26 ANNUAL REPORT 
2. The hydraulic limes may be considered as being simply impure 
limestones, consisting of carbonate of lime intimately blended by nature 
with clay and some sand. The amount of carbonate of calcium and 
magnesium 1s invariably more than 70 per cent. When burnt and made 
up with water, they slake and gradually harden to a compact mass which 
attains maximum hardness in water. Owing to their self-slaking, pul- 
verization 1s unnecessary. 
3. Roman cements are likewise naturally blended mixtures of calcium 
and magnesium carbonates, and clay or earthy matter. The carbonates 
compose from 50 to 70 per cent. of the mass. On burning they form a 
porous and friable mass which, however, does not slake in water in the 
lumpy condition, but must be ground to a powder, which is readily 
accomplished by means of cheap machinery. The clay content of Roman 
cement is greater than that of hydraulic limes, the percentage being be- 
tween 50 and 30 per cent. This limit of composition and the fact that it 
must be ground distinguishes it clearly from the hydraulic limes. The 
strength of Roman cement is greater than that of hydraulic lime, though 
inferior to that of the next higher series, the Portland cements. 
4. Highest in the scale of usefulness we find the Portland cements, 
which, unlike the preceding, are purely artificial materials. In chemical 
composition they range between 59 and 65 per cent. of calcium oxide, the 
remainder consisting of the ordinary constitutents of clay, viz., silica, 
alumina and iron. 
In the artificial preparation of this cement the constituents, lime- 
carrying material of any kind whatever and clay, are mixed and ground 
together most intimately and burned to a high temperature so that vitrifica- 
tion ensues. The resulting mass is, as a rule, of a dark color, dense and 
hard. On grinding it to a powder it shows striking hydraulic properties, 
exhibiting a high tensile and crushing strength and a high cementing 
power. Both the intimate grinding of the raw materials and the pulveriz- 
ing of the hard vitrified cement (clinker) stand for a considerable expendi- 
ture of power, while the high temperature to be reached in burning means 
a large consumption of fuel. It is natural, hence, that this cement should 
be a higher priced mortar material, whose greater cost of production 1s, 
however, balanced by the increased strength, uniformity and regularity 
of its behavior in use. 
In these four classes of hydraulic materials we have thus a chain 
reaching from the raw mixture of hydrous silicious rocks with hydrated 
lime to the highly calcareous silicates formed at a high heat, approaching 
igneous fusion, as in Portland cement. From the low tensile strength 
and low specific gravity of the pozzuolane cement we rise, through the 
hydraulic lime and Roman cement, to the great strength and high specific 
gravity of the Portland cement. 
In the pozzuolane class the reaction is brought about at the ordinary 
temperature, and the weakly acid character of hydrous silicic acid is made 
use of, while in the burning of the Roman cement the strong acidity of 
anhydrous silicic acid at higher temperatures is made evident, resulting 
