Zircon-based sintered concrete

Abstract

A sintered concrete having the following mean chemical composition, as mass percentages on the basis of the oxides and for a total of 100%; ZrO.sub.2: 55 to 70%, SiO.sub.2: 25 to 40%, P.sub.2O.sub.5: 0.2 to 9.0%, Al.sub.2O.sub.3: 0.5 to 7.0%, CaO: >0.2%, CaO+MgO+B.sub.2O.sub.3+Fe.sub.2O.sub.3: 0.2 to 10.0%, MgO+B.sub.2O.sub.3+Fe.sub.2O.sub.3; 7.5%, B.sub.2O.sub.3+MgO: 4.5%, ZrO.sub.2+SiO.sub.2+P.sub.2O.sub.5+Al.sub.2O.sub.3+CaO+MgO+B.sub.2O.sub.3+Fe.sub.2O.sub.3: 95.0%, and containing more than 70% of zircon, as a mass percentage on the basis of the mass of the crystalline phases.

Claims

1. A sintered concrete having the following chemical composition, as mass percentages on the basis of the oxides and for a total of 100%: ZrO.sub.2: 55 to 70%, SiO.sub.2: 25 to 40%, P.sub.2O.sub.5: 0.2 to 9.0%, Al.sub.2O.sub.3: 0.5 to 7.0%, CaO: >0.2%, CaO+MgO+B.sub.2O.sub.3+Fe.sub.2O.sub.3: 0.2 to 10.0%, MgO+B.sub.2O.sub.3+Fe.sub.2O.sub.3: 7.5%, B.sub.2O.sub.3+MgO: 4.5%, ZrO.sub.2+SiO.sub.2+P.sub.2O.sub.5+Al.sub.2O.sub.3+CaO+MgO+B.sub.2O.sub.3+Fe.sub.2O.sub.3: 95.0%, said sintered concrete having crystalline phases and containing more than 70% of zircon, as a mass percentage on the basis of the mass of the crystalline phases, the oxides representing more than 98.5% of the mass of the sintered concrete.

2. The sintered concrete as claimed in claim 1, said composition being such that: the content of ZrO.sub.2 is greater than 57% and less than 67%; and/or the content of SiO.sub.2 is greater than 26% and less than 37%; and/or the content of P.sub.2O.sub.5 is greater than 0.3% and less than 8.5%; and/or the sum of the contents of CaO, MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is greater than 0.3% and less than 9.5%; and/or the content of CaO is greater than 0.3% and less than 7.9%; and/or the sum of the contents of MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is less than 7.0%; and/or the sum of the contents of MgO and B.sub.2O.sub.3 is less than 4%; and/or the sum of the contents of oxides other than ZrO.sub.2, SiO.sub.2, P.sub.2O.sub.5, Al.sub.2O.sub.3, CaO, MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is less than 4.5%.

3. The sintered concrete as claimed in claim 1, said composition being such that: the content of ZrO.sub.2 is greater than 58% and less than 65%; and/or the content of SiO.sub.2 is greater than 27% and less than 35%; and/or the content of P.sub.2O.sub.5 is greater than 0.7% and less than 5.0%; and/or the sum of the contents of CaO, MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is greater than 0.5% and less than 5.9%; and/or the content of CaO is greater than 0.4% and less than 5.0%; and/or the sum of the contents of MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is less than 3.5%; and/or the sum of the contents of MgO and B.sub.2O.sub.3 is less than 2.4%; and/or the sum of the contents of oxides other than ZrO.sub.2, SiO.sub.2, P.sub.2O.sub.5, Al.sub.2O.sub.3, CaO, MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is less than 3.0%.

4. The sintered concrete as claimed in claim 1, in which the content of zircon, as a mass percentage on the basis of the crystalline phases, is greater than 80%.

5. The sintered concrete as claimed in claim 1, which is in the form of a block, all the dimensions of which are greater than 1 mm.

6. The sintered concrete as claimed in claim 1, having the following chemical composition, as mass percentages on the basis of the oxides and for a total of 100%: ZrO.sub.2: 55 to 70%, SiO.sub.2: 25 to 40%, CaO: 0.2 to 3.0%, Al.sub.2O.sub.3: 0.5 to 7.0%, MgO: 0.1 to 3.0%, P.sub.2O.sub.5: 0.3 to 9.0%, other oxides: <5.0%.

7. The sintered concrete as claimed in claim 6, said composition being such that: ZrO.sub.2>59%, and/or SiO.sub.2>27%, and/or CaO>0.4%, and/or Al.sub.2O.sub.3>1.0%, and/or MgO>0.3%, and/or P.sub.2O.sub.5>0.7%.

8. The sintered concrete as claimed in claim 7, said composition being such that: ZrO.sub.2>61%, and/or SiO.sub.2>29%, and/or CaO>0.5%, and/or Al.sub.2O.sub.3>1.5%, and/or MgO>0.4%, and/or P.sub.2O.sub.5>1.2%.

9. The sintered concrete as claimed in claim 1, said composition being such that: ZrO.sub.2<67%, and/or SiO.sub.2<35%, and/or CaO<2.0%, and/or Al.sub.2O.sub.3<6.0%, and/or MgO<2.1%, and/or P.sub.2O.sub.5<5.0%, and/or other oxides <3.5%.

10. The sintered concrete as claimed in claim 9, said composition being such that: ZrO.sub.2<65%, and/or SiO.sub.2<31%, and/or CaO<1.2%, and/or Al.sub.2O.sub.3<4.0%, and/or MgO<0.9%, and/or P.sub.2O.sub.5<3.0%, and/or other oxides <2%.

11. The sintered concrete as claimed in claim 1, in which: the content of P.sub.2O.sub.5 is greater than 0.3% and less than 5.8%, and the content of Fe.sub.2O.sub.3 is greater than 0.1% and less than 6.5%, and the content of MgO is less than 0.5%, and the content of B.sub.2O.sub.3 is less than 0.5%, and the content of CaO is greater than 0.3% and less than 3%, and the sum of the contents of MgO and B.sub.2O.sub.3 is less than 1%, and the sum of the contents of CaO, MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is greater than 0.3% and less than 10%, and the sum of the contents of MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is greater than 0.1% and less than 7.0%.

12. The sintered concrete as claimed in claim 11, in which: the content of P.sub.2O.sub.5 is greater than 0.5% and less than 3.0%, and/or the content of Fe.sub.2O.sub.3 is greater than 0.5% and less than 3.5%, and/or the content of MgO is less than 0.3%, and/or the content of B.sub.2O.sub.3 is less than 0.3%, and/or the content of CaO is greater than 0.4% and less than 2.0%, and/or the sum of the contents of MgO and B.sub.2O.sub.3 is less than 0.6%, and/or the sum of the contents of CaO, MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is greater than 0.7% and less than 5.1%, and/or the sum of the contents of MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is greater than 0.5% and less than 4.4%.

13. The sintered concrete as claimed in claim 1, in which: the content of P.sub.2O.sub.5 is greater than 0.25% and less than 8.3%, and the content of B.sub.2O.sub.3 is greater than 0.05% and less than 4.0%, and the content of MgO is less than 0.5%, and the content of Fe.sub.2O.sub.3 is less than 0.5%, and the content of CaO is greater than 0.3% and less than 3%, and the sum of the contents of CaO, MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is greater than 0.3% and less than 8.0%, and the sum of the contents of MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is greater than 0.05% and less than 5.0%, and the sum of the contents of MgO and B.sub.2O.sub.3 is greater than 0.05% and less than 4.5%.

14. The sintered concrete as claimed in claim 13, in which: the content of P.sub.2O.sub.5 is greater than 0.7% and less than 4.5%, and/or the content of B.sub.2O.sub.3 is greater than 0.2% and less than 2.0%, and/or the content of MgO is less than 0.3%, and/or the content of Fe.sub.2O.sub.3 is less than 0.3%, and/or the content of CaO is greater than 0.4% and less than 1.5%, and/or the sum of the contents of CaO, MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is greater than 0.5% and less than 4.2%, and/or the sum of the contents of MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is greater than 0.2% and less than 3.0%, and/or the sum of the contents of MgO and B.sub.2O.sub.3 is greater than 0.2% and less than 3.0%.

15. The sintered concrete as claimed in claim 1, in which: the content of P.sub.2O.sub.5 is greater than 0.25% and less than 7.0%, and the content of CaO is greater than 0.3% and less than 8.4%, and the content of B.sub.2O.sub.3 is less than 0.5%, and the content of MgO is less than 0.5%, and the content of Fe.sub.2O.sub.3 is less than 0.5%, and the sum of the contents of CaO, MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is greater than 0.3% and less than 9.5%, and the sum of the contents of MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is less than 1.5%, and the sum of the contents of MgO and B.sub.2O.sub.3 is less than 1.0%.

16. The sintered concrete as claimed in claim 15, in which: the content of P.sub.2O.sub.5 is greater than 0.4% and less than 3.5%, and/or the content of CaO is greater than 0.8% and less than 5.0%, and/or the content of B.sub.2O.sub.3 is less than 0.3%, and/or the content of MgO is less than 0.3%, and/or the content of Fe.sub.2O.sub.3 is less than 0.3%, and/or the sum of the contents of CaO, MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is greater than 0.7% and less than 4.5%, and/or the sum of the contents of MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is less than 0.5%, and/or the sum of the contents of MgO and B.sub.2O.sub.3 is less than 0.5%.

17. The sintered concrete as claimed in claim 1, in which: the content of P.sub.2O.sub.5 is greater than 0.25% and less than 6.5%, and the content of Al.sub.2O.sub.3 is greater than 0.8% and less than 6.5%, and the content of B.sub.2O.sub.3 is less than 0.5%, and the content of MgO is less than 0.5%, and the content of Fe.sub.2O.sub.3 is less than 0.5%, and the content of CaO is greater than 0.3% and less than 3%, and the sum of the contents of CaO, MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is greater than 0.2% and less than 4.5%, and the sum of the contents of MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is less than 1.5%, and the sum of the contents of MgO and B.sub.2O.sub.3 is less than 1.0%.

18. The sintered concrete as claimed in claim 17, in which: the content of P.sub.2O.sub.5 is greater than 0.6, and/or the content of Al.sub.2O.sub.3 is greater than 1.5% and less than 6.0%, and/or the content of B.sub.2O.sub.3 is less than 0.3%, and/or the content of MgO is less than 0.3%, and/or the content of Fe.sub.2O.sub.3 is less than 0.3%, and/or the content of CaO is greater than 0.4% and less than 2.0%, and/or the sum of the contents of CaO, MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is greater than 0.3% and less than 3.0%, and/or the sum of the contents of MgO, B.sub.2O.sub.3 and Fe.sub.2O.sub.3 is less than 1.0%, and/or the sum of the contents of MgO and B.sub.2O.sub.3 is less than 0.5%.

19. A process for manufacturing a sintered concrete, said process comprising the following successive steps: a) mixing particulate starting materials to form a feedstock, b) activating said feedstock so as to obtain a fresh concrete, c) forming said fresh concrete, d) hardening said fresh concrete so as to obtain a hardened concrete, e) sintering said hardened concrete so as to obtain said sintered concrete, the feedstock being adapted so that said sintered concrete is in accordance with claim 1.

20. The process as claimed in claim 19, in which the feedstock includes between 1.0% and 6.0% by weight of a hydraulic binder, and more than 0.25% of a phosphate chosen from magnesium phosphates, iron phosphates, boron phosphates, calcium phosphates, aluminum phosphates and mixtures thereof, said phosphate providing more than 50% of the phosphorus of the feedstock.

21. The process as claimed in claim 19, in which the feedstock is such that, as mass percentages on the basis of the feedstock, the mass amount of zircon particles is greater than 75% and less than 95%, and/or the mass amount of free zirconia particles is greater than 1.0% and less than 20.0%, and/or the mass amount of hydraulic binder in the feedstock is greater than 1.5% and less than 5.0%, and/or the mass amount of said phosphate is greater than 0.3% and less than 14.0%.

22. The process as claimed in claim 21, in which the feedstock is such that, as mass percentages on the basis of the feedstock, the mass amount of zircon particles is less than 90%, and/or the mass amount of free zirconia particles is greater than 5.0% and less than 15.0%, and/or the mass amount of hydraulic binder in the feedstock is greater than 2.0% and less than 4.0%, and/or the mass amount of said phosphate is greater than 1.0% and less than 5.0%.

23. The process as claimed in claim 20, in which the hydraulic binder is a cement.

24. The process as claimed in claim 23, in which the cement is an aluminous cement.

25. The process as claimed in claim 20, in which the alumina content of the hydraulic binder is between 50% and 85%.

26. The process as claimed in claim 20, in which the only phosphate present in the feedstock is Mg(PO.sub.3).sub.2.nH.sub.2O or FePO.sub.4.nH.sub.2O or BPO.sub.4.nH.sub.2O or Ca.sub.2P.sub.2O.sub.7.nH.sub.2O or AlPO.sub.4.nH.sub.2O, with n0.

27. A glass production unit, including a component made of a sintered concrete as claimed in any one of claims 1 to 18.

28. A glass production unit including a component made of a sintered concrete as claimed in claim 1, said component being selected from the group consisting of: a plate block, a floor slab, a superstructure component, a channel block of a feed channel, a burner unit, an expandable, a mandrel used in the manufacture of glass tubes according to the Danner process, and an electrode holder block.

29. The glass production unit as claimed in claim 28, in which the expendable is a tube, a plunger, a stirrer, a rotor, an orifice ring, a spout.

30. The process as claimed in claim 21, in which said phosphate is Mg(PO.sub.3).sub.2.

31. The process as claimed in claim 22, in which said phosphate is Mg(PO.sub.3).sub.2.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Other features and advantages of the invention will emerge more clearly on reading the detailed description which follows and on examining the detailed drawing in which FIGS. 1 and 2 are photographs of glass surfaces observed for example 1, outside the invention, and example 3, according to the invention, respectively.

(2) The photographs were taken at the same magnification.

DETAILED DESCRIPTION

(3) Steps similar to those of a conventional process for manufacturing a sintered concrete intended for applications in glass melting furnaces may be performed.

(4) In particular, a concrete according to the invention may be manufactured according to steps a) to c) described above.

(5) In step a), a dry particulate feedstock is prepared as a function of the desired composition and porosity.

(6) Composition

(7) The manner of determining the proportions of the constituents of the feedstock is entirely known to those skilled in the art. In particular, a person skilled in the art knows that the zirconium, silicon, calcium, aluminum, magnesium, iron, boron and phosphorus present in the feedstock are found in the sintered concrete. He also knows how to determine which constituents will be transformed to constitute the matrix.

(8) The feedstock preferably includes more than 0.25%, preferably more than 0.5%, preferably more than 1% of a phosphate chosen from magnesium phosphates, iron phosphates, boron phosphates, calcium phosphates, aluminum phosphates and mixtures thereof, preferably a magnesium phosphate, preferably Mg(H.sub.2PO.sub.4).sub.2, MgHPO.sub.4, Mg.sub.3(PO.sub.4).sub.2, Mg(PO.sub.3).sub.2, Mg.sub.2P.sub.2O.sub.7 and mixtures thereof, said compounds possibly incorporating water molecules, preferably Mg(PO.sub.3).sub.2, said phosphate providing more than 50%, preferably more than 60%, preferably more than 70%, preferably more than 80%, of the phosphorus of the feedstock. Surprisingly, and as demonstrated by the examples below, the inventors have discovered that the combination of magnesium and/or iron and/or aluminum and/or boron and/or calcium with phosphorus in the form of a magnesium phosphate and/or of an iron phosphate and/or of an aluminum phosphate and/or of a boron phosphate and/or of a calcium phosphate substantially improves the possibilities for forming the fresh concrete, notably by vibration, while at the same time limiting the bubbling of the sintered concrete when it is in contact with molten glass.

(9) Preferably, the feedstock is constituted of zircon particles, of hydraulic binder, preferably of a cement, of magnesium phosphate and/or of iron phosphate and/or of aluminum phosphate and/or of boron phosphate and/or of calcium phosphate, of fumed silica, of zirconia and of a forming additive.

(10) Preferably, more than 70%, more than 80% of the zirconium, expressed in the form of zirconia ZrO.sub.2, as a mass percentage, is provided in the form of zircon, the remainder being provided in the form of free zirconia.

(11) Preferably, the feedstock includes not more than 5% of free silica, i.e. silica which is not associated, for instance in zircon. Preferably, the free silica is fumed silica. Preferably, the fumed silica contains more than 93% of silica (SiO.sub.2), the fumed silica powder having a median size of between 0.1 and 0.8 m, preferably between 0.2 and 0.7 m.

(12) The hydraulic binder is a cement, preferably an aluminous cement, preferably a calcium aluminate cement.

(13) Preferably, more than 60%, more than 65%, more than 70% of the aluminum, expressed in the form of alumina Al.sub.2O.sub.3, as a mass percentage, is provided in the form of a hydraulic binder, preferably in the form of an aluminous cement, preferably in the form of a calcium aluminate cement.

(14) In a preferred embodiment, the oxides other than ZrO.sub.2, SiO.sub.2, CaO, Al.sub.2O.sub.3, MgO, Fe.sub.2O.sub.3, B.sub.2O.sub.3 and P.sub.2O.sub.5, preferably the oxides other than ZrO.sub.2, SiO.sub.2, CaO, Al.sub.2O.sub.3, MgO and P.sub.2O.sub.5 are impurities, i.e. inevitable constituents, necessarily introduced with the starting materials. By way of example, mention may be made of Na.sub.2O.

(15) Preferably, the content of Na.sub.2O is less than 0.7%, preferably less than 0.5%, preferably less than 0.3%, preferably less than 0.1%.

(16) The feedstock preferably contains a forming additive, preferably temporarily (i.e. removed during the sintering), preferably in a mass amount of less than 3.0%, preferably less than 2.0%, preferably less than 1.0%, preferably less than 0.5% and preferably greater than 0.05%, preferably greater than 0.1%.

(17) Preferably, the forming additive is chosen from plasticizers, such as polyethylene glycol (PEG) or polyvinyl alcohol (PVA), organic temporary binders such as resins, lignosulfonates, dextrin and alginates, deflocculants, such as alkali metal polyphosphates, alkali metal polyacrylates, polycarboxylates, and mixtures of these products.

(18) In one embodiment, the feedstock includes fibers, preferably organic fibers, preferably of vinyl or polypropylene type, preferably in a mass amount of between 0.01% and 0.1%, preferably in a mass amount of between 0.01% and 0.03%. Preferably, the mean (arithmetic mean) length of these fibers is greater than 6 mm, preferably between 18 and 24 mm. These fibers advantageously facilitate the removal of water during drying.

(19) In another embodiment, the feedstock does not include any fibers.

(20) Particle Size Distribution

(21) Preferably, the maximum size of the feedstock particles is less than or equal to 15 mm, preferably less than or equal to 10 mm, preferably less than or equal to 8 mm, preferably less than or equal to 5 mm, or even less than 4 mm, or even less than 3 mm and preferably greater than 0.4 mm, preferably greater than 1 mm, or even greater than 2 mm.

(22) Preferably, more than 30%, more than 35%, or even more than 45% of the zircon particles are aggregate particles, i.e. particles with a size of between 150 m and 15 mm.

(23) Preferably, more than 45%, more than 50% and less than 70%, less than 65% of the zircon particles are fine particles, i.e. particles with a size of less than 150 m.

(24) Preferably, more than 90%, more than 95%, or even 100% of the particles including more than 15% by mass of CaO and/or of the particles including more than 40% by mass of Al.sub.2O.sub.3 and/or of the particles including more than 15% by mass of MgO and/or of the particles including more than 50% by mass of P.sub.2O.sub.5 and/or of the particles including more than 40% by mass of Fe.sub.2O.sub.3 and/or of the particles including more than 30% by mass of B.sub.2O.sub.3 are fine particles, i.e. particles with a size of less than 150 m.

(25) Preferably, the zirconia particle powder (providing free zirconia) has a median size of less than 10 m, preferably less than 8 m, preferably less than 5 m, and/or greater than 1 m, preferably greater than 2 m.

(26) The Andrasen or Fuller-Bolomey compaction models may be used to adapt the particle size distribution of the feedstock to the desired porosity. Compaction models are notably described in the publication entitled Trait de cramiques et matriaux minraux [Treatise on ceramics and mineral materials]; C. A. Jouenne, Editions Septima. Paris (1984), pages 403 to 405.

(27) In step b), preferably after having sufficiently dry-blended to obtain a homogeneous mix, water is conventionally added to the feedstock. Preferably, at least 3% and less than 8%, preferably less than 7% of water, as mass percentages relative to the mineral mass of the feedstock, are added, besides the water. The amount of water depends on the technique used in step c). For example, for a step c) performed by vibration casting, addition of an amount of water of between 3% and 6%, as mass percentages on the basis of the feedstock, is preferred.

(28) The water is preferably added gradually to the mixer while it is running, until a substantially homogeneous wet mix is obtained. The addition of water brings about activation of the feedstock, i.e. it engages its process of setting to a solid.

(29) In step c), the fresh concrete obtained in step b) may be poured into a mold, in order to be formed, so as to form a raw component.

(30) Preferably, the mold is conformed so that the sintered concrete obtained is in the form of a block with a mass of greater than 5 kg, preferably 10 kg. Such blocks are well suited to the intended applications.

(31) The forming may result from casting, vibro-casting, vibro-compacting, pressing or a combination of these techniques, preferably from casting, vibro-casting or a combination of these techniques.

(32) In step d), the fresh concrete sets to a solid so as to obtain a hardened concrete. The hardened concrete may undergo a drying step, so as to remove part of the water which was used for the forming. Such a step is entirely known to those skilled in the art.

(33) In step e), the hardened concrete is sintered at a temperature preferably between 900 C. and 1600 C., preferably between 1300 C. and 1600 C., preferably in air, preferably at atmospheric pressure. The sintering time is adapted as a function of the dimensions of the hardened concrete to be sintered. The duration of the sintering stage is generally between 1 and 20 hours, preferably between 5 and 10 hours. In applications in which, in its service position, the hardened concrete may be subjected to heating conditions liable to sinter it, the hardened concrete is preferably placed in position without having been sintered, and is then sintered in situ.

(34) On conclusion of step e), a sintered concrete according to the invention is obtained. Preferably, the oxides represent more than 98.5%, more than 99% or even substantially 100% of the mass of a sintered concrete according to the invention.

Examples

(35) The nonlimiting examples that follow are given for the purposes of illustrating the invention. In these examples, the following starting materials used were chosen, the percentages given being mass percentages: a zircon aggregate having the following chemical analysis, as mass percentages: ZrO.sub.2: 66%, SiO.sub.2: 33%, Al.sub.2O.sub.3: 0.3%, P.sub.2O.sub.5: 0.3%, Fe.sub.2O.sub.3: <0.1%, TiO.sub.2: <0.1%, less than 0.2% of other compounds, a size of between 0.5 and 2 mm and a median size (D.sub.50) equal to 0.9 mm, a zircon aggregate having the following mean chemical analysis, as mass percentages: ZrO.sub.2: 66%, SiO.sub.2: 33%, Al.sub.2O.sub.3: 0.3%, P.sub.2O.sub.5: 0.3%, Fe.sub.2O.sub.3:<0.1%, TiO.sub.2: <0.1%, less than 0.2% of other compounds, a size of between 0 and 0.5 mm, and a median size (D.sub.50) equal to 0.35 mm, a zircon sand having the following mean chemical analysis, as mass percentages: ZrO.sub.2: 66.8%, SiO.sub.2: 32.9%, Al.sub.2O.sub.3: 0.52%, P.sub.2O.sub.5: 0.08%, Fe.sub.2O.sub.3: <0.1%, TiO.sub.2<0.1%, less than 0.2% of other compounds and a median size (D.sub.50) equal to 170 m, a zircon flour having the following mean chemical analysis, as mass percentages: ZrO.sub.2: 66.4%, SiO.sub.2: 32.7%, Al.sub.2O.sub.3: 0.16%, P.sub.2O.sub.5: 0.12%, Fe.sub.2O.sub.3: <0.1%, TiO.sub.2: 0.10%, and less than 0.2% of other compounds, and a median size (D.sub.50) equal to 10.9 m, a micronized zircon powder having the following mean chemical analysis, as mass percentages: ZrO.sub.2: 63.6%, SiO.sub.2: 34.1%, Al.sub.2O.sub.3: 0.94%, P.sub.2O.sub.5: 0.13%, Fe.sub.2O.sub.3: 0.06%, TiO.sub.2:

(36) 0.10%, and less than 0.2% of other compounds, and a median size (D.sub.50) equal to 1.4 m, zirconia, sold by the company Socit Europenne des Produits Rfractaires under the name CC10, the median size of which is equal to 3.5 m, and having a mass content of zirconia of greater than 98.5%, fumed silica, the median size of which is equal to 0.5 m, and having a silica content of greater than 93.5%, a magnesium phosphate Mg(PO.sub.3).sub.2 powder, having a median size equal to 14 m, an iron phosphate FePO.sub.4.nH.sub.2O powder, having a median size equal to 8 m, and having a loss on ignition at 1000 C. equal to 20%, a boron phosphate BPO.sub.4 powder, having a median size equal to 8 m, a calcium phosphate Ca.sub.2P.sub.2O.sub.7 powder, having a median size equal to 8 m, a calcium aluminate cement CA25R sold by the company Almatis, having a median size (D.sub.50) equal to 9 m, a sodium polyphosphate powder, a modified polycarboxylate ester.

(37) Sintered concrete blocks were manufactured according to a process in accordance with the invention.

(38) In step a), the starting materials were measured out and mixed so as to form a feedstock. In step b), the feedstock was placed in a mixer and an amount of water as described in table 1 was added. After mixing for a time of 10 minutes, the fresh concrete is obtained.

(39) In step c), the fresh concrete is vibration-cast (50 Hz, 0.3 mm of double amplitude) in a wooden mold.

(40) In step d), after setting, a hardened concrete is obtained, and is removed from the mold.

(41) In step e), the hardened concrete is sintered in the following thermal cycle: raising from ambient temperature to 1560 C. at a rate of 100 C./h, maintenance at 1560 C. for 6 hours, lowering the temperature at a rate equal to 100 C./h down to 500 C., followed by free lowering to ambient temperature.

(42) Table 1 below summarizes, for each example, the composition of the feedstock, the amount of water used in step b) and the possibility of laying the concrete by vibration.

(43) TABLE-US-00001 TABLE 1 Example Example Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Example 9 10 11 Zircon aggregate 28.85 29.15 29.05 28.65 28.7 27.4 26.9 26.1 28.3 28.3 28.3 0.5-2 mm (%) Zircon aggregate 10 9.8 9.7 9.6 9.95 9.5 9.3 9.05 9.8 9.8 9.8 0-0.5 mm (%) Zircon sand (%) 24 23.3 23.3 23.1 23.9 22.8 22.35 21.7 23.5 23.5 23.5 Zircon flour (%) 14 13.7 13.6 13.5 13.9 13.3 13 12.65 13.7 13.7 13.7 Micronized 7 6.8 6.8 6.7 7 6.65 6.5 6.3 6.85 6.85 6.85 zircon (%) Zirconia (%) 10 9.8 9.7 9.6 9.9 9.5 9.3 9.05 9.8 9.8 9.8 Fumed silica (%) 3 2.9 2.9 2.9 3 3 3 3 3 3 3 Cement CA25R 3 2.9 2.9 2.9 3 3 3 3 3 3 3 (%) Mg(PO.sub.3).sub.2 (%) 1.9 2.9 0.5 4.7 6.5 9 FePO.sub.4nH.sub.2O (%) 1.9 BPO.sub.4 (%) 1.9 Ca.sub.2P.sub.2O.sub.7 (%) 1.9 Sodium 1.5 polyphosphate (%) Modified 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 polycarboxylate ether (%) Water (%) 3.7 4.1 4.1 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3 Possibility of yes no yes yes yes yes yes yes yes yes yes laying the fresh concrete by vibration

(44) Table 1 shows that the presence of Mg(PO.sub.3).sub.2 in amounts equal to 1.9% and 2.9% modifies little the behavior of the fresh concrete, which remains vibration-castable. On the other hand, the addition of sodium polyphosphate does not allow vibration casting.

(45) This is likewise the case for the presence of Mg(PO.sub.3).sub.2 in amounts equal to 0.5%, 4.7%, 6.5% and 9%, and also for FePO.sub.4 in an amount equal to 1.9%, BPO.sub.4 in an amount equal to 1.9%, and Ca.sub.2P.sub.2O.sub.7 in an amount equal to 1.9%.

(46) The apparent density and the apparent porosity of the sintered concretes of examples 1 and 3 to 11 were measured according to the standard ISO5017.

(47) The chemical analyses were performed by X-ray fluorescence.

(48) The bubbling behavior on contact with molten glass of the sintered concretes of examples 1 and 3 to 11 were evaluated via the following method:

(49) Crucibles with an outside diameter equal to 50 mm, a total height equal to 40 mm, a concentric hole with an outside diameter having a diameter equal to 30 mm and a base with a thickness equal to 10 mm are machined in the sintered concretes of the test examples.

(50) Each crucible is filled with 30 g of a clear borosilicate glass powder whose median size is equal to 1 mm, whose maximum size is equal to 2 mm, and having the following chemical analysis by mass: SiO.sub.2: 73%, B.sub.2O.sub.3: 10%, Al.sub.2O.sub.3: 5%, Na.sub.2O+K.sub.2O: 7.5%, other oxides: 4.5%. The crucible and the glass as a whole is then placed in an electric oven and undergoes the following heat treatment, in air: rise to 1180 C. at a speed equal to 500 C./h, maintenance at 1180 C. for 72 hours, lowering to 785 C. at a speed equal to 500 C./h, lowering to 640 C. at a speed equal to 20 C./h maintenance at 640 C. for 5 hours, lowering to ambient temperature at a speed equal to 8 C./h.

(51) The ratio of the area of the bubbles generated during the test and of the area of glass observed taken into account may be evaluated via the following nonlimiting method.

(52) After cooling, the resin is poured into the crucible so as to totally fill the crucible. The crucible is then cut so as to obtain a slice with a thickness equal to 7 mm, said slice containing the vertical axis of symmetry of the crucible and having a height equal to that of the crucible.

(53) The slice is then polished so as to make the glass transparent and to facilitate the observations, said polishing being performed at the minimum with a 1200 grade paper, preferably with diamond paste.

(54) Images are then taken using a light microscope, a source of light illuminating the glass opposite the observation (back-lighting). This back-lighting reveals the bubbles contained in the glass. The development, notably the aperture, is performed so that all the bubbles contained in the glass slice appear sharp.

(55) The magnification used is the highest possible magnification allowing observation of the entire surface of the glass of the slice, in a single image.

(56) The image is then analyzed using the imageJ software available on the website http://rsbweb.nih.gov/ij/ according to the following method: open the image in imageJ; delete any previous results via the Analyse>Clear Results function; define the magnitude to be measured, in other words the area, by checking only the Area box in Analyze>Set measurements, and then by confirming with OK; adjust the brightness via the Image>Adjust>Brightness/contrast function, then click on Auto; apply a Gaussian blur with a sigma (or radius) of a value equal to 2.00 using the Process>Filters>Gaussian blur function, then validate by means of the OK button; convert the number of levels of colors/gray into 8 bits via the Image>Type>8-bit function; binarize the image via the Image>Adjust>Threshold>Auto function, with the Dark Background box checked, the scroll menu corresponding to the type of threshold being on Default, the red threshold color being selected by means of the scroll menu on Red, do not check Stack histogram, and click on Apply and then close the window; using the Freehand tool selected by means of the dedicated icon, define by means of the mouse the zone of glass to be analyzed, this zone not containing bubbles in contact with the inner surfaces of the crucible; measure the area of said zone, Z.sub.A, with the Analyse>Measure tool. The area value is displayed in the Area column of a window which opens. Note the value and close the window; delete the part of the image which is outside the zone of glass to be analyzed using the Edit>Clear outside tool, and then deselect the zone of glass to be analyzed previously selected with the Edit>Selection>Select None tool and delete the results with the Analyse>Clear results tool; select the interior of the zone of glass to be analyzed, the zones not to be taken into account, for instance the cracks which may appear during the cooling of the glass. These selections are made using the Freehand tool and its dedicated icon; determine the area Z.sub.i of each of the zones i not to be taken into account, successively, by means of the following sequence of commands: Analyse>Measure and then Analyse>Clear results and then Edit>Clear and then Edit>Selection>Select None. Repeat this sequence i times. Z.sub.B is the sum of the areas Z.sub.i; invert the black and white zones of the image using the Process>Binary>Make Binary tool. The bubbles then appear black on a white background (value 255 for white, 0 for black); certain bubbles may appear in the form of empty circles (white circles with a black central part). For these bubbles, transform the black color of the central part to white using the Process>Binary>Fill holes function; determine the area of the bubbles by means of the following commands: Analyze>Analyze Particles . . . indicating in the Size zone: 0-infinity, in the Circularity zone: 0.00-1.00, in the Show zone: Nothing, and then check only the boxes: Display results, Clear results, In situ Show and click OK; save the results file Results.xls with the File>Save As . . . command; open the results file Results.xls and obtain the sum Z.sub.c of the figures in the Area column representing the area of each bubble of the analyzed zone; calculate the area of glass observed taken into account, equal to the area of glass observed Z.sub.A minus the area Z.sub.B of the excluded zones, Z.sub.AZ.sub.B; calculate the ratio of the area of the bubbles Z.sub.c, and of the area of glass taken into account Z.sub.AZ.sub.B, Z.sub.C/(Z.sub.AZ.sub.B).

(57) This ratio characterizes the bubbling behavior of the sintered concrete in contact with molten glass used in the test.

(58) FIGS. 1 and 2 show the areas of glass observed in example 1 outside the invention and example 3 according to the invention, respectively. The bubbles generated by contact of the sintered concrete with the molten glass appear white.

(59) Table 2 below summarizes the features obtained after sintering.

(60) TABLE-US-00002 TABLE 2 Example 1 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Example 9 Example 10 Example 11 ZrO.sub.2 (%) 65.2 64.0 63.3 64.9 62.3 61.2 59.8 62.1 62.1 62.1 SiO.sub.2 (%) 29.6 29.0 28.7 29.4 28.3 27.8 27.1 28.2 28.2 28.2 CaO (%) 0.71 0.69 0.69 0.71 0.68 0.67 0.65 0.68 0.68 2.78 Al.sub.2O.sub.3 (%) 3.66 3.56 3.55 3.64 3.5 3.44 3.36 3.49 3.49 3.49 MgO (%) 0.05 0.52 0.77 0.17 1.17 1.57 2.11 0.05 0.05 0.05 P.sub.2O.sub.5 (%) 0.08 1.51 2.26 0.45 3.45 4.64 6.27 2.31 3.27 2.74 Fe.sub.2O.sub.3 (%) 2.52 0 0 B.sub.2O.sub.3 (%) 0 1.57 0 Other oxides (%) 0.70.sup.(1) 0.72.sup.(1) 0.73.sup.(1) 0.73.sup.(1) 0.6.sup.(1) 0.69.sup.(1) 0.71.sup.(1) 0.66.sup.(2) 0.66.sup.(2) 0.66.sup.(2) % of zircon, as a mass 87 97 98 93 98 98 98 97 97 97 percentage on the basis of the mass of the crystalline phases Apparent density (g/cm.sup.3) 3.73 3.65 3.64 3.67 3.58 3.58 3.56 3.66 3.58 3.67 Apparent porosity (%) 16.5 17.9 17.5 18.4 18.1 16.4 4.5 19 19.1 17.4 Area of bubbles/area of 27.9 7.1 9.4 8.7 7.6 9.5 8.3 11.8 12.1 8.1 glass observed taken into account (%) .sup.(1)oxides other than ZrO.sub.2, SiO.sub.2, MgO, CaO, Al.sub.2O.sub.3 and P.sub.2O.sub.5 .sup.(2)oxides other than ZrO.sub.2, SiO.sub.2, MgO, CaO, Al.sub.2O.sub.3, P.sub.2O.sub.5, Fe.sub.2O.sub.3 and B.sub.2O.sub.3.

(61) As seen in the results indicated in table 2, after contact at 1180 C. for 72 hours with clear borosilicate glass, the ratio of the area of bubbles and of the area of glass observed taken into account, expressed as a percentage, is smaller for the concretes of examples 3 to 11 according to the invention than for that of the concrete of example 1 outside the invention.

(62) Table 2 shows, in effect, that the concretes of examples 3 and 4 according to the invention have a ratio of the area of bubbles and of the area of glass observed taken into account equal to 7.1% and 9.4%, respectively, which is very much smaller than the ratio of the area of bubbles and of the area of glass observed taken into account for the concrete of example 1 outside the invention, equal to 27.9%.

(63) This is likewise the case for the concretes of examples 5, 6, 7, 8, 9, 10 and 11 according to the invention, which have a ratio of the area of bubbles and of the area of glass observed taken into account equal to 8.7%, 7.6%, 9.5%, 8.3%, 11.8%, 12.1% and 8.1%, respectively, which are very much smaller than the ratio of the area of bubbles and of the area of glass observed taken into account for the concrete of example 1 outside the invention, equal to 27.9%.

(64) The amount of defects generated in the glass articles manufactured using the sintered concretes according to the invention is thus smaller than that for glass articles manufactured using the sintered concrete of example 1. The manufacturing yield for the glass articles is thus increased.

(65) The product of example 3 is the product according to the invention that is preferred amongst all.

(66) Needless to say, the present invention is not limited to the described embodiments, which are given as nonlimiting illustrative examples.

(67) In particular, the concretes according to the invention are not limited to particular shapes or sizes.