Concrete corrosion

Abstract

The present invention relates to a mixture for preventing the formation of ettringite in concrete, to the use of such a mixture for forming a barrier layer against sulphate attack in concrete and to concrete containing a corresponding barrier layer.

Description

[0001] Concrete corrosion

[0002] The present invention relates to a mixture for preventing the formation of ettringite in concrete, the use of such a mixture for forming a barrier layer against sulphate attack in concrete and concrete containing a corresponding barrier layer.

[0003] Livestock are usually kept in stables whose floors are made of concrete. Calcium sulfoaluminate (3CaO-AI O23 -3CaSO4 -32 H2 0), which is also known as ettringite, is often formed during the dehydration of cement, i.e. during the production of the floor. The formation of ettringite is harmless at the time of production, i.e. during curing.

[0004] If SOT ions are later added to the hardened concrete, for example from the groundwater, ettringite can form again, creating a crystallization pressure that can crush the cement paste. This is known as sulphate attack. When keeping livestock, various additives are used to improve animal welfare. This can also result in contact with the concrete surface. So far, care has been taken to ensure that such additives do not contain sulphate ions. There is therefore a need to provide a protective layer for concrete that enables the formation of ettringite, i.e. sulphate attack, in concrete to be avoided as far as possible, particularly in the context of livestock farming.

[0005] Surprisingly, it has been shown that a sulphate-containing mixture enables the formation of a barrier layer in concrete, so that sulphate drift is prevented. In a first embodiment, the problem underlying the present invention is therefore solved by a mixture comprising poultry excrement and sodium bisulphate to prevent ettringite formation in concrete, wherein the weight ratio of excrement to sodium bisulphate is 1 : 10 to 10: 1.

[0006] Concrete in the sense of the present invention is hardened concrete. Sodium bisulfate (sodium hydrogen sulfate (NaHSC )) is commonly used in acidic cleaners, in the food industry and also as a pH regulator in various fields. If concrete comes into contact with sodium bisulphate, ettringite formation can be observed.

[0007] Surprisingly, it has now been shown that when sodium bisulphate is combined with poultry excrement and the resulting mixture is brought into contact with concrete, no sulphate attack can be observed. On the contrary, it has been shown that a barrier layer is formed which prevents the penetration of sulphate ions into the concrete surface or the concrete pores.

[0008] Poultry excrement refers in particular to excrement from chickens and / or turkeys.

[0009] In a further embodiment, the problem underlying the present invention is solved by using a mixture of poultry excrement and sodium bisulphate to form a barrier layer against sulphate attack in concrete. The barrier layer is formed in particular in the area of the surface of concrete and extends to a depth of in particular 10 mm or less. Preferably, the barrier layer has a thickness of 2 to 5 mm and is located at a depth of 1 mm to 8 mm, preferably from 3 mm to 7 mm.

[0010] The thickness of the barrier layer and its penetration depth in the concrete depend in particular on the type of concrete and its porosity.

[0011] The barrier layer is formed by bringing concrete into contact with a mixture as described above. In particular, contact is made by providing an aqueous slurry of the mixture and bringing it into contact with the concrete. This can be done by applying the slurry to the concrete surface. It is also possible for concrete to be immersed in the slurry.

[0012] Contact is maintained until the barrier layer is formed. The duration of the formation of the barrier layer and thus the duration of the contact application depends on the exact properties of the concrete. In particular, contact is made over a period of at least 24 hours up to a week. A longer contact period of up to 6 weeks, for example, does not lead to the formation of an improved barrier layer. Shorter contact times can result in the barrier layer not forming completely.

[0013] Investigations have shown that crystals are present in the area of the barrier layer, which fill the pores of the concrete. However, this is not secondary ettringite formation, as this process would cause a compressive to cracking effect on the concrete structure. Microdefects or microcracks from the filled pores could not be observed, however. On the contrary, EDX analyses proved that the pores of the concrete are filled with calcium carbonate. Surprisingly, there is no formation of gypsum or other attacking sulphates.

[0014] In a further embodiment, the problem underlying the present invention is solved by concrete, wherein the concrete contains a barrier layer against ettringite formation, wherein this barrier layer is made possible by bringing concrete into contact with a mixture described above.

[0015] In the following embodiments, the present invention is further explained in a non-limiting manner.

[0016] Examples Concrete samples were brought into contact with chicken faeces and sodium bisulfate in sealable plastic Euro containers measuring 80 cm x 80 cm x 12 cm and then analyzed.

[0017] For this purpose, concrete slabs of grade C3037, which is often used as a concrete floor in stables, were tested in the Euro containers. The aim was to determine the damage to the concrete caused by sodium bisulphate over time. Four different tests were carried out for each type of concrete. The period of the respective treatment was 41 days, which corresponds to the average fattening period of poultry.

[0018] Three Euro containers with lids from Auer Packaging were provided and filled as follows:

[0019] Box 1 : dried chicken faeces

[0020] Box 2: dried chicken faeces and sodium bisulphate at a rate of 250 g / m2Box 3: dried chicken faeces and sodium bisulphate at a rate of 500 g / m2

[0021] The dried chicken faeces were provided by Osnabruck University of Applied Sciences. Sodium bisulphate from Grillo Werke AG (trade name: Improbed), Duisburg, Germany, was used. Boxes 1, 2 and 3 were filled with dried chicken fae- cesto a height of 25 mm. The corresponding amount of sodium bisulphate was distributed in boxes 2 and 3. The boxes were then filled to a height of 50 mm with drinking water.

[0022] Concrete samples were placed in the boxes and left there for 41 days at room temperature. The specimens were then removed and rinsed with tap water.

[0023] The pH value was measured continuously in the boxes to document the development over the 41 days. The results are shown in Fig. la, lb and lc. The results from 3 test runs are shown. It can be seen that the pH value in boxes 1, 2 and 3 increased slightly from a value of 6-7 to a value of 7 to 8 over the 41- day period.

[0024] Drill dust samples were taken. For this purpose, boreholes were drilled at different depths in the sample and the drill dust obtained was analyzed. The sulphate content was determined using inductively coupled plasma-atomic emission spectrometry (DIN EN ISO 11885: 2009-09 (E22)). The sulphate content was analyzed at a depth of 0-10 mm of the sample, 10-20 mm of the sample and 20-30 mm of the sample.

[0025] Sulphate was detected in the surface area (0-10 mm) of all samples. The content was around 10,000 to 15,000 mg / kg at an initial bisulphate concentration of 250 g / m2(box 2) and around 17,000 to 20,000 mg / kg at an initial bisulphate concentration of 500 g / m2(box 3).

[0026] A content of 7,500 mg / kg was measured in the 0 sample (comparative test, box 1).

[0027] In the samples taken at a depth of 10-20 mm from the surface or 20-30 mm, the bisulphate content was the same in all samples. Therefore, no bisulphate could penetrate into deeper layers of the concrete, which could have triggered ettringite formation or sulphate attack.

[0028] Surprisingly, a combination of excrements with sodium bisulphate led to the formation of a barrier layer. This prevented the formation of ettringite even on porous concrete surfaces.

[0029] Example 2:

[0030] Concrete samples of grading curve AB16 with a cement content of 320 kg / m3were brought into contact with chicken faeces and water (each 50 % by volume) and 250 g / m2sodium bisulphate or 500 g / m2sodium bisulphate over a period of 4 loading cycles of 40 days each. The samples were then analyzed using SEM (ZEISS Gemini SEM 200 NanoVP low-vacuum SEM) and EDX.

[0031] It has been shown that the surface of the concrete samples appears loosened. Underneath is a compacted layer (barrier layer) in which the pores of the concrete are filled with calcium carbonate. Such a layer could not be detected in samples without sodium bisulphate (box 1 from example 1).

[0032] There was no penetration of the sulphur or sodium of the sodium bisulphate into the concrete below the barrier layer.

[0033] Fig. 2a shows an SEM image of a sample that was brought into contact with chicken faecesand water (50 % by volume each) and 250 g / m2sodium bisulphate. Fig. 2b shows the corresponding EDX image of the sample, which was used to analyze the contents of the pore filling. The results of the EDX measurement are shown in the table below:

[0034] Table 1 : Results of the EDX analysis according to Fig. 2b; all values in atom.%

[0035] Fig. 3a shows an SEM image of a sample that was brought into contact with chicken faeces and water (50 % by volume each) and 500 g / m2sodium bisulphate. Fig. 3b shows the corresponding EDX image of the sample, which was used to analyze the contents of the pore filling. The results of the EDX measurement are shown in the table below:

[0036] Table 2: Results of the EDX analysis according to Fig. 3b; all values in atom.%

[0037] The samples were treated with impregnating resin for the EDX analysis. Therefore the carbon content is, as usual, increased.

[0038] The analyzed filled pores are located within the barrier layer.

[0039] Example 3:

[0040] Concrete samples as described in example 1 were brought into contact with chicken faeces and sodium bisulphate in an aqueous slurry over a period of 42 days. After 42 days, the concrete samples were rinsed with tap water. They were then again brought into contact with an aqueous slurry containing sodium bisulphate and chicken faeces for a period of 42 days, then rinsed again and the process repeated. A total of 4 runs (4 times in the aqueous slurry for a period of 42 days each and subsequent rinsing with tap water) were carried out.

[0041] The following quantities of sodium bisulphate were used: Sample A: dried chicken faeces, no sodium bisulfate

[0042] Sample B: dried chicken faeces and sodium bisulfate in a quantity of 250 g / m2Sample E: dried chicken faeces and sodium bisulfate in a quantity of 500 g / m2From this, an aqueous slurry was prepared by adding tap water as described in example 1.

[0043] Samples B and E were analysed using a polarizing microscope on thin sections. An EDX analysis (Hitachi S-2700 scanning electron microscope with tungsten cathode) was also carried out. Where necessary, the samples were coated with carbon to improve conductivity. The acceleration voltage was 20 kV for all measurements.

[0044] Fig. 4a (linear polarization (II-pole)) and 4b (crossed polarization (x-pole)) show polarization micrographs of sample B. Directly on the surface there is a fine crust of crystalline material with a brownish intrinsic color (II pole) and bright interference colors. This crust is a maximum of 50 pm thick. The impacted surface is still completely preserved. Removals or breakouts of material were only rarely observed and are comparable with sample A, which was not brought into contact with sodium bisulphate.

[0045] EDX analyses were also carried out on samples A, B and E at different points. The material composition of pore fillings, which were also visible in polarization microscopy, was examined. In addition, the crust and the concrete itself were analyzed in the area near the surface and at a depth of about 100 pm to about 300 pm. Fig. 5 shows the positions of the EDX measurement of sample B as an example. The results of the EDX analysis were confirmed by means of element mapping.

[0046] The pore fillings are calcium carbonate. A similar composition was also found on the crust of the surface. It is therefore assumed that this also consists of calcium carbonate. The cement paste immediately below the crust as well as below the barrier layer contains more calcium, silicon and some aluminum compared to the pore filling and the crust, which corresponds to the typical composition of concrete.

[0047] The sodium and sulphur contents are unremarkable, but the crust contains slightly more sulphur than the cement paste below. The sulphur content in sample A was comparable to that in sample B and sample E, so that it can be assumed that the sulphur originates from the chicken faeces. There was no evidence of sodium bisulphate penetrating into deeper layers of the concrete, so that a kind of barrier layer was also present here to prevent this. Sulphate attack was successfully prevented.

[0048] In all samples, the cement paste was completely preserved and no anomalies were visible in the microstructure. A clear enrichment of sulphur or sodium was not recognizable. There was no depletion of calcium.

Claims

Claims1. A mixture comprising poultry excrement and sodium bisulfate for preventing ettringite formation in concrete, wherein the weight ratio of excrement to sodium bisulfate is 1 : 10 to 10: 1.

2. Use of a mixture according to claim 1 to form a barrier layer against sulphate attacking in concrete.

3. Use according to claim 2, characterized in that the barrier layer is formed in the area of the surface to a depth of 10 mm or less.

4. Use according to at least claim 2 or 3, characterized in that the barrier layer is obtained by bringing concrete into contact with a mixture according to claim 1.

5. Use according to claim 4, characterized in that the mixture is present as a slurry, in particular an aqueous slurry.

6. Use according to claim 5, characterized in that the slurry is applied to the concrete surface or the concrete is immersed in the slurry.

7. Concrete containing a barrier layer against ettringite formation obtained by bringing concrete into contact with a mixture according to claim 1.

8. Concrete according to claim 7, characterized in that the contacting takes place over a period of 24 h to 1 week.

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