118 The Philippine Journal of Science ms 



hardening, set-kiln products often gave better results in time. 

 Recently engineers have begun to realize that ordinary cement 

 contains only about two-thirds of its weight of actual cement, 

 and that the remainder is about as inert as ordinary sand because 

 it has not been sufficiently pulverized, and there has been a grow- 

 ing tendency to award a bonus for extreme fineness. 4 This is a 

 step in the right direction, but many comprehensive tests show 

 that the strength developed by different cements does not depend 

 upon the degree of fineness. 



Other conditions being the same, the degree of fineness affects 

 the quantity of active material in a given bulk rather than the 

 quality ; and as an underburned Portland cement is easier to pul- 

 verize than a hard-burned product, the manufacturer can supply 

 a more finely ground cement without a corresponding increase in 

 strength. The strength developed and maintained by Port- 

 land cement is the net result of many factors including, besides 

 fineness, the influences of chemical composition, burning, and 

 seasoning. 



The present state of our knowledge does not enable us to 

 specify either the conditions of manufacture or the properties of 

 a cement having the greatest possible value; and, even if other- 

 wise, manufacturing conditions, the nature of the available raw 

 materials, and the bulkiness of the product are such that it would 

 be impracticable to demand an ideal product. On the other hand, 

 our present knowledge is sufficient to enable us to specify and 

 identify the characteristics of Portland cements which give the 

 greatest efficiency which modern improvements for burning and 

 grinding have made practicable, to promote and support the 

 best manufacturing practices, and, accordingly, to secure Port- 

 land cement having the greatest uniformity and efficiency con- 

 sistent with the principles of true economy. 



The efficiency of Portland cement depends primarily upon the 

 thorough sintering or fusing of the raw materials, and for 

 best results it is very essential to burn at a high temperature 

 and eliminate free lime. The strongest cements require the 

 highest burning temperatures because the formation of the high- 

 calcium silicates and the low-calcium aluminates requires greater 

 heat than is necessary to form the much weaker low-limed 

 silicates and high-limed aluminates. 5 The effects of the different 



'Eng. News (1909), 62, 105, 179, 230, 358. 



6 Schott, Cement & Eng. News (1910), 22, Nos. 9 to 12. 



