CN109095834B

CN109095834B - Method for preparing recycled concrete by using unqualified premixed concrete and recycled powder

Info

Publication number: CN109095834B
Application number: CN201810835024.7A
Authority: CN
Inventors: 肖建庄; 隋同波; 李坛; 易卓然
Original assignee: Tongji University
Current assignee: Tongji University
Priority date: 2018-07-26
Filing date: 2018-07-26
Publication date: 2021-10-08
Anticipated expiration: 2038-07-26
Also published as: CN109095834A

Abstract

The invention relates to a method for preparing recycled concrete by using unqualified premixed concrete and recycled powder, which comprises the following steps: adding a retarder which accounts for 0.06-0.13% of the mass of the unqualified premixed concrete into the unqualified premixed concrete and storing the retarder; preparing new concrete according to a standard, and mixing unqualified premixed concrete into the new concrete according to the mass ratio of 10-50%; and then adding a water reducing agent accounting for 0.16-0.31% of the total mass of the mixture and regenerated powder accounting for 10-40 w%, and mechanically stirring uniformly to obtain the regenerated concrete. The concrete prepared by the invention can better solve the problem of treatment of unqualified premixed concrete in concrete production, does not pollute the environment, reduces the material consumption of concrete production, and can meet the performance requirement of the concrete before being replaced. Therefore, the invention has obvious economic benefit, environmental benefit and social benefit.

Description

Method for preparing recycled concrete by using unqualified premixed concrete and recycled powder

Technical Field

The invention belongs to the technical field of building materials, and particularly relates to a method for preparing recycled concrete by using unqualified premixed concrete and recycled powder.

Background

Commercial concrete mixing plants inevitably produce a lot of off-spec ready-mixed concrete. For example, due to problems during transportation and problems with construction structures on site, the concrete cannot be poured as desired, and a surplus of ready-mixed concrete and concrete with too little slump are produced. And discharging a large amount of unqualified ready-mixed concrete when the cement mixing transport vehicle at the construction site finishes unloading and returns to the station is used for washing the concrete. If unqualified premixed concrete is buried or piled after final setting, a large amount of land is occupied, sand and stone separation treatment is carried out, various waste slurries generated in the production process are randomly discharged to cause secondary pollution, and the influence on the environment is very serious.

The unqualified ready-mixed concrete has larger viscosity which is more than 1000g/L because of long time of storage after being mixed, at the moment, early C-S-H is formed, hydration products are in the process of growing outwards from the early C-S-H, and cement slurry gradually loses fluidity. This period is also a period of CH formation, and large-particle-size CH crystals are enriched on the surface of the aggregate to form an interface transition region. Therefore, when the unqualified premixed concrete is used for replacing the common concrete, the following problems exist: 1) the unqualified premixed concrete has larger consistency, and can influence the working performance of recycled concrete, so that the slump of the recycled concrete is reduced, and the pumping is difficult; meanwhile, the unqualified premixed concrete has higher viscosity, so that the unqualified premixed concrete is difficult to be uniformly mixed in the mixing process, and the regenerated concrete has a layering phenomenon; in addition, viscous premixed concrete also influences the vibration compaction of recycled concrete in a member with dense reinforcing steel bars, and gaps are formed to reduce the performance of the member. 2) And the unqualified premixed concrete is in the interface forming period, and the concrete which is not fully and uniformly mixed with the new concrete can form a new interface on the interface of the new concrete and the unqualified premixed concrete. The interface area distribution between the massive unqualified premixed concrete and the new concrete is wider, and the large-area interface area becomes a weak area of the concrete and cracks under a smaller load, so that the quality and the performance of the new concrete are seriously influenced.

The patent publication No. CN106365484A discloses a process for recycling waste commercial concrete, which comprises the steps of cleaning the commercial concrete with clear water, separating waste residue and waste water into sand and waste slurry, and recycling the sand, the slurry and water respectively. The waste residue and the slurry recycled by the invention can replace partial artificial sand and water to mix the commercial concrete, so that the consumption of the aggregate and the water of the fresh concrete is saved, and the material cost is further saved. The separation process needs a plurality of steps, is relatively tedious, and in addition, can only extract the raw materials from the waste concrete and utilize, can not utilize waste commodity concrete itself.

The patent of publication No. CN103693875A discloses a recycling production process of premixed concrete solid waste, which comprises the steps of crushing the concrete solid waste into blocks with the particle size of 0-30 mm by a gear roller crusher, screening coarse aggregate with the particle size of 10-30 mm, and crushing and screening the rest materials into fine aggregate with the particle size of 1-5 mm and active ultrafine powder with the particle size of 0-0.025 mm by a conical ball mill and an ultrafine grinding machine. The recycling process provided by the invention can be used for efficiently analyzing and recycling the coarse aggregate, the fine aggregate and the active powder in the concrete solid waste residue, and the total energy consumption of the production process is reduced. However, the recycling process can only recycle one hardened premixed concrete solid waste residue, and requires a site to stack the returned premixed concrete, thereby occupying the land.

The patent of publication No. CN104446211A discloses a wall brick prepared from premixed concrete waste slurry, which is mainly prepared from the following components in parts by weight: 600-800 parts of filter-pressing slurry, 40-140 parts of cement, 1-1.4 parts of calcium lignosulfonate, 25-125 parts of water, 800-950 parts of artificial sand and 400-550 parts of iron tailing slag. The wall brick provided by the invention utilizes slurry produced by premixed concrete enterprises and waste liquid produced in the paper making process, has low cost, environmental protection and high economic benefit, and the prepared wall brick has high compressive strength and good frost resistance.

Disclosure of Invention

Aiming at the problem of how to treat unqualified premixed concrete, the invention provides a method for preparing recycled concrete by using unqualified premixed concrete and recycled powder, which can directly and effectively recycle the unqualified premixed concrete and recycle solid waste residues or waste concrete in the form of the recycled powder.

The purpose of the invention is realized by the following technical scheme:

a method for preparing recycled concrete by using unqualified premixed concrete comprises the following steps:

(1) adding a retarder which accounts for 0.06-0.13% of the mass of the unqualified premixed concrete into the unqualified premixed concrete and storing the retarder;

(2) preparing new concrete according to a standard, and mixing unqualified premixed concrete into the new concrete according to the mass ratio of 10-50%;

(3) and then adding a water reducing agent accounting for 0.16-0.31% of the total mass of the mixture and regenerated powder accounting for 10-40 w%, and mechanically stirring uniformly to obtain the regenerated concrete.

Preferably, the retarder is sodium citrate, wherein the sodium citrate is white crystal particles or powder and has the density of 1.79 g/cm; the content of sodium citrate is more than or equal to 98 percent, the content of chloride is less than or equal to 0.07 percent, the content of arsenic salt is less than or equal to 3P.P.M, the content of lead salt is less than or equal to 10P.P.M, the content of heavy metal is less than or equal to 20P.P.M, the content of sulfate is less than or equal to 0.05 percent, and the content of reducing substance is less than or equal to 0.5 percent.

Preferably, the water reducing agent is polycarboxylic acid, wherein the polycarboxylic acid is white powder in appearance, the bulk density is 550-600kg/m3, the active ingredient is not less than 25%, the pH (20% concentration) is 5-7, the water reducing rate is not less than 14%, the bleeding rate ratio is not more than 90%, and the gas content is not less than 3.0%; the difference between the coagulation times was-90. + -. 120 min.

Preferably, the regenerated powder is an active ultrafine powder with the particle size of less than 0.025mm, and can be prepared by grinding premixed concrete solid waste residues or waste concrete for other structures.

Preferably, the unqualified ready-mixed concrete and the new concrete are the same type of concrete or have similar mixture ratio.

Preferably, the unqualified premixed concrete is returned concrete which is qualified in the raw material test, but the storage time of the unqualified premixed concrete is not longer than the initial setting time of the returned concrete which is unqualified in the fresh concrete test on the construction site.

Preferably, the consumption of the retarder accounts for 0.08-0.12% of the quality of the unqualified ready-mixed concrete.

Preferably, the amount of the water reducing agent accounts for 0.2-0.3% of the mass of the recycled concrete.

Preferably, the regenerated powder accounts for 10-30% of the mass of the regenerated concrete.

The specific preparation method of the invention is designed based on the following principle:

1. consistency control

The unqualified ready-mixed concrete consumes part of water in hydration reaction in the early stage of hydration, and the other part of water is wrapped in the ready-mixed concrete by the C-S-H gel generated in the early stage, so that the ready-mixed concrete is thickened, the mixing of the ready-mixed concrete and the new concrete is prevented, and the moisture in the new concrete is absorbed, so that the consistency of the finally-mixed ready-mixed concrete is increased. By stirring off-specification ready-mixed concrete at a relatively high rate (10-15 rpm) in the early stage of mixing, the formation of the C-S-H gel skeleton in the early stage is hindered, enabling it to form a stable structure with fresh concrete in the later stage. After the unqualified premixed concrete is doped into the concrete, the water reducing agent is doped, so that the new concrete releases more water, and the unqualified premixed concrete is diluted and lubricated, so that the unqualified premixed concrete is dispersed into the new concrete, and the slump loss caused by the unqualified premixed concrete is compensated.

2. Hydration control

The off-spec ready mixed concrete incorporated in the new concrete is at a near initial set, which is the accelerated stage of cement hydration. In this stage, which is also the initial C-S-H gel formation stage, the initial C-S-H gel formed with cement particles as cores is a crystal nucleus and develops all around to form a network C-S-H gel. The C-S-H gel at this stage is in the process of forming a whole from dispersed gel bodies to interconnection, and thus still has fluidity. However, as time progresses, by the initial setting stage, the C-S-H gel skeleton begins to form, mobility is lost, and strength begins to develop. The added retarder generates retardation by adsorbing on the hydration product and inhibiting the continuous growth of the hydration product. In the mixing process of the unqualified ready-mixed concrete and the new concrete, the water reducing agent is added to disperse the unqualified ready-mixed concrete, the new concrete is hydrated to continue to grow by taking the dispersed C-S-H gel as a core, and a new C-S-H framework can be quickly formed. Because the unqualified premixed concrete C-S-H gel skeleton before initial setting is not formed, the C-S-H gel in the new and old concrete is well bonded, and the interface effect of CH enrichment can not occur.

3. Pore optimization

The unqualified ready-mixed concrete mixed in the new concrete can hinder the discharge of gas in the recycled concrete during the mixing time due to the larger consistency, so that the recycled concrete contains more pores. The volcanic ash effect, the filling effect and the micro-aggregate effect of the regenerated powder refine the internal pore structure of the concrete, divide the original large pores into small pores which are fine and not communicated, and play a role in obviously reducing the porosity of the concrete and increasing the compactness, thereby improving the strength of the new concrete.

By the method and the consistency adjustment, hydration control and pore optimization, the unqualified premixed concrete and the new concrete are hydrated and consolidated into a uniform whole together, thereby overcoming the problems of construction and structural performance caused by mixing different concretes.

Compared with the prior art, the invention has the following beneficial effects:

1. high substitution rate: replacing 10-50% of the concrete with unqualified premixed concrete in an equivalent manner according to the mass ratio, wherein the strength reduction of the mixed concrete is lower than 5% in 7 days and 28 days; when the replacement ratio is less than 20%, the strength is hardly decreased.

2. The mixing method is simple and is beneficial to industrial production and application.

3. The cost of the raw materials is low: the unqualified ready-mixed concrete is unqualified ready-mixed material, is a burden for enterprises, and is in urgent need of treatment, so that the material is easy to obtain and the cost is extremely low.

4. No toxic and side effects and good durability: the concrete mixed by the invention adopts unqualified premixed concrete which meets the national standard, does not contain harmful components, and the concrete does not have the adverse effect of alkali-aggregate reaction.

5. Wide applicability: the method can be used for various premixed concretes, and the patent technology is applicable to the same type of concrete as long as the actual strength grade of the unqualified premixed concrete is not lower than that of the freshly mixed concrete.

6. Obvious economic benefit, environmental benefit and social benefit: the unqualified premixed concrete is adopted to replace part of the new concrete, and the regenerated powder is adopted to replace the cementing material, so that the over-fast consumption of non-renewable resources can be reduced, and the environmental damage caused by the discharge of the unqualified premixed concrete is eliminated. The raw materials of the invention have low cost, the cost of each cubic meter of concrete can be saved by 10-20% calculated by mixing C30 concrete and replacing 20% of concrete, and if unqualified premixed concrete of 3000-4000 million cubic meters is treated every year, the economic benefit and the social benefit brought are very obvious.

Drawings

FIG. 1 is a graph showing the 7-day strength of recycled powdered concrete prepared according to the present invention;

FIG. 2 is a graph showing the 28-day strength of recycled powdered concrete prepared according to the present invention;

FIG. 3 shows slump of recycled powder concrete prepared according to the present invention.

Detailed Description

The technical solution in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. It is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments, and all other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention are within the scope of the present invention without any inventive work.

The new concrete in the raw materials used by the invention comprises the following components in parts by weight: 0.7 of cement, 2.28 of sand, 2.91 of broken stone, 0.5 of water and a water reducing agent accounting for 0.43 percent of the mass of the new concrete, wherein the unqualified premixed concrete and the new concrete have the same mixing ratio. Unqualified premixed concrete is used as a raw material, a polycarboxylate water reducer and a sodium citrate retarder are added, new concrete is replaced in the process of mixing and preparing recycled concrete, and the standing time of the unqualified premixed concrete is 3 hours or 4.5 hours.

Example 1

The preparation method comprises the following specific steps: (1) storing unqualified premixed concrete for 3 hours; (2) 0.06 percent of sodium citrate retarder is doped into the unqualified premixed concrete, and the mixture is stirred at the speed of 10-15 r/min; (3) mixing new concrete according to a reference mixing proportion, and mechanically stirring for 5-10 min; (4) keeping the new concrete in a stirring state, slowly pouring 30% of unqualified premixed concrete into the new concrete according to the mass ratio, adding regenerated powder accounting for 40% of the total mass of the mixture and 0.16% of polycarboxylic acid water reducing agent, and stirring for 5-10 min.

Example 2

The preparation method comprises the following specific steps: (1) storing unqualified premixed concrete for 3 hours; (2) 0.06 percent of sodium citrate retarder is doped into the unqualified premixed concrete, and the mixture is stirred at the speed of 10-15 r/min; (3) mixing new concrete according to a reference mixing proportion, and mechanically stirring for 5-10 min; (4) keeping the new concrete in a stirring state, slowly pouring 30% of unqualified premixed concrete into the new concrete according to the mass ratio, adding regenerated powder accounting for 10% of the total mass of the mixture and 0.16% of polycarboxylic acid water reducing agent, and stirring for 5-10 min.

Example 3

The preparation method comprises the following specific steps: (1) storing unqualified premixed concrete for 4.5 hours; (2) 0.13 percent of sodium citrate retarder is added into the unqualified premixed concrete, and the mixture is stirred at the speed of 10-15 r/min; (3) mixing new concrete according to a reference mixing proportion, and mechanically stirring for 5-10 min; (1) keeping the new concrete in a stirring state, slowly pouring 30% of unqualified premixed concrete into the new concrete according to the mass ratio, adding regenerated powder accounting for 40% of the total mass of the mixture and 0.31% of polycarboxylic acid water reducing agent, and stirring for 5-10 min.

Example 4

The preparation method comprises the following specific steps: (1) storing unqualified premixed concrete for 4.5 hours; (2) 0.13 percent of sodium citrate retarder is added into the unqualified premixed concrete, and the mixture is stirred at the speed of 10-15 r/min; (3) mixing new concrete according to a reference mixing proportion, and mechanically stirring for 5-10 min; (4) keeping the new concrete in a stirring state, slowly pouring 30% of unqualified premixed concrete into the new concrete according to the mass ratio, adding regenerated powder accounting for 10% of the total mass of the mixture and 0.31% of polycarboxylic acid water reducing agent, and stirring for 5-10 min.

Comparative example

The new concrete comprises the following components in parts by weight: 0.7 percent of cement, 2.28 percent of sand, 2.91 percent of gravel, 0.5 percent of water and 0.43 percent of water reducing agent. The preparation method comprises the following specific steps: and (3) putting the dry materials into a stirrer, mechanically stirring for 5min, adding water and a water reducing agent, and mechanically stirring for 7min to form the premixed concrete under the reference mixing ratio.

The concrete test blocks obtained in the examples and the comparative examples are tested and compared in terms of mechanical properties and working properties, and the properties of the concrete test blocks meet the corresponding standard requirements, and the comparison of the properties is shown in Table 1. FIG. 1 is the 7-day strength of the recycled powdered concrete prepared according to the present invention, FIG. 2 is the 28-day strength of the recycled powdered concrete prepared according to the present invention, and FIG. 3 is the slump of the recycled powdered concrete prepared according to the present invention.

TABLE 1 comparison of test data for examples and comparative examples

Therefore, the strength of the recycled concrete mixed by the invention is not obviously reduced, and the recycled concrete has better flowing property.

Claims

1. A method for preparing recycled concrete by using unqualified premixed concrete and recycled powder is characterized by comprising the following steps:

(3) then, adding a water reducing agent accounting for 0.16-0.31% of the total mass of the mixture and regenerated powder accounting for 10-40 w%, and mechanically stirring uniformly to obtain regenerated concrete;

the retarder is citric acid or salt thereof;

the water reducing agent is a high-efficiency water reducing agent;

the regenerated powder is an active ultrafine powder with the particle size of less than 0.025 mm;

the unqualified premixed concrete and the new concrete are the same type of concrete or have similar proportion;

the unqualified premixed concrete is returned concrete with qualified raw material test, but unqualified fresh concrete test on a construction site, and the storage time of the unqualified premixed concrete is not longer than the initial setting time;

the retarder accounts for 0.08-0.12% of the quality of the unqualified premixed concrete;

the amount of the water reducing agent accounts for 0.2-0.3% of the mass of the recycled concrete;

the regenerated powder accounts for 10-30% of the mass of the regenerated concrete.