Gypsum board, manufacturing method of gypsum board

Abstract

It has a gypsum core, The gypsum core is a hardened body of a gypsum slurry containing a gypsum raw material including recycled gypsum recovered from gypsum board waste, water, and foam obtained by previously foaming a foaming agent, The gypsum board has an average diameter of bubbles contained in the gypsum core of 50 μm or more and 650 μm or less.

Description

[Technical Field]

[0001] The present invention relates to a gypsum board and a method for manufacturing a gypsum board. [Background technology]

[0002] Patent Document 1 discloses a method for producing a gypsum board, which includes the steps of preparing a slurry by mixing a gypsum raw material containing 70% by weight or more of recycled gypsum, 0.1% by weight to 1.0% by weight of a water-reducing agent relative to the gypsum raw material, 0.01% by weight to 0.10% by weight of a foaming agent relative to the gypsum raw material, and water, and hardening the slurry, in which the gypsum raw material accounts for 90% by weight or more of the total amount of the slurry excluding the water content. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-63165 Summary of the Invention [Problem to be solved by the invention]

[0004] BACKGROUND ART Gypsum board waste has been conventionally discharged as waste.

[0005] Gypsum board waste includes scraps generated during gypsum board production and new building interior construction. Most of the scraps generated during gypsum board production are recycled by gypsum board manufacturers.

[0006] Another example of gypsum board waste is waste gypsum board generated during building renovations, demolition work, etc. Most of the waste gypsum board generated during building renovations, demolition work, etc. is not recycled, but is disposed of in landfills. Furthermore, waste gypsum board generated during building renovations, demolition work, etc. accounts for a large proportion of all waste gypsum board generated.

[0007] It is expected that the amount of gypsum board waste generated will continue to increase in the future as building demolition increases, but from the perspective of protecting the natural environment and reducing costs, there is a need to reduce the amount of gypsum board waste that is disposed of in landfills.

[0008] Therefore, as disclosed in Patent Document 1 and the like, gypsum boards such as gypsum boards manufactured using waste gypsum boards have been studied for some time.

[0009] However, in order to manufacture the recycled gypsum used in the gypsum board disclosed in Patent Document 1, many steps are required, which poses problems in terms of manufacturing costs and productivity.

[0010] Therefore, it is conceivable to produce gypsum boards using recycled gypsum produced by crushing and calcining waste gypsum boards as the gypsum source. However, waste gypsum boards contain components other than gypsum, and there is a risk that components other than gypsum may be mixed into the recycled gypsum, which is a calcined product of crushed waste gypsum boards.

[0011] Since gypsum boards are used as building materials, etc., they are required to have sufficient compressive strength in order to increase the screw holding power, etc., of screws used to fasten the gypsum boards, etc. However, there was concern that if gypsum boards were manufactured using recycled gypsum as the gypsum source, it would be impossible to manufacture gypsum boards with sufficient compressive strength to withstand practical use.

[0012] In view of the problems of the above-mentioned conventional technology, one aspect of the present invention aims to provide a gypsum board having sufficient compressive strength while using recycled gypsum obtained from waste gypsum board materials as the gypsum raw material. [Means for solving the problem]

[0013] In order to solve the above problem, according to one aspect of the present invention, there is provided a gypsum core, The gypsum core is a hardened body of a gypsum slurry containing a gypsum raw material including recycled gypsum recovered from gypsum board waste, water, and foam obtained by previously foaming a foaming agent, The gypsum board has an average diameter of bubbles contained in the gypsum core of 50 μm or more and 650 μm or less. [Effects of the Invention]

[0014] According to one embodiment of the present invention, a gypsum board having sufficient compressive strength can be provided while using recycled gypsum obtained from waste gypsum board materials as the gypsum raw material. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is an explanatory diagram of a gypsum board according to an embodiment of the present invention. [Figure 2] FIG. 2 is an explanatory diagram of the relationship between the proportion of recycled gypsum in the gypsum raw material and the compressive strength of the obtained gypsum hardened body. [Figure 3A] FIG. 3A is a cross-sectional SEM image of the gypsum core of the gypsum board obtained in Example 1-4. [Figure 3B] FIG. 3B is a cross-sectional SEM image of the gypsum core of the gypsum board obtained in Example 1-4. [Figure 4A] FIG. 4A is a cross-sectional SEM image of the gypsum core of the gypsum board obtained in Comparative Example 1. [Figure 4B] FIG. 4B is a cross-sectional SEM image of the gypsum core of the gypsum board obtained in Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiments, and various modifications and substitutions can be made to the following embodiments without departing from the scope of the present invention. [Gypsum board] Hereinafter, the gypsum board of this embodiment will be described with reference to the drawings.

[0017] FIG. 1 shows a schematic diagram of a gypsum board 10 of this embodiment.

[0018] As shown in FIG. 1, a gypsum board 10 of this embodiment has a gypsum core 11.

[0019] The gypsum board 10 of this embodiment can have a plate-like shape as shown in Fig. 1. The gypsum core 11 has an upper surface 11A and a lower surface 11B located opposite the upper surface 11A. The upper surface 11A and the lower surface 11B are surfaces located at both ends along the thickness.

[0020] A side surface 11C is disposed between the upper surface 11A and the lower surface 11B.

[0021] In Figure 1, the gypsum board 10 and the gypsum core 11 are shown in a rectangular parallelepiped shape, but the gypsum board 10 including the gypsum core 11 can be used as a building material, etc., and is not limited to such a shape, but can be made into an appropriate shape depending on the application.

[0022] 1 shows an example in which the gypsum board 10 is made up of only a gypsum core 11, but is not limited to this form. The gypsum board 10 of this embodiment can be any one selected from, for example, a gypsum board defined in JIS A 6901 (2014), a gypsum board that is lighter or heavier than the gypsum board defined in JIS A 6901 (2014) (hereinafter, the gypsum board defined in the above JIS and the gypsum board that is lighter or heavier than the gypsum board defined in the above JIS are also collectively referred to as "gypsum board"), a glass mat gypsum board, a gypsum-containing board with glass fiber nonwoven fabric, etc.

[0023] Therefore, in the gypsum board 10 of this embodiment, depending on the shape of the gypsum board, for example, board base paper, glass mat, or the like can be further placed as a surface material on the upper surface 11A or the lower surface 11B of the gypsum core 11. Also, glass fiber nonwoven fabric (glass tissue), or the like can be embedded in the upper surface 11A or the lower surface 11B. As shown in FIG. 1, the gypsum board 10 can also be made of only the gypsum core 11, without placing a surface material on the upper surface 11A or the lower surface 11B of the gypsum core 11.

[0024] One method for reusing waste gypsum boards is to use recycled gypsum obtained by crystallizing the gypsum in the waste gypsum boards as a raw material. However, to produce recycled gypsum, the waste gypsum boards must be crushed and fired to form a gypsum slurry, and then crystallization must be carried out in the gypsum slurry, which requires many steps for production. This creates problems in terms of production costs, etc.

[0025] Therefore, the inventors of the present invention have studied gypsum boards using recycled gypsum obtained from waste gypsum boards. The recycled gypsum is gypsum obtained by crushing and calcining waste gypsum boards to produce hemihydrate gypsum. In other words, the recycled gypsum is a calcined product of crushed waste gypsum boards.

[0026] Since recycled gypsum is obtained by crushing and calcining waste gypsum board, the number of manufacturing steps is fewer than that of recycled gypsum, and the energy required in the manufacturing process can also be reduced. Therefore, recycled gypsum can reduce manufacturing costs, etc., compared to recycled gypsum. However, according to the research of the inventors of the present invention, when a gypsum board is manufactured using a gypsum raw material containing recycled gypsum, the compressive strength of the gypsum board may be reduced.

[0027] (1) Examination of compressive strength of gypsum board Therefore, the inventors of the present invention have investigated the cause of the decrease in compressive strength in gypsum boards using recycled gypsum. (1-1) Study on the effect of organic fiber mixing on the compressive strength of gypsum hardened body (Experimental Example 1) First, the inventors of the present invention have investigated the influence of organic fibers, such as paper, attached to recycled gypsum on the compressive strength of a gypsum board made from recycled gypsum.

[0028] To accurately examine the influence of organic fibers, the researchers used recycled gypsum, from which the organic fiber paper had been completely removed, and evaluated the compressive strength of gypsum hardened bodies made by hardening gypsum slurries produced by varying the amount of paper added.

[0029] Specifically, in Experimental Examples 1-1 to 1-4, recycled gypsum, paper, and water were mixed together in the blending ratios shown in Table 1 below to prepare gypsum slurries.

[0030] The water added when preparing the gypsum slurry is added so that the specific gravity of the gypsum hardened body obtained by hardening the gypsum slurry becomes 0.65.

[0031] The gypsum slurry was then poured into a cubic formwork with a side length of 40 mm and dried to a constant weight at 40°C to prepare a hardened gypsum specimen.

[0032] The recycled gypsum was prepared by crushing gypsum board to such an extent that the attached paper could be removed, and then completely removing the paper by sieving, etc. The paper used was paper recovered from the recycled gypsum. (Method for evaluating the compressive strength of hardened gypsum body)

[0033] The compressive strength of the resulting gypsum hardened specimens was measured using an autograph (Shimadzu Corporation, AG-X plus) at a loading rate of 1 mm / min. The compressive strength of each experimental example listed in the table below was measured for six specimens prepared under the same conditions, and the sum of the measured values ​​for the six specimens was divided by 6, the number of specimens, to obtain the average (arithmetic mean) compressive strength of the six specimens. In the other experimental examples below, the compressive strength of the gypsum hardened specimens was evaluated under the same conditions.

[0034] The evaluation results are shown in Table 1.

[0035] [Table 1] The results shown in Table 1 confirmed that the compressive strength of the gypsum hardened body tends to improve by reducing the paper content. However, even when the paper content was 2.5 parts by mass, i.e., 2.5% by mass relative to the gypsum raw material, the compressive strength of the gypsum hardened body was 25 kgf / cm 2It was confirmed that the gypsum hardened body had sufficient compressive strength. 2 By having a compressive strength of 25 kgf / cm or more, when screws are driven into the gypsum hardened body, the gypsum hardened body is prevented from being damaged by excessive driving of the screws, and has enough strength to hold the driven screws. 2 Having the above compressive strength means that when used in a gypsum board, the board has a compressive strength that is sufficient for practical use. (1-2) Examination of the effect of the proportion of recycled gypsum in the gypsum raw material on the compressive strength of the gypsum hardened body (Experimental Example 2) Next, the inventors of the present invention studied the compressive strength of the gypsum hardened body when the ratio of recycled gypsum in the gypsum raw material was changed.

[0036] The mass ratio of recycled gypsum in the gypsum raw material was changed to produce gypsum slurries, and the compressive strength of the gypsum hardened bodies obtained by hardening the gypsum slurries was evaluated.

[0037] Specifically, in Experimental Examples 2-1 to 2-10, gypsum slurries were prepared by mixing new gypsum, recycled gypsum, and water to obtain the blending ratios shown in Table 2 below. Note that water was added when preparing the gypsum slurries so that the specific gravity of the gypsum hardened body obtained by hardening the gypsum slurry would be a predetermined value. Then, gypsum hardened bodies were produced under the same conditions as in Experimental Example 1 except that the above gypsum slurries were used, and the compressive strength was evaluated.

[0038] The recycled gypsum used is recycled gypsum with an attached paper content of 2.5% by mass, while the new gypsum uses a new gypsum raw material that is not recycled.

[0039] The evaluation results are shown in Table 2. Furthermore, Figure 2 shows the relationship between the proportion of recycled gypsum in the gypsum raw material and the compressive strength of the obtained gypsum hardened body.

[0040] [Table 2] According to the results shown in Table 2, it was confirmed that the compressive strength of the gypsum hardened body tends to decrease as the content ratio of recycled gypsum increases. However, even in Experimental Example 2-10, where the content ratio of recycled gypsum in the gypsum source was 100 mass%, the compressive strength of the gypsum hardened body was 25 kgf / cm 2 It was confirmed that the compressive strength was above the above level and was sufficient.

[0041] (1-3) Examination of the cause of change in compressive strength of gypsum hardened body due to increase in the content of recycled gypsum in gypsum raw materials

[0042] However, it was confirmed that the compressive strength of the gypsum hardened body drops sharply when the proportion of recycled gypsum in the gypsum raw material exceeds 30% by mass, as shown in Figure 2. One possible reason for this is an increase in the proportion of paper in the gypsum raw material, but because recycled gypsum contains a certain amount of paper, this is unlikely to be the cause of the sudden drop in compressive strength of the gypsum hardened body when the proportion of recycled gypsum in the gypsum raw material exceeds 30% by mass.

[0043] Therefore, the inventors of the present invention investigated the cause of the sudden decrease in compressive strength of the gypsum hardened body when the proportion of recycled gypsum in the gypsum raw material exceeds 30 mass%.

[0044] Specifically, the content of recycled gypsum in the gypsum raw material was set to 100% by mass, and a gypsum board production line was used to produce gypsum boards in which board base paper was placed on the surface of a gypsum core.

[0045] When the cross section of the gypsum core of the manufactured gypsum board was observed, it was confirmed that there were many bubbles with a diameter of approximately 2 mm or more, which were not observed in the 40 mm square gypsum hardened body.

[0046] In addition, test pieces were cut out from multiple locations on the manufactured gypsum board and the compressive strength was measured. The compressive strength values ​​varied widely, with the minimum value being less than half of the maximum value, at 25 kgf / cm. 2 There were also some points like the following.

[0047] Therefore, a scanning electron microscope was used to observe the cross section of the gypsum core of the manufactured gypsum board. The results confirmed that, despite the absence of foaming agents, in addition to bubbles visible to the naked eye with diameters of approximately 2 mm or more, there were also bubbles of various diameters, including fine bubbles with diameters of 50 μm or less. Furthermore, it was confirmed that bubbles of non-uniform size were randomly arranged within the gypsum core, resulting in a heterogeneous gypsum core and gypsum board. Therefore, it is believed that the localized decrease in compressive strength occurs in areas where bubbles are densely packed.

[0048] It is believed that the bubbles of various diameters contained in the gypsum core of the manufactured gypsum board were generated when a foaming agent attached to the recycled gypsum foamed when the raw materials were mixed in a mixer to prepare the gypsum slurry.

[0049] Therefore, the inventors of the present invention conducted further research and investigated a gypsum board that suppresses the effect of the reduction in strength caused by foaming of the foaming agent contained in recycled gypsum.

[0050] Initially, an attempt was made to reduce or remove bubbles caused by a foaming agent contained in recycled gypsum in the gypsum slurry (hereinafter also referred to as "foams caused by recycled gypsum") by adding an antifoaming agent to the gypsum slurry. However, the method of adding an antifoaming agent to the gypsum slurry was unable to eliminate bubbles with non-uniform diameters.

[0051] Next, when preparing a gypsum slurry to be used as a gypsum core, the researchers attempted to add pre-foamed foam, which was prepared by foaming a foaming agent in advance, to the gypsum slurry. The foam addition conditions, such as the amount of foaming agent used, were adjusted so that the bubbles would have an average diameter of 50 μm or more and 650 μm or less after the gypsum slurry hardened. As a result, the researchers found that adding pre-foamed foam could reduce the impact of bubbles originating from recycled gypsum, leading to the completion of the present invention.

[0052] The inventors of the present invention believe that the reason why the influence of bubbles caused by recycled gypsum can be reduced by adding bubbles formed in advance using a foaming agent to a gypsum slurry is as follows.

[0053] It is believed that bubbles formed in advance with a foaming agent are stable bubbles that are less likely to break than large bubbles resulting from the foaming agent contained in recycled gypsum. Large bubbles resulting from the foaming agent contained in recycled gypsum, which are relatively unstable and easily broken, are dispersed in the gypsum slurry by being kneaded together with bubbles formed in advance with a foaming agent in the gypsum slurry. It is believed that when the gypsum slurry is kneaded, large bubbles resulting from the foaming agent contained in recycled gypsum, which are relatively unstable and easily broken, absorb small bubbles and grow larger by repeatedly coming into contact with relatively stable bubbles formed in advance with a foaming agent, thereby breaking down.

[0054] Therefore, by adding bubbles previously foamed with a foaming agent to the gypsum slurry, large bubbles resulting from the foaming agent contained in the recycled gypsum can be broken down, leaving mainly the added bubbles. As a result, it is believed that a gypsum board having a gypsum core obtained by hardening the gypsum slurry can be produced that has sufficient compressive strength while using recycled gypsum recovered from waste gypsum boards as the gypsum raw material.

[0055] As described above, the variation in compressive strength within a gypsum board may become large, and the average compressive strength of the entire gypsum board may also decrease. Therefore, when evaluating the compressive strength of a gypsum board, if multiple test specimens for evaluating compressive strength can be taken, it is preferable to cut multiple test specimens, for example, 2 to 10, from the gypsum board and evaluate the compressive strength. If the compressive strength of all of the multiple test specimens evaluated is 25 kgf / cm 2 In the above cases, the gypsum board can be said to have particularly sufficient compressive strength.

[0056] (2) About gypsum boards Based on the above study results, in the gypsum board 10 of this embodiment, the gypsum core 11 can be a hardened body of gypsum slurry containing gypsum raw material including recycled gypsum recovered from gypsum board waste, water, and foam created by previously foaming a foaming agent.

[0057] By making the gypsum core 11 a hardened body of gypsum slurry containing a gypsum raw material including recycled gypsum, water, and foams previously foamed with a foaming agent, it is possible to produce a gypsum board having sufficient compressive strength while using recycled gypsum recovered from waste gypsum boards as the gypsum raw material. (2-1) Components of gypsum slurry The gypsum core 11 contained in the gypsum board 10 of this embodiment is a hardened body of gypsum slurry. Components that can be contained in the gypsum slurry will be described below. (Gypsum raw material) The gypsum raw material means a raw material containing gypsum, such as calcined gypsum, used to prepare a gypsum slurry.

[0058] The gypsum feedstock can include recycled gypsum, as described above.

[0059] As mentioned above, recycled gypsum is a calcined product of crushed gypsum board waste. Therefore, recycled gypsum can contain calcined gypsum, i.e., calcium sulfate hemihydrate. Regarding the gypsum component, recycled gypsum can be composed only of calcined gypsum, but gypsum dihydrate may also remain. Because recycled gypsum is made from gypsum board waste, it may contain components other than gypsum, such as paper derived from gypsum board waste.

[0060] The recycled gypsum may contain dry-ground gypsum board waste. The recycled gypsum may be composed of dry-ground gypsum board waste. However, even in this case, it does not exclude the possibility of unavoidable impurities being mixed in during the manufacturing process. Compared to wet-ground gypsum board waste, dry-ground gypsum board waste can suppress the adhesion and mixing of moisture to the ground material during grinding. Therefore, when the recycled gypsum contains dry-ground gypsum board waste, the mixing of gypsum dihydrate can be suppressed. Note that dry-ground gypsum board waste refers to a material obtained by firing ground material obtained by dry-grinding gypsum board waste, i.e., a fired product.

[0061] The content of recycled gypsum in the gypsum raw material is not particularly limited, but while conventional gypsum boards using recycled gypsum have not been able to achieve sufficient strength, the gypsum board of this embodiment can increase strength regardless of the content of recycled gypsum. Therefore, since the gypsum board of this embodiment can achieve particularly high effects, it is preferable that the content of recycled gypsum in the gypsum raw material is high. Therefore, the content of recycled gypsum in the gypsum raw material is preferably, for example, 30% by mass or more, more preferably 50% by mass or more, and even more preferably 65% ​​by mass or more.

[0062] The gypsum raw material can also be composed solely of recycled gypsum, so the content of recycled gypsum in the gypsum raw material can be 100 mass % or less.

[0063] Therefore, for example, the content of recycled gypsum in the gypsum raw material is preferably 30% by mass or more and 100% by mass or less, more preferably 50% by mass or more and 100% by mass or less, and even more preferably 65% ​​by mass or more and 100% by mass or less.

[0064] In addition to recycled gypsum, the gypsum raw material may also contain virgin gypsum raw material, that is, a new gypsum raw material that is not a recycled product.

[0065] As the virgin gypsum raw material, β-type calcined gypsum or α-type calcined gypsum alone, or a mixture of both, can be used. β-type calcined gypsum is obtained by calcining in the atmosphere gypsum that is either natural gypsum, by-product gypsum, flue gas desulfurization gypsum, etc., alone or a mixture of any of these. α-type calcined gypsum is obtained by calcining in water (including in steam) gypsum that is either natural gypsum, by-product gypsum, flue gas desulfurization gypsum, etc., alone or a mixture of any of these. (Foaming agent) The type of foaming agent (foaming agent) to be added to the gypsum slurry to generate foam is not particularly limited. For example, the foaming agent preferably includes one or more selected from alkyl ether sulfates and alkyl sulfates.

[0066] By including one or more types of foaming agent selected from alkyl ether sulfates and alkyl sulfates, it is possible to particularly easily break down large bubbles resulting from the foaming agent contained in recycled gypsum, which are relatively unstable and easily broken down, thereby particularly increasing the compressive strength of the gypsum board 10. The foaming agent may be composed of one or more types selected from alkyl ether sulfates and alkyl sulfates.

[0067] The amount of foaming agent contained in the gypsum slurry is not particularly limited. For example, the amount of foaming agent used to generate bubbles can be selected so that the average diameter of the bubbles contained in the gypsum core 11 falls within the range described below, and the foaming agent can be contained in the gypsum slurry.

[0068] The gypsum slurry can contain a foaming agent in a proportion of, for example, 0.01% by mass or more and 0.1% by mass or less. This is because, when the gypsum slurry contains 0.01% by mass or more of the foaming agent used to generate foam, bubbles of a desired size are generated in a gypsum hardened body such as a gypsum core obtained by hardening the gypsum slurry, thereby particularly increasing the compressive strength of the gypsum board. However, even if a large amount of foaming agent is used to generate foam and the amount of foaming agent contained in the gypsum slurry is increased, there is no significant change in the effect of increasing the compressive strength of the gypsum board, so it is preferable that the gypsum slurry contain the foaming agent in a proportion of 0.1% by mass or less. (additives) The gypsum slurry may further contain various additives.

[0069] Examples of additives include one or more selected from inorganic fibers such as glass fibers, organic fibers, lightweight aggregates, fire-resistant materials such as vermiculite, setting retarders, setting accelerators, water-reducing agents, bubble size adjusters such as sulfosuccinate surfactants, water repellents such as silicone and paraffin, organic carboxylic acids, organic carboxylates, and organic phosphate compounds.

[0070] For example, when the gypsum slurry contains a bubble diameter adjusting agent, the size of the bubbles and air bubbles contained in the gypsum slurry and the gypsum core 11 can be adjusted particularly easily.

[0071] When the gypsum board has a surface base paper such as a gypsum board, an adhesion improver such as starch or polyvinyl alcohol that improves the adhesion between the gypsum core 11 and the gypsum board base paper can be used as an additive. (2-2) Average diameter of air bubbles contained in the gypsum core The average diameter of the bubbles contained in the gypsum core 11 is preferably 50 μm or more and 650 μm or less.

[0072] By making the average diameter of the bubbles contained in the gypsum core 11 equal to or greater than 50 μm, the compressive strength of the gypsum board 10 can be particularly increased.

[0073] By setting the average diameter of the bubbles contained in the gypsum core 11 to 650 μm or less, it is possible to prevent large voids from occurring on the surface of the gypsum core 11, for example, on the upper surface 11A or the lower surface 11B. Therefore, when the gypsum board is a gypsum board, it is possible to improve the adhesion between the gypsum core 11 and the gypsum board base paper. Furthermore, by setting the average diameter of the bubbles contained in the gypsum core 11 to 650 μm or less, it is possible to ensure a sufficient amount of gypsum in the gypsum core 11 and increase the compressive strength of the gypsum board 10.

[0074] From the viewpoint of particularly increasing the compressive strength of the gypsum board 10 and preventing large voids from occurring on the surface of the gypsum core 11, it is more preferable that the average diameter of the bubbles contained in the gypsum core 11 be 150 μm or more and 400 μm or less.

[0075] The average diameter of the bubbles contained in the gypsum core 11 can be calculated from an image of a cross section of the gypsum core 11. Specifically, the average diameter of the bubbles contained in the gypsum core 11 can be obtained, for example, by the following procedure.

[0076] (How to determine the average bubble diameter) First, a scanning electron microscope photograph is taken of a cross section of the gypsum core 11 at a magnification such that 25 to 35 air bubbles are included in one photograph. Note that a microscope or the like can be used instead of the scanning electron microscope.

[0077] For each bubble in the captured image, bubbles with a length of 30 μm or more along a predetermined measurement direction are extracted. The measurement direction is not particularly limited. For example, since the captured image is usually rectangular, the measurement direction can be the direction along the long side of the rectangle. Furthermore, the gypsum core obtained by hardening the gypsum slurry to which bubbles have been added contains bubbles generated by the elimination of water and bubbles derived from the bubbles added to the gypsum slurry. However, the bubbles generated by the elimination of water are small, and their length along the measurement direction is shorter than 30 μm. Therefore, by extracting and evaluating bubbles with a length of 30 μm or more along the measurement direction, the bubbles derived from the bubbles added to the gypsum slurry can be evaluated.

[0078] Then, for all bubbles extracted in the captured image, the diameter of the bubble, which is the maximum length along the measurement direction, is measured, and the average value (arithmetic mean value) of the diameters of all the measured bubbles in the image is taken as the average diameter of the bubbles in the captured image.

[0079] Using the same procedure, a total of three cross-sectional images are taken from different observation locations for one gypsum core 11, and the average diameter of the bubbles is determined for each image. The average value (arithmetic mean value) of the average diameters of the bubbles in the three images is then calculated, and this is used as the average diameter of the bubbles contained in the gypsum core 11 to be evaluated. (2-3) Organic fibers contained in the gypsum core The gypsum core 11 may also contain organic fibers.

[0080] The organic fibers are mainly derived from paper and the like contained in the recycled gypsum. In order to adjust the content of organic fibers contained in the gypsum core 11, organic fibers can be added to the gypsum slurry when preparing the gypsum slurry, as needed. In addition, the proportion of organic fibers contained in the recycled gypsum can be adjusted by adjusting the degree of pulverization (crushing) of the recycled gypsum and, as needed, removing paper by sieving or the like.

[0081] The gypsum core 11 preferably contains organic fibers in a proportion of 0.1 mass % to 2.5 mass %.

[0082] When the gypsum core 11 contains 0.1% by mass or more of organic fibers, the shear strength, bending fracture load, and the like of the gypsum core 11 and the gypsum board 10 can be increased.

[0083] By setting the organic fiber content of the gypsum core 11 to 2.5% by mass or less, the fluidity of the gypsum slurry can be increased when manufacturing a gypsum board, thereby increasing the productivity of the gypsum board. Furthermore, by setting the organic fiber content of the gypsum core 11 to 2.5% by mass or less, the non-combustibility of the gypsum board 10 can be improved.

[0084] The inventors of the present invention have studied the influence of the content of organic fibers contained in the gypsum core 11 of the gypsum board 10 on the total calorific value.

[0085] Specifically, in Experimental Examples 3-1 to 3-6, new gypsum or recycled gypsum, paper, and water were mixed to prepare gypsum slurries in the proportions shown in Table 3 below. Water was added so that the specific gravity of the gypsum hardened body obtained by hardening the gypsum slurry would be 0.65. The gypsum slurry was then mixed with a gypsum slurry having a basis weight of 200 g / m. 2 The gypsum board was then placed between sheets of board base paper and molded to a thickness of 12.5 mm to prepare a gypsum board, which was the gypsum board 10.

[0086] The recycled gypsum was prepared by crushing gypsum board to such an extent that the attached paper could be removed, and then completely removing the paper by sieving, etc. The paper used was paper recovered from the recycled gypsum. (Pyrogenicity test)

[0087] The obtained test specimen, the gypsum board, was subjected to a heat generation test in accordance with the ISO5660-1 cone calorimeter method, and the total heat generation amount after a heating time of 20 minutes was measured.

[0088] The evaluation results are shown in Table 3.

[0089] [Table 3] According to the results shown in Table 3, when the paper content is 3% by mass or less, the total heat generation rate for 20 minutes is 8 MJ / m 2 Therefore, it was confirmed that the gypsum core 11 has excellent non-combustibility when the organic fiber content is set to 3.0% by mass or less, and in particular, the non-combustibility can be improved by setting the organic fiber content to 2.5% by mass or less.

[0090] The proportion of organic fibers contained in the gypsum core 11 can be measured and calculated, for example, by the following procedure.

[0091] First, the gypsum core 11 is heated to 150°C and then pulverized to obtain a pulverized product. The degree of pulverization is not particularly limited, and the pulverization size can be selected so as to reduce the time required for dissolving the gypsum component during the following water washing on a sieve.

[0092] The resulting pulverized material is then washed with water on a 100-mesh sieve. The material remaining on the sieve is the organic fiber contained in the gypsum core, and the mass is measured after drying to determine the organic fiber mass. The mass ratio of the organic fiber mass to the pulverized material supplied on the sieve is then calculated, which can be used as the mass ratio of the organic fiber contained in the gypsum core 11.

[0093] In addition, when the gypsum board 10 has a surface material, the above evaluation can be performed on the gypsum core 11 after removing the portion of the gypsum board 10 that includes the surface material. (2-4) Total heat generation of gypsum board The gypsum board 10 of this embodiment has a total heat output of 8 MJ / m for 20 minutes. 2 It is preferable that:

[0094] The total heat output of 10 gypsum boards for 20 minutes is 8MJ / m 2 The following conditions can result in a gypsum board with excellent non-combustible properties. The total calorific value of the gypsum board 10 can be selected by adjusting the content of organic components in the gypsum board 10, and can be adjusted, for example, by the degree of paper removal when producing recycled gypsum.

[0095] For example, as described in "(2-3) Regarding organic fibers contained in the gypsum core," by setting the content of organic fibers contained in the gypsum core 11 to 3.0 mass% or less, the total heat generation amount of the gypsum board 10 for 20 minutes can be reduced to 8 MJ / m 2 The total calorific value can be particularly suppressed by setting the content of organic fibers in the gypsum core 11 to 2.5 mass % or less. (2-5) Specific gravity of gypsum board The specific gravity of the gypsum board is not particularly limited, but may be, for example, 0.65 or more. By making the specific gravity of the gypsum board 0.65 or more, the strength of the gypsum board can be particularly increased.

[0096] The upper limit of the specific gravity of the gypsum board is not particularly limited, but from the viewpoint of improving the ease of handling at the construction site, it may be less than 1.1 or may be 1.0 or less.

[0097] The specific gravity of the gypsum board can be evaluated according to the method described in JIS A 6901 (2014). [Manufacturing method of gypsum board] The method for manufacturing a gypsum board of the present embodiment can include a gypsum slurry preparation step, a molding step, and a hardening step.

[0098] The gypsum board according to one aspect of the present disclosure can be manufactured by the gypsum board manufacturing method of the present embodiment. Therefore, some of the matters already described for the gypsum board will not be described again. (Gypsum slurry preparation process) In the gypsum slurry preparation step, a gypsum raw material containing recycled gypsum recovered from waste gypsum boards, water, and foam obtained by previously foaming a foaming agent are mixed together to prepare a gypsum slurry.

[0099] For example, these raw material components can be kneaded using a mixer or the like to prepare a gypsum slurry. Various additives can also be added to the gypsum slurry as needed. Since the raw materials that can be suitably used for the gypsum slurry have already been described, the description thereof will be omitted here.

[0100] The raw materials for the gypsum slurry may be kneaded all at once or in multiple stages. For example, after only the solid components are mixed to form a gypsum composition, the gypsum composition and the remaining liquid components may be kneaded to form a gypsum slurry.

[0101] The timing of adding and mixing the foam into the gypsum slurry is not particularly limited. For example, the raw materials for the gypsum slurry, including the foam, may be mixed together. Alternatively, the foam may be added to the gypsum slurry after preparing a gypsum slurry containing components other than the foam.

[0102] In the gypsum slurry preparation process, bubbles can be added so that the average diameter of the bubbles contained in the gypsum core 11 of the gypsum board 10 obtained after the hardening process is 50 μm or more and 650 μm or less. For this reason, for example, a preliminary test can be conducted to manufacture a gypsum board using bubbles generated by changing the amount of foaming agent used, and the conditions for foam generation and addition can be selected based on the results of the preliminary test.

[0103] In the gypsum slurry preparation process, multiple types of gypsum slurries may be prepared with different bubble generation and addition conditions, such as bubble diameter and bubble addition amount. In this case, a laminate of multiple types of gypsum slurries can be formed in the molding process. In the molding process, the thickness of each layer can be selected so that the average bubble diameter is within a predetermined range throughout the gypsum core 11 in the gypsum board 10 obtained after the hardening process. (molding process) In the molding step, the gypsum slurry prepared in the gypsum slurry preparation step is molded to produce a gypsum slurry molded body.

[0104] Specifically, for example, when a gypsum board is manufactured as the gypsum board 10, a gypsum slurry molded body can be manufactured by placing gypsum slurry between base papers for gypsum board and passing the gypsum slurry through a molding machine.

[0105] When multiple types of gypsum slurries with different foam addition conditions are prepared in the gypsum slurry preparation step, the gypsum slurries may be supplied in a desired layering order in the molding step to form a layered body of gypsum slurries. (hardening process) In the hardening step, the gypsum slurry molded body obtained in the molding step can be hardened.

[0106] The hardening process can be carried out by the hydration reaction of calcined gypsum (hemihydrate gypsum) contained in the gypsum raw material in the gypsum slurry, which generates needle-like crystals of gypsum dihydrate and coagulates and solidifies. Therefore, within the gypsum slurry formed body formed in the molding process, a reaction occurs between the calcined gypsum and water in the gypsum raw material added to the gypsum slurry, and the hydration reaction of the calcined gypsum progresses, thereby carrying out the hardening process.

[0107] The method for manufacturing a gypsum board according to the present embodiment may further include any steps. Specifically, the method may include, for example, the following cutting step, drying step, recycled gypsum manufacturing step, etc. (cutting process) In the cutting step, the gypsum slurry molded body can be cut by a cutting device.

[0108] After the gypsum slurry molded body is formed in the molding step, the gypsum slurry gradually hardens. Therefore, the cutting step can be performed, for example, during the hardening step or after the hardening step is completed. However, it is preferable to perform the cutting step after the hardening step has progressed to an extent that the gypsum slurry molded body can be cut.

[0109] The cutting step can be performed multiple times. Therefore, the method for manufacturing a gypsum board of this embodiment can also have a first cutting step, which can be called a rough cutting step, for example. It can also have a second cutting step.

[0110] In the first cutting step, the gypsum slurry molded body can be cut to a desired size depending on, for example, the size of a dryer used in the drying step described below.

[0111] The second cutting step can be carried out, for example, after the drying step, and the product can be cut to a desired size. (drying process) In the drying step, the gypsum slurry molded body can be dried. In the drying step, excess moisture contained in the gypsum slurry molded body can be dried. Note that it is preferable to supply a gypsum slurry molded body that has completed the hardening step to the drying step. The drying step can be performed by force-drying the gypsum slurry molded body using a dryer.

[0112] The method for forcibly drying the gypsum slurry molded bodies with a dryer is not particularly limited, but for example, a dryer can be provided on a transport path of the gypsum slurry molded bodies, and the gypsum slurry molded bodies can be continuously dried by passing through the dryer. Alternatively, the gypsum slurry molded bodies can be transported into the dryer and dried in batches. (Recycled gypsum manufacturing process) In the recycled gypsum manufacturing process, recycled gypsum can be produced by crushing and firing gypsum board waste. The produced recycled gypsum can be subjected to a gypsum slurry preparation process.

[0113] In the recycled gypsum manufacturing process, the means for pulverizing the gypsum board waste is not particularly limited, but for example, dry pulverization can be used. Compared to wet pulverization, dry pulverization of gypsum board waste can suppress adhesion of moisture to and contamination of the pulverized material during pulverization. Therefore, dry pulverization of gypsum board waste can reduce energy consumption during firing and suppress contamination of gypsum dihydrate.

[0114] Therefore, the recycled gypsum to be subjected to the gypsum slurry preparation step may contain dry-ground waste gypsum board material, or may be composed of dry-ground waste gypsum board material.

[0115] In the recycled gypsum manufacturing process, sieving, magnetic separation, etc. can be performed as needed to remove components other than gypsum, such as metals and paper fragments.

[0116] According to the method for manufacturing a gypsum board of this embodiment, a gypsum board having sufficient compressive strength can be obtained while using recycled gypsum recovered from waste gypsum boards as the gypsum raw material. [Example]

[0117] The present invention will be explained below by giving specific examples, but the present invention is not limited to these examples. [Reference example 1-1~Reference example 1-6] Gypsum boards were manufactured by the following procedure, and their compressive strength was evaluated and their adhesiveness was tested. (1) Manufacturing conditions Specifically, in Reference Examples 1-1 to 1-6, gypsum slurries were prepared by mixing novel gypsum, water, foam, and starch as an adhesion improver so that the gypsum raw material and paper had the blending ratios shown in the following Table 4 (gypsum slurry preparation step). Note that water was added so that the specific gravity of the gypsum hardened body obtained by hardening the gypsum slurry would be 0.65.

[0118] The new gypsum is made from new gypsum raw materials that are not recycled.

[0119] When foam was added to the gypsum slurry, the amount of foaming agent used to generate the foam was adjusted to foam the gypsum slurry so that the average diameter of the bubbles contained in the gypsum core would be the value shown in Table 5. In Reference Example 1-1, no foam was added.

[0120] The obtained gypsum slurry was placed between sheets of gypsum board base paper and molded into a board having a thickness of 12.5 mm, thereby producing a gypsum slurry molded body (molding step).

[0121] Thereafter, the gypsum slurry molded body was hardened (hardening step).

[0122] After the hardening step, the gypsum slurry molded body was cut and dried to produce a gypsum board (cutting step, drying step).

[0123] (2) Evaluation method

[0124] (2-1) Compressive strength To evaluate the compressive strength of the resulting gypsum boards, a total of 10 specimens measuring 40 mm in length, 40 mm in width, and 12.5 mm in thickness were cut out from different locations within the same gypsum board. The specimens were then dried at a constant temperature of 40°C, after which the compressive strength of each specimen was measured at a loading rate of 1 mm / min using an autograph (Shimadzu Corporation, AG-X plus), and the average value and standard deviation were calculated.

[0125] The compressive strength of each test specimen is shown in the columns for Test Specimen 1 to Test Specimen 10 in Table 4. The average value is shown in the "Average" column, and the standard deviation is shown in the "σ" column. (2-2) Average bubble diameter The average diameter of the bubbles was measured and calculated using the same procedure as in "(2-2) Average diameter of bubbles contained in the gypsum core" (Method for determining the average diameter of bubbles), except that the gypsum core was observed and photographed using a microscope. The direction along the long side of the photographed image was the measurement direction. This also applies to the following examples and comparative examples. (2-3) Specific gravity The specific gravity of the gypsum board was evaluated according to the method described in JIS A 6901 (2014). (2-4) Adhesion test When conducting the adhesiveness test, first, two test pieces measuring 300 mm in length, 910 mm in width, and 12.5 mm in thickness were taken from the manufactured gypsum board.

[0126] A cut was made in the base paper for the board on the backside of one of the collected test pieces, along its length, at a location 50 mm from the edge along its width, using a cutter.

[0127] Next, the test piece was folded by applying force from the back surface to the front surface at both ends along the width of the test piece, centered on the slit, so that the test piece would split along the slit. Then, the gypsum core was completely separated from the board base paper located on the front surface of the test piece.

[0128] The surface of the gypsum core located on the surface side of the test piece was observed, and the percentage of the area where the board base paper remained was calculated.

[0129] The other test piece was cut lengthwise under the same conditions as the first test piece, except that the cut was made in the board base paper on the front side. Next, the test piece was folded by applying force from the front to the back side to both ends along the width of the test piece, centered on the cut, so that the test piece would split open along the cut. The gypsum core was then completely separated from the board base paper located on the back side of the test piece.

[0130] Thereafter, the surface of the gypsum core located on the back side of the test piece was observed, and the percentage of the area where the board base paper remained was calculated.

[0131] The average of the area ratios of the board base paper remaining in the gypsum core calculated for the two test pieces was calculated and defined as the remaining ratio of the board base paper for the gypsum board.

[0132] When the remaining ratio of the board base paper was 60% or less, it was rated as C. When the remaining ratio of the board base paper was more than 60% but less than 90%, it was rated as B. When the remaining ratio of the board base paper was 90% or more, it was rated as A.

[0133] The adhesion of the board base paper to the gypsum core is sufficiently high in case A, and decreases in the order of B and C.

[0134] The evaluation results are shown in Table 5. [Examples 1-1 to 1-5, Comparative Example 1] Gypsum boards were manufactured by the following procedure, and their compressive strength was evaluated and their adhesiveness was tested. (1) Manufacturing conditions Specifically, in Examples 1-1 to 1-5 and Comparative Example 1, recycled gypsum, paper, water, foam, and starch as an adhesion improver were mixed to prepare gypsum slurries (gypsum slurry preparation step) so that the gypsum raw materials and paper had the blending ratios shown in Table 4 below. Water was added so that the specific gravity of the gypsum hardened body obtained by hardening the gypsum slurry would be 0.65. Alkyl ether sulfate was used as a foaming agent to form foam.

[0135] Furthermore, when foam was added to the gypsum slurry, the amount of foaming agent used to generate the foam was adjusted to foam the gypsum slurry so that the average diameter of the bubbles in the gypsum core would be the value shown in Table 5. In all of the examples in which foam was added to the gypsum slurry, the gypsum slurry contained 0.01 mass % or more and 0.1 mass % or less of the foaming agent. In Comparative Example 1, no foam was added.

[0136] The recycled gypsum was prepared by dry-crushing gypsum board to the extent that the attached paper could be removed, and then calcining the gypsum board after completely removing the paper by sieving, etc. The paper used was paper recovered from the recycled gypsum.

[0137] The obtained gypsum slurry was placed between sheets of gypsum board base paper and molded into a board having a thickness of 12.5 mm, thereby producing a gypsum slurry molded body (molding step).

[0138] Thereafter, the gypsum slurry molded body was hardened (hardening step).

[0139] After the hardening step, the gypsum slurry molded body was cut and dried to produce a gypsum board (cutting step, drying step).

[0140] The obtained gypsum boards were subjected to evaluation of compressive strength, average bubble diameter, specific gravity, and adhesiveness tests in the same manner as in Reference Examples 1-1 to 1-6. The evaluation results are shown in Tables 4 and 5.

[0141] In addition, SEM images were observed for the cross sections of the gypsum cores of the obtained gypsum boards of Examples 1-4 and Comparative Example 1. The obtained SEM images of Examples 1-4 are shown in Figures 3A and 3B, and the SEM images of Comparative Example 1 are shown in Figures 4A and 4B, respectively.

[0142] Although the gypsum cores of the gypsum boards obtained in Examples 1-1 to 1-5 contain an adhesion improver and the like, the amount contained is so small that the gypsum cores can also be considered to be composed of gypsum derived from recycled gypsum and paper. In this case, since the gypsum derived from recycled gypsum is gypsum dihydrate, the content of paper, which is an organic fiber, in the gypsum core is 2.06% by mass.

[0143] [Table 4]

[0144] [Table 5] According to the results shown in Table 4, it was confirmed that the average compressive strength of the gypsum board of Comparative Example 1, which used recycled gypsum and did not add foam, decreased and the standard deviation also increased. Furthermore, for the first, sixth, and eighth test specimens, the compressive strength was 25 kgf / cm 2 It was confirmed that it was less than

[0145] In contrast, in the gypsum boards of Examples 1-1 to 1-5, bubbles were added, the amount of foaming agent used when generating the bubbles was adjusted, and the average diameter of the bubbles in the gypsum core was set to 50 μm or more and 650 μm or less, and it was confirmed that the average value of the compressive strength was higher than that of Comparative Example 1. Also, in the gypsum boards of Examples 1-1 to 1-5, the compressive strength of all 10 test specimens was 25 kgf / cm 2 The standard deviation of the compressive strength is 10 kgf / cm 2 It was also confirmed that the average values ​​of compressive strength for the gypsum boards of Examples 1-1 to 1-5 were sufficiently large values ​​comparable to the average values ​​of compressive strength for the gypsum boards of Reference Examples 1-1 to 1-6, which used novel gypsum.

[0146] 3A and 3B, which are SEM images of the gypsum core of Example 1-4, were compared with FIGS. 4A and 4B, which are SEM images of the gypsum core of Comparative Example 1. As a result, it was confirmed that the shape of the bubbles in the gypsum core was irregular in Comparative Example 1, whereas in Example 1-4 the bubbles were nearly spherical and smaller than in Comparative Example 1. Note that the shape and size of the bubbles in the gypsum core of Comparative Example 1 varied greatly, so the average diameter was not measured.

[0147] It was also confirmed that the adhesion between the gypsum core and the base paper for the board can be improved by increasing the average diameter of the air bubbles in the gypsum core to 50 μm or more.

[0148] Gypsum dihydrate, which is obtained by rehydrating recycled gypsum obtained by crushing and calcining waste gypsum, has crystal morphology and size that are almost unchanged from the fine needle-like crystals contained in the waste gypsum, and remains as fine needle-like crystals. The fine needle-like crystals of gypsum dihydrate and their aggregates obtained by rehydrating recycled gypsum are weakly entangled with paper fibers, which is thought to result in reduced adhesion between the gypsum core manufactured using recycled gypsum and the base paper for boards. However, the above evaluation results confirmed that by setting the average diameter of the air bubbles in the gypsum core to between 50 μm and 650 μm, adhesion can be improved to the same level as when virgin gypsum is used.

[0149] In Examples 1-1 to 1-5, examples were shown in which the proportion of recycled gypsum in the gypsum raw material was 100% by mass. However, as is clear from the results of Reference Examples 1-2 to 1-6, even when part of the recycled gypsum was replaced with new gypsum, it was confirmed that the compressive strength of the gypsum board was sufficiently high by setting the average diameter of the air bubbles contained in the gypsum core within a predetermined range. In addition, it was confirmed that in this case, the variation in compressive strength between multiple test specimens cut out from the gypsum board could be reduced.

[0150] [Note] (1) A gypsum board according to one embodiment of the present disclosure has a gypsum core, The gypsum core is a hardened body of a gypsum slurry containing a gypsum raw material including recycled gypsum recovered from gypsum board waste, water, and foam obtained by previously foaming a foaming agent, The average diameter of the bubbles contained in the gypsum core is 50 μm or more and 650 μm or less. (2) In the above (1), the foaming agent may contain one or more selected from alkyl ether sulfates and alkyl sulfates. (3) In the above (1) or (2), the gypsum slurry may contain the foaming agent in a proportion of 0.01% by mass or more and 0.1% by mass or less. (4) In any one of the above (1) to (3), the gypsum raw material may contain the recycled gypsum in a proportion of 30% by mass or more and 100% by mass or less. (5) In any of the above (1) to (4), the gypsum core may contain organic fibers in a proportion of 0.1% by mass or more and 2.5% by mass or less. (6) In any one of the above (1) to (5), the recycled gypsum may include dry-ground waste gypsum board. (7) In any of the above items (1) to (6), the specific gravity may be 0.65 or more. (8) In any of the above (1) to (7), the total heat output for 20 minutes is 8 MJ / m 2 It may be the following. (9) A method for manufacturing a gypsum board according to one embodiment of the present disclosure includes: a gypsum slurry preparation step of mixing a gypsum raw material including recycled gypsum recovered from gypsum board waste, water, and foam obtained by previously foaming a foaming agent to prepare a gypsum slurry; a molding step of producing a gypsum slurry molded body by molding the gypsum slurry; and a hardening step of hardening the gypsum slurry molded body, In the gypsum slurry preparation step, the bubbles are added so that the average diameter of the bubbles contained in the gypsum core obtained after the hardening step is 50 μm or more and 650 μm or less. (10) In the above (9), the foaming agent may contain one or more selected from alkyl ether sulfates and alkyl sulfates. (11) In the above (9) or (10), the gypsum slurry may contain the foaming agent in a proportion of 0.01% by mass or more and 0.1% by mass or less. (12) In any one of the above (9) to (11), the gypsum raw material may contain the recycled gypsum in a proportion of 30% by mass or more and 100% by mass or less. (13) In any of the above (9) to (12), the gypsum core may contain organic fibers in a proportion of 0.1% by mass or more and 2.5% by mass or less. (14) In any of the above (9) to (13), the recycled gypsum may include dry-ground waste gypsum board.

[0151] Although the gypsum board and the manufacturing method of the gypsum board have been described above in the embodiments, the present invention is not limited to the above embodiments, etc. Various modifications and changes are possible within the scope of the gist of the present invention described in the claims.

[0152] This application claims priority based on Japanese Patent Application No. 2024-012314, filed with the Japan Patent Office on January 30, 2024, the entire contents of which are incorporated herein by reference. [Explanation of symbols]

[0153] 10 Gypsum board 11 Gypsum core 11A Top 11B Bottom side 11C Side

Claims

1. It has a gypsum core, The gypsum core is a hardened body of a gypsum slurry containing a gypsum raw material including recycled gypsum recovered from gypsum board waste, water, and foam obtained by previously foaming a foaming agent, The average diameter of the bubbles contained in the gypsum core is 50 μm or more and 650 μm or less, The recycled gypsum is a gypsum board that is a fired product of the crushed gypsum board waste material.

2. The gypsum board according to claim 1 , wherein the foaming agent comprises one or more selected from alkyl ether sulfates and alkyl sulfates.

3. The gypsum board according to claim 1 or 2, wherein the gypsum slurry contains the foaming agent in a proportion of 0.01% by mass or more and 0.1% by mass or less.

4. The gypsum board according to claim 1 or 2, wherein the gypsum raw material contains the recycled gypsum in a proportion of 30% by mass or more and 100% by mass or less.

5. The gypsum board according to claim 1 or 2, wherein the gypsum core contains organic fibers in a proportion of 0.1 mass% or more and 2.5 mass% or less.

6. The gypsum board according to claim 1 or 2, wherein the recycled gypsum includes dry-ground gypsum board waste.

7. The gypsum board according to claim 1 or 2, having a specific gravity of 0.65 or more.

8. Total heat output for 20 minutes is 8MJ / m ２ The gypsum board according to claim 1 or claim 2, wherein:

9. a gypsum slurry preparation step of mixing a gypsum raw material containing recycled gypsum recovered from gypsum board waste, water, and foam obtained by previously foaming a foaming agent to prepare a gypsum slurry; a molding step of producing a gypsum slurry molded body by molding the gypsum slurry; and a hardening step of hardening the gypsum slurry molded body, In the gypsum slurry preparation step, the bubbles are added so that the average diameter of the bubbles contained in the gypsum core obtained after the hardening step is 50 μm or more and 650 μm or less, The method for producing a gypsum board, wherein the recycled gypsum is a fired product of crushed waste gypsum board material.

10. The method for producing a gypsum board according to claim 9, wherein the foaming agent comprises at least one selected from alkyl ether sulfates and alkyl sulfates.

11. The method for manufacturing a gypsum board according to claim 9 or 10, wherein the gypsum slurry contains the foaming agent in a proportion of 0.01% by mass or more and 0.1% by mass or less.

12. The method for manufacturing a gypsum board according to claim 9 or 10, wherein the gypsum raw material contains the recycled gypsum in a proportion of 30% by mass or more and 100% by mass or less.

13. The method for manufacturing a gypsum board according to claim 9 or 10, wherein the gypsum core contains organic fibers in a proportion of 0.1 mass% or more and 2.5 mass% or less.

14. The method for manufacturing a gypsum board according to claim 9 or 10, wherein the recycled gypsum includes a dry-ground product of the gypsum board waste material.

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