POZZUOLANO. A material used by the ancient Roman, and even by modern, hydraulic engineers for the purpose of preparing an artificial cement for works constructed In the sea. This material was originally obtained from the volcanic formations of the Bay of Puteoh, near Naples (from which circumstance the name itself is derived); but similar substances have been discovered in different parts of Italy and in other countries, and have been applied in the arts with equal success. Iii fact the pozzuolano is a dehydrized silicate of alumina, In which the silica exists in a state easily attackable by caustic alkalies, and Is therefore capable at once of entering into a combination with the lime presented to it In the preparation of mortars, forming with it a double silicate 'of lime and alumina of an Insolnble character ; and substances of this description are very commonly met with in volcanic) districts, wherein lava floods of a basaltic character have been poured out upon argillaceous beds. Thus in the environs of Rome itself the celebrated Catacombs were excavated in a large deposit of this argillaceous sand ; in the island of Sardinia ; in the south of France; in the north of Germany, near Andernach ; and no doubt in countless other localities, the pozzuolano has been found under favourable economical conditions. Tho German material of this description is known in commerce under the name of Cress, or Dutch terms.
Both the tress and the pozzuolano are used with the most favourable results when mixed with the purest and richest hydrates of lime ; but the success of their application depends entirely upon the intimate nature of the mixture of the various Ingredients, and upon the perfec tion of the hydration. These conditions can only be secured by the
most perfect manipulation; and that again by the low price of manual labour ; and it Is on this account that the use of the natural cements, or of the artificial hydraulic cements, has been latterly introduced in most cases instead of that of the pozzuolano mortars which Smeaton and the earlier engineers so strongly recommended. But it is essential to remark (as was indeed before stated under lloterans) that there are some conditions with respect to the state of the silicates of alumina entering into the composition of artificial cements, which are essential for their success ; and, as far as our present means of analysis allow us to form an opinion on the subject, it would seem that these conditions are the most satisfactorily fulfilled when the combinations between the silica, the lime, and the alumina, are effected under great heat. This is actually the case with the Portland cements ; and the very different results obtained by the mixture of the volcanically dehydrized silicates of alumina from those obtained by the mixture of underburut brick earths, their chemical equivalents in nearly all respects, tend to confirm this opinion. It may be added that in Holland, Northern Germany, and in Italy, the pozzuolano class of materials is still used ; but that in England they have so entirely been laid aside of late years as not easily to be obtained in the ordinary market.
Berthier gives the following analysis of the tress of Andenetch and of the pozzuolano of Civita Vecchia :