A second-generation C50 recycled concrete applicable to severe cold regions and its preparation method

Abstract

The present invention discloses a C50 second-generation recycled concrete applicable to severe cold regions and a preparation method thereof. The recycled concrete is composed of the following raw materials in parts by weight: 385-400 parts of cement, 5-10 parts of nano-silica, 65-75 parts of silica fume, 660-680 parts of river sand, 900-920 parts of second-generation recycled coarse aggregate, 5-7 parts of basalt fiber, 130-150 parts of water, 2.3 parts of water reducer, 0.02 parts of defoamer and 0.02-0.04 parts of air-entraining agent. By crushing the waste recycled concrete into second-generation recycled coarse aggregate and then using it in concrete, the present invention effectively alleviates the problem of resource shortage, reduces carbon emissions, and promotes the green and healthy development of the construction industry. The combined use of nano-silica and silica fume synergistically enhances the crack resistance of the recycled concrete. At the same time, the addition of basalt fiber improves the integrity of the recycled concrete, can inhibit the cracks caused by hydration heat to a certain extent, and improves the obvious defects of the second-generation recycled coarse aggregate.

Description

Technical Field

[0001] The present invention belongs to the technical field of building material design, and particularly relates to a C50 second-generation recycled concrete applicable to cold regions and a preparation method thereof. Background Art

[0002] With the development of recycled concrete preparation technology, the utilization rate of recycled concrete in building structures has been continuously improved. The first batch of structures built with recycled concrete in China has reached the service life or failed, facing the fate of being demolished, which will lead to environmental pollution and waste of resources. Crushing waste recycled concrete into second-generation recycled coarse aggregate and then using it in concrete can not only alleviate the problem of resource shortage, but also reduce carbon emissions and promote the green and healthy development of the construction industry.

[0003] Compared with the first-generation recycled coarse aggregate, the second-generation recycled coarse aggregate has obvious defects, including multiple interfacial transition zones, multiple pores, multiple cracks, etc. Due to the use of air-entraining agents in recycled concrete in cold regions, there are more harmful capillary pores, connected pores and weak interfacial transition zones in the mortar adhered to the surface of the second-generation recycled coarse aggregate. These serious defects make it difficult for the second-generation recycled coarse aggregate to be applied to freeze-resistant structural concrete again.

[0004] Multiple types of freeze-resistant concrete are provided in the prior art. For example, the patent application with the publication number CN115321914A discloses a preparation method of freeze-thaw resistant recycled concrete. It first strengthens the recycled coarse aggregate, and then combines it with water, cement, natural sand, basalt fiber and water reducer, which can withstand 200 freeze-thaw cycles and the strength can reach 35.61 MPa. The patent application with the publication number CN109574567A discloses a freeze-resistant recycled concrete and its preparation method, which is prepared by mixing cement, coarse aggregate, recycled coarse aggregate, fine aggregate, fly ash, slag powder, silica fume, recycled rubber powder, air-entraining agent, water reducer, modified polypropylene fiber, lignin fiber and water, and can withstand 335 freeze-thaw cycles and the strength can reach 35.5 MPa. The recycled coarse aggregate in the above patents is all the first-generation recycled coarse aggregate, and the strength of the recycled concrete is far less than 50 MPa, and it is difficult to be used for 50 years in cold regions, that is, to withstand 400 freeze-thaw cycles. Summary of the Invention

[0005] The purpose of this part is to outline some aspects of the embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this part, as well as in the abstract and title of the present application, to avoid obscuring the purpose of this part, the abstract and the title, and such simplifications or omissions cannot be used to limit the scope of the present invention.

[0006] In view of the above and / or problems existing in the prior art, the present invention is proposed.

[0007] Therefore, the object of the present invention is to overcome the deficiencies in the prior art and provide a C50 second-generation recycled concrete suitable for severe cold regions.

[0008] To solve the above technical problems, the present invention provides the following technical solutions: The concrete raw material components include

[0009] 385 - 400 parts of cement, 5 - 10 parts of nano-silica, 65 - 75 parts of silica fume, 660 - 680 parts of river sand, 900 - 920 parts of second-generation recycled coarse aggregate, 5 - 7 parts of basalt fiber, 130 - 150 parts of water, 2.3 parts of water reducer, 0.02 parts of defoamer, and 0.02 - 0.04 parts of air-entraining agent.

[0010] As a preferred embodiment of the C50 second-generation recycled concrete suitable for severe cold regions of the present invention, wherein: The components of the cement include CaO, SiO 2 , Fe 2 O 3 , Al 2 O 3 .

[0011] As a preferred embodiment of the C50 second-generation recycled concrete suitable for severe cold regions of the present invention, wherein: The particle size of the nano-silica is 5 - 50 nm, and the specific surface area > 300000 m 2 / kg.

[0012] As a preferred embodiment of the C50 second-generation recycled concrete suitable for severe cold regions of the present invention, wherein: The average particle size of the silica fume is 2.54 μm, and the content of silicon dioxide > 96%.

[0013] As a preferred embodiment of the C50 second-generation recycled concrete suitable for severe cold regions of the present invention, wherein: The components of the second-generation recycled coarse aggregate include CaCO 3 , Ca(OH) 2 , SiO 2 .

[0014] As a preferred embodiment of the C50 second-generation recycled concrete suitable for severe cold regions of the present invention, wherein: The water reducer is a polycarboxylate water reducer, and the defoamer is an organosilicon defoamer.

[0015] As a preferred embodiment of the C50 second-generation recycled concrete suitable for severe cold regions of the present invention, wherein: The air-entraining agent is a dodecylbenzenesulfonate air-entraining agent.

[0016] Another object of the present invention is to overcome the deficiencies in the prior art and provide a preparation method for the C50 second-generation recycled concrete suitable for severe cold regions.

[0017] To solve the above technical problems, the present invention provides the following technical solutions: pre-treat the secondary recycled coarse aggregate;

[0018] Fully mix the water reducing agent, air entraining agent, defoaming agent and water to form a mixed solution;

[0019] Pour the cement, nano-silica, silica fume and 2 / 3 of the mixed solution into a mixer and stir well to form a mixed slurry;

[0020] Pour the pre-treated secondary recycled coarse aggregate into the mixer and stir until it is completely coated with the mixed slurry;

[0021] Pour the river sand, basalt fiber and the remaining 1 / 3 of the mixed solution into the mixer and stir well, then pour into a mold and cure to obtain concrete.

[0022] As a preferred embodiment of the method for preparing C50 secondary recycled concrete applicable to cold regions according to the present invention, wherein: the pre-treatment includes washing the residual powder on the surface of the secondary recycled coarse aggregate with tap water, and placing the washed secondary recycled coarse aggregate in water and soaking for 24 h until it reaches a saturated state.

[0023] As a preferred embodiment of the method for preparing C50 secondary recycled concrete applicable to cold regions according to the present invention, wherein: the curing condition is curing in a high-temperature water curing box at a temperature of 70 - 80 °C for 5 d, then spraying a layer of silane impregnating agent on its surface, covering the surface of the specimen with plastic wrap, and taking it out after curing for another 5 d.

[0024] Advantages of the present invention:

[0025] In the present invention, the waste recycled concrete is crushed into secondary recycled coarse aggregate and then used in concrete, effectively alleviating the problem of resource shortage, reducing carbon emissions, and promoting the green and healthy development of the construction industry. The combined use of nano-silica and silica fume synergistically enhances the crack resistance of the recycled concrete. At the same time, with the addition of basalt fiber, silica fume and nano-silica can adsorb on the surface of the basalt fiber and undergo a secondary hydration reaction. The high-strength C-S-H gel produced by the secondary hydration wraps around the basalt fiber, improving the integrity of the recycled concrete, suppressing the cracks caused by hydration heat to a certain extent, and improving the obvious defects of the secondary recycled coarse aggregate. The recycled concrete prepared by the present invention has a compressive strength of up to 53.4 MPa, can withstand 434 freeze-thaw cycles, and can serve in cold regions for 50 years. The effective utilization of secondary recycled coarse aggregate greatly reduces environmental pollution and contributes to the green and sustainable development of the construction industry. Description of the Drawings

[0026] To more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative labor, other drawings can also be obtained based on these drawings. Among them:

[0027] Figure 1 Schematic diagram of the second-generation recycled concrete test block prepared by the present invention.

[0028] Figure 2 Schematic diagram of the second-generation recycled coarse aggregate used in the embodiments of the present invention. Specific embodiments

[0029] To make the above objects, features, and advantages of the present invention more obvious and understandable, the following will provide a detailed description of the specific embodiments of the present invention in conjunction with the embodiments of the specification.

[0030] In the following description, many specific details are set forth in order to fully understand the present invention. However, the present invention can also be implemented in other ways different from those described herein. Those skilled in the art can make similar promotions without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0031] Secondly, the so-called "one embodiment" or "embodiment" herein refers to a specific feature, structure, or characteristic that can be included in at least one implementation manner of the present invention. The "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor is it a separate or selectively exclusive embodiment from other embodiments.

[0032] Prepare the first-generation recycled concrete:

[0033] Weigh the following raw material components according to the formula by weight fraction;

[0034] 397 parts of ordinary Portland cement, 59 parts of silica fume, 685 parts of river sand, 970 parts of recycled coarse aggregate, 164 parts of water, 2.28 parts of polycarboxylate superplasticizer, 0.02 parts of dodecylbenzenesulfonate air-entraining agent, 0.02 parts of silicone defoamer.

[0035] Pour the recycled coarse aggregate, ordinary Portland cement, silica fume and two-thirds of the water into a blender and stir for 90 s. Subsequently, pour the river sand into the blender and stir for 60 s. Finally, mix the water reducer, air-entraining agent, defoaming agent and the remaining one-third of the water and pour them into the blender and stir for 120 s. Put the prepared concrete into a prism mold and seal it with plastic wrap. Demold after 24 h, and cure the specimens in a standard curing room at a temperature of 20 ± 2 °C and a relative humidity of more than 95%. After curing for 24 days, take the specimens out of the control room and soak them in water for 4 days to make them completely saturated, obtaining the first-generation recycled concrete.

[0036] Prepare the second-generation recycled coarse aggregate: Put the first-generation recycled concrete that has undergone 400 rapid freeze-thaw cycles into a jaw crusher and crush it using a two-stage crushing method to obtain the second-generation recycled coarse aggregate.

[0037] The second-generation recycled coarse aggregate used in the present invention is Class III recycled coarse aggregate specified in GB / T 25177—2010, with an apparent density of 2312 kg / m 3 , a water absorption rate of 6.78%, a crushing value of 24.12%, an adhered mortar content of 36.2%, and a particle size of 6 mm.

[0038] The composition of the cement used in the present invention is: 53.81% CaO, 21.33% SiO 2 , 9.60% Al 2 O 3 , 5.90 Fe 2 O 3 , 3.27% SO3, 3.13% MgO.

[0039] The river sand used in the present invention is medium sand with a fineness modulus of 2.4.

[0040] The present invention measures the compressive strength of concrete with reference to "Standard Test Method for Mechanical Properties of Ordinary Concrete GB / T 50081—2019";

[0041] Measure the mass loss rate and relative dynamic elastic modulus of the concrete after 400 rapid freeze-thaw tests with reference to "Standard Test Method for Long-Term Performance and Durability of Ordinary Concrete GB / T 50082—2009".

[0042] The raw materials used in the present invention are all ordinary commercially available products in the field without special instructions.

[0043] Example 1

[0044] This example provides a preparation method of C50 second-generation recycled concrete applicable to cold regions, specifically:

[0045] 1) Weigh the raw materials according to the following formula:

[0046] 920 parts of second-generation recycled coarse aggregate, 2.3 parts of water reducer (polycarboxylate water reducer), 0.04 parts of air entraining agent (dodecylbenzene sulfonate air entraining agent), 0.02 parts of defoaming agent (organic silicon defoaming agent), 150 parts of water, 390 parts of cement, 10 parts of nano-silicon dioxide, 75 parts of silica fume, 680 parts of river sand (medium sand) and 5 parts of basalt fiber.

[0047] 2) Wash off the powder remaining on the surface of the second-generation recycled coarse aggregate with tap water, and soak the washed second-generation recycled coarse aggregate in water for 24 hours until saturated;

[0048] 3) Fully mix the water reducing agent, air entraining agent, defoaming agent and water to form a mixed solution;

[0049] 4) Pour cement, nano-silicon dioxide, silica fume and 2 / 3 of the mixed solution into a mixer and stir thoroughly to form a mixed slurry;

[0050] 5) Pour the second-generation recycled coarse aggregate into a mixer and stir until it is completely coated with the mixed slurry;

[0051] 6) Pour river sand, basalt fiber and the remaining 1 / 3 of the mixed solution into a blender and stir thoroughly, pour into a mold and shape it 24 hours later;

[0052] 7) The specimen was placed in a high-temperature water curing box at 70°C for 5 days, after which a layer of silane impregnating agent was sprayed on its surface, and the specimen surface was covered with plastic wrap. After being placed in a standard curing room for 5 days, the aforementioned C50 second-generation recycled concrete suitable for severe cold regions was obtained.

[0053] Example 2

[0054] The difference between this embodiment and embodiment 1 is that the raw material formula of the second-generation recycled concrete is adjusted, specifically:

[0055] 910 parts of second-generation recycled coarse aggregate, 2.3 parts of water reducer (polycarboxylate water reducer), 0.03 parts of air entraining agent (dodecylbenzene sulfonate air entraining agent), 0.02 parts of defoaming agent (organic silicon defoaming agent), 140 parts of water, 385 parts of cement, 7 parts of nano-silicon dioxide, 70 parts of silica fume, 680 parts of river sand (medium sand) and 7 parts of basalt fiber.

[0056] The rest of the preparation method is the same as that in Example 1 to obtain the second-generation recycled concrete of this example.

[0057] Example 3

[0058] The difference between this embodiment and embodiment 1 is that the raw material formula of concrete is adjusted, specifically:

[0059] 900 parts of second-generation recycled coarse aggregate, 2.3 parts of water reducer (polycarboxylate water reducer), 0.03 parts of air-entraining agent (dodecylbenzenesulfonate air-entraining agent), 0.02 parts of defoaming agent (organosilicon defoaming agent), 130 parts of water, 400 parts of cement, 5 parts of nano-silica, 65 parts of silica fume, 660 parts of river sand (medium sand) and 5 parts of basalt fiber.

[0060] The remaining preparation methods are the same as those in Example 1, and the second-generation recycled concrete of this example is prepared.

[0061] The performance of the second-generation recycled concrete prepared in Examples 1 to 3 was tested, and the results are shown in Table 1:

[0062] Table 1 Test results of the performance of recycled concrete with different proportions

[0063]

[0064] Observing the results in Table 1, it can be seen that:

[0065] The compressive strength of the recycled concrete prepared in Examples 1 to 3 of the present invention is greater than 50 MPa, and it can withstand more than 400 freeze-thaw cycles, having excellent frost resistance. This is because the nano-scale particles of nano-silica effectively fill the micro-cracks and through capillary pores of the second-generation recycled coarse aggregate, and hydrate in these capillary pores to form C-S-H with a low Ca / Si ratio, effectively improving the performance of the second-generation recycled coarse aggregate.

[0066] Comparative Example 1

[0067] Comparative Example 1 is based on Example 1. The difference between Comparative Example 1 and Example 1 is that: the raw material formula of the concrete is adjusted, nano-silica is not added, and more silica fume is added. Specifically:

[0068] 900 parts of second-generation recycled coarse aggregate, 2.3 parts of water reducer (polycarboxylate water reducer), 0.03 parts of air-entraining agent (dodecylbenzenesulfonate air-entraining agent), 0.02 parts of defoaming agent (organosilicon defoaming agent), 130 parts of water, 400 parts of cement, 82 parts of silica fume, 660 parts of river sand (medium sand) and 5 parts of basalt fiber.

[0069] The remaining preparation methods are the same as those in Example 1, and the second-generation recycled concrete of this comparative example is prepared.

[0070] Comparative Example 2

[0071] Comparative Example 2 is based on Example 1. The difference between Comparative Example 2 and Example 1 is that: the raw material formula of the concrete is adjusted, and the dosage of silica fume is reduced. Specifically:

[0072] 900 parts of second-generation recycled coarse aggregate, 2.3 parts of water reducer (polycarboxylate water reducer), 0.03 parts of air-entraining agent (dodecylbenzene sulfonate air-entraining agent), 0.02 parts of defoaming agent (organosilicon defoaming agent), 130 parts of water, 400 parts of cement, 10 parts of nano-silica, 40 parts of silica fume, 660 parts of river sand (medium sand), and 5 parts of basalt fiber.

[0073] The remaining preparation methods are the same as those in Example 1, and the second-generation recycled concrete of this comparative example is prepared.

[0074] Comparative Example 3

[0075] Comparative Example 3 is based on Example 1. The difference between Comparative Example 3 and Example 1 is that: the raw material formula of the concrete is adjusted, and the amount of silica fume is increased. Specifically:

[0076] 900 parts of second-generation recycled coarse aggregate, 2.3 parts of water reducer (polycarboxylate water reducer), 0.03 parts of air-entraining agent (dodecylbenzene sulfonate air-entraining agent), 0.02 parts of defoaming agent (organosilicon defoaming agent), 130 parts of water, 400 parts of cement, 10 parts of nano-silica, 85 parts of silica fume, 660 parts of river sand (medium sand), and 5 parts of basalt fiber.

[0077] The remaining preparation methods are the same as those in Example 1, and the second-generation recycled concrete of this comparative example is prepared.

[0078] The performance of the second-generation recycled concrete prepared in Comparative Examples 1 to 3 was tested and compared with that of Example 1. The results are shown in Table 2:

[0079] Table 2 Test results of the performance of recycled concrete prepared with different proportions of silica fume

[0080]

[0081] It can be seen from the comparison results in Table 2 that when the amount of silica fume added to the recycled concrete is small, the strength and frost resistance are difficult to meet the requirements. At the same time, excessive silica fume will also lead to a decrease in strength and frost resistance. This is because silica fume plays a role in filling and hydration reactions in the concrete. An appropriate amount of silica fume can fill the voids in the concrete, increase the compactness of the concrete, and improve the strength and frost resistance. However, when the amount of silica fume is small, the voids cannot be fully filled, resulting in insufficient compactness of the concrete, and the strength and frost resistance are difficult to meet the requirements. When the silica fume is excessive, the silica in the silica fume cannot react completely, resulting in incomplete formation of calcium silicate gel in the concrete, weakening the strength and frost resistance of the concrete.

[0082] Comparative Example 4

[0083] Comparative Example 4 is based on Example 1. The difference between Comparative Example 4 and Example 1 is that the raw material formula of the concrete is adjusted, and the defoaming agent (silicone defoaming agent) is not added. Specifically:

[0084] 920 parts of secondary recycled coarse aggregate, 2.3 parts of water reducing agent (polycarboxylate water reducing agent), 0.04 parts of air entraining agent (dodecyl benzene sulfonate air entraining agent), 150 parts of water, 390 parts of cement, 10 parts of nano-silica, 75 parts of silica fume, 680 parts of river sand (medium sand), and 5 parts of basalt fiber.

[0085] The remaining preparation methods are the same as those in Example 1, and the secondary recycled concrete of this comparative example is prepared.

[0086] Comparative Example 5

[0087] Comparative Example 5 is based on Example 1. The difference between Comparative Example 5 and Example 1 is that the raw material formula of the concrete is adjusted, and the dosage of the defoaming agent (silicone defoaming agent) is reduced. Specifically:

[0088] 920 parts of secondary recycled coarse aggregate, 2.3 parts of water reducing agent (polycarboxylate water reducing agent), 0.04 parts of air entraining agent (dodecyl benzene sulfonate air entraining agent), 0.01 parts of defoaming agent (silicone defoaming agent), 150 parts of water, 390 parts of cement, 10 parts of nano-silica, 75 parts of silica fume, 680 parts of river sand (medium sand), and 5 parts of basalt fiber.

[0089] The remaining preparation methods are the same as those in Example 1, and the secondary recycled concrete of this comparative example is prepared.

[0090] Comparative Example 6

[0091] Comparative Example 6 is based on Example 1. The difference between Comparative Example 6 and Example 1 is that the raw material formula of the concrete is adjusted, and the dosage of the defoaming agent (silicone defoaming agent) is increased. Specifically:

[0092] 920 parts of secondary recycled coarse aggregate, 2.3 parts of water reducing agent (polycarboxylate water reducing agent), 0.04 parts of air entraining agent (dodecyl benzene sulfonate air entraining agent), 0.04 parts of defoaming agent (silicone defoaming agent), 150 parts of water, 390 parts of cement, 10 parts of nano-silica, 75 parts of silica fume, 680 parts of river sand (medium sand), and 5 parts of basalt fiber.

[0093] The remaining preparation methods are the same as those in Example 1, and the secondary recycled concrete of this comparative example is prepared.

[0094] The performance of the secondary recycled concrete prepared in Comparative Examples 4 to 6 was tested and compared with that in Example 1. The results are shown in Table 3:

[0095] Table 3 Performance test results of recycled concrete prepared with different proportions of defoaming agent

[0096]

[0097] It can be seen from the comparison results in Table 3 that: compared with Example 1, the compressive strength of Comparative Example 4 decreased by 10.6%, and the maximum number of freeze-thaw cycles that can be endured decreased by 74 times. This is because an appropriate amount of defoamer can effectively eliminate some harmful pores, thereby improving the frost resistance of recycled concrete; the compressive strength of Comparative Example 5 decreased by 7.8%, and the maximum number of freeze-thaw cycles that can be endured decreased by 54 times. This is because a small amount of defoamer can eliminate some harmful pores in recycled concrete; the compressive strength of Comparative Example 6 decreased by 6.5%, and the maximum number of freeze-thaw cycles that can be endured decreased by 32 times. This is because an excessive amount of defoamer reduced most of the introduced air bubbles in recycled concrete, but at the same time damaged the pore structure inside the concrete, thereby reducing its strength and frost resistance.

[0098] Comparative Example 7

[0099] Based on Example 1, the difference between Comparative Example 7 and Example 1 is that: the raw material formula of the concrete is adjusted, and basalt fiber is not added. Specifically:

[0100] 920 parts of second-generation recycled coarse aggregate, 2.3 parts of water reducing agent (polycarboxylate water reducing agent), 0.04 part of air-entraining agent (dodecylbenzenesulfonate air-entraining agent), 0.02 part of defoamer (organosilicon defoamer), 150 parts of water, 390 parts of cement, 10 parts of nano-silica, 75 parts of silica fume, 680 parts of river sand (medium sand).

[0101] The remaining preparation methods are the same as those in Example 1, and the second-generation recycled concrete of this comparative example is prepared.

[0102] The performance of the second-generation recycled concrete prepared in Comparative Example 7 was tested and compared with that in Example 1. The results are shown in Table 4:

[0103] Table 4 Test results of the performance of recycled concrete prepared with different proportions of defoamer

[0104]

[0105] It can be seen from the comparison results in Table 4 that: compared with Example 1, the compressive strength of Comparative Example 7 decreased by 6.6%, and the maximum number of freeze-thaw cycles that can be endured decreased by 55 times. Basalt fiber can form a fine fiber network structure in recycled concrete, improving its cohesive strength and toughness. Nano-silica and silica fume can be adsorbed on the surface of basalt fiber, and the generated C-S-H fills in the fiber network structure, which can prevent the penetration of water and the expansion of micro-cracks caused by freeze-thaw cycles, thereby improving the frost resistance of concrete.

[0106] It can be seen from Comparative Examples 1 to 7 that only when nano-silica, defoaming agent and basalt fiber are compounded can the overall improvement of the comprehensive performance of recycled concrete be achieved.

[0107] Comparative Example 8

[0108] Comparative Example 8 is based on Example 1. The difference between Comparative Example 8 and Example 1 is that step 2) is omitted, the second-generation recycled coarse aggregate is not cleaned and soaked, and the rest of the preparation methods are the same as those in Example 1 to obtain the second-generation recycled concrete of this comparative example.

[0109] Comparative Example 9

[0110] Comparative Example 9 is based on Example 1. The difference between Comparative Example 9 and Example 1 is that step 7) is adjusted and the silane impregnating agent is not sprayed after curing.

[0111] Comparative Example 10

[0112] Comparative Example 10 is based on Example 1. The difference between Comparative Example 10 and Example 1 is that the preparation process is different, specifically:

[0113] 1) Weigh the raw materials according to the following formula:

[0114] 920 parts of second-generation recycled coarse aggregate, 2.3 parts of water reducer (polycarboxylate water reducer), 0.04 parts of air entraining agent (dodecylbenzene sulfonate air entraining agent), 0.02 parts of defoaming agent (organic silicon defoaming agent), 150 parts of water, 390 parts of cement, 10 parts of nano-silicon dioxide, 75 parts of silica fume, 680 parts of river sand (medium sand) and 5 parts of basalt fiber.

[0115] 2) Wash away the powder remaining on the surface of the second-generation recycled coarse aggregate with tap water, and soak the washed second-generation recycled coarse aggregate in water for 24 hours until saturated;

[0116] 3) Fully mix the water reducing agent, air entraining agent, defoaming agent and water to form a mixed solution;

[0117] 4) Pour the second generation recycled coarse aggregate and river sand (medium sand) into the mixer and mix;

[0118] 4) Pour cement, nano-silicon dioxide, silica fume, basalt fiber and mixed solution into a mixer and stir thoroughly, pour into a mold and form for 24 hours;

[0119] 7) The specimen was placed in a high-temperature water curing box at 70°C for 5 days, after which a layer of silane impregnating agent was sprayed on its surface, and the specimen surface was covered with plastic wrap. After being placed in a standard curing room for 5 days, the aforementioned C50 second-generation recycled concrete suitable for severe cold regions was obtained.

[0120] The properties of the second-generation recycled concrete prepared in Comparative Examples 8 to 10 were tested and compared with those of Example 1. The results are shown in Table 5 as follows:

[0121] Table 5 Test results of the properties of recycled concrete prepared by different preparation processes

[0122]

[0123]

[0124] It can be seen from the comparison results in Table 5 that: compared with Comparative Example 1, the compressive strength of Comparative Example 9 decreased by 5.9%, and the maximum number of freeze-thaw cycles it can withstand decreased by 50 times. This is because nano-silica and silica fume were added to the product, which would generate more hydration heat, thus leading to the generation of micro-cracks. These micro-cracks would develop along the unhydrated nano-silica and silica fume particles, and the silane impregnating agent could penetrate into the cracks and react with these SiO 2 to carry out cross-linking reaction to form a denser and stronger silicate gel, enhancing the compactness of the concrete. In addition, the silane impregnating agent also has a waterproof function and can form a tight coating to prevent water from penetrating into the concrete, effectively reducing the water content inside the concrete, thereby reducing freeze-thaw damage. The compressive strength of Comparative Example 10 decreased by 15.6%, and the maximum number of freeze-thaw cycles it can withstand decreased by 102 times. This is because during the preparation of recycled concrete, the treatment method of coating a layer of slurry on the surface of the second-generation recycled coarse aggregate first can improve the strength and frost resistance of the concrete. The slurry preferentially penetrates into the second-generation recycled coarse aggregate, making the structure denser. If the second-generation recycled aggregate and river sand are first mixed and stirred, it will cause the surface of the second-generation recycled concrete to be covered by the powder in the river sand, thus preventing the penetration of the slurry.

[0125] The recycled concrete prepared in the present invention uses second-generation recycled coarse aggregate, while the first-generation recycled coarse aggregate is used in the prior art. Compared with the first-generation recycled coarse aggregate, the second-generation recycled coarse aggregate has worse properties, resulting in low strength and poor durability of the prepared second-generation recycled concrete. After being combined with the materials mentioned in the present invention, the compressive strength of the second-generation recycled concrete can reach C50 and can meet the requirement of being used in cold regions for 50 years, having good strength and durability.

[0126] Particularly in Example 1 of the present invention, when 10 parts of nano-silica and 75 parts of silica fume are added, the maximum number of freeze-thaw cycles of the obtained recycled concrete is the largest. This is because silica fume and nano-silica effectively fill the pores and cracks at the micron and nano scales respectively, and a large amount of low Ca / Si ratio and high-density C-S-H generated by secondary hydration significantly reduces the porosity of recycled coarse aggregates and recycled concrete. Secondly, the incorporation of basalt fibers connects the high-strength C-S-H together, increasing the tensile strength and toughness of the concrete. When the concrete is subjected to external forces, the basalt fibers can effectively disperse the stress, increase the crack resistance of the concrete, and prevent the expansion of cracks. Then, the use of defoamers effectively reduces the number of large pores in the concrete, and the combination of water reducers, air-entraining agents and defoamers makes the pore distribution in the concrete more reasonable. Finally, the silane impregnating agent can penetrate from the outside of the concrete. Since nano-silica and silica fume are added to the product, more hydration heat will be generated, which will lead to the generation of micro-cracks. These micro-cracks will develop along the unhydrated nano-silica and silica fume particles, and the silane impregnating agent can penetrate into the cracks and react with these SiO2 to form a denser and stronger silicate gel.

[0127] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered by the scope of the claims of the present invention.

Claims

1. A second-generation recycled concrete C50 applicable to severe cold regions, characterized in that: The second-generation recycled concrete is made by crushing waste recycled concrete into second-generation recycled coarse aggregate and then using it in concrete. The raw material components of the second-generation recycled concrete include, 385 - 400 parts of cement, 5 - 10 parts of nano-silica, 65 - 75 parts of silica fume, 660 - 680 parts of river sand, 900 - 920 parts of second-generation recycled coarse aggregate, 5 - 7 parts of basalt fiber, 130 - 150 parts of water, 2.3 parts of water reducer, 0.02 parts of defoamer, and 0.02 - 0.04 parts of air-entraining agent; The preparation method of the second-generation recycled concrete C50 applicable to severe cold regions includes, Pre-treating the second-generation recycled coarse aggregate; Fully mixing the water reducer, air-entraining agent, defoamer and water to form a mixed solution; Pouring the cement, nano-silica, silica fume and 2 / 3 of the mixed solution into a mixer and stirring fully to form a mixed slurry; Pouring the pre-treated second-generation recycled coarse aggregate into the mixer and stirring until it is completely coated with the mixed slurry; Pouring the river sand, basalt fiber and the remaining 1 / 3 of the mixed solution into the mixer and stirring fully, then pouring into a mold and curing to obtain the concrete; The pre-treatment includes washing the powder remaining on the surface of the second-generation recycled coarse aggregate with tap water, and soaking the washed second-generation recycled coarse aggregate in water for 24 h until it reaches a saturated state; The curing condition is curing in a high-temperature water curing box at a temperature of 70 - 80 °C for 5 d, then spraying a layer of silane impregnating agent on its surface, covering the specimen surface with plastic wrap, and taking it out after curing for another 5 d.

2. The second-generation recycled concrete C50 applicable to severe cold regions as described in claim 1, characterized in that: The components of the cement include CaO, SiO 2 , Fe 2 O 3 , Al 2 O 3 .

3. The second-generation recycled concrete C50 applicable to severe cold regions as described in claim 1, characterized in that: The particle size of the nano-silica is 5 to 50 nm, and the specific surface area > 300,000 m 2 / kg.

4. The second-generation recycled concrete C50 applicable to severe cold regions as described in claim 1, characterized in that: The average particle size of the silica fume is 2.54 μm, and the content of silicon dioxide > 96%.

5. The second-generation recycled concrete C50 applicable to severe cold regions as described in claim 1, characterized in that: The components of the second-generation recycled coarse aggregate include CaCO 3 , Ca(OH) 2 , SiO 2 .

6. The second-generation recycled concrete C50 applicable to severe cold regions as described in claim 1, characterized in that: The water reducer is a polycarboxylic acid water reducer, and the defoamer is an organosilicon defoamer.

7. The second-generation recycled concrete C50 applicable to severe cold regions as described in claim 1, characterized in that: The air-entraining agent is a dodecyl benzene sulfonate air-entraining agent.

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