Method for recovering cement paste from waste concrete

Abstract

To provide a method for recovering high-quality cement paste from waste concrete.SOLUTION: A method for recovering cement paste from waste concrete has a first step for bringing waste concrete into contact with a carbon dioxide-containing gas to fix carbon dioxide thereon, a second step for grinding the waste concrete after the first step, and a third step for sorting the waste concrete after the second step to recover fine powder with a particle size of less than 1.2 mm.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for recovering cement paste from waste concrete. [Background technology]

[0002] Waste concrete generated during the demolition of reinforced concrete buildings, etc., is crushed to a particle size of about 40 mm or less after removing foreign objects other than concrete such as rebar, and used as roadbed material, backfill material, etc. However, the demand for roadbed material, etc. is on the decline, and the remaining unused waste concrete must be disposed of, which is causing problems from the perspective of industrial waste treatment. In addition, river aggregate used for concrete is becoming difficult to obtain due to collection restrictions due to the problem of depletion, so technology to recycle concrete aggregate from waste concrete is being considered.

[0003] For example, there has been proposed a method of recycling aggregate by subjecting crushed waste concrete obtained by crushing waste concrete, which has a maximum dimension of 5 mm or more, to a heating treatment using hot air, and then rubbing with a rubbing medium to remove the cement paste (Patent Document 1); a method of producing recycled aggregate by rubbing together waste concrete roughly crushed to 40 mm or less with another piece of waste concrete roughly crushed to 40 mm or less against a strong rough surface under high pressure to frictionally grind the mortar adhering to the aggregate surface (Patent Document 2); and a method of using an eccentric rotor to rub crushed waste concrete by rotating the rotor, and stirring to separate water-containing deposits from the aggregate, and then sieving to recover the aggregate (Patent Document 3). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2003-26459 A [Patent Document 2] Japanese Patent Application Publication No. 10-231153 [Patent Document 3] JP 2002-210380 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, all of the above-mentioned prior arts aim to recover high-quality recycled aggregate by removing cement paste from the aggregate surface by rubbing the crushed waste concrete, and do not consider the recovery of cement paste at all, or even if they do, they do not assume the improvement of the quality of the cement paste. The present inventors analyzed the quality of the cement paste recovered by the above-mentioned prior art and found that the cement paste contains a large amount of aggregate due to rubbing the crushed waste concrete, has a low calcium content and a high alkali equivalent, and thus has a problem in that its reuse is limited. Therefore, an object of the present invention is to provide a method for recovering high-quality cement paste from waste concrete. [Means for solving the problem]

[0006] The present inventors have studied ways to improve the quality of cement paste recovered from waste concrete. As a result, they have found that by fixing carbon dioxide in waste concrete, the waste concrete shrinks slightly, and stress acts on the interface between the cement paste and aggregate, making it easier to peel them off. They have also found that by grinding the waste concrete after fixing carbon dioxide to selectively peel the cement paste from the aggregate surface, and by selecting fine powder of a predetermined particle size from the ground waste concrete, it is possible to efficiently recover high-quality cement paste rich in calcium.

[0007] That is, the present invention provides the following [1] to [6]. [1] A first step of contacting waste concrete with a carbon dioxide-containing gas to fix carbon dioxide; A second step of grinding the waste concrete after the first step; The third process is to separate the waste concrete after the second process and collect fine powder with a particle size of less than 1.2 mm. A method for recovering cement paste from waste concrete comprising the steps of: [2] The method for recovering waste concrete according to [1], wherein in the first step, carbon dioxide is fixed until the mass of the waste concrete increases by 0.5 mass% or more. [3] The method for recovering the raw material according to [1] or [2] above, wherein in the second step, the raw material is ground by one or more methods selected from a mechanical grinding method, a screw grinding method, and an eccentric rotor method. [4] The method according to any one of [1] to [3] above, wherein in the third step, the separation is carried out by one or more methods selected from sieve separation and gravity separation. [5] The method according to any one of the above [1] to [4], wherein the cement paste has a calcium oxide content of 18 mass% or more. [6] The method according to any one of the above [1] to [5], wherein the cement paste is a cement raw material or an admixture. Effect of the Invention

[0008] According to the present invention, it is possible to efficiently recover cement paste from waste concrete, and the recovered cement paste is rich in calcium and has high quality, so it can be reused without any particular restrictions. Furthermore, according to the present invention, the waste concrete remaining after recovering the cement paste can be recycled as recycled roadbed material or recycled aggregate. Therefore, the method of the present invention is also useful as a method for recycling waste concrete. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The method for recovering cement paste according to the present invention includes a first step, a second step, and a third step. Each step will be described below.

[0010] (First step) This process is a process of fixing carbon dioxide by contacting the waste concrete with a carbon dioxide-containing gas. By contacting the waste concrete with a carbon dioxide-containing gas, calcium compounds such as calcium oxide, calcium hydroxide, and calcium silicate compounds contained in the cement paste in the waste concrete can be converted into calcium carbonate. The carbonation of the calcium compounds causes the waste concrete to shrink slightly, which creates stress at the interface between the cement paste and the aggregate, making it easier to peel off the cement paste from the aggregate surface in the next process. On the other hand, the shrinkage also acts as a densification, improving the strength of the cement paste and preventing it from being atomized more than necessary and becoming dust. Furthermore, since the present invention does not require high-temperature heating of the waste concrete, it is not only suppressed from emitting carbon dioxide during heating, but is also useful as a carbon dioxide recycling method.

[0011] In this process, first, waste concrete is prepared. Examples of waste concrete include dismantled concrete generated during civil engineering works or the demolition of structures, and surplus concrete generated during the construction of buildings. The waste concrete may be concrete blocks that have been broken into small pieces, magnetically separated, or hand-sorted for the purpose of separating foreign objects such as reinforcing bars. The size of the concrete blocks is usually 100 to 400 mm square, and preferably about 300 mm square.

[0012] Furthermore, in this step, in order to increase the contact area with the carbon dioxide-containing gas and increase the amount of carbon dioxide that is fixed, the particle size of the waste concrete may be adjusted using various crushers and classifiers. Known crushers can be used, including, for example, a jaw crusher that compresses and crushes using a fixed blade and a moving blade fixed to a frame, an impact crusher that crushes by impacting a striking plate rotated at high speed with a repulsion plate fixed to the device, a cone crusher that compresses and crushes using a conical concave and an eccentric mantle, a centrifugal crusher that crushes concrete blocks by colliding them with each other using the centrifugal force caused by rotation, and a gyratory crusher that compresses and crushes the concrete by biting it between the concave and mantle. The crushing process may be carried out two or more times, and when it is carried out two or more times, the same or different crushers can be used, and can be selected appropriately depending on the size of the waste concrete. As the classifier, any suitable classification device can be selected and used, for example, a vibrating sieve such as a ripple flow type or a low head type, a rotary sieve such as a trommel, a dry classifier such as an air separator or a cyclone, or a wet classifier such as a spiral classifier.

[0013] The particle size of the waste concrete with adjusted particle size is preferably less than 40 mm, more preferably less than 30 mm, even more preferably less than 20 mm, and even more preferably less than 10 mm, from the viewpoint of increasing the amount of carbon dioxide immobilized and improving the quality of the cement paste. The lower limit of the particle size of the waste concrete is not particularly limited, but from the viewpoint of production efficiency, it is preferably 1 mm or more, more preferably 2 mm or more, and even more preferably 3 mm or more. The "particle size" here refers to the maximum dimension of the crushed material (for example, the dimension of the major axis when the cross section is elliptical).

[0014] Next, the waste concrete is brought into contact with a carbon dioxide-containing gas to fix the carbon dioxide. The carbon dioxide-containing gas is not particularly limited as long as it contains carbon dioxide, and examples thereof include carbon dioxide filled in a cylinder and exhaust gas from a factory. Examples of factory exhaust gas include exhaust gas from a cement factory and exhaust gas from a thermal power plant. The factory exhaust gas may be treated with a separation membrane or an adsorbent to increase the carbon dioxide concentration.

[0015] The carbon dioxide-containing gas may contain gases other than carbon dioxide, such as carbon monoxide, hydrocarbons, nitrogen oxides, sulfur oxides, etc. These gases are usually contained in the exhaust gas from factories. Furthermore, the carbon dioxide-containing gas may contain an inert gas, such as nitrogen gas or argon gas. The total amount of these other gases, in terms of volume fraction, in the carbon dioxide-containing gas is preferably 99% or less, more preferably 98% or less, and further preferably 97% or less.

[0016] From the viewpoint of increasing the amount of fixed carbon dioxide, the ratio of carbon dioxide in the carbon dioxide-containing gas is preferably 1% or more, more preferably 2% or more, even more preferably 3% or more, and even more preferably 20% or more, as a volume fraction value. The upper limit of the ratio of carbon dioxide is not particularly limited, and may be 100%. From the viewpoint of promoting fixation of carbon dioxide, the relative humidity of the carbon dioxide-containing gas is preferably 20% or more, more preferably 30% or more, and even more preferably 40% or more. The moisture content can be measured by the method described in "7. Measurement of moisture content in exhaust gas" of "JIS Z 8808:2013 Measurement method for dust concentration in exhaust gas."

[0017] Methods for fixing carbon dioxide include, for example, a method of injecting carbon dioxide gas into a device containing waste concrete, and a method of injecting carbon dioxide gas into water in which waste concrete is immersed, but are not limited to these as long as carbonation can be promoted. As a device for injecting carbon dioxide gas, for example, a carbonation treatment curing tank in which the atmosphere such as temperature and humidity is controlled, a carbonation treatment tank for water-impregnated waste concrete, or a tubular electric furnace into which water vapor and carbon dioxide gas can be introduced can be used.

[0018] From the viewpoint of promoting the separation of the cement paste and the aggregate, the fixation of carbon dioxide is carried out until the mass of the waste concrete increases by preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 1.5% by mass or more. The upper limit of the increase is not particularly limited, but from the viewpoint of production efficiency, it is preferably 10% by mass or less, more preferably 7% by mass or less, and even more preferably 5% by mass or less with respect to the mass of the waste concrete.

[0019] The contact temperature and contact time with the carbon dioxide-containing gas can be appropriately selected as long as the amount of carbon dioxide fixed falls within the above-mentioned ranges, and are, for example, as follows. From the viewpoint of promoting fixation of carbon dioxide, the contact temperature with the carbon dioxide-containing gas is preferably 10° C. or higher, more preferably 15° C. or higher, even more preferably 20° C. or higher, and is preferably 200° C. or lower, more preferably 150° C. or lower. Moreover, the contact time with the carbon dioxide-containing gas is preferably 30 minutes or more, more preferably 1 hour or more, and even more preferably 3 hours or more, from the viewpoint of promoting fixation of carbon dioxide, and is preferably 60 days or less, more preferably 40 days or less, and even more preferably 30 days or less, from the viewpoint of treatment efficiency.

[0020] (Second step) This step is a step of grinding the waste concrete after the first step. This allows the cement paste to be selectively peeled off from the aggregate surface. Here, in this specification, "grinding" means scraping off the particle surface by friction. The grinding method is not particularly limited as long as it can scrape off the particle surfaces by friction, and examples thereof include one or more selected from a mechanical rubbing method, a screw grinding method, and an eccentric rotor method. The mechanical grinding method may be, for example, an autogenous grinding mill. The screw grinding method is not particularly limited as long as it is scraped off by the friction of a rotating screw, and any known device may be used. The eccentric rotor method is not particularly limited as long as it is scraped off by the friction caused by the rotation of the rotor, and any known device may be appropriately selected.

[0021] In this process, the particle diameter D of the waste concrete before the second process is 50 (A) and the particle size D of the waste concrete after the second process 50 The grinding is continued until the ratio of (B) [(B) / (A)] (hereinafter referred to as the "grinding ratio") is preferably less than 1, more preferably less than 0.9, and even more preferably less than 0.8. From the viewpoint of suppressing alkali increase, the lower limit of the ratio [(B) / (A)] is preferably 0.5 or more, more preferably 0.6 or more, and even more preferably 0.65 or more. In the present specification, the "particle diameter D 50 " is the particle diameter (D ) that corresponds to 50% of the cumulative distribution curve when the particle size distribution of the sample is created on a volume basis in accordance with JIS R 1629. 50 ) As the particle size distribution measuring device, for example, Microtrac (manufactured by Nikkiso Co., Ltd.) can be used.

[0022] (Third step) This process involves separating the waste concrete from the second process and recovering fine powder with a particle size of less than 1.2 mm. The method for sorting the waste concrete after the second step is not particularly limited as long as it can recover fine powder of a desired particle size, but examples of the method include sieve sorting and gravity sorting. Note that sieve sorting and gravity sorting may be performed alone or in combination. For the sieve separation, a known classifier can be used, for example, a dry or wet vibrating sieve, or a dry or wet cyclone classifier. A known specific gravity separator can be used for the specific gravity separation, and examples of such separators include dry and wet specific gravity separators. More specifically, examples of dry specific gravity separators include an air table separator that separates particles with different specific gravities by vibrating an inclined deck back and forth from the bottom of the deck. Examples of wet specific gravity separators include a jig separator that applies a pulsating water flow up and down to a particle layer on a fixed net in water, stratifying and separating particles on the net according to their specific gravities, a table separator (James table, Willfrey table) that separates particles with different specific gravities by the horizontal movement and water flow of an inclined deck, and a centrifugal force separator that applies centrifugal force to particles to separate particles with different specific gravities. The specific gravity separator may be equipped with a bag filter for collecting dust generated during specific gravity separation.

[0023] The particle size of the fine powder recovered in this step is less than 1.2 mm, but from the viewpoint of recovering calcium-rich cement paste, it is preferably less than 1.0 mm, more preferably less than 0.8 mm, and even more preferably less than 0.6 mm.

[0024] The cement paste recovered by the method of the present invention is rich in calcium, and for example, the calcium oxide content can be preferably 18 mass% or more, more preferably 19 mass% or more, and even more preferably 21 mass% or more.

[0025] The cement paste recovered by the method of the present invention has an alkali equivalent of usually 3% or less, preferably 2.9% or less, and more preferably 2.8% or less. The lower limit of the alkali equivalent is not particularly limited and may be 0%, but is usually 0.1% or more, preferably 0.3% or more, and more preferably 0.5% or more. In this specification, the term "alkali equivalent" refers to the value calculated by the following formula (1) based on the results of chemical analysis of a sample.

[0026] Alkaline equivalent (Na2Oeq) = Na2O + 0.658K2O (1)

[0027] (In the formula, Na2O is the content (%) of sodium oxide in the sample, and K2O is the content (%) of potassium oxide in the sample.)

[0028] As described above, the cement paste recovered by the present invention is rich in calcium and can be reused as a cement raw material. Also, since the cement paste recovered by the present invention is obtained from carbonated waste concrete and has a pozzolanic reaction, it can be reused as a mineral admixture. Furthermore, the residue from the third step is made up of the cement paste of the waste concrete removed, and can therefore be reused as recycled roadbed material or recycled aggregate. EXAMPLES

[0029] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to the following examples.

[0030] The devices used in the examples are as follows: 1) Jaw crusher: BB300 (made by Recce) 2) Asahi Carbonation Tester: ACT-800N (manufactured by Asahi Scientific) 3) Grinding machine: Nitto Scientific Co., Ltd. Tabletop pot mill with rotating table ANZ-51D 4) Grinding container: Stainless steel can (φ80×135mm, 600mL, equipped with 4 x 20mm aluminum plates) 5) JIS sieve: 0.3mm, 0.6mm, 1.18mm, 4.75mm 6) X-ray fluorescence analyzer: ZSX Primus II (Rigaku Corporation)

[0031] 1. Analysis of alkali equivalent (Na2Oeq) The alkali equivalent was calculated from the results of chemical analysis of the sample using the following formula (1).

[0032] Alkaline equivalent (Na2Oeq) = Na2O + 0.658K2O (1)

[0033] (In the formula, Na2O is the content (%) of sodium oxide in the sample, and K2O is the content (%) of potassium oxide in the sample.)

[0034] 2. Particle size D 50 Measurement A Microtrac (manufactured by Nikkiso Co., Ltd.) was used as a particle size distribution measuring device to create a volume-based particle size distribution in accordance with JIS R 1629, and the particle diameter D corresponding to 50% of the cumulative distribution curve was calculated. 50 asked for.

[0035] Example 1 (First step) The waste concrete was crushed with a jaw crusher, sieved through a 4.75 mm sieve, and the under-sieve was carbonated for 28 days in a carbonation test device at 20°C, a CO2 concentration of 5%, and a humidity of 55%. The mass of the waste concrete before and after the carbonation treatment is shown in Table 1. It can be seen from Table 1 that the mass of the waste concrete increased by 2% by mass due to the carbonation treatment.

[0036] [Table 1] (Second step) The waste concrete after the first process was placed in a stainless steel grinding container and self-grinded for 24 hours using a tabletop pot mill rotating table at a roller rotation speed of 150 rpm. The particle diameter D 50 is 1.29 mm, and the particle diameter after the second process D 50 Since the particle diameter of the crushed waste concrete was 0.95 mm, D 50 (A) and the particle size D of the waste concrete before grinding 50 The ratio to (B) [(B) / (A)] was confirmed to be 0.74. (Third step) The material was sieved using JIS sieves with 1.2mm, 0.6mm, and 0.3mm meshes, and the chemical composition was analyzed by particle size using X-ray fluorescence analysis (XRF). The results are shown in Table 2. In Table 1, "-0.3mm" means the undersize of the 0.3mm mesh, and "-0.6mm" and "-1.2mm" are interpreted similarly. Also, "1.2mm-" means the oversize of the 1.2mm mesh.

[0037] Comparative Example 1 The waste concrete was crushed with a jaw crusher, sieved through a 4.75 mm mesh, and the under-sieve was sieved through JIS sieves with 1.2 mm, 0.6 mm, and 0.3 mm mesh, and the chemical composition was analyzed by particle size using X-ray fluorescence analysis (XRF). The results are shown in Table 2.

[0038] Comparative Example 2 The same procedure as in Example 1 was carried out except that the first step was not carried out and the second and third steps were carried out, and the chemical composition was analyzed by particle size by X-ray fluorescence analysis (XRF). The results are shown in Table 2.

[0039] [Table 2]

[0040] As shown in Table 2, Example 1 has a higher calcium content and a lower alkali equivalent than Comparative Example 2, which shows that the cement paste was selectively peeled off from the aggregate surface by the carbonation treatment in the first step, and high-quality cement paste was recovered. In addition, the cement paste recovered in the third step has a higher calcium content and a lower alkali equivalent content as the particle size decreases, which indicates that the quality of the cement paste is improved as the particle size decreases.

Claims

1. A first step of contacting waste concrete with a carbon dioxide-containing gas; A second step of grinding the waste concrete after the first step; A third step of separating the waste concrete after the second step and recovering fine powder having a particle size of less than 0.6 mm. Equipped with In the first step, the waste concrete is contacted with a carbon dioxide-containing gas having a relative humidity of 40% or more and a carbon dioxide ratio of 3% or more until the mass of the waste concrete increases by 0.5% by mass or more compared to the waste concrete before the first step. A method for recovering cement paste from waste concrete.

2. 2. The method according to claim 1, wherein in the second step, the pulverization is carried out by at least one method selected from the group consisting of a mechanical rubbing method, a screw pulverization method, and an eccentric rotor method.

3. The method according to claim 1 or 2, wherein in the third step, the separation is carried out by one or more methods selected from sieve separation and gravity separation.

4. The method according to any one of claims 1 to 3, wherein the cement paste has a calcium oxide content of 18 mass% or more.

5. The method according to any one of claims 1 to 4, wherein the cement paste is a cement raw material or an admixture.

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