Calcium carbonate, method for producing same, and concrete production method

Abstract

The present disclosure provides: a calcium carbonate that is derived from gypsum and has a carbon dioxide concentration of at most 0.7 mass% as measured using a non-dispersive infrared absorption method when heated to 500°C in a nitrogen atmosphere; a method for producing the same; and a concrete production method. The purpose of the present disclosure is to provide: a calcium carbonate that can be easily produced using less amount of water, and that can be used as a cement auxiliary raw material to obtain concrete having sufficient strength; a method for producing the same; and a concrete production method.

Description

Calcium carbonate, method for producing the same, and method for producing concrete

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

[0002] Conventionally, waste gypsum boards are utilized as ground improvement materials or auxiliary raw materials for cement after removing surface paper and crushing the boards. However, when used as a ground improvement material, there are problems such as difficulty in obtaining sufficient strength and fluorine elution. In addition, when used as an auxiliary raw material for cement, there are problems such as the generation of hydrogen sulfide. For this reason, most waste gypsum boards are discarded without being recycled. On the other hand, the discharge amount of gypsum boards is expected to increase significantly year by year, and there is a demand for establishing effective utilization methods for waste gypsum boards.

[0003] For example, Patent Document 1 discloses a method for producing Portland cement, which comprises: a crushing step of crushing a waste gypsum board such that particles having 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 crushed cement clinker.

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

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

[0006] For example, when a gypsum-containing material such as waste gypsum board is recycled and used as an auxiliary material for concrete such as calcium carbonate, strength reduction may be caused by components other than gypsum contained in the gypsum-containing material. Waste gypsum board contains organic components, and when it is attempted to be reused as an auxiliary material for mortar or the like, the residual organic components derived from the gypsum board may cause a reduction in strength. In Patent Document 1, organic components (surfactants) contained in recovered gypsum are reduced by crushing waste gypsum board and washing the obtained crushed product with water. Therefore, the above water washing requires a large amount of water, which causes an increase in production cost. In addition, there are many processes such as the crushing step and the water washing step, which complicates the operation.

[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.

[0008] This invention has been made in view of the above circumstances, and aims to provide calcium carbonate, a method for producing the same, and a method for producing concrete, which can be manufactured simply by reducing the amount of water used and can be used as a cement auxiliary material to obtain concrete with sufficient strength.

[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. To solve the above problem, the inventors diligently conducted research and found that by drying the gypsum-derived calcium carbonate at a predetermined temperature and using it as a cement auxiliary ingredient, concrete with sufficient strength can be obtained. Further research revealed that when the calcium carbonate is heated at a predetermined temperature and the carbon dioxide concentration is analyzed, concrete with sufficient strength can be obtained when the carbon dioxide concentration is below a predetermined level.

[0010] Based on the above considerations, the present invention provides the following calcium carbonate, a method for producing the same, and a method for producing concrete.

[0011] The calcium carbonate in this embodiment is calcium carbonate derived from gypsum. Hereinafter, "derived from gypsum" means a compound manufactured from a gypsum-containing substance (gypsum-containing substance).

[0012] [1] Calcium carbonate derived from gypsum, wherein the carbon dioxide concentration measured by non-dispersive infrared absorption spectroscopy when heated to 500°C under a nitrogen atmosphere is 0.7% by mass or less. [2] The calcium carbonate according to [1], wherein the gypsum is waste gypsum board. [3] The calcium carbonate according to [1] or [2], wherein the carbon dioxide concentration is 0.5% by mass or less.

[0013] [4] A method for producing calcium carbonate, comprising a drying step of drying calcium carbonate derived from gypsum at a heating temperature of 230°C to 450°C, wherein the carbon dioxide concentration measured by non-dispersive infrared absorption spectroscopy when the calcium carbonate after the drying step is heated to 500°C under a nitrogen atmosphere is 0.7% by mass or less. [5] The method for producing calcium carbonate according to [4], wherein the calcium carbonate derived from gypsum is obtained by contacting a gypsum-containing material with a solution containing at least one alkali metal salt of alkali metal carbonate and alkali metal bicarbonate. [6] The method for producing calcium carbonate according to [5], wherein the gypsum-containing material is waste gypsum board.

[0014] [7] A method for producing concrete, comprising the step of mixing calcium carbonate derived from gypsum, cement, fine aggregate and water, wherein the calcium carbonate derived from gypsum has a carbon dioxide concentration of 0.7% by mass or less when heated to 500°C under a nitrogen atmosphere and measured using a non-dispersive infrared absorption method. [8] The method for producing concrete according to [7], wherein the gypsum is waste gypsum board.

[0015] According to the present invention, it is possible to provide calcium carbonate, a method for producing the same, and a method for producing concrete, which can be manufactured simply by reducing the amount of water used and which can be used as a cement auxiliary material to obtain concrete with sufficient strength.

[0016] These are the IR spectra measured by non-dispersive infrared absorption spectroscopy when the calcium carbonate from Examples 1 and 2, Comparative Examples 1 and 2, and Reference Example 1, as well as the waste gypsum powder from Reference Example 2, were heated in a nitrogen atmosphere in the temperature range of 50°C to 500°C.

[0017] 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.

[0018] [Calcium Carbonate] The calcium carbonate of this embodiment is derived from gypsum and has a carbon dioxide concentration of 0.7% by mass or less, as measured by non-dispersive infrared absorption (NDIR) when heated to 500°C under a nitrogen atmosphere. When the above carbon dioxide concentration is 0.7% by mass or less, concrete with sufficient strength can be obtained when the calcium carbonate of this embodiment is used as a cement auxiliary material. This indicates that the lower the carbon dioxide concentration measured above, the less organic matter derived from gypsum is contained in the calcium carbonate of this embodiment. From the viewpoint of obtaining concrete with more sufficient strength, the above carbon dioxide concentration is preferably 0.6% by mass or less, more preferably 0.5% by mass or less, and even more preferably 0.45% by mass or less. Furthermore, there is no particular limit on the lower limit of the above carbon dioxide concentration, and it may be 0% by mass.

[0019] The above measurement is preferably performed by heating to 450°C, from the viewpoint of preventing the decomposition of calcium carbonate.

[0020] The above carbon dioxide concentration can be obtained, for example, by heating to 500°C under a nitrogen atmosphere using a solid total organic carbon analyzer, performing the analysis, and then analyzing it using the analysis software attached to the analyzer. Specifically, it can be measured by the method described in the examples.

[0021] Furthermore, the IR spectrum obtained by the above measurement may have one peak or two or more peaks. If there are two or more peaks, the carbon dioxide concentration is calculated from the sum of the areas of each peak.

[0022] The calcium carbonate in this embodiment is calcium carbonate derived from gypsum. Calcium carbonate derived from gypsum can be obtained, for example, by the manufacturing method described later. According to the manufacturing method described later, the calcium carbonate of this embodiment that satisfies the above carbon dioxide concentration can be easily produced.

[0023] Gypsum-containing materials are materials containing gypsum, which is at least one of calcium sulfate and calcium sulfate hydrate. Examples include products using gypsum such as gypsum boards and waste gypsum boards, or their waste; recovered materials from flue gas desulfurization treatment (also called "flue gas desulfurization gypsum," which are recovered materials from 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, products using gypsum such as gypsum boards and waste gypsum boards, or their waste, are preferred, and furthermore, considering the industrial advantage of reusing gypsum, waste gypsum boards are more preferred.

[0024] [Method for producing calcium carbonate] The method for producing calcium carbonate according to this embodiment includes a drying step in which calcium carbonate derived from gypsum is dried at a heating temperature of 230°C to 450°C, and the carbon dioxide concentration measured by non-dispersive infrared absorption spectroscopy when the calcium carbonate after the drying step is heated to 500°C under a nitrogen atmosphere is 0.7% by mass or less.

[0025] [Drying Process] The calcium carbonate production method of this embodiment includes a drying process in which calcium carbonate derived from gypsum is dried at a heating temperature of 230°C to 450°C. When the heating temperature during the drying of calcium carbonate derived from gypsum is within the above range, it is possible to make it difficult for organic matter derived from gypsum to remain in the dried calcium carbonate. Therefore, the dried calcium carbonate has less organic matter derived from gypsum, and when used as a cement auxiliary material, concrete with sufficient strength can be obtained. From the viewpoint of obtaining calcium carbonate that yields concrete with even greater strength, the heating temperature is preferably 250°C to 440°C, more preferably 280°C to 420°C, and even more preferably 300°C to 400°C. The drying time is preferably 10 to 48 hours, more preferably 10 to 24 hours, and even more preferably 10 to 18 hours.

[0026] 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.

[0027] The calcium carbonate after the above drying process has a carbon dioxide concentration of 0.7% by mass or less, as measured by non-dispersive infrared absorption spectroscopy when heated to 500°C. Therefore, the calcium carbonate after the above drying process has a low organic content derived from gypsum, and when used as a cement auxiliary material, it can produce concrete with sufficient strength. Furthermore, by keeping the heating temperature during the drying of the calcium carbonate within the above range, the carbon dioxide concentration can be reduced to 0.7% by mass or less.

[0028] [Step to obtain calcium carbonate derived from gypsum] The calcium carbonate derived from gypsum can be obtained, for example, by 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.

[0029] (Gypsum-containing material) The gypsum-containing material is a material containing gypsum, which is at least one of calcium sulfate and calcium sulfate hydrate. Examples and preferred embodiments of the gypsum-containing material are as described above.

[0030] 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.

[0031] 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.

[0032] (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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] (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.

[0038] 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.

[0039] 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.

[0040] The reaction temperature for the reaction that occurs when a gypsum-containing material comes into contact with a solution containing an alkali metal salt is not particularly limited, and may be between 10 and 80°C, for example. 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 between 0.5 and 5 hours, for example. From the viewpoint of more efficiently carrying out the reaction, it is preferably between 1 and 3 hours.

[0041] 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 or filtration. The recovered calcium carbonate is then subjected to the drying process described above.

[0042] [Method for Producing Concrete] The method for producing concrete according to the present embodiment includes a step of kneading calcium carbonate derived from gypsum, cement, fine aggregate and water, wherein the calcium carbonate derived from gypsum has a carbon dioxide concentration of 0.7% by mass or less as measured by a non-dispersive infrared absorption method when heated to 500°C in a nitrogen atmosphere.

[0043] According to the method for producing concrete of the present embodiment, concrete with sufficient strength can be obtained because calcium carbonate derived from gypsum, which has a carbon dioxide concentration of not more than the predetermined value as measured by the non-dispersive infrared absorption method when heated at the predetermined temperature as described above, is used.

[0044] In consideration of the industrial advantage of reusing gypsum, the calcium carbonate derived from gypsum is preferably calcium carbonate derived from waste gypsum board. The calcium carbonate derived from gypsum can be obtained by the above-mentioned [Method for Producing Calcium carbonate].

[0045] In the step of kneading calcium carbonate derived from gypsum, cement, aggregate and water, calcium carbonate derived from gypsum, cement and aggregate contained in a concrete composition are kneaded with water to produce a concrete kneaded product. The above-mentioned solids 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 capable of uniformly kneading even when all raw materials including each solid and water are mixed at once. The amount of water used for kneading can be changed depending on the types and formulations of the materials used, so it is not uniquely determined. However, the water-cement ratio is preferably 25% by mass or more and 70% by mass or less, more preferably 50% by mass or more and 65% by mass or less. In addition, there are no limitations on kneading conditions, the type of kneader, etc., and conventional kneaders can be used. The obtained concrete kneaded product can be cured by, for example, steam curing, underwater curing or the like to obtain concrete.

[0046] (Concrete Composition) The concrete composition used in the present embodiment comprises calcium carbonate derived from gypsum, cement, fine aggregate and water. The blending amount of calcium carbonate derived from gypsum in the concrete composition is per 1 m volume of the concrete composition ３ , it may be 10 kg or more and 450 kg or less, may be 50 kg or more and 300 kg or less, or may be 100 kg or more and 250 kg or less.

[0047] The type of cement contained in the concrete composition is not particularly limited, and examples thereof include ordinary Portland cement, high-early-strength Portland cement, moderate-heat Portland cement, low-heat Portland cement, and the like. The blending amount of cement in the concrete composition is per 1 m volume of the concrete composition ３ , it may be 170 kg or more and 750 kg or less, may be 300 kg or more and 400 kg or less, or may be 340 kg or more and 380 kg or less.

[0048] The fine aggregate contained in the concrete composition 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 volume of the concrete composition ３ , it 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 per 1 m volume of the concrete composition ３ , it 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.

[0049] 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.

[0050] The details of each component used in the production of calcium carbonate, preparation of mortar specimens, and measurement of unit weight are as follows: • Alkali metal salt: Sodium carbonate; "Soda Ash Light," manufactured by Tokuyama Corporation • Gypsum-containing material: Waste gypsum powder (pulverized using a ball mill after peeling the board paper from waste gypsum board; average particle size: 1 mm) • Ordinary Portland cement (NC): Manufactured by Sumitomo Osaka Cement Co., Ltd. • Calcium carbonate: • Calcium carbonate obtained in Examples 1 and 2 and Comparative Examples 1 and 2; "LP-90," manufactured by Omi Mining Co., Ltd. • Standard sand: Standard sand conforming to JIS R 5201:2015 "Physical Test Methods for Cement"

[0051] (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, and the mixture was stirred and mixed at a temperature of 20°C for 1 hour. The resulting reaction product was dried at a heating temperature of 250°C for 18 hours to obtain the calcium carbonate of Example 1.

[0052] (Example 2 and Comparative Examples 1 and 2: Production of calcium carbonate) Calcium carbonate for Example 2 and Comparative Examples 1 and 2 was obtained in the same manner as in Example 1, except that the heating temperature (drying temperature) during drying was changed as shown in Table 1.

[0053] (Reference Example 1) As calcium carbonate, "LP-90" manufactured by Omi Mining Co., Ltd. was prepared.

[0054] (Reference Example 2) After peeling the base paper from the waste gypsum board, it was crushed using a ball mill to obtain the waste gypsum powder (average particle size: 1 mm) shown in Reference Example 2.

[0055] (Reference Example 3) After peeling the base paper from the waste gypsum board, the material was crushed using a ball mill and then classified to obtain the waste gypsum powder (average particle size: 1 mm) shown in Reference Example 3.

[0056] (Preparation of mortar specimens for Examples 1 and 2 and Comparative Examples 1 and 2) 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). The resulting mixture was poured into three mortar molds in accordance with JIS R 5201:2015 "Physical testing methods for cement," and after 72 hours, the molds were removed to produce three mortar specimens for each example and comparative example. Subsequently, the specimens were cured in water at 20°C for 7 days and 28 days, respectively, to obtain the mortar specimens for each example and comparative example.

[0057] (Preparation of mortar specimens for Reference Example 1) 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). The resulting mixture was poured into three mortar molds conforming to JIS R 5201:2015 "Physical testing methods for cement," and after 72 hours, the molds were removed to produce three mortar specimens. Subsequently, the specimens were cured in water at 20°C for 7 days and 28 days to obtain the mortar specimens for Reference Example 1.

[0058] (Preparation of mortar specimens for Reference Example 3) 337.5 g of ordinary Portland cement (NC), 112.5 g of waste gypsum powder obtained in Reference Example 3, 1350 g of standard sand, and 225 g of water were added to a mixer and mixed (water-cement ratio: 50% by mass). The resulting mixture was poured into three mortar molds in accordance with JIS R 5201:2015 "Physical testing methods for cement," and after 72 hours, the molds were removed to produce three mortar specimens. Subsequently, the specimens were cured in water at 20°C for 7 days and 28 days to obtain the mortar specimens for Reference Example 3.

[0059] (Measurement of carbon dioxide concentration) Using a solid total organic carbon analyzer (soliTOC® cube, manufactured by Elementor Japan Co., Ltd.), approximately 50 mg of the sample was placed in a steel pan and heated under a nitrogen atmosphere in the temperature range of 50°C to 500°C under the temperature conditions shown in Figure 1, and analysis was performed. The analysis was performed using the analysis software accompanying the above instrument (soliTOC Software for Windows® V2.1.2(9102fb2), Qt5.11.0 2022-02-14, manufactured by Elementor Japan Co., Ltd.), and the IR (infrared absorption) spectrum and carbon dioxide concentration were obtained. The carbon dioxide concentration is shown in Table 1, and the IR spectrum is shown in Figure 1.

[0060] (Measurement of Unit Weight) The unit weight was measured in Reference Example 1, Example 1, and Comparative Example 2. 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 mortar in a 500 cc container and taking the average of the two weight measurements. The results are shown in Table 1.

[0061] (Mortar Strength) The compressive strength of the mortar was measured in accordance with JIS R 5201:2015 "Physical Test Methods for Cement: 10.5 Measurement".

[0062]

[0063] Examples 1 and 2, which use calcium carbonate in which the carbon dioxide concentration measured by non-dispersive infrared absorption spectroscopy when heated at a predetermined temperature is below a predetermined amount, show higher compressive strength of the mortar after 3, 7, and 28 days compared to Comparative Examples 1 and 2, in which the carbon dioxide concentration exceeds the predetermined amount. Furthermore, Examples 1 and 2 exhibit compressive strength comparable to Reference Example 1, which uses commercially available calcium carbonate. Therefore, it has been confirmed that mortar with sufficient strength can be obtained by using the calcium carbonate of this embodiment. It can also be seen that the unit weight of the mortar in Example 1 is greater than that of the mortars in Comparative Example 2 and Reference Example 1. This is presumed to be because, in Example 1, drying the calcium carbonate at a predetermined temperature removed organic matter derived from gypsum by evaporation, reducing the amount of organic matter remaining in the dried calcium carbonate. Furthermore, Reference Example 3, which uses waste gypsum powder obtained by performing multiple water washing steps as described in Patent Document 1, achieves a compressive strength comparable to Reference Example 1, but requires multiple water washing steps, resulting in high water usage and a complicated operation.

[0064] The calcium carbonate of this embodiment can be easily manufactured with reduced water usage, and by using it as a cement auxiliary ingredient, concrete with sufficient strength can be obtained. Therefore, it can be suitably used as a material for cement compositions, mortar, and concrete. It can also be used as a filler, building material, etc.

Claims

1. Calcium carbonate derived from gypsum, wherein the carbon dioxide concentration, as measured by non-dispersive infrared absorption spectroscopy when heated to 500°C under a nitrogen atmosphere, is 0.7% by mass or less.

2. The calcium carbonate according to claim 1, wherein the gypsum is waste gypsum board.

3. The calcium carbonate according to claim 1 or 2, wherein the carbon dioxide concentration is 0.5% by mass or less.

4. A method for producing calcium carbonate, comprising a drying step of drying calcium carbonate derived from gypsum at a heating temperature of 230°C to 450°C, wherein the carbon dioxide concentration measured by non-dispersive infrared absorption spectroscopy when the calcium carbonate after the drying step is heated to 500°C under a nitrogen atmosphere is 0.7% by mass or less.

5. The method for producing calcium carbonate according to claim 4, wherein the calcium carbonate derived from gypsum is obtained by contacting a gypsum-containing material with a solution containing at least one alkali metal salt of an alkali metal carbonate and an alkali metal bicarbonate.

6. The method for producing calcium carbonate according to claim 5, wherein the gypsum-containing material is waste gypsum board.

7. A method for producing concrete, comprising the step of mixing calcium carbonate derived from gypsum, cement, fine aggregate, and water, wherein the calcium carbonate derived from gypsum has a carbon dioxide concentration of 0.7% by mass or less, as measured by a non-dispersive infrared absorption method when heated to 500°C under a nitrogen atmosphere.

8. The method for producing concrete according to claim 7, wherein the gypsum is waste gypsum board.

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