JP7793433B2 - Ground improvement material and ground improvement method using the same - Google Patents

Description

The present invention relates to a ground improvement material and a ground improvement method using the same.

A known method for improving ground is to supply a cement-based solidifying material to the ground and then stir and mix it to increase the strength of the ground (for example, uniaxial compressive strength).
On the other hand, cement clinker, which is the main raw material of cement, is produced by burning various raw materials such as limestone, clay, silica stone, and iron raw materials at high temperatures using a rotary kiln, etc. In cement clinker produced by such a method, chromium, which is inevitably contained in the raw materials, may exist in the form of harmful hexavalent chromium.
Usually, hexavalent chromium contained in cement is fixed as cement hydrate during the hardening process of a cement composition (e.g., concrete) containing the cement, and therefore, hexavalent chromium is not eluted. However, when cement is used as a ground improvement material, depending on the soil to be improved, sufficient cement hydrate for fixing hexavalent chromium is not produced, resulting in the problem of elution of hexavalent chromium from the improved ground.

Conventionally, methods for solving this problem have been proposed.
For example, Patent Document 1 describes a method for preventing the elution of hexavalent chromium from a solidified material containing hexavalent chromium using cement, characterized in that the cement is adjusted to have a total amount of C ₃ S and C ₃ A of 70% by weight or more, and that 25 to 80 parts by weight of slag is blended with 100 parts by weight of cement, followed by solidification.
Furthermore, Patent Document 2 describes a ground improvement material containing a hydraulic material, blast furnace slag, and gypsum, which is characterized in that the elution of hexavalent chromium is reduced by the action of the blast furnace slag.

Japanese Patent Application Laid-Open No. 2000-308863 Japanese Patent Application Laid-Open No. 2001-348571

The object of the present invention is to provide a ground improvement material that can suppress the amount of hexavalent chromium leaching from improved ground and has excellent strength development, as well as a ground improvement method using the same.

As a result of extensive research to solve the above problems, the present inventors have found that the above object can be achieved by using a ground improvement material containing a cement-based solidification material and molybdenum, and have completed the present invention.
That is, the present invention provides the following [1] to [6].
[1] A ground improvement material comprising a cement-based solidification material and molybdenum.
[2] The soil improvement material according to [1], wherein the amount of molybdenum is 0.01 to 5.0 parts by mass per 100 parts by mass of the cement-based solidification material.
[3] The cement-based solidification material contains cement and gypsum, and the proportion of the gypsum in the cement-based solidification material is 2 to 20 mass% in terms of anhydride. [1] or [2]. The ground improvement material according to.
[4] The ground improvement material according to any one of [1] to [3], wherein the molybdenum is in powder form and has a median diameter (D50) of 5.00 μm or less.
[5] The cement-based solidification material contains ground granulated blast furnace slag, and the proportion of the ground granulated blast furnace slag in the cement-based solidification material is 5 to 40% by mass. [1] - [4] The ground improvement material according to any one of [4] to [4].
[6] A method for improving ground using the ground improvement material according to any one of [1] to [5], wherein the ground improvement material is supplied to the ground, stirred and mixed, and improved ground is obtained.

The soil improvement material and soil improvement method using the same of the present invention can reduce the amount of hexavalent chromium leaching from improved soil, thereby increasing the strength (e.g., uniaxial compressive strength) of the improved soil.

The soil improvement material of the present invention contains a cement-based solidification material and molybdenum.
The cement-based solidification material used in the present invention refers to a powdery material that contains cement as the main ingredient (usually 40% by mass or more, preferably 50% by mass or more, more preferably 55% by mass or more, even more preferably 60% by mass or more, even more preferably 70% by mass or more, and particularly preferably 80% by mass or more) and also contains admixtures that can be optionally blended.
Examples of cements used in cement-based solidification materials include various types of Portland cement such as ordinary Portland cement, high-early-strength Portland cement, moderate-heat Portland cement, low-heat Portland cement, and sulfate-resistant Portland cement; blended cements such as blast-furnace cement, fly ash cement, and silica cement; ecocement; white cement; and ultra-rapid-hardening cement.
Among these, various types of Portland cement are preferred from the viewpoint of ease of availability.
From the viewpoint of improving strength development, the proportion of cement in the ground improvement material is preferably 40% by mass or more, more preferably 50% by mass or more, even more preferably 55% by mass or more, even more preferably 60% by mass or more, even more preferably 70% by mass or more, and particularly preferably 80% by mass or more.

The cement-based solidification material may contain an admixture. Examples of the admixture include ground granulated blast furnace slag, gypsum, quicklime, slaked lime, fly ash, ground limestone, and silica fume. These may be used alone or in combination of two or more.
In particular, it is preferable that the cement-based solidification material contains ground granulated blast furnace slag as a cement admixture, from the viewpoint that the amount of hexavalent chromium leaching can be sufficiently suppressed even if the amount of molybdenum is reduced in order to reduce the cost of molybdenum.
When the cement-based solidification material contains ground granulated blast furnace slag, the proportion of ground granulated blast furnace slag in the cement-based solidification material is preferably 5 to 40% by mass, more preferably 6 to 30% by mass, even more preferably 8 to 25% by mass, and particularly preferably 10 to 15% by mass. If the proportion is 5% by mass or more, the amount of molybdenum can be reduced while the amount of hexavalent chromium elution is sufficiently suppressed. If the proportion is 30% by mass or less, the amount of cement is relatively large, thereby making it possible to further increase the strength (e.g., uniaxial compressive strength) of the improved soil.
In addition, when the cement contained in the cement-based solidification material is blast furnace cement, the proportion of the ground granulated blast furnace slag includes the ground granulated blast furnace slag contained in the blast furnace cement.

Furthermore, from the viewpoint of improving strength development, it is preferable that the cement-based solidification material contains additionally added gypsum (hereinafter referred to as "additional gypsum") in addition to the gypsum contained in the cement.
The content of the additional gypsum in the cement-based solidifying material is preferably 2 to 20 mass %, more preferably 3 to 18 mass %, even more preferably 4 to 16 mass %, and particularly preferably 8 to 12 mass %, calculated on an anhydrous basis.
Examples of the additional gypsum include anhydrous gypsum, hemihydrate gypsum, dihydrate gypsum, or mixtures thereof.
The proportion of admixture in the cement-based solidification material (the total proportion when two or more types of admixtures are included) is preferably 60 mass% or less, more preferably 50 mass% or less, even more preferably 40 mass% or less, even more preferably 30 mass% or less, and particularly preferably 20 mass% or less, from the viewpoint of relatively increasing the proportion of cement and improving strength development.

From the viewpoint of further suppressing the amount of hexavalent chromium leaching, the amount of molybdenum per 100 parts by mass of cement-based solidification material is preferably 0.01 parts by mass or more, more preferably 0.05 parts by mass or more, even more preferably 0.3 parts by mass or more, even more preferably 0.8 parts by mass or more, and particularly preferably 1.2 parts by mass or more. Furthermore, from the viewpoint of preventing excessive increases in costs, the above amount is preferably 5.0 parts by mass or less, more preferably 4.0 parts by mass or less, even more preferably 3.0 parts by mass or less, and particularly preferably 2.5 parts by mass or less.

Molybdenum is preferably in powder form, from the viewpoint that the molybdenum can be uniformly mixed into the improved ground when the soil improvement material is supplied to the ground and stirred and mixed, and the elution of hexavalent chromium can be further suppressed.
The median diameter (D50) of the powdered molybdenum is preferably 5.00 μm or less, more preferably 3.00 μm or less, and particularly preferably 2.20 μm or less. If the median diameter (D50) is 5.00 μm or less, the elution of hexavalent chromium can be further suppressed. From the viewpoint of the ease of obtaining powdered molybdenum, the median diameter is preferably 1.00 μm or more, more preferably 1.50 μm or more.
In this specification, the term "median diameter (D50)" refers to a specific particle size when, when a powder is divided into particles having particle sizes smaller than a specific particle size (an aggregate of small particle sizes) and particles having particle sizes larger than the specific particle size (an aggregate of large particle sizes), the aggregate of small particle sizes and the aggregate of large particle sizes are in equal amounts (50% by mass each).
The median diameter (D50) was measured using a laser diffraction scattering particle size distribution analyzer or "JIS
It can be obtained by creating a mass cumulative distribution using a sieving method in accordance with "JIS Z 8815-1994 (General rules for sieving test methods)."

According to the soil improvement material of the present invention, the strength of the ground (for example, uniaxial compressive strength) can be increased, and the amount of hexavalent chromium eluted from the ground can be suppressed.
One example of a method for improving ground using the soil improvement material is a method in which the soil improvement material is supplied to the ground to be improved, stirred and mixed, and improved ground is obtained.
Examples of methods for supplying and stirring and mixing a ground improvement material to the ground include a dry addition method in which the ground improvement material is supplied in powder form to the ground and stirred and mixed, and a slurry addition method in which water is added to the ground improvement material to make a slurry, and then the slurry is supplied to the ground and stirred and mixed.
The ground to be improved is not particularly limited, but examples include volcanic ash clay, sandy soil, clay, and organic soil (for example, humus soil with an organic matter content of 5% by mass or more).

The amount of soil improvement material to be supplied per 1 ^m3 of ground to be improved varies depending on the properties of the ground, the amount of hexavalent chromium contained in the ground, the construction conditions, and the strength required of the improved ground obtained after treatment, but is preferably 50 to 450 kg, more preferably 100 to 400 kg, and particularly preferably 150 to 380 kg per 1 ^m3 of ground to be improved.
When the amount is 50 kg or more, the strength of the improved ground can be increased and the elution of hexavalent chromium can be further suppressed, and when the amount is 450 kg or less, an excessive increase in costs can be prevented.

The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.
[Materials used]
(1) Cement: ordinary Portland cement manufactured by Taiheiyo Cement Corporation. (2) Gypsum: anhydrous gypsum. (3) Ground granulated blast furnace slag (shown as "blast furnace slag" in Table 2): manufactured by DC Corporation under the trade name "Cerament."
(4) Molybdenum powder A; A.L.M.T. Corporation, reagent, purity: 99% by mass, median diameter: 1.99 μm
(5) Molybdenum Powder B; manufactured by Kanto Chemical Co., Ltd., reagent, purity: 98% by mass, median diameter: 2.50 μm
(6) Soil A: Volcanic ash clayey soil (loam)
(7) Soil B: Sandy soil

[Examples 1 to 3, Comparative Examples 1 to 4]
A cement-based solidification material containing cement, blast furnace slag powder, and anhydrous gypsum in the proportions shown in Table 1 was mixed with molybdenum powder A in the amount shown in Table 1 to obtain a ground improvement material.
The soil improvement material was mixed with soil A (volcanic ash clayey soil) in an amount of 200 kg per 1 ^m3 of soil to obtain improved soil. The amount added was 200 kg, which is lower than the general amount (usually 350 kg/ ^m3 ), in order to make it easier to confirm the tendency of the amount of hexavalent chromium leaching.
The unconfined compressive strength of the obtained ground improvement soil at ages of 7 days and 28 days was measured in accordance with "JIS A 1216:2020 (Unconfined compression test method for soil)".
In addition, the soil improved after the unconfined compressive strength measurement was subjected to a hexavalent chromium elution test in accordance with Environment Agency Notification No. 46, and the amount of hexavalent chromium eluted from the soil improved after 7 days and 28 days of age was measured in accordance with "JIS K 0102:2016 (Factory wastewater testing method)." The results are shown in Table 1.

[Example 4, Comparative Example 5]
The proportions of each material contained in the cement-based solidification material were as shown in Table 2, and molybdenum powder A was mixed in the amount shown in Table 2. Soil B was used instead of soil A, and the same procedure as in Example 1 was used to obtain ground improvement soil.
The unconfined compressive strength and other properties of the improved soil were measured in the same manner as in Example 1. The results are shown in Table 2.

[Example 5]
The proportions of each material contained in the cement-based solidification material were as shown in Table 3, molybdenum powder was mixed in the types and amounts shown in Table 3, and soil B was used instead of soil A. The same procedure as in Example 1 was used to obtain ground improvement soil.
The unconfined compressive strength and other properties of the obtained improved soil were measured in the same manner as in Example 1.
[Example 6]
A soil improved by this method was obtained in the same manner as in Example 5, except that molybdenum powder B was used instead of molybdenum powder A.
The unconfined compressive strength and other properties of the obtained improved soil were measured in the same manner as in Example 1.
The results are shown in Table 3.

Comparing Examples 1 to 3 and Comparative Examples 1 to 4 from Table 1, it can be seen that the amount of hexavalent chromium eluted when a molybdenum-containing soil improvement material was used (Examples 1 to 3) (7 days: 0.04 to 0.05 mg/L, 28 days: 0.01 to 0.03 mg/L) is smaller than the amount of hexavalent chromium eluted when a molybdenum-free soil improvement material was used (Comparative Examples 1 to 4) (7 days: 0.05 to 0.23 mg/L, 28 days: 0.04 to 0.16 mg/L). This shows that the amount of hexavalent chromium eluted can be reduced when a molybdenum-containing soil improvement material is used.
A similar tendency was also observed in the comparison between Example 4 and Comparative Example 5 in Table 2 (targeting sandy soil).
Furthermore, from Table 3, a comparison of Example 5 (in which the molybdenum powder had a median diameter of 1.99 μmm) and Example 6 (in which the molybdenum powder had a median diameter of 2.50 μmm) reveals that the amount of hexavalent chromium eluted from Example 5 at a material age of 7 days (0.03 mg/liter) is smaller than the amount of hexavalent chromium eluted from Example 6 at a material age of 7 days (0.04 mg/liter).

Claims (5)

Contains cement-based solidification material and molybdenum,
The ground improvement material is characterized in that the molybdenum is in powder form and has a median diameter (D50) of 5.00 μm or less . The ground improvement material according to claim 1, wherein the amount of molybdenum is 0.01 to 5.0 parts by mass per 100 parts by mass of the cement-based solidification material. The ground improvement material according to claim 1 or 2, wherein the cement-based solidification material contains cement and gypsum, and the proportion of the gypsum in the cement-based solidification material is 2 to 20 mass% on an anhydrous basis. The ground improvement material according to any one of claims 1 to 3 , wherein the cement-based solidification material contains ground granulated blast furnace slag, and the proportion of the ground granulated blast furnace slag in the cement-based solidification material is 5 to 40 mass%. A method for improving ground using the ground improvement material according to any one of claims 1 to 4 ,
A ground improvement method characterized by supplying the above ground improvement material to the ground, stirring and mixing it, and obtaining improved ground.