Use of zeolitic rocks in ceramic tile production

Neapolitan yellow tuff is a pyroclastic rock mainly consisting of zeolites (phillipsite, analcime, chabazite) with a chemical composition similar to many quartz-feldspathic fluxes currently used in tile making. It is characterised by remarkable fusibility and cation exchange capacity, and by a low cost. This zeolite was investigated, on a laboratory scale, to produce dry-pressed, single-fired stoneware tiles. The replacement of potassic feldspar with zeolite tuff in stoneware bodies (up to 15 wt-%) slightly affected the technological properties of the product (slight increase of shrinkage and/or water absorption). On the other hand, the presence of zeolite improved the grindability and the rheological behaviour of the bodies during the wet grinding and spray drying processes. This would make it possible to lower grinding times and water content of the slips. The main limit for the use of neapolitan yellow tuff seems to be its significant content of iron oxide (about 4 wt-%).