Method of preparing a novel cement particularly suitable as a refractory cement
The present invention relates to a process for preparing a novel cement particularly suitable as a refractory cement. It is also related to the cement obtained by such process, as well as the clinkers used when carrying out the same. 
     The cement according to the invention is a double clinker cement containing anhydrous calcium sulphonate (designated herein-below by the abridged formula C 4  A 3  S) and a conventional aluminous clinker. 
     The novel cement enables refractory concretes to be made with improved mechanical properties and, possibly, improved refractory properties as compared to conventional aluminous cements. 
     The C 4  A 3  S cement, the formula of which written in full is 4CaO,3Al 2  O 3 ,SO 3 , is a product well-known per se; it constitutes essentially the expanding complex added to Portland cement either by mixing or by adding calcium sulphate to the Portland cement raw materials, with a view to obtaining so-called &#34;shrinkage compensating cements&#34;. 
     According to a known method, an aluminous clinker containing C 4  A 3  S can be made by adding calcium sulphate to aluminous cement, raw materials and by sintering said mixture, enlarging or broadening the sintering range. 
     When a double clinker cement according to the invention is to be prepared, a C 4  A 3  S clinker and an aluminous clinker are mixed in a ratio comprised between 10/90 and 30/70. 
     Preferably the aluminous clinker contains 30 to 70% CA(CaO, Al 2  O 3 ). Such clinkers are used for making cements of the type sold under the commercial designation &#34;Fondu&#34; or &#34;Secar&#34; by the Lafarge Fondu International Company. 
     Depending on the particular case envisaged, the clinkers are either mixed then ground together to about 4000 cm 2  /g Blaine specific surface, or are ground separately to the required granolumetric particle size prior to mixing the thus ground clinkers. In this latter case the granolumetric data (the particle size) of the constituents should be comprised between 2600 cm 2  /g Blaine S.S. and 5000 cm 2  /g (preferalby 3000 cm 2  /g S.S) for the aluminous clinker, and between about 5000 and 8000 cm 2  /g Blaine S.S. (preferably 7000 cm 2  /g Blaine S.S.) for the C 4  A 3  S clinker. 
     The chemical composition of the C 4  A 3  S clinker is as follows (in % by weight): 
     
         ______________________________________                                    
. CaO           =          35-37                                          
. Al.sub.2 O.sub.3                                                        
                =          48-54                                          
. SO.sub.3      =           9-14                                          
. Fe.sub.2 O.sub.3                                                        
                =           0-4                                           
. SiO.sub.2     =            0-4.                                         
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     As a matter of fact the raw materials used are substances proper to provide the required CaO, Al 2  O 3  and SO 3 . Preferably the following substances are used: 
     substances providing CaO: limestone, burnt lime, slaked lime; 
     substances providing Al 2  O 3  : alumina, hydrated alumina, white bauxite, red bauxite; 
     substances providing SO 3  : gypsum, plaster, calcium sulphate, semi-hydrate, anhydride and aluminium sulphate. 
     The raw materials are mixed and ground until 95% of the particles have a size of less than 100 μum, and are then burnt until the sintering stage is reached. Preferably the sintering temperature of C 4  A 3  S is equal to or higher than 1400° C.; an advantageous sintering temperature is equal to about 1500° C. 
     As will be explained herein-below, by mixing clinkers of the kind defined herein-above, cements having particularly advantageous mechanical properties can be obtained. 
     According to one embodiment of the invention a C 4  A 3  S clinker was prepared by the instant method using the following raw materials : 
     
         ______________________________________                                    
. CaCO.sub.3      = 38.51%                                                
. CaSO.sub.4&#39; 2H.sub.2 O                                                  
                  = 23.13%                                                
. Alumina ground to                                                       
6000 cm.sup.2 /g Blaine S.S.                                              
                  = 39.36%                                                
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     The mixture of these constituents was ground until its granulometry was such that 95% of the particles forming the mixture had a size of less than 100 μm; the mixture was then moulded into bricks with 25 to 30% water in the presence of 2.5% of a lignosulphonate. The bricks thus obtained were progressively heated to a temperature of 160° in an oven, and then maintained during 90 minutes at various sintering temperatures. 
     Appended Table I lists the chemical and mineralogical properties of the resulting C 4  A 3  S as a function of the soak temperature. 
     Table I shows that the range of C 4  A 3  S soak temperatures within which no volatization of SO 3  occurs is very broad. 
     The phases other than C 4  A 3  S disappear when the temperature exceeds 1400° C.; at a sintering temperature higher than 1500° C. the sulphate will volatilize. 
     The invention is not limited to the process described herein-before; the C 4  A 3  S clinker may be prepared by a dry or semi-dry process in accordance with the methods well known in the cement making industry. 
    
    
     The invention will be described in a more detailed manner herein-below with reference to several examples which are given by way of illustration, but which are not limiting. When not otherwise specified, the proportions are indicated in percentages by weight. 
     
                       TABLE I                                                     
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     Average chemical                                                     
Soak composition of the                                                   
tem- C.sub.4 A.sub.3 -S clinkers                                          
pera-              Im-                                                    
ture               puri-  X-ray diffraction Intensities.sup.1             
°C.                                                                
     CaO    Al.sub.2 O.sub.3                                              
                    SO.sub.3                                              
                         ties C.sub.4 A.sub.3 -S                          
                                    CA   CaSO.sub.4                       
                                               C.sub.12 A.sub.7           
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1350 37.15  49.90   12.80                                                 
                         0.15 F     m    mf                               
1370 &#34;      &#34;       &#34;    &#34;    TF    f    --                               
1400 &#34;      &#34;       &#34;    &#34;    TF    f    --                               
1430 &#34;      &#34;       &#34;    &#34;    TF    --   --                               
1500 &#34;      &#34;       &#34;    &#34;    TF    --   --                               
1550 42.60  57.20   --              F          F                          
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 .sup.1 TF = very strong F = strong m = medium mf = medium to low  f = low
 
    
     EXAMPLES 1 to 5 
     A double clinker cement was prepared according to the present invention by mixing a C 4  A 3  S clinker as defined herein-above, ground to a fineness of 7000 cm 2  /g Blaine S.S. with an aluminous clinker containing 40% CA (Fondu clinker) ground to a 3000 cm 2  /g Blaine S.S. 
     The final chemical composition of this double clinker cement was as follows: 
     
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     Fondu/C.sub.4 A.sub.3 -S                                             
                       Iron                                               
Cement                                                                    
     mixture:                                                             
             SiO.sub.2                                                    
                Al.sub.2 O.sub.3                                          
                    CaO                                                   
                       oxides                                             
                           TiO.sub.2                                      
                              SO.sub.3                                    
                                 Impurities                               
__________________________________________________________________________
a    85  15  3.23                                                         
                40.39                                                     
                    38.38                                                 
                       13.90                                              
                           1.74                                           
                              1.92                                        
                                 0.44                                     
b    80  20  3.04                                                         
                40.94                                                     
                    38.31                                                 
                       13.08                                              
                           1.64                                           
                              2.56                                        
                                 0.43                                     
c    75  25  2.85                                                         
                41.51                                                     
                    38.24                                                 
                       12.26                                              
                           1.54                                           
                              3.25                                        
                                 0.35                                     
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     Table II appended hereto indicates the properties of concretes prepared on the basis of the double clinker cement (Fondu - C 4  A 3  S) and shows: 
     the W/C ratio (water/cement ratio by weight) as determined by the so-called &#34;ball-in-the-hand&#34; method) 
     the modulus of rupture (F) and compressive strength (C) after heating the concretes to about 800° C. in conformity with ASTM standard C 268, for various C 4  A 3  S sintering temperatures and various Fondu-C 4  A 3  S ratios of the cement composition. 
     The concretes examined were concretes with calcined fire-clay (chamotte) aggregates containing 42 to 44% Al 2  O 3  having a particle size of 0 to 5 mm according to the Fuller curve, the cement content was 500 kg per m 3  concrete, and the indicated properties are compared to those of concretes prepared from Fondu only. 
     Appended Table II shows that the properties of the concrete were strongly influenced by the quality of the C 4  A 3  S. 
     It can be seen that the improvement of the mechanical strength after heating was obtained when the C 4  A 3  S was soaked at a temperature of more than 1400° C., more particularly at 1500° C. 
     The Table shows that the W/C ratio decreased while the consistency (determined by the ASTM &#34;ball&#34; method) remained constant; it should be noted that this ratio decreased as the C 4  A 3  S sintering temperature increased. 
     
                       TABLE II                                                    
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                                Mechanical                                
                                properties of the                         
Soak                            concrete after                            
temperature  Composition of     heating to 800° C.                 
for the      the cement, %      MPa                                       
Exam- C.sub.4 A.sub.3 -S                                                  
                 Fondu   C.sub.4 A.sub.3 -S                               
                               W/C  F.sup.1                               
                                          C.sup.2                         
ples  preparation                                                         
                 100     0     0.42 4.3   40.0                            
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1     1350° C.                                                     
                 85      15    0.40 4.3   42.0                            
                 80      20    0.41 4.5   37.5                            
                 75      25    0.41 3.8   43.5                            
2     1370° C.                                                     
                 85      15    0.44 4.3   42.0                            
                 80      20    0.45 4.2   44.0                            
                 75      25    0.45 3.8   43.5                            
3     1400° C.                                                     
                 85      15    0.39 5.4   51.5                            
                 80      20    0.39 5.6   57.5                            
                 75      25    0.39 5.8   55.0                            
4     1430° C.                                                     
                 85      15    0.39 6.1   55.5                            
                 80      20    0.39 6.3   56.0                            
                 75      25    0.39 6.7   58.5                            
5     1500° C.                                                     
                 80      20    0.37 6.9   62.0                            
                 75      25    0.37 7.1   65.0                            
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 .sup.1 F = modulus of rupture                                            
 .sup.2 C = Compression                                                   
 
    
     
                       TABLE III                                                   
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Refractories under load:                                                  
                            Modulus                                       
                            of rup-                                       
         Temperature for movement under                                   
                            ture at                                       
         load of indicated % age                                          
                            1000° C.                               
         0.5%   1%     2%     5%   10%  MPa                               
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. Fondu        1195° C.                                            
                        1250 1350 1360 1390 3.3                           
. Fondu 80%                                                               
               1185     1230 1300 1355 1400 6.0                           
C.sub.4 A.sub.3 -S-20%                                                    
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                                           TABLE IV                                
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                    chamotte concrete 42-44% Al.sub.2 O.sub.3 (0-5 mm)    
                    Cement content 500 kg/m.sup.3                         
                    Cured at 20° C.                                
                    MPa     After heating MPa                             
                    24 h    110° C.                                
                                  800° C.                          
                                       1100° C.                    
Examples         W/C                                                      
                    F   C   F  C  F C  F C                                
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     FR® 100%                                                         
                 0.39                                                     
                    12.8                                                  
                        115.0                                             
                             8.1                                          
                               103.0                                      
                                  4.4                                     
                                    60.0                                  
                                       5.1                                
                                         36.0                             
6    FR 80% + C.sub.4 A.sub.3 -S 20%.sup.1                                
                 0.39                                                     
                    13.8                                                  
                        100.5                                             
                            10.3                                          
                               100.0                                      
                                  6.5                                     
                                    61.5                                  
                                       5.1                                
                                         35.0                             
7    FR 80% + C.sub.4 A.sub.3 -S 20%.sup.2                                
                 0.38                                                     
                    11.1                                                  
                        105.0                                             
                            11.6                                          
                               101.5                                      
                                  7.3                                     
                                    72.5                                  
                                       5.8                                
                                         37.0                             
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 .sup.1 C.sub.4 A.sub.3 -S soaked at 1430° C.                      
 .sup.2 C.sub.4 A.sub.3 -S soaked at 1500° C.                      
 FR = Fondu R                                                             
 
    
     In appended Table III the properties of the chamotted concretes similar to those descried herein-above and obtained, respectively by using Fondu and a double clinker according to the invention, said double clinker containing 80% Fondu and 20% C 4  A 3  S sintered at 1500° C. 
     It can be seen that the modulus of rupture of the double clinker cement (Fondu+C 4  A 3  S) as measured at 1000° C., was improved, compared to Fondu alone. Furthermore, the addition of C 4  A 3  S to the Fondu (40% CA) had no deleterious effect on the refractory properties as tested by measuring the amount of movement under a load of 0.2 MPa in accordance with French standard B 49 105. 
     EXAMPLES 6 AND 7 
     Double clinker cement prepared by mixing: 
     Fondu, 95% of the particles of which have a size lower than 40 μm (Fondu R); its Blaine specific surface being 4950 cm 2  /g; 
     C 4  A 3  S sintered at 1430° C. to 1500° C.and ground to a fineness of 7000 cm 2  /g Blaine S.S. 
     Appended Table IV illustrates the influence of the C 4  A 3  S clinker on the mechanical properties of the concretes prepared from double clinker cements: 
     after curing for 24 hours at 20° C., 
     after heating the concrete to temperatures of 110°-800°-1100° C. 
     This Table shows that adding C 4  A 3  S soaked at 1500° C. resulted in an improvement of the properties after heating compared to Fondu R alone, particularly after heating the same to a temperature of 800° C. 
     EXAMPLES 8 TO 10 p Double clinker cement prepared from C 4  A 3  S and Secar (about 70% CA). According to another embodiment of the invention the double clinker cement contained: 
     a C 4  A 3  S soaked at 1500° C. and ground to 7000 cm 2  /g Blaine S.S. 
     a white aluminous cement clinker (Secar) ground to 3600 cm 2  /g Blaine S.S. 
     The average composition (in % by weight) of the Secar used was as follows: 
     
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. Al.sub.2 O.sub.3                                                        
           =              70.85                                           
. CaO      =              28.60                                           
. SiO.sub.2                                                               
           =              0.10                                            
. Na.sub.2 O                                                              
           =              0.45                                            
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     The results yielded by the chamotte concrete (42-44% Al 2  O 3  0-5 mm Fuller grinding) are listed in the appended Table V, for a 500 kg/m 3  cement content and various C 4  A 3  S proportions. 
     
                       TABLE V                                                     
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                 chamotte                                                 
                 concrete 42-44% Al.sub.2 O.sub.3                         
                 Mechanical properties                                    
                 after heating, MPa                                       
                 800° C.                                           
                          1100° C.                                 
Examples                                                                  
       SECAR    C.sub.4 A.sub.3 -S                                        
                        W/C  F    C     F    C                            
______________________________________                                    
       100%      0%     0.42 4.2  71.5  4.2  50.5                         
8      95%       5%     0.39 7.4  78.5  5.8  47.5                         
9      90%      10%     0.39 8.6  93.0  6.2  50.5                         
10     85%      15%     0.39 7.9  87.0  6.1  50.0                         
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     Table V shows that particularly remarkable results were obtained after heating at 800° C., with a C 4  A 3  S concentration of 10%. 
     EXAMPLES 11 AND 12 
     These examples related respectively to the preparation of double clinker cement by simultaneously grinding the clinkers (Example 12) and by mixing the previously ground clinkers (Example 11). 
     The appended Table VI lists the results obtained which show that simultaneous grinding or separate grinding followed by mixing give equivalent results. 
     Generally speaking, the mechanical performances after heating of the concretes prepared from double clinker in accordance with the invention were considerably improved when the admixed C 4  A 3  S clinker was sintered at an elevated temperature, preferably at 1500° C. 
     
                                           TABLE VI                                
__________________________________________________________________________
                       Chamotte concrete                                  
                       42-44% Al.sub.2 O.sub.3 500 kg/m.sup.2             
                       20° C.                                      
                             After heating to                             
                       24 h  800° C.                               
                                  1100° C.                         
                       MPa   MPa  MPa                                     
EXAMPLES            W/C                                                   
                       F  C  F C  F C                                     
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       MIXTURE:                                                           
       Fondu 80%(3000 SSB)                                                
11                  0.37                                                  
                       12.3                                               
                          80.0                                            
                             6.9                                          
                               62.0                                       
                                  4.4                                     
                                    28.5                                  
       C.sub.4 A.sub.3 -S 20%(7000 SSB)                                   
       SIMULTANEOUS AND                                                   
       JOINT GRINDING:                                                    
       Fondu 80%                                                          
12                  0.38                                                  
                       12.5                                               
                          80.5                                            
                             5.5                                          
                               58.0                                       
                                  4.3                                     
                                    28.5                                  
       C.sub.4 A.sub.3 -S 20%                                             
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     The invention is not limited to the embodiments described herein-above; many variants and modifications may be envisaged by those skilled in the art within the scope of the invention as defined by the following claims.