Gypsum dihydrate and method for producing the same

Abstract

To provide large-sized gypsum dihydrate, and a novel method for producing the same.SOLUTION: A method for producing gypsum dihydrate using a gypsum board waste material includes: a heat treatment step of generating hemihydrate gypsum from gypsum dihydrate contained in the gypsum board waste material, by heating the gypsum board waste material at 180°C or less; and a hydration step of generating gypsum dihydrate from the hemihydrate gypsum by suspending the hemihydrate gypsum in an aqueous medium, adding seed crystals of gypsum dihydrate having a major axis of 20 μm or more, and maintaining a temperature of a resulting suspension at 77°C or less. The gypsum dihydrate having a major axis of 160 μm or more and a minor axis of 100 μm or more is also provided.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to gypsum dihydrate and a method for producing the same. [Background technology]

[0002] Gypsum board is made by laminating gypsum board base paper on both sides of a board-shaped molded body of gypsum dihydrate (CaSO4·2H2O), and is lightweight and strong. Gypsum board also has excellent non-combustibility, fire resistance, heat insulation, and sound insulation properties, and because of these excellent properties, it is widely used as a building material.

[0003] However, the majority of the gypsum used to make gypsum boards is imported from natural gypsum, which has led to the problem of unstable supply. Furthermore, the consumption of imported natural gypsum is undesirable from the perspective of protecting natural resources. On the other hand, while gypsum boards are mass-produced, a large amount of gypsum board waste is also generated, and there is a desire to reduce the amount of waste material that must be disposed of.

[0004] One way to effectively solve these problems is to reuse waste gypsum board. For example, if the gypsum dihydrate contained in waste gypsum board could be reused to manufacture new gypsum boards, the above problems could be solved. One technology that enables the reuse of gypsum dihydrate is a method for regenerating gypsum dihydrate, which involves heat-treating the gypsum dihydrate in waste gypsum board at a temperature of about 200°C to convert it into gypsum hemihydrate (CaSO4·1 / 2H2O), suspending this hemihydrate in an aqueous medium, and maintaining it at a temperature of about 80°C to hydrate it and precipitate gypsum dihydrate crystals (see Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6195238 Summary of the Invention [Problem to be solved by the invention]

[0006] It is desirable that the particle diameter of gypsum dihydrate used in gypsum boards be large (sometimes referred to as "large" in this specification), but the particle diameter of the gypsum dihydrate obtained by the method disclosed in Patent Document 1 still leaves room for improvement.

[0007] An object of the present invention is to provide large-sized gypsum dihydrate and a novel method for producing the same. [Means for solving the problem]

[0008] In order to solve the above problems, the present invention employs the following configuration. [1] A method for producing gypsum dihydrate using waste gypsum board, the method comprising: a heat treatment step of producing gypsum hemihydrate from the gypsum dihydrate in the waste gypsum board by heat treating the waste gypsum board at a temperature of 180°C or less; and a hydration step of producing gypsum dihydrate from the gypsum hemihydrate by suspending the gypsum hemihydrate in an aqueous medium and adding gypsum dihydrate seed crystals having a major axis of 20 μm or more, and maintaining the temperature of the resulting suspension at 90°C or less. [2] The method for producing gypsum dihydrate according to [1], wherein the temperature of the suspension is maintained at 65 to 90°C for 2 hours or more in the hydration step. [3] The method for producing gypsum dihydrate according to [1] or [2], wherein the seed crystals are added only once in the hydration step. [4] Gypsum dihydrate, with a major axis of 160 μm or more and a minor axis of 100 μm or more. [Effects of the Invention]

[0009] According to the present invention, large-sized gypsum dihydrate and a novel method for producing the same are provided. [Brief explanation of the drawings]

[0010] [Figure 1] 1 shows XRD spectrum data of the crystals obtained in Example 1. [Figure 2] 1 shows SEM image data of the crystals obtained in Example 1. [Figure 3] 1 shows XRD spectrum data of the crystals obtained in Example 2. [Figure 4] 1 shows XRD spectrum data of the crystals obtained in Example 3. [Figure 5] 1 shows XRD spectrum data of the crystals obtained in Example 4. [Figure 6] 1 shows SEM image data of crystals obtained in Examples 1 to 4 after adding seed crystals and before lowering the temperature of the suspension. [Figure 7] 1 shows SEM image data of crystals obtained in Examples 1 to 3 by adding seed crystals and then lowering the temperature of the suspension to 66° C. FIG. [Figure 8] 1 shows SEM image data of crystals obtained in Examples 1 to 3 by adding seed crystals and then lowering the temperature of the suspension to 54° C. FIG. [Figure 9] 1 shows SEM image data of crystals obtained in Examples 1 to 4 by adding seed crystals and then lowering the temperature of the suspension to 30° C. FIG. [Figure 10] 1 shows XRD spectrum data of the crystals obtained in Example 5. [Figure 11] FIG. 1 shows SEM image data of crystals obtained after adding seed crystals and before lowering the temperature of the suspension, and crystals obtained by lowering the temperature of the suspension to 66° C., 54° C., and 40° C. in Example 5. [Figure 12] 1 shows XRD spectrum data of the crystals obtained in Example 6. [Figure 13] FIG. 1 shows SEM image data of crystals obtained after adding seed crystals and before lowering the temperature of the suspension, and crystals obtained by lowering the temperature of the suspension to 66° C., 54° C., and 30° C. in Example 6. [Figure 14] 1 shows XRD spectrum data of the crystals obtained in Example 7. [Figure 15]FIG. 1 shows SEM image data of crystals obtained after adding seed crystals and before lowering the temperature of the suspension, and crystals obtained by lowering the temperature of the suspension to 66° C., 54° C., and 30° C. in Example 7. [Figure 16] 1 shows XRD spectrum data of the crystals obtained in Example 8. [Figure 17] 1 shows SEM image data of the crystals obtained in Example 8. [Figure 18] 1 shows XRD spectrum data of the crystals obtained in Example 9. [Figure 19] 1 shows SEM image data of the crystals obtained in Example 9. [Figure 20] 1 shows XRD spectrum data of the crystals obtained in Example 10. [Figure 21] 1 shows SEM image data of the crystals obtained in Example 10. [Figure 22] 1 shows XRD spectrum data of the crystals obtained in Example 11. [Figure 23] 1 shows SEM image data of the crystals obtained in Example 11. [Figure 24] 1 shows XRD spectrum data of the crystals obtained in Example 12. [Figure 25] 1 shows SEM image data of the crystals obtained in Example 12. [Figure 26] 1 is a graph showing the relationship between the time in the hydration step, the temperature in the hydration step, and the surface area of ​​the crystals in Example 12. DETAILED DESCRIPTION OF THE INVENTION

[0011] ◇Production method of dihydrate gypsum A method for producing gypsum dihydrate (calcium sulfate dihydrate, CaSO·2H0) according to one embodiment of the present invention is a method for producing gypsum dihydrate using waste gypsum board, and the method includes: a heat treatment step of producing hemihydrate gypsum (calcium sulfate hemihydrate, CaSO·2H0) from the gypsum dihydrate in the waste gypsum board by heat treating the waste gypsum board at a temperature of 180°C or less; and a hydration step of producing gypsum dihydrate from the hemihydrate by suspending the hemihydrate in an aqueous medium and adding gypsum dihydrate seed crystals having a major axis of 20 μm or more, and maintaining the temperature of the resulting suspension at 90°C or less. According to the manufacturing method of this embodiment, gypsum dihydrate in waste gypsum boards is reused to obtain large-sized gypsum dihydrate.

[0012] <<Heat treatment process>> In the heat treatment step, the waste gypsum board is heat treated at a temperature of 180° C. or less to produce gypsum hemihydrate from gypsum dihydrate in the waste gypsum board. While the conventional heat treatment temperature for waste gypsum boards is about 200°C or higher, the heat treatment temperature for waste gypsum boards in the manufacturing method of this embodiment is lower, at 180°C or lower.

[0013] <Waste gypsum board> Examples of the gypsum board waste include used gypsum board generated during building renovation or demolition work; scraps and leftover gypsum board generated during the manufacture of gypsum board or the construction of buildings; unused gypsum board and processed products thereof; and the like.

[0014] In the present embodiment, the gypsum board may be a known one. For example, the gypsum board may be one formed by bonding gypsum board base paper to both sides of a board-shaped gypsum dihydrate molded body.

[0015] It is preferable that the gypsum board base paper has been removed from the waste gypsum board material. The waste gypsum board is preferably crushed, and more preferably pulverized into particles. The removal of gypsum board liner paper from waste gypsum board materials and the crushing of waste gypsum board materials can both be performed by known methods. For example, the removal of gypsum board liner paper from waste gypsum board materials can be performed by the method disclosed in JP-A-10-286553 or JP-A-2000-254531.

[0016] In gypsum board waste, the ratio of the gypsum dihydrate content to the total mass of the gypsum board waste ([gypsum dihydrate content in gypsum board waste (parts by mass)] / [total mass of gypsum board waste (parts by mass)]×100) is not particularly limited, but is preferably 50% by mass or more, and may be, for example, any of 60% by mass or more, 70% by mass or more, 80% by mass or more, and 90% by mass or more. On the other hand, the proportion may be 100% by mass or less, for example, 95% by mass or less.

[0017] The waste gypsum board is preferably in the form of particles. By using the waste gypsum board in the form of particles, gypsum hemihydrate can be produced more efficiently from gypsum dihydrate.

[0018] The average particle size of the waste gypsum board is preferably 0.1 to 10 mm, more preferably 0.1 to 5 mm, and even more preferably 0.5 to 1 mm. Waste gypsum board having an average particle size equal to or greater than the lower limit can be more easily produced or obtained. By using waste gypsum board having an average particle size equal to or less than the upper limit, gypsum hemihydrate can be produced more efficiently from gypsum dihydrate.

[0019] The average particle size of the waste gypsum board can be, for example, an arithmetic mean value obtained by observing the particles of the waste gypsum board and measuring the major axes of 100 particles randomly selected from the observed image. The "major axis of the particles" will be explained later.

[0020] The particles of the waste gypsum board, the seed crystals described below, and the crystals of gypsum dihydrate described below can all be observed using a scanning electron microscope (SEM).

[0021] Gypsum board waste having an average particle size within a specific range can be obtained, for example, by passing crushed gypsum board waste through a sieve having meshes of the desired size.

[0022] <Heat treatment conditions> In the heat treatment step, the temperature at which the waste gypsum board is heated is 180°C or lower, which is lower than conventional methods. By keeping the heat treatment temperature below the upper limit, the resulting hemihydrate gypsum is more easily hydrated in the hydration step, making it easier to obtain large-sized gypsum dihydrate. Furthermore, by keeping the heat treatment temperature below the upper limit, it is possible to reduce energy consumption compared to conventional methods, and in addition, the time required for the process can be shortened compared to conventional methods.

[0023] For example, in gypsum boards, when gypsum board base paper is bonded to a gypsum dihydrate molded body with an adhesive, the gypsum board waste may contain the adhesive. The adhesive may also contain starch. When using gypsum board waste containing such an adhesive, if the heat treatment temperature of the gypsum board waste is high (e.g., 200°C or higher), the starch may adhere to the surface of the generated gypsum hemihydrate, which may hinder the generation of gypsum dihydrate from the gypsum hemihydrate in the hydration process. In contrast, in this embodiment, the temperature during the heat treatment of the gypsum board waste is 180°C or lower, which is lower than conventional methods. This avoids the influence of such adhesives, and does not hinder the generation of gypsum dihydrate from the gypsum hemihydrate in the hydration process. As a result, large-sized gypsum dihydrate can be obtained more easily.

[0024] In order to further enhance the above-mentioned effects, the heat treatment temperature of the waste gypsum board may be, for example, any one of 175°C or less, 165°C or less, 155°C or less, and 145°C or less. On the other hand, in order to more efficiently produce gypsum hemihydrate from gypsum dihydrate, the heat treatment temperature of the waste gypsum board is preferably 135°C or higher, and may be, for example, 140°C or higher.

[0025] In the heat treatment step, the time for heat treatment of the waste gypsum board is not particularly limited as long as sufficient gypsum hemihydrate is produced, but is preferably 10 minutes or more, more preferably 30 minutes or more, and even more preferably 50 minutes or more. By setting the heat treatment time to the above lower limit or more, the amount of gypsum hemihydrate produced increases. On the other hand, the heat treatment time for waste gypsum board is preferably 10 hours or less, more preferably 7 hours or less, and even more preferably 4 hours or less, in order to avoid excessive treatment. The heat treatment time shown here is particularly preferable when the heat treatment temperature of the waste gypsum board is within the above-mentioned range.

[0026] The heat treatment of the waste gypsum board can be carried out by using, for example, a hot air dryer, a conductive electric heat dryer, or the like as a heating device.

[0027] <<Hydration process>> In the hydration step, the gypsum hemihydrate is suspended in an aqueous medium, and gypsum dihydrate seed crystals having a major axis of 20 μm or more are added to obtain a suspension. The temperature of the suspension is maintained at 90° C. or lower, thereby producing gypsum dihydrate from the gypsum hemihydrate. In the production method of this embodiment, by using the seed crystals and setting the temperature of the suspension to 90°C or less, the hydration of gypsum hemihydrate easily proceeds and the amount of large-sized gypsum dihydrate produced increases.

[0028] In this specification, the term "aqueous medium" refers to a liquid medium containing water. The aqueous medium may be, for example, water or a mixed solvent containing water and a solvent other than water.

[0029] In this specification, the term "solvent" is a concept that encompasses both a component that is liquid at room temperature and that dissolves a solute, and a component that is liquid at room temperature and that functions as a dispersion medium for dispersing a dispersoid. In this specification, "room temperature" means a temperature that is neither particularly cold nor hot, that is, an ordinary temperature, and examples thereof include temperatures of 15 to 25°C.

[0030] In the aqueous medium, the ratio of the water content to the total mass of the aqueous medium ([water content of the aqueous medium (parts by mass)] / [total mass of the aqueous medium (parts by mass)]×100) is preferably 80% by mass or more, more preferably 90% by mass or more, and may be, for example, either 95% by mass or more or 97% by mass or more. When the ratio is equal to or more than the lower limit, the generation of gypsum dihydrate (hydration of gypsum hemihydrate) proceeds more efficiently. On the other hand, the proportion is 100% by mass or less. It is particularly preferable that the proportion is 100% by mass, that is, the aqueous medium is water.

[0031] In the hydration step, a suspension is prepared by suspending gypsum hemihydrate in an aqueous medium, and seed crystals of gypsum dihydrate are added to the suspension. The suspension before the addition of the seed crystals can be prepared at room temperature.

[0032] In the suspension before the addition of seed crystals in the hydration step, the ratio of the content of gypsum hemihydrate to the total mass of the suspension ([gypsum hemihydrate content (parts by mass) in the suspension before the addition of seed crystals] / [total mass (parts by mass) of the suspension before the addition of seed crystals]×100, sometimes referred to in this specification as "gypsum hemihydrate concentration") is not particularly limited, but is preferably 10% by mass or more, and may be, for example, any one of 13% by mass or more, 16% by mass or more, and 19% by mass or more. When the concentration of gypsum hemihydrate is equal to or more than the lower limit, the amount of large-sized gypsum dihydrate produced further increases. The upper limit of the concentration of gypsum hemihydrate is not particularly limited. For example, the concentration of gypsum hemihydrate is preferably 40% by mass or less, in order to improve the uniformity of the suspension and allow the production of gypsum dihydrate (hydration of gypsum hemihydrate) to proceed more smoothly.

[0033] In the hydration step, the temperature of the suspension when the seed crystals are added is preferably 60° C. or higher, and may be, for example, any one of 68° C. or higher, 76° C. or higher, and 83° C. or higher. By adding the seed crystals to the suspension at such a temperature, the generation of large gypsum dihydrate (hydration of gypsum hemihydrate) proceeds more smoothly. On the other hand, in the hydration step, the temperature of the suspension when the seed crystals are added is preferably 90° C. or lower, and may be, for example, either 87° C. or lower or 85° C. or lower. Adding the seed crystals to the suspension at such a temperature further enhances the effect obtained by using the seed crystals.

[0034] The seed crystals used have a major axis of 20 μm or more. By adding seed crystals having a size equal to or larger than a specific value, the generation of large gypsum dihydrate (hydration of gypsum hemihydrate) in the suspension is promoted.

[0035] In this specification, not only in the case of a seed crystal, the "major axis" of a particle means the maximum value of a line segment connecting any two points on the particle's outline (the line representing the outer surface of the particle) in an observation image of the particle, and the "minor axis" means the maximum value of the particle's diameter in the direction perpendicular to the major axis in the observation image.

[0036] In order to obtain the above-mentioned effects more significantly, the major axis of the seed crystal may be, for example, any one of 50 μm or more, 100 μm or more, and 150 μm or more.

[0037] On the other hand, the major axis of the seed crystal is preferably 250 μm or less, and may be, for example, 220 μm or less. When the major axis of the seed crystal is the above upper limit or less, the crystal growth of gypsum dihydrate becomes better, larger gypsum dihydrate is produced more quickly, and the amount of gypsum produced increases.

[0038] The minor axis of the seed crystal is not particularly limited, but is preferably 3 to 110 μm.

[0039] The seed crystal may be one obtained by the manufacturing method of this embodiment, or may be one obtained by another manufacturing method.

[0040] In the hydration step, seed crystals may be added two or more times, but are preferably added only once. In conventional production of gypsum dihydrate (hydration of gypsum hemihydrate), seed crystals are often added two or more times to increase the amount of gypsum dihydrate produced, but this results in an increased amount of seed crystals being added. In contrast, in the present embodiment, the amount of large gypsum dihydrate produced can be sufficiently increased even by adding seed crystals only once.

[0041] In the hydration step, the ratio of the amount of seed crystals added to the amount of gypsum hemihydrate in the suspension before the addition of the seed crystals ([amount of seed crystals added (parts by mass)] / [content of gypsum hemihydrate in the suspension before the addition of the seed crystals (parts by mass)]×100, sometimes referred to herein as "ratio of the amount of seed crystals added") is not particularly limited, but is preferably 0.7% by mass or more, and may be, for example, either 2% by mass or more or 4% by mass or more. When the ratio of the amount of seed crystals added is equal to or greater than the lower limit, the effect obtained by using the seed crystals is further enhanced. The upper limit of the proportion of the amount of seed crystals added is not particularly limited. In order to prevent excessive use of seed crystals, the proportion of the amount of seed crystals added is preferably 10% by mass or less. The proportion of the amount of seed crystals added may be, for example, 6% by mass or less, 4% by mass or less, or 2% by mass or less.

[0042] In the hydration step, the temperature of the suspension obtained by adding the seed crystals is maintained at 90°C or less. To this end, if the temperature of the suspension obtained by adding the seed crystals is above 90°C (higher than 90°C), the temperature of the suspension is lowered to 90°C, and from this point on, the temperature of the suspension is started to be maintained at 90°C or less. If the temperature of the suspension obtained by adding the seed crystals is already 90°C or less, the temperature of the suspension is started to be maintained at 90°C or less from the point on which the seed crystals are added.

[0043] After starting to maintain the temperature of the suspension after the addition of the seed crystals at 90°C or less, the temperature of the suspension may be, for example, maintained constant, or may be maintained while being intermittently changed (increased or decreased) within a range of 90°C or less, or may be maintained while being continuously decreased; however, it is preferable to maintain the temperature while being intermittently or continuously decreased. That is, it is preferable that the temperature of the suspension obtained by adding the seed crystals is continuously decreased, or that the temperature is maintained constant and then continuously decreased, each of which is repeated once or twice or more times. In this way, by lowering the temperature of the suspension after the addition of the seed crystals to a temperature lower than that at the time of the addition of the seed crystals, the amount of large-sized gypsum dihydrate produced can be further increased.

[0044] In the hydration step, the time for which the temperature of the suspension after the addition of the seed crystals is maintained at 90°C or less is preferably 1 hour or more, more preferably 3 hours or more, and even more preferably 4 hours or more, and may be, for example, 8 hours or more or 10 hours or more. When the time is equal to or greater than the lower limit, the amount of enlarged gypsum dihydrate produced can be further increased. On the other hand, the time is preferably 25 hours or less in order to avoid an excessively long time.

[0045] In the hydration step, once the temperature of the suspension after the addition of the seed crystals has started to be maintained at 90°C or less, it is preferable to maintain the temperature of the suspension after the addition of the seed crystals at 65 to 90°C for 2 hours or more, regardless of whether the temperature of the suspension is kept constant or is changed intermittently or continuously. By maintaining the suspension at 65°C or higher for 2 hours or more, the amount of large-sized gypsum dihydrate produced can be further increased.

[0046] As described above, when the suspension after the addition of seed crystals is maintained at a temperature of 65 to 90° C. for 2 hours or longer, the temperature is preferably 68 to 90° C., and may be, for example, any one of 73 to 90° C., 78 to 90° C., and 82 to 90° C. The higher the lower limit of the temperature, the more likely it is that large-sized gypsum dihydrate can be obtained.

[0047] In the hydration step, as described above, it is more preferable to hold the suspension after adding the seed crystals at a temperature of 65 to 90° C. for 2 hours or more, and then further hold the temperature of the suspension within a range of room temperature to 60° C. By doing so, the amount of enlarged gypsum dihydrate produced can be further increased.

[0048] In the hydration step, the minimum temperature of the suspension after the addition of the seed crystals is preferably within a range of, for example, room temperature (e.g., 15°C) to 66°C, and may be, for example, any one of 25 to 66°C, 30 to 66°C, 35 to 66°C, and 40 to 66°C. By setting the minimum temperature within such a range, the amount of enlarged gypsum dihydrate produced can be further increased. Setting the temperature higher within the range of 30 to 66°C is more advantageous for enlarging the gypsum dihydrate.

[0049] In the hydration step, after starting to maintain the temperature of the suspension at 90°C or less after the addition of seed crystals, the temperature of the suspension is preferably maintained at 75 to 90°C for a certain period of time, and after maintaining the temperature at 75 to 90°C for a certain period of time, the temperature may be further maintained at 35 to 74°C (for example, 60 to 74°C) for a certain period of time. By maintaining the temperature of the suspension for a certain period of time in this manner, the amount of gypsum dihydrate produced is further increased. When the temperature of the suspension is maintained at 35 to 74°C for a certain period of time, it is preferable to limit the fluctuation range of the temperature of the suspension to within 5°C.

[0050] In the hydration step, as described above, the time for which the temperature of the suspension after the addition of seed crystals is maintained at 75 to 90°C (sometimes referred to in this specification as the "maintenance time before temperature reduction") is preferably 1 hour or longer, and may be, for example, 1 hour or longer, 3 hours or longer, or 5 hours or longer. When the maintenance time before temperature reduction is equal to or longer than the lower limit, the amount of enlarged gypsum dihydrate produced increases. On the other hand, in order to prevent the hydration step from being excessively long, the holding time before cooling is preferably 10 hours or less.

[0051] In the hydration step, as described above, the temperature of the suspension after the addition of seed crystals is maintained at 75 to 90°C for a certain period of time, and then further maintained at 35 to 74°C for a certain period of time, which is preferably 1 to 5 hours, and may be 2 to 4 hours. By maintaining the temperature of the suspension for a certain period of time in this manner, the amount of enlarged gypsum dihydrate produced increases. This period of time is particularly suitable for suppressing the temperature fluctuation range of the suspension to within 5°C as described above.

[0052] The presence or absence of gypsum dihydrate in the hydration step can be confirmed by a known method. For example, the crystals obtained in the hydration step can be analyzed by X-ray diffraction (XRD), and the generation of gypsum dihydrate can be confirmed by the disappearance of peaks derived from gypsum hemihydrate and the appearance and increase in peak intensity of peaks derived from gypsum dihydrate in the obtained spectrum.

[0053] The gypsum dihydrate produced by the hydration step can be extracted by removing liquid components such as the aqueous medium using a known method, such as filtration, decantation, or centrifugation. Filtration can be carried out using a filtering device such as a rotary screen, a drum filter, a disc filter, a Nutsche filter, a filter press, a screw press, or a tube press. The centrifugation can be carried out using, for example, a decanter-type centrifuge such as a screw decanter or a screen decanter. In particular, since the obtained gypsum dihydrate has large particles, deliquoring (removal of liquid components) during filtration of the suspension is extremely easy, filterability is excellent, and gypsum dihydrate can be extracted extremely efficiently by filtration.

[0054] The removed gypsum dihydrate may be further washed with water, an organic solvent, or the like. The organic solvent is preferably one in which gypsum dihydrate is insoluble and has a low boiling point, and examples of such organic solvents include alcohols such as methanol and ethanol.

[0055] In the present embodiment, the conversion rate of gypsum hemihydrate to gypsum dihydrate (hydration rate of gypsum hemihydrate) can be calculated, for example, by performing thermogravimetry (TG) on a sample of gypsum hemihydrate used for suspension in an aqueous medium and a sample of gypsum dihydrate, which is the target product, and further performing thermogravimetry on the crystals obtained in the hydration step, and quantifying the amount of gypsum hemihydrate in the crystals obtained in the hydration step using the data obtained on the sample. In addition, by using these data, the purity of the gypsum dihydrate obtained in the hydration process can also be calculated.

[0056] The major axis of the gypsum dihydrate obtained by the above production method is preferably 160 μm or more, and may be, for example, 180 μm or more or 200 μm or more. Gypsum dihydrate with such a major axis has not been known in the past. The minor axis of the gypsum dihydrate obtained by the above production method is preferably 100 μm or more. Gypsum dihydrate with such a minor axis has not been known before. It is preferable that the gypsum dihydrate obtained by the above production method satisfies both of these conditions for the major diameter and the minor diameter.

[0057] The surface area of ​​the gypsum obtained by the above manufacturing method is 5000 μm 2 It is preferable that the thickness is equal to or larger than 10000 μm, for example. 2 More than 13000μm 2 or more, and 16000 μm 2 Any of the above may be used. The larger the size of the gypsum dihydrate, the larger the surface area of ​​the gypsum dihydrate tends to be. The upper limit of the surface area of ​​the gypsum dihydrate is not particularly limited. 2 Gypsum dihydrate, which is less than 100%, is easier to manufacture. The surface area of ​​gypsum dihydrate can be calculated, for example, from the measured value of the surface area per unit mass (specific surface area).

[0058] ◇Gypsum dihydrate Gypsum dihydrate according to one embodiment of the present invention has a major axis of 160 μm or more and a minor axis of 100 μm or more. Such large-sized gypsum dihydrate has not been known in the past. The gypsum dihydrate of this embodiment is particularly suitable for use in producing gypsum boards.

[0059] The gypsum dihydrate of this embodiment can be produced by the method for producing gypsum dihydrate according to one embodiment of the present invention described above.

[0060] The major axis of the gypsum dihydrate of this embodiment may be either 180 μm or more or 200 μm or more, and the minor axis may be 120 μm or more.

[0061] The surface area of ​​the gypsum dihydrate in this embodiment is, for example, as described above. [Example]

[0062] The present invention will be described in more detail below with reference to specific examples, although the present invention is not limited to the examples shown below.

[0063] <<Production and evaluation of gypsum dihydrate>> [Example 1] Gypsum board waste with an average particle size of approximately 1 mm, from which the gypsum board base paper had been removed, was heated at 160°C for 1 hour to produce gypsum hemihydrate from the gypsum dihydrate in the gypsum board waste (heat treatment process).

[0064] Next, pure water (850 mL) was added to the obtained gypsum hemihydrate (150 g) at room temperature, and the mixture was stirred to suspend the gypsum hemihydrate in water. The resulting suspension was then heated to 84°C and stirred at 200 rpm using a stirring blade. Gypsum dihydrate seed crystals (7.5 g) were added thereto, and stirring was continued for 2 hours while maintaining the temperature at 84°C. The gypsum dihydrate seed crystals were obtained by heat-treating waste gypsum board at 140°C, extracting gypsum dihydrate from the waste gypsum board, and hydrating the resulting hemihydrate at 70°C for 24 hours. The major axis of the seed crystals was 10 to 200 μm, and the minor axis was 5 to 100 μm. Then, a portion of the suspension was collected, filtered, and washed with methanol to obtain crystals. Next, while continuing to stir the suspension, the temperature of the suspension was lowered to 66°C over 4 hours, and a portion of the suspension was collected, filtered, and washed with methanol to obtain crystals. Immediately thereafter, while continuing to stir the suspension, the temperature of the suspension was lowered to 54°C over 5 hours, and a portion of the suspension was collected, filtered, and washed with methanol to obtain crystals. Immediately after stirring the suspension, the temperature was lowered to 30°C over 8 hours or more, and a portion of the suspension was collected, filtered, and washed with methanol to obtain crystals. The time required from the addition of the seed crystals to the collection of the suspension was 24 hours. As a result of the above, gypsum dihydrate was produced from gypsum hemihydrate (hydration process). The conditions for the heat treatment step and hydration step are shown in Table 1.

[0065] The four crystals obtained above were analyzed by XRD. The X-ray diffraction spectrum data obtained at this time is shown in Figure 1. Furthermore, these four crystals were analyzed by SEM. The SEM image data obtained at this time is shown in Figure 2. Furthermore, the conversion rate of gypsum hemihydrate to gypsum dihydrate (hydration rate of gypsum hemihydrate) was calculated by thermogravimetry (TG), and the conversion rates were 46.7% (84°C), 80.3% (66°C), 81.6% (54°C), and 87.9% (30°C).

[0066] As is clear from Fig. 1, the spectrum of gypsum hemihydrate was not observed at the stage when the temperature of the suspension was lowered to 66°C after the addition of the seed crystals. As is clear from Fig. 2, large gypsum dihydrate was produced at the stage when the temperature of the suspension was lowered to 66°C.

[0067] [Example 2] As in Example 1, a heat treatment step was carried out. Next, in the same manner as in Example 1, gypsum hemihydrate was suspended in water, and the resulting suspension was heated to a temperature of 84°C and stirred, and seed crystals (7.5 g) were added thereto. Then, a portion of the suspension was immediately collected, filtered, and washed with methanol to obtain crystals. Thereafter, the hydration step was carried out in the same manner as in Example 1 to obtain three types of crystals, and a total of four types of crystals were evaluated. The X-ray diffraction spectrum data obtained at this time is shown in FIG. 3. Furthermore, SEM image data of these four types of crystals are shown in FIGS. 6 to 9 together with the data from Example 1. Furthermore, the conversion rates of gypsum hemihydrate to gypsum dihydrate (hydration rate of gypsum hemihydrate) were calculated to be 9.6% (84°C), 84.6% (66°C), 87.2% (54°C), and 90.5% (30°C).

[0068] [Example 3] Four types of crystals were obtained and evaluated in the same manner as in Example 1, except that after the addition of the seed crystals, the temperature of the suspension was maintained at 84°C and stirring was continued for 4 hours instead of 2 hours. X-ray diffraction spectrum data obtained at this time is shown in FIG. 4. Furthermore, SEM image data of these four types of crystals are shown in FIGS. 6 to 9. Furthermore, the conversion rates of gypsum hemihydrate to gypsum dihydrate (hydration rate of gypsum hemihydrate) were calculated to be 64.6% (84°C), 81.8% (66°C), 86.5% (54°C), and 86.5% (30°C).

[0069] [Example 4] As in Example 1, a heat treatment step was carried out. Next, pure water (850 mL) was added to the obtained gypsum hemihydrate (150 g) at room temperature, and the mixture was stirred to suspend the gypsum hemihydrate in water. The resulting suspension was then heated to 84°C and stirred at 200 rpm using a stirring blade. The same seed crystals (7.5 g) as used in Example 1 were added thereto, and stirring was continued for 6 hours while maintaining the temperature at 84°C. Then, a portion of the suspension was immediately collected, filtered, and washed with methanol to obtain crystals. Immediately after stirring the suspension, the temperature was lowered to 30°C over 8 hours or more, and a portion of the suspension was collected, filtered, and washed with water to obtain crystals. The time required from the addition of the seed crystals to the collection of the suspension was 24 hours. As a result of the above, gypsum dihydrate was produced from gypsum hemihydrate (hydration process).

[0070] The two types of crystals obtained above were analyzed by XRD in the same manner as in Example 1. The X-ray diffraction spectrum data obtained at this time is shown in FIG. 5. Furthermore, these two types of crystals were analyzed by SEM in the same manner as in Example 1. The SEM image data obtained at this time are shown in FIGS. 6 to 9. Furthermore, the conversion rate of gypsum hemihydrate to gypsum dihydrate (hydration rate of gypsum hemihydrate) was calculated to be 62.5% (84°C) and 84.8% (30°C).

[0071] 3 to 5, regardless of whether the time for which the temperature of the suspension was maintained at 84° C. after the addition of the seed crystals (maintenance time before cooling) was 0, 4, or 6 hours (Examples 2 to 4), the spectrum of gypsum hemihydrate was not observed when the temperature of the suspension was lowered to 30° C. In Examples 2 and 3, the spectrum of gypsum hemihydrate was already not observed when the temperature of the suspension was lowered to 66° C. 6 to 9, large particles of gypsum dihydrate were formed when the temperature of the suspension was lowered. When the holding time before temperature lowering was 4 hours and 6 hours (Examples 3 and 4), large particles of gypsum dihydrate were already formed before the temperature was lowered (84°C). As is clear from the above results of the conversion rate of gypsum hemihydrate to gypsum dihydrate, there was a tendency that the conversion rate of gypsum hemihydrate to gypsum dihydrate increased as the temperature of the suspension was lowered, regardless of the time for which the temperature of the suspension was held at 84°C (holding time before temperature drop).

[0072] [Example 5] As in Example 1, a heat treatment step was carried out. Next, in the same manner as in Example 1, gypsum hemihydrate was suspended in water, the temperature of the resulting suspension was increased to 84°C, and the suspension was stirred. To the suspension, seed crystals (7.5 g) were added, and the temperature was maintained at 86°C, followed by stirring for 2 hours. Commercially available crystals of gypsum dihydrate (manufactured by Kanto Chemical Co., Inc.) were used as seed crystals. The major axis of these seed crystals was clearly smaller than the major axis of the seed crystals used in Example 1, being less than 40 µm. Then, a portion of the suspension was collected, filtered, and washed with methanol to obtain crystals. Immediately thereafter, while continuing to stir the suspension, the temperature of the suspension was lowered to 66°C over 4 hours, and a portion of the suspension was collected, filtered, and washed with methanol to obtain crystals. Immediately thereafter, while continuing to stir the suspension, the temperature of the suspension was lowered to 54°C over 5 hours, and a portion of the suspension was collected, filtered, and washed with methanol to obtain crystals. Immediately thereafter, while continuing to stir the suspension, the temperature of the suspension was lowered to 40°C over 7 hours, and a portion of the suspension was collected, filtered, and washed with methanol to obtain crystals. Immediately thereafter, while continuing to stir the suspension, the temperature of the suspension was lowered to 30°C over 5 hours or more, and a portion of the suspension was collected, filtered, and washed with methanol to obtain crystals. As a result of the above, gypsum dihydrate was produced from gypsum hemihydrate (hydration process).

[0073] The five types of crystals obtained above were analyzed by XRD in the same manner as in Example 1. The X-ray diffraction spectrum data obtained at this time is shown in Figure 10. Furthermore, of these five types of crystals, SEM image data of four types of crystals obtained by lowering the temperature of the suspension to 40°C is shown in Figure 11.

[0074] As is clear from Fig. 10, the spectrum of gypsum hemihydrate was not observed at the stage when the temperature of the suspension was lowered to 66°C after the addition of the seed crystals. As is clear from Fig. 11, at the stage when the temperature of the suspension was lowered to 66°C, large-sized gypsum dihydrate was mainly produced. As is clear from FIGS. 2 and 11, Example 1, in which seed crystals with a large major axis were used, tended to produce gypsum dihydrate with a larger major axis than Example 5.

[0075] [Example 6] Four types of crystals were obtained and evaluated in the same manner as in Example 3, except that the amount of seed crystals added was 1.5 g instead of 7.5 g. The X-ray diffraction spectrum data obtained at this time is shown in FIG. 12. Furthermore, SEM image data of these four types of crystals is shown in FIG. 13. Furthermore, the conversion rates of gypsum hemihydrate to gypsum dihydrate (hydration rate of gypsum hemihydrate) were calculated to be 6.7% (84°C), 49% (66°C), 86.7% (54°C), and 88.3% (30°C).

[0076] [Example 7] Four types of crystals were obtained and evaluated in the same manner as in Example 3, except that the amount of seed crystals added was 4.5 g instead of 7.5 g. The X-ray diffraction spectrum data obtained at this time is shown in FIG. 14. Furthermore, SEM image data of these four types of crystals is shown in FIG. 15. Furthermore, the conversion rates of gypsum hemihydrate to gypsum dihydrate (hydration rate of gypsum hemihydrate) were calculated to be 22.7% (84°C), 85.6% (66°C), 86.9% (54°C), and 91.1% (30°C).

[0077] As is clear from Fig. 12, in Example 6, the spectrum of gypsum hemihydrate was not observed at the stage when the temperature of the suspension was lowered to 54°C after the seed crystals were added. As is clear from Fig. 13, large-sized gypsum dihydrate was mainly produced at the stage when the temperature of the suspension was lowered to 54°C. As is clear from Fig. 14, in Example 7, the spectrum of gypsum hemihydrate was not observed at the stage when the temperature of the suspension was lowered to 66°C after the seed crystals were added. As is clear from Fig. 15, at the stage when the temperature of the suspension was lowered to 66°C, large-sized gypsum dihydrate was mainly produced.

[0078] As is clear from FIGS. 6 to 9, in Example 3 in which the amount of seed crystals added was 7.5 g, large gypsum dihydrate particles were already formed at the stage before the temperature of the suspension was lowered (84° C.). That is, a comparison of Examples 3, 6, and 7 showed that the larger the amount of seed crystals added, the more likely large gypsum dihydrate particles were to be produced, even if the temperature of the suspension after the addition of the seed crystals was high.

[0079] As is clear from the above results of the conversion rate of gypsum hemihydrate to gypsum dihydrate, there was a tendency that the conversion rate of gypsum hemihydrate to gypsum dihydrate increased as the temperature of the suspension was lowered, regardless of the amount of seed crystals added.

[0080] [Example 8] As in Example 1, a heat treatment step was carried out. Next, pure water (850 mL) was added to the obtained gypsum hemihydrate (150 g) at room temperature, and the mixture was stirred to suspend the gypsum hemihydrate in water. The resulting suspension was then heated to 84°C and stirred at 200 rpm using a stirring blade. The same seed crystals (4.5 g) as used in Example 1 were added thereto, and stirring was continued for 4 hours while maintaining the temperature at 84°C. Then, a portion of the suspension was immediately collected, filtered, and washed with methanol to obtain crystals. Immediately thereafter, while continuing to stir the suspension, the temperature of the suspension was lowered to 66°C over 2 hours, and a portion of the suspension was collected, filtered, and washed with methanol to obtain crystals. Next, while continuing to stir the suspension, the temperature of the suspension was maintained at 66° C. for 3 hours. During this time, stirring was continued and at the stages of 1 hour, 2 hours, and 3 hours, portions of the suspension were sampled, filtered, and washed with methanol to obtain crystals. Immediately thereafter, while continuing to stir the suspension, the temperature of the suspension was lowered to 30°C over 5 hours, and a portion of the suspension was collected, filtered, and washed with methanol to obtain crystals. As a result of the above, gypsum dihydrate was produced from gypsum hemihydrate (hydration process).

[0081] The six crystals obtained above were analyzed by XRD. The X-ray diffraction spectrum data obtained at this time is shown in Figure 16. Furthermore, these six crystals were analyzed by SEM. The SEM image data obtained at this time is shown in Figure 17.

[0082] [Example 9] Six types of crystals were obtained and evaluated in the same manner as in Example 8, except that the amount of seed crystals added was changed from 4.5 g to 7.5 g. The X-ray diffraction spectrum data obtained at this time is shown in Figure 18. Furthermore, SEM image data of these six types of crystals is shown in Figure 19.

[0083] As is clear from Fig. 16 and Fig. 18, regardless of the amount of seed crystals added (in any of Examples 8 and 9), the spectrum of gypsum hemihydrate was not observed immediately after the temperature of the suspension was lowered to 66°C after the seed crystals were added. 17 and 19, regardless of the amount of seed crystals added, a large amount of large-sized gypsum dihydrate was produced immediately after the temperature of the suspension was lowered to 66° C. When Examples 8 and 9 were compared, no clear difference was observed in the rate of production of large-sized gypsum dihydrate.

[0084] [Example 10] The heat treatment step was carried out in the same manner as in Example 8, and then seed crystals (4.5 g) were added to the suspension, and the temperature was maintained at 84° C. while stirring was continued for 4 hours. Then, a portion of the suspension was immediately collected, filtered, and washed with methanol to obtain crystals. Immediately thereafter, while continuing to stir the suspension, the temperature of the suspension was lowered to 73°C over 1 hour, and a portion of the suspension was collected, filtered, and washed with methanol to obtain crystals. Next, while continuing to stir the suspension, the temperature of the suspension was maintained at 73° C. for 3 hours. During this time, stirring was continued and at the stages of 1 hour, 2 hours, and 3 hours, portions of the suspension were sampled, filtered, and washed with methanol to obtain crystals. Immediately thereafter, while continuing to stir the suspension, the temperature of the suspension was lowered to 30°C over 6 hours, and a portion of the suspension was collected, filtered, and washed with methanol to obtain crystals. As a result of the above, gypsum dihydrate was produced from gypsum hemihydrate (hydration process). These six types of crystals were analyzed by XRD. The X-ray diffraction spectrum data obtained at this time is shown in Figure 20. Furthermore, these six types of crystals were analyzed by SEM. The SEM image data obtained at this time is shown in Figure 21.

[0085] As is clear from FIG. 20, after the seed crystals were added, the temperature of the suspension was lowered to 73° C. and this temperature was maintained for 1 hour, and then the spectrum of gypsum hemihydrate was not observed. As is clear from FIG. 21, when the temperature of the suspension was lowered to 73° C. and maintained at this temperature for 1 hour, a large amount of large gypsum dihydrate was produced.

[0086] 16 and 17 and FIGS. 20 and 21, it was found that when the temperature of the suspension was initially lowered (first-stage temperature lowering temperature), i.e., in Example 8, large particles of gypsum dihydrate were produced more quickly than in Example 10.

[0087] [Example 11] As in Example 1, a heat treatment step was carried out. Next, pure water (750 mL) was added to the obtained gypsum hemihydrate (250 g) at room temperature, and the mixture was stirred to suspend the gypsum hemihydrate in water. The resulting suspension was then heated to 84°C and stirred at 200 rpm using a stirring blade. The same seed crystals (7.5 g) as used in Example 1 were added thereto, and stirring was continued for 4 hours while maintaining the temperature at 84°C. Then, a portion of the suspension was immediately collected, filtered, and washed with methanol to obtain crystals. Next, while continuing to stir the suspension, the temperature of the suspension was lowered to 66°C over 2 hours, and a portion of the suspension was collected, filtered, and washed with methanol to obtain crystals. Next, while continuing to stir the suspension, the temperature of the suspension was maintained at 66° C. for 3 hours. During this time, stirring was continued and at the stages of 1 hour, 2 hours, and 3 hours, portions of the suspension were sampled, filtered, and washed with methanol to obtain crystals. Immediately thereafter, while continuing to stir the suspension, the temperature of the suspension was lowered to 30°C over 5 hours, and a portion of the suspension was collected, filtered, and washed with methanol to obtain crystals. As a result of the above, gypsum dihydrate was produced from gypsum hemihydrate (hydration process).

[0088] The six crystals obtained above were analyzed by XRD. The X-ray diffraction spectrum data obtained at this time is shown in Figure 22. Furthermore, these six crystals were analyzed by SEM. The SEM image data obtained at this time is shown in Figure 23.

[0089] As is clear from FIG. 22, after the seed crystals were added, the temperature of the suspension was lowered to 66° C. and this temperature was maintained for 1 hour, and then the spectrum of gypsum hemihydrate was not observed. As is clear from FIG. 23, when the temperature of the suspension was lowered to 66° C. and maintained at this temperature for 1 hour, a large amount of large gypsum dihydrate was produced.

[0090] 16 and 22, it was found that when the concentration of gypsum hemihydrate in the suspension was lower, that is, in Example 8, larger particles of gypsum dihydrate tended to be produced more quickly than in Example 11.

[0091] [Example 12] As in Example 1, a heat treatment step was carried out. Next, pure water (850 mL) was added to the obtained gypsum hemihydrate (150 g) at room temperature, and the mixture was stirred to suspend the gypsum hemihydrate in water. The resulting suspension was then heated to 84°C and stirred at 200 rpm using a stirring blade. The same seed crystals (4.5 g) as used in Example 1 were added thereto, and stirring was continued for 4 hours while maintaining the temperature at 84°C. Then, a portion of the suspension was immediately collected, filtered, and washed with methanol to obtain crystals. Immediately thereafter, while continuing to stir the suspension, the temperature of the suspension was lowered to 66°C over 2 hours, and a portion of the suspension was collected, filtered, and washed with methanol to obtain crystals. Next, while continuing to stir the suspension, the temperature of the suspension was maintained at 66° C. for 20 hours. During this time, stirring was continued, and at the time points of 1 hour, 2 hours, 3 hours, and 20 hours, portions of the suspension were sampled, filtered, and washed with methanol to obtain crystals. As a result of the above, gypsum dihydrate was produced from gypsum hemihydrate (hydration process).

[0092] The six crystals obtained above were analyzed by XRD. The X-ray diffraction spectrum data obtained at this time is shown in Figure 24. Furthermore, these six crystals were analyzed by SEM. The SEM image data obtained at this time is shown in Figure 25.

[0093] Furthermore, the specific surface areas of these six crystals were measured, and the surface areas of the crystals were calculated from the measured values. Figure 26 shows a graph showing the relationship between the time during the hydration process (hydration time), the temperature during the hydration process (hydration temperature), and the surface area of ​​the crystals.

[0094] As is clear from FIG. 24, after the seed crystals were added, the temperature of the suspension was lowered to 66° C. and this temperature was maintained for 1 hour, and then the spectrum of gypsum hemihydrate was not observed. As is clear from FIG. 25, when the temperature of the suspension was lowered to 66° C. and maintained at this temperature for 1 hour, a large amount of large gypsum dihydrate was produced. As is clear from Figure 26, the surface area of ​​gypsum dihydrate crystals tended to increase with increasing hydration time. In the early stage of the hydration process, the surface area of ​​the crystals increased rapidly with a rapid drop in hydration temperature, and after the hydration temperature became constant, the surface area of ​​the crystals increased gradually.

[0095] [Comparative Example 1] Gypsum board waste with an average particle size of approximately 1 mm, from which the gypsum board base paper had been removed, was heated at 220°C for 1 hour to produce gypsum hemihydrate from the gypsum dihydrate in the gypsum board waste. Next, pure water was added to the obtained gypsum hemihydrate at room temperature, and the mixture was stirred to suspend the gypsum hemihydrate in water, thereby obtaining a suspension with a gypsum hemihydrate concentration of 28 mass %. The resulting suspension was then heated to 85° C. and stirred with a stirring blade at a rotation speed of 200 rpm for 8 hours. As a result, gypsum dihydrate was produced from gypsum hemihydrate. Then, a portion of the suspension was collected, filtered, and washed with methanol to obtain crystals. The obtained crystals were analyzed by XRD, and no spectrum of gypsum hemihydrate was observed. However, the crystals were analyzed by SEM, and the major axis was less than 160 μm, and the enlargement of the gypsum dihydrate was inferior to that in Examples 1 to 12.

[0096] [Table 1]

[0097] In Examples 1 to 12, large gypsum dihydrate was obtained using gypsum dihydrate in waste gypsum board material via gypsum hemihydrate. In Examples 1 to 12, the generation of gypsum dihydrate with a major axis of 160 μm or more and a minor axis of 100 μm or more was confirmed from other data in addition to the data shown in the drawings as SEM imaging data of the obtained gypsum dihydrate crystals.

[0098] In contrast, in Comparative Example 1, the degree of enlargement of gypsum dihydrate was inferior to that in Examples 1 to 12. In Comparative Example 1, the heat treatment of the waste gypsum board was more severe than in Examples 1 to 12, and no seed crystals were used for the suspension containing gypsum hemihydrate. [Industrial Applicability]

[0099] INDUSTRIAL APPLICABILITY The present invention can be used to regenerate gypsum dihydrate contained in waste gypsum board materials and to manufacture gypsum boards using this regenerated gypsum dihydrate.

Claims

1. A method for producing gypsum dihydrate using gypsum board waste, The manufacturing method includes a heat treatment step of heat-treating the gypsum board waste at a temperature of 180 ° C. or less to generate gypsum hemihydrate from gypsum dihydrate in the gypsum board waste; a hydration step of suspending the gypsum hemihydrate in an aqueous medium and adding gypsum dihydrate seed crystals having a major axis of 20 μm or more, and maintaining the temperature of the suspension obtained at 90° C. or less to produce gypsum dihydrate from the gypsum hemihydrate.

2. The method for producing dihydrate gypsum according to claim 1, wherein in the hydration step, the temperature of the suspension is maintained at 65 to 90°C for 2 hours or more.

3. The method for producing dihydrate gypsum according to claim 1 or 2, wherein the seed crystals are added only once in the hydration step.

4. Gypsum dihydrate having a major axis of 160 μm or more and a minor axis of 100 μm or more.

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