JP7834289B2 - Method for sequestrating carbon dioxide into concrete, and concrete structures containing concrete. - Google Patents

Description

One embodiment of the present invention relates to a method for fixing carbon dioxide into concrete contained in a concrete structure, and to a concrete structure to which this method is applied.

Concrete is mainly composed of cement hydrate, aggregate, water, and additives, and due to its excellent mechanical properties, weather resistance, ease of handling, and economic efficiency, it is widely used in various fields as one of the important structural materials for creating social production and economic foundations. Cement is known to emit large amounts of carbon dioxide during its manufacture, which is cited as one of the causes of the greenhouse effect. Therefore, in order to contribute to solving this problem, for example, Patent Document 1 discloses a method for fixing carbon dioxide in concrete by bringing ready-mixed concrete into contact with the concrete before it hardens when constructing a concrete structure (hereinafter referred to as a concrete structure). Patent Document 2 discloses a design method for concrete structures that is effective in promoting the absorption of carbon dioxide into concrete.

Patent No. 5957283 Patent No. 4822373

One embodiment of the present invention aims to provide a method for efficiently fixing carbon dioxide in concrete. For example, one embodiment of the present invention aims to provide a method for fixing carbon dioxide in concrete contained in an existing concrete structure. Alternatively, one embodiment of the present invention aims to provide a concrete structure obtained by applying the above method.

One embodiment of the present invention is a method for fixing carbon dioxide in concrete. This method includes forming gas injection holes in concrete contained in a concrete structure, introducing gas containing carbon dioxide into the gas injection holes, and, after introducing the gas, capping one end of the gas injection holes to seal the gas inside the holes.

One embodiment of the present invention is a concrete structure. This structure comprises a first concrete and a second concrete. The second concrete differs in composition from the first concrete, is surrounded by the first concrete, and extends in a first direction toward the interior of the first concrete. The first concrete includes a first zone in contact with the second concrete and a second zone surrounding the first zone. The calcium carbonate concentration in the first zone decreases continuously in a second direction perpendicular to the first direction as the distance from the interface between the first and second concrete increases. The calcium carbonate concentration in the second zone remains constant in the second direction.

One embodiment of the present invention is a concrete structure. This structure includes concrete having a bottomed hole or through hole extending in a first direction. The concrete includes a first zone constituting the side wall of the bottomed hole or through hole, and a second zone surrounding the first zone. The concentration of calcium carbonate in the first zone decreases continuously as the distance from the side wall increases in a second direction perpendicular to the first direction. The concentration of calcium carbonate in the second zone is constant in the second direction.

A perspective view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A cross-sectional view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A cross-sectional view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A perspective view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A perspective view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A perspective view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A top view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A perspective view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A top view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A perspective view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A perspective view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A side view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A perspective view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A side view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A cross-sectional view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A cross-sectional view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A perspective view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A cross-sectional view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A cross-sectional view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A cross-sectional view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A schematic diagram showing the composition within concrete obtained by a method for fixing carbon dioxide into concrete, which is one embodiment of the present invention. A schematic diagram showing the composition within concrete obtained by a method for fixing carbon dioxide into concrete, which is one embodiment of the present invention. A cross-sectional view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A cross-sectional view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A perspective view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A top view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A perspective view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A side view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A cross-sectional view illustrating one embodiment of the present invention: a method for fixing carbon dioxide into concrete. A perspective view of the concrete structure used in the example. Cross-sectional view of the concrete structure used in the example.

The embodiments of the present invention will be described below with reference to the drawings and other materials. However, the present invention can be implemented in various forms without departing from its essence, and is not intended to be limited to the embodiments described below.

The drawings may schematically represent the width, thickness, shape, etc., of each part in order to clarify the explanation, but these are merely examples and do not limit the interpretation of the present invention. In this specification and each drawing, elements having the same function as those described with respect to previously shown drawings are denoted by the same reference numerals, and redundant explanations may be omitted.

In the following, the expression "one structure is exposed from another structure" means a part of one structure that is not covered by another structure, and this part that is not covered by another structure also includes the case where it is covered by yet another structure.

Hereinafter, "concrete" refers to a material in which cement, one of the raw materials, reacts with water to produce hydrates that harden and do not exhibit fluidity. On the other hand, a mixture containing cement and water that has not completely hardened and retains fluidity is referred to as ready-mixed concrete (also called fresh concrete).

The following describes, with reference to the attached drawings, a method for sequestrating carbon dioxide using an existing concrete structure, and a concrete structure to which this method is applied, according to one embodiment of the present invention. In these drawings, for convenience, the xy plane is considered the horizontal plane, and the z direction is considered the vertical direction.

1. Overview In one embodiment of the present invention, a method for fixing carbon dioxide is used, in which a concrete structure containing hardened concrete is used as a substrate for carbon dioxide fixation. That is, carbon dioxide is brought into contact with the concrete contained in the concrete structure to perform carbon dioxide fixation (carbonation of concrete). There are no limitations on the type, shape, use, or installation location of the concrete structure, and any existing concrete structure can be used. Examples of existing concrete structures include building columns and foundation beams, bridge piers and abutments, embankments and breakwaters installed in rivers and harbors, wave-dissipating blocks, and lining concrete used in roads and tunnels. Alternatively, movable property (concrete products) containing concrete, such as rectangular or U-shaped concrete blocks or foundation stones, may also be used.

2. Method for fixing carbon dioxide 2-1. Formation of gas injection holes Figure 1A shows a schematic perspective view of the concrete structure 100. The concrete structure 100 shown here schematically represents all or part of various concrete structures, and as mentioned above, there are no limitations on the type, shape, or use of the concrete structure.

In one embodiment of the present invention, a method for fixing carbon dioxide is used, in which first, gas injection holes 104 are formed in the concrete 102 constituting the concrete structure 100. As shown in the schematic cross-sectional views (Figures 1B and 1C), the gas injection holes 104 may be through holes penetrating the concrete structure 100 from one surface to the opposite surface of the concrete 102 (Figure 1B), or they may be closed holes with one end closed (Figure 1C). The gas injection holes 104 may be formed to extend linearly, or they may include a bent shape, although these are not shown. Furthermore, as shown in Figures 2A and 2B, multiple gas injection holes 104 may be provided. In this case, the directions in which the multiple gas injection holes 104 extend may be the same, or the extension direction of at least one gas injection hole 104 may be different from that of the other gas injection holes 104. As shown in Figure 2A, the extension direction of all gas injection holes 104 may be perpendicular to the outer surface of the concrete structure 100, or, as shown in Figure 2B, some or all of the gas injection holes 104 may extend in a direction inclined to the outer surface of the concrete structure 100. Although not shown, multiple gas injection holes 104 may intersect. That is, the gas injection holes 104 may be formed in a network shape within the concrete 102.

The surface on which the gas injection hole 104 is provided is not limited to a surface perpendicular to the horizontal plane; it may also be a surface parallel to the horizontal plane (for example, the top surface of the concrete structure 100), or the gas injection hole 104 may be provided on a surface inclined at an angle of less than 90° from the horizontal plane.

The cross-sectional area of the gas injection hole 104, that is, the opening area of the gas injection hole 104 on the outer surface of the concrete 102, can be set arbitrarily. For example, if the cross-sectional shape of the gas injection hole 104 (shape on the outer surface of the concrete structure 100) is circular, its diameter may be 1 mm or more and 100 mm or 2 mm or more and 150 mm or less, and the opening area may be 0.785 ^mm² or more and 78.5 ^cm² or less and 3.14 ^mm² or more and 177 ^cm² or less. By selecting the above range, the gas injection hole 104 can be formed without significantly impairing the strength or aesthetics of the concrete structure 100.

The gas injection hole 104 can be formed by using a drill, such as a rotary hammer drill or a hammer drill, to work from the outer surface of the concrete 102 inward.

Alternatively, the gas injection holes 104 may be formed at the same time as the concrete structure 100 is formed. Specifically, a formwork 150 is made to determine the shape of the concrete structure 100 (Figure 3A). At this time, one or more core materials 152 are placed inside the formwork 150 to form a space in the concrete 102 corresponding to the gas injection holes 104 (Figures 3A and 3B). The core materials 152 may be provided so that a part of them is exposed from the formwork 150. The shape of the core material 152 can be determined considering the shape of the gas injection holes 104, and it may be a straight rod shape, or it may be partially or entirely bent. Also, the core material 152 may be a hollow tube or it may not be hollow (solid). There are no restrictions on the material contained in the core material 152, and it may be a metal material such as aluminum, iron, or stainless steel, wood, or resin. As for the resin, it may be a fiber-reinforced plastic composite with fibers such as glass fiber or carbon fiber. A release agent (mold release agent) or a hardening retarder may be applied to the outer surface of the core material 152. By applying a release agent or hardening retarder, the core material 152 can be easily removed from the concrete 102 to form the gas injection hole 104, as described later.

Next, concrete 102 is poured. That is, ready-mixed concrete 154 is poured into the formwork 150 (Figures 4A and 4B). At this time, the ready-mixed concrete 154 is poured so that at least a portion of the core material 152 is embedded in the formwork 150 by the ready-mixed concrete 154. The ready-mixed concrete 154 contains at least water and cement, but may also contain aggregates such as sand, gravel, pebbles, rocks, crushed stone, and crushed sand, as well as additives such as AE agents (air-enhancing agents), fluidizers, thickeners, and quick-setting agents.

The structure 100 is formed by hardening the ready-mixed concrete 154 and removing the formwork 150. Furthermore, by removing the core material 152, gas injection holes 104 can be formed in the concrete structure 100 (Figure 5). However, it is not always necessary to remove the core material 152. For example, when forming gas injection holes 104 in a network pattern, it may be difficult to remove the core material 152. In such cases, a porous material or a tubular core material 152 with fine openings can be used, and carbon dioxide can be brought into contact with the concrete 102 through the core material 152.

2-2. Introduction of Carbon Dioxide Next, in order to fix carbon dioxide in the concrete 102, a gas containing carbon dioxide is supplied to the gas injection hole 104, and the carbon dioxide is brought into contact with the side wall of the gas injection hole 104. Specifically, a carbon dioxide supply source 120 is connected to one end (inlet) of the gas injection hole 104 via a carbon dioxide line 122, and the gas containing carbon dioxide is introduced into the gas injection hole 104 (Figure 6A). Although not shown, if multiple gas injection holes 104 are provided, the same number of carbon dioxide supply sources 120 as the number of gas injection holes 104 may be used, and the carbon dioxide supply source 120 corresponding to each gas injection hole 104 may be connected, or a number of carbon dioxide supply sources 120 fewer than the number of gas injection holes 104 may be connected to the gas injection holes 104 using branched carbon dioxide lines 122. The carbon dioxide line 122 may be equipped with a pressure gauge 124 and/or a flow meter 125 for measuring the pressure of the gas containing carbon dioxide introduced into the gas injection hole 104 (i.e., the pressure inside the gas injection hole 104).

The method of connecting the carbon dioxide line 122 to the gas injection hole 104 can be arbitrarily selected; it is sufficient to simply insert the tip of the carbon dioxide line 122 into the gas injection hole 104. Alternatively, to maintain a stable connection, for example, as shown in Figure 7A, an adapter 110 with an opening having a female thread structure can be attached to the injection port, and a joint 126 having a male thread structure that engages with the female thread structure can be attached to the tip of the carbon dioxide line 122. By screwing the joint 126 into the adapter 110, the carbon dioxide line 122 can be reliably connected to the gas injection hole 104. As an optional configuration, a resin O-ring (or packing) 112 may be provided between the adapter 110 and the concrete 102 to prevent leakage of gas containing carbon dioxide.

If the gas injection hole 104 is a through hole, the other end (discharge port) can be capped to seal in the gas containing carbon dioxide, thereby preventing leakage of the gas containing carbon dioxide and enabling more effective contact between the concrete 102 and carbon dioxide. For example, the discharge port may be sealed using adhesive tape, an elastic material such as rubber, or, as shown in Figures 6B and 7A, the gas injection hole 104 may be sealed using a plate 106 containing a resin such as acrylic resin, epoxy resin, polyester resin, or polyimide resin, or a metal material such as iron, stainless steel, or aluminum. Fiber-reinforced plastic may be used as the resin. The plate 106 may be fixed using bolts or screws, or a plate 106 that matches the shape of the discharge port may simply be inserted into the discharge port. Alternatively, the plate 106 may be fixed using adhesive or adhesive tape. Furthermore, as shown in Figure 8A, in order to improve the airtightness of the gas injection hole 104, the outlet may be closed with a packing 108 containing an elastic material such as rubber, and a plate 106 may be provided so as to sandwich the packing 108 between the concrete 102.

The gas containing carbon dioxide may be pure carbon dioxide (e.g., 99% purity or higher) or a mixture of carbon dioxide and another gas. When using a mixture, the other gas may be air, oxygen, nitrogen, etc. The concentration of carbon dioxide in the mixture can be set arbitrarily, but to efficiently bring the concrete 102 into contact with the carbon dioxide, it is preferable that it be higher than the concentration of carbon dioxide in the atmosphere (approximately 420 ppm). For example, the carbon dioxide concentration can be set to any concentration between 1% and 100% by volume, 10% and 50% by volume, or 10% and 20% by volume.

The carbon dioxide supply source 120 only needs to have the function of supplying gas containing carbon dioxide to the gas injection hole 104, and examples include gas cylinders or tanks containing carbon dioxide as shown in Figure 6A. The carbon dioxide supply source 120 is connected to a regulator (not shown) to regulate the pressure of the gas containing carbon dioxide. Alternatively, if there is a facility that emits a large amount of carbon dioxide (chemical plant, waste incineration plant, thermal power plant, and various other factories) already located near the concrete structure 100, the gas emitted from these facilities, or purified carbon dioxide obtained by dedusting, desulfurizing, denitrifying, etc., of the exhaust gas may be used. In this case, since these facilities function as the carbon dioxide supply source 120, the cost of transporting carbon dioxide is reduced, and further emissions of carbon dioxide associated with transportation are prevented.

The gas containing carbon dioxide should be introduced so that the pressure inside the gas injection port 104 is higher than 0 MPa and 2 MPa or less. If the pressure inside the gas injection port 104 is lower than atmospheric pressure (for example, 1 atmosphere or 0.101 MPa), a vacuum pump 130 such as a rotary oil pump or dry pump should be connected to the carbon dioxide line 122 to reduce the pressure inside the gas injection port 104 before introducing the gas containing carbon dioxide. Conversely, if a pressure increase is required, a compressor or the like may be used.

The gas containing carbon dioxide may be introduced continuously or intermittently during the period of carbon dioxide fixation. In the latter case, after supplying the gas containing carbon dioxide, the carbon dioxide line 122 can be removed from the gas injection port 104, and the injection port can be closed to seal the gas injection port 104. For example, the injection port may be sealed with adhesive tape, or an elastic material such as rubber may be inserted into the injection port to seal it. Alternatively, as shown in Figure 8B, the gas injection port 104 may be sealed using a cap 114 having a male screw structure that engages with the adapter 110. By sealing the gas injection port 104, carbon dioxide can be reliably retained inside the gas injection port 104, and leakage of carbon dioxide can be prevented.

As an optional configuration, a concentration meter 128 for measuring carbon dioxide concentration may be installed inside the gas injection port 104 (for example, on the side wall or cap 114 of the gas injection port 104), and the carbon dioxide concentration may be measured periodically. This allows for monitoring of changes in carbon dioxide concentration.

2-3. Humidity Adjustment The rate of carbon dioxide fixation in concrete also depends on humidity, and it is known that carbon dioxide is fixed at a high rate when the humidity is about 50%. For this reason, in one embodiment of the present invention, the humidity inside the gas injection hole 104 may be adjusted. Specifically, a water supply source 140 may be provided, and water may be supplied into the gas injection hole 104 via the carbon dioxide line 122 (Figure 6A). Although not shown, the water supply source 140 may be equipped with a heating device or a cooling device to control the temperature of the supplied water. Alternatively, gas containing carbon dioxide may be supplied from the carbon dioxide supply source 120 to the water supply source 140, and water containing carbon dioxide may be supplied to the gas injection hole 104.

As an optional configuration, a hygrometer 129 for measuring humidity may be installed inside the gas injection hole 104 (for example, on the side wall or cap 114 of the gas injection hole 104), and the humidity may be measured periodically (Figure 7B). This allows for monitoring of changes in humidity.

On the other hand, if the humidity inside the gas injection hole 104 is high, if a large amount of water is adsorbed onto the side wall of the gas injection hole 104, if the side wall of the gas injection hole 104 is frozen, or if the concrete 102 contains a large amount of water, the inside of the gas injection hole 104 may be depressurized using a vacuum pump 130 to remove some of the water inside the gas injection hole 104. After that, gas containing carbon dioxide can be supplied into the gas injection hole 104.

The contact time between the concrete 102 and the gas containing carbon dioxide depends on the length and cross-sectional area (i.e., the volume of the gas injection hole 104), the temperature, and the concentration of carbon dioxide in the gas containing carbon dioxide, but it can be set to, for example, 1 hour to 20 years, 1 day to 10 years, 10 weeks to 5 years, or 1 year to 3 years.

The supply of carbon dioxide and water may be carried out using a control device 142, as shown in Figure 7A. The control device 142 is supplied with carbon dioxide from a carbon dioxide supply source 120 and water from a water supply source 140, respectively. The control device 142 includes a mechanism (e.g., a blower pump) for supplying a gas containing carbon dioxide to the gas injection port 104. The control device 142 may also be configured to prepare a gas containing carbon dioxide with appropriate humidity using the supplied water and carbon dioxide, and to supply this gas to the gas injection port 104. Alternatively, the control device 142 may be configured to control the temperature of the gas containing carbon dioxide supplied to the gas injection port 104. By providing such a function to the control device 142, a gas containing carbon dioxide can be supplied into the gas injection port 104 at a temperature and humidity that yields a high carbon dioxide fixation rate.

Furthermore, as shown in Figure 7B, the gas containing carbon dioxide may be circulated between the control device 142 and the gas injection port 104. In this case, the control device 142 is equipped with a mechanism, such as a circulation pump, to recover the gas discharged from the gas injection port 104 and supply it back to the gas injection port 104. The control device 142 may also be configured to measure the concentration and humidity of carbon dioxide contained in the gas discharged from the gas injection port 104 and to add carbon dioxide or water to the gas as appropriate based on the obtained data. This makes it possible to maintain conditions in the gas injection port 104 that allow for an optimal carbon dioxide fixation rate on a steady basis.

Through the steps taken so far, carbon dioxide is fixed in the concrete 102.

2-4. Concrete Placement into Gas Injection Holes As an optional step, new repair concrete 103 may be placed into the gas injection holes 104 of the concrete structure 100 in which carbon dioxide has been fixed. Specifically, as shown in Figures 9A to 9C, ready-mixed concrete is filled into the gas injection holes 104 and allowed to harden. In the concrete structure 100, the repair concrete 103 extending into the interior of the concrete 102 is surrounded by the concrete 102. The area of the cross-section of the repair concrete 103 (the cross-section perpendicular to the direction in which the repair concrete 103 extends) is the same as or substantially the same as the opening area of the gas injection hole 104, for example, 0.785 ^mm² or more and 177 ^cm² or less.

The repair concrete 103 may be poured over the entire gas injection hole 104, or, although not shown, it may be poured only over a portion of the injection port side. By pouring the repair concrete 103, it is possible to compensate for the structural strength lost by forming the gas injection hole 104, and to maintain the aesthetic appearance of the concrete structure 100.

As will be explained in more detail later, the concrete 102 contained in the concrete structure 100 has an increased calcium carbonate composition because carbon dioxide is fixed from the side walls of the gas injection holes 104. Therefore, the compositions of concrete 102 and repair concrete 103 may differ. In this case, the concentration of calcium carbonate is higher in the former, while the concentration of calcium hydroxide is higher in the latter.

2-5. Concrete Structure with Fixed Carbon Dioxide When carbon dioxide is brought into contact with concrete 102 contained in concrete structure 100 using the method described above, calcium hydroxide, calcium silicate (C-S-H), monosulfate, ettringite, and unhydrated cement contained in concrete 102 react with carbon dioxide and change into calcium carbonate. As a result, the introduced carbon dioxide is fixed in concrete 102 as calcium carbonate. This reaction is also called carbonation, and it first occurs from the side wall of the gas injection hole 104 and proceeds from the side wall into the interior of concrete 102. Therefore, although it depends on the density and local composition of concrete 102, as shown in the cross-sectional view perpendicular to the extension direction of the gas injection hole 104 (Figure 10A), carbonation proceeds isotropically in principle in the direction perpendicular to the extension direction of the gas injection hole 104 (i.e., in the xz plane) (see solid arrow in Figure 10A). The degree of carbonation is higher the closer to the gas injection hole 104. Therefore, the concrete 102 has a first zone 102a (the zone enclosed by the dashed circle in Figure 10A) which has a relatively high calcium carbonate concentration and constitutes the side wall of the gas injection hole 104, and a second zone 102b (the zone outside the circle) which has a lower calcium carbonate concentration than that of the first zone 102a and surrounds the first zone 102a. The first zone 102a and the second zone 102b are in contact with each other.

Therefore, as schematically shown in Figure 10B, in the first zone 102a, the calcium carbonate concentration in the concrete 102 continuously decreases as the distance from the side wall of the gas injection hole 104 increases in the direction perpendicular to the y direction, i.e., any direction in the xz plane (see arrow a in Figure 10A, for example). When filling with repair concrete 103, the calcium carbonate concentration in the concrete 102 continuously increases as the distance from the interface between the repair concrete 103 and the concrete 102 increases. The zone where this increase stops and the calcium carbonate concentration becomes constant or substantially constant is the second zone 102b. Although not shown, the calcium carbonate concentration is constant or substantially constant within the repair concrete 103.

On the other hand, since calcium hydroxide is consumed in the reaction between concrete and carbon dioxide, in the first zone 102a, the concentration of calcium hydroxide in concrete 102 increases continuously in any direction in the xz plane as the distance from the side wall of the gas injection hole 104 increases. When filling with repair concrete 103, the concentration of calcium hydroxide in concrete 102 decreases continuously as the distance from the interface between the repair concrete 103 and concrete 102 increases. In the second zone 102b, virtually no increase in the concentration of calcium hydroxide is observed, and the concentration of calcium hydroxide is constant or substantially constant. Although not shown, the concentration of calcium hydroxide is constant or substantially constant within the repair concrete 103.

Similarly, in the direction from the outer surface of concrete 102 toward the opposite outer surface (see arrow b in Figure 10A), the calcium carbonate concentration plot against distance from the outer surface can show three stages. Specifically, as shown in Figure 10C, the first stage corresponds to the second zone 102b from the outer surface to the interface between the first zone 102a and the second zone 102b, where the concentrations of calcium carbonate and calcium hydroxide are constant or substantially constant. The second stage corresponds to the first zone 102a from passing through the interface between the first zone 102a and the second zone 102b to the interface between the first zone 102a and the second zone 102b again, where the concentration of calcium carbonate continuously increases and then continuously decreases, and the concentration of calcium hydroxide continuously decreases and then continuously increases. The third stage corresponds to the second zone 102b, which is reached after passing through the interface between the first zone 102a and the second zone 102b for the second time, and is the outer surface of the concrete structure 100, where the concentrations of calcium carbonate and calcium hydroxide are constant or substantially constant.

In the carbon dioxide fixation method described above, the concrete 102 of the concrete structure 100 can come into contact with carbon dioxide at an extremely high concentration compared to the carbon dioxide concentration in the atmosphere within the gas injection hole 104. Therefore, the carbon dioxide introduced into the gas injection hole 104 can efficiently come into contact with the concrete 102. Furthermore, by sealing the gas injection hole 104, leakage of carbon dioxide is prevented, ensuring a high level of safety during operation.

Cement, the raw material for concrete 102, releases a large amount of carbon dioxide during its manufacture. However, by applying this carbon dioxide fixation method, concrete 102 will fix a large amount of carbon dioxide. Therefore, the carbon dioxide fixation method according to the embodiment of the present invention can contribute to reducing carbon dioxide in the atmosphere and mitigating global warming. Furthermore, concrete 102 can have a high concentration of calcium carbonate, which is produced by the reaction between calcium hydroxide, generated during the hydration of cement, and carbon dioxide. As a result of fixing carbon dioxide, the density of concrete 102 increases, and consequently, its compressive strength increases. In fact, the inventors have confirmed that when approximately 20% (60 kg/ ^m³ ) of the cement's carbon dioxide is fixed, the compressive strength of the concrete increases by about 8% to 10%. Therefore, by applying this carbon dioxide fixation method, it is also possible to increase the strength of existing concrete structures 100.

Methods for fixing carbon dioxide in concrete include placing hardened concrete in a curing tank filled with carbon dioxide and bringing it into contact with the concrete, bringing carbon dioxide into contact with the surface of hardened porous concrete, supplying carbon dioxide into formwork for pouring ready-mixed concrete, and bringing the concrete structure into contact with the atmosphere. However, methods using curing tanks require a curing tank to contain the concrete for carbon dioxide fixation, making them unsuitable for structures such as buildings, columns, and tunnels. Methods involving bringing carbon dioxide into contact with the porous concrete surface also require a means of sealing the concrete surface, making them impractical for large concrete structures. Methods supplying carbon dioxide into formwork require the formwork to be formed as a closed space, making them suitable for small structures but unsuitable for buildings. Furthermore, methods utilizing contact with the atmosphere are inefficient due to the extremely low concentration of carbon dioxide in the air.

In contrast, the carbon dioxide fixation method according to one embodiment can be applied to various existing concrete structures 100 without any particular restrictions on type, size, shape, use, or construction site, as long as gas injection holes 104 can be formed. Furthermore, since there is no need to use new cement except for the step of filling with repair concrete 103, there is almost no need to directly or indirectly utilize the cement production step that generates large amounts of carbon dioxide. This means that not only is it unnecessary to create a new reaction substrate for carbon dioxide fixation, but a vast amount of reaction substrate already exists on the ground. Therefore, the carbon dioxide fixation method according to one embodiment can fix an extremely large amount of carbon dioxide and can be said to be useful as an effective tool for curbing global warming.

3. Application Example 3-1. Application to Tunnel Lining Concrete As described above, the carbon dioxide fixation method according to one embodiment of the present invention can be applied to various concrete structures 100. For example, as shown in Figure 11A and its enlarged view Figure 11B, the method can be applied even if the concrete structure 100 is a tunnel, and carbon dioxide can be fixed using the concrete (tunnel lining concrete) 102 that constitutes the inner wall of the tunnel. In this case, a plurality of gas injection holes 104 are formed in the concrete 102, and gas containing carbon dioxide is supplied to the gas injection holes 104. The extension direction of the gas injection holes 104 is arbitrary and may be in the vertical direction (z direction), or it may be inclined from the vertical direction. For example, it may be parallel to the normal of the inner wall of the tunnel, or the gas injection holes 104 may be formed so as to be inclined from the normal. Also, the gas injection holes 104 may be through holes penetrating the concrete 102, or they may be bottomed holes. Furthermore, even if the gas injection hole 104 is a through hole, its outlet is in bedrock or ground in contact with the concrete 102, so it is not necessary to close the outlet when supplying gas containing carbon dioxide.

3-2. Application to Concrete Structures Including Reinforcement This carbon dioxide fixation method can also be applied to concrete structures containing reinforcement (reinforced concrete structures). For example, when fixing carbon dioxide to a concrete structure 100 having multiple reinforcing bars 160 and concrete 102 poured to embed the reinforcing bars 160, as shown in Figure 12A, one or more gas injection holes 104 are formed so as to avoid the reinforcing bars 160. That is, each gas injection hole 104 is provided so that the reinforcing bars 160 are not exposed within the gas injection hole 104. In the example shown in Figure 12B, multiple linear gas injection holes 104 are provided so as to extend in the y direction between adjacent reinforcing bars 160.

If the concrete structure 100 is a building, this carbon dioxide fixation method can be applied to the reinforced concrete that makes up the columns and beams. When applied to columns, gas injection holes 104 are provided so as to avoid the reinforcing bars 160 (see Figure 13A) that make up the main reinforcement bars and lateral reinforcement bars of the column (Figure 13B).

Concrete is alkaline, but when it reacts with carbon dioxide to produce calcium carbonate, it gradually becomes acidic. When concrete becomes acidic, corrosion of the reinforcing steel occurs, and the expansion of the reinforcing steel due to corrosion can induce deterioration such as cracking and damage to the concrete. Therefore, when using a concrete structure 100 containing reinforcing steel, it is preferable that carbon dioxide is not fixed in the concrete located near the reinforcing steel.

In a concrete structure 100 containing reinforcing bars, the reinforcing bars are not placed in the center of the concrete structure 100, but rather in a zone relatively close to the outer surface. For example, as shown in Figure 12A, the reinforcing bars 160 are positioned to enclose the central part of the concrete structure, and the concrete 102 is constructed to embed the reinforcing bars 160. The same applies to reinforced concrete columns; among the reinforcing bars 160, those parallel to the extension direction of the column (main column reinforcement) are positioned to surround the central axis of the column, while horizontally extending reinforcing bars, called lateral reinforcement bars, are positioned to surround the main column reinforcement. The part of the concrete 102 outside of the reinforcing bars is called cover concrete.

Therefore, to prevent carbon dioxide fixation into the concrete near the reinforcing bars, a protective tube 162 may be provided to cover the portion of the side wall of the gas injection hole 104 that is made up of concrete cover, as shown in Figure 14. The protective tube 162 can be provided on the injection side and/or outlet side of the gas injection hole 104, and its end reaches the outer surface of the concrete structure 100. Therefore, a portion of the side wall covered by the protective tube 162 reaches one end (injection or outlet) of the gas injection hole 104. Although not shown, a portion of the protective tube 162 may protrude outward from the gas injection hole 104.

The length L of the protective tube 162 in the direction in which the gas injection hole 104 extends is preferably equal to or greater than the thickness of the concrete cover. More specifically, the length L of the protective tube 162 is preferably equal to or greater than the sum S of the shortest distance D from the reinforcing bar 160 located in the innermost part of the concrete structure 100 to the outer surface of the concrete 102 and the maximum length of the cross-section of the reinforcing bar 160 (e.g., the diameter d of the cross-section). Alternatively, the length L is preferably selected from a range of 2 to 5 times or 1.5 to 3 times the sum S.

There are no restrictions on the materials used in the protective tube 162; for example, it may be made of metal materials such as iron, aluminum, or stainless steel, resins such as epoxy resin, silicone resin, or acrylic resin, or wood. For example, the protective tube 162 may be formed by applying an epoxy adhesive or an acrylic adhesive to the inlet and outlet sides of the gas injection hole 104 to a length L, and then allowing it to cure.

By positioning the protective tube 162 and preventing contact between the side wall of length L and carbon dioxide from the inlet and outlet of the gas injection hole 104, the side wall exposed from the protective tube 162 selectively comes into contact with carbon dioxide. As a result, carbon dioxide fixation (carbonation) begins from the side wall exposed from the protective tube 162, and then the carbonation progresses from the side wall into the concrete 102. As a result, as shown in Figure 14, it is possible to prevent the first zone 102a where carbon dioxide is fixed from expanding to the reinforcing bar 160, and the reinforcing bar 160 can be kept in the second zone 102b where carbon dioxide is not fixed (i.e., not carbonated). Since the reinforcing bar 160 is included in the second zone 102b, corrosion of the reinforcing bar 160 and the resulting deterioration of the concrete 102 can be prevented.

When using the protective tube 162, the repair concrete 103 may be poured after removing the protective tube 162, or it may be poured with the protective tube 162 remaining in the gas injection hole 104. In the latter case, the concrete structure 100 includes the protective tube 162, and the protective tube 162 covers at least a portion of the surface of the repair concrete 103.

In this embodiment, as a model experiment of a carbon dioxide fixation method according to one embodiment of the present invention, carbon dioxide was supplied to a gas injection hole provided in a rectangular prism-shaped concrete, and the carbonation of the concrete was evaluated.

1. Example 1-1. Preparation of concrete with gas injection holes A formwork with an internal volume of 100 mm × 100 mm × 400 mm was prepared, and a metal rod with a cross-sectional diameter of 9 mm and a length of 400 mm was placed parallel to the longitudinal direction of the formwork. The metal rod was positioned so as to pass through the center of the cross-section perpendicular to the longitudinal direction of the internal volume of the formwork. Ready-mixed concrete was poured into this formwork. The ready-mixed concrete was prepared using ordinary Portland cement manufactured by Taiheiyo Cement Corporation, with a unit water content of 170 kg/ ^m³ and a unit cement content of 340 kg/ ^m³ (water-cement ratio of 50%). After 24 hours, the formwork and metal rod were removed to obtain concrete 102. A schematic perspective view of concrete 102 is shown in Figure 15. As shown in Figure 15, concrete 102 had a volume of 100 mm × 100 mm × 400 mm and had through holes parallel to its longitudinal direction as gas injection holes 104.

1-2. Carbon Dioxide Fixation A carbon dioxide cylinder was connected to one end (inlet) of the gas injection hole 104 via a rubber hose, and carbon dioxide was continuously supplied to the gas injection hole 104 at a pressure of 0.1 MPa. After supplying carbon dioxide, the concrete was cut, and the properties of the pore solution in the cross section perpendicular to the longitudinal direction were evaluated. The properties were evaluated by spraying a 1% ethanol solution of phenolphthalein as an indicator onto the cross section and checking whether or not the indicator changed color.

Twenty-four hours after the start of carbon dioxide supply, as schematically shown in Figure 15B, a circular area with a diameter of approximately 60 mm centered on the gas injection hole 104 (a through-hole) did not show any coloration with the indicator and was colorless, while the area outside this region showed red coloration with the indicator. Seventy-two hours after the start of carbon dioxide supply, the entire cross-section did not show any coloration with the indicator and was colorless.

2. Comparative Example On the other hand, as a comparative example, concrete of the same size but without the gas injection holes 104 was prepared and placed in a constant temperature and humidity chamber under a relative humidity of 60% and a carbon dioxide concentration of 5% to evaluate carbonation. When carbonation was evaluated in the same manner as described above, after 4 weeks from the start of carbonation, a region up to 12.8 mm from the outer surface did not show coloration by the indicator, and after 8 weeks, a region up to 18 mm from the outer surface did not show coloration by the indicator, while red coloration by the indicator was observed in the other areas.

3. Discussion When the pH of the concrete pore solution exceeds approximately 10, the indicator turns red, and when the pH falls below 8.3, it remains colorless without showing any color change. Therefore, in the regions where red color change is observed, the pH of the pore solution exceeds 10, indicating that carbonation, i.e., carbon dioxide fixation, is not progressing, or that a sufficient amount of carbon dioxide is not being fixed. On the other hand, in the regions where no color change is observed, the pH of the pore solution is below 8.3, indicating that carbonation is progressing sufficiently.

The results of the example show that approximately 30% of the concrete was carbon dioxide fixed 24 hours after the start of carbon dioxide supply, and that carbon dioxide was fixed throughout the entire concrete after 72 hours. On the other hand, in the comparative example, approximately 45% was carbonized after 4 weeks (i.e., approximately 672 hours), but even after 8 weeks (approximately 1344 hours), only 59% was carbonized. From these results, it was confirmed that by providing gas injection holes in the concrete and supplying gas containing high concentration of carbon dioxide through the gas injection holes, carbon dioxide can be efficiently fixed without using a device to contain the entire concrete, such as a constant temperature bath. This indicates that the carbon dioxide fixation method according to one embodiment of the present invention is an extremely useful method for carbon dioxide fixation using concrete structures, especially large concrete structures.

The embodiments described above as embodiments of the present invention can be combined and implemented as appropriate, insofar as they do not contradict each other. Additions, deletions, or design modifications of components based on these embodiments, made by those skilled in the art, are also included within the scope of the present invention, as long as they retain the essence of the present invention.

Any effects or benefits other than those brought about by the embodiments described above, if they are clear from the description herein or easily predictable to those skilled in the art, are naturally understood to be brought about by the present invention.

100: Concrete structure, 102: Concrete, 102a: First zone, 102b: Second zone, 103: Repair concrete, 104: Gas injection hole, 106: Plate, 108: Packing, 110: Adapter, 112: O-ring, 114: Cap, 120: Carbon dioxide supply source, 122: Carbon dioxide line, 124: Pressure gauge, 125: Flow meter, 126: Joint, 128: Concentration meter, 129: Hygrometer, 130: Vacuum pump, 140: Water supply source, 142: Control device, 150: Formwork, 152: Core material, 154: Ready-mixed concrete, 160: Reinforcement, 162: Protective tube

Claims (15)

Forming gas injection holes in the concrete contained in the concrete structure that is to be carbonated ,
Introducing a gas containing carbon dioxide into the aforementioned gas injection port,
A method for fixing carbon dioxide, comprising: introducing the gas, capping one end of the gas injection hole to seal the gas in the gas injection hole, and pouring concrete into the gas injection hole after the gas has been sealed. Forming gas injection holes in the concrete contained in the concrete structure that is to be carbonated ,
A method for fixing carbon dioxide, comprising introducing a gas containing carbon dioxide into the gas injection hole, and pouring concrete into the gas injection hole after the introduction of the gas. The method according to claim 1 or 2, further comprising depressurizing the gas injection port before introducing the gas. The method according to claim 1 or 2, wherein the gas further comprises water. The aforementioned gas injection hole is a through hole,
The method according to claim 1 or 2, further comprising capping the other end of the gas injection hole before introducing the gas. The method according to claim 1 or 2, wherein the gas injection hole is a bottomed hole. The method according to claim 1 or 2, wherein the gas injection hole extends perpendicularly to the outer surface of the concrete structure. The method according to claim 1 or 2, wherein the gas injection hole extends in a direction inclined with respect to the outer surface of the concrete structure. The further includes covering a portion of the side wall of the gas injection hole with a protective tube, wherein the portion reaches the one end of the gas injection hole.
The aforementioned concrete structure includes reinforcing steel,
The method according to claim 1, wherein the length of the protective tube in the direction in which the gas injection hole extends is greater than the sum of the shortest distance from the outer surface of the concrete structure to the reinforcing bar and the diameter of the reinforcing bar. The formation of the aforementioned gas injection hole is
Pouring ready-mixed concrete into a formwork in which the core material is placed, such that at least a portion of the core material is embedded.
The method according to claim 1 or 2, wherein the ready-mixed concrete is hardened and the core material is removed. The material comprises a first concrete, and a second concrete having a different composition from the first concrete, surrounded by the first concrete, and extending in a first direction from the outside to the inside of the first concrete.
The first concrete includes a first zone in contact with the second concrete, and a second zone surrounding the first zone.
The concentration of calcium carbonate in the first zone decreases continuously in a second direction perpendicular to the first direction as the distance from the interface between the first concrete and the second concrete increases.
A concrete structure in which the concentration of calcium carbonate in the second zone is constant in the second direction. The concrete structure according to claim 11, wherein the second zone includes reinforcing bars. The concrete structure according to claim 11, further comprising a protective tube that is in contact with the first concrete and covers at least a portion of the surface of the second concrete. The method according to claim 1 or 2, wherein the concrete in which the gas injection hole is formed is lining concrete. The method according to claim 1 or 2, wherein the concrete structure is selected from building columns, building foundation beams, bridge piers, column abutments, embankments, breakwaters, and wave-dissipating blocks.