Alkali-activated modified recycled coarse aggregate and preparation method thereof

Abstract

The invention belongs to the field of building materials, and discloses alkali-activated modified recycled coarse aggregate and a preparation method thereof. The method comprises the following steps: taking waste concrete as a raw material, crushing and screening to obtain recycled coarse aggregate; soaking in a sodium silicate solution, coating with volcanic ash slurry, and carrying out constant-temperature thermocuring treatment, so as to obtain the modified recycled coarse aggregate. Wherein the sodium silicate solution has the effects of an alkali activator and a cementing agent, the volcanic ash slurry provides an active component for the aggregate, and the alkali activation reaction process is enhanced through constant-temperature thermocuring. The water absorption rate of the prepared modified coarse aggregate is as low as 1.25%, the crushing value of the aggregate is reduced to 10.1%, and the relative maximum reduction amplitude of the porosity can reach 18.8%. When the concrete is used for preparing C60-grade concrete, the compressive strength at the age of 7d, 28d and 90d is respectively improved by 27.4%, 17.1% and 19.3% at most. According to the modification method disclosed by the invention, the physical property of the recycled coarse aggregate can be improved, the porosity is reduced, and the mechanical property of the recycled concrete at different ages is enhanced.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of solid waste resource utilization of building materials, and particularly relates to an alkali-activated modified recycled coarse aggregate and a preparation method thereof. BACKGROUND

[0002] Concrete is a basic material for construction engineering, and its preparation consumes a large amount of natural coarse aggregate. However, the reserves of natural coarse aggregate are increasingly scarce, and under this background, reasonable utilization of waste concrete recycled coarse aggregate has become an important way to reduce the consumption of natural resources and improve the resource utilization rate of construction waste. Waste concrete can be prepared into recycled coarse aggregate of different particle sizes through processes such as disassembly, crushing and screening. However, compared with natural coarse aggregate, recycled coarse aggregate has many inherent defects, such as high water absorption, large crushing index and weak interface transition zone performance, which greatly restricts its large-scale promotion and engineering application in concrete production.

[0003] In order to effectively solve the above problems, scholars have carried out extensive research on the modification methods of recycled coarse aggregate in recent years. At present, the main modification methods of aggregate are mainly divided into mechanical modification, chemical modification, microbial modification and heat treatment. The above modification methods can greatly reduce the water absorption and porosity of recycled coarse aggregate, and improve the apparent density of the aggregate, which is beneficial to the efficient recycling of waste concrete resources and alleviates the shortage of natural sand resources. However, single modification method has unavoidable limitations, such as low mechanical modification efficiency, uneven chemical modification effect, high heat treatment process cost and strict microbial modification operation conditions.

[0004] At present, there are related explorations on the composite modification method of recycled coarse aggregate, but most of the researches rarely introduce factory solid waste such as volcanic ash material as a modified component, and the composite modification technology considering solid waste resource utilization and modification effect is relatively scarce. The alkali activation of chemical modification and volcanic ash material can realize the multiple effects of old mortar pore filling and weak interface strengthening of recycled coarse aggregate, greatly improve the modification efficiency, and has the advantages of controllable cost and low energy consumption, which is an important development direction for the modification of recycled coarse aggregate in the future. SUMMARY

[0005] The purpose of this invention is to provide a low-cost, high-performance alkali-activated modified recycled coarse aggregate and its preparation method. This method, through composite modification involving alkali solution soaking and coating with pozzolanic material slurry, effectively solves the problem of poor mechanical properties in recycled concrete caused by directly using untreated recycled coarse aggregate. The modified recycled coarse aggregate exhibits significantly superior physical properties compared to unmodified recycled coarse aggregate. Moreover, using this modified recycled coarse aggregate in concrete preparation not only greatly improves the mechanical and durability properties of concrete but also reduces the extraction of natural sand and gravel resources and cement usage, thereby lowering carbon emissions throughout the entire life cycle of concrete production. This has significant practical implications for promoting the green and low-carbon transformation of the construction industry and achieving sustainable development.

[0006] To achieve the above objectives, the present invention employs the following technical solution: a method for preparing alkali-activated modified recycled coarse aggregate, comprising the following steps: (1) Select low-strength waste concrete as raw material, and process it by crushing, screening and impurity removal to obtain recycled coarse aggregate; (2) Soak the recycled coarse aggregate in an industrial grade sodium silicate solution with a concentration of 2-6 wt% and a modulus of 3.0-3.2 at room temperature for 8-10 hours. After soaking, remove the aggregate and filter out the excess solution to obtain the recycled coarse aggregate pretreated with sodium silicate solution. (3) Mix the pretreated recycled coarse aggregate with the activated volcanic ash slurry. After the aggregate surface is evenly and fully coated with the slurry, remove it, remove the excess flowing slurry from the surface, and then heat and solidify it to prepare alkali-activated modified recycled coarse aggregate.

[0007] Further improvements to the preparation method of alkali-activated modified recycled coarse aggregate: Preferably, in step (1), the strength grade of the low-strength waste concrete is C25-C40.

[0008] Preferably, in step (1), the particle size range of the waste concrete before crushing is 30-50mm.

[0009] Preferably, in step (1), the impurities include wood and metal impurities.

[0010] Preferably, in step (1), the particle size range of the recycled coarse aggregate is 4.75-16mm.

[0011] Preferably, in step (3), the activated pozzolanic slurry is a pozzolanic slurry prepared from high-purity silica fume, with a water-to-ash ratio of 2.2-2.4, a SiO2 content of ≥96% in the silica fume, and a specific surface area of ​​20,000-25,000 m². 2 / kg.

[0012] Preferably, in step (3), the activated pozzolanic slurry is a pozzolanic slurry prepared from ultrafine fly ash, with a water-ash ratio of 0.8-0.9, an ultrafine fly ash fineness of 2-4 micrometers, and a bulk density of 550-700 kg / m³. 3 .

[0013] Preferably, in step (3), the mixing mass ratio of pretreated recycled coarse aggregate to activated pozzolanic slurry is (1.0-1.2):1.

[0014] Preferably, in step (3), the heating and curing temperature is 60-65℃ and the time is 6-8 hours.

[0015] The second objective of this invention is to provide an alkali-activated modified recycled coarse aggregate prepared by any of the above-mentioned methods.

[0016] As a further technical solution for the above-mentioned alkali-activated modified recycled coarse aggregate: Preferably, the alkali-activated modified recycled coarse aggregate is used to replace natural aggregate in concrete.

[0017] The advantages of this invention compared to the prior art are as follows: (1) This invention provides a low-cost, high-performance alkali-activated modified recycled coarse aggregate preparation method. First, waste concrete is crushed and screened to obtain untreated recycled coarse aggregate. Then, the untreated recycled coarse aggregate is soaked in solution and coated with slurry to obtain modified recycled coarse aggregate.

[0018] Industrial-grade sodium silicate solution was selected for the soaking solution, as it exhibits good alkali activation and is cost-effective. Different sodium silicate concentrations were used to explore the optimal alkali activation concentration. Lower concentrations made it difficult for the solution to penetrate the micropores on the aggregate surface, while higher concentrations might create new weak layers on the recycled coarse aggregate, negatively impacting overall aggregate density. Sufficient soaking time was designed to ensure the solution fully penetrated the pores of the recycled coarse aggregate and initiated a reaction. During the modification process, sodium silicate effectively dissolved residual active components inside and on the surface of the recycled coarse aggregate, providing a sufficient material basis for subsequent pozzolanic reactions and creating a stable alkaline reaction environment. Furthermore, the sodium silicate solution penetrated the cracks and pores inside the recycled coarse aggregate, generating viscous silica gel on its interior and surface, thereby repairing micro-cracks in the old mortar on the aggregate surface and reducing the porosity and water absorption rate of the recycled coarse aggregate.

[0019] The coating slurry uses activated volcanic ash material to improve the utilization rate of solid waste in the factory. The soaked recycled coarse aggregate is mixed with the volcanic ash slurry, and an appropriate mixing method is used to ensure that the slurry fully coats the aggregate. This facilitates a more complete alkali activation reaction or hydration of the aggregate during the subsequent heating and curing process, resulting in better modification of the recycled coarse aggregate. The water-cement ratio and coating time of the slurry are designed to ensure complete slurry coverage on the surface of the recycled coarse aggregate after mixing.

[0020] (2) The method for preparing alkali-activated modified recycled coarse aggregate described in this invention has the dual advantages of low cost and excellent modification effect. This method utilizes the synergistic effect of alkali solution soaking and pozzolanic slurry coating to drive the hydration reaction to proceed fully. Through the generation and deposition of hydration products, the internal pores of the aggregate are filled and micro-cracks are repaired, making the aggregate structure more compact, thereby significantly reducing the water absorption, crushing index, and porosity of the recycled coarse aggregate. Using this modified aggregate to prepare recycled concrete can significantly improve the mechanical properties of the recycled concrete. At the same time, the preparation method has a simple process flow and is suitable for large-scale industrial production. Attached Figure Description

[0021] Figure 1 It is the water absorption rate of the recycled coarse aggregates prepared in Examples 1-6 and Comparative Examples 1-5.

[0022] Figure 2 It is the crushing value of the recycled coarse aggregate prepared in Examples 1-6 and Comparative Examples 1-5.

[0023] Figure 3 It refers to the porosity of the recycled coarse aggregates prepared in Examples 2-3, 5-6 and Comparative Examples 4-5.

[0024] Figure 4 This is a comparison chart of the 7-day compressive strength of concrete samples prepared in Examples 7-12, Comparative Examples 6-10, Control Group, and Blank Group.

[0025] Figure 5 This is a comparison chart of the 28-day compressive strength of concrete samples prepared in Examples 7-12, Comparative Examples 6-10, Control Group, and Blank Group.

[0026] Figure 6 This is a comparison chart of the 90-day compressive strength of concrete samples prepared in Examples 7-12, Comparative Examples 6-10, Control Group, and Blank Group. Detailed Implementation

[0027] The present invention will now be described in detail through specific embodiments. These embodiments are provided to enable a more thorough understanding of the present invention and to fully convey the scope of the invention to those skilled in the art. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art. Unless otherwise specified, the experimental methods in the following embodiments are conventional methods. Unless otherwise specified, the reagents and materials used are commercially available.

[0028] The recycled coarse aggregate used in this embodiment comes from waste bridge concrete with a strength grade of C30. After crushing, screening and removal of impurities, recycled coarse aggregate with a particle size range of 4.75-16 mm is obtained. The main components are hardened cement mortar and original aggregate mineral components, such as hydrated calcium silicate, calcium hydroxide, quartz, calcite, etc.

[0029] Henan Borun Foundry Materials Co., Ltd. uses two types of pozzolanic slurry: one is a pozzolanic slurry (denoted as SF) prepared with high-purity silica fume, with a water-to-ash ratio of 2.4, a SiO2 content of ≥96% in the silica fume, and a specific surface area of ​​20,000-25,000 m². 2 / kg; a slurry prepared from ultrafine fly ash (denoted as FA) has a water-cement ratio of 0.8, an ultrafine fly ash fineness of 3 micrometers, and a bulk density of 550-700 kg / m³. 3 .

[0030] The cement is Conch PO 42.5 silicate cement, purchased from Anhui Conch Cement Ningguo Cement Plant.

[0031] The river sand was purchased locally and has a fineness modulus of approximately 2.82.

[0032] The water-reducing agent selected is a polycarboxylate-based water-reducing agent with a water reduction rate of not less than 30%, which was purchased from Subote New Materials Co., Ltd.

[0033] The natural coarse aggregate is natural limestone, produced by Hebei Hongyao Minerals Co., Ltd., with a particle size range of 4.75-16 mm.

[0034] Example 1 This embodiment provides a method for preparing alkali-activated modified recycled coarse aggregate, including the following steps: (1) Collect waste concrete (strength grade C30) with a particle size of 30-50 mm, crush the waste concrete with a small crusher, remove wood and metal impurities by screening, and finally obtain recycled coarse aggregate with a particle size range of 4.75-16 mm. (2) Soak the recycled coarse aggregate in an industrial grade fast-dissolving sodium silicate solution with a concentration of 2% and a modulus of 3.0 for 8 hours. Stir the recycled coarse aggregate every 2 hours to release surface bubbles. Take it out and filter out excess solution until the recycled coarse aggregate reaches a surface dry-wet state to obtain the soaked recycled coarse aggregate. (3) Take the pozzolanic slurry (water-ash ratio of 0.8) prepared with ultrafine fly ash. The fineness of the ultrafine fly ash is 3 micrometers and the bulk density is 550 kg / m³. 3 The soaked recycled coarse aggregate was mixed with volcanic ash slurry at a mass ratio of 1:1 to ensure that the aggregate surface was fully coated with slurry. After coating, the coarse aggregate was removed and excess flowing slurry was removed. The aggregate was then placed in a 60°C constant temperature oven for heating and curing for 8 hours to finally obtain alkali-activated modified recycled coarse aggregate 1, which corresponds to 2%-FA in the attached figure.

[0035] Example 2 This embodiment provides a method for preparing alkali-activated modified recycled coarse aggregate. The specific steps are the same as in Example 1, except that in step (2), the recycled coarse aggregate is immersed in an industrial-grade sodium silicate solution with a concentration of 4 wt% and a modulus of 3.1 for 9 hours. Finally, alkali-activated modified recycled coarse aggregate 2 is obtained, corresponding to 4%-FA in the attached figure.

[0036] Example 3 This embodiment provides a method for preparing alkali-activated modified recycled coarse aggregate. The specific steps are the same as in Example 1, except that in step (2), the recycled coarse aggregate is soaked in an industrial-grade sodium silicate solution with a concentration of 6 wt% and a modulus of 3.2 for 10 hours. Finally, alkali-activated modified recycled coarse aggregate 3 is obtained, corresponding to 6%-FA in the attached figure.

[0037] Example 4 This embodiment provides a method for preparing alkali-activated modified recycled coarse aggregate. The specific steps are the same as in Example 1, except that step (3) uses high-purity silica fume to prepare pozzolanic slurry (water-ash ratio of 2.4), with SiO2 content ≥96% and specific surface area of ​​22000 m². 2 / kg. The final product is alkali-activated modified recycled coarse aggregate 4, corresponding to 2%-SF in the attached figure.

[0038] Example 5 This embodiment provides a method for preparing alkali-activated modified recycled coarse aggregate. The specific steps are the same as in Example 4, except that in step (2), the recycled coarse aggregate is soaked in an industrial-grade sodium silicate solution with a concentration of 4 wt% and a modulus of 3.0 for 8 hours. Furthermore, in step (3), a volcanic ash slurry (water-ash ratio of 2.4) prepared from high-purity silica fume is used, with a SiO2 content ≥96% and a specific surface area of ​​20000 m². 2 / kg. The final product is alkali-activated modified recycled coarse aggregate 5, corresponding to 4%-SF in the attached figure.

[0039] Example 6 This embodiment provides a method for preparing alkali-activated modified recycled coarse aggregate. The specific steps are the same as in Example 4, except that in step (2), the recycled coarse aggregate is soaked in an industrial-grade sodium silicate solution with a concentration of 6 wt% and a modulus of 3.0 for 8 hours. Furthermore, in step (3), a volcanic ash slurry (water-ash ratio of 2.4) prepared from high-purity silica fume is used, with a SiO2 content ≥96% and a specific surface area of ​​25000 m². 2 / kg. The final product is alkali-activated modified recycled coarse aggregate 6, corresponding to 6%-SF in the attached figure.

[0040] Comparative Example 1 This comparative example provides a method for processing recycled coarse aggregate. The specific steps are the same as in Example 1. The only difference is that the recycled coarse aggregate after soaking in step (2) is not coated with volcanic ash slurry in step (3). Instead, it is placed directly in a constant temperature box and heated and cured at 60°C for 8 hours to obtain recycled coarse aggregate 1, which corresponds to 2 in the attached figure.

[0041] Comparative Example 2 This comparative example provides a method for processing recycled coarse aggregate. The specific steps are the same as in Example 2. The only difference is that the recycled coarse aggregate after soaking in step (2) is not coated with volcanic ash slurry in step (3), but is directly placed in a 60°C constant temperature box for heating and curing for 8 hours to obtain recycled coarse aggregate 2, which corresponds to 4 in the figure.

[0042] Comparative Example 3 This comparative example provides a method for processing recycled coarse aggregate. The specific steps are as described in Example 3. The only difference is that the recycled coarse aggregate after soaking in step (2) is not coated with volcanic ash slurry in step (3), but is directly placed in a 60°C constant temperature oven for heating and curing for 8 hours to obtain recycled coarse aggregate 3, which corresponds to 6 in the figure.

[0043] Comparative Example 4 This comparative example provides a method for processing recycled coarse aggregate. The specific steps are the same as in Example 1. The only difference is that the recycled coarse aggregate in step (1) is not soaked in solution in step (2), but is directly coated with volcanic ash slurry in step (3) to obtain recycled coarse aggregate 4, which corresponds to 0%-FA in the figure.

[0044] Comparative Example 5 This comparative example provides a method for processing recycled coarse aggregate. The specific steps are the same as in Example 4, except that the recycled coarse aggregate from step (1) is not subjected to the solution soaking treatment in step (2), but is directly subjected to the pozzolanic slurry coating treatment in step (3). In step (3), a pozzolanic slurry (water-ash ratio of 2.4) prepared with high-purity silica fume is used. Finally, recycled coarse aggregate 5 is obtained, corresponding to 0%-SF in the attached figure.

[0045] Table 1 Comparison of treatments in Examples 1-6 and Comparative Examples 1-5

[0046] Figure 1 The graph shows the aggregate water absorption test values ​​for the control group (natural coarse aggregate, collected from Hefei Cement Institute, sourced from a demolished old bridge, design grade C30, denoted as NA), the blank group (recycled coarse aggregate prepared in step (1) of Example 1, denoted as RA), Examples 1-6, and Comparative Examples 1-5. The water absorption test was conducted in two parallel groups according to GB / T 14685-2022 "Construction Gravel and Crushed Stone" standard. Figure 1 It can be seen that the aggregate water absorption rate of Examples 1-6 and Comparative Examples 1-5 is lower than that of the blank group, while only Examples 3, 5 and 6 have water absorption rates lower than that of the control group.

[0047] Figure 2 The graph shows the aggregate crushing value test values ​​of the control group (natural coarse aggregate, collected from Hefei Cement Institute, sourced from a demolished old bridge, design grade C30, denoted as NA), the blank group (recycled coarse aggregate prepared in step (1) of Example 1, denoted as RA), Examples 1-6, and Comparative Examples 1-5. According to GB / T 14685-2022 "Construction Gravel and Crushed Stone" standard, two parallel crushing value experiments were conducted. Figure 2 It can be seen that the aggregate crushing values ​​of Examples 1-6 and Comparative Examples 1-5 are all lower than those of the blank group, with only Examples 3 and 6 showing crushing values ​​lower than the control group. The reduction in water absorption and crushing value indicates a significant decrease in porosity and microcracks on the surface of the alkali-activated modified aggregate, thereby promoting the improvement of the aggregate's own strength. Water absorption experiments show that the water absorption of the virgin aggregate is reduced to a maximum of 1.25% under the modification process, and crushing value experiments show that the crushing value of the virgin aggregate can be as low as 10.1% under the modification process, lower than the crushing value of natural limestone aggregate.

[0048] To investigate the effect of synergistic modification on the porosity of recycled coarse aggregate, mercury intrusion porosimetry was performed on the aggregates from the control group, blank group, Examples 2-3, 5-6, and Comparative Examples 4-5 to test the aggregate porosity and void ratio. The results are as follows: Figure 3 As shown. By Figure 3It can be seen that, compared with the blank group, the porosity of the recycled coarse aggregate in Comparative Examples 4-5 remained almost unchanged, corresponding to an increase in the proportion of macropores; while in Examples 2-3 and 5-6, the porosity of the recycled coarse aggregate decreased relatively, and the proportion of macropores decreased while the proportion of micropores increased. The decrease in porosity and the reduction in the proportion of macropores both indicate that the pore structure of the alkali-activated modified aggregate is improved, resulting in a denser overall structure. Conversely, an increase in porosity indicates a poorer pore structure and a more porous overall structure.

[0049] control group Concrete specimens were prepared with dimensions of 100mm × 100mm × 100mm, a water-cement ratio of 0.35, a sand ratio of 0.36, and cured under standard conditions for 28 days. The raw materials are as follows: Take 450g of PO 42.5 cement, 650g of river sand, 158g of water, 4.5g of water-reducing agent, and 1150g of natural coarse aggregate (collected from Hefei Cement Institute, sourced from a demolished old bridge, design grade C30, denoted as NA), mix and mold to prepare a 100mm×100mm×100mm cubic concrete specimen.

[0050] Blank group Concrete specimens were prepared with dimensions of 100mm × 100mm × 100mm, a water-cement ratio of 0.35, a sand ratio of 0.36, and cured under standard conditions for 28 days. The raw materials are as follows: Take 450g of PO 42.5 cement, 650g of river sand, 158g of water, 4.5g of water-reducing agent, and 1150g of recycled coarse aggregate (denoted as RA) prepared in step (1) of Example 1, mix and mold to prepare concrete samples.

[0051] Examples 7-12 The alkali-activated modified recycled coarse aggregates 1-6 obtained in Examples 1-6 were used to replace all natural aggregates in the preparation of concrete specimens. The concrete specimens were 100mm × 100mm × 100mm in size, with a water-cement ratio of 0.35 and a sand ratio of 0.36, and were cured under standard conditions for 28 days. The raw materials are as follows: Take 450g of PO 42.5 cement, 650g of river sand, 158g of water, 4.5g of water-reducing agent, and 1150g of one of the alkali-activated modified recycled coarse aggregates 1-6 prepared in Examples 1-6, mix and mold to prepare concrete samples.

[0052] Comparative Examples 6-10 Recycled coarse aggregate 1-5 obtained from Comparative Examples 1-5 was used to replace all natural aggregates in the preparation of concrete specimens. The concrete specimens were 100mm × 100mm × 100mm in size, with a water-cement ratio of 0.35 and a sand ratio of 0.36. Curing was carried out under standard conditions for 28 days. The raw materials are as follows: Take 450g of PO 42.5 cement, 650g of river sand, 158g of water, 4.5g of water-reducing agent, and 1150g of one of the recycled coarse aggregates 1-5 prepared in comparative examples 1-5, mix and mold to prepare concrete samples.

[0053] The concrete samples prepared in Examples 7-12 and Comparative Examples 6-10 were tested for compressive strength at 7 days, 28 days, and 90 days under standard curing conditions. The test results are shown in the table below. Strength diagrams at different ages are also provided for reference, as attached. Figures 4-6 As shown.

[0054] Table 2. Strength comparison of concrete prepared in Examples 7-12 and Comparative Examples 6-10

[0055] Figure 4 This figure shows the 7-day compressive strength test values ​​of concrete specimens prepared in the control group, blank group, Examples 7-12, and Comparative Examples 6-10. The items in the figure are represented by the abbreviations of the coarse aggregate used. The testing procedure followed GB / T 50081-2019 "Standard for Test Methods of Physical and Mechanical Properties of Concrete". Three 100mm cube specimens from each group were tested using a compression testing machine, and the average value of the test results was taken. Figure 4 It can be seen that the compressive strength of the concrete in the comparative example and the embodiment is higher than that of the blank group, and the compressive strength of the concrete in Examples 8-12 is much higher than that of the control group.

[0056] Figure 5 This figure shows the 28-day compressive strength test values ​​of concrete specimens prepared in the control group, blank group, Examples 7-12, and Comparative Examples 6-10. The items in the figure are represented by abbreviations for the coarse aggregate used. The testing procedure followed GB / T 50081-2019 "Standard for Test Methods of Physical and Mechanical Properties of Concrete". Three 100mm cube specimens from each group were tested using a compression testing machine, and the average value of the test results was taken. Figure 5 It can be seen that the compressive strength of the examples and comparative examples is higher than that of the blank group. The compressive strength of concrete in comparative examples 7, 8, 10 and examples 9, 11, 12 is much higher than that of the control group.

[0057] Figure 6 This figure shows the 90-day compressive strength test values ​​of concrete specimens prepared in the control group, blank group, Examples 7-12, and Comparative Examples 6-10. The items in the figure are represented by the abbreviations of the coarse aggregate used. The testing procedure followed GB / T 50081-2019 "Standard for Test Methods of Physical and Mechanical Properties of Concrete". Three 100mm cube specimens from each group were tested using a compression testing machine, and the average value of the test results was taken. Figure 6It can be seen that the compressive strength of the examples and comparative examples is higher than that of the blank group, and the compressive strength of the concrete in Examples 7-12 is much higher than that of the control group.

[0058] As can be seen from Table 2 and Figures 4–6, if unmodified recycled coarse aggregate is used to completely replace natural coarse aggregate during the concrete preparation process, the concrete strength will decrease significantly. However, after treatment with the composite modification process proposed in this study, the engineering application value of recycled coarse aggregate can be effectively improved, and its inherent defects such as microcracks and high porosity caused by the crushing process can be compensated, so that the performance of modified recycled coarse aggregate is close to that of natural coarse aggregate.

[0059] Specifically, the blank group used unmodified recycled coarse aggregate to replace 100% of natural coarse aggregate in the concrete preparation. Compared with the control group, its compressive strength showed a significant decreasing trend. The experimental results of concrete specimens from the control group, blank group, Examples 7–12, and Comparative Examples 6–10 showed that different modification methods could improve the mechanical properties of concrete to varying degrees: compared with the unmodified group's recycled coarse aggregate prepared concrete, the modified group's recycled concrete showed the highest increase in compressive strength at 7 days (27.4%), 28 days (17.1%), and 90 days (19.3%). Among them, the recycled coarse aggregate used in Comparative Examples 6–10 was modified by soaking in sodium silicate solution or coating with pozzolanic slurry alone. The improvement in compressive strength of the concrete at different ages was relatively limited, far less than the improvement achieved by a combination of sodium silicate solution and pozzolanic slurry modification. The reason for this is that alkaline sodium silicate solution can effectively activate the active components in pozzolanic materials, promote the activation reaction process on the surface of recycled coarse aggregate, and the corresponding activation products generated can repair the microcracks in the recycled aggregate itself, thereby further enhancing the aggregate performance. In summary, the synergistic modification effect of the two methods is significantly better than that of a single modification method, fully verifying that the composite modification method proposed in this study has the technical advantages of synergistic efficiency.

[0060] The modification method employed in this invention has the advantages of low cost and low energy consumption. The modifying materials used are readily available and have been experimentally verified to have excellent modification effects, making them suitable for large-scale industrial production. The above description is merely a preferred embodiment of this application and is not intended to limit the application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A method for producing alkali-activated modified recycled coarse aggregate, characterized by, The method comprises the following steps: (1) selecting low-strength grade waste concrete as raw material, crushing, screening and removing impurities to obtain recycled coarse aggregate; (2) placing the recycled coarse aggregate in an industrial-grade sodium silicate solution with a concentration of 2-6 wt% and a modulus of 3.0-3.2 for room temperature soaking for 8-10 hours, taking out after soaking and filtering out the excess solution, to obtain the recycled coarse aggregate pretreated by the sodium silicate solution; (3) mixing and stirring the pretreated recycled coarse aggregate with active volcanic ash slurry, taking out after the aggregate surface is uniformly and fully coated with the slurry, removing the excess flowing slurry on the surface, and then performing heating and curing treatment to obtain the alkali-activated modified recycled coarse aggregate.

2. The method for producing alkali-activated modified recycled coarse aggregate according to claim 1, characterized by, In step (1), the low-strength grade waste concrete has a strength grade of C25-C40.

3. The method for producing alkali-activated modified recycled coarse aggregate according to claim 1, characterized by, In step (1), the particle size of the waste concrete before crushing ranges from 30 mm to 50 mm.

4. The method for producing alkali-activated modified recycled coarse aggregate according to claim 1, characterized by, In step (1), the impurities include wood and metal impurities.

5. The method for producing alkali-activated modified recycled coarse aggregate according to claim 1, characterized by, In step (1), the particle size of the recycled coarse aggregate ranges from 4.75 mm to 16 mm.

6. The method for producing alkali-activated modified recycled coarse aggregates according to claim 1, characterized in that, In step (3), the active volcanic ash slurry is a high-purity silica ash prepared volcanic ash slurry, with a water-cement ratio of 2.2-2.4, a SiO2 content in the silica ash ≥ 96%, and a specific surface area of 20,000-25,000 m 2 / kg.

7. The method for producing alkali-activated modified recycled coarse aggregates according to claim 1, characterized by, In step (3), the active volcanic ash slurry is a volcanic ash slurry prepared from superfine fly ash, with a water-cement ratio of 0.8-0.9, a fineness of the superfine fly ash of 2-4 microns, and a bulk density of 550-700 kg / m 3 .

8. The method for producing alkali-activated modified recycled coarse aggregate according to claim 1 or 6 or 7, characterized in that, In step (3), the mixing mass ratio of the pretreated recycled coarse aggregate to the active volcanic ash slurry is (1.0-1.2):

1.

9. The method for producing alkali-activated modified recycled coarse aggregates according to claim 1, characterized by, In step (3), the heating and curing temperature is 60-65°C, and the time is 6-8 hours.

10. The alkali-activated modified recycled coarse aggregate prepared by the method according to any one of claims 1-9.

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