Reducing material and its manufacturing method, cement composition, ground improvement material, and method for suppressing hydrogen sulfide gas generation

Abstract

To provide a reduction material that can inhibit the generation of hydrogen sulfide gas while containing calcium sulfide.SOLUTION: A reduction material contains calcium sulfide, and one or both of free lime and magnesium oxide. In the case of containing the free lime, the content of the free lime is 2.8 pts.mass or more relative to the calcium sulfide 100 pts.mass. In the case of containing the magnesium oxide, the content of the magnesium oxide is 1 pt.mass or more relative to the calcium sulfide 100 pts.mass.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a reducing agent and a method for producing the same, a cement composition, a soil improvement material, and a method for suppressing the generation of hydrogen sulfide gas. [Background technology]

[0002] Cement clinker is produced using limestone, clay, silica, iron oxide, and the like as main raw materials. In addition to these main raw materials, various industrial by-products and industrial waste are also effectively utilized as raw materials and fuels in the production of cement clinker. For this reason, depending on the selection of raw materials, cement clinker may be contaminated with small amounts of heavy metals such as cadmium, chromium, lead, and molybdenum derived from the various raw materials and fuels. Therefore, techniques are known for reducing heavy metal ions derived from cement clinker using various reducing agents to reduce the amount of elution (Patent Documents 1 to 4).

[0003] For example, Patent Document 1 proposes a technology for reducing hexavalent chromium to trivalent chromium and suppressing elution of hexavalent chromium by using a mixture of ferrous sulfate and sulfite as a solidification material when solidifying soft ground. Patent Document 2 proposes a technology for suppressing elution of hexavalent chromium by using a ground improvement material that has a specific pH and oxidation-reduction potential and contains a predetermined amount of MgO. Patent Documents 3 and 4 propose technologies for suppressing elution of heavy metals by using a heavy metal immobilization agent containing calcium sulfide.

[0004] The surface of calcium sulfide may decompose due to moisture and carbon dioxide in the air, generating trace amounts of hydrogen sulfide. In response to this, Patent Document 4 proposes a technology for reducing the generation of hydrogen sulfide by adding a specific proportion of calcium sulfate to calcium sulfide. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-201406 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-155141 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-102643 [Patent Document 4] Japanese Patent Application Laid-Open No. 2005-306911 Summary of the Invention [Problem to be solved by the invention]

[0006] If the generation of hydrogen sulfide gas caused by calcium sulfide could be further reduced, the working environment could be further improved. Therefore, the present disclosure provides a reducing agent that contains calcium sulfide but can sufficiently suppress the generation of hydrogen sulfide gas, a method for producing the same, a cement composition, and a ground improvement material. Also provided is a method for suppressing the generation of hydrogen sulfide gas by using such a reducing agent. [Means for solving the problem]

[0007] In one aspect, the present disclosure provides a reducing material that contains calcium sulfide and one or both of free lime and magnesium oxide, and when the free lime is contained, the content of the free lime per 100 parts by mass of calcium sulfide is 2.8 parts by mass or more, and when the magnesium oxide is contained, the content of the magnesium oxide per 100 parts by mass of calcium sulfide is 1 part by mass or more.

[0008] The reducing agent contains calcium sulfide and a predetermined amount of either or both of free lime and magnesium oxide, thereby making it possible to sufficiently suppress the generation of hydrogen sulfide gas while containing calcium sulfide.

[0009] The reducing agent preferably contains 20% by mass or more of calcium sulfide. Even if the calcium sulfide content is high, the generation of hydrogen sulfide gas can be sufficiently suppressed. By increasing the calcium sulfide content, the reduction effect can be further enhanced.

[0010] The reducing agent preferably contains a calcium sulfide-containing material containing a carbon-reduced product of gypsum and an additive. More preferably, the calcium sulfide-containing material contains 50 mass % or more of calcium sulfide, and the additive contains at least a portion of free lime and magnesium oxide. Such a reducing agent is easy to manufacture at low manufacturing cost and has a high reducing effect.

[0011] In the reducing agent, it is preferable that the additive contains 20 mass % or more of free lime, which makes it possible to more effectively suppress the generation of hydrogen sulfide gas.

[0012] The additive preferably includes one or both of clinker dust and dehydrated cake of Ca-containing slurry, which can sufficiently reduce the production cost of the reducing material.

[0013] The gypsum content is preferably less than 57% by mass, which allows the reducing agent to achieve high levels of both reduction and hydrogen sulfide gas suppression.

[0014] In one aspect, the present disclosure provides a cement composition containing any one of the reducing agents described above. Because such a cement composition contains the reducing agent, it is possible to sufficiently suppress the generation of hydrogen sulfide gas while containing calcium sulfide.

[0015] In one aspect, the present disclosure provides a soil improvement material containing any one of the reducing agents described above. Because such soil improvement material contains the reducing agent, it can sufficiently suppress the generation of hydrogen sulfide gas while containing calcium sulfide.

[0016] In one aspect, the present disclosure provides a method for producing a reducing material, the method comprising: an evaluation step of evaluating components contained in a calcium sulfide-containing material and an additive containing one or both of free lime and magnesium oxide; and a blending step of blending the calcium sulfide-containing material with the additive containing one or both of free lime and magnesium oxide to obtain a reducing material, wherein in the blending step, the calcium sulfide-containing material and the additive are blended so that the amount of free lime is 2.8 parts by mass or more and / or the amount of magnesium oxide is 1 part by mass or more per 100 parts by mass of calcium sulfide based on the evaluation results in the evaluation step.

[0017] According to the above-described manufacturing method, a reducing material having a high reducing effect can be stably manufactured. The reducing material obtained by this manufacturing method contains calcium sulfide and a predetermined amount of either or both of free lime and magnesium oxide. This makes it possible to sufficiently suppress the generation of hydrogen sulfide gas while containing calcium sulfide.

[0018] The production method preferably includes a reduction step of heating a raw material containing a gypsum source and a carbon source to reduce at least a portion of the gypsum and obtain a calcium sulfide-containing material. This allows the production of the reducing material at a sufficiently low production cost.

[0019] In one aspect, the present disclosure provides a method for suppressing the generation of hydrogen sulfide gas, the method comprising the step of blending a calcium sulfide-containing material with an additive containing one or both of free lime and magnesium oxide, the calcium sulfide-containing material and the additive being blended so that the free lime is 2.8 parts by mass or more and / or the magnesium oxide is 1 part by mass or more per 100 parts by mass of calcium sulfide. This method can sufficiently suppress the generation of hydrogen sulfide gas, for example, when it is necessary to suppress the elution of heavy metal ions such as hexavalent chromium from soil, cement, etc. [Effects of the Invention]

[0020] According to the present disclosure, it is possible to provide a reducing agent that contains calcium sulfide but is capable of sufficiently suppressing the generation of hydrogen sulfide gas, a method for producing the same, a cement composition, and a ground improvement material, and also to provide a method for suppressing the generation of hydrogen sulfide gas by using such a reducing agent. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a diagram for explaining a method for evaluating the amount of hydrogen sulfide generated. [Figure 2] FIG. 1 is a diagram showing a procedure for measuring the amount of hydrogen sulfide generated. DETAILED DESCRIPTION OF THE INVENTION

[0022] Embodiments of the present disclosure will be described below. However, the following embodiments are merely examples for explaining the present disclosure, and are not intended to limit the present disclosure to the following contents.

[0023] The reducing material according to one embodiment contains calcium sulfide and one or both of free lime (f.CaO) and magnesium oxide. When free lime is contained, the content of the free lime relative to 100 parts by mass of calcium sulfide is 2.8 parts by mass or more.

[0024] The content of calcium sulfide in the reducing agent is preferably 20% by mass or more, more preferably 30% by mass or more, even more preferably 40% by mass or more, and particularly preferably 50% by mass or more, from the viewpoint of sufficiently increasing the reducing action and sufficiently suppressing the elution of hexavalent chromium and the like. From the viewpoint of ensuring the content of at least one of free lime and magnesium oxide, the content of calcium sulfide may be, for example, 95% by mass or less, or may be 90% by mass or less. The content of calcium sulfide can be determined, for example, by the XRD-Rietveld method.

[0025] The content of free lime relative to 100 parts by mass of calcium sulfide is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 12 parts by mass or more, from the viewpoint of sufficiently and stably suppressing the generation of hydrogen sulfide gas. The content of free lime may be, for example, less than 40 parts by mass or less than 30 parts by mass, from the viewpoint of ensuring a high reducing action.

[0026] The content of free lime in the reducing agent may be, for example, 0.5% by mass or more, 2% by mass or more, or 3% by mass or more, from the viewpoint of sufficiently suppressing the generation of hydrogen sulfide gas. The content of free lime in the reducing agent may be, for example, less than 30% by mass, less than 20% by mass, or less than 15% by mass, from the viewpoint of sufficiently exerting a high reduction effect.

[0027] The free lime (f.CaO) content may be a value (measured value) measured by the Cement Association Standard Test Method (JCAS I-01-1997 "Quantitative Method for Free Calcium Oxide"). It may also be the sum (α + β) of the CaO content (α) determined by the XRD-Rietveld method and the value (β) obtained by converting the Ca(OH)2 content determined by the XRD-Rietveld method to CaO content. The measured value, the sum, and the calculated value based on these values ​​determined by the above methods may be within the above-mentioned numerical ranges.

[0028] When the reducing agent contains magnesium oxide, the content of magnesium oxide per 100 parts by mass of calcium sulfide is 1 part by mass or more. From the viewpoint of sufficiently and stably suppressing the generation of hydrogen sulfide gas, the content of magnesium oxide is preferably 2 parts by mass or more, more preferably 2.8 parts by mass or more, even more preferably 5 parts by mass or more, and particularly preferably 10 parts by mass or more. From the viewpoint of ensuring a high reduction action, the content of magnesium oxide may be, for example, less than 40 parts by mass, less than 30 parts by mass, or less than 20 parts by mass.

[0029] The content of magnesium oxide in the reducing agent may be, for example, 0.5% by mass or more, 2% by mass or more, or 5% by mass or more, from the viewpoint of sufficiently suppressing the generation of hydrogen sulfide gas. The content of magnesium oxide in the reducing agent may be, for example, less than 30% by mass, less than 20% by mass, or less than 15% by mass, from the viewpoint of sufficiently exerting a high reduction effect. The content of magnesium oxide can be determined, for example, by the XRD-Rietveld method.

[0030] The reducing agent may contain both free lime and magnesium oxide. In this case, the total amount of free lime and magnesium oxide per 100 parts by mass of calcium sulfide is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 12 parts by mass or more, from the viewpoint of sufficiently and stably suppressing the generation of hydrogen sulfide gas. From the viewpoint of the strength development of the cement composition and the ground improvement material, the total amount of free lime and magnesium oxide per 100 parts by mass of calcium sulfide may be, for example, less than 40 parts by mass or less than 30 parts by mass.

[0031] The total content of free lime and magnesium oxide in the reducing agent may be, for example, 0.5% by mass or more, 2% by mass or more, or 3% by mass or more, from the viewpoint of sufficiently suppressing the generation of hydrogen sulfide gas. The total content of free lime and magnesium oxide in the reducing agent may be, for example, less than 30% by mass, less than 20% by mass, or less than 15% by mass, from the viewpoint of sufficiently exerting a high reduction effect.

[0032] The reducing agent may contain components other than calcium sulfide, free lime, and magnesium oxide. However, the gypsum (CaSO4) content in the reducing agent is preferably less than 57 mass%, more preferably less than 40 mass%, and even more preferably less than 35 mass%, calculated on an anhydrous basis. This allows the reducing agent to achieve both a high level of reduction activity and a high level of hydrogen sulfide gas suppression activity.

[0033] The reducing agent may include a calcium sulfide-containing material containing a carbon-reduced product of gypsum. The carbon-reduced product of gypsum includes calcium sulfide. The calcium sulfide-containing material containing a carbon-reduced product of gypsum may be a heat-treated product obtained by heating a composition containing a gypsum source and a carbon source. The gypsum source is not particularly limited, and examples include waste gypsum dihydrate by-products from thermal power plants, natural gypsum dihydrate, natural anhydrous gypsum, and waste gypsum board. Of these, waste gypsum board is preferably used from the perspective of waste utilization. Examples of carbon sources include carbon-containing materials such as paper, waste plastic, carbon fiber, biomass, charcoal, coal, coal gasification slag, and petroleum. The calcium sulfide-containing material containing a carbon-reduced product of gypsum may be obtained by heating the above composition in the air or in a reducing atmosphere with an oxygen concentration lower than that of the air at a temperature range of 500 to 1500°C.

[0034] The calcium sulfide content in the calcium sulfide-containing material may be, for example, 40% by mass or more, 50% by mass or more, or 60% by mass or more from the viewpoint of providing a reducing agent with a high reducing effect. From the viewpoint of ease of production, the calcium sulfide content in the calcium sulfide-containing material may be less than 90% by mass.

[0035] The ratio of calcium sulfide contained in the calcium sulfide-containing material to the total calcium sulfide contained in the reducing agent (the ratio of calcium sulfide derived from the calcium sulfide-containing material) is preferably 50% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, thereby making it possible to sufficiently reduce the production cost of the reducing agent.

[0036] The reducing material may contain an additive containing free lime (f.CaO) and / or magnesium oxide in addition to the calcium sulfide-containing material. From the viewpoint of reducing the production cost of the reducing material, clinker dust and dehydrated cake of Ca-containing slurry are preferably used as the additive. Clinker dust is recovered, for example, from the chlorine bypass facility of a cement factory. On the other hand, the dehydrated cake of Ca-containing slurry (hereinafter sometimes referred to as "dehydrated cake") is not particularly limited, and examples thereof include hydrocake, which is a residue generated when producing magnesium hydroxide from seawater, and dehydrated cake, which is a by-product of the desulfurization process. The dehydrated cake may be dried as needed.

[0037] The free lime contained in the reducing agent may preferably be 10 mass % or more, more preferably 20 mass % or more, and even more preferably 25 mass % or more, of the free lime contained in the reducing agent, thereby further enhancing the effect of suppressing the generation of hydrogen sulfide gas.

[0038] The Blaine specific surface area of ​​the additive is preferably 2000 cm 2 / g or more, and more preferably 4000 cm 2 / g or more, more preferably 6000 cm 2 / g or more. This makes it possible to more effectively suppress the generation of hydrogen sulfide gas. This Blaine specific surface area can be measured in accordance with JIS R5201:2015 "Physical Testing Methods for Cement."

[0039] Clinker dust contains free lime. From the viewpoint of providing a reducing agent that has a high reducing effect while sufficiently suppressing the generation of hydrogen sulfide gas, the content of free lime in the clinker dust is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, and particularly preferably 35% by mass or more. From the viewpoint of ease of production of clinker dust, the content of free lime may be, for example, less than 60% by mass or less than 50% by mass. The content of free lime in the clinker dust is measured according to the Cement Association Standard Test Method (JCAS I-01-1997 "Method for Determining Free Calcium Oxide"). When the clinker dust contains magnesium oxide, it is preferable that the total content of free lime and magnesium oxide is within the above-mentioned range. The content of magnesium oxide in the clinker dust can be determined by the XRD-Rietveld method.

[0040] The dehydrated cake contains free lime and / or magnesium oxide. From the viewpoint of providing a reducing agent with high reducing activity while sufficiently suppressing the generation of hydrogen sulfide gas, the content of free lime or magnesium oxide in the dehydrated cake is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, and particularly preferably 40% by mass or more. From the viewpoint of easy availability of the dehydrated cake, the content of free lime or magnesium oxide may be, for example, less than 95% by mass or less than 80% by mass. The content of free lime in the dehydrated cake is measured according to the Cement Association Standard Test Method (JCAS I-01-1997 "Method for Determining Free Calcium Oxide"). When the dehydrated cake contains both free lime and magnesium oxide, the total content of free lime and magnesium oxide is preferably within the above-mentioned range. The content of magnesium oxide in the dehydrated cake can be determined by the XRD-Rietveld method.

[0041] The sulfate content in the clinker dust and dewatered cake is preferably low. For example, the calcium sulfate content in the clinker dust may be less than 20 mass%, less than 15 mass%, or less than 10 mass%. The total content of calcium sulfate and magnesium sulfate in the dewatered cake may be less than 40 mass%, or less than 20 mass%. The sulfate content can be determined by the XRD-Rietveld method.

[0042] The content of clinker dust in the reducing material may be 3% by mass or more, 5% by mass or more, or 8% by mass or more, from the viewpoint of further suppressing the generation of hydrogen sulfide gas while sufficiently reducing the manufacturing cost of the reducing material. The content of clinker dust in the reducing material may be less than 40% by mass or less than 30% by mass, from the viewpoint of maintaining a sufficiently high reduction action. The numerical range of the content of dehydrated cake in the reducing material may also be the same as the above-mentioned numerical range of the content of clinker dust. Furthermore, the numerical range of the total content when both dehydrated cake and clinker dust are included may also be the same as the above-mentioned numerical range of the content of clinker dust.

[0043] The additive is not limited to clinker dust and dewatered cake, and may contain various inorganic compounds either alone or in combination with one or both of these. Examples of inorganic compounds include chlorides, oxides, hydroxides, sulfates, sulfites, and carbonates. Specific examples include calcium hydroxide, ferrous chloride, calcium carbonate, magnesium hydroxide, ferrous sulfate, and calcium sulfate. Calcium sulfate may include any of the dihydrate (gypsum dihydrate), hemihydrate (gypsum hemihydrate), and anhydrite (gypsum anhydrite). The reducing agent may contain at least one of the above components.

[0044] The sulfite contained in the reducing agent is not particularly limited, and may include at least one selected from the group consisting of calcium sulfite (e.g., hemihydrate), sodium sulfite, magnesium sulfite, calcium bisulfite (Ca(HSO3)2), sodium bisulfite, and magnesium bisulfite. Of these, it is preferable to include calcium sulfite. This can further reduce the elution of hexavalent chromium when used as a reducing agent for soil improvement materials.

[0045] Each sulfite may be a commercially available chemically synthesized product or a naturally occurring product. Calcium sulfite may be, for example, calcium sulfite anhydride and / or calcium sulfite hemihydrate contained in gypsum generated in a flue gas desulfurization process or the like.

[0046] The hydration number of calcium sulfite contained in the reducing agent is not particularly limited, and may be, for example, hemihydrate or anhydrous. The calcium sulfite content in the present disclosure is the content as hemihydrate and is determined by converting all calcium sulfite into hemihydrate. From the viewpoint of sufficiently reducing the amount of hydrogen sulfide gas generated, the calcium sulfite content in the reducing agent is preferably 0.5% by mass or more, more preferably 1% by mass or more, even more preferably 3% by mass or more, and particularly preferably 5% by mass or more. From the viewpoint of maintaining a high reduction activity, the calcium sulfite content in the reducing agent is preferably 30% by mass or less, more preferably 20% by mass or less, even more preferably 15% by mass or less, and particularly preferably 12% by mass or less.

[0047] The reducing agent may contain calcium carbonate. From the viewpoint of sufficiently reducing the amount of hydrogen sulfide gas generated, the content of calcium carbonate in the reducing agent is preferably 0.5% by mass or more, more preferably 1% by mass or more, even more preferably 3% by mass or more, and particularly preferably 5% by mass or more. From the viewpoint of maintaining a high reduction activity, the content of calcium carbonate in the reducing agent is preferably 30% by mass or less, more preferably 20% by mass or less, even more preferably 15% by mass or less, and particularly preferably 12% by mass or less. The calcium carbonate contained in the reducing agent may be a commercially available product (reagent), may be chemically synthesized, or may be naturally occurring. For example, it may be limestone or may be contained in clinker dust. The content of calcium carbonate in the reducing agent can be determined by the XRD-Rietveld method.

[0048] The reducing material may contain sulfur. The sulfur is not particularly limited, but elemental sulfur can be used. Examples of elemental sulfur include sulfur produced from natural sulfur, sulfur obtained from pyrite, and sulfur obtained as a by-product of the desulfurization process during petroleum refining. If elemental sulfur obtained from a by-product is contained among these, production costs can be reduced. Sulfur may be contained in a calcium sulfide-containing material.

[0049] The reducing agent can sufficiently suppress the generation of hydrogen sulfide gas, and when used in cement compositions, soil improvement materials, etc., can sufficiently suppress the elution of heavy metals such as hexavalent chromium.

[0050] The method for producing the reducing agent includes a reduction step of heating a raw material containing a gypsum source and a carbon source to reduce at least a portion of the gypsum contained in the gypsum source and obtain a calcium sulfide-containing material containing calcium sulfide, an evaluation step of evaluating components contained in the calcium sulfide-containing material and an additive containing one or both of free lime and magnesium oxide, and a blending step of blending the calcium sulfide-containing material with the additive to obtain a reducing agent. In the blending step, the calcium sulfide-containing material and the additive are blended so that the amount of free lime is 2.8 parts by mass or more and / or the amount of magnesium oxide is 1 part by mass or more per 100 parts by mass of calcium sulfide based on the evaluation results of each component in the evaluation step.

[0051] The method for evaluating each component in the evaluation step is not particularly limited. For example, the contents of calcium sulfide, free lime, and magnesium oxide may be measured using an analytical device, or data correlating with these contents may be derived. The evaluation results may be specific analytical values ​​or data derived as described above. Any evaluation method may be used without particular limitation as long as it can calculate or estimate the blending ratio of calcium sulfide to free lime and / or magnesium oxide.

[0052] The above-described production method can stably produce the above-described reducing material. The gypsum source and carbon source are as described above. This production method can be carried out based on the description of the above-described embodiment of the reducing material. The above description of the reducing material applies to this production method.

[0053] The reducing agent obtained by the above-described production method can sufficiently suppress the generation of hydrogen sulfide gas, and when used in cement compositions, soil improvement materials, etc., can sufficiently suppress the elution of heavy metals such as hexavalent chromium.

[0054] A method for suppressing the generation of hydrogen sulfide gas according to one embodiment includes a step of blending a calcium sulfide-containing material with an additive containing one or both of free lime and magnesium oxide, and suppressing the generation of hydrogen sulfide gas by blending the calcium sulfide-containing material with the additive so that the free lime is 2.8 parts by mass or more and / or the magnesium oxide is 1 part by mass or more per 100 parts by mass of calcium sulfide. In addition to this blending step, other steps described in the above-mentioned method for producing a reducing agent may be included. This method for suppressing the generation of hydrogen sulfide gas can be carried out in the same manner as the above-mentioned embodiment of the reducing agent and the embodiment of the method for producing a reducing agent. The above-mentioned description of the reducing agent and the method for producing the reducing agent applies to this method for suppressing the generation of hydrogen sulfide gas.

[0055] The cement composition and ground improvement material according to one embodiment may contain the above-mentioned reducing agent, cement, and gypsum. By including the reducing agent, the cement composition and ground improvement material can maintain the amount of hexavalent chromium elution at a sufficiently low level from short-term to long-term ages. The content of the reducing agent in the cement composition and ground improvement material is preferably 0.1 to 30% by mass, more preferably 0.5 to 20% by mass, and even more preferably 1.0 to 10% by mass. This can sufficiently suppress the elution of hexavalent chromium from the resulting mortar, concrete, and ground improvement soil.

[0056] The content of calcium sulfide in the cement composition and the ground improvement material may be, for example, 1% by mass or more, 2% by mass or more, 3% by mass or more, or 4% by mass or more. By including calcium sulfide in such a range, it is possible to sufficiently suppress the elution of hexavalent chromium from the resulting mortar, concrete, and ground improvement soil. From the viewpoint of the strength development of the cement composition and the ground improvement material, the content of calcium sulfide in the cement composition and the ground improvement material may be less than 30% by mass, less than 20% by mass, or less than 10% by mass.

[0057] The gypsum may be any of gypsum dihydrate, gypsum hemihydrate, and gypsum anhydride. From the viewpoint of the strength development of the ground improvement material, it is preferable to contain gypsum dihydrate or gypsum anhydride. For example, when obtaining a ground improvement material, gypsum obtained by mixing gypsum dihydrate and gypsum anhydride may be used. From the viewpoint of the strength development of the ground improvement soil, the content of gypsum in the ground improvement material, calculated as an anhydride, is, for example, 1 to 25 mass%, preferably 3 to 20 mass%, more preferably 4 to 15 mass%, and even more preferably 5 to 12 mass%. The range of the gypsum content in the cement composition may be the same.

[0058] The cement may be any of the various Portland cements specified in JIS R5210:2003 "Portland Cement." Among these, ordinary Portland cement or high-early-strength Portland cement is preferred from the viewpoints of availability and increasing compressive strength at short ages. The total chromium content in the cement may be, for example, 30 to 250 mg / kg or 50 to 200 mg / kg from the viewpoint of availability. Similarly, the amount of water-soluble hexavalent chromium in the cement may be, for example, 3 to 40 mg / kg or 4 to 30 mg / kg from the viewpoint of similar availability. The total chromium content of the cement is measured in accordance with the method described in JIS R5202:2010, and the amount of water-soluble hexavalent chromium is measured in accordance with the method described in Cement Association Standard Test Method I-51-1981.

[0059] The cement content in the cement composition and ground improvement material is, for example, 50 to 98 mass%, preferably 70 to 95 mass%, and more preferably 75 to 90 mass%. If the cement content is less than 50 mass%, the cement composition and ground improvement soil tend to have difficulty in developing strength. On the other hand, if the cement content exceeds 98 mass%, the amount of hexavalent chromium eluted from mortar, concrete, and ground improvement soil may increase depending on the hexavalent chromium content of the cement.

[0060] Cement clinker can also be used as a substitute for cement. When using cement clinker, it is preferable to use it after adjusting it to an appropriate fineness.

[0061] The cement composition and the ground improvement material may further contain blast furnace slag powder. The content of the blast furnace slag powder may be, for example, 1 to 50 mass%, 5 to 30 mass%, or 10 to 20 mass%. When the blast furnace slag is contained in such a range, the amount of hexavalent chromium eluted from the resulting mortar, concrete, and ground improvement soil can be further suppressed, and the amount of reducing agent used can be reduced.

[0062] The fineness of the cement composition and the ground improvement material is not particularly limited, and the Blaine specific surface area is, for example, 1000 to 6000 cm 2 / g, and 2000 to 5500 cm 2 / g, and 3000 to 5000 cm 2 / g, and 4000 to 4500 cm 2 / g. Within this range, the amount of hexavalent chromium elution can be suppressed while maintaining the strength of the resulting mortar, concrete, and ground improvement soil. This Blaine specific surface area can be measured in accordance with JIS R5201:2015 "Physical Testing Methods for Cement."

[0063] There are no particular limitations on the method for producing the cement composition and the ground improvement material, and they may be produced by mixing raw materials adjusted to a predetermined fineness, or by mixing and grinding the raw materials.

[0064] The improved soil contains the above-mentioned soil improvement material and the soil to be improved. Such improved soil is obtained by mixing the above-mentioned soil improvement material and the soil to be improved. 3 The content of the soil improvement material relative to the above may be, for example, 20 to 500 kg, 50 to 450 kg, 50 to 400 kg, or 100 to 350 kg.

[0065] The soil to be improved is not particularly limited, and may be volcanic ash clayey soil (e.g., Kanto loam), which is relatively difficult to suppress the elution of hexavalent chromium. By using the ground improvement material according to this embodiment, it is possible to sufficiently suppress the elution of hexavalent chromium from the improved soil while maintaining a high compressive strength of the improved soil.

[0066] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments. For example, the use of the reducing material is not limited to a soil improvement material or a cement composition, and the reducing material may be mixed with incineration ash, construction soil, etc. [Example]

[0067] The present disclosure will be described in detail below with reference to examples, comparative examples, and reference examples, but the present disclosure is not limited to the following examples.

[0068] [Manufacturing reducing materials] The following raw materials were prepared: ·Waste gypsum board ·charcoal ·Anhydrite Coal gasification slag Calcium sulfite hemihydrate: Wako Pure Chemical Industries, Ltd., chemical grade (10 g of this product dispersed in 100 g of distilled water, the supernatant pH: 8.40, oxidation-reduction potential (ORP): 260 mV) Magnesium oxide: Wako Pure Chemical Industries, Ltd., chemical grade Calcium oxide: Calcium carbonate is heated to 900°C for 1 hour and then crushed in a mortar. Calcium hydroxide: Wako Pure Chemical Industries, Ltd., chemical grade Calcium carbonate: Wako Pure Chemical Industries, Ltd., chemical grade Clinker dust (1) and (2): Recovered from the chlorine bypass facility of a cement factory.

[0069] Charcoal and waste gypsum board were mixed and heated to 950°C in a small test kiln to obtain calcium sulfide-containing material A (Ca sulfide-containing material A) containing a carbon-reduced product of gypsum. Furthermore, anhydrous gypsum was mixed with coal gasification slag and heated to 950°C in an electric furnace to obtain a heated product containing a carbon-reduced product of gypsum. The heated product was sieved using a sieve with 90 μm openings, and the fine particles that passed through the sieve were designated calcium sulfide-containing material B (Ca sulfide-containing material B). The obtained calcium sulfide-containing materials A and B were each pulverized using a planetary mill.

[0070] Table 1 shows the results of quantitative analysis of calcium sulfide-containing materials A and B and the above raw materials by XRD-Rietveld method, as well as the free lime content (f.CaO). For composition analysis by XRD-Rietveld method, an X-ray diffractometer (manufactured by Bruker AXS, accelerating voltage: 30 kV, current: 10 mA, tube: Cu) was used. Rietveld analysis was performed on the obtained powder X-ray diffraction patterns to quantify each component. Analysis software (manufactured by Bruker AXS, Topas) was used for Rietveld analysis.

[0071] Regarding the free lime content (f.CaO), Table 1 shows the value measured according to the Cement Association's standard test method (JCAS I-01-1997 "Quantitative Method for Free Calcium Oxide") as the "measured value." The sum (α + β) of the CaO content (α) determined by XRD Rietveld analysis and the value (β) obtained by converting the Ca(OH)2 content determined by XRD Rietveld analysis to CaO content is shown as the "calculated value." The conversion from Ca(OH)2 content to CaO content was performed using the following formula: In the following formula, the molecular weight of Ca is set to 56 and the molecular weight of Ca(OH)2 is set to 74. CaO content = Ca(OH)2 content x CaO molecular weight / Ca(OH)2 molecular weight

[0072] [Table 1]

[0073] The raw materials shown in Table 1 were added as additives to the crushed calcium sulfide-containing material A or calcium sulfide-containing material B in the proportions shown in Table 2 to prepare reducing materials. The mass-based content of each component was as shown in Table 3. The content of each component based on CaS (100 parts by mass) was as shown in Table 4. The content of each component in Tables 3 and 4 is a calculated value based on the composition shown in Table 1 and the blending amount in Table 2. Tables 3 and 4 also show the content of f.CaO by origin (the content of f.CaO derived from calcium sulfide-containing materials A and B, and the content of f.CaO derived from additives (1) and (2)), calculated based on the f.CaO value shown in Table 1 and the blending amount in Table 2.

[0074] [Table 2]

[0075] [Table 3]

[0076] [Table 4]

[0077] [Evaluation of hydrogen sulfide gas generation rate] The amount of hydrogen sulfide gas generated was evaluated using the apparatus shown in FIG. 1 and the procedure shown in FIG. 2. 30 g of sample 30 (reducing material) (approximately 30 cm) was placed in a 250 ml polyethylene container 10. 3The container 10 was placed in a thermo-hygrostat set at 60°C and 60% RH with the opening covered with cloth 20. The container was then left to stand for a predetermined period (1 day, 3 days, 7 days, or 14 days) in a thermo-hygrostat set at 60°C and 60% RH. After the predetermined period, the container 10 was removed from the thermo-hygrostat and cooled at room temperature for 10 minutes. The cloth 20 was then removed, and a detector tube was inserted into the container 10 through the opening. The detector tube used was a GV-110,4LT (detection range: 0.1 to 4 ppm, suction volume: 100 mL) manufactured by Gastec Corporation. One minute after the detector tube was inserted, the gas in the container was aspirated with the detector tube at a position approximately 10 mm above the top surface of the sample 30 in the container 10 for 90 seconds. After 1.5 minutes of suction, the value on the detector tube was read to determine the H2S concentration. The H2S concentrations determined for each reducing agent are shown in Table 5.

[0078] [Table 5]

[0079] Table 5 also shows the proportion of f.CaO by origin based on the total amount of f.CaO (the proportion of f.CaO derived from calcium sulfide-containing materials A and B, and the proportion of f.CaO derived from additives (1) and (2)).

[0080] Compared with the reducing materials of Comparative Examples 1 to 3, which used only calcium sulfide-containing materials, it was confirmed that the reducing materials of Examples 1 to 11, which contained various additives and a predetermined amount of free lime (f.CaO) or MgO, could sufficiently suppress the amount of hydrogen sulfide gas generated.

[0081] [Preparation of ground improvement material] The following raw materials were prepared: Cement: Ordinary Portland cement (total chromium content: 63.5 mg / kg, water-soluble hexavalent chromium content: 4.5 mg / kg) ·Anhydrite: Natural anhydrite - Soil to be improved: Kanto loam

[0082] The above-mentioned raw materials and the reducing materials of Examples 1 and 5 and Comparative Example 2 after evaluating the amount of hydrogen sulfide gas generated (the number of days in parentheses is the period of storage in a thermo-hygrostat) were mixed in the proportions shown in Table 6 to prepare ground improvement materials (1) to (3).

[0083] [Preparation and evaluation of ground improvement soil] The amount of soil improvement material mixed for the target soil (Kanto loam) is 3 300kg / m 3 The soil improvement materials (1) to (3) were mixed so that the above values ​​were obtained, and mixed in a Hobart mixer. Mixing was carried out for a total of 3 minutes, with the soil adhering to the paddle and ball being scraped off at 1 minute 30 seconds intervals. After mixing was completed, the mixture was packed into a cylindrical formwork measuring 50 mm in diameter and 100 mm in height using a rammer in three layers, and then sealed and cured at 20°C for 7 and 28 days.

[0084] For each of the improved soils at the above ages, a leaching test was conducted in accordance with Environment Agency Notification No. 46 (August 23, 1991), and the amount of hexavalent chromium leached was measured. The improved soil samples used for the measurements were dried overnight by vacuum degassing using an aspirator. The amount of hexavalent chromium leached was determined by quantifying the hexavalent chromium concentration in the filtrate after immersion using the diphenylcarbazide spectrophotometric method in accordance with JIS K0102:2016, section 65.2.1. During the quantitative measurement procedure, the interval between the addition of 3 mL of sulfuric acid (1+9) and the addition of 1 mL of diphenylcarbazide solution (10 g / L) was kept within 20 seconds.

[0085] [Table 6]

[0086] Table 6 also shows the CaS content in the ground improvement material calculated based on the CaS content in the reducing material shown in Table 3 and the mix proportions of the ground improvement material in Table 6. As shown in Table 6, the amount of Cr(VI) eluted from the ground improvement soil at each age was below the detection limit. From these results, it was confirmed that the reducing materials of each Example not only suppress the amount of hydrogen sulfide gas generated during storage, but also sufficiently reduce the amount of Cr(VI) eluted from the ground improvement soil even when added to the ground improvement material after reducing the amount of hydrogen sulfide gas generated. [Explanation of symbols]

[0087] 10...container, 20...cloth, 30...sample.

Claims

1. A reducing material comprising a calcium sulfide-containing material containing a carbon-reduced product of gypsum and clinker dust, The content of calcium sulfide in the reducing material is 50% by mass or more, The reducing material has a free lime content of 2.8 parts by mass or more relative to 100 parts by mass of calcium sulfide.

2. 2. The reducing material according to claim 1, wherein the content of the clinker dust is 5% by mass or more and less than 40% by mass.

3. 50 mass% or more of the calcium sulfide contained in the reducing material is contained in the calcium sulfide-containing material, The reducing material according to claim 1 or 2, wherein at least a portion of the free lime is contained in the clinker dust.

4. The reducing material according to any one of claims 1 to 3, wherein 20 mass% or more of the free lime contained in the reducing material is contained in the clinker dust.

5. The reducing material according to any one of claims 1 to 4, wherein the content of the free lime in the clinker dust is 30 mass% or more.

6. A reducing material according to any one of claims 1 to 5, having a CaSO4 content of 31.5 mass% or less.

7. A cement composition comprising the reducing material according to any one of claims 1 to 6.

8. A ground improvement material comprising the reducing material according to any one of claims 1 to 6.

9. an evaluation step of evaluating the calcium sulfide-containing material containing the carbon-reduced product of gypsum and the components contained in the clinker dust; a blending step of blending the calcium sulfide-containing material with the clinker dust to obtain a reducing material having a calcium sulfide content of 50 mass% or more, In the blending step, the calcium sulfide-containing material and the clinker dust are blended together so that the amount of free lime is 2.8 parts by mass or more per 100 parts by mass of calcium sulfide, based on the evaluation results in the evaluation step.

10. The method for producing a reducing material according to claim 9, comprising a reduction step of heating a raw material containing a gypsum source and a carbon source to reduce at least a portion of the gypsum, thereby obtaining the calcium sulfide-containing material.

11. The method includes a step of blending a calcium sulfide-containing material containing a carbon-reduced product of gypsum with clinker dust, In the step, the calcium sulfide-containing material and the clinker dust are blended so that the calcium sulfide content is 50 mass% or more and the free lime is 2.8 mass parts or more per 100 mass parts of the calcium sulfide.

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