Method of immobilizing carbon dioxide

Abstract

To provide a method of immobilizing carbon dioxide present in gas containing carbon dioxide, using ready-mixed concrete sludge, which is of high processing efficiency and can immobilize a large amount of carbon dioxide.SOLUTION: The method of immobilizing carbon dioxide comprises the steps of preparing ready-mixed concrete using a material at least including cement, fine aggregate, coarse aggregate and water, recovering ready mixed concrete sludge which is the residue of the ready mixed concrete and has a higher water / cement ratio relative to the ready mixed concrete, and immobilizing carbon dioxide present in gas containing carbon dioxide by supplying the recovered ready mixed concrete sludge or the water-content adjusted product thereof with gas containing carbon dioxide, when a time in a range of 1.0 to 12 times the time from the preparation of the ready mixed concrete to the beginning (JIS R5201) of the setting of the cement has passed.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for immobilizing carbon dioxide, and more particularly to a method for immobilizing carbon dioxide (carbon dioxide gas) contained in carbon dioxide-containing gases such as exhaust gases from factories in ready-mixed concrete sludge. [Background technology]

[0002] In recent years, reducing carbon dioxide emissions has become an important issue in order to curb global warming. In this regard, various techniques have been developed to absorb and immobilize carbon dioxide contained in exhaust gases from cement manufacturing plants and the like into unhardened or hardened cement-containing compositions. For example, Patent Document 1 describes a cement composition production system for producing a cement composition (e.g., concrete) containing cement and water, the cement composition production system including a cement slurry preparation device that mixes a portion of the cement with the water by stirring and supplies carbon dioxide gas to obtain a carbonated cement slurry that absorbs and fixes the carbon dioxide gas, and a cement composition preparation device that kneads the remainder of the cement with the carbonated cement slurry obtained in the cement slurry preparation device by stirring to obtain the cement composition, wherein the cement slurry preparation device is equipped with a specific device.

[0003] Patent Document 2 describes a method for producing a cement admixture, which includes a slurrying step in which fresh concrete sludge or fine waste concrete powder is heat-treated and then mixed with water to form a slurry, and a carbonation step in which carbon dioxide gas is passed through the slurry to carbonate it. Furthermore, Patent Document 2 describes that after the carbonation step, the cement mixture is dehydrated by heating or the like, the dehydrated cement mixture is fed into a cooler of a cement manufacturing apparatus to produce a cement raw material containing cement clinker and the cement mixture, and the cement raw material, gypsum, and, if necessary, a grinding aid are fed into a mill to be ground and mixed. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2023-20598 [Patent Document 2] Japanese Patent Publication No. 2021-138574 Summary of the Invention [Problem to be solved by the invention]

[0005] The technology described in the above-mentioned Patent Document 1 does not use ready-mixed concrete sludge as the object to be treated with carbon dioxide gas. In Patent Document 1, in the production of a cement composition such as concrete, carbon dioxide gas is supplied to a slurry containing cement and water, and carbon dioxide is fixed in the slurry.

[0006] On the other hand, the technology described in the above-mentioned Patent Document 2 uses ready-mixed concrete sludge as the object to be treated with carbon dioxide gas. In the technology of Patent Document 2, fresh concrete sludge is heat-treated, for example, at 100 to 300°C, and then the resulting heat-treated fresh concrete sludge is mixed with water to obtain a slurry. Carbon dioxide gas is then passed through this slurry to carbonate it (in other words, fix the carbon dioxide), thereby obtaining a cement admixture. Furthermore, since the obtained cement mixture is in the form of a slurry, it is subjected to a heat treatment, such as drying at 105°C for 24 hours, as described in Example 1, before being mixed with cement. The technology described in Patent Document 2 requires heat treatment of the ready-mixed concrete sludge, and there is room for improvement in terms of treatment efficiency.

[0007] An object of the present invention is to provide a method for immobilizing carbon dioxide (carbon dioxide gas) in a carbon dioxide-containing gas using ready-mixed concrete sludge, which method has high treatment efficiency and immobilizes a large amount of carbon dioxide. [Means for solving the problem]

[0008] As a result of intensive research into solving the above-mentioned problems, the present inventors have found that a large amount of carbon dioxide can be fixed even if the carbon dioxide-containing gas is supplied for a short time, by a method including: a ready-mixed concrete preparation step of preparing ready-mixed concrete; a sludge recovery step of recovering ready-mixed concrete sludge, which is a residue of the ready-mixed concrete and has a higher water-to-cement ratio than the ready-mixed concrete; and a gas supply step of supplying a carbon dioxide-containing gas to the ready-mixed concrete sludge or a product thereof with its moisture content adjusted recovered in the sludge recovery step, when a time period within a range of 1.0 to 12 times the time period from the time of preparation of the ready-mixed concrete in the ready-mixed concrete preparation step to the onset of setting defined in accordance with "JIS R 5201" (however, this setting refers to setting when a cement paste is prepared using the above cement), and thus completed the present invention.

[0009] The present invention provides the following [1] to [6]. [1] A method for immobilizing carbon dioxide, comprising: a ready-mixed concrete preparation step of preparing ready-mixed concrete using materials for ready-mixed concrete containing at least cement, fine aggregate, coarse aggregate, and water; a sludge recovery step of recovering ready-mixed concrete sludge, which is a residue of the ready-mixed concrete and has a higher water-cement ratio than the ready-mixed concrete; and a gas supply step of supplying a carbon dioxide-containing gas to the ready-mixed concrete sludge or a moisture-content-adjusted product thereof recovered in the sludge recovery step when a time within a range of 1.0 to 12 times the time from the time of preparation of the ready-mixed concrete in the ready-mixed concrete preparation step to the onset of setting (however, the setting refers to setting when a cement paste is prepared using the cement) as defined in accordance with "JIS R 5201:2015 (Physical Testing Methods for Cement)," thereby immobilizing the carbon dioxide in the carbon dioxide-containing gas in the ready-mixed concrete sludge or a moisture-content-adjusted product thereof, thereby obtaining ready-mixed concrete sludge or a moisture-content-adjusted product thereof in which the carbon dioxide has been immobilized. [2] The method for fixation of carbon dioxide according to [1] above, further comprising a gas supply start time determination step, which is performed before the gas supply step, to determine the start time of supplying the carbon dioxide-containing gas in the gas supply step based on the initial value of setting in the gas supply step, the start time being determined depending on the time of preparation of the ready-mixed concrete in the ready-mixed concrete preparation step and the type of cement, which is one of the materials of the ready-mixed concrete. [3] The method for fixation of carbon dioxide according to [2] above, further comprising, before the gas supply step, a gas supply time determination step of determining in advance the supply time of the carbon dioxide-containing gas in the gas supply step based on the start time of supply of the carbon dioxide-containing gas determined in the gas supply start time determination step. [4] The method for fixation of carbon dioxide according to any one of the above [1] to [3], comprising, between the sludge recovery step and the gas supply step, a water supply step of supplying water to the ready-mixed concrete sludge or a solid matter thereof after dewatering, to obtain a ready-mixed concrete sludge with an adjusted moisture content. [5] The method for fixation of carbon dioxide according to any one of [1] to [4] above, wherein in the gas supply step, the ready-mixed concrete sludge or the product thereof with its moisture content adjusted has a mass ratio of water to the total of the cement and the fine aggregate (water × 100 / (cement + fine aggregate); unit: %) of 100 to 500%. [6] A method for producing ready-mixed concrete, comprising obtaining ready-mixed concrete sludge or a product thereof with an adjusted moisture content in which carbon dioxide has been immobilized by the method for immobilizing carbon dioxide according to any one of [1] to [5] above, and then using the ready-mixed concrete sludge or the product thereof with an adjusted moisture content in which carbon dioxide has been immobilized as is as a raw material for ready-mixed concrete to produce ready-mixed concrete. [Effects of the Invention]

[0010] The method of the present invention is highly efficient in that it can shorten the time required to supply a carbon dioxide-containing gas to fresh concrete sludge or a product thereof with its moisture content adjusted. Furthermore, the method of the present invention allows a large amount of carbon dioxide to be fixed even if the supply time of the carbon dioxide-containing gas is short. Furthermore, the ready-mixed concrete sludge or its moisture content adjusted product (in which carbon dioxide has been fixed) obtained by the method of the present invention can be used as it is as a raw material for ready-mixed concrete. DETAILED DESCRIPTION OF THE INVENTION

[0011] The carbon dioxide fixation method of the present invention (hereinafter sometimes abbreviated as "the method of the present invention") includes a ready-mixed concrete preparation step, a sludge recovery step, and a gas supply step. The method of the present invention can include a water supplying step of supplying water to the fresh concrete sludge or the solid matter thereof after dewatering (for example, sludge cake, which is a solid matter obtained by subjecting fresh concrete sludge to solid-liquid separation (dewatering) using a filter press) between the sludge recovery step and the gas supplying step. The method of the present invention may include a step of determining a gas supply start time before the gas supply step. In this case, the method of the present invention may further include a step of determining a gas supply time. Each step constituting the method of the present invention will be described in detail below in the order of ready-mixed concrete preparation step, sludge recovery step, water supply step, gas supply step, gas supply start time determination step, and gas supply time determination step.

[0012] [Ready-mix concrete preparation process] This step is a step of preparing ready-mixed concrete using cement, fine aggregate, coarse aggregate, water, and other materials that are mixed as necessary. Examples of cement include various types of Portland cement such as ordinary Portland cement, high-early-strength Portland cement, and low-heat Portland cement, various types of blended cement such as blast-furnace cement, and ecocement. Other materials include admixtures such as ground granulated blast furnace slag, and various types of water-reducing agents and other admixtures. In this specification, "ready-mixed concrete" refers to concrete that has enough fluidity to be transported using an agitator truck (in other words, concrete that is in an unhardened state before being poured on-site). The term "ready-mixed concrete" also encompasses ready-mixed concrete.

[0013] [Sludge collection process] This step is a step of recovering ready-mixed concrete sludge, which is a residue of the ready-mixed concrete prepared in the previous ready-mixed concrete preparation step and has a higher water-cement ratio than the ready-mixed concrete. Examples of ready-mixed concrete sludge include (i) the sludge remaining after the coarse aggregate has been recovered from the material (consisting of sludge, fine aggregate, and coarse aggregate) obtained by washing the inside of a mixer in a ready-mix concrete plant with water, and (ii) the sludge remaining after the coarse aggregate has been recovered from the material (consisting of sludge, fine aggregate, and coarse aggregate) obtained by washing the inside of the drum of an agitator vehicle with water. "Water-cement ratio" refers to the mass ratio of water to cement expressed as a percentage ([mass of water] × 100 ÷ [mass of cement]). Ready-mixed concrete sludge is made by adding water to ready-mixed concrete and then removing the coarse aggregate, and therefore has a higher water-cement ratio than ready-mixed concrete.

[0014] In the fresh concrete sludge, the mass ratio of water to the total of cement and fine aggregate (water×100 / (cement+fine aggregate); unit: %) is preferably 100 to 500%, more preferably 100 to 400%. If the ratio is less than 100%, the amount of carbon dioxide immobilized in the ready-mixed concrete sludge may decrease sharply.If the ratio exceeds 500%, the amount of carbon dioxide immobilized in the ready-mixed concrete sludge will plateau, and the amount of water used will increase, requiring an increase in the capacity of a storage tank or the like in the carbon dioxide immobilization system described below, which is undesirable. In addition, the mass ratio of water to the total of cement and fine aggregate in ready-mixed concrete (water×100 / (cement+fine aggregate); unit: %) is usually about 12 to 18%. Hereinafter, the mass ratio of water to the total of cement and fine aggregate (water×100 / (cement+fine aggregate); unit: %) may be referred to as the "liquid-solid ratio."

[0015] In the present invention, the recovered fresh concrete sludge is dehydrated using a solid-liquid separation means such as a filter press to obtain a sludge cake (a solid material after dehydration), and then water is added to the sludge cake to obtain a fresh concrete sludge with an adjusted moisture content. In this case, the preferred liquid-solid ratio of the water content adjusted product of ready-mixed concrete sludge is the same as the above-mentioned liquid-solid ratio (preferably 100 to 500%, more preferably 100 to 400%).

[0016] [Water supply process] The method of the present invention can include a water supplying step of supplying water to the fresh concrete sludge obtained in the sludge recovery step or the solid matter thereof after dewatering, to obtain a fresh concrete sludge with an adjusted moisture content, with the aim of obtaining the above-mentioned mass ratio (preferably 100 to 500%). When the ready-mixed concrete sludge obtained in the sludge recovery step satisfies the above-mentioned mass ratio (preferably 100 to 500%), the water supply step can be omitted. However, to obtain a more preferable mass ratio as described above, the water supply step may be performed. For example, when the above-mentioned mass ratio is 100%, water can be added so that the mass ratio becomes 300% in order to increase the amount of carbon dioxide fixed.

[0017] [Gas supply process] This process is a process in which, when a time within a range of 1.0 to 12 times the time from the preparation of ready-mixed concrete in the ready-mixed concrete preparation process (the mixing of the various ingredients) to the start of setting (hereinafter sometimes abbreviated as the "initial setting time") as defined in accordance with "JIS R 5201:2015 (Physical testing methods for cement)" has elapsed (however, this setting refers to the setting that occurs when cement paste is prepared using the cement that is an ingredient of the ready-mixed concrete prepared in the ready-mixed concrete preparation process), a carbon dioxide-containing gas is supplied to the ready-mixed concrete sludge or a product thereof with its moisture content adjusted (hereinafter sometimes abbreviated as "ready-mixed concrete sludge, etc.") recovered in the sludge recovery process, thereby immobilizing the carbon dioxide in the carbon dioxide-containing gas in the ready-mixed concrete sludge, etc., and obtaining ready-mixed concrete sludge, etc. in which the carbon dioxide has been immobilized. In this specification, "when a time period within a range of 1.0 to 12 times has elapsed" does not mean a point in time (a specific time), but a specific time period determined within a range of 1.0 to 12. This "specific time period" is determined by the start time of supplying the carbon dioxide-containing gas and the supply time of the gas (however, the supply time may be continuous or intermittent).

[0018] One example of a method for supplying carbon dioxide-containing gas to fresh concrete sludge includes: (a) a first tank (storage tank) that is open at the top and that stores the fresh concrete sludge; (b) a sealed second tank (reaction tank) that has an upper space filled with carbon dioxide (carbon dioxide gas) and a lower space that stores the fresh concrete sludge; (c) a first pipeline that guides the fresh concrete sludge in the first tank (storage tank) to the upper space of the second tank (reaction tank); and (d) a second tank. An example of such a method is to use a carbon dioxide fixation system that includes: (a) a second pipeline for guiding the fresh concrete sludge in the (reaction tank) to the first tank (storage tank); (e) a pump (for example, installed in the middle of the first pipeline) for circulating the fresh concrete sludge through the first tank (storage tank), the first pipeline, the second tank (reaction tank), and the second pipeline in that order; and (f) a carbon dioxide supply device for supplying carbon dioxide (carbon dioxide gas) to the space above the second tank (reaction tank).

[0019] [Gas supply start time determination process] This process is a process that precedes the gas supply process, and is a process in which the start time of supplying the carbon dioxide-containing gas in the gas supply process is determined in advance based on the value of the start of setting (start of setting) in the gas supply process, which is determined depending on the time of preparation of fresh concrete in the fresh concrete preparation process and the type of cement, which is one of the materials of the fresh concrete. For example, when ready-mixed concrete contains high-early-strength Portland cement as cement, the initial setting time is earlier than when ordinary Portland cement is contained, so it is desirable to start supplying the carbon dioxide-containing gas earlier than when ordinary Portland cement is contained.

[0020] [Gas supply time determination process] This step is a step preceding the gas supply step, in which the supply time of the carbon dioxide-containing gas is determined in advance based on the start time of the supply of the carbon dioxide-containing gas determined in the gas supply start time determination step described above. For example, assuming that the initial setting time of cement is 2 hours and 30 minutes, as described above, the supply of carbon dioxide-containing gas should be carried out within a specific time period determined within the range of 2 hours and 30 minutes (1.0 times) to 30 hours (12 times), starting from the time when the ready-mixed concrete is prepared. As an example, the start time of the supply of the carbon dioxide-containing gas is set to a point 12 hours after the preparation of the ready-mixed concrete, and the supply time of the carbon dioxide-containing gas is set to 2 hours and 30 minutes (see Example 2 described later). In this case, the end time of the supply of the carbon dioxide-containing gas is a point 14 hours and 30 minutes after the preparation of the ready-mixed concrete.

[0021] In the present invention, the earlier the start time of the carbon dioxide-containing gas supply, the longer the supply time of the carbon dioxide-containing gas for obtaining a desired amount of fixed carbon dioxide tends to be. On the other hand, the later the start time of the carbon dioxide-containing gas supply, the shorter the time for supplying the carbon dioxide-containing gas to obtain a desired amount of carbon dioxide fixation, which is an advantage in that the processing efficiency of the gas supply step is increased, but the later the end time of the carbon dioxide-containing gas supply is, the lower the processing efficiency of the entire method of the present invention is.

[0022] Next, a method for producing ready-mixed concrete using ready-mixed concrete sludge (in which carbon dioxide has been immobilized) obtained by the method of the present invention will be described. The method for producing ready-mixed concrete of the present invention (hereinafter sometimes abbreviated as "the production method of the present invention") involves obtaining ready-mixed concrete sludge or the like in which carbon dioxide has been immobilized by the gas supply step described above, and then using this ready-mixed concrete sludge or the like as is as a material for ready-mixed concrete to produce ready-mixed concrete. The ready-mixed concrete sludge in which carbon dioxide is immobilized is an unhardened cement-containing composition containing cement, fine aggregate, etc., and therefore can be mixed with other cement, fine aggregate, coarse aggregate, water, etc. to produce ready-mixed concrete. [Example]

[0023] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. [material] The following materials were used: (1) Cement: Ordinary Portland cement (using cement paste with a water-cement ratio of 28%, when the setting test is conducted in accordance with JIS R 5201:2015 (Physical Testing Methods for Cement) the initial setting time is 2 hours and 25 minutes) (2) Fine aggregate: Mountain sand (surface dry density: 2.57g / cm 3 ) (3) Coarse aggregate: crushed stone (surface dry density: 2.65 g / cm 3 ) (4) Water: Tap water

[0024] [Example 1] (a) Preparation of fresh concrete sludge The amounts of each material (water, cement, fine aggregate, and coarse aggregate) shown in Table 1 were all charged into a tank and mixed to prepare ready-mixed concrete with a liquid-solid ratio (mass ratio of water to the total of cement and fine aggregate) of 15%, and then water was immediately added to the ready-mixed concrete in an amount to make the liquid-solid ratio 300%, to obtain a cement-containing composition. The coarse aggregate was removed from this cement-containing composition using a sieve with 5 mm openings, to obtain ready-mixed concrete sludge.

[0025] (b) Carbon dioxide fixation The obtained fresh concrete sludge was subjected to carbon dioxide fixation by the above-mentioned gas supply step using the above-mentioned carbon dioxide fixation system (see the description of the above-mentioned gas supply step). In this case, the time from the preparation of ready-mixed concrete to the start of carbon dioxide (carbon dioxide gas) supply was set to 24 hours (see the "Time until gas supply starts (hr)" column in Table 2). This value (24 hours) corresponds to 9.9 times the time until the initial setting (2 hours 25 minutes) mentioned above (see the "Ratio (times) to the initial setting time" column in Table 2). The supply of carbon dioxide (carbon dioxide gas) was stopped when the pH of the fresh concrete sludge became less than 7. At this point, the pH of the fresh concrete sludge was 6.8 (see the "pH of sample" column in Table 2). The time from the start to the end of the supply of carbon dioxide (carbon dioxide gas) was 1.8 hours (see the column "treatment time (hr)" in Table 2).

[0026] (c) Calculation of the amount of carbon dioxide fixed The fresh concrete sludge (in which carbon dioxide had been immobilized) obtained in the gas supply process was heated to a constant weight at temperatures of 500°C and 800°C, and the difference (XY) between the mass at 500°C (X) and the mass at 800°C (Y) was taken as the amount of immobilized carbon dioxide, and the amount of immobilized carbon dioxide (kg) per ton of cement was calculated.

[0027] [Examples 2 to 4, Comparative Examples 1 and 2] As shown in Table 2, the experiment was carried out in the same manner as in Example 1, except that the time until the start of gas supply was changed. The results are shown in Table 2. In Comparative Example 2, the fresh concrete sludge hardened during the experiment, so the fixation of carbon dioxide by the gas supplying step was not carried out.

[0028] [Table 1]

[0029] [Table 2]

[0030] From Table 2, it can be seen that the treatment time required to obtain an immobilized amount of 320 to 335 kg was 1.8 to 4 hours in Examples 1 to 4, and the treatment efficiency was high. On the other hand, in Comparative Example 1, the ratio of the time from preparation of ready-mixed concrete to the time until the initial setting of cement ("initial setting time" in Table 2) was 0.2, which is below the 1.0 to 12 times specified in the present invention, and therefore the treatment time required to obtain an immobilized amount of 332 kg was 7 hours, and the treatment efficiency was low.

[0031] [Example 5] 50 kg of water was added to 500 kg of ready-mixed concrete sludge (275 kg of cement and fine aggregate in total, 225 kg of water) to adjust the liquid-solid ratio to 100%, to obtain ready-mixed concrete sludge. The materials used (cement, etc.) were the same as those in Example 1. An experiment was conducted on the obtained fresh concrete sludge in the same manner as in Example 1, using a carbon dioxide fixation system similar to that in Example 1, under conditions where the time until the start of gas supply was 18 hours (intermediate between Examples 1 and 2) and the treatment time was approximately 2 hours (pH at the end of gas supply: less than 7). [Examples 6 to 9] The experiment was carried out in the same manner as in Example 5, except that the liquid-solid ratio was changed as shown in Table 3. The results are shown in Table 3. In Table 3, "amount of carbon dioxide fixed (kg)" indicates the amount of carbon dioxide fixed (kg) per ton of cement, as in Table 2.

[0032] [Table 3]

[0033] Table 3 shows that when the liquid-solid ratio is in the range of 100 to 500%, the amount of carbon dioxide fixed is 295 to 338 kg per ton of cement, which is similar to the amount fixed (320 to 335 kg) in Examples 1 to 4 (liquid-solid ratio: 300%).

Claims

1. A ready-mixed concrete preparation step of preparing ready-mixed concrete using ready-mixed concrete materials including at least cement, fine aggregate, coarse aggregate, and water; a sludge recovery step of recovering fresh concrete sludge, which is a residue of the fresh concrete and has a higher water-cement ratio than the fresh concrete; a gas supplying step of supplying a carbon dioxide-containing gas to the fresh concrete sludge or a moisture-content-adjusted product thereof recovered in the sludge recovering step when a time within a range of 1.0 to 12 times the time from the preparation of the fresh concrete in the fresh concrete preparing step to the onset of setting (however, the setting refers to setting when a cement paste is prepared using the cement) defined in accordance with "JIS R 5201:2015 (Physical testing methods for cement)" has elapsed, thereby immobilizing the carbon dioxide in the carbon dioxide-containing gas in the fresh concrete sludge or a moisture-content-adjusted product thereof, thereby obtaining fresh concrete sludge or a moisture-content-adjusted product thereof in which the carbon dioxide has been immobilized; A method for fixation of carbon dioxide, comprising:

2. 2. The method for fixation of carbon dioxide according to claim 1, further comprising, before the gas supply step, a gas supply start time determination step of determining in advance the start time of supplying the carbon dioxide-containing gas in the gas supply step based on the value of the initial setting in the gas supply step, the value being determined depending on the time of preparation of the ready-mixed concrete in the ready-mixed concrete preparation step and the type of cement, which is one of the materials of the ready-mixed concrete.

3. 3. The method for fixation of carbon dioxide according to claim 2, further comprising, before the gas supply step, a gas supply time determination step of determining in advance a supply time of the carbon dioxide-containing gas in the gas supply step based on the start time of supply of the carbon dioxide-containing gas determined in the gas supply start time determination step.

4. 2. The method for fixation of carbon dioxide according to claim 1, further comprising, between the sludge recovery step and the gas supply step, a water supply step of supplying water to the ready-mixed concrete sludge or a solid matter thereof after dewatering to obtain a product with an adjusted moisture content of the ready-mixed concrete sludge.

5. 2. The method for fixation of carbon dioxide according to claim 1, wherein, in the gas supply step, the ready-mixed concrete sludge or the product thereof with a moisture content adjusted therefor has a mass ratio of water to the total mass of the cement and the fine aggregate (water × 100 / (cement + fine aggregate); unit: %) of 100 to 500%.

6. A method for producing ready-mixed concrete, comprising obtaining ready-mixed concrete sludge or a product thereof with an adjusted moisture content in which carbon dioxide has been immobilized by the method for immobilizing carbon dioxide according to any one of claims 1 to 5, and then using the ready-mixed concrete sludge or the product thereof with an adjusted moisture content in which carbon dioxide has been immobilized as is as a raw material for ready-mixed concrete to produce ready-mixed concrete.

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