IE42172B1 - Calcium sulphate hemihydrate compositions containing potassium sulphate - Google Patents

Description

This invention relates to calcium sulphate a-hemihydrate - containing compositions for admixture with water to form hardenable mixtures. Hardenable mixtures of calcium sulphate α-hemihydrate and water are convenient5 ly pourable whereby the mixtures flow during casting to provide a flat surface before commencement of hardening. Calcium sulphate α-hemihydrate compositions which upon admixture with water give such pourable mixtures are hereinafter referred to as pourable compositions.

Pourable compositions based on calcium sulphate ce-hemihydrate are already known. Thus, for example, a pourable gypsum composition consisting of calcium sulphate α-heraihydrate and an aqueous resin emulsion is described in German OS 2 223 188. Moreover, from German OS 2 247 756, a composition for use in the production of flooring materials is known which contains, in addition to calcium, sulphate c-hemihydrate, 0.4 to 1.5% by weight of liquefier, (i.e. a compound which serves to improve flow properties), 0.001 to 0.05% by weight of hardening retarders, 5 to % by weight -of white lime hydrate and building sand and which, after water has been added, can be poured in the required place prior to commencement of hardening. - 2 42172 A substantial disadvantage of the previously known pourable compositions lies in the fact that, once hardening has been effected, they have a tendency to crack and also in many cases have an inadequate surface hard3 ness, particularly when used to produce floor coverings. Also, when calcium sulphate α-hemihydrate which has been prepared by hydrothermal recrystallisation of phosphoric acid gypsum is used, flooring materials produced from the pourable compositions have been found to exhibit detrimental blistering, owing to their white lime content.

One object of the present invention is to provide new pourable compositions based on calcium sulphate a-hemihydrate which can be used in the production of flooring materials.

According .to one feature of'the present invention, there ϊέ provided a calcium sulphate a-hemihydrate containing composition for admixture with water to form a hardenable mixture, the said composition comprising calcium sulphate α-hemihydrate at least 85% by weight of which has a particle size between 6 and 60pm and 0.2 to 0.9% by weight (based upon the weight of ahemihydrate) of potassium sulphate.

According to a further feature of the present invention, there is provided a hardenable mixture of a calcium sulphate α-hemihydrate - containing composition according to the invention as hereinbefore defined, in water. The said mixture preferably contains the said composition and water in a weight ratio of from 100 : 24 to 100 : 40, advantageously from 100 : 26 to 100 : 36.

According to a still further feature of the present invention, there is provided a process for the production of gypsum articles which comprises casting and then hardening a hardenable mixture according to the invention as hereinbefore defined, the potassium sulphate being dissolved in the start of the hardening process.

The calcium sulphate α-hemihydrate- containing compositions for admixture with water to form a hardenable mixture in accordance with the present invention preferably also contain compounds which improve the flow properties of mixtures obtained upon admixture with water (i.e. liquefiers) and compounds which decrease the rate of hardening of mixtures obtained upon admixture with water (i.e. hardening retarders).

By working in accordance with the present invention, we have succeeded in producing pourable compositions which provide mixtures having desirable flow characteristics, which are self-levelling upon casting, and which also produce gypsum articles with good strength values.

Thus for example we have produced pourable compositions which can be used with advantage in the preparation of flooring materials.

The present invention is based on the realisation that, to obtain better strength, not only should a smaller water gypsum value (as defined below) be chosen, but also a-hemihydrate with a specific particle size distribution must be used. It has th^is been found that the sedimentation of the α-hemihydrate particles can be prevented by a controlled distribution of particle sizes. Moreover, · by these measures, the quantities of liquefierjs needed to produce desired flow properties can be considerably reduced.

Preferably at least 85% by weight-of the calcium sulphate α-hemihydrate used in the compositions accord2θ ing to the invention has a particle size between 6 and pm, and advantageously at least 50% and more especially at least 85% by weight of the α-hemihydrate has a particle - 5 4 217 2 size between 20 and 30 pm. The finest portion of particle size less than 6 pm, should preferably not exceed 5% by weight, and preferably not more than 10% by weight should be above 60 pm. If these values are not adhered to, the strength values of gypsum articles produced from the pour able compositions may be reduced. Moreover, too high a proportion of coarse particles may result in sedimentation during casting with the result that the surface properties of the gypsum articles thereby produced can be impaired.

The calcium sulphate α-hemihydrate used is preferably that obtained by the hydrothermal recrystallisation of phosphoric acid gypsum. Since α-hemihydrates of this kind are advantageously produced in a coarse-crystalline form, with particle sizes of approximately 100 to 200 pm, the starting material for producing compositions according to the present invention first has to be ground to the desired particle size. This can be done in ball mills, in a known manner.

The compositions according to the invention contain potassium sulphate as a further component. As a result of the addition of potassium sulphate, it has been found that the occurrence of defects such as distortion ’ and bulging, and the formation of cracks on the surface of gypsum articles produced (frequently found with calcium sulphate α-hemihydrate as a result of the high linear expansion coefficient)can be reduced or prevented. The potassium 'sulphate should be incorporated in the composition in quantities of 0.2 to 0.9 % by weight, based on the calcium sulphate α-hemihydrate. Particularly good results have been obtained with quantities of 0.4 to 0.6 % by weight. With the quantities given, no loss in strength in hardened floor coverings produced from compositions according to the invention could be detected. The particle size is not critical, but the potassium . sulphate should be present in dissolved form at the start of the hardening process.

The flow properties of the compositions can be improved by adding known liquefiers, those based on melamine and phenolic resins such as Melrrient F 10 and - 7 43172 Hydrisol DS 3022 having proved especially suitable. te (Melment F 10 is a trade mark of Suddeutsche Kalkstickstoffwerke A.G., Trostberg and Hydrisol DS 3022 v is a trade mark of BASF, Ludwigshafen). Using the particle S sizes according to the invention 0,3 to 0.7 % by weight of liquefier, based on the weight of α-hemihydrate used and depending on the desired density, has been found sufficient particularly with calcium sulphate α-hemihydrates which have been obtained from phosphoric acid gypsum by the hydrothermal recrystallisation method. For producing flooring bases (under-floors) with a density of e.g. 1.85 g/cc, 0.77= by weight of liquefier II II II II It 1.75 g/cc, 0.5% II It II It II It li II II 1.65 g/ cc, 0.3% » II II tt are necessary. The quantities of liquefier can therefore be reduced by more than half the quantity required hitherto.

As stated above, the compositions-according to the invention preferably contain a hardening retarder. Advantageously a retarder is chosen which not only guarantees a I long enough flow time for processing the composition but also prevents hardening from occurring boo rapidly after - 8 42172 the flow time has ended. In this way, excessive heating during the hardening process can be prevented and the danger of crack formation on the surface of the cast compounds can be reduced still further. The flow time and final hardening time must, of course, be adapted to suit the intended purpose. As a particularly suitable retarder, Recordal P (a trade mark of Schieferit- Werke, Meinbrexen/Weser) is used, in quantities of about 0.05 to 0.4 % by weight based on the weight of α-hemihydrate.

To improve the processing methods and properties of the gypsum articles produced, all the known auxiliary substances such as for example antifoam agents, methyl cellulose, dyes, powdered quartz or fillers, may be added to the composition according to the inven15 tion. In the case of the laying of floors, processing may take place both on the construction site and also in a processing plant. The latter method will be used, for example, when the compositions according to the invention are for use in coating prefabricated construe2o tional parts, such'as brick walls. 43172 The strength of gypsum articles which can he obtained with the compositions according to the invention are affected chiefly by the density chosen and thus also by the water jypsum value used. The bending strength and compressive strength increase as the water gypsum value decreases, as can be seen from the following table. The water gypsum value is the ratio 100 g water/grams material added. Bending strength is measured by German standard DIN 1168. Water Bending Compressive gypsum value strength Strength 0.28 120 kp/cm2 720 kp/cm2 0.32 102 kp/cm2 580 kp/cm2 0.40 , 75 kp/cm2 450 kp/cm2 The following Examples illustrates the invention. Unless otherwise stated, all percentages are by weight based on the weight of the calcium sulphate hemihydrate.

EXAMPLE 1 Calcium sulphate α-hemihydrate with a particle size of 100 to 200pm prepared by hydrothermal recrystallization from phos phoric acid gypsum, is ground, in its dry state, in a ball mill.

The amplitude and throughput of the mill are adjusted such that the ground α-hemihydrate has an average particle size of 30pm and the distribution of particles is within the range from 1 to 60pm. The finest portion, of particle size less than 6pm, is about 4% by weight. t of the ground calcium sulphate α-hemihydrate are mixed, in a mixer, with 0.7% by weight of Melment F 10, 0.5% by weight of potassium sulphate with a particle size of less than 250pm, 0.3% by weight of Recordal P and 0.015% of methyl cellulose. The viscosity of the methyl cellulose, measured in a 2% solution according to Hopnler, is 3000 cp. As antifoam agent, 0.03% by weight of Etingal (Trade Mark) are sprayed on. After thorough mixing, the pourable composition is transported in sacks or containers to the processing point.

On the construction site, the powdered pourable composition is mixed with water in a weight ratio of 100:32; in the present example, this is effected in a continuously operating mixing machine, as used in the processing of finished plaster. The flow time of the mixture is 60 minutes and the expansion during hardening is about 1 mm/m. The size of the areas to be cast is chosen to be such that each area can be cast within the flow time of the pourable composition. This is necessary because the cast surface must not be moved again after the flow time has ended.

If the rehydration process is distrubuted by vibrations or like occurrences, crack formation and thus a reduction in strength generally occur. The casting itself is effected in known manner on floor bases provided with foil or waterrepellent paint. The compressive strength of the cast surface, which could be walked on after about 3 hours, was 580 kp/cm and the bending strength 2 was about 102 kp/cm .

EXAMPLE 2 Preparation and grinding of the calcium sulphate α-hemihydrate were effected as in Example 1. However, to reduce the plasticity of the pourable composition, 1 t of α-hemihydrate was replaced by 1 t of powdered quartz (average particle size 30 pm) in the mixing process. With this mixture, too, a crack-free floor with high strength values was obtained. The compressive strength in this case was 400 kp/cm and the bending strength was 90 kp/cm2.

Claims (20)

1. What we claim is:1. A calcium sulphate α-hemihydrate - containing composition for admixture with water to form a hardenable mixture, the said composition comprising calcium sulphate 5 α-hemihydrate at least 50% by weight of which has a particle size between 6 and 60 pm and 0.2 to 0.9% by weight (based upon the weight of α-hemihydrate) of potassium sulphate.

2. A composition as claimed in claim 1 wherein at least 10 85% by weight of the calcium sulphate α-hemihydrate has a particle size between 6 and 60 pm.

3. A composition is claimed in claim 1 or claim 2 wherein at least 50% by weight of the calcium sulphate α-hemihydrate has a particle size between 20 and 30 pm. 15

4. A composition as claimed in claim 3 wherein at least 85% by weight of the calcium sulphate ,p>.hemihydrate has a particle size between 20 and 30'pm.

5. A composition as claimed in any of the preceding claims wherein not mote than 5% by weight of the calcium 2Q sulphate α-hemihydrate has a particle size of less than 6 pm.

6. A composition as claimed in any of the preceding claims wherein not more than 10% by weight of the calcium sulphate α-hemihydrate has a particle size above 60 pm. 2g

7. A composition as claimed In any of the preceding claims which contains from 0.4 to 0.6% by weight of potassium sulphate.

8. A composition as claimed in any of the preceding claims which further contains a compound which improves the flow properties of mixtures obtained upon admixture with water.

9. A composition as claimed in claim 8 wherein said compound comprises a melamine or phenolic resin.

10. A composition as claimed in claim 8 or claim 9 which contains from 0.3 to 0.7% by weight (based on the weight of α-hemihydrate) of the said compound.

11. A composition as claimed in any of the preceding claims which contains a hardening retarder which decreases the rate of hardening of mixtures obtained upon admixture with water.

12. A composition as claimed in claim 1 substantially as herein described.

13. A calcium sulphate α-hemihydrate-containing composition for admixture with water to form a hardenable mixture, substantially as herein described in either of the Examples.

14. A hardenable mixture of a calcium sul phate α-hemi hydratecontaining composition as claimed in any of the preceding claims, in water.

15. A mixture as claimed in claim 14 which contains the said compositions and water in a weight ratio of from 100:24 to 100:40.

16. A mixture as claimed in claim 15 wherein the said weight ratio is from 100:26 to 100:36.

17. A process for the production of gypsum articles which comprises casting and then hardening a mixture as claimed in any of claims 14 to 16, the potassium sulphate being dissolved at the start of the hardening process.

18. A process as claimed in claim 17 substantially as herein described.

19. A process for the production of gypsum articles, substantially as herein described in either of the Examples

20. Gypsum articles when produced by a process as claimed in any of claims 17 to 19.