PHYSICAL AND CHEMICAL PROPERTIES. PARTS IV, V. 185 
(c) Since with the silicates the strength increases as the lime 
increases while with the aluminates the opposite is true, 
and since the high silicates and the low aluminates require 
the greater heat, it follows that the strongest cements 
i require the highest burning temperatures. 
(d) The differences in the physical properties of the various 
calcium compounds account for the failure of ultimate 
chemical analyses to reveal the true nature of commercial 
cements. 
(e) Combined magnesia like combined lime has no injurious 
effect in cement. 
(f) The hardening process of hydraulic calcium compounds is 
to a large extent not limited to exposed surfaces, but takes 
place throughout the mass, which is one of the reasons 
that good Portland cements give more constant and 
reliable results than other calcareous cements. 
(14) The effects of the condition of free and combined lime 
in hydraulic cements: 
(a) From the natural pozzuolane cements, with their low gravity and 
slow, unreliable hardening properties, there is a rise in efficiency 
and usefulness through hydraulic lime, slag, and natural 
cements to the great and reliable strength of good Portland 
cement, solely by reason of the condition of the free and com- 
bined lime content. 
(6) Overburned natural cement is essentially underburned Portland 
cement, and the evil effects resulting from either process are 
due to the production of a maximum amount of slow slaking, 
sintered free lime, and free magnesia. 
(c) A more efficient natural (or Roman) cement than that produced at 
present by burning cement rock in set-kilns could be manufac- 
tured by blending clay and limestones in proper proportion and 
then burning the mixture at a slow temperature in a rotary kiln. 
(d) Rapid cooling is an essential to the efficiency of slag cement. 
(e) Rapid cooling is not essential to the preservation of high limed 
silicates and low limed aluminates in Portland cement. 
(f) Owing to the unequal degree of burning to which the raw material 
in different parts of the set kiln is subjected and to fuel con- 
tamination, the aggregate produced by this process usually 
consists of a mixture of all five classes of hydraulic cements in 
which the well-sintered Portland cement clinker, typical of 
good rotary practise, predominates. 
(g) The characteristic low early strength of set-kiln cement is due to 
the presence of the underburned material and not to the slow 
cooling of the clinker. 
(hk) Portland cements so bad as utterly to disintegrate when sub- 
jected to the normal tests for soundness need no further con- 
sideration until additional seasoning enables them to remain 
sound in cold water or air. 
