PART IV. MATERIALS AND MANUFACTURE OF PUZZOLAN 
CEMENTS. 
PUZZOLANIC MATERIALS. 
Puzzolanic materials include all those natural or artificial substances 
hat are capable of forming hydraulic cements on being simply mixed 
vith lime, without the use of heat. Many materials possess this prop- 
erty, but relatively few have ever attained sufficient commercial 
mportance to be discussed here. In composition the puzzolanic 
materials are largely made up of silica and alumina, usually with more 
>r less iron oxide; some, as the slags used in cement manufacture, carry 
so notable percentages of lime. As might be inferred from this com- 
>osition, most of the puzzolanic materials possess hydraulicity to a 
reater or less degree, but the addition of lime usually greatly increases 
heir hydraulic power. 
NATURAL PUZZOLANIC MATERIALS. 
Natural puzzolanic materials are widely distributed, though they 
lave never attained much commercial importance save in Europe, 
hey ma} 7 be divided into two classes, according to origin. In the first 
slass may be included all those which are the direct products of vol- 
:anic action, the material being a fine volcanic ash or dust deposited 
dther on the slopes of the volcano or carried by the wind to lakes or 
itreams in which the ash is deposited. This group includes the more 
ictive puzzolanic materials, its chief 1 epresentatives being pozzuolana 
)roper, santorin, tosca, tetin, and trass. It may be noted that in 
>rigin materials of this class resemble closely the granulated slags used 
n slag-cement manufacture, both volcanic ashes and granulated slags 
)eing due to the two processes of (1) fusion of a silico-aluminous 
nate rial, and (2) rapid cooling of the resulting product by ejection 
nto air or immersion in water. The second class includes a number 
)f less important (because less active) hydraulic materials, such as 
irenes, psammites, etc., which are materials resulting from the decay 
)f certain igneous rocks. 
Pozzuolana derives its names from the little town of Pozzuoli, located 
i few miles west of Maples, at which point the material was first 
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