Roadbed material manufacturing method

Abstract

Provided is a method for easily producing a roadbed material in which a crushed product of cured cement containing hexavalent chromium is used a raw material, and which less readily elutes hexavalent chromium.　This roadbed material production method includes a carbonation treatment step for subjecting crushed cured cement containing hexavalent chromium to a carbonation treatment using carbon dioxide-containing gas having a temperature of 50-140°C and moisture content of 1.5% or higher to obtain a roadbed material that elutes the hexavalent chromium less readily than before the treatment. The production method includes, preferably prior to the carbonation treatment step, a crushing step for crushing cured cement containing hexavalent chromium to obtain a crushed product in which particles having a particle size of 100 mm or less account for 50 mass% or more, and particles having a particle size of 2.36 mm or less account for 1 mass% or more.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a roadbed material.

Background Art

[0002] Conventionally, it has been known to reuse waste concrete and the like generated in the demolition of structures as geotechnical materials such as roadbed materials and backfill materials. On the other hand, cementitious hardened bodies (paste, mortar, or concrete) have a problem that the elution amount of hexavalent chromium increases as neutralization progresses. As a method for suppressing the elution of hexavalent chromium from industrial waste such as concrete waste, Patent Document 1 describes a hexavalent chromium treatment method in which industrial waste containing hexavalent chromium or industrial wastewater is brought into contact with slag mainly composed of converter decarburization slag to reduce hexavalent chromium in the industrial waste or the industrial wastewater. Further, as a method for manufacturing a roadbed material or a geotechnical material in which the elution of hexavalent chromium from concrete waste is suppressed, Patent Document 2 describes a method for manufacturing a roadbed material or the like from concrete waste, characterized in that blast furnace slow-cooled slag is added to the concrete waste and crushed together with the blast furnace slow-cooled slag in the step of crushing the concrete waste into a roadbed material and / or a geotechnical material.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] When reusing concrete waste as roadbed material, it is necessary to suppress the leaching of hexavalent chromium contained in the concrete waste (hereinafter also referred to as "waste concrete"). As waste concrete undergoes carbonation, hexavalent chromium becomes more likely to leach out, so measures to suppress hexavalent chromium leaching are necessary, such as by mixing in treatment materials with reducing properties. The object of the present invention is to provide a method for manufacturing roadbed material that allows for the easy production of roadbed material in which hexavalent chromium is less likely to leach, using crushed cementitious material containing hexavalent chromium as a raw material. [Means for solving the problem]

[0005] As a result of diligent research to solve the above problems, the inventors of the present invention have found that the above objective can be achieved by a method for producing roadbed material that includes a step of carbonation treatment using a carbon dioxide-containing gas having a temperature of 50 to 140°C and a moisture content of 1.5% or more on crushed cementitious hardened material containing hexavalent chromium, thereby obtaining roadbed material in which hexavalent chromium is less likely to leach out compared to before the treatment, and have completed the present invention. The present invention relates to the following [1]~[ 8 This provides [the following]. [1] A method for manufacturing roadbed material using crushed cementitious material containing hexavalent chromium as a raw material, comprising a carbonation treatment step in which the crushed cementitious material is subjected to a carbonation treatment using a carbon dioxide-containing gas having a temperature of 50 to 140°C and a moisture content of 1.5% or more, thereby obtaining roadbed material in which hexavalent chromium is less likely to leach out compared to before the treatment. [2] Prior to the carbonation treatment process described above, the cementitious material containing hexavalent chromium is crushed to include at least 50% by mass of material with a particle size of 100 mm or less, and at least 1% by mass of material with a particle size of 2.36 mm or less. the above A method for manufacturing roadbed material according to [1], comprising a crushing step for obtaining crushed material. [3] A method for manufacturing roadbed material according to [1], comprising a crushing step, prior to the carbonization treatment step, of crushing the cementitious hardened body containing the hexavalent chromium to obtain crushed material containing 50% by mass or more of particles with a particle size of 100 mm or less and 1% by mass or more of particles with a particle size of 2.36 mm or less. [4]Between the crushing step and the carbonation treatment step, the process includes a classification step in which the crushed material obtained in the crushing step is classified to obtain crushed material containing at least 80% by mass of particles with a particle size of 100 mm or less, and at least 5% by mass of particles with a particle size of 2.36 mm or less. Furthermore, in the carbonation treatment process described above, the crushed material obtained in the classification process described above is subjected to carbonation treatment. The aforementioned [ 3 A method for manufacturing roadbed material as described in [ ]. [ 5 The above cementitious hardened body is waste material made of concrete or waste material made of mortar [1]~[ 4 A method for manufacturing roadbed material as described in any of the following: [6] The method for manufacturing roadbed material according to any one of [1] to [4] above, wherein the crushed material subjected to the carbonation treatment step contains 80% by mass or more of material with a particle size of 100 mm or less, and 30% by mass or more of material with a particle size of 2.36 mm or less. [7] A method for manufacturing roadbed material according to any one of [1] to [4] above, wherein the carbonation treatment step is performed for 5 minutes or more. [8] A method for manufacturing roadbed material according to any one of [1] to [4] above, wherein the proportion of carbon dioxide in the carbon dioxide-containing gas is 5% by volume or more. [Effects of the Invention]

[0006] According to the present invention, a roadbed material that is less prone to leaching of hexavalent chromium can be easily manufactured using crushed cementitious material containing hexavalent chromium as a raw material. Furthermore, since the leaching of hexavalent chromium from the roadbed material itself can be reduced, there is no need to take measures to suppress hexavalent chromium, such as mixing in a reducing agent when using the roadbed material. Moreover, because there is no need to use a reducing agent, problems such as the leaching of the reducing agent when the roadbed material comes into contact with water after use do not occur. [Modes for carrying out the invention]

[0007] The present invention relates to a method for manufacturing roadbed material, which uses crushed cementitious material containing hexavalent chromium as a raw material. The method includes a carbonation treatment step in which the crushed cementitious material is subjected to a carbonation treatment using a carbon dioxide-containing gas having a temperature of 50 to 140°C and a moisture content of 1.5% or more, thereby obtaining a roadbed material in which hexavalent chromium is less likely to leach out compared to before the treatment. The method will be described in detail below. In this specification, a cementitious body means a body formed by the hardening of a composition containing cement and water, and more specifically, it means any of a hardened body made of concrete, a hardened body made of mortar, or a hardened body made of cement paste. Furthermore, in this specification, the term "cementaceous material" includes not only fully hardened material but also partially hardened material (in other words, material that is in the process of hardening; for example, concrete sludge).

[0008] As for the cementitious hardened material, even if the cementitious hardened material has undergone significant carbonation and is prone to hexavalent chromium leaching, it is possible to manufacture a roadbed material that is less prone to hexavalent chromium leaching, and from the viewpoint of promoting the utilization of waste, waste material consisting of cementitious hardened material is preferably used. Examples of waste materials consisting of cementitious hardened materials include recycled aggregate, waste materials consisting of concrete or mortar, waste materials consisting of cement paste, and sludge generated in ready-mixed concrete (either fully hardened or semi-hardened sludge after dewatering). In particular, from the standpoint of ease of acquisition, waste materials made of concrete or waste materials made of mortar are preferred.

[0009] [Carbonation curing process] This process involves carbonation treatment of crushed cementitious material containing hexavalent chromium using a carbon dioxide-containing gas with a temperature of 50 to 140°C and a moisture content of 1.5% or more, in order to obtain a roadbed material in which hexavalent chromium is less likely to leach out compared to before the treatment. In this specification, carbon dioxide-containing gas means a gas that contains carbon dioxide (CO2, which is a gas). Examples of gases containing carbon dioxide include factory exhaust gases. Examples of factory exhaust gases include those from cement plants, coal-fired power plants, and exhaust gases generated during exhaust treatment at paint factories. Furthermore, as the exhaust gas from the factory, a highly purified gas obtained by separating and recovering the exhaust gas from the factory can also be used. In the carbonation curing process, since carbon dioxide gas is immobilized in the crushed material of the cementitious hardened body, by using the exhaust gas of the factory as the carbon dioxide-containing gas, the carbon dioxide in the exhaust gas can be immobilized in the cementitious hardened body, thereby reducing the amount of carbon dioxide discharged into the atmosphere.

[0010] The ratio of carbon dioxide gas in the carbon dioxide-containing gas, as a volume fraction value, is preferably 5% or more, more preferably 10% or more, still more preferably 20% or more, still more preferably 40% or more, still more preferably 50% or more, and particularly preferably 60% or more. When the ratio is 5% or more, the amount of carbon dioxide immobilized in the roadbed material becomes larger, and the elution property of hexavalent chromium in the roadbed material can be reduced in a shorter time. In addition, the amount of carbon dioxide discharged into the atmosphere can be further reduced.

[0011] In the present invention, the carbon dioxide-containing gas satisfies the condition that the temperature is 50 to 140°C. The above temperature is 50 to 140°C, preferably 55 to 130°C, more preferably 65 to 125°C, still more preferably 75 to 110°C, still more preferably 80 to 100°C, and particularly preferably 85 to 95°C. When the above temperature is outside the range of 50 to 140°C, the amount of carbon dioxide gas (carbon dioxide) to be immobilized becomes small. When the temperature of the carbon dioxide-containing gas (for example, the exhaust gas of the factory) assumed as the object to be applied in the method of the present invention is less than 50°C or exceeds 140°C, the carbon dioxide-containing gas can be heated or cooled to adjust the temperature of the gas to a desired value within the range of 50 to 140°C.

[0012] In the present invention, the carbon dioxide-containing gas satisfies the condition that the moisture content is 1.5% or more. The moisture content is the ratio of water vapor in the carbon dioxide-containing gas and means a volume fraction (unit: %). The moisture content is 1.5% or more, preferably 2% or more, more preferably 3% or more, even more preferably 4% or more, and particularly preferably 4.5% or more. If the moisture content is less than 1.5%, the amount of carbon dioxide fixed will be small. In other words, even when using the same amount of carbon dioxide-containing gas with the same proportion (volume fraction) of carbon dioxide, if the moisture content is 1.5% or more, the amount of carbon dioxide fixed will be larger, and the elution of hexavalent chromium from the roadbed material can be reduced more quickly and more effectively. Furthermore, the amount of carbon dioxide emitted into the atmosphere can be reduced even further.

[0013] The above water content can be set even larger than the preferred range described above, if we consider only the increase in the amount of carbon dioxide fixed, without considering the increased burden on the water supply. In this case, the moisture content is preferably 10% or more, more preferably 15% or more, even more preferably 20% or more, and particularly preferably 25% or more. Increasing the moisture content in this way allows for more efficient and larger-scale fixation of carbon dioxide. The upper limit of the moisture content is not particularly limited, but the greater the moisture content, the greater the burden on the water supply (in particular, the need for high-performance water supply equipment and a large amount of water). Therefore, it is preferably 70%, more preferably 60%, and most preferably 50%. If the moisture content of a carbon dioxide-containing gas (for example, factory exhaust gas) that is intended to be the target of the method of the present invention is less than 1.5%, moisture can be supplied to the carbon dioxide-containing gas to adjust its moisture content to a desired value of 1.5% or more. Furthermore, the above moisture content can be measured using the method described in "7. Measurement of moisture content in exhaust gas" of "JIS Z 8808:2013 Method for measuring dust concentration in exhaust gas".

[0014] One preferred embodiment of the carbon dioxide-containing gas used in the present invention is a gas containing water vapor, carbon dioxide, and an inert gas. Examples of inert gases include nitrogen gas and argon gas. The proportion of inert gas in the carbon dioxide-containing gas is preferably 10% or more, more preferably 20% or more, even more preferably 30% or more, and particularly preferably 35% or more, as a volume fraction. A proportion of 10% or more is preferable because such carbon dioxide-containing gas is readily available.

[0015] Examples of other components of the carbon dioxide-containing gas used in this invention (components other than water vapor, carbon dioxide, and inert gases) include carbon monoxide, hydrocarbons, nitrogen oxides, and sulfur oxides. These examples of other components are typically found in factory exhaust gases, etc. The proportion of other components in the carbon dioxide-containing gas is preferably 30% or less, more preferably 20% or less, even more preferably 10% or less, and particularly preferably 5% or less, as expressed by volume fraction. A proportion of 30% or less is preferable because such carbon dioxide-containing gas is readily available.

[0016] The carbonation treatment is carried out by contacting the crushed cementitious material containing hexavalent chromium with the carbon dioxide-containing gas mentioned above. In the present invention, the time for which the crushed material is in contact with the carbon dioxide-containing gas is preferably 5 minutes or more, more preferably 10 minutes or more, even more preferably 20 minutes or more, even more preferably 30 minutes or more, and particularly preferably 40 minutes or more, from the viewpoint of increasing the amount of carbon dioxide fixed. The upper limit of this time is preferably 48 hours, more preferably 36 hours, even more preferably 24 hours, even more preferably 12 hours, even more preferably 6 hours, and particularly preferably 3 hours, from the viewpoint that if the time is too long, the efficiency of the carbonation treatment will decrease and the amount of carbon dioxide fixed will plateau.

[0017] Furthermore, the conditions for the carbonation treatment (temperature, time, etc.) may be adjusted as appropriate depending on the circumstances of the carbonation treatment. For example, from the viewpoint of further improving the efficiency of the carbonation treatment, it is preferable to perform the carbonation treatment at a temperature of 75 to 130°C (preferably 80 to 125°C) for 30 minutes to 3 hours (preferably 40 minutes to 2 hours). Furthermore, from the viewpoint of reducing the cost required for heating and increasing the amount of carbon dioxide fixed, it is preferable to perform the carbonation treatment at a temperature of 50°C or higher and less than 75°C (preferably 55 to 70°C) for 12 to 36 hours (preferably 20 to 30 hours).

[0018] Prior to the carbonation treatment process, a crushing process may be performed in which the cementitious hardened body containing hexavalent chromium is crushed to obtain crushed material containing at least 50% by mass of particles with a particle size of 100 mm or less, and at least 1% by mass of particles with a particle size of 2.36 mm or less. By performing a crushing process before the carbonation process, the efficiency of the carbonation process can be further improved. Furthermore, the particle size distribution of the roadbed material obtained in the carbonation process can be made to a desired level. In this specification, "particle size" refers to the size corresponding to the mesh size of a sieve. For example, a particle size of 100 mm or less means that the particles pass through a sieve with a mesh size of 100 mm.

[0019] The means for crushing the cementitious hardened material described above is not particularly limited as long as it can produce cementitious hardened material of the desired particle size, and examples include jaw crushers and roll crushers. The crushed material obtained by crushing the above-mentioned cementitious hardened body will vary depending on the preferred particle size of the target roadbed material, but considering the normal particle size distribution of roadbed material, it is preferable that it contains 50% or more by mass (preferably 60% or more by mass, more preferably 70% or more by mass) of particles with a particle size of 100 mm or less. Furthermore, the crushed material obtained by crushing the above-mentioned cementitious hardened body contains, from the viewpoint of further reducing the elution of hexavalent chromium from the roadbed material, 1% by mass or more, preferably 5% by mass or more, more preferably 10% by mass or more, and particularly preferably 20% by mass or more, of particles with a particle size of 2.36 mm or less.

[0020] Furthermore, from the viewpoint of obtaining a roadbed material having a desired particle size distribution, a classification step may be performed between the crushing step and the carbonation treatment step to classify the crushed material obtained in the crushing step and obtain crushed material having a desired particle size distribution (for example, crushed material containing 80% or more by mass of particles with a particle size of 100 mm or less, and 5% or more by mass of particles with a particle size of 2.36 mm or less). The crushed material after classification will vary depending on the preferred particle size of the target roadbed material, but considering the normal particle size distribution of roadbed material, it is preferable that it contains 80% or more by mass (preferably 90% or more by mass, more preferably 100% by mass) of particles with a particle size of 100 mm or less. Furthermore, from the viewpoint of further reducing the elution of hexavalent chromium from the roadbed material, the crushed material after classification preferably contains 5% or more by mass, more preferably 10% or more by mass, even more preferably 20% or more by mass, and particularly preferably 30% or more by mass, of particles with a particle size of 2.36 mm or less. Furthermore, any crushed material that does not pass through the sieve during classification may be subjected to the crushing process again. [Examples]

[0021] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. [Example 1] Commercially available recycled crushed stone (RC-40: made from crushed waste concrete with a particle size of 40 mm or less) was classified to obtain crushed material with a particle size of 0.6 to 1.18 mm. The crushed material was subjected to a carbonation treatment by contacting it with a carbon dioxide-containing gas having a carbon dioxide content of 71.4%, a moisture content of 28.6%, and a temperature of 120°C for one hour. The above moisture content was measured in accordance with the method described in "7. Measurement of moisture content in exhaust gas" of "JIS Z 8808:2013 Method for measuring dust concentration in exhaust gas".

[0022] [Measurement of the proportion of calcium carbonate] For the crushed material after carbonation treatment, the mass loss was determined when the material was heated in an electric furnace at 500°C and 800°C. The mass loss from 500°C to 800°C was obtained by subtracting the mass loss at 500°C from the mass loss at 800°C. The proportion of calcium carbonate was calculated from this mass loss. Furthermore, the decrease in mass between 500°C and 800°C indicates that the calcium carbonate contained in the crushed material has undergone decarboxylation (in other words, CaCO3 has been converted to CaO). Therefore, the amount of calcium carbonate (CaCO3) before decarboxylation can be calculated based on the degree of this mass decrease (amount of CO2).

[0023] [Measurement of hexavalent chromium leaching] The amount of hexavalent chromium leached from the crushed material after carbonation treatment was measured using the leaching test method in accordance with the "Hexavalent Chromium" section of the attached table in the Environmental Agency Notification No. 46 of August 23, 1991, "Regarding Environmental Standards for Soil Contamination." In addition, the pH of the leached solution obtained by the above leaching test method was measured.

[0024] [Example 2] The crushed material was subjected to carbonation treatment in the same manner as in Example 1, except that the temperature of the carbon dioxide-containing gas was changed from 120°C to 90°C. [Example 3] With regard to the carbon dioxide-containing gas, the carbonation treatment of the crushed material was carried out in the same manner as in Example 1, except that the proportion of carbon dioxide was changed from 71.4% to 80%, the moisture content was changed from 28.6% to 18.7%, and the temperature was changed from 120°C to 60°C.

[0025] [Comparative Example 1] With regard to the carbon dioxide-containing gas, the carbonation treatment of the crushed material was carried out in the same manner as in Example 1, except that the proportion of carbon dioxide was changed from 71.4% to 5±0.2%, the moisture content was changed from 28.6% to 1.4%, and the temperature was changed from 120°C to 20±2°C. [Comparative Example 2] The crushed material was subjected to carbonation treatment in the same manner as in Comparative Example 1, except that the contact time with carbon dioxide-containing gas was changed from 24 hours to 48 hours. [Comparative Example 3] The crushed material was subjected to carbonation treatment in the same manner as in Comparative Example 1, except that the contact time with carbon dioxide-containing gas was changed from 24 hours to 72 hours. For each of the crushed materials obtained after carbonation treatment in Examples 2-3 and Comparative Examples 1-3, the proportion of calcium carbonate and the amount of hexavalent chromium eluted were measured in the same manner as in Example 1. The results are shown in Table 1.

[0026] [Table 1]

[0027] Table 1 shows that the amount of hexavalent chromium leached from Examples 1-3 (0.028-0.034 mg / liter) was smaller than the amount of hexavalent chromium leached from Comparative Examples 1-3 (0.045-0.055 mg / liter), indicating that the roadbed materials in Examples 1-3 were less prone to leaching hexavalent chromium compared to Comparative Examples 1-3. Furthermore, the proportion of calcium carbonate in Examples 1-3 (10.0-15.3%) is lower than that in Comparative Examples 1-2 (8.1-9.7%), indicating that more carbon dioxide is fixed in a shorter time.

Claims

1. A method for manufacturing roadbed material using crushed cementitious material containing hexavalent chromium as a raw material, A carbonation treatment process is performed on the crushed cementitious material described above using a carbon dioxide-containing gas with a temperature of 50°C or higher and less than 75°C, and a moisture content of 1.5% by volume or higher, for 12 to 36 hours to obtain a roadbed material in which hexavalent chromium is less likely to leach compared to before the treatment. Includes, A method for manufacturing roadbed material, characterized in that the proportion of carbon dioxide in the above-mentioned carbon dioxide-containing gas is 60% by volume or more.

2. Before the above carbonation treatment process, A crushing step to crush the cementitious hardened body containing the above-mentioned hexavalent chromium to obtain the crushed material containing at least 50% by mass of particles with a particle size of 100 mm or less, and at least 1% by mass of particles with a particle size of 2.36 mm or less. A method for manufacturing roadbed material according to claim 1, including the method described above.

3. Before the above carbonation treatment process, A crushing step to crush a cementitious hardened body containing the above-mentioned hexavalent chromium, thereby obtaining crushed material containing at least 50% by mass of particles with a particle size of 100 mm or less, and at least 1% by mass of particles with a particle size of 2.36 mm or less. A method for manufacturing roadbed material according to claim 1, including the method described above.

4. Between the crushing process and the carbonation process, A classification process to obtain crushed material in which the crushed material obtained in the above crushing process is classified to obtain crushed material containing at least 80% by mass of particles with a particle size of 100 mm or less, and at least 5% by mass of particles with a particle size of 2.36 mm or less. Includes, The method for manufacturing roadbed material according to claim 3, wherein the carbonation treatment step involves performing a carbonation treatment on the crushed material obtained in the classification step.

5. A method for manufacturing roadbed material according to any one of claims 1 to 4, wherein the cementitious hardened body is waste material made of concrete or waste material made of mortar.

6. A method for manufacturing roadbed material according to any one of claims 1 to 4, wherein in the carbonation treatment step described above, the crushed material subjected to carbonation treatment contains 80% by mass or more of material with a particle size of 100 mm or less, and 30% by mass or more of material with a particle size of 2.36 mm or less.

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