Hydraulic binder system based on aluminum oxide

Abstract

The present invention relates to a hydraulic binder system based on calcined aluminum oxide for use in refractory materials. The invention further relates to a process for producing the hydraulic binder system and also to the use thereof.

Claims

1. A hydraulic binder system for use in refractory materials, comprising a) from 90.0 to 99.99% by weight of at least one calcined aluminum oxide A having an average particle size of from 0.3 to 25.0 m and a BET surface area of from 0.5 to 30.0 m.sup.2/g; and b) from 0.01 to 10.0% by weight of at least one component B selected from the group consisting of magnesium hydroxide, calcium hydroxide, strontium hydroxide, barium hydroxide and hydrates thereof, magnesium oxide, calcium oxide, strontium oxide and barium oxide; where the respective proportions by weight are based on the total amount of the hydraulic binder system.

2. The binder as claimed in claim 1, characterized in that from 0.05 to 4.95% by weight of at least one plasticizer C is additionally present.

3. The binder system as claimed in claim 1, characterized in that the at least one calcined aluminum oxide A is a) at least one calcined aluminum oxide A1 having an average particle size of from 1.8 to 8.0 m and a specific BET surface area of from 0.5 to 2.0 m.sup.2/g; or b) at least one calcined aluminum oxide A2 having an average particle size of from 0.3 to 1.7 m and a specific BET surface area of from 2.0 to 10.0 m.sup.2/g.

4. The binder system as claimed in claim 1, characterized in that at least two different calcined aluminum oxides A are present.

5. The binder system as claimed in claim 4, characterized in that a) at least one calcined aluminum oxide A1 having an average particle size of from 1.8 to 8.0 m and a specific BET surface area of from 0.5 to 2.0 m.sup.2/g; and b) at least one calcined aluminum oxide A2 having an average particle size of from 0.3 to 1.7 m and a specific BET surface area of from 2.0 to 10.0 m.sup.2/g are present.

6. The binder system as claimed in claim 1, characterized in that at least one aluminum oxide A is coated.

7. The binder system as claimed in claim 1, characterized in that at least one aluminum oxide A is coated, with the coating agent being phosphoric acid.

8. The binder system as claimed in claim 1, characterized in that the component B has an average particle size of from 1.0 to 20.0 m and a BET surface area of from 0.5 to 50.0 m.sup.2/g.

9. The binder system as claimed in claim 1, characterized in that the at least one component B is a magnesium oxide.

10. The binder system as claimed in claim 1, characterized in that the at least one component B is a calcium hydroxide.

11. The binder system as claimed in claim 1, characterized in that the at least one component B is a strontium hydroxide or hydrate thereof.

12. The binder system as claimed in claim 1, characterized in that the at least one component B is a barium hydroxide or hydrate thereof.

13. The binder system as claimed in claim 2, characterized in that the at least one plasticizer C is trisodium citrate.

14. A process for producing a hydraulic binder system as claimed in claim 1, comprising the step: a) mixing of from 90.0 to 99.99% by weight of at least one calcined aluminum oxide A having an average particle size of from 0.3 to 25.0 m and a BET surface area of from 0.5 to 30.0 m.sup.2/g; with from 0.01 to 10.0% by weight of at least one component B, selected from the group consisting of magnesium hydroxide, calcium hydroxide, strontium hydroxide, barium hydroxide and hydrates thereof, magnesium oxide, calcium oxide, strontium oxide and barium oxide; where the respective proportions by weight are based on the total amount of the hydraulic binder system.

15. The process as claimed in claim 14, characterized in that from 0.05 to 4.95% by weight of at least one plasticizer C is added during or after step a).

16. The process as claimed in claim 14, characterized in that the mixing of the at least one calcined aluminum oxide A with the at least one component B and optionally at least one plasticizer C in step a) is carried out by joint milling.

17. The use method of using a hydraulic binder system as claimed in claim 1 in refractory materials and fine-ceramic materials.

Description

(1) The following examples illustrate the invention.

(2) Unless explicitly indicated otherwise, the particle sizes mentioned are average particle sizes D.sub.50. For the indicated value, 50% of all particles are larger and 50% of all particles are smaller. The determination of the particle sizes was carried out by means of laser granulometry in accordance with ISO 13320.

(3) The determination of the specific surface area was carried out by nitrogen adsorption (BET) in accordance with DIN ISO 9277.

(4) The setting time was determined in accordance with EN 196 (Vicat test), and the determination of the cold compressive strength was carried out in accordance with EN 1402.

(5) A) Production of the Starting Materials

(6) The starting materials AO1 to AO3 were produced from commercial Bayer process aluminum hydroxide having an average particle size of 80 m and a technical purity of Al(OH).sub.3>99% (for example obtainable from Aluminiumoxid Stade, Stade.

(7) 1. Production of Aluminum Oxide AO1

(8) Calcination:

(9) The thermal treatment of the aluminum hydroxide was carried out in an electrically heated furnace from Nabertherm, using an aluminum oxide crucible having a volume of about 21 as calcination vessel. The calcination was carried out for four hours at a temperature of 1300 C.

(10) Milling:

(11) The milling of the calcination product was carried out in a planetary ball mill PM 100 from Retsch. Here, 60 g of calcined material were milled for 20 minutes using 200 g of aluminum oxide milling balls (diameter 0.5-1 cm).

(12) The resulting aluminum oxide had a specific BET surface area of 7.0 m.sup.2/g and an average particle size of 0.8 m. The purity was 99.6% of Al.sub.2O.sub.3. The -alumina content was >90%.

(13) 2. Production of Aluminum Oxide AO2

(14) Calcination:

(15) The thermal treatment of the aluminum hydroxide was carried out in an electrically heated furnace from Nabertherm, using an aluminum oxide crucible having a volume of about 21 as calcination vessel. The calcination was carried out for four hours at a calcination temperature of 1650 C.

(16) Milling:

(17) The milling of the calcination product was carried out in a planetary ball mill PM 100 from Retsch. Here, 60 g of calcined material were milled for five minutes using 200 g of aluminum oxide milling balls (diameter 0.5-1 cm).

(18) The resulting aluminum oxide had a specific BET surface area of 1.0 m.sup.2/g and an average particle size of 4.0 m. The purity was 99.6% of Al.sub.2O.sub.3. The -alumina content was >90%.

(19) 3. Production of Aluminum Oxide AO3

(20) Calcination:

(21) The thermal treatment of the aluminum hydroxide was carried out in an electrically heated furnace from Nabertherm, using an aluminum oxide crucible having a volume of about 21 as calcination vessel. The calcination was carried out for four hours at a calcination temperature of 1650 C.

(22) Milling:

(23) The milling of the calcination product was carried out in a planetary ball mill PM 100 from Retsch. Here, 60 g of calcined material were milled for ten minutes using 200 g of aluminum oxide milling balls (diameter 0.5-1 cm).

(24) The resulting aluminum oxide had a specific BET surface area of 1.7 m.sup.2/g and an average particle size of 2.0 m. The purity was 99.6% of Al.sub.2O.sub.3. The -alumina content was >90%.

B) USE EXAMPLES

Example 1: Setting Behavior of a Fine-Grained Base Composition Composed of Calcined Aluminas

(25) TABLE-US-00001 TABLE 1 Constitution of the fine-grained base composition Constitution of base composition [% by weight] Constituents 84.30 Binder system as per Table 2 15.70 Make-up water

(26) TABLE-US-00002 TABLE 2 Constitution of the hydraulic binder system Constitution Particle Specific BET of binder size surface area [% by weight] Constituents D.sub.50 [m.sup.2/g] 99.64 A AO1 0.8 m 7.0 0.28 B Magnesium oxide.sup.1 3.0 m 16.3 0.07 C Plasticizer system.sup.2 0.8 m 7.0 .sup.1caustic calcined, from Nedmag, the Netherlands .sup.2plasticizer system (72% of citric acid, 28% of acetic acid; both reagent grade from Merck)
Experimental Procedure

(27) The calcined alumina was converted into a slip by addition of magnesium oxide, plasticizer system and water and intensive mixing. The finished mixture was poured into plastic molds and stored in a closed container at room temperature for 27 hours. The setting time of the mixture was determined by means of the Vicat test. Apparent density (from the weight and volume of the test speciment) and cold compressive strength were determined on the set and demolded test specimens (4646 mm). The results obtained are shown in Table 3.

(28) TABLE-US-00003 TABLE 3 Setting behavior of the fine-grained base composition containing magnesium oxide Setting time Apparent density Cold compressive [min] [g/cm.sup.3] strength [N/mm.sup.2] 120 2.19 2.0

(29) The demolded test specimens were fired at 1625 C. for 3 hours. Table 4 shows the properties of the sintered aluminum oxide ceramic:

(30) TABLE-US-00004 TABLE 4 Properties of the sintered aluminum oxide ceramic Drying shrinkage Sintering shrinkage Sintered density (110 C.) (1625 C.) (1625 C.) 1.46% 14.95% 3.57 g/cm.sup.3

Example 2: Influence of the Amount of Magnesium Oxide on the Setting of an -Alumina Concrete

(31) TABLE-US-00005 TABLE 5 Constitution of the self-flowing -alumina concrete Constitution of concrete Particle size [% by weight] Constituents D.sub.50 39.0 Sintered -alumina T60.sup.1 1-3 mm 7.8 Sintered -alumina T60.sup.1 0.5-1 mm 15.0 Sintered -alumina T60.sup.1 <0.5 8.86 Sintered -alumina T60.sup.1 <45 m 23.45 Binder system as per Table 6 5.63-5.65 Make-up water .sup.1from Almatis GmbH, Germany

(32) TABLE-US-00006 TABLE 6 Constitution of the hydraulic binder system Constitution Particle Specific BET of binder size surface area [% by weight] Constituents D.sub.50 [m.sup.2/g] 9.08 A AO1 0.8 m 7.0 90.26-88.70 A AO2 4.0 m 1.0 0.08-2.00 B Magnesium oxide.sup.2 3.0 m 16.3 0.21 C Plasticizer system.sup.3 0.8 m 7.0 .sup.2caustic calcined, from Nedmag, the Netherlands .sup.3trisodium citrate, reagent grade
Experimental Procedure

(33) A homogeneous slip was firstly produced from the calcined aluminas by addition of plasticizer system and make-up water and intensive mixing. This slip was subsequently mixed with the sintered -alumina particles. The magnesium oxide was subsequently added. The finished concrete mixture was poured into plastic molds and stored at room temperature in a closed container for 27 hours. The setting time of the mixture was determined by means of the Vicat test. The cold compressive strength was determined on the set and demolded test specimens (4646 mm). The results are shown in Table 7.

(34) TABLE-US-00007 TABLE 7 Influence of the amount of magnesium oxide on the setting of an -alumina concrete Magnesium oxide in Cold compres- concrete/in binder Setting time sive strength [% by weight] [min] [N/mm.sup.2] 0.02/0.08 5500 1.0 0.06/0.26 1500 5.8 0.24/1.02 960 7.3 0.35/1.49 490 6.4 0.47/2.00 210 7.7

(35) It was able to be shown that the addition of even a small amount of magnesium oxide brought about setting of the -alumina concrete. Furthermore, it was able to be shown that the setting time and the cold compressive strength of the set test specimens can be set via the amount added.

Example 3: Influence of the BET Surface Area of Magnesium Oxide on the Setting of an -Alumina Concrete

(36) TABLE-US-00008 TABLE 8 Constitution of the self-flowing -alumina concretes Constitution of concrete Particle size [% by weight] Constituents D.sub.50 38.9 Sintered -alumina T60.sup.1 1.0-3.0 mm 7.9 Sintered -alumina T60.sup.1 0.5-1.0 mm 14.9 Sintered -alumina T60.sup.1 <0.5 mm 8.9 Sintered -alumina T60.sup.1 <45 m 23.8 Binder system as per Table 9 5.6 Make-up water .sup.1from Almatis GmbH, Germany

(37) TABLE-US-00009 TABLE 9 Constitution of the hydraulic binder system Composition Particle Specific BET of binder size surface area [% by weight] Constituents D.sub.50 [m.sup.2/g] 88.3 A AO2 4.0 m 1.0 10.1 A AO1 0.8 m 7.0 0.8 B Magnesium oxide.sup.2 see Table 6 0.8 C Plasticizer system.sup.3 .sup.2caustic calcined, from Nedmag, the Netherlands .sup.3from 65 to 87.5% by weight of polycarboxylic acid ether (PCE), from 12 to 32.5% by weight of citric acid, from 0.1 to 1.4% by weight of amidosulfonic acid, from 0.1 to 1.4% by weight of silicon dioxide (for example 66.7% by weight of CASTAMENT FS 20 and 33.3% by weight of Castament FS 10, both obtainable from BASF SE).

(38) -Alumina concretes containing untreated magnesium oxide which had been heated at 600 C. and at 1000 C. were examined. As a measure of the reactivity, the specific surface area was determined by the BET method.

(39) TABLE-US-00010 TABLE 10 Parameters of the magnesium oxide species examined Heating Specific BET temperature of Particle size distribution surface area magnesium oxide D.sub.90 [m] D.sub.50 [m] D.sub.10 [m] [m.sup.2/g] not heated 11.0 3.0 0.7 16.3 600 C. 11.0 3.0 0.7 14.5 1000 C. 11.0 3.0 0.7 2.0
Experimental Procedure

(40) A clear solution was produced from plasticizer system and make-up water. The other constituents of the formulation were firstly premixed dry for one minute, of the water/plasticizer mixture were subsequently added and the mixture was mixed for three minutes. Finally, the remainder of the water/plasticizer mixture was added and the mixture was mixed until the composition became flowable (about 2 minutes).

(41) The concrete mixtures prepared in this way were poured into plastic molds and stored at room temperature in a closed container for 24 hours. The setting time of the concrete mixtures was determined by means of the Vicat test. The test specimens were dried at 110 C. for 24 hours. Apparent density (from the weight and volume of the test specimen) and cold compressive strength were determined on set, dried test specimens (4646 mm). The results are shown in Table 11.

(42) TABLE-US-00011 TABLE 11 Influence of the specific surface area of magnesium oxide on the setting of an -alumina concrete Specific BET Apparent Cold compres- Heating surface area Setting time density sive strength temperature [m.sup.2/g] [min] [g/cm.sup.3] [N/mm.sup.2] not heated 16.3 960 3.00 7.3 600 C. 14.5 1560 3.03 8.3 1000 C. 2.0 2760 2.94 2.5

(43) It was able to be shown that the setting time of the self-flowing -alumina concrete is shortened by increasing the BET surface area of the magnesium oxide.

Example 4: Influence of Various Alkaline Earth Metal Oxides/Hydroxides on the Setting of an -Alumina Concrete

(44) a) Magnesium Oxide

(45) TABLE-US-00012 TABLE 12 Constitution of the self-flowing -alumina concretes Constitution of concrete Particle size [% by weight] Constituents D.sub.50 46.8 Sintered -alumina T60.sup.1 <0.5 mm 47.6 Binder system as per Table 13 5.6 Make-up water .sup.1from Almatis GmbH, Germany

(46) TABLE-US-00013 TABLE 13 Constitution of the hydraulic binder system Constitution Particle Specific BET of binder size surface area [% by weight] Constituents D.sub.50 [m.sup.2/g] 32.4 A AO1 0.8 m 7.0 66.0 A AO2 4.0 m 1.0 1.0 B Magnesium oxide.sup.1 3.0 m 16.3 0.6 C Plasticizer system.sup.2 .sup.1from Nedmag, The Netherlands

(47) .sup.266.7% by weight of polycarboxylic acid ether (PCE, for example obtainable as Viscocrete 225P (Sika) and 33.3% by weight of citric acid (reagent grade, Merck KGaA)

(48) b) Calcium Hydroxide

(49) TABLE-US-00014 TABLE 14 Constitution of the self-flowing -alumina concretes Constitution of concrete Particle size [% by weight] Constituents D.sub.50 46.8 Sintered -alumina T60.sup.1 <0.5 mm 47.6 Binder system as per Table 15 5.6 Make-up water .sup.1from Almatis GmbH, Germany

(50) TABLE-US-00015 TABLE 15 Constitution of the hydraulic binder system Constitution Particle Specific BET of binder size surface area [% by weight] Constituents D.sub.50 [m.sup.2/g] 32.4 A AO1 0.8 m 7.0 65.7 A AO2 4.0 m 1.0 1.3 B Calcium hydroxide.sup.1 3.6 m 13.0 0.6 C Plasticizer system.sup.2 .sup.1reagent grade, Nekablanc Kalkfabrik Netstal AG, Switzerland .sup.266.7% by weight of polycarboxylic acid ether (PCE, for example obtainable as Viscocrete 225P (Sika) and 33.3% by weight of citric acid (reagent grade, Merck KGaA)
c) Strontium Hydroxide

(51) TABLE-US-00016 TABLE 16 Constitution of the self-flowing -alumina concretes Constitution of concrete Particle size [% by weight] Constituents D.sub.50 46.5 Sintered -alumina T60.sup.1 <0.5 mm 47.9 Binder system as per Table 17 5.6 Make-up water .sup.1from Almatis GmbH, Germany

(52) TABLE-US-00017 TABLE 17 Constitution of the hydraulic binder system Constitution Particle Specific BET of binder size surface area [% by weight] Constituents D.sub.50 [m.sup.2/g] 32.4 A AO1 0.8 m 7.0 65.8 A AO2 4.0 m 1.0 2.5* B Sr(OH).sub.2 8 H.sub.2O.sup.1 13.0 m 5.0 0.6 C Plasticizer system.sup.2 *corresponds to 1.1% by weight of hydrate-free Sr(OH).sub.2 .sup.1reagent grade .sup.266.7% by weight of polycarboxylic acid ether (PCE, for example obtainable as Viscocrete 225P (Sika) and 33.3% by weight of citric acid (reagent grade (Merck)
d) Barium Hydroxide

(53) TABLE-US-00018 TABLE 18 Constitution of the self-flowing -alumina concretes Constitution of concrete Particle size [% by weight] Constituents D.sub.50 46.5 Sintered -alumina T60.sup.1 <0.5 mm 47.9 Binder system as per Table 19 5.6 Make-up water

(54) TABLE-US-00019 TABLE 19 Constitution of the hydraulic binder system Constitution Particle Specific BET of binder size surface area [% by weight] Constituents D.sub.50 [m.sup.2/g] 32.4 A AO1 0.8 m 7.0 65.8 A AO2 4.0 m 1.0 2.1* B Ba(OH).sub.2 8 H.sub.2O.sup.1 12.0 m 8.0 0.6 C Plasticizer system.sup.2 *corresponds to 1.1% by weight of hydrate-free Ba(OH).sub.2 .sup.1reagent grade .sup.266.7% by weight of polycarboxylic acid ether (PCE, for example obtainable as Viscocrete 225P (Sika) and 33.3% by weight of citric acid (reagent grade, Merck KGaA)
Experimental Procedure

(55) A homogeneous slip was firstly produced from the calcined aluminas by addition of plasticizer system and make-up water and intensive mixing. This slip was subsequently mixed with the sintered -alumina particles. The alkaline earth metal component was subsequently added. For test purposes, a mixture without addition of activator was also produced. The finished concrete mixture was poured into plastic molds and stored at room temperature in a closed container for 24 hours. The setting time of the mixture was determined by means of the Vicat test. Apparent density (from the weight and volume of the test specimen) and cold compressive strength were determined on the set and demolded test specimens (4646 mm). The results achieved are summarized in Table 20.

(56) TABLE-US-00020 TABLE 20 Influence of various alkaline earth metal oxides/hydroxides on the setting behavior of -alumina concretes Solu- Amount bility added to in H.sub.2O Oxide/ concrete [g/l at ST CCS AD hydroxide [% by weight] 20 C.] [min] [N/mm.sup.2] [g/cm.sup.3] MgO 0.5 0.009 1440 2.8 3.17 Ca(OH).sub.2 0.6 1.7 20 2.2 3.11 Sr(OH).sub.2 8 H.sub.2O 1.1 20.0 210 1.6 3.12 Ba(OH).sub.2 8 H.sub.2O 1.1 72.0 360 1.3 3.12 ST = setting time, CCS = cold compressive strength, AD = apparent density

(57) It was able to be shown that no setting of the -alumina concrete occurred without addition of an activator, but setting occurred in every case with addition of an activator. Here, magnesium oxide developed the greatest binding force (greatest compressive strength). A reduction in the binding force and lengthening of the setting time were associated with increasing solubility of the oxides/hydroxides.

Example 5: Influence of the Amount of Calcium Hydroxide on the Setting Behavior of -Alumina Concrete

(58) TABLE-US-00021 TABLE 21 Constitution of the self-flowing -alumina concretes Constitution of concrete Particle size [% by weight] Constituents D.sub.50 46.58-46.47 Sintered -alumina T60.sup.1 <0.5 mm 46.90-47.03 Binder system as per Table 22 6.52-6.50 Make-up water .sup.1from Almatis GmbH, Germany

(59) TABLE-US-00022 TABLE 22 Constitution of the hydraulic binder system Constitution Particle Specific BET of binder size surface area [% by weight] Constituents D.sub.50 [m.sup.2/g] 66.14-65.81 A AO3 2 m 1.7 33.16-33.00 A AO1 0.8 m 7.0 0.11-0.59 B Ca(OH).sub.2.sup.1 3.6 m 13.0 0.60-0.59 C Plasticizer system.sup.2 .sup.1Nekablanc Kalkfabrik Netstal AG, Switzerland .sup.2from 65 to 87.5% by weight of polycarboxylic acid ether (PCE), from 12 to 32.5% by weight of citric acid, from 0.1 to 1.4% by weight of amidosulfonic acid, from 0.1 to 1.4% by weight of silicon dioxide (for example 66.7% by weight of CASTAMENT FS 20 and 33.3% by weight of Castament FS 10, both obtainable from BASF SE).
Experimental Procedure

(60) A homogeneous slip was firstly produced from the calcined aluminas by addition of plasticizer system and make-up water and intensive mixing. This slip was subsequently mixed with the sintered -alumina particles. The Ca(OH).sub.2 activator was subsequently added. The finished concrete mixture was poured into plastic molds and stored at room temperature in a closed container for 24 hours. The setting time of the mixture was determined by means of the Vicat test. Apparent density (from the weight and volume of the test specimen) and cold compressire strength were determined on the set and demolded test specimens (4646 mm). The results are shown in Table 23.

(61) TABLE-US-00023 TABLE 23 Influence of the amount of Ca(OH).sub.2 added on the properties of the -alumina concretes Amount of Cold Ca(OH).sub.2 added Setting Apparent compressive to concrete time density strength [% by weight] [min] [g/cm.sup.3] [N/mm.sup.2] 0.05 1500 3.08 0.9 0.09 390 3.08 1.8 0.19 45 3.01 1.8 0.23 35 3.08 2.3 0.28 10 3.03 2.2

(62) It was able to be shown that even small amounts of Ca(OH).sub.2 bring about solidification of the -alumina concrete. The setting time and the strength of the set test specimens could be set via the amount added.

Example 6: Influence of Coating of the Calcined Aluminas

(63) TABLE-US-00024 TABLE 24 Constitution of the self-flowing -alumina concrete Constitution of concrete Particle size [% by weight] Constituents D.sub.50 46.8 Sintered -alumina T60.sup.1 <0.5 mm 47.6 Binder system as per Table 25 5.6 Make-up water .sup.1from Almatis GmbH, Germany

(64) TABLE-US-00025 TABLE 25 Constitution of the hydraulic binder system Specific Constitution of BET surface binder Particle size area [% by weight] Constituents D.sub.50 [m.sup.2/g] 32.4 A AO1, coated.sup.1 0.8 m 7.0 66.0 A AO2, coated.sup.1 4.0 m 1.0 1.0 B Magnesium oxide.sup.2 3.0 m 16.3 0.6 C Plasticizer system.sup.3 .sup.1phosphorus content 0.063% by weight .sup.2caustic calcined, from Nedmag, the Netherlands .sup.3from 65 to 87.5% by weight of polycarboxylic acid ether, from 12 to 32.5% by weight of citric acid, from 0.1 to 1.4% by weight of amidosulfonic acid, from 0.1 to 1.4% by weight of silicon dioxide (for example 66.7% by weight of CASTAMENT FS 20 and 33.3% by weight of CASTAMENT FS 10, both obtainable from BASF SE).
Experimental Procedure

(65) A mixture of AO1 and AO2 in a weight ratio of 16.5:33.5 was homogeneously mixed at room temperature with a dilute technical-grade phosphoric acid having a concentration of 0.74%. The amount of dilute H.sub.3PO.sub.4 was selected so that 0.002 g of H.sub.3PO.sub.4 were used per g of Al.sub.2O.sub.3. The mixture was subsequently dried at 110 C. and used for producing the concrete test specimens.

(66) A homogeneous slip was firstly produced from the coated calcined alumina mixture by addition of plasticizer system and make-up water and intensive mixing. This slip was subsequently mixed with the sindered -alumina particles. The magnesium oxide was subsequently added. The finished concrete mixture was poured into plastic molds and stored at room temperature in a closed container for 27 hours. The setting time of the mixture was determined by means of the Vicat test. Apparent density (from the weight and volume of the test specimen) and cold compressive strength were determined on the set and demolded test specimens (4646 mm). For comparison, an -alumina concrete made from the corresponding uncoated aluminas was examined. The results achieved are shown in Table 26.

(67) TABLE-US-00026 TABLE 26 Influence of phosphoric acid on the setting behavior in an -alumina concrete Cold Setting Calcined compressive strength Apparent density time aluminum oxides [N/mm.sup.2] [g/cm.sup.3] [min] uncoated 3.6 3.10 70 coated* 4.8 3.11 120 *corresponds to 0.068% by weight of P.sub.2O.sub.5 in the -alumina concrete

(68) It was able to be shown that the use of coated calcined aluminas brings about a considerable improvement in the binding force. This becomes apparent from the increase in the cold compressive strength of the set concretes by more than 30%. It was also able to be shown that the setting time of the concrete is appreciably increased by use of a coated alumina.

Example 7: Characterization of the Binder System of the Invention in Terms of the Use Properties of -Alumina Concretes Produced Therefrom

(69) TABLE-US-00027 TABLE 27 Constitution of the -alumina concrete Novel hydraulic binder 1% MgO Constitution of concrete Particle size [% by weight] Constituents D.sub.50 29.2 Sintered -alumina T60.sup.1 1-3 mm 21.37 Sintered -alumina T60.sup.1 0.5-1 mm 4.58 Sintered -alumina T60.sup.1 <0.5 12.25 Sintered -alumina T60.sup.1 <45 m 28.02 Binder system as per Table 7.b 4.58 Make-up water .sup.1from Almatis GmbH, Germany

(70) TABLE-US-00028 TABLE 28 Constitution of the hydraulic binder system of the invention Constitution of Particle Specific binder size BET surface [% by weight] Constituents D.sub.50 area [m.sup.2/g] 32.6 A AO1 0.8 m 7.0 66.2 A AO2 4.0 m 1.0 1.0 B Magnesium oxide.sup.2 3.0 m 16.3 0.2 C Plasticizer system.sup.3 0.8 m 7.0 .sup.2caustic calcined, from Nedmag, the Netherlands .sup.350% by weight of trisodium citrate, reagent grade, 50% by weight of polycarboxylic acid ether (PCE, for example obtainable as Viscocrete 225 P (Sika))

(71) TABLE-US-00029 TABLE 29 Constitution of the -alumina concrete Novel hydraulic binder 3% MgO Constitution of concrete Particle size [% by weight] Constituents D.sub.50 29.2 Sintered -alumina T60.sup.1 1-3 mm 21.37 Sintered -alumina T60.sup.1 0.5-1 mm 4.58 Sintered -alumina T60.sup.1 <0.5 12.25 Sintered -alumina T60.sup.1 <45 m 28.02 Binder system as per Table 7.d 4.58 Make-up water .sup.1from Almatis GmbH, Germany

(72) TABLE-US-00030 TABLE 30 Constitution of the hydraulic binder system of the invention Constitution Particle Specific of binder [% size BET surface by weight] Constituents D.sub.50 area [m.sup.2/g] 31.83 A AO1 0.8 m 7.0 64.6 A AO2 4.0 m 1.0 3.33 B Magnesium oxide.sup.2 3.0 m 16.3 0.24 C Plasticizer system.sup.3 0.8 m 7.0 .sup.2caustic calcined, from Nedmag, the Netherlands .sup.357% by weight of trisodium citrate, reagent grade, 43% by weight of polycarboxylic acid ether (PCE, for example obtainable as Viscocrete 225 P (Sika))
Experimental Procedure

(73) All constituents of the formulation apart from water were firstly premixed dry for one minute, of the water was subsequently added and the mixture was mixed for 3 minutes. Finally, the remainder of the water was added and the mixture was mixed until the composition became flowable (about 2 minutes).

(74) Consistency and Setting Time:

(75) The consistency of the vibration concretes prepared in this way was determined by means of the slump flow (DIN EN 1402-4). After determination of the consistency, the concrete was taken up and stored in a closed plastic container. The determination of the consistency was repeated every 30 minutes until the concrete had set.

(76) TABLE-US-00031 TABLE 31 Consistency and setting time of the -alumina concretes Designation of the concrete Slump flow [%] Setting time [h] Novel hydraulic binder 1% MgO 130 4.5 Novel hydraulic binder 3% MgO 130 10

(77) As can be seen from Table 31, the -alumina concretes display practicable processing properties when they are produced using the hydraulic binder system of the invention.

(78) Drying Behavior:

(79) 1 kg of fresh concrete was in each case introduced into a plastic bucket and allowed to cure at room temperature with exclusion of air for 24 hours. The buckets were then opened and dried to constant weight at 110 C. in a drying oven. During this, the mass of the concrete samples was determined regularly.

(80) TABLE-US-00032 TABLE 32 Water addition, total loss on drying and chemically bound water in the set -alumina concretes (the latter determined from the difference between the first two.) Water Total loss on Chemically addition drying bound water Designation of the concrete [%] [%] [%] Novel hydraulic binder 1% MgO 4.58 4.25 0.33 Novel hydraulic binder 3% MgO 4.58 3.99 0.59

(81) The binder system of the invention binds significantly less water in hydrates than does cement.

(82) The novel, hydraulic binder system of the invention leads to a concrete having a high permeability of the microstructure. Drying was concluded after only 24 hours. This property of the binder system of the invention allows short manufacturing cycles for finished refractory concrete parts and rapid heating-up curves in the case of monolithic refractory linings.

(83) Density and Strength:

(84) In addition, the concrete mixtures which had been prepared in this way were poured into plastic molds and stored at room temperature in a closed container for 24 hours. The test specimens were dried at 110 C. for 24 hours. Part of the test specimens were then fired for three hours at 1000 C. or 1500 C. Apparent density (from the weight and volume of the test specimen), the shrinkage and cold compressive strength were determined on dried and fired test specimens (4646 mm).

(85) TABLE-US-00033 TABLE 33 Cold compressive strength, apparent density and drying shrinkage of the -alumina concretes dried at 110 C. Cold compressive Apparent strength density Drying Designation of the concrete [N/mm.sup.2] [g/cm.sup.3] shrinkage [%] Novel hydraulic binder 1% 7 3.06 0.02 MgO Novel hydraulic binder 3% 18 3.04 0.05 MgO

(86) TABLE-US-00034 TABLE 34 Cold compressive strength, apparent density and drying shrinkage of the -alumina concretes fired at 1000 C. Cold compressive Sintered Designation strength density Firing of the concrete [N/mm.sup.2] [g/cm.sup.3] shrinkage [%] Novel hydraulic binder 1% MgO 16 3.06 0.04 Novel hydraulic binder 3% MgO 23 3.04 0.05

(87) TABLE-US-00035 TABLE 35 Cold compressive strength, apparent density and drying shrinkage of the -alumina concretes fired at 1500 C. Cold compressive Sintered Designation strength density Firing of the concrete [N/mm.sup.2] [g/cm.sup.3] shrinkage [%] Novel hydraulic binder 1% MgO 110 3.08 0.11 Novel hydraulic binder 3% MgO 130 3.10 0.09

(88) The apparent density of the concretes containing the binder system of the invention is at the level of other -alumina concretes according to the prior art. The drying shrinkage is minimal. Growth due to hydration, as occurs at high contents of rho alumina, does not occur in the case of the binder system of the invention. The strength of the concrete produced using the hydraulic binder system of the invention is in the low, practicable range. After firing at 1000 C. or 1500 C., the strength of the concretes produced using the hydraulic binder system of the invention is comparable to the strength of concretes produced using binders according to the prior art. The low firing shrinkage has a positive effect on the stability of refractory linings since cracks can be caused by an excessive firing shrinkage.

(89) High-Temperature Strength:

(90) Test cubes having an edge length of 25 mm were prepared from test specimens fired at 1500 C. using a diamond saw in order to determine the hot compressive strength. The hot compressive strength was determined at 1450 C.

(91) TABLE-US-00036 TABLE 36 Hot compressive strength measured at 1450 C. of the -alumina concretes fired at 1500 C. Hot compressive strength Designation of the concrete [N/mm.sup.2] Novel hydraulic binder 1% MgO 31.3 Novel hydraulic binder 3% MgO 59.2

(92) The hot compressive strength of all concretes is at a very high level.

(93) Corrosion Resistance:

(94) In addition, the concrete mixtures which had been prepared in this way were poured into cylindrical crucible molds having a diameter of 50 mm and a height of 65 mm. The cylindrical depression in the crucibles had a diameter of 23 mm and a depth of 35 mm. The crucibles were cured at room temperature for 24 hours, dried at 110 C. for 24 hours and fired at 1500 C. for 3 hours. The crucibles were then filled with 17 g of slag and once again maintained at 1500 C. for 3 hours. The crucibles were subsequently cut through by means of a diamond saw and the cut surfaces were evaluated. Corrosion depth and infiltration depth of the slag into the refractory material were measured in 8 places and the arithmetic mean was formed.

(95) TABLE-US-00037 TABLE 37 Chemical analysis of the slags used: Oxide Content [%] Fe.sub.2O.sub.3 36 CaO 29 SiO.sub.2 20 PO.sub.4 6 Al.sub.2O.sub.3 6 Mn.sub.2O.sub.3 1.5 MgO 1.5

(96) TABLE-US-00038 TABLE 38 Results of the slag tests Designation of the concrete Corrosion depth Infiltration depth Novel hydraulic binder 1% MgO <0.1 mm 4.2 mm Novel hydraulic binder 3% MgO <0.1 mm 3.1 mm

(97) The crucibles containing the binder system of the invention display little reaction with the slag since these concretes contain no cement and thus no calcium oxide. The volume of the slag has remained virtually completely in the crucible and not penetrated into the refractory material. The infiltration depth is very small and corrosion has effectively not taken place.