EP1521727A1

EP1521727A1 - Method for producing aerated concrete

Info

Publication number: EP1521727A1
Application number: EP03735664A
Authority: EP
Inventors: Horst Lohrmann
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-07-16
Filing date: 2003-06-23
Publication date: 2005-04-13
Also published as: DE10232180B4; AU2003238027A1; DE10232180A1; WO2004007393A1

Abstract

The invention relates to a method for producing aerated concrete on the basis of a mixture that comprises quartziferous sand and/or quartziferous stone dust, a quicklime-containing binder, water and a pore forming material and optionally cement and/or anhydrite. The invention is characterized in that the quicklime is substantially completely slaked with water before the pore forming material is added to the mixture, thereby making it possible for the pore formation to be carried out in a mixture that has a constant temperature and a constantly high consistency. The inventive method allows for easily reproducible aeration results and gives an aerated concrete with a very low dry bulk density and high compression strength.

Description

METHOD FOR THE PRODUCTION OF POROUS CONCRETE

The invention relates to a method for producing aerated concrete and in particular to a method which allows better control of the porosity process and thus the production of aerated concrete of low bulk density with increased compressive strength.

In recent times, there has been an increasing demand for aerated concrete that has an even higher porosity, that is, a lower bulk density, and high thermal insulation (see, for example, DIN 4165 E from March 2001). However, the production of cellular concrete of low bulk density of constant quality is difficult in practice, on the one hand because of the fluctuations in the starting materials, and on the other hand and mainly because of the difficulties in reproducibly carrying out the driving process.

In the known processes for the production of aerated concrete, the starting materials - usually quicklime (fine lime), cement, ground quartz-containing sand, water and a pore former such as aluminum powder, optionally in the presence of other additives such as anhydride (calcium sulfate) - are short in a defined sequence placed successively in a mixer and mixed within a total mixing time of a few minutes. When the mixture is drained, it has a temperature of around 35 to 40 ° C, partly because an initial reaction of the quicklime with the formation of hydrated lime cannot be avoided.

After the pourable mixture has been poured into a casting mold (volume, for example, about 4 m ³ ), the mixture temperature rises steadily due to the strongly exothermic reaction of the quicklime with water and, depending on the recipe, reaches values of around 75 to 95 ° C. At the same time, the mixture increasingly stiffens, both through the binding of free water to the calcium oxide of quicklime and through the formation of highly dispersed hydrated lime. Cement in the mixture delays the lime and aluminum reaction. However, the solidifying effect in the mixture only begins noticeably later, when the porosity has already taken place. One of the goals here is to achieve a certain sawing maturity of the mixture at a desired point in time.

It is characteristic of the production of aerated concrete according to the known methods that the reaction of the porogen (the aluminum powder) with the calcium hydroxide takes place in the course of increasing mixing temperature and increasing consistency of the casting compound. The porosity therefore takes place under constantly changing physical, chemical and rheological conditions. This means that in the overall solid-liquid-gaseous system, highly complex intermediate stages are involved both with regard to the formation of the solid-liquid-gaseous phase boundaries and with regard to the gas phase itself, the temperature and pressure, and consequently also the volume, of the constantly increasing temperature of the casting compound are to be run through until the driving maximum is reached. In addition, there is a constantly increasing, because also temperature-dependent, water vapor partial pressure, which together with the hydrogen formed from the aluminum reaction forms the total pressure in the pore.

The formation of gas is particularly critical for the aerated concrete in the early phase of lime hydration, because no stable phase boundaries have yet been formed and therefore a diffuse gas distribution leads to the finest pores in the entire casting compound, which impairs the strength of the later solid framework. The blowing height achieved is often not stable, especially with very light aerated concrete, for example the bulk density class 0.40 according to DIN 4165, with dry bulk densities between 0.35 and 0.40 kg dm ³ , because gas blows out and the mass up to several percent the total height coincides. This increases the bulk density again.

In addition to the temperature fluctuations, the fluctuations in the consistency of the casting compound during the porosity process are also problematic for the blowing process. The change in consistency from the initially low consistency of the mixture to high consistency at the end of the blowing process, like the constant rise in temperature, forces the aluminum powder to be overdosed in order to achieve the desired blowing height. The higher the initial consistency, the lower the water content of the mixture, the greater the overdose. The consequence of overdosing is gas outbreaks, falling back of the mass, structural disorders and quality defects. As a result, the water content is usually as high as possible selected for the mixture (with water solids values around 0.6) in order to be able to carry out the porosity with the lowest possible consistency values. However, the result is an increase in the capillary pores in the solid framework and a decrease in the strength of the hardened aerated concrete.

There was therefore a need for a process which leads to aerated concrete, in particular aerated concrete with a low dry bulk density, of consistently high quality. In particular, there was a need for such a method, which allows the porosity process to be carried out under controllable conditions.

It is accordingly object of the invention to provide such a method.

This object is achieved with the method according to claim 1. Further method variants are described in the subclaims.

The process according to the invention differs from the processes of the prior art in that the burnt lime is first of all completely extinguished with water before the pore former is added to the mixture for producing the aerated concrete. The procedure according to the invention leads to the fact that the hydration of lime and the formation of the highly disperse hydrated lime have already been essentially completed before the pore former is added. In contrast, in conventional methods, lime hydration takes place at the same time as the driving process. The progressive crystallization of the successive lime hydrate leads to new tooth structures, especially in the later stage of lime hydration. A so-called tempering value becomes effective rheologically; a certain additional force is therefore required for pore enlargement to take place. In the conventional mortar systems for the production of the aerated concrete, the solid-liquid system therefore has a different constitution and thus a different internal structure of the system from solid particles and water than is the case in the mortar system of the method according to the invention.

In the process according to the invention, there are constant conditions for the formation of the phase boundaries between the solid-liquid system on the one hand and the gas phase on the other hand for pore formation over the entire porosity process. The highly disperse lime hydrate, which is essentially completely available from the start, in cooperation with the surface tension of the water. In the process according to the invention, the pores are formed with a constant consistency of the mixture. Every change in the raw materials and the recipe therefore has clear effects on the porosity, so that there are targeted control options via the composition of the mixture.

In addition, the porosity process can be carried out at a substantially constant temperature. In contrast to the methods of the prior art, in which temperature increases of 40 to 50 ° C. are the rule during the porosity process, the method according to the invention can be carried out in such a way that the temperature fluctuation during pore formation is only a maximum of approximately ± 2 ° C. , The reason for this is that the exothermic lime hydration in the process according to the invention is essentially completed before the porosity process is triggered. As a result of the pore formation taking place at a constant temperature, the intensity of the gas formation depends solely on the pore former and its specific surface and can also be influenced in a targeted manner by setting the temperature of the mixture to a suitable temperature during the pore formation. Compared to the conventional method, there is the clear advantage that the desired increase in volume of the mixture is practically proportional to the dosage of the aluminum powder.

Another decisive advantage over conventional methods is that the porosity can be carried out with a high consistency of the mixture. The water contents or the water-solids values can therefore be significantly lower than the previously practicable values. The strength of the finished, steam-hardened aerated concrete increases accordingly. Specifically, for example, in the process according to the invention for the low aerated concrete bulk density classes, 0.30 to 0.40 water / solids values of 0.40 to 0.55 and in particular 0.45 to 0.55 can be achieved, while values in the known processes are in use by 0.6. Significantly higher compressive strengths are achieved than was previously possible in the prior art. For example, aerated concretes can be produced by the process according to the invention, which have a compressive strength of at least 2.0 N / mm ² with a dry bulk density according to DIN 4165 of 0.30 or 0.35 kg dm ³ . So far, about 1.6 N / mm ^{2 was} possible here. While in the prior art for the bulk density class 0.40 DIN 4165 compressive strengths of approximately 2.0 N / mm ^{2 have been} achieved to date, the method according to the invention achieves compressive strengths of at least 2.5 N / mm ^{2 here} .

In principle, all pore formers and blowing agents customary in the prior art can be used as pore formers in the process according to the invention. Aluminum is particularly suitable, for example in the form of aluminum powder or aluminum paste. Passivated aluminum is particularly preferred, the reactivity of which is sufficiently slowed down in order to be able to carry out the driving process under particularly constant conditions. Because of the constant conditions in the course of the porosity, however, other pore formers such as foaming agents can also be used in the process according to the invention.

In principle, the same starting materials can be used in the process according to the invention that have been used up to now in the production of aerated concrete. In addition to quartz-containing sand and / or quartz-containing rock flour, burnt lime-containing binder, water and a pore former, the mixture used in the process according to the invention can contain further additives customary in the production of aerated concrete, such as cement and / or anhydrite. Cement in the mixture serves to stiffen the mass until it is ready for sawing and is used in such a way that the strengthening effect only begins to be noticeable after the driving process has been completed. Return material, that is, uncured material recovered from the production of cellular concrete, can also be added to the mixture.

It is basically possible to extinguish the burnt lime in the presence of all other components of the mixture with the exception of the pore former. However, it is preferred to first quench the burnt lime alone with water, without further additions, and only then to add the further components to the mixture. In this way, agglomeration in the lime hydrate formed can be avoided in a particularly advantageous manner, and the subsequent calcium silicate hydrate formation is particularly uniform. If return material is used in the mixture, however, it may also be advantageous to mix the return material with the quick lime at the beginning and to extinguish the quick lime in the presence of the return goods. The advantage of this procedure is that the water contained in the return material can be taken into account in the amount of water required to extinguish the quicklime, and so the amount of water in the mortar is not greater than necessary. When the quicklime is quenched with water, the temperature of the premix produced increases due to the exothermic reaction of the components. If the quicklime has been extinguished with water either alone or only in the presence of the return material, the temperature in the suspension drops again by mixing in the other components such as cement or anhydrite. The process according to the invention is preferably carried out in such a way that the porosity process can take place at a temperature between 60 and 90 ° C. and in particular between 70 and 85 ° C. In the finished mixture, which contains all components including the pore former, the temperature practically no longer changes during the porosity process. The temperature of the mixture remains constant in a maximum of ± 2 ° C until the end of the porosity. These constant conditions ensure controllable, uniform and easily reproducible pore formation.

As already mentioned, not only is the temperature constant during the porosity process according to the invention, but the consistency of the mixture, once set, practically does not change in the course of the porosity. The porosity can be carried out from the start in a mixture of flowable or plastic consistency. The water content in the mixture is preferably 40 to 60% by mass, in particular 45 to 55% by mass, of the solid starting materials. In order to ensure uniform porosity, in particular with a plastic consistency of the mixture, it can be advantageous to carry out the driving process using a vibrator. The porosity process itself takes place in a manner known per se in casting molds, into which the mixture to be porosified is expediently introduced as a flowable, self-mollifying suspension. The further process steps, including the steam hardening of the aerated concrete, are also carried out in a manner known per se in the prior art.

The process according to the invention can readily be carried out using the devices and equipment which have hitherto been customary in the production of aerated concrete. Existing devices for the production of aerated concrete can continue to be used. Advantageously, they are preceded only by a pre-mixer in which the quicklime is extinguished in accordance with the method according to the invention. The still hot premix is then transferred to a main mixer in order to be mixed there with the remaining constituents of the mixture and then to be processed further in a generally customary manner. The ingredients are mixed in with the other commonly used ingredients Dosing. If only one pre-mixer is used to extinguish the quicklime, the method according to the invention is carried out intermittently. Further processing depends on the reaction time of the quicklime during the extinguishing process.

It is particularly expedient to use several premixers for the upstream hydration of the quicklime, in which the quicklime is removed at different times. The number of premixers required depends on the hydration time of the quicklime, which is usually about 12 to 20 minutes, and the pouring rhythm of the production can also be taken into account. The process can be carried out particularly efficiently if the premix is passed into the main mixing cycle before the maximum quenching temperature is reached. However, care must be taken to avoid agglomeration of the lime hydrate formed and an excessive reduction in the dispersity of the mixture.

The invention will be explained in more detail below with the aid of examples.

embodiments

All examples are based on 100 kg of total solid (without aluminum powder).

To stabilize the porosity process and to increase the strength of the steam-hardened aerated concrete, an increase in the quicklime content or, after exhausting the possible temperature rise, an increase in the lime hydrate addition, in each case in exchange for sand, is provided in all examples.

In the case of premixing, the temperature differences ΔT relate to the heating that the components of quicklime (Brk), water (W) and, if applicable, return material, which have been mixed together at ambient temperature, have experienced after the extinguishing process has been completed. In the case of the main mixture, the temperature difference relates to the temperature increase of the other feed materials by adding the hot premix. After the mixing process has been completed, the temperature of the mixture remains constant during the porosity within an interval of ± 2 ° C. "Concrete 0.30" denotes a cellular concrete with a dry bulk density according to DIN 4165 of 0.30 kg / dm ³ .

1. PRE-MIXING (without heat loss)

2. MAIN MIX (without heat loss)

The degree of purity of the aluminum powder used must also be taken into account

The demand is given in the form of a basic value and a factor. The basic value gives the gas volume at reaction temperature and atmospheric pressure, corresponding to the required volume increase of the mixture. The factor takes into account the consistency of the mixture, its average load on the pore and the enlargement of the pores against atmospheric pressure. The factor can only be estimated and is to be determined empirically.

Claims

1. A method for producing aerated concrete from a mixture comprising quartz-containing sand and or quartz-containing rock flour, burnt lime-containing binder, water and a pore former, characterized in that the burnt lime is substantially completely extinguished with water before the

Pore former is added to the mixture.

2. The method according to claim 1, characterized in that the mixture additionally comprises cement and / or anhydrite.

3. The method according to claim 1 or 2, characterized in that the mixture further contains return material.

4. The method according to any one of claims 1 to 3, characterized in that the burnt lime is extinguished alone or in the presence of the return material with water before the other components of the mixture are added.

5. The method according to any one of claims 1 to 4, characterized in that the pore former is mixed in at a temperature between 60 and 90 ° C and in particular between 70 and 85 ° C.

6. The method according to any one of claims 1 to 5, characterized in that the porosity takes place at a substantially constant temperature.

7. The method according to any one of claims 1 to 6, characterized in that the water content of the mixture is 40 to 60% by mass, in particular 45 to 55% by mass, based on the mass of the solid components of the mixture.

8. The method according to any one of claims 1 to 7, characterized in that the porosity takes place with a plastic consistency of the mixture, in particular using a vibrator.

9. The method according to any one of claims 1 to 8, characterized in that the mixture to be porosized is poured into a casting mold as a flowable, self-mollifying suspension.

10. The method according to any one of claims 1 to 9, characterized in that a foam agent or aluminum powder, in particular a passivated aluminum powder, is used as the pore former.

11. The method according to any one of claims 1 to 10 for the production of aerated concrete with a dry bulk density according to DIN 4165 from 0.30 to 0.35 kg / dm ³ with a compressive strength of at least 2.0 N / mm ² or a dry bulk density of 0, 40 kg / dm ³ with a compressive strength of at least 2.5 N / mm ² .

12. The method according to any one of claims 1 to 11, characterized in that the burnt lime is quenched in a pre-mixer and then in a main mixer is transferred to be mixed there with the remaining components of the mixture.

13. The method according to claim 12, characterized in that the extinguishing of the burnt lime is carried out in several premixers at different times.