CLINKER COOLING, GRINDING, AND STORAGE. 535 



time tests. In larger quantities, its retarding influence becomes less, 

 and finally negative, while a decided weakening of the cement is notice- 

 able. 



The more theoretical part of the discussion, relating to the form in 

 which the sulphate is applied, and the influence of various percentages 

 of sulphate on the set and strength of the cement, will be first presented : 

 after which the actual methods of application, with analyses of gypsums 

 and plasters used in practice, will be discussed. 



Form in which calcium sulphate is used. The requisite calcium 

 sulphate may be added to the cement in one of three forms: as crude 

 gypsum, as calcined plaster, or as dead-burnt (anhydrous) plaster. 

 For a full description of the manufacture and properties of these three 

 products the reader is referred to Part I of this volume. In the pres- 

 ent place their essential characters can be briefly stated as follows: 

 Crude gypsum is a natural hydrous sulphate of lime, corresponding to 

 the formula CaS04+2H 2 O, and to the composition calcium sulphate 

 79.1 per cent, water 20.9 per cent. Calcined plaster, or plaster of Paris, 

 is obtained by heating gypsum at temperatures of 350-400 F., the 

 result being that three fourths of the combined water is driven off. 

 The resulting plaster has the formula CaSO 4 + H 2 0, corresponding 

 to the composition calcium sulphate 93.8 per cent, water 6.2 per cent. 

 If gypsum be calcined at temperatures much above 400 F., all of its 

 combined water will be expelled, leaving dead-burnt or anhydrous 

 plaster, which is' simply CaS04. 



Considerable discussion has been aroused over the question, which 

 of these three forms of calcium sulphate is the more advantageous for 

 use : but few satisfactory series of experiments are on record in regard 

 to this point. A misleading statement often made is that plaster of 

 Paris, because of its greater chemical activity, will naturally be much 

 more effective than gypsum, weight for weight. The fallacy involved 

 in this statement is revealed when it is considered that the calcium 

 sulphate added to the cement has absolutely no effect until the mixture 

 is gauged with water; and that this addition of water will naturally 

 reconvert the plaster immediately into the hydrous lime sulphate, 

 gypsum. Any argument based on relative chemical activity, so-called, 

 is therefore fallacious. 



The results of a few recorded experiments, on the comparative effects 

 of the various forms of calcium sulphate, on the set and strength of 

 the cement, will be given below: after which the conclusions which 

 may be drawn from these experiments and from commercial conditions 

 and actual practice will be summarized. 



