Roadbed material production 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

Manufacturing method of roadbed material

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

[0002] It has been known to reuse waste concrete generated during the demolition of structures as earthwork materials, such as roadbed materials and backfill materials. However, cementitious hardened bodies (pastes, mortars, or concrete) have a problem in that the amount of hexavalent chromium leaching increases as carbonation progresses. Patent Document 1 describes a hexavalent chromium treatment method for reducing the elution of hexavalent chromium from industrial waste, such as waste concrete. Patent Document 1 describes a hexavalent chromium treatment method for reducing the elution of hexavalent chromium from industrial waste, such as waste concrete, by contacting the industrial waste or industrial wastewater containing hexavalent chromium with slag mainly composed of converter decarbonization slag. Patent Document 2 describes a method for producing roadbed materials and earthwork materials from waste concrete, in which the elution of hexavalent chromium from the waste concrete is reduced. Patent Document 2 describes a method for producing roadbed materials and earthwork materials from waste concrete, in which the process of crushing the waste concrete to produce roadbed materials and / or earthwork materials involves adding cooled blast furnace slag to the waste concrete and crushing the slag together.

[0003] JP 2017-70896 A JP 2008-127968 A

[0004] When concrete waste and the like is reused as roadbed material, it is necessary to suppress the elution of hexavalent chromium contained in the concrete waste (hereinafter also referred to as "waste concrete"). Because hexavalent chromium is more likely to leach from waste concrete and the like as carbonation progresses, it is necessary to take measures to suppress the elution of hexavalent chromium, such as by mixing a treatment material with a reducing action. An object of the present invention is to provide a method for producing roadbed material that is less likely to leach hexavalent chromium, using crushed cementitious hardened body containing hexavalent chromium as a raw material.

[0005] As a result of intensive research to solve the above-mentioned problems, the present inventors have found that the above-mentioned object can be achieved by a method for producing a roadbed material, which includes a step of carbonating crushed material of a cementitious hardened body containing hexavalent chromium using a carbon dioxide-containing gas at a temperature of 50 to 140°C and having a moisture content of 1.5% or more to obtain a roadbed material from which hexavalent chromium is less likely to be eluted than before the treatment, and have completed the present invention. That is, the present invention provides the following [1] to [4]. [1] A method for producing a roadbed material using crushed material of a cementitious hardened body containing hexavalent chromium as a raw material, the method comprising: a carbonation treatment step of carbonating crushed material of the cementitious hardened body using a carbon dioxide-containing gas at a temperature of 50 to 140°C and having a moisture content of 1.5% or more to obtain a roadbed material from which hexavalent chromium is less likely to be eluted than before the treatment. [2] The method for producing a roadbed material according to [1], which includes a crushing step of crushing the cementitious hardened body containing hexavalent chromium to obtain crushed material containing 50% by mass or more of particles having a particle size of 100 mm or less and 1% by mass or more of particles having a particle size of 2.36 mm or less, prior to the carbonation treatment step. [3] The method for producing a roadbed material according to [2], which includes a classification step of classifying the crushed material obtained in the crushing step between the crushing step and the carbonation treatment step to obtain crushed material containing 80% by mass or more of particles having a particle size of 100 mm or less and 5% by mass or more of particles having a particle size of 2.36 mm or less. [4] The method for producing a roadbed material according to any of [1] to [3], wherein the cementitious hardened body is a waste material made of concrete or a waste material made of mortar.

[0006] According to the present invention, a roadbed material that is less susceptible to hexavalent chromium leaching can be easily produced using crushed cementitious hardened bodies containing hexavalent chromium as a raw material. Furthermore, because 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 a treatment material with a reducing action, when using the roadbed material. Furthermore, because there is no need to use a treatment material with a reducing action, there is no risk of the treatment material with a reducing action being washed out when the roadbed material comes into contact with water or the like after use.

[0007] The method for producing a roadbed material of the present invention is a method for producing a roadbed material using crushed cementitious hardened body containing hexavalent chromium as a raw material, and includes a carbonation treatment step in which the crushed cementitious hardened body is carbonated using a carbon dioxide-containing gas having a temperature of 50 to 140°C and a moisture content of 1.5% or more to obtain a roadbed material from which hexavalent chromium is less likely to leach out than before the treatment. This is described in detail below. In this specification, the term "cementitious hardened body" refers to a hardened composition containing cement and water, and specifically refers to any of a hardened body made of concrete, a hardened body made of mortar, and a hardened body made of cement paste. In addition, in this specification, the term "cementitious hardened body" encompasses not only a completely hardened hardened body but also a semi-hardened hardened body (in other words, a body in the process of hardening; for example, concrete sludge).

[0008] As the cementitious hardened body, even if the cementitious hardened body has progressed in neutralization and has become susceptible to hexavalent chromium elution, it is possible to produce a roadbed material from which hexavalent chromium is less likely to elute, and from the viewpoint of promoting the utilization of waste, waste materials made from cementitious hardened bodies are preferably used. Examples of waste materials made from cementitious hardened bodies include recycled aggregate, waste materials made from concrete or mortar, waste materials made from cement paste, and sludge generated in ready-mix concrete (fully hardened or semi-hardened sludge after dehydration treatment). Among these, waste materials made from concrete or waste materials made from mortar are preferred from the viewpoint of ease of acquisition, etc.

[0009] [Carbonation Curing Step] This step is a step in which crushed cementitious hardened body containing hexavalent chromium 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 to obtain a roadbed material from which hexavalent chromium is less likely to be eluted than before the treatment. In this specification, the carbon dioxide-containing gas refers to carbon dioxide gas (CO 2 , which is a gas). ２) is used. Examples of carbon dioxide-containing gases include factory exhaust gases. Examples of factory exhaust gases include exhaust gases from cement factories, exhaust gases from coal-fired power plants, and exhaust gases generated during exhaust treatment in paint factories. Highly purified gases separated and recovered from factory exhaust gases can also be used as factory exhaust gases. In the carbonation curing step, carbon dioxide gas is immobilized in the crushed cementitious hardened body. Therefore, by using factory exhaust gas 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 emitted into the atmosphere.

[0010] The proportion of carbon dioxide in the carbon dioxide-containing gas, as a volume fraction, is preferably 5% or more, more preferably 10% or more, even more preferably 20% or more, even more preferably 40% or more, even more preferably 50% or more, and particularly preferably 60% or more. When the proportion is 5% or more, the amount of carbon dioxide fixed in the roadbed material increases, and the leaching of hexavalent chromium in the roadbed material can be reduced more in a shorter time. In addition, the amount of carbon dioxide emitted 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 temperature is 50 to 140°C, preferably 55 to 130°C, more preferably 65 to 125°C, even more preferably 75 to 110°C, still more preferably 80 to 100°C, and particularly preferably 85 to 95°C. If the temperature is outside the range of 50 to 140°C, the amount of carbon dioxide gas (carbon dioxide) immobilized will be small. If the temperature of the carbon dioxide-containing gas (e.g., factory exhaust gas) to which the method of the present invention is intended to be applied is less than 50°C or more than 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 proportion of water vapor in the carbon dioxide-containing gas, and means the 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 gas (carbon dioxide) immobilized will be small. In other words, even when using carbon dioxide-containing gas with the same amount and carbon dioxide gas proportion (volume fraction), if the moisture content is 1.5% or more, the amount of carbon dioxide immobilized will be large, and the leaching of hexavalent chromium in the roadbed material can be reduced more quickly. In addition, carbon dioxide emissions into the atmosphere can be further reduced.

[0013] When only the increase in the amount of carbon dioxide immobilization is considered, without considering the increase in the burden of moisture supply, the moisture content can be set to be even greater than the above-mentioned preferred range. 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 enables more efficient and larger-scale immobilization of carbon dioxide gas (carbon dioxide). The upper limit of the moisture content is not particularly limited, but since the greater the moisture content, the greater the burden of moisture supply (particularly, the need for higher performance moisture supply equipment and a larger amount of water), it is preferably 70%, more preferably 60%, and particularly preferably 50%. When the moisture content of the carbon dioxide-containing gas (e.g., factory exhaust gas) intended as the target for application of the method of the present invention is less than 1.5%, moisture can be supplied to the carbon dioxide-containing gas to adjust the moisture content of the gas to a desired value of 1.5% 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."

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

[0015] Examples of other components (components other than water vapor, carbon dioxide gas, and inert gas) in the carbon dioxide-containing gas used in the present invention include carbon monoxide, hydrocarbons, nitrogen oxides, sulfur oxides, etc. Examples of these other components are typically contained in factory exhaust gases, etc. The proportion of other components in the carbon dioxide-containing gas, as a volume fraction value, is preferably 30% or less, more preferably 20% or less, even more preferably 10% or less, and particularly preferably 5% or less. A proportion of 30% or less is preferred because such carbon dioxide-containing gas is easily available.

[0016] The carbonation treatment is carried out by contacting crushed material of the cementitious hardened body containing hexavalent chromium with the above-mentioned carbon dioxide-containing gas. In the present invention, the time for contacting the crushed material 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 fixation. The upper limit of the 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 decreases and the amount of carbon dioxide fixation reaches a plateau.

[0017] The conditions of the carbonation treatment (temperature, time, etc.) may be adjusted as appropriate depending on the implementation status of the carbonation treatment. For example, from the viewpoint of further improving the efficiency of the carbonation treatment, it is preferable to carry out 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). From the viewpoint of reducing the cost required for heating and increasing the amount of carbon dioxide fixed, it is preferable to carry out the carbonation treatment at a temperature of 50°C or higher but lower than 75°C (preferably 55 to 70°C) for 12 to 36 hours (preferably 20 to 30 hours).

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

[0019] The means for crushing the cementitious hardened body is not particularly limited as long as it can obtain a cementitious hardened body of the desired particle size, and examples thereof include a jaw crusher and a roll crusher. The crushed material obtained by crushing the cementitious hardened body varies depending on the preferred particle size of the target roadbed material, but considering the normal particle size distribution of roadbed materials, it preferably contains particles having a particle size of 100 mm or less in a proportion of 50 mass% or more (preferably 60 mass% or more, more preferably 70 mass% or more). Furthermore, from the viewpoint of further reducing the elution of hexavalent chromium from the roadbed material, the crushed material obtained by crushing the cementitious hardened body contains particles having a particle size of 2.36 mm or less in a proportion of 1 mass% or more, preferably 5 mass% or more, more preferably 10 mass% or more, and particularly preferably 20 mass% or more.

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

[0021] The present invention will be described in more 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: 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. Carbonation treatment was carried out by contacting the crushed material with a carbon dioxide-containing gas having a carbon dioxide ratio of 71.4% and a moisture content of 28.6% at a temperature of 120°C for 1 hour. The moisture content was measured in accordance with the method described in "7. Measurement of moisture content in flue gas" of "JIS Z 8808:2013 Method for measuring dust concentration in flue gas."

[0022] [Measurement of the percentage of calcium carbonate] The crushed material after carbonation treatment was heated at temperatures of 500°C and 800°C using an electric furnace to determine the amount of mass loss at each temperature. The amount of mass loss from 500°C to 800°C was determined by subtracting the amount of mass loss at 500°C from the amount of mass loss at 800°C. The percentage of calcium carbonate was calculated from this amount of loss. The mass loss from 500°C to 800°C indicates that calcium carbonate contained in the crushed material was decarbonated (in other words, CaCO ３ In other words, the degree of mass reduction (CO ２ Based on the amount of calcium carbonate (CaCO ) before decarbonation, ３ ) can be calculated.

[0023] [Measurement of the amount of eluted hexavalent chromium] The amount of eluted hexavalent chromium from the crushed material after carbonation treatment was measured by the elution test method in accordance with "Hexavalent chromium" in the appendix to "Environmental Standards for Soil Contamination," Notification No. 46 of the Environment Agency, dated August 23, 1991. In addition, the pH of the eluted test solution obtained by the above elution test method was measured.

[0024] [Example 2] Carbonation treatment of crushed material was carried out 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] Carbonation treatment of crushed material was carried out in the same manner as in Example 1, except that the carbon dioxide content of the carbon dioxide-containing gas 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] The crushed material was carbonated in the same manner as in Example 1, except that the carbon dioxide content of the carbon dioxide-containing gas 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 carbonated in the same manner as in Comparative Example 1, except that the contact time with the carbon dioxide-containing gas was changed from 24 hours to 48 hours. [Comparative Example 3] The crushed material was carbonated in the same manner as in Comparative Example 1, except that the contact time with the carbon dioxide-containing gas was changed from 24 hours to 72 hours. For each of the crushed materials obtained after carbonation in Examples 2 to 3 and Comparative Examples 1 to 3, the calcium carbonate content 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]

[0027] From Table 1, it can be seen that the amount of hexavalent chromium eluted in Examples 1 to 3 (0.028 to 0.034 mg / liter) is smaller than the amount of hexavalent chromium eluted in Comparative Examples 1 to 3 (0.045 to 0.055 mg / liter), and that the roadbed materials of Examples 1 to 3 are less susceptible to elution of hexavalent chromium than Comparative Examples 1 to 3. Furthermore, the proportion of calcium carbonate in Examples 1 to 3 (10.0 to 15.3%) is smaller than the proportion of calcium carbonate in Comparative Examples 1 and 2 (8.1 to 9.7%), and that more carbon dioxide is fixed in a shorter time.

Claims

1. A method for producing a roadbed material using crushed cementitious hardened body containing hexavalent chromium as a raw material, comprising: a carbonation treatment step of performing a carbonation treatment on the crushed material of the cementitious hardened body using a carbon dioxide-containing gas having a temperature of 50 to 140° C. and a moisture content of 1.5% or more to obtain a roadbed material from which hexavalent chromium is less likely to be eluted than before the treatment; A method for producing a roadbed material, comprising:

2. Before the carbonation treatment step, a crushing step of crushing the cementitious hardened body containing hexavalent chromium to obtain the crushed material containing particles having a particle size of 100 mm or less in a proportion of 50% by mass or more and particles having a particle size of 2.36 mm or less in a proportion of 1% by mass or more; The method for producing a roadbed material according to claim 1, comprising:

3. Prior to the carbonation treatment step, a crushing step of crushing the cementitious hardened body containing hexavalent chromium to obtain a crushed product containing particles having a particle size of 100 mm or less in a proportion of 50% by mass or more and particles having a particle size of 2.36 mm or less in a proportion of 1% by mass or more; The method for producing a roadbed material according to claim 1, comprising:

4. Between the crushing step and the carbonation step, A classification step of classifying the crushed material obtained in the crushing step to obtain a crushed material containing 80% by mass or more of particles having a particle size of 100 mm or less and 5% by mass or more of particles having a particle size of 2.36 mm or less; Including, The method for manufacturing a roadbed material according to claim 3, wherein in the carbonation treatment step, the crushed material obtained in the classification step is carbonated.

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

6. A method for manufacturing a roadbed material described in any one of claims 1 to 4, wherein in the carbonation treatment process, the crushed material to be carbonated contains at least 80 mass% of material having a particle size of 100 mm or less, and at least 30 mass% of material having a particle size of 2.36 mm or less.

7. A method for manufacturing a roadbed material described in any one of claims 1 to 4, wherein in the carbonation treatment step, the carbonation treatment is carried out for 5 minutes or more.

8. A method for manufacturing a roadbed material described in any one of claims 1 to 4, wherein the proportion of carbon dioxide in the carbon dioxide-containing gas is 5 volume % or more.