The present invention is related to a plaster resistant to high temperatures in the order of 1350.degree. to 1400.degree. C; in other words, the invention is concerned with a refractory plaster material.
The conventional plasters constitute excellent insulating materials, but they are not adapted to be used at elevated temperatures. The CaSO.sub.4,1/2H.sub.2 O semi-hydrate, when hydrated to form plaster, will show cracks when exposed to temperatures of 200.degree. C. This deterioration will be accentuated, and the network formed by the cracks become most important, when the temperature reaches a value of approximately 900.degree. C, the material then losing all of its cohesion. At temperatures of about 1200.degree. C, SO.sub.2 will volatilize and the plaster will be structurally impaired by inflation or swelling due to re-hydration when the lime formed cools.
So-called "fire-retardant plasters" are known which, by dehydratation of the gypsum (CaSO.sub.4,2H.sub.2 O) and by their insulating power due to the presence of a light-weight additive (such as vermiculite, for example) exhibit effective fire-retardant properties. However they undergo rapid destruction when the temperature reaches values over 1000.degree. C.
It is an object of the present invention to provide a material containing plaster, overcomes the above-described drawbacks of the known materials and which exhibit satisfactory properties at temperature of up to 1350.degree.-1400.degree. C.
The material according to the invention retains its dimensional stability and is not altered when subjected to high temperatures, even when the latter reach 1400.degree. C. The novel material further exhibits excellent thermal shock-resisting properties under conditions of sudden temperature rise, and is explosion proof.
The thermically isolating and fire-retarding properties of a plaster according to the present invention are such that the latter is particularly well adapted to be used in the building industry. It may be used for the protection of walls and/or metallic beams, for the manufacture of structural squares or panels, etc.
The material according to the present invention comprises CaSO.sub.4 1/2H.sub.2 O, alumious filler and mineral fibres. Said material is a filler-containing plaster and it can be used in a manner similar to that applied to the known plasters, as far as moulding and the projecting techniques are concerned. The presence of the aluminous filler leads to the formation of stable C.sub.4 A.sub.3 S at temperatures up to 1400.degree. C. When an excess amount of alumina is present and when the temperature rises to values of above 1400.degree. C, formation of CA.sub.2 and CA.sub.6, which are restistant against humidity, is observed after evaporation or volatilization of SO.sub.3.
In the present description, C.sub.3 A.sub.4 S designates anhydrous calcium sulphoaluminate (4CaO, 3Al.sub.2 O.sub.3 SO.sub.3), while CA.sub.2 and CA.sub.6 designate, respectively, calcium dialuminate (CaO, 2Al.sub.2 O.sub.3) and calcium hexa-aluminate (CaO, 6Al.sub.2 0.sub.3).
The proportion of calcium sulfate with respect to the amount of filler material is defined by the ratio "Al.sub.2 O.sub.3 /CaO" which may vary from 1.20 to 12.
On the other hand, the fibres serve to reinforce the structure of the material and to avoid cracking; the proportion of fibres used can vary between 2 and 15%.
The aluminous filler material may be alumina or bauxite, or a mixture thereof. Preferably the ratio of Al.sub.2 O.sub.3 /CaO contained in the novel plaster is between 1.2 and 12 (by weight), and more preferably between about 2.5 and 5.
The calcium sulphate utilized in accordance with the present invention is preferably a calcium sulphate of a fine granulometric grade such that on sieving, using 100 micron meshes, the reject does not exceed a proportion of 25%; for example, a semi-hydrated C calcium sulphate or a B calcium sulphate may be used, i.e. a product the granulometric characteristics of which as determined by a method using a laser, correspond to those indicated in Table A herein below:
TABLE A ______________________________________ Passing through 100.mu. Laser granulometry mesh 2.mu. 4.mu. 8.mu. 16.mu. 32.mu. 64.mu. sieve ______________________________________ C 6 14 24 36 53 77 97 B 9 20 32 45 59 80 98 ______________________________________
Said calcium sulphate may also still subsist in the form of phosphogypsum, i.e. in the form of a plaster obtained from phosphogypsum, which is a by-product well-known in the phosphoric acid-producing industry.
The aluminous filler according to the invention can be constituted by any aluminous material having, after calcination of the material considered, an alumina content of more than 80% and preferably at least 85%, this alumina component being more particularly alumina A and/or white bauxite re-crushed to 6000 cm.sup.2 /g Blaine Specific Surface, the reject, when sieving through a 100 micron mesh sieve being less than 10%.
The fibres used in a proportion comprised between 2 and 25% and preferably 2 and 15% by weight of the mixture, and most preferably used in a proportion comprised between 4 and 6% are silicon-containing mineral fibres, silicon-and aluminum containing fibres which are currently used in the technical fields of building and refractory materials. Said fibres may also be constituted by silicon and calcium containing fibres.
Examples of convenient fibres are the fibres marketed under the commerical designations FIBRAL [by the firm named Societe d'Etude de Produits Refractaires (SEPR)], SAFFIL (ICI), KERLANE (SEPR), SEM-FIL (of FIBER GLASS), STRATIFIL (of Saint-Gobain),
Preferably a plaster according to the present invention is prepared by incorporating the various constituents of which it is composed:
-directly in the form of flakes, during mixing the plaster,
-or after de-carding or unballasting of the dry mixture.
The invention will be described in a more detailed manner herein-below with reference to the following Examples which are given by way of illustration, but not of limitation.