A high-strength recycled aggregate concrete and its preparation method

Abstract

This application relates to the technical field of concrete, and specifically discloses a high-strength recycled aggregate concrete and a preparation method thereof. A high-strength recycled aggregate concrete comprises raw materials in the following parts by weight: 120-180 parts of cement; 150-170 parts of water; 60-100 parts of fly ash; 70-100 parts of slag powder; 800-1000 parts of sand; 700-900 parts of recycled brick waste; 5-10 parts of water reducer; 50-60 parts of modified biochar; 70-80 parts of porous carbon material. The high-strength recycled aggregate concrete of this application has the advantage of improving the strength of recycled brick aggregate concrete.

Description

Technical Field

[0001] The present invention relates to the technical field of concrete, and particularly relates to a high-strength recycled aggregate concrete and a preparation method thereof. Background Art

[0002] According to statistics, the output of construction waste in China basically accounts for about 40% of the total urban waste. For every 10,000 square meters of old buildings demolished, 7,000 - 12,000 tons of construction waste will be generated. Among them, preparing recycled brick aggregate concrete by crushing waste bricks is an exploration of its resource utilization. Based on relevant experimental results, the following main disadvantages exist in all-recycled brick aggregate concrete: the strength of all-recycled brick aggregate concrete is relatively low, which limits its application in the engineering field, thus seriously restricting the resource utilization of waste bricks; the shrinkage rate of all-recycled brick aggregate concrete is large and it is prone to cracking; the comprehensive utilization rate of cement in all-recycled brick aggregate concrete is relatively low, and generally only one hydration reaction occurs and then basically terminates, resulting in the failure to fully exert the strength and performance of cement. How to both make full use of the physical and chemical properties of waste bricks and greatly improve the flexural strength and compressive strength of concrete to achieve its resource-based and rational utilization is a key problem that urgently needs to be solved. Summary of the Invention

[0003] In order to improve the strength of recycled brick aggregate concrete, the present application provides a high-strength recycled aggregate concrete and a preparation method thereof.

[0004] A high-strength recycled aggregate concrete provided by the present application adopts the following technical solutions:

[0005] A high-strength recycled aggregate concrete includes the following raw materials in parts by weight:

[0006] 120 - 180 parts of cement;

[0007] 150 - 170 parts of water;

[0008] 60 - 100 parts of fly ash;

[0009] 70 - 100 parts of mineral powder;

[0010] 800 - 1000 parts of sand;

[0011] 700 - 900 parts of recycled brick slag;

[0012] 5 - 10 parts of water reducing agent;

[0013] 50 - 60 parts of modified biochar;

[0014] 70 - 80 parts of porous carbon material;

[0015] The modified biochar is obtained by pre-treating straw powder in an aqueous nitric acid solution, then impregnating it in an alumina sol, and carbonizing it at 300 - 400 °C for 2 - 3 h;

[0016] The porous carbon material is obtained by adding calcium lignosulfonate to an oxalic acid solution, stirring, drying, and carbonizing it at 680 - 710 °C for 0.5 - 1 h.

[0017] By adopting the above technical solution, since the recycled brick waste, a solid waste, is used as the coarse aggregate of concrete, it can not only achieve resource recycling and reduce the pressure of environmental governance, but also the recycled brick waste can partially replace cement. However, the content of cement mortar in the recycled aggregate is relatively high, and its surface is much rougher than that of natural aggregate. Due to the generation of a large number of fine cracks inside the recycled aggregate during the disintegration and crushing process, its water absorption rate is much larger than that of natural aggregate. Therefore, the hydration process of concrete is incomplete, resulting in a decrease in the strength of concrete and easy cracking of the concrete. Thus, in this application, the modified biochar and the porous carbon material are compounded to solve the defects brought by adding the recycled aggregate.

[0018] First, the straw powder is impregnated in the alumina sol. After high-temperature carbonization, alumina is loaded on the surface of the biochar. The biochar obtained by carbonization at 300 - 400 °C has a disordered and amorphous structure. The porous carbon material is obtained by carbonizing calcium lignosulfonate at high temperature, and its structure is an ordered and layered structure. The prepared modified biochar material is loaded with a certain amount of alumina. Calcium hydroxide generated during the cement hydration process can form calcium aluminate hydrate on the surface of nano-alumina particles, and alumina is easily chemically bonded to the hydration products of cement, enabling the hydration products to form between the layers of the porous carbon material, thereby improving the density and strength of the concrete. The modified biochar and the porous carbon material are compounded, enabling the modified biochar to be dispersed between the layers of the porous carbon material. The uncompletely carbonized organic groups on the surface of the biochar form a binding force with the surface of the porous carbon material, reducing the interfacial defects, enabling the two to form a close packing in the concrete, and improving the compressive strength and flexural strength of the concrete.

[0019] With the incorporation of modified biochar and porous carbon fiber, the compounding of the two enhances the flexural toughness of the concrete matrix. Due to the close packing of the layers, the force required to resist flexural fracture increases. Therefore, after the composite material reaches the maximum failure load, its continuous load-bearing capacity increases, and the flexural toughness of the material increases. In addition, the low thermal conductivity of biochar gives it the effect of heat preservation and insulation. The combination of the two makes the chemical properties highly stable and not easily cause chemical corrosion of the internal structure of cement. This is because of the presence of fixed carbon and the removal of active groups inside biochar after high-temperature pyrolysis. The water retention property of biochar and porous carbon materials due to their own pore structures can absorb part of the water content added during cement mixing, thereby reducing the content of free water in cement and further reducing the pores generated due to water evaporation. At the same time, the water absorbed inside biochar and porous carbon materials can be used for internal curing of cement, making the internal structure of cement denser and increasing the strength of cement.

[0020] Optionally, the mass concentration of nitric acid in the nitric acid aqueous solution is 55 - 64%, the straw powder is added to the nitric acid aqueous solution for pretreatment for 10 - 15 h, and then added to the alumina sol for impregnation for 20 - 24 h.

[0021] By adopting the above technical solution, pretreating the straw powder in the nitric acid aqueous solution oxidizes the surface of the straw powder in the acidic solution to generate active sites, which is beneficial to more alumina being loaded on the surface of biochar during the carbonization stage.

[0022] Optionally, the calcium lignosulfonate is carbonized at 680 - 710 °C for 0.5 - 1 h and then soaked in 0.5 - 1 mol / L hydrochloric acid aqueous solution for 8 - 10 h.

[0023] By adopting the above technical solution, at high temperature, calcium lignosulfonate forms a porous carbon material with a layered ordered packing. After soaking in hydrochloric acid, the porous carbon material is activated, and a pore structure is formed on the surface of the layered porous carbon material, improving the water retention inside the concrete.

[0024] Optionally, the water is the wastewater from the concrete mixing plant.

[0025] By adopting the above technical solution, since porous materials are incorporated into the concrete material, the porous materials can adsorb harmful metal ions in the wastewater from the mixing plant, enabling the direct reuse of the wastewater from the mixing plant and avoiding direct discharge, which causes environmental pollution.

[0026] Optionally, the particle size of the recycled brick waste is 5 - 30 mm.

[0027] By adopting the above technical solution, the recycled brick waste, as coarse aggregate, acts as the skeleton material of the concrete, forming a particle size grading with fine aggregate sand and gravel, thereby improving the strength of the overall concrete material.

[0028] Optionally, the straw is any one of hemp straw, wheat straw, corn straw, sesame straw, and rice straw.

[0029] By adopting the above technical solution, straw burning will produce carbon emissions. By preparing biochar by high-temperature carbonization of straw in an anaerobic environment, carbon dioxide emissions are reduced. Using biochar to partially replace cement in building materials not only reduces the amount of cement used but also, as a means of carbon sequestration, additionally reduces greenhouse gas emissions in the construction industry.

[0030] Optionally, the density of the S95 mineral powder ≥ 2.8 g / cm 3 , the specific surface area ≥ 400 m 2 / kg, the water content ≤ 1.0, and the loss on ignition ≤ 3.0.

[0031] By adopting the above technical solution, S95 mineral powder is compounded with fly ash to prepare concrete. By making the S95 mineral powder meet certain physical property indexes, the workability of the concrete mixture is good, the cohesion is strong, the thermal shrinkage cracks of the concrete can be reduced, and the later strength of the concrete is enhanced.

[0032] In a second aspect, the present application provides a method for preparing high-strength recycled aggregate concrete, adopting the following technical solution:

[0033] A method for preparing high-strength recycled aggregate concrete, comprising the following steps:

[0034] Step 1, mixing cement, water, water reducer, fly ash, S95 mineral powder, sand, and recycled brick waste evenly to obtain a mixed slurry;

[0035] Step 2, stirring and mixing the modified biochar and the porous carbon material, then adding them to the mixed slurry, stirring evenly, pouring and molding, and curing to obtain the concrete.

[0036] By adopting the above technical solution, using S95 mineral powder compounded with fly ash, recycled brick waste as coarse aggregate, sand as fine aggregate, and mixing with a water reducer to form a slurry, and then adding modified biochar and porous carbon material to mix, the obtained concrete realizes resource recycling while meeting certain requirements for the compressive strength and flexural strength of the concrete.

[0037] In summary, the present application has the following beneficial effects:

[0038] 1. Since the present application uses modified biochar and porous carbon material for compounding to solve the defects brought by adding recycled aggregate. The modified biochar is dispersed between the layers of the porous carbon material. The organic groups on the surface of the biochar that are not completely carbonized form a binding force with the surface of the porous carbon material, reducing interface defects, and enabling the two to form a close packing in the concrete, thereby improving the compressive strength and flexural strength of the concrete.

[0039] 2. In this application, since porous materials are incorporated into the concrete material, the porous materials can adsorb harmful metal ions in the wastewater of the mixing plant, enabling the direct reuse of the wastewater from the mixing plant and avoiding direct discharge, which may cause environmental pollution.

[0040] 3. This application uses S95 blast furnace slag powder and fly ash in combination to prepare concrete. By making the S95 blast furnace slag powder meet certain physical property indicators, the workability of the concrete mixture is good and the cohesion is strong, which can reduce the thermal shrinkage cracks of the concrete and enhance the later strength of the concrete. Specific Embodiments

[0041] The following further elaborates on this application in combination with examples and comparative examples.

[0042] Preparation Examples of Raw Materials

[0043] Preparation Example 1

[0044] Preparation of modified biochar: Hemp straw was crushed into straw powder, the straw powder was added to a nitric acid aqueous solution with a mass concentration of 60% for pretreatment for 12 h, then added to alumina sol for impregnation for 22 h, and carbonized at 350 °C for 2.5 h to obtain;

[0045] Preparation of porous carbon material: Calcium lignosulfonate was added to an oxalic acid aqueous solution with a mass concentration of 50%, stirred, dried, carbonized at 700 °C for 0.8 h, then soaked in a 0.8 mol / L hydrochloric acid aqueous solution for 9 h, and dried to obtain.

[0046] Preparation Example 2

[0047] Preparation of modified biochar: Corn straw was crushed into straw powder, the straw powder was added to a nitric acid aqueous solution with a mass concentration of 55% for pretreatment for 10 h, then added to alumina sol for impregnation for 20 h, and carbonized at 300 °C for 2 h to obtain;

[0048] Preparation of porous carbon material: Calcium lignosulfonate was added to an oxalic acid aqueous solution with a mass concentration of 50%, stirred, dried, carbonized at 680 °C for 0.5 h, soaked in a 0.5 mol / L hydrochloric acid aqueous solution for 8 h, and dried to obtain.

[0049] Preparation Example 3

[0050] Preparation of modified biochar: Wheat straw was crushed into straw powder, the straw powder was added to a nitric acid aqueous solution with a mass concentration of 64% for pretreatment for 15 h, then added to alumina sol for impregnation for 24 h, and carbonized at 400 °C for 3 h to obtain;

[0051] Preparation of porous carbon material: Add calcium lignosulfonate to an oxalic acid aqueous solution with a mass concentration of 50%, stir, dry, carbonize at 710 °C for 1 h, and soak in 1 mol / L hydrochloric acid aqueous solution for 10 h. Obtained by drying.

[0052] Preparation Example 4

[0053] The difference from Preparation Example 1 is that the mass concentration of nitric acid in the nitric acid aqueous solution is 50%, the straw powder is added to the nitric acid aqueous solution for pretreatment for 8 h, and then added to the alumina sol for impregnation for 18 h.

[0054] Preparation Example 5

[0055] The difference from Preparation Example 1 is that the mass concentration of nitric acid in the nitric acid aqueous solution is 68%, the straw powder is added to the nitric acid aqueous solution for pretreatment for 18 h, and then added to the alumina sol for impregnation for 27 h.

[0056] Preparation Example 6

[0057] The difference from Preparation Example 1 is that after carbonization of calcium lignosulfonate, it is soaked in 0.3 mol / L hydrochloric acid aqueous solution for 6 h.

[0058] Preparation Example 7

[0059] The difference from Preparation Example 1 is that after carbonization of calcium lignosulfonate, it is soaked in 1.3 mol / L hydrochloric acid aqueous solution for 12 h.

[0060] Examples

[0061] Example 1

[0062] A preparation method of high-strength recycled aggregate concrete, comprising the following steps:

[0063] Step 1, stir cement, mixing plant wastewater, polycarboxylate water reducer, fly ash, slag powder, sand and recycled brick waste for 10 min to obtain a uniformly mixed slurry. The particle size of the recycled brick waste is 20 mm, and the recycled brick waste is obtained from the demolition site. The density of S95 slag powder ≥ 2.8 g / cm 3 , specific surface area ≥ 400 m 2 / kg, water content ≤ 1.0, loss on ignition ≤ 3.0, fineness modulus of sand is 2.5, fly ash is Class II fly ash, and cement is ordinary Portland cement;

[0064] Step 2, stir the modified biochar and the porous carbon material for 10 min, then add them to the mixed slurry and stir for 10 min, then cast and cure to obtain the concrete, wherein the modified biochar is prepared by Preparation Example 1 and the porous carbon material is prepared by Preparation Example 1.

[0065] Example 2

[0066] A preparation method of high-strength recycled aggregate concrete, which is different from Example 1 in that the modified biochar is prepared by Preparation Example 2, the porous carbon material is prepared by Preparation Example 2, and the particle size of the recycled brick slag is 5 mm.

[0067] Example 3

[0068] A preparation method of high-strength recycled aggregate concrete, which is different from Example 1 in that the modified biochar is prepared by Preparation Example 3, the porous carbon material is prepared by Preparation Example 3, and the particle size of the recycled brick slag is 30 mm.

[0069] The components and their corresponding weight parts of the raw materials in Examples 1-3 are shown in Table 1.

[0070] Table 1 Raw materials and their weights (kg) in Examples 1-3

[0071]

[0072]

[0073] Example 4

[0074] A preparation method of high-strength recycled aggregate concrete, which is different from Example 1 in that the modified biochar is prepared by Preparation Example 4, and the porous carbon material is prepared by Preparation Example 4.

[0075] Example 5

[0076] A preparation method of high-strength recycled aggregate concrete, which is different from Example 1 in that the modified biochar is prepared by Preparation Example 5, and the porous carbon material is prepared by Preparation Example 5.

[0077] Example 6

[0078] A preparation method of high-strength recycled aggregate concrete, which is different from Example 1 in that the modified biochar is prepared by Preparation Example 6, and the porous carbon material is prepared by Preparation Example 6.

[0079] Example 7

[0080] A preparation method of high-strength recycled aggregate concrete, which is different from Example 1 in that the modified biochar is prepared by Preparation Example 7, and the porous carbon material is prepared by Preparation Example 7.

[0081] Comparative Example

[0082] Comparative Example 1

[0083] A preparation method of high-strength recycled aggregate concrete, which is different from Example 1 in that the raw materials do not include modified biochar. It includes the following steps: Step 1, stir 160 kg of cement, 140 kg of mixing plant wastewater, 8 kg of polycarboxylate water reducer, 80 kg of fly ash, 85 kg of blast furnace slag, 900 kg of sand and 800 kg of recycled brick waste for 10 min, and mix evenly to obtain a mixed slurry. Step 2, stir 75 kg of porous carbon material for 10 min, then add it to the mixed slurry and stir for 10 min, then cast and cure to obtain the concrete, where the porous carbon material is prepared by Preparation Example 1.

[0084] Comparative Example 2

[0085] A preparation method of high-strength recycled aggregate concrete, which is different from Example 1 in that the raw materials do not include porous carbon material. It includes the following steps: Step 1, stir 160 kg of cement, 140 kg of mixing plant wastewater, 8 kg of polycarboxylate water reducer, 80 kg of fly ash, 85 kg of blast furnace slag, 900 kg of sand and 800 kg of recycled brick waste for 10 min, and mix evenly to obtain a mixed slurry. Step 2, stir 55 kg of modified biochar for 10 min, then add it to the mixed slurry and stir for 10 min, then cast and cure to obtain the concrete, where the modified biochar is prepared by Preparation Example 1.

[0086] Comparative Example 3

[0087] A preparation method of high-strength recycled aggregate concrete, which is different from Example 1 in that the raw materials do not include modified biochar and porous carbon material. It includes the following steps: Step 1, stir 160 kg of cement, 140 kg of mixing plant wastewater, 8 kg of polycarboxylate water reducer, 80 kg of fly ash, 85 kg of blast furnace slag, 900 kg of sand and 800 kg of recycled brick waste for 10 min, and mix evenly to obtain a mixed slurry, then cast and cure to obtain the concrete.

[0088] Performance detection test

[0089] Detection method

[0090] The concrete prepared in Examples 1-7 and Comparative Examples 1-3 was made into cube standard test blocks of 150 mm×150 mm×150 mm as test specimens. Three standard test blocks were cast in each group, and then the standard test blocks were placed in a standard curing room for curing. After 28 days, a pressure testing machine was used to test the three standard test blocks, and the arithmetic mean of the three test values was taken as the test value of the group of recycled concrete test blocks. The test was carried out according to the compressive strength test in the "Standard for Test Methods of Physical and Mechanical Properties of Concrete" GB / T 50081-2019; the test was carried out according to the flexural strength test in the "Standard for Test Methods of Physical and Mechanical Properties of Concrete" GB / T 50081-2019.

[0091] Table 2 Results of Performance Detection Tests

[0092] Compressive strength / MPa Flexural strength / MPa Example 1 51.1 5.5 Example 2 51. 5.3 Example 3 49 5.1 Example 4 46.1 4.8 Example 5 46.5 4.7 Example 6 47 4.5 Example 7 47.8 4.51 Comparative Example 1 33.1 3.1 Comparative Example 2 34.5 2.9 Comparative Example 3 22.9 2

[0093] The compressive strength of recycled aggregate concrete is generally greater than 40 Mpa, reaching the strength grade of ordinary concrete C40. The better the mechanical properties of the concrete, the greater the compressive strength and flexural strength numerically.

[0094] Combined with Example 1 and Examples 4-5 and Table 2, it can be seen that by fully oxidizing the straw powder in nitric acid solution and fully impregnating it in alumina sol, a certain amount of alumina is loaded on the prepared modified biochar material. Calcium hydroxide generated during the cement hydration process can form calcium aluminate hydrate on the surface of nano-alumina particles, and alumina is easily chemically bonded to the hydration products of cement, enabling the hydration products to form between the porous carbon material layers, improving the density and strength of the concrete.

[0095] Combined with Example 1 and Examples 6-7 and Table 2, it can be seen that by fully activating the carbonized porous carbon material in hydrochloric acid solution, a pore structure is formed on the layer surface, enabling water molecules to be retained in the interlayer pores, reducing the loss of free water in the concrete, and improving the later strength of the concrete.

[0096] Combined with Example 1 and Comparative Examples 1-3 and Table 2, it can be seen that by compounding the modified biochar and the porous carbon material, the compressive strength and flexural strength are improved significantly compared to single addition. It can be thus shown that the combination of the two can not only improve the defects of recycled aggregates but also ensure the strength of the concrete.

[0097] This specific embodiment is only an interpretation of the present application and is not a limitation thereof. Those skilled in the art can make modifications without creative contributions to this embodiment according to needs after reading this specification, but as long as it is within the scope of the claims of the present application, it is protected by the patent law.

Claims

1. A high-strength recycled aggregate concrete, characterized in that: It includes raw materials in the following parts by weight: 120 - 180 parts of cement; 150 - 170 parts of water; 60 - 100 parts of fly ash; 70 - 100 parts of slag powder; 800 - 1000 parts of sand; 700 - 900 parts of recycled brick waste; 5 - 10 parts of water reducing agent; 50 - 60 parts of modified biochar; 70 - 80 parts of porous carbon material; The modified biochar is obtained by adding straw powder into nitric acid aqueous solution for pretreatment, then adding it into alumina sol for impregnation, and carbonizing at 300 - 400 °C for 2 - 3 h; The porous carbon material is obtained by adding calcium lignosulfonate into oxalic acid solution, stirring, drying, and carbonizing at 680 - 710 °C for 0.5 - 1 h.

2. The high-strength recycled aggregate concrete according to claim 1, wherein: The mass concentration of nitric acid in the nitric acid aqueous solution is 55 - 64%. The straw powder is added into the nitric acid aqueous solution for pretreatment for 10 - 15 h, and then added into alumina sol for impregnation for 20 - 24 h.

3. A high-strength recycled aggregate concrete according to claim 1, characterized in that: The calcium lignosulfonate is added into the oxalic acid aqueous solution with a mass concentration of 50%, carbonized at 680 - 710 °C for 0.5 - 1 h, and then soaked in 0.8 mol / L hydrochloric acid aqueous solution for 9 h.

4. A high-strength recycled aggregate concrete according to claim 1, characterized in that: The calcium lignosulfonate is added into the oxalic acid aqueous solution with a mass concentration of 50%, carbonized at 680 - 710 °C for 0.5 - 1 h, and then soaked in 1 mol / L hydrochloric acid aqueous solution for 10 h.

5. A high-strength recycled aggregate concrete according to claim 1, characterized in that: The calcium lignosulfonate is added into the oxalic acid aqueous solution with a mass concentration of 50%, carbonized at 680 - 710 °C for 0.5 - 1 h, and then soaked in 0.5 mol / L hydrochloric acid aqueous solution for 8 h.

6. A high-strength recycled aggregate concrete according to claim 1, characterized in that: The water is the wastewater from a concrete mixing plant.

7. A high-strength recycled aggregate concrete according to claim 1, characterized in that: The particle size of the recycled brick waste is 5 - 30 mm.

8. A high-strength recycled aggregate concrete according to claim 1, characterized in that: The straw is any one of hemp straw, wheat straw, corn straw, sesame straw, and rice straw.

9. A high-strength recycled aggregate concrete according to claim 1, wherein: The density of the mineral powder is ≥ 2.8 g / cm 3 , the specific surface area is ≥ 400 m 2 / kg, the water content is ≤ 1.0, and the loss on ignition is ≤ 3.

0.

10. The preparation method of the high-strength recycled aggregate low-carbon concrete according to any one of claims 1-9, characterized in that: It includes the following steps: Step 1, mix cement, water, water reducing agent, fly ash, slag powder, sand, and recycled brick waste evenly to obtain a mixed slurry; Step 2, stir and mix the modified biochar and the porous carbon material, then add them into the mixed slurry, stir evenly, pour and mold, and cure to obtain the concrete.

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