Optimization of sulfate content in cement

Abstract

The invention relates to a method for producing a cement mixture, said method comprising the following steps: a) providing at least one first reactant and a sulfate carrier, b) mixing the at least one first reactant and the sulfate carrier for the cement mixture according to a specified mixing ratio, c) collecting a sample of the cement mixture from step b), d) mixing the sample with water and recording the heat released by the reaction occurring, e) analyzing the heat released recorded in a time window from 10 minutes to 60 minutes after mixing the sample with water and differentiating between scene I in which the heat released stably decreases in the time window and scene II in which the heat released stably decreases in the time window, and scene II in which the heat released stably decreases in the time window. The heat is at least temporarily increased in the context I, and wherein in the context II, the heat is at least temporarily increased, f) adjusting the mixing ratio, and wherein in the context I, the mixing ratio is adjusted in order to reduce the amount of the added sulfate carrier, and in the context II, the mixing ratio is adjusted in order to increase the amount of the added sulfate carrier.

Description

[0001] The invention relates to a method for optimizing the sulfate content in cement.

[0002] In cement, the content of tricalcium aluminate (C3A for short) is particularly relevant for the setting and hardening behavior. When used, this can lead to too rapid solidification ("spoon binder"). In order to prevent this, a sulfate carrier is added, usually calcium sulfate. In order to determine the appropriate content of the sulfate carrier, cement mixtures with different sulfate contents are accordingly produced in recent times, mixed into a mortar by adding water and adding sand, and then investigated, for example, the compressive strength at the time intervals usually specified in standards (for example ASTM, EN 196). While this method is very precise, the length of the setting process means that it is very time-consuming and thus cannot be used for actual process control. At the same time, however, the sulfate carrier is a cost material that should be used as sparingly as possible.

[0003] WO 2020 / 091821 A1 discloses a cement production.

[0004] CN 111551698 A discloses an online detection method for production quality of cement.

[0005] CN 104965532 A discloses a control system for cement raw materials and a method thereof.

[0006] It is therefore desirable to add the necessary amount of sulfate carrier to the cement, but this must be used as sparingly as possible and it is therefore necessary to determine the just sufficient addition in a system that can be used in real time.

[0007] It is the object of the invention to ensure a method for interactive control of the sufficient addition of sulfate carrier during cement production.

[0008] This object is achieved by the method having the features specified in claim 1. Extensions of the advantageous effects are apparent from the dependent claims, from the description below and from the figures.

[0009] The method according to the invention is used for producing a cement mixture, thus for example a mixture of clinker and sulfate carrier, preferably further components (for example artificial pozzolans). These methods are generally known to the person skilled in the art and also the possible mixtures that lead to different, mostly standardized products. The method according to the invention comprises the following steps: a) providing at least one first reactant and a sulfate carrier, b) mixing the at least one first reactant and the sulfate carrier for the cement mixture in the specified mixing ratio, c) collecting a sample of the cement mixture from step b), d) mixing the sample with water to form a slurry and recording the heat emitted by the reaction taking place, e) analyzing the heat emitted recorded in a time window of 7 minutes to 60 minutes after mixing the sample with water and distinguishing between scenario I and scenario II, wherein in the scenario I the heat emitted steadily decreases in the time window and wherein in the scenario II the heat at least temporarily increases, f) adjusting the mixing proportions of the cement mixture, wherein in the scenario I the mixing proportions are adjusted to decrease the amount of added sulphate carrier and in the scenario II the mixing proportions are adjusted to increase the amount of added sulphate carrier.

[0010] Steps a) and b) are carried out in a manner known and customary in the prior art. As a starting value, mixing proportions are usually specified which guarantee a reliable setting behavior even at high C3A contents. For example, a mixing proportion of 10% by weight of sulphate carrier can be specified as a starting value. However, since this mixing proportion is adjusted in the method according to the application, the original starting value is less relevant.

[0011] Typically, a plurality of reactants is provided in step a). The first reactant is usually clinker, the other reactants are selected from the list comprising fly ash, granulated blast furnace slag, other slags, active clay, artificial and natural pozzolans, old cement or limestone. Then in step b) the different components are mixed, for example and in particular according to EN 197-1.

[0012] Preferably, the sampling in step c) is carried out continuously or periodically. For example, every 30 minutes a sample is taken from the product stream. The sample can also be transported to the laboratory, for example by a pneumatic tube, which is usually customary.

[0013] In step d) or optionally after step c) and before step d), sand and other aggregates can be additionally added in the mortar or concrete. This allows to readjust the setting behavior at the time of use.

[0014] Step d) then comprises the addition of water and mixing. This is preferably done quickly in order to allow the measurement to be started as early as possible. There are various ways in which the heat emitted by the reaction taking place can be recorded. This can take place, for example, isothermally, for example by means of differential scanning calorimetry (DSC); or adiabatically, for example by means of differential thermal analysis (DTA). Preferably, the measurement is carried out in a calorimeter, into which the (usually slurry-like) mixture of water and sample is introduced, and the temperature is recorded under adiabatic conditions. Here, the exact recording method is not so important, since the absolute values, specific energies or similar values are evaluated in step e).

[0015] Step e) then comprises the analysis of the heat emitted by the reaction taking place, which was recorded in step d). The analysis is limited to whether a peak of uncontrolled hydration of C3A can be detected. If there is sufficient or too much sulphate carrier, an initial peak is first obtained, in which the first peak has a maximum at about 2 to 5 minutes, which in particular includes the first reaction of burnt lime (CaO) to slaked lime (Ca(OH)2) and the first reaction of C3A with the existing sulphate carrier to ettringite. After the maximum, the heat then steadily decreases (strictly monotonically falling curve). However, it has now surprisingly been found that, depending on the amount of sulphate carrier and the degree of grinding (and thus the available C3A surface area), a second maximum occurs in the interval between 10 and 30 minutes after the addition of water. This effect is currently being used for a quick and easy evaluation. Thus, if the sulphate carrier dose is insufficient, a second maximum of C3A hydration is obtained in the interval between 10 and 30 minutes. As a key element of the present application, this is achieved by reducing the sulphate carrier until the second peak indicates an insufficient dose, and then increasing the amount of sulphate carrier again in order to precisely avoid the second peak, thus avoiding uncontrolled hydration of C3A. This allows the process to be checked in a simple and quick manner (about 1 hour after sampling) and the mixing ratio to be adjusted in step f). Step e) thus allows a significant simplification and faster analysis, since for the very early event occurring within one hour after the start of hydration, it is only necessary to distinguish between two simple scenarios. In scenario I, the heat emitted steadily decreases within the time window. There is thus sufficient sulphate carrier, and no uncontrolled hydration of C3A takes place. On the other hand, in scenario II, the heat at least temporarily increases. A second exothermic reaction thus takes place, which can be associated with uncontrolled hydration of C3A after all the sulphate carrier has been used up. If scenario II, in which the heat emitted increases a second time, is observed during the measurement of the sample, the amount of sulphate carrier must be increased such that the second exothermic reaction in the interval between 10 and 30 minutes no longer takes place.

[0016] In another embodiment of the present application, the adjustment in step f) is preferably carried out in a percentage-wide manner based on the sulphate carrier content. Thus, in the case of high contents, the reduction is faster than in the case of low contents.

[0017] In another embodiment of the present application, the adjustment in step f) is initially carried out in a first increment. Once scenario II has been detected for the first time, the mixing ratio is adjusted using a second increment. For example, the second increment is half the magnitude of the first increment. If scenario II is detected again, the increment can be further adjusted, for example halved each time (for example down to a specified technically appropriate minimum increment, which is usually selected in accordance with the accuracy of the production unit). This makes it possible to approach the optimum mixing ratio as quickly as possible while at the same time being good and reliable.

[0018] In another embodiment of the present application, the analysis in step e) is limited to a time window of 5 minutes to 40 minutes. According to this method, as the sulphate carrier content decreases, the limit is approached at which uncontrolled C3A hydration occurs, so the maximum initially occurs in a region at a long time interval from the initial hydration peak. In this very low-interference region, the second maximum can be reliably and extremely easily detected.

[0019] In another embodiment of the present application, the analysis in step e) is limited to a time window of 10 minutes to 40 minutes. According to this method, as the sulphate carrier content decreases, the limit is approached at which uncontrolled C3A hydration occurs, so the maximum initially occurs in a region at a long time interval from the initial hydration peak. In this very low-interference region, the second maximum can be reliably and extremely easily detected.

[0020] In another embodiment of the present application, the analysis in step e) is limited to a time window of 20 minutes to 40 minutes. According to this method, as the sulphate carrier content decreases, the limit is approached at which uncontrolled C3A hydration occurs, so the maximum initially occurs in a region at a long time interval from the initial hydration peak. In this very low-interference region, the second maximum can be reliably and extremely easily detected.

[0021] In another embodiment of the present application, the analysis in step e) is limited to determining the slope at two time points, for example 20 minutes and 40 minutes. From the ratio of the two slopes alone, it is possible to detect the beginning of a dose deficiency of the sulphate carrier (scenario II) in an extremely simple manner.

[0022] In a further embodiment of the application, the heat released in step d) is recorded in the form of the sample temperature. Since the evaluation is relatively robust, it is not necessary to carry out the measurement isothermally or adiabatically as with DTA and DSC. It is therefore sufficient to measure the temperature of the sample in a slightly thermally insulated measurement region in which the sample is located. In the absence of an exothermic or endothermic reaction, the less perfect insulation leads to an exponential temperature regulation to the ambient level, and then the exothermic reaction is distinguished in the form of a temperature increase, which is the two reactions considered here (initial peak, hydration of CaO and hydration of C3A). This makes it possible to use a relatively simple measurement setup, which in turn makes a large number of samples cost-effective and thus simplifies the applicability for controlling the production process.

[0023] In another embodiment of the application, the sample is ground between step c) and step d). It has been found that the finer the material, the more pronounced the observed effect, since the grinding also increases the surface area. Thus, for example, the sample is ground for 2 to 5 minutes with a vibrating disc mill. This makes it easier to detect a dosage deficiency of the sulphate carrier more quickly.

[0024] In another embodiment of the application, the mixing ratio is suddenly changed for a short time, so that a reliable dosage excess of the sulphate carrier occurs, for example to a mixing ratio of 10% by weight of sulphate carrier. This ensures that the sulphate carrier content does not drop too much by accident. Subsequently, the method according to the application is reduced again to the necessary minimum, from which an increase in heat can be observed within an interval of 10 to 30 minutes.

[0025] The method according to the application is explained in more detail below with reference to the working examples shown in the accompanying drawings.

[0026] Figure 1 Flow chart Reference will be made to Figure 1 The method is explained by way of example. By way of example, cement is mixed from clinker (fired from limestone, clay, sand and iron ore) and artificial pozzolana (active clay). However, other reactants can also be mixed, for example limestone or granulated blast furnace slag, but this has no influence on the application. The clinker comes from a clinker store 10, the artificial pozzolana from a pozzolana store 12. In addition, sulphate carrier is metered according to the mixing ratio from a sulphate carrier store 11, the addition being controlled by a control unit 30 on the basis of the mixing ratio. The reactant streams are mixed and ground together in a grinding machine 13. A sample is taken from the product stream leaving the grinding machine 13 in a sampling unit 14, for example every 30 minutes, while the remaining product stream is fed to a cement store 15.

[0027] The sample from the sampling unit 14 is ground in the laboratory mill 20 for 4 minutes, mixed with water from the water supply 21 and introduced into the calorimeter 22 in a well mixed and rapid manner, in which the temperature of the sample is recorded. This temperature is recorded by or transmitted to the control unit 30 and the temperature increase is determined from the increase between 20 and 40 minutes after the addition of water to the sample. If there is a steady decrease (scenario I), the mixing ratio in the control unit 30 is adjusted so that the proportion of sulphate carrier is reduced. This continues until the temperature increases again between 20 and 40 minutes after the addition of water to the sample (scenario II, indicating that the amount of sulphate carrier is too low). The control unit 30 accordingly adjusts the mixing ratio in order to increase the amount of sulphate carrier again.

[0028] Furthermore, there is a specified unit 31 which sets the mixture ratio, for example to a sulphate carrier content of 10% by weight, for example at start-up and periodically (for example once a week), so that it is ensured that there is always sufficient sulphate carrier.

[0029] Reference signs 10 clinker store 11 sulphate carrier store 12 pozzolana store 13 mill 14 sampling unit 15 cement store 20 laboratory mill 21 water supply 22 calorimeter 30 control unit 31 specified unit

Claims

1. A method for producing a cement mixture, wherein the method comprises the following steps: a) Provide at least one first reactant and a sulfate carrier. b) Mix at least one first reactant and a sulfate carrier for the cement mixture in the specified mixing ratio. c) Collect a sample of the cement mixture from step b). d) Mix the sample with water and record the heat released by the reaction. e) Analyze the heat released recorded within a time window of 10 to 60 minutes after the sample is mixed with water, and distinguish between scenario I and scenario II, wherein in scenario I, the heat released decreases steadily within the time window, and wherein in scenario II, the heat release increases at least temporarily. f) Adjusting the mixing ratio, wherein in scenario I, the mixing ratio is adjusted to reduce the amount of added sulfate carrier, and in scenario II, the mixing ratio is adjusted to increase the amount of added sulfate carrier.

2. The method according to claim 1, characterized in that, The analysis in step e) is limited to a time window of 20 to 40 minutes.

3. The method according to any one of the preceding claims, characterized in that, The heat released in step d) is recorded as the sample temperature.

4. The method according to any one of the preceding claims, characterized in that, In step d), or after step c) and before step d), additional sand and other aggregates may be added to the mortar or concrete.

5. The method according to any one of the preceding claims, characterized in that, Between step c) and step d), the sample is ground.

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