DK165782B - PROCEDURE FOR STABILIZING A SUSPENSION OF MICROSOFT IN WATER - Google Patents

Description

in

DK 165782B

The present invention relates to a method for stabilizing a suspension of micro dust in water.

In the preparation of concrete mixtures containing microsilica as an additive, it is common to incorporate dry silica dust together with plasticizers and the other components of the concrete. However, methods are also known for preparing these mixtures in which a premix of silica dust and optionally plasticizer is prepared shortly before the premix is added to the concrete. In fact, the premix has poor storage stability in that it gelatins within a few hours and achieves a consistency that is so rigid that the premix is not practically usable.

A major problem with the use of silica dust 15 is its transport to the place of use. Although the density of the silica dust 3 is approx. 2.2 g / cm, it is far from possible to achieve a density of the silica dust during packaging of this magnitude, and one must usually accept a 3-5 times greater volume than theoretically possible. In addition, when pouring or leaking the packaging, the silicate dust will readily be mixed into the ambient air, and special precautions must therefore be taken to avoid such mixing.

It is common for silica dust to be delivered either in small sacks containing 20 - 30 kg, in large sacks containing approx. 500 25 kg or in special tankers. All these modes of transport are expensive. The first because no packaging machines have been developed that can effectively handle the silica dust due to its special nature, the second because special handling equipment is required at the point of use, and the three-way because the special tank trucks are not usable for transporting other estate.

However, the transport problem can be solved in many conditions conveniently if the silica dust is supplied in an aqueous slurry, but in order to make this option practically usable, a considerable storage stability of the suspension must be required.

The object of the invention is therefore to provide a process by which silica dust suspension is prepared.

DK 165782B

2 down with considerable storage stability.

This object is met by a method peculiar to that of claim 1.

In the process of the invention, the rate at which the suspension gelatines is reduced to such a degree that the gelatinization after 28 days is substantially less than that of a comparable suspension of silica without additives and pH control. Therefore, in the process of the invention, suspensions can be made at the silica dust production site and of sufficient durability to allow the aqueous suspension to be distributed to the consumable sites where it can be added without dust problems to the concrete mixture.

It has also been found that the silica dust in the suspended state is relatively compact and only fills approximately the same as dry silica dust can be compacted at best.

According to an advantageous embodiment, the plasticizer itself can be used as the pH-regulating agent.

The additive mixture is added to the concrete at any time during the mixing process. A premix of the ingredients included in the additive mixture offers great advantages over the usual mixing method, with each ingredient being individually mixed in the concrete. When premixed with water, the plasticizer will tend to cover the silica dust particles and disperse them homogeneously in the pulp. The result is that flocculations in the material break down. Such flocculations often occur when the ingredients are mixed together separately. Flocculations can present a serious disadvantage when it comes to uniform strength and durability in the cast concrete. Once flocculations are formed in the concrete mix, a long-lasting mechanical mix is required to disperse them. This may cause an over-mix which may destroy the processability and moldability of the mixture.

The plasticizing and possibly accelerating and retarding agents used according to the present invention

DK 165782B

3 of the present invention are well-known conventional agents currently used in high strength concrete having a compressive strength of 40 - 80 MPa.

These substances are chemical compounds which liquefy the concrete for a certain period of time, whereby normal workability can be achieved with a much lower water / cement number than normal, or cause a very workable, so-called "floating concrete". The substances available on the market are divided into six different groups: 1. Hydroxylated carboxylic acids and their salts 2. Modifications and derivatives of hydroxylated carboxylic acids and their salts; 3. Inorganic materials such as zinc salts, borates, phosphates and chlorides; 4. Carbohydrates, polysaccharides and sugars; 5. Amines and their derivatives and polymeric compounds such as cellulose ethers and silicones; 6. Certain modified lignosulfonic acids.

The term plasticizer used in the present specification and claims means one or more of the constituents of the six groups of usual substances mentioned above, either alone or in combination.

Superplasticizers commonly used commercially today include: 2- 1. Lignosulfonic acid and its salts and modifications and derivatives thereof; 2. Melamine derivatives; 3. Naf calendar ivater.

The term superplasticizer is used herein to mean one or more of the constituents of the above three groups of substances either alone or in combination.

There are at least 12 commonly used superplasticizers. Eight of these fall into the above categories 2) and 3). The preferred substance of category 2) is a generally sulfonated condensate of melamine and formaldehyde, which is marketed under the trademark Melment. The preferred substance of category 3) is a sulfonated condensate of naphthalene and formaldehyde.

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Concrete containing superplasticizers is widely used in castings that require good flow properties, for example in areas of high reinforcement density, pump concrete and in complicated molding. Some of the benefits obtained by using superplasticizers in prefabricated concrete elements and factory concrete are a) increased strength at all curing rates, b) improved resistance to sulfate attack, c) better bonding to reinforced steel, and d) reduced water permeability.

10 When superplasticizer is added to the concrete portion, the plasticization lasts approx. 30 - 60 minutes depending on site conditions. Therefore, the fabric should be added on the construction site when it comes to factory concrete.

Concrete containing one or more superplasticizers is described in "Super Plasticized Concrete", ACI Journal, May 1977, pages N6-N11 with literature references.

It has been found that a premixed addition to mortar and concrete in the form of a slurry containing 10 - 80 wt% silica dust, 0.5 - 40 wt% plasticizer 20 and / or superplasticizer and the remainder water and having a pH value within the range of 3.0 - 7.5, increases the density and impermeability of the mortar and concrete by up to several orders of magnitude. In fact, it has been found that non-aerated concrete made with the additive mixture of the present invention virtually eliminates the ingress of water which can freeze and aggressive liquids. Concrete containing the silica dust additive has a freeze-thaw resistance that is equal to or better than is the case with concrete which has an actual air mixture and has the same or higher strength. Loss of air or increased distance between the pores is offset by the beneficial effects of the silica dust addition with respect to fundamental changes in the pore structure of the glue phase in concrete. The result is that the adhesive phase is more evenly distributed and that it has a significantly finer pore structure.

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The silica dust used in this invention is an amorphous silica by-product of the production of ferrosilicon and silicon metal. This is achieved by collecting the finely divided particles in the flue gas from electric furnaces. The Sili-5 cast is a puzzolan, ie. it incorporates compounds with lime and moisture at usual temperatures and forms compounds that have binder properties. The main constituent is silica (SiO2). Usually, the dust contains at least 60% SiO 2, but the best results are obtained with this invention when the 10 SiO 2 content is at least approx. 85% by weight.

15

The following tables provide chemical and physical data for 20 typical samples of the silica referred to in this specification.

TABLE 1

Dust collected in bag filters during Si-metal production 25

Component weight%

SiO2 94 - 98

SiC 0.2 - 0.7 Fe2 ° 3 0.05 "0.15

TiO2 0.01 - 0.02 Al2 O3 0.1 - 0.3

MgO 0.2-0.8

CaO 0.1 - 0.3 Na 2 O 0.3 - 0.5 K 2 O 0.2 - 0.6 6 '

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(Table 1 continued):

Component weight%

Mn 0.003 - 0.01 Cu Cu 0.002 - 0.005

Zn 0.005 - 0.01

Ni 0.001 - 0.002 S 0.1 - 0.3 C 0.2 - 1.0 10 P 0.03 - 0.06

Glow loss (1000 ° C) 0.8 - 1.5

Room weight from filter 200 - 300 g / l

Compact weight, 500 - 700 g / l

Solid weight range 2.20-2.25 g / cm3 2 Specific surface area 18 - 22 m / g

Primary particle size: 90% <1 µl TABLE 2 20 Dust collected in bag filters by producing 75% FeSi

Component weight%

SiO2 86 - 90

SiC 0.1 - 0.4 Fe2 O2 0.3 - 0.9

TiO2 0.02 - 0.06 Al2 O3 0.2 - 0.6

MgO 2.5 - 3.5

CaO 0.2 - 0.5 Na 2 O 0.9 - 1.8 K 2 O 2.5-3.5

Mn Cu Zn 35 Ni S 0.2 - 0.4 C 0.8 - 2.0 P 0.03 - 0.08

Glow loss (1000 ° C). 2.4-4.0 *

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7 (Table 2 continued)

Component weight% 5 Room weight, from filter 200 - 300 g / l

Compact weight, 500 - 700 g / l 3

Solid density 2.20 - 2.25 g / cm 2

Specific surface area 18 - 22 m / g

Primary particle size: 90% <Ija 10

It is best to use dust collected from an electric furnace producing ferrosilicon containing at least 75% silicon, but also dust collected from an electric furnace producing ferrosilicon having a 50% silicon content can be used in this invention.

It is also possible to recover the amorphous silica as a main product by adjusting the reaction conditions. Amorphous silica of this type can also be produced synthetically without reduction and re-oxidation.

The amorphous si'ika, c. = R * r.v = r.des in the present invention is mainly composed of spherical particles measuring less than 1 ^ in diameters. The sphere shape, fineness and Puzzolan reactivity make the amorphous silica very useful for the present invention.

For example, the amorphous silica may comprise at least 60-90% by weight 3

Si02. It will have a solids density of 2.20 - 2.25 g / cm.

The particles are mostly spherical and at least 90% by weight of the primary particles have a particle size of less than 1 µl. Of course, these values may vary. For example, silica may have a lower SiC 2 content. The particle size distribution can also be adjusted. For example, it is possible to remove coarser particles 2. The dust has a specific surface of 18 - 22 m / g.

The amorphous silica appears dark gray due to the carbon content. However, the carbon can be burned away, for example 35 at temperatures above 400 ° C. According to the present invention, the mixture is prepared by mixing the main constituents in an aqueous slurry, thereby obtaining good contact between the constituents and a uniform, homogeneous distribution. The slurry addition mixture may contain from 8

DK 165782 B

10 to 80 wt.% Of silica dust and from 0.5 to 40 wt.% (Dry weight) of one or more superplasticizing or plasticizing agents alone or in combination (preferably from 1 to 20 wt.% Of said superplasticizer or plasticizer) alone or in combination). The rest is water. According to an example, 20 kg of silica dust, 1.3 kg of commercially available sulfonated condensate of naphthalene-formaldehyde (superplasticizer) and 1.3 of a commercially available cellulose ether (plasticizer) are distributed uniformly and homogeneously in 20 liters. water. The pH of the slurry is adjusted with the usual mineral acid or alkali to be in the range of 3.0 to 7.5, preferably between ca. 5.0 to 6.0, to obtain a slurry having a consistency suitable for transport and admixture in the concrete mix. In addition to or instead of adjusting the pH of the slurry, dispersants such as phosphates, citric acid, polyacrylate or glycerine may be used to obtain the desired consistency of the slurry. It is most economical to use water when preparing the additive composition of the present invention, but an organic liquid can also be used provided it is compatible with the concrete and not in any other way harmful.

The relative viscosity of the subject epidemic blend was measured with a Haake viscometer using an E.30 sensor following the procedure described by the manufacturer. The slurry addition was compared to a "zero mixture" water-soluble addition which contained the same amount of silica without any plasticizer. In both samples, 30 silica dust accounted for 65% by weight. The samples provided the following data:

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Trial Reverse speed - 1 hour 7 days 28 days for sensor-measured-measured-measured-rotation rotation-turning-turning-turning-torque_ torque_ torque_

Zero mixture 32 53> 150> 150. 5. 16 56> 150> 150 8 60> 150> 150 4 64> 150> 150 2 69> 150> 150 1 78> 150> 150

Yield point 49> 150> 150

Sample A ligno 32 4 8 11 sulfonate 16 4 11 16 2.5 wt% 8 5 11 17 ("Drill press 4 6 12 18 NA®. 2 7 14 2 3 1 9 16 23

Yield Point 2 3.5 15.0 15 Sample B Sulfonated 32 18 17 26 Naphthalene 16 20 21 27 Formaldehyde Concentrate 8 25 23 26 Density 2.5% by weight 4 27 24 25 ("Mighty" ® 2 32 2 8 27 1 39 34 33

Yield point 28 28 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Sample C Sulfonated 32 21 57 55 2 Nourished Melamine 16 32 63 61 3 Phrmaldehyde Cr - 8 3 3 5 8 6-in 4 Density 2.5% by weight 4 36 74 69 ("Rescon Η P" ®) 2 42 61 76 1 49 91 S8 5

Yield point 32 S3 72 6 7

As can be seen from the table, silica dust tends to form a thixotropic mixture in water. This often causes the 9 slurry to harden (gelatinize). It was well over 10 surprisingly and unexpectedly to find that the superplasticizer in 11 Sample A above effectively reduced the slurry's tendency to gelatin, which otherwise often occurs down silica slurry alone.

13

It is believed that the superplasticizer and plasticizer 14 tend to coat the surface of the silica dust particles during mixing. This reduces the tendency of the slurry to gelatinize. Experience has shown that when the yield point in the table above is close to approx. 25, the slurry is excellent for use in accordance with the present invention. The slurry is satisfactory

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10 up to a yield point of approx. 75. When the yield point in the slurry is above approx. 100, it becomes difficult to pump and the slurry is not considered satisfactory for use in the present invention. 150 represents the upper measuring limit of the viscometer and corresponds to the consistency of a rigid paste.

According to the invention, a slurry with silica dust can be stabilized and the tendency for gelatinization can be substantially reduced or eliminated by adding to the slurry from about 10%. 0.1 to approx. 10% by weight, preferably from approx. 2 to approx. 5% by weight of a superplasticizer or plasticizer based on the dry weight of silica dust in the slurry. Generally, the amount of silica dust in the slurry will consist of from as little as 10% to as much as · approx. 80 weight. One or more super-plasticizers or plasticizers can be used alone or in combination to stabilize the silica slurry. When the slurry mixture is to be used as an additive to concrete or mortar, the amount of superplasticizer or plasticizer may exceed 10% by weight and, as specified herein, may be from about 10% by weight. 0.5 to approx. 40% by weight of the slurry mixture.

The sufficient amount of the mixture according to the invention is usually added to the fresh concrete or mortar portion to give it from approx. 2 to approx. 100% by weight and most preferably from approx.

25 2 to 25% by weight of silica dust based on the weight of cement in the concrete portion and from 0.1 to approx. 5% by weight of superplasticizer or plasticizer alone or in combination based on the amount of cement in the concrete or mortar portion.

Experiments simulating climatic conditions and construction methods at the casting site determine the optimal amounts of the constituents of the mixture and the amount of the mixture itself added to the concrete with the available working materials. This is in accordance with the usual industrial methods. Conventional tests show the effect of the mixture 35 on the concrete in terms of the concrete's air content, consistency, water separation and possible fresh air loss, 11

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curing speed, compressive and bending tensile strength, resistance to freezing and thawing, shrinkage during drying and permissible chloride content.

The commonly used method of adding superplasticizer and plasticizers often causes the concrete to disproportionately separate water and to separate. This results in a thin watery paste which fails to keep the coarse aggregate in suspension. It is also known that most superplasticizers and plasticizers 10 can cause plasticization by reducing the surface tension of the water component of the concrete mix. This can lead to the separation of coarse aggregate particles and can lead to low resistance to freezing and thawing, loss of pumping properties, poor wear resistance, difficulties in surface smoothing and poor surface in molding work.

The silica dust in the blend according to the invention has great fineness and increases the surface area of the solids per unit volume of water. This provides better separation and suspension of coarse aggregate particles. This results in increased plasticity and workability due to change in particle interference. Because cement, water and additive mixture in accordance with the invention contain more solids per unit volume, the paste becomes less aqueous and therefore will not readily separate. Water separation is therefore reduced by the silica dust by holding the water in the paste. This gives a homogeneous, highly workable and pump-friendly mixture with a reduced tendency for water separation.

The compressive strength of concrete containing the additive preparation according to the invention is generally higher than one would expect if the individual increases in firmness obtained when each component is added separately are added. The reason for this is not entirely clear, but it is believed that the superplastic or plasticizers provide a better distribution of the silica dust particles throughout the concrete mass and that there are certain synergistic effects between the constituents of the additive mixture.

The additive preparation according to the invention is particularly good in conventional fresh concrete mixtures and can be mixed into the concrete mass by means of commonly used

DK 165782B

12 methods. For example, a slurry containing 20.5 kg of silica dust, 3.6 kg of dry "Lomar D" (superplasticizer, sulfonated condensate of naphthalene and formaldehyde) and 20.8 l of water can be added to a conventional fresh concrete containing 205 kg 5 Portland cement type I without other additives. The concrete mixture obtained at a water / cement ratio of 0.35 by weight has a good workability, consistency and no water separation in fresh condition. In cured state, the compressive strength after 28 days will typically be high, of the order of 85 MPa.

10 The freeze-thaw resistance is astonishingly high even without mixed air. The concrete mixture contains 10% silica dust (dry) and approx. 1.5% "Lomar D" (dry) based on the weight of cement in the concrete mix. The increased density of the concrete obtained increases the resistance to penetration of water and aggressive chemicals. This provides improved freeze-thaw properties over a concrete or mortar mixture which does not contain said slurry with silica dust.

The best ratio of the components of this additive mixture prepared according to the invention is determined by 20 standard industrial tests. The constituents remain and therefore only one add-on operation is needed at the casting site, as opposed to conventional practice, which requires three or four add-on operations. All the additives can be mixed at once. In the addition mixture according to the invention, all ingredients are uniformly and homogeneously dispersed and can be mixed at once, in contrast to conventional practice, with each ingredient being added sequentially to avoid flocculation. The additive composition made in accordance with the invention saves loading time and reduces 30 errors since only one additive additive is needed instead of three or four. The need for storage options is reduced and quality control is improved by only one manufacturer supplying all the additives in the one additive according to the invention. This also eliminates the problem of contaminating the storage container. Another advantage of the present slurry mixture is that the fine particle dust is avoided on the casting site.

These are the beneficial effects of silica dust in concrete.

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13, the admixture made according to the invention gives increased sulfate resistance and increased resistance to alkali-silica reaction in concrete containing the additive mixture.

Since the additive composition in question can be used in conditions which could extend the curing time to an unacceptable degree, accelerating substances can be added to the mixture to give the best possible cure and early firmness. It can also be used where it is desirable to retard the cure time for fresh 10 concrete, eg in a bridge deck, so that curing takes place only after the casting and final operations are completed.

The additive composition according to the invention is "tailor-made" in that it contains the constituents and quantities of constituents that are best for the concrete to be used in a current construction work. Any known additive usually used in concrete or mortar can be incorporated into the additive composition of the invention.

Known accelerating agents such as calcium chloride, calcium nitrate and calcium formate can be incorporated into the additive mixture together with the main constituents of the quantities now used in the industry. These quantities are determined by standard tests for the optimal quantity for the current purpose. One or more accelerating substances may comprise from ca. 5 to 25 approx. 20% by weight based on the weight of silica dust in the addition mixture.

Retardants such as glucose in the form of glucose or sucrose, which are usually used in concrete or mortar portions, can also be incorporated into the additive mixture in optimum amounts determined by standard experiments. One or more retarding agents may be added in amounts from ca. 5 to approx.

20% by weight based on the amount of silica dust in the addition mixture.

(S)

Air admixtures, such as vinyl sol resin or "Darex", which is a sulfonated fatty acid derived from fats, may be incorporated, if desired, into the additive preparation according to the invention for special applications where a given level of admixed air may be desired. One or more air admixtures may be present in amounts of approx.

0.5 to approx. 3 weight based on the weight of silica dust.

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One or more additives alone or in combination with other additives can be incorporated together with the main ingredients of the additive preparation according to the invention. The compatibility and consistency of the additive mixture is determined by routine standard tests, as is the final effect it has on the concrete or mortar portion to be used in the casting work.

The additive preparation according to the invention may contain various amounts of selected ingredients, but in order to fully benefit from the advantages obtained by the invention, the amounts of the additive mixture mixed in an ordinary fresh concrete portion will be one which yields from approx. 2 to approx. 100 wt% silica dust based on the weight of cement and from 0.1 to approx. 5% by weight of one or more superplasticizers or plasticizers alone or in combination based on the weight of the cement. Either water or an organic liquid compatible with fresh concrete is mixed into the additive mixture in sufficient quantities to produce a slurry in which the main constituents are uniformly and homogeneously dispersed. Accelerating and retarding agents, air admixtures and any other additives are mixed with the main ingredients of the additive preparation according to the invention in an amount sufficient to give the desired concentration in the fresh concrete portion. In all cases, the optimum amount of constituents in the additive composition according to the invention is determined by standard testing under simulated climatic conditions according to the working materials and the construction methods to be used.

Claims (2)

A method of stabilizing a suspension of 5 silica dust in water which contains 10-80 wt% silica dust, characterized in that the suspension contains 0.5 -40 wt% plasticizer and adjusting the pH of the mixture with a pH regulating addition in an amount such that the pH of the mixture is in the range of 3.0 - 7.5. Process according to Claim 1, characterized in that as a pH-regulating additive a plasticizer whose pH is in the range of 3.0 - 7.5 is used. 20