THE GEOLOGY OF CEMENTS 187 
from the London clay septaria, and was called Roman cement 
from the erroneous tradition that the Romans made cement 
from them. They consist of nodules of calcareous clay 
traversed by septa of calcite; their usual composition is 
about silica 18 per cent., alumina 3 to 5 per cent., iron oxide, 
5 per cent., lime 30 per cent., and carbon dioxide 31 per cent. 
Hydraulic cement passes by the reduction in uncombined 
lime into Portland cement, which is made from a mixture 
of finely ground limestone and clay; the mixture is heated to 
a little below its fusion point—usually to between 2600° and 
3000° F.—when the materials by diffusion form new com- 
pounds. As the most important constituent in Portland 
cement is the tricalcic silicate, 3Ca0, SiO,, its ideal composi- 
tion is lime 73-6 per cent., silica 26:4 per cent. The constitu- 
ents have to be mixed in precise proportions, and therefore 
soft pure materials are desirable, such as chalk and river clay. 
The third group, including the Roman Pozzolana cement, 
are made from volcanic tuff such as the Trass of the Rhine. 
The glass in these tuffs is unstable and combines with lime 
to form silicates of lime and alumina without the use of 
heat. Blast furnace slag, which is also a silicate glass, and 
various organic materials, such as diatom earth, which consist 
of unstable amorphous silica, make similar cement when 
ground with lime. 
The three types of hydraulic cement depend upon analo- 
gous chemical processes. The modern interpretation of the 
constitution of Portland cement was founded by H. Le 
Chatelier (1883, etc, and his Constitution of Hydraulic 
Mortars, New York, 1905). He regarded Portland cement 
as consisting of crystals of tricalcic silicate in a crystalline 
ground mass. Later researches show that the constitution 
is more complex, and is dependent upon the interaction of 
several lime silicates and lime aluminates, each of which 
has a definite composition, specific gravity, and optical 
properties. These artificial mineral species at a high tem- 
perature, below that of fusion, combine in solid solution 
and form what Tornebohm called alite and celite. Alite 
is a solid solution of tricalcic aluminate (3Ca0, AlOg) in 
tricalcic silicate (3Ca0, Si0,), and celite of dicalcic aluminate 
(2Ca0, Al, Oy) in dicalcic silicate (2Ca0, SiO). 
According to. Cl. Richardson (Eng. News., liii, 1905,