EP4352028A1

EP4352028A1 - Cement composition and method for producing a cement element

Info

Publication number: EP4352028A1
Application number: EP23757585.7A
Authority: EP
Inventors: Joshua Ryan ONOFRIO
Original assignee: Refraforce GmbH
Current assignee: Refraforce GmbH
Priority date: 2022-08-16
Filing date: 2023-08-14
Publication date: 2024-04-17
Also published as: WO2024038020A1

Abstract

The invention relates to a refractory cement composition comprising: - a proportion of 35 to 70 wt.% of a coarse-grain fraction of at least one refractory raw material having grain sizes of at least 0.5 mm; - a proportion of 15 to 30 wt.% of a fine-grain fraction of the at least one refractory raw material having grain sizes smaller than 0.5 mm; - a proportion of 0 to 20 wt.% of fine-grain calcined aluminium oxide having grain sizes smaller than 0.1 mm; - a proportion of 0 to 2 wt.% of fine-grain magnesium oxide powder having grain sizes smaller than 0.1 mm; - a proportion of 2 to 7 wt.% fine-grain aluminium metal powder having grain sizes smaller than 100 µm; - a proportion of 2 to 8 wt.% of at least one fine-grain carbon carrier having grain sizes smaller than 100 µm, wherein the at least one fine grain carbon carrier contains or is carbon black and/or graphite; - a proportion of 4 to 20 wt.% of an aqueous colloidal silica sol suspension, wherein the silica sol suspension contains a solids proportion of 30 to 50 wt.% of amorphous nanoscaled silicon dioxide particles. The invention also relates to a method for producing a fired cement element from a refractory cement composition.

Description

CONCRETE COMPOSITION AND PRODUCTION METHOD FOR A CONCRETE ELEMENT

FIELD OF THE INVENTION

The invention relates to a fireproof concrete composition. The invention further relates to a concrete element which is made from a fireproof concrete composition and to a method for producing a fired concrete element.

STATE OF THE ART

Numerous recipes for producing refractory linings are already known from the prior art for refractory products. For carbon-containing refractory products, it is known to use so-called antioxidants in the mixture, for example in the form of metal powders. These additives have the advantage of protecting the carbon from oxidation by forming voluminous metal oxides at higher temperatures and when there is little oxygen available, which reduce the pore space of the refractory products. However, these metal powders cannot be used as antioxidants or can only be used to a limited extent if the mixture or the mass formed from it contains an aqueous phase. In this case, due to the reactivity of metal powder with water and the associated hydrogen formation, there is a risk of explosion and therefore a high safety risk. From the group of metal powders, aluminum metal powder in particular is very reactive. When using reactive aluminum metal powder, contact with water or Humidity should be avoided, as large amounts of hydrogen can be formed, which pose a high safety risk due to the risk of explosion. In contrast, silicon metal powder, for example, shows a significantly lower reactivity in contact with water and is therefore less critical to handle.

For example, from US 8, 450, 229 B2 is a mixture for producing carbon-bonded magnesia bricks, i.e. basic refractory bricks, for use, for example, in a Converter has become known, the mixture comprising magnesium oxide as the main component as a refractory rock raw material, as well as pyrogenically obtained silicon dioxide powder, at least one synthetic resin as a binder and at least one metal-based powdered antioxidant and the proportion of silicon dioxide powder being between 0.01 and 5 wt. % based on the proportion of magnesium oxide is. The pyrogenic silicon dioxide powder is presented in an organic dispersion, for example in alcohol, in order to avoid the problems mentioned above when metal powders come into contact with water.

In general, US 8, 450, 229 B2 recommends working as water-free as possible. It is pointed out that carbide phases can form during the burning process of carbon-containing refractory bricks that contain metallic antioxidants, especially aluminum metal. Which is why applications in which the shaped bodies cool down to a considerable extent after in-situ carbonization and can then absorb water, as can occur, for example, when production is interrupted, lead to decomposition of the carbides formed during carbonization with an accompanying change in volume and thus destruction of the molding can lead.

Furthermore, for example, from US 2012/0142518 Al a cement-free, fireproof composition has become known, the composition containing aluminum oxide, silicon carbide, dry fumed silica, aluminum metal, a carbon-containing material, reactive aluminum oxide and additionally an antioxidant in the form of boron carbide, silicon, or mixtures contains from it. References to pyrogenic silica as a component of the fireproof composition in US 2012/0142518 Al explicitly refer to the use of dry, pyrogenically produced silica particles with grain sizes in the micrometer range (so-called "microsilica" particles), in contrast to colloidal silica. The The proportion of aluminum metal powder in this fireproof composition is stated to be at most 1.5% by weight, preferably at most 1% by weight, the fireproof composition possibly even without the addition of aluminum metal powder due to the fumed silica it necessarily contains can be produced. In the event that aluminum metal is used to produce the refractory composition, the addition of aluminum metal powder with grain sizes of approximately 200 pm is recommended, otherwise the reaction kinetics will be affected when using a finer-grained metal powder with smaller grain sizes below 200 pm, especially in the presence of water difficult to control in the refractory composition.

As can also be seen from US 2012/0142518 Al, both tests were carried out with recipes according to the fireproof composition proposed in US 2012/0142518 Al using dry, fumed silica (see recipes “Mix 1” and “Mix 2” in Tables II and III), as well as comparative experiments with recipes in which an aqueous suspension of colloidal silica was used instead of fumed silica (see recipes “Mix 3” and “Mix 4” in Tables II and III). An appropriate proportion of water was added to the recipes using dry, fumed silica.

From the comparison of the fired shaped bricks produced with the different recipes, it is known that their hot flexural strength (English: Hot MOR, Hot Modulus of Rupture; measurement method according to ASTM C583), measured at a temperature of 2700°F (corresponding to approximately 1500°C) , in the tests with the fireproof composition proposed in US 2012/0142518 Al using dry, pyrogenic silica (recipes "Mix 1" and "Mix 2") was approximately twice as high as in the corresponding comparison tests in which one aqueous suspension of colloidal silica was used instead of pyrogenic silica (recipes “Mix 3” and “Mix 4”). The maximum value of the hot bending strength at 2700 ° F (i.e. at around 1500 ° C) was set for a fireproof composition proposed in US 2012/0142518 Al (see Tables II and III: recipe "Mix 1"; with 0.5% by weight of aluminum metal powder and 6% by weight of fumed silica in the form of "microsilica" particles) a value of 742 psi (pounds per square inch) was obtained, which corresponds to a value of approximately 51 bar or 5.1 MPa. In a corresponding comparison test (see Tables II and III: Recipe "Mix 3"; with 0.5% by weight of aluminum metal powder and 8% by weight of an aqueous suspension of colloidal silica), in which an aqueous suspension of colloidal silica was used instead of fumed silica under otherwise identical experimental conditions, was just one A value of 366 psi was obtained for the hot bending strength at 2700 ° F, which corresponds to approximately 25 bar or 2.5 MPa. From US 2012/0142518 Al it follows that the use of silicon dioxide particles in a colloidal silica sol suspension In any case, it is disadvantageous compared to the use of fumed silica in the form of “microsilica” particles - regardless of whether aluminum metal powder is used or not.

The disadvantage of the refractory compositions known from US 2012/0142518 Al with the recipes “Mix 1” to “Mix 4” is at least that the strength values obtained from fired shaped bricks produced with these compositions are comparatively low.

Tables II and III also show another comparative test "Mix 5" with conventional, cement-bound concrete, whose hot bending strength at 2700 ° F is only 122 psi (the equivalent of about 8.4 bar or 0.84 MPa) and is therefore clear has worse strength values than the recipes "Mix 1" to "Mix 4". In this conventional concrete composition according to the recipe "Mix 5", which has a high proportion of 3.3% by weight of calcium aluminate cement, only a low proportion of 0, 1% by weight of aluminum metal powder and a proportion of 2.5% by weight of pyrogenically produced silica particles with grain sizes in the micrometer range ("microsilica" particles), the addition of water (according to "Mix 5" results in 5% by weight. % water added) inevitably leads to an exothermic hydration reaction of the aluminum metal powder with water. However, this reaction disadvantageously forms hydrogen gas (see also paragraph [0046] of US 2012/0142518 A1), whereby the aluminum metal powder reacts to form aluminum oxide and is therefore no longer available as a reactant in the structure of the concrete composition. When adding higher proportions of aluminum metal powder, for example 1% by weight or more, it would occur when adding Water in "Mix 5" inevitably leads to the formation of oxyhydrogen gas, which is why the "Mix 5" recipe is not suitable for the safe production of high-strength, particularly dimensionally stable refractory concrete.

Fireproof concrete compositions for the iron industry are known from KR 101 047 358 Bl, with improved corrosion resistance and high spray adhesion of silica sol being achieved using anion- and cation-based hardening agents. The concrete compositions of exemplary embodiments 1 to 8 mentioned in the two tables 1 and 2 in KR 101 047 358 Bl are each based on a fireproof starting material in the form of brown corundum (brown fused alumina) with coarse grain proportions of 71% by weight to 79.5% by weight. out of. Information on fine grain proportions of the refractory starting material with grain sizes smaller than 0.5 mm is missing. In the recipes of exemplary embodiments 7 and 8, for example, 5% by weight of carbon black, 5% by weight of aluminum metal powder with a grain size of approximately 0.2 mm, and 7.4% by weight of silica sol with a proportion of 0.3% by weight .% of an anionic dispersant used. The disadvantage of these concrete compositions is that, due to the lack of fine grain content, there is no binding matrix to embed the high coarse grain content of granular coarse components with grain sizes of up to 10 mm. In addition, it is disadvantageous to use aluminum metal powder with a grain size of up to 0.2 mm, since such a metal powder is less reactive due to its grain size, causes local concentration differences in the structure and therefore cannot be distributed as evenly in the concrete compositions as a fine-grained aluminum metal powder even smaller grain sizes. The hot flexural strengths (Hot Modulus of Rupture) of the concrete compositions given in the table were determined at a temperature of 1400 ° C and amount to a maximum of 78 kg / cm ² in the case of the recipe of exemplary embodiment 7 or a maximum of 77 kg / cm 2 in exemplary embodiment 8. cm ² (equivalent to 7.6 MPa), which is comparatively low.

However, the hot bending strengths of such refractory products are usually given at 1500 ° C (as previously in the case of US 2012/0142518 Al), with the person skilled in the art knowing, for example, from the publication by Ham cek, J. et al. : “On the high temperature bending strength of castables" [Ceramics - Silik ty 56 (3) 198-203 (2012)] it is known that the hot bending strengths, in particular of refractory materials with a low cement content (ULCC, English: Ultra-Low Cement Castables) or without cement content (NCC, English: No-Cement Castables) decrease as the firing temperature increases. It can therefore be assumed that the maximum hot bending strengths specified in KR 101 047 358 Bl at 1400 ° C are extrapolated at a temperature of 1500 ° C in any case smaller than 7, 6 MPa and are therefore comparatively low. Disadvantageously, the strength values specified in KR 101 047 358 B1, in particular hot bending strengths, of fired shaped bricks that were produced with compositions according to exemplary embodiments 7 and 8 are comparatively low.

CN 110 240 486 B relates to a conventional cement-bound concrete mass with a significant proportion of at least 4 to 6 weight. % of calcium aluminate cement, which, among other things, accounts for 4 to 6 wt. % of aluminum metal powder contains. As already mentioned, in a conventional cement-bound concrete mass, particularly in larger quantities or Aluminum powder added to higher concentrations forms gases very violently with water and hydrogen - up to the formation of oxyhydrogen. The purpose of the manufacturing process for the cement-bound concrete mass mentioned in CN 110 240 486 B is to enclose the aluminum powder through a complex surface treatment and calcination process using ceramic membrane microcapsules, so that such an inerted, metallic material with an oxidic surface does not form when added to the cement-bound concrete mass Reacts with water. In the first process step, the aluminum metal powder is inerted by the formation of surface aluminum oxide. In the second process step, the surface-corroded aluminum metal powder is placed in alkaline silica sol. Finally, the surface-treated aluminum metal powder is calcined at 500 to 700 ° C, whereby at higher temperatures from 1000 ° C, the oxide layers made of aluminum oxide (A1 ₂ O ₃ ) and silicon dioxide (SiO ₂ ) form ceramic membranes in the form of in-situ formed mullite. These ceramic membranes form microcapsules that surround the internal aluminum powder and inert it. The silicon dioxide particles from the Silica sol are bound in the microcapsules. In summary, in CN 110 240 486 B the cement-bound concrete mass with a high cement content of at least 4 wt. % neither fine-grained aluminum metal powder nor an aqueous colloidal silica sol suspension was added, but rather microcapsules surrounded by a ceramic membrane in which the inerted aluminum metal powder is enclosed and silicon dioxide particles are bound therein.

The disadvantage of this process mentioned in CN 110 240 486 B is that the separate production of the ceramic microcapsules that are added to the cement-bound concrete mass is very complex. The in-situ formed mullite whiskers described also have the disadvantage that in-situ formed mullite in the presence of higher proportions of calcium aluminate cement leads to low-melting phases with impaired hot strength properties in the fired concrete element. This is known to those skilled in the art.

In order to obtain high-strength, particularly dimensionally stable refractory concrete, the corresponding concrete composition should be made as cement-free as possible or at least contain a significantly lower proportion of cement binder than that in the

CN 110 240 486 B specified at least 4 wt. % calcium aluminate cement. Furthermore, when firing such a concrete element, the in-situ formation of ceramic mullite phases must be avoided and a high-strength, particularly dimensionally stable concrete element with high hot bending strength should therefore be as free as possible from in-situ formed ceramic mullite phases.

Recipes for producing a castable refractory material for refractory linings are already known from JP 2001 114571 A, with 0.5 to 10 weight in the concrete composition. % of aluminum powder and converted from 0.1 to a maximum of 2 wt. % solids content of amorphous silicon dioxide particles are added. The amorphous silicon dioxide particles are produced using an aqueous colloidal silica sol suspension with a solids content of 20% by weight. % of silicon dioxide particles provided. The addition of a higher proportion of the aqueous colloidal silica sol suspension (with 20% silicon dioxide particles) with a Solid content of amorphous silicon dioxide particles greater than 2 wt. % in the concrete composition is described as disadvantageous because it reduces the corrosion resistance of the refractory material. Information on the strength of fired concrete elements cannot be found in JP 2001 114571 A.

The disadvantage of the recipes known from JP 2001 114571 A is at least that the proportion of nanoscale silicon dioxide particles that are introduced into the respective concrete compositions via the silica sol suspension is very low. Concrete elements that are particularly dimensionally stable after firing cannot be produced with these recipes.

Concrete compositions are also known from US 2014/0291904 Al in which 0.1 to 5 wt. % pyrogenically produced silica particles (so-called "microsilica" particles) are used. Water is added as the mixing liquid. In the concrete compositions, which can contain up to 0.5% by weight of cement binder, there is deliberately no aluminum metal powder present in order to produce hydrogen To avoid gas formation when mixing with water. The addition of a portion of a colloidal silica sol suspension is not mentioned in US 2014/0291904 Al. However, concrete elements that are particularly dimensionally stable after firing cannot be produced with these recipes.

OBJECT OF THE INVENTION

It is therefore an object of the invention to improve the disadvantages known from the prior art and to provide a composition for the production of high-strength, particularly dimensionally stable refractory concrete, which contains carbon, which is protected from oxidation with appropriate metal powders, in particular with aluminum metal powder is, but the composition can still be processed with an aqueous liquid without safety risk in order to produce concrete elements suitable for fireproof applications with improved strength, in particular with improved hot bending strength. Furthermore, a composition for the production of refractory concrete is to be specified, with which concrete elements are produced can be made that are particularly dimensionally stable after firing. A further object of the invention is to provide a method for producing such fired concrete elements.

PRESENTATION OF THE INVENTION

To solve the problem according to the invention, a fireproof concrete composition is specified, the concrete composition comprising:

- a proportion of 35 to 70% by weight, preferably 50 to

65% by weight, of a coarse grain fraction of at least one refractory raw material with grain sizes of at least 0.5 mm, preferably with grain sizes of 0.5 mm to 12 mm;

- a proportion of 15 to 30% by weight, preferably 20 to 25% by weight, of a fine grain fraction of the at least one refractory raw material with grain sizes smaller than 0.5 mm;

- wherein the at least one refractory raw material is selected from the group consisting of: sintered alumina, precious corundum, brown corundum, gray corundum, magnesium aluminum spinel, mullite, bauxite, andalusite, chamotte, and/or silicon carbide, as well as mixtures of the aforementioned substances;

- a proportion of 0 to 20% by weight, preferably 1 to 7% by weight, of fine-grained calcined alumina with grain sizes smaller than 0.1 mm, preferably with grain sizes smaller than 50 pm;

- a proportion of 0 to 2% by weight of fine-grained magnesium oxide powder with grain sizes smaller than 0.1 mm;

- a proportion of 2 to 7% by weight, preferably 3 to 6% by weight, of fine-grained aluminum metal powder with grain sizes smaller than 100 pm, preferably with grain sizes smaller than 63 pm;

- a proportion of 2 to 8% by weight of at least one fine-grained carbon carrier with grain sizes smaller than 100 pm, the at least one fine-grained carbon carrier containing or being soot and/or graphite;

- a proportion of 4 to 20% by weight of an aqueous colloidal silica sol suspension, the silica sol suspension containing a solid proportion of 30 to 50% by weight of amorphous nanoscaled silicon dioxide particles. In the refractory concrete composition according to the invention, the proportion of fine-grain components with grain sizes smaller than 0.5 mm, in particular the fine-grain fraction of the at least one refractory raw material, forms a binding matrix for the coarse-grain fraction of the at least one refractory raw material with grain sizes of at least 0.5 mm and larger. The granular coarse components with grain sizes of at least 0.5 mm are embedded in the binding matrix formed by the fine fraction, which is advantageous for the subsequent production of particularly dimensionally stable concrete elements.

Surprisingly, it has been shown in the fireproof concrete composition according to the invention that - in contrast to the teaching of

US 2012/0142518 Al - the use of nanoscaled silicon dioxide (SiO ₂ ) particles, which are presented in an aqueous colloidal silica sol suspension, in cooperation with the other components of the fireproof concrete composition according to the invention is particularly advantageous in order to produce particularly dimensionally stable, high-strength fired ones To be able to produce concrete elements.

The term "silica sol" is understood to mean an aqueous colloidal suspension of mostly spherical polysilicic acid molecules with 30% by weight to a maximum of 60% silicon dioxide. This term is made up of the terms "silica" for silica and "sol", a synonym for colloid The approximately spherical polysilicic acid particles are connected via oxygen bridges to form an amorphous silica, also known as silica gel.

Due to their comparatively small particle sizes on the nanometer scale, the nanoscaled silicon dioxide (SiO ₂ ) particles could only be insufficiently evenly distributed as a dry, water-free solid component, i.e. as dry silicon dioxide particles, in the rest of the concrete composition with a mixture of substances with otherwise consistently larger grain sizes Micrometer scale or millimeter scale can be mixed in. In addition, when storing a dry mix for producing the refractory concrete composition, the dry silicon dioxide particles contained therein would separate. However, the nanosized silicon dioxide particles are already advantageously present in the aqueous colloidal silica sol suspension Spherical individual particles that are distributed as homogeneously as possible and are not cross-linked with each other and are hydroxylated on their surface.

Introducing a sufficient amount of the aqueous colloidal silica sol suspension as a mixing liquid into the refractory concrete composition, if necessary with mixing in a suitable concrete mixer, offers the advantage that as soon as the silica sol suspension evenly wets the mixture of the concrete composition, the nanosized silicon dioxide particles are also in the fireproof concrete composition as evenly as possible. are homogeneously distributed and the concrete composition is flowable and can be poured. This also ensures that, particularly after mixing the components of the refractory concrete composition, the other fine-grained components such as the fine-grained aluminum powder and the fine-grained carbon carrier are mixed as evenly as possible. homogeneously distributed in the concrete composition. In the following, the term “homogeneously distributed” is understood to mean the distribution that is as uniform as possible, in particular of fine-grained components with comparatively small grain sizes on the nanometer scale or on the micrometer scale, within the mixture of the refractory concrete composition. The more evenly or homogeneously distributed the fine-grained components in particular within the mixture of the refractory concrete composition with different grain sizes, the more uniform properties also have concrete elements that are produced with a concrete composition according to the invention.

Without being bound to a theory, the refractory concrete composition according to the invention appears to involve, in particular, the interaction of a sufficient amount of nanoscaled silicon dioxide (SiO ₂ ) particles presented as a silica sol, which are particularly reactive and in the refractory concrete composition in the form of the aqueous colloidal silica sol -Suspension particularly uniform or are homogeneously distributed, being particularly advantageous in combination with a sufficient amount of fine-grained aluminum metal powder and with a sufficient proportion of a fine-grained carbon carrier with grain sizes smaller than 100 pm. In our own preliminary tests it could be shown that when a concrete element is fired that is produced with a concrete composition according to the invention, which has a proportion of 2 weight in the concrete composition. % to 7 wt. % of fine-grained aluminum metal powder with grain sizes smaller than 100 pm in combination with the other components of the concrete composition, the in-situ formation of high-strength, ceramic phases occurs. Depending on the respective firing temperature, these ceramic phases contain or are aluminum in the form of one or more oxides, carbides, mixed carbides, oxycarbides, nitrides, and/or oxynitrides and/or corresponding mixtures thereof.

It could be shown that the in-situ phase formation of the aluminum-based, ceramic phases is advantageous through an interaction of the aluminum metal powder, in particular with sufficiently present, homogeneously distributed nanoscaled silicon dioxide (SiO ₂ ) particles in the concrete composition, as well as with a sufficient proportion of soot, graphite or mixtures thereof, which are distributed as fine-grained carbon carriers as homogeneously as possible in the concrete composition.

These high-strength, ceramic phases, which form in-situ when the concrete composition is fired at temperatures of approximately 800 ° C and which contain aluminum or aluminum compounds in the form of one or more oxides, carbides, mixed carbides, oxycarbides, nitrides, and/or oxynitrides, and/ or corresponding mixtures thereof, are advantageously structured in small pieces in a fired concrete element on a micrometer scale and/or on a nanometer scale and are therefore largely homogeneously distributed in the fired concrete element. Thus, fired concrete elements can be produced with the concrete composition according to the invention, which have the most homogeneous strength properties possible due to the homogeneously distributed ceramic phases and are particularly dimensionally stable.

It is particularly advantageous if the proportion of fine-grained aluminum metal powder in the concrete composition according to the invention is 3% by weight. % to 6 wt. % is set so that in the fired concrete product, which is made from the concrete composition according to the invention is produced, a sufficient proportion of such aluminum-based ceramic new phase formations is distributed. The preliminary tests showed that, for example, by selecting a proportion of 5 wt. % of fine-grained aluminum metal powder in the concrete composition, the strength of fired concrete elements made from it can be further increased, since the proportion of in-situ formed, aluminum-based ceramic phases or Structures in the fired concrete element further increased. From the current perspective, a proportion of 5% by weight seems to be the case, particularly for reasons of economic efficiency. % of fine-grained aluminum metal powder in the concrete composition achieves an optimum cost/benefit. With a further increased proportion up to about 7 wt. % of aluminum metal powder in the concrete composition according to the invention, the technical advantages mentioned continue to increase, but the material costs for producing the concrete composition according to the invention also increase.

The total proportion of the at least one refractory raw material or refractory raw material mixture in the refractory concrete composition is from 65 to 90 weight. % . A grain size distribution of this at least one refractory raw material or raw material mixture is determined by the proportion of 35 to 70 weight. % of a coarse grain fraction with grain sizes of at least 0.5 mm, and by the proportion of 15 to 30 wt. % of a fine grain fraction with grain sizes smaller than 0.5 mm. The granular coarse components with grain sizes of at least 0.5 mm are advantageously embedded in the binding matrix, which is formed by the fine-grain fraction of the at least one refractory raw material and the proportions of the other fine-grained components of the refractory concrete composition with grain sizes smaller than 0.5 mm.

Carbon black, industrial carbon black, bright carbon black, amorphous graphite, and mixtures of the aforementioned substances can be used as fine-grained carbon carriers. The term “industrial carbon black” refers to carbon black produced specifically as an industrial raw material. Bright carbon black is carbon black released from incineration plants. The possible proportion of 0 to 2 wt. % of fine-grained magnesium oxide powder (MgO) in the concrete composition serves as a setting accelerator to shorten the setting time in refractory concrete production. Depending on the fineness of the material, it may be sufficient for the fireproof concrete composition according to the invention to contain only very small proportions of magnesium oxide powder, for example around 0.05% by weight. %, contains in order to achieve a comparatively quick setting time of the refractory concrete.

In a preferred embodiment of the invention, it can be particularly useful if the solids content of amorphous nanoscale silicon dioxide particles in the concrete composition is 2 to 7 wt. % amounts . This proportion corresponds to the dry, anhydrous solid content of silicon dioxide particles contained in the aqueous colloidal silica sol suspension provided. Reactive silicon dioxide (SiO ₂ ) is thus made available in sufficient, but not too large, quantities in order to achieve the in-situ formation of the advantageous ceramic phases in an appropriate quantity in a fired concrete element produced according to the invention.

A particularly high reactivity, in particular for the desired new phase formation of ceramic phases during subsequent firing, can be achieved in a refractory concrete composition according to the invention if the amorphous nanoscaled silicon dioxide particles have grain sizes of 2 nm to 100 nm, preferably grain sizes of 5 nm to 75 nm, exhibit . A correspondingly high reactivity, ensured by correspondingly small particle sizes, causes rapid phase formation even at low temperatures.

In contrast to the concrete composition according to the invention, for example, amorphous microsilica with particle sizes in the micrometer (pm) range do not show sufficient reactivity, which is why the advantageous ceramic phases in such a structure are not obtained when such a mixture is subsequently fired with microsilica particles in the micrometer range. In order to be able to provide a fireproof concrete composition according to the invention, which can be used as flexibly as possible for different applications, the at least one fireproof raw material is selected from the group consisting of: sintered alumina, precious corundum, brown corundum, gray corundum, magnesium aluminum spinel, mullite, bauxite, andalusite , fireclay, and/or silicon carbide, as well as mixtures of the aforementioned substances.

Fireclay is a rock-like, man-made, fireproof material containing 10 to 45 percent aluminum oxide (A1 ₂ O ₃ ). Fireclay does not refer to other fireproof building materials.

Depending on the area of application and the respective requirements for the refractory products, different refractory raw materials or raw material mixtures can be used. For example, for refractory products that are subject to very high levels of stress, in particular those subject to high thermal and/or mechanical stress, such as those required for the steel production process, comparatively higher quality refractory raw materials can be used, such as sintered clay, high-grade corundum or magnesium aluminum spinel or Mixtures of them.

In other areas of application in which refractory products are subject to less stress and are without direct contact with steel and slag, comparatively cheaper refractory raw materials such as bauxite, andalusite and fireclay can be used, which are more advantageous in terms of their cost/performance ratio than, for example, the higher-quality refractory raw materials mentioned above .

It can be particularly advantageous for the desired in-situ phase regeneration of ceramic phases in the fired concrete element if, according to the invention, the amorphous nanoscaled silicon dioxide particles and/or the at least one fine-grained carbon carrier and/or the fine-grained aluminum metal powder is homogeneously distributed in the concrete composition or are . Due to the homogeneous distribution of these fine-grained Components, in particular the nanoscale silicon dioxide particles, ensure that the new ceramic phases that form in-situ during the firing process of the refractory concrete composition are correspondingly homogeneously distributed in the fired concrete material. This means that material properties that are as homogeneous as possible can be achieved in all areas of the fired concrete material produced according to the invention.

In order to achieve the most even or To ensure homogeneous distribution of one or more of the fine-grained components mentioned in the concrete composition, it may be advisable to use the dry starting material for producing the fireproof concrete composition in a suitable concrete mixer or before adding silica sol as a mixing liquid. Compulsory mixer to mix dry. Additionally or as an alternative, the dry mix for producing the refractory concrete composition can then be mixed during the addition of the appropriate amount of aqueous colloidal silica sol suspension in order to ensure a homogeneous distribution of one or more of the fine-grained components mentioned in the concrete composition.

In an alternative embodiment of the invention, the refractory concrete composition can be free of a setting accelerator, in particular free of magnesium oxide. As mentioned at the beginning, the optional addition of magnesium oxide in the concrete composition serves as a setting accelerator. The concrete composition according to the invention can also be prepared or prepared without magnesium oxide. are processed . If necessary, you can work without adding a setting accelerator. Alternatively, for example, lime, Portland cement, calcium aluminate cements, magnesium chloride and/or other magnesium salts can be used alone or in mixtures as setting accelerators instead of magnesium oxide.

However, the addition of at least one of the setting accelerators mentioned can cause a reduction in the fire resistance and thus a reduction in the application limit temperature of the fireproof concrete composition according to the invention. With the aforementioned Setting accelerators are basic components, but the fireproof raw materials mentioned at the beginning of the fireproof concrete composition according to the invention are, in contrast, non-basic. This combination can lead to the formation of low-melting phases at elevated temperatures. Accordingly, a reduction or Complete omission of the setting accelerators mentioned in the refractory concrete composition has application advantages.

From a technical perspective, it is in principle possible to use higher proportions of basic refractory raw materials such as magnesia (MgO) or dolomite in a refractory raw material mixture. However, this means that with higher proportions of basic refractory raw materials in the concrete composition - especially if they are fine-grained - the sol-gel reaction occurs all the more quickly. In such a case, the concrete composition would set particularly quickly and the processing time of the concrete composition for casting the concrete elements would thus be shortened. However, such a concrete element shows poorer hot strength properties due to the low-melting phases that occur when the temperature increases or when the basic oxides (e.g.: MgO, CaO) and the non-basic oxides (e.g.: SiO ₂ , A1 ₂ O ₃ ) present in the structure together would form.

In a further development of the fireproof concrete composition according to the invention, the concrete composition can further comprise at least one dispersant, wherein the at least one dispersant preferably contains or is sodium polynaphthalene sulfonate. By adding a suitable dispersant, the flowability of the concrete can be increased, whereby the required amount of mixing liquid, in this case the required amount of aqueous colloidal silica sol suspension, can be reduced. The addition of a suitable dispersant can be particularly advantageous, particularly for carbon-containing concretes, since carbon is generally difficult to wet with water. The object according to the invention mentioned at the outset is also achieved by a concrete element produced by casting a fireproof concrete composition according to the invention according to one of claims 1 to 7. As already mentioned at the beginning, the proportions of fine-grained aluminum metal powder (Al), fine-grained carbon carrier (C) and nanosized silicon dioxide (SiO ₂ ) particles present in the concrete composition are still contained in the structure of a concrete element according to the invention. The fine-grained ones mentioned are advantageous. nanoscale components are available as reaction partners distributed as evenly as possible in the structure for the new phase formation during the subsequent firing. The concrete composition provided according to the invention is flowable due to the added proportion of aqueous colloidal silica sol suspension.

In order to ensure the most even distribution of the silica sol suspension as a mixing liquid in the concrete composition, it can be advantageous if the fireproof concrete composition provided is mixed accordingly before the concrete element is poured into a formwork mold. This ensures that the fine-grained components required for ceramic phase formation in the fired concrete material, in particular the aluminum powder, the nanosized silicon dioxide particles and the carbon carrier material, are also distributed as evenly as possible in the structure of the concrete element.

Fireproof concrete elements and components offer advantages for the end customer because the necessary, sometimes time-consuming and complicated manufacturing steps have already been carried out beforehand.

Particularly advantageous or may be necessary if a concrete element produced with the refractory concrete composition according to the invention is essentially free of water.

Such a concrete element can be made by pouring and drying a concrete composition according to one of claims 1 to 7 or can be produced by drying a concrete element according to the invention according to claim 8. The drying temperature is advantageously chosen in the range from 110 ° C to 350 ° C in order to achieve this Concrete composition essentially dry or dry the water introduced with the aqueous silica sol suspension. to remove this from the concrete composition.

In addition to its function as a binder and source for reactive nanoscale amorphous silicon dioxide, the silica sol also offers advantages in this regard. In the reaction-based sol-gel process, which essentially causes the green binding of the refractory concrete, extremely small amounts of chemically bound hydrate phases are created and the gel structure also has a high permeability compared to other binding systems. The drying process can therefore take place very quickly and briefly at relatively low temperatures, which reduces economic efficiency and energy consumption.

The object according to the invention mentioned at the outset is also achieved by a fired concrete element, wherein the fired concrete element is made from a fireproof concrete composition according to the invention, and wherein the concrete element is fired at a firing temperature of 800 ° C to 1600 ° C and at least one or more ceramic Has phases with needle-shaped structures.

The special morphology of the ceramic phases formed in-situ results in a type of ceramic reinforcement of the concrete. Stresses in fired concrete elements that occur due to volume changes due to abrupt temperature changes can therefore be caught and absorbed more easily. This is evident from the significantly improved resistance to temperature changes of fired concrete elements produced according to the invention. The fired concrete material shows significantly better durability than conventional refractory materials when exposed to frequent temperature changes. In addition to their high mechanical strength, the in-situ formed ceramic phases are extremely resistant to chemical attacks such as contact with liquid slags.

In contrast to the method of CN 110 240 486 B, the fired concrete element produced according to the invention forms due to the Ceramic microcapsules missing in the refractory concrete composition, no mullite whiskers formed in situ.

In order to ensure ceramic phase formation, which is distributed as evenly as possible as ceramic reinforcement throughout the entire structure of the fired concrete element, it may be advisable if the concrete element is fired for a sufficiently long time at a selected firing temperature in order to achieve the most uniform possible temperature distribution with as small as possible during the firing process To ensure temperature gradients within the concrete element. The burning time required to achieve the most uniform possible temperature distribution within the concrete element during the burning process depends largely on the size and geometric shape, in particular the wall thickness, of the concrete element to be burned. The relevant test standards for dense refractory products specify the firing temperatures and holding times required to fully heat a test specimen to the prescribed firing temperature. For example, the holding time of 5 hours specified in the DIN EN 993-10 standard for determining the permanent change in length of dense, shaped refractory products (in the case of testing according to DIN EN 993-10 at 1700) can be used as a guideline for the burning time at the respective selected firing temperature ° C).

Particularly advantageous material properties can result from a fired concrete element according to the invention if the at least one ceramic phase contains or is aluminum in the form of one or more oxides, carbides, mixed carbides, oxycarbides, nitrides, and/or oxynitrides, and/or mixtures thereof.

Surprisingly, it was found that the at least one or more ceramic phases preferably form when the concrete product is at a firing temperature in the range from 800 ° C to 1600 ° C, preferably at a firing temperature of at least 1000 ° C, particularly preferably at a firing temperature of at least 1250 ° C, has been pretreated or burned.

Table 1 below provides an overview of in-situ formed ceramic phases or Phase groups listed, starting from A dried concrete element produced according to the invention (see the left column in Table 1) arises during a temperature pretreatment at the minimum temperature specified in each case and which can in turn change into other phases with a further increase in temperature. The ceramic phases formed or Phase groups are all mechanically and chemically extremely stable and some have needle-like structures. The fired refractory concrete produced according to the invention, which contains such ceramic phases or Contains phase groups, is therefore advantageously ceramic-reinforced, and has exceptionally good strength, high thermal shock resistance during temperature changes and high chemical stability towards aggressive media such as liquid slags.

A dried concrete element, which is produced with the fireproof concrete composition according to the invention and has been dried accordingly at a drying temperature of 110 ° C to 350 ° C, contains in its structure, among other things, the proportions of fine-grained, homogeneously distributed aluminum metal powder required for the new phase formation during the subsequent firing ( Al), fine-grained, homogeneously distributed carbon support material (C) and nanoscale silicon dioxide (SiO ₂ ) particles. Such a dried fireproof concrete element can already be delivered to customers.

In detail, new phase formations were determined in a fired concrete element produced according to the invention at a firing temperature of at least 700 ° C, which contained, for example, aluminum oxide in the form of A1 ₂ O ₃ , aluminum carbide in the form of A1 ₄ C ₃ , and silicon (Si). contain .

After increasing the temperature of the examined concrete element to a firing temperature of at least 800 ° C, new phase formations could be determined at 800 ° C, for example aluminum oxide (A1 ₂ O ₃ ), silicon (Si), aluminum carbide (A1 ₄ C ₃ ) and Contain aluminum-silicon (Al-Si) mixed carbides.

After a further increase in temperature of the examined

Concrete element to a firing temperature of at least 1300 ° C New phase formations can be determined at 1300 ° C, which contain, for example, aluminum oxide (A1 ₂ O ₃ ), aluminum carbide (A1 ₄ C ₃ ), aluminum nitride (AIN), aluminum oxycarbides and Al-Si mixed carbides.

Aluminum carbides decompose at higher temperatures above 1300 ° C and form Al-Si mixed carbides.

With a further increase in temperature to a firing temperature of at least 1600 ° C, new phase formations could be determined in the concrete element according to the invention examined at 1600 ° C, for example aluminum oxide (Ä1 ₂ O ₃ ), aluminum nitride (AIN), aluminum oxynitride (A1ON). , aluminum oxycarbonitrides (A1CON) and silicon aluminum oxide nitride (SiAlON) ceramic phases.

Table 1: in-situ new phase formations (phase groups) in the concrete material produced according to the invention at different firing temperatures (700 ° C to 1600 ° C)

The newly formed phase structures not only show excellent mechanical resistance, they are also chemically very stable and particularly resistant to slag attack, i.e. chemically resistant to contact with liquid slags. This fact, coupled with a very good infiltration resistance due to a special pore size distribution, results in an extraordinarily good slagging resistance to various slags that often come into contact with refractory materials in the steel production process. In contrast to conventional and known materials, the open pores of the invention show Refractory concrete has significantly smaller pore dimensions, which means that significantly lower capillary forces occur when it comes into contact with liquid phases, such as aggressive slags, and the infiltration of these media into the fired concrete material according to the invention is not possible. only takes place to a small extent compared to previously known refractory materials. This fact significantly improves the slagging resistance of fired concrete elements produced according to the invention.

Another technical advantage of fired concrete elements produced according to the invention, which is significant in most steel production applications, lies in the relatively low thermal conductivity of the concrete elements produced with the fireproof concrete composition according to the invention compared to conventional carbon-bonded materials. This circumstance can be explained by the relatively low carbon content in the concrete composition according to the invention. At an ambient temperature of 1000 ° C, the thermal conductivity of a fired concrete element produced according to the invention is, for example, around 4 W/mK, while the thermal conductivity, for example in conventional isostatically pressed products, such as those used in the continuous casting process, is at values greater than 10 W/mK lies . Heat losses can thus be significantly reduced with concrete elements produced according to the invention.

In our own preliminary tests, it was surprisingly possible to show that a fired concrete element produced according to the invention, after pretreatment at a firing temperature of at least 1000 ° C, has a cold compressive strength (according to DIN EN 993-5) of at least 140 MPa, preferably of at least 160 MPa, and/or can have a cold bending strength (according to DIN EN 993-6) of at least 20 MPa. These very high values are due to the mechanically extremely stable and sometimes needle-like or needle-shaped structures of the newly formed phases.

In addition, it was shown in preliminary tests that a concrete element produced according to the invention can have a hot bending strength, measured at 1500 ° C, of at least 15 MPa (determined according to ISO 5013 or DIN EN 993-7). Such a high one Hot bending strength indicates exceptional mechanical stability of the material at high temperatures.

A fired concrete element produced according to the invention can be used particularly flexibly if the fired concrete element is essentially stable in volume and a permanent change in length (determined according to DIN EN 993-10) of the fired concrete element after cooling is of -0.1% to +0.1 % relative to its initial length before burning. This geometric stability of the concrete material according to the invention even at and after very high firing temperatures offers, for example, great advantages with regard to the stability and longevity of fireproof brick linings.

The inventive object mentioned at the outset is also achieved by a method according to the invention for producing a fired concrete element, the method comprising the following steps:

Providing a refractory concrete composition according to the invention;

Pouring the refractory concrete composition into a formwork mold, whereby the concrete composition solidifies into a concrete element;

Removing the concrete element from the formwork form; optionally drying the concrete element, preferably choosing a drying temperature of 110 ° C to 350 ° C for drying;

Firing the concrete element at a firing temperature of 800 ° C to 1600 ° C, preferably at a firing temperature of at least 1000 ° C, particularly preferably at a firing temperature of at least 1250 ° C.

As part of the manufacturing process according to the invention, it may be necessary to additionally mix the components of the fireproof concrete composition before providing the refractory concrete composition. For example, it can be useful if the dry starting material for producing the fireproof concrete composition is initially mixed with an appropriate amount of an aqueous colloidal silica sol. Suspension is mixed and mixed homogeneously in a suitable mixer. In this case, the dry starting material and the aqueous colloidal silica sol suspension can each be stored separately from one another. When mixing the dry starting material with the aqueous colloidal silica sol suspension as the mixing liquid, the amounts and weight proportions of the components to be mixed can advantageously be precisely controlled and adjusted. The proportion of water or Humidity can be precisely adjusted and controlled in this way. be logged.

This ensures that a fireproof concrete composition is provided according to the recipe according to the invention, which is flowable and which also contains the correspondingly desired proportion of water or contains moisture.

The fireproof concrete composition provided is then poured as a flowable mass into a suitable formwork mold. The casting can advantageously be carried out by shaking and/or vibrating the corresponding formwork form in order to compact the cast concrete composition in the formwork form by means of vibration energy introduced. For example, several prepared formwork forms can be placed on a vibrating table, and several concrete elements can thus be produced at the same time. After the concrete has hardened, the concrete elements are removed from the formwork forms. The formwork forms are removed accordingly in order to preserve the hardened concrete elements.

If necessary, the concrete elements or Shaped bodies in a suitable drying unit preferably at temperatures of 110 ° C to 350 ° C.

The advantages of the final firing of a concrete element according to the invention at a firing temperature of at least 800 ° C have already been pointed out earlier. The high-strength, ceramic phases that form when the concrete composition is fired in-situ at temperatures of around 800 ° C and the aluminum or aluminum compounds in The form of one or more oxides, carbides, mixed carbides, oxycarbides, nitrides, and/or oxynitrides, and/or corresponding mixtures thereof are contained or are advantageously structured in small pieces in a fired concrete element on a micrometer scale and/or on a nanometer scale and thus largely homogeneously distributed in the fired concrete element. Thus, fired concrete elements can be produced with the concrete composition according to the invention, which have the most homogeneous strength properties possible due to the homogeneously distributed ceramic phases and are particularly dimensionally stable.

BRIEF DESCRIPTION OF THE FIGURE

Figure 1 shows the course of the cold compressive strength (measured according to DIN EN 993-5) of concrete material produced according to the invention as a function of the pretreatment temperature.

The sudden increase in the determined cold compressive strength (given in MPa) at a pretreatment temperature between 700 ° C and 800 ° C, which can be seen in Figure 1, is due to the in-situ new phase formation that occurs at this pretreatment temperature from 700 ° C during the firing process of a fired concrete element use the invention. For this purpose, concrete elements which were produced from concrete compositions according to the invention were examined. The recipe information for the concrete compositions according to the invention used for the strength tests correspond to the values given in Table 2 for exemplary embodiments 2 and 3.

The mechanically extraordinarily stable ceramic phases formed in-situ during the firing process, some with needle-like structures, bring about an extraordinarily high mechanical stability and strength of the refractory concrete material according to the invention over a very wide temperature range from 700 ° C to 1600 ° C and beyond. Cold compressive strengths of at least 140 MPa were measured in a temperature range from 800 ° C to 1300 ° C . At a pretreatment temperature of 1000 ° C, cold compressive strengths of 160 MPa could even be determined in fired concrete elements produced according to the invention. WAYS OF CARRYING OUT THE INVENTION

Table 2 below lists some exemplary embodiments according to the invention in the form of possible recipes for producing a fireproof concrete composition.

The exemplary embodiments 1 to 5 of refractory concrete compositions according to the invention given in Table 2 each serve for the subsequent production of refractory concrete elements, which can be delivered to customers after appropriate drying. Usually, the dried concrete elements are first burned on site at the customer's site in order to have the aforementioned advantages according to the invention as fired refractory concrete elements.

The recipes for refractory concrete compositions according to exemplary embodiments 1, 4 and 5 listed in Table 2 are typically used for applications in which the refractory concrete material produced therefrom comes into contact with pig iron. These are, for example, refractory applications for refractory concrete elements in the area of pig iron ladle, blast furnace troughs, and/or torpedo ladle in pig iron production.

The refractory concrete compositions according to exemplary embodiments 2 and 3 in Table 2 are typically used for applications in steel production, for example for refractory concrete blocks in the area of the steel ladle, the ladle bottom and the walls of the steel ladle, for perforated bricks, as well as for various functional refractory products, which are: Continuous casting processes are required. The ones in Fig. 1 shown values for the cold compressive strength (in MPa) were determined using fired concrete elements that were produced from refractory concrete compositions according to exemplary embodiments 2 and 3 of Table 2. The other specified strength values (cold bending strength, hot bending strength) were also determined using these refractory concrete compositions according to exemplary embodiments 2 and 3 of Table 2.

Table 2: Recipes (Examples 1 to 5) for refractory concrete compositions according to the invention

In order to provide a fireproof concrete composition according to the invention, in the recipes according to exemplary embodiments 1 to 5, the dry mixtures of the raw material components were first prepared without silica sol as the mixing liquid. For this purpose, the dry mixtures prepared in each case were mixed homogeneously according to the respective recipe. The dry mixtures were then each mixed with the in

Amounts of aqueous colloidal silica sol stated in Table 2 Suspension mixed, the silica sol suspension here each having a solid content of amorphous nanoscaled silicon dioxide particles of 40 wt. % (based on the silica sol suspension) was provided.

The homogeneous mixing of the dry mixture of the raw material components with the silica sol suspension was carried out in a compulsory mixer, maintaining a mixing time of 3 to 5 minutes.

The fireproof concrete composition according to the invention thus provided was flowable and could then be cast into suitable formwork forms. The formwork forms were advantageously vibrated during the casting of the refractory concrete composition in order to compress the casting compound accordingly and to obtain refractory concrete elements that were as pore-free as possible. After the concrete had remained in the formwork for an appropriate period of time until the setting and hardening reaction of the binder (silica sol) was complete, the concrete elements produced could be removed from the formwork. After subsequent drying, for example at 200° C., a fireproof concrete element according to the invention was obtained. After firing the refractory concrete elements at a firing temperature of 800 ° C to 1600 ° C, a fired concrete element was obtained, which was produced according to one of the concrete compositions according to the invention specified in exemplary embodiments 1 to 5.

Claims

CLAIMS Fireproof concrete composition comprising:

- a proportion of 35 to 70% by weight, preferably 50 to

- a proportion of 15 to 30% by weight, preferably 20 to

25% by weight, a fine grain fraction of the at least one refractory raw material with grain sizes smaller than 0.5 mm;

- a proportion of 4 to 20% by weight of an aqueous colloidal silica sol suspension, the silica sol suspension containing a solid proportion of 30 to 50% by weight of amorphous nanoscaled silicon dioxide particles. Fireproof concrete composition according to claim 1, characterized in that in the concrete composition the solid content of amorphous nanoscale silicon dioxide particles is from 2 to 7% by weight. Refractory concrete composition according to claim 1 or 2, characterized in that the amorphous nanoscaled silicon dioxide Particles have grain sizes of 2 nm to 100 nm, preferably grain sizes of 5 nm to 75 nm. Refractory concrete composition according to one of claims 1 to 3, characterized in that the total proportion of at least one refractory raw material or refractory

Raw material mixture in the concrete composition totaling 65 to 90 weight. % amounts . Fireproof concrete composition according to one of claims 1 to 4, characterized in that after mixing the components of the fireproof concrete composition, the amorphous nanoscaled silicon dioxide particles and / or the at least one fine-grained carbon carrier and / or the fine-grained aluminum metal powder is homogeneously distributed in the concrete composition or . are . Fireproof concrete composition according to one of claims 1 to 5, characterized in that the concrete composition is free of a setting accelerator, in particular is free of magnesium oxide. Refractory concrete composition according to one of claims 1 to 6, characterized in that the concrete composition further comprises at least one dispersant, wherein the at least one dispersant preferably contains or is sodium polynaphthalene sulfonate. Concrete element, produced by casting a fireproof concrete composition according to one of claims 1 to 7, characterized in that the structure of the concrete element contains the proportions of fine-grained aluminum metal powder, fine-grained carbon carrier and nanoscaled silicon dioxide particles present in the concrete composition. Concrete element according to claim 8, characterized in that the concrete element is free of water, in particular after drying at a drying temperature of 110 ° C to 350 ° C.

. Fired concrete element, made from a fireproof concrete composition according to one of claims 1 to 7, characterized in that the fired concrete element is fired at a firing temperature of 800°C to 1600°C and has at least one or more ceramic phases with needle-shaped structures. . Fired concrete element according to claim 10, characterized in that the at least one ceramic phase contains or is aluminum in the form of one or more oxides, carbides, mixed carbides, oxycarbides, nitrides, and/or oxynitrides, and/or mixtures thereof. . Fired concrete element according to claim 10 or 11, characterized in that the fired concrete element after pretreatment at a firing temperature of at least 1000 ° C has a cold compressive strength according to DIN EN 993-5 of at least

140 MPa, preferably at least 160 MPa, and / or a cold bending strength according to DIN EN 993-6 of at least 20 MPa. . Fired concrete element according to one of claims 10 to 12, characterized in that the fired concrete element has a hot bending strength, measured according to ISO 5013 or DIN EN 993-7 at 1500 ° C, of at least 15 MPa. . Fired concrete element according to one of claims 10 to 13, characterized in that the fired concrete element is essentially stable in volume and a permanent change in length according to DIN EN 993-10 of the fired concrete element after cooling of -0.1% to +0.1% relative to its initial length before firing. . Method for producing a fired concrete element according to one of claims 10 to 14, comprising the following steps:

- Providing a refractory concrete composition according to any one of claims 1 to 7; - Pouring the refractory concrete composition into a formwork mold, whereby the concrete composition solidifies into a concrete element;

- Removing the concrete element from the formwork form; - optionally drying the concrete element, preferably choosing a drying temperature of 110°C to 350°C for drying;

- Firing the concrete element at a firing temperature of 800°C to 1600°C, preferably at a firing temperature of at least 1000°C, particularly preferably at a firing temperature of at least 1250°C.