Abstract:
An improved accelerator is produced by impact and ball mill grinding gypsum board so that gypsum particles have size of less than one micron in diameter, in the absence of an additive to inhibit calcination.

Description:
BACKGROUND OF THE INVENTION 
     The purpose and function of an accelerator for hemihydrate gypsum is described at length in U.S. Pat. Nos. 3,262,799 and 3,573,947. These patents describe the highlights in the history of using land plaster and rehydrated gypsum accelerators and use of sugar as an anti-calcination agent. U.S. Pat. No. 3,314,613 and 3,337,191 disclose preparation of an accelerator from gypsum board by impact grinding. 
     SUMMARY OF THE INVENTION 
     I have discovered that a fully satisfactory and highly desirable gypsum hemihydrate accelerator can be produced by impact and ball mill grinding gypsum so that gypsum particles have fineness on the order of less than one micron in diameter without the necessity of incorporating an anti-calcination agent in the grinding mixture. I have also discovered that when ground gypsum board is used instead of land plaster, ground gypsum board does not cake and plug up the ball mill as land plaster does, and also that at this degree of fineness the fibrous appearance of the fibers of the gypsum face papers disappears and the ground gypsum board ceases to be difficult to handle. 
     The essential object of the present invention is to provide an improved accelerator for hemihydrated gypsum which consists essentially of gypsum board ground to a fineness on the order of one micron. 
    
    
     DESCRIPTION OF THE INVENTION 
     The preferred starting material is gypsum board, i.e. waste or cull gypsum board, or end trim from gypsum board, or gypsum board itself. The material is first ground up in an impact mill to about 100 mesh particle size. It is then reground for approximately two hours in a ball mill to the point where the average particle size is one micron in diameter or smaller. 
     When the gypsum board material is ground in the hammer mill only, it has a B.E.T. (Brunauer, Emmett and Teller) surface area of 13,000 sq. cm/gram and an equivalent average particle size of 4.6 microns. After a further grinding for about two hours in the laboratory size ball mill, the material has a B.E.T. surface area of approximately 130,000 and an equivalent average particle size of 0.46 microns. At the end of the ball mill grinding operation, the material is ready for use as an accelerator. It is added directly to hemihydrate gypsum or a slurry of hemihydrate gypsum. B.E.T. test procedure is described in the Journal of American Chemical Society 59,1553 (1937); 60,309 (1938); and 62,1723 (1940). 
     One gram of subject impact and ball milled gypsum board accelerator mixed with 200 grams of hemihydrate gypsum and 192 cc of water and agitated for 10 seconds at high speed in a Waring blender resulted in a setting time of 3 minutes after a ball mill grinding time of 1.5 hours. At the end of the impact milling and prior to ball mill grinding, one gram (regular accelerator) produced a setting time of 6 minutes. 
     The following data is furnished from gypsum plant trial runs: 
     
         ______________________________________      Control                  Control      before early in late in  after      Test Run             Test Run Test Run Test Run______________________________________Type of Accelerator        Regular  Subject  Subject                                 RegularAccelerator usage,lbs./MSF     26       3.7      4.48   26Board dry weight,lbs./MSF     1870     1760     1815   1850Weight corrected tocaliper      1885     1756     1845   1865Thickness, ins.        .496     .501     .492   .496Board moisture, %        5-15     5-15     5-15   5-15Bond, dry, %  Face       100      100      100    100  Back       100      100      100    100Bond, HighHumid, %  Face       100      98       100    100  Back       98       96       99     98Flexural strengths:Dry, across, lbs.        145.5    146      150.5  143.5Deflection, ins.        .49      .48      .47    .44at Hi Humid,across, lbs. 99.5     95.5     102.5  100Deflection, in.        .71      .70      .68    .67Dry, parallel, lbs.        50.5     49       51.5   57Deflection, in.        .39      .35      .25    .25at Hi Humid,parallel, lbs.        37.5     33       35     40Deflection, in.        .85      1.0      1.0    .9Twist failure, ins.  Face       3/8      15/8     2/8    0  Back       1/8      16/8     3/8    5/8Punch, lbs.  Dry        830      647      817    828  Hi Humid   565      427      553    560Nail Pull through, lbs.  Dry        146      123      146    136.5  Hi Humid   111.5    81.5     121.5  107.5Field Hardness(Hunter), lbs.  Dry        17.1     13.4     16.5   15.9  Hi Humid   11.6     10.1     12.1   13.1A.S.T.M. C-473 Tests:Core hardness, lbs.  Dry        35.5     29.5     40     36  Hi Humid   31.3     21.6     27     27End hardness, lbs.  Dry        33.3     28.8     36.7   38.3  Hi Humid   26.8     23.7     28.3   32.2Edge hardness, lbs.  Dry        34.6     29.3     39.8   40.3  Hi Humid   27.2     21.7     28.5   30______________________________________ 
    
     
         ______________________________________            With Regular AcceleratorCORE SLURRY TESTS            After test run______________________________________Wet density, lbs/cu.ft.            66.0Dry density, lbs/cu.ft.            47.8Dry Compressive Strength, psi            629Slurry setting time, Vicat, sec.            160          With Subject Accelerator            One-third of                        Two-thirds of            way through way through            test run    test run______________________________________Wet density, lbs/cu.ft.            63.8        64.3Dry density, lbs/cu.ft.            44.9        44.0Dry compressive strength, psi            693         694Slurry setting time, Vicat, sec.            120         150______________________________________ 
    
     
         ______________________________________ACCELERATOR      (Machine mix set with 200 gEFFICIENCY TEST  hemihydrate gypsum)______________________________________Regular Accelerator            5 grams yielded                        180 sec. set timeSubject Accelerator            1 gram yielded                        230 sec. set time______________________________________ 
    
     It will be seen that the dry compressive strengths of cubes taken when the subject accelerator of the invention was in use were increased by 10% over cubes taken just after the test run. At the same time, the cube density was decreased by 7.5%. Taken together, these two factors indicate a significant increase in strength. 
     The regular accelerator referred to in the test data is the material as it is at the end of the impact mill treatment, i.e. prior to any ball mill grinding.