DD212953A5 - CONCRETE ADDITIVE IN MICROSILICIUM DIOXIDE COMPOSITION, IN THE FORM OF A MULTICOMPONENT MIXTURE, METHOD FOR THE PRODUCTION THEREOF AND CONCRETE PRODUCED BY SUCH ADDITIVE AGENT - Google Patents

Abstract

The invention relates to a concrete additive of a multi-component mixture containing microsilica and at least one water-reducing agent or at least one high-grade water-reducing agent. Advantageously, the mixture contains one or more water reducers in combination with one or more high grade water reducers along with the microsilica. As additional additives can optionally accelerator, Verzoegerer u. Like. Be added alone or in combination. The concrete admixture is used as a homogeneous mixture, preferably in the form of a slurry in water or an organic liquid. The invention overcomes the disadvantages caused by quality losses of the air pore system in air-entrained concrete due to the use of water-reducing agents. In this case, according to the invention, the other advantageous properties with regard to processability, strength and malleability of the concrete were retained.

Description

Concrete additive in the form of a multi-component mixture containing microsilica, process for its preparation and concrete produced with this additive.

Field of application of the invention

The invention relates to a concrete additive of a multi-component mixture containing microsilica and at least one water-reducing agent or at least one high-grade water-reducing agent. Advantageously, the mixture contains one or more water reducing agents in combination with one or more high-grade water reducing agents together with the microsilica. As further additives can optionally accelerator, retarder, etc. be added alone or in combination.

Object of the invention

It is an object of the invention to overcome the disadvantages caused by quality losses of the air pore system in aerated concrete due to the use of water reducing agents. In this case, according to the invention, the other advantageous properties with regard to processability, strength and malleability of the concrete are to be retained.

Explanation of the essence of the invention

According to the invention, a premix of microsilica and at least one water-reducing agent and optionally further additives is prepared and this premix is added to the concrete mixture at a desired stage and mixed. The use of a masterbatch therefore has great advantages over the usual practice in which each admixture is added separately to the concrete batch because the water reducer during pre-mixing acts to uniformly disperse and homogeneously disperse the particles of microsilica, thereby also reducing flocculation in the Break material. 'Flakes of the material, as formed in the usual individual addition of the additives, have a very adverse effect on the uniform strength and durability of the cast concrete. When flakes are formed in the batch, this requires a prolonged mixing operation to disperse the flakes, but excessive mixing may degrade the processability and moldability of the concrete.

While the mechanism of premixing is not yet fully understood, it is believed that one obtains a synergistic effect on plasticity and processability, as well as increased strength over conventional concrete approaches where the additives were added separately.

Water-reducing agents which can be used according to the invention and optionally applicable accelerators, retarders and the like. are known and are constantly being used for high strength concrete, e.g. for those with one

Compressive strength up to about 422 to 844 kg / cm.

Characteristic of the known technical solutions

One of the biggest advances in concrete technology in recent decades has been the formation of air entrainment to protect the concrete against freeze and thaw damage in the presence of deicing chemicals. Air-entrained concrete is recommended for almost all uses. Tests have shown that concrete with an air content of about 5 to 7.5 + 1 % by volume endures up to about 1900 freeze-thaw cycles, while a concrete that is identical in all other respects but does not contain air pockets has a maximum about 150 freeze-thaw cycles endure (see eg "Air-Entrained Concrete", Portland Cement Association, Document ISO 45.02 T, 1967).

Among the many benefits of air-entrained concrete are: improved processability, increased resistance to deicing agents such as calcium chloride as well as to sulfate and improved water impermeability.

In a widely used process for producing air-entrained cement, the introduction of an air-entraining agent is used during the mixing of the concrete. Experience has shown that the mixing process is the most important factor in the production of the air pore cement, and in this case the uniform distribution of the introduced pores is essential for the production of an insensitive and resistant concrete. Unevenness is always to be expected if the introduced air is not adequately finely dispersed during mixing. Factors such as the amount of concrete mixture used, the conditions imposed by the mixer used, and the mixing speed are also important. Excessive mixing also leads to a loss of part of the

brought air. These circumstances are known to the person skilled in the art and require no further explanation.

As air entraining agents, a number of different products are commercially available, e.g. thermoplastic resins containing phenolaldehyde and ether groups, and their salts and soaps.

The most commonly used airborne agents include the Vinsol resins. Vinsol resins are thermoplastic resins derived from pinewood containing phenolic aldehyde and ether groups. The sodium soap of Vinsol resin is a particularly effective air entraining agent. Only about 0.15% by weight of the cement suffice for the formation of the air pores in a conventional concrete mixture. Another much used air entraining agent is DAREX AEA. This is a sulfonated hydrocarboxyl derivative of fats (manufactured by Dewey and Almy Chemical Co.).

A concrete produced using known air entraining agents contains a large number of self-contained, spherical micro-air pores. The average pore diameter is generally between 0.076 and 0.152 mm and since there are 300 to 500 billion air voids in 0.73 m air-entrained concrete, one has a pore content of 4 to 6 percent by volume. These pores do not embrittle each other and are well distributed in the cement / water phase. The distances between the pores is an important factor for the freeze-thaw resistance of the hardened concrete. A distance of less than 0.2 mm, measured according to ASTM C 457, is considered essential for achieving the required frost-thaw resistance.

One of the most significant advances since air entrainment has been the use of so-called water reducers.

Water reducing agents are chemical compounds that liquefy the concrete for a period of time, so that

1) the normal processability is maintained at a lower water-cement ratio concrete or

2) an extremely processable "flowable concrete" is obtained without undesirable side effects such as segregation, low durability, low abrasion resistance, precipitates, etc., or

3) a combination of the properties mentioned under 1) and 2) is achieved.

There are six different classes of known commercial water reducers:

1. hydroxycarboxylic acids and their string,

2. Modifications and derivatives of hydroxycarboxylic acids and their salts,

3. Inorganic substances, such as zinc salts, borates, phosphates and chlorides,

4. carbohydrates, polysaccharides and sugar acids,

5. Amines and their derivatives and polymeric compounds, such as cellulose ethers and silicones,

6. Certain modified lignosulfonic acids.

The products referred to in the above six classes may be used alone or in combination as water reducing agents for the purpose of the invention;

Among the common high-grade water-reducing agents are the following:

1. lignosulfonic acids, their salts and modifications, and their derivatives,

2. thiamine derivatives,

3. naphthalene derivatives.

These substances can also be used according to the invention alone or in combination as high-grade water reducing agents.

There are at least twelve widely used high-grade water reducers, eight of them from categories 2 and 3 above. The preferred Category 2 material is a common sulfonated condensate of melamine and formaldehyde (commercially known as Melment). The preferred Category 3 material is a sulfonated condensate of naphthalene and formaldehyde.

The high-grade water reducers have a much greater plasticizing effect on conventional concrete formulations. The best results in terms of processability, moldability and strength are also achieved in the practice of the invention using high-grade water-reducing agents.

Concrete with high-grade water-reducing agents is widely used for in-situ poured concrete structures where extreme flow properties are required, especially for high density areas of reinforcements, good pumping properties, and intricate structures.

Among the advantages of using high-grade water reducers for precast concrete and ready-mix concrete are:

a) increased strength at any age,

b) improved resistance to attack by sulphates,

c) increased binding to the rebar,

d) improved processability and moldability and

e) increased tightness against the ingress of water.

When a high grade water reducer is added to a concrete mix, the plasticizing effect lasts for about 30 to 60 minutes, depending on the working conditions. Accordingly, the addition must be made when using ready-mixed concrete.

Concrete with one or more water reducers is described in detail in "Super Plasticized Concrete", ACI Journal, May 1977, pp. N 6 - N 11.

Although concrete classified either as air-entrained concrete or as water- reducible plasticized concrete is eminently suitable for many applications requiring only the properties attributable to air entrainment or plasticization, the contractor may encounter difficulties attempts to use both an air entraining agent and a water reducer for plasticizing the concrete.

It is generally accepted today that the air-pore system of hardened air-entrained concrete is a high-grade water-reducing agent and

containing neutralized Vinsol resin. The air gap factor is greater than 0.2 nrim and there is the possibility of air loss of the fresh concrete. The air pore parameters and especially the pore factor of the air pore system is a main criterion for the prediction of the anticipated performance of the concrete with regard to the resistance to repeated freeze-thaw cycles.

Thus, there is a problem to produce a concrete having the desired freeze-thawing resistance and air entrained beneficial properties associated with excellent processability and increased strength of a concrete plasticized with a water reducer (concrete plasticizer).

Another object of the invention is to provide a water-reducing and plasticizing concrete admixture, by which the freeze-thaw resistance is not reduced, but increased.

It has now been found that a premixed concrete admixture of microsilica and a water-reducing agent, preferably a high-grade water-reducing agent - as Eünzelprodukt or in combination of several products - when added to mortar and concrete increases their density and impermeability by several orders of magnitude. However, it has also been found that an air-pore-free concrete produced with the microsilica additive according to the invention is in fact impermeable to the penetration of free water and aggressive liquids. Concrete containing the microsilica additive has a freeze-thaw resistance equal to or better than that of concrete with air voids and has the same or higher strength. It

·: J , '"

Thus, it can be seen that the damage to the air pore system which experience has shown to occur with the use of normal and high grade water reducers does not occur in this case, since it prevents loss of luster or enlargement of the well spacing, thanks to the beneficial effects of the microsilica additive with regard to significant changes in the pore structure in the setting phase of the concrete. Namely, there is a more even distribution with a much finer pore structure.

The microsilica to be used in the present invention is an amorphous by-product in the production of ferrosilicon and silicon metal, whereby the finely divided particles of the exhaust gases of the electric arc furnace are collected. Microsilica is a pozzolan, that is, it combines at normal temperatures with lime and moisture to form compounds with cementitious properties. The main constituent is silicon dioxide (SiO ₂ ), generally at least 60 %. For the purposes of the invention, however, the best results are obtained when the SiC.sub.2 content is at least about 85% by weight.

An amorphous silica excellent for use in the present invention is obtained as a by-product of the production of silicon metal and ferrosilicon in an electric reduction furnace. These processes produce large amounts of dust collected in filters or other deposition equipment. Such silica is supplied by Elkem a / s, Norway.

The analysis and physical data of typical samples of said silicon dioxide product are given in the following tables.

Table 1 Dust from the bag filter in the production of Si metal

Components% by weight

SiO ₂

SiC Fe ₂ O ₃

TiO ₂

Al ₂ O ₃

MgO CaO Na ₂ O

K ₂ O

Ignition loss (1000 ^cc ) Bulk density, bunker, g / l Bulk density, compact, g / l real density, g / cm specific surface area, mVg primary particle size, percentage <1 μm

.2

94 - 98 0.2 - 0.7 0.05 - 0.15 0.01 - 0.02 '0.1 - 0.3 0.2 - 0.8 0.1 - 0.3 0.3 - 0.5 , 0.2 - 0.6 0,003 - 0,01 0,002 - 0.005 0.005 - 0,01 0.001 - 0,002 0.1 - 0.3 0.2 - 1.0 0.03 - 0.06 0.8 - 1.5 200 - 300 500 - 700 2.20 - 2.25 18 - 22

Table 2 Dust from the bag filter in the production of 75% FeSi

Components wt. %

SiO ₂ SiC

Fe ₂ ° 3 - TiO 2 Al ₂ ° 3 MgO CaO Loss on ignition (1000 ⁰ C) Na ₂ 0 Bulk density, bunker, g / l K ₂ O Mn Cu Zn Ni S C P

Bulk density, compact, g / l

3 real density, g / cm

specific surface area, m / g primary particle size, percentage <1 μm

86 - 90 0.1 - 0,4 0.3 - 0.9 0.02 0.06 0.2 - 0.6 2.5. - 3,5 0.2 - 0.5 0.9 - 1.8 2.5 - 3,5

0.2 - 0,4 0.8 - 2.0 0.03 - 0.08 2.4 - 4.0 200 -300 500 - 700 2.20 - 2.25 18 - 22

90

Anorphic silica of the above type can also be obtained in other preparations of Si and FeSi, e.g. in the production of silicon by reduction of silica (crude silica, e.g., quartz) by means of coal. Iron is added when the alloy ferrosilicon is to be produced. Part of the product of this reduction of silica may be reoxidized in the vapor phase (e.g., in air) to form finely divided silica. The from an electric oven to

Production of ferrosilicon-collected dust having a Si 1iciumdioxidgehalt of at least 75 % is preferable for the purposes of the invention. The Staub.aus an electric furnace for the production of 50 % Ferrosilicium is usable.

It is also possible to obtain amorphous silica not as a by-product but as a main peak coduct by changing the reaction conditions accordingly. Anorphic silica of the type required can also be prepared synthetically without reduction and reoxidation.

The amorphous silica to be used in the present invention consists essentially of submicron spherical particles. The spherical shape together with the fineness and pozzolana ami activity surprisingly lead to the erfins' successes.

The amorphous silica particles may, for example, consist of at least 60 to 90% by weight SiO ₂ , have a real density of 2.20 to 2.25 g / cm ³ and a specific effective surface area of 18 to 22 m / g. The particles are substantially spherical and at least 90% by weight of the primary particles have a size smaller than 1 micrometer. However, deviations from these values are possible. For example, the

Particles have a lower SiCl content. The size distribution may optionally be regulated by classification to remove the coarser particles.

The amorphous silica may have a dark gray color due to a carbon content. However, the coal can be burned away at temperatures above 400 ⁰ C. It is also possible to use the procedure for manufacturing

Γ 'development to modify silicon and Ferrosilicium'in such a way as to obtain silica in a comparatively white form which is identical, but in other respects with the normally occurring gray mold. The process changes consist in a reduction in the amount of coal or in the discharge of coal from the charge. A consequence of this process change is a change in the ratio of silica produced to the amount of silicon or ferrosilicon produced.

The term microsilica used in this specification refers to particulate amorphous silica resulting from a process in which silica is reduced and the vapor phase reduction product is reoxidized in air. The term microsilica also includes the same type of silica synthesized without reduction and reoxidation. Most conveniently, the microsilica used in the present invention is obtained from the exhaust gases of electrical reduction furnaces for the production of silicon and ferrosilicon.

The concrete admixture of the invention contains from about 30 to about 98 weight percent microsilica and from about 2 to 50 percent of one or more water reducing agents, based on the weight of the microsilicon dioxide

in the additive. One or more high-grade water reducers may be used, either alone or in combination with one or more normal water reducers. These are the essential ingredients of the microsilica dioxide additive in which the selected high grade or normal water reducer is uniformly and homogeneously dispersed.

The ingredients may be mixed in any suitable known mixer. For example, For example, 22.68 kg of microsilica and 2.27 kg of high grade water reducer (Melment) are uniformly and homogeneously dispersed in a batch rotary dryer drum mixer so that the individual particles come into intimate contact. The best results are obtained when the essential ingredients are mixed in an aqueous slurry. The aqueous slurried admixture may contain: about 10 to 80% by weight of microsilica, preferably about 40 to about 60% by weight of microsilica, about 0.5 to about 40% by weight (dry weight) of one or more high grade or normal water reducers - singly or in combination - and preferably about 1.0 to about 20% by weight of said water reducing agents or combinations thereof. The rest is water.

According to one embodiment, 20.4 kg of microsilica, 1.36 kg of commercially available sulfonated naphthalene-formaldehyde condensate (high-grade water reducer) and 1.36 kg of cellulose ether (water reducer) are uniformly and homogeneously dispersed in 25 liters of water, preferably in a Benbury mixer. The pH of the aqueous slurry may be adjusted to between about 3.0 and 7.5 by conventional mineral acids or alkalis.

preferably about 5.0 to 6.0. be adjusted so that the slurry gets a proper for transport and interference in the concrete approach consistency. In addition to or instead of adjusting the pH of the slurry, one may use dispersants such as phosphates, citric acid, polyacrylates or glycerin to obtain the desired consistency. Water is the cheapest liquid for making the slurry, but if desired, one can also use an organic liquid provided it is compatible with the concrete and otherwise not harmful.

The relative viscosity of the aqueous slurry additive of the present invention was measured with a Haake viscometer using an E.30 sensor and the standard method described by the manufacturer and compared to an aqueous slurry containing the same amount of microsilica. however, did not contain a water reducer. In each sample for comparison, the slurry contained 65% by weight of microsilica. The data recorded in these tests are shown in Table 3 below.

Tabel change Ie 3 7 days 28 days sample geschwin 1 H. precisely measured precisely measured speed precisely measured a shot a shot the sensor _: a shot moment moment rotation moment 32 > 150 > 150 blank 16 53 > 150 > 150 8th 56 > 150 > 150 4 60 > 150 > 150 2 54 > 150 > 150 1 , 69 > 150 > 150 78 > 150 > 150th yield 32 49 8th 11 Sample -A- 16 8 4 2 4 11 11 12 14 16 17 18 23 Lignin sulfonate 2.5% by weight (Borresperse NA) 1 4 5 6 7 16 23 ' 9 3.5 15.0 yield 32 2 17 26 Sample -B- 16 8 4 2 18 21 23 24 28 27 26 25 27 sulfonated condensate of naphthalene and 1 20 25 27 32 34 C ι 33 Formaldehyde 2.5% by weight 39 (Mighty) 28 43 · yield 32 28 57 55. Sample -C- 16 8 4 2 1 21 63 68 74 81 91 61 64 69 76 88 sulfonated condensate of melamine and formaldehyde 2.5% by weight 32 33 36 42 49 (REscon H P) 63 72 yield 32

As can be seen from the above data, the microsilica tends to form a thixotropic mixture in water, often resulting in gelation. When the slurry comes to gel, this is not desirable because pumping from the reservoir is then extremely difficult. It was now surprising and unpredictable to find that

the high grade water reducer of samples B and C and the normal water reducer of sample A act to reduce the tendency for gelation in the aqueous slurry, as experience has shown that the presence of microsilica in aqueous slurry causes gelation.

Thus, it is believed that during mixing, the high grade and normal water reducing agents coat the surface of the particles of microsilica and thereby effectively reduce the tendency for gelation. Experience has proved the following. When the yield value is in the vicinity of about 25, the slurry is excellent for the use according to the invention. The slurry is satisfactory to a yield value of about 75. If the yield value is above about 100, there is difficulty in pumperting and the slurry is no longer satisfactory in the use of the present invention.

According to the invention, the aqueous slurry of microsilica is stabilized and the tendency to gelation is substantially limited or eliminated by removing from about 0.1 to 10.0% by weight, preferably from about 2.0 to 5.0% by weight (based on dry matter and the weight of microsilica) of a high or normal water reducer is dispersed in the aqueous slurry. In general, the amount of microsilica in the aqueous slurry ranges from 5% to about 80% by weight. For stabilization, high grade and / or normal water reducer is blended singly or in combination as described. To use the slurry as

Admixture for concrete or mortar may exceed the amount of high or normal water reducer 10% by weight and be about 0.5 to 40% by weight.

The amount of concrete admixture to be added to the fresh concrete or mortar mix depends on the use of these building materials and depends on the cement weight of the mixture.

In general, a sufficient amount of the additive is about 2.0 to 100 wt%, preferably about 2 to 25 wt%, of dry microsilica calculated on the weight of the cement in the concrete batch, and about 0.1 to 5 wt% high grade and / or normal water reducer, also based on cement weight.

In accordance with the standards of industrial practice, the optimum amount of additive constituents and the amount of additive added to the concrete in a specified range shall be determined by experiments simulating the environmental conditions and the conditions foreseen or expected to be built , Conventional tests are used to demonstrate the effects of the additive on the concrete, as it is important for the work, in particular with regard to the air content of the concrete, the consistency, the water permeability and the possible loss of air of the fresh concrete, the hardening rate, the compressive and flexural strength, the freeze-thaw resistance, the shrinkage on drying and the permissible chloride content.

Conventional addition of high grade or normal water reducers often causes excessive bleeding and segregation. This is evidenced by a thin aqueous paste which is incapable of keeping roughly aggregated particles in suspension. It is also known that most high-grade and normal water-reducing agents cause plasticization by lowering the surface tension of the aqueous component of the concrete mixture. This may result in separation of the coarsely aggregated particles and causes low freeze-thaw cycling, loss of pumpability, difficulty in finishing, and poor texture of the molded articles.

The addition of microsilica from the concrete admixture of the present invention with its high degree of fineness increases the surface area of solids per unit of water volume. This achieves a better separation and suspension of the coarse particles and this leads to an increased plasticity and processability by changing the mutual influence of the particles. If the cement mixture, water and additive according to the invention contain more solids per unit volume, the mass is less aqueous and less prone to separation. Bleeding is reduced because the microsilica keeps the water in the mass. The result is a homogeneous, pumpable, highly processable mixture with reduced bleeding properties.

The compressive strength of concrete with the additive according to the invention is generally higher than one would expect from the addition of the individual constituents as gain in strength. The reasons are not without

however, it can be understood that the high grade or normal water reducing agents will give better dispersion of the microsilica particles throughout the concrete mass and that certain synergistic effects will occur between the ingredients of the admixture.

The concrete admixture according to the invention is useful for use in conventional fresh concrete mixtures. The. Admixture can be carried out by the usual methods of concrete mixing technology, for example according to the following example.

The additive in the form of an aqueous slurry containing 20.4 kg of microsilica, 3.6 kg of dry sulfonated condensate of naphthalene and formaldehyde (Lomar D) as a high-grade water reducer and 25 liters of water, becomes a conventional fresh concrete batch with 204 kg Portland cement type I without other additives added and mixed. The resulting concrete mixture with a water-cement ratio of 0.35 (by weight) showed good processability and consistency and no segregation in the fresh state. When cured after 28 days, compressive strength was typically high and on the order of 844 kg / cm. The frost-thaw corrosion resistance was surprisingly high, even in the absence of air pores. The bed mix contained 10 % microsilica (dry) and about 1.5 % of said water reducer (dry), calculated on the cement weight of the concrete mix. The reduced permeability of this concrete increases the resistance to penetration of water and aggressive chemicals. This also results in the improvement of the freeze-thaw Wechselbeständikeit compared to a concrete or mortar, which does not contain the additive of the invention.

The multicomponent additive according to the invention is premixed in optimum proportions according to industry standards, so that one has a single storage or dispensing system, in contrast to the previous practice in which three or four dispensing systems must be present at the workplace. So all additives are entered at the same time, since the product according to the invention contains all the necessary active ingredients in a homogeneous dispersion. According to previous practice, the ingredients were added separately and sequentially to flocculates and the like. to avoid. The additive according to the invention saves transport and prevents errors, since only one addition instead of previously three or four is required. Stock keeping is easier and quality control is improved. Since only one reservoir is required, the risk of contamination and confusion is eliminated. Another advantage of the aqueous slurry is that it does not cause dust at the work site. However, it can also be supplied for smaller jobs a dry mix in bags.

The ability of the additive to give the concrete a higher sulphate resistance and higher resistance to an alkali-silica reaction is due to the beneficial effects of the microsilica blend.

Since the additive can be used under conditions where the setting time is significantly prolonged, accelerators can be added to the additive to achieve optimum setting and early strength. On the other hand, it may be desirable to delay the setting time of the fresh concrete.

The additive of the invention is made to measure and desire with optimal ingredients and in optimal proportions for a concrete for construction. All conventional concrete admixtures can be incorporated into the additive according to the invention. :

Accelerators, such as calcium chloride, calcium nitrate and calcium formate, may be incorporated into the admixture with its major constituents, in the usual amounts given by industry standards and experience. For example, one or more accelerators may be added at about 5 to 20 weight percent based on the weight of the microsilicon dioxide.

Retarders, such as sugars in the form of glucose or sucrose, may also be mixed in the known optimal amounts. For example, one or more retarders may be added in amounts of about 5 to 20 percent by weight, based on the weight of the microsilicon dioxide.

Also, air entraining agents such as sulfonated fatty acids (Vinsol resin, Darex) can be mixed into the admixture. One or more air entraining agents may e.g. in amounts of about 0.5 to 20 wt.%, Based on the weight of the microsilica present.

The additional additives provided in addition to the main components can also be mixed in combinations.

The additive may be prepared in various proportions of the selected ingredients. To take advantage of the invention, the additive in fresh concrete approach in the following

Amounts applied: about 2 to 100% by weight of microsilica based on the cement weight, and about 0.1 to 5% by weight of one or more high-grade and / or normal water-reducing agents. Preferably, water or an organic liquid compatible with the fresh concrete is added to form a slurry with uniform and homogeneous dispersion of the ingredients. Accelerators, retarders, air entraining agents and other common additives may be mixed with the main components of the present invention, each in the amounts required for the desired concentration in fresh concrete. In all cases, the optimum amount is determined according to the standards and usual test methods.

Claims (13)

invention claims A concrete admixture in the form of a multicomponent mixture, characterized in that it contains as essential constituents one or more water reducing agents and / or high-grade water-reducing agents dispersed in microsilica, the microsilica being in an amount of about 30 to 98% by weight and the water reducing agent resp. the mixture of a plurality of water-reducing agents in an amount of about 2 to 50 wt.%, Based on the weight of the microsilica are present. 2. concrete admixtures according to point 1, characterized in that the multicomponent mixture is slurried in water or an organic liquid and in this slurry about 20 to 80 wt. % solids are uniformly and homogeneously suspended. 3. concrete admixtures according to point 1 or 2, characterized by a content of one or more retarders and / or one or more air entraining agents. 4. concrete admixtures according to item 1, 2 or 3, characterized by the use of a microsilica derived from the exhaust gases of electric furnaces for the production of silicon metal or ferrosilicon. 5. concrete admixture according to point 2, 3 or 4, characterized in that in the aqueous slurry, the microsilica having a solids content of 10 wt.% And the or the water reducing agent having a solids content of about 0.4 to 40 % are included. 6. concrete admixture according to any one of items 2 to 5, characterized in that the aqueous slurry has a pH of about 3.0 to 7; comprising. 5 7. concrete admixture according to any one of items 1 to 6, characterized in that in addition to the essential components it contains other conventional concrete admixtures. 8. .... Process for the preparation of a concrete admixture according to the Points 1 to 7, characterized by mixing one or more water-reducing agents and / or high-grade water-reducing agents with microsilica, wherein the microsilica dioxide in an amount of about 30 to 98 wt.% and the water-reducing agent or a combination of several water-reducing agents in an amount of about 2 to 50 wt .% ,. based on microsilica, and that the mixing work is continued until at least one of the ingredients is uniformly dispersed in the microsilica. 9. Procedure according to point 8., characterized in that the mixing in an aqueous solution slurrying, wherein the microsilica in an amount of about 10 to 80 wt.% And the other components in an amount of about 0.5 to 40 wt.% (dry weight) are homogeneously mixed. 10. Procedure according to point 9., characterized in that the aqueous slurry is adjusted to a pH of about 3.0 to 7.5. 11. Procedure according to item 9. or 10., characterized in that in addition conventional concrete admixtures, retarders and air entraining agents mixed. 12. Use of the concrete admixture according to items 1 to 9, characterized in that admixing the concrete admixture a conventional concrete batch in such an amount that the microsilica to about 2 to 100 wt.%, Calculated on cement weight, and the water reduction component to about 0.10 to 5.0 wt.%, Based on cement weight, present in the concrete mixture .sind. 13. Hardened concrete component, characterized in that it is made using a concrete admixture according to items 1 to 12.