CEMENT MATERIALS. 
73 
The almost infinite number of raw materials which are theoretically available are, how- 
ever, reduced to a very few in practice under existing commercial conditions. The necessitv 
for making the mixture as cheaply as possible rules out of consideration a large number of 
materials which would be considered available if chemical composition was the only thing to 
be taken into account. Some materials otherwise suitable are too scarce; some are too 
difficult to pulverize. In consequence, a comparatively few combinations of raw materials 
are actually used in practice. 
In certain localities deposits of argillaceous (clayey) limestone or "cement rock" occur, in 
which the lime, silica, alumina, and iron oxide exist in so nearly the proper proportions that 
only a relatively small amount (say 10 per cent or so) of other material is required in order 
to make a mixture of correct composition. 
In the majority of plants, however, most or all of the necessary lime is furnished by one 
raw material, while silica, alumina, and iron oxide are largely or entirely derived from an- 
other raw material. The raw material which furnishes the lime is limestone, chalk, or marl, 
while the silica, alumina, and iron oxide of the mixture are derived from clay, shale, or slate. 
Determining the possible value for Portland cement manufacture of a deposit of raw mate- 
rial is a complex problem, depending on a number of distinct factors, all of which must be 
given due consideration. The more important of the factors are: 
1. Chemical composition of the material. 
2. Physical character of the material. 
3. Amount of material available. 
4. Location of the deposit with respect to transportation routes. 
5. Location of the deposit with relation to fuel supplies. 
6. Location of the deposit with respect to markets. 
Ignorance of the respective importance of these factors frequently leads to an overestimate 
of the value of a deposit of raw material. Their effects may be briefly stated as follows: 
CHEMICAL COMPOSITION. 
The raw material must be of correct chemical composition for use as a cement material. 
This implies that the material, if a limestone, must contain as small a percentage as possible 
of magnesium carbonate. Under the present conditions 5 or 6 per cent is the maximum per- 
missible. Free silica, in the form of chert, flint, or sand must be absent, or present only in 
small quantities, say 1 per cent or less. If the limestone is a clayey limestone or "cement 
rock," the proportion between its silica and its alumina and iron should fall within the limits: 
SiO, 
2 and 
SiO, 
3.5 
A clay or shale should satisfy the above requirement and should be free from sand, gravel, 
stc. Alkalies and sulphates should, if present, not exceed 3 per cent or so. 
The nearer a limestone approaches in composition to the mixture used in Portland cement 
manufacture the greater its value for that purpose, for it will require the addition of less 
extraneous material to make the mixture absolutely correct in composition. The following 
ire analyses of Portland cement mixtures, ready for burning, as used at various lar^e cement 
plants in the United States: 
Analyses of Portland cement mixtures. 
1. 
2. 
3. 
4. 
Silica (Si0 2 ) 
12.85 
4.92 
1.21 
76.36 
2.13 
12.92 
4.83 
1.77 
75. 53 
4.34 
13. 52 
6.56 
75.13 
4.32 
14.94 
Alumina ( AI2O3) 
2.66 
Iron oxide (Fe 2 3 ) 
1.10 
Lime carbonate (CaCOs) ... 
75.59 
Magnesium carbonate (MgCC>3) 
4.64 
