Pozzuolana is a name given to several substances which somewhat resemble each other; including the Pozzuolana proper, also Trass, Arenes, Psammites, etc.
In consequence of the amount of clay they contain they confer hydraulic properties upon the lime to a very considerable degree.
The Italian pozzuolana may with advantage be used with fat lime and sand in the following proportions : - 2
12 Pozzuolana well pulverised. 6 Quartzose sand well washed. 9 Rich lime recently slaked.
Natural Pozzuolana is a naturally-burnt earth of volcanic origin, found at Pozzuoli, near Vesuvius, and in other parts of southern Europe.
Trass is also a naturally-burnt argillaceous earth, found on the sites of extinct volcanoes, chiefly near Andernach on the Rhine.
It occurs in lumps of a greyish colour and earthy appearance, is used in the same way as pozzuolana, and confers hydraulic properties upon fat limes.
Arenes are natural mixtures of sand and clay. They appear not to have been subjected to heat, but they confer hydraulic properties upon fat lime, probably because they contain a large proportion of soluble silica.
Psammites may be considered as "very feeble pozzuolanas in the crude state, and acquire but a slight increase of hydraulic energy by any degree of calcination.
"Even their feeble powers, however, confer upon them this advantage, that for mortars not absolutely immersed in water when green, and when there is ample time for their properties to develop themselves before submersion, they can be employed in larger proportions than any species of sand wholly inert would admit of." 2
Artificial Pozzuolanas are prepared from clays of suitable composition by a slight calcination.
"Briquettes made from crushed syenite from which the impalpable powder had been removed were 18 per cent stronger than those in which it had not been removed." - M.P.I.C.E., vol. lxxxvii. p. 203.
1 Ferric oxide. 2 Gillmore.