| 摘要 |
<p>A cementitious material which sets when mixed with water, and can be used as plaster, mortar and the like, comprises limestone dried but not calcined, powdered, e.g. by spraying, treated with a small proportion of hydrochloric acid, sulphuric acid or a fluoborate, e.g. calcium fluoborate, and mixed with porous siliceous granular material such as pumice grains or foamed slag. The amount of hydrochloric or sulphuric acid may be between ,5 and 2 per cent on the weight of the limestone, and of fluoborate between ,2 and 2 per cent. A small amount of tannic acid may be added to prevent frothing during the spraying operation. To improve the colour, e.g. when grey limestones are used, a heated naturally occurring or synthetic material comprising gypsum associated with dolomite or calcite or a mixture of calcium sulphate with magnesia, magnesium sulphate, or sulphite may be added in amounts up to 50 per cent of the final mixture, but 20 per cent is normally sufficient. The material is heated between 110 and 1500 DEG C., preferably between 120 and 240 DEG C. A small amount, ,5-2 per cent of the limestone, of finely divided asbestos may be added, and also ,5-2 per cent of cellulosic fibres. Lime or Portland cement may be blended with the treated limestone in amounts up to about 20 per cent of the limestone. In an example, limestone in pieces 7 1/2 inch to 3 inches is dried at about 240 DEG F. for ten minutes and ground to pass a 50 mesh 1 M.M. screen. 800 lb. is placed in a mixer comprising a fixed nearly horizontal closed cylinder having rotary agitators which turn over and shower the powder. A mixture of hydrochloric acid and tannic acid is sprayed by compressed air through a hole in each of the end walls of the mixer. The weight of hydrochloric acid is ,75 per cent of the weight of the limestone and of tannic acid ,25 per cent. To the treated limestone are added 9 per cent of Portland cement, 4 per cent of asbestos, ,2 per cent of dried and powdered paper pulp and 4 per cent of pumice grains. When a porous plaster is required a borate, e.g. ammonium, aluminium, calcium or magnesium borate, in amount between 1 and 5 per cent of the weight of the plaster, or an alkali metal peroxide, between ,02 and 1,5 per cent of the plaster, may be added.ALSO:A mixer for treating powdered cementitious material with liquid comprises a fixed nearly horizontal closed cylinder having rotary agitators which turn over and shower the powder. Liquid is sprayed by compressed air through a hole in each of the end walls of the mixer.</p> |
