SE460288B - ADDITIVE MIXTURE FOR CONCRETE AND USE, PROCEDURE FOR ITS PREPARATION AND APPLICATION OF ITS - Google Patents

Description

460 288 2 b! U! 40 wm materials that were used in the past in high-strength concrete with a compressive strength of up to 40-80 MPa.

One of the greatest advances in concrete technology in recent decades has been the development of aerated concrete. Air mixing protects the concrete from damage caused by freezing and thawing of water in the concrete when it is above its typical water saturation point, and when it contains de-icing substances. The use of mixed air is usually recommended for concrete for almost all purposes.

Tests have shown that concrete containing about 5% to 7.5%, in 1% by volume air content, will withstand up to 1900 freeze-thaw cycles as opposed to a maximum of 150 cycles in non-aerated concrete which is otherwise completely identical.

See, for example, "Air-entrained Concretc," Portland Cement Association Document ISO 45.02T, 1967.

Air-mixed concrete has many other advantages, such as better machinability, increased resistance to de-icing substances such as calcium chloride, increased sulphate resistance and improved water tightness.

A commonly used production method for producing aerated concrete consists in adding an air mixing substance during the mixing of the concrete. Experience has shown that the mixing process is the most important factor in the production of aerated concrete. In this context, a similar dispersion of the air pores involved is of the utmost importance for producing concrete that resists flaking. Inequality is always a risk if the air involved is not sufficiently dispersed during the mixing process. Factors such as the size of the concrete batch, the condition of the mixing machine and the duration of the mixing process are also important. Overmixing can even lead to loss of some of the air involved.

The technology and the best procedures for the mixing phase in aerated concrete are now quite well known and further elaboration of this substance is not considered necessary by those who know how to do it. Several air mixtures based on many different materials are available on the market today, for example thermoplastic resins containing phenol-aldehydes and ether linkages as well as salts and soaps thereof.

Wine sol resins are without a doubt the most commonly used ß. '13 40 s 460 288 air blend in the United States. Wine sun resins are a thermoplastic gharts extracted from pine wood. They contain phenol aldehydes and ether linkages. Sodium resin soda soap is a particularly effective air blend. Only about 0.15% of the cement weight is sufficient to mix air into the concrete batch in the usual way. DAREX AEA is another commonly used air mixture. It is a sulfonated carboxylic acid derivative of fats and oils and is sold by Dewey and Almy Chemical Co.

The air-mixed concrete obtained by using well-known air-mixing products contains a large number of extremely small air pores. The average pore diameter usually ranges from eight hundredths to sixteen hundredths of a mm. There can be as many as 400 to 600 billion pores in a cubic meter of aerated concrete with an air content in the range of 4 to 6% by volume and with a maximum ballast size of 32 mm. The pores are not interconnected and are well dispersed throughout the water-cement phase. Distance between the air pores is an important factor with regard to the freezing-thawing resistance of hardened concrete. A distance of less than 0.2 mm measured according to ASTM C 457 standard is considered absolutely necessary to achieve the desired freeze-thaw resistance.

One of the most important developments in concrete technology since aerated concrete emerged in the mid-1930s is the use of the so-called plasticizing additives.

Plastic additives are chemical compounds that make the concrete liquid for a certain time. This means that 1) normal machinability can be achieved in the concrete which has a much lower water-cement number or normal or Z) one can get a highly machinable "liquid concrete" (which is actually self-leveling without the occurrence of undesirable side effects such as separation, low independence, low separation), or 3) a combination of (1) and (2). Additives are known additives for abrasion resistance and water-repellent plastic. The substances available on the market are divided into six different groups: 1. Hydroxylated carboxylic acids and their salts; 2. modifications and derivatives of hydroxylerado carboxylic acids and their salts; 3. Inorganic materials such as zinc salts, borate, phosphate and chloride; 40 carbohydrates, polysaccharides and sugar acids; S. amines and their derivatives and polymeric compounds such as cellulose ethers and silicones; 6. certain modified lignosulfonic acids.

The term plasticizing additives as used herein means one or more ingredients in the six above-mentioned groups of said substances either alone or in kmm fi natimr Superplasticized additives commonly used commercially include; 1. Lignosulfonic acids and their salts and modifications and derivatives thereof; Z. Melamine derivative; 3. naphthalene derivatives.

The term superplasticizing additive is used herein to give meaning to one or more of the excipients in the above-mentioned three groups of substances either alone or in combination.

There are twelve commonly used superplasticizing additives. Eight of these belong to those under categories (2) and (3) above. The preferred substances in category (2) are a common sulfonated condensate of melamine and formaldehyde which is pre-treated under the brand name Melmet. The preferred substance in category (3) is a sulfonated condensate of naphthalene and formaldehyde.

Superplasticity additives have a much greater plasticizing effect in ordinary concrete mixtures. The best results in terms of processability, castability and casting strength are achieved by using superplasticity additives in accordance with the present invention.

Concrete containing superplasticity additives is widely used in casting work that requires high flow properties, for example in areas with high reinforcement density, pumped concrete and in complicated moldings. Some of the advantages obtained by using the superplasticizers 5 in prefabricated concrete blocks and ready-mixed concrete are a) increased strength through all degrees of hardness, b) improved resistance to attack by sulphates, c) better bonding ability to reinforced steel, and d) reduced water penetration. ! When a superplacing additive is added, the concrete batch lasts about 30-60 minutes, depending on the construction site conditions. Therefore, the agent should be grooved on the construction site 460 288 in the case of ready-mixed concrete.

Concrete containing one or more superplasticizing additives is described in "Super Plasticized Concrete," ACI Journal, May 1977, pages N6-N11 including literature references. Although aerated concrete or plasticized concrete has proven to be very useful for many purposes that require only the qualities that can be attributed to either air mixing or plasticization, it has proved difficult when the contractor has tried both air mixing additives and a superplasticizing additive to plastic concrete.

It is today agreed that the air pore system in a hardened air-mixed concrete containing a superplastic resin additive and neutralized Vinsol resin is very poor because the distance factors between the air pores are greater than 0.2 mm, and since there is a potential loss of air from the fresh bed - tongues. As mentioned, the air pore parameters, especially the distance factors in the air pore system, are one of the most important criteria for determining the concrete's probable resistance to repeated freezing-thawing cycles.

The industry's problem is therefore to produce a concrete that combines the freezing-thawing properties (and the properties associated with them) in aerated concrete and the good machinability and increased strength in plasticized concrete.

Another object of this invention is therefore to provide a plasticizing additive to concrete which will not reduce but improve the freezing-thawing resistance.

It has been found that a premixed mortar and concrete additive consisting of silicate dust and one or more plasticizing additives, preferably a superplasticizing additive used alone or in combination, increases the density and impermeability of the mortar and concrete by several orders of magnitude. In fact, it has been found that non-aerated concrete produced with the silica additive of this invention virtually eliminates the penetration of freezing water and aggressive liquids. Concrete containing silica additive having a freeze-thaw resistance equal to or better than concrete having an actual air mixture and having the same or higher strength. The degradation of the air pore system usually experienced with plasticizing and superplasticizing additives is therefore irrelevant.

The loss of air or increased distance between the pores is offset by the beneficial effects that the silica additives give with regard to fundamental changes in the pore structure of the adhesive phase in the concrete. The result is that the adhesive phase becomes more evenly distributed and that it has a significantly finer pore structure. The present additive mixture is characterized in that it is in the form of a slurry, which contains 10-80% by weight of silica dust, 0.5-40% by weight of plasticizing and / or superplacing agents based on dry weight and the balance water and having a pH in the range of 3.0-7.5.

The present process for preparing the additive mixture is characterized in that one or more plasticizers and / or superplasticizers are mixed with water, after which silica dust is added so that the mixture contains 10-80% by weight of silica dust and 0.5-40% by weight. cent placentating and / or superplasticizing agent based on dry weight and the pH of the mixture is adjusted to a value of 3.0-7.5.

According to the present invention, an additive is used as an additive in a mixture, as defined herein, tong and mortar, so that the concrete or mortar contains 2-100% by weight of silica dust and 0.1-5% by weight of plasticizer and / or superplasticizer, calculated on the amount of .e- cement.

The silica dust used in the present invention is an amorphous silica complex from the production of ferro-silicon and silicon metal. It arises by collecting the finely divided particles in the flue gases from electric furnaces. The silica dust is a pozzolan, ie it includes a compound with lime and moisture at normal temperatures and forms compounds that have binder properties. The main constituent is silica (SiO2). Generally, the substance contains at least 60% of the invention when the Si content is at least S5% I.

S102 but the best results are obtained according to the present of the weight.

An amorphous silica which is excellent for use in this invention is obtained as a by-product in the production of silicon metal or ferro-silicon in electric reduction furnaces. In these processes, quite large amounts of silica are formed in the form of dust, which is collected in filters or other collection devices. Such silica can be obtained from Elkem A / S, Norway.

The following tables provide chemical and physical data for typical silicon dioxide samples as described.

Table 1 Dust collected in bag filter during production of silicon metal Component Weight percentage SiO2 94 ~ 98 Si2 0.2 - 0.7 FezO3 0.05 - 0.15 TiO2 0.01 - 0.02 AIZOS 0.1 - 0.3 MgO 0 .2 - 0.8 CaO 0.1 - 0.3 Nazo 0.3 '- 0.5 KZO 0.2 - 0.6 Mn 0.003 - 0.01 cu 0.002 - 0.005 Zn 0.005 - 0.01 Ni 0.001 ~ 0.002 S 0.1 - 0.03 C 0.2 - 1.0 P 0.03 - 0.06 Annealing loss _, (100 ° C) 0.8 - 1.5 Scratch density from filter g / 1,200 - 300 bulk density , compact g / 1 500 - 700 density of phasoms, g / cms 2.20 - 2.25 specific surface area, mz / g 18 - 22 primary particle size percent <I um 90 460 288 Table 2 Dust collected in bag filters during production of 75 Percent FeSi Component Weight percentage SiO 2 86 - 90 Si 2 2 0.1 - 0.4 Fe 2 O 3 0.3 - 0.9 Tio 2 0.02 - 0.06 m 2 O 2, 2 ~ om Mg0 2.5 - 3.5 CaO 0, 2 - 0.5 Na 2 O 0.9 - 1.8 K 2 O, - 3.5 Mn - Cu - Zn - Ni - S 0.2 - 0.4 c 0.8 - 2.0 P 0.03 - 0, 08 Annealing loss (1000 ° C) 2.4 - 4.0 bulk density from filter g / l 200 - 300 bulk density f compacted g / l 500 - 700 solids density g / cm š 2.20 - 2.25 - * specific surface area m2 / g 18 - zz primary particle size percent <I um 90 Amorphous silica of the above type can be obtained from other producers of Si and FeSi. The production of silicon involves, for example, the reduction of silica (coarse-grained quartz) with carbon. Iron is involved in the production of ferro-silicon alloy no.

Part of the product From this reduction of carbon dioxide can be reoxidized in the gas phase (ie air] to form the fine particulate silica useful in connection with this invention.It is best to use dust collected from an electric furnace which produces Ierrokiscl Containing at least 75% silicon, dust collected from an electric furnace producing 9% silicon with 50% silicon content may also be used in accordance with this invention.

It is also possible to obtain 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 reoxidation.

The amorphous silica used in the present invention is mainly composed of spherical particles having a diameter below 1 μm. The spherical shape, fineness and pozzolanic reactivity make this amorphous silica very useful according to this invention.

The amorphous silica may, for example, consist of at least 60-90% by weight of SiO 2. It will have a solids density of 2.20-2.25 g / cm 2.

The particles are mostly spherical and at least 90% of the weight of the primary particles has a particle size of less than 1 μm. Of course, these values may vary. Silica, for example, may have a lower SiO2 content. The particle size distribution can also be adjusted. For example, it is possible to remove coarser particles. The fabric has a specific surface area of 18-22 cm2 / g.

The amorphous silica has a dark gray color due to the carbon content. However, the carbon can be burned off, for example at temperatures above 40006. It is also possible to modify the production processes for silicon and ferro-silicon and thereby obtain a silica which is relatively white but in other ways practically identical to the gray silica which is usually produced. The modification mainly consists of reducing the amount of carbon or eliminating carbon from the raw material. Another possibility is to change the ratio between the amount of silicon dioxide produced and the amount of silicon or ferro-silicon. In other words, the ratio of silica to silicon and ferro-silicon will be higher in the modified process.

The term silica dust used herein means the particulate amorphous silica formed in a process where silica is reduced and where the reduction product is oxidized in the gas phase in air. The term silica dust also includes the same type of amorphous silica that is produced synthetically without reduction and reoxidation. The silica dust as 460 238 1Û 3U 'electric reduction furnaces. The additive mixture used in according to this invention can be most easily obtained from the exhaust gas from silicon metal or ferro-silicon prepared in according to this invention containing from about 30 to about 98% by weight of silica dust and from about Z to 50% of one or more plasticizing additives but based on the weight of silica dust. in the additive mixture. One or more superplasticizing additives may be used either alone or in combination with one or more plasticizing additives. These are the most important constituents in the silica additive where the superplasticizing or plasticizing additives are similar and homogeneously distributed.

The ingredients can be mixed in any conventional mixer. For example, 25 kg of silica dust and 2.5 kg of Melment superplasticizing additives can be fed into a rotating drum mixer. In this case, the particulate constituents are distributed evenly and homogeneously so that they come into close contact in the additive mixture in accordance with this invention.

The best results are obtained by mixing the main constituents in a water porridge to ensure that there is a good "rate" between the constituents and a similar, homogeneous distribution.

The porridge additive mixture may contain from about 10 to 80% by weight of silica dust (preferably from about 40 to 60% by weight of the silica dust) and from about 0.5 to about 40% by weight (dry weight) of one or more superplasticizing or plasticizing groove additives alone or in combination (preferably from about 1% to about 20% of said superplasticizing or plasticizing additive alone or in combination). the chest is water. In one example, 20 kg of silica dust, ln quality (superplasticizing additive) and 1.3 kg of, - 1 kg of sulphonated condensate of naphthalene formaldehyde are distributed on the market. Banbury hlandare. The pH values in the porridge can be adjusted with ordinary mineral acid or alkali to between 3.0 and about 7.5, preferably between about .0 to about 6.0 to get a porridge with a consistency that is suitable for transport and for mixing in bctongsatsen. In addition or instead of adjusting the pH of the porridge, dispersants such as phosphates, citric acid, polyacrylate or glyccrine can be used to obtain the desired consistency of n. (Ss. 1U 1'- is most economical to use water when preparing additive porridge according to the present invention, but an organic liquid can also be used provided that it is compatible with the concrete and not otherwise harmful.

The relative viscosity of the porridge mixture in this invention was measured with a Haake viscometer using a 15.30 sensor, according to the procedure described by the manufacturer. The porridge additive was compared with a "zero-mixture" water-dissolved additive containing the same amount of silica without any plasticizer additive. In both samples, silica dust accounted for 65 'å of the weight. The tests gave the following data: The Inversa test speed- 1 hour. up- 7 days 28 days the heat on sensed measured torque measured measured narrotation torque torque torque Zero mixture 32 53> 150> 150 16 56> 150> 150 8 60> 150> 150 4 64> 150> 150 2 69> 150 > 15 1 78> 150> 150 Tensile strength 49> 150> 150 Sample -A- 32 4 8 11 lignosulfonate 16 4 11 16 2.5% by weight 8 5 11 17 (Borresperse NA) 4 6 12 18 2 7 14 23 1 9 16 Stretch limit 2 3, 5 15, 0 Sample -B- Sulfonated 32 18 17; 26 naphthalene-formal- 16 20 21 27 dchyrí condensate 8 25 23 26 2.5% by weight 4 27 24 25 (Mighty) 2 32 28 27 1 S9 3-1 53 Tensile strength 28 28 43 Sample -C- sulfonated 32 21 57 55 xxxelamine -formal- 16 32 63 151 delrvd condensate 8 33 (18 15-1 2.5% by weight 4 31) 74 09 (liescon 11 P) 2 43 81 To 1 49 91 SS Stršíckgråíns 32 63 TI As can be seen from the table, the silica dust has a tundens to form a thixotropic solution in water. l1ettzx göi- often that the porridge stwnar tgelz-xtinornr). This is unsatisfactory, since the container crucible is practically extremely difficult to pump out of the storage containers. It is quite surprising and unexpected to find that superplasticizing additives in sample A above effectively reduce the porridge's tendency to gelatinize, which otherwise often happens with silica dust alone in vegetables - TIBII. 43 It is believed that the superplasticizing and plasticizing additives have a tendency to coat the surface of the silica particles during mixing. This reduces the porridge's tendency to gelatinize. Experience has shown that when the yield strength in the table is close to about 25, the porridge is excellent for use in accordance with the present invention. The porridge is satisfactory up to a yield strength of about 75. When the yield strength of the porridge is above about 100, it becomes difficult to pump and the porridge is not considered satisfactory for use according to the present invention.

In accordance with this invention, the porridge with silica dust can be stabilized and the tendency to gelatinize can be substantially reduced or eliminated by adding the porridge from about 0.1 to about 10% and most preferably from about 2 to 5% of a superplasticizing or plasticizing additive of the weight (dry base) based on the weight of the silica dust in the porridge. In general, the amount of silica dust in the porridge U will consist of from as little as 5% to as much as about 80 a by weight. One or more superplasticizing or plasticizing additives may be used alone or in combination to stabilize the silica porridge. When the silica mixture is to be used as an additive to concrete or mortar, the amount of superplasticizing or plasticizing additive 0 may exceed 10% by weight and as specified here it may be from about 0.5 to about 40% of the weight of the porridge mixture.

The amount of porridge to be added to a conventional fresh concrete mix or mortar according to the present invention will vary depending on the particular use. The amount of porridge to be added is determined by the weight of cement in the concrete or mortar set.

The sufficient amount of the mixture according to the present invention is usually added to the fresh concrete or mortar to give it from about 2 to about 100% and F preferably from about 2 to ZS% of silica dust based on the weight of 50 460 288 13 0. of cement in the concrete batch and from about 0.1 to about 5 a of the super-plasticizing or plasticizing bile alone or in combination based on the weight of cement in the concrete or mortar batch.

Experiments that simulate climatic conditions and construction procedures on castings determine the optimal amount of the components in the mixture and the amount of the mixture itself to be added to the concrete with the working materials available. This is in accordance with standard industrial methods. Conventional tests show the effect of the mixture on the concrete with regard to the concrete's air content, consistency, water separation and possible air loss from fresh concrete, hardening speed, compressive and flexural strength, resistance to freezing and thawing, loss on drying and permissible chloride content.

The commonly used method of adding superplasticity and plasticizing additives often causes the concrete to separate incomparably much water and to separate. This results in a thin water-like paste which is unable to keep the coarse ballast articles in suspension. It is also known that most superplasticizing and plasticizing additives can cause plasticizing by reducing the surface tension of the water component of the concrete mix. This can lead to the separation of coarse ballast particles and can lead to low resistance to freezing and thawing, loss of pumping properties, poor abrasion resistance, difficulties in surface smoothing and poor surface during molding work.

The silica dust in the mixture according to this invention has great fineness and increases the surface area of the solids per unit of water volume. Thereby better separation and suspension of coarse ballast particles is achieved. This results in increased plasticity and workability due to changes in the article interference. Since the mixture of cement, water and additive mixture according to the present invention contains several solids per unit volume, the paste becomes less water-like and will not be so easy to separate. The intrinsic excretion is therefore reduced by the silica dust by holding water in the 460 288 1U lb 2U ÅS 14 paste. This gives a homogeneous, very workable and pump-friendly mixture with a reduced tendency to water separation.

The compressive strength of the concrete containing the additive mixture of this invention is generally higher than would be expected if the individual increases in strength obtained when each component is added separately were added together.

The basis for this is not entirely clear, but it is assumed that the superplasticizing or plasticizing additives give the silica dust particles a better distribution in the entire concrete mass and that there are certain synergistic effects between the components in the additive mixture.

The additive mixture of the present invention is particularly good in conventional fresh concrete mixtures and can be mixed into the concrete mass by commonly used methods. For example, a porridge containing 20.5 kg of silica dust, 3.6 kg of dry Lomar D (superplasticizing additive, sulfonated condensate of naphthalene and formaldehyde) and 20.8 l of water can be added to a conventional fresh concrete containing 205 kg of Portland cement type. In without other additives. The concrete mixture obtained by a water-cement ratio of 0.35 by weight has good workability, consistency and no water separation in the fresh state. In the cured state, the compressive strength after 28 days will usually be high, of the order of 85 MPa.

The freeze-thaw resistance is surprisingly high even without the air involved. The concrete mixture contains 10% silica dust (dry) and about 1.5% Lomar D (dry) based on the weight of cement in the hetone mixture. The increased density of the concrete obtained increases the resistance to penetration of water and aggressive chemicals. This gives improved freezing-thawing properties in relation to a concrete or mortar mixture which does not contain said porridge with silica dust.

The best ratios of the ingredients according to the additive mixture of this invention are determined by standard industrial tests. The ingredients are mixed in advance and you therefore only need an addition operation at the casting site in contrast to usual practice which requires 3 or 4 addition operations. All additives can be mixed at once.

In the additive mixture of the present invention, all the ingredients are similarly homogeneously dispersed and they can be mixed in at once as opposed to conventional practice where each individual ingredient is added in turn to avoid flocculation. The additive mixture according to the invention saves loading time and reduces errors because only one batch is necessary to add instead of three or four. The need for storage facilities is reduced and the quality control is improved by only one manufacturer delivering all the additives in the additive mixture according to the present invention. This also eliminates the problem of contamination of the storage container. Another advantage of the porridge mixture according to the present invention is that the fine particle dust is avoided at the casting site. For smaller tasks, however, dry mixture can be packed in 35-45 kilogram bags and delivered to the casting site.

It is the beneficial effects of silica dust in the concrete mixture that make this additive mixture according to the invention provide increased resistance to sulphate and increased resistance to alkali-silica reaction in concrete which contains the additive mixture.

Since this additive mixture according to the invention can be used under conditions which may prolong the ordering time to an unacceptable degree, accelerating additives in the mixture can be made to give the best possible hardening and early firmness. It can also be used where it is desirable to delay the curing time for fresh concrete, for example in a bridge track such that curing only takes place after the casting and final operations have been completed.

The additive mixture according to the present invention is "tailored" in that it contains the constituents and amounts of constituents which are best for the concrete to be used in a particular construction work. Any known additive commonly used in concrete or mortar can be incorporated into this additive mixture according to the invention.

Known accelerating additives such as calcium chloride, calcium nitrate and calcium formate can be incorporated into the additive mixture together with the main constituent in the amounts now used in industry. These quantities are determined by standard tests for optimal quantities for the purpose in question.

One or more accelerators may comprise from about 5 to about 20% by weight based on the weight of silica dust in the additive mixture.

Retardants such as sugar in the form of glucose or sucrose commonly used in concrete or mortar kits may also be incorporated into the additive mixture in optimum amounts g as determined by standard experiments. One or more retarding additives may be present in amounts from about 5 to about 20% by weight based on the weight of silica dust in the additive mixture.

Air blend additives such as Vínsol resin or Darex which is a sulfonated fatty acid derived from fats may, if desired, be incorporated into this additive blend of the invention for particular uses where a given level of admixture may be desired. One or more air admixture additives may be present in amounts from about 0.5 to about 3% by weight based on the weight of silica dust.

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

This additive mixture according to the invention may contain different amounts of selected ingredients, but in order to take full advantage of the advantage of the present invention, the amount of additive mixture mixed into a regular fresh concrete batch should be about 100% by weight of cement and from about 0.1 to be sufficient to give from about 2 of the silica dust based on about S% of one or more substances alone or in combination based on the weight of cement. superplacing or plasticizing Either water or an organic liquid compatible with fresh concrete is mixed into the additive mixture in sufficient quantity to produce a porridge in which the main constituents are similar and homogeneously dispersed. Accelerating and retarding additives, air mixing additives and any other common additives are mixed together with the main constituents of the additive mixture of this invention in an amount sufficient to give the desired concentration in the fresh concrete mix. In all cases, the optimum amount of the constituents of the additive mixture according to the present invention is determined by standard testing under simulated climatic conditions depending on the working materials and construction methods to be used.

We have chosen above to use some concrete examples of the invention. It is to be understood that the intention is to embrace all changes and modifications of these selected examples which do not depart from the spirit or scope of the invention.

Claims (3)

_t e c k n a d thereof, 460 288 12 Patent claims Additive mixture for concrete and use. know - that it is in the form of a slurry, which contains 10-80% by weight of silica, 0.5-40% by weight of plasticizers and / or superplasticizers based on dry weight and the remainder water and which has a pH value within range 3.0-7.5. Process for preparing an additive mixture according to claim 1, characterized in that one or more plasticizing and / or superplasticizing agents are mixed with water, after which silica dust is added such that the mixture contains 10-80% by weight of silica dust and 0.5 -40% by weight of plasticizer and / or superplasticizer based on dry weight and the pH of the mixture is adjusted to a value of 3.0-7.5. Use of an additive mixture according to claim 1 as an additive in concrete and mortar said that the concrete or mortar contains 2-100% by weight of silica dust and 0.1-5% by weight of plasticizer and / or euperplasticizer, calculated on the amount of cement. L '7'