Soil modifier and method for manufacturing modified soil

Abstract

This invention provides a soil modifier that allows for the efficient use of woody biomass combustion ash in high-volume processing. [Solution] The soil modifier is a soil modifier that modifies soil, wherein the soil has a water content of 100% or more, contains woody biomass combustion ash, and the woody biomass combustion ash is fly ash.

Description

[Technical Field]

[0001] This invention relates to a soil modifier and a method for producing modified soil. [Background technology]

[0002] While combustion ash discharged from woody biomass power plants (hereinafter referred to as woody biomass combustion ash) is utilized as recycled material for civil engineering and other purposes, fly ash from woody biomass combustion ash is currently disposed of in landfills without being utilized as recycled material. One of the reasons why fly ash from woody biomass combustion ash is not utilized as recycled material is that harmful heavy metals exceeding the standards set by the Soil Contamination Countermeasures Act leach from the fly ash of woody biomass combustion ash.

[0003] Furthermore, unlike coal ash discharged from coal-fired power plants, the use of woody biomass combustion ash as a substitute for cement has not progressed because fly ash contains a large amount of salts such as sodium and potassium, which could have adverse effects if used in concrete.

[0004] As a technology for utilizing woody biomass combustion ash as a recycled material, a technique is known in which a soil conditioner containing palm kernel shell combustion ash is mixed with or spread on soil. In this soil conditioner, blast furnace slag fine powder is added to the palm kernel shell combustion ash to prevent harmful heavy metals contained in the palm kernel shell combustion ash from leaching into the soil, and paper sludge ash is added to the palm kernel shell combustion ash to absorb water contained in the soil (see, for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 6808883 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] When using woody biomass combustion ash as a recycled material, considering the woody biomass combustion ash as waste that would otherwise be disposed of, it is desirable that the usage amount be as large as possible.

[0007] An object of the present invention is to provide a soil modifier that can utilize woody biomass combustion ash at a high throughput.

Means for Solving the Problems

[0008] One aspect of the present invention is a soil modifier for modifying soil, wherein the water content ratio of the soil is 100% or more, the soil modifier contains woody biomass combustion ash, and the woody biomass combustion ash is fly ash.

[0009] Another aspect of the present invention is a method for producing modified soil by mixing soil and a soil modifier, wherein the water content ratio of the soil is 100% or more, the soil modifier contains fly ash of biomass combustion ash, and the blending amount of the soil modifier is determined based on a threshold value of a parameter correlated with the cone penetration amount by a fall cone.

Effects of the Invention

[0010] According to one aspect of the present invention, it is possible to provide a soil modifier that can utilize woody biomass combustion ash at a high throughput.

Brief Description of the Drawings

[0011] [Figure 1] It is a table showing an example of the composition of woody biomass combustion ash contained in the soil modifier. [Figure 2] It is a graph showing the correlation between the fall cone penetration amount and the cone index. [Figure 3] It is a graph showing the correlation between the fall cone penetration amount and the uniaxial compression strength. [Figure 4] It is a flowchart showing an example of the method for producing modified soil. [Figure 5]It is a table showing the results of the elution test of woody biomass combustion ash. [Figure 6] It is a table showing the conditions of the soil. [Figure 7] It is a table showing the composition of an example of each of the magnesium insolubilizer and cement added to the modified soil. [Figure 8] It is a table showing experimental examples of the modified soil.

Mode for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present invention will be described in detail.

[0013] <Soil modifier> The soil modifier of the present disclosure is a soil modifier for modifying soil. A soil modifier indicates a material for modifying the properties of soil.

[0014] The soil is high-moisture soil with a moisture content of 100% or more. High-moisture soil is, for example, dredged soil and sand in harbors, rivers, etc. Note that dredged soil and sand are organic sludge in a high-moisture state that is difficult to be used as construction materials by modification or solidification, and most of them are landfilled.

[0015] From the viewpoint of increasing the amount of woody biomass combustion ash used in the soil modifier, the lower limit of the moisture content of the soil is preferably 150% or more, more preferably 200% or more. Also, the upper limit of the moisture content of the soil is not limited, but from the viewpoint of ensuring the strength of the soil modified by the soil modifier, it is preferably 350% or less, more preferably 300% or less, and even more preferably 250% or less.

[0016] The soil modifier of the present disclosure contains woody biomass combustion ash. Woody biomass combustion ash is combustion ash discharged as a by-product from a woody biomass power plant. Biomass combustion ash is, for example, combustion ash recovered when burning a single fuel with PKS (Palm Kernel Shell) or a mixed fuel of PKS and wood chips as fuel types in a circulating fluidized bed boiler.

[0017] Furthermore, woody biomass combustion ash is fly ash. Fly ash, also known as fly ash, is the ash collected by an electrostatic precipitator from the combustion ash. Fly ash from woody biomass combustion ash has high water absorption properties and, through its physical water absorption action, has the function of reducing the water content of high-moisture-content soil.

[0018] Furthermore, the bottom ash (main ash) of woody biomass combustion ash is mainly composed of silica sand (SiO2) and exhibits almost no water absorption, making it unsuitable as a material for soil modification.

[0019] The water absorption capacity of fly ash from woody biomass combustion can be predicted from the water absorption ratio. The water absorption ratio of fly ash from woody biomass combustion is not particularly limited, but is preferably 80% or more, more preferably 90% or more, and even more preferably 100% or more. The water absorption ratio can be measured by referring to the water absorption ratio test method described in Kato, Yusuke, Imai, Goro, Omukai, Naoki, Mochizuki, Midoshi, Saito, Etsuro, and Yoshino, Hiroshi: Study on soil improvement by PS ash addition, 40th Proceedings of the Japanese Geotechnical Society, pp. 677-678, 2005.

[0020] Figure 1 is a table showing the composition of A ash, B ash, and C ash in terms of oxides, as an example of fly ash from woody biomass combustion. From Figure 1, it can be seen that fly ash from woody biomass combustion contains a large amount of calcium (Ca). It is thought that by adding a large amount of such fly ash from woody biomass combustion to high-moisture-content soil, the water, silica (Si), sulfur (S), etc. in the soil react with the Ca in the fly ash of the woody biomass combustion ash to form ettringite (3CaO·Al2O3·3CaOSO4·32H2O), which then weakly solidifies after curing for several days, thereby imparting strength to the soil.

[0021] The amount of soil modifier used for soil with a water content of 100% or more is not particularly limited, but for example, 1 m of soil 3 In contrast, the amount of fly ash used from woody biomass combustion ash is 500 kg / m³. 3 More than 2500kg / m 3It can be as follows, preferably 750 kg / m 3 or more and 2000 kg / m 3 or less, more preferably 1000 kg / m 3 or more and 1500 kg / m 3 or less. When the usage amount of the soil modifier is 500 kg / m 3 or more and 2500 kg / m 3 or less in terms of the usage amount conversion of the fly ash of the woody biomass combustion ash, the soil with a water content ratio of 100% or more can be solidified.

[0022] As described above, the soil modifier of the present disclosure contains fly ash of woody biomass combustion ash with soil having a water content ratio of 100% or more as the object of modification, so that the woody biomass combustion ash can be utilized at a high usage amount. Further, the soil modifier of the present disclosure is used for soil having a water content ratio of 100% or more, so that the obtained modified soil can fix harmful heavy metals such as hexavalent chromium while exhibiting a predetermined strength.

[0023] The soil modifier of the present disclosure preferably further contains a magnesium-based insolubilizing material. The magnesium-based insolubilizing material contains magnesium oxide (MgO) as the main component and does not contain harmful heavy metals, and is a material that controls the elution of harmful heavy metals and the like from an object such as soil by mixing it with the object.

[0024] The content of MgO in the magnesium-based insolubilizing material is not particularly limited. For example, it is 25% by mass or more and 50% by mass or less, preferably 30% by mass or more and 45% by mass or less, and more preferably 35% by mass or more and 40% by mass or less. When the content of MgO contained in the magnesium-based insolubilizing material is 25% by mass or more and 50% by mass or less, the elution of harmful heavy metals such as selenium that cannot be insolubilized only by solidifying the soil with the woody biomass combustion ash can be suppressed.

[0025] The magnesium-based immobilizing agent may contain components other than MgO. These other components may include, for example, aluminum oxide (Al2O3), silicon dioxide (SiO2), sulfur trioxide (SO3), calcium oxide (CaO), ferric oxide (Fe2O3), and a reducing agent. The reducing agent is an optional component.

[0026] The Al2O3 content in the magnesium-based insolubilizing agent is not particularly limited, but is, for example, 0.1% by mass or more and 2% by mass or less, preferably 0.3% by mass or more and 1.5% by mass or less, and more preferably 0.5% by mass or more and 1% by mass or less.

[0027] The SiO2 content in the magnesium-based immobilizing agent is not particularly limited, but is, for example, 0.5% by mass or more and 3% by mass or less, preferably 1% by mass or more and 2.5% by mass or less, and more preferably 1.5% by mass or more and 2% by mass or less.

[0028] The SO3 content in the magnesium-based immobilizing agent is not particularly limited, but is, for example, 3% by mass or more and 20% by mass or less, preferably 5% by mass or more and 15% by mass or less, and more preferably 8% by mass or more and 12% by mass or less.

[0029] The CaO content in the magnesium-based immobilizing agent is not particularly limited, but is, for example, 3% by mass or more and 20% by mass or less, preferably 5% by mass or more and 15% by mass or less, and more preferably 8% by mass or more and 12% by mass or less.

[0030] The Fe2O3 content in the magnesium-based insolubilizing agent is not particularly limited, but is, for example, 10% by mass or more and 40% by mass or less, preferably 15% by mass or more and 45% by mass or less, and more preferably 20% by mass or more and 30% by mass or less.

[0031] The content of the reducing agent in the magnesium-based insolubilizing agent is not particularly limited, but for example, it is 10% by mass or more and 40% by mass or less, preferably 15% by mass or more and 45% by mass or less, and more preferably 20% by mass or more and 25% by mass or less.

[0032] The content of magnesium-based immobilizer in the soil modifier is not particularly limited, but for example, it is 1% by mass or more and 20% by mass or less relative to the woody biomass combustion ash in the soil modifier, preferably 2% by mass or more and 15% by mass or less, and more preferably 3% by mass or more and 10% by mass or less.

[0033] The soil modifier disclosed herein contains such a magnesium-based immobilizer, which makes it possible to suppress the leaching of harmful heavy metals such as selenium that cannot be immobilized by solidification of soil with woody biomass combustion ash alone.

[0034] The soil modifier of this disclosure preferably further contains cement. The cement refers to a powder that hardens through a chemical reaction with water, mainly composed of limestone, clay, silica, iron oxide raw materials, etc.

[0035] The type of cement is not particularly limited, but various types of Portland cement can be used, such as ordinary Portland cement, rapid-hardening Portland cement, ultra-rapid-hardening Portland cement, moderate-heat Portland cement, low-heat Portland cement, sulfate-resistant Portland cement, blended cements such as blast furnace cement, silica cement, and fly ash cement, and special cements such as alumina cement. These can be used individually or in combination. Among these, Portland cement and blast furnace cement are preferred, and Portland cement is more preferred.

[0036] The cement content is not particularly limited, but for example, it is 3% by mass or more and 20% by mass or less relative to the woody biomass combustion ash in the soil modifier, preferably 4% by mass or more and 15% by mass or less, and more preferably 5% by mass or more and 10% by mass or less.

[0037] The soil modifier of this disclosure contains such cement, which makes it possible to suppress the leaching of harmful heavy metals such as boron that cannot be immobilized by solidification of the soil with woody biomass combustion ash alone.

[0038] <Method for manufacturing modified soil> The method for producing modified soil described herein is a method for producing modified soil by mixing mud and a mud modifier. In this specification, modified soil is obtained by mixing mud with a mud modifier to modify its properties.

[0039] The mud used in the soil modification method of this disclosure is a high-moisture-content mud with a water content of 100% or more. This mud is the same as the mud targeted for modification by the mud modifier of this embodiment.

[0040] The soil modifier used in the soil modification method of this disclosure is fly ash from biomass combustion ash. This soil modifier is the same as the fly ash from biomass combustion ash contained in the soil modifier of this embodiment.

[0041] In the method for producing modified soil according to this disclosure, the amount of soil modifier mixed in is not particularly limited, but for example, 1 m of soil 3 In contrast, the amount of fly ash used from woody biomass combustion ash is 500 kg / m³. 3 More than 2500kg / m 3 It can be as follows, preferably 750 kg / m 3 More than 2000kg / m 3 The following, and more preferably 1000 kg / m 3 More than 1500kg / m 3 The following applies: The amount of soil modifier used is equivalent to 500 kg / m³ of fly ash from woody biomass combustion ash. 3 More than 2500kg / m 3 The following conditions allow for the solidification of mud with a water content of 100% or more.

[0042] In the soil modification method described herein, the amount of soil modifier to be mixed is determined based on a threshold parameter correlated with the cone penetration amount in a fall cone test. The fall cone test refers to a soil liquidity limit test using a fall cone in accordance with JGS 0142-2020. The cone penetration amount refers to the penetration amount due to the free fall of the cone.

[0043] Parameters correlated with cone penetration include, for example, unconfined compressive strength and cone index. The cone index represents the strength of the modified soil immediately after modification, as determined by the cone index test of compacted soil in accordance with JIS A 1228. Figure 2 is a graph showing the correlation between fall cone penetration and cone index. Figure 3 is a graph showing the correlation between fall cone penetration and unconfined compressive strength.

[0044] The threshold for the parameter correlated with the cone penetration is a predetermined standard value for the strength of the parameter correlated with the cone penetration. For example, in the case shown in Figure 2, it is the standard value for unconfined compressive strength (strength of modified soil), and in the case shown in Figure 3, it is the standard value for the cone index (strength of modified soil immediately after modification).

[0045] "Determining the amount of soil modifier based on parameter thresholds" means increasing the amount of soil modifier until the parameter thresholds are met. For example, in the case shown in Figure 2, the amount of soil modifier is adjusted to meet the standard value for unconfined compressive strength. In the case shown in Figure 3, the amount of soil modifier is adjusted to meet the standard value for the cone index.

[0046] Figure 4 is a flowchart showing an example of a method for producing modified soil. In the method for producing modified soil described herein, first, a test is performed to determine the leaching of harmful heavy metals from the fly ash of woody biomass combustion ash contained in the soil modifier (Step S1).

[0047] The heavy metal leaching test targets nine heavy metals (cadmium, hexavalent chromium, total cyanide, selenium, lead, total mercury, arsenic, fluorine, and boron) in accordance with the soil environmental standards based on the Soil Contamination Countermeasures Act (Ministry of the Environment Notifications No. 18 and No. 46) (see Figure 5). The heavy metal leaching test is performed in accordance with the test solution preparation methods of JIS K0125, JIS K012, etc.

[0048] Next, a soil modifier containing fly ash from woody biomass combustion ash is mixed with the soil to be modified (target soil) (Step S2). The soil modified by mixing the target soil with the soil modifier becomes the modified soil. The amount of soil modifier to be mixed is arbitrary; for example, 1 m of soil 3 In contrast, the amount of soil modifier converted to the fly ash content of wood-based biomass combustion ash is 500 kg / m³. 3 Let's assume that.

[0049] For the modified soil obtained, the cone penetration depth is measured by a fall cone test, and the strength of the measured cone penetration depth is confirmed in a predetermined parameter that correlates with the cone penetration depth (Step S3).

[0050] Step S4 confirms whether the confirmed strength satisfies the predetermined strength standard value (target strength). If it does not meet the target strength, the amount of soil modifier added is increased (Step S2). The amount of soil modifier added is arbitrary; for example, 1 m of soil 3 In contrast, the amount of soil modifier converted to the fly ash content of wood-based biomass combustion ash is 250 kg / m³. 3 Let's assume that.

[0051] The following steps S2 to S4 are repeated, increasing the amount of soil modifier in step S4 until the target strength is satisfied (step S2). If the target strength is satisfied in step S4, proceed to step S5 described below.

[0052] In the soil modification method of this embodiment, as described above, modified soil is produced by mixing mud with a water content of 100% or more with a mud modifier containing fly ash from biomass combustion ash. This makes it possible to provide modified soil in which the leaching of harmful heavy metals such as hexavalent chromium is suppressed while utilizing a high amount of woody biomass combustion ash. Furthermore, by determining the amount of mud modifier to be mixed based on a threshold parameter correlated with the cone penetration amount in the fall cone test, the strength of the modified soil can be efficiently evaluated with a small sample.

[0053] The method for producing modified soil according to this disclosure further incorporates a magnesium-based immobilizer. The magnesium-based immobilizer used in the method for producing modified soil according to this disclosure is the same as the magnesium-based immobilizer contained in the soil modifier of this embodiment.

[0054] In the method for producing modified soil according to this disclosure, the amount of magnesium-based immobilizing agent added is not particularly limited, but for example, it is 1% by mass or more and 20% by mass or less relative to the woody biomass combustion ash in the soil modifier, preferably 2% by mass or more and 15% by mass or less, and more preferably 3% by mass or more and 10% by mass or less.

[0055] In the soil modification method described herein, the amount of magnesium-based immobilizer to be added is determined based on the results of a leaching test of soil that is 7 days old or older. The leaching test is the same as the leaching test for harmful heavy metals in the soil modifier described above. The soil that is 7 days old or older and used in the leaching test is, for example, soil that is 28 days old.

[0056] "Determining the amount of magnesium-based immobilizer to add based on the elution test results" means, for example, increasing the amount of magnesium-based immobilizer to add until the results of the leaching test of harmful heavy metals in modified soil that is 7 days old or older satisfy the leaching standards based on the soil environmental standards under the Soil Contamination Countermeasures Act (Ministry of the Environment Notification No. 18 and No. 46), or until the amount of each harmful heavy metal leached decreases compared to the results of the harmful heavy metal leaching test in the aforementioned mud modifier.

[0057] Specifically, returning to the flowchart in Figure 4, which shows an example of a soil modification method, if the target strength is satisfied in step S4, a magnesium-based immobilizer is added as an auxiliary agent (step S5). The amount of magnesium-based immobilizer added is arbitrary; for example, it may be 3% by mass relative to the woody biomass combustion ash in the soil modifier.

[0058] The modified soil containing magnesium-based immobilizer will be cured. Curing methods include sealed curing and open curing. For sealed curing, for example, the modified soil is stored in a double-layered polyethylene bag with the air removed and placed in a constant temperature bath at 20°C. For open curing, for example, the modified soil is spread to a thickness of about 10 mm in a plastic container and placed in a constant temperature bath at 20°C without a lid (Step S7).

[0059] Next, a heavy metal leaching test is performed on the modified soil after 28 days of age, and the results of the heavy metal leaching test are compared with the leaching standards based on the soil environmental standards under the Soil Contamination Countermeasures Act (Environmental Agency Notification No. 18 and No. 46) (Step S7).

[0060] The results of the leaching test for harmful heavy metals in the obtained modified soil are checked to see if they meet the leaching standards (Step S8). If the leaching standards are not met, the amount of magnesium-based immobilizer added is increased (Step S5). The amount of magnesium-based immobilizer to be increased is arbitrary; for example, it may be 2% by mass relative to the woody biomass combustion ash in the soil modifier.

[0061] The following steps S5 to S8 are repeated, increasing the amount of magnesium-based immobilizing agent added in step S5 until the elution criteria are met in step S8. If the elution criteria are met in step S8, the process is terminated.

[0062] In the soil modification method of this embodiment, as described above, by further mixing in a magnesium-based immobilizer, it is possible to provide modified soil in which the leaching of harmful heavy metals such as selenium, which cannot be immobilized by solidification of mud with woody biomass combustion ash alone, is suppressed, while utilizing a high amount of woody biomass combustion ash. Furthermore, by determining the amount of magnesium-based immobilizer to be added based on the leaching test results of modified soil that is 7 days old or older, it is possible to efficiently determine the amount of magnesium-based immobilizer to be added that suppresses the leaching of harmful heavy metals such as selenium.

[0063] The method for producing modified soil according to this disclosure further incorporates cement. The cement used in the method for producing modified soil according to this disclosure is the same as the cement contained in the soil modifier of this embodiment.

[0064] In the method for producing modified soil according to this disclosure, the amount of cement added is not particularly limited, but for example, it is 3% by mass or more and 20% by mass or less relative to the woody biomass combustion ash in the soil modifier, preferably 4% by mass or more and 15% by mass or less, and more preferably 5% by mass or more and 10% by mass or less.

[0065] In the method for producing modified soil described herein, the amount of cement to be added is determined based on the results of a leaching test of modified soil containing a magnesium-based immobilizer that is 7 days old or older. The leaching test is the same as the leaching test for harmful heavy metals in the mud modifier described above. The modified soil that is 7 days old or older and used in the leaching test is, for example, modified soil that is 28 days old.

[0066] "Determining the amount of cement to add based on the leaching test results" means, for example, increasing the amount of cement added until the results of the leaching test of harmful heavy metals in modified soil mixed with magnesium-based immobilizers that are 7 days old or older satisfy the leaching standards based on the soil environmental standards under the Soil Contamination Countermeasures Act (Ministry of the Environment Notification No. 18 and No. 46), or until the amount of each harmful heavy metal leached decreases compared to the results of the leaching test of harmful heavy metals in the aforementioned mud modifier.

[0067] Specifically, returning to the flowchart in Figure 4, which shows an example of a soil modification method, in step S4, under conditions that satisfy the target strength, cement is further added as an additive (step S5). The amount of cement added is arbitrary; for example, it may be 5% by mass relative to the woody biomass combustion ash in the soil modifier.

[0068] The modified soil mixed with cement is cured. Curing methods include sealed curing and open curing. For example, in sealed curing, the modified soil is stored in a double-layered polyethylene bag with the air removed and placed in a constant temperature bath at 20°C. For open curing, for example, the modified soil is spread to a thickness of about 10 mm in a plastic container and placed in a constant temperature bath at 20°C without a lid (Step S7).

[0069] The results of the leaching test for harmful heavy metals in the obtained modified soil are checked to see if the leaching standard is met (Step S8). If the leaching standard is not met, the amount of cement added is increased (Step S5). The amount of cement added is arbitrary, for example, 5% by mass relative to the woody biomass combustion ash in the soil modifier.

[0070] The following steps S5 to S8 are repeated, increasing the amount of cement added in step S5 until the elution criteria are met in step S8. If the elution criteria are met in step S8, the process is terminated.

[0071] In the soil modification method of this embodiment, as described above, by further mixing in cement, it is possible to provide modified soil in which the leaching of harmful heavy metals such as boron, which cannot be immobilized by solidification of the mud with woody biomass combustion ash alone, is suppressed, while utilizing a high amount of woody biomass combustion ash. Furthermore, by determining the amount of cement to be added based on the leaching test results of modified soil that is 7 days old or older, it is possible to efficiently determine the amount of cement to be added that suppresses the leaching of harmful heavy metals such as boron. [Examples]

[0072] Embodiments of the present invention will be further described below using experimental examples. Various tests and evaluations will be conducted according to the methods described below. In the following, unitless numerical values, "%", or "parts" refer to mass unless otherwise specified.

[0073] <Mud soil modification material> As a soil modifier, fly ash from woody biomass combustion ash (hereinafter referred to as biomass combustion ash), corresponding to ash C in Figure 1, was used. The biomass combustion ash was fly ash recovered from a circulating fluidized bed boiler using PKS and wood pellets as fuel, with a soil particle density of 2.725 Mg / m³. 3 , minimum density 0.595Mg / m 3 , maximum density 0.873Mg / m 3 The 50% particle size was 0.0118 mm, pH 13.0, and water absorption ratio 124%. The composition of the biomass combustion ash, in terms of oxides, was SiO2: 32.8%, CaO: 29.2%, K2O: 7.1%, SO3: 4.7%, P2O55: 2.1%, MgO: 3.4%, Fe2O3: 4.7%, Al2O3: 5.1%, and CO2: 6.3%.

[0074] <Leaching test of biomass combustion ash> A leaching test was conducted on biomass combustion ash (Ash C in Figure 1) used in soil modifiers. The leaching test targeted nine hazardous heavy metals (cadmium, hexavalent chromium, total cyanide, selenium, lead, total mercury, arsenic, fluorine, and boron) in accordance with the soil environmental standards based on the Soil Contamination Countermeasures Act (Ministry of the Environment Notification No. 18 and No. 46). The leaching test was performed in accordance with the test solution preparation methods of JIS K0125, JIS K012, etc. In Ash C (Figure 5), hexavalent chromium, selenium, lead, and fluorine exceeded the standard values ​​of the above soil environmental standards (hereinafter referred to as the leaching standard values).

[0075] <Fall Cone Test> For biomass combustion ash, a soil liquid limit test (fall cone test) was conducted using a fall cone in accordance with JGS 0142-2020, and the fall cone penetration depth was measured. Uniaxial compressive strength and cone index, which correlate with the fall cone penetration depth, were prepared in advance, and the amount of biomass combustion ash to be blended was determined so that the uniaxial compressive strength and cone index corresponding to the measured fall cone penetration depth met predetermined standard values.

[0076] <Target mud> The target soil for modification with the soil modifier was soil under the conditions shown in Figure 6. The target soil was soil collected from Bay D, with a soil particle density ρs: 2.509 Mg / m³ 3 Natural water content Wn: 194%, liquid limit W L :132%, plastic limit W P The material composition was 41.5%, plasticity index Ip: 90.4%, particle size composition: gravel 0.0%, sand 6.9%, silt 31.9%, clay 60.4%, ignition loss IL: 19%, and pH: 7.8.

[0077] <Magnesium-based insolubilizing agent> As a magnesium-based immobilizer, the MgO-based immobilizer shown in Figure 7 was used. The composition of the MgO-based immobilizer, in terms of oxides, was MgO: 38.1%, Al2O3: 0.6%, SiO2: 1.6%, SO3: 10.6%, CaO: 9.6%, Fe2O3: 22.3%, and others: 17.2%.

[0078] <Cement> As cement, blast furnace cement type B (hereinafter referred to as BB) and ordinary Portland cement (hereinafter referred to as N) were used. The composition of BB, on an oxide basis, was Na2O: 0.2%, MgO: 2.1%, Al2O3: 6.6%, SiO2: 20.1%, SO3: 2.9%, K2O: 0.5%, CaO: 59.9%, TiO2: 0.4%, Fe2O3: 2.5%, and others: 4.9%. The composition of N, on an oxide basis, was Na2O: 0.3%, MgO: 0.8%, Al2O3: 3.8%, SiO2: 15.4%, SO3: 3.5%, K2O: 0.3%, CaO: 67.6%, TiO2: 0.4%, Fe2O3: 2.8%, and others: 5.3%.

[0079] <Composition analysis> The compositions of biomass combustion ash, magnesium-based immobilizer, and cement were measured by energy-dispersive X-ray spectroscopy (SEM-EDX) compositional analysis.

[0080] <Modified soil> Following the flowchart shown in Figure 4, modified soil, prepared by mixing the target soil with a soil modifier, was cured. Optionally, magnesium-based immobilizers and cement were added to the modified soil. Two curing methods were employed: sealed curing and open curing. For sealed curing, the modified soil was stored in a double-layered, deaerated polyethylene bag and cured in a 20°C constant temperature chamber for 28 days. For open curing, the modified soil was spread to a thickness of approximately 10 mm in a plastic container and cured in a 20°C constant temperature chamber for 28 days without a lid.

[0081] <Test for leaching harmful heavy metals> A heavy metal leaching test was conducted on the modified soil after curing. The test targeted nine heavy metals (cadmium, hexavalent chromium, total cyanide, selenium, lead, total mercury, arsenic, fluorine, and boron) in accordance with the soil environmental standards based on the Soil Contamination Countermeasures Act (Ministry of the Environment Notification No. 18 and No. 46). The heavy metal leaching test was performed in accordance with the test solution preparation methods of JIS K0125, JIS K012, etc.

[0082] [Experimental Example 1] 1 m of mud adjusted to a water content of 245% 3 For this, 1500 kg / m³ of soil modifier was applied. 3 Modified soil mixed in the specified ratio was sealed and cured to obtain modified soil at 28 days of age. In the modified soil of Experimental Example 1, the leaching test results showed that cadmium, hexavalent chromium, total cyanide, total mercury, lead, arsenic, fluorine, and boron were below the leaching standard value, while selenium exceeded the leaching standard value. The results of Experimental Example 1 are shown in Figure 8.

[0083] [Experimental Example 2] Except for the addition of 5% magnesium-based immobilizer, modified soil was obtained in the same manner as in Experimental Example 1. In the modified soil of Experimental Example 2, elution tests showed that cadmium, hexavalent chromium, total cyanide, total mercury, selenium, lead, arsenic, fluorine, and boron were all below the elution standard values. The results of Experimental Example 2 are shown in Figure 8.

[0084] [Experimental Example 3] Except for the addition of 5% blast furnace cement (BB), modified soil was obtained in the same manner as in Experimental Example 2. In the modified soil of Experimental Example 3, the leaching test results showed that cadmium, hexavalent chromium, total cyanide, total mercury, selenium, lead, arsenic, fluorine, and boron were all below the leaching standard values, and in particular, the amount of selenium leached decreased. The results of Experimental Example 3 are shown in Figure 8.

[0085] [Experimental Example 4] 1 m of mud adjusted to a water content of 240% 3 For this, 1000 kg / m³ of soil modifier is applied. 3 Modified soil mixed in the specified ratio was sealed and cured to obtain modified soil at 28 days of age. In the modified soil of Experimental Example 4, the leaching test results showed that cadmium, hexavalent chromium, total cyanide, total mercury, lead, arsenic, and fluorine were below the leaching standard values, while selenium and boron exceeded the leaching standard values. The results of Experimental Example 4 are shown in Figure 8.

[0086] [Experimental Example 5] Except for curing with an open container instead of a sealed container, modified soil was obtained in the same manner as in Experimental Example 4. In the modified soil of Experimental Example 5, the leaching test results showed that cadmium, hexavalent chromium, total cyanide, total mercury, lead, arsenic, and fluorine were below the leaching standard values, while selenium and boron exceeded the leaching standard values. The results of Experimental Example 5 are shown in Figure 8.

[0087] [Experimental Example 6] Furthermore, modified soil was obtained in the same manner as in Experimental Example 4, except that 5% magnesium-based immobilizer was added. In the modified soil of Experimental Example 2, the elution test results showed that cadmium, hexavalent chromium, total cyanide, total mercury, selenium, lead, arsenic, and fluorine were below the elution standard value, while boron exceeded the elution standard value. The results of Experimental Example 6 are shown in Figure 8.

[0088] [Experimental Example 7] Except for curing with an open container instead of a sealed container, modified soil was obtained in the same manner as in Experimental Example 6. In the modified soil of Experimental Example 7, the leaching test results showed that cadmium, hexavalent chromium, total cyanide, total mercury, selenium, lead, arsenic, and fluorine were below the leaching standard value, while boron exceeded the leaching standard value. The results of Experimental Example 7 are shown in Figure 8.

[0089] [Experimental Example 8] Furthermore, modified soil was obtained in the same manner as in Experimental Example 6, except that 5% ordinary Portland cement (N) was added. In the modified soil of Experimental Example 8, the leaching tests showed that cadmium, hexavalent chromium, total cyanide, total mercury, selenium, lead, arsenic, fluorine, and boron were all below the leaching standard values. The results of Experimental Example 8 are shown in Figure 8.

[0090] [Experimental Example 9] Except for curing with an open container instead of a sealed container, modified soil was obtained in the same manner as in Experimental Example 8. In the modified soil of Experimental Example 9, the leaching test results showed that cadmium, hexavalent chromium, total cyanide, total mercury, selenium, lead, arsenic, fluorine, and boron were below the leaching standard value. Although boron slightly exceeded the leaching standard value, the amount of boron leached was reduced compared to Experimental Example 7, which did not contain ordinary Portland cement (N). The results of Experimental Example 9 are shown in Figure 8.

[0091] [Experimental Example 10] Modified soil was obtained in the same manner as in Experimental Example 8, except that the amount of ordinary Portland cement (N) added was 10%. In the modified soil of Experimental Example 10, the leaching test results showed that cadmium, hexavalent chromium, total cyanide, total mercury, selenium, lead, arsenic, fluorine, and boron were all below the leaching standard values. The results of Experimental Example 10 are shown in Figure 8.

[0092] [Experimental Example 11] Except for curing with an open container instead of a sealed container, modified soil was obtained in the same manner as in Experimental Example 10. In the modified soil of Experimental Example 11, the leaching tests showed that cadmium, hexavalent chromium, total cyanide, total mercury, selenium, lead, arsenic, fluorine, and boron were all below the leaching standard values. The results of Experimental Example 11 are shown in Figure 8.

[0093] Experimental examples 1-11 showed that in modified soil obtained by mixing mud with a water content of 100% or more with a mud modifier containing fly ash from biomass combustion ash, the leaching of harmful heavy metals such as hexavalent chromium was suppressed while utilizing a high amount of woody biomass combustion ash. Furthermore, it was found that by determining the amount of mud modifier to mix based on a threshold parameter correlated with the cone penetration amount in the fall cone test, the strength of the modified soil could be efficiently evaluated with a small sample size.

[0094] Furthermore, from experimental examples 2, 3, and 6-11, it was found that in modified soil to which magnesium-based immobilizers were added, the leaching of harmful heavy metals such as selenium, which cannot be immobilized by solidification of the mud with woody biomass combustion ash alone, was suppressed, even while utilizing a high amount of woody biomass combustion ash. In addition, it was found that by determining the amount of magnesium-based immobilizer to add based on the leaching test results of modified soil that was 7 days old or older, the amount of magnesium-based immobilizer that suppresses the leaching of harmful heavy metals such as selenium can be determined, thus enabling the efficient production of modified soil.

[0095] Furthermore, from Experimental Examples 3, 8-11, it was found that in modified soil with added cement, the leaching of harmful heavy metals such as boron, which cannot be immobilized by solidification of the mud with woody biomass combustion ash alone, was suppressed, even while utilizing a high amount of woody biomass combustion ash. In addition, it was found that by determining the amount of cement to add based on the leaching test results of modified soil that is 7 days old or older, the amount of cement that suppresses the leaching of harmful heavy metals such as boron can be determined, thus enabling the efficient production of modified soil.

[0096] The embodiments disclosed above include, for example, the following aspects:

[0097] <1> A soil modifier that modifies mud, The water content of the aforementioned mud is 100% or more. It contains woody biomass combustion ash, The aforementioned woody biomass combustion ash is fly ash. Mud soil modification material.

[0098] <2> Furthermore, it contains a magnesium-based insolubilizing agent. The aforementioned <1> The soil modifier described in [reference].

[0099] <3> Furthermore, it contains cement, The aforementioned <1> or <2> The soil modifier according to claim 2.

[0100] <4> The cement in question is Portland cement. The aforementioned <3> The soil modifier described in [reference].

[0101] <5> A method for producing modified soil by mixing mud and mud modifier, The water content of the aforementioned mud is 100% or more. The soil modifier contains fly ash from biomass combustion ash. The amount of the soil modifier to be mixed is determined based on a threshold parameter that correlates with the amount of cone penetration obtained by the fall cone test. Method for producing modified soil.

[0102] <6> Further mixing in a magnesium-based insolubilizing agent, The amount of magnesium-based immobilizer to be added is determined based on the elution test results of the modified soil that is 7 days old or older. The aforementioned <5> A method for producing modified soil as described above.

[0103] <7> Further mix the cement, The amount of cement to be added is determined based on the elution test results of the modified soil, which is 7 days old or older. The aforementioned <5> or <6> A method for producing modified soil as described above.

[0104] Although embodiments of the present invention have been described above, the present invention is not limited to any particular embodiment, and various modifications and changes are possible within the scope of the invention as described in the claims.

Claims

1. A soil modifier that modifies mud, The water content of the aforementioned mud is 100% or more. It contains woody biomass combustion ash, The aforementioned woody biomass combustion ash is fly ash. Mud soil modification material.

2. Furthermore, it contains a magnesium-based insolubilizing agent. The soil modifier according to claim 1.

3. Furthermore, it contains cement, The soil modifier according to claim 2.

4. The cement in question is Portland cement. The soil modifier according to claim 3.

5. A method for producing modified soil by mixing mud and mud modifier, The water content of the aforementioned mud is 100% or more. The soil modifier contains fly ash from biomass combustion ash. The amount of the soil modifier to be mixed is determined based on a threshold parameter that correlates with the amount of cone penetration obtained by the fall cone test. Method for producing modified soil.

6. Further mixing in a magnesium-based insolubilizing agent, The amount of magnesium-based immobilizer to be added is determined based on the elution test results of the modified soil that is 7 days old or older. The method for producing modified soil according to claim 5.

7. Further mix the cement, The amount of cement to be added is determined based on the elution test results of the modified soil, which is 7 days old or older. The method for producing modified soil according to claim 6.

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