DE1920943A1 - Gypsum mortar - Google Patents

Description

Description of the patent application

BPB Industries limited,

Ferguson House, Marylebone Road, London, N.W. 1, England

concerning

Gypsum mortar

The invention relates to plastering and mortar compounds containing gypsum, which are suitable for application to a substrate of high absorption or absorbency, such as plasterboard or gas cement, are suitable. It works on such a ground Rapid penetration of water from the plaster into the substrate before the setting process is complete, leading to the appearance of imperfections in the plaster layer.

It has already been proposed to incorporate 30 to 60 $> ground calcium carbonate and a cellulose ether into mixtures of slaked lime and calcium sulfate hemihydrate plaster, with the aim of trying to improve the behavior of the compound applied to a strongly absorbent substrate. The cellulose ether reduces the rate of water loss by increasing the viscosity of the aqueous phase, but leads to increasing difficulties in moistening and mixing the mass with water. These difficulties will be

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reduced by the addition of caleium carbonate, which, however, if the amount exceeds $ 30 to $ 35, leads to a decrease in the bond strength.

It has now been found that when moistened and. Mixing a gypsum mortar with water encountered difficulties without loss of bond strength and without premature Significantly reduce drying out on absorbent surfaces.

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Can be changed if you use a hemihydrate plaster for the mortar is used which, when water is added, greatly increases the specific surface area (i.e. under a substantial Reduction in grain size) can be dispersed quickly and easily.

All hemihydrate plasters suffer a certain degree of breakdown of their particles when water is added j with some types of piasters, however, e.g. those obtained from certain gypsum stones by suitable firing, when wetting, a relatively large decrease in grain size takes place. With the help of such highly dispersible plasters to build mortars that contain less calcium carbonate than previously thought necessary to ensure good wetting and properties of the mass To give mixing properties, these mortars at the same time retain a high degree of water retention when applied to a highly absorbent surface. The scope up to which a given plaster will disperse upon wetting and thus its suitability for purposes of the invention can be understood by those skilled in the art with the aid of what is described below Testes can be easily determined.

The mortars according to the invention contain, calculated on the dry weight of the mass, $ 50 to $ 95> calcium sulfate hemihydrate plaster with a specific surface area in the dry state

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from 2500 to 5000 cm / g and a dispersion factor (as defined below) of at least 4.6; 0 to 4 $ slaked lime; up to 45 % ground calcium carbonate; and 0.05 to 0.5 # cellulose ether.

• The dispersion factor of a plaster is used for purposes of Invention expressed as the ratio of the specific surface area of the plaster after this 1 minute at 20 C vigorously was stirred with water, to the specific surface area of the same plaster before wetting. The ones preferred in practice Moistening conditions consist in that the plaster under the action of a rapidly rotating blade with five times the weight of water containing some sodium citrate, mixed. The specific surface area of the plaster samples can be measured using any known standard technique However, it is of course advisable if the same method is used within a series of determinations.

A common method for dispersing a plaster under standardized conditions is as follows: 500 ml of water at 20 °, in which 0.2 <f ₀ sodium citrate are dissolved, are introduced into a household liquefier equipped with a small, high-speed stirrer. Then 100 g of the plaster to be examined are added and the mixture is immediately stirred at 12,000 rpm for the required period of time. The plaster is then filtered off with suction and, in order to remove the water, washed with water-miscible organic solvents, for which purpose, for example, 300 ml of isopropanol and then 300 ml of acetone are used. The total time between the initial addition of the piaster to the water and the complete removal of water should not exceed 6 to 7 minutes. The plaster sample is then placed in an oven at 35-4O ⁰ C

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dried and easily driven through a sieve, the openings of which ^{correspond to the British Standard Sieve No. 52 H} (mesh opening approx. 0.25 mm), whereupon its specific surface area is determined.

• With the help of this method, a plaster with the typical specific surface area (3000 cm / g) tests carried out, to determine the lowest dispersibility ratios, the results are still good in the batches given above result. It was found that the minimum values vary with the dispersion time as follows:

Dispersion time dispersibility ratio

30th sec 60 η 120 Il 180 n

3.0 4.6 5.2 5.7

At present, a dispersion time of 1 minute is preferred as a standard for determining the dispersion factor of a plaster.

When applying the invention in practice, the plasters available must be examined in order to determine whether they are sufficiently dispersible in water that the advantages of the invention can be achieved. this can be done using the test procedure just described.

In addition to the above-mentioned ingredients, the mortars preferred according to the invention also contain 0.15 to 0.25 tfo powder

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sized animal glue and 0 to 5 # ground gypsum stone. You can also use a retarder (e.g. citric or tartaric acid) and / or an accelerator (e.g. potassium or Aluminum sulfate) to regulate the setting time and to stabilize the plaster.

The calcium carbonate is preferably ground limestone, the degree of which is set in such a way that about 85 # pass a British * standard sieve of 200 meshes (mesh size approx. 0.6 mm). The powdered animal glue is preferably a finely ground water-soluble material. The cellulose ether is preferably methyl cellulose with a viscosity of 6,000 cp in 2 #iger aqueous solution at 2O ⁰ C. The lime is a commercially available hydrated lime, and the gypsum is preferably finely ground that it about 80 $> a British Standard Sieve of 100 meshes (mesh size approx. 0.125 mm) pass,

Examples of gypsum mortar are given below to illustrate the advantages of the invention should.

The following approach is preferred:

Parts by weight

Hemihydrate plaster 73t00

ground limestone 24.90

Glue powder 0.20

Slaked lime 0.80

ground gypsum stone 0.83

Cellulose ether 0.25

Tartaric acid 0.03

100.01

First there were two different hemihydrate plasters (A and B) compared.

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The plasters to be compared were made from two different gypsum stones by batch-wise dry-burning in a conventional cooker. After the burn both were on:
been grind.

were both reduced to a specific surface area of 3000 cm / g

Determinations of the dispersion factor with the aid of the above method showed that plaster "A" had a factor of 6.12 and "B ^{n" had} a factor of 3143.

A number of gypsum mortars have been prepared using various proportions of the two Plaster. It was found that plaster "A" alone and mixtures with a dispersion factor above 4.6 has proven itself when applied to highly absorbent surfaces · Im In contrast, it was found that plaster "B" gave a non-workable mortar and that mixtures with dispersion factors below 4.6 in. Various degrees unsatisfactory was.

These observations were confirmed by a further comparison with the same approaches on a highly absorbent Background between three different sculptures made in the usual way, each of which has a specific surface of about 3000 cm / g possessed. One of them had one Dispersion factor of 6.25 and showed excellent processing properties on the highly absorbent substrate and good water retention during the final leveling. A second plaster had a factor of 4.4 and also showed good processing properties, but dried too quickly for really satisfactory results. The third plaster had a factor of 3.5 and could not be used on the highly absorbent surface

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Claims (1)

DR.ING. F. WUKSTUOFF β MUNICH DIP ₁ L. ING. G. PUI-S, «CHWEIGERSTRASSE DR.E.V.FECHMANN tblxfox SSOS DR. ING. D. BEHRENS · iiuoMinuiiimii it hCkoih 1A-36 Patent claims f 1 © J Gypsum mortar made of, based on the dry weight,, 50 - 95%> calcium sulfate hemihydrate, 0 - 4 # slaked lime, 0 - 45 i » ground calcium carbonate and 0.05 - 0.5 # cellulose ether, marked with this that the calcium sulfate hemihydrate in the dry state has a specific surface of 2500-5000 cm / g and that its dispersion factor (ratio of the specific surface after wetting to the specific surface before wetting) is at least 4.6 2β gypsum mortar according to claim 1, characterized ' by an additional content of 0.15-0.25% by weight of powdered animal glue and, if necessary, up to 5 wt .-? 6 ground mineral plaster. 3. plaster mortar according to claim 1 and 2, characterized by the following composition in parts by weight: Hemihydrate pin 73.0 ground limestone 24.9 Glue powder 0.2 Slaked lime 0.8 ground gypsum stone 0.83 Cellulose ether 0.25 Tartaric acid 0.03 8644 '. 909845 // 111S2