JP7844220B2 - Method for forming a carbonated lightweight cellular cementum hardened body - Google Patents

Description

This invention relates to a method for forming a carbonated lightweight cellular cementum hardened body.

In recent years, reducing carbon dioxide emissions has become a crucial issue in order to curb global warming.
In this regard, technologies are being considered to fixate carbon dioxide recovered from exhaust gases and other sources generated at cement manufacturing plants.
Patent Document 1 describes a method for improving the strength and durability of improved ground by injecting carbon dioxide, in which a cement-based material is mixed with the soil of the target ground by a mechanical mixing method or a jet mixing method, wherein γ-belite is added to the cement-based material, and after a framework is formed in the mixing section, before hardening is complete, a _CO2 source is injected into the section through an injection pipe.
Furthermore, as a ground improvement method utilizing carbon dioxide, Patent Document 2 describes a ground improvement method utilizing carbon dioxide, characterized by comprising: a spraying and mixing step of spraying a carbon dioxide absorbent or an aqueous solution containing a carbon dioxide absorbent that reacts with carbon dioxide and solidifies, onto the surface of the ground to be improved or mixing it into the ground; a penetration step of allowing the carbon dioxide absorbent to penetrate into the gaps in the ground; and a reaction and solidification step of reacting the carbon dioxide absorbent with carbon dioxide or a gas containing carbon dioxide to solidify the carbon dioxide absorbent and leave it in the gaps as a binder.

Japanese Patent Publication No. 2007-204941 Japanese Patent Publication No. 2019-163623

The object of this invention is to provide a method for forming a carbonated lightweight cellular cementitious hardened body that can fix a larger amount of carbon dioxide.

As a result of diligent research to solve the above problems, the inventors of the present invention have found that the above objectives can be achieved by a method comprising the steps of: mixing cement and water for cement-containing mixture to obtain a cement-containing mixture; mixing a foaming agent with water for foaming agent-containing water to obtain foam, and mixing gas; mixing the cement-containing mixture with the foam to obtain an unhardened cement composition; casting the unhardened cement composition to form a lightweight buoyant cementitious hardened body; and blowing carbon dioxide gas at a rate of 1 to 50 liters/minute for 20 to 420 minutes during the period from 0.75 days to 15 days of age to form a carbonated lightweight buoyant cementitious hardened body, thereby completing the present invention.
In other words, the present invention provides the following [1] to [5].
[1] A method for forming a carbonated lightweight foamed cementite hardened body comprising a cement composition containing cement, a foaming agent, and water, and having carbon dioxide fixed, comprising: a cement-containing mixture preparation step of mixing the cement with water for a cement-containing mixture which is a part of the water contained in the cement composition to obtain a cement-containing mixture; a foam preparation step of mixing the foaming agent with water for a foaming agent-containing mixture which is a part of the water contained in the cement composition to obtain foam, and a gas to obtain foam; and mixing the cement-containing mixture with the foam. A method for forming a carbonated lightweight foamed cementitious hardened body, comprising: a cement composition preparation step of obtaining the unhardened cement composition; a casting step of casting the unhardened cement composition to form a lightweight foamed cementitious hardened body; and a carbon dioxide spraying step of spraying carbon dioxide, which is the gaseous state of carbon dioxide to be fixed, onto the lightweight foamed cementitious hardened body at a rate of 1 to 50 liters/minute for 20 to 420 minutes during a period from 0.75 to 15 years of age to form the carbonated lightweight foamed cementitious hardened body.

[2] A method for forming a carbonated lightweight cellular cementitious body according to [1], wherein the density of the unhardened cement composition is 0.40 to 2.00 g/ ^cm³ .
[3] The method for forming a carbonated lightweight cellular cementitious hardened body according to [1] or [2], wherein the carbon dioxide spraying step is performed at a temperature of 5 to 30°C.
[4] A method for forming a carbonated lightweight foamed cementitious body according to any one of [1] to [3] above, wherein in the cement-containing mixture preparation step, a portion of the cement and a portion or all of the water for the cement-containing mixture are mixed to obtain a first cement-containing mixture, then carbon dioxide gas, which is the gaseous state of carbon dioxide to be fixed, is supplied to the first cement-containing mixture to carbonize the first cement-containing mixture, and then the carbonated first cement-containing mixture is mixed with the remainder of the cement and the remainder of the water for the cement-containing mixture, or the carbonated first cement-containing mixture is mixed with the remainder of the cement to obtain the cement-containing mixture.
[5] A method for forming a carbonated lightweight foamed cementitious hardened body according to any one of [1] to [4] above, wherein the proportion of carbon dioxide in the carbonated lightweight foamed cementitious hardened body is 7.0 to 20.0% by mass.

According to the method for forming a carbonated lightweight cellular cementum hardened body of the present invention, it is possible to form a carbonated lightweight cellular cementum hardened body in which a larger amount of carbon dioxide is immobilized.

The present invention provides a method for forming a carbonated lightweight foamed cementitious hardened body, comprising a cement composition containing cement, a foaming agent, and water, and having carbon dioxide fixed within it, comprising: a cement-containing mixture preparation step of mixing cement with water for a cement-containing mixture, which is a portion of the water contained in the cement composition, to obtain a cement-containing mixture; and a foaming agent-containing water obtained by mixing a foaming agent with water for a foaming agent-containing water, which is a portion of the water contained in the cement composition, and gas to obtain bubbles. The method includes a foam preparation step, a cement composition preparation step of mixing a cement-containing mixture with foam to obtain an unhardened cement composition, a casting step of casting the unhardened cement composition to form a lightweight foamed cementitious hardened body, and a carbon dioxide spraying step of spraying carbon dioxide, which is the gaseous form of carbon dioxide to be fixed, at a rate of 1 to 50 liters/minute for 20 to 420 minutes during a period from 0.75 to 15 years of age to form a carbonated lightweight foamed cementitious hardened body.
The following explains each step in detail.

[Preparation process for cement-containing mixtures]
This process involves mixing cement with water, which is a portion of the water contained in the cement composition, to obtain a cement-containing mixture.
The cements mentioned above are not particularly limited and include, for example, various types of Portland cement such as ordinary Portland cement, rapid-hardening Portland cement, moderate-heat Portland cement, and low-heat Portland cement, as well as blended cements such as blast furnace cement and fly ash cement, and eco-cements. These may be used individually or in combination of two or more types.
Among these, ordinary Portland cement is preferred from the standpoint of versatility and other factors.

The type of water used is not particularly limited and includes, for example, tap water, sludge water, etc.
In this process, the phrase "water for cement-containing mixture, which is part of the water contained in the cement composition" means a portion of the total amount of water used to mix with the cement in the cement composition (the total amount of water minus the water used for foaming agent-containing water).
The proportion of water for the cement-containing mixture in the total amount of water (100% by mass) in the cement composition is preferably 30 to 90% by mass, more preferably 50 to 80% by mass, and particularly preferably 60 to 75% by mass. If the above proportion is 30% by mass or more, the fluidity of the cement-containing mixture is further improved, and workability is improved. Also, when carbon dioxide is supplied in this process (described later), it becomes easier to supply carbon dioxide homogeneously to the cement-containing mixture. If the above proportion is 90% by mass or less, the amount of water contained in the foaming agent-containing water becomes relatively larger, and the amount of bubbles can be increased.

The water-cement ratio of the cement-containing mixture is preferably 40-500%, more preferably 45-300%, even more preferably 50-150%, even more preferably 55-120%, and particularly preferably 60-100%. If the above ratio is 40% or higher, the fluidity of the cement-containing mixture is further improved, and workability is enhanced. Also, when carbon dioxide is supplied in this process (described later), it becomes easier to supply carbon dioxide uniformly to the cement-containing mixture. If the above ratio is 500% or lower, the strength of the carbonated lightweight cellular cementitious hardened body can be further increased.
The water-cement ratio is the mass ratio of water to cement (water/cement) expressed as a percentage (%).
In this process, the method of mixing cement and a portion of the water is not particularly limited. For example, a portion of the water may be added to a mixing tank, then the cement may be added and mixed, or the cement and a portion of the water may be added to a mixing tank simultaneously and then mixed.

From the viewpoint of fixing carbon dioxide in the cement composition and increasing the amount of carbon dioxide fixed to the final carbonated lightweight cellular cementitious hardened body, carbon dioxide may be supplied to the cement-containing mixture in this process to carbonize the cement-containing mixture.
An example of a method for supplying carbon dioxide to a cement-containing mixture is to first obtain a cement-containing mixture by mixing a portion of the cement with a portion or all of the water for the cement-containing mixture, then supply carbon dioxide gas, which is the carbon dioxide to be fixed, into the first cement-containing mixture to carbonize it, and then mix the carbonated first cement-containing mixture with the remaining cement and the remaining water for the cement-containing mixture, or mix the carbonated first cement-containing mixture with the remaining cement to obtain a cement-containing mixture. According to this method, a larger amount of carbon dioxide can be fixed to the cement-containing mixture.

In the method of supplying carbon dioxide to the cement-containing mixture described above, the proportion of a portion of the total amount of cement (the total amount of cement contained in the cement-containing mixture) is preferably 1 to 50% by mass, more preferably 3 to 40% by mass, even more preferably 5 to 30% by mass, even more preferably 8 to 25% by mass, even more preferably 10 to 20% by mass, and particularly preferably 12 to 18% by mass. If the above proportion is 1% by mass or more, the amount of carbon dioxide immobilized in the cement-containing mixture will be greater. If the above proportion is 50% by mass or less, the strength of the carbonated lightweight cellular cementitious hardened body will be greater.

In a method for supplying carbon dioxide to a cement-containing mixture, when mixing a portion of the cement with a portion of the water for the cement-containing mixture, the proportion of the portion of water in the total amount of water in the cement-containing mixture is preferably 50 to 99% by mass, preferably 60 to 98% by mass, more preferably 70 to 97% by mass, and particularly preferably 75 to 96% by mass. If the proportion is 50% by mass or more, the fluidity of the first cement-containing mixture is further improved, making it easier to uniformly supply carbon dioxide to the first cement-containing mixture. If the proportion is 99% by mass or less, the amount of water in the cement-containing mixture obtained in the cement-containing mixture preparation step becomes larger, further improving the fluidity of the cement-containing mixture.

The water-cement ratio of the first cement-containing mixture is preferably 100-500%, more preferably 150-450%, even more preferably 200-400%, and particularly preferably 250-350%. If the ratio is 100% or higher, the fluidity of the first cement-containing mixture is further improved, making it easier to uniformly supply carbon dioxide to the mixture. If the ratio is 500% or lower, the amount of carbon dioxide immobilized in the first cement-containing mixture increases.

As a method for carbonizing a first cement-containing mixture by supplying carbon dioxide, which is the gaseous state of carbon dioxide to be fixed, into the first cement-containing mixture, it is preferable to supply the carbon dioxide while the first mixture is flowing, from the viewpoint of supplying the carbon dioxide homogeneously to the first cement-containing mixture (hereinafter also referred to as the "first mixture").
Methods for supplying carbon dioxide into the first mixture include, for example, the following methods (i) to (iii).
(i) A method of supplying carbon dioxide gas into the first kneaded product, wherein a carbon dioxide gas supply means for supplying carbon dioxide gas into the first kneaded product is installed in a stirring tank for obtaining the first kneaded product.
A more specific example is a method in which a first cement-containing mixture is obtained by stirring a portion of cement and a portion or all of the water for mixing the cement to obtain a first mixture, and a carbon dioxide supply means (for example, a diffuser plate) is installed in a space for containing the mixture in the stirring tank, and carbon dioxide gas is blown into the mixture using the carbon dioxide supply means while stirring the first mixture, thereby obtaining a first cement-containing mixture that has been carbonated (hereinafter also referred to as "carbonated mixture").

(ii) A method of transferring a first kneaded material from a stirring tank for containing the first kneaded material into a device having carbon dioxide supply means (e.g., a diffuser plate) for supplying carbon dioxide gas, blowing carbon dioxide gas into the first kneaded material in the device to obtain a carbonated kneaded material, and then transferring the carbonated kneaded material to a carbonated kneaded material tank for containing the carbonated kneaded material.
The stirring tank and the carbonation and kneading tank may be the same or different.

A more specific example is a method in which a first mixture is contained in a stirring tank for stirring and mixing a portion of cement and a portion or all of the water for cement mixing to obtain a first mixture, and the first mixture is transferred to a device having a carbon dioxide supply means for supplying carbon dioxide gas using a pump or the like through a first flow passage for supplying the first mixture from the stirring tank to the device, and then carbon dioxide gas is supplied to the first mixture while stirring the first mixture in the device, and then the first mixture after carbon dioxide gas supply (carbonated mixture) is transferred to the stirring tank using a pump or the like through a second flow passage for supplying the carbonated mixture from the device to the stirring tank to obtain a carbonated mixture.
Furthermore, the first kneaded material may be repeatedly circulated in the following order: the stirring tank, the first flow passage, the apparatus, and the second flow passage, until a sufficient amount of carbon dioxide is supplied.
Furthermore, after obtaining the carbonated mixture, the carbonated mixture may be temporarily stored in a storage tank separate from the carbonated mixture tank, and then supplied from the storage tank to the carbonated mixture tank as needed.

(iii) A method of transferring the first kneaded material from a stirring tank for containing the first kneaded material to a carbonated kneaded material tank for containing the carbonated kneaded material, by blowing carbon dioxide gas into the first kneaded material.
A more specific example is a method in which a first mixture is placed in a stirring tank for stirring and mixing a portion of cement and a portion or all of the water for cement mixing to obtain a first mixture, and while the first mixture supplied from the stirring tank is circulated through a pipeline for circulating the first mixture, carbon dioxide gas is supplied to the first mixture and stirred to obtain a carbonated mixture, and then the carbonated mixture is transferred to the carbonated mixture tank.

Examples of the above-mentioned pipeline include one having a carbon dioxide supply port for supplying the carbon dioxide gas to a first kneaded material circulating within the pipeline, and having a configuration that allows for stirring and mixing of the first kneaded material and the carbon dioxide gas. Specifically, examples include a pipeline equipped with aeration means (e.g., aeration plates) and stirring means (e.g., line mixers or static mixers) for supplying the carbon dioxide gas.
Alternatively, after supplying carbon dioxide gas to the first mixture, the carbonated mixture may be returned to the stirring tank without being transferred to the carbonated mixture tank. Furthermore, the first mixture may be repeatedly circulated through the stirring tank and the pipeline in that order to supply a sufficient amount of carbon dioxide gas, and then the first mixture may be transferred to the carbonated mixture tank as a carbonated mixture.
Furthermore, in the above method, after obtaining the carbonated kneaded product, it may be temporarily stored in a carbonated kneaded product storage tank for storing carbonated kneaded products, and the carbonated kneaded product may be supplied from the storage tank to the carbonated kneaded product tank as appropriate.

From the viewpoint of increasing the amount of carbon dioxide fixed in the first compound, the supply of carbon dioxide may be carried out under pressurized conditions at the liquid surface of the first compound (for example, by increasing the internal pressure of the tank containing the first compound to 1,200 hPa or higher, which is above atmospheric pressure).
Therefore, a carbon dioxide supply means for supplying carbon dioxide is preferably one that can be pressurized.

Furthermore, carbon dioxide gas may be supplied to the first compound as a gas consisting solely of carbon dioxide, but from the viewpoint of ease of availability, etc., it may also be supplied to the first compound as a gas containing carbon dioxide.
The proportion of carbon dioxide in the gas containing carbon dioxide is preferably 5% by volume or more, more preferably 10% by volume or more, even more preferably 20% by volume or more, even more preferably 50% by volume or more, even more preferably 80% by volume or more, and particularly preferably 90% by volume or more. If the proportion is 5% by volume or more, the amount of carbon dioxide fixed in the first kneaded product can be increased. In addition, the time required to supply carbon dioxide can be shortened.
Examples of gases containing carbon dioxide include exhaust gas generated in the cement manufacturing process (carbon dioxide concentration: approximately 20% by volume), exhaust gas generated in the steelmaking process (carbon dioxide concentration: approximately 20% by volume), exhaust gas generated in the thermal power generation process (carbon dioxide concentration: approximately 10% by volume), and gas separated and recovered from these exhaust gases (carbon dioxide concentration: approximately 100% by volume).

The supply of carbon dioxide is carried out so that the pH of the first compound obtained by carbonation is preferably within the range of 5.0 to 11.5, more preferably 5.5 to 11.0, even more preferably 6.0 to 10.0, and particularly preferably 6.5 to 9.5. When carbon dioxide is supplied so that the pH is 5.0 or higher, the strength of the carbonated lightweight cellular cementitious hardened body is increased. In addition, the time required for supplying carbon dioxide is shortened, and the manufacturing efficiency is further improved. When carbon dioxide is supplied so that the pH is 11.5 or lower, the amount of carbon dioxide fixed in the first compound is increased. Note that the pH of the first compound decreases when carbon dioxide is supplied.
The amount of carbon dioxide that needs to be fixed in the first compound to be sufficiently large varies depending on the water-cement ratio, the carbon dioxide supply means, and the carbon dioxide concentration of the gas containing carbon dioxide supplied by said means. For this reason, it is preferable to determine the timing of ending the carbon dioxide supply based on the measured pH value of the carbonated compound.

After obtaining a carbonated mixture (a first cement-containing mixture obtained by carbonation), a cement-containing mixture can be obtained by mixing the carbonated mixture with the remaining cement and the remaining water for the cement-containing mixture, or by mixing the carbonated mixture with the remaining cement.
The proportion of the remaining cement in the total amount of cement (the total amount of cement contained in the cement-containing mixture) is preferably 50 to 99% by mass, more preferably 60 to 97% by mass, even more preferably 70 to 95% by mass, even more preferably 75 to 92% by mass, even more preferably 80 to 90% by mass, and particularly preferably 82 to 88% by mass. If the above proportion is 50% by mass or more, the strength of the carbonated lightweight cellular cementaceous hardened body will be greater. If the above proportion is 99% by mass or less, the amount of carbon dioxide immobilized in the cement composition will be greater.

When mixing the first cement-containing mixture, which has been carbonated, with the remaining cement and the remaining water for the cement-containing mixture, the proportion of the remaining water in the total amount of water for mixing the cement-containing mixture (total amount of water contained in the cement-containing mixture) is preferably 1 to 50% by mass, more preferably 2 to 40% by mass, even more preferably 3 to 30% by mass, and particularly preferably 4 to 25% by mass. If the above proportion is 1% by mass or more, the amount of water contained in the cement-containing mixture increases, improving the fluidity of the cement composition. If the above proportion is 50% by mass or less, the fluidity of the first mixture is relatively improved, making it easier to uniformly supply carbon dioxide to the first mixture.

[Bubble preparation process]
This process involves mixing a foaming agent with water containing the foaming agent, which is a portion of the water contained in the cement composition, and then mixing this water with gas to obtain bubbles.
The foaming agent is not particularly limited; commercially available foaming agents for air mortar or air milk can be used.
Examples of foaming agents include surfactant-based foaming agents such as anionic surfactants, cationic surfactants, amphoteric surfactants, and nonionic surfactants, as well as protein-based foaming agents.
In this process, the phrase "water for foaming agent-containing water, which is part of the water contained in the cement composition" means a portion of the total amount of water mixed with the cement in the cement composition (the total amount of water minus the water used for the cement-containing mixture).

The proportions of foaming agent and water are determined appropriately according to the target density and mass of the unhardened cement composition, the foaming ratio of the foaming agent, and the dilution ratio of the foaming agent.
Specifically, it can be calculated by following the steps (i) to (ii) below.
(i) Assuming that the amount of unhardened cement composition obtained in the cement composition preparation process (described later) is ^{1 m³} , the volume (volume of air bubbles) is calculated by subtracting the volume of the cement-containing mixture obtained in the cement-containing mixture preparation process from the volume of the unhardened cement composition (1 m³).
(ii) The amount of foaming agent can be calculated from the volume of bubbles, the foaming ratio of the foaming agent, and the dilution ratio using the following formula.
Amount of foaming agent (kg) = Volume of bubbles / Foaming ratio of the foaming agent / Dilution ratio × 1000
The amount of water can be calculated from the amount of foaming agent and the dilution ratio.
For example, when producing 1 ^m³ of unhardened cement composition, if the volume of the cement-containing mixture is 0.34 ^m³ , the volume of air bubbles will be 0.66 ^m³ (660 liters). In this case, if a foaming agent with a foaming ratio of 20 and a dilution ratio of 20 is used, the amount of foaming agent is 1.65 kg (0.66 / 20 / 20 × 1000 = 1.65 kg).

One example of a method for obtaining bubbles by mixing a foaming agent with foaming agent-containing water (a mixture of foaming agent-containing water, which is a portion of the water contained in the cement composition) and gas is to prepare bubbles by mixing (kneading) water, foaming agent, and gas (usually compressed gas) in a foaming apparatus. Here, compressed gas (usually compressed air) can be produced using a device capable of compressing gas, such as a compressor.

[Cement composition preparation process]
This step involves mixing the cement-containing mixture obtained in the above-mentioned cement-containing mixture preparation step with the bubbles obtained in the above-mentioned bubbles preparation step to obtain an unhardened cement composition.
The cement-containing mixture obtained in the cement-containing mixture preparation process and the bubbles obtained in the bubbles preparation process are usually prepared separately and then mixed using a mixing device such as a mixer.
Alternatively, an apparatus combining a mixing means and a bubble dispensing device may be used. Specifically, one method involves mixing components other than the foaming agent using the mixing means while adding the foaming agent from the bubble dispensing device, thereby generating bubbles in the mixture during mixing. This form is also included in the present invention. In this case, the cement-containing mixture preparation step, the bubble preparation step, and the cement composition preparation step are carried out continuously within the same apparatus.

The water-cement ratio (the ratio of the total amount of water in the cement composition to the amount of cement) of the cement composition is preferably 40-500%, more preferably 50-300%, even more preferably 60-200%, and particularly preferably 65-100%. If the above ratio is 40% or higher, the fluidity of the unhardened cement composition is further improved, and workability during placement and other processes is further improved. If the above ratio is 500% or lower, the strength of the hardened carbonated lightweight cellular cementitious body is increased. Furthermore, material segregation becomes less likely.

The density of the uncured cement composition is preferably 0.40 to 2.00 g/ ^cm³ , more preferably 0.45 to 1.50 g/ ^cm³ , even more preferably 0.50 to 1.00 g/ ^cm³ , even more preferably 0.52 to 0.70 g/ ^cm³ , and particularly preferably 0.54 to 0.65 g/ ^cm³ . If the density is 0.40 g/ ^cm³ or higher, the strength of the carbonated lightweight cellular cementitious hardened body can be increased. If the density is 2.00 g/ ^cm³ or lower, the mass of the carbonated lightweight cellular cementitious hardened body becomes smaller, improving workability and other properties.
The amount of gas (usually air) in the uncured cement composition is preferably 50 to 80 volume%, more preferably 55 to 75 volume%, even more preferably 58 to 70 volume%, even more preferably 60 to 68 volume%, and particularly preferably 61 to 66 volume%. If the amount of gas is 50 volume% or more, the carbonated lightweight cellular cementitious hardened body can be made lighter. If the amount of gas is 80 volume% or less, the strength of the carbonated lightweight cellular cementitious hardened body can be made greater.
The density and gas content of the uncured lightweight cellular cementitious body can be adjusted by the amount of bubbles, etc.

The cement composition may contain aggregate. The aggregate may consist of fine aggregate alone, or a combination of fine aggregate and coarse aggregate. Natural aggregate, artificial aggregate, or recycled aggregate can also be used.
The fine aggregate is not particularly limited and includes, for example, river sand, mountain sand, land sand, sea sand, crushed sand, silica sand, slag fine aggregate, and lightweight fine aggregate, or a mixture consisting of two or more types selected from these.
The coarse aggregate is not particularly limited and may include, for example, river gravel, mountain gravel, land gravel, sea gravel, crushed stone, slag coarse aggregate, and lightweight coarse aggregate, or a mixture consisting of two or more types selected from these.
The amount of aggregate is not particularly limited and can be any amount that is typical for lightweight cellular cementitious cementitious bodies (e.g., lightweight embankments). For example, the amount of aggregate (total amount if fine aggregate and coarse aggregate are included) is preferably 10 to 350 parts by mass, more preferably 100 to 300 parts by mass, and particularly preferably 150 to 250 parts by mass, per 100 parts by mass of cement. If the above amount is 10 parts by mass or more, the strength of the carbonated lightweight cellular cementitious cementitious body will be increased. If the above amount is 350 parts by mass or less, the mass of the carbonated lightweight cellular cementitious body can be reduced further.
When a cement composition contains aggregate, the aggregate may be supplied and mixed in any of the following steps: the cement-containing mixture preparation step, the foam preparation step, or the cement composition preparation step. However, it is usually supplied and mixed in the cement-containing mixture preparation step.

The cement composition may optionally contain (i) various admixtures such as fly ash, silica fume, and blast furnace slag powder, (ii) various admixtures such as cement dispersants such as water-reducing agents, AE water-reducing agents, high-performance water-reducing agents, and high-performance AE water-reducing agents, as well as AE agents, defoamers, and fluidizers, and (iii) soil. The method of mixing the various admixtures, admixtures, and soil is not particularly limited, but from the viewpoint of manufacturing efficiency, it is preferable that they be supplied and mixed in the cement-containing mixture preparation process.
Furthermore, as the soil, locally sourced soil obtained at the site where the carbonated lightweight cellular cementitious hardened body (for example, lightweight embankment) is formed can be used. The proportion of the above soil in the lightweight cellular cementitious hardened body is preferably 60% by mass or less, more preferably 5 to 40% by mass, and particularly preferably 10 to 30% by mass, from the viewpoint of strength development of the lightweight cellular cementitious hardened body.

[Concrete pouring process]
This process involves placing the unhardened cement composition obtained in the cement composition preparation process to form a lightweight, cellular cementitious hardened body. Possible placement methods include, for example, pumping the unhardened cement composition.
The shape of the formed lightweight cellular cementum hardened body is not particularly limited, and the shape may be determined as appropriate depending on the application.
The density of the lightweight cellular cementum hardened material is preferably 0.40 to 2.00 g/ ^cm³ , more preferably 0.45 to 1.50 g/ ^cm³ , even more preferably 0.50 to 1.00 g/ ^cm³ , even more preferably 0.52 to 0.70 g/ ^cm³ , and particularly preferably 0.54 to 0.65 g/ ^cm³ . If the density is 0.40 g/ ^cm³ or higher, the strength of the carbonated lightweight cellular cementum hardened material can be increased. If the density is 2.00 g/ ^cm³ or lower, the mass of the carbonated lightweight cellular cementum hardened material becomes smaller, improving workability and other properties.

[Carbon dioxide spraying process]
This process involves spraying carbon dioxide gas, which is the target of fixation, onto the lightweight cellular cementitious cementitious hardened body cast in the casting process at a rate of 1 to 50 liters/minute for 20 to 420 minutes during the period from 0.75 days to 15 days of age, in order to form a carbonated lightweight cellular cementitious hardened body.
It is preferable to spray carbon dioxide gas during the period from 0.75 to 15 years of age, preferably from 2 to 14 years of age, more preferably from 3 to 12 years of age, and preferably from 4 to 10 years of age. By spraying carbon dioxide gas during the above period, more carbon dioxide can be fixed.

It is preferable to blow carbon dioxide at a rate of 1 to 50 liters/minute, preferably 3 to 40 liters/minute, more preferably 5 to 30 liters/minute, and particularly preferably 8 to 20 liters/minute, for a period of 20 to 420 minutes, preferably 30 to 360 minutes, more preferably 40 to 300 minutes, and particularly preferably 90 to 240 minutes. By blowing carbon dioxide at a rate of 1 liter/minute or more for 20 minutes or more, more carbon dioxide can be fixed. Furthermore, by blowing carbon dioxide at a rate of 50 liters/minute or less for 420 minutes or less, the production efficiency can be further improved.
As the carbon dioxide gas, the same type of carbon dioxide gas used in the cement-containing mixture preparation process described above, when carbonizing the first cement-containing mixture, can be used.

The carbon dioxide spraying process is preferably carried out at a temperature of 5 to 30°C, more preferably 10 to 28°C, and particularly preferably 15 to 25°C. Carrying out the above process at a temperature of 5°C or higher allows for the fixation of more carbon dioxide. Carrying out the above process at a temperature of 30°C or lower allows for greater strength of the carbonated lightweight cellular cementum hardened body.
Furthermore, from the viewpoint of efficiently fixing the injected carbon dioxide gas into the lightweight foamed cementum hardened body, it is preferable to cover the lightweight foamed cementum hardened body with a covering material such as vinyl, and then inject carbon dioxide gas between the lightweight foamed cementum hardened body and the covering material.

The proportion of carbon dioxide in the carbonated lightweight foamed cementum hardened body (the proportion of carbon dioxide immobilized in the carbonated lightweight foamed cementum hardened body) is preferably 7.0 to 20.0% by mass, more preferably 7.5 to 19.0% by mass, and particularly preferably 8.0 to 18.0% by mass. If the above proportion is 7.0% by mass or more, more carbon dioxide can be immobilized, and carbon dioxide emissions can be further reduced. Carbonated lightweight foamed cementum hardened body with a above proportion exceeding 20.0% by mass may be difficult to manufacture.
The compressive strength of carbonated lightweight cellular cementitious hardened material, measured in accordance with "JIS A 1108:2018 (Compression Test Method for Concrete)," is preferably 400 to 1,300 kN/ ^m² , more preferably 420 to 1,250 kN/ ^m² , and even more preferably 600 to 1,200 kN/ ^m² .

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.
[Materials used]
(1) Cement: Ordinary Portland cement manufactured by Taiheiyo Cement Corporation (2) Foaming agent: Anionic surfactant, manufactured by Onoda Chemico Corporation, product name: OFA-S
(3) Water; tap water

[Example 1]
As part of the cement-containing mixture preparation process, a portion of the cement and a portion of the water for the cement-containing mixture were mixed in a container using a hand mixer for 60 seconds to obtain a mixture (first cement-containing mixture, temperature: 23°C). The amounts of the cement and water were such that the water-cement ratio was 300% (the unit amount of the cement portion was 60.5 kg/ ^m³ , and the unit amount of the water for the cement-containing mixture was 181.6 kg/ ^m³ ).
A carbon dioxide-containing gas (temperature: 20°C) with a carbon dioxide gas concentration of 99.6% by volume was supplied to the mixture in the above container at a rate of 30 liters/minute using a carbon dioxide cylinder. The supply of carbon dioxide-containing gas continued until the pH of the first cement-containing mixture reached 9.0.
Next, the remaining cement and the first cement-containing mixture were mixed using a hand mixer for 60 seconds to obtain a mixture. Then, after scraping off the mixture adhering to the inner wall of the mixer, the remaining water for the cement-containing mixture was added and mixed for 60 seconds to obtain a cement-containing mixture. The amounts of the remaining cement and water were set so that the water-cement ratio of the cement-containing mixture was 64% (the unit amount of the remaining cement was 292.5 kg/ ^m³ , and the unit amount of the remaining water for the cement-containing mixture was 45.4 kg/ ^m³ ).
Note that the unit quantity refers to the amount of mixture based on mass per unit volume of lightweight cellular cementitious cementitious material (1 ^m³) .

On the other hand, bubbles were prepared by kneading a foaming agent with water for the foaming agent-containing solution and kneading while supplying compressed air using a compressor.
The amounts of foaming agent and the water used were determined to be such that the density of the unhardened cement composition would be 0.61 g/ ^cm³ and the air content of the unhardened cement composition would be 62.8% by volume (specifically, the unit amount of foaming agent would be 1.65 kg/ ^m³ and the unit amount of water for the foaming agent-containing water would be 31.35 kg/ ^m³ ).
Next, the cement-containing mixture and air bubbles were mixed for 60 seconds using a forced mixer to obtain an unhardened cement composition.
The density of the unhardened cement composition was calculated by filling a 1-liter container with the cement composition, leveling the surface, measuring the mass, and then dividing the resulting mass by the volume (1 liter).
Furthermore, the air content of the unhardened cement composition was measured in accordance with the Japan Highway Public Corporation standard "Test Methods for Air Mortar and Air Milk (JHS A 313-1992)".

A cement composition was poured into a formwork to produce a lightweight, aerated cementitious hardened body (embankment structure) measuring 20 cm x 40 cm x 8 cm.
In the carbon dioxide spraying process, at the ages shown in Table 1, a formwork filled with lightweight cellular cementitious cementitious material was placed in a synthetic resin container with an open top, and the opening was covered with a sheet. Next, carbon dioxide (carbon dioxide content 99.6 vol%) was sprayed onto the lightweight cellular cementitious cementitious material inside the container at a rate of 10 liters/minute for the time shown in Table 1. After 28 days, a carbonated lightweight cellular cementitious cementitious material was obtained.
These operations were carried out under normal temperature conditions (approximately 20-25°C).

The compressive strength of the obtained carbonated lightweight cellular cementitious cementitious body at 28 days was measured in accordance with "JIS A 1108:2018 (Test method for compressive strength of concrete)".
Furthermore, the percentage of carbon dioxide (mass%) in the mortar portion of the 28-day-old specimen used to measure compressive strength was determined using thermogravimetric differential thermal analysis (TG-DTA). Specifically, after crushing the specimen, thermogravimetric differential thermal analysis (TG-DTA) was performed on the sample (mortar portion). From the measurement results, the decrease in mass in the endothermic peak range of approximately 550 to 800°C was determined to be due to the decarboxylation of calcium carbonate contained in the mortar portion. From the amount of this decrease, the percentage of carbon dioxide (mass%) in the mortar portion of the specimen was calculated.

[Examples 2-11]
In the cement-containing mixture preparation process, cement and water for the cement-containing mixture were mixed in a container using a hand mixer for 60 seconds to obtain a mixture (cement-containing mixture, temperature: 23°C). The amount of cement and water used was such that the water-cement ratio was 64% (the unit amount of a portion of the cement was 353 kg/ ^m³ , and the unit amount of a portion of the water for the cement-containing mixture was 227 kg/ ^m³ ).
On the other hand, bubbles were prepared by mixing compressed gas (air) into foaming agent-containing water, which was obtained by mixing a foaming agent with water for foaming agent-containing water.
The amounts of foaming agent and the water used were determined to be such that the density of the unhardened cement composition would be 0.61 g/ ^cm³ and the air content of the unhardened cement composition would be 62.8% by volume (specifically, the unit amount of foaming agent would be 1.65 kg/ ^m³ and the unit amount of water for the foaming agent-containing water would be 31.35 kg/ ^m³ ).

Next, the cement-containing mixture and air bubbles were mixed for 60 seconds using a forced mixer to obtain an unhardened cement composition.
The density and air content of the uncured cement composition were measured in the same manner as in Example 1.
Furthermore, in the same manner as in Example 1, carbon dioxide gas was sprayed onto the lightweight cellular cementum hardened body in the container for the time shown in Table 1. After 28 days, a carbonated lightweight cellular cementum hardened body was obtained.
These operations were carried out under normal temperature conditions (approximately 20-25°C).
The compressive strength of the obtained carbonated lightweight cellular cementitious hardened body at 28 days was measured in the same manner as in Example 1.

[Comparative Example 1]
A lightweight cellular cementitious hardened body was obtained at an age of 28 days in the same manner as in Example 2, except that the carbon dioxide spraying process was omitted.
The density and air content of the uncured cement composition, and the compressive strength of the obtained lightweight cellular cementitious cementitious body at 28 days of age were measured in the same manner as in Example 1.
[Comparative Example 2]
A lightweight cellular cementitious hardened body was obtained at 28 days of age in the same manner as in Comparative Example 1, except that a foaming agent and water for foaming agent-containing water were mixed, and compressed carbon dioxide was supplied using a carbon dioxide cylinder during the mixing process.
The density and air content of the uncured cement composition, and the compressive strength of the obtained lightweight cellular cementitious cementitious body at 28 days of age were measured in the same manner as in Example 1.
[Comparative Examples 3-5]
A carbonated lightweight cellular cementum hardened body was obtained at an age of 28 days in the same manner as in Example 2.
The density and air content of the uncured cement composition, and the compressive strength of the obtained carbonated lightweight cellular cementum hardened body at 28 days of age were measured in the same manner as in Example 1.
The results for each are shown in Table 1.

A comparison of Examples 1-11 and Comparative Examples 1-2 in Table 1 shows that a larger amount of carbon dioxide can be immobilized in the carbonated lightweight foamed cementum hardened body by performing a carbon dioxide spraying process.
Furthermore, a comparison of Examples 1 to 11 and Comparative Examples 3 to 5 shows that by spraying carbon dioxide gas at a rate of 10 liters/minute for 30 to 360 minutes during the period from age 1 to age 13, a large amount of carbon dioxide can be immobilized in the carbonated lightweight cellular cementum hardened body.

Claims (4)

A method for forming a carbonated lightweight cellular cementitious hardened body comprising a cement composition containing cement, a foaming agent, and water, and having carbon dioxide fixed in it,
A cement-containing mixture preparation step involves mixing the above-mentioned cement with water for the cement-containing mixture, which is a portion of the water contained in the above-mentioned cement composition, to obtain a cement-containing mixture.
A foam preparation step involves mixing the above-mentioned foaming agent with foaming agent-containing water, which is the remaining water in the above-mentioned cement composition, and gas to obtain bubbles.
A cement composition preparation step involves mixing the above-mentioned cement-containing mixture with the above-mentioned air bubbles to obtain the unhardened cement composition,
A casting step in which the unhardened cement composition is cast to form a lightweight aerated cementitious hardened body,
A carbon dioxide spraying process is performed on the above-mentioned lightweight cellular cementitious cementitious hardened body, during a period from 0.75 days to 15 days of age, in which carbon dioxide, which is the gaseous form of carbon dioxide to be fixed, is sprayed so that the proportion of carbon dioxide in the carbonated lightweight cellular cementitious hardened body is 7.0 to 20.0% by mass , thereby forming the carbonated lightweight cellular cementitious hardened body.
A method for forming a carbonated lightweight cellular cementitious hardened body, characterized by containing the following: A method for forming a carbonated lightweight cellular cementitious hardened body according to claim ¹ , wherein the density of the unhardened cement composition is 0.40 to 2.00 g/cm³. The method for forming a carbonated lightweight cellular cementitious hardened body according to claim 1 or 2, wherein the above carbon dioxide spraying step is performed at a temperature of 5 to 30°C. A method for forming a carbonated lightweight cellular cementitious hardened body according to any one of claims 1 to 3, wherein, in the cement-containing mixture preparation step described above, a portion of the cement and a portion or all of the water for the cement-containing mixture are mixed to obtain a first cement-containing mixture; carbon dioxide, which is the gaseous form of carbon dioxide to be fixed, is supplied to the first cement-containing mixture to carbonize it; and then the carbonated first cement-containing mixture is mixed with the remaining cement and the remaining water for the cement-containing mixture, or the carbonated first cement-containing mixture is mixed with the remaining cement to obtain the cement-containing mixture.