Concrete composition and method for producing concrete elements

Abstract

The present invention provides - coarse fraction of at least one refractory raw material having a particle size of less than 0.5 mm in a proportion of 35-70% by weight; - a fine fraction of at least one refractory raw material having a particle size of less than 0.5 mm in a proportion of 15-30% by weight; - fine-grained calcined alumina with a particle size of less than 0.1 mm in a proportion of 0-20% by weight; - Fine-grained magnesium oxide powder with a particle size of less than 0.1 mm in a proportion of 0-2% by weight; - fine-grained aluminum metal powder with a particle size of less than 100 μm in a proportion of 2-7% by weight; - at least one fine-grained carbon support having a particle size of less than 100 μm, in a proportion of 2 to 8% by weight, which comprises or is carbon black and / or graphite; - an aqueous colloidal silica sol suspension in a proportion of 4 to 20% by weight, wherein the silica sol suspension contains a solid content of amorphous nanoscale silicon dioxide particles of 30 to 50% by weight; The present invention relates to a refractory concrete composition comprising: Additionally, a method for producing fired concrete elements from the refractory concrete composition is provided.

Description

[Technical Field]

[0001] The present invention relates to a refractory concrete composition. Furthermore, the present invention relates to a concrete element produced from the refractory concrete composition, as well as to a method for producing a fired concrete element. [Background technology]

[0002] Numerous formulations for producing refractory linings are known in the prior art of 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 large amounts of metal oxides at high temperatures and with a low oxygen supply, which reduces the pore space of the refractory product. However, these metal powders cannot be used as antioxidants, or can only be used to a limited extent, if the mixture or resulting mass contains an aqueous phase. In this case, there is a risk of explosion due to the reactivity of the metal powder with water and the resulting generation of hydrogen, which therefore poses a high safety risk. Among metal powders, aluminum metal powder is particularly highly reactive. When using reactive aluminum metal powder, contact with water or moisture must be avoided because large amounts of hydrogen can be formed, which poses a high safety risk due to the risk of explosion. In contrast, for example, silicon metal powder exhibits very low reactivity when in contact with water, and therefore is less dangerous to handle.

[0003] For example, U.S. Patent No. 8,450,229 (B2) discloses a mixture for producing carbon-bonded magnesium blocks, i.e., basic refractory blocks, for use in converters, for example. The mixture contains, as its main components, magnesium oxide as the refractory rock raw material, as well as pyrogenically prepared silicon dioxide powder, at least one synthetic resin as a binder, and at least one metal-based powdered antioxidant, the proportion of silicon dioxide powder being 0.01 to 5 wt. % based on the proportion of magnesium oxide. The pyrogenic silicon dioxide powder is present in an organic dispersion, such as alcohol, to avoid the above-mentioned problems associated with contact between the metal powder and water.

[0004] In general, U.S. Patent No. 8,450,229 (B2) recommends working without using water whenever possible. It is therefore noted that carbide phases may form during the firing process of carbon-containing refractory blocks containing metal antioxidants, especially aluminum metal. Therefore, if the mold is cooled significantly after in-situ carbonation and then absorbs moisture, as may occur during a production interruption, this may result in the decomposition of the carbide formed during carbonation, resulting in a volume change and thus the destruction of the molded body.

[0005] Furthermore, for example, cement-free refractory compositions are known from US Patent Application Publication No. 2012 / 0142518(A1), which contain aluminum oxide, silicon carbide, dried pyrogenic silica, aluminum metal, carbon-containing materials, reactive aluminum oxide, and an antioxidant in the form of boron carbide, silicon, or a mixture thereof. The reference to pyrogenic silica as a component of the refractory composition in US Patent Application Publication No. 2012 / 0142518(A1) explicitly refers to the use of dry, pyrogenically prepared silica particles (so-called "microsilica" particles) having a particle size in the micrometer range, as opposed to colloidal silica. The proportion of aluminum metal powder in the refractory composition is specified as up to 1.5% by weight, preferably up to 1% by weight, and the refractory composition can be prepared without the addition of aluminum metal powder, due to the essential pyrogenic silica contained therein. If aluminum metal is used in the preparation of the refractory composition, the addition of aluminum metal powder having a particle size of about 200 μm is recommended. This is because, otherwise, when using fine-grained metal powders having particle sizes less than 200 μm, the reaction rate of the refractory composition is difficult to control, especially when water is present.

[0006] As further indicated in US Patent Application Publication No. 2012 / 0142518(A1), tests were conducted comparing formulations according to the fire-resistant compositions proposed in US Patent Application Publication No. 2012 / 0142518(A1) using dried pyrogenic silica (see formulations "Mix 1" and "Mix 2" in Tables II and III) with formulations using an aqueous suspension of colloidal silica instead of pyrogenic silica (see formulations "Mix 3" and "Mix 4" in Tables II and III), in which a corresponding proportion of water was added using dried pyrogenic silica.

[0007] A comparison of fired molded blocks produced with different formulations showed that their hot flexural strength (Hot MOR, Hot Modulus of Rupture, measured by the method of ASTM C583), measured at a temperature of 2700°F (corresponding to approximately 1500°C), was approximately twice as high in tests with the refractory compositions proposed in US Patent Application Publication No. 2012 / 0142518(A1) using dried pyrogenic silica (recipes "Mix 4" and "Mix 2") as in corresponding comparative tests in which an aqueous suspension of colloidal silica was used instead of pyrogenic silica (formulations "Mix 3" and "Mix 1"). For the refractory composition proposed in US Patent Application Publication No. 2012 / 0142518(A1) (see Tables II and III: formulation "Mix 1"; containing 0.5% by weight of aluminum metal powder and 6% by weight of pyrogenic silica in the form of "microsilica" particles), a maximum hot bending strength of 742 psi (pounds per square inch) was obtained at 2700°F (i.e., about 1500°C), which corresponds to a value of about 51 bar or 5.1 MPa, respectively. In a corresponding comparative test (see Tables II and III: formulation "Mix 3"; containing 0.5% by weight of aluminum metal powder and 8% by weight of an aqueous suspension of colloidal silica), under the same test conditions but using an aqueous suspension of colloidal silica instead of pyrogenic silica, a value of only 2.5 psi was obtained for the hot bending strength at 2700°F, which corresponds to a value of about 25 bar or 366 MPa, respectively. According to US Patent Application Publication No. 2012 / 0142518 A1, the use of silicon dioxide particles in colloidal silica sol suspensions is in any case disadvantageous compared to the use of pyrogenic silica in the form of "microsilica" particles, regardless of whether aluminum metal powder is used or not.

[0008] The drawback of the fire-resistant compositions of the formulations "Mix 1" to "Mix 4" known from US Patent Application Publication No. 2012 / 0142518(A1) is at least the relatively low strength values of fired molded blocks produced using these compositions.

[0009] Tables II and III also show the results of "Mix 5" with conventional cement-bonded concrete. Further comparative testing shows that the hot flexural strength of this concrete at 2700°F is only 122 psi (equivalent to approximately 8.4 bar or 0.84 MPa, respectively), thus significantly lower than the strength values of the "Mix 1" to "Mix 4" mixes. This conventional concrete composition, "Mix 5," contains a high proportion of alumina cement (3.3 wt.%), a small proportion of aluminum metal powder (0.1 wt.%), and 2.5 wt.% of pyrogenically prepared silica particles ("microsilica" particles) with particle sizes in the micrometer range. The addition of water (5 wt.% water is added in "Mix 5") inevitably results in an exothermic hydration reaction between the aluminum metal powder and water. However, this reaction undesirably produces hydrogen gas (see also paragraph

[0046] of U.S. Patent Application Publication No. 2012 / 0142518(A1)), and the aluminum metal powder reacts to form aluminum oxide, making it unavailable as a reactant in the microstructure of the concrete composition. If larger amounts of aluminum metal powder are added, for example more than 1% by weight, the addition of water in "Mix 5" inevitably leads to the formation of oxyhydrogen, and therefore the "Mix 5" formulation is not suitable for the safe production of high strength, especially dimensionally stable, refractory concrete.

[0010] Korean Patent No. 101047358(B1) discloses a refractory concrete composition for the steel industry, in which improved corrosion resistance and high spray adhesion of silica sol are achieved using anionic and cationic hardeners. The concrete compositions of exemplary embodiments 1 to 8 listed in Tables 1 and 2 of Korean Patent No. 101047358(B1) each assume a refractory starting material in the form of brown corundum (brown fused alumina) with a coarse particle content of 71% to 79.5% by weight. Information regarding the fine particle content of the refractory raw material, which has a particle size of less than 0.5 mm, is lacking. For example, the formulations of Examples 7 and 8 each use 7.4% by weight of silica sol containing 5% by weight of carbon black, 5% by weight of aluminum metal powder with a particle size of approximately 0.2 mm, and 0.3% by weight of an anionic dispersant. A drawback of these concrete compositions is at least the inability of the binding matrix to embed a high coarse fraction of the coarse fraction having a particle size of up to 10 mm due to the lack of a fine fraction. Furthermore, the use of aluminum metal powders having a particle size of up to 0.2 mm is disadvantageous, since such metal powders are less reactive due to their particle size, which leads to local concentration differences in the microstructure and therefore cannot be dispersed as uniformly in the concrete composition as fine-grained aluminum metal powders having even smaller particle sizes. The hot flexural strength (also called hot modulus of rupture) specified in the tables of concrete compositions is measured in each case at a temperature of 1400°C (2552°F) and is up to 77 kg / cm for the mix of embodiment 7. 2 In the case of embodiment 8, the maximum is 78 kg / cm 2 (corresponding to 7.6 MPa, respectively), which is relatively low.

[0011] However, in most cases, the hot bending fracture strength of such refractory products is indicated at 1500°C (2732°F) (as in U.S. Patent Application Publication No. 2012 / 0142518(A1)), and those skilled in the art know, for example from Hamacek, J. et al.'s publication "On the high temperature bending strength of castables" [Ceramics-Silikaty 56(3)198-203(2012)], that the hot bending strength, especially of refractories with low cement content (ULCC, ultra-low cement castable) or no cement content (NCC, no cement castable), decreases with increasing firing temperature. Therefore, the maximum hot bending strength specified at 1400°C in Korean Patent Publication No. 101047358(B1) is extrapolated to a temperature of 1500°C, in any case, less than 7.6 MPa, and therefore is estimated to be a relatively low value. Advantageously, the strength values, especially the hot bending strength, as defined in Korean Patent Publication No. 101047358(B1) of the sintered molded blocks prepared using the compositions according to embodiments 7 and 8 are relatively low.

[0012] Chinese Patent No. 110240486(B) relates to a conventional cement-bonded concrete mass containing a significant proportion of at least 4-6% by weight of calcium aluminate cement, which, among other things, contains 4-6% by weight of aluminum metal powder. As already mentioned, aluminum powder added to conventional cement-bonded concrete mass, especially in larger amounts or at higher concentrations, reacts very strongly with water, resulting in the formation of hydrogen gas, including oxyhydrogen formation. The method for producing cement-bonded concrete mass specified in Chinese Patent No. 110240486(B) therefore aims to encapsulate aluminum powder through a complex surface treatment and calcination process using ceramic membrane microcapsules, so that metal materials with passivated oxide surfaces in this way do not react with water when added to the cement-bonded concrete mass. In the first process step, the aluminum metal powder is passivated by the formation of aluminum oxide on its surface. In the second process, the surface-etched aluminum metal powder is immersed in an alkaline silica sol. Finally, the surface-treated aluminum metal powder is calcined at 500-700°C (932-1292°F), and then at temperatures above 1000°C (1832°F), an oxide layer of aluminum oxide (Al2O3) and silicon dioxide (SiO2) forms a ceramic membrane in the form of in-situ formed mullite. These ceramic membranes form microcapsules that surround and inactivate the aluminum powder inside. Silicon dioxide particles from the silica sol are bound within the microcapsules. In summary, the cement-bound concrete mass with a high cement content of at least 4% by weight described in Chinese Patent No. 110240486(B) does not contain fine aluminum metal powder or an aqueous colloidal silica sol suspension, but rather contains microcapsules surrounded by a ceramic membrane, in which inert aluminum metal powder is encapsulated and silicon dioxide particles are bound.

[0013] In any case, the drawback of the method described in CN Patent No. 110240486(B) is that the separate production of the ceramic microcapsules added to the cement-bonded concrete mass is very complicated. The described in-situ formed mullite whiskers also have the drawback that in the presence of a high proportion of calcium aluminate cement, the in-situ formed mullite leads to a low melting point phase, which reduces the high temperature strength properties of the fired concrete element. This is known to those skilled in the art.

[0014] In order to obtain high-strength, particularly dimensionally stable, refractory concrete, the corresponding concrete composition must therefore be produced as cement-free as possible, or at least contain a proportion of cement binder significantly lower than the at least 4% by weight of calcium aluminate specified in Chinese Patent No. 110240486(B). Furthermore, the in-situ formation of ceramic mullite phase must be avoided when firing such concrete elements, and high-strength, particularly dimensionally stable concrete elements with high hot flexural strength must therefore be free as far as possible from ceramic mullite phase formed in situ.

[0015] JP 2001114571(A) discloses a formulation for producing a castable refractory material for refractory linings. The formulation contains 0.5 to 10% by weight of aluminum powder and 0.1% to a maximum of 2% by weight of amorphous silicon dioxide particles added to a concrete composition. The amorphous silicon dioxide particles are provided by an aqueous colloidal silica sol suspension with a solids content of 20% by weight of silicon dioxide particles. The formulation states that adding a high proportion of aqueous colloidal silica sol suspension with a solids content of more than 2% by weight of amorphous silicon dioxide particles (20% silicon dioxide particles) to the concrete composition is disadvantageous because it reduces the corrosion resistance of the refractory material. No information regarding the strength of the fired concrete elements is provided in JP 2001114571(A).

[0016] The disadvantage of the formulations known from JP 2001114571(A) is at least the very low proportion of nanoscale silicon dioxide particles introduced into the respective concrete composition via a silica sol suspension, and concrete elements that are particularly dimensionally stable after firing cannot be produced using these formulations.

[0017] U.S. Patent Application Publication No. 2014 / 0291904(A1) also discloses concrete compositions that use 0.1-5 wt. % pyrogenically prepared silica particles (so-called "microsilica" particles). Water is added to the mixture. In concrete compositions that can contain up to 0.5 wt. % cement binder, aluminum metal powder is intentionally absent to prevent the formation of hydrogen gas upon mixing with water. The addition of small amounts of colloidal silica sol suspensions is not mentioned in U.S. Patent Application Publication No. 2014 / 0291904(A1). Furthermore, concrete elements that are particularly dimensionally stable after firing cannot be produced using these formulations.

[0018] Object of the invention The object of the present invention is therefore to overcome the drawbacks of the prior art and to provide a composition for producing high-strength, particularly dimensionally stable, fire-resistant concrete, comprising carbon protected from oxidation by a suitable metal powder, particularly aluminum metal powder, which composition can be processed with an aqueous liquid without safety risks to produce concrete elements suitable for fire-resistant applications with improved strength, particularly improved hot bending strength. Furthermore, a composition for producing fire-resistant concrete should be provided, which can be used to produce concrete elements that are dimensionally stable, particularly after firing. Another object of the present invention is to provide a method for producing such fired concrete elements. Summary of the Invention

[0019] In order to solve the problem according to the present invention, a refractory concrete composition is provided, the concrete composition comprising: - a proportion of 35 to 70% by weight, preferably 50 to 65% by weight, of a coarse fraction of at least one refractory raw material having a particle size of at least 0.5 mm, preferably 0.5 mm to 12 mm; a fine fraction of at least one refractory raw material having a particle size of less than 0.5 mm in a proportion of 15 to 30% by weight, preferably 20 to 25% by weight; The at least one refractory raw material is a fine fraction selected from the group consisting of sintered alumina, fine corundum, brown corundum, gray corundum, magnesium aluminum spinel, mullite, bauxite, andalusite, fire clay and / or silicon carbide, and mixtures of the above materials. - a proportion of 0 to 20% by weight, preferably 1 to 7% by weight, of fine-grained calcined alumina having a particle size of less than 0.1 mm, preferably less than 50 μm; - Fine-grained magnesium oxide powder with a particle size of less than 0.1 mm, in a proportion of 0-2% by weight; - 2 to 7% by weight, preferably 3 to 6% by weight, of fine-grained aluminum metal powder having a particle size of less than 100 μm, preferably less than 63 μm; - at least one fine-grained carbon support having a particle size of less than 100 μm, which contains or is carbon black and / or graphite, in a proportion of 2 to 8% by weight; - an aqueous colloidal silica sol suspension in a proportion of 4 to 20% by weight, the silica sol suspension comprising a solids content of amorphous nanoscale silica particles of 30 to 50% by weight.

[0020] In the refractory concrete composition according to the invention, a certain proportion of fine components, in particular the fine fraction of at least one refractory raw material, having a particle size of less than 0.5 mm, forms a binding matrix for at least one coarse fraction of the refractory raw material having a particle size of at least 0.5 mm. The granular coarse components having a particle size of at least 0.5 mm are embedded in the binding matrix formed by the fine components, which is particularly advantageous for the subsequent production of dimensionally stable concrete elements.

[0021] Surprisingly, the refractory concrete composition according to the invention has shown, in contrast to the teaching of US 2012 / 0142518 A1, that the use of nanoscale silicon dioxide (SiO2) particles present in an aqueous colloidal silica sol suspension in combination with other components of the refractory concrete composition according to the invention is particularly advantageous for producing particularly dimensionally stable, high-strength fired concrete elements.

[0022] The term "silica sol" refers to an aqueous colloidal suspension of primarily spherical polysilicic acid molecules containing 30% to up to 60% silicon dioxide by weight. The term is composed of "silica," which refers to silicic acid, and "sol," which is a synonym for colloid. Here, the roughly spherical polysilicic acid particles are connected by oxygen bridges to form amorphous silicic acid (also known as silica gel).

[0023] Due to their relatively small particle size on the nanometer scale, nanoscale silicon dioxide (SiO2) particles cannot be dispersed uniformly enough in the remaining concrete composition as a dry, anhydrous solid component, i.e., as dry silicon dioxide particles, into a mixture of materials with consistently larger particle sizes on the micrometer or millimeter scale. Furthermore, storage of the dry mixture for producing a fire-resistant concrete composition will result in the separation of the dry silicon dioxide particles contained therein. However, in an aqueous colloidal silica sol suspension, the nanoscale silica particles are already present in an advantageous manner as homogeneously dispersed, spherical, individual particles that are not crosslinked to each other and are hydroxylated on their surfaces.

[0024] Introducing a sufficient amount of aqueous colloidal silica sol suspension into a refractory concrete composition as a mixing fluid, optionally by mixing in a suitable concrete mixer, offers the advantage that, once the silica sol suspension uniformly wets the concrete composition mixture, the nanoscale silicon dioxide particles are dispersed as uniformly or homogeneously as possible in the refractory concrete composition, and the concrete composition is fluid and can be cast. Therefore, it is ensured that, especially after mixing the components of the refractory concrete composition, other fine-grained components, such as fine-grained aluminum powder and fine-grained carbon carriers, are also dispersed as uniformly or homogeneously as possible in the concrete composition. Hereinafter, the term "homogeneously dispersed" is understood to mean the most uniform dispersion of fine-grained components, especially those with relatively small particle sizes on the nanometer or micrometer scale, within the refractory concrete composition mixture. The more uniformly or homogeneously dispersed fine-grained components, especially those with different particle sizes, are within the refractory concrete composition mixture, the more uniform the properties of the concrete elements prepared with the concrete composition according to the present invention.

[0025] Without being bound by any theory, it appears that for the refractory concrete composition according to the invention, the interaction of a sufficient amount of nanoscale silicon dioxide (SiO) particles present as a silica sol, which are particularly reactive and are particularly uniformly or homogeneously dispersed in the refractory concrete composition in the form of an aqueous colloidal silica sol suspension, in combination with a sufficient amount of fine-grained aluminum metal powder and with a sufficient proportion of fine-grained carbon carrier having a particle size of less than 100 μm, is particularly advantageous.

[0026] Independent preliminary tests have shown that high-strength ceramic phases are formed in situ when concrete elements prepared using the concrete composition according to the invention are fired, which, in combination with the other components of the concrete composition, contain 2% to 7% by weight of fine-grained aluminum metal powder with a particle size of less than 100 μm. 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 their corresponding mixtures.

[0027] It can be shown that the in-situ phase formation of aluminum-based ceramic phases is advantageously promoted by the interaction of the aluminum metal powder, in particular with the homogeneously dispersed nanoscale silicon dioxide (SiO) particles present in the concrete composition, as well as with a sufficient proportion of carbon black, graphite or mixtures thereof, dispersed as homogeneously as possible in the concrete composition as a fine carbon carrier.

[0028] These high-strength ceramic phases are formed in situ upon firing of the concrete composition at temperatures of about 800°C (1472°F), and they comprise or are in the form of one or more oxides, carbides, mixed carbides, oxycarbides, nitrides, and / or oxynitrides, and / or their corresponding mixtures, and are advantageously structured in the fired concrete element on a small scale, on the micrometer and / or nanometer scale, and thus are approximately homogeneously dispersed in the fired concrete element. Thus, fired concrete elements can be produced using the concrete composition according to the invention, which, due to the homogeneously dispersed ceramic phase, have strength properties that are as homogeneous as possible and are particularly dimensionally stable.

[0029] It is particularly advantageous if the proportion of fine-grained aluminum metal powder in the concrete composition according to the invention is adjusted to 3% to 6% by weight so that a sufficient proportion of aluminum-based ceramic new phase formers is dispersed in the fired concrete product produced from the concrete composition according to the invention. Preliminary tests have shown that the strength of fired concrete elements produced from this concrete composition can be further increased by selecting, for example, a proportion of fine-grained aluminum metal powder of 5% by weight in the concrete composition, because the proportion of in-situ formed aluminum-based ceramic phases or structures in the fired concrete element is further increased. From the current perspective, it is believed that the optimal cost-effectiveness ratio is achieved, particularly for reasons of economic efficiency, with a proportion of fine-grained aluminum metal powder of 5% by weight in the concrete composition. With a further increased proportion of aluminum metal powder in the concrete composition according to the invention, up to approximately 7% by weight, the aforementioned technical advantages are still increased, but the material costs for producing the concrete composition according to the invention also increase.

[0030] The total proportion of the at least one refractory raw material or refractory raw material mixture in the refractory concrete composition is 65-90% by weight, and the particle size distribution of this at least one refractory raw material or raw material mixture is provided by a coarse fraction having a particle size of at least 0.5 mm in a proportion of 35-70% by weight and a fine fraction having a particle size of less than 0.5 mm in a proportion of 15-30% by weight, whereby the granular coarse component having a particle size of at least 0.5 mm is advantageously embedded in a binding matrix formed by the fine fraction of the at least one refractory raw material and a predetermined proportion of further fine components of the refractory concrete composition having a particle size of less than 0.5 mm.

[0031] Soot, carbon black, shiny soot, amorphous graphite, and mixtures of the above materials can be used as fine carbon supports. The term "carbon black" refers to soot specially produced as an industrial raw material. Shiny soot is soot produced by combustion plants.

[0032] Fine-grained magnesium oxide powder (MgO), in a possible proportion of 0-2% by weight, in the concrete composition acts as a setting accelerator to shorten the setting time in the production of refractory concrete. Depending on the fineness of the material, it may be sufficient if the refractory concrete composition according to the invention contains only a very small proportion of magnesium oxide powder, for example approximately 0.05% by weight, to achieve a relatively fast setting time of the refractory concrete.

[0033] In a preferred embodiment of the present invention, it may be particularly useful if the solids content of amorphous nanoscale silicon dioxide particles in the concrete composition is 2-7 wt. %. This proportion corresponds to the dry anhydrous solids content of silicon dioxide particles contained in the aqueous colloidal silica sol suspension provided. Thus, reactive silicon dioxide (SiO) is provided in a sufficient, but not excessive, amount to achieve the in-situ formation of advantageous ceramic phases in the fired concrete elements produced according to the present invention in a corresponding amount.

[0034] Particularly high reactivity for the formation of new desired ceramic phases, especially during subsequent firing, can be achieved with the refractory concrete compositions according to the invention when the amorphous nanoscale silicon dioxide particles have a particle size of 2 nm to 100 nm, preferably 5 nm to 75 nm. The correspondingly high reactivity is ensured by the correspondingly small particle size, which leads to rapid phase formation even at low temperatures.

[0035] In contrast to the concrete compositions according to the invention, for example, amorphous microsilica having a particle size on the micrometer (μm) scale does not exhibit sufficient reactivity, so that the advantageous ceramic phase of such a microstructure is not obtained with micrometer-scale microsilica particles, even if such a mixture is subsequently fired.

[0036] In order to provide a refractory concrete composition according to the invention that can be used as flexibly as possible for different applications, the at least one refractory raw material is selected from the group consisting of sintered alumina, fine corundum, brown corundum, grey corundum, magnesium aluminium spinel, mullite, bauxite, andalusite, fireclay and / or silicon carbide, and mixtures of the aforementioned substances.

[0037] Fireclay is a rock-like, artificially produced refractory material containing 10-45% aluminum oxide (Al2O3). Fireclay does not refer to other refractory building materials.

[0038] Depending on the field of application and the respective requirements for the refractory product, different refractory raw materials or raw material mixtures may be used. For example, relatively high quality refractory raw materials such as sintered alumina, fine corundum, magnesium aluminum spinel, or mixtures thereof may be used for refractory products that are subject to high stresses, especially those that are subject to high thermal and / or mechanical stresses such as those required in steel manufacturing processes.

[0039] In other fields of application where the refractory product is subjected to less stress and where there is no direct contact between steel and slag, relatively cost-effective refractory raw materials such as bauxite, andalusite, and fireclay can be used, which offer a more advantageous cost / performance ratio than, for example, the high-quality refractory raw materials mentioned above.

[0040] Homogeneous dispersion of amorphous nanoscale silicon dioxide particles and / or at least one fine-grained carbon carrier and / or fine-grained aluminum metal powder in the concrete composition used to produce the concrete element can be particularly advantageous for the desired in-situ formation of new ceramic phases in the fired concrete element. Homogeneous dispersion of these fine-grained components, especially nanoscale silicon dioxide particles, ensures that the newly formed ceramic phases in situ during the firing process of the refractory concrete composition are evenly dispersed in the fired concrete material. Therefore, material properties as homogeneous as possible can be achieved in all areas of the fired concrete material produced according to the present invention.

[0041] To ensure the most uniform or homogeneous dispersion of one or more of the fine grain components in the concrete composition, it may be advantageous to dry mix the dry starting materials for the preparation of the refractory concrete composition in a suitable concrete mixer or pug mill mixer, further prior to adding the silica sol as a mixing fluid. Additionally or alternatively, the dry mix for the preparation of the refractory concrete composition may then be mixed between the addition of a corresponding amount of aqueous colloidal silica sol suspension to ensure homogeneous dispersion of one or more of the fine grain components in the concrete composition.

[0042] In another embodiment of the present invention, the refractory concrete composition may be free of set accelerators, particularly magnesium oxide. As previously mentioned, magnesium oxide, optionally added to the concrete composition, functions as a set accelerator. However, the concrete composition according to the present invention may also be produced or processed without magnesium oxide. If necessary, it is also possible to work without adding a set accelerator. Alternatively, for example, lime, Portland cement, calcium aluminate cement, magnesium chloride, and / or other magnesium salts may be used alone or in a mixture as a set accelerator instead of magnesium oxide.

[0043] However, the addition of at least one of the aforementioned set accelerators can cause a decrease in the fire resistance and therefore the application limit temperature of the fireproof concrete composition according to the present invention. The set accelerators are basic components, while the fireproof raw materials of the fireproof concrete composition according to the present invention are non-basic. This combination can lead to the formation of low-melting-point phases at high temperatures. Therefore, reducing or completely omitting the set accelerators in the fireproof concrete composition can be advantageous in terms of application technology.

[0044] From a technical standpoint, it is theoretically possible to use a higher proportion of basic refractory raw materials, such as magnesia (MgO) or dolomite, in the refractory raw material mixture. However, this results in the sol-gel reaction occurring more quickly with a higher proportion of basic refractory raw materials in the concrete composition, especially if they are finely divided. In such cases, the concrete composition sets particularly quickly, thus shortening the processing time for casting the concrete components. However, such concrete components exhibit poorer thermal strength characteristics due to the low-melting-point phases that are generated when the temperature is increased or when basic oxides (e.g., MgO, CaO) and non-basic oxides (e.g., SiO2, Al2O3) present in the microstructure are fired.

[0045] In a further development of the fireproof concrete composition according to the present invention, the concrete composition may further comprise at least one dispersant, preferably comprising or being sodium polynaphthalenesulfonate. The addition of a suitable dispersant can increase the fluidity of the concrete, thereby reducing the amount of mixing fluid required to be added, in this case, the amount of aqueous colloidal silica sol suspension required. In particular, the addition of a suitable dispersant can be particularly advantageous in the case of carbonaceous concrete, since carbon is generally difficult to wet with water.

[0046] The above-mentioned object of the present invention is also achieved by a concrete element produced by casting the refractory concrete composition according to any one of claims 1 to 7. As described above, the fine-grained aluminum metal powder (Al), the fine-grained carbon carrier (C), and the nanoscale silicon dioxide (SiO2) particles are still contained in the microstructure of the concrete element according to the present invention in the above-mentioned proportions. Advantageously, for the subsequent formation of new phases during firing, the above-mentioned fine or nanoscale components are available as reactants, dispersed as uniformly as possible in the microstructure. The concrete composition provided according to the present invention has fluidity due to the addition of the proportion of aqueous colloidal silica sol suspension.

[0047] To ensure the most uniform dispersion of the silica sol suspension as a mixing fluid in the concrete composition, it may be advantageous if the provided refractory concrete composition is still properly mixed before pouring the concrete elements into the formwork molds. This ensures that the fine grain components required for the formation of the ceramic phase in the fired concrete material, in particular aluminum powder, nanoscale silicon dioxide particles, and carbon carrier materials, are also dispersed as uniformly as possible in the microstructure of the concrete elements.

[0048] Refractory concrete elements and building components offer advantages to the end customer, as the necessary, sometimes time-consuming and complex manufacturing steps have already been carried out in advance.

[0049] It may be particularly advantageous or necessary if the concrete elements prepared using the refractory concrete composition according to the invention are substantially free of water.

[0050] Such concrete elements can be produced by casting and drying a concrete composition according to any one of claims 1 to 7, or by drying a concrete element according to the invention according to claim 8. Advantageously, the drying temperature is chosen to be in the range of 110°C to 350°C (230°F to 662°F) in order to dry out the water introduced into the concrete composition, essentially an aqueous silica sol suspension, or to remove the water introduced from the concrete composition.

[0051] In addition to its function as a binder and a source of reactive nanoscale amorphous silicon dioxide, silica sol also offers advantages in this respect. The sol-gel process, which follows the reaction that essentially causes the green bond of refractory concrete, produces a very small amount of chemically bonded hydrated phase, and the gel structure has high permeability compared to other bonding systems. The drying process therefore occurs very quickly and in a short time at a relatively low temperature, which reduces economic efficiency and energy consumption.

[0052] The above-mentioned objects of the present invention are also achieved by a fired concrete element, which is made from a refractory concrete composition according to the present invention, and the concrete element is fired at a firing temperature of 800°C to 1600°C (1472°F to 2912°F) and has at least one ceramic phase with an acicular structure.

[0053] The special morphology of the in-situ formed ceramic phase provides a kind of ceramic reinforcement for the concrete. The tension in the fired concrete element arising due to volume changes caused by sudden temperature changes can therefore be more easily absorbed. This can be confirmed by the significantly improved thermal shock resistance of the fired concrete element produced according to the invention. The fired concrete material exhibits significantly better durability than conventional refractory materials in frequent temperature changes. In addition to their high mechanical strength, the in-situ formed ceramic phase is extremely resistant to chemical attack, such as contact with liquid slag.

[0054] In contrast to the method of Chinese Patent No. 110240486(B), in-situ formed mullite whiskers are not formed in the fired concrete elements produced according to the present invention because there are no ceramic microcapsules in the refractory concrete composition.

[0055] To ensure the formation of ceramic phases as uniformly distributed as possible throughout the microstructure of the fired concrete element as a ceramic reinforcement, it may be advantageous for the concrete element to be fired at the selected firing temperature for a sufficient time to ensure the most uniform temperature distribution during the firing process with the smallest possible temperature gradient within the concrete element. The firing time required to ensure the most uniform temperature distribution within the concrete element during the firing process depends largely on the size and geometry, particularly the wall thickness, of the fired concrete element. Relevant test standards for high-density refractory products specify the firing temperature and the hold time required to fully heat the test specimen to the specified firing temperature. For example, the 5-hour hold time specified in standard DIN EN 993-10 for measuring the permanent change in length of high-density molded refractory products (for tests according to DIN EN 993-10 at 1700°C (3092°F)) can be used as a guideline for the firing time at the selected firing temperature.

[0056] Particularly advantageous material properties may result from the fired concrete element according to the invention if at least one ceramic phase comprises or is aluminium in the form of one or more oxides, carbides, mixed carbides, oxidized carbides, nitrides and / or oxidized nitrides and / or mixtures thereof.

[0057] Surprisingly, it has been found that at least one ceramic phase preferably forms when the concrete product is pretreated and / or fired at a firing temperature in the range of 800°C to 1600°C (1472°F to 2912°F), preferably at a firing temperature of at least 1000°C (1832°F), and particularly preferably at a firing temperature of at least 1250°C (2282°F).

[0058] Table 1 below lists an overview of the ceramic phases or phase groups formed in situ in dried concrete elements produced according to the invention during temperature pretreatment at the minimum temperature specified in each case (see the left column of Table 1), which may then undergo transition to other phases upon further temperature increase. All of the formed ceramic phases or phase groups are mechanically and chemically very stable and partially have an acicular structure. Refractory concrete produced according to the invention containing such ceramic phases or phase groups is therefore advantageously ceramic-reinforced and has very good strength, high thermal shock resistance during temperature changes, and high chemical stability against aggressive media such as liquid slag.

[0059] Concrete elements produced using the refractory concrete composition according to the present invention and properly dried at a drying temperature of 110°C to 350°C (230°F to 662°F) contain in their microstructure, inter alia, finely and homogeneously dispersed aluminum metal powder (Al), finely and homogeneously dispersed carbon carrier material (C), and nanoscale silicon dioxide (SiO2) particles in predetermined proportions required for the new phase structure during subsequent firing. Such dried refractory concrete elements are already available to customers.

[0060] In particular, new phase structures are determined in fired concrete elements produced according to the invention at firing temperatures of at least 700°C (1292°F), which include, for example, aluminum oxide in the form of Al2O3, aluminum carbide in the form of Al4C3, and silicon (Si).

[0061] After raising the temperature of the concrete elements under test to a firing temperature of at least 800°C, new phase structures can be determined at 800°C (1472°F), including, for example, aluminum oxide (Al2O3), silicon (Si), aluminum carbide (Al4C3), and aluminum silicon (Al-Si) mixed carbides.

[0062] After further increasing the temperature of the tested concrete elements to a firing temperature of at least 1300°C (2372°F), new phase structures can be determined at 1300°C (2372°F), including, for example, aluminum oxide (Al2O3), aluminum carbide (Al4C3), aluminum nitride (AlN), aluminum oxycarbide, and Al-Si mixed carbide. Aluminum carbide decomposes at temperatures above 1300°C (2372°F) to form Al-Si mixed carbide.

[0063] Upon further increasing the temperature to a firing temperature of at least 1600°C (2912°F), new phase structures can be determined in concrete elements according to the invention tested at 1600°C (2912°F), including, for example, aluminum oxide (Al2O3), aluminum nitride (AlN), aluminum oxynitride (AlON), aluminum oxycarbonitride (AlCON), and silicon aluminum oxynitride (SiAlON) ceramic phases. [Table 1]

[0064] The newly formed phase structures not only exhibit excellent mechanical resistance, but are also very chemically stable, particularly resistant to slag attack, i.e., chemically resistant to contact with liquid slag. This situation, combined with the excellent penetration resistance due to the special pore size distribution, results in excellent slag resistance against the various slags that frequently come into contact with refractory materials in the steel production process. In contrast to previously known materials, the open pores of the refractory concrete according to the present invention exhibit significantly smaller pore sizes, which results in significantly lower capillary forces upon contact with liquid phases, such as corrosive slags, and penetration of these media into the sintered concrete material according to the present invention does not occur or occurs only to a lesser extent compared to previously known refractory materials. This situation significantly improves the slag resistance of the sintered concrete elements produced according to the present invention.

[0065] A further technical advantage of the sintered concrete elements produced according to the present invention, which is evident in most steel manufacturing applications, is the relatively low thermal conductivity of the concrete elements produced using the refractory concrete composition according to the present invention compared to conventional carbon-bonded materials. This situation can be explained by the relatively low carbon content in the concrete composition according to the present invention. At an ambient temperature of 1000°C (1832°F), the thermal conductivity of the sintered concrete elements produced according to the present invention is, for example, about 4 W / mK, while the thermal conductivity of conventional isostatically pressed products used in continuous casting processes is, for example, more than 10 W / mK. Heat loss can therefore be significantly reduced with the concrete elements produced according to the present invention.

[0066] Own preliminary tests have surprisingly shown that fired concrete elements produced according to the invention after pretreatment at a firing temperature of at least 1000°C (1832°F) can have a cold compressive strength (according to DIN EN 993-5) of at least 140 MPa, preferably at least 160 MPa, and / or a cold flexural strength (according to DIN EN 993-6) of at least 20 MPa. These very high values arise due to the presence of newly formed phases that are mechanically very stable and sometimes have an acicular or needle-shaped structure.

[0067] Furthermore, preliminary testing has shown that concrete elements produced according to the present invention can have a hot flexural strength, measured at 1500°C (2732°F), of at least 15 MPa (determined according to ISO 5013 or DIN EN 993-7, respectively). Such high hot flexural strength indicates the exceptional mechanical stability of the material at high temperatures.

[0068] The fired concrete elements produced according to the invention can be used particularly flexibly if they are substantially volume-stable and if the permanent change in length of the fired concrete elements after cooling (measured according to DIN EN 993-10) is between -0.1% and +0.1% of their initial length before firing. This geometric stability of the concrete material according to the invention, even at and after very high firing temperatures, offers great advantages, for example, in terms of the stability and lifespan of refractory linings. The above-mentioned object of the invention is also achieved by a method according to the invention for producing a fired concrete element, the method comprising the steps of: - providing a refractory concrete composition according to the invention; - pouring the refractory concrete composition into a formwork mould and allowing the concrete composition to set into a concrete element; - removing the concrete element from the formwork mould; - optionally drying the concrete elements, preferably a drying temperature between 110°C and 350°C (230°F and 662°F) being selected for drying; firing the concrete elements at a firing temperature between -800°C and 1600°C (1472°F and 2912°F), preferably at a firing temperature of at least 1000°C (1832°F), particularly preferably at a firing temperature of at least 1250°C (2282°F); Includes.

[0069] In the context of the manufacturing method according to the present invention, further mixing of the components of the refractory concrete composition may be optionally necessary before providing the refractory concrete composition. For example, it may be convenient to first mix the dry starting materials for producing the refractory concrete composition with a corresponding amount of an aqueous colloidal silica sol suspension and homogeneously mix them in a suitable mixer. In this case, the dry starting materials and the aqueous colloidal silica sol suspension can be advantageously stored separately from each other. When mixing the dry starting materials and the aqueous colloidal silica sol suspension as a mixture, the amounts and weight ratios of the mixed components can be advantageously controlled and precisely adjusted. In this way, the amount of water or moisture introduced by the aqueous colloidal silica sol suspension can also be precisely adjusted, controlled, and recorded, respectively.

[0070] It is therefore ensured that a refractory concrete composition which is fluid and also contains the corresponding desired proportion of water or moisture, respectively, is provided according to the formulation of the present invention.

[0071] The provided refractory concrete composition is then poured into a suitable formwork mold as a fluid mass. Advantageously, casting can be carried out while rocking and / or vibrating the corresponding formwork mold to compress the cast concrete composition in the formwork mold by the applied vibration energy. For example, multiple prepared formwork molds can be placed on a vibrating table, thereby allowing multiple concrete elements to be produced simultaneously. After the concrete has hardened, the concrete elements are removed from the formwork molds to obtain the hardened concrete elements, or the formwork molds are removed as appropriate.

[0072] Subsequent drying of the concrete elements or compacts in a suitable drying unit, in any suitable manner, is preferably carried out at a temperature of between 110°C and 350°C (230°F and 662°F).

[0073] The advantages of final firing of concrete elements according to the invention at firing temperatures of at least 800°C (1472°F) have already been demonstrated. The high-strength ceramic phases formed in situ during firing of the concrete composition at temperatures of about 800°C and containing or in the form of one or more oxides, carbides, mixed carbides, oxycarbides, nitrides, and / or oxynitrides, and / or their corresponding mixtures, are advantageously structured on a small scale in the fired concrete elements, on the micrometer and / or nanometer scale, and are therefore more or less homogeneously dispersed in the fired concrete elements. Thus, fired concrete elements can be produced using the concrete composition according to the invention, which, due to the homogeneously dispersed ceramic phases, have strength properties that are as homogeneous as possible and are particularly dimensionally stable. [Brief explanation of the drawings]

[0074] [Figure 1] 2 shows the course of the cold compressive strength (measured in accordance with DIN EN 993-5) of concrete materials produced according to the invention as a function of the pretreatment temperature.

[0075] The sharp increase in cold compressive strength (expressed in MPa) measured at pretreatment temperatures of 700°C to 800°C (1292°F to 1472°F), which is evident in Figure 1, is due to a new phase structure that occurs in situ at this pretreatment temperature from 700°C (1292°F) during the firing process of the fired concrete elements according to the invention. For this purpose, concrete elements produced from concrete compositions according to the invention were investigated. The mix specifications of the concrete compositions according to the invention used for strength testing correspond to the values shown in Table 2 for exemplary embodiments 2 and 3.

[0076] The in situ formed, mechanically highly stable ceramic phase during firing, has a partially acicular structure and provides the exceptionally high mechanical stability and strength of the refractory concrete material according to the invention over a very wide temperature range of 700°C to 1600°C (1292°F to 2912°F). Cold compressive strengths of at least 140 MPa have been measured over a temperature range of 800°C to 1300°C (1472°F to 2372°F). At a pretreatment temperature of 1000°C (1832°F), a cold compressive strength of 160 MPa was even measured for fired concrete elements produced according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0077] Table 2 below lists some exemplary embodiments according to the present invention in the form of possible formulations for producing refractory concrete compositions.

[0078] Exemplary embodiments 1 to 5 of the refractory concrete compositions according to the invention are used for the subsequent production of refractory concrete elements, as specified in Table 2, which are available to the customer after appropriate drying. Typically, the dried concrete elements are first fired in situ at the customer's premises so as to have the advantages of the invention as fired refractory concrete elements.

[0079] The refractory concrete composition formulations according to exemplary embodiments 1, 4, and 5 listed in Table 2 are typically used in applications where the refractory concrete materials produced therefrom come into contact with pig iron. These are, for example, refractory applications of refractory concrete elements in the field of pig iron ladles, blast furnace troughs, and / or torpedo ladles in pig iron production.

[0080] Refractory concrete compositions according to Examples 2 and 3 of Table 2 are typically used for steel production applications, i.e., for refractory concrete blocks in the area of steel ladles, ladle bottoms, and steel ladle walls, for perforated blocks, and for various functional refractory products required in continuous casting processes. The cold compressive strength (MPa) values shown in Figure 1 were determined based on fired concrete elements made with the refractory concrete compositions according to the exemplary embodiments 2 and 3 of Table 2. Other specified strength values (cold bending strength, hot bending strength) were also determined based on these refractory concrete compositions according to the exemplary embodiments 2 and 3 of Table 2. [Table 2]

[0081] To provide the fireproof concrete compositions according to the present invention, in the formulations of Exemplary Embodiments 1 to 5, dry mixtures of the raw material components were first homogeneously prepared without using silica sol as a mixing fluid. To this end, the prepared dry mixtures were homogeneously mixed according to the respective formulations. The dry mixtures were then mixed with the amounts of aqueous colloidal silica sol suspensions specified in Table 2, which in each case had a solids content of amorphous nanoscale silica particles of 40 wt. % (based on the silica sol suspension).

[0082] Homogeneous mixing of the dry mixture of raw ingredients and the silica sol suspension was carried out in a pugmill mixer, with mixing times of 3-5 minutes each.

[0083] The refractory concrete composition thus provided according to the invention was fluid and could then be poured into a suitable formwork mold. The formwork mold was advantageously vibrated during the casting of the refractory concrete composition in order to appropriately compact the cast mass and obtain a refractory concrete element as free as possible from pores. After a corresponding holding time of the concrete in the formwork mold until the setting and hardening reaction of the binder (silica sol) was complete, the produced concrete element could be removed from the formwork mold. After subsequent drying, for example at 200°C (392°F), a refractory concrete element according to the invention was obtained. After firing the refractory concrete element at a firing temperature of 800°C to 1600°C (1472°C to 2912°F), a fired concrete element prepared according to one of the concrete compositions according to the invention specified in exemplary embodiments 1 to 5 was obtained.

Claims

1. - a proportion of 35 to 70% by weight, preferably 50 to 65% by weight, of a coarse fraction of at least one refractory raw material having a particle size of at least 0.5 mm, preferably a particle size of 0.5 mm to 12 mm; - a fine fraction of said at least one refractory raw material having a particle size of less than 0.5 mm in a proportion of 15 to 30% by weight, preferably 20 to 25% by weight; - a fine fraction of refractory raw material, wherein said at least one refractory raw material is selected from the group consisting of sintered alumina, fine corundum, brown corundum, grey corundum, magnesium aluminium spinel, mullite, bauxite, andalusite, fireclay and / or silicon carbide, and mixtures of said substances; - a proportion of 0 to 20% by weight, preferably 1 to 7% by weight, of fine-grained calcined alumina having a particle size of less than 0.1 mm, preferably less than 50 μm; - a proportion of 0 to 2% by weight of fine-grained magnesium oxide powder having a particle size of less than 0.1 mm; - a proportion of 2 to 7% by weight, preferably 3 to 6% by weight, of fine-grained aluminum metal powder having a particle size of less than 100 μm, preferably having a particle size of less than 63 μm; - at least one fine-grained carbon support having a particle size of less than 100 μm, in a proportion of 2 to 8% by weight, which comprises or is carbon black and / or graphite; - an aqueous colloidal silica sol suspension in a proportion of 4-20% by weight, the silica sol suspension comprising a solids content of amorphous nanoscale silicon dioxide particles of 30-50% by weight; 1. A refractory concrete composition comprising:

2. 2. The refractory concrete composition of claim 1, wherein the solids content of the amorphous nanoscale silicon dioxide particles in the concrete composition is 2 to 7 wt. %.

3. 3. The refractory concrete composition according to claim 1 or 2, characterized in that the amorphous nanoscale silicon dioxide particles have a particle size of 2 nm to 100 nm, preferably 5 nm to 75 nm.

4. 4. The refractory concrete composition according to claim 1, wherein the total proportion of the at least one refractory raw material or refractory raw material mixture in the concrete composition is in a range of from 65 to 90% by weight.

5. 5. The refractory concrete composition according to claim 1, wherein after mixing of the components of the refractory concrete composition, the amorphous nanoscale silica particles and / or the at least one fine-grained carbon carrier and / or the fine-grained aluminum metal powder are homogeneously dispersed in the concrete composition.

6. Refractory concrete composition according to any one of claims 1 to 5, characterized in that the concrete composition is free of set accelerators and in particular free of magnesium oxide.

7. 7. The refractory concrete composition according to any one of claims 1 to 6, characterized in that the concrete composition further comprises at least one dispersant, the at least one dispersant preferably comprising or being sodium polynaphthalene sulfonate.

8. 8. A concrete element produced by casting the refractory concrete composition of any one of claims 1 to 7, characterized in that the microstructure of the concrete element comprises fine-grained aluminum metal powder, fine-grained carbon carrier, and nanoscale silicon dioxide particles in the proportions indicated in the concrete composition.

9. 9. A concrete element according to claim 8, characterized in that the concrete element is free of water after drying, in particular at drying temperatures of 110°C to 350°C (230°F to 662°F).

10. 8. A fired concrete element produced from the refractory concrete composition according to any 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 (1472°F to 2912°F) and has at least one ceramic phase with an acicular structure.

11. 11. A fired concrete element according to claim 10, characterized in that the at least one ceramic phase comprises or is aluminium in the form of one or more oxides, carbides, mixed carbides, oxycarbides, nitrides and / or oxynitrides and / or mixtures thereof.

12. 12. The calcined concrete element according to claim 10 or 11, characterized in that after pretreatment at a calcination temperature of at least 1000°C (1832°F), the calcined concrete element 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 flexural strength according to DIN EN 0993-6 of at least 20 MPa.

13. 13. The fired concrete element according to any one of claims 10 to 12, characterized in that the fired concrete element has a hot flexural strength at 1500°C (1732°F) of at least 15 MPa, measured according to ISO 5013 and DIN EN 993-7, respectively.

14. 14. The fired concrete element according to any one of claims 10 to 13, characterized in that the fired concrete element is substantially volume-stable and has 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.

15. A method for producing a fired concrete element according to any one of claims 10 to 14, comprising the steps of: - providing a refractory concrete composition according to any one of claims 1 to 7; - pouring said refractory concrete composition into a formwork mould and allowing said concrete composition to set into a concrete element; - removing the concrete element from the formwork mould; - optionally drying the concrete elements, preferably a drying temperature between 110°C and 350°C (230°F and 662°F) being selected for drying; firing the concrete elements at a firing temperature of between -800°C and 1600°C (1472°F and 2912°F), preferably at a firing temperature of at least 1000°C (1832°F), particularly preferably at a firing temperature of at least 1250°C (2282°F); A method comprising:

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