STATE GEOLOGIST. 159 
As has already been indicated, the importance of natural pozzuolanes 
consists principally in the fact that when added to Portland cement they 
improve the resisting quality of the latter, especially against the action 
of sea water; but pozzuolane mortar alone, owing to its porosity, 1s poor- 
ly suited for sea water work, and could not be used with any degree of 
success. ‘The experiments in regard to the benefit of pozzuolane addi- 
tions are, however, by no means completed. 
In some districts along the Rhine and in many localities in Italy 
the pozzuolanes are simply screened and made up with lime like ordinary 
sand, this method being quite satisfactory for ordinary work. In the 
building of Austrian railways the proportion of 1 part lime to 3 parts 
_ pozzuolane was found to be the best. 
SLAG CEMENT. 
General Considerations. —When fused basic blast furnace slag of 
the composition already indicated in the chapter on “Raw Materials” is 
suddenly quenched in water, the rate of cooling is, of course, the most 
rapid possible. If, as is generally assumed, slow cooling is associated 
with the breaking down or dissociation of complex mineral compounds 
into simpler ones it is evident that the quenched slag is not given a 
chance to do this. It must, of necessity, remain the complex glass it 
was when in fusion. This granulated slag on being mixed with lime 
produces, as we know, a hydraulic cement of considerable strength. Yet 
the same slag cooled slowly shows but little hydraulic strength. We are 
evidently in this instance dealing with a far-reaching molecular change. 
Work of Zulkowski.—The first to realize that he was dealing here 
with isomeric compounds and to scientifically investigate these was Pro- 
fessor Zulkowski, of the University of Prague.* Though his work has 
been considerably criticised, it must be acknowledged that it brings out 
fundamental principles which cannot be ignored. They bear important 
relations to the general question of hydraulicity whether referring to slag 
or to Portland cement. 
Glass, an amorphous mixture of silicates, or a solution, as we may 
call it, when heated for some time at a low temperature, will be found to 
crystallize, as we can observe when the glass combustion-tube used in 
chemical work assumes a porcelain-like appearance after long use. On 
examination these crystals have been found to be the simple mono- 
calcium silicate, wollastonite. All slags, whether basic or acid, may 
likewise be called glasses with the same tendency to crystallize on slow 
cooling. In recent experiments** definite minerals have been recognized 
as uniformly appearing in all basic slags, some of them predominating in 
strongly hydraulic and others in feebly hydraulic slag. And it is of 
*Die Chemische Industrie, 1889, p. 69. 
** Dr. Passow, Stahl und Hisen, Aug. Ist, 1908. 
