STATE GEOLOGIST. 25 
GHAPTER I: 
GENERAL CONSIDERATIONS ON THE HYDRAULIC 
CEMENTS. 
The term cement in its present meaning stands for a mortar which 
hardens in water as well as in air, in distinction from the air. mortars, 
the ordinary lime mortar, plaster and the so-called Sorel cements. Based 
on this definition, we know four distinct kinds of hydraulic materials which 
form the basis of all hydraulic mortars. ‘These are: 
tr. Pozzuolane cements. 
2. Hydraulic limes. 
By Noman cement. 
4. Portland cement. 
All harden by a process of hydration. They all require water for the 
initial hardening. 
1. Pozzuolane cements are the oldest known hydraulic mortars, hav- 
ing been used by the ancients most extensively, especially by the Romans. 
The Pozzuolane of the ancients, pulvis puteolis, consists of a volcanic 
tufa, of a porous, open-grained structure which, with the slaked lime, pro- 
duced the wonderfully durable cement used by the Romans for their 
gigantic aqueducts and hydraulic work of all sorts. Alone, the pozzuolane 
materials do not produce a cement, the presence of the slaked lime being 
invariably necessary. ‘The activity of the volcanic materials depends on 
the presence of soluble silicic acid, or hydrous silicic acid, which readily 
enters into combination with the lime hydrate. [or this reason any ma- 
terial furnishing silicic acid readily available for reaction with a strong 
base like lime may be used for this purpose. Even dehydrated silicate 
of alumina, or lightly burnt clay, will react sufficiently to form with lime 
hydrate a cement, which has been applied practically by mixing brick dust 
with slaked lime, resulting in a kind of a hydraulic mortar. 
A modern kind of artificial pozzuolane has found extensive applica- 
tion, namely, slag cement, which is made from granulated blast furnace 
slag ground together with dried quicklime or lime hydrate. Blast furnace 
slag cooled slowly represents an artificial basic rock with but slight hy- 
draulic properties. When granulated, or cooled quickly from the liquid 
condition by means of cold water, it possesses the characteristics of the 
true natural pozzuolane, and has considerable “soluble” or available silicic 
acid, and when intimately mixed with lime hydrate gives rise to a strong 
hydraulic cement. 
