Calcium carbonate production method, concrete production method, and concrete composition

Abstract

The purpose of the present invention is to provide a calcium carbonate production method, a concrete production method, and a concrete composition, with all of which it is possible to reduce water usage, simplify production steps, and obtain a concrete having sufficient strength by using obtained calcium carbonate as a cement auxiliary raw material. One aspect of the present invention pertains to a calcium carbonate production method comprising a step for obtaining calcium carbonate by bringing a gypsum-containing material into contact with a solution containing an alkali metal salt which is at least one of an alkali metal carbonate and an alkali metal bicarbonate. In the method, an antifoaming agent is added at a certain time point during the step for obtaining calcium carbonate.

Description

Method for producing calcium carbonate, method for producing concrete, and concrete composition

[0001] The present invention relates to a method for producing calcium carbonate, a method for producing concrete, and a concrete composition.

[0002] Conventionally, waste gypsum board is crushed after removing paper materials on the surface, and then used as a ground improvement material or an auxiliary raw material for cement. However, when used as a ground improvement material, it has problems such as difficulty in obtaining strength and fluorine elution. In addition, as an auxiliary raw material for cement, it has problems such as hydrogen sulfide generation. Therefore, most waste gypsum board is discarded without being recycled. On the other hand, the discharge amount of gypsum board is expected to increase significantly year by year, and there is a demand for establishing an effective utilization method of waste gypsum board.

[0003] For example, Patent Document 1 discloses a method for producing Portland cement characterized by comprising: a crushing step of crushing waste gypsum board such that particles with a particle diameter of 5 mm or more account for 10% by weight or less to obtain a crushed product; a water washing step of washing the crushed product with water to obtain recovered gypsum; and a mixing step of mixing the recovered gypsum with a pulverized cement clinker.

[0004] On the other hand, Patent Document 2 describes a method for producing calcium carbonate by extracting calcium components contained in waste gypsum board and reacting the extracted calcium components with carbon dioxide.

[0005] Japanese Patent Laid-Open No.2002-255598; Japanese Patent Laid-Open No.2023-051282

[0006] For example, when reusing gypsum-containing materials such as waste gypsum board, and using them as an auxiliary material for concrete such as calcium carbonate, a decrease in strength may occur due to components other than gypsum contained in the gypsum-containing material. Waste gypsum board contains organic matter, and when it is to be reused as an auxiliary material such as mortar, the remaining organic matter derived from the gypsum board may cause a decrease in strength. Patent Document 1 describes a method of reducing the organic matter (surfactant) contained in the recovered gypsum by crushing the waste gypsum board and washing the resulting crushed material with water. However, this washing requires a large amount of water, which increases manufacturing costs. In addition, the number of steps, such as the crushing process and the washing process, is large, making the operation complicated.

[0007] Similarly, when the calcium carbonate obtained in Patent Document 2 is used as an auxiliary material for concrete, a decrease in strength may occur due to components other than gypsum contained in the gypsum-containing material. Furthermore, although the surfactants contained in the gypsum-containing material are reduced by washing with water, as is done with waste gypsum board used as a raw material, a large amount of water is required, which increases manufacturing costs.

[0008] This invention has been made in view of the above circumstances, and aims to provide a method for producing calcium carbonate, a method for producing concrete, and a concrete composition that reduce the amount of water used, have a simple manufacturing process, and allow for the production of concrete with sufficient strength by using the resulting calcium carbonate as a cement auxiliary material.

[0009] Through their research, the inventors discovered that when calcium carbonate produced from gypsum-containing materials such as waste gypsum board is mixed into cement as a cement auxiliary ingredient, the strength of the cement decreases. They determined that this is due to organic components such as surfactants contained in the waste gypsum board remaining in the calcium carbonate. Therefore, the inventors diligently studied to solve the above problem and conceived the idea of ​​using an antifoaming agent when producing calcium carbonate from gypsum-containing materials such as waste gypsum board, leading to the present invention.

[0010] To solve the above problems, the present invention provides the following methods for producing calcium carbonate, methods for producing concrete, and concrete compositions.

[0011] [1] A method for producing calcium carbonate, comprising the step of contacting a gypsum-containing material with a solution containing at least one alkali metal salt, an alkali metal carbonate, and an alkali metal bicarbonate, to obtain calcium carbonate, wherein an antifoaming agent is added at any point during the step of obtaining the calcium carbonate. [2] The method for producing calcium carbonate according to [1], wherein the antifoaming agent is a nonionic antifoaming agent. [3] The method for producing calcium carbonate according to [1] or [2], further comprising the step of drying the calcium carbonate at a heating temperature of 160°C or lower after the step of obtaining the calcium carbonate. [4] The method for producing calcium carbonate according to [3], wherein the heating temperature is 110°C or lower. [5] The method for producing calcium carbonate according to any one of [1] to [4], wherein the content of the antifoaming agent is 0.5% by mass or more and 15% by mass or less with respect to the total amount of the reaction solution of the gypsum-containing material and the solution containing the alkali metal salt. [6] A method for producing calcium carbonate according to any one of [1] to [5], wherein in the step of obtaining the calcium carbonate, the defoaming agent is added immediately after contacting the gypsum-containing material with the solution containing the alkali metal salt. [7] A method for producing calcium carbonate according to any one of [1] to [6], wherein the gypsum-containing material is waste gypsum board.

[0012] [8] A method for producing concrete, comprising the step of mixing calcium carbonate derived from gypsum, an antifoaming agent, cement, aggregate and water. [9] The method for producing concrete according to [8], wherein the gypsum is waste gypsum board.

[0013]

[10] A concrete composition comprising calcium carbonate derived from gypsum, an antifoaming agent, cement, aggregate, and water.

[0014] According to the present invention, it is possible to provide a method for producing calcium carbonate, a method for producing concrete, and a concrete composition that reduce the amount of water used, have a simple manufacturing process, and can produce concrete with sufficient strength by using the resulting calcium carbonate as a cement auxiliary material.

[0015] These are the IR spectra of calcium carbonate obtained in Examples 1-3 and untreated calcium carbonate when heated at 450°C under a nitrogen atmosphere.

[0016] The following describes embodiments of the present invention (which may be referred to as "these embodiments"). The present invention is not limited to the following embodiments and can be modified and implemented as such without hindering the effects of the invention. In this specification, the numerical range notation "AA to BB" means "AA or greater and BB or less". In this specification, the numerical values ​​related to "greater than or equal to", "less than or equal to", and "~" in the description of numerical ranges can be any combination. For example, if a certain numerical range is described as "CC to DD" and "EE to FF", then numerical ranges such as "CC to FF" and "EE to DD" are also included.

[0017] [Method for producing calcium carbonate] The method for producing calcium carbonate in this embodiment includes the step of contacting a gypsum-containing material with a solution containing at least one alkali metal salt, such as an alkali metal carbonate or an alkali metal bicarbonate, to obtain calcium carbonate, and adding an antifoaming agent at some point during the process of obtaining the calcium carbonate.

[0018] In this embodiment, the method for producing calcium carbonate involves contacting a gypsum-containing material with a solution containing the alkali metal salt, and by adding an antifoaming agent at some point during the process of obtaining calcium carbonate, it is possible to obtain calcium carbonate that yields concrete with sufficient strength when used as a cement auxiliary material. Thus, the method for producing calcium carbonate in this embodiment reduces the amount of water used and simplifies the manufacturing process.

[0019] (Gypsum-containing material) Gypsum-containing material is a material containing gypsum, which is at least one of calcium sulfate and calcium sulfate hydrate. Examples include gypsum-containing products or their waste, such as gypsum board and waste gypsum board; recovered material from flue gas desulfurization treatment (also called "flue gas desulfurization gypsum," which is recovered material during flue gas desulfurization treatment in copper refining, for example); hydrofluoric acid gypsum (a by-product in the process of producing hydrogen fluoride); phosphate gypsum (a by-product in the process of producing wet phosphoric acid); titanium gypsum (a by-product in the process of producing titanium oxide); activated silicate gypsum (a by-product in the process of producing activated silica); and other by-products produced in the manufacturing process of chemical products. Among these, from the viewpoint of better demonstrating the effects of the present invention, gypsum-containing products or their waste, such as gypsum board and waste gypsum board, are preferred, and furthermore, considering the industrial advantage of reusing gypsum, waste gypsum board is more preferred.

[0020] For ease of handling, the average particle size of the gypsum-containing material should be 10 mm or less, 1.0 mm or less, and 0.5 mm or less. In this specification, the average particle size of the gypsum-containing material is measured by sieving, using a rotary tap type automatic sieving machine with standard sieves conforming to the provisions of JIS Z 8801:2019. The sieves are stacked in order from smallest to largest mesh size, and the sample remaining on each sieve is weighed. The particle size at which the cumulative total reaches 50% is taken as the average particle size.

[0021] The gypsum-containing material may be pre-ground using a pulverizer such as a roller mill to achieve the above particle size. Furthermore, when using waste gypsum board as the gypsum-containing material, it is preferable to remove any wallpaper or other surface coatings beforehand before grinding.

[0022] (Solution containing alkali metal salts) A solution containing alkali metal salts is a solution containing at least one alkali metal salt, such as an alkali metal carbonate or an alkali metal bicarbonate. Preferably, the alkali metals in the alkali metal salt are lithium, sodium, and potassium, and more preferably sodium and potassium. These alkali metals can be used individually or in combination. Considering ease of handling, it is preferable to use one alkali metal.

[0023] Regarding alkali metal carbonates and alkali metal bicarbonates, any alkali metal salt can be used, but from the viewpoint of more efficiently promoting the reaction between gypsum (calcium sulfate) and the alkali metal salt, it is preferable to use alkali metal carbonates. Preferred alkali metal carbonates include lithium carbonate, sodium carbonate, and potassium carbonate, and more preferably sodium carbonate and potassium carbonate.

[0024] Alkali metal carbonates can be used alone, in which case one alkali metal carbonate or multiple alkali metal carbonates may be used. The same applies when using alkali metal bicarbonates. Alkali metal carbonates and alkali metal bicarbonates may also be used in combination, in which case one alkali metal carbonate, one alkali metal bicarbonate, multiple alkali metal carbonates, or multiple alkali metal bicarbonates may be used. Considering ease of handling, it is preferable to use one alkali metal carbonate and one alkali metal bicarbonate.

[0025] The solution containing the alkali metal salt is preferably an aqueous solution. Using an aqueous solution has advantages in that it eliminates the need for special consideration regarding the specifications of the equipment used, and also in terms of environmental impact. When using an aqueous solution, there are no particular restrictions on the water used as the medium; various types of water can be used, such as tap water, distilled water, deionized water, or industrial water.

[0026] From the viewpoint of more efficiently carrying out the reaction, the concentration of the solution containing the alkali metal salt is preferably 0.01 to 0.15% by mass, more preferably 0.05 to 0.12% by mass, and even more preferably 0.8 to 1.0% by mass.

[0027] (Contact) In this process, calcium carbonate is obtained by contacting a gypsum-containing material with a solution containing an alkali metal salt. Since the gypsum-containing material is a solid and the solution containing the alkali metal salt is a liquid, the contact between the gypsum-containing material and the solution containing the alkali metal salt is a solid-liquid contact. This contact is carried out by adding the gypsum-containing material to the solution containing the alkali metal salt, or by adding the solution containing the alkali metal salt to the gypsum-containing material and mixing.

[0028] For contact between the gypsum-containing material and the solution containing the alkali metal salt, for example, a reactor equipped with a container capable of holding the gypsum-containing material and a supply port for supplying the solution containing the alkali metal salt, or a reactor equipped with a container capable of holding the solution containing the alkali metal salt and a supply port for supplying the gypsum-containing material, can be used. From the viewpoint of promoting contact between the gypsum-containing material and the solution containing the alkali metal salt and obtaining calcium carbonate more efficiently, it is preferable that the reactor be equipped with a stirrer.

[0029] The contact between the gypsum-containing material and the solution containing the alkali metal salt may be carried out in a continuous flow manner or in a batch manner.

[0030] The reaction temperature for the reaction that occurs when the gypsum-containing material comes into contact with the solution containing the alkali metal salt is not particularly limited, and may be, for example, between 10 and 100°C. From the viewpoint of more efficiently carrying out the reaction, it is preferably between 15 and 60°C, and more preferably between 20 and 45°C. The reaction time is not particularly limited, and may be, for example, between 0.5 and 5 hours. From the viewpoint of more efficiently carrying out the reaction, it is preferably between 1 and 3 hours.

[0031] (Addition of defoaming agent) In the calcium carbonate production method of this embodiment, it is necessary to add a defoaming agent at some point during the process of obtaining calcium carbonate. By adding a defoaming agent during the process of obtaining calcium carbonate, it is possible to obtain calcium carbonate that will produce concrete with sufficient strength when used as a cement auxiliary material. The defoaming agent may be added before contacting the gypsum-containing material with the solution containing the alkali metal salt, immediately after contacting the gypsum-containing material with the solution containing the alkali metal salt, or after contacting the gypsum-containing material with the solution containing the alkali metal salt and calcium carbonate has been formed. More specifically, the defoaming agent may be added to the solution containing the alkali metal salt and then the gypsum-containing material may be added, or the defoaming agent may be added immediately after adding the gypsum-containing material to the solution containing the alkali metal salt, or the defoaming agent may be added after mixing the gypsum-containing material with the solution containing the alkali metal salt and calcium carbonate has been formed. When adding the defoaming agent after the calcium carbonate has been formed, it is preferable to mix further after adding the defoaming agent. Among the above, adding an antifoaming agent immediately after contacting the gypsum-containing material with the alkali metal salt solution is preferable from the viewpoint of obtaining calcium carbonate that yields concrete with sufficient strength when used as a cement auxiliary material.

[0032] As an antifoaming agent, any type of antifoaming agent can be used, including ionic antifoaming agents (anionic antifoaming agents, cationic antifoaming agents, and amphoteric antifoaming agents) and nonionic antifoaming agents (nonionic antifoaming agents). However, from the viewpoint of obtaining calcium carbonate that yields concrete with sufficient strength when used as a cement auxiliary material, nonionic antifoaming agents are preferred.

[0033] Examples of nonionic defoamers include lower alcohol-based defoamers; mineral oil-based defoamers; polyalkylene glycol-based defoamers such as polyethylene glycol, polypropylene glycol, polybutylene glycol, and polyoxyethylene polyoxypropylene glycol; fatty acid ester-based defoamers such as sorbitan fatty acid esters, sucrose fatty acid esters, fatty acid monoglycerides, and polyglycerin fatty acid esters; ether-based defoamers such as polyalkylene glycol ethers; and silicone-based defoamers such as polydimethylsiloxane and polymethylphenylsiloxane. Among these, polyalkylene glycol-based defoamers and ether-based defoamers are preferred, and polyalkylene glycol-based defoamers are more preferred, from the viewpoint of obtaining calcium carbonate that yields concrete with sufficient strength when used as a cement auxiliary material. The defoamers may be the above nonionic defoamers used alone or in combination of several types.

[0034] Examples of commercially available nonionic antifoaming agents include "Sika Control 404 AER" manufactured by Sika Japan Co., Ltd.

[0035] The amount of the defoaming agent is preferably 0.5% to 15% by mass, more preferably 0.8% to 14% by mass, and even more preferably 0.8% to 12% by mass, relative to the total volume of the reaction solution of the gypsum-containing material and the alkali metal salt. When the amount of the defoaming agent is within the above range, calcium carbonate can be obtained that yields concrete with more sufficient strength when used as a cement auxiliary material.

[0036] (Calcium Carbonate) The calcium carbonate produced in this process is sparingly soluble in water and therefore forms a precipitate. The precipitated calcium carbonate may be recovered. Calcium carbonate can be recovered by methods such as decantation and filtration, and may also be dried by heating as described later. The recovered calcium carbonate can be used in cement compositions, mortar and concrete materials, fillers, building materials, etc.

[0037] [Drying Process] In this embodiment, the method for producing calcium carbonate preferably includes a step of drying the calcium carbonate at a heating temperature of 160°C or lower after the step of obtaining the calcium carbonate, from the viewpoint of better demonstrating the effects of the present invention. When the heating temperature when drying the calcium carbonate is 160°C or lower, the effect of the defoaming agent is better demonstrated, and it is possible to make it difficult for organic matter contained in gypsum to remain in the obtained calcium carbonate.

[0038] The heating temperature is preferably 150°C or lower, more preferably 120°C or lower, and even more preferably 110°C or lower. The lower limit of the heating temperature is not particularly limited, but is preferably 40°C or higher. The drying time is preferably 10 to 48 hours, more preferably 10 to 24 hours, and even more preferably 10 to 18 hours.

[0039] The drying equipment used to dry the calcium carbonate mentioned above is not particularly limited and may be batch or continuous. Examples of drying equipment include airflow dryers, band dryers, spray dryers, and rotary dryers. Among these, airflow dryers are preferred from the viewpoint of removing water more efficiently. An example of an airflow dryer is a device that supplies heated gas to a drying tube (which may also be a cylindrical tank) and supplies calcium carbonate (the material to be dried) to the drying tube.

[0040] From the viewpoint of better demonstrating the effects of the present invention, it is preferable that the calcium carbonate obtained by the calcium carbonate production method of this embodiment has an antifoaming agent attached to its surface or interior. The presence of an antifoaming agent on the calcium carbonate can be inferred, for example, by analyzing the calcium carbonate using a solid total organic carbon analyzer under a nitrogen atmosphere at a temperature range of 450°C to 500°C, and observing that the amount of carbon dioxide (mass%) of the obtained calcium carbonate is increased compared to the amount of carbon dioxide (mass%) of untreated calcium carbonate as a reference. The amount of carbon dioxide (mass%) of the calcium carbonate can be specifically observed by the method described in the examples.

[0041] From the viewpoint of further exerting the effect of the present invention, the adhesion rate of the antifoaming agent adhering to the calcium carbonate is preferably 0.2% by mass or more and 5.0% by mass or less, more preferably 0.3% by mass or more and 4.0% by mass or less, and still more preferably 0.4% by mass or more and 3.0% by mass or less.

[0042] [Concrete Composition] The concrete composition of the present embodiment contains calcium carbonate derived from gypsum, an antifoaming agent, cement, aggregate, and water. In the concrete composition of the present embodiment, concrete with sufficient strength can be obtained by using the calcium carbonate obtained by the method for producing calcium carbonate described above as the calcium carbonate derived from gypsum.

[0043] The calcium carbonate derived from gypsum described above may have an antifoaming agent attached thereto. The antifoaming agent contained in the concrete composition of the present embodiment is the same as that described in the method for producing calcium carbonate above. The total blending amount of calcium carbonate derived from gypsum and the antifoaming agent in the concrete composition is per 1 m ３ of the volume of the concrete composition, it may be 10 kg or more and 450 kg or less, may be 50 kg or more and 300 kg or less, and may be 100 kg or more and 250 kg or less.

[0044] The type of cement contained in the concrete composition of the present embodiment is not particularly limited, and examples thereof include ordinary Portland cement, high-early-strength Portland cement, moderate-heat Portland cement, and low-heat Portland cement. The blending amount of cement in the concrete composition is per 1 m ３ of the volume of the concrete composition, it may be 170 kg or more and 750 kg or less, may be 300 kg or more and 400 kg or less, and may be 340 kg or more and 380 kg or less.

[0045] The fine aggregate contained in the concrete composition of the present embodiment has a particle size of 5 mm or less, and for example, known materials such as river sand, mountain sand, land sand, crushed sand, sea sand, and silica sand can be used. The blending amount of fine aggregate in the concrete composition is per 1 m ３Per 1 m of the volume of the concrete composition, the content may be 500 kg or more and 1500 kg or less, may be 700 kg or more and 1000 kg or less, or may be 800 kg or more and 900 kg or less. The blending amount of coarse aggregate in the concrete composition is ３ Per unit, the content may be 500 kg or more and 1500 kg or less, may be 875 kg or more and 1025 kg or less, or may be 900 kg or more and 1000 kg or less.

[0046] [Method for Producing Concrete] The method for producing concrete according to the present embodiment includes a step of kneading calcium carbonate derived from gypsum, a defoaming agent, cement, aggregate and water. According to the method for producing concrete of the present embodiment, concrete with sufficient strength can be obtained because the aforementioned concrete composition containing calcium carbonate derived from gypsum, a defoaming agent, cement, aggregate and water is used. In consideration of the industrial advantage of reusing gypsum, the calcium carbonate derived from gypsum is preferably calcium carbonate derived from waste gypsum board.

[0047] In the step of kneading calcium carbonate derived from gypsum, a defoaming agent, cement, aggregate and water, the solid matter contained in the concrete composition and water are kneaded to produce a concrete kneaded product. The solid matter contained in the concrete composition may be prepared in advance and then mixed with water to obtain a concrete kneaded product, and there is no particular limitation as long as it is a method that can uniformly knead even if all raw materials including each solid matter contained in the concrete composition and water are mixed at once. The amount of water used for kneading can be changed depending on the type and blending of the materials used, so it is not uniquely determined. However, the water-cement ratio is preferably 25 mass% or more and 70 mass% or less, and more preferably 50 mass% or more and 65 mass% or less. In addition, there are no limitations on kneading conditions, the type of kneader, etc., and a conventional kneader can be used. The obtained concrete kneaded product can be cured by, for example, steam curing, underwater curing or the like to obtain concrete.

[0048] Next, the present invention will be specifically described by way of examples, but the present invention is not limited in any way by these examples.

[0049] The details of each component used in the production of calcium carbonate and mortar are as follows: • Alkali metal salt: Sodium carbonate; "Soda Ash Light," manufactured by Tokuyama Corporation • Gypsum content: Waste gypsum powder (pulverized using a ball mill after peeling the board paper from waste gypsum board; average particle size: 1 mm) • Antifoaming agent 1: Polyalkylene glycol derivative; "Sika Control 404 AER," manufactured by Sika Japan Co., Ltd. • Ordinary Portland cement (NC): Manufactured by Sumitomo Osaka Cement Co., Ltd. • Calcium carbonate: Calcium carbonate obtained in Examples 1-7 • Standard sand: Standard sand conforming to JIS R 5201:2015 "Physical testing methods for cement"

[0050] (Example 1: Production of calcium carbonate) 100 mL of an aqueous solution containing sodium carbonate (concentration: 11% by mass) was added to a stirring vessel, then 17 g of waste gypsum powder (crushed waste gypsum board) was added, followed immediately by 1 mL of a 10-fold dilution of defoaming agent 1 (polyalkylene glycol derivative) (defoaming agent content relative to the total volume of reaction solution: 1% by mass), and the mixture was stirred at a temperature of 20°C for 1 hour. The resulting reaction product was dried at a heating temperature of 105°C for 18 hours to obtain the calcium carbonate of Example 1.

[0051] (Examples 2-5: Production of calcium carbonate) Calcium carbonate of Examples 2-5 was obtained in the same manner as in Example 1, except that the content (mass%) of the defoaming agent and the heating temperature during drying (drying temperature) were changed as shown in Table 2.

[0052] (Example 6: Production of calcium carbonate) Calcium carbonate for Example 6 was obtained in the same manner as in Example 1, except that 100 mL of an aqueous solution containing calcium carbonate (concentration: 95% by mass) was added to a stirring vessel, then 10 mL of a 10-fold dilution of defoaming agent 1 (polyalkylene glycol derivative) was added (defoaming agent content relative to the total amount of reaction solution: 10% by mass), and then 17.2 g of waste gypsum powder (pulverized waste gypsum board) was added.

[0053] (Example 7: Production of calcium carbonate) 100 mL of an aqueous solution containing calcium carbonate (concentration: 11% by mass) was added to a stirring vessel, then 17 g of waste gypsum powder (crushed waste gypsum board) was added, and the mixture was stirred at 20°C for 1 hour. 10 mL of a 10-fold dilution of defoaming agent 1 (polyalkylene glycol derivative) (defoaming agent content relative to the total volume of the reaction solution: 10% by mass) was added to the resulting reaction mixture, and the mixture was stirred at 20°C for another hour. After that, the mixture was dried at a heating temperature of 105°C for 18 hours to obtain the calcium carbonate of Example 7.

[0054] (Measurement of carbon dioxide content) As sample powders, calcium carbonate obtained in Examples 1 to 3 and untreated calcium carbonate ("LP-90", manufactured by Omi Mining Co., Ltd.) were prepared. Using a solid total organic carbon analyzer (elementar Japan Co., Ltd., soliTOC® cube), approximately 50 mg of the sample was placed in a steel pan and heated under a nitrogen atmosphere at the temperature conditions shown in Figure 1 for analysis. The analysis was performed using the analysis software accompanying the above instrument (elementar Japan Co., Ltd., soliTOC Software for Windows® V2.1.2(9102fb2), Qt5.11.0 2022-02-14), and the IR (infrared absorption) spectrum and carbon dioxide content were obtained. The carbon dioxide content is shown in Table 1, and the IR spectrum is shown in Figure 1.

[0055]

[0056] The calcium carbonate obtained in Examples 1 to 3 showed an increase of 0.5% by mass or more in carbon dioxide content compared to untreated calcium carbonate, suggesting that an antifoaming agent was attached to the calcium carbonate.

[0057] (Measurement of Unit Weight of Mortar) (Examples 1-7) 337.5 g of ordinary Portland cement (NC) and 112.5 g of calcium carbonate were added to a mixer and mixed uniformly. Then, 1350 g of standard sand and 225 g of water were added and mixed (water-cement ratio: 50% by mass). Mortar was prepared from the resulting mixture in accordance with JIS R 5201:2015 "Physical Test Methods for Cement". The unit weight of the mortar was determined by placing the above mortar into a 500 cc container and taking the average of the two measurements of the weight of the mortar. The results are shown in Table 2.

[0058] (Reference Example 1) Mortar was prepared using 450 g of ordinary Portland cement (NC) in accordance with JIS R 5201:2015 "Physical Test Methods for Cement," and the unit weight of the mortar was measured in the same manner as in Examples 1 to 7 above. The results are shown in Table 2.

[0059] (Reference Example 2) 337.5 g of ordinary Portland cement (NC), 112.5 g of calcium carbonate ("LP-90", manufactured by Omi Mining Co., Ltd.), 1350 g of standard sand, and 225 g of water were added to a mixer and mixed (water-cement ratio: 50% by mass). Mortar was prepared from the resulting mixture in accordance with JIS R 5201:2015 "Physical Test Methods for Cement," and the unit weight of the mortar was measured in the same manner as in Examples 1 to 7 above. The results are shown in Table 2.

[0060]

[0061] The unit weights of the mortars containing calcium carbonate derived from gypsum in Examples 1 to 7 are all comparable to or greater than the unit weights of the mortars in Reference Example 1 (using only ordinary Portland cement) and Reference Example 2 (using commercially available calcium carbonate). Therefore, it has been confirmed that when calcium carbonate derived from gypsum obtained by the manufacturing method of this embodiment is used, mortar with strength comparable to or greater than that of conventional products can be obtained.

Claims

1. A method for producing calcium carbonate, comprising the step of contacting a gypsum-containing material with a solution containing at least one alkali metal salt, such as an alkali metal carbonate or an alkali metal bicarbonate, to obtain calcium carbonate, wherein an antifoaming agent is added at any point during the step of obtaining the calcium carbonate.

2. The method for producing calcium carbonate according to claim 1, wherein the defoaming agent is a nonionic defoaming agent.

3. The method for producing calcium carbonate according to claim 1, further comprising the step of drying the calcium carbonate at a heating temperature of 160°C or lower, after the step of obtaining the calcium carbonate.

4. The method for producing calcium carbonate according to claim 3, wherein the heating temperature is 110°C or lower.

5. The method for producing calcium carbonate according to claim 1, wherein the amount of the defoaming agent is 0.5% by mass or more and 15% by mass or less with respect to the total amount of the reaction solution of the gypsum-containing material and the alkali metal salt solution.

6. The method for producing calcium carbonate according to claim 1, wherein, in the step of obtaining the calcium carbonate, the defoaming agent is added immediately after contacting the gypsum-containing material with the solution containing the alkali metal salt.

7. The method for producing calcium carbonate according to claim 1, wherein the gypsum-containing material is waste gypsum board.

8. A method for producing concrete, comprising the step of mixing calcium carbonate derived from gypsum, an antifoaming agent, cement, aggregate, and water.

9. The method for producing concrete according to claim 8, wherein the gypsum is waste gypsum board.

10. A concrete composition comprising calcium carbonate derived from gypsum, an antifoaming agent, cement, aggregate, and water.

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