Preparation method of modified potassium magnesium phosphate cement recycled concrete based on secondary stirring method

Abstract

The invention discloses a preparation method of modified potassium magnesium phosphate cement recycled concrete based on a secondary stirring method. The modified potassium magnesium phosphate cement recycled concrete is mainly prepared from 16% of dead burnt magnesium oxide, 11% of monopotassium phosphate, 5-10% of a retarder, 3% of fly ash, 5% of silica fume, 0-16% of recycled aggregate, 25-41% of natural aggregate, 0.08% of a polycarboxylic acid water reducer and 6.6% of water. On one hand, a proper amount of retarder is doped, so that the hydration speed of the magnesium potassium phosphate cement-based material is slowed down, and the magnesium potassium phosphate cement-based material has relatively high early strength and relatively high mechanical properties; on the other hand, the fluidity and the volume stability of the concrete are improved by doping the water reducing agent and the admixture; meanwhile, the secondary stirring method ensures that the potassium magnesium phosphate cement can inhibit the hydration speed in the stirring process, so that the hydration reaction is more completely carried out. The invention is mainly suitable for repairing structural concrete and high-performance recycled concrete, and can be used for practical engineering under the conditions of seawater erosion environment, sulfate erosion environment, chloride ion erosion environment and the like. A large amount of waste concrete is used as recycled aggregate, so that the loss of natural resources is reduced, and the effects of environmental protection and solid waste circulation are achieved.

Description

Technical Field

[0001] The invention belongs to the field of building materials and concrete, and specifically relates to a method for preparing modified potassium magnesium phosphate cement recycled concrete based on a secondary mixing method. Background Art

[0002] With the development of my country's urbanization process, a large number of waste concretes generated by old buildings and new buildings require not only spatial locations to dispose of excess construction solid waste, but also an urgent need for a way to utilize solid waste. Potassium magnesium phosphate cement is a type of cementitious material generated by acid-base neutralization reaction of heavy burned magnesium oxide, potassium dihydrogen phosphate and admixtures in a certain proportion under acidic conditions. It has the characteristics of fast hydration, high early strength, strong bonding performance and good volume stability. Since the hydration products generated at the interface between potassium magnesium phosphate cement and larger aggregates are stable, and the bonding performance between the hydration products and aggregates is better, the mechanical properties of the interface transition zone can be improved.

[0003] In addition, based on the fact that Portland cement is a common matrix for recycled aggregate utilization, existing studies have shown that recycled aggregate has a negative impact on the mechanical and durability properties of Portland cement. Potassium magnesium phosphate cement has better bonding properties than Portland cement, can strengthen the cohesion between aggregates, and also has the function of solidifying waste to form a dense matrix, which is conducive to alleviating the negative impact of the dosage and weak physical properties of recycled coarse aggregate on concrete. The two have a theoretical basis for interaction.

[0004] Therefore, efforts to promote the application scope of recycled aggregates, improve the utilization rate of recycled aggregates, and reduce the loss of natural resources are not only beneficial to the green cycle of the environment, but also to the development of the current recycled concrete field. This new type of cementitious material is used to repair and reinforce the properties of concrete structures, combined with recycled aggregates from crushed construction solid waste to prepare a new type of recycled concrete. Summary of the invention

[0005] The purpose of the present invention is to solve the deficiencies of the prior art. Aiming at the problems of low strength, poor durability and low replacement rate of recycled aggregates in silicate recycled concrete, the present invention provides a preparation method of modified potassium magnesium phosphate cement recycled concrete based on secondary mixing method, which has the characteristics of high early strength, good volume stability and high bonding strength, and can be applied to actual engineering conditions in seawater erosion environment, sulfate erosion environment and high temperature environment. The preparation method is green and low-carbon, which has a huge driving effect on the clay resources and environmental protection required for silicate cement, and is conducive to energy conservation and emission reduction.

[0006] The present invention provides a method for preparing modified potassium magnesium phosphate cement recycled concrete based on a secondary mixing method, wherein the mass percentage of each component includes the following raw materials:

[0007] Dead-burned magnesium oxide 16%, potassium dihydrogen phosphate 11%, composite retarder 5-10%, fly ash 3%, silica fume 5%, recycled aggregate 16%, natural aggregate 25%, river sand 16.7%, polycarboxylate water reducer 0.08%, water 6.6%;

[0008] Preferably, the dead-burned magnesium oxide has 200 mesh, 92 purity, and a specific surface area of ​​217 m 2 kg -1 , the average particle size is 60.05μm, and the specific gravity is 3.14g / cm 3 .

[0009] Preferably, the composite retarder is characterized in that it is composed of sodium tetraborate, disodium hydrogen phosphate dodecahydrate, and barium chloride.

[0010] Preferably, the natural aggregate and recycled aggregate are characterized by a particle size of 5 to 25 mm. The recycled aggregate is characterized by a recycled aggregate replacement rate of 40%.

[0011] Preferably, the river sand is characterized by being continuously graded river sand with a particle size of 1 to 2.5 mm.

[0012] Preferably, the water reducer is a polycarboxylic acid high performance and high efficiency water reducer.

[0013] Preferably, the method for preparing modified potassium magnesium phosphate cement recycled concrete based on the secondary mixing method is characterized by comprising the following steps:

[0014] S1: Determine the proportion and dosage of each group of concrete according to claim 1;

[0015] S2: First wet the mixer surface, then add natural aggregate, recycled aggregate and river sand, and mix the three for 2 minutes;

[0016] S3: Add dead-burned magnesium oxide, potassium dihydrogen phosphate and composite retarder and stir for 2 minutes;

[0017] S4: Finally, add fly ash and silica fume into the mixture and stir for 1 min;

[0018] S5: Then mix the liquid polycarboxylate water reducer into the water, add half of the water for the first stirring, stir for 30 seconds, then pour the remaining water into the stirred mixture, continue stirring for 2 minutes, stir evenly and pour into the mold to form.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] Compared with the prior art, the present invention has the following advantages: first, potassium magnesium phosphate cement is used, which has a fast hydration speed, high early strength, good volume stability, and high bonding strength with many substrates. The selected composite retarder can provide sufficient setting time to allow the hydrated or unhydrated silicate powder on the surface of the recycled aggregate to react, and the polycarboxylic acid high-performance water reducer can have sufficient fluidity while ensuring a low water-cement ratio, so that the concrete can be applied to actual projects. The two composite mineral admixtures can form a dense interface transition zone at the interface transition zone, which will significantly improve the mechanical properties and durability of the concrete.

[0021] The modified potassium magnesium phosphate cement recycled concrete provided by the present invention can adjust the setting time to a certain extent by adjusting the amount of retarder and mineral admixture, and obtain higher early strength under the premise of meeting the construction requirements, so as to meet the requirements of rapid repair projects. DETAILED DESCRIPTION

[0022] The present invention will be further described below in conjunction with specific embodiments.

[0023] Example 1: This example provides a modified potassium magnesium phosphate cement recycled concrete based on a secondary mixing method, which is composed of the following components by mass percentage: 16% dead-burned magnesium oxide, 11% potassium dihydrogen phosphate, 2.0% composite retarder, 3% fly ash, 5% silica fume, 16% recycled aggregate, 25% natural aggregate, 16.7% river sand, 0.08% polycarboxylic acid water reducer, and 6.6% water.

[0024] The method for preparing the modified potassium magnesium phosphate cement recycled concrete comprises the following steps:

[0025] S1: Determine the proportion and dosage of each group of concrete according to claim 1;

[0026] S2: First wet the mixer surface, then add natural aggregate, recycled aggregate and river sand, and mix the three for 2 minutes;

[0027] S3: Add dead-burned magnesium oxide, potassium dihydrogen phosphate and composite retarder and stir for 2 minutes;

[0028] S4: Finally, add fly ash and silica fume into the mixture and stir for 1 min;

[0029] S5: Then mix the liquid polycarboxylate water reducer into the water, add half of the water for the first stirring, stir for 30 seconds, then pour the remaining water into the stirred mixture, continue stirring for 2 minutes, stir evenly and pour into the mold to form.

[0030] Comparative Example 1: The preparation method of the modified potassium magnesium phosphate cement recycled concrete in this embodiment is the same as that in the above-mentioned embodiment 1, except that 2.0% of the retarder sodium tetraborate is used.

[0031] Comparative Example 2: The preparation method of the modified potassium magnesium phosphate cement recycled concrete in this embodiment is the same as that in the above-mentioned embodiment 1, except that 2.0% of the retarder boric acid is used.

[0032] Comparative Example 3: This embodiment has the same preparation method as the modified potassium magnesium phosphate cement recycled concrete of the above-mentioned embodiment 1, and is composed of the following components by mass percentage: 16% dead-burned magnesium oxide, 11% potassium dihydrogen phosphate, 2.0% composite retarder, 3% fly ash, 5% silica fume, 8.2% recycled aggregate, 33% natural aggregate, 16.7% river sand, 0.08% polycarboxylate water reducer, and 6.6% water.

[0033] Comparative Example 4: This embodiment has the same preparation method as the modified potassium magnesium phosphate cement recycled concrete of the above-mentioned Example 1, and is composed of the following components by mass percentage: 16% dead-burned magnesium oxide, 11% potassium dihydrogen phosphate, 2.0% composite retarder, 3% fly ash, 5% silica fume, 41% natural aggregate, 16.7% river sand, 0.08% polycarboxylate water reducer, and 6.6% water.

[0034] Comparative Example 5: This embodiment has the same preparation method as the modified potassium magnesium phosphate cement recycled concrete of the above-mentioned Example 1, and is composed of the following components by mass percentage: 16% dead-burned magnesium oxide, 11% potassium dihydrogen phosphate, 1.6% composite retarder, 3% fly ash, 5% silica fume, 16% recycled aggregate, 25% natural aggregate, 16.7% river sand, 0.08% polycarboxylate water reducer, and 6.6% water.

[0035] Comparative Example 6: This embodiment has the same preparation method as the modified potassium magnesium phosphate cement recycled concrete of the above-mentioned Example 1, and is composed of the following components by mass percentage: 16% dead-burned magnesium oxide, 11% potassium dihydrogen phosphate, 1% composite retarder, 3% fly ash, 5% silica fume, 16% recycled aggregate, 25% natural aggregate, 16.7% river sand, 0.08% polycarboxylate water reducer, and 6.6% water.

[0036] Comparative Example 7: This embodiment has the same composition as the modified potassium magnesium phosphate cement recycled concrete of the above embodiment 1, except that the preparation method of the modified potassium magnesium phosphate cement recycled concrete comprises the following steps:

[0037] S1: Determine the proportion and dosage of each group of concrete according to claim 1;

[0038] S2: First wet the mixer surface, then add natural aggregate, recycled aggregate and river sand, and mix the three for 2 minutes;

[0039] S3: Add dead-burned magnesium oxide, potassium dihydrogen phosphate and composite retarder and stir for 2 minutes;

[0040] S4: Finally, add fly ash and silica fume into the mixture and stir for 1 min;

[0041] S5: Then mix the liquid polycarboxylate water reducer into the water, pour all the water into the stirred mixture, continue stirring for 2 minutes, and pour into the mold after stirring evenly.

[0042] Performance Testing

[0043] 1. Test methods

[0044] After the modified potassium magnesium phosphate cement recycled concrete prepared in the embodiment and the comparative example was vibrated into shape, the specimens were placed in an indoor environment for one day, and their surfaces were covered with a waterproof plastic film; after being placed for 24 hours, the molds were removed, and the specimens were placed in an indoor environment for natural curing, and the specimens were taken out after curing for 3 days and 28 days.

[0045] Cube compressive strength test: Based on the "Concrete Physical and Mechanical Properties Test Method Standard" GB / T 50081-2019. Equipment: Italian MATEST C088-01 material testing machine.

[0046] 2. Test results

[0047] Table 1 Test results of embodiments and comparative examples

[0048]

[0049]

[0050] It can be seen from the test results in Table 1 that after 3 days of curing, the strength of Example 1 reached 68% of the strength after 28 days of curing, meeting the requirements of actual engineering for early strength of concrete; compared with the strength of Comparative Examples 1 and 2 after 3 days and 28 days of curing, it was found that the use of the composite retarder can significantly improve the early strength, and the maximum strength can reach 63.82 MPa.

[0051] In addition, in Comparative Examples 3 and 4 corresponding to the replacement rates of recycled aggregates of 0 and 20%, it can be seen that the strength of concrete added with recycled aggregates after 28 days of curing is generally above 58 MPa, and is not negatively affected by the replacement rate of recycled aggregates. On the contrary, in terms of early strength, the strength of concrete with a replacement rate of 20% recycled aggregates is 1.12 times that of concrete using only natural aggregates, which verifies the above hypothesis, that is, it is helpful to alleviate the negative effects of the dosage and weak physical properties of recycled coarse aggregates on concrete, and the two have a theoretical basis for interaction.

[0052] In comparing Example 1 with Comparative Examples 5 and 6, it was found that the strength improvement effect of the modified potassium magnesium phosphate cement recycled concrete was better with the increase of the amount of composite retarder, which was increased by 6.46MPa and 6.89MPa respectively. The different mixing processes resulted in different strength developments between Example 1 and Comparative Example 7, with little difference in early strength. The strength of the modified potassium magnesium phosphate cement recycled concrete by the secondary mixing method after 28 days of curing was increased by 5.18MPa.

[0053] Experiments show that the cement-based materials of the modified potassium magnesium phosphate cement recycled concrete based on the secondary mixing method can form an interaction with the recycled aggregate that is beneficial to the development of strength, and the secondary mixing method can provide appropriate moisture for the hydration process of potassium magnesium phosphate cement, and combine with the composite retarder to inhibit its hydration rate, so that it has sufficient hydration reaction time and hydration process, reducing the concentrated release of hydration heat.

[0054] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A modified potassium magnesium phosphate cement recycled concrete based on a secondary mixing method, characterized in that: The components of the mix are as follows: dead-burned magnesium oxide, potassium dihydrogen phosphate, composite retarder, fly ash, silica fume, recycled aggregate, natural aggregate, polycarboxylic acid water reducer and water. The mass ratio of each component is 16% dead-burned magnesium oxide, 11% potassium dihydrogen phosphate, 5-10% composite retarder, 3% fly ash, 5% silica fume, 0-16% recycled aggregate, 25-41% natural aggregate, 16.7% river sand, 0.08% polycarboxylic acid water reducer and 6.6% water.

2. A modified potassium magnesium phosphate cement recycled concrete according to claim 1, characterized in that: The dead-burned magnesium oxide has 200 mesh, 92 purity, and a specific surface area of ​​217m 2 kg -1 , the average particle size is 60.05μm, and the specific gravity is 3.14g / cm 3 .

3. The composite retarder according to claim 1, characterized in that The composition comprises sodium tetraborate, disodium hydrogen phosphate dodecahydrate and barium chloride.

4. The natural aggregate and recycled aggregate according to claim 1, characterized in that The particle size is 5 to 25 mm.

5. The river sand according to claim 1, characterized in that It is continuously graded river sand with a particle size of 1 to 2.5 mm.

6. The recycled aggregate according to claim 4, characterized in that The replacement rate of recycled aggregate is 0 to 40%.

7. The modified potassium magnesium phosphate cement recycled concrete according to claim 1, characterized in that: The water reducer is a polycarboxylic acid high performance and high efficiency water reducer.

8. The modified potassium magnesium phosphate cement recycled concrete according to claim 1, characterized in that: The curing method is natural curing.

9. The method for preparing modified potassium magnesium phosphate cement recycled concrete based on the secondary mixing method according to claim 1, characterized in that: The steps include: S1: Determine the proportion and dosage of each group of concrete according to claim 1; S2: First wet the mixer surface, then add natural aggregate, recycled aggregate and river sand, and mix the three for 2 minutes; S3: Add dead-burned magnesium oxide, potassium dihydrogen phosphate and composite retarder, Stir for 2 minutes; S4: Finally, add fly ash and silica fume into the mixture and stir for 1 min; S5: Then mix the liquid polycarboxylate water reducer into the water, add half of the water for the first stirring, stir for 30 seconds, then pour the remaining water into the stirred mixture, continue stirring for 2 minutes, stir evenly and pour into the mold to form.

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