JP7746485B2 - Neutral solidification material and soil processing method - Google Patents

Description

The present invention relates to a neutral solidification material and a soil treatment method.

Soil solidification materials are used to improve soil strength and facilitate the removal and reuse of soft soil and construction waste soil. Soil solidification materials are classified into cement-based, lime-based, and gypsum-based types depending on the components they contain. Gypsum-based soil solidification materials are typically used, as they can maintain the pH of the soil near neutral after soil improvement, taking into consideration the preservation of the surrounding environment, such as water quality and vegetation. However, gypsum-based soil solidification materials sometimes do not develop sufficient strength compared to cement-based materials, and there is room for improvement in this regard.

Patent Document 1 discloses a soil solidification material consisting of magnesium oxide, aluminum sulfate and/or iron sulfate, with the remainder being gypsum, with the aim of improving the strength of improved soil when using a soil solidification material. Patent Document 2 also discloses a soil improvement material that contains metal salts such as metal sulfates, magnesium-containing substances, and polymer-based thickening materials, as well as auxiliary materials such as gypsum hemihydrate.

Japanese Patent Application Laid-Open No. 2000-109829 Japanese Patent Application Laid-Open No. 2008-100313

However, while the soil improvement materials described in Patent Documents 1 and 2 were able to maintain the pH of the improved soil near neutral, they did not adequately develop strength, and sometimes were unable to improve the soil to the desired strength.

The improved soil obtained by solidifying construction waste soil may be reused at the site where it was generated, or at a different site. In the latter case, the improved soil is typically cured for a certain period of time at the site where it was generated, and then transported to the site where it will be reused. During this process, the strength of the improved soil may decrease due to processes such as excavation and loosening. As a result, the soil may not be able to achieve sufficient strength at the site where it is to be reused. The soil improvement materials described in Patent Documents 1 and 2 do not address this issue at all.

The present invention aims to provide a neutral solidification material that can produce improved soil that maintains a pH close to neutral and is less likely to lose strength due to excavation or loosening.

After extensive research to solve the above problems, the inventors discovered that by using a polymer flocculant with a specific composition, it is possible to maintain the pH of improved soil while also increasing its strength, leading to the creation of the present invention.

That is, the present invention relates to a composition comprising magnesium oxide and
at least one sulfate selected from aluminum sulfate, ferrous sulfate, and gypsum;
a polymer flocculant having an acrylamide unit and a sodium acrylate unit,
The present invention provides a neutral solidification material in which the composition ratio of the acrylamide units constituting the polymer flocculant is 55 to 90 mol %.

A preferred embodiment of the present invention provides a neutral solidification material containing 1 to 8 mass % of the polymer flocculant.

A preferred embodiment of the present invention provides a neutral solidification material containing 10 to 50 mass % of the magnesium oxide.

Furthermore, the present invention provides a method for treating soil and a neutral solidification material, the method comprising the step of curing the mixture until the cone index measured in accordance with JIS A1228 is 800 kN/ ^m2 or more;
The neutral solidification material contains magnesium oxide, at least one sulfate selected from aluminum sulfate, ferrous sulfate, and gypsum, and a polymer flocculant containing polyacrylamide and sodium acrylate as main components, and the composition ratio of the acrylamide units constituting the polymer flocculant is 55 to 90 mol %.

A further preferred embodiment of the present invention provides a soil treatment method in which the soil to be treated contains an aerating agent.

This invention allows soil to be improved to have high strength while maintaining a near-neutral pH, allowing the improved soil to be reused for a wide range of purposes, leading to more effective use of resources.

FIG. 1 is a cross-sectional schematic diagram showing an example of a method for treating soil using a neutral solidification material, together with an example of a shield machine used in the air bubble shield method.

Preferred embodiments of the present invention are described below. However, the present invention is not limited to the following embodiments. In the following description, when it is stated that "X to Y[Z]" (X and Y are arbitrary numbers, and [Z] is a unit), it means "greater than or equal to X[Z] and less than or equal to Y[Z]" unless otherwise specified.

The neutral solidification material of the present invention is a material that can be mixed with soil to be improved to maintain a neutral pH and obtain soil with high strength. Examples of soil to be improved include wet soil such as soft soil containing water, or mud, such as construction waste soil and construction sludge that are generated secondarily during various construction works, such as road construction, tunnel construction using the shield tunneling method, and building construction. In other words, the neutral solidification material of the present invention is suitable for solidifying at least one of wet soil, construction waste soil, and construction sludge.
In the following description, "improved soil" refers to soil that has been improved by adding the neutral solidification material of the present invention to the soil to be improved.

In this invention, "neutral" refers to "the pH of the improved soil being 5.8 or higher and 8.6 or lower." This is the value shown in the section on "those discharged into public waters other than the sea" in the pH standards for wastewater stipulated in the Water Pollution Control Act. Therefore, improved soil containing the neutral solidification material of this invention complies with the wastewater standards, even if, for example, rainwater that has permeated the improved soil flows into groundwater or a river. Therefore, the improved soil can be reused with reduced environmental impact, regardless of its intended use or location.

The neutral solidification material of the present invention contains the following components (1) to (3).
(1) magnesium oxide; (2) at least one sulfate selected from aluminum sulfate, ferrous sulfate, and gypsum; and (3) a polymer flocculant having an acrylamide unit and a sodium acrylate unit.

The neutral solidification material of the present invention contains magnesium oxide, which is a component that is mainly blended to solidify the soil to be improved.
Examples of magnesium oxide include lightly burned magnesium oxide obtained by burning magnesium hydroxide or magnesium carbonate at 600 to 900°C.

The magnesium oxide content in the neutral solidification material is preferably 10 to 50% by mass, more preferably 10 to 47% by mass, and even more preferably 10 to 45% by mass, calculated on anhydrous basis. By using this amount of magnesium oxide, it is possible to achieve a high level of both the strength development of the soil to be improved and the maintenance of a neutral pH. In addition, when improving soil contaminated with heavy metals, the heavy metals can be insolubilized, preventing their leaching from the improved soil. This has the added benefit of preserving the surrounding environment when the improved soil is used.

The neutral solidification material of the present invention contains at least one sulfate selected from the group consisting of aluminum sulfate, ferrous sulfate, and gypsum. The sulfate is a component that is primarily added to maintain the pH of the soil to be improved at a neutral level.

The sulfate contained in the neutral solidification material may be any one of aluminum sulfate, ferrous sulfate, or gypsum, or may contain two of aluminum sulfate, ferrous sulfate, and gypsum, or may contain all of aluminum sulfate, ferrous sulfate, and gypsum.
From the viewpoint of achieving a high level of strength development in the soil to be improved while maintaining a neutral pH, the sulfates contained in the neutral solidification material preferably contain at least two of aluminum sulfate, ferrous sulfate, and gypsum, more preferably contain at least aluminum sulfate and ferrous sulfate, and even more preferably contain all of aluminum sulfate, ferrous sulfate, and gypsum.

The total content of sulfates in the neutral solidifying material is preferably 42 to 89 mass %, more preferably 44 to 87 mass %, and even more preferably 46 to 84 mass %, calculated on an anhydrous basis.
By adjusting the sulfate content to the above range, it is possible to develop strength in the soil to be improved while maintaining a neutral and stable pH.

When aluminum sulfate is contained as the sulfate, the content of aluminum sulfate in the neutral solidification material is preferably 0 to 65 mass %, more preferably 0 to 60 mass %, and even more preferably 0 to 55 mass %, calculated on an anhydrous basis. In other words, as long as the total content of sulfates satisfies the above range, aluminum sulfate may not be contained.
By adjusting the aluminum sulfate content to the above range, it is possible to develop strength in the soil to be improved while maintaining a neutral and stable pH.

When ferrous sulfate is contained as a sulfate, the content of ferrous sulfate in the neutral solidification material is preferably 0 to 30% by mass, more preferably 0 to 25% by mass, and even more preferably 0 to 20% by mass, calculated on an anhydrous basis. In other words, as long as the total content of sulfates satisfies the above range, ferrous sulfate may not be contained.
By adjusting the ferrous sulfate content to the above range, it is possible to maintain the pH of the soil to be improved at a neutral level while increasing the strength of the soil. In addition, when improving soil contaminated with heavy metals, the heavy metals can be reduced and insolubilized, thereby suppressing the elution of heavy metals from the improved soil.

When gypsum is contained as the sulfate, at least one of gypsum anhydride, gypsum hemihydrate, gypsum dihydrate, etc. can be used as the gypsum. Gypsum specified in JIS R9151 can also be used. Of these, it is preferable to use gypsum hemihydrate as the gypsum from the viewpoint of improving the strength and maintaining the pH of the soil to be improved, as well as reducing production costs.
When gypsum is contained as a sulfate, the gypsum content in the neutral solidification material is preferably 0 to 75 mass %, more preferably 0 to 70 mass %, and even more preferably 0 to 65 mass %, calculated on an anhydrous basis. In other words, as long as the total content of sulfates satisfies the above range, gypsum may not be contained.
By adjusting the content of gypsum to the above range, the hydraulic properties of the gypsum itself can be used to further increase the strength of the improved soil.

The neutral solidification material of the present invention contains a polymer flocculant having an acrylamide unit and a sodium acrylate unit.
The polymer flocculant is a component that is mainly added to agglomerate the soil particles to be improved and increase the strength of the improved soil, and is preferably a single component or a mixture of organic polymers. The total content of the polymer flocculant in the neutral solidification material is preferably 1 to 8 mass%, more preferably 1 to 7 mass%, and even more preferably 1 to 6 mass%.
In the present invention, the term "unit" refers to a monomer unit that constitutes a polymer.

Specific examples of the polymer flocculant contained in the neutral solidification material include a copolymer formed by polymerizing acrylamide, which is a first monomer, and sodium acrylate, which is a second monomer, and the polymer flocculant contains an acrylamide unit and a sodium acrylate unit.
The copolymer may be in at least one of the following forms: an alternating copolymer in which each monomer is polymerized alternately; a random copolymer in which the arrangement of each monomer is random; a block copolymer having structural units in which the same type of monomer is polymerized successively; and a graft copolymer in which a polymer made of one monomer is branched and bonded to a polymer made of the other monomer.

One of the characteristics of the polymer flocculant contained in the neutral solidification material is that the acrylamide units constituting the polymer flocculant have a specific composition ratio.
Specifically, the composition ratio of acrylamide units contained in the polymer flocculant is preferably 55 to 90 mol%, more preferably 60 to 85 mol%, and even more preferably 60 to 80 mol%. By using a polymer flocculant with acrylamide units in such a ratio, it is possible to achieve a high level of both the strength development of the soil to be improved and the maintenance of a neutral pH.

The reason why the use of a polymer flocculant containing acrylamide units in the above-mentioned ratio can be modified to improve strength is not clear, but the present inventors speculate as follows.
Generally, in the neutral to alkaline pH range (i.e., a pH range of 5.8 or higher), the mineral particles that make up soil particles have a low isoelectric point, which causes the particles to be negatively charged, and electrostatic repulsion easily occurs between water and the soil particles. As a result, the soil particles are less likely to aggregate, such as by flocculation, and as a result, the strength of the soil as a whole tends to decrease.
When the above-mentioned polymer flocculant is added, first, the amide groups derived from the acrylamide units adsorb to the soil particle surfaces, and then the carboxyl groups derived from the sodium acrylate units repel each other, widening the molecular chain of the polymer flocculant, which then embraces (takes in) the soil particles and water, effectively agglomerating the soil particles and retaining water between the soil particles, which is thought to improve the strength of the improved soil. In particular, the strength development process caused by the addition of the above-mentioned polymer flocculant is thought to proceed in the following order: (i) adsorption of the flocculant to the soil particle surfaces, (ii) cross-linking between soil particles, and (iii) agglomeration of the soil particles. By preferably setting the composition ratio of acrylamide units in the polymer flocculant to 55 to 90 mol%, it is thought that the above processes (i) and (ii), which are important for strength development, can be progressed in a balanced manner.

The carbon, hydrogen, and nitrogen contained in a polymer flocculant can be measured, for example, by the following method. Specifically, approximately 2 mg of the polymer flocculant to be measured is precisely weighed out and introduced into an elemental analyzer (e.g., J Science Lab's Microcorder JM10) to quantitatively measure the mass of each element. For example, antipyrine (carbon element: 70.2% by mass, hydrogen element: 6.4% by mass, nitrogen element: 14.9% by mass) can be used as a standard.

The mass ratio (% by mass) of carbon element measured by the above method is designated as C1, and the mass ratio (% by mass) of nitrogen element is designated as N1. From these values, the composition ratios of acrylamide units and sodium acrylate units are calculated by the following method.
First, the acrylamide unit has the chemical formula "-CH ₂ -CH(CO-NH ₂ )-", and the unit contains three carbon atoms and one nitrogen atom.
The sodium acrylate unit has the chemical formula "--CH ₂ --CH(COONa)--", and the number of carbon atoms in the unit is 3 and the number of nitrogen atoms in the unit is zero.

The composition ratio (mol%) of the acrylamide units in the polymer flocculant is "A," and the composition ratio (mol%) of the sodium acrylate units in the polymer flocculant is "100 - A." Based on the number of carbon atoms and nitrogen atoms in each unit, the carbon composition ratio (mol%) of the acrylamide units is expressed as "3 x A," and the nitrogen composition ratio (mol%) of the acrylamide units is expressed as "A." Furthermore, the carbon composition ratio (mol%) of the sodium acrylate units is expressed as "3 x (100 - A)."

Based on the above-mentioned proportions, the ratio (C5/N5) of the carbon element composition ratio (C5) to the nitrogen element composition ratio (N5) in the polymer flocculant is calculated by the following formula (a).
(C5/N5)={(3×A)+3×(100-A)}/A=300/A...(a)

Here, C5/N5 is expressed as "{(C1)/(carbon atom amount)}/{(N1)/(nitrogen atom amount)}" (carbon atom amount = 12, nitrogen atom amount = 14), and therefore the composition ratio A of the acrylamide unit is calculated by the following formula (b).
Composition ratio A of acrylamide units (mol%)
= 300/{(C1)/(amount of carbon atoms)}/{(N1)/(amount of nitrogen atoms)} (b)

The composition ratio of the sodium acrylate unit is calculated by the following formula (c).
Composition ratio of sodium acrylate unit (mol%)=100−A (c)

The composition ratio of sodium acrylate units contained in the polymer flocculant is preferably 10 to 45 mol%, more preferably 15 to 40 mol%, and even more preferably 20 to 35 mol%.

The magnesium oxide, sulfate, and polymer flocculant may each independently be in the form of a powder, or may be in the form of a liquid or slurry dissolved or dispersed in a solvent such as water. The magnesium oxide and sulfate may each independently be in the form of an anhydride or a hydrate.
From the viewpoint of improving the ease of handling during transportation and use of the neutral solidification material, it is preferable that the magnesium oxide, sulfate, and polymer flocculant are all in powder form.
From the same viewpoint, the neutral solidification material of the present invention is preferably in powder form.

The Blaine specific surface area of magnesium oxide is preferably 6,000 to 20,000 ^{cm 2} /g, more preferably 7,000 to 20,000 ^{cm 2} /g, and even more preferably 8,000 to 20,000 cm ² /g.
By setting the Blaine specific surface area within this range, the workability of the neutral solidification material can be improved, and the high hydration activity can increase the solidification of the soil to be improved, thereby improving the strength of the improved soil. Furthermore, it is preferable that the above-mentioned Blaine specific surface area range is satisfied when light-burned magnesium oxide is used as the magnesium oxide, since this allows for a high level of both improvement in workability and improvement in the strength of the improved soil.
The Blaine specific surface area of magnesium oxide can be measured using a Blaine air permeability tester in accordance with, for example, JIS R5201:1997 "Physical testing methods for cement."

The BET specific surface area of magnesium oxide is preferably 5 to 30 m ² /g, more preferably 7 to 30 m ² /g, and even more preferably 8 to 30 m ² /g.
By setting the range in this way, the workability of the neutral solidification material can be improved, and the high hydration activity can increase the solidification of the soil to be improved, thereby improving the strength of the improved soil. Furthermore, it is preferable that the above-mentioned range of Blaine's specific surface area is satisfied when light-burned magnesium oxide is used as the magnesium oxide, since this allows for a high level of both improvement in workability and improvement in the strength of the improved soil.
The BET specific surface area of magnesium oxide can be measured by a constant volume gas adsorption method using, for example, a high precision gas adsorption apparatus (BELSORP-mini, manufactured by BEL Japan Co., Ltd.).

The MgO content in the magnesium oxide is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more. This content ratio helps prevent a weakening of the pH buffering capacity of the environment surrounding the improved soil, and also improves the strength of the improved soil.

The CaO content in the magnesium oxide is preferably 3% by mass or less, more preferably 2% by mass or less, and even more preferably 1% by mass or less. This content ratio makes it easier to maintain a neutral pH in the environment surrounding the improved soil, and also improves the strength of the improved soil.

The MgO and CaO contents of the magnesium oxide can be measured in accordance with JIS M 8853:1998 "Methods for chemical analysis of aluminosilicate raw materials for ceramics."

As long as the effects of the present invention are achieved, the neutral solidification material may further contain additives depending on the properties of the soil to be improved. Examples of additives that can be used include at least one of calcium aluminate minerals such as hydrotalcite and hydrocalumite, silicon-containing minerals such as silica stone, carbonates such as magnesium carbonate and limestone, porous materials such as sepiolite, perlite, zeolite and silica, and chelating agents.
The various additives may be in the form of powder, or may be in the form of a liquid or slurry dissolved or dispersed in a solvent such as water.
The various additives may be independently in the form of an anhydride or a hydrate.
The presence or absence of various additives and the amount of each additive to be added are preferably determined based on the results of a blending test carried out in advance.

The neutral solidification material of the present invention can be used in a solidification treatment method in which the neutral solidification material is mixed with soil to be improved and solidified.
The neutral solidification material may be in the form of a powder or a slurry when mixed with the soil to be improved.
In addition, when mixing the neutral solidification material with the soil to be improved, the neutral solidification material and the soil to be improved may be mixed simultaneously, or one of the neutral solidification material and the soil to be improved may be added to the other.

The amount of neutral solidification material to be added per cubic ^meter of soil to be improved can be changed as appropriate depending on the type and properties of the soil to be improved and the desired developed strength. From the viewpoint of achieving uniform mixing, improved developed strength, and reduced processing costs, the amount is preferably 20 to 200 kg/ ^m3 , more preferably 20 to 150 kg/ ^m3 , and even more preferably 20 to 75 kg/ ^m3 .

The equipment used to mix the neutral solidification material with the soil to be improved can be any equipment commonly used in the relevant technical field, such as a backhoe, a backhoe with a mixing bucket, a stabilizer, a self-propelled soil improvement machine, a stationary mixer, a trencher-type agitator/mixer, a deep mixing machine, a power blender, and a plant mixer.

An example of a soil treatment method using the neutral solidification material of the present invention will be described below with reference to Figure 1. The neutral solidification material of the present invention can be used for excavated soil such as mud discharged by shield tunneling methods such as the mud pressure shield tunneling method and the air bubble shield tunneling method, as the soil to be treated for improvement.
The shield machine 1 shown in Figure 1 is typically used in the air bubble shield tunneling method. The shield machine 1 includes a disk-shaped cutter 11 with multiple holes extending in the excavation direction, and a motor 12 connected to the cutter 11 for rotating the cutter 11. The direction of the drive shaft of the motor 12 coincides with the excavation direction. The shield machine 1 shown in the figure also includes a screw conveyor 3 located behind the cutter 11 in the excavation direction, for transporting the excavated soil excavated by the cutter 11 backward in the excavation direction.

First, the ground is excavated using the shield machine 1. In this embodiment, an air bubble injection pipe 15 is provided to inject air bubbles into the excavated soil in order to increase the fluidity of the excavated soil when excavating the ground. One end of the air bubble injection pipe 15 is connected to an air bubble supply device (not shown) such as a pump, and the other end opens to the cutter 11. The air bubble supply device may supply air bubbles together with an air foaming agent, for example.

Excavated soil generated by excavating the ground is transported in the opposite direction to the excavation direction by the chamber 2 and screw conveyor 3 of the shield machine 1 through holes provided in the cutter 11. Excavated soil containing an aerating agent has high fluidity and is easy to transport.
During transport by the screw conveyor 3, in order to prevent eruptions due to pressure changes when the excavated soil is discharged from the screw conveyor 3, an anti-eruption material may be added to the inside of the screw conveyor 3 using a dosing device 4 provided on the front side of the screw conveyor 3 in the excavation direction, and the material may be mixed with the excavated soil 10 while being transported.
Next, the excavated soil 10 transported rearward in the excavation direction by the screw conveyor 3 is discharged onto the belt conveyor 5 through a gate 31 located on the rear side of the screw conveyor 3 in the excavation direction and transported there.

The excavated soil 10 transported by the belt conveyor is then placed in a bucket 6, and the neutral solidification agent of the present invention is added to the excavated soil 10 using a solidification agent adding device 20. The soil is then mixed using a mixer 30, and the soil is then preferably cured for a certain period of time. These operations may be carried out after the excavated soil 10 has been transported to the surface via a dump or shaft, or may be carried out at the tunnel construction site. The curing period can be adjusted as needed depending on the desired strength and the acceptance conditions at the destination, but is preferably set to approximately 1 to 14 days, and more preferably 1 to 7 days.

Through the above process, improved soil can be obtained, which has developed strength from the soil being treated. The cured soil is then transported to the desired location, such as a treatment plant or recycling site, using transport equipment such as a dump truck or truck, and is either reused or disposed of as needed. Because the improved soil's strength has been improved to a level that allows it to be reused, it can be transported as construction material and put to effective use.

In this way, by using the neutral solidification material of the present invention, construction waste soil, which in conventional technology would have been treated as industrial waste or soil with limited uses due to its lack of strength, can be solidified before being transported outside the construction site where it was generated, and the improved soil can be reused as construction material at the same or a different construction site.

In particular, when improved soil is used at a construction site different from the one where construction waste soil was generated, the improved soil is cured for a certain period of time before being transported to the other construction site. However, improved soil solidified using the neutral solidification agent of the present invention can be transported with its strength fully maintained, even if it is excavated or loosened during transportation. Furthermore, the improved soil can be reused as is at the other construction site where it was transported, without any special processes.

Furthermore, because soil improved with the neutral solidification material of the present invention exhibits high strength, it can be suitably used, for example, as "Type 2 construction generated soil" as defined in the soil classification standards of the Ministry of Land, Infrastructure, Transport and Tourism's notification "Regarding Generated Soil Utilization Standards," or as "Type 2 treated soil" as defined in the quality standards of the Ministry of Land, Infrastructure, Transport and Tourism's notification "Construction Sludge Treatment Level Utilization Technical Standards." In particular, soil improved with the neutral solidification material of the present invention can be suitably used in a wide range of civil engineering and construction applications, such as backfilling of structures, backfilling of buildings, backfilling of civil engineering structures, road embankments, embankments, land development, railway embankments, airport embankments, and water surface reclamation.

The present invention will be described in more detail below using examples. The scope of the present invention is not limited to these examples. In the following description, "%" represents "% by mass" unless otherwise specified.

[Examples 1 to 6 and Comparative Examples 1 to 5]
<1. Preparation of construction waste soil samples>
Powdered mudstone was used as the raw soil. This mudstone had a water content of 29.9% measured in accordance with JIS A1203 (hereinafter simply referred to as "water content"), a wet density of 1.75 g/ ^cm3 measured in accordance with JIS A1210, and a pH of 8.4 measured in accordance with JGS 0211-2000.
Next, water, an aerating agent (OK-1, manufactured by Pacific Shield Mechanics Co., Ltd.), and an eruption prevention material (SP-α, manufactured by Pacific Shield Mechanics Co., Ltd.) were added to the mudstone in the proportions shown in Table 1 below to obtain two types of construction generated soil replica samples (mud-like samples) with different water contents. Hereinafter, these construction generated soil replica samples will also be referred to as "generated soil sample 1" and "generated soil sample 2."

<2. Preparation of neutral solidification material>
As raw materials, magnesium oxide, aluminum sulfate, and ferrous sulfate shown below were mixed with polymer flocculants containing different mol% of acrylamide units and sodium acrylate units in the proportions shown in Table 2 below to prepare a total of 10 types of powdered neutral solidification materials.
Magnesium oxide: Light-burned magnesium oxide manufactured by Ube Material Industries, Ltd. Aluminum sulfate: Aluminum sulfate tetratetrahydrate manufactured in China Ferrous sulfate: Ferrous sulfate monohydrate manufactured in China Polymer flocculant:
(1) Neutral solidification materials B and H: Acofloc A-150UH, manufactured by MT Aquapolymer Co., Ltd.
(2) Neutral solidification material C: Sanflock AH-9S, manufactured by Sanyo Chemical Industries, Ltd.
(3) Neutral solidification materials D and I: Sanflock AH-4SFA, manufactured by Sanyo Chemical Industries, Ltd.
(4) Neutral solidification materials E and J: Acofloc A-125, manufactured by MT Aquapolymer Co., Ltd.
(5) Neutral solidification material F: Waterflock PA-041, manufactured by Technica Co., Ltd.
(6) Neutral solidification material G: Acofloc A-220, manufactured by MT Aquapolymer Co., Ltd.

<3. Preparation of improved soil>
The powdered neutral solidification material was added and mixed at a rate of 30 kg/ ^m³ to generated soil sample 1, and at a rate of 40 kg/ ^m³ to generated soil sample 2, to obtain the desired improved soil. The pH of each improved soil was maintained at neutral.

[Soil strength evaluation]
The cone index was measured for the generated soil samples and the improved soil in accordance with JIS A1228.
For the excavated soil samples, specimens for measuring the cone index were prepared immediately after mixing each material, and the cone index of each was measured.
In order to confirm the effect of re-kneading on the improved soil, the same improved soil was repeatedly used after 1 day of curing and after 7 days of curing, and specimens for measuring the cone index were prepared for each curing period immediately before the test, and the cone index was measured for each period.
To judge the strength of the soil, the cone index value measured after the improved soil had been cured indoors for the aforementioned period was multiplied by 0.9, taking into consideration the effect of strength reduction during excavation and loosening of the improved soil, and differences in the surrounding environment. The cone index value was used as the estimated strength value of the improved soil at the site, and soils with an estimated value of 800 kN/ ^m2 or more (class 2 construction generated soil or more) were rated as good (shown as "O" in Table 3), and soils with an estimated value of less than 800 kN/ ^m2 were rated as poor (shown as "X" in Table 3). The results are shown in Tables 1 and 3.

As shown in Table 3, the neutral solidification material of the example, which uses magnesium oxide, sulfate, and a polymer flocculant with a specified acrylamide unit composition ratio, is able to fully develop the strength of improved soil, regardless of the curing period, compared to the comparative example. This is supported by the fact that the cone index of the generated soil sample, which was at a level that would have been classified as "Class 4 construction generated soil" in the soil quality classification standard of the notification "Regarding Generated Soil Utilization Standards," was improved to an improved soil with a cone index equivalent to Class 2 construction generated soil by adding the neutral solidification material.
In particular, the neutral solidification materials of Examples 1 to 4, which have a suitable range of polymer coagulant content in the neutral solidification material, can further increase the strength of the improved soil and are less susceptible to strength reduction due to excavation or loosening, so it can be seen that the improved soil can be suitably reused even if it is cured for a certain period of time after the addition of the neutral solidification material.

As described above, the neutral solidification material of the present invention can improve the soil to be improved, maintaining the pH at a near-neutral level while also improving the soil to develop high strength, allowing the improved soil to be reused for a wide range of applications, leading to effective use of resources.

Claims (5)

Magnesium oxide,
at least one sulfate selected from aluminum sulfate, ferrous sulfate, and gypsum;
a polymer flocculant having an acrylamide unit and a sodium acrylate unit,
The composition ratio of the acrylamide units constituting the polymer flocculant is 55 to 90 mol %. The neutral solidification material according to claim 1, containing 1 to 8 mass% of the polymer flocculant. The neutral solidification material according to claim 1 or 2, containing 10 to 50 mass% of magnesium oxide. The method comprises a step of curing a mixture of the soil to be treated and the neutral solidification material until the cone index measured in accordance with JIS A1228 is 800 kN/m2 or more;
The soil treatment method includes the neutral solidification material comprising magnesium oxide, at least one sulfate selected from aluminum sulfate, ferrous sulfate, and gypsum, and a polymer flocculant having an acrylamide unit and a sodium acrylate unit, and the composition ratio of the acrylamide unit constituting the polymer flocculant is 55 to 90 mol %. The soil treatment method according to claim 4, wherein the soil to be treated is one containing an aerating agent.