Concrete having improved heat resistance

Abstract

The invention relates to concrete having improved heat resistance, the concrete being cement- free and comprising: at least 55 wt.% of steel slag aggregate; 15 to 40 wt.% of a binder consisting of ground steel slag; up to 1.0 wt.% of a chelating agent; up to 2 wt.% of admixtures such as plasticizers or superplasticizers; water in a range of up to 9 wt.%, and wherein the water to dry binder ratio (W / B) is in a range of less than 0.40; and wherein the combined steel slag has a particle size distribution build up comprising of: a D10 in a range of 5 to 50 micron, a D50 in a range of 0.9 to 3 mm, and a D90 in a range of 5 to 20 mm. The invention also relates to the use of this concrete as energy storage material in a thermal energy storage and recovery system.

Description

[0001] CONCRETE HAVING IMPROVED HEAT RESISTANCE

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a concrete having an improved heat resistance, in particular to temperatures up to about 450°C. The invention also relates to a method of preparing a concrete element made from the heatproof concrete and to the use of the concrete as energy storage material in a thermal or an electro-thermal energy storage and recovery system.

[0004] BACKGROUND TO THE INVENTION

[0005] Renewable energy sources for generating electricity are intermittent by nature. Wind speed and sun shine are neither a constant over the day nor over the seasons. If electric power generation from such intermittent energy sources has to be continuous, an energy storage unit is required where energy is stored during periods of abundant supply and retrieved from as electricity demand exceeds supply. Furthermore, if periods of insufficient supply last very long, an energy storage unit of formidable size might be required to sustain power during these periods, in consequence, such power generation systems are often complemented with conventional power generation from fossil fuels which can be easily stored in large quantity.

[0006] As a first example, power generation systems for island grids of up to 100 kW interconnecting a few remote households comprise a small wind turbine, large quantities of batteries and backup diesel generators. As a second example, solar-thermal power plants convert solar radiation into heat which is transferred by a thermal fluid to a thermodynamic machine such as a steam turbine to generate electricity. In order to extend the operational time of such plants, the thermal fluid may also transfer heat to a heat storage unit. This heat is later retrieved from the heat storage unit. Furthermore, steam generation may be augmented by combustion of fossil fuel to further extend the operational time of such plants and render them completely independent of weather conditions.

[0007] The beneficial use of energy storage is, however, not limited to the case of electricity generation from wind or sun. In large interconnected power grids, electricity is abundant and cheap during the night when baseload power plants such as nuclear power plants continue to supply at full capacity while the demand for electricity is low. During a few hours of the day electricity demand and prices peak, and peak load power plants operate. Albeit on a different time scale, this situation is somewhat similar to the case of abundant and lacking wind power.

[0008] Hydrostorage power plants store electricity during periods of abundance and supply electricity during peak hours. Hydrostorage is the most developed large scale electric energy storage technology, but it is limited to few suitable geographic locations and large plant sizes. So-called electro-thermal energy storage as a complement to intermittently available energy sources and / or periodically peaking energy demands is also a feasible option. Electrothermal energy storage generally comprises the steps of converting electricity to heat, storing the heat, and converting the heat back to electricity. While electro-thermal energy storage seems to lack some technical elegance, it is a serious option to consider from an economic point of view, as it has a peculiar cost structure compared to other electrical energy storage technologies. Heat is stored relatively easily and as a consequence, electro-thermal energy storage has low cost per kWh ("energy capacity cost"). It has also moderate cost per kW ("conversion cost" or "power cost"). As the conversion of electricity to heat and vice versa is purely of a physical kind without any (electro-) chemical conversion involved, a long cycle life for a storage device based thereupon can be expected. Unlike hydro-storage, it is location independent and relatively compact, i.e. has a high energy density. In addition, electro-thermal energy storage generally follows favourable scaling laws for up scaling to larger power and capacity levels.

[0009] Electro-thermal energy storage is a viable option for the storage of electrical energy if a large capacity is needed and if the primary source of power is inexpensive relative to the energy storage device. The latter is the case in particular for natural renewable energy sources and their basically free "fuels" solar and wind power or for inexpensive base load electricity during periods of low demand. Beyond a certain hydrocarbon fuel price level (for example reflecting a remote geographic location with high fossil fuel transport costs or a CO2tax), electro-thermal energy storage is for example a good complement to wind or solar power generation.

[0010] Such a system for storage of electrical energy and thermal energy is for example known from patent document EP3130875-A1. A key aspect for such a system is the choice for the energy storage materials. In this patent document the energy storage material include granular material, such as (dry) sand or sand grit, and pieces of basalt. Other materials known in the art include magnesia refractory bricks and alumina refractory bricks. However, such materials may have a relative high porosity level adversely affecting the energy storage capacity. And depending on the heat transport fluid used in such electro-thermal energy storage and recovery system, the energy storage material may be exposed to temperatures between about 300 and 350°C, or even up to about 500°C, for long periods of time. It has been found that over time many of the materials currently used or proposed tend to form local cracks when exposed for long periods of time to such high temperatures. The formation of cracks within the energy storage materials adversely affects the heat transfer capacity of such materials. There is a demand for materials having a good heat resistance in particular that do not form cracks when exposed to temperatures up to about 450°C and which may be used in electro-thermal energy storage systems.

[0011] DESCRIPTION OF THE INVENTION

[0012] As will be appreciated herein, for any description of compositions or preferred compositions, all references to percentages are by weight percent (wt.%) unless otherwise indicated.

[0013] The term “up to” and “up to about”, as employed herein, explicitly includes, but is not limited to, the possibility of zero weight-percent of the particular component to which it refers. For example, up to 2 wt.% TiO2may include a steel slag composition having no TiO2.

[0014] It is an object of the invention to provide a concrete having improved heat resistance.

[0015] It is an object of the invention to provide a concrete having a good heat resistance in particular that does not form cracks when exposed to temperatures up to about 450°C.

[0016] It is another object of the invention to provide an electro-thermal energy storage system using the improved concrete of this invention as energy storage material.

[0017] These and other objects and further advantages are met or exceeded by the present invention according to claim 1 and with preferred embodiments set out in the dependent claims.

[0018] In order to achieve one or more of these objects, the present invention proposes, in a first aspect, a concrete having improved heat resistance, the concrete being cement-free and comprising: at least 55 wt.% of steel slag aggregate, preferably ground or milled steel slag aggregate, and preferably at least 60 wt.% of steel slag aggregate;

[0019] 15 to 40 wt.% of a binder consisting of ground or milled steel slag; up to 1.0 wt.% of a chelating agent; up to 2 wt.% of admixtures such as plasticizers or superplasticizers; water in a range of up to 9 wt.%, and wherein the water to dry binder ratio (W / B) is in a range of less than 0.40; and wherein the steel slag combined (i.e. aggregate and binder) has a particle size distribution build up comprising of: a D10 in a range of 5 to 50 micron, preferably of 6 to 30 micron; a D50 in a range of 0.9 to 3 mm, preferably of 1 to 2.5 mm; and a D90 in a range of 5 to 20 mm, preferably of 6 to 16 mm. An “D90” of 5 mm means that 90% of the sample mass consists of particles smaller than 5 mm and 10% are bigger. An “D10” of 50 micron means that 10% of the sample mass consists of particles smaller than 50 micron and 90% are bigger. The term “D50” refers to the mean or average particle diameter, considered to be the average particle size by mass. An “D50” of 0.9 mm means the average particle diameter is 0.9 mm; it also means that 50% of the sample mass consists of particles smaller than 0.9 mm and 50% are bigger.

[0020] The particle size distribution of steel slag fractions may be measured for example by laser diffraction technique (e.g., Mastersizer2000, Malvern).

[0021] The heatproof concrete according to the invention combines a set of important engineering properties, in particular a high density, low porosity, high heat conductivity, good compactibility at room temperature working conditions and high heat capacity. Importantly it has been found that the concrete does not form cracks when exposed for a long period of time to high temperatures, e.g. the storage period can be a period in the order of a day, a period in the order of a year or a period of different interrupted duration. This means it does not form cracks when exposed to temperatures of more than about 300°C or even at least up to about 450°C. This makes the concrete a very good candidate for applications requiring a good heat resistance. In particular this makes the concrete suitable for being used in large scale systems for storing and recovering of thermal energy and also for systems for storing and recovering electricity using thermal energy.

[0022] It is an important feature of the concrete according to the invention that the presence of cement, such as for example Portland cement, is limited so as obtain a concrete composition that is cement-free. In practice this means that there is no purposive addition of any cement to the concrete mixture. In accordance with the invention it has been found that a cement presence would lead to fractures in a concrete structure or shape when at temperatures above 300°C, possibly due to water initiating cracks due to the high vapour pressure when present in pores of the resultant cement stone.

[0023] Whereas steel slag may comprise some free-lime, but this reacts only very slowly over time with any water present in the concrete. This reaction occurs over a period of years when at a temperature in a range of more than about 180°C to about 500°C. As this reaction is limited and very slow in time, it contributes to the thermal stability of the concrete at elevated temperatures without the formation of cracks in a concrete structure or shape.

[0024] It is another important feature of the invention that the heatproof concrete contains a chelating agent up to about 1.0 wt.%. In an embodiment the heatproof concrete contains at least 0.05 wt.%, and preferably of at least 0.10 wt.%, of a chelating agent. In an embodiment the amount of chelating agent is 0.5% to 9.0%, preferably 0.5% to 5.0%, relative to the amount of steel slag binder, on a dry basis.

[0025] The chelating agent is preferably selected from the group consisting of: polycarboxylic acid salts, preferably tricarboxylic acid salts, more preferably alkali salts of citric acid, more preferably potassium citrate or sodium citrate, and most preferably tri-potassium citrate monohydrate. The presence of a small amount of chelating agent in the concrete mixture, which acts as an activating agent as well as superplasticizer, results in a high reduction of water demand of the steel slag, and in turn a reduction of the amount of water needed in the concrete mixture paste. This causes high performance of the resulting concrete product, in particular a low water content leads to less porosity and thereby a significant reduced risk of cracking when exposed to temperatures at least up to about 450°C. By making use of the chelating agent, the amount of water required to obtain concrete mixture with a good flowability is considerably reduced so initially a lower amount of water is needed.

[0026] In a preferred embodiment the chelating agent is potassium citrate, and more preferably is tri-potassium citrate monohydrate. In a preferred embodiment the amount of potassium citrate, more preferably of the tri-potassium citrate monohydrate, is 0.5% to 9.0%, preferably 0.5% to 5.0%, and more preferably 0.5% to 2.5%, relative to the amount of steel slag binder, on a dry basis.

[0027] In the cement-free heatproof concrete according to the invention the amount of water in the preparation of the concrete is limited and is in a range of up to 9 wt.%, and wherein also the water to dry steel slag binder ratio (W / B) is in a range of less than 0.40. Too high an amount of water results also in a decrease of the compressive strength of the concrete after curing. In an embodiment the concrete the amount of water is at least 2 wt.%, and preferably at least 3 wt.%. In an embodiment the amount of water is limited to a range of up to 6 wt.%, and wherein the water to dry steel slag binder ratio (W / B) is in a range of less than 0.40, and preferably less than 0.30.

[0028] It is another important feature of the invention that the heatproof concrete has a binder consisting of ground or milled steel slag in a range of 15 to 40 wt.%, and preferably in a range of 20 to 35 wt.%, and more preferably of 20 to 30 wt.%.

[0029] In an embodiment the binder consisting of the ground or milled steel slag has a particle size distribution build up wherein the D50 is in a range of 5 to 30 micron, and preferably of 5 to 20 micron. In a preferred embodiment the binder consisting of the ground or milled steel slag has a particle size distribution build up of: a D10 in a range of 1 to 5 micron; a D50 in a range of 5 to 30 micron, preferably of 5 to 20 micron; and a DOO in a range of 10 to 100 micron, preferably 20 to 80 micron.

[0030] In an embodiment of the heatproof concrete the ground or milled steel slag comprises in a range of 10 to 25%, and preferably of 10 to 20%, of steel slag particles in a sieve fraction ranging from 8 to 25 mm, and preferably of 8 to 16 mm. The use of too many too large a steel slag particles may give raise to a higher pore formation adversely affecting various properties of the heatproof concrete.

[0031] In an embodiment the ground or milled steel slag comprises in a range of 10% to 25% of steel slag particles in a sieve fraction ranging from of 4 to 8 mm.

[0032] In an embodiment the ground or milled steel slag comprises up to 80%, and preferably of up to 75%, of steel slag particles in a sieve fraction range of 0 to 4 mm. The use of too much of too small a steel slag particles may give raise to increased difficulties in the processing of the concrete and may adversely increase the water demand. A very limited use of water in the concrete is desired to maintain the favourable improved heat resistance after prolonged exposure time at elevated temperature by avoiding the formation of pores which might result in increased risk of crack in a concrete structure or shape. Also the grinding or milling of the steel slag particles into very small dimensions increases the costs.

[0033] In an embodiment of the invention the ground steel slag is selected from the group of basic oxygen furnace (BOF) slag, ladle (LD) slag, electric arc furnace (EAF) slag, and mixtures thereof. "Steel slag" herein refers to the slag by-product produced from steel-making manufacturers. Steel slag may include slag produced from Basic Oxygen Furnaces (BOF). Steel slag may also include slag produced from Electric Arc Furnaces (EAF). Steel slag as used herein may further include ladle slag. "Ladle slag" herein refers to a type of steel slag. Ladle slag is produced as a by-product from a ladle refining operation. In various steel making processes, molten steel produced in an EAF or BOF process undergoes an additional refining process based on the quality of the desired steel. Additional fluxes and alloys are added to a ladle to remove the impurities within the steel and to produce steel with the desired properties. This operation is known as ladle refining, because it is executed in the transfer ladle. During this process, additional steel slags are generated, which are ladle slags.

[0034] In a preferred embodiment of the invention the ground steel slag is basic oxygen furnace (BOF) slag.

[0035] In an embodiment according to the invention the steel slag used (both for the aggregate and the binder) is BOF steel slag comprising: 10 - 30 wt.% brownmillerite, up to 15 wt.% magnetite, 25 - 60 wt.% C2S, 10 - 30 wt.% Mg-Wuestite, up to 20 wt.% C3S, and up to about 6 wt.% free-CaO. C2S or belite and brownmillerite are commonly present in converter steel slag.

[0036] In an embodiment according to the invention the steel slag used (both for the aggregate and the binder) is BOF steel slag comprising: 35 - 60 wt.% CaO, 8 - 17 wt.% SiO2, 15 - 35 wt.% of Fe Oxides, 1 - 5 wt.% AI2O3, 1 - 13 wt.% MgO, up to 4 wt.% P2O5, up to 2 wt.% TiO2, and balance impurities.

[0037] Basic oxygen steelmaking (commonly abbreviated as BOS, BOP, or BOF), also known as Linz-Donawitz steelmaking or the oxygen converter process, is a method of primary steelmaking in which carbon-rich molten pig iron is made into steel. Blowing oxygen through a lance over the molten pig iron inside the converter lowers the carbon content of the alloy and changes it into low-carbon steel. The process is known as “basic” because fluxes of burnt lime or dolomite, which are chemical basis, are added to promote the removal of impurities and protect the lining of the converter.

[0038] It will be understood that "steel slag" as used herein excludes iron slag and blast furnace slag that are typically generated during iron production. Blast furnace slag is distinct from BOF steel slag. Where blast furnace slag results from smelting iron ore, coke, and fluxes during the operations of extracting iron from the iron ore, the BOF steel slag is formed during refining operations converting the crude iron into steel by combining fluxes with the nonferrous oxides and other unwanted elements in the raw materials under molten state. Blast furnace slag is composed of: CaO 34-42 wt.%, SiO228-38 wt.%, AI2O38-20 wt.%, MgO 6-12 wt.%, and a very low total Fe-content expressed as FeO (FeO, Fe2O3) of less than 2 wt.%.

[0039] Furthermore, it is possible to add admixtures to the heatproof concrete that modify properties of the final binder and building material made from it already to the concrete fines feed (usually those will be added to the binder). Often used admixtures are water reducing agents and plasticizers like for example, but not exclusively, organic compounds with one or more from carboxylate, sulfonate, phosphonate, phosphate or alcohol functional groups. These serve to achieve a good consistency, i.e. flowability, of the concrete paste with a smaller amount of water. Since a decrease of water to binder ratio normally provides an increase of strength, such admixtures are commonly used. Preferably the admixtures used in this invention should meet the requirements of ASTM C494 (Standard Specification for Chemical Admixtures for Concrete).

[0040] Other useful admixtures that influence workability are retarders. These mainly aim at prolonging the time that a specified consistency is maintained. Retarders slow the setting and / or hardening of the binder paste. Suitable substances are for example, but not exclusively, phosphates, borates, salts of Pb, Zn, Cu, As, Sb, lignosulphonates, hydroxycarboxylic acids and their salts, phosphonates, sugars (saccharides).

[0041] In a preferred embodiment the heatproof concrete comprises up to about 2 wt.%, preferably in a range of 0.01 to 2.0 wt.% of admixtures to modify the rheology properties of the concrete to control the setting time. In particular plasticizers and superplasticizers (e.g., lignosulphates, based on polycarboxylic acid polymers, etc.) may be used. Plasticizers and superplasticizers also facilitate the good flowing behaviour and mould filling of the concrete around the tubular members used for the flow through of a heat transport fluid when constructing an electro-thermal energy storage and recovery system or a thermal energy storage and recovery system.

[0042] In an embodiment the heatproof concrete according to this invention after being set and cured has a density or more than about 3000 kg / m3, and more preferably of at least about 3100 kg / m3. It is known that the density of the steel slag particles ranges between about 3200 and 3600 kg / m3due to fluctuations in composition.

[0043] In an embodiment the heatproof concrete according to this invention has a 7-days compressive strength and a 28-days compressive strength of at least 16 MPa and 20 MPa, respectively. The 7-days and 28-days compressive strength are determined according to EN 196-1 , in three replicates (cubes) at room temperature. In an embodiment the concrete has a 28-days compressive strength of at least 22 MPa.

[0044] In an embodiment, and depending on the application or final use of the concrete according to this invention, it may further comprise further non-steel slag aggregates. Such further non-steel slag aggregates can be either natural or artificial. Examples of further nonsteel slag aggregates include sand, normal weight aggregate, gravel aggregates, limestone aggregate and secondary aggregate. Preferably, the further non-steel slag aggregate is granite or limestone aggregate. When used in an application requiring a very high heat capacity, the use of such aggregates is preferably limited and its use is more preferably avoided.

[0045] In a preferred embodiment the sole aggregate present in the heatproof concrete according to this invention is formed by steel slag.

[0046] In a preferred embodiment of the cement-free concrete having improved heat resistance, the concrete has a composition consisting of: at least 55 wt.% of ground or milled steel slag aggregate, and preferably at least 60 wt.%;

[0047] 15 to 40 wt.% of a binder consisting of ground or milled steel slag; up to 1.0 wt.% of a chelating agent; up to 2 wt.% of admixtures such as plasticizers or superplasticizers; water in a range of up to 9 wt.%, and wherein the water to dry binder ratio (W / B) is in a range of up to 0.40; and wherein the steel slag combined (i.e. aggregate and binder) has a particle size distribution build up comprising of: a D10 in a range of 5 to 50 micron, preferably of 6 to 30 micron; a D50 in a range of 0.9 to 3 mm, preferably of 1 to 2.5 mm; and a D90 in a range of 5 to 20 mm, preferably of 6 to 16 mm; and with preferred embodiments for the various components and particle dimensions as herein described and claimed.

[0048] In another aspect of the invention there is provided a method for preparing a cement-free heatproof concrete element from the concrete according to this invention, the method comprising the steps of: a) providing the fractions of ground steel slag, preferably the steel slag is BOF steel slag, the water, the binder and the chelating agent; b) optionally providing non-steel slag aggregates (such as sand, gravel and / or limestone); and c) optionally providing admixtures such as plasticizers or superplasticizers; d) mixing the ingredients of step a) to c) to obtain a concrete mixture paste according to the specification as herein set out or claimed; e) pouring the concrete mixture or concrete mixture paste in a mould for forming a concrete element and which can be in various forms and sizes; f) optionally compacting of the concrete mixture or concrete mixture paste; the step of compacting can be effected by any conventional means such as compaction / vibration or static compression, allowing to achieve the required density. g) curing the concrete mixture or concrete mixture paste to obtain the heatproof concrete element. Curing of the concrete may occur in regular atmospheric conditions.

[0049] In an embodiment of the method, it is preferred that dry parts comprising the ground steel slag, the binder and chelating agent and the optional non-steel slag aggregate(s) be mixed separately from the liquid part comprising water and any chemical admixture (such as plasticizers or superplasticizers). The dry materials can be mixed until uniformity is achieved. The chemical admixture can be added to water. Liquid and dry materials are mixed also until a suitable dispersion is achieved. In another aspect of the invention it relates to a premix kit, i.e. in a dry form, for obtaining a concrete element, the premix kit comprising the fractions of ground steel slag (steel slag aggregate and binder), water, the chelating agent, and optionally aggregates. The premix kit can be used to instantaneously prepare a concrete element by adding water and the optional admixtures (such as plasticizers or superplasticizers), and mixing and curing as known in the art.

[0050] In another aspect of the invention it relates to the use of the concrete according to this invention as energy storage material in a thermal energy storage and recovery system. Preferably tubular elements for the flow through of a heat transport fluid and made of metal, e.g. stainless steel, are embedded in the energy storage material and in direct contact with the energy storage material for an optimum energy exchange connection.

[0051] In a preferred embodiment it relates to the use of the concrete according to this invention as energy storage material in an electro-thermal energy storage and recovery system. Preferably tubular elements for the flow through of a heat transport fluid and made of metal, e.g. stainless steel, are embedded in the energy storage material and in direct contact with the energy storage material for an optimum energy exchange connection.

[0052] In an embodiment it relates to an electro-thermal energy storage and recovery system for storage of electrical energy, the system comprising comprising: a reservoir built or to be built on the ground; a quantity of energy storage material arranged in the reservoir; a system of tubular elements embedded in the energy storage material, wherein the tubular elements are manufactured from heat and electricity-conducting material; a circulating unit for circulating a heat transport fluid, preferably water or steam, through the tubular elements; a heating unit configured to convert electrical energy into heat in order to heat the heat transport fluid in the tubular elements, wherein the heating unit comprises a number of connecting elements which are attached to one or more of the tubular elements and which are configured to connect one or more of the tubular elements to at least one external electrical energy source, arranged such that an electric current can be carried over a predetermined length of the one or more tubular elements for the purpose of heating the one or more tubular elements in a first mode of use, transferring heat from the one or more tubular elements to the heat transport medium and transferring heat from the heated heat transport medium to the energy storage material for storage thereof in this material, and wherein the energy storage material consists of the heatproof concrete according to this invention as set out in the claims and the description.

[0053] The reservoir is constructed on the ground, for instance a piece of land close to a power station. The reservoir comprises for instance a building or an area surrounded by walls or dikes in which a large number of tubular elements can be disposed. The tubular elements are embedded in the energy storage material consisting of the heatproof concrete according to this invention. The embedding is preferably such that the tubular elements are in direct contact with the energy storage material over essentially their whole tubular outer surface to enable a good and efficient energy transfer to and from the energy storage material.

[0054] BRIEF DESCRIPTION OF THE FIGURE

[0055] The invention will now be explained by means of the following non-limiting figure.

[0056] Fig. 1 shows a plot of the target line for cumulative (aggregate and binder) steel slag particle size distribution of the concrete according to this invention as well as an example of the particle size distribution of solely the steel slag binder.

[0057] The invention will now be illustrated with reference to non-limiting comparative and examples according to the invention.

[0058] EXAMPLES

[0059] Three cement-free concrete mixtures in accordance with the invention have been composed consisting of: BOF steel slag aggregate made from commercial available Ekoliet® of different sieve fractions, viz. 0 / 4 mm, 4 / 8 mm, and 8 / 16 mm, Ekoliet® steel slag binder having a D50 of 11 micron, water, tri-potassium citrate monohydrate as the chelating agent, and a small amount of a commercial available superplasticizer. All steel slag additions are based on dry weight. Ekoliet® is a registered trademark of Pelt & Hooykaas B.V. The composition of the three concrete mixtures are listed in Table 1 and having a W / B ratio of less than 0.40.

[0060] The cement mixtures have been mixed and poured into moulds to form a concrete element, compacted by means of vibration and cured in regular atmosphere. Some properties of the fresh concrete have been determined (shaking measure F4, the slump, and the volumetric density). The shaking measure has been measured in accordance with EN-NEN- 12350-5 (2019), the slump has been measured in a flow test in accordance with EN-NEN- 12350-2 (2019). Also the 7-days and 28-days compressive strength have been determined according to EN 196-1 , in three replicates (cubes) at room temperature. The results are summarized in Table 2.

[0061] Table 1 . The components of three cement-free concrete mixtures.

[0062] Table 2. Properties of the three mixtures of Table 1 .

[0063] From the results of Table 2 it can be seen that the concrete according to this invention provides good compactibility at room temperature, has a high volumetric mass, and high 28- days compressive strength. Concrete mixture 2 is a preferred mixture in composition as it provides the best balance compactibility at room temperature, high volumetric mass, and 28- days compressive strength.

[0064] The concrete elements of dimensions 10x10x10 cm have been tested by exposing these elements for 4 days at 100°C to dry them to remove as much moisture as possible, next ramping up in about 3 hours to 450°C, and keeping the elements for 5 days at 450°C. No cracks formed in the elements after exposure at these temperatures. The loss is compressive strength after 5 days exposure to 450°C was less than 10%.

[0065] The absence of any cracks within the concrete elements when exposed to high temperatures and maintenance of high compressive strength makes the concrete in particular suitable for being used as energy storage material in large scale systems for storing and recovering of thermal energy and also for systems for storing and recovering electricity using thermal energy.

[0066] Also three comparative concrete mixtures A to C have been prepared and tested. The composition of the three comparative concrete mixtures are listed in Table 3. All three mixtures comprised cement of the type CEMIII / B 42.5N as defined in standard EN197-1. The Ekoliet® steel slag binder had a D50 of 11 micron. No chelating agent had been added to these mixtures.

[0067] Table 3. The components of three comparative concrete mixtures.

[0068] The cement mixtures have been mixed and poured into moulds to form a concrete element, compacted by means of vibration and cured in regular atmosphere. Some properties of the fresh concrete have been determined (slump and the volumetric density). The slump has been measured in a flow test in accordance with EN-NEN-12350-2 (2019). Also the 7-days and 28-days compressive strength have been determined according to EN 196-1 , in three replicates (cubes) at room temperature. The results are summarized in Table 4.

[0069] And the concrete elements of dimensions 10x10x10 cm have been tested also by exposing these elements for 4 days at 100°C to dry them to remove as much moisture as possible, next ramping up in about 3 hours to 450°C, and keeping the elements for 5 days at 450°C. All concrete elements tested started to show serious cracks at their surface once the temperature was above about 300°C. The loss is compressive strength after 5 days exposure at 450°C was more than 80% for all three concrete mixtures.

[0070] Table 4. Test results for the three mixtures of Table 3.

[0071] From the results of Table 4 and the tests at elevated temperatures it can be seen that increasing the amount of steel slag as aggregate increases the volumetric mass of the concrete mixture, which is favourable when using the concrete mixture as energy storage material in large scale systems for storing and recovering of thermal energy or for systems for storing and recovering electricity using thermal energy. Furthermore, the compressive strength levels of the various concrete mixtures are good, but unfortunately these fully collapse after exposure to a high temperature, in the case at hand to more than 300°C thereby rendering these concrete mixtures not suitable for use at elevated temperatures. More importantly, all three comparative concrete mixtures, thus irrespective of the presence of steel slag aggregate, showed severe formation of cracks when at temperatures above about 300°C due to the presence of substantial amounts of water in the concrete mixture (despite the drying step at 100°C) and due to the formation of cement stone because of the presence of cement in the concrete mixture. At elevated temperatures there are complex dissociation reactions of amongst others the calcium hydroxide in the cement stone resulting in that the cohesive effect of the cement stone is deteriorating over time.

Claims

CLAIMS1. Concrete having improved heat resistance, the concrete being cement-free and comprising: at least 55 wt.% of steel slag aggregate;15 to 40 wt.% of a binder consisting of ground steel slag; up to 1.0 wt.% of a chelating agent; up to 2 wt.% of admixtures such as plasticizers or superplasticizers; water in a range of up to 9 wt.%, and wherein the water to dry binder ratio (W / B) is in a range of less than 0.40; and wherein the steel slag has a particle size distribution build up comprising of: a D10 in a range of 5 to 50 micron, preferably of 6 to 30 micron; a D50 in a range of 0.9 to 3 mm, preferably of 1 to 2.5 mm; and a D90 in a range of 5 to 20 mm, preferably of 6 to 16 mm.

2. Concrete according to claim 1 , wherein the chelating agent is selected from the group of polycarboxylic acid salts, preferably tricarboxylic acid salts, more preferably alkali salts of citric acid, more preferably potassium citrate or sodium citrate, most preferably tripotassium citrate monohydrate.

3. Concrete according to claim 1 or 2, wherein the chelating agent is potassium citrate, preferably is tri-potassium citrate monohydrate.

4. Concrete according to any one of claims 1 to 3, wherein the amount of chelating agent is 0.5% to 9.0%, preferably 0.5% to 5.0%, relative to the amount of dry ground steel slag binder.

5. Concrete according to any one of claims 1 to 4, wherein the steel slag is selected from the group consisting of basic oxygen furnace (BOF) slag, ladle (LD) slag, electric arc furnace (EAF) slag, and mixtures thereof.

6. Concrete according to any one of claims 1 to 5, wherein the steel slag comprises in a range of 10% to 25% of particles in a sieve fraction range of 8 to 16 mm.

7. Concrete according to any one of claims 1 to 6, wherein the steel slag comprises in a range of 10% to 25% of particles in a sieve fraction range of 4 to 8 mm.

8. Concrete according to any one of claims 1 to 7, wherein the steel slag comprises up to 80% of particles in a sieve fraction range of 0 to 4 mm.

9. Concrete according to any one of claims 1 to 8, wherein the binder consisting of ground steel slag has a mean particle size D50 in a range of 5 to 30 micron, and preferably a particle size distribution build up of: a D10 in a range of 1 to 5 micron, a D50 in a range of 5 to 30 micron, and a D90 in a range of 10 to 100 micron.

10. Concrete according to any one of claims 1 to 9, wherein the concrete has a density of more than 3000 kg / m3, and preferably of more than 3100 kg / m3.

11. Concrete according to any one of claims 1 to 10, comprising further aggregates such as sand, gravel and / or limestone.

12. Concrete according to any one of claims 1 to 11 , wherein the ground steel slag is ground Basic Oxygen Furnace (BOF) steel slag comprises of: 10 - 30 wt.% brownmillerite, 0 - 15 wt.% magnetite, 25 - 60 wt.% C2S, 10 - 30 wt.% Mg-Wuestite 0 - 20 wt.% C3S, and 0 - 6 wt.% free-CaO.

13. Concrete according to any one of claims 1 to 11 , wherein the ground steel slag is ground Basic Oxygen Furnace (BOF) steel slag comprises of: 35 - 60 wt.% CaO, 10 - 17 wt.% SiO2, 15 - 35 wt.% of ZFe Oxides, 1 - 5 wt.% AI2O3, 1 - 13 wt.% MgO, 0 - 4 wt.% P2O5, 0 - 2 wt.% TiO2.

14. Method for preparing a heatproof concrete element from the concrete according to any one of claims 1 to 13, the method comprising the steps of: a) providing the fractions of steel slag aggregate, preferably of ground Basic Oxygen Furnace (BOF) steel slag, water, the binder, and the chelating agent; b) optionally providing non-steel slag aggregates; andc) optionally providing admixtures; d) mixing the ingredients of step a) to c) to obtain a concrete mixture paste; e) pouring the concrete mixture or concrete mixture paste in a mould for forming a concrete element; f) optionally compacting; g) curing the concrete mixture or concrete mixture paste to obtain the heatproof concrete element.

15. Use of a concrete according to any one of claims 1 to 13 as energy storage material in an electro-thermal energy storage and recovery system or in an thermal energy storage and recovery system.

16. Electro-thermal energy storage and recovery system comprising tubular elements embedded in the energy storage material made from the concrete according to any one of claims 1 to 13.

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