AU2014368363B2

AU2014368363B2 - Method for producing a gunned-concrete accelerator using new raw materials

Info

Publication number: AU2014368363B2
Application number: AU2014368363A
Authority: AU
Inventors: Benedikt Lindlar; Christian STENGER; Martin Weibel
Original assignee: Sika Technology AG
Current assignee: Sika Technology AG
Priority date: 2013-12-20
Filing date: 2014-12-19
Publication date: 2018-11-22
Anticipated expiration: 2034-12-19
Also published as: WO2015092004A2; WO2015092004A3; ES2764834T3; BR112016014218A2; BR112016014218B1; AU2014368363A1; AU2018260805B2; AU2018260805A1; EP3083525A2; EP3083525B1; EP3636623A1; PT3083525T; HUE047779T2

Abstract

The invention relates to a method for producing a gunned-concrete accelerator from a mixture a), b), or c), which method comprises mixing a) at least one aluminum compound selected from an aluminum sulfate and/or an aluminum hydroxy sulfate, at least one calcium compound, and optionally water, or b) at least one aluminum compound selected from aluminum sulfate and/or aluminum hydroxy sulfate, at least one alkali aluminate, and optionally water, wherein the alkali content of the mixture is at most 10 mass% Na

Description

METHOD FOR PRODUCING A GUNNED-CONCRETE ACCELERATOR USING NEW RAW MATERIALS

Technical field

The invention relates to a method for producing a sprayed-concrete accelerator with new raw materials, to the sprayed-concrete accelerator obtainable therefrom, and to the use thereof.

Prior art

The person skilled in the art is well aware that the setting and hardening of a hydraulic binder or a mixture comprising a hydraulic binder can be accelerated through the addition of an accelerator. Typical examples of hydraulic binders are cement such as Portland cement, blended Portland cements, lime, slaked limes, and mixtures thereof, and typical examples of mixtures which comprise a hydraulic binder of this kind and further components are mortar and concrete.

Sprayed concrete or mortar is concrete or mortar which is conveyed to the installation site in a closed hose or pipe line, where it is applied by spraying and at the same time compacted. Sprayed concrete or sprayed mortar must set and harden rapidly, so that sufficient strength is developed very quickly in the sprayed mixture to ensure that the sprayed layer adheres correctly to the surface to which the sprayed concrete or mortar has been applied. Consequently, accelerators for sprayed concrete and sprayed mortar are of special significance.

Sodium aluminate is a standard industrial product and is used as accelerator for hydraulic binders. Customarily a relatively large amount of sodium aluminate is required, and this may significantly increase the alkali metal content of the concrete. In many parts of the world this is unwanted, or the maximum amounts allowable are subject to statutory restriction. In some cases the use of alkali metal aluminate-based accelerators is no longer accepted on account of their high pH (> 12) and hence on account of their hazard potential for eyes, skin, and lungs.

Mixtures based on aluminum sulfate are frequently used as accelerators, since they have the best price/performance ratio. Within the first few minutes of setting, however, they are frequently too slow. This drawback can be overcome by raising the aluminum content of the mixture and/or by setting the molar ratio of aluminum to sulfate at greater than 2/3 (« 0.66). In aluminum sulfate with the formula AhjSCUb, the stoichiometric molar ratio of Al to sulfate is 2 : 3. Aluminum sulfate can be prepared by reaction of aluminum hydroxide and sulfuric acid.

Viewed chemically, mixtures, used as accelerator, with a molar ratio of Al/sulfate of greater than 2/3 comprise aluminum hydroxysulfate, a compound also known as basic aluminum sulfate. Aqueous commercial mixtures having a molar ratio of Al to sulfate in the range from about 0.66 to about 1 are commonly in suspension form, those with a ratio of greater than 1 commonly in solution form.

What have been found appropriate in practice for increasing the aluminum content of mixtures containing aluminum sulfate, as a sulfate-free or low-sulfate aluminum source, are amorphous aluminum hydroxides, aluminum hydroxysulfates, aluminum hydroxycarbonates, or aluminum nitrate, which may be added as powder or suspension. Commercial amorphous aluminum hydroxysulfate commonly has a ratio of AI:SC>4 of greater than 1 or very much greater than 1 and is expensive. Other known sources of aluminum are even more expensive, inadequately reactive, or contain disruptive components.

Exposition of the invention

The problem addressed with the present invention was therefore that of overcoming the aforementioned problems associated with the prior art. In particular the problem lay in the provision of a method for producing accelerators suitable for sprayed concrete or sprayed mortar, and preferably comprising basic aluminum sulfate, that allows more cost-effective production as compared with the prior art.

This has surprisingly been able to be achieved through the use of raw materials which, relatively, are less expensive in order to produce the accelerator. For this purpose, either the aluminum content of mixtures comprising aluminum sulfate and/or aluminum hydroxysulfate was increased by addition of alkali metal aluminate, or the sulfate content available for a rapid reaction in mixtures comprising aluminum sulfate and/or an aluminum hydroxysulfate was reduced by addition of a calcium compound.

The sulfate content available for a rapid reaction is significant, because the setting of cement or of another hydraulic binder on spraying takes only a few minutes. The precipitation of gypsum, which occurs on addition of the calcium compound, removes free sulfate from the accelerator. This sulfate, bound in the form of calcium sulfate, is withdrawn from the direct setting reaction, a desirable feature, and is not released again until after a delay of some hours, i.e., during the hardening, in a controlled process of redissolution of the gypsum. This too is generally desirable. Without wanting to be tied to any one theory, it is thought that the calcium released in this process accelerates hardening.

In an alternative embodiment, aluminum sulfate with increased aluminum content was obtained directly by reaction of aluminum hydroxide, preferably cheap, crystalline aluminum hydroxide, with substoichiometric quantities of sulfuric acid.

The invention therefore relates to methods for producing a sprayed-concrete accelerator from a mixture a), b) or c), comprising the mixing of a) at least one aluminum compound selected from an aluminum sulfate and/or an aluminum hydroxysulfate, at least one calcium compound, and optionally water, or b) at least one aluminum compound selected from an aluminum sulfate and/or an aluminum hydroxysulfate, at least one alkali metal aluminate and optionally water, the alkali metal content of the mixture not being more than 10 mass% Na2O equivalent, preferably not more than 5 mass% Na2O equivalent, more preferably not more than 2 mass% Na2O equivalent, very preferably not more than 1 mass% Na2O equivalent, or c) concentrated sulfuric acid and aluminum hydroxide, preferably crystalline aluminum hydroxide, the sulfuric acid being present in a stoichiometric deficit and the mixture being reacted at a temperature of above 100°C, preferably under overpressure, i.e., a pressure greater than 1 bar, where the sprayed-concrete accelerator or, where it is a solid sprayed-concrete accelerator, a mixture of the solid sprayed-concrete accelerator in water has a pH of preferably below 4.

Through the use of the favorably priced raw materials it is possible to produce mixtures based on aluminum sulfates and/or aluminum hydroxysulfates, with an increased molar ratio of aluminum to sulfate, at lower costs than before; for this molar ratio, consideration is given only to readily available aluminum and sulfate, meaning that sulfate bound as a CaSCL is disregarded. There are various hydrates of CaSO4; in general, it precipitates in the form of the dihydrate. These mixtures are suitable for use as sprayed-concrete accelerators.

Through the use of the mixtures of the invention as sprayed-concrete accelerators it is possible to improve the development of strength in the sprayed concrete. Furthermore, the sprayed-concrete accelerator exhibits a reduced viscosity. It has surprisingly been found that the addition of the calcium compound in the mixture a) also results in improved storage stability of the sprayed-concrete accelerator.

Preferred embodiments of the method are reproduced in the dependent claims. The invention also relates to the sprayed-concrete accelerator obtainable from the method, and to the use thereof as a sprayed-concrete accelerator. The invention is elucidated in detail below.

Way of performing the invention

Accelerators are substances or mixtures which accelerate the setting and/or hardening of a hydraulic binder or of a mixture comprising a hydraulic binder. Typical examples of hydraulic binders are cement, lime, slaked limes, and mixtures thereof, and typical examples of mixtures which comprise a binder of this kind and further components are cement suspensions, such as for injections, for example, mortar, and concrete. The mixture in question is preferably a cement suspension, a concrete or mortar, which comprises cement, more particularly Portland cement and/or blended Portland cements. Hydraulic binders, especially cements, usually contain calcium sulfate in the form of gypsum, hemihydrate and/or anhydrite as setting regulator and/or sulfate source.

Sprayed concrete is used generally as a generic term for sprayed concrete and sprayed mortar. Accelerators for sprayed concrete or sprayed mortar are generally referred to as sprayed-concrete accelerators. Unless expressly indicated otherwise, accordingly, the expression “sprayed concrete” in the present application, including the claims, always also includes sprayed mortar. Correspondingly, the expression “sprayed-concrete accelerator” in the present application and in the appended claims embraces accelerators for sprayed concrete and accelerators for sprayed mortar, and also accelerators for cementitious injections.

Sprayed concrete is a concrete or mortar which is conveyed to the installation site in a closed hose or pipe line, where it is applied by spraying and at the same time is compacted. There is a fundamental distinction between two spraying processes: the dry spraying and the wet spraying process. With the dry spraying process, an earth-moist or dry concrete or mortar is supplied to the conveying line and is conveyed generally with compressed air to the spraying nozzle. There, mixing water and optionally concrete admixtures such as, for example, sprayed-concrete accelerators or rebound reducers are added. In the case of the wet spraying process, a wet base mix comprising a concrete or mortar is conveyed to the nozzle via the conveying line and is sprayed generally by means of compressed air or driven air. In the nozzle region, if desired, concrete admixtures such as, for example, sprayed-concrete accelerators may be added. Wet spraying processes can be carried out by the thin-stream method or the dense-stream method.

Spraying processes are used, for example, when shuttering is not a possibility, or for the provision of linings and reinforcing layers in the production of civil engineering constructions, or when cavities must be provided with a reinforcing layer, such as in tunnels, galleries, and pipes, such as in mining, for example. The spraying process may further serve for solidifying or stabilizing or compacting natural soils, such as cliffs, slopes or excavations, subsoil and bedrock.

In accordance with the method of the invention, a sprayed-concrete accelerator of a mixture a), b) or c) is produced. The method for producing the respective mixture a), b) or c) which constitutes the sprayed-concrete accelerator is elucidated comprehensively below.

The sprayed-concrete accelerator comprising the mixture a), b) or c) may take the form of a solid sprayed-concrete accelerator, i.e., a solids mixture, preferably a powder. It is preferred, however, for the sprayed-concrete accelerator produced by the method of the invention comprising the mixture a), b) or c) to be an aqueous mixture, more particularly an aqueous solution or an aqueous suspension. An aqueous suspension is a dispersion of particulate solids in a liquid phase. A solid sprayed-concrete accelerator can be readily converted, by addition of water, into an aqueous mixture, more particularly an aqueous solution or suspension of the sprayed-concrete accelerator. It may, for example, be judicious to transport the sprayed-concrete accelerator to the site of use in solid form in order to save space, and to convert it only when on site into an aqueous solution or suspension, which can then be used as a sprayed-concrete accelerator.

The sprayed-concrete accelerator produced by the method of the invention and comprising the mixture a), b) or c) can be used for the dry spraying process or for the wet spraying process. Using the sprayed-concrete accelerator in the form of an aqueous mixture, more particularly an aqueous solution or suspension, is preferred.

The sprayed-concrete accelerator in accordance with the mixture a), b) or c), present in the form of an aqueous mixture, more particularly an aqueous solution or suspension, has a pH of below 4, as for example a pH in the range from 2 to below 4. Where it is a solid sprayed-concrete accelerator, a mixture of the solid sprayed-concrete accelerator in water likewise has a pH of below 4, as for example a pH in the range from 2 to below 4. The mixture of the solid sprayed-concrete accelerator in water can be obtained for example by dissolving or suspending 100 g of the solid sprayed-concrete accelerator in 100 g of water. It may take several minutes or hours for an equilibrium and a stable pH to establish. All of the pH values reported here, as is customary, relate to the pH of the aqueous mixture in the equilibrium state.

The addition of water to the dry mixture is accompanied by dissolution events and other reactions, which can lead, for example, to the precipitation of solid substances. Depending on raw materials employed, it may take several minutes or hours for the mixture to react fully and for the equilibrium to become established. In the course of the full reaction, the pH may change, and so a pH that is constant over time is only established after full reaction. The establishment of equilibrium is therefore also easy to determine from a pH which no longer undergoes any substantial change. The pH is determined at room temperature (20°C).

The aqueous mixture of the mixture a), b) or c), more particularly in the form of an aqueous solution or aqueous suspension, may have a water content, for example, in the range from 20 to 90 wt%, more particularly from 40 to 80 wt%, preferably from 50 to 70 wt%, and more preferably from 60 to 65 wt%. The above water content relates to the total water content, thus including any water of crystallization, if present. Some starting materials, especially aluminum sulfate, may contain water, especially in the form of water of crystallization. There are certain industrial products in which the water of crystallization may in fact make up to almost 50 wt% of the total weight, a factor which must be taken into account. The solids content without water of crystallization in the sprayed-concrete accelerator comprising the mixture a), b) or c) is more preferably 30 to 45 wt% and more particularly 35 to 40 wt%.

It is preferred for a sprayed-concrete accelerator comprising the mixture a), b) and/or c) to be free from chloride or substantially free from chloride, having for example a chloride content of not more than 0.1 wt%, based on the mixture a), b) and/or c). It is further preferred for the sprayed-concrete accelerator comprising the mixture a), b) and/or c) to be free from alkali metal or substantially free from alkali metal, having for example an alkali metal content of not more than 1 mass% Na2O equivalent, based on the mixture a), b) or c). However, higher amounts of chloride and/or alkali metal are also possible. The Na2O equivalent is elucidated later on below. A sprayed-concrete accelerator comprising the mixture a), b) or c) is preferably an aqueous suspension, the aqueous suspension more preferably comprising jurbanite. Jurbanite is AI(SO4)(OH) · 5 H2O and is present as a solid in the suspension.

In the mixtures a), b) or c) the molar ratio of aluminum to (sulfate - calcium), i.e., molAi/(molso4 - molca), is situated for example in the range from 0.66 to 2.2 or 2/3 to 2.2, more particularly greater than 2/3, preferably 0.67 to 1.9, more preferably in the range from 0.68 to 1.6, more preferably in the range from 0.69 to 1.3, more preferably in the range from 0.7 to 1, more preferably in the range from 0.71 to 0.9, and very preferably in the range from 0.72 to 0.8, with the amount of Ca in the mixtures b) and c), if it is present at all, being commonly only negligibly small, as for example in traces or as impurity, with the consequence that the above molar ratio there often corresponds essentially to the molar ratio of aluminum to sulfate, molAi/molso4- In a mixture a) it should be borne in mind that some of the sulfate is bound in the form of calcium sulfate if water is present. In aqueous solution, CaSO4 2H2O (“gypsum”) precipitates. The aqueous sprayed-concrete accelerators comprising the mixtures a), b) or c) preferably comprise aluminum hydroxysulfate in solution or in precipitated form as a suspension, in which case the molar ratio of aluminum to (sulfate -calcium) as defined above is preferably greater than 2/3 and more preferably greater than 2/3 and not more than 1.

For the molar ratio, the entire aluminum, sulfate, and calcium in the mixture are considered, irrespective of the form in which they are present, as for example in solution and/or in bound form as solid. Calcium is taken into consideration, since sulfate which is bound in the form of sparingly soluble calcium sulfate, commonly gypsum, is not available for the reaction by which the sprayed concrete becomes set. This setting generally takes just a few minutes. In the subsequent curing of the sprayed concrete, which may take hours or possibly days, the gypsum is not disruptive and may even be useful. In general it dissolves in the process and is incorporated into the hydrate phases of the cement.

In the case of an aqueous mixture a) it may generally be assumed that, leaving aside a small concentration of calcium ions in the mixture below the solubility limit of calcium sulfate, the remaining amount of calcium from the calcium compound is converted into calcium sulfate. The amount of Ca ions which is present in solution on account of the solubility limit of CaSCU is very small, owing to the relatively sparing solubility of calcium sulfate (solubility product at 25°C according to R.C. Weast (Ed.), Handbook of Chemistry and Physics, 56th edition, CRC Press, Cleveland USA 1975: 2.45 χ 10'5 mol2!'2).

The amounts of calcium, aluminum, and sulfate and the corresponding molar ratio molAi/(molso4 - molca) in the mixtures a), b) or c) can be easily determined or calculated from the amounts of the starting compounds used, on the basis of their composition. Where necessary, it is possible optionally for the amount of the relevant elements and/or ions in the starting components and/or in the mixture to be determined quantitatively by means of established analytical methods as well, prominent among them being ICP-OES (optical emission spectroscopy by means of inductively coupled plasma). In general, sulfur is present only in the form of sulfate, and so the amount of sulfur determined by the analysis can also be taken as a direct determination of the amount of sulfate. Where sulfur is also present in a form other than sulfate, something which is usually not the case, however, this must be taken into account separately when determining the sulfate content. Further examples of customary analytical methods which may possibly be used are, for example, ion chromatography or electrophoresis for dissolved ions, or, for example, XRD (X-ray diffraction) for crystalline phases.

The method for producing the mixture a) as sprayed-concrete accelerator comprises the mixing of at least one aluminum compound selected from an aluminum sulfate and/or an aluminum hydroxysulfate, at least one calcium compound, and optionally water. The aluminum compound is an aluminum sulfate and/or an aluminum hydroxysulfate, an aluminum sulfate being preferred. It is possible to use only aluminum sulfate, only aluminum hydroxysulfate, or aluminum sulfate and aluminum hydroxysulfate.

As aluminum sulfate it is possible to use all customary products, including technical aluminum sulfate. These products are available commercially. It is possible to employ anhydrous aluminum sulfate, aluminum sulfate hydrates, or mixtures thereof. In practice, aluminum sulfates with different hydrate and/or water contents are sold, which are suitable for the present invention. Generally speaking, aluminum sulfate hydrates are preferred, since they have better solubility. Technical aluminum sulfate may possibly have an increased sulfate content, with the ratio AI/SO4 being less than 2/3. This may come about, for example, from the fact that technical aluminum sulfate frequently includes a small excess (e.g., < 1%, usually not more than 0.3%) of sulfuric acid. Also possible is the additional adding of sulfate compounds such as sodium sulfate or of further sulfuric acid, in amounts for example of up to 5 wt% or more, but generally this is not preferred.

Suitable aluminum hydroxysulfates are stoichiometric and nonstoichiometric aluminum hydroxysulfates, which may be present in each case in anhydrous form or as a hydrate. In the aluminum hydroxysulfates, some of the sulfate ions in aluminum sulfate are replaced by hydroxyl groups. Aluminum hydroxysulfates are often also called basic aluminum sulfate. Stoichiometric aluminum hydroxysulfate, AI(OH)(SC>4), which optionally has water of hydration, is also referred to as aluminum hydroxide sulfate. In the stoichiometric aluminum hydroxysulfate, the molar ratio AI/SO4 is equal to 1.

An example of this is jurbanite. Nonstoichiometric aluminum hydroxysulfates have a lower hydroxide content than the stoichiometric aluminum hydroxysulfate. For charge compensation, the sulfate content is increased accordingly. Therefore the molar ratio AI/SO4 in nonstoichiometric aluminum hydroxysulfates is smaller than 1 and greater than 2/3. As nonstoichiometric aluminum hydroxysulfate it is possible, for example, to use the basic aluminum sulfate obtained from the method of the invention from mixture c).

As stoichiometric or nonstoichiometric aluminum hydroxysulfates it is possible to use anhydrous aluminum hydroxysulfates, aluminum hydroxysulfate hydrates, or mixtures thereof.

One or more customary calcium compounds may be used as calcium compound. These may be inorganic or organic calcium compounds, with inorganic calcium compounds being preferred. Without wishing to be tied to any one theory, the calcium compound serves commonly as a source of calcium ions to scavenge sulfate in the mixture, with the formation of sparingly soluble calcium sulfate.

The calcium compound used may be an artificial or natural (e.g., mineral) compound which is wholly or partly water-soluble at least in the acidic pH range.

In one preferred variant, the calcium compound is used in an amount such that the calcium concentration in the accelerator is only just above or even below the solubility limit of gypsum. Without wishing to be tied to any one theory, it is thought that mixtures of concrete and accelerators of this kind are saturated from the start in terms of Ca and therefore set more rapidly.

Surprisingly it has been ascertained that even small amounts of Ca ions or Ca-containing crystal nuclei, still present in solution, are active and also improve the storage stability. The calcium compound can therefore also be added in small amounts below the solubility limit of gypsum in the mixture. In this case, there is no precipitation of gypsum, and the molar ratio of Al to sulfate in the solution remains unchanged.

The at least one calcium compound is preferably a calcium compound which is soluble in water at a pH < 7, i.e., in the acidic range, this solubility possibly also including a reaction. Calcium sulfate is therefore, for example, not preferred.

Examples of preferred calcium compounds are calcium oxide, calcium hydroxide, calcium carbonate, cement, especially Portland cement, and mixtures thereof. Furthermore, calcium halides such as calcium chloride and calcium nitrate are likewise suitable, but not preferred, since halides such as chloride, or nitrate, if present in too great a concentration, may adversely affect certain properties of the concrete or mortar and are restricted in terms of their amount in the majority of countries for numerous applications. Particularly the use of calcium nitrate, however, may be entirely judicious in certain circumstances. Further examples of suitable calcium compounds are calcium formate and calcium acetate. In one embodiment a mixture of a basic calcium compound and calcium carbonate may be used as calcium compound. Examples of basic calcium compounds are calcium oxide, calcium hydroxide, or cement.

Preferred examples of the cement are, for example, a cement in accordance with EN standard 197 CEM I, CEM II, CEM III, CEM IV or CEM V, or in accordance with ASTM standard C150 type I, type II, type III, type IV or type V.

It is generally preferred for the added calcium compound to react substantially completely to form calcium sulfate. In certain cases, however, it may also be advantageous for the calcium compound not to react fully, and for residues or inert fractions to remain.

Through the addition of the calcium compound to an aluminum sulfate and/or an aluminum hydroxysulfate in aqueous mixture it is possible, as a result of precipitation of calcium sulfate (gypsum), for some of the sulfate present to be bound and so for the ratio of aluminum to readily available sulfate in the accelerator to be increased. In the case of the reaction of an aluminum sulfate and/or an aluminum hydroxysulfate with calcium carbonate, furthermore, carbon dioxide is released. The addition of cement, especially Portland cement, leads not only to the deposition of gypsum, but also to the addition of aluminum present in the cement to the mixture. Other substances present in the cement as well, such as iron or silicate, may be useful.

Water may optionally be added to the mixture. The addition of water is particularly preferred. It is possible to use water alone or water containing one or more additives. Examples of suitable additives are those identified below. Present in the water, for example, may be at least one stabilizer, more particularly a stabilizer for the storage stability. It may be a soluble stabilizer, such as phosphoric acid, or preferably an insoluble stabilizer, such as sepiolite or bentonite, for example.

The mixture may further be admixed, as and when required, with one or more additives. These may be customary additives which are used in sprayed-concrete accelerators. Examples of such additives are one or more aluminum compounds, more particularly sulfate-free or low-sulfate aluminum compounds, in order to increase the aluminum content further; one or more dispersants; one or more additives which improve the setting characteristics; and/or one or more stabilizers. Examples of stabilizers have been given above. Examples of additives employed with preference which in suitable amount improve the setting characteristics are Mg2+ compounds or magnesium salts, such as magnesium oxide, magnesium hydroxide or magnesium carbonate, diethanolamine (DEA) and fluorine compounds such as hydrofluoric acid, alkali metal fluorides, and fluoro complexes. In the accelerator they may improve the setting and in some cases also the storage stability.

The mixture a) may further be admixed with at least one alkali metal aluminate as additive as well. As examples of suitable alkali metal aluminates, reference is made to the examples given below for mixture b). The alkali metal aluminate is preferably a sodium aluminate, a potassium aluminate, or a mixture thereof.

If at least one alkali metal aluminate is added, the alkali metal content of the mixture a) is for example not more than 10 mass%, preferably not more than 5 mass%, preferably not more than 2 mass%, more preferably not more than 1 mass% Na2O equivalent. These preferred ranges for the alkali metal content are also applicable, moreover, if no alkali metal aluminate is added to the mixture a).

Examples of sulfate-free or low-sulfate aluminum compounds are amorphous aluminum hydroxides, aluminum nitrate or aluminum hydroxycarbonates. Examples of dispersants are polyacrylic acid, polyacrylates, derivatives of phosphonic acids, and mixtures comprising two or more of the stated components.

The ratio of aluminum sulfate and/or aluminum hydroxysulfate to the at least one calcium compound may vary within wide ranges. It is preferred for the molar ratio Ca : Al in the mixture a) to be in the range from 0.002 to 2, preferably from 0.01 to 1, more preferably from 0.05 to 0.5.

For producing the mixture a) it is possible for example, based on the total weight of the starting components, including any water of crystallization, if present therein but without any liquid water optionally added, to add 50 to 99.95 wt% and preferably 80 to 99.5 wt% of aluminum sulfate and/or aluminum hydroxysulfate and 0.05 to 50 wt%, preferably 0.2 to 20 wt%, more preferably 0.5 to 10 wt%, and very preferably 1 to 5 wt%, of at least one calcium compound; here it is optionally also possible to add one or more additives, in a fraction for example of 0 to 50 wt%, preferably 0.01 to 20 wt%, more preferably of 0.5 to 10 wt%.

The mixture a) may be present in the form of a solid, more particularly a powder, or, preferably, in the form of an aqueous mixture, more particularly an aqueous solution or aqueous suspension. The water content of the aqueous mixture a) may be for example in the range from 20 to 90 wt%, more particularly 40 to 80 wt%, preferably from 50 to 70 wt%, and more preferably 60 to 65 wt%. The above water content relates to the total water content, thus including any water of crystallization, if present.

The sequence in which the individual components are added to the mixture is arbitrary. To produce a dry mixture, the starting components can simply be mixed in dry form and further additives can optionally be added. In one preferred embodiment, the aluminum sulfate and/or aluminum hydroxysulfate is dissolved and/or suspended in water and the at least one calcium compound is subsequently added in solid form, the components being combined with stirring. In the case of basic calcium compounds, however, this embodiment may possibly result in lumps being formed. In order to avoid this, it is possible when using basic calcium compounds such as Ca(OH)2 for example, or cement, such as aluminous cement, for this basic powder to be mixed with lime (CaCC>3). When this mixture is dissolved, gas (CO2) is produced, which counteracts the formation of lumps.

In a further embodiment, the calcium compound is first dissolved and/or suspended in water and then aluminum sulfate and/or aluminum hydroxysulfate in powder form or as aqueous solution or suspension are admixed, or the calcium compound in solution and/or suspension in water is added to the aluminum sulfate and/or aluminum hydroxysulfate as powder, suspension or solution.

In one particularly preferred embodiment, solid aluminum sulfate and/or aluminum hydroxysulfate and the at least one calcium compound in solid form are mixed, and the resulting solids mixture is added to water, which judiciously is vigorously stirred. In the case of the preferred embodiments, optional additives may be added in any desired way - for example, as a separate component or as a constituent of the initial water introduced.

The mixing of the components may take place using customary mixing and/or stirring apparatus or else manually with suitable mixing and/or stirring means for dry mixtures or aqueous mixtures. In this way a homogeneous mixture of the components can be obtained.

In the case of the mixing of aluminum sulfate and/or aluminum hydroxysulfate and of the at least one calcium compound in water, there is generally an exothermic reaction. The mixture may optionally be heated, but heating is generally necessary only when the dissolution or reaction of the raw materials is otherwise too slow. In general the components require only brief mixing. Particularly if, as preferred, water is added to the mixture as well, the mixing and, where appropriate, subsequent stirring may take place for example over a period of 10 minutes to 48 hours, preferably 10 minutes to 24 hours, or 6 hours to 48 hours, in order to give the completed sprayed-concrete accelerator.

At least part of the calcium sulfate formed in the mixture may be separated off, by filtration, for example. It may, however, be judicious to leave it in the mixture, since it may have an additional accelerating effect on the setting and hardening of the concrete or mortar. Optionally it is also possible for a part or the total amount of water added to be removed again, by evaporation, for example; in general, however, this is not preferred.

The method for producing the mixture b) as sprayed-concrete accelerator comprises the mixing of at least one aluminum compound selected from an aluminum sulfate and/or an aluminum hydroxysulfate, at least one alkali metal aluminate, and optionally water, the alkali metal content of the mixture being not more than 10 mass%, preferably not more than 5 mass%, more preferably not more than 2 mass%, and very preferably not more than 1 mass% of Na2O equivalent. This figure is based on the total weight of the aqueous or dry mixture.

As is generally customary in the cement industry, the alkali metal content is expressed as Na2O equivalent, calculated according to the formula below (data in mass%):

Na2O equivalent = Na2O + 0.658 · K2O As can be seen from the formula, the alkali metal content relates only to Na and K.

Products contemplated as aluminum sulfate and/or aluminum hydroxysulfate for the aluminum compound are all customary products which can also be used for the mixture a). Reference is therefore made to the details above.

As alkali metal aluminate it is possible, for example, to use lithium aluminates, sodium aluminates, potassium aluminates, or a mixture of two or more of these, with preference being given to sodium aluminate and potassium aluminate and mixtures thereof and particular preference to sodium aluminate; the stated aluminates may also be present in the form of their hydrates. The alkali metal aluminates are available commercially. Sodium aluminate is an industrial product. The majority of commercial alkali metal aluminates, with or without water of hydration, are nonstoichiometric - for example, potassium aluminates are sold with a stoichiometry of 1.4 K2O + 1 AI2C>3. In addition to the stoichiometric alkali metal aluminates, therefore, the alkali metal aluminates may also be nonstoichiometric alkali metal aluminates, of the kind customary in commerce. If suitable for the desired method regime, it is also possible with advantage to use the aqueous solutions or suspensions of these alkali metal aluminates directly, such as commercial sodium aluminate solutions, for example. The addition of an alkali metal aluminate raises the aluminum content of the mixture.

Water may optionally be added to the mixture b). The addition of water is particularly preferred. It is possible to use water alone or water containing one or more additives. Examples of suitable additives are those identified below. Present in the water, for example, may be at least one stabilizer, more particularly a stabilizer for the storage stability. It may be a soluble stabilizer, such as phosphoric acid, or preferably an insoluble stabilizer, such as sepiolite or bentonite, for example.

The mixture b) may further be admixed, as and when required, with one or more additives. These may be customary additives which are used in sprayed-concrete accelerators. Examples of such additives are the same as stated above for the mixture a) to which reference may therefore be made.

The ratio of aluminum sulfate and/or aluminum hydroxysulfate to the at least one alkali metal aluminate may vary within wide ranges. It is preferred, for the mixture b), for aluminum sulfate and/or aluminum hydroxysulfate and the at least one alkali metal aluminate to be mixed in a weight ratio of the at least one alkali metal aluminate, including any water of crystallization, if present, to the aluminum sulfate and/or aluminum hydroxysulfate, including any water of crystallization, if present, in the range from 0.001 to 1, preferably from 0.005 to 0.2, and more preferably from 0.01 to 0.1.

For producing the mixture b) it is possible for example, based on the total weight of the starting components, including any water of crystallization, if present therein but without any liquid water added, for from 50 to 99.95 wt%, preferably from 80 to 99.5 wt%, and more preferably 90 to 99.5 wt% of aluminum sulfate and/or aluminum hydroxysulfate and 0.05 to 50 wt%, preferably from 0.2 to 20 wt%, and more preferably from 0.5 to 10 wt% of at least one alkali metal aluminate to be added, in which case optionally there may also be one or more additives added, in a fraction, for example, of 0 to 50 wt%, preferably 0.01 to 20 wt%, more preferably of 0.5 to 10 wt%.

The mixture b) may be present in the form of a solid, more particularly a powder, or, preferably, an aqueous mixture, more particularly an aqueous solution or aqueous suspension. The water content of the aqueous mixture b) may be for example in the range from 20 to 90 wt%, more particularly 40 to 80 wt%, preferably from 50 to 70 wt%, and more preferably from 60 to 65 wt%. The above water content relates to the total water content, thus including any water of crystallization, if present. Where the sprayed-concrete accelerator comprising the mixture b) constitutes an aqueous mixture, it preferably has a pH in the range from 2 to below 4. Where it is a solid sprayed-concrete accelerator comprising the mixture b), a mixture of the solid sprayed-concrete accelerator in water preferably has a pH in the range from 2 to below 4.

The sequence in which the individual components are added to the mixture is arbitrary. In order to produce a dry mixture, the starting components in dry form can be mixed and optionally further additives can be added. In one embodiment the aluminum sulfate and/or aluminum hydroxysulfate are dissolved and/or suspended in water and the at least one alkali metal aluminate is then added in solid form and the components are stirred, or, conversely, the at least one alkali metal aluminate is dissolved and/or suspended in water and then aluminum sulfate and/or aluminum hydroxysulfate in solid form are added and the components are stirred. It is of course also possible to stir together aqueous solutions or suspensions in each case of aluminum sulfate and/or aluminum hydroxysulfate and of the at least one alkali metal aluminate. In a further embodiment, solid aluminum sulfate and/or aluminum hydroxysulfate and the at least one alkali metal aluminate in solid form are mixed and water is added to the resulting solids mixture and the components are stirred, or the resulting solids mixture is preferably stirred into water. With the stated embodiments, optional additives may be added in any desired way, as for example as a separate component or as a constituent of the water added.

In the case of the mixing of aluminum sulfate and/or aluminum hydroxysulfate and of the at least one alkali metal aluminate in water, an exothermic reaction may take place. The mixture may optionally be heated, but heating is generally not necessary. In general the components require only brief mixing. Particularly if, as preferred, water is added to the mixture as well, the mixing and, where appropriate, subsequent stirring may take place for example over a period of 10 minutes to 48 hours, preferably 10 minutes to 24 hours, or 6 hours to 48 hours, in order to give the completed sprayed-concrete accelerator. Optionally it is also possible for a part or the total amount of water added to be removed again, by evaporation, for example; in general, however, this is not preferred.

The method for producing the mixture c) encompasses the reaction of concentrated sulfuric acid and aluminum hydroxide, preferably crystalline aluminum hydroxide, the sulfuric acid being present in a stoichiometric deficit and the mixture being reacted, preferably under overpressure (> 1 bar), at a temperature of above 100°C. Optionally, after the reaction, water may be added for dilution.

The sulfuric acid is present in a stoichiometric deficit or in substoichiometric amount, this relationship referring to the stoichiometry of the reaction between aluminum hydroxide and sulfuric acid for the preparation of aluminum sulfate in accordance with the following reaction equation: 2 AI(OH)3 + 3 H2SO4 -+ AI2(SO4)3 + 6 H20

Accordingly, sulfuric acid is present in a stoichiometric deficit when the molar ratio of H2SO4 to AI(OH)3 is less than 1.5. In the production of the mixture c), the molar ratio of H2SO4 to AI(OH)3 is preferably less than 1.48, more preferably not more than 1.45, and very preferably not more than 1.4. It is preferred, furthermore, for the molar ratio of H2SO4 to AI(OH)3 to be at least 1 and more preferably at least 1.2. It is preferred for concentrated sulfuric acid and aluminum hydroxide, preferably crystalline aluminum hydroxide, to be mixed in a molar ratio of sulfuric acid to aluminum hydroxide, preferably crystalline aluminum hydroxide, in a range from 1.45 to 1, preferably from 1.45 to 1.2.

Concentrated sulfuric acid used may be all commercially customary concentrated sulfuric acids. Aqueous sulfuric acid solutions are defined by their H2SO4 content in mass percent. Judiciously a concentrated sulfuric acid is used that has a sulfuric acid content of, for example, at least 50 mass%. Particularly suitable, for example, are sulfuric acids having a sulfuric acid content in the range from 60 to 90 mass%.

The aluminum hydroxide used is preferably crystalline aluminum hydroxide.

The crystalline aluminum hydroxide may optionally include fractions of amorphous aluminum hydroxide, e.g., less than 10 wt%, commonly less than 1 wt%. Various crystal modifications of aluminum hydroxide are suitable. Crystalline aluminum hydroxide is a cheap mass product. Suitability is possessed for example by crystalline aluminum hydroxides which are obtained during the Bayer Process for refining bauxites. The crystalline aluminum hydroxide may optionally also be bauxite. Natural bauxite may optionally include various aluminum hydroxides. Examples are aluminum orthohydroxide AI(OH)3 such as γ-ΑΙ(ΟΗ)3 (monoclinic, known as mineral gibbsite (hydrargillite)), β-ΑΙ(ΟΗ)3 (hexagonal, known as mineral bayerite) and triclinic aluminum orthohydroxide, known as mineral nordstrandite, and relatively low-water-content aluminum metahydroxide or aluminum oxide hydroxide AIO(OH) such as oc-AIO(OH) (orthorhombic, known as mineral diaspore) ory-AIO(OH) (orthorhombic, known as mineral boehmite). Other modifications with or without water of crystallization are possible. Crystalline aluminum hydroxide is insoluble in water. The crystalline aluminum hydroxide may be used dry or in moist form. When moist aluminum hydroxide is used, account must be taken of the water it contains, in order to obtain the desired molar ratio of aluminum hydroxide to sulfuric acid and also to obtain the high concentration that is needed for the reaction - in other words, the sulfuric acid must be used in correspondingly more concentrated form and hence in lesser amount.

The reaction may be carried out in such a way, for example, that the concentrated sulfuric acid in a stoichiometric deficit is charged to a reactor and the aluminum hydroxide, preferably the crystalline aluminum hydroxide, is added dry or in moist form, it being possible for the sulfuric acid to be preheated before being added. The mixture is stirred, and, depending on the heat and the concentration of the sulfuric acid used, an exothermic reaction starts after a certain time, and consequently a melt of the reaction products may be formed.

The reaction can be carried out without overpressure, but preferably takes place under overpressure, i.e., at a pressure of above 1 bar, more preferably at a pressure of at least 3 bar. The reaction takes place for example preferably at a pressure in the range from 3 to 15 bar, more preferably from 5 to 15 bar.

The reaction takes place at an elevated temperature of more than 100°C, preferably at least 150°C. The reaction may be carried out for example at a temperature in a range from 120 to 220°C, preferably from 160 to 190°C.

The reaction mixture is suitably stirred at the stated temperatures and pressures for a certain time following addition of the sulfuric acid, for example for a time of at least 10 minutes, preferably at least 15 minutes, the stirring time possibly being, for example, 10 to 60 minutes and preferably 15 to 30 minutes.

After the end of the reaction, the reaction mixture can be cooled and any water present can be removed by evaporation, under reduced pressure, for example, or by filtration, to give a solid product. In general, however, the reaction mixture solidifies completely in the course of cooling. After the end of the reaction, water is preferably added. Unreacted starting products may optionally be removed by filtration.

One or more additives may also be added, as and when required, to the mixture c). These may be customary additives which are used in sprayed-concrete accelerators. Examples of such additives are those stated above for the mixture a), and so reference may be made thereto.

The mixture c) may be admixed with at least one additive selected from a calcium compound and an aluminum compound and/or with one or more other additives. Examples of suitable calcium compounds are those stated for the mixture a). Examples of suitable aluminum compounds are the abovementioned sulfate-free or low-sulfate aluminum compounds such as amorphous aluminum hydroxides, aluminum hydroxysulfates, aluminum nitrate or aluminum hydroxycarbonates, or else, optionally, alkali metal aluminates as used for the mixture b).

The additives, if used, are added to the mixture c) preferably after reaction of the concentrated sulfuric acid with aluminum hydroxide, preferably crystalline aluminum hydroxide.

The reaction produces a basic aluminum sulfate in solid form, for example as powder, or preferably as aqueous mixture, more particularly as aqueous solution or aqueous suspension. The water content of the mixture c) may be situated for example in the range from 20 to 90 wt%, more particularly 40 to 80 wt%, preferably from 50 to 70 wt%, and very preferably from 60 to 65 wt%. The above water content is based on the total water content, hence including any water of crystallization, if present. Where the sprayed-concrete accelerator comprising the mixture c) constitutes an aqueous mixture, it preferably has a pH in the range from 2 to below 4. Where it is a solid sprayed-concrete accelerator comprising the mixture c), a mixture of the solid sprayed-concrete accelerator in water preferably has a pH in the range from 2 to below 4.

If aluminous cement is used as calcium compound, the production of the mixture a) is difficult, since the high reactivity of aluminous cement frequently causes the formation of lumps in the mixture before reaction has proceeded to completion. These problems can be avoided, however, if one of two or both specific method regimes already described above are selected, in which the starting components are first mixed dry and then the dry mixture is mixed with water.

In this case, the starting components are firstly mixed dry and then the dry mixture is added to water. The dry mixture in this case is preferably stirred gradually into the water.

Alternatively, aluminous cement as a basic component (powder) alone, or a powder mixture which comprises aluminous cement, may be admixed with calcium carbonate before being added to the water or to the aqueous solution. The CO2 gas which is released in acidic medium (pH 2 to below 4) counteracts the formation of lumps and is able to prevent lump formation.

The invention also relates, accordingly, to a method for producing a sprayed-concrete accelerator, comprising the mixing of at least one aluminum compound selected from an aluminum sulfate and an aluminum hydroxysulfate and aluminous cement, and optionally calcium carbonate in solid form, to give a solids mixture, and the addition of the solids mixture to water which optionally comprises dissolved and/or dispersed calcium carbonate, to give an aqueous mixture, more particularly an aqueous solution or suspension, the sprayed-concrete accelerator having a pH of below 4.

Products contemplated as aluminum sulfate and/or aluminum hydroxysulfate are all customary products which can also be used for the mixture a).

Reference is therefore made to the details above. Suitable aluminous cement comprises all aluminous cements that are available commercially. All details given above and below in relation to the mixture a) apply equally to this method, except that the at least one calcium compound used in the mixture a) is to be replaced by aluminous cement, and the above method is used. Reference is therefore made to these details.

The invention also relates to the sprayed-concrete accelerators which are obtainable by the above-described methods and which comprise a mixture a), b) or c).

The invention also relates to the use of a mixture a), b) or c) as accelerator for sprayed concrete or sprayed mortar, or as sprayed-concrete accelerator. Through the sprayed-concrete accelerator it is possible to accelerate the setting and/or hardening of the sprayed concrete or sprayed mortar. This sprayed-concrete accelerator is suitable for the above-described dry spraying processes and wet spraying processes, and is used more preferably in a wet spraying process.

The sprayed-concrete accelerator comprising the mixture a), b) or c) may be added as powder or as an aqueous mixture, more particularly as aqueous solution or suspension, to the sprayed concrete or sprayed mortar, with the sprayed-concrete accelerator being used preferably as an aqueous solution or suspension, especially in the case of the wet spraying process. As mentioned above, the aqueous mixture may also be completed only at the site of use, from the solid sprayed-concrete accelerator, by addition of water and any further additives, such as diethanolamine, for example. Commercial DEA usually contains 10 - 20% water and can be used in this form.

In the case of the dry spraying process, the sprayed-concrete accelerator of the invention is added to the dry or moist sprayed concrete or sprayed mortar prior to or simultaneously with the addition of the mixing water. In the case of the wet spraying process, the sprayed-concrete accelerator of the invention is added to the wet base mixture of concrete or mortar and mixing water.

The sprayed-concrete accelerator of the invention can be added at any stage of the spraying operation to the mixture that is to be sprayed, in other words to the dry or moist sprayed concrete or mortar or to the wet base mixture; for example, it may be added in the pump which transports the mixture, in the line in which the mixture is transported, in the preliminary wetting nozzle or in the spraying nozzle, or together with the air used for spraying, or together with the water which is added in the spraying nozzle when a dry spraying process is carried out. The sprayed-concrete accelerator of the invention is added preferably in the spraying nozzle, particularly in the case of the wet spraying process.

The sprayed-concrete accelerator of the invention is added preferably using a metering unit suitable for introducing a predetermined quantity. The sprayed-concrete accelerator of the invention may be added for example in a quantity of 0.1 to 15 wt%, preferably 2 to 8 wt%, based on the amount of hydraulic binder in the cement suspension, in the sprayed concrete or sprayed mortar.

Examples

Accelerators were produced in accordance with the following formulation:

Aluminum sulfate (AhjSCUb with 17% of AI2O3) 58 wt%

Water 39 wt%

Sepiolite (Stabilizer) 1 wt%

Calcium compound 2 wt%

Examples of suitable calcium compound for use include the following:

Calcium oxide e.g., CaO, burnt lime, etc.

Calcium hydroxide e.g., Ca(OH)2, slaked lime, etc.

Calcium carbonate e.g., CaCO3, limestone, marble, dolomite, etc. (in ground form)

Calcium nitrate e.g., Ca(NC>3)2, Ca(NC>3)2 4 H2O etc.

Calcium chloride e.g., CaCk 2 H2O, CaCk 4 H2O, CaCk 6 H2O etc.

Calcium formate e.g., Ca(CHC>2)2,

Calcium acetate e.g., Ca(C2H3O2)2, calcium acetate hydrate, etc.

The accelerator may optionally be further admixed with at least one additive, for example an additive referred to above in the description, in which case generally the water content or the aluminum sulfate content is reduced accordingly. It is possible, for example, for the mixture stated above to be admixed with 1 wt% of one or more additives, in which case the water content is reduced accordingly to 38 wt%.

Throughout this specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

Claims

1. A method for producing a sprayed-concrete accelerator from a mixture a), comprising the mixing of a) at least one aluminum compound selected from an aluminum sulfate and/or an aluminum hydroxysulfate, at least one calcium compound selected from calcium oxide, calcium hydroxide, calcium carbonate, calcium nitrate, calcium formate, and calcium acetate, and optionally water, where in the mixture a) the molar ratio Ca:AI is in the range from 0.002 to 0.5 and the sprayed-concrete accelerator or, where it is a solid sprayed-concrete accelerator, a mixture of the solid sprayed-concrete accelerator in water has a pH of below 4.
2. The method as claimed in claim 1, where, in the mixture a), the molar ratio Ca:AI is in the range from 0.01 to 0.5, preferably from 0.05 to 0.5.
3. The method as claimed in any one of claims 1 or 2, where for the mixture a) the at least one aluminum compound selected from aluminum sulfate and aluminum hydroxysulfate is dissolved and/or suspended in water and the at least one calcium compound is added in solid form, or the at least one aluminum compound selected from aluminum sulfate and aluminum hydroxysulfate and the at least one calcium compound, preferably a basic calcium compound, are mixed in solid form and the resulting solids mixture is added to water, or the at least one aluminum compound selected from aluminum sulfate and aluminum hydroxysulfate and at least one basic calcium compound and optionally calcium carbonate are mixed in solid form, and the resulting solids mixture is added to water which optionally contains dissolved and/or suspended calcium carbonate, with calcium carbonate being present at least in one component selected from the solids mixture and the water, or the calcium compound is first dissolved and/or suspended in water and then aluminum sulfate and/or aluminum hydroxysulfate are admixed as powder or as aqueous solution or suspension.
4. The method as claimed in any one of claims 1 to 3, at least a portion of the calcium sulfate formed in the mixture a) being separated off and/or, in the mixture a), at least a portion of the added water being removed again.
5. The method as claimed in any one of claims 1 to 4, the sprayed-concrete accelerator or, where it is a solid sprayed-concrete accelerator, a mixture of the solid sprayed-concrete accelerator in water having a pH in the range from 2 to below 4.
6. The method as claimed in any one of claims 1 to 5, the sprayed-concrete accelerator being present as powder, aqueous solution or aqueous suspension, preferably aqueous solution or suspension.
7. The method as claimed in claim 6, the sprayed-concrete accelerator being an aqueous suspension which comprises jurbanite.
8. The method as claimed in any one of claims 1 to 7, the molar ratio of aluminum to (sulfate - calcium), molAi/(molso4 - molca), in the mixture a), being in the range from 0.66 to 2.2 or 2/3 to 2.2, more particularly greater than 2/3, preferably in the range from 0.67 to 1.9, more preferably from 0.68 to 1.6, still more preferably from 0.69 to 1.3, still more preferably from 0.7 to 1, still more preferably from 0.71 to 0.9, and very preferably in the range from 0.72 to 0.8.
9. The method as claimed in any one of claims 1 to 8, where to the mixture a), further, one or more additives are added, wherein an additive may be, for example, at least one alkali metal aluminate.
10. The method as claimed in any one of claims 1 to 9, where, based on the total weight of the starting components, including any water of crystallization, if present therein but without any water optionally added, in the preparation of the mixture a), from 50 to 99.95 wt%, preferably 80 to 99.5 wt%, of aluminum sulfate and/or aluminum hydroxysulfate and 0.05 to 50 wt%, preferably 0.2 to 20 wt%, of at least one calcium compound are added, wherein, further, one or more additives may be added to the mixture a).
11. A sprayed-concrete accelerator of a mixture a), obtained by a method as claimed in any one of claims 1 to 10.
12. The use of a mixture a), as claimed in claim 11 as accelerator for cement suspensions, sprayed concrete or sprayed mortar, particularly as accelerator in a wet spraying process.
13. A method for producing a sprayed-concrete accelerator according to any one of the claims 1 to 10, comprising the mixing of at least one aluminum compound selected from an aluminum sulfate and/or an aluminum hydroxysulfate, and optionally calcium carbonate in solid form, to give a solids mixture, and the addition of the solids mixture to water which optionally comprises dissolved and/or dispersed calcium carbonate, to give an aqueous mixture, more particularly an aqueous solution or suspension.