Regenerated coarse aggregate early-strength low-resilience sprayed concrete and preparation method thereof

Abstract

The invention discloses recycled coarse aggregate early-strength low-resilience sprayed concrete and a preparation method thereof, and belongs to the technical field of building materials. The recycled coarse aggregate early-strength low-resilience sprayed concrete is prepared from the following components in parts by weight: 450 to 500 parts of cement, 800 to 910 parts of fine aggregate, 800 to 920 parts of coarse aggregate, 15 to 25 parts of an exciting agent, 160 to 190 parts of water, 4.5 to 10 parts of a water reducing agent and 27 to 45 parts of an accelerator, the used coarse aggregate is regenerated hardened concrete, and the exciting agent is a special material. The recycled coarse aggregate early-strength low-resilience shotcrete provided by the invention is high in cohesiveness, strong in cohesion and high in early strength, is more easily adhered to the surface of surrounding rock when being sprayed to tunnel surrounding rock or a side slope, is not easy to fall off, and is low in resilience rate.

Description

Technical Field

[0001] The invention belongs to the technical field of building materials, and in particular relates to a recycled coarse aggregate early-strength low-rebound shotcrete and a preparation method thereof. Background Art

[0002] As a key structure of tunnel support, shotcrete plays an important role in restraining surrounding rock deformation and ensuring safe tunnel construction.

[0003] The existing shotcrete has the following common problems: (1) High rebound rate: Most wet shotcrete has a rebound rate of more than 10% even when the material is good and the construction process is appropriate. In most cases, the rebound rate is around 15%. Dry shotcrete is even more prominent, often exceeding 20%. (2) The early strength is not high enough to meet the early strength requirements of shotcrete in poor low geological sections to ensure construction safety.

[0004] At present, with the increasing acceleration of urban and rural modernization, the construction industry has also developed rapidly. At the same time, a large number of old buildings are also facing demolition, and the accompanying construction waste is also increasing day by day. The stacking of construction waste occupies a large amount of land resources, causing waste of resources and bringing negative impacts on the economy, ecology and society. Summary of the invention

[0005] The purpose of the present invention is to provide a recycled coarse aggregate early strength low rebound shotcrete and a preparation method thereof. Using waste concrete as coarse aggregate to prepare an early strength low rebound shotcrete not only makes use of construction waste, but also can effectively improve the performance of shotcrete, reduce the waste of shotcrete, and increase the safety of tunnel construction.

[0006] To achieve the above object, the present invention provides the following technical solutions: One of the technical solutions of the present invention is to provide a recycled coarse aggregate early-strength low-rebound shotcrete, which comprises, by weight: 450-500 parts of cement, 800-910 parts of fine aggregate, 800-920 parts of coarse aggregate, 15-25 parts of activator, 160-190 parts of water, 4.5-10 parts of water reducer and 27-45 parts of accelerator; The coarse aggregate is recycled hardened concrete blocks with a particle size of 5-10 mm; The preparation steps of the activator include: adding water into a reactor and preheating to 60-80° C., then simultaneously adding a solution 1 containing calcium nitrate tetrahydrate and aluminum nitrate nonahydrate and a solution 2 containing sodium carbonate and sodium hydroxide, the adding time being 2-3 hours, to obtain a mixed system, and controlling the mass fraction of the solute in the mixed system to be 10%; after the adding is completed, stirring is continued at 60-80° C. for 2-3 hours, and then aging is performed for 10-12 hours, filtering, and drying to obtain the activator.

[0007] Preferably, the fine aggregate is natural sand or artificial sand.

[0008] Preferably, the water reducer is a standard polycarboxylic acid high performance water reducer.

[0009] Preferably, the accelerating setting agent is an alkali-free accelerating setting agent.

[0010] Preferably, the mass ratio of the calcium nitrate tetrahydrate, aluminum nitrate nonahydrate, sodium carbonate and sodium hydroxide is 40-50:20-30:10-20:4-8.

[0011] Preferably, the mass fraction of the solute in the solution 1 is 25%; the mass fraction of the solute in the solution 2 is 20%.

[0012] The second technical solution of the present invention is to provide a method for preparing the above-mentioned recycled coarse aggregate early-strength low-rebound shotcrete, the steps comprising: firstly mixing all raw materials except the accelerator and stirring them evenly to obtain normal concrete, and during construction, mixing the normal concrete and the accelerator by wind pressure to obtain the recycled coarse aggregate early-strength low-rebound shotcrete.

[0013] In concrete, the interface strength between mortar and coarse aggregate has always been the weak point of concrete, especially in the early stage of concrete, when the strength is relatively low, and concrete damage is mainly caused by the fracture of the interface layer between mortar and coarse aggregate. In the recycled coarse aggregate early strength low rebound shotcrete provided by the present invention, the coarse aggregate is made of recycled hardened concrete, and the surface of the aggregate is a porous structure, which has a larger real specific surface area relative to ordinary coarse aggregate. At this time, the mortar and coarse aggregate have more contact surfaces, and the more contact surfaces, the more mortar-aggregate bonding surfaces, the greater the cohesion, and the higher the interface strength. Secondly, since the aggregate surface is a porous structure, the mortar or cement paste can also be immersed in the gaps of the recycled aggregate. As the slurry hardens, the slurry immersed in the gaps of the recycled aggregate is anchored in the coarse aggregate like a wedge, further improving the interface force between the slurry and the coarse aggregate, and improving the early strength of the concrete. In addition, an activator is added to the recycled coarse aggregate early-strength low-rebound shotcrete provided by the present invention. Since the coarse aggregate of the present invention is recycled hardened concrete, some incompletely hydrated cementitious materials still remain on the surface of the aggregate. After the activator is added, the unhydrated cementitious materials on the surface of the aggregate can be hydrated again, thereby reacting with the cement in the present invention, promoting the combination of cement slurry and coarse aggregate through chemical bonds, so that the combination is more firm and the interface strength between the slurry and the aggregate is higher.

[0014] The beneficial technical effects of the present invention are as follows: In the recycled coarse aggregate early-strength low-rebound shotcrete provided by the present invention, the coarse aggregate is made of recycled hardened concrete. Compared with conventional coarse aggregate, the recycled hardened concrete aggregate has a porous structure and a large specific surface area, and has a greater binding and absorption capacity for mixing water and cement slurry in the concrete. Combined with the special activator of the present invention, the prepared shotcrete can have high viscosity, strong cohesion, and high early strength. When sprayed onto the tunnel surrounding rock or the slope, it is easier to adhere to the surrounding rock surface, is not easy to fall off, and has a low rebound rate. DETAILED DESCRIPTION

[0015] Now, various exemplary embodiments of the present invention are described in detail, and this detailed description should not be considered as a limitation of the present invention, but should be understood as a more detailed description of certain aspects, characteristics and embodiments of the present invention. It should be understood that the terms described in the present invention are only for describing specific embodiments and are not used to limit the present invention.

[0016] In addition, for the numerical range in the present invention, it is understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. The intermediate value in any stated value or stated range, and each smaller range between any other stated value or intermediate value in the range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0017] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the invention pertains. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention.

[0018] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0019] The cement used in the examples and comparative examples of the present invention is ordinary Portland cement (Esheng PO • 42.5).

[0020] The fine aggregate used in the embodiments and comparative examples of the present invention is natural sand (fineness modulus is 2.8).

[0021] The coarse aggregate used in the embodiments of the present invention and some comparative examples is recycled hardened concrete with a particle size of 5 to 10 mm.

[0022] The water reducing agent used in the examples and comparative examples of the present invention is a standard polycarboxylic acid high performance water reducing agent (model GK-3000).

[0023] The quick-setting agent used in the embodiments of the present invention and the comparative examples is an alkali-free quick-setting agent (model: GK-3B).

[0024] Unless otherwise specified, "parts" in the Examples and Comparative Examples refer to "parts by weight".

[0025] Example 1 Preparation of shotcrete: Weigh 40 parts of calcium nitrate tetrahydrate and 22 parts of aluminum nitrate nonahydrate to prepare a 25wt.% solution 1; weigh 10 parts of sodium carbonate and 5 parts of sodium hydroxide to prepare a 20wt.% solution 2; add a certain amount of water to the reactor, control the temperature in the reactor to 60°C, and simultaneously add solution 1 and solution 2 dropwise for 2h. After the addition is completed, the solute concentration in the final system is 10wt.%. After the addition is completed, continue stirring at 60°C for 2h, then stop stirring, keep the temperature unchanged, and naturally cool to room temperature after aging reaction for 10h, filter and vacuum dry at 60°C to obtain an activator.

[0026] Prepare the raw materials in the proportion of 450 parts of cement, 837 parts of fine aggregate, 805 parts of coarse aggregate, 17 parts of activator, 170 parts of water, 6 parts of water reducer and 36 parts of accelerator. First, mix all the raw materials except the accelerator and stir them evenly to obtain normal concrete. During construction, mix the obtained normal concrete and the accelerator by wind pressure to obtain shotcrete.

[0027] Example 2 Preparation of shotcrete: Weigh 43 parts of calcium nitrate tetrahydrate and 23 parts of aluminum nitrate nonahydrate to prepare a 25wt.% solution 1; weigh 14 parts of sodium carbonate and 6 parts of sodium hydroxide to prepare a 20wt.% solution 2; add a certain amount of water to the reactor, control the temperature in the reactor to 65°C, and simultaneously add solution 1 and solution 2 dropwise for 2.5 hours. After the addition is completed, the solute concentration in the final system is 10wt.%. After the addition is completed, continue stirring at 65°C for 2 hours, then stop stirring, keep the temperature unchanged, and age the reaction for 11 hours before naturally cooling to room temperature, filter and vacuum dry at 60°C to obtain an activator.

[0028] Prepare the raw materials in the proportion of 470 parts of cement, 815 parts of fine aggregate, 815 parts of coarse aggregate, 20 parts of activator, 178 parts of water, 6.5 parts of water reducer and 38 parts of accelerator. First, mix all the raw materials except the accelerator and stir them evenly to obtain normal concrete. During construction, mix the obtained normal concrete and the accelerator by wind pressure to obtain shotcrete.

[0029] Example 3 Preparation of shotcrete: Weigh 45 parts of calcium nitrate tetrahydrate and 25 parts of aluminum nitrate nonahydrate to prepare a 25wt.% solution 1; weigh 15 parts of sodium carbonate and 6 parts of sodium hydroxide to prepare a 20wt.% solution 2; add a certain amount of water to the reactor, control the temperature in the reactor to 70°C, and simultaneously add solution 1 and solution 2 dropwise for 2h. The final solute concentration in the system after the addition is 10wt.%. After the addition is completed, continue stirring at 70°C for 3h, then stop stirring, keep the temperature unchanged, and naturally cool to room temperature after aging reaction for 11h, filter and vacuum dry at 60°C to obtain an activator.

[0030] Prepare the raw materials in the proportion of 480 parts of cement, 800 parts of fine aggregate, 825 parts of coarse aggregate, 21 parts of activator, 180 parts of water, 6.6 parts of water reducer and 40 parts of accelerator. First, mix all the raw materials except the accelerator and stir them evenly to obtain normal concrete. During construction, mix the obtained normal concrete and the accelerator by wind pressure to obtain shotcrete.

[0031] Example 4 Preparation of shotcrete: Weigh 50 parts of calcium nitrate tetrahydrate and 30 parts of aluminum nitrate nonahydrate to prepare a 25wt.% solution 1; weigh 20 parts of sodium carbonate and 8 parts of sodium hydroxide to prepare a 20wt.% solution 2; add a certain amount of water to the reactor, control the temperature in the reactor to 80°C, and simultaneously add solution 1 and solution 2 dropwise for 3 hours. After the addition is completed, the solute concentration in the final system is 10wt.%. After the addition is completed, continue stirring at 80°C for 3 hours, then stop stirring, keep the temperature unchanged, and age the reaction for 12 hours before naturally cooling to room temperature, filter and vacuum dry at 60°C to obtain an activator.

[0032] Prepare the raw materials in the proportion of 500 parts of cement, 800 parts of fine aggregate, 800 parts of coarse aggregate, 25 parts of activator, 190 parts of water, 7 parts of water reducer and 42 parts of accelerator. First, mix all the raw materials except the accelerator and stir them evenly to obtain normal concrete. During construction, mix the obtained normal concrete and the accelerator by wind pressure to obtain shotcrete.

[0033] Comparative Example 1 Preparation of shotcrete: Based on Example 2, the coarse aggregate is conventional gravel of equal mass (particle size 5-10 mm), and no activator is added.

[0034] Comparative Example 2 Preparation of shotcrete: Based on Example 2, no activator is added.

[0035] Comparative Example 3 Preparation of shotcrete: Based on Example 2, the coarse aggregate was replaced with conventional gravel of equal mass (particle size 5-10 mm).

[0036] Comparative Example 4 Preparation of shotcrete: Based on Example 2, the activator was replaced by a mixture of equal masses of sodium silicate and sodium hydroxide in a mass ratio of 4:1.

[0037] The performance test of the shotcrete prepared in Examples 1 to 4 and Comparative Examples 1 to 4 was carried out, and the test standard for compressive strength was GB / T 50081-2019. The rebound rate test method is as follows: before the shotcrete construction, a colored strip of cloth (large enough to collect all the fallen concrete) is laid under the construction site. After the shotcrete construction is completed, the fallen concrete is collected and weighed. Rebound rate = weight of fallen concrete ÷ total weight of shotcrete used in construction. Total shotcrete weight = concrete density × total shotcrete volume used in construction.

[0038] The test results are shown in Table 1.

[0039] Table 1

[0040] It can be seen from Table 1 that the addition of the activator of the present invention can make the prepared shotcrete have higher early strength and lower rebound rate (compare Example 2 with Comparative Example 2); when the homemade activator of the present invention is replaced with a common alkali activator, the early strength and rebound rate of the prepared shotcrete will be significantly reduced (compare Example 2 with Comparative Example 4). Compared with the conventional shotcrete prepared in Comparative Example 1, the recycled coarse aggregate early strength low rebound shotcrete prepared in Examples 1 to 4 of the present invention has higher early strength and lower rebound rate. The use of recycled hardened concrete coarse aggregate in the present invention also helps to improve the early strength of the prepared shotcrete and reduce its rebound rate (compare Example 2 with Comparative Example 3).

[0041] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A recycled coarse aggregate early strength low rebound shotcrete, characterized in that: By weight, the components include: 450-500 parts of cement, 800-910 parts of fine aggregate, 800-920 parts of coarse aggregate, 15-25 parts of activator, 160-190 parts of water, 4.5-10 parts of water reducer and 27-45 parts of accelerator; The coarse aggregate is recycled hardened concrete blocks with a particle size of 5-10 mm; The preparation steps of the activator include: adding water into a reactor and preheating to 60-80° C., then simultaneously adding a solution 1 containing calcium nitrate tetrahydrate and aluminum nitrate nonahydrate and a solution 2 containing sodium carbonate and sodium hydroxide, the adding time being 2-3 hours, to obtain a mixed system, and controlling the mass fraction of the solute in the mixed system to be 10%; after the adding is completed, stirring is continued at 60-80° C. for 2-3 hours, and then aging is performed for 10-12 hours, filtering, and drying to obtain the activator.

2. The recycled coarse aggregate early strength low rebound shotcrete according to claim 1, characterized in that: The fine aggregate is natural sand or artificial sand.

3. The recycled coarse aggregate early strength low rebound shotcrete according to claim 1, characterized in that: The water reducer is a standard polycarboxylic acid high performance water reducer.

4. The recycled coarse aggregate early strength low rebound shotcrete according to claim 1, characterized in that: The accelerating setting agent is an alkali-free accelerating setting agent.

5. The recycled coarse aggregate early strength low rebound shotcrete according to claim 1, characterized in that: The mass ratio of the calcium nitrate tetrahydrate, aluminum nitrate nonahydrate, sodium carbonate and sodium hydroxide is 40-50:20-30:10-20:4-8.

6. The recycled coarse aggregate early strength low rebound shotcrete according to claim 1, characterized in that: The mass fraction of the solute in the solution 1 is 25%; the mass fraction of the solute in the solution 2 is 20%.

7. A method for preparing the recycled coarse aggregate early-strength low-rebound shotcrete according to any one of claims 1 to 6, characterized in that: The steps include: First, all raw materials except the accelerating setting agent are mixed and stirred evenly to obtain normal concrete. During construction, the normal concrete and the accelerating setting agent are mixed evenly by wind pressure to obtain the recycled coarse aggregate early-strength low-rebound shotcrete.

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