Optimized sulfate content in cement

Abstract

The invention relates to a method for producing a cement mixture, said method having the following steps: a) providing at least one reactant and a sulfate carrier, b) mixing the at least one first reactant and the sulfate carrier according to a specified mixture ratio in order to form the cement mixture, c) removing a sample of the cement mixture from step b), d) mixing the sample with water and detecting the heat generated by the proceeding reaction, e) analyzing the detected generated heat in a time frame of 10 min to 60 min after mixing the sample with the water and differentiating between an event I and an event II, wherein in event I, the generated heat decreases constantly during the time frame, and in event II, the heat increases at least temporarily, and f) adapting the mixture ratio. In case of event I, the mixture ratio is adapted in order to reduce the supplied quantity of sulfate carrier, and in case of event II, the mixture ratio is adapted in order to increase the supplied quantity of sulfate carrier.

Description

[0001] Optimized sulfate content in cement

[0002] The invention relates to a method for optimizing the sulfate content in cement.

[0003] In cement, the content of tricalcium aluminate, or C3A for short, is particularly relevant for stiffening and setting behavior. This can lead to excessively rapid setting during use (spoon binder). To prevent this, a sulfate carrier, typically calcium sulfate, is added. To determine the appropriate sulfate carrier content, cement mixtures with different sulfate contents are produced today. They are mixed to form a mortar by adding water and sand, and then tested for compressive strength, for example, at the intervals usually specified in the standards (e.g., ASTM, EN 196). Although this method is very accurate, it takes a very long time due to the setting time and therefore cannot be used for true process control. At the same time, the sulfate carrier is a cost factor that one would like to use as sparingly as possible.

[0004] A cement production process is known from WO 2020 / 091 821 A1.

[0005] From CN 111551698 A a method for online detection of cement production quality is known.

[0006] From CN 104965532 A a control system for cement raw materials and a method therefor are known.

[0007] There is therefore a desire to add the required amount of sulfate carrier to the cement, but to use it as sparingly as possible and therefore to determine the just sufficient addition in a system that can be used in real time.

[0008] The object of the invention is to provide a method for interactively monitoring the sufficient addition of sulfate carriers during cement production. This object is achieved by the method having the features specified in claim 1. Advantageous further developments emerge from the subclaims, the following description, and the drawings.

[0009] The method according to the invention is used to produce a cement mixture, for example, by mixing clinker with a sulfate carrier, ideally with other components, such as artificial pozzolans. These methods are known in principle to those skilled in the art, as are the possible mixtures that lead to various, mostly standardized products. The method according to the invention comprises the following steps: a) Providing at least one first reactant and one sulfate carrier, b) Mixing the at least one first reactant and the sulfate carrier to form the cement mixture according to a predetermined mixing ratio, c) Taking a sample of the cement mixture from step b), d) Mixing the sample with water to form a paste and recording the heat generated by the ongoing reaction, e) Analyzing the recorded heat generated in the time window from 7 minutes to 60 minutes after mixing the sample with the water, distinguishing between Case I and Case II,wherein in case I the heat generated is constantly decreasing in the time window and wherein in case II the heat is at least temporarily increasing, f) adjusting the mixing ratio of the cement mixture, wherein in case I the mixing ratio is adjusted to reduce the added amount of the sulfate carrier and wherein in case II the mixing ratio is adjusted to increase the added amount of the sulfate carrier.

[0010] Steps a) and b) are carried out as is known and customary in the art. A mixing ratio is typically specified as the starting value, which guarantees reliable setting behavior even with a high C3A content. For example, a mixing ratio of 10 wt.% sulfate carrier can be used as the starting value. However, since this mixing ratio is adjusted in the process according to the invention, the original starting value is less relevant. Typically, several reactants are provided in step a). The first reactant is usually clinker, while the other reactants are selected from a list comprising fly ash, granulated blast furnace slag, slag, activated clay, artificial and natural pozzolans, waste cement, or limestone. These are then mixed in step b) in various compositions, for example and in particular in accordance with EN 197-1.

[0011] Sampling in step c) is preferably carried out continuously or periodically. For example, a sample is taken from the product stream every 30 minutes. The sample can also be transported to a laboratory via pneumatic tube, as is often the case.

[0012] In step d), or optionally after step c) and before step d), additional sand and other aggregates can be added to a mortar or concrete. This allows the setting behavior to be simulated as it occurs during use.

[0013] In step d), water is added and the mixture is mixed. This is preferably done quickly to allow the measurement to begin as early as possible. The heat generated by the ongoing reaction can be recorded in various ways. For example, it can be done isothermally, for example using differential scanning calorimetry (DSC), or adiabatically, for example using differential thermal analysis (DTA). The measurement is preferably carried out in a calorimeter, into which the (usually paste-like) mixture of water and the sample is introduced and the temperature is recorded under thermal insulation. The exact recording method is less important here, since in step e) absolute values, concrete energies, or the like are evaluated.

[0014] In step e), the heat generated by the ongoing reaction, recorded in step d), is then analyzed. This analysis is limited to determining whether a peak for the uncontrolled hydration of the C3A can be detected. Is there sufficient or too much sulfate carrier present? Initially, an initial peak occurs, with a first peak having a maximum at approximately 2 to 5 minutes, which specifically encompasses the first reaction of burnt lime (CaO) to slaked lime (Ca(OH)2) and of the C3A with the already available sulfate carrier to form ettringite. After the maximum, the heat then decreases steadily (strictly monotonically decreasing curve). However, it has now surprisingly been shown that, depending on the amount of sulfate carrier and the degree of grinding (and thus the available surface area of ​​the C3A), a second maximum forms between 10 and 30 minutes after the addition of water. This effect is now exploited for quick and easy evaluation.If there is an underdosage of sulfate carrier, a second maximum in the hydration of the C3A occurs in the range between 10 and 30 minutes. This is now exploited in a key way according to the invention by reducing the sulfate carrier until this second peak indicates the underdosage and then increasing the amount of sulfate carrier again to avoid precisely this second peak and thus uncontrolled hydration of the C3A. This allows the process to be regulated quickly and easily (approximately one hour after sampling) and the mixing ratio to be adjusted in step f). In step e), the analysis can be dramatically simplified and accelerated, since only two simple cases need to be distinguished at an event occurring very early within one hour after the start of hydration. In case I, the heat generated is constantly decreasing within the time window.Thus, sufficient sulfate carrier is present, preventing uncontrolled hydration of the C3A. In case II, however, the heat increases at least temporarily, thus resulting in a second exothermic reaction that can be attributed to uncontrolled hydration of the C3A after the sulfate carrier has been completely consumed. If case II, with a second increase in the generated heat, is observed during the measurement of a sample, the amount of sulfate carrier must be increased so that the second exothermic reaction no longer occurs within 10 to 30 minutes.

[0015] In a further embodiment of the invention, the adjustment in step f) is preferably made as a percentage based on the sulfate carrier content. Thus, a faster decrease occurs at a high content than at lower contents.

[0016] In a further embodiment of the invention, the adjustment in step f) is initially carried out with a first increment. After Case II is first determined, a second increment is used to adjust the mixing ratio. For example, the second increment is half the size of the first increment. If Case II is determined again, the increment can be further adjusted, for example, halved each time (for example, up to a predetermined technically reasonable minimum increment, which is typically selected depending on the accuracy of the system). This enables the fastest, most accurate, and most reliable approximation to the optimal mixing ratio.

[0017] In a further embodiment of the invention, the analysis in step e) is limited to the time window of 5 to 40 minutes. Since the process approaches the limit at which uncontrolled C3A hydration occurs with decreasing sulfate carrier content, the maximum is initially located in the range of longer times, from the initial hydration peak. In this very low-interference range, the second maximum can be reliably and extremely easily detected.

[0018] In a further embodiment of the invention, the analysis in step e) is limited to the time window of 10 to 40 minutes. Since the process approaches the limit at which uncontrolled C3A hydration occurs with decreasing sulfate carrier content, the maximum is initially located in the range of longer times, from the initial hydration peak. In this very low-interference range, the second maximum can be reliably and extremely easily detected.

[0019] In a further embodiment of the invention, the analysis in step e) is limited to the time window of 20 to 40 minutes. Since the process approaches the limit at which uncontrolled C3A hydration occurs with decreasing sulfate carrier content, the maximum is initially located in the range of longer times, from the initial hydration peak. In this very low-interference range, the second maximum can be reliably and extremely easily detected.

[0020] In a further embodiment of the invention, the analysis in step e) is limited to determining the gradient exclusively at two points in time, for example, 20 min and 40 min. The ratio of the two gradients alone makes it extremely easy to detect an incipient underdosage of the sulfate carrier (case II). In a further embodiment of the invention, the heat generated in step d) is recorded in the form of the sample temperature. Since the evaluation is comparatively robust, the measurement does not have to be carried out isothermally or adiabatically, as is the case, for example, with DTA or DSC. It is therefore sufficient to measure the sample temperature in a thermally somewhat insulated measuring area in which the sample is located.Without an exothermic or endothermic reaction, the imperfect insulation leads to an exponential temperature adaptation to the ambient level, from which exothermic reactions then stand out as temperature increases, which are the two reactions considered here (initial peak, hydration of CaO and hydration of C3A). This enables a comparatively simple measurement setup, which in turn enables a large number of samples to be collected cost-effectively and thus simplifies applicability for controlling the manufacturing process.

[0021] In a further embodiment of the invention, the sample is ground between step c) and step d). It has been found that the observed effect is more pronounced the finer the material, as this also increases the surface area. Therefore, the sample is ground, for example, using a vibrating disc mill for 2 to 5 minutes. This makes underdosing of the sulfate carrier faster and easier to detect.

[0022] In a further embodiment of the invention, the mixing ratio is temporarily changed abruptly so that a safe overdose of the sulfate carrier occurs, for example, to a mixing ratio of 10 wt.% sulfate carrier. This ensures that the sulfate carrier content does not accidentally drop too much. Subsequently, according to the method according to the invention, the mixture is reduced again to the necessary minimum, from which a heat increase in the range of 10 to 30 min can be observed.

[0023] The method according to the invention is explained in more detail below using an embodiment shown in the drawing.

[0024] Fig. 1 Flow diagram The method is illustrated by way of example in Fig. 1. For example, the cement is mixed from clinker (burnt from limestone, clay, sand and iron ore) and artificial pozzolan (activated clay). However, other reactants such as limestone or granulated blast furnace slag could also be added, although this would have no influence on the invention. The clinker comes from a clinker store 10, the artificial pozzolan from a pozzolan store 12. In addition, a sulfate carrier from a sulfate carrier store 11 is metered in according to a mixing ratio, wherein the addition is regulated by a control unit 30 on the basis of a mixing ratio. The reactant streams are mixed and ground together in the mill 13. A sample is taken from the product stream leaving the mill 13 in the sampling device 14, for example every 30 minutes, while the remaining product stream is fed to a cement store 15.

[0025] The sample from sampling point 14 is ground in a laboratory mill 20 for 4 minutes, mixed with water from water supply 21, and quickly introduced into a calorimeter 22, where the sample temperature is recorded. This temperature is recorded by or transmitted to the control unit 30, and the temperature gradient is determined from the increase between 20 minutes and 40 minutes after the water was added to the sample. If this temperature is constantly decreasing (case I), the mixing ratio in the control unit 30 is adjusted such that the proportion of sulfate carrier is reduced. This process continues until a renewed increase in temperature between 20 minutes and 40 minutes after the water was added to the sample (case II), indicating that the amount of sulfate carrier is too low. The control unit 30 therefore adjusts the mixing ratio accordingly to increase the amount of sulfate carrier again.

[0026] In addition, a setting unit 31 is provided, which, for example, during start-up and cyclically, for example once a week, sets the mixing ratio to a sulfate carrier content of 10 wt.%, thus ensuring that sufficient sulfate carrier is always available. Reference symbol

[0027] 10 clinker warehouses

[0028] 11 sulfate carrier deposits

[0029] 12 Pozzolana storage 13 Mill

[0030] 14 Sampling

[0031] 15 cement storage facilities

[0032] 20 laboratory mill

[0033] 21 Water supply 22 Calorimeter

[0034] 30 Control unit

[0035] 31 default unit

Claims

Patent claims 1. A method for producing a cement mixture, the method comprising the following steps: a) providing at least one first reactant and one sulfate carrier, b) mixing the at least one first reactant and the sulfate carrier to the cement mixture according to a predetermined mixing ratio, c) taking a sample of the cement mixture from step b), d) mixing the sample with water and detecting the heat generated by the ongoing reaction, e) analyzing the detected heat generated in the time window from 10 minutes to 60 minutes after mixing the sample with the water and distinguishing between case I and case II, wherein in case I the heat generated is constantly decreasing in the time window and wherein in case II the heat is at least temporarily increasing, f) adjusting the mixing ratio,wherein in case I the mixing ratio is adjusted to reduce the added amount of sulfate carrier and wherein in case II the mixing ratio is adjusted to increase the added amount of sulfate carrier.

2. Method according to claim 1, characterized in that the analysis in step e) is limited to the time window 20 min to 40 min.

3. Method according to one of the preceding claims, characterized in that the detection of the heat generated in step d) takes place in the form of the temperature of the sample.

4. Method according to one of the preceding claims, characterized in that in step d) or after step c) and before step d) an additional addition of sand and other aggregates in a mortar or concrete can take place.

5. Method according to one of the preceding claims, characterized in that the sample is ground between step c) and step d).

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