WO2008152111A2 - Method for producing a concrete mixture - Google Patents

Abstract

The invention relates to a novel method for producing a concrete mixture. According to said method, in a preliminary mixing stage, water and fine-grained concrete components, at least cement and as required sand, in a quantity of between 2 and 97 % of the total sand fraction in the concrete mixture, filler and a water-reducing admixture are pre-mixed in a mixing process and by ultrasonic treatment and then in a final mixing stage the mixture produced from water and fine-grained concrete components, at least consisting of cement, is subsequently mixed into the remaining sand and the coarse crushed rock. The advantages of the method are characterised by reduced energy expenditure during the mixing of the concrete and by an increased strength of the concrete.

Description

 Description Method for producing concrete mix

The invention relates to processes for producing concrete mixture by metering and mixing the mixing components.

It is a process for producing concrete mix by dosing and mixing the mixing components in a mixer until the concrete mix becomes homogeneous, with subsequent introduction of the homogeneous concrete mix into a vehicle mixer, in which during transport to the work site at the same time to avoid the Entomogenisieren further mixed, known. The disadvantages of the mentioned method are the following: 1. Very long mixing time with the resulting high energy consumption. 2. In producing the high performance concrete mix for high strength concretes, it is necessary to use cements with high fineness (up to 6000 cm ² / g Blaine), which also requires a significantly higher energy requirement for grinding cement and concrete mixes. The document "Concrete" no. 12, 2005, pp 614-617 a method for the production of self-compacting concretes is known in which the fluidity of the concrete mixture by using the highly effective flow agents, by high proportion of finely dispersed components (cement + 4 minutes, and due to the low water contents and high additive dosages with regard to the flour grain content, more energy is required in the production of the self-compacted concretes, which reduces the capacity of the mixing plants and increases the capacity Power consumption.

A method is known which increases the capacity of the concrete plant by using a "hybrid" mixing process (Druckschrift "Beton" No. 12, 2005, p.617). The purpose of the "hybrid" mixing process is to pre-mix and pre-mix in a step-wise process.After premixing in the mixing plant, all mixing components are premixed only briefly (up to 30 seconds), after which the mixture is loaded into the vehicle mixer where the final mixing Final mixture is carried out at low mixing intensity. The disadvantages of the "hybrid" mixing process are that the capacity of the mixing plants still low and the energy consumption is still too high., The reason is that the premixing all mixing components, such as the fine (sand, filler, cement), as well as the coarse (gravel, grit), which accounts for the largest mass fraction (more than 60%) of total blended components in the blending plant .. When producing the high performance concrete mix for high strength concretes, it is also necessary to use cements with high fineness (up to 6000 cm ² / g according to Blaine), which also requires a much higher energy requirement for the grinding of cement and concrete mixtures.

The known methods are disadvantageous in that the energy required for mixing the concrete components is still too great and thus the throughput of the mixing plant is still too low. The performance of the concrete mix produced by the known processes and, as a consequence, the structural properties of concrete, such as strength, remain low when using the coarse cements. The high mixing time of the mixing components results in increased abrasion of the mixer. The cement fraction with grain sizes over 80 μm does not load to 100%. Therefore, the core of coarse cement particles in the concrete block remains as an innermost filler. This means that the entire effort and above all the energy used in cement production can not be fully implemented as a binder because of incomplete hydration. In this way, up to 10% of cement can be lost.

The invention has for its object to overcome the above-mentioned disadvantages in the manufacture of concrete mix. By premixing first of the fine-grained constituents of the concrete mixture and their simultaneous mechanical and / or ultrasound-related activation during the step-by-step concrete mixture with final final mixture of entire mixing components, it is possible in individual, the To increase the early and final strength of concrete especially when using the coarsely ground cements and thus to realize the production of high-strength concretes by the use of coarsely ground cements to increase the efficiency of the concrete mix or the strength of the concretes, the energy required for concrete mixing reduce mixing time and thereby increase the throughput capacity of the mixing plant while still improving cement and thus concrete quality and reducing wear on the mixer casing and mixing paddles. Among other things, it is possible by means of the invention to use coarsely ground cements for the production of the high-strength concretes and nevertheless to increase the potential capability of the binder to 100%. The structural cement and concrete properties are improved.

The solution of this object is achieved according to claim 1. Preferred embodiments are the subject of claims 2 to 8.

The phenomena occurring when using the new method are explained as follows:

The later and especially earlier degree of hydration of cement, and thus the strength of concrete, are determined primarily by the fineness of the cement, including its hydraulic and non-hydraulic additives. Therefore, it is desired to increase the cement fineness. The grindability of the cement clinker is relatively low, which causes a very high energy consumption in extreme fine grinding up to 6000 cm ² / g according to Blaine and above. The formation of a water-impermeable gel on the surfaces of the cement particles after water treatment minimizes the energy efficiency of the fine grinding. Decisive for this are the reactions occurring in the first few minutes after mixing with water. These are primarily the chemical reactions of the tricalcium aluminate in the clinker with the calcium sulfate added to ettringite as a solidification regulator. The neoplasms formed during water tapping are extremely soft, and therefore can with minimal energy consumption from the surface of the cement particles under a corresponding mechanical and / or ultrasound conditional action are removed. In this way, cement particles can be reground with minimal energy consumption during the concrete mixing.

The familiar mixing of all concrete components from the beginning leads to enormous energy expenditure and reduced energy efficiency. The cement particles can not be effectively treated sufficiently effective in order to rub off the angular edges of the cement particles and to crush them even further. Especially the efficient process of the mentioned phenomena can accelerate the liquefaction of the fine concrete components and thus the homogenization by improved cement particle packing as well as reduce the water consumption of concrete mix.

From the above described representations, it is to be expected that it is precisely the homogenization of the fine constituents of the concrete mixture that decisively determines the mixing time. For these reasons, it is expedient to treat only cement with filler, superplasticizer and a proportion of fine components of the concrete mixture, such as sand. Of course, the grains of sand and filler remaining in the water as abrasive material can significantly accelerate the abrasion of the new growths from the surface of the cement particles. This is possible under an intense mechanical and ultrasound-related effect.

According to the currently known knowledge, this phenomenon can be explained as follows: By separately activated mixing of the fine fraction of the concrete mixture, the amount of chemical water demand required for the initial hydration is increased but the amount of physically incorporated water in the electrical double layer envelopes covering the cement particles and for the Liquefaction necessary amount of free water, however, reduced. The entire water requirement decreases. It is conceivable that the concentration of high-grade ions, such as Ca ⁺² , Al ⁺³ , Fe ⁺³ , is markedly increased in the electrical double-layer cladding covering the cement particles. This results in a reduction of the diffusion layer thickness of the electric double layer, resulting in a first reduction of the physically caused amount of water triggers. A further decrease in the physical amount of water is continued by the resulting accelerated Etringitbildung.

This can be explained as follows: As is known, the formation of a particularly swelling gel of etringite can temporarily make the cement particles impermeable to water. In this way, the early hydration can be determined not only by the fine grinding of the clinker, including its hydraulic and non-hydraulic additives, but also by the possibility of forming a water impermeable, difficult to dissolve gel on the surfaces of the cement particles. The hydrated cement particles behave temporarily like a chemically inert filler.

By rapidly minimizing the thickness of the electric double layer, the distance between cement particles can be reduced, so that the cement particles are stored closer together, and the voids between the particles are reduced. This results in a corresponding increase in the chemically induced and a rapid decrease in the physically integrated in the diffusion layers amount of water and reducing the necessary amount of water for liquefaction. This reduces the total water requirement of the cement.

The fact that the liquefaction of the concrete mixture, when activated by intensive mixing, has a low water requirement and that the liquefaction proceeds abruptly during mixing, shows that the above considerations are correct.

On a large scale, the concrete mix is produced in two operations as follows:

In the first concrete mixing process, a homogeneous and activated cementitious glue is made with the filler and a sand portion. In the second process, a mortar or concrete mixture of activated cementitious glue is made with appropriate Gesteinkörnungen.

In the first process, only the finest components of the concrete mix, namely cement filler and sand, are mixed together with water. In order to effectively activate the coarsely ground cement, the water cement value must be in the range 0.10 and 0.30. At such low water content, the friction between the coarsely ground cement particles, on the one hand, and between the cement particles and the filler and sand, on the other hand, especially large. The necessary flowability of the cement glue can be achieved by using highly effective flow agents and by introducing it into the mixture of a certain liquefaction energy. These

Liquefaction energy is introduced into the mixture in the conventional mixers by an extended mixing time. This can be achieved in a significantly shorter time with a much greater energy efficiency in mixers at high speed and / or with the ultrasound treatment.

For a short, high-speed mixing of the cement glue, the moistened cement granules are ground thin. That increases its hydraulic activity. Particularly great effectiveness of mixing is achieved when using coarsely ground cements.

In the second process, the activated cementitious glue is mixed together with the coarse aggregates of the mortar or concrete mixture, that is to say with the sand residue and the gravel (chippings). Since in the first process of producing the activated cementitious glue very good mixing between the cement, filler and sand content is achieved with the water and superplasticizer is the second process after the addition of the sand residue and the coarse rock graining (gravel, grit) for the Homogeneity of the mortar or concrete mix in the conventional mixers a relatively short time of 15-20 s necessary. The new process can be realized in the concrete and precast plants by incorporation into the concrete mixing plant of a small cement activator (a high speed mixer and / or an ultrasonic generator) from which the activated cementitious glue is loaded into the mixer of the concrete mixing plant simultaneously with the rock granules , In ready-mix concrete plants, the new process can be realized without concrete mixers of the mixing plant. The required amount of aggregates (fine and coarse) and the activated cement-containing glue are loaded directly into the vehicle mixer. The slow but long mixing during transportation to the construction site with pre-discharge intensive mixing at least within 5 minutes guarantees a homogeneous mixing of the rock grains with the cement-containing glue.

The new process allows to grant energy-efficient use especially of coarsely ground cements for mortar and concrete mixes. In addition, a rubbing off of the first hydrate phases in the concrete is conceivable, whereby new reaction surfaces arise. These processes result in a progressive increase in the surface area and thus in an increase in the content of water and of the fluid, leading to a drop in the flowability. In vibrating concretes, it has been found that the refinement of the cement due to long mixing makes the cement paste smoother and stickier and ultimately leads to an increase in the strength of the concrete.

Embodiments of the invention are shown according to claim 1 below and are described in more detail below:

The binders used for the experiments were CEM I 32.5 R and for comparison CEM I 52.5 R.

In order to compare the results of the new process with the known processes, reference concrete mixtures were prepared according to the conventional method in a conventional concrete mixer in which all constituents were homogenized by single-stage mixing for 5 minutes. In this way, a conventional method was simulated. For the new process, two-stage mixing consisting of premix and final mixing stage was used. In the pre-mixing stage, according to the calculated mass ratios, the fine-grained concrete components, cement, mickrosilica and superplasticizer were activated to 100% with a 10% sand content together in a high-speed mixer through extremely intense mixing in aisle III for 3 minutes. In this way, only up to 20-30% of the mass fraction of the total concrete mix is included in the mix in the mix. This makes it possible to save energy in the pre-mixing stage to 70-80%.

The homogenization of the entire components of the concrete mixture in the final mixing stage was achieved in a conventional concrete mixer according to calculated mass ratios with a short mixing time of 30s for concrete and 60 s for mortar. Thus, the entire mixing time from 5 minutes to 3.5 minutes, d. H. shorten by 1, 5 min. This in turn means additional energy consumption of up to 19% lower. When producing ready-mixed concrete, the final mixing stage can be shortened up to 15 s. This makes it possible to save even more energy. The quality of the concrete mixtures produced was assessed according to the compressive strength.

1. Experiments with concrete mixtures:

To test the new process, CEM I 32.5 R cement, sand in an amount of 10% of the total sand content in the concrete mixture, which corresponds to a sand / cement ratio of 1: 5, and a superplasticizer in a high speed mixer were used in the pre-mixing stage in the course 3 for 3 min mixed intensively. In the final mixing step, the mixture was continued with the remainder of the sand to the gravel and gravel of the concrete mix at conventional intensity for a further 30 seconds.

According to the known method, all concrete components were used in full.

The mass fractions of the total concrete mixture are shown in Table 1.

Table 1: Mass fractions of the concrete mix and results of compressive strength according to the new and known methods.

From Table 1 it follows that the ratios between concrete components in all mixtures are identical. The concrete mixtures produced differ in the processes by which they are manufactured.

The results presented show that the premixing of the fine-grained concrete component significantly increases the strength of the new process and that the energy requirement falls by more than 70%.

2 trials with mortar mixtures.

In order to test the new method for mortar production, in the premix cement CEM I 32.5 R, sand at a sand / cement ratio of 1: 5 and a superplasticizer were intensively mixed using a high speed mixer at aisle 3 for 3 min. In the final mixing step, the mixture was continued with the remainder of the sand of the mortar mix for a further 60 seconds.

According to the known method, all mortar components were used in full amount for one-stage mixing for 6 min.

The mass fractions of the total mortar mixture are shown in Table 2.

Table 2:. The mass ratios of the component in prepared mortar mixtures.

From Table 2 it can be seen that the ratios between components in facing mixtures are identical. The mortar mixtures produced differ only in the processes by which they are manufactured.

The results presented show that the premixing of the fine-grained concrete component significantly increases the strength of the new process and that the energy requirement falls by more than 70%.

The new method may be particularly effective if premixing of the fine concrete components is carried out in a mixer with ultrasonic treatment.

The components or method steps to be used in accordance with the invention are not subject to special exceptions in terms of their size and shape design, material selection and technical conception or their process conditions, so that the selection criteria known in the respective field of application can be used without restriction.

Claims

claims A process for producing concrete mixture by metering and stepwise mixing of the concrete components by premixing and final mixing stage, characterized in that in a premixing water and fine-grained concrete components, but at least cement and sand as required in an amount of 2 to 97% of the total sand content in the concrete mixture and optionally fillers and superplasticizers are premixed by mixing and optionally by ultrasound treatment and then in a final mixing step the mixture of water and fine concrete components made at least cement is finally mixed with other concrete components, at least the remaining sand and coarse rock grains. 2. The method according to claim 1, characterized in that the mixing in the premix is carried out to liquefaction. 3. The method according to claim 1, characterized in that the mixing is not carried out in the premixing stage to liquefaction. 4. The method according to any one of claims 1 to 3, characterized in that for the premixing step sand fraction <0.5 to <4 mm is used. 5. The method according to any one of claims 1 to 4, characterized in that the flow agent is introduced from the beginning in the premix stage. 6. The method according to any one of claims 1 to 5, characterized in that the flow agent is introduced directly before the liquefaction of the concrete mixture in the premix. 7. The method according to any one of claims 1 to 6, characterized in that the pore-forming agent is introduced directly in the final mixing stage. 8. The method according to any one of claims 1 to 7, characterized in that the sand in an amount of 10 to 70 wt .-%, preferably 10 to 40 wt .-% of the total sand content is used.