Poz′zu-o-la′na.
A volcanic earth composed mainly of silica and alumina, and shipped from Pozzuoli.
A portion analyzed by
M. Berthier was constituted as follows:---
| Silica. | 44.5 |
| Alumina. | 15 |
| Lime. | 8.8 |
| Magnesia. | 4.7 |
| Oxide of iron. | 12 |
| Soda | 4 |
| Potash | 1.4 |
| Water | 9.2 |
| — |
| 99.6 |
Strabo says of this earth:---
“Puteoli has become a place of extensive commerce, having artificially constructed harbors, which were much facilitated by the facile nature of the sand [degraded tufa], which contains much gypsum, and will cement and consolidate thoroughly.
For mixing this sand with limestones they construct moles in the sea, thus forming bays along the open coast in which the largest transport-ships may safely ride.”
It was obtained by the Romans near
Baiae, and is still exported largely from that part of
Italy.
Mixed with lime and water, it hardens anywhere, — in the air or under water.
It is invaluable, and the permanency of the great sea-walls and other maritime
[
1782]
structures of the Romans is largely due to its use. It is commonly known as
Roman cement.
Many attempts have been made to form an artificial pozzuolana by combinations of sand, clay, lime, and iron.
Ochreous clays are especially suitable.
General Treussart's system for making artificial pozzuolana is to reduce soft red bricks to powder, and mix them (2 parts) with lime that has been some time slaked (1 part); water to incorporate.
See cement.
The preparation of the natural material is by pounding or grinding and sifting, whereby it is reduced to powder, in which state it is beaten to a proper consistency with a due proportion of lime.
Artificial pozzuolana is produced by pulverizing the clay, the psammite, or the arene, which is wetted, and then strewing a layer of it, about four tenths of an inch, on a plate of iron heated to a point between a cherry-red and forging-heat.
It is left till it be raised to the same degree, for a space of time which varies for each kind of material, from five to twenty minutes. It must be continually stirred with a small rod, in order that the whole of the particles may be uniformly calcined.
Hydraulic limes which harden under water are dependent for their quality upon the presence of a certain amount of clay.
In
France the manufacture of artificial hydraulic lime is largely carried on. See lime-kiln.
