They are mostly of volcanic origin, and include pumice, tufa, santorin earth, trass and pozzuolana itself.
They are not usually manufactured by the careful grinding together of the pozzuolana and the lime, but are mixed roughly, a great excess of pozzuolana being employed.
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.