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.
Were hydraulic; it was also known that this property could be conferred on ordinary lime by admixture of silicious materials such as pozzuolana or t.ufa.
They are mostly of volcanic origin, and include pumice, tufa, santorin earth, trass and pozzuolana itself.
The following analyses show their general composition: An artificial product which serves perfectly as a pozzuolana is granulated blast-furnace slag.
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.