The principles which govern the preparation and setting of the other class of calcium sulphate cements, that is, cements of the Keene class, are not fully understood, but there is a fair amount of knowledge on the subject, both empirical and scientific. The essential difference between the setting of Keene's cement and that of plaster of Paris is that the former takes place much more slowly, occupying hours instead of minutes, and the considerable heating and expansion which characterize the setting of plaster of Paris are much less marked.
After the calcium sulphate has settled, the potassium chromate solution is converted into bichromate by the action of sulphuric acid, and the salt is allowed to crystallize.
When calcium sulphate is present, the nascent methane induces the formation of calcium carbonate, sulphuretted hydrogen and water.
The carbonates are dissolved in hydrochloric acid, and calcium sulphate solution is added to a portion of the solution.
And filtered, and neutralized with powdered chalk and a little milk of lime; the precipitate of calcium citrate so obtained is decomposed with dilute sulphuric acid, the solution filtered, evaporated to remove calcium sulphate and concentrated, preferably in vacuum pans.
In solution, barium salts may be detected by the immediate precipitate they give on the addition of calcium sulphate (this serves to distinguish barium salts from calcium salts), and by the yellow precipitate of barium chromate formed on the addition of potassium chromate.
When the former is used it is roasted with calcium sulphate or alkali waste to form a matte which is then blown in a Bessemer converter or heated in a reverberatory furnace with a siliceous flux with the object of forming a rich nickel sulphide.
All cements having calcium sulphate as their base are suitable only for indoor work because of the solubility of this substance.
From the heating of native calcium sulphate and carbon is obtained calx sulphurata (U.S. and B.P.), or sulphurated lime, a greyish-white powder.
This solution is allowed to stand for some time (in order that any calcium sulphate and basic ferric sulphate may separate), and is then evaporated until ferrous sulphate crystallizes on cooling; it is then drawn off and evaporated until it attains a specific gravity of 1.40.