When lime is used as a matrix, certain natural earths such as pozzuolana or trass, or, failing these, powdered bricks or tiles, may be used instead of sand with great advantage.
They are mostly of volcanic origin, and include pumice, tufa, santorin earth, trass and pozzuolana itself.
Generally speaking, they are only of local importance, their cheapness depending largely on the nearness and abundance of some suitable volcanic deposit of the trass or tufa class.
It is evident that this will form a pozzuolanic cement if a suitable silicious material such as trass is added to the cement.
The use of trass in this manner as an adjunct to Portland cement has been advocated by W.
Ordinary mortar failed in such positions; mortar made from lime prepared from limestones or chalks containing a little clay was found to stand; mortar made from lime mixed with trass or similar active silicious material was also found to stand.