of the concrete.

[0069] Table 2 28d compressive strength test results

[0070] The compressive strength of the three sets of sample examples remained stable at 3.82-3.95 MPa, balancing lightweight characteristics with structural load-bearing capacity. Comparative Example 1, with its unreacted fly ash, lacked sufficient activity to fully participate in the cement hydration reaction, resulting in weak bonding between the powder and the matrix, making it prone to cracking under stress and exhibiting the largest decrease in compressive strength. Comparative Example 2 lacked a composite foam stabilizer, leading to uneven cell structure and large pores becoming weak points under stress, further reducing compressive strength. Comparative Example 3, without the addition of polypropylene fibers, lost the crack-resistant and toughening effect of the fibers, causing rapid crack propagation in the concrete under load, resulting in significantly lower compressive strength than the examples. Comparative Example 4, with its fly ash lacking a surface whisker anchoring structure, exhibited insufficient bonding between the filler and the cement matrix, making the interface prone to micro-cracks, and its compressive strength was lower than the sample formulated in this invention.

[0071] Experiment 3: Drying shrinkage rate test: Test method: The sample was molded to a size of 40mm×40mm×160mm. After standard curing for 7 days, it was placed in a dry environment with a temperature of 23℃ and an ambient relative humidity of 50% for 90 days. The length change of the sample was measured using a length comparator, and the drying shrinkage rate was calculated. The smaller the shrinkage rate, the better the dimensional stability of the concrete.

[0072] Table 3 Results of 90-day drying shrinkage rate test

[0073] The embodiments of this invention, through four-stage modification of fly ash and control of fiber composite formulation, achieve a drying shrinkage rate controlled at 1.75-1.82‰, exhibiting excellent dimensional stability. Comparative Example 1, with its virgin fly ash containing numerous impurities and delayed hydration, resulted in uneven volume deformation due to continuous internal hydration, leading to a significantly excessive drying shrinkage rate. Comparative Example 2 exhibited an unbalanced pore structure, with pore collapse inducing overall shrinkage. Comparative Example 3 lacked the binding and restraining effect of polypropylene fibers, resulting in free shrinkage of the matrix after concrete dehydration and a significantly increased shrinkage rate. Comparative Example 4, with its fly ash lacking surface whisker anchoring, exhibited insufficient bonding strength of the filler within the cement matrix, leading to interface debonding during the alternating wet and dry process and a higher shrinkage rate than the three examples.

[0074] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A lightweight thermal insulation concrete based on fly ash, characterized in that, It is prepared from the following raw materials in parts by weight: 100-120 parts cement clinker, 60-80 parts modified fly ash microspheres, 30-50 parts ceramsite, 10-15 parts nano silica fume, 5-8 parts redispersible latex powder, 2-4 parts polypropylene fiber, 1-3 parts polycarboxylate superplasticizer, 0.5-1.5 parts composite foam stabilizer, 0.8-1.2 parts synthetic foaming agent, and 40-55 parts deionized water; The modified fly ash microspheres were prepared using a modification process involving gradient activation grinding, surface hydroxyl activation-room-temperature covalent grafting, atmospheric pressure core-shell coating, and low-temperature directional whisker growth.

2. The lightweight thermal insulation concrete based on fly ash according to claim 1, characterized in that, The ceramsite has a particle size of 5-10mm and a bulk density of ≤300kg / m³. 3 The nano-silica particles have a diameter of 20-50 nm and a silica purity of ≥98%.

3. The lightweight thermal insulation concrete based on fly ash according to claim 1, characterized in that, The polypropylene fibers are 6-12 mm in length and 20-40 μm in diameter.

4. The lightweight thermal insulation concrete based on fly ash according to claim 1, characterized in that: The alkali-resistant composite foam stabilizer is prepared by compounding stearic acid and potassium silicate in a mass ratio of 1:4; the alkali-resistant synthetic foaming agent is sodium fatty alcohol polyoxyethylene ether sulfate anionic foaming agent.

5. The lightweight thermal insulation concrete based on fly ash according to claim 1, characterized in that, The gradient activation grinding process of the modified fly ash microspheres is as follows: select Grade I raw fly ash, add 0.5-1% of triethanolamine grinding aid by weight of fly ash, grind in stages under high-purity nitrogen protection, pre-grind at low speed of 200r / min for 2min to break up powder agglomeration, and finely grind at high speed of 600r / min for 8-10min to obtain ultrafine fly ash powder.

6. The lightweight thermal insulation concrete based on fly ash according to claim 1, characterized in that, The surface hydroxyl activation and silane covalent grafting process is as follows: First, prepare a weakly alkaline sodium hydroxide activation solution with pH 11-12. Mix ultrafine fly ash with the activation solution at a solid-liquid ratio of 1:3-5 g / mL, ultrasonically disperse at 300W for 30 min, filter, and drain at room temperature for later use. Then, prepare a silane hydrolysis modification solution, which consists of anhydrous ethanol, deionized water, glacial acetic acid, and KH-570 silane coupling agent. The volume ratio of ethanol to water is 3:

1. Adjust the pH to 3.5-4.0 with glacial acetic acid. Add 3-5% KH-570 by weight of ultrafine fly ash and stir at low speed at room temperature for 20 min to complete the controlled hydrolysis. The activated fly ash powder was immersed in the hydrolysis modification solution, and the system temperature was controlled at 40-50℃ and the reaction was carried out at 200-300r / min for 2 hours. After the reaction was completed, the mixture was filtered and dried at 105℃ to constant weight to obtain silane-grafted fly ash.

7. The lightweight thermal insulation concrete based on fly ash according to claim 1, characterized in that, The ambient temperature and pressure core-shell coating process is as follows: prepare a 5-6% sodium alginate aqueous solution, and mix silane-grafted fly ash with the solution at a solid-liquid ratio of 1:4-5 g / mL. Stir for 20 minutes to mix evenly. Add an 8% calcium chloride crosslinking solution, with the volume of the calcium chloride crosslinking solution being 25%-30% of the volume of the sodium alginate aqueous solution. Add the solution at a uniform rate over 15 minutes. Crosslink and cure at ambient temperature and pressure for 40 minutes. Then, air dry at 25℃ and 55% relative humidity for 8 hours.

8. The lightweight thermal insulation concrete based on fly ash according to claim 1, characterized in that, The low-temperature directional whisker growth process is as follows: a total ion concentration of 0.8 mol / L and Mg... 2+ SiO3 2- A magnesium silicate precursor solution with a molar ratio of 1:1.2, containing magnesium chloride hexahydrate and anhydrous sodium silicate, with 0.1 mol / L sodium bicarbonate added as a pH buffer and ion penetration aid; fly ash microspheres coated with the gel layer with a solid-liquid ratio of 1:6-8 g / mL to the precursor solution, stabilized at pH 9-10, and subjected to a low-temperature, normal-pressure hydrothermal reaction at 120-130℃ for 6-8 h; magnesium silicate whiskers are grown in situ on the surface of the gel layer.

9. A method for preparing lightweight thermal insulation concrete based on fly ash as described in any one of claims 1-8, characterized in that, Includes the following steps: S1. Powder premixing: Weigh cement clinker, modified fly ash microspheres, and nano silica fume according to the proportion, put them into the mixing equipment, dry mix at 350-550r / min for 4min to obtain a uniform dry powder mixture. S2. Aggregate and fiber blending: Sprinkle the ceramsite and polypropylene fiber evenly into the dry powder mixture and stir at a low speed of 200r / min for 3min. S3. Functional liquid foaming modification: Polycarboxylate superplasticizer, composite foam stabilizer and synthetic foaming agent are dissolved in deionized water in sequence and stirred at room temperature for 2 minutes to obtain functional mixture; the mixture is poured into the mixture and stirred at high speed of 900 r / min for 6 minutes, the ambient temperature is controlled at 20-25℃ and the relative humidity is ≥60%, and the final pH of the slurry is adjusted to be stable at 10.5-11.

5. S4. Gradient curing during pouring: The foamed mixture is poured into the mold at a uniform speed and left to stand for 2 hours to set; a film covering for moisture retention is adopted for curing, with a total curing period of 28 days, to obtain finished concrete.

10. The preparation method according to claim 9, characterized in that, The moisturizing and maintenance process is as follows: in the early stage, maintain at room temperature (20-25℃, relative humidity ≥90%) for 7 days; in the later stage, maintain at constant temperature (20-25℃, relative humidity 95%-98%) for 21 days.

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