High-specific-gravity gypsum hardened body and method for producing high-specific-gravity gypsum hardened body

Abstract

The present invention develops simple technology for making it possible to achieve a novel gypsum hardened body with increased compressive strength that is not achievable in conventional gypsum boards, without undermining the advantages of a gypsum board as a building material, namely fire prevention / resistance properties, sound insulation, dimensional stability, and ease of construction.　Provided is a high-specific-gravity gypsum hardened body filled with raw gypsum particles, said high-specific-gravity gypsum hardened body being a plate-shaped molded product formed of a mixture that includes: a calcined gypsum slurry which comprises calcined gypsum and water; and raw gypsum particles, wherein the dry specific gravity of said molded product is 1.46-1.9, and the raw gypsum particles have a grain size based on sieving whereby the grain size of particles retained on a granularity sieve is not less than 0.3 mm and particles having sizes up to not more than 0.3 times the thickness of the plate-shaped molded product are included. Also provided is a method for producing the high-specific-gravity gypsum hardened body.

Description

High-specific-gravity gypsum hardened body and method for producing high-specific-gravity gypsum hardened body

[0001] The present invention relates to a high-specific-gravity gypsum hardened body containing gypsum raw stone particles and a method for manufacturing the high-specific-gravity gypsum hardened body. More specifically, the present invention relates to a technology for providing a gypsum hardened body having a high dry specific gravity of 1.46 to 1.9 compared to conventional gypsum boards, which is formed by molding a mixture containing calcined gypsum slurry and gypsum raw stone particles sieved to include particles with a particle size of 0.3 mm or more remaining on the sieve and up to 0.3 times the thickness of the plate-shaped gypsum hardened body (plate-shaped molded product). The molded product, which is the high-specific-gravity gypsum hardened body of the present invention, is not limited to gypsum boards having gypsum base paper above and below the gypsum layer, but can also be applied to gypsum boards without base paper. The following description will mainly use gypsum boards as a representative example.

[0002] Gypsum board products are widely used for the interior walls and ceilings of buildings such as houses, schools, and buildings. Gypsum board is made of inorganic gypsum as the core material, and both sides are covered with gypsum board base paper. It has excellent features such as fire resistance, sound insulation, dimensional stability, and ease of construction, and is also economical, making it the most widely used building material as an "indispensable building material." For this reason, standardization by JIS is progressing so that gypsum board products of the correct quality can be selected depending on the application and purpose of construction. For example, thickness (mm) and mass per unit area (kg / m ２ Product standards for gypsum boards are established based on factors such as the specific gravity, bending breaking load (N), flame retardancy, and heat generation. Standardized gypsum board products are mass-produced efficiently as high-quality products by an established gypsum board manufacturing method (see Patent Documents 1 to 3, etc.) that uses calcined gypsum slurry and gypsum base paper, making it possible to construct many high-quality buildings.

[0003] JP 2000-71218 A JP 2001-300933 A JP 2006-82487 A

[0004] As described above, gypsum board products are extremely useful building materials, possessing excellent characteristics such as fire resistance, sound insulation, dimensional stability, and ease of construction that cannot be obtained with other building materials. However, although they have a certain degree of strength, they suffer from the problem of being inferior to other building materials such as wood in terms of strength, such as compressive strength. The inventors recognized that if the strength of conventional gypsum board products could be increased without impairing the properties of the products, the value of gypsum board as a building material could be further increased, and that if this could be achieved, it would be possible to provide buildings using gypsum board of more optimal quality, which would be a useful technology.

[0005] Therefore, an object of the present invention is to develop a simple technology that can realize a new gypsum hardened body (gypsum board and gypsum plate) that has increased compressive strength not achieved by conventional gypsum boards, without compromising the advantages of conventional gypsum boards as a building material, such as fire resistance, sound insulation, dimensional stability, and ease of construction. Another object of the present invention is to develop a technology that can easily provide a gypsum hardened body (a plate-shaped molded product such as a gypsum board or gypsum plate) with an appropriately adjusted compressive strength. A further object of the present invention is to develop a technology that can easily provide a gypsum hardened body (a plate-shaped molded product such as a gypsum board or gypsum plate) that is also highly recyclable.

[0006] The above object is achieved by the present invention described below. Specifically, the present invention provides the following high-specific-gravity gypsum hardened body (a plate-shaped molded product such as a gypsum board or a gypsum plate): [1] A high-specific-gravity gypsum hardened body having gypsum raw stone particles filled therein, the high-specific-gravity gypsum hardened body being a plate-shaped molded product made of a mixture containing a calcined gypsum slurry made of calcined gypsum and water and gypsum raw stone particles, the molded product having a dry specific gravity of 1.46 to 1.9, and the gypsum raw stone particles have a particle size of 0.3 mm or more remaining on a particle size sieve and have been sieved to include particles having a size of up to 0.3 times the thickness of the plate-shaped molded product.

[0007] Preferred embodiments of the high-specific-gravity gypsum hardened body of the present invention include the following. [2] The high-specific-gravity gypsum hardened body according to the above item [1], wherein the particles of the gypsum raw stone having a size of up to 0.3 times the thickness of the molded product have a particle size of 3 mm or less. [3] The high-specific-gravity gypsum hardened body according to the above item [1] or [2], wherein the filling rate of the gypsum raw stone particles in the molded product is 15 volume % or more and 60 volume % or less. [4] The high-specific-gravity gypsum hardened body according to any one of the above items [1] to [3], wherein the calcined gypsum is beta-type hemihydrate gypsum. [5] The high-specific-gravity gypsum hardened body according to any one of the above items [1] to [4], wherein the particles of the gypsum raw stone are at least one selected from particles of anhydrous gypsum and particles of gypsum dihydrate. [6] The high-specific-gravity gypsum hardened body according to any one of the above items [1] to [5], wherein the compressive strength of the molded product is 25 MPa or more. [7] The high-specific-gravity gypsum hardened body according to [6] above, in which the compressive strength is proportional to the square of the dry specific gravity. [8] The high-specific-gravity gypsum hardened body according to any one of [1] to [7] above, in which the volume change rate when the molded product is heated to 700°C and then cooled to room temperature is 25% or less. [9] The high-specific-gravity gypsum hardened body according to any one of [1] to [8] above, in which the calcined molded product becomes a recyclable material as calcined gypsum.

[10] The high-specific-gravity gypsum hardened body according to any one of [1] to [9] above, in which all cleavage planes of the gypsum raw stone particles are filled with needle-shaped crystals derived from the calcined gypsum slurry without gaps.

[11] The high-specific-gravity gypsum hardened body according to any one of [1] to

[10] above, in which the gypsum raw stone particles further include coarse particles having a size exceeding 0.3 times the thickness of the molded product.

[12] The high-specific-gravity gypsum hardened body according to any one of [1] to

[11] above, wherein the gypsum raw stone particles further contain coarse particles having a particle size exceeding 3 mm.

[0008] As another embodiment, the present invention provides the following method for producing a high-specific-gravity gypsum hardened body.

[13] A method for producing a high-specific-gravity gypsum hardened body having a dry specific gravity of 1.46 to 1.9, in which gypsum raw stone particles are filled inside, the method comprising the steps of: spraying a part of a 0.1 to 3.0% solution of an anionic surfactant or a nonionic surfactant onto the particle surfaces of the gypsum raw stone to wet them; adding a part or all of the remaining part of the 0.1 to 3.0% solution of the anionic surfactant or the nonionic surfactant to the calcined gypsum powder, or adding a part or all of the remaining solution and water, or adding water and mixing to form a calcined gypsum slurry; a premixing step of mixing a part of the calcined gypsum slurry before hardening obtained in the calcined gypsum slurry-forming step with the gypsum raw stone particles to form a premixed slurry; and a mixing step of mixing the premixed slurry obtained in the premixing step with the remaining calcined gypsum slurry remaining after use in the premixing step to form a mixed slurry. The method for producing a high-specific-gravity gypsum hardened body includes: a pouring step of pouring the mixed slurry obtained in the mixing step into a molding member; and a hardening step of allowing the mixed slurry poured in the pouring step to stand and harden.

[0009] Preferred embodiments of the method for producing a high-specific-gravity gypsum hardened body of the present invention include the following:

[14] The method for producing a high-specific-gravity gypsum hardened body according to the above item

[13] , wherein the molding member is a formwork, and the method includes the steps of covering the inner surface of the formwork with a first surface material before pouring the mixed slurry into the formwork, pouring the mixed slurry into the formwork to fill it, and covering the opening of the formwork filled with the mixed slurry with a second surface material.

[15] The method for producing a high-specific-gravity gypsum hardened body according to the above item

[12] or

[13] , wherein the high-specific-gravity gypsum hardened body is a plate-shaped molded product, and the gypsum raw stone particles are sieved to have a particle size of 0.3 mm or more remaining on a particle size sieve and to include particles having a size of 0.3 times or less the thickness of the molded product.

[16] The method for producing a high-specific-gravity gypsum hardened body according to any one of

[13] to

[15] above, wherein the particles of the gypsum raw stone have a particle size of 0.3 mm or more remaining on a particle size sieve and are sieved with the particle size sieve to have a particle size of 3 mm or less.

[0010] According to the present invention, a simple technique is provided that enables the production of new gypsum hardened bodies (gypsum boards and gypsum panels) with increased compressive strength and a dry specific gravity of 1.46 to 1.9, which is not possible with conventional gypsum boards, without compromising the advantages of conventional gypsum boards and gypsum panels as building materials, such as fire resistance, sound insulation, dimensional stability, and ease of construction. Furthermore, according to the present invention, by adjusting the blending amount of gypsum raw stone particles, it is possible to provide a variety of gypsum hardened body products (plate-shaped molded products such as gypsum boards and gypsum panels) with appropriately adjusted compressive strength. Furthermore, according to the present invention, by heating a gypsum hardened body product (plate-shaped molded product such as gypsum boards and gypsum panels) at, for example, 150°C for 3 hours, the entire core can be recycled as calcined gypsum, resulting in a useful effect in terms of effective resource utilization. Furthermore, according to the present invention, by crushing and sieving a gypsum board containing particles of gypsum raw stone, the particles of gypsum raw stone can be easily separated from the remaining matrix portion (core portion), and the extracted particles of gypsum raw stone can be reused as they are, without any processing, as a raw material for high-specific-gravity gypsum hardened body products (gypsum boards and gypsum boards) filled with the particles of the gypsum raw stone of the present invention. This provides a useful effect.

[0011] According to the technology of the present invention, by varying the packing ratio of gypsum raw stone particles of a specific particle size specified in the present invention incorporated into a calcined gypsum slurry, it is possible to provide gypsum boards and gypsum sheets (molded products) that are high-specific-gravity gypsum hardened bodies with various specific gravities. For example, the higher the packing ratio of the gypsum raw stone particles, the more the amount of calcined gypsum powder and the amount of water mixed can be reduced. Reducing the amount of calcined gypsum powder means that the amount of gypsum dihydrate, the raw material for the calcined gypsum used in the calcined gypsum slurry, can be reduced. This saves on the gypsum raw material and significantly reduces the fuel cost for dry-process calcination of the powder to produce calcined gypsum, resulting in an economical advantage that is extremely important in practical use. According to the present invention, it is possible to provide various molded products that can be used for various purposes, for example, with a packing ratio of gypsum raw stone particles of 15% by volume or more and 60% by volume or less, depending on the intended use.

[0012] 1 is a graph showing the relationship between the specific gravity and compressive strength of gypsum board, which is a hardened gypsum body, when gypsum raw stone particles are not used as a raw material and when gypsum raw stone particles are used as a raw material. In the figure, ■ marks are graphs for gypsum boards containing 1 to 3 mm gypsum raw stone, and ● marks are graphs for gypsum boards without raw stone. This is an electron microscope photograph showing a portion of the interface between gypsum raw stone particles (a) and a needle-like crystal layer (b) derived from a calcined gypsum slurry, in which the cleavage plane of the gypsum raw stone particles (a) is filled without gaps with the needle-like crystals (b) derived from the calcined gypsum slurry. This is an electron microscope photograph showing another portion of the interface between gypsum raw stone particles (a) and a needle-like crystal layer (b) derived from a calcined gypsum slurry, in which the cleavage plane of the gypsum raw stone particles (a) is filled without gaps with the needle-like crystals (b) derived from the calcined gypsum slurry. This is a diagram showing the appearance of a plate-shaped gypsum hardened body (gypsum board) with a specific gravity of 1.72, in which gypsum raw stone particles having a particle size of 1.0 mm or more and 2.0 mm or less are filled so that the filling rate is 42.0%.

[0013] The present invention will be described in more detail below with reference to preferred embodiments. The present inventors have conducted extensive research to solve the problems of the prior art described above, and have arrived at the present invention. In response to the above problems, the present inventors investigated the possibility of increasing the compressive strength of gypsum boards by increasing the specific gravity of the boards. As a result, they discovered that the compressive strength of gypsum boards and gypsum plates can be effectively increased by blending gypsum raw stone particles of a specific particle size (particle size) with a calcined gypsum slurry conventionally used in the manufacture of gypsum boards, thereby achieving a dry specific gravity of 1.46 to 1.9. Hereinafter, gypsum boards will be described as a representative example.

[0014] In conventional technology, there is a limit to how much water can be mixed to turn calcined gypsum powder into a slurry, making it impossible to produce gypsum board products with a specific gravity of 1.46 or higher. Conversely, even if ultra-lightweight products with a specific gravity of less than 0.5 could be produced, they would be too fragile to maintain their shape and would be unusable and therefore impractical. According to the inventors' research, even if strength was increased by adding, for example, a few percent of glass chops, the shape would not be maintained. In this regard, the inventors believe that because conventional gypsum boards maintain their shape solely with raw materials containing calcined gypsum, the effect of adding reinforcing materials on improving strength is limited. Because gypsum boards are used in large quantities as a building material, standardization is underway in an effort to standardize building quality. For the reasons explained above, the specific gravity of many gypsum board products that meet JIS standards is low, ranging from 0.65 to 0.90, 0.90 to 1.45, or 0.75 to 0.95, with a maximum of 1.45. According to the studies of the present inventors, the specific gravity of general-purpose gypsum hardened bodies (gypsum boards and gypsum plates) actually available on the market is 0.5 to 1.2.

[0015] Lighter building materials are desired from the perspectives of transportability and workability. For example, as described in Patent Publication No. 5,894,356, a foaming agent is added and stirred to mix gas into the gypsum layer, forming unstable bubbles in the slurry and reducing the specific gravity. This technique creates unstable bubbles in the slurry, which can lead to problems of filler detachment, uneven distribution, and shape change over time. In contrast, the inventors have developed a gypsum board made by blending gypsum raw stone particles of a specific particle size with the calcined gypsum slurry used in gypsum board production. This eliminates the problems of filler detachment, uneven distribution, and shape change over time, resulting in a building material with stable durability and quality. On the other hand, as mentioned above, it is not desirable to increase the specific gravity too much for gypsum-based molded products such as gypsum boards and gypsum panels, which are building materials that require lightweight construction. The present inventors have conducted extensive research into increasing the strength, such as compressive strength, of a gypsum hardened body without making the specific gravity too high, and as a result have arrived at the present invention.

[0016] FIG. 1 is a graph showing the relationship between the specific gravity and compressive strength of a gypsum hardened body (gypsum board) obtained through the inventors' research. As shown in FIG. 1, the inventors have newly discovered that the compressive strength of a gypsum hardened body (gypsum board) is proportional to the square of the specific gravity. Until now, the relationship between the specific gravity and compressive strength of gypsum boards has been considered to be linearly proportional, and the above findings obtained through the present invention are contrary to conventional technical common sense. The inventors consider this point as follows. As mentioned above, gypsum board products are required to be lightweight, and the majority of conventional gypsum board products that have been studied to date have specific gravities in the low specific gravity range of 0.65 or more and 1.2 or less, and this range is narrow. Due to this, it is believed that the relationship between specific gravity and compressive strength has been generally understood to be linearly proportional, as can be seen from the graph marked with a black circle in FIG. 1, which shows the relationship between specific gravity and compressive strength of conventional gypsum board products. In the following description, the gypsum hardened body (a plate-shaped gypsum molded product) will also be simply referred to as gypsum board.

[0017] The present inventors have investigated a method for increasing the specific gravity of gypsum board by filling the interior with gypsum raw stone particles to achieve a high specific gravity. Specifically, they conducted extensive research into the possibility of effectively increasing the compressive strength of gypsum board without increasing the specific gravity too much. As a result, they discovered the following and arrived at the present invention. Specifically, they discovered that by blending gypsum raw stone particles within a specific particle size range, sieved to have a particle size of 0.3 mm or more remaining on a particle size sieve and 0.3 times or less the thickness of the gypsum board (a plate-shaped gypsum molded product), with a calcined gypsum slurry used in manufacturing gypsum board, and then hydrating and hardening the mixture to produce a gypsum board, it is possible to provide a gypsum board with a dry specific gravity of 1.46 to 1.9, inclusive, and with increased compressive strength without increasing the specific gravity too much.

[0018] FIG. 1 is a graph showing the relationship between the specific gravity and compressive strength of gypsum boards, based on the results of measuring the compressive strength [MPa] of each of the following gypsum boards with different configurations. The results indicated by ● in FIG. 1 are for gypsum boards with a dry specific gravity of 0.67 to 1.29 and no gypsum ore particles filled inside. The results indicated by ■ in FIG. 1 are for gypsum boards with a dry specific gravity increased from 1.46 to 1.88 by filling the interior with gypsum ore particles with a particle size of 1.0 mm or more and 3.0 mm or less, which falls within the specific particle size range specified in the present invention. As shown in FIG. 1, it was confirmed that, with the high correlation shown below, it can be concluded that the compressive strength y of gypsum boards is proportional to the square of the specific gravity x under each condition (the presence or absence of filled ore particles or different particle sizes).

[0019] (1) Conventional structure without filling gypsum raw stone particles inside (shown by ● in Figure 1, dry specific gravity 0.67 to 1.29) y = 51.572x ２ -70.287x-26.915(R ２ (2) The structure of the present invention in which particles of gypsum raw stone with a particle size of 1.0 mm to 3.0 mm are filled inside (shown by ■ in Figure 1, the dry specific gravity is increased to 1.46 to 1.88) y = 39.313x ２-85.964x-68.138(R ２ = 0.952)

[0020] As shown in Figure 1, a comparison between a conventional configuration in which gypsum raw stone particles are not filled inside and a configuration of the present invention in which gypsum raw stone particles with a particle size of 1.0 mm to 3.0 mm are filled inside shows that increasing the specific gravity of a gypsum board to 1.46 or more and 1.9 or less by using a configuration in which gypsum raw stone particles within a specific size range specified in the present invention are filled inside greatly contributes to improving the compressive strength of the gypsum board.

[0021] The high-specific-gravity gypsum hardened body of the present invention is characterized by its specific gravity being adjusted to a high level of 1.46 to 1.9 by using gypsum raw stone particles of a specific particle size sieved to contain gypsum raw stone particles with a particle size of 0.3 mm or more remaining on a particle size sieve and up to 0.3 times the thickness of the high-specific-gravity gypsum hardened body (such as gypsum board) in combination with a calcined gypsum slurry, but otherwise is the same as conventional gypsum boards, with no particular difference in the raw materials used. According to the inventors' studies, a preferred form that enables the desired specific gravity and strength to be appropriately obtained in the plate-shaped molded product of the present invention, whose specific gravity is specified to be 1.46 to 1.9, is to set the packing rate of the specific gypsum raw stone particles that characterize the present invention and that constitute the molded product to approximately 15% to 60% by volume. In order to further increase the compressive strength of the hardened body, for example, it is advisable to set the filling rate of the specific gypsum raw stone particles that make up the molded product (hardened body) to about 37% by volume or more and 60% by volume or less. The gypsum raw stone particles that characterize the present invention and the effects of incorporating these particles will be described below.

[0022] The gypsum raw stone particles are not particularly limited, and for example, any one selected from anhydrous gypsum particles and dihydrate gypsum particles can be used, or these can be used in combination. According to the studies of the present inventors, the gypsum raw stone particles used in the present invention can be used even if they are low in purity. Therefore, gypsum raw stone particles prepared as described below can also be used. For example, the high-specific-gravity gypsum board of the present invention, which is filled with gypsum raw stone particles, can be easily separated into the gypsum raw stone particles and the matrix portion (core portion) consisting of the calcined gypsum slurry by crushing and sieving. Furthermore, since the purity of the gypsum raw stone particles constituting the present invention does not matter, the extracted coarse particles can be reused as they are to manufacture gypsum boards containing gypsum raw stone particles without any processing.

[0023] Furthermore, when the gypsum raw stone particles constituting the present invention are gypsum dihydrate, the following advantages are obtained. In the above-described configuration, when a gypsum board is formed, the calcined gypsum in the calcined gypsum slurry hydrates and hardens with water to form gypsum dihydrate, so that the core portion of the gypsum board is formed entirely of gypsum dihydrate. As a result, when the gypsum board is recycled, by calcining the gypsum board (for example, by heating at 150°C for about 3 hours in a small calcination furnace), the gypsum dihydrate in the core portion is dehydrated and converted into calcined gypsum. Therefore, the above-described configuration is convenient for reuse as a raw material for manufacturing gypsum board.

[0024] Furthermore, calcined gypsum is also called hemihydrate gypsum, and there are two types: alpha (α) hemihydrate gypsum and beta (β) hemihydrate gypsum. However, it is preferable to use β hemihydrate gypsum as the calcined gypsum used in the present invention. That is, according to the studies of the present inventors, when α hemihydrate gypsum is used as the calcined gypsum, a gypsum hardened body with a higher specific gravity can be formed, but the compressive strength of the gypsum hardened body tends to decrease and cracks tend to occur more easily. For this reason, it is preferable to refrain from using α hemihydrate gypsum in the present invention and use β hemihydrate gypsum as the main component. It is also possible to use α hemihydrate gypsum together with the main component β hemihydrate gypsum, as long as it does not impair the object of the present invention.

[0025] The gypsum particles characterizing the present invention are characterized by gypsum particles of a specific particle size that have been sieved to include gypsum particles having a particle size of 0.3 mm or more remaining on a particle size sieve and a size of up to 0.3 times the thickness of a high-specific-gravity gypsum hardened body (such as a gypsum board), which is a plate-shaped molded product. According to the inventors' studies, the gypsum particles constituting the present invention, although depending on the thickness of the gypsum hardened body (such as a gypsum board), are preferably gypsum particles having a particle size of 0.3 mm or more remaining on a particle size sieve and a size of up to 3 mm. Furthermore, in order to more stably obtain the effects of the present invention, it is preferable to prepare the high-specific-gravity gypsum hardened body (such as a gypsum board) of the present invention using gypsum particles having a particle size of 1.0 mm or more remaining on a particle size sieve and a size of up to 3.0 mm.

[0026] After extensive research, the inventors discovered that by setting the particle size of the gypsum raw stone characteristic of the present invention to 0.3 mm or more and 0.3 times or less the thickness of the resulting gypsum hardened body (plate-shaped molded product) of gypsum board (e.g., 3 mm or less for a 10 mm thickness), the gypsum raw stone particles filled inside the gypsum hardened body (e.g., gypsum board) can be easily arranged in three or more rows in the thickness direction of the gypsum hardened body (e.g., gypsum board). According to the inventors' research, this results in a gypsum hardened body with a high specific gravity that is resistant to stress from any direction. The thickness of the molded product used to determine the upper limit of the gypsum raw stone particles to be filled inside the plate-shaped molded product is determined, for example, in the case of gypsum board, by using a desired product available. Gypsum boards of the following thicknesses are standardized by JIS, and standardized products having these thicknesses are available. Ordinary gypsum boards are standardized in thicknesses of 9.5 mm, 12.5 mm, and 15.0 mm, while reinforced gypsum boards are standardized in thicknesses of 12.5 mm, 15.0 mm, 16.0 mm, 18.0 mm, 21.0 mm, and 25.0 mm, and are sold as products.

[0027] In contrast to the above, if the particle size of the gypsum raw stone particles filled inside the gypsum board is less than 0.3 mm, it is difficult to mix the calcined gypsum, water, and gypsum raw stone particles to form a slurry, and the dihydrate gypsum particles derived from the calcined gypsum used together, which have a particle size of about 0.1 mm, are difficult to fill between the particles of the coexisting gypsum raw stone, which is not preferable. Also, if the particle size of the gypsum raw stone particles filled inside the gypsum board is coarse and exceeds 0.3 times the thickness of the obtained gypsum hardened body (such as a gypsum board), it is difficult to arrange the gypsum raw stone particles in three or more rows in the thickness direction of the gypsum hardened body, which is not preferable because it may result in poor handleability during production, or the large gypsum raw stone particles may not be properly fixed by the dihydrate gypsum particles derived from the calcined gypsum, which may have an adverse effect on the strength of the gypsum hardened body. The high-specific-gravity gypsum hardened body of the present invention (for example, high-specific-gravity gypsum board) may be filled therein with an appropriate amount of gypsum rough stone particles having a particle size of 0.3 mm to 0.3 times or less the thickness of the gypsum hardened body (such as a gypsum board) as specified in the present invention. Gypsum rough stone particles having a particle size of less than 0.3 mm or coarse gypsum rough stone particles having a particle size of more than 0.3 times the thickness of the resulting gypsum hardened body (such as a gypsum board) may be added, as long as the amount is within a range that does not adversely affect the production, strength, etc. of the gypsum hardened body.

[0028] In the present invention, the amount of gypsum raw stone particles having the above-mentioned specific particle size to be blended into the calcined gypsum slurry must be determined so that the resulting gypsum hardened body, i.e., gypsum board or gypsum plate, has a high specific gravity of 1.46 to 1.9. According to the inventors' studies, by appropriately blending gypsum raw stone particles having a particle size of 0.3 mm or more and 0.3 times or less the thickness of the resulting gypsum hardened body (e.g., gypsum board), the specific gravity of the resulting gypsum board can be freely adjusted to be within the range of 1.46 to 1.9. This makes it possible to flexibly adjust the physical properties, such as the strength, of the gypsum board or gypsum plate. Furthermore, when adjusting the specific gravity of a gypsum board or the like to be within the range of 1.46 to 1.9, a preferred embodiment of the present invention is to adjust the specific gravity using gypsum raw stone particles of two particle sizes: fine particles having a particle size distribution of 1 mm to 3 mm and coarse particles having a particle size distribution of more than 3 mm.

[0029] According to the inventors' investigations, a high-specific-gravity gypsum hardened body, as defined in the present invention, formed by molding a mixture containing calcined gypsum slurry and gypsum raw stone particles with a particle size of 0.3 mm or more and a thickness of 0.3 times or less the thickness of the high-specific-gravity gypsum hardened body (such as gypsum board), and having a specific gravity adjusted to 1.46 to 1.9, can produce a gypsum board or gypsum plate with an unprecedented compressive strength of 25 MPa or more. Furthermore, the high-specific-gravity gypsum hardened body of the present invention has a volume change rate (volume shrinkage rate) of 25% or less when heated to 700°C and then cooled to room temperature. As described below, it has also been found that this has the effect of reducing the volume change rate (volume shrinkage rate) compared to conventional gypsum boards. As mentioned above, the plate-shaped molded product (hardened product) of the present invention preferably has a packing rate of the unique gypsum raw stone particles that characterize the present invention, comprising the molded product, of approximately 15% to 60% by volume.

[0030] 2-1 and 2-2 are electron microscope photographs showing the interface between the gypsum raw stone particles (a) constituting the high-specific-gravity gypsum hardened body of the present invention and the needle-like crystal layer (b) derived from the calcined gypsum slurry. As shown in FIGS. 2-1 and 2-2, in the high-specific-gravity gypsum hardened body of the present invention, the needle-like crystal layer (b) derived from the calcined gypsum slurry densely penetrates and interlocks with all the irregularities (cleavage planes) on the surface of the gypsum raw stone particles (a), and the needle-like crystals derived from the calcined gypsum slurry are packed tightly at the interface of the gypsum raw stone particles. The inventors believe that this is a physical factor that allows the present invention to produce a gypsum board or gypsum plate with improved compressive strength. In addition to the above, in the case of the high-specific-gravity gypsum hardened body of the present invention, the gypsum raw stone particles and the matrix (portion derived from the calcined gypsum slurry) constituting the gypsum layer are made from the same raw materials. Therefore, it is expected that recombination of large-grained gypsum raw stone particles and fine gypsum particles consisting of needle-like crystals can occur at the interface between the gypsum raw stone particles and the matrix. Therefore, the present inventors believe that such chemical factors are also responsible for the remarkable improvement in compressive strength achieved by the configuration of the present invention.

[0031] The gypsum board, which is a high-specific-gravity gypsum hardened body of the present invention that provides the above-mentioned unprecedented effects, can be easily obtained by the manufacturing method of the present invention. The manufacturing method of the present invention is a method for manufacturing a high-specific-gravity gypsum hardened body to obtain a high-specific-gravity gypsum hardened body having a specific gravity of 1.46 to 1.9, but is basically the same as a conventional method for manufacturing gypsum board, except that gypsum raw stone particles of the specific particle size specified in the present invention are used as raw materials. Specifically, it can be prepared as follows. The calcined gypsum slurry used in the step of forming a gypsum layer by pouring calcined gypsum slurry onto gypsum base paper to form a gypsum layer between the top and bottom sheets of gypsum base paper is prepared by mixing calcined gypsum powder with at least gypsum raw stone particles of the specific particle size specified in the present invention and kneading water, and the amount of gypsum raw stone particles constituting the calcined gypsum slurry is adjusted so that the specific gravity of the gypsum board obtained in the step of forming the gypsum layer is 1.46 to 1.9.

[0032] The method for producing a high-specific-gravity gypsum hardened body of the present invention, which is intended to obtain a high-specific-gravity gypsum hardened body having a specific gravity of 1.46 to 1.9, is useful in that it enables material savings and reduced production costs, as described below. Specifically, according to the inventors' studies, a gypsum hardened body produced by adding gypsum ore particles to a calcined gypsum slurry can be obtained with a smaller amount of calcined gypsum slurry than when producing a conventional gypsum hardened body. This allows for reduced amounts of water and water-reducing agent. Furthermore, because the gypsum ore itself can be used as the raw material for the gypsum hardened body, the production volume required for producing calcined gypsum powder by crushing, heating, and dehydrating the gypsum ore, which is the raw material for the calcined gypsum slurry, can be reduced compared to conventional production methods. As a result, material savings and reduced production costs are possible. In addition to the manufacturing advantages described above, the manufacturing method of the present invention also improves the properties and durability (strength) of the resulting gypsum hardened body, as described above. Therefore, the method for producing a high-specific-gravity gypsum hardened body of the present invention is useful for efficient and sustainable construction and manufacturing processes.

[0033] A specific manufacturing method for obtaining the high-specific-gravity gypsum hardened body of the present invention can be carried out, for example, by the following procedure. (1) Prepare a 0.1-3.0% solution of an anionic surfactant or nonionic surfactant, for example, a solution adjusted to a concentration of approximately 0.5%. (2) Next, use a portion of this solution to evenly spray and wet the surfaces of gypsum raw stone particles. This operation allows gypsum raw stone particles of a specific particle size to blend well with the calcined gypsum slurry used in combination. (3) To prepare the calcined gypsum slurry, add the remaining anionic surfactant or nonionic surfactant solution used in (2) to the calcined gypsum, add additional water as needed, or in some cases add only water without the surfactant solution, and knead to form a calcined gypsum slurry. (4) Then, mix a portion of the calcined gypsum slurry that has not yet begun to harden onto the surfaces of the moistened gypsum raw stone particles that will serve as nuclei, so that it coats them. This process may be repeated several times. (5) After the particles of gypsum raw stone are sprinkled on the calcined gypsum slurry and blended, the remaining calcined gypsum slurry is added and mixed to obtain a slurry mixture. (6) The slurry mixture thus obtained is poured into a molding member, for example, a form, filled therewith, allowed to stand, hardened, and then dried to obtain the high-specific-gravity gypsum hardened body of the present invention.

[0034] Before pouring the slurry mixture into the formwork in step (6) above, a first surface material may be applied to cover the inner surface of the formwork, which is a molding component (hereinafter, the formwork will be described as a representative example of a molding component). Furthermore, after pouring and filling the slurry mixture, a second surface material may be applied to cover the opening of the formwork from above the mixture. In this manner, the high-specific-gravity gypsum hardened body has its entire surface covered with the surface material. The first and second surface materials may be the same or different. The surface material may be appropriately selected from, for example, gypsum board base paper, glass fiber nonwoven fabric, glass mat, etc. Among these, it is preferable to use gypsum board base paper for both. Figure 3 shows the appearance of an example of a high-specific-gravity gypsum hardened body (gypsum board) obtained in this manner, with a specific gravity of 1.72, a thickness of 10 mm, and a gypsum raw stone particle filling rate of 42.0%.

[0035] Next, the present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0036] <Particle Size Adjustment of Gypsum Raw Stone Particles> First, gypsum raw stone particles adjusted to have specific particle size distributions to be used in each of the Examples and Comparative Examples were prepared by sieving as described below.

[0037] (Particle Size Adjustment of Gypsum Raw Stone Particles Having a Particle Size Distribution of 1 mm to 3 mm) The gypsum dihydrate raw stone particles were sieved through a sieve with mesh openings of 3 mm, and the gypsum dihydrate raw stone particles that fell below the sieve were further sieved through a sieve with mesh openings of 1 mm, and the particles that remained on the sieve were collected, thereby obtaining gypsum raw stone particles having a particle size distribution of 1 mm to 3 mm.

[0038] (Particle size adjustment of gypsum raw stone particles having a particle size distribution of less than 0.3 mm) The dihydrate gypsum raw stone particles were sieved through a sieve with mesh openings of 0.3 mm, and the particles that remained below the sieve were collected to obtain gypsum raw stone particles having a particle size distribution of less than 0.3 mm.

[0039] <Preparation of Surfactant Solution> Next, a surfactant solution was prepared by adding water to the surfactant to adjust the concentration to about 0.5% as described below. Specifically, when the entire amount of the obtained surfactant solution was added to hemihydrate gypsum to form a calcined gypsum slurry, the amount of water relative to 100% by mass of powdered hemihydrate gypsum was the amount of mixed water shown in Table 1 by mass, and the surfactant solution was adjusted to a concentration of about 0.5% by mass. As shown in Table 1, for example, in all Examples, the surfactant solution was added and kneaded in an amount equivalent to 53% by mass of the weight (mass) of the hemihydrate gypsum in terms of mixed water.

[0040] <Cained gypsum slurry used> The main component of the calcined gypsum slurry used in the present invention is calcined gypsum. Calcined gypsum is classified into α-type hemihydrate gypsum and β-type hemihydrate gypsum depending on the calcination method. α-type hemihydrate gypsum is produced by a wet method and is obtained by calcining gypsum dihydrate in water (including in steam). β-type hemihydrate gypsum is produced by a dry method and is obtained by calcining gypsum dihydrate in the atmosphere. In the following examples, calcined gypsum slurries were prepared using α-type hemihydrate gypsum or β-type hemihydrate gypsum used in the production of ordinary gypsum boards, and a high-specific-gravity gypsum hardened body, which is a plate-shaped molded product, was obtained from a mixture containing the calcined gypsum slurry and gypsum raw stone particles according to the procedure of the production method of the present invention described above as follows.

[0041] [Examples 1 to 5, Comparative Examples 1 to 7] In the examples and comparative examples, high-specific-gravity gypsum hardened bodies were obtained as follows. An outline of this process will be described below. First, gypsum dihydrate raw stone particles, the particle size distribution of which had been adjusted in the above-described <Particle Size Adjustment of Gypsum Raw Stone Particles>, were prepared in the amounts shown in Table 1 for use in each of Examples 1 to 5 and Comparative Example 7. Then, a portion of the surfactant solution obtained in the above-described <Preparation of Surfactant Solution> was sprayed onto each of the gypsum dihydrate raw stone particles using a sprayer to evenly wet their surfaces.

[0042] Next, starch was added as an adhesive aid to the calcined gypsum in the formulation shown in Table 1, and the remaining surfactant solution after spraying was added and stirred to obtain a calcined gypsum slurry. The types and amounts of calcined gypsum used above, as well as specific numerical values ​​for the amount of starch, are shown in Table 1. In the examples of the present invention, as described below, a premixed slurry was obtained by premixing a portion of the calcined gypsum slurry obtained above with gypsum raw stone particles of a specific particle size specified in the present invention, and the remaining calcined gypsum slurry was mixed to obtain a mixed slurry. The resulting mixed slurry was used to prepare a high-specific-gravity gypsum hardened body using the procedure described above. In Comparative Examples 1 to 5, since gypsum raw stone particles were not blended, there was no need to use a premixed slurry. In Comparative Examples 1 to 5, a normal mixed slurry for obtaining a hardened body could be obtained with the formulation shown in Table 1.

[0043] (Comparative Example 6) In this comparative example, as shown in Table 1, in order to obtain a dried hardened body with a target specific gravity of 1.6 without blending particles of gypsum dihydrate raw stone, 123 g of β-hemihydrate gypsum was added with 36 mass% of a surfactant solution (converted to the weight of the β-hemihydrate gypsum) and kneaded. However, lumps formed and the mixture could not be kneaded properly, resulting in powder remaining and making it impossible to form a calcined gypsum slurry. For this reason, in this comparative example, a molded body required to obtain a hardened body could not be produced.

[0044] Comparative Example 7 As shown in Table 1, in Comparative Example 7, ultrafine gypsum dihydrate raw stone particles of less than 0.3 mm, which are outside the range specified in the present invention and were obtained by sieving, were blended. Then, in order to obtain a dried hardened body with a target specific gravity of 1.63, 67 g of β-hemihydrate gypsum and 67 g of the ultrafine gypsum dihydrate raw stone particles were added with the surfactant solution in an amount of 53 mass% based on the weight of the gypsum hemihydrate, calculated as a mixed water amount, and kneaded. However, lumps formed, making it difficult to knead properly, and powder remained, making it impossible to form a calcined gypsum slurry. Therefore, in this Comparative Example, it was not possible to produce a molded body to obtain a hardened body.

[0045] Examples 1 to 5 In each example, a calcined gypsum slurry was mixed with gypsum raw stone particles of a specific particle size specified in the present invention as shown in Table 1 as described below to obtain a mixed slurry, and a high-specific-gravity gypsum hardened body was prepared using the obtained mixed slurry. First, a portion of the calcined gypsum slurry in a state before hardening, obtained in the above-mentioned step of obtaining a calcined gypsum slurry, was gradually mixed with the dihydrate gypsum raw stone particles that had been wetted by spraying a surfactant solution in the above-mentioned spraying and wetting step, and the calcined gypsum slurry was sprinkled on the dihydrate gypsum raw stone particles and blended to obtain a premixed slurry. The remaining calcined gypsum slurry was further mixed with the obtained premixed slurry to obtain a mixed slurry that was a slurry-like mixture.

[0046] Next, in each of Examples 1 to 5, the mixed slurry obtained as described above was used to prepare a hardened product as follows. First, a form for obtaining a plate-shaped molded product measuring 80 mm x 240 mm x 10 mm was prepared for each example. The inner surface of the form was then covered with gypsum board base paper, and the mixed slurry obtained above was poured into the form to fill it. The top surface of the form was then leveled with a trowel so that the top surface of the formwork was flush with the top surface of the hardened product to be prepared. The gypsum board base paper was then placed over the top of the mixture, pressing it down so as not to dent the top surface, and the mixture was allowed to stand for 20 minutes to harden the mixed slurry to a certain extent. The partially hardened mixture was then removed from the form, turned over, and allowed to stand for another 20 minutes to completely harden the mixture, yielding a wet gypsum hardened product.

[0047] Next, the entire surface of the wet gypsum hardened body obtained above was wrapped in kraft paper, and in this state, the wet gypsum hardened body was placed in a high-temperature oven and heated for 20 minutes at 220° C. After heating, the gypsum hardened body was removed from the high-temperature oven, and the kraft paper was removed. Then, the gypsum hardened body was dried for 16 hours in a constant-temperature dryer at 40° C. to obtain fully dried gypsum hardened bodies of Examples 1 to 5.

[0048] (Comparative Examples 1 to 5) Gypsum hardened bodies of the comparative examples were obtained in the same manner as in Examples 1 to 5, except that the material poured into the formwork was a calcined gypsum slurry not mixed with particles of gypsum raw stone. As mentioned above, in Comparative Examples 6 and 7, it was not possible to produce a molded body necessary to obtain a hardened body.

[0049]

[0050] [Evaluation] The cured bodies of Examples 1 to 5 and Comparative Examples 1 to 5 obtained as described above were 10 mm thick plates with base paper used in manufacturing gypsum boards attached to both sides, and the properties were measured as described below, and the volumetric shrinkage rate and fire resistance were evaluated. The results are summarized in Table 2.

[0051] <Measurement of specific gravity of dried hardened body> Each of the gypsum boards, which were dried gypsum hardened bodies measuring 80 mm × 240 mm × 10 mm obtained in Examples 1 to 5 and Comparative Examples 1 to 5, was placed in a dryer at 40°C for 24 hours, and then its weight and dimensions were measured, and the specific gravity was calculated from the measured values. The obtained results are shown in Table 2.

[0052] <Measurement of Actual Packing Ratio of Gypsum Raw Stone Particles> For each of the high-specific-gravity gypsum hardened bodies (gypsum boards) of Examples 1 to 5 and Comparative Examples 1 to 5, which were filled with gypsum raw stone particles of a specific particle size obtained as described above, the actual packing ratio of the gypsum raw stone particles was calculated using the following formula. The obtained results are shown in Table 2.

[0053] The filling rate x of the gypsum raw stone particles constituting the high-specific-gravity gypsum hardened body of the present invention relative to the hardened calcined gypsum slurry (matrix = gypsum core) is calculated from Ax + B(1-x) = C by any of the following calculation formulas. [In the above formula, A, B, and C respectively represent the following: A: specific gravity of gypsum raw stone particles, B: specific gravity of the dried and hardened body when hardened using only calcined gypsum slurry (no gypsum raw stone), C: specific gravity of the entire dried and hardened body]

[0054] <Measurement of compressive strength> After measuring the <specific gravity of the dried hardened body> described above, four 30 mm x 30 mm x 10 mm test pieces were cut from each of the gypsum boards, which were 80 mm x 240 mm x 10 mm dried gypsum hardened bodies of Examples 1 to 5 and Comparative Examples 1 to 5, to prepare test samples. Each test sample obtained was pressurized at 2 N / s using an Amsler testing machine (manufactured by Mayekawa Testing Machinery Co., Ltd., product name: TYPE MS.10C.BCTW) to measure the compressive strength. The arithmetic mean value of each measured value for the four test samples was calculated, and the results obtained are summarized in Table 2 as compressive strength.

[0055] <Measurement of Total Water Absorption> After measuring the specific gravity in the above-mentioned <Measurement of Specific Gravity of Dried Hardened Body>, a test sample measuring 70 mm x 80 mm x 10 mm was cut from each gypsum board, which was the 80 mm x 240 mm x 10 mm dried gypsum hardened body of Examples 1 to 4 and Comparative Examples 1 and 4. Each test sample was placed in a dryer at 40°C for 24 hours, and then the weight of the test sample (before immersion in water) was measured. After immersion in room temperature water for 24 hours, the weight of the test sample (after immersion in water) was measured. Using these measured values, the total water absorption was calculated by {weight of test sample (after immersion in water) - weight of test sample (before immersion in water) / weight of test sample (before immersion in water)} x 100. The results are summarized in Table 2.

[0056] <Fire Resistance Test> After measuring the <Total Water Absorption> described above, a fire resistance test was carried out as follows using each of the test sample specimens for Examples 1 to 4 and Comparative Examples 1 and 4 used in the measurement. Each specimen was dried in a dryer at 40°C for 48 hours, and then heated in a high-speed heating oven according to the heating program of the ISO 834 fire resistance test. The time required to reach 220°C, the temperature at which combustion begins on the back surface, and the temperature of the heated surface at that time were measured. The results obtained are summarized in Table 2.

[0057]

[0058] As shown in Table 2, it was confirmed that all of the high-specific-gravity gypsum hardened bodies of the examples of the present invention exhibited a high dry specific gravity of 1.46 to 1.9, both of which were not achieved by conventional techniques, and a high compressive strength of 25 MPa or more. Furthermore, as shown in Table 2, it was confirmed that the gypsum hardened bodies of Comparative Examples 1 to 5, which did not contain gypsum raw stone particles, were unable to exhibit a high compressive strength of 25 MPa or more. In addition, it was found that the gypsum hardened bodies of Comparative Examples 1 to 4, which used β-type hemihydrate gypsum in the matrix portion and did not contain gypsum raw stone particles, were unable to achieve the dry specific gravity of 1.46 or more that is the objective of the present invention. The dry specific gravity of the gypsum hardened body of Comparative Example 5 was 1.90, confirming that the use of α-type hemihydrate gypsum resulted in a gypsum hardened body with a high specific gravity. However, it was found that the compressive strength of the hardened body could not be increased by using gypsum raw stone particles.

[0059] 3 shows the appearance of a 10 mm thick, plate-shaped gypsum board with a specific gravity of 1.72, which corresponds to an example of the present invention. This gypsum board is constructed by filling the interior of the board with gypsum raw stone particles, which have been sieved to include particles of a specific particle size of 1.0 mm or more and 2.0 mm or less, and then filling the gypsum board with the particles at a filling rate of 42.0%. This gypsum board was manufactured in the same manner as in the example described above.

[0060] a: Gypsum raw stone particles b: Needle crystal layer derived from calcined gypsum slurry

Claims

1. A high-specific-gravity gypsum hardened body with gypsum raw stone particles filled inside, characterized in that the high-specific-gravity gypsum hardened body is a plate-shaped molded product made from a mixture containing a calcined gypsum slurry made of calcined gypsum and water and gypsum raw stone particles, the dry specific gravity of the molded product being 1.46 to 1.9, and the gypsum raw stone particles have a particle size of 0.3 mm or more remaining on a particle size sieve and are sieved to include particles up to 0.3 times the thickness of the plate-shaped molded product.

2. The high-specific-gravity gypsum hardened body according to claim 1, wherein the particles of the gypsum raw stone having a size of up to 0.3 times the thickness of the molded product have a particle size of 3 mm or less.

3. A high-specific-gravity gypsum hardened body according to claim 1 or 2, wherein the filling rate of the gypsum raw stone particles in the molded product is 15% by volume or more and 60% by volume or less.

4. A high-specific-gravity gypsum hardened body according to any one of claims 1 to 3, wherein the calcined gypsum is beta-type hemihydrate gypsum.

5. A high-specific-gravity gypsum hardened body according to any one of claims 1 to 4, wherein the gypsum raw stone particles are at least one selected from the group consisting of gypsum anhydride particles and gypsum dihydrate particles.

6. A high-specific-gravity gypsum hardened body according to any one of claims 1 to 5, wherein the compressive strength of the molded product is 25 MPa or more.

7. The high-specific-gravity gypsum hardened body according to claim 6, wherein the compressive strength is proportional to the square of the dry specific gravity.

8. A high-specific-gravity gypsum hardened body according to any one of claims 1 to 7, wherein the volume change rate when the molded product is heated to 700°C and then cooled to room temperature is 25% or less.

9. The high-specific-gravity gypsum hardened body according to any one of claims 1 to 8, wherein the calcined molded product becomes a recyclable material as calcined gypsum.

10. A high-specific-gravity gypsum hardened body according to any one of claims 1 to 9, in which all cleavage planes of the gypsum raw stone particles are filled without gaps with needle-shaped crystals derived from the calcined gypsum slurry.

11. A high-specific-gravity gypsum hardened body according to any one of claims 1 to 10, wherein the particles of the gypsum raw stone further contain coarse particles having a thickness exceeding 0.3 times the thickness of the molded product.

12. A high-specific-gravity gypsum hardened body according to any one of claims 1 to 11, wherein the gypsum raw stone particles further contain coarse particles having a particle size exceeding 3 mm.

13. A method for producing a high-specific-gravity gypsum hardened body having a dry specific gravity of 1.46 to 1.9, in which gypsum raw stone particles are filled inside, comprising the steps of: spraying a portion of a 0.1 to 3.0% solution of an anionic surfactant or a nonionic surfactant onto the surface of the gypsum raw stone particles to wet them; adding a portion or all of the remaining portion of the 0.1 to 3.0% solution of the anionic surfactant or the nonionic surfactant to the calcined gypsum powder, or adding a portion or all of the remaining solution and water, or adding water and mixing to form a calcined gypsum slurry; a premixing step of mixing a portion of the calcined gypsum slurry obtained in the calcined gypsum slurry-forming step with the gypsum raw stone particles before hardening begins to obtain a premixed slurry; and a mixing step of mixing the premixed slurry obtained in the premixing step with the remaining calcined gypsum slurry remaining after use in the premixing step to obtain a mixed slurry. The method for producing a high-specific-gravity gypsum hardened body includes: a pouring step of pouring the mixed slurry obtained in the mixing step into a molding member; and a hardening step of allowing the mixed slurry poured in the pouring step to stand and harden.

14. A method for producing a high-specific-gravity gypsum hardened body as described in claim 13, wherein the molding member is a formwork, and before pouring the mixed slurry into the formwork, the inner surface of the formwork is covered with a first surface material, the mixed slurry is then poured into the formwork to fill it, and the opening of the formwork filled with the mixed slurry is covered with a second surface material.

15. A method for producing a high-specific-gravity gypsum hardened body according to claim 13 or 14, wherein the high-specific-gravity gypsum hardened body is a plate-shaped molded product, and the particles of the gypsum raw stone are sieved to include particles with a particle size of 0.3 mm or more remaining on a particle size sieve and having a size of up to 0.3 times the thickness of the molded product.

16. A method for producing a high-specific-gravity gypsum hardened body according to any one of claims 13 to 15, wherein the particles of the gypsum raw stone have a particle size of 0.3 mm or more remaining on a particle size sieve and are sieved with the particle size sieve to have a particle size of 3 mm or less.

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