A tea stem cellulose and tea saponin-based coating slurry recycled aggregate and a low-alkali ecological preparation process thereof

Abstract

This invention discloses a coated recycled aggregate based on tea stem cellulose and tea saponin, and its low-alkali ecological preparation process. The invention utilizes the resource-based graded treatment of tea stems, employing acid hydrolysis to prepare tea stem cellulose and ethanol immersion to purify tea saponin, which are then synergistically applied to a surface modification system for recycled aggregates. By weight fraction, the coating slurry mainly consists of 50 parts ordinary silicate cement, 20 parts silica fume, 30 parts fly ash, 10 parts polyvinyl alcohol fiber, 10 parts carbon fiber, and 0.3 parts tea stem cellulose. A wet coating process using a granulator forms a dense interface layer on the surface of the recycled aggregate. The surface activity of tea saponin strengthens the interfacial bond between tea stem cellulose and the cement matrix. Combined with carbonation curing, this process simultaneously promotes the reaction of cement hydration products Ca(OH)2 and CO2 to generate CaCO3, significantly reducing the pH value of the system and achieving low-alkali control. The modified coated recycled aggregate not only possesses superior mechanical properties but also improved durability, making it a viable alternative to natural aggregates in the preparation of high-performance concrete. This invention addresses the issues of insufficient mechanical properties and alkaline substance accumulation in recycled aggregates by utilizing waste tea stems at high value, providing an innovative path for the resource utilization of construction solid waste that combines low carbon emissions and ecological benefits.

Description

Technical Field

[0001] This invention relates to the field of concrete technology, and in particular to mortar-coated recycled aggregate and its low-alkali ecological preparation process. Background Technology

[0002] With the acceleration of urbanization, the construction industry is undergoing increasingly frequent upgrades and replacements, leading to a year-on-year increase in construction waste. While using crushed concrete to produce recycled aggregate is an important method for the resource utilization of construction waste and boasts high resource utilization efficiency, the recycled aggregate suffers from low strength due to the presence of old mortar adhering to its surface and numerous micro-cracks within. Currently, methods to improve recycled aggregate mainly include mechanical treatment, soaking treatment, and acid corrosion. Although these methods can partially improve the strength of recycled aggregate, they still present risks of incomplete interface strengthening and alkali enrichment. Furthermore, traditional coating pastes have weak interfacial adhesion to recycled aggregates, failing to effectively seal micro-cracks, resulting in limited improvement in mechanical properties. Additionally, the hydration of cement-based materials generates a large amount of Ca(OH)2, resulting in a system pH value as high as 12-13, which easily triggers alkali-aggregate reactions in concrete, reducing durability.

[0003] Therefore, existing technologies lack efficient recycled aggregate modification technologies that combine mechanical strengthening, low-alkali regulation, and solid waste synergy.

[0004] Furthermore, with the rapid development of the tea processing industry, the annual output of tea stems, a major byproduct, has exceeded ten million tons. Traditional disposal methods mostly involve open-air burning or simple landfilling, which not only wastes biomass resources but also releases large amounts of greenhouse gases and dust pollutants. Although existing tea stem resource utilization technologies involve activated carbon preparation or composting, they generally suffer from bottlenecks such as low product added value and high energy consumption, making it difficult to achieve efficient graded extraction and functional application of organic components, thus hindering the construction of a circular economy system for the tea industry. Summary of the Invention

[0005] This invention provides a highly efficient recycled aggregate modification technology based on tea stem cellulose and tea saponin, which combines mechanical strengthening, low-alkali regulation, and solid waste synergy, overcoming the limitations of existing technologies. The technology for strengthening recycled aggregate by adding tea stem cellulose and tea saponin employs a mechanical granulation process to form a coating slurry layer on the aggregate surface to seal microcracks. This process achieves uniform coating by controlling the wetting and particle adsorption processes in stages, and combined with carbonization curing technology, promotes the mineralization reaction between CO2 and the slurry, simultaneously improving the mechanical properties of the recycled aggregate and the durability of concrete, while also achieving permanent CO2 sequestration, effectively reducing the carbon emissions throughout the entire lifecycle of construction solid waste resource utilization.

[0006] The slurry-coated recycled aggregate of the present invention is composed of the following raw materials in parts by weight: 80-120 parts of recycled aggregate with a particle size of 4.75-9.5mm, 40-55 parts of cement, 15-20 parts of silica fume, 30-40 parts of fly ash, 10-15 parts of polyvinyl alcohol fiber, 8-10 parts of carbon fiber, 0.1-0.5 parts of tea stem cellulose, 0.01-0.05 parts of tea saponin, and water.

[0007] Preferably, silicate cement, silica fume, fly ash, polyvinyl alcohol fiber, carbon fiber, and tea stalk cellulose are mixed to obtain a coated powder. Tea saponin is prepared into a tea saponin solution with a concentration of 0.1%-0.5%. The recycled aggregate is sprayed with the tea saponin solution once in a granulator, and then the coated powder is added again. This process is repeated multiple times until the tea saponin solution and the coated powder are completely added, resulting in coated recycled aggregate.

[0008] Preferably, the tea stem cellulose in the components is obtained by purifying tea stems through acid hydrolysis, washing with water, freeze-drying, and grinding; the tea saponin is obtained by defatting tea stems, extracting with alcohol, and filtering.

[0009] Preferably, the extraction steps of tea stem cellulose are as follows: (1) Raw material pretreatment: Dehydrate the tea stems to a moisture content of <5%, crush them into 80-100 mesh powder by a pulverizer, then sieve to remove impurities and collect the sieve material; (2) Acid hydrolysis purification: Put sulfuric acid solution and tea stem powder into a reaction vessel and continuously stir the reaction in a constant temperature water bath at 30-55℃ to destroy the lignin-hemicellulose composite structure; (3) Water washing: Transfer the acid hydrolysis residue into distilled water to further remove residual acid from the surface of the residue; (4) Finished product preparation: Use freeze drying process to solidify the fiber, and then use a ball mill to dry grind it to obtain tea stem cellulose powder.

[0010] Preferably, the extraction steps of tea saponin are as follows: (1) Acetone defatting pretreatment: tea stem powder and acetone are loaded into a filter paper tube at a weight ratio of 1:4-6, and extracted in a water bath at 60-100℃ to remove oily impurities. The extracted tea stem powder is collected and dried until there is no acetone odor; (2) Ethanol gradient extraction: defatted tea powder and 60% ethanol are added at a solid-liquid ratio of 1:8-15 and extracted with ultrasonic enhancement; (3) Product preparation: the extract is filtered to obtain a solid, and saturated sodium chloride solution is added at a volume ratio of solid to saturated sodium chloride solution of 1:1-2. The precipitate is collected by centrifugation.

[0011] Preferably, the cement is silicate cement, the fly ash is grade I fly ash, the polyvinyl alcohol fiber is a high modulus type with a single filament diameter of 20-40 μm, and the carbon fiber is a polyacrylonitrile-based carbon fiber with a single filament diameter of 7-12 μm.

[0012] This invention also provides a low-alkali, eco-friendly preparation process for the above-mentioned coated recycled aggregate, comprising the following steps:

[0013] S1. First, mix 40-55 parts of silicate cement, 15-20 parts of silica fume, 30-40 parts of fly ash, 10-15 parts of polyvinyl alcohol fiber, 8-10 parts of carbon fiber, and 0.1-0.5 parts of tea stalk cellulose to form a coated powder.

[0014] S2. Place the recycled aggregate into the turned-on granulator and spray it with a 0.1%-0.5% tea saponin solution using a water sprayer to make the surface of the recycled aggregate evenly coated with the solution.

[0015] S3. Add the coated powder and granulate. Thereafter, repeat the spraying and powder addition granulation process in S2-S3. That is, before each addition of coated powder, spray the surface of the recycled aggregate with tea saponin solution using a spray bottle.

[0016] S4. After multiple coating processes, the treated slurry-coated recycled aggregate is obtained and then subjected to carbonization curing. Preferably, the pelletizer speed in step S3 is 30-50 r / min.

[0017] Preferably, in step S1, during the mixing process, the stirring rate is controlled to ensure that each component can be fully and evenly dispersed to form a uniformly coated powder, wherein the stirring speed is 30 r / min and the time is 1 min.

[0018] Preferably, in step S2, the recycled aggregate is placed into the granulator, the tea saponin solution is sprayed with water from a spray bottle, and the granulator is turned on so that the surface of the recycled aggregate is evenly coated with the solution. The tea saponin solution can achieve dynamic spreading on the surface of the recycled aggregate. The granulator speed is set to 10 r / min and the time is 30 s.

[0019] Preferably, in step S3, before each addition of the coating powder, the surface of the recycled aggregate is moistened with a water sprayer. The purpose of moistening is to form a water film on the surface of the recycled aggregate, enhancing the adhesion between the coating powder and the recycled aggregate, and also helping to achieve uniform distribution of the powder on the aggregate surface. By adding the powder in small amounts multiple times, the thickness of the coating layer can be gradually increased, ensuring the uniformity and density of the coating layer.

[0020] Preferably, in step S4, after multiple coating operations, the treated coated recycled aggregate is obtained. At this point, the surface of the recycled aggregate has been uniformly coated with a composite slurry composed of cement, silica fume, fly ash, polyvinyl alcohol fiber, carbon fiber, and tea stalk cellulose. This coating layer can effectively improve the surface properties of the recycled aggregate, enhance its mechanical properties and durability, and provide a good foundation for subsequent carbonization and curing processes. Then, the prepared coated recycled aggregate is placed in a carbonization curing chamber. The carbon dioxide partial pressure should be controlled at 0.2-0.8 MPa to ensure sufficient carbon dioxide concentration to promote the carbonization reaction. The carbonization curing time is typically 1-5 days.

[0021] The present invention also provides the application of the above-mentioned coated recycled aggregate in construction, for example, in the preparation of concrete, the preparation of environmentally friendly building materials, and the preparation of load-bearing structures.

[0022] The research approach of this invention is as follows:

[0023] The tea stem cellulose introduced in this process improves the performance of recycled aggregates through the following synergistic mechanisms: On the one hand, the abundant hydroxyl groups on the surface of tea stem cellulose chemically bond with cement minerals C3S and C2S, promoting heterogeneous nucleation of calcium silicate hydrate (CSH) gel and reducing the porosity of the coating layer; on the other hand, the carboxyl functional groups (-COOH) on the surface of tea stem cellulose preferentially react with CO2 during the carbonation curing stage to generate nano-calcium carbonate crystal nuclei, accelerating the conversion of Ca(OH)2, thereby reducing the pH value of the system.

[0024] Furthermore, research has found that tea saponin can be mixed with water in advance to form a spray solution, which can greatly reduce the amount of tea saponin used by spraying.

[0025] Experimental results showed that the spreading speed of tea saponin solution on the surface of recycled aggregate was significantly improved compared with water alone, achieving dynamic spreading. Furthermore, tea saponin and tea stem cellulose formed a "surfactant-fiber" composite carrier, driving the slurry to capillary infiltration along cracks.

[0026] Furthermore, the powder used to coat the recycled aggregate in this invention is a composite powder of cement, fly ash, silica fume, polyvinyl alcohol fiber, carbon fiber, and tea stalk cellulose. Silica fume has high pozzolanic activity and can react with calcium hydroxide, a cement hydration product, to generate more hydrated calcium silicate gel, filling the pores created by the crushing of the recycled aggregate and significantly improving its strength. Secondly, fly ash can replace part of the cement, reducing the cost of the coating slurry while improving the workability of the recycled aggregate, increasing the fluidity and pumpability of the coating slurry, and reducing water consumption.

[0027] Furthermore, the polyvinyl alcohol fiber is a polymer fiber made from polyvinyl alcohol through a spinning process, and its length is generally 6-12 mm. The polyvinyl alcohol fiber in the coating material has good hydrophilicity and chemical stability, and can form a good interfacial bond with the cement matrix.

[0028] Meanwhile, polyvinyl alcohol fibers have a synergistic effect with silica fume and fly ash. The interfacial bonding strength between polyvinyl alcohol fibers and cement matrix is ​​affected by fly ash and silica fume. Fly ash can reduce the chemical bonding and interfacial friction between fibers and matrix, thereby inhibiting premature fiber breakage during pull-out and improving the ductility of composite materials. The addition of silica fume helps to enhance the interfacial bonding strength.

[0029] Furthermore, carbon fiber is a fibrous material with a carbon content of over 90%, made from polyacrylonitrile, pitch, or viscose fibers through high-temperature carbonization. The main role of carbon fiber in coating materials for encapsulating powder is to improve mechanical properties, enhance durability, and increase crack resistance. These effects help reduce maintenance needs and cement usage, thereby indirectly reducing carbon dioxide emissions.

[0030] Furthermore, the tea stem cellulose encapsulated in powder forms a three-level reinforcement system with polyvinyl alcohol fiber and carbon fiber. The synergistic effect is specifically manifested as follows: the hydroxyl groups on the surface of tea stem cellulose form a hydrogen bond network with the polyvinyl alcohol segments of PVA fiber; tea stem cellulose bridges nanoscale cracks; PVA fiber dissipates the energy of mesoscale cracks through plastic deformation; and carbon fiber inhibits the propagation of macroscopic cracks. The three-level reinforcement system can greatly improve tensile strength compared with a single reinforcement system.

[0031] Further research revealed that the rolling and rubbing mechanical action of the pelletizer selectively arranges the recycled aggregate according to the largest contact area, further compressing the particles on the inner side of the pellets and allowing the thin film water layers to come into contact. In subsequent processes, wetting and mechanical action play a dominant role, further compressing the surface of the recycled aggregate coated with the slurry. The performance of the pelletizer is mainly determined by the rotation time and rotation speed, with a rotation speed of 30-50 r / min.

[0032] This invention employs a composite powder material reinforced with tea stem cellulose to coat the surface of recycled aggregate. By optimizing the powder particle size distribution and using a tea saponin-assisted wetting process, a uniform and dense reinforcing layer is formed on the aggregate surface. In the controlled granulation process, the granulator operates at a specific rotation speed (30-50 r / min), combined with a tea saponin-modified water spray system, promoting adhesion between the tea stem cellulose and the powder. During the curing stage, the tea stem cellulose undergoes a directional mineralization reaction with CO2, filling surface cracks while reconstructing the transition zone at the old mortar interface, forming a carbonized product layer with a fiber-reinforced effect. This allows for a simultaneous improvement in the mechanical properties and durability of the recycled aggregate.

[0033] The beneficial effects of this invention are as follows:

[0034] (1) This invention innovatively realizes the synergistic utilization of tea stem cellulose and tea saponin in tea stems. Through acid hydrolysis-ethanol extraction combined technology, tea stem cellulose and tea saponin are extracted from waste tea stems in stages and input, which solves the problems of resource waste and pollution in traditional tea stem processing.

[0035] (2) Tea stem cellulose, with its high specific surface area and abundant hydroxyl / carboxyl functional groups, forms a three-dimensional network structure in the slurry, which enhances the interfacial bonding of cement matrix through hydrogen bonding and promotes the directional growth of calcium carbonate crystals as a mineralization nucleation site; tea saponin plays the role of surfactant, reduces the surface tension of the solution, drives tea stem cellulose to penetrate along the cracks of recycled aggregate, and inhibits the precipitation of alkaline substances through the chelation of saponin molecules with calcium ions.

[0036] (3) This invention constructs a dual mechanism for improving the performance of recycled aggregates by coupling the wet coating process of the granulator with the carbonization curing technology. The granulator uses a gradient speed of 30-50 r / min in conjunction with the tea saponin modified wetting process to form a dense and uniform composite coating layer on the surface of the recycled aggregates through the "dynamic wetting-layer coating" mode, which effectively seals microcracks and reconstructs the interface transition zone.

[0037] (4) Through the above-mentioned technological innovations, the low-alkali ecological recycled aggregate prepared has the following alkalinity regulation mechanism: tea stem cellulose accelerates the CO2 mineralization reaction to consume alkaline substances, and carbonization maintenance promotes the reduction of pH value of the system. This technology can dispose of a large amount of tea stem waste every year, while reducing the amount of construction solid waste landfill by tens of millions of tons, solving the problem of poor mechanical properties of recycled aggregate, and providing the construction industry with a three-in-one sustainable development path of "solid waste resource utilization - carbon sequestration - low alkali".

[0038] (5) The carbonization curing method of the present invention is to perform carbonization curing after the coated recycled aggregate is formed. Compared with the traditional standard curing, it can fix carbon dioxide and is environmentally friendly. Secondly, the calcium carbonate formed by carbonization curing can further fill the gap between the coated slurry and the recycled aggregate, further enhancing the strength of the recycled aggregate.

[0039] This invention comprehensively constructs a co-processing system for tea stems and construction solid waste. It achieves efficient separation of tea stem cellulose and tea saponins through a combined acid hydrolysis-ethanol extraction technology, and develops a synergistically enhanced composite modified slurry. This preparation process combines the toughening effect of tea stem cellulose with the surface activity of tea saponins, forming a dense interfacial transition layer during wet granulation. Combined with a carbonization curing process, it promotes the directional transformation of hydration products, simultaneously improving the mechanical properties of recycled aggregates and controlling the pH value of the system. This process not only provides a new approach to the disposal of tea stem waste and the utilization of recycled aggregates, but also offers low-alkali ecological benefits, thus possessing broad application prospects. Attached Figure Description

[0040] Figure 1 A flowchart of the preparation process provided by the present invention.

[0041] Figure 2 This refers to the steps involved in extracting cellulose from tea stems.

[0042] Figure 3 This refers to the extraction steps of tea saponins. Detailed Implementation

[0043] This invention provides a coated recycled aggregate based on tea stem cellulose and tea saponin and its low-alkali ecological preparation. The invention will be further described below with reference to specific embodiments, but these are not intended to limit the invention.

[0044] In the following examples, both tea stem cellulose and tea saponin are derived from waste tea stems, and the methods for obtaining them are as follows:

[0045] (1) Extraction of cellulose from tea stems: Tea stems were placed in a 60℃ drying oven to dehydrate until the moisture content was less than 5%, and then crushed into 80-100 mesh powder by a pulverizer. The powder was then vibrated and sieved for 5 minutes using a 0.15mm standard sieve to remove inorganic impurities such as sand and soil, and the sieve material was collected. A 3wt% sulfuric acid solution was prepared, and the tea stem powder was mixed with the solid-liquid ratio at 1:15 and added to the reaction vessel. The mixture was continuously mechanically stirred in a 45℃ constant temperature water bath for 30 minutes. The acid hydrolysis residue was then transferred to distilled water for 30 minutes to further remove residual acid from the surface of the residue. The fiber was solidified by freeze drying and then dry-milled in a ball mill for 15 minutes to obtain tea stem cellulose powder with good flowability.

[0046] (2) Extraction of tea saponins: 100g of tea stem powder was weighed and placed into a filter paper tube, 500mL of acetone was added, and the mixture was extracted in an 85℃ water bath for 1 hour to remove oily impurities. The extracted tea stem powder was collected and dried in a forced-air dryer at 60℃ for 1 hour until there was no acetone odor. Defatted tea powder and 60% ethanol were added at a solid-liquid ratio of 1:12 and ultrasonically extracted for 30min. The extract was vacuum filtered through a Buchner funnel, and saturated sodium chloride solution was added at a volume ratio of 1:1.2. The mixture was allowed to stand for 12h and the precipitate was collected by centrifugation.

[0047] Example 1

[0048] (1) Preparation of coating powder: Weigh 50 parts of silicate cement, 20 parts of silica fume, 30 parts of fly ash, 10 parts of polyvinyl alcohol fiber, 10 parts of carbon fiber and 0.1 parts of tea stem cellulose, mix them in advance, stir them in a mixer at 30 r / min for 1 min to make coating powder.

[0049] (2) Wet granulation process: 100 parts of recycled aggregate were placed into the granulator and the granulator was turned on. The coating powder was added in 5 batches. Before each addition of the coating powder, the surface of the recycled aggregate or granules was sprayed with a 0.1% tea saponin solution to wet it once. The total number of spraying times in the entire wet granulation process was 5, with each spray amount of about 2g; the granulator speed was 45r / min and the duration was 3min.

[0050] (3) Carbonation curing process: After multiple wrappings, the treated slurry-coated recycled aggregate is obtained and carbonized and cured, with the carbon dioxide partial pressure controlled at 0.5 MPa for 3 days.

[0051] Example 2

[0052] (1) Preparation of coating powder: Weigh 50 parts of silicate cement, 20 parts of silica fume, 30 parts of fly ash, 10 parts of polyvinyl alcohol fiber, 10 parts of carbon fiber and 0.3 parts of tea stem cellulose, mix them in advance, stir them in a mixer at 30 r / min for 1 min to make coating powder.

[0053] (2) Wet granulation process: The concentration of the sprayed tea saponin solution is 0.3%, and the rest of the process is the same as in Example 1.

[0054] (3) Carbonization curing process: After multiple wrappings, the treated slurry-coated recycled aggregate is obtained and carbonized and cured, with the carbon dioxide partial pressure controlled at 0.5 MPa for 3 days.

[0055] Example 3

[0056] (1) Preparation of coating powder: Weigh 50 parts of silicate cement, 20 parts of silica fume, 30 parts of fly ash, 10 parts of polyvinyl alcohol fiber, 10 parts of carbon fiber and 0.5 parts of tea stem cellulose, mix them in advance, stir them in a mixer at 30 r / min for 1 min to make coating powder.

[0057] (2) Wet granulation process: The concentration of the sprayed tea saponin solution is 0.5%, and the rest of the process is the same as in Example 1.

[0058] (3) Carbonation curing process: After multiple wrappings, the treated slurry-coated recycled aggregate is obtained and carbonized and cured, with the carbon dioxide partial pressure controlled at 0.5 MPa for 3 days.

[0059] Example 4

[0060] (1) Preparation of coated powder: Weigh 50 parts of silicate cement, 20 parts of silica fume, 30 parts of fly ash, 10 parts of polyvinyl alcohol fiber, 10 parts of carbon fiber and 0.3 parts of tea stem cellulose, mix them in advance, stir them in a mixer at 30 r / min for 1 min to make coated powder.

[0061] (2) Wet granulation process: The granulator speed is 30 r / min, and the rest of the process is the same as in Example 1.

[0062] (3) Carbonation curing process: After multiple wrappings, the treated slurry-coated recycled aggregate is obtained and carbonized and cured, with the carbon dioxide partial pressure controlled at 0.5 MPa for 3 days.

[0063] Example 5

[0064] (1) Preparation of coating powder: Weigh 50 parts of silicate cement, 20 parts of silica fume, 30 parts of fly ash, 10 parts of polyvinyl alcohol fiber, 10 parts of carbon fiber and 0.3 parts of tea stem cellulose, mix them in advance, stir them in a mixer at 30 r / min for 1 min to make coating powder.

[0065] (2) Wet granulation process: The granulator speed is 50 r / min, and the rest of the process is the same as in Example 1.

[0066] (3) Carbonation curing process: After multiple wrappings, the treated slurry-coated recycled aggregate is obtained and carbonized and cured, with the carbon dioxide partial pressure controlled at 0.5 MPa for 3 days.

[0067] Example 6

[0068] (1) Preparation of coating powder: Weigh 40 parts of silicate cement, 15 parts of silica fume, 30 parts of fly ash, 12 parts of polyvinyl alcohol fiber, 8 parts of carbon fiber and 0.3 parts of tea stem cellulose, mix them in advance, stir them in a mixer at 30 r / min for 1 min to make coating powder.

[0069] (2) Wet granulation process: 80 portions of recycled aggregate were placed into the granulator, and the process was the same as in Example 1.

[0070] (3) Carbonation curing process: After multiple wrappings, the treated slurry-coated recycled aggregate is obtained and carbonized and cured, with the carbon dioxide partial pressure controlled at 0.5 MPa for 3 days.

[0071] Example 7

[0072] (1) Preparation of coated powder: Weigh 45 parts of silicate cement, 17 parts of silica fume, 30 parts of fly ash, 14 parts of polyvinyl alcohol fiber, 9 parts of carbon fiber and 0.3 parts of tea stem cellulose, mix them in advance, stir them in a mixer at 30 r / min for 1 min to make coated powder.

[0073] (2) Wet granulation process: 90 portions of recycled aggregate were placed into the granulator, and the process was the same as in Example 1.

[0074] (3) Carbonation curing process: After multiple wrappings, the treated slurry-coated recycled aggregate is obtained and carbonized and cured, with the carbon dioxide partial pressure controlled at 0.5 MPa for 3 days.

[0075] Example 8

[0076] (1) Preparation of coated powder: Weigh 55 parts of silicate cement, 20 parts of silica fume, 40 parts of fly ash, 15 parts of polyvinyl alcohol fiber, 10 parts of carbon fiber and 0.3 parts of tea stem cellulose, mix them in advance, stir them in a mixer at 30 r / min for 1 min to make coated powder.

[0077] (2) Wet granulation process: 120 portions of recycled aggregate were placed into the granulator, and the process was the same as in Example 1.

[0078] (3) Carbonation curing process: After multiple wrappings, the treated slurry-coated recycled aggregate is obtained and carbonized and cured, with the carbon dioxide partial pressure controlled at 0.5 MPa for 3 days.

[0079] Comparative Example 1

[0080] (1) Preparation of coating powder: Weigh 50 parts of silicate cement, 20 parts of silica fume, 30 parts of fly ash, 10 parts of polyvinyl alcohol fiber and 10.3 parts of carbon fiber, mix them in advance, stir them in a mixer at 30 r / min for 1 min to make coating powder.

[0081] (2) Wet granulation process: spray distilled water, the process is the same as in Example 1.

[0082] (3) Carbonation curing process: After multiple wrappings, the treated slurry-coated recycled aggregate is obtained and carbonized and cured, with the carbon dioxide partial pressure controlled at 0.5 MPa for 3 days.

[0083] Comparative Example 2

[0084] (1) Preparation of coating powder: Weigh 50 parts of silicate cement, 20 parts of silica fume, 30 parts of fly ash, 10 parts of polyvinyl alcohol fiber, 10 parts of carbon fiber and 0.33 parts of tea stem cellulose, mix them in advance, stir them in a mixer at 30 r / min for 1 min to make coating powder.

[0085] (2) Wet granulation process: spray distilled water, and the rest of the process is the same as in Example 1.

[0086] (3) Carbonation curing process: After multiple wrappings, the treated slurry-coated recycled aggregate is obtained and carbonized and cured, with the carbon dioxide partial pressure controlled at 0.5 MPa for 3 days.

[0087] Comparative Example 3

[0088] (1) Preparation of coating powder: Weigh 50 parts of silicate cement, 20 parts of silica fume, 30 parts of fly ash, 10 parts of polyvinyl alcohol fiber and 10.3 parts of carbon fiber, mix them in advance, stir them in a mixer at 30 r / min for 1 min to make coating powder.

[0089] (2) Wet granulation process: The concentration of the sprayed tea saponin solution is 3.3%, and the rest of the process is the same as in Example 1.

[0090] (3) Carbonation curing process: After multiple wrappings, the treated slurry-coated recycled aggregate is obtained and carbonized and cured, with the carbon dioxide partial pressure controlled at 0.5 MPa for 3 days.

[0091] Comparative Example 4

[0092] (1) Preparation of coating powder: Weigh 50 parts of silicate cement, 20 parts of silica fume, 30 parts of fly ash, 10 parts of polyvinyl alcohol fiber, 10 parts of carbon fiber and 0.3 parts of tea stem cellulose, mix them in advance, stir them in a mixer at 30 r / min for 1 min to make coating powder.

[0093] (2) Wet granulation process: The process is the same as in Example 1.

[0094] (3) Natural curing process: After multiple wrappings, the treated slurry-coated recycled aggregate is obtained. The temperature is controlled at 20℃ and the relative humidity is 95% for 3 days.

[0095] Performance testing process

[0096] (1) Compressive strength test: The aggregate after carbonization curing is subjected to compressive strength test according to the "Test Method for Strength of Cement Mortar" (GB / T 17671-2021);

[0097] (2) Water absorption test: Take another 20g of aggregate and place it in a constant temperature drying oven. Set the temperature to 105±5℃ and dry for 24 hours until constant weight. Weigh the dried sample (M1) using an electronic balance. Then, completely immerse the sample in 25℃ deionized water. After 6 hours, take out the sample and gently wipe the surface water with moistened filter paper. Immediately weigh the sample after water absorption (M2).

[0098] (3) Dynamic pH monitoring: The treated recycled aggregate was soaked in deionized water (solid-liquid ratio 1:5), and the pH value was measured using a pH meter after 28 days. The performance test results of the above examples and comparative examples are summarized in Table 1.

[0099] Table 1. Proportioning and Performance Testing of Grating Materials

[0100]

[0101] As shown in Table 1, comparing Examples 1-3, the compressive strength significantly increased with the addition of tea stem cellulose from 0.1 parts to 0.3 parts, indicating that tea stem cellulose improves mechanical properties by filling pores and enhancing interfacial bonding. However, when the addition increased to 0.5 parts, the compressive strength decreased to 46.1 MPa, indicating that excessive tea stem cellulose led to agglomeration and weakened structural density. Furthermore, the pH of the extract decreased from 10.5 to 9.5 with the increase of tea saponin concentration from 0.1% to 0.5%, indicating that tea saponin chelates free Ca2+. 2+Inhibit the precipitation of cement hydration product Ca(OH)2, thereby reducing the alkalinity of the system.

[0102] As shown in Table 1, comparing Examples 2 and 4-5, the optimal dispersion of tea saponin and tea stem cellulose at a rotation speed of 45 r / min results in a dense three-dimensional network structure, effectively enhancing the mechanical properties of the aggregate coating. A rotation speed of 30 r / min, being too low, leads to uneven particle mixing and localized agglomeration of the coating slurry, weakening compressive strength. A rotation speed of 50 r / min, being too high, may cause excessive shearing, destroying the reinforcing effect of the coating slurry. As shown in Table 1, comparing Examples 2 and 6-8, the material combinations within the scope of claim 1 can all produce qualified recycled aggregates with high strength, low water absorption, and low alkalinity.

[0103] As shown in Table 1, comparing Example 2 with Comparisons 1-3, the synergistic enhancing effect of tea stem cellulose and tea saponin is evident. Example 2 (containing 0.3 parts tea stem cellulose + 0.3% tea saponin) exhibits a compressive strength of 49.7 MPa, significantly higher than Comparisons 1-3. Tea saponin chelates Ca with carboxylic acid groups via hydroxyl groups. 2+ It inhibits the formation of Ca(OH)2; the dense cellulose structure of tea stems hinders the formation of OH. - Diffusion: The pH of the leachate from Example 2 was 9.8, significantly better than the 12.3, 11.8, and 10.9 of the comparative examples. The cellulose in tea stems and tea saponins synergistically reduced the alkalinity of the system. Examples 2 and 4 show that carbonization curing improves compressive strength and inhibits water absorption. The water absorption rate of Example 2 was only 1.9%, a 39% decrease compared to 3.1% in Comparative Example 4. Furthermore, it regulates pH; the pH of the leachate from Example 2 was 9.8, while the pH of Comparative Example 4 (uncarbonized) was as high as 12.1.

[0104] The above specific embodiments are used to explain and illustrate the present invention, but not to limit the present invention. Any modifications and changes made to the present invention within the spirit and scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A type of coated recycled aggregate based on tea stem cellulose and tea saponin, characterized in that, It is composed of the following raw materials in parts by weight: 80-120 parts of recycled aggregate with a particle size of 4.75-9.5mm, 40-55 parts of cement, 15-20 parts of silica fume, 30-40 parts of fly ash, 10-15 parts of polyvinyl alcohol fiber, 8-10 parts of carbon fiber, 0.1-0.5 parts of tea stem cellulose, 0.01-0.05 parts of tea saponin, and water; The cement is silicate cement. The silicate cement, silica fume, fly ash, polyvinyl alcohol fiber, carbon fiber and tea stem cellulose are mixed to obtain the coated powder. The tea saponin is prepared into a tea saponin solution with a concentration of 0.1%-0.5%. The recycled aggregate is sprayed with the tea saponin solution once in the granulator and then the coated powder is added again. This cycle is repeated multiple times until the tea saponin solution and the coated powder are completely fed to obtain the coated recycled aggregate. The tea stem cellulose in the components is obtained by acid hydrolysis purification, water washing, freeze drying and grinding of tea stems; the tea saponin is obtained by defatting tea stems, alcohol extraction and filtration.

2. The slurry-coated recycled aggregate according to claim 1, characterized in that, The extraction steps of tea stem cellulose are as follows: (1) Raw material pretreatment: Dehydrate the tea stem to a moisture content of <5%, crush it into 80-100 mesh powder by a pulverizer, then sieve to remove impurities and collect the sieve material; (2) Acid hydrolysis purification: Put sulfuric acid solution and tea stem powder into a reaction vessel and continuously stir mechanically in a constant temperature water bath at 30-55℃ to destroy the lignin-hemicellulose composite structure; (3) Water washing: Transfer the acid hydrolysis residue into distilled water to further remove residual acid from the surface of the residue; (4) Finished product preparation: Use freeze drying process to solidify the fiber, and then use a ball mill to dry grind it to obtain tea stem cellulose powder.

3. The slurry-coated recycled aggregate according to claim 1, characterized in that, The extraction steps of tea saponin are as follows: (1) Acetone defatting pretreatment: tea stem powder and acetone are loaded into filter paper tubes at a weight ratio of 1:4-6, and extracted in a water bath at 60-100℃ to remove oily impurities. The extracted tea stem powder is collected and dried until there is no acetone odor. (2) Ethanol gradient extraction: defatted tea powder and 60% ethanol are added at a solid-liquid ratio of 1:8-15 and extracted with ultrasonic enhancement. (3) Product preparation: the extract is filtered to obtain solid. Saturated sodium chloride solution is added at a volume ratio of solid to saturated sodium chloride solution of 1:1-2, and the precipitate is collected by centrifugation.

4. The slurry-coated recycled aggregate according to claim 1, characterized in that, The fly ash is grade I fly ash, the polyvinyl alcohol fiber is a high modulus type with a single filament diameter of 20-40μm, and the carbon fiber is a polyacrylonitrile-based carbon fiber with a single filament diameter of 7-12μm.

5. A method for preparing low-alkali recycled aggregate according to any one of claims 1-4, characterized in that, Includes the following steps: S1. First, mix 40-55 parts of silicate cement, 15-20 parts of silica fume, 30-40 parts of fly ash, 10-15 parts of polyvinyl alcohol fiber, 8-10 parts of carbon fiber, and 0.1-0.5 parts of tea stalk cellulose to form a coated powder. S2. Place the recycled aggregate into the turned-on pellet mill and spray it with a 0.1%-0.5% tea saponin solution using a water sprayer. S3. Add the coating powder to granulate. After that, repeat the spraying and powder addition granulation process in S2-S3. That is, before each addition of coating powder, spray the surface of the recycled aggregate with tea saponin solution using a water sprayer. S4. After multiple wrapping processes, the treated slurry-coated recycled aggregate is obtained and then subjected to carbonization curing.

6. The preparation method according to claim 5, characterized in that, In step S3, the granulator speed is 30-50 r / min.

7. The preparation method according to claim 6, characterized in that, In step S4, the carbonization curing method involves controlling the partial pressure of carbon dioxide in the carbonization curing chamber at 0.2-0.8 MPa and the curing time at 1-5 days.

8. The application of the mortar-coated recycled aggregate according to any one of claims 1-4 in construction.

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