WO2020202258A1 - Soil improvement cement composition, soil improvement construction method and soil improvement body - Google Patents

Abstract

The present invention is a soil improvement cement composition at least containing Portland cement and fly ash that satisfies all of conditions (F1)-(F5) below, wherein the fly ash content is 10-40 mass% when the total of the Portland cement and the fly ash is taken to be 100 mass%.　(F1) The Blaine specific surface area of the fly ash is 2,500-6,000 cm²/g. (F2) The mass loss of the fly ash after the fly ash is heated for 15 minutes at 975±25°C is 5 mass% or less. (F3) The SiO₂ content in the fly ash is 50 mass% or more. (F4) The sphere-equivalent specific surface area of particles in which iron oxide and amorphous material are present in mixture in the fly ash is 2,800-11,000 cm²/cm³. (F5) The sphere-equivalent specific surface area of amorphous particles containing Ca (calcium) in the fly ash is 2,100-22,500 cm²/cm³.

Description

Cement composition for ground improvement, ground improvement method, and ground improvement body

The present invention relates to a cement composition for ground improvement having high workability and strength development even in hot weather and areas, a ground improvement method using the cement composition, and a ground improvement body. In addition, in this specification, "high temperature" means that the annual average temperature is 25 degreeC or more.

Cement-based ground improvement materials are often used for improving the foundation ground of structures, constructing temporary ground, taking measures against liquefaction, improving soil generated from construction and construction sludge, and solidifying contaminated soil. This cement-based ground improvement material can be imparted with various functions by adding various auxiliary materials to Portland cement, which is the main material, depending on the target soil and the purpose of improvement.
In recent years, from the viewpoint of effective utilization of industrial waste, ground improvement materials utilizing a large amount of by-products emitted from the steel industry and the electric power industry have been developed. For example, in Patent Documents 1 and 2, coal ash having a specific brightness obtained by crushing coal ash containing a large amount of unburned carbon and a cement composition containing cement are used for ground improvement of cohesive soil. Coal ash, a method for producing coal ash, and a cement composition that can improve the uniformity at the time of improvement (mixing) are described.

Japanese Unexamined Patent Publication No. 2018-12226 Japanese Unexamined Patent Publication No. 2018-12229

By the way, when ground improvement is carried out in hot weather or areas, the retention time of fluidity of the mixture of ground improvement material and water (hereinafter referred to as "ground improvement material slurry") becomes short, so if the ground improvement work is delayed. In some cases, the viscosity of the ground improvement material slurry becomes too high and construction cannot be performed. In this case, the ground improvement material slurry has to be discarded, but this is a problem from the viewpoint of further delaying the construction and promoting the effective utilization of the waste.
If the fluidity retention time of the ground improvement material slurry can be lengthened, workability can be improved, material loss in ground improvement work can be reduced, and effective utilization of waste (by-products) such as coal ash can be promoted. ..
Therefore, an object of the present invention is to provide a cement composition for ground improvement, which can hold the fluidity of the ground improvement material slurry for a longer time even in a high temperature weather or region, and has high strength development. ..

Therefore, the present inventor has diligently studied a cement composition for ground improvement capable of achieving the above object, and found that a cement composition for ground improvement containing fly ash satisfying a specific condition can achieve the above object. Was completed.
That is, the present invention is a cement composition for ground improvement having the following constitution.

[1] Contains at least fly ash and Portland cement that satisfy all of the following conditions (F1) to (F5).
A cement composition for ground improvement, wherein the total of the fly ash and Portland cement is 100% by mass, and the content of the fly ash is 10 to 40% by mass.
(F1) Fly ash has a specific surface area of 2500 to 6000 cm ² / g.
(F2) The mass reduction rate of fly ash after heating the fly ash at 975 ± 25 ° C. for 15 minutes is 5% by mass or less (F3) The content of SiO _{2 in} the fly ash is 50% by mass or more (F4) Fly The sphere-equivalent specific surface area of particles in which iron oxide and amorphous are mixed in ash is 2800 to 11000 cm ² / cm ³
(F5) The spherical specific surface area of amorphous particles containing Ca (calcium) in fly ash is 2100 to 22500 cm ² / cm ³
[2] The cement composition for ground improvement according to the above [1], wherein the fly ash further satisfies the following condition (F6).
(F6) The sphere-equivalent specific surface area of particles in which mullite and amorphous particles are mixed in fly ash is 1900 to 9500 cm ² / cm ³
[3] The cement composition for ground improvement according to the above [1] or [2], wherein the fly ash further satisfies the following conditions (F7).
(F7) The sphere-equivalent specific surface area of Ca-free amorphous particles in fly ash is 2100 to 9000 cm ² / cm ³
[4] A cement composition for ground improvement containing one or more types of gypsum selected from anhydrous gypsum, hemihydrate gypsum, and dihydrate gypsum.
Portland cement, fly ash, and the sum of the gypsum as 100 mass%, the content of the gypsum is not more than 6.0 mass% converted to SO _3, as described in any one of [1] to [3] Cement composition for ground improvement.
[5] A ground improvement method for improving the ground by mixing soil with a ground improvement material slurry containing at least the ground improvement cement composition according to any one of [1] to [4] and water.
[6] A ground improvement body obtained by mixing a cohesive soil satisfying all of the following conditions (S1) to (S3) with the ground improvement material slurry according to the above [5].
(S1) Moisture content ratio is 70 to 150%
(S2) The allophane content measured by the acidic oxalate extraction method is 5.0% or more. (S3) In the cohesive soil when a slurry containing 150 mg of Ca (OH) ₂ and 100 ml of water is mixed. Adsorption amount of Ca (OH) ₂ is 80 mg or more per 1 g of cohesive soil

The cement composition for ground improvement of the present invention can prolong the retention time of fluidity of the ground improvement material slurry even in hot weather and regions, and has high strength development. Further, according to the ground improvement method of the present invention, workability can be improved, material loss can be reduced, and effective utilization of waste can be promoted. Further, the ground improvement body of the present invention has high strength.

As described above, the present invention is a cement composition for ground improvement, a ground improvement method, and a ground improvement body.
First, the cement composition for ground improvement of the present invention will be described.
1. 1. Ground improvement cement composition The cement composition contains at least fly ash and Portland cement satisfying the above specific conditions, and the content of the fly ash is 10 to 40 with the total of fly ash and Portland cement as 100% by mass. It is mass%. If the content of fly ash is less than 10% by mass, the workability of the ground improvement material slurry is lowered, and if it exceeds 40% by mass, the strength development of the ground improving cement composition is lowered. The content of the fly ash is preferably 12 to 35% by mass, more preferably 14 to 30% by mass.

(I) Portland cement The Portland cement used in the present invention is ordinary Portland cement, early-strength Portland cement, moderate heat Portland cement, and moderate heat Portland cement specified in R 5210 "Portland cement" of Japanese Industrial Standards (hereinafter referred to as "JIS"). One or more selected from low heat Portland cement can be mentioned. Among these, the Portland cement is preferably ordinary Portland cement and / or early-strength Portland cement because the strength development of the ground improvement cement composition is improved.

(Ii) Fly ash The fly ash used in the present invention is a fly ash that satisfies all of the following conditions (F1) to (F5).
(F1) Fly ash has a specific surface area of 2500 to 6000 cm ² / g.
(F2) The mass reduction rate of fly ash after heating the fly ash at 975 ± 25 ° C. for 15 minutes is 5% by mass or less (F3) The content of SiO _{2 in} the fly ash is 50% by mass or more (F4) Fly The sphere-equivalent specific surface area of particles in which iron oxide and amorphous are mixed in ash is 2800 to 11000 cm ² / cm ³
(F5) Sphere-equivalent specific surface area of amorphous particles containing Ca in fly ash is 2100 to 22500 cm ² / cm ³

Next, the conditions of the fly ash of (F1) to (F5) will be described in detail.
(F1): The Blaine specific surface area of 2500cm less than ² / g of the fly ash, and reduced strength development of soil improvement cement composition exceeds 6000 cm ² / g, the viscosity of the soil improvement material slurry is increased In addition, it is difficult to obtain fly ash with a brain specific surface area of more than 6000 cm ² / g. The specific surface area of the brain is preferably 2700 to 5000 cm ² / g, and more preferably 2900 to 4000 cm ² / g.

(F2): When the mass reduction rate of the fly ash exceeds 5% by mass after heating the fly ash at 975 ± 25 ° C. for 15 minutes, the strength development of the ground improvement cement composition decreases. The mass reduction rate is preferably 4.3% by mass or less, more preferably 3.5% by mass or less, from the viewpoint of easy availability and strength development.

(F3): When the content of SiO _{2 in} the fly ash is less than 50% by mass, the strength development of the ground improvement cement composition is lowered. The content of SiO ₂ is preferably 52 to 70% by mass, more preferably 54 to 65% by mass, from the viewpoint of easy availability and strength development.

(F4): When the sphere-equivalent specific surface area of the particles in which iron oxide and amorphous particles are mixed in the fly ash is outside the range of 2800 to 11000 cm ² / cm ³ , the workability of the ground improvement material slurry and the cement for ground improvement The strength development of the composition may decrease. The sphere-equivalent specific surface area of the particles in which iron oxide and amorphous are mixed is preferably 4000 to 10000 cm ² / cm ³ , and more preferably 5000 to 9000 cm ² / cm ³ .

(F5): When the spherical equivalent specific surface area of the amorphous particles containing Ca in the fly ash is outside the range of 2100 to 22500 cm ² / cm ³ , the workability of the ground improvement material slurry and the cement composition for ground improvement The strength development may decrease. The spherical specific surface area of the amorphous particles containing Ca is preferably 4000 to 20000 cm ² / cm ³ , more preferably 7000 to 18500 cm ² / cm ³ .

In addition, in order to improve the workability of the ground improvement material slurry, especially the fluidity and the strength development of the ground improvement cement composition, the sphere conversion ratio of the particles in which mullite and amorphous are mixed in the fly ash. The surface area is preferably 1900 to 9500 cm ² / cm ³ , more preferably 3000 to 9000 cm ² / cm ³ , and even more preferably 4500 to 9000 cm ² / cm ³ . Also. For the same reason, the spherical specific surface area of Ca-free amorphous particles in fly ash is preferably 2100 to 9000 cm ² / cm ³ , more preferably 3000 to 8500 cm ² / cm ³ , and even more preferably 4500 to 8500 cm. it is a ² / cm ^3.

The fly ash usually contains 5 to 25% by mass of quartz, and the lattice volume of quartz in the fly ash used in the present invention is a value obtained by using the Rietveld analysis method, preferably 113. It is 5 to 114.5 Å ³ . When the lattice volume of quartz is within the above range, the fluidity of the ground improvement material slurry and the strength development of the ground improvement cement composition are further improved. The lattice volume of quartz is more preferably 113.6 to 114.4 Å ³ , and even more preferably 113.7 to 114.3 Å ³ .
The Rietveld analysis of quartz in fly ash is based on the X-ray diffraction pattern of fly ash, for example, Bruker's analysis software (Topas ver. 2.1) and 331161 (Quartz) as crystal structure data (ICDD number). Can be done using.
Further, the spherical specific surface area of the quartz particles in the fly ash is preferably 1100 to 12500 cm ² / cm ³ , more preferably 2500 to 10000 cm ² / cm ³ , and further preferably 4000, because the strength development and the like are improved. It is ~ 10000 cm ² / cm ³ .

The fly ash particles undergo the following steps (1) to (4) to obtain (i) particles in which iron oxide and amorphous are mixed, (ii) particles in which mulite and amorphous are mixed, and (iii) Ca. It is classified into five types: amorphous particles that do not contain, (iv) amorphous particles that contain Ca, and (v) quartz particles.
(1) Sample Preparation Step This step is a step of mixing fly ash and resin to prepare a cured test piece. By dispersing the fly ash in the resin, the fly ash particles do not overlap, and each particle can be accurately extracted and its characteristic value can be measured at the time of particle analysis described later.
The resin may be any resin that shrinks less in the curing process and does not crack, and examples thereof include epoxy resins, acrylic resins, polyester resins, and methacrylic resins. The mixing ratio of the resin is preferably 0.8 to 4 in terms of volume ratio with respect to fly ash. When the mixing ratio of the resin is within this range, a plurality of particles are dispersed without contacting each other, and polishing described later can be performed to obtain cut surfaces of many particles.

Next, the imaging surface of the cured test piece is polished. If the image plane is uneven or the cut surface of the particles does not appear sufficiently, the particle size and shape of the particles cannot be measured accurately, and the accuracy of the particle analysis described later is lowered.
The polishing device for the imaging surface of the test piece is not particularly limited, and a commonly used polishing device can be used. The abrasive used in the polishing step is not particularly limited, and examples thereof include a silicon carbide abrasive, a boron carbide abrasive, a diamond paste, and an alumina powder. Further, the polishing is preferably buffing using alumina powder having a diameter of 0.3 to 3 μm or the like as an abrasive, and since there are few irregularities on the image surface, a cross section using an argon ion beam is more preferable. Polishing with a polisher.

Finally, a thin-film deposition film is formed on the surface of the test piece whose imaging surface has been polished to impart conductivity to the test piece. In the particle analysis described later, the test piece is irradiated with an electron beam, but since fly ash and resin do not have conductivity, the surface of the test piece is charged when a reflected electron image is obtained without forming a vapor deposition film on the test piece. However, an accurate reflected electron image cannot be obtained. Therefore, in order to obtain an accurate reflected electron image, a thin-film film having conductivity is formed on the surface of the test piece.
The vapor-deposited film is not particularly limited as long as it can impart conductivity to the surface of the test piece, and examples thereof include a thin-film film of carbon, platinum-palladium, gold, or the like. Further, the method for forming the thin-film deposition film is not particularly limited, and a known method can be used.

(2) Fly ash particle analysis step The step is a step of first obtaining a reflected electron image (BSE) and a chemical composition of the test piece prepared in the sample preparation step using an electron microscope. Since the electron microscope only needs to be able to measure the reflected electron image and the chemical composition of a minute region, a scanning electron microscope (SEM), an electron probe microanalyzer (EPMA), or the like can be used. The backscattered electron image is displayed brighter as the average atomic number of the elements constituting the region is larger. Examples of the chemical composition acquisition device include a wavelength dispersive X-ray spectroscope (WDS) and an energy dispersive X-ray spectroscope (EDS), but energy dispersive X-rays are preferable because the chemical composition can be acquired in a short time. It is a spectroscope (EDS).
In order to obtain a reflected electron image having a high resolution, it is preferable to set the acceleration voltage to about 10 to 15 keV, the irradiation current to about 200 to 1000 pA, and the observation magnification to about 500 to 2000 times.

In performing the particle analysis of fly ash, the reflected electron image is obtained from the test piece of fly ash, and the fly ash particles and the reflected electron image of the resin are compared with each other visually and the luminance histogram is referred to. Determine the threshold of brightness that can separate the resin from. Then, using the threshold value, binarization processing is performed to extract fly ash particles. For the extracted fly ash particles, a geometric measurement value is measured for each particle. Examples of this geometric measurement value include a circularity coefficient, a circle-equivalent diameter (diameter of a circle having an area equal to the cross-sectional area of the particle), an aspect ratio, and the like.

(3) Chemical analysis step of fly ash particles The step is a step of chemically analyzing fly ash particles to grasp the chemical composition of fly ash. In order to quickly obtain the chemical composition of fly ash with high accuracy using an energy-dispersed X-ray spectrometer, the accelerating voltage is about 10 to 15 keV, the irradiation current is about 200 to 1000 pA, and the analysis time is one analysis. It is recommended to set it to 5 to 10 seconds per point. The diameter of the analysis area is preferably the entire individual particles.
The order of the chemical analysis and the measurement of the geometrically measured value does not matter. The number of fly ash particles to be measured is preferably 1000 or more, more preferably 2000 or more, in order to reduce the measurement error between the chemical analysis and the geometric measurement value. The number of X-ray counts per particle is preferably 5000 counts or more, more preferably 10,000 counts or more, and further preferably 100,000 counts or more. For the convenience of image processing in particle analysis, the divided particles at the edge of the image are joined together in the analysis and counted as one particle. In particle analysis, the area and chemical composition of particles are acquired as characters for each particle.

(4) Classification step of fly ash particles In this step, (i) iron oxide and amorphous are mixed using the above chemical composition of fly ash particles and the threshold of the chemical composition of fly ash particles shown in Table 1. Any of the following particles: (ii) a mixture of mulite and amorphous particles, (iii) Ca-free amorphous particles, (iv) Ca-containing amorphous particles, and (v) quartz particles. This is the process of classifying into classes. The extraction of fly ash particles, chemical analysis, and calculation of geometric measurement values for classification can be performed automatically by using the particle analysis software attached to the electron microscope, which is convenient.

The sphere-equivalent specific surface area is calculated according to the following.
First, for each fly ash particle classified into each of the above classes, assuming that all the particles are spheres, the equivalent circle diameter D is calculated from the cross-sectional area S of the particles using the following equation (1).

Next, the surface area A and volume V of the particles when the particles are assumed to be spheres are calculated from the calculated equivalent circle diameters using the equations (2) and (3).

Finally, to calculate the sum of the total surface area and the volume of fly ash particles in each class to calculate the spherical equivalent specific surface area S _A using (4).

In order to improve the workability of the ground improvement material slurry and the strength development of the ground improvement cement composition, (Na ₂ O + 0.658 × K ₂ O) / (MgO + SO ₃ + TiO ₂ + P ₂ ) in the fly ash. The mass ratio of O ₅ + MnO) is preferably 0.2 to 1.0, more preferably 0.25 to 0.8, still more preferably 0.28 to 0.7, and particularly preferably 0.3 to 0. It is 6.
Further, in order to improve the workability of the ground improvement material slurry and the strength development of the ground improvement cement composition, the compaction density of the fly ash calculated by measuring by the following method is preferably 1.0. ~ 1.5cm ³ / g, more preferably 1.05 ~ 1.45cm ³ / g, more preferably from 1.1 ~ 1.4cm ³ / g.
[Measurement method of compaction density]
Using a powder tester PT-D manufactured by Hosokawa Micron, the fly ash is filled in a 100 cm ³ cup, the cup is tapped 180 times in 180 seconds, and then the mass of the fly ash compacted in the cup is measured. Measure and calculate the compaction density. The volume (denominator) used in the density calculation formula is the volume of the cup.

Further, the cement composition for ground improvement of the present invention further contains one or more gypsum selected from anhydrous gypsum, hemihydrate gypsum, and dihydrate gypsum in order to improve the strength development of the cement composition for ground improvement. It may be. The content of gypsum, gypsum, fly ash, and the sum of the Portland cement as 100 mass%, or less 6.0 wt% converted to SO _3. When the content of gypsum exceeds 6.0% by mass in terms of SO ₃ , the fluidity of the ground improvement material slurry decreases. Incidentally, the content of the gypsum is converted to SO _3, more preferably 0.3 to 5.0 wt%, more preferably 0.6 to 4.0 wt% or less, particularly preferably 1.0-3.0 It is mass%.
The gypsum is preferably anhydrous gypsum or dihydrate gypsum in order to improve the strength development of the ground improvement cement composition. The specific surface area of the gypsum brain is preferably 3000 to 15000 cm ² / g, more preferably 3500 to 13000 cm ² / g in order to improve the strength development.
The cement composition for ground improvement of the present invention may further contain one or more inorganic powders selected from blast furnace slag powder, limestone powder, and silica stone powder as arbitrary constituent components. Examples of the mixing device for the cement composition for ground improvement include a mixer, a mixer, and a blender generally used for mixing powders.

2. Ground improvement method Next, the ground improvement method of the present invention will be described.
The construction method is a construction method for improving the ground by mixing the soil with the ground improvement material slurry containing at least the ground improvement cement composition and water.
Tap water or the like can be used as the water, and the blending amount of the water is preferably 65 to 120 parts by mass, more preferably 70 to 110 parts by mass, and further preferably 75 parts by mass with respect to 100 parts by mass of the ground improvement cement composition. It is up to 100 parts by mass, particularly preferably 75 to 90 parts by mass. Even after the ground improvement material slurry is stored for a relatively long time with a water content of 65 parts by mass or more, the slurry and soil can be mixed uniformly, and if it exceeds 120 parts by mass, it is sufficient to mix with soil having a high water content. The ground improvement effect may not be obtained.
Further, when the air temperature is 30 ° C. or higher, a water reducing agent or a retarding agent may be used in combination in order to secure the fluidity of the ground improving material slurry. Examples of the water reducing agent include a water reducing agent, an AE water reducing agent, and a high-performance AE water reducing agent, which are generally used for cement and concrete. Examples of the retarding agent include organic acids such as oxycarboxylic acid, ketocarboxylic acid, aminocarboxylic acid, and high molecular weight organic acid, salts thereof, sugars, and sugar alcohols.
For kneading the ground improvement material slurry, a mixer or a hand mixer generally used for kneading the slurry can be used.

3. 3. Ground improvement body The ground improvement body of the present invention is an improved soil obtained by mixing the cohesive soil satisfying all the conditions (S1) to (S3) and the ground improvement material slurry.
If the ground improvement material slurry is applied to the cohesive soil satisfying all the conditions (S1) to (S3) to improve the ground, a ground improvement body having high strength can be obtained.

Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited to these Examples.
1. 1. Materials used The various materials used in the test are shown below.
(1) Ordinary Portland cement Brain The specific surface area is 3310 cm ² / g and is manufactured by Taiheiyo Cement.
(2) Anhydrous gypsum brain The specific surface area is 8250 cm ² / g, manufactured by Nacord.
(3) Water Tap water.
The characteristics of the fly ash (FA) used in the test are shown in Tables 2 and 3. It should be noted that FA1 to 5 satisfy all of the above conditions (F1) to (F5), but FA6 to 8 do not satisfy any of these conditions. Further, FA1 does not satisfy the condition of (F6) and FA2 does not satisfy the case of (F7), but FA3 and FA4 satisfy both the conditions of (F6) and (F7). Further, the spherical specific surface area of the quartz particles in FA5 is out of the range of 1100 to 12500 cm ² / cm ³ .

2. Production of Cement Composition for Ground Improvement Fly ash (FA), cement (C), and anhydrous gypsum (G), which is an optional component, were mixed according to the formulation shown in Table 4 to produce a cement composition.

By the way, in Table 3,
(I) In Examples 1 to 7, the content of fly ash is 10 to 40% by mass, where the total of fly ash FA1 to 5 and Portland cement satisfying all the conditions (F1) to (F5) is 100% by mass. Is the cement composition of the present invention within the range of.
(Ii) Examples 8 to 10 are the cement compositions of the present invention in which anhydrous gypsum is further mixed so that the SO ₃ conversion is 6% by mass or less.
(Iii) Comparative Examples 1 to 4 are cement compositions containing no fly ash.
(IV) Comparative Examples 5 to 7 are cement compositions containing FA6 to 8 which do not satisfy any of the above conditions (F1) to (F5).
(V) Comparative Examples 8 and 9 are cement compositions in which the content of fly ash is out of the range of 10 to 40% by mass.

3. 3. Measurement of pot life of ground improvement material slurry When the environmental temperature (ambient temperature) is 27 ° C, 2 kg of the ground improvement cement composition (powder) and 1.5 kg of water shown in Table 2 are put into a 5 L plastic cup (water). The powder ratio was 75%), and the mixture was kneaded for 2 minutes using a hand mixer to prepare a ground improvement material slurry, and then the P funnel flow time of the slurry was measured twice to determine the average value of these.
Next, the ground improvement material slurry was returned to a 5 L plastic cup, sealed with a wrap film to prevent the dissipation of water, and allowed to stand. When 2 hours and 3 hours have passed after standing, the ground improvement material slurry is similarly kneaded with a hand mixer, and the P funnel flow time is measured again in the same manner to determine the time course of the flow time immediately after the kneading. It was. Based on the description in Document A below, it was determined that the P-rohto flow time of 12 seconds or more after 3 hours had passed immediately after the kneading was rejected.
Reference A: "Engineering Properties of Improved Soil by FGC Deep Mixing Method Using Coal Ash and Future Prospects", Proceedings of the 6th Coal Utilization Technology Conference, pp145-157, 1996

4. Measurement of Strength of Ground Improvement Material 10 kg of cement composition for ground improvement and 7.5 kg of water were put into a toslon can (water powder ratio was 75%) and kneaded with a hand mixer to prepare a ground improvement material slurry.
Next, the slurry, a volcanic ash cohesive soil (Kanto loam) having a water content of 75.1%, an allophane content of 18.3% measured by an acidic oxalate extraction method, and a Ca (OH) ₂ adsorption amount of 90 mg. In accordance with the Japanese Geotechnical Society standard (JGS0821-2009) "Method for preparing specimens without compacting stable treated soil", the soil was mixed at a temperature of 20 ° C. to provide a ground improvement body with a diameter of 5 cm and a height of 10 cm. A specimen was prepared.
Further, the specimen prepared here was cured for 7 days, 28 days, and 91 days, and then the uniaxial compression strength was measured in accordance with JIS A1216 "Soil uniaxial compression test method" to determine the strength development. confirmed.
As for the uniaxial compressive strength, based on the description in Document B below, an indoor compounding test was considered for the improved uniaxial compressive strength of 50 to 100 kN / m ² , which is generally required for the improvement of generated soil (on-site). / Indoor) Assuming that the strength ratio is 0.5, the maximum value of 200 (= 100 / 0.5) kN / m ² or more was accepted.
Reference B: "Ground improvement manual for cement-based solidifying materials, 4th edition" (published by Cement Association)
Table 5 shows the measurement results of the P funnel flow time and the uniaxial compression strength.

5. Evaluation of Measurement Results (i) Retention Time of Fluidity As shown in Table 5, the ground improvement material slurry using the ground improvement cement composition (Examples 1 to 10) of the present invention was prepared at an environmental temperature of 27 ° C. The P funnel flow time after kneading is as short as 12 seconds or less even after 3 hours, and the fluidity retention time is long.
On the other hand, the slurries using the cement compositions of Comparative Examples 1 to 4 and Comparative Example 8 tend to have a short fluidity retention time at an environmental temperature of 27 ° C.
(Ii) Uniaxial compressive strength The uniaxial compressive strength of the ground improvement product using the cement composition for ground improvement (Examples 1 to 10) of the present invention on the 28th day of the material age exceeds 200 kN / m ^2. In particular, in Examples 8 to 10, the strength can be increased to the same level as that of ordinary Portland cement (Comparative Example 1) by adding gypsum, and the pot life can be secured.
On the other hand, in Comparative Examples 5 to 7, the increase in strength was smaller than in Examples 1 to 10, and in Comparative Examples 2 to 4, although the strength was increased by the addition of gypsum, the flow time was longer. It is difficult to secure pot life in hot weather and areas.

Claims (6)

1. It contains at least fly ash and Portland cement that satisfy all of the following conditions (F1) to (F5).
   A cement composition for ground improvement, wherein the total of the fly ash and Portland cement is 100% by mass, and the content of the fly ash is 10 to 40% by mass.
   (F1) Fly ash has a specific surface area of 2500 to 6000 cm ² / g.
   (F2) The mass reduction rate of fly ash after heating the fly ash at 975 ± 25 ° C. for 15 minutes is 5% by mass or less (F3) The content of SiO _{2 in} the fly ash is 50% by mass or more (F4) Fly The sphere-equivalent specific surface area of particles in which iron oxide and amorphous are mixed in ash is 2800 to 11000 cm ² / cm ³
   (F5) The spherical specific surface area of amorphous particles containing Ca (calcium) in fly ash is 2100 to 22500 cm ² / cm ³
2. The cement composition for ground improvement according to claim 1, wherein the fly ash further satisfies the following condition (F6).
   (F6) The sphere-equivalent specific surface area of particles in which mullite and amorphous particles are mixed in fly ash is 1900-9500 cm ² / cm ³
3. The cement composition for ground improvement according to claim 1 or 2, wherein the fly ash further satisfies the following condition (F7).
   (F7) The sphere-equivalent specific surface area of Ca-free amorphous particles in fly ash is 2100 to 9000 cm ² / cm ³
4. A cement composition for ground improvement containing one or more types of gypsum selected from anhydrous gypsum, semi-hydrated gypsum, and dihydrate gypsum.
   Portland cement, fly ash, and the total 100 mass% of the gypsum content of the gypsum is not more than 6.0 mass% converted to SO _3, ground according to any one of claims 1 to 3 Improvement cement composition.
5. A ground improvement method for improving the ground by mixing soil with a ground improvement material slurry containing at least the cement composition for ground improvement and water according to any one of claims 1 to 4.
6. A ground improvement body obtained by mixing cohesive soil satisfying all of the following conditions (S1) to (S3) with the ground improvement material slurry according to claim 5.
   (S1) Moisture content ratio is 70 to 150%
   (S2) The allophane content measured by the acidic oxalate extraction method is 5.0% or more. (S3) In the cohesive soil when a slurry containing 150 mg of Ca (OH) ₂ and 100 ml of water is mixed. Adsorption amount of Ca (OH) ₂ is 80 mg or more per 1 g of cohesive soil