JP6900992B2 - Calcium sulfite-containing gypsum, cement composition, ground improvement material and ground improvement soil - Google Patents

Description

The present disclosure relates to calcium sulfite-containing gypsum, cement compositions, ground improvement materials and ground improvement soils.

Cement clinker is produced using limestone, clay, silica stone, iron oxide and the like as main raw materials. In the production of cement clinker, in addition to these main raw materials, various industrial by-products and industrial wastes are effectively used as raw materials and fuels. However, depending on the selection of raw materials, heavy metals such as cadmium, chromium, and lead derived from various raw materials may be mixed in the cement clinker in a very small amount.

Among heavy metals, hexavalent chromium, unlike other heavy metals, present in the form of a stable oxo anions such chromate ions (CrO 4 ^_2-), even at high pH conditions hardly soluble Does not form hydroxides. Therefore, when ground improvement is carried out using a cement-based ground improvement material, it is necessary to control the elution amount of hexavalent chromium according to the conditions of use such as soil quality, addition amount, and strength development. Therefore, various studies have been conducted on methods for suppressing the elution of hexavalent chromium from the ground-improved soil using a cement-based ground-improving material.

For example, Patent Documents 1 and 2 propose a method of reducing the elution of hexavalent chromium from solidified soil by adding a reducing agent such as sulfite to cement. On the other hand, as for calcium sulfite, a technique for producing calcium sulfite by reacting sulfur gas with slaked lime or sodium sulfite with slaked lime in a liquid phase is known (see Patent Document 3).

JP-A-2010-222795 Japanese Unexamined Patent Publication No. 2016-121047 Japanese Unexamined Patent Publication No. 7-215718

According to the study by the present inventors, when a commercially available commercially available calcium sulfite is blended with the soil to be treated in order to reduce the elution amount of hexavalent chromium, a phenomenon occurs in which the compressive strength is lowered. It turns out that there are cases. Therefore, the present disclosure provides a ground improvement material and a ground improvement soil capable of maintaining a high uniaxial compressive strength while reducing the elution of hexavalent chromium. The present disclosure also provides calcium sulfite-containing gypsum and cement compositions capable of obtaining such ground improvement materials and soil improvement soils.

In one aspect, the present disclosure is a calcium sulfite-containing gypsum containing calcium sulfite, wherein the peak intensity ratio in the diffraction pattern obtained by powder XRD using CuKα ray satisfies the formula (1). provide.
Ia / Ib <2 ... (1)
However, Ia is the peak intensity at 2θ = 15.76 to 16.25 °, and Ib is the peak intensity at 2θ = 16.26 to 16.75 °.

The calcium sulfite-containing gypsum can maintain a high uniaxial compressive strength while reducing the elution of hexavalent chromium. The reason is not always clear, but the present inventors infer as follows. Ia contained in the above formula (1) reflects the intensity of the peak derived from the (110) plane of the space group Pbcn representing the crystal structure of calcium sulfite and the (101) plane of the space group Pban. On the other hand, Ia included in the above formula (1) reflects the intensity of the peak derived from the (200) plane of the space group Pbcn and the (002) plane of the space group Pban. Calcium sulfite-containing gypsum in which the ratio of these peak intensities (Ia / Ib) satisfies the above formula (1) contains calcium sulfite, which has a different crystalline state from commercially available calcium sulfite, and when mixed with soil, it contains calcium sulfite. It is considered to be a factor to increase the uniaxial compressive strength.

The present disclosure, in one aspect, is a calcium sulfite-containing gypsum containing calcium sulfite, the content of calcium sulfite (X [mass%]) determined by at least one of the thermal weight analysis and the XRD-Rietveld method. Provided is a calcium sulfite-containing gypsum in which the sulfur dioxide content (Y [mass%]) measured based on "16 Sulfur dioxide quantification method" of JIS R 9101: 2018 satisfies the relationship of the formula (2). To do.
Y <0.4737X ... (2)

Calcium sulfite usually exists as a hemihydrate, but the ratio of the molecular weight of calcium sulfite hemihydrate to sulfur trioxide is about 0.5. Therefore, theoretically, the relationship of Y = 0.5X is established. However, in the calcium sulfite-containing gypsum satisfying the above formula (2), the sulfur dioxide content measured by "16 Sulfur dioxide quantification method" of JIS R 9101: 2018 is the calcium sulfite content (X [mass%]. ]) Is lower than the theoretical sulfur dioxide content calculated from the ratio of the above molecular weights. It is considered that this is because a part of the sulfite ion contained in the calcium sulfite-containing gypsum is replaced with the sulfate ion and is dissolved in the crystal structure of calcium sulfite. The calcium sulfite contained in the calcium sulfite-containing gypsum satisfying the above formula (2) is thus different from ordinary calcium sulfite in the ionic state. This is considered to be a factor that increases the uniaxial compressive strength when mixed with soil.

The present disclosure is, in one aspect, a calcium sulfite-containing gypsum containing calcium sulfite, which contains plate-like crystals, the thickness of the plate-like crystals is 5 μm or less, and the length of the main surface is 5 to 100 μm or less. , Calcium sulfite-containing gypsum. The calcium sulfite-containing gypsum has excellent reactivity because of its small crystal size. Therefore, when mixed with soil, the uniaxial compressive strength can be maintained high while reducing the elution of hexavalent chromium. Further, the calcium sulfite-containing gypsum is excellent in pulverizability, and can be easily formed into fine crystals having a main surface length of 10 μm or less with a small pulverization energy.

Each of the above-mentioned calcium sulfite-containing gypsum may contain at least one of dihydrate gypsum, hemihydrate gypsum and anhydrous gypsum. As a result, calcium sulfite having a different crystal state from the usual state can be stably present as calcium sulfite-containing gypsum.

The calcium sulfite-containing gypsum described above may have a calcium sulfite content (X [mass%]) of 2% by mass or more, which is obtained by at least one of the thermal weight analysis and the XRD-Rietveld method. As a result, even if the amount of calcium sulfite-containing gypsum is reduced, the elution of hexavalent chromium from the ground-improved soil can be sufficiently suppressed.

The average particle size of the above-mentioned calcium sulfite-containing gypsum in the laser diffraction / scattering method may be 10 μm or less. As a result, the uniaxial compressive strength can be further increased when used for the solidification treatment of soil.

The present disclosure provides, in another aspect, a cement composition comprising any of the calcium sulfite-containing gypsum described above. Since this cement composition contains the above-mentioned calcium sulfite-containing gypsum, it can be suitably used as concrete or mortar while reducing the elution of hexavalent chromium. The content of calcium sulfite in the cement composition may be 0.2% by mass or more. Thereby, the elution of hexavalent chromium can be further reduced. The content is a value measured by at least one of thermogravimetric analysis, XRD-Rietveld analysis and wet analysis described in Examples.

The present disclosure provides, in yet another aspect, a ground improvement material containing any of the calcium sulfite-containing gypsum described above. Since the ground improvement soil improved by using such a ground improvement material contains the above-mentioned calcium sulfite-containing gypsum, it is possible to maintain a high uniaxial compressive strength while reducing the elution of hexavalent chromium. The content of calcium sulfite in the ground improvement material may be 2% by mass or more. As a result, the uniaxial compressive strength can be sufficiently increased while reducing the elution of hexavalent chromium.

The present disclosure provides, in yet another aspect, a ground improvement soil containing the above-mentioned ground improvement material and soil. Since the ground improvement soil containing the above-mentioned ground improvement material contains the above-mentioned calcium sulfite-containing gypsum, it is possible to maintain a high uniaxial compressive strength while reducing the elution of hexavalent chromium.

The present disclosure can provide a ground improvement material and a ground improvement soil capable of maintaining a high uniaxial compressive strength while reducing the elution of hexavalent chromium. The present disclosure can also provide calcium sulfite-containing gypsum and cement compositions capable of obtaining such ground improvement materials and soil improvement soils.

FIG. 1 is a diagram showing an example of a process for producing calcium sulfite. FIG. 2A is a diffraction chart of the powder XRD of Example 1 (calcium sulfite-containing gypsum A). FIG. 2B is a diffraction chart of the powder XRD of Example 2 (calcium sulfite-containing gypsum B). FIG. 3A is a diffraction chart of the powder XRD of Example 3 (calcium sulfite-containing gypsum C). FIG. 3B is a diffraction chart of the powder XRD of Example 4 (calcium sulfite-containing gypsum D). FIG. 4A is a diffraction chart of the powder XRD of Example 5 (calcium sulfite-containing gypsum E). FIG. 4B is a diffraction chart of the powder XRD of Comparative Example 1 (calcium sulfite as a reagent). FIG. 5 is an enlarged view showing a portion of 2θ = 15.5 to 17 ° in the diffraction charts of Example 1, Example 2, and Comparative Example 1. FIG. 6 is an enlarged view showing a portion of 2θ = 15.5 to 17 ° in the diffraction charts of Examples 3, 4 and 5. FIG. 7 is a graph showing the relationship between the calcium sulfite content determined by thermogravimetric analysis and the peak intensity ratio (Ia / Ib). FIG. 8 is a graph showing the relationship between the calcium sulfite content (X) determined by XRD-Rietveld analysis or thermogravimetric analysis and the sulfur dioxide content (Y) determined by wet analysis. is there. FIG. 9 is a graph showing the results of the amount of Cr (VI) eluted from the solidified soil of Examples 6 to 13 and Comparative Example 2. FIG. 10 is a graph showing the results of the uniaxial compressive strength of the solidified soil of Examples 6 to 13 and Comparative Example 2. FIG. 11 is an SEM (scanning electron microscope) photograph of the calcium sulfite-containing gypsum of Example 5. FIG. 12 is a SEM (scanning electron microscope) photograph of calcium sulfite-containing gypsum of another example.

Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings as the case may be. However, the following embodiments are examples for explaining the present disclosure, and are not intended to limit the present disclosure to the following contents. In the description, the same reference numerals are used for the same elements or elements having the same function, and duplicate description may be omitted in some cases.

[First Embodiment of Calcium Sulfite-Containing Gypsum]
The calcium sulfite-containing gypsum according to the first embodiment of the present disclosure contains calcium sulfite, and the peak intensity ratio in the diffraction pattern obtained by powder XRD using CuKα rays satisfies the formula (1).
Ia / Ib <2 ... (1)
However, Ia is the peak intensity at 2θ = 15.76 to 16.25 °, and Ib is the peak intensity at 2θ = 16.26 to 16.75 °.

A calcium sulfite-containing gypsum satisfying the above formula (1) contains calcium sulfite having a crystal state different from that of commercially available calcium sulfite. Therefore, for example, when used as a ground improving material, the uniaxial compressive strength can be maintained higher than that of commercially available calcium sulfite while reducing the elution of hexavalent chromium to the same level as that of commercially available calcium sulfite. From the viewpoint of further increasing the uniaxial compressive strength, Ia / Ib is preferably 1.8 or less, more preferably 1 or less, still more preferably 0.7 or less, and particularly preferably 0.5 or less. Is. From the above viewpoint, the lower limit of Ia / Ib is not particularly limited, but may be, for example, 0.05 or more, 0.1 or more, or 0.2 or more.

"Calcium sulfite" in the present disclosure is a hemihydrate unless otherwise specified. However, the calcium sulfite-containing gypsum may contain a hydrate other than the hemihydrate, such as tetrahydrate and anhydrous, in addition to the calcium sulfite hemihydrate. The content of calcium sulfite in calcium sulfite-containing gypsum can be measured by thermogravimetric analysis or the XRD-Rietveld method.

Calcium sulfite is an active ingredient that reduces the amount of hexavalent chromium eluted when used as a ground improvement material, for example, and improves the uniaxial compressive strength. Therefore, it is preferable that the calcium sulfite content in the calcium sulfite-containing gypsum is high. When the content of calcium sulfite in the calcium sulfite-containing gypsum measured by thermoweight analysis and at least one of the XRD-Rietveld methods is X [mass%], X is preferably 2% by mass or more, and more. It is preferably 20% by mass or more, more preferably 28% by mass or more, and particularly preferably 40% by mass or more. The upper limit of X is not limited, but may be, for example, 99% by mass or less, 98% by mass or less, 90% by mass or less, or 80% by mass or less.

Gypsum containing calcium sulfite contains gypsum. The gypsum may contain at least one of dihydrate gypsum, hemihydrate gypsum and anhydrous gypsum. The term "gypsum" in the present disclosure is a comprehensive concept including dihydrate gypsum, anhydrous gypsum and hemihydrate gypsum. The gypsum content in calcium sulfite-containing gypsum can be measured by thermogravimetric analysis or the XRD-Rietveld method. When the content of gypsum in the calcium sulfite-containing gypsum is high, the amount of gypsum to be separately blended when preparing the cement composition can be reduced.

However, if the content of gypsum becomes too high, the amount of calcium sulfite-containing gypsum blended is limited in order to prevent the blended amount of gypsum in the cement composition from becoming too high. From such a viewpoint, the content of gypsum (total of each hydrate) in the calcium sulfite-containing gypsum is preferably 98% by mass or less, more preferably 70% by mass or less, and further preferably 50% by mass. Hereinafter, it is particularly preferably 30% by mass or less. The lower limit of the gypsum content in the calcium sulfite-containing gypsum may be, for example, 1% by mass or 5% by mass. The gypsum content is measured by thermogravimetric analysis or the XRD-Rietveld method. Of these, the value may be a value measured by the same analytical method as the content of calcium sulfite.

The calcium sulfite-containing gypsum has a sulfur dioxide content of preferably 1 to 45% by mass, more preferably 5 to 43% by mass, as measured based on "16 Method for Quantifying Sulfur Dioxide" of JIS R 9101: 2018. %, More preferably 10 to 40% by mass, and particularly preferably 20 to 40% by mass. The sulfur dioxide content (referred to as Y [mass%]) is the ratio of the molecular weight of sulfur dioxide to the molecular weight of calcium sulfite hemihydrate (about 0.5), thermal weight analysis, and the XRD-Riet belt method. It is preferable that the content is smaller than the sulfur dioxide content calculated from the calcium sulfite content X [mass%] obtained on at least one of the above. Y / X is preferably less than 0.47, more preferably less than 0.465, still more preferably less than 0.45, and particularly preferably less than 0.42.

The above-mentioned numerical range of Y / X is either when the content of calcium sulfite obtained by the thermal weight analysis and the XRD-Rietveld method is different (for example, X ₁ [mass%] ≠ X ₂ [mass%]). It suffices to satisfy with the calcium sulfite content obtained by one method (for example, X ₁ [mass%]), and is satisfied with the calcium sulfite content (for example, X ₂ [mass%]) obtained by the other method. It does not have to be. Of course, both X ₁ [mass%] and X ₂ [mass%] may satisfy the above-mentioned numerical range of Y / X.

The content of sulfur trioxide measured based on JIS R 5202: 2010 "12 Method for Quantifying Sulfur Trioxide" is preferably 5 to 40% by mass, more preferably 10 to 30% by mass.

[Second Embodiment of Calcium Sulfite-Containing Gypsum]
The calcium sulfite-containing gypsum according to the second embodiment of the present disclosure contains calcium sulfite, and has the calcium sulfite content (X [mass%]) obtained by at least one of the thermal weight analysis and the XRD-Rietveld method, and JIS R. The sulfur dioxide content (Y [mass%]) measured based on "16 Sulfur dioxide quantification method" of 9101: 2018 satisfies the relationship of the formula (2).
Y <0.4737X ... (2)

It is considered that the calcium sulfite contained in the calcium sulfite-containing gypsum satisfying the above formula (2) has an ionic state different from that of normal calcium sulfite. This is considered to be one of the factors that increase the uniaxial compressive strength when mixed with soil. From the viewpoint of obtaining a ground improving material capable of maintaining a higher uniaxial compressive strength while reducing the elution of hexavalent chromium, it is preferable to satisfy the following formula (3), and the following formula (4) is satisfied. Is more preferable, it is more preferable to satisfy the following formula (5), and it is particularly preferable to satisfy the following formula (6).

Y <0.47X ... (3)
Y <0.465X ... (4)
Y <0.45X ... (5)
Y <0.42X ... (6)

Y in the formulas (2), (3), (4), (5) or (6) may exceed 0.1X (0.1X <Y) and may exceed 0.2X (0. It may exceed 2X <Y) and 0.3X (0.3X <Y). The above formulas (2), (3), (4), (5) or (6) are when the calcium sulfite content determined by the thermoweight analysis and the XRD-Rietveld method is different (for example, X ₁ [mass]. %] ≠ X ₂ [mass%]), it is sufficient that the calcium sulfite content obtained by either method (for example, X ₁ [mass%]) is satisfied, and the calcium sulfite content obtained by the other method is sufficient. It does not have to be satisfied by the rate (for example, X _{2 [mass%]).} Of course, both X ₁ [mass%] and X ₂ [mass%] may satisfy the numerical range of the above formulas (2), (3), (4), (5) or (6).

The calcium sulfite-containing gypsum according to the second embodiment may have the properties described in the first embodiment. For example, the above formula (1) may be satisfied, or the above-mentioned preferable properties described in the first embodiment may be provided. That is, the description of the first embodiment can be applied to calcium sulfite of the second embodiment.

[Third Embodiment of calcium sulfite-containing gypsum]
The calcium sulfite-containing gypsum according to the third embodiment of the present disclosure contains calcium sulfite. The thickness of the plate-like crystals contained in the calcium sulfite-containing gypsum is 5 μm or less, and the length of the main surface is 5 to 100 μm or less. The plate crystals may be calcium sulfite. The thickness and length of the plate crystals can be measured based on the observation image by SEM. The length of the main surface means the maximum distance among the distances connecting two points at different positions selected on the outer edge of the main surface on the main surface (plane having the largest area) of the plate-shaped crystal. For example, when the main surface is square, the length of the diagonal line is the length of the main surface. The thickness of the plate-shaped crystal is the maximum length of the lengths along the direction orthogonal to the main surface.

The thickness of the plate-like crystal is preferably 4 μm or less, and more preferably 3 μm or less. The length of the main surface of the plate crystal is preferably 90 μm or less, more preferably 80 μm or less, further preferably 70 μm or less, and particularly preferably 60 μm or less. The length of the main surface of the plate-shaped crystal is preferably 8 μm or more, more preferably 10 μm or more, and further preferably 20 μm or more.

The calcium sulfite-containing gypsum according to the third embodiment may have the properties described in the first embodiment and / or the second embodiment. For example, the above formula (1) may be satisfied, or the above-mentioned preferable properties described in the first embodiment may be provided. Further, at least one of the above formulas (2) to (6) may be satisfied. That is, the contents of the description of the first embodiment and the second embodiment can be applied to the calcium sulfite of the third embodiment.

[Modification of calcium sulfite-containing gypsum]
The calcium sulfite-containing gypsum of this modification is the calcium sulfite-containing gypsum of the first embodiment, the second embodiment or the third embodiment, and further, the average particle size in the laser diffraction / scattering method is 10 μm or less. While having such an average particle size, it also has the properties described in the first embodiment, the second embodiment, or the third embodiment. It should be noted that all the properties of the first embodiment and the second embodiment may be combined, or all the properties of the first embodiment, the second embodiment and the third embodiment may be combined. That is, the contents of the description of the first embodiment, the second embodiment and the third embodiment can be applied to the calcium sulfite of this modified example.

The average particle size in the present disclosure is the median diameter (D50: particle size when the cumulative frequency is 50%) measured using a laser diffraction type particle size distribution measuring device. As the laser diffraction type particle size distribution measuring device, SALD-2200 (device name) manufactured by Shimadzu Corporation can be used.

By having the above-mentioned average particle size, the calcium sulfite-containing gypsum can sufficiently increase the uniaxial compressive strength of the solidified soil when used as a solidifying material as an example of a soil conditioner. From the viewpoint of further increasing the uniaxial compressive strength, the average particle size of the calcium sulfite-containing gypsum is preferably 7 μm or less, more preferably 5 μm or less, still more preferably 1 μm or less, and particularly preferably 0. It is 5 μm or less.

[Cement composition]
The cement composition according to one embodiment of the present disclosure includes calcium sulfite-containing gypsum according to any one of the above-described embodiments or modifications. The cement composition may include cement clinker. Moreover, gypsum different from the gypsum containing calcium sulfite may be contained. The type of cement composition is not particularly limited, and ordinary Portland cement, early-strength Portland cement, moderate-heat Portland cement, ultra-early-strength Portland cement, sulfate-resistant Portland cement, moderate-heat Portland cement, low-heat Portland cement, blast furnace cement, and frying It may be any of ash cement, silica fume cement, alumina cement and the like.

Since the cement composition contains calcium sulfite-containing gypsum, when used as a solidifying material as an example of a ground improvement material, it increases the uniaxial compressive strength while reducing the elution of hexavalent chromium from the solidified soil. Can be done. The content of calcium sulfite in the cement composition is preferably 0.2% by mass or more, more preferably 1% by mass or more, further preferably 2% by mass or more, and particularly preferably 4% by mass or more. is there. As a result, the uniaxial compressive strength can be further increased while further reducing the elution of hexavalent chromium. The upper limit of the content of calcium sulfite in the cement composition may be 20% by mass or 10% by mass. From the same viewpoint, the content of calcium sulfite-containing gypsum in the cement composition is preferably 0.2% by mass or more, more preferably 1% by mass or more, still more preferably 2% by mass or more, and particularly preferably 2% by mass or more. It is 4% by mass or more.

The content of gypsum in the cement composition is converted to SO ₃ may be, for example, 1 to 20 wt%, may be 2 to 15 mass%. When the above-mentioned "another gypsum" is included, the content of the gypsum in the cement composition is preferably in the above-mentioned numerical range in total of the gypsum derived from the calcium sulfite-containing gypsum and the "another gypsum".

The form of gypsum contained in the cement composition is not particularly limited, and may include, for example, dihydrate gypsum, and may contain both dihydrate gypsum and anhydrous gypsum. The proportion of dihydrate gypsum in the gypsum contained in the cement composition is preferably 10% by mass or more, more preferably 20% by mass or more, and further preferably 40% by mass or more.

The brain specific surface area of the cement composition is preferably 3000 to 8000 cm ² / g, more preferably 4000 to 7000 cm ² / g, and even more preferably 5000 to 7000 cm ² / g. When the brain specific surface area is 3000 cm ² / g or more, it becomes easy to achieve excellent strength development. On the other hand, when the brain specific surface area is 8000 cm ² / g or less, it is easy to control the viscosity when used as a concrete or solidifying material slurry within a suitable range. The use of the cement composition is not particularly limited, but it may be used as a ground improving material or a solidifying material.

[Ground improvement material and ground improvement soil]
The ground improvement material according to one embodiment of the present disclosure may contain the above cement composition, or may contain commercially available Portland cement and calcium sulfite-containing gypsum. Therefore, it can be said that it is a cement-based ground improvement material. Such a ground improvement material can maintain a high uniaxial compressive strength of the ground improvement soil while reducing the elution of hexavalent chromium from the ground improvement soil. The ground improving material of this embodiment may be used as a cement-based solidifying material.

The soil improvement soil according to the embodiment of the present disclosure includes soil (soil to be improved) and the above-mentioned soil improvement material. The ground improvement soil may be solidified soil that has been solidified. Since this ground improvement soil contains the above-mentioned ground improvement material, it is possible to increase the uniaxial compressive strength while suppressing the elution of hexavalent chromium when the solidification treatment is performed.

[Method for manufacturing calcium sulfite-containing gypsum and cement composition]
Next, an example of the method for producing the calcium sulfite-containing gypsum of the present disclosure will be described. FIG. 1 is an apparatus for producing calcium sulfite of this example. The manufacturing apparatus 100 includes a cement clinker section 10 for firing a sulfur-containing raw material and a clinker raw material to produce a cement clinker, a bypass section 12 for extracting exhaust gas, and a preheater section 16 having one or more cyclones. It has 70 and a desulfurization section 60 that absorbs sulfite gas contained in exhaust gas from a cement clinker firing section 70 into a raw material slurry to obtain a sulfite slurry containing calcium sulfite-containing gypsum.

Sulfur-containing wastes include those containing at least one selected from waste gypsum board, waste tires, desulfurized slag, gypsum, and cokes. The waste gypsum board may contain paper components adhering to the surface of the gypsum board.

Sulfur-containing waste and clinker raw material are supplied to the preheater unit 16. The preheater section 16 brings the raw material into contact with the exhaust gas from the cement kiln section 10 to preheat the raw material. The bypass section 12 extracts exhaust gas containing sulfurous acid gas from between the kiln bottom 10a of the cement kiln section 10 and the bottom cyclone (the cyclone at the bottom) or the calcining furnace (not shown) of the preheater section 16. The bypass portion 12 may be a chlorine bypass portion. The cement clinker produced in the cement kiln section 10 is cooled to 40 to 220 ° C. in the cooling section 11 and introduced into the silo 18. This cement clinker may be used in the production of cement compositions.

In the cement kiln section 10, coal, petroleum coke, and the like are burned and the raw materials react to generate exhaust gas containing sulfur dioxide. The exhaust gas extracted from the bypass portion 12 and the exhaust gas extracted from the kiln tail 10a contain sulfur dioxide. These exhaust gases are cooled in the cooling unit 17 to, for example, 400 ° C. or lower, preferably 100 to 200 ° C. The cooled exhaust gas is introduced into the dust collecting unit 19, and dust particles containing chlorine compounds are removed. The exhaust gas containing sulfurous acid gas from which the dust particles have been removed is introduced into the slurry preparation unit 30.

In the slurry preparation unit 30, the raw material slurry containing calcium carbonate and the exhaust gas containing sulfurous acid gas come into contact with each other to generate a sulfurous acid slurry containing calcium sulfite-containing gypsum. Here, the sulfur dioxide gas is mixed with the sulfur dioxide gas-containing exhaust gas from the sulfur dioxide gas concentration adjusting unit 20 before being introduced into the slurry preparation unit 30. Thereby, the sulfur dioxide gas concentration of the exhaust gas introduced into the slurry preparation unit 30 can be adjusted. The sulfur dioxide gas concentration adjusting unit 20 may be, for example, a cylinder filled with sulfur dioxide gas, or another plant that generates a mixed gas containing a high concentration of sulfur dioxide gas. The concentration of sulfur dioxide in the exhaust gas introduced into the slurry preparation unit may be, for example, about 100 ppm to 5% on a volume basis (standard state). The concentration of sulfur dioxide in the present disclosure is a volume-based (standard state) concentration.

The sulfite slurry obtained in the slurry preparation unit 30 is dried to obtain a calcium sulfite-containing gypsum. In this case, the particle size may be adjusted by pulverizing after drying or while drying. The drying temperature is preferably 40 to 100 ° C, more preferably 50 to 80 ° C. Drying may be performed under vacuum or reduced pressure. Grinding can be carried out while drying, for example by heating and / or under reduced pressure. The pulverization method is not particularly limited, but it is preferable to use a high-speed mixer, for example. As a result, the dehydrated cake of calcium sulfite-containing gypsum can be efficiently dried and pulverized to a sufficiently small particle size. The gas after the sulfurous acid content is recovered by the slurry preparation unit 30 is introduced into the kiln again via the cooling unit 11.

Although an example of the method for producing calcium sulfite-containing gypsum of the present disclosure has been described above, the method for producing calcium sulfite-containing gypsum is not limited to this. For example, instead of the exhaust gas generated from the cement clinker manufacturing equipment, the exhaust gas generated at the thermal power plant may be used. A gas containing sulfur dioxide can be used as appropriate.

The cement composition may be produced by blending the cement clinker produced by the production apparatus 100 and the calcium sulfite-containing gypsum. Further, it may be produced by further blending gypsum. Further, commercially available Portland cement and the above-mentioned calcium sulfite-containing gypsum may be blended and prepared, or gypsum may be further blended and prepared. The cement composition may be used as it is as a soil conditioner, or may be mixed with gypsum to be a soil conditioner. The soil-improved soil may be prepared by blending the soil to be improved, the cement composition and gypsum.

In another example, the solid content of the sulfite slurry obtained in the slurry preparation unit 30 may be concentrated to obtain a concentrated slurry, or the sulfite slurry may be dehydrated to prepare a dehydrated cake. Sulfurous acid slurry, concentrated slurry or dehydrated cake and other raw materials such as cement clinker and gypsum may be introduced into a crusher (for example, a mill) and mixed while crushing to prepare a cement composition. Thereby, the particle size of the calcium sulfite-containing gypsum can be adjusted while mixing. If a high temperature cement clinker is introduced into a crusher, drying, mixing and crushing can be performed in parallel to produce a cement composition.

Although some embodiments have been described above, the present disclosure is not limited to the above-described embodiments.

The contents of the present disclosure will be described in more detail with reference to the examples, but the present disclosure is not limited to the following examples.

[Manufacture of gypsum containing calcium sulfite]
A commercially available CaCO ₃ powder and water were mixed to prepare a raw material slurry having a solid content ratio of 20% by mass. _{Exhaust gas (containing SO 2} ) generated from a cement kiln for producing cement clinker was brought into contact with a raw material slurry to prepare a slurry of calcium sulfite-containing gypsum (lime-gypsum method).

By changing the properties of the raw material and fuel burned in the cement kiln _{and adjusting the SO 2} concentration in the exhaust gas generated from the cement kiln, five types of calcium sulfite-containing gypsum slurries having different calcium sulfite contents were produced. Of these, four types of slurries were filtered and dehydrated, and the obtained solid content was dried at 60 ° C. to obtain calcium sulfite-containing gypsum A to D.

Another type of slurry was filtered, dehydrated, and then dried using a high-speed mixer (manufactured by EarthTechnica Co., Ltd., trade name: FS10, capacity: 11 L). The dry powdering was carried out by stirring the inside of the kettle of the high-speed mixer at 60 ° C. under reduced pressure (60 kPa) while purging with nitrogen. As a result, calcium sulfite-containing gypsum E, which was more atomized than calcium sulfite-containing gypsum A to D, was obtained.

For comparison, a commercially available reagent calcium sulfite hemihydrate (manufactured by Wako Pure Chemical Industries, Ltd.) was prepared.

[Analysis of gypsum containing calcium sulfite]
Thermal weight analysis, powder XRD analysis, XRD-Rietveld analysis, wet analysis of sulfur dioxide and sulfur trioxide, and average particle size measurement of calcium sulfite-containing gypsum A to E were performed. The conditions for each analysis are as follows. The results of each analysis are as shown in Table 1. The powder XRD analysis of calcium sulfite as a reagent and the measurement of the average particle size were also performed in the same manner.

<Thermogravimetric analysis>
Using a commercially available thermogravimetric analyzer, the change in thermogravimetric analysis when heated to 200 ° C., 600 ° C. and 900 ° C. was measured. At 200 ° C., dihydrate gypsum is dehydrated. Calcium sulfite oxidizes at 600 ° C. Calcium carbonate decomposes at 900 ° C. Using the change in mass at each temperature, the contents of dihydrate gypsum, calcium sulfite and calcium carbonate were calculated based on the following formulas. It should be noted that the holding at each temperature was set until the weight change disappeared. The holding time at each temperature was 1 hour. In addition, the atmosphere during heating was air.

-Content rate of dihydrate gypsum (mass%) = mass loss when heated to 200 ° C x ([molecular weight of dihydrate gypsum] / [molecular weight of water of crystallization contained in dihydrate gypsum]) / sample mass x 100
-Calcium carbonate content (mass%) = (mass of sample after heating to 900 ° C and no change in mass) x [mass of sample after heating to -600 ° C and no change in mass) x [CaCO ₃ Molecular weight] / [Molecular weight of CO ₂ ] / Mass of sample x 100
-Calcium sulfite content (mass%) = 100- (dihydrate gypsum content + calcium carbonate content)

<Powder XRD>
An X-ray diffractometer (manufactured by Bruker AXS Co., Ltd., acceleration voltage: 30 kV, current: 10 mA, tube: Cu) was used. From the obtained diffraction patterns, the peak intensities at 2θ = 15.76 to 16.25 ° are shown in Table 1 as Ia, and the peak intensities at 2θ = 16.26 to 16.75 ° are shown as Ib. In addition, Ia and Ib are peak intensities calculated by using the above-mentioned X-ray diffractometer.

<Analysis by XRD-Rietveld method>
Rietveld analysis of the diffraction pattern obtained by powder XRD was performed. Analysis software (Topas, manufactured by Bruker AXS Co., Ltd.) was used for the Rietveld analysis. The contents of dihydrate gypsum, calcium sulfite and calcium carbonate obtained from this analysis result are as shown in Table 1.

<Wet analysis>
The sulfur dioxide content was measured based on "16 Sulfur Dioxide Quantification Method" of JIS R 9101: 2018. The content of sulfur trioxide was measured based on "12 Method for Quantifying Sulfur Trioxide" of JIS R 5202: 2010. In addition, the contents of calcium sulfite and dihydrate gypsum were calculated based on these measurement results and the molecular weights of sulfur dioxide, sulfur trioxide, calcium sulfite hemihydrate and dihydrate gypsum. These results are as shown in the column of wet analysis in Table 1.

<Average particle size>
Water was used as the dispersion medium, and ultrasonic dispersion was performed to prepare a measurement sample. The particle size distribution was measured using a laser diffraction type particle size distribution measuring device (manufactured by Shimadzu Corporation, device name: SALD-2200). From the obtained particle size distribution, the median diameter (D50: particle size when the cumulative frequency was 50%) was determined and used as the average particle size. The results are as shown in Table 1.

As shown in Table 1, it was confirmed that Examples 1 to 5 (calcium sulfite-containing gypsum A to E) contained calcium sulfite. The calcium sulfite in Table 1 is the content of hemihydrate.

FIG. 2A is a diffraction chart of the powder XRD of Example 1 (calcium sulfite-containing gypsum A). FIG. 2B is a diffraction chart of the powder XRD of Example 2 (calcium sulfite-containing gypsum B). FIG. 3A is a diffraction chart of the powder XRD of Example 3 (calcium sulfite-containing gypsum C). FIG. 3B is a diffraction chart of the powder XRD of Example 4 (calcium sulfite-containing gypsum D). FIG. 4A is a diffraction chart of the powder XRD of Example 5 (calcium sulfite-containing gypsum E). FIG. 4B is a diffraction chart of the powder XRD of Comparative Example 1 (calcium sulfite as a reagent). As shown in FIGS. 2, 3 and 4, the calcium sulfite-containing gypsum A to D and the reagent calcium sulfite were all analyzed by powder XRD, and 2θ = 15.76 to 16.25 ° and 16.26 to 16.26. It was confirmed that each of 16.75 ° had a diffraction peak derived from calcium sulfite.

FIG. 5 is an enlarged view showing a portion of 2θ = 15.5 to 17 ° in the diffraction charts of Example 1, Example 2, and Comparative Example 1. FIG. 6 is an enlarged view showing a portion of 2θ = 15.5 to 17 ° in the diffraction charts of Examples 3, 4 and 5. As shown in FIGS. 5, 6 and 1, the reagent calcium sulfite at 2θ = 15.76 to 16.25 ° as compared with the peak intensity Ib of the diffraction peak B at 2θ = 16.26 to 16.75 °. The peak intensity Ia of the diffraction peak A was considerably larger, and Ia / Ib was 3.06 (Comparative Example 1 in FIG. 5). On the other hand, all of the calcium sulfite-containing gypsums A to E had Ia / Ib of less than 2 (Examples 1 to 5 in FIGS. 5 and 6).

FIG. 7 is a graph showing the relationship between the calcium sulfite content obtained by thermogravimetric analysis and the peak intensity ratio (Ia / Ib) in Examples 1 to 4 and Comparative Example 1. As shown in FIG. 7, for Examples 1 to 4 produced by the same process, it is considered that there is a correlation between the calcium sulfite content and the peak intensity ratio (Ia / Ib). According to this correlation, it can be said that the peak intensity ratio (Ia / Ib) tends to decrease as the calcium sulfite content increases.

FIG. 8 is a graph showing the relationship between the calcium sulfite content (X) determined by XRD-Rietveld analysis or thermogravimetric analysis and the sulfur dioxide content (Y) determined by wet analysis. is there. In this graph, in addition to the calcium sulfite-containing gypsum A to E of Examples 1 to 5, calcium sulfite-containing gypsum produced in the same manner as the calcium sulfite-containing gypsum A to E and a commercially available reagent are blended in the following proportions. The results of the two mixed samples (Comparative Examples a and b) obtained in the above process were also plotted. FIG. 8 also shows a straight line with Y = 0.4737X. This straight line is the straight line having the smallest slope among the straight lines passing through the origin (0,0) and any of the analysis results of Comparative Examples a and b.

(Comparative example a)
A mixed sample was prepared by blending calcium sulfite hemihydrate, dihydrate gypsum and calcium carbonate in a mass ratio of 2: 2: 6.
(Comparative Example b)
A mixed sample was prepared by blending calcium sulfite hemihydrate, dihydrate gypsum and calcium carbonate in a mass ratio of 4: 4: 2.

As shown in FIG. 8, it was confirmed that the plots of Examples 1 to 5 and the calcium sulfite-containing gypsum produced in the same manner were below the theoretical value shown by the straight line of Y = 0.4737X. It was. That is, it was confirmed that it was in the range of Y <0.4737X. On the other hand, the reagent calcium sulfite was in the range of Y ≧ 0.4737X.

[Manufacturing of cement composition]
A cement composition was prepared using the calcium sulfite-containing gypsum of Examples 3, 4 and 5 and Comparative Example 1. Other raw materials used are as follows.
・ Ordinary Portland cement / dihydrate gypsum (exhaust dihydrate gypsum)

Each raw material was mixed using a locking mixer in the formulation shown in Table 2 to prepare a cement composition for ground improvement of each Example and Comparative Example. In the formulation, the amount of calcium sulfite-containing gypsum was adjusted so that the content of calcium sulfite (content by thermogravimetric analysis) contained in the cement composition for improving each place was set to a predetermined value. _{In addition, the SO 3} equivalent content of gypsum in the cement composition, which is calculated based on the content of sulfur trioxide contained in calcium sulfite-containing gypsum and the content of sulfur trioxide contained in ordinary Portland cement, is almost constant. The blending amount of the desorbed dihydrate gypsum was adjusted so as to be. Table 2 shows the calculation results of _{the SO 3} equivalent content of gypsum in the cement composition.

The content of calcium sulfite in the cement compositions of Comparative Examples 2 to 6 in Table 2 is the same as the blending amount of calcium sulfite as a reagent. _{The SO 3} equivalent content of gypsum in the cement compositions of Comparative Examples 2 to 6 was calculated based on the blending ratio of the blended drainage dihydrate gypsum. The content of calcium sulfite in the cement compositions of Examples 6 to 14 was calculated by multiplying the blending ratio of calcium sulfite-containing gypsum by the content of calcium sulfite by thermal weight analysis of calcium sulfite-containing gypsum. _{The SO 3} conversion content of gypsum in the cement compositions of Comparative Examples 2 to 6 is the total value of _{the SO 3} conversion values of gypsum derived from ordinary Portland cement and drained dihydrate gypsum. _{The SO 3} conversion content of gypsum in the cement compositions of Examples 6 to 14 is the total value of _{the SO 3} conversion values of gypsum derived from calcium sulfite-containing gypsum, ordinary Portland cement and gypsum drainage dihydrate gypsum.

[Evaluation of solidified soil]
Using the cement compositions of each Example and Comparative Example, soil ground improvement was carried out in the following manner, and its performance was evaluated. Kanto loam (wet density: 1.36 cm ³ / g) was prepared as the soil to be treated. The cement compositions of each Example and Comparative Example ^{were blended with the soil to be treated at a ratio of 300 kg / m 3} and mixed using a hovert mixer to obtain a mixture.

The prepared mixture was packed in three layers using a rammer in a cylindrical frame having a diameter of 50 mm and a height of 100 mm, and then sealed and cured at 20 ° C. until the age of 7 days. Made. The solidified soil produced was subjected to an elution test in accordance with Notification No. 46 of the Environment Agency (August 23, 1991), and the amount of hexavalent chromium [Cr (VI)] eluted was determined. The amount of hexavalent chromium eluted was determined by quantifying the hexavalent chromium concentration in the filtrate after shaking by the diphenylcarbazide absorptiometry. In addition, the uniaxial compressive strength of the prepared solidified soil at ages 7, 28 and 91 days was measured according to JIS A 1216 “Soil uniaxial compression test method”. The results of these measurements are shown in Table 3.

In Table 3, when the amount of Cr (VI) eluted was displayed as "<0.002", it indicates that it was less than the lower limit of detection. Example 14 using the cement composition containing calcium sulfite-containing gypsum E of Example 5 showed the highest uniaxial compressive strength. In addition, the amount of Cr (VI) eluted in Example 14 was also relatively low.

FIG. 9 is a graph showing the results of the amount of Cr (VI) eluted from the solidified soil shown in Table 3 for each content of “calcium sulfite” shown in the “composition of cement composition” shown in Table 2. The amount of hexavalent chromium eluted from the solidified soil using the cement composition of each example was almost the same as that of the solidified soil using calcium sulfite as a reagent.

FIG. 10 is a graph showing the results of the uniaxial compressive strength of the solidified soil shown in Table 3 for each content of “calcium sulfite” shown in the “composition of cement composition” shown in Table 2. In Comparative Examples 3 to 6 using the reagent calcium sulfite, the uniaxial compressive strength tended to decrease when the content of calcium sulfite in the cement composition was 4% by mass or more. On the other hand, the cement compositions of Examples 6 to 9 containing the calcium sulfite-containing gypsum of Example 3 and the cement compositions of Examples 10 to 13 containing the calcium sulfite-containing gypsum of Example 4 are cement compositions. Even if the content of calcium sulfite in the cement exceeded 4% by mass, the uniaxial compressive strength was maintained higher than that of the additive-free one (Comparative Example 2).

FIG. 11 is an SEM photograph of the calcium sulfite-containing gypsum of Example 5 used for preparing the cement composition of Example 14. On the other hand, FIG. 12 is an SEM photograph of calcium sulfite-containing gypsum of another example produced in the same manner as in Examples 1 to 4. As shown in FIG. 11, the calcium sulfite-containing gypsum of Example 5 prepared using a high-speed mixer had a sufficiently small particle size. The calcium sulfite-containing gypsum of another example shown in FIG. 12 contained plate-like crystals having a main surface length of 10 to 30 μm and a thickness of 1 μm. Such crystals can be atomized by pulverizing them using a normal pulverizer.

According to the present disclosure, it is possible to provide a ground improvement material and a ground improvement soil capable of maintaining a high uniaxial compressive strength while reducing the elution of hexavalent chromium. The present disclosure can also provide calcium sulfite-containing gypsum and cement compositions capable of obtaining such ground improvement materials and soil improvement soils.

10 ... cement kiln part, 10a ... kiln butt, 11 ... cooling part, 12 ... bypass part, 16 ... preheater part, 17 ... cooling part, 18 ... silo, 19 ... dust collecting part, 20 ... desulfurization gas concentration adjusting part, 30 ... Slurry preparation section, 60 ... Desulfurization section, 70 ... Cement clinker firing section, 100 ... Manufacturing equipment.

Claims (10)

Calcium sulfite-containing gypsum containing calcium sulfite
Peak intensity ratio meets the equation (1) in the diffraction pattern obtained by powder XRD using CuKα line,
A calcium sulfite-containing gypsum having a calcium sulfite content (X [mass%]) of 2% by mass or more, which is obtained by at least one of the thermal weight analysis and the XRD-Rietveld method.
Ia / Ib <2 ... (1)
(However, Ia is the peak intensity at 2θ = 15.76 to 16.25 °, and Ib is the peak intensity at 2θ = 16.26 to 16.75 °.) Calcium sulfite-containing gypsum containing calcium sulfite
Calcium sulfite content (X [mass%]) determined by at least one of thermogravimetric analysis and XRD-Rietveld method, and sulfur dioxide measured based on JIS R 9101: 2018 "16 Sulfur Dioxide Quantification Method" the content of sulfur and (Y [wt%]) but meets the relationship of formula (2),
A gypsum containing calcium sulfite, wherein the content (X [mass%]) of calcium sulfite is 2% by mass or more.
Y <0.4737X ... (2) Calcium sulfite-containing gypsum containing calcium sulfite
Contains plate crystals,
The plate thickness of the crystal is 5μm or less, and the length of the main surface Ri 5~100μm der,
A calcium sulfite-containing gypsum having a calcium sulfite content (X [mass%]) of 2% by mass or more, which is obtained by at least one of the thermal weight analysis and the XRD-Rietveld method. (Ii) The calcium sulfite-containing gypsum according to any one of claims 1 to 3, which comprises at least one of water gypsum, hemihydrate gypsum and anhydrous gypsum. The calcium sulfite-containing gypsum according to any one of claims 1 to 4, wherein the content (X [mass%]) of calcium sulfite is 20% by mass or more . The calcium sulfite-containing gypsum according to any one of claims 1 to 5, wherein the average particle size in the laser diffraction / scattering method is 10 μm or less. A cement composition comprising the calcium sulfite-containing gypsum according to any one of claims 1 to 6. The cement composition according to claim 7, wherein the content of calcium sulfite is 0.2% by mass or more. A ground improving material containing the calcium sulfite-containing gypsum according to any one of claims 1 to 6. A soil improvement soil containing the soil improvement material according to claim 9.

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