STATE GEOLOGIST: 23 
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CHAPTER VI. 
THE COMPOUNDING OF PORTLAND CEMENT MIXTURES. 
In the selection of the raw materials the following factors must be 
considered : 
| THE CLAYS, 
1. The clay must have a percentage ratio of silica to alumina of from 
3:1 to 4:1. A clay should be found, if possible, which is naturally fine; 
but if such a material cannot be found, either a coarse clay of the proper 
composition must be selected and ground fine, or an addition of ground 
sand or sandstone must be given a more aluminous but otherwise suitable 
clay. 
In examining many Portland cements this point seems to be the chief 
difficulty with a number of brands. Owing to the high content of alumina, 
the cement not only sets too fast, but also shows an inherent weakness. 
Matters are made still worse by the addition of gypsum, which is a 
grave source of deleterious influences. At the same time, the clay 
should not be too silicious, but should le between the above limits. The 
sum of the silica and alumina percentages in the hydrous clay should 
not exceed 87 per cent., thus leaving at least 13 per cent. for combined 
water and fluxes like ferric oxide, potash and soda. 
2. The clay must not contain so much magnesia that the magnesia 
content of the cement rises above 3 per cent. 
3. The clay should contain at least 3 per cent of ferric oxide, but 
not sufficient to raise the ferric oxide content of the cement over 4 per 
cent. 
4. The clay should be low in sulphur and should not contain more 
than 1 per cent. of sulphur in any form. 
5. Lhe material should be free from any irregularly distributed 
matter, be it concretionary nodules of ferrous carbonate, calcium carbonate 
as found in the older sedimentary clays, or lumps of these or similar 
minerals as found in glacial clays. 
Examples of how these considerations are neglected might be cited 
in considerable number, it having been the writer’s task to examine 
numerous Portland cements. In one case a new plant used a clay of the 
following composition for its clav base: | - 
