NO153566B - ADDITIONAL MIXTURE FOR CONCRETE AND MORTAL, PROCEDURE FOR PREPARING THE MIXTURE, AND USE THEREOF. - Google Patents

Description

This invention relates to an additive mixture in the form of a slurry for concrete and mortar. It comprises a multi-component mixture containing silica dust and at least one water-reducing additive, as well as water. The mixture can advantageously contain one or more plasticizers combined with one or more superplasticizers and silica dust. If you want, you can also mix in accelerating or retarding additives, either individually or combined.

It is previously known to use silica dust as an additive to concrete. Thus, a cement mixture consisting of cement, silica dust and possibly a plasticizer is known from Swiss patent no. 574,880. However, silica dust and plasticizer are added separately to the cement.

Furthermore, a method for producing a cement mixture with a low content of aluminate is known from British patent no. 1,532,178. The mixture consists of silica dust, cement and possibly a plasticising agent. The patent also states that cement, silica dust and any plasticizer can be mixed before they are added to the concrete. However, this is a dry mixture and thus a sufficiently homogeneous mixture of silica dust and plasticizer is not achieved.

It has now been discovered that significantly better results are achieved by adding an additive landing in the form of a slurry containing silica dust, plasticizer and water to concrete or mortar, while surprisingly it has been shown that such a slurry does not tend to gel and thus can stored for a long time. In addition, the mixture has a number of beneficial properties that will be described later.

The present invention thus relates to an additive mixture for concrete and mortar and is characterized by the fact that the additive mixture exists as a slurry containing 10-80% by weight silica dust, 0.5-40% by weight plasticizing and/or superplasticizing agent and the rest water and has a pH value within the range 3.0-7.5.

The invention further relates to a method for producing an additive mixture for concrete and mortar, which method is characterized by mixing one or more plasticizing and/or superplasticizing agents with water, after which silica dust is added in such an amount that the mixture contains 10 - 80% by weight of silica dust and 0 .5 - 40% by weight plasticising and/or superplasticising agent, and that the pH value in the mixture is adjusted to within the range 3.0 - 7.5..

Finally, the invention relates to the use of the additive mixture as an additive to concrete and mortar in such an amount that the concrete or mortar contains from 2 - 100% by weight of silica dust and 0.1 - 5.0% by weight of plasticizer and/or superplasticizer based on the weight of cement. •

The mixture can be added to the concrete batch at any point in the mixing process. Such pre-mixing has a major advantage over the usual mixing method for concrete where each component is mixed into the concrete separately. When premixed with water, the plasticizer will tend to coat the silica dust particles and disperse them homogeneously in the mass. The result is that flocculations in the material are broken down. Such flocculations often occur when the components are mixed in separately. Flocculations can mean a serious disadvantage in terms of uniform strength and durability of the poured concrete. Once flocculations have formed in the concrete batch, prolonged mechanical mixing is required to disperse them. This can lead to overmixing which can destroy the workability and castability of the batch.

Although it is not fully understood what happens during premixing, it is assumed that premixing gives a synergistic effect to the concrete batch's plasticity and workability. At the same time, a greater firmness is achieved than in ordinary concrete batches where the components are added separately, like so. is usually done.

The plasticizing and possibly accelerating and decelerating substances used in this invention are known, common agents that are currently used in high-strength concrete with a compressive strength of 40 - 80 MPa.

One of the most important developments in concrete technology since air-entrained concrete appeared in the mid-1930s is the use of so-called plasticizers.

Plasticizers are chemical compounds that make the concrete liquid for a certain time. This means that 1) normal workability can be achieved in concrete that has a much lower water-cement ratio than normal, or 2) you can get a very workable "flowable concrete", or 3) a combination of 1) and 2).

Plasticizers are well-known additives to 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 sugar acids; 5. Amines and their derivatives and polymeric compounds such as cellulose ethers and silicones;

6. Lignosulfonates.

The term plasticizer used here means one or more of the constituents in the six above-mentioned groups of common substances, either alone or in combination.

Superplasticizers that are usually used commercially today include: 1. Lignosulfonic acids and their salts and modifications and derivatives thereof; 2. Melamine derivatives;

3. Naphthalene derivatives.

The term superplasticizer is used here to mean one or more of the components in the three groups of substances mentioned above, either alone or in combination.

There are at least twelve commonly used superplasticizers.

Eight of these fall under category (2) above. The preferred substance in category (2) is a common sulphonated condensate of melamine and formaldehyde which is sold under the trademark Melment. The preferred substance in category (3) is a sulphonated condensate of naphthalene and formaldehyde.

Concrete containing superplasticizers is largely used in casting work that requires good flow properties, for example in areas with high reinforcement density, pumped concrete, and in complicated form-casting. Ncen of the advantages of using superplasticizers in prefabricated concrete elements and factory concrete are a) increased strength at all degrees of hardening, b) improved resistance to attack by sulfates, c) better bonding to reinforced steel, and d! reduced water permeability.

When a superplasticizer is added to the concrete batch, the plasticization lasts approximately 30 - 60 minutes, depending on the construction site conditions. Therefore, the substance should be added on the construction site in the case of factory-made concrete.

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

It has been found that a forblancet addition to mortar and concrete

in the form of a slurry containing 10 - 80% by weight silica dust,

0.5 - 40% by weight of plasticizers and/or superplasticizers and the rest water, and which have a pH value in the range 3.0 - 7.5 increases the density and impermeability of the mortar and concrete by several orders of magnitude. In fact, it has been found that non-entrained concrete produced with the admixture of this invention virtually eliminates the ingress of freezable water and aggressive liquids. Concrete containing the silica dust addition has a freeze-thaw resistance equal to or better than concrete that has an actual air entrainment and it has the same or higher strength. Loss of air or increased distance between the pores is offset by the beneficial effects that the addition of silica dust provides with respect to fundamental changes in the pore structure of the lime phase in concrete. The result is that the glue phase is more uniformly distributed and that it gets a significantly finer pore structure.

The silica dust used in this invention is an amorphous silica by-product from the production of ferrosilicon and silicon metal. It is produced by collecting the finely divided particles in the flue gas from electric melting furnaces. - ■ The silica dust is a pozzolan, i.e. it combines with lime and moisture at normal temperatures and forms compounds that have binder properties. The main component is silicon dioxide (SiC^). Usually the dust contains at least 60% SiC^» but the best results are obtained in this invention when the Si02~ content is at least approx. 85% of the weight.

The following tables give chemical and physical dates for typical samples of silica of this description. It is best to use dust collected from an electrothermal furnace that produces ferrosilicon containing at least 75% silicon. But also dust collected from an electrothermal furnace that produces ferrosilicon with a 50% silicon content can be used in 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 this invention is composed primarily of spherical particles measuring less than one micron in diameter. The spherical shape, the fineness and the pozzolanic reactivity make this amorphous silica very useful in this invention.

The amorphous silica can, 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. The particles are mostly spherical, and at least 90% by weight of the primary particles have a particle size of less than one m. Of course, these values can vary. Silica can, for example, have a lower SiC^ content. The particle size distribution can also be adjusted. It is, for example, possible to remove coarse particles. The dust has a specific

2

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 at temperatures above 400°C. According to the present invention, the mixture is prepared by mixing the main components in a water slurry, whereby a good contact between the components and a uniform, homogeneous distribution is achieved. The slurry additive mixture may contain from 10% to 80% by weight of silica dust and from 0.5% to 40% by weight (dry weight) of one or more superplasticizers or plasticizers alone or in combination (preferably from 1% to 20% by weight of said superplasticizer - or plasticizers alone or in combination.) The rest is water. In an example, 20 kg of silica dust, 1.3 kg of market-quality sulfonated condensate of naphthalene-formaldehyde (superplasticizer) and 1.3 of a market-quality cellulose ether (plasticizer) are distributed uniformly and homogeneously in 20 1 of water. The pH value in the slurry is adjusted with ordinary mineral acid or alkali to between 3.0 and 7.5, preferably between about 5.0 to about 6.0, in order to obtain a slurry with a consistency that is suitable for transport and for mixing in the concrete batch. IN

in addition to, or instead of, adjusting the pH value in the slurry, dispersants such as phosphates, citric acid, polyacrylate or glycerin can be used to obtain the desired consistency of the slurry. It is most economical to use water when making the additive slurry in this invention, but an organic liquid can also be used provided it is compatible with the concrete and not otherwise harmful.

The relative viscosity of the slurry mixture in this invention was measured with a Haake viscometer using an E.30 sensor, according to the method described by the manufacturer. The slurry mixture was compared to a "zero mixture" water-dissolved additive containing the same amount of silica without any plasticizer. In both samples, silica dust made up 65% of the weight.

The tests yielded the following data:

As can be seen from the table, silica dust tends to form a thixotropic mixture in water. This often causes the slurry to harden (gelatinize).

This is not satisfactory, because gelatinized slurry is practically extremely difficult to pump out of the storage container.

It was rather surprising and unexpected to find that the plasticizer in sample A above effectively reduced the tendency of the slurry to gel, which otherwise often happens with silica dust alone in slurry.

It is believed that superplasticizers and plasticizers 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 above table is in the vicinity of 25, the slurry is excellent for use in accordance with this invention. The slurry is satisfactory up to a flow limit of approx. 75. When the yield point in the slurry is over 100, it becomes difficult to pump, and the slurry is not considered satisfactory for use according to this invention. 150 represents the viscometer's upper measurement limit and corresponds to the consistency of a stiff paste.\

According to this invention, slurry with silica dust can be stabilized, and the tendency to gelatinization can be substantially reduced or eliminated by adding to the slurry from 0.1% to 10%, and preferably from 2% to 5% of a superplasticizer or plasticizer by weight (dry basis ) based on the weight of silica dust in the slurry: In general, the amount of silica dust in the slurry will consist of from as little as 10% to as much as 80% by weight. One or more superplasticizers or plasticizers may be used alone or in combination to stabilize the silica slurry. When the slurry mixture is to be used as an addition to concrete or mortar, the amount of superplasticizing or plasticizing substances may exceed 10% of the weight and, as specified here, it may amount to from 0.5% to 40% of the weight of the slurry mixture.

The sufficient amount of the mixture of this invention is usually added to the fresh concrete or mortar batch to give it from 2% to 100%, and preferably from 2% to 25% of silica dust based on the weight of cement in the concrete batch, and from 0.1% to 5% of superplasticizer or plasticizer alone or in combination based on the weight of cement in the concrete or mortar batch.

Experiments that simulate climatic conditions and

construction procedures at the casting site determine the optimum amounts of the constituents of 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 normal 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, curing speed, compressive and flexural strength, resistance to freezing and thawing, shrinkage during drying and permissible chloride content.

The commonly used method of adding superplasticizers and plasticizers often causes the concrete to excrete a disproportionate amount of water and to separate. This results in a thin watery paste that is unable to hold the coarse aggregate particles in suspension. It is also known that most superplasticizers and plasticizers can cause plasticization by reducing the surface tension of the water component of the concrete mixture. 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 connection with surface smoothing and poor surface in mold casting work.

The silica dust in the mixture in this invention is very fine and increases the surface area of the solids per unit volume of water. This results in less separation and better suspension of coarse aggregate particles. This results in increased plasticity and machinability due to changes in the particle interference. Because the mixture of cement, water and admixture in this invention contains more solids per unit volume, the paste is less watery and will not separate as easily. Water separation is therefore reduced by the silica dust as this keeps the water in the paste. This gives a homogeneous, highly workable and pump-friendly mixture with a reduced tendency to water separation.

The compressive strength of concrete containing the admixture of this invention is generally higher than would be expected if one added together the individual increases in strength achieved when each component is added separately. The reason for this is not entirely clear, but it is believed that the superplasticizers or plasticizers give the silica dust particles a better distribution throughout the concrete mass and that there are certain synergistic effects between the components of the additive mixture.

This invention's additive mixture is particularly good in conventional fresh concrete mixtures and can be mixed into the concrete mass using commonly used methods. For example, a slurry containing 20.5 kg of silica dust, 3.6 kg of dry Lomar D (super plasticizer, sulphonated condensate of naphthalene and formaldehyde) and 20.8 1 of water can be added to a conventional fresh concrete containing 205 kg of Portland cement Type I without other additives..The concrete mixture with a water-cement ratio of 0.35 by weight, you get good workability, consistency and no water separation in the fresh state.

In the hardened state, the compressive strength after 28 days will typically be high, in the order of 85 MPa. The freeze-thaw resistance is astonishingly high even without entrained air. The concrete mix 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 achieved increases its resistance to the penetration of water and aggressive chemicals.

This gives improved freeze-thaw properties compared to a concrete or mortar mixture that does not contain the aforementioned slurry with silica dust.

The best ratio of the ingredients in the admixture of this invention is determined by standard industrial tests. The components are pre-mixed and therefore only one addition operation is required at the casting site, in contrast to common practice which requires three or four addition operations. All the additives can be mixed in at once. In this invention's additive mixture, all components are uniformly and homogeneously dispersed and they can be mixed in at once, in contrast to common practice where each individual component is added in turn to avoid flocculation. The admixture of this invention saves loading time and reduces errors since only one batch addition is required instead of three or four. The need for storage facilities is reduced and quality control is improved by having one manufacturer supply all the additives in this invention's one additive mixture. This also eliminates the problem of contamination of the storage container. Another advantage of the slurry mixture in this invention is that fine particle dust is avoided at the casting site.

It is the beneficial effects of silica dust in the concrete mixture which means that the admixture of this invention provides increased sulphate resistance and increased resistance to alkali-silica reaction in concrete containing the admixture.

Because the admixture of this invention can be used under conditions which could prolong the curing time to an unacceptable degree, accelerating substances can be added to the mixture to give the best possible curing and early firmness. It can also be used where it is desirable to slow down the setting time for fresh concrete, for example in a bridge deck, so that setting only takes place after the casting and finishing operations have been completed.

The admixture of this invention is "tailor-made," in that it contains the ingredients and amounts of ingredients that are best for the concrete to be used in a particular construction job. Any known admixture commonly used in concrete or mortar may be incorporated into the admixture of this invention.

Known accelerating substances such as calcium chloride, calcium nitrate and calcium formate can be incorporated into the additive mixture together with the main ingredients in the quantities currently used in industry.

These amounts are determined through standard tests for optimal amounts for the purpose in question. One or more accelerating substances may comprise from 5% to 20% of the weight based on the weight of silica dust in the additive mixture.

Retardants such as sugars in the form of glucose or sucrose commonly used in concrete or mortar mixes can also be incorporated into the admixture in optimum amounts determined through standard testing. One or more retarding agents may be present in amounts from 5% to 20% by weight based on the weight of silica dust in the admixture.

Air entrainment substances such as Vinsol resin or Darex, which are sulfonated fatty acids derived from fats, may, if desired, be incorporated into this invention's additive mixture for special applications where a given level of entrained air may be desirable. One or more air entraining agents may be present in amounts from 0.5 to 3% by weight based on the weight of silica fume.

The admixture of this invention may contain varying amounts of selected ingredients, but to obtain the full benefit of this invention, the amount of admixture mixed into a normal fresh concrete batch will be sufficient to provide from 2% to 100% of silica dust based on the weight of cement, and from 0..1% to 5% of one or more superplasticizers or plasticizers alone or in combination based on the weight of cement. Either water or an organic liquid compatible with fresh concrete is mixed into the admixture in sufficient quantities to make a slurry in which the main ingredients are uniformly and homogeneously dispersed. Accelerating and retarding substances, air entraining substances and any other common admixtures are mixed together with the main ingredients of the admixture of this invention in an amount sufficient to give the desired concentration in the fresh concrete batch. In all cases, the optimal amount of ingredients in this invention's admixture is determined through standard testing under simulated climatic conditions according to which working materials and which

construction methods to be used.

Above, we have chosen to use some concrete examples of the invention. It is to be understood that the purpose is to include all changes and modifications of these selected examples which do not deviate from the spirit and scope of the invention.

Claims (1)

Method for manufacturing an accumulator electrode which consists of a number of conductive rods arranged next to each other which are joined at a distance from each other above and below by cross connections, of which at least one is made of conductive material, which conductive rods are in contact with active material which is surrounded by a sheath of electrolyte-permeable, insulating material that is resistant to corrosive attack and to the electrolyte, characterized by two perforated plastic foils being laid together with a fibrous intermediate layer so that the perforations essentially cover each other, after which the foils are joined together to form a laminate in such a way that the plastic in the foils at least partially penetrates the fibrous layer, and that the laminate is then shaped in some manner known per se into tube casings, each of which is brought to enclose at least one conducting rod and active material.