CHAPTER III. 

 COMPOSITION, PROPERTIES, AND TESTS OF PLASTERS. 



IN the present chapter the chemical composition and physical prop- 

 erties of plaster of Paris and cement plasters will be taken up in the 

 order named. 



Chemical Composition. 



Theoretical composition. A theoretically pure plaster of Paris, 

 being a definite chemical compound (CaSO4+iH 2 0), would have the 

 composition: lime sulphate, 93.8 per cent; water, 6.2 per cent. This 

 composition is approached quite closely in plasters made from a pure 

 rock gypsum. 



Cement plasters, as has been described on earlier pages, can be made 

 in two different ways, which give two different products so far as com- 

 position is concerned. (1) Cement plasters may be made by adding 

 retarders to a pure plaster of Paris. As the retarder is organic matter 

 and rarely amounts to over 1 per cent of the total mass the resulting 

 product will on analysis differ very little from the plaster of Paris of 

 which it was made. (2) Cement plaster may also be made by burning 

 an impure gypsum, with or without the addition of retarder. In this 

 case analysis would show the presence of a large percentage of clayey 

 matter, etc., and a cement plaster of this type will therefore have a 

 composition very different from that of a pure plaster of Paris. Exam- 

 ples of both these types will be found in Table 11, opposite. 



Actual composition of plaster of Paris. The composition of the pure 

 quick-setting plasters here grouped as plaster of Paris, as these plasters 

 appear in commerce, is shown by the following table (10) of represen- 

 tative analyses. 



Actual composition of cement plasters. As noted earlier, a 

 cement plaster when ready for sale differs from the gypsum of which 

 it was made only in the loss of water and the addition of a very small 

 proportion (-$% to 1%) of retarder. The difference between the analy- 

 ses of any two samples or brands of cement plaster will therefore depend 

 on the difference in composition between the gypsums from which the 

 two samples were made. 



56 



