JP4758963B2 - Coated silica sand - Google Patents

Description

  The present invention relates to a structure of resin or rubber-coated silica sand having an elastic film, which can provide an artificial turf structure having excellent impact absorbability and a comfortable elasticity.

  Artificial turf with sand filled with silica sand is used in tennis stadiums, futsal courts, ballparks, schools and kindergarten grounds.

  Artificial grass with sand is said to have good cushioning properties. The cushioning property of the artificial turf with sand is due to the displacement and movement of the sand, but since the filler is quartz sand, the cushioning property is limited. That is, the friction during sanding due to the load during motion and impact causes a cementation phenomenon in which particles are crushed and solidified by moisture such as rain, and the entire layer becomes hard and the shock absorption becomes extremely low, which is dangerous. is there. Small crushed sand is scattered around and becomes a source of pollution.

  In addition, long pile artificial turf filled with inexpensive hard filler (silica sand) and elastic filler (rubber chips) inside a long pile artificial turf has an appearance similar to natural turf and excellent shock absorption, It has the advantage of low maintenance costs, and is rapidly spreading in soccer fields, baseball fields, etc., instead of natural grass, which requires a lot of maintenance costs.

  However, because of the large gap between the rubber chips of the long pile artificial turf, silica sand falls and concentrates in the lower layer, and even though the rubber chips are passed, the dynamic movement impacts the entire weight, and the sand is still pulverized. The phenomenon similar to the artificial turf is concerned, and some have been pointed out even though the history is still short.

  As means for solving such a problem, various methods for covering the surface of silica sand filled in the artificial turf with rubber / plastic have been proposed.

  Patent Documents 1 to 4 describe, for example, spraying a solution / emulsion of an elastic body such as butadiene synthetic rubber, polyolefin, polyurethane, etc., and coating the silica sand surface by dipping, but the silica sand surface can be coated. It is difficult to maintain impact absorption and comfortable elasticity for a long period of time with rubber and resin alone at low cost.

  In addition, the method according to Patent Document 5 can actually produce inexpensive and good coated silica sand, and can sufficiently prevent the pulverization of the sand, but it is a further level in terms of shock absorption and comfortable elasticity. Is requested.

JP 61-103417 A JP-A-1-131705 JP-A-6-15065 JP-A-6-235206 Japanese Patent Laid-Open No. 2003-041151

  In view of the above-mentioned conventional problems, the present invention provides a silica sand coated with a resin or rubber film imparted with elasticity to obtain an artificial turf structure capable of obtaining a sustainable impact absorption and comfortable elasticity. With the goal.

  The present inventor can improve the impact absorption and elasticity without increasing the thickness of the resin or rubber coating layer, which dramatically increases the material cost, by including the microballoon in the coating layer. As a result, the present invention has been completed.

That is, the coated silica sand of the present invention is a coated silica sand for artificial turf filling having a coating layer made of a resin or rubber and containing a microballoon in the coating layer.
In addition, the method for producing the coated silica sand of the present invention comprises 8 to 40 parts by weight of resin or rubber, 100 parts by weight of silica sand, and 20 to 100 parts by weight of an inorganic microballoon or 100 parts by weight of the resin or rubber. 1-6 parts by weight of plastic-based microballoon is added and stirred and mixed, and when silica sand begins to aggregate, inorganic powder is added, and stirring and mixing are continued to release the aggregation of dredged sand. It is a manufacturing method of the covering silica sand for turf filling.

  The coated silica sand of the present invention can improve impact absorption and elasticity without increasing the thickness of the resin or rubber coating layer that dramatically increases the material cost, and is excellent in economic efficiency.

  Therefore, if the coated silica sand of the present invention is used, such as an artificial turf structure with sand, a long pile artificial turf structure, etc. that has excellent shock absorption, has a comfortable elasticity, and can maintain a comfortable playability for a long period of time. Various artificial turf structures can be provided at low cost.

  Silica sand includes natural sand such as mountain sand, river sand, sea sand, and hard granular materials such as sand, glass, ceramics and fly ash generated during rock crushing. Silica sand having a diameter of 2.5 mm or less is preferred.

  The coating layer covering the silica sand is a layer made of resin or rubber and contains microballoons. The thickness of the coating layer is not particularly limited and depends on the particle size of the silica sand, but is preferably about 30 to 100 μm in consideration of the elastic coating layer containing microballoons.

  The resin or rubber is not particularly limited, but it is liquid before coating, flexible after curing, excellent adhesion to silica sand, strength enough to withstand intense movements in player shoes, and outdoors. It is preferable to have durability that is resistant to aging in direct sunlight and other climates. A water-dispersed resin or a solvent solution resin may be used, but a 100% solid content liquid resin is preferable in consideration of economy. Specifically, a thermosetting resin or a thermoplastic resin such as an epoxy resin, an acrylic resin, a polyester resin, a polyurethane resin, or a polybutadiene rubber can be given, and a polyurethane resin is preferable.

  The resin or rubber preferably has a 20% tensile stress after curing of 0.1 to 1.2 MPa, and more preferably 0.3 to 0.8 MPa. When the pressure is less than 0.1 MPa, the coating layer is too soft, and there is a concern that problems such as impact and wear may occur. On the other hand, if it exceeds 1.2 MPa, the coating layer is too hard, and the effect of adding microballoons cannot be exhibited, impact absorption is reduced, and there is a concern that the coating layer has poor elasticity.

  Examples of the microballoons include inorganic microballoons such as silica balloons, ceramic balloons, and aluminosilicate balloons, plastic microballoons such as vinylidene chloride resin balloons, acrylonitrile resin balloons, and phenol resin balloons.

  Inorganic microballoons and plastic microballoons may be used alone, but as the amount of inorganic microballoons increases, the apparent hardness of the coating layer becomes harder (it becomes harder and absorbs shocks). It is necessary to consider the balance with elasticity. On the other hand, plastic microballoons are advantageous in that respect, but are very expensive. Therefore, both of these characteristics may be used in combination.

  Depending on the manufacturing method, there is a concern that the microballoon has a high rate of destruction during the process (decreases the survival rate of the balloon). Therefore, it is preferable to select a microballoon having as large a compressive strength as possible. Although it depends on the material and shape of the balloon, the larger the particle size, the weaker the strength, so the smaller the particle size is preferable. The particle size of the microballoon is preferably within the thickness of the coating layer. Specifically, the average particle size is preferably 20 to 70 μm, and more preferably 30 to 50 μm. When the average particle size is less than 20 μm, the coating layer becomes hard, and there is a concern that the impact absorption and elasticity will be poor. When the average particle size exceeds 70 μm, there is a high possibility that the microballoon will jump out of the coating layer, and the compressive strength becomes low as described above. There is a concern that will become scarce.

  It is preferable that the coating layer contains a pigment for providing concealability to improve weather resistance and for coloring. The pigment varies depending on the color.For example, iron oxide is used for brick, brown and yellow, chromium oxide is used for green, phthalocyanine organic pigment is used, and titanium oxide is used for white. Various colors can be prepared by combining. Moreover, when providing a heat-shielding effect, you may use a heat-shielding pigment with large heat reflectivity. In addition to titanium oxide, which has high solar heat reflecting ability as a pigment, AR300 brown (manufactured by Kawamura Chemical Co., Ltd.) is used for brown, AY-610 yellow (manufactured by Kawamura Chemical) is used for yellow, and green There is Fastogen Green 2YK (Dainippon Ink Chemical Co., Ltd.) and the like.

  The coating layer may contain other compounding agents as necessary.

  For example, in order to improve the weather resistance, ozone resistance, and other durability of the coating layer, Irganox 1010 (manufactured by Ciba Geigy), benzotriazole type such as Tinuvin P-327, benzophenone type ultraviolet absorber such as Tomissorb 800, An anti-aging agent can be added.

  Depending on the application, it can be dispersed in an appropriate plasticizer in advance, and antibacterial and antifungal agents such as zinc pyrithione and zinc 2-pyridinethiol-1 oxide can be added to suppress the generation of bacteria and mold.

  In addition, inorganic powders such as calcium carbonate, talc, silica, clay and the like may be added as necessary for the purpose of reinforcing physical properties, extenders and the like.

  Moreover, if rubber fine powder that has good adhesiveness and can be stably supplied, such as an inexpensive recycled product, is contained, the film can be made thick at low cost, and an effect close to that of adding a microballoon can be obtained. At this time, the amount of microballoon added can be reduced.

  The method for producing the coated quartz sand of the present invention is not particularly limited, and can be produced, for example, by stirring and mixing quartz sand, resin or rubber, and microballoons. At this time, in order to prevent the microballoon from being broken during the manufacturing process, it is preferable to first add and mix the resin, and then add and mix the microballoon.

  Moreover, in order to improve the adhesiveness between the coating layer and the surface of the silica sand, pretreatment of the silica sand may be performed using a silane coupling agent such as KBE-9007 (manufactured by Shin-Etsu Chemical Co., Ltd.).

  The amount of resin or rubber added is not particularly limited. For example, in the case of silica sand having an average particle diameter of 1.5 mm, it is preferably 8 to 40 parts by weight, and 12 to 30 parts by weight with respect to 100 parts by weight of silica sand. It is more preferable.

  The addition amount of the microballoon is not particularly limited, but the true specific gravity of the inorganic microballoon is slightly different depending on the material, but is within the range of 0.6 to 0.9 in the above preferred particle size range. When the balloon is used alone, the addition amount is preferably 20 to 100 parts by weight, more preferably 40 to 60 parts by weight with respect to 100 parts by weight of the resin or rubber.

  Moreover, when using a plastic microballoon independently, it is preferable that it is 1-6 weight part with respect to 100 weight part of resin or rubber | gum, and it is more preferable that it is 2-4 weight part. In addition, plastic microballoons are very lightweight powders with a true specific gravity of 0.08 or less that are very easy to scatter. Therefore, in order to prevent scattering, inorganic powder can be adhered to the sphere surface of the balloon, or it can be hydrated. The amount added is expressed as the amount of pure plastic microballoons before such treatment. Therefore, when using the balloon after processing, it is necessary to consider by adding the weight of the processing.

  In the case where an inorganic microballoon and a plastic microballoon are used in combination, the addition amount in the case of using the above alone may be appropriately selected. That is, when 60% by weight of inorganic microballoon and 40% by weight of plastic microballoon are used, 60% of the addition amount is used for inorganic microballoon and 40% of the addition amount is used for plastic microballoon.

  If the amount of microballoon added is less than the above range, impact absorption and elasticity may be poor, and if it is too much, there is a concern that the coating layer may be insufficient in strength or that the inside of the coating layer cannot be filled. Arise.

  When the amount of microballoon added is small and it is difficult to agglomerate and decompose even after addition, for example, the method described in Patent Document 5, that is, a resin or rubber and a film forming material containing microballoon are added to cinnabar. Then, the mixture is stirred and mixed, and when it starts to aggregate, inorganic powder is added, and stirring and mixing are continued to release the agglomeration of dredged sand. As the inorganic powder, those described above can be used.

  Specifically, the addition timing is slightly delayed so as not to impair the wetness of the microballoon with the coating resin, and even if the mixture is agglomerated, the microballoon works in the direction of promoting the decomposition of the agglomerate, so a small amount of inorganic By simply adding the powder and continuing stirring for a short time, it can be decomposed relatively easily in a short time.

  In addition, the amount of resin per silica sand is less when coated in multiple steps than when it is manufactured in only one step, so the degree of aggregation is less and it can be decomposed more easily, and the microballoon survival rate Highly covering work can be done.

  In addition, since the microballoons are also densely present in the outermost part of the coating layer, when there is a concern about the strength of the coating layer, by further providing a coating layer made only of a resin, I can do it.

  The coated silica sand of the present invention is used alone or as a silica sand of an artificial turf-containing turf (pile length of about 20 mm) structure used for tennis courts, futsal, school grounds, etc. Untreated silica sand) can be mixed or laminated up to about 1/3. As a result, an artificial turf structure with sand that has excellent impact absorbability and excellent elasticity can be obtained.

  A long pile artificial turf (pile length: 50 to 70 mm) structure used in a soccer field, a baseball field, etc. is filled with a hard filler such as silica sand and a soft (elastic) filler such as a rubber chip. The coated quartz sand of the present invention is mixed as a hard filler for a long pile artificial turf structure with rubber chips alone or with untreated quartz sand in a weight ratio (coated quartz sand / untreated quartz sand) to about 1/3 or It can be used by laminating. As a result, a long pile artificial turf structure having excellent impact absorbability and a comfortable elasticity can be provided over a long period of time, and comfortable playability can be maintained. Further, the artificial turf structure filled with the coated silica sand of the present invention instead of the rubber chip can prevent defects such as odor problems when the waste tire ground rubber chip is used.

  Hereinafter, the present invention will be described in more detail with reference to examples. In the examples, “part” means “part by weight”.

<Materials used>
(1) Silica sand Silica sand a: No. 4 silica sand (manufactured by Tokai Ritec Co., Ltd., Toki City, Gifu Prefecture, average particle size 1.5 mm)
Silica sand b: Kashima 3B silica sand (manufactured by Takano Shoji Co., Ltd., Kashima, Ibaraki, average particle size 1.8mm)

(2) Coating resin An average molecular weight of 3000, 300 parts of bifunctional polyoxypropylene glycol and 120 parts of diphenylmethane diisocyanate were reacted at 80 ° C. for 4 hours in a nitrogen stream, isocyanate group content 7.4%, viscosity A polyurethane resin of 4,100 mPa · s / 25 ° C. was used.

  The 20% tensile stress of this resin was 0.41 MPa. In addition, this stress is measured in accordance with JIS K 6251 (Tensile test method for vulcanized rubber) after vacuum defoaming, making a non-foamed film of about 10 μm with a doctor blade, curing at room temperature for 7 days, and curing. Measured.

(3) Catalyst Dabco 33LV (manufactured by Air Products Co., Ltd., tertiary amine)

(4) Microballoon inorganic balloon: Cenolite FS-150 (Sakai Kogyo Co., Ltd., ceramics), average particle size 40 μm
Plastic balloon: EMC40 (B) (Nippon Philite Co., Ltd., acrylonitrile resin system), average particle size 45 μm, true specific gravity average value 0.05, 2.2 times the amount of inorganic powder adhered to the surface to prevent scattering Things (true specific gravity 0.16)

(5) Pigment F3 Green (manufactured by Toda Kogyo Co., Ltd.) Green pigment

(6) Inorganic powder Inorganic powder a: Super SSS (manufactured by Maruo Calcium Co., Ltd., calcium carbonate)
Inorganic powder b: Talc DNB (manufactured by Nippon Talc Co., Ltd., talc)

(7) Artificial turf Sand-filled artificial turf for tennis courts: Pile length 20 mm (manufactured by Quanzhou Rug Co., Ltd.)
Long pile artificial turf: split yarn pile, pile length 60mm (manufactured by Otsuka Furniture Manufacturing and Sales Co., Ltd.)

<Evaluation method>
(1) Floor hardness test A head model weighing 3.8 kg was tested using a test machine in accordance with JIS A 6519 (steel floor base material for gymnasium) 8.7 (floor hardness test). From the height, the maximum acceleration (G) when free-falling and colliding with the center of a wooden plywood disk on the following specimen was measured. Note that when the tip load dropped, the sand was thin, so the tip sunk into the sand and scattered at the same time, and an accurate value was not obtained. Therefore, a wooden plywood disc was placed as a slightly harder material. The maximum acceleration when measured with only a wooden plywood disk was 83G.

  Specimen I: Silica sand to be evaluated was filled to a thickness of 20 mm in an iron cylindrical container having an inner diameter of 72 mm and a height of 50 mm, and was flatly consolidated, and a 13 mm thick and 70 mm diameter wooden plywood disk was placed thereon. This was installed on concrete and used for measurement.

  Specimen II: A tennis court artificial turf with a pile length of 20 mm was pasted in a square rigid vinyl chloride case with a square of 15 cm square and a height of 20 mm, and this was filled with quartz sand to be evaluated up to a height of 20 mm. Then, it was compacted flatly, and a wooden plywood having a thickness of 13 mm and a diameter of 70 mm was placed in the upper central part, and this was placed on concrete and used for measurement.

  Specimen III: A long pile artificial turf with a pile length of 60 mm is pasted in a square styrene case with a square of 20 cm square and a height of 60 mm, silica sand a is 20 mm in this, and silica sand or rubber chip to be evaluated is placed thereon. Filled with 20 mm and compacted flatly, a 13 mm thick and 70 mm diameter wooden plywood was placed in the upper center, and this was placed on concrete for measurement.

(2) Silica sand impact pulverization test The silica sand impact pulverization test was conducted using the testing machine shown in FIG.

  Into a cylindrical container 1 with a diameter of 110 mm, silica sand to be evaluated is put so as to have a thickness of 20 mm, and from this, a drop load of 1.5 kg, a drop height of 12 cm, a drop bottom surface φ = 6.5 cm, an iron disk 3 having A rubber plate 4 having a thickness of 3 mm and a hardness of 60 (JIS K 6253, A-type hardness meter) was applied with an impact for 6 hours by free fall at 50 cycles per minute, and the particle size before and after the test was 0. The amount of fine powder of 5 mm or less was examined, and the degree of pulverization resistance by impact was examined.

<Example 1>
To a stirring vessel, 400 parts of silica sand a, 60 parts of a coating resin preliminarily mixed with 1% of catalyst, 8 parts of pigment, and 24 parts of inorganic powder a are added and stirred. Next, 18 parts of the inorganic balloon and 4.0 parts of the plastic balloon (4.0 / 3.2 parts of the plastic balloon itself) are added, and the speed is reduced and the mixture is stirred slowly. Since the whole is slightly agglomerated, 15 parts of inorganic powder a is added, and stirring is stopped when each silica sand particle is separated into simple substances. The resulting coated silica sand is allowed to cure at room temperature for 5 days.

  Evaluation was performed using this coated silica sand. The results are shown in Table 1.

<Example 2>
Evaluation was made using mixed silica sand in which the coated silica sand prepared in Example 1 and untreated silica sand a were well mixed at a weight ratio of 1: 1. The results are shown in Table 1.

<Example 3>
To the stirring vessel, 400 parts of silica sand b, 48 parts of a coating resin preliminarily mixed with 1% of catalyst and 6.7 parts of pigment are added and stirred. Next, 18 parts of the inorganic balloon, 4.0 parts of the plastic balloon (4.0 / 3.2 parts of the plastic balloon itself) and 18 parts of the inorganic powder a are added, and the mixture is slowly stirred at a reduced speed. Since the whole is slightly agglomerated, 10 parts of inorganic powder a is added, and once the silica sand particles are separated into simple substances, the stirring is stopped once. Next, again add 12 parts of a coating resin preliminarily mixed with 1% of catalyst, 1.3 parts of pigment, and 6 parts of inorganic powder a, and slowly agitate to make each silica sand particle alone without agglomeration. Make sure that the mixer is turned off. The resulting coated silica sand is allowed to cure at room temperature for 5 days.

  Evaluation was made using this coated silica sand. The results are shown in Table 1.

<Example 4>
To the stirring vessel, 400 parts of silica sand b, 48 parts of a coating resin preliminarily mixed with 1% of catalyst and 6.7 parts of pigment are added and stirred. Next, 8.2 parts of plastic balloon (8.2 / 3.2 parts of plastic balloon itself), 18 parts of inorganic powder a, and 18 parts of inorganic powder b are added, and the speed is lowered and stirred slowly. Since the whole is slightly agglomerated, 15 parts of inorganic powder b is added, and once each silica sand particle is separated into simple substances, stirring is stopped once. Next, again add 12 parts of a coating resin premixed with 1% of catalyst, 1.3 parts of pigment, and 12 parts of inorganic powder b, and slowly agitate to separate each particle of silica sand without agglomeration. Make sure that the mixer is turned off. The resulting coated silica sand is allowed to cure at room temperature for 5 days.

  Evaluation was made using this coated silica sand. The results are shown in Table 1.

<Comparative Example 1>
Evaluation was made using untreated silica sand a. The results are shown in Table 1.

<Comparative example 2>
Into a stirring vessel, 400 parts of silica sand a, 60 parts of a coating resin premixed with 1% of a catalyst in advance and 6 parts of a pigment were added to start stirring, and the resin gradually thickened and gelation started. When agglomeration begins to aggregate, 50 parts of inorganic powder a is added little by little. After confirming that each particle of the silica sand is separated and separated from the silica sand alone, stirring is stopped. The resulting coated silica sand is allowed to cure at room temperature for 5 days.

  Evaluation was made using this coated silica sand. The results are shown in Table 1.

<Comparative Example 3>
Coated silica sand was obtained in the same manner as in Comparative Example 2 except that the addition amount was 90 parts of coating resin, 9 parts of pigment, and 75 parts of inorganic powder a.

  Evaluation was made using this coated silica sand. The results are shown in Table 1.

<Comparative example 4>
Evaluation was made using mixed silica sand in which the coated silica sand prepared in Comparative Example 2 and untreated silica sand a were well mixed at a weight ratio of 1: 1. The results are shown in Table 1.

<Comparative Example 5>
Evaluation was made using a waste tire grinding chip RG-814 (Muraoka Rubber Industries, Ltd., 1 to 3 mmφ). The results are shown in Table 1.

<Reference Example 1>
For reference, a floor hardness test was performed on a color elastic pavement of standard hardness.

  80 parts of EPDM color rubber chip “RG-41 brick” (manufactured by USS Toyo Co., Ltd.) and 18 parts of urethane binder “SBU Isocyanate 0620” (Sumika Bayer Urethane Co., Ltd.) are mixed for 3 minutes with a mixer. The mold was flattened on a 10 mm thick mold with a plastering iron and cured for 7 days.

  When the floor hardness test was conducted in the same manner as the above evaluation method except that this EPDM color rubber chip layer was used instead of the silica sand of the test specimen I, the maximum acceleration was 70 G.

(1) Floor hardness test (Specimen I)
From Table 1, the coated silica sands of Examples 1 and 3 are excellent in impact absorption and have a soft elasticity. In Example 4 where the plastic balloon is used alone, the effect is even more remarkable.

  Further, Example 2 using a 1/1 mixture of coated silica sand and untreated silica sand has a value similar to that of coated silica sand not containing microballoons (Comparative Examples 2 and 3). It has better shock absorption than 1).

  Note that Comparative Example 3 in which the amount of the coating resin was increased by 50% compared to Comparative Example 2 was not as high as that of Example 1, and even if the amount of resin was increased, it did not lead to a dramatic improvement as in the present invention.

(2) Floor hardness test (Specimen II)
According to Table 1, the same tendency as that of Specimen I is observed in Specimen II, but there is a gap in the artificial turf structure between piles, between pile / silica sand, pile / base cloth, etc. It is relaxed, the value is lower than that of the specimen I, and the difference from the comparative example is also reduced. However, in actuality, it is considered that the difference from the comparative example becomes larger as the playing time elapses.

(3) Floor hardness test (Specimen III)
From Table 1, the long pile artificial turf structure filled with silica sand of Example 4 does not reach the rubber chip filling type of the standard structure of Comparative Example 5, but compared with the silica sand single filling structure of Comparative Example 1. It is a structure excellent in shock absorption as it is located at a value close to Comparative Example 5 and uses coated silica sand.

(4) Impact crushing test of silica sand From Table 1, the coated silica sand of Example 1 has good crushing impact resistance as well as the coated silica sand containing no microballoon (Comparative Example 2).

  In addition, when mixed in the same amount (weight) with untreated silica sand, Example 2 using the coated silica sand of Example 1 is slightly better than Comparative Example 4 using the coated silica sand of Comparative Example 1. ing. Further, the amount of increase in fine powder is good at about 1/5 or less compared to untreated silica sand (Comparative Example 1). Most of the fine powder is obtained by breaking and pulverizing No. 4 silica sand, and when mixed with coated silica sand, the amount of breaking and grinding dramatically decreases. Moreover, it can be easily estimated that this effect becomes even more remarkable when coated silica sand having higher elasticity is used.

It is the schematic of the testing machine used in the Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Cylindrical container 2 Clay sand 3 Iron disk 4 Rubber plate

Claims (4)

  A coated silica sand for filling artificial turf, which has a coating layer made of resin or rubber, and contains a microballoon in the coating layer.   The coated silica sand for artificial turf filling according to claim 1, wherein the resin or rubber is a resin or rubber having a 20% tensile stress after curing of 0.1 MPa to 1.2 MPa.   The coated silica sand for artificial turf filling according to claim 1 or 2, wherein the microballoon has an average particle size of 20 to 70 µm. 8 to 40 parts by weight of resin or rubber is added to 100 parts by weight of silica sand, and 20 to 100 parts by weight of inorganic microballoon or 1 to 6 parts by weight of plastic microballoon is added to 100 parts by weight of the resin or rubber. Then, the mixture is stirred and mixed, and when the silica sand starts to aggregate, the inorganic powder is added, and stirring and mixing are continued to release the aggregation of dredged sand.

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