JP7702907B2 - Neutral regenerated soil manufacturing method using low-concentration carbon dioxide - Google Patents

Description

The present invention relates to a method for producing neutral reclaimed soil using low-concentration carbon dioxide, which provides a method for reforming weak alkaline construction sludge and efficiently fixing low-concentration carbon dioxide in the reformed soil as calcium carbonate to produce neutral reclaimed soil.

Construction sludge discharged from construction sites often contains cement and lime components, and is usually alkaline, with a pH of 11 or higher. This type of construction sludge is brought to an intermediate treatment plant, where quicklime or cement-based solidification materials are added and the sludge is modified before being reused as reclaimed soil, or disposed of in landfills at final disposal sites. When reclaimed soil is used, it often has a strong alkalinity with a pH of over 11, which has been pointed out as limiting its uses.

To address these issues, the inventors have already obtained a patent for a "reclaimed soil manufacturing system and manufacturing method" (see Patent Document 1) and a patent for a "neutral reclaimed soil manufacturing system and manufacturing method using carbon dioxide fixation technology" (see Patent Document 2).

Patent No. 6559644 Patent No. 6975757

This invention proposes a solution to the problem of _CO2 concentration that was not solved by the prior patent application. That is, the above two patent inventions devised a method of producing neutral regenerated soil by contacting alkaline construction soil with high-purity (concentration of 95% or more) carbon dioxide. Therefore, there was a problem that it was difficult to neutralize efficiently and at a practical level with the relatively low concentration (concentration of 10% or less) of _CO2 emitted from factory chimneys, etc.

The object of the present invention is to provide a method for producing neutral reclaimed soil using low-concentration carbon dioxide gas, which does not require concentrating low-concentration carbon dioxide gas (10% or less), and can produce a large amount of neutral reclaimed soil with a pH of 8.6 or less in a short period of time.

That is, the present invention includes the following.
According to the method for producing neutral reclaimed soil using low-concentration carbon dioxide gas of the present invention,
A modification and solidification process in which a PS (paper sludge) ash-based modifier is added to construction sludge or construction generated soil and mixed to produce modified and solidified soil;
A loosening and granulation process for producing a loosened and granulated modified soil by loosening and granulating the modified and solidified soil;
A classification step of generating classified soil by classifying the loosened granulated improved soil through a sieve;
a laying step of laying the classified soil in layers in a pit;
a neutralization step of supplying low-concentration carbon dioxide gas having a concentration of 10% or less to the bottom of the pit to neutralize the classified soil;
The present invention is characterized by comprising:

By going through the above process, it is possible to produce large quantities of neutral reclaimed soil with a pH of 8.6 or less in a short period of time without concentrating low-concentration carbon dioxide gas (concentration of 10% or less).

In addition, according to the method for producing neutral reclaimed soil using low-concentration carbon dioxide gas of the present invention,
The laying step is characterized in that the classified soil is laid in the pit from the bottom up in descending order of grain size.

This results in larger voids between the classified soils (smaller dry density of the classified soils) and increased breathability compared to when the particle size of each layer is uniform and not classified, shortening the neutralization time.

In addition, according to the method for producing neutral reclaimed soil using low-concentration carbon dioxide gas of the present invention,
The classified soil laid in the top layer in the laying step is characterized in that the particle size of the classified soil laid in the bottom layer is less than 10 mm, and the particle size of the classified soil laid in the bottom layer is 40 mm or more and less than 75 mm.

In this way, by laying coarse-grained classified soil in the lower layer, the neutralization property that the longer the time required for neutralization of the coarser classified soil is utilized, and the lower the layer, the longer the contact time of _CO2 is secured, thereby improving the neutralization efficiency. Furthermore, by laying fine-grained modified soil on the top, the upper part of the soil layer is sealed more tightly, and it can function as a lid to prevent the escape of _CO2 from the surface layer.

In addition, according to the method for producing neutral reclaimed soil using low-concentration carbon dioxide gas of the present invention,
The neutralization step comprises:
Assuming that the volume of the classified soil laid in the pit in the laying step is V,
an initial supply step of supplying the low concentration carbon dioxide gas at a volumetric flow rate of 5 V to 20 V for 0.1 to 1 hour from the start of supply;
a later supply step of supplying the low-concentration carbon dioxide gas at a volumetric flow rate of 8 V to 40 V after the low-concentration carbon dioxide gas fills the pit;
a middle-stage supply step of supplying the low-concentration carbon dioxide gas at a volumetric flow rate of 80 V to 320 V between the initial supply step and the later supply step,
The total supply time of the low concentration carbon dioxide gas is 4 to 24 hours.

In this way, by adjusting the amount of low-concentration carbon dioxide gas supplied and the contact time with the classified soil, neutralization of the classified soil can be achieved efficiently in a short period of time.

In addition, according to the method for producing neutral reclaimed soil using low-concentration carbon dioxide gas of the present invention,
The method is characterized in that after the neutralization step, a plowing step is included in which the classified soil having a particle size of less than 20 mm is plowing off.

As a result, the removed classified soil is transferred to a product stockyard and used as neutral reclaimed soil.

In addition, according to the method for producing neutral reclaimed soil using low-concentration carbon dioxide gas of the present invention,
The method is characterized in that the classified soil having a particle size of 20 mm or more that has undergone multiple cycles of neutralization treatment is mixed with neutral reclaimed soil obtained by the method for producing neutral reclaimed soil using the low concentration carbon dioxide gas.

This allows the construction sludge received in the pit to be used without any waste.

In addition, according to the method for producing neutral reclaimed soil using low-concentration carbon dioxide gas of the present invention,
The modified solidified treated soil is characterized by being strongly alkaline with a pH of 11 or higher.

In other words, according to the present invention, it is possible to neutralize such strongly alkaline construction sludge.

The present invention provides a method for producing neutral reclaimed soil using low-concentration carbon dioxide gas, which does not require concentrating low-concentration carbon dioxide gas of 10% or less, and can produce a large amount of neutral reclaimed soil with a pH of 8.6 or less in a short period of time.

FIG. 2 is a schematic diagram showing a pit in which classified soil is laid in layers and neutralized according to an embodiment of the present invention. FIG. 1 is a schematic diagram showing an incubator in which a test specimen containing a sample of classified soil is placed. This is a graph showing the relationship between the neutralization completion time and _CO2 concentration of neutral reclaimed soil for each blend sample. 1 is a graph showing neutralization time for different dry densities of simulated alkaline sludge. 13 is a graph showing the relationship between the neutralization completion time of neutral reclaimed soil and particle size. FIG. 1 is a conceptual diagram of a column-type neutralization and fixation test. 1 is a graph showing the relationship between the flow rate of carbon dioxide and neutralization time. 1 is a graph showing the relationship between the flow rate of carbon dioxide and the fixation rate in soil relative to the inflow amount.

Hereinafter, a method for producing neutral reclaimed soil using low-concentration carbon dioxide gas according to an embodiment of the present invention will be described with reference to the drawings. First, construction sludge discharged from a construction site is transported to an intermediate treatment facility. The construction sludge transported to the intermediate treatment facility is industrial waste that exhibits strong alkalinity. The cone index (qc) of this construction sludge is less than 200 kN/ ^m2 .

(Step S1: Modification and solidification process)
Here, the alkaline construction sludge brought to the intermediate treatment facility will eventually solidify if left as is. However, due to space restrictions at the intermediate treatment facility, it cannot be left as is (cured) for a long period of time. Therefore, it is common to add a strong alkaline solidification material to modify the sludge so that it can be handled quickly. In this case, there are cases where the pH becomes 12 or higher due to the addition of the solidification material.

In the present invention, this conventional technology is applied as is to modify and solidify construction sludge. That is, in the modification and solidification treatment area (not shown) of the intermediate treatment facility, an alkaline improver is added to the construction sludge to modify and solidify it. As the alkaline improver, a PS (paper sludge) ash-based water-absorbent improver is used.

As will be explained in Example 1 below, the results of tests showed that the neutralization time of construction sludge can be significantly shortened by adding a PS ash-based modifier, so it is essential for the present invention to use a PS ash-based modifier as an alkalinity improver.

The amount of PS ash-based modifier added depends on the water content of the construction sludge, but is usually in the range of 5 to 30% by mass relative to the solid components in the construction sludge, and 10 to 20% is more preferable. It has been confirmed in laboratory experiments that construction sludge to which 20% or more of PS ash-based modifier has been added does not re-mutate due to the generation of water after the neutralization reaction with _CO2 . In addition, there is no restriction on the pH value of the construction sludge after the addition of PS ash-based modifier (hereinafter referred to as modified solidification treated soil).

In this modification and solidification process, the addition of an alkaline solidification agent to the construction sludge ensures that the soft construction sludge is modified and solidified, producing semi-solid modified and solidified soil.

(Step S2: Loosening and granulation process)
The semi-solidified soil is undergoing a hydration reaction and will solidify in the form of lumps if left as is. Therefore, the semi-solidified soil is crushed and granulated using a granulation technique (see, for example, JP 2015-127050 A).

Here, the suitability of the loosening granulation is judged by the cone index test method (JIS A 1228). Usually, if the cone index of the modified solidification treated soil is 100 kN/ ^m2 or more, loosening granulation is possible, but to produce more granulated construction sludge (hereinafter referred to as loosened granulated modified soil) by loosening granulation, it is preferable that the cone index of the modified solidification treated soil is 400 kN/ ^m2 or more and less than 1800 kN/ ^m2 . The machine used for the loosening granulation process (loosening granulation means) may be a backhoe, but a crusher such as a soil improvement machine (plant) with a crushing function is preferable.

The loosened and granulated improved soil obtained in this loosening and granulation process is granulated, which increases the surface area of the particles, making them more susceptible to contact with air and carbon dioxide, and adjusting the particle size to an appropriate level.

Specifically, because the PS ash-based modifier is added to the modified solidification treated soil, granulation is promoted, and the particle size of most of the loosened granulated modified soil is 40 mm or less.
Here, it has been confirmed that when loosened granulated modified soil is produced in an actual plant, the estimated proportions are 10% with a particle size of 40 mm or more and less than 75 mm, approximately 10% with a particle size of 40 to 20 mm, approximately 40% with a particle size of 20 to 10 mm, and approximately 40% with a particle size of 10 mm or less.

In addition, the increased surface area of loosened, granulated, improved soil makes it easier to dry. At the same time, it also reacts with carbon dioxide in the air, making it easier for the pH to decrease.

(Step S3: Classification process)
Next, the loosened granulated modified soil is sieved to classify it. Specifically, sieves of 40 mm, 20 mm, and 10 mm are prepared, and the loosened granulated modified soil is classified into four different particle sizes: classified soil with a particle size of 40 mm or more and less than 75 mm (hereinafter referred to as 40 mm over material), classified soil with a particle size of less than 40 and 20 mm or more (hereinafter referred to as 40-20 mm material), classified soil with a particle size of less than 20 and 10 mm or more (hereinafter referred to as 20-10 mm material), and classified soil with a particle size of less than 10 mm (hereinafter referred to as 10 mm under material). A vibrating sieve is suitable as the sieving machine.

(Step S4: Laying process)
Fig. 1 is a schematic diagram showing a pit into which classified soil is poured according to an embodiment of the present invention. As shown in Fig. 1(a), the pit 2 is a rectangular space formed on the ground. An inlet pipe 4 for supplying carbon dioxide gas, which is made of a heat-resistant pipe or the like, is buried in the bottom of the pit 2. A blower 8 for supplying carbon dioxide gas to the inlet pipe 4 is connected to the above-ground end of the inlet pipe 4.

Next, as shown in FIG. 1(b), the classified soil is laid in the pit 2 in the following order: 40 mm over material 10, 40 to 20 mm material 12, 20 to 10 mm material 14, and 10 mm under material 16, in a loosely packed state.

In this way, by classifying the loosened granulated soil and laying it in the pit 2 in a loosely packed state from the bottom up in order of grain size, the carbon dioxide gas permeability of the classified soil can be increased. In other words, because the classified soil has a uniform grain size for each layer, the voids between the classified soil are larger than when the grain size is not classified (the dry density of the classified soil is smaller), and the increased permeability leads to a shorter neutralization time.

It is also a commonly known fact that the larger the particle size of the classified soil, the better the air permeability (sand is more air permeable than clay, and gravel is more air permeable than sand). Therefore, by laying coarse-grained materials in the lower layer and fine-grained materials in the upper layer, a soil layer space with better air permeability can be created in the lower layer. As a result, forced injection of _CO2 from an inlet pipe laid in the lower layer (see Patent No. 6975757) is not necessary. In addition, by laying coarse-grained classified soil in the lower layer, the neutralization characteristic that the longer the time required for neutralization of the coarser-grained classified soil is taken advantage of, and the lower the layer, the longer the contact time of _CO2 is secured, thereby improving the neutralization efficiency. Furthermore, by laying fine-grained modified soil at the top, the degree of sealing of the upper part of the soil layer is increased, and it can function as a lid to prevent _CO2 from dissipating from the surface layer.

(Step S5: Neutralization process)
In this state, the blower 8 is driven to supply low-concentration carbon dioxide gas of 10% or less (of course, carbon dioxide gas of 10% or more may also be used) to the inlet pipe 4 as shown in FIG. 1(c).

Here, assuming that the volume of the classified soil laid in the pit 2 in the laying process (step S4) is V, in the initial stage (0.1 to 1 hour after the start of supply), the pressure of the blower 8 is reduced so that the low-concentration carbon dioxide gas is slowly supplied at a volumetric flow rate of 5V to 20V (initial supply process), and then the pressure of the blower 8 is increased to gradually increase the supply amount of the low-concentration carbon dioxide gas and supply it at a volumetric flow rate of 80V to 320V (middle supply process), and when the low-concentration carbon dioxide gas fills the pit 2, the pressure of the blower 8 is reduced again so that the low-concentration carbon dioxide gas is supplied at a volumetric flow rate of 10V to 40V (late supply process). Note that the fact that the low-concentration carbon dioxide gas has filled the pit 2 may be detected by a detection device and an instruction to automatically reduce the pressure may be given to the blower 8.

After the low concentration carbon dioxide gas fills the pit 2, the blower 8 is turned off after a specified time has elapsed, and the supply of low concentration carbon dioxide gas stops. The time from when the supply starts to when it stops is approximately 4 to 24 hours. Therefore, the time required for neutralization of the classified soil is also approximately 4 to 24 hours.

In this way, by adjusting the amount of low-concentration carbon dioxide gas supplied and the contact time with the classified soil, neutralization of the classified soil can be achieved efficiently in a short period of time.

In addition, by supplying low-concentration carbon dioxide gas, which is heavier than air, from an inlet pipe 4 buried in the bottom of the pit 2, the low-concentration carbon dioxide gas gradually accumulates in the pit 2, and the low-concentration carbon dioxide is distributed evenly throughout the pit 2. Therefore, when the low-concentration carbon dioxide gas has been supplied to the upper layer, neutralization is completed simultaneously from the lower layer to the upper layer.

(Step S6: Scraping process)
Next, the supply of low-concentration carbon dioxide gas is stopped, and the 10 mm under material 16 and the 20-10 mm material 14 are shoveled out and removed from the pit 2. The removed classified soil is transferred to a product stockyard and used as neutral reclaimed soil.

The 40-20mm material 12 with a low neutralization rate remaining in the lower layer is reused in the neutralization process of the next and subsequent cycles (Step S4: Laying process, Step S5: Neutralization process) and continues to be neutralized. The finally neutralized 40-20mm material 12 is crushed to less than 20mm using a crusher, and mixed with the 10mm under material 16 and 20-10mm material 14 removed in Step S6: Plowing process to be used as neutral reclaimed soil.

The bottom layer of material 10 over 40 mm thick is generated in small quantities, so it continues to be used as an aeration layer. By using it continuously for a certain period of time in this way, the pH drops to 8.6 or less. Finally, it is crushed into pieces less than 20 mm using a crusher and used as neutral reclaimed soil.

Therefore, it becomes unnecessary to purchase single-grain crushed stone separately as in the prior art (see Japanese Patent No. 6975757), and it becomes possible to recycle classified soil having a large grain size.
According to the invention relating to this embodiment, a method for producing neutral reclaimed soil using low-concentration carbon dioxide ^can be provided, which makes it possible to mass-produce neutral reclaimed soil with a pH of 8.6 or less in a short period of time, within 24 hours, without concentrating low-concentration carbon dioxide with a Cone Index of 800 kN/m2 or more and a concentration of 10% or less.

In addition, by supplying low-concentration carbon dioxide slowly without pressurizing it, it is possible to increase the fixation efficiency of _CO2 and minimize the dissipation of CO2 outside the pit 2. Furthermore, the construction sludge received in the pit 2 can be used without being left behind.

In addition, the prior invention (see Patent No. 6975757) claimed that it was possible to neutralize reclaimed soil up to a maximum soil layer thickness of about 3m (about ^150m3 per pit) by supplying high-concentration carbon dioxide gas with a purity of nearly 100% together with high-pressure air from the bottom of the pit. However, in reality, the practical level was up to a soil layer thickness of 1.5m. In contrast, the invention related to this embodiment is capable of neutralizing construction sludge with a soil layer thickness of 3m or more, even though the carbon dioxide gas concentration is one-tenth or less of the conventional level.

Compared to conventional methods, the following advantages can be expected:
(1) It is a low-cost technology. Conventional technology has proposed a neutralization method in which industrial carbon dioxide gas with an increased purity, such as by concentrating the carbon dioxide gas, is purchased and used. In contrast, the present invention can use low-concentration carbon dioxide gas for neutralization as is. For example, the carbon dioxide gas concentration in the exhaust gas discharged from an oil refinery plant is said to be about 10%, and if the _CO2 in such exhaust gas can be used directly without concentrating it, it will lead to significant cost reductions. Furthermore, since there is no need for equipment or power to inject _CO2 , neutralization can be achieved at a lower cost than with conventional technology.

(2) It is a technology that contributes to carbon neutrality.
Carbon dioxide emitted from factories is a greenhouse gas and is one of the causes of global warming. By using this carbon dioxide to neutralize regenerated soil, _CO2 is fixed in the soil as carbonate, making it a carbon recycling technology. Furthermore, by efficiently and in large quantities fixing low-concentration carbon dioxide in classified soil, it leads to carbon neutrality (and in some cases carbon negative), which strongly contributes to society toward the realization of a decarbonized society.

(3) The reuse rate of construction sludge will increase.
By modifying alkaline construction sludge, which currently has limited uses, into high-quality neutral reclaimed soil, it can be used not only as a wide range of ground materials but also as a vegetation base material. In addition, reclaimed soil with a particle size of over 20 mm, which has been discarded without being used until now, can be used as neutral reclaimed soil, so it is expected that the reuse rate of construction sludge will increase.

(4) It is highly versatile as it does not require special machinery.
Compared to the prior patent application (Patent No. 6975757), the only additional machinery required is a classifier (such as a vibrating sieve) for classifying the soil and a crusher for crushing the granulated coarse-grained modified soil, so neutral regenerated soil can be produced by applying general-purpose machinery.

(5) Reclaimed soil is neutral and of high quality. Reclaimed soil has a neutral pH of 5.8 to 8.6 and a cone index of 800 kN/ ^m2 or more. Furthermore, it is possible to produce high-quality reclaimed soil that does not easily re-turn into mud.

In this embodiment of the invention, an example is shown in which construction sludge is neutralized to produce neutral reclaimed soil, but the present invention can also be applied to the case in which construction waste soil, which is a by-product of construction work, is used instead of construction sludge.

In addition, in the present invention, in order to achieve neutralization of regenerated soil using low-concentration _CO2 , a neutralization test was conducted in which low-concentration carbon dioxide of 10% or less was reacted, and as a result, the following new findings were obtained (details are shown in the examples).
1) Increasing the amount of PS ash-based modifier added dramatically shortens the neutralization time.
2) The smaller the dry density of the modified soil (the larger the pores), the shorter the neutralization time.
3) The finer the particle size of the modified soil, the shorter the neutralization time.
4) Even if the _CO2 concentration is 10% or less, neutralization will occur within 24 hours (shortest of several hours) of contact time if the above conditions 1) to 3) are met.
5) The smaller the _CO2 flow rate, the longer the neutralization time, but the greater the _CO2 fixation rate.
The above findings are the basis for the necessity of steps S1 to S6 in the present invention.

Next, examples of experiments carried out using the invention according to the embodiment will be described.
Example 1: Neutralization promotion effect of PS ash-based modifier To blue clay with an initial water content w = _1.0wL (liquid limit _wL = 40.7%, plastic limit _wP = 23.7%), blast furnace cement type B was added at 3% and 6% by dry mass ratio to prepare simulated sludge. For the sample with 3% added cement, PS ash-based modifier was further added at 0 to 20% of the dry mass of the blue clay to prepare each sample. The prepared samples were sealed and cured for 5 days, and for the sample without added PS ash-based modifier, soil particles of 9.5 mm or more were removed using a sieve.

Each sample was loosely packed in a plastic cup with a diameter of 82 mm to a layer thickness of 30 mm to prepare a test specimen. Ten test specimens were prepared for each sample, and neutralized by placing them in an incubator as shown in Figure 2 and curing them under conditions of different _CO2 concentrations. In this test, the test specimens were taken out of the incubator one by one at any time and pH was measured based on the standards of the Geotechnical Society of Japan. In this experiment, the time when pH fell below 8.6 was considered to be the completion of neutralization, and was defined as the neutralization completion time _tN .

Figure 3 shows the relationship between the carbonation completion time _tN and the _CO2 concentration for each blended sample. In all blending conditions, _tN tends to be longer as the _CO2 concentration decreases. However, in the cases where 10% or 20% PS ash-based modifier was added, the carbonation completion time was dramatically shorter than in the cases where no PS ash-based modifier was added. In addition, in the range of _CO2 concentrations from 2 to 10%, the carbonation completion time was within 24 hours in all cases where PS ash-based modifier was added. From the above, it was found that adding PS ash-based modifier is extremely effective in neutralization with low-concentration _CO2 gas.

[Example 2] Effect of density Alkaline simulated sludge was prepared by adding 3% and 20% of blast furnace cement type B and PS ash-based modifier in dry mass ratio to blue clay with w = _1.0wL . The simulated sludge was packed in a cup with a diameter of 82 mm to h = 30 mm in different ways to prepare specimens (dry density ρ _d = 0.74, 0.93, 1.16 g/cm ³ ). The specimens were incubated at a CO ₂ concentration of 10% and neutralized. Approximately 10 specimens were prepared for each density, and the pH was measured by taking them out one by one during neutralization. The results are shown in Figure 4.
This test confirmed that the smaller the dry density _ρd , the shorter the neutralization time, and that the greater the gaps are made to improve breathability, the greater the efficiency of neutralization by _CO2 .

[Example 3] Effect of particle size Alkaline sludge was prepared by adding 3% and 10% dry mass of blast furnace cement type B and PS ash-based modifier to Kasaoka clay with initial water content w = 30, 40, and 50% (relatively sticky clay, liquid limit w _L = 60.4%, plastic limit w _P = 26.0%). The samples were disintegrated and classified with a sieve, and the samples that passed through each sieve were cured in an incubator with a CO ₂ concentration of 10% and neutralized. The results are shown in Figure 5. It was confirmed that the time t _N required to complete neutralization was shorter for modified soil with smaller particle size.

[Example 4] Effect of _CO2 flow rate Alkaline sludge was prepared by adding 3% and 20% of blast furnace cement type B and PS ash-based modifier, respectively, in dry mass ratio to blue clay of w= _1.0wL . Using this sludge, 10% _CO2 gas was passed through a 1L test piece (dry density _ρd = 0.75-0.77g/ ^cm3 ) with φ=100mm and h=127mm at flow rates Q=0.5, 1.0, and 1.5L/min to perform neutralization (see Figure 6). In the test, the neutralization time and the amount of fixed _CO2 were also evaluated from the _CO2 concentration ratio _Cout / _Cin measured at the inflow and outflow. It can be seen from Figure 7 that the neutralization time _tEON to reach _Cout / _Cin = 1.0 becomes shorter as the flow rate Q increases. In addition, the _CO2 fixation rate F (%) relative to the inflowing _CO2 was calculated, and F was plotted against Q in Figure 8. It was found that the _CO2 fixation rate F decreased with an increase in Q.

Therefore, in order to speed up the neutralization time, it is better to increase the flow rate of _CO2 , while on the other hand, it is important to reduce the flow rate in order to increase the rate of _CO2 fixation in the soil. In the same way as cooking rice, alkaline soil can be efficiently neutralized with _CO2 by adjusting the flow rate.

The above examples confirmed that it is possible to reliably lower the pH of even strongly alkaline construction sludge and produce high-quality neutral reclaimed soil.

2 Pit 4 Inlet pipe 8 Blower 10 40mm over material 12 40-20mm material 14 20-10mm material 16 10mm under material

Claims (4)

A modification and solidification process in which a PS (paper sludge) ash-based modifier is added to construction sludge or construction generated soil and mixed to produce modified and solidified soil;
A loosening and granulation process for producing a loosened and granulated modified soil by loosening and granulating the modified and solidified soil;
A classification step of generating classified soil by classifying the loosened granulated improved soil through a sieve;
a laying step of laying the classified soil in layers from the bottom in descending order of grain size on the bottom of a pit where no single-grain crushed stone has been laid ;
a neutralization step of supplying low-concentration carbon dioxide gas having a concentration of 10% or less to the bottom of the pit to neutralize the classified soil;
A method for producing neutral reclaimed soil using low-concentration carbon dioxide, comprising a step of raking off the neutralized classified soil having a particle size of less than 20 mm,
A method for producing neutral reclaimed soil using low concentration carbon dioxide, characterized in that the classified soil with a particle size of 20 mm or more that has undergone multiple cycles of the neutralization process is crushed to a particle size of less than 20 mm, and then mixed with the classified soil with a particle size of less than 20 mm that has been neutralized in the plowing process, and reused as neutral reclaimed soil. The classified soil laid in the top layer in the laying step has a particle size of less than 10 mm,
2. The method for producing neutral reclaimed soil using low-concentration carbon dioxide gas according to claim 1, wherein the classified soil laid in the bottom layer has a particle size of 40 mm or more and less than 75 mm. The neutralization step comprises:
Assuming that the volume of the classified soil laid in the pit in the laying step is V,
an initial supply step of supplying the low concentration carbon dioxide gas at a volumetric flow rate of 5 V to 20 V for 0.1 to 1 hour from the start of supply;
a later supply step of supplying the low-concentration carbon dioxide gas at a volumetric flow rate of 8 V to 40 V after the low-concentration carbon dioxide gas fills the pit;
a middle-stage supply step of supplying the low-concentration carbon dioxide gas at a volumetric flow rate of 80 V to 320 V between the initial supply step and the later supply step,
3. The method for producing neutral regenerated soil using low-concentration carbon dioxide gas according to claim 1 or 2, characterized in that the total supply time of the low-concentration carbon dioxide gas is 4 to 24 hours. The method for producing neutral reclaimed soil using low-concentration carbon dioxide gas according to any one of claims 1 to 3, characterized in that the modified solidified treated soil exhibits a strong alkalinity with a pH of 11 or more.