128 The Philippine Journal of Science im 



obtained, as in this instance, by submitting each combination of 

 different raw materials to only one burning test, A burning 

 test may produce excellent cement, but the results obtained 

 might be so seriously affected by unavoidable variations in 

 factory conditions as to exclude the possibility of their successful 

 commercial use. On the other hand, a burning test may give 

 very unsatisfactory results, and yet a thorough study of the 

 raw materials used may reveal commercially practicable condi- 

 tions of manufacture which will produce extremely good results. 



Slight variations in composition, degree of burning, and 

 pulverization are unavoidable both in laboratory and manufactur- 

 ing practices, and such changes modify the quality of the 

 product more or less according to the nature of the raw materials. 

 Therefore, it is necessary to ascertain the effects produced by 

 reasonable modifications in the hydraulic index, degree of 

 vitrification, fineness, etc. This is especially true if, as in this 

 instance, we desire to know the relative merits of available raw 

 materials and the conditions of manufacture which will produce 

 the best results from the standpoint of manufacturing efficiency. 

 The changes brought about by well-planned modifications in 

 the conditions of manufacture will be the more significant the 

 more closely the experimental conditions approach those of the 

 best commercial practices. This is especially true with respect 

 to the degree of vitrification. 



A very objectionable feature of many reports is that the results 

 were obtained with underburned or unsound cement. Such 

 products have no definite characteristics in setting or hardening 

 properties. They may develop considerable strength in seven 

 or twenty-eight days only to disintegrate entirely after a few 

 months, and both their set and strength may change from a 

 satisfactory to an unsatisfactory one, or vice versa, overnight. 

 In reality they are not true Portland cements but a mixture 

 of lime ; hydraulic limes ; and Roman, slag, and Portland cements. 

 Obviously, it is misleading to judge the efficiency of raw materials 

 from results obtained by improper burning. It is much more 

 logical to ascertain the possibilities of well-sintered mixtures, 

 to observe whether good burning would be practicable under 

 commercial conditions, and then to take into consideration such 

 allowance as must be made for unavoidable variations and 

 imperfections in commercial manufacture. 



If the clinker is allowed to cool slowly with the experimental 

 kiln, the results obtained may be characteristic only of the set- 



