JP6973443B2 - Gypsum manufacturing method and cement composition manufacturing method - Google Patents

Description

The present invention relates to gypsum and a method for producing the same, a cement composition and a method for producing the same, and a ground improving material.

When the ground is solidified using a cement-based ground improvement material, hexavalent chromium may elute depending on the conditions of use such as soil quality, amount of addition, and strength development, and there is concern about the impact on the environment. Many studies have been conducted on methods for suppressing the elution of hexavalent chromium from solidified soil using cement-based solidifying materials. Basically, by adding various reducing agents to cement, hexavalent chromium from solidified soil has been studied. A method for reducing the elution of hexavalent chromium is known (see Patent Documents 1 to 5).

Japanese Unexamined Patent Publication No. 2010-22795 Japanese Unexamined Patent Publication No. 2012-201765 Japanese Unexamined Patent Publication No. 2016-121047 Japanese Unexamined Patent Publication No. 2016-216548 Japanese Unexamined Patent Publication No. 2016-169317

By the way, the use of a specific type of gypsum as a reducing agent is also being considered. For example, Patent Documents 1, 3 and 4 describe that calcium sulfite (gypsum sulfite) can be used as a reducing agent to be blended in a ground improving material. However, sulfurous acid gypsum is known to cause a delay in cement setting and a decrease in strength development as compared with dihydrate gypsum, and is not used for cement and is limited to use as a ground improvement material. ..

The present invention is a method for producing gypsum that is added to cement clinker and can form a cement composition together with cement clinker. It is an object of the present invention to provide a method for producing gypsum useful for sufficiently suppressing elution of chrome. Another object of the present invention is to provide such gypsum, a cement composition containing the same, a method for producing the same, and a ground improving material containing the gypsum.

The method for producing gypsum according to the present invention is a method for producing gypsum through a gas-liquid contact step between a gas containing sulfur dioxide and a slurry containing at least calcium sulfate and calcium hydroxide, and the gypsum contained in the above slurry. Assuming that the amount of calcium is 100 parts by mass, the amount of calcium hydroxide contained in the slurry is 0.2 to 10 parts by mass, and the gypsum produced is quantified according to the method described in JIS R9101-1995. The content of sulfur dioxide is 0.5 to 15% by mass.

The gas-liquid contact step of the above-mentioned production method is a step of absorbing sulfur dioxide in a slurry in which calcium sulfate and a predetermined amount of calcium hydroxide coexist based on the amount of calcium sulfate. By carrying out this step, the sulfur dioxide content quantified according to the above JIS method (hereinafter, in some cases, simply referred to as "sulfur dioxide content") is 0.5 to 15% by mass. Gypsum is obtained. When the gypsum has a sulfur dioxide content of 0.5% by mass or more, even if the cement clinker has a relatively high chromium content when the gypsum and the cement clinker are mixed to produce a cement composition. , Elution of hexavalent chromium from the hardened cement can be sufficiently suppressed. On the other hand, when the sulfur dioxide content of gypsum is 15% by mass or less, it is possible to sufficiently suppress the delay in condensation of cement and the decrease in strength development.

The method for producing gypsum according to the present invention may be the following embodiment using magnesium hydroxide. That is, the method for producing gypsum according to the present invention is a first step of preparing a first slurry containing at least magnesium hydroxide, calcium sulfate, and calcium hydroxide, a gas containing sulfur dioxide, and a first step. of by the slurry contacting gas-liquid, and magnesium ions, sulfate ions, and at least one ion of sulfite ion and sulfite hydrogen ions, and a second step of obtaining a second slurry containing the gypsum, the second by slurrying the solid-liquid separation, including with recovering gypsum, magnesium ions, and a third step of obtaining a third slurry containing at least one ion of sulfate ions, sulfite ions and nitrous hydrogen sulfate ions, Assuming that the amount of calcium sulfate contained in the first slurry is 100 parts by mass, the amount of calcium hydroxide contained in the first slurry is 0.2 to 10 parts by mass, and the gypsum obtained in the third step is obtained. The content of sulfur dioxide quantified according to the method described in JIS R9101-1995 is 0.5 to 15% by mass.

This production method is a modification of the "magnesium hydroxide-gypsum method", which is one aspect of the flue gas desulfurization process, so that gypsum that can suppress the elution of hexavalent chromium can be produced. That is, the second step of this production method is a step of absorbing sulfur dioxide in a first slurry in which calcium sulfate and a predetermined amount of calcium hydroxide coexist based on the amount of calcium sulfate. By carrying out this step, gypsum having a sulfur dioxide content of 0.5 to 15% by mass can be obtained.

The method for producing gypsum according to the above embodiment using magnesium hydroxide preferably further includes a step of regenerating magnesium hydroxide that has become magnesium sulfate by contact with sulfur dioxide. In the method for producing gypsum in such an embodiment, the fourth step of obtaining a fourth slurry containing magnesium sulfate and the fourth slurry are obtained by subjecting at least a part of the third slurry to an oxidation treatment. A fifth step of obtaining a fifth slurry containing magnesium hydroxide regenerated from magnesium sulfate, dihydrate gypsum, and calcium hydroxide by adding calcium hydroxide is included in the first slurry. The calcium sulfate is at least a part of the dihydrate gypsum contained in the fifth slurry.

The present invention provides gypsum having a sulfur dioxide content of 0.5 to 15% by mass, which is quantified according to the method described in JIS R9101-1995. The present invention also provides a cement composition comprising the gypsum and a cement clinker. Further, the present invention provides a method for producing a cement composition, which comprises a step of mixing the gypsum with a cement clinker. From the viewpoint of further reducing the elution of hexavalent chromium, it is preferable to carry out a pulverization treatment as well as a mixing treatment in the step of mixing gypsum and cement clinker.

Cement composition according to the present invention, JIS R5202: SO ₃ amount to be quantified according to the method described in 2010 is 1.5 to 15 wt%, the particles constituting the cement composition following particle size 10μm _{When the amount of SO 3} contained in the entire cement composition is 100 parts by mass, it is preferable that the amount of _{SO 3} contained in the particles having a particle size of 10 μm or less is 30 parts by mass or more. According to the cement composition satisfying these conditions, the elution of hexavalent chromium can be further reduced.

The present invention provides a ground improving material containing the above cement composition and at least one of anhydrous gypsum and blast furnace slag.

According to the present invention, it is a method for producing gypsum which is added to cement clinker and can form a cement composition together with cement clinker. A method for producing gypsum useful for sufficiently suppressing the elution of hexavalent chromium is provided. Further, according to the present invention, such gypsum, a cement composition containing the same, a method for producing the same, and a ground improving material containing the gypsum are provided.

FIG. 1 is a schematic view showing a flow of a method for producing gypsum according to the present invention. FIG. 2 is an X-ray diffraction pattern of gypsum according to Examples and Comparative Examples.

Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the following embodiments.

<Gypsum manufacturing method>
FIG. 1 is a schematic diagram showing a flow of a method for producing gypsum according to the present embodiment. This production method is a modification of the "magnesium hydroxide-gypsum method", which is one aspect of the flue gas desulfurization process, to enable the production of gypsum capable of suppressing the elution of hexavalent chromium, and is the following step. including.
-A first step of preparing a first slurry containing at least magnesium hydroxide, dihydrate gypsum (calcium sulfate), and calcium hydroxide.
A gas containing sulfur dioxide, by contacting a first gas-liquid and slurry, and magnesium ions, ions, and at least one ion of sulfite ion and sulfite hydrogen ions, the second containing the gypsum sulfate The second step of obtaining the slurry.
- By the second slurry to solid-liquid separation, thereby recovering gypsum, and magnesium ions, the third step of obtaining a third slurry containing at least one ion of sulfate ions, sulfite ions and sulfite hydrogen ions ..
A fourth step of obtaining a fourth slurry containing magnesium sulfate by subjecting at least a part of the third slurry to an oxidation treatment.
A fifth step of obtaining a fifth slurry containing magnesium hydroxide regenerated from magnesium sulfate, dihydrate gypsum, and calcium hydroxide by adding calcium hydroxide to the fourth slurry.

(First step)
The first step is preparing a magnesium hydroxide (Mg _{(OH) 2),} gypsum dihydrate _{(CaSO 4 · 2H 2 O)} , a first slurry containing at least a calcium hydroxide (Ca _{(OH) 2)} It is a process to do. The first slurry according to the present embodiment is obtained by adjusting the components of the fifth slurry obtained in the fifth step as necessary, and is for absorbing sulfur dioxide contained in the gas. (See reaction formulas (1) to (4) in the second step). An example of a gas containing sulfur dioxide is the exhaust gas of a coal-fired power plant.

Assuming that the mass of water contained in the first slurry is 100 parts by mass, the amount of magnesium hydroxide is preferably 3 to 40 parts by mass from the viewpoint of efficiently absorbing sulfur dioxide in the second step. It is more preferably about 20 parts by mass, and even more preferably 4 to 10 parts by mass.

Assuming that the amount of dihydrate gypsum contained in the first slurry is 100 parts by mass, the amount of calcium hydroxide contained in the first slurry is quantified according to the method described in JIS R9101-1995 in the second step. that from the viewpoint of the content of sulfur dioxide (SO ₂ amount) to obtain a gypsum which is a predetermined amount, is from 0.2 to 10 parts by weight, preferably from 0.5 to 7 parts by weight, 1.0 to 5 parts by weight More preferred.

(Second step)
The second step includes a gas containing sulfur dioxide, by contacting a first gas-liquid and slurry, and magnesium ions, sulfate ions, and at least one ion of sulfite ion and sulfite hydrogen ions, and a plaster This is a step of obtaining a second slurry containing the mixture. In other words, the second step corresponds to the absorption step in the "magnesium hydroxide-gypsum method", and the sulfur dioxide contained in the gas is theoretically the reaction of (1) to (4) and (5). ) Is a step of absorbing into the slurry by proceeding at the same time. This step can be carried out by supplying the gas and the first slurry to the absorption tower used in the magnesium hydroxide-gypsum method, respectively. Theoretically, the reaction for producing sulfite gypsum proceeds as in (5), but in the process of this embodiment, the dihydrate gypsum contained in the slurry is used as a nucleus, and the sulfite ions are contained in the crystals of the dihydrate gypsum. and SO ₄ ^2-ions and SO ₃ ^2-ions are incorporated in the form of substitutions, presumably a form that does not adversely affect the cement quality.
H ₂ O + SO ₂ → H ₂ SO ₃ (1)
H ₂ SO ₃ + Mg (OH) ₂ → ו ₃ + 2H ₂ O (2)
0054 ₃ + SO ₂ + H ₂ O → Mg (HSO ₃ ) ₂ (3)
Mg (HSO ₃ ) ₂ + Mg (OH) ₂ → _{2ו 3} + 2H ₂ O (4)
_{SO 2 + Ca (OH) 2} → CaSO 3 · 1 / 2H 2 O + 1 / 2H 2 O (5)

(Third step)
The third step, by solid-liquid separation of the second slurry, the content of sulfur dioxide to be quantified according to the method described in JIS R9101-1995 (SO ₂ amount) at 0.5 to 15 mass% with the recovery of certain gypsum, it is a step of obtaining a magnesium ion, a third slurry containing at least one ion of sulfate ions, sulfite ions and sulfite hydrogen ions. The solid-liquid separation treatment can be carried out using, for example, a hydrocyclone, a dehydrator, a filter press, or the like, and one of these may be used alone or a plurality of of these may be arranged in series. Often, two or more types may be used in combination.

Gypsum obtained in the third step, the content of sulfur dioxide to be quantified according to the method described in JIS R9101-1995 (SO ₂ amount) is 0.5 to 15 wt%, 1.5 to 5 mass %, More preferably 1.6 to 4% by mass, and even more preferably 2 to 3% by mass. When the content of sulfur dioxide in the gypsum is 0.5% by mass or more, the gypsum obtained by washing the mixture and the cement clinker are mixed to produce a cement composition, and the chrome of the cement clinker is produced. Even if the content is relatively high, the elution of hexavalent chromium from the hardened cement can be sufficiently suppressed. On the other hand, when the sulfur dioxide content of gypsum is 15% by mass or less, it is possible to sufficiently suppress the delay in condensation of cement and the decrease in strength development.

(4th step)
The fourth step is a step of obtaining a fourth slurry containing magnesium sulfate (magnesium ion and sulfate ion) by subjecting at least a part of the third slurry to an oxidation treatment. This step can be carried out by supplying a third slurry to the oxide column used in the magnesium hydroxide-gypsum method.
Л ₃ + 1 / 2O ₂ → Л ₄ (6)
(SO ₃ ^2- + 1 / 2O ₂ → SO ₄ ^2- )
Mg (HSO ₃ ) ₂ + O ₂ → ו ₄ + H ₂ SO ₄ (7)
(2HSO ₃ ⁻ + O ₂ → SO ₄ ^2- + H ₂ SO ₄ )

(Fifth step)
The fifth step is a step of adding calcium hydroxide to the fourth slurry to obtain a fifth slurry containing magnesium hydroxide regenerated from magnesium sulfate, dihydrate gypsum, and calcium hydroxide. .. This step can be carried out by supplying a fourth slurry to the metathesis tank used in the magnesium hydroxide-gypsum method.
_{MgSO 4 + Ca (OH) 2} + H 2 O → CaSO 4 · 2H 2 O + Mg (OH) 2 (8)

In the fifth step, by adding an excess amount of calcium hydroxide in terms of molar ratio with respect to magnesium sulfate in the fourth slurry, magnesium hydroxide, dihydrate gypsum, and calcium hydroxide are contained in the fifth step. A slurry can be obtained. The first slurry can be obtained by adjusting the amount of the components contained in the fifth slurry as necessary.

<Gypsum>
The gypsum according to the present embodiment is recovered as a solid content in the above-mentioned third step and contains dihydrate gypsum as a main component. As described above, this gypsum has a sulfur dioxide content in a predetermined range (0.5 to 15% by mass), and when the gypsum and the cement clinker are mixed to produce a cement composition, the cement clinker Even if the chromium content is relatively high, the elution of hexavalent chromium from the hardened cement can be sufficiently suppressed. As long as the sulfur dioxide content in the entire gypsum is within the predetermined range (0.5 to 15% by mass), the gypsum having a sulfur dioxide content of less than 0.5% by mass or more than 15% by mass is used. May be further added.

The gypsum according to the present embodiment preferably has a magnesium oxide (MgO) content of less than 0.3% by mass, more preferably less than 0.2% by mass. When the content of magnesium oxide is less than 0.3% by mass, the effect of avoiding adverse effects such as delayed condensation can be achieved.

<Cement composition>
A cement composition can be produced by mixing the above gypsum with a cement clinker. From the viewpoint of further reducing the elution of hexavalent chromium, it is preferable to carry out a pulverization treatment as well as a mixing treatment in the step of mixing gypsum and cement clinker. The blending amount of gypsum may be about 1.5 to 20.0 parts by mass, preferably about 1.5 to 6.0 parts by mass with respect to 100 parts by mass of cement clinker. As described above, according to the gypsum according to the present embodiment, the elution of hexavalent chromium from the hardened cement can be sufficiently suppressed, so that the total chromium content of the cement clinker is, for example, 50 to 250 mg / kg. It may be 100 to 200 mg / kg, and the amount of water-soluble hexavalent chromium may be, for example, 3 to 40 mg / kg, 10 to 40 mg / kg, or 20 to 20 to 40 mg / kg. It may be 30 mg / kg.

Cement composition according to the present embodiment, JIS R5202: SO ₃ amount to be quantified according to the method described in 2010 is 1.5 to 15 wt%, the particles diameter 10μm constituting the cement composition _{Assuming that the amount of SO 3} contained in the entire cement composition containing the following particles is 100 parts by mass, it is preferable that the amount of _{SO 3} contained in the particles having a particle size of 10 μm or less is 30 parts by mass or more. According to the cement composition satisfying these conditions, the elution of hexavalent chromium can be further reduced.

Blaine specific surface area of the cement composition after pulverization treatment 2500~6000cm ² / g are preferred, 3000~5000cm ² / g is more preferable. By Blaine specific surface area of 2500 cm 2 / ^g or more, excellent strength development of easy to achieve, on the other hand, it is less 6000 cm 2 / ^g, viscosity on construction when used as a concrete or solidifying material slurry Easy to control within a range that does not cause any problems.

The type of cement composition is not particularly limited, and ordinary Portland cement, early-strength Portland cement, ultra-early-strength Portland cement, sulfate-resistant Portland cement, moderate heat Portland cement, low heat Portland cement, blast furnace cement, and fly ash cement. , Silica fume cement, alumina cement and the like may be used.

<Ground improvement material>
The ground improvement material according to the present embodiment contains the above cement composition and at least one of anhydrous gypsum and blast furnace slag. When one of the anhydrous gypsum and the blast furnace slag is blended, assuming that the amount of the cement composition is 100 parts by mass, the blending amount of the anhydrous gypsum is preferably 3 to 20 parts by mass, preferably 5 to 15 parts by mass. The blending amount of the blast furnace slag is preferably 2 to 50 parts by mass, preferably 5 to 30 parts by mass. When the anhydrous gypsum and the blast furnace slag are used in combination, assuming that the amount of the cement composition is 100 parts by mass, the total blending amount of both is preferably 5 to 60 parts by mass, and preferably 20 to 50 parts by mass. The blending amount of the blast furnace slag is preferably 2 to 40 parts by mass, preferably 5 to 30 parts by mass.

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments.

<Manufacturing of plaster>
(Examples 1 and 2)
Flue gas desulfurization gypsum was continuously produced from exhaust gas containing sulfur dioxide from a coal-fired power plant by the production method according to the above embodiment. In the gypsum according to Examples 1 and 2, the amount of calcium hydroxide contained in the first slurry is in the range of 1 to 10 parts by mass with respect to 100 parts by mass of the dihydrate gypsum contained in the first slurry. In addition, it was collected at different timings during operation by adjusting the amount of calcium hydroxide added in the fifth step. The amount of calcium hydroxide to be added was mainly adjusted according to the fluctuation of the amount of sulfur dioxide contained in the exhaust gas.

(Comparative Examples 1 to 3)
In the gypsum according to Comparative Examples 1 to 3, the amount of calcium hydroxide contained in the first slurry is less than 0.5 parts by mass with respect to 100 parts by mass of the dihydrate gypsum contained in the first slurry. It was collected at different timings during operation by adjusting the amount of calcium hydroxide added in the fifth step.

(Comparative Examples 4 to 12)
The gypsum according to Comparative Examples 4 to 12 was produced by the so-called lime / gypsum method, respectively. That is, an exhaust gas containing sulfur dioxide was passed through a slurry containing fine powder of lime, and this was oxidized with air to obtain dihydrate gypsum.

<Evaluation of plaster>
The following evaluations were made on the gypsum according to the examples and comparative examples. The results are shown in Table 1.
(1) Measurement of ignition loss (ig.loss) The ignition loss (ig.loss) of gypsum is described in "5.2 Ignition cement and blast furnace" in "5. Quantification method of ignition loss" of JIS R5202: 2010. Measured according to the method described in "For non-slag".
(2) SO ₃ content of SO ₃ of the measurement gypsum, JIS R 5202: This was measured according to the method described in 2010.
(3) SO ₂ amount measured gypsum sulfur dioxide content (SO ₂ amount) was quantified according to the method described in JIS R9101-1995. When the X-ray diffraction of all the gypsum according to the examples and the comparative examples was confirmed by an X-ray diffractometer (manufactured by Rigaku Co., Ltd., model number RINT2000), no clear diffraction peak was observed (see FIG. 2).
(4) Measurement of Magnesium Oxide Content (MgO Amount) The MgO amount was measured according to the method described in JIS R5202: 2010.

<Manufacturing of cement composition>
(Examples 1 and 2 and Comparative Examples 1 to 12)
Add 3.5 parts by mass of the gypsum according to the examples and comparative examples to the cement clinker having a total Cr content of 101 mg / kg and a water-soluble Cr (VI) content of 13 mg / kg when no gypsum is added. A total of 14 types of cement compositions were produced. Cement was pulverized by mixing clinker of 5 mm or less and gypsum with a vibration mill. The brain specific surface area was 3200 ± 150 cm ² / g.

(Example 3)
The cement composition according to Example 3 was prepared in the same manner as in Example 1 except that the crushing method was changed. That is, in this embodiment, instead of mixing and crushing gypsum and gypsum, first, gypsum and clinker are separately ^{crushed so as to have a brain specific surface area of 3200 ± 150 cm 2} / g, and then both are mixed. bottom.

_{Assuming that the amount of SO 3} contained in the entire cement composition is 100 parts by mass, the cement composition according to Example 1 has an _{amount of SO 3} contained in particles having a particle size of 10 μm or less of 30 parts by mass or more. The cement composition according to Example 3 contained less than 20 parts by mass of _{SO 3 in} particles having a particle size of 10 μm or less. The particles having a particle size of 10 μm or less were obtained by passing the cement composition through a sieve net having a mesh size of 10 μm. The SO ₃ content in the following particles with particle sizes of 10μm JIS R5202: was measured according to the method described in 2010.

<Quantification of water-soluble Cr (VI)>
The amount of water-soluble Cr (VI) in cement was measured according to the method described in Cement Kyokai Standard Test Method I-51-1981. The results are shown in Table 1.

(Examples 4 to 7 and Comparative Examples 13 to 17)
And plaster was prepared in the same manner as in Examples 1 and 2, gypsum produced by a general method (SO ₂ amount: 0 wt% or more and less than 0.5 wt%) were prepared and these two types of gypsum A total of 9 types of gypsum mixtures having different mixing ratios and the like were continuously prepared. Next, 3.5% by mass of the gypsum mixture and 4.5% by mass of limestone were mixed with each of the clinker fired so as to have the composition of ordinary Portland cement clinker, and the brain specific surface area was 3150 ±. The cement composition of Examples 4 to 7 and Comparative Examples 13 to 17 was produced by pulverizing to 150 cm ^{2 / g.} For the obtained cement composition, the amount of water-soluble Cr (VI) was determined according to the method described in the Cement Association Standard Test Method I-51-1981, and the setting and mortar compressive strength were determined by JIS R5201 "Physical test method for cement". Each was measured according to the above. The relationship between the amount of water-soluble Cr (VI) and the amount of SO _{2 in} gypsum is derived from Examples 1 and 2 and Comparative Examples 1 to 3, and the cement compositions of Examples 4 to 7 and Comparative Examples 13 to 17 are derived from the relationship. _{The amount of SO 2} contained in the gypsum mixture inside was estimated. The results are shown in Table 2. From Table 2, it can be seen that the cement compositions of Examples 4 to 7 have a setting time and compressive strength comparable to those of Comparative Examples 13 to 17 in which the effect of reducing water-soluble Cr (VI) is not observed. ..

Claims (4)

It is a method for producing gypsum through a gas-liquid contact step between a gas containing sulfur dioxide and a slurry containing at least calcium sulfate and calcium hydroxide.
Assuming that the amount of calcium sulfate contained in the slurry is 100 parts by mass, the amount of calcium hydroxide contained in the slurry is 0.5 to 10 parts by mass.
Assuming that the amount of water contained in the slurry is 100 parts by mass, the amount of magnesium hydroxide contained in the slurry is 3 to 40 parts by mass.
The gypsum is a method for producing gypsum, wherein the content of sulfur dioxide quantified according to the method described in JIS R9101-1995 is 0.5 to 15% by mass. A first step of preparing a first slurry containing at least magnesium hydroxide, calcium sulfate, and calcium hydroxide.
A gas containing sulfur dioxide, by contacting the first gas-liquid and slurry, and magnesium ions, ions, and at least one ion of sulfite ion and sulfite hydrogen ions, the second containing the gypsum sulfate The second step to obtain the slurry and
By solid-liquid separating the second slurry, with the recovery of gypsum, magnesium ion, a third step of obtaining a third slurry containing at least one ion of sulfate ions, sulfite ions and sulfite hydrogen ions When,
Including
Assuming that the amount of calcium sulfate contained in the first slurry is 100 parts by mass, the amount of calcium hydroxide contained in the first slurry is 0.5 to 10 parts by mass.
Assuming that the amount of water contained in the slurry is 100 parts by mass, the amount of magnesium hydroxide contained in the slurry is 3 to 40 parts by mass.
The gypsum is a method for producing gypsum, wherein the content of sulfur dioxide quantified according to the method described in JIS R9101-1995 is 0.5 to 15% by mass. A fourth step of obtaining a fourth slurry containing magnesium sulfate by subjecting at least a part of the third slurry to an oxidation treatment.
A fifth step of obtaining a fifth slurry containing magnesium hydroxide regenerated from magnesium sulfate, dihydrate gypsum, and calcium hydroxide by adding calcium hydroxide to the fourth slurry.
Including
The method for producing gypsum according to claim 2, wherein the calcium sulfate contained in the first slurry is at least a part of the dihydrate gypsum contained in the fifth slurry. A method for producing a cement composition, which comprises a step of mixing and pulverizing the gypsum obtained by the production method according to any one of claims 1 to 3 and a cement clinker.

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