Preparation method of low-shrinkage recycled concrete with whole-process curing

Abstract

The application discloses a kind of low shrinkage recycled concrete whole-process curing preparation methods, whole-process curing includes (I) aggregate system stage carbon dioxide curing;(II) aggregate system and cementing system mixing stage carbon dioxide curing;(III) recycled concrete after pouring carbon dioxide curing.For the same proportion of recycled concrete, the strength and shrinkage performance of recycled concrete cured by whole-process are better than those of recycled concrete cured by ordinary method, and the 7d dry shrinkage rate is reduced by 21.66% and the 28d dry shrinkage rate is reduced by 19.35% after being treated by the curing method of the application, and the comprehensive performance is significantly improved.The recycled concrete cured by whole-process proposed by the application is better than the recycled concrete cured by single process under the same conditions in terms of compressive strength and dry shrinkage rate, and further better than the performance of recycled concrete cured by ordinary method.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of recycled concrete shrinkage control and curing, and particularly relates to a low-shrinkage recycled concrete whole-process curing preparation method. BACKGROUND

[0002] Recycled concrete refers to concrete prepared by replacing part or all of natural aggregates such as sand and gravel with recycled concrete aggregates (referred to as recycled aggregates) formed by mixing recycled concrete blocks after crushing, washing and grading according to a certain proportion and grading. The recycled concrete technology can realize the recycling of waste concrete, form new structural materials, realize the resource utilization of building solid waste, reduce environmental pollution, save construction funds and speed up the reconstruction work after major natural disasters.

[0003] At present, the low-carbon development of the concrete industry focuses on carbon emission reduction of concrete raw materials. A large number of scholars have evaluated the environmental impact of recycled concrete. Studies have shown that recycled concrete has carbon emission reduction potential in terms of avoiding waste concrete landfill, reducing transportation distance, carbon absorption during recycling, etc. At the same time, the carbon sink capacity of concrete cannot be ignored. The annual average carbon sink capacity of concrete in unit area during the construction, demolition and recycling stages can reach 0.001425tCO2 / (m 2 2), and the ratio of carbon sink to carbon emission of concrete is much higher than that of wood and steel. Further, carbon dioxide curing and carbon modification have become a very potential carbon capture and storage technology. In the concrete block, carbon dioxide curing is proposed as an alternative to traditional curing methods (i.e. water curing, steam curing and heat curing). Carbonated modified recycled aggregates have the benefits of carbon absorption, improved waste recycling rate and strength.

[0004] It is difficult to scientifically consider the defects of large shrinkage and easy cracking of the material itself in the design of recycled concrete and its structure by using the current formula or theory. Establishing the basic theory and method of low-shrinkage full-recycled concrete and its structure will help improve the performance of recycled concrete structures. SUMMARY

[0005] The application is proposed to overcome the shortcomings in the prior art, and aims to provide a low-shrinkage recycled concrete whole-process curing preparation method.

[0006] To achieve the above-mentioned purpose, the application is implemented by the following technical scheme:

[0007] A low-shrinkage recycled concrete whole-process curing preparation method comprises:

[0008] (I) Carbon dioxide curing in the aggregate system stage

[0009] The aggregate system is soaked in a calcium hydroxide solution, while continuously introducing nano-carbon dioxide into the solution, the length of the introduction is controlled by the pH of the solution, when the pH reaches 3.0, stop aeration; then take out and dry to constant weight, the drying temperature does not exceed 60℃;

[0010] (Ⅱ) Carbon dioxide curing of the aggregate system and the cementitious system mixing stage

[0011] The aggregate system after drying in step (I) is fully stirred and mixed with a certain amount of cementitious system, and placed in a pressure curing box containing carbon dioxide atmosphere for carbonation curing;

[0012] (Ⅲ) Carbon dioxide curing of the recycled concrete after pouring

[0013] The aggregate system and the cementitious system after curing in step (II) are fully mixed with the remaining weight of the cementitious system and other additives, and the water amount designed in the mix proportion is added, and after standard curing in the formwork, the formwork is removed, and then placed in a nano-carbon dioxide containing aqueous solution for carbonation water curing.

[0014] In the above technical solution, the recycled concrete comprises an aggregate system, a cementitious system, water and other additives, the aggregate system comprises coarse aggregate and fine aggregate, the coarse aggregate is a mixture of recycled aggregate and natural aggregate in a certain proportion, and the fine aggregate is a mixture of recycled fine aggregate and natural fine aggregate in a certain proportion, the cementitious system comprises cement, and the other additives comprise superplasticizer.

[0015] In the above technical solution, the concentration of the calcium hydroxide solution in step (I) is 0.02-0.06 mol / L; further preferably, the concentration of the calcium hydroxide solution in the aggregate system curing is 0.03-0.05 mol / L; more preferably, the concentration of the calcium hydroxide solution in the aggregate system curing is 0.035-0.045 mol / L.

[0016] In the above technical solution, the equivalent diameter of the nano-carbon dioxide bubbles in the aggregate system curing in step (I) is 0.1-30 nm; further preferably, the equivalent diameter of the nano-carbon dioxide bubbles in the aggregate system curing is 1-20 nm; more preferably, the equivalent diameter of the nano-carbon dioxide bubbles in the aggregate system curing is 5-15 nm.

[0017] In the above technical solution, the weight of the cementitious system in step (II) is 20%-50% of the amount of the cementitious system in the mix proportion design; further preferably, the weight of the cementitious system used in step (II) is 30%-40% of the amount of the cementitious system in the mix proportion design.

[0018] In the above technical solution, the carbon dioxide concentration in the curing box in step (II) is 1% to 50%; further preferably, the carbon dioxide concentration in the curing box in step (II) is 5% to 40%; and more preferably, the carbon dioxide concentration in the curing box in step (II) is 5% to 20%.

[0019] In the above technical solution, the carbon dioxide pressure in the curing box in step (II) is 0.1 MPa to 1 MPa; further preferably, the carbon dioxide pressure in the curing box is 0.1 MPa to 0.75 MPa; and more preferably, the carbon dioxide pressure in the curing box is 0.2 MPa to 0.5 MPa.

[0020] In the above technical solution, the relative humidity of carbon dioxide in the curing box in step (II) is 50% to 95%; further preferably, the relative humidity of carbon dioxide in the curing box is 60% to 85%; and more preferably, the relative humidity of carbon dioxide in the curing box is 65% to 80%.

[0021] In the above technical solution, the curing time in the curing box in step (II) is 6 h to 24 h; further preferably, the curing time in the curing box is 12 h to 24 h; and more preferably, the curing time in the curing box is 18 h to 24 h.

[0022] In the above technical solution, the water solution containing nano-carbon dioxide in step (III) is a saturated nano-carbon dioxide solution.

[0023] In the above technical solution, the pH of the water solution containing nano-carbon dioxide in step (III) is controlled to be 3.5 to 5.0; further preferably, the pH of the water solution containing nano-carbon dioxide is controlled to be 4.0 to 5.0; and more preferably, the pH of the water solution containing nano-carbon dioxide is controlled to be 4.25 to 4.75.

[0024] In the above technical solution, the carbonation water curing time in the carbonation curing of the concrete product in step (III) is 3 d to 7 d.

[0025] In the above technical solution, the standard curing time in the mold in the carbonation curing of the concrete product in step (III) is 1 d.

[0026] In the above technical solution, the concrete product is further cured in a standard curing environment for 28 d after the carbonation curing in step (III), and the temperature of the standard curing environment is 20°C and the humidity is 95% RH.

[0027] The beneficial effects of the present application are:

[0028] The application provides a whole-process curing method for shrinkage control in material preparation, mixing and pouring forming stages in a low-shrinkage recycled concrete preparation process, and the shrinkage performance of the prepared concrete is better than that of untreated recycled concrete, 7d dry shrinkage rate is reduced by 21.66%, 28d dry shrinkage rate is reduced by 19.35%, the comprehensive performance is significantly improved, the basic requirements of recycled concrete are met, and the method has strong practical significance. DETAILED DESCRIPTION

[0029] In order to enable personnel in the technical field to better understand the technical solutions of the application, the technical solutions of the application are further described below through specific embodiments.

[0030] The components of the recycled concrete applied in the application include an aggregate system and a gel system, the aggregate system includes coarse aggregate and fine aggregate, the coarse aggregate is prepared by mixing recycled coarse aggregate and natural coarse aggregate at a certain ratio, and the fine aggregate is prepared by mixing recycled fine aggregate and natural fine aggregate at a certain ratio; the gel system includes cement, and the specific component composition is as follows: natural coarse aggregate 0-50 parts, recycled coarse aggregate 10-50 parts, natural fine aggregate 0-15 parts, recycled fine aggregate 0-15 parts, water 6-14 parts, and high-efficiency water reducing agent 1-2 parts.

[0031] The recycled concrete in the following embodiment 1 and comparative examples 1-3 all adopts the same formula, and the components of the recycled concrete and the component numbers of the mass parts of each component are as follows: recycled coarse aggregate 50 parts, recycled fine aggregate 25 parts, cement 20 parts, water 10 parts, and high-efficiency water reducing agent 2 parts.

[0032] Embodiment 1

[0033] A whole-process curing preparation method of low-shrinkage recycled concrete, comprising:

[0034] (I) carbon dioxide curing in the aggregate system stage

[0035] The aggregate system includes recycled coarse aggregate and recycled fine aggregate, the recycled coarse aggregate and the recycled fine aggregate are mixed at a ratio of 2:1, then are soaked in a 0.06mol / L calcium hydroxide solution, and continuously introduced with 20% CO2 at a flow rate of 100ml / min for one week, and then are taken out and dried to constant weight in an oven at a temperature of 60℃.

[0036] (II) carbon dioxide curing in the mixing stage of the aggregate system and the gel system

[0037] The dried aggregate is fully mixed with 30% of the designed mass of cement (i.e. 6 parts of cement), and is placed in a pressure curing box containing a carbon dioxide atmosphere for carbonation curing; wherein the carbon dioxide concentration in the curing box is 20%, the pressure is 0.05MPa, and the curing time in the curing box is 12h.

[0038] (III) carbonation curing after recycled concrete is cast

[0039] The aggregate system and cementitious system after curing are fully mixed with 70% of the remaining weight of cement (14 parts of cement), 10 parts of water and 2 parts of high efficiency water reducing agent to prepare recycled concrete. The formwork is removed after 1d. After removal, carbonation water curing is carried out in a water solution containing nano carbon dioxide, and the carbon dioxide solution is introduced with a PH of 4.5. The curing time is 7d. Then the sample is moved to a standard curing environment (temperature 20℃, humidity 95% RH) for curing until the 28d age.

[0040] The cementitious system (cement) is mixed in two times, and the hydration of the cementitious system (cement) in the second mixing is effectively maintained; if the first cementitious system (cement) is fully carbonated, the cementitious system (cement) and water cannot guarantee sufficient hydration reaction in the second mixing, thereby reducing the final strength of the concrete.

[0041] The carbonation curing of step (III) of the application adopts carbonated water curing, which can ensure that carbon dioxide fully enters the inside of the test piece and fully reacts, and also ensures sufficient water. The curing in the pressure tank cannot ensure sufficient water entering the pore inside the test piece.

[0042] In order to compare the influence of the whole process curing method of the application on the performance of the finally prepared recycled concrete, comparative examples 1-3 are set up:

[0043] Comparative example 1

[0044] This comparative example adopts the same oxidation method as the aggregate system stage of example 1, does not perform carbonation curing in the aggregate system stage, and adopts a standard method for curing after the recycled concrete is prepared and cast.

[0045] (I) carbonation curing in the aggregate system stage: the aggregate system includes recycled coarse aggregate and recycled fine aggregate, the recycled coarse aggregate and the recycled fine aggregate are mixed in a ratio of 2:1, then soaked in a 0.06mol / L calcium hydroxide solution, and continuously introduced with 20% CO2 with a flow rate of 100ml / min for one week, and then taken out and dried to constant weight in an oven at a temperature of 60℃.

[0046] (II) standard curing after preparation and casting of recycled concrete

[0047] The dried aggregate is fully mixed with the designed cement, water and high efficiency water reducing agent to prepare recycled concrete. The formwork is removed after 1d, and then moved to a standard curing environment (temperature 20℃, humidity 95% RH) for curing until the 28d age.

[0048] Comparative Example 2

[0049] This comparative example is the same as Example 1 except that no carbon dioxide curing is performed on the aggregate system phase.

[0050] (I) Aggregate system and cementitious system mixed curing: the aggregate and 30% of the designed cement mass are fully mixed, and then placed in a pressure curing box containing a carbon dioxide atmosphere for carbonation curing; the carbon dioxide concentration in the curing box is 20%, and the pressure is 0.05 MPa. The curing time in the curing box is 12 h.

[0051] (II) Recycled concrete post-pouring curing: the cured aggregate system and cementitious system are fully mixed with the remaining 70% of the cement and superplasticizer to prepare recycled concrete. The mold is removed after 1 d. After removal, the sample is placed in a carbonated water solution containing nano carbon dioxide for carbonation water curing, and the carbon dioxide carbonation water is introduced at a PH of 4.5. The curing time is 7 d. Then the sample is moved to a standard curing environment (temperature 20°C, humidity 95% RH) for curing until the 28 d age.

[0052] Comparative Example 3

[0053] This comparative example only uses the existing standard curing method, and the recycled concrete is cured for the same length of time as Example 1 after pouring.

[0054] Recycled concrete preparation and curing: the aggregate, cementitious material, and water specified in the mix design are fully mixed to prepare recycled concrete. The mold is removed after 1 d, and then the sample is moved to a standard curing environment (temperature 20°C, humidity 95% RH) for curing until the 28 d age.

[0055] Table 1 Comparison of performance test results of concrete products obtained by different curing types

[0056]

[0057] As can be seen from the comparison of the example 1 with the comparative examples 1-3, the strength and shrinkage performance of the recycled concrete cured in the whole process are superior to those of the recycled concrete cured in different processes, and the 28d strength of the recycled concrete cured in the whole process is 15.31% higher than that of the recycled concrete cured in the standard curing condition, the 7d dry shrinkage rate is reduced by 21.66%, and the 28d dry shrinkage rate is reduced by 19.35%. For the single process curing in the comparative examples, the 28d strength of the example is 7.30% and 5.36% higher than that of the comparative examples 1 and 2, respectively, the 7d dry shrinkage rate is reduced by 13.00% and 7.60%, respectively, and the 28d dry shrinkage rate is reduced by 7.95% and 5.74%, respectively. Therefore, the recycled concrete cured in the whole process according to the application can be superior to the recycled concrete cured in a single process under the same conditions in terms of compressive strength and dry shrinkage rate, and further superior to the performance of the recycled concrete cured in the ordinary conditions.

[0058] The application is directed to the problem of large shrinkage and cracking risk in the application process of recycled concrete, and proposes a low-shrinkage recycled concrete whole-process curing preparation method. In each stage of concrete preparation: aggregate system curing, aggregate system and cementitious system mixing curing, and recycled concrete pouring post-curing, carbon dioxide curing is carried out respectively, so as to ensure that the prepared concrete maintains a low shrinkage rate and absorbs carbon dioxide at the same time. The curing method has the characteristics of strong practicability and sufficient theoretical basis.

[0059] It should be noted that the embodiments in the application and the features in the embodiments can be combined with each other without conflict.

[0060] The applicant declares that the above description is only a specific embodiment of the application, but the protection scope of the application is not limited thereto. It should be understood by those skilled in the art that any changes or replacements within the technical scope disclosed by the application can be easily thought out by those skilled in the art, and all fall within the protection scope and disclosure scope of the application.

Claims

1. A method for the production of low shrinkage recycled concrete with full process curing, characterized by: The application relates to a carbonation curing method for recycled concrete. (Ⅰ) carbon dioxide curing of an aggregate system The aggregate system is soaked in a calcium hydroxide solution, and nano carbon dioxide is continuously introduced into the solution, and then the aggregate system is taken out and dried to constant weight; The equivalent diameter of the nano carbon dioxide bubbles in the step (I) is 0.1 nm-30 nm; the carbon dioxide concentration is 1%-100%, and the gas flow range is 5 ml / min-500 ml / min; (Ⅱ) carbon dioxide curing of a mixing stage of the aggregate system and a cementitious system The dried aggregate system in the step (I) is fully stirred and mixed with a certain amount of the cementitious system, and is placed in a pressure curing box containing a carbon dioxide atmosphere for carbonation curing; The carbon dioxide mass concentration in the curing box in the step (II) is 1%-50%, the carbon dioxide pressure range is 0.1 MPa-1 MPa, the carbon dioxide relative humidity range is 50%-95%, and the curing time is 6 h-24 h; The mass of the cementitious system added in the step (II) is 20%-50% of the total mass of the cementitious system; (Ⅲ) carbon dioxide curing of a poured recycled concrete The aggregate system and the cementitious system after the curing in the step (II) are fully stirred and mixed with the remaining amount of the cementitious system, and the water amount designed according to the mixing proportion is added, the concrete is standardly cured in a formwork and then is demoulded, and is placed in a nano carbon dioxide-containing water solution for carbonation water curing; The nano carbon dioxide-containing water solution in the step (III) is a saturated nano carbon dioxide solution, and the pH of the solution is 3.5-5.0; The carbonation water curing time in the step (III) is 3 d-7 d.

2. The method of claim 1, wherein the method further comprises: The recycled concrete comprises an aggregate system, a cementitious system, water and other additives, the aggregate system comprises coarse aggregate and fine aggregate, the coarse aggregate is made of recycled coarse aggregate and natural coarse aggregate according to a proportion, the fine aggregate is made of recycled fine aggregate and natural fine aggregate according to a proportion, the cementitious system comprises cement, and the other additives comprise a high-efficiency water reducing agent.

3. The method of claim 1, wherein the method further comprises: The concentration of the calcium hydroxide solution in the step (I) is 0.02 mol / L-0.06 mol / L.

4. The method of claim 1, wherein the method further comprises: The standard curing time of the concrete product in the step (III) in the formwork is 1 d, and the temperature and humidity of the standard curing environment are 20 DEG C and 95% RH respectively.

5. The method of claim 1, wherein the method further comprises: After the carbonation curing of the concrete product in the step (III), the concrete product is continuously cured in a standard curing environment to a 28 d age, and the temperature and humidity of the standard curing environment are 20 DEG C and 95% RH respectively.

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