Product having a high alumina content

Abstract

A sintered refractory product having the form of a block and consisting of a granulate formed by all the grains having a size larger than 100 m, referred to as coarse grains, and a matrix binding the coarse grains and consisting of the grains having a size smaller than or equal to 100 m, the granulate representing between 45% and 90% by mass of the product, the product having a composition such that, in a mass percentage based on the oxides: Al.sub.2O.sub.3>80%, SiO.sub.2<15%, Na.sub.2O<0.15%, Fe.sub.2O.sub.3<0.05%, CaO<0.1%, the other oxides forming the remainder up to 100%, and the Na.sub.2O content in the matrix being greater than 0.010%, in a mass percentage based on the mass of the product.

Claims

1. A sintered refractory product having the shape of a block and consisting of: grains having a size of greater than 100 m, referred to as coarse grains, the coarse grains forming an aggregate, and a matrix bonding said coarse grains and consisting of grains having a size of less than or equal to 100 m, the aggregate representing between 45% and 90% by weight of the product, said product having a composition such that, as a weight percentage on the basis of the oxides: Al.sub.2O.sub.3>80%, SiO.sub.2<15%, Na.sub.2O<0.15% Fe.sub.2O.sub.3<0.05%, CaO<0.1%, the other oxides constituting the balance to 100%, the content of Na.sub.2O in the matrix being greater than 0.010%, as a weight percentage on the basis of the weight of the product, the product having crystalline phases.

2. The product as claimed claim 1, wherein: Na.sub.2O+K.sub.2O<0.20%.

3. The product as claimed in claim 1, wherein: Al.sub.2O.sub.3>85% and/or 3%<SiO.sub.2<10% and/or Na.sub.2O+K.sub.2O<0.15% and/or Fe.sub.2O.sub.3<0.03% and/or CaO<0.05% and/or ZrO.sub.2<5% and/or TiO.sub.2<5%.

4. The product as claimed claim 1, wherein: Al.sub.2O.sub.3>90% and/or SiO.sub.2>7% and/or Na.sub.2O<0.12% and/or ZrO.sub.2<1% and/or TiO.sub.2<1%.

5. The product as claimed in claim 1, wherein the content of Na.sub.2O in the aggregate is less than 0.070%, as a weight percentage on the basis of the weight of the aggregate.

6. The product as claimed in claim 1, wherein a total amount of alumina and of mullite and/or spinel and/or mullite-zirconia is greater than 95%, as a weight percentage on the basis of the weight of the crystalline phases.

7. The product as claimed in claim 1, wherein the amount of mullite is greater than 10% and less than 50%, as a weight percentage on the basis of the weight of the crystalline phases.

8. The product as claimed in claim 1, having an open porosity of greater than 10% and less than 30% by volume.

9. The product as claimed in claim 1, having an open porosity of less than 20% by volume.

10. The product as claimed in claim 1, having a pore size distribution, measured using a mercury porosimeter, such that: P.sub.50 is greater than 1 m and less than 20 m, and/or P.sub.80 is greater than 10 m and less than 50 m, and/or P.sub.90 is greater than 30 m and less than 70 m.

11. The product as claimed in claim 1, wherein more than 90% by weight of the aggregate consists of coarse mullite grains, coarse alumina grains, coarse spinel grains, coarse mullite-zirconia grains, or any combination thereof.

12. The product as claimed in claim 1, wherein more than 90% by weight of the matrix consists of alumina.

13. The product as claimed in claim 1, wherein the grains of the matrix having a size greater than 10 m represent more than 10% and less than 30% of the weight of the product and/or the grains of the matrix having a size less than or equal to 10 m represent more than 10% and less than 30% of the weight of the product.

14. A device selected from the group formed by a gas turbine, a chemical reactor, a firing support and a glass furnace superstructure, said device comprising one wall at least partly formed by a product according to claim 1.

15. The device as claimed in claim 14, said chemical reactor being a secondary reforming reactor for the manufacture of ammonia or methane, and the firing support being a firing support for ceramic parts.

Description

DETAILED DESCRIPTION

(1) Product

(2) The product may in particular be in the form of a block, and in particular in the form of a plate or a tile. Preferably, the thickness of the block is less than 60 mm, less than 50 mm, less than 40 mm, or even less than 30 mm and/or greater than 10 mm, preferably greater than 15 mm. Preferably, the length and/or the width of the block is greater than 10 cm, greater than 20 cm and/or less than 50 cm, preferably less than 40 cm.

(3) The block may also be in the form of a brick, and in particular a substantially parallelepipedal brick. The length of the brick may be greater than 15 cm, or even greater than 20 cm and/or less than 35 cm, less than 30 cm, or even less than 25 cm. The width and/or the thickness of the brick is preferably greater than 4 cm, greater than 5 and/or less than 10 cm, or even less than 8 cm.

(4) Composition

(5) Preferably, the oxides represent more than 90%, more than 95%, more than 99%, or even substantially 100% of the weight of the product.

(6) SiO.sub.2, Na.sub.2O, K.sub.2O, Fe.sub.2O.sub.3, and CaO are optional oxides.

(7) Preferably, 80%<Al.sub.2O.sub.3<95%, 2%<SiO.sub.2<15%, 0.01%<Fe.sub.2O.sub.3<0.05%, 0.05%<Na.sub.2O+K.sub.2O<0.20%, with 0.03%<Na.sub.2O<0.15%, CaO<0.1%,
the other oxides constituting the balance to 100%.

(8) Preferably, Al.sub.2O.sub.3>85% and/or SiO.sub.2<10% and/or Na.sub.2O+K.sub.2O<0.15% and/or Fe.sub.2O.sub.3<0.03% and/or CaO<0.05% and/or Na.sub.2O<0.15% and/or ZrO.sub.2<5% and/or TiO.sub.2<5%.

(9) Preferably, SiO.sub.2>3% and/or Na.sub.2O<0.12% and/or ZrO.sub.2<1% and/or TiO.sub.2<1%.

(10) In one embodiment Al.sub.2O.sub.3>90%.

(11) Preferably, SiO.sub.2>3%, preferably SiO.sub.2>5%, preferably SiO.sub.2>7%.

(12) Preferably, less than 20%, less than 10%, less than 5%, or even less than 1% of the silica is free, as a weight percentage on the basis of the silica. Preferably the non-free silica is present, for more than 90% by weight, or even substantially 100% by weight, in the form of a refractory alumina silicate, more particularly in the form of mullite.

(13) Preferably, the other oxides are less reducible than Al.sub.2O.sub.3. They are preferably selected from ZrO.sub.2, TiO.sub.2, MgO, and rare earth elements such as Y.sub.2O.sub.3, CeO.sub.2 and HfO.sub.2.

(14) Preferably, MgO<1%, preferably MgO<0.5%, preferably MgO<0.2%, preferably MgO<0.1%, preferably MgO<0.05%, preferably MgO<0.01%.

(15) Preferably, ZrO.sub.2<0.01%, or even ZrO.sub.2<0.005%.

(16) Preferably, TiO.sub.2<0.01%, preferably TiO.sub.2<0.004%.

(17) Microstructure

(18) A microprobe analysis of a product according to the invention shows that the matrix is less rich in sodium than a product having an overall Na.sub.2O content of greater than 0.15%. Without being able to explain it by a theory, it appears that the sodium diffuses during the firing or the sintering from the tabular alumina grains to the matrix, without interacting with the mullite.

(19) Surprisingly, the inventors have however discovered that the presence of a minimum content of Na.sub.2O specifically in the matrix was advantageous for the flexural strength. Preferably, the total amount of alumina on the one hand and of mullite and/or spinel and/or mullite-zirconia on the other hand, preferably the total amount of alumina and of mullite, is greater than 95%, preferably greater than 99%, or even substantially 100%, on the basis of the total weight of the sintered refractory product.

(20) Preferably, the amount of mullite is greater than 10%, greater than 20%, greater than 30% and/or less than 50%, less than 40%.

(21) The presence of mullite is particularly advantageous for an application in a gas turbine. Preferably, the mullite is an electrocast mullite.

(22) Preferably, the amount of alumina (Al.sub.2O.sub.3 phase) is greater than 50%, greater than 55%, greater than 60% and/or less than 80%, less than 70%.

(23) The open porosity is preferably less than 20% by volume, and/or preferably greater than 10% by volume, or even greater than 15% by volume. The resistance to thermal shocks and to thermal cycling is thereby improved.

(24) Advantageously, the pore size distribution, measured using a mercury porosimeter, is such that: P.sub.50 is greater than 1 m, greater than 5 m, greater than 10 m, and/or less than 20 m, less than 15 m, preferably between 10 and 20 m, and/or P.sub.80 is greater than 10 m, greater than 20 m, greater than 25 m, and/or less than 50 m, less than 40 m, or even less than 35 m, and/or P.sub.90 is greater than 30 m, greater than 40 m, greater than 45 m, and/or less than 70 m, less than 60 m, or even less than 55 m.

(25) Aggregate

(26) The presence of an amount of aggregate of greater than 45% advantageously improves the thermal shock resistance.

(27) The aggregate represents preferably more than 50%, more than 55% and/or less than 85%, less than 80%, less than 70%, less than 60% of the weight of the product.

(28) Composition

(29) Preferably, the aggregate consists, for more than 90%, more than 95%, or even substantially 100% by weight, of coarse grains of mullite (72-80% Al.sub.2O.sub.3/20-28% SiO.sub.2 approximately) and/or of coarse grains of virtually pure alumina (>99.5% Al.sub.2O.sub.3), in particular of tabular alumina, and/or of coarse grains of spinel and/or of coarse grains of mullite-zirconia.

(30) Preferably, more than 80%, more than 90%, more than 95%, or even substantially 100% of the mullite and/or of the spinel (MgAl.sub.2O.sub.4) and/or of the mullite-zirconia (37% ZrO.sub.2; 17% SiO.sub.2; 46% Al.sub.2O.sub.3 as weight percentages) of the product is present in the aggregate.

(31) The content of Na.sub.2O+K.sub.2O, preferably the content of Na.sub.2O, in the aggregate is preferably less than 0.14%, preferably less than 0.12%, preferably less than or equal to 0.11%, or even less than 0.09%, as a weight percentage on the basis of the weight of the product.

(32) The content of Na.sub.2O+K.sub.2O, preferably the content of Na.sub.2O, in the aggregate, preferably the content in the alumina aggregate, in particular tabular alumina aggregate, is preferably less than 0.070%, preferably less than 0.050%, preferably less than 0.040%, preferably less than 0.030%, as a weight percentage on the basis of the weight of the aggregate. Advantageously, a very pure aggregate gives improved performances.

(33) Preferably, more than 90%, more than 95%, or even substantially 100% by weight of the SiO.sub.2 of the product is in the aggregate, preferably in coarse mullite grains. This implies in particular that little or no clay is used in the feedstock intended for the manufacture of the product according to the invention.

(34) Particle Size Distribution

(35) Preferably, the percentile D.sub.90 of the aggregate is less than 5 mm, preferably less than 3 mm, preferably less than 2 mm, preferably less than 1 mm and/or greater than 0.2 mm, preferably greater than 0.3 mm, preferably greater than 0.4 mm.

(36) The median size D.sub.50 of the aggregate is preferably less than 2 mm, preferably less than 1 mm and/or greater than 0.2 mm, preferably greater than 0.3 mm.

(37) Matrix

(38) Composition

(39) The matrix preferably consists, for more than 90%, more than 95%, or even substantially 100% by weight, of alumina, preferably of tabular and/or calcined alumina. Preferably, the matrix comprises substantially no silica.

(40) The content of Na.sub.2O in the matrix is preferably greater than 0.015%, preferably greater than 0.020%, and/or preferably less than 0.14%, preferably less than 0.1%, preferably less than 0.09%, preferably less than 0.08%, as a weight percentage on the basis of the weight of the product. Such an Na.sub.2O content range makes it possible to obtain a product having satisfactory thermomechanical properties, under economic sintering conditions (atmospheric pressure, reduced temperature, typically at a temperature of less than 1700 C., or even less than or equal to 1650 C., in an oxidizing atmosphere, preferably in air, in a batch or continuous furnace).

(41) Particle Size Distribution

(42) The grains having a size of less than or equal to 100 m and greater than 10 m represent preferably more than 10%, more than 15% and/or less than 25%, less than 20% of the weight of the product.

(43) The grains having a size of less than or equal to 10 m represent preferably more than 10%, more than 15% or even more than 20% and/or less than 30%, less than 27% of the weight of the product.

(44) Manufacturing Process

(45) Steps a) to e) are steps conventionally used for manufacturing sintered products.

(46) In step a), the feedstock contains a particulate mixture of oxides consisting of ultrafine particles, fine particles and coarse particles.

(47) The way of determining the amounts of the oxides in the feedstock as a function of their contents in the product to be manufactured is fully known to a person skilled in the art. In particular, a person skilled in the art knows that the refractory oxides present in the feedstock are found in the sintered refractory product. For one and the same refractory product, the composition of the feedstock may however vary, especially as a function of the amounts and of the nature of the additives present in the feedstock.

(48) Preferably, the feedstock comprises, as a percentage by weight on the basis of the oxides of the feedstock: more than 10%, preferably more than 20% and/or less than 35%, less than 30%, or even less than 25% of calcined alumina and/or of reactive alumina, and/or more than 30%, more than 35%, more than 40% and/or less than 55%, less than 50%, or even less than 45% of tabular alumina.

(49) The presence of calcined and/or reactive alumina advantageously makes it possible to limit the sintering temperature.

(50) The feedstock comprises preferably more than 1%, preferably more than 1.5%, as percentages by weight on the basis of the oxides, of tabular alumina particles having a size of less than 2 mm.

(51) Preferably, the feedstock contains less than 1% by weight of MgO, on the basis of the oxides, more preferably contains no MgO, except in the form of impurities, at contents of less than 0.5%, preferably less than 0.2%. The process is thereby simplified.

(52) Coarse Fraction

(53) The coarse particles of tabular alumina, or even all of the coarse particles, have a weight content of Na.sub.2O+K.sub.2O, preferably of Na.sub.2O, of less than 1500 ppm, preferably less than 1000 ppm and preferably less than 500 ppm, as percentages by weight on the basis of the oxides of the feedstock. Without being bound by this theory, the inventors consider that this low content prevents a reaction which would lead to a prejudicial phase.

(54) The average weight content of Fe.sub.2O.sub.3 of the coarse particles is preferably less than 500 ppm, or even less than 300 ppm.

(55) The coarse fraction preferably comprises, as percentages by weight on the basis of the oxides of the feedstock: more than 10%, more than 15%, more than 20%, and/or less than 45%, less than 35%, less than 30%, or even less than 25% of alumina particles, preferably of tabular alumina particles, and/or more than 1%, preferably more than 5%, more than 15%, more than 25%, more than 30% and/or less than 45%, less than 40% of mullite particles.

(56) The coarse particles preferably have a size of less than 2 mm.

(57) Fine Fraction

(58) The fine fraction preferably comprises, as percentages by weight on the basis of the oxides of the feedstock, more than 1%, more than 5%, more than 10%, more than 15%, and/or less than 25%, less than 20% of tabular alumina particles, said particles preferably having a size of greater than 10 m and less than 100 m.

(59) The ultrafine fraction preferably comprises, as percentages by weight on the basis of the oxides of the feedstock, more than 5%, preferably more than 10%, more than 15%, more than 20% and/or less than 30%, less than 25% of calcined or reactive alumina particles.

(60) Preferably, all of the calcined alumina particles and/or reactive alumina particles have a median size D.sub.50 of less than 10 m, preferably less than 5 m and/or greater than 1 micron, preferably greater than 2 m.

(61) Additives

(62) The feedstock may also contain one or more additives, optionally in particulate form, conventionally used for giving the feedstock a sufficient plasticity during the shaping step b) and for imparting a sufficient mechanical strength to the preform. The amounts of additives are not limiting. In particular, the amounts conventionally used in the known sintering processes are suitable. The additives are however selected so that their compositions, and in particular their contents of alkali metals and alkaline-earth metals, are compatible with the manufacture of a refractory product in accordance with the invention.

(63) Certain oxides may be introduced by the additives. As examples of additives that can be used, mention may be made, nonlimitingly, of: temporary (i.e. completely or partly eliminated during the drying and firing steps) organic binders, such as resins, derivatives of cellulose or of lignin, such as carboxymethyl cellulose and dextrin, and polyvinyl alcohols, etc. Preferably, the amount of temporary binder is between 0.1% and 6% by weight relative to the weight of the particulate mixture of oxides of the feedstock; chemical binders, such as phosphoric acid or aluminum monophosphate; hydraulic binders, such as aluminous cements, for instance the SECAR 71 cement, or cement of CaO aluminate type; deflocculants, such as alkali metal polyphosphates or methacrylate derivatives; sintering promoters such as titanium dioxide (in a proportion that does not exceed 2% approximately of the weight of the feedstock) or magnesium hydroxide; shaping agents, such as magnesium stearate or calcium stearate; additions of natural silico-aluminate type, for example clays, or of synthetic silico-aluminates. These additions, in particular the natural clays, may introduce alumina, silica and some alkali metal or alkaline-earth metal oxides, or even iron oxide.

(64) In the feedstock, water is also conventionally added. The amount of water added, on the basis of the particulate mixture of the oxides, is preferably less than 5%, less than 4%, or even less than 3%.

(65) The mixing of the various constituents of the feedstock is continued until a substantially homogeneous feedstock is obtained.

(66) In step b), the feedstock is placed in a mold, then compacted, preferably by vibration and/or pressing and/or tamping, so as to form a preform.

(67) In the case of shaping by pressing, a specific pressure of 400 to 800 kg/cm.sup.2 is suitable for obtaining a non-plastic paste. The pressing is preferably carried out uniaxially or isostatically, for example by means of a hydraulic press. It may advantageously be preceded by a manual or pneumatic ramming operation and/or a vibrating operation.

(68) In step d), the drying is preferably carried out in air or in a humidity-controlled atmosphere, preferably at a temperature between ambient temperature and 200 C. It preferably lasts until the residual moisture of the preform is less than 0.5%, conventionally between 10 hours and a week depending on the format of the preform.

(69) In step e), the dried preform is fired so as to be sintered. The sintering operation is well known to a person skilled in the art. The sintering corresponds to the thermal consolidation of the material. It is generally accompanied by a reduction in the porosity and by a dimensional shrinkage.

(70) The sintering temperature depends on the composition of the feedstock. A temperature between 1300 C. and 1800 C. is preferred. The sintering is preferably carried out in an oxidizing atmosphere, more preferably in air. It is preferably carried out at atmospheric pressure. The duration of the firing, preferably between 1 and 15 days approximately from cold to cold, varies as a function of the materials but also of the size and of the shape of the refractory products to be manufactured.

(71) Step e) converts the preform into a refractory product according to the invention, particularly useful as a refractory tile of a combustion chamber of an industrial plant. Unlike products that are sintered in situ, that is to say sintered after having been placed in their service position, for example after having been sprayed onto a wall to be protected, a product according to the invention is preferably sintered in a kiln, so that each of its faces is substantially heated in the same manner, before it is placed in its service position. The temperature gradient is thus prevented from being dependent on the point under consideration on the outer surface of the block. Unlike products that are sintered in situ, the product according to the invention thus has a density and a microstructure that are homogeneous throughout the product, which makes it possible, in service, to limit the local thermomechanical stresses and the points of corrosion, erosion or recession, and therefore to increase the service life of the product.

(72) Applications

(73) The side wall of a gas turbine conventionally comprises a plurality of blocks, preferably in the form of tiles, which are assembled.

(74) In order to form this side wall, a product according to the invention may be used directly or be assembled by means of appropriate expansion joints, according to techniques well known to a person skilled in the art.

Examples

(75) The following examples are provided for illustrative purposes and do not limit the invention. They were manufactured according to steps a) to e) described above. More specifically, the raw materials were mixed in an Eirich-type mixer. The feedstock thus obtained was pressed uniaxially in order to obtain preforms with dimensions of 21020040 mm.sup.3, the relative dry density of which was greater than 95%. The sintering was carried out at 1630 C. in air.

(76) For examples 2 to 4, the aggregate consists of tabular alumina (T60/T64 grades supplied by Almatis, which have an Na.sub.2O content of between 1800 and 3000 ppm, i.e. between 0.18% and 0.3% by weight) and electrocast mullite 10F supplied by Washington Mills. In example 1, the grades of tabular alumina aggregate supplied by Almatis were replaced by the corresponding C99LS grades supplied by Aluchem Inc. which have an Na.sub.2O content of between 350 and 520 ppm, i.e. between 0.035% and 0.052% by weight.

(77) Comparative example 3 differs from example 2 in that it comprises a matrix with a higher loading of Na.sub.2O via the use of a sodium solution (soda) in the mixture before shaping.

(78) Comparative example 4 differs from example 2 in that it comprises a source of very pure fine alumina (PFR40/P122B) supplied by RTA (Rio Tinto Alcan) containing in particular less Na.sub.2O than the source of fine alumina from example 2 (A10/A15SG). Comparative example 5 differs from example 1 in that it has a very pure matrix due to the fact that it uses the same source of fine alumina as that used in comparative example 4.

(79) For the examples, the aggregate represents 57% of the weight of the product, the balance to 100% consisting, by definition, of the matrix.

(80) The following table indicates the raw materials, the characteristics of the products obtained and the results obtained.

(81) TABLE-US-00001 TABLE 1 Examples 1 2* 3* 4* 5* Raw materials that introduce the oxides Mixture of tabular alumina particles: median size 350 m; 42 42 42 maximum size 1.5 mm from T60/64 grades supplied by Almatis Mixture of tabular alumina particles of median size 42 42 350 microns, maximum size 1.5 mm from C99LS grades supplied by Aluchem Inc. Mixture of calcined alumina particles (D.sub.50 = 3.5 microns) from 23 23 23 A10/15SG grades supplied by Almatis Mixture of calcined alumina particles (D.sub.50 = 3.5 microns) from 23 23 PFR40/P122B grades supplied by Rio Tinto Alcan Electrocast mullite 10 F grade from Washington Mills 35 35 35 35 35 Total mineral particles 100 100 100 100 100 Raw materials that introduce the oxides Organic and/or mineral binders +1.5 +1.5 +1.5 +1.5 +1.5 Sodium solution 1.5 g/liter 0 0 +2.6 0 0 Water +2.6 +2.6 +2.6 +2.6 Chemical composition of the product Al.sub.2O.sub.3 + SiO.sub.2 (%) >99.8 >99.7 NM >99.7 >99.8 Na.sub.2O (%) 0.10 0.17 0.68 0.16 0.085 K.sub.2O (%) 0.004 0.004 NM 0.004 0.004 Fe.sub.2O.sub.3 (%) 0.016 0.023 NM 0.017 0.007 CaO (%) 0.03 0.03 NM <0.03 0.01 MgO (%) 0.008 0.008 NM 0.007 0.003 ZrO.sub.2 (%) 0.006 0.004 NM 0.006 0.005 Chemical composition of the matrix as a weight percentage of the product Na.sub.2O (%) 0.02 0.03 0.54 0.02 0.009 Product properties Wet green density (g/cm.sup.3) 2.99 2.99 2.98 2.98 2.98 Moisture (% water) 2.5 2.5 2.5 NM 2.5 Bulk density (g/cm.sup.3) 2.93 2.92 2.96 2.90 2.87 Open porosity (%) 17.5 18.0 16.74 18.7 19.6 P.sub.50 pores (Hg porosimetry) - microns 13.5 24.1 NM NM NM P.sub.80 pores in microns (Hg porosimetry) - microns 30 60 NM NM NM P.sub.90 pores in microns (Hg porosimetry) - microns 50 100 NM NM NM MOR measured at 20 C. (MPa) 10.5 9.9 11.7 11.0 7.5 MOR (MPa) measured at 20 C. after three thermal shocks 4.3 3.1 2.8 3.5 3.3 at 1200 C. MOR loss after three thermal shocks at 1200 C. (%) 59 69 76 68 56 MOR measured at 1200 C. (MPa) 8.7 8.5 11.2 NM 6.2 MOR measured at 1400 C. (MPa) 8.0 6.5 4.5 NM 5.1 Steam corrosion test 21 26 NM NM NM MOR loss before/after corrosion (%) *examples outside of the invention; NM: not measured

(82) The density and the open porosity are measured according to the ISO 5017 standard on a test specimen with dimensions of 202080 mm.sup.3 taken from the core of the block.

(83) The weight contents of oxide Na.sub.2O, CaO, K.sub.2O, MgO, ZrO.sub.2, Fe.sub.2O.sub.3 and other minor oxides are measured by inductively coupled plasma (ICP) emission spectrometry, the SiO.sub.2 content and the Al.sub.2O.sub.3 content are determined by X-ray fluorescence (XRF) spectroscopy.

(84) The modulus of rupture at 20 C. (MOR 20 C.) is measured in air on a test specimen with dimensions of 802020 mm.sup.3. The 3-point bending fixture is assembled with a distance of 60 mm between the two lower supports and the descent rate of the punch is equal to 0.5 mm/min. The value is an average resulting from three successive measurements.

(85) The modulus of rupture at 1200 C. or at 1400 C. (MOR 1200 C. and MOR 1400 C.) is measured in air on a test specimen with dimensions of 802020 mm.sup.3. The 3-point bending fixture is assembled with a distance of 70 mm between the two lower supports and the descent rate of the punch is equal to 0.5 mm/min. The value is an average resulting from three successive measurements.

(86) The relative loss of flexural strength (MOR loss) was measured according to the standardized PRE III.26/PRE/R.1.78 test. This test makes it possible to determine the thermal shock behavior by the relative loss of flexural strength (MOR) after one or more cycles that consist in heating the test specimen from ambient temperature to a temperature of 1200 C., in keeping the test specimen at this temperature for 30 minutes, then in submerging the test specimen in a cold water quench tank. The MOR values before and after the thermal shocks are measured according to the protocol described above. The relative loss of flexural strength is the ratio of the difference between these MOR values to the MOR value before application of the thermal shocks.

(87) The steam oxidation test makes it possible to internally evaluate the behavior of the products under gas turbine application conditions. The oxidation test is carried out at 1400 C. for 500 hours in steam with a constant throughput of 32 kg/m.sup.3/h. the corrosion resistance is obtained by measuring the relative loss of flexural strength between a sound sample and a sample that has undergone the oxidation test. Example 1 according to the invention has an MOR at 20 C. of greater than 10 MPa and provides the best results for the residual MOR after three thermal shocks and the MOR at 1400 C. while maintaining a good corrosion resistance. In this example, the content of Na.sub.2O in the aggregate is less than 0.09%, as a weight percentage on the basis of the weight of the product, and the content of Na.sub.2O in the tabular alumina aggregate is less than 0.05% as a weight percentage on the basis of the weight of the aggregate. The comparison of example 1 on the one hand and examples 2 and 3 on the other hand shows that it is preferable to reduce the Na.sub.2O content.

(88) Example 4 with a matrix having a very low Na.sub.2O content has a residual MOR after three thermal shocks that is significantly lower than that of the product from example 1 according to the invention which has a purer aggregate. The comparison of examples 1 and 4 thus shows that it is preferable to purify the aggregate rather than the matrix.

(89) Comparative example 5 has a matrix which comprises less than 0.010% of Na.sub.2O, unlike example 1. This results in a higher open porosity and a lower MOR at 20 C. The MOR at 20 C. and the residual MOR after three thermal shocks are too low for the targeted applications.

(90) As is now clearly apparent, the invention provides a product that is perfectly well suited to the targeted applications, and in particular to a gas turbine.

(91) The invention is not however limited to this application.